Science.gov

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. Electronic structure of few-layer epitaxial graphene on Ru(0001).

    PubMed

    Sutter, P; Hybertsen, M S; Sadowski, J T; Sutter, E

    2009-07-01

    The electronic structure of epitaxial monolayer, bilayer, and trilayer graphene on Ru(0001) was determined by selected-area angle-resolved photoelectron spectroscopy (micro-ARPES). Micro-ARPES band maps provide evidence for a strong electronic coupling between monolayer graphene and the adjacent metal, which causes the complete disruption of the graphene pi-bands near the Fermi energy. However, the perturbation by the metal decreases rapidly with the addition of further graphene sheets, and already an epitaxial graphene bilayer on Ru recovers the characteristic Dirac cones of isolated monolayer graphene. A graphene trilayer on Ru behaves like free-standing bilayer graphene. Density-functional theory based calculations show that this decoupling is due to the efficient passivation of metal d-states by the interfacial graphene layer.

  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.

  4. Symmetry breaking in few layer graphene films

    NASA Astrophysics Data System (ADS)

    Bostwick, Aaron; Ohta, Taisuke; McChesney, Jessica L.; Emtsev, Konstantin V.; Seyller, Thomas; Horn, Karsten; Rotenberg, Eli

    2007-10-01

    Recently, it was demonstrated that the quasiparticle dynamics, the layer-dependent charge and potential, and the c-axis screening coefficient could be extracted from measurements of the spectral function of few layer graphene films grown epitaxially on SiC using angle-resolved photoemission spectroscopy (ARPES). In this paper we review these findings, and present detailed methodology for extracting such parameters from ARPES. We also present detailed arguments against the possibility of an energy gap at the Dirac crossing ED.

  5. 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.

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

    PubMed

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

    2014-12-17

    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. 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.

  7. 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.

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

    DOE PAGES

    Liu, Ying; Huxtable, Scott T.; Yang, Bao; ...

    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

  9. 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.

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

    PubMed

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

    2016-02-18

    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.

  11. 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.

  12. 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.

  13. Synthesis, properties, and dispersion of few-layer graphene fluoride.

    PubMed

    Grayfer, Ekaterina D; Makotchenko, Viktor G; Kibis, Lidiya S; Boronin, Andrei I; Pazhetnov, Egor M; Zaikovskii, Vladimir I; Fedorov, Vladimir E

    2013-09-01

    We have fluorinated few-layer graphene (FLG) by using a low-temperature fluorination route with gaseous ClF3. The treatment process resulted in a new graphene derivative with a finite approximate composition of C2F. TEM studies showed that the product consisted of thin transparent sheets with no more than 10 fluorographene layers stacked together. Spectroscopic methods revealed a predominantly covalent nature of the C-F bonds in the as-synthesized product and we found no evidence for the existence of so-called "semi-ionic" C-F bonds, as observed in bulk C(x)F. In contrast to the case of graphite and typical (thick) expanded graphites, fluorination of FLG did not lead to the intercalation of ClF3 molecules, owing to the lack of a 3D layered structure. The approximate "critical" number of graphene layers that were necessary to form a phase of intercalated compound was estimated to be more than 12, thus providing a "chemical proof" of the difference between the properties of few-layered graphenes and bulk graphites. Fluorographene C2F was successfully delaminated into thinner layers in organic solvents, which is an important property for its integration into electronic devices, nanohybrids, etc.

  14. 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.

  15. 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

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

    PubMed

    Dávila, María Eugenia; Le Lay, Guy

    2016-02-10

    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.

  17. Spectroscopic investigation of thermal conductivity in few-layer graphene

    NASA Astrophysics Data System (ADS)

    Denison, Joseph C., Jr.

    Carbon is an extremely versatile element due to the ability of its electronic structure to allow strong bonds with many elements including other carbon atoms. This allows for the formation of many types of large and complex architectures, such as fullerenes and carbon nanotubes, at the nanoscale. One of the most fascinating allotropes of carbon is graphene, a two-dimensional honeycomb lattice with carbon in sp2 hybridization, which building block for layered graphite and other nanocarbons.[1] Because of its unique structure, graphene displays several interesting properties including high thermal[2-4] and electrical mobility and conductivity[1,5]. The initial studies on graphene were performed on mechanically exfoliated samples, which were limited to few microns in size. In the recent years, large areas of single- and few-layer graphene (˜few cm x cm) are being produced by chemical vapor deposition technique for practical applications. However, chemical vapor deposition grown graphene is highly polycrystalline with interfaces such as edges, grain boundaries, dislocations, and point defects. This inevitable presence of defects in graphene influences its electrical and thermal transport. While many studies have previously focused on the influence of defects on electrical mobility and conductivity, there is little information on the influence of defects on the thermal properties of graphene. This study specifically investigates the effect of both intrinsic and extrinsic defects on the in-plane thermal properties of graphene using micro-Raman spectroscopy. The in-plane thermal conductivity of few-layered graphene (FLG) was measured using Raman spectroscopy, following the work of Balandin et al. [4]The thermal conductivity was estimated from a shift of the characteristic G-band of graphene as a function of the excitation laser power. The graphene samples were synthesized on nickel substrates using chemical vapor deposition, and transferred to copper TEM grids and

  18. Long Spin Diffusion Length in Few-Layer Graphene Flakes

    NASA Astrophysics Data System (ADS)

    Yan, W.; Phillips, L. C.; Barbone, M.; Hämäläinen, S. J.; Lombardo, A.; Ghidini, M.; Moya, X.; Maccherozzi, F.; van Dijken, S.; Dhesi, S. S.; Ferrari, A. C.; Mathur, N. D.

    2016-09-01

    We report a spin valve with a few-layer graphene flake bridging highly spin-polarized La0.67Sr0.33MnO3 electrodes, whose surfaces are kept clean during lithographic definition. Sharp magnetic switching is verified using photoemission electron microscopy with x-ray magnetic circular dichroism contrast. A naturally occurring high interfacial resistance ˜12 M Ω facilitates spin injection, and a large resistive switching (0.8 M Ω at 10 K) implies a 70 - 130 μ m spin diffusion length that exceeds previous values obtained with sharp-switching electrodes.

  19. 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.

  20. Stacking-dependent transport properties in few-layers graphene

    NASA Astrophysics Data System (ADS)

    Lima, Matheus Paes; Padilha, José Eduardo; Pontes, Renato Borges; Fazzio, Adalberto; Silva, Antônio José Roque da

    2017-01-01

    By performing ab initio electronic structure and transport calculations, we investigated the effects of the stacking order (Bernal (AB) and rhombohedral (ABC)) as well as the number of layers, in the electronic structure and charge transport of few-layers graphene (FLG). We observed that for the ABC stack the transport properties are derived from surface states close to the Fermi level connected to dispersive states with an exponential penetration towards the inner layers, whereas for the AB stacking the transport is distributed over all layers. We present a simple model for the resistances as a function of the number of layers which contemplates the different contribution of the surface and inner layers for the transport. However, even if the stackings AB and ABC present completely different electronic and transport properties, both present the same cohesive energies, showing the absence of a thermodynamical preference for a given kind of stacking.

  1. Strain engineering of Kapitza resistance in few-layer graphene.

    PubMed

    Chen, Jie; Walther, Jens H; Koumoutsakos, Petros

    2014-02-12

    We demonstrate through molecular dynamics simulations that the Kapitza resistance in few-layer graphene (FLG) can be controlled by applying mechanical strain. For unstrained FLG, the Kapitza resistance decreases with the increase of thickness and reaches an asymptotic value of 6 × 10(-10) m(2)K/W at a thickness about 16 nm. Uniaxial cross-plane strain is found to increase the Kapitza resistance in FLG monotonically, when the applied strain varies from compressive to tensile. Moreover, uniaxial strain couples the in-plane and out-of-plane strain/stress when the surface of FLG is buckled. We find that with a compressive cross-plane stress of 2 GPa, the Kapitza resistance is reduced by about 50%. On the other hand it is almost tripled with a tensile cross-plane stress of 1 GPa. Remarkably, compressive in-plane strain can either increase or reduce the Kapitza resistance, depending on the specific way it is applied. Our study suggests that graphene can be exploited for both heat dissipation and insulation through strain engineering.

  2. 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.

  3. Oxidation and disorder in few-layered graphene induced by the electron-beam irradiation

    SciTech Connect

    Xu Zhiwei; Wang Rui; Qian Xiaoming; Chen Lei; Li Jialu; Song Xiaoyan; Liu Liangsen; Chen Guangwei

    2011-05-02

    Structural changes caused by an electron beam with the high irradiation energy of 5 MeV were investigated in few-layered graphene. Both the original and the irradiated few-layered graphene were characterized by x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy. It was found that a typical diffraction peak of graphene oxide emerged and this may be attributed to a partial oxidation in few-layered graphene which was induced by the irradiation. In addition, the graphitic structure of few-layered graphene was found to be disordered according to the increased intensity ratio of D to G band.

  4. Epitaxial deposition of silver ultra-fine nano-clusters on defect-free surfaces of HOPG-derived few-layer graphene in a UHV multi-chamber by in situ STM, ex situ XPS, and ab initio calculations

    PubMed Central

    2012-01-01

    The growth of three-dimensional ultra-fine spherical nano-particles of silver on few layers of graphene derived from highly oriented pyrolytic graphite in ultra-high vacuum were characterized using in situ scanning tunneling microscopy (STM) in conjunction with X-ray photoelectron spectroscopy. The energetics of the Ag clusters was determined by DFT simulations. The Ag clusters appeared spherical with size distribution averaging approximately 2 nm in diameter. STM revealed the preferred site for the position of the Ag atom in the C-benzene ring of graphene. Of the three sites, the C-C bridge, the C-hexagon hollow, and the direct top of the C atom, Ag prefers to stay on top of the C atom, contrary to expectation of the hexagon-close packing. Ab initio calculations confirm the lowest potential energy between Ag and the graphene structure to be at the exact site determined from STM imaging. PMID:22395057

  5. Epitaxial deposition of silver ultra-fine nano-clusters on defect-free surfaces of HOPG-derived few-layer graphene in a UHV multi-chamber by in situ STM, ex situ XPS, and ab initio calculations

    NASA Astrophysics Data System (ADS)

    Ndlovu, Gebhu F.; Roos, Wiets D.; Wang, Zhiming M.; Asante, Joseph KO; Mashapa, Matete G.; Jafta, Charl J.; Mwakikunga, Bonex W.; Hillie, Kenneth T.

    2012-03-01

    The growth of three-dimensional ultra-fine spherical nano-particles of silver on few layers of graphene derived from highly oriented pyrolytic graphite in ultra-high vacuum were characterized using in situ scanning tunneling microscopy (STM) in conjunction with X-ray photoelectron spectroscopy. The energetics of the Ag clusters was determined by DFT simulations. The Ag clusters appeared spherical with size distribution averaging approximately 2 nm in diameter. STM revealed the preferred site for the position of the Ag atom in the C-benzene ring of graphene. Of the three sites, the C-C bridge, the C-hexagon hollow, and the direct top of the C atom, Ag prefers to stay on top of the C atom, contrary to expectation of the hexagon-close packing. Ab initio calculations confirm the lowest potential energy between Ag and the graphene structure to be at the exact site determined from STM imaging.

  6. Epitaxial deposition of silver ultra-fine nano-clusters on defect-free surfaces of HOPG-derived few-layer graphene in a UHV multi-chamber by in situ STM, ex situ XPS, and ab initio calculations.

    PubMed

    Ndlovu, Gebhu F; Roos, Wiets D; Wang, Zhiming M; Asante, Joseph Ko; Mashapa, Matete G; Jafta, Charl J; Mwakikunga, Bonex W; Hillie, Kenneth T

    2012-03-06

    The growth of three-dimensional ultra-fine spherical nano-particles of silver on few layers of graphene derived from highly oriented pyrolytic graphite in ultra-high vacuum were characterized using in situ scanning tunneling microscopy (STM) in conjunction with X-ray photoelectron spectroscopy. The energetics of the Ag clusters was determined by DFT simulations. The Ag clusters appeared spherical with size distribution averaging approximately 2 nm in diameter. STM revealed the preferred site for the position of the Ag atom in the C-benzene ring of graphene. Of the three sites, the C-C bridge, the C-hexagon hollow, and the direct top of the C atom, Ag prefers to stay on top of the C atom, contrary to expectation of the hexagon-close packing. Ab initio calculations confirm the lowest potential energy between Ag and the graphene structure to be at the exact site determined from STM imaging.

  7. Heat Conduction across Monolayer and Few-Layer Graphenes

    DTIC Science & Technology

    2010-05-01

    film. We note that even though the metal films were deposited in vacuum , water vapor that adsorbs on the substrate during the air exposure after the... developed approach based on Raman spectroscopy16 to count the number of layers n of the graphene flakes. In this approach, n is determined from the ratio...Au/Ti, we coated a monolayer graphene (1- LG) sample with a semitransparent layer of Au (8 nm)/Ti (2 nm) and compared the Raman spectrum of the sample

  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. Optical properties of few layered graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Pratap Choudhary, Raghvendra; Shukla, Shobha; Vaibhav, Kumar; Bhagwan Pawar, Pranav; Saxena, Sumit

    2015-09-01

    Quantum dots provide a unique opportunity to study the confinement effects of electronic wave function on the properties of materials. We have investigated the optical properties of graphene quantum dots synthesized using ultra-fast light-matter interactions followed by one step reduction process. Atomic-scale morphological information suggests the presence of both zigzag and armchair edges in these quantum dots. Optical characterizations were performed using absorption, photoluminescence, and infrared spectroscopy. A shift in the emission spectrum and disappearance of n → π* transition in the absorption spectrum on reduction of the ablated samples confirmed the formation of graphene quantum dots. First principles calculations are in good agreement with the experimentally reported infrared data.

  11. Stacking sequence and interlayer coupling in few-layer graphene revealed by in situ imaging

    SciTech Connect

    Wang, Zhu-Jun; Dong, Jichen; Cui, Yi; Eres, Gyula; Timpe, Olaf; Fu, Qiang; Ding, Feng; Willinger, Marc-Georg; Schloegl, R.

    2016-10-19

    In the transition from graphene to graphite, the addition of each individual graphene layer modifies the electronic structure and produces a different material with unique properties. Controlled growth of few-layer graphene is therefore of fundamental interest and will provide access to materials with engineered electronic structure. Here we combine isothermal growth and etching experiments with in situ scanning electron microscopy to reveal the stacking sequence and interlayer coupling strength in few-layer graphene. The observed layer-dependent etching rates reveal the relative strength of the graphene graphene and graphene substrate interaction and the resulting mode of adlayer growth. Scanning tunnelling microscopy and density functional theory calculations confirm a strong coupling between graphene edge atoms and platinum. Simulated etching confirms that etching can be viewed as reversed growth. This work demonstrates that real-time imaging under controlled atmosphere is a powerful method for designing synthesis protocols for sp2 carbon nanostructures in between graphene and graphite.

  12. Stacking sequence and interlayer coupling in few-layer graphene revealed by in situ imaging

    NASA Astrophysics Data System (ADS)

    Wang, Zhu-Jun; Dong, Jichen; Cui, Yi; Eres, Gyula; Timpe, Olaf; Fu, Qiang; Ding, Feng; Schloegl, R.; Willinger, Marc-Georg

    2016-10-01

    In the transition from graphene to graphite, the addition of each individual graphene layer modifies the electronic structure and produces a different material with unique properties. Controlled growth of few-layer graphene is therefore of fundamental interest and will provide access to materials with engineered electronic structure. Here we combine isothermal growth and etching experiments with in situ scanning electron microscopy to reveal the stacking sequence and interlayer coupling strength in few-layer graphene. The observed layer-dependent etching rates reveal the relative strength of the graphene-graphene and graphene-substrate interaction and the resulting mode of adlayer growth. Scanning tunnelling microscopy and density functional theory calculations confirm a strong coupling between graphene edge atoms and platinum. Simulated etching confirms that etching can be viewed as reversed growth. This work demonstrates that real-time imaging under controlled atmosphere is a powerful method for designing synthesis protocols for sp2 carbon nanostructures in between graphene and graphite.

  13. Work function of few layer graphene covered nickel thin films measured with Kelvin probe force microscopy

    SciTech Connect

    Eren, B.; Gysin, U.; Marot, L. Glatzel, Th.; Steiner, R.; Meyer, E.

    2016-01-25

    Few layer graphene and graphite are simultaneously grown on a ∼100 nm thick polycrystalline nickel film. The work function of few layer graphene/Ni is found to be 4.15 eV with a variation of 50 meV by local measurements with Kelvin probe force microscopy. This value is lower than the work function of free standing graphene due to peculiar electronic structure resulting from metal 3d-carbon 2p(π) hybridization.

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

    SciTech Connect

    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, our investigation sheds new light on the significance of the electrode dielectric screening on the capacitance of few-layer graphene electrodes.

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

    DOE PAGES

    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

  16. Reversible loss of Bernal stacking during the deformation of few-layer graphene in nanocomposites.

    PubMed

    Gong, Lei; Young, Robert J; Kinloch, Ian A; Haigh, Sarah J; Warner, Jamie H; Hinks, Jonathan A; Xu, Ziwei; Li, Li; Ding, Feng; Riaz, Ibtsam; Jalil, Rashid; Novoselov, Kostya S

    2013-08-27

    The deformation of nanocomposites containing graphene flakes with different numbers of layers has been investigated with the use of Raman spectroscopy. It has been found that there is a shift of the 2D band to lower wavenumber and that the rate of band shift per unit strain tends to decrease as the number of graphene layers increases. It has been demonstrated that band broadening takes place during tensile deformation for mono- and bilayer graphene but that band narrowing occurs when the number of graphene layers is more than two. It is also found that the characteristic asymmetric shape of the 2D Raman band for the graphene with three or more layers changes to a symmetrical shape above about 0.4% strain and that it reverts to an asymmetric shape on unloading. This change in Raman band shape and width has been interpreted as being due to a reversible loss of Bernal stacking in the few-layer graphene during deformation. It has been shown that the elastic strain energy released from the unloading of the inner graphene layers in the few-layer material (~0.2 meV/atom) is similar to the accepted value of the stacking fault energies of graphite and few layer graphene. It is further shown that this loss of Bernal stacking can be accommodated by the formation of arrays of partial dislocations and stacking faults on the basal plane. The effect of the reversible loss of Bernal stacking upon the electronic structure of few-layer graphene and the possibility of using it to modify the electronic structure of few-layer graphene are discussed.

  17. Reversible Loss of Bernal Stacking during the Deformation of Few-Layer Graphene in Nanocomposites

    PubMed Central

    2013-01-01

    The deformation of nanocomposites containing graphene flakes with different numbers of layers has been investigated with the use of Raman spectroscopy. It has been found that there is a shift of the 2D band to lower wavenumber and that the rate of band shift per unit strain tends to decrease as the number of graphene layers increases. It has been demonstrated that band broadening takes place during tensile deformation for mono- and bilayer graphene but that band narrowing occurs when the number of graphene layers is more than two. It is also found that the characteristic asymmetric shape of the 2D Raman band for the graphene with three or more layers changes to a symmetrical shape above about 0.4% strain and that it reverts to an asymmetric shape on unloading. This change in Raman band shape and width has been interpreted as being due to a reversible loss of Bernal stacking in the few-layer graphene during deformation. It has been shown that the elastic strain energy released from the unloading of the inner graphene layers in the few-layer material (∼0.2 meV/atom) is similar to the accepted value of the stacking fault energies of graphite and few layer graphene. It is further shown that this loss of Bernal stacking can be accommodated by the formation of arrays of partial dislocations and stacking faults on the basal plane. The effect of the reversible loss of Bernal stacking upon the electronic structure of few-layer graphene and the possibility of using it to modify the electronic structure of few-layer graphene are discussed. PMID:23899378

  18. 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.

  19. 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.

  20. 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.

  1. 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.

  2. 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.

  3. 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.

  4. Gamma ray-assisted irradiation of few-layer graphene films: a Raman spectroscopy study

    NASA Astrophysics Data System (ADS)

    Kleut, D. N.; Marković, Z. M.; Holclajtner Antunović, I. D.; Dramićanin, M. D.; Kepić, D. P.; Todorović Marković, B. M.

    2014-09-01

    This paper represents results of a Raman spectroscopy study of gamma-irradiated few-layer graphene thin films at three different doses: 25, 50 and 110 kGy. Graphene thin films were deposited by the vacuum filtration method and then transferred onto glass substrate. Raman spectroscopy and atomic force microscopy analysis have shown that the average in-plane crystallite size La of graphene thin films varies slightly when an irradiation dose is applied. Raman spectroscopy revealed that gamma irradiation of graphene thin films resulted in slight p-doping of the graphene thin film surface. It was found that during gamma irradiation at a dose of 110 kGy, the graphene sheets merged. As a result, the number of incorporated defects in the graphene structure was reduced (the ID/IG ratio decreased with the increase in the applied dose).

  5. Stacking sequence and interlayer coupling in few-layer graphene revealed by in situ imaging

    DOE PAGES

    Wang, Zhu-Jun; Dong, Jichen; Cui, Yi; ...

    2016-10-19

    In the transition from graphene to graphite, the addition of each individual graphene layer modifies the electronic structure and produces a different material with unique properties. Controlled growth of few-layer graphene is therefore of fundamental interest and will provide access to materials with engineered electronic structure. Here we combine isothermal growth and etching experiments with in situ scanning electron microscopy to reveal the stacking sequence and interlayer coupling strength in few-layer graphene. The observed layer-dependent etching rates reveal the relative strength of the graphene graphene and graphene substrate interaction and the resulting mode of adlayer growth. Scanning tunnelling microscopy andmore » density functional theory calculations confirm a strong coupling between graphene edge atoms and platinum. Simulated etching confirms that etching can be viewed as reversed growth. This work demonstrates that real-time imaging under controlled atmosphere is a powerful method for designing synthesis protocols for sp2 carbon nanostructures in between graphene and graphite.« less

  6. Low temperature synthesis and field emission characteristics of single to few layered graphene grown using PECVD

    NASA Astrophysics Data System (ADS)

    Kumar, Avshish; Khan, Sunny; Zulfequar, M.; Harsh; Husain, Mushahid

    2017-04-01

    In this work, high-quality graphene has successfully been synthesized on copper (Cu) coated Silicon (Si) substrate at very large-area by plasma enhanced chemical vapor deposition system. This method is low cost and highly effective for synthesizing graphene relatively at low temperature of 600 °C. Electron microscopy images have shown that surface morphology of the grown samples is quite uniform consisting of single layered graphene (SLG) to few layered graphene (FLG). Raman spectra reveal that graphene has been grown with high-quality having negligible defects and the observation of G and G' peaks is also an indicative of stokes phonon energy shift caused due to laser excitation. Scanning probe microscopy image also depicts the synthesis of single to few layered graphene. The field emission characteristics of as-grown graphene samples were studied in a planar diode configuration at room temperature. The graphene samples were observed to be a good field emitter having low turn-on field, higher field amplification factor and long term emission current stability.

  7. 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.

  8. Stacking sequence and interlayer coupling in few-layer graphene revealed by in situ imaging

    PubMed Central

    Wang, Zhu-Jun; Dong, Jichen; Cui, Yi; Eres, Gyula; Timpe, Olaf; Fu, Qiang; Ding, Feng; Schloegl, R.; Willinger, Marc-Georg

    2016-01-01

    In the transition from graphene to graphite, the addition of each individual graphene layer modifies the electronic structure and produces a different material with unique properties. Controlled growth of few-layer graphene is therefore of fundamental interest and will provide access to materials with engineered electronic structure. Here we combine isothermal growth and etching experiments with in situ scanning electron microscopy to reveal the stacking sequence and interlayer coupling strength in few-layer graphene. The observed layer-dependent etching rates reveal the relative strength of the graphene–graphene and graphene–substrate interaction and the resulting mode of adlayer growth. Scanning tunnelling microscopy and density functional theory calculations confirm a strong coupling between graphene edge atoms and platinum. Simulated etching confirms that etching can be viewed as reversed growth. This work demonstrates that real-time imaging under controlled atmosphere is a powerful method for designing synthesis protocols for sp2 carbon nanostructures in between graphene and graphite. PMID:27759024

  9. Resistance and rupture analysis of single- and few-layer graphene nanosheets impacted by various projectiles

    NASA Astrophysics Data System (ADS)

    Sadeghzadeh, Sadegh; Liu, Ling

    2016-09-01

    In this paper, a quasi-classical model for the collision of various nanoparticles with single- and few-layer graphene nanosheets was introduced as a multi-scale approach that couples non-equilibrium molecular dynamics with the Finite Element Method. As a resistance criterion, it was observed that the coefficient of restitution and the induced stresses depend on the impact velocity of projectile. These parameters were evaluated computationally, and it was revealed that certain resulting behaviors differ from behaviors at the macro scale. By obtaining an out-of-plane yield stress limit of 1.0 TPa for graphene, the stress analysis of single- and multi-layer graphene sheets revealed that the limit projectile velocity needed for the yielding of graphene sheets increases with the increase in the number of layers. For aluminum nanoparticles, this increase is almost linear, and for other metals, it slightly deviates from the linear trend. It was also observed that the graphene sheets have a different rupture form when impacted by gaseous molecules than by metal particles. Considering the very high momentum of gas molecules and their shock-like behavior during high-speed collisions with a graphene sheet, pores with a size of one carbon atom can be created in graphene sheets. Since a single-layer graphene sheet can withstand a projectile which is 3.64 times larger than a projectile impacting a 20-layer graphene sheet, spaced graphene sheets seem to be more effective in absorbing the impact energy of projectiles than conventional few-layer graphene sheets.

  10. Surface potentials and layer charge distributions in few-layer graphene films.

    PubMed

    Datta, Sujit S; Strachan, Douglas R; Mele, E J; Johnson, A T Charlie

    2009-01-01

    Graphene-derived nanomaterials are emerging as ideal candidates for postsilicon electronics. Elucidating the electronic interaction between an insulating substrate and few-layer graphene (FLG) films is crucial for device applications. Here, we report electrostatic force microscopy (EFM) measurements revealing that the FLG surface potential increases with film thickness, approaching a "bulk" value for samples with five or more graphene layers. This behavior is in sharp contrast with that expected for conventional conducting or semiconducting films, and derives from unique aspects of charge screening by graphene's relativistic low energy carriers. EFM measurements resolve previously unseen electronic perturbations extended along crystallographic directions of structurally disordered FLGs, likely resulting from long-range atomic defects. These results have important implications for graphene nanoelectronics and provide a powerful framework by which key properties can be further investigated.

  11. Colloidal properties and stability of aqueous suspensions of few-layer graphene: Importance of graphene concentration.

    PubMed

    Su, Yu; Yang, Guoqing; Lu, Kun; Petersen, Elijah J; Mao, Liang

    2017-01-01

    Understanding the colloidal stability of graphene is essential for predicting its transport and ecological risks in aquatic environments. We investigated the agglomeration of (14)C-labeled few-layer graphene (FLG) at concentrations spanning nearly four orders of magnitude (2 μg/L to 10 mg/L) using dynamic light scattering and sedimentation measurements. FLG agglomerates formed rapidly in deionized water at concentrations >3 mg/L. From 1 mg/L to 3 mg/L, salt-induced agglomeration was decreased with dilution of FLG suspensions; the critical coagulation concentration of the more concentrated suspension (3 mg/L) was significantly lower than the dilute suspension (1 mg/L) in the presence of NaCl (1.6 mmol/L and 10 mmol/L, respectively). In contrast, FLG underwent slow agglomeration and settling at concentrations ≤0.1 mg/L in NaCl solutions and ambient waters with low ionic strength (<10 mmol/L). FLG nanoparticles with smaller lateral sizes (25 nm-75 nm) were shown to agglomerate more slowly than larger FLG, and these small FLG particles exhibited greater bioaccumulation in zebrafish embryo and stronger chorion penetration ability than larger FLG particles. These findings suggest that FLG at more environmentally relevant concentration is relatively stable and may have implications for exposure of small FLG to ecological receptors.

  12. Hybridization of conductive few-layer graphene with well-dispersed Pd nanocrystals

    NASA Astrophysics Data System (ADS)

    Qian, Wen; Cottingham, Steven; Jiao, Jun

    2013-06-01

    To improve the activity of Pd nanocrystals (NCs) and maximize the applicability, we used a very simple, low-cost and environmentally benign method to hybridize conductive few-layer graphene with Pd NCs. The TEM results indicated that the monodispersed Pd NCs were well distributed on the graphene surface. The particle size and loading density can be easily tailored by varying reaction times. The XRD and Raman spectrum clearly demonstrated that the pristine exfoliated few-layer graphene are well-crystallized with very low defects, and still preserved the high crystalline structure after the chemical deposition of Pd NCs. Furthermore, this effective process does not require the use of surfactants during the entire reaction, resulting in a clean interface between Pd NCs and graphene substrate, with improved electron transmission. This work presents not only a promising methodology for the mass production of Pd@graphene hybrids, but also opening up the opportunity to develop graphene-Pd based devices for applications in catalysts, sensors and hydrogen storage.

  13. 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

  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. 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.

  16. 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.

  17. Fabrication and characterization of nanometer-sized gaps in suspended few-layer graphene devices

    NASA Astrophysics Data System (ADS)

    Lumetti, S.; Martini, L.; Candini, A.

    2017-02-01

    Graphene nanodevices, such as ultra-narrow constrictions and nanometer-spaced gaps, are emerging as appealing candidates for various applications, ranging from advanced quantum devices to single-molecule junctions and even DNA sequencing. Here, we present the realization and characterization of nanometer-sized gaps in suspended few-layer graphene devices via feedback-controlled electroburning at room temperature. By analyzing the electrical behavior after the electroburning process, we identify two distinct regimes for the resulting devices, deriving a simple yet effective quantitative criterion to determine the complete opening of the nanogaps.

  18. 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.

  19. 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

  20. 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

  1. Raman spectroscopy of few-layer graphene prepared by C2-C6 cluster ion implantation

    NASA Astrophysics Data System (ADS)

    Wang, Z. S.; Zhang, R.; Zhang, Z. D.; Huang, Z. H.; Liu, C. S.; Fu, D. J.; Liu, J. R.

    2013-07-01

    Few-layer graphene has been prepared on 300 nm-thick Ni films by C2-C6 cluster ion implantation at 20 keV/cluster. Raman spectroscopy reveals significant influence of the number of atoms in the cluster, the implantation dose, and thermal treatment on the structure of the graphene layers. In particular, the graphene samples exhibit a sharp G peak at 1584 cm-1 and 2D peaks at 2711-2717 cm-1. The IG/I2D ratios higher than 1.70 and IG/ID ratio as high as 1.95 confirm that graphene sheets with low density of defects have been synthesized with much improved quality by ion implantation with larger clusters of C4-C6.

  2. 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.

  3. 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.

  4. Enhanced photocatalytic properties of graphene modified few-layered WSe2 nanosheets

    NASA Astrophysics Data System (ADS)

    Yu, Bo; Zheng, Binjie; Wang, Xinqiang; Qi, Fei; He, Jiarui; Zhang, Wanli; Chen, Yuanfu

    2017-04-01

    Reduced graphene oxide (RGO) modified few-layered WSe2 nanosheets have been synthesized through a facile one-pot solvothermal reaction. Compared with bare WSe2 nanosheets with relatively poor photocatalytic activity, WSe2/RGO composite demonstrates significant enhancement in photocatalytic degradation of organic dye RhB under visible light irradiation. The k value of WSe2/RGO is ∼1.9 times larger than that of bare WSe2 nanosheets, which is attributed to the excellent charge separation feature and electronic transport ability of graphene nanosheets, leading to highly reduced electron-hole pair recombination of graphene on WSe2 nanosheets thus strongly enhancing the photocatalytic performance. The WSe2/RGO composite is a promising catalyst for photocatalytic degradation of organic pollutants by solar energy.

  5. 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.

  6. Few-Layer Graphene Kills Selectively Tumor Cells from Myelomonocytic Leukemia Patients.

    PubMed

    Russier, Julie; León, Verónica; Orecchioni, Marco; Hirata, Eri; Virdis, Patrizia; Fozza, Claudio; Sgarrella, Francesco; Cuniberti, Gianaurelio; Prato, Maurizio; Vázquez, Ester; Bianco, Alberto; Delogu, Lucia G

    2017-03-06

    In the cure of cancer, a major cause of today's mortality, chemotherapy is the most common treatment, though serious frequent challenges are encountered by current anticancer drugs. We discovered that few-layer graphene (FLG) dispersions have a specific killer action on monocytes, showing neither toxic nor activation effects on other immune cells. We confirmed the therapeutic application of graphene on an aggressive type of cancer that is myelomonocytic leukemia, where the monocytes are in their malignant form. We demonstrated that graphene has the unique ability to target and boost specifically the necrosis of monocytic cancer cells. Moreover, the comparison between FLG and a common chemotherapeutic drug, etoposide, confirmed the higher specificity and toxicity of FLG. Since current chemotherapy treatments of leukemia still cause serious problems, these findings open the way to new and safer therapeutic approaches.

  7. Few layer graphene-polypropylene nanocomposites: the role of flake diameter.

    PubMed

    Vallés, Cristina; Abdelkader, Amr M; Young, Robert J; Kinloch, Ian A

    2014-01-01

    Graphene shows excellent potential as a structural reinforcement in polymer nanocomposites due to its exceptional mechanical properties. We have shown previously that graphene composites can be analysed using conventional composite theory with the graphene flakes acting as short fillers which have a critical length of ∼3 μm which is required for good reinforcement. Herein, polypropylene (PP) nanocomposites were prepared using electrochemically-exfoliated few layer graphene (FLG) with two different flake diameters (5 μm and 20 μm). The crystallization temperature and degree of crystallinity of the PP were found to increase with the loading of FLG, which suggests that the flakes acted as crystallisation nucleation sites. Mechanical testing showed that the 5 μm flakes behaved as short fillers and reinforced the PP matrix poorly. The modulus of the 20 μm flake composites, however, increased linearly with loading up to 20 wt%, without any of the detrimental aggregation effects seen in other graphene systems. The mechanical data were compared with our previous work on other graphene composite systems and the apparent need to balance the degree of functionalization to improve matrix compatibility whilst not encouraging aggregation is discussed.

  8. A novel approach towards selective bulk synthesis of few-layer graphenes in an electric arc

    NASA Astrophysics Data System (ADS)

    Karmakar, Soumen; Kulkarni, Naveen V.; Nawale, Ashok B.; Lalla, Niranjan P.; Mishra, Ratikant; Sathe, V. G.; Bhoraskar, S. V.; Das, A. K.

    2009-06-01

    The paper demonstrates the selective bulk synthesis of few-layer graphenes by optimizing an external magnetic field assisted electric arc. An ultra-high purity glassy graphite anode was sublimated in an argon atmosphere, and carbon nanotubes (CNTs), along with graphene sheets, were found inside the deposit formed on the cathode. Both the high purity CNTs and the graphene sheets, with minimal structural defects, were synthesized separately by varying the strength and orientation of the external magnetic field. The as-synthesized carbonaceous samples were characterized with the help of transmission electron microscopy, selected area electron diffraction (SAED), Raman spectroscopy and thermogravimetry with the objective of optimizing the highest selective production of 2D graphene structures. The as-synthesized graphene sheets exhibited a relatively high degree of graphitization and low structural defect density as confirmed by Raman spectroscopy. They were found to exhibit higher oxidation temperature (767 °C) than that of the carbon nanocrystalline particles (690 °C), as inferred from the thermogravimatric analysis. Moreover, they were found to roll up at their edges on account of their surface energy minimization. This was confirmed by the SAED analysis. With this new technique, we could successfully synthesize 2D graphene structures at the rate of a few g h-1.

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

    NASA Astrophysics Data System (ADS)

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

    2014-07-01

    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.

  10. 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.

  11. Self-organized arrays of graphene and few-layer graphene quantum dots in fluorographene matrix: Charge transient spectroscopy

    SciTech Connect

    Antonova, Irina V.; Nebogatikova, Nadezhda A.; Prinz, Victor Ya.

    2014-05-12

    Arrays of graphene or few-layer graphene quantum dots (QDs) embedded in a partially fluorinated graphene matrix were created by chemical functionalization of layers. Charge transient spectroscopy employed for investigation of obtained QD systems (size 20–70 nm) has allowed us to examine the QD energy spectra and the time of carrier emission (or charge relaxation) from QDs as a function of film thickness. It was found that the characteristic time of carrier emission from QDs decreased markedly (by about four orders of magnitude) on increasing the QD thickness from one graphene monolayer to 3 nm. Daylight-assisted measurements also demonstrate a strong decrease of the carrier emission time.

  12. 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.

  13. 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).

  14. 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.

  15. Thermal transport in three-dimensional foam architectures of few-layer graphene and ultrathin graphite.

    PubMed

    Pettes, Michael Thompson; Ji, Hengxing; Ruoff, Rodney S; Shi, Li

    2012-06-13

    At a very low solid concentration of 0.45 ± 0.09 vol %, the room-temperature thermal conductivity (κ(GF)) of freestanding graphene-based foams (GF), comprised of few-layer graphene (FLG) and ultrathin graphite (UG) synthesized through the use of methane chemical vapor deposition on reticulated nickel foams, was increased from 0.26 to 1.7 W m(-1) K(-1) after the etchant for the sacrificial nickel support was changed from an aggressive hydrochloric acid solution to a slow ammonium persulfate etchant. In addition, κ(GF) showed a quadratic dependence on temperature between 11 and 75 K and peaked at about 150 K, where the solid thermal conductivity (κ(G)) of the FLG and UG constituents reached about 1600 W m(-1) K(-1), revealing the benefit of eliminating internal contact thermal resistance in the continuous GF structure.

  16. 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

  17. Facile synthesis of few-layer graphene with a controllable thickness using rapid thermal annealing.

    PubMed

    Chu, Jae Hwan; Kwak, Jinsung; Kwon, Tae-Yang; Park, Soon-Dong; Go, Heungseok; Kim, Sung Youb; Park, Kibog; Kang, Seoktae; Kwon, Soon-Yong

    2012-03-01

    Few-layer graphene films with a controllable thickness were grown on a nickel surface by rapid thermal annealing (RTA) under vacuum. The instability of nickel films in air facilitates the spontaneous formation of ultrathin (<2-3 nm) carbon- and oxygen-containing compounds on a nickel surface; thus, the high-temperature annealing of the nickel samples without the introduction of intentional carbon-containing precursors results in the formation of graphene films. From annealing temperature and ambient studies during RTA, it was found that the evaporation of oxygen atoms from the surface is the dominant factor affecting the formation of graphene films. The thickness of the graphene layers is strongly dependent on the RTA temperature and time, and the resulting films have a limited thickness (<2 nm), even for an extended RTA time. The transferred films have a low sheet resistance of ~0.9 ± 0.4 kΩ/sq, with ~94% ± 2% optical transparency, making them useful for applications as flexible transparent conductors.

  18. MgO-decorated few-layered graphene as an anode for li-ion batteries.

    PubMed

    Petnikota, Shaikshavali; Rotte, Naresh K; Reddy, M V; Srikanth, Vadali V S S; Chowdari, B V R

    2015-02-04

    Combustion of magnesium in dry ice and a simple subsequent acid treatment step resulted in a MgO-decorated few-layered graphene (FLG) composite that has a specific surface area of 393 m(2)/g and an average pore volume of 0.9 cm(3)/g. As an anode material in Li-ion batteries, the composite exhibited high reversible capacity and excellent cyclic performance in spite of high first-cycle irreversible capacity loss. A reversible capacity as high as 1052 mAh/g was measured during the first cycle. Even at the end of the 60th cycle, more than 83% of the capacity could be retained. Cyclic voltammetry results indicated pseudocapacitance behavior due to electrochemical absorption and desorption of lithium ions onto graphene. An increase in the capacity has been observed during long-term cycling owing to electrochemical exfoliation of graphene sheets. Owing to its good thermal stability and superior cyclic performance with high reversible capacities, MgO-decked FLG can be an excellent alternative to graphite as an anode material in Li-ion batteries, after suitable modifications.

  19. Layer-by-Layer Insight into Electrostatic Charge Distribution of Few-Layer Graphene

    NASA Astrophysics Data System (ADS)

    Rokni, Hossein; Lu, Wei

    2017-02-01

    In few-layer graphene (FLG) systems on a dielectric substrate such as SiO2, the addition of each extra layer of graphene can drastically alter their electronic and structural properties. Here, we map the charge distribution among the individual layers of finite-size FLG systems using a novel spatial discrete model that describes both electrostatic interlayer screening and fringe field effects. Our results reveal that the charge density in the region very close to the edges is screened out an order of magnitude more weakly than that across the central region of the layers. Our discrete model suggests that the interlayer charge screening length in 1–8 layer thick graphene systems depends mostly on the overall gate/molecular doping level rather than on temperature, in particular at an induced charge density >5 × 1012 cm‑2, and can reliably be determined to be larger than half the interlayer spacing but shorter than the bilayer thickness. Our model can be used for designing FLG-based devices, and offers a simple rule regarding the charge distribution in FLG: approximately 70%, 20%, 6% and 3% (99% overall) of the total induced charge density reside within the four innermost layers, implying that the gate-induced electric field is not definitely felt by >4th layer.

  20. Direct Preparation of Few Layer Graphene Epoxy Nanocomposites from Untreated Flake Graphite.

    PubMed

    Throckmorton, James; Palmese, Giuseppe

    2015-07-15

    The natural availability of flake graphite and the exceptional properties of graphene and graphene-polymer composites create a demand for simple, cost-effective, and scalable methods for top-down graphite exfoliation. This work presents a novel method of few layer graphite nanocomposite preparation directly from untreated flake graphite using a room temperature ionic liquid and laminar shear processing regimen. The ionic liquid serves both as a solvent and initiator for epoxy polymerization and is incorporated chemically into the matrix. This nanocomposite shows low electrical percolation (0.005 v/v) and low thickness (1-3 layers) graphite/graphene flakes by TEM. Additionally, the effect of processing conditions by rheometry and comparison with solvent-free conditions reveal the interactions between processing and matrix properties and provide insight into the theory of the chemical and physical exfoliation of graphite crystals and the resulting polymer matrix dispersion. An interaction model that correlates the interlayer shear physics of graphite flakes and processing parameters is proposed and tested.

  1. Layer-by-Layer Insight into Electrostatic Charge Distribution of Few-Layer Graphene

    PubMed Central

    Rokni, Hossein; Lu, Wei

    2017-01-01

    In few-layer graphene (FLG) systems on a dielectric substrate such as SiO2, the addition of each extra layer of graphene can drastically alter their electronic and structural properties. Here, we map the charge distribution among the individual layers of finite-size FLG systems using a novel spatial discrete model that describes both electrostatic interlayer screening and fringe field effects. Our results reveal that the charge density in the region very close to the edges is screened out an order of magnitude more weakly than that across the central region of the layers. Our discrete model suggests that the interlayer charge screening length in 1–8 layer thick graphene systems depends mostly on the overall gate/molecular doping level rather than on temperature, in particular at an induced charge density >5 × 1012 cm−2, and can reliably be determined to be larger than half the interlayer spacing but shorter than the bilayer thickness. Our model can be used for designing FLG-based devices, and offers a simple rule regarding the charge distribution in FLG: approximately 70%, 20%, 6% and 3% (99% overall) of the total induced charge density reside within the four innermost layers, implying that the gate-induced electric field is not definitely felt by >4th layer. PMID:28220816

  2. Kapitza Resistance between Few-Layer Graphene and Water: Liquid Layering Effects.

    PubMed

    Alexeev, Dmitry; Chen, Jie; Walther, Jens H; Giapis, Konstantinos P; Angelikopoulos, Panagiotis; Koumoutsakos, Petros

    2015-09-09

    The Kapitza resistance (RK) between few-layer graphene (FLG) and water was studied using molecular dynamics simulations. The RK was found to depend on the number of the layers in the FLG though, surprisingly, not on the water block thickness. This distinct size dependence is attributed to the large difference in the phonon mean free path between the FLG and water. Remarkably, RK is strongly dependent on the layering of water adjacent to the FLG, exhibiting an inverse proportionality relationship to the peak density of the first water layer, which is consistent with better acoustic phonon matching between FLG and water. These findings suggest novel ways to engineer the thermal transport properties of solid-liquid interfaces by controlling and regulating the liquid layering at the interface.

  3. Few layers graphene as thermally activated optical modulator in the visible-near IR spectral range.

    PubMed

    Benítez, J L; Hernández-Cordero, Juan; Muhl, S; Mendoza, D

    2016-01-01

    We report the temperature modulation of the optical transmittance of a few layers of graphene (FLG). The FLG was heated either by the Joule effect of the current flowing between coplanar electrodes or by the absorption of a continuous-wave 532 nm laser. The optical signals used to evaluate the modulation of the FLG were at 633, 975, and 1550 nm; the last wavelengths are commonly used in optical communications. We also evaluated the effect of the substrate on the modulation effect by comparing the performance of a freely suspended FLG sample with one mounted on a glass substrate. Our results show that the modulation of the optical transmittance of FLG can be from millihertz to kilohertz.

  4. Optical methods for determining thicknesses of few-layer graphene flakes

    NASA Astrophysics Data System (ADS)

    Ouyang, Wengen; Liu, Xin-Z.; Li, Qunyang; Zhang, Yingying; Yang, Jiarui; Zheng, Quan-shui

    2013-12-01

    Optical microscopy (OM) methods have been commonly used as a convenient means for locating and identifying few-layer graphene (FLG) on SiO2/Si substrates. However, it is less clear how reliably optical images of FLG could be used to determine the sample thickness. In this work, various OM methods based on color differences and color contrasts are presented and their reliabilities are evaluated. Our analysis shows that these color-based OM methods depend sensitively on certain parameters of the measuring system, particularly the light source and the reference substrate. These parameters have usually been overlooked and less controlled in routine experiments. From evaluating the performance of these OM methods with both virtual and real FLG samples, we propose some practical guidelines for minimizing the impact of these less-controlled experimental parameters and provide a user-friendly MATLAB script for facilitating the implementation.

  5. Enhancement of plasma illumination characteristics of few-layer graphene-diamond nanorods hybrid

    NASA Astrophysics Data System (ADS)

    Jothiramalingam Sankaran, Kamatchi; Yeh, Chien-Jui; Drijkoningen, Sien; Pobedinskas, Paulius; Van Bael, Marlies K.; Leou, Keh-Chyang; Lin, I.-Nan; Haenen, Ken

    2017-02-01

    Few-layer graphene (FLG) was catalytically formed on vertically aligned diamond nanorods (DNRs) by a high temperature annealing process. The presence of 4-5 layers of FLG on DNRs was confirmed by transmission electron microscopic studies. It enhances the field electron emission (FEE) behavior of the DNRs. The FLG-DNRs show excellent FEE characteristics with a low turn-on field of 4.21 V μm-1 and a large field enhancement factor of 3480. Moreover, using FLG-DNRs as cathode markedly enhances the plasma illumination behavior of a microplasma device, viz not only the plasma current density is increased, but also the robustness of the devices is improved.

  6. Enhancement of plasma illumination characteristics of few-layer graphene-diamond nanorods hybrid.

    PubMed

    Sankaran, Kamatchi Jothiramalingam; Yeh, Chien-Jui; Drijkoningen, Sien; Pobedinskas, Paulius; Van Bael, Marlies K; Leou, Keh-Chyang; Lin, I-Nan; Haenen, Ken

    2017-02-10

    Few-layer graphene (FLG) was catalytically formed on vertically aligned diamond nanorods (DNRs) by a high temperature annealing process. The presence of 4-5 layers of FLG on DNRs was confirmed by transmission electron microscopic studies. It enhances the field electron emission (FEE) behavior of the DNRs. The FLG-DNRs show excellent FEE characteristics with a low turn-on field of 4.21 V μm(-1) and a large field enhancement factor of 3480. Moreover, using FLG-DNRs as cathode markedly enhances the plasma illumination behavior of a microplasma device, viz not only the plasma current density is increased, but also the robustness of the devices is improved.

  7. 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.

  8. In situ nitrogenated graphene-few-layer WS2 composites for fast and reversible Li+ storage

    NASA Astrophysics Data System (ADS)

    Chen, Dongyun; Ji, Ge; Ding, Bo; Ma, Yue; Qu, Baihua; Chen, Weixiang; Lee, Jim Yang

    2013-08-01

    Two-dimensional nanosheets can leverage on their open architecture to support facile insertion and removal of Li+ as lithium-ion battery electrode materials. In this study, two two-dimensional nanosheets with complementary functions, namely nitrogen-doped graphene and few-layer WS2, were integrated via a facile surfactant-assisted synthesis under hydrothermal conditions. The layer structure and morphology of the composites were confirmed by X-ray diffraction, scanning electron microscopy and high-resolution transmission microscopy. The effects of surfactant amount on the WS2 layer number were investigated and the performance of the layered composites as high energy density lithium-ion battery anodes was evaluated. The composite formed with a surfactant : tungsten precursor ratio of 1 : 1 delivered the best cyclability (average of only 0.08% capacity fade per cycle for 100 cycles) and good rate performance (80% capacity retention with a 50-fold increase in current density from 100 mA g-1 to 5000 mA g-1), and may find uses in power-oriented applications.Two-dimensional nanosheets can leverage on their open architecture to support facile insertion and removal of Li+ as lithium-ion battery electrode materials. In this study, two two-dimensional nanosheets with complementary functions, namely nitrogen-doped graphene and few-layer WS2, were integrated via a facile surfactant-assisted synthesis under hydrothermal conditions. The layer structure and morphology of the composites were confirmed by X-ray diffraction, scanning electron microscopy and high-resolution transmission microscopy. The effects of surfactant amount on the WS2 layer number were investigated and the performance of the layered composites as high energy density lithium-ion battery anodes was evaluated. The composite formed with a surfactant : tungsten precursor ratio of 1 : 1 delivered the best cyclability (average of only 0.08% capacity fade per cycle for 100 cycles) and good rate performance (80

  9. Ultrathin ultra-broadband electro-absorption modulator based on few-layer graphene based anisotropic metamaterial

    NASA Astrophysics Data System (ADS)

    Sayem, Ayed Al; Mahdy, M. R. C.; Jahangir, Ifat; Rahman, Md. Saifur

    2017-02-01

    In this article, a few-layered graphene-dielectric multilayer (metamaterial) electro-optic modulator has been proposed in the mid and far infrared range that works on electro-absorption mechanism. Graphene, both mono layer and few layer, is an actively tunable optical material that allows control of inter-band and intra-band transition by tuning its chemical potential. Utilizing this unique feature of graphene, we propose a multilayer graphene dielectric stack where few layer graphene is preferred over mono layer graphene. Although the total thickness of the stack still remains in the nanometer range, this device can exhibit superior performances in terms of (i) high modulation depth, (ii) ultra-broadband performance, (iii) ultra-low insertion loss due to inherent metamaterial properties, (iv)nano-scale footprint, (v) polarization independence and (vi) capability of being integrated to a silicon waveguide. Interestingly, these superior performances, achievable by using few layer graphene with carefully designed metamaterial, may not be possible with mono layer graphene. Our proposals have been validated by both the effective medium theory and general transfer matrix method.

  10. Stacking Structures of Few-Layer Graphene Revealed by Phase-Sensitive Infrared Nanoscopy.

    PubMed

    Kim, Deok-Soo; Kwon, Hyuksang; Nikitin, Alexey Yu; Ahn, Seongjin; Martín-Moreno, Luis; García-Vidal, Francisco J; Ryu, Sunmin; Min, Hongki; Kim, Zee Hwan

    2015-07-28

    The stacking orders in few-layer graphene (FLG) strongly influences the electronic properties of the material. To explore the stacking-specific properties of FLG in detail, one needs powerful microscopy techniques that visualize stacking domains with sufficient spatial resolution. We demonstrate that infrared (IR) scattering scanning near-field optical microscopy (sSNOM) directly maps out the stacking domains of FLG with a nanometric resolution, based on the stacking-specific IR conductivities of FLG. The intensity and phase contrasts of sSNOM are compared with the sSNOM contrast model, which is based on the dipolar tip-sample coupling and the theoretical conductivity spectra of FLG, allowing a clear assignment of each FLG domain as Bernal, rhombohedral, or intermediate stacks for tri-, tetra-, and pentalayer graphene. The method offers 10-100 times better spatial resolution than the far-field Raman and infrared spectroscopic methods, yet it allows far more experimental flexibility than the scanning tunneling microscopy and electron microscopy.

  11. Mechanical and electronic coupling in few-layer graphene and hBN wrinkles: a first-principles study

    NASA Astrophysics Data System (ADS)

    Guo, Yufeng; Qiu, Jiapeng; Guo, Wanlin

    2016-12-01

    Wrinkle engineering is an important pathway to develop novel functional devices of two-dimensional materials. By combining first-principles calculations and continuum mechanics modelling, we have investigated the wrinkling of few-layer graphene and hexagonal boron nitride (hBN) and provide a way to estimate their bending stiffness. For few-layer wrinkles under the same strain, the magnitude of structural deformation of each constituent layer gradually decreases from bottom to top layers, while interlayer interaction increases with increasing layer number. Comparing with monolayer wrinkles, the electronic properties of few-layer wrinkles are more sensitive to bending deformation as mechanical and electronic coupling induce charge redistribution at the wrinkles, making few-layer graphene and hBN wrinkles suitable for electromechanical system application.

  12. 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.

  13. 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.

  14. Removal of Pb(II) ions from aqueous solutions on few-layered graphene oxide nanosheets.

    PubMed

    Zhao, Guixia; Ren, Xuemei; Gao, Xing; Tan, Xiaoli; Li, Jiaxing; Chen, Changlun; Huang, Yuying; Wang, Xiangke

    2011-11-07

    Few-layered graphene oxide (FGO) was synthesized from graphite by using the modified Hummers method, and was characterized by scanning electron microscopy, atomic force microscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. The prepared FGO was used to adsorb Pb(II) ions from aqueous solutions. The abundant oxygen-containing groups on the surfaces of FGO played an important role in Pb(II) ion adsorption on FGO. The adsorption of Pb(II) ions on FGO was dependent on pH values and independent of ionic strength. The adsorption of Pb(II) ions on FGO was mainly dominated by strong surface complexation. From the adsorption isotherms, the maximum adsorption capacities (C(smax)) of Pb(II) ions on FGO calculated from the Langmuir model were about 842, 1150, and 1850 mg g(-1) at 293, 313, and 333 K, respectively, higher than any currently reported. The FGO had the highest adsorption capacities of today's nanomaterials. The thermodynamic parameters calculated from the temperature dependent adsorption isotherms indicated that the adsorption of Pb(II) ions on FGO was a spontaneous and endothermic process.

  15. 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.

  16. In situ nitrogenated graphene-few-layer WS2 composites for fast and reversible Li+ storage.

    PubMed

    Chen, Dongyun; Ji, Ge; Ding, Bo; Ma, Yue; Qu, Baihua; Chen, Weixiang; Lee, Jim Yang

    2013-09-07

    Two-dimensional nanosheets can leverage on their open architecture to support facile insertion and removal of Li(+) as lithium-ion battery electrode materials. In this study, two two-dimensional nanosheets with complementary functions, namely nitrogen-doped graphene and few-layer WS2, were integrated via a facile surfactant-assisted synthesis under hydrothermal conditions. The layer structure and morphology of the composites were confirmed by X-ray diffraction, scanning electron microscopy and high-resolution transmission microscopy. The effects of surfactant amount on the WS2 layer number were investigated and the performance of the layered composites as high energy density lithium-ion battery anodes was evaluated. The composite formed with a surfactant : tungsten precursor ratio of 1 : 1 delivered the best cyclability (average of only 0.08% capacity fade per cycle for 100 cycles) and good rate performance (80% capacity retention with a 50-fold increase in current density from 100 mA g(-1) to 5000 mA g(-1)), and may find uses in power-oriented applications.

  17. Layer-to-layer compression and enhanced optical properties of few-layer graphene nanosheet induced by ion irradiation

    NASA Astrophysics Data System (ADS)

    Shang, Zhen; Tan, Yang; Zhou, Shengqiang; Chen, Feng

    2016-08-01

    We report on the first experimental study of the layer-to-layer compression and enhanced optical properties of few-layer graphene nanosheet by applying ion irradiation. The deformation of graphene layers is investigated both theoretically and experimentally. It is observed that after the irradiation of energetic ion beams, the space between separate graphene layers is reduced due to layer-to-layer compression, resulting in tighter contact of the graphene sheet with the surface of the substrate. This processing enables enhanced interaction of the graphene with the evanescent-field wave near the surface, which induces reinforced polarization-dependent light absorption of the graphene. Utilizing the ion-bombarded graphene nanosheets as saturable absorbers, we have realized efficient Q-switched waveguide lasing with enhanced performance through the interaction of the graphene and evanescent field.

  18. 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

  19. Epitaxial Graphene Quantum Electronics

    DTIC Science & Technology

    2014-05-19

    ferromagnetism with spintronics potential. * We have achieved the highest operational frequency in graphene transistors. Epitaxial graphene; quantum transport...important discovery with implications for spintronics . * We have found that ballistic transport most likely involves non-conventional charge carriers

  20. Electrostatic transfer of epitaxial graphene to glass.

    SciTech Connect

    Ohta, Taisuke; Pan, Wei; Howell, Stephen Wayne; Biedermann, Laura Butler; Beechem Iii, Thomas Edwin; Ross, Anthony Joseph, III

    2010-12-01

    We report on a scalable electrostatic process to transfer epitaxial graphene to arbitrary glass substrates, including Pyrex and Zerodur. This transfer process could enable wafer-level integration of graphene with structured and electronically-active substrates such as MEMS and CMOS. We will describe the electrostatic transfer method and will compare the properties of the transferred graphene with nominally-equivalent 'as-grown' epitaxial graphene on SiC. The electronic properties of the graphene will be measured using magnetoresistive, four-probe, and graphene field effect transistor geometries [1]. To begin, high-quality epitaxial graphene (mobility 14,000 cm2/Vs and domains >100 {micro}m2) is grown on SiC in an argon-mediated environment [2,3]. The electrostatic transfer then takes place through the application of a large electric field between the donor graphene sample (anode) and the heated acceptor glass substrate (cathode). Using this electrostatic technique, both patterned few-layer graphene from SiC(000-1) and chip-scale monolayer graphene from SiC(0001) are transferred to Pyrex and Zerodur substrates. Subsequent examination of the transferred graphene by Raman spectroscopy confirms that the graphene can be transferred without inducing defects. Furthermore, the strain inherent in epitaxial graphene on SiC(0001) is found to be partially relaxed after the transfer to the glass substrates.

  1. Few-layers graphene oxide for NO2 gas sensor on plastic

    NASA Astrophysics Data System (ADS)

    Ramli, Muhammad M.; Isa, Siti S. Mat; Jamlos, M. F.; Murad, S. A. Z.; Isa, M. Mohamad; Kasjoo, S. R.; Ahmad, N.; Nor, N. I. M.; Khalid, N.

    2017-03-01

    Vacuum filtration method was used in order to fabricate a homogeneous and uniform thin film of multi-layer graphene oxide on plastic substrate. This self-regulating technique allows the number of graphene oxide layer to be controlled thus controlling the film thickness by simply varying either the concentration of the graphene oxide in the suspension or the filtration volume. Measurement of the sheet resistance as a function of graphene oxide concentration in solution shows the percolation behavior of multi-layer films. The device was then exposed in nitrogen dioxide (NO2) environment at room temperature and 200 °C, under atmospheric pressure. Results demonstrate that the graphene oxide film shows good sensitivity and excellent recovery time using plastic substrate.

  2. 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.

  3. Lithium-Intercalated Few Layer Graphene: Approaching the Limits of Transparency and Conductivity in Graphene-based Materials

    NASA Astrophysics Data System (ADS)

    Bao, Wenzhong; Wan, Jiayu; Han, Xiaogang; Cai, Xinghan; Zhu, Hongli; Kim, Dohun; Xu, Yunlu; Munday, Jeremy; Drew, H. Dennis; Fuhrer, Michael; Hu, Liangbing; University Of Maryland College Park Collaboration

    2014-03-01

    We measure simultaneous in situ optical transmittance spectra and electrical transport properties of few-layer graphene (FLG) nanostructures upon electrochemical lithiation/delithiation. Reversible Li-intercalation stages and a two-phase boundary are observed optically. Due to the unusual electronic structure of FLG, upon intercalation we observe a simultaneous increase of both optical transmittance and DC conductivity, strikingly different from other materials. Transmission as high as 91.7% for sheet resistance of 3.0 Ω/square is achieved for 19 layer LiC6, corresponding to a figure of merit (FOM) σdc/σopt = 1400, five times higher than any previously demonstrated for a continuous transparent electrode. The unconventional modification of FLG optoelectronic properties is explained by the suppression of interband optical transitions and a small intraband Drude conductivity near the interband edge. Our techniques can enable investigation of other aspects of intercalation in nanostructures, for example intercalation dynamics and solid-electrolyte interface formation.

  4. 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.

  5. Estimating the elastic properties of few-layer graphene from the free-standing indentation response.

    PubMed

    Zhou, Lixin; Wang, Yugang; Cao, Guoxin

    2013-11-27

    Using molecular mechanics simulations, the elastic properties of multi-layer graphene (MLG) are investigated; this includes both the linear analysis based on the indentation load-displacement relationship and the nonlinear analysis based on the strain energy. The elastic properties of graphene layers in MLG are similar to each other and also quite close to those of monolayer graphene. The van der Waals (VDW) interaction between graphene layers (interlayer interaction) will create a difference between the indenter tip displacement and the deviation of MLG in indentation, which will cause an overestimation of the elastic modulus of MLG based on classic indentation analysis. This overestimation can be as high as 20%. In addition, the interlayer interaction will significantly affect the nonlinear elastic behavior of MLG in free-standing indentation. With an increase in the number of layers of MLG, the second-order elastic stiffness of MLG is very sensitive to the indentation loading range, and the third-order nonlinear elastic constant is significantly increased.

  6. 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.

  7. Powder, paper and foam of few-layer graphene prepared in high yield by electrochemical intercalation exfoliation of expanded graphite.

    PubMed

    Wu, Liqiong; Li, Weiwei; Li, Peng; Liao, Shutian; Qiu, Shengqiang; Chen, Mingliang; Guo, Yufen; Li, Qi; Zhu, Chao; Liu, Liwei

    2014-04-09

    A facile and high-yield approach to the preparation of few-layer graphene (FLG) by electrochemical intercalation exfoliation (EIE) of expanded graphite in sulfuric acid electrolyte is reported. Stage-1 H2SO4-graphite intercalation compound is used as a key intermediate in EIE to realize the efficient exfoliation. The yield of the FLG sheets (<7 layers) with large lateral sizes (tens of microns) is more than 75% relative to the total amount of starting expanded graphite. A low degree of oxygen functionalization existing in the prepared FLG flakes enables them to disperse effectively, which contributes to the film-forming characteristics of the FLG flakes. These electrochemically exfoliated FLG flakes are integrated into several kinds of macroscopic graphene structures. Flexible and freestanding graphene papers made of the FLG flakes retain excellent conductivity (≈24,500 S m(-1)). Three-dimensional (3D) graphene foams with light weight are fabricated from the FLG flakes by the use of Ni foams as self-sacrifice templates. Furthermore, 3D graphene/Ni foams without any binders, which are used as supercapacitor electrodes in aqueous electrolyte, provide the specific capacitance of 113.2 F g(-1) at a current density of 0.5 A g(-1), retaining 90% capacitance after 1000 cycles.

  8. Field emission from few-layer graphene nanosheets produced by liquid phase exfoliation of graphite.

    PubMed

    Dong, Jianhui; Zeng, Baoqing; Lan, Yucheng; Tian, Shikai; Shan, Yun; Liu, Xingchong; Yang, Zhonghai; Wang, Hui; Ren, Z F

    2010-08-01

    Graphene nanosheets have been synthesized from commercial expandable graphite by heating in a microwave oven and dispersing in ethanol by ultrasonication. Scanning and transmission electron microscopy and electron energy-loss spectroscopy and atomic force microscope showed that the nanosheets were about 2 nm in thickness and 10 microm in diameter. The field emission of the graphene sheets has been investigated. An emission current density of 1 mA/cm2 has been achieved at an electric field of 3.7 V/microm with a turn-on field of 1.7 V/microm at 0.01 mA/cm2. The annealing of the samples at 400 degrees C in vacuum greatly improved the field emission performance.

  9. 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.

  10. Facile and Scalable Synthesis Method for High-Quality Few-Layer Graphene through Solution-Based Exfoliation of Graphite.

    PubMed

    Wee, Boon-Hong; Wu, Tong-Fei; Hong, Jong-Dal

    2017-02-08

    Here we describe a facile and scalable method for preparing defect-free graphene sheets exfoliated from graphite using the positively charged polyelectrolyte precursor poly(p-phenylenevinylene) (PPV-pre) as a stabilizer in an aqueous solution. The graphene exfoliated by PPV-pre was apparently stabilized in the solution as a form of graphene/PPV-pre (denoted to GPPV-pre), which remains in a homogeneous dispersion over a year. The thickness values of 300 selected 76% GPPV-pre flakes ranged from 1 to 10 nm, corresponding to between one and a few layers of graphene in the lateral dimensions of 1 to 2 μm. Furthermore, this approach was expected to yield a marked decrease in the density of defects in the electronic conjugation of graphene compared to that of graphene oxide (GO) obtained by Hummers' method. The positively charged GPPV-pre was employed to fabricate a poly(ethylene terephthalate) (PET) electrode layer-by-layer with negatively charged GO, yielding (GPPV-pre/GO)n film electrode. The PPV-pre and GO in the (GPPV-pre/GO)n films were simultaneously converted using hydroiodic acid vapor to fully conjugated PPV and reduced graphene oxide (RGO), respectively. The electrical conductivity of (GPPV/RGO)23 multilayer films was 483 S/cm, about three times greater than that of the (PPV/RGO)23 multilayer films (166 S/cm) comprising RGO (prepared by Hummers method). Furthermore, the superior electrical properties of GPPV were made evident, when comparing the capacitive performances of two supercapacitor systems; (polyaniline PANi/RGO)30/(GPPV/RGO)23/PET (volumetric capacitance = 216 F/cm(3); energy density = 19 mWh/cm(3); maximum power density = 498 W/cm(3)) and (PANi/RGO)30/(PPV/RGO)23/PET (152 F/cm(3); 9 mWh/cm(3); 80 W/cm(3)).

  11. 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

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

    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.

  13. 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.

  14. 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

  15. Physical defect formation in few layer graphene-like carbon on metals: influence of temperature, acidity, and chemical functionalization.

    PubMed

    Schumacher, Christoph M; Grass, Robert N; Rossier, Michael; Athanassiou, Evagelos K; Stark, Wendelin J

    2012-03-06

    A systematical examination of the chemical stability of cobalt metal nanomagnets with a graphene-like carbon coating is used to study the otherwise rather elusive formation of nanometer-sized physical defects in few layer graphene as a result of acid treatments. We therefore first exposed the core-shell nanomaterial to well-controlled solutions of altering acidity and temperature. The release of cobalt into these solutions over time offered a simple tool to monitor the progress of particle degradation. The results suggested that the oxidative damage of the graphene-like coatings was the rate-limiting step during particle degradation since only fully intact or entirely emptied carbon shells were found after the experiments. If ionic noble metal species were additionally present in the acidic solutions, the noble metal was found to reduce on the surface of specific, defective particles. The altered electrochemical gradients across the carbon shells were however not found to lead to a faster release of cobalt from the particles. The suggested mechanistic insight was further confirmed by the covalent chemical functionalization of the particle surface with chemically inert aryl species, which leads to an additional thickening of the shells. This leads to reduced cobalt release rates as well as slower noble metal reduction rates depending on the augmentation of the shell thickness.

  16. 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.

  17. 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

  18. 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

  19. 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

  20. Local solid phase growth of few-layer graphene on silicon carbide from nickel silicide supersaturated with carbon

    SciTech Connect

    Escobedo-Cousin, Enrique; Vassilevski, Konstantin; Hopf, Toby; Wright, Nick; O'Neill, Anthony; Horsfall, Alton; Goss, Jonathan; Cumpson, Peter

    2013-03-21

    Patterned few-layer graphene (FLG) films were obtained by local solid phase growth from nickel silicide supersaturated with carbon, following a fabrication scheme, which allows the formation of self-aligned ohmic contacts on FLG and is compatible with conventional SiC device processing methods. The process was realised by the deposition and patterning of thin Ni films on semi-insulating 6H-SiC wafers followed by annealing and the selective removal of the resulting nickel silicide by wet chemistry. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to confirm both the formation and subsequent removal of nickel silicide. The impact of process parameters such as the thickness of the initial Ni layer, annealing temperature, and cooling rates on the FLG films was assessed by Raman spectroscopy, XPS, and atomic force microscopy. The thickness of the final FLG film estimated from the Raman spectra varied from 1 to 4 monolayers for initial Ni layers between 3 and 20 nm thick. Self-aligned contacts were formed on these patterned films by contact photolithography and wet etching of nickel silicide, which enabled the fabrication of test structures to measure the carrier concentration and mobility in the FLG films. A simple model of diffusion-driven solid phase chemical reaction was used to explain formation of the FLG film at the interface between nickel silicide and silicon carbide.

  1. Passively mode-locked fiber laser based on a hollow-core photonic crystal fiber filled with few-layered graphene oxide solution

    NASA Astrophysics Data System (ADS)

    Liu, Zhi-Bo; He, Xiaoying; Wang, D. N.

    2011-08-01

    We demonstrate a nanosecond-pulse erbium-doped fiber laser that is passively mode locked by a hollow-core photonic crystal fiber filled with few-layered graphene oxide solution. Owing to the good solution processing capability of few-layered graphene oxide, which can be filled into the core of a hollow-core photonic crystal fiber through a selective hole filling process, a graphene saturable absorber can be successfully fabricated. The output pulses obtained have a center wavelength, pulse width, and repetition rate of 1561.2nm, 4.85ns, and 7.68MHz, respectively. This method provides a simple and efficient approach to integrate the graphene into the optical fiber system.

  2. Passively mode-locked fiber laser based on a hollow-core photonic crystal fiber filled with few-layered graphene oxide solution.

    PubMed

    Liu, Zhi-Bo; He, Xiaoying; Wang, D N

    2011-08-15

    We demonstrate a nanosecond-pulse erbium-doped fiber laser that is passively mode locked by a hollow-core photonic crystal fiber filled with few-layered graphene oxide solution. Owing to the good solution processing capability of few-layered graphene oxide, which can be filled into the core of a hollow-core photonic crystal fiber through a selective hole filling process, a graphene saturable absorber can be successfully fabricated. The output pulses obtained have a center wavelength, pulse width, and repetition rate of 1561.2 nm, 4.85 ns, and 7.68 MHz, respectively. This method provides a simple and efficient approach to integrate the graphene into the optical fiber system.

  3. Formation and photovoltaic performance of few-layered graphene-decorated TiO2 nanocrystals used in dye-sensitized solar cells.

    PubMed

    Liu, Yueli; Cheng, Yuqing; Shu, Wei; Peng, Zhuoyin; Chen, Keqiang; Zhou, Jing; Chen, Wen; Zakharova, Galina S

    2014-06-21

    Few-layer graphene/TiO2 nanocrystal composites are successfully in situ synthesized at a low temperature of 400 °C using C28H16Br2 as the precursor. Raman mapping images show that the TiO2 nanocrystals are very uniformly dispersed in the composite films, and the in situ coating during the thermal decomposition process will favor the formation of a good interface combination between the few-layered graphene and the TiO2 nanocrystals. The few-layer graphene/TiO2 nanocrystal composites are used as photoanodes in dye-sensitized solar cells (DSSCs), and the conversion efficiency of 8.25% is obtained under full sun irradiation (AM 1.5), which increases by 65% compared with that of the pure TiO2 nanocrystal DSSCs (5.01%). It is found that the good interface combination between few-layered graphene and TiO2 nanocrystals may improve the electric conductivity and lifetime of photoinduced electrons in DSSCs. Moreover, some carbon atoms are doped into the crystal structure of the TiO2 nanocrystals during the thermal decomposition process, which will enhance the light absorption by narrowing the band gap and favor the improvement of the photovoltaic efficiency.

  4. 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.

  5. 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).

  6. Nickel Nanoparticles Encapsulated in Few-Layer Nitrogen-Doped Graphene Derived from Metal-Organic Frameworks as Efficient Bifunctional Electrocatalysts for Overall Water Splitting.

    PubMed

    Xu, You; Tu, Wenguang; Zhang, Bowei; Yin, Shengming; Huang, Yizhong; Kraft, Markus; Xu, Rong

    2017-03-01

    Nickel nanoparticles encapsulated in few-layer nitrogen-doped graphene (Ni@NC) are synthesized by using a Ni-based metal-organic framework as the precursor for high-temperature annealing treatment. The resulting Ni@NC materials exhibit highly efficient and ultrastable electrocatalytic activity toward the hydrogen evolution reaction and the oxygen evolution reaction as well as overall water splitting in alkaline environment.

  7. Direct physical exfoliation of few-layer graphene from graphite grown on a nickel foil using polydimethylsiloxane with tunable elasticity and adhesion

    NASA Astrophysics Data System (ADS)

    Yoo, Kwanghyun; Takei, Yusuke; Kim, Sungjin; Chiashi, Shohei; Maruyama, Shigeo; Matsumoto, Kiyoshi; Shimoyama, Isao

    2013-05-01

    We firstly introduce a facile method for the site-specific direct physical exfoliation of few-layer graphene sheets from cheap and easily enlargeable graphite grown on a Ni foil using an optimized polydimethylsiloxane (PDMS) stamp. By decreasing the PDMS cross-linking time, the PDMS elasticity is reduced to ˜52 kPa, similar to that of a typical gel. As a result of this process, the PDMS becomes more flexible yet remains in a handleable state as a stamp. Furthermore, the PDMS adhesion to a graphite/Ni surface, as measured by the peel strength, increases to ˜5.1 N m-1, which is approximately 17 times greater than that of typical PDMS. These optimized properties allow the PDMS stamp to have improved contact with the graphite/Ni surface, including the graphite wrinkles. This process is verified, and changes in surface morphology are observed using a 3D laser scanning microscope. Under conformal contact, the optimized PDMS stamp demonstrates the site-specific direct physical exfoliation of few-layer graphene sheets including mono- and bi-layer graphene sheets from the graphite/Ni substrate without the use of special equipment, conditions or chemicals. The number of layers of the exfoliated graphene and its high quality are revealed by the measured Raman spectroscopy. The exfoliation method using tunable elasticity and adhesion of the PDMS stamp can be used not only for cost-effective mass production of defect-less few-layer graphene from the graphite substrate for micro/nano device arrays but also for nano-contact printing of various structures, devices and cells.

  8. Direct physical exfoliation of few-layer graphene from graphite grown on a nickel foil using polydimethylsiloxane with tunable elasticity and adhesion.

    PubMed

    Yoo, Kwanghyun; Takei, Yusuke; Kim, Sungjin; Chiashi, Shohei; Maruyama, Shigeo; Matsumoto, Kiyoshi; Shimoyama, Isao

    2013-05-24

    We firstly introduce a facile method for the site-specific direct physical exfoliation of few-layer graphene sheets from cheap and easily enlargeable graphite grown on a Ni foil using an optimized polydimethylsiloxane (PDMS) stamp. By decreasing the PDMS cross-linking time, the PDMS elasticity is reduced to ∼52 kPa, similar to that of a typical gel. As a result of this process, the PDMS becomes more flexible yet remains in a handleable state as a stamp. Furthermore, the PDMS adhesion to a graphite/Ni surface, as measured by the peel strength, increases to ∼5.1 N m⁻¹, which is approximately 17 times greater than that of typical PDMS. These optimized properties allow the PDMS stamp to have improved contact with the graphite/Ni surface, including the graphite wrinkles. This process is verified, and changes in surface morphology are observed using a 3D laser scanning microscope. Under conformal contact, the optimized PDMS stamp demonstrates the site-specific direct physical exfoliation of few-layer graphene sheets including mono- and bi-layer graphene sheets from the graphite/Ni substrate without the use of special equipment, conditions or chemicals. The number of layers of the exfoliated graphene and its high quality are revealed by the measured Raman spectroscopy. The exfoliation method using tunable elasticity and adhesion of the PDMS stamp can be used not only for cost-effective mass production of defect-less few-layer graphene from the graphite substrate for micro/nano device arrays but also for nano-contact printing of various structures, devices and cells.

  9. 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

  10. 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.

  11. 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

  12. 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.

  13. 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

  14. 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

  15. Mixed phase, sp2-sp3 bonded, and disordered few layer graphene-like nanocarbon: Synthesis and characterizations

    NASA Astrophysics Data System (ADS)

    Kumar, Arvind; Patil, Sumati; Joshi, Anupama; Bhoraskar, Vasant; Datar, Suwarna; Alegaonkar, Prashant

    2013-04-01

    We report on a method for the synthesis of Graphene-like nanocarbon (GNC) sheets. The obtained GNC contain mixed phase, sp2-sp3 bonded, and few atom layer of disordered carbon network. Initially, the mixture of soft wood charcoal (C), potassium nitrate (KNO3), and sulfur (S) was subjected to the combustion process. The obtained (as-synthesized) samples were intercalated (in H2SO4 (98%): HNO3 (60%), 48 h at 300 K) and subsequently annealed at 1000 °C for ˜60 s. The as-synthesized, intercalated, and annealed samples were studied using number of characterization techniques. The Raman spectra (at λ = 514 nm) recorded for the as-synthesized samples showed five prominent peaks, namely, for amorphous-Carbon (a-C), D doublet, small G, and 2D band. The position and intensity of the recorded peaks were varied for the intercalated and annealed samples. Our SEM analysis revealed that, the area of the GNC sheets varied typically in the range of 10 to 20 μm2. The analysis of HRTEM/SAED together showed that the intercalated samples contained a-C phase, whereas, short range ordering was observed for the annealed samples. The tunneling spectra recorded for the annealed GNC sheets showed V-shaped local density of states with the peak present near the minima. Our analysis revealed that, the sp2 chains and the polycyclic carbon rings (PCR) could be formed in the host ta-C matrix generating mixed sp2-sp3 bonded carbon network along with the local disorder. Details are presented.

  16. Binder free three-dimensional sulphur/few-layer graphene foam cathode with enhanced high-rate capability for rechargeable lithium sulphur batteries.

    PubMed

    Xi, Kai; Kidambi, Piran R; Chen, Renjie; Gao, Chenlong; Peng, Xiaoyu; Ducati, Caterina; Hofmann, Stephan; Kumar, R Vasant

    2014-06-07

    A novel ultra-lightweight three-dimensional (3-D) cathode system for lithium sulphur (Li-S) batteries has been synthesised by loading sulphur on to an interconnected 3-D network of few-layered graphene (FLG) via a sulphur solution infiltration method. A free-standing FLG monolithic network foam was formed as a negative of a Ni metallic foam template by CVD followed by etching away of Ni. The FLG foam offers excellent electrical conductivity, an appropriate hierarchical pore structure for containing the electro-active sulphur and facilitates rapid electron/ion transport. This cathode system does not require any additional binding agents, conductive additives or a separate metallic current collector thus decreasing the weight of the cathode by typically ∼20-30 wt%. A Li-S battery with the sulphur-FLG foam cathode shows good electrochemical stability and high rate discharge capacity retention for up to 400 discharge/charge cycles at a high current density of 3200 mA g(-1). Even after 400 cycles the capacity decay is only ∼0.064% per cycle relative to the early (e.g. the 5th cycle) discharge capacity, while yielding an average columbic efficiency of ∼96.2%. Our results indicate the potential suitability of graphene foam for efficient, ultra-light and high-performance batteries.

  17. Binder free three-dimensional sulphur/few-layer graphene foam cathode with enhanced high-rate capability for rechargeable lithium sulphur batteries

    NASA Astrophysics Data System (ADS)

    Xi, Kai; Kidambi, Piran R.; Chen, Renjie; Gao, Chenlong; Peng, Xiaoyu; Ducati, Caterina; Hofmann, Stephan; Kumar, R. Vasant

    2014-05-01

    A novel ultra-lightweight three-dimensional (3-D) cathode system for lithium sulphur (Li-S) batteries has been synthesised by loading sulphur on to an interconnected 3-D network of few-layered graphene (FLG) via a sulphur solution infiltration method. A free-standing FLG monolithic network foam was formed as a negative of a Ni metallic foam template by CVD followed by etching away of Ni. The FLG foam offers excellent electrical conductivity, an appropriate hierarchical pore structure for containing the electro-active sulphur and facilitates rapid electron/ion transport. This cathode system does not require any additional binding agents, conductive additives or a separate metallic current collector thus decreasing the weight of the cathode by typically ~20-30 wt%. A Li-S battery with the sulphur-FLG foam cathode shows good electrochemical stability and high rate discharge capacity retention for up to 400 discharge/charge cycles at a high current density of 3200 mA g-1. Even after 400 cycles the capacity decay is only ~0.064% per cycle relative to the early (e.g. the 5th cycle) discharge capacity, while yielding an average columbic efficiency of ~96.2%. Our results indicate the potential suitability of graphene foam for efficient, ultra-light and high-performance batteries.A novel ultra-lightweight three-dimensional (3-D) cathode system for lithium sulphur (Li-S) batteries has been synthesised by loading sulphur on to an interconnected 3-D network of few-layered graphene (FLG) via a sulphur solution infiltration method. A free-standing FLG monolithic network foam was formed as a negative of a Ni metallic foam template by CVD followed by etching away of Ni. The FLG foam offers excellent electrical conductivity, an appropriate hierarchical pore structure for containing the electro-active sulphur and facilitates rapid electron/ion transport. This cathode system does not require any additional binding agents, conductive additives or a separate metallic current collector

  18. 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.

  19. Morphology and Electrical Characterization of Reduced Epitaxial Graphene Oxide

    NASA Astrophysics Data System (ADS)

    Hu, Yike; Wu, Xiaosong; Sprinkle, Michael; Madiomanana, Nerasoa; Ruan, Ming; Berger, Claire; de Heer, Walter

    2009-03-01

    We present results for on-chip oxidation of epitaxial graphene and sequential reduction of the insulating graphene oxide layers. In our previous work , we have used the Hummer's method to oxidize epitaxial graphene and used electron beam exposure and heat treatment to reduce the epitaxial graphene oxide (EGO) band gap by changing the degree of oxidation. Here we further explore various oxidation and reduction methods on epitaxial graphene. EGO is characterized by atomic force microscopy, low-energy electron diffraction, ellipsometry, and Raman Spectrometry. Mobility measurements of patterned structures are presented where epitaxial graphene layers pads are seamlessly connected to EGO ribbons.

  20. Epitaxial growth of large area single-crystalline few-layer MoS{sub 2} with high space charge mobility of 192 cm{sup 2} V{sup −1} s{sup −1}

    SciTech Connect

    Ma, Lu; Yu, Mingzhe; Wu, Yiying E-mail: wu@chemistry.ohio-state.edu; Nath, Digbijoy N.; Lee, Edwin W.; Lee, Choong Hee; Arehart, Aaron; Rajan, Siddharth E-mail: wu@chemistry.ohio-state.edu

    2014-08-18

    We report on the vapor-solid growth of single crystalline few-layer MoS{sub 2} films on (0001)-oriented sapphire with excellent structural and electrical properties over centimeter length scale. High-resolution X-ray diffraction scans indicated that the films had good out-of-plane ordering and epitaxial registry. A carrier density of ∼2 × 10{sup 11 }cm{sup −2} and a room temperature mobility of 192 cm{sup 2}/Vs were extracted from space-charge limited transport regime in the films. The electron mobility was found to exhibit in-plane anisotropy with a ratio of ∼1.8. Theoretical estimates of the temperature-dependent electron mobility including optical phonon, acoustic deformation potential, and remote ionized impurity scattering were found to satisfactorily match the measured data. The synthesis approach reported here demonstrates the feasibility of device quality few-layer MoS{sub 2} films with excellent uniformity and high quality.

  1. Selective epitaxial growth of graphene on SiC

    NASA Astrophysics Data System (ADS)

    Camara, N.; Rius, G.; Huntzinger, J.-R.; Tiberj, A.; Mestres, N.; Godignon, P.; Camassel, J.

    2008-09-01

    We present a method of selective epitaxial growth of few layers graphene (FLG) on a "prepatterned" silicon carbide (SiC) substrate. The methods involves, successively, the sputtering of a thin aluminium nitride (AlN) layer on top of a monocrystalline SiC substrate and, then, patterning it with e-beam lithography and wet etching. The sublimation of few atomic layers of Si from the SiC substrate occurs only through the selectively etched AlN layer. The presence of the Raman G-band at ˜1582cm-1 in the AlN-free areas is used to validate the concept. It gives absolute evidence of selective FLG growth.

  2. Selective epitaxial growth of graphene on SiC

    SciTech Connect

    Camara, N.; Rius, G.; Godignon, P.; Huntzinger, J.-R.; Tiberj, A.; Camassel, J.

    2008-09-22

    We present a method of selective epitaxial growth of few layers graphene (FLG) on a ''prepatterned'' silicon carbide (SiC) substrate. The methods involves, successively, the sputtering of a thin aluminium nitride (AlN) layer on top of a monocrystalline SiC substrate and, then, patterning it with e-beam lithography and wet etching. The sublimation of few atomic layers of Si from the SiC substrate occurs only through the selectively etched AlN layer. The presence of the Raman G-band at {approx}1582 cm{sup -1} in the AlN-free areas is used to validate the concept. It gives absolute evidence of selective FLG growth.

  3. Tunable bands in biased multilayer epitaxial graphene.

    PubMed

    Williams, Michael D; Samarakoon, Duminda K; Hess, Dennis W; Wang, Xiao-Qian

    2012-04-28

    We have studied the electronic characteristics of multilayer epitaxial graphene under a perpendicularly applied electric bias. Ultraviolet photoemission spectroscopy measurements reveal that there is notable variation of the electronic density-of-states in valence bands near the Fermi level. Evolution of the electronic structure of graphite and rotational-stacked multilayer epitaxial graphene as a function of the applied electric bias is investigated using first-principles density-functional theory including interlayer van der Waals interactions. The experimental and theoretical results demonstrate that the tailoring of electronic band structure correlates with the interlayer coupling tuned by the applied bias. The implications of controllable electronic structure of rotationally fault-stacked epitaxial graphene grown on the C-face of SiC for future device applications are discussed.

  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

    2016-05-06

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-03-01

    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.

  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. A study on the physicochemical properties of hydroalcoholic solutions to improve the direct exfoliation of natural graphite down to few-layers graphene

    NASA Astrophysics Data System (ADS)

    Fedi, Filippo; Miglietta, Maria Lucia; Polichetti, Tiziana; Ricciardella, Filiberto; Massera, Ettore; Ninno, Domenico; Di Francia, Girolamo

    2015-03-01

    Straightforward methods to produce pristine graphene flakes in large quantities are based on the liquid-phase exfoliation processes. These one-step physical transformations of graphite into graphene offer many unique advantages. To date, a large number of liquids have been employed as exfoliation media exploiting their thermodynamic and chemical features as compared to those of graphene. Here, we pursued the goal of realizing water based mixtures to exfoliate graphite and disperse graphene without the aid of surfactants. To this aim, aqueous mixtures with suitable values of surface tension and Hansen solubility parameters (HSPs), were specifically designed and used. The very high water surface tension was decreased by the addition of solvents with lower surface tensions such as alcohols, obtaining, in this way, more favourable HSP distances. The specific role of each of these thermodynamic features was finally investigated. The results showed that the designed hydroalcoholic solutions were effective in both the graphite exfoliation and dispersion without the addition of any surfactants or other stabilizing agents. Stable graphene suspensions were obtained at concentration comparable to those produced with low-boiling solvents and water/surfactants.

  8. Tunable interfacial properties of epitaxial graphene on metal substrates

    NASA Astrophysics Data System (ADS)

    Gao, Min; Pan, Yi; Zhang, Chendong; Hu, Hao; Yang, Rong; Lu, Hongliang; Cai, Jinming; Du, Shixuan; Liu, Feng; Gao, H.-J.

    2010-02-01

    We report on tuning interfacial properties of epitaxially-grown graphenes with different kinds of metal substrates based on scanning tunneling microscopy experiments and density functional theory calculations. Three kinds of metal substrates, Ni(111), Pt(111), and Ru(0001), show different interactions with the epitaxially grown graphene at the interfaces. The different interfacial interaction making graphene n-type and p-type doped, leads to the polarity change of the thermoelectric property of the graphene/metal systems. These findings may give further insights to the interfacial interactions in the graphene/metal systems and promote the use of graphene-based heterostructures in devices.

  9. STM Properties and Manipulation of Epitaxial Graphene

    NASA Astrophysics Data System (ADS)

    Thibado, Paul

    2014-03-01

    Epitaxial graphene grown on SiC has been identified as one of the most likely avenues to graphene-based electronics. Understanding how morphology affects electronic properties is therefore important. In our work, epitaxial graphene was grown on the polar and non-polar a-, m-, and r-crystallographic oriented surfaces of SiC, and was investigated using scanning tunneling microscopy (STM). Bunched nano-ridges ten times smaller than previously recorded were observed throughout the surface. A new STM technique called electrostatic-manipulation scanning tunneling microscopy (EM-STM) was performed to modify the morphology of the nano-ridges. By modeling the electrostatics involved in the EM-STM measurement, we estimate that a force of 5 nN and energy of 10 eV was required to alter the local interfacial bonding. At the atomic scale, STM images of Moire patterns reveal low-angle, twisted bi-layer graphene, grain boundaries, and an apparent lattice constant dilation. We will show that this dilation is due to the STM tip electrostatically dragging the graphene surface. Collaborators: P. Xu, D. Qi, M.L. Ackerman, S.D. Barber, J.K. Schoelz, and J. Thompson, Department of Physics, University of Arkansas, Fayetteville, AR, 72701, USA; V.D. Wheelr, R.L. Myers-Ward, C.R. Eddy, Jr., and D.K. Gaskill, U.S. Naval Research Laboratory, Washington, DC 20375, USA; and L.O. Nyakiti, Texas A&M University. Department of Physics, University of Arkansas, Fayetteville, AR, 72701, USA.

  10. 100-GHz Transistors from Wafer-Scale Epitaxial Graphene

    NASA Astrophysics Data System (ADS)

    Lin, Y.-M.; Dimitrakopoulos, C.; Jenkins, K. A.; Farmer, D. B.; Chiu, H.-Y.; Grill, A.; Avouris, Ph.

    2010-02-01

    The high carrier mobility of graphene has been exploited in field-effect transistors that operate at high frequencies. Transistors were fabricated on epitaxial graphene synthesized on the silicon face of a silicon carbide wafer, achieving a cutoff frequency of 100 gigahertz for a gate length of 240 nanometers. The high-frequency performance of these epitaxial graphene transistors exceeds that of state-of-the-art silicon transistors of the same gate length.

  11. 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.

  12. Effect of vertically oriented few-layer graphene on the wettability and interfacial reactions of the AgCuTi-SiO2f/SiO2 system.

    PubMed

    Sun, Z; Zhang, L X; Qi, J L; Zhang, Z H; Hao, T D; Feng, J C

    2017-03-22

    With the aim of expanding their applications, particularly when joining metals, a simple but effective method is reported whereby the surface chemical reactivity of SiO2f/SiO2 (SiO2f/SiO2 stands for silica fibre reinforced silica based composite materials and f is short for fibre) composites with vertically oriented few-layer graphene (VFG, 3-10 atomic layers of graphene vertically oriented to the substrate) can be tailored. VFG was uniformly grown on the surface of a SiO2f/SiO2 composite by using plasma enhanced chemical vapour deposition (PECVD). The wetting experiments were conducted by placing small pieces of AgCuTi alloy foil on SiO2f/SiO2 composites with and without VFG decoration. It was demonstrated that the contact angle dropped from 120° (without VFG decoration) to 50° (with VFG decoration) when the holding time was 10 min. The interfacial reaction layer in SiO2f/SiO2 composites with VFG decoration became continuous without any unfilled gaps compared with the composites without VFG decoration. High-resolution transmission electron microscopy (HRTEM) was employed to investigate the interaction between VFG and Ti from the AgCuTi alloy. The results showed that VFG possessed high chemical reactivity and could easily react with Ti even at room temperature. Finally, a mechanism of how VFG promoted the wetting of the SiO2f/SiO2 composite by the AgCuTi alloy is proposed and thoroughly discussed.

  13. Precise Identification of Graphene's Crystal Structures by Removable Nanowire Epitaxy.

    PubMed

    Kim, Jonghyeok; Lim, Kitaek; Lee, Yangjin; Kim, Jongin; Kim, Kihwan; Park, Jungwon; Kim, Kwanpyo; Lee, Won Chul

    2017-03-16

    Monitoring crystallographic orientations of graphene is important for the reliable generation of graphene-based nanostructures such as van der Waals heterostructures and graphene nanoribbons because their physical properties are dependent on crystal structures. However, facile and precise identification of graphene's crystallographic orientations is still challenging because the majority of current tools rely on complex atomic-scale imaging. Here, we present an identification method for the crystal orientations and grain boundaries of graphene using the directional alignment between epitaxially grown AuCN nanowires and the underlying graphene. Because the nanowires are visible in scanning electron microscopy, crystal orientations of graphene can be inspected with simple procedures. Kernel density estimation that we used in analyzing the nanowire directions enables precise measurement of graphene's crystal orientations. We also confirm that the imaged nanowires can be simply removed without degrading graphene's quality, thus showing that the present method can be practically used for measuring graphene's crystal structures.

  14. 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.

  15. 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.

  16. Wafer bonding solution to epitaxial graphene-silicon integration

    NASA Astrophysics Data System (ADS)

    Dong, Rui; Guo, Zelei; Palmer, James; Hu, Yike; Ruan, Ming; Hankinson, John; Kunc, Jan; Bhattacharya, Swapan K.; Berger, Claire; de Heer, Walt A.

    2014-03-01

    A new strategy for the integration of graphene electronics with silicon complementary metal-oxide-semiconductor (Si-CMOS) technology is demonstrated that requires neither graphene transfer nor patterning. Inspired by silicon-on-insulator and three-dimensional device hyper-integration techniques, a thin monocrystalline silicon layer ready for CMOS processing is bonded to epitaxial graphene (EG) on SiC. The parallel Si and graphene electronic platforms are interconnected by metal vias. In this method, EG is grown prior to bonding so that the process is compatible with EG high temperature growth and preserves graphene integrity and nano-structuring.

  17. 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.

  18. Raman spectra of epitaxial graphene on SiC and of epitaxial graphene transferred to SiO2.

    PubMed

    Lee, Dong Su; Riedl, Christian; Krauss, Benjamin; von Klitzing, Klaus; Starke, Ulrich; Smet, Jurgen H

    2008-12-01

    Raman spectra were measured for mono-, bi-, and trilayer graphene grown on SiC by solid state graphitization, whereby the number of layers was preassigned by angle-resolved ultraviolet photoemission spectroscopy. It was found that the only unambiguous fingerprint in Raman spectroscopy to identify the number of layers for graphene on SiC(0001) is the line width of the 2D (or D*) peak. The Raman spectra of epitaxial graphene show significant differences as compared to micromechanically cleaved graphene obtained from highly oriented pyrolytic graphite crystals. The G peak is found to be blue-shifted. The 2D peak does not exhibit any obvious shoulder structures, but it is much broader and almost resembles a single-peak even for multilayers. Flakes of epitaxial graphene were transferred from SiC onto SiO2 for further Raman studies. A comparison of the Raman data obtained for graphene on SiC with data for epitaxial graphene transferred to SiO2 reveals that the G peak blue-shift is clearly due to the SiC substrate. The broadened 2D peak however stems from the graphene structure itself and not from the substrate.

  19. Structural Consequences of Hydrogen Intercalation of Epitaxial Graphene on SiC(0001)

    DTIC Science & Technology

    2014-10-23

    Structural consequences of hydrogen intercalation of epitaxial graphene on SiC(0001) Jonathan D. Emery,1,a) Virginia H. Wheeler,2 James E. Johns,1...the interface between epitaxial graphene (EG) and its SiC substrate is known to significantly influence the electronic properties of the graphene ...from that of the overlying graphene layers. This newly formed graphene layer becomes decoupled from the SiC substrate and, along with the other graphene

  20. Quantum Hall effect in epitaxial graphene with permanent magnets

    PubMed Central

    Parmentier, F. D.; Cazimajou, T.; Sekine, Y.; Hibino, H.; Irie, H.; Glattli, D. C.; Kumada, N.; Roulleau, P.

    2016-01-01

    We have observed the well-kown quantum Hall effect (QHE) in epitaxial graphene grown on silicon carbide (SiC) by using, for the first time, only commercial NdFeB permanent magnets at low temperature. The relatively large and homogeneous magnetic field generated by the magnets, together with the high quality of the epitaxial graphene films, enables the formation of well-developed quantum Hall states at Landau level filling factors v = ±2, commonly observed with superconducting electro-magnets. Furthermore, the chirality of the QHE edge channels can be changed by a top gate. These results demonstrate that basic QHE physics are experimentally accessible in graphene for a fraction of the price of conventional setups using superconducting magnets, which greatly increases the potential of the QHE in graphene for research and applications. PMID:27922114

  1. Quantum Hall effect in epitaxial graphene with permanent magnets.

    PubMed

    Parmentier, F D; Cazimajou, T; Sekine, Y; Hibino, H; Irie, H; Glattli, D C; Kumada, N; Roulleau, P

    2016-12-06

    We have observed the well-kown quantum Hall effect (QHE) in epitaxial graphene grown on silicon carbide (SiC) by using, for the first time, only commercial NdFeB permanent magnets at low temperature. The relatively large and homogeneous magnetic field generated by the magnets, together with the high quality of the epitaxial graphene films, enables the formation of well-developed quantum Hall states at Landau level filling factors v = ±2, commonly observed with superconducting electro-magnets. Furthermore, the chirality of the QHE edge channels can be changed by a top gate. These results demonstrate that basic QHE physics are experimentally accessible in graphene for a fraction of the price of conventional setups using superconducting magnets, which greatly increases the potential of the QHE in graphene for research and applications.

  2. Quantum Hall effect in epitaxial graphene with permanent magnets

    NASA Astrophysics Data System (ADS)

    Parmentier, F. D.; Cazimajou, T.; Sekine, Y.; Hibino, H.; Irie, H.; Glattli, D. C.; Kumada, N.; Roulleau, P.

    2016-12-01

    We have observed the well-kown quantum Hall effect (QHE) in epitaxial graphene grown on silicon carbide (SiC) by using, for the first time, only commercial NdFeB permanent magnets at low temperature. The relatively large and homogeneous magnetic field generated by the magnets, together with the high quality of the epitaxial graphene films, enables the formation of well-developed quantum Hall states at Landau level filling factors v = ±2, commonly observed with superconducting electro-magnets. Furthermore, the chirality of the QHE edge channels can be changed by a top gate. These results demonstrate that basic QHE physics are experimentally accessible in graphene for a fraction of the price of conventional setups using superconducting magnets, which greatly increases the potential of the QHE in graphene for research and applications.

  3. 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.

  4. 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.

  5. 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.

  6. Substrate-induced magnetism in epitaxial graphene buffer layers.

    PubMed

    Ramasubramaniam, A; Medhekar, N V; Shenoy, V B

    2009-07-08

    Magnetism in graphene is of fundamental as well as technological interest, with potential applications in molecular magnets and spintronic devices. While defects and/or adsorbates in freestanding graphene nanoribbons and graphene sheets have been shown to cause itinerant magnetism, controlling the density and distribution of defects and adsorbates is in general difficult. We show from first principles calculations that graphene buffer layers on SiC(0001) can also show intrinsic magnetism. The formation of graphene-substrate chemical bonds disrupts the graphene pi-bonds and causes localization of graphene states near the Fermi level. Exchange interactions between these states lead to itinerant magnetism in the graphene buffer layer. We demonstrate the occurrence of magnetism in graphene buffer layers on both bulk-terminated as well as more realistic adatom-terminated SiC(0001) surfaces. Our calculations show that adatom density has a profound effect on the spin distribution in the graphene buffer layer, thereby providing a means of engineering magnetism in epitaxial graphene.

  7. Disordered Fermi liquid in epitaxial graphene from quantum transport measurements.

    PubMed

    Lara-Avila, Samuel; Tzalenchuk, Alexander; Kubatkin, Sergey; Yakimova, Rositza; Janssen, T J B M; Cedergren, Karin; Bergsten, Tobias; Fal'ko, Vladimir

    2011-10-14

    We have performed magnetotransport measurements on monolayer epitaxial graphene and analyzed them in the framework of the disordered Fermi liquid theory. We have separated the electron-electron and weak-localization contributions to resistivity and demonstrated the phase coherence over a micrometer length scale, setting the limit of at least 50 ps on the spin relaxation time in this material.

  8. 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

  9. Infrared fingerprints of few-layer black phosphorus

    PubMed Central

    Zhang, Guowei; Huang, Shenyang; Chaves, Andrey; Song, Chaoyu; Özçelik, V. Ongun; Low, Tony; Yan, Hugen

    2017-01-01

    Black phosphorus is an infrared layered material. Its bandgap complements other widely studied two-dimensional materials: zero-gap graphene and visible/near-infrared gap transition metal dichalcogenides. Although highly desirable, a comprehensive infrared characterization is still lacking. Here we report a systematic infrared study of mechanically exfoliated few-layer black phosphorus, with thickness ranging from 2 to 15 layers and photon energy spanning from 0.25 to 1.36 eV. Each few-layer black phosphorus exhibits a thickness-dependent unique infrared spectrum with a series of absorption resonances, which reveals the underlying electronic structure evolution and serves as its infrared fingerprints. Surprisingly, unexpected absorption features, which are associated with the forbidden optical transitions, have been observed. Furthermore, we unambiguously demonstrate that controllable uniaxial strain can be used as a convenient and effective approach to tune the electronic structure of few-layer black phosphorus. Our study paves the way for black phosphorus applications in infrared photonics and optoelectronics. PMID:28059084

  10. Infrared fingerprints of few-layer black phosphorus.

    PubMed

    Zhang, Guowei; Huang, Shenyang; Chaves, Andrey; Song, Chaoyu; Özçelik, V Ongun; Low, Tony; Yan, Hugen

    2017-01-06

    Black phosphorus is an infrared layered material. Its bandgap complements other widely studied two-dimensional materials: zero-gap graphene and visible/near-infrared gap transition metal dichalcogenides. Although highly desirable, a comprehensive infrared characterization is still lacking. Here we report a systematic infrared study of mechanically exfoliated few-layer black phosphorus, with thickness ranging from 2 to 15 layers and photon energy spanning from 0.25 to 1.36 eV. Each few-layer black phosphorus exhibits a thickness-dependent unique infrared spectrum with a series of absorption resonances, which reveals the underlying electronic structure evolution and serves as its infrared fingerprints. Surprisingly, unexpected absorption features, which are associated with the forbidden optical transitions, have been observed. Furthermore, we unambiguously demonstrate that controllable uniaxial strain can be used as a convenient and effective approach to tune the electronic structure of few-layer black phosphorus. Our study paves the way for black phosphorus applications in infrared photonics and optoelectronics.

  11. Infrared fingerprints of few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Zhang, Guowei; Huang, Shenyang; Chaves, Andrey; Song, Chaoyu; Özçelik, V. Ongun; Low, Tony; Yan, Hugen

    2017-01-01

    Black phosphorus is an infrared layered material. Its bandgap complements other widely studied two-dimensional materials: zero-gap graphene and visible/near-infrared gap transition metal dichalcogenides. Although highly desirable, a comprehensive infrared characterization is still lacking. Here we report a systematic infrared study of mechanically exfoliated few-layer black phosphorus, with thickness ranging from 2 to 15 layers and photon energy spanning from 0.25 to 1.36 eV. Each few-layer black phosphorus exhibits a thickness-dependent unique infrared spectrum with a series of absorption resonances, which reveals the underlying electronic structure evolution and serves as its infrared fingerprints. Surprisingly, unexpected absorption features, which are associated with the forbidden optical transitions, have been observed. Furthermore, we unambiguously demonstrate that controllable uniaxial strain can be used as a convenient and effective approach to tune the electronic structure of few-layer black phosphorus. Our study paves the way for black phosphorus applications in infrared photonics and optoelectronics.

  12. Oxygen switching of the epitaxial graphene-metal interaction.

    PubMed

    Larciprete, Rosanna; Ulstrup, Søren; Lacovig, Paolo; Dalmiglio, Matteo; Bianchi, Marco; Mazzola, Federico; Hornekær, Liv; Orlando, Fabrizio; Baraldi, Alessandro; Hofmann, Philip; Lizzit, Silvano

    2012-11-27

    Using photoemission spectroscopy techniques, we show that oxygen intercalation is achieved on an extended layer of epitaxial graphene on Ir(111), which results in the "lifting" of the graphene layer and in its decoupling from the metal substrate. The oxygen adsorption below graphene proceeds as on clean Ir(111), giving only a slightly higher oxygen coverage. Upon lifting, the C 1s signal shows a downshift in binding energy, due to the charge transfer to graphene from the oxygen-covered metal surface. Moreover, the characteristic spectral signatures of the graphene-substrate interaction in the valence band are removed, and the spectrum of strongly hole-doped, quasi free-standing graphene with a single Dirac cone around the K point is observed. The oxygen can be deintercalated by annealing, and this process takes place at around T = 600 K, in a rather abrupt way. A small amount of carbon atoms is lost, implying that graphene has been etched. After deintercalation graphene restores its interaction with the Ir(111) substrate. Additional intercalation/deintercalation cycles readily occur at lower oxygen doses and temperatures, consistently with an increasingly defective lattice. Our findings demonstrate that oxygen intercalation is an efficient method for fully decoupling an extended layer of graphene from a metal substrate, such as Ir(111). They pave the way for the fundamental research on graphene, where extended, ordered layers of free-standing graphene are important and, due to the stability of the intercalated system in a wide temperature range, also for the advancement of next-generation graphene-based electronics.

  13. The invention of graphene electronics and the physics of epitaxial graphene on silicon carbide

    NASA Astrophysics Data System (ADS)

    de Heer, Walt A.

    2012-01-01

    Graphene electronics was officially invented at the Georgia Institute of Technology in 2003 after experimental and theoretical research on graphene properties starting from 2001. This paper focuses on the motivation and events leading to the invention of graphene electronics, as well as on recent developments. Graphene electronics was originally conceived as a new electronics paradigm to incorporate the room-temperature ballistic and coherent properties of carbon nanotubes in a patternable electronic material. Graphene on silicon carbide was chosen as the most suitable material. Other electronics schemes, involving transferred (exfoliated and chemical vapor deposition-produced) graphitic materials, that operate in the diffusive regime may not be competitive with standard methods and may therefore not significantly impact electronics. In recent years, epitaxial graphene has improved to the point where graphene electronics according to the original concept appears to be within reach. Beyond electronics, epitaxial graphene research has led to important developments in graphene physics in general and has become a leading platform for graphene science as well.

  14. 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.

  15. Thermal stability of corrugated epitaxial graphene grown on Re(0001).

    PubMed

    Miniussi, E; Pozzo, M; Baraldi, A; Vesselli, E; Zhan, R R; Comelli, G; Menteş, T O; Niño, M A; Locatelli, A; Lizzit, S; Alfè, D

    2011-05-27

    We report on a novel approach to determine the relationship between the corrugation and the thermal stability of epitaxial graphene grown on a strongly interacting substrate. According to our density functional theory calculations, the C single layer grown on Re(0001) is strongly corrugated, with a buckling of 1.6 Å, yielding a simulated C 1s core level spectrum which is in excellent agreement with the experimental one. We found that corrugation is closely knit with the thermal stability of the C network: C-C bond breaking is favored in the strongly buckled regions of the moiré cell, though it requires the presence of diffusing graphene layer vacancies.

  16. 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.

  17. Epitaxial nucleation of CVD bilayer graphene on copper.

    PubMed

    Song, Yenan; Zhuang, Jianing; Song, Meng; Yin, Shaoqian; Cheng, Yu; Zhang, Xuewei; Wang, Miao; Xiang, Rong; Xia, Yang; Maruyama, Shigeo; Zhao, Pei; Ding, Feng; Wang, Hongtao

    2016-12-08

    Bilayer graphene (BLG) has emerged as a promising candidate for next-generation electronic applications, especially when it exists in the Bernal-stacked form, but its large-scale production remains a challenge. Here we present an experimental and first-principles calculation study of the epitaxial chemical vapor deposition (CVD) nucleation process for Bernal-stacked BLG growth on Cu using ethanol as a precursor. Results show that a carefully adjusted flow rate of ethanol can yield a uniform BLG film with a surface coverage of nearly 90% and a Bernal-stacking ratio of nearly 100% on ordinary flat Cu substrates, and its epitaxial nucleation of the second layer is mainly due to the active CH3 radicals with the presence of a monolayer-graphene-covered Cu surface. We believe that this nucleation mechanism will help clarify the formation of BLG by the epitaxial CVD process, and lead to many new strategies for scalable synthesis of graphene with more controllable structures and numbers of layers.

  18. 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.

  19. 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.

  20. Implanted bottom gate for epitaxial graphene on silicon carbide

    NASA Astrophysics Data System (ADS)

    Waldmann, D.; Jobst, J.; Fromm, F.; Speck, F.; Seyller, T.; Krieger, M.; Weber, H. B.

    2012-04-01

    We present a technique to tune the charge density of epitaxial graphene via an electrostatic gate that is buried in the silicon carbide substrate. The result is a device in which graphene remains accessible for further manipulation or investigation. Via nitrogen or phosphor implantation into a silicon carbide wafer and subsequent graphene growth, devices can routinely be fabricated using standard semiconductor technology. We have optimized samples for room temperature as well as for cryogenic temperature operation. Depending on implantation dose and temperature we operate in two gating regimes. In the first, the gating mechanism is similar to a MOSFET, the second is based on a tuned space charge region of the silicon carbide semiconductor. We present a detailed model that describes the two gating regimes and the transition in between.

  1. Growing Epitaxial Graphene on an Insulator by MBE

    NASA Astrophysics Data System (ADS)

    Mohapatra, Chandra; Eckstein, James

    2009-03-01

    We have used electron beam evaporation of solid carbon (C) to deposit graphene on MgO <111> at 850C. The growth appears epitaxial as observed by in-situ RHEED which also reveals that the hot scattering surface transitions from an insulator to a conductor after deposition of 1 monolayer of C. Growth at higher temperatures gives better crystallinity. We further characterize the film by ex-situ Raman spectroscopy, AFM and transport. Raman reveals all the characteristic G, D and 2D peaks of graphene and the 2D peak can be fit to a single lorentzian typical for graphene. AFM pictures show that the surface consists of flat connected domains, which are uniform across the substrate. Electrical transport shows insulating behavior with resistance (R) varying as 1/T^2. This work was supported by the DOE BES at the F. Seitz Materials Research Laboratory at the University of Illinois, Urbana.

  2. 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.

  3. Top-gate dielectric induced doping and scattering of charge carriers in epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Puls, Conor P.; Staley, Neal E.; Moon, Jeong-Sun; Robinson, Joshua A.; Campbell, Paul M.; Tedesco, Joseph L.; Myers-Ward, Rachael L.; Eddy, Charles R.; Gaskill, D. Kurt; Liu, Ying

    2011-07-01

    We show that an e-gun deposited dielectric impose severe limits on epitaxial graphene-based device performance based on Raman spectroscopy and low-temperature transport measurements. Specifically, we show from studies of epitaxial graphene Hall bars covered by SiO2 that the measured carrier density is strongly inhomogenous and predominantly induced by charged impurities at the grapheme/dielectric interface that limit mobility via Coulomb interactions. Our work emphasizes that material integration of epitaxial graphene and a gate dielectric is the next major road block towards the realization of graphene-based electronics.

  4. Graphene Substrate for van der Waals Epitaxy of Layer-Structured Bismuth Antimony Telluride Thermoelectric Film.

    PubMed

    Kim, Eun Sung; Hwang, Jae-Yeol; Lee, Kyu Hyoung; Ohta, Hiromichi; Lee, Young Hee; Kim, Sung Wng

    2017-02-01

    Graphene as a substrate for the van der Waals epitaxy of 2D layered materials is utilized for the epitaxial growth of a layer-structured thermoelectric film. Van der Waals epitaxial Bi0.5 Sb1.5 Te3 film on graphene synthesized via a simple and scalable fabrication method exhibits good crystallinity and high thermoelectric transport properties comparable to single crystals.

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

    PubMed

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

    2015-10-07

    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°.

  6. 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

  7. 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.

  8. Chemically Resolved Interface Structure of Epitaxial Graphene on SiC(0001)

    DTIC Science & Technology

    2013-11-19

    Chemically Resolved Interface Structure of Epitaxial Graphene on SiC(0001) Jonathan D. Emery,1 Blanka Detlefs,2 Hunter J. Karmel,1 Luke O. Nyakiti,3...unique properties that can be significantly modified through interaction with an underlying support. For epitaxial graphene on SiC(0001), the...interface strongly influences the electronic properties of the overlaying graphene . We demonstrate a novel combination of x-ray scattering and spectroscopy

  9. Magnetic Oscillations and Landau Quantization in Decoupled Epitaxial Graphene Multilayers*

    NASA Astrophysics Data System (ADS)

    Stroscio, Joseph A.

    2009-03-01

    A fundamental challenge to the development of a new electronics based on single atomic sheets of carbon, known as graphene, is to realize a large-area production platform that can produce a carbon system with the same intrinsic properties as a single sheet of graphene. Multi-layer epitaxial graphene (MEG) grown on SiC substrates has been proposed as a possible platform to this end [1]. The central question is, Can MEG behave as single layer graphene with the same intrinsic electrical characteristics? In this talk we show that MEG graphene on SiC exhibits single layer graphene properties through new tunneling magnetic measurements. The circular motion of electrons in a magnetic field has historically been a powerful probe of the Fermi surface properties of materials. Oscillations in many measureable properties, such as magnetization, thermal conductivity, and resistance, all reflect the Landau quantization of the electron energy levels. In this talk we show the ability to observe tunneling magneto-conductance oscillations (TMCOs) in the tunneling differential conductance as a function of both magnetic field and electron energy. The TMCO arise from intense Dirac quantization of the 2-dimensional Dirac electron and hole quasiparticles in MEG grown on SiC substrates. Spatial profiles of the Landau quantization demonstrate the high quality of MEG on SiC with carrier concentrations that vary less than 10% over hundreds of nm. The single layer quantization observed in these multi-layer samples is attributed to observed rotational stacking domains that effectively decouple the carbon layers in MEG on SiC, thereby yielding single layer graphene properties in a large area carbon production method. *In collaboration with Lee Miller, Kevin Kubista, Gregory M. Rutter, Ming Ruan, Mike Sprinkle, Claire Berger, Walt A. de Heer, and Phillip N. First, Georgia Institute of Technology [1] W.A. de Heer et. al., Solid State Comm. 143, 92 (2007).

  10. 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.

  11. Bilayer-induced asymmetric quantum Hall effect in epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Iagallo, Andrea; Tanabe, Shinichi; Roddaro, Stefano; Takamura, Makoto; Sekine, Yoshiaki; Hibino, Hiroki; Miseikis, Vaidotas; Coletti, Camilla; Piazza, Vincenzo; Beltram, Fabio; Heun, Stefan

    2015-05-01

    The transport properties of epitaxial graphene on SiC(0001) at quantizing magnetic fields are investigated. Devices patterned perpendicularly to SiC terraces clearly exhibit bilayer inclusions distributed along the substrate step edges. We show that the transport properties in the quantum Hall regime are heavily affected by the presence of bilayer inclusions, and observe a significant departure from the conventional quantum Hall characteristics. In particular, we observe anomalous values of the quantized resistance and a peculiar asymmetry with magnetic field which was not observed before for graphene on SiC. A quantitative model involving enhanced inter-channel scattering mediated by the presence of bilayer inclusions is presented that successfully explains the observed symmetry properties.

  12. Molecular beam epitaxial growth of graphene and ridge-structure networks of graphene

    NASA Astrophysics Data System (ADS)

    Maeda, Fumihiko; Hibino, Hiroki

    2011-11-01

    By gas-source molecular beam epitaxy (MBE) using cracked ethanol, we grew graphene at substrate temperatures between 600 and 915 °C on graphene formed on SiC(0 0 0 1) by thermal decomposition. To investigate the substrate temperature dependence of graphene growth we analysed the MBE-grown graphene by Raman spectroscopy and in situ x-ray photoelectron spectroscopy (XPS) and observed it by atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM). Analyses using the G-band peak and the peak intensity ratio between D- and G-band peaks in the Raman spectra revealed that growth at higher temperatures improved the crystallinity and increased the domain size. Although the growth rate decreased at higher temperatures, as revealed by XPS, these results indicated that growth at a higher temperature is effective in obtaining graphene of higher quality. Furthermore, the AFM and TEM observations revealed a network of fin-like ridge structures of graphene sticking out from the surface. The presence of these 'graphene nanofins' indicated that two-dimensional islands of graphene are surrounded by the nanofins, and the island size was estimated to be 67 nm using the average distance between the nanofins.

  13. 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.

  14. Tuning epitaxial graphene sensitivity to water by hydrogen intercalation.

    PubMed

    Melios, C; Winters, M; Strupiński, W; Panchal, V; Giusca, C E; Imalka Jayawardena, K D G; Rorsman, N; Silva, S Ravi P; Kazakova, O

    2017-03-09

    The effects of humidity on the electronic properties of quasi-free standing one layer graphene (QFS 1LG) are investigated via simultaneous magneto-transport in the van der Pauw geometry and local work function measurements in a controlled environment. QFS 1LG on 4H-SiC(0001) is obtained by hydrogen intercalation of the interfacial layer. In this system, the carrier concentration experiences a two-fold increase in sensitivity to changes in relative humidity as compared to the as-grown epitaxial graphene. This enhanced sensitivity to water is attributed to the lowering of the hydrophobicity of QFS 1LG, which results from spontaneous polarization of 4H-SiC(0001) strongly influencing the graphene. Moreover, the superior carrier mobility of the QFS 1LG system is retained even at the highest humidity. The work function maps constructed from Kelvin probe force microscopy also revealed higher sensitivity to water for 1LG compared to 2LG in both QFS 1LG and as-grown systems. These results point to a new field of applications for QFS 1LG, i.e., as humidity sensors, and the corresponding need for metrology in calibration of graphene-based sensors and devices.

  15. 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.

  16. 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.

  17. Oxynitride and Silicates at Epitaxial Graphene on SiC (0001)

    NASA Astrophysics Data System (ADS)

    Sirikumara, Hansika; Bohorquez, Jaime; Jayasekera, Thushari

    2014-03-01

    Epitaxial graphene, the sp2-hybridized network of carbon grown on another material is one way of creating large-scale graphene. Intercalated oxygen at the interface has shown to saturate the Si dangling bonds, and is a promising way of tuning the charge density in epitaxial graphene on SiC. It would be interesting to investigate how oxy-nitrides and silicates at the SiC/graphene interface can change the electronic properties of the graphene layer. Based on the first principles density functional theory calculations, we discuss the electronic and structural properties of epitaxial graphene on SiC with Si2O5 and SiON layers at the interface.

  18. 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.

  19. 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

  20. Real-time observation of epitaxial graphene domain reorientation

    DOE PAGES

    Thuermer, Konrad; Foster, Michael E.; Bartelt, Norman Charles; ...

    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

  1. 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.

  2. 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.

  3. Graphene growth by molecular beam epitaxy on the carbon-face of SiC

    SciTech Connect

    Moreau, E.; Godey, S.; Ferrer, F. J.; Vignaud, D.; Wallart, X.; Avila, J.; Asensio, M. C.; Bournel, F.; Gallet, J.-J.

    2010-12-13

    Graphene layers have been grown by molecular beam epitaxy (MBE) on the (0001) C-face of SiC and have been characterized by atomic force microscopy, low energy electron diffraction (LEED), and UV photoelectron spectroscopy. Contrary to the graphitization process, the step-terrace structure of SiC is fully preserved during the MBE growth. LEED patterns show multiple orientation domains which are characteristic of graphene on SiC (0001), indicating non-Bernal rotated graphene planes. Well-defined Dirac cones, typical of single-layer graphene, have been observed in the valence band for few graphene layers by synchrotron spectroscopy, confirming the electronic decoupling of graphene layers.

  4. The impact of substrate selection for the controlled growth of graphene by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Schumann, T.; Lopes, J. M. J.; Wofford, J. M.; Oliveira, M. H.; Dubslaff, M.; Hanke, M.; Jahn, U.; Geelhaar, L.; Riechert, H.

    2015-09-01

    We examine how substrate selection impacts the resulting film properties in graphene growth by molecular beam epitaxy (MBE). Graphene growth on metallic as well as dielectric templates was investigated. We find that MBE offers control over the number of atomic graphene layers regardless of the substrate used. High structural quality could be achieved for graphene prepared on Ni (111) films which were epitaxially grown on MgO (111). For growth either on Al2O3 (0001) or on (6√3×6√3)R30°-reconstructed SiC (0001) surfaces, graphene with a higher density of defects is obtained. Interestingly, despite their defective nature, the layers possess a well defined epitaxial relation to the underlying substrate. These results demonstrate the feasibility of MBE as a technique for realizing the scalable synthesis of this two-dimensional crystal on a variety of substrates.

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

    PubMed

    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

    2015-09-24

    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.

  6. 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

  7. Magnetoresistive junctions based on epitaxial graphene and h-BN

    NASA Astrophysics Data System (ADS)

    Yazyev, Oleg; Pasquarello, Alfredo

    2009-03-01

    Using a first-principles approach, we investigate the structural, magnetic and transport properties of interfaces based on epitaxially grown monolayer graphene and hexagonal boron nitride (h-BN) in combination with ferromagnetic transition metals (Fe, Co and Ni). Such structurally well defined interfaces based on (111) fcc or (0001) hcp transition metals can be produced using simple manufacturing processes. Our calculations predict magnetoresistance ratios over 100% for certain junction compositions. In addition, such systems feature strong antiparallel (Fe and Co) and parallel (Ni) exchange coupling across the interface combined with low junction resistance. The predicted properties position such magnetoresistive junctions as an interesting alternative to the currently used giant and tunneling magnetoresistance systems and make them suitable for practical applications.

  8. Influence of Carbon in Metalorganic Chemical Vapor Deposition of Few-Layer WSe2 Thin Films

    NASA Astrophysics Data System (ADS)

    Zhang, Xiaotian; Al Balushi, Zakaria Y.; Zhang, Fu; Choudhury, Tanushree H.; Eichfeld, Sarah M.; Alem, Nasim; Jackson, Thomas N.; Robinson, Joshua A.; Redwing, Joan M.

    2016-12-01

    Metalorganic chemical vapor deposition (MOCVD) is a promising technique to form large-area, uniform films of monolayer or few-layer transition metal dichalcogenide (TMD) thin films; however, unintentional carbon incorporation is a concern. In this work, we report the presence of a defective graphene layer that forms simultaneously during MOCVD growth of tungsten diselenide (WSe2) on sapphire at high growth temperature and high Se:W ratio when using tungsten hexacarbonyl (W(CO)6) and dimethyl selenide ((CH3)2Se, DMSe) as precursors. The graphene layer alters the surface energy of the substrate reducing the lateral growth and coalescence of WSe2 domains. The use of hydrogen selenide (H2Se) instead of DMSe eliminates the defective graphene layer enabling coalesced monolayer and few-layer WSe2 films.

  9. 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.

  10. 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.

  11. 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.

  12. Transfer-free electrical insulation of epitaxial graphene from its metal substrate.

    PubMed

    Lizzit, Silvano; Larciprete, Rosanna; Lacovig, Paolo; Dalmiglio, Matteo; Orlando, Fabrizio; Baraldi, Alessandro; Gammelgaard, Lauge; Barreto, Lucas; Bianchi, Marco; Perkins, Edward; Hofmann, Philip

    2012-09-12

    High-quality, large-area epitaxial graphene can be grown on metal surfaces, but its transport properties cannot be exploited because the electrical conduction is dominated by the substrate. Here we insulate epitaxial graphene on Ru(0001) by a stepwise intercalation of silicon and oxygen, and the eventual formation of a SiO(2) layer between the graphene and the metal. We follow the reaction steps by X-ray photoemission spectroscopy and demonstrate the electrical insulation using a nanoscale multipoint probe technique.

  13. Porphyrin molecules boost the sensitivity of epitaxial graphene for NH3 detection

    NASA Astrophysics Data System (ADS)

    Iezhokin, I.; den Boer, D.; Offermans, P.; Ridene, M.; Elemans, J. A. A. W.; Adriaans, G. P.; Flipse, C. F. J.

    2017-02-01

    The sensitivity of quasi-free standing epitaxial graphene for NH3 detection is strongly enhanced by chemical functionalization with cobalt porphyrins resulting in a detection limit well below 100 ppb. Hybridization between NH3 and cobalt porphyrins induces a charge transfer to graphene and results in a shift of the graphene Fermi-level as detected by Hall measurements and theoretically explained by electronic structure calculations.

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

    PubMed Central

    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

  15. Van der Waals epitaxy and characterization of hexagonal boron nitride nanosheets on graphene

    NASA Astrophysics Data System (ADS)

    Song, Yangxi; Zhang, Changrui; Li, Bin; Ding, Guqiao; Jiang, Da; Wang, Haomin; Xie, Xiaoming

    2014-07-01

    Graphene is highly sensitive to environmental influences, and thus, it is worthwhile to deposit protective layers on graphene without impairing its excellent properties. Hexagonal boron nitride (h-BN), a well-known dielectric material, may afford the necessary protection. In this research, we demonstrated the van der Waals epitaxy of h-BN nanosheets on mechanically exfoliated graphene by chemical vapor deposition, using borazine as the precursor to h-BN. The h-BN nanosheets had a triangular morphology on a narrow graphene belt but a polygonal morphology on a larger graphene film. The h-BN nanosheets on graphene were highly crystalline, except for various in-plane lattice orientations. Interestingly, the h-BN nanosheets preferred to grow on graphene than on SiO2/Si under the chosen experimental conditions, and this selective growth spoke of potential promise for application to the preparation of graphene/h-BN superlattice structures fabricated on SiO2/Si.

  16. Van der Waals epitaxy and characterization of hexagonal boron nitride nanosheets on graphene.

    PubMed

    Song, Yangxi; Zhang, Changrui; Li, Bin; Ding, Guqiao; Jiang, Da; Wang, Haomin; Xie, Xiaoming

    2014-01-01

    Graphene is highly sensitive to environmental influences, and thus, it is worthwhile to deposit protective layers on graphene without impairing its excellent properties. Hexagonal boron nitride (h-BN), a well-known dielectric material, may afford the necessary protection. In this research, we demonstrated the van der Waals epitaxy of h-BN nanosheets on mechanically exfoliated graphene by chemical vapor deposition, using borazine as the precursor to h-BN. The h-BN nanosheets had a triangular morphology on a narrow graphene belt but a polygonal morphology on a larger graphene film. The h-BN nanosheets on graphene were highly crystalline, except for various in-plane lattice orientations. Interestingly, the h-BN nanosheets preferred to grow on graphene than on SiO2/Si under the chosen experimental conditions, and this selective growth spoke of potential promise for application to the preparation of graphene/h-BN superlattice structures fabricated on SiO2/Si.

  17. 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

    2015-04-08

    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.

  18. Experimental and theoretical investigations of monolayer and few-layer talc

    NASA Astrophysics Data System (ADS)

    Alencar, Ananias B.; Barboza, Ana Paula M.; Archanjo, Bráulio S.; Chacham, Helio; Neves, Bernardo R. A.

    2015-03-01

    We report experimental and theoretical investigations of a new nanomaterial: monolayer and few-layer talc. We show, through atomic force microscopy (AFM) measurements, that natural talc mineral can be mechanically exfoliated down to monolayer flakes. Our AFM-based mechanical characterization also shows that single- and few-layer talc flakes, of several square-microns, present properties similar to those of graphene, BN and MoS2, including the existence of folds and the recently reported negative dynamic compressibility. From first principles calculations, we also predict the mechanical properties of monolayer talc. We obtain theoretical values of monolayer talc breaking strength that are near graphene’s record values and its 2D elastic modulus. We also predict that the flexural rigidity of monolayer talc should be more than thirty times larger than that of graphene, but that it could still be bent to very small curvatures without fracturing.

  19. Formation of monolayer and few-layer hexagonal boron nitride nanosheets via surface segregation.

    PubMed

    Xu, Mingsheng; Fujita, Daisuke; Chen, Hongzheng; Hanagata, Nobutaka

    2011-07-01

    We report that few-layer hexagonal boron nitride (h-BN) nanosheets can be produced by using a surface segregation method. The formation of h-BN sheets is via an intermediate boron-nitrogen buffer layer. Our results suggest that surface segregation of boron and nitrogen from a solid source is an alternative approach to tailoring synthesis of h-BN sheets for potential applications such as in graphene electronics.

  20. 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

    2016-06-01

    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.

  1. Looking behind the scenes: Raman spectroscopy of top-gated epitaxial graphene through the substrate

    NASA Astrophysics Data System (ADS)

    Fromm, F.; Wehrfritz, P.; Hundhausen, M.; Seyller, Th

    2013-11-01

    Raman spectroscopy is frequently used to study the properties of epitaxial graphene grown on silicon carbide (SiC). In this work, we present a confocal micro-Raman study of epitaxial graphene on SiC(0001) in top-down geometry, i.e. in a geometry where both the primary laser light beam as well as the back-scattered light is guided through the SiC substrate. Compared to the conventional top-up configuration, in which confocal micro-Raman spectra are measured from the air side, we observe a significant intensity enhancement in top-down configuration, indicating that most of the Raman-scattered light is emitted into the SiC substrate. The intensity enhancement is explained in terms of dipole radiation at a dielectric surface. The new technique opens the possibility to probe graphene layers in devices where the graphene layer is covered by non-transparent materials. We demonstrate this by measuring gate-modulated Raman spectra of a top-gated epitaxial graphene field effect device. Moreover, we show that these measurements enable us to disentangle the effects of strain and charge on the positions of the prominent Raman lines in epitaxial graphene on SiC.

  2. 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

  3. 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

  4. 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.

  5. 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.

  6. Precise control of epitaxy of graphene by microfabricating SiC substrate

    NASA Astrophysics Data System (ADS)

    Fukidome, H.; Kawai, Y.; Fromm, F.; Kotsugi, M.; Handa, H.; Ide, T.; Ohkouchi, T.; Miyashita, H.; Enta, Y.; Kinoshita, T.; Seyller, Th.; Suemitsu, M.

    2012-07-01

    Epitaxial graphene (EG) on SiC is promising owing to a capability to produce high-quality film on a wafer scale. One of the remaining issues is microscopic thickness variation of EG near surface steps, which induces variations in its electronic properties and device characteristics. We demonstrate here that the variations of layer thickness and electronic properties are minimized by using microfabricated SiC substrates which spatially confines the epitaxy. This technique will contribute to the realization of highly reliable graphene devices.

  7. 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

  8. Comeback of epitaxial graphene for electronics: large-area growth of bilayer-free graphene on SiC

    NASA Astrophysics Data System (ADS)

    Kruskopf, Mattias; Momeni Pakdehi, Davood; Pierz, Klaus; Wundrack, Stefan; Stosch, Rainer; Dziomba, Thorsten; Götz, Martin; Baringhaus, Jens; Aprojanz, Johannes; Tegenkamp, Christoph; Lidzba, Jakob; Seyller, Thomas; Hohls, Frank; Ahlers, Franz J.; Schumacher, Hans W.

    2016-12-01

    We present a new fabrication method for epitaxial graphene on SiC which enables the growth of ultra-smooth defect- and bilayer-free graphene sheets with an unprecedented reproducibility, a necessary prerequisite for wafer-scale fabrication of high quality graphene-based electronic devices. The inherent but unfavorable formation of high SiC surface terrace steps during high temperature sublimation growth is suppressed by rapid formation of the graphene buffer layer which stabilizes the SiC surface. The enhanced nucleation is enforced by decomposition of deposited polymer adsorbate which acts as a carbon source. Unique to this method are the conservation of mainly 0.25 and 0.5 nm high surface steps and the formation of bilayer-free graphene on an area only limited by the size of the sample. This makes the polymer-assisted sublimation growth technique a promising method for commercial wafer scale epitaxial graphene fabrication. The extraordinary electronic quality is evidenced by quantum resistance metrology at 4.2 K showing ultra-high precision and high electron mobility on mm scale devices comparable to state-of-the-art graphene.

  9. Native point defects in few-layer phosphorene

    NASA Astrophysics Data System (ADS)

    Wang, V.; Kawazoe, Y.; Geng, W. T.

    2015-01-01

    Using hybrid density functional theory combined with a semiempirical van der Waals dispersion correction, we have investigated the structural and electronic properties of vacancies and self-interstitials in defective few-layer phosphorene. We find that both a vacancy and a self-interstitial defect are more stable in the outer layer than in the inner layer. The formation energy and transition energy of both a vacancy and a self-interstitial P defect decrease with increasing film thickness, mainly due to the upward shift of the host valence band maximum in reference to the vacuum level. Consequently, both vacancies and self-interstitials could act as shallow acceptors, and this well explains the experimentally observed p -type conductivity in few-layer phosphorene. On the other hand, since these native point defects have moderate formation energies and are stable in negatively charged states, they could also serve as electron compensating centers in n -type few-layer phosphorene.

  10. 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.

  11. Charge neutrality in epitaxial graphene on 6 H -SiC(0001) via nitrogen intercalation

    NASA Astrophysics Data System (ADS)

    Caffrey, Nuala M.; Armiento, Rickard; Yakimova, Rositsa; Abrikosov, Igor A.

    2015-08-01

    The electronic properties of epitaxial graphene grown on SiC(0001) are known to be impaired relative to those of freestanding graphene. This is due to the formation of a carbon buffer layer between the graphene layers and the substrate, which causes the graphene layers to become strongly n -doped. Charge neutrality can be achieved by completely passivating the dangling bonds of the clean SiC surface using atomic intercalation. So far, only one element, hydrogen, has been identified as a promising candidate. We show, using first-principles density functional calculations, how it can also be accomplished via the growth of a thin layer of silicon nitride on the SiC surface. The subsequently grown graphene layers display the electronic properties associated with charge neutral graphene. We show that the surface energy of this structure is considerably lower than that of others with intercalated atomic nitrogen and determine how its stability depends on the N2 chemical potential.

  12. Small scale rotational disorder observed in epitaxial graphene on SiC(0001)

    NASA Astrophysics Data System (ADS)

    Walter, Andrew L.; Bostwick, Aaron; Speck, Florian; Ostler, Markus; Kim, Keun Su; Chang, Young Jun; Moreschini, Luca; Innocenti, Davide; Seyller, Thomas; Horn, Karsten; Rotenberg, Eli

    2013-02-01

    Interest in the use of graphene in electronic devices has motivated an explosion in the study of this remarkable material. The simple, linear, Dirac cone band structure offers a unique possibility to investigate its finer details by angle-resolved photoelectron spectroscopy (ARPES). Indeed, ARPES has been performed on graphene grown on metal substrates but electronic applications require an insulating substrate. Epitaxial graphene grown by the thermal decomposition of silicon carbide (SiC) is an ideal candidate for this due to the large scale, uniform, graphene layers produced. The experimental spectral function of epitaxial graphene on SiC has been extensively studied. However, until now the cause of an anisotropy in the spectral width of the Fermi surface has not been determined. In the current work we show, by comparison of the spectral function to a semi-empirical model, that the anisotropy is due to small scale rotational disorder (˜± 0.15°) of graphene domains in graphene grown on SiC(0001) samples. The complicated shape described by the line-width is accurately reproduced by the semi-empirical model only when rotational disorder is included. While spectra from rare regions of the sample containing only one or two rotational domains is also presented. In addition to the direct benefit in the understanding of graphene's electronic structure this work suggests a mechanism to explain similar variations in related ARPES data.

  13. Single orientation graphene synthesized on iridium thin films grown by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Dangwal Pandey, A.; Krausert, K.; Franz, D.; Grânäs, E.; Shayduk, R.; Müller, P.; Keller, T. F.; Noei, H.; Vonk, V.; Stierle, A.

    2016-08-01

    Heteroepitaxial iridium thin films were deposited on (0001) sapphire substrates by means of molecular beam epitaxy, and subsequently, one monolayer of graphene was synthesized by chemical vapor deposition. The influence of the growth parameters on the quality of the Ir films, as well as of graphene, was investigated systematically by means of low energy electron diffraction, x-ray reflectivity, x-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, and atomic force microscopy. Our study reveals (111) oriented iridium films with high crystalline quality and extremely low surface roughness, on which the formation of large-area epitaxial graphene is achieved. The presence of defects, like dislocations, twins, and 30° rotated domains in the iridium films is also discussed. The coverage of graphene was found to be influenced by the presence of 30° rotated domains in the Ir films. Low iridium deposition rates suppress these rotated domains and an almost complete coverage of graphene was obtained. This synthesis route yields inexpensive, air-stable, and large-area graphene with a well-defined orientation, making it accessible to a wider community of researchers for numerous experiments or applications, including those which use destructive analysis techniques or irreversible processes. Moreover, this approach can be used to tune the structural quality of graphene, allowing a systematic study of the influence of defects in various processes like intercalation below graphene.

  14. Electrostatic transfer of patterned epitaxial graphene from SiC(0001) to glass

    NASA Astrophysics Data System (ADS)

    Biedermann, Laura B.; Beechem, Thomas E.; Ross, Anthony J.; Ohta, Taisuke; Howell, Stephen W.

    2010-12-01

    We report on a scalable electrostatic process to transfer epitaxial graphene onto alkali-containing glass substrates. Multilayer epitaxial graphene (MEG) was grown by heating silicon carbide (000\\bar{1} ) to high temperatures (1650-1700 °C) in an argon-mediated environment. Optical lithography was used to define patterned graphene regions, typically 20×20 μm2, which were then transferred to Pyrex substrates. For the electrostatic transfer, a large electric potential (1.2 kV) was applied between the donor MEG sample (anode) and the heated acceptor glass substrate (cathode). Atomic force microscopy scans of the transferred graphene showed that the morphology of the transferred multilayer graphene resembles that of the donor MEG. Raman spectroscopy analysis confirmed that the graphene can be transferred without inducing defects. The sheet resistance of the transferred graphene was as low as 150 Ω/squ. The transfer of small (1-2 μm wide) and large (~70×70 μm2) graphene patterns to Zerodur demonstrates the versatility of this transfer technique.

  15. Large area and structured epitaxial graphene produced by confinement controlled sublimation of silicon carbide.

    PubMed

    de Heer, Walt A; Berger, Claire; Ruan, Ming; Sprinkle, Mike; Li, Xuebin; Hu, Yike; Zhang, Baiqian; Hankinson, John; Conrad, Edward

    2011-10-11

    After the pioneering investigations into graphene-based electronics at Georgia Tech, great strides have been made developing epitaxial graphene on silicon carbide (EG) as a new electronic material. EG has not only demonstrated its potential for large scale applications, it also has become an important material for fundamental two-dimensional electron gas physics. It was long known that graphene mono and multilayers grow on SiC crystals at high temperatures in ultrahigh vacuum. At these temperatures, silicon sublimes from the surface and the carbon rich surface layer transforms to graphene. However the quality of the graphene produced in ultrahigh vacuum is poor due to the high sublimation rates at relatively low temperatures. The Georgia Tech team developed growth methods involving encapsulating the SiC crystals in graphite enclosures, thereby sequestering the evaporated silicon and bringing growth process closer to equilibrium. In this confinement controlled sublimation (CCS) process, very high-quality graphene is grown on both polar faces of the SiC crystals. Since 2003, over 50 publications used CCS grown graphene, where it is known as the "furnace grown" graphene. Graphene multilayers grown on the carbon-terminated face of SiC, using the CCS method, were shown to consist of decoupled high mobility graphene layers. The CCS method is now applied on structured silicon carbide surfaces to produce high mobility nano-patterned graphene structures thereby demonstrating that EG is a viable contender for next-generation electronics. Here we present for the first time the CCS method that outperforms other epitaxial graphene production methods.

  16. Large area and structured epitaxial graphene produced by confinement controlled sublimation of silicon carbide

    PubMed Central

    de Heer, Walt A.; Berger, Claire; Ruan, Ming; Sprinkle, Mike; Li, Xuebin; Hu, Yike; Zhang, Baiqian; Hankinson, John; Conrad, Edward

    2011-01-01

    After the pioneering investigations into graphene-based electronics at Georgia Tech, great strides have been made developing epitaxial graphene on silicon carbide (EG) as a new electronic material. EG has not only demonstrated its potential for large scale applications, it also has become an important material for fundamental two-dimensional electron gas physics. It was long known that graphene mono and multilayers grow on SiC crystals at high temperatures in ultrahigh vacuum. At these temperatures, silicon sublimes from the surface and the carbon rich surface layer transforms to graphene. However the quality of the graphene produced in ultrahigh vacuum is poor due to the high sublimation rates at relatively low temperatures. The Georgia Tech team developed growth methods involving encapsulating the SiC crystals in graphite enclosures, thereby sequestering the evaporated silicon and bringing growth process closer to equilibrium. In this confinement controlled sublimation (CCS) process, very high-quality graphene is grown on both polar faces of the SiC crystals. Since 2003, over 50 publications used CCS grown graphene, where it is known as the “furnace grown” graphene. Graphene multilayers grown on the carbon-terminated face of SiC, using the CCS method, were shown to consist of decoupled high mobility graphene layers. The CCS method is now applied on structured silicon carbide surfaces to produce high mobility nano-patterned graphene structures thereby demonstrating that EG is a viable contender for next-generation electronics. Here we present for the first time the CCS method that outperforms other epitaxial graphene production methods. PMID:21960446

  17. 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.

  18. On the theory of electronic states of the "epitaxial graphene-quantum-well film" system

    NASA Astrophysics Data System (ADS)

    Alisultanov, Z. Z.; Meilanov, R. P.

    2012-07-01

    The problem of an epitaxial graphene formed on a thin metal film in an external magnetic field has been considered. It has been shown that the problem can be solved using the Green's function method within the Kadanoff-Baym formalism. Analytical expressions for the transferred charge as a function of the magnetic field and the thickness of the film have been obtained.

  19. Express optical analysis of epitaxial graphene on SiC: impact of morphology on quantum transport.

    PubMed

    Yager, Tom; Lartsev, Arseniy; Mahashabde, Sumedh; Charpentier, Sophie; Davidovikj, Dejan; Danilov, Andrey; Yakimova, Rositza; Panchal, Vishal; Kazakova, Olga; Tzalenchuk, Alexander; Lara-Avila, Samuel; Kubatkin, Sergey

    2013-09-11

    We show that inspection with an optical microscope allows surprisingly simple and accurate identification of single and multilayer graphene domains in epitaxial graphene on silicon carbide (SiC/G) and is informative about nanoscopic details of the SiC topography, making it ideal for rapid and noninvasive quality control of as-grown SiC/G. As an illustration of the power of the method, we apply it to demonstrate the correlations between graphene morphology and its electronic properties by quantum magneto-transport.

  20. A method to extract pure Raman spectrum of epitaxial graphene on SiC

    NASA Astrophysics Data System (ADS)

    Kunc, J.; Hu, Y.; Palmer, J.; Berger, C.; de Heer, W. A.

    2013-11-01

    The Raman spectrum of epitaxial graphene on SiC is generally obtained by simply subtracting a SiC spectra from the experimental data, which results in noisy spectrum and negative intensity. By using a Non-negative Matrix Factorization (NMF) method, we obtain pure graphene spectra, even for monolayer graphene and sub-micron size patterned features, as well as in spatial mapping and depth profile. We show that the NMF method is efficient in data smoothing and for signal deconvolution with no assumption required for the functional form of the signals.

  1. Long Spin Relaxation Times in Wafer Scale Epitaxial Graphene on SiC(0001)

    NASA Astrophysics Data System (ADS)

    Maassen, Thomas; van den Berg, J. Jasper; Ijbema, Natasja; Fromm, Felix; Seyller, Thomas; Yakimova, Rositza; van Wees, Bart J.

    2012-03-01

    We developed an easy, upscalable process to prepare lateral spin-valve devices on epitaxially grown monolayer graphene on SiC(0001) and perform nonlocal spin transport measurements. We observe the longest spin relaxation times tau_S in monolayer graphene, while the spin diffusion coefficient D_S is strongly reduced compared to typical results on exfoliated graphene. The increase of tau_S is probably related to the changed substrate, while the cause for the small value of D_S remains an open question.

  2. A switch for epitaxial graphene electronics: Utilizing the silicon carbide substrate as transistor channel

    NASA Astrophysics Data System (ADS)

    Krach, F.; Hertel, S.; Waldmann, D.; Jobst, J.; Krieger, M.; Reshanov, S.; Schöner, A.; Weber, H. B.

    2012-03-01

    Due to the lack of graphene transistors with large on/off ratio, we propose a concept employing both epitaxial graphene and its underlying substrate silicon carbide (SiC) as electronic materials. We demonstrate a simple, robust, and scalable transistor, in which graphene serves as electrodes and SiC as a semiconducting channel. The common interface has to be chosen such that it provides favorable charge injection. The insulator and gate functionality is realized by an ionic liquid gate for convenience but could be taken over by a solid gate stack. On/off ratios exceeding 44000 at room temperature are found.

  3. Epitaxial Growth and Electronic Properties of Large Hexagonal Graphene Domains on Cu(111) Thin Film

    NASA Astrophysics Data System (ADS)

    Ago, Hiroki; Kawahara, Kenji; Ogawa, Yui; Tanoue, Shota; Bissett, Mark A.; Tsuji, Masaharu; Sakaguchi, Hidetsugu; Koch, Roland J.; Fromm, Felix; Seyller, Thomas; Komatsu, Katsuyoshi; Tsukagoshi, Kazuhito

    2013-07-01

    Large hexagonal single-crystalline domains of single-layer graphene are epitaxially grown by ambient-pressure chemical vapor deposition over a thin Cu(111) film deposited on c-plane sapphire. The hexagonal graphene domains with a maximum size of 100 µm are oriented in the same direction due to the epitaxial growth. Reflecting high crystallinity, a clear band structure with the Dirac cone is observed by angle-resolved photoelectron spectroscopy (ARPES), and a high carrier mobility exceeding 4,000 cm2 V-1 s-1 is obtained on SiO2/Si at room temperature. Our epitaxial approach combined with large domain growth is expected to contribute to future electronic applications.

  4. 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.

  5. 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

  6. Tunneling Spectroscopy Studies of Epitaxial Graphene on Silicon Carbide(0001) and Its Interfaces

    NASA Astrophysics Data System (ADS)

    Sandin, Andreas Axel Tomas

    A two dimensional network of sp2 bonded carbon atoms is defined as graphene. This novel material possesses remarkable electronic properties due to its unique band structure at the vicinity of the Fermi energy. The toughest challenge to bring use of graphene electronic properties in device geometries is that graphene is exceptionally sensitive to its electrical environment for integration into macroscopic system of electrical contacts and substrates. One of the most promising substrates for graphene is the polar surfaces of SiC for the reason it can be grown epitaxially by sublimating Si from the top-most SiC atomic layers. In this work, the interfaces of graphene grown on the Si-terminated polar surface SiC(0001) is studied in UHV using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), low energy electron diffraction (LEED) and auger electron Spectroscopy (AES). STM is used image the graphene surface and interfaces with the capability of atomic resolution. LEED is used to study surface atomic reciprocal ordering and AES is used to determine surface atomic composition during the graphene formation. Interfacial layer (Buffer layer), Single layer graphene and bilayer graphene are identified electronically by means of probing the first member of the image potential derived state. This state is found by dZ/dV spectroscopy in the high energy unoccupied states and is exceptionally sensitive to electrostatic changes to the surface which is detected by energy shifts of image potential states (IPS). This sensitivity is utilized to probe the graphene screening of external electric fields by varying the electric field in the tunneling junction and addresses the fact that charged impurity scattering is likely to be crucial for epitaxial graphene on SiC(0001) when it comes to transport parameters. Shifts of IPS energy position has also been used verify work function changes for identification of several Sodium Intercalation structures of epitaxial

  7. Substrate-induced band gap opening in epitaxial graphene

    SciTech Connect

    Zhou, S.Y.; Gweon, G.-H.; Fedorov, A.V.; First, P.N.; de Heer,W.A.; Lee, D.-H.; Guinea, F.; Castro Neto, A.H.; Lanzara, A.

    2007-09-08

    Graphene has shown great application potential as the hostmaterial for next-generation electronic devices. However, despite itsintriguing properties, one of the biggest hurdles for graphene to beuseful as an electronic material is the lack of an energy gap in itselectronic spectra. This, for example, prevents the use of graphene inmaking transistors. Although several proposals have been made to open agap in graphene's electronic spectra, they all require complexengineering of the graphene layer. Here, we show that when graphene isepitaxially grown on SiC substrate, a gap of ~;0.26 eV is produced. Thisgap decreases as the sample thickness increases and eventually approacheszero when the number of layers exceeds four. We propose that the originof this gap is the breaking of sublattice symmetry owing to thegraphene-substrate interaction. We believe that our results highlight apromising direction for band gap engineering of graphene.

  8. Strain control of vibrational properties of few layer phosphorene

    NASA Astrophysics Data System (ADS)

    Tokár, K.; Derian, R.; Brndiar, J.; Štich, I.

    2016-11-01

    Using density functional theory techniques, we study lattice vibrational Raman and infrared modes in strained few-layer phosphorene. We find very significant frequency shifts, up to ≈ 100 cm-1 in the applied strain range of ±6%, of the Raman activities in both high- and low-frequency region and infrared activities in the low-frequency region. The type of applied strain, that is, armchair and zigzag, selectively affects specific vibrational modes. Combined with high spatial-resolution Raman/infrared scattering experiments, our calculated results can aid understanding of the complicated inhomogeneous strain distributions in few-layer phosphorene or manufacturing of materials with desired electronic properties via strain or layer engineering.

  9. 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

  10. Growth protocols and characterization of epitaxial graphene on SiC elaborated in a graphite enclosure

    NASA Astrophysics Data System (ADS)

    Kumar, B.; Baraket, M.; Paillet, M.; Huntzinger, J.-R.; Tiberj, A.; Jansen, A. G. M.; Vila, L.; Cubuku, M.; Vergnaud, C.; Jamet, M.; Lapertot, G.; Rouchon, D.; Zahab, A.-A.; Sauvajol, J.-L.; Dubois, L.; Lefloch, F.; Duclairoir, F.

    2016-01-01

    The epitaxial growth of graphene by the sublimation of Si-terminated silicon carbide (SiC) is studied inside a graphite enclosure in a radio-frequency furnace by comparing different in situ processes involving hydrogen etching or not and different growth conditions. For the growth under vacuum, even with the surface preparation of hydrogen etching, the morphology of the synthesized graphene is found full of voids and defects in the form of a multilayer graphene film. For the growth under Ar, the hydrogen etching plays a vital role to improve the graphene quality in terms of surface roughness, the number of graphene layers and the domain size. For the graphene samples grown with the proposed protocol, the original combination of micro-probe Raman spectroscopy and simultaneous optical transmission and reflection measurements reveals a detailed spatially resolved image of the graphene domains with monolayer domain size of ~5×5 μm2 on about 2/3 of the total sample surface. The magnetotransport data yield charge-carrier mobilities up to 2900 cm2/Vs as found for high quality graphene on the Si-face of SiC. The observed magnetoquantum oscillations in the magnetoresistance confirm the expected behavior of single-layer graphene.

  11. Temperature dependent phonon shifts in few-layer black phosphorus.

    PubMed

    Late, Dattatray J

    2015-03-18

    Atomically thin two-dimensional (2D) sheets of black phosphorus have attracted much attention due to their potential for future nanoelectronic and photonics device applications. Present investigations deal with the temperature dependent phonon shifts in a few-layer black phosphorus nanosheet sample prepared using micromechanical exfoliation on a 300 nm SiO2/Si substrate. The temperature dependent Raman spectroscopy experiments were carried out on a few-layer black phosphorus sample, which depicts softening of Ag(1), B2g, and Ag(2) modes as temperature increases from 77 to 673 K. The calculated temperature coefficients for Ag(1), B2g, and Ag(2) modes of the few-layer black phosphorus nanosheet sample were observed to be -0.01, -0.013, and -0.014 cm(-1) K(-1), respectively. The temperature dependent softening modes of black phosphorus results were explained on the basis of a double resonance process which is more active in an atomically thin sample. This process can also be fundamentally pertinent in other promising and emerging 2D ultrathin layer and heterostructured materials.

  12. High van Hove singularity extension and Fermi velocity increase in epitaxial graphene functionalized by intercalated gold clusters

    NASA Astrophysics Data System (ADS)

    Nair, M. N.; Cranney, M.; Vonau, F.; Aubel, D.; Le Fèvre, P.; Tejeda, A.; Bertran, F.; Taleb-Ibrahimi, A.; Simon, L.

    2012-06-01

    Gold intercalation between a buffer layer and a graphene monolayer of epitaxial graphene on SiC(0001) leads to the formation of small aggregates of clusters. Angle-resolved photoemission spectroscopy measurements reveal that these clusters preserve the linear dispersion of the graphene quasiparticles and surprisingly increase their Fermi velocity. They also strongly modify the band structure of graphene around the van Hove singularities by a strong extension without charge transfer. These results give new insight on the role of the intercalant in the renormalization of the bare electronic band structure of graphene usually observed in graphite and graphene intercalation compounds.

  13. 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.

  14. Electronic transport at monolayer-bilayer junctions in epitaxial graphene on SiC

    NASA Astrophysics Data System (ADS)

    Giannazzo, F.; Deretzis, I.; La Magna, A.; Roccaforte, F.; Yakimova, R.

    2012-12-01

    Two-dimensional maps of the electronic conductance in epitaxial graphene grown on SiC were obtained by calibrated conductive atomic force microscopy. The correlation between morphological and electrical maps revealed the local conductance degradation in epitaxial graphene over the SiC substrate steps or at the junction between monolayer (1L) and bilayer (2L) graphene regions. The effect of steps strongly depends on the charge transfer phenomena between the step sidewall and graphene, whereas the resistance increase at the 1L/2L junction is a purely quantum-mechanical effect independent on the interaction with the substrate. First-principles transport calculations indicate that the weak wave-function coupling between the 1L π/π* bands with the respective first bands of the 2L region gives rise to a strong suppression of the conductance for energies within ±0.48 eV from the Dirac point. Conductance degradation at 1L/2L junctions is therefore a general issue for large area graphene with a certain fraction of inhomogeneities in the layer number, including graphene grown by chemical vapor deposition on metals.

  15. van der Waals epitaxy of CdTe thin film on graphene

    NASA Astrophysics Data System (ADS)

    Mohanty, Dibyajyoti; Xie, Weiyu; Wang, Yiping; Lu, Zonghuan; Shi, Jian; Zhang, Shengbai; Wang, Gwo-Ching; Lu, Toh-Ming; Bhat, Ishwara B.

    2016-10-01

    van der Waals epitaxy (vdWE) facilitates the epitaxial growth of materials having a large lattice mismatch with the substrate. Although vdWE of two-dimensional (2D) materials on 2D materials have been extensively studied, the vdWE for three-dimensional (3D) materials on 2D substrates remains a challenge. It is perceived that a 2D substrate passes little information to dictate the 3D growth. In this article, we demonstrated the vdWE growth of the CdTe(111) thin film on a graphene buffered SiO2/Si substrate using metalorganic chemical vapor deposition technique, despite a 46% large lattice mismatch between CdTe and graphene and a symmetry change from cubic to hexagonal. Our CdTe films produce a very narrow X-ray rocking curve, and the X-ray pole figure analysis showed 12 CdTe (111) peaks at a chi angle of 70°. This was attributed to two sets of parallel epitaxy of CdTe on graphene with a 30° relative orientation giving rise to a 12-fold symmetry in the pole figure. First-principles calculations reveal that, despite the relatively small energy differences, the graphene buffer layer does pass epitaxial information to CdTe as the parallel epitaxy, obtained in the experiment, is energetically favored. The work paves a way for the growth of high quality CdTe film on a large area as well as on the amorphous substrates.

  16. The investigation of cobalt intercalation underneath epitaxial graphene on 6H-SiC(0 0 0 1)

    NASA Astrophysics Data System (ADS)

    Zhang, Yuxi; Zhang, Hanjie; Cai, Yiliang; Song, Junjie; He, Pimo

    2017-02-01

    The intercalation behaviour of cobalt underneath both epitaxial graphene monolayer and bilayer on 6H-SiC(0001) have been investigated by scanning tunneling microscopy (STM) and density functional theory (DFT). Upon deposition, cobalt atoms prefer to agglomerate into clusters on the epitaxial graphene. After annealing the sample to 850 °C, the intercalation of the adsorbed cobalt atoms into both monolayer and bilayer epitaxial graphene on SiC takes place, as observed by the atomically resolved STM images. Further studies based on DFT modeling and simulated STM images show that, resulting from the interplay between the intercalated cobalt atoms and the carbon layers sandwiching it, the most energetically favourable intercalation sites of cobalt atoms underneath monolayer and bilayer graphene differ. Furthermore, the results show energy barriers of 0.60 eV and 0.41 eV for cobalt penetration through mono-vacancy defects at monolayer and bilayer graphene.

  17. Quasi-free-standing bilayer epitaxial graphene field-effect transistors on 4H-SiC (0001) substrates

    SciTech Connect

    Yu, C.; Li, J.; Song, X. B.; Liu, Q. B.; Cai, S. J.; Feng, Z. H.; He, Z. Z.

    2016-01-04

    Quasi-free-standing epitaxial graphene grown on wide band gap semiconductor SiC demonstrates high carrier mobility and good material uniformity, which make it promising for graphene-based electronic devices. In this work, quasi-free-standing bilayer epitaxial graphene is prepared and its transistors with gate lengths of 100 nm and 200 nm are fabricated and characterized. The 100 nm gate length graphene transistor shows improved DC and RF performances including a maximum current density I{sub ds} of 4.2 A/mm, and a peak transconductance g{sub m} of 2880 mS/mm. Intrinsic current-gain cutoff frequency f{sub T} of 407 GHz is obtained. The exciting DC and RF performances obtained in the quasi-free-standing bilayer epitaxial graphene transistor show the great application potential of this material system.

  18. 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.

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

    PubMed

    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

    2016-01-07

    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 cm(2)·V(-1)·s(-1) at 4 K.

  20. 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

  1. 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.

  2. 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.

  3. 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

    2016-05-05

    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.

  4. Epitaxial growth of crystalline polyaniline on reduced graphene oxide.

    PubMed

    Majumdar, Dipanwita; Baskey, Moni; Saha, Shyamal K

    2011-08-17

    Due to its unique electronic properties, graphene has already been identified as a promising material for future carbon based electronics. To develop graphene technology, the fabrication of a high quality P-N junction is a great challenge. Here, we describe a general technique to grow single crystalline polyaniline (PANI) films on graphene sheets using in situ polymerization via the oxidation-reduction of aniline monomer and graphene oxide, respectively, to fabricate a high quality P-N junction, which shows diode-like behavior with a remarkably low turn-on voltage (60 mV) and high rectification ratio (1880:1) up to a voltage of 0.2 V. The origin of these superior electronic properties is the preferential growth of a highly crystalline PANI film as well as lattice matching between the d-values [∼2.48 Å] of graphene and {120} planes of PANI.

  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. 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

    2016-03-01

    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.

  7. 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.

  8. Influence of Impurity Spin Dynamics on Quantum Transport in Epitaxial Graphene.

    PubMed

    Lara-Avila, Samuel; Kubatkin, Sergey; Kashuba, Oleksiy; Folk, Joshua A; Lüscher, Silvia; Yakimova, Rositza; Janssen, T J B M; Tzalenchuk, Alexander; Fal'ko, Vladimir

    2015-09-04

    Experimental evidence from both spin-valve and quantum transport measurements points towards unexpectedly fast spin relaxation in graphene. We report magnetotransport studies of epitaxial graphene on SiC in a vector magnetic field showing that spin relaxation, detected using weak-localization analysis, is suppressed by an in-plane magnetic field B(∥), and thereby proving that it is caused at least in part by spinful scatterers. A nonmonotonic dependence of the effective decoherence rate on B(∥) reveals the intricate role of the scatterers' spin dynamics in forming the interference correction to the conductivity, an effect that has gone unnoticed in earlier weak localization studies.

  9. 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.

  10. 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.

  11. Mechanical and Electrical Anisotropy of Few-Layer Black Phosphorus.

    PubMed

    Tao, Jin; Shen, Wanfu; Wu, Sen; Liu, Lu; Feng, Zhihong; Wang, Chao; Hu, Chunguang; Yao, Pei; Zhang, Hao; Pang, Wei; Duan, Xuexin; Liu, Jing; Zhou, Chongwu; Zhang, Daihua

    2015-11-24

    We combined reflection difference microscopy, electron transport measurements, and atomic force microscopy to characterize the mechanical and electrical anisotropy of few-layer black phosphorus. We were able to identify the lattice orientations of the two-dimensional material and construct suspended structures aligned with specific crystal axes. The approach allowed us to probe the anisotropic mechanical and electrical properties along each lattice axis in separate measurements. We measured the Young's modulus of few-layer black phosphorus to be 58.6 ± 11.7 and 27.2 ± 4.1 GPa in zigzag and armchair directions. The breaking stress scaled almost linearly with the Young's modulus and was measured to be 4.79 ± 1.43 and 2.31 ± 0.71 GPa in the two directions. We have also observed highly anisotropic transport behavior in black phosphorus and derived the conductance anisotropy to be 63.7%. The test results agreed well with theoretical predictions. Our work provided very valuable experimental data and suggested an effective characterization means for future studies on black phosphorus and anisotropic two-dimensional nanomaterials in general.

  12. Terahertz detection by epitaxial-graphene field-effect-transistors on silicon carbide

    NASA Astrophysics Data System (ADS)

    Bianco, F.; Perenzoni, D.; Convertino, D.; De Bonis, S. L.; Spirito, D.; Perenzoni, M.; Coletti, C.; Vitiello, M. S.; Tredicucci, A.

    2015-09-01

    We report on room temperature detection of terahertz radiation by means of antenna-coupled field effect transistors (FETs) fabricated using epitaxial graphene grown on silicon carbide. The achieved photoresponsivity (˜0.25 V/W) and noise equivalent power (˜80 nW/ √{ Hz } ) result from the combined effect of two independent detection mechanisms: over-damped plasma wave rectification and thermoelectric effects, the latter ascribed to the presence of carrier density junctions along the FET channel. The calculated plasmonic and thermoelectric response reproduces qualitatively well the measured photovoltages; the experimentally observed sign-switch demonstrates the stronger contribution of plasmonic detection compared to the thermoelectric one. These results unveil the potential of plasmonic detectors exploiting epitaxial graphene on silicon carbide for fast large area imaging of macroscopic samples.

  13. 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).

  14. 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

    2015-06-04

    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.

  15. 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

  16. 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.

  17. 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.

  18. Landau level splitting in nitrogen-seeded epitaxial graphene

    DOE PAGES

    Rothwell, S. L.; Wang, F.; Liu, G.; ...

    2016-07-01

    We present a new form of semiconducting graphene grown on C-face silicon carbide, SiC(0001), seeded with a sub-monolayer of nitrogen. This graphene exhibits a gap of 0.3-0.7 eV from the Fermi level to the valence band dependent on lm thickness as measured via angle resolved photo-emission spectroscopy (ARPES). Scanning tunneling microscopy (STM) images imply that the bandgap may be the result of strain-induced confinement. STM indicates that much of the graphene consists of wide at hexagonal plateaus, 8-20 nm2 on average, surrounded by both smooth and disordered folds of length scales from 0.5-2 nm tall, 1-4 nm thick, and 1-20more » nm long. The remainder of the surface is covered in smooth or disordered ripples and folds intermixed. Scanning tunneling spectroscopy (STS) measurements on all features show peaks suggestive of Landau levels, and have been analyzed to give pseudo-magnetic field magnitudes. The magnetic lengths associated with these fields are less than the average plateau diameter but comparable to typical fold widths. We consider a growth process whereby the graphene grows pinned to the substrate by the interface nitrogen. The graphene experiences compressive strain as a result of both this pinning as well as competing thermal expansion forces between the substrate and lm. As a result, graphene on nitrogen-seeded SiC has a more concentrated network of strained ripples and folds than seen on C-face SiC graphene without nitrogen.« less

  19. Landau level splitting in nitrogen-seeded epitaxial graphene

    SciTech Connect

    Rothwell, S. L.; Wang, F.; Liu, G.; Xu, C.; Feldman, L. C.; Conrad, E. H.; Guisinger, N. P.; Cohen, P. I.

    2016-07-01

    We present a new form of semiconducting graphene grown on C-face silicon carbide, SiC(0001), seeded with a sub-monolayer of nitrogen. This graphene exhibits a gap of 0.3-0.7 eV from the Fermi level to the valence band dependent on lm thickness as measured via angle resolved photo-emission spectroscopy (ARPES). Scanning tunneling microscopy (STM) images imply that the bandgap may be the result of strain-induced confinement. STM indicates that much of the graphene consists of wide at hexagonal plateaus, 8-20 nm2 on average, surrounded by both smooth and disordered folds of length scales from 0.5-2 nm tall, 1-4 nm thick, and 1-20 nm long. The remainder of the surface is covered in smooth or disordered ripples and folds intermixed. Scanning tunneling spectroscopy (STS) measurements on all features show peaks suggestive of Landau levels, and have been analyzed to give pseudo-magnetic field magnitudes. The magnetic lengths associated with these fields are less than the average plateau diameter but comparable to typical fold widths. We consider a growth process whereby the graphene grows pinned to the substrate by the interface nitrogen. The graphene experiences compressive strain as a result of both this pinning as well as competing thermal expansion forces between the substrate and lm. As a result, graphene on nitrogen-seeded SiC has a more concentrated network of strained ripples and folds than seen on C-face SiC graphene without nitrogen.

  20. Wafer scale millimeter-wave integrated circuits based on epitaxial graphene in high data rate communication

    PubMed Central

    Habibpour, Omid; He, Zhongxia Simon; Strupinski, Wlodek; Rorsman, Niklas; Zirath, Herbert

    2017-01-01

    In recent years, the demand for high data rate wireless communications has increased dramatically, which requires larger bandwidth to sustain multi-user accessibility and quality of services. This can be achieved at millimeter wave frequencies. Graphene is a promising material for the development of millimeter-wave electronics because of its outstanding electron transport properties. Up to now, due to the lack of high quality material and process technology, the operating frequency of demonstrated circuits has been far below the potential of graphene. Here, we present monolithic integrated circuits based on epitaxial graphene operating at unprecedented high frequencies (80–100 GHz). The demonstrated circuits are capable of encoding/decoding of multi-gigabit-per-second information into/from the amplitude or phase of the carrier signal. The developed fabrication process is scalable to large wafer sizes. PMID:28145513

  1. Wafer scale millimeter-wave integrated circuits based on epitaxial graphene in high data rate communication

    NASA Astrophysics Data System (ADS)

    Habibpour, Omid; He, Zhongxia Simon; Strupinski, Wlodek; Rorsman, Niklas; Zirath, Herbert

    2017-02-01

    In recent years, the demand for high data rate wireless communications has increased dramatically, which requires larger bandwidth to sustain multi-user accessibility and quality of services. This can be achieved at millimeter wave frequencies. Graphene is a promising material for the development of millimeter-wave electronics because of its outstanding electron transport properties. Up to now, due to the lack of high quality material and process technology, the operating frequency of demonstrated circuits has been far below the potential of graphene. Here, we present monolithic integrated circuits based on epitaxial graphene operating at unprecedented high frequencies (80–100 GHz). The demonstrated circuits are capable of encoding/decoding of multi-gigabit-per-second information into/from the amplitude or phase of the carrier signal. The developed fabrication process is scalable to large wafer sizes.

  2. High-quality epitaxial graphene devices with low carrier density for resistance metrology

    NASA Astrophysics Data System (ADS)

    Yang, Yanfei; Huang, Lung-I.; Newell, David; Real, Mariano; Elmquist, Randolph

    2014-03-01

    Epitaxially grown graphene on silicon carbide (SiC) is a promising material for both quantum resistance metrology and wafer-scale electronics. However, monolayers are typically found to be heavily n-doped due to the charge exchange between the graphene and the non-conducting buffer layer beneath that is covalently bonded to the SiC substrate. Carrier densities are usually in the range of 1012 ~ 1013 cm-2, where heavy doping shifts the quantized Hall resistance plateau to high magnetic field values. Various gating methods have been developed to reduce the carrier density, but require lithography processes that increase the probability of contamination that degrades the performance of the devices. Recently, we fabricated high-quality Hall devices on diced semi-insulating SiC wafers, obtaining carrier densities in the range of 1010 ~ 1011 cm-2 and mobility above 104 cm2V-1s-1 without gating. Graphene is grown on the Si face of SiC(0001) substrates and devices are fabricated using a metal layer subtractive process without organic chemical contamination of the graphene. We measure well-developed quantum Hall plateaus with filling factor ν = 2, the fingerprint for monolayer graphene, at magnetic fields below 2 T at liquid helium temperature. A variety of quantum phenomena are observed in these clean, high quality graphene devices. NIST and Georgetown University.

  3. 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

  4. 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

  5. Plasma-Modified, Epitaxial Fabricated Graphene on SiC for the Electrochemical Detection of TNT

    PubMed Central

    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

  6. 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.

  7. 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.

  8. Long- versus Short-Range Scattering in Doped Epitaxial Graphene.

    PubMed

    Straßer, C; Ludbrook, B M; Levy, G; Macdonald, A J; Burke, S A; Wehling, T O; Kern, K; Damascelli, A; Ast, C R

    2015-05-13

    Tuning the electronic properties of graphene by adatom deposition unavoidably introduces disorder into the system, which directly affects the single-particle excitations and electrodynamics. Using angle-resolved photoemission spectroscopy (ARPES) we trace the evolution of disorder in graphene by thallium adatom deposition and probe its effect on the electronic structure. We show that the signatures of quasiparticle scattering in the photoemission spectral function can be used to identify thallium adatoms, although charged, as efficient short-range scattering centers. Employing a self-energy model for short-range scattering, we are able to extract a δ-like scattering potential δ = -3.2 ± 1 eV. Therefore, isolated charged scattering centers do not necessarily act just as good long-range (Coulomb) scatterers but can also act as efficient short-range (δ-like) scatterers; in the case of thallium, this happens with almost equal contributions from both mechanisms.

  9. 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.

  10. 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

  11. Rapid epitaxy-free graphene synthesis on silicidated polycrystalline platinum

    NASA Astrophysics Data System (ADS)

    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

    2015-07-01

    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.

  12. 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

  13. Rapid epitaxy-free graphene synthesis on silicidated polycrystalline platinum.

    PubMed

    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

    2015-07-15

    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.

  14. STM imaging, spectroscopy and manipulation of a self-assembled PTCDI monolayer on epitaxial graphene.

    PubMed

    Yang, H; Mayne, A J; Comtet, G; Dujardin, G; Kuk, Y; Sonnet, Ph; Stauffer, L; Nagarajan, S; Gourdon, A

    2013-04-14

    Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), and manipulation studies were performed on an ordered self-assembled monolayer (SAM) of N,N'-bis(1-hexylheptyl)perylene-3,4:9,10-bis(dicarboximide) molecules on epitaxial graphene on hexagonal silicon carbide - SiC(0001). Four novel aspects of the molecular SAM on graphene are presented. Molecules adsorb in both armchair and zig-zag configurations, giving rise to six orientations of the molecular layer with respect to the underlying substrate. The interaction between the molecules and the graphene surface shifts the LUMO towards the Fermi level, inducing a charge transfer and the opening of a band gap in the graphene, with the LUMO inside. This decouples the LUMO from the surface rendering it invisible in the dI/dV spectroscopy. The HOMO only becomes visible at short tip-surface distances, as its energy lies within the band gap of the SiC substrate. Finally, the observed molecular defects are very particular, being composed exclusively of molecular dimers. These molecular dimers have a stronger interaction with the graphene than other molecules.

  15. 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

    2016-03-09

    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.

  16. 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)

  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. 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.

  19. Enhanced crystallinity of epitaxial graphene grown on hexagonal SiC surface with molybdenum plate capping.

    PubMed

    Jin, Han Byul; Jeon, Youngeun; Jung, Sungchul; Modepalli, Vijayakumar; Kang, Hyun Suk; Lee, Byung Cheol; Ko, Jae-Hyeon; Shin, Hyung-Joon; Yoo, Jung-Woo; Kim, Sung Youb; Kwon, Soon-Yong; Eom, Daejin; Park, Kibog

    2015-04-24

    The crystallinity of epitaxial graphene (EG) grown on a Hexagonal-SiC substrate is found to be enhanced greatly by capping the substrate with a molybdenum plate (Mo-plate) during vacuum annealing. The crystallinity enhancement of EG layer grown with Mo-plate capping is confirmed by the significant change of measured Raman spectra, compared to the spectra for no capping. Mo-plate capping is considered to induce heat accumulation on SiC surface by thermal radiation mirroring and raise Si partial pressure near surface by confining the sublimated Si atoms between SiC substrate and Mo-plate, which would be the essential contributors of crystallinity enhancement.

  20. 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.

  1. 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.

  2. Acousto-electric transport in epitaxial monolayer graphene on SiC

    NASA Astrophysics Data System (ADS)

    Santos, P. V.; Schumann, T.; Oliveira, M. H.; Lopes, J. M. J.; Riechert, H.

    2013-06-01

    We report on the piezoelectric excitation and acoustic charge transport by gigahertz surface acoustic waves (SAWs) in epitaxial monolayer graphene (EG) on SiC. The GHz SAWs frequencies were generated by interdigital transducers fabricated on a piezoelectric island on the SiC substrate. Acoustic transport studies in a Hall bar geometry show that the propagating SAW field transports carriers in EG, the transport direction being determined by the direction of the acoustic beam. Carrier transport is driven by drift in the piezoelectric field induced by the SAW in EG.

  3. 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.

  4. 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

  5. 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.

  6. 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

  7. Nonequilibrium mesoscopic conductance fluctuations as the origin of 1 /f noise in epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Kalmbach, C.-C.; Ahlers, F. J.; Schurr, J.; Müller, A.; Feilhauer, J.; Kruskopf, M.; Pierz, K.; Hohls, F.; Haug, R. J.

    2016-11-01

    We investigate the 1 /f noise properties of epitaxial graphene devices at low temperatures as a function of temperature, current, and magnetic flux density. At low currents, an exponential decay of the 1 /f noise power spectral density with increasing temperature is observed that indicates mesoscopic conductance fluctuations as the origin of 1 /f noise at temperatures below 50 K. At higher currents, deviations from the typical quadratic current dependence and the exponential temperature dependence occur as a result of nonequilibrium conditions due to current heating. By applying the Kubakaddi theory [S. S. Kubakaddi, Phys. Rev. B 79, 075417 (2009), 10.1103/PhysRevB.79.075417], a model describing the 1 /f noise power spectral density of nonequilibrium mesoscopic conductance fluctuations in epitaxial graphene is developed and used to determine the energy loss rate per carrier. In the regime of Shubnikov-de Haas oscillations, a strong increase of 1 /f noise is observed, which we attribute to an additional conductance fluctuation mechanism due to localized states in quantizing magnetic fields. When the device enters the regime of quantized Hall resistance, the 1 /f noise vanishes. It reappears if the current is increased and quantum Hall breakdown sets in.

  8. Topological phase transition and quantum spin Hall edge states of antimony few layers

    PubMed Central

    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

  9. 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

  10. 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

    2016-09-14

    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.

  11. 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

    2015-12-09

    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.

  12. Effect of in-situ oxygen on the electronic properties of graphene grown by carbon molecular beam epitaxy grown

    SciTech Connect

    Park, Jeongho; Mitchel, W. C.; Back, Tyson C.; Elhamri, Said

    2012-03-26

    We report that graphene grown by molecular beam epitaxy from solid carbon (CMBE) on (0001) SiC in the presence of unintentional oxygen exhibits a small bandgap on the order of tens of meV. The presence of bandgaps is confirmed by temperature dependent Hall effect and resistivity measurements. X-ray photoelectron spectroscopy (XPS) measurements suggest that oxygen incorporates into the SiC substrate in the form of O-Si-C and not into the graphene as graphene oxide or some other species. The effect is independent of the carrier type of the graphene. Temperature dependent transport measurements show the presence of hopping conduction in the resistivity and a concurrent disappearance of the Hall voltage. Interactions between the graphene layers and the oxidized substrate are believed to be responsible for the bandgap.

  13. 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.

  14. CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY: Epitaxial Growth of Graphene on 6H-SiC (0001) by Thermal Annealing

    NASA Astrophysics Data System (ADS)

    Tang, Jun; Liu, Zhong-Liang; Kang, Chao-Yang; Pan, Hai-Bin; Wei, Shi-Qiang; Xu, Peng-Shou; Gao, Yu-Qiang; Xu, Xian-Gang

    2009-08-01

    An epitaxial graphene (EG) layer is successfully grown on a Si-terminated 6H-SiC (0001) substrate by the method of thermal annealing in an ultrahigh vacuum molecular beam epitaxy chamber. The structure and morphology of the EG sample are characterized by reflection high energy diffraction (RHEED), Raman spectroscopy and atomic force microscopy (AFM). Graphene diffraction streaks can be seen in RHEED. The G and 2D peaks of graphene are clearly observed in the Raman spectrum. The AFM results show that the graphene nominal thickness is about 4-10 layers.

  15. Ultrafast electron dynamics in epitaxial graphene investigated with time- and angle-resolved photoemission spectroscopy

    NASA Astrophysics Data System (ADS)

    Ulstrup, Søren; Johannsen, Jens Christian; Crepaldi, Alberto; Cilento, Federico; Zacchigna, Michele; Cacho, Cephise; Chapman, Richard T.; Springate, Emma; Fromm, Felix; Raidel, Christian; Seyller, Thomas; Parmigiani, Fulvio; Grioni, Marco; Hofmann, Philip

    2015-04-01

    In order to exploit the intriguing optical properties of graphene it is essential to gain a better understanding of the light-matter interaction in the material on ultrashort timescales. Exciting the Dirac fermions with intense ultrafast laser pulses triggers a series of processes involving interactions between electrons, phonons and impurities. Here we study these interactions in epitaxial graphene supported on silicon carbide (semiconducting) and iridium (metallic) substrates using ultrafast time- and angle-resolved photoemission spectroscopy (TR-ARPES) based on high harmonic generation. For the semiconducting substrate we reveal a complex hot carrier dynamics that manifests itself in an elevated electronic temperature and an increase in linewidth of the π band. By analyzing these effects we are able to disentangle electron relaxation channels in graphene. On the metal substrate this hot carrier dynamics is found to be severely perturbed by the presence of the metal, and we find that the electronic system is much harder to heat up than on the semiconductor due to screening of the laser field by the metal.

  16. Dirac fermion heating, current scaling, and direct insulator-quantum Hall transition in multilayer epitaxial graphene.

    PubMed

    Liu, Fan-Hung; Hsu, Chang-Shun; Chuang, Chiashain; Woo, Tak-Pong; Huang, Lung-I; Lo, Shun-Tsung; Fukuyama, Yasuhiro; Yang, Yanfei; Elmquist, Randolph E; Liang, Chi-Te

    2013-08-22

    We have performed magnetotransport measurements on multilayer epitaxial graphene. By increasing the driving current I through our graphene devices while keeping the bath temperature fixed, we are able to study Dirac fermion heating and current scaling in such devices. Using zero-field resistivity as a self thermometer, we are able to determine the effective Dirac fermion temperature (TDF) at various driving currents. At zero field, it is found that TDF ∝ I≈1/2. Such results are consistent with electron heating in conventional two-dimensional systems in the plateau-plateau transition regime. With increasing magnetic field B, we observe an I-independent point in the measured longitudinal resistivity ρxx which is equivalent to the direct insulator-quantum Hall (I-QH) transition characterized by a temperature-independent point in ρxx. Together with recent experimental evidence for direct I-QH transition, our new data suggest that such a transition is a universal effect in graphene, albeit further studies are required to obtain a thorough understanding of such an effect.

  17. 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.

  18. 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.

  19. High performance and bendable few-layered InSe photodetectors with broad spectral response.

    PubMed

    Tamalampudi, Srinivasa Reddy; Lu, Yi-Ying; Kumar U, Rajesh; Sankar, Raman; Liao, Chun-Da; Moorthy B, Karukanara; Cheng, Che-Hsuan; Chou, Fang Cheng; Chen, Yit-Tsong

    2014-05-14

    Two-dimensional crystals with a wealth of exotic dimensional-dependent properties are promising candidates for next-generation ultrathin and flexible optoelectronic devices. For the first time, we demonstrate that few-layered InSe photodetectors, fabricated on both a rigid SiO2/Si substrate and a flexible polyethylene terephthalate (PET) film, are capable of conducting broadband photodetection from the visible to near-infrared region (450-785 nm) with high photoresponsivities of up to 12.3 AW(-1) at 450 nm (on SiO2/Si) and 3.9 AW(-1) at 633 nm (on PET). These photoresponsivities are superior to those of other recently reported two-dimensional (2D) crystal-based (graphene, MoS2, GaS, and GaSe) photodetectors. The InSe devices fabricated on rigid SiO2/Si substrates possess a response time of ∼50 ms and exhibit long-term stability in photoswitching. These InSe devices can also operate on a flexible substrate with or without bending and reveal comparable performance to those devices on SiO2/Si. With these excellent optoelectronic merits, we envision that the nanoscale InSe layers will not only find applications in flexible optoelectronics but also act as an active component to configure versatile 2D heterostructure devices.

  20. 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.

  1. 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.

  2. Approaching the Dirac Point in High-Mobility Multilayer Epitaxial Graphene

    NASA Astrophysics Data System (ADS)

    Orlita, M.; Faugeras, C.; Plochocka, P.; Neugebauer, P.; Martinez, G.; Maude, D. K.; Barra, A.-L.; Sprinkle, M.; Berger, C.; de Heer, W. A.; Potemski, M.

    2008-12-01

    Multilayer epitaxial graphene is investigated using far infrared transmission experiments in the different limits of low magnetic fields and high temperatures. The cyclotron-resonance-like absorption is observed at low temperature in magnetic fields below 50 mT, probing the nearest vicinity of the Dirac point. The carrier mobility is found to exceed 250000cm2/(V·s). In the limit of high temperatures, the well-defined Landau level quantization is observed up to room temperature at magnetic fields below 1 T, a phenomenon unusual in solid state systems. A negligible increase in the width of the cyclotron resonance lines with increasing temperature indicates that no important scattering mechanism is thermally activated.

  3. Synthesis of Few-Layer, Large Area Hexagonal-Boron Nitride by Pulsed Laser Deposition (POSTPRINT)

    DTIC Science & Technology

    2014-09-01

    AFRL-RX-WP-JA-2015-0050 SYNTHESIS OF FEW-LAYER, LARGE AREA HEXAGONAL-BORON NITRIDE BY PULSED LASER DEPOSITION (POSTPRINT) Nicholas R Glavin...2014 4. TITLE AND SUBTITLE SYNTHESIS OF FEW-LAYER, LARGE AREA HEXAGONAL- BORON NITRIDE BY PULSED LASER DEPOSITION (POSTPRINT) 5a. CONTRACT NUMBER...deposition (PLD) has been investigated as a technique for synthesis of ultra-thin, few-layer hexagonal boron nitride (ɦ-BN) thin films on crystalline

  4. The influence of selective chemical doping on clean, low-carrier density SiC epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Chuang, Chiashain; Yang, Yanfei; Huang, Lung-I.; Liang, Chi-Te; Elmquist, Randolph E.; National Institute of of Standards; Technology Collaboration; National Taiwan University, Department of Physics Collaboration

    2015-03-01

    The charge-transfer effect of ambient air on magneto-transport in polymer-free SiC graphene was investigated. Interestingly, adsorption of atmospheric gas molecules on clean epitaxial graphene can reduce the carrier density to near charge neutrality, allowing observation of highly precise v = 2 quantum Hall plateaus. The atmospheric adsorbates were reproducibly removed and pure gases (N2, O2, CO2, H2O) were used to form new individual adsorbates on SiC graphene. Our experimental results (τt/τq ~ 2) support the theoretical predictions for the ratio of transport relaxation time τt to quantum lifetime τq in clean graphene. The analysis of Shubnikov-de Haas oscillations at intermediate doping levels indicates that the carrier scattering is reduced by water and oxygen so as to increase both the classical and quantum mobility. This study points to the key dopant gases in ambient air and also paves the way towards extremely precise quantized Hall resistance standards in epitaxial graphene systems with carrier density tuned by exposure to highly pure gases and vacuum annealing treatment. National Institute of Standard and Technology.

  5. Contribution of the buffer layer to the Raman spectrum of epitaxial graphene on SiC(0001)

    NASA Astrophysics Data System (ADS)

    Fromm, F.; Oliveira, M. H., Jr.; Molina-Sánchez, A.; Hundhausen, M.; Lopes, J. M. J.; Riechert, H.; Wirtz, L.; Seyller, T.

    2013-04-01

    We report a Raman study of the so-called buffer layer with (6\\sqrt 3\\times 6\\sqrt 3)R30^{\\circ } periodicity which forms the intrinsic interface structure between epitaxial graphene and SiC(0001). We show that this interface structure leads to a non-vanishing signal in the Raman spectrum at frequencies in the range of the D- and G-band of graphene and discuss its shape and intensity. Ab initio phonon calculations reveal that these features can be attributed to the vibrational density of states of the buffer layer.

  6. Growth of low-threading-dislocation-density GaN on graphene by hydride vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    He, Shunyu; Xu, Yu; Qi, Lin; Li, Zongyao; Cao, Bing; Wang, Chinhua; Zhang, Jicai; Wang, Jianfeng; Xu, Ke

    2017-03-01

    Recently, gallium nitride (GaN) films grown on graphene have been widely studied. Here, we have grown low-threading-dislocation-density GaN films on graphene by hydride vapor phase epitaxy (HVPE). The full widths at half maximum (FWHMs) of X-ray rocking curves (XRCs) of the GaN films were 276 and 350 arcsec at the 0002 and 10\\bar{1}2 reflections, respectively. This shows that the threading dislocation densities are on the order of magnitude of 108 cm‑2, which is consistent with the results of cathodoluminescence (CL).

  7. Epitaxial graphene growth and shape dynamics on copper: phase-field modeling and experiments.

    PubMed

    Meca, Esteban; Lowengrub, John; Kim, Hokwon; Mattevi, Cecilia; Shenoy, Vivek B

    2013-01-01

    The epitaxial growth of graphene on copper foils is a complex process, influenced by thermodynamic, kinetic, and growth parameters, often leading to diverse island shapes including dendrites, squares, stars, hexagons, butterflies, and lobes. Here, we introduce a phase-field model that provides a unified description of these diverse growth morphologies and compare the model results with new experiments. Our model explicitly accounts for the anisotropies in the energies of growing graphene edges, kinetics of attachment of carbon at the edges, and the crystallinity of the underlying copper substrate (through anisotropy in surface diffusion). We show that anisotropic diffusion has a very important, counterintuitive role in the determination of the shape of islands, and we present a "phase diagram" of growth shapes as a function of growth rate for different copper facets. Our results are shown to be in excellent agreement with growth shapes observed for high symmetry facets such as (111) and (001) as well as for high-index surfaces such as (221) and (310).

  8. 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.

  9. Air-stable few-layer black phosphorus phototransistor for near-infrared detection

    NASA Astrophysics Data System (ADS)

    Na, Junhong; Park, Kichul; Kim, Jin Tae; Choi, Won Kook; Song, Yong-Won

    2017-02-01

    We have demonstrated a few-layer black phosphorus (BP) phototransistor of stable operation in ambient air environment and at near-infrared light (λ = 1550 nm). The air-stable electronic and optoelectronic properties of the few-layer BP phototransistor have been achieved by a proper Al2O3 passivation. The optical identification method and qualitative and quantitative electrical characterizations of the few-layer BP phototransistor in dark state confirmed that the device performance was robust in ambient air, to further chemical treatments, and storage of more than six months. In addition, the low-frequency noise characterizations had revealed that the noise spectral density related to the sensitivity of phototransistor was reduced. Owing to the suppression of interaction between few-layer BP and adsorbates arising from the Al2O3 passivation, a fast rise time of the few-layer BP phototransistor, less than 100 μs, had been observed, demonstrating the intrinsic photoresponse properties of few-layer BP. The low dark current of ˜4 nA at the operation bias and the reasonable responsivity of ˜6 mA W-1 were obtained under the condition lacking adsorbates interactions. Internally, the dark current and responsivity level was tunable by changing the operation bias. Our results are close to the intrinsic properties of the few-layer BP phototransistor, implying that it can be a building block of functioned few-layer BP photodetectors.

  10. 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.

  11. 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).

  12. Structural and electronic properties of epitaxial graphene on SiC(0 0 0 1): a review of growth, characterization, transfer doping and hydrogen intercalation

    NASA Astrophysics Data System (ADS)

    Riedl, C.; Coletti, C.; Starke, U.

    2010-09-01

    Graphene, a monoatomic layer of graphite, hosts a two-dimensional electron gas system with large electron mobilities which makes it a prospective candidate for future carbon nanodevices. Grown epitaxially on silicon carbide (SiC) wafers, large area graphene samples appear feasible and integration in existing device technology can be envisioned. This paper reviews the controlled growth of epitaxial graphene layers on SiC(0 0 0 1) and the manipulation of their electronic structure. We show that epitaxial graphene on SiC grows on top of a carbon interface layer that—although it has a graphite-like atomic structure—does not display the linear π-bands typical for graphene due to a strong covalent bonding to the substrate. Only the second carbon layer on top of this interface acts like monolayer graphene. With a further carbon layer, a graphene bilayer system develops. During the growth of epitaxial graphene on SiC(0 0 0 1) the number of graphene layers can be precisely controlled by monitoring the π-band structure. Experimental fingerprints for in situ growth control could be established. However, due to the influence of the interface layer, epitaxial graphene on SiC(0 0 0 1) is intrinsically n-doped and the layers have a long-range corrugation in their density of states. As a result, the Dirac point energy where the π-bands cross is shifted away from the Fermi energy, so that the ambipolar properties of graphene cannot be exploited. We demonstrate methods to compensate and eliminate this structural and electronic influence of the interface. We show that the band structure of epitaxial graphene on SiC(0 0 0 1) can be precisely tailored by functionalizing the graphene surface with tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) molecules. Charge neutrality can be achieved for mono- and bilayer graphene. On epitaxial bilayer graphene, where a band gap opens due to the asymmetric electric field across the layers imposed by the interface, the magnitude of this band

  13. Van der Waals Epitaxy of Two-Dimensional MoS2-Graphene Heterostructures in Ultrahigh Vacuum.

    PubMed

    Miwa, Jill A; Dendzik, Maciej; Grønborg, Signe S; Bianchi, Marco; Lauritsen, Jeppe V; Hofmann, Philip; Ulstrup, Søren

    2015-06-23

    In this work, we demonstrate direct van der Waals epitaxy of MoS2-graphene heterostructures on a semiconducting silicon carbide (SiC) substrate under ultrahigh vacuum conditions. Angle-resolved photoemission spectroscopy (ARPES) measurements show that the electronic structure of free-standing single-layer (SL) MoS2 is retained in these heterostructures due to the weak van der Waals interaction between adjacent materials. The MoS2 synthesis is based on a reactive physical vapor deposition technique involving Mo evaporation and sulfurization in a H2S atmosphere on a template consisting of epitaxially grown graphene on SiC. Using scanning tunneling microscopy, we study the seeding of Mo on this substrate and the evolution from nanoscale MoS2 islands to SL and bilayer (BL) MoS2 sheets during H2S exposure. Our ARPES measurements of SL and BL MoS2 on graphene reveal the coexistence of the Dirac states of graphene and the expected valence band of MoS2 with the band maximum shifted to the corner of the Brillouin zone at K̅ in the SL limit. We confirm the 2D character of these electronic states via a lack of dispersion with photon energy. The growth of epitaxial MoS2-graphene heterostructures on SiC opens new opportunities for further in situ studies of the fundamental properties of these complex materials, as well as perspectives for implementing them in various device schemes to exploit their many promising electronic and optical properties.

  14. Quantum Conductance Probing of Oxygen Vacancies in SrTiO3 Epitaxial Thin Film using Graphene.

    PubMed

    Kang, Kyeong Tae; Kang, Haeyong; Park, Jeongmin; Suh, Dongseok; Choi, Woo Seok

    2017-03-16

    Quantum Hall conductance in monolayer graphene on an epitaxial SrTiO3 (STO) thin film is studied to understand the role of oxygen vacancies in determining the dielectric properties of STO. As the gate-voltage sweep range is gradually increased in the device, systematic generation and annihilation of oxygen vacancies, evidenced from the hysteretic conductance behavior in the graphene, are observed. Furthermore, based on the experimentally observed linear scaling relation between the effective capacitance and the voltage sweep range, a simple model is constructed to manifest the relationship among the dielectric properties of STO with oxygen vacancies. The inherent quantum Hall conductance in graphene can be considered as a sensitive, robust, and noninvasive probe for understanding the electronic and ionic phenomena in complex transition-metal oxides without impairing the oxide layer underneath.

  15. 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

  16. 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

    2016-02-08

    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.

  17. Synthesis, characterization, and properties of few-layer MoO3.

    PubMed

    Sreedhara, M B; Matte, H S S Ramakrishna; Govindaraj, A; Rao, C N R

    2013-10-01

    Nanosheets of MoO3 that consist of only a few layers have been prepared by using four methods, including the oxidation of MoS2 nanosheets, intercalation with LiBr, and ultrasonication. These nanosheets have been characterized by atomic force microscopy and other techniques. Besides showing a blue-shift of the optical absorption band compared to the bulk sample, few-layer MoO3 exhibits enhanced photocatalytic activity. In combination with a borocarbonitride, few-layer MoO3 shows good performance characteristics as a supercapacitor electrode.

  18. Electronic energy loss spectra from mono-layer to few layers of phosphorene

    NASA Astrophysics Data System (ADS)

    Mohan, Brij; Thakur, Rajesh; Ahluwalia, P. K.

    2016-05-01

    Using first principles calculations, electronic and optical properties of few-layers phosphorene has been investigated. Electronic band structure show a moderate band gap of 0.9 eV in monolayer phosphorene which decreases with increasing number of layers. Optical properties of few-layers of phosphorene in infrared and visible region shows tunability with number of layers. Electron energy loss function has been plotted and huge red shift in plasmonic behaviours is found. These tunable electronic and optical properties of few-layers of phosphorene can be useful for the applications of optoelectronic devices.

  19. 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

  20. Van der Waals epitaxial growth of two-dimensional single-crystalline GaSe domains on graphene

    DOE PAGES

    Li, Xufan; Basile, Leonardo; Huang, Bing; ...

    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

  1. 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.

  2. 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.

  3. 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.

  4. Environmental Instability and Degradation of Single- and Few-Layer WTe2 Nanosheets in Ambient Conditions.

    PubMed

    Ye, Fan; Lee, Jaesung; Hu, Jin; Mao, Zhiqiang; Wei, Jiang; Feng, Philip X-L

    2016-09-08

    Ambient environmental instability and degradation mechanism of single- and few-layer WTe2 are investigated. Oxidation of W and Te atoms appears to be a main reason for degradation. Single-layer samples' Raman signals disappear within 20 min in air. Few-layer WTe2 exhibits saturating degradation behavior: only the top layer WTe2 is oxidized; the degraded layer can protect inner layers from further degradation.

  5. Tuning a Schottky barrier of epitaxial graphene/4H-SiC (0001) by hydrogen intercalation

    SciTech Connect

    Dharmaraj, P.; Justin Jesuraj, P.; Jeganathan, K.

    2016-02-01

    We report the electron transport properties of epitaxial graphene (EG) grown on 4H-SiC (0001) by low energy electron-beam irradiation. As-grown EG (AEG) on SiC interface exhibits rectifying current-voltage characteristics with a low Schottky barrier (SB) of 0.55 ± 0.05 eV and high reverse current leakage. The SB of AEG/SiC junction is extremely impeded by the Fermi level pinning (FLP) above the Dirac point due to charged states at the interface. Nevertheless, a gentle hydrogen intercalation at 900 °C enables the alleviation of both FLP and carrier scattering owing to the saturation of dangling bonds as evidenced by the enhancement of SB (0.75 ± 0.05 eV) and high electron mobility well excess of 6000 cm{sup 2} V{sup −1} s{sup −1}.

  6. Toward epitaxially grown two-dimensional crystal hetero-structures: Single and double MoS2/graphene hetero-structures by chemical vapor depositions

    NASA Astrophysics Data System (ADS)

    Lin, Meng-Yu; Chang, Chung-En; Wang, Cheng-Hung; Su, Chen-Fung; Chen, Chi; Lee, Si-Chen; Lin, Shih-Yen

    2014-08-01

    Uniform large-size MoS2/graphene hetero-structures fabricated directly on sapphire substrates are demonstrated with layer-number controllability by chemical vapor deposition (CVD). The cross-sectional high-resolution transmission electron microscopy (HRTEM) images provide the direct evidences of layer numbers of MoS2/graphene hetero-structures. Photo-excited electron induced Fermi level shift of the graphene channel are observed on the single MoS2/graphene hetero-structure transistors. Furthermore, double hetero-structures of graphene/MoS2/graphene are achieved by CVD fabrication of graphene layers on top of the MoS2, as confirmed by the cross-sectional HRTEM. These results have paved the possibility of epitaxially grown multi-hetero-structures for practical applications.

  7. 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.

  8. 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.

  9. Detecting the local transport properties and the dimensionality of transport of epitaxial graphene by a multi-point probe approach

    NASA Astrophysics Data System (ADS)

    Barreto, Lucas; Perkins, Edward; Johannsen, Jens; Ulstrup, Søren; Fromm, Felix; Raidel, Christian; Seyller, Thomas; Hofmann, Philip

    2013-01-01

    The electronic transport properties of epitaxial monolayer graphene (MLG) and hydrogen-intercalated quasi free-standing bilayer graphene (QFBLG) on SiC(0001) are investigated by micro multi-point probes. Using a probe with 12 contacts, we perform four-point probe measurements with the possibility to effectively vary the contact spacing over more than one order of magnitude, allowing us to establish that the transport is purely two-dimensional. Combined with the carrier density obtained by angle-resolved photoemission spectroscopy, we find the room temperature mobility of MLG to be (870±120) cm2/V s. The transport in QFBLG is also found to be two-dimensional with a mobility of (1600±160) cm2/V s.

  10. Molecular beam epitaxy growth of SrO buffer layers on graphite and graphene for the integration of complex oxides

    SciTech Connect

    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.

  11. Hanle precession in the presence of energy-dependent coupling between localized states and an epitaxial graphene spin channel

    NASA Astrophysics Data System (ADS)

    van den Berg, J. J.; Kaverzin, A.; van Wees, B. J.

    2016-12-01

    Hanle spin precession measurements are a common method to extract the spin transport properties of graphene. In epitaxial graphene on silicon carbide, these measurements show unexpected behavior, due to presumed localized states in the carbon buffer layer that is present between the channel and the substrate. As a consequence, the Hanle curve narrows in its magnetic field dependence and can show an unconventional shape, which has been experimentally observed and modeled in previous studies. Here, we extend the previously developed model by assuming that the localized states are charge traps, that have a power-law distribution of trapping times. Our simulations show that the energy dependence of these trapping times can be extracted from the temperature evolution of the Hanle curve, which was previously observed in experiments. Our extended model gives better insight into what processes play a role when a spin channel is coupled to localized states and their relation to the experimental observations.

  12. Spectroscopic Ellipsometry, Auger and STM Characterization of Epitaxial Graphene grown on 6H-SiC (0001)

    NASA Astrophysics Data System (ADS)

    Nelson, Florence; Diebold, Alain C.; Sandin, Andreas; Dougherty, Dan; Aspnes, Dave; Rowe, Jack

    2012-02-01

    Graphene grown by the thermal decomposition of SiC has become of interest to the semiconductor industry due to its unique, high-mobility electronic structure. The growth is of a more scalable nature when compared to exfoliated flakes produced from the ``scotch tape'' method. The resulting film rests on a ``buffer layer'' separating the graphene from the underlying substrate, which is thought to consist of a mixture of sp^2 and non-sp^2 bonding due to the sp^3 bonding of the SiC substrate. The mobilities of the graphene layer have previously been shown to differ from that of the interface layer. We investigate the difference in the optical response of the two layers using Spectroscopic Ellipsometry and find a red-shift of the ˜4.5 eV absorbance found in graphene due to the exciton-domianted transition at the M point of the Brilloun Zone. The structural characterization of the films are performed through Auger and STM on substrates which were cleaned by CMP and chemical etching methods prior to the epitaxial growth in UHV.

  13. 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.

  14. Atomic-scale imaging of few-layer black phosphorus and its reconstructed edge

    NASA Astrophysics Data System (ADS)

    Lee, Yangjin; Yoon, Jun-Yeong; Scullion, Declan; Jang, Jeongsu; Santos, Elton J. G.; Jeong, Hu Young; Kim, Kwanpyo

    2017-03-01

    Black phosphorus (BP) has recently emerged as an alternative 2D semiconductor owing to its fascinating electronic properties such as tunable bandgap and high charge carrier mobility. The structural investigation of few-layer BP, such as identification of layer thickness and atomic-scale edge structure, is of great importance to fully understand its electronic and optical properties. Here we report atomic-scale analysis of few-layered BP performed by aberration corrected transmission electron microscopy (TEM). We establish the layer-number-dependent atomic resolution imaging of few-layer BP via TEM imaging and image simulations. The structural modification induced by the electron beam leads to revelation of crystalline edge and formation of BP nanoribbons. Atomic resolution imaging of BP clearly shows the reconstructed zigzag (ZZ) edge structures, which is also corroborated by van der Waals first principles calculations on the edge stability. Our study on the precise identification of BP thickness and atomic-resolution imaging of edge structures will lay the groundwork for investigation of few-layer BP, especially BP in nanostructured forms.

  15. Selective Epitaxy of InP on Si and Rectification in Graphene/InP/Si Hybrid Structure.

    PubMed

    Niu, Gang; Capellini, Giovanni; Hatami, Fariba; Di Bartolomeo, Antonio; Niermann, Tore; Hussein, Emad Hameed; Schubert, Markus Andreas; Krause, Hans-Michael; Zaumseil, Peter; Skibitzki, Oliver; Lupina, Grzegorz; Masselink, William Ted; Lehmann, Michael; Xie, Ya-Hong; Schroeder, Thomas

    2016-10-12

    The epitaxial integration of highly heterogeneous material systems with silicon (Si) is a central topic in (opto-)electronics owing to device applications. InP could open new avenues for the realization of novel devices such as high-mobility transistors in next-generation CMOS or efficient lasers in Si photonics circuitry. However, the InP/Si heteroepitaxy is highly challenging due to the lattice (∼8%), thermal expansion mismatch (∼84%), and the different lattice symmetries. Here, we demonstrate the growth of InP nanocrystals showing high structural quality and excellent optoelectronic properties on Si. Our CMOS-compatible innovative approach exploits the selective epitaxy of InP nanocrystals on Si nanometric seeds obtained by the opening of lattice-arranged Si nanotips embedded in a SiO2 matrix. A graphene/InP/Si-tip heterostructure was realized on obtained materials, revealing rectifying behavior and promising photodetection. This work presents a significant advance toward the monolithic integration of graphene/III-V based hybrid devices onto the mainstream Si technology platform.

  16. 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.

  17. Synthesis and electrochemical property of few-layer molybdenum disulfide nanosheets

    NASA Astrophysics Data System (ADS)

    Fu, Yanjue; Wang, Chunrui; Wang, Linlin; Peng, Xia; Wu, Binhe; Sun, Xingqu; Chen, Xiaoshuang

    2016-12-01

    Large-scale few-layer MoS2 nanosheets have been fabricated via a simple hydrothermal route using molybdenum powder as precursors. The as-prepared MoS2 samples were characterized by X-ray powder diffraction (XRD) analysis, transmission electron microscopy (TEM), and Raman and photoluminescence (PL) spectral analyses at room temperature. The results confirm that the as-prepared MoS2 displays a sheet-like morphology with a thickness of few (bi- to tri-) layers. Electrochemical measurements showed that the as-prepared few-layer MoS2 exhibited the highest reversible capacity of 1127 mAh g-1 and a stable reversible capacity of 1057 mAh g-1 after 30 cycles.

  18. Defining the role of humidity in the ambient degradation of few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Walia, Sumeet; Sabri, Ylias; Ahmed, Taimur; Field, Matthew R.; Ramanathan, Rajesh; Arash, Aram; Bhargava, Suresh K.; Sriram, Sharath; Bhaskaran, Madhu; Bansal, Vipul; Balendhran, Sivacarendran

    2017-03-01

    Few-layer black phosphorus (BP) is an emerging material of interest for applications in electronics. However, lack of ambient stability is hampering its incorporation in practical devices as it demands for an inert operating environment. Here, we study the individual effects of key environmental factors, such as temperature, light and humidity on the deterioration of BP. It is shown that humidity on its own does not cause material degradation. In fact, few-layer BP is employed as a recoverable humidity sensor. This study eliminates humidity as an active parameter in BP degradation. Hence, by simply isolating BP from light, its lifetime can be prolonged even in the presence of O2. As such, this study opens the pathway for devising new strategies for the practical implementation of BP.

  19. Few-layer SnSe{sub 2} transistors with high on/off ratios

    SciTech Connect

    Pei, Tengfei; Bao, Lihong Wang, Guocai; Ma, Ruisong; Yang, Haifang; Li, Junjie; Gu, Changzhi; Du, Shixuan; Gao, Hong-jun; Pantelides, Sokrates

    2016-02-01

    We report few-layer SnSe{sub 2} field effect transistors (FETs) with high current on/off ratios. By trying different gate configurations, 300 nm SiO{sub 2} and 70 nm HfO{sub 2} as back gate only and 70 nm HfO{sub 2} as back gate combined with a top capping layer of polymer electrolyte, few-layer SnSe{sub 2} FET with a current on/off ratio of 10{sup 4} can be obtained. This provides a reliable solution for electrically modulating quasi-two-dimensional materials with high electron density (over 10{sup 13} cm{sup −2}) for field-effect transistor applications.

  20. 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

  1. Low-Frequency Interlayer Breathing Modes in Few-Layer Black Phosphorus

    DOE PAGES

    Ling, Xi; Liang, Liangbo; Huang, Shengxi; ...

    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

  2. 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.

  3. 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.

  4. 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.

  5. A hybrid MBE-based growth method for large-area synthesis of stacked hexagonal boron nitride/graphene heterostructures.

    PubMed

    Wofford, Joseph M; Nakhaie, Siamak; Krause, Thilo; Liu, Xianjie; Ramsteiner, Manfred; Hanke, Michael; Riechert, Henning; J Lopes, J Marcelo

    2017-02-27

    Van der Waals heterostructures combining hexagonal boron nitride (h-BN) and graphene offer many potential advantages, but remain difficult to produce as continuous films over large areas. In particular, the growth of h-BN on graphene has proven to be challenging due to the inertness of the graphene surface. Here we exploit a scalable molecular beam epitaxy based method to allow both the h-BN and graphene to form in a stacked heterostructure in the favorable growth environment provided by a Ni(111) substrate. This involves first saturating a Ni film on MgO(111) with C, growing h-BN on the exposed metal surface, and precipitating the C back to the h-BN/Ni interface to form graphene. The resulting laterally continuous heterostructure is composed of a top layer of few-layer thick h-BN on an intermediate few-layer thick graphene, lying on top of Ni/MgO(111). Examinations by synchrotron-based grazing incidence diffraction, X-ray photoemission spectroscopy, and UV-Raman spectroscopy reveal that while the h-BN is relaxed, the lattice constant of graphene is significantly reduced, likely due to nitrogen doping. These results illustrate a different pathway for the production of h-BN/graphene heterostructures, and open a new perspective for the large-area preparation of heterosystems combining graphene and other 2D or 3D materials.

  6. A hybrid MBE-based growth method for large-area synthesis of stacked hexagonal boron nitride/graphene heterostructures

    PubMed Central

    Wofford, Joseph M.; Nakhaie, Siamak; Krause, Thilo; Liu, Xianjie; Ramsteiner, Manfred; Hanke, Michael; Riechert, Henning; J. Lopes, J. Marcelo

    2017-01-01

    Van der Waals heterostructures combining hexagonal boron nitride (h-BN) and graphene offer many potential advantages, but remain difficult to produce as continuous films over large areas. In particular, the growth of h-BN on graphene has proven to be challenging due to the inertness of the graphene surface. Here we exploit a scalable molecular beam epitaxy based method to allow both the h-BN and graphene to form in a stacked heterostructure in the favorable growth environment provided by a Ni(111) substrate. This involves first saturating a Ni film on MgO(111) with C, growing h-BN on the exposed metal surface, and precipitating the C back to the h-BN/Ni interface to form graphene. The resulting laterally continuous heterostructure is composed of a top layer of few-layer thick h-BN on an intermediate few-layer thick graphene, lying on top of Ni/MgO(111). Examinations by synchrotron-based grazing incidence diffraction, X-ray photoemission spectroscopy, and UV-Raman spectroscopy reveal that while the h-BN is relaxed, the lattice constant of graphene is significantly reduced, likely due to nitrogen doping. These results illustrate a different pathway for the production of h-BN/graphene heterostructures, and open a new perspective for the large-area preparation of heterosystems combining graphene and other 2D or 3D materials. PMID:28240323

  7. Fast and broadband photoresponse of few-layer black phosphorus field-effect transistors.

    PubMed

    Buscema, Michele; Groenendijk, Dirk J; Blanter, Sofya I; Steele, Gary A; van der Zant, Herre S J; Castellanos-Gomez, Andres

    2014-06-11

    Few-layer black phosphorus, a new elemental two-dimensional (2D) material recently isolated by mechanical exfoliation, is a high-mobility layered semiconductor with a direct bandgap that is predicted to strongly depend on the number of layers, from 0.35 eV (bulk) to 2.0 eV (single layer). Therefore, black phosphorus is an appealing candidate for tunable photodetection from the visible to the infrared part of the spectrum. We study the photoresponse of field-effect transistors (FETs) made of few-layer black phosphorus (3-8 nm thick), as a function of excitation wavelength, power, and frequency. In the dark state, the black phosphorus FETs can be tuned both in hole and electron doping regimes allowing for ambipolar operation. We measure mobilities in the order of 100 cm(2)/V s and a current ON/OFF ratio larger than 10(3). Upon illumination, the black phosphorus transistors show a response to excitation wavelengths from the visible region up to 940 nm and a rise time of about 1 ms, demonstrating broadband and fast detection. The responsivity reaches 4.8 mA/W, and it could be drastically enhanced by engineering a detector based on a PN junction. The ambipolar behavior coupled to the fast and broadband photodetection make few-layer black phosphorus a promising 2D material for photodetection across the visible and near-infrared part of the electromagnetic spectrum.

  8. 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.

  9. New Method to Determine the Schottky Barrier in Few-Layer Black Phosphorus Metal Contacts.

    PubMed

    Lee, Su Yeong; Yun, Won Seok; Lee, J D

    2017-03-01

    Schottky barrier height and carrier polarity are seminal concepts for a practical device application of the interface between semiconductor and metal electrode. Investigation of those concepts is usually made by a conventional method such as the Schottky-Mott rule, incorporating the metal work function and semiconductor electron affinity, or the Fermi level pinning effect, resulting from the metal-induced gap states. Both manners are, however, basically applied to the bulk semiconductor metal contacts. To explore few-layer black phosphorus metal contacts far from the realm of bulk, we propose a new method to determine the Schottky barrier by scrutinizing the layer-by-layer phosphorus electronic structure from the first-principles calculation combined with the state-of-the-art band unfolding technique. In this study, using the new method, we calculate the Schottky barrier height and determine the contact polarity of Ti, Sc, and Al metal contacts to few-layer (mono-, bi-, tri-, and quadlayer) black phosphorus. This gives a significant physical insight toward the utmost layer-by-layer manipulation of electronic properties of few-layer semiconductor metal contacts.

  10. Gate-Tunable Giant Stark Effect in Few-Layer Black Phosphorus.

    PubMed

    Liu, Yanpeng; Qiu, Zhizhan; Carvalho, Alexandra; Bao, Yang; Xu, Hai; Tan, Sherman J R; Liu, Wei; Castro Neto, A H; Loh, Kian Ping; Lu, Jiong

    2017-03-08

    Two-dimensional black phosphorus (BP) has sparked enormous research interest due to its high carrier mobility, layer-dependent direct bandgap and outstanding in-plane anisotropic properties. BP is one of the few two-dimensional materials where it is possible to tune the bandgap over a wide energy range from the visible up to the infrared. In this article, we report the observation of a giant Stark effect in electrostatically gated few-layer BP. Using low-temperature scanning tunnelling microscopy, we observed that in few-layer BP, when electrons are injected, a monotonic reduction of the bandgap occurs. The injected electrons compensate the existing defect-induced holes and achieve up to 35.5% bandgap modulation in the light-doping regime. When probed by tunnelling spectroscopy, the local density of states in few-layer BP shows characteristic resonance features arising from layer-dependent sub-band structures due to quantum confinement effects. The demonstration of an electrical gate-controlled giant Stark effect in BP paves the way to designing electro-optic modulators and photodetector devices that can be operated in a wide electromagnetic spectral range.

  11. Intrinsic transport of h-BN encapsulated few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Arefe, Ghidewon; Kim, Young Duck; Chenet, Daniel; Cui, Xu; Chang, Damien; Hone, James

    2015-03-01

    Few-layer black phosphorus (BP) is an exciting two-dimensional material with ambipolar behavior, large on/off ratio, and high mobility with a direct bandgap. The anisotropic atomic nature of black phosphorus exhibits unique angle dependent electronic and optical features. One of the primary difficulties in fabricating few-layer BP devices to study transport is the reactive nature of the material in ambient conditions as it degrades in the presence of air and moisture. In order to characterize the intrinsic physical properties of BP, we fabricated few-layer BP flakes that are fully encapsulated by hexagonal boron nitride (h-BN) with a clean stacking technique. We also characterized the electrical transport of h-BN encapsulated BP devices that show greatly improved environmental stability and high mobility at low temperature due to the suppression of extrinsic scattering effects such as charge impurities, surface polar optical phonons, and absorbents from air. H-BN encapsulated BP devices will be an essential platform for the observation of new physics from BP and realization of BP based advanced opto-electronic application devices body.

  12. Quantum-confinement and Structural Anisotropy result in Electrically-Tunable Dirac Cone in Few-layer Black Phosphorous.

    PubMed

    Dolui, Kapildeb; Quek, Su Ying

    2015-07-01

    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, E ext, 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 E ext 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.

  13. Generation of Anisotropic Massless Dirac Fermions and Asymmetric Klein Tunneling in Few-Layer Black Phosphorus Superlattices.

    PubMed

    Li, Zhenglu; Cao, Ting; Wu, Meng; Louie, Steven G

    2017-03-02

    Artificial lattices have been employed in a broad range of two-dimensional systems, including those with electrons, atoms, and photons, in the quest for massless Dirac fermions with high flexibility and controllability. Establishing triangular or hexagonal symmetry, from periodically patterned molecule assembly or electrostatic gating as well as from moiré pattern induced by substrate, has produced electronic states with linear dispersions from two-dimensional electron gas (2DEG) residing in semiconductors, metals, and graphene. Different from the commonly studied isotropic host systems, here we demonstrate that massless Dirac fermions with tunable anisotropic characteristics can, in general, be generated in highly anisotropic 2DEG under slowly varying external periodic potentials. In the case of patterned few-layer black phosphorus superlattices, the new chiral quasiparticles exist exclusively in certain isolated energy window and inherit the strong anisotropic properties of pristine black phosphorus. These states exhibit asymmetric Klein tunneling, in which the transmission probability of the wave packets with normal incidence is no longer unity and can be tuned and controlled. In general, the direction of wave packet incidence for perfect transmission and that of the normal incidence are different, and the difference can reach more than 50° under an appropriate barrier orientation in black phosphorus superlattices. Our findings provide insight into the understanding and possible utilization of these novel emergent chiral quasiparticles.

  14. Sub-nanometer-gap tip-enhanced nanoimaging of few-layer MoS2(Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Voronine, Dmitri V.

    2016-10-01

    Two-dimensional (2D) materials beyond graphene such as transition metal dichalcogenides (TMDs) have unique mechanical, optical and electronic properties with promising applications in flexible devices, catalysis and sensing. Optical imaging of TMDs using photoluminescence and Raman spectroscopy can reveal the effects of structure, strain, doping, defects, edge states, grain boundaries and surface functionalization. However, Raman signals are inherently weak and so far have been limited in spatial resolution in TMDs to a few hundred nanometres which is much larger than the intrinsic scale of these effects. Here we overcome the diffraction limit by using resonant tip-enhanced Raman scattering (TERS) of few-layer MoS2, and obtain nanoscale optical images with 20 nm spatial resolution. This becomes possible due to electric field enhancement in an optimized subnanometre-gap resonant tip-substrate configuration. We investigate the limits of signal enhancement by varying the tip-sample gap with sub-Angstrom precision and observe a quantum quenching behavior, as well as a Schottky-Ohmic transition, for subnanometre gaps, which enable surface mapping based on this new contrast mechanism. This quantum regime of plasmonic gap-mode enhancement with a few nanometre thick MoS2 junction may be used for designing new quantum optoelectronic devices and sensors.

  15. 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.

  16. High-Mobility Transport Anisotropy in Few-Layer MoO3 and Its Origin.

    PubMed

    Zhang, Wei-Bing; Qu, Qian; Lai, Kang

    2017-01-18

    The novel two-dimensional semiconductors with high carrier mobility and excellent stability are essential to the next-generation high-speed and low-power nanoelectronic devices. Because of the natural abundance, intrinsic gap, and chemical stability, metal oxides were also recently suggested as potential candidates for electronic materials. However, their carrier mobilities are typically on the order of tens of square centimeters per volt per second, much lower than that for commonly used silicon. By using first-principles calculations and deformation potential theory, we have predicted few-layer MoO3 as chemically stable wide-band-gap semiconductors with a considerably high acoustic-phonon-limited carrier mobility above 3000 cm(2) V(-1) s(-1), which makes them promising candidates for both electron- and hole-transport applications. Moreover, we also find a large in-plane anisotropy of the carrier mobility with a ratio of about 20-30 in this unusual system. Further analysis indicates that, because of the unique charge density distribution of whole valence electrons and the states near the band edge, both the elastic modulus and deformation potential are strongly directionally dependent. Also, the predicted high-mobility transport anisotropy of few-layer MoO3 can be attributed to the synergistic effect of the anisotropy of the elastic modulus and deformation potential. Our results not only give an insightful understanding for the high carrier mobility observed in few-layer MoO3 systems but also reveal the importance of the carrier-transport direction to the device performance.

  17. 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.

  18. 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.

  19. Third order nonlinear optical response exhibited by mono- and few-layers of WS2

    DOE PAGES

    Torres-Torres, Carlos; Perea-López, Néstor; Elías, Ana Laura; ...

    2016-04-13

    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. In conclusion, we envision applications for developing all-optical bidimensional nonlinear optical devices.

  20. Strain-engineering the anisotropic electrical conductance of few-layer black phosphorus.

    PubMed

    Fei, Ruixiang; Yang, Li

    2014-05-14

    Newly fabricated few-layer black phosphorus and its monolayer structure, phosphorene, are expected to be promising for electronic and optical applications because of their finite direct band gaps and sizable but anisotropic electronic mobility. By first-principles simulations, we show that this unique anisotropic free-carrier mobility can be controlled by using simple strain conditions. With the appropriate biaxial or uniaxial strain (4-6%), we can rotate the preferred conducting direction by 90°. This will be useful for exploring unusual quantum Hall effects and exotic electronic and mechanical applications based on phosphorene.

  1. Strain-Engineering the Anisotropic Electrical Conductance of Few-Layer Black Phosphorus

    NASA Astrophysics Data System (ADS)

    Fei, Ruixiang; Yang, Li

    2014-05-01

    Newly fabricated monolayer phosphorene and its few-layer structures are expected to be promising for electronic and optical applications because of their finite direct band gaps and sizable but anisotropic electronic mobility. By first-principles simulations, we show that this unique anisotropic conductance can be controlled by using simple strain conditions. With the appropriate biaxial or uniaxial strain, we can rotate the preferred conducting direction by 90 degrees. This will be of useful for exploring quantum Hall effects, and exotic electronic and mechanical applications based on phosphorene.

  2. Tunneling transport of mono- and few-layers magnetic van der Waals MnPS3

    NASA Astrophysics Data System (ADS)

    Lee, Sungmin; Choi, Ki-Young; Lee, Sangik; Park, Bae Ho; Park, Je-Geun

    2016-08-01

    We have investigated the tunneling transport of mono- and few-layers of MnPS3 by using conductive atomic force microscopy. Due to the band alignment of indium tin oxide/MnPS3/Pt-Ir tip junction, the key features of both Schottky junction and Fowler-Nordheim tunneling (FNT) were observed for all the samples with varying thickness. Using the FNT model and assuming the effective electron mass (0.5 me) of MnPS3, we estimate the tunneling barrier height to be 1.31 eV and the dielectric breakdown strength as 5.41 MV/cm.

  3. New First Order Raman-active Modes in Few Layered Transition Metal Dichalcogenides

    PubMed Central

    Terrones, H.; Corro, E. Del; Feng, S.; Poumirol, J. M.; Rhodes, D.; Smirnov, D.; Pradhan, N. R.; Lin, Z.; Nguyen, M. A. T.; Elías, A. L.; Mallouk, T. E.; Balicas, L.; Pimenta, M. A.; Terrones, M.

    2014-01-01

    Although the main Raman features of semiconducting transition metal dichalcogenides are well known for the monolayer and bulk, there are important differences exhibited by few layered systems which have not been fully addressed. WSe2 samples were synthesized and ab-initio calculations carried out. We calculated phonon dispersions and Raman-active modes in layered systems: WSe2, MoSe2, WS2 and MoS2 ranging from monolayers to five-layers and the bulk. First, we confirmed that as the number of layers increase, the E′, E″ and E2g modes shift to lower frequencies, and the A′1 and A1g modes shift to higher frequencies. Second, new high frequency first order A′1 and A1g modes appear, explaining recently reported experimental data for WSe2, MoSe2 and MoS2. Third, splitting of modes around A′1 and A1g is found which explains those observed in MoSe2. Finally, exterior and interior layers possess different vibrational frequencies. Therefore, it is now possible to precisely identify few-layered STMD. PMID:24572993

  4. 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

    2014-08-28

    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.

  5. Ionic-Liquid Gated Few-layer MoS2 Field-Effect Transistors

    NASA Astrophysics Data System (ADS)

    Perera, Meeghage; Lin, Ming-Wei; Chuang, Hsun-Jen; Chamlagain, Bhim; Wang, Chongyu; Tan, Xuebin; Cheng, Mark Ming-Cheng; Zhou, Zhixian

    2013-03-01

    We report the electrical characterization of ionic-liquid-gated bilayer and few-layer MoS2 field-effect transistors. The extrinsic mobility of our ionic-liquid-gated devices exceeds 70 cm2V-1S-1 at 250 K, which is 1-2 orders of magnitude higher than that measured in the Si back-gate configuration (without ionic liquid). These devices also show ambipolar behavior with a high ON-OFF current ratio of > 107 for electrons and > 106 for holes, and a near ideal subthreshold swing (SS) of ~ 50 mV/decade at 250 K for the electron channel. More significantly, we show that the mobility increases from ~ 100 cm2V-1S-1 at 180 K to ~ 220 cm2V-1S-1 at 77K as the temperature decreases following a μ ~ T-γ dependence with γ ~ 1, indicating that the intrinsic phonon-limited mobility can be achieved in few-layer MoS2 FETs. We attribute the enhanced device performance to the drastic reduction of the Schottky barrier width (thus higher tunneling efficiency) via highly efficient band bending at the MoS2/metal interface afforded by the extremely large electrical double layer capacitance of the ionic liquid. This work was supported by NSF (No. ECCS-1128297).

  6. 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.

  7. 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.

  8. 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.

  9. Observation of the Intrinsic Bandgap Behavior in As-Grown Epitaxial Twisted Graphene (Postprint)

    DTIC Science & Technology

    2015-01-06

    sp3 C C bonding carbon. Discussion The sp3 C C interlayer bonding has been experimentally observed in graphite and carbon nanotubes modified by...carbon nanotube fibers. ACS Nano 5, 1921 1927 (2011). 19. Hass, J. et al. Structural properties of the multilayer graphene/4H-SiC ð0001Þ system as...nanoribborns. Phys. Rev. Lett. 104, 056801 (2010). 24. Shimizu, T. et al. Large intrinsic energy bandgaps in annealed nanotube -derived graphene

  10. Growth and characterization of Al2O3 films on fluorine functionalized epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Robinson, Zachary R.; Jernigan, Glenn G.; Wheeler, Virginia D.; Hernández, Sandra C.; Eddy, Charles R.; Mowll, Tyler R.; Ong, Eng Wen; Ventrice, Carl A.; Geisler, Heike; Pletikosic, Ivo; Yang, Hongbo; Valla, Tonica

    2016-08-01

    Intelligent engineering of graphene-based electronic devices on SiC(0001) requires a better understanding of processes used to deposit gate-dielectric materials on graphene. Recently, Al2O3 dielectrics have been shown to form conformal, pinhole-free thin films by functionalizing the top surface of the graphene with fluorine prior to atomic layer deposition (ALD) of the Al2O3 using a trimethylaluminum (TMA) precursor. In this work, the functionalization and ALD-precursor adsorption processes have been studied with angle-resolved photoelectron spectroscopy, low energy electron diffraction, and X-ray photoelectron spectroscopy. It has been found that the functionalization process has a negligible effect on the electronic structure of the graphene, and that it results in a twofold increase in the adsorption of the ALD-precursor. In situ TMA-dosing and XPS studies were also performed on three different Si(100) substrates that were terminated with H, OH, or dangling Si-bonds. This dosing experiment revealed that OH is required for TMA adsorption. Based on those data along with supportive in situ measurements that showed F-functionalization increases the amount of oxygen (in the form of adsorbed H2O) on the surface of the graphene, a model for TMA-adsorption on graphene is proposed that is based on a reaction of a TMA molecule with OH.

  11. 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

  12. Frenkel-like Wannier-Mott excitons in few-layer Pb I2

    NASA Astrophysics Data System (ADS)

    Toulouse, Alexis S.; Isaacoff, Benjamin P.; Shi, Guangsha; Matuchová, Marie; Kioupakis, Emmanouil; Merlin, Roberto

    2015-04-01

    Optical measurements and first-principles calculations of the band structure and exciton states in direct-gap bulk and few-layer Pb I2 indicate that the n =1 exciton is Frenkel-like in nature in that its energy exhibits a weak dependence on thickness down to atomic-length scales. Results reveal large increases in the gap and exciton binding energy with a decreasing number of layers and a transition of the fundamental gap, which becomes indirect for one and two monolayers. Calculated values are in reasonable agreement with a particle-in-a-box model relying on the Wannier-Mott theory of exciton formation. General arguments and existing data suggest that the Frenkel-like character of the lowest exciton is a universal feature of wide-gap layered semiconductors whose effective masses and dielectric constants give bulk Bohr radii that are on the order of the layer spacing.

  13. Environmental effects in mechanical properties of few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Moreno-Moreno, Miriam; Lopez-Polin, Guillermo; Castellanos-Gomez, Andres; Gomez-Navarro, Cristina; Gomez-Herrero, Julio

    2016-09-01

    We report on the mechanical properties of few-layer black phosphorus (BP) nanosheets, in high vacuum and as a function of time of exposure to atmospheric conditions. BP flakes with thicknesses ranging from 4 to 30 nm suspended over circular holes are characterized by nanoindentations using an atomic force microscope tip. From measurements in high vacuum an elastic modulus of 46 ± 10 GPa and breaking strength of 2.4 ± 1 GPa are estimated. Both magnitudes are independent of the thickness of the flakes. Our results show that the exposure to air has substantial influence in the mechanical response of flakes thinner than 6 nm but small effects on thicker flakes.

  14. Quasiparticle energies, excitons, and optical spectra of few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Tran, Vy; Fei, Ruixiang; Yang, Li

    2015-12-01

    We report first-principles GW-Bethe-Salpeter-equation (BSE) studies of excited-state properties of few-layer black phosphorus (BP) (phosphorene). With improved GW computational methods, we obtained converged quasiparticle band gaps and optical absorption spectra by the single-shot (G0W0) procedure. Moreover, we reveal fine structures of anisotropic excitons, including the series of one-dimensional like wave functions, spin singlet-triplet splitting, and electron-hole binding energy spectra by solving BSE. An effective-mass model is employed to describe these electron-hole pairs, shedding light on estimating the exciton binding energy of anisotropic two-dimensional semiconductors without expensive ab initio simulations. Finally, the anisotropic optical response of BP is explained by using optical selection rules based on the projected single-particle density of states at band edges.

  15. 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.

  16. Few-layer black phosphorus based saturable absorber mirror for pulsed solid-state lasers.

    PubMed

    Ma, Jie; Lu, Shunbin; Guo, Zhinan; Xu, Xiaodong; Zhang, Han; Tang, Dingyuan; Fan, Dianyuan

    2015-08-24

    We experimentally demonstrated that few-layer black phosphorus (BP) could be used as an optical modulator for solid-state lasers to generate short laser pulses. The BP flakes were fabricated by the liquid phase exfoliation method and drop-casted on a high-reflection mirror to form a BP-based saturable absorber mirror (BP-SAM). Stable Q-switched pulses with a pulse width of 620 ns at the wavelength of 1046 nm were obtained in a Yb:CaYAlO(4) (Yb:CYA) laser with the BP-SAM. The generated pulse train has a repetition rate of 113.6 kHz and an average output power of 37 mW. Our results show that the BP-SAMs could have excellent prospective for ultrafast photonics applications.

  17. 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.

  18. Transitions metal dichalcogenides: Growth, fermiology studies, and few-layered transport properties

    NASA Astrophysics Data System (ADS)

    Rhodes, Daniel

    Transition metal dichalcogenides (TMDs or TMDCs) have garnered much interest recently due to their weakly layered structures, allowing for mechanical exfoliation down to a single atomic layer. As such, it is pertinent to re-examine the bulk properties of these materials in order to completely understand and predict what is happening in the few-layered limit. A large majority of these systems were first investigated in the 1950s and 1960s. As such, many of the current growth methods rely on these reports, making new growth techniques for lowering defects of importance as well. In this thesis, both topics are taken into consideration and discussed, though the latter remains to be investigated in much more detail and should be the work of future research efforts. (Abstract shortened by ProQuest.).

  19. 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.

  20. Controlled Gas Exfoliation of Boron Nitride into Few-Layered Nanosheets

    SciTech Connect

    Zhu, Wenshuai; Gao, Xiang; Li, Qian; Li, Hongping; Chao, Yanhong; Li, Meijun; Mahurin, Shannon M.; Li, Huaming; Zhu, Huiyuan; Dai, Sheng

    2016-07-22

    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) via 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 high a mass yield of 16~20%. N2 sorption and desorption isotherms show that the BNNS-10 exhibited a much higher specific surface area of 278 m2/g–1 than that of bulk BN (10 m2/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. In conclusion, 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 with high yeilds.

  1. Controlled Gas Exfoliation of Boron Nitride into Few-Layered Nanosheets

    DOE PAGES

    Zhu, Wenshuai; Gao, Xiang; Li, Qian; ...

    2016-07-22

    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) via 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 high amore » mass yield of 16~20%. N2 sorption and desorption isotherms show that the BNNS-10 exhibited a much higher specific surface area of 278 m2/g–1 than that of bulk BN (10 m2/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. In conclusion, 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 with high yeilds.« less

  2. Quasi-freestanding epitaxial graphene transistor with silicon nitride top gate

    NASA Astrophysics Data System (ADS)

    Wehrfritz, Peter; Fromm, Felix; Malzer, Stefan; Seyller, Thomas

    2014-07-01

    We report on top-gated field effect devices built from quasi-freestanding monolayer graphene (QFMLG) on 6H-SiC(0001) in combination with a silicon nitride (SiN) gate dielectric. SiN was grown by plasma enhanced chemical vapour deposition. The composition of the dielectric was investigated by x-ray photoelectron spectroscopy (XPS). Spectroscopic and electrical characterization of the graphene layers were done by XPS, Raman spectroscopy and Hall effect measurements before and after SiN deposition. In contrast to previous reports on SiN/graphene, we observe that our dielectric layer induces strong n-type doping. With such a gate insulator, the neutrality level of the QFMLG could be accessed by an appropriate gate voltage.

  3. Micro-Raman and micro-transmission imaging of epitaxial graphene grown on the Si and C faces of 6H-SiC

    PubMed Central

    2011-01-01

    Micro-Raman and micro-transmission imaging experiments have been done on epitaxial graphene grown on the C- and Si-faces of on-axis 6H-SiC substrates. On the C-face it is shown that the SiC sublimation process results in the growth of long and isolated graphene ribbons (up to 600 μm) that are strain-relaxed and lightly p-type doped. In this case, combining the results of micro-Raman spectroscopy with micro-transmission measurements, we were able to ascertain that uniform monolayer ribbons were grown and found also Bernal stacked and misoriented bilayer ribbons. On the Si-face, the situation is completely different. A full graphene coverage of the SiC surface is achieved but anisotropic growth still occurs, because of the step-bunched SiC surface reconstruction. While in the middle of reconstructed terraces thin graphene stacks (up to 5 layers) are grown, thicker graphene stripes appear at step edges. In both the cases, the strong interaction between the graphene layers and the underlying SiC substrate induces a high compressive thermal strain and n-type doping. PMID:21801347

  4. Effects of aluminum on epitaxial graphene grown on C-face SiC

    SciTech Connect

    Xia, Chao Johansson, Leif I.; Hultman, Lars; Virojanadara, Chariya; Niu, Yuran

    2015-05-21

    The effects of Al layers deposited on graphene grown on C-face SiC substrates are investigated before and after subsequent annealing using low energy electron diffraction (LEED), photoelectron spectroscopy, and angle resolved photoemission. As-deposited layers appear inert. Annealing at a temperature of about 400 °C initiates migration of Al through the graphene into the graphene/SiC interface. Further annealing at temperatures from 500 °C to 700 °C induces formation of an ordered compound, producing a two domain √7× √7R19° LEED pattern and significant changes in the core level spectra that suggest formation of an Al-Si-C compound. Decomposition of this compound starts after annealing at 800 °C, and at 1000 °C, Al is no longer possible to detect at the surface. On Si-face graphene, deposited Al layers did not form such an Al-Si-C compound, and Al was still detectable after annealing above 1000 °C.

  5. Two-photon photoemission from image-potential states of epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Gugel, Dieter; Niesner, Daniel; Eickhoff, Christian; Wagner, Stefanie; Weinelt, Martin; Fauster, Thomas

    2015-12-01

    Using angle- and time-resolved two-photon photoelectron spectroscopy we observe a single series of image-potential states of graphene on monolayer (MLG) and bilayer graphene (BLG) on SiC(0001). The first image-potential state on MLG (BLG) has a binding energy of 0.93 eV (0.84 eV). Lifetimes of the first three image-potential states of MLG are 9, 44 and 110 fs. On hydrogen-intercalated, quasi-freestanding graphene no unoccupied states are observed. We attribute this to the absence of occupied initial states for direct transitions into image-potential states at photon energies below the work function used in two-photon photoemission. The work function varies between 4.14 and 4.79 eV, but the vacuum level stays ∼4.5 eV above the Dirac point for all surfaces studied. This finding suggests that direct excitation of image-potential states cannot be achieved by doping and the electron dynamics for free-standing graphene is not accessible by two-photon photoemission using photon energies below the work function.

  6. Insulator-quantum Hall transition and Dirac fermion heating in low-carrier-density monolayer epitaxial graphene

    NASA Astrophysics Data System (ADS)

    Huang, Lung-I.; Yang, Yanfei; Elmquist, Randolph; Newell, David; Liang, Chi-Te

    2014-03-01

    We present magneto-transport measurements on ungated, low-carrier-density epitaxial graphene Hall devices at low temperatures T. At T = 4.25 K the carrier density and mobility of one device are 1.38x1011 cm-2 and 6500 cm2V-1s-1, respectively. At low magnetic fields B, this device shows insulating behavior in the sense that the measured resistivity ρxx increases with decreasing T. A highly developed quantum Hall (QH) resistivity plateau ρxy ~h/2e2 corresponding to a Landau-level filling factor ν = 2 in monolayer graphene can be observed at magnetic fields B >= 1.5 T. Between the low-field insulator regime and the ν = 2 QH state we observe a T-independent point in ρxx which corresponds to the insulator-quantum Hall (I-QH) transition. This transition, like those in semiconductor-based two-dimensional (2D) systems, can be also observed by increasing the driving current I at fixed ambient temperature. However, the measured ρxx at the I-QH transition is close to h/4e2 , rather than h/2e2 as expected by conventional I-QH theory. Furthermore, ρxx is substantially higher than ρxy at the crossing point. By using the zero-field resistivity and weak localization effect as two independent thermometers to determine effective Dirac fermion temperature (TDF) at various I, we find that TDF ~ I 0 . 5, consistent with those obtained in various 2D systems. NIST and National Taiwan University.

  7. Probing the Role of Interlayer Coupling and Coulomb Interactions on Electronic Structure in Few-Layer MoSe 2 Nanostructures

    DOE PAGES

    Bradley, Aaron J.; M. Ugeda, Miguel; da Jornada, Felipe H.; ...

    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

  8. 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.

  9. Wide-range temperature dependence of epitaxial graphene growth on 4H-SiC (0 0 0 -1): A study of ridge structures formation dynamics associated with temperature

    NASA Astrophysics Data System (ADS)

    Ushio, Shoji; Yoshii, Arata; Tamai, Naoto; Ohtani, Noboru; Kaneko, Tadaaki

    2011-03-01

    We investigated the temperature dependence (1200-2100 °C) of morphological features in the growth of epitaxial graphene on 4H-SiC (0 0 0 -1) C-face in ultra high vacuum (UHV). As a SiC initial surface preparation technique prior to the epitaxial graphene growth, Si-vapor etching was conducted to ensure a uniform step-terrace structure at 4H-SiC (0 0 0 -1) consisting of 0.5 nm step height. The epitaxial graphene growth has been performed in our original experimental setup with a TaC container inside an all metal chamber with radiative heating system, which allows to increase the graphitization temperature up to 2100 °C in UHV. The surface morphology of the epitaxially grown multilayer graphene, characterized by atomic force microscopy (AFM), exhibited a drastic change as a function of temperature. Ridge structures of the topmost graphene layer(s) appeared above 1600 °C, where the well-ordered step-terrace initial structure of the SiC vanished. In this high temperature range, the network of the ridges spread over the surface with an increase in temperature. Raman topography mapping for characterizing site dependent 2D-spectrum implies that rotational stacking disorder of graphene layers is accompanied by relatively small ridge network on the surface in a specific temperature range.

  10. Rapid and Nondestructive Identification of Polytypism and Stacking Sequences in Few-Layer Molybdenum Diselenide by Raman Spectroscopy

    DOE PAGES

    Lu, Xin; Utama, M. Iqbal Bakti; Lin, Junhao; ...

    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.

  11. 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

  12. Spectral and total temperature-dependent emissivities of few-layer structures on a metallic substrate.

    PubMed

    Blandre, Etienne; Chapuis, Pierre-Olivier; Vaillon, Rodolphe

    2016-01-25

    We investigate the thermal radiative emission of few-layer structures deposited on a metallic substrate and its dependence on temperature with the Fluctuational Electrodynamics approach. We highlight the impact of the variations of the optical properties of metallic layers on their temperature-dependent emissivity. Fabry-Pérot spectral selection involving at most two transparent layers and one thin reflective layer leads to well-defined peaks and to the amplification of the substrate emission. For a single Fabry-Pérot layer on a reflective substrate, an optimal thickness that maximizes the emissivity of the structure can be determined at each temperature. A thin lossy layer deposited on the previous structure can enhance interference phenomena, and the analysis of the participation of each layer to the emission shows that the thin layer is the main source of emission. Eventually, we investigate a system with two Fabry-Pérot layers and a metallic thin layer, and we show that an optimal architecture can be found. The total hemispherical emissivity can be increased by one order of magnitude compared to the substrate emissivity.

  13. Thermal transport across few-layer boron nitride encased by silica

    SciTech Connect

    Ni, Yuxiang; Dumitricǎ, Traian; Jiang, Jiechao; Meletis, Efstathios

    2015-07-20

    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 (SiO{sub 2}). We report large interfacial thermal resistances, of about 2.2 × 10{sup −8} m{sup 2} K W{sup −1}, which are not sensitive to the number of h-BN layers or the SiO{sub 2} crystallinity. The h-BN/SiO{sub 2} 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.

  14. 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.

  15. 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

  16. Surface transfer doping induced effective modulation on ambipolar characteristics of few-layer black phosphorus.

    PubMed

    Xiang, Du; Han, Cheng; Wu, Jing; Zhong, Shu; Liu, Yiyang; Lin, Jiadan; Zhang, Xue-Ao; Ping Hu, Wen; Özyilmaz, Barbaros; Neto, A H Castro; Wee, Andrew Thye Shen; Chen, Wei

    2015-03-12

    Black phosphorus, a fast emerging two-dimensional material, has been configured as field effect transistors, showing a hole-transport-dominated ambipolar characteristic. Here we report an effective modulation on ambipolar characteristics of few-layer black phosphorus transistors through in situ surface functionalization with caesium carbonate (Cs2CO3) and molybdenum trioxide (MoO3), respectively. Cs2CO3 is found to strongly electron dope black phosphorus. The electron mobility of black phosphorus is significantly enhanced to ~27 cm(2) V(-1) s(-1) after 10 nm Cs2CO3 modification, indicating a greatly improved electron-transport behaviour. In contrast, MoO3 decoration demonstrates a giant hole-doping effect. In situ photoelectron spectroscopy characterization reveals significant surface charge transfer occurring at the dopants/black phosphorus interfaces. Moreover, the surface-doped black phosphorus devices exhibit a largely enhanced photodetection behaviour. Our findings coupled with the tunable nature of the surface transfer doping scheme ensure black phosphorus as a promising candidate for further complementary logic electronics.

  17. 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.

  18. Few-layer Phosphorene: An Ideal 2D Material For Tunnel Transistors.

    PubMed

    Ameen, Tarek A; Ilatikhameneh, Hesameddin; Klimeck, Gerhard; Rahman, Rajib

    2016-06-27

    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.

  19. Few-layer Phosphorene: An Ideal 2D Material For Tunnel Transistors

    PubMed Central

    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 ~ 106 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

  20. Structural Phase Transition and Material Properties of Few-Layer Monochalcogenides

    NASA Astrophysics Data System (ADS)

    Mehboudi, Mehrshad; Fregoso, Benjamin M.; Yang, Yurong; Zhu, Wenjuan; van der Zande, Arend; Ferrer, Jaime; Bellaiche, L.; Kumar, Pradeep; Barraza-Lopez, Salvador

    2016-12-01

    GeSe and SnSe monochalcogenide monolayers and bilayers undergo a two-dimensional phase transition from a rectangular unit cell to a square unit cell at a critical temperature Tc well below the melting point. Its consequences on material properties are studied within the framework of Car-Parrinello molecular dynamics and density-functional theory. No in-gap states develop as the structural transition takes place, so that these phase-change materials remain semiconducting below and above Tc. As the in-plane lattice transforms from a rectangle into a square at Tc, the electronic, spin, optical, and piezoelectric properties dramatically depart from earlier predictions. Indeed, the Y and X points in the Brillouin zone become effectively equivalent at Tc, leading to a symmetric electronic structure. The spin polarization at the conduction valley edge vanishes, and the hole conductivity must display an anomalous thermal increase at Tc. The linear optical absorption band edge must change its polarization as well, making this structural and electronic evolution verifiable by optical means. Much excitement is drawn by theoretical predictions of giant piezoelectricity and ferroelectricity in these materials, and we estimate a pyroelectric response of about 3 ×10-12 C /K m here. These results uncover the fundamental role of temperature as a control knob for the physical properties of few-layer group-IV monochalcogenides.

  1. 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.

  2. Growth and spectroscopic characterization of monolayer and few-layer hexagonal boron nitride on metal substrates

    NASA Astrophysics Data System (ADS)

    Feigelson, Boris N.; Bermudez, Victor M.; Hite, Jennifer K.; Robinson, Zachary R.; Wheeler, Virginia D.; Sridhara, Karthik; Hernández, Sandra C.

    2015-02-01

    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.

  3. Surface transfer doping induced effective modulation on ambipolar characteristics of few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Xiang, Du; Han, Cheng; Wu, Jing; Zhong, Shu; Liu, Yiyang; Lin, Jiadan; Zhang, Xue-Ao; Ping Hu, Wen; Özyilmaz, Barbaros; Neto, A. H. Castro; Wee, Andrew Thye Shen; Chen, Wei

    2015-03-01

    Black phosphorus, a fast emerging two-dimensional material, has been configured as field effect transistors, showing a hole-transport-dominated ambipolar characteristic. Here we report an effective modulation on ambipolar characteristics of few-layer black phosphorus transistors through in situ surface functionalization with caesium carbonate (Cs2CO3) and molybdenum trioxide (MoO3), respectively. Cs2CO3 is found to strongly electron dope black phosphorus. The electron mobility of black phosphorus is significantly enhanced to ~27 cm2 V-1 s-1 after 10 nm Cs2CO3 modification, indicating a greatly improved electron-transport behaviour. In contrast, MoO3 decoration demonstrates a giant hole-doping effect. In situ photoelectron spectroscopy characterization reveals significant surface charge transfer occurring at the dopants/black phosphorus interfaces. Moreover, the surface-doped black phosphorus devices exhibit a largely enhanced photodetection behaviour. Our findings coupled with the tunable nature of the surface transfer doping scheme ensure black phosphorus as a promising candidate for further complementary logic electronics.

  4. 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.

  5. 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.

  6. Photoconductivity of few-layered p-WSe2 phototransistors via multi-terminal measurements

    NASA Astrophysics Data System (ADS)

    Pradhan, Nihar R.; Garcia, Carlos; Holleman, Joshua; Rhodes, Daniel; Parker, Chason; Talapatra, Saikat; Terrones, Mauricio; Balicas, Luis; McGill, Stephen A.

    2016-12-01

    Recently, two-dimensional materials and in particular transition metal dichalcogenides (TMDs) have been extensively studied because of their strong light-matter interaction and the remarkable optoelectronic response of their field-effect transistors (FETs). Here, we report a photoconductivity study from FETs built from few-layers of p-WSe2 measured in a multi-terminal configuration under illumination by a 532 nm laser source. The photogenerated current was measured as a function of the incident optical power, of the drain-to-source bias and of the gate voltage. We observe a considerably larger photoconductivity when the phototransistors were measured via a four-terminal configuration when compared to a two-terminal one. For an incident laser power of 248 nW, we extract 18 A W-1 and ˜4000% for the two-terminal responsivity (R) and the concomitant external quantum efficiency (EQE) respectively, when a bias voltage V ds = 1 V and a gate voltage V bg = 10 V are applied to the sample. R and EQE are observed to increase by 370% to ˜85 A W-1 and ˜20 000% respectively, when using a four-terminal configuration. Thus, we conclude that previous reports have severely underestimated the optoelectronic response of transition metal dichalcogenides, which in fact reveals a remarkable potential for photosensing applications.

  7. Stacking-dependent electronic property of trilayer graphene epitaxially grown on Ru(0001)

    SciTech Connect

    Que, Yande; Xiao, Wende E-mail: hjgao@iphy.ac.cn; Chen, Hui; Wang, Dongfei; Du, Shixuan; Gao, Hong-Jun E-mail: hjgao@iphy.ac.cn

    2015-12-28

    The growth, atomic structure, and electronic property of trilayer graphene (TLG) on Ru(0001) were studied by low temperature scanning tunneling microscopy and spectroscopy in combined with tight-binding approximation (TBA) calculations. TLG on Ru(0001) shows a flat surface with a hexagonal lattice due to the screening effect of the bottom two layers and the AB-stacking in the top two layers. The coexistence of AA- and AB-stacking in the bottom two layers leads to three different stacking orders of TLG, namely, ABA-, ABC-, and ABB-stacking. STS measurements combined with TBA calculations reveal that the density of states of TLG with ABC- and ABB-stacking is characterized by one and two sharp peaks near to the Fermi level, respectively, in contrast to the V-shaped feature of TLG with ABA-stacking. Our work demonstrates that TLG on Ru(0001) might be an ideal platform for exploring stacking-dependent electronic properties of graphene.

  8. Strain in epitaxial Bi2Se3 grown on GaN and graphene substrates: A reflection high-energy electron diffraction study

    NASA Astrophysics Data System (ADS)

    Li, Bin; Guo, Xin; Ho, Wingkin; Xie, Maohai

    2015-08-01

    Topological insulator (TI) has been one of the focus research themes in condensed matter physics in recent years. Due to the relatively large energy bandgap, Bi2Se3 has been identified as one of the most promising three-dimensional TIs with application potentials. Epitaxial Bi2Se3 by molecular-beam epitaxy has been reported by many groups using different substrates. A common feature is that Bi2Se3 grows readily along the c-axis direction irrespective of the type and condition of the substrate. Because of the weak van deer Waals interaction between Bi2Se3 quintuple layers, the grown films are reported to be strain-free, taking the lattice constant of the bulk crystal. At the very initial stage of Bi2Se3 deposition, however, strain may still exist depending on the substrate. Strain may bring some drastic effects to the properties of the TIs and so achieving strained TIs can be of great fundamental interests as well as practical relevance. In this work, we employ reflection high-energy electron diffraction to follow the lattice constant evolution of Bi2Se3 during initial stage depositions on GaN and graphene, two very different substrates. We reveal that epitaxial Bi2Se3 is tensile strained on GaN but strain-free on graphene. Strain relaxation on GaN is gradual.

  9. THz Plasmonics of Quasi-freestanding Bilayer Epitaxial Graphene via H-intercalation

    NASA Astrophysics Data System (ADS)

    Daniels, Kevin; Boyd, Anthony; Nath, Anindya; Jadidi, Mohammad; Sushkov, Andrei; Drew, Dennis; Myers-Ward, Rachael; Gaskill, Kurt

    Graphene plasmonics has attracted attention as a suitable platform for tunable THz optoelectronics. THz plasmonic resonances in conventional large-area graphene, however, suffer from low quality factor (Q) because of high carrier scattering rate. This low Q is attributed to charge carrier induced scattering and lower carrier mobility caused by the partially covalent bonding between the silicon carbide (SiC) substrate and the 6 √3 buffer layer between the substrate and EG. Improving the Q of plasmons makes stronger THz resonance effects and also enable THz optoelectronics with fine tunability in frequency via gating. EG on Si-face, semi-insulating 6H-SiC was intercalated in-situ by hydrogen (H2) , releasing the buffer layer from SiC forming quasi-freestanding bilayer graphene. H-intercalation time was varied from 0 - 75 minutes and structural, electrical and optical properties were explored, revealing at long H-intercalation durations high carrier mobility (3000-4000 cm2/Vs) and high sheet carrier concentration (1E13 cm-2) independent of carrier mobility. Far IR simultaneous transmission/reflection measurements revealed a narrow frequency response with line widths (γ) smaller in H-intercalated EG (30cm-1) than observed in pristine EG (>100cm-1) consistent with the improved mobility.

  10. Improvement in Mobility and Conductivity of Few-Layer MoS2 Films

    NASA Astrophysics Data System (ADS)

    Ma, Xiying; He, Jie; Chen, Kangye; Lin, La

    2014-12-01

    We present a study of the influence of annealing and doping on the electrical properties of few-layer (FL) MoS2 films on Si and quartz substrates deposited using a self-designed metal sulfide chemical vapor deposition (MSCVD) system. FL MoS2 slices obtained through MSCVD, in the size range of 50-200 nm, were found to be uniformly scattered on the substrates. The conductivity and mobility of these films are greatly enhanced after annealing at 650-850°C. The largest mobility measured for pure MoS2 on quartz substrate is 6.4×103cm2/Vs, almost two orders of magnitude larger than that of bulk MoS2 (500 cm2/Vs). We deduce that the superior charge carrier mobility in our sample is mainly attributed to reduced phonon scattering because of a lower carrier density (1010-1011 cm-2) compared to previously documented values (1012-1013cm-2). Additionally, the conductivity and carrier concentration of FL MoS2 films were enhanced by about two orders of magnitude compared to those of the as-grown films doped with Cu, Na and Ag ions but not doped with B ions. The films doped with Na and Ag exhibit characteristic p-type conductivity, while those doped with Cu and B exhibit n-type conductivity. Moreover, the MoS2/Si heterojunction exhibited good rectification characteristics and excellent conductivity, indicating that the FL MoS2 films will find many applications in high-efficiency nanodevices.

  11. Epitaxial Graphene Surface Preparation for Atomic Layer Deposition of Al2O3

    DTIC Science & Technology

    2011-06-01

    j dielectrics such as Al2O3 , HfO2, Ta2O5, and TiO2 , are important for the realization of graphene-based top-gated electronic devices including field... ALD pulse sequencing of NO2-trimethylaluminum (TMA); 16 oxidation of electron beam evaporated metallic Al, Hf, Ti, Ta;17,18 and spin- coating of a... ALD of Al2O3 films in promoting uni- form, high quality oxide deposition. Initial treatments resulted in partial coverage, while the optimized treatment

  12. Selective area growth of Bernal bilayer epitaxial graphene on 4H-SiC (0001) substrate by electron-beam irradiation

    NASA Astrophysics Data System (ADS)

    Dharmaraj, P.; Jeganathan, K.; Parthiban, S.; Kwon, J. Y.; Gautam, S.; Chae, K. H.; Asokan, K.

    2014-11-01

    We report selective area growth of large area homogeneous Bernal stacked bilayer epitaxial graphene (BLEG) on 4H-SiC (0001) substrate by electron-beam irradiation. Sublimation of Si occurs by energetic electron irradiations on SiC surface via breaking of Si-C bonds in the localized region, which allows the selective growth of graphene. Raman measurements ensure the formation of homogeneous BLEG with weak compressive strain of -0.08%. The carrier mobility of large area BLEG is ˜5100 cm2 V-1 s-1 with a sheet carrier density of 2.2 × 1013 cm-2. Current-voltage measurements reveal that BLEG on 4H-SiC forms a Schottky junction with an operation at mA level. Our study reveals that the barrier height at the Schottky junction is low (˜0.58 eV) due to the Fermi-level pinning above the Dirac point.

  13. Odd-Integer Quantum Hall States and Giant Spin Susceptibility in p -Type Few-Layer WSe2

    NASA Astrophysics Data System (ADS)

    Xu, Shuigang; Shen, Junying; Long, Gen; Wu, Zefei; Bao, Zhi-qiang; Liu, Cheng-Cheng; Xiao, Xiao; Han, Tianyi; Lin, Jiangxiazi; Wu, Yingying; Lu, Huanhuan; Hou, Jianqiang; An, Liheng; Wang, Yuanwei; Cai, Yuan; Ho, K. M.; He, Yuheng; Lortz, Rolf; Zhang, Fan; Wang, Ning

    2017-02-01

    We fabricate high-mobility p -type few-layer WSe2 field-effect transistors and surprisingly observe a series of quantum Hall (QH) states following an unconventional sequence predominated by odd-integer states under a moderate strength magnetic field. By tilting the magnetic field, we discover Landau level crossing effects at ultralow coincident angles, revealing that the Zeeman energy is about 3 times as large as the cyclotron energy near the valence band top at the Γ valley. This result implies the significant roles played by the exchange interactions in p -type few-layer WSe2 , in which itinerant or QH ferromagnetism likely occurs. Evidently, the Γ valley of few-layer WSe2 offers a unique platform with unusually heavy hole carriers and a substantially enhanced g factor for exploring strongly correlated phenomena.

  14. Structural investigation of nanocrystalline graphene grown on (6√3 × 6√3)R30°-reconstructed SiC surfaces by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Schumann, T.; Dubslaff, M.; Oliveira, M. H., Jr.; Hanke, M.; Fromm, F.; Seyller, T.; Nemec, L.; Blum, V.; Scheffler, M.; Lopes, J. M. J.; Riechert, H.

    2013-12-01

    Growth of nanocrystalline graphene films on (6√3 × 6√3)R30°-reconstructed SiC surfaces was achieved by molecular beam epitaxy, enabling the investigation of quasi-homoepitaxial growth. The structural quality of the graphene films, which is investigated by Raman spectroscopy, increases with growth time. X-ray photoelectron spectroscopy proves that the SiC surface reconstruction persists throughout the growth process and that the synthesized films consist of sp2-bonded carbon. Interestingly, grazing incidence x-ray diffraction measurements show that the graphene domains possess one single in-plane orientation, are aligned to the substrate, and offer a noticeably contracted lattice parameter of 2.450 Å. We correlate this contraction with theoretically calculated reference values (all-electron density functional calculations based on the van der Waals corrected Perdew-Burke-Ernzerhof functional) for the lattice parameter contraction induced in ideal, free-standing graphene sheets by: substrate-induced buckling, the edges of limited-size flakes and typical point defects (monovacancies, divacancies, Stone-Wales defects).

  15. Controlled growth of few-layer hexagonal boron nitride on copper foils using ion beam sputtering deposition.

    PubMed

    Wang, Haolin; Zhang, Xingwang; Meng, Junhua; Yin, Zhigang; Liu, Xin; Zhao, Yajuan; Zhang, Liuqi

    2015-04-01

    Ion beam sputtering deposition (IBSD) is used to synthesize high quality few-layer hexagonal boron nitride (h-BN) on copper foils. Compared to the conventional chemical vapor deposition, the IBSD technique avoids the use of unconventional precursors and is much easier to control, which should be very useful for the large-scale production of h-BN in the future.

  16. Strong enhancement of photoresponsivity with shrinking the electrodes spacing in few layer GaSe photodetectors

    NASA Astrophysics Data System (ADS)

    Cao, Yufei; Cai, Kaiming; Hu, Pingan; Zhao, Lixia; Yan, Tengfei; Luo, Wengang; Zhang, Xinhui; Wu, Xiaoguang; Wang, Kaiyou; Zheng, Houzhi

    2015-01-01

    A critical challenge for the integration of optoelectronics is that photodetectors have relatively poor sensitivities at the nanometer scale. Generally, a large electrodes spacing in photodetectors is required to absorb sufficient light to maintain high photoresponsivity and reduce the dark current. However, this will limit the optoelectronic integration density. Through spatially resolved photocurrent investigation, we find that the photocurrent in metal-semiconductor-metal (MSM) photodetectors based on layered GaSe is mainly generated from the region close to the metal-GaSe interface with higher electrical potential. The photoresponsivity monotonically increases with shrinking the spacing distance before the direct tunneling happens, which was significantly enhanced up to 5,000 AW-1 for the bottom Ti/Au contacted device. It is more than 1,700-fold improvement over the previously reported results. The response time of the Ti/Au contacted devices is about 10-20 ms and reduced down to 270 μs for the devices with single layer graphene as metallic electrodes. A theoretical model has been developed to well explain the photoresponsivity for these two types of device configurations. Our findings realize reducing the size and improving the performance of 2D semiconductor based MSM photodetectors simultaneously, which could pave the way for future high density integration of optoelectronics with high performances.

  17. Strong enhancement of photoresponsivity with shrinking the electrodes spacing in few layer GaSe photodetectors

    PubMed Central

    Cao, Yufei; Cai, Kaiming; Hu, Pingan; Zhao, Lixia; Yan, Tengfei; Luo, Wengang; Zhang, Xinhui; Wu, Xiaoguang; Wang, Kaiyou; Zheng, Houzhi

    2015-01-01

    A critical challenge for the integration of optoelectronics is that photodetectors have relatively poor sensitivities at the nanometer scale. Generally, a large electrodes spacing in photodetectors is required to absorb sufficient light to maintain high photoresponsivity and reduce the dark current. However, this will limit the optoelectronic integration density. Through spatially resolved photocurrent investigation, we find that the photocurrent in metal-semiconductor-metal (MSM) photodetectors based on layered GaSe is mainly generated from the region close to the metal-GaSe interface with higher electrical potential. The photoresponsivity monotonically increases with shrinking the spacing distance before the direct tunneling happens, which was significantly enhanced up to 5,000 AW−1 for the bottom Ti/Au contacted device. It is more than 1,700-fold improvement over the previously reported results. The response time of the Ti/Au contacted devices is about 10–20 ms and reduced down to 270 μs for the devices with single layer graphene as metallic electrodes. A theoretical model has been developed to well explain the photoresponsivity for these two types of device configurations. Our findings realize reducing the size and improving the performance of 2D semiconductor based MSM photodetectors simultaneously, which could pave the way for future high density integration of optoelectronics with high performances. PMID:25632886

  18. Strong enhancement of photoresponsivity with shrinking the electrodes spacing in few layer GaSe photodetectors.

    PubMed

    Cao, Yufei; Cai, Kaiming; Hu, Pingan; Zhao, Lixia; Yan, Tengfei; Luo, Wengang; Zhang, Xinhui; Wu, Xiaoguang; Wang, Kaiyou; Zheng, Houzhi

    2015-01-30

    A critical challenge for the integration of optoelectronics is that photodetectors have relatively poor sensitivities at the nanometer scale. Generally, a large electrodes spacing in photodetectors is required to absorb sufficient light to maintain high photoresponsivity and reduce the dark current. However, this will limit the optoelectronic integration density. Through spatially resolved photocurrent investigation, we find that the photocurrent in metal-semiconductor-metal (MSM) photodetectors based on layered GaSe is mainly generated from the region close to the metal-GaSe interface with higher electrical potential. The photoresponsivity monotonically increases with shrinking the spacing distance before the direct tunneling happens, which was significantly enhanced up to 5,000 AW(-1) for the bottom Ti/Au contacted device. It is more than 1,700-fold improvement over the previously reported results. The response time of the Ti/Au contacted devices is about 10-20 ms and reduced down to 270 μs for the devices with single layer graphene as metallic electrodes. A theoretical model has been developed to well explain the photoresponsivity for these two types of device configurations. Our findings realize reducing the size and improving the performance of 2D semiconductor based MSM photodetectors simultaneously, which could pave the way for future high density integration of optoelectronics with high performances.

  19. Synthesis and electronic properties of chemically functionalized graphene on metal surfaces

    NASA Astrophysics Data System (ADS)

    Grüneis, Alexander

    2013-01-01

    A review on the electronic properties, growth and functionalization of graphene on metals is presented. Starting from the derivation of the electronic properties of an isolated graphene layer using the nearest neighbor tight-binding (TB) approximation for π and σ electrons, the TB model is then extended to third-nearest neighbors and interlayer coupling. The latter is relevant to few-layer graphene and graphite. Next, the conditions under which epitaxial graphene can be obtained by chemical vapor deposition are reviewed with a particular emphasis on the Ni(111) surface. Regarding functionalization, I first discuss the intercalation of monolayer Au into the graphene/Ni(111) interface, which renders graphene quasi-free-standing. The Au intercalated quasi-free-standing graphene is then the basis for chemical functionalization. Functionalization of graphene is classified into covalent, ionic and substitutional functionalization. As archetypical examples for these three possibilities I discuss covalent functionalization by hydrogen, ionic functionalization by alkali metals and substitutional functionalization by nitrogen heteroatoms.

  20. Strain in epitaxial Bi{sub 2}Se{sub 3} grown on GaN and graphene substrates: A reflection high-energy electron diffraction study

    SciTech Connect

    Li, Bin; Guo, Xin; Ho, Wingkin; Xie, Maohai

    2015-08-24

    Topological insulator (TI) has been one of the focus research themes in condensed matter physics in recent years. Due to the relatively large energy bandgap, Bi{sub 2}Se{sub 3} has been identified as one of the most promising three-dimensional TIs with application potentials. Epitaxial Bi{sub 2}Se{sub 3} by molecular-beam epitaxy has been reported by many groups using different substrates. A common feature is that Bi{sub 2}Se{sub 3} grows readily along the c-axis direction irrespective of the type and condition of the substrate. Because of the weak van der Waals interaction between Bi{sub 2}Se{sub 3} quintuple layers, the grown films are reported to be strain-free, taking the lattice constant of the bulk crystal. At the very initial stage of Bi{sub 2}Se{sub 3} deposition, however, strain may still exist depending on the substrate. Strain may bring some drastic effects to the properties of the TIs and so achieving strained TIs can be of great fundamental interests as well as practical relevance. In this work, we employ reflection high-energy electron diffraction to follow the lattice constant evolution of Bi{sub 2}Se{sub 3} during initial stage depositions on GaN and graphene, two very different substrates. We reveal that epitaxial Bi{sub 2}Se{sub 3} is tensile strained on GaN but strain-free on graphene. Strain relaxation on GaN is gradual.

  1. Tip induced mechanical deformation of epitaxial graphene grown on reconstructed 6H-SiC(0001) surface during scanning tunneling and atomic force microscopy studies

    NASA Astrophysics Data System (ADS)

    Morán Meza, José Antonio; Lubin, Christophe; Thoyer, François; Cousty, Jacques

    2015-06-01

    The structural and mechanical properties of an epitaxial graphene (EG) monolayer thermally grown on top of a 6H-SiC(0001) surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM). Experimental STM, dynamic STM and AFM images of EG on 6H-SiC(0001) show a lattice with a 1.9 nm period corresponding to the (6 × 6) quasi-cell of the SiC surface. The corrugation amplitude of this (6 × 6) quasi-cell, measured from AFM topographies, increases with the setpoint value of the frequency shift Δf (15-20 Hz, repulsive interaction). Excitation variations map obtained simultaneously with the AFM topography shows that larger dissipation values are measured in between the topographical bumps of the (6 × 6) quasi-cell. These results demonstrate that the AFM tip deforms the graphene monolayer. During recording in dynamic STM mode, a frequency shift (Δf) map is obtained in which Δf values range from 41 to 47 Hz (repulsive interaction). As a result, we deduced that the STM tip, also, provokes local mechanical distortions of the graphene monolayer. The origin of these tip-induced distortions is discussed in terms of electronic and mechanical properties of EG on 6H-SiC(0001).

  2. Tip induced mechanical deformation of epitaxial graphene grown on reconstructed 6H-SiC(0001) surface during scanning tunneling and atomic force microscopy studies.

    PubMed

    Meza, José Antonio Morán; Lubin, Christophe; Thoyer, François; Cousty, Jacques

    2015-01-26

    The structural and mechanical properties of an epitaxial graphene (EG) monolayer thermally grown on top of a 6H-SiC(0001) surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM). Experimental STM, dynamic STM and AFM images of EG on 6H-SiC(0001) show a lattice with a 1.9 nm period corresponding to the (6 × 6) quasi-cell of the SiC surface. The corrugation amplitude of this (6 × 6) quasi-cell, measured from AFM topographies, increases with the setpoint value of the frequency shift Δf (15-20 Hz, repulsive interaction). Excitation variations map obtained simultaneously with the AFM topography shows that larger dissipation values are measured in between the topographical bumps of the (6 × 6) quasi-cell. These results demonstrate that the AFM tip deforms the graphene monolayer. During recording in dynamic STM mode, a frequency shift (Δf) map is obtained in which Δf values range from 41 to 47 Hz (repulsive interaction). As a result, we deduced that the STM tip, also, provokes local mechanical distortions of the graphene monolayer. The origin of these tip-induced distortions is discussed in terms of electronic and mechanical properties of EG on 6H-SiC(0001).

  3. Few-Layer MoS2-Organic Thin-Film Hybrid Complementary Inverter Pixel Fabricated on a Glass Substrate.

    PubMed

    Lee, Hee Sung; Shin, Jae Min; Jeon, Pyo Jin; Lee, Junyeong; Kim, Jin Sung; Hwang, Hyun Chul; Park, Eunyoung; Yoon, Woojin; Ju, Sang-Yong; Im, Seongil

    2015-05-13

    Few-layer MoS2-organic thin-film hybrid complementary inverters demonstrate a great deal of device performance with a decent voltage gain of ≈12, a few hundred pW power consumption, and 480 Hz switching speed. As fabricated on glass, this hybrid CMOS inverter operates as a light-detecting pixel as well, using a thin MoS2 channel.

  4. Controlled synthesis and transfer of large-area WS2 sheets: from single layer to few layers.

    PubMed

    Elías, Ana Laura; Perea-López, Néstor; Castro-Beltrán, Andrés; Berkdemir, Ayse; Lv, Ruitao; Feng, Simin; Long, Aaron D; Hayashi, Takuya; Kim, Yoong Ahm; Endo, Morinobu; Gutiérrez, Humberto R; Pradhan, Nihar R; Balicas, Luis; Mallouk, Thomas E; López-Urías, Florentino; Terrones, Humberto; Terrones, Mauricio

    2013-06-25

    The isolation of few-layered transition metal dichalcogenides has mainly been performed by mechanical and chemical exfoliation with very low yields. In this account, a controlled thermal reduction-sulfurization method is used to synthesize large-area (~1 cm(2)) WS2 sheets with thicknesses ranging from monolayers to a few layers. During synthesis, WOx thin films are first deposited on Si/SiO2 substrates, which are then sulfurized (under vacuum) at high temperatures (750-950 °C). An efficient route to transfer the synthesized WS2 films onto different substrates such as quartz and transmission electron microscopy (TEM) grids has been satisfactorily developed using concentrated HF. Samples with different thicknesses have been analyzed by Raman spectroscopy and TEM, and their photoluminescence properties have been evaluated. We demonstrated the presence of single-, bi-, and few-layered WS2 on as-grown samples. It is well known that the electronic structure of these materials is very sensitive to the number of layers, ranging from indirect band gap semiconductor in the bulk phase to direct band gap semiconductor in monolayers. This method has also proved successful in the synthesis of heterogeneous systems of MoS2 and WS2 layers, thus shedding light on the controlled production of heterolayered devices from transition metal chalcogenides.

  5. Effect of aging-induced disorder on the quantum transport properties of few-layer WTe2

    NASA Astrophysics Data System (ADS)

    Lai Liu, Wei; Chen, Mao Lin; Li, Xiao Xi; Dubey, Sudipta; Xiong, Ting; Dai, Zhi Ming; Yin, Jun; Guo, Wan Lin; Ma, Jin Long; Chen, Ya Ni; Tan, Jun; Li, Da; Wang, Zhen Hua; Li, Wu; Bouchiat, Vincent; Sun, Dong Ming; Han, Zheng; Zhang, Zhi Dong

    2017-03-01

    The emerging physical phenomena found in transition metal dicalcogenides (TMDCs) have triggered vast investigations in recent years. Among them, nanoelectronics in WTe2 devices have attracted particular attentions due to its exotic band structure that leads to exciting phenomena such as the predicted type-II Weyl semimetallic state. However, the thickness dependence of its quantum transport properties in the two-dimensional limit remains under debate. The major missing ingredient in the previous studies is the aging-induced disorder, as atomically thin layers of TMDCs are often known to be metastable in the ambient atmosphere. Here, we show systematic performance of low temperature quantum electronic transport of few-layer WTe2. It is observed that aging-induced localized electronic states explains the low temperature Coulomb gap in transport measurements, leading to the anomalous magnetotransport which appears to be extrinsic. While few-layered WTe2 shows clear metallic tendency in the fresh state, degraded devices first exhibited a re-entrant insulating behavior, and finally entered a fully insulating state. Correspondingly, a crossover from parabolic to linear magnetoresistance, and, upon further aging, leads to the observation of weak anti-localization. Our study reveals for the first time the correlation between the unusual magnetotransport and disorder in few-layered WTe2, which is indispensable in providing guidance on its future device applications.

  6. Few-Layer MoSe2 Possessing High Catalytic Activity towards Iodide/Tri-iodide Redox Shuttles

    PubMed Central

    Lee, Lawrence Tien Lin; He, Jian; Wang, Baohua; Ma, Yaping; Wong, King Young; Li, Quan; Xiao, Xudong; Chen, Tao

    2014-01-01

    Due to the two-dimensional confinement of electrons, single- and few-layer MoSe2 nanostructures exhibit unusual optical and electrical properties and have found wide applications in catalytic hydrogen evolution reaction, field effect transistor, electrochemical intercalation, and so on. Here we present a new application in dye-sensitized solar cell as catalyst for the reduction of I3− to I− at the counter electrode. The few-layer MoSe2 is fabricated by surface selenization of Mo-coated soda-lime glass. Our results show that the few-layer MoSe2 displays high catalytic efficiency for the regeneration of I− species, which in turn yields a photovoltaic energy conversion efficiency of 9.00%, while the identical photoanode coupling with “champion” electrode based on Pt nanoparticles on FTO glass generates efficiency only 8.68%. Thus, a Pt- and FTO-free counter electrode outperforming the best conventional combination is obtained. In this electrode, Mo film is found to significantly decrease the sheet resistance of the counter electrode, contributing to the excellent device performance. Since all of the elements in the electrode are of high abundance ratios, this type of electrode is promising for the fabrication of large area devices at low materials cost. PMID:24525919

  7. Emergence of Two-Dimensional Massless Dirac Fermions, Chiral Pseudospins, and Berry's Phase in Potassium Doped Few-Layer Black Phosphorus.

    PubMed

    Baik, Seung Su; Kim, Keun Su; Yi, Yeonjin; Choi, Hyoung Joon

    2015-12-09

    Thin flakes of black phosphorus (BP) are a two-dimensional (2D) semiconductor whose energy gap is predicted being sensitive to the number of layers and external perturbations. Very recently, it was found that a simple method of potassium (K) doping on the surface of BP closes its band gap completely, producing a Dirac semimetal state with a linear band dispersion in the armchair direction and a quadratic one in the zigzag direction. Here, based on first-principles density functional calculations, we predict that, beyond the critical K density of the gap closure, 2D massless Dirac Fermions (i.e., Dirac cones) emerge in K-doped few-layer BP, with linear band dispersions in all momentum directions, and the electronic states around Dirac points have chiral pseudospins and Berry's phase. These features are robust with respect to the spin-orbit interaction and may lead to graphene-like electronic transport properties with greater flexibility for potential device applications.

  8. Atomic and electronic structure of trilayer graphene/SiC(0001): Evidence of Strong Dependence on Stacking Sequence and charge transfer

    NASA Astrophysics Data System (ADS)

    Pierucci, Debora; Brumme, Thomas; Girard, Jean-Christophe; Calandra, Matteo; Silly, Mathieu G.; Sirotti, Fausto; Barbier, Antoine; Mauri, Francesco; Ouerghi, Abdelkarim

    2016-09-01

    The transport properties of few-layer graphene are the directly result of a peculiar band structure near the Dirac point. Here, for epitaxial graphene grown on SiC, we determine the effect of charge transfer from the SiC substrate on the local density of states (LDOS) of trilayer graphene using scaning tunneling microscopy/spectroscopy and angle resolved photoemission spectroscopy (ARPES). Different spectra are observed and are attributed to the existence of two stable polytypes of trilayer: Bernal (ABA) and rhomboedreal (ABC) staking. Their electronic properties strongly depend on the charge transfer from the substrate. We show that the LDOS of ABC stacking shows an additional peak located above the Dirac point in comparison with the LDOS of ABA stacking. The observed LDOS features, reflecting the underlying symmetry of the two polytypes, were reproduced by explicit calculations within density functional theory (DFT) including the charge transfer from the substrate. These findings demonstrate the pronounced effect of stacking order and charge transfer on the electronic structure of trilayer or few layer graphene. Our approach represents a significant step toward understand the electronic properties of graphene layer under electrical field.

  9. Atomic and electronic structure of trilayer graphene/SiC(0001): Evidence of Strong Dependence on Stacking Sequence and charge transfer

    PubMed Central

    Pierucci, Debora; Brumme, Thomas; Girard, Jean-Christophe; Calandra, Matteo; Silly, Mathieu G.; Sirotti, Fausto; Barbier, Antoine; Mauri, Francesco; Ouerghi, Abdelkarim

    2016-01-01

    The transport properties of few-layer graphene are the directly result of a peculiar band structure near the Dirac point. Here, for epitaxial graphene grown on SiC, we determine the effect of charge transfer from the SiC substrate on the local density of states (LDOS) of trilayer graphene using scaning tunneling microscopy/spectroscopy and angle resolved photoemission spectroscopy (ARPES). Different spectra are observed and are attributed to the existence of two stable polytypes of trilayer: Bernal (ABA) and rhomboedreal (ABC) staking. Their electronic properties strongly depend on the charge transfer from the substrate. We show that the LDOS of ABC stacking shows an additional peak located above the Dirac point in comparison with the LDOS of ABA stacking. The observed LDOS features, reflecting the underlying symmetry of the two polytypes, were reproduced by explicit calculations within density functional theory (DFT) including the charge transfer from the substrate. These findings demonstrate the pronounced effect of stacking order and charge transfer on the electronic structure of trilayer or few layer graphene. Our approach represents a significant step toward understand the electronic properties of graphene layer under electrical field. PMID:27629702

  10. Evidence of minority carrier injection efficiency >90% in an epitaxial graphene/SiC Schottky emitter bipolar junction phototransistor for ultraviolet detection

    SciTech Connect

    Chava, Venkata S. N. Omar, Sabih U.; Brown, Gabriel; Shetu, Shamaita S.; Andrews, J.; Sudarshan, T. S.; Chandrashekhar, M. V. S.

    2016-01-25

    In this letter, we report the UV detection characteristics of an epitaxial graphene (EG)/SiC based Schottky emitter bipolar phototransistor (SEPT) with EG on top as the transparent Schottky emitter layer. Under 0.43 μW UV illumination, the device showed a maximum common emitter current gain of 113, when operated in the Schottky emitter mode. We argue that avalanche gain and photoconductive gain can be excluded, indicating minority carrier injection efficiency, γ, as high as 99% at the EG/p-SiC Schottky junction. This high γ is attributed to the large, highly asymmetric barrier, which EG forms with the p-SiC. The maximum responsivity of the UV phototransistor is estimated to be 7.1 A/W. The observed decrease in gain with increase in UV power is attributed to recombination in the base region, which reduces the minority carrier lifetime.

  11. Correlating low-energy electron microscopy and micro-Raman imaging of epitaxial graphene on SiC

    NASA Astrophysics Data System (ADS)

    Cheng, Guangjun; Calizo, Irene; Meade, Patrick; He, Guowei; Real, M. A.; Elmquist, R. E.; Feenstra, R. M.; Hight Walker, A. R.

    2013-03-01

    Several techniques exist for determining the number of graphene layers grown on SiC such as low-energy electron microscopy (LEEM) and Raman spectroscopy. The method which is arguably the most definitive for SiC-grown graphene isLEEM. Low-energy (0 - 10 eV) electrons interfere with the graphene layers, yielding minima in the electron reflectivity vs. energy curve that can be used to determine the layer number.1 LEEM also provides the means of collecting selected-area diffraction on ?m-size surface regions (micro-LEED), giving access to further useful structural information. While Raman spectroscopy is also commonly used to determine graphene layer number on SiC substrates; such measurements have no definitive calibration for large-area graphene on SiC, unlike the case of exfoliated graphene on SiO2. In this talk, results of correlated LEEM/micro-Raman imaging of large-area, mono and multilayer graphene samples are presented. These initial findings show that LEEM can show the contrast between terrace regions and step edges at particular areas of monolayer-graphene surfaces. Micro-Raman imaging of these same locations show Raman shifts in the G' (2D) band. The influence of heterogeneities on electrical behavior of graphene will be discussed. Comparative studies of multilayer graphene are in progress, and will also be reported. 1. H. Hibino, et al., Phys. Rev. B 77, 075413 (2008). 2. L. I. Johansson, et al., Phys. Rev. B 84, 125405 (2011).

  12. Vector soliton fiber laser passively mode locked by few layer black phosphorus-based optical saturable absorber.

    PubMed

    Song, Yufeng; Chen, Si; Zhang, Qian; Li, Lei; Zhao, Luming; Zhang, Han; Tang, Dingyuan

    2016-11-14

    We report on the optical saturable absorption of few-layer black phosphorus nanoflakes and demonstrate its application for the generation of vector solitons in an erbium-doped fiber laser. By incorporating the black phosphorus nanoflakes-based saturable absorber (SA) into an all-fiber erbium-doped fiber laser cavity, we are able to obtain passive mode-locking operation with soliton pulses down to ~670 fs. The properties and dynamics of the as-generated vector solitons are experimentally investigated. Our results show that BP nanoflakes could be developed as an effective SA for ultrashort pulse fiber lasers, particularly for the generation of vector soliton pulses in fiber lasers.

  13. Third order nonlinear optical response exhibited by mono- and few-layers of WS2

    SciTech Connect

    Torres-Torres, Carlos; Perea-López, Néstor; Elías, Ana Laura; Gutiérrez, Humberto R.; Cullen, David A.; Berkdemir, Ayse; López-Urías, Florentino; Terrones, Humberto; Terrones, Mauricio

    2016-04-13

    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. In conclusion, we envision applications for developing all-optical bidimensional nonlinear optical devices.

  14. Supercritical carbon dioxide-assisted rapid synthesis of few-layer black phosphorus for hydrogen peroxide sensing.

    PubMed

    Yan, Shancheng; Wang, Bojun; Wang, Zhulan; Hu, Dong; Xu, Xin; Wang, Junzhuan; Shi, Yi

    2016-06-15

    Solutions with large-scale dispersions of 2D black phosphorus (BP), often referred to as phosphorene, are obtained through solvent exfoliation. But, rapid phosphorene synthesis remains a challenge. Furthermore, although the chemical sensing capability of BP-based sensors has been theoretically predicted, its experimental verification remains lacking. In this study, we demonstrate the use of supercritical carbon dioxide-assisted rapid synthesis (5h) of few-layer BP. In addition, we construct a non-enzymatic hydrogen peroxide (H2O2) sensor based on few-layer BP for the first time to utilize BP degradation under ambient conditions. The proposed H2O2 sensor exhibits a considerably lower detection limit of 1 × 10(-7) M compared with the general detection limit of 1 × 10(-7) M-5 × 10(-5)M via electrochemical methods. Overall, the results of this study will not only expand the coverage of BP research but will also identify the important sensing characteristics of BP.

  15. Synthesis and characterization of single- and few-layer mica nanosheets by the microwave-assisted solvothermal approach

    NASA Astrophysics Data System (ADS)

    Van Khai, Tran; Gil Na, Han; Kwak, Dong Sub; Kwon, Yong Jung; Ham, Heon; Shim, Kwang Bo; Kim, Hyoun Woo

    2013-04-01

    We have successfully fabricated single- and few-layer mica nanosheets, by means of using a solvothermal method in conjunction with a microwave irradiated expansion process. In the solvothermal process, dissolved potassium ions were intercalated onto the interlayer space of the mica. Following this, microwave irradiation facilitates the exfoliation of individual nanosheets. The synthesized products were characterized by field emission scanning electron microscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM), x-ray diffraction, x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy measurements. AFM and TEM studies claimed the existence of single-layer mica. High-resolution TEM (HR-TEM) investigations revealed that the exfoliated product corresponded to a crystalline mica structure, being comprised of Si, Al, O, and K elements. XPS spectra exhibited the major constituent peaks, including O 1s, Si 2p, Al 2p, and K 2p. In addition, C atomic concentration has been slightly increased by the contamination during exfoliation, presumably due to the increase of the exposed mica surface. The C 1s XPS spectra revealed that the C-C bonding in organic surface contaminants was broken, whereas the Si-C bonding was enhanced, by the exfoliation process. The O 1s XPS spectra revealed that the Si-O bonding in mica was broken, generating the O-Si-C bonding. This study paves the way towards the fabrication of single- or few-layer inorganic nanosheets of desired materials, via a convenient and efficient route.

  16. Synthesis and characterization of single- and few-layer mica nanosheets by the microwave-assisted solvothermal approach.

    PubMed

    Van Khai, Tran; Gil Na, Han; Sub Kwak, Dong; Jung Kwon, Yong; Ham, Heon; Bo Shim, Kwang; Woo Kim, Hyoun

    2013-04-12

    We have successfully fabricated single- and few-layer mica nanosheets, by means of using a solvothermal method in conjunction with a microwave irradiated expansion process. In the solvothermal process, dissolved potassium ions were intercalated onto the interlayer space of the mica. Following this, microwave irradiation facilitates the exfoliation of individual nanosheets. The synthesized products were characterized by field emission scanning electron microscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM), x-ray diffraction, x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy measurements. AFM and TEM studies claimed the existence of single-layer mica. High-resolution TEM (HR-TEM) investigations revealed that the exfoliated product corresponded to a crystalline mica structure, being comprised of Si, Al, O, and K elements. XPS spectra exhibited the major constituent peaks, including O 1s, Si 2p, Al 2p, and K 2p. In addition, C atomic concentration has been slightly increased by the contamination during exfoliation, presumably due to the increase of the exposed mica surface. The C 1s XPS spectra revealed that the C-C bonding in organic surface contaminants was broken, whereas the Si-C bonding was enhanced, by the exfoliation process. The O 1s XPS spectra revealed that the Si-O bonding in mica was broken, generating the O-Si-C bonding. This study paves the way towards the fabrication of single- or few-layer inorganic nanosheets of desired materials, via a convenient and efficient route.

  17. Mechanically delaminated few layered MoS2 nanosheets based high performance wire type solid-state symmetric supercapacitors

    NASA Astrophysics Data System (ADS)

    Krishnamoorthy, Karthikeyan; Pazhamalai, Parthiban; Veerasubramani, Ganesh Kumar; Kim, Sang Jae

    2016-07-01

    Two dimensional nanostructures are increasingly used as electrode materials in flexible supercapacitors for portable electronic applications. Herein, we demonstrated a ball milling approach for achieving few layered molybdenum disulfide (MoS2) via exfoliation from their bulk. Physico-chemical characterizations such as X-ray diffraction, field emission scanning electron microscope, and laser Raman analyses confirmed the occurrence of exfoliated MoS2 sheets with few layers from their bulk via ball milling process. MoS2 based wire type solid state supercapacitors (WSCs) are fabricated and examined using cyclic voltammetry (CV), electrochemical impedance spectroscopy, and galvanostatic charge discharge (CD) measurements. The presence of rectangular shaped CV curves and symmetric triangular shaped CD profiles suggested the mechanism of charge storage in MoS2 WSC is due to the formation of electrochemical double layer capacitance. The MoS2 WSC device delivered a specific capacitance of 119 μF cm-1, and energy density of 8.1 nW h cm-1 with better capacitance retention of about 89.36% over 2500 cycles, which ensures the use of the ball milled MoS2 for electrochemical energy storage devices.

  18. Observation of resistively detected hole spin resonance and zero-field pseudo-spin splitting in epitaxial graphene

    PubMed Central

    Mani, Ramesh G.; Hankinson, John; Berger, Claire; de Heer, Walter A.

    2012-01-01

    Electronic carriers in graphene show a high carrier mobility at room temperature. Thus, this system is widely viewed as a potential future charge-based high-speed electronic material to complement–or replace–silicon. At the same time, the spin properties of graphene have suggested improved capability for spin-based electronics or spintronics and spin-based quantum computing. As a result, the detection, characterization and transport of spin have become topics of interest in graphene. Here we report a microwave photo-excited transport study of monolayer and trilayer graphene that reveals an unexpectedly strong microwave-induced electrical response and dual microwave-induced resonances in the dc resistance. The results suggest the resistive detection of spin resonance, and provide a measurement of the g-factor, the spin relaxation time and the sub-lattice degeneracy splitting at zero magnetic field. PMID:22871815

  19. Effects of Pretreatment on the Electronic Properties of Plasma Enhanced Chemical Vapor Deposition Hetero-Epitaxial Graphene Devices

    NASA Astrophysics Data System (ADS)

    Zhang, Lian-Chang; Shi, Zhi-Wen; Yang, Rong; Huang, Jian

    2014-09-01

    Quasi-monolayer graphene is successfully grown by the plasma enhanced chemical vapor deposition heteroepitaxial method we reported previously. To measure its electrical properties, the prepared graphene is fabricated into Hall ball shaped devices by the routine micro-fabrication method. However, impurity molecules adsorbed onto the graphene surface will impose considerable doping effects on the one-atom-thick film material. Our experiment demonstrates that pretreatment of the device by heat radiation baking and electrical annealing can dramatically influence the doping state of the graphene and consequently modify the electrical properties. While graphene in the as-fabricated device is highly p-doped, as confirmed by the position of the Dirac point at far more than +60 V, baking treatment at temperatures around 180°C can significantly lower the doping level and reduce the conductivity. The following electrical annealing is much more efficient to desorb the extrinsic molecules, as confirmed by the in situ measurement, and as a result, further modify the doping state and electrical properties of the graphene, causing a considerable drop of the conductivity and a shifting of Dirac point from beyond +60 V to 0 V.

  20. Evolution of electronic structure of few-layer phosphorene from angle-resolved photoemission spectroscopy of black phosphorous

    NASA Astrophysics Data System (ADS)

    Ehlen, N.; Senkovskiy, B. V.; Fedorov, A. V.; Perucchi, A.; Di Pietro, P.; Sanna, A.; Profeta, G.; Petaccia, L.; Grüneis, A.

    2016-12-01

    A complete set of tight-binding parameters for the description of the quasiparticle dispersion relations of black phosphorous (BP) and N -layer phosphorene with N =1 ...∞ is presented. The parameters, which describe valence and conduction bands, are fit to angle-resolved photoemission spectroscopy (ARPES) data of pristine and lithium doped BP. We show that zone-folding of the experimental three-dimensional electronic band structure of BP is a simple and intuitive method to obtain the layer-dependent two-dimensional electronic structure of few-layer phosphorene. Zone folding yields the band gap of N -layer phosphorene in excellent quantitative agreement to experiments and ab initio calculations. A combined analysis of optical absorption and ARPES spectra of pristine and doped BP is used to estimate a value for the exciton binding energy of BP.

  1. Simultaneous generation of high-efficiency broadband asymmetric anomalous refraction and reflection waves with few-layer anisotropic metasurface

    PubMed Central

    Li, Zhancheng; Liu, Wenwei; Cheng, Hua; Liu, Jieying; Chen, Shuqi; Tian, Jianguo

    2016-01-01

    Optical metasurfaces consisting of single-layer nanostructures have immensely promising applications in wavefront control because they can be used to arbitrarily manipulate wave phase, and polarization. However, anomalous refraction and reflection waves have not yet been simultaneously and asymmetrically generated, and the limited efficiency and bandwidth of pre-existing single-layer metasurfaces hinder their practical applications. Here, a few-layer anisotropic metasurface is presented for simultaneously generating high-efficiency broadband asymmetric anomalous refraction and reflection waves. Moreover, the normal transmission and reflection waves are low and the anomalous waves are the predominant ones, which is quite beneficial for practical applications such as beam deflectors. Our work provides an effective method of enhancing the performance of anomalous wave generation, and the asymmetric performance of the proposed metasurface shows endless possibilities in wavefront control for nanophotonics device design and optical communication applications. PMID:27762286

  2. Structural, electronic and vibrational properties of few-layer 2H-and 1T-TaSe2

    DOE PAGES

    Yan, Jia -An; Dela Cruz, Mack A.; Cook, Brandon G.; ...

    2015-11-16

    Two-dimensional metallic transition metal dichalcogenides (TMDs) are of interest for studying phenomena such as charge-density wave (CDW) and superconductivity. Few-layer tantalum diselenides (TaSe2) are typical metallic TMDs exhibiting rich CDW phase transitions. However, a description of the structural, electronic and vibrational properties for different crystal phases and stacking configurations, essential for interpretation of experiments, is lacking. We present first principles calculations of structural phase energetics, band dispersion near the Fermi level, phonon properties and vibrational modes at the Brillouin zone center for different layer numbers, crystal phases and stacking geometries. Evolution of the Fermi surfaces as well as the phononmore » dispersions as a function of layer number reveals dramatic dimensionality effects in this CDW material. Lastly, our results indicate strong electronic interlayer coupling, detail energetically possible stacking geometries, and provide a basis for interpretation of Raman spectra.« less

  3. Simultaneous generation of high-efficiency broadband asymmetric anomalous refraction and reflection waves with few-layer anisotropic metasurface.

    PubMed

    Li, Zhancheng; Liu, Wenwei; Cheng, Hua; Liu, Jieying; Chen, Shuqi; Tian, Jianguo

    2016-10-20

    Optical metasurfaces consisting of single-layer nanostructures have immensely promising applications in wavefront control because they can be used to arbitrarily manipulate wave phase, and polarization. However, anomalous refraction and reflection waves have not yet been simultaneously and asymmetrically generated, and the limited efficiency and bandwidth of pre-existing single-layer metasurfaces hinder their practical applications. Here, a few-layer anisotropic metasurface is presented for simultaneously generating high-efficiency broadband asymmetric anomalous refraction and reflection waves. Moreover, the normal transmission and reflection waves are low and the anomalous waves are the predominant ones, which is quite beneficial for practical applications such as beam deflectors. Our work provides an effective method of enhancing the performance of anomalous wave generation, and the asymmetric performance of the proposed metasurface shows endless possibilities in wavefront control for nanophotonics device design and optical communication applications.

  4. Fabrication and independent control of patterned polymer gate for a few-layer WSe2 field-effect transistor

    NASA Astrophysics Data System (ADS)

    Hong, Sung Ju; Park, Min; Kang, Hojin; Lee, Minwoo; Jeong, Dae Hong; Park, Yung Woo

    2016-08-01

    We report the fabrication of a patterned polymer electrolyte for a two-dimensional (2D) semiconductor, few-layer tungsten diselenide (WSe2) field-effect transistor (FET). We expose an electron-beam in a desirable region to form the patterned structure. The WSe2 FET acts as a p-type semiconductor in both bare and polymer-covered devices. We observe a highly efficient gating effect in the polymer-patterned device with independent gate control. The patterned polymer gate operates successfully in a molybdenum disulfide (MoS2) FET, indicating the potential for general applications to 2D semiconductors. The results of this study can contribute to large-scale integration and better flexibility in transition metal dichalcogenide (TMD)-based electronics.

  5. Thickness-dependent electron mobility of single and few-layer MoS2 thin-film transistors

    NASA Astrophysics Data System (ADS)

    Kim, Ji Heon; Kim, Tae Ho; Lee, Hyunjea; Park, Young Ran; Choi, Woong; Lee, Cheol Jin

    2016-06-01

    We investigated the dependence of electron mobility on the thickness of MoS2 nanosheets by fabricating bottom-gate single and few-layer MoS2 thin-film transistors with SiO2 gate dielectrics and Au electrodes. All the fabricated MoS2 transistors showed on/off-current ratio of ˜107 and saturated output characteristics without high-k capping layers. As the MoS2 thickness increased from 1 to 6 layers, the field-effect mobility of the fabricated MoS2 transistors increased from ˜10 to ˜18 cm2V-1s-1. The increased subthreshold swing of the fabricated transistors with MoS2 thickness suggests that the increase of MoS2 mobility with thickness may be related to the dependence of the contact resistance and the dielectric constant of MoS2 layer on its thickness.

  6. Superelastic Few-Layer Carbon Foam Made from Natural Cotton for All-Solid-State Electrochemical Capacitors.

    PubMed

    Lin, Tianquan; Liu, Fengxin; Xu, Feng; Bi, Hui; Du, Yahui; Tang, Yufeng; Huang, Fuqiang

    2015-11-18

    Flexible/stretchable devices for energy storage are essential for future wearable and flexible electronics. Electrochemical capacitors (ECs) are an important technology for supplement batteries in the energy storage and harvesting field, but they are limited by relatively low energy density. Herein, we report a superelastic foam consisting of few-layer carbon nanowalls made from natural cotton as a good scaffold to growth conductive polymer polyaniline for stretchable, lightweight, and flexible all-solid-state ECs. As-prepared superelastic bulk tubular carbon foam (surface area ∼950 m(2)/g) can withstand >90% repeated compression cycling and support >45,000 times its own weight but no damage. The flexible device has a high specific capacitance of 510 F g(-1), a specific energy of 25.5 Wh kg(-1) and a power density of 28.5 kW kg(-1) in weight of the total electrode materials and withstands 5,000 charging/discharging cycles.

  7. Spin-resolved photoemission study of epitaxially grown MoSe 2 and WSe 2 thin films

    DOE PAGES

    Mo, Sung-Kwan; Hwang, Choongyu; Zhang, Yi; ...

    2016-09-12

    Few-layer thick MoSe2 and WSe2 possess non-trivial spin textures with sizable spin splitting due to the inversion symmetry breaking embedded in the crystal structure and strong spin–orbit coupling. Here, we report a spin-resolved photoemission study of MoSe2 and WSe2 thin film samples epitaxially grown on a bilayer graphene substrate. Furthermore, we only found spin polarization in the single- and trilayer samples—not in the bilayer sample—mostly along the out-of-plane direction of the sample surface. The measured spin polarization is found to be strongly dependent on the light polarization as well as the measurement geometry, which reveals intricate coupling between the spinmore » and orbital degrees of freedom in this class of material.« less

  8. Electronic and Optical Properties of Few Layer Black Phosphorus and Black Phosphorus Nanoribbons from First Principles Calculations

    NASA Astrophysics Data System (ADS)

    Tran, Vy

    Recently, a new semiconducting 2D material, black phosphorus, has piqued the interest of research groups in the field. In its bulk form, black phosphorus was synthesized over a century ago and in 2014 devices based on thin flakes of black phosphorus were successfully realized. This was a crucial step towards the exploration and characterization of this material. However, because this material was virtually ignored until this point, many open questions needed to be quickly addressed. Fundamental properties such as the band gap, carrier mobility, optical spectrum, and thermal transport had not been established. Furthermore, the effect of extrinsic factors such as the number of layers, external electric fields, and applied strain had not been explored. How these extrinsic factors affect the tunability of the aforementioned physical properties is of utmost importance for device engineers. Using first principle computations based on density functional theory and the GW approximation including many-electron effects, we calculate the fundamental electronic and optical properties of few-layer black phosphorus. Beyond basic calculations, such as the band structure, quasiparticle band gap, and optical absorption spectrum, we dig deeper to explore the origin and nature of some of black phosphorus' unusual and surprising properties. These properties include the existence of relativistic Dirac fermions as charge carriers, a highly anisotropic band structure, an anisotropic optical absorption spectrum, quasi-1D excitonic features, and an ultra-high sensitivity to a gate electric field. In the first chapter, we discuss the properties of few-layer black phosphorus. We calculate the quasiparticle band gap, and excitonic optical spectra for 1-4 layers. We provide an empirical formula in the form of a power law to fit the calculated results and predict the values for larger layer numbers. We also propose an effective mass hydrogenic model to describe the excitonic spectra calculated

  9. Probing the electronic properties of graphene on C-face SiC down to single domains by nanoresolved photoelectron spectroscopies

    NASA Astrophysics Data System (ADS)

    Razado-Colambo, I.; Avila, J.; Chen, C.; Nys, J.-P.; Wallart, X.; Asensio, M.-C.; Vignaud, D.

    2015-07-01

    Graphene samples with thicknesses ranging from monolayer to few layer graphene grown on the C-face of SiC by Si flux-assisted molecular beam epitaxy were studied to understand their stacking structure. Particular attention was put on determining the size, thickness, spatial distribution, and orientation relative to the SiC of the graphene domains. A complete electronic characterization of the graphene films down to submicrometer grains was obtained by using synchrotron-based conventional and nanoresolved photoelectron spectroscopies. These measurements were completed with scanning probe techniques like atomic force and scanning tunneling microscopies. By probing exactly the same region of the samples using angular-resolved and core-level photoelectron spectroscopy imaging and point modes, we were able to identify two types of grains constituting the graphene films with radically different thickness, stacking and orientation. The size, distribution, and registry with the substrate for each type of grain were determined. Most interestingly, we have evidenced that multilayer graphene grains with Bernal stacking coexist with areas composed of twisted bilayer graphene grains.

  10. Erbium-doped fiber laser passively mode locked with few-layer WSe2/MoSe2 nanosheets

    PubMed Central

    Mao, Dong; She, Xiaoyang; Du, Bobo; Yang, Dexing; Zhang, Wending; Song, Kun; Cui, Xiaoqi; Jiang, Biqiang; Peng, Tao; Zhao, Jianlin

    2016-01-01

    Few-layer transition-metal dichalcogenide WSe2/MoSe2 nanosheets are fabricated by a liquid exfoliation technique using sodium deoxycholate bile salt as surfactant, and their nonlinear optical properties are investigated based on a balanced twin-detector measurement scheme. It is demonstrated that both types of nanosheets exhibit nonlinear saturable absorption properties at the wavelength of 1.55 μm. By depositing the nanosheets on side polished fiber (SPF) or mixing the nanosheets with polyvinyl alcohol (PVA) solution, SPF-WSe2 saturable absorber (SA), SPF-MoSe2 SA, PVA-WSe2 SA, and PVA-MoSe2 SA are successfully fabricated and further tested in erbium-doped fiber lasers. The SPF-based SA is capable of operating at the high pump regime without damage, and a train of 3252.65 MHz harmonically mode-locked pulses are obtained based on the SPF-WSe2 SA. Soliton mode locking operations are also achieved in the fiber laser separately with other three types of SAs, confirming that the WSe2 and MoSe2 nanosheets could act as cost-effective high-power SAs for ultrafast optics. PMID:27010509

  11. Erbium-doped fiber laser passively mode locked with few-layer WSe2/MoSe2 nanosheets.

    PubMed

    Mao, Dong; She, Xiaoyang; Du, Bobo; Yang, Dexing; Zhang, Wending; Song, Kun; Cui, Xiaoqi; Jiang, Biqiang; Peng, Tao; Zhao, Jianlin

    2016-03-24

    Few-layer transition-metal dichalcogenide WSe2/MoSe2 nanosheets are fabricated by a liquid exfoliation technique using sodium deoxycholate bile salt as surfactant, and their nonlinear optical properties are investigated based on a balanced twin-detector measurement scheme. It is demonstrated that both types of nanosheets exhibit nonlinear saturable absorption properties at the wavelength of 1.55 μm. By depositing the nanosheets on side polished fiber (SPF) or mixing the nanosheets with polyvinyl alcohol (PVA) solution, SPF-WSe2 saturable absorber (SA), SPF-MoSe2 SA, PVA-WSe2 SA, and PVA-MoSe2 SA are successfully fabricated and further tested in erbium-doped fiber lasers. The SPF-based SA is capable of operating at the high pump regime without damage, and a train of 3252.65 MHz harmonically mode-locked pulses are obtained based on the SPF-WSe2 SA. Soliton mode locking operations are also achieved in the fiber laser separately with other three types of SAs, confirming that the WSe2 and MoSe2 nanosheets could act as cost-effective high-power SAs for ultrafast optics.

  12. Erbium-doped fiber laser passively mode locked with few-layer WSe2/MoSe2 nanosheets

    NASA Astrophysics Data System (ADS)

    Mao, Dong; She, Xiaoyang; Du, Bobo; Yang, Dexing; Zhang, Wending; Song, Kun; Cui, Xiaoqi; Jiang, Biqiang; Peng, Tao; Zhao, Jianlin

    2016-03-01

    Few-layer transition-metal dichalcogenide WSe2/MoSe2 nanosheets are fabricated by a liquid exfoliation technique using sodium deoxycholate bile salt as surfactant, and their nonlinear optical properties are investigated based on a balanced twin-detector measurement scheme. It is demonstrated that both types of nanosheets exhibit nonlinear saturable absorption properties at the wavelength of 1.55 μm. By depositing the nanosheets on side polished fiber (SPF) or mixing the nanosheets with polyvinyl alcohol (PVA) solution, SPF-WSe2 saturable absorber (SA), SPF-MoSe2 SA, PVA-WSe2 SA, and PVA-MoSe2 SA are successfully fabricated and further tested in erbium-doped fiber lasers. The SPF-based SA is capable of operating at the high pump regime without damage, and a train of 3252.65 MHz harmonically mode-locked pulses are obtained based on the SPF-WSe2 SA. Soliton mode locking operations are also achieved in the fiber laser separately with other three types of SAs, confirming that the WSe2 and MoSe2 nanosheets could act as cost-effective high-power SAs for ultrafast optics.

  13. Few-Layer MoS₂ p-Type Devices Enabled by Selective Doping Using Low Energy Phosphorus Implantation.

    PubMed

    Nipane, Ankur; Karmakar, Debjani; Kaushik, Naveen; Karande, Shruti; Lodha, Saurabh

    2016-02-23

    P-type doping of MoS2 has proved to be a significant bottleneck in the realization of fundamental devices such as p-n junction diodes and p-type transistors due to its intrinsic n-type behavior. We report a CMOS compatible, controllable and area selective phosphorus plasma immersion ion implantation (PIII) process for p-type doping of MoS2. Physical characterization using SIMS, AFM, XRD and Raman techniques was used to identify process conditions with reduced lattice defects as well as low surface damage and etching, 4X lower than previous plasma based doping reports for MoS2. A wide range of nondegenerate to degenerate p-type doping is demonstrated in MoS2 field effect transistors exhibiting dominant hole transport. Nearly ideal and air stable, lateral homogeneous p-n junction diodes with a gate-tunable rectification ratio as high as 2 × 10(4) are demonstrated using area selective doping. Comparison of XPS data from unimplanted and implanted MoS2 layers shows a shift of 0.67 eV toward lower binding energies for Mo and S peaks indicating p-type doping. First-principles calculations using density functional theory techniques confirm p-type doping due to charge transfer originating from substitutional as well as physisorbed phosphorus in top few layers of MoS2. Pre-existing sulfur vacancies are shown to enhance the doping level significantly.

  14. Exfoliation and Raman Spectroscopic Fingerprint of Few-Layer NiPS3 Van der Waals Crystals

    PubMed Central

    Kuo, Cheng-Tai; Neumann, Michael; Balamurugan, Karuppannan; Park, Hyun Ju; Kang, Soonmin; Shiu, Hung Wei; Kang, Jin Hyoun; Hong, Byung Hee; Han, Moonsup; Noh, Tae Won; Park, Je-Geun

    2016-01-01

    The range of mechanically cleavable Van der Waals crystals covers materials with diverse physical and chemical properties. However, very few of these materials exhibit magnetism or magnetic order, and thus the provision of cleavable magnetic compounds would supply invaluable building blocks for the design of heterostructures assembled from Van der Waals crystals. Here we report the first successful isolation of monolayer and few-layer samples of the compound nickel phosphorus trisulfide (NiPS3) by mechanical exfoliation. This material belongs to the class of transition metal phosphorus trisulfides (MPS3), several of which exhibit antiferromagnetic order at low temperature, and which have not been reported in the form of ultrathin sheets so far. We establish layer numbers by optical bright field microscopy and atomic force microscopy, and perform a detailed Raman spectroscopic characterization of bilayer and thicker NiPS3 flakes. Raman spectral features are strong functions of excitation wavelength and sample thickness, highlighting the important role of interlayer coupling. Furthermore, our observations provide a spectral fingerprint for distinct layer numbers, allowing us to establish a sensitive and convenient means for layer number determination. PMID:26875451

  15. Ab initio simulation of single- and few-layer MoS2 transistors: Effect of electron-phonon scattering

    NASA Astrophysics Data System (ADS)

    Szabó, Áron; Rhyner, Reto; Luisier, Mathieu

    2015-07-01

    In this paper, we present full-band atomistic quantum transport simulations of single- and few-layer MoS2 field-effect transistors (FETs) including electron-phonon scattering. The Hamiltonian and the electron-phonon coupling constants are determined from ab initio density-functional-theory calculations. It is observed that the phonon-limited electron mobility is enhanced with increasing layer thicknesses and decreases at high charge concentrations. The electrostatic control is found to be crucial even for a single-layer MoS2 device. With a single-gate configuration, the double-layer MoS2 FET shows the best intrinsic performance with an ON current, ION=685 μ A /μ m , but with a double-gate contact the transistor with a triple-layer channel delivers the highest current with ION=1850 μ A /μ m . The charge in the channel is almost independent of the number of MoS2 layers, but the injection velocity increases significantly with the channel thickness in the double-gate devices due to the reduced electron-phonon scattering rates in multilayer structures. We demonstrate further that the ballistic limit of transport is not suitable for the simulation of MX 2 FETs because of the artificial negative differential resistance it predicts.

  16. Controlled Scalable Synthesis of Uniform, High-Quality Monolayer and Few-layer MoS2 Films

    PubMed Central

    Yu, Yifei; Li, Chun; Liu, Yi; Su, Liqin; Zhang, Yong; Cao, Linyou

    2013-01-01

    Two dimensional (2D) materials with a monolayer of atoms represent an ultimate control of material dimension in the vertical direction. Molybdenum sulfide (MoS2) monolayers, with a direct bandgap of 1.8 eV, offer an unprecedented prospect of miniaturizing semiconductor science and technology down to a truly atomic scale. Recent studies have indeed demonstrated the promise of 2D MoS2 in fields including field effect transistors, low power switches, optoelectronics, and spintronics. However, device development with 2D MoS2 has been delayed by the lack of capabilities to produce large-area, uniform, and high-quality MoS2 monolayers. Here we present a self-limiting approach that can grow high quality monolayer and few-layer MoS2 films over an area of centimeters with unprecedented uniformity and controllability. This approach is compatible with the standard fabrication process in semiconductor industry. It paves the way for the development of practical devices with 2D MoS2 and opens up new avenues for fundamental research. PMID:23689610

  17. Hall and field-effect mobilities in few layered p-WSe2 field-effect transistors

    NASA Astrophysics Data System (ADS)

    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.

    2015-03-01

    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 cm2/Vs at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm2/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.

  18. Layer-number dependent high-frequency vibration modes in few-layer transition metal dichalcogenides induced by interlayer couplings

    NASA Astrophysics Data System (ADS)

    Tan, Qing-Hai; Zhang, Xin; Luo, Xiang-Dong; Zhang, Jun; Tan, Ping-Heng

    2017-03-01

    Two-dimensional transition metal dichalcogenides (TMDs) have attracted extensive attention due to their many novel properties. The atoms within each layer in two-dimensional TMDs are joined together by covalent bonds, while van der Waals interactions combine the layers together. This makes its lattice dynamics layer-number dependent. The evolutions of ultralow frequency (< 50 cm‑1) modes, such as shear and layer-breathing modes have been well-established. Here, we review the layer-number dependent high-frequency (> 50 cm‑1) vibration modes in few-layer TMDs and demonstrate how the interlayer coupling leads to the splitting of high-frequency vibration modes, known as Davydov splitting. Such Davydov splitting can be well described by a van der Waals model, which directly links the splitting with the interlayer coupling. Our review expands the understanding on the effect of interlayer coupling on the high-frequency vibration modes in TMDs and other two-dimensional materials. Project supported by the National Basic Research Program of China (No. 2016YFA0301200), the National Natural Science Foundation of China (Nos. 11225421, 11474277, 11434010, 61474067, 11604326, 11574305 and 51527901), and the National Young 1000 Talent Plan of China.

  19. Tuning the charge carriers in epitaxial graphene on SiC(0001) from electron to hole via molecular doping with C60F48

    NASA Astrophysics Data System (ADS)

    Tadich, A.; Edmonds, M. T.; Ley, L.; Fromm, F.; Smets, Y.; Mazej, Z.; Riley, J.; Pakes, C. I.; Seyller, Th.; Wanke, M.

    2013-06-01

    We demonstrate that the intrinsic electron doping of monolayer epitaxial graphene on SiC(0001) can be tuned in a controlled fashion to holes via molecular doping with the fluorinated fullerene C60F48. In situ angle-resolved photoemission is used to measure an upward shift of (0.6 ± 0.05) eV in the Dirac point from -0.43 eV to +0.17 eV relative to the Fermi level. The carrier density is observed to change from n ˜ (1 × 1013 ± 0.1 × 1013) cm-2 to p ˜ (2 × 1012 ± 1 × 1012) cm-2. We introduce a doping model employing Fermi-Dirac statistics which explicitly takes temperature and the highly correlated nature of molecular orbitals into account. The model describes the observed doping behaviour in our experiment and readily explains why net p-type doping was not achieved in a previous study [Coletti et al., Phys. Rev. B 81, 8 (2010)] which used tetrafluorotetra-cyanoquinodimethane (F4-TCNQ).

  20. Fabrication and transfer of flexible few-layers MoS2 thin film transistors to any arbitrary substrate.

    PubMed

    Salvatore, Giovanni A; Münzenrieder, Niko; Barraud, Clément; Petti, Luisa; Zysset, Christoph; Büthe, Lars; Ensslin, Klaus; Tröster, Gerhard

    2013-10-22

    Recently, transition metal dichalcogenides (TMDCs) have attracted interest thanks to their large field effective mobility (>100 cm(2)/V · s), sizable band gap (around 1-2 eV), and mechanical properties, which make them suitable for high performance and flexible electronics. In this paper, we present a process scheme enabling the fabrication and transfer of few-layers MoS2 thin film transistors from a silicon template to any arbitrary organic or inorganic and flexible or rigid substrate or support. The two-dimensional semiconductor is mechanically exfoliated from a bulk crystal on a silicon/polyvinyl alcohol (PVA)/polymethyl methacrylane (PMMA) stack optimized to ensure high contrast for the identification of subnanometer thick flakes. Thin film transistors (TFTs) with structured source/drain and gate electrodes are fabricated following a designed procedure including steps of UV lithography, wet etching, and atomic layer deposited (ALD) dielectric. Successively, after the dissolution of the PVA sacrificial layer in water, the PMMA film, with the devices on top, can be transferred to another substrate of choice. Here, we transferred the devices on a polyimide plastic foil and studied the performance when tensile strain is applied parallel to the TFT channel. We measured an electron field effective mobility of 19 cm(2)/(V s), an I(on)/I(off)ratio greater than 10(6), a gate leakage current as low as 0.3 pA/μm, and a subthreshold swing of about 250 mV/dec. The devices continue to work when bent to a radius of 5 mm and after 10 consecutive bending cycles. The proposed fabrication strategy can be extended to any kind of 2D materials and enable the realization of electronic circuits and optical devices easily transferrable to any other support.

  1. Focused helium-ion beam irradiation effects on electrical transport properties of few-layer WSe2: Enabling nanoscale direct write homo-junctions

    SciTech Connect

    Stanford, Michael; Noh, Joo Hyon; Koehler, Michael R.; Mandrus, David G.; Duscher, Gerd; Rondinone, Adam Justin; Ivanov, Ilia N.; Ward, Thomas Zac; Rack, Philip D.; Pudasaini, Pushpa Raj; Belianinov, Alex; Cross, Nicholas

    2016-06-06

    Atomically thin transition metal dichalcogenides (TMDs) are currently receiving significant attention due to their promising opto-electronic properties. Tuning optical and electrical properties of mono and few-layer TMDs, such as tungsten diselenide (WSe2), by controlling the defects, is an intriguing opportunity to synthesize next generation two dimensional material opto-electronic devices. Here, we report the effects of focused helium ion beam irradiation on the structural, optical and electrical properties of few-layer WSe2, via high resolution scanning transmission electron microscopy, Raman spectroscopy, and electrical transport measurements. By controlling the ion irradiation dose, we selectively introduce precise defects in few-layer WSe2 thereby locally tuning the resistivity and transport properties of the material. Hole transport in the few layer WSe2 is degraded more severely relative to electron transport after helium ion irradiation. Moreover, by selectively exposing material with the ion beam, we demonstrate a simple yet highly tunable method to create lateral homo-junctions in few layer WSe2 flakes, which constitutes an important advance towards two dimensional opto-electronic devices.

  2. Focused helium-ion beam irradiation effects on electrical transport properties of few-layer WSe2: enabling nanoscale direct write homo-junctions

    NASA Astrophysics Data System (ADS)

    Stanford, Michael G.; Pudasaini, Pushpa Raj; Belianinov, Alex; Cross, Nicholas; Noh, Joo Hyon; Koehler, Michael R.; Mandrus, David G.; Duscher, Gerd; Rondinone, Adam J.; Ivanov, Ilia N.; Ward, T. Zac; Rack, Philip D.

    2016-06-01

    Atomically thin transition metal dichalcogenides (TMDs) are currently receiving significant attention due to their promising opto-electronic properties. Tuning optical and electrical properties of mono and few-layer TMDs, such as tungsten diselenide (WSe2), by controlling the defects, is an intriguing opportunity to synthesize next generation two dimensional material opto-electronic devices. Here, we report the effects of focused helium ion beam irradiation on the structural, optical and electrical properties of few-layer WSe2, via high resolution scanning transmission electron microscopy, Raman spectroscopy, and electrical transport measurements. By controlling the ion irradiation dose, we selectively introduce precise defects in few-layer WSe2 thereby locally tuning the resistivity and transport properties of the material. Hole transport in the few layer WSe2 is degraded more severely relative to electron transport after helium ion irradiation. Furthermore, by selectively exposing material with the ion beam, we demonstrate a simple yet highly tunable method to create lateral homo-junctions in few layer WSe2 flakes, which constitutes an important advance towards two dimensional opto-electronic devices.

  3. Focused helium-ion beam irradiation effects on electrical transport properties of few-layer WSe2: Enabling nanoscale direct write homo-junctions

    DOE PAGES

    Stanford, Michael; Noh, Joo Hyon; Koehler, Michael R.; ...

    2016-06-06

    Atomically thin transition metal dichalcogenides (TMDs) are currently receiving significant attention due to their promising opto-electronic properties. Tuning optical and electrical properties of mono and few-layer TMDs, such as tungsten diselenide (WSe2), by controlling the defects, is an intriguing opportunity to synthesize next generation two dimensional material opto-electronic devices. Here, we report the effects of focused helium ion beam irradiation on the structural, optical and electrical properties of few-layer WSe2, via high resolution scanning transmission electron microscopy, Raman spectroscopy, and electrical transport measurements. By controlling the ion irradiation dose, we selectively introduce precise defects in few-layer WSe2 thereby locally tuningmore » the resistivity and transport properties of the material. Hole transport in the few layer WSe2 is degraded more severely relative to electron transport after helium ion irradiation. Moreover, by selectively exposing material with the ion beam, we demonstrate a simple yet highly tunable method to create lateral homo-junctions in few layer WSe2 flakes, which constitutes an important advance towards two dimensional opto-electronic devices.« less

  4. Atmospheric pressure route to epitaxial nitrogen-doped trilayer graphene on 4H-SiC (0001) substrate

    SciTech Connect

    Boutchich, M.; Arezki, H.; Alamarguy, D.; Güneş, F.; Alvarez, J.; Kleider, J. P.; Ho, K.-I.; Lai, C. S.; Sediri, H.; Ouerghi, A.

    2014-12-08

    Large-area graphene film doped with nitrogen is of great interest for a wide spectrum of nanoelectronics applications, such as field effect devices, super capacitors, and fuel cells among many others. Here, we report on the structural and electronic properties of nitrogen doped trilayer graphene on 4H-SiC (0001) grown under atmospheric pressure. The trilayer nature of the growth is evidenced by scanning transmission electron microscopy. X-ray photoelectron spectroscopy shows the incorporation of 1.2% of nitrogen distributed in pyrrolic-N, and pyridinic-N configurations as well as a graphitic-N contribution. This incorporation causes an increase in the D band on the Raman signature indicating that the nitrogen is creating defects. Ultraviolet photoelectron spectroscopy shows a decrease of the work function of 0.3 eV due to the N-type doping of the nitrogen atoms in the carbon lattice and the edge defects. A top gate field effect transistor device has been fabricated and exhibits carrier mobilities up to 1300 cm{sup 2}/V s for holes and 850 cm{sup 2}/V s for electrons at room temperature.

  5. Gate-tunable and thickness-dependent electronic and thermoelectric transport in few-layer MoS2

    NASA Astrophysics Data System (ADS)

    Kayyalha, Morteza; Maassen, Jesse; Lundstrom, Mark; Shi, Li; Chen, Yong P.

    2016-10-01

    Over the past few years, there has been a growing interest in layered transition metal dichalcogenides such as molybdenum disulfide (MoS2). Most studies so far have focused on the electronic and optoelectronic properties of single-layer MoS2, whose band structure features a direct bandgap, in sharp contrast to the indirect bandgap of thicker MoS2. In this paper, we present a systematic study of the thickness-dependent electrical and thermoelectric properties of few-layer MoS2. We observe that the electrical conductivity ( σ) increases as we reduce the thickness of MoS2 and peaks at about two layers, with six-times larger conductivity than our thickest sample (23-layer MoS2). Using a back-gate voltage, we modulate the Fermi energy ( E F ) of the sample where an increase in the Seebeck coefficient ( S ) is observed with decreasing gate voltage ( E F ) towards the subthreshold (OFF state) of the device, reaching as large as 500 μ V / K in a four-layer MoS2. While previous reports have focused on a single-layer MoS2 and measured Seebeck coefficient in the OFF state, which has vanishing electrical conductivity and thermoelectric power factor ( P F = S 2 σ ), we show that MoS2-based devices in their ON state can have P F as large as > 50 /μ W cm K 2 in the two-layer sample. The P F increases with decreasing thickness and then drops abruptly from double-layer to single-layer MoS2, a feature we suggest as due to a change in the energy dependence of the electron mean-free-path according to our theoretical calculation. Moreover, we show that care must be taken in thermoelectric measurements in the OFF state to avoid obtaining erroneously large Seebeck coefficients when the channel resistance is very high. Our study paves the way towards a more comprehensive examination of the thermoelectric performance of two-dimensional (2D) semiconductors.

  6. Spin-resolved photoemission study of epitaxially grown MoSe 2 and WSe 2 thin films

    SciTech Connect

    Mo, Sung-Kwan; Hwang, Choongyu; Zhang, Yi; Fanciulli, Mauro; Muff, Stefan; Hugo Dil, J.; Shen, Zhi-Xun; Hussain, Zahid

    2016-09-12

    Few-layer thick MoSe2 and WSe2 possess non-trivial spin textures with sizable spin splitting due to the inversion symmetry breaking embedded in the crystal structure and strong spin–orbit coupling. Here, we report a spin-resolved photoemission study of MoSe2 and WSe2 thin film samples epitaxially grown on a bilayer graphene substrate. Furthermore, we only found spin polarization in the single- and trilayer samples—not in the bilayer sample—mostly along the out-of-plane direction of the sample surface. The measured spin polarization is found to be strongly dependent on the light polarization as well as the measurement geometry, which reveals intricate coupling between the spin and orbital degrees of freedom in this class of material.

  7. Tunneling Plasmonics in Bilayer Graphene.

    PubMed

    Fei, Z; Iwinski, E G; Ni, G X; Zhang, L M; Bao, W; Rodin, A S; Lee, Y; Wagner, M; Liu, M K; Dai, S; Goldflam, M D; Thiemens, M; Keilmann, F; Lau, C N; Castro-Neto, A H; Fogler, M M; Basov, D N

    2015-08-12

    We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At subnanometer separation, such layers can form either a strongly coupled bilayer graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nanoimaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single- and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene, yet another implication of interlayer tunneling. Our work uncovers essential plasmonic properties in bilayer graphene and suggests a possibility to achieve novel plasmonic functionalities in graphene few-layers.

  8. Tunneling Plasmonics in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Fei, Z.; Iwinski, E. G.; Ni, G. X.; Zhang, L. M.; Bao, W.; Rodin, A. S.; Lee, Y.; Wagner, M.; Liu, M. K.; Dai, S.; Goldflam, M. D.; Thiemens, M.; Keilmann, F.; Lau, C. N.; Castro-Neto, A. H.; Fogler, M. M.; Basov, D. N.

    2015-08-01

    We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At sub-nanometer separation, such layers can form either a strongly coupled bilayer graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nano-imaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single- and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene: yet another implication of interlayer tunneling. Our work uncovers essential plasmonic properties in bilayer graphene and suggests a possibility to achieve novel plasmonic functionalities in graphene few-layers.

  9. All-Graphene Photodetectors

    PubMed Central

    2013-01-01

    We investigate the optoelectronic properties of novel graphene/FeCl3-intercalated few-layer graphene (FeCl3–FLG, dubbed graphexeter) heterostructures using photovoltage spectroscopy. We observe a prominent photovoltage signal generated at the graphene/FeCl3–FLG and graphene/Au interfaces, whereas the photovoltage at the FeCl3–FLG/Au interface is negligible. The sign of the photovoltage changes upon sweeping the chemical potential of the pristine graphene through the charge neutrality point, and we show that this is due to the photothermoelectric effect. Our results are a first step toward all-graphene-based photodetectors and photovoltaics. PMID:23597182

  10. A graphene electron lens

    NASA Astrophysics Data System (ADS)

    Gerhard, L.; Moyen, E.; Balashov, T.; Ozerov, I.; Portail, M.; Sahaf, H.; Masson, L.; Wulfhekel, W.; Hanbücken, M.

    2012-04-01

    An epitaxial layer of graphene was grown on a pre patterned 6H-SiC(0001) crystal. The graphene smoothly covers the hexagonal nano-holes in the substrate without the introduction of small angle grain boundaries or dislocations. This is achieved by an elastic deformation of the graphene by ≈0.3% in accordance to its large elastic strain limit. This elastic stretching of the graphene leads to a modification of the band structure and to a local lowering of the electron group velocity of the graphene. We propose to use this effect to focus two-dimensional electrons in analogy to simple optical lenses.

  11. Seeing Many-Body Effects in Single- and Few-Layer Graphene: Observation of Two-Dimensional Saddle-Point Excitons

    DTIC Science & Technology

    2010-01-01

    of the Fermi velocity [3], and of plasmarons [4]. The role of electron-hole interactions, as are relevant for the optical response, has remained...somewhat paradoxical . On the one hand, theoretical studies have predicted strong excitonic corrections to the optical response [5-9], as well as BCS...layer and from the increased density of states at the Fermi energy [31]. Thus, excitonic effects are expected to diminish with increasing layer number

  12. Electrostatic force assisted deposition of graphene

    DOEpatents

    Liang, Xiaogan [Berkeley, CA

    2011-11-15

    An embodiment of a method of depositing graphene includes bringing a stamp into contact with a substrate over a contact area. The stamp has at least a few layers of the graphene covering the contact area. An electric field is developed over the contact area. The stamp is removed from the vicinity of the substrate which leaves at least a layer of the graphene substantially covering the contact area.

  13. Layer-Controlled Chemical Vapor Deposition Growth of MoS2 Vertical Heterostructures via van der Waals Epitaxy.

    PubMed

    Samad, Leith; Bladow, Sage M; Ding, Qi; Zhuo, Junqiao; Jacobberger, Robert M; Arnold, Michael S; Jin, Song

    2016-07-26

    The fascinating semiconducting and optical properties of monolayer and few-layer transition metal dichalcogenides, as exemplified by MoS2, have made them promising candidates for optoelectronic applications. Controllable growth of heterostructures based on these layered materials is critical for their successful device applications. Here, we report a direct low temperature chemical vapor deposition (CVD) synthesis of MoS2 monolayer/multilayer vertical heterostructures with layer-controlled growth on a variety of layered materials (SnS2, TaS2, and graphene) via van der Waals epitaxy. Through precise control of the partial pressures of the MoCl5 and elemental sulfur precursors, reaction temperatures, and careful tracking of the ambient humidity, we have successfully and reproducibly grown MoS2 vertical heterostructures from 1 to 6 layers over a large area. The monolayer MoS2 heterostructure was verified using cross-sectional high resolution transmission electron microscopy (HRTEM) while Raman and photoluminescence spectroscopy confirmed the layer-controlled MoS2 growth and heterostructure electronic interactions. Raman, photoluminescence, and energy dispersive X-ray spectroscopy (EDS) mappings verified the uniform coverage of the MoS2 layers. This reaction provides an ideal method for the scalable layer-controlled growth of transition metal dichalcogenide heterostructures via van der Waals epitaxy for a variety of optoelectronic applications.

  14. Origins of Ripples in CVD-Grown Few-layered MoS2 Structures under Applied Strain at Atomic Scales

    PubMed Central

    Wang, Jin; Namburu, Raju R.; Dubey, Madan; Dongare, Avinash M.

    2017-01-01

    The potential of the applicability of two-dimensional molybdenum disulfide (MoS2) structures, in various electronics, optoelectronics, and flexible devices requires a fundamental understanding of the effects of strain on the electronic, magnetic and optical properties. Particularly important is the recent capability to grow large flakes of few-layered structures using chemical vapor deposition (CVD) wherein the top layers are relatively smaller in size than the bottom layers, resulting in the presence of edges/steps across adjacent layers. This paper investigates the strain response of such suspended few-layered structures at the atomic scales using classic molecular dynamics (MD) simulations. MD simulations suggest that the suspended CVD-grown structures are able to relax the applied in-plane strain through the nucleation of ripples under both tensile and compressive loading conditions. The presence of terraced edges in these structures is the cause for the nucleation of ripples at the edges that grow towards the center of the structure under applied in-plane strains. The peak amplitudes of ripples observed are in excellent agreement with the experimental observations. The study provides critical insights into the mechanisms of strain relaxation of suspended few-layered MoS2 structures that determine the interplay between the mechanical response and the electronic properties of CVD-grown structures. PMID:28102351

  15. Origins of Ripples in CVD-Grown Few-layered MoS2 Structures under Applied Strain at Atomic Scales

    NASA Astrophysics Data System (ADS)

    Wang, Jin; Namburu, Raju R.; Dubey, Madan; Dongare, Avinash M.

    2017-01-01

    The potential of the applicability of two-dimensional molybdenum disulfide (MoS2) structures, in various electronics, optoelectronics, and flexible devices requires a fundamental understanding of the effects of strain on the electronic, magnetic and optical properties. Particularly important is the recent capability to grow large flakes of few-layered structures using chemical vapor deposition (CVD) wherein the top layers are relatively smaller in size than the bottom layers, resulting in the presence of edges/steps across adjacent layers. This paper investigates the strain response of such suspended few-layered structures at the atomic scales using classic molecular dynamics (MD) simulations. MD simulations suggest that the suspended CVD-grown structures are able to relax the applied in-plane strain through the nucleation of ripples under both tensile and compressive loading conditions. The presence of terraced edges in these structures is the cause for the nucleation of ripples at the edges that grow towards the center of the structure under applied in-plane strains. The peak amplitudes of ripples observed are in excellent agreement with the experimental observations. The study provides critical insights into the mechanisms of strain relaxation of suspended few-layered MoS2 structures that determine the interplay between the mechanical response and the electronic properties of CVD-grown structures.

  16. Casein mediated green synthesis and decoration of reduced graphene oxide.

    PubMed

    Maddinedi, Sireesh Babu; Mandal, Badal Kumar; Vankayala, Raviraj; Kalluru, Poliraju; Tammina, Sai Kumar; Kiran Kumar, H A

    2014-05-21

    This research is mainly focusing on one-step biosynthesis of graphene from graphene oxide and its stabilization using naturally occurring milk protein, casein. The synthesis of casein reduced graphene oxide (CRGO) was completed within 7h under reflux at 90°C with the formation of few layered fine graphene nanosheets. UV-Vis, XRD, XPS analysis data revealed the reduction process of the graphene oxide. Results of FT-IR, HPLC and TEM analysis have shown that the ensuing material consists of graphene decorated with casein molecules. Aspartic acid and glutamic acid residue present in casein molecules are responsible for the reduction of graphene oxide.

  17. Casein mediated green synthesis and decoration of reduced graphene oxide

    NASA Astrophysics Data System (ADS)

    Maddinedi, Sireesh Babu; Mandal, Badal Kumar; Vankayala, Raviraj; Kalluru, Poliraju; Tammina, Sai Kumar; Kiran Kumar, H. A.

    This research is mainly focusing on one-step biosynthesis of graphene from graphene oxide and its stabilization using naturally occurring milk protein, casein. The synthesis of casein reduced graphene oxide (CRGO) was completed within 7 h under reflux at 90 °C with the formation of few layered fine graphene nanosheets. UV-Vis, XRD, XPS analysis data revealed the reduction process of the graphene oxide. Results of FT-IR, HPLC and TEM analysis have shown that the ensuing material consists of graphene decorated with casein molecules. Aspartic acid and glutamic acid residue present in casein molecules are responsible for the reduction of graphene oxide.

  18. High quality epitaxial graphene by hydrogen-etching of 3C-SiC(111) thin-film on Si(111)

    NASA Astrophysics Data System (ADS)

    Mondelli, Pierluigi; Gupta, Bharati; Grazia Betti, Maria; Mariani, Carlo; Lipton Duffin, Josh; Motta, Nunzio

    2017-03-01

    Etching with atomic hydrogen, as a preparation step before the high-temperature growth process of graphene onto a thin 3C-SiC film grown on Si(111), greatly improves the structural quality of topmost graphene layers. Pit formation and island coalescence, which are typical of graphene growth by SiC graphitization, are quenched and accompanied by widening of the graphene domain sizes to hundreds of nanometers, and by a significant reduction in surface roughness down to a single substrate bilayer. The surface reconstructions expected for graphene and the underlying layer are shown with atomic resolution by scanning tunnelling microscopy. Spectroscopic features typical of graphene are measured by core-level photoemission and Raman spectroscopy.

  19. High quality epitaxial graphene by hydrogen-etching of 3C-SiC(111) thin-film on Si(111).

    PubMed

    Mondelli, Pierluigi; Gupta, Bharati; Betti, Maria Grazia; Mariani, Carlo; Duffin, Josh Lipton; Motta, Nunzio

    2017-03-17

    Etching with atomic hydrogen, as a preparation step before the high-temperature growth process of graphene onto a thin 3C-SiC film grown on Si(111), greatly improves the structural quality of topmost graphene layers. Pit formation and island coalescence, which are typical of graphene growth by SiC graphitization, are quenched and accompanied by widening of the graphene domain sizes to hundreds of nanometers, and by a significant reduction in surface roughness down to a single substrate bilayer. The surface reconstructions expected for graphene and the underlying layer are shown with atomic resolution by scanning tunnelling microscopy. Spectroscopic features typical of graphene are measured by core-level photoemission and Raman spectroscopy.

  20. Control of Superhydrophilic and Superhydrophobic Graphene Interface

    PubMed Central

    Dong, Jing; Yao, Zhaohui; Yang, Tianzhong; Jiang, Lili; Shen, Chengmin

    2013-01-01

    Superhydrophobic and superhydrophilic properties of chemically-modified graphene have been achieved in larger-area vertically aligned few-layer graphene nanosheets (FLGs), prepared on Si (111) substrate by microwave plasma chemical vapor deposition (MPCVD). Furthermore, in order to enhance wettability, silicon wafers with microstructures were fabricated, on which graphene nanosheets were grown and modified by a chemical method to form hydrophilic and hydrophobic structures. A superhydrophilic graphene surface (contact angle 0°) and a superhydrophobic graphene surface (contact angle 152.0°) were obtained. The results indicate that the microstructured silicon enhances the hydrophilic and hydrophobic wettabilities significantly.