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Sample records for few-layer epitaxial graphene

  1. Broadband electromagnetic response and ultrafast dynamics of few-layer epitaxial graphene

    SciTech Connect

    Choi, Hyunyong; Borondics, Ferenc; Siegel, David A.; Zhou, Shuyun Y.; Martin, Michael C.; Lanzara, Alessandra; Kaindl, Robert A.

    2009-03-26

    We study the broadband optical conductivity and ultrafast carrier dynamics of epitaxial graphene in the few-layer limit. Equilibrium spectra of nominally buffer, monolayer, and multilayer graphene exhibit significant terahertz and near-infrared absorption, consistent with a model of intra- and interband transitions in a dense Dirac electron plasma. Non-equilibrium terahertz transmission changes after photoexcitation are shown to be dominated by excess hole carriers, with a 1.2-ps mono-exponential decay that refects the minority-carrier recombination time.

  2. Electroburning of few-layer graphene flakes, epitaxial graphene, and turbostratic graphene discs in air and under vacuum

    PubMed Central

    Richter, Nils; Convertino, Domenica; Coletti, Camilla; Balestro, Franck; Wernsdorfer, Wolfgang; Kläui, Mathias; Affronte, Marco

    2015-01-01

    Summary Graphene-based electrodes are very promising for molecular electronics and spintronics. Here we report a systematic characterization of the electroburning (EB) process, leading to the formation of nanometer-spaced gaps, on different types of few-layer graphene (namely mechanically exfoliated graphene on SiO2, graphene epitaxially grown on the C-face of SiC and turbostratic graphene discs deposited on SiO2) under air and vacuum conditions. The EB process is found to depend on both the graphene type and on the ambient conditions. For the mechanically exfoliated graphene, performing EB under vacuum leads to a higher yield of nanometer-gap formation than working in air. Conversely, for graphene on SiC the EB process is not successful under vacuum. Finally, the EB is possible with turbostratic graphene discs only after the creation of a constriction in the sample using lithographic patterning. PMID:25821711

  3. Vertically aligned GaAs nanowires on graphite and few-layer graphene: generic model and epitaxial growth.

    PubMed

    Munshi, A Mazid; Dheeraj, Dasa L; Fauske, Vidar T; Kim, Dong-Chul; van Helvoort, Antonius T J; Fimland, Bjørn-Ove; Weman, Helge

    2012-09-12

    By utilizing the reduced contact area of nanowires, we show that epitaxial growth of a broad range of semiconductors on graphene can in principle be achieved. A generic atomic model is presented which describes the epitaxial growth configurations applicable to all conventional semiconductor materials. The model is experimentally verified by demonstrating the growth of vertically aligned GaAs nanowires on graphite and few-layer graphene by the self-catalyzed vapor-liquid-solid technique using molecular beam epitaxy. A two-temperature growth strategy was used to increase the nanowire density. Due to the self-catalyzed growth technique used, the nanowires were found to have a regular hexagonal cross-sectional shape, and are uniform in length and diameter. Electron microscopy studies reveal an epitaxial relationship of the grown nanowires with the underlying graphitic substrates. Two relative orientations of the nanowire side-facets were observed, which is well explained by the proposed atomic model. A prototype of a single GaAs nanowire photodetector demonstrates a high-quality material. With GaAs being a model system, as well as a very useful material for various optoelectronic applications, we anticipate this particular GaAs nanowire/graphene hybrid to be promising for flexible and low-cost solar cells. PMID:22889019

  4. Symmetry Breaking in Few Layer Graphene Films

    SciTech Connect

    Bostwick, A.; Ohta, T.; McChesney, J.L.; Emtsev, K.; Seyller,Th.; Horn, K.; Rotenberg, E.

    2007-05-25

    Recently, it was demonstrated that the quasiparticledynamics, the layer-dependent charge and potential, and the c-axisscreening coefficient could be extracted from measurements of thespectral function of few layer graphene films grown epitaxially on SiCusing angle-resolved photoemission spectroscopy (ARPES). In this articlewe review these findings, and present detailed methodology for extractingsuch parameters from ARPES. We also present detailed arguments againstthe possibility of an energy gap at the Dirac crossing ED.

  5. Electrical breakdown of graphene and few-layer graphene structures

    NASA Astrophysics Data System (ADS)

    Sundararajan, Abhishek; Hunley, D. Patrick; Strachan, Douglas. R.

    2012-02-01

    The electrical breakdown of graphene and few-layer graphene (FLG) structures are investigated. To better understand the dynamics of these nano-scale thermal effects, we investigate graphene and FLG structures of various dimensions and find that significant joule heating occurs inducing the structures to evolve. A distinct change in the behavior during electrical stressing indicates that different mechanisms and geometrical effects occur at the various stages of evolution. The results could have implications on the development of high current carrying nanoscale graphene devices. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program through award EPS-0814194, and the University of Kentucky Center for Advanced Materials.

  6. Electrical breakdown of graphene and few-layer graphene structures

    NASA Astrophysics Data System (ADS)

    Sundararajan, Abhishek; Johnson, Stephen; Hunley, D. Patrick; Flores, Roel; Johnson, A. T. Charlie; Strachan, Douglas

    2011-03-01

    The electrical breakdown of graphene and few-layer graphene (FLG) structures are investigated. To better understand the dynamics of these nano-scale thermal effects, we investigate graphene and FLG nanowires of various dimensions and find that significant joule heating occurs inducing the structures to evolve. A distinct change in the behavior during electrical stressing indicates that different mechanisms occur at the various stages of evolution. The results are compared to detailed thermal modeling of our structures and could have implications on the development of high current carrying nanoscale graphene devices. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering.

  7. Ultrahigh conductivity of large area suspended few layer graphene films

    NASA Astrophysics Data System (ADS)

    Rouhi, Nima; Wang, Yung Yu; Burke, Peter J.

    2012-12-01

    Room-temperature (atmospheric-pressure) electrical conductivity measurements of wafer-scale, large-area suspended (few layer) graphene membranes with areas up to 1000 μm2 (30 μm × 30 μm) are presented. Multiple devices on one wafer can be fabricated with high yield from the same chemical vapor deposition grown graphene sheet, transferred from a nickel growth substrate to large opening in a suspended silicon nitride support membrane. This represents areas two to orders of magnitude larger than prior transport studies on any suspended graphene device (single or few layer). We find a sheet conductivity of ˜2500 e2/h (or about 10 Ω/sq) of the suspended graphene, which is an order of magnitude higher than any previously reported sheet conductance of few layer graphene.

  8. Chemical storage of hydrogen in few-layer graphene.

    PubMed

    Subrahmanyam, K S; Kumar, Prashant; Maitra, Urmimala; Govindaraj, A; Hembram, K P S S; Waghmare, Umesh V; Rao, C N R

    2011-02-15

    Birch reduction of few-layer graphene samples gives rise to hydrogenated samples containing up to 5 wt % of hydrogen. Spectroscopic studies reveal the presence of sp(3) C-H bonds in the hydrogenated graphenes. They, however, decompose readily on heating to 500 °C or on irradiation with UV or laser radiation releasing all the hydrogen, thereby demonstrating the possible use of few-layer graphene for chemical storage of hydrogen. First-principles calculations throw light on the mechanism of dehydrogenation that appears to involve a significant reconstruction and relaxation of the lattice. PMID:21282617

  9. Chemical storage of hydrogen in few-layer graphene

    PubMed Central

    Subrahmanyam, K. S.; Kumar, Prashant; Maitra, Urmimala; Govindaraj, A.; Hembram, K. P. S. S.; Waghmare, Umesh V.; Rao, C. N. R.

    2011-01-01

    Birch reduction of few-layer graphene samples gives rise to hydrogenated samples containing up to 5 wt % of hydrogen. Spectroscopic studies reveal the presence of sp3 C-H bonds in the hydrogenated graphenes. They, however, decompose readily on heating to 500 °C or on irradiation with UV or laser radiation releasing all the hydrogen, thereby demonstrating the possible use of few-layer graphene for chemical storage of hydrogen. First-principles calculations throw light on the mechanism of dehydrogenation that appears to involve a significant reconstruction and relaxation of the lattice. PMID:21282617

  10. Nonlocal thermal transport across embedded few-layer graphene sheets

    DOE PAGESBeta

    Liu, Ying; Huxtable, Scott T.; Yang, Bao; Sumpter, Bobby G.; Qiao, Rui

    2014-11-13

    Thermal transport across the interfaces between few-layer graphene sheets and soft materials exhibits intriguing anomalies when interpreted using the classical Kapitza model, e.g., the conductance of the same interface differs greatly for different modes of interfacial thermal transport. Using atomistic simulations, we show that such thermal transport follows a nonlocal flux-temperature drop constitutive law and is characterized jointly by a quasi-local conductance and a nonlocal conductance instead of the classical Kapitza conductance. Lastly, the nonlocal model enables rationalization of many anomalies of the thermal transport across embedded few-layer graphene sheets and should be used in studies of interfacial thermal transportmore » involving few-layer graphene sheets or other ultra-thin layered materials.« less

  11. Nonlocal thermal transport across embedded few-layer graphene sheets

    SciTech Connect

    Liu, Ying; Huxtable, Scott T.; Yang, Bao; Sumpter, Bobby G.; Qiao, Rui

    2014-11-13

    Thermal transport across the interfaces between few-layer graphene sheets and soft materials exhibits intriguing anomalies when interpreted using the classical Kapitza model, e.g., the conductance of the same interface differs greatly for different modes of interfacial thermal transport. Using atomistic simulations, we show that such thermal transport follows a nonlocal flux-temperature drop constitutive law and is characterized jointly by a quasi-local conductance and a nonlocal conductance instead of the classical Kapitza conductance. Lastly, the nonlocal model enables rationalization of many anomalies of the thermal transport across embedded few-layer graphene sheets and should be used in studies of interfacial thermal transport involving few-layer graphene sheets or other ultra-thin layered materials.

  12. Characterization of few-layered graphene grown by carbon implantation

    SciTech Connect

    Lee, Kin Kiong; McCallum, Jeffrey C.; Jamieson, David N.

    2014-02-21

    Graphene is considered to be a very promising material for applications in nanotechnology. The properties of graphene are strongly dependent on defects that occur during growth and processing. These defects can be either detrimental or beneficial to device performance depending on defect type, location and device application. Here we present experimental results on formation of few-layered graphene by carbon ion implantation into nickel films and characteristics of graphene devices formed by graphene transfer and lithographic patterning. Micro-Raman spectroscopy was used to determine the number of graphene layers formed and identify defects arising from the device processing. The graphene films were cleaned by annealing in vacuum. Transport properties of cleaned graphene films were investigated by fabrication of back-gated field-effect transistors, which exhibited high hole and electron mobility of 1935 and 1905 cm2/Vs, respectively.

  13. Interfacial Atomic Structure of Twisted Few-Layer Graphene

    PubMed Central

    Ishikawa, Ryo; Lugg, Nathan R.; Inoue, Kazutoshi; Sawada, Hidetaka; Taniguchi, Takashi; Shibata, Naoya; Ikuhara, Yuichi

    2016-01-01

    A twist in bi- or few-layer graphene breaks the local symmetry, introducing a number of intriguing physical properties such as opening new bandgaps. Therefore, determining the twisted atomic structure is critical to understanding and controlling the functional properties of graphene. Combining low-angle annular dark-field electron microscopy with image simulations, we directly determine the atomic structure of twisted few-layer graphene in terms of a moiré superstructure which is parameterized by a single twist angle and lattice constant. This method is shown to be a powerful tool for accurately determining the atomic structure of two-dimensional materials such as graphene, even in the presence of experimental errors. Using coincidence-site-lattice and displacement-shift-complete theories, we show that the in-plane translation state between layers is not a significant structure parameter, explaining why the present method is adequate not only for bilayer graphene but also a few-layered twisted graphene. PMID:26888259

  14. Interfacial Atomic Structure of Twisted Few-Layer Graphene.

    PubMed

    Ishikawa, Ryo; Lugg, Nathan R; Inoue, Kazutoshi; Sawada, Hidetaka; Taniguchi, Takashi; Shibata, Naoya; Ikuhara, Yuichi

    2016-01-01

    A twist in bi- or few-layer graphene breaks the local symmetry, introducing a number of intriguing physical properties such as opening new bandgaps. Therefore, determining the twisted atomic structure is critical to understanding and controlling the functional properties of graphene. Combining low-angle annular dark-field electron microscopy with image simulations, we directly determine the atomic structure of twisted few-layer graphene in terms of a moiré superstructure which is parameterized by a single twist angle and lattice constant. This method is shown to be a powerful tool for accurately determining the atomic structure of two-dimensional materials such as graphene, even in the presence of experimental errors. Using coincidence-site-lattice and displacement-shift-complete theories, we show that the in-plane translation state between layers is not a significant structure parameter, explaining why the present method is adequate not only for bilayer graphene but also a few-layered twisted graphene. PMID:26888259

  15. Detection of interlayer interaction in few-layer graphene

    NASA Astrophysics Data System (ADS)

    Wu, Zefei; Han, Yu; Lin, Jiangxiazi; Zhu, Wei; He, Mingquan; Xu, Shuigang; Chen, Xiaolong; Lu, Huanhuan; Ye, Weiguang; Han, Tianyi; Wu, Yingying; Long, Gen; Shen, Junying; Huang, Rui; Wang, Lin; He, Yuheng; Cai, Yuan; Lortz, Rolf; Su, Dangsheng; Wang, Ning

    2015-08-01

    Bernal-stacked few-layer graphene has been investigated by analyzing its Landau-level spectra through quantum capacitance measurements. We find that surface relaxation, which is insignificant in trilayer graphene, starts to manifest in Bernal-stacked tetralayer graphene. In trilayer graphene, the interlayer interaction parameters are generally similar to those of graphite. However, in tetralayer graphene, the hopping parameters of the two bulk layers are quite different from those of the two outer layers. This represents direct evidence of the surface relaxation phenomenon. Traditionally, the van der Waals interaction between the carbon layers is thought to be insignificant. However, we suggest that the interlayer interaction is an important factor in explaining the observed results, and the symmetry-breaking effects in graphene sublattice are not negligible.

  16. Imaging Stacking Order in Few-Layer Graphene

    SciTech Connect

    C Lui; Z Li; Z Chen; P Klimov; L Brus; T Heinz

    2011-12-31

    Few-layer graphene (FLG) has been predicted to exist in various crystallographic stacking sequences, which can strongly influence the material's electronic properties. We demonstrate an accurate and efficient method to characterize stacking order in FLG using the distinctive features of the Raman 2D-mode. Raman imaging allows us to visualize directly the spatial distribution of Bernal (ABA) and rhombohedral (ABC) stacking in tri- and tetralayer graphene. We find that 15% of exfoliated graphene tri- and tetralayers is composed of micrometer-sized domains of rhombohedral stacking, rather than of usual Bernal stacking. These domains are stable and remain unchanged for temperatures exceeding 800 C.

  17. Few layer epitaxial germanene: a novel two-dimensional Dirac material

    NASA Astrophysics Data System (ADS)

    Dávila, María Eugenia; Le Lay, Guy

    2016-02-01

    Monolayer germanene, a novel graphene-like germanium allotrope akin to silicene has been recently grown on metallic substrates. Lying directly on the metal surfaces the reconstructed atom-thin sheets are prone to lose the massless Dirac fermion character and unique associated physical properties of free standing germanene. Here, we show that few layer germanene, which we create by dry epitaxy on a gold template, possesses Dirac cones thanks to a reduced interaction. This finding established on synchrotron-radiation-based photoemission, scanning tunneling microscopy imaging and surface electron diffraction places few layer germanene among the rare two-dimensional Dirac materials. Since germanium is currently used in the mainstream Si-based electronics, perspectives of using germanene for scaling down beyond the 5 nm node appear very promising. Other fascinating properties seem at hand, typically the robust quantum spin Hall effect for applications in spintronics and the engineering of Floquet Majorana fermions by light for quantum computing.

  18. Few layer epitaxial germanene: a novel two-dimensional Dirac material.

    PubMed

    Dávila, María Eugenia; Le Lay, Guy

    2016-01-01

    Monolayer germanene, a novel graphene-like germanium allotrope akin to silicene has been recently grown on metallic substrates. Lying directly on the metal surfaces the reconstructed atom-thin sheets are prone to lose the massless Dirac fermion character and unique associated physical properties of free standing germanene. Here, we show that few layer germanene, which we create by dry epitaxy on a gold template, possesses Dirac cones thanks to a reduced interaction. This finding established on synchrotron-radiation-based photoemission, scanning tunneling microscopy imaging and surface electron diffraction places few layer germanene among the rare two-dimensional Dirac materials. Since germanium is currently used in the mainstream Si-based electronics, perspectives of using germanene for scaling down beyond the 5 nm node appear very promising. Other fascinating properties seem at hand, typically the robust quantum spin Hall effect for applications in spintronics and the engineering of Floquet Majorana fermions by light for quantum computing. PMID:26860590

  19. Few layer epitaxial germanene: a novel two-dimensional Dirac material

    PubMed Central

    Dávila, María Eugenia; Le Lay, Guy

    2016-01-01

    Monolayer germanene, a novel graphene-like germanium allotrope akin to silicene has been recently grown on metallic substrates. Lying directly on the metal surfaces the reconstructed atom-thin sheets are prone to lose the massless Dirac fermion character and unique associated physical properties of free standing germanene. Here, we show that few layer germanene, which we create by dry epitaxy on a gold template, possesses Dirac cones thanks to a reduced interaction. This finding established on synchrotron-radiation-based photoemission, scanning tunneling microscopy imaging and surface electron diffraction places few layer germanene among the rare two-dimensional Dirac materials. Since germanium is currently used in the mainstream Si-based electronics, perspectives of using germanene for scaling down beyond the 5 nm node appear very promising. Other fascinating properties seem at hand, typically the robust quantum spin Hall effect for applications in spintronics and the engineering of Floquet Majorana fermions by light for quantum computing. PMID:26860590

  20. Few layer graphene based superlattices as efficient thermal insulators

    NASA Astrophysics Data System (ADS)

    Ni, Yuxiang; Chalopin, Yann; Volz, Sebastian

    2013-09-01

    While graphene and few layer graphene (FLG) are considered as having the highest thermal conductivity in their in-plane directions, our molecular dynamics (MD) simulations however show that those systems are also characterized by a superior thermal contact resistance, which could be largely tuned with the layer number when in contact with a silica substrate. Taking advantages of such a resistive interface, MD simulations show that SiO2/FLG superlattices have a thermal conductivity as low as 0.30 W/m K, exhibiting a promising prospect in nano-scale thermal insulation. These findings pave the way for an improved thermal management of nanoscale systems such as thermal barrier coatings and phase change memory materials with atomic-scale super-insulators.

  1. Magnetoelectric effect in functionalized few-layer graphene

    NASA Astrophysics Data System (ADS)

    Santos, Elton J. G.

    2013-04-01

    We show that the spin moment induced by sp3-type defects created by different covalent functionalizations on a few-layer graphene structure can be controlled by an external electric field. Based on ab initio density functional calculations, including van der Waals interactions, we find that this effect has a dependence on the number of stacked layers and concentration of point defects, but the interplay of both with the electric field drives the system to a half-metallic state. The calculated magnetoelectric coefficient α has a value comparable to those found for ferromagnetic thin films (e.g., Fe, Co, Ni) and magnetoelectric surfaces (e.g., CrO2). The value of α also agrees with the universal value predicted for ferromagnetic half-metals and also points to a novel route to induce half-metallicity in graphene using surface decoration.

  2. Synthesis and characterization of intercalated few-layer graphenes

    NASA Astrophysics Data System (ADS)

    Sato, Shogo; Ichikawa, Hiroaki; Iwata, Nobuyuki; Yamamoto, Hiroshi

    2014-02-01

    Toward achieving room-temperature superconductivity, FeCl3-intercalated few-layer graphenes (FeCl3-FLGs) and Ca-intercalated few-layer graphenes (Ca-FLGs) were synthesized. FeCl3-FLGs were synthesized by the two-zone method and Ca-FLGs were synthesized using Ca-Li alloy. The Raman spectra of the FeCl3-FLGs showed a lower-intensity peak at 1607 cm-1 than that of the corresponding bare G. The peak at 1607 cm-1 suggested that the sample was stage 4-5 FeCl3-FLGs. The room-temperature electrical resistivity of FeCl3-FLGs was 2.65 × 10-5 Ω·m, which linearly decreased with decreasing temperature with a marked change occurring at approximately 200 K. From a XRD pattern of Ca-FLGs, we concluded that Ca is intercalated in FLGs. The room-temperature resistivity of Ca-FLGs was 3.45 × 10-5 Ω·m, which increased with decreasing temperature.

  3. Structural distortions in few-layer graphene creases.

    PubMed

    Robertson, Alex W; Bachmatiuk, Alicja; Wu, Yimin A; Schäffel, Franziska; Büchner, Bernd; Rümmeli, Mark H; Warner, Jamie H

    2011-12-27

    Folds and creases are frequently found in graphene grown by chemical vapor deposition (CVD), due to the differing thermal expansion coefficients of graphene from the growth catalyst and the flexibility of the sheet during transfer from the catalyst. The structure of a few-layer graphene (FLG) crease is examined by aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM). A study of 2D fast Fourier transforms (FFTs) taken about the region of the crease allowed for the crystal stacking structure of the system to be elucidated. It was found that strain-induced stacking faults were created in the AB Bernal-stacked FLG bulk around the region proximal to the crease termination; this is of interest as the stacking order of FLG is known to have an effect on its electronic properties and thus should be considered when transferring CVD-grown FLG to alternate substrates for electronic device fabrication. The FFTs, along with analysis of the real space images, were used to determine the configuration of the layers in the crease itself and were corroborated by multislice atomistic TEM simulations. The termination of the crease part way through the FLG sheet is also examined and is found to show strong out of plane distortions in the area about it. PMID:22122696

  4. Mesoporous Few-Layer Graphene Platform for Affinity Biosensing Application.

    PubMed

    Ali, Md Azahar; Singh, Chandan; Mondal, Kunal; Srivastava, Saurabh; Sharma, Ashutosh; Malhotra, Bansi D

    2016-03-30

    A label-free, highly reproducible, sensitive, and selective biosensor is proposed using antiapolipoprotein B 100 (AAB) functionalized mesoporous few-layer reduced graphene oxide and nickel oxide (rGO-NiO) nanocomposite for detection of low density lipoprotein (LDL) molecules. The formation of mesoporous rGO-NiO composite on indium tin oxide conductive electrode has been accomplished via electrophoretic technique using colloidal suspension of rGO sheets and NiO nanoparticles. This biosensor shows good stability obtained by surface conjugation of antibody AAB molecules with rGO-NiO matrix by EDC-NHS coupling chemistry. The defect-less few layer rGO sheets, NiO nanoparticles (nNiO) and formation of nanocomposite has been confirmed by Raman mapping, electron microscopic studies, X-ray diffraction, and electrochemical techniques. The synthesized rGO-NiO composite is mesoporous dominated with a small percentage of micro and macroporous structure as is evident by the results of Brunauer-Emmett-Teller experiment. Further, the bioconjugation of AAB with rGO-NiO has been investigated by Fourier transform-infrared spectroscopy studies. The kinetic studies for binding of antigen-antibody (LDL-AAB) and analytical performance of this biosensor have been evaluated by the impedance spectroscopic method. This biosensor exhibits an excellent sensitivity of 510 Ω (mg/dL)(-1) cm(-2) for detection of LDL molecules and is sensitive to 5 mg/dL concentration of LDL in a wide range of 0-130 mg/dL. Thus, this fabricated biosensor is an efficient and highly sensitive platform for the analysis of other antigen-antibody interactions and biomolecules detection. PMID:26950488

  5. Raman Scattering from few-layer Graphene Films

    NASA Astrophysics Data System (ADS)

    Gupta, A.; Joshi, P.; Srinivas, T.; Eklund, Peter

    2006-03-01

    Few layer-graphene sheet (nGL's) films, where n is the number of graphene layers, are new two-dimensional sp^2 carbon systems that have been shown to produce exciting Fractional Quantum Hall phenomena. We report here on the first Raman scattering (RS) results of nGLs. nGLs with lateral dimensions of ˜1-3 μm were prepared by chemical delamination of graphite flake or HOPG and then transferred from solution onto substrates (mica, pyrex,In/pyrex and Au/pyrex). RS spectra have been collected on nGL's with n=1, 2, 3 and compared with the graphite. Graphite exhibits two E2g interlayer modes at 42 cm-1 and 1582 cm-1. The Raman spectra of (n=1-3) nGLs were found to exhibit peaks at 1350 cm-1 and 1620 cm-1, i.e., near frequencies associated with high phonon density of states. The high frequency E2g band is found to split into two bands when the nGL is supported on metallic substrates (In,Au). In both these cases, we observe bands at 1583 cm-1, ˜1592 cm-1 rather than one band at 1581 cm-1 when the nGL is on insulating pyrex. The splitting of the interlayer band when on metallic substrates is identified with charge transfer between the nGL and the substrate. The phonon density of states scattering observed does not appear to be due to disorder in the basal plane.

  6. Ordered carbon nanotube growth on graphene and few-layer graphene

    NASA Astrophysics Data System (ADS)

    Hunley, D. Patrick; Johnson, Stephen; Stieha, Joseph; Sundararajan, Abhishek; Meacham, Aaron; Strachan, Douglas

    2011-03-01

    Carbon nanotubes are grown on graphene and few-layer graphene films through chemical vapor deposition. The nanotube growth is found to depend on the thickness of the few-layer graphene films. The thinnest films show significant alignment of the nanotubes with the crystallographic axes of the graphene. This alignment is compared to the orientation of the crystallographic etch tracks, permitting the orientation of the nanotubes to be determined. Related nanotube/graphene structures will also be presented and discussed. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering.

  7. Few layer graphene synthesis via SiC decomposition at low temperature and low vacuum

    NASA Astrophysics Data System (ADS)

    Kayali, Emre; Mercan, Elif; Emre Oren, Ersin; Cambaz Buke, Goknur

    2016-04-01

    Based on the large-scale availability and good electrical properties, the epitaxial graphene (EG) on SiC exhibits a big potential for future electronic devices. However, it is still necessary to work continuously on lowering the formation temperature and vacuum values of EG while improving the quality and increasing the lateral size to fabricate high-performance electronic devices at reduced processing costs. In this study, we investigated the effect of the presence of Mo plate and hydrogen atmosphere as well as the vacuum annealing durations on SiC decomposition. Our studies showed that the graphene layers can be produced at lower annealing temperatures (1200 °C) and vacuum values (10-4 Torr) in the presence of Mo plate and hydrogen. For high quality continuous graphene formation, Mo plate should be in contact with SiC. If there is a gap between Mo and SiC, non-wetting oxide droplets on few layer graphene (FLG) are recorded. Moreover, it is found that the morphology of these islands can be controlled by changing the annealing time and atmosphere conditions, and applying external disturbances such as vibration.

  8. Tribological characteristics of few-layer graphene over Ni grain and interface boundaries

    NASA Astrophysics Data System (ADS)

    Tripathi, Manoj; Awaja, Firas; Paolicelli, Guido; Bartali, Ruben; Iacob, Erica; Valeri, Sergio; Ryu, Seunghwa; Signetti, Stefano; Speranza, Giorgio; Pugno, Nicola Maria

    2016-03-01

    The tribological properties of metal-supported few-layered graphene depend strongly on the grain topology of the metal substrate. Inhomogeneous distribution of graphene layers at such regions led to variable landscapes with distinguishable roughness. This discrepancy in morphology significantly affects the frictional and wetting characteristics of the FLG system. We discretely measured friction characteristics of FLG covering grains and interfacial grain boundaries of polycrystalline Ni metal substrate via an atomic force microscopy (AFM) probe. The friction coefficient of FLG covered at interfacial grain boundaries is found to be lower than that on grains in vacuum (at 10-5 Torr pressure) and similar results were obtained in air condition. Sliding history with AFM cantilever, static and dynamic pull-in and pull-off adhesion forces were addressed in the course of friction measurements to explain the role of the out-of-plane deformation of graphene layer(s). Finite element simulations showed good agreement with experiments and led to a rationalization of the observations. Thus, with interfacial grain boundaries the FLG tribology can be effectively tuned.The tribological properties of metal-supported few-layered graphene depend strongly on the grain topology of the metal substrate. Inhomogeneous distribution of graphene layers at such regions led to variable landscapes with distinguishable roughness. This discrepancy in morphology significantly affects the frictional and wetting characteristics of the FLG system. We discretely measured friction characteristics of FLG covering grains and interfacial grain boundaries of polycrystalline Ni metal substrate via an atomic force microscopy (AFM) probe. The friction coefficient of FLG covered at interfacial grain boundaries is found to be lower than that on grains in vacuum (at 10-5 Torr pressure) and similar results were obtained in air condition. Sliding history with AFM cantilever, static and dynamic pull-in and pull

  9. Near-field scanning microwave microscopy of few-layer graphene.

    SciTech Connect

    Kalugin, Nikolai G.; Gonzales, Edward; Kalichava, Irakli; Gin, Aaron V.; Wickey, Lee; Del Barga, Christopher; Talanov, Vladimir V.; Shaner, Eric Arthur

    2010-08-01

    Near-field microwave microscopy can be used as an alternative to atomic-force microscopy or Raman microscopy in determination of graphene thickness. We evaluated the values of AC impedance for few layer graphene. The impedance of mono and few-layer graphene at 4GHz was found predominantly active. Near-field microwave microscopy allows simultaneous imaging of location, geometry, thickness, and distribution of electrical properties of graphene without device fabrication. Our results may be useful for design of future graphene-based microwave devices.

  10. Direct Synthesis of Few-Layer Graphene on NaCl Crystals.

    PubMed

    Shi, Liurong; Chen, Ke; Du, Ran; Bachmatiuk, Alicja; Rümmeli, Mark Hermann; Priydarshi, Manish Kumar; Zhang, Yanfeng; Manivannan, Ayyakkannu; Liu, Zhongfan

    2015-12-16

    Chemical vapor deposition is used to synthesize few-layer graphene on micro crystalline sodium chloride (NaCl) powder. The water-soluble nature of NaCl makes it convenient to produce free standing graphene layers via a facile and low-cost approach. Unlike traditional metal-catalyzed or oxygen-aided growth, the micron-size NaCl crystal planes play an important role in the nucleation and growth of few-layer graphene. Moreover, the possibility of synthesizing cuboidal graphene is also demonstrated in the present approach for the first time. Raman spectroscopy, optical microscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy are used to evaluate the quality and structure of the few-layer graphene along with cuboidal graphene obtained in this process. The few-layer graphene synthesized using the present method has an adsorption ability for anionic and cationic dye molecules in water. The present synthesis method may pave a facile way for manufacturing few-layer graphene on a large scale. PMID:26524105

  11. Facile Large Scale Production of Few-Layer Graphene Sheets by Shear Exfoliation in Volatile Solvent.

    PubMed

    Akhtar, M Wasim; Park, Chan Woo; Kim, Youn Sop; Kim, Jong Seok

    2015-12-01

    Few layer graphene sheets were synthesized from natural graphite through mechanical shear mixer in 1-butanol as solvent. The liquid phase exfoliation of graphite through the shear mixer generated incising forces for 20 minutes which changed the large amount of graphite's flake into few layer graphene. The removal of solvent from the deposited dispersion was performed immediately by keeping at the room temperature. The deposited graphene thin films were characterized by AFM, HR-TEM, XRD, FT-IR and Raman Spectroscopy. The HR-TEM results showed the formation of few layers and well dispersed graphene. The Raman spectroscopy and XRD characterization confirmed the good quality and non-oxidized state of graphene. PMID:26682388

  12. Synthesis of few layer graphene by non-transferred arc plasma system.

    PubMed

    Baek, Jong-Jun; Kim, Tae-Hee; Park, Dong-Wha

    2013-11-01

    Graphene has recently been the focus of a great deal of attention owing to its outstanding properties, which include high mobility, high thermal conductivity and high structural stability. In this study, a few layer graphene was successfully synthesized from methane gas using a non-transferred direct current arc plasma system. Non-transferred thermal plasma offers high temperature, steep temperature gradient and high enthalpy to enhance the reaction kinetics of graphene synthesis. In order to prepare high quality few layer graphene, graphene products synthesized under several conditions was analyzed comparatively. Effects of gap distance between the plasma torch and graphite substrate, the flow rate of additional reactant gas, and different types of plasma forming gas on the synthesis of few layer graphene were investigated. Methane gas was injected into the plasma jet as a carbon source for the synthesis of graphene and a thermal plasma jet was generated by pure argon or a mixture of argon-hydrogen. The results revealed that hydrogen gas improved the quality of few layer graphene by inducing surface etching and increasing plasma power. PMID:24245266

  13. Contribution of dielectric screening to the total capacitance of few-layer graphene electrodes

    DOE PAGESBeta

    Zhan, Cheng; Jiang, De-en

    2016-02-17

    We apply joint density functional theory (JDFT), which treats the electrode/electrolyte interface self-consistently, to an electric double-layer capacitor (EDLC) based on few-layer graphene electrodes. The JDFT approach allows us to quantify a third contribution to the total capacitance beyond quantum capacitance (CQ) and EDL capacitance (CEDL). This contribution arises from the dielectric screening of the electric field by the surface of the few-layer graphene electrode, and we therefore term it the dielectric capacitance (CDielec). We find that CDielec becomes significant in affecting the total capacitance when the number of graphene layers in the electrode is more than three. In conclusion,more » our investigation sheds new light on the significance of the electrode dielectric screening on the capacitance of few-layer graphene electrodes.« less

  14. Robust adhesion of flower-like few-layer graphene nanoclusters

    PubMed Central

    Tian, Shibing; Li, Lin; Sun, Wangning; Xia, Xiaoxiang; Han, Dong; Li, Junjie; Gu, Changzhi

    2012-01-01

    Nanostructured surface possessing ultrahigh adhesion like “gecko foot” or “rose petal” can offer more opportunities for bionic application. We grow flower-like few-layer graphene on silicon nanocone arrays to form graphene nanoclusters, showing robust adhesion. Their contact angle (CA) is 164° with a hysteresis CA of 155° and adhesive force for a 5 μL water droplet is about 254 μN that is far larger than present reported results. We bring experimental evidences that this great adhesion depends on large-area plentiful edges of graphene nanosheets tuned by conical nanostructure and intrinsic wetting features of graphene. Such new hierarchical few-layer graphene nanostructure provides a feasible strategy to understand the ultra-adhesive mechanism of the “gecko effect” or “rose effect” and enhance the wettability of graphene for many practical applications. PMID:22803004

  15. Robust adhesion of flower-like few-layer graphene nanoclusters

    NASA Astrophysics Data System (ADS)

    Tian, Shibing; Li, Lin; Sun, Wangning; Xia, Xiaoxiang; Han, Dong; Li, Junjie; Gu, Changzhi

    2012-07-01

    Nanostructured surface possessing ultrahigh adhesion like ``gecko foot'' or ``rose petal'' can offer more opportunities for bionic application. We grow flower-like few-layer graphene on silicon nanocone arrays to form graphene nanoclusters, showing robust adhesion. Their contact angle (CA) is 164° with a hysteresis CA of 155° and adhesive force for a 5 μL water droplet is about 254 μN that is far larger than present reported results. We bring experimental evidences that this great adhesion depends on large-area plentiful edges of graphene nanosheets tuned by conical nanostructure and intrinsic wetting features of graphene. Such new hierarchical few-layer graphene nanostructure provides a feasible strategy to understand the ultra-adhesive mechanism of the ``gecko effect'' or ``rose effect'' and enhance the wettability of graphene for many practical applications.

  16. Robust adhesion of flower-like few-layer graphene nanoclusters.

    PubMed

    Tian, Shibing; Li, Lin; Sun, Wangning; Xia, Xiaoxiang; Han, Dong; Li, Junjie; Gu, Changzhi

    2012-01-01

    Nanostructured surface possessing ultrahigh adhesion like "gecko foot" or "rose petal" can offer more opportunities for bionic application. We grow flower-like few-layer graphene on silicon nanocone arrays to form graphene nanoclusters, showing robust adhesion. Their contact angle (CA) is 164° with a hysteresis CA of 155° and adhesive force for a 5 μL water droplet is about 254 μN that is far larger than present reported results. We bring experimental evidences that this great adhesion depends on large-area plentiful edges of graphene nanosheets tuned by conical nanostructure and intrinsic wetting features of graphene. Such new hierarchical few-layer graphene nanostructure provides a feasible strategy to understand the ultra-adhesive mechanism of the "gecko effect" or "rose effect" and enhance the wettability of graphene for many practical applications. PMID:22803004

  17. Thermoacoustic and photoacoustic characterizations of few-layer graphene by pulsed excitations

    NASA Astrophysics Data System (ADS)

    Wang, Xiong; Witte, Russell S.; Xin, Hao

    2016-04-01

    We characterized the thermoacoustic and photoacoustic properties of large-area, few-layer graphene by pulsed microwave and optical excitations. Due to its high electric conductivity and low heat capacity per unit area, graphene lends itself to excellent microwave and optical energy absorption and acoustic signal emanation due to the thermoacoustic effect. When exposed to pulsed microwave or optical radiation, distinct thermoacoustic and photoacoustic signals generated by the few-layer graphene are obtained due to microwave and laser absorption of the graphene, respectively. Clear thermoacoustic and photoacoustic images of large-area graphene sample are achieved. A numerical model is developed and the simulated results are in good accordance with the measured ones. This characterization work may find applications in ultrasound generator and detectors for microwave and optical radiation. It may also become an alternative characterization approach for graphene and other types of two-dimensional materials.

  18. Monolayer and/or few-layer graphene on metal or metal-coated substrates

    DOEpatents

    Sutter, Peter Werner; Sutter, Eli Anguelova

    2015-04-14

    Disclosed is monolayer and/or few-layer graphene on metal or metal-coated substrates. Embodiments include graphene mirrors. In an example, a mirror includes a substrate that has a surface exhibiting a curvature operable to focus an incident beam onto a focal plane. A graphene layer conformally adheres to the substrate, and is operable to protect the substrate surface from degradation due to the incident beam and an ambient environment.

  19. Few layered graphene Sheet decorated by ZnO Nanoparticles for anti-bacterial application

    NASA Astrophysics Data System (ADS)

    Bykkam, Satish; Narsingam, Sowmya; Ahmadipour, Mohsen; Dayakar, T.; Venkateswara Rao, K.; Shilpa Chakra, Ch.; Kalakotla, Shanker

    2015-07-01

    A widely soluble few layered graphene (FLG) sheets decorated by ZnO nano particles were prepared through hydrothermal method using ethylene glycol as a solvent and a reducing agent. The obtained FLG/ZnO composite material was characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM) and Raman spectroscopy. The anti-bacterial properties of ZnO nano particles decorated few layered graphenes were tested against Escherichia coli and Salmonella typhi by using well diffusion method. The results confirmed that FLG/ZnO has significant antibacterial activity more against S. typhi than E. coli. The obtained results from the current research work conclusively states that the ZnO nano particles which were decorated by few layered graphene possess a significant anti-bacterial activity.

  20. Low-frequency phonons of few-layer graphene within a tight-binding model

    NASA Astrophysics Data System (ADS)

    Popov, Valentin N.; Van Alsenoy, Christian

    2014-12-01

    Few-layer graphene is a layered carbon material with covalent bonding in the layers and weak van der Waals interactions between the layers. The interlayer energy is more than two orders of magnitude smaller than the intralayer one, which hinders the description of the static and dynamic properties within electron band structure models. We overcome this difficulty by introducing two sets of matrix elements—one set for the covalent bonds in the graphene layers and another one for the van der Waals interactions between adjacent graphene layers in a tight-binding model of the band structure. Both sets of matrix elements are derived from an ab initio study on carbon dimers. The matrix elements are applied in the calculation of the phonon dispersion of graphite and few-layer graphene with AB and ABC layer stacking. The results for few-layer graphene with AB stacking agree well with the available experimental data, which justifies the application of the matrix elements to other layered carbon structures with van der Waals interactions such as few-layer graphene nanoribbons, multiwall carbon nanotubes, and carbon onions.

  1. High yield production and purification of few layer graphene by Gum Arabic assisted physical sonication

    PubMed Central

    Chabot, Victor; Kim, Brian; Sloper, Brent; Tzoganakis, Costas; Yu, Aiping

    2013-01-01

    Exploiting the emulsification properties of low cost, environmentally safe Gum Arabic we demonstrate a high yield process to produce a few layer graphene with a low defect ratio, maintaining the pristine graphite structure. In addition, we demonstrate the need for and efficacy of an acid hydrolysis treatment to remove the polymer residues to produce 100% pure graphene. The scalable process gives yield of up to 5 wt% graphene based on 10 g starting graphite. The graphene product is compared with reduced graphene oxide produced through Hummer's method using UV-visible spectroscopy, SEM, TEM, and Raman spectroscopy. The two graphene materials show significant difference in these characterizations. Further, the film fabricated from this graphene exhibits 20 times higher electrical conductivity than that of the reduced graphene oxide. Sonication processing of graphite with environmentally approved biopolymers such as Gum Arabic opens up a scalable avenue for production of cheap graphene. PMID:23478744

  2. Direct synthesis of few-layer graphene supported platinum nanocatalyst for methanol oxidation

    NASA Astrophysics Data System (ADS)

    Tan, Hong; Ma, Xiaohui; Sheng, Leimei; An, Kang; Yu, Liming; Zhao, Hongbin; Xu, Jiaqiang; Ren, Wei; Zhao, Xinluo

    2014-11-01

    High-crystalline few-layer graphene supported Pt nanoparticles have been synthesized by arc discharge evaporation of carbon electrodes containing Pt element. A high-temperature treatment under hydrogen atmosphere has been carried out to obtain a new type of Pt/graphene catalyst for methanol oxidation in direct methanol fuel cell. The morphology and structure characterizations of as-grown few-layer graphene supported Pt nanoparticles and Pt/graphene catalysts have been studied by Raman spectroscopy, scanning electron microscopy with energy-dispersive spectroscopy, and high-resolution transmission electron microscopy. Cyclic voltammograms and chronoamperometric curves show that our present Pt/graphene catalysts have larger current density for methanol oxidation, higher tolerance to carbon monoxide poisoning, and better stability during the operating procedure, compared to commercial Pt/C catalysts.

  3. Magneto-transport properties of a random distribution of few-layer graphene patches

    NASA Astrophysics Data System (ADS)

    Iacovella, Fabrice; Trinsoutrot, Pierre; Mitioglu, Anatolie; Conédéra, Véronique; Pierre, Mathieu; Raquet, Bertrand; Goiran, Michel; Vergnes, Hugues; Caussat, Brigitte; Plochocka, Paulina; Escoffier, Walter

    2014-11-01

    In this study, we address the electronic properties of conducting films constituted of an array of randomly distributed few layer graphene patches and investigate on their most salient galvanometric features in the moderate and extreme disordered limit. We demonstrate that, in annealed devices, the ambipolar behaviour and the onset of Landau level quantization in high magnetic field constitute robust hallmarks of few-layer graphene films. In the strong disorder limit, however, the magneto-transport properties are best described by a variable-range hopping behaviour. A large negative magneto-conductance is observed at the charge neutrality point, in consistency with localized transport regime.

  4. Magneto-transport properties of a random distribution of few-layer graphene patches

    SciTech Connect

    Iacovella, Fabrice; Mitioglu, Anatolie; Pierre, Mathieu; Raquet, Bertrand; Goiran, Michel; Plochocka, Paulina; Escoffier, Walter; Trinsoutrot, Pierre; Vergnes, Hugues; Caussat, Brigitte

    2014-11-21

    In this study, we address the electronic properties of conducting films constituted of an array of randomly distributed few layer graphene patches and investigate on their most salient galvanometric features in the moderate and extreme disordered limit. We demonstrate that, in annealed devices, the ambipolar behaviour and the onset of Landau level quantization in high magnetic field constitute robust hallmarks of few-layer graphene films. In the strong disorder limit, however, the magneto-transport properties are best described by a variable-range hopping behaviour. A large negative magneto-conductance is observed at the charge neutrality point, in consistency with localized transport regime.

  5. Synthesis and characterization of porous, mixed phase, wrinkled, few layer graphene like nanocarbon from charcoal

    NASA Astrophysics Data System (ADS)

    Manoj, B.

    2015-12-01

    A technique to synthesis wrinkled graphene like nano carbon (GNC) from charcoal is reported in the current study. The charcoal produced by thermal decomposition and is intercalated by Hummers method. It is separated by centrifugation and sonication to get few layer graphene sheets. The structural and chemical changes of the nanostructure is elucidated by Raman spectroscopy, TEM, SEM-EDS and XPS. Raman spectra revealed the existence of highly graphitized amorphous carbon, which is confirmed by the appearance of five peaks in the deconvoluted first order Raman spectra. The SEM analysis reveals the formation of large area graphene sheets with nano-porous structure in it. The TEM/SAED analysis exhibits the presence of short range few layer graphene.

  6. Optical limiting action of few layered graphene oxide dispersed in different solvents

    NASA Astrophysics Data System (ADS)

    Liaros, N.; Iliopoulos, K.; Stylianakis, M. M.; Koudoumas, E.; Couris, S.

    2013-11-01

    In this work, we report on new results regarding the nonlinear optical response and the optical limiting action of few layered graphene oxide dispersed in various organic solvents, such as N,N-Dimethylformamide (DMF), Tetrahydrofuran (THF), (N-Methyl-2-Pyrrolidone (NMP), Chlorobenzene (CB), 1,2-Dichlorobenzene (o-DCB), and 1-Chloronaphthalene (CN). Few layered graphene oxide was found to exhibit saturable absorption for low incident intensity and reverse saturable absorption at higher intensity. Its nonlinear absorption was found to be significantly larger than that of single layer graphene oxide, depending upon the solvent used becoming maximum for heavier solvents like e.g. CN and o-DCB. The present results are discussed and compared with other literature reports.

  7. Ultrafast terahertz response in photoexcited, vertically grown few-layer graphene

    NASA Astrophysics Data System (ADS)

    Fu, Maixia; Quan, Baogang; He, Jingwen; Yao, Zehan; Gu, Changzhi; Li, Junjie; Zhang, Yan

    2016-03-01

    The terahertz (THz) response from vertically aligned few-layer graphene samples with and without femtosecond optical excitation was investigated. The frequency-dependent optical conductivity of the photoexcited vertically aligned few-layer graphene had a strong free carrier response. Upon photoexcitation, a transient decrease in THz transmission on the subpicosecond timescale was observed. A modulation depth of nearly 16% was demonstrated in the range of the photoexcitation power used. The photoinduced ultrafast response presented here is distinct from previous studies on horizontally grown graphene. The mechanism underlying this photoconductive ultrafast response was investigated by measuring the transmission properties and by calculating the carrier density. The results of these studies are promising for the development of high-performance THz modulators and ultrafast switchable THz photoelectric devices.

  8. A 3D insight on the catalytic nanostructuration of few-layer graphene

    PubMed Central

    Melinte, G.; Florea, I.; Moldovan, S.; Janowska, I.; Baaziz, W.; Arenal, R.; Wisnet, A.; Scheu, C.; Begin-Colin, S.; Begin, D.; Pham-Huu, C.; Ersen, O.

    2014-01-01

    The catalytic cutting of few-layer graphene is nowadays a hot topic in materials research due to its potential applications in the catalysis field and the graphene nanoribbons fabrication. We show here a 3D analysis of the nanostructuration of few-layer graphene by iron-based nanoparticles under hydrogen flow. The nanoparticles located at the edges or attached to the steps on the FLG sheets create trenches and tunnels with orientations, lengths and morphologies defined by the crystallography and the topography of the carbon substrate. The cross-sectional analysis of the 3D volumes highlights the role of the active nanoparticle identity on the trench size and shape, with emphasis on the topographical stability of the basal planes within the resulting trenches and channels, no matter the obstacle encountered. The actual study gives a deep insight on the impact of nanoparticles morphology and support topography on the 3D character of nanostructures built up by catalytic cutting. PMID:24916201

  9. Synthesis of few layer graphene by direct exfoliation of graphite and a Raman spectroscopic study

    SciTech Connect

    Gayathri, S.; Jayabal, P.; Ramakrishnan, V.; Kottaisamy, M.

    2014-02-15

    The exfoliation of graphene from pristine graphite in a liquid phase was achieved successfully via sonication followed by centrifugation method. Ultraviolet–visible (UV–vis) spectra of the obtained graphene dispersions at different exfoliation time indicated that the concentration of graphene dispersion increased markedly with increasing exfoliation time. The sheet-like morphology of the exfoliated graphene was revealed by Scanning Electron Microscopy (SEM) image. Further, the morphological change in different exfoliation time was investigated by Atomic Force Microscopy (AFM). A complete structural and defect characterization was probed using micro-Raman spectroscopic technique. The shape and position of the 2D band of Raman spectra revealed the formation of bilayer to few layer graphene. Also, Raman mapping confirmed the presence of uniformly distributed bilayer graphene sheets on the substrate.

  10. Synthesis of Few-Layer Graphene Using DC PE-CVD

    NASA Astrophysics Data System (ADS)

    Kim, Jeong Hyuk; Castro, Edward Joseph D.; Hwang, Yong Gyoo; Lee, Choong Hun

    2011-12-01

    Few layer graphene (FLG) had been successfully grown on polycrystalline Ni films or foils on a large scale using DC Plasma Enhanced Chemical Vapor Deposition (DC PE-CVD) as a result of the Raman spectra drawn out of the sample. The size of graphene films is dependent on the area of the Ni film as well as the DC PE-CVD chamber size. Synthesis time has an effect on the quality of graphene produced. However, further analysis and experiments must be pursued to further identify the optimum settings and conditions of producing better quality graphene. Applied plasma voltage on the other hand, had an influence on the minimization of defects in the graphene grown. It has also presented a method of producing a free standing PMMA/graphene membrane on a FeCl3(aq) solution which could then be transferred to a desired substrate.

  11. Hydrothermal synthesis of highly nitrogen-doped few-layer graphene via solid–gas reaction

    SciTech Connect

    Liang, Xianqing; Zhong, Jun; Shi, Yalin; Guo, Jin; Huang, Guolong; Hong, Caihao; Zhao, Yidong

    2015-01-15

    Highlights: • A novel approach to synthesis of N-doped few-layer graphene has been developed. • The high doping levels of N in products are achieved. • XPS and XANES results reveal a thermal transformation of N bonding configurations. • The developed method is cost-effective and eco-friendly. - Abstract: Nitrogen-doped (N-doped) graphene sheets with high doping concentration were facilely synthesized through solid–gas reaction of graphene oxide (GO) with ammonia vapor in a self-designed hydrothermal system. The morphology, surface chemistry and electronic structure of N-doped graphene sheets were investigated by TEM, AFM, XRD, XPS, XANES and Raman characterizations. Upon hydrothermal treatment, up to 13.22 at% of nitrogen could be introduced into the crumpled few-layer graphene sheets. Both XPS and XANES analysis reveal that the reaction between oxygen functional groups in GO and ammonia vapor produces amide and amine species in hydrothermally treated GO (HTGO). Subsequent thermal annealing of the resultant HTGO introduces a gradual transformation of nitrogen bonding configurations in graphene sheets from amine N to pyridinic and graphitic N with the increase of annealing temperature. This study provides a simple but cost-effective and eco-friendly method to prepare N-doped graphene materials in large-scale for potential applications.

  12. Few-layer graphene as a support film for transmission electron microscopy imaging of nanoparticles.

    PubMed

    McBride, James R; Lupini, Andrew R; Schreuder, Michael A; Smith, Nathanael J; Pennycook, Stephen J; Rosenthal, Sandra J

    2009-12-01

    One consistent limitation for high-resolution imaging of small nanoparticles is the high background signal from the amorphous carbon support film. With interest growing for smaller and smaller nanostructures, state of the art electron microscopes are becoming necessary for rudimentary tasks, such as nanoparticle sizing. As a monolayer of carbon, free-standing graphene represents the ultimate support film for nanoparticle imaging. In this work, conventional high-resolution transmission electron microscopy (HRTEM) and aberration-corrected scanning transmission electron microscopy (STEM) were used to assess the benefits and feasibility of few-layer graphene support films. Suspensions of few-layer graphene to produce the support films were prepared by simple sonication of exfoliated graphite. The greatest benefit was observed for conventional HRTEM, where lattice resolved imaging of sub 2 nm CdSe nanocrystals was achieved. The few-layer graphene films were also used as a support film for C(s)-corrected STEM and electron energy loss spectroscopy of CuInSe(2) nanocrystals. PMID:20356171

  13. Effect of surface nitridation on the epitaxial growth of few-layer sp2 BN

    NASA Astrophysics Data System (ADS)

    Snure, Michael; Paduano, Qing; Kiefer, Arnold

    2016-02-01

    Boron Nitride is a promising 2D dielectric material for use in numerous electronic applications. In order to realize this potential, a process for producing atomically thin layers on microelectronics-compatible substrates is desirable. In this paper we describe an approach to epitaxially grow few-layer sp2 BN directly on an insulating substrate, using metal-organic chemical vapor deposition (MOCVD). We also elucidate the effect of sapphire surface nitridation on the growth characteristics. We compare the effect of nitridation on the growth rate, surface morphology and structure across a wide range of V/III ratios. Depending on the V/III ratio, two different growth modes were identified: at low V/III 3D island growth is dominant and at high V/III the growth transitions to a self-terminating mode. Under self-terminating growth a film thickness of 1.5 nm is typically achieved. Surface nitridation was found to improve nucleation, promoting self-terminating growth, and resulting in atomically smooth films. Reflection high energy electron diffraction (RHEED) patterns reveal the epitaxial relationship between BN and sapphire to be [1-100]‖[11-20] and [0001]‖[0001]. Growth at low V/III ratios without surface nitridation produced films with large hexagonal holes, which could not be completely filled by extending the growth time. Through surface nitridation, these holes were eliminated, producing continuous smooth films.

  14. Few-layer graphene growth from polystyrene as solid carbon source utilizing simple APCVD method

    NASA Astrophysics Data System (ADS)

    Ahmadi, Shahrokh; Afzalzadeh, Reza

    2016-07-01

    This research article presents development of an economical, simple, immune and environment friendly process to grow few-layer graphene by controlling evaporation rate of polystyrene on copper foil as catalyst and substrate utilizing atmospheric pressure chemical vapor deposition (APCVD) method. Evaporation rate of polystyrene depends on molecular structure, amount of used material and temperature. We have found controlling rate of evaporation of polystyrene by controlling the source temperature is easier than controlling the material weight. Atomic force microscopy (AFM) as well as Raman Spectroscopy has been used for characterization of the layers. The frequency of G‧ to G band ratio intensity in some samples varied between 0.8 and 1.6 corresponding to few-layer graphene. Topography characterization by atomic force microscopy confirmed Raman results.

  15. Room-temperature gating of molecular junctions using few-layer graphene nanogap electrodes

    NASA Astrophysics Data System (ADS)

    Barreiro, Amelia; Prins, Ferry; Ruitenberg, Justus; Seldenthuis, Johannes; Aliaga-Alcalde, Nuria; Vandersypen, Lieven; van der Zant, Herre

    2012-02-01

    We report on a new method based on feedback controlled electroburning to controllably form nanogaps in few-layer graphene [1]. The gaps have separations on the order of 1-2 nm as estimated from a Simmons model for tunneling. Furthermore, molecules are deposited in the nanogaps. These molecular junctions display gateable IV-characteristics at room temperature. Gateable transport through molecules contacted between the electrodes demonstrates the potential of room-temperature operation of our molecular devices. Combined with the observed stability in time, our study shows that few-layer graphene nanogaps are an interesting alternative to metal electrodes. [1] Ferry Prins, Amelia Barreiro, Justus Ruitenberg, Johannes Seldenthuis, N'uria Aliaga-Alcalde, Lieven Vandersypen, Herre van der Zant, Nanoletters 11 (2011) 4607 - 4611

  16. Unforeseen high temperature and humidity stability of FeCl3 intercalated few layer graphene.

    PubMed

    Wehenkel, Dominique Joseph; Bointon, Thomas Hardisty; Booth, Tim; Bøggild, Peter; Craciun, Monica Felicia; Russo, Saverio

    2015-01-01

    We present the first systematic study of the stability of the structure and electrical properties of FeCl3 intercalated few-layer graphene to high levels of humidity and high temperature. Complementary experimental techniques such as electrical transport, high resolution transmission electron microscopy and Raman spectroscopy conclusively demonstrate the unforseen stability of this transparent conductor to a relative humidity up to 100% at room temperature for 25 days, to a temperature up to 150°C in atmosphere and to a temperature as high as 620°C in vacuum, that is more than twice higher than the temperature at which the intercalation is conducted. The stability of FeCl3 intercalated few-layer graphene together with its unique values of low square resistance and high optical transparency, makes this material an attractive transparent conductor in future flexible electronic applications. PMID:25567796

  17. Unforeseen high temperature and humidity stability of FeCl3 intercalated few layer graphene

    PubMed Central

    Wehenkel, Dominique Joseph; Bointon, Thomas Hardisty; Booth, Tim; Bøggild, Peter; Craciun, Monica Felicia; Russo, Saverio

    2015-01-01

    We present the first systematic study of the stability of the structure and electrical properties of FeCl3 intercalated few-layer graphene to high levels of humidity and high temperature. Complementary experimental techniques such as electrical transport, high resolution transmission electron microscopy and Raman spectroscopy conclusively demonstrate the unforseen stability of this transparent conductor to a relative humidity up to 100% at room temperature for 25 days, to a temperature up to 150°C in atmosphere and to a temperature as high as 620°C in vacuum, that is more than twice higher than the temperature at which the intercalation is conducted. The stability of FeCl3 intercalated few-layer graphene together with its unique values of low square resistance and high optical transparency, makes this material an attractive transparent conductor in future flexible electronic applications. PMID:25567796

  18. EDITORIAL: Epitaxial graphene Epitaxial graphene

    NASA Astrophysics Data System (ADS)

    de Heer, Walt A.; Berger, Claire

    2012-04-01

    Graphene is widely regarded as an important new electronic material with interesting two-dimensional electron gas properties. Not only that, but graphene is widely considered to be an important new material for large-scale integrated electronic devices that may eventually even succeed silicon. In fact, there are countless publications that demonstrate the amazing applications potential of graphene. In order to realize graphene electronics, a platform is required that is compatible with large-scale electronics processing methods. It was clear from the outset that graphene grown epitaxially on silicon carbide substrates was exceptionally well suited as a platform for graphene-based electronics, not only because the graphene sheets are grown directly on electronics-grade silicon carbide (an important semiconductor in its own right), but also because these sheets are oriented with respect to the semiconductor. Moreover, the extremely high temperatures involved in production assure essentially defect-free and contamination-free materials with well-defined interfaces. Epitaxial graphene on silicon carbide is not a unique material, but actually a class of materials. It is a complex structure consisting of a reconstructed silicon carbide surface, which, for planar hexagonal silicon carbide, is either the silicon- or the carbon-terminated face, an interfacial carbon rich layer, followed by one or more graphene layers. Consequently, the structure of graphene films on silicon carbide turns out to be a rich surface-science puzzle that has been intensively studied and systematically unravelled with a wide variety of surface science probes. Moreover, the graphene films produced on the carbon-terminated face turn out to be rotationally stacked, resulting in unique and important structural and electronic properties. Finally, in contrast to essentially all other graphene production methods, epitaxial graphene can be grown on structured silicon carbide surfaces to produce graphene

  19. Direct exfoliation of natural graphite into micrometer size few layers graphene sheets using ionic liquids

    SciTech Connect

    Wang, Xiqing; Fulvio, Pasquale F; Baker, Gary A; Veith, Gabriel M; Unocic, Raymond R; Mahurin, Shannon Mark; Dai, Sheng

    2010-01-01

    Stable high-concentration suspensions (up to 0.95 mg mL{sup -1}) of non-oxidized few layer graphene (FLG), five or less sheets, with micrometre-long edges were obtained via direct exfoliation of natural graphite flakes in ionic liquids, such as 1-butyl-3-methyl-imidazolium bis(trifluoro-methane-sulfonyl)imide ([Bmim]-[Tf{sub 2}N]), by tip ultrasonication.

  20. Direct exfoliation of natural graphite into micrometer size few layers graphene sheets using ionic liquids

    SciTech Connect

    Wang, X.; Fulvio, P. F.; Baker, G. A.; Veith, G. M.; Unocic, R. R.; Mahurin, S., M.; Chi, M.; Dai, S.

    2010-01-01

    Stable high-concentration suspensions (up to 0.95 mg mL-1) of non-oxidized few layer graphene (FLG), five or less sheets, with micrometre-long edges were obtained via direct exfoliation of natural graphite flakes in ionic liquids, such as 1-butyl-3-methyl-imidazolium bis(trifluoro-methane-sulfonyl)imide ([Bmim]-[Tf2N]), by tip ultrasonication.

  1. Dynamic negative compressibility of few-layer graphene, h-BN, and MoS2.

    PubMed

    Barboza, Ana Paula M; Chacham, Helio; Oliveira, Camilla K; Fernandes, Thales F D; Ferreira, Erlon H Martins; Archanjo, Braulio S; Batista, Ronaldo J C; de Oliveira, Alan B; Neves, Bernardo R A

    2012-05-01

    We report a novel mechanical response of few-layer graphene, h-BN, and MoS(2) to the simultaneous compression and shear by an atomic force microscope (AFM) tip. The response is characterized by the vertical expansion of these two-dimensional (2D) layered materials upon compression. Such effect is proportional to the applied load, leading to vertical strain values (opposite to the applied force) of up to 150%. The effect is null in the absence of shear, increases with tip velocity, and is anisotropic. It also has similar magnitudes in these solid lubricant materials (few-layer graphene, h-BN, and MoS(2)), but it is absent in single-layer graphene and in few-layer mica and Bi(2)Se(3). We propose a physical mechanism for the effect where the combined compressive and shear stresses from the tip induce dynamical wrinkling on the upper material layers, leading to the observed flake thickening. The new effect (and, therefore, the proposed wrinkling) is reversible in the three materials where it is observed. PMID:22468807

  2. Environmental Synthesis of Few Layers Graphene Sheets Using Ultrasonic Exfoliation with Enhanced Electrical and Thermal Properties

    PubMed Central

    Noroozi, Monir; Zakaria, Azmi; Radiman, Shahidan; Abdul Wahab, Zaidan

    2016-01-01

    In this paper, we report how few layers graphene that can be produced in large quantity with low defect ratio from exfoliation of graphite by using a high intensity probe sonication in water containing liquid hand soap and PVP. It was founded that the graphene powder obtained by this simple exfoliation method after the heat treatment had an excellent exfoliation into a single or layered graphene sheets. The UV-visible spectroscopy, FESEM, TEM, X-ray powder diffraction and Raman spectroscopy was used to analyse the graphene product. The thermal diffusivity of the samples was analysed using a highly accurate thermal-wave cavity photothermal technique. The data obtained showed excellent enhancement in the thermal diffusivity of the graphene dispersion. This well-dispersed graphene was then used to fabricate an electrically conductive polymer-graphene film composite. The results demonstrated that this low cost and environmental friendly technique allowed to the production of high quality layered graphene sheets, improved the thermal and electrical properties. This may find use in the wide range of applications based on graphene. PMID:27064575

  3. Environmental Synthesis of Few Layers Graphene Sheets Using Ultrasonic Exfoliation with Enhanced Electrical and Thermal Properties.

    PubMed

    Noroozi, Monir; Zakaria, Azmi; Radiman, Shahidan; Abdul Wahab, Zaidan

    2016-01-01

    In this paper, we report how few layers graphene that can be produced in large quantity with low defect ratio from exfoliation of graphite by using a high intensity probe sonication in water containing liquid hand soap and PVP. It was founded that the graphene powder obtained by this simple exfoliation method after the heat treatment had an excellent exfoliation into a single or layered graphene sheets. The UV-visible spectroscopy, FESEM, TEM, X-ray powder diffraction and Raman spectroscopy was used to analyse the graphene product. The thermal diffusivity of the samples was analysed using a highly accurate thermal-wave cavity photothermal technique. The data obtained showed excellent enhancement in the thermal diffusivity of the graphene dispersion. This well-dispersed graphene was then used to fabricate an electrically conductive polymer-graphene film composite. The results demonstrated that this low cost and environmental friendly technique allowed to the production of high quality layered graphene sheets, improved the thermal and electrical properties. This may find use in the wide range of applications based on graphene. PMID:27064575

  4. Atomic-scale observation of rotational misorientation in suspended few-layer graphene sheets.

    PubMed

    Singh, Manoj K; Titus, Elby; Gonçalves, Gil; Marques, Paula A A P; Bdikin, Igor; Kholkin, Andrei L; Gracio, José J A

    2010-05-01

    Single or few-layer graphene (FLG) sheets offer extraordinary electronic, thermal and mechanical properties and are expected to find a variety of applications. Fully exploiting the properties of graphene will require a method for the production of high-quality graphene sheets (almost pristine graphene) in large quantities. In this regard, we report a two-step method for obtaining a homogenous colloidal suspension of single or FLG sheets up to 0.15 mg ml(-1) in N,N-dimethylformamide solution. The graphene nanostructures are directly imaged using a high-resolution transmission electron microscope (HRTEM) operated at 200 kV with a point resolution of 0.16 nm. We observed rotational misorientation within the flake in the HRTEM images of 2, 4 and 6 layers of graphene sheets, giving rise to Moiré patterns. By filtering in the frequency domain using a Fourier transform, we reconstruct the graphene lattice of each sheet and determine the relative rotation between consecutive graphene layers up, to six separate sheets. Direct evidence is obtained for FLG sheets with packing that is different to the standard AB Bernal packing of bulk graphite. Furthermore, we observed periodic ripples in suspended graphene sheets in our TEM measurements. Electrostatic force microscopy was used to characterize the electric potential distribution on the surface of FLG sheets on SiO2/Si substrates in ambient conditions. The FLG sheets were found to exhibit a conducting nature with small potential variations on the surface. PMID:20648314

  5. Extremely large magnetoresistance in few-layer graphene/boron–nitride heterostructures

    PubMed Central

    Gopinadhan, Kalon; Shin, Young Jun; Jalil, Rashid; Venkatesan, Thirumalai; Geim, Andre K.; Neto, Antonio H. Castro; Yang, Hyunsoo

    2015-01-01

    Understanding magnetoresistance, the change in electrical resistance under an external magnetic field, at the atomic level is of great interest both fundamentally and technologically. Graphene and other two-dimensional layered materials provide an unprecedented opportunity to explore magnetoresistance at its nascent stage of structural formation. Here we report an extremely large local magnetoresistance of∼2,000% at 400 K and a non-local magnetoresistance of >90,000% in an applied magnetic field of 9 T at 300 K in few-layer graphene/boron–nitride heterostructures. The local magnetoresistance is understood to arise from large differential transport parameters, such as the carrier mobility, across various layers of few-layer graphene upon a normal magnetic field, whereas the non-local magnetoresistance is due to the magnetic field induced Ettingshausen–Nernst effect. Non-local magnetoresistance suggests the possibility of a graphene-based gate tunable thermal switch. In addition, our results demonstrate that graphene heterostructures may be promising for magnetic field sensing applications. PMID:26388149

  6. The effect of spin-orbit coupling in band structure of few-layer graphene

    SciTech Connect

    Sahdan, Muhammad Fauzi Darma, Yudi

    2014-03-24

    Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducting states on their edge or surface. This can be happened due to spin-orbit coupling and time-reversal symmetry. Moreover, the edge current flows through their edge or surface depends on its spin orientation and also it is robust against non-magnetic impurities. Therefore, topological insulators are predicted to be useful ranging from spintronics to quantum computation. Graphene was first predicted to be the precursor of topological insulator by Kane-Mele. They developed a Hamiltonian model to describe the gap opening in graphene. In this work, we investigate the band structure of few-layer graphene by using this model with analytical approach. The results of our calculations show that the gap opening occurs at K and K’ point, not only in single layer, but also in bilayer and trilayer graphene.

  7. Large-area few-layered graphene film determination by multispectral imaging microscopy.

    PubMed

    Wang, Hsiang-Chen; Huang, Shih-Wei; Yang, Jhe-Ming; Wu, Guan-Huang; Hsieh, Ya-Ping; Feng, Shih-Wei; Lee, Min Kai; Kuo, Chie-Tong

    2015-05-21

    A multispectral imaging method for the rapid and accurate identification of few-layered graphene using optical images is proposed. Commonly rapid identification relies on optical interference effects which limits the choice of substrates and light sources. Our method is based on the comparison of spectral characteristics with principle components from a database which is populated by correlation of micro-Raman registration, spectral characteristics, and optical microscopy. Using this approach the thickness and extent of different graphene layers can be distinguished without the contribution of the optical interference effects and allows characterization of graphene on glass substrates. The high achievable resolution, easy implementation and large scale make this approach suitable for the in-line metrology of industrial graphene production. PMID:25921320

  8. Determination of the stacking order of curved few-layered graphene systems

    NASA Astrophysics Data System (ADS)

    Hayashi, Takuya; MuramatsuCurrent Affiliation: Department Of Materials Science; Technology, Nagaoka University Of Technology, 1603-1, Kamitomioka, Nagaoka, 940-2188, Japan, Hiroyuki; ShimamotoCurrent Affiliation: Advanced Manufacturing Research Institute, Aist, 2266-98 Anagahora, Shimoshidami, Moriyama-Ku, Nagoya 463-8560, Japan, Daisuke; Fujisawa, Kazunori; Tojo, Tomohiro; Muramoto, Yoshitaka; Yokomae, Takuya; Asaoka, Toru; Kim, Yoong Ahm; Terrones, Mauricio; Endo, Morinobu

    2012-09-01

    We report a facile method to efficiently visualize the atomic carbon network of curved few-layered graphitic systems including folded bi-layer graphene, nanoribbon edges and multi-walled carbon nanotubes (straight and bent), via the processing of aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) images. This technique is also able to atomically resolve the structure of overlapping graphene layers with different orientations, thus enabling us to determine the stacking order of multiple graphene layers. To the best of our knowledge, we are the first to identify the stacking order of a misoriented 4-layer closed-edge graphene and a metal-semiconductor double-walled carbon nanotube junction.

  9. Preconcentration of U(VI) ions on few-layered graphene oxide nanosheets from aqueous solutions.

    PubMed

    Zhao, Guixia; Wen, Tao; Yang, Xin; Yang, Shubin; Liao, Jiali; Hu, Jun; Shao, Dadong; Wang, Xiangke

    2012-05-28

    Graphene oxide nanosheets have attracted multidisciplinary attention due to their unique physicochemical properties. Herein, few-layered graphene oxide nanosheets were synthesized from graphite using a modified Hummers method and were characterized by TEM, AFM, Raman spectroscopy, XPS, FTIR spectroscopy, TG-DTA and acid-base titrations. The prepared few-layered graphene oxide nanosheets were used as adsorbents for the preconcentration of U(VI) ions from large volumes of aqueous solutions as a function of pH, ionic strength and temperature. The sorption of U(VI) ions on the graphene oxide nanosheets was strongly dependent on pH and independent of the ionic strength, indicating that the sorption was mainly dominated by inner-sphere surface complexation rather than by outer-sphere surface complexation or ion exchange. The abundant oxygen-containing functional groups on the surfaces of the graphene oxide nanosheets played an important role in U(VI) sorption. The sorption of U(VI) on graphene oxide nanosheets increased with an increase in temperature and the thermodynamic parameters calculated from the temperature-dependent sorption isotherms suggested that the sorption of U(vi) on graphene oxide nanosheets was an endothermic and spontaneous process. The maximum sorption capacities (Q(max)) of U(VI) at pH 5.0 ± 0.1 and T = 20 °C was 97.5 mg g(-1), which was much higher than any of the currently reported nanomaterials. The graphene oxide nanosheets may be suitable materials for the removal and preconcentration of U(VI) ions from large volumes of aqueous solutions, for example, U(VI) polluted wastewater, if they can be synthesized in a cost-effective manner on a large scale in the future. PMID:22473651

  10. Controlling single and few-layer graphene crystals growth in a solid carbon source based chemical vapor deposition

    SciTech Connect

    Papon, Remi; Sharma, Subash; Shinde, Sachin M.; Vishwakarma, Riteshkumar; Tanemura, Masaki; Kalita, Golap

    2014-09-29

    Here, we reveal the growth process of single and few-layer graphene crystals in the solid carbon source based chemical vapor deposition (CVD) technique. Nucleation and growth of graphene crystals on a polycrystalline Cu foil are significantly affected by the injection of carbon atoms with pyrolysis rate of the carbon source. We observe micron length ribbons like growth front as well as saturated growth edges of graphene crystals depending on growth conditions. Controlling the pyrolysis rate of carbon source, monolayer and few-layer crystals and corresponding continuous films are obtained. In a controlled process, we observed growth of large monolayer graphene crystals, which interconnect and merge together to form a continuous film. On the other hand, adlayer growth is observed with an increased pyrolysis rate, resulting few-layer graphene crystal structure and merged continuous film. The understanding of monolayer and few-layer crystals growth in the developed CVD process can be significant to grow graphene with controlled layer numbers.

  11. Controllable chemical vapor deposition growth of few layer graphene for electronic devices.

    PubMed

    Wei, Dacheng; Wu, Bin; Guo, Yunlong; Yu, Gui; Liu, Yunqi

    2013-01-15

    Because of its atomic thickness, excellent properties, and widespread applications, graphene is regarded as one of the most promising candidate materials for nanoelectronics. The wider use of graphene will require processes that produce this material in a controllable manner. In this Account, we focus on our recent studies of the controllable chemical vapor deposition (CVD) growth of graphene, especially few-layer graphene (FLG), and the applications of this material in electronic devices. CVD provides various means of control over the morphologies of the produced graph ene. We studied several variables that can affect the CVD growth of graphene, including the catalyst, gas flow rate, growth time, and growth temperature and successfully achieved the controlled growth of hexagonal graphene crystals. Moreover, we developed several modified CVD methods for the controlled growth of FLGs. Patterned CVD produced FLGs with desired shapes in required areas. By introducing dopant precursor in the CVD process, we produced substitutionally doped FLGs, avoiding the typically complicated post-treatment processes for graphene doping. We developed a template CVD method to produce FLG ribbons with controllable morphologies on a large scale. An oxidation-activated surface facilitated the CVD growth of polycrystalline graphene without the use of a metal catalyst or a complicated postgrowth transfer process. In devices, CVD offers a controllable means to modulate the electronic properties of the graphene samples and to improve device performance. Using CVD-grown hexagonal graphene crystals as the channel materials in field-effect transistors (FETs), we improved carrier mobility. Substitutional doping of graphene in CVD opened a band gap for efficient FET operation and modulated the Fermi energy level for n-type or p-type features. The similarity between the chemical structure of graphene and organic semiconductors suggests potential applications of graphene in organic devices. We

  12. Structural and optical properties of cobalt slanted nanopillars conformally coated with few-layer graphene

    SciTech Connect

    Wilson, Peter M.; Lipatov, Alexey; Schmidt, Daniel; Schubert, Eva; Schubert, Mathias; Hofmann, Tino E-mail: thofmann@engr.unl.edu; Sinitskii, Alexander E-mail: thofmann@engr.unl.edu

    2015-06-08

    Optical characterization of anisotropic multicomponent nanostructures is generally not a trivial task, since the relation between a material's structural properties and its permittivity tensor is nonlinear. In this regard, an array of slanted cobalt nanopillars that are conformally coated with few-layer graphene is a particularly challenging object for optical characterization, as it has a complex anisotropic geometry and comprises several materials with different topologies and filling fractions. Normally, a detailed characterization of such complex nanostructures would require a combination of several microscopic and spectroscopic techniques. In this letter, we demonstrate that the important structural parameters of these graphene-coated sculptured thin films can be determined using a fast and simple generalized spectroscopic ellipsometry test combined with an anisotropic Bruggeman effective medium approximation. The graphene coverage as well as structural parameters of nanostructured thin films agree excellently with electron microscopy and Raman spectroscopy observations. The demonstrated optical approach may also be applied to the characterization of other nanostructured materials.

  13. Crystallographic growth and alignment of carbon nanotubes on few-layer graphene

    NASA Astrophysics Data System (ADS)

    Arash, Aram; Hunley, Patrick D.; Nasseri, Mohsen; Boland, Mathias J.; Sundararajan, Abhishek; Hudak, Bethany M.; Guiton, Beth S.; Strachan, Douglas R.

    2015-03-01

    Hybrid carbon nanotube and graphene structures are emerging as an exciting material system built from a common sp2 carbon backbone. Such hybrid systems have promise for use in improving the performance of energy storage and high-speed electronic applications. Towards the attainment of such hybrid materials, the catalytic growth and crystallographic alignment of these integrated structures are investigated along with the atomic-scale features of their interfaces. The catalytic activity of nanoparticles to form carbon nanotubes on the surface of few-layer graphene is tuned through precise feedstock application. Through careful materials synthesis, the interfaces of these hybrid carbon nanotube - graphene systems are investigated through ultra-high resolution electron microscopy.

  14. Wafer-scale arrayed p-n junctions based on few-layer epitaxial GaTe

    NASA Astrophysics Data System (ADS)

    Yuan, Xiang; Tang, Lei; Hu, Weida; Xiu, Faxian

    2015-03-01

    Two dimensional (2D) materials have showed appealing applications in electronics and optoelectronics. Gapless graphene presents ultra-broadband and fast photoresponse while the 2D semiconducting MoS2 and GaTe exhibit highly sensitive and tunable responsivity to the visible light. However, the device yield and its repeatability call for a further improvement of 2D materials to render large-scale uniformity. Here we report a layer-by-layer growth of the wafer-scale GaTe by molecular beam epitaxy. To develop the arrayed p-n junctions, the few-layer GaTe was grew on three-inch Si wafers. The resultant diodes reveal good rectifying characteristics and photoresponse with maximum photodetection responsivity of 2.74 A/W and photovoltaic external quantum efficiency up to 62%. The photocurrent reaches saturation very fast within 22 μs and shows no sign of device degradation after 1.37 million cycles of operation. Most strikingly, such high performance has been achieved across the entire wafer, making the volume production of devices accessible. Finally, several photo-images was acquired by using these photodiodes with a reasonable contrast and resolution, demonstrating for the first time the potential for these 2D technology coming into the real life.

  15. Structure-Dependent Fano Resonances in the Infrared Spectra of Phonons in Few-Layer Graphene

    SciTech Connect

    Li Z.; Carr G.; Chun Hung,L.; Cappelluti, E.; Benfatto, L.; Mak, K.F.; Jie, S.; Heinz, T.F.

    2012-04-10

    The in-plane optical phonons around 200 meV in few-layer graphene are investigated utilizing infrared absorption spectroscopy. The phonon spectra exhibit unusual asymmetric features characteristic of Fano resonances, which depend critically on the layer thickness and stacking order of the sample. The phonon intensities in samples with rhombohedral (ABC) stacking are significantly higher than those with Bernal (AB) stacking. These observations reflect the strong coupling between phonons and interband electronic transitions in these systems and the distinctive variation in the joint density of electronic states in samples of differing thickness and stacking order.

  16. Influence of irradiation upon few-layered graphene using electron-beams and gamma-rays

    SciTech Connect

    Wang, Yuqing; Feng, Yi Mo, Fei; Qian, Gang; Chen, Yangming; Yu, Dongbo; Wang, Yang; Zhang, Xuebin

    2014-07-14

    Few-layered graphene (FLG) is irradiated by electron beams and gamma rays. After 100 keV electron irradiation, the edges of FLG start bending, shrinking, and finally generate gaps and carbon onions due to sputtering and knock-on damage mechanism. When the electron beam energy is increased further to 200 keV, FLG suffers rapid and catastrophic destruction. Unlike electron irradiation, Compton effect is the dominant damage mechanism in gamma irradiation. The irradiation results indicate the crystallinity of FLG decreases first, then restores as increasing irradiation doses, additionally, the ratio (O/C) of FLG surface and the relative content of oxygen groups increases after irradiation.

  17. Low defect concentration few-layer graphene using a two-step electrochemical exfoliation

    NASA Astrophysics Data System (ADS)

    Huang, Xuhua; Li, Senlin; Qi, Zhiqiang; Zhang, Wei; Ye, Wei; Fang, Yanyan

    2015-03-01

    Low defect concentration few-layer graphene (FLG) sheets were fabricated by a two-step electrochemical intercalation exfoliation, including a graphite foil pretreatment in sodium hydroxide solution and a subsequent further exfoliation in sulfuric acid solution. During the process, the pretreatment results in the expansion of the graphite foil and in turn facilitates the final exfoliation in sulfuric acid solution. The results show that the ID/IG of the obtained FLG sheets is as low as 0.29 while maintaining relatively high yield, more than 56%. In addition, the oxygen content in the FLG sheets is 8.32% with the C/O ratio of 11.02.

  18. High-yield synthesis of few-layer graphene flakes through electrochemical expansion of graphite in propylene carbonate electrolyte.

    PubMed

    Wang, Junzhong; Manga, Kiran Kumar; Bao, Qiaoliang; Loh, Kian Ping

    2011-06-15

    High-yield production of few-layer graphene flakes from graphite is important for the scalable synthesis and industrial application of graphene. However, high-yield exfoliation of graphite to form graphene sheets without using any oxidation process or super-strong acid is challenging. Here we demonstrate a solution route inspired by the lithium rechargeable battery for the high-yield (>70%) exfoliation of graphite into highly conductive few-layer graphene flakes (average thickness <5 layers). A negative graphite electrode can be electrochemically charged and expanded in an electrolyte of Li salts and organic solvents under high current density and exfoliated efficiently into few-layer graphene sheets with the aid of sonication. The dispersible graphene can be ink-brushed to form highly conformal coatings of conductive films (15 ohm/square at a graphene loading of <1 mg/cm(2)) on commercial paper. PMID:21557613

  19. Few layers isolated graphene domains grown on copper foils by microwave surface wave plasma CVD using camphor as a precursor

    NASA Astrophysics Data System (ADS)

    Ram Aryal, Hare; Adhikari, Sudip; Uchida, Hideo; Wakita, Koichi; Umeno, Masayoshi

    2016-03-01

    Few layers isolated graphene domains were grown by microwave surface wave plasma CVD technique using camphor at low temperature. Graphene nucleation centers were suppressed on pre-annealed copper foils by supplying low dissociation energy. Scanning electron microscopy study of time dependent growth reveals that graphene nucleation centers were preciously suppressed, which indicates the possibility of controlled growth of large area single crystal graphene domains by plasma processing. Raman spectroscopy revealed that the graphene domains are few layered which consist of relatively low defects.

  20. Few-layered graphene oxide nanosheets as superior sorbents for heavy metal ion pollution management.

    PubMed

    Zhao, Guixia; Li, Jiaxing; Ren, Xuemei; Chen, Changlun; Wang, Xiangke

    2011-12-15

    Graphene has attracted multidisciplinary study because of its unique physicochemical properties. Herein, few-layered graphene oxide nanosheets were synthesized from graphite using the modified Hummers method, and were used as sorbents for the removal of Cd(II) and Co(II) ions from large volumes of aqueous solutions. The effects of pH, ionic strength, and humic acid on Cd(II) and Co(II) sorption were investigated. The results indicated that Cd(II) and Co(II) sorption on graphene oxide nanosheets was strongly dependent on pH and weakly dependent on ionic strength. The abundant oxygen-containing functional groups on the surfaces of graphene oxide nanosheets played an important role on Cd(II) and Co(II) sorption. The presence of humic acid reduced Cd(II) and Co(II) sorption on graphene oxide nanosheets at pH < 8. The maximum sorption capacities (C(smax)) of Cd(II) and Co(II) on graphene oxide nanosheets at pH 6.0 ± 0.1 and T = 303 K were about 106.3 and 68.2 mg/g, respectively, higher than any currently reported. The thermodynamic parameters calculated from temperature-dependent sorption isotherms suggested that Cd(II) and Co(II) sorptions on graphene oxide nanosheets were endothermic and spontaneous processes. The graphene oxide nanosheets may be suitable materials in heavy metal ion pollution cleanup if they are synthesized in large scale and at low price in near future. PMID:22070750

  1. Chemical Vapor Deposited Few-Layer Graphene as an Electron Field Emitter.

    PubMed

    Behural, Sanjay K; Nayak, Sasmita; Yang, Qiaoqin; Hirose, Akira; Janil, Omkar

    2016-01-01

    Chemical vapor deposition (CVD) growth of graphene on polycrystalline copper (Cu) foil in a low pressure conditions has been presented, aiming to achieve the highest quality with large-scale fabrications, which requires comprehensive understanding and effective controlling of the growth process. Herein, few-layer graphene (FLG) films with large-domain sizes were grown on Cu metal catalyst substrates using a vertical mass-flow hot-filament CVD reactor, with the intention of large scale production, by optimizing the CVD system and three of the process parameters: (i) gas flow compositions, (ii) substrate annealing time and (iii) graphene deposition time. The detailed scanning electron microscope and Raman spectroscopy analysis indicate that all the above mentioned process parameters affect growth of FLG film on Cu substrate. The presence of two intense peaks, G and 2D-band at 1583.6 and 2702.6 cm⁻¹ for synthesized sample at optimized conditions (H₂/CH₄ ratio of 50:1 at graphene deposition time of 10 minutes and substrate annealed time for 20 minutes) revealed the formation of FLG films with large domain size. These graphene films on Cu have shown the room temperature field electron emission characteristics, hence appears to be prospective candidate for vacuum nanoelectronics. PMID:27398456

  2. Transparent actuator made with few layer graphene electrode and dielectric elastomer, for variable focus lens

    NASA Astrophysics Data System (ADS)

    Hwang, Taeseon; Kwon, Hyeok-Yong; Oh, Joon-Suk; Hong, Jung-Pyo; Hong, Seung-Chul; Lee, Youngkwan; Ryeol Choi, Hyouk; Jin Kim, Kwang; Hossain Bhuiya, Mainul; Nam, Jae-Do

    2013-07-01

    A transparent dielectric elastomer actuator driven by few-layer-graphene (FLG) electrode was experimentally investigated. The electrodes were made of graphene, which was dispersed in N-methyl-pyrrolidone. The transparent actuator was fabricated from developed FLG electrodes. The FLG electrode with its sheet resistance of 0.45 kΩ/sq (80 nm thick) was implemented to mask silicone elastomer. The developed FLG-driven actuator exhibited an optical transparency of over 57% at a wavenumber of 600 nm and produced bending displacement performance ranging from 29 to 946 μm as functions of frequency and voltage. The focus variation was clearly demonstrated under actuation to study its application-feasibility in variable focus lens and various opto-electro-mechanical devices.

  3. Preparation of few-layer graphene-capped boron nanowires and their field emission properties

    NASA Astrophysics Data System (ADS)

    Yong-Xin, Zhang; Fei, Liu; Cheng-Min, Shen; Tian-Zhong, Yang; Jun, Li; Shao-Zhi, Deng; Ning-Sheng, Xu; Hong-Jun, Gao

    2016-07-01

    Large-area boron nanowire (BNW) films were fabricated on the Si(111) substrate by chemical vapor deposition (CVD). The average diameter of the BNWs is about 20 nm, with lengths of 5–10 μm. Then, graphene-capped boron nanowires (GC-BNWs) were obtained by microwave plasma chemical vapor deposition (MPCVD). Characterization by scanning electron microscopy indicates that few-layer graphene covers the surface of the boron nanowires. Field emission measurements of the BNWs and GC-BNW films show that the GC-BNW films have a lower turn-on electric field than the BNW films. Project supported by the National Basic Research Program of China (Grant No. 2013CB933604), the National Natural Science Foundation of China (Grant No. 51572290), and the Chinese Academy of Sciences (Grant Nos. 1731300500015 and XDB07030100).

  4. Tunable photoresponse with small drain voltage in few-layer graphene-WSe2 heterostructures

    NASA Astrophysics Data System (ADS)

    Luo, Wei; Qin, Shiqiao; Long, Mingsheng; Liu, Erfu; Fu, Yajun; Zhou, Wei; Miao, Feng; Zhang, Sen; Zhang, Renyan; Zhang, Xue-Ao

    2016-07-01

    Two-dimensional layered heterostructures show great potential to develop optoelectronic systems. Here, we have investigated the photoresponse properties of two contact interfaces in few-layer graphene-WSe2 heterostructures. The photoresponsivity of graphene-WSe2 contact interface is about 2.67 mA/W, and the photoresponsivity of WSe2-metal contact interface is about 0.2 mA/W. Photocurrent images show that the two contact interfaces behave differently under drain voltage from - 0.5 V to 0.5 V. The photoresponsivity of one contact interface increases with the drain voltage, and that of the other decreases with the drain voltage. Experimental results and band diagram studies prove that the photoresponse properties of contact interfaces are tuned by small drain voltage. This study will be beneficial for understanding the effect of drain voltage on the heterostructures.

  5. Spectroscopic characterization of charge carrier anisotropic motion in twisted few-layer graphene

    NASA Astrophysics Data System (ADS)

    Kandyba, Viktor; Yablonskikh, Mikhail; Barinov, Alexei

    2015-11-01

    Graphene, a layer of carbon atoms in a honeycomb lattice, captures enormous interest as probably the most promising component of future electronics thanks to its mechanical robustness, flexibility, and unique charge carrier quasiparticles propagating like massless high energy Dirac fermions. If several graphene layers form a stack, the interaction between them is, on the one hand, weak, allowing realization of various registries between the layers and, on the other hand, strong enough for a wide range tuning of the electronic properties. Here we grow few layer graphene with various number of layers and twist configurations and address the electronic properties of individual atomic layers in single microscopic domains using angle-resolved photoelectron spectromicroscopy. The dependence of the interlayer coupling on the twist angle is analyzed and, in the domains with tri-layers and more, if different rotations are present, the electrons in weaker coupled adjacent layers are shown to have different properties manifested by coexisting van Hove singularities, moiré superlattices with corresponding superlattice Dirac points, and charge carrier group velocity renormalizations. Moreover, pronounced anisotropy in the charge carrier motion, opening a possibility to transform strongly coupled graphene bilayers into quasi one-dimensional conductors, is observed.

  6. Interlayer catalytic exfoliation realizing scalable production of large-size pristine few-layer graphene.

    PubMed

    Geng, Xiumei; Guo, Yufen; Li, Dongfang; Li, Weiwei; Zhu, Chao; Wei, Xiangfei; Chen, Mingliang; Gao, Song; Qiu, Shengqiang; Gong, Youpin; Wu, Liqiong; Long, Mingsheng; Sun, Mengtao; Pan, Gebo; Liu, Liwei

    2013-01-01

    Mass production of reduced graphene oxide and graphene nanoplatelets has recently been achieved. However, a great challenge still remains in realizing large-quantity and high-quality production of large-size thin few-layer graphene (FLG). Here, we create a novel route to solve the issue by employing one-time-only interlayer catalytic exfoliation (ICE) of salt-intercalated graphite. The typical FLG with a large lateral size of tens of microns and a thickness less than 2 nm have been obtained by a mild and durative ICE. The high-quality graphene layers preserve intact basal crystal planes owing to avoidance of the degradation reaction during both intercalation and ICE. Furthermore, we reveal that the high-quality FLG ensures a remarkable lithium-storage stability (>1,000 cycles) and a large reversible specific capacity (>600 mAh g(-1)). This simple and scalable technique acquiring high-quality FLG offers considerable potential for future realistic applications. PMID:23355949

  7. Spectroscopic characterization of charge carrier anisotropic motion in twisted few-layer graphene

    PubMed Central

    Kandyba, Viktor; Yablonskikh, Mikhail; Barinov, Alexei

    2015-01-01

    Graphene, a layer of carbon atoms in a honeycomb lattice, captures enormous interest as probably the most promising component of future electronics thanks to its mechanical robustness, flexibility, and unique charge carrier quasiparticles propagating like massless high energy Dirac fermions. If several graphene layers form a stack, the interaction between them is, on the one hand, weak, allowing realization of various registries between the layers and, on the other hand, strong enough for a wide range tuning of the electronic properties. Here we grow few layer graphene with various number of layers and twist configurations and address the electronic properties of individual atomic layers in single microscopic domains using angle-resolved photoelectron spectromicroscopy. The dependence of the interlayer coupling on the twist angle is analyzed and, in the domains with tri-layers and more, if different rotations are present, the electrons in weaker coupled adjacent layers are shown to have different properties manifested by coexisting van Hove singularities, moiré superlattices with corresponding superlattice Dirac points, and charge carrier group velocity renormalizations. Moreover, pronounced anisotropy in the charge carrier motion, opening a possibility to transform strongly coupled graphene bilayers into quasi one-dimensional conductors, is observed. PMID:26548567

  8. Interlayer catalytic exfoliation realizing scalable production of large-size pristine few-layer graphene

    NASA Astrophysics Data System (ADS)

    Geng, Xiumei; Guo, Yufen; Li, Dongfang; Li, Weiwei; Zhu, Chao; Wei, Xiangfei; Chen, Mingliang; Gao, Song; Qiu, Shengqiang; Gong, Youpin; Wu, Liqiong; Long, Mingsheng; Sun, Mengtao; Pan, Gebo; Liu, Liwei

    2013-01-01

    Mass production of reduced graphene oxide and graphene nanoplatelets has recently been achieved. However, a great challenge still remains in realizing large-quantity and high-quality production of large-size thin few-layer graphene (FLG). Here, we create a novel route to solve the issue by employing one-time-only interlayer catalytic exfoliation (ICE) of salt-intercalated graphite. The typical FLG with a large lateral size of tens of microns and a thickness less than 2 nm have been obtained by a mild and durative ICE. The high-quality graphene layers preserve intact basal crystal planes owing to avoidance of the degradation reaction during both intercalation and ICE. Furthermore, we reveal that the high-quality FLG ensures a remarkable lithium-storage stability (>1,000 cycles) and a large reversible specific capacity (>600 mAh g-1). This simple and scalable technique acquiring high-quality FLG offers considerable potential for future realistic applications.

  9. Electronic properties of impurity-infected few-layer graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Mousavi, Hamze; Bagheri, Mehran

    2015-02-01

    Spurred by achievements in devising different multilayered graphene-based nano-systems, based on the random tight-binding Hamiltonian model and within the coherent potential approximation, the influence of varying the number of layers and the effect of doping by the boron and nitrogen impurities on the density of states of a mono- and few-layer armchair- and zigzag-edge graphene nanoribbons are theoretically investigated. When the nanoribbons are pristine, with increasing the number of layers the band gap of the armchair nanoribbons is decreased, yet the zigzag ribbons remain metallic and depending on the number of the layers few peaks are appeared around the zero-energy level. Moreover, in the presence of impurities, the band gap of the armchair nanoribbons is decreased for each number of layers. The Van-Hove singularities are steadily broadened and the density of states move to a higher (lower) value of the energy as a result of doping with boron (nitrogen) atoms. This study could provide with us to explore and devise new optoelectronic devices based on the impurity-infected graphene nanoribbons with tunable widths and edges.

  10. Electrostatic force microscopy and electrical isolation of etched few-layer graphene nano-domains

    SciTech Connect

    Hunley, D. Patrick; Sundararajan, Abhishek; Boland, Mathias J.; Strachan, Douglas R.

    2014-12-15

    Nanostructured bi-layer graphene samples formed through catalytic etching are investigated with electrostatic force microscopy. The measurements and supporting computations show a variation in the microscopy signal for different nano-domains that are indicative of changes in capacitive coupling related to their small sizes. Abrupt capacitance variations detected across etch tracks indicates that the nano-domains have strong electrical isolation between them. Comparison of the measurements to a resistor-capacitor model indicates that the resistance between two bi-layer graphene regions separated by an approximately 10 nm wide etch track is greater than about 1×10{sup 12} Ω with a corresponding gap resistivity greater than about 3×10{sup 14} Ω⋅nm. This extremely large gap resistivity suggests that catalytic etch tracks within few-layer graphene samples are sufficient for providing electrical isolation between separate nano-domains that could permit their use in constructing atomically thin nanogap electrodes, interconnects, and nanoribbons.

  11. Tribological characteristics of few-layer graphene over Ni grain and interface boundaries.

    PubMed

    Tripathi, Manoj; Awaja, Firas; Paolicelli, Guido; Bartali, Ruben; Iacob, Erica; Valeri, Sergio; Ryu, Seunghwa; Signetti, Stefano; Speranza, Giorgio; Pugno, Nicola Maria

    2016-03-17

    The tribological properties of metal-supported few-layered graphene depend strongly on the grain topology of the metal substrate. Inhomogeneous distribution of graphene layers at such regions led to variable landscapes with distinguishable roughness. This discrepancy in morphology significantly affects the frictional and wetting characteristics of the FLG system. We discretely measured friction characteristics of FLG covering grains and interfacial grain boundaries of polycrystalline Ni metal substrate via an atomic force microscopy (AFM) probe. The friction coefficient of FLG covered at interfacial grain boundaries is found to be lower than that on grains in vacuum (at 10(-5) Torr pressure) and similar results were obtained in air condition. Sliding history with AFM cantilever, static and dynamic pull-in and pull-off adhesion forces were addressed in the course of friction measurements to explain the role of the out-of-plane deformation of graphene layer(s). Finite element simulations showed good agreement with experiments and led to a rationalization of the observations. Thus, with interfacial grain boundaries the FLG tribology can be effectively tuned. PMID:26948836

  12. Ultrafast Solvent-Assisted Sodium Ion Intercalation into Highly Crystalline Few-Layered Graphene.

    PubMed

    Cohn, Adam P; Share, Keith; Carter, Rachel; Oakes, Landon; Pint, Cary L

    2016-01-13

    A maximum sodium capacity of ∼35 mAh/g has hampered the use of crystalline carbon nanostructures for sodium ion battery anodes. We demonstrate that a diglyme solvent shell encapsulating a sodium ion acts as a "nonstick" coating to facilitate rapid ion insertion into crystalline few-layer graphene and bypass slow desolvation kinetics. This yields storage capacities above 150 mAh/g, cycling performance with negligible capacity fade over 8000 cycles, and ∼100 mAh/g capacities maintained at currents of 30 A/g (∼12 s charge). Raman spectroscopy elucidates the ordered, but nondestructive cointercalation mechanism that differs from desolvated ion intercalation processes. In situ Raman measurements identify the Na(+) staging sequence and isolates Fermi energies for the first and second stage ternary intercalation compounds at ∼0.8 eV and ∼1.2 eV. PMID:26618985

  13. Nanobubbles stability and multiwall carbon nanotubes straightening on few-layer graphene surfaces

    NASA Astrophysics Data System (ADS)

    Al-Harthi, S. H.; Al-Barwani, M.; Elzain, M.; Al-Naamani, N.; Hysen, T.

    2011-08-01

    The formation of nanobubbles, straightening, and orientation of multiwalled carbon nanotubes (MWCNTs) are investigated by preparing thin films employing dip casting of colloidal solutions on few-layer graphene (FLG) surfaces. In contrast to what is known, it is observed that nanobubbles are formed on an ultrathin dewetted layer of 0.5 nm thickness and exhibit unusual stability at ultrahigh vacuum conditions. Further, nanobubble rings are observed at atmospheric pressure. The results provide direct experimental evidence of the interaction between MWCNTs and nanobubbles at air-liquid-solid triple contact line and at lower step edge of the FLG, which together lead to the alignment of MWCNTs. Due to the balance between the surface tension forces and the strain forces arising from tube bending, MWCNTs attained straight rod-like shape. This enabled us to evaluate the elastic modulus of the MWCNT as E = 0.9 TPa.

  14. Field emission properties of hybrid few-layer graphene-carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Lei Qi, Jun; Zhang, Fu; Xia Zhang, Li; Cao, Jian; Cai Feng, Ji

    2014-04-01

    Few-layer graphene (FLG) and carbon nanotube (CNT) hybrid is prepared by in situ growth of FLG on the walls of CNTs, using PECVD, without catalyst. The amount and size of FLG can be controlled by total gas pressure and growth time. The field emission (FE) characteristics of CNTs coated with different-density FLG were studied, and an FE phenomenon schematic and electrostatic field equipotential model of these FLG-CNTs were proposed. These results show that the geometrical morphology of FLG plays an important role in the FE property of hybrid FLG-CNTs. The medium-density FLG on the CNTs exhibits excellent FE properties, with a low turn-on electric field and threshold field, as well as large field enhancement factor, which are much better than those of the as-grown CNTs. The excellent FE properties of the FLG-CNT hybrids make them promising candidates for high-performance FE emitters.

  15. Mono- to few-layered graphene oxide embedded randomness assisted microcavity amplified spontaneous emission source.

    PubMed

    Das, Pratyusha; Maiti, Rishi; Barman, Prahalad K; Ray, Samit K; Shivakiran, Bhaktha B N

    2016-02-01

    The realization of optoelectronic devices using two-dimensional materials such as graphene and its intermediate product graphene oxide (GO) is extremely challenging owing to the zero band gap of the former. Here, a novel amplified spontaneous emission (ASE) system based on a GO-embedded all-dielectric one-dimensional photonic crystal (1DPhC) micro-resonator is presented. The mono- to few-layered GO sheet is inserted within a microcavity formed by two 5-bilayered SiO2/SnO2 Bragg reflectors. Significantly enhanced photoluminescence (PL) emission of GO embedded in 1DPhC is explicated by studying the electric field confined within the micro-resonator using the transfer matrix method. The inherent randomness, due to fabrication limitations, in the on-average periodic 1DPhC is exploited to further enhance the PL of the optically active micro-resonator. The 1DPhC and randomness assisted field confinement reduces the ASE threshold of the mono- to few-layered weak emitter making the realization of an ASE source feasible. Consequently, ASE at the microcavity resonance and at the low-frequency band-edge of photonic stop-band is demonstrated. Variation of the detection angle from 5° to 30°, with respect to the sample surface normal allows reallocation of the defect mode ASE peak over a spectral range of 558-542 nm, making the GO-incorporated 1DPhC a novel and attractive system for integrated optic applications. PMID:26670725

  16. Mono- to few-layered graphene oxide embedded randomness assisted microcavity amplified spontaneous emission source

    NASA Astrophysics Data System (ADS)

    Das, Pratyusha; Maiti, Rishi; Barman, Prahalad K.; Ray, Samit K.; Shivakiran, Bhaktha B. N.

    2016-02-01

    The realization of optoelectronic devices using two-dimensional materials such as graphene and its intermediate product graphene oxide (GO) is extremely challenging owing to the zero band gap of the former. Here, a novel amplified spontaneous emission (ASE) system based on a GO-embedded all-dielectric one-dimensional photonic crystal (1DPhC) micro-resonator is presented. The mono- to few-layered GO sheet is inserted within a microcavity formed by two 5-bilayered SiO2/SnO2 Bragg reflectors. Significantly enhanced photoluminescence (PL) emission of GO embedded in 1DPhC is explicated by studying the electric field confined within the micro-resonator using the transfer matrix method. The inherent randomness, due to fabrication limitations, in the on-average periodic 1DPhC is exploited to further enhance the PL of the optically active micro-resonator. The 1DPhC and randomness assisted field confinement reduces the ASE threshold of the mono- to few-layered weak emitter making the realization of an ASE source feasible. Consequently, ASE at the microcavity resonance and at the low-frequency band-edge of photonic stop-band is demonstrated. Variation of the detection angle from 5° to 30°, with respect to the sample surface normal allows reallocation of the defect mode ASE peak over a spectral range of 558-542 nm, making the GO-incorporated 1DPhC a novel and attractive system for integrated optic applications.

  17. Thermal diffusivity of few-layers graphene measured by an all-optical method

    NASA Astrophysics Data System (ADS)

    Cabrera, H.; Mendoza, D.; Benítez, J. L.; Bautista Flores, C.; Alvarado, S.; Marín, E.

    2015-11-01

    We report on the measurement of the thermal diffusivity, D, of few-layers graphene obtained by chemical vapor deposition, using a noncontact optical microscopy method based on a mode mismatched thermal lens technique in a pump-probe two-laser beams configuration. It takes advantage of the coaxial counter propagating tightly focused excitation and collimated probe beams in an improved thermal lens microscopy setup. The obtained results: D  =  (6.5  ±  0.09)  ×  10-4 m2 s-1, D  =  (1.9  ±  0.07)  ×  10-4 m2 s-1 and D  =  (1.3  ±  0.05)  ×  10-4 m2 s-1 for four (on a glass slide), eight and sixteen graphene (freestanding) layers are reasonable values, as can be concluded from reported literature data.

  18. Anchoring Nanostructured Manganese Fluoride on Few-Layer Graphene Nanosheets as Anode for Enhanced Lithium Storage.

    PubMed

    Rui, Kun; Wen, Zhaoyin; Lu, Yan; Shen, Chen; Jin, Jun

    2016-01-27

    Manganese fluoride (MnF2)/few-layer graphene nanosheets (GNS) composites are successfully prepared via a facile solvothermal method. It is found that in situ formed tetragonal MnF2 submicron crystals (50-200 nm) with good crystallinity anchoring homogeneously onto conducting GNS, allows the electrically insulating MnF2 particles to be wired up to the current collector with enhanced electron transport pathway. The MnF2/GNS composites act as anode in LIBs and display prominently improved electrochemical performance in comparison to that of pure MnF2, on account of the close interactions between the underlying graphene nanosheets and MnF2 particles grown atop. Distinctly enhanced capacity as high as 489 mAh g(-1) after 100 cycles can be obtained at 600 mA g(-1), while the self-activation process can be greatly accelerated at 6000 mA g(-1) with a maximum specific capacity of 530 mAh g(-1). With long cycling stability for 4000 cycles at 6000 mA g(-1), the MnF2/GNS composite can be deemed as an attractive candidate anode for high-capacity, long cycle life, and environmentally friendly LIBs. PMID:26727406

  19. Investigations of the Electronic, Vibrational and Structural Properties of Single and Few-Layer Graphene

    NASA Astrophysics Data System (ADS)

    Lui, Chun Hung

    Single and few-layer graphene (SLG and FLG) have stimulated great scientific interest because of their distinctive properties and potential for novel applications. In this dissertation, we investigate the mechanical, electronic and vibrational properties of these remarkable materials by various techniques, including atomic-force microscopy (AFM) and Raman, infrared (IR), and ultrafast optical spectroscopy. With respect to its mechanical properties, SLG is known to be capable of undergoing significant mechanical deformation. We have applied AFM to investigate how the morphology of SLG is influenced by the substrate on which it is deposited. We have found that SLG is strongly affected by the morphology of the underlying supporting surface. In particular, SLG deposited on atomically flat surfaces of mica substrates exhibits an ultraflat morphology, with height variation essentially indistinguishable from that observed for the surface of cleaved graphite. One of the most distinctive aspects of SLG is its spectrum of electronic excitations, with its characteristic linear energy-momentum dispersion relation. We have examined the dynamics of the corresponding Dirac fermions by optical emission spectroscopy. By analyzing the spectra of light emission induced in the spectral visible range by 30-femtosecond laser pulses, we find that the charge carriers in graphene cool by the emission of strongly coupled optical phonons in a few 10's of femtoseconds and thermalize among themselves even more rapidly. The charge carriers and the strongly coupled optical phonons are thus essentially in thermal equilibrium with one another on the picosecond time scale, but can be driven strongly out of equilibrium with the other phonons in the system. Temperatures exceeding 3000 K are achieved for the subsystem of the charge carriers and optical phonons under femtosecond laser excitation. While SLG exhibits remarkable physical properties, its few-layer counterparts are also of great interest

  20. AB stacked few layer graphene growth by chemical vapor deposition on single crystal Rh(1 1 1) and electronic structure characterization

    NASA Astrophysics Data System (ADS)

    Kordatos, Apostolis; Kelaidis, Nikolaos; Giamini, Sigiava Aminalragia; Marquez-Velasco, Jose; Xenogiannopoulou, Evangelia; Tsipas, Polychronis; Kordas, George; Dimoulas, Athanasios

    2016-04-01

    Graphene synthesis on single crystal Rh(1 1 1) catalytic substrates is performed by Chemical Vapor Deposition (CVD) at 1000 °C and atmospheric pressure. Raman analysis shows full substrate coverage with few layer graphene. It is found that the cool-down rate strongly affects the graphene stacking order. When lowered, the percentage of AB (Bernal) -stacked regions increases, leading to an almost full AB stacking order. When increased, the percentage of AB-stacked graphene regions decreases to a point where almost a full non AB-stacked graphene is grown. For a slow cool-down rate, graphene with AB stacking order and good epitaxial orientation with the substrate is achieved. This is indicated mainly by Raman characterization and confirmed by Reflection high-energy electron diffraction (RHEED) imaging. Additional Scanning Tunneling Microscopy (STM) topography data confirm that the grown graphene is mainly an AB-stacked structure. The electronic structure of the graphene/Rh(1 1 1) system is examined by Angle resolved Photo-Emission Spectroscopy (ARPES), where σ and π bands of graphene, are observed. Graphene's ΓK direction is aligned with the ΓK direction of the substrate, indicating no significant contribution from rotated domains.

  1. Salt-assisted direct exfoliation of graphite into high-quality, large-size, few-layer graphene sheets.

    PubMed

    Niu, Liyong; Li, Mingjian; Tao, Xiaoming; Xie, Zhuang; Zhou, Xuechang; Raju, Arun P A; Young, Robert J; Zheng, Zijian

    2013-08-21

    We report a facile and low-cost method to directly exfoliate graphite powders into large-size, high-quality, and solution-dispersible few-layer graphene sheets. In this method, aqueous mixtures of graphite and inorganic salts such as NaCl and CuCl2 are stirred, and subsequently dried by evaporation. Finally, the mixture powders are dispersed into an orthogonal organic solvent solution of the salt by low-power and short-time ultrasonication, which exfoliates graphite into few-layer graphene sheets. We find that the as-made graphene sheets contain little oxygen, and 86% of them are 1-5 layers with lateral sizes as large as 210 μm(2). Importantly, the as-made graphene can be readily dispersed into aqueous solution in the presence of surfactant and thus is compatible with various solution-processing techniques towards graphene-based thin film devices. PMID:23824229

  2. Cyclododecane as support material for clean and facile transfer of large-area few-layer graphene

    SciTech Connect

    Capasso, A.; Leoni, E.; Dikonimos, T.; Buonocore, F.; Lisi, N.; De Francesco, M.; Lancellotti, L.; Bobeico, E.; Sarto, M. S.; Tamburrano, A.; De Bellis, G.

    2014-09-15

    The transfer of chemical vapor deposited graphene is a crucial process, which can affect the quality of the transferred films and compromise their application in devices. Finding a robust and intrinsically clean material capable of easing the transfer of graphene without interfering with its properties remains a challenge. We here propose the use of an organic compound, cyclododecane, as a transfer material. This material can be easily spin coated on graphene and assist the transfer, leaving no residues and requiring no further removal processes. The effectiveness of this transfer method for few-layer graphene on a large area was evaluated and confirmed by microscopy, Raman spectroscopy, x-ray photoemission spectroscopy, and four-point probe measurements. Schottky-barrier solar cells with few-layer graphene were fabricated on silicon wafers by using the cyclododecane transfer method and outperformed reference cells made by standard methods.

  3. Stability of few layer graphene films doped with gold (III) chloride

    NASA Astrophysics Data System (ADS)

    Abdullah-Al-Galib, Mir; Hou, Bo; Shahriad, Tahmeed; Zivanovic, Sandra; Radadia, Adarsh D.

    2016-03-01

    In this paper we study the stability of few layer graphene (5-7 layers) doped with gold nanoparticles through spin coating of a gold (III) chloride solution. Specifically sheet resistance, optical transmittance and surface morphology were monitored over a period of four weeks. Through scanning electron microscopy we observed that the gold nanoparticles of 29.1 ± 1.3 nm diameters, which were formed on surfaces freshly doped with a 20 mM solution, agglomerate and fuse over the period of four weeks into larger particles of 50-110 nm diameters. At the end of four weeks of aging, regardless in air or vacuum, the optical transmittance at 550 nm for the doped samples resumed a value close to that of undoped samples. During these four weeks, the sheet resistances of the samples doped with 20 mM gold chloride also increased from 130 ohm/sq to 300 ohm/sq, but stayed comparable to indium tin oxide. In summary, despite the instability of doped FLG surfaces obtained using gold (III) chloride solutions, this study warrants the use of doped FLG films for building the next generation photovoltaics.

  4. Tribological properties of few-layer graphene oxide sheets as oil-based lubricant additives

    NASA Astrophysics Data System (ADS)

    Chen, Zhe; Liu, Yuhong; Luo, Jianbin

    2016-03-01

    The performance of a lubricant largely depends on the additives it involves. However, currently used additives cause severe pollution if they are burned and exhausted. Therefore, it is necessary to develop a new generation of green additives. Graphene oxide (GO) consists of only C, H and O and thus is considered to be environmentally friendly. So the tribological properties of the few-layer GO sheet as an additive in hydrocarbon base oil are investigated systematically. It is found that, with the addition of GO sheets, both the coefficient of friction (COF) and wear are decreased and the working temperature range of the lubricant is expanded in the positive direction. Moreover, GO sheets has better performance under higher sliding speed and the optimized concentration of GO sheets is determined to be 0.5wt%. After rubbing, GO is detected on the wear scars through Raman spectroscopy. And it is believed that, during the rubbing, GO sheets adhere to the sliding surfaces, behaving like protective films and preventing the sliding surfaces from contacting with each other directly. This paper proves that the GO sheet is an effective lubricant additive, illuminates the lubrication mechanism, and provides some critical parameters for the practical application of GO sheets in lubrication.

  5. Optical reflectivity and Raman scattering in few-layer-thick graphene highly doped by K and Rb.

    PubMed

    Jung, Naeyoung; Kim, Bumjung; Crowther, Andrew C; Kim, Namdong; Nuckolls, Colin; Brus, Louis

    2011-07-26

    We report the optical reflectivity and Raman scattering of few layer (L) graphene exposed to K and Rb vapors. Samples many tens of layers thick show the reflectivity and Raman spectra of the stage 1 bulk alkali intercalation compounds (GICs) KC(8) and RbC(8). However, these bulk optical and Raman properties only begin to appear in samples more than about 15 graphene layers thick. The 1 L to 4 L alkali exposed graphene Raman spectra are profoundly different than the Breit-Wigner-Fano (BWF) spectra of the bulk stage 1 compounds. Samples less than 10 layers thick show Drude-like plasma edge reflectivity dip in the visible; alkali exposed few layer graphenes are significantly more transparent than intrinsic graphene. Simulations show the in-plane free electron density is lower than in the bulk stage 1 GICs. In few layer graphenes, alkalis both intercalate between layers and adsorb on the graphene surfaces. Charge transfer electrically dopes the graphene sheets to densities near and above 10(+14) electrons/cm(2). New intrinsic Raman modes at 1128 and 1264 cm(-1) are activated by in-plane graphene zone folding caused by strongly interacting, locally crystalline alkali adlayers. The K Raman spectra are independent of thickness for L = 1-4, indicating that charge transfer from adsorbed and intercalated K layers are similar. The Raman G mode is downshifted and significantly broadened from intrinsic graphene. In contrast, the Rb spectra vary strongly with L and show increased doping by intercalated alkali as L increases. Rb adlayers appear to be disordered liquids, while intercalated layers are locally crystalline solids. A significant intramolecular G mode electronic resonance Raman enhancement is observed in K exposed graphene, as compared with intrinsic graphene. PMID:21682332

  6. Magnetotransport properties of a few-layer graphene-ferromagnetic metal junctions in vertical spin valve devices

    SciTech Connect

    Entani, Shiro Naramoto, Hiroshi; Sakai, Seiji

    2015-05-07

    Magnetotransport properties were studied for the vertical spin valve devices with two junctions of permalloy electrodes and a few-layer graphene interlayer. The graphene layer was directly grown on the bottom electrode by chemical vapor deposition. X-ray photoelectron spectroscopy showed that the permalloy surface fully covered with a few-layer graphene is kept free from oxidation and contamination even after dispensing and removing photoresist. This enabled fabrication of the current perpendicular to plane spin valve devices with a well-defined interface between graphene and permalloy. Spin-dependent electron transport measurements revealed a distinct spin valve effect in the devices. The magnetotransport ratio was 0.8% at room temperature and increased to 1.75% at 50 K. Linear current-voltage characteristics and resistance increase with temperature indicated that ohmic contacts are realized at the relevant interfaces.

  7. Magnetotransport properties of a few-layer graphene-ferromagnetic metal junctions in vertical spin valve devices

    NASA Astrophysics Data System (ADS)

    Entani, Shiro; Naramoto, Hiroshi; Sakai, Seiji

    2015-05-01

    Magnetotransport properties were studied for the vertical spin valve devices with two junctions of permalloy electrodes and a few-layer graphene interlayer. The graphene layer was directly grown on the bottom electrode by chemical vapor deposition. X-ray photoelectron spectroscopy showed that the permalloy surface fully covered with a few-layer graphene is kept free from oxidation and contamination even after dispensing and removing photoresist. This enabled fabrication of the current perpendicular to plane spin valve devices with a well-defined interface between graphene and permalloy. Spin-dependent electron transport measurements revealed a distinct spin valve effect in the devices. The magnetotransport ratio was 0.8% at room temperature and increased to 1.75% at 50 K. Linear current-voltage characteristics and resistance increase with temperature indicated that ohmic contacts are realized at the relevant interfaces.

  8. Few-layer graphene sheets with embedded gold nanoparticles for electrochemical analysis of adenine

    PubMed Central

    Biris, Alexandru R; Pruneanu, Stela; Pogacean, Florina; Lazar, Mihaela D; Borodi, Gheorghe; Ardelean, Stefania; Dervishi, Enkeleda; Watanabe, Fumiya; Biris, Alexandru S

    2013-01-01

    This work describes the synthesis of few-layer graphene sheets embedded with various amounts of gold nanoparticles (Gr-Au-x) over an Aux/MgO catalytic system (where × = 1, 2, or 3 wt%). The sheet-like morphology of the Gr-Au-x nanostructures was confirmed by transmission electron microscopy and high resolution transmission electron microscopy, which also demonstrated that the number of layers within the sheets varied from two to seven. The sample with the highest percentage of gold nanoparticles embedded within the graphitic layers (Gr-Au-3) showed the highest degree of crystallinity. This distinct feature, along with the large number of edge-planes seen in high resolution transmission electron microscopic images, has a crucial effect on the electrocatalytic properties of this material. The reaction yields (40%–50%) and the final purity (96%–98%) of the Gr-Au-x composites were obtained by thermogravimetric analysis. The Gr-Au-x composites were used to modify platinum substrates and subsequently to detect adenine, one of the DNA bases. For the bare electrode, no oxidation signal was recorded. In contrast, all of the modified electrodes showed a strong electrocatalytic effect, and a clear peak for adenine oxidation was recorded at approximately +1.05 V. The highest increase in the electrochemical signal was obtained using a platinum/Gr-Au-3-modified electrode. In addition, this modified electrode had an exchange current density (I0, obtained from the Tafel plot) one order of magnitude higher than that of the bare platinum electrode, which also confirmed that the transfer of electrons took place more readily at the Gr-Au-3-modified electrode. PMID:23610521

  9. Structural Analysis and Direct Imaging of Rotational Stacking Faults in Few-Layer Graphene Synthesized from Solid Botanical Precursor

    NASA Astrophysics Data System (ADS)

    Kalita, Golap; Wakita, Koichi; Umeno, Masayoshi

    2011-07-01

    Here, we report the structural analysis and rotational stacking faults of few-layer graphene sheets derived by the controlled pyrolysis of the solid botanical derivative camphor (C10H16O). The second-order Raman spectra of the sheets show that the graphene layers are more than one single layer, and the numbers of layers can be controlled by adjusting the amount of camphor pyrolyzed. Transmission electron microscopy images show a minimum of 3 layers for thinner graphene sheets and a maximum of 12 layers for thicker graphene sheets. Low-voltage aberration-corrected high-resolution transmission electron microscopy is also carried out to gain insight into the hexagonal structure and stacking of graphene layers. The transmission electron microscopy study showed the presence of moiré patterns with a relative rotation between graphene layers.

  10. The production of concentrated dispersions of few-layer graphene by the direct exfoliation of graphite in organosilanes

    PubMed Central

    2012-01-01

    We report the formation and characterization of graphene dispersions in two organosilanes, 3-glycidoxypropyl trimethoxysilane (GPTMS) and phenyl triethoxysilane (PhTES) as new reactive solvents. The preparation method was mild and easy and does not produce any chemical modification. The dispersions, which exhibit the Tyndall effect, were characterized by TEM and Raman spectroscopy to confirm the presence of few-layer graphene. Concentrations as high as 0.66 and 8.00 mg/ml were found for PhTES and GPTMS, respectively. The latter is one of the highest values reported for a dispersion of graphene obtained by any method. This finding paves the way for the direct synthesis of polymer nanofiller-containing composites consisting of graphene and reactive silanes to be used in sol–gel synthesis, without any need for solvent removal, thus preventing graphene reaggregation to form graphite flakes. PMID:23237423

  11. The influence of few-layer graphene on the gas permeability of the high-free-volume polymer PIM-1

    PubMed Central

    Althumayri, Khalid; Harrison, Wayne J.; Shin, Yuyoung; Gardiner, John M.; Casiraghi, Cinzia; Bernardo, Paola; Clarizia, Gabriele

    2016-01-01

    Gas permeability data are presented for mixed matrix membranes (MMMs) of few-layer graphene in the polymer of intrinsic microporosity PIM-1, and the results compared with previously reported data for two other nanofillers in PIM-1: multiwalled carbon nanotubes functionalized with poly(ethylene glycol) (f-MWCNTs) and fused silica. For few-layer graphene, a significant enhancement in permeability is observed at very low graphene content (0.05 vol.%), which may be attributed to the effect of the nanofiller on the packing of the polymer chains. At higher graphene content permeability decreases, as expected for the addition of an impermeable filler. Other nanofillers, reported in the literature, also give rise to enhancements in permeability, but at substantially higher loadings, the highest measured permeabilities being at 1 vol.% for f-MWCNTs and 24 vol.% for fused silica. These results are consistent with the hypothesis that packing of the polymer chains is influenced by the curvature of the nanofiller surface at the nanoscale, with an increasingly pronounced effect on moving from a more-or-less spherical nanoparticle morphology (fused silica) to a cylindrical morphology (f-MWCNT) to a planar morphology (graphene). While the permeability of a high-free-volume polymer such as PIM-1 decreases over time through physical ageing, for the PIM-1/graphene MMMs a significant permeability enhancement was retained after eight months storage. PMID:26712643

  12. The influence of few-layer graphene on the gas permeability of the high-free-volume polymer PIM-1.

    PubMed

    Althumayri, Khalid; Harrison, Wayne J; Shin, Yuyoung; Gardiner, John M; Casiraghi, Cinzia; Budd, Peter M; Bernardo, Paola; Clarizia, Gabriele; Jansen, Johannes C

    2016-02-13

    Gas permeability data are presented for mixed matrix membranes (MMMs) of few-layer graphene in the polymer of intrinsic microporosity PIM-1, and the results compared with previously reported data for two other nanofillers in PIM-1: multiwalled carbon nanotubes functionalized with poly(ethylene glycol) (f-MWCNTs) and fused silica. For few-layer graphene, a significant enhancement in permeability is observed at very low graphene content (0.05 vol.%), which may be attributed to the effect of the nanofiller on the packing of the polymer chains. At higher graphene content permeability decreases, as expected for the addition of an impermeable filler. Other nanofillers, reported in the literature, also give rise to enhancements in permeability, but at substantially higher loadings, the highest measured permeabilities being at 1 vol.% for f-MWCNTs and 24 vol.% for fused silica. These results are consistent with the hypothesis that packing of the polymer chains is influenced by the curvature of the nanofiller surface at the nanoscale, with an increasingly pronounced effect on moving from a more-or-less spherical nanoparticle morphology (fused silica) to a cylindrical morphology (f-MWCNT) to a planar morphology (graphene). While the permeability of a high-free-volume polymer such as PIM-1 decreases over time through physical ageing, for the PIM-1/graphene MMMs a significant permeability enhancement was retained after eight months storage. PMID:26712643

  13. Evolution of surface morphology and electronic structure of few layer graphene after low energy Ar{sup +} ion irradiation

    SciTech Connect

    Al-Harthi, S. H.; Kara'a, A.; Elzain, M.; Hysen, T.; Al-Hinai, A. T.; Myint, M. T. Z.

    2012-11-19

    We report on co-existing dual anisotropy ripple formation, sp bonding transformation, and variation in the delocalized {pi} electron system in 1 keV Ar{sup +} ion irradiated few-layer graphene surfaces. Ripples in directions, perpendicular and parallel to the ion beam were found. The irradiation effect and the transition from the sp{sup 2}-bonding to sp{sup 3}-hybridized state were analyzed from the deconvolution of the C (1s) peak and from the shape of the derivative of the Auger transition spectra. The results suggest a plausible mechanism for tailoring of few-layer graphene electronic band structure with interlayer coupling tuned by the ion irradiation.

  14. Thermal conductivity of giant mono- to few-layered CVD graphene supported on an organic substrate.

    PubMed

    Liu, Jing; Wang, Tianyu; Xu, Shen; Yuan, Pengyu; Xu, Xu; Wang, Xinwei

    2016-05-21

    The thermal conductivity (k) of supported graphene is a critical property that reflects the graphene-substrate interaction, graphene structure quality, and is needed for thermal design of a graphene device. Yet the related k measurement has never been a trivial work and very few studies are reported to date, only at the μm level. In this work, for the first time, the k of giant chemical vapor decomposition (CVD) graphene supported on poly(methyl methacrylate) (PMMA) is characterized using our transient electro-thermal technique based on a differential concept. Our graphene size is ∼mm, far above the samples studied in the past. This giant graphene measurement eliminates the thermal contact resistance problems and edge phonon scattering encountered in μm-scale graphene k measurement. Such mm-scale measurement is critical for device/system-level thermal design since it reflects the effect of abundant grains in graphene. The k of 1.33-layered, 1.53-layered, 2.74-layered and 5.2-layered supported graphene is measured as 365 W m(-1) K(-1), 359 W m(-1) K(-1), 273 W m(-1) K(-1) and 33.5 W m(-1) K(-1), respectively. These values are significantly lower than the k of supported graphene on SiO2, and are about one order of magnitude lower than the k of suspended graphene. We speculate that the abundant C atoms in the PMMA promote more ready energy and momentum exchange with the supported graphene, and give rise to more phonon scattering than the SiO2 substrate. This leads to a lower k of CVD graphene on PMMA than that on SiO2. We attribute the existence of disorder in the sp(2) domain, graphene oxide (GO) and stratification in the 5.2-layered graphene to its more k reduction. The Raman linewidth (G peak) of the 5.2-layered graphene is also twice larger than that of the other three kinds of graphene, indicating the much more phonon scattering and shorter phonon lifetime in it. Also the electrical conductivity of the 5.2-layered graphene is about one-fifth of that for the

  15. Thermal conductivity of giant mono- to few-layered CVD graphene supported on an organic substrate

    NASA Astrophysics Data System (ADS)

    Liu, Jing; Wang, Tianyu; Xu, Shen; Yuan, Pengyu; Xu, Xu; Wang, Xinwei

    2016-05-01

    The thermal conductivity (k) of supported graphene is a critical property that reflects the graphene-substrate interaction, graphene structure quality, and is needed for thermal design of a graphene device. Yet the related k measurement has never been a trivial work and very few studies are reported to date, only at the μm level. In this work, for the first time, the k of giant chemical vapor decomposition (CVD) graphene supported on poly(methyl methacrylate) (PMMA) is characterized using our transient electro-thermal technique based on a differential concept. Our graphene size is ~mm, far above the samples studied in the past. This giant graphene measurement eliminates the thermal contact resistance problems and edge phonon scattering encountered in μm-scale graphene k measurement. Such mm-scale measurement is critical for device/system-level thermal design since it reflects the effect of abundant grains in graphene. The k of 1.33-layered, 1.53-layered, 2.74-layered and 5.2-layered supported graphene is measured as 365 W m-1 K-1, 359 W m-1 K-1, 273 W m-1 K-1 and 33.5 W m-1 K-1, respectively. These values are significantly lower than the k of supported graphene on SiO2, and are about one order of magnitude lower than the k of suspended graphene. We speculate that the abundant C atoms in the PMMA promote more ready energy and momentum exchange with the supported graphene, and give rise to more phonon scattering than the SiO2 substrate. This leads to a lower k of CVD graphene on PMMA than that on SiO2. We attribute the existence of disorder in the sp2 domain, graphene oxide (GO) and stratification in the 5.2-layered graphene to its more k reduction. The Raman linewidth (G peak) of the 5.2-layered graphene is also twice larger than that of the other three kinds of graphene, indicating the much more phonon scattering and shorter phonon lifetime in it. Also the electrical conductivity of the 5.2-layered graphene is about one-fifth of that for the other three. This

  16. The photoelectric characteristics of a few-layer graphene/Si Schottky junction solar cell

    NASA Astrophysics Data System (ADS)

    Ma, Xiying; Gu, Weixia

    2015-10-01

    We present a study of the photovoltaic effects of a graphene/n- Si Schottky junction solar cell. The graphene/Si solar cell was prepared by means of rapid chemical vapor deposition, while the graphene films were grown with a CH4/Ar mixed gas under a constant flow at 950°C and then annealed at 1000°C. It was found that the junction between the graphene film and the n-Si structure played an important role in determining the device performance. An energy conversion efficiency of 2.1% was achieved under an optical illumination of 100 mW. The strong photovoltaic effects of the cell were due to device junction's ability to efficiently generate and separate electron-hole pairs.

  17. Few layer graphene matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

    PubMed

    Cho, Donghyun; Hong, Sangsu; Shim, Sangdeok

    2013-08-01

    We present the employment of few layer graphene (FLG) as a matrix for the analysis of low molecular weight polymeric compounds using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The practicality of FLG as a matrix for MALDI experiments is demonstrated by analyzing low molecular weight polymers, polar polyethylene glycol (PEG) of 1000 Da and nonpolar polymethylmethacrylate (PMMA) of 650 Da. The high quality MS spectra without low-mass interference signals without any further sampling procedure were acquired. PMID:23882840

  18. The influence of nanoscale roughness and substrate chemistry on the frictional properties of single and few layer graphene

    NASA Astrophysics Data System (ADS)

    Spear, Jessica C.; Custer, James P.; Batteas, James D.

    2015-05-01

    Nanoscale carbon lubricants such as graphene, have garnered increased interest as protective surface coatings for devices, but its tribological properties have been shown to depend on its interactions with the underlying substrate surface and its degree of surface conformity. This conformity is especially of interest as real interfaces exhibit roughness on the order of ~10 nm that can dramatically impact the contact area between the graphene film and the substrate. To examine the combined effects of surface interaction strength and roughness on the frictional properties of graphene, a combination of Atomic Force Microscopy (AFM) and Raman microspectroscopy has been used to explore substrate interactions and the frictional properties of single and few-layer graphene as a coating on silica nanoparticle films, which yield surfaces that mimic the nanoscaled asperities found in realistic devices. The interactions between the graphene and the substrate have been controlled by comparing their binding to hydrophilic (silanol terminated) and hydrophobic (octadecyltrichlorosilane modified) silica surfaces. AFM measurements revealed that graphene only partially conforms to the rough surfaces, with decreasing conformity, as the number of layers increase. Under higher mechanical loading the graphene conformity could be reversibly increased, allowing for a local estimation of the out-of-plane bending modulus of the film. The frictional properties were also found to depend on the number of layers, with the largest friction observed on single layers, ultimately decreasing to that of bulk graphite. This trend however, was found to disappear, depending on the tip-sample contact area and interfacial shear strain of the graphene associated with its adhesion to the substrate.Nanoscale carbon lubricants such as graphene, have garnered increased interest as protective surface coatings for devices, but its tribological properties have been shown to depend on its interactions with the

  19. Surface-enhanced Raman scattering studies of few-layer graphene on silver substrate with 514 nm excitation

    NASA Astrophysics Data System (ADS)

    Ouyang, Yu; Chen, Li

    2011-04-01

    Few-layer graphene was prepared by deoxidizing graphite oxide. Surface-enhanced Raman scattering (SERS) spectrum of graphene on Ag substrate was presented with 514 nm excitation. More than nine Raman lines are shown in the 100-3200 cm -1. The intensities of all lines are enhanced. Besides D mode at 1347 cm -1, G peak at 1581 cm -1, G ∗ band at ˜2458 cm -1, and G' band at ˜2691 cm -1, there are two features in low frequency area at about 239 cm -1 and 992 cm -1 separately, one line in high frequency area at about 2900 cm -1 with a shoulder appearing at ˜2860 cm -1. According to SERS, the structure of graphene was analyzed. Furthermore, the increase of D/G intensity ratio in SERS shows the SERS has much potential to be applied in controlling synthesis quality of graphene. In addition, low frequency Raman lines presented in SERS show application potential in analyzing the layer number, stacked style and edge of graphene.

  20. Co3O4 nanocubes homogeneously assembled on few-layer graphene for high energy density lithium-ion batteries

    NASA Astrophysics Data System (ADS)

    Xu, Junming; Wu, Jinsong; Luo, Langli; Chen, Xinqi; Qin, Huibin; Dravid, Vinayak; Mi, Shaobo; Jia, Chunlin

    2015-01-01

    Graphene-based nanocomposites have been synthesized and tested as electrode materials for high power lithium-ion batteries. In the synthesis of such nanocomposites, graphene is generally introduced by either thermally or chemically reduced graphite oxide (GO), which has poorer electric conductivity and crystallinity than mechanically exfoliated graphene. Here, we prepare few-layer graphene sheet (FLGS) with high electric conductivity, by sonicating expanded graphite in DMF solvent, and develop a simple one-pot hydrothermal method to fabricate monodispersed and ultrasmall Co3O4 nanocubes (about 4 nm in size) on the FLGS. This composite, consisting of homogeneously assembled and high crystalline Co3O4 nanocubes on the FLGS, has shown higher capacity and much better cycling stability than counterparts synthesized using GO as a precursor. The products in different synthesis stages have been characterized by TEM, FTIR and XPS to investigate the nanocube growth mechanism. We find that Co(OH)2 initially grew homogeneously on the graphene surface, then gradually oxidized to form Co3O4 nanoparticle seeds, and finally converted to Co3O4 nanocubes with caboxylated anion as surfactant. This work explores the mechanism of nanocrystal growth and its impact on electrochemical properties to provide further insights into the development of nanostructured electrode materials for high power energy storage.

  1. Electron Hopping Through Single-to-Few-Layer Graphene Oxide Films. Side-Selective Photocatalytic Deposition of Metal Nanoparticles.

    PubMed

    Lightcap, Ian V; Murphy, Sean; Schumer, Timothy; Kamat, Prashant V

    2012-06-01

    Single- to few-layer graphene oxide (GO) sheets have been successfully anchored onto TiO2 films using electrophoretic deposition. Upon UV illumination of TiO2-GO films, photogenerated electrons from TiO2 are captured by GO. These electrons are initially used in GO's reduction, while additional electron transfer results in storage across its sp(2) network. In the presence of silver ions, deposition of silver nanoparticles (NPs) is accomplished on the GO surface opposite the TiO2, thus confirming the ability of GO to transport electrons through its plane. Illumination-controlled reduction of silver ions allows for simple selection of particle size and loading, making these semiconductor-graphene-metal (SGM) films ideal for custom catalysis and sensor applications. Initial testing of SGM films as surface-enhanced resonance Raman (SERRS) sensors produced significant target molecule signal enhancements, enabling detection of nanomolar concentrations. PMID:26285621

  2. Sequential Electron Transport and Vibrational Excitations in an Organic Molecule Coupled to Few-Layer Graphene Electrodes.

    PubMed

    Burzurí, Enrique; Island, Joshua O; Díaz-Torres, Raúl; Fursina, Alexandra; González-Campo, Arántzazu; Roubeau, Olivier; Teat, Simon J; Aliaga-Alcalde, Núria; Ruiz, Eliseo; van der Zant, Herre S J

    2016-02-23

    Graphene electrodes are promising candidates to improve reproducibility and stability in molecular electronics through new electrode-molecule anchoring strategies. Here we report sequential electron transport in few-layer graphene transistors containing individual curcuminoid-based molecules anchored to the electrodes via π-π orbital bonding. We show the coexistence of inelastic co-tunneling excitations with single-electron transport physics due to an intermediate molecule-electrode coupling; we argue that an intermediate electron-phonon coupling is the origin of these vibrational-assisted excitations. These experimental observations are complemented with density functional theory calculations to model electron transport and the interaction between electrons and vibrational modes of the curcuminoid molecule. We find that the calculated vibrational modes of the molecule are in agreement with the experimentally observed excitations. PMID:26841282

  3. Towards Lego Snapping; Integration of Carbon Nanotubes and Few-Layer Graphene

    NASA Astrophysics Data System (ADS)

    Nasseri, Mohsen; Boland, Mathias; Farrokhi, M. Javad; Strachan, Douglas

    Integration of semiconducting, conducting, and insulating nanomaterials into precisely aligned complicated systems is one of the main challenges to the ultimate size scaling of electronic devices, which is a key goal in nanoscience and nanotechnology. This integration could be made more effective through controlled alignment of the crystallographic lattices of the nanoscale components. Of the vast number of materials of atomically-thin materials, two of the sp2 bonded carbon structures, graphene and carbon nanotubes, are ideal candidates for this type of application since they are built from the same backbone carbon lattice. Here we report carbon nanotube and graphene hybrid nanostructures fabricated through their catalytic synthesis and etching. The growth formations we have investigated through various high-resolution microscopy techniques provide evidence of lego-snapped interfaces between nanotubes and graphene into device-relevant orientations. We will finish with a discussion of the various size and energy regimes relevant to these lego-snapped interfaces and their implications on developing these integrated formations.

  4. Observation of spin-glass behavior in nickel adsorbed few layer graphene

    SciTech Connect

    Mitra, Sreemanta; Mondal, Oindrila; Banerjee, Sourish; Chakravorty, Dipankar

    2013-01-14

    Nickel-adsorbed graphene was prepared by first synthesizing graphite oxide (GO) by modified Hummers' method and then reducing a solution containing both GO and Ni{sup 2+}. Energy dispersive X-ray spectroscopy analysis showed 31 at. % nickel was present. Magnetization measurements under both dc and ac magnetic fields were carried out in the temperature range 2 K to 300 K. The zero field cooled and field cooled magnetization data showed a pronounced irreversibility at a temperature around 20 K. The analysis of the ac susceptibility data was carried out by both Vogel-Fulcher as well as power law. From dynamic scaling analysis, the microscopic flipping time {tau}{sub 0}{approx}10{sup -13}s and critical exponent z{nu}=5.9{+-}0.1 were found, indicating the presence of conventional spin glass in the system. The spin glass transition temperature was estimated as 19.5 K. Decay of thermoremanent magnetization was explained by stretched exponential function with a value of the exponent as 0.6. From the results, it is concluded that nickel adsorbed graphene behaves like a spin-glass.

  5. Nanoscale measurements of unoccupied band dispersion in few-layer graphene

    PubMed Central

    Jobst, Johannes; Kautz, Jaap; Geelen, Daniël; Tromp, Rudolf M.; van der Molen, Sense Jan

    2015-01-01

    The properties of any material are fundamentally determined by its electronic band structure. Each band represents a series of allowed states inside a material, relating electron energy and momentum. The occupied bands, that is, the filled electron states below the Fermi level, can be routinely measured. However, it is remarkably difficult to characterize the empty part of the band structure experimentally. Here, we present direct measurements of unoccupied bands of monolayer, bilayer and trilayer graphene. To obtain these, we introduce a technique based on low-energy electron microscopy. It relies on the dependence of the electron reflectivity on incidence angle and energy and has a spatial resolution ∼10 nm. The method can be easily applied to other nanomaterials such as van der Waals structures that are available in small crystals only. PMID:26608712

  6. Simulation of High Density Lipoprotein Behavior on a Few Layer Graphene Undergoing Non-Uniform Mechanical Load.

    PubMed

    Glukhova, Olga E; Prytkova, Tatiana R; Savostyanov, George V

    2016-04-21

    Effect of a nonuniform external mechanical load on high density lipoprotein (HDL) in aqueous medium was investigated using course-grained molecular dynamics simulations. The nonuniform load was achieved by a few layer graphene on one side and closed single-walled carbon nanotube (SWNT) (7, 7) on the opposite side of lipoprotein. The tube had a diameter of 1 nm and was oriented perpendicularly to the graphene. HDL was located between them. The tube was approaching to HDL on graphene deforming it. We considered two cases of the tube movement with velocities of 20 and 5 m/s. Coarse-grained (CG) molecular dynamics with application of the MARTINI force field for HDL and coarse-grained model with an all-atom (AA)/CG mapping ratio of 1.5 for carbon nanotube (CNT) (each CG bead was modeled by the 4-site CG benzene) were used. Coarse-grained model of HDL was received by method of self-assembly. HDL was static but not fixed that gave the possibility to compensate its external influence in some way. It was established that in water medium HDL interacted with graphene substrate. It was established that in water HDL interacts with graphene substrate, slightly flattening but retaining its shape of the whole. It was also observed that during the calculations HDL partially dodged nanotube. Lipoprotein belts unfolded on the graphene substrate in the way of the best compensation for the impact of nanotubes. Finally, we observed that the approaching tube has passed through the less dense medium of dipalmitoylphosphatidylcholine (DPPC) and its pressure on the macromolecule decreased. Inhomogeneity of the external exposure deformed HDL at approximately 10-50%. The character of deformation demonstrated that lipoprotein has viscoelastic properties similar to a fluid. The discovered ability of lipoprotein may help to establish mechanism of interaction of lipoproteins with arterial walls and dynamic behavior of lipoproteins in arterial intima. PMID:27046673

  7. Temperature Dependence of Electric Transport in Few-layer Graphene under Large Charge Doping Induced by Electrochemical Gating

    PubMed Central

    Gonnelli, R. S.; Paolucci, F.; Piatti, E.; Sharda, Kanudha; Sola, A.; Tortello, M.; Nair, Jijeesh R.; Gerbaldi, C.; Bruna, M.; Borini, S.

    2015-01-01

    The temperature dependence of electric transport properties of single-layer and few-layer graphene at large charge doping is of great interest both for the study of the scattering processes dominating the conductivity at different temperatures and in view of the theoretically predicted possibility to reach the superconducting state in such extreme conditions. Here we present the results obtained in 3-, 4- and 5-layer graphene devices down to 3.5 K, where a large surface charge density up to about 6.8·1014 cm−2 has been reached by employing a novel polymer electrolyte solution for the electrochemical gating. In contrast with recent results obtained in single-layer graphene, the temperature dependence of the sheet resistance between 20 K and 280 K shows a low-temperature dominance of a T2 component – that can be associated with electron-electron scattering – and, at about 100 K, a crossover to the classic electron-phonon regime. Unexpectedly, this crossover does not show any dependence on the induced charge density, i.e. on the large tuning of the Fermi energy. PMID:25906088

  8. Thermal conductivity measurement of few layer graphene film by a micropipette sensor with laser point heating source

    NASA Astrophysics Data System (ADS)

    Jeong, J. Y.; Lee, K. M.; Shrestha, R.; Horne, K.; Das, S.; Choi, W.; Kim, M.; Choi, T. Y.

    2016-05-01

    We report a thermal characterization method for a large-scale free-standing chemical vapor deposited few layer graphene (FLG), in which a micropipette temperature sensor with an inbuilt laser point heating source was used. The technique is unique as it exhibits in general the characteristic features of high accuracy measurement of thermal conductivity of free-standing ultrathin films. Using the micropipette sensor we successfully implemented the characterization technique to show high thermal transport behavior in free-standing graphene. For accurate and successful measurement of thermal conductivity, FLG grown on Ni was transferred to a polycarbonate (PC) membrane with holes (average diameter of 100 μm) in order to isolate the graphene film from heat spreading through the bottom of the film by the laser point heating. The thermal conductivity of FLG by this method was measured at 2868 ± 932 W/m °C. The large uncertainty of 32% in thermal conductivity measurement is mainly due to the non-uniform (∼30% deviation) thickness of the film.

  9. Liquid-Phase Exfoliation of Graphite into Single- and Few-Layer Graphene with α-Functionalized Alkanes.

    PubMed

    Haar, Sébastien; Bruna, Matteo; Lian, Jian Xiang; Tomarchio, Flavia; Olivier, Yoann; Mazzaro, Raffaello; Morandi, Vittorio; Moran, Joseph; Ferrari, Andrea C; Beljonne, David; Ciesielski, Artur; Samorì, Paolo

    2016-07-21

    Graphene has unique physical and chemical properties, making it appealing for a number of applications in optoelectronics, sensing, photonics, composites, and smart coatings, just to cite a few. These require the development of production processes that are inexpensive and up-scalable. These criteria are met in liquid-phase exfoliation (LPE), a technique that can be enhanced when specific organic molecules are used. Here we report the exfoliation of graphite in N-methyl-2-pyrrolidinone, in the presence of heneicosane linear alkanes terminated with different head groups. These molecules act as stabilizing agents during exfoliation. The efficiency of the exfoliation in terms of the concentration of exfoliated single- and few-layer graphene flakes depends on the functional head group determining the strength of the molecular dimerization through dipole-dipole interactions. A thermodynamic analysis is carried out to interpret the impact of the termination group of the alkyl chain on the exfoliation yield. This combines molecular dynamics and molecular mechanics to rationalize the role of functionalized alkanes in the dispersion and stabilization process, which is ultimately attributed to a synergistic effect of the interactions between the molecules, graphene, and the solvent. PMID:27349897

  10. Vacancies in epitaxial graphene

    SciTech Connect

    Davydov, S. Yu.

    2015-08-15

    The coherent-potential method is used to consider the problem of the influence of a finite concentration of randomly arranged vacancies on the density of states of epitaxial graphene. To describe the density of states of the substrate, simple models (the Anderson model, Haldane-Anderson model, and parabolic model) are used. The electronic spectrum of free single-sheet graphene is considered in the low-energy approximation. Charge transfer in the graphene-substrate system is discussed. It is shown that, in all cases, the density of states of epitaxial graphene decreases proportionally to the vacancy concentration. At the same time, the average charge transferred from graphene to the substrate increases.

  11. Direct Synthesis of Few-Layer F-Doped Graphene Foam and Its Lithium/Potassium Storage Properties.

    PubMed

    Ju, Zhicheng; Zhang, Shuai; Xing, Zheng; Zhuang, Quanchao; Qiang, Yinghuai; Qian, Yitai

    2016-08-17

    Heteroatom-doped graphene is considered a potential electrode materials for lithium-ion batteries (LIBs). However, potassium-ion batteries (PIBs) systems are possible alternatives due to the comparatively higher abundance. Here, a practical solid-state method is described for the preparation of few-layer F-doped graphene foam (FFGF) with thickness of about 4 nm and high surface area (874 m(2)g(-1)). As anode material for LIBs, FFGF exhibits 800 mAh·g(-1) after 50 cycles at a current density of 100 mA·g(-1) and 555 mAh·g(-1) after 100 cycles at 200 mA·g(-1) as well as remarkable rate capability. FFGF also shows 165.9 mAh·g(-1) at 500 mA·g(-1) for 200 cycles for PIBs. Research suggests that the multiple synergistic effects of the F-modification, high surface area, and mesoporous membrane structures endow the ions and electrons throughout the electrode matrix with fast transportation as well as offering sufficient active sites for lithium and potassium storage, resulting in excellent electrochemical performance. Furthermore, the insights obtained will be of benefit to the design of reasonable electrode materials for alkali metal ion batteries. PMID:27467782

  12. Spectroscopic metrics allow in situ measurement of mean size and thickness of liquid-exfoliated few-layer graphene nanosheets

    NASA Astrophysics Data System (ADS)

    Backes, Claudia; Paton, Keith R.; Hanlon, Damien; Yuan, Shengjun; Katsnelson, Mikhail I.; Houston, James; Smith, Ronan J.; McCloskey, David; Donegan, John F.; Coleman, Jonathan N.

    2016-02-01

    Liquid phase exfoliation is a powerful and scalable technique to produce defect-free mono- and few-layer graphene. However, samples are typically polydisperse and control over size and thickness is challenging. Notably, high throughput techniques to measure size and thickness are lacking. In this work, we have measured the extinction, absorption, scattering and Raman spectra for liquid phase exfoliated graphene nanosheets of various lateral sizes (90 <= <= 810 nm) and thicknesses (2.7 <= <= 10.4). We found all spectra to show well-defined dependences on nanosheet dimensions. Measurements of extinction and absorption spectra of nanosheet dispersions showed both peak position and spectral shape to vary with nanosheet thickness in a manner consistent with theoretical calculations. This allows the development of empirical metrics to extract the mean thickness of liquid dispersed nanosheets from an extinction (or absorption) spectrum. While the scattering spectra depended on nanosheet length, poor signal to noise ratios made the resultant length metric unreliable. By analyzing Raman spectra measured on graphene nanosheet networks, we found both the D/G intensity ratio and the width of the G-band to scale with mean nanosheet length allowing us to establish quantitative relationships. In addition, we elucidate the variation of 2D/G band intensities and 2D-band shape with the mean nanosheet thickness, allowing us to establish quantitative metrics for mean nanosheet thickness from Raman spectra.Liquid phase exfoliation is a powerful and scalable technique to produce defect-free mono- and few-layer graphene. However, samples are typically polydisperse and control over size and thickness is challenging. Notably, high throughput techniques to measure size and thickness are lacking. In this work, we have measured the extinction, absorption, scattering and Raman spectra for liquid phase exfoliated graphene nanosheets of various lateral sizes (90 <= <= 810 nm) and

  13. Microwave-Assisted Synthesis of Highly-Crumpled, Few-Layered Graphene and Nitrogen-Doped Graphene for Use as High-Performance Electrodes in Capacitive Deionization.

    PubMed

    Amiri, Ahmad; Ahmadi, Goodarz; Shanbedi, Mehdi; Savari, Maryam; Kazi, S N; Chew, B T

    2015-01-01

    Capacitive deionization (CDI) is a promising procedure for removing various charged ionic species from brackish water. The performance of graphene-based material in capacitive deionization is lower than the expectation of the industry, so highly-crumpled, few-layered graphene (HCG) and highly-crumpled nitrogen-doped graphene (HCNDG) with high surface area have been introduced as promising candidates for CDI electrodes. Thus, HCG and HCNDG were prepared by exfoliation of graphite in the presence of liquid-phase, microwave-assisted methods. An industrially-scalable, cost-effective, and simple approach was employed to synthesize HCG and HCNDG, resulting in few-layered graphene and nitrogen-doped graphene with large specific surface area. Then, HCG and HCNDG were utilized for manufacturing a new class of carbon nanostructure-based electrodes for use in large-scale CDI equipment. The electrosorption results indicated that both the HCG and HCNDG have fairly large specific surface areas, indicating their huge potential for capacitive deionization applications. PMID:26643279

  14. Microwave-Assisted Synthesis of Highly-Crumpled, Few-Layered Graphene and Nitrogen-Doped Graphene for Use as High-Performance Electrodes in Capacitive Deionization

    PubMed Central

    Amiri, Ahmad; Ahmadi, Goodarz; Shanbedi, Mehdi; Savari, Maryam; Kazi, S. N.; Chew, B. T.

    2015-01-01

    Capacitive deionization (CDI) is a promising procedure for removing various charged ionic species from brackish water. The performance of graphene-based material in capacitive deionization is lower than the expectation of the industry, so highly-crumpled, few-layered graphene (HCG) and highly-crumpled nitrogen-doped graphene (HCNDG) with high surface area have been introduced as promising candidates for CDI electrodes. Thus, HCG and HCNDG were prepared by exfoliation of graphite in the presence of liquid-phase, microwave-assisted methods. An industrially-scalable, cost-effective, and simple approach was employed to synthesize HCG and HCNDG, resulting in few-layered graphene and nitrogen-doped graphene with large specific surface area. Then, HCG and HCNDG were utilized for manufacturing a new class of carbon nanostructure-based electrodes for use in large-scale CDI equipment. The electrosorption results indicated that both the HCG and HCNDG have fairly large specific surface areas, indicating their huge potential for capacitive deionization applications. PMID:26643279

  15. Microwave-Assisted Synthesis of Highly-Crumpled, Few-Layered Graphene and Nitrogen-Doped Graphene for Use as High-Performance Electrodes in Capacitive Deionization

    NASA Astrophysics Data System (ADS)

    Amiri, Ahmad; Ahmadi, Goodarz; Shanbedi, Mehdi; Savari, Maryam; Kazi, S. N.; Chew, B. T.

    2015-12-01

    Capacitive deionization (CDI) is a promising procedure for removing various charged ionic species from brackish water. The performance of graphene-based material in capacitive deionization is lower than the expectation of the industry, so highly-crumpled, few-layered graphene (HCG) and highly-crumpled nitrogen-doped graphene (HCNDG) with high surface area have been introduced as promising candidates for CDI electrodes. Thus, HCG and HCNDG were prepared by exfoliation of graphite in the presence of liquid-phase, microwave-assisted methods. An industrially-scalable, cost-effective, and simple approach was employed to synthesize HCG and HCNDG, resulting in few-layered graphene and nitrogen-doped graphene with large specific surface area. Then, HCG and HCNDG were utilized for manufacturing a new class of carbon nanostructure-based electrodes for use in large-scale CDI equipment. The electrosorption results indicated that both the HCG and HCNDG have fairly large specific surface areas, indicating their huge potential for capacitive deionization applications.

  16. Free-standing few-layered graphene oxide films: selective, steady and lasting permeation of organic molecules with adjustable speeds

    NASA Astrophysics Data System (ADS)

    Huang, Tao; An, Qi; Luan, Xinglong; Zhang, Qian; Zhang, Yihe

    2016-01-01

    A variety of small molecules with diameters around 1 nm possess a range of functions, such as antibiotic, antimicrobic, anticoagulant, pesticidal and chemotherapy effects, making these molecules especially useful in various applications ranging from medical treatment to environmental microbiological control. However, the long-term steady delivery (release or permeation) of these small molecules with adjustable and controllable speeds has remained an especially challenging task. In this study, we prepared covalently cross-linked free-standing few-layered GO films using a layer-by-layer technique in combination with photochemical cross-linkages, and achieved a controlled release of positively charged, negatively charged, and zwitterionic small molecules with adjustable and controllable speeds. The steady delivery of the small molecule lasted up to 9 days. Other functionalities, such as graphene-enhanced Raman spectra and electrochemical properties that could also be integrated or employed in delivery systems, were also studied for our films. We expect the special molecular delivery properties of our films to lead to new possibilities in drug/fertilizer delivery and environmental microbiological control applications.A variety of small molecules with diameters around 1 nm possess a range of functions, such as antibiotic, antimicrobic, anticoagulant, pesticidal and chemotherapy effects, making these molecules especially useful in various applications ranging from medical treatment to environmental microbiological control. However, the long-term steady delivery (release or permeation) of these small molecules with adjustable and controllable speeds has remained an especially challenging task. In this study, we prepared covalently cross-linked free-standing few-layered GO films using a layer-by-layer technique in combination with photochemical cross-linkages, and achieved a controlled release of positively charged, negatively charged, and zwitterionic small molecules with

  17. Free-standing few-layered graphene oxide films: selective, steady and lasting permeation of organic molecules with adjustable speeds.

    PubMed

    Huang, Tao; An, Qi; Luan, Xinglong; Zhang, Qian; Zhang, Yihe

    2016-01-28

    A variety of small molecules with diameters around 1 nm possess a range of functions, such as antibiotic, antimicrobic, anticoagulant, pesticidal and chemotherapy effects, making these molecules especially useful in various applications ranging from medical treatment to environmental microbiological control. However, the long-term steady delivery (release or permeation) of these small molecules with adjustable and controllable speeds has remained an especially challenging task. In this study, we prepared covalently cross-linked free-standing few-layered GO films using a layer-by-layer technique in combination with photochemical cross-linkages, and achieved a controlled release of positively charged, negatively charged, and zwitterionic small molecules with adjustable and controllable speeds. The steady delivery of the small molecule lasted up to 9 days. Other functionalities, such as graphene-enhanced Raman spectra and electrochemical properties that could also be integrated or employed in delivery systems, were also studied for our films. We expect the special molecular delivery properties of our films to lead to new possibilities in drug/fertilizer delivery and environmental microbiological control applications. PMID:26698634

  18. Comparison study of few-layered graphene supported platinum and platinum alloys for methanol and ethanol electro-oxidation

    NASA Astrophysics Data System (ADS)

    Shen, Yi; Xiao, Kaijun; Xi, Jingyu; Qiu, Xinping

    2015-03-01

    Pt, PtRu, PtNi and PtRuNi nanoparticles are assembled to few-layered graphene (FLG) and the resulting hybrids are examined as catalysts for the electro-oxidation of methanol and ethanol. The structures of the catalysts are characterized using transmission electron microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy. The compositional and electronic properties of the nanoparticles are analyzed using X-ray photoelectron spectroscopy. The activity of the catalysts towards methanol and ethanol electro-oxidation is studied by cyclic voltammetry, linear sweeping voltammetry and chronoamperometry. It is found that the activity of the catalysts follows the sequence of Pt/FLG < PtNi/FLG < PtRu/FLG < PtRuNi/FLG. The activity of the catalysts is well correlated with the structural characteristics. The superior activity of the PtRuNi/FLG catalyst is attributed to the synergistic effects of Pt, Ru and Ni, as explained by the bi-functional, ligand and strain effects.

  19. Synthesis of few-layered, high-purity graphene oxide sheets from different graphite sources for biology

    NASA Astrophysics Data System (ADS)

    Jasim, Dhifaf A.; Lozano, Neus; Kostarelos, Kostas

    2016-03-01

    This work aimed to interrogate the role that the starting graphitic material played on the physicochemical properties of graphene oxide (GO) sheets and their impact on mammalian cell viability following exposure to those flakes. Three different GO thin sheets were synthesised from three starting graphite material: flakes (GO-f), ground (GO-g) and powder (GO-p) using a modified Hummers’ method. The synthetic yield of this methodology was found to differ according to type of starting material, with GO-p resulting in most efficient yields. Structural and morphological comparison of the three GO sheet types were carried out using transmission electron microscopy and atomic force microscopy. Optical properties were measured using UV/visible and fluorescence spectroscopy. Surface characteristics and chemistry were determined using a battery of techniques. Exposure to human cells was studied using the human A549 lung epithelial cultures. Our results revealed that all three GO samples were composed of few-layer sheets with similar physicochemical and surface characteristics. However, significant differences were observed in terms of their lateral dimensions with GO-p, prepared from graphite powder, being the largest among the GOs. No cytotoxicity was detected for any of the GO samples following exposure onto A549 cells up to 48 h. In conclusion, the form and type of the starting graphite material is shown to be an important factor that can determine the synthetic yield and the structural characteristics of the resulting GO sheets.

  20. Understanding the role of few-layer graphene nanosheets in enhancing the hydrogen sorption kinetics of magnesium hydride.

    PubMed

    Liu, Guang; Wang, Yijing; Jiao, Lifang; Yuan, Huatang

    2014-07-23

    The catalytic effects of few-layer, highly wrinkled graphene nanosheet (GNS) addition on the dehydrogenation/rehydrogenation performance of MgH2 were investigated. It was found that MgH2-5 wt %GNSs nanocomposites prepared by ball milling exhibit relatively lower sorption temperature, faster sorption kinetics, and more stable cycling performance than that of pure-milled MgH2. The dehydrogenation step confirms that the Avrami exponent n increases from 1.22 to 2.20 by the Johnson-Mehl-Avrami (JMA) formalism when the desorption temperature is reduced from 350 °C to 320 °C and 300 °C, implying that a change in the decomposition temperature can alter the mechanism during the dehydrogenation process. For rehydrogenation, the Avrami value n is close to 1; further study by several models coincident with n = 1 reveals that the absorption process of the MgH2-5 wt %GNSs sample conforms to the Mampel equation formulated through the random nucleation approach and that the nature of the absorption mechanism does not change within the temperature range studied. Furthermore, microstructure analysis demonstrated that the defective GNSs are distributed uniformly among the MgH2 particles and that the grain size of the MgH2-5 wt %GNSs nanocomposite is approximately 5-9 nm. The efficient metal-free catalytic dehydrogenation/rehydrogenation of MgH2 can be attributed to the coupling of the nanosize effect and defective GNSs. PMID:24941011

  1. Vanadium Oxyfluoride/Few-Layer Graphene Composite as a High-Performance Cathode Material for Lithium Batteries.

    PubMed

    Cambaz, Musa Ali; Vinayan, B P; Clemens, Oliver; Munnangi, Anji Reddy; Chakravadhanula, Venkata Sai Kiran; Kübel, Christian; Fichtner, Maximilian

    2016-04-18

    Metal oxyfluoride compounds are gathering significant interest as cathode materials for lithium ion batteries at the moment because of their high theoretical capacity and resulting high energy density. In this regard, a new and direct approach is presented to synthesize phase-pure vanadium oxyfluoride (VO2F). The structure of VO2F was identified by Rietveld refinement of the powder X-ray diffraction (XRD) pattern. It crystallizes in a perovskite-type structure with disorder of the oxide and fluoride ions. The as-synthesized VO2F was tested as a cathode material for lithium ion batteries after being surface-coated with few-layer graphene. The VO2F delivered a first discharge capacity of 254 mA h g(-1) and a reversible capacity of 208 mA h g(-1) at a rate of C/20 for the first 20 cycles with an average discharge voltage of 2.84 V, yielding an energy density of 591 W h kg(-1). Improved rate capability that outperforms the previous report has been achieved, showing a discharge capacity of 150 mA h g(-1) for 1 C. The structural changes during lithium insertion and extraction were monitored by ex-situ XRD analysis of the electrodes discharged and charged to various stages. Lithium insertion results in an irreversible structural change of the anion lattice from (3)/4 cubic close packing to hexagonal close packing to accommodate the inserted lithium ions while keeping the overall space-group symmetry. For the first time we have revealed a structural change for the ReO3-type structure of as-prepared VO2F to the RhF3 structure after lithiation/delithiation, with structural changes that have not been observed in previous reports. Furthermore, the new synthetic approach described here would be a platform for the synthesis of new oxyfluoride compounds. PMID:27018603

  2. Effect of nitriding/nanostructuration of few layer graphene supported iron-based particles; catalyst in graphene etching and carbon nanofilament growth.

    PubMed

    Baaziz, Walid; Melinte, Georgian; Ersen, Ovidiu; Pham-Huu, Cuong; Janowska, Izabela

    2014-08-14

    Stable, highly faceted and dispersed iron nitride particles supported on few layer graphene are obtained by ammonia decomposition on iron-based particles at the temperature commonly used for the synthesis of N-doped CNTs and graphene etching. The TEM/EELS analysis reveals nitrogen diffusion in a bulk of the particles. The resulting facet FeNx catalyst exhibits high activity in the etching of graphene, which is assisted by catalyst reorganization. Ammonia decomposition is used for the first time for graphene etching, while the highly faceted catalyst has an impact on the etched channels structures. According to the shape of the active planes of the catalyst, the etching results in sharp "V" channel ends and often "step-like" edges. The FeNx morphology proves previously reported triangularisation of arches in highly N-doped carbon nanotubes. The conditioning of the catalyst by its shaping and nitrogen incorporation is investigated additionally in the carbon nanostructure formation, for decomposition of ethane. The herringbone CNFs, "hollow" bamboo-like CNFs/CNTs or CNTs are effectively observed. PMID:24964374

  3. Ultrafine nickel oxide quantum dots enbedded with few-layer exfoliative graphene for an asymmetric supercapacitor: Enhanced capacitances by alternating voltage

    NASA Astrophysics Data System (ADS)

    Jing, Mingjun; Wang, Chiwei; Hou, Hongshuai; Wu, Zhibin; Zhu, Yirong; Yang, Yingchang; Jia, Xinnan; Zhang, Yan; Ji, Xiaobo

    2015-12-01

    A green and one-step method of electrochemical alternating voltage has been utilized to form NiO quantum dots/graphene flakes (NiO-dots/Gh) for supercapacitor applications. NiO quantum dots (∼3 nm) are uniformly deposited on few-layer graphene surfaces by oxygen functional groups on graphene surface that is naturally utilized to bridge NiO and graphene through Ni-O-C bands, which exhibits outstanding specific capacitance 1181.1 F g-1 at a current density of 2.1 A g-1 and rate behavior 66.2% at 42 A g-1 as NiO dots can be fleetly wired up to current collector through the underlying graphene two-dimensional layers. The NiO-dots/Gh composite is further undertaken in asymmetric supercapacitors with high energy density (27.3 Wh kg-1 at 1562.6 W kg-1).

  4. Cellular and molecular mechanistic insight into the DNA-damaging potential of few-layer graphene in human primary endothelial cells.

    PubMed

    Sasidharan, Abhilash; Swaroop, Siddharth; Chandran, Parwathy; Nair, Shantikumar; Koyakutty, Manzoor

    2016-07-01

    Despite graphene being proposed for a multitude of biomedical applications, there is a dearth in the fundamental cellular and molecular level understanding of how few-layer graphene (FLG) interacts with human primary cells. Herein, using human primary umbilical vein endothelial cells as model of vascular transport, we investigated the basic mechanism underlying the biological behavior of graphene. Mechanistic toxicity studies using a battery of cell based assays revealed an organized oxidative stress paradigm involving cytosolic reactive oxygen stress, mitochondrial superoxide generation, lipid peroxidation, glutathione oxidation, mitochondrial membrane depolarization, enhanced calcium efflux, all leading to cell death by apoptosis/necrosis. We further investigated the effect of graphene interactions using cDNA microarray analysis and identified potential adverse effects by down regulating key genes involved in DNA damage response and repair mechanisms. Single cell gel electrophoresis assay/Comet assay confirmed the DNA damaging potential of graphene towards human primary cells. PMID:26970024

  5. Van der Waals stacks of few-layer h-AlN with graphene: an ab initio study of structural, interaction and electronic properties

    NASA Astrophysics Data System (ADS)

    dos Santos, Renato B.; de Brito Mota, F.; Rivelino, R.; Kakanakova-Georgieva, A.; Gueorguiev, G. K.

    2016-04-01

    Graphite-like hexagonal AlN (h-AlN) multilayers have been experimentally manifested and theoretically modeled. The development of any functional electronics applications of h-AlN would most certainly require its integration with other layered materials, particularly graphene. Here, by employing vdW-corrected density functional theory calculations, we investigate structure, interaction energy, and electronic properties of van der Waals stacking sequences of few-layer h-AlN with graphene. We find that the presence of a template such as graphene induces enough interlayer charge separation in h-AlN, favoring a graphite-like stacking formation. We also find that the interface dipole, calculated per unit cell of the stacks, tends to increase with the number of stacked layers of h-AlN and graphene.

  6. Laser-induced etching of few-layer graphene synthesized by Rapid-Chemical Vapour Deposition on Cu thin films.

    PubMed

    Piazzi, Marco; Croin, Luca; Vittone, Ettore; Amato, Giampiero

    2012-12-01

    The outstanding electrical and mechanical properties of graphene make it very attractive for several applications, Nanoelectronics above all. However a reproducible and non destructive way to produce high quality, large-scale area, single layer graphene sheets is still lacking. Chemical Vapour Deposition of graphene on Cu catalytic thin films represents a promising method to reach this goal, because of the low temperatures (T < 950°C-1000°C) involved during the process and of the theoretically expected monolayer self-limiting growth. On the contrary such self-limiting growth is not commonly observed in experiments, thus making the development of techniques allowing for a better control of graphene growth highly desirable. Here we report about the local ablation effect, arising in Raman analysis, due to the heat transfer induced by the laser incident beam onto the graphene sample. PMID:23503582

  7. Tunable plasmons in few-layer nitrogen-doped graphene nanostructures: A time-dependent density functional theory study

    NASA Astrophysics Data System (ADS)

    Shu, Xiao-qin; Zhang, Hong; Cheng, Xin-lu; Miyamoto, Yoshiyuki

    2016-05-01

    Compared with conventional metal plasmonic materials, surface plasmons in graphene are advantageous in terms of higher confinement, relative low loss, flexible featuring, and good tunability. However, the working frequencies of the pristine graphene (undoped graphene) surface plasmons are located in the terahertz and infrared regions, which limit their applications. Here we show high-frequency plasmons in nitrogen (N)-doped graphene nanostructures investigated by time-dependent density functional theory. We found the optical absorption strength of systems containing two layers to be more than twofold stronger than that of systems with monolayers. The optical absorption strength increases as the interlayer distance increases, and the absorption spectra are red-shifted for impulse excitations polarized in the armchair edge direction (x axis). For microstructures of more than two layers, the optical absorption strength increases as number of layers of the N-doped graphene nanostructures increases. In addition, when the number of layers becomes elevated at low-energy resonances, the absorption spectra are seen to blue-shift. The plasmon energy resonance points are located in the visible and ultraviolet regions. The N-doped graphene provides an effective strategy for nanoscale plasmon devices in the visible and ultraviolet regions, despite their weaker absorption intensities when compared with the pristine graphene.

  8. CVD synthesis of mono- and few-layer graphene using alcohols at low hydrogen concentration and atmospheric pressure

    NASA Astrophysics Data System (ADS)

    Campos-Delgado, Jessica; Botello-Méndez, Andrés R.; Algara-Siller, Gerardo; Hackens, Benoit; Pardoen, Thomas; Kaiser, Ute; Dresselhaus, Mildred S.; Charlier, Jean-Christophe; Raskin, Jean-Pierre

    2013-10-01

    An original and easy route to produce mono-, bi- and tri-layer graphene is proposed using the chemical vapor deposition technique. The synthesis is carried out at atmospheric pressure using liquid precursors, copper as catalyst, and a single gas injection line consisting of a very diluted mixture of H2 in Argon (H2: 5%). Two different alcohols are investigated as possible sources of carbon: 2-phenylethanol and ethanol. The characterization of the samples with SEM, TEM and Raman spectroscopy confirms the presence of graphene on top of copper, and yields a detailed picture of the structure of the produced graphene layers.

  9. Few-layer MoS2-anchored graphene aerogel paper for free-standing electrode materials.

    PubMed

    Lee, Wee Siang Vincent; Peng, Erwin; Loh, Tamie Ai Jia; Huang, Xiaolei; Xue, Jun Min

    2016-04-21

    To reduce the reliance on polymeric binders, conductive additives, and metallic current collectors during the electrode preparation process, as well as to assess the true performance of lithium ion battery (LIB) anodes, a free-standing electrode has to be meticulously designed. Graphene aerogel is a popular scaffolding material that has been widely used with embedded nanoparticles for application in LIB anodes. However, the current graphene aerogel/nanoparticle composite systems still involve decomposition into powder and the addition of additives during electrode preparation because of the thick aerogel structure. To further enhance the capacity of the system, MoS2 was anchored onto a graphene aerogel paper and the composite was used directly as an LIB anode. The resultant additive-free MoS2/graphene aerogel paper composite exhibited long cyclic performance with 101.1% retention after 700 cycles, which demonstrates the importance of free-standing electrodes in enhancing cyclic stability. PMID:27020143

  10. Few-layer MoS2-anchored graphene aerogel paper for free-standing electrode materials

    NASA Astrophysics Data System (ADS)

    Lee, Wee Siang Vincent; Peng, Erwin; Loh, Tamie Ai Jia; Huang, Xiaolei; Xue, Jun Min

    2016-04-01

    To reduce the reliance on polymeric binders, conductive additives, and metallic current collectors during the electrode preparation process, as well as to assess the true performance of lithium ion battery (LIB) anodes, a free-standing electrode has to be meticulously designed. Graphene aerogel is a popular scaffolding material that has been widely used with embedded nanoparticles for application in LIB anodes. However, the current graphene aerogel/nanoparticle composite systems still involve decomposition into powder and the addition of additives during electrode preparation because of the thick aerogel structure. To further enhance the capacity of the system, MoS2 was anchored onto a graphene aerogel paper and the composite was used directly as an LIB anode. The resultant additive-free MoS2/graphene aerogel paper composite exhibited long cyclic performance with 101.1% retention after 700 cycles, which demonstrates the importance of free-standing electrodes in enhancing cyclic stability.To reduce the reliance on polymeric binders, conductive additives, and metallic current collectors during the electrode preparation process, as well as to assess the true performance of lithium ion battery (LIB) anodes, a free-standing electrode has to be meticulously designed. Graphene aerogel is a popular scaffolding material that has been widely used with embedded nanoparticles for application in LIB anodes. However, the current graphene aerogel/nanoparticle composite systems still involve decomposition into powder and the addition of additives during electrode preparation because of the thick aerogel structure. To further enhance the capacity of the system, MoS2 was anchored onto a graphene aerogel paper and the composite was used directly as an LIB anode. The resultant additive-free MoS2/graphene aerogel paper composite exhibited long cyclic performance with 101.1% retention after 700 cycles, which demonstrates the importance of free-standing electrodes in enhancing cyclic

  11. Simple, green and high-yield production of single- or few-layer graphene by hydrothermal exfoliation of graphite

    NASA Astrophysics Data System (ADS)

    Liu, Xiangrong; Zheng, Mingtao; Xiao, Ke; Xiao, Yong; He, Chenglong; Dong, Hanwu; Lei, Bingfu; Liu, Yingliang

    2014-04-01

    Graphene is widely used as promising electronic material and devices, owing to its exceptional electronic and optoelectronic properties. Up to now, defect-free graphene has been limited to the method for controllable, reproducible and scalable mass production. A simple, green, and nontoxic approach for large-scale preparation of high quality graphene is produced by exfoliation of graphite sheets collaborated with intercalant (FeCl2) under hydrothermal conditions, the absence of defects or oxides in graphene with a yield up to 10 wt% can be a practical application and industrial process such as optical limiters, transparent conductors, and sensors. This new process could potentially be improved to give a yield of up to 35 wt% of the starting graphite mass with sediment recycling. We show with experiments and theories that exfoliation graphene is the result of a combined action by diminishing the van der Waals interactions between graphite layers and the shear force drove by the Brownian motion of H2O and FeCl2 molecules. Hydrothermal exfoliation has potential applications in the exfoliation of other layered materials (e.g. BN, MoS2) and carbon nantubes, and in the synthesis of intercalation compounds, nanoribbons, and nanoparticles, thus opening new ways of exfoliation engineering.Graphene is widely used as promising electronic material and devices, owing to its exceptional electronic and optoelectronic properties. Up to now, defect-free graphene has been limited to the method for controllable, reproducible and scalable mass production. A simple, green, and nontoxic approach for large-scale preparation of high quality graphene is produced by exfoliation of graphite sheets collaborated with intercalant (FeCl2) under hydrothermal conditions, the absence of defects or oxides in graphene with a yield up to 10 wt% can be a practical application and industrial process such as optical limiters, transparent conductors, and sensors. This new process could potentially be

  12. Layer Number Dependence of Li(+) Intercalation on Few-Layer Graphene and Electrochemical Imaging of Its Solid-Electrolyte Interphase Evolution.

    PubMed

    Hui, Jingshu; Burgess, Mark; Zhang, Jiarui; Rodríguez-López, Joaquín

    2016-04-26

    A fundamental question facing electrodes made out of few layers of graphene (FLG) is if they display chemical properties that are different to their bulk graphite counterpart. Here, we show evidence that suggests that lithium ion intercalation on FLG, as measured via stationary voltammetry, shows a strong dependence on the number of layers of graphene that compose the electrode. Despite its extreme thinness and turbostratic structure, Li ion intercalation into FLG still proceeds through a staging process, albeit with different signatures than bulk graphite or multilayer graphene. Single-layer graphene does not show any evidence of ion intercalation, while FLG with four graphene layers displays limited staging peaks, which broaden and increase in number as the layer number increases to six. Despite these mechanistic differences on ion intercalation, the formation of a solid-electrolyte interphase (SEI) was observed on all electrodes. Scanning electrochemical microscopy (SECM) in the feedback mode was used to demonstrate changes in the surface conductivity of FLG during SEI evolution. Observation of ion intercalation on large area FLG was conditioned to the fabrication of "ionic channels" on the electrode. SECM measurements using a recently developed Li-ion sensitive imaging technique evidenced the role of these channels in enabling Li-ion intercalation through localized flux measurements. This work highlights the impact of nanostructure and microstructure on macroscopic electrochemical behavior and provides guidance to the mechanistic control of ion intercalation using graphene, an atomically thin interface where surface and bulk reactivity converge. PMID:26943950

  13. One-Minute Room-Temperature Transfer-Free Production of Mono- and Few-Layer Polycrystalline Graphene on Various Substrates

    NASA Astrophysics Data System (ADS)

    Jiang, Shenglin; Zeng, Yike; Zhou, Wenli; Miao, Xiangshui; Yu, Yan

    2016-01-01

    Graphene deposited on various substrates has attracted the attention of the scientific and technical communities for use in a wide range of applications. Graphene on substrates is commonly produced by two types of methods, namely, methods that require a transfer step and transfer-free methods. Compared with methods that require a transfer step, transfer-free methods have a simpler procedure and a lower cost. Thus, transfer-free methods have considerable potential to meet the industrial and commercial demands of production methods. However, some limitations of the current transfer-free methods must be overcome, such as the high temperatures encountered during production, the relatively long manufacturing times, incompatibilities for both rigid and flexible substrates, and an inability to extend the process to other two-dimensional (2-D) atomic crystals. In this work, a room-temperature rubbing method is developed for the rapid transfer-free production of defect-free polycrystalline graphene on rigid and flexible substrates. Starting with inexpensive commercially obtained graphite powder, mono- and few-layer graphene can be fabricated directly on various substrates, with an average production time of less than one minute (from raw graphite to graphene on the substrate). Importantly, this method can be extended to other 2-D atomic crystals.

  14. One-Minute Room-Temperature Transfer-Free Production of Mono- and Few-Layer Polycrystalline Graphene on Various Substrates

    PubMed Central

    Jiang, Shenglin; Zeng, Yike; Zhou, Wenli; Miao, Xiangshui; Yu, Yan

    2016-01-01

    Graphene deposited on various substrates has attracted the attention of the scientific and technical communities for use in a wide range of applications. Graphene on substrates is commonly produced by two types of methods, namely, methods that require a transfer step and transfer-free methods. Compared with methods that require a transfer step, transfer-free methods have a simpler procedure and a lower cost. Thus, transfer-free methods have considerable potential to meet the industrial and commercial demands of production methods. However, some limitations of the current transfer-free methods must be overcome, such as the high temperatures encountered during production, the relatively long manufacturing times, incompatibilities for both rigid and flexible substrates, and an inability to extend the process to other two-dimensional (2-D) atomic crystals. In this work, a room-temperature rubbing method is developed for the rapid transfer-free production of defect-free polycrystalline graphene on rigid and flexible substrates. Starting with inexpensive commercially obtained graphite powder, mono- and few-layer graphene can be fabricated directly on various substrates, with an average production time of less than one minute (from raw graphite to graphene on the substrate). Importantly, this method can be extended to other 2-D atomic crystals. PMID:26763292

  15. Fabrication of few-layer graphene film based field effect transistor and its application for trace-detection of herbicide atrazine

    NASA Astrophysics Data System (ADS)

    Thanh Cao, Thi; Chuc Nguyen, Van; Binh Nguyen, Hai; Thang Bui, Hung; Thu Vu, Thi; Phan, Ngoc Hong; Thang Phan, Bach; Hoang, Le; Bayle, Maxime; Paillet, Matthieu; Sauvajol, Jean Louis; Phan, Ngoc Minh; Tran, Dai Lam

    2016-09-01

    We describe the fabrication of highly sensitive graphene-based field effect transistor (FET) enzymatic biosensor for trace-detection of atrazine. The few-layers graphene films were prepared on polycrystalline copper foils by atmospheric pressure chemical vapor deposition method using an argon/hydrogen/methane mixture. The characteristics of graphene films were investigated by scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The results indicated low uniformity of graphene layers, which is probably induced by heterogeneous distribution of graphene nucleation sites on the Cu surface. The pesticide detection is accomplished through the measurement of the drain-source current variations of the FET sensor upon the urea enzymatic hydrolysis reaction. The obtained biosensor is able to detect atrazine with a sensitivity of 56 μA/logCATZ in range between 2 × 10‑4 and 20 ppb and has a limit of detection as low as 0.05 ppt. The elaboration of such highly sensitive biosensors will provide better biosensing performances for the detection of biochemical targets.

  16. CO2 enhanced chemical vapor deposition growth of few-layer graphene over NiO(x).

    PubMed

    Son, In Hyuk; Song, Hyun Jae; Kwon, Soonchul; Bachmatiuk, Alicja; Lee, Seung Jae; Benayad, Anass; Park, Jong Hwan; Choi, Jae-Young; Chang, Hyuk; Rümmeli, Mark H

    2014-09-23

    The use of mild oxidants in chemical vapor deposition (CVD) reactions has proven enormously useful. This was also true for the CVD growth of carbon nanotubes. As yet though, the use of mild oxidants in the CVD of graphene has remained unexplored. Here we explore the use of CO2 as a mild oxidant during the growth of graphene over Ni with CH4 as the feedstock. Both our experimental and theoretical findings provide in-depth insight into the growth mechanisms and point to the mild oxidants playing multiple roles. Mild oxidants lead to the formation of a suboxide in the Ni, which suppresses the bulk diffusion of C species suggesting a surface growth mechanism. Moreover, the formation of a suboxide leads to enhanced catalytic activity at the substrate surface, which allows reduced synthesis temperatures, even as low as 700 °C. Even at these low temperatures, the quality of the graphene is exceedingly high as indicated by a negligible D mode in the Raman spectra. These findings suggest the use of mild oxidants in the CVD fabrication as a whole could have a positive impact. PMID:25171048

  17. Dissolution-and-reduction CVD synthesis of few-layer graphene on ultra-thin nickel film lifted off for mode-locking fiber lasers

    PubMed Central

    Peng, Kaung-Jay; Lin, Yung-Hsiang; Wu, Chung-Lun; Lin, Sheng-Fong; Yang, Chun-Yu; Lin, Shih-Meng; Tsai, Din-Ping; Lin, Gong-Ru

    2015-01-01

    The in-situ dissolution-and-reduction CVD synthesized few-layer graphene on ultra-thin nickel catalyst film is demonstrated at temperature as low as 550 °C, which can be employed to form transmission-type or reflection-type saturable absorber (SA) for mode-locking the erbium-doped fiber lasers (EDFLs). With transmission-type graphene SA, the EDFL shortens its pulsewidth from 483 to 441 fs and broadens its spectral linewidth from 4.2 to 6.1 nm with enlarging the pumping current from 200 to 900 mA. In contrast, the reflection-type SA only compresses the pulsewidth from 875 to 796 fs with corresponding spectral linewidth broadened from 2.2 to 3.3 nm. The reflection-type graphene mode-locker increases twice of its equivalent layer number to cause more insertion loss than the transmission-type one. Nevertheless, the reflection-type based saturable absorber system can generate stabilized soliton-like pulse easier than that of transmission-type system, because the nonlinearity induced self-amplitude modulation depth is simultaneously enlarged when passing through the graphene twice under the retro-reflector design. PMID:26328535

  18. Engineering epitaxial graphene with oxygen

    NASA Astrophysics Data System (ADS)

    Kimouche, Amina; Martin, Sylvain; Winkelmann, Clemens; Fruchart, Olivier; Courtois, Hervé; Coraux, Johann; Hybrid system at low dimension Team

    2013-03-01

    Almost free-standing graphene can be obtained on metals by decoupling graphene from its substrate, for instance by intercalation of atoms beneath graphene, as it was shown with oxygen atoms. We show that the interaction of oxygen with epitaxial graphene on iridium leads to the formation of an ultrathin crystalline oxide extending between graphene and the metallic substrate via the graphene wrinkles. Graphene studied in this work was prepared under ultra-high vacuum by CVD. The samples were studied by combining scanning probe microscopy (STM, AFM) and spatially resolved spectroscopy (Raman, STS). The ultrathin oxide forms a decoupling barrier layer between graphene and Ir, yielding truly free-standing graphene whose hybridization and charge transfers with the substrate have been quenched. Our work presents novel types of graphene-based nanostructures, and opens the route to the transfer-free preparation of graphene directly onto an insulating support contacted to the metallic substrate which could serve as a gate electrode. Work supported by the EU-NMP GRENADA project

  19. Degradation of (14)C-labeled few layer graphene via Fenton reaction: Reaction rates, characterization of reaction products, and potential ecological effects.

    PubMed

    Feng, Yiping; Lu, Kun; Mao, Liang; Guo, Xiangke; Gao, Shixiang; Petersen, Elijah J

    2015-11-01

    Graphene has attracted considerable commercial interest due to its numerous potential applications. It is inevitable that graphene will be released into the environment during the production and usage of graphene-enabled consumer products, but the potential transformations of graphene in the environment are not well understood. In this study, (14)C-labeled few layer graphene (FLG) enabled quantitative measurements of FLG degradation rates induced by the iron/hydrogen peroxide induced Fenton reaction. Quantification of (14)CO2 production from (14)C-labeled FLG revealed significant degradation of FLG after 3 days with high H2O2 (200 mmol L(-1)) and iron (100 μmol L(-1)) concentrations but substantially lower rates under environmentally relevant conditions (0.2-20 mmol L(-1) H2O2 and 4 μmol L(-1) Fe(3+)). Importantly, the carbon-14 labeling technique allowed for quantification of the FLG degradation rate at concentrations nearly four orders of magnitude lower than those typically used in other studies. These measurements revealed substantially faster degradation rates at lower FLG concentrations and thus studies with higher FLG concentrations may underestimate the degradation rates. Analysis of structural changes to FLG using multiple orthogonal methods revealed significant FLG oxidation and multiple reaction byproducts. Lastly, assessment of accumulation of the degraded FLG and intermediates using aquatic organism Daphnia magna revealed substantially decreased body burdens, which implied that the changes to FLG caused by the Fenton reaction may dramatically impact its potential ecological effects. PMID:26210029

  20. Sulfur and nitrogen co-doped, few-layered graphene oxide as a highly efficient electrocatalyst for the oxygen-reduction reaction.

    PubMed

    Xu, Jiaoxing; Dong, Guofa; Jin, Chuanhong; Huang, Meihua; Guan, Lunhui

    2013-03-01

    S and N co-doped, few-layered graphene oxide is synthesized by using pyrimidine and thiophene as precursors for the application of the oxygen reduction reaction (ORR). The dual-doped catalyst with pyrrolic/graphitic N-dominant structures exhibits competitive catalytic activity (10.0 mA cm(-2) kinetic-limiting current density at -0.25 V) that is superior to that for mono N-doped carbon nanomaterials. This is because of a synergetic effect of N and S co-doping. Furthermore, the dual-doped catalyst also shows an efficient four-electron-dominant ORR process, which has excellent methanol tolerance and improved durability in comparison to commercial Pt/C catalysts. PMID:23404829

  1. Dielectric properties of novel composites prepared with few layered graphene (FLG)-lithium triniobate (LiNb{sub 3}O{sub 8})

    SciTech Connect

    Tumuluri, Anil; Raju, K. C. James; Marka, Sandeep Kumar; Srikanth, V. V. S. S.

    2015-08-28

    Lithium triniobate (LiNb{sub 3}O{sub 8}) has been prepared using microwave assisted combustion synthesis at a low temperature of 400 °C. Graphite oxide is synthesized using Hummers method and thereafter it is thermally exfoliated at 500 and 700 °C to obtain two samples of Few Layered Graphene (FLG). Then, FLG-LiNb{sub 3}O{sub 8} composites prepared through a simple ultra-sonication assisted solution mixing process. The obtained samples are named as FLN5 and FLN7 based on the temperature used to obtain FLG. Low frequency dielectric measurements are recorded for both the samples and the measured dielectric constant is observed to be more for FLN5 (250 at 60 Hz) than FLN7 (115 at 60 Hz) whilst the dielectric loss is less in case of FLN5 (1.525 at 60 Hz). The enhancement in dielectric properties for FLN5 is explained in terms of polarization in FLG-LiNb{sub 3}O{sub 8} composites.

  2. Spin transport in epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Tbd, -

    2014-03-01

    Spintronics is a paradigm focusing on spin as the information vector in fast and ultra-low-power non volatile devices such as the new STT-MRAM. Beyond its widely distributed application in data storage it aims at providing more complex architectures and a powerful beyond CMOS solution for information processing. The recent discovery of graphene has opened novel exciting opportunities in terms of functionalities and performances for spintronics devices. We will present experimental results allowing us to assess the potential of graphene for spintronics. We will show that unprecedented highly efficient spin information transport can occur in epitaxial graphene leading to large spin signals and macroscopic spin diffusion lengths (~ 100 microns), a key enabler for the advent of envisioned beyond-CMOS spin-based logic architectures. We will also show that how the device behavior is well explained within the framework of the Valet-Fert drift-diffusion equations. Furthermore, we will show that a thin graphene passivation layer can prevent the oxidation of a ferromagnet, enabling its use in novel humide/ambient low-cost processes for spintronics devices, while keeping its highly surface sensitive spin current polarizer/analyzer behavior and adding new enhanced spin filtering property. These different experiments unveil promising uses of graphene for spintronics.

  3. Epitaxial graphene: the material for graphene electronics

    SciTech Connect

    Sprinkle, M.; Soukiassian, P.; de Heer, W.A.; Berger, C.; Conrad, E.H.

    2009-12-10

    The search for an ideal graphene sheet has been a quest driving graphene research. While most research has focused on exfoliated graphene, intrinsic substrate interactions and mechanical disorder have precluded the observation of a number of graphene's expected physical properties in this material. The only graphene candidate that has demonstrated all the essential properties of an ideal sheet is multilayer graphene grown on the SiC(000) surface. Its unique stacking allows nearly all the sheets in the stack to behave like isolated graphene, while the weak graphene-graphene interaction prevents any significant doping or distortion in the band near the Fermi level.

  4. Exfoliating and Dispersing Few-Layered Graphene in Low-Boiling-Point Organic Solvents towards Solution-Processed Optoelectronic Device Applications.

    PubMed

    Zhang, Lu; Miao, Zhongshuo; Hao, Zhen; Liu, Jun

    2016-05-01

    With normal organic surfactants, graphene can only be dispersed in water and cannot be dispersed in low-boiling-point organic solvents, which hampers its application in solution-processed organic optoelectronic devices. Herein, we report the exfoliation of graphite into graphene in low-boiling-point organic solvents, for example, methanol and acetone, by using edge-carboxylated graphene quantum dots (ECGQD) as the surfactant. The great capability of ECGQD for graphene dispersion is due to its ultralarge π-conjugated unit that allows tight adhesion on the graphene surface through strong π-π interactions, its edge-carboxylated structure that diminishes the steric effects of the oxygen-containing functional groups on the basal plane of ECGQD, and its abundance of carboxylic acid groups for solubility. The graphene dispersion in methanol enables the application of graphene:ECGQD as a cathode interlayer in polymer solar cells (PSCs). Moreover, the PSC device performance of graphene:ECGQD is better than that of Ca, the state-of-the-art cathode interlayer material. PMID:26957045

  5. Hetero epitaxial graphene on various substrates

    NASA Astrophysics Data System (ADS)

    Harris, Gary; Kaut, Gurpreet; Taylor, Crawford

    2015-03-01

    Large-scale production of graphene is pivotal for the development of graphene-based electronics. These results focus on the synthesis and characterization of graphene layers. Two methods were used to grow graphene films. First, graphene films were epitaxially grown on silicon carbide substrates by thermal decomposition of SiC at high temperature and low pressure. In-house built reactor consisting of induction furnace was used to form epitaxial films for electronic applications. Second, chemical vapor deposition method was used for direct graphene synthesis on 3C-SiC with the use of copper as a catalyst. In thermal CVD process, hydrogen and methane gases were used as precursors. Methane acts as a carbon source and annealing and cooling were done hydrogen environment. Different polytypes of silicon carbide (6H-SiC and 3C-SiC) and their crystal orientations were exploited as substrates to form epitaxial graphene. Hetero epitaxial 3C-SiC epilayer was first deposited on Si substrate using chemical vapor deposition technique in cold wall, low pressure, and horizontal CVD reactor. The reactor temperature, argon pressure, flow rates and concentration of different gases (propane, silane, hydrogen and argon) was investigated to control the growth of 3C-SiC and silicon sublimation rate. The resulting graphene films were confirmed using Raman spectroscopy. Further, graphene films have been characterized with the tools of atomic force microscopy (AFM) and scanning electron microscopy (SEM). Mobility, electrical resistivity and carrier density measurements were taken using hall measurements. NSF_PRDM

  6. New X-ray insight into oxygen intercalation in epitaxial graphene grown on 4H-SiC(0001)

    SciTech Connect

    Kowalski, G. Tokarczyk, M.; Dąbrowski, P.; Ciepielewski, P.; Możdżonek, M.; Strupiński, W.; Baranowski, J. M.

    2015-03-14

    Efficient control of intercalation of epitaxial graphene by specific elements is a way to change properties of the graphene. Results of several experimental techniques, such as X-ray photoelectron spectroscopy, micro-Raman mapping, reflectivity, attenuated total reflection, X-ray diffraction, and X-ray reflectometry, gave a new insight into the intercalation of oxygen in the epitaxial graphene grown on 4H-SiC(0001). These results confirmed that oxygen intercalation decouples the graphene buffer layer from the 4H-SiC surface and converts it into the graphene layer. However, in contrast to the hydrogen intercalation, oxygen does not intercalate between carbon planes (in the case of few layer graphene) and the interlayer spacing stays constant at the level of 3.35–3.32 Å. Moreover, X-ray reflectometry showed the presence of an oxide layer having the thickness of about 0.8 Å underneath the graphene layers. Apart from the formation of the nonuniform thin oxide layer, generation of defects in graphene caused by oxygen was also evidenced. Last but not least, water islands underneath defected graphene regions in both intercalated and non-intercalated samples were most probably revealed. These water islands are formed in the case of all the samples stored under ambient laboratory conditions. Water islands can be removed from underneath the few layer graphene stacks by relevant thermal treatment or by UV illumination.

  7. Local transport measurements on epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Baringhaus, J.; Edler, F.; Neumann, C.; Stampfer, C.; Forti, S.; Starke, U.; Tegenkamp, C.

    2013-09-01

    Growth of large-scale graphene is still accompanied by imperfections. By means of a four-tip scanning tunneling and electron microscope (4-tip STM/SEM), the local structure of graphene grown on SiC(0001) was correlated with scanning electron microscope images and spatially resolved transport measurements. The systematic variation of probe spacings and substrate temperature has clearly revealed two-dimensional transport regimes of Anderson localization as well as of diffusive transport. The detailed analysis of the temperature dependent data demonstrates that the local on-top nano-sized contacts do not induce significant strain to the epitaxial graphene films.

  8. Thickness-Dependent Hydrophobicity of Epitaxial Graphene.

    PubMed

    Munz, Martin; Giusca, Cristina E; Myers-Ward, Rachael L; Gaskill, D Kurt; Kazakova, Olga

    2015-08-25

    This article addresses the much debated question whether the degree of hydrophobicity of single-layer graphene (1LG) is different from that of double-layer graphene (2LG). Knowledge of the water affinity of graphene and its spatial variations is critically important as it can affect the graphene properties as well as the performance of graphene devices exposed to humidity. By employing chemical force microscopy with a probe rendered hydrophobic by functionalization with octadecyltrichlorosilane (OTS), the adhesion force between the probe and epitaxial graphene on SiC has been measured in deionized water. Owing to the hydrophobic attraction, a larger adhesion force was measured on 2LG Bernal-stacked domains of graphene surfaces, thus showing that 2LG is more hydrophobic than 1LG. Identification of 1LG and 2LG domains was achieved through Kelvin probe force microscopy and Raman spectral mapping. Approximate values of the adhesion force per OTS molecule have been calculated through contact area analysis. Furthermore, the contrast of friction force images measured in contact mode was reversed to the 1LG/2LG adhesion contrast, and its origin was discussed in terms of the likely water depletion over hydrophobic domains as well as deformation in the contact area between the atomic force microscope tip and 1LG. PMID:26218503

  9. Proximity induced Superconductivity in Epitaxial Graphene

    NASA Astrophysics Data System (ADS)

    Natterer, Fabian D.; Ha, Jeonghoon; Baek, Hongwoo; Zhang, Duming; Cullen, William; Zhitenev, Nikolai B.; Kuk, Young; Stroscio, Joseph A.

    The intimate electrical contact of a superconductor with a normal metal leads to an exchange of carriers through their boundary. Cooper pairs leak into the normal metal via Andreev reflection and enable the normal metal to acquire superconducting-like properties. The electron-hole conversion process in graphene is prominent due to relativistic quantum mechanics governing low energy chiral carriers in a multi-valley system. In the present experiment, we reveal spatial measurements of the proximity effect in graphene from a graphene-superconductor interface. Superconducting aluminum films were grown on epitaxial graphene on SiC. The aluminum films were discontinuous with networks of trenches in the film morphology reaching down to the substrate to exposed graphene terraces. Scanning tunneling spectra measured on the graphene terraces show a clear decay of the superconducting gap width with increasing separation from the graphene-aluminum edges. The decay length for the superconducting energy gap extends beyond 400 nm. Subtle deviations in the exponentially decaying energy gap were also observed on a much smaller length scale of tens of nanometers. Funding from SNSF (project 158468), NIST/CNST Grant 70NANB10H193, and KRF-2010-00349.

  10. Observation of the intrinsic bandgap behaviour in as-grown epitaxial twisted graphene.

    PubMed

    Park, Jeongho; Mitchel, William C; Elhamri, Said; Grazulis, Lawrence; Hoelscher, John; Mahalingam, Krishnamurthy; Hwang, Choongyu; Mo, Sung-Kwan; Lee, Jonghoon

    2015-01-01

    Twisted graphene is of particular interest due to several intriguing characteristics, such as its the Fermi velocity, van Hove singularities and electronic localization. Theoretical studies recently suggested the possible bandgap opening and tuning. Here, we report a novel approach to producing epitaxial twisted graphene on SiC (0001) and the observation of its intrinsic bandgap behaviour. The direct deposition of C60 on pre-grown graphene layers results in few-layer twisted graphene confirmed by angular resolved photoemission spectroscopy and Raman analysis. The strong enhanced G band in Raman and sp(3) bonding characteristic in X-ray photoemission spectroscopy suggests the existence of interlayer interaction between adjacent graphene layers. The interlayer spacing between graphene layers measured by transmission electron microscopy is 0.352 ± 0.012 nm. Thermal activation behaviour and nonlinear current-voltage characteristics conclude that an intrinsic bandgap is opened in twisted graphene. Low sheet resistance (~ 160 Ω □(-1) at 10 K) and high mobility (~2,000 cm(2) V(-1) s(-1) at 10 K) are observed. PMID:25562285

  11. Epitaxial graphene nanoribbon array fabrication using BCP-assisted nanolithography.

    PubMed

    Liu, Guanxiong; Wu, Yanqing; Lin, Yu-Ming; Farmer, Damon B; Ott, John A; Bruley, John; Grill, Alfred; Avouris, Phaedon; Pfeiffer, Dirk; Balandin, Alexander A; Dimitrakopoulos, Christos

    2012-08-28

    A process for fabricating dense graphene nanoribbon arrays using self-assembled patterns of block copolymers on graphene grown epitaxially on SiC on the wafer scale has been developed. Etching masks comprising long and straight nanoribbon array structures with linewidths as narrow as 10 nm were fabricated, and the patterns were transferred to graphene. Our process combines both top-down and self-assembly steps to fabricate long graphene nanoribbon arrays with low defect counts. These are the narrowest nanoribbon arrays of epitaxial graphene on SiC fabricated to date. PMID:22780305

  12. On the density of states of disordered epitaxial graphene

    SciTech Connect

    Davydov, S. Yu.

    2015-05-15

    The study is concerned with two types of disordered epitaxial graphene: (i) graphene with randomly located carbon vacancies and (ii) structurally amorphous graphene. The former type is considered in the coherent potential approximation, and for the latter type, a model of the density of states is proposed. The effects of two types of substrates, specifically, metal and semiconductor substrates are taken into account. The specific features of the density of states of epitaxial graphene at the Dirac point and the edges of the continuous spectrum are analyzed. It is shown that vacancies in epitaxial graphene formed on the metal substrate bring about logarithmic nulling of the density of states of graphene at the Dirac point and the edges of the continuous spectrum. If the Dirac point corresponds to the middle of the band gap of the semiconductor substrate, the linear trend of the density of states to zero in the vicinity of the Dirac point in defect-free graphene transforms into a logarithmic decrease in the presence of vacancies. In both cases, the graphene-substrate interaction is assumed to be weak (quasi-free graphene). In the study of amorphous epitaxial graphene, a simple model of free amorphous graphene is proposed as the initial model, in which account is taken of the nonzero density of states at the Dirac point, and then the interaction of the graphene sheet with the substrate is taken into consideration. It is shown that, near the Dirac point, the quadratic behavior of the density of states of free amorphous graphene transforms into a linear dependence for amorphous epitaxial graphene. In the study, the density of states of free graphene corresponds to the low-energy approximation of the electron spectrum.

  13. Electronic states in epitaxial graphene fabricated on silicon carbide

    SciTech Connect

    Davydov, S. Yu.

    2011-08-15

    An analytical expression for the density of states of a graphene monolayer interacting with a silicon carbide surface (epitaxial graphene) is derived. The density of states of silicon carbide is described within the Haldane-Anderson model. It is shown that the graphene-substrate interaction results in a narrow gap of {approx}0.01-0.06 eV in the density of states of graphene. The graphene atom charge is estimated; it is shown that the charge transfer from the substrate is {approx}10{sup -3}-10{sup -2}e per graphene atom.

  14. Epitaxy of GaN Nanowires on Graphene.

    PubMed

    Kumaresan, Vishnuvarthan; Largeau, Ludovic; Madouri, Ali; Glas, Frank; Zhang, Hezhi; Oehler, Fabrice; Cavanna, Antonella; Babichev, Andrey; Travers, Laurent; Gogneau, Noelle; Tchernycheva, Maria; Harmand, Jean-Christophe

    2016-08-10

    Epitaxial growth of GaN nanowires on graphene is demonstrated using molecular beam epitaxy without any catalyst or intermediate layer. Growth is highly selective with respect to silica on which the graphene flakes, grown by chemical vapor deposition, are transferred. The nanowires grow vertically along their c-axis and we observe a unique epitaxial relationship with the ⟨21̅1̅0⟩ directions of the wurtzite GaN lattice parallel to the directions of the carbon zigzag chains. Remarkably, the nanowire density and height decrease with increasing number of graphene layers underneath. We attribute this effect to strain and we propose a model for the nanowire density variation. The GaN nanowires are defect-free and they present good optical properties. This demonstrates that graphene layers transferred on amorphous carrier substrates is a promising alternative to bulk crystalline substrates for the epitaxial growth of high quality GaN nanostructures. PMID:27414518

  15. On the kinetic barriers of graphene homo-epitaxy

    SciTech Connect

    Zhang, Wei; Yu, Xinke; Xie, Ya-Hong; Cahyadi, Erica; Ratsch, Christian

    2014-12-01

    The diffusion processes and kinetic barriers of individual carbon adatoms and clusters on graphene surfaces are investigated to provide fundamental understanding of the physics governing epitaxial growth of multilayer graphene. It is found that individual carbon adatoms form bonds with the underlying graphene whereas the interaction between graphene and carbon clusters, consisting of 6 atoms or more, is very weak being van der Waals in nature. Therefore, small carbon clusters are quite mobile on the graphene surfaces and the diffusion barrier is negligibly small (∼6 meV). This suggests the feasibility of high-quality graphene epitaxial growth at very low growth temperatures with small carbon clusters (e.g., hexagons) as carbon source. We propose that the growth mode is totally different from 3-dimensional bulk materials with the surface mobility of carbon hexagons being the highest over graphene surfaces that gradually decreases with further increase in cluster size.

  16. Scanning Tunneling Spectroscopy of Proximity Superconductivity in Epitaxial Multilayer Graphene

    PubMed Central

    Natterer, Fabian D.; Ha, Jeonghoon; Baek, Hongwoo; Zhang, Duming; Cullen, William; Zhitenev, Nikolai B.; Kuk, Young; Stroscio, Joseph A.

    2016-01-01

    We report on spatial measurements of the superconducting proximity effect in epitaxial graphene induced by a graphene-superconductor interface. Superconducting aluminum films were grown on epitaxial multilayer graphene on SiC. The aluminum films were discontinuous with networks of trenches in the film morphology reaching down to exposed graphene terraces. Scanning tunneling spectra measured on the graphene terraces show a clear decay of the superconducting energy gap with increasing separation from the graphene-aluminum edges. The spectra were well described by Bardeen-Cooper-Schrieffer (BCS) theory. The decay length for the superconducting energy gap in graphene was determined to be greater than 400 nm. Deviations in the exponentially decaying energy gap were also observed on a much smaller length scale of tens of nanometers. PMID:27088134

  17. Growth of Epitaxial Oxide Thin Films on Graphene.

    PubMed

    Zou, Bin; Walker, Clementine; Wang, Kai; Tileli, Vasiliki; Shaforost, Olena; Harrison, Nicholas M; Klein, Norbert; Alford, Neil M; Petrov, Peter K

    2016-01-01

    The transfer process of graphene onto the surface of oxide substrates is well known. However, for many devices, we require high quality oxide thin films on the surface of graphene. This step is not understood. It is not clear why the oxide should adopt the epitaxy of the underlying oxide layer when it is deposited on graphene where there is no lattice match. To date there has been no explanation or suggestion of mechanisms which clarify this step. Here we show a mechanism, supported by first principles simulation and structural characterisation results, for the growth of oxide thin films on graphene. We describe the growth of epitaxial SrTiO3 (STO) thin films on a graphene and show that local defects in the graphene layer (e.g. grain boundaries) act as bridge-pillar spots that enable the epitaxial growth of STO thin films on the surface of the graphene layer. This study, and in particular the suggestion of a mechanism for epitaxial growth of oxides on graphene, offers new directions to exploit the development of oxide/graphene multilayer structures and devices. PMID:27515496

  18. Growth of Epitaxial Oxide Thin Films on Graphene

    PubMed Central

    Zou, Bin; Walker, Clementine; Wang, Kai; Tileli, Vasiliki; Shaforost, Olena; Harrison, Nicholas M.; Klein, Norbert; Alford, Neil M.; Petrov, Peter K.

    2016-01-01

    The transfer process of graphene onto the surface of oxide substrates is well known. However, for many devices, we require high quality oxide thin films on the surface of graphene. This step is not understood. It is not clear why the oxide should adopt the epitaxy of the underlying oxide layer when it is deposited on graphene where there is no lattice match. To date there has been no explanation or suggestion of mechanisms which clarify this step. Here we show a mechanism, supported by first principles simulation and structural characterisation results, for the growth of oxide thin films on graphene. We describe the growth of epitaxial SrTiO3 (STO) thin films on a graphene and show that local defects in the graphene layer (e.g. grain boundaries) act as bridge-pillar spots that enable the epitaxial growth of STO thin films on the surface of the graphene layer. This study, and in particular the suggestion of a mechanism for epitaxial growth of oxides on graphene, offers new directions to exploit the development of oxide/graphene multilayer structures and devices. PMID:27515496

  19. Graphene nanoribbons epitaxy on boron nitride

    NASA Astrophysics Data System (ADS)

    Lu, Xiaobo; Yang, Wei; Wang, Shuopei; Wu, Shuang; Chen, Peng; Zhang, Jing; Zhao, Jing; Meng, Jianling; Xie, Guibai; Wang, Duoming; Wang, Guole; Zhang, Ting Ting; Watanabe, Kenji; Taniguchi, Takashi; Yang, Rong; Shi, Dongxia; Zhang, Guangyu

    2016-03-01

    In this letter, we report a pilot study on epitaxy of monolayer graphene nanoribbons (GNRs) on hexagonal boron nitride (h-BN). We found that GNRs grow preferentially from the atomic steps of h-BN, forming in-plane heterostructures. GNRs with well-defined widths ranging from ˜15 nm to ˜150 nm can be obtained reliably. As-grown GNRs on h-BN have high quality with a carrier mobility of ˜20 000 cm2 V-1 s-1 for ˜100-nm-wide GNRs at a temperature of 1.7 K. Besides, a moiré pattern induced quasi-one-dimensional superlattice with a periodicity of ˜15 nm for GNR/h-BN was also observed, indicating zero crystallographic twisting angle between GNRs and h-BN substrate. The superlattice induced band structure modification is confirmed by our transport results. These epitaxial GNRs/h-BN with clean surfaces/interfaces and tailored widths provide an ideal platform for high-performance GNR devices.

  20. Self-doping effects in epitaxially grown graphene

    SciTech Connect

    Siegel, David A.; Zhou, Shuyun Y.; El Gabaly, Farid; Fedorov, Alexei V.; Schmid, Andreas K.; Lanzara, Alessandra

    2008-09-19

    Self-doping in graphene has been studied by examining single-layer epitaxially grown graphene samples with differing characteristic lateral terrace widths. Low energy electron microscopy was used to gain real-space information about the graphene surface morphology, which was compared with data obtained by angle-resolved photoemission spectroscopy to study the effect of the monolayer graphene terrace width on the low energy dispersions. By altering the graphene terrace width, we report significant changes in the electronic structure and quasiparticle relaxation time of the material, in addition to a terrace width-dependent doping effect.

  1. Electronic Band Engineering of Epitaxial Graphene by Atomic Intercalation

    NASA Astrophysics Data System (ADS)

    Jayasekera, Thushari; Sandin, Andreas; Xu, Shu; Wheeler, Virginia; Gaskill, D. K.; Rowe, J. E.; Kim, K. W.; Dougherty, Daniel B.; Buongiorno Nardelli, M.

    2012-02-01

    Using calculations from first principles, we have investigated possible ways of engineering the electronic band structure of epitaxial graphene on SiC. In particular, intercalation of different atomic species, such as Hydrogen, Fluorine, Sodium, Germanium, Carbon and Silicon is shown to modify and tune the interface electronic properties and band alignments. Our results suggest that intercalation in graphene is quite different from that in graphite, and could provide a fundamentally new way to achieve electronic control in graphene electronics.

  2. Localized States Influence Spin Transport in Epitaxial Graphene

    NASA Astrophysics Data System (ADS)

    Maassen, T.; van den Berg, J. J.; Huisman, E. H.; Dijkstra, H.; Fromm, F.; Seyller, T.; van Wees, B. J.

    2013-02-01

    We developed a spin transport model for a diffusive channel with coupled localized states that result in an effective increase of spin precession frequencies and a reduction of spin relaxation times in the system. We apply this model to Hanle spin precession measurements obtained on monolayer epitaxial graphene on SiC(0001). Combined with newly performed measurements on quasi-free-standing monolayer epitaxial graphene on SiC(0001) our analysis shows that the different values for the diffusion coefficient measured in charge and spin transport measurements on monolayer epitaxial graphene on SiC(0001) and the high values for the spin relaxation time can be explained by the influence of localized states arising from the buffer layer at the interface between the graphene and the SiC surface.

  3. Vertical conduction mechanism of the epitaxial graphene/n-type 4H-SiC heterojunction at cryogenic temperatures

    NASA Astrophysics Data System (ADS)

    Tadjer, M. J.; Anderson, T. J.; Hobart, K. D.; Nyakiti, L. O.; Wheeler, V. D.; Myers-Ward, R. L.; Gaskill, D. K.; Eddy, C. R.; Kub, F. J.; Calle, F.

    2012-05-01

    Vertical diodes of epitaxial graphene on n- 4H-SiC were investigated. The graphene Raman spectra exhibited a higher intensity in the G-line than the 2D-line, indicative of a few-layer graphene film. Rectifying properties improved at low temperatures as the reverse leakage decreased over six orders of magnitude without freeze-out in either material. Carrier concentration of ˜1016 cm-3 in the SiC remained stable down to 15 K, while accumulation charge decreased and depletion width increased in forward bias. The low barrier height of 0.08 eV and absence of recombination-induced emission indicated majority carrier field emission as the dominant conduction mechanism.

  4. Weak Localization in few layer Black Phosphorus

    NASA Astrophysics Data System (ADS)

    Gillgren, Nathaniel; Shi, Yanmeng; Espiritu, Timothy; Watanabe, Kenji; Taniguchi, Takahashi; Lau, Chun Ning (Jeanie)

    Few-layer black phosphorus has recently attracted interest from the scientific community due to its high mobility, tunable band gap, and large anisotropy. Recent experiments have demonstrated that black phosphorus provides a promising candidate to explore the physics of 2D semiconductors. In this study we explore the magnetotransport of few-layer black phosphorus-boron nitride hetereostructure devices at low magnetic fields. Weak localization is observed at low temperatures. We extract the dephasing length and measure its dependence on temperature, carrier density and electric field.

  5. Highly ordered growth of PTCDA on epitaxial bilayer graphene

    NASA Astrophysics Data System (ADS)

    Meissner, Matthias; Gruenewald, Marco; Sojka, Falko; Udhardt, Christian; Forker, Roman; Fritz, Torsten

    2012-11-01

    For using the unique electronic properties of graphene in future nanoelectronic devices, control of the band structure is essential. While it has been shown already in the literature that this can be achieved by the deposition of organic molecules, little attention has been paid so far to the precise structural characterization of the interface. Here, we report on the epitaxial growth of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) layers on graphene, epitaxially grown on silicon carbide (SiC). The description of low energy electron diffraction (LEED) patterns of graphene on SiC by multiscattering is revisited. By means of a home-made algorithm used to correct radial distortions of the LEED images we are able to provide precise structural data of the PTCDA layers. By that, two different point-on-line types of PTCDA could be identified, one of which has neither been reported on graphite nor on graphene before.

  6. Argon-assisted growth of epitaxial graphene on Cu(111)

    NASA Astrophysics Data System (ADS)

    Robinson, Zachary R.; Tyagi, Parul; Mowll, Tyler R.; Ventrice, Carl A., Jr.; Hannon, James B.

    2012-12-01

    The growth of graphene by catalytic decomposition of ethylene on Cu(111) in an ultrahigh vacuum system was investigated with low-energy electron diffraction, low-energy electron microscopy, and atomic force microscopy. Attempts to form a graphene overlayer using ethylene at pressures as high as 10 mTorr and substrate temperatures as high as 900 ∘C resulted in almost no graphene growth. By using an argon overpressure, the growth of epitaxial graphene on Cu(111) was achieved. The suppression of graphene growth without the use of an argon overpressure is attributed to Cu sublimation at elevated temperatures. During the initial stages of growth, a random distribution of rounded graphene islands is observed. The predominant rotational orientation of the islands is within ±1∘ of the Cu(111) substrate lattice.

  7. Microwave studies of weak localization and antilocalization in epitaxial graphene

    SciTech Connect

    Drabińska, Aneta; Kamińska, Maria; Wołoś, Agnieszka; Baranowski, J. M.

    2013-12-04

    A microwave detection method was applied to study weak localization and antilocalization in epitaxial graphene sheets grown on both polarities of SiC substrates. Both coherence and scattering length values were obtained. The scattering lengths were found to be smaller for graphene grown on C-face of SiC. The decoherence rate was found to depend linearly on temperature, showing the electron-electron scattering mechanism.

  8. Growth and Features of Epitaxial Graphene on SiC

    NASA Astrophysics Data System (ADS)

    Kusunoki, Michiko; Norimatsu, Wataru; Bao, Jianfeng; Morita, Koichi; Starke, Ulrich

    2015-12-01

    Recent progress of epitaxial graphene on SiC was reviewed, focusing on its growth and structural and electronic features. Homogeneous graphene can be grown on SiC(0001) on a wafer scale, however on SiC(000bar{1}) multilayer but rotationally stacked graphene with monolayer like electronic property grows. HRTEM revealed the formation mechanism and structural features of graphene on the both surfaces. The high structural and electronic quality of the grown graphene is monitored by Raman spectroscopy and magneto-transport characterization. High-resolution ARPES measurements of the electronic dispersion around the bar{text{K}}-point retrieved the ABA and ABC stacked trilayer graphene. The measurements also directly revealed that electronic structures of graphene were manipulated by transfer doping and atomic intercalation. In particular, p- and n-doped regions on a meso-scale and the p-n junctions prepared on SiC via controlling intercalation of Ge exhibited ballistic transport and Klein tunneling, which predicted novel potentials on to epitaxial graphene on SiC.

  9. Charged nano-domes and bubbles in epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Ben Gouider Trabelsi, A.; Kusmartsev, F. V.; Robinson, B. J.; Ouerghi, A.; Kusmartseva, O. E.; Kolosov, O. V.; Mazzocco, R.; Gaifullin, Marat B.; Oueslati, M.

    2014-04-01

    For the first time, new epitaxial graphene nano-structures resembling charged ‘bubbles’ and ‘domes’ are reported. A strong influence, arising from the change in morphology, on the graphene layer’s electronic, mechanical and optical properties has been shown. The morphological properties of these structures have been studied with atomic force microscopy (AFM), ultrasonic force microscopy (UFM) and Raman spectroscopy. After initial optical microscopy observation of the graphene, a detailed description of the surface morphology, via AFM and nanomechanical UFM measurements, was obtained. Here, graphene nano-structures, domes and bubbles, ranging from a few tens of nanometres (150-200 nm) to a few μm in size have been identified. The AFM topographical and UFM stiffness data implied the freestanding nature of the graphene layer within the domes and bubbles, with heights on the order of 5-12 nm. Raman spectroscopy mappings of G and 2D bands and their ratio confirm not only the graphene composition of these structures but also the existence of step bunching, defect variations and the carrier density distribution. In particular, inside the bubbles and substrate there arises complex charge redistribution; in fact, the graphene bubble-substrate interface forms a charged capacitance. We have determined the strength of the electric field inside the bubble-substrate interface, which may lead to a minigap of the order of 5 meV opening for epitaxial graphene grown on 4H-SiC face-terminated carbon.

  10. Charged nano-domes and bubbles in epitaxial graphene.

    PubMed

    Trabelsi, A Ben Gouider; Kusmartsev, F V; Robinson, B J; Ouerghi, A; Kusmartseva, O E; Kolosov, O V; Mazzocco, R; Gaifullin, Marat B; Oueslati, M

    2014-04-25

    For the first time, new epitaxial graphene nano-structures resembling charged 'bubbles' and 'domes' are reported. A strong influence, arising from the change in morphology, on the graphene layer's electronic, mechanical and optical properties has been shown. The morphological properties of these structures have been studied with atomic force microscopy (AFM), ultrasonic force microscopy (UFM) and Raman spectroscopy. After initial optical microscopy observation of the graphene, a detailed description of the surface morphology, via AFM and nanomechanical UFM measurements, was obtained. Here, graphene nano-structures, domes and bubbles, ranging from a few tens of nanometres (150–200 nm) to a few μm in size have been identified. The AFM topographical and UFM stiffness data implied the freestanding nature of the graphene layer within the domes and bubbles, with heights on the order of 5–12 nm. Raman spectroscopy mappings of G and 2D bands and their ratio confirm not only the graphene composition of these structures but also the existence of step bunching, defect variations and the carrier density distribution. In particular, inside the bubbles and substrate there arises complex charge redistribution; in fact, the graphene bubble–substrate interface forms a charged capacitance. We have determined the strength of the electric field inside the bubble–substrate interface, which may lead to a minigap of the order of 5 meV opening for epitaxial graphene grown on 4H-SiC face-terminated carbon. PMID:24675237

  11. The effect of adsorption on static conductivity of epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Davydov, S. Yu.

    2014-12-01

    An analytical expression for adsorption-induced changes in the static conductivity of the adlayer-single-sheet-graphene-substrate system has been obtained using the Kubo-Greenwood formalism with allowance for the dipole-dipole repulsion in the adlayer. The cases of both metal and semiconductor substrates have been considered. The results are applied to description of the hydrogen monolayer-single-sheet-graphene-tungsten substrate system. Numerical estimations show that the magnitude of the effect of the adsorbed hydrogen monolayer on the static conductivity σeg of epitaxial graphene on tungsten is on the order of the σeg value.

  12. Epitaxially grown strained pentacene thin film on graphene membrane.

    PubMed

    Kim, Kwanpyo; Santos, Elton J G; Lee, Tae Hoon; Nishi, Yoshio; Bao, Zhenan

    2015-05-01

    Organic-graphene system has emerged as a new platform for various applications such as flexible organic photovoltaics and organic light emitting diodes. Due to its important implication in charge transport, the study and reliable control of molecular packing structures at the graphene-molecule interface are of great importance for successful incorporation of graphene in related organic devices. Here, an ideal membrane of suspended graphene as a molecular assembly template is utilized to investigate thin-film epitaxial behaviors. Using transmission electron microscopy, two distinct molecular packing structures of pentacene on graphene are found. One observed packing structure is similar to the well-known bulk-phase, which adapts a face-on molecular orientation on graphene substrate. On the other hand, a rare polymorph of pentacene crystal, which shows significant strain along the c-axis, is identified. In particular, the strained film exhibits a specific molecular orientation and a strong azimuthal correlation with underlying graphene. Through ab initio electronic structure calculations, including van der Waals interactions, the unusual polymorph is attributed to the strong graphene-pentacene interaction. The observed strained organic film growth on graphene demonstrates the possibility to tune molecular packing via graphene-molecule interactions. PMID:25565340

  13. Terahertz and mid-infrared reflectance of epitaxial graphene.

    PubMed

    Santos, Cristiane N; Joucken, Frédéric; De Sousa Meneses, Domingos; Echegut, Patrick; Campos-Delgado, Jessica; Louette, Pierre; Raskin, Jean-Pierre; Hackens, Benoit

    2016-01-01

    Graphene has emerged as a promising material for infrared (IR) photodetectors and plasmonics. In this context, wafer scale epitaxial graphene on SiC is of great interest in a variety of applications in optics and nanoelectronics. Here we present IR reflectance spectroscopy of graphene grown epitaxially on the C-face of 6H-SiC over a broad optical range, from terahertz (THz) to mid-infrared (MIR). Contrary to the transmittance, reflectance measurements are not hampered by the transmission window of the substrate, and in particular by the SiC Reststrahlen band in the MIR. This allows us to present IR reflectance data exhibiting a continuous evolution from the regime of intraband to interband charge carrier transitions. A consistent and simultaneous analysis of the contributions from both transitions to the optical response yields precise information on the carrier dynamics and the number of layers. The properties of the graphene layers derived from IR reflection spectroscopy are corroborated by other techniques (micro-Raman and X-ray photoelectron spectroscopies, transport measurements). Moreover, we also present MIR microscopy mapping, showing that spatially-resolved information can be gathered, giving indications on the sample homogeneity. Our work paves the way for a still scarcely explored field of epitaxial graphene-based THz and MIR optical devices. PMID:27102827

  14. Terahertz and mid-infrared reflectance of epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Santos, Cristiane N.; Joucken, Frédéric; de Sousa Meneses, Domingos; Echegut, Patrick; Campos-Delgado, Jessica; Louette, Pierre; Raskin, Jean-Pierre; Hackens, Benoit

    2016-04-01

    Graphene has emerged as a promising material for infrared (IR) photodetectors and plasmonics. In this context, wafer scale epitaxial graphene on SiC is of great interest in a variety of applications in optics and nanoelectronics. Here we present IR reflectance spectroscopy of graphene grown epitaxially on the C-face of 6H-SiC over a broad optical range, from terahertz (THz) to mid-infrared (MIR). Contrary to the transmittance, reflectance measurements are not hampered by the transmission window of the substrate, and in particular by the SiC Reststrahlen band in the MIR. This allows us to present IR reflectance data exhibiting a continuous evolution from the regime of intraband to interband charge carrier transitions. A consistent and simultaneous analysis of the contributions from both transitions to the optical response yields precise information on the carrier dynamics and the number of layers. The properties of the graphene layers derived from IR reflection spectroscopy are corroborated by other techniques (micro-Raman and X-ray photoelectron spectroscopies, transport measurements). Moreover, we also present MIR microscopy mapping, showing that spatially-resolved information can be gathered, giving indications on the sample homogeneity. Our work paves the way for a still scarcely explored field of epitaxial graphene-based THz and MIR optical devices.

  15. Terahertz and mid-infrared reflectance of epitaxial graphene

    PubMed Central

    Santos, Cristiane N.; Joucken, Frédéric; De Sousa Meneses, Domingos; Echegut, Patrick; Campos-Delgado, Jessica; Louette, Pierre; Raskin, Jean-Pierre; Hackens, Benoit

    2016-01-01

    Graphene has emerged as a promising material for infrared (IR) photodetectors and plasmonics. In this context, wafer scale epitaxial graphene on SiC is of great interest in a variety of applications in optics and nanoelectronics. Here we present IR reflectance spectroscopy of graphene grown epitaxially on the C-face of 6H-SiC over a broad optical range, from terahertz (THz) to mid-infrared (MIR). Contrary to the transmittance, reflectance measurements are not hampered by the transmission window of the substrate, and in particular by the SiC Reststrahlen band in the MIR. This allows us to present IR reflectance data exhibiting a continuous evolution from the regime of intraband to interband charge carrier transitions. A consistent and simultaneous analysis of the contributions from both transitions to the optical response yields precise information on the carrier dynamics and the number of layers. The properties of the graphene layers derived from IR reflection spectroscopy are corroborated by other techniques (micro-Raman and X-ray photoelectron spectroscopies, transport measurements). Moreover, we also present MIR microscopy mapping, showing that spatially-resolved information can be gathered, giving indications on the sample homogeneity. Our work paves the way for a still scarcely explored field of epitaxial graphene-based THz and MIR optical devices. PMID:27102827

  16. Growth-induced electronic properties of epitaxial graphene

    NASA Astrophysics Data System (ADS)

    First, Phillip

    2012-02-01

    The growth of epitaxial graphene on silicon carbide is challenging to understand and control, yet rife with scientific and technological opportunities. This is due in part to different growth-induced structures such as the ``moire'' alignment of graphene layers in multilayer epitaxial graphene on SiC(0001) and the formation of sidewall ribbons at natural and lithographically-defined SiC(0001) step-bunches (nanofacets). We apply scanning tunneling microscopy (STM) and spectroscopy (STS) to probe the local energy bands of such growth-induced structures. STS at cryogenic temperatures and large magnetic fields creates a comb of discrete Landau level energies that we use to quantitatively characterize the local electronic properties.

  17. Carrier Transport in Epitaxial Multi-layer Graphene

    NASA Astrophysics Data System (ADS)

    Lin, Yu-Ming; Dimitrakopoulos, Christos; Farmer, Damon; Han, Shu-Jen; Wu, Yanqing; Zhu, Wenjuan; Gaskill, D. Kurt; Tedesco, Joseph; Myers-Ward, Rachael; Eddy, Charles, Jr.; Grill, Alfred; Avouris, Phaedon; Ibm Team; Nrl Team

    2011-03-01

    Significant attention has been focused recently on the electrical properties of graphene grown epitaxially on SiC substrates, because it offers an ideal platform for carbon-based electronics using conventional top-down lithography techniques. The transport properties of graphene are usually studied via Hall effect measurements, which provide information on the carrier mobility and density. Hall measurements performed at a single magnetic field yield a weighted average of carrier mobility and density, and are strictly applicable to homogeneous samples. In this study, we performed variable-field Hall and resistivity measurements on epitaxial graphene, and the results were analyzed with a multi-carrier model. Good agreements were obtained between experimental data and the model, providing further evidence of multi-carrier transport in the C-face grown MLG. This work is supported by DARPA under contract FA8650-08-C-7838 through the CERA program and by the Office of Naval Research.

  18. Few-layer black phosphorus nanoparticles.

    PubMed

    Sofer, Zdenek; Bouša, Daniel; Luxa, Jan; Mazanek, Vlastimil; Pumera, Martin

    2016-01-28

    Herein, black phosphorus quantum dots and nanoparticles of a few layer thickness were prepared and characterized using STEM, AFM, dynamic light scattering, X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy and photoluminescence. Impact electrochemistry of the induvidual black phosphorus nanoparticles allows their size determination. The centrifugation of colloidal black phosphorus nanoparticles allowed separation of quantum dots with sizes up to 15 nm. These black phosphorus nanoparticles exhibit a large band gap and are expected to find a wide range of applications from semiconductors to biomolecule tags. The use of black phosphorus nanoparticles for vapour sensing was successfully demonstrated. PMID:26691583

  19. Edge promoted ultrasensitive electrochemical detection of organic bio-molecules on epitaxial graphene nanowalls.

    PubMed

    Kumar Roy, Pradip; Ganguly, Abhijit; Yang, Wei-Hsun; Wu, Chien-Ting; Hwang, Jih-Shang; Tai, Yian; Chen, Kuei-Hsien; Chen, Li-Chyong; Chattopadhyay, Surojit

    2015-08-15

    We report the simultaneous electrochemical detection of dopamine (DA), uric acid (UA) and ascorbic acid (AA) on three dimensional (3D) unmodified 'as-grown' epitaxial graphene nanowall arrays (EGNWs). The 3D few layer EGNWs, unlike the 2D planar graphene, offers an abundance of vertically oriented nano-graphitic-edges that exhibit fast electron-transfer kinetics and high electroactive surface area to geometrical area (EAA/GA≈134%), as evident from the Fe(CN)6(3-/4-) redox kinetic study. The hexagonal sp(2)-C domains, on the basal plane of the EGNWs, facilitate efficient adsorption via spontaneous π-π interaction with the aromatic rings in DA and UA. Such affinity together with the fast electron kinetics enables simultaneous and unambiguous identification of individual AA, DA and UA from their mixture. The unique edge dominant EGNWs result in an unprecedented low limit of detection (experimental) of 0.033 nM and highest sensitivity of 476.2 µA/µM/cm(2), for UA, which are orders of magnitude higher than comparable existing reports. A reaction kinetics based modeling of the edge-oriented 3D EGNW system is proposed to illustrate the superior electro-activity for bio-sensing applications. PMID:25801954

  20. Low contact resistance in epitaxial graphene devices for quantum metrology

    SciTech Connect

    Yager, Tom E-mail: ywpark@snu.ac.kr; Lartsev, Arseniy; Lara-Avila, Samuel; Kubatkin, Sergey; Cedergren, Karin; Yakimova, Rositsa; Panchal, Vishal; Kazakova, Olga; Tzalenchuk, Alexander; Kim, Kyung Ho; Park, Yung Woo E-mail: ywpark@snu.ac.kr

    2015-08-15

    We investigate Ti/Au contacts to monolayer epitaxial graphene on SiC (0001) for applications in quantum resistance metrology. Using three-terminal measurements in the quantum Hall regime we observed variations in contact resistances ranging from a minimal value of 0.6 Ω up to 11 kΩ. We identify a major source of high-resistance contacts to be due bilayer graphene interruptions to the quantum Hall current, whilst discarding the effects of interface cleanliness and contact geometry for our fabricated devices. Moreover, we experimentally demonstrate methods to improve the reproducibility of low resistance contacts (<10 Ω) suitable for high precision quantum resistance metrology.

  1. Chemical gating of epitaxial graphene through ultrathin oxide layers

    NASA Astrophysics Data System (ADS)

    Larciprete, Rosanna; Lacovig, Paolo; Orlando, Fabrizio; Dalmiglio, Matteo; Omiciuolo, Luca; Baraldi, Alessandro; Lizzit, Silvano

    2015-07-01

    We achieved a controllable chemical gating of epitaxial graphene grown on metal substrates by exploiting the electrostatic polarization of ultrathin SiO2 layers synthesized below it. Intercalated oxygen diffusing through the SiO2 layer modifies the metal-oxide work function and hole dopes graphene. The graphene/oxide/metal heterostructure behaves as a gated plane capacitor with the in situ grown SiO2 layer acting as a homogeneous dielectric spacer, whose high capacity allows the Fermi level of graphene to be shifted by a few hundreds of meV when the oxygen coverage at the metal substrate is of the order of 0.5 monolayers. The hole doping can be finely tuned by controlling the amount of interfacial oxygen, as well as by adjusting the thickness of the oxide layer. After complete thermal desorption of oxygen the intrinsic doping of SiO2 supported graphene is evaluated in the absence of contaminants and adventitious adsorbates. The demonstration that the charge state of graphene can be changed by chemically modifying the buried oxide/metal interface hints at the possibility of tuning the level and sign of doping by the use of other intercalants capable of diffusing through the ultrathin porous dielectric and reach the interface with the metal.We achieved a controllable chemical gating of epitaxial graphene grown on metal substrates by exploiting the electrostatic polarization of ultrathin SiO2 layers synthesized below it. Intercalated oxygen diffusing through the SiO2 layer modifies the metal-oxide work function and hole dopes graphene. The graphene/oxide/metal heterostructure behaves as a gated plane capacitor with the in situ grown SiO2 layer acting as a homogeneous dielectric spacer, whose high capacity allows the Fermi level of graphene to be shifted by a few hundreds of meV when the oxygen coverage at the metal substrate is of the order of 0.5 monolayers. The hole doping can be finely tuned by controlling the amount of interfacial oxygen, as well as by adjusting

  2. Multilayer epitaxial graphene grown on the SiC (000- 1) surface; structure and electronic properties

    SciTech Connect

    Sprinkle, M.; Hicks, J.; Tejeda, A.; Taleb-Ibrahimi, A.; Le Fevre, P.; Bertran, F.; Tinkey, H.; Clark, M.C.; Soukiassian, P.; Martinotti, D.; Hass, J.; Conrad, E.H.

    2010-10-22

    We review the progress towards developing epitaxial graphene as a material for carbon electronics. In particular, we discuss improvements in epitaxial graphene growth, interface control and the understanding of multilayer epitaxial graphene's (MEG's) electronic properties. Although graphene grown on both polar faces of SiC will be discussed, our discussions will focus on graphene grown on the (000{bar 1}) C-face of SiC. The unique properties of C-face MEG have become apparent. These films behave electronically like a stack of nearly independent graphene sheets rather than a thin Bernal stacked graphite sample. The origins of multilayer graphene's electronic behaviour are its unique highly ordered stacking of non-Bernal rotated graphene planes. While these rotations do not significantly affect the inter-layer interactions, they do break the stacking symmetry of graphite. It is this broken symmetry that leads to each sheet behaving like isolated graphene planes.

  3. Single and Few Layer Silicene: Structural, Electronic and Transport Properties

    NASA Astrophysics Data System (ADS)

    Carey, J. David; Roome, Nathanael

    Single layer silicene has weaker π bonding that graphene resulting in buckling of the Si atoms in different sub-lattices. Despite the loss of planarity, a linear bandstructure emerges where we find a Fermi velocity of about 5.3 x 105 m/s. Determination of the phonon dispersion characteristics reveals a Γ point optical phonon with an energy of 69 meV and a K point optical phonon with an energy of 62 meV. In graphene these phonons play important role in scattering electrons, and in Raman spectroscopy, but have larger energies of 194 and 166 meV, respectively. The lower phonon energies in silicene, arising from the higher atomic masses, would be expected to scatter carriers efficiently and limit carrier mobility. We have calculated, however, that the electron-optical phonon coupling matrix elements are about a factor of 25 times smaller than in graphene and this important result will help with the further development of silicene based devices due to reduced phonon scattering. The two stable stacking configurations of bilayer silicene, AA and AB, now have to account of the position of the atomic buckling in the two layers, leading to four possible atomic configurations with the buckling between the layers being in- or out-of-phase with each other. We find that in contrast to graphene, the two stable configurations are based on AA type stacking being about 70 meV per atom more stable than AB stacking. The potential for elemental layered materials beyond graphene for device applications will also be discussed. Single and Few Layer Silicene: Structural, Electronic and Transport Properties.

  4. Epitaxial growth mechanisms of graphene and effects of substrates

    NASA Astrophysics Data System (ADS)

    Özçelik, V. Ongun; Cahangirov, S.; Ciraci, S.

    2012-06-01

    The growth process of single layer graphene with and without substrate is investigated using ab initio, finite temperature molecular dynamic calculations within density functional theory. An understanding of the epitaxial graphene growth mechanisms in the atomic level is provided by exploring the transient stages which occur at the growing edges of graphene. These stages are formation and collapse of large carbon rings together with the formation and healing of Stone-Wales like pentagon-heptagon defects. The activation barriers for the healing of these growth induced defects on various substrates are calculated using the climbing image nudge elastic band method and compared with that of the Stone-Wales defect. It is found that the healing of pentagon-heptagon defects occurring near the edge in the course of growth is much easier than that of Stone-Wales defect. The role of the substrate in the epitaxial growth and in the healing of defects are also investigated in detail, along with the effects of using carbon dimers as the building blocks of graphene growth.

  5. 1/f Noise in Gated Epitaxial Graphene Nanoribbons

    NASA Astrophysics Data System (ADS)

    Vail, Owen; Yang, Jeremy; Miettinen, Anna; Hankinson, John; Berger, Claire; de Heer, Walter; Jiang, Zhigang

    Epitaxial Graphene Nanoribbons (EGNR) grown on sidewall SiC have gained interest as a high-quality interconnect enabling room temperature ballistic transport over micron lengths. To be useful as an interconnect a proper characterization of the noise level in the EGNR needs to be determined. Toward this end, we fabricated EGNR devices with an Aluminum-Oxide top gate and use field effect to tune the fermi energy in the graphene channel. Our studies of the electronic noise and its dependence on the charge density in the ribbon reveal information about the subband structure of the density of states in addition to the ribbon's spectral density at low frequencies. Comparisons to the widely reported 1/f noise in silicon and other forms of graphene provide strong references for analyzing our results.

  6. Chemical gating of epitaxial graphene through ultrathin oxide layers.

    PubMed

    Larciprete, Rosanna; Lacovig, Paolo; Orlando, Fabrizio; Dalmiglio, Matteo; Omiciuolo, Luca; Baraldi, Alessandro; Lizzit, Silvano

    2015-08-01

    We achieved a controllable chemical gating of epitaxial graphene grown on metal substrates by exploiting the electrostatic polarization of ultrathin SiO2 layers synthesized below it. Intercalated oxygen diffusing through the SiO2 layer modifies the metal-oxide work function and hole dopes graphene. The graphene/oxide/metal heterostructure behaves as a gated plane capacitor with the in situ grown SiO2 layer acting as a homogeneous dielectric spacer, whose high capacity allows the Fermi level of graphene to be shifted by a few hundreds of meV when the oxygen coverage at the metal substrate is of the order of 0.5 monolayers. The hole doping can be finely tuned by controlling the amount of interfacial oxygen, as well as by adjusting the thickness of the oxide layer. After complete thermal desorption of oxygen the intrinsic doping of SiO2 supported graphene is evaluated in the absence of contaminants and adventitious adsorbates. The demonstration that the charge state of graphene can be changed by chemically modifying the buried oxide/metal interface hints at the possibility of tuning the level and sign of doping by the use of other intercalants capable of diffusing through the ultrathin porous dielectric and reach the interface with the metal. PMID:26148485

  7. Hot carrier relaxation of Dirac fermions in bilayer epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Huang, J.; Alexander-Webber, J. A.; Janssen, T. J. B. M.; Tzalenchuk, A.; Yager, T.; Lara-Avila, S.; Kubatkin, S.; Myers-Ward, R. L.; Wheeler, V. D.; Gaskill, D. K.; Nicholas, R. J.

    2015-04-01

    Energy relaxation of hot Dirac fermions in bilayer epitaxial graphene is experimentally investigated by magnetotransport measurements on Shubnikov-de Haas oscillations and weak localization. The hot-electron energy loss rate is found to follow the predicted Bloch-Grüneisen power-law behaviour of T4 at carrier temperatures from 1.4 K up to ˜100 K, due to electron-acoustic phonon interactions with a deformation potential coupling constant of 22 eV. A carrier density dependence n_e-1.5 in the scaling of the T4 power law is observed in bilayer graphene, in contrast to the n_e-0.5 dependence in monolayer graphene, leading to a crossover in the energy loss rate as a function of carrier density between these two systems. The electron-phonon relaxation time in bilayer graphene is also shown to be strongly carrier density dependent, while it remains constant for a wide range of carrier densities in monolayer graphene. Our results and comparisons between the bilayer and monolayer exhibit a more comprehensive picture of hot carrier dynamics in graphene systems.

  8. Controlled epitaxial graphene growth within removable amorphous carbon corrals

    SciTech Connect

    Palmer, James; Hu, Yike; Hankinson, John; Guo, Zelei; Heer, Walt A. de; Kunc, Jan; Berger, Claire

    2014-07-14

    We address the question of control of the silicon carbide (SiC) steps and terraces under epitaxial graphene on SiC and demonstrate amorphous carbon (aC) corrals as an ideal method to pin SiC surface steps. aC is compatible with graphene growth, structurally stable at high temperatures, and can be removed after graphene growth. For this, aC is first evaporated and patterned on SiC, then annealed in the graphene growth furnace. There at temperatures above 1200 °C, mobile SiC steps accumulate at the aC corral that provide effective step flow barriers. Aligned step free regions are thereby formed for subsequent graphene growth at temperatures above 1330 °C. Atomic force microscopy imaging supports the formation of step-free terraces on SiC with the step morphology aligned to the aC corrals. Raman spectroscopy indicates the presence of good graphene sheets on the step-free terraces.

  9. Controlled epitaxial graphene growth within removable amorphous carbon corrals

    NASA Astrophysics Data System (ADS)

    Palmer, James; Kunc, Jan; Hu, Yike; Hankinson, John; Guo, Zelei; Berger, Claire; de Heer, Walt A.

    2014-07-01

    We address the question of control of the silicon carbide (SiC) steps and terraces under epitaxial graphene on SiC and demonstrate amorphous carbon (aC) corrals as an ideal method to pin SiC surface steps. aC is compatible with graphene growth, structurally stable at high temperatures, and can be removed after graphene growth. For this, aC is first evaporated and patterned on SiC, then annealed in the graphene growth furnace. There at temperatures above 1200 °C, mobile SiC steps accumulate at the aC corral that provide effective step flow barriers. Aligned step free regions are thereby formed for subsequent graphene growth at temperatures above 1330 °C. Atomic force microscopy imaging supports the formation of step-free terraces on SiC with the step morphology aligned to the aC corrals. Raman spectroscopy indicates the presence of good graphene sheets on the step-free terraces.

  10. Interlayer electronic hybridization leads to exceptional thickness-dependent vibrational properties in few-layer black phosphorus.

    PubMed

    Hu, Zhi-Xin; Kong, Xianghua; Qiao, Jingsi; Normand, Bruce; Ji, Wei

    2016-02-01

    Stacking two-dimensional (2D) materials into multi-layers or heterostructures, known as van der Waals (vdW) epitaxy, is an essential degree of freedom for tuning their properties on demand. Few-layer black phosphorus (FLBP), a material with high potential for nano- and optoelectronics applications, appears to have interlayer couplings much stronger than graphene and other 2D systems. Indeed, these couplings call into question whether the stacking of FLBP can be governed only by vdW interactions, which is of crucial importance for epitaxy and property refinement. Here, we perform a theoretical investigation of the vibrational properties of FLBP, which reflect directly its interlayer coupling, by discussing six Raman-observable phonons, including three optical, one breathing and two shear modes. With increasing sample thickness, we find anomalous redshifts of the frequencies for each optical mode but a blueshift for the armchair shear mode. Our calculations also show splitting of the phonon branches, due to anomalous surface phenomena, and strong phonon-phonon coupling. By computing uniaxial stress effects, inter-atomic force constants and electron densities, we provide a compelling demonstration that these properties are the consequence of strong and highly directional interlayer interactions arising from the electronic hybridization of the lone electron-pairs of FLBP, rather than from vdW interactions. This exceptional interlayer coupling mechanism controls the stacking stability of BP layers and thus opens a new avenue beyond vdW epitaxy for understanding the design of 2D heterostructures. PMID:26763557

  11. Structure and transport of topological insulators on epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Kally, James; Reifsnyder Hickey, Danielle; Lin, Yu-Chuan; Richardella, Anthony; Lee, Joon Sue; Robinson, Joshua; Mkhoyan, K. Andre; Samarth, Nitin

    Recent advancements in spintronics have shown that a class of materials, topological insulators (TI), can be used as a spin-current generator or detector. Topological insulators have protected surface states with the electron's spin locked to its momentum. To access these surface states, (Bi, Sb)2Te3 can be grown by molecular beam epitaxy to have the Fermi energy near the Dirac point so that transport occurs only through the spin-dependent surface states. Graphene is another 2D material of great interest for spintronics because of its very long spin diffusion length. This is an ideal material to act as a spin channel in devices. The van der Waals nature of the growth exhibited by 2D materials such as (Bi, Sb)2Te3 and graphene allows heterostructures to be formed despite the large lattice mismatch. We explore the structure and transport of (Bi, Sb)2Te3 grown on epitaxial graphene on 6H-SiC substrates for spintronic applications. This work was supported in part by C-SPIN and LEAST, two of the six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA.

  12. In-situ epitaxial growth of graphene/h-BN van der Waals heterostructures by molecular beam epitaxy

    PubMed Central

    Zuo, Zheng; Xu, Zhongguang; Zheng, Renjing; Khanaki, Alireza; Zheng, Jian-Guo; Liu, Jianlin

    2015-01-01

    Van der Waals materials have received a great deal of attention for their exceptional layered structures and exotic properties, which can open up various device applications in nanoelectronics. However, in situ epitaxial growth of dissimilar van der Waals materials remains challenging. Here we demonstrate a solution for fabricating van der Waals heterostructures. Graphene/hexagonal boron nitride (h-BN) heterostructures were synthesized on cobalt substrates by using molecular beam epitaxy. Various characterizations were carried out to evaluate the heterostructures. Wafer-scale heterostructures consisting of single-layer/bilayer graphene and multilayer h-BN were achieved. The mismatch angle between graphene and h-BN is below 1°. PMID:26442629

  13. Decoupling of epitaxial graphene via gold intercalation probed by dispersive Raman spectroscopy

    SciTech Connect

    Pillai, P. B. E-mail: m.desouza@sheffield.ac.uk; DeSouza, M. E-mail: m.desouza@sheffield.ac.uk; Narula, R.; Reich, S.; Wong, L. Y.; Batten, T.; Pokorny, J.

    2015-05-14

    Signatures of a superlattice structure composed of a quasi periodic arrangement of atomic gold clusters below an epitaxied graphene (EG) layer are examined using dispersive Raman spectroscopy. The gold-graphene system exhibits a laser excitation energy dependant red shift of the 2D mode as compared to pristine epitaxial graphene. The phonon dispersions in both the systems are mapped using the experimentally observed Raman signatures and a third-nearest neighbour tight binding electronic band structure model. Our results reveal that the observed excitation dependent Raman red shift in gold EG primarily arise from the modifications of the phonon dispersion in gold-graphene and shows that the extent of decoupling of graphene from the underlying SiC substrate can be monitored from the dispersive nature of the Raman 2D modes. The intercalated gold atoms restore the phonon band structure of epitaxial graphene towards free standing graphene.

  14. Electronic property of Na-doped epitaxial graphenes on SiC

    NASA Astrophysics Data System (ADS)

    Choi, Seon-Myeong; Jhi, Seung-Hoon

    2009-04-01

    The electronic property of epitaxial graphenes with Na adsorption or intercalation is studied with the use of pseudopotential density functional method. It is found that the charge transfer and the Na binding energy show strong coverage dependence. Calculated energetics shows that Na prefers the intercalation between the buffer and top graphene layers to the adsorption on top graphene layer. The buffer layer is inert to Na adsorption on top graphene layer but it is charged when Na atoms are intercalated. This indicates that the conduction of epitaxial graphenes can be affected significantly by Na intercalation.

  15. Epitaxial graphene quantum dots for high-performance terahertz bolometers

    NASA Astrophysics Data System (ADS)

    El Fatimy, Abdel; Myers-Ward, Rachael L.; Boyd, Anthony K.; Daniels, Kevin M.; Gaskill, D. Kurt; Barbara, Paola

    2016-04-01

    Light absorption in graphene causes a large change in electron temperature due to the low electronic heat capacity and weak electron–phonon coupling. This property makes graphene a very attractive material for hot-electron bolometers in the terahertz frequency range. Unfortunately, the weak variation of electrical resistance with temperature results in limited responsivity for absorbed power. Here, we show that, due to quantum confinement, quantum dots of epitaxial graphene on SiC exhibit an extraordinarily high variation of resistance with temperature (higher than 430 MΩ K‑1 below 6 K), leading to responsivities of 1 × 1010 V W‑1, a figure that is five orders of magnitude higher than other types of graphene hot-electron bolometer. The high responsivity, combined with an extremely low electrical noise-equivalent power (∼2 × 10‑16 W Hz‑1/2 at 2.5 K), already places our bolometers well above commercial cooled bolometers. Additionally, we show that these quantum dot bolometers demonstrate good performance at temperature as high as 77 K.

  16. Epitaxial graphene quantum dots for high-performance terahertz bolometers.

    PubMed

    El Fatimy, Abdel; Myers-Ward, Rachael L; Boyd, Anthony K; Daniels, Kevin M; Gaskill, D Kurt; Barbara, Paola

    2016-04-01

    Light absorption in graphene causes a large change in electron temperature due to the low electronic heat capacity and weak electron-phonon coupling. This property makes graphene a very attractive material for hot-electron bolometers in the terahertz frequency range. Unfortunately, the weak variation of electrical resistance with temperature results in limited responsivity for absorbed power. Here, we show that, due to quantum confinement, quantum dots of epitaxial graphene on SiC exhibit an extraordinarily high variation of resistance with temperature (higher than 430 MΩ K(-1) below 6 K), leading to responsivities of 1 × 10(10) V W(-1), a figure that is five orders of magnitude higher than other types of graphene hot-electron bolometer. The high responsivity, combined with an extremely low electrical noise-equivalent power (∼2 × 10(-16) W Hz(-1/2) at 2.5 K), already places our bolometers well above commercial cooled bolometers. Additionally, we show that these quantum dot bolometers demonstrate good performance at temperature as high as 77 K. PMID:26727199

  17. Real-time observation of epitaxial graphene domain reorientation

    NASA Astrophysics Data System (ADS)

    Rogge, Paul C.; Thürmer, Konrad; Foster, Michael E.; McCarty, Kevin F.; Dubon, Oscar D.; Bartelt, Norman C.

    2015-04-01

    Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, `ripening' domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene-substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials.

  18. Understanding controls on interfacial wetting at epitaxial graphene: Experiment and Theory

    SciTech Connect

    Zhou, Hua; Ganesh, Panchapakesan; Presser, Volker; Wander, Matthew C; Fenter, Paul; Kent, Paul R; Jiang, Deen; Chialvo, Ariel A; Mcdonough, John; Shuford, Kevin L; Gogotsi, Yury G.

    2012-01-01

    The interaction of interfacial water with graphitic carbon at the atomic scale is studied as a function of the hydrophobicity of epitaxial graphene. High resolution x-ray reflectivity shows that the graphene-water contact angle is controlled by the average graphene thickness, due to the fraction of the film surface expressed as the epitaxial buffer layer whose contact angle (contact angle c = 73 ) is substantially smaller than that of multilayer graphene ( c = 93 ). Classical and ab initio molecular dynamics simulations show that the reduced contact angle of the buffer layer is due to both its epitaxy with the SiC substrate and the presence of interfacial defects. This insight clarifies the relationship between interfacial water structure and hydrophobicity, in general, and suggests new routes to control interface properties of epitaxial graphene.

  19. Understanding controls on interfacial wetting at epitaxial graphene: Experiment and Theory

    SciTech Connect

    Kent, Paul R

    2011-01-01

    The interaction of interfacial water with graphitic carbon at the atomic scale is studied as a function of the hydrophobicity of epitaxial graphene. High resolution x-ray reflectivity shows that the graphene-water contact angle is controlled by the average graphene thickness, due to the fraction of the film surface expressed as the epitaxial buffer layer whose contact angle (contact angle {Theta}{sub c} = 73{sup o}) is substantially smaller than that of multilayer graphene ({Theta}{sub c} = 93{sup o}). Classical and ab initio molecular dynamics simulations show that the reduced contact angle of the buffer layer is due to both its epitaxy with the SiC substrate and the presence of interfacial defects. This insight clarifies the relationship between interfacial water structure and hydrophobicity, in general, and suggests new routes to control interface properties of epitaxial graphene.

  20. Understanding controls on interfacial wetting at epitaxial graphene: Experiment and theory

    NASA Astrophysics Data System (ADS)

    Zhou, Hua; Ganesh, P.; Presser, Volker; Wander, Matthew C. F.; Fenter, Paul; Kent, Paul R. C.; Jiang, De-En; Chialvo, Ariel A.; McDonough, John; Shuford, Kevin L.; Gogotsi, Yury

    2012-01-01

    The interaction of interfacial water with graphitic carbon at the atomic scale is studied as a function of the hydrophobicity of epitaxial graphene. High resolution x-ray reflectivity shows that the graphene-water contact angle is controlled by the average graphene thickness, due to the fraction of the film surface expressed as the epitaxial buffer layer whose contact angle (contact angle θc = 73°) is substantially smaller than that of multilayer graphene (θc = 93°). Classical and ab initio molecular dynamics simulations show that the reduced contact angle of the buffer layer is due to both its epitaxy with the SiC substrate and the presence of interfacial defects. This insight clarifies the relationship between interfacial water structure and hydrophobicity, in general, and suggests new routes to control interface properties of epitaxial graphene.

  1. Density functional theory modeling of multilayer "epitaxial" graphene oxide.

    PubMed

    Zhou, Si; Bongiorno, Angelo

    2014-11-18

    CONSPECTUS: Graphene oxide (GO) is a complex material of both fundamental and applied interest. Elucidating the structure of GO is crucial to achieve control over its properties and technological applications. GO is a nonstoichiometric and hygroscopic material with a lamellar structure, and its physical chemical properties depend critically on synthesis procedures and postsynthesis treatments. Numerous efforts are in place to both understand and exploit this versatile layered carbon material. This Account reports on recent density functional theory (DFT) studies of "epitaxial" graphene oxide (hereafter EGO), a type of GO obtained by oxidation of graphene films grown epitaxially on silicon carbide. Here, we rely on selected X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and X-ray diffraction (XRD) measurements of EGO, and we discuss in great detail how we utilized DFT-based techniques to project out from the experimental data basic atomistic information about the chemistry and structure of these films. This Account provides an example as to how DFT modeling can be used to elucidate complex materials such as GO from a limited set of experimental information. EGO exhibits a uniform layered structure, consisting of a stack of graphene planes hosting predominantly epoxide and hydroxyl groups, and water molecules intercalated between the oxidized carbon layers. Here, we first focus on XPS measurements of EGO, and we use DFT to generate realistic model structures, calculate core-level chemical shifts, and through the comparison with experiment, gain insight on the chemical composition and metastability characteristics of EGO. DFT calculations are then used to devise a simplistic but accurate simulation scheme to study thermodynamic and kinetic stability and to predict the intralayer structure of EGO films aged at room temperature. Our simulations show that aged EGO encompasses layers with nanosized oxidized domains presenting a high concentration of

  2. Real-time observation of epitaxial graphene domain reorientation

    PubMed Central

    Rogge, Paul C.; Thürmer, Konrad; Foster, Michael E.; McCarty, Kevin F.; Dubon, Oscar D.; Bartelt, Norman C.

    2015-01-01

    Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, ‘ripening'; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene–substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials. PMID:25892219

  3. Real-time observation of epitaxial graphene domain reorientation

    SciTech Connect

    Thuermer, Konrad; Foster, Michael E.; Bartelt, Norman Charles; Rogge, Paul C.; McCarty, Kevin F.; Dubon, Oscar D.; Bartelt, Norman C.

    2015-04-20

    Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, ‘ripening’; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene–substrate interaction reproduces the results. This study suggests new strategies for improving the van der Waals epitaxy of 2D materials.

  4. Real-time observation of epitaxial graphene domain reorientation

    DOE PAGESBeta

    Thuermer, Konrad; Foster, Michael E.; Bartelt, Norman Charles; Rogge, Paul C.; Lawrence Berkeley National Lab.; McCarty, Kevin F.; Dubon, Oscar D.; Lawrence Berkeley National Lab.; Bartelt, Norman C.

    2015-04-20

    Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, ‘ripening’; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV permore » C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene–substrate interaction reproduces the results. This study suggests new strategies for improving the van der Waals epitaxy of 2D materials.« less

  5. Understanding controls on interfacial wetting at epitaxial graphene: Experiment and Theory

    SciTech Connect

    Zhou, Hua; Ganesh, Panchapakesan; Presser, Volker; Wander, Matthew C; Fenter, Paul; Kent, Paul R; Jiang, Deen; Chialvo, Ariel A; Mcdonough, John; Shuford, Kevin L; Gogotsi, Yury G.

    2012-01-01

    The interaction of water with graphitic carbon at the atomic scale is studied as a function of the hydrophobicity of epitaxial graphene. High resolution X-ray reflectivity combined with both classical and ab initio molecular dynamics simulations allows us to understand how the graphene-water interface changes as the hydrophobicity of intrinsic multilayer graphene (contact angle c = 93 ) is reduced by the presence of substrate and surface defect sites, leading to increased hydrophilicity ( c = 73 ) for zero-layer graphene (i.e., the epitaxial buffer layer). The relationship between water depletion and hydrophobicity (i.e., the hydrophobic gap) is also clarified by the current findings.

  6. Suppression of inhomogeneous segregation in graphene growth on epitaxial metal films.

    PubMed

    Yoshii, Shigeo; Nozawa, Katsuya; Toyoda, Kenji; Matsukawa, Nozomu; Odagawa, Akihiro; Tsujimura, Ayumu

    2011-07-13

    Large-scale uniform graphene growth was achieved by suppressing inhomogeneous carbon segregation using a single domain Ru film epitaxially grown on a sapphire substrate. An investigation of how the metal thickness affected growth and a comparative study on metals with different crystal structures have revealed that locally enhanced carbon segregation at stacking domain boundaries of metal is the origin of inhomogeneous graphene growth. Single domain Ru film has no stacking domain boundary, and the graphene growth on it is mainly caused not by segregation but by a surface catalytic reaction. Suppression of local segregation is essential for uniform graphene growth on epitaxial metal films. PMID:21648391

  7. Epitaxial graphene on silicon carbide: Low-vacuum growth, characterization, and device fabrication

    NASA Astrophysics Data System (ADS)

    Sprinkle, Michael W.

    In the past several years, epitaxial graphene on silicon carbide has been transformed from an academic curiosity of social scientists to a leading candidate material to replace silicon in post-CMOS electronics. This has come with rapid development of growth technologies, improved understanding of epitaxial graphene on the polar faces of silicon carbide, and new device fabrication techniques. The contributions of this thesis include refinement and improved understanding of graphene growth on the silicon- and carbon-faces in the context of managed local silicon partial pressure, high-throughput epitaxial graphene thickness measurement and uniformity characterization by ellipsometry, observations of nearly ideal graphene band structures on rotationally stacked carbon-face multilayer epitaxial graphene, presentation of initial experiments on localized in situ chemical modification of epitaxial graphene for an alternate path to semiconducting behavior, and novel device fabrication methods to exploit the crystal structure of the silicon carbide substrate. The latter is a particularly exciting foray into three dimensional patterning of the substrate that may eliminate the critical problem of edge roughness in graphene nanoribbons.

  8. Direct growth of graphene on in situ epitaxial hexagonal boron nitride flakes by plasma-assisted molecular beam epitaxy

    SciTech Connect

    Xu, Zhongguang; Zheng, Renjing; Khanaki, Alireza; Zuo, Zheng; Liu, Jianlin

    2015-11-23

    Hexagonal boron nitride (h-BN) single-crystal domains were grown on cobalt (Co) substrates at a substrate temperature of 850–900 °C using plasma-assisted molecular beam epitaxy. Three-point star shape h-BN domains were observed by scanning electron microscopy, and confirmed by Raman and X-ray photoelectron spectroscopy. The h-BN on Co template was used for in situ growth of multilayer graphene, leading to an h-BN/graphene heterostructure. Carbon atoms preferentially nucleate on Co substrate and edges of h-BN and then grow laterally to form continuous graphene. Further introduction of carbon atoms results in layer-by-layer growth of graphene on graphene and lateral growth of graphene on h-BN until it may cover entire h-BN flakes.

  9. Electronic cooling via interlayer Coulomb coupling in multilayer epitaxial graphene

    PubMed Central

    Mihnev, Momchil T.; Tolsma, John R.; Divin, Charles J.; Sun, Dong; Asgari, Reza; Polini, Marco; Berger, Claire; de Heer, Walt A.; MacDonald, Allan H.; Norris, Theodore B.

    2015-01-01

    In van der Waals bonded or rotationally disordered multilayer stacks of two-dimensional (2D) materials, the electronic states remain tightly confined within individual 2D layers. As a result, electron–phonon interactions occur primarily within layers and interlayer electrical conductivities are low. In addition, strong covalent in-plane intralayer bonding combined with weak van der Waals interlayer bonding results in weak phonon-mediated thermal coupling between the layers. We demonstrate here, however, that Coulomb interactions between electrons in different layers of multilayer epitaxial graphene provide an important mechanism for interlayer thermal transport, even though all electronic states are strongly confined within individual 2D layers. This effect is manifested in the relaxation dynamics of hot carriers in ultrafast time-resolved terahertz spectroscopy. We develop a theory of interlayer Coulomb coupling containing no free parameters that accounts for the experimentally observed trends in hot-carrier dynamics as temperature and the number of layers is varied. PMID:26399955

  10. Microfiber-based few-layer black phosphorus saturable absorber for ultra-fast fiber laser.

    PubMed

    Luo, Zhi-Chao; Liu, Meng; Guo, Zhi-Nan; Jiang, Xiao-Fang; Luo, Ai-Ping; Zhao, Chu-Jun; Yu, Xue-Feng; Xu, Wen-Cheng; Zhang, Han

    2015-07-27

    Few-layer black phosphorus (BP), as the most alluring graphene analogue owing to its similar structure as graphene and thickness dependent direct band-gap, has now triggered a new wave of research on two-dimensional (2D) materials based photonics and optoelectronics. However, a major obstacle of practical applications for few-layer BPs comes from their instabilities of laser-induced optical damage. Herein, we demonstrate that, few-layer BPs, which was fabricated through the liquid exfoliation approach, can be developed as a new and practical saturable absorber (SA) by depositing few-layer BPs with microfiber. The saturable absorption property of few-layer BPs had been verified through an open-aperture z-scan measurement at the telecommunication band. The microfiber-based BP device had been found to show a saturable average power of ~4.5 mW and a modulation depth of 10.9%, which is further confirmed through a balanced twin detection measurement. By integrating this optical SA device into an erbium-doped fiber laser, it was found that it can deliver the mode-locked pulse with duration down to 940 fs with central wavelength tunable from 1532 nm to 1570 nm. The prevention of BP from oxidation through the "lateral interaction scheme" owing to this microfiber-based few-layer BP SA device might partially mitigate the optical damage problem of BP. Our results not only demonstrate that black phosphorus might be another promising SA material for ultrafast photonics, but also provide a practical solution to solve the optical damage problem of black phosphorus by assembling with waveguide structures such as microfiber. PMID:26367661

  11. Interlayer electronic hybridization leads to exceptional thickness-dependent vibrational properties in few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Hu, Zhi-Xin; Kong, Xianghua; Qiao, Jingsi; Normand, Bruce; Ji, Wei

    2016-01-01

    Stacking two-dimensional (2D) materials into multi-layers or heterostructures, known as van der Waals (vdW) epitaxy, is an essential degree of freedom for tuning their properties on demand. Few-layer black phosphorus (FLBP), a material with high potential for nano- and optoelectronics applications, appears to have interlayer couplings much stronger than graphene and other 2D systems. Indeed, these couplings call into question whether the stacking of FLBP can be governed only by vdW interactions, which is of crucial importance for epitaxy and property refinement. Here, we perform a theoretical investigation of the vibrational properties of FLBP, which reflect directly its interlayer coupling, by discussing six Raman-observable phonons, including three optical, one breathing and two shear modes. With increasing sample thickness, we find anomalous redshifts of the frequencies for each optical mode but a blueshift for the armchair shear mode. Our calculations also show splitting of the phonon branches, due to anomalous surface phenomena, and strong phonon-phonon coupling. By computing uniaxial stress effects, inter-atomic force constants and electron densities, we provide a compelling demonstration that these properties are the consequence of strong and highly directional interlayer interactions arising from the electronic hybridization of the lone electron-pairs of FLBP, rather than from vdW interactions. This exceptional interlayer coupling mechanism controls the stacking stability of BP layers and thus opens a new avenue beyond vdW epitaxy for understanding the design of 2D heterostructures.Stacking two-dimensional (2D) materials into multi-layers or heterostructures, known as van der Waals (vdW) epitaxy, is an essential degree of freedom for tuning their properties on demand. Few-layer black phosphorus (FLBP), a material with high potential for nano- and optoelectronics applications, appears to have interlayer couplings much stronger than graphene and other 2D

  12. Evolution of interfacial intercalation chemistry on epitaxial graphene/SiC by surface enhanced Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Ferralis, Nicola; Carraro, Carlo

    2014-11-01

    A rapid and facile evaluation of the effects of physical and chemical processes on the interfacial layer between epitaxial graphene monolayers on SiC(0 0 0 1) surfaces is essential for applications in electronics, photonics, and optoelectronics. Here, the evolution of the atomic scale epitaxial graphene-buffer-layer-SiC interface through hydrogen intercalation, thermal annealings, UV-ozone etching and oxygen exposure is studied by means of single microparticle mediated surface enhanced Raman spectroscopy (smSERS). The evolution of the interfacial chemistry in the buffer layer is monitored through the Raman band at 2132 cm-1 corresponding to the Sisbnd H stretch mode. Graphene quality is monitored directly by the selectively enhanced Raman signal of graphene compared to the SiC substrate signal. Through smSERS, a simultaneous correlation between optimized hydrogen intercalation in epitaxial graphene/SiC and an increase in graphene quality is uncovered. Following UV-ozone treatment, a fully hydrogen passivated interface is retained, while a moderate degradation in the quality of the hydrogen intercalated quasi-freestanding graphene is observed. While hydrogen intercalated defect free quasi-freestanding graphene is expected to be robust upon UV-ozone, thermal annealing, and oxygen exposure, ozonolytic reactivity at the edges of H-intercalated defected graphene results in enhanced amorphization of the quasi-freestanding (compared to non-intercalated) graphene, leading ultimately to its complete etching.

  13. Acoustoelectric transport at gigahertz frequencies in coated epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Hernández-Mínguez, A.; Tahraoui, A.; Lopes, J. M. J.; Santos, P. V.

    2016-05-01

    Epitaxial graphene (EG) produced from SiC surfaces by silicon sublimation is emerging as a material for electronic applications due to its good electronic properties and availability over large areas on a semiconducting substrate. In this contribution, we report on the transport of charge carriers in EG on SiC using high-frequency (>1 GHz) surface acoustic waves (SAWs). In our devices, the EG is coated with hydrogen-silsesquioxane, SiO2, and a ZnO layer. This allows the efficient generation of SAWs and is compatible with the deposition of a metal top gate. Measurements of frequency- and time-resolved power scattering parameters confirm the generation and propagation of SAWs with frequencies of up to more than 7 GHz. Furthermore, the ZnO coating enhances the acoustoelectric currents by two orders of magnitude as compared to our previous uncoated samples. These results are an important step towards the dynamic acoustic control of charge carriers in graphene at gigahertz frequencies.

  14. Mini array of quantum Hall devices based on epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Novikov, S.; Lebedeva, N.; Hämäläinen, J.; Iisakka, I.; Immonen, P.; Manninen, A. J.; Satrapinski, A.

    2016-05-01

    Series connection of four quantum Hall effect (QHE) devices based on epitaxial graphene films was studied for realization of a quantum resistance standard with an up-scaled value. The tested devices showed quantum Hall plateaux RH,2 at a filling factor v = 2 starting from a relatively low magnetic field (between 4 T and 5 T) when the temperature was 1.5 K. The precision measurements of quantized Hall resistance of four QHE devices connected by triple series connections and external bonding wires were done at B = 7 T and T = 1.5 K using a commercial precision resistance bridge with 50 μA current through the QHE device. The results showed that the deviation of the quantized Hall resistance of the series connection of four graphene-based QHE devices from the expected value of 4×RH,2 = 2 h/e2 was smaller than the relative standard uncertainty of the measurement (<1 × 10-7) limited by the used resistance bridge.

  15. Polarized micro Raman scattering spectroscopy for curved edges of epitaxial graphene

    SciTech Connect

    Islam, Md. Sherajul Makino, T.; Hashimoto, A.; Bhuiyan, A. G.; Tanaka, S.

    2014-12-15

    This letter performed polarized microscopic laser Raman scattering spectroscopy on the curved edges of transferred epitaxial graphene on SiO{sub 2}/Si. The intensity ratio between the parallel and perpendicular polarized D band is evolved, providing a spectroscopy-based technique to probe the atomic-scale edge structures in graphene. A detailed analysis procedure for non-ideal disordered curved edges of graphene is developed combining the atomic-scale zigzag and armchair edge structures along with some point defects. These results could provide valuable information of the realistic edges of graphene at the atomic-scale that can strongly influence the performance of graphene-based nanodevices.

  16. Giant quantum Hall plateaus generated by charge transfer in epitaxial graphene.

    PubMed

    Alexander-Webber, J A; Huang, J; Maude, D K; Janssen, T J B M; Tzalenchuk, A; Antonov, V; Yager, T; Lara-Avila, S; Kubatkin, S; Yakimova, R; Nicholas, R J

    2016-01-01

    Epitaxial graphene has proven itself to be the best candidate for quantum electrical resistance standards due to its wide quantum Hall plateaus with exceptionally high breakdown currents. However one key underlying mechanism, a magnetic field dependent charge transfer process, is yet to be fully understood. Here we report measurements of the quantum Hall effect in epitaxial graphene showing the widest quantum Hall plateau observed to date extending over 50 T, attributed to an almost linear increase in carrier density with magnetic field. This behaviour is strong evidence for field dependent charge transfer from charge reservoirs with exceptionally high densities of states in close proximity to the graphene. Using a realistic framework of broadened Landau levels we model the densities of donor states and predict the field dependence of charge transfer in excellent agreement with experimental results, thus providing a guide towards engineering epitaxial graphene for applications such as quantum metrology. PMID:27456765

  17. Giant quantum Hall plateaus generated by charge transfer in epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Alexander-Webber, J. A.; Huang, J.; Maude, D. K.; Janssen, T. J. B. M.; Tzalenchuk, A.; Antonov, V.; Yager, T.; Lara-Avila, S.; Kubatkin, S.; Yakimova, R.; Nicholas, R. J.

    2016-07-01

    Epitaxial graphene has proven itself to be the best candidate for quantum electrical resistance standards due to its wide quantum Hall plateaus with exceptionally high breakdown currents. However one key underlying mechanism, a magnetic field dependent charge transfer process, is yet to be fully understood. Here we report measurements of the quantum Hall effect in epitaxial graphene showing the widest quantum Hall plateau observed to date extending over 50 T, attributed to an almost linear increase in carrier density with magnetic field. This behaviour is strong evidence for field dependent charge transfer from charge reservoirs with exceptionally high densities of states in close proximity to the graphene. Using a realistic framework of broadened Landau levels we model the densities of donor states and predict the field dependence of charge transfer in excellent agreement with experimental results, thus providing a guide towards engineering epitaxial graphene for applications such as quantum metrology.

  18. Atomically thin epitaxial template for organic crystal growth using graphene with controlled surface wettability.

    PubMed

    Nguyen, Nguyen Ngan; Jo, Sae Byeok; Lee, Seong Kyu; Sin, Dong Hun; Kang, Boseok; Kim, Hyun Ho; Lee, Hansol; Cho, Kilwon

    2015-04-01

    A two-dimensional epitaxial growth template for organic semiconductors was developed using a new method for transferring clean graphene sheets onto a substrate with controlled surface wettability. The introduction of a sacrificial graphene layer between a patterned polymeric supporting layer and a monolayer graphene sheet enabled the crack-free and residue-free transfer of free-standing monolayer graphene onto arbitrary substrates. The clean graphene template clearly induced the quasi-epitaxial growth of crystalline organic semiconductors with lying-down molecular orientation while maintaining the "wetting transparency", which allowed the transmission of the interaction between organic molecules and the underlying substrate. Consequently, the growth mode and corresponding morphology of the organic semiconductors on graphene templates exhibited distinctive dependence on the substrate hydrophobicity with clear transition from lateral to vertical growth mode on hydrophilic substrates, which originated from the high surface energy of the exposed crystallographic planes of the organic semiconductors on graphene. The optical properties of the pentacene layer, especially the diffusion of the exciton, also showed a strong dependency on the corresponding morphological evolution. Furthermore, the effect of pentacene-substrate interaction was systematically investigated by gradually increasing the number of graphene layers. These results suggested that the combination of a clean graphene surface and a suitable underlying substrate could serve as an atomically thin growth template to engineer the interaction between organic molecules and aromatic graphene network, thereby paving the way for effectively and conveniently tuning the semiconductor layer morphologies in devices prepared using graphene. PMID:25798655

  19. The decoupling of epitaxial graphene on SiC by hydrogen intercalation: an ab initio study

    NASA Astrophysics Data System (ADS)

    Nemec, Lydia; Rinke, Patrick; Blum, Volker; Scheffler, Matthias

    2015-03-01

    Large-scale ordered epitaxial graphene can be grown on various substrates, out of which silicon carbide (SiC) is one of the most promising. The exact material properties of graphene depend on the growth conditions and its interaction with the substrate. By hydrogen intercalation of epitaxial graphene on the Si-face of SiC the graphene layer decouples from the substrate forming quasi-free-standing monolayer graphene (QFMLG). We performed an density functional theory study of QFMLG on the polar 6H-SiC(0001) surface based on a van der Waals corrected semi-local exchange-correlation functional using the all-electron numeric atom-centered basis function code FHI-aims. We find an adsorption height in excellent agreement with X-ray standing wave experiments, a very low buckling of the graphene layer, and a very homogeneous electron density at the interface. All these features improve the electronic properties of QFMLG compared to epitaxial graphene. Using the insight gleaned on the Si-face, we present the structure of a hypothetical QFMLG phase on the C-face of SiC. We find that hydrogen intercalation is a promising option to control the SiC-graphene interface.

  20. Characterization of selective epitaxial graphene growth on silicon carbide: Limitations and opportunities

    NASA Astrophysics Data System (ADS)

    Zaman, Farhana

    The need for post-CMOS nanoelectronics has led to the investigation of innovative device structures and materials. Graphene, a zero bandgap semiconductor with ballistic transport properties, has great potential to extend diversification and miniaturization beyond the limits of CMOS. The goal of this work is to study the growth of graphene on SiC using the novel method of selective graphitization. The major contributions of this research are as follows — First, epitaxial graphene is successfully grown on selected regions of SiC not capped by AlN deposited by molecular beam epitaxy. This contribution enables the formation of electronic-grade graphene in desired patterns without having to etch the graphene or expose it to any detrimental contact with external chemicals. Etching of AlN opens up windows to the SiC in desirable patterns for subsequent graphitization without leaving etch-residues (determined by XPS). Second, the impact of process parameters on the growth of graphene is investigated. Temperature, time, and argon pressure are the primary growth-conditions altered. A temperature of 1400°C in 1 mbar argon for 20 min produced the most optimal graphene growth without significant damage to the AlN capping-layer. Third, first-ever electronic transport measurements are achieved on the selective epitaxial graphene. Hall mobility of about 1550 cm2/Vs has been obtained to date. Finally, the critical limitations of the selective epitaxial graphene growth are enumerated. The advent of enhanced processing techniques that will overcome these limitations will create a multitude of opportunities for applications for graphene grown in this manner. It is envisaged to be a viable approach to fabrication of radio-frequency field-effect transistors.

  1. Large area molybdenum disulphide- epitaxial graphene vertical Van der Waals heterostructures

    PubMed Central

    Pierucci, Debora; Henck, Hugo; Naylor, Carl H.; Sediri, Haikel; Lhuillier, Emmanuel; Balan, Adrian; Rault, Julien E.; Dappe, Yannick J.; Bertran, François; Fèvre, Patrick Le; Johnson, A. T. Charlie; Ouerghi, Abdelkarim

    2016-01-01

    Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design. PMID:27246929

  2. Large area molybdenum disulphide- epitaxial graphene vertical Van der Waals heterostructures.

    PubMed

    Pierucci, Debora; Henck, Hugo; Naylor, Carl H; Sediri, Haikel; Lhuillier, Emmanuel; Balan, Adrian; Rault, Julien E; Dappe, Yannick J; Bertran, François; Fèvre, Patrick Le; Johnson, A T Charlie; Ouerghi, Abdelkarim

    2016-01-01

    Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design. PMID:27246929

  3. Large area molybdenum disulphide- epitaxial graphene vertical Van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Pierucci, Debora; Henck, Hugo; Naylor, Carl H.; Sediri, Haikel; Lhuillier, Emmanuel; Balan, Adrian; Rault, Julien E.; Dappe, Yannick J.; Bertran, François; Fèvre, Patrick Le; Johnson, A. T. Charlie; Ouerghi, Abdelkarim

    2016-06-01

    Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design.

  4. Energy Bandgap and Edge States in an Epitaxially Grown Graphene/h-BN Heterostructure.

    PubMed

    Hwang, Beomyong; Hwang, Jeongwoon; Yoon, Jong Keon; Lim, Sungjun; Kim, Sungmin; Lee, Minjun; Kwon, Jeong Hoon; Baek, Hongwoo; Sung, Dongchul; Kim, Gunn; Hong, Suklyun; Ihm, Jisoon; Stroscio, Joseph A; Kuk, Young

    2016-01-01

    Securing a semiconducting bandgap is essential for applying graphene layers in switching devices. Theoretical studies have suggested a created bulk bandgap in a graphene layer by introducing an asymmetry between the A and B sub-lattice sites. A recent transport measurement demonstrated the presence of a bandgap in a graphene layer where the asymmetry was introduced by placing a graphene layer on a hexagonal boron nitride (h-BN) substrate. Similar bandgap has been observed in graphene layers on metal substrates by local probe measurements; however, this phenomenon has not been observed in graphene layers on a near-insulating substrate. Here, we present bulk bandgap-like features in a graphene layer epitaxially grown on an h-BN substrate using scanning tunneling spectroscopy. We observed edge states at zigzag edges, edge resonances at armchair edges, and bandgap-like features in the bulk. PMID:27503427

  5. Energy Bandgap and Edge States in an Epitaxially Grown Graphene/h-BN Heterostructure

    PubMed Central

    Hwang, Beomyong; Hwang, Jeongwoon; Yoon, Jong Keon; Lim, Sungjun; Kim, Sungmin; Lee, Minjun; Kwon, Jeong Hoon; Baek, Hongwoo; Sung, Dongchul; Kim, Gunn; Hong, Suklyun; Ihm, Jisoon; Stroscio, Joseph A.; Kuk, Young

    2016-01-01

    Securing a semiconducting bandgap is essential for applying graphene layers in switching devices. Theoretical studies have suggested a created bulk bandgap in a graphene layer by introducing an asymmetry between the A and B sub-lattice sites. A recent transport measurement demonstrated the presence of a bandgap in a graphene layer where the asymmetry was introduced by placing a graphene layer on a hexagonal boron nitride (h-BN) substrate. Similar bandgap has been observed in graphene layers on metal substrates by local probe measurements; however, this phenomenon has not been observed in graphene layers on a near-insulating substrate. Here, we present bulk bandgap-like features in a graphene layer epitaxially grown on an h-BN substrate using scanning tunneling spectroscopy. We observed edge states at zigzag edges, edge resonances at armchair edges, and bandgap-like features in the bulk. PMID:27503427

  6. Energy Bandgap and Edge States in an Epitaxially Grown Graphene/h-BN Heterostructure

    NASA Astrophysics Data System (ADS)

    Hwang, Beomyong; Hwang, Jeongwoon; Yoon, Jong Keon; Lim, Sungjun; Kim, Sungmin; Lee, Minjun; Kwon, Jeong Hoon; Baek, Hongwoo; Sung, Dongchul; Kim, Gunn; Hong, Suklyun; Ihm, Jisoon; Stroscio, Joseph A.; Kuk, Young

    2016-08-01

    Securing a semiconducting bandgap is essential for applying graphene layers in switching devices. Theoretical studies have suggested a created bulk bandgap in a graphene layer by introducing an asymmetry between the A and B sub-lattice sites. A recent transport measurement demonstrated the presence of a bandgap in a graphene layer where the asymmetry was introduced by placing a graphene layer on a hexagonal boron nitride (h-BN) substrate. Similar bandgap has been observed in graphene layers on metal substrates by local probe measurements; however, this phenomenon has not been observed in graphene layers on a near-insulating substrate. Here, we present bulk bandgap-like features in a graphene layer epitaxially grown on an h-BN substrate using scanning tunneling spectroscopy. We observed edge states at zigzag edges, edge resonances at armchair edges, and bandgap-like features in the bulk.

  7. Electronic structure of epitaxial graphene layers on SiC: effects of the substrate

    SciTech Connect

    Varchon, F.; Feng, R.; Hass, J.; Li, X.; Nguyen, B. Ngoc; Naud, C.; Mallet, P.; Veuillen, J.-Y.; Berger, C.; Conrad, E.H.; Magaud, L.

    2008-10-17

    A strong substrate-graphite bond is found in the first all-carbon layer by density functional theory calculations and x-ray diffraction for few graphene layers grown epitaxially on SiC. This first layer is devoid of graphene electronic properties and acts as a buffer layer. The graphene nature of the film is recovered by the second carbon layer grown on both the (0001) and (0001{sup -}) 4H-SiC surfaces. We also present evidence of a charge transfer that depends on the interface geometry. Hence the graphene is doped and a gap opens at the Dirac point after three Bernal stacked carbon layers are formed.

  8. Weak localization in few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Du, Yuchen; Neal, Adam T.; Zhou, Hong; Ye, Peide D.

    2016-06-01

    We have conducted a comprehensive investigation into the magneto-transport properties of few-layer black phosphorus in terms of phase coherence length, phase coherence time, and mobility via weak localization measurement and Hall-effect measurement. We present magnetoresistance data showing the weak localization effect in bare p-type few-layer black phosphorus and reveal its strong dependence on temperature and carrier concentration. The measured weak localization agrees well with the Hikami-Larkin-Nagaoka model and the extracted phase coherence length of 104 nm at 350 mK, decreasing as ˜T-0.513+-0.053 with increased temperature. Weak localization measurement allows us to qualitatively probe the temperature-dependent phase coherence time τ ϕ , which is in agreement with the theory of carrier interaction in the diffusive regime. We also observe the universal conductance fluctuation phenomenon in few-layer black phosphorus within moderate magnetic field and low temperature regime.

  9. Room-temperature molecular-resolution characterization of self-assembled organic monolayers on epitaxial graphene.

    PubMed

    Wang, Qing Hua; Hersam, Mark C

    2009-06-01

    Graphene, a two-dimensional sheet of carbon atoms, is a promising material for next-generation technology because of its advantageous electronic properties, such as extremely high carrier mobilities. However, chemical functionalization schemes are needed to integrate graphene with the diverse range of materials required for device applications. In this paper, we report self-assembled monolayers of the molecular semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) formed on epitaxial graphene grown on the SiC(0001) surface. The molecules possess long-range order with a herringbone arrangement, as shown by ultra-high vacuum scanning tunnelling microscopy at room temperature. The molecular ordering is unperturbed by defects in the epitaxial graphene or atomic steps in the underlying SiC surface. Scanning tunnelling spectra of the PTCDA monolayer show distinct features that are not observed on pristine graphene. The demonstration of robust, uniform organic functionalization of epitaxial graphene presents opportunities for graphene-based molecular electronics and sensors. PMID:21378849

  10. The correlation of epitaxial graphene properties and morphology of SiC (0001)

    SciTech Connect

    Guo, Y.; Guo, L. W. E-mail: xlchen@iphy.ac.cn; Huang, J.; Jia, Y. P.; Lin, J. J.; Lu, W.; Li, Z. L.; Yang, R.; Chen, X. L. E-mail: xlchen@iphy.ac.cn

    2014-01-28

    The electronic properties of epitaxial graphene (EG) on SiC (0001) depend sensitively on the surface morphology of SiC substrate. Here, 2–3 layers of graphene were grown on on-axis 6H-SiC with different step densities realized through controlling growth temperature and ambient pressure. We show that epitaxial graphene on SiC (0001) with low step density and straight step edge possesses fewer point defects laying mostly on step edges and higher carrier mobility. A relationship between step density and EG mobility is established. The linear scan of Raman spectra combined with the atomic force microscopy morphology images revealed that the Raman fingerprint peaks are nearly the same on terraces, but shift significantly while cross step edges, suggesting the graphene is not homogeneous in strain and carrier concentration over terraces and step edges of substrates. Thus, control morphology of epitaxial graphene on SiC (0001) is a simple and effective method to pursue optimal route for high quality graphene and will be helpful to prepare wafer sized graphene for device applications.

  11. Epitaxial graphene on SiC: from carrier density engineering to quasi-free standing graphene by atomic intercalation

    NASA Astrophysics Data System (ADS)

    Forti, S.; Starke, U.

    2014-03-01

    Epitaxial graphene (EG) on SiC has been proven to be an excellent material to investigate the fundamental physical properties of graphene and also to directly implement new findings into devices realized on the versatile platform of SiC. Within this framework, this work aims to review some of the recent major achievements accomplished in the field of EG on SiC, related to the growth of EG on the SiC(0 0 0 1) surface, the control of its doping level, the decoupling of the graphene from the substrate and the intercalation of foreign atomic species at the interface.

  12. Epitaxial Ultrathin Organic Crystals on Graphene for High-Efficiency Phototransistors.

    PubMed

    Liu, Xiaolong; Luo, Xiaoguang; Nan, Haiyan; Guo, Hui; Wang, Peng; Zhang, Linglong; Zhou, Minmin; Yang, Ziyi; Shi, Yi; Hu, Weida; Ni, Zhenhua; Qiu, Teng; Yu, Zongfu; Xu, Jian-Bin; Wang, Xinran

    2016-07-01

    Epitaxially grown ultrathin organic semiconductors on graphene show great promise as highly efficient phototransistors. The devices exhibit a strong photoresponse down to the limit of a monolayer organic crystal, with a photoresponsivity higher than 10(4) A W(-1) and a photoconductive gain over 10(8) . The excellent performance is attributed to the high quality of the organic crystal and interface, a unique feature of van der Waals epitaxy. PMID:27146896

  13. Resonant optical third-harmonic generation in few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    de Matos, Christiano J. S.; Rodrigues, Manuel J. L. F.; de Oliveira, Rafael E. P.; Peixoto, Hélder X. P.; Wu, Hsin-Yu; Wei, Ho Y.; Castro Neto, Antonio H.; Viana-Gomes, José C.

    Black phosphorus (BP), a layered monoatomic anisotropic crystal, has recently re-emerged due to demonstrations of its exfoliation down to few-layer thicknesses. It has been shown that BP remains a direct bandgap semiconductor from the bulk to the monolayer, which has triggered interest in its optoelectronic applications. However, optical characterization has been largely restricted to the linear regime, with nonlinear characterization limited to z-scan and saturable absorption measurements. In this work, we show optical third-harmonic generation measurements in bulk and few-layer BP. Results indicate a resonant increase in the generation efficiency of the latter, with signal intensities reaching values three orders of magnitude higher then those of graphene. The mechanisms leading to the resonant increase will be discussed. This work is supported by Fapesp (2012/50259-8 and 2015/11779-4), MackPesquisa, NRF-CRP (R-144-000-295-281), and NRF - Medium Sized Centre Programme.

  14. Epitaxial Graphene on SiC(0001): More than Just Honeycombs

    NASA Astrophysics Data System (ADS)

    Qi, Y.; Rhim, S. H.; Sun, G. F.; Weinert, M.; Li, L.

    2010-08-01

    Using scanning tunneling microscopy with Fe-coated W tips and first-principles calculations, we show that the interface of epitaxial graphene/SiC(0001) is a warped graphene layer with hexagon-pentagon-heptagon (H5,6,7) defects that break the honeycomb symmetry, thereby inducing a gap and states below EF near the K point. Although the next graphene layer assumes the perfect honeycomb lattice, its interaction with the warped layer modifies the dispersion about the Dirac point. These results explain recent angle-resolved photoemission and carbon core-level shift data and solve the long-standing problem of the interfacial structure of epitaxial graphene on SiC(0001).

  15. Epitaxial graphene on SiC(0001): More than just honeycombs

    NASA Astrophysics Data System (ADS)

    Li, L.; Qi, Y.; Rhim, R. H.; Sun, G. F.; Weinert, M.

    2011-03-01

    Combing scanning tunneling microscopy using transition-metal (Fe, Cr)-coated W tips and first-principles calculations, we show that the interface of epitaxial graphene/SiC(0001) is a warped graphene layer with periodic inclusions of hexagon-pentagon-heptagon (H5 , 6 , 7) defects. These defects break the six-fold honeycomb symmetry, thereby inducing a gap and two states below EF near the Dirac point. Furthermore, we show that the next graphene layer assumes the perfect honeycomb lattice, but its interaction with the warped interfacial layer modifies the linear dispersion about the Dirac point, leading to parabolic dispersion and an apparent gap of ~ 0.25 eV. These results explain recent angle-resolved photoemission and carbon core-level shift data, and resolve the long-standing issue of the interfacial structure of epitaxial graphene on SiC(0001).

  16. High mobility Single Layer Epitaxial Graphene on 4H-SiC (000-1)

    NASA Astrophysics Data System (ADS)

    Hu, Yike; Guo, Zelei; Ruan, Ming; Hankinson, John; Palmer, James; Zhang, Baiqian; Dong, Rui; Kunc, Jan; Berger, Claire; Deheer, Walt

    2012-02-01

    Multi-layer Epitaxial Graphene on 4H-SiC (000-1) has demonstrated very high mobility up to˜27,000 cm^2/Vs [1]. Recently single layer graphene grown by the Confinement Control Growth method [2] exhibits mobility up to ˜ 25,000cm^2/V.s at 4K and 13,000 cm^2/V.s at 300K with p=3 x 10^12 cm-2 The relation between Raman G peak features (FWHM and position) and carrier density of Epitaxial Graphene on carbon face is revealed. Quantum Hall Effect [3] is observed both for p and n type carriers on top gated sample. This indicates that top gated single layer graphene can be produced on the Carbon face with high quality and high carrier mobility. [4pt] [1] Science 312, 1191 (2006) [0pt] [2] PNAS 108 (41) 16900 (2011) [0pt] [3] APL 95, 223108 (2009)

  17. Nanofabrication of heteromolecular organic nanostructures on epitaxial graphene via room temperature feedback-controlled lithography.

    PubMed

    Wang, Qing Hua; Hersam, Mark C

    2011-02-01

    Nanoscale control of surface chemistry holds promise for tailoring the electronic, optical, and chemical properties of graphene. Toward this end, the nanofabrication of sub-5-nm heteromolecular organic nanostructures is demonstrated on epitaxial graphene using room temperature ultrahigh vacuum scanning tunneling microscopy. In particular, monolayers of the organic semiconductor 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) are nanopatterned on epitaxial graphene using feedback-controlled lithography (FCL) and then used as chemical resists to template the deposition of N,N'-dioctyl-3,4,9,10-perylene-tetracarboxylic diimide (PTCDI-C8). The generality of this FCL-based nanofabrication procedure suggests its applicability to a wide range of fundamental studies and prototype device fabrication on chemically functionalized graphene. PMID:21166423

  18. Tuning electronic transport in epitaxial graphene-based van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Lin, Yu-Chuan; Li, Jun; de La Barrera, Sergio C.; Eichfeld, Sarah M.; Nie, Yifan; Addou, Rafik; Mende, Patrick C.; Wallace, Robert M.; Cho, Kyeongjae; Feenstra, Randall M.; Robinson, Joshua A.

    2016-04-01

    Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe2-graphene interface. Electrical transport measurements reveal a lower resistance between WSe2 and fully hydrogenated epitaxial graphene (EGFH) compared to WSe2 grown on partially hydrogenated epitaxial graphene (EGPH). Using low-energy electron microscopy and reflectivity on these samples, we extract the work function difference between the WSe2 and graphene and employ a charge transfer model to determine the WSe2 carrier density in both cases. The results indicate that WSe2-EGFH displays ohmic behavior at small biases due to a large hole density in the WSe2, whereas WSe2-EGPH forms a Schottky barrier junction.Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe2-graphene interface. Electrical transport measurements reveal a lower resistance between WSe2 and fully hydrogenated epitaxial graphene (EGFH) compared to WSe2 grown on partially hydrogenated epitaxial graphene (EGPH). Using low

  19. Tuning carrier density across Dirac point in epitaxial graphene on SiC by corona discharge

    NASA Astrophysics Data System (ADS)

    Lartsev, Arseniy; Yager, Tom; Bergsten, Tobias; Tzalenchuk, Alexander; Janssen, T. J. B. M.; Yakimova, Rositza; Lara-Avila, Samuel; Kubatkin, Sergey

    2014-08-01

    We demonstrate reversible carrier density control across the Dirac point (Δn ˜ 1013 cm-2) in epitaxial graphene on SiC (SiC/G) via high electrostatic potential gating with ions produced by corona discharge. The method is attractive for applications where graphene with a fixed carrier density is needed, such as quantum metrology, and more generally as a simple method of gating 2DEGs formed at semiconductor interfaces and in topological insulators.

  20. Growth of atomically smooth MgO films on graphene by molecular beam epitaxy

    SciTech Connect

    Wang, W. H.; Han, W.; Pi, K.; McCreary, K. M.; Miao, F.; Bao, W.; Lau, C. N.; Kawakami, R. K.

    2008-11-03

    We investigate the growth of MgO films on graphene by molecular beam epitaxy and find that surface diffusion promotes a rough morphology. To reduce the mobility of surface atoms, the graphene surface is dressed by Ti atoms prior to MgO deposition. With as little as 0.5 ML (monolayer) of Ti, the MgO overlayer becomes atomically smooth. Furthermore, no aggregation of MgO is observed at the edges of the graphene sheet. These results are important for the fabrication of nanoscale electronic and spintronic devices.

  1. van der Waals Epitaxy of MoS2 Layers Using Graphene As Growth Templates

    SciTech Connect

    Shi, Yumeng; Zhou, Wu; Lu, Ang-Yu; Fang, Wenjing; Lee, Yi-Hsien; Hsu, Allen Long; Kim, Soo Min; Kim, Ki Kang; Yang, Hui Ying; Liang, Lain-Jong; Idrobo Tapia, Juan C; Kong, Jing

    2012-01-01

    We present a method for synthesizing MoS{sub 2}/Graphene hybrid heterostructures with a growth template of graphene-covered Cu foil. Compared to other recent reports, a much lower growth temperature of 400 C is required for this procedure. The chemical vapor deposition of MoS{sub 2} on the graphene surface gives rise to single crystalline hexagonal flakes with a typical lateral size ranging from several hundred nanometers to several micrometers. The precursor (ammonium thiomolybdate) together with solvent was transported to graphene surface by a carrier gas at room temperature, which was then followed by post annealing. At an elevated temperature, the precursor self-assembles to form MoS{sub 2} flakes epitaxially on the graphene surface via thermal decomposition. With higher amount of precursor delivered onto the graphene surface, a continuous MoS{sub 2} film on graphene can be obtained. This simple chemical vapor deposition method provides a unique approach for the synthesis of graphene heterostructures and surface functionalization of graphene. The synthesized two-dimensional MoS{sub 2}/Graphene hybrids possess great potential toward the development of new optical and electronic devices as well as a wide variety of newly synthesizable compounds for catalysts.

  2. Transport measurements on monolayer and few-layer WSe2

    NASA Astrophysics Data System (ADS)

    Palomaki, Tauno; Zhao, Wenjin; Finney, Joe; Fei, Zaiyao; Nguyen, Paul; McKay, Frank; Cobden, David

    The behavior of the electrical contacts often dominates transport measurements in mono and few-layer transition metal dichalcogenide (TMD) devices. Creating good contacts for some TMDs is particularly challenging since the fabrication procedure should prevent the TMD from oxidizing or chemically interacting with the contacts. In this talk, we discuss our progress on creating mono and few-layer WSe2 devices with both good electrical contacts and minimal effects from the substrate, polymer contamination, oxidation and other chemistry. For example, we have developed a technique for encapsulating metallic contacts and WSe2 flakes together in hexagonal boron nitride with multiple gates to separate and control the contributions from the channel and the Schottky barriers at the contacts. Research supported in part by Samsung GRO grant US 040814

  3. Electron-Transport Properties of Few-Layer Black Phosphorus.

    PubMed

    Xu, Yuehua; Dai, Jun; Zeng, Xiao Cheng

    2015-06-01

    We perform the first-principles computational study of the effect of number of stacking layers and stacking style of the few-layer black phosphorus (BPs) on the electronic properties, including transport gap, current-voltage (i-v) relation, and differential conductance. Our computation is based on the nonequilibrium Green's function approach combined with density functional theory calculations. Specifically, we compute electron-transport properties of monolayer BP, bilayer BP, and trilayer BP as well as bilayer BPs with AB-, AA-, or AC-stacking. We find that the stacking number has greater influence on the transport gap than the stacking type. Conversely, the stacking type has greater influence on i-v curve and differential conductance than on the transport gap. This study offers useful guidance for determining the number of stacking layers and the stacking style of few-layer BP sheets in future experimental measurements and for potential applications in nanoelectronic devices. PMID:26266491

  4. Tuning electronic transport in epitaxial graphene-based van der Waals heterostructures.

    PubMed

    Lin, Yu-Chuan; Li, Jun; de la Barrera, Sergio C; Eichfeld, Sarah M; Nie, Yifan; Addou, Rafik; Mende, Patrick C; Wallace, Robert M; Cho, Kyeongjae; Feenstra, Randall M; Robinson, Joshua A

    2016-04-21

    Two-dimensional tungsten diselenide (WSe2) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe2-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe2-graphene interface. Electrical transport measurements reveal a lower resistance between WSe2 and fully hydrogenated epitaxial graphene (EGFH) compared to WSe2 grown on partially hydrogenated epitaxial graphene (EGPH). Using low-energy electron microscopy and reflectivity on these samples, we extract the work function difference between the WSe2 and graphene and employ a charge transfer model to determine the WSe2 carrier density in both cases. The results indicate that WSe2-EGFH displays ohmic behavior at small biases due to a large hole density in the WSe2, whereas WSe2-EGPH forms a Schottky barrier junction. PMID:27073972

  5. High resolution study of structural and electronic properties of epitaxial graphene grown on off-axis 4H-SiC (0001)

    NASA Astrophysics Data System (ADS)

    Giannazzo, F.; Deretzis, I.; Nicotra, G.; Fisichella, G.; Ramasse, Q. M.; Spinella, C.; Roccaforte, F.; La Magna, A.

    2014-05-01

    In this paper, the structural and electronic properties of epitaxial graphene (EG) grown on 8°-off 4H-SiC (0001) by high temperature thermal processes have been extensively investigated by a combination of several high resolution characterization techniques. The increase in the number of graphene layers with the growth temperature (from 1600 to 1700 °C) was studied by microRaman spectroscopy and high resolution transmission electron microscopy (HRTEM) on cross-sectioned samples. The few layers of graphene reside on a stepped SiC surface with alternating (0001) terraces and (11-2n) facets. Peculiar corrugations (wrinkles) in the graphene membrane preferentially oriented perpendicularly to the substrate steps were also observed. Motivated by recent atomic resolution studies of the EG/SiC interface revealing a local delamination of the interfacial C buffer from the (11-2n) facets, we searched for a correlation of these interfacial structural properties with the macroscopic electronic transport in EG field effect transistors (FETs). In particular, electrical characterization of EG top gated FETs fabricated with the channel length parallel or perpendicular to the substrate steps revealed a peculiar anisotropy of the channel conductance with respect to the steps' orientation. This effect was explained in terms of a local enhancement of EG resistance on the (11-2n) facets with respect to the (0001) basal plane, which is consistent with a reduced doping due to the local buffer layer delamination from those facets. Furthermore, scanning probe microscopy-based local electron mean free path measurements on EG showed a ~3× enhancement of mean free path on the buffer-layer-free (11-2n) facets with respect to (0001) terraces, probably associated to a strong reduction of Coulomb scattering effects on graphene's electrons.

  6. Emergent Functionality and Controllability in Few-Layer Metasurfaces.

    PubMed

    Cheng, Hua; Liu, Zhaocheng; Chen, Shuqi; Tian, Jianguo

    2015-09-23

    Recent progress in metamaterial research has successfully exceeded the limitations imposed by conventional materials and optical devices, enabling the manipulation of electromagnetic waves as desired. The distinct characteristics and controlling abilities of metamaterials make them ideal candidates for novel photonics devices not only in traditional optics but also for biological detection, medical science, and metrology. However, the controllability and functionality of both single-layer metasurfaces and bulk metamaterials are not sufficient to meet the requirements of emerging technologies; hence, new solutions must be found. As such technologies advance, new functionalities will emerge as different or identical single-layer metasurfaces are combined. Thus, innovation in few-layer metasurfaces will become an increasingly important line of research. Here, these metasurfaces are classified according to their functionalities and the few-layer metasurfaces that have been proposed up to now are presented in a clear sequence. It is expected that, with further development in this area, few-layer metasurfaces will play an important role in the family of optical materials. PMID:26265505

  7. Epitaxial graphene on SiC formed by the surface structure control technique

    NASA Astrophysics Data System (ADS)

    Aritsuki, Takuya; Nakashima, Takeshi; Kobayashi, Keisuke; Ohno, Yasuhide; Nagase, Masao

    2016-06-01

    The thermal decomposition of silicon carbide (SiC) is a promising method for producing wafer-scale single-crystal graphene. The optimal growth condition for high-mobility epitaxial graphene fabricated by infrared rapid thermal annealing is discussed in this paper. The surface structures, such as step–terrace and graphene coverage structures, on a non-off-axis SiC(0001) substrate were well controlled by varying the annealing time in a range below 10 min. The mobility of graphene grown at 1620 °C for 5 min in 100 Torr Ar ambient had a maximum value of 2089 cm2 V‑1 s‑1. We found that the causes of the mobility reduction were low graphene coverage, high sheet carrier density, and nonuniformity of the step structure.

  8. Flower-Shaped Domains and Wrinkles in Trilayer Epitaxial Graphene on Silicon Carbide

    PubMed Central

    Lalmi, B.; Girard, J. C.; Pallecchi, E.; Silly, M.; David, C.; Latil, S.; Sirotti, F.; Ouerghi, A.

    2014-01-01

    Trilayer graphene is of particular interest to the 2D materials community because of its unique tunable electronic structure. However, to date, there is a lack of fundamental understanding of the properties of epitaxial trilayer graphene on silicon carbide. Here, following successful synthesis of large-area uniform trilayer graphene, atomic force microscopy (AFM) showed that the trilayer graphene on 6H-SiC(0001) was uniform over a large scale. Additionally, distinct defects, identified as flower-shaped domains and isolated wrinkle structures, were observed randomly on the surface using scanning tunneling microscopy and spectroscopy (STM/STS). These carbon nanostructures formed during growth, has different structural and electronic properties when compared with the adjacent flat regions of the graphene. Finally, using low temperature STM/STS at 4K, we found that the isolated wrinkles showed an irreversible rotational motion between two 60° configurations at different densities of states. PMID:24513669

  9. Hofstadter Butterfly and Many-Body Effects in Epitaxial Graphene Superlattice.

    PubMed

    Yang, Wei; Lu, Xiaobo; Chen, Guorui; Wu, Shuang; Xie, Guibai; Cheng, Meng; Wang, Duoming; Yang, Rong; Shi, Dongxia; Watanabe, Kenji; Taniguchi, Takashi; Voisin, Christophe; Plaçais, Bernard; Zhang, Yuanbo; Zhang, Guangyu

    2016-04-13

    Graphene placed on hexagonal boron nitride (h-BN) has received a wide range of interest due to the improved electrical performance and rich physics from the interface, especially the emergence of superlattice Dirac points as well as Hofstadter butterfly in high magnetic field. Instead of transferring graphene onto h-BN, epitaxial growth of graphene directly on a single-crystal h-BN provides an alternative and promising way to study these interesting superlattice effects due to their precise lattice alignment. Here we report an electrical transport study on epitaxial graphene superlattice on h-BN with a period of ∼15.6 nm. The epitaxial graphene superlattice is clean, intrinsic, and of high quality with a carrier mobility of ∼27 000 cm(2) V(-1) s(-1), which enables the observation of Hofstadter butterfly features originated from the superlattice at a magnetic field as low as 6.4 T. A metal-insulator transition and magnetic field dependent Fermi velocity were also observed, suggesting prominent electron-electron interaction-induced many-body effects. PMID:26950258

  10. Nanoselective area growth of GaN by metalorganic vapor phase epitaxy on 4H-SiC using epitaxial graphene as a mask

    NASA Astrophysics Data System (ADS)

    Puybaret, Renaud; Patriarche, Gilles; Jordan, Matthew B.; Sundaram, Suresh; El Gmili, Youssef; Salvestrini, Jean-Paul; Voss, Paul L.; de Heer, Walt A.; Berger, Claire; Ougazzaden, Abdallah

    2016-03-01

    We report the growth of high-quality triangular GaN nanomesas, 30-nm thick, on the C-face of 4H-SiC using nanoselective area growth (NSAG) with patterned epitaxial graphene grown on SiC as an embedded mask. NSAG alleviates the problems of defects in heteroepitaxy, and the high mobility graphene film could readily provide the back low-dissipative electrode in GaN-based optoelectronic devices. A 5-8 graphene-layer film is first grown on the C-face of 4H-SiC by confinement-controlled sublimation of silicon carbide. Graphene is then patterned and arrays of 75-nm-wide openings are etched in graphene revealing the SiC substrate. A 30-nm-thick GaN is subsequently grown by metal organic vapor phase epitaxy. GaN nanomesas grow epitaxially with perfect selectivity on SiC, in the openings patterned through graphene. The up-or-down orientation of the mesas on SiC, their triangular faceting, and cross-sectional scanning transmission electron microscopy show that they are biphasic. The core is a zinc blende monocrystal surrounded with single-crystal wurtzite. The GaN crystalline nanomesas have no threading dislocations or V-pits. This NSAG process potentially leads to integration of high-quality III-nitrides on the wafer scalable epitaxial graphene/silicon carbide platform.

  11. Fabrication and characterization of nanopatterned epitaxial graphene films for carbon based electronics

    NASA Astrophysics Data System (ADS)

    Song, Zhimin

    In this thesis, we show that planar graphene ribbons have properties similar to those of nanotubes. Both exhibit semiconducting or metallic properties depending on crystal orientation. The band gap varies approximately as the inverse of the ribbon width. Both can be doped and gated. Due to these similarities, the patterned graphene also has nanotube like transport properties, which include coherent transport, ballistic transport, and high current capabilities. In essential contrast to nanotubes, graphene ribbons can be rationally patterned using standard electron beam lithography methods, functional graphene devices could be fabricated eliminating the need for metal interconnects on the wafer. This would remove many obstacles faced by carbon nanotubes, while retaining the benefits of high carrier mobility and quasi-1D transport. We have produced ultrathin epitaxial graphite films on single-crystal silicon carbide by vacuum graphitization, which show remarkable 2D electron gas (2DEG) behavior. The most highly ordered samples exhibit Shubnikov-de Haas oscillations that correspond to nonlinearities observed in the Hall resistance, indicating a potential new quantum Hall system. The transport properties, which are closely related to those of carbon nanotubes, are dominated by the single epitaxial graphene layer at the silicon carbide interface and reveal the Dirac nature of the charge carriers. Patterned structures show quantum confinement of electrons and phase coherence lengths beyond 1 micrometer at 4 kelvin, with mobilities exceeding 2.5 square meters per volt-second. We show that the high-mobility films can be patterned via conventional lithographic techniques, and we demonstrate modulation of the film conductance using a top-gate electrode. These key elements suggest electronic device applications based on nanopatterned epitaxial graphene (NPEG) with the potential for large-scale integration. The research created a foundation for graphene science and technology

  12. Epitaxial graphene on SiC(0001): It takes a Si jump

    NASA Astrophysics Data System (ADS)

    Sun, G. F.; Liu, Y.; Rhim, S. H.; Jia, J. F.; Xue, Q. K.; Weinert, M.; Li, L.

    2011-03-01

    Using scanning tunneling microscopy with transition metal (Fe, Cr)-coated W tips and first-principles calculations, we have recently shown that interface of epitaxial graphene/SiC(0001) is a warped graphene layer with periodic inclusions of hexagon-pentagon-heptagon (H5 , 6 , 7) defects that break the six-fold honeycomb symmetry. Here we show that this unique structure facilitate a novel pathway for the disposal of Si during growth: the diffusion of Si vertically through the warped interfacial layer via a series of configurations that involve the dissociation and formation of C-C and Si-C bonds within the pentagon and heptagon of the H5 , 6 , 7 complex. The calculated energy barrier for this diffusion path is 4.7 eV. These results and their implications on the self-limiting growth of epitaxial graphene on SiC(0001) will be presented at the meeting.

  13. Epitaxial assembly of graphene on face (0001) of silicon carbide: Modeling by semiempirical methods

    NASA Astrophysics Data System (ADS)

    Alekseev, N. I.; Kal'nin, A. A.; Karmanov, D. D.; Luchinin, V. V.; Tarasov, S. A.; Charykov, N. A.

    2013-10-01

    The epitaxial growth of graphene on silicon face (0001) of silicon carbide is simulated using the semiempirical methods of quantum chemistry. The experimental conditions for the epitaxial growth of graphene on SiC, at which the probability of seams and similar defects appearing is reduced to a minimum, are formulated. Possible ways of the emergence of reconstructions of the singular carbon and silicon SiC faces during the synthesis of graphenes are investigated as a test of the approach's efficiency. It is noted that simulation reproduces the reconstruction periods experimentally determined for both faces, and yields the most likely atomic arrangements in cases where the experimental formula of the superstructure allows different versions of such arrangements.

  14. Electronic structures of single- and multi-layer epitaxial graphene on SiC (0001)

    NASA Astrophysics Data System (ADS)

    Kim, Seungchul; Ihm, Jisoon; Son, Young-Woo

    2009-03-01

    The electronic structures of single- and multi-layered epitaxial graphene on silicon carbide (0001) surface are studied theoretically. To calculate energy bands of the systems, we construct the simple Hamiltonian with tight-binding approximations. We confirm that the present simple model do give identical electronic structure to the previous ab-initio study on the single layer case [1]. We extend the model up to four epitaxial graphene layers to explain various interesting experimental findings. The roles of the coupling between graphenes and the buffer layer, and their large scale reconstructions to the electronic structures are also investigated. [1] S. Kim, J. Ihm, H. J. Choi, Y.-W. Son, Phys. Rev. Lett. 100, 176802 (2008).

  15. Few-layer III-VI and IV-VI 2D semiconductor transistors

    NASA Astrophysics Data System (ADS)

    Sucharitakul, Sukrit; Liu, Mei; Kumar, Rajesh; Sankar, Raman; Chou, Fang C.; Chen, Yit-Tsong; Gao, Xuan

    Since the discovery of atomically thin graphene, a large variety of exfoliable 2D materials have been thoroughly explored for their exotic transport behavior and promises in technological breakthroughs. While most attention on 2D materials beyond graphene is focused on transition metal-dichalcogenides, relatively less attention is paid to layered III-VI and IV-VI semiconductors such as InSe, SnSe etc which bear stronger potential as 2D materials with high electron mobility or thermoelectric figure of merit. We will discuss our recent work on few-layer InSe 2D field effect transistors which exhibit carrier mobility approaching 1000 cm2/Vs and ON-OFF ratio exceeding 107 at room temperature. In addition, the fabrication and device performance of transistors made of mechanically exfoliated multilayer IV-VI semiconductor SnSe and SnSe2 will be discussed.

  16. Asymmetric Electron Transport at Monolayer-Bilayer Heterojunctions of Epitaxial Graphene

    SciTech Connect

    Li, An-Ping; Clark, Kendal W; Zhang, Xiaoguang; Gu, Gong; He, Guowei; Feenstra, Randall

    2014-01-01

    The symmetry of the graphene honeycomb lattice is a key element determining many of graphene s unique electronic properties, such as the linear energy-momentum dispersion and the suppressed backscattering 1,2. However, line defects in large-scale epitaxial graphene films, such as grain boundaries, edges, surface steps, and changes in layer thickness, often break the sublatttice symmetry and can impact transport properties of graphene profoundly 3-6. Here we report asymmetric electron transport upon polarity reversal at individual monolayer-bilayer (ML-BL) boundaries in epitaxial graphene on SiC (0001), revealed by scanning tunneling potentiometry. A greater voltage drop is observed when the current flows from BL to ML graphene than in the reverse direction, and the difference remains nearly unchanged with increasing current. This is not a typical nonlinear conductance due to electron transmission through an asymmetric potential. Rather, it indicates the opening of a dynamic energy gap at the Fermi energy due to the Coulomb interaction between the injected nonequilibrium electron density and the pseudospin polarized Friedel oscillation charge density at the boundary. This intriguing heterojunction transport behavior opens a new avenue towards novel quantum functions such as quantum switching.

  17. Liquid-Gated Epitaxial Graphene: How Leakage Currents Affect Ionic Strength Sensing

    NASA Astrophysics Data System (ADS)

    Bedoya, Mauricio D.; Metaxas, Peter J.; Scrimgeour, Jan; Hu, Yike; Dong, Rui; Berger, Claire; de Heer, Walt A.; Curtis, Jennifer E.

    2013-03-01

    Graphene is a promising material for the fabrication of miniaturized biological and chemical sensors. Epitaxial graphene is an exciting candidate due to its compatibility with standard processing techniques and its intrinsic robustness. We have fabricated liquid-gated FET-like devices based upon sub-millimeter wide epitaxial graphene strips defined using optical lithography methods. The devices exhibit a bipolar conductance versus gate voltage behavior with the minimum conductance point being dependent upon the ionic strength of a KCl solution. Measurements of the graphene conductance and gate-leakage currents during the stepping of the gate voltage demonstrate the presence of time dependent nA-scale leakage currents which limit signal stability at short times. Notably, these currents depend upon the gate voltage and the composition of the gate electrode. These and other electrode dependent effects have ramifications for graphene sensor design and implementation such as the need to limit gate voltage operating windows as and carefully design electrodes. With high transconductance and controlled doping, such devices should be able to function at low gate voltages if a full understanding of charge and charge transport at the graphene interface is obtained. NSF Grant No. DMR-0820382. PJM thanks the ANN and DIISR.

  18. Raman and morphology visualization in epitaxial graphene on 4H-SiC by Nitrogen or Argon ion irradiation

    NASA Astrophysics Data System (ADS)

    Zhao, Jin-Hua; Qin, Xi-Feng; Wang, Feng-Xiang; Fu, Gang; Wang, Xue-Lin

    2015-12-01

    Graphene is a one-atom-thick planar sheet of carbon atoms that are densely packed into a honeycomb crystal lattice and is attracting tremendous interest since being discovered in 2004. Epitaxial growth of graphene on silicon carbide (SiC) is an effective method to obtain high quality layers. In this work, the effects of irradiation on epitaxial SiC/graphene were studied. The samples were irradiated with Nitrogen and Argon ions at an energy of 200 keV and different fluence with 4 × 1012 ions/cm2 to 1 × 1013 ions/cm2. The results of Raman measurements indicate that ion beam irradiation causes defects and disorder in the graphene crystal structure, and the level of defects increases with increasing ion fluence. Surface morphology images are obtained by atomic force microscope (AFM). This work is valuable for the potential application of epitaxial graphene on SiC in the field of optoelectronics devices.

  19. Epitaxial growth and electrochemical transfer of graphene on Ir(111)/α-Al2O3(0001) substrates

    NASA Astrophysics Data System (ADS)

    Koh, Shinji; Saito, Yuta; Kodama, Hideyuki; Sawabe, Atsuhito

    2016-07-01

    Low-pressure chemical vapor deposition growth of graphene on Iridium (Ir) layers epitaxially deposited on α-Al2O3 (0001) substrates was investigated. The X-ray diffraction, Raman and reflection high energy electron diffraction characterizations revealed that graphene films were epitaxially grown on Ir(111) layers, and the in-plane epitaxial relationship between graphene, Ir(111), and α-Al2O3(0001) was graphene ⟨ 1 1 ¯ 00 ⟩//Ir⟨ 11 2 ¯ ⟩//α-Al2O3⟨ 11 2 ¯ 0 ⟩. The graphene on Ir(111) was electrochemically transferred onto SiO2/Si substrates. We also demonstrated the reuse of the Ir(111)/α-Al2O3(0001) substrates in multiple growth and transfer cycles.

  20. A prototype of RK/200 quantum Hall array resistance standard on epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Lartsev, A.; Lara-Avila, S.; Danilov, A.; Kubatkin, S.; Tzalenchuk, A.; Yakimova, R.

    2015-07-01

    Epitaxial graphene on silicon carbide is a promising material for the next generation of quantum Hall resistance standards. Single Hall bars made of graphene have already surpassed their state-of-the-art GaAs based counterparts as an R K / 2 ( R K = h / e 2 ) standard, showing at least the same precision and higher breakdown current density. Compared to single devices, quantum Hall arrays using parallel or series connection of multiple Hall bars can offer resistance values spanning several orders of magnitude and (in case of parallel connection) significantly larger measurement currents, but impose strict requirements on uniformity of the material. To evaluate the quality of the available material, we have fabricated arrays of 100 Hall bars connected in parallel on epitaxial graphene. One out of four devices has shown quantized resistance that matched the correct value of R K / 200 within the measurement precision of 10 - 4 at magnetic fields between 7 and 9 T. The defective behaviour of other arrays is attributed mainly to non-uniform doping. This result confirms the acceptable quality of epitaxial graphene, pointing towards the feasibility of well above 90% yield of working Hall bars.

  1. High Electron Mobility in Epitaxial Trilayer Graphene on Off-axis SiC(0001)

    PubMed Central

    Hajlaoui, Mahdi; Sediri, Haikel; Pierucci, Debora; Henck, Hugo; Phuphachong, Thanyanan; Silly, Mathieu G.; de Vaulchier, Louis-Anne; Sirotti, Fausto; Guldner, Yves; Belkhou, Rachid; Ouerghi, Abdelkarim

    2016-01-01

    The van de Waals heterostructure formed by an epitaxial trilayer graphene is of particular interest due to its unique tunable electronic band structure and stacking sequence. However, to date, there has been a lack in the fundamental understanding of the electronic properties of epitaxial trilayer graphene. Here, we investigate the electronic properties of large-area epitaxial trilayer graphene on a 4° off-axis SiC(0001) substrate. Micro-Raman mappings and atomic force microscopy (AFM) confirmed predominantly trilayer on the sample obtained under optimized conditions. We used angle-resolved photoemission spectroscopy (ARPES) and Density Functional Theory (DFT) calculations to study in detail the structure of valence electronic states, in particular the dispersion of π bands in reciprocal space and the exact determination of the number of graphene layers. Using far-infrared magneto-transmission (FIR-MT), we demonstrate, that the electron cyclotron resonance (CR) occurs between Landau levels with a (B)1/2 dependence. The CR line-width is consistent with a high Dirac fermions mobility of ~3000 cm2·V−1·s−1 at 4 K. PMID:26739366

  2. Controlling the growth of epitaxial graphene on metalized diamond (111) surface

    SciTech Connect

    Cooil, S. P.; Wells, J. W.; Hu, D.; Evans, D. A.; Niu, Y. R.; Zakharov, A. A.; Bianchi, M.

    2015-11-02

    The 2-dimensional transformation of the diamond (111) surface to graphene has been demonstrated using ultrathin Fe films that catalytically reduce the reaction temperature needed for the conversion of sp{sup 3} to sp{sup 2} carbon. An epitaxial system is formed, which involves the re-crystallization of carbon at the Fe/vacuum interface and that enables the controlled growth of monolayer and multilayer graphene films. In order to study the initial stages of single and multilayer graphene growth, real time monitoring of the system was preformed within a photoemission and low energy electron microscope. It was found that the initial graphene growth occurred at temperatures as low as 500 °C, whilst increasing the temperature to 560 °C was required to produce multi-layer graphene of high structural quality. Angle resolved photoelectron spectroscopy was used to study the electronic properties of the grown material, where a graphene-like energy momentum dispersion was observed. The Dirac point for the first layer is located at 2.5 eV below the Fermi level, indicating an n-type doping of the graphene due to substrate interactions, while that of the second graphene layer lies close to the Fermi level.

  3. Large Ultraviolet Photoresponsivity of Few-layer Black Phosphorus

    NASA Astrophysics Data System (ADS)

    Wu, Jing; Kok Wai Koon, Gavin; Xiang, Du; Castro Neto, Antonio H.; Özyilmaz, Barbaros; Centre of Advanced 2D Materials Team

    Black phosphorus has recently gained much attention in the scientific community. Black phosphorus can be seen as a crystal generated by periodic repetition of tetraphosphorus (P4) molecules. It is known that tetraphosphorus P4 can be transformed temporarily to diphosphorus P2 upon ultraviolet (UV) irradiation. Thus, it is expected that the P4 structured black phosphorus also has strong interaction with light especially in the UV range. Here we report on the optoelectronic characteristics of few-layer black phosphorus field effect transistors (FETs) ranging from the UV to the near infrared (NIR). We demonstrate that black phosphorus is an excellent ultraviolet (UV) photodetector with a specific detectivity ~3x1013 Jones. We report also an exceptional photo responsivity of 107 times higher than previously reported values for black phosphorus visible light photodetectors. We attribute such a colossal UV photo responsivity to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusually high density of states for high-efficient UV absorption due to their singularity nature. Large Ultraviolet Photoresponsivity of Few-layer Black Phosphorus.

  4. Mini-Dirac cones in the band structure of a copper intercalated epitaxial graphene superlattice

    NASA Astrophysics Data System (ADS)

    Forti, S.; Stöhr, A.; Zakharov, A. A.; Coletti, C.; Emtsev, K. V.; Starke, U.

    2016-09-01

    The electronic band structure of an epitaxial graphene superlattice, generated by intercalating a monolayer of Cu atoms, is directly imaged by angle-resolved photoelectron spectroscopy. The 3.2 nm lateral period of the superlattice is induced by a varying registry between the graphene honeycomb and the Cu atoms as imposed by the heteroepitaxial interface Cu/SiC. The carbon atoms experience a lateral potential across the supercell of an estimated value of about 65 meV. The potential leads to strong energy renormalization in the band structure of the graphene layer and the emergence of mini-Dirac cones. The mini-cones’ band velocity is reduced to about half of graphene's Fermi velocity. Notably, the ordering of the interfacial Cu atoms can be reversibly blocked by mild annealing. The superlattice indeed disappears at ∼220 °C.

  5. Structural investigations of hydrogenated epitaxial graphene grown on 4H-SiC (0001)

    SciTech Connect

    Tokarczyk, M.; Kowalski, G. Stępniewski, R.; Możdżonek, M.; Strupiński, W.; Ciepielewski, P.; Borysiuk, J.

    2013-12-09

    Structural investigations of hydrogenated epitaxial graphene grown on SiC(0001) are presented. It is shown that hydrogen plays a dual role. In addition to contributing to the well-known removal of the buffer layer, it goes between the graphene planes, resulting in an increase of the interlayer spacing to 3.6 Å–3.8 Å. It is explained by the intercalation of molecular hydrogen between carbon planes, which is followed by H{sub 2} dissociation, resulting in negatively charged hydrogen atoms trapped between the graphene layers, with some addition of covalent bonding to carbon atoms. Negatively charged hydrogen may be responsible for p-doping observed in hydrogenated multilayer graphene.

  6. In silico carbon molecular beam epitaxial growth of graphene on the h-BN substrate: carbon source effect on van der Waals epitaxy.

    PubMed

    Lee, Jonghoon; Varshney, Vikas; Park, Jeongho; Farmer, Barry L; Roy, Ajit K

    2016-05-01

    Against the presumption that hexagonal boron-nitride (h-BN) should provide an ideal substrate for van der Waals (vdW) epitaxy to grow high quality graphene films, carbon molecular beam epitaxy (CMBE) techniques using solid carbon sublimation have reported relatively poor quality of the graphene. In this article, the CMBE growth of graphene on the h-BN substrate is numerically studied in order to identify the effect of the carbon source on the quality of the graphene film. The carbon molecular beam generated by the sublimation of solid carbon source materials such as graphite and glassy carbon is mostly composed of atomic carbon, carbon dimers and carbon trimers. Therefore, the graphene film growth becomes a complex process involving various deposition characteristics of a multitude of carbon entities. Based on the study of surface adsorption and film growth characteristics of these three major carbon entities comprising graphite vapour, we report that carbon trimers convey strong traits of vdW epitaxy prone to high quality graphene growth, while atomic carbon deposition is a surface-reaction limited process accompanied by strong chemisorption. The vdW epitaxial behaviour of carbon trimers is found to be substantial enough to nucleate and develop into graphene like planar films within a nanosecond of high flux growth simulation, while reactive atomic carbons tend to impair the structural integrity of the crystalline h-BN substrate upon deposition to form an amorphous interface between the substrate and the growing carbon film. The content of reactive atomic carbons in the molecular beam is suspected to be the primary cause of low quality graphene reported in the literature. A possible optimization of the molecular beam composition towards the synthesis of better quality graphene films is suggested. PMID:27108606

  7. Aqueous dispersions of few-layer-thick chemically modified magnesium diboride nanosheets by ultrasonication assisted exfoliation.

    PubMed

    Das, Saroj Kumar; Bedar, Amita; Kannan, Aadithya; Jasuja, Kabeer

    2015-01-01

    The discovery of graphene has led to a rising interest in seeking quasi two-dimensional allotropes of several elements and inorganic compounds. Boron, carbon's neighbour in the periodic table, presents a curious case in its ability to be structured as graphene. Although it cannot independently constitute a honeycomb planar structure, it forms a graphenic arrangement in association with electron-donor elements. This is exemplified in magnesium diboride (MgB2): an inorganic layered compound comprising boron honeycomb planes alternated by Mg atoms. Till date, MgB2 has been primarily researched for its superconducting properties; it hasn't been explored for the possibility of its exfoliation. Here we show that ultrasonication of MgB2 in water results in its exfoliation to yield few-layer-thick Mg-deficient hydroxyl-functionalized nanosheets. The hydroxyl groups enable an electrostatically stabilized aqueous dispersion and create a heterogeneity leading to an excitation wavelength dependent photoluminescence. These chemically modified MgB2 nanosheets exhibit an extremely small absorption coefficient of 2.9 ml mg(-1) cm(-1) compared to graphene and its analogs. This ability to exfoliate MgB2 to yield nanosheets with a chemically modified lattice and properties distinct from the parent material presents a fundamentally new perspective to the science of MgB2 and forms a first foundational step towards exfoliating metal borides. PMID:26041686

  8. Aqueous dispersions of few-layer-thick chemically modified magnesium diboride nanosheets by ultrasonication assisted exfoliation

    PubMed Central

    Das, Saroj Kumar; Bedar, Amita; Kannan, Aadithya; Jasuja, Kabeer

    2015-01-01

    The discovery of graphene has led to a rising interest in seeking quasi two-dimensional allotropes of several elements and inorganic compounds. Boron, carbon’s neighbour in the periodic table, presents a curious case in its ability to be structured as graphene. Although it cannot independently constitute a honeycomb planar structure, it forms a graphenic arrangement in association with electron-donor elements. This is exemplified in magnesium diboride (MgB2): an inorganic layered compound comprising boron honeycomb planes alternated by Mg atoms. Till date, MgB2 has been primarily researched for its superconducting properties; it hasn’t been explored for the possibility of its exfoliation. Here we show that ultrasonication of MgB2 in water results in its exfoliation to yield few-layer-thick Mg-deficient hydroxyl-functionalized nanosheets. The hydroxyl groups enable an electrostatically stabilized aqueous dispersion and create a heterogeneity leading to an excitation wavelength dependent photoluminescence. These chemically modified MgB2 nanosheets exhibit an extremely small absorption coefficient of 2.9 ml mg−1 cm−1 compared to graphene and its analogs. This ability to exfoliate MgB2 to yield nanosheets with a chemically modified lattice and properties distinct from the parent material presents a fundamentally new perspective to the science of MgB2 and forms a first foundational step towards exfoliating metal borides. PMID:26041686

  9. Atomistic view in the initial stages of growth of epitaxial graphene on metal substrates

    NASA Astrophysics Data System (ADS)

    Zhang, Zhenyu

    2011-03-01

    For both fundamental studies and potential development of graphene electronics, it is pressing to search for reliable methods for mass production of quality graphene. Epitaxial growth of graphene on catalytic metal substrates combined with post-growth transfer has become a promising route towards this goal [1,2]. However, to better control the quality and yield of graphene, a comprehensive understanding of the growth kinetics is essential. In particular, how the carbon atoms adsorbed on the metal surface (or dissolved into the metal) meet to nucleate into stable carbon islands will greatly influence both the growth rate and quality of larger carbon entities such as graphene sheets. In this talk, we first show that the delicate competition between carbon-carbon bonding and carbon-metal bonding dictates the initial nucleation sites of graphene on metal surfaces. These results are discussed in connection with the experimental findings that on Ir(111) and Ru(0001) substrates graphene nucleates from the step edges [4,5]. We also predict that on Cu(111) nucleation should take place everywhere on a terrace. Next we study larger carbon clusters on Cu(111) and explicitly compare the stability of linear and compact structures. We find that the linear carbon ``nanoarches'' are more stable than compact islands consisting of up to 13 carbon atoms, and these nanoarched structures may serve as the missing bridge between carbon dimers and larger graphene nanodomes. Based on these improved understanding of the atomistic rate processes involved, we propose a few kinetic pathways that may lead to better growth control of bilayer graphene and graphene nanoribbons as elemental building blocks for developing graphene electronics. Work done in collaboration with Hua Chen, Wenguang Zhu, Robert Van Wesep, Wei Chen, Ping Cui, and Haiping Lan, and supported by USDOE, USNSF, and NNSF of China.

  10. Graphene nanoribbon field-effect transistors on wafer-scale epitaxial graphene on SiC substrates

    SciTech Connect

    Hwang, Wan Sik E-mail: djena@nd.edu; Zhao, Pei; Tahy, Kristof; Xing, Huili; Seabaugh, Alan; Jena, Debdeep E-mail: djena@nd.edu; Nyakiti, Luke O.; Wheeler, Virginia D.; Myers-Ward, Rachael L.; Eddy, Charles R.; Gaskill, D. Kurt; Robinson, Joshua A.; Haensch, Wilfried

    2015-01-01

    We report the realization of top-gated graphene nanoribbon field effect transistors (GNRFETs) of ∼10 nm width on large-area epitaxial graphene exhibiting the opening of a band gap of ∼0.14 eV. Contrary to prior observations of disordered transport and severe edge-roughness effects of graphene nanoribbons (GNRs), the experimental results presented here clearly show that the transport mechanism in carefully fabricated GNRFETs is conventional band-transport at room temperature and inter-band tunneling at low temperature. The entire space of temperature, size, and geometry dependent transport properties and electrostatics of the GNRFETs are explained by a conventional thermionic emission and tunneling current model. Our combined experimental and modeling work proves that carefully fabricated narrow GNRs behave as conventional semiconductors and remain potential candidates for electronic switching devices.

  11. Strain-Engineered Graphene Grown on Hexagonal Boron Nitride by Molecular Beam Epitaxy

    PubMed Central

    Summerfield, Alex; Davies, Andrew; Cheng, Tin S.; Korolkov, Vladimir V.; Cho, YongJin; Mellor, Christopher J.; Foxon, C. Thomas; Khlobystov, Andrei N.; Watanabe, Kenji; Taniguchi, Takashi; Eaves, Laurence; Novikov, Sergei V.; Beton, Peter H.

    2016-01-01

    Graphene grown by high temperature molecular beam epitaxy on hexagonal boron nitride (hBN) forms continuous domains with dimensions of order 20 μm, and exhibits moiré patterns with large periodicities, up to ~30 nm, indicating that the layers are highly strained. Topological defects in the moiré patterns are observed and attributed to the relaxation of graphene islands which nucleate at different sites and subsequently coalesce. In addition, cracks are formed leading to strain relaxation, highly anisotropic strain fields, and abrupt boundaries between regions with different moiré periods. These cracks can also be formed by modification of the layers with a local probe resulting in the contraction and physical displacement of graphene layers. The Raman spectra of regions with a large moiré period reveal split and shifted G and 2D peaks confirming the presence of strain. Our work demonstrates a new approach to the growth of epitaxial graphene and a means of generating and modifying strain in graphene. PMID:26928710

  12. Strain-Engineered Graphene Grown on Hexagonal Boron Nitride by Molecular Beam Epitaxy

    NASA Astrophysics Data System (ADS)

    Summerfield, Alex; Davies, Andrew; Cheng, Tin S.; Korolkov, Vladimir V.; Cho, Yongjin; Mellor, Christopher J.; Foxon, C. Thomas; Khlobystov, Andrei N.; Watanabe, Kenji; Taniguchi, Takashi; Eaves, Laurence; Novikov, Sergei V.; Beton, Peter H.

    2016-03-01

    Graphene grown by high temperature molecular beam epitaxy on hexagonal boron nitride (hBN) forms continuous domains with dimensions of order 20 μm, and exhibits moiré patterns with large periodicities, up to ~30 nm, indicating that the layers are highly strained. Topological defects in the moiré patterns are observed and attributed to the relaxation of graphene islands which nucleate at different sites and subsequently coalesce. In addition, cracks are formed leading to strain relaxation, highly anisotropic strain fields, and abrupt boundaries between regions with different moiré periods. These cracks can also be formed by modification of the layers with a local probe resulting in the contraction and physical displacement of graphene layers. The Raman spectra of regions with a large moiré period reveal split and shifted G and 2D peaks confirming the presence of strain. Our work demonstrates a new approach to the growth of epitaxial graphene and a means of generating and modifying strain in graphene.

  13. Strain-Engineered Graphene Grown on Hexagonal Boron Nitride by Molecular Beam Epitaxy.

    PubMed

    Summerfield, Alex; Davies, Andrew; Cheng, Tin S; Korolkov, Vladimir V; Cho, YongJin; Mellor, Christopher J; Foxon, C Thomas; Khlobystov, Andrei N; Watanabe, Kenji; Taniguchi, Takashi; Eaves, Laurence; Novikov, Sergei V; Beton, Peter H

    2016-01-01

    Graphene grown by high temperature molecular beam epitaxy on hexagonal boron nitride (hBN) forms continuous domains with dimensions of order 20 μm, and exhibits moiré patterns with large periodicities, up to ~30 nm, indicating that the layers are highly strained. Topological defects in the moiré patterns are observed and attributed to the relaxation of graphene islands which nucleate at different sites and subsequently coalesce. In addition, cracks are formed leading to strain relaxation, highly anisotropic strain fields, and abrupt boundaries between regions with different moiré periods. These cracks can also be formed by modification of the layers with a local probe resulting in the contraction and physical displacement of graphene layers. The Raman spectra of regions with a large moiré period reveal split and shifted G and 2D peaks confirming the presence of strain. Our work demonstrates a new approach to the growth of epitaxial graphene and a means of generating and modifying strain in graphene. PMID:26928710

  14. Electric Field Effects on Electronic Structures of Epitaxial Graphene on SiC

    NASA Astrophysics Data System (ADS)

    Lee, Hyungjun; Kim, Seungchul; Ihm, Jisoon; Son, Young-Woo; Choi, Hyoung Joon

    2011-03-01

    We report first-principles calculations of atomic and electronic structures of epitaxial single-layer graphene on Si-faced SiC(0001) surface under homogeneous transverse electric fields. We find that atomic positions are insensitive to applied electric fields, but the electronic band structures of the graphene layer are shifted in energy, depending strongly on the applied electric fields, while those of the buffer layer are almost unchanged. This effect finally results in field-induced closing of the energy gap at the Dirac energy point and recovery of the conic feature of the low-energy band structures of free-standing graphene, which are verified and analyzed further with a tight-binding model. The recovery of conical dispersion of the single-layer graphene and ambipolar field-effect behavior makes epitaxial single-layer graphene one of the promising alternatives to current state-of-the-art transistors for radiofrequency applications. This work was supported by the NRF of Korea (Grant No. 2009-0081204). Computational resources have been provided by KISTI Supercomputing Center (KSC-2008-S02-0004).

  15. Tuning carrier density across Dirac point in epitaxial graphene on SiC by corona discharge

    SciTech Connect

    Lartsev, Arseniy; Yager, Tom; Lara-Avila, Samuel Kubatkin, Sergey; Bergsten, Tobias; Tzalenchuk, Alexander; Janssen, T. J. B. M; Yakimova, Rositza

    2014-08-11

    We demonstrate reversible carrier density control across the Dirac point (Δn ∼ 10{sup 13 }cm{sup −2}) in epitaxial graphene on SiC (SiC/G) via high electrostatic potential gating with ions produced by corona discharge. The method is attractive for applications where graphene with a fixed carrier density is needed, such as quantum metrology, and more generally as a simple method of gating 2DEGs formed at semiconductor interfaces and in topological insulators.

  16. Atomic oxidation of large area epitaxial graphene on 4H-SiC(0001)

    SciTech Connect

    Velez-Fort, E.; Ouerghi, A.; Silly, M. G.; Sirtti, F.; Eddrief, M.; Marangolo, M.; Shukla, A.

    2014-03-03

    Structural and electronic properties of epitaxial graphene on 4H-SiC were studied before and after an atomic oxidation process. X-ray photoemission spectroscopy indicates that oxygen penetrates into the substrate and decouples a part of the interface layer. Raman spectroscopy demonstrates the increase of defects due to the presence of oxygen. Interestingly, we observed on the near edge x-ray absorption fine structure spectra a splitting of the π* peak into two distinct resonances centered at 284.7 and 285.2 eV. This double structure smears out after the oxidation process and permits to probe the interface architecture between graphene and the substrate.

  17. Voltage-controlled inversion of tunnel magnetoresistance in epitaxial nickel/graphene/MgO/cobalt junctions

    SciTech Connect

    Godel, F.; Doudin, B.; Henry, Y.; Halley, D. E-mail: dayen@ipcms.unistra.fr; Dayen, J.-F. E-mail: dayen@ipcms.unistra.fr; Venkata Kamalakar, M.

    2014-10-13

    We report on the fabrication and characterization of vertical spin-valve structures using a thick epitaxial MgO barrier as spacer layer and a graphene-passivated Ni film as bottom ferromagnetic electrode. The devices show robust and scalable tunnel magnetoresistance, with several changes of sign upon varying the applied bias voltage. These findings are explained by a model of phonon-assisted transport mechanisms that relies on the peculiarity of the band structure and spin density of states at the hybrid graphene|Ni interface.

  18. Carbon dimers as the dominant feeding species in epitaxial growth and morphological phase transition of graphene on different Cu substrates.

    PubMed

    Wu, Ping; Zhang, Yue; Cui, Ping; Li, Zhenyu; Yang, Jinlong; Zhang, Zhenyu

    2015-05-29

    Cu substrates are highly preferred for the potential mass production of high-quality graphene, yet many of the important aspects of the atomistic growth mechanisms involved remain to be explored. Using multiscale modeling, we identify C-C dimers as the dominant feeding species in the epitaxial growth of graphene on both Cu(111) and Cu(100) substrates. By contrasting the different activation energies involved in C-C dimer diffusion on terraces and its attachment at graphene island edges, we further reveal why graphene growth is diffusion limited on Cu(111), but attachment limited on Cu(100). We also find even higher potential energy barriers against dimer diffusion along the island edges; consequently, a dendritic-to-compact transition is predicted to take place during graphene enlargement on either substrate, but at different growth temperatures. These findings serve as new insights for better control of epitaxial graphene growth. PMID:26066446

  19. Carbon Dimers as the Dominant Feeding Species in Epitaxial Growth and Morphological Phase Transition of Graphene on Different Cu Substrates

    NASA Astrophysics Data System (ADS)

    Wu, Ping; Zhang, Yue; Cui, Ping; Li, Zhenyu; Yang, Jinlong; Zhang, Zhenyu

    2015-05-01

    Cu substrates are highly preferred for the potential mass production of high-quality graphene, yet many of the important aspects of the atomistic growth mechanisms involved remain to be explored. Using multiscale modeling, we identify C-C dimers as the dominant feeding species in the epitaxial growth of graphene on both Cu(111) and Cu(100) substrates. By contrasting the different activation energies involved in C-C dimer diffusion on terraces and its attachment at graphene island edges, we further reveal why graphene growth is diffusion limited on Cu(111), but attachment limited on Cu(100). We also find even higher potential energy barriers against dimer diffusion along the island edges; consequently, a dendritic-to-compact transition is predicted to take place during graphene enlargement on either substrate, but at different growth temperatures. These findings serve as new insights for better control of epitaxial graphene growth.

  20. Dielectric and Conductivity Mapping of Few-Layer Metal Chalcogenides

    NASA Astrophysics Data System (ADS)

    Lai, Keji; Wu, Di; Liu, Yingnan; Ren, Yuan; Lin, Min; Peng, Hailin; Ismach, Ariel; Ghosh, Rudresh; Ruoff, Rodney

    2014-03-01

    A novel microwave impedance microscope was used to spatially map the local dielectric constant and conductivity of few-layered metal chalcogenides without the need of contact electrodes. For phase-change In2Se3 nanoplates grown on mica substrates, our results showed a sudden drop of permittivity from the bulk value for thicknesses below 5 layers and strong dielectric inhomogeneity around 4 and 5 layers. For CVD-grown MoS2 flakes on SiO2/Si wafers, we observed highly conductive localized regions within monolayer islands. These regions, which can be imaged by scanning electron microscopy and atomic force microscopy, show enhanced Raman signals and PL signal quenching. Continued imaging effort is expected to shed some light on the growth mechanism and electron physics of these quasi-2D chalcogenides.

  1. Few-layer HfS2 transistors.

    PubMed

    Kanazawa, Toru; Amemiya, Tomohiro; Ishikawa, Atsushi; Upadhyaya, Vikrant; Tsuruta, Kenji; Tanaka, Takuo; Miyamoto, Yasuyuki

    2016-01-01

    HfS2 is the novel transition metal dichalcogenide, which has not been experimentally investigated as the material for electron devices. As per the theoretical calculations, HfS2 has the potential for well-balanced mobility (1,800 cm(2)/V·s) and bandgap (1.2 eV) and hence it can be a good candidate for realizing low-power devices. In this paper, the fundamental properties of few-layer HfS2 flakes were experimentally evaluated. Micromechanical exfoliation using scotch tape extracted atomically thin HfS2 flakes with varying colour contrasts associated with the number of layers and resonant Raman peaks. We demonstrated the I-V characteristics of the back-gated few-layer (3.8 nm) HfS2 transistor with the robust current saturation. The on/off ratio was more than 10(4) and the maximum drain current of 0.2 μA/μm was observed. Moreover, using the electric double-layer gate structure with LiClO4:PEO electrolyte, the drain current of the HfS2 transistor significantly increased to 0.75 mA/μm and the mobility was estimated to be 45 cm(2)/V·s at least. This improved current seemed to indicate superior intrinsic properties of HfS2. These results provides the basic information for the experimental researches of electron devices based on HfS2. PMID:26926098

  2. Few-layer HfS2 transistors

    PubMed Central

    Kanazawa, Toru; Amemiya, Tomohiro; Ishikawa, Atsushi; Upadhyaya, Vikrant; Tsuruta, Kenji; Tanaka, Takuo; Miyamoto, Yasuyuki

    2016-01-01

    HfS2 is the novel transition metal dichalcogenide, which has not been experimentally investigated as the material for electron devices. As per the theoretical calculations, HfS2 has the potential for well-balanced mobility (1,800 cm2/V·s) and bandgap (1.2 eV) and hence it can be a good candidate for realizing low-power devices. In this paper, the fundamental properties of few-layer HfS2 flakes were experimentally evaluated. Micromechanical exfoliation using scotch tape extracted atomically thin HfS2 flakes with varying colour contrasts associated with the number of layers and resonant Raman peaks. We demonstrated the I-V characteristics of the back-gated few-layer (3.8 nm) HfS2 transistor with the robust current saturation. The on/off ratio was more than 104 and the maximum drain current of 0.2 μA/μm was observed. Moreover, using the electric double-layer gate structure with LiClO4:PEO electrolyte, the drain current of the HfS2 transistor significantly increased to 0.75 mA/μm and the mobility was estimated to be 45 cm2/V·s at least. This improved current seemed to indicate superior intrinsic properties of HfS2. These results provides the basic information for the experimental researches of electron devices based on HfS2. PMID:26926098

  3. High temperature characteristics of bilayer epitaxial graphene field-effect transistors on SiC Substrate

    NASA Astrophysics Data System (ADS)

    Ze-Zhao, He; Ke-Wu, Yang; Cui, Yu; Qing-Bin, Liu; Jing-Jing, Wang; Jia, Li; Wei-Li, Lu; Zhi-Hong, Feng; Shu-Jun, Cai

    2016-06-01

    In this paper, high temperature direct current (DC) performance of bilayer epitaxial graphene device on SiC substrate is studied in a temperature range from 25 °C to 200 °C. At a gate voltage of ‑8 V (far from Dirac point), the drain-source current decreases obviously with increasing temperature, but it has little change at a gate bias of +8 V (near Dirac point). The competing interactions between scattering and thermal activation are responsible for the different reduction tendencies. Four different kinds of scatterings are taken into account to qualitatively analyze the carrier mobility under different temperatures. The devices exhibit almost unchanged DC performances after high temperature measurements at 200 °C for 5 hours in air ambience, demonstrating the high thermal stabilities of the bilayer epitaxial graphene devices. Project supported by the National Natural Science Foundation of China (Grant No. 61306006).

  4. Atomically-Smooth MgO films grown on Epitaxial Graphene by Pulsed Laser Deposition

    NASA Astrophysics Data System (ADS)

    Stuart, Sean; Sandin, Andreas; Rowe, Jack; Dougherty, Dan; Ulrich, Marc

    2013-03-01

    The growth of high quality insulating films on graphene is a crucial materials science task for graphene electronic and spintronic applications. It has been demonstrated that direct spin injection from a magnetic electrode to graphene is possible using MgO tunnel barriers of sufficient quality. We have used pulsed laser deposition (PLD) to grow thin magnesium oxide films directly on epitaxial graphene on SiC(0001). We observe very smooth film morphologies (typical rms roughness of ~ 0.4 nm) that are nearly independent of film thickness and conform to the substrate surface which had ~ 0.2 nm rms roughness. Surface roughness of 0.04 nm have been recorded for ~ 1nm films with no pinholes seen by AFM. XPS and XRD data show non crystalline, hydroxylated MgO films with uniform coverage. This work shows that PLD is a good technique to produce graphene-oxide interfaces without pre-deposition of an adhesion layer or graphene functionalization. The details and kinetics of the deposition process will be described with comparisons being made to other dielectric-on-graphene deposition approaches. Funded by ARO Staff Research Contract # W911NF.

  5. Bipolar Carrier Transfer Channels in Epitaxial Graphene/SiC Core-Shell Heterojunction for Efficient Photocatalytic Hydrogen Evolution.

    PubMed

    Lu, Wei; Wang, Da; Guo, Liwei; Jia, Yuping; Ye, Manping; Huang, Jiao; Li, Zhilin; Peng, Yuan; Yuan, Wenxia; Chen, Xiaolong

    2015-12-22

    Bipolar carrier transfer channels exist in the in situ epitaxial-graphene-wrapped 6H-SiC core-shell heterojunction due to the self-doping of graphene. Due to the special interface structure and high graphene quality, this material exhibits significant photocatalytic enhancement. Its hydrogen evolution efficiency is greater than that of the Pt/SiC composite. This micrometer-sized metal-free photocatalyst exhibits an activity comparable to that of metal-based nanophotocatalysts. PMID:26541083

  6. Nitrogen-induced structures in epitaxial graphene on 6H-SiC(0001)

    NASA Astrophysics Data System (ADS)

    Sun, Guofeng; Rhim, Sung-Hyon; Qi, Yun; Weinert, Michael; Li, Lian

    2009-03-01

    Nitrogen-induced structures on epitaxial graphene grown on 6H- SiC(0001) are studied by scanning tunneling microscopy (STM) and first-principles calculations. Several defect structures produced by nitrogen incorporation are observed by STM. Calculations of the energetics of nitrogen substitution at various sites neighboring a carbon vacancy indicate that nitrogen prefers to be at the site nearest to the vacancy, consistent with the STM observations.

  7. Vertical Single-Crystalline Organic Nanowires on Graphene: Solution-Phase Epitaxy and Optical Microcavities.

    PubMed

    Zheng, Jian-Yao; Xu, Hongjun; Wang, Jing Jing; Winters, Sinéad; Motta, Carlo; Karademir, Ertuğrul; Zhu, Weigang; Varrla, Eswaraiah; Duesberg, Georg S; Sanvito, Stefano; Hu, Wenping; Donegan, John F

    2016-08-10

    Vertically aligned nanowires (NWs) of single crystal semiconductors have attracted a great deal of interest in the past few years. They have strong potential to be used in device structures with high density and with intriguing optoelectronic properties. However, fabricating such nanowire structures using organic semiconducting materials remains technically challenging. Here we report a simple procedure for the synthesis of crystalline 9,10-bis(phenylethynyl) anthracene (BPEA) NWs on a graphene surface utilizing a solution-phase van der Waals (vdW) epitaxial strategy. The wires are found to grow preferentially in a vertical direction on the surface of graphene. Structural characterization and first-principles ab initio simulations were performed to investigate the epitaxial growth and the molecular orientation of the BPEA molecules on graphene was studied, revealing the role of interactions at the graphene-BPEA interface in determining the molecular orientation. These free-standing NWs showed not only efficient optical waveguiding with low loss along the NW but also confinement of light between the two end facets of the NW forming a microcavity Fabry-Pérot resonator. From an analysis of the optical dispersion within such NW microcavities, we observed strong slowing of the waveguided light with a group velocity reduced to one-tenth the speed of light. Applications of the vertical single-crystalline organic NWs grown on graphene will benefit from a combination of the unique electronic properties and flexibility of graphene and the tunable optical and electronic properties of organic NWs. Therefore, these vertical organic NW arrays on graphene offer the potential for realizing future on-chip light sources. PMID:27438189

  8. Scalable Clean Exfoliation of High-Quality Few-Layer Black Phosphorus for a Flexible Lithium Ion Battery.

    PubMed

    Chen, Long; Zhou, Guangmin; Liu, Zhibo; Ma, Xiaomeng; Chen, Jing; Zhang, Zhiyong; Ma, Xiuliang; Li, Feng; Cheng, Hui-Ming; Ren, Wencai

    2016-01-20

    Few-layer black phosphorus (BP) nanosheets that are clean and of high quality, are efficiently produced by exfoliating bulk BP crystals, which are prepared by a scalable gas-phase catalytic transformation method in water. They are stable enough in water for further processing and applications. As an example, these BP nanosheets are combined with graphene to give high-performance flexible lithium-ion batteries. PMID:26584241

  9. Hydrogen assisted growth of high quality epitaxial graphene on the C-face of 4H-SiC

    SciTech Connect

    Cai, Tuocheng; Jia, Zhenzhao; Yan, Baoming; Yu, Dapeng; Wu, Xiaosong

    2015-01-05

    We demonstrate hydrogen assisted growth of high quality epitaxial graphene on the C-face of 4H-SiC. Compared with the conventional thermal decomposition technique, the size of the growth domain by this method is substantially increased and the thickness variation is reduced. Based on the morphology of epitaxial graphene, the role of hydrogen is revealed. It is found that hydrogen acts as a carbon etchant. It suppresses the defect formation and nucleation of graphene. It also improves the kinetics of carbon atoms via hydrocarbon species. These effects lead to increase of the domain size and the structure quality. The consequent capping effect results in smooth surface morphology and suppression of multilayer growth. Our method provides a viable route to fine tune the growth kinetics of epitaxial graphene on SiC.

  10. Anisotropic diffusion of oxygen on a few layers of black phosphorous

    NASA Astrophysics Data System (ADS)

    Fernández-Escamilla, Hector Noe; González-Chávez, Víctor Hugo; Martínez-Guerra, Eduardo; Garay-Tapia, Andrés; Martínez-Guerra, Edgar

    Recently, phosporene has also been scored well as a functional material for two-dimensional electronic and optoelectronic devices. That is, because in contrast to graphene, black phosphorous has an inherent, direct and appreciable band gap that can be modulated with the numbers of layes. However, the presence of exposed lone pairs at the surface makes phosphorous very reactive to air and humidity and consequently, degradation of its properties. No such fundamental explanation have been made, thus corresponding first principle predictions to evaluate diffusion of O over and along a mono- and a few layers are indispensable. Energy barriers and the mechanisms of oxygen diffusion on mono- and a few layer of black phosphorous were calculated using the NEB(Nudge Elastic band) method as implemented in Quantum Espresso. The electronic states are expanded in plane waves with kinetic-energy cutoffs of 25 and 200 Ry for the wave function and charge density, respectively. Also, as the H2O and O2 are polar molecules, spin-polarized calculations have been carried out. We evaluated the diffusion barriers for O2 and H2O on phosphorene along zigzag, armchair and intermediated directions. Our calculations show that diffusion of O is preferred on zigzag directions and dissociation of O2 is favored as a result of energy gains of about 2 eV. Also, apparently diffusion pathways are blocked along layers.

  11. Effects of surface impurities on epitaxial graphene growth

    NASA Astrophysics Data System (ADS)

    del Campo, Valeria; Henríquez, Ricardo; Häberle, Patricio

    2013-01-01

    The focus of this report is to explore the large scale growth of graphene on Ru(0 0 0 1) and verify the possible effects of crystallographic defects and impurities in the quality of the synthesized material. After a Low Pressure Chemical Vapor Deposition (LP-CVD) process we obtained a graphene film accompanied by other types of graphitic structures. Impurities on the ruthenium surface behaved as nucleation sites in the formation of carbon islands several micrometers wide. The morphological structure of these islands is constituted by carbon discs with diameters in the range of few to several hundred nanometers and thicknesses always below 1 nm.

  12. Tailoring low-dimensional structures of bismuth on monolayer epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Chen, H.-H.; Su, S. H.; Chang, S.-L.; Cheng, B.-Y.; Chen, S. W.; Chen, H.-Y.; Lin, M.-F.; Huang, J. C. A.

    2015-06-01

    To improve graphene-based multifunctional devices at nanoscale, a stepwise and controllable fabrication procedure must be elucidated. Here, a series of structural transition of bismuth (Bi) adatoms, adsorbed on monolayer epitaxial graphene (MEG), is explored at room temperature. Bi adatoms undergo a structural transition from one-dimensional (1D) linear structures to two-dimensional (2D) triangular islands and such 2D growth mode is affected by the corrugated substrate. Upon Bi deposition, a little charge transfer occurs and a characteristic peak can be observed in the tunneling spectrum, reflecting the distinctive electronic structure of the Bi adatoms. When annealed to ~500 K, 2D triangular Bi islands aggregate into Bi nanoclusters (NCs) of uniform size. A well-controlled fabrication method is thus demonstrated. The approaches adopted herein provide perspectives for fabricating and characterizing periodic networks on MEG and related systems, which are useful in realizing graphene-based electronic, energy, sensor and spintronic devices.

  13. Tailoring low-dimensional structures of bismuth on monolayer epitaxial graphene

    PubMed Central

    Chen, H.-H.; Su, S. H.; Chang, S.-L.; Cheng, B.-Y.; Chen, S. W.; Chen, H.-Y.; Lin, M.-F.; Huang, J. C. A.

    2015-01-01

    To improve graphene-based multifunctional devices at nanoscale, a stepwise and controllable fabrication procedure must be elucidated. Here, a series of structural transition of bismuth (Bi) adatoms, adsorbed on monolayer epitaxial graphene (MEG), is explored at room temperature. Bi adatoms undergo a structural transition from one-dimensional (1D) linear structures to two-dimensional (2D) triangular islands and such 2D growth mode is affected by the corrugated substrate. Upon Bi deposition, a little charge transfer occurs and a characteristic peak can be observed in the tunneling spectrum, reflecting the distinctive electronic structure of the Bi adatoms. When annealed to ~500 K, 2D triangular Bi islands aggregate into Bi nanoclusters (NCs) of uniform size. A well-controlled fabrication method is thus demonstrated. The approaches adopted herein provide perspectives for fabricating and characterizing periodic networks on MEG and related systems, which are useful in realizing graphene-based electronic, energy, sensor and spintronic devices. PMID:26100604

  14. Theory of the Growth of Epitaxial Graphene on Close-Packed Metals

    NASA Astrophysics Data System (ADS)

    Zangwill, Andrew; Vvedensky, Dimitri

    2011-03-01

    We present a simple rate theory of epitaxial graphene growth on close-packed metals. Motivated by recent low-energy electron microscopy experiments [E. Loginova, N.C.Bartelt, P.J. Feibelman, and K.F. McCarty, New Journal of Physics, 10, 093026 (2008)], our theory supposes that graphene islands grow predominantly by the addition of five-atom clusters, rather than solely by the capture of diffusing carbon atoms. With suitably chosen kinetic parameters, we find quantitative agreement with (i) the measured time-evolution of the adatom density and (ii) the measured temperature-dependence of the adatom density at the onset of nucleation by assuming that the smallest stable precursor to graphene growth is an immobile island composed of six five-atom clusters.

  15. Evidence for Charge Transfer at the Interface between Hybrid Phosphomolybdate and Epitaxial Graphene.

    PubMed

    Huder, Loïc; Rinfray, Corentin; Rouchon, Denis; Benayad, Anass; Baraket, Mira; Izzet, Guillaume; Lipp-Bregolin, Felipe; Lapertot, Gérard; Dubois, Lionel; Proust, Anna; Jansen, Louis; Duclairoir, Florence

    2016-05-17

    The interfacing of polyoxometalates and graphene can be considered to be an innovative way to generate hybrid structures that take advantage of the properties of both components. Polyoxometalates are redox-sensitive and photosensitive compounds with high temperature stability (up to 400 °C for some), showing tunable properties depending on the metal incorporated inside the complex. Graphene has a unique electronic band structure combined with good material properties for electrical and optical applications. The spontaneous, rather than electrochemical, functionalization of epitaxial graphene on SiC with Keggin phosphomolybdate derivative TBA3[PMo11O39{Sn(C6H4)C≡C(C6H4)N2}] (named K(Mo)Sn[N2(+)]) bearing a phenyl diazonium unit is investigated. Graphene decoration is evidenced by means of AFM, Raman, XPS, and cyclic voltammetry, indicating a successful immobilization of the polyoxomolybdate. The covalent bonding of the polyoxometalate to the graphene substrate can be deduced from the appearance of a D band in the Raman spectra and from the loss of mobility in the electrical conduction. High-resolution XPS spectra reveal an electron transfer from the graphene to the Mo complex. The comparison of charge-carrier density measurements before and after grafting supports the p-type doping effect, which is further evidenced by work function UPS measurements. PMID:27118296

  16. Structural consequences of hydrogen intercalation of epitaxial graphene on SiC(0001)

    SciTech Connect

    Emery, Jonathan D. E-mail: bedzyk@northwestern.edu; Johns, James E.; McBriarty, Martin E.; Hersam, Mark C.; Wheeler, Virginia H.; Kurt Gaskill, D.; Detlefs, Blanka; Bedzyk, Michael J. E-mail: bedzyk@northwestern.edu

    2014-10-20

    The intercalation of various atomic species, such as hydrogen, to the interface between epitaxial graphene (EG) and its SiC substrate is known to significantly influence the electronic properties of the graphene overlayers. Here, we use high-resolution X-ray reflectivity to investigate the structural consequences of the hydrogen intercalation process used in the formation of quasi-free-standing (QFS) EG/SiC(0001). We confirm that the interfacial layer is converted to a layer structurally indistinguishable from that of the overlying graphene layers. This newly formed graphene layer becomes decoupled from the SiC substrate and, along with the other graphene layers within the film, is vertically displaced by ∼2.1 Å. The number of total carbon layers is conserved during the process, and we observe no other structural changes such as interlayer intercalation or expansion of the graphene d-spacing. These results clarify the under-determined structure of hydrogen intercalated QFS-EG/SiC(0001) and provide a precise model to inform further fundamental and practical understanding of the system.

  17. Wafer-scale epitaxial graphene on SiC for sensing applications

    NASA Astrophysics Data System (ADS)

    Karlsson, Mikael; Wang, Qin; Zhao, Yichen; Zhao, Wei; Toprak, Muhammet S.; Iakimov, Tihomir; Ali, Amer; Yakimova, Rositza; Syväjärvi, Mikael; Ivanov, Ivan G.

    2015-12-01

    The epitaxial graphene-on-silicon carbide (SiC-G) has advantages of high quality and large area coverage owing to a natural interface between graphene and SiC substrate with dimension up to 100 mm. It enables cost effective and reliable solutions for bridging the graphene-based sensors/devices from lab to industrial applications and commercialization. In this work, the structural, optical and electrical properties of wafer-scale graphene grown on 2'' 4H semi-insulating (SI) SiC utilizing sublimation process were systemically investigated with focus on evaluation of the graphene's uniformity across the wafer. As proof of concept, two types of glucose sensors based on SiC-G/Nafion/Glucose-oxidase (GOx) and SiC-G/Nafion/Chitosan/GOx were fabricated and their electrochemical properties were characterized by cyclic voltammetry (CV) measurements. In addition, a few similar glucose sensors based on graphene by chemical synthesis using modified Hummer's method were also fabricated for comparison.

  18. Approach to multifunctional device platform with epitaxial graphene on transition metal oxide

    PubMed Central

    Park, Jeongho; Back, Tyson; Mitchel, William C.; Kim, Steve S.; Elhamri, Said; Boeckl, John; Fairchild, Steven B.; Naik, Rajesh; Voevodin, Andrey A.

    2015-01-01

    Heterostructures consisting of two-dimensional materials have shown new physical phenomena, novel electronic and optical properties, and new device concepts not observed in bulk material systems or purely three dimensional heterostructures. These new effects originated mostly from the van der Waals interaction between the different layers. Here we report that a new optical and electronic device platform can be provided by heterostructures of 2D graphene with a metal oxide (TiO2). Our novel direct synthesis of graphene/TiO2 heterostructure is achieved by C60 deposition on transition Ti metal surface using a molecular beam epitaxy approach and O2 intercalation method, which is compatible with wafer scale growth of heterostructures. As-grown heterostructures exhibit inherent photosensitivity in the visible light spectrum with high photo responsivity. The photo sensitivity is 25 times higher than that of reported graphene photo detectors. The improved responsivity is attributed to optical transitions between O 2p orbitals in the valence band of TiO2 and C 2p orbitals in the conduction band of graphene enabled by Coulomb interactions at the interface. In addition, this heterostructure provides a platform for realization of bottom gated graphene field effect devices with graphene and TiO2 playing the roles of channel and gate dielectric layers, respectively. PMID:26395160

  19. Atomically Sharp Interface in an h-BN-epitaxial graphene van der Waals Heterostructure

    PubMed Central

    Sediri, Haikel; Pierucci, Debora; Hajlaoui, Mahdi; Henck, Hugo; Patriarche, Gilles; Dappe, Yannick J.; Yuan, Sheng; Toury, Bérangère; Belkhou, Rachid; Silly, Mathieu G.; Sirotti, Fausto; Boutchich, Mohamed; Ouerghi, Abdelkarim

    2015-01-01

    Stacking various two-dimensional atomic crystals is a feasible approach to creating unique multilayered van der Waals heterostructures with tailored properties. Herein for the first time, we present a controlled preparation of large-area h-BN/graphene heterostructures via a simple chemical deposition of h-BN layers on epitaxial graphene/SiC(0001). Van der Waals forces, which are responsible for the cohesion of the multilayer system, give rise to an abrupt interface without interdiffusion between graphene and h-BN, as shown by X-ray Photoemission Spectroscopy (XPS) and direct observation using scanning and High-Resolution Transmission Electron Microscopy (STEM/HRTEM). The electronic properties of graphene, such as the Dirac cone, remain intact and no significant charge transfer i.e. doping, is observed. These results are supported by Density Functional Theory (DFT) calculations. We demonstrate that the h-BN capped graphene allows the fabrication of vdW heterostructures without altering the electronic properties of graphene. PMID:26585245

  20. Plasma-Modified, Epitaxial Fabricated Graphene on SiC for the Electrochemical Detection of TNT.

    PubMed

    Trammell, Scott A; Hernández, Sandra C; Myers-Ward, Rachael L; Zabetakis, Daniel; Stenger, David A; Gaskill, D Kurt; Walton, Scott G

    2016-01-01

    Using square wave voltammetry, we show an increase in the electrochemical detection of trinitrotoluene (TNT) with a working electrode constructed from plasma modified graphene on a SiC surface vs. unmodified graphene. The graphene surface was chemically modified using electron beam generated plasmas produced in oxygen or nitrogen containing backgrounds to introduce oxygen or nitrogen moieties. The use of this chemical modification route enabled enhancement of the electrochemical signal for TNT, with the oxygen treatment showing a more pronounced detection than the nitrogen treatment. For graphene modified with oxygen, the electrochemical response to TNT can be fit to a two-site Langmuir isotherm suggesting different sites on the graphene surface with different affinities for TNT. We estimate a limit of detection for TNT equal to 20 ppb based on the analytical standard S/N ratio of 3. In addition, this approach to sensor fabrication is inherently a high-throughput, high-volume process amenable to industrial applications. High quality epitaxial graphene is easily grown over large area SiC substrates, while plasma processing is a rapid approach to large area substrate processing. This combination facilitates low cost, mass production of sensors. PMID:27529251

  1. Transition metal dichalcogenides and beyond: synthesis, properties, and applications of single- and few-layer nanosheets.

    PubMed

    Lv, Ruitao; Robinson, Joshua A; Schaak, Raymond E; Sun, Du; Sun, Yifan; Mallouk, Thomas E; Terrones, Mauricio

    2015-01-20

    CONSPECTUS: In the wake of the discovery of the remarkable electronic and physical properties of graphene, a vibrant research area on two-dimensional (2D) layered materials has emerged during the past decade. Transition metal dichalcogenides (TMDs) represent an alternative group of 2D layered materials that differ from the semimetallic character of graphene. They exhibit diverse properties that depend on their composition and can be semiconductors (e.g., MoS2, WS2), semimetals (e.g., WTe2, TiSe2), true metals (e.g., NbS2, VSe2), and superconductors (e.g., NbSe2, TaS2). The properties of TMDs can also be tailored according to the crystalline structure and the number and stacking sequence of layers in their crystals and thin films. For example, 2H-MoS2 is semiconducting, whereas 1T-MoS2 is metallic. Bulk 2H-MoS2 possesses an indirect band gap, but when 2H-MoS2 is exfoliated into monolayers, it exhibits direct electronic and optical band gaps, which leads to enhanced photoluminescence. Therefore, it is important to learn to control the growth of 2D TMD structures in order to exploit their properties in energy conversion and storage, catalysis, sensing, memory devices, and other applications. In this Account, we first introduce the history and structural basics of TMDs. We then briefly introduce the Raman fingerprints of TMDs of different layer numbers. Then, we summarize our progress on the controlled synthesis of 2D layered materials using wet chemical approaches, chemical exfoliation, and chemical vapor deposition (CVD). It is now possible to control the number of layers when synthesizing these materials, and novel van der Waals heterostructures (e.g., MoS2/graphene, WSe2/graphene, hBN/graphene) have recently been successfully assembled. Finally, the unique optical, electrical, photovoltaic, and catalytic properties of few-layered TMDs are summarized and discussed. In particular, their enhanced photoluminescence (PL), photosensing, photovoltaic conversion, and

  2. Raman scattering of few-layers MoTe2

    NASA Astrophysics Data System (ADS)

    Grzeszczyk, M.; Gołasa, K.; Zinkiewicz, M.; Nogajewski, K.; Molas, M. R.; Potemski, M.; Wysmołek, A.; Babiński, A.

    2016-06-01

    We report on room-temperature Raman scattering measurements in few-layer crystals of exfoliated molybdenum ditelluride (MoTe2) performed with the use of 632.8 nm (1.96 eV) laser light excitation. In agreement with a recent study reported by Froehlicher et al (2015 Nano Lett. 15 6481) we observe a complex structure of the out-of-plane vibrational modes ({{{A}}}1{{g}}{/{{A}}}1\\prime ), which can be explained in terms of interlayer interactions between single atomic planes of MoTe2. In the case of low-energy shear and breathing modes of rigid interlayer vibrations, it is shown that their energy evolution with the number of layers can be well reproduced within a linear chain model with only the nearest neighbor interaction taken into account. Based on this model the corresponding in-plane and out-of-plane force constants are determined. We also show that the Raman scattering in MoTe2 measured using 514.5 nm (2.41 eV) laser light excitation results in much simpler spectra. We argue that the rich structure of the out-of-plane vibrational modes observed in Raman scattering spectra excited with the use of 632.8 nm laser light results from its resonance with the electronic transition at the M point of the MoTe2 first Brillouin zone.

  3. Colossal Ultraviolet Photoresponsivity of Few-Layer Black Phosphorus.

    PubMed

    Wu, Jing; Koon, Gavin Kok Wai; Xiang, Du; Han, Cheng; Toh, Chee Tat; Kulkarni, Eeshan S; Verzhbitskiy, Ivan; Carvalho, Alexandra; Rodin, Aleksandr S; Koenig, Steven P; Eda, Goki; Chen, Wei; Neto, A H Castro; Özyilmaz, Barbaros

    2015-08-25

    Black phosphorus has an orthorhombic layered structure with a layer-dependent direct band gap from monolayer to bulk, making this material an emerging material for photodetection. Inspired by this and the recent excitement over this material, we studied the optoelectronics characteristics of high-quality, few-layer black phosphorus-based photodetectors over a wide spectrum ranging from near-ultraviolet (UV) to near-infrared (NIR). It is demonstrated for the first time that black phosphorus can be configured as an excellent UV photodetector with a specific detectivity ∼3 × 10(13) Jones. More critically, we found that the UV photoresponsivity can be significantly enhanced to ∼9 × 10(4) A W(-1) by applying a source-drain bias (VSD) of 3 V, which is the highest ever measured in any 2D material and 10(7) times higher than the previously reported value for black phosphorus. We attribute such a colossal UV photoresponsivity to the resonant-interband transition between two specially nested valence and conduction bands. These nested bands provide an unusually high density of states for highly efficient UV absorption due to the singularity of their nature. PMID:26207324

  4. Charge trap memory based on few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Feng, Qi; Yan, Faguang; Luo, Wengang; Wang, Kaiyou

    2016-01-01

    Atomically thin layered two-dimensional materials, including transition-metal dichalcogenide (TMDC) and black phosphorus (BP), have been receiving much attention, because of their promising physical properties and potential applications in flexible and transparent electronic devices. Here, for the first time we show nonvolatile charge-trap memory devices, based on field-effect transistors with large hysteresis, consisting of a few-layer black phosphorus channel and a three dimensional (3D) Al2O3/HfO2/Al2O3 charge-trap gate stack. An unprecedented memory window exceeding 12 V is observed, due to the extraordinary trapping ability of the high-k HfO2. The device shows a high endurance of over 120 cycles and a stable retention of ~30% charge loss after 10 years, even lower than the reported MoS2 flash memory. The high program/erase current ratio, large memory window, stable retention and high on/off current ratio, provide a promising route towards flexible and transparent memory devices utilising atomically thin two-dimensional materials. The combination of 2D materials with traditional high-k charge-trap gate stacks opens up an exciting field of nonvolatile memory devices.

  5. Charge trap memory based on few-layer black phosphorus.

    PubMed

    Feng, Qi; Yan, Faguang; Luo, Wengang; Wang, Kaiyou

    2016-02-01

    Atomically thin layered two-dimensional materials, including transition-metal dichalcogenide (TMDC) and black phosphorus (BP), have been receiving much attention, because of their promising physical properties and potential applications in flexible and transparent electronic devices. Here, for the first time we show nonvolatile charge-trap memory devices, based on field-effect transistors with large hysteresis, consisting of a few-layer black phosphorus channel and a three dimensional (3D) Al2O3/HfO2/Al2O3 charge-trap gate stack. An unprecedented memory window exceeding 12 V is observed, due to the extraordinary trapping ability of the high-k HfO2. The device shows a high endurance of over 120 cycles and a stable retention of ∼30% charge loss after 10 years, even lower than the reported MoS2 flash memory. The high program/erase current ratio, large memory window, stable retention and high on/off current ratio, provide a promising route towards flexible and transparent memory devices utilising atomically thin two-dimensional materials. The combination of 2D materials with traditional high-k charge-trap gate stacks opens up an exciting field of nonvolatile memory devices. PMID:26758336

  6. Spin transport in epitaxial graphene on the C-terminated ( 000 1 ¯ )-face of silicon carbide

    NASA Astrophysics Data System (ADS)

    van den Berg, J. J.; Yakimova, R.; van Wees, B. J.

    2016-07-01

    We performed a temperature dependent study of the charge and spin transport properties of epitaxial graphene on the C-terminated ( 000 1 ¯ ) face of silicon carbide (SiC), a system without a carbon buffer layer between the graphene and the SiC. Using spin Hanle precession in the nonlocal geometry, we measured a spin relaxation length of λS = 0.7 μm at room temperature, lower than in exfoliated graphene. We show that the charge and spin diffusion coefficient, DC and DS, respectively, increasingly deviate from each other during electrical measurements up to a difference of a factor 4. Thus, we show that a model of localized states that was previously used to explain DC ≠ DS, can also be applied to epitaxial graphene systems without a carbon buffer layer. We attribute the effect to charge trap states in the interface between the graphene and the SiC.

  7. X-ray radiation effects in multilayer epitaxial graphene

    SciTech Connect

    Hicks, Jeremy; Tinkey, Holly; Hankinson, John; Heer, Walt A. de; Conrad, Edward H.; Arora, Rajan; Kenyon, Eleazar; Chakraborty, Partha S.; Cressler, John D.; Berger, Claire

    2011-12-05

    We characterize multilayer graphene grown on C-face SiC before and after exposure to a total ionizing dose of 12 Mrad(SiO{sub 2}) using a 10 keV x-ray source. While we observe the partial peeling of the top graphene layers and the appearance of a modest Raman D-peak, we find that the electrical characteristics (mobility, sheet resistivity, free carrier concentration) of the material are mostly unaffected by radiation exposure. Combined with x-ray photoelectron spectroscopy data showing numerous carbon-oxygen bonds after irradiation, we conclude that the primary damage mechanism is through surface etching from reactive oxygen species created by the x-rays.

  8. Low carrier density epitaxial graphene devices on SiC.

    PubMed

    Yang, Yanfei; Huang, Lung-I; Fukuyama, Yasuhiro; Liu, Fan-Hung; Real, Mariano A; Barbara, Paola; Liang, Chi-Te; Newell, David B; Elmquist, Randolph E

    2015-01-01

    The transport characteristics of graphene devices with low n- or p-type carrier density (∼10(10) -10(11) cm(-2) ), fabricated using a new process that results in minimal organic surface residues, are reported. The p-type molecular doping responsible for the low carrier densities is initiated by aqua regia. The resulting devices exhibit highly developed ν = 2 quantized Hall resistance plateaus at magnetic field strengths of less than 4 T. PMID:25136792

  9. Rapid epitaxy-free graphene synthesis on silicidated polycrystalline platinum

    PubMed Central

    Babenko, Vitaliy; Murdock, Adrian T.; Koós, Antal A.; Britton, Jude; Crossley, Alison; Holdway, Philip; Moffat, Jonathan; Huang, Jian; Alexander-Webber, Jack A.; Nicholas, Robin J.; Grobert, Nicole

    2015-01-01

    Large-area synthesis of high-quality graphene by chemical vapour deposition on metallic substrates requires polishing or substrate grain enlargement followed by a lengthy growth period. Here we demonstrate a novel substrate processing method for facile synthesis of mm-sized, single-crystal graphene by coating polycrystalline platinum foils with a silicon-containing film. The film reacts with platinum on heating, resulting in the formation of a liquid platinum silicide layer that screens the platinum lattice and fills topographic defects. This reduces the dependence on the surface properties of the catalytic substrate, improving the crystallinity, uniformity and size of graphene domains. At elevated temperatures growth rates of more than an order of magnitude higher (120 μm min−1) than typically reported are achieved, allowing savings in costs for consumable materials, energy and time. This generic technique paves the way for using a whole new range of eutectic substrates for the large-area synthesis of 2D materials. PMID:26175062

  10. Rb and Cs deposition on epitaxial graphene grown on 6H -SiC(0001)

    NASA Astrophysics Data System (ADS)

    Watcharinyanon, S.; Virojanadara, C.; Johansson, L. I.

    2011-11-01

    Epitaxial graphene grown on the silicon-terminated SiC(0001) is doped by alkali metals deposited on the surface. The synchrotron radiation based photoelectron spectroscopy results presented reveal that Rb and Cs deposited on monolayer graphene samples, grown on the silicon-terminated SiC(0001), gives rise to n-type doping, i.e. electron transfer from the metal to the graphene layer. The Dirac point of the single π-cone is found to shift downwards away from the Fermi level by ca. 1.0 eV after alkali metal depositions. The adsorbed Rb and Cs atoms do initially disrupt the bonds in the graphene layer but after heating the bonds appear to be recreated when the alkali metal coverage start to decrease due to thermal desorption. None of these two alkali metals do intercalate into the graphene and buffer layer after depositing at room temperature or after heating. This is contrary to the case of Li where intercalation occurred directly after deposition.

  11. Incorporating isolated molybdenum (Mo) atoms into Bilayer Epitaxial Graphene on 4H-SiC(0001)

    NASA Astrophysics Data System (ADS)

    Huang, Han; Wan, Wen; Li, Hui; Wong, Swee Liang; Lv, Lu; Gao, Yongli; Wee, Andrew T. S.

    2014-03-01

    The atomic structures and electronic properties of isolated Mo atoms in bilayer epitaxial graphene (BLEG) on 4H-SiC(0001) are investigated by low temperature scanning tunneling microscopy (LT-STM). LT-STM results reveal that isolated Mo dopants prefer to substitute C atoms at α-sites, and preferentially locate between the graphene bilayers. First-principles calculations confirm that the embedding of single Mo dopants within BLEG is energetically favorable as compared to monolayer graphene. The calculated bandstructures show that Mo-doped BLEG is n-doped, and each Mo atom introduces a local magnetic moment of 1.81 μB. Our findings demonstrate a simple and stable method to incorporate single transition metal dopants into the graphene lattice to tune its electronic and magnetic properties for possible use in graphene spin devices. NRF-CRP (Singapore) grants R-143-000-360-281and R-144-000-295-281. ``Shenghua Professorship'' startup funding from CSU and the support from the NSF of China (Grant No.11304398).

  12. Interface structure for growth of epitaxial graphene on SiC(0001)

    NASA Astrophysics Data System (ADS)

    Rhim, S. H.; Sun, G.; Li, L.; Weinert, M.

    2009-03-01

    In spite of the enormous effort devoted to the study of the epitaxial growth of graphene on SiC, there is not yet a consensus regarding the structure of the interface between graphene and the substrate. There have been a long standing discrepancy between low energy electron diffraction (LEED) and STM patterns regarding the periodicity of graphene on SiC(0001); the theoretical studies of the of 6√3x6√3 ootnotetextS. Kim, J. Ihm, H. J. Choi, and Y. W. Son, Phys. Rev. Lett. 100, 176802 (2008). or √3x√3 ootnotetextF. Varchon et al, Phys. Rev. Lett. 99, 126805 (2007); A. Mattausch and O. Pankratov, Phys. Rev. Lett. 99, 076802 (2007 periodicity, while describing some aspects, disagree in important details with scanning tunneling microscopy (STM) images. We present a combined theoretical and experimental study, employing density functional calculations and STM, to investigate this issue. We propose the formation of a defected graphene layer at the interface, and then subsequent growth of graphene. The calculated bias-dependent STM images are in good agreement with our STM images, and provide insight into the details of the interface structure.

  13. Voltage Scaling of Graphene Device on SrTiO3 Epitaxial Thin Film.

    PubMed

    Park, Jeongmin; Kang, Haeyong; Kang, Kyeong Tae; Yun, Yoojoo; Lee, Young Hee; Choi, Woo Seok; Suh, Dongseok

    2016-03-01

    Electrical transport in monolayer graphene on SrTiO3 (STO) thin film is examined in order to promote gate-voltage scaling using a high-k dielectric material. The atomically flat surface of thin STO layer epitaxially grown on Nb-doped STO single-crystal substrate offers good adhesion between the high-k film and graphene, resulting in nonhysteretic conductance as a function of gate voltage at all temperatures down to 2 K. The two-terminal conductance quantization under magnetic fields corresponding to quantum Hall states survives up to 200 K at a magnetic field of 14 T. In addition, the substantial shift of charge neutrality point in graphene seems to correlate with the temperature-dependent dielectric constant of the STO thin film, and its effective dielectric properties could be deduced from the universality of quantum phenomena in graphene. Our experimental data prove that the operating voltage reduction can be successfully realized due to the underlying high-k STO thin film, without any noticeable degradation of graphene device performance. PMID:26855043

  14. The metastable chemical gallery of the oxide of epitaxial graphene at room temperature

    NASA Astrophysics Data System (ADS)

    Kim, Suenne; Zhou, Si; Hu, Yike; Berger, Claire; de Heer, Walt; Bongiorno, Angelo; Riedo, Elisa

    2013-03-01

    Insights in the chemistry of graphene oxide and its response to external stimuli are crucial to control its electronic and optical properties, thus enabling future applications of this material. Here, we present a combined experimental and density functional theory study concerning the compositional and structural properties of the oxide of epitaxial graphene (OeG) as a function of time and temperature. Our result indicates that OeG synthesized by oxidizing epitaxial graphene grown on SiC via the Hummers method is a metastable material whose structure and chemistry evolve with a notable degree at room temperature. XPS studies reveal, metastable OeG reaches a nearly stable reduced O/C ratio of 0.37 with a featured relaxation time of a month. Initially the most enriched epoxide groups decrease with time while hydroxyl groups increase. In addition to this, further XPS study of OeG as a function of temperature shows heating above 120 C in air can abruptly deteriorate the OeG structure. Our calculations show that the availability of hydrogen atoms could be the key factor in tuning structural and chemical properties at relatively low temperatures. National Science Foundation (CMMI-1100290/DMR-0820382)

  15. Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene.

    PubMed

    Liu, Xiaolong; Balla, Itamar; Bergeron, Hadallia; Campbell, Gavin P; Bedzyk, Michael J; Hersam, Mark C

    2016-01-26

    Atomically thin MoS2/graphene heterostructures are promising candidates for nanoelectronic and optoelectronic technologies. Among different graphene substrates, epitaxial graphene (EG) on SiC provides several potential advantages for such heterostructures, including high electronic quality, tunable substrate coupling, wafer-scale processability, and crystalline ordering that can template commensurate growth. Exploiting these attributes, we demonstrate here the thickness-controlled van der Waals epitaxial growth of MoS2 on EG via chemical vapor deposition, giving rise to transfer-free synthesis of a two-dimensional heterostructure with registry between its constituent materials. The rotational commensurability observed between the MoS2 and EG is driven by the energetically favorable alignment of their respective lattices and results in nearly strain-free MoS2, as evidenced by synchrotron X-ray scattering and atomic-resolution scanning tunneling microscopy (STM). The electronic nature of the MoS2/EG heterostructure is elucidated with STM and scanning tunneling spectroscopy, which reveals bias-dependent apparent thickness, band bending, and a reduced band gap of ∼0.4 eV at the monolayer MoS2 edges. PMID:26565112

  16. Band Engineering and Magnetic Doping of Epitaxial Graphene on SiC (0001)

    NASA Astrophysics Data System (ADS)

    Kong, Byoung Don; Jayasekera, Thushari; Kim, Ki Wook; Buongiorno Nardelli, M.

    2010-03-01

    Advances in the epitaxial growth of graphene films on SiC have the potential to open new classes of device applications that may revolutionize the semiconductor roadmap for future decades. However, this progress will require an in-depth understanding and utilization of the electronic processes that take place at the nanoscale, in particular the role of the interface buffer layer, where most of the electronic properties of the system can be controlled. In analogy with the formation of the Schottky barrier in metal-insulator interfaces (the energetic barrier the electrons have to overcome to go from the valence band of the metal to the conduction band of the insulator) here we demonstrate the ability to tune and control the band alignment and the magnetic doping at the heterojunction between graphene and SiC, a fundamental requirement for improving device efficiency and applicability. Using first principles calculations, we will show how the surface electrostatic distribution can be used to tune the valence band offset by introducing surface impurities such as B, Al, N, and P. Similarly, we will demonstrate how the introduction of magnetic impurities in the buffer layer can tune the spintronic behavior of the epitaxial graphene layer. This work was supported, in part, by the NERC/NIST SWAN-NRI and the DARPA/HRL CERA programs.

  17. Simulating structural transitions with kinetic Monte Carlo: The case of epitaxial graphene on SiC

    NASA Astrophysics Data System (ADS)

    Deretzis, I.; La Magna, A.

    2016-03-01

    We have developed a kinetic Monte Carlo numerical scheme, specifically suited to simulate structural transitions in crystalline materials, and implemented it for the case of epitaxial graphene on SiC. In this process, surface Si atoms selectively sublimate, while the residual C atoms rearrange from a position occupied in the SiC hexagonal lattice to the graphene honeycomb structure, modifying their hybridization (from s p3 to s p2 ) and bond partners (from Si-C to C-C). The model is based on the assumption that the Monte Carlo particles follow the evolution of their reference crystal until they experience a thermally activated reversible transition to another crystal structure. We demonstrate that, in a formulation based on three parallel lattices, the method is able to recover the complex evolution steps of epitaxial graphene on SiC. Moreover, the simulation results are in noteworthy agreement with the overall experimental scenario, both when varying the structural properties of the material (e.g., the initial surface configuration or polarity) as well as the process conditions (e.g., the temperature and pressure).

  18. Thickness determination of few-layer hexagonal boron nitride films by scanning electron microscopy and Auger electron spectroscopy

    SciTech Connect

    Sutter, P. Sutter, E.

    2014-09-01

    We assess scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) for thickness measurements on few-layer hexagonal boron nitride (h-BN), the layered dielectric of choice for integration with graphene and other two-dimensional materials. Observations on h-BN islands with large, atomically flat terraces show that the secondary electron intensity in SEM reflects monolayer height changes in films up to least 10 atomic layers thickness. From a quantitative analysis of AES data, the energy-dependent electron escape depth in h-BN films is deduced. The results show that AES is suitable for absolute thickness measurements of few-layer h-BN of 1 to 6 layers.

  19. Role of silicon dangling bonds in the electronic properties of epitaxial graphene on silicon carbide.

    PubMed

    Ridene, Mohamed; Kha, Calvin S; Flipse, Cees F J

    2016-03-29

    In this paper, we study the electronic properties of epitaxial graphene (EG) on silicon carbide by means of ab initio calculations based on the local spin density approximation + U method taking into account the Coulomb interaction between Si localized electrons. We show that this interaction is not completely suppressed but is screened by carbon layers grown on-top of silicon carbide. This finding leads to a good qualitative understanding of the experimental results reported on EG on silicon carbide. Our results highlight the presence of the Si localized states and might explain the anomalous Hanle curve and the high values of spin relaxation time in EG. PMID:26891244

  20. Role of silicon dangling bonds in the electronic properties of epitaxial graphene on silicon carbide

    NASA Astrophysics Data System (ADS)

    Ridene, Mohamed; Kha, Calvin S.; Flipse, Cees F. J.

    2016-03-01

    In this paper, we study the electronic properties of epitaxial graphene (EG) on silicon carbide by means of ab initio calculations based on the local spin density approximation + U method taking into account the Coulomb interaction between Si localized electrons. We show that this interaction is not completely suppressed but is screened by carbon layers grown on-top of silicon carbide. This finding leads to a good qualitative understanding of the experimental results reported on EG on silicon carbide. Our results highlight the presence of the Si localized states and might explain the anomalous Hanle curve and the high values of spin relaxation time in EG.

  1. SU(4) symmetry breaking revealed by magneto-optical spectroscopy in epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Tan, Liang Z.; Orlita, Milan; Potemski, Marek; Palmer, James; Berger, Claire; de Heer, Walter A.; Louie, Steven G.; Martinez, Gérard

    2015-06-01

    Refined infrared magnetotransmission experiments have been performed in magnetic fields B up to 35 T on a series of multilayer epitaxial graphene samples. Following the main optical transition involving the n =0 Landau level (LL), we observe a new absorption transition increasing in intensity with magnetic fields B ≥26 T. Our analysis shows that this is a signature of the breaking of the SU(4) symmetry of the n =0 LL. Using a quantitative model, we show that the only symmetry-breaking scheme consistent with our experiments is a charge density wave (CDW).

  2. Resistivity anisotropy measured using four probes in epitaxial graphene on silicon carbide

    NASA Astrophysics Data System (ADS)

    Kobayashi, Keisuke; Tanabe, Shinichi; Tao, Takuto; Okumura, Toshio; Nakashima, Takeshi; Aritsuki, Takuya; O, Ryong-Sok; Nagase, Masao

    2015-03-01

    The electronic transport of epitaxial graphene on silicon carbide is anisotropic because of the anisotropy of the surface structure of the substrate. In this Letter, we present a new method for measuring anisotropic transport based on the van der Pauw method. This method can measure anisotropic transport on the macroscopic scale without special equipment or device fabrication. We observe an anisotropic resistivity with a ratio of maximum to minimum of 1.62. The calculated maximum mobility is 2876 cm2·V-1·s-1, which is 1.43 times higher than that obtained by the standard van der Pauw method.

  3. Moiré induced organization of size-selected Pt clusters soft landed on epitaxial graphene

    PubMed Central

    Linas, Sébastien; Jean, Fabien; Zhou, Tao; Albin, Clément; Renaud, Gilles; Bardotti, Laurent; Tournus, Florent

    2015-01-01

    Two-dimensional hexagonal arrays of Pt nanoparticles (1.5 nm diameter) have been obtained by deposition of preformed and size selected Pt nanoparticles on graphene. This original self-organization is induced, at room temperature, by the 2D periodic undulation (the moiré pattern) of graphene epitaxially grown on the Ir(111) surface. By means of complementary techniques (scanning tunneling microscopy, grazing incidence X ray scattering), the Pt clusters shapes and organization are characterized and the structural evolution during annealing is investigated. The soft-landed clusters remain quasi-spherical and a large proportion appears to be pinned on specific moiré sites. The quantitative determination of the proportion of organized clusters reveals that the obtained hexagonal array of the almost spherical nanoparticles is stable up to 650 K, which is an indication of a strong cluster-surface interaction. PMID:26278787

  4. Influence of defects in SiC (0001) on epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Guo, Yu; Guo, Li-Wei; Lu, Wei; Huang, Jiao; Jia, Yu-Ping; Sun, Wei; Li, Zhi-Lin; Wang, Yi-Fei

    2014-08-01

    Defects in silicon carbide (SiC) substrate are crucial to the properties of the epitaxial graphene (EG) grown on it. Here we report the effect of defects in SiC on the crystalline quality of EGs through comparative studies of the characteristics of the EGs grown on SiC (0001) substrates with different defect densities. It is found that EGs on high quality SiC possess regular steps on the surface of the SiC and there is no discernible D peak in its Raman spectrum. Conversely, the EG on the SiC with a high density of defects has a strong D peak, irregular stepped morphology and poor uniformity in graphene layer numbers. It is the defects in the SiC that are responsible for the irregular stepped morphology and lead to the small domain size in the EG.

  5. Large-scale epitaxial growth kinetics of graphene: A kinetic Monte Carlo study

    SciTech Connect

    Jiang, Huijun; Hou, Zhonghuai

    2015-08-28

    Epitaxial growth via chemical vapor deposition is considered to be the most promising way towards synthesizing large area graphene with high quality. However, it remains a big theoretical challenge to reveal growth kinetics with atomically energetic and large-scale spatial information included. Here, we propose a minimal kinetic Monte Carlo model to address such an issue on an active catalyst surface with graphene/substrate lattice mismatch, which facilitates us to perform large scale simulations of the growth kinetics over two dimensional surface with growth fronts of complex shapes. A geometry-determined large-scale growth mechanism is revealed, where the rate-dominating event is found to be C{sub 1}-attachment for concave growth-front segments and C{sub 5}-attachment for others. This growth mechanism leads to an interesting time-resolved growth behavior which is well consistent with that observed in a recent scanning tunneling microscopy experiment.

  6. Adsorption and intercalation of Cs atoms on epitaxial graphene on Ir(111)

    NASA Astrophysics Data System (ADS)

    Lazic, Predrag; Petrovic, Marin; Srut, Iva; Pletikosic, Ivo; Milun, Milorad; Pervan, Petar; Runte, Sven; Busse, Carsten; Michely, Thomas; Sokcevic, Damir; Brako, Radovan; Atodiresei, Nicolae; Sadowski, Jurek; Pan, Zhi-Hu; Valla, Tonica; Kralj, Marko

    2013-03-01

    From the experimental studies of surface adsorption of Cs atoms and their intercalation under epitaxial graphene on Ir(111) it is known that both - adsorbed and intercalated phase of Cs atoms coexist. However, adsorbed phase is realized as a diluted superlattice adlayer of Cs atom while intercalated phase is a dense Cs layer. The preference for intercalated phase at large Cs layer densities can not be obtained from the DFT calculations with semilocal (GGA) functionals. Only after the van der Waals interaction is taken into account the agreement with experiment is achieved. From the results of calculations it follows that the main energy contribution responsible for the switching of preference from adsorption to intercalation is the graphene delamination energy from the Ir(111) surface which is dominantly of the van der Waals nature.

  7. Epitaxial growth of large-area bilayer graphene on Ru(0001)

    SciTech Connect

    Que, Yande; Xiao, Wende E-mail: hjgao@iphy.ac.cn; Fei, Xiangmin; Chen, Hui; Du, S. X.; Gao, H.-J. E-mail: hjgao@iphy.ac.cn; Huang, Li

    2014-03-03

    Large-area bilayer graphene (BG) is grown epitaxially on Ru(0001) surface and characterized by low temperature scanning tunneling microscopy. The lattice of the bottom layer of BG is stretched by 1.2%, while strain is absent from the top layer. The lattice mismatch between the two layers leads to the formation of a moiré pattern with a periodicity of ∼21.5 nm and a mixture of AA- and AB-stacking. The √3 × √3 superstructure around atomic defects is attributed to the inter-valley scattering of the delocalized π-electrons, demonstrating that the as-grown BG behaves like intrinsic free-standing graphene.

  8. Anomalous thermomagnetic effects in an epitaxial and irradiated graphene monolayer

    NASA Astrophysics Data System (ADS)

    Zhou, Xingfei; Xu, Yafang; Jin, Guojun

    2015-12-01

    We study valley-dependent anomalous thermomagnetic effects, including the Nernst and Ettingshausen effects, in a graphene monolayer that is subjected to a staggered sublattice potential and off-resonant circularly polarized light. It is found that a topological phase transition in this system can significantly affect the signs of the Nernst conductivity as well as the Ettingshausen thermal conductivity, which provides an alternative method to characterize the phase transition between band and topological insulators. At the topological phase-transition point, pure valley-polarized electric and heat currents are generated. In contrast to traditional thermomagnetism, an anomalous thermomagnetic figure of merit is formulated and used to characterize conversion efficiency. The theoretical approach, including numerical calculations and analytical treatment, can also be used to study the same properties of other graphenelike materials.

  9. Investigation of structural and electronic properties of epitaxial graphene on 3C–SiC(100)/Si(100) substrates

    PubMed Central

    Gogneau, Noelle; Ben Gouider Trabelsi, Amira; Silly, Mathieu G; Ridene, Mohamed; Portail, Marc; Michon, Adrien; Oueslati, Mehrezi; Belkhou, Rachid; Sirotti, Fausto; Ouerghi, Abdelkarim

    2014-01-01

    Graphene has been intensively studied in recent years in order to take advantage of its unique properties. Its synthesis on SiC substrates by solid-state graphitization appears a suitable option for graphene-based electronics. However, before developing devices based on epitaxial graphene, it is desirable to understand and finely control the synthesis of material with the most promising properties. To achieve these prerequisites, many studies are being conducted on various SiC substrates. Here, we review 3C–SiC(100) epilayers grown by chemical vapor deposition on Si(100) substrates for producing graphene by solid state graphitization under ultrahigh-vacuum conditions. Based on various characterization techniques, the structural and electrical properties of epitaxial graphene layer grown on 3C–SiC(100)/Si(100) are discussed. We establish that epitaxial graphene presents properties similar to those obtained using hexagonal SiC substrates, with the advantage of being compatible with current Si-processing technology. PMID:25339846

  10. Two Dimensional Epitaxial Water Adlayer on Mica with Graphene Coating: An ab Initio Molecular Dynamics Study.

    PubMed

    Li, Hui; Zeng, Xiao Cheng

    2012-09-11

    Motivated by a recent atomic-force-microscopy (AFM) study of water adlayers on mica by Heath and co-workers (Graphene Visualizes the First Water Adlayers on Mica at Ambient Conditions. Science2010, 329, 1188), we performed an ab initio molecular dynamics study of structural and dynamic properties of monolayer, bilayer, and trilayer water adlayers on the muscovite mica (001) surface with and without a graphene coating. We find that in the first epitaxial water adlayer, water molecules that form strong hydrogen bonds with the oxygen on the mica surface show little motions, thereby solid-like, while those "bridging" water molecules on top of the first water adlayer exhibit "itinerant" behavior, thereby liquid-like. Overall, the Born-Oppenheim molecular dynamics (BOMD) simulations (based on the BLYP-D functional) show that the first water adlayer on mica exhibits a unique hybrid solid-liquid-like behavior with a very low diffusion coefficient at ambient conditions. In particular, no dangling hydrogen bonds are found in the first water adlayer on mica. Moreover, the bilayer and trilayer water adlayers show slightly higher structural stability than the first water adlayer. A graphene coating on the water adlayer further enhances stability of the water adlayers. Most importantly, the bilayer water adlayer on mica with the graphene coating becomes fully solid-like, the structure of which is the same as the bilayer slice of ice-Ih with a thickness of 7.4 Å, consistent with the AFM measurement. PMID:26605715

  11. Controlling the spatial arrangement of organic magnetic anions adsorbed on epitaxial graphene on Ru(0001).

    PubMed

    Stradi, Daniele; Garnica, Manuela; Díaz, Cristina; Calleja, Fabián; Barja, Sara; Martín, Nazario; Alcamí, Manuel; Vazquez de Parga, Amadeo L; Miranda, Rodolfo; Martín, Fernando

    2014-12-21

    Achieving control over the self-organization of functional molecules on graphene is critical for the development of graphene technology in organic electronic and spintronic. Here, by using a scanning tunneling microscope (STM), we show that the electron acceptor molecule 7,7',8,8'-tetracyano-p-quinodimethane (TCNQ) and its fluorinated derivative 2,3,5,6-tetrafluoro-7,7',8,8'-tetracyano-p-quinodimethane (F4-TCNQ), co-deposited on the surface of epitaxial graphene on Ru(0001), transform spontaneously into their corresponding magnetic anions and self-organize in two remarkably different structures. TCNQ forms densely packed linear magnetic arrays, while F4-TCNQ molecules remain as isolated non interacting magnets. With the help of density functional theory (DFT) calculations, we trace back the origin of this behavior in the competition between the intermolecular repulsion experienced by the individual charged anions, which tends to separate the molecules, and the delocalization of the electrons transferred from the surface to the molecules, which promotes the formation of molecular oligomers. Our results demonstrate that it is possible to control the spatial arrangement of organic magnetic anions co-adsorbed on a surface by means of chemical substitution, paving the way for the design of two-dimensional fully organic magnetic structures on graphene and on other surfaces. PMID:25382549

  12. Emergence of an Out-of-Plane Optical Phonon (ZO) Kohn Anomaly in Quasifreestanding Epitaxial Graphene.

    PubMed

    Politano, Antonio; de Juan, Fernando; Chiarello, Gennaro; Fertig, Herbert A

    2015-08-14

    In neutral graphene, two prominent cusps known as Kohn anomalies are found in the phonon dispersion of the highest optical phonon at q=Γ (LO branch) and q=K (TO branch), reflecting a significant electron-phonon coupling (EPC) to undoped Dirac electrons. In this work, high-resolution electron energy loss spectroscopy is used to measure the phonon dispersion around the Γ point in quasifreestanding graphene epitaxially grown on Pt(111). The Kohn anomaly for the LO phonon is observed at finite momentum q~2k_{F} from Γ, with a shape in excellent agreement with the theory and consistent with known values of the EPC and the Fermi level. More strikingly, we also observe a Kohn anomaly at the same momentum for the out-of-plane optical phonon (ZO) branch. This observation is the first direct evidence of the coupling of the ZO mode with Dirac electrons, which is forbidden for freestanding graphene but becomes allowed in the presence of a substrate. Moreover, we estimate the EPC to be even greater than that of the LO mode, making graphene on Pt(111) an optimal system to explore the effects of this new coupling in the electronic properties. PMID:26317732

  13. In silico carbon molecular beam epitaxial growth of graphene on the h-BN substrate: carbon source effect on van der Waals epitaxy

    NASA Astrophysics Data System (ADS)

    Lee, Jonghoon; Varshney, Vikas; Park, Jeongho; Farmer, Barry L.; Roy, Ajit K.

    2016-05-01

    Against the presumption that hexagonal boron-nitride (h-BN) should provide an ideal substrate for van der Waals (vdW) epitaxy to grow high quality graphene films, carbon molecular beam epitaxy (CMBE) techniques using solid carbon sublimation have reported relatively poor quality of the graphene. In this article, the CMBE growth of graphene on the h-BN substrate is numerically studied in order to identify the effect of the carbon source on the quality of the graphene film. The carbon molecular beam generated by the sublimation of solid carbon source materials such as graphite and glassy carbon is mostly composed of atomic carbon, carbon dimers and carbon trimers. Therefore, the graphene film growth becomes a complex process involving various deposition characteristics of a multitude of carbon entities. Based on the study of surface adsorption and film growth characteristics of these three major carbon entities comprising graphite vapour, we report that carbon trimers convey strong traits of vdW epitaxy prone to high quality graphene growth, while atomic carbon deposition is a surface-reaction limited process accompanied by strong chemisorption. The vdW epitaxial behaviour of carbon trimers is found to be substantial enough to nucleate and develop into graphene like planar films within a nanosecond of high flux growth simulation, while reactive atomic carbons tend to impair the structural integrity of the crystalline h-BN substrate upon deposition to form an amorphous interface between the substrate and the growing carbon film. The content of reactive atomic carbons in the molecular beam is suspected to be the primary cause of low quality graphene reported in the literature. A possible optimization of the molecular beam composition towards the synthesis of better quality graphene films is suggested.Against the presumption that hexagonal boron-nitride (h-BN) should provide an ideal substrate for van der Waals (vdW) epitaxy to grow high quality graphene films, carbon

  14. In silico carbon molecular beam epitaxial growth of graphene on the h-BN substrate: carbon source effect on van der Waals epitaxy

    NASA Astrophysics Data System (ADS)

    Lee, Jonghoon; Varshney, Vikas; Park, Jeongho; Farmer, Barry L.; Roy, Ajit K.

    2016-05-01

    Against the presumption that hexagonal boron-nitride (h-BN) should provide an ideal substrate for van der Waals (vdW) epitaxy to grow high quality graphene films, carbon molecular beam epitaxy (CMBE) techniques using solid carbon sublimation have reported relatively poor quality of the graphene. In this article, the CMBE growth of graphene on the h-BN substrate is numerically studied in order to identify the effect of the carbon source on the quality of the graphene film. The carbon molecular beam generated by the sublimation of solid carbon source materials such as graphite and glassy carbon is mostly composed of atomic carbon, carbon dimers and carbon trimers. Therefore, the graphene film growth becomes a complex process involving various deposition characteristics of a multitude of carbon entities. Based on the study of surface adsorption and film growth characteristics of these three major carbon entities comprising graphite vapour, we report that carbon trimers convey strong traits of vdW epitaxy prone to high quality graphene growth, while atomic carbon deposition is a surface-reaction limited process accompanied by strong chemisorption. The vdW epitaxial behaviour of carbon trimers is found to be substantial enough to nucleate and develop into graphene like planar films within a nanosecond of high flux growth simulation, while reactive atomic carbons tend to impair the structural integrity of the crystalline h-BN substrate upon deposition to form an amorphous interface between the substrate and the growing carbon film. The content of reactive atomic carbons in the molecular beam is suspected to be the primary cause of low quality graphene reported in the literature. A possible optimization of the molecular beam composition towards the synthesis of better quality graphene films is suggested.Against the presumption that hexagonal boron-nitride (h-BN) should provide an ideal substrate for van der Waals (vdW) epitaxy to grow high quality graphene films, carbon

  15. One-step Synthesis of Few-layer WS2 by Pulsed Laser Deposition

    NASA Astrophysics Data System (ADS)

    Loh, Tamie A. J.; Chua, Daniel H. C.; Wee, Andrew T. S.

    2015-12-01

    Atomically thin tungsten disulfide (WS2) has attracted much attention in recent years due its indirect-to-direct band gap transition, band gap tunability, and giant spin splitting. However, the fabrication of atomically thin WS2 remains largely underdeveloped in comparison to its structural analogue MoS2. Here we report the direct fabrication of highly crystalline few-layer WS2 on silver substrates by pulse laser deposition at the relatively low temperature of 450 °C. The growth takes places by conventional epitaxy, through the in-situ formation of nearly lattice-matching Ag2S on the silver surface. Intriguingly, it was observed that the resulting film was composed of not only the usual semiconducting 2H-WS2 structure but also the less common metallic 1T-WS2. Modifications of the synthesis parameters allow for control over the crystalline quality, film thickness and crystal phase composition of the resulting WS2 film.

  16. Thickness-Dependent Dielectric Constant of Few-Layer In₂Se₃ Nanoflakes.

    PubMed

    Wu, Di; Pak, Alexander J; Liu, Yingnan; Zhou, Yu; Wu, Xiaoyu; Zhu, Yihan; Lin, Min; Han, Yu; Ren, Yuan; Peng, Hailin; Tsai, Yu-Hao; Hwang, Gyeong S; Lai, Keji

    2015-12-01

    The dielectric constant or relative permittivity (ε(r)) of a dielectric material, which describes how the net electric field in the medium is reduced with respect to the external field, is a parameter of critical importance for charging and screening in electronic devices. Such a fundamental material property is intimately related to not only the polarizability of individual atoms but also the specific atomic arrangement in the crystal lattice. In this Letter, we present both experimental and theoretical investigations on the dielectric constant of few-layer In2Se3 nanoflakes grown on mica substrates by van der Waals epitaxy. A nondestructive microwave impedance microscope is employed to simultaneously quantify the number of layers and local electrical properties. The measured ε(r) increases monotonically as a function of the thickness and saturates to the bulk value at around 6-8 quintuple layers. The same trend of layer-dependent dielectric constant is also revealed by first-principles calculations. Our results of the dielectric response, being ubiquitously applicable to layered 2D semiconductors, are expected to be significant for this vibrant research field. PMID:26575786

  17. Thickness-dependent Dielectric Constant of Few-layer In2Se3 Nano-flakes

    NASA Astrophysics Data System (ADS)

    Wu, Di; Pak, Alexander; Liu, Yingnan; Wu, Xiaoyu; Ren, Yuan; Tsai, Yu-Hao; Lin, Min; Peng, Hailin; Hwang, Gyeong; Lai, Keji

    2015-03-01

    The dielectric constant or relative permittivity of active materials in electronic devices is a critical parameter for charging and screening effects. For layered two-dimensional (2D) materials, it is of great interest to understand how their dielectric constants depend on dimensionalities and the arrangement of crystal lattices. Here we present both experimental and theoretical investigations on the dielectric constant of few-layer In2Se3 nano-flakes grown on mica substrates by van der Waals epitaxy. A nondestructive microwave impedance microscope (MIM) is employed to simultaneously quantify the number of layers and local electrical and optical properties. The measured effective dielectric constant increases monotonically as a function of the thickness and saturates to the bulk value at around 6-8 quintuple layers. The same trend of layer-dependent dielectric constant is also revealed through a density functional theory approach. Our results of the dielectric response are expected to be significant for the applications of layered materials in nano-devices.

  18. One-step Synthesis of Few-layer WS2 by Pulsed Laser Deposition.

    PubMed

    Loh, Tamie A J; Chua, Daniel H C; Wee, Andrew T S

    2015-01-01

    Atomically thin tungsten disulfide (WS2) has attracted much attention in recent years due its indirect-to-direct band gap transition, band gap tunability, and giant spin splitting. However, the fabrication of atomically thin WS2 remains largely underdeveloped in comparison to its structural analogue MoS2. Here we report the direct fabrication of highly crystalline few-layer WS2 on silver substrates by pulse laser deposition at the relatively low temperature of 450 °C. The growth takes places by conventional epitaxy, through the in-situ formation of nearly lattice-matching Ag2S on the silver surface. Intriguingly, it was observed that the resulting film was composed of not only the usual semiconducting 2H-WS2 structure but also the less common metallic 1T-WS2. Modifications of the synthesis parameters allow for control over the crystalline quality, film thickness and crystal phase composition of the resulting WS2 film. PMID:26657172

  19. Topological phase transition and quantum spin Hall edge states of antimony few layers.

    PubMed

    Kim, Sung Hwan; Jin, Kyung-Hwan; Park, Joonbum; Kim, Jun Sung; Jhi, Seung-Hoon; Yeom, Han Woong

    2016-01-01

    While two-dimensional (2D) topological insulators (TI's) initiated the field of topological materials, only very few materials were discovered to date and the direct access to their quantum spin Hall edge states has been challenging due to material issues. Here, we introduce a new 2D TI material, Sb few layer films. Electronic structures of ultrathin Sb islands grown on Bi2Te2Se are investigated by scanning tunneling microscopy. The maps of local density of states clearly identify robust edge electronic states over the thickness of three bilayers in clear contrast to thinner islands. This indicates that topological edge states emerge through a 2D topological phase transition predicted between three and four bilayer films in recent theory. The non-trivial phase transition and edge states are confirmed for epitaxial films by extensive density-functional-theory calculations. This work provides an important material platform to exploit microscopic aspects of the quantum spin Hall phase and its quantum phase transition. PMID:27624972

  20. One-step Synthesis of Few-layer WS2 by Pulsed Laser Deposition

    PubMed Central

    Loh, Tamie A. J.; Chua, Daniel H. C.; Wee, Andrew T. S.

    2015-01-01

    Atomically thin tungsten disulfide (WS2) has attracted much attention in recent years due its indirect-to-direct band gap transition, band gap tunability, and giant spin splitting. However, the fabrication of atomically thin WS2 remains largely underdeveloped in comparison to its structural analogue MoS2. Here we report the direct fabrication of highly crystalline few-layer WS2 on silver substrates by pulse laser deposition at the relatively low temperature of 450 °C. The growth takes places by conventional epitaxy, through the in-situ formation of nearly lattice-matching Ag2S on the silver surface. Intriguingly, it was observed that the resulting film was composed of not only the usual semiconducting 2H-WS2 structure but also the less common metallic 1T-WS2. Modifications of the synthesis parameters allow for control over the crystalline quality, film thickness and crystal phase composition of the resulting WS2 film. PMID:26657172

  1. Imaging epitaxial graphene on SiC(0001) using STM with functionalized W tips

    NASA Astrophysics Data System (ADS)

    Rhim, S. H.; Qi, Y.; Sun, G. F.; Liu, Y.; Weinert, M.; Li, L.

    2012-02-01

    Epitaxial graphene on SiC(0001) is studied using scanning tunneling microscopy with W tips functionalized by transition-metal (Cr, Fe) coatings, enabling the imaging of states within a few meV of the Fermi level that are not accessible with conventional W tips. First-principles modeling of these tips as pyramidal structures on W(110) indicates that an apex atom is stable for the Cr/W(110) tip but not for the Fe/W(110) or W/W(110) tips. Further calculations of the tunneling current show that the Cr- and Fe-coated tips can get significantly closer to the substrate than a bare W tip at a given current, and that the Cr (Fe) tip states contributing to the tunneling at low bias are spatially more localized than the W tip states. These characteristics lead to increased resolution, making possible the selective imaging of the complex electronic properties of the epitaxial graphene on SiC(0001)1,2.

  2. van der Waals epitaxial growth of graphene on sapphire by chemical vapor deposition without a metal catalyst.

    PubMed

    Hwang, Jeonghyun; Kim, Moonkyung; Campbell, Dorr; Alsalman, Hussain A; Kwak, Joon Young; Shivaraman, Shriram; Woll, Arthur R; Singh, Arunima K; Hennig, Richard G; Gorantla, Sandeep; Rümmeli, Mark H; Spencer, Michael G

    2013-01-22

    van der Waals epitaxial growth of graphene on c-plane (0001) sapphire by CVD without a metal catalyst is presented. The effects of CH(4) partial pressure, growth temperature, and H(2)/CH(4) ratio were investigated and growth conditions optimized. The formation of monolayer graphene was shown by Raman spectroscopy, optical transmission, grazing incidence X-ray diffraction (GIXRD), and low voltage transmission electron microscopy (LVTEM). Electrical analysis revealed that a room temperature Hall mobility above 2000 cm(2)/V·s was achieved, and the mobility and carrier type were correlated to growth conditions. Both GIXRD and LVTEM studies confirm a dominant crystal orientation (principally graphene [10-10] || sapphire [11-20]) for about 80-90% of the material concomitant with epitaxial growth. The initial phase of the nucleation and the lateral growth from the nucleation seeds were observed using atomic force microscopy. The initial nuclei density was ~24 μm(-2), and a lateral growth rate of ~82 nm/min was determined. Density functional theory calculations reveal that the binding between graphene and sapphire is dominated by weak dispersion interactions and indicate that the epitaxial relation as observed by GIXRD is due to preferential binding of small molecules on sapphire during early stages of graphene formation. PMID:23244231

  3. Large Frequency Change with Thickness in Interlayer Breathing Mode--Significant Interlayer Interactions in Few Layer Black Phosphorus.

    PubMed

    Luo, Xin; Lu, Xin; Koon, Gavin Kok Wai; Castro Neto, Antonio H; Özyilmaz, Barbaros; Xiong, Qihua; Quek, Su Ying

    2015-06-10

    Bulk black phosphorus (BP) consists of puckered layers of phosphorus atoms. Few-layer BP, obtained from bulk BP by exfoliation, is an emerging candidate as a channel material in post-silicon electronics. A deep understanding of its physical properties and its full range of applications are still being uncovered. In this paper, we present a theoretical and experimental investigation of phonon properties in few-layer BP, focusing on the low-frequency regime corresponding to interlayer vibrational modes. We show that the interlayer breathing mode A(3)g shows a large redshift with increasing thickness; the experimental and theoretical results agree well. This thickness dependence is two times larger than that in the chalcogenide materials, such as few-layer MoS2 and WSe2, because of the significantly larger interlayer force constant and smaller atomic mass in BP. The derived interlayer out-of-plane force constant is about 50% larger than that of graphene and MoS2. We show that this large interlayer force constant arises from the sizable covalent interaction between phosphorus atoms in adjacent layers and that interlayer interactions are not merely of the weak van der Waals type. These significant interlayer interactions are consistent with the known surface reactivity of BP and have been shown to be important for electric-field induced formation of Dirac cones in thin film BP. PMID:25939057

  4. Direct growth of single- and few-layer MoS2 on h-BN by CVD method

    NASA Astrophysics Data System (ADS)

    Yan, Aiming; Velasco, Jairo, Jr.; Kahn, Salman; Watanabe, Kenji; Taniguchi, Takashi; Wang, Feng; Crommie, Michael; Zettl, Alex

    As a promising candidate for the next-generation electronics, large-scale single- and few-layer molybdenum disulfide (MoS2) grown by CVD method is an important advancement towards technological implementation of this material. However, the choice of substrate can significantly affect the performance of MoS2 based devices. An attractive insulating substrate or mate for MoS2 (and related materials such as graphene) is hexagonal boron nitride (h-BN). Stacked heterostructures of MoS2 and h-BN have been produced by manual transfer methods, but a more efficient and scalable assembly method is needed. Here we demonstrate the direct growth of single- and few-layer MoS2 on h-BN by chemical vapor deposition (CVD) method. The growth mechanisms for single- and few-layer samples are found to be distinct, and for single-layer samples low relative rotation angles (<5°) between the MoS2 and h-BN lattices prevail. In addition, MoS2 directly grown on h-BN maintains its intrinsic 1.89 eV bandgap. Our CVD synthesis method presents a viable path towards high-quality MoS2 based field effect transistors in a controllable and scalable fashion. Acknowledgement: the U.S. Department of Energy under Contract DE-AC02-05CH11231; NSF Grant DMR-1206512.

  5. Blinking suppression of CdTe quantum dots on epitaxial graphene and the analysis with Marcus electron transfer

    SciTech Connect

    Hirose, Takuya; Tamai, Naoto; Kutsuma, Yasunori; Kurita, Atsusi; Kaneko, Tadaaki

    2014-08-25

    We have prepared epitaxial graphene by a Si sublimation method from 4H-SiC. Single-particle spectroscopy of CdTe quantum dots (QDs) on epitaxial graphene covered with polyvinylpyrrolidone (PVP) or polyethylene glycol (PEG) showed the suppression of luminescence blinking and ∼10 times decreased luminescence intensity as compared with those on a glass. The electronic coupling constant, H{sub 01}, between CdTe QDs and graphene was calculated to be (3.3 ± 0.4) × 10{sup 2 }cm{sup −1} in PVP and (3.7 ± 0.8) × 10{sup 2 }cm{sup −1} in PEG based on Marcus theory of electron transfer and Tang-Marcus model of blinking with statistical distribution.

  6. Superior mechanical flexibility of phosphorene and few-layer black phosphorus

    SciTech Connect

    Wei, Qun; Peng, Xihong

    2014-06-23

    Recently, fabricated two dimensional (2D) phosphorene crystal structures have demonstrated great potential in applications of electronics. Mechanical strain was demonstrated to be able to significantly modify the electronic properties of phosphorene and few-layer black phosphorus. In this work, we employed first principles density functional theory calculations to explore the mechanical properties of phosphorene, including ideal tensile strength and critical strain. It was found that a monolayer phosphorene can sustain tensile strain up to 27% and 30% in the zigzag and armchair directions, respectively. This enormous strain limit of phosphorene results from its unique puckered crystal structure. We found that the tensile strain applied in the armchair direction stretches the pucker of phosphorene, rather than significantly extending the P-P bond lengths. The compromised dihedral angles dramatically reduce the required strain energy. Compared to other 2D materials, such as graphene, phosphorene demonstrates superior flexibility with an order of magnitude smaller Young's modulus. This is especially useful in practical large-magnitude-strain engineering. Furthermore, the anisotropic nature of phosphorene was also explored. We derived a general model to calculate the Young's modulus along different directions for a 2D system.

  7. Tunable charge-trap memory based on few-layer MoS2.

    PubMed

    Zhang, Enze; Wang, Weiyi; Zhang, Cheng; Jin, Yibo; Zhu, Guodong; Sun, Qingqing; Zhang, David Wei; Zhou, Peng; Xiu, Faxian

    2015-01-27

    Charge-trap memory with high-κ dielectric materials is considered to be a promising candidate for next-generation memory devices. Ultrathin layered two-dimensional (2D) materials like graphene and MoS2 have been receiving much attention because of their fantastic physical properties and potential applications in electronic devices. Here, we report on a dual-gate charge-trap memory device composed of a few-layer MoS2 channel and a three-dimensional (3D) Al2O3/HfO2/Al2O3 charge-trap gate stack. Because of the extraordinary trapping ability of both electrons and holes in HfO2, the MoS2 memory device exhibits an unprecedented memory window exceeding 20 V. Importantly, with a back gate the window size can be effectively tuned from 15.6 to 21 V; the program/erase current ratio can reach up to 10(4), allowing for multibit information storage. Moreover, the device shows a high endurance of hundreds of cycles and a stable retention of ∼ 28% charge loss after 10 years, which is drastically lower than ever reported MoS2 flash memory. The combination of 2D materials with traditional high-κ charge-trap gate stacks opens up an exciting field of nonvolatile memory devices. PMID:25496773

  8. 3D Epitaxy of Graphene nanostructures in the Matrix of Ag, Al and Cu

    NASA Astrophysics Data System (ADS)

    Salamanca-Riba, Lourdes; Isaacs, Romaine; Wuttig, Manfred; Lemieux, Melburne; Hu, Liangbing; Iftekhar, Jaim; Rashkeev, Sergey; Kukla, Maija; Rabin, Oded; Mansour, Azzam

    2015-03-01

    Graphene nanostructures in the form ribbons were embedded in the lattice of metals such as Ag, Cu, and Al in concentrations up to 36.4 at.%, 21.8 at% and 10.5 at.%, respectively. These materials are called covetics. Raman scattering from Ag and Al covetics indicate variations in the intensity of peaks at ~ 1,300 cm-1 and 1,600 cm-1 with position on the sample. These peaks are associated with the D (defects) and G (graphite E2g mode) peaks of graphitic carbon with sp2 bonding and reveal various degrees of imperfections in the graphene layers. First principles calculations of the dynamic matrix of Ag and Al covetics show bonding between C and the metal. EELS mapping of the C-K edge and high resolution lattice images show that the graphene-like regions form ribbons with epitaxial orientation with the metal lattice of Ag and Al. The temperature dependences of the resistivites of Ag and Cu covetics are similar to those of the pure metals with only slight increase in resistivity. Films of Cu covetic deposited by e-beam evaporation and PLD show higher transmittance and resistance to oxidation than pure metal films of the same thickness indicating that copper covetic films can be used for transparent electrodes. Funded by DARPA/ARL Grant No. W911NF-13-1-0058, and ONR Award No N000141410042.

  9. Vibrational spectra of nanowires measured using laser doppler vibrometry and STM studies of epitaxial graphene : an LDRD fellowship report.

    SciTech Connect

    Biedermann, Laura Butler

    2009-09-01

    , their vibration spectra was more extensively studied. The thermal vibration spectra of Ag{sub 2}Ga nanoneedles was measured under both ambient and low-vacuum conditions. The operational deflection shapes of the vibrating Ag{sub 2}Ga nanoneedles was also measured, allowing confirmation of the eigenmodes of vibration. The modulus of the crystalline nanoneedles was 84.3 {+-} 1.0 GPa. Gas damping is the dominate mechanism of energy loss for nanowires oscillating under ambient conditions. The measured quality factors, Q, of oscillation are in line with theoretical predictions of air damping in the free molecular gas damping regime. In the free molecular regime, Q{sub gas} is linearly proportional to the density and diameter of the nanowire and inversely proportional to the air pressure. Since the density of the Ag{sub 2}Ga nanoneedles is three times that of the MWNTs, the Ag{sub 2}Ga nanoneedles have greater Q at atmospheric pressures. Our initial measurements of Q for Ag{sub 2}Ga nanoneedles in low-vacuum (10 Torr) suggest that the intrinsic Q of these nanoneedles may be on the order of 1000. The epitaxial carbon that grows after heating (000{bar 1}) silicon carbide (SiC) to high temperatures (1450-1600) in vacuum was also studied. At these high temperatures, the surface Si atoms sublime and the remaining C atoms reconstruct to form graphene. X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were used to characterize the quality of the few-layer graphene (FLG) surface. The XPS studies were useful in confirming the graphitic composition and measuring the thickness of the FLG samples. STM studies revealed a wide variety of nanometer-scale features that include sharp carbon-rich ridges, moire superlattices, one-dimensional line defects, and grain boundaries. By imaging these features with atomic scale resolution, considerable insight into the growth mechanisms of FLG on the carbon-face of SiC is obtained.

  10. Direct growth of hexagonal boron nitride/graphene heterostructures on cobalt foil substrates by plasma-assisted molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Xu, Zhongguang; Khanaki, Alireza; Tian, Hao; Zheng, Renjing; Suja, Mohammad; Zheng, Jian-Guo; Liu, Jianlin

    2016-07-01

    Graphene/hexagonal boron nitride (G/h-BN) heterostructures have attracted a great deal of attention because of their exceptional properties and wide variety of potential applications in nanoelectronics. However, direct growth of large-area, high-quality, and stacked structures in a controllable and scalable way remains challenging. In this work, we demonstrate the synthesis of h-BN/graphene (h-BN/G) heterostructures on cobalt (Co) foil by sequential deposition of graphene and h-BN layers using plasma-assisted molecular beam epitaxy. It is found that the coverage of h-BN layers can be readily controlled on the epitaxial graphene by growth time. Large-area, uniform-quality, and multi-layer h-BN films on thin graphite layers were achieved. Based on an h-BN (5-6 nm)/G (26-27 nm) heterostructure, capacitor devices with Co(foil)/G/h-BN/Co(contact) configuration were fabricated to evaluate the dielectric properties of h-BN. The measured breakdown electric field showed a high value of ˜2.5-3.2 MV/cm. Both I-V and C-V characteristics indicate that the epitaxial h-BN film has good insulating characteristics.

  11. Magnetism of nitrogen-vacancy complexes in epitaxial graphene on SiC(0001)

    NASA Astrophysics Data System (ADS)

    Rhim, S. H.; Qi, Y.; Sun, G. F.; Liu, Y.; Weinert, M.; Li, L.

    2010-03-01

    We present a study of the magnetism of nitrogen impurity--vacancy complexes in epitaxial graphene on SiC(0001) using a combination of density functional calculations and scanning tunneling microscopy. Nitrogen-vacancy complexes are created by means of a nitrogen plasma. From the calculated energetics and by comparison with STM, the most probable configuration is found to have the vacancy and substitutional nitrogen at the nearest neighbor site. This configuration is non-magnetic, whereas other higher energy N-vacancy configurations (and the isolated vacancy) are magnetic. Adatoms such as H, C, and N, in combination with the nitrogen-vacancy complex, show a variety of magnetic behaviors, depending on both the adatom and its position.

  12. Structural and electronic properties of PTCDA thin films on epitaxial graphene.

    PubMed

    Huang, Han; Chen, Shi; Gao, Xingyu; Chen, Wei; Wee, Andrew Thye Shen

    2009-11-24

    In situ low-temperature scanning tunneling microscopy is used to study the growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on epitaxial graphene (EG) on 6H-SiC(0001), as well as on HOPG for comparison. PTCDA adopts a layer-by-layer growth mode, with its molecular plane lying flat on both surfaces. The PTCDA films grow continuously over the EG step edges, but not on HOPG. STS performed on single-layer PTCDA on monolayer EG shows a wide band gap larger than 3.3 eV, consistent with pristine PTCDA films. Synchrotron-based high-resolution photoemission spectroscopy reveals weak charge transfer between PTCDA and EG. This suggests weak electronic coupling between PTCDA and the underlying EG layer. PMID:19852489

  13. Formation of nitrogen-vacancy complexes during plasma-assisted nitrogen doping of epitaxial graphene on SiC(0001)

    NASA Astrophysics Data System (ADS)

    Rhim, S. H.; Qi, Y.; Liu, Y.; Weinert, M.; Li, L.

    2012-06-01

    Doping epitaxial graphene on SiC(0001) using nitrogen plasma leads to N-vacancy complexes. Based on the calculated energetics by density functional theory and comparison with scanning tunneling microscopy observations, the most probable configuration is determined to be a nonmagnetic complex consisting of substitutional nitrogen next to a carbon vacancy. Further calculations show that other N-vacancy complexes, where the substituted N and the vacancy are the second and third nearest neighbors, do exhibit localized moments. These results indicate that the electronic and magnetic properties of graphene can be further tailored by plasma-assisted nitrogen doping.

  14. Atmospheric doping effects in epitaxial graphene: correlation of local and global electrical studies

    NASA Astrophysics Data System (ADS)

    Panchal, Vishal; Giusca, Cristina E.; Lartsev, Arseniy; Martin, Nicholas A.; Cassidy, Nathan; Myers-Ward, Rachael L.; Gaskill, D. Kurt; Kazakova, Olga

    2016-03-01

    We directly correlate the local (20 nm scale) and global electronic properties of a device containing mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H -SiC(0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions we investigate the doping effects of N2, O2, water vapour and NO2 at concentrations representative of the ambient air. We show that presence of O2, water vapour and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected. Furthermore, we demonstrate that the general consensus of O2 and water vapour present in ambient air providing majority of the p-doping to graphene is a common misconception. We experimentally show that even the combined effect of O2, water vapour, and NO2 at concentrations higher than typically present in the atmosphere does not fully replicate p-doping from ambient air. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly separate them from those coming from targeted species.

  15. SU(4) symmetry breaking revealed by magneto-optical spectroscopy in epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Tan, Liang Z.; Orlita, Milan; Potemski, Marek; Sprinkle, Mike; Berger, Claire; de Heer, Walter; Louie, Steven; Martinez, Gerard

    2015-03-01

    Electron-electron and electron-phonon interactions break the spin and valley degeneracies of the lowest Landau level (LL) in graphene. Multiple theoretical models have been proposed for the broken symmetry ground state. Previous tilted magnetic field transport experiments have obtained partial information on the ground state by probing the spin degree of freedom. In this work, we show that, via the valley-dependent electron-phonon interaction, symmetry breaking of the valley degree of freedom can be detected in infra-red transmission signatures close to magneto-phonon resonances. We have performed infra-red magneto-transmission experiments on multi-layer epitaxial graphene samples in magnetic fields up to 35 T. Following the main optical transition involving the lowest LL, we observe a new absorption transition increasing in intensity with magnetic fields greater than 26 T. Our theoretical calculations quantitatively explain these features, and unambiguously identify the charge density wave as the ground state in our samples. This work was supported by National Science Foundation Grant No. DMR10-1006184, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by the NSF through XSEDE resources at NICS.

  16. Role of atomic terraces and steps in the electron transport properties of epitaxial graphene grown on SiC

    NASA Astrophysics Data System (ADS)

    Kuramochi, H.; Odaka, S.; Morita, K.; Tanaka, S.; Miyazaki, H.; Lee, M. V.; Li, S.-L.; Hiura, H.; Tsukagoshi, K.

    2012-03-01

    Thermal decomposition of vicinal SiC substrates with self-organized periodic nanofacets is a promising method to produce large graphene sheets toward the commercial exploitation of graphene's superior electronic properties. The epitaxial graphene films grown on vicinal SiC comprise two distinct regions of terrace and step; and typically exhibit anisotropic electron transport behavior, although limited areas in the graphene film showed ballistic transport. To evaluate the role of terraces and steps in electron transport properties, we compared graphene samples with terrace and step regions grown on 4H-SiC(0001). Arrays of field effect transistors were fabricated on comparable graphene samples with their channels parallel or perpendicular to the nanofacets to identify the source of measured reduced mobility. Minimum conductivity and electron mobility increased with the larger proportional terrace region area; therefore, the terrace region has superior transport properties to step regions. The measured electron mobility in the terrace region, ˜1000 cm2/Vs, is 10 times larger than that in the step region, ˜100 cm2/Vs. We conclusively determine that parasitic effects originate in regions of graphene that grow over step edges in 4H-SiC(0001).

  17. Observing hot carrier distribution in an n-type epitaxial graphene on a SiC substrate

    SciTech Connect

    Someya, T.; Ishida, Y.; Yoshida, R.; Iimori, T.; Yukawa, R.; Akikubo, K.; Yamamoto, Sh.; Yamamoto, S.; Kanai, T.; Itatani, J.; Komori, F.; Shin, S.; Matsuda, I.; Fukidome, H.; Funakubo, K.; Suemitsu, M.; Yamamoto, T.

    2014-04-21

    Hot carrier dynamics in the Dirac band of n-type epitaxial graphene on a SiC substrate were traced in real time using femtosecond-time-resolved photoemission spectroscopy. The spectral evolution directly reflects the energetically linear density of states superimposed with a Fermi–Dirac distribution. The relaxation time is governed by the internal energy dissipation of electron–electron scattering, and the observed electronic temperature indicates cascade carrier multiplication.

  18. SiC Homoepitaxy, Etching and Graphene Epitaxial Growth on SiC Substrates Using a Novel Fluorinated Si Precursor Gas (SiF4)

    NASA Astrophysics Data System (ADS)

    Rana, Tawhid; Chandrashekhar, M. V. S.; Daniels, Kevin; Sudarshan, Tangali

    2016-04-01

    Tetrafluorosilane (SiF4 or TFS), a novel precursor gas, has been demonstrated to perform three primary operations of silicon carbide-related processing: SiC etching, SiC epitaxial growth and graphene epitaxial growth. TFS etches SiC substrate vigorously in a H2 ambient by efficient Si removal from the surface, where SiC etch rate is a function of TFS gas concentration. In this SiC etching process, Si is removed by TFS and C is removed by H2. When propane is added to a H2 and TFS gas mixture, etching is halted and high-quality SiC epitaxy takes place in a Si droplet-free condition. TFS's ability to remove Si can also be exploited to grow epitaxial graphene in a controllable manner in an inert (Ar) ambient. Here, TFS enhances graphene growth by selective etching of Si from the SiC surface.

  19. Probing the role of interlayer coupling and coulomb interactions on electronic structure in few-layer MoSe₂ nanostructures.

    PubMed

    Bradley, Aaron J; Ugeda, Miguel M; da Jornada, Felipe H; Qiu, Diana Y; Ruan, Wei; Zhang, Yi; Wickenburg, Sebastian; Riss, Alexander; Lu, Jiong; Mo, Sung-Kwan; Hussain, Zahid; Shen, Zhi-Xun; Louie, Steven G; Crommie, Michael F

    2015-04-01

    Despite the weak nature of interlayer forces in transition metal dichalcogenide (TMD) materials, their properties are highly dependent on the number of layers in the few-layer two-dimensional (2D) limit. Here, we present a combined scanning tunneling microscopy/spectroscopy and GW theoretical study of the electronic structure of high quality single- and few-layer MoSe2 grown on bilayer graphene. We find that the electronic (quasiparticle) bandgap, a fundamental parameter for transport and optical phenomena, decreases by nearly one electronvolt when going from one layer to three due to interlayer coupling and screening effects. Our results paint a clear picture of the evolution of the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. This demonstrates the importance of layer number and electron-electron interactions on van der Waals heterostructures and helps to clarify how their electronic properties might be tuned in future 2D nanodevices. PMID:25775022

  20. Controllable synthesis of few-layered and hierarchically porous boron nitride nanosheets.

    PubMed

    Xiao, Feng; Chen, Zhixin; Casillas, Gilberto; Richardson, Christopher; Li, Huijun; Huang, Zhenguo

    2016-03-11

    Few-layered porous boron nitride nanosheets (BNNS) have been prepared using a dynamic magnesium diboride (MgB2) template and ammonium chloride (NH4Cl) etchant. Magnesium-based intermediates serve as layer separators in the synthesis and prevent extensive aggregation, resulting in few-layered BNNS. The resultant BNNS are hierarchically porous and show good CO2/N2 adsorption selectivity. PMID:26871737

  1. Role of the Pinning Points in epitaxial Graphene Moiré Superstructures on the Pt(111) Surface

    PubMed Central

    Martínez, José I.; Merino, Pablo; Pinardi, Anna L.; Gonzalo, Otero-Irurueta; López, María F.; Méndez, Javier; Martín-Gago, José A.

    2016-01-01

    The intrinsic atomic mechanisms responsible for electronic doping of epitaxial graphene Moirés on transition metal surfaces is still an open issue. To better understand this process we have carried out a first-principles full characterization of the most representative Moiré superstructures observed on the Gr/Pt(111) system and confronted the results with atomically resolved scanning tunneling microscopy experiments. We find that for all reported Moirés the system relaxes inducing a non-negligible atomic corrugation both, at the graphene and at the outermost platinum layer. Interestingly, a mirror “anti-Moiré” reconstruction appears at the substrate, giving rise to the appearance of pinning-points. We show that these points are responsible for the development of the superstructure, while charge from the Pt substrate is injected into the graphene, inducing a local n-doping, mostly localized at these specific pinning-point positions. PMID:26852920

  2. Role of the Pinning Points in epitaxial Graphene Moiré Superstructures on the Pt(111) Surface.

    PubMed

    Martínez, José I; Merino, Pablo; Pinardi, Anna L; Gonzalo, Otero-Irurueta; López, María F; Méndez, Javier; Martín-Gago, José A

    2016-01-01

    The intrinsic atomic mechanisms responsible for electronic doping of epitaxial graphene Moirés on transition metal surfaces is still an open issue. To better understand this process we have carried out a first-principles full characterization of the most representative Moiré superstructures observed on the Gr/Pt(111) system and confronted the results with atomically resolved scanning tunneling microscopy experiments. We find that for all reported Moirés the system relaxes inducing a non-negligible atomic corrugation both, at the graphene and at the outermost platinum layer. Interestingly, a mirror "anti-Moiré" reconstruction appears at the substrate, giving rise to the appearance of pinning-points. We show that these points are responsible for the development of the superstructure, while charge from the Pt substrate is injected into the graphene, inducing a local n-doping, mostly localized at these specific pinning-point positions. PMID:26852920

  3. Microscopically-Tuned Band Structure of Epitaxial Graphene through Interface and Stacking Variations Using Si Substrate Microfabrication

    PubMed Central

    Fukidome, Hirokazu; Ide, Takayuki; Kawai, Yusuke; Shinohara, Toshihiro; Nagamura, Naoka; Horiba, Koji; Kotsugi, Masato; Ohkochi, Takuo; Kinoshita, Toyohiko; Kumighashira, Hiroshi; Oshima, Masaharu; Suemitsu, Maki

    2014-01-01

    Graphene exhibits unusual electronic properties, caused by a linear band structure near the Dirac point. This band structure is determined by the stacking sequence in graphene multilayers. Here we present a novel method of microscopically controlling the band structure. This is achieved by epitaxy of graphene on 3C-SiC(111) and 3C-SiC(100) thin films grown on a 3D microfabricated Si(100) substrate (3D-GOS (graphene on silicon)) by anisotropic etching, which produces Si(111) microfacets as well as major Si(100) microterraces. We show that tuning of the interface between the graphene and the 3C-SiC microfacets enables microscopic control of stacking and ultimately of the band structure of 3D-GOS, which is typified by the selective emergence of semiconducting and metallic behaviours on the (111) and (100) portions, respectively. The use of 3D-GOS is thus effective in microscopically unlocking various potentials of graphene depending on the application target, such as electronic or photonic devices. PMID:24903119

  4. Layer-dependent Band Alignment and Work Function of Few-Layer Phosphorene

    PubMed Central

    Cai, Yongqing; Zhang, Gang; Zhang, Yong-Wei

    2014-01-01

    Using first-principles calculations, we study the electronic properties of few-layer phosphorene focusing on layer-dependent behavior of band gap, work function band alignment and carrier effective mass. It is found that few-layer phosphorene shows a robust direct band gap character, and its band gap decreases with the number of layers following a power law. The work function decreases rapidly from monolayer (5.16 eV) to trilayer (4.56 eV), and then slowly upon further increasing the layer number. Compared to monolayer phosphorene, there is a drastic decrease of hole effective mass along the ridge (zigzag) direction for bilayer phosphorene, indicating a strong interlayer coupling and screening effect. Our study suggests that 1). Few-layer phosphorene with a layer-dependent band gap and a robust direct band gap character is promising for efficient solar energy harvest. 2). Few-layer phosphorene outperforms monolayer counterpart in terms of a lighter carrier effective mass, a higher carrier density and a weaker scattering due to enhanced screening. 3). The layer-dependent band edges and work functions of few-layer phosphorene allow for modification of Schottky barrier with enhanced carrier injection efficiency. It is expected that few-layer phosphorene will present abundant opportunities for a plethora of new electronic applications. PMID:25327586

  5. Van der Waals epitaxial growth of two-dimensional single-crystalline GaSe domains on graphene

    DOE PAGESBeta

    Li, Xufan; Basile, Leonardo; Huang, Bing; Ma, Cheng; Lee, Jaekwang; Vlassiouk, Ivan V.; Puretzky, Alexander A.; Lin, Ming -Wei; Chi, Miaofang; Idrobo Tapia, Juan Carlos; et al

    2015-07-22

    Two-dimensional (2D) van der Waals (vdW) heterostructures are a family of artificially-structured materials that promise tunable optoelectronic properties for devices with enhanced functionalities. Compared to stamping, direct epitaxy of vdW heterostructures is ideal for clean interlayer interfaces and scalable device fabrication. Here, we explore the synthesis and preferred orientations of 2D GaSe atomic layers on graphene (Gr) by vdW epitaxy. Guided by the wrinkles on graphene, GaSe nuclei form that share a predominant lattice orientation. Due to vdW epitaxial growth many nuclei grow as perfectly aligned crystals and coalesce to form large (tens of microns), single-crystal flakes. Through theoretical investigationsmore » of interlayer energetics, and measurements of preferred orientations by atomic-resolution STEM and electron diffraction, a 10.9 interlayer rotation of the GaSe lattice with respect to the underlying graphene is found to be the most energetically preferred vdW heterostructure with the largest binding energy and the longest-range ordering. These GaSe/Gr vdW heterostructures exhibit an enhanced Raman E21g band of monolayer GaSe along with highly-quenched photoluminescence due to strong charge transfer. Despite the very large lattice mismatch of GaSe/Gr through vdW epitaxy, the predominant orientation control and convergent formation of large single-crystal flakes demonstrated here is promising for the scalable synthesis of large-area vdW heterostructures for the development of new optical and optoelectronic devices.« less

  6. Van der Waals epitaxial growth of two-dimensional single-crystalline GaSe domains on graphene

    SciTech Connect

    Li, Xufan; Basile, Leonardo; Huang, Bing; Ma, Cheng; Lee, Jaekwang; Vlassiouk, Ivan V.; Puretzky, Alexander A.; Lin, Ming -Wei; Chi, Miaofang; Idrobo Tapia, Juan Carlos; Rouleau, Christopher M.; Sumpter, Bobby G.; Yoon, Mina; Geohegan, David B.; Xiao, Kai

    2015-07-22

    Two-dimensional (2D) van der Waals (vdW) heterostructures are a family of artificially-structured materials that promise tunable optoelectronic properties for devices with enhanced functionalities. Compared to stamping, direct epitaxy of vdW heterostructures is ideal for clean interlayer interfaces and scalable device fabrication. Here, we explore the synthesis and preferred orientations of 2D GaSe atomic layers on graphene (Gr) by vdW epitaxy. Guided by the wrinkles on graphene, GaSe nuclei form that share a predominant lattice orientation. Due to vdW epitaxial growth many nuclei grow as perfectly aligned crystals and coalesce to form large (tens of microns), single-crystal flakes. Through theoretical investigations of interlayer energetics, and measurements of preferred orientations by atomic-resolution STEM and electron diffraction, a 10.9 interlayer rotation of the GaSe lattice with respect to the underlying graphene is found to be the most energetically preferred vdW heterostructure with the largest binding energy and the longest-range ordering. These GaSe/Gr vdW heterostructures exhibit an enhanced Raman E21g band of monolayer GaSe along with highly-quenched photoluminescence due to strong charge transfer. Despite the very large lattice mismatch of GaSe/Gr through vdW epitaxy, the predominant orientation control and convergent formation of large single-crystal flakes demonstrated here is promising for the scalable synthesis of large-area vdW heterostructures for the development of new optical and optoelectronic devices.

  7. Atomic Layer Epitaxy of h-BN(0001) Multilayers on Co(0001) and Molecular Beam Epitaxy Growth of Graphene on h-BN(0001)/Co(0001).

    PubMed

    Driver, M Sky; Beatty, John D; Olanipekun, Opeyemi; Reid, Kimberly; Rath, Ashutosh; Voyles, Paul M; Kelber, Jeffry A

    2016-03-22

    The direct growth of hexagonal boron nitride (h-BN) by industrially scalable methods is of broad interest for spintronic and nanoelectronic device applications. Such applications often require atomically precise control of film thickness and azimuthal registry between layers and substrate. We report the formation, by atomic layer epitaxy (ALE), of multilayer h-BN(0001) films (up to 7 monolayers) on Co(0001). The ALE process employs BCl3/NH3 cycles at 600 K substrate temperature. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) data show that this process yields an increase in h-BN average film thickness linearly proportional to the number of BCl3/NH3 cycles, with BN layers in azimuthal registry with each other and with the Co(0001) substrate. LEED diffraction spot profile data indicate an average BN domain size of at least 1900 Å. Optical microscopy data indicate the presence of some domains as large as ∼20 μm. Transmission electron microscopy (TEM) and ambient exposure studies demonstrate macroscopic and microscopic continuity of the h-BN film, with the h-BN film highly conformal to the Co substrate. Photoemission data show that the h-BN(0001) film is p-type, with band bending near the Co/h-BN interface. Growth of graphene by molecular beam epitaxy (MBE) is observed on the surface of multilayer h-BN(0001) at temperatures of 800 K. LEED data indicate azimuthal graphene alignment with the h-BN and Co(0001) lattices, with domain size similar to BN. The evidence of multilayer BN and graphene azimuthal alignment with the lattice of the Co(0001) substrate demonstrates that this procedure is suitable for scalable production of heterojunctions for spintronic applications. PMID:26940024

  8. Van der Waals Epitaxial Growth of Two-Dimensional Single-Crystalline GaSe Domains on Graphene.

    PubMed

    Li, Xufan; Basile, Leonardo; Huang, Bing; Ma, Cheng; Lee, Jaekwang; Vlassiouk, Ivan V; Puretzky, Alexander A; Lin, Ming-Wei; Yoon, Mina; Chi, Miaofang; Idrobo, Juan C; Rouleau, Christopher M; Sumpter, Bobby G; Geohegan, David B; Xiao, Kai

    2015-08-25

    Two-dimensional (2D) van der Waals (vdW) heterostructures are a family of artificially structured materials that promise tunable optoelectronic properties for devices with enhanced functionalities. Compared to transferring, direct epitaxy of vdW heterostructures is ideal for clean interlayer interfaces and scalable device fabrication. Here we report the synthesis and preferred orientations of 2D GaSe atomic layers on graphene (Gr) by vdW epitaxy. GaSe crystals are found to nucleate predominantly on random wrinkles or grain boundaries of graphene, share a preferred lattice orientation with underlying graphene, and grow into large (tens of micrometers) irregularly shaped, single-crystalline domains. The domains are found to propagate with triangular edges that merge into the large single crystals during growth. Electron diffraction reveals that approximately 50% of the GaSe domains are oriented with a 10.5 ± 0.3° interlayer rotation with respect to the underlying graphene. Theoretical investigations of interlayer energetics reveal that a 10.9° interlayer rotation is the most energetically preferred vdW heterostructure. In addition, strong charge transfer in these GaSe/Gr vdW heterostructures is predicted, which agrees with the observed enhancement in the Raman E(2)1g band of monolayer GaSe and highly quenched photoluminescence compared to GaSe/SiO2. Despite the very large lattice mismatch of GaSe/Gr through vdW epitaxy, the predominant orientation control and convergent formation of large single-crystal flakes demonstrated here is promising for the scalable synthesis of large-area vdW heterostructures for the development of new optical and optoelectronic devices. PMID:26202730

  9. Molecular beam epitaxy growth of high quality p-doped SnS van der Waals epitaxy on a graphene buffer layer

    SciTech Connect

    Wang, W.; Leung, K. K.; Fong, W. K.; Wang, S. F.; Surya, C.; Hui, Y. Y.; Lau, S. P.; Chen, Z.; Shi, L. J.; Cao, C. B.

    2012-05-01

    We report on the systematic investigation of optoelectronic properties of tin (IV) sulfide (SnS) van der Waals epitaxies (vdWEs) grown by molecular beam epitaxy (MBE) technique. Energy band simulation using commercial CASTEP code indicates that SnS has an indirect bandgap of size 0.982 eV. Furthermore, our simulation shows that elemental Cu can be used as a p-type dopant for the material. Growth of high quality SnS thin films is accomplished by MBE technique using graphene as the buffer layer. We observed significant reduction in the rocking curve FWHM over the existing published values. Crystallite size in the range of 2-3 {mu}m is observed which is also significantly better than the existing results. Measurement of the absorption coefficient, {alpha}, is performed using a Hitachi U-4100 Spectrophotometer system which demonstrate large values of {alpha} of the order of 10{sup 4} cm{sup -1}. Sharp cutoff in the values of {alpha}, as a function of energy, is observed for the films grown using a graphene buffer layer indicating low concentration of localized states in the bandgap. Cu-doping is achieved by co-evaporation technique. It is demonstrated that the hole concentration of the films can be controlled between 10{sup 16} cm{sup -3} and 5 x 10{sup 17}cm{sup -3} by varying the temperature of the Cu K-cell. Hole mobility as high as 81 cm{sup 2}V{sup -1}s{sup -1} is observed for SnS films on graphene/GaAs(100) substrates. The improvements in the physical properties of the films are attributed to the unique layered structure and chemically saturated bonds at the surface for both SnS and the graphene buffer layer. Consequently, the interaction between the SnS thin films and the graphene buffer layer is dominated by van der Waals force and structural defects at the interface, such as dangling bonds or dislocations, are substantially reduced.

  10. Probing the extended-state width of disorder-broadened Landau levels in epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Takase, K.; Hibino, H.; Muraki, K.

    2015-09-01

    We experimentally investigate the width of extended states in disorder-broadened Landau levels (LLs) in top-gated epitaxial graphene on silicon carbide using two different methods: gated transport spectroscopy and activation gap measurements on integer quantum Hall states. The transport spectroscopy reveals that the widths of the extended states in the zero-energy (N =0 ) and first excited (N =1 ) LLs are of similar magnitude over the ranges of magnetic field (4-16 T) and temperature studied (1.6-150 K). Under certain assumptions we find that the extended-state width follows a power-law temperature dependence with the exponent η ˜0.3 in the N =0 (N =1 ) LL, with almost no (very weak) magnetic-field dependence. Activation gap measurements at the filling factors of ν =2 and 6 give results consistent with transport spectroscopy for the N =1 LL, but indicate a larger broadening for the N =0 LL than deduced from the spectroscopy.

  11. Magnetotransport of Epitaxial Graphene on Hexagonal SiC Surface Grown with Metal Plate Capping

    NASA Astrophysics Data System (ADS)

    Park, Kibog; Jin, Han Byul; Jung, Sungchul; Kim, Junhyoung; Chae, Dong-Hun; Kim, Wan-Seop; Park, Jaesung

    High quality epitaxial graphene (EG) was grown on a Si-face hexagonal SiC substrate by capping the surface with a metal plate (Molybdenum, Tungsten) during UHV annealing. The growth temperature was ~ 950 degree C, significantly lower than the conventional UHV annealing. The crystallinity of EG film was examined with Raman spectrum measurements. Almost no D-peak and a large narrow 2D-peak ensure that a thin (mono- or bi-layer) EG film was grown with a negligible number of defects. The electrical properties of EG film were also characterized by performing magnetotransport measurements with Hall-bar structures. The carrier type was found to be n-type, the sheet carrier density be (3.6-9.2)x1012 /cm2, and the Hall mobility be ~2100 cm2/Vs. Due to the relatively high carrier density, the Quantum Hall Effect was observed only for high filling factors up to 14 T. However, clear Shubnikov-de-Hass oscillations were observed, indicating that the random carrier scattering due to impurities or defects is minimal in the EG film grown with metal plate capping. Supported by NRF in South Korea (2014M2B2A9031944).

  12. Generic epitaxial graphene biosensors for ultrasensitive detection of cancer risk biomarker

    NASA Astrophysics Data System (ADS)

    Tehrani, Z.; Burwell, G.; Mohd Azmi, M. A.; Castaing, A.; Rickman, R.; Almarashi, J.; Dunstan, P.; Miran Beigi, A.; Doak, S. H.; Guy, O. J.

    2014-09-01

    A generic electrochemical method of ‘bioreceptor’ antibody attachment to phenyl amine functionalized graphitic surfaces is demonstrated. Micro-channels of chemically modified multi-layer epitaxial graphene (MLEG) have been used to provide a repeatable and reliable response to nano-molar (nM) concentrations of the cancer risk (oxidative stress) biomarker 8-hydroxydeoxyguanosine (8-OHdG). X-ray photoelectron spectroscopy, Raman spectroscopy are used to characterize the functionalized MLEG. Confocal fluorescence microscopy using fluorescent-labelled antibodies indicates that the anti-8-OHdG antibody selectively binds to the phenyl amine-functionalized MLEG’s channel. Current-voltage measurements on functionalized channels showed repeatable current responses from antibody-biomarker binding events. This technique is scalable, reliable, and capable of providing a rapid, quantitative, label-free assessment of biomarkers at nano-molar (<20 nM) concentrations in analyte solutions. The sensitivity of the sensor device was investigated using varying concentrations of 8-OHdG, with changes in the sensor’s channel resistance observed upon exposure to 8-OHdG. Detection of 8-OHdG concentrations as low as 0.1 ng ml-1 (0.35 nM) has been demonstrated. This is five times more sensitive than reported enzyme linked immunosorbent assay tests (0.5 ng ml-1).

  13. Near-edge X-ray absorption spectroscopy signature of image potential states in multilayer epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Coelho, P. M.; dos Reis, D. D.; Matos, M. J. S.; Mendes-de-Sa, T. G.; Goncalves, A. M. B.; Lacerda, R. G.; Malachias, A.; Magalhaes-Paniago, R.

    2016-02-01

    Single layer behavior in multilayer epitaxial graphene has been a matter of intense investigation. This is due to the layer decoupling that occurs during growth of graphene on some types of substrates, such as carbon-terminated silicon carbide. We show here that near-edge X-ray absorption spectroscopy can be used to observe the signature of this decoupling. To this end, samples of multilayer graphene from silicon carbide sublimation were grown with different degrees of decoupling. Raman spectroscopy was used to infer the degree of structural decoupling. X-ray grazing-incidence diffraction and scanning tunneling microscopy showed that growth initiates with the presence of bilayer graphene commensurate structures, while layer decoupling is associated to the formation of incommensurate structures observed for longer sublimation time. Near-edge X-ray absorption spectroscopy was used to probe the electronic states above the Fermi energy. Besides the σ* and π* empty states, image potential states are observed and show a clear change of intensity as a function of incident angle. These image potential states evolve from a graphite- to graphene-like behavior as a function of growth time and can be used to infer the degree of structural coupling among layers.

  14. Epitaxial Growth of Thin Ferroelectric Polymer Films on Graphene Layer for Fully Transparent and Flexible Nonvolatile Memory.

    PubMed

    Kim, Kang Lib; Lee, Wonho; Hwang, Sun Kak; Joo, Se Hun; Cho, Suk Man; Song, Giyoung; Cho, Sung Hwan; Jeong, Beomjin; Hwang, Ihn; Ahn, Jong-Hyun; Yu, Young-Jun; Shin, Tae Joo; Kwak, Sang Kyu; Kang, Seok Ju; Park, Cheolmin

    2016-01-13

    Enhancing the device performance of organic memory devices while providing high optical transparency and mechanical flexibility requires an optimized combination of functional materials and smart device architecture design. However, it remains a great challenge to realize fully functional transparent and mechanically durable nonvolatile memory because of the limitations of conventional rigid, opaque metal electrodes. Here, we demonstrate ferroelectric nonvolatile memory devices that use graphene electrodes as the epitaxial growth substrate for crystalline poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) polymer. The strong crystallographic interaction between PVDF-TrFE and graphene results in the orientation of the crystals with distinct symmetry, which is favorable for polarization switching upon the electric field. The epitaxial growth of PVDF-TrFE on a graphene layer thus provides excellent ferroelectric performance with high remnant polarization in metal/ferroelectric polymer/metal devices. Furthermore, a fully transparent and flexible array of ferroelectric field effect transistors was successfully realized by adopting transparent poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] semiconducting polymer. PMID:26618802

  15. Effects of Pb Intercalation on the Structural and Electronic Properties of Epitaxial Graphene on SiC.

    PubMed

    Yurtsever, Ayhan; Onoda, Jo; Iimori, Takushi; Niki, Kohei; Miyamachi, Toshio; Abe, Masayuki; Mizuno, Seigi; Tanaka, Satoru; Komori, Fumio; Sugimoto, Yoshiaki

    2016-08-01

    The effects of Pb intercalation on the structural and electronic properties of epitaxial single-layer graphene grown on SiC(0001) substrate are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, Kelvin probe force microscopy (KPFM), X-ray photoelectron spectroscopy, and angle-resolved photoemission spectroscopy (ARPES) methods. The STM results show the formation of an ordered moiré superstructure pattern induced by Pb atom intercalation underneath the graphene layer. ARPES measurements reveal the presence of two additional linearly dispersing π-bands, providing evidence for the decoupling of the buffer layer from the underlying SiC substrate. Upon Pb intercalation, the Si 2p core level spectra show a signature for the existence of PbSi chemical bonds at the interface region, as manifested in a shift of 1.2 eV of the bulk SiC component toward lower binding energies. The Pb intercalation gives rise to hole-doping of graphene and results in a shift of the Dirac point energy by about 0.1 eV above the Fermi level, as revealed by the ARPES measurements. The KPFM experiments have shown that decoupling of the graphene layer by Pb intercalation is accompanied by a work function increase. The observed increase in the work function is attributed to the suppression of the electron transfer from the SiC substrate to the graphene layer. The Pb intercalated structure is found to be stable in ambient conditions and at high temperatures up to 1250 °C. These results demonstrate that the construction of a graphene-capped Pb/SiC system offers a possibility of tuning the graphene electronic properties and exploring intriguing physical properties such as superconductivity and spintronics. PMID:27295020

  16. Seeding atomic layer deposition of high-k dielectrics on epitaxial graphene with organic self-assembled monolayers.

    PubMed

    Alaboson, Justice M P; Wang, Qing Hua; Emery, Jonathan D; Lipson, Albert L; Bedzyk, Michael J; Elam, Jeffrey W; Pellin, Michael J; Hersam, Mark C

    2011-06-28

    The development of high-performance graphene-based nanoelectronics requires the integration of ultrathin and pinhole-free high-k dielectric films with graphene at the wafer scale. Here, we demonstrate that self-assembled monolayers of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) act as effective organic seeding layers for atomic layer deposition (ALD) of HfO(2) and Al(2)O(3) on epitaxial graphene on SiC(0001). The PTCDA is deposited via sublimation in ultrahigh vacuum and shown to be highly ordered with low defect density by molecular-resolution scanning tunneling microscopy. Whereas identical ALD conditions lead to incomplete and rough dielectric deposition on bare graphene, the chemical functionality provided by the PTCDA seeding layer yields highly uniform and conformal films. The morphology and chemistry of the dielectric films are characterized by atomic force microscopy, ellipsometry, cross-sectional scanning electron microscopy, and X-ray photoelectron spectroscopy, while high-resolution X-ray reflectivity measurements indicate that the underlying graphene remains intact following ALD. Using the PTCDA seeding layer, metal-oxide-graphene capacitors fabricated with a 3 nm Al(2)O(3) and 10 nm HfO(2) dielectric stack show high capacitance values of ∼700 nF/cm(2) and low leakage currents of ∼5 × 10(-9) A/cm(2) at 1 V applied bias. These results demonstrate the viability of sublimated organic self-assembled monolayers as seeding layers for high-k dielectric films in graphene-based nanoelectronics. PMID:21553842

  17. Two-dimensional massless Dirac fermions, chiral pseudo-spins, and Berry's phase in few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Baik, Seung Su; Choi, Hyoung Joon

    Black phosphorus (BP) and its two-dimensional (2D) derivative phosphorene are rapidly emerging nanoelectronic materials with potential applicability to field effect transistors and optoelectronic devices. Unlike the gapless semiconductor graphene, multilayer BP has a substantial band gap of 0.2 eV, and this band-gap size is predicted being sensitive to the external perturbations such as pressure, strain, and electric field. Very recently, a semiconductor-semimetal transition in BP was realized by the surface potassium (K) doping, producing a Dirac semimetal state with a linear dispersion in the armchair direction and a quadratic one in the zigzag direction. Here, based on first-principles density functional calculations, we present that beyond the critical K density, 2D massless Dirac fermions emerge in K-doped few-layer BP, and the electronic states around Dirac points have chiral pseudo-spins and Berry's phase. These features are robust with respect to the spin-orbit interaction. The switchable massless Dirac fermions discussed here may open a new way for the development of high performance devices in 2D materials beyond graphene. This work was supported by NRF of Korea (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2015-C3-039).

  18. Few-layer SnSe2 transistors with high on/off ratios

    NASA Astrophysics Data System (ADS)

    Pei, Tengfei; Bao, Lihong; Wang, Guocai; Ma, Ruisong; Yang, Haifang; Li, Junjie; Gu, Changzhi; Pantelides, Sokrates; Du, Shixuan; Gao, Hong-jun

    2016-02-01

    We report few-layer SnSe2 field effect transistors (FETs) with high current on/off ratios. By trying different gate configurations, 300 nm SiO2 and 70 nm HfO2 as back gate only and 70 nm HfO2 as back gate combined with a top capping layer of polymer electrolyte, few-layer SnSe2 FET with a current on/off ratio of 104 can be obtained. This provides a reliable solution for electrically modulating quasi-two-dimensional materials with high electron density (over 1013 cm-2) for field-effect transistor applications.

  19. Quantum transport measurement of few-layer WTe2 field effect devices

    NASA Astrophysics Data System (ADS)

    Chen, Jianhao; Liu, Xin; Tian, Shibing; Zhang, Chenglong; Jia, Shuang

    2015-03-01

    We have performed systematic quantum transport measurement on field effect devices fabricated from few-layer WTe2 single crystals. We found that the magnetoresistance of few-layer WTe2 could be very different from that of bulk samples, which may arise from the imbalance of electron and hole carriers in the samples. We shall discuss our findings in more details in light of recent progress in our experiment. This work is supported by National Natural Science Foundation of China (11374021 and 11327406); by China Ministry of Science and Technology under Contract # 2014CB920900 and 2013CB921900; and by the Young 1000-Talent Program of China.

  20. The nature of the interlayer interaction in bulk and few-layer phosphorus

    DOE PAGESBeta

    Shulenburger, Luke; Baczewski, A. D.; Zhu, Z.; Guan, J.; Tomanek, David

    2015-11-02

    Sensitive dependence of the electronic structure on the number of layers in few-layer phosphorene raises a question about the true nature of the interlayer interaction in so-called van der Waals (vdW) solids . We performed quantum Monte Carlo calculations and found that the interlayer interaction in bulk black phosphorus and related few-layer phosphorene is associated with a significant charge redistribution that is incompatible with purely dispersive forces and not captured by density functional theory calculations with different vdW corrected functionals. Lastly, these findings confirm the necessity of more sophisticated treatment of nonlocal electron correlation in total energy calculations.

  1. Second and third harmonic generation in few-layer gallium telluride characterized by multiphoton microscopy

    NASA Astrophysics Data System (ADS)

    Susoma, Jannatul; Karvonen, Lasse; Säynätjoki, Antti; Mehravar, Soroush; Norwood, Robert A.; Peyghambarian, Nasser; Kieu, Khanh; Lipsanen, Harri; Riikonen, Juha

    2016-02-01

    We report on the nonlinear optical properties of few-layer GaTe studied by multiphoton microscopy. Second and third harmonic generation from few-layer GaTe flakes were observed in this study with the laser pump wavelength of 1560 nm. These processes were found to be sensitive to the number of GaTe layers. The second- and third-order nonlinear susceptibilities of 2.7 × 10-9 esu (1.15 pm/V) and 1.4 × 10-8 esu (2 × 10-16 m2/V2) were estimated, respectively.

  2. Molecular-beam epitaxy and robust superconductivity of stoichiometric FeSe crystalline films on bilayer graphene

    SciTech Connect

    Song Canli; Jiang Yeping; Xue Qikun; Wang Yilin; Li Zhi; Wang Lili; He Ke; Ma Xucun; Chen Xi

    2011-07-01

    We report on molecular beam epitaxy growth of stoichiometric and superconducting FeSe crystalline thin films on double-layer graphene. Layer-by-layer growth of high-quality films has been achieved in a well-controlled manner by using Se-rich condition, which allow us to investigate the thickness-dependent superconductivity of FeSe. In situ low-temperature scanning tunneling spectra reveal that the local superconducting gap in the quasiparticle density of states is visible down to two triple layers for the minimum measurement temperature of 2.2 K, and that the transition temperature T{sub c} scales inversely with film thickness.

  3. Molecular beam epitaxy growth of SrO buffer layers on graphite and graphene for the integration of complex oxides

    DOE PAGESBeta

    Ahmed, Adam S.; Wen, Hua; Ohta, Taisuke; Pinchuk, Igor V.; Zhu, Tiancong; Beechem, Thomas; Kawakami, Roland K.

    2016-04-27

    Here, we report the successful growth of high-quality SrO films on highly-ordered pyrolytic graphite (HOPG) and single-layer graphene by molecular beam epitaxy. The SrO layers have (001) orientation as confirmed by X-ray diffraction (XRD) while atomic force microscopy measurements show continuous pinhole-free films having rms surface roughness of <1.5 Å. Moreover, transport measurements of exfoliated graphene, after SrO deposition, show a strong dependence between the Dirac point and Sr oxidation. As a result, the SrO is leveraged as a buffer layer for more complex oxide integration via the demonstration of (001) oriented SrTiO3 grown atop a SrO/HOPG stack.

  4. Molecular beam epitaxy growth of SrO buffer layers on graphite and graphene for the integration of complex oxides

    NASA Astrophysics Data System (ADS)

    Ahmed, Adam S.; Wen, Hua; Ohta, Taisuke; Pinchuk, Igor V.; Zhu, Tiancong; Beechem, Thomas; Kawakami, Roland K.

    2016-08-01

    We report the successful growth of high-quality SrO films on highly-ordered pyrolytic graphite (HOPG) and single-layer graphene by molecular beam epitaxy. The SrO layers have (001) orientation as confirmed by X-ray diffraction (XRD) while atomic force microscopy measurements show continuous pinhole-free films having rms surface roughness of <1.5 Å. Transport measurements of exfoliated graphene after SrO deposition show a strong dependence between the Dirac point and Sr oxidation. Subsequently, the SrO is leveraged as a buffer layer for more complex oxide integration via the demonstration of (001) oriented SrTiO3 grown atop a SrO/HOPG stack.

  5. Single- and few-layer WTe2 and their suspended nanostructures: Raman signatures and nanomechanical resonances

    NASA Astrophysics Data System (ADS)

    Lee, Jaesung; Ye, Fan; Wang, Zenghui; Yang, Rui; Hu, Jin; Mao, Zhiqiang; Wei, Jiang; Feng, Philip X.-L.

    2016-04-01

    Single crystal tungsten ditelluride (WTe2) has recently been discovered to exhibit non-saturating extreme magnetoresistance in bulk; it has also emerged as a new layered material from which atomic layer crystals can be extracted. While atomically thin WTe2 is attractive for its unique properties, little research has been conducted on single- and few-layer WTe2. Here we report the isolation of single- and few-layer WTe2, as well as the fabrication and characterization of the first WTe2 suspended nanostructures. We have observed new Raman signatures of single- and few-layer WTe2 that have been theoretically predicted but have not been reported to date, in both on-substrate and suspended WTe2 flakes. We have further probed the nanomechanical properties of suspended WTe2 structures by measuring their flexural resonances, and obtain a Young's modulus of EY ~ 80 GPa for the suspended WTe2 flakes. This study paves the way for future investigations and utilizations of the multiple new Raman fingerprints of single- and few-layer WTe2, and for explorations of mechanical control of WTe2 atomic layers.

  6. Single- and few-layer WTe2 and their suspended nanostructures: Raman signatures and nanomechanical resonances.

    PubMed

    Lee, Jaesung; Ye, Fan; Wang, Zenghui; Yang, Rui; Hu, Jin; Mao, Zhiqiang; Wei, Jiang; Feng, Philip X-L

    2016-04-14

    Single crystal tungsten ditelluride (WTe2) has recently been discovered to exhibit non-saturating extreme magnetoresistance in bulk; it has also emerged as a new layered material from which atomic layer crystals can be extracted. While atomically thin WTe2 is attractive for its unique properties, little research has been conducted on single- and few-layer WTe2. Here we report the isolation of single- and few-layer WTe2, as well as the fabrication and characterization of the first WTe2 suspended nanostructures. We have observed new Raman signatures of single- and few-layer WTe2 that have been theoretically predicted but have not been reported to date, in both on-substrate and suspended WTe2 flakes. We have further probed the nanomechanical properties of suspended WTe2 structures by measuring their flexural resonances, and obtain a Young's modulus of EY ≈ 80 GPa for the suspended WTe2 flakes. This study paves the way for future investigations and utilizations of the multiple new Raman fingerprints of single- and few-layer WTe2, and for explorations of mechanical control of WTe2 atomic layers. PMID:27030574

  7. Isolation and characterization of nanosheets containing few layers of the Aurivillius family of oxides and metal-organic compounds

    SciTech Connect

    Sreedhara, M.B.; Prasad, B.E.; Moirangthem, Monali; Murugavel, R.; Rao, C.N.R.

    2015-04-15

    Nanosheets containing few-layers of ferroelectric Aurivillius family of oxides, Bi{sub 2}A{sub n−1}B{sub n}O{sub 3n+3} (where A=Bi{sup 3+}, Ba{sup 2+} etc. and B=Ti{sup 4+}, Fe{sup 3+} etc.) with n=3, 4, 5, 6 and 7 have been prepared by reaction with n-butyllithium, followed by exfoliation in water. The few-layer samples have been characterized by Tyndall cones, atomic force microscopy, optical spectroscopy and other techniques. The few-layer species have a thickness corresponding to a fraction of the c-parameter along which axis the perovskite layers are stacked. Magnetization measurements have been carried out on the few-layer samples containing iron. Few-layer species of a few layered metal-organic compounds have been obtained by ultrasonication and characterized by Tyndall cones, atomic force microscopy, optical spectroscopy and magnetic measurements. Significant changes in the optical spectra and magnetic properties are found in the few-layer species compared to the bulk samples. Few-layer species of the Aurivillius family of oxides may find uses as thin layer dielectrics in photovoltaics and other applications. - Graphical abstract: Exfoliation of the layered Aurivillius oxides into few-layer nanosheets by chemical Li intercalation using n-BuLi followed by reaction in water. Exfoliation of the layered metal-organic compounds into few-layer nanosheets by ultrasonication. - Highlights: • Few-layer nanosheets of Aurivillius family of oxides with perovskite layers have been generated by lithium intercalation. • Few-layer nanosheets of few layered metal-organic compounds have been generated by ultrasonication. • Few-layer nanosheets of the Aurivillius oxides have been characterized by AFM, TEM and optical spectroscopy. • Aurivillius oxides containing Fe show layer dependent magnetic properties. • Exfoliated few-layer metal-organic compounds show changes in spectroscopic and magnetic properties compared with bulk materials.

  8. Three-fold diffraction symmetry in epitaxial graphene and the SiC substrate

    SciTech Connect

    Siegel, D A; Zhou, S Y; El Gabaly, F; Schmid, A K; McCarty, K F; Lanzara, A

    2009-12-10

    The crystallographic symmetries and spatial distribution of stacking domains in graphene films on 6H-SiC(0001) have been studied by low energy electron diffraction (LEED) and dark field imaging in a low energy electron microscope (LEEM). We find that the graphene diffraction spots from 2 and 3 atomic layers of graphene have 3-fold symmetry consistent with AB (Bernal or rhombohedral) stacking of the layers. On the contrary, graphene diffraction spots from the buffer layer and monolayer graphene have apparent 6-fold symmetry, although the 3-fold nature of the satellite spots indicates a more complex periodicity in the graphene sheets.

  9. Synthesis of nitrogen-doped epitaxial graphene via plasma-assisted method: Role of the graphene-substrate interaction

    NASA Astrophysics Data System (ADS)

    Orlando, Fabrizio; Lacovig, Paolo; Dalmiglio, Matteo; Baraldi, Alessandro; Larciprete, Rosanna; Lizzit, Silvano

    2016-01-01

    Functionalization of graphene by substitution of carbon with nitrogen atoms is a promising way to tailor its electronic properties, but a good control over the heteroatomic configuration in the graphene network is most often a difficult task. In this paper, the synthesis of N-doped graphene by nitrogen plasma treatment of graphene/Ir(111) is presented. The formation of substitutional, pyrrolic and pyridinic nitrogen is analyzed by means of X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD). The graphene-Ir interaction is suggested to control the variation in the relative concentration of the nitrogen species. Annealing of the sample also leads to modifications of the nitrogen species incorporated in the graphene layer. Furthermore, the connection of the substitutional nitrogen arrangement with its corresponding spectroscopic fingerprint is unequivocally confirmed by XPD measurements, which give also a direct insight on the local geometry of the nitrogen atoms incorporated in the carbon network.

  10. High photosensitivity few-layered MoSe2 back-gated field-effect phototransistors

    NASA Astrophysics Data System (ADS)

    Abderrahmane, A.; Ko, P. J.; Thu, T. V.; Ishizawa, S.; Takamura, T.; Sandhu, A.

    2014-09-01

    In this paper, we report on the fabrication and optoelectronic properties of high sensitive phototransistors based on few-layered MoSe2 back-gated field-effect transistors, with a mobility of 19.7 cm2 V-1 s-1 at room temperature. We obtained an ultrahigh photoresponsivity of 97.1 AW-1 and an external quantum efficiency (EQE) of 22 666% using 532 nm laser excitation at room temperature. The photoresponsivity was improved near the threshold gate voltage; however, the selection of the silicon dioxide as a gate oxide represents a limiting factor in the ultimate performance. Thanks to their high photoresponsivity and external quantum efficiency, the few-layered MoSe2-based devices are promising for photoelectronic applications.

  11. Femtosecond solid-state laser based on a few-layered black phosphorus saturable absorber.

    PubMed

    Su, Xiancui; Wang, Yiran; Zhang, Baitao; Zhao, Ruwei; Yang, Kejian; He, Jingliang; Hu, Qiangqiang; Jia, Zhitai; Tao, Xutang

    2016-05-01

    In this Letter, a high-quality, few-layered black phosphorus (BP) saturable absorber (SA) was fabricated successfully, and a femtosecond solid-state laser modulated by BP-SA was experimentally demonstrated for the first time, to the best of our knowledge. Pulses as short as 272 fs were achieved with an average output power of 0.82 W, corresponding to the pulse energy of 6.48 nJ and peak power of 23.8 MW. So far, these represent the shortest pulse duration and highest output power ever obtained with a BP-based mode-locked solid-state laser. The results indicate the promising potential of few-layered BP-SA for applications in solid-state femtosecond mode-locked lasers. PMID:27128045

  12. Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics

    PubMed Central

    Hanlon, Damien; Backes, Claudia; Doherty, Evie; Cucinotta, Clotilde S.; Berner, Nina C.; Boland, Conor; Lee, Kangho; Harvey, Andrew; Lynch, Peter; Gholamvand, Zahra; Zhang, Saifeng; Wang, Kangpeng; Moynihan, Glenn; Pokle, Anuj; Ramasse, Quentin M.; McEvoy, Niall; Blau, Werner J.; Wang, Jun; Abellan, Gonzalo; Hauke, Frank; Hirsch, Andreas; Sanvito, Stefano; O'Regan, David D.; Duesberg, Georg S.; Nicolosi, Valeria; Coleman, Jonathan N.

    2015-01-01

    Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement. PMID:26469634

  13. Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics.

    PubMed

    Hanlon, Damien; Backes, Claudia; Doherty, Evie; Cucinotta, Clotilde S; Berner, Nina C; Boland, Conor; Lee, Kangho; Harvey, Andrew; Lynch, Peter; Gholamvand, Zahra; Zhang, Saifeng; Wang, Kangpeng; Moynihan, Glenn; Pokle, Anuj; Ramasse, Quentin M; McEvoy, Niall; Blau, Werner J; Wang, Jun; Abellan, Gonzalo; Hauke, Frank; Hirsch, Andreas; Sanvito, Stefano; O'Regan, David D; Duesberg, Georg S; Nicolosi, Valeria; Coleman, Jonathan N

    2015-01-01

    Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement. PMID:26469634

  14. Spiral Growth of Few-Layer MoS2 by Chemical Vapor Deposition

    NASA Astrophysics Data System (ADS)

    Dong, Xi; Tomer, Dushyant; Li, Lian

    Monolayer and few-layer transition metal dichalcogenide MoS2 are grown by chemical vapor deposition on SiO2/Si substrates using MoO3 and S powder as precursors. Before growth, the substrates are pretreated with perylene-3, 4, 9, 10-tetracarboxylic acid tetrapotassium salt to promote nucleation. Monolayer MoS2 islands are triangularly shaped with sizes ranging from a few to tens of micrometers, which also exhibits the characteristic Raman bands at 403.36 and 385.05 cm-1 corresponding to the A1g and E2g modes, respectively. Atomic force microscopy imaging further confirms the monolayer thickness to be 0.8 nm. For few-layer MoS2 films, triangular spirals are observed with both left- and right-handed chirality. Raman spectra showed interesting features of these growth spirals, the details of which will be presented at the meeting. NSF DMR-1508560.

  15. Low-Frequency Interlayer Breathing Modes in Few-Layer Black Phosphorus

    DOE PAGESBeta

    Ling, Xi; Liang, Liangbo; Huang, Shengxi; Puretzky, Alexander A.; Geohegan, David B.; Sumpter, Bobby G.; Kong, Jing; Meunier, Vincent; Dresselhaus, Mildred S.

    2015-05-08

    As a new two-dimensional layered material, black phosphorus (BP) is a very promising material for nanoelectronics and nano-optoelectronics. We use Raman spectroscopy and first-principles theory to characterize and understand low-frequency (LF) interlayer breathing modes (<100 cm-1) in few-layer BP for the first time. Using laser polarization dependence study and group theory analysis the breathing modes are assigned to Ag symmetry. Compared to the high-frequency (HF) Raman modes, the LF breathing modes are considerably more sensitive to interlayer coupling and thus their frequencies show stronger dependence on the number of layers. Hence, they constitute an effective means to probe both themore » crystalline orientation and thickness of few-layer BP. Furthermore, the temperature dependence shows that the breathing modes have a harmonic behavior, in contrast to HF Raman modes which exhibit anharmonicity.« less

  16. Low-Frequency Interlayer Breathing Modes in Few-Layer Black Phosphorus

    SciTech Connect

    Ling, Xi; Liang, Liangbo; Huang, Shengxi; Puretzky, Alexander A.; Geohegan, David B.; Sumpter, Bobby G.; Kong, Jing; Meunier, Vincent; Dresselhaus, Mildred S.

    2015-05-08

    As a new two-dimensional layered material, black phosphorus (BP) is a very promising material for nanoelectronics and nano-optoelectronics. We use Raman spectroscopy and first-principles theory to characterize and understand low-frequency (LF) interlayer breathing modes (<100 cm-1) in few-layer BP for the first time. Using laser polarization dependence study and group theory analysis the breathing modes are assigned to Ag symmetry. Compared to the high-frequency (HF) Raman modes, the LF breathing modes are considerably more sensitive to interlayer coupling and thus their frequencies show stronger dependence on the number of layers. Hence, they constitute an effective means to probe both the crystalline orientation and thickness of few-layer BP. Furthermore, the temperature dependence shows that the breathing modes have a harmonic behavior, in contrast to HF Raman modes which exhibit anharmonicity.

  17. Few-layer MoS2: a promising layered semiconductor.

    PubMed

    Ganatra, Rudren; Zhang, Qing

    2014-05-27

    Due to the recent expanding interest in two-dimensional layered materials, molybdenum disulfide (MoS2) has been receiving much research attention. Having an ultrathin layered structure and an appreciable direct band gap of 1.9 eV in the monolayer regime, few-layer MoS2 has good potential applications in nanoelectronics, optoelectronics, and flexible devices. In addition, the capability of controlling spin and valley degrees of freedom makes it a promising material for spintronic and valleytronic devices. In this review, we attempt to provide an overview of the research relevant to the structural and physical properties, fabrication methods, and electronic devices of few-layer MoS2. Recent developments and advances in studying the material are highlighted. PMID:24660756

  18. Near-field optical microscopy and spectroscopy of few-layer black phosphorous

    NASA Astrophysics Data System (ADS)

    Frenzel, A. J.; Tran, S.; Hinton, J. P.; Sternbach, A. J.; Yang, J.; Gillgren, N.; Lau, C. N.; Basov, D. N.

    Few-layer black phosphorous is a recent addition to the family of two-dimensional (2D) materials which exhibits strongly anisotropic transport and optical properties due to its puckered honeycomb structure. It was recently predicted that this intrinsic anisotropy should manifest in the plasmon dispersion. Additionally, tuning layer number and carrier density can control the dispersion of these collective modes. Scanning near-field optical microscopy (SNOM) has been demonstrated as a powerful method to probe electronic properties, including propagating collective modes, in layered 2D materials. We used SNOM to investigate anisotropic carrier response in few-layer black phosphorous encapsulated by hexagonal boron nitride. In addition to exploring gate-voltage tunability of the electronic response, we demonstrate effective modulation of the near-field signal by ultrafast photoexcitation.

  19. Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics

    NASA Astrophysics Data System (ADS)

    Hanlon, Damien; Backes, Claudia; Doherty, Evie; Cucinotta, Clotilde S.; Berner, Nina C.; Boland, Conor; Lee, Kangho; Harvey, Andrew; Lynch, Peter; Gholamvand, Zahra; Zhang, Saifeng; Wang, Kangpeng; Moynihan, Glenn; Pokle, Anuj; Ramasse, Quentin M.; McEvoy, Niall; Blau, Werner J.; Wang, Jun; Abellan, Gonzalo; Hauke, Frank; Hirsch, Andreas; Sanvito, Stefano; O'Regan, David D.; Duesberg, Georg S.; Nicolosi, Valeria; Coleman, Jonathan N.

    2015-10-01

    Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement.

  20. Strain- and twist-engineered optical absorption of few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Jia, Qian; Kong, XiangHua; Qiao, JingSi; Ji, Wei

    2016-09-01

    Density functional and many-body perturbation theories calculations were carried out to investigate fundamental and optical bandgap, exciton binding energy and optical absorption property of normal and strain- and twist-engineered few-layer black phosphorus (BP). We found that the fundamental bandgaps of few layer BP can be engineered by layer stacking and in-plane strain, with linear relationships to their associated exciton binding energies. The strain-dependent optical absorption behaviors are also anisotropic that the position of the first absorption peak monotonically blue-shifts as the strain applies to either direction for incident light polarized along the armchair direction, but this is not the case for that along the zigzag direction. Given those striking properties, we proposed two prototype devices for building potentially more balanced light absorbers and light filter passes, which promotes further applications and investigations of BP in nanoelectronics and optoelectronics.

  1. Large-Area Synthesis of High-Quality Uniform Few-Layer MoTe2.

    PubMed

    Zhou, Lin; Xu, Kai; Zubair, Ahmad; Liao, Albert D; Fang, Wenjing; Ouyang, Fangping; Lee, Yi-Hsien; Ueno, Keiji; Saito, Riichiro; Palacios, Tomás; Kong, Jing; Dresselhaus, Mildred S

    2015-09-23

    The controlled synthesis of large-area, atomically thin molybdenum ditelluride (MoTe2) crystals is crucial for its various applications based on the attractive properties of this emerging material. In this work, we developed a chemical vapor deposition synthesis to produce large-area, uniform, and highly crystalline few-layer 2H and 1T' MoTe2 films. It was found that these two different phases of MoTe2 can be grown depending on the choice of Mo precursor. Because of the highly crystalline structure, the as-grown few-layer 2H MoTe2 films display electronic properties that are comparable to those of mechanically exfoliated MoTe2 flakes. Our growth method paves the way for the large-scale application of MoTe2 in high-performance nanoelectronics and optoelectronics. PMID:26305492

  2. Low-Frequency Interlayer Breathing Modes in Few-Layer Black Phosphorus.

    PubMed

    Ling, Xi; Liang, Liangbo; Huang, Shengxi; Puretzky, Alexander A; Geohegan, David B; Sumpter, Bobby G; Kong, Jing; Meunier, Vincent; Dresselhaus, Mildred S

    2015-06-10

    As a new two-dimensional layered material, black phosphorus (BP) is a very promising material for nanoelectronics and optoelectronics. We use Raman spectroscopy and first-principles theory to characterize and understand the low-frequency (LF) interlayer breathing modes (<100 cm(-1)) in few-layer BP for the first time. Using a laser polarization dependence study and group theory analysis, the breathing modes are assigned to Ag symmetry. Compared to the high-frequency (HF) Raman modes, the LF breathing modes are considerably more sensitive to interlayer coupling and, thus, their frequencies show a stronger dependence on the number of layers. Hence, they constitute an effective means to probe both the crystalline orientation and thickness of few-layer BP. Furthermore, the temperature dependence shows that in the temperature range -150 to 30 °C, the breathing modes have a weak anharmonic behavior, in contrast to the HF Raman modes that exhibit strong anharmonicity. PMID:25955659

  3. Visualization of Grain Structure and Boundaries of Polycrystalline Graphene and Two-Dimensional Materials by Epitaxial Growth of Transition Metal Dichalcogenides.

    PubMed

    Ago, Hiroki; Fukamachi, Satoru; Endo, Hiroko; Solís-Fernández, Pablo; Yunus, Rozan Mohamad; Uchida, Yuki; Panchal, Vishal; Kazakova, Olga; Tsuji, Masaharu

    2016-03-22

    The presence of grain boundaries in two-dimensional (2D) materials is known to greatly affect their physical, electrical, and chemical properties. Given the difficulty in growing perfect large single-crystals of 2D materials, revealing the presence and characteristics of grain boundaries becomes an important issue for practical applications. Here, we present a method to visualize the grain structure and boundaries of 2D materials by epitaxially growing transition metal dichalcogenides (TMDCs) over them. Triangular single-crystals of molybdenum disulfide (MoS2) epitaxially grown on the surface of graphene allowed us to determine the orientation and size of the graphene grains. Grain boundaries in the polycrystalline graphene were also visualized reflecting their higher chemical reactivity than the basal plane. The method was successfully applied to graphene field-effect transistors, revealing the actual grain structures of the graphene channels. Moreover, we demonstrate that this method can be extended to determine the grain structure of other 2D materials, such as tungsten disulfide (WS2). Our visualization method based on van der Waals epitaxy can offer a facile and large-scale labeling technique to investigate the grain structures of various 2D materials, and it will also contribute to understand the relationship between their grain structure and physical properties. PMID:26943750

  4. Interlayer breathing and shear modes in few-layer black phosphorus.

    PubMed

    Jiang, Jin-Wu; Wang, Bing-Shen; Park, Harold S

    2016-04-27

    The interlayer breathing and shear modes in few-layer black phosphorus are investigated for their symmetry and lattice dynamical properties. The symmetry groups for the even-layer and odd-layer few-layer black phosphorus are utilized to determine the irreducible representation and the infrared and Raman activity for the interlayer modes. The valence force field model is applied to calculate the eigenvectors and frequencies for the interlayer breathing and shear modes, which are explained using the atomic chain model. The anisotropic puckered configuration for black phosphorus leads to a highly anisotropic frequency for the two interlayer shear modes. More specifically, the frequency for the shear mode in the direction perpendicular to the pucker is less than half of the shear mode in the direction parallel with the pucker. We also report a set of specular interlayer modes having the same frequency for all few-layer black phosphorus with layer numbers N being a multiple of 3, because these modes manifest themselves as collective vibrations of atoms in specific layers. The optical activity of the collective modes enables possible experimental identification for these modes. PMID:26988113

  5. Interlayer breathing and shear modes in few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Jiang, Jin-Wu; Wang, Bing-Shen; Park, Harold S.

    2016-04-01

    The interlayer breathing and shear modes in few-layer black phosphorus are investigated for their symmetry and lattice dynamical properties. The symmetry groups for the even-layer and odd-layer few-layer black phosphorus are utilized to determine the irreducible representation and the infrared and Raman activity for the interlayer modes. The valence force field model is applied to calculate the eigenvectors and frequencies for the interlayer breathing and shear modes, which are explained using the atomic chain model. The anisotropic puckered configuration for black phosphorus leads to a highly anisotropic frequency for the two interlayer shear modes. More specifically, the frequency for the shear mode in the direction perpendicular to the pucker is less than half of the shear mode in the direction parallel with the pucker. We also report a set of specular interlayer modes having the same frequency for all few-layer black phosphorus with layer numbers N being a multiple of 3, because these modes manifest themselves as collective vibrations of atoms in specific layers. The optical activity of the collective modes enables possible experimental identification for these modes.

  6. Growth mechanism of pulsed laser fabricated few-layer MoS₂ on metal substrates.

    PubMed

    Loh, Tamie A J; Chua, Daniel H C

    2014-09-24

    Pulsed laser deposition (PLD) on metal substrates has recently been discovered to present an alternative method for producing highly crystalline few-layer MoS2. However, not every metal behaves in the same manner during film growth, and hence, it is crucial that the ability of various metals to produce crystalline MoS2 be thoroughly investigated. In this work, MoS2 was deposited on metal substrates, Al, Ag, Ni, and Cu, using a pulsed laser. Highly crystalline few-layer MoS2 was successfully grown on Ag, but is absent in Al, Ni, and Cu under specific growth conditions. This discrepancy was attributed to either excessively strong or insufficient adlayer-substrate interactions. In the case of Al, the effects of the strong interface interactions can be offset by increasing the amount of source atoms supplied, thereby producing semicrystalline few-layer MoS2. The results show that despite PLD being a physical vapor deposition technique, both physical and chemical processes play an important role in MoS2 growth on metal substrates. PMID:25203278

  7. Anisotropic optical properties of few-layer transition metal dichalcogenide ReS2

    NASA Astrophysics Data System (ADS)

    Li, Zhenglu; Cao, Ting; da Jornada, Felipe H.; Wu, Meng; Louie, Steven G.

    We present first-principles (DFT, GW and GW-BSE) calculations of the electronic and optical properties of few-layer rhenium disulfide (ReS2). Monolayer ReS2 shows strong many-electron effects with a fundamental quasiparticle band gap of 2.38 eV based on G0W0 calculation and a large exciton binding energy of 690 meV based on solving the Bethe-Salpeter equation. Highly anisotropic linear-polarized optical absorptions are revealed for few-layer and bulk ReS2. The band gap shows a decreasing trend with the optical polarization direction near the absorption edge gradually rotating from around 67 degree in the monolayer to 85 degree in the bulk, referencing to the Re-chain. Our calculations are consistent with recent experimental data and theoretical studies, and provide a systematic understanding of the electronic and optical properties in few-layer ReS2. This work was supported by National Science Foundation Grant No. DMR15-1508412 and the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at Lawrence Berkeley National Laboratory's NERSC facility.

  8. Metalorganic chemical vapor deposition of few-layer sp2 bonded boron nitride films

    NASA Astrophysics Data System (ADS)

    Paduano, Qing; Snure, Michael; Weyburne, David; Kiefer, Arnold; Siegel, Gene; Hu, Jianjun

    2016-09-01

    A systematic study of the growth of atomically smooth few-layer sp2 bonded BN on 50 mm sapphire substrates by metalorganic chemical vapor deposition (MOCVD) using Triethylboron (TEB) and NH3 as precursors is described. Based on the experimental results obtained using Raman spectroscopy, atomic force microscopy (AFM), X-ray reflectance measurements and transmission electron microscopy, we explored the growth parameter space and identified three different growth modes: random three-dimensional (3D) growth, a self-terminating few-layer growth mode, and a very slow layer-by-layer mode. The growth mode depends on the temperature, pressure, V/III ratio, and surface nitridation conditions, as follows: 3D island growth is dominant in the low V/III range and is characterized by a decreasing growth rate with increasing deposition temperature. When the V/III ratio is increased this 3D island growth mode transitions to a self-terminating few-layer growth mode. An additional transition from self-terminating growth to 3D growth occurs when the growth pressure is increased. Very slow layer by layer growth is found at high temperature and low pressure. Finally, substrate surface nitridation promotes self-terminating growth that results in atomically smooth films.

  9. 1.06 μm Q-switched ytterbium-doped fiber laser using few-layer topological insulator Bi₂Se₃ as a saturable absorber.

    PubMed

    Luo, Zhengqian; Huang, Yizhong; Weng, Jian; Cheng, Huihui; Lin, Zhiqing; Xu, Bin; Cai, Zhiping; Xu, Huiying

    2013-12-01

    Passive Q-switching of an ytterbium-doped fiber (YDF) laser with few-layer topological insulator (TI) is, to the best of our knowledge, experimentally demonstrated for the first time. The few-layer TI: Bi₂Se₃ (2-4 layer thickness) is firstly fabricated by the liquid-phase exfoliation method, and has a low saturable optical intensity of 53 MW/cm² measured by the Z-scan technique. The optical deposition technique is used to induce the few-layer TI in the solution onto a fiber ferrule for successfully constructing the fiber-integrated TI-based saturable absorber (SA). By inserting this SA into the YDF laser cavity, stable Q-switching operation at 1.06 μm is achieved. The Q-switched pulses have the shortest pulse duration of 1.95 μs, the maximum pulse energy of 17.9 nJ and a tunable pulse-repetition-rate from 8.3 to 29.1 kHz. Our results indicate that the TI as a SA is also available at 1 μm waveband, revealing its potential as another broadband SA (like graphene). PMID:24514504

  10. Quantum-confinement and Structural Anisotropy result in Electrically-Tunable Dirac Cone in Few-layer Black Phosphorous

    PubMed Central

    Dolui, Kapildeb; Quek, Su Ying

    2015-01-01

    Two-dimensional (2D) materials are well-known to exhibit interesting phenomena due to quantum confinement. Here, we show that quantum confinement, together with structural anisotropy, result in an electric-field-tunable Dirac cone in 2D black phosphorus. Using density functional theory calculations, we find that an electric field, Eext, applied normal to a 2D black phosphorus thin film, can reduce the direct band gap of few-layer black phosphorus, resulting in an insulator-to-metal transition at a critical field, Ec. Increasing Eext beyond Ec can induce a Dirac cone in the system, provided the black phosphorus film is sufficiently thin. The electric field strength can tune the position of the Dirac cone and the Dirac-Fermi velocities, the latter being similar in magnitude to that in graphene. We show that the Dirac cone arises from an anisotropic interaction term between the frontier orbitals that are spatially separated due to the applied field, on different halves of the 2D slab. When this interaction term becomes vanishingly small for thicker films, the Dirac cone can no longer be induced. Spin-orbit coupling can gap out the Dirac cone at certain electric fields; however, a further increase in field strength reduces the spin-orbit-induced gap, eventually resulting in a topological-insulator-to-Dirac-semimetal transition. PMID:26129645

  11. Thermal expansion, anharmonicity and temperature-dependent Raman spectra of single- and few-layer MoSe₂ and WSe₂.

    PubMed

    Late, Dattatray J; Shirodkar, Sharmila N; Waghmare, Umesh V; Dravid, Vinayak P; Rao, C N R

    2014-06-01

    We report the temperature-dependent Raman spectra of single- and few-layer MoSe2 and WSe2 in the range 77-700 K. We observed linear variation in the peak positions and widths of the bands arising from contributions of anharmonicity and thermal expansion. After characterization using atomic force microscopy and high-resolution transmission electron microscopy, the temperature coefficients of the Raman modes were determined. Interestingly, the temperature coefficient of the A(2)(2u) mode is larger than that of the A(1g) mode, the latter being much smaller than the corresponding temperature coefficients of the same mode in single-layer MoS2 and of the G band of graphene. The temperature coefficients of the two modes in single-layer MoSe2 are larger than those of the same modes in single-layer WSe2. We have estimated thermal expansion coefficients and temperature dependence of the vibrational frequencies of MoS2 and MoSe2 within a quasi-harmonic approximation, with inputs from first-principles calculations based on density functional theory. We show that the contrasting temperature dependence of the Raman-active mode A(1g) in MoS2 and MoSe2 arises essentially from the difference in their strain-phonon coupling. PMID:24692405

  12. Few-layered MoS2 nanosheets wrapped ultrafine TiO2 nanobelts with enhanced photocatalytic property.

    PubMed

    Li, Haidong; Wang, Yana; Chen, Guohui; Sang, Yuanhua; Jiang, Huaidong; He, Jiating; Li, Xu; Liu, Hong

    2016-03-10

    Photocatalytic materials comprised of semiconductor nanostructures have attracted tremendous scientific and technological interest over the last 30 years. This is due to the fact that these photocatalytic materials have unique properties that allow for an effective direct energy transfer from light to highly reactive chemical species which are applicable in the remediation of environmental pollutants and photocatalytic hydrogen generation. Heterostructured photocatalysts are a promising type of photocatalyst which can combine the properties of different components to generate a synergic effect, resulting in a high photocatalytic activity. In this work, a heterostructured photocatalyst comprised of few-layered MoS2 nanosheets coated on a TiO2 nanobelts surface was synthesized through a simple hydrothermal treatment. The hybrid heterostructures with enhanced broad spectrum photocatalytic properties can harness UV and visible light energy to decompose organic contaminants in aqueous solutions as well as split water to hydrogen and oxygen. The mechanism of the enhancement is that the MoS2/TiO2 nanobelts heterostructure can enhance the separation of the photo-induced carriers, which results in a higher photocurrent due to the special electronic characteristics of the graphene-like layered MoS2 nanosheets. This methodology is potentially applicable to the synthesis of a range of hybrid nanostructures with promising applications in photocatalysis and other relevant areas. PMID:26932455

  13. Integration of hexagonal boron nitride with quasi-freestanding epitaxial graphene: toward wafer-scale, high-performance devices.

    PubMed

    Bresnehan, Michael S; Hollander, Matthew J; Wetherington, Maxwell; LaBella, Michael; Trumbull, Kathleen A; Cavalero, Randal; Snyder, David W; Robinson, Joshua A

    2012-06-26

    Hexagonal boron nitride (h-BN) is a promising dielectric material for graphene-based electronic devices. Here we investigate the potential of h-BN gate dielectrics, grown by chemical vapor deposition (CVD), for integration with quasi-freestanding epitaxial graphene (QFEG). We discuss the large scale growth of h-BN on copper foil via a catalytic thermal CVD process and the subsequent transfer of h-BN to a 75 mm QFEG wafer. X-ray photoelectron spectroscopy (XPS) measurements confirm the absence of h-BN/graphitic domains and indicate that the film is chemically stable throughout the transfer process, while Raman spectroscopy indicates a 42% relaxation of compressive stress following removal of the copper substrate and subsequent transfer of h-BN to QFEG. Despite stress-induced wrinkling observed in the films, Hall effect measurements show little degradation (<10%) in carrier mobility for h-BN coated QFEG. Temperature dependent Hall measurements indicate little contribution from remote surface optical phonon scattering and suggest that, compared to HfO(2) based dielectrics, h-BN can be an excellent material for preserving electrical transport properties. Graphene transistors utilizing h-BN gates exhibit peak intrinsic cutoff frequencies >30 GHz (2.4× that of HfO(2)-based devices). PMID:22545808

  14. Determination of the Schottky barrier height of ferromagnetic contacts to few-layer phosphorene

    SciTech Connect

    Anugrah, Yoska; Robbins, Matthew C.; Koester, Steven J.; Crowell, Paul A.

    2015-03-09

    Phosphorene, the 2D analogue of black phosphorus, is a promising material for studying spin transport due to its low spin-orbit coupling and its ½ nuclear spin, which could allow the study of hyperfine effects. In this work, the properties of permalloy (Py) and cobalt (Co) contacts to few-layer phosphorene are presented. The Schottky barrier height was extracted and determined as a function of gate bias. Flat-band barrier heights, relative to the valence band edge, of 110 meV and 200 meV were determined for Py and Co, respectively. These results are important for future studies of spin transport in phosphorene.

  15. Third order nonlinear optical response exhibited by mono- and few-layers of WS2

    NASA Astrophysics Data System (ADS)

    Torres-Torres, Carlos; Perea-López, Néstor; Elías, Ana Laura; Gutiérrez, Humberto R.; Cullen, David A.; Berkdemir, Ayse; López-Urías, Florentino; Terrones, Humberto; Terrones, Mauricio

    2016-06-01

    In this work, strong third order nonlinear optical properties exhibited by WS2 layers are presented. Optical Kerr effect was identified as the dominant physical mechanism responsible for these third order optical nonlinearities. An extraordinary nonlinear refractive index together with an important contribution of a saturated absorptive response was observed to depend on the atomic layer stacking. Comparative experiments performed in mono- and few-layer samples of WS2 revealed that this material is potentially capable of modulating nonlinear optical processes by selective near resonant induced birefringence. We envision applications for developing all-optical bidimensional nonlinear optical devices.

  16. Stark Effect Spectroscopy of Mono- and Few-Layer MoS2.

    PubMed

    Klein, J; Wierzbowski, J; Regler, A; Becker, J; Heimbach, F; Müller, K; Kaniber, M; Finley, J J

    2016-03-01

    We demonstrate electrical control of the A-exciton interband transition in mono- and few-layer MoS2 crystals embedded into photocapacitor devices via the DC Stark effect. Electric field-dependent low-temperature photoluminescence spectroscopy reveals a significant tuneability of the A-exciton transition energy up to ∼ 16 meV from which we extract the mean DC exciton polarizability ⟨β̅N⟩ = (0.58 ± 0.25) × 10(-8) Dm V(-1). The exciton polarizability is shown to be layer-independent, indicating a strong localization of both electron and hole wave functions in each individual layer. PMID:26845085

  17. Weak localization and electron–electron interactions in few layer black phosphorus devices

    NASA Astrophysics Data System (ADS)

    Shi, Yanmeng; Gillgren, Nathaniel; Espiritu, Timothy; Tran, Son; Yang, Jiawei; Watanabe, Kenji; Taniguchi, Takahashi; Lau, Chun Ning

    2016-09-01

    Few layer phosphorene (FLP) devices are extensively studied due to their unique electronic properties and potential applications on nano-electronics. Here we present magnetotransport studies which reveal electron–electron interactions as the dominant scattering mechanism in hexagonal boron nitride-encapsulated FLP devices. From weak localization measurements, we estimate the electron dephasing length to be 30 to 100 nm at low temperatures, which exhibits a strong dependence on carrier density n and a power-law dependence on temperature (∼T ‑0.4). These results establish that the dominant scattering mechanism in FLP is electron–electron interactions.

  18. Surface-Induced Orientation Control of CuPc Molecules for the Epitaxial Growth of Highly Ordered Organic Crystals on Graphene

    SciTech Connect

    Xiao, Kai; Deng, Wan; Keum, Jong Kahk; Yoon, Mina; Vlassiouk, Ivan V; Clark, Kendal W; Li, An-Ping; Kravchenko, Ivan I; Gu, Gong; Payzant, E Andrew; Sumpter, Bobby; Smith, Sean C; Browning, Jim; Geohegan, David B

    2013-01-01

    The epitaxial growth and preferred molecular orientation of copper phthalocyanine (CuPc) molecules on graphene has been systematically investigated and compared with growth on Si substrates, demonstrating the role of surface-mediated interactions in determining molecular orientation. X-ray scattering and diffraction, scanning tunneling microscopy, scanning electron microscopy, and first-principles theoretical calculations were used to show that the nucleation, orientation and packing of CuPc molecules on films of graphene are fundamentally different compared to those grown on Si substrates. Interfacial dipole interactions induced by charge transfer between CuPc molecules and graphene are shown to epitaxially align the CuPc mole-cules in a face-on orientation in a series of ordered superstructures. At high temperatures, CuPc molecules lie flat with respect to the graphene substrate to form strip-like CuPc crystals with micron sizes containing monocrystalline grains. Such large epitaxial crystals may potentially enable bulk-like properties to improve the device properties in organic electronics, which charge transport, exciton diffusion and dissociation are currently limited by grain size effects and molecular orientation.

  19. Unveiling the Mechanisms Leading to H2 Production Promoted by Water Decomposition on Epitaxial Graphene at Room Temperature.

    PubMed

    Politano, Antonio; Cattelan, Mattia; Boukhvalov, Danil W; Campi, Davide; Cupolillo, Anna; Agnoli, Stefano; Apostol, Nicoleta G; Lacovig, Paolo; Lizzit, Silvano; Farías, Daniel; Chiarello, Gennaro; Granozzi, Gaetano; Larciprete, Rosanna

    2016-04-26

    By means of a combination of surface-science spectroscopies and theory, we investigate the mechanisms ruling the catalytic role of epitaxial graphene (Gr) grown on transition-metal substrates for the production of hydrogen from water. Water decomposition at the Gr/metal interface at room temperature provides a hydrogenated Gr sheet, which is buckled and decoupled from the metal substrate. We evaluate the performance of Gr/metal interface as a hydrogen storage medium, with a storage density in the Gr sheet comparable with state-of-the-art materials (1.42 wt %). Moreover, thermal programmed reaction experiments show that molecular hydrogen can be released upon heating the water-exposed Gr/metal interface above 400 K. The Gr hydro/dehydrogenation process might be exploited for an effective and eco-friendly device to produce (and store) hydrogen from water, i.e., starting from an almost unlimited source. PMID:27054462

  20. Strain sensing through the optical properties of graphene: Comparing indentation of epitaxial- and CVD-grown graphene

    NASA Astrophysics Data System (ADS)

    Wood, Erin L.; Yang, Yanfei; Gannett, Will; Shaw, Gordon A.; Elmquist, Randolph E.; Keller, Mark W.; Hight Walker, Angela R.

    The unprecedented mechanical and electrical properties of graphene have garnered great interest, yet critical understanding of deformation processes is needed before robust devices are realized. Raman spectroscopy is an information rich, non-destructive probe of mechanical, structural, and electrical properties of graphene through analysis of the prominent bands; D, G, and G'. Previous reports on strained graphene have been largely limited to graphene transferred to flexible substrates and have produced divergent results regarding shifting and splitting in the G band. To systematically evaluate strain, we compare as-grown graphene on either Cu or SiC to the blank substrates which are well understood. Strain was applied by micro- or nano-indentation and Raman mapping was collected of the deformed area providing validation of the applied strain. Confocal Raman microscopy is diffraction limited, however, and localized strain cannot be spatially resolved at the nanoscale. To overcome this, an AFM probe was co-located within the Raman laser focus to obtain sub-diffraction spatial resolution. This also increases the sensitivity to the surface, allowing for observation of the D peak within a micron of nano-indents, which was unseen in confocal Raman spectroscopy.

  1. Ripples near edge terminals in MoS2 few layers and pyramid nanostructures

    NASA Astrophysics Data System (ADS)

    Mills, Adam; Yu, Yifei; Chen, Chuanhui; Huang, Bevin; Cao, Linyou; Tao, Chenggang

    2016-02-01

    Atomically thin transition-metal dichalcogenides are of great interest due to their intriguing physical properties and potential applications. Here, we report our findings from scanning tunneling microscopy and spectroscopy investigations on molybdenum disulfide (MoS2) mono- to few-layers and pyramid nanostructures synthesized through chemical vapor deposition. On the few-layered MoS2 nanoplatelets grown on gallium nitride (GaN) and pyramid nanostructures on highly oriented pyrolytic graphite, we observed an intriguing curved region near the edge terminals. The measured band gap on these curved regions is 1.96 ± 0.10 eV, consistent with the value of the direct band gap in MoS2 monolayers. The curved features near the edge terminals and the associated electronic properties may contribute to the catalytic behaviors of MoS2 nanostructures and have potential applications in future electronic devices and energy-related products based on MoS2 nanostructures.

  2. Photovoltaic effect in few-layer black phosphorus PN junctions defined by local electrostatic gating

    NASA Astrophysics Data System (ADS)

    Buscema, Michele; Groenendijk, Dirk J.; Steele, Gary A.; van der Zant, Herre S. J.; Castellanos-Gomez, Andres

    2014-08-01

    In conventional photovoltaic solar cells, photogenerated carriers are extracted by the built-in electric field of a semiconductor PN junction, defined by ionic dopants. In atomically thin semiconductors, the doping level can be controlled by the field effect, enabling the implementation of electrically tunable PN junctions. However, most two-dimensional (2D) semiconductors do not show ambipolar transport, which is necessary to realize PN junctions. Few-layer black phosphorus (b-P) is a recently isolated 2D semiconductor with direct bandgap, high mobility, large current on/off ratios and ambipolar operation. Here we fabricate few-layer b-P field-effect transistors with split gates and hexagonal boron nitride dielectric. We demonstrate electrostatic control of the local charge carrier type and density in the device. Illuminating a gate-defined PN junction, we observe zero-bias photocurrents and significant open-circuit voltages due to the photovoltaic effect. The small bandgap of the material allows power generation for illumination wavelengths up to 940 nm, attractive for energy harvesting in the near-infrared.

  3. Photovoltaic effect in few-layer black phosphorus PN junctions defined by local electrostatic gating.

    PubMed

    Buscema, Michele; Groenendijk, Dirk J; Steele, Gary A; van der Zant, Herre S J; Castellanos-Gomez, Andres

    2014-01-01

    In conventional photovoltaic solar cells, photogenerated carriers are extracted by the built-in electric field of a semiconductor PN junction, defined by ionic dopants. In atomically thin semiconductors, the doping level can be controlled by the field effect, enabling the implementation of electrically tunable PN junctions. However, most two-dimensional (2D) semiconductors do not show ambipolar transport, which is necessary to realize PN junctions. Few-layer black phosphorus (b-P) is a recently isolated 2D semiconductor with direct bandgap, high mobility, large current on/off ratios and ambipolar operation. Here we fabricate few-layer b-P field-effect transistors with split gates and hexagonal boron nitride dielectric. We demonstrate electrostatic control of the local charge carrier type and density in the device. Illuminating a gate-defined PN junction, we observe zero-bias photocurrents and significant open-circuit voltages due to the photovoltaic effect. The small bandgap of the material allows power generation for illumination wavelengths up to 940 nm, attractive for energy harvesting in the near-infrared. PMID:25164986

  4. High-Performance Few-layer Mo-doped ReSe2 Nanosheet Photodetectors

    PubMed Central

    Yang, Shengxue; Tongay, Sefaattin; Yue, Qu; Li, Yongtao; Li, Bo; Lu, Fangyuan

    2014-01-01

    Transition metal dichalcogenides (TMDCs) have recently been the focus of extensive research activity owing to their fascinating physical properties. As a new member of TMDCs, Mo doped ReSe2 (Mo:ReSe2) is an octahedral structure semiconductor being optically biaxial and highly anisotropic, different from most of hexagonal layered TMDCs with optically uniaxial and relatively high crystal symmetry. We investigated the effects of physisorption of gas molecule on the few-layer Mo:ReSe2 nanosheet based photodetectors. We compared the photoresponse of the as-exfoliated device with annealed device both in air or ammonia (NH3) environment. After annealing at sub-decomposition temperatures, the Mo:ReSe2 photodetectors show a better photoresponsivity (~55.5 A/W) and higher EQE (10893%) in NH3 than in air. By theoretical investigation, we conclude that the physisorption of NH3 molecule on Mo:ReSe2 monolayer can cause the charge transfer between NH3 molecule and Mo:ReSe2 monolayer, increasing the n-type carrier density of Mo:ReSe2 monolayer. The prompt photoswitching, high photoresponsivity and different sensitivity to surrounding environment from the few-layer anisotropic Mo:ReSe2 can be used to design multifunctional optoelectronic and sensing devices. PMID:24962077

  5. Electronic and thermoelectric properties of few-layer transition metal dichalcogenides

    SciTech Connect

    Wickramaratne, Darshana; Lake, Roger K.; Zahid, Ferdows

    2014-03-28

    The electronic and thermoelectric properties of one to four monolayers of MoS{sub 2}, MoSe{sub 2}, WS{sub 2}, and WSe{sub 2} are calculated. For few layer thicknesses, the near degeneracies of the conduction band K and Σ valleys and the valence band Γ and K valleys enhance the n-type and p-type thermoelectric performance. The interlayer hybridization and energy level splitting determine how the number of modes within k{sub B}T of a valley minimum changes with layer thickness. In all cases, the maximum ZT coincides with the greatest near-degeneracy within k{sub B}T of the band edge that results in the sharpest turn-on of the density of modes. The thickness at which this maximum occurs is, in general, not a monolayer. The transition from few layers to bulk is discussed. Effective masses, energy gaps, power-factors, and ZT values are tabulated for all materials and layer thicknesses.

  6. Characterizing and tuning excitons in monolayer and few-layer MoS 2

    NASA Astrophysics Data System (ADS)

    Qiu, Diana Y.; da Jornada, Felipe H.; Louie, Steven G.

    2015-03-01

    We use the GW-BSE method to study excitons arising from transitions in different regions of momentum space in mono- and few-layer MoS2 and consider mechanisms to fundamentally change the features and character of the optical spectra. Our calculations show that sharp spatial variations in dielectric screening make 2D systems, such as MoS2 , computationally challenging, requiring very fine k-space sampling to resolve the structure of excitonic wave functions and converge binding energies. In highly converged calculations, we identify a series of excitons arising from transitions at the K/K' valleys in the Brillouin zone, a higher energy series arising from transitions in the valley of a Mexican hat potential centered at the Γ point, and transitions at the indirect gap from Γ to Λ in few-layer MoS2 . As layer number changes, these states, which have varying character, momentum-space structure and real-space locations, are affected differently by changes in confinement and hybridization. By tuning layer number and strain, we find that we not only can tune the excitation energies but can also change the relative energies of the various excitonic series, allowing for movement of the lowest energy exciton between different regions of the Brillouin zone This work was supported by NSF Grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231.

  7. Contacts and transport characteristics of few-layer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Wang, Junjie; Li, Jing; Shevrin, Jacob; Nguyen, An; Mallouk, Tom; Zhu, J.; Rhodes, Daniel; Balicas, Luis; Watanabe, K.; Taniguchi, T.

    2014-03-01

    Two-dimensional layered transition metal dichalcogenides (TMDs) are potentially useful for electronic and optoelectronic applications. However, the lack of reliable methods to make ohmic contacts has been a major challenge. This work addresses two aspects of this challenge, i.e. interface cleanness and conductivity of the material in the contact area. Using gentle Ar ion milling immediately before the deposition of metal electrodes, we can completely remove polymer residue from prior lithography without significantly damaging the few-layer TMD sheet. Gate stacks made of Au and HfO2 films can inject carriers up to 3 ×1013 cm-2. We make van der Pauw devices of few-layer (< 5 L) TMD (MoS2, WS2, WSe2) sheets using Ti/Au contacts with area < 2 (um)2 and observe contact resistance less than 10 k Ω at high carrier densities, where the sheet conductance is well above 2e2/h. We eliminate hysteresis in the transfer curve of TMD devices by pulsing the gate voltage. Ambipolar conduction is observed in WSe2 devices, with an on/off ratio exceeding 106 for both electrons and holes. WSe2 devices supported on h-BN show field-effect (hole) mobility > 100 cm2/(Vs) at 300K. We discuss the effects of the various approaches taken above.

  8. Synthesis of few-layer MoS2 nanosheet-loaded Ag3PO4 for enhanced photocatalytic activity.

    PubMed

    Song, Yanhua; Lei, Yucheng; Xu, Hui; Wang, Cheng; Yan, Jia; Zhao, Haozhu; Xu, Yuanguo; Xia, Jiexiang; Yin, Sheng; Li, Huaming

    2015-02-21

    Novel few-layer MoS2/Ag3PO4 composites were fabricated. The results indicated that Ag3PO4 nanoparticles were directly formed on the surface of few-layer MoS2. The physical and chemical properties of the few-layer MoS2/Ag3PO4 composite photocatalysts were tested in order to investigate the effects of few-layer MoS2 on the photocatalytic activity of Ag3PO4. The photocatalytic activity of the few-layer MoS2/Ag3PO4 composites was evaluated by the photocatalytic degradation of Rhodamine B (RhB) and bisphenol A (BPA) under visible light irradiation. The photocatalytic activity of the few-layer MoS2/Ag3PO4 composites was higher than that of pure Ag3PO4. The optimal few-layer MoS2 content for the organic pollutant degradation of the heterojunction structures was determined. The synergic effect between few-layer MoS2 and Ag3PO4 was found to lead to an improved photogenerated carrier separation. The stability and the possible photocatalytic mechanism of the composites were also discussed. PMID:25567674

  9. Epitaxial B-Graphene: Large-Scale Growth and Atomic Structure.

    PubMed

    Usachov, Dmitry Yu; Fedorov, Alexander V; Petukhov, Anatoly E; Vilkov, Oleg Yu; Rybkin, Artem G; Otrokov, Mikhail M; Arnau, Andrés; Chulkov, Evgueni V; Yashina, Lada V; Farjam, Mani; Adamchuk, Vera K; Senkovskiy, Boris V; Laubschat, Clemens; Vyalikh, Denis V

    2015-07-28

    Embedding foreign atoms or molecules in graphene has become the key approach in its functionalization and is intensively used for tuning its structural and electronic properties. Here, we present an efficient method based on chemical vapor deposition for large scale growth of boron-doped graphene (B-graphene) on Ni(111) and Co(0001) substrates using carborane molecules as the precursor. It is shown that up to 19 at. % of boron can be embedded in the graphene matrix and that a planar C-B sp(2) network is formed. It is resistant to air exposure and widely retains the electronic structure of graphene on metals. The large-scale and local structure of this material has been explored depending on boron content and substrate. By resolving individual impurities with scanning tunneling microscopy we have demonstrated the possibility for preferential substitution of carbon with boron in one of the graphene sublattices (unbalanced sublattice doping) at low doping level on the Ni(111) substrate. At high boron content the honeycomb lattice of B-graphene is strongly distorted, and therefore, it demonstrates no unballanced sublattice doping. PMID:26121999

  10. Few-layered MoS2 nanosheets wrapped ultrafine TiO2 nanobelts with enhanced photocatalytic property

    NASA Astrophysics Data System (ADS)

    Li, Haidong; Wang, Yana; Chen, Guohui; Sang, Yuanhua; Jiang, Huaidong; He, Jiating; Li, Xu; Liu, Hong

    2016-03-01

    Photocatalytic materials comprised of semiconductor nanostructures have attracted tremendous scientific and technological interest over the last 30 years. This is due to the fact that these photocatalytic materials have unique properties that allow for an effective direct energy transfer from light to highly reactive chemical species which are applicable in the remediation of environmental pollutants and photocatalytic hydrogen generation. Heterostructured photocatalysts are a promising type of photocatalyst which can combine the properties of different components to generate a synergic effect, resulting in a high photocatalytic activity. In this work, a heterostructured photocatalyst comprised of few-layered MoS2 nanosheets coated on a TiO2 nanobelts surface was synthesized through a simple hydrothermal treatment. The hybrid heterostructures with enhanced broad spectrum photocatalytic properties can harness UV and visible light energy to decompose organic contaminants in aqueous solutions as well as split water to hydrogen and oxygen. The mechanism of the enhancement is that the MoS2/TiO2 nanobelts heterostructure can enhance the separation of the photo-induced carriers, which results in a higher photocurrent due to the special electronic characteristics of the graphene-like layered MoS2 nanosheets. This methodology is potentially applicable to the synthesis of a range of hybrid nanostructures with promising applications in photocatalysis and other relevant areas.Photocatalytic materials comprised of semiconductor nanostructures have attracted tremendous scientific and technological interest over the last 30 years. This is due to the fact that these photocatalytic materials have unique properties that allow for an effective direct energy transfer from light to highly reactive chemical species which are applicable in the remediation of environmental pollutants and photocatalytic hydrogen generation. Heterostructured photocatalysts are a promising type of

  11. Interface structure of epitaxial graphene grown on 4H-SiC(0001)

    SciTech Connect

    Hass, J.; Millán-Otoya, J.E.; First, P.N.; Conrad, E.H.

    2009-06-12

    We present a structural analysis of the graphene-4HSiC(0001) interface using surface x-ray reflectivity. We find that the interface is composed of an extended reconstruction of two SiC bilayers. The interface directly below the first graphene sheet is an extended layer that is more than twice the thickness of a bulk SiC bilayer ({approx}1.7 {angstrom} compared to 0.63 {angstrom}). The distance from this interface layer to the first graphene sheet is much smaller than the graphite interlayer spacing but larger than the same distance measured for graphene grown on the (000{bar 1}) surface, as predicted previously by ab initio calculations.

  12. Silicon layer intercalation of centimeter-scale, epitaxially grown monolayer graphene on Ru(0001)

    NASA Astrophysics Data System (ADS)

    Mao, Jinhai; Huang, Li; Pan, Yi; Gao, Min; He, Junfeng; Zhou, Haitao; Guo, Haiming; Tian, Yuan; Zou, Qiang; Zhang, Lizhi; Zhang, Haigang; Wang, Yeliang; Du, Shixuan; Zhou, Xingjiang; Castro Neto, A. H.; Gao, Hong-Jun

    2012-02-01

    We develop a strategy for graphene growth on Ru(0001) followed by silicon-layer intercalation that not only weakens the interaction of graphene with the metal substrate but also retains its superlative properties. This G/Si/Ru architecture, produced by silicon-layer intercalation approach (SIA), was characterized by scanning tunneling microscopy/spectroscopy and angle resolved electron photoemission spectroscopy. These experiments show high structural and electronic qualities of this new composite. The SIA allows for an atomic control of the distance between the graphene and the metal substrate that can be used as a top gate. Our results show potential for the next generation of graphene-based materials with tailored properties.

  13. Role of functionalized transition-metal coated W tips in STM imaging: Application to epitaxial graphene on SiC(0001)

    NASA Astrophysics Data System (ADS)

    Rhim, S. H.; Qi, Y.; Sun, G. F.; Liu, Y.; Weinert, M.; Li, L.

    2011-09-01

    Scanning tunneling microscopy using W tips functionalized with transition-metal (Cr, Fe) coatings is demonstrated to facilitate the imaging of states within a few millielectron volts (meV) of the Fermi level for epitaxial graphene on SiC(0001), which are not accessible with bare W tips. First-principles modeling of these probe tips as pyramidal structures on W(110) indicates that an apex atom is stable for the Cr/W(110) tip but not for the Fe/W(110) or W/W(110) tips. This difference in their atomic structures, together with the variation in the extent of the 3d and 5d orbitals, is found to be responsible for the capability of Cr and Fe functionalized tips to selectively image the complex electronic properties of epitaxial graphene on SiC(0001).

  14. Step edge influence on barrier height and contact area in vertical heterojunctions between epitaxial graphene and n-type 4H-SiC

    SciTech Connect

    Tadjer, M. J. Nyakiti, L. O.; Robinson, Z.; Anderson, T. J.; Myers-Ward, R. L.; Wheeler, V. D.; Eddy, C. R.; Gaskill, D. K.; Koehler, A. D.; Hobart, K. D.; Kub, F. J.

    2014-02-17

    Vertical rectifying contacts of epitaxial graphene grown by Si sublimation on the Si-face of 4H-SiC epilayers were investigated. Forward bias preferential conduction through the step edges was correlated by linear current density normalization. This phenomenon was observed on samples with 2.7–5.8 monolayers of epitaxial graphene as determined by X-ray photoelectron spectroscopy. A modified Richardson plot was implemented to extract the barrier height (0.81 eV at 290 K, 0.99 eV at 30 K) and the electrically dominant SiC step length of a Ti/Al contact overlapping a known region of approximately 0.52 μm wide SiC terraces.

  15. Onset of two-dimensional superconductivity in space charge doped few-layer molybdenum disulfide

    NASA Astrophysics Data System (ADS)

    Biscaras, Johan; Chen, Zhesheng; Paradisi, Andrea; Shukla, Abhay

    2015-11-01

    Atomically thin films of layered materials such as molybdenum disulfide (MoS2) are of growing interest for the study of phase transitions in two-dimensions through electrostatic doping. Electrostatic doping techniques giving access to high carrier densities are needed to achieve such phase transitions. Here we develop a method of electrostatic doping which allows us to reach a maximum n-doping density of 4 × 1014 cm-2 in few-layer MoS2 on glass substrates. With increasing carrier density we first induce an insulator to metal transition and subsequently an incomplete metal to superconductor transition in MoS2 with critical temperature ~10 K. Contrary to earlier reports, after the onset of superconductivity, the superconducting transition temperature does not depend on the carrier density. Our doping method and the results we obtain in MoS2 for samples as thin as bilayers indicates the potential of this approach.

  16. Thermal Conductivity and Phonon Transport in Suspended Few-Layer Hexagonal Boron Nitride

    NASA Astrophysics Data System (ADS)

    Jo, Insun; Pettes, Michael Thompson; Kim, Jaehyun; Watanabe, Kenji; Taniguchi, Takashi; Yao, Zhen; Shi, Li

    2013-02-01

    The thermal conductivity of suspended few-layer hexagonal boron nitride (h-BN) was measured using a micro-bridge device with built-in resistance thermometers. Based on the measured thermal resistance values of 11-12 atomic layer h-BN samples with suspended length ranging between 3 and 7.5 um, the room-temperature thermal conductivity of a 11-layer sample was found to be about 360 Wm-1K-1, approaching the basal plane value reported for bulk h-BN. The presence of a polymer residue layer on the sample surface was found to decrease the thermal conductivity of a 5-layer h-BN sample to be about 250 Wm-1K-1 at 300 K. Thermal conductivities for both the 5 layer and the 11 layer samples are suppressed at low temperatures, suggesting increasing scattering of low frequency phonons in thin h-BN samples by polymer residue.

  17. Coherent Lattice Vibrations in Mono- and Few-Layer WSe2.

    PubMed

    Jeong, Tae Young; Jin, Byung Moon; Rhim, Sonny H; Debbichi, Lamjed; Park, Jaesung; Jang, Yu Dong; Lee, Hyang Rok; Chae, Dong-Hun; Lee, Donghan; Kim, Yong-Hoon; Jung, Suyong; Yee, Ki Ju

    2016-05-24

    We report the observation of coherent lattice vibrations in mono- and few-layer WSe2 in the time domain, which were obtained by performing time-resolved transmission measurements. Upon the excitation of ultrashort pulses with the energy resonant to that of A excitons, coherent oscillations of the A1g optical phonon and longitudinal acoustic phonon at the M point of the Brillouin zone (LA(M)) were impulsively generated in monolayer WSe2. In multilayer WSe2 flakes, the interlayer breathing mode (B1) is found to be sensitive to the number of layers, demonstrating its usefulness in characterizing layered transition metal dichalcogenide materials. On the basis of temperature-dependent measurements, we find that the A1g optical phonon mode decays into two acoustic phonons through the anharmonic decay process. PMID:27102714

  18. Photoluminescence of freestanding single- and few-layer MoS2

    NASA Astrophysics Data System (ADS)

    Scheuschner, Nils; Ochedowski, Oliver; Kaulitz, Anne-Marie; Gillen, Roland; Schleberger, Marika; Maultzsch, Janina

    2014-03-01

    We present a photoluminescence study of freestanding and Si/SiO2 supported single- and few-layer MoS2. The single-layer exciton peak (A) is only observed in freestanding MoS2. The photoluminescence of supported single-layer MoS2 instead originates from the A- (trion) peak as the MoS2 is n-type doped from the substrate. In bilayer MoS2, the van der Waals interaction with the substrate decreases the indirect band gap energy by up to ≈80 meV. Furthermore, the photoluminescence spectra of suspended MoS2 can be influenced by interference effects.

  19. Light-Induced Ambient Degradation of Few-Layer Black Phosphorus: Mechanism and Protection.

    PubMed

    Zhou, Qionghua; Chen, Qian; Tong, Yilong; Wang, Jinlan

    2016-09-12

    The environmental instability of single- or few-layer black phosphorus (BP) has become a major hurdle for BP-based devices. The degradation mechanism remains unclear and finding ways to protect BP from degradation is still highly challenging. Based on ab initio electronic structure calculations and molecular dynamics simulations, a three-step picture on the ambient degradation of BP is provided: generation of superoxide under light, dissociation of the superoxide, and eventual breakdown under the action of water. The well-matched band gap and band-edge positions for the redox potential accelerates the degradation of thinner BP. Furthermore, it was found that the formation of P-O-P bonds can greatly stabilize the BP framework. A possible protection strategy using a fully oxidized BP layer as the native capping is thus proposed. Such a fully oxidization layer can resist corrosion from water and leave the BP underneath intact with simultaneous high hole mobility. PMID:27529543

  20. Anomalous Lattice Vibrations of Single- and Few-Layer MoS{sub 2}

    SciTech Connect

    Lee, Changgu; Yan, Hugen; Brus, Louis; Heinz, Tony; Hone, James; Ryu, Sunmin

    2010-01-01

    Molybdenum disulfide (MoS{sub 2}) of single- and few-layer thickness was exfoliated on SiO{sub 2}/Si substrate and characterized by Raman spectroscopy. The number of S−Mo−S layers of the samples was independently determined by contact-mode atomic force microscopy. Two Raman modes, E{sup 1}{sub 2g} and A{sub 1g}, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.

  1. Vacuum ultraviolet excitation luminescence spectroscopy of few-layered MoS2.

    PubMed

    Pankratov, V; Hoszowska, J; Dousse, J-Cl; Huttula, M; Kis, A; Krasnozhon, D; Zhang, M; Cao, W

    2016-01-13

    We report on vacuum ultraviolet (VUV) excited photoluminescence (PL) spectra emitted from a chemical vapor deposited MoS2 few-layered film. The excitation spectrum was recorded by monitoring intensities of PL spectra at ~1.9 eV. A strong wide excitation band peaking at 7 eV was found in the excitation. The PL excitation band is most intensive at liquid helium temperature and completely quenched at 100 K. Through first-principles calculations of photoabsorption in MoS2, the excitation was explicated and attributed to transitions of electrons from p- and d- type states in the valence band to the d- and p-type states in the conduction band. The obtained photon-in/photon-out results clarify the excitation and emission behavior of the low dimensional MoS2 when interacting with the VUV light sources. PMID:26648394

  2. Structure and Electronic Properties of Single- to Few Layers Molybdenum Disulfide Films

    NASA Astrophysics Data System (ADS)

    Trainer, D.; Putilov, A.; Wolak, M.; Chandrasena, R. U.; Kronast, F.; Gray, A. X.; Xi, X. X.; Iavarone, M.

    Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS) we have investigated the electronic properties of mono- to few layers molybdenum disulfide films grown on HOPG using ambient pressure chemical vapor deposition (APCVD). Atomic force microscopy and STM show that this growth technique produces crystalline triangular and hexagonal islands with varying thicknesses in 1ML increments. The films exhibited a suppression of quasiparticle band-gap as a function of layer number as measured by local spectroscopy. Changes in the valence band edge were supported by photoemission electron microscopy (PEEM) measurements. We also report on a strain-induced contraction of the quasiparticle band-gap in proximity to grain boundaries and defects. This work was supported as part of the Center for the Computational Design of Functional Layered Materials, an Energy Frontier Research Center funded by the U.S. DOE, BES under Award DE-SC0012575.

  3. Large-area few-layer MoS2 deposited by sputtering

    NASA Astrophysics Data System (ADS)

    Huang, Jyun-Hong; Chen, Hsing-Hung; Liu, Pang-Shiuan; Lu, Li-Syuan; Wu, Chien-Ting; Chou, Cheng-Tung; Lee, Yao-Jen; Li, Lain-Jong; Chang, Wen-Hao; Hou, Tuo-Hung

    2016-06-01

    Direct magnetron sputtering of transition metal dichalcogenide targets is proposed as a new approach for depositing large-area two-dimensional layered materials. Bilayer to few-layer MoS2 deposited by magnetron sputtering followed by post-deposition annealing shows superior area scalability over 20 cm2 and layer-by-layer controllability. High crystallinity of layered MoS2 was confirmed by Raman, photo-luminescence, and transmission electron microscopy analysis. The sputtering temperature and annealing ambience were found to play an important role in the film quality. The top-gate field-effect transistor by using the layered MoS2 channel shows typical n-type characteristics with a current on/off ratio of approximately 104. The relatively low mobility is attributed to the small grain size of 0.1–1 μm with a trap charge density in grain boundaries of the order of 1013 cm–2.

  4. Stacking Fault Enriching the Electronic and Transport Properties of Few-Layer Phosphorenes and Black Phosphorus.

    PubMed

    Lei, Shuangying; Wang, Han; Huang, Lan; Sun, Yi-Yang; Zhang, Shengbai

    2016-02-10

    Interface engineering is critical for enriching the electronic and transport properties of two-dimensional materials. Here, we identify a new stacking, named Aδ, in few-layer phosphorenes (FLPs) and black phosphorus (BP) based on first-principles calculation. With its low formation energy, the Aδ stacking could exist in FLPs and BP as a stacking fault. The presence of the Aδ stacking fault induces a direct to indirect transition of the band gap in FLPs. It also affects the carrier mobilities by significantly increasing the carrier effective masses. More importantly, the Aδ stacking enables the fabrication of a whole spectrum of lateral junctions with all the type-I, II, and III alignments simply through the manipulation of the van der Waals stacking without resorting to any chemical modification. This is achieved by the widely tunable electron affinity and ionization potential of FLPs and BP with the Aδ stacking. PMID:26799596

  5. Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals

    NASA Astrophysics Data System (ADS)

    Aslan, Ozgur Burak; Ruppert, Claudia; Heinz, Tony

    2015-03-01

    Single- and few-layer crystals of exfoliated MoTe2 have been characterized spectroscopically by photoluminescence, Raman scattering, and optical absorption measurements. We find that MoTe2 in the monolayer limit displays strong photoluminescence. On the basis of complementary optical absorption results, we conclude that monolayer MoTe2 is a direct-gap semiconductor with an optical band gap of 1.10 eV. This new monolayer material extends the spectral range of atomically thin direct-gap materials from the visible to the near-infrared. Supported by the NSF through Grant DMR-1124894 for sample preparation and characterization by the O?ce of Naval Research for analysis. C.R. acknowledges support from the Alexander von Humboldt Foundation.

  6. Thermal transport across few-layer boron nitride encased by silica

    NASA Astrophysics Data System (ADS)

    Ni, Yuxiang; Jiang, Jiechao; Meletis, Efstathios; Dumitric, Traian

    2015-07-01

    Two dimensional hexagonal boron nitride (h-BN) attracted attention for use in applications. Using equilibrium molecular dynamics, we examine the phonon transport in few-layer h-BN encased by silica (SiO2). We report large interfacial thermal resistances, of about 2.2 × 10-8 m2 K W-1, which are not sensitive to the number of h-BN layers or the SiO2 crystallinity. The h-BN/SiO2 superlattices exhibit ultra-low thermal conductivities across layers, as low as 0.3 W/m K. They are structurally stable up to 2000 K while retaining the low-thermal conductivity attributes. Our simulations indicate that incorporation of h-BN layers and nanoparticles in silica could establish thermal barriers and heat spreading paths, useful for high performance coatings and electronic device applications.

  7. Controlled Synthesis of ZrS2 Monolayer and Few Layers on Hexagonal Boron Nitride.

    PubMed

    Zhang, Mei; Zhu, Yiming; Wang, Xinsheng; Feng, Qingliang; Qiao, Shanlin; Wen, Wen; Chen, Yanfeng; Cui, Menghua; Zhang, Jin; Cai, Congzhong; Xie, Liming

    2015-06-10

    Group IVB transition metal (Zr and Hf) dichalcogenide (TMD) monolayers can have higher carrier mobility and higher tunneling current density than group VIB (Mo and W) TMD monolayers. Here we report the synthesis of hexagonal ZrS2 monolayer and few layers on hexagonal boron nitride (BN) using ZrCl4 and S as precursors. The domain size of ZrS2 hexagons is around 1-3 μm. The number of layers of ZrS2 was controlled by tuning the evaporation temperature of ZrCl4. The stacking angle between ZrS2 and BN characterized by transmission electron microscopy shows a preferred stacking angle of near 0°. Field-effect transistors (FETs) fabricated on ZrS2 flakes showed n-type transport behavior with an estimated mobility of 0.1-1.1 cm(2) V(-1) s(-1). PMID:25996159

  8. Double resonance Raman modes in monolayer and few-layer MoTe2

    NASA Astrophysics Data System (ADS)

    Guo, Huaihong; Yang, Teng; Yamamoto, Mahito; Zhou, Lin; Ishikawa, Ryo; Ueno, Keiji; Tsukagoshi, Kazuhito; Zhang, Zhidong; Dresselhaus, Mildred S.; Saito, Riichiro

    2015-05-01

    We study the second-order Raman process of mono- and few-layer MoTe2, by combining ab initio density functional perturbation calculations with experimental Raman spectroscopy using 532, 633, and 785 nm excitation lasers. The calculated electronic band structure and the density of states show that the resonance Raman process occurs at the M point in the Brillouin zone, where a strong optical absorption occurs due to a logarithmic Van Hove singularity of the electronic density of states. The double resonance Raman process with intervalley electron-phonon coupling connects two of the three inequivalent M points in the Brillouin zone, giving rise to second-order Raman peaks due to the M -point phonons. The calculated vibrational frequencies of the second-order Raman spectra agree with the observed laser-energy-dependent Raman shifts in the experiment.

  9. Basal-plane thermal conductivity of few-layer molybdenum disulfide

    SciTech Connect

    Jo, Insun; Ou, Eric; Shi, Li; Pettes, Michael Thompson; Wu, Wei

    2014-05-19

    We report the in-plane thermal conductivity of suspended exfoliated few-layer molybdenum disulfide (MoS{sub 2}) samples that were measured by suspended micro-devices with integrated resistance thermometers. The obtained room-temperature thermal conductivity values are (44–50) and (48–52) W m{sup −1} K{sup −1} for two samples that are 4 and 7 layers thick, respectively. For both samples, the peak thermal conductivity occurs at a temperature close to 120 K, above which the thermal conductivity is dominated by intrinsic phonon-phonon scattering although phonon scattering by surface disorders can still play an important role in these samples especially at low temperatures.

  10. Controlled van der Waals epitaxy of monolayer MoS2 triangular domains on graphene.

    PubMed

    Ago, Hiroki; Endo, Hiroko; Solís-Fernández, Pablo; Takizawa, Rina; Ohta, Yujiro; Fujita, Yusuke; Yamamoto, Kazuhiro; Tsuji, Masaharu

    2015-03-11

    Multilayered heterostructures of two-dimensional materials have recently attracted increased interest because of their unique electronic and optical properties. Here, we present chemical vapor deposition (CVD) growth of triangular crystals of monolayer MoS2 on single-crystalline hexagonal graphene domains which are also grown by CVD. We found that MoS2 grows selectively on the graphene domains rather than on the bare supporting SiO2 surface. Reflecting the heteroepitaxy of the growth process, the MoS2 domains grown on graphene present two preferred equivalent orientations. The interaction between the MoS2 and the graphene induced an upshift of the Raman G and 2D bands of the graphene, while significant photoluminescence quenching was observed for the monolayer MoS2. Furthermore, photoinduced current modulation along with an optical memory effect was demonstrated for the MoS2-graphene heterostructure. Our work highlights that heterostructures synthesized by CVD offer an effective interlayer van der Waals interaction which can be developed for large-area multilayer electronic and photonic devices. PMID:25695865

  11. Controlled Gas Exfoliation of Boron Nitride into Few-Layered Nanosheets.

    PubMed

    Zhu, Wenshuai; Gao, Xiang; Li, Qian; Li, Hongping; Chao, Yanhong; Li, Meijun; Mahurin, Shannon M; Li, Huaming; Zhu, Huiyuan; Dai, Sheng

    2016-08-26

    The controlled exfoliation of hexagonal boron nitride (h-BN) into single- or few-layered nanosheets remains a grand challenge and becomes the bottleneck to essential studies and applications of h-BN. Here, we present an efficient strategy for the scalable synthesis of few-layered h-BN nanosheets (BNNS) using a novel gas exfoliation of bulk h-BN in liquid N2 (L-N2 ). The essence of this strategy lies in the combination of a high temperature triggered expansion of bulk h-BN and the cryogenic L-N2 gasification to exfoliate the h-BN. The produced BNNS after ten cycles (BNNS-10) consisted primarily of fewer than five atomic layers with a high mass yield of 16-20 %. N2 sorption and desorption isotherms show that the BNNS-10 exhibited a much higher specific surface area of 278 m(2)  g(-1) than that of bulk BN (10 m(2)  g(-1) ). Through the investigation of the exfoliated intermediates combined with a theoretical calculation, we found that the huge temperature variation initiates the expansion and curling of the bulk h-BN. Subseqently, the L-N2 penetrates into the interlayers of h-BN along the curling edge, followed by an immediate drastic gasification of L-N2 , further peeling off h-BN. This novel gas exfoliation of high surface area BNNS not only opens up potential opportunities for wide applications, but also can be extended to produce other layered materials in high yields. PMID:27444210

  12. Few-layer bismuth selenides exfoliated by hemin inhibit amyloid-β1–42 fibril formation

    PubMed Central

    Peng, Jian; Xiong, Yunjing; Lin, Zhiqin; Sun, Liping; Weng, Jian

    2015-01-01

    Inhibiting amyloid-β (Aβ) fibril formation is the primary therapeutic strategy for Alzheimer’s disease. Several small molecules and nanomaterials have been used to inhibit Aβ fibril formation. However, insufficient inhibition efficiency or poor metabolization limits their further applications. Here, we used hemin to exfoliate few-layer Bi2Se3 in aqueous solution. Then we separated few-layer Bi2Se3 with different sizes and thicknesses by fractional centrifugation, and used them to attempt to inhibit Aβ1-42 aggregation. The results show that smaller and thinner few-layer Bi2Se3 had the highest inhibition efficiency. We further investigated the interaction between few-layer Bi2Se3 and Aβ1-42 monomers. The results indicate that the inhibition effect may be due to the high adsorption capacity of few-layer Bi2Se3 for Aβ1−42 monomers. Few-layer Bi2Se3 also decreased Aβ-mediated peroxidase-like activity and cytotoxicity according to in vitro neurotoxicity studies under physiological conditions. Therefore, our work shows the potential for applications of few-layer Bi2Se3 in the biomedical field. PMID:26018135

  13. Few-layer bismuth selenides exfoliated by hemin inhibit amyloid-β1-42 fibril formation

    NASA Astrophysics Data System (ADS)

    Peng, Jian; Xiong, Yunjing; Lin, Zhiqin; Sun, Liping; Weng, Jian

    2015-05-01

    Inhibiting amyloid-β (Aβ) fibril formation is the primary therapeutic strategy for Alzheimer’s disease. Several small molecules and nanomaterials have been used to inhibit Aβ fibril formation. However, insufficient inhibition efficiency or poor metabolization limits their further applications. Here, we used hemin to exfoliate few-layer Bi2Se3 in aqueous solution. Then we separated few-layer Bi2Se3 with different sizes and thicknesses by fractional centrifugation, and used them to attempt to inhibit Aβ1-42 aggregation. The results show that smaller and thinner few-layer Bi2Se3 had the highest inhibition efficiency. We further investigated the interaction between few-layer Bi2Se3 and Aβ1-42 monomers. The results indicate that the inhibition effect may be due to the high adsorption capacity of few-layer Bi2Se3 for Aβ1-42 monomers. Few-layer Bi2Se3 also decreased Aβ-mediated peroxidase-like activity and cytotoxicity according to in vitro neurotoxicity studies under physiological conditions. Therefore, our work shows the potential for applications of few-layer Bi2Se3 in the biomedical field.

  14. Observation of the retarded transportation of a photogenerated hole on epitaxial graphene.

    PubMed

    Wang, Shujie; Yuan, Xizhi; Bi, Xiaofeng; Wang, Xiaomei; Huang, Qingsong

    2015-10-01

    Graphene is usually adopted as an assistant additive for catalysts in photocatalytic processes, because of its ability to accelerate the separation of photogenerated charge carriers. To elucidate the mechanism, hydrogen peroxide is adopted to convert the O2(-)˙ active species into OH˙ for degradation of an organic dye. If the pH value is less than 7, the concentration of the OH˙ species can be reduced more quickly with the addition of graphene than without, because negatively charged electrons can be transported quickly on graphene. If the pH value is larger than 7, the concentration of OH˙ can be promoted by the catalyst SiC with photogenerated h(+) release and reaction with OH(-), however the concentration is reduced if the SiC catalyst is covered by a graphene sheet, as it retards h(+) release from the SiC substrate. Our findings have provided a certification for the role of graphene in photo-catalytic processes. PMID:26308861

  15. Large energy pulse generation modulated by graphene epitaxially grown on silicon carbide.

    PubMed

    Yu, Haohai; Chen, Xiufang; Zhang, Huaijin; Xu, Xiangang; Hu, Xiaobo; Wang, Zhengping; Wang, Jiyang; Zhuang, Shidong; Jiang, Minhua

    2010-12-28

    Graphene grown by thermal decomposition of a two-inch 6H silicon carbide (SiC) wafers surface was used to modulate a large energy pulse laser. Because of its saturable absorbing properties, graphene was used as a passive Q-switcher, and because of its high refractive index the SiC substrate was used as an output coupler. Together they formed a setup where the passively Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal laser was realized with the pulse energy of 159.2 nJ. Our results illustrate the feasibility of using graphene as an inexpensive Q-switcher for solid-state lasers and its promising applications in integrated optics. PMID:21058692

  16. Direct Growth of Single- and Few-Layer MoS2 on h-BN with Preferred Relative Rotation Angles.

    PubMed

    Yan, Aiming; Velasco, Jairo; Kahn, Salman; Watanabe, Kenji; Taniguchi, Takashi; Wang, Feng; Crommie, Michael F; Zettl, Alex

    2015-10-14

    Monolayer molybdenum disulfide (MoS2) is a promising two-dimensional direct-bandgap semiconductor with potential applications in atomically thin and flexible electronics. An attractive insulating substrate or mate for MoS2 (and related materials such as graphene) is hexagonal boron nitride (h-BN). Stacked heterostructures of MoS2 and h-BN have been produced by manual transfer methods, but a more efficient and scalable assembly method is needed. Here we demonstrate the direct growth of single- and few-layer MoS2 on h-BN by chemical vapor deposition (CVD) method, which is scalable with suitably structured substrates. The growth mechanisms for single-layer and few-layer samples are found to be distinct, and for single-layer samples low relative rotation angles (<5°) between the MoS2 and h-BN lattices prevail. Moreover, MoS2 directly grown on h-BN maintains its intrinsic 1.89 eV bandgap. Our CVD synthesis method presents an important advancement toward controllable and scalable MoS2-based electronic devices. PMID:26317240

  17. Probing the Role of Interlayer Coupling and Coulomb Interactions on Electronic Structure in Few-Layer MoSe2Nanostructures

    NASA Astrophysics Data System (ADS)

    Bradley, Aaron J.; M. Ugeda, Miguel; da Jornada, Felipe H.; Qiu, Diana Y.; Ruan, Wei; Zhang, Yi; Wickenburg, Sebastian; Riss, Alexander; Lu, Jiong; Mo, Sung-Kwan; Hussain, Zahid; Shen, Zhi-Xun; Louie, Steven G.; Crommie, Michael F.

    2015-04-01

    Despite the weak nature of interlayer forces in transition metal dichalcogenide (TMD) materials, their properties are highly dependent on the number of layers in the few-layer two-dimensional (2D) limit. Here, we present a combined scanning tunneling microscopy/spectroscopy and GW theoretical study of the electronic structure of high quality single- and few-layer MoSe2 grown on bilayer graphene. We find that the electronic (quasiparticle) bandgap, a fundamental parameter for transport and optical phenomena, decreases by nearly one electronvolt when going from one layer to three due to interlayer coupling and screening effects. Our results paint a clear picture of the evolution of the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. This demonstrates the importance of layer number and electron-electron interactions on van der Waals heterostructures, and helps to clarify how their electronic properties might be tuned in future 2D nanodevices.

  18. Probing the Role of Interlayer Coupling and Coulomb Interactions on Electronic Structure in Few-Layer MoSe2 Nanostructures

    PubMed Central

    2015-01-01

    Despite the weak nature of interlayer forces in transition metal dichalcogenide (TMD) materials, their properties are highly dependent on the number of layers in the few-layer two-dimensional (2D) limit. Here, we present a combined scanning tunneling microscopy/spectroscopy and GW theoretical study of the electronic structure of high quality single- and few-layer MoSe2 grown on bilayer graphene. We find that the electronic (quasiparticle) bandgap, a fundamental parameter for transport and optical phenomena, decreases by nearly one electronvolt when going from one layer to three due to interlayer coupling and screening effects. Our results paint a clear picture of the evolution of the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. This demonstrates the importance of layer number and electron–electron interactions on van der Waals heterostructures and helps to clarify how their electronic properties might be tuned in future 2D nanodevices. PMID:25775022

  19. Probing the Role of Interlayer Coupling and Coulomb Interactions on Electronic Structure in Few-Layer MoSe 2 Nanostructures

    DOE PAGESBeta

    Bradley, Aaron J.; M. Ugeda, Miguel; da Jornada, Felipe H.; Qiu, Diana Y.; Ruan, Wei; Zhang, Yi; Wickenburg, Sebastian; Riss, Alexander; Lu, Jiong; Mo, Sung-Kwan; et al

    2015-03-16

    Despite the weak nature of interlayer forces in transition metal dichalcogenide (TMD) materials, their properties are highly dependent on the number of layers in the few-layer two-dimensional (2D) limit. Here, we present a combined scanning tunneling microscopy/spectroscopy and GW theoretical study of the electronic structure of high quality single- and few-layer MoSe2 grown on bilayer graphene. We find that the electronic (quasiparticle) bandgap, a fundamental parameter for transport and optical phenomena, decreases by nearly one electronvolt when going from one layer to three due to interlayer coupling and screening effects. Our results paint a clear picture of the evolution ofmore » the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. This demonstrates the importance of layer number and electron-electron interactions on van der Waals heterostructures and helps to clarify how their electronic properties might be tuned in future 2D nanodevices.« less

  20. Probing the Role of Interlayer Coupling and Coulomb Interactions on Electronic Structure in Few-Layer MoSe 2 Nanostructures

    SciTech Connect

    Bradley, Aaron J.; M. Ugeda, Miguel; da Jornada, Felipe H.; Qiu, Diana Y.; Ruan, Wei; Zhang, Yi; Wickenburg, Sebastian; Riss, Alexander; Lu, Jiong; Mo, Sung-Kwan; Hussain, Zahid; Shen, Zhi-Xun; Louie, Steven G.; Crommie, Michael F.

    2015-03-16

    Despite the weak nature of interlayer forces in transition metal dichalcogenide (TMD) materials, their properties are highly dependent on the number of layers in the few-layer two-dimensional (2D) limit. Here, we present a combined scanning tunneling microscopy/spectroscopy and GW theoretical study of the electronic structure of high quality single- and few-layer MoSe2 grown on bilayer graphene. We find that the electronic (quasiparticle) bandgap, a fundamental parameter for transport and optical phenomena, decreases by nearly one electronvolt when going from one layer to three due to interlayer coupling and screening effects. Our results paint a clear picture of the evolution of the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. This demonstrates the importance of layer number and electron-electron interactions on van der Waals heterostructures and helps to clarify how their electronic properties might be tuned in future 2D nanodevices.

  1. Exciton and charge carrier dynamics in few-layer WS2

    NASA Astrophysics Data System (ADS)

    Vega-Mayoral, Victor; Vella, Daniele; Borzda, Tetiana; Prijatelj, Matej; Tempra, Iacopo; Pogna, Eva A. A.; Dal Conte, Stefano; Topolovsek, Peter; Vujicic, Natasa; Cerullo, Giulio; Mihailovic, Dragan; Gadermaier, Christoph

    2016-03-01

    Semiconducting transition metal dichalcogenides (TMDs) have been applied as the active layer in photodetectors and solar cells, displaying substantial charge photogeneration yields. However, their large exciton binding energy, which increases with decreasing thickness (number of layers), as well as the strong resonance peaks in the absorption spectra suggest that excitons are the primary photoexcited states. Detailed time-domain studies of the photoexcitation dynamics in TMDs exist mostly for MoS2. Here, we use femtosecond optical spectroscopy to study the exciton and charge dynamics following impulsive photoexcitation in few-layer WS2. We confirm excitons as the primary photoexcitation species and find that they dissociate into charge pairs with a time constant of about 1.3 ps. The better separation of the spectral features compared to MoS2 allows us to resolve a previously undetected process: these charges diffuse through the samples and get trapped at defects, such as flake edges or grain boundaries, causing an appreciable change of their transient absorption spectra. This finding opens the way to further studies of traps in TMD samples with different defect contents.Semiconducting transition metal dichalcogenides (TMDs) have been applied as the active layer in photodetectors and solar cells, displaying substantial charge photogeneration yields. However, their large exciton binding energy, which increases with decreasing thickness (number of layers), as well as the strong resonance peaks in the absorption spectra suggest that excitons are the primary photoexcited states. Detailed time-domain studies of the photoexcitation dynamics in TMDs exist mostly for MoS2. Here, we use femtosecond optical spectroscopy to study the exciton and charge dynamics following impulsive photoexcitation in few-layer WS2. We confirm excitons as the primary photoexcitation species and find that they dissociate into charge pairs with a time constant of about 1.3 ps. The better

  2. Rapid and Nondestructive Identification of Polytypism and Stacking Sequences in Few-Layer Molybdenum Diselenide by Raman Spectroscopy

    SciTech Connect

    Lu, Xin; Utama, M. Iqbal Bakti; Lin, Junhao; Luo, Xin; Zhao, Yanyuan; Zhang, Jun; Pantelides, Sokrates T.; Zhou, Wu; Quek, Su Ying; Xiong, Qihua

    2015-07-02

    Various combinations of interlayer shear modes emerge in few-layer molybdenum diselenide grown by chemical vapor deposition depending on the stacking configuration of the sample. Raman measurements may also reveal polytypism and stacking faults, as supported by first principles calculations and high-resolution transmission electron microscopy. Thus, Raman spectroscopy is an important tool in probing stacking-dependent properties in few-layer 2D materials.

  3. Understanding the STM images of epitaxial graphene on a reconstructed 6H-SiC(0001) surface: the role of tip-induced mechanical distortion of graphene.

    PubMed

    Morán-Meza, José A; Cousty, Jacques; Lubin, Christophe; Thoyer, François

    2016-06-01

    Epitaxial graphene (EG) grown on an annealed 6H-SiC(0001) surface has been studied under ultra-high vacuum (UHV) conditions by using a combined dynamic-scanning tunneling microscope/frequency modulation-atomic force microscope (dynamic-STM/FM-AFM) platform based on a qPlus probe. STM and AFM images independently recorded present the same hexagonal lattice of bumps with a 1.9 nm lattice period, which agrees with density functional theory (DFT) calculations and experimental results previously reported, attributed to the (6 × 6) quasi-cell associated with the 6H-SiC(0001) reconstruction. However, topographic bumps in AFM images and maxima in the simultaneously recorded mean-tunneling-current map do not overlap but appear to be spaced typically by about 1 nm along the [11] direction of the (6 × 6) quasi-cell. A similar shift is observed between the position of maxima in dynamic-STM images and those in the simultaneously recorded frequency shift map. The origin of these shifts is discussed in terms of electronic coupling variations between the local density of states (LDOS) of EG and the LDOS of the buffer layer amplified by mechanical distortions of EG induced by the STM or AFM tip. Therefore, a constant current STM image of EG on a reconstructed 6H-SiC(0001) surface does not reproduce its real topography but corresponds to the measured LDOS modulations, which depend on the variable tip-induced graphene distortion within the (6 × 6) quasi-cell. PMID:27165124

  4. Preparation of Few-Layer Bismuth Selenide by Liquid-Phase-Exfoliation and Its Optical Absorption Properties

    PubMed Central

    Sun, Liping; Lin, Zhiqin; Peng, Jian; Weng, Jian; Huang, Yizhong; Luo, Zhengqian

    2014-01-01

    Bismuth selenide (Bi2Se3), a new topological insulator, has attracted much attention in recent years owing to its relatively simple band structure and large bulk band gap. Compared to bulk, few-layer Bi2Se3 is recently considered as a highly promising material. Here, we use a liquid-phase exfoliation method to prepare few-layer Bi2Se3 in N-methyl-2-pyrrolidone or chitosan acetic solution. The resulted few-layer Bi2Se3 dispersion demonstrates an interesting absorption in the visible light region, which is different from bulk Bi2Se3 without any absorption in this region. The absorption spectrum of few-layer Bi2Se3 depends on its size and layer number. At the same time, the nonlinear and saturable absorption of few-layer Bi2Se3 thin film in near infrared is also characterized well and further exploited to generate laser pulses by a passive Q-switching technique. Stable Q-switched operation is achieved with a lower pump threshold of 9.3 mW at 974 nm, pulse energy of 39.8 nJ and a wide range of pulse-repetition-rate from 6.2 to 40.1 kHz. Therefore, the few-layer Bi2Se3 may excite a potential applications in laser photonics and optoelectronic devices. PMID:24762534

  5. Photocatalytic Stability of Single- and Few-Layer MoS₂.

    PubMed

    Parzinger, Eric; Miller, Bastian; Blaschke, Benno; Garrido, Jose A; Ager, Joel W; Holleitner, Alexander; Wurstbauer, Ursula

    2015-11-24

    MoS2 crystals exhibit excellent catalytic properties and great potential for photocatalytic production of solar fuels such as hydrogen gas. In this regard, the photocatalytic stability of exfoliated single- and few-layer MoS2 immersed in water is investigated by μ-Raman spectroscopy. We find that while the basal plane of MoS2 can be treated as stable under photocatalytic conditions, the edge sites and presumably also defect sites are highly affected by a photoinduced corrosion process. The edge sites of MoS2 monolayers are significantly more resistant to photocatalytic degradation compared to MoS2 multilayer edge sites. The photostability of MoS2 edge sites depends on the photon energy with respect to the band gap in MoS2 and also on the presence of oxygen in the electrolyte. These findings are interpreted in the framework of an oxidation process converting MoS2 into MoOx in the presence of oxygen and photoinduced charge carriers. The high stability of the MoS2 basal plane under photocatalytic treatment under visible light irradiation of extreme light intensities on the order of P ≈ 10 mW/μm(2) substantiates MoS2's potential as photocatalyst for solar hydrogen production. PMID:26536283

  6. Determination of the thickness and orientation of few-layer tungsten ditelluride using polarized Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Kim, Minjung; Han, Songhee; Kim, Jung Hwa; Lee, Jae-Ung; Lee, Zonghoon; Cheong, Hyeonsik

    2016-09-01

    Orthorhombic tungsten ditelluride (WTe2), with a distorted 1T structure, exhibits a large magnetoresistance that depends on the orientation, and its electrical characteristics changes from semimetallic to insulating as the thickness decreases. Through polarized Raman spectroscopy in combination with transmission electron diffraction, we establish a reliable method to determine the thickness and crystallographic orientation of few-layer WTe2. The Raman spectrum shows a pronounced dependence on the polarization of the excitation laser. We found that the separation between two Raman peaks at ∼90 cm‑1 and at 80–86 cm‑1, depending on thickness, is a reliable fingerprint for determination of the thickness. For determination of the crystallographic orientation, the polarization dependence of the A 1 modes, measured with the 632.8 nm excitation, turns out to be the most reliable. We also discovered that the polarization behaviors of some of the Raman peaks depend on the excitation wavelength as well as thickness, indicating a close interplay between the band structure and anisotropic Raman scattering cross section.

  7. Indirect-to-direct band gap crossover in few-layer MoTe₂.

    PubMed

    Lezama, Ignacio Gutiérrez; Arora, Ashish; Ubaldini, Alberto; Barreteau, Céline; Giannini, Enrico; Potemski, Marek; Morpurgo, Alberto F

    2015-04-01

    We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temperature microreflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications. PMID:25803208

  8. Few-layer Phosphorene: An Ideal 2D Material For Tunnel Transistors.

    PubMed

    Ameen, Tarek A; Ilatikhameneh, Hesameddin; Klimeck, Gerhard; Rahman, Rajib

    2016-01-01

    2D transition metal dichalcogenides (TMDs) have attracted a lot of attention recently for energy-efficient tunneling-field-effect transistor (TFET) applications due to their excellent gate control resulting from their atomically thin dimensions. However, most TMDs have bandgaps (Eg) and effective masses (m(*)) outside the optimum range needed for high performance. It is shown here that the newly discovered 2D material, few-layer phosphorene, has several properties ideally suited for TFET applications: 1) direct Eg in the optimum range ~1.0-0.4 eV, 2) light transport m(*) (0.15 m0), 3) anisotropic m(*) which increases the density of states near the band edges, and 4) a high mobility. These properties combine to provide phosphorene TFET outstanding ION ~ 1 mA/um, ON/OFF ratio ~ 10(6) for a 15 nm channel and 0.5 V supply voltage, thereby significantly outperforming the best TMD-TFETs and CMOS in many aspects such as ON/OFF current ratio and energy-delay products. Furthermore, phosphorene TFETS can scale down to 6 nm channel length and 0.2 V supply voltage within acceptable range in deterioration of the performance metrics. Full-band atomistic quantum transport simulations establish phosphorene TFETs as serious candidates for energy-efficient and scalable replacements of MOSFETs. PMID:27345020

  9. Tunable Lattice Constant and Band Gap of Single- and Few-Layer ZnO.

    PubMed

    Lee, Junseok; Sorescu, Dan C; Deng, Xingyi

    2016-04-01

    Single and few-layer ZnO(0001) (ZnO(nL), n = 1-4) grown on Au(111) have been characterized via scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. We find that the in-plane lattice constants of the ZnO(nL, n ≤ 3) are expanded compared to that of the bulk wurtzite ZnO(0001). The lattice constant reaches a maximum expansion of 3% in the ZnO(2L) and decreases to the bulk wurtzite ZnO value in the ZnO(4L). The band gap decreases monotonically with increasing number of ZnO layers from 4.48 eV (ZnO(1L)) to 3.42 eV (ZnO(4L)). These results suggest that a transition from a planar to the bulk-like ZnO structure occurs around the thickness of ZnO(4L). The work also demonstrates that the lattice constant and the band gap in ultrathin ZnO can be tuned by controlling the number of layers, providing a basis for further investigation of this material. PMID:27003692

  10. Tuning excitons in monolayer and few-layer MoS2

    NASA Astrophysics Data System (ADS)

    Qiu, Diana Y.; da Jornada, Felipe H.; Louie, Steven G.

    2014-03-01

    Our recent ab initio GW-BSE calculations showed that monolayer MoS2 is a computationally challenging system, requiring a large number of empty bands and very fine k-point sampling to converge its quasiparticle band structure and optical properties. Careful convergence of a GW-BSE calculation reveals that MoS2 has a large number of bound excitons with varying k-space characteristics. Specifically, there are two series of excitons: a low-energy series with k-space wavefunctions localized at the K/K' valleys in the Brillouin zone and a higher energy series localized in a ring around the Γ point. There is very little hybridization between these two exciton series in monolayer MoS2, but changes in electronic structure and screening due to additional layers, strain, or doping can lead to changes in exciton binding energies, character, and hybridization. Thus, we have carried out ab initio GW-BSE calculations to study the excitonic properties of few-layer MoS2. We find that layering and straining MoS2 systematically changes the exciton binding energies, the peak positions and amount of absorbance in the optical spectrum, and the character and hybridization of the excitons near Γ. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231.

  11. Large-scale arrays of single- and few-layer MoS2 nanomechanical resonators.

    PubMed

    Jia, Hao; Yang, Rui; Nguyen, Ariana E; Alvillar, Sahar Naghibi; Empante, Thomas; Bartels, Ludwig; Feng, Philip X-L

    2016-05-19

    We report the fabrication of large-scale arrays of suspended molybdenum disulfide (MoS2) atomic layers, as two-dimensional (2D) MoS2 nanomechanical resonators. We employ a water-assisted lift-off process to release chemical vapor deposited (CVD) MoS2 atomic layers from a donor substrate, followed by an all-dry transfer onto microtrench arrays. The resultant large arrays of suspended single- and few-layer MoS2 drumhead resonators (0.5-2 μm in diameter) offer fundamental resonances (f0) in the very high frequency (VHF) band (up to ∼120 MHz) and excellent figures-of-merit up to f0 × Q ≈ 3 × 10(10) Hz. A stretched circular diaphragm model allows us to estimate low pre-tension levels of typically ∼15 mN m(-1) in these devices. Compared to previous approaches, our transfer process features high yield and uniformity with minimal liquid and chemical exposure (only involving DI water), resulting in high-quality MoS2 crystals and exceptional device performance and homogeneity; and our process is readily applicable to other 2D materials. PMID:27150738

  12. Onset of two-dimensional superconductivity in space charge doped few-layer molybdenum disulfide

    PubMed Central

    Biscaras, Johan; Chen, Zhesheng; Paradisi, Andrea; Shukla, Abhay

    2015-01-01

    Atomically thin films of layered materials such as molybdenum disulfide (MoS2) are of growing interest for the study of phase transitions in two-dimensions through electrostatic doping. Electrostatic doping techniques giving access to high carrier densities are needed to achieve such phase transitions. Here we develop a method of electrostatic doping which allows us to reach a maximum n-doping density of 4 × 1014 cm−2 in few-layer MoS2 on glass substrates. With increasing carrier density we first induce an insulator to metal transition and subsequently an incomplete metal to superconductor transition in MoS2 with critical temperature ≈10 K. Contrary to earlier reports, after the onset of superconductivity, the superconducting transition temperature does not depend on the carrier density. Our doping method and the results we obtain in MoS2 for samples as thin as bilayers indicates the potential of this approach. PMID:26525386

  13. Growth and spectroscopic characterization of monolayer and few-layer hexagonal boron nitride on metal substrates.

    PubMed

    Feigelson, Boris N; Bermudez, Victor M; Hite, Jennifer K; Robinson, Zachary R; Wheeler, Virginia D; Sridhara, Karthik; Hernández, Sandra C

    2015-02-28

    Atomically thin two dimensional hexagonal boron nitride (2D h-BN) is one of the key materials in the development of new van der Waals heterostructures due to its outstanding properties including an atomically smooth surface, high thermal conductivity, high mechanical strength, chemical inertness and high electrical resistance. The development of 2D h-BN growth is still in the early stages and largely depends on rapid and accurate characterization of the grown monolayer or few layers h-BN films. This paper demonstrates a new approach to characterizing monolayer h-BN films directly on metal substrates by grazing-incidence infrared reflection absorption spectroscopy (IRRAS). Using h-BN films grown by atmospheric-pressure chemical vapor deposition on Cu and Ni substrates, two new sub-bands are found for the A2u out-of-plane stretching mode. It is shown, using both experimental and computational methods, that the lower-energy sub-band is related to 2D h-BN coupled with substrate, while the higher energy sub-band is related to decoupled (or free-standing) 2D h-BN. It is further shown that this newly-observed fine structure in the A2u mode can be used to assess, quickly and easily, the homogeneity of the h-BN-metal interface and the effects of metal surface contamination on adhesion of the layer. PMID:25640166

  14. Interlayer Interaction that is Decisive in the Energy Gap of a Few Layer Phosphorene

    NASA Astrophysics Data System (ADS)

    Sugihara, Yuki; Oshiyama, Atsushi

    We report on our first-principles calculations that clarify the microscopic origin of the band-gap variation in a few-layer phosphorene (i.e. layered phosphorous) and also rectify a prevailed picture of the electronic structure of this new layered material. Calculations have been done either using GGA with inclusion of van der Waals correction in the density-functional theory or GW approximation in the self-energy. We unequivocally reveal that the interlayer interaction causes the bonding-antibonding splitting of the highest valence band state, thus reducing the fundamental energy gap. This is due to the highest state consists mainly of p orbitals along the direction perpendicular to the layers. It was predicted that phosphorene has four polytypes named α (black), β (blue), γ and δ and all these polytypes exhibit common feature of the band-gap variation. Especially, γ phosphorene is proposed to show the metal-insulator transition from the semiconductor mono-layer to the metal bi-layer. We reveal that this transition takes place in thicker region.

  15. Metal to insulator quantum-phase transition in few-layered ReS2

    NASA Astrophysics Data System (ADS)

    Pradhan, Nihar; Rhodes, Daniel; Lu, Zhenguang; Smirnov, Dmitry; Manousakis, Efstratios; Dobrosavljevic, Vladimir; Balicas, Luis; McCreary, Amber; Feng, Simin; Terrones, Maurico; Namburu, Raju; Dubey, Madan; Hight Walker, Angela; Terrones, Humberto

    ReS2 a layer-independent direct band-gap semiconductor of 1.5 eV implies a potential for its use in optoelectronic applications. Here, we present an overall evaluation of transport and anisotropic Raman of few-layered ReS2 FET. ReS2 exfoliated on SiO2 behaves as an n-type semiconductor with an intrinsic carrier mobility surpassing μi ~ 30cm2 /Vs at T = 300 K which increases up to ~ 350cm2 /vs at 2 K. Semiconducting behavior is observed at low electron densities n, but at high values of nthe resistivity decreases by a factor > 7 upon cooling to 2 K and displays a metallicT2-dependence. The electric-field induced metallic state observed in MoS2 was recently claimed to result from a percolation type of transition. Instead, through a scaling analysis of the conductivity as a function of Tand n, we find that the metallic state of ReS2 results from a second-order metal to insulator transition driven by electronic correlations. Supported by U.S. Army Research Office MURI Grant No. W911NF-11-1-0362.

  16. Metal to insulator quantum-phase transition in few-layered ReS2

    NASA Astrophysics Data System (ADS)

    Pradhan, Nihar; McCreary, Amber; Rhodes, Daniel; Lu, Zhenguang; Smirnov, Dmitry; Manousakis, Efstratios; Feng, Simin; Namburu, Raju; Dubey, Madan; Hight Walker, Angela; Terrones, Humberto; Terrones, Mauricio; Dobrosavljevic, Vladimir; Balicas, Luis

    ReS2 a layer-independent direct band-gap semiconductor of 1.5 eV implies a potential for its use in optoelectronic applications. Here, we present an overall evaluation of transport and anisotropic Raman of few-layered ReS2 FET. ReS2 exfoliated on SiO2 behaves as an n-type semiconductor with an intrinsic carrier mobility surpassing μi ~30 cm2/Vs at T = 300 K which increases up to ~350 cm2/vs at 2 K. Semiconducting behavior is observed at low electron densities n, but at high values of nthe resistivity decreases by a factor >7 upon cooling to 2 K and displays a metallicT2 -dependence. The electric-field induced metallic state observed in MoS2 was recently claimed to result from a percolation type of transition. Instead, through a scaling analysis of the conductivity as a function of Tand n, we find that the metallic state of ReS2 results from a second-order metal to insulator transition driven by electronic correlations. Supported by U.S. Army Research Office MURI Grant No. W911NF-11-1-0362.

  17. Anisotropic in-plane thermal conductivity observed in few-layer black phosphorus

    PubMed Central

    Luo, Zhe; Maassen, Jesse; Deng, Yexin; Du, Yuchen; Garrelts, Richard P.; Lundstrom, Mark S; Ye, Peide D.; Xu, Xianfan

    2015-01-01

    Black phosphorus has been revisited recently as a new two-dimensional material showing potential applications in electronics and optoelectronics. Here we report the anisotropic in-plane thermal conductivity of suspended few-layer black phosphorus measured by micro-Raman spectroscopy. The armchair and zigzag thermal conductivities are ∼20 and ∼40 W m−1 K−1 for black phosphorus films thicker than 15 nm, respectively, and decrease to ∼10 and ∼20 W m−1 K−1 as the film thickness is reduced, exhibiting significant anisotropy. The thermal conductivity anisotropic ratio is found to be ∼2 for thick black phosphorus films and drops to ∼1.5 for the thinnest 9.5-nm-thick film. Theoretical modelling reveals that the observed anisotropy is primarily related to the anisotropic phonon dispersion, whereas the intrinsic phonon scattering rates are found to be similar along the armchair and zigzag directions. Surface scattering in the black phosphorus films is shown to strongly suppress the contribution of long mean-free-path acoustic phonons. PMID:26472191

  18. Hall and field-effect mobilities in few layered p-WSe₂ field-effect transistors.

    PubMed

    Pradhan, N R; Rhodes, D; Memaran, S; Poumirol, J M; Smirnov, D; Talapatra, S; Feng, S; Perea-Lopez, N; Elias, A L; Terrones, M; Ajayan, P M; Balicas, L

    2015-01-01

    Here, we present a temperature (T) dependent comparison between field-effect and Hall mobilities in field-effect transistors based on few-layered WSe2 exfoliated onto SiO2. Without dielectric engineering and beyond a T-dependent threshold gate-voltage, we observe maximum hole mobilities approaching 350 cm(2)/Vs at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm(2)/Vs as T is lowered below ~150 K, indicating that insofar WSe2-based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. The gate capacitance, as extracted from the Hall-effect, reveals the presence of spurious charges in the channel, while the two-terminal sheet resistivity displays two-dimensional variable-range hopping behavior, indicating carrier localization induced by disorder at the interface between WSe2 and SiO2. We argue that improvements in the fabrication protocols as, for example, the use of a substrate free of dangling bonds are likely to produce WSe2-based FETs displaying higher room temperature mobilities, i.e. approaching those of p-doped Si, which would make it a suitable candidate for high performance opto-electronics. PMID:25759288

  19. Thermal effects on the Raman phonon of few-layer phosphorene

    SciTech Connect

    Ling, Zhi-Peng; Ang, Kah-Wee

    2015-12-01

    Two-dimensional phosphorene is a promising channel material for next generation transistor applications due to its superior carrier transport property. Here, we report the influence of thermal effects on the Raman phonon of few-layer phosphorene formed on hafnium-dioxide (HfO{sub 2}) high-k dielectric. When annealed at elevated temperatures (up to 200 °C), the phosphorene film was found to exhibit a blue shift in both the out-of-plane (A{sup 1}{sub g}) and in-plane (B{sub 2g} and A{sup 2}{sub g}) phonon modes as a result of compressive strain effect. This is attributed to the out-diffusion of hafnium (Hf) atoms from the underlying HfO{sub 2} dielectric, which compresses the phosphorene in both the zigzag and armchair directions. With a further increase in thermal energy beyond 250 °C, strain relaxation within phosphorene eventually took place. When this happens, the phosphorene was unable to retain its intrinsic crystallinity prior to annealing, as evident from the broadening of full-width at half maximum of the Raman phonon. These results provide an important insight into the impact of thermal effects on the structural integrity of phosphorene when integrated with high-k gate dielectric.

  20. A first principles investigation of point defects in monolayer, few-layer, and bulk WS2

    NASA Astrophysics Data System (ADS)

    Li, Wun-Fan; Fang, Changming; Dijkstra, Marjolein; van Huis, Marijn A.; Soft Condensed Matter Team

    We present the results of a systematic study of physics of point defects in 2D WS2 materials conducted by means of density functional theory. First, we investigate the physics of point defects in monolayer (ML) WS2. Second, we examine the impact of point defects on the physical properties of multi-layer defective WS2 as a function of slab thickness. The studied point defects are: monovacancies, interstitials and anti-sites, and the considered physical properties include local geometry, defect formation energy, electronic structure and magnetism. Van der Waals interaction, spin-polarization and spin-orbit coupling effects are also incorporated in the calculations to ensure accurate results. In a ML WS2, we predict that IS is the most favorable defect inside WS2 having a low formation energy of 1.21 eV. WS and WS2 anti-sites result in a total magnetic moment of 2 μB. By studying ML, few-layer (up to 4 layers), and bulk WS2 slabs we find that, all point defects cause only localized perturbation, thus have little influence on the thickness-dependent evolution of the physical properties. The depth-dependence of the defect formation energy is also found: VS prefers to stay on the surface, while VW prefers the slab center. This work is supported by the Dutch Science Foundation NWO, VIDI Grant (Grant Nr. 723.012.006).