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.

Epitaxial graphene-encapsulated surface reconstruction of Ge(110)

NASA Astrophysics Data System (ADS)

Campbell, Gavin P.; Kiraly, Brian; Jacobberger, Robert M.; Mannix, Andrew J.; Arnold, Michael S.; Hersam, Mark C.; Guisinger, Nathan P.; Bedzyk, Michael J.

2018-04-01

Understanding and engineering the properties of crystalline surfaces has been critical in achieving functional electronics at the nanoscale. Employing scanning tunneling microscopy, surface x-ray diffraction, and high-resolution x-ray reflectivity experiments, we present a thorough study of epitaxial graphene (EG)/Ge(110) and report a Ge(110) "6 × 2" reconstruction stabilized by the presence of epitaxial graphene unseen in group-IV semiconductor surfaces. X-ray studies reveal that graphene resides atop the surface reconstruction with a 0.34 nm van der Waals (vdW) gap and provides protection from ambient degradation.

State memory in solution gated epitaxial graphene

NASA Astrophysics Data System (ADS)

Butko, A. V.; Butko, V. Y.; Lebedev, S. P.; Lebedev, A. A.; Davydov, V. Y.; Smirnov, A. N.; Eliseyev, I. A.; Dunaevskiy, M. S.; Kumzerov, Y. A.

2018-06-01

We studied electrical transport in transistors fabricated on a surface of high quality epitaxial graphene with density of defects as low as 5·1010 cm-2 and observed quasistatic hysteresis with a time constant in a scale of hours. This constant is in a few orders of magnitude greater than the constant previously reported in CVD graphene. The hysteresis observed here can be described as a shift of ∼+2V of the Dirac point measured during a gate voltage increase from the position of the Dirac point measured during a gate voltage decrease. This hysteresis can be characterized as a nonvolatile quasistatic state memory effect in which the state of the gated graphene is determined by its initial state prior to entering the hysteretic region. Due to this effect the difference in resistance of the gated graphene measured in the hysteretic region at the same applied voltages can be as high as 70%. The observed effect can be explained by assuming that charge carriers in graphene and oppositely charged molecular ions from the solution form quasistable interfacial complexes at the graphene interface. These complexes likely preserve the initial state by preventing charge carriers in graphene from discharging in the hysteretic region.

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

GaN epitaxial layers grown on multilayer graphene by MOCVD

NASA Astrophysics Data System (ADS)

Li, Tianbao; Liu, Chenyang; Zhang, Zhe; Yu, Bin; Dong, Hailiang; Jia, Wei; Jia, Zhigang; Yu, Chunyan; Gan, Lin; Xu, Bingshe

2018-04-01

In this study, GaN epitaxial layers were successfully deposited on a multilayer graphene (MLG) by using metal-organic chemical vapor deposition (MOCVD). Highly crystalline orientations of the GaN films were confirmed through electron backscatter diffraction (EBSD). An epitaxial relationship between GaN films and MLG is unambiguously established by transmission electron microscope (TEM) analysis. The Raman spectra was used to analyze the internal stress of GaN films, and the spectrum shows residual tensile stress in the GaN films. Moreover, the results of the TEM analysis and Raman spectra indicate that the high quality of the MLG substrate is maintained even after the growth of the GaN film. This high-quality MLG makes it possible to easily remove epitaxial layers from the supporting substrate by micro-mechanical exfoliation technology. This work can aid in the development of transferable devices using GaN films.

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.

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pillai, P. B., E-mail: p.pillai@sheffield.ac.uk, E-mail: m.desouza@sheffield.ac.uk; DeSouza, M., E-mail: p.pillai@sheffield.ac.uk, E-mail: m.desouza@sheffield.ac.uk; Narula, R.

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 showsmore » 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.« less

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.

Epitaxially Self-Assembled Alkane Layers for Graphene Electronics.

PubMed

Yu, Young-Jun; Lee, Gwan-Hyoung; Choi, Ji Il; Shim, Yoon Su; Lee, Chul-Ho; Kang, Seok Ju; Lee, Sunwoo; Rim, Kwang Taeg; Flynn, George W; Hone, James; Kim, Yong-Hoon; Kim, Philip; Nuckolls, Colin; Ahn, Seokhoon

2017-02-01

The epitaxially grown alkane layers on graphene are prepared by a simple drop-casting method and greatly reduce the environmentally driven doping and charge impurities in graphene. Multiscale simulation studies show that this enhancement of charge homogeneity in graphene originates from the lifting of graphene from the SiO 2 surface toward the well-ordered and rigid alkane self-assembled layers. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Direct growth of hexagonal boron nitride/graphene heterostructures on cobalt foil substrates by plasma-assisted molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Zhongguang; Khanaki, Alireza; Tian, Hao

2016-07-25

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 layersmore » 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.« less

Extraordinary epitaxial alignment of graphene islands on Au(111)

NASA Astrophysics Data System (ADS)

Wofford, Joseph M.; Starodub, Elena; Walter, Andrew L.; Nie, Shu; Bostwick, Aaron; Bartelt, Norman C.; Thürmer, Konrad; Rotenberg, Eli; McCarty, Kevin F.; Dubon, Oscar D.

2012-05-01

Pristine, single-crystalline graphene displays a unique collection of remarkable electronic properties that arise from its two-dimensional, honeycomb structure. Using in situ low-energy electron microscopy, we show that when deposited on the (111) surface of Au carbon forms such a structure. The resulting monolayer, epitaxial film is formed by the coalescence of dendritic graphene islands that nucleate at a high density. Over 95% of these islands can be identically aligned with respect to each other and to the Au substrate. Remarkably, the dominant island orientation is not the better lattice-matched 30° rotated orientation but instead one in which the graphene [01] and Au [011] in-plane directions are parallel. The epitaxial graphene film is only weakly coupled to the Au surface, which maintains its reconstruction under the slightly p-type doped graphene. The linear electronic dispersion characteristic of free-standing graphene is retained regardless of orientation. That a weakly interacting, non-lattice matched substrate is able to lock graphene into a particular orientation is surprising. This ability, however, makes Au(111) a promising substrate for the growth of single crystalline graphene films.

Quasi van der Waals epitaxy of copper thin film on single-crystal graphene monolayer buffer

NASA Astrophysics Data System (ADS)

Lu, Zonghuan; Sun, Xin; Washington, Morris A.; Lu, Toh-Ming

2018-03-01

Quasi van der Waals epitaxial growth of face-centered cubic Cu (~100 nm) thin films on single-crystal monolayer graphene is demonstrated using thermal evaporation at an elevated substrate temperature of 250 °C. The single-crystal graphene was transferred to amorphous (glass) and crystalline (quartz) SiO2 substrates for epitaxy study. Raman analysis showed that the thermal evaporation method had minimal damage to the graphene lattice during the Cu deposition. X-ray diffraction and electron backscatter diffraction analyses revealed that both Cu films are single-crystal with (1 1 1) out-of-plane orientation and in-plane Σ3 twin domains of 60° rotation. The crystallinity of the SiO2 substrates has a negligible effect on the Cu crystal orientation during the epitaxial growth, implying the strong screening effect of graphene. We also demonstrate the epitaxial growth of polycrystalline Cu on a commercial polycrystalline monolayer graphene consisting of two orientation domains offset 30° to each other. It confirms that the crystal orientation of the epitaxial Cu film follows that of graphene, i.e. the Cu film consists of two orientation domains offset 30° to each other when deposited on polycrystalline graphene. Finally, on the contrary to the report in the literature, we show that the direct current and radio frequency flip sputtering method causes significant damage to the graphene lattice during the Cu deposition process, and therefore neither is a suitable method for Cu epitaxial growth on graphene.

Graphene nanoribbons epitaxy on boron nitride

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lu, Xiaobo; Wang, Shuopei; Wu, Shuang

2016-03-14

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 cm{sup 2} V{sup −1} s{sup −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-BNmore » 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.« less

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

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.

Nucleation of uniform mono- and bilayer epitaxial graphene on SiC(0001)

NASA Astrophysics Data System (ADS)

Wu, Xiaosong; Zhang, Rui; Dong, Yunliang; Guo, Shuai; Kong, Wenjie; Liao, Zhimin; Yu, Dapeng

2012-02-01

Early stage of epitaxial graphene growth on SiC(0001) has been investigated. Using the confinement controlled sublimation (CCS) method, we has achieved well controlled growth and been able to see the formation of mono- and bilayer graphene islands. The growth features reveal the intriguing growth mechanism. In particular, a new ``stepdown'' growth mode has been identified. Graphene can propagate tens of micrometers across many SiC steps, while, most importantly, step bunching is avoided and the initial regular stepped SiC surface morphology is preserved. The stepdown growth demonstrates a route towards uniform epitaxial graphene in wafer size without sacrificing the initial substrate surface morphology.

Epitaxial growth mechanisms of graphene and effects of substrates

NASA Astrophysics Data System (ADS)

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

2012-06-01

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

Spin transport in epitaxial graphene

NASA Astrophysics Data System (ADS)

Tbd, -

2014-03-01

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

Van der Waals Epitaxy of GaSe/Graphene Heterostructure: Electronic and Interfacial Properties.

PubMed

Ben Aziza, Zeineb; Henck, Hugo; Pierucci, Debora; Silly, Mathieu G; Lhuillier, Emmanuel; Patriarche, Gilles; Sirotti, Fausto; Eddrief, Mahmoud; Ouerghi, Abdelkarim

2016-10-07

Stacking two-dimensional materials in so-called van der Waals (vdW) heterostructures, like the combination of GaSe and graphene, provides the ability to obtain hybrid systems which are suitable to design optoelectronic devices. Here, we report the structural and electronic properties of the direct growth of multilayered GaSe by Molecular beam Epitaxy (MBE) on graphene. Reflection high-energy electron diffraction (RHEED) images exhibited sharp streaky features indicative of high quality GaSe layer produced via a vdW epitaxy. Micro-Raman spectroscopy showed that, after the vdW hetero-interface formation, the Raman signature of pristine graphene is preserved. However, the GaSe film tuned the charge density of graphene layer by shifting the Dirac point by about 80 meV toward lower binding energies, attesting an electron transfer from graphene to GaSe. Angle-resolved photoemission spectroscopy (ARPES) measurements showed that the maximum of the valence band of few layers of GaSe are located at the Γ point at a binding energy of about -0.73 eV relatively to the Fermi level (p-type doping). From the ARPES measurements, a hole effective mass defined along the ΓM direction and equal to about m*/m0 = -1.1 was determined. By coupling the ARPES data with high resolution X-ray photoemission spectroscopy (HR-XPS) measurements, the Schottky interface barrier height was estimated to be 1.2 eV. These findings allow deeper understanding of the interlayer interactions and the electronic structure of GaSe/graphene vdW heterostructure.

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 Bi 0.5 Sb 1.5 Te 3 film on graphene synthesized via a simple and scalable fabrication method exhibits good crystallinity and high thermoelectric transport properties comparable to single crystals. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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

PubMed

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.

Spin-injection into epitaxial graphene on silicon carbide

NASA Astrophysics Data System (ADS)

Konishi, Keita; Cui, Zhixin; Hiraki, Takahiro; Yoh, Kanji

2013-09-01

We have studied the spin-injection properties in epitaxial graphene on SiC. The ferromagnetic metal (FM) electrodes were composed of a tunnel barrier layer AlOx (14 Å) and a ferromagnetic Co (600 Å) layer. We have successfully observed the clear resistance peaks indicating spin-injection both in the "local" and "non-local" spin measurement set-ups at low temperatures. We estimate spin-injection rate of 1% based on "non-local" measurement and 1.6% based on local measurements. Spin-injection rate of multilayer graphene by mechanical exfoliation method was twice as high as single layer graphene on SiC based on "local" measurement.

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

PubMed Central

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

2016-01-01

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

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yu, C.; Li, J.; Song, X. B.

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-standingmore » bilayer epitaxial graphene transistor show the great application potential of this material system.« less

Molecular beam epitaxy of graphene on ultra-smooth nickel: growth mode and substrate interactions

NASA Astrophysics Data System (ADS)

Wofford, J. M.; Oliveira, M. H., Jr.; Schumann, T.; Jenichen, B.; Ramsteiner, M.; Jahn, U.; Fölsch, S.; Lopes, J. M. J.; Riechert, H.

2014-09-01

Graphene is grown by molecular beam epitaxy using epitaxial Ni films on MgO(111) as substrates. Raman spectroscopy and scanning tunneling microscopy reveal the graphene films to have few crystalline defects. While the layers are ultra-smooth over large areas, we find that Ni surface features lead to local non-uniformly thick graphene inclusions. The influence of the Ni surface structure on the position and morphology of these inclusions strongly suggests that multilayer graphene on Ni forms at the interface of the first complete layer and metal substrate in a growth-from-below mechanism. The interplay between Ni surface features and graphene growth behavior may facilitate the production of films with spatially resolved multilayer inclusions through engineered substrate surface morphology.

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

Isolation of high quality graphene from Ru by solution phase intercalation

NASA Astrophysics Data System (ADS)

Koren, E.; Sutter, E.; Bliznakov, S.; Ivars-Barcelo, F.; Sutter, P.

2013-09-01

We introduce a method for isolating graphene grown on epitaxial Ru(0001)/α-Al2O3. The strong graphene/Ru(0001) coupling is weakened by electrochemically driven intercalation of hydrogen underpotentially deposited in aqueous KOH solution, which allows the penetration of water molecules at the graphene/Ru(0001) interface. Following these electrochemically driven processes, the graphene can be isolated by electrochemical hydrogen evolution and transferred to arbitrary supports. Raman and transport measurements demonstrate the high quality of the transferred graphene. Our results show that intercalation, typically carried out in vacuum, can be extended to solution environments for graphene processing under ambient conditions.

Microwave studies of weak localization and antilocalization in epitaxial graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Drabińska, Aneta; Kamińska, Maria; Wołoś, Agnieszka

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.

The role of electron-electron repulsion in the problem of epitaxial graphene on a metal: Simple estimates

NASA Astrophysics Data System (ADS)

Davydov, S. Yu.

2017-08-01

For single-layer graphene placed on a metal substrate, the influence of intra- and interatomic Coulomb repulsion of electrons ( U and G, respectively) on its phase diagram is considered in the framework of an extended Hartree-Fock theory. The general solution of the problem is presented, on the basis of which special cases allowing for analytical consideration are analyzed: free and epitaxial graphene with and without regard for the energy of the electron transition between neighboring atoms of graphene. Three regions of the phase diagram are considered: spin and charge density waves (SDW and CDW, respectively) and the semimetal (SM) state uniform in the spin and charge. The main attention is paid to undoped graphene. It is shown that the allowance for the interaction with a metal substrate expands the SM existence domain. However, in all the considered cases, the boundary between the SDW and CDW states is described by the equation U = zG, where z = 3 is the number of nearest neighbors in graphene. The widening of the SM state region also results from the doping of graphene, and the effect is independent of the sign of free carriers introduced into epitaxial graphene by the substrate. According to estimates made, the only state possible in the buffer layer is the metal-type SM state, whereas, in epitaxial graphene, the CDW state is possible. The influence of temperature on the phase diagram of epitaxial graphene is discussed.

Epitaxial ferromagnetic single clusters and smooth continuous layers on large area MgO/CVD graphene substrates

NASA Astrophysics Data System (ADS)

Godel, Florian; Meny, Christian; Doudin, Bernard; Majjad, Hicham; Dayen, Jean-François; Halley, David

2018-02-01

We report on the fabrication of ferromagnetic thin layers separated by a MgO dielectric barrier from a graphene-covered substrate. The growth of ferromagnetic metal layers—Co or Ni0.8Fe0.2—is achieved by Molecular Beam Epitaxy (MBE) on a 3 nm MgO(111) epitaxial layer deposited on graphene. In the case of a graphene, grown by chemical vapor deposition (CVD) over Ni substrates, an annealing at 450 °C, under ultra-high-vacuum (UHV) conditions, leads to the dewetting of the ferromagnetic layers, forming well-defined flat facetted clusters whose shape reflects the substrate symmetry. In the case of CVD graphene transferred on SiO2, no dewetting is observed after same annealing. We attribute this difference to the mechanical stress states induced by the substrate, illustrating how it matters for epitaxial construction through graphene. Controlling the growth parameters of such magnetic single objects or networks could benefit to new architectures for catalysis or spintronic applications.

Lattice-Matched Epitaxial Graphene Grown on Boron Nitride.

PubMed

Davies, Andrew; Albar, Juan D; Summerfield, Alex; Thomas, James C; Cheng, Tin S; Korolkov, Vladimir V; Stapleton, Emily; Wrigley, James; Goodey, Nathan L; Mellor, Christopher J; Khlobystov, Andrei N; Watanabe, Kenji; Taniguchi, Takashi; Foxon, C Thomas; Eaves, Laurence; Novikov, Sergei V; Beton, Peter H

2018-01-10

Lattice-matched graphene on hexagonal boron nitride is expected to lead to the formation of a band gap but requires the formation of highly strained material and has not hitherto been realized. We demonstrate that aligned, lattice-matched graphene can be grown by molecular beam epitaxy using substrate temperatures in the range 1600-1710 °C and coexists with a topologically modified moiré pattern with regions of strained graphene which have giant moiré periods up to ∼80 nm. Raman spectra reveal narrow red-shifted peaks due to isotropic strain, while the giant moiré patterns result in complex splitting of Raman peaks due to strain variations across the moiré unit cell. The lattice-matched graphene has a lower conductance than both the Frenkel-Kontorova-type domain walls and also the topological defects where they terminate. We relate these results to theoretical models of band gap formation in graphene/boron nitride heterostructures.

Epitaxial graphene growth on FIB patterned 3C-SiC nanostructures on Si (111): reducing milling damage.

PubMed

Amjadipour, Mojtaba; MacLeod, Jennifer; Lipton-Duffin, Josh; Iacopi, Francesca; Motta, Nunzio

2017-08-25

Epitaxial growth of graphene on SiC is a scalable procedure that does not require any further transfer step, making this an ideal platform for graphene nanostructure fabrication. Focused ion beam (FIB) is a very promising tool for exploring the reduction of the lateral dimension of graphene on SiC to the nanometre scale. However, exposure of graphene to the Ga + beam causes significant surface damage through amorphisation and contamination, preventing epitaxial graphene growth. In this paper we demonstrate that combining a protective silicon layer with FIB patterning implemented prior to graphene growth can significantly reduce the damage associated with FIB milling. Using this approach, we successfully achieved graphene growth over 3C-SiC/Si FIB patterned nanostructures.

STM/STS study of ridges on epitaxial graphene/SiC

NASA Astrophysics Data System (ADS)

Li, Y. Y.; Liu, Y.; Weinert, M.; Li, L.

2012-02-01

The graphitization of hexagonal SiC surfaces provides a viable alternative for the synthesis of wafer-sized graphene for mass device production. During later stages of growth, ridges are often observed on the graphene layers as a result of bending and buckling to relieve the strain between the graphene and SiC substrate. In this work, we show, by atomic resolution STM/STS, that these ridges are in fact bulged regions of the graphene layer, forming one-dimentional (nanowire) and zero-dimentional (quantum dot) nanostructures. We further show that their structures can be manipulated by the pressure exerted by the STM tip during imaging. These results and their impact on the electronic properties of epitaxial graphene on SiC(0001) will be presented at the meeting.

Controlled epitaxial graphene growth within removable amorphous carbon corrals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Palmer, James; Hu, Yike; Hankinson, John

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 atmore » 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.« less

Intercalated europium metal in epitaxial graphene on SiC

DOE PAGES

Anderson, Nathaniel; Hupalo, Myron; Keavney, David; ...

2017-10-25

X-ray magnetic circular dichroism (XMCD) reveals the magnetic properties of intercalated europium metal under graphene on SiC(0001). Intercalation of Eu nanoclusters (average size 2.5 nm) between graphene and SiC substate are formed by deposition of Eu on epitaxially grown graphene that is subsequently annealed at various temperatures while keeping the integrity of the graphene layer. Using sum-rules analysis of the XMCD of Eu M 4,5 edges at T = 15 K, our samples show paramagnetic-like behavior with distinct anomaly at T ≈ 90 K, which may be related to the Nèel transition, T N = 91 K, of bulk metalmore » Eu. Here, we find no evidence of ferromagnetism due to EuO or antiferromagnetism due to Eu 2 O 3, indicating that the graphene layer protects the intercalated metallic Eu against oxidation over months of exposure to atmospheric environment.« less

Intercalated europium metal in epitaxial graphene on SiC

DOE Office of Scientific and Technical Information (OSTI.GOV)

Anderson, Nathaniel; Hupalo, Myron; Keavney, David

X-ray magnetic circular dichroism (XMCD) reveals the magnetic properties of intercalated europium metal under graphene on SiC(0001). Intercalation of Eu nanoclusters (average size 2.5 nm) between graphene and SiC substate are formed by deposition of Eu on epitaxially grown graphene that is subsequently annealed at various temperatures while keeping the integrity of the graphene layer. Using sum-rules analysis of the XMCD of Eu M 4,5 edges at T = 15 K, our samples show paramagnetic-like behavior with distinct anomaly at T ≈ 90 K, which may be related to the Nèel transition, T N = 91 K, of bulk metalmore » Eu. Here, we find no evidence of ferromagnetism due to EuO or antiferromagnetism due to Eu 2 O 3, indicating that the graphene layer protects the intercalated metallic Eu against oxidation over months of exposure to atmospheric environment.« less

Resonant tunneling structures based on epitaxial graphene on SiC

NASA Astrophysics Data System (ADS)

Nguyen, V. Hung; Bournel, A.; Dollfus, P.

2011-12-01

Recently some experiments have suggested that graphene epitaxially grown on SiC can exhibit an energy bandgap of 260 meV, which enhances the potential of this material for electronic applications. On this basis, we propose to use spatial doping to generate graphene-on-SiC double-barrier structures. The non-equilibrium Green's function technique for solving the massive Dirac model is applied to highlight typical transport phenomena such as the electron confinement and the resonant tunneling effects. The I-V characteristics of graphene resonant tunneling diodes were then investigated and the effect of different device parameters was discussed. It is finally shown that this kind of double-barrier junction provides an efficient way to confine the charge carriers in graphene and to design graphene resonant tunneling structures.

Growth of boron-doped few-layer graphene by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Soares, G. V.; Nakhaie, S.; Heilmann, M.; Riechert, H.; Lopes, J. M. J.

2018-04-01

We investigated the growth of boron-doped few-layer graphene on α-Al2O3 (0001) substrates by molecular beam epitaxy using two different growth approaches: one where boron was provided during the entire graphene synthesis and the second where boron was provided only during the second half of the graphene growth run. Electrical measurements show a higher p-type carrier concentration for samples fabricated utilizing the second approach, with a remarkable modulation in the carrier concentration of almost two orders of magnitude in comparison to the pristine graphene film. The results concerning the influence of the boron flux at different growth stages of graphene on the electrical and physicochemical properties of the films are presented.

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Zhongguang; Zheng, Renjing; Khanaki, Alireza

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-BNmore » until it may cover entire h-BN flakes.« less

Epitaxially grown strained pentacene thin film on graphene membrane.

PubMed

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

2015-05-06

Organic-graphene system has emerged as a new platform for various applications such as flexible organic photovoltaics and organic light emitting diodes. Due to its important implication in charge transport, the study and reliable control of molecular packing structures at the graphene-molecule interface are of great importance for successful incorporation of graphene in related organic devices. Here, an ideal membrane of suspended graphene as a molecular assembly template is utilized to investigate thin-film epitaxial behaviors. Using transmission electron microscopy, two distinct molecular packing structures of pentacene on graphene are found. One observed packing structure is similar to the well-known bulk-phase, which adapts a face-on molecular orientation on graphene substrate. On the other hand, a rare polymorph of pentacene crystal, which shows significant strain along the c-axis, is identified. In particular, the strained film exhibits a specific molecular orientation and a strong azimuthal correlation with underlying graphene. Through ab initio electronic structure calculations, including van der Waals interactions, the unusual polymorph is attributed to the strong graphene-pentacene interaction. The observed strained organic film growth on graphene demonstrates the possibility to tune molecular packing via graphene-molecule interactions. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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

Nickel enhanced graphene growth directly on dielectric substrates by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Wofford, Joseph M.; Speck, Florian; Seyller, Thomas; Lopes, Joao Marcelo J.; Riechert, Henning

2016-07-01

The efficacy of Ni as a surfactant to improve the crystalline quality of graphene grown directly on dielectric Al2O3(0001) substrates by molecular beam epitaxy is examined. Simultaneously exposing the substrate to a Ni flux throughout C deposition at 950 °C led to improved charge carrier mobility and a Raman spectrum indicating less structural disorder in the resulting nanocrystalline graphene film. X-ray photoelectron spectroscopy confirmed that no residual Ni could be detected in the film and showed a decrease in the intensity of the defect-related component of the C1s level. Similar improvements were not observed when a lower substrate temperature (850 °C) was used. A close examination of the Raman spectra suggests that Ni reduces the concentration of lattice vacancies in the film, possibly by catalytically assisting adatom incorporation.

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 SiO 2 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.

Properties of copper (fluoro-)phthalocyanine layers deposited on epitaxial graphene.

PubMed

Ren, Jun; Meng, Sheng; Wang, Yi-Lin; Ma, Xu-Cun; Xue, Qi-Kun; Kaxiras, Efthimios

2011-05-21

We investigate the atomic structure and electronic properties of monolayers of copper phthalocyanines (CuPc) deposited on epitaxial graphene substrate. We focus in particular on hexadecafluorophthalocyanine (F(16)CuPc), using both theoretical and experimental (scanning tunneling microscopy - STM) studies. For the individual CuPc and F(16)CuPc molecules, we calculated the electronic and optical properties using density functional theory (DFT) and time-dependent DFT and found a red-shift in the absorption peaks of F(16)CuPc relative to those of CuPc. In F(16)CuPc, the electronic wavefunctions are more polarized toward the electronegative fluorine atoms and away from the Cu atom at the center of the molecule. When adsorbed on graphene, the molecules lie flat and form closely packed patterns: F(16)CuPc forms a hexagonal pattern with two well-ordered alternating α and β stripes while CuPc arranges into a square lattice. The competition between molecule-substrate and intermolecular van der Waals interactions plays a crucial role in establishing the molecular patterns leading to tunable electron transfer from graphene to the molecules. This transfer is controlled by the layer thickness of, or the applied voltage on, epitaxial graphene resulting in selective F(16)CuPc adsorption, as observed in STM experiments. In addition, phthalocyanine adsorption modifies the electronic structure of the underlying graphene substrate introducing intensity smoothing in the range of 2-3 eV below the Dirac point (E(D)) and a small peak in the density of states at ∼0.4 eV above E(D). © 2011 American Institute of Physics.

Molecular Beam Epitaxial Growth and Characterization of Graphene and Hexagonal Boron Nitride Two-Dimensional Layers

NASA Astrophysics Data System (ADS)

Zheng, Renjing

Van der Waals (vdW) materials (also called as two-dimensional (2D) material in some literature) systems have received extensive attention recently due to their potential applications in next-generation electronics platform. Exciting properties have been discovered in this field, however, the performance and properties of the systems rely on the materials' quality and interface significantly, leading to the urgent need for scalable synthesis of high-quality vdW crystals and heterostructures. Toward this direction, this dissertation is devoted on the study of Molecular Beam Epitaxy (MBE) growth and various characterization of vdW materials and heterostructures, especially graphene and hexagonal boron nitride (h-BN). The goal is to achieve high-quality vdW materials and related heterostructures. There are mainly four projects discussed in this dissertation. The first project (Chapter 2) is about MBE growth of large-area h-BN on copper foil. After the growth, the film was transferred onto SiO2 substrate for characterization. It is observed that as-grown film gives evident h-BN Raman spectrum; what's more, h-BN peak intensity and position is dependent on film thickness. N-1s and B-1s XPS peaks further suggest the formation of h-BN. AFM and SEM images show the film is flat and continuous over large area. Our synthesis method shows it's possible to use MBE to achieve h-BN growth and could also pave a way for some unique structure, such as h-BN/graphene heterostructures and doped h-BN films by MBE. The second project (Chapter 3) is focused on establishment of grapehene/h-BN heterostructure on cobalt (Co) film. In-situ epitaxial growth of graphene/h-BN heterostructures on Co film substrate was achieved by using plasma-assisted MBE. The direct graphene/h-BN vertical stacking structures were demonstrated and further confirmed by various characterizations, such as Raman spectroscopy, SEM, XPS and TEM. Large area heterostructures consisting of single- /bilayer graphene and

Chemical gating of epitaxial graphene through ultrathin oxide layers.

PubMed

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

2015-08-07

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

Ion sensitivity of large-area epitaxial graphene film on SiC substrate

NASA Astrophysics Data System (ADS)

Mitsuno, Takanori; Taniguchi, Yoshiaki; Ohno, Yasuhide; Nagase, Masao

2017-11-01

We investigated the intrinsic ion sensitivity of graphene field-effect transistors (FETs) fabricated by a resist-free stencil mask lithography process from a large-scale graphene film epitaxially grown on a SiC substrate. A pH-adjusted phosphate-buffered solution was used for the measurement to eliminate the interference of other ions on the graphene FET's ion sensitivity. The charge neutrality point shifted negligibly with changing pH for the pH-adjusted phosphate-buffered solution, whereas for the mixed buffer solution, it shifted toward the negative gate voltage owing to the decrease in the concentration of phthalate ions. This phenomenon is contrary to that observed in previous reports. Overall, our results indicate that the graphene film is intrinsically insensitive to ions except for those with functional groups that interact with the graphene surface.

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.

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 E 2 1g 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

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Xufan; Basile, Leonardo; Huang, Bing

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 E 2 1g 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

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.

Blinking suppression of CdTe quantum dots on epitaxial graphene and the analysis with Marcus electron transfer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hirose, Takuya; Tamai, Naoto, E-mail: tamai@kwansei.ac.jp; Kutsuma, Yasunori

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.

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kowalski, G., E-mail: kowal@fuw.edu.pl; Tokarczyk, M.; Dąbrowski, P.

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) andmore » 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.« less

Structural consequences of hydrogen intercalation of epitaxial graphene on SiC(0001)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Emery, Jonathan D., E-mail: jdemery@anl.gov, E-mail: bedzyk@northwestern.edu; Johns, James E.; McBriarty, Martin E.

2014-10-20

The intercalation of various atomic species, such as hydrogen, to the interface between epitaxial graphene (EG) and its SiC substrate is known to significantly influence the electronic properties of the graphene overlayers. Here, we use high-resolution X-ray reflectivity to investigate the structural consequences of the hydrogen intercalation process used in the formation of quasi-free-standing (QFS) EG/SiC(0001). We confirm that the interfacial layer is converted to a layer structurally indistinguishable from that of the overlying graphene layers. This newly formed graphene layer becomes decoupled from the SiC substrate and, along with the other graphene layers within the film, is vertically displacedmore » by ∼2.1 Å. The number of total carbon layers is conserved during the process, and we observe no other structural changes such as interlayer intercalation or expansion of the graphene d-spacing. These results clarify the under-determined structure of hydrogen intercalated QFS-EG/SiC(0001) and provide a precise model to inform further fundamental and practical understanding of the system.« less

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

DOE PAGES

Ahmed, Adam S.; Wen, Hua; Ohta, Taisuke; ...

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.

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ahmed, Adam S.; Wen, Hua; Ohta, Taisuke

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.

Planar edge Schottky barrier-tunneling transistors using epitaxial graphene/SiC junctions.

PubMed

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

2014-09-10

A purely planar graphene/SiC field effect transistor is presented here. The horizontal current flow over one-dimensional tunneling barrier between planar graphene contact and coplanar two-dimensional SiC channel exhibits superior on/off ratio compared to conventional transistors employing vertical electron transport. Multilayer epitaxial graphene (MEG) grown on SiC(0001̅) was adopted as the transistor source and drain. The channel is formed by the accumulation layer at the interface of semi-insulating SiC and a surface silicate that forms after high vacuum high temperature annealing. Electronic bands between the graphene edge and SiC accumulation layer form a thin Schottky barrier, which is dominated by tunneling at low temperatures. A thermionic emission prevails over tunneling at high temperatures. We show that neglecting tunneling effectively causes the temperature dependence of the Schottky barrier height. The channel can support current densities up to 35 A/m.

STM/STS investigation of edge structure in epitaxial graphene

NASA Astrophysics Data System (ADS)

Ridene, M.; Girard, J. C.; Travers, L.; David, C.; Ouerghi, A.

2012-08-01

In this paper, we have used low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) to study zigzag or armchair edges of epitaxial graphene on 6H-SiC (0001). The monolayer carbon structures exhibit occasionally one-dimensional ridge (1D) in close vicinity to step edge. This ridge exhibits different edges orientations in armchair-zigzag transition which give rise to different local density of states (LDOS) along this 1D structure. This ridge formation is likely explained by residual compressive in-plane stresses.

The Ordering and Electronic Structure of Multilayer Epitaxial Graphene on SiC

NASA Astrophysics Data System (ADS)

Conrad, Edward

2011-03-01

The structural definition of graphene as a single sheet of hexagonal carbon limits how we view this material. It is the electronic properties of a single isolated graphene sheet that actually defines and motivates current graphene research. Remarkably, the best example of the idealized band structure of graphene comes does not come from a single graphene layer but from multilayer films grown on SiC. Multilayer epitaxial graphene (MEG) not only shows all the 2D properties expected for an isolated graphene sheet, but it the scalability to large scale integrated carbon circuits. I will show that the reason for this remarkable property, i.e. that a multilayer graphene films behaving like a single graphene sheet, is due to MEG's unique stacking. MEG films have a quasi-ordered rotational stacking that breaks the Bernal stacking symmetry associated with graphite. Angle resolved photoemission spectroscopy (ARPES) data demonstrates that the bands are linear at the K-point of these films. We can also show that the rotated stacking is highly ordered and that less than 20% of the graphene sheets in the film are Bernal stacked. I will also show that ARPES measurements on MEG films demonstrate serious inadequacies with both tight binding and ab initio formalisms. In particular the data shows no reductions in the Fermi velocity or the formation of Van Hove singularity that have been consistently predicted for this material. I wish to acknowledge funding from the NSF under Grants No. DMR-0820382 and DMR-1005880.

Free-standing epitaxial graphene on silicon carbide and transport barriers in layered materials

NASA Astrophysics Data System (ADS)

Shivaraman, Shriram

This thesis is based on the topic of layered materials, in which different layers interact with each other via van der Waals forces. The majority of this thesis deals with epitaxial graphene (EG) obtained from silicon carbide (SiC). Free-standing epitaxial graphene (FSEG) structures are produced from EG using a photoelectrochemical (PEC) etching process developed for making suspended graphene structures on a large-scale. These structures are investigated for their mechanical and electrical properties. For doubly-clamped FSEG structures, a unique U-beam effect is observed which causes orders of magnitude increase in their mechanical resonance frequency compared to that expected using simple beam theory. Combined magnetotransport and Raman spectroscopy studies reveal that FSEG devices produced from nominally monolayer graphene on the Si-face of SiC exhibit properties of an inhomogeneously doped bilayer after becoming suspended. This suggests that the buffer layer which precedes graphene growth on the Si-face of SiC gets converted to a graphene layer after the PEC etching process. In the second theme of this thesis, transport barriers in layered materials are investigated. The EG-SiC interface is studied using a combination of electrical (I-V, C-V) and photocurrent spectroscopy techniques. It is shown that the interface may be described as having a Schottky barrier for electron transport with a Gaussian distribution of barrier heights. Another interface explored in this work is that between different layers of MoS 2, a layered material belonging to the class of transition metal dichalcogenides. This interface maybe thought of as a one-dimensional junction. Four-point transport measurements indicate the presence of a barrier for electron transport at this interface. A simple model of the junction as a region with an increased threshold voltage and degraded mobility is suggested. The final chapter is a collection of works based on the topic of layered materials, which

Epitaxial Growth of Thin Ferroelectric Polymer Films on Graphene Layer for Fully Transparent and Flexible Nonvolatile Memory.

PubMed

Kim, Kang Lib; Lee, Wonho; Hwang, Sun Kak; Joo, Se Hun; Cho, Suk Man; Song, Giyoung; Cho, Sung Hwan; Jeong, Beomjin; Hwang, Ihn; Ahn, Jong-Hyun; Yu, Young-Jun; Shin, Tae Joo; Kwak, Sang Kyu; Kang, Seok Ju; Park, Cheolmin

2016-01-13

Enhancing the device performance of organic memory devices while providing high optical transparency and mechanical flexibility requires an optimized combination of functional materials and smart device architecture design. However, it remains a great challenge to realize fully functional transparent and mechanically durable nonvolatile memory because of the limitations of conventional rigid, opaque metal electrodes. Here, we demonstrate ferroelectric nonvolatile memory devices that use graphene electrodes as the epitaxial growth substrate for crystalline poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) polymer. The strong crystallographic interaction between PVDF-TrFE and graphene results in the orientation of the crystals with distinct symmetry, which is favorable for polarization switching upon the electric field. The epitaxial growth of PVDF-TrFE on a graphene layer thus provides excellent ferroelectric performance with high remnant polarization in metal/ferroelectric polymer/metal devices. Furthermore, a fully transparent and flexible array of ferroelectric field effect transistors was successfully realized by adopting transparent poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] semiconducting polymer.

Graphene-silicon layered structures on single-crystalline Ir(111) thin films

DOE PAGES

Que, Yande D.; Tao, Jing; Zhang, Yong; ...

2015-01-20

Epitaxial growth of graphene on transition metal crystals, such as Ru,⁽¹⁻³⁾ Ir,⁽⁴⁻⁶⁾ and Ni,⁽⁷⁾ provides large-area, uniform graphene layers with controllable defect density, which is crucial for practical applications in future devices. To decrease the high cost of single-crystalline metal bulks, single-crystalline metal films are strongly suggested as the substrates for epitaxial growth large-scale high-quality graphene.⁽⁸⁻¹⁰⁾ Moreover, in order to weaken the interactions of graphene with its metal host, which may result in a suppression of the intrinsic properties of graphene,⁽¹¹ ¹²⁾ the method of element intercalation of semiconductors at the interface between an epitaxial graphene layer and a transitionmore » metal substrate has been successfully realized.⁽¹³⁻¹⁶⁾« less

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lartsev, Arseniy; Yager, Tom; Lara-Avila, Samuel, E-mail: samuel.lara@chalmers.se

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

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.

An atomic carbon source for high temperature molecular beam epitaxy of graphene.

PubMed

Albar, J D; Summerfield, A; Cheng, T S; Davies, A; Smith, E F; Khlobystov, A N; Mellor, C J; Taniguchi, T; Watanabe, K; Foxon, C T; Eaves, L; Beton, P H; Novikov, S V

2017-07-26

We report the use of a novel atomic carbon source for the molecular beam epitaxy (MBE) of graphene layers on hBN flakes and on sapphire wafers at substrate growth temperatures of ~1400 °C. The source produces a flux of predominantly atomic carbon, which diffuses through the walls of a Joule-heated tantalum tube filled with graphite powder. We demonstrate deposition of carbon on sapphire with carbon deposition rates up to 12 nm/h. Atomic force microscopy measurements reveal the formation of hexagonal moiré patterns when graphene monolayers are grown on hBN flakes. The Raman spectra of the graphene layers grown on hBN and sapphire with the sublimation carbon source and the atomic carbon source are similar, whilst the nature of the carbon aggregates is different - graphitic with the sublimation carbon source and amorphous with the atomic carbon source. At MBE growth temperatures we observe etching of the sapphire wafer surface by the flux from the atomic carbon source, which we have not observed in the MBE growth of graphene with the sublimation carbon source.

In situ fabrication of quasi-free-standing epitaxial graphene nanoflakes on gold.

PubMed

Leicht, Philipp; Zielke, Lukas; Bouvron, Samuel; Moroni, Riko; Voloshina, Elena; Hammerschmidt, Lukas; Dedkov, Yuriy S; Fonin, Mikhail

2014-04-22

Addressing the multitude of electronic phenomena theoretically predicted for confined graphene structures requires appropriate in situ fabrication procedures yielding graphene nanoflakes (GNFs) with well-defined geometries and accessible electronic properties. Here, we present a simple strategy to fabricate quasi-free-standing GNFs of variable sizes, performing temperature programmed growth of graphene flakes on the Ir(111) surface and subsequent intercalation of gold. Using scanning tunneling microscopy (STM), we show that epitaxial GNFs on a perfectly ordered Au(111) surface are formed while maintaining an unreconstructed, singly hydrogen-terminated edge structure, as confirmed by the accompanying density functional theory (DFT) calculations. Using tip-induced lateral displacement of GNFs, we demonstrate that GNFs on Au(111) are to a large extent decoupled from the Au(111) substrate. The direct accessibility of the electronic states of a single GNF is demonstrated upon analysis of the quasiparticle interference patterns obtained by low-temperature STM. These findings open up an interesting playground for diverse investigations of graphene nanostructures with possible implications for device fabrication.

A structural and electronic comparison of armchair and zigzag epitaxial graphene sidewall nanoribbons

NASA Astrophysics Data System (ADS)

Nevius, Meredith; Wang, F.; Palacio, I.; Celis, A.; Tejeda, A.; Taleb-Ibrahimi, A.; de Heer, W.; Berger, C.; Conrad, E.

2014-03-01

Graphene grown on sidewalls of trenches etched in SiC shows particular promise as a candidate for post-Si CMOS electronics because of its ballistic transport, exceptional mobilities, low intrinsic doping, and the opening of a large band gap. However, before definitive progress can be made toward epitaxial graphene-based transistors, we must fully understand the nuances of graphene ribbon growth on different SiC facets. We have now confirmed that sidewall ribbons grown in graphene's two primary crystallographic directions (``armchair'' and ``zigzag'') differ greatly in both structure and electronic band-structure. We present data from both geometries obtained using low-energy electron microscopy (LEEM), low-energy electron diffraction (LEED), angle-resolved photoemission spectroscopy (ARPES), photoemission electron microscopy (PEEM), micro-ARPES and dark-field micro-ARPES. We demonstrate that while graphene grows on stable facets of trenches oriented for armchair edge growth, trenches oriented for zigzag edge growth prefer narrow ribbons of graphene on the (0001) surface near the trench edge. The structure of these zigzag edge graphene ribbons is complex and paramount to understanding their transport. This work was supported by the NSF under grants DMR-1005880 and DMR-0820382, the W. M. Keck Foundation and the Partner University Fund from the Embassy of France

Incorporating isolated molybdenum (Mo) atoms into Bilayer Epitaxial Graphene on 4H-SiC(0001)

NASA Astrophysics Data System (ADS)

Huang, Han; Wan, Wen; Li, Hui; Wong, Swee Liang; Lv, Lu; Gao, Yongli; Wee, Andrew T. S.

2014-03-01

The atomic structures and electronic properties of isolated Mo atoms in bilayer epitaxial graphene (BLEG) on 4H-SiC(0001) are investigated by low temperature scanning tunneling microscopy (LT-STM). LT-STM results reveal that isolated Mo dopants prefer to substitute C atoms at α-sites, and preferentially locate between the graphene bilayers. First-principles calculations confirm that the embedding of single Mo dopants within BLEG is energetically favorable as compared to monolayer graphene. The calculated bandstructures show that Mo-doped BLEG is n-doped, and each Mo atom introduces a local magnetic moment of 1.81 μB. Our findings demonstrate a simple and stable method to incorporate single transition metal dopants into the graphene lattice to tune its electronic and magnetic properties for possible use in graphene spin devices. NRF-CRP (Singapore) grants R-143-000-360-281and R-144-000-295-281. ``Shenghua Professorship'' startup funding from CSU and the support from the NSF of China (Grant No.11304398).

Quasi free-standing epitaxial graphene fabrication on 3C-SiC/Si(111)

NASA Astrophysics Data System (ADS)

Amjadipour, Mojtaba; Tadich, Anton; Boeckl, John J.; Lipton-Duffin, Josh; MacLeod, Jennifer; Iacopi, Francesca; Motta, Nunzio

2018-04-01

Growing graphene on SiC thin films on Si is a cheaper alternative to the growth on bulk SiC, and for this reason it has been recently intensively investigated. Here we study the effect of hydrogen intercalation on epitaxial graphene obtained by high temperature annealing on 3C-SiC/Si(111) in ultra-high vacuum. By using a combination of core-level photoelectron spectroscopy, low energy electron diffraction, and near-edge x-ray absorption fine structure (NEXAFS) we find that hydrogen saturates the Si atoms at the topmost layer of the substrate, leading to free-standing graphene on 3C-SiC/Si(111). The intercalated hydrogen fully desorbs after heating the sample at 850 °C and the buffer layer appears again, similar to what has been reported for bulk SiC. However, the NEXAFS analysis sheds new light on the effect of hydrogen intercalation, showing an improvement of graphene’s flatness after annealing in atomic H at 600 °C. These results provide new insight into free-standing graphene fabrication on SiC/Si thin films.

Identification of dominant scattering mechanism in epitaxial graphene on SiC

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lin, Jingjing; Guo, Liwei, E-mail: lwguo@iphy.ac.cn, E-mail: chenx29@aphy.iphy.ac.cn; Jia, Yuping

2014-05-05

A scheme of identification of scattering mechanisms in epitaxial graphene (EG) on SiC substrate is developed and applied to three EG samples grown on SiC (0001), (112{sup ¯}0), and (101{sup ¯}0) substrates. Hall measurements combined with defect detection technique enable us to evaluate the individual contributions to the carrier scatterings by defects and by substrates. It is found that the dominant scatterings can be due to either substrate or defects, dependent on the substrate orientations. The EG on SiC (112{sup ¯}0) exhibits a better control over the two major scattering mechanisms and achieves the highest mobility even with a highmore » carrier concentration, promising for high performance graphene-based electronic devices. The method developed here will shed light on major aspects in governing carrier transport in EG to harness it effectively.« less

Step edge influence on barrier height and contact area in vertical heterojunctions between epitaxial graphene and n-type 4H-SiC

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tadjer, M. J., E-mail: marko.tadjer.ctr@nrl.navy.mil; Nyakiti, L. O.; Robinson, Z.

2014-02-17

Vertical rectifying contacts of epitaxial graphene grown by Si sublimation on the Si-face of 4H-SiC epilayers were investigated. Forward bias preferential conduction through the step edges was correlated by linear current density normalization. This phenomenon was observed on samples with 2.7–5.8 monolayers of epitaxial graphene as determined by X-ray photoelectron spectroscopy. A modified Richardson plot was implemented to extract the barrier height (0.81 eV at 290 K, 0.99 eV at 30 K) and the electrically dominant SiC step length of a Ti/Al contact overlapping a known region of approximately 0.52 μm wide SiC terraces.

Mini array of quantum Hall devices based on epitaxial graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Novikov, S.; Lebedeva, N.; Hämäläinen, J.

2016-05-07

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 R{sub H,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 thatmore » the deviation of the quantized Hall resistance of the series connection of four graphene-based QHE devices from the expected value of 4×R{sub H,2} = 2 h/e{sup 2} was smaller than the relative standard uncertainty of the measurement (<1 × 10{sup −7}) limited by the used resistance bridge.« less

Epitaxial growth of aligned atomically precise chevron graphene nanoribbons on Cu(111).

PubMed

Teeter, Jacob D; Costa, Paulo S; Mehdi Pour, Mohammad; Miller, Daniel P; Zurek, Eva; Enders, Axel; Sinitskii, Alexander

2017-07-25

Atomically precise chevron graphene nanoribbons (GNRs) have been synthesized on Cu(111) substrates by the surface-assisted coupling of 6,11-dibromo-1,2,3,4-tetraphenyltriphenylene (C 42 Br 2 H 26 ) and thermal cyclodehydrogenation of the resulting polymer. The GNRs form on Cu(111) epitaxially along the 〈112〉 crystallographic directions, which was found to be in agreement with the computational results, and at lower temperatures than on Au(111). This work demonstrates that the substrate plays an important role in the on-surface synthesis of GNRs and can result in new assembly modes of GNR structures.

Emergence of an Out-of-Plane Optical Phonon (ZO) Kohn Anomaly in Quasifreestanding Epitaxial Graphene.

PubMed

Politano, Antonio; de Juan, Fernando; Chiarello, Gennaro; Fertig, Herbert A

2015-08-14

In neutral graphene, two prominent cusps known as Kohn anomalies are found in the phonon dispersion of the highest optical phonon at q=Γ (LO branch) and q=K (TO branch), reflecting a significant electron-phonon coupling (EPC) to undoped Dirac electrons. In this work, high-resolution electron energy loss spectroscopy is used to measure the phonon dispersion around the Γ point in quasifreestanding graphene epitaxially grown on Pt(111). The Kohn anomaly for the LO phonon is observed at finite momentum q~2k_{F} from Γ, with a shape in excellent agreement with the theory and consistent with known values of the EPC and the Fermi level. More strikingly, we also observe a Kohn anomaly at the same momentum for the out-of-plane optical phonon (ZO) branch. This observation is the first direct evidence of the coupling of the ZO mode with Dirac electrons, which is forbidden for freestanding graphene but becomes allowed in the presence of a substrate. Moreover, we estimate the EPC to be even greater than that of the LO mode, making graphene on Pt(111) an optimal system to explore the effects of this new coupling in the electronic properties.

Direct Transformation of Amorphous Silicon Carbide into Graphene under Low Temperature and Ambient Pressure

PubMed Central

Peng, Tao; Lv, Haifeng; He, Daping; Pan, Mu; Mu, Shichun

2013-01-01

A large-scale availability of the graphene is critical to the successful application of graphene-based electronic devices. The growth of epitaxial graphene (EG) on insulating silicon carbide (SiC) surfaces has opened a new promising route for large-scale high-quality graphene production. However, two key obstacles to epitaxial growth are extremely high requirements for almost perfectly ordered crystal SiC and harsh process conditions. Here, we report that the amorphous SiC (a-Si1−xCx) nano-shell (nano-film) can be directly transformed into graphene by using chlorination method under very mild reaction conditions of relative low temperature (800°C) and the ambient pressure in chlorine (Cl2) atmosphere. Therefore, our finding, the direct transformation of a-Si1−xCx into graphene under much milder condition, will open a door to apply this new method to the large-scale production of graphene at low costs. PMID:23359349

Tension-controlled single-crystallization of copper foils for roll-to-roll synthesis of high-quality graphene films

NASA Astrophysics Data System (ADS)

Jo, Insu; Park, Subeom; Kim, Dongjin; San Moon, Jin; Park, Won Bae; Kim, Tae Hyeong; Hyoun Kang, Jin; Lee, Wonbae; Kim, Youngsoo; Lee, Dong Nyung; Cho, Sung-Pyo; Choi, Hyunchul; Kang, Inbyeong; Park, Jong Hyun; Lee, Jeong Soo; Hong, Byung Hee

2018-04-01

It has been known that the crystalline orientation of Cu substrates plays a crucial role in chemical vapor deposition (CVD) synthesis of high-quality graphene. In particular, Cu (1 1 1) surface showing the minimum lattice mismatch with graphene is expected to provide an ideal catalytic reactivity that can minimize the formation of defects, which also induces larger single-crystalline domain sizes of graphene. Usually, the Cu (1 1 1) substrates can be epitaxially grown on single-crystalline inorganic substrates or can be recrystallized by annealing for more than 12 h, which limits the cost and time-effective synthesis of graphene. Here, we demonstrate a new method to optimize the crystalline orientations of vertically suspended Cu foils by tension control during graphene growth, resulting in large-area recrystallization into Cu (1 1 1) surface as the applied tension activates the grain boundary energy of Cu and promotes its abnormal grain growth to single crystals. In addition, we found a clue that the formation of graphene cooperatively assists the recrystallization into Cu (1 1 1) by minimizing the surface energy of Cu. The domain sizes and charge carrier mobility of graphene grown on the single-crystalline Cu (1 1 1) are 5 times and ~50% increased, respectively, in comparison with those of graphene from Cu (1 0 0), indicating that the less lattice mismatch and the lower interaction energy between Cu (1 1 1) and graphene allows the growth of larger single-crystalline graphene with higher charge carrier mobility. Thus, we believe that our finding provides a crucial idea to design a roll-to-roll (R2R) graphene synthesis system where the tension control is inevitably involved, which would be of great importance for the continuous production of high-quality graphene in the future.

Room temperature deintercalation of alkali metal atoms from epitaxial graphene by formation of charge-transfer complexes

NASA Astrophysics Data System (ADS)

Shin, H.-C.; Ahn, S. J.; Kim, H. W.; Moon, Y.; Rai, K. B.; Woo, S. H.; Ahn, J. R.

2016-08-01

Atom (or molecule) intercalations and deintercalations have been used to control the electronic properties of graphene. In general, finite energies above room temperature (RT) thermal energy are required for the intercalations and deintercalations. Here, we demonstrate that alkali metal atoms can be deintercalated from epitaxial graphene on a SiC substrate at RT, resulting in the reduction in density of states at the Fermi level. The change in density of states at the Fermi level at RT can be applied to a highly sensitive graphene sensor operating at RT. Na atoms, which were intercalated at a temperature of 80 °C, were deintercalated at a high temperature above 1000 °C when only a thermal treatment was used. In contrast to the thermal treatment, the intercalated Na atoms were deintercalated at RT when tetrafluorotetracyanoquinodimethane (F4-TCNQ) molecules were adsorbed on the surface. The RT deintercalation occurred via the formation of charge-transfer complexes between Na atoms and F4-TCNQ molecules.

Integration of the ferromagnetic insulator EuO onto graphene.

PubMed

Swartz, Adrian G; Odenthal, Patrick M; Hao, Yufeng; Ruoff, Rodney S; Kawakami, Roland K

2012-11-27

We have demonstrated the deposition of EuO films on graphene by reactive molecular beam epitaxy in a special adsorption-controlled and oxygen-limited regime, which is a critical advance toward the realization of the exchange proximity interaction (EPI). It has been predicted that when the ferromagnetic insulator (FMI) EuO is brought into contact with graphene, an overlap of electronic wave functions at the FMI/graphene interface can induce a large spin splitting inside the graphene. Experimental realization of this effect could lead to new routes for spin manipulation, which is a necessary requirement for a functional spin transistor. Furthermore, EPI could lead to novel spintronic behavior such as controllable magnetoresistance, gate tunable exchange bias, and quantized anomalous Hall effect. However, experimentally, EuO has not yet been integrated onto graphene. Here we report the successful growth of high-quality crystalline EuO on highly oriented pyrolytic graphite and single-layer graphene. The epitaxial EuO layers have (001) orientation and do not induce an observable D peak (defect) in the Raman spectra. Magneto-optic measurements indicate ferromagnetism with a Curie temperature of 69 K, which is the value for bulk EuO. Transport measurements on exfoliated graphene before and after EuO deposition indicate only a slight decrease in mobility.

Calculation of electron spectra and some problems in the thermodynamics of graphene layers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alisultanov, Z. Z., E-mail: zaur0102@gmail.com

The expressions for the energy spectra of monolayer, bilayer, and multilayer graphene, as well as epitaxial graphene, are derived using the quantum Green’s functions method. Analytic expressions are obtained for the densities of states of these systems. It is shown that a bandgap can appear the spectrum of an epitaxial graphene bilayer. A number of problems in the thermodynamics of electrons in free and epitaxial graphene layers are considered as applications. Analytic expressions are obtained for the chemical potential and heat capacity in the limiting cases of low and high temperatures. Quantum oscillations of heat capacity in graphene are analyzedmore » taking into account the Coulomb interaction. The Berry phase of epitaxial graphene is investigated.« less

Surface conductance of graphene from non-contact resonant cavity.

PubMed

Obrzut, Jan; Emiroglu, Caglar; Kirillov, Oleg; Yang, Yanfei; Elmquist, Randolph E

2016-06-01

A method is established to reliably determine surface conductance of single-layer or multi-layer atomically thin nano-carbon graphene structures. The measurements are made in an air filled standard R100 rectangular waveguide configuration at one of the resonant frequency modes, typically at TE 103 mode of 7.4543 GHz. Surface conductance measurement involves monitoring a change in the quality factor of the cavity as the specimen is progressively inserted into the cavity in quantitative correlation with the specimen surface area. The specimen consists of a nano-carbon-layer supported on a low loss dielectric substrate. The thickness of the conducting nano-carbon layer does not need to be explicitly known, but it is assumed that the lateral dimension is uniform over the specimen area. The non-contact surface conductance measurements are illustrated for a typical graphene grown by chemical vapor deposition process, and for a high quality monolayer epitaxial graphene grown on silicon carbide wafers for which we performed non-gated quantum Hall resistance measurements. The sequence of quantized transverse Hall resistance at the Landau filling factors ν = ±6 and ±2, and the absence of the Hall plateau at ν = 4 indicate that the epitaxially grown graphene is a high quality mono-layer. The resonant microwave cavity measurement is sensitive to the surface and bulk conductivity, and since no additional processing is required, it preserves the integrity of the conductive graphene layer. It allows characterization with high speed, precision and efficiency, compared to transport measurements where sample contacts must be defined and applied in multiple processing steps.

Point Defects and Grain Boundaries in Rotationally Commensurate MoS 2 on Epitaxial Graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Xiaolong; Balla, Itamar; Bergeron, Hadallia

2016-03-28

With reduced degrees of freedom, structural defects are expected to play a greater role in two-dimensional materials in comparison to their bulk counterparts. In particular, mechanical strength, electronic properties, and chemical reactivity are strongly affected by crystal imperfections in the atomically thin limit. Here, ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) are employed to interrogate point and line defects in monolayer MoS2 grown on epitaxial graphene (EG) at the atomic scale. Five types of point defects are observed with the majority species showing apparent structures that are consistent with vacancy and interstitial models. The total defect densitymore » is observed to be lower than MoS2 grown on other substrates and is likely attributed to the van der Waals epitaxy of MoS2 on EG. Grain boundaries (GBs) with 30° and 60° tilt angles resulting from the rotational commensurability of MoS2 on EG are more easily resolved by STM than atomic force microscopy at similar scales due to the enhanced contrast from their distinct electronic states. For example, band gap reduction to ~0.8 and ~0.5 eV is observed with STS for 30° and 60° GBs, respectively. In addition, atomic resolution STM images of these GBs are found to agree well with proposed structure models. This work offers quantitative insight into the structure and properties of common defects in MoS2 and suggests pathways for tailoring the performance of MoS2/graphene heterostructures via defect engineering.« less

Room temperature deintercalation of alkali metal atoms from epitaxial graphene by formation of charge-transfer complexes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shin, H.-C.; Ahn, S. J.; Kim, H. W.

2016-08-22

Atom (or molecule) intercalations and deintercalations have been used to control the electronic properties of graphene. In general, finite energies above room temperature (RT) thermal energy are required for the intercalations and deintercalations. Here, we demonstrate that alkali metal atoms can be deintercalated from epitaxial graphene on a SiC substrate at RT, resulting in the reduction in density of states at the Fermi level. The change in density of states at the Fermi level at RT can be applied to a highly sensitive graphene sensor operating at RT. Na atoms, which were intercalated at a temperature of 80 °C, were deintercalatedmore » at a high temperature above 1000 °C when only a thermal treatment was used. In contrast to the thermal treatment, the intercalated Na atoms were deintercalated at RT when tetrafluorotetracyanoquinodimethane (F4-TCNQ) molecules were adsorbed on the surface. The RT deintercalation occurred via the formation of charge-transfer complexes between Na atoms and F4-TCNQ molecules.« less

Measuring the dielectric and optical response of millimeter-scale amorphous and hexagonal boron nitride films grown on epitaxial graphene.

PubMed

Rigosi, Albert F; Hill, Heather M; Glavin, Nicholas R; Pookpanratana, Sujitra J; Yang, Yanfei; Boosalis, Alexander G; Hu, Jiuning; Rice, Anthony; Allerman, Andrew A; Nguyen, Nhan V; Hacker, Christina A; Elmquist, Randolph E; Hight Walker, Angela R; Newell, David B

2018-01-01

Monolayer epitaxial graphene (EG), grown on the Si face of SiC, is an advantageous material for a variety of electronic and optical applications. EG forms as a single crystal over millimeter-scale areas and consequently, the large scale single crystal can be utilized as a template for growth of other materials. In this work, we present the use of EG as a template to form millimeter-scale amorphous and hexagonal boron nitride ( a -BN and h -BN) films. The a -BN is formed with pulsed laser deposition and the h -BN is grown with triethylboron (TEB) and NH 3 precursors, making it the first metal organic chemical vapor deposition (MOCVD) process of this growth type performed on epitaxial graphene. A variety of optical and non-optical characterization methods are used to determine the optical absorption and dielectric functions of the EG, a -BN, and h -BN within the energy range of 1 eV to 8.5 eV. Furthermore, we report the first ellipsometric observation of high-energy resonant excitons in EG from the 4H polytype of SiC and an analysis on the interactions within the EG and h -BN heterostructure.

Comparison study of PE epitaxy on carbon nanotubes and graphene oxide and PE/graphene oxide as amphiphilic molecular structure for solvent separation

NASA Astrophysics Data System (ADS)

He, Linghao; Zheng, Xiaoli; Xu, Qun; Chen, Zhimin; Fu, Jianwei

2012-03-01

Carbon nanotubes (CNTs) and graphene nanosheets, as one-dimensional and two-dimensional carbon-based nanomaterials respectively, have different abilities to induce the polymer crystallization. In this study, hybrid materials, polyethylene (PE) decorating on CNTs and graphene oxide (GO), were prepared by a facile and efficient method using supercritical carbon dioxide (SC CO2) as anti-solvent. And the morphology and crystallization behavior of PE on CNTs and GO were investigated by transmission electron microscopy, Fourier transform infrared spectroscopy, Raman spectra, wide angle X-ray diffraction, and differential scanning calorimetry. Although both CNTs and GO could act as nucleating agents to induce PE epitaxial growth, CNTs were decorated by PE lamellar crystals forming nanohybrid "shish-kebab" (NHSK) structure, whereas GO sheets were only decorated with petal-like PE crystals. The varying morphologies of the nanohybrids depend on the PE epitaxy and the interactions between polymer chains and substrates. High surface curvature and the perfect ordered crystal structure of CNTs make PE crystals periodically grow on CNTs. While PE crystals grow and form multiple orientation-lamellae on GO due to the lattice matching and complex interactions between PE chains and GO. In addition, our experimental results show an interesting and evident stratification phenomenon for the PE/GO hybrid material, implying that GO decorated by PE have a screening function for the solvents. We anticipate that this work can widen the area of functionalization of carbon-based nanomaterials with a controlled means by an environmentally benign method, which are important for the functional design in nanodevice applications.

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

In situ observations of the atomistic mechanisms of Ni catalyzed low temperature graphene growth.

PubMed

Patera, Laerte L; Africh, Cristina; Weatherup, Robert S; Blume, Raoul; Bhardwaj, Sunil; Castellarin-Cudia, Carla; Knop-Gericke, Axel; Schloegl, Robert; Comelli, Giovanni; Hofmann, Stephan; Cepek, Cinzia

2013-09-24

The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 °C are directly revealed via complementary in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 °C, two different surface carbide (Ni2C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 °C, graphene predominantly grows directly on Ni(111) via replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes.

Controlling the morphology of MBE-grown WSe2 on epitaxial graphene/SiC(0001).

NASA Astrophysics Data System (ADS)

Liu, Liwei; Moghadam, Afsaneh; Weinert, Michael; Li, Lian

Controlling the morphology of transition metal dichalcogenides (TMDs) during molecular beam epitaxy is critical for their potential device applications. In this work, by systematically changing the substrate temperature and W/Se flux ratio, the growth of sub-monolayer to few layers WSe2 on graphene/SiC(0001) is investigated by in situ scanning tunneling microscopy, x-ray photoelectron spectroscopy, and Raman spectroscopy. The results indicate that the morphology of the WSe2 films can be controlled from fractal to compact triangular. These findings and their implication for the controlled growth of TMD heterostructures will be discussed at the meeting. This research was supported by NSF (DMR-1508560).

Capture Zone Distributions and Island Morphologies in Organic Epitaxy and Graphene Formation

NASA Astrophysics Data System (ADS)

Pimpinelli, Alberto; Einstein, T. L.

2013-03-01

Stating that island nucleation is an essential step in the formation of an epitaxial or supported layer may appear trivially obvious. However, less trivial is the observation that the size of the critical nucleus plays a crucial role in that it determines both the island density (and therefore the size of domains) and the evolution of the island morphology. In this talk we will describe recent developments in the analysis of capture zone distributions (CZD) specifically tailored for application to organic materials. We will also describe specific features of organic and graphene island morphologies, and discuss how they are related to the nucleation process and to the size of the critical nucleus. Work at UMD supported by NSF-MRSEC, Grant DMR 05-20471 and NSF CHE 07-49949

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

Constructing molecular structures on periodic superstructure of graphene/Ru(0001)

PubMed Central

Li, Geng; Huang, Li; Xu, Wenyan; Que, Yande; Zhang, Yi; Lu, Jianchen; Du, Shixuan; Liu, Yunqi; Gao, Hong-Jun

2014-01-01

We review the way to fabricate large-scale, high-quality and single crystalline graphene epitaxially grown on Ru(0001) substrate. A moiré pattern of the graphene/Ru(0001) is formed due to the lattice mismatch between graphene and Ru(0001). This superstructure gives rise to surface charge redistribution and could behave as an ordered quantum dot array, which results in a perfect template to guide the assembly of organic molecular structures. Molecules, for example iron phthalocyanine and C60, on this template show how the molecule–substrate interaction makes different superstructures. These results show the possibility of constructing ordered molecular structures on graphene/Ru(0001), which is helpful for practical applications in the future. PMID:24615151

Effects of UV light intensity on electrochemical wet etching of SiC for the fabrication of suspended graphene

NASA Astrophysics Data System (ADS)

O, Ryong-Sok; Takamura, Makoto; Furukawa, Kazuaki; Nagase, Masao; Hibino, Hiroki

2015-03-01

We report on the effects of UV light intensity on the photo assisted electrochemical wet etching of SiC(0001) underneath an epitaxially grown graphene for the fabrication of suspended structures. The maximum etching rate of SiC(0001) was 2.5 µm/h under UV light irradiation in 1 wt % KOH at a constant current of 0.5 mA/cm2. The successful formation of suspended structures depended on the etching rate of SiC. In the Raman spectra of the suspended structures, we did not observe a significant increase in the intensity of the D peak, which originates from defects in graphene sheets. This is most likely explained by the high quality of the single-crystalline graphene epitaxially grown on SiC.

Graphene-Based Flexible and Transparent Tunable Capacitors.

PubMed

Man, Baoyuan; Xu, Shicai; Jiang, Shouzheng; Liu, Aihua; Gao, Shoubao; Zhang, Chao; Qiu, Hengwei; Li, Zhen

2015-12-01

We report a kind of electric field tunable transparent and flexible capacitor with the structure of graphene-Bi1.5MgNb1.5O7 (BMN)-graphene. The graphene films with low sheet resistance were grown by chemical vapor deposition. The BMN thin films were fabricated on graphene by using laser molecular beam epitaxy technology. Compared to BMN films grown on Au, the samples on graphene substrates show better quality in terms of crystallinity, surface morphology, leakage current, and loss tangent. By transferring another graphene layer, we fabricated flexible and transparent capacitors with the structure of graphene-BMN-graphene. The capacitors show a large dielectric constant of 113 with high dielectric tunability of ~40.7 % at a bias field of 1.0 MV/cm. Also, the capacitor can work stably in the high bending condition with curvature radii as low as 10 mm. This flexible film capacitor has a high optical transparency of ~90 % in the visible light region, demonstrating their potential application for a wide range of flexible electronic devices.

Self-heating and failure in scalable graphene devices

DOE PAGES

Beechem, Thomas E.; Shaffer, Ryan A.; Nogan, John; ...

2016-06-09

Self-heating induced failure of graphene devices synthesized from both chemical vapor deposition (CVD) and epitaxial means is compared using a combination of infrared thermography and Raman imaging. Despite a larger thermal resistance, CVD devices dissipate >3x the amount of power before failure than their epitaxial counterparts. The discrepancy arises due to morphological irregularities implicit to the graphene synthesis method that induce localized heating. As a result, morphology, rather than thermal resistance, therefore dictates power handling limits in graphene devices.

Infrared spectroscopy of wafer-scale graphene.

PubMed

Yan, Hugen; Xia, Fengnian; Zhu, Wenjuan; Freitag, Marcus; Dimitrakopoulos, Christos; Bol, Ageeth A; Tulevski, George; Avouris, Phaedon

2011-12-27

We report spectroscopy results from the mid- to far-infrared on wafer-scale graphene, grown either epitaxially on silicon carbide or by chemical vapor deposition. The free carrier absorption (Drude peak) is simultaneously obtained with the universal optical conductivity (due to interband transitions) and the wavelength at which Pauli blocking occurs due to band filling. From these, the graphene layer number, doping level, sheet resistivity, carrier mobility, and scattering rate can be inferred. The mid-IR absorption of epitaxial two-layer graphene shows a less pronounced peak at 0.37 ± 0.02 eV compared to that in exfoliated bilayer graphene. In heavily chemically doped single-layer graphene, a record high transmission reduction due to free carriers approaching 40% at 250 μm (40 cm(-1)) is measured in this atomically thin material, supporting the great potential of graphene in far-infrared and terahertz optoelectronics.

Ultrahigh-mobility graphene devices from chemical vapor deposition on reusable copper

PubMed Central

Banszerus, Luca; Schmitz, Michael; Engels, Stephan; Dauber, Jan; Oellers, Martin; Haupt, Federica; Watanabe, Kenji; Taniguchi, Takashi; Beschoten, Bernd; Stampfer, Christoph

2015-01-01

Graphene research has prospered impressively in the past few years, and promising applications such as high-frequency transistors, magnetic field sensors, and flexible optoelectronics are just waiting for a scalable and cost-efficient fabrication technology to produce high-mobility graphene. Although significant progress has been made in chemical vapor deposition (CVD) and epitaxial growth of graphene, the carrier mobility obtained with these techniques is still significantly lower than what is achieved using exfoliated graphene. We show that the quality of CVD-grown graphene depends critically on the used transfer process, and we report on an advanced transfer technique that allows both reusing the copper substrate of the CVD growth and making devices with mobilities as high as 350,000 cm2 V–1 s–1, thus rivaling exfoliated graphene. PMID:26601221

Oxidation-assisted graphene heteroepitaxy on copper foil.

PubMed

Reckinger, Nicolas; Tang, Xiaohui; Joucken, Frédéric; Lajaunie, Luc; Arenal, Raul; Dubois, Emmanuel; Hackens, Benoît; Henrard, Luc; Colomer, Jean-François

2016-11-10

We propose an innovative, easy-to-implement approach to synthesize aligned large-area single-crystalline graphene flakes by chemical vapor deposition on copper foil. This method doubly takes advantage of residual oxygen present in the gas phase. First, by slightly oxidizing the copper surface, we induce grain boundary pinning in copper and, in consequence, the freezing of the thermal recrystallization process. Subsequent reduction of copper under hydrogen suddenly unlocks the delayed reconstruction, favoring the growth of centimeter-sized copper (111) grains through the mechanism of abnormal grain growth. Second, the oxidation of the copper surface also drastically reduces the nucleation density of graphene. This oxidation/reduction sequence leads to the synthesis of aligned millimeter-sized monolayer graphene domains in epitaxial registry with copper (111). The as-grown graphene flakes are demonstrated to be both single-crystalline and of high quality.

Preparation and electrical transport properties of quasi free standing bilayer graphene on SiC (0001) substrate by H intercalation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yu, Cui; Liu, Qingbin; Li, Jia

2014-11-03

We investigate the temperature dependent electrical transport properties of quasi-free standing bilayer graphene on 4H-SiC (0001) substrate. Three groups of monolayer epitaxial graphene and corresponding quasi-free standing bilayer graphene with different crystal quality and layer number homogeneity are prepared. Raman spectroscopy and atomic-force microscopy are used to obtain their morphologies and layer number, and verify the complete translation of buffer layer into graphene. The highest room temperature mobility reaches 3700 cm{sup 2}/V·s for the quasi-free standing graphene. The scattering mechanism analysis shows that poor crystal quality and layer number inhomogeneity introduce stronger interacting of SiC substrate to the graphene layer andmore » more impurities, which limit the carrier mobility of the quasi-free standing bilayer graphene samples.« less

Narrow plasmon resonances enabled by quasi-freestanding bilayer epitaxial graphene

NASA Astrophysics Data System (ADS)

Daniels, Kevin M.; Jadidi, M. Mehdi; Sushkov, Andrei B.; Nath, Anindya; Boyd, Anthony K.; Sridhara, Karthik; Drew, H. Dennis; Murphy, Thomas E.; Myers-Ward, Rachael L.; Gaskill, D. Kurt

2017-06-01

Exploiting the underdeveloped terahertz range (~1012-1013 Hz) of the electromagnetic spectrum could advance many scientific fields (e.g. medical imaging for the identification of tumors and other biological tissues, non-destructive evaluation of hidden objects or ultra-broadband communication). Despite the benefits of operating in this regime, generation, detection and manipulation have proven difficult, as few materials have functional interactions with THz radiation. In contrast, graphene supports resonances in the THz regime through structural confinement of surface plasmons, which can lead to enhanced absorption. In prior work, the achievable plasmon resonances in such structures have been limited by multiple electron scattering mechanisms (i.e. large carrier scattering rates) which greatly broaden the resonance (>100 cm-1 3 THz). We report the narrowest room temperature Drude response to-date, 30 cm-1 (0.87 THz), obtained using quasi-free standing bilayer epitaxial graphene (QFS BLG) synthesized on (0 0 0 1)6H-SiC. This narrow response is due to a 4-fold increase in carrier mobility and improved thickness and electronic uniformity of QFS BLG. Moreover, QFS BLG samples patterned into microribbons targeting 1.8-5.7 THz plasmon resonances also exhibit low scattering rates (37-53 cm-1). Due to the improved THz properties of QFS BLG, the effects of e-beam processing on carrier scattering rates was determined and we found that fabrication conditions can be tuned to minimize the impact on optoelectronic properties. In addition, electrostatic gating of patterned QFS BLG shows narrow band THz amplitude modulation. Taken together, these properties of QFS BLG should facilitate future development of THz optoelectronic devices for monochromatic applications.

High-quality GaN epitaxially grown on Si substrate with serpentine channels

NASA Astrophysics Data System (ADS)

Wei, Tiantian; Zong, Hua; Jiang, Shengxiang; Yang, Yue; Liao, Hui; Xie, Yahong; Wang, Wenjie; Li, Junze; Tang, Jun; Hu, Xiaodong

2018-06-01

A novel serpentine-channeled mask was introduced to Si substrate for low-dislocation GaN epitaxial growth and the fully coalesced GaN film on the masked Si substrate was achieved for the first time. Compared with the epitaxial lateral overgrowth (ELOG) growth method, this innovative mask only requires one-step epitaxial growth of GaN which has only one high-dislocation region per mask opening. This new growth method can effectively reduce dislocation density, thus improving the quality of GaN significantly. High-quality GaN with low dislocation density ∼2.4 × 107 cm-2 was obtained, which accounted for about eighty percent of the GaN film in area. This innovative technique is promising for the growth of high-quality GaN templates and the subsequent fabrication of high-performance GaN-based devices like transistors, laser diodes (LDs), and light-emitting diodes (LEDs) on Si substrate.

Tuning ultrafast electron injection dynamics at organic-graphene/metal interfaces.

PubMed

Ravikumar, Abhilash; Kladnik, Gregor; Müller, Moritz; Cossaro, Albano; Bavdek, Gregor; Patera, Laerte L; Sánchez-Portal, Daniel; Venkataraman, Latha; Morgante, Alberto; Brivio, Gian Paolo; Cvetko, Dean; Fratesi, Guido

2018-05-03

We compare the ultrafast charge transfer dynamics of molecules on epitaxial graphene and bilayer graphene grown on Ni(111) interfaces through first principles calculations and X-ray resonant photoemission spectroscopy. We use 4,4'-bipyridine as a prototypical molecule for these explorations as the energy level alignment of core-excited molecular orbitals allows ultrafast injection of electrons from a substrate to a molecule on a femtosecond timescale. We show that the ultrafast injection of electrons from the substrate to the molecule is ∼4 times slower on weakly coupled bilayer graphene than on epitaxial graphene. Through our experiments and calculations, we can attribute this to a difference in the density of states close to the Fermi level between graphene and bilayer graphene. We therefore show how graphene coupling with the substrate influences charge transfer dynamics between organic molecules and graphene interfaces.

Graphene: from functionalization to devices

NASA Astrophysics Data System (ADS)

Tejeda, Antonio; Soukiassian, Patrick G.

2014-03-01

The year 2014 marks the first decade of the rise of graphene. Graphene, a single atomic layer of carbon atoms in sp2 bonding configuration having a honeycomb structure, has now become a well-known and well-established material. Among some of its many outstanding fundamental properties, one can mention a very high carrier mobility, a very large spin diffusion length, unsurpassed mechanical properties as graphene is the strongest material ever measured and an exceptional thermal conductivity scaling more than one order of magnitude above that of copper. After the first years of the graphene rush, graphene growth is now well controlled using various methods like epitaxial growth on silicon carbide substrate, chemical vapour deposition (CVD) or plasma techniques on metal, insulator or semiconductor substrates. More applied research is now taking over the initial studies on graphene production. Indeed, graphene is a promising material for many advanced applications such as, but not limited to, electronic, spintronics, sensors, photonics, micro/nano-electromechanical (MEMS/NEMS) systems, super-capacitors or touch-screen technologies. In this context, this Special Issue of the Journal of Physics D: Applied Physics on graphene reviews some of the recent achievements, progress and prospects in this field. It includes a collection of seventeen invited articles covering the current status and future prospects of some selected topics of strong current interest. This Special Issue is organized in four sections. The first section is dedicated to graphene devices, and opens with an article by de Heer et al on an investigation of integrating graphene devices with silicon complementary metal-oxide-semiconductor (CMOS) technology. Then, a study by Svintsov et al proposes a lateral all-graphene tunnel field-effect transistor (FET) with a high on/off current switching ratio. Next, Tsukagoshi et al present how a band-gap opening occurs in a graphene bilayer by using a perpendicular

Atomistic Simulations of Graphene Growth: From Kinetics to Mechanism.

PubMed

Qiu, Zongyang; Li, Pai; Li, Zhenyu; Yang, Jinlong

2018-03-20

Epitaxial growth is a promising strategy to produce high-quality graphene samples. At the same time, this method has great flexibility for industrial scale-up. To optimize growth protocols, it is essential to understand the underlying growth mechanisms. This is, however, very challenging, as the growth process is complicated and involves many elementary steps. Experimentally, atomic-scale in situ characterization methods are generally not feasible at the high temperature of graphene growth. Therefore, kinetics is the main experimental information to study growth mechanisms. Theoretically, first-principles calculations routinely provide atomic structures and energetics but have a stringent limit on the accessible spatial and time scales. Such gap between experiment and theory can be bridged by atomistic simulations using first-principles atomic details as input and providing the overall growth kinetics, which can be directly compared with experiment, as output. Typically, system-specific approximations should be applied to make such simulations computationally feasible. By feeding kinetic Monte Carlo (kMC) simulations with first-principles parameters, we can directly simulate the graphene growth process and thus understand the growth mechanisms. Our simulations suggest that the carbon dimer is the dominant feeding species in the epitaxial growth of graphene on both Cu(111) and Cu(100) surfaces, which enables us to understand why the reaction is diffusion limited on Cu(111) but attachment limited on Cu(100). When hydrogen is explicitly considered in the simulation, the central role hydrogen plays in graphene growth is revealed, which solves the long-standing puzzle into why H 2 should be fed in the chemical vapor deposition of graphene. The simulation results can be directly compared with the experimental kinetic data, if available. Our kMC simulations reproduce the experimentally observed quintic-like behavior of graphene growth on Ir(111). By checking the

Buffer-eliminated, charge-neutral epitaxial graphene on oxidized 4H-SiC (0001) surface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sirikumara, Hansika I., E-mail: hansi.sirikumara@siu.edu; Jayasekera, Thushari, E-mail: thushari@siu.edu

Buffer-eliminated, charge-neutral epitaxial graphene (EG) is important to enhance its potential in device applications. Using the first principles Density Functional Theory calculations, we investigated the effect of oxidation on the electronic and structural properties of EG on 4H-SiC (0001) surface. Our investigation reveals that the buffer layer decouples from the substrate in the presence of both silicate and silicon oxy-nitride at the interface, and the resultant monolayer EG is charge-neutral in both cases. The interface at 4H-SiC/silicate/EG is characterized by surface dangling electrons, which opens up another route for further engineering EG on 4H-SiC. Dangling electron-free 4H-SiC/silicon oxy-nitride/EG is idealmore » for achieving charge-neutral EG.« less

Highly selective covalent organic functionalization of epitaxial graphene

NASA Astrophysics Data System (ADS)

Bueno, Rebeca A.; Martínez, José I.; Luccas, Roberto F.; Del Árbol, Nerea Ruiz; Munuera, Carmen; Palacio, Irene; Palomares, Francisco J.; Lauwaet, Koen; Thakur, Sangeeta; Baranowski, Jacek M.; Strupinski, Wlodek; López, María F.; Mompean, Federico; García-Hernández, Mar; Martín-Gago, José A.

2017-05-01

Graphene functionalization with organics is expected to be an important step for the development of graphene-based materials with tailored electronic properties. However, its high chemical inertness makes difficult a controlled and selective covalent functionalization, and most of the works performed up to the date report electrostatic molecular adsorption or unruly functionalization. We show hereafter a mechanism for promoting highly specific covalent bonding of any amino-terminated molecule and a description of the operating processes. We show, by different experimental techniques and theoretical methods, that the excess of charge at carbon dangling-bonds formed on single-atomic vacancies at the graphene surface induces enhanced reactivity towards a selective oxidation of the amino group and subsequent integration of the nitrogen within the graphene network. Remarkably, functionalized surfaces retain the electronic properties of pristine graphene. This study opens the door for development of graphene-based interfaces, as nano-bio-hybrid composites, fabrication of dielectrics, plasmonics or spintronics.

Identifying suitable substrates for high-quality graphene-based heterostructures

NASA Astrophysics Data System (ADS)

Banszerus, L.; Janssen, H.; Otto, M.; Epping, A.; Taniguchi, T.; Watanabe, K.; Beschoten, B.; Neumaier, D.; Stampfer, C.

2017-06-01

We report on a scanning confocal Raman spectroscopy study investigating the strain-uniformity and the overall strain and doping of high-quality chemical vapour deposited (CVD) graphene-based heterostuctures on a large number of different substrate materials, including hexagonal boron nitride (hBN), transition metal dichalcogenides, silicon, different oxides and nitrides, as well as polymers. By applying a hBN-assisted, contamination free, dry transfer process for CVD graphene, high-quality heterostructures with low doping densities and low strain variations are assembled. The Raman spectra of these pristine heterostructures are sensitive to substrate-induced doping and strain variations and are thus used to probe the suitability of the substrate material for potential high-quality graphene devices. We find that the flatness of the substrate material is a key figure for gaining, or preserving high-quality graphene.

Atomistic mechanisms for bilayer growth of graphene on metal substrates

DOE PAGES

Chen, Wei; Cui, Ping; Zhu, Wenguang; ...

2015-01-08

Epitaxial growth on metal substrates has been shown to be the most powerful approach in producing large-scale high-quality monolayer graphene, yet it remains a major challenge to realize uniform bilayer graphene growth. Here we carry out a comparative study of the atomistic mechanisms for bilayer graphene growth on the (111) surfaces of Cu and Ni, using multiscale approaches combining first-principles calculations and rate-equation analysis. We first show that the relatively weak graphene-Cu interaction enhances the lateral diffusion and effective nucleation of C atoms underneath the graphene island, thereby making it more feasible to grow bilayer graphene on Cu. In contrast,more » the stronger graphene-Ni interaction suppresses the lateral mobility and dimerization of C atoms underneath the graphene, making it unlikely to achieve controlled growth of bilayer graphene on Ni. We then determine the critical graphene size beyond which nucleation of the second layer will take place. Intriguingly, the critical size exhibits an effective inverse "Ehrlich-Schwoebel barrier" effect, becoming smaller for faster C migration from the Cu surface to the graphene-Cu interface sites across the graphene edge. Lastly, these findings allow us to propose a novel alternating growth scheme to realize mass production of bilayer graphene.« less

Atomically thin heterostructures based on single-layer tungsten diselenide and graphene.

PubMed

Lin, Yu-Chuan; Chang, Chih-Yuan S; Ghosh, Ram Krishna; Li, Jie; Zhu, Hui; Addou, Rafik; Diaconescu, Bogdan; Ohta, Taisuke; Peng, Xin; Lu, Ning; Kim, Moon J; Robinson, Jeremy T; Wallace, Robert M; Mayer, Theresa S; Datta, Suman; Li, Lain-Jong; Robinson, Joshua A

2014-12-10

Heterogeneous engineering of two-dimensional layered materials, including metallic graphene and semiconducting transition metal dichalcogenides, presents an exciting opportunity to produce highly tunable electronic and optoelectronic systems. In order to engineer pristine layers and their interfaces, epitaxial growth of such heterostructures is required. We report the direct growth of crystalline, monolayer tungsten diselenide (WSe2) on epitaxial graphene (EG) grown from silicon carbide. Raman spectroscopy, photoluminescence, and scanning tunneling microscopy confirm high-quality WSe2 monolayers, whereas transmission electron microscopy shows an atomically sharp interface, and low energy electron diffraction confirms near perfect orientation between WSe2 and EG. Vertical transport measurements across the WSe2/EG heterostructure provides evidence that an additional barrier to carrier transport beyond the expected WSe2/EG band offset exists due to the interlayer gap, which is supported by theoretical local density of states (LDOS) calculations using self-consistent density functional theory (DFT) and nonequilibrium Green's function (NEGF).

Ultrahard carbon film from epitaxial two-layer graphene

NASA Astrophysics Data System (ADS)

Gao, Yang; Cao, Tengfei; Cellini, Filippo; Berger, Claire; de Heer, Walter A.; Tosatti, Erio; Riedo, Elisa; Bongiorno, Angelo

2018-02-01

Atomically thin graphene exhibits fascinating mechanical properties, although its hardness and transverse stiffness are inferior to those of diamond. So far, there has been no practical demonstration of the transformation of multilayer graphene into diamond-like ultrahard structures. Here we show that at room temperature and after nano-indentation, two-layer graphene on SiC(0001) exhibits a transverse stiffness and hardness comparable to diamond, is resistant to perforation with a diamond indenter and shows a reversible drop in electrical conductivity upon indentation. Density functional theory calculations suggest that, upon compression, the two-layer graphene film transforms into a diamond-like film, producing both elastic deformations and sp2 to sp3 chemical changes. Experiments and calculations show that this reversible phase change is not observed for a single buffer layer on SiC or graphene films thicker than three to five layers. Indeed, calculations show that whereas in two-layer graphene layer-stacking configuration controls the conformation of the diamond-like film, in a multilayer film it hinders the phase transformation.

Growth of high-quality AlN epitaxial film by optimizing the Si substrate surface

NASA Astrophysics Data System (ADS)

Huang, Liegen; Li, Yuan; Wang, Wenliang; Li, Xiaochan; zheng, Yulin; Wang, Haiyan; Zhang, Zichen; Li, Guoqiang

2018-03-01

High-quality AlN epitaxial films have been grown on Si substrates by optimizing the hydrofluoric acid (HF) solution for cleaning of Si substrates. Effect of the Si substrate surface on the surface morphology and structural property of AlN epitaxial films is investigated in detail. It is revealed that as the concentration of HF solution increases from 0 to 2.0%, the surface morphology and the crystalline quality are initially improved and then get worse, and show an optimized value at 1.5%. The as-grown ∼200 nm-thick AlN epitaxial films on Si substrates grown with HF solution of 1.5% reveal the root-mean-square (RMS) surface roughness of 0.49 nm and the full-width at half-maximum for AlN(0002) X-ray rocking curve of 0.35°, indicating the smooth surface morphology and the high crystalline quality. The corresponding mechanism is proposed to interpret the effect of Si substrate surface on surface morphology and structural property of AlN epitaxial films, and provides an effective approach for the perspective fabrication of AlN-based devices.

Metal-Free CVD Graphene Synthesis on 200 mm Ge/Si(001) Substrates.

PubMed

Lukosius, M; Dabrowski, J; Kitzmann, J; Fursenko, O; Akhtar, F; Lisker, M; Lippert, G; Schulze, S; Yamamoto, Y; Schubert, M A; Krause, H M; Wolff, A; Mai, A; Schroeder, T; Lupina, G

2016-12-14

Good quality, complementary-metal-oxide-semiconductor (CMOS) technology compatible, 200 mm graphene was obtained on Ge(001)/Si(001) wafers in this work. Chemical vapor depositions were carried out at the deposition temperatures of 885 °C using CH 4 as carbon source on epitaxial Ge(100) layers, which were grown on Si(100), prior to the graphene synthesis. Graphene layer with the 2D/G ratio ∼3 and low D mode (i.e., low concentration of defects) was measured over the entire 200 mm wafer by Raman spectroscopy. A typical full-width-at-half-maximum value of 39 cm -1 was extracted for the 2D mode, further indicating that graphene of good structural quality was produced. The study also revealed that the lack of interfacial oxide correlates with superior properties of graphene. In order to evaluate electrical properties of graphene, its 2 × 2 cm 2 pieces were transferred onto SiO 2 /Si substrates from Ge/Si wafers. The extracted sheet resistance and mobility values of transferred graphene layers were ∼1500 ± 100 Ω/sq and μ ≈ 400 ± 20 cm 2 /V s, respectively. The transferred graphene was free of metallic contaminations or mechanical damage. On the basis of results of DFT calculations, we attribute the high structural quality of graphene grown by CVD on Ge to hydrogen-induced reduction of nucleation probability, explain the appearance of graphene-induced facets on Ge(001) as a kinetic effect caused by surface step pinning at linear graphene nuclei, and clarify the orientation of graphene domains on Ge(001) as resulting from good lattice matching between Ge(001) and graphene nucleated on such nuclei.

High-quality uniform dry transfer of graphene to polymers.

PubMed

Lock, Evgeniya H; Baraket, Mira; Laskoski, Matthew; Mulvaney, Shawn P; Lee, Woo K; Sheehan, Paul E; Hines, Daniel R; Robinson, Jeremy T; Tosado, Jacob; Fuhrer, Michael S; Hernández, Sandra C; Walton, Scott G

2012-01-11

In this paper we demonstrate high-quality, uniform dry transfer of graphene grown by chemical vapor deposition on copper foil to polystyrene. The dry transfer exploits an azide linker molecule to establish a covalent bond to graphene and to generate greater graphene-polymer adhesion compared to that of the graphene-metal foil. Thus, this transfer approach provides a novel alternative route for graphene transfer, which allows for the metal foils to be reused. © 2011 American Chemical Society

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

Epitaxial CuInSe2 thin films grown by molecular beam epitaxy and migration enhanced epitaxy

NASA Astrophysics Data System (ADS)

Abderrafi, K.; Ribeiro-Andrade, R.; Nicoara, N.; Cerqueira, M. F.; Gonzalez Debs, M.; Limborço, H.; Salomé, P. M. P.; Gonzalez, J. C.; Briones, F.; Garcia, J. M.; Sadewasser, S.

2017-10-01

While CuInSe2 chalcopyrite materials are mainly used in their polycrystalline form to prepare thin film solar cells, epitaxial layers have been used for the characterization of defects. Typically, epitaxial layers are grown by metal-organic vapor phase epitaxy or molecular beam epitaxy (MBE). Here we present epitaxial layers grown by migration enhanced epitaxy (MEE) and compare the materials quality to MBE grown layers. CuInSe2 layers were grown on GaAs (0 0 1) substrates by co-evaporation of Cu, In, and Se using substrate temperatures of 450 °C, 530 °C, and 620 °C. The layers were characterized by high resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and atomic force microscopy (AFM). HR-XRD and HR-TEM show a better crystalline quality of the MEE grown layers, and Raman scattering measurements confirm single phase CuInSe2. AFM shows the previously observed faceting of the (0 0 1) surface into {1 1 2} facets with trenches formed along the [1 1 0] direction. The surface of MEE-grown samples appears smoother compared to MBE-grown samples, a similar trend is observed with increasing growth temperature.

Probing the dielectric response of the interfacial buffer layer in epitaxial graphene via optical spectroscopy

NASA Astrophysics Data System (ADS)

Hill, Heather M.; Rigosi, Albert F.; Chowdhury, Sugata; Yang, Yanfei; Nguyen, Nhan V.; Tavazza, Francesca; Elmquist, Randolph E.; Newell, David B.; Hight Walker, Angela R.

2017-11-01

Monolayer epitaxial graphene (EG) is a suitable candidate for a variety of electronic applications. One advantage of EG growth on the Si face of SiC is that it develops as a single crystal, as does the layer below, referred to as the interfacial buffer layer (IBL), whose properties include an electronic band gap. Although much research has been conducted to learn about the electrical properties of the IBL, not nearly as much work has been reported on the optical properties of the IBL. In this work, we combine measurements from Mueller matrix ellipsometry, differential reflectance contrast, atomic force microscopy, and Raman spectroscopy, as well as calculations from Kramers-Kronig analyses and density-functional theory, to determine the dielectric function of the IBL within the energy range of 1 eV to 8.5 eV.

A new approach to epitaxially grow high-quality GaN films on Si substrates: the combination of MBE and PLD.

PubMed

Wang, Wenliang; Wang, Haiyan; Yang, Weijia; Zhu, Yunnong; Li, Guoqiang

2016-04-22

High-quality GaN epitaxial films have been grown on Si substrates with Al buffer layer by the combination of molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) technologies. MBE is used to grow Al buffer layer at first, and then PLD is deployed to grow GaN epitaxial films on the Al buffer layer. The surface morphology, crystalline quality, and interfacial property of as-grown GaN epitaxial films on Si substrates are studied systematically. The as-grown ~300 nm-thick GaN epitaxial films grown at 850 °C with ~30 nm-thick Al buffer layer on Si substrates show high crystalline quality with the full-width at half-maximum (FWHM) for GaN(0002) and GaN(102) X-ray rocking curves of 0.45° and 0.61°, respectively; very flat GaN surface with the root-mean-square surface roughness of 2.5 nm; as well as the sharp and abrupt GaN/AlGaN/Al/Si hetero-interfaces. Furthermore, the corresponding growth mechanism of GaN epitaxial films grown on Si substrates with Al buffer layer by the combination of MBE and PLD is hence studied in depth. This work provides a novel and simple approach for the epitaxial growth of high-quality GaN epitaxial films on Si substrates.

Theoretical and experimental study of highly textured GaAs on silicon using a graphene buffer layer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alaskar, Yazeed; Arafin, Shamsul; Lin, Qiyin

2015-09-01

A novel heteroepitaxial growth technique, quasi-van der Waals epitaxy, promises the ability to deposit three-dimensional GaAs materials on silicon using two-dimensional graphene as a buffer layer by overcoming the lattice and thermal expansion mismatch. In this study, density functional theory (DFT) simulations were performed to understand the interactions at the GaAs/graphene hetero-interface as well as the growth orientations of GaAs on graphene. To develop a better understanding of the molecular beam epitaxy-grown GaAs films on graphene, samples were characterized by x-ray diffraction (..theta..-2..theta.. scan, ..omega..-scan, grazing incidence XRD and pole figure measurement) and transmission electron microscopy. The realizations of smoothmore » GaAs films with a strong (111) oriented fiber-texture on graphene/silicon using this deposition technique are a milestone towards an eventual demonstration of the epitaxial growth of GaAs on silicon, which is necessary for integrated photonics application.« less

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dharmaraj, P.; Jeganathan, K., E-mail: kjeganathan@yahoo.com; Parthiban, S.

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 cm{sup 2} V{sup −1} s{sup −1} with a sheet carrier density of 2.2 × 10{sup 13} cm{sup −2}. Current-voltage measurements reveal that BLEG on 4H-SiC forms a Schottky junction with an operation at mAmore » 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.« less

Generic epitaxial graphene biosensors for ultrasensitive detection of cancer risk biomarker

NASA Astrophysics Data System (ADS)

Tehrani, Z.; Burwell, G.; Mohd Azmi, M. A.; Castaing, A.; Rickman, R.; Almarashi, J.; Dunstan, P.; Miran Beigi, A.; Doak, S. H.; Guy, O. J.

2014-09-01

A generic electrochemical method of ‘bioreceptor’ antibody attachment to phenyl amine functionalized graphitic surfaces is demonstrated. Micro-channels of chemically modified multi-layer epitaxial graphene (MLEG) have been used to provide a repeatable and reliable response to nano-molar (nM) concentrations of the cancer risk (oxidative stress) biomarker 8-hydroxydeoxyguanosine (8-OHdG). X-ray photoelectron spectroscopy, Raman spectroscopy are used to characterize the functionalized MLEG. Confocal fluorescence microscopy using fluorescent-labelled antibodies indicates that the anti-8-OHdG antibody selectively binds to the phenyl amine-functionalized MLEG’s channel. Current-voltage measurements on functionalized channels showed repeatable current responses from antibody-biomarker binding events. This technique is scalable, reliable, and capable of providing a rapid, quantitative, label-free assessment of biomarkers at nano-molar (<20 nM) concentrations in analyte solutions. The sensitivity of the sensor device was investigated using varying concentrations of 8-OHdG, with changes in the sensor’s channel resistance observed upon exposure to 8-OHdG. Detection of 8-OHdG concentrations as low as 0.1 ng ml-1 (0.35 nM) has been demonstrated. This is five times more sensitive than reported enzyme linked immunosorbent assay tests (0.5 ng ml-1).

CVD growth of large-grain graphene on Cu(111) thin films

NASA Astrophysics Data System (ADS)

Miller, David L.; Diederichsen, Kyle M.; Keller, Mark W.

2013-03-01

Chemical vapor deposition of graphene on polycrystalline Cu foils has produced high quality films with carrier mobility approaching that of exfoliated graphene. Growth on single-crystal films of Cu has received less attention, despite its potential advantages for graphene quality and its importance for eventual applications. This is likely due to the difficulty of obtaining large (>= 1 mm) grains in Cu thin films, as well as dewetting and roughening of Cu films at temperatures near the Cu melting point (1084 C). We found that 450 nm of Cu(111), epitaxially grown by sputtering onto Al2O3(0001), formed > 1 mm grains when annealed at 1065 C for 40 minutes in 40 Torr of Ar and 2.5 mTorr of H2. After this annealing, adding 3 mTorr of CH4 for 8 minutes produced a monolayer graphene film covering > 99 % of the Cu surface. Stopping growth after 4 minutes produced dendritic graphene islands with 6-fold symmetry and diameter of 20 μm to 100 μm . After growth, the Cu film remained smooth except for thermal grooving at grain boundaries and a few holes of diameter ~ 10 μm where Cu dewetted completely (~ 10 holes on each 5 mm x 6 mm chip).

Dislocations in bilayer graphene

NASA Astrophysics Data System (ADS)

Butz, Benjamin; Dolle, Christian; Niekiel, Florian; Weber, Konstantin; Waldmann, Daniel; Weber, Heiko B.; Meyer, Bernd; Spiecker, Erdmann

2014-01-01

Dislocations represent one of the most fascinating and fundamental concepts in materials science. Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals. In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy. However, in layered crystals such as graphite, dislocation movement is mainly restricted to the basal plane. Thus, the dislocations cannot escape, enabling their confinement in crystals as thin as only two monolayers. To explore the nature of dislocations under such extreme boundary conditions, the material of choice is bilayer graphene, the thinnest possible quasi-two-dimensional crystal in which such linear defects can be confined. Homogeneous and robust graphene membranes derived from high-quality epitaxial graphene on silicon carbide provide an ideal platform for their investigation. Here we report the direct observation of basal-plane dislocations in freestanding bilayer graphene using transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. Our investigation reveals two striking size effects. First, the absence of stacking-fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern that corresponds to an alternating ABAC change of the stacking order. Second, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane that results directly from accommodation of strain. In fact, the buckling changes the strain state of the bilayer graphene and is of key importance for its electronic properties. Our findings will contribute to the understanding of dislocations and of their role in the structural, mechanical and electronic properties of bilayer and

Dislocations in bilayer graphene.

PubMed

Butz, Benjamin; Dolle, Christian; Niekiel, Florian; Weber, Konstantin; Waldmann, Daniel; Weber, Heiko B; Meyer, Bernd; Spiecker, Erdmann

2014-01-23

Dislocations represent one of the most fascinating and fundamental concepts in materials science. Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals. In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy. However, in layered crystals such as graphite, dislocation movement is mainly restricted to the basal plane. Thus, the dislocations cannot escape, enabling their confinement in crystals as thin as only two monolayers. To explore the nature of dislocations under such extreme boundary conditions, the material of choice is bilayer graphene, the thinnest possible quasi-two-dimensional crystal in which such linear defects can be confined. Homogeneous and robust graphene membranes derived from high-quality epitaxial graphene on silicon carbide provide an ideal platform for their investigation. Here we report the direct observation of basal-plane dislocations in freestanding bilayer graphene using transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. Our investigation reveals two striking size effects. First, the absence of stacking-fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern that corresponds to an alternating AB B[Symbol: see text]AC change of the stacking order. Second, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane that results directly from accommodation of strain. In fact, the buckling changes the strain state of the bilayer graphene and is of key importance for its electronic properties. Our findings will contribute to the understanding of dislocations and of their role in the structural, mechanical and electronic

Atomically Thin Heterostructures Based on Single-Layer Tungsten Diselenide and Graphene [Plus Supplemental Information

DOE PAGES

Lin, Yu-Chuan; Chang, Chih-Yuan S.; Ghosh, Ram Krishna; ...

2014-11-10

Heterogeneous engineering of two-dimensional layered materials, including metallic graphene and semiconducting transition metal dichalcogenides, presents an exciting opportunity to produce highly tunable electronic and optoelectronic systems. We report the direct growth of highly crystalline, monolayer tungsten diselenide (WSe 2) on epitaxial graphene (EG). Raman spectroscopy and photoluminescence confirms high-quality WSe 2 monolayers; while transmission electron microscopy shows an atomically sharp interface and low energy electron diffraction confirms near perfect orientation between WSe 2 and EG. Vertical transport measurements across the WSe 2/EG heterostructure provides evidence that a tunnel barrier exists due to the van der Waals gap, and is supportedmore » by density functional theory that predicts a 1.6 eV barrier for transport from WSe 2 to graphene.« less

Spatially resolved mapping of electrical conductivity across individual domain (grain) boundaries in graphene.

PubMed

Clark, Kendal W; Zhang, X-G; Vlassiouk, Ivan V; He, Guowei; Feenstra, Randall M; Li, An-Ping

2013-09-24

All large-scale graphene films contain extended topological defects dividing graphene into domains or grains. Here, we spatially map electronic transport near specific domain and grain boundaries in both epitaxial graphene grown on SiC and CVD graphene on Cu subsequently transferred to a SiO2 substrate, with one-to-one correspondence to boundary structures. Boundaries coinciding with the substrate step on SiC exhibit a significant potential barrier for electron transport of epitaxial graphene due to the reduced charge transfer from the substrate near the step edge. Moreover, monolayer-bilayer boundaries exhibit a high resistance that can change depending on the height of substrate step coinciding at the boundary. In CVD graphene, the resistance of a grain boundary changes with the width of the disordered transition region between adjacent grains. A quantitative modeling of boundary resistance reveals the increased electron Fermi wave vector within the boundary region, possibly due to boundary induced charge density variation. Understanding how resistance change with domain (grain) boundary structure in graphene is a crucial first step for controlled engineering of defects in large-scale graphene films.

Atomic Scale Study on Growth and Heteroepitaxy of ZnO Monolayer on Graphene.

PubMed

Hong, Hyo-Ki; Jo, Junhyeon; Hwang, Daeyeon; Lee, Jongyeong; Kim, Na Yeon; Son, Seungwoo; Kim, Jung Hwa; Jin, Mi-Jin; Jun, Young Chul; Erni, Rolf; Kwak, Sang Kyu; Yoo, Jung-Woo; Lee, Zonghoon

2017-01-11

Atomically thin semiconducting oxide on graphene carries a unique combination of wide band gap, high charge carrier mobility, and optical transparency, which can be widely applied for optoelectronics. However, study on the epitaxial formation and properties of oxide monolayer on graphene remains unexplored due to hydrophobic graphene surface and limits of conventional bulk deposition technique. Here, we report atomic scale study of heteroepitaxial growth and relationship of a single-atom-thick ZnO layer on graphene using atomic layer deposition. We demonstrate atom-by-atom growth of zinc and oxygen at the preferential zigzag edge of a ZnO monolayer on graphene through in situ observation. We experimentally determine that the thinnest ZnO monolayer has a wide band gap (up to 4.0 eV), due to quantum confinement and graphene-like structure, and high optical transparency. This study can lead to a new class of atomically thin two-dimensional heterostructures of semiconducting oxides formed by highly controlled epitaxial growth.

Supercapacitors based on high-quality graphene scrolls.

PubMed

Zeng, Fanyan; Kuang, Yafei; Liu, Gaoqin; Liu, Rui; Huang, Zhongyuan; Fu, Chaopeng; Zhou, Haihui

2012-07-07

High-quality graphene scrolls (GSS) with a unique scrolled topography are designed using a microexplosion method. Their capacitance properties are investigated by cyclic voltammetry, galvanostatic charge-discharge and electrical impedance spectroscopy. Compared with the specific capacity of 110 F g(-1) for graphene sheets, a remarkable capacity of 162.2 F g(-1) is obtained at the current density of 1.0 A g(-1) in 6 M KOH aqueous solution owing to the unique scrolled structure of GSS. The capacity value is increased by about 50% only because of the topological change of graphene sheets. Meanwhile, GSS exhibit excellent long-term cycling stability along with 96.8% retained after 1000 cycles at 1.0 A g(-1). These encouraging results indicate that GSS based on the topological structure of graphene sheets are a kind of promising material for supercapacitors.

Supercapacitors based on high-quality graphene scrolls

NASA Astrophysics Data System (ADS)

Zeng, Fanyan; Kuang, Yafei; Liu, Gaoqin; Liu, Rui; Huang, Zhongyuan; Fu, Chaopeng; Zhou, Haihui

2012-06-01

High-quality graphene scrolls (GSS) with a unique scrolled topography are designed using a microexplosion method. Their capacitance properties are investigated by cyclic voltammetry, galvanostatic charge-discharge and electrical impedance spectroscopy. Compared with the specific capacity of 110 F g-1 for graphene sheets, a remarkable capacity of 162.2 F g-1 is obtained at the current density of 1.0 A g-1 in 6 M KOH aqueous solution owing to the unique scrolled structure of GSS. The capacity value is increased by about 50% only because of the topological change of graphene sheets. Meanwhile, GSS exhibit excellent long-term cycling stability along with 96.8% retained after 1000 cycles at 1.0 A g-1. These encouraging results indicate that GSS based on the topological structure of graphene sheets are a kind of promising material for supercapacitors.

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dharmaraj, P.; Justin Jesuraj, P.; Jeganathan, K., E-mail: kjeganathan@yahoo.com

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 highmore » electron mobility well excess of 6000 cm{sup 2} V{sup −1} s{sup −1}.« less

Nanopatched Graphene with Molecular Self-Assembly Toward Graphene-Organic Hybrid Soft Electronics.

PubMed

Kang, Boseok; Lee, Seong Kyu; Jung, Jaehyuck; Joe, Minwoong; Lee, Seon Baek; Kim, Jinsung; Lee, Changgu; Cho, Kilwon

2018-06-01

Increasing the mechanical durability of large-area polycrystalline single-atom-thick materials is a necessary step toward the development of practical and reliable soft electronics based on these materials. Here, it is shown that the surface assembly of organosilane by weak epitaxy forms nanometer-thick organic patches on a monolayer graphene surface and dramatically increases the material's resistance to harsh postprocessing environments, thereby increasing the number of ways in which graphene can be processed. The nanopatched graphene with the improved mechanical durability enables stable operation when used as transparent electrodes of wearable strain sensors. Also, the nanopatched graphene applied as an electrode modulates the molecular orientation of deposited organic semiconductor layers, and yields favorable nominal charge injection for organic transistors. These results demonstrate the potential for use of self-assembled organic nanopatches in graphene-based soft electronics. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Determining the Mechanism of Low Temperature Graphene Growth

DTIC Science & Technology

2014-05-27

layer graphene over cobalt film crystallized on sapphire. ACS Nano, 2010. 4: p. 7407-7414. 8. Sutter, P.W., J. I. Flege and E. A. Sutter, Epitaxial...111). Journal of Physics, 2009. 11: p. 1-25. 15. Sukhdeo, D., Large area CVD of graphene over thin films of cobalt . 2009, Columbia University. p...B.R. Luo, W.P. Hu, G. Yu, Y.Q. Liu, Low Temperature Growth of Highly Nitrogen- doped Single Crystal Graphene Arrays by Chemical Vapor Deposition

Polymer coating and stress test for carrier density stabilization in epitaxial graphene

NASA Astrophysics Data System (ADS)

Rigosi, Albert; Liu, Chieh-I.; Yang, Yanfei; Obrzut, Jan; Lee, Hsin Yen; Bittle, Emily; Elmquist, Randolph

Homogeneous monolayer epitaxial graphene (EG) is an ideal candidate for the development of a quantum Hall resistance (QHR) standard. A clean fabrication process was used to produce EG-QHR devices with a n-type doping level of order 1011 cm-2, which delivers the metrological accuracy at the ν = 2 plateau in a moderate magnetic field (<9 T). However, the ν = 2 plateau deviates from h/2e2 quickly as the carrier density shifts close to the Dirac point (<1010 cm-2) , and this observation occurs over time as EG is exposed to air, allowing for complexation with p-type molecular dopants. Here we report experimental results on the use of parylene C as an encapsulation layer, whereby EG can maintain its carrier density level under ambient laboratory conditions for a few months. Furthermore, we varied the parylene C thicknesses and the controllable temperatures (up to 85° C) and humidities (up to 85%). We monitored the electronic properties of our EG samples by low temperature magnetotransport measurements in a 9 T superconducting magnet cryostat, and room temperature surface conductance in a resonant microwave cavity. We will compare parylene C, Cytop, and PMMA and show that polymer encapsulation may offer a solution to the problem of carrier density instability from atmospheric doping.

High quality atomically thin PtSe2 films grown by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Yan, Mingzhe; Wang, Eryin; Zhou, Xue; Zhang, Guangqi; Zhang, Hongyun; Zhang, Kenan; Yao, Wei; Lu, Nianpeng; Yang, Shuzhen; Wu, Shilong; Yoshikawa, Tomoki; Miyamoto, Koji; Okuda, Taichi; Wu, Yang; Yu, Pu; Duan, Wenhui; Zhou, Shuyun

2017-12-01

Atomically thin PtSe2 films have attracted extensive research interests for potential applications in high-speed electronics, spintronics and photodetectors. Obtaining high quality thin films with large size and controlled thickness is critical. Here we report the first successful epitaxial growth of high quality PtSe2 films by molecular beam epitaxy. Atomically thin films from 1 ML to 22 ML have been grown and characterized by low-energy electron diffraction, Raman spectroscopy and x-ray photoemission spectroscopy. Moreover, a systematic thickness dependent study of the electronic structure is revealed by angle-resolved photoemission spectroscopy (ARPES), and helical spin texture is revealed by spin-ARPES. Our work provides new opportunities for growing large size single crystalline films to investigate the physical properties and potential applications of PtSe2.

Graphene-Mesoporous Si Nanocomposite as a Compliant Substrate for Heteroepitaxy.

PubMed

Boucherif, Abderrahim Rahim; Boucherif, Abderraouf; Kolhatkar, Gitanjali; Ruediger, Andreas; Arès, Richard

2017-05-01

The ultimate performance of a solid state device is limited by the restricted number of crystalline substrates that are available for epitaxial growth. As a result, only a small fraction of semiconductors are usable. This study describes a novel concept for a tunable compliant substrate for epitaxy, based on a graphene-porous silicon nanocomposite, which extends the range of available lattice constants for epitaxial semiconductor alloys. The presence of graphene and its effect on the strain of the porous layer lattice parameter are discussed in detail and new remarkable properties are demonstrated. These include thermal stability up to 900 °C, lattice tuning up to 0.9 % mismatch, and compliance under stress for virtual substrate thicknesses of several micrometers. A theoretical model is proposed to define the compliant substrate design rules. These advances lay the foundation for the fabrication of a compliant substrate that could unlock the lattice constant restrictions for defect-free new epitaxial semiconductor alloys and devices. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Towards functionalization of graphene: in situ study of the nucleation of copper-phtalocyanine on graphene

NASA Astrophysics Data System (ADS)

Schwarz, Daniel; Henneke, Caroline; Kumpf, Christian

2016-02-01

Molecular films present an elegant way for the uniform functionalization or doping of graphene. Here, we present an in situ study on the initial growth of copper phthalocyanine (CuPc) on epitaxial graphene on Ir(111). We followed the growth up to a closed monolayer with low energy electron microscopy and selected area electron diffraction (μLEED). The molecules coexist on graphene in a disordered phase without long-range order and an ordered crystalline phase. The local topography of the graphene substrate plays an important role in the nucleation process of the crystalline phase. Graphene flakes on Ir(111) feature regions that are under more tensile stress than others. We observe that the CuPc molecules form ordered domains initially on those graphene regions that are closest to the fully relaxed lattice. We attribute this effect to a stronger influence of the underlying Ir(111) substrate for molecules adsorbed on those relaxed regions.

Electromigration in epitaxial Cu(001) lines

NASA Astrophysics Data System (ADS)

Ramanath, G.; Kim, H.; Goindi, H. S.; Frederick, M. J.; Shin, C.-S.; Goswami, R.; Petrov, I.; Greene, J. E.

2002-04-01

We report the electromigration (EM) response of single-domain epitaxial Cu(001) lines on layers of Ta, TaN, and TiN. Epitaxial Cu(001) lines on nitride layers exhibit nearly two orders of magnitude higher mean-time-to-failure (MTTF) values than those on Ta, indicating the strong influence of the underlayer. The activation energy of EM for Cu on the nitrides is ˜0.8-1.2 eV, and that of Cu on Ta is ˜0.2 eV, for 200-300 °C. Our results also indicate that the MTTF values correlate inversely to the crystal quality of the Cu layers measured by X-ray diffraction. The EM resistance of epitaxial Cu lines with different crystal quality on TaN were measured to separate the effects of interface chemistry and crystal quality. While higher quality epitaxial films reveal a higher EM resistance, the magnitude of the change is smaller than that obtained by changing the interface chemistry. Epitaxial lines exhibit more than 3-4 orders of magnitude higher MTTF than polycrystalline lines on the same underlayer. Based upon our results, we propose that the Cu/underlayer interface chemistry and presence of grain boundary diffusion play important roles in unpassivated Cu films.

Temperature-triggered chemical switching growth of in-plane and vertically stacked graphene-boron nitride heterostructures

PubMed Central

Gao, Teng; Song, Xiuju; Du, Huiwen; Nie, Yufeng; Chen, Yubin; Ji, Qingqing; Sun, Jingyu; Yang, Yanlian; Zhang, Yanfeng; Liu, Zhongfan

2015-01-01

In-plane and vertically stacked heterostructures of graphene and hexagonal boron nitride (h-BN-G and G/h-BN, respectively) are both recent focuses of graphene research. However, targeted synthesis of either heterostructure remains a challenge. Here, via chemical vapour deposition and using benzoic acid precursor, we have achieved the selective growth of h-BN-G and G/h-BN through a temperature-triggered switching reaction. The perfect in-plane h-BN-G is characterized by scanning tunnelling microscopy (STM), showing atomically patched graphene and h-BN with typical zigzag edges. In contrast, the vertical alignment of G/h-BN is confirmed by unique lattice-mismatch-induced moiré patterns in high-resolution STM images, and two sets of aligned selected area electron diffraction spots, both suggesting a van der Waals epitaxial mechanism. The present work demonstrates the chemical designability of growth process for controlled synthesis of graphene and h-BN heterostructures. With practical scalability, high uniformity and quality, our approach will promote the development of graphene-based electronics and optoelectronics. PMID:25869236

Effects of a modular two-step ozone-water and annealing process on silicon carbide graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Webb, Matthew J., E-mail: matthew.webb@cantab.net; Lundstedt, Anna; Grennberg, Helena

By combining ozone and water, the effect of exposing epitaxial graphene on silicon carbide to an aggressive wet-chemical process has been evaluated after high temperature annealing in ultra high vacuum. The decomposition of ozone in water produces a number of oxidizing species, however, despite long exposure times to the aqueous-ozone environment, no graphene oxide was observed after the two-step process. The systems were comprehensively characterized before and after processing using Raman spectroscopy, core level photoemission spectroscopy, and angle resolved photoemission spectroscopy together with low energy electron diffraction, low energy electron microscopy, and atomic force microscopy. In spite of the chemicalmore » potential of the aqueous-ozone reaction environment, the graphene domains were largely unaffected raising the prospect of employing such simple chemical and annealing protocols to clean or prepare epitaxial graphene surfaces.« less

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).

High-quality graphene flakes exfoliated on a flat hydrophobic polymer

NASA Astrophysics Data System (ADS)

Pedrinazzi, Paolo; Caridad, José M.; Mackenzie, David M. A.; Pizzocchero, Filippo; Gammelgaard, Lene; Jessen, Bjarke S.; Sordan, Roman; Booth, Timothy J.; Bøggild, Peter

2018-01-01

We show that graphene supported on a hydrophobic and flat polymer surface results in flakes with extremely low doping and strain as assessed by their Raman spectroscopic characteristics. We exemplify this technique by micromechanical exfoliation of graphene on flat poly(methylmethacrylate) layers and demonstrate Raman peak intensity ratios I(2D)/I(G) approaching 10, similar to pristine freestanding graphene. We verify that these features are not an artifact of optical interference effects occurring at the substrate: they are similarly observed when varying the substrate thickness and are maintained when the environment of the graphene flake is completely changed, by encapsulating preselected flakes between hexagonal boron nitride layers. The exfoliation of clean, pristine graphene layers directly on flat polymer substrates enables high performance, supported, and non-encapsulated graphene devices for flexible and transparent optoelectronic studies. We additionally show that the access to a clean and supported graphene source leads to high-quality van der Waals heterostructures and devices with reproducible carrier mobilities exceeding 50 000 cm2 V-1 s-1 at room temperature.

Molecular adsorption on graphene

NASA Astrophysics Data System (ADS)

Kong, Lingmei; Enders, Axel; Rahman, Talat S.; Dowben, Peter A.

2014-11-01

Current studies addressing the engineering of charge carrier concentration and the electronic band gap in epitaxial graphene using molecular adsorbates are reviewed. The focus here is on interactions between the graphene surface and the adsorbed molecules, including small gas molecules (H2O, H2, O2, CO, NO2, NO, and NH3), aromatic, and non-aromatic molecules (F4-TCNQ, PTCDA, TPA, Na-NH2, An-CH3, An-Br, Poly (ethylene imine) (PEI), and diazonium salts), and various biomolecules such as peptides, DNA fragments, and other derivatives. This is followed by a discussion on graphene-based gas sensor concepts. In reviewing the studies of the effects of molecular adsorption on graphene, it is evident that the strong manipulation of graphene’s electronic structure, including p- and n-doping, is not only possible with molecular adsorbates, but that this approach appears to be superior compared to these exploiting edge effects, local defects, or strain. However, graphene-based gas sensors, albeit feasible because huge adsorbate-induced variations in the relative conductivity are possible, generally suffer from the lack of chemical selectivity.

Wrinkles of graphene on Ir(111): Macroscopic network ordering and internal multi-lobed structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Petrovic, Marin; Sadowski, Jerzy T.; Siber, Antonio

The large-scale production of graphene monolayer greatly relies on epitaxial samples which often display stress-relaxation features in the form of wrinkles. Wrinkles of graphene on Ir(111) are found to exhibit a fairly well ordered interconnecting network which is characterized by low-energy electron microscopy (LEEM). The high degree of quasi-hexagonal network arrangement for the graphene aligned to the underlying substrate can be well described as a (non-Poissonian) Voronoi partition of a plane. The results obtained strongly suggest that the wrinkle network is frustrated at low temperatures, retaining the order inherited from elevated temperatures when the wrinkles interconnect in junctions which mostmore » often join three wrinkles. Such frustration favors the formation of multi-lobed wrinkles which are found in scanning tunneling microscopy (STM) measurements. The existence of multiple lobes is explained within a model accounting for the interplay of the van der Waals attraction between graphene and iridium and bending energy of the wrinkle. The presented study provides new insights into wrinkling of epitaxial graphene and can be exploited to further expedite its application.« less

Wrinkles of graphene on Ir(111): Macroscopic network ordering and internal multi-lobed structure

DOE PAGES

Petrovic, Marin; Sadowski, Jerzy T.; Siber, Antonio; ...

2015-07-17

The large-scale production of graphene monolayer greatly relies on epitaxial samples which often display stress-relaxation features in the form of wrinkles. Wrinkles of graphene on Ir(111) are found to exhibit a fairly well ordered interconnecting network which is characterized by low-energy electron microscopy (LEEM). The high degree of quasi-hexagonal network arrangement for the graphene aligned to the underlying substrate can be well described as a (non-Poissonian) Voronoi partition of a plane. The results obtained strongly suggest that the wrinkle network is frustrated at low temperatures, retaining the order inherited from elevated temperatures when the wrinkles interconnect in junctions which mostmore » often join three wrinkles. Such frustration favors the formation of multi-lobed wrinkles which are found in scanning tunneling microscopy (STM) measurements. The existence of multiple lobes is explained within a model accounting for the interplay of the van der Waals attraction between graphene and iridium and bending energy of the wrinkle. The presented study provides new insights into wrinkling of epitaxial graphene and can be exploited to further expedite its application.« less

Role of topographical defects in organic film growth of 4,4' -biphenyldicarboxylic acid on graphene: A low-energy electron microscopy study

NASA Astrophysics Data System (ADS)

Khokhar, Fawad S.; van Gastel, Raoul; Poelsema, Bene

2010-11-01

We have used low-energy electron microscopy (LEEM) to study the formation of self-assembled molecular networks on graphene sheets. 4,4' -biphenyldicarboxylic acid (BDA) molecular networks were grown using organic molecular beam epitaxy. LEEM images provide direct insight in the growth dynamics and show that defects in the graphene play a crucial role in the final morphology of the molecular film that forms. BDA is demonstrated to form hydrogen bond-stabilized chains on graphene. Dark-field LEEM images reveal that the same defects that determine the morphology of the film, also direct the orientation of the domains, highlighting the importance of understanding the role of defects in epitaxial processes on graphene.

Fabrication of precision high quality facets on molecular beam epitaxy material

DOEpatents

Petersen, Holly E.; Goward, William D.; Dijaili, Sol P.

2001-01-01

Fabricating mirrored vertical surfaces on semiconductor layered material grown by molecular beam epitaxy (MBE). Low energy chemically assisted ion beam etching (CAIBE) is employed to prepare mirrored vertical surfaces on MBE-grown III-V materials under unusually low concentrations of oxygen in evacuated etching atmospheres of chlorine and xenon ion beams. UV-stabilized smooth-surfaced photoresist materials contribute to highly vertical, high quality mirrored surfaces during the etching.

Fabry-Perot enhanced Faraday rotation in graphene.

PubMed

Ubrig, Nicolas; Crassee, Iris; Levallois, Julien; Nedoliuk, Ievgeniia O; Fromm, Felix; Kaiser, Michl; Seyller, Thomas; Kuzmenko, Alexey B

2013-10-21

We demonstrate that giant Faraday rotation in graphene in the terahertz range due to the cyclotron resonance is further increased by constructive Fabry-Perot interference in the supporting substrate. Simultaneously, an enhanced total transmission is achieved, making this effect doubly advantageous for graphene-based magneto-optical applications. As an example, we present far-infrared spectra of epitaxial multilayer graphene grown on the C-face of 6H-SiC, where the interference fringes are spectrally resolved and a Faraday rotation up to 0.15 radians (9°) is attained. Further, we discuss and compare other ways to increase the Faraday rotation using the principle of an optical cavity.

Epitaxial solar cells fabrication

NASA Technical Reports Server (NTRS)

Daiello, R. V.; Robinson, P. H.; Kressel, H.

1975-01-01

Silicon epitaxy has been studied for the fabrication of solar cell structures, with the intent of optimizing efficiency while maintaining suitability for space applications. SiH2CL2 yielded good quality layers and junctions with reproducible impurity profiles. Diode characteristics and lifetimes in the epitaxial layers were investigated as a function of epitaxial growth conditions and doping profile, as was the effect of substrates and epitaxial post-gettering on lifetime. The pyrolytic decomposition of SiH4 was also used in the epitaxial formation of highly doped junction layers on bulk Si wafers. The effects of junction layer thickness and bulk background doping level on cell performance, in particular, open-circuit voltage, were investigated. The most successful solar cells were fabricated with SiH2 CL2 to grow p/n layers on n(+) substrates. The best performance was obtained from a p(+)/p/n/n(+) structure grown with an exponential grade in the n-base layer.

Molecular Beam Epitaxy Growth of High Crystalline Quality LiNbO3

NASA Astrophysics Data System (ADS)

Tellekamp, M. Brooks; Shank, Joshua C.; Goorsky, Mark S.; Doolittle, W. Alan

2016-12-01

Lithium niobate is a multi-functional material with wide reaching applications in acoustics, optics, and electronics. Commercial applications for lithium niobate require high crystalline quality currently limited to bulk and ion sliced material. Thin film lithium niobate is an attractive option for a variety of integrated devices, but the research effort has been stagnant due to poor material quality. Both lattice matched and mismatched lithium niobate are grown by molecular beam epitaxy and studied to understand the role of substrate and temperature on nucleation conditions and material quality. Growth on sapphire produces partially coalesced columnar grains with atomically flat plateaus and no twin planes. A symmetric rocking curve shows a narrow linewidth with a full width at half-maximum (FWHM) of 8.6 arcsec (0.0024°), which is comparable to the 5.8 arcsec rocking curve FWHM of the substrate, while the film asymmetric rocking curve is 510 arcsec FWHM. These values indicate that the individual grains are relatively free of long-range disorder detectable by x-ray diffraction with minimal measurable tilt and twist and represents the highest structural quality epitaxial material grown on lattice mismatched sapphire without twin planes. Lithium niobate is also grown on lithium tantalate producing high quality coalesced material without twin planes and with a symmetric rocking curve of 193 arcsec, which is nearly equal to the substrate rocking curve of 194 arcsec. The surface morphology of lithium niobate on lithium tantalate is shown to be atomically flat by atomic force microscopy.

Utilization of plasmas for graphene synthesis

NASA Astrophysics Data System (ADS)

Shashurin, Alexey; Keidar, Michael

2013-10-01

Graphene is a one-atom-thick planar sheet of carbon atoms that are densely packed in a honeycomb crystal lattice. Grapheen has tremendous range of potential applications ranging from high-speed transistors to electrochemical energy storage devices and biochemical sensors. Methods of graphene synthesis include mechanical exfoliation, epitaxial growth on SiC, CVD and colloidal suspensions. In this work the utilization of plasmas in synthesis process is considered. Types of carbonaceous structures produced by the anodic arc and regions of their synthesis were studied. Ultimate role of substrate temperature and transformations occurring with various carbonaceous structures generated in plasma discharge were considered. Formation of graphene film on copper substrate was detected at temperatures around the copper melting point. The film was consisted of several layers graphene flakes having typical sizes of about 200 nm. Time required for crystallization of graphene on externally heated substrates was determined. This work was supported by National Science Foundation (NSF Grant No. CBET-1249213).

Side-gate modulation effects on high-quality BN-Graphene-BN nanoribbon capacitors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Yang; Chen, Xiaolong; Ye, Weiguang

High-quality BN-Graphene-BN nanoribbon capacitors with double side-gates of graphene have been experimentally realized. The double side-gates can effectively modulate the electronic properties of graphene nanoribbon capacitors. By applying anti-symmetric side-gate voltages, we observed significant upward shifting and flattening of the V-shaped capacitance curve near the charge neutrality point. Symmetric side-gate voltages, however, only resulted in tilted upward shifting along the opposite direction of applied gate voltages. These modulation effects followed the behavior of graphene nanoribbons predicted theoretically for metallic side-gate modulation. The negative quantum capacitance phenomenon predicted by numerical simulations for graphene nanoribbons modulated by graphene side-gates was not observed,more » possibly due to the weakened interactions between the graphene nanoribbon and side-gate electrodes caused by the Ga{sup +} beam etching process.« less

The evolving quality of frictional contact with graphene.

PubMed

Li, Suzhi; Li, Qunyang; Carpick, Robert W; Gumbsch, Peter; Liu, Xin Z; Ding, Xiangdong; Sun, Jun; Li, Ju

2016-11-24

Graphite and other lamellar materials are used as dry lubricants for macroscale metallic sliding components and high-pressure contacts. It has been shown experimentally that monolayer graphene exhibits higher friction than multilayer graphene and graphite, and that this friction increases with continued sliding, but the mechanism behind this remains subject to debate. It has long been conjectured that the true contact area between two rough bodies controls interfacial friction. The true contact area, defined for example by the number of atoms within the range of interatomic forces, is difficult to visualize directly but characterizes the quantity of contact. However, there is emerging evidence that, for a given pair of materials, the quality of the contact can change, and that this can also strongly affect interfacial friction. Recently, it has been found that the frictional behaviour of two-dimensional materials exhibits traits unlike those of conventional bulk materials. This includes the abovementioned finding that for few-layer two-dimensional materials the static friction force gradually strengthens for a few initial atomic periods before reaching a constant value. Such transient behaviour, and the associated enhancement of steady-state friction, diminishes as the number of two-dimensional layers increases, and was observed only when the two-dimensional material was loosely adhering to a substrate. This layer-dependent transient phenomenon has not been captured by any simulations. Here, using atomistic simulations, we reproduce the experimental observations of layer-dependent friction and transient frictional strengthening on graphene. Atomic force analysis reveals that the evolution of static friction is a manifestation of the natural tendency for thinner and less-constrained graphene to re-adjust its configuration as a direct consequence of its greater flexibility. That is, the tip atoms become more strongly pinned, and show greater synchrony in their stick

Detachment of CVD-grown graphene from single crystalline Ni films by a pure gas phase reaction

NASA Astrophysics Data System (ADS)

Zeller, Patrick; Henß, Ann-Kathrin; Weinl, Michael; Diehl, Leo; Keefer, Daniel; Lippmann, Judith; Schulz, Anne; Kraus, Jürgen; Schreck, Matthias; Wintterlin, Joost

2016-11-01

Despite great previous efforts there is still a high need for a simple, clean, and upscalable method for detaching epitaxial graphene from the metal support on which it was grown. We present a method based on a pure gas phase reaction that is free of solvents and polymer supports and avoids mechanical transfer steps. The graphene was grown on 150 nm thick, single crystalline Ni(111) films on Si(111) wafers with YSZ buffer layers. Its quality was monitored by using low energy electron diffraction and scanning tunneling microscopy. The gas phase etching uses a chemical transport reaction, the so-called Mond process, based on the formation of gaseous nickel tetracarbonyl in 1 bar of CO at 75 °C and by adding small amounts of sulfide catalysts. X-ray photoelectron spectroscopy, Raman spectroscopy and scanning electron microscopy were used to characterize the detached graphene. It was found that the method successfully removes the nickel from underneath the graphene layer, so that the graphene lies on the insulating oxide buffer layer. Small residual particles of nickel sulfide and cracks in the obtained graphene layer were identified. The defect concentrations were comparable to graphene samples obtained by wet chemical etching and by the bubbling transfer.

High-quality infrared imaging with graphene photodetectors at room temperature.

PubMed

Guo, Nan; Hu, Weida; Jiang, Tao; Gong, Fan; Luo, Wenjin; Qiu, Weicheng; Wang, Peng; Liu, Lu; Wu, Shiwei; Liao, Lei; Chen, Xiaoshuang; Lu, Wei

2016-09-21

Graphene, a two-dimensional material, is expected to enable broad-spectrum and high-speed photodetection because of its gapless band structure, ultrafast carrier dynamics and high mobility. We demonstrate a multispectral active infrared imaging by using a graphene photodetector based on hybrid response mechanisms at room temperature. The high-quality images with optical resolutions of 418 nm, 657 nm and 877 nm and close-to-theoretical-limit Michelson contrasts of 0.997, 0.994, and 0.996 have been acquired for 565 nm, 1550 nm, and 1815 nm light imaging measurements by using an unbiased graphene photodetector, respectively. Importantly, by carefully analyzing the results of Raman mapping and numerical simulations for the response process, the formation of hybrid photocurrents in graphene detectors is attributed to the synergistic action of photovoltaic and photo-thermoelectric effects. The initial application to infrared imaging will help promote the development of high performance graphene-based infrared multispectral detectors.

Method for synthesis of high quality graphene

DOEpatents

Lanzara, Alessandra [Piedmont, CA; Schmid, Andreas K [Berkeley, CA; Yu, Xiaozhu [Berkeley, CA; Hwang, Choonkyu [Albany, CA; Kohl, Annemarie [Beneditkbeuern, DE; Jozwiak, Chris M [Oakland, CA

2012-03-27

A method is described herein for the providing of high quality graphene layers on silicon carbide wafers in a thermal process. With two wafers facing each other in close proximity, in a first vacuum heating stage, while maintained at a vacuum of around 10.sup.-6 Torr, the wafer temperature is raised to about 1500.degree. C., whereby silicon evaporates from the wafer leaving a carbon rich surface, the evaporated silicon trapped in the gap between the wafers, such that the higher vapor pressure of silicon above each of the wafers suppresses further silicon evaporation. As the temperature of the wafers is raised to about 1530.degree. C. or more, the carbon atoms self assemble themselves into graphene.

Electronic and mechanical properties of graphene-germanium interfaces grown by chemical vapor deposition

DOE PAGES

Kiraly, Brian T.; Jacobberger, Robert M.; Mannix, Andrew J.; ...

2015-10-27

Epitaxially oriented wafer-scale graphene grown directly on semiconducting Ge substrates is of high interest for both fundamental science and electronic device applications. To date, however, this material system remains relatively unexplored structurally and electronically, particularly at the atomic scale. To further understand the nature of the interface between graphene and Ge, we utilize ultrahigh vacuum scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) along with Raman and X-ray photoelectron spectroscopy to probe interfacial atomic structure and chemistry. STS reveals significant differences in electronic interactions between graphene and Ge(110)/Ge(111), which is consistent with a model of stronger interaction on Ge(110)more » leading to epitaxial growth. Raman spectra indicate that the graphene is considerably strained after growth, with more point-to-point variation on Ge(111). Furthermore, this native strain influences the atomic structure of the interface by inducing metastable and previously unobserved Ge surface reconstructions following annealing. These nonequilibrium reconstructions cover >90% of the surface and, in turn, modify both the electronic and mechanical properties of the graphene overlayer. Finally, graphene on Ge(001) represents the extreme strain case, where graphene drives the reorganization of the Ge surface into [107] facets. From this study, it is clear that the interaction between graphene and the underlying Ge is not only dependent on the substrate crystallographic orientation, but is also tunable and strongly related to the atomic reconfiguration of the graphene–Ge interface.« less

Selective growth of Pb islands on graphene/SiC buffer layers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, X. T.; Miao, Y. P.; Ma, D. Y.

2015-02-14

Graphene is fabricated by thermal decomposition of silicon carbide (SiC) and Pb islands are deposited by Pb flux in molecular beam epitaxy chamber. It is found that graphene domains and SiC buffer layer coexist. Selective growth of Pb islands on SiC buffer layer rather than on graphene domains is observed. It can be ascribed to the higher adsorption energy of Pb atoms on the 6√(3) reconstruction of SiC. However, once Pb islands nucleate on graphene domains, they will grow very large owing to the lower diffusion barrier of Pb atoms on graphene. The results are consistent with first-principle calculations. Sincemore » Pb atoms on graphene are nearly free-standing, Pb islands grow in even-number mode.« less

Helium ion microscopy of graphene: beam damage, image quality and edge contrast

NASA Astrophysics Data System (ADS)

Fox, D.; Zhou, Y. B.; O'Neill, A.; Kumar, S.; Wang, J. J.; Coleman, J. N.; Duesberg, G. S.; Donegan, J. F.; Zhang, H. Z.

2013-08-01

A study to analyse beam damage, image quality and edge contrast in the helium ion microscope (HIM) has been undertaken. The sample investigated was graphene. Raman spectroscopy was used to quantify the disorder that can be introduced into the graphene as a function of helium ion dose. The effects of the dose on both freestanding and supported graphene were compared. These doses were then correlated directly to image quality by imaging graphene flakes at high magnification. It was found that a high magnification image with a good signal to noise ratio will introduce very significant sample damage. A safe imaging dose of the order of 1013 He+ cm-2 was established, with both graphene samples becoming highly defective at doses over 5 × 1014 He+ cm-2. The edge contrast of a freestanding graphene flake imaged in the HIM was then compared with the contrast of the same flake observed in a scanning electron microscope and a transmission electron microscope. Very strong edge sensitivity was observed in the HIM. This enhanced edge sensitivity over the other techniques investigated makes the HIM a powerful nanoscale dimensional metrology tool, with the capability of both fabricating and imaging features with sub-nanometre resolution.

Growth of Graphene by Catalytic Dissociation of Ethylene on CuNi(111)

NASA Astrophysics Data System (ADS)

Tyagi, Parul; Mowll, Tyler; Robinson, Zachary; Ventrice, Carl

2013-03-01

Copper foil is one of the most common substrates for growing large area graphene films. The main reason for this is that Cu has a very low carbon solubility, which results in the self-termination of a single layer of graphene when grown using hydrocarbon precursors at low pressure. Our previous results on Cu(111) substrates has found that temperatures of at least 900 °C are needed to form single domain epitaxial films. By using a CuNi alloy, the catalytic activity of the substrate is expected to increase, which will allow the catalytic decomposition of the hydrocarbon precursor at lower temperatures. In this study, the growth of graphene by the catalytic decomposition of ethylene on a 90:10 CuNi(111) substrate was attempted. The growths were done in an ultra-high vacuum system by either heating the substrate to the growth temperature followed by introducing the ethylene precursor or by introducing the ethylene precursor and subsequently heating it to the growth temperature. The growth using the former method results in a two-domain epitaxial graphene overlayer. However, introducing the ethylene before heating the substrate resulted in considerable rotational disorder within the graphene film. This has been attributed to the deposition of carbon atoms on the surface at temperatures too low for the carbon to crystallize into graphene. This research was supported by the NSF (DMR-1006411).

Controlled fabrication of high-quality carbon nanoscrolls from monolayer graphene.

PubMed

Xie, Xu; Ju, Long; Feng, Xiaofeng; Sun, Yinghui; Zhou, Ruifeng; Liu, Kai; Fan, Shoushan; Li, Qunqing; Jiang, Kaili

2009-07-01

We report a simple and effective way of fabricating high-quality carbon nanoscrolls (CNSs), using isopropyl alcohol solution to roll up monolayer graphene predefined on SiO(2)/Si substrates. Transmission electron microscopy studies reveal that the CNS has a tube-like structure with a hollow core surrounded by graphene walls 0.35 nm apart. Raman spectroscopy studies show that the CNS is free of significant defects, and the electronic structure and phonon dispersion are slightly different from those of two-dimensional graphene. Finally, the CNS-based device is fabricated, directly on the SiO(2)/Si substrate. Electrical-transport measurements show that its resistance is weakly gate-dependent but strongly temperature-dependent. In addition, the CNS can sustain a high current density up to 5 x 10(7) A/cm(2), indicating that it is a good candidate for microcircuit interconnects. The controlled fabrication of high-quality CNSs may open up new opportunities for both fundamental and applied research of CNSs.

Epitaxial graphene/SiC Schottky ultraviolet photodiode with orders of magnitude adjustability in responsivity and response speed

NASA Astrophysics Data System (ADS)

Yang, Junwei; Guo, Liwei; Guo, Yunlong; Hu, Weijie; Zhang, Zesheng

2018-03-01

A simple optical-electronic device that possesses widescale adjustability in its performance is specially required for realizing multifunctional applications as in optical communication and weak signal detectors. Here, we demonstrate an epitaxial graphene (EG)/n-type SiC Schottky ultraviolet (UV) photodiode with extremely widescale adjustability in its responsivity and response speed. It is found that the response speed of the device can be modulated over seven orders of magnitude from tens of nanoseconds to milliseconds by changing its working bias from 0 to -5 V, while its responsivity can be varied by three orders of magnitude. A 2.18 A/W responsivity is observed at -5 V when a 325 nm laser is irradiated on, corresponding to an external quantum efficiency over 800% ascribed to the trap induced internal gain mechanism. These performances of the EG/SiC Schottky photodiode are far superior to those based on traditional metal/SiC and indicate that the EG/n-type SiC Schottky diode is a good candidate for application in UV photodetection.

Electronic structure of monolayer 1T'-MoTe2 grown by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Tang, Shujie; Zhang, Chaofan; Jia, Chunjing; Ryu, Hyejin; Hwang, Choongyu; Hashimoto, Makoto; Lu, Donghui; Liu, Zhi; Devereaux, Thomas P.; Shen, Zhi-Xun; Mo, Sung-Kwan

2018-02-01

Monolayer transition metal dichalcogenides (TMDCs) in the 1T' structural phase have drawn a great deal of attention due to the prediction of quantum spin Hall insulator states. The band inversion and the concomitant changes in the band topology induced by the structural distortion from 1T to 1T' phases are well established. However, the bandgap opening due to the strong spin-orbit coupling (SOC) is only verified for 1T'-WTe2 recently and still debated for other TMDCs. Here we report a successful growth of high-quality monolayer 1T'-MoTe2 on a bilayer graphene substrate through molecular beam epitaxy. Using in situ angle-resolved photoemission spectroscopy (ARPES), we have investigated the low-energy electronic structure and Fermi surface topology. The SOC-induced breaking of the band degeneracy points between the valence and conduction bands is clearly observed by ARPES. However, the strength of SOC is found to be insufficient to open a bandgap, which makes monolayer 1T'-MoTe2 on bilayer graphene a semimetal.

Electronic structure of monolayer 1T'-MoTe 2 grown by molecular beam epitaxy

DOE PAGES

Tang, Shujie; Zhang, Chaofan; Jia, Chunjing; ...

2017-11-14

Monolayer transition metal dichalcogenides (TMDCs) in the 1T' structural phase have drawn a great deal of attention due to the prediction of quantum spin Hall insulator states. The band inversion and the concomitant changes in the band topology induced by the structural distortion from 1T to 1T' phases are well established. However, the bandgap opening due to the strong spin-orbit coupling (SOC) is only verified for 1T'-WTe 2 recently and still debated for other TMDCs. Here we report a successful growth of high-quality monolayer 1T'-MoTe 2 on a bilayer graphene substrate through molecular beam epitaxy. Using in situ angle-resolved photoemissionmore » spectroscopy (ARPES), we have investigated the low-energy electronic structure and Fermi surface topology. The SOC-induced breaking of the band degeneracy points between the valence and conduction bands is clearly observed by ARPES. However, the strength of SOC is found to be insufficient to open a bandgap, which makes monolayer 1T'-MoTe 2 on bilayer graphene a semimetal.« less

Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene

DTIC Science & Technology

2015-11-13

at the monolayer MoS2 edges. KEYWORDS: transition metal dichalcogenide, silicon carbide , scanning tunneling microscopy, synchrotron X-ray scattering... silicon from SiC not only offers uniform large-area synthesis of graphene but also provides technological advantages over alternative methods such as...Röhrl, J.; et al. Towards Wafer-Size Graphene Layers by Atmospheric Pressure Graphitization of Silicon Carbide . Nat. Mater. 2009, 8, 203−207. (18) Çelebi

Efficient etching-free transfer of high quality, large-area CVD grown graphene onto polyvinyl alcohol films

NASA Astrophysics Data System (ADS)

Marta, Bogdan; Leordean, Cosmin; Istvan, Todor; Botiz, Ioan; Astilean, Simion

2016-02-01

Graphene transfer is a procedure of paramount importance for the production of graphene-based electronic devices. The transfer procedure can affect the electronic properties of the transferred graphene and can be detrimental for possible applications both due to procedure induced defects which can appear and due to scalability of the method. Hence, it is important to investigate new transfer methods for graphene that are less time consuming and show great promise. In the present study we propose an efficient, etching-free transfer method that consists in applying a thin polyvinyl alcohol layer on top of the CVD grown graphene on Cu and then peeling-off the graphene onto the polyvinyl alcohol film. We investigate the quality of the transferred graphene before and after the transfer, using Raman spectroscopy and imaging as well as optical and atomic force microscopy techniques. This simple transfer method is scalable and can lead to complete transfer of graphene onto flexible and transparent polymer support films without affecting the quality of the graphene during the transfer procedure.

Observation of the retarded transportation of a photogenerated hole on epitaxial graphene.

PubMed

Wang, Shujie; Yuan, Xizhi; Bi, Xiaofeng; Wang, Xiaomei; Huang, Qingsong

2015-10-07

Graphene is usually adopted as an assistant additive for catalysts in photocatalytic processes, because of its ability to accelerate the separation of photogenerated charge carriers. To elucidate the mechanism, hydrogen peroxide is adopted to convert the O2(-)˙ active species into OH˙ for degradation of an organic dye. If the pH value is less than 7, the concentration of the OH˙ species can be reduced more quickly with the addition of graphene than without, because negatively charged electrons can be transported quickly on graphene. If the pH value is larger than 7, the concentration of OH˙ can be promoted by the catalyst SiC with photogenerated h(+) release and reaction with OH(-), however the concentration is reduced if the SiC catalyst is covered by a graphene sheet, as it retards h(+) release from the SiC substrate. Our findings have provided a certification for the role of graphene in photo-catalytic processes.

Graphene plasmonic nanogratings for biomolecular sensing in liquid

NASA Astrophysics Data System (ADS)

Chorsi, Meysam T.; Chorsi, Hamid T.

2017-12-01

We design a surface plasmon resonance (SPR) molecular sensor based on graphene and biomolecule adsorption at graphene-liquid interfaces. The sensor configuration consists of two opposing arrays of graphene nanograting mounted on a substrate, with a liquid-phase sensing medium confined between them. We characterize the design in simulation on a variety of substrates by altering the refractive index of the sensing medium and varying the absorbance-transmittance characteristics. The influence of various parameters on the biosensor's performance, including the Fermi level of graphene, the dielectric constant of the substrate, and the incident angle for plasmon excitation, is investigated. Numerical simulations demonstrate the sensitivity higher than 3000 nm/RIU (refractive index unit). The device supports a wide range of substrates in which graphene can be epitaxially grown. The proposed biosensor works independent of the incident angle and can be tuned to cover a broadband wavelength range.

PREFACE: Ultrathin layers of graphene, h-BN and other honeycomb structures Ultrathin layers of graphene, h-BN and other honeycomb structures

NASA Astrophysics Data System (ADS)

Geber, Thomas; Oshima, Chuhei

2012-08-01

nanometer scale. This special section contains interesting papers on graphene, h-BN and related 'honeycomb' compounds on solid surfaces, which are currently in development. Interfacial interaction strongly modifies the electronic and atomic structures of these overlayer systems and substrate surfaces. In addition, one can recognize a variety of growth phenomena by changing the surface and growth conditions, which are promising as regards fabricating those noble nanosystems. We have great pleasure in acknowledging the enthusiastic response and participation of our invited authors and their diligent preparation of the manuscripts. Ultrathin layers of graphene, h-BN and other honeycomb structures contents Ultrathin layers of graphene, h-BN and other honeycomb structuresThomas Geber and Chuhei Oshima Templating of arrays of Ru nanoclusters by monolayer graphene/Ru Moirés with different periodicitiesEli Sutter, Bin Wang, Peter Albrecht, Jayeeta Lahiri, Marie-Laure Bocquet and Peter Sutter Controllable p-doping of graphene on Ir(111) by chlorination with FeCl3N A Vinogradov, K A Simonov, A V Generalov, A S Vinogradov, D V Vyalikh, C Laubschat, N Mårtensson and A B Preobrajenski Optimizing long-range order, band gap, and group velocities for graphene on close-packed metal surfacesF D Natterer, S Rusponi, M Papagno, C Carbone and H Brune Epitaxial growth of graphene on transition metal surfaces: chemical vapor deposition versus liquid phase depositionSamuel Grandthyll, Stefan Gsell, Michael Weinl, Matthias Schreck, Stefan Hüfner and Frank Müller High-yield boron nitride nanosheets from 'chemical blowing': towards practical applications in polymer compositesXuebin Wang, Amir Pakdel, Chunyi Zhi, Kentaro Watanabe, Takashi Sekiguchi, Dmitri Golberg and Yoshio Bando BCx layers with honeycomb lattices on an NbB2(0001) surfaceChuhei Oshima Epitaxial growth of boron-doped graphene by thermal decomposition of B4CWataru Norimatsu, Koichiro Hirata, Yuta Yamamoto, Shigeo Arai and Michiko

Vibrational Excitations and Low Energy Electronic Structure of Epoxide-decorated Graphene

PubMed Central

Mattson, E.C.; Johns, J.E.; Pande, K.; Bosch, R.A.; Cui, S.; Gajdardziska-Josifovska, M.; Weinert, M.; Chen, J.H.; Hersam, M.C.; Hirschmugl, C.J.

2014-01-01

We report infrared studies of adsorbed atomic oxygen (epoxide functional groups) on graphene. Two different systems are used as a platform to explore these interactions, namely, epitaxial graphene/SiC(0001) functionalized with atomic oxygen (graphene epoxide, GE) and chemically reduced graphene oxide (RGO). In the case of the model GE system, IR reflectivity measurements show that epoxide groups distort the graphene π bands around the K-point, imparting a finite effective mass and contributing to a band gap. In the case of RGO, epoxide groups are found to be present following the reduction treatment by a combination of polarized IR reflectance and transmittance measurements. Similar to the GE system, a band gap in the RGO sample is observed as well. PMID:24563725

Vibrational Excitations and Low Energy Electronic Structure of Epoxide-decorated Graphene.

PubMed

Mattson, E C; Johns, J E; Pande, K; Bosch, R A; Cui, S; Gajdardziska-Josifovska, M; Weinert, M; Chen, J H; Hersam, M C; Hirschmugl, C J

2014-01-02

We report infrared studies of adsorbed atomic oxygen (epoxide functional groups) on graphene. Two different systems are used as a platform to explore these interactions, namely, epitaxial graphene/SiC(0001) functionalized with atomic oxygen (graphene epoxide, GE) and chemically reduced graphene oxide (RGO). In the case of the model GE system, IR reflectivity measurements show that epoxide groups distort the graphene π bands around the K-point, imparting a finite effective mass and contributing to a band gap. In the case of RGO, epoxide groups are found to be present following the reduction treatment by a combination of polarized IR reflectance and transmittance measurements. Similar to the GE system, a band gap in the RGO sample is observed as well.

Epitaxial Deposition Of Germanium Doped With Gallium

NASA Technical Reports Server (NTRS)

Huffman, James E.

1994-01-01

Epitaxial layers of germanium doped with gallium made by chemical vapor deposition. Method involves combination of techniques and materials used in chemical vapor deposition with GeH4 or GeCl4 as source of germanium and GaCl3 as source of gallium. Resulting epitaxial layers of germanium doped with gallium expected to be highly pure, with high crystalline quality. High-quality material useful in infrared sensors.

Approach to Multifunctional Device Platform with Epitaxial Graphene on Transition Metal Oxide (Postprint)

DTIC Science & Technology

2015-09-23

with a metal oxide ( TiO2 ). Our novel direct synthesis of graphene/ TiO2 heterostructure is achieved by C60 deposition on transition Ti metal surface...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

Near-equilibrium chemical vapor deposition of high-quality single-crystal graphene directly on various dielectric substrates.

PubMed

Chen, Jianyi; Guo, Yunlong; Jiang, Lili; Xu, Zhiping; Huang, Liping; Xue, Yunzhou; Geng, Dechao; Wu, Bin; Hu, Wenping; Yu, Gui; Liu, Yunqi

2014-03-05

By using near-equilibrium chemical vapor deposition, it is demonstrated that high-quality single-crystal graphene can be grown on dielectric substrates. The maximum size is about 11 μm. The carrier mobility can reach about 5650 cm(2) V(-1) s(-1) , which is comparable to those of some metal-catalyzed graphene crystals, reflecting the good quality of the graphene lattice. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Water on graphene: review of recent progress

NASA Astrophysics Data System (ADS)

Melios, C.; Giusca, C. E.; Panchal, V.; Kazakova, O.

2018-04-01

The sensitivity of graphene to the surrounding environment is given by its π electrons, which are directly exposed to molecules in the ambient air. The high sensitivity of graphene to the local environment has shown to be both advantageous and problematic for graphene-based devices, such as transistors and sensors, where the graphene carrier concentration and mobility changes due to ambient humidity variations. In this review, recent progress is presented in understanding the effects of water on different types of graphene: epitaxially grown and quasi-free standing on SiC(0 0 0 1), grown by chemical vapour deposition and transfered on SiO2, and exfoliated flakes. It is demonstrated that water withdraws electrons from graphene, but the graphene-water interaction highly depends on the thickness, layer stacking, underlying substrate and substrate-induced doping. Moreover, we highlight the importance of clear and unambiguous description of the environmental conditions (i.e. relative humidity) whenever a routine characterisation for carrier concentration and mobility is reported (often presented as a simple figure-of-merit), as these electrical characteristics are highly dependent on the adsorbed molecules and the surrounding environment.

Controllable poly-crystalline bilayered and multilayered graphene film growth by reciprocal chemical vapor deposition

NASA Astrophysics Data System (ADS)

Wu, Qinke; Jung, Seong Jun; Jang, Sung Kyu; Lee, Joohyun; Jeon, Insu; Suh, Hwansoo; Kim, Yong Ho; Lee, Young Hee; Lee, Sungjoo; Song, Young Jae

2015-06-01

We report the selective growth of large-area bilayered graphene film and multilayered graphene film on copper. This growth was achieved by introducing a reciprocal chemical vapor deposition (CVD) process that took advantage of an intermediate h-BN layer as a sacrificial template for graphene growth. A thin h-BN film, initially grown on the copper substrate using CVD methods, was locally etched away during the subsequent graphene growth under residual H2 and CH4 gas flows. Etching of the h-BN layer formed a channel that permitted the growth of additional graphene adlayers below the existing graphene layer. Bilayered graphene typically covers an entire Cu foil with domain sizes of 10-50 μm, whereas multilayered graphene can be epitaxially grown to form islands a few hundreds of microns in size. This new mechanism, in which graphene growth proceeded simultaneously with h-BN etching, suggests a potential approach to control graphene layers for engineering the band structures of large-area graphene for electronic device applications.We report the selective growth of large-area bilayered graphene film and multilayered graphene film on copper. This growth was achieved by introducing a reciprocal chemical vapor deposition (CVD) process that took advantage of an intermediate h-BN layer as a sacrificial template for graphene growth. A thin h-BN film, initially grown on the copper substrate using CVD methods, was locally etched away during the subsequent graphene growth under residual H2 and CH4 gas flows. Etching of the h-BN layer formed a channel that permitted the growth of additional graphene adlayers below the existing graphene layer. Bilayered graphene typically covers an entire Cu foil with domain sizes of 10-50 μm, whereas multilayered graphene can be epitaxially grown to form islands a few hundreds of microns in size. This new mechanism, in which graphene growth proceeded simultaneously with h-BN etching, suggests a potential approach to control graphene layers for

Robust spin-valley polarization in commensurate Mo S2 /graphene heterostructures

NASA Astrophysics Data System (ADS)

Du, Luojun; Zhang, Qian; Gong, Benchao; Liao, Mengzhou; Zhu, Jianqi; Yu, Hua; He, Rui; Liu, Kai; Yang, Rong; Shi, Dongxia; Gu, Lin; Yan, Feng; Zhang, Guangyu; Zhang, Qingming

2018-03-01

The investigation and control of quantum degrees of freedom (DoFs) of carriers lie at the heart of condensed-matter physics and next-generation electronics/optoelectronics. van der Waals heterostructures stacked from distinct two-dimensional (2D) crystals offer an unprecedented platform for combining the superior properties of individual 2D materials and manipulating spin, layer, and valley DoFs. Mo S2 /graphene heterostructures, harboring prominent spin-transport properties of graphene, giant spin-orbit coupling, and spin-valley polarization of Mo S2 , are predicted as a perfect venue for optospintronics. Here, we report the epitaxial growth of commensurate Mo S2 on graphene with high quality by chemical vapor deposition, and demonstrate robust temperature-independent spin-valley polarization at off-resonant excitation. We further show that the helicity of B exciton is larger than that of A exciton, allowing the manipulation of spin bits in the commensurate heterostructures by both optical helicity and wavelength. Our results open a window for controlling spin DoF by light and pave a way for taking spin qubits as information carriers in the next-generation valley-controlled optospintronics.

STM/STS studies of Ca-intercalated bilayer graphene

NASA Astrophysics Data System (ADS)

Shimizu, Ryota; Sugawara, Katsuaki; Kanetani, Kohei; Iwaya, Katsuya; Sato, Takafumi; Takahashi, Takashi; Hitosugi, Taro

2013-03-01

We have performed low temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements on a two-dimensional Ca-intercalated bilayer graphene epitaxially grown on a 6H-SiC(0001) substrate. The STM topographic images clearly resolve each intercalated Ca atom with graphene-based honeycomb lattice. Furthermore, we found a clear ×2.5 modulation in the topography, implying charge density wave or Moiré pattern originated from the interaction with the SiC substrate. Comparison with ARPES measurements provided us of further insight into the Fermi surface deduced from STS.

Improved quality of graphene in the absence of hydrogen in a low-temperature growth process using an alcohol precursor

NASA Astrophysics Data System (ADS)

Choi, Kyeonggon; Lee, Kiyeol; Jeong, Jaehoon; Ye, Jongpil

2017-03-01

We present the results of low-temperature growth of graphene on polycrystalline copper foil surfaces at 800 °C by using low-pressure chemical-vapor deposition of alcohol precursors. The structural quality of the graphene sample was found to depend significantly on the ambient conditions during the annealing and the growth processes. The improved quality of graphene grown in an oxidizing environment was found to be associated with a lower nucleation density, suggesting that chemisorbed oxygen atoms play a critical role in determining the quality of graphene.

Thermally activated hysteresis in high quality graphene/h-BN devices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cadore, A. R., E-mail: alissoncadore@gmail.com, E-mail: lccampos@fisica.ufmg.br; Mania, E.; Lacerda, R. G.

2016-06-06

We report on gate hysteresis of resistance in high quality graphene/hexagonal boron nitride (h-BN) devices. We observe a thermally activated hysteretic behavior in resistance as a function of the applied gate voltage at temperatures above 375 K. In order to investigate the origin of the hysteretic phenomenon, we compare graphene/h-BN heterostructure devices with SiO{sub 2}/Si back gate electrodes to devices with graphite back gate electrodes. The gate hysteretic behavior of the resistance is present only in devices with an h-BN/SiO{sub 2} interface and is dependent on the orientation of the applied gate electric field and sweep rate. We describe a phenomenologicalmore » model which captures all of our findings based on charges trapped at the h-BN/SiO{sub 2} interface. Such hysteretic behavior in graphene resistance must be considered in high temperature applications for graphene devices and may open new routes for applications in digital electronics and memory devices.« less

High quality factor whispering gallery modes from self-assembled hexagonal GaN rods grown by metal-organic vapor phase epitaxy.

PubMed

Tessarek, C; Sarau, G; Kiometzis, M; Christiansen, S

2013-02-11

Self-assembled GaN rods were grown on sapphire by metal-organic vapor phase epitaxy using a simple two-step method that relies first on a nitridation step followed by GaN epitaxy. The mask-free rods formed without any additional catalyst. Most of the vertically aligned rods exhibit a regular hexagonal shape with sharp edges and smooth sidewall facets. Cathodo- and microphotoluminescence investigations were carried out on single GaN rods. Whispering gallery modes with quality factors greater than 4000 were measured demonstrating the high morphological and optical quality of the self-assembled GaN rods.

Epitaxial growth of single-orientation high-quality MoS2 monolayers

NASA Astrophysics Data System (ADS)

Bana, Harsh; Travaglia, Elisabetta; Bignardi, Luca; Lacovig, Paolo; Sanders, Charlotte E.; Dendzik, Maciej; Michiardi, Matteo; Bianchi, Marco; Lizzit, Daniel; Presel, Francesco; De Angelis, Dario; Apostol, Nicoleta; Das, Pranab Kumar; Fujii, Jun; Vobornik, Ivana; Larciprete, Rosanna; Baraldi, Alessandro; Hofmann, Philip; Lizzit, Silvano

2018-07-01

We present a study on the growth and characterization of high-quality single-layer MoS2 with a single orientation, i.e. without the presence of mirror domains. This single orientation of the MoS2 layer is established by means of x-ray photoelectron diffraction. The high quality is evidenced by combining scanning tunneling microscopy with x-ray photoelectron spectroscopy measurements. Spin- and angle-resolved photoemission experiments performed on the sample revealed complete spin-polarization of the valence band states near the K and -K points of the Brillouin zone. These findings open up the possibility to exploit the spin and valley degrees of freedom for encoding and processing information in devices that are based on epitaxially grown materials.

Fast Batch Production of High-Quality Graphene Films in a Sealed Thermal Molecular Movement System.

PubMed

Xu, Jianbao; Hu, Junxiong; Li, Qi; Wang, Rubing; Li, Weiwei; Guo, Yufen; Zhu, Yongbo; Liu, Fengkui; Ullah, Zaka; Dong, Guocai; Zeng, Zhongming; Liu, Liwei

2017-07-01

Chemical vapor deposition (CVD) growth of high-quality graphene has emerged as the most promising technique in terms of its integrated manufacturing. However, there lacks a controllable growth method for producing high-quality and a large-quantity graphene films, simultaneously, at a fast growth rate, regardless of roll-to-roll (R2R) or batch-to-batch (B2B) methods. Here, a stationary-atmospheric-pressure CVD (SAPCVD) system based on thermal molecular movement, which enables fast B2B growth of continuous and uniform graphene films on tens of stacked Cu(111) foils, with a growth rate of 1.5 µm s -1 , is demonstrated. The monolayer graphene of batch production is found to nucleate from arrays of well-aligned domains, and the films possess few defects and exhibit high carrier mobility up to 6944 cm 2 V -1 s -1 at room temperature. The results indicate that the SAPCVD system combined with single-domain Cu(111) substrates makes it possible to realize fast batch-growth of high-quality graphene films, which opens up enormous opportunities to use this unique 2D material for industrial device applications. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Transfer of Graphene Layers Grown on SiC Wafers to Other Substrates and Their Integration into Field Effect Transistors

NASA Astrophysics Data System (ADS)

Unarunotai, Sakulsuk; Murata, Yuya; Chialvo, Cesar; Kim, Hoon-Sik; MacLaren, Scott; Mason, Nadya; Petrov, Ivan; Rogers, John

2010-03-01

An approach to produce graphene films by epitaxial growth on silicon carbide substrate is promising, but its current implementation requires the use of SiC as the device substrate. We present a simple method for transferring epitaxial sheets of graphene on SiC to other substrates. The graphene was grown on the (0001) face of 6H-SiC by thermal annealing in a hydrogen atmosphere. Transfer was accomplished using a peeling process with a bilayer film of Gold/polyimide, to yield graphene with square millimeters of coverage on the target substrate. Back gated field-effect transistors fabricated on oxidized silicon substrates with Cr/Au as source-drain electrodes exhibited ambipolar characteristics with hole mobilities of ˜100 cm^2/V-s, and negligible influence of resistance at the contacts. This work was supported by the U.S. DOE, under Award No. DE-FG02-07ER46471, through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign.

High-Quality Reduced Graphene Oxide by a Dual-Function Chemical Reduction and Healing Process

PubMed Central

Some, Surajit; Kim, Youngmin; Yoon, Yeoheung; Yoo, HeeJoun; Lee, Saemi; Park, Younghun; Lee, Hyoyoung

2013-01-01

A new chemical dual-functional reducing agent, thiophene, was used to produce high-quality reduced graphene oxide (rGO) as a result of a chemical reduction of graphene oxide (GO) and the healing of rGO. Thiophene reduced GO by donation of electrons with acceptance of oxygen while it was converted into an intermediate oxidised polymerised thiophene that was eventually transformed into polyhydrocarbon by loss of sulphur atoms. Surprisingly, the polyhydrocarbon template helped to produce good-quality rGOC (chemically reduced) and high-quality rGOCT after thermal treatment. The resulting rGOCT nanosheets did not contain any nitrogen or sulphur impurities, were highly deoxygenated and showed a healing effect. Thus the electrical properties of the as-prepared rGOCT were superior to those of conventional hydrazine-produced rGO that require harsh reaction conditions. Our novel dual reduction and healing method with thiophene could potentially save energy and facilitate the commercial mass production of high-quality graphene. PMID:23722643

Magnetoresistance of vertical Co-graphene-NiFe junctions controlled by charge transfer and proximity-induced spin splitting in graphene

NASA Astrophysics Data System (ADS)

Asshoff, P. U.; Sambricio, J. L.; Rooney, A. P.; Slizovskiy, S.; Mishchenko, A.; Rakowski, A. M.; Hill, E. W.; Geim, A. K.; Haigh, S. J.; Fal'ko, V. I.; Vera-Marun, I. J.; Grigorieva, I. V.

2017-09-01

Graphene is hailed as an ideal material for spintronics due to weak intrinsic spin-orbit interaction that facilitates lateral spin transport and tunability of its electronic properties, including a possibility to induce magnetism in graphene. Another promising application of graphene is related to its use as a spacer separating ferromagnetic metals (FMs) in vertical magnetoresistive devices, the most prominent class of spintronic devices widely used as magnetic sensors. In particular, few-layer graphene was predicted to act as a perfect spin filter. Here we show that the role of graphene in such devices (at least in the absence of epitaxial alignment between graphene and the FMs) is different and determined by proximity-induced spin splitting and charge transfer with adjacent ferromagnetic metals, making graphene a weak FM electrode rather than a spin filter. To this end, we report observations of magnetoresistance (MR) in vertical Co-graphene-NiFe junctions with 1-4 graphene layers separating the ferromagnets, and demonstrate that the dependence of the MR sign on the number of layers and its inversion at relatively small bias voltages is consistent with spin transport between weakly doped and differently spin-polarized layers of graphene. The proposed interpretation is supported by the observation of an MR sign reversal in biased Co-graphene-hBN-NiFe devices and by comprehensive structural characterization. Our results suggest a new architecture for vertical devices with electrically controlled MR.

Imaging chiral symmetry breaking from Kekule bond order in graphene

DOE PAGES

Gutiérrez, Christopher; Kim, Cheol -Joo; Brown, Lola; ...

2016-05-23

Chirality—or ‘handedness’—is a symmetry property crucial to fields as diverse as biology, chemistry and high-energy physics. In graphene, chiral symmetry emerges naturally as a consequence of the carbon honeycomb lattice. This symmetry can be broken by interactions that couple electrons with opposite momenta in graphene. Here we directly visualize the formation of Kekulé bond order, one such phase of broken chiral symmetry, in an ultraflat graphene sheet grown epitaxially on a copper substrate. We show that its origin lies in the interactions between individual vacancies in the copper substrate that are mediated electronically by the graphene. We show that thismore » interaction causes the bonds in graphene to distort, creating a phase with broken chiral symmetry. Furthermore, the Kekulé ordering is robust at ambient temperature and atmospheric conditions, indicating that intercalated atoms may be harnessed to drive graphene and other two-dimensional materials towards electronically desirable and exotic collective phases.« less

High-quality nonpolar a-plane GaN epitaxial films grown on r-plane sapphire substrates by the combination of pulsed laser deposition and metal–organic chemical vapor deposition

NASA Astrophysics Data System (ADS)

Yang, Weijia; Zhang, Zichen; Wang, Wenliang; Zheng, Yulin; Wang, Haiyan; Li, Guoqiang

2018-05-01

High-quality a-plane GaN epitaxial films have been grown on r-plane sapphire substrates by the combination of pulsed laser deposition (PLD) and metal–organic chemical vapor deposition (MOCVD). PLD is employed to epitaxial growth of a-plane GaN templates on r-plane sapphire substrates, and then MOCVD is used. The nonpolar a-plane GaN epitaxial films with relatively small thickness (2.9 µm) show high quality, with the full-width at half-maximum values of GaN(11\\bar{2}0) along [1\\bar{1}00] direction and GaN(10\\bar{1}1) of 0.11 and 0.30°, and a root-mean-square surface roughness of 1.7 nm. This result is equivalent to the quality of the films grown by MOCVD with a thickness of 10 µm. This work provides a new and effective approach for achieving high-quality nonpolar a-plane GaN epitaxial films on r-plane sapphire substrates.

Precise, Self-Limited Epitaxy of Ultrathin Organic Semiconductors and Heterojunctions Tailored by van der Waals Interactions.

PubMed

Wu, Bing; Zhao, Yinghe; Nan, Haiyan; Yang, Ziyi; Zhang, Yuhan; Zhao, Huijuan; He, Daowei; Jiang, Zonglin; Liu, Xiaolong; Li, Yun; Shi, Yi; Ni, Zhenhua; Wang, Jinlan; Xu, Jian-Bin; Wang, Xinran

2016-06-08

Precise assembly of semiconductor heterojunctions is the key to realize many optoelectronic devices. By exploiting the strong and tunable van der Waals (vdW) forces between graphene and organic small molecules, we demonstrate layer-by-layer epitaxy of ultrathin organic semiconductors and heterostructures with unprecedented precision with well-defined number of layers and self-limited characteristics. We further demonstrate organic p-n heterojunctions with molecularly flat interface, which exhibit excellent rectifying behavior and photovoltaic responses. The self-limited organic molecular beam epitaxy (SLOMBE) is generically applicable for many layered small-molecule semiconductors and may lead to advanced organic optoelectronic devices beyond bulk heterojunctions.

Epitaxial CoSi2 on MOS devices

DOEpatents

Lim, Chong Wee; Shin, Chan Soo; Petrov, Ivan Georgiev; Greene, Joseph E.

2005-01-25

An Si.sub.x N.sub.y or SiO.sub.x N.sub.y liner is formed on a MOS device. Cobalt is then deposited and reacts to form an epitaxial CoSi.sub.2 layer underneath the liner. The CoSi.sub.2 layer may be formed through a solid phase epitaxy or reactive deposition epitaxy salicide process. In addition to high quality epitaxial CoSi.sub.2 layers, the liner formed during the invention can protect device portions during etching processes used to form device contacts. The liner can act as an etch stop layer to prevent excessive removal of the shallow trench isolation, and protect against excessive loss of the CoSi.sub.2 layer.

Epitaxial thin films

DOEpatents

Hunt, Andrew Tye; Deshpande, Girish; Lin, Wen-Yi; Jan, Tzyy-Jiuan

2006-04-25

Epitatial thin films for use as buffer layers for high temperature superconductors, electrolytes in solid oxide fuel cells (SOFC), gas separation membranes or dielectric material in electronic devices, are disclosed. By using CCVD, CACVD or any other suitable deposition process, epitaxial films having pore-free, ideal grain boundaries, and dense structure can be formed. Several different types of materials are disclosed for use as buffer layers in high temperature superconductors. In addition, the use of epitaxial thin films for electrolytes and electrode formation in SOFCs results in densification for pore-free and ideal gain boundary/interface microstructure. Gas separation membranes for the production of oxygen and hydrogen are also disclosed. These semipermeable membranes are formed by high-quality, dense, gas-tight, pinhole free sub-micro scale layers of mixed-conducting oxides on porous ceramic substrates. Epitaxial thin films as dielectric material in capacitors are also taught herein. Capacitors are utilized according to their capacitance values which are dependent on their physical structure and dielectric permittivity. The epitaxial thin films of the current invention form low-loss dielectric layers with extremely high permittivity. This high permittivity allows for the formation of capacitors that can have their capacitance adjusted by applying a DC bias between their electrodes.

Large-scale synthesis of high-quality hexagonal boron nitride nanosheets for large-area graphene electronics.

PubMed

Lee, Kang Hyuck; Shin, Hyeon-Jin; Lee, Jinyeong; Lee, In-yeal; Kim, Gil-Ho; Choi, Jae-Young; Kim, Sang-Woo

2012-02-08

Hexagonal boron nitride (h-BN) has received a great deal of attention as a substrate material for high-performance graphene electronics because it has an atomically smooth surface, lattice constant similar to that of graphene, large optical phonon modes, and a large electrical band gap. Herein, we report the large-scale synthesis of high-quality h-BN nanosheets in a chemical vapor deposition (CVD) process by controlling the surface morphologies of the copper (Cu) catalysts. It was found that morphology control of the Cu foil is much critical for the formation of the pure h-BN nanosheets as well as the improvement of their crystallinity. For the first time, we demonstrate the performance enhancement of CVD-based graphene devices with large-scale h-BN nanosheets. The mobility of the graphene device on the h-BN nanosheets was increased 3 times compared to that without the h-BN nanosheets. The on-off ratio of the drain current is 2 times higher than that of the graphene device without h-BN. This work suggests that high-quality h-BN nanosheets based on CVD are very promising for high-performance large-area graphene electronics. © 2012 American Chemical Society

Origin of the mosaicity in graphene grown on Cu(111)

NASA Astrophysics Data System (ADS)

Nie, Shu; Wofford, Joseph M.; Bartelt, Norman C.; Dubon, Oscar D.; McCarty, Kevin F.

2011-10-01

We use low-energy electron microscopy to investigate how graphene grows on Cu(111). Graphene islands first nucleate at substrate defects such as step bunches and impurities. A considerable fraction of these islands can be rotationally misaligned with the substrate, generating grain boundaries upon interisland impingement. New rotational boundaries are also generated as graphene grows across substrate step bunches. Thus, rougher substrates lead to higher degrees of mosaicity than do flatter substrates. Increasing the growth temperature improves crystallographic alignment. We demonstrate that graphene growth on Cu(111) is surface diffusion limited by comparing simulations of the time evolution of island shapes with experiments. Islands are dendritic with distinct lobes, but unlike the polycrystalline, four-lobed islands observed on (100)-textured Cu foils, each island can be a single crystal. Thus, epitaxial graphene on smooth, clean Cu(111) has fewer structural defects than it does on Cu(100).

Large energy pulse generation modulated by graphene epitaxially grown on silicon carbide.

PubMed

Yu, Haohai; Chen, Xiufang; Zhang, Huaijin; Xu, Xiangang; Hu, Xiaobo; Wang, Zhengping; Wang, Jiyang; Zhuang, Shidong; Jiang, Minhua

2010-12-28

Graphene grown by thermal decomposition of a two-inch 6H silicon carbide (SiC) wafers surface was used to modulate a large energy pulse laser. Because of its saturable absorbing properties, graphene was used as a passive Q-switcher, and because of its high refractive index the SiC substrate was used as an output coupler. Together they formed a setup where the passively Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal laser was realized with the pulse energy of 159.2 nJ. Our results illustrate the feasibility of using graphene as an inexpensive Q-switcher for solid-state lasers and its promising applications in integrated optics.

Exceptionally large migration length of carbon and topographically-facilitated self-limiting molecular beam epitaxial growth of graphene on hexagonal boron nitride

DOE Office of Scientific and Technical Information (OSTI.GOV)

Plaut, Annette S.; Wurstbauer, Ulrich; Wang, Sheng

We demonstrate growth of single-layer graphene (SLG) on hexagonal boron nitride (h-BN) by molecular beam epitaxy (MBE), only limited in area by the finite size of the h-BN flakes. Using atomic force microscopy and micro-Raman spectroscopy, we show that for growth over a wide range of temperatures (500 °C – 1000 °C) the deposited carbon atoms spill off the edge of the h-BN flakes. We attribute this spillage to the very high mobility of the carbon atoms on the BN basal plane, consistent with van der Waals MBE. The h-BN flakes vary in size from 30 μm to 100 μm,more » thus demonstrating that the migration length of carbon atoms on h-BN is greater than 100 μm. When sufficient carbon is supplied to compensate for this loss, which is largely due to this fast migration of the carbon atoms to and off the edges of the h-BN flake, we find that the best growth temperature for MBE SLG on h-BN is ~950 °C. Self-limiting graphene growth appears to be facilitated by topographic h-BN surface features: We have thereby grown MBE self-limited SLG on an h-BN ridge. This opens up future avenues for precisely tailored fabrication of nano- and hetero-structures on pre-patterned h-BN surfaces for device applications.« less

Epitaxy of semiconductor-superconductor nanowires

NASA Astrophysics Data System (ADS)

Krogstrup, P.; Ziino, N. L. B.; Chang, W.; Albrecht, S. M.; Madsen, M. H.; Johnson, E.; Nygård, J.; Marcus, C. M.; Jespersen, T. S.

2015-04-01

Controlling the properties of semiconductor/metal interfaces is a powerful method for designing functionality and improving the performance of electrical devices. Recently semiconductor/superconductor hybrids have appeared as an important example where the atomic scale uniformity of the interface plays a key role in determining the quality of the induced superconducting gap. Here we present epitaxial growth of semiconductor-metal core-shell nanowires by molecular beam epitaxy, a method that provides a conceptually new route to controlled electrical contacting of nanostructures and the design of devices for specialized applications such as topological and gate-controlled superconducting electronics. Our materials of choice, InAs/Al grown with epitaxially matched single-plane interfaces, and alternative semiconductor/metal combinations allowing epitaxial interface matching in nanowires are discussed. We formulate the grain growth kinetics of the metal phase in general terms of continuum parameters and bicrystal symmetries. The method realizes the ultimate limit of uniform interfaces and seems to solve the soft-gap problem in superconducting hybrid structures.

Spontaneous doping on high quality talc-graphene-hBN van der Waals heterostructures

NASA Astrophysics Data System (ADS)

Mania, E.; Alencar, A. B.; Cadore, A. R.; Carvalho, B. R.; Watanabe, K.; Taniguchi, T.; Neves, B. R. A.; Chacham, H.; Campos, L. C.

2017-09-01

Steady doping, added to its remarkable electronic properties, would make graphene a valuable commodity in the solar cell market, as energy power conversion could be substantially increased. Here we report a graphene van der Waals heterostructure which is able to spontaneously dope graphene (p-type) up to n ~ 2.2  ×  1013 cm-2 while providing excellent charge mobility (μ ~ 25 000 cm2 V-1 s-1). Such properties are achieved via deposition of graphene on atomically flat layered talc, a natural and abundant dielectric crystal. Raman investigation shows a preferential charge accumulation on graphene-talc van der Waals heterostructures, which are investigated through the electronic properties of talc/graphene/hBN heterostructure devices. These heterostructures preserve graphene’s good electronic quality, verified by the observation of quantum Hall effect at low magnetic fields (B  =  0.4 T) at T  =  4.2 K. In order to investigate the physical mechanisms behind graphene-on-talc p-type doping, we performed first-principles calculations of their interface structural and electronic properties. In addition to potentially improving solar cell efficiency, graphene doping via van der Waals stacking is also a promising route towards controlling the band gap opening in bilayer graphene, promoting a steady n or p type doping in graphene and, eventually, providing a new path to access superconducting states in graphene, predicted to exist only at very high doping.

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xia, Chao, E-mail: chaxi@ifm.liu.se; Johansson, Leif I.; Hultman, Lars

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, andmore » 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.« less

Tailoring the graphene/silicon carbide interface for monolithic wafer-scale electronics.

PubMed

Hertel, S; Waldmann, D; Jobst, J; Albert, A; Albrecht, M; Reshanov, S; Schöner, A; Krieger, M; Weber, H B

2012-07-17

Graphene is an outstanding electronic material, predicted to have a role in post-silicon electronics. However, owing to the absence of an electronic bandgap, graphene switching devices with high on/off ratio are still lacking. Here in the search for a comprehensive concept for wafer-scale graphene electronics, we present a monolithic transistor that uses the entire material system epitaxial graphene on silicon carbide (0001). This system consists of the graphene layer with its vanishing energy gap, the underlying semiconductor and their common interface. The graphene/semiconductor interfaces are tailor-made for ohmic as well as for Schottky contacts side-by-side on the same chip. We demonstrate normally on and normally off operation of a single transistor with on/off ratios exceeding 10(4) and no damping at megahertz frequencies. In its simplest realization, the fabrication process requires only one lithography step to build transistors, diodes, resistors and eventually integrated circuits without the need of metallic interconnects.

Directed Assembly of Molecules on Graphene/Ru(0001)

NASA Astrophysics Data System (ADS)

Zhang, L. Z.; Zhang, H. G.; Sun, J. T.; Pan, Y.; Liu, Q.; Mao, J. H.; Zhou, H. T.; Low, T.; Guo, H. M.; Du, S. X.; Gao, H.-J.

2012-02-01

Recently, the graphene monolayers have been seen to adopt a superstructure - moir'e pattern - on Ru(0001). By using low temperature scanning tunneling spectroscopy, we identified the laterally localized electronic states on this system. The individual states are separated by 3 nm and comprise regions of about 90 carbon atoms. This constitutes a highly regular quantum dot-array with molecular precision. It is evidenced by quantum well resonances with energies that relate to the corrugation of the graphene layer. By using scanning tunneling microscopy/spectroscopy, we demonstrate the selective adsorption and formation of ordered molecular arrays of FePc and pentacene molecules on the graphene/Ru(0001) templates. With in-depth investigations of the molecular adsorption and assembly processes we reveal the existence lateral electric dipoles in the epitaxial graphene monolayers and the capability of the dipoles in directing and driving the molecular adsorption and assembly. When increasing the molecular coverage, we observed the formation of regular Kagome lattices that duplicate the lattice of the moir'e pattern of monolayer graphene.

High quality factor graphene varactors for wireless sensing applications

NASA Astrophysics Data System (ADS)

Koester, Steven J.

2011-10-01

A graphene wireless sensor concept is described. By utilizing thin gate dielectrics, the capacitance in a metal-insulator-graphene structure varies with charge concentration through the quantum capacitance effect. Simulations using realistic structural and transport parameters predict quality factors, Q, >60 at 1 GHz. When placed in series with an ideal inductor, a resonant frequency tuning ratio of 25% (54%) is predicted for sense charge densities ranging from 0.32 to 1.6 μC/cm2 at an equivalent oxide thickness of 2.0 nm (0.5 nm). The resonant frequency has a temperature sensitivity, df/dT, less than 0.025%/K for sense charge densities >0.32 μC/cm2.

Thermoelectric properties of epitaxial β-FeSi2 thin films grown on Si(111) substrates with various film qualities

NASA Astrophysics Data System (ADS)

Watanabe, Kentaro; Taniguchi, Tatsuhiko; Sakane, Shunya; Aoki, Shunsuke; Suzuki, Takeyuki; Fujita, Takeshi; Nakamura, Yoshiaki

2017-05-01

Si-based epitaxial β-FeSi2 thin films are attractive as materials for on-chip thermoelectric power generators. We investigated the structure, crystallinity, and thermoelectric properties of β-FeSi2 thin films epitaxially grown on Si(111) substrates by using three different techniques: conventional reactive deposition epitaxy followed by molecular beam epitaxy (RDE+MBE), solid phase epitaxy (SPE) based on codeposition of Fe and Si presented previously, and SPE followed by MBE (SPE+MBE) presented newly by this work. Their epitaxial growth temperatures were fixed at 530 °C for comparison. RDE+MBE thin films exhibited high crystalline quality, but rough surfaces and rugged β-FeSi2/Si(111) interfaces. On the other hand, SPE thin films showed flat surfaces and abrupt β-FeSi2/Si(111) interfaces but low crystallinity. We found that SPE+MBE thin films realized crystallinity higher than SPE thin films, and also had flatter surfaces and sharper interfaces than RDE+MBE thin films. In SPE+MBE thin film growth, due to the initial SPE process with low temperature codeposition, thermal interdiffusion of Fe and Si was suppressed, resulting in the surface flatness and abrupt interface. Second high temperature MBE process improved the crystallinity. We also investigated thermoelectric properties of these β-FeSi2 thin films. Structural factors affecting the thermoelectric properties of RDE+MBE, SPE, and SPE+MBE thin films were investigated.

Penetration of alkali atoms throughout a graphene membrane: theoretical modeling

NASA Astrophysics Data System (ADS)

Boukhvalov, D. W.; Virojanadara, C.

2012-02-01

Theoretical studies of penetration of various alkali atoms (Li, Na, Rb, Cs) throughout a graphene membrane grown on a silicon carbide substrate are reported and compared with recent experimental results. Results of first principles modeling demonstrate a rather low (about 0.8 eV) energy barrier for the formation of temporary defects in the carbon layer required for the penetration of Li at a high concentration of adatoms, a higher (about 2 eV) barrier for Na, and barriers above 4 eV for Rb and Cs. Experiments prove migration of lithium adatoms from the graphene surface to the buffer layer and SiC substrate at room temperature, sodium at 100 °C and impenetrability of the graphene membrane for Rb and Cs. Differences between epitaxial and free-standing graphene for the penetration of alkali ions are also discussed.

Penetration of alkali atoms throughout a graphene membrane: theoretical modeling.

PubMed

Boukhvalov, D W; Virojanadara, C

2012-03-07

Theoretical studies of penetration of various alkali atoms (Li, Na, Rb, Cs) throughout a graphene membrane grown on a silicon carbide substrate are reported and compared with recent experimental results. Results of first principles modeling demonstrate a rather low (about 0.8 eV) energy barrier for the formation of temporary defects in the carbon layer required for the penetration of Li at a high concentration of adatoms, a higher (about 2 eV) barrier for Na, and barriers above 4 eV for Rb and Cs. Experiments prove migration of lithium adatoms from the graphene surface to the buffer layer and SiC substrate at room temperature, sodium at 100 °C and impenetrability of the graphene membrane for Rb and Cs. Differences between epitaxial and free-standing graphene for the penetration of alkali ions are also discussed.

Ultra-broadband photodetectors based on epitaxial graphene quantum dots

NASA Astrophysics Data System (ADS)

El Fatimy, Abdel; Nath, Anindya; Kong, Byoung Don; Boyd, Anthony K.; Myers-Ward, Rachael L.; Daniels, Kevin M.; Jadidi, M. Mehdi; Murphy, Thomas E.; Gaskill, D. Kurt; Barbara, Paola

2018-03-01

Graphene is an ideal material for hot-electron bolometers due to its low heat capacity and weak electron-phonon coupling. Nanostructuring graphene with quantum-dot constrictions yields detectors of electromagnetic radiation with extraordinarily high intrinsic responsivity, higher than 1×109 V W-1 at 3 K. The sensing mechanism is bolometric in nature: the quantum confinement gap causes a strong dependence of the electrical resistance on the electron temperature. Here, we show that this quantum confinement gap does not impose a limitation on the photon energy for light detection and these quantum-dot bolometers work in a very broad spectral range, from terahertz through telecom to ultraviolet radiation, with responsivity independent of wavelength. We also measure the power dependence of the response. Although the responsivity decreases with increasing power, it stays higher than 1×108 V W-1 in a wide range of absorbed power, from 1 pW to 0.4 nW.

Characterization of Graphene Grown Directly on Crystalline Substrates

NASA Astrophysics Data System (ADS)

Rothwell, Sara L.

Graphene has become one of the most popular materials under research, particularly since the 2010 Nobel Prize in Physics. Many visions posit that graphene electronics will be some of the fastest and smallest circuitry physically feasible, however before this becomes reality the scientific community must gain a firm handle on the creation of semiconducting varieties of graphene. In addition, well understood epitaxial growth of graphene on insulating materials will add to the facility of fabricating all-carbon electronics. This thesis presents experimental work detailing the growth of pristine graphene grown on sapphire (GOS) through the thermal decomposition of acetylene, and the electronic characterization of graphene grown on nitrogen-seeded silicon carbide (NG), a semiconducting variety of graphene grown in collaboration with researchers at Georgia Institute of Technology and Rutgers University. GOS displays turbostratic stacking and characteristics of monolayer graphene as analyzed by Raman spectroscopy and atomic force microscopy. Scanning tunneling microscopy characterization of NG illustrates a topography of pleats from 0.5-2 nm tall, 1-4 nm thick, and 1-20 nm long, as well as atomically flat plateaus and other areas of intermixed features. Scanning tunneling spectroscopy measurements across NG features show peaks interpreted as Landau levels induced by strain. Analysis of these Landau levels in coordination with previous characterization concludes that a model employing a bandgap fits best.

Graphene based d-character Dirac Systems

NASA Astrophysics Data System (ADS)

Li, Yuanchang; Zhang, S. B.; Duan, Wenhui

From graphene to topological insulators, Dirac material continues to be the hot topics in condensed matter physics. So far, almost all of the theoretically predicted or experimentally observed Dirac materials are composed of sp -electrons. By using first-principles calculations, we find the new Dirac system of transition-metal intercalated epitaxial graphene on SiC(0001). Intrinsically different from the conventional sp Dirac system, here the Dirac-fermions are dominantly contributed by the transition-metal d-electrons, which paves the way to incorporate correlation effect with Dirac-cone physics. Many intriguing quantum phenomena are proposed based on this system, including quantum spin Hall effect with large spin-orbital gap, quantum anomalous Hall effect, 100% spin-polarized Dirac fermions and ferromagnet-to-topological insulator transition.

Controllable Edge Oxidation and Bubbling Exfoliation Enable the Fabrication of High Quality Water Dispersible Graphene

PubMed Central

Tian, Suyun; Sun, Jing; Yang, Siwei; He, Peng; Wang, Gang; Di, Zengfeng; Ding, Guqiao; Xie, Xiaoming; Jiang, Mianheng

2016-01-01

Despite significant progresses made on mass production of chemically exfoliated graphene, the quality, cost and environmental friendliness remain major challenges for its market penetration. Here, we present a fast and green exfoliation strategy for large scale production of high quality water dispersible few layer graphene through a controllable edge oxidation and localized gas bubbling process. Mild edge oxidation guarantees that the pristine sp2 lattice is largely intact and the edges are functionalized with hydrophilic groups, giving rise to high conductivity and good water dispersibility at the same time. The aqueous concentration can be as high as 5.0 mg mL−1, which is an order of magnitude higher than previously reports. The water soluble graphene can be directly spray-coated on various substrates, and the back-gated field effect transistor give hole and electron mobility of ~496 and ~676 cm2 V−1 s−1, respectively. These results achieved are expected to expedite various applications of graphene. PMID:27666869

Controllable Edge Oxidation and Bubbling Exfoliation Enable the Fabrication of High Quality Water Dispersible Graphene.

PubMed

Tian, Suyun; Sun, Jing; Yang, Siwei; He, Peng; Wang, Gang; Di, Zengfeng; Ding, Guqiao; Xie, Xiaoming; Jiang, Mianheng

2016-09-26

Despite significant progresses made on mass production of chemically exfoliated graphene, the quality, cost and environmental friendliness remain major challenges for its market penetration. Here, we present a fast and green exfoliation strategy for large scale production of high quality water dispersible few layer graphene through a controllable edge oxidation and localized gas bubbling process. Mild edge oxidation guarantees that the pristine sp 2 lattice is largely intact and the edges are functionalized with hydrophilic groups, giving rise to high conductivity and good water dispersibility at the same time. The aqueous concentration can be as high as 5.0 mg mL -1 , which is an order of magnitude higher than previously reports. The water soluble graphene can be directly spray-coated on various substrates, and the back-gated field effect transistor give hole and electron mobility of ~496 and ~676 cm 2 V -1 s -1 , respectively. These results achieved are expected to expedite various applications of graphene.

Controllable Edge Oxidation and Bubbling Exfoliation Enable the Fabrication of High Quality Water Dispersible Graphene

NASA Astrophysics Data System (ADS)

Tian, Suyun; Sun, Jing; Yang, Siwei; He, Peng; Wang, Gang; di, Zengfeng; Ding, Guqiao; Xie, Xiaoming; Jiang, Mianheng

2016-09-01

Despite significant progresses made on mass production of chemically exfoliated graphene, the quality, cost and environmental friendliness remain major challenges for its market penetration. Here, we present a fast and green exfoliation strategy for large scale production of high quality water dispersible few layer graphene through a controllable edge oxidation and localized gas bubbling process. Mild edge oxidation guarantees that the pristine sp2 lattice is largely intact and the edges are functionalized with hydrophilic groups, giving rise to high conductivity and good water dispersibility at the same time. The aqueous concentration can be as high as 5.0 mg mL-1, which is an order of magnitude higher than previously reports. The water soluble graphene can be directly spray-coated on various substrates, and the back-gated field effect transistor give hole and electron mobility of ~496 and ~676 cm2 V-1 s-1, respectively. These results achieved are expected to expedite various applications of graphene.

Transferring-free and large-area graphitic carbon film growth by using molecular beam epitaxy at low growth temperature

NASA Astrophysics Data System (ADS)

Lin, Meng-Yu; Wang, Cheng-Hung; Pao, Chun-Wei; Lin, Shih-Yen

2015-09-01

Graphitic carbon films prepared by using molecular beam epitaxy (MBE) on metal templates with different thicknesses deposited on SiO2/Si substrates are investigated in this paper. With thick Cu templates, only graphitic carbon flakes are obtained near the Cu grain boundaries at low growth temperatures on metal/SiO2 interfaces. By replacing the Cu templates with thin Ni templates, complete graphitic carbon films with superior crystalline quality is obtained at 600 °C on SiO2/Si substrates after removing the Ni templates. The enhanced attachment of the graphitic carbon film to the SiO2/Si substrates with reduced Ni thickness makes the approach a promising approach for transferring-free graphene preparation at low temperature by using MBE.

Tunable magnetotransport in Fe/hBN/graphene/hBN/Pt(Fe) epitaxial multilayers

NASA Astrophysics Data System (ADS)

Magnus Ukpong, Aniekan

2018-03-01

Theoretical and computational analysis of the magnetotransport properties and spin-transfer torque field-induced switching of magnetization density in vertically-stacked multilayers is presented. Using epitaxially-capped free layers of Pt and Fe, atom-resolved magnetic moments and spin-transfer torques are computed at finite bias. The calculations are performed within linear response approximation to the spin-density reformulation of the van der Waals density functional theory. Dynamical spin excitations are computed as a function of a spin-transfer torque induced magnetic field along the magnetic easy axis, and the corresponding spin polarization perpendicular to the easy axis is obtained. Bias-dependent giant anisotropic magnetoresistance of up to 3200% is obtained in the nonmagnetic-metal-capped Fe/hBN/graphene/hBN/Pt multilayer architecture. Since this specific heterostructure is not yet fabricated and characterized, the predicted high performance has not been demonstrated experimentally. Nevertheless, similar calculations performed on the Fe/hBN/Co stack show that the tunneling magnetoresistance obtained at the Fermi-level is in excellent agreement with results of recent magnetotransport measurements on magnetic tunnel junctions that contain the monolayer hBN tunnel region. The magnitude of the spin-transfer torque is found to increase as the tunneling spin current increases, and this activates the magnetization switching process due to increased charge accumulation. This mechanism causes substantial spin backflow, which manifests as rapid undulations in the bias-dependent tunneling spin currents. The implication of these findings on the design of nanoscale spintronic devices with spin-transfer torque tunable magnetization density is discussed. Insights derived from this study are expected to enhance the prospects for developing and integrating artificially assembled van der Waals multilayer heterostructures as the preferred material platform for efficient

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.

Development of High Quality 4H-SiC Thick Epitaxy for Reliable High Power Electronics Using Halogenated Precursors

DTIC Science & Technology

2016-08-02

epitaxy platform, it is essential that malignant defects, such as in-grown stacking faults (IGSFs) and basal plane dislocations (BPDs), be...crystal quality. (5) Even though the inlet C/Si ratio is kept fixed , the C/Si ratio at the growth surface varies depending on the different gas...morphology, and quality (generation of additional defects). Two CVD reactor types, a chimney reactor and an inverted chimney reactor, are assembled; the

Hybrid Molecular Beam Epitaxy for High Quality Strontium Titanate

NASA Astrophysics Data System (ADS)

Jalan, Bharat

2011-12-01

Advancement in thin film growth techniques drives new physics and technologies. Thin film growth approaches and characterization techniques have become more crucial than ever to design and evaluate many emerging materials systems, such as complex oxides. Complex oxides with the perovskite and related structures are fundamentally different from conventional semiconductors and exhibit much richer phenomena as diverse as ferroelectricity, superconductivity, and strongly-correlated Mott-Hubbard-type insulator characteristics. The structural quality of oxide films grown by molecular beam epitaxy (MBE) now matches that of epitaxial semiconductors. Stoichiometry control, however, remains a major challenge. The presence of large (˜tens of ppm) amounts of point defects and impurities, which are commonly present in thin films, has often made the realization and interpretation of intrinsic phenomena difficult. In this dissertation we first describe our work in the development of a hybrid MBE approach for the growth of high quality insulating SrTiO 3 films. The approach uses a combination of solid and metal-organic sources to supply the metals. Films grow in layer-by-layer and step-flow growth modes, with atomically smooth surfaces and an excellent structural quality that is only limited by those of the substrates. A major as- pect of this MBE technique is that it provides a route to stoichiometric SrTiO3. This is achieved by growing films within a "MBE growth window", in which the stoichiome- try is self-regulating, independent of the precise metal flux ratios. Despite the use of a chemical precursor that supply Ti, the carbon incorporation in the films remains below or in the low ppm range. This was achieved by growing films at relatively high temper- atures. We will discuss the transport properties of MBE grown SrTiO3 film. We show that excellent stoichiometry control and low intrinsic defect concentrations, afforded by MBE, allow for the high electron mobility in n

Fabrication of Si(111) crystalline thin film on graphene by aluminum-induced crystallization

DOE Office of Scientific and Technical Information (OSTI.GOV)

Høiaas, I. M.; Kim, D. C., E-mail: dc.kim@crayonano.com, E-mail: helge.weman@ntnu.no; Weman, H., E-mail: dc.kim@crayonano.com, E-mail: helge.weman@ntnu.no

2016-04-18

We report the fabrication of a Si(111) crystalline thin film on graphene by the aluminum-induced crystallization (AIC) process. The AIC process of Si(111) on graphene is shown to be enhanced compared to that on an amorphous SiO{sub 2} substrate, resulting in a more homogeneous Si(111) thin film structure as revealed by X-ray diffraction and atomic force microscopy measurements. Raman measurements confirm that the graphene is intact throughout the process, retaining its characteristic phonon spectrum without any appearance of the D peak. A red-shift of Raman peaks, which is more pronounced for the 2D peak, is observed in graphene after themore » crystallization process. It is found to correlate with the red-shift of the Si Raman peak, suggesting an epitaxial relationship between graphene and the adsorbed AIC Si(111) film with both the graphene and Si under tensile strain.« less

EDITORIAL: Special issue on Graphene Special issue on Graphene

NASA Astrophysics Data System (ADS)

Morpurgo, Alberto F.; Trauzettel, Björn

2010-03-01

and effectively reflect the status of different areas of graphene research. The excitonic condensation in a double graphene system is discussed by Kharitonov and Efetov. Borca et al report on a method to fabricate and characterize graphene monolayers epitaxially grown on Ru(0001). Furthermore, the energy and transport gaps in etched graphene nanoribbons are analyzed experimentally by Molitor et al. Mucha-Kruczyński et al review the tight-binding model of bilayer graphene, whereas Wurm et al focus on a theoretical description of the Aharonov-Bohm effect in monolayer graphene rings. Screening effects and collective excitations are studied by Roldán et al. Subsequently, Palacios et al review the electronic and magnetic structures of graphene nanoribbons, a problem that is highly relevant for graphene-based transistors. Klein tunneling in single and multiple barriers in graphene is the topic of the review article by Pereira Jr et al, while De Martino and Egger discuss the spectrum of a magnetic quantum dot in graphene. Titov et al study the effect of resonant scatterers on the local density of states in a rectangular graphene setup with metallic leads. Finally, the resistance modulation of multilayer graphene controlled by gate electric fields is experimentally analyzed by Miyazaki et al. We would like to thank all the authors for their contributions, which combine new results and pedagogical discussions of the state-of-the-art in different areas: it is this combination that most often adds to the value of topical issues. Special thanks also goes to the staff of Institute of Physics Publishing for contributing to the success of this effort.

Delaminated Transfer of CVD Graphene

NASA Astrophysics Data System (ADS)

Clavijo, Alexis; Mao, Jinhai; Tilak, Nikhil; Altvater, Michael; Andrei, Eva

Single layer graphene is commonly synthesized by dissociation of a carbonaceous gas at high temperatures in the presence of a metallic catalyst in a process known as Chemical Vapor Deposition or CVD. Although it is possible to achieve high quality graphene by CVD, the standard transfer technique of etching away the metallic catalyst is wasteful and jeopardizes the quality of the graphene film by contamination from etchants. Thus, development of a clean transfer technique and preservation of the parent substrate remain prominent hurdles to overcome. In this study, we employ a copper pretreatment technique and optimized parameters for growth of high quality single layer graphene at atmospheric pressure. We address the transfer challenge by utilizing the adhesive properties between a polymer film and graphene to achieve etchant-free transfer of graphene films from a copper substrate. Based on this concept we developed a technique for dry delamination and transferring of graphene to hexagonal boron nitride substrates, which produced high quality graphene films while at the same time preserving the integrity of the copper catalyst for reuse. DOE-FG02-99ER45742, Ronald E. McNair Postbaccalaureate Achievement Program.

Graphene enhanced field emission from InP nanocrystals.

PubMed

Iemmo, L; Di Bartolomeo, A; Giubileo, F; Luongo, G; Passacantando, M; Niu, G; Hatami, F; Skibitzki, O; Schroeder, T

2017-12-08

We report the observation of field emission (FE) from InP nanocrystals (NCs) epitaxially grown on an array of p-Si nanotips. We prove that FE can be enhanced by covering the InP NCs with graphene. The measurements are performed inside a scanning electron microscope chamber with a nano-controlled W-thread used as an anode. We analyze the FE by Fowler-Nordheim theory and find that the field enhancement factor increases monotonically with the spacing between the anode and the cathode. We also show that InP/p-Si junction has a rectifying behavior, while graphene on InP creates an ohmic contact. Understanding the fundamentals of such nanojunctions is key for applications in nanoelectronics.

Graphene enhanced field emission from InP nanocrystals

NASA Astrophysics Data System (ADS)

Iemmo, L.; Di Bartolomeo, A.; Giubileo, F.; Luongo, G.; Passacantando, M.; Niu, G.; Hatami, F.; Skibitzki, O.; Schroeder, T.

2017-12-01

We report the observation of field emission (FE) from InP nanocrystals (NCs) epitaxially grown on an array of p-Si nanotips. We prove that FE can be enhanced by covering the InP NCs with graphene. The measurements are performed inside a scanning electron microscope chamber with a nano-controlled W-thread used as an anode. We analyze the FE by Fowler-Nordheim theory and find that the field enhancement factor increases monotonically with the spacing between the anode and the cathode. We also show that InP/p-Si junction has a rectifying behavior, while graphene on InP creates an ohmic contact. Understanding the fundamentals of such nanojunctions is key for applications in nanoelectronics.

Gate-independent energy gap in noncovalently intercalated bilayer graphene on SiC(0001)

NASA Astrophysics Data System (ADS)

Li, Yuanchang

2016-12-01

Our first-principles calculations show that an energy gap around 0.12-0.25 eV can be engineered in epitaxial graphene on SiC(0001) through the noncovalent intercalation of transition or alkali metals but originated from the distinct mechanisms. The former is attributed to the combined effects of a metal-induced perpendicular electric field and interaction, while the latter is solely attributed to the built-in electric field. A great advantage of this scheme is that the gap size is almost independent of the gate voltage up to 1 V/nm, thus reserving the electric means to tune the Fermi level of graphene when configured as field-effect transistors. Given the recent progress in experimental techniques for intercalated graphene, our findings provide a practical way to incorporate graphene in the current semiconductor industry.

Oriented graphene nanoribbons embedded in hexagonal boron nitride trenches

PubMed Central

Chen, Lingxiu; He, Li; Wang, Hui Shan; Wang, Haomin; Tang, Shujie; Cong, Chunxiao; Xie, Hong; Li, Lei; Xia, Hui; Li, Tianxin; Wu, Tianru; Zhang, Daoli; Deng, Lianwen; Yu, Ting; Xie, Xiaoming; Jiang, Mianheng

2017-01-01

Graphene nanoribbons (GNRs) are ultra-narrow strips of graphene that have the potential to be used in high-performance graphene-based semiconductor electronics. However, controlled growth of GNRs on dielectric substrates remains a challenge. Here, we report the successful growth of GNRs directly on hexagonal boron nitride substrates with smooth edges and controllable widths using chemical vapour deposition. The approach is based on a type of template growth that allows for the in-plane epitaxy of mono-layered GNRs in nano-trenches on hexagonal boron nitride with edges following a zigzag direction. The embedded GNR channels show excellent electronic properties, even at room temperature. Such in-plane hetero-integration of GNRs, which is compatible with integrated circuit processing, creates a gapped channel with a width of a few benzene rings, enabling the development of digital integrated circuitry based on GNRs. PMID:28276532

Epitaxial thin film growth in outer space

NASA Technical Reports Server (NTRS)

Ignatiev, Alex; Chu, C. W.

1988-01-01

A new concept for materials processing in space exploits the ultravacuum component of space for thin-film epitaxial growth. The unique LEO space environment is expected to yield 10-ftorr or better pressures, semiinfinite pumping speeds, and large ultravacuum volume (about 100 cu m) without walls. These space ultravacuum properties promise major improvement in the quality, unique nature, and throughput of epitaxially grown materials, including semiconductors, magnetic materials, and thin-film high-temperature superconductors.

Molecular beam epitaxial growth of high-quality InSb on InP and GaAs substrates

NASA Technical Reports Server (NTRS)

Oh, J. E.; Bhattacharya, P. K.; Chen, Y. C.; Tsukamoto, S.

1989-01-01

Epitaxial layers of InSb were grown on InP and GaAs substrates by molecular beam epitaxy. The dependence of the epilayer quality on flux ratio, J sub Sb4/J sub In, was studied. Deviation from an optimum value of J sub Sb4/J sub In (approx. 2) during growth led to deterioration in the surface morphology and the electrical and crystalline qualities of the films. Room temperature electron mobilities as high as 70,000 and 53,000 sq cm /V-s were measured in InSb layers grown on InP and GaAs substrates, respectively. Unlike the previous results, the conductivity in these films is n-type even at T = 13 K, and no degradation of the electron mobility due to the high density of dislocations was observed. The measured electron mobilities (and carrier concentrations) at 77 K in InSb layers grown on InP and GaAs substrates are 110,000 sq cm/V-s (3 x 10(15) cm(-3)) and 55,000 sq cm/V-s (4.95 x 10(15) cm(-3)), respectively, suggesting their application to electronic devices at cryogenic temperatures.

Reliable Exfoliation of Large-Area High-Quality Flakes of Graphene and Other Two-Dimensional Materials.

PubMed

Huang, Yuan; Sutter, Eli; Shi, Norman N; Zheng, Jiabao; Yang, Tianzhong; Englund, Dirk; Gao, Hong-Jun; Sutter, Peter

2015-11-24

Mechanical exfoliation has been a key enabler of the exploration of the properties of two-dimensional materials, such as graphene, by providing routine access to high-quality material. The original exfoliation method, which remained largely unchanged during the past decade, provides relatively small flakes with moderate yield. Here, we report a modified approach for exfoliating thin monolayer and few-layer flakes from layered crystals. Our method introduces two process steps that enhance and homogenize the adhesion force between the outermost sheet in contact with a substrate: Prior to exfoliation, ambient adsorbates are effectively removed from the substrate by oxygen plasma cleaning, and an additional heat treatment maximizes the uniform contact area at the interface between the source crystal and the substrate. For graphene exfoliation, these simple process steps increased the yield and the area of the transferred flakes by more than 50 times compared to the established exfoliation methods. Raman and AFM characterization shows that the graphene flakes are of similar high quality as those obtained in previous reports. Graphene field-effect devices were fabricated and measured with back-gating and solution top-gating, yielding mobilities of ∼4000 and 12,000 cm(2)/(V s), respectively, and thus demonstrating excellent electrical properties. Experiments with other layered crystals, e.g., a bismuth strontium calcium copper oxide (BSCCO) superconductor, show enhancements in exfoliation yield and flake area similar to those for graphene, suggesting that our modified exfoliation method provides an effective way for producing large area, high-quality flakes of a wide range of 2D materials.

Symmetry Breaking in Few Layer Graphene Films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bostwick, A.; Ohta, T.; McChesney, J.L.

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.

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.

High Yield Chemical Vapor Deposition Growth of High Quality Large-Area AB Stacked Bilayer Graphene

PubMed Central

Liu, Lixin; Zhou, Hailong; Cheng, Rui; Yu, Woo Jong; Liu, Yuan; Chen, Yu; Shaw, Jonathan; Zhong, Xing; Huang, Yu; Duan, Xiangfeng

2012-01-01

Bernal stacked (AB stacked) bilayer graphene is of significant interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical electrical field. Mechanical exfoliation can be used to produce AB stacked bilayer graphene flakes but typically with the sizes limited to a few micrometers. Chemical vapor deposition (CVD) has been recently explored for the synthesis of bilayer graphene but usually with limited coverage and a mixture of AB and randomly stacked structures. Herein we report a rational approach to produce large-area high quality AB stacked bilayer graphene. We show that the self-limiting effect of graphene growth on Cu foil can be broken by using a high H2/CH4 ratio in a low pressure CVD process to enable the continued growth of bilayer graphene. A high temperature and low pressure nucleation step is found to be critical for the formation of bilayer graphene nuclei with high AB stacking ratio. A rational design of a two-step CVD process is developed for the growth of bilayer graphene with high AB stacking ratio (up to 90 %) and high coverage (up to 99 %). The electrical transport studies demonstrated that devices made of the as-grown bilayer graphene exhibit typical characteristics of AB stacked bilayer graphene with the highest carrier mobility exceeding 4,000 cm2/V·s at room temperature, comparable to that of the exfoliated bilayer graphene. PMID:22906199

A novel grating-imaging method to measure carrier diffusion coefficient in graphene

NASA Astrophysics Data System (ADS)

Chen, Ke; Wang, Yaguo; Akinwande, Deji; Bank, Seth; Lin, Jung-Fu

Similar to carrier mobility, carrier diffusion coefficient in graphene determines the response rate of future graphene-based electronics. Here we present a simple, sensitive and non-destructive technique integrated with ultrafast pump-probe spectroscopy to measure carrier diffusion in CVD-grown graphene. In the method, the pump and the probe beams pass through the same area of a photomask with metal strips i.e. a transmission amplitude grating, and get diffracted. The diffracted light is collected by an objective lens and focused onto the sample to generate carrier density grating. Relaxation of this carrier density grating is governed by both carrier recombination and carrier diffusion in the sample. Transient transmission change of the probe beams, which reflects this relaxation process, is recorded. The measured diffusion coefficients of multilayer and monolayer CVD-grown graphene are 2000cm2/s and 10000cm2/s, respectively, comparable with the reported values of epitaxial graphene and reduced graphene. This transmission grating technique can be used to measure carrier dynamics in versatile 2D materials.

Graphene growth from reduced graphene oxide by chemical vapour deposition: seeded growth accompanied by restoration

NASA Astrophysics Data System (ADS)

Chang, Sung-Jin; Hyun, Moon Seop; Myung, Sung; Kang, Min-A.; Yoo, Jung Ho; Lee, Kyoung G.; Choi, Bong Gill; Cho, Youngji; Lee, Gaehang; Park, Tae Jung

2016-03-01

Understanding the underlying mechanisms involved in graphene growth via chemical vapour deposition (CVD) is critical for precise control of the characteristics of graphene. Despite much effort, the actual processes behind graphene synthesis still remain to be elucidated in a large number of aspects. Herein, we report the evolution of graphene properties during in-plane growth of graphene from reduced graphene oxide (RGO) on copper (Cu) via methane CVD. While graphene is laterally grown from RGO flakes on Cu foils up to a few hundred nanometres during CVD process, it shows appreciable improvement in structural quality. The monotonous enhancement of the structural quality of the graphene with increasing length of the graphene growth from RGO suggests that seeded CVD growth of graphene from RGO on Cu surface is accompanied by the restoration of graphitic structure. The finding provides insight into graphene growth and defect reconstruction useful for the production of tailored carbon nanostructures with required properties.

Graphene growth from reduced graphene oxide by chemical vapour deposition: seeded growth accompanied by restoration.

PubMed

Chang, Sung-Jin; Hyun, Moon Seop; Myung, Sung; Kang, Min-A; Yoo, Jung Ho; Lee, Kyoung G; Choi, Bong Gill; Cho, Youngji; Lee, Gaehang; Park, Tae Jung

2016-03-10

Understanding the underlying mechanisms involved in graphene growth via chemical vapour deposition (CVD) is critical for precise control of the characteristics of graphene. Despite much effort, the actual processes behind graphene synthesis still remain to be elucidated in a large number of aspects. Herein, we report the evolution of graphene properties during in-plane growth of graphene from reduced graphene oxide (RGO) on copper (Cu) via methane CVD. While graphene is laterally grown from RGO flakes on Cu foils up to a few hundred nanometres during CVD process, it shows appreciable improvement in structural quality. The monotonous enhancement of the structural quality of the graphene with increasing length of the graphene growth from RGO suggests that seeded CVD growth of graphene from RGO on Cu surface is accompanied by the restoration of graphitic structure. The finding provides insight into graphene growth and defect reconstruction useful for the production of tailored carbon nanostructures with required properties.

Graphene growth from reduced graphene oxide by chemical vapour deposition: seeded growth accompanied by restoration

PubMed Central

Chang, Sung-Jin; Hyun, Moon Seop; Myung, Sung; Kang, Min-A; Yoo, Jung Ho; Lee, Kyoung G.; Choi, Bong Gill; Cho, Youngji; Lee, Gaehang; Park, Tae Jung

2016-01-01

Understanding the underlying mechanisms involved in graphene growth via chemical vapour deposition (CVD) is critical for precise control of the characteristics of graphene. Despite much effort, the actual processes behind graphene synthesis still remain to be elucidated in a large number of aspects. Herein, we report the evolution of graphene properties during in-plane growth of graphene from reduced graphene oxide (RGO) on copper (Cu) via methane CVD. While graphene is laterally grown from RGO flakes on Cu foils up to a few hundred nanometres during CVD process, it shows appreciable improvement in structural quality. The monotonous enhancement of the structural quality of the graphene with increasing length of the graphene growth from RGO suggests that seeded CVD growth of graphene from RGO on Cu surface is accompanied by the restoration of graphitic structure. The finding provides insight into graphene growth and defect reconstruction useful for the production of tailored carbon nanostructures with required properties. PMID:26961409

Characterization Of Graphene-Ferroelectric Superlattice Hybrid Devices

NASA Astrophysics Data System (ADS)

Yusuf, Mohammed; Du, Xu; Dawber, Matthew

2013-03-01

Ferroelectric materials possess a spontaneous electrical polarization, which can be controlled by an electric field. A good interface between ferroelectric surface and graphene sheets can introduce a new generation of multifunctional devices, in which the ferroelectric material can be used to control the properties of graphene. In our approach, problems encountered in previous efforts to combine ferroelectric/carbon systems are overcome by the use of artificially layered superlattice materials grown in the form of epitaxial thin films. In these materials the phase transition temperature and dielectric response of the material can be tailored, allowing us to avoid polarization screening by surface absorbates, whilst maintaining an atomically smooth surface and optimal charge doping properties. Using ferroelectric PbTiO3/SrTiO3 superlattices, we have shown ultra-low-voltage operation of graphene field effect devices within +/- 1 V at room temperature. The switching of the graphene field effect transistors is characterized by pronounced resistance hysteresis, suitable for ultra-fast non-volatile electronics. Low temperature characterization confirmed that the coercive field required for the ferroelectric domain switching increases significantly with decreasing temperatures. National Science Foundation (NSF) (grant number 1105202)

Moving graphene devices from lab to market: advanced graphene-coated nanoprobes

NASA Astrophysics Data System (ADS)

Hui, Fei; Vajha, Pujashree; Shi, Yuanyuan; Ji, Yanfeng; Duan, Huiling; Padovani, Andrea; Larcher, Luca; Li, Xiao Rong; Xu, Jing Juan; Lanza, Mario

2016-04-01

After more than a decade working with graphene there is still a preoccupying lack of commercial devices based on this wonder material. Here we report the use of high-quality solution-processed graphene sheets to fabricate ultra-sharp probes with superior performance. Nanoprobes are versatile tools used in many fields of science, but they can wear fast after some experiments, reducing the quality and increasing the cost of the research. As the market of nanoprobes is huge, providing a solution for this problem should be a priority for the nanotechnology industry. Our graphene-coated nanoprobes not only show enhanced lifetime, but also additional unique properties of graphene, such as hydrophobicity. Moreover, we have functionalized the surface of graphene to provide piezoelectric capability, and have fabricated a nano relay. The simplicity and low cost of this method, which can be used to coat any kind of sharp tip, make it suitable for the industry, allowing production on demand.After more than a decade working with graphene there is still a preoccupying lack of commercial devices based on this wonder material. Here we report the use of high-quality solution-processed graphene sheets to fabricate ultra-sharp probes with superior performance. Nanoprobes are versatile tools used in many fields of science, but they can wear fast after some experiments, reducing the quality and increasing the cost of the research. As the market of nanoprobes is huge, providing a solution for this problem should be a priority for the nanotechnology industry. Our graphene-coated nanoprobes not only show enhanced lifetime, but also additional unique properties of graphene, such as hydrophobicity. Moreover, we have functionalized the surface of graphene to provide piezoelectric capability, and have fabricated a nano relay. The simplicity and low cost of this method, which can be used to coat any kind of sharp tip, make it suitable for the industry, allowing production on demand. Electronic

Vertically Oriented Growth of GaN Nanorods on Si Using Graphene as an Atomically Thin Buffer Layer.

PubMed

Heilmann, Martin; Munshi, A Mazid; Sarau, George; Göbelt, Manuela; Tessarek, Christian; Fauske, Vidar T; van Helvoort, Antonius T J; Yang, Jianfeng; Latzel, Michael; Hoffmann, Björn; Conibeer, Gavin; Weman, Helge; Christiansen, Silke

2016-06-08

The monolithic integration of wurtzite GaN on Si via metal-organic vapor phase epitaxy is strongly hampered by lattice and thermal mismatch as well as meltback etching. This study presents single-layer graphene as an atomically thin buffer layer for c-axis-oriented growth of vertically aligned GaN nanorods mediated by nanometer-sized AlGaN nucleation islands. Nanostructures of similar morphology are demonstrated on graphene-covered Si(111) as well as Si(100). High crystal and optical quality of the nanorods are evidenced through scanning transmission electron microscopy, micro-Raman, and cathodoluminescence measurements supported by finite-difference time-domain simulations. Current-voltage characteristics revealed high vertical conduction of the as-grown GaN nanorods through the Si substrates. These findings are substantial to advance the integration of GaN-based devices on any substrates of choice that sustains the GaN growth temperatures, thereby permitting novel designs of GaN-based heterojunction device concepts.

Chemically engineered graphene-based 2D organic molecular magnet.

PubMed

Hong, Jeongmin; Bekyarova, Elena; de Heer, Walt A; Haddon, Robert C; Khizroev, Sakhrat

2013-11-26

Carbon-based magnetic materials and structures of mesoscopic dimensions may offer unique opportunities for future nanomagnetoelectronic/spintronic devices. To achieve their potential, carbon nanosystems must have controllable magnetic properties. We demonstrate that nitrophenyl functionalized graphene can act as a room-temperature 2D magnet. We report a comprehensive study of low-temperature magnetotransport, vibrating sample magnetometry (VSM), and superconducting quantum interference (SQUID) measurements before and after radical functionalization. Following nitrophenyl (NP) functionalization, epitaxially grown graphene systems can become organic molecular magnets with ferromagnetic and antiferromagnetic ordering that persists at temperatures above 400 K. The field-dependent, surface magnetoelectric properties were studied using scanning probe microscopy (SPM) techniques. The results indicate that the NP-functionalization orientation and degree of coverage directly affect the magnetic properties of the graphene surface. In addition, graphene-based organic magnetic nanostructures were found to demonstrate a pronounced magneto-optical Kerr effect (MOKE). The results were consistent across different characterization techniques and indicate room-temperature magnetic ordering along preferred graphene orientations in the NP-functionalized samples. Chemically isolated graphene nanoribbons (CINs) were observed along the preferred functionality directions. These results pave the way for future magnetoelectronic/spintronic applications based on promising concepts such as current-induced magnetization switching, magnetoelectricity, half-metallicity, and quantum tunneling of magnetization.

Growth of Graphene by Catalytic Dissociation of Ethylene on CuNi(111)

NASA Astrophysics Data System (ADS)

Ventrice, Carl A., Jr.; Tyagi, Parul; Golden, Max; Mowll, Tyler

2015-03-01

The growth of graphene by the catalytic decomposition of ethylene on a 90:10 CuNi(111) substrate was performed. The growths were done in a UHV system by either heating the substrate to the growth temperature followed by introducing the ethylene precursor or by introducing the ethylene precursor and subsequently heating it to the growth temperature. The growth using the former method results in a two-domain epitaxial graphene overlayer at temperatues as low as 550 °C. However, introducing the ethylene before heating the substrate resulted in considerable rotational disorder within the graphene film. This has been attributed to the formation of a carbide phase below 550 °C. This research was supported by the NSF (DMR-1006411).

Molecular-beam epitaxy of (Zn,Mn)Se on Si(100)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Slobodskyy, T.; Ruester, C.; Fiederling, R.

2004-12-20

We have investigated the growth by molecular-beam epitaxy of the II-VI diluted magnetic semiconductor (Zn,Mn)Se on As-passivated Si(100) substrates. The growth start has been optimized by using low-temperature epitaxy. Surface properties were assessed by Nomarski and scanning electron microscopy. Optical properties of (Zn,Mn)Se have been studied by photoluminescence and a giant Zeeman splitting of up to 30 meV has been observed. Our observations indicate a high crystalline quality of the epitaxial films.

Influence of in-situ deposited SiNx interlayer on crystal quality of GaN epitaxial films

NASA Astrophysics Data System (ADS)

Fan, Teng; Jia, Wei; Tong, Guangyun; Zhai, Guangmei; Li, Tianbao; Dong, Hailiang; Xu, Bingshe

2018-05-01

GaN epitaxial films with SiNx interlayers were prepared by metal organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates. The influences of deposition times and locations of SiNx interlayers on crystal quality of GaN epitaxial films were studied. Under the optimal growth time of 120 s for the SiNx interlayer, the dislocation density of GaN film is reduced to 4.05 × 108 cm-2 proved by high resolution X-ray diffraction results. It is found that when the SiNx interlayer deposits on the GaN nucleation islands, the subsequent GaN film has the lowest dislocation density of only 2.89 × 108 cm-2. Moreover, a model is proposed to illustrate the morphological evolution and associated propagation processes of TDs in GaN epi-layers with SiNx interlayers for different deposition times and locations.

Gallenene epitaxially grown on Si(1 1 1)

NASA Astrophysics Data System (ADS)

Tao, Min-Long; Tu, Yu-Bing; Sun, Kai; Wang, Ya-Li; Xie, Zheng-Bo; Liu, Lei; Shi, Ming-Xia; Wang, Jun-Zhong

2018-07-01

Gallenene, an analogue of graphene composed of gallium, is epitaxially grown on Si(1 1 1) surface and studied by low temperature scanning tunneling microscopy (LT-STM). The STM images display that the buffer layer has a superstructure with respect to the substrate lattice and the gallenene layer has a hexagonal honeycomb structure. The scanning tunneling spectra (STS) of the gallenene show that it behaves as a metallic layer. First-principles calculations give the proposed configuration. Our results provide a method to synthesize the gallenene and shed important light on the growth mechanism of it.

Electrolytic exfoliation of graphite in water with multifunctional electrolytes: en route towards high quality, oxide-free graphene flakes.

PubMed

Munuera, J M; Paredes, J I; Villar-Rodil, S; Ayán-Varela, M; Martínez-Alonso, A; Tascón, J M D

2016-02-07

Electrolytic--usually referred to as electrochemical--exfoliation of graphite in water under anodic potential holds enormous promise as a simple, green and high-yield method for the mass production of graphene, but currently suffers from several drawbacks that hinder its widespread adoption, one of the most critical being the oxidation and subsequent structural degradation of the carbon lattice that is usually associated with such a production process. To overcome this and other limitations, we introduce and implement the concept of multifunctional electrolytes. The latter are amphiphilic anions (mostly polyaromatic hydrocarbons appended with sulfonate groups) that play different relevant roles as (1) an intercalating electrolyte to trigger exfoliation of graphite into graphene flakes, (2) a dispersant to afford stable aqueous colloidal suspensions of the flakes suitable for further use, (3) a sacrificial agent to prevent graphene oxidation during exfoliation and (4) a linker to promote nanoparticle anchoring on the graphene flakes, yielding functional hybrids. The implementation of this strategy with some selected amphiphiles even furnishes anodically exfoliated graphenes of a quality similar to that of flakes produced by direct, ultrasound- or shear-induced exfoliation of graphite in the liquid phase (i.e., almost oxide- and defect-free). These high quality materials were used for the preparation of catalytically efficient graphene-Pt nanoparticle hybrids, as demonstrated by model reactions (reduction of nitroarenes). The multifunctional performance of these electrolytes is also discussed and rationalized, and a mechanistic picture of their oxidation-preventing ability is proposed. Overall, the present results open the prospect of anodic exfoliation as a competitive method for the production of very high quality graphene flakes.

A proposal for epitaxial thin film growth in outer space

NASA Technical Reports Server (NTRS)

Ignatiev, Alex; Chu, C. W.

1988-01-01

A new concept for materials processing in space exploits the ultravacuum component of space for thin film epitaxial growth. The unique low earth orbit space environment is expected to yield 10 to the -14th torr or better pressures, semiinfinite pumping speeds, and large ultravacuum volume without walls. These space ultravacuum properties promise major improvement in the quality, unique nature, and the throughput of epitaxially grown materials. Advanced thin film materials to be epitaxially grown in space include semiconductors, magnetic materials, and thin film high temperature superconductors.

Growing vertical ZnO nanorod arrays within graphite: efficient isolation of large size and high quality single-layer graphene.

PubMed

Ding, Ling; E, Yifeng; Fan, Louzhen; Yang, Shihe

2013-07-18

We report a unique strategy for efficiently exfoliating large size and high quality single-layer graphene directly from graphite into DMF dispersions by growing ZnO nanorod arrays between the graphene layers in graphite.

Observation of room-temperature high-energy resonant excitonic effects in graphene

NASA Astrophysics Data System (ADS)

Santoso, I.; Gogoi, P. K.; Su, H. B.; Huang, H.; Lu, Y.; Qi, D.; Chen, W.; Majidi, M. A.; Feng, Y. P.; Wee, A. T. S.; Loh, K. P.; Venkatesan, T.; Saichu, R. P.; Goos, A.; Kotlov, A.; Rübhausen, M.; Rusydi, A.

2011-08-01

Using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry, we observe a resonant exciton at an unusually high energy of 6.3 eV in epitaxial graphene. Surprisingly, the resonant exciton occurs at room temperature and for a very large number of graphene layers N≈75, thus suggesting a poor screening in graphene. The optical conductivity (σ1) of a resonant exciton scales linearly with the number of graphene layers (up to at least 8 layers), implying the quantum character of electrons in graphene. Furthermore, a prominent excitation at 5.4 eV, which is a mixture of interband transitions from π to π* at the M point and a π plasmonic excitation, is observed. In contrast, for graphite the resonant exciton is not observable but strong interband transitions are seen instead. Supported by theoretical calculations, for N⩽ 28 the σ1 is dominated by the resonant exciton, while for N> 28 it is a mixture between exitonic and interband transitions. The latter is characteristic for graphite, indicating a crossover in the electronic structure. Our study shows that important elementary excitations in graphene occur at high binding energies and elucidate the differences in the way electrons interact in graphene and graphite.

Understanding the growth mechanism of graphene on Ge/Si(001) surfaces

NASA Astrophysics Data System (ADS)

Dabrowski, J.; Lippert, G.; Avila, J.; Baringhaus, J.; Colambo, I.; Dedkov, Yu S.; Herziger, F.; Lupina, G.; Maultzsch, J.; Schaffus, T.; Schroeder, T.; Kot, M.; Tegenkamp, C.; Vignaud, D.; Asensio, M.-C.

2016-08-01

The practical difficulties to use graphene in microelectronics and optoelectronics is that the available methods to grow graphene are not easily integrated in the mainstream technologies. A growth method that could overcome at least some of these problems is chemical vapour deposition (CVD) of graphene directly on semiconducting (Si or Ge) substrates. Here we report on the comparison of the CVD and molecular beam epitaxy (MBE) growth of graphene on the technologically relevant Ge(001)/Si(001) substrate from ethene (C2H4) precursor and describe the physical properties of the films as well as we discuss the surface reaction and diffusion processes that may be responsible for the observed behavior. Using nano angle resolved photoemission (nanoARPES) complemented by transport studies and Raman spectroscopy as well as density functional theory (DFT) calculations, we report the direct observation of massless Dirac particles in monolayer graphene, providing a comprehensive mapping of their low-hole doped Dirac electron bands. The micrometric graphene flakes are oriented along two predominant directions rotated by 30° with respect to each other. The growth mode is attributed to the mechanism when small graphene “molecules” nucleate on the Ge(001) surface and it is found that hydrogen plays a significant role in this process.

Understanding the growth mechanism of graphene on Ge/Si(001) surfaces

PubMed Central

Dabrowski, J.; Lippert, G.; Avila, J.; Baringhaus, J.; Colambo, I.; Dedkov, Yu S.; Herziger, F.; Lupina, G.; Maultzsch, J.; Schaffus, T.; Schroeder, T.; Kot, M.; Tegenkamp, C.; Vignaud, D.; Asensio, M.-C.

2016-01-01

The practical difficulties to use graphene in microelectronics and optoelectronics is that the available methods to grow graphene are not easily integrated in the mainstream technologies. A growth method that could overcome at least some of these problems is chemical vapour deposition (CVD) of graphene directly on semiconducting (Si or Ge) substrates. Here we report on the comparison of the CVD and molecular beam epitaxy (MBE) growth of graphene on the technologically relevant Ge(001)/Si(001) substrate from ethene (C2H4) precursor and describe the physical properties of the films as well as we discuss the surface reaction and diffusion processes that may be responsible for the observed behavior. Using nano angle resolved photoemission (nanoARPES) complemented by transport studies and Raman spectroscopy as well as density functional theory (DFT) calculations, we report the direct observation of massless Dirac particles in monolayer graphene, providing a comprehensive mapping of their low-hole doped Dirac electron bands. The micrometric graphene flakes are oriented along two predominant directions rotated by 30° with respect to each other. The growth mode is attributed to the mechanism when small graphene “molecules” nucleate on the Ge(001) surface and it is found that hydrogen plays a significant role in this process. PMID:27531322

Understanding the growth mechanism of graphene on Ge/Si(001) surfaces.

PubMed

Dabrowski, J; Lippert, G; Avila, J; Baringhaus, J; Colambo, I; Dedkov, Yu S; Herziger, F; Lupina, G; Maultzsch, J; Schaffus, T; Schroeder, T; Kot, M; Tegenkamp, C; Vignaud, D; Asensio, M-C

2016-08-17

The practical difficulties to use graphene in microelectronics and optoelectronics is that the available methods to grow graphene are not easily integrated in the mainstream technologies. A growth method that could overcome at least some of these problems is chemical vapour deposition (CVD) of graphene directly on semiconducting (Si or Ge) substrates. Here we report on the comparison of the CVD and molecular beam epitaxy (MBE) growth of graphene on the technologically relevant Ge(001)/Si(001) substrate from ethene (C2H4) precursor and describe the physical properties of the films as well as we discuss the surface reaction and diffusion processes that may be responsible for the observed behavior. Using nano angle resolved photoemission (nanoARPES) complemented by transport studies and Raman spectroscopy as well as density functional theory (DFT) calculations, we report the direct observation of massless Dirac particles in monolayer graphene, providing a comprehensive mapping of their low-hole doped Dirac electron bands. The micrometric graphene flakes are oriented along two predominant directions rotated by 30° with respect to each other. The growth mode is attributed to the mechanism when small graphene "molecules" nucleate on the Ge(001) surface and it is found that hydrogen plays a significant role in this process.

Surface-directed molecular assembly of pentacene on monolayer graphene for high-performance organic transistors.

PubMed

Lee, Wi Hyoung; Park, Jaesung; Sim, Sung Hyun; Lim, Soojin; Kim, Kwang S; Hong, Byung Hee; Cho, Kilwon

2011-03-30

Organic electronic devices that use graphene electrodes have received considerable attention because graphene is regarded as an ideal candidate electrode material. Transfer and lithographic processes during fabrication of patterned graphene electrodes typically leave polymer residues on the graphene surfaces. However, the impact of these residues on the organic semiconductor growth mechanism on graphene surface has not been reported yet. Here, we demonstrate that polymer residues remaining on graphene surfaces induce a stand-up orientation of pentacene, thereby controlling pentacene growth such that the molecular assembly is optimal for charge transport. Thus, pentacene field-effect transistors (FETs) using source/drain monolayer graphene electrodes with polymer residues show a high field-effect mobility of 1.2 cm(2)/V s. In contrast, epitaxial growth of pentacene having molecular assembly of lying-down structure is facilitated by π-π interaction between pentacene and the clean graphene electrode without polymer residues, which adversely affects lateral charge transport at the interface between electrode and channel. Our studies provide that the obtained high field-effect mobility in pentacene FETs using monolayer graphene electrodes arises from the extrinsic effects of polymer residues as well as the intrinsic characteristics of the highly conductive, ultrathin two-dimensional monolayer graphene electrodes.

Abbreviated epitaxial growth mode (AGM) method for reducing cost and improving quality of LEDs and lasers

DOEpatents

Tansu, Nelson; Chan, Helen M; Vinci, Richard P; Ee, Yik-Khoon; Biser, Jeffrey

2013-09-24

The use of an abbreviated GaN growth mode on nano-patterned AGOG sapphire substrates, which utilizes a process of using 15 nm low temperature GaN buffer and bypassing etch-back and recovery processes during epitaxy, enables the growth of high-quality GaN template on nano-patterned AGOG sapphire. The GaN template grown on nano-patterned AGOG sapphire by employing abbreviated growth mode has two orders of magnitude lower threading dislocation density than that of conventional GaN template grown on planar sapphire. The use of abbreviated growth mode also leads to significant reduction in cost of the epitaxy. The growths and characteristics of InGaN quantum wells (QWs) light emitting diodes (LEDs) on both templates were compared. The InGaN QWs LEDs grown on the nano-patterned AGOG sapphire demonstrated at least a 24% enhancement of output power enhancement over that of LEDs grown on conventional GaN templates.

GaN/NbN epitaxial semiconductor/superconductor heterostructures

NASA Astrophysics Data System (ADS)

Yan, Rusen; Khalsa, Guru; Vishwanath, Suresh; Han, Yimo; Wright, John; Rouvimov, Sergei; Katzer, D. Scott; Nepal, Neeraj; Downey, Brian P.; Muller, David A.; Xing, Huili G.; Meyer, David J.; Jena, Debdeep

2018-03-01

Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors—silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor—an electronic gain element—to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance—a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family.

GaN/NbN epitaxial semiconductor/superconductor heterostructures.

PubMed

Yan, Rusen; Khalsa, Guru; Vishwanath, Suresh; Han, Yimo; Wright, John; Rouvimov, Sergei; Katzer, D Scott; Nepal, Neeraj; Downey, Brian P; Muller, David A; Xing, Huili G; Meyer, David J; Jena, Debdeep

2018-03-07

Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors-silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor-an electronic gain element-to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance-a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family.

Investigating the Non-Covalent Functionalization and Chemical Transformation of Graphene

NASA Astrophysics Data System (ADS)

Sham, Chun-Hong

Trend in device miniatures demands capabilities to produce rationally designed patterns in ever-shrinking length scale. The research community has examined various techniques to push the current lithography resolution to sub-10nm scale. One of the ideas is to utilize the natural nanoscale patterns of molecular assemblies. In this thesis, the self-assembling phenomenon of a photoactive molecule on epitaxial graphene (EG) grown on SiC was discussed. This molecular assembly enables manipulation of chemical contrast in nanoscale through UV exposure or atomic layer deposition. Future development of nanoelectronics industry will be fueled by innovations in electronics materials, which could be discovered through covalent modification of graphene. In a study reported in this thesis, silicon is deposited onto EG. After annealing, a new surface reconstruction, identified to be (3x3)-SiC, was formed. Raman spectroscopy finds no signature of graphene after annealing, indicating a complete chemical transformation of graphene. DFT calculations reveal a possible conversion mechanism. Overall, these studies provide insights for future device miniaturization; contribute to the search of novel materials and help bridging the gap between graphene and current silicon-based industrial infrastructures.

Vertically grown nanowire crystals of dibenzotetrathienocoronene (DBTTC) on large-area graphene

DOE PAGES

Kim, B.; Chiu, C. -Y.; Kang, S. J.; ...

2016-06-01

Here we demonstrate controlled growth of vertical organic crystal nanowires on single layer graphene. Using Scanning Electron Microscopy (SEM), high-resolution transition electron microscopy (TEM), and Grazing Incidence X-ray Diffraction (GIXD), we probe the microstructure and morphology of dibenzotetrathienocoronene (DBTTC) nanowires epitaxially grown on graphene. The investigation is performed at both the ensemble and single nanowire level, and as function of growth parameters, providing insight of and control over the formation mechanism. Finally, the size, density and height of the nanowires can be tuned via growth conditions, opening new avenues for tailoring three-dimensional (3-D) nanostructured architectures for organic electronics with improvedmore » functional performance.« less

Molecular beam epitaxial growth, transmittance and photoluminescence spectra of zinc-blende CdTe thin films with high-quality on perovskite SrTiO3 (1 1 1) substrates

NASA Astrophysics Data System (ADS)

Song, Kun; Zhu, Xuanting; Tang, Kai; Bai, W.; Zhu, Liangqing; Yang, Jing; Zhang, Yuanyuan; Tang, Xiaodong; Chu, Junhao

2018-03-01

High-crystalline quality CdTe thin films are grown on the largely lattice-mismatched SrTiO3 (STO) (1 1 1) substrates by molecular beam epitaxy. A transformation from a three dimensional regime to a two dimensional one is observed by the reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM). The formation of an elastic deformation CdTe layer on STO (1 1 1), namely a pseudomorphic growth mode with a critical thickness of ∼40 nm, is supported by the RHEED, AFM and X-ray diffraction. Crystal structures and epitaxial relationships of CdTe epitaxial films on STO (1 1 1) are characterized by 2θ-ω scans and reciprocal space mapping. Two strong absorption peaks at the energies of ∼1.621 eV and ∼1.597 eV at 5 K are clearly observed for a ∼120 nm thick CdTe epitaxial film, which are proposed to be ascribed to the strained and unstrained epitaxial CdTe layers, respectively. Moreover, the presence of the exciton band while the absence of deep level defect states for the ∼120 nm thick CdTe film characterized by the temperature dependent photoluminescence spectra further supports the high-crystalline quality.

Wafer-Scale and Wrinkle-Free Epitaxial Growth of Single-Orientated Multilayer Hexagonal Boron Nitride on Sapphire.

PubMed

Jang, A-Rang; Hong, Seokmo; Hyun, Chohee; Yoon, Seong In; Kim, Gwangwoo; Jeong, Hu Young; Shin, Tae Joo; Park, Sung O; Wong, Kester; Kwak, Sang Kyu; Park, Noejung; Yu, Kwangnam; Choi, Eunjip; Mishchenko, Artem; Withers, Freddie; Novoselov, Kostya S; Lim, Hyunseob; Shin, Hyeon Suk

2016-05-11

Large-scale growth of high-quality hexagonal boron nitride has been a challenge in two-dimensional-material-based electronics. Herein, we present wafer-scale and wrinkle-free epitaxial growth of multilayer hexagonal boron nitride on a sapphire substrate by using high-temperature and low-pressure chemical vapor deposition. Microscopic and spectroscopic investigations and theoretical calculations reveal that synthesized hexagonal boron nitride has a single rotational orientation with AA' stacking order. A facile method for transferring hexagonal boron nitride onto other target substrates was developed, which provides the opportunity for using hexagonal boron nitride as a substrate in practical electronic circuits. A graphene field effect transistor fabricated on our hexagonal boron nitride sheets shows clear quantum oscillation and highly improved carrier mobility because the ultraflatness of the hexagonal boron nitride surface can reduce the substrate-induced degradation of the carrier mobility of two-dimensional materials.

Monolayer graphene/SiC Schottky barrier diodes with improved barrier height uniformity as a sensing platform for the detection of heavy metals

PubMed Central

Eriksson, Jens; Khranovskyy, Volodymyr; Iakimov, Tihomir; Lloyd Spetz, Anita; Yakimova, Rositsa

2016-01-01

A vertical diode structure comprising homogeneous monolayer epitaxial graphene on silicon carbide is fabricated by thermal decomposition of a Si-face 4H-SiC wafer in argon atmosphere. Current–voltage characteristics of the graphene/SiC Schottky junction were analyzed by applying the thermionic-emission theory. Extracted values of the Schottky barrier height and the ideality factor are found to be 0.4879 ± 0.013 eV and 1.01803 ± 0.0049, respectively. Deviations of these parameters from average values are smaller than those of previously observed literature data, thereby implying uniformity of the Schottky barrier height over the whole diode area, a stable rectifying behaviour and a good quality of ohmic palladium–graphene contacts. Keeping in mind the strong sensitivity of graphene to analytes we propose the possibility to use the graphene/SiC Schottky diode as a sensing platform for the recognition of toxic heavy metals. Using density functional theory (DFT) calculations we gain insight into the nature of the interaction of cadmium, mercury and lead with graphene as well as estimate the work function and the Schottky barrier height of the graphene/SiC structure before and after applying heavy metals to the sensing material. A shift of the I–V characteristics of the graphene/SiC-based sensor has been proposed as an indicator of presence of the heavy metals. Since the calculations suggested the strongest charge transfer between Pb and graphene, the proposed sensing platform was characterized by good selectivity towards lead atoms and slight interferences from cadmium and mercury. The dependence of the sensitivity parameters on the concentration of Cd, Hg and Pb is studied and discussed. PMID:28144530

Monolayer graphene/SiC Schottky barrier diodes with improved barrier height uniformity as a sensing platform for the detection of heavy metals.

PubMed

Shtepliuk, Ivan; Eriksson, Jens; Khranovskyy, Volodymyr; Iakimov, Tihomir; Lloyd Spetz, Anita; Yakimova, Rositsa

2016-01-01

A vertical diode structure comprising homogeneous monolayer epitaxial graphene on silicon carbide is fabricated by thermal decomposition of a Si-face 4H-SiC wafer in argon atmosphere. Current-voltage characteristics of the graphene/SiC Schottky junction were analyzed by applying the thermionic-emission theory. Extracted values of the Schottky barrier height and the ideality factor are found to be 0.4879 ± 0.013 eV and 1.01803 ± 0.0049, respectively. Deviations of these parameters from average values are smaller than those of previously observed literature data, thereby implying uniformity of the Schottky barrier height over the whole diode area, a stable rectifying behaviour and a good quality of ohmic palladium-graphene contacts. Keeping in mind the strong sensitivity of graphene to analytes we propose the possibility to use the graphene/SiC Schottky diode as a sensing platform for the recognition of toxic heavy metals. Using density functional theory (DFT) calculations we gain insight into the nature of the interaction of cadmium, mercury and lead with graphene as well as estimate the work function and the Schottky barrier height of the graphene/SiC structure before and after applying heavy metals to the sensing material. A shift of the I - V characteristics of the graphene/SiC-based sensor has been proposed as an indicator of presence of the heavy metals. Since the calculations suggested the strongest charge transfer between Pb and graphene, the proposed sensing platform was characterized by good selectivity towards lead atoms and slight interferences from cadmium and mercury. The dependence of the sensitivity parameters on the concentration of Cd, Hg and Pb is studied and discussed.

Equilibrium chemical vapor deposition growth of Bernal-stacked bilayer graphene.

PubMed

Zhao, Pei; Kim, Sungjin; Chen, Xiao; Einarsson, Erik; Wang, Miao; Song, Yenan; Wang, Hongtao; Chiashi, Shohei; Xiang, Rong; Maruyama, Shigeo

2014-11-25

Using ethanol as the carbon source, self-limiting growth of AB-stacked bilayer graphene (BLG) has been achieved on Cu via an equilibrium chemical vapor deposition (CVD) process. We found that during this alcohol catalytic CVD (ACCVD) a source-gas pressure range exists to break the self-limitation of monolayer graphene on Cu, and at a certain equilibrium state it prefers to form uniform BLG with a high surface coverage of ∼94% and AB-stacking ratio of nearly 100%. More importantly, once the BLG is completed, this growth shows a self-limiting manner, and an extended ethanol flow time does not result in additional layers. We investigate the mechanism of this equilibrium BLG growth using isotopically labeled (13)C-ethanol and selective surface aryl functionalization, and results reveal that during the equilibrium ACCVD process a continuous substitution of graphene flakes occurs to the as-formed graphene and the BLG growth follows a layer-by-layer epitaxy mechanism. These phenomena are significantly in contrast to those observed for previously reported BLG growth using methane as precursor.

Graphene growth with ‘no’ feedstock

NASA Astrophysics Data System (ADS)

Qing, Fangzhu; Jia, Ruitao; Li, Bao-Wen; Liu, Chunlin; Li, Congzhou; Peng, Bo; Deng, Longjiang; Zhang, Wanli; Li, Yanrong; Ruoff, Rodney S.; Li, Xuesong

2017-06-01

Synthesis of graphene by chemical vapor deposition (CVD) from hydrocarbons on Cu foil substrates can yield high quality and large area graphene films. In a typical CVD process, a hydrocarbon in the gas phase is introduced for graphene growth and hydrogen is usually required to achieve high quality graphene. We have found that in a low pressure CVD system equipped with an oil mechanical vacuum pump located downstream, graphene can be grown without deliberate introduction of a carbon feedstock but with only trace amounts of C present in the system, the origin of which we attribute to the vapor of the pump oil. This finding may help to rationalize the differences in graphene growth reported by different research groups. It should also help to gain an in-depth understanding of graphene growth mechanisms with the aim to improve the reproducibility and structure control in graphene synthesis, e.g. the formation of large area single crystal graphene and uniform bilayer graphene.

Interface and interaction of graphene layers on SiC(0001[combining macron]) covered with TiC(111) intercalation.

PubMed

Wang, Lu; Wang, Qiang; Huang, Jianmei; Li, Wei-Qi; Chen, Guang-Hui; Yang, Yanhui

2017-10-11

It is important to understand the interface and interaction between the graphene layer, titanium carbide [TiC(111)] interlayer, and silicon carbide [SiC(0001[combining macron])] substrates in epitaxial growth of graphene on silicon carbide (SiC) substrates. In this study, the fully relaxed interfaces which consist of up to three layers of TiC(111) coatings on the SiC(0001[combining macron]) as well as the graphene layers interactions with these TiC(111)/SiC(0001[combining macron]) were systematically studied using the density functional theory-D2 (DFT-D2) method. The results showed that the two layers of TiC(111) coating with the C/C-terminated interfaces were thermodynamically more favorable than one or three layers of TiC(111) on the SiC(0001[combining macron]). Furthermore, the bonding of the Ti-hollow-site stacked interfaces would be a stronger link than that of the Ti-Fcc-site stacked interfaces. However, the formation of the C/Ti/C and Ti/C interfaces implied that the first upper carbon layer can be formed on TiC(111)/SiC(0001[combining macron]) using the decomposition of the weaker Ti-C and C-Si interfacial bonds. When growing graphene layers on these TiC(111)/SiC(0001[combining macron]) substrates, the results showed that the interaction energy depended not only on the thickness of the TiC(111) interlayer, but also on the number of graphene layers. Bilayer graphene on the two layer thick TiC(111)/SiC(0001[combining macron]) was thermodynamically more favorable than a monolayer or trilayer graphene on these TiC(111)/SiC(0001[combining macron]) substrates. The adsorption energies of the bottom graphene layers with the TiC(111)/SiC(0001[combining macron]) substrates increased with the decrease of the interface vertical distance. The interaction energies between the bottom, second and third layers of graphene on the TiC(111)/SiC(0001[combining macron]) were significantly higher than that of the freestanding graphene layers. All of these findings provided

Thermoelectric La-doped SrTiO3 epitaxial layers with single-crystal quality: from nano to micrometers.

PubMed

Apreutesei, Mihai; Debord, Régis; Bouras, Mohamed; Regreny, Philippe; Botella, Claude; Benamrouche, Aziz; Carretero-Genevrier, Adrian; Gazquez, Jaume; Grenet, Geneviève; Pailhès, Stéphane; Saint-Girons, Guillaume; Bachelet, Romain

2017-01-01

High-quality thermoelectric La 0.2 Sr 0.8 TiO 3 (LSTO) films, with thicknesses ranging from 20 nm to 0.7 μm, have been epitaxially grown on SrTiO 3 (001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 × 10 -4 Ω cm at room temperature), one order of magnitude lower than standard commercial Nb-doped SrTiO 3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of approximately -60 μV/K have been recorded for all films. These LSTO films can be integrated on Si for non-volatile memory structures or opto-microelectronic devices, functioning as transparent conductors or thermoelectric elements.

Thermoelectric La-doped SrTiO3 epitaxial layers with single-crystal quality: from nano to micrometers

NASA Astrophysics Data System (ADS)

Apreutesei, Mihai; Debord, Régis; Bouras, Mohamed; Regreny, Philippe; Botella, Claude; Benamrouche, Aziz; Carretero-Genevrier, Adrian; Gazquez, Jaume; Grenet, Geneviève; Pailhès, Stéphane; Saint-Girons, Guillaume; Bachelet, Romain

2017-12-01

High-quality thermoelectric La0.2Sr0.8TiO3 (LSTO) films, with thicknesses ranging from 20 nm to 0.7 μm, have been epitaxially grown on SrTiO3(001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 × 10-4 Ω cm at room temperature), one order of magnitude lower than standard commercial Nb-doped SrTiO3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of approximately -60 μV/K have been recorded for all films. These LSTO films can be integrated on Si for non-volatile memory structures or opto-microelectronic devices, functioning as transparent conductors or thermoelectric elements.

Copper intercalation at the interface of graphene and Ir(111) studied by scanning tunneling microscopy

NASA Astrophysics Data System (ADS)

Sicot, M.; Fagot-Revurat, Y.; Kierren, B.; Vasseur, G.; Malterre, D.

2014-11-01

We report on the intercalation of a submonolayer of copper at 775 K underneath graphene epitaxially grown on Ir(111) studied by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) at 77 K. Nucleation and growth dynamics of Cu below graphene have been investigated, and, most importantly, the intercalation mechanism has been identified. First, LEED patterns reveal the pseudomorphic growth of Cu on Ir under the topmost graphene layer resulting in a large Cu in-plane lattice parameter expansion of about 6% compared to Cu(111). Second, large-scale STM topographs as a function of Cu coverage show that Cu diffusion on Ir below graphene exhibits a low energy barrier resulting in Cu accumulation at Ir step edges. As a result, the graphene sheet undergoes a strong edges reshaping. Finally, atomically-resolved STM images reveal a damaged graphene sheet at the atomic scale after metal intercalation. Point defects in graphene were shown to be carbon vacancies. According to these results, a Cu penetration path beneath graphene is proposed to occur via metal aided defect formation with no or poor self healing of the graphene sheet. This work illustrates the fact that Cu intercalation is harmful for graphene grown on Ir(111) at the atomic scale.

Fabrication of high-quality single-crystal Cu thin films using radio-frequency sputtering.

PubMed

Lee, Seunghun; Kim, Ji Young; Lee, Tae-Woo; Kim, Won-Kyung; Kim, Bum-Su; Park, Ji Hun; Bae, Jong-Seong; Cho, Yong Chan; Kim, Jungdae; Oh, Min-Wook; Hwang, Cheol Seong; Jeong, Se-Young

2014-08-29

Copper (Cu) thin films have been widely used as electrodes and interconnection wires in integrated electronic circuits, and more recently as substrates for the synthesis of graphene. However, the ultra-high vacuum processes required for high-quality Cu film fabrication, such as molecular beam epitaxy (MBE), restricts mass production with low cost. In this work, we demonstrated high-quality Cu thin films using a single-crystal Cu target and radio-frequency (RF) sputtering technique; the resulting film quality was comparable to that produced using MBE, even under unfavorable conditions for pure Cu film growth. The Cu thin film was epitaxially grown on an Al2O3 (sapphire) (0001) substrate, and had high crystalline orientation along the (111) direction. Despite the 10(-3) Pa vacuum conditions, the resulting thin film was oxygen free due to the high chemical stability of the sputtered specimen from a single-crystal target; moreover, the deposited film had >5× higher adhesion force than that produced using a polycrystalline target. This fabrication method enabled Cu films to be obtained using a simple, manufacturing-friendly process on a large-area substrate, making our findings relevant for industrial applications.

Vibrational spectra of nanowires measured using laser doppler vibrometry and STM studies of epitaxial graphene : an LDRD fellowship report.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Biedermann, Laura Butler

2009-09-01

MWNTs, 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.« less

Synthesis and characterization of 2D graphene sheets from graphite powder

NASA Astrophysics Data System (ADS)

Patel, Rakesh V.; Patel, R. H.; Chaki, S. H.

2018-05-01

Graphene is 2D material composed of one atom thick hexagonal layer. This material has attracted great attention among scientific community because of its high surface area, excellent mechanical properties and conductivity due to free electrons in the 2D lattice. There are various approaches to prepare graphene nanosheets such as top-down approach where graphite exfoliation and nanotube unwrapping can be done. The bottom up approach involves deposition of hydrocarbon through CVD, epitaxial method and organo-synthesis etc.. In present studies top down approach method was used to prepare graphene. The graphite powder with around 20 µm to 150µm particle size was subjected to concentrated strong acid in presence of strong oxidizing agent in order to increase the d-spacing between layers which leads to the disruption of crystal lattice as confirmed by XRD (X'pert Philips). FT Raman spectra taken via (Renishaw InVia microscope) of pristine powder and Graphene oxide revealed the increase in D-band and reduction in G-Band. These exfoliated sheets have oxygen rich complexes at the surface of the layers as characterised by FTIR technique. The GO powder was ultrasonicated to prepare the stable suspension of Graphene. The graphene layers were observed under TEM (Philips Tecnai 20) as 2dimensional sheets with around 1µm sizes.

Tailoring Graphene Morphology and Orientation on Cu(100), Cu(110), and Cu(111)

NASA Astrophysics Data System (ADS)

Jacobberger, Robert; Arnold, Michael

2013-03-01

Graphene CVD on Cu is phenomenologically complex, yielding diverse crystal morphologies, such as lobes, dendrites, stars, and hexagons, of various orientations. We present a comprehensive study of the evolution of these morphologies as a function of Cu surface orientation, pressure, H2:CH4, and nucleation density. Growth was studied on ultra-smooth, epitaxial Cu films inside Cu enclosures to minimize factors that normally complicate growth. With low H2:CH4, Mullins-Sekerka instabilities propagate to form dendrites, indicating transport limited growth. In LPCVD, the dendrites extend hundreds of microns in the 100, 111, and 110 directions on Cu(100), (110), and (111) and are perturbed by twin boundaries. In APCVD, multiple preferred dendrite orientations exist. With increasing H2:CH4, the dendritic nature of growth is suppressed. In LPCVD, square, rectangle, and hexagon crystals form on Cu(100), (110) and (111), reflecting the Cu crystallography. In APCVD, the morphology becomes hexagonal on each surface. If given ample time, every growth regime yields high-quality monolayers with D:G Raman ratio <0.1. The understanding gained here provides a framework to rationally tailor the graphene crystal morphology and orientation.

Low-temperature plasma-deposited silicon epitaxial films: Growth and properties

DOE Office of Scientific and Technical Information (OSTI.GOV)

Demaurex, Bénédicte, E-mail: benedicte.demaurex@epfl.ch; Bartlome, Richard; Seif, Johannes P.

2014-08-07

Low-temperature (≤200 °C) epitaxial growth yields precise thickness, doping, and thermal-budget control, which enables advanced-design semiconductor devices. In this paper, we use plasma-enhanced chemical vapor deposition to grow homo-epitaxial layers and study the different growth modes on crystalline silicon substrates. In particular, we determine the conditions leading to epitaxial growth in light of a model that depends only on the silane concentration in the plasma and the mean free path length of surface adatoms. For such growth, we show that the presence of a persistent defective interface layer between the crystalline silicon substrate and the epitaxial layer stems not only frommore » the growth conditions but also from unintentional contamination of the reactor. Based on our findings, we determine the plasma conditions to grow high-quality bulk epitaxial films and propose a two-step growth process to obtain device-grade material.« less

Low-temperature plasma-deposited silicon epitaxial films: Growth and properties

DOE PAGES

Demaurex, Bénédicte; Bartlome, Richard; Seif, Johannes P.; ...

2014-08-05

Low-temperature (≤ 180 °C) epitaxial growth yields precise thickness, doping, and thermal-budget control, which enables advanced-design semiconductor devices. In this paper, we use plasma-ehanced chemical vapor deposition to grow homo-epitaxial layers and study the different growth modes on crystalline silicon substrates. In particular, we determine the conditions leading to epitaxial growth in light of a model that depends only on the silane concentration in the plasma and the mean free path length of surface adatoms. For such growth, we show that the presence of a persistent defective interface layer between the crystalline silicon substrate and the epitaxial layer stems notmore » only from the growth conditions but also from unintentional contamination of the reactor. As a result of our findings, we determine the plasma conditions to grow high-quality bulk epitaxial films and propose a two-step growth process to obtain device-grade material.« less

The effect of adsorbates on the electrical stability of graphene studied by transient photocurrent spectroscopy

NASA Astrophysics Data System (ADS)

Kalkan, S. B.; Aydın, H.; Özkendir, D.; ćelebi, C.

2018-01-01

Adsorbate induced variations in the electrical conductivity of graphene layers with two different types of charge carriers are investigated by using the Transient Photocurrent Spectroscopy (TPS) measurement technique. In-vacuum TPS measurements taken for a duration of 5 ks revealed that the adsorption/desorption of atmospheric adsorbates leads to more than a 110% increment and a 45% decrement in the conductivity of epitaxial graphene (n-type) and chemical vapor deposition graphene (p-type) layers on semi-insulating silicon carbide (SiC) substrates, respectively. The graphene layers on SiC are encapsulated and passivated with a thin SiO2 film grown by the Pulsed Electron Deposition method. The measurements conducted for short periods and a few cycles showed that the encapsulation process completely suppresses the time dependent conductivity instability of graphene independent of its charge carrier type. The obtained results are used to construct an experimental model for identifying adsorbate related conductivity variations in graphene and also in other 2D materials with an inherently high surface-to-volume ratio.

Raman spectroscopy of graphene-based materials and its applications in related devices.

PubMed

Wu, Jiang-Bin; Lin, Miao-Ling; Cong, Xin; Liu, He-Nan; Tan, Ping-Heng

2018-03-05

Graphene-based materials exhibit remarkable electronic, optical, and mechanical properties, which has resulted in both high scientific interest and huge potential for a variety of applications. Furthermore, the family of graphene-based materials is growing because of developments in preparation methods. Raman spectroscopy is a versatile tool to identify and characterize the chemical and physical properties of these materials, both at the laboratory and mass-production scale. This technique is so important that most of the papers published concerning these materials contain at least one Raman spectrum. Thus, here, we systematically review the developments in Raman spectroscopy of graphene-based materials from both fundamental research and practical (i.e., device applications) perspectives. We describe the essential Raman scattering processes of the entire first- and second-order modes in intrinsic graphene. Furthermore, the shear, layer-breathing, G and 2D modes of multilayer graphene with different stacking orders are discussed. Techniques to determine the number of graphene layers, to probe resonance Raman spectra of monolayer and multilayer graphenes and to obtain Raman images of graphene-based materials are also presented. The extensive capabilities of Raman spectroscopy for the investigation of the fundamental properties of graphene under external perturbations are described, which have also been extended to other graphene-based materials, such as graphene quantum dots, carbon dots, graphene oxide, nanoribbons, chemical vapor deposition-grown and SiC epitaxially grown graphene flakes, composites, and graphene-based van der Waals heterostructures. These fundamental properties have been used to probe the states, effects, and mechanisms of graphene materials present in the related heterostructures and devices. We hope that this review will be beneficial in all the aspects of graphene investigations, from basic research to material synthesis and device applications.

Thermoelectric La-doped SrTiO3 epitaxial layers with single-crystal quality: from nano to micrometers

PubMed Central

Apreutesei, Mihai; Debord, Régis; Bouras, Mohamed; Regreny, Philippe; Botella, Claude; Benamrouche, Aziz; Carretero-Genevrier, Adrian; Gazquez, Jaume; Grenet, Geneviève; Pailhès, Stéphane; Saint-Girons, Guillaume; Bachelet, Romain

2017-01-01

Abstract High-quality thermoelectric La0.2Sr0.8TiO3 (LSTO) films, with thicknesses ranging from 20 nm to 0.7 μm, have been epitaxially grown on SrTiO3(001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 × 10−4 Ω cm at room temperature), one order of magnitude lower than standard commercial Nb-doped SrTiO3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of approximately –60 μV/K have been recorded for all films. These LSTO films can be integrated on Si for non-volatile memory structures or opto-microelectronic devices, functioning as transparent conductors or thermoelectric elements. PMID:28740558

Magnetite nano-islands on silicon-carbide with graphene

DOE PAGES

Anderson, Nathaniel A.; Zhang, Qiang; Hupalo, Myron; ...

2017-01-05

X-ray magnetic circular dichroism (XMCD) measurements of iron nano-islands grown on graphene and covered with a Au film for passivation reveal that the oxidation through defects in the Au film spontaneously leads to the formation of magnetite nano-particles (i.e, Fe 3O 4). The Fe nano-islands (20 and 75 monolayers; MLs) are grown on epitaxial graphene formed by thermally annealing 6HSiC( 0001) and subsequently covered, in the growth chamber, with nominal 20 layers of Au. Our X-ray absorption spectroscopy and XMCD measurements at applied magnetic fields show that the thin film (20 ML) is totally converted to magnetite whereas the thickermore » lm (75 ML) exhibits properties of magnetite but also those of pure metallic iron. Temperature dependence of the XMCD signal (of both samples) shows a clear transition at T V ≈ 120 K consistent with the Verwey transition of bulk magnetite. These results have implications on the synthesis of magnetite nano-crystals and also on their regular arrangements on functional substrates such as graphene.« less

Magnetite nano-islands on silicon-carbide with graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Anderson, Nathaniel A.; Zhang, Qiang; Hupalo, Myron

X-ray magnetic circular dichroism (XMCD) measurements of iron nano-islands grown on graphene and covered with a Au film for passivation reveal that the oxidation through defects in the Au film spontaneously leads to the formation of magnetite nano-particles (i.e, Fe 3O 4). The Fe nano-islands (20 and 75 monolayers; MLs) are grown on epitaxial graphene formed by thermally annealing 6HSiC( 0001) and subsequently covered, in the growth chamber, with nominal 20 layers of Au. Our X-ray absorption spectroscopy and XMCD measurements at applied magnetic fields show that the thin film (20 ML) is totally converted to magnetite whereas the thickermore » lm (75 ML) exhibits properties of magnetite but also those of pure metallic iron. Temperature dependence of the XMCD signal (of both samples) shows a clear transition at T V ≈ 120 K consistent with the Verwey transition of bulk magnetite. These results have implications on the synthesis of magnetite nano-crystals and also on their regular arrangements on functional substrates such as graphene.« less

Choice of Substrate Material for Epitaxial CdTe Solar Cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, Tao; Kanevce, Ana; Sites, James R.

2015-06-14

Epitaxial CdTe with high quality, low defect density, and high carrier concentration should in principle yield high-efficiency photovoltaic devices. However, insufficient effort has been given to explore the choice of substrate for high-efficiency epitaxial CdTe solar cells. In this paper, we use numerical simulations to investigate three crystalline substrates: silicon (Si), InSb, and CdTe each substrate material are generally discussed.

Sub-bandgap response of graphene/SiC Schottky emitter bipolar phototransistor examined by scanning photocurrent microscopy

NASA Astrophysics Data System (ADS)

Barker, Bobby G., Jr.; Chava, Venkata Surya N.; Daniels, Kevin M.; Chandrashekhar, M. V. S.; Greytak, Andrew B.

2018-01-01

Graphene layers grown epitaxially on SiC substrates are attractive for a variety of sensing and optoelectronic applications because the graphene acts as a transparent, conductive, and chemically responsive layer that is mated to a wide-bandgap semiconductor with large breakdown voltage. Recent advances in control of epitaxial growth and doping of SiC epilayers have increased the range of electronic device architectures that are accessible with this system. In particular, a recently-introduced Schottky-emitter bipolar phototransistor (SEPT) based on an epitaxial graphene (EG) emitter grown on a p-SiC base epilayer has been found to exhibit a maximum common emitter current gain of 113 and a UV responsivity of 7.1 A W-1. The behavior of this device, formed on an n +-SiC substrate that serves as the collector, was attributed to a very large minority carrier injection efficiency at the EG/p-SiC Schottky contact. This large minority carrier injection efficiency is in turn related to the large built-in potential found at a EG/p-SiC Schottky junction. The high performance of this device makes it critically important to analyze the sub bandgap visible response of the device, which provides information on impurity states and polytype inclusions in the crystal. Here, we employ scanning photocurrent microscopy (SPCM) with sub-bandgap light as well as a variety of other techniques to clearly demonstrate a localized response based on the graphene transparent electrode and an approximately 1000-fold difference in responsivity between 365 nm and 444 nm excitation. A stacking fault propagating from the substrate/epilayer interface, assigned as a single layer of the 8H-SiC polytype within the 4H-SiC matrix, is found to locally increase the photocurrent substantially. The discovery of this polytype heterojunction opens the potential for further development of heteropolytype devices based on the SEPT architecture.

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.

Epitaxy of GaN in high aspect ratio nanoscale holes over silicon substrate

NASA Astrophysics Data System (ADS)

Wang, Kejia; Wang, Anqi; Ji, Qingbin; Hu, Xiaodong; Xie, Yahong; Sun, Ying; Cheng, Zhiyuan

2017-12-01

Dislocation filtering in gallium nitride (GaN) by epitaxial growth through patterned nanoscale holes is studied. GaN grown from extremely high aspect ratio holes by metalorganic chemical vapor deposition is examined by transmission electron microscopy and high-resolution transmission electron microscopy. This selective area epitaxial growth method with a reduced epitaxy area and an increased depth to width ratio of holes leads to effective filtering of dislocations within the hole and improves the quality of GaN significantly.

Improved heat dissipation in gallium nitride light-emitting diodes with embedded graphene oxide pattern.

PubMed

Han, Nam; Cuong, Tran Viet; Han, Min; Ryu, Beo Deul; Chandramohan, S; Park, Jong Bae; Kang, Ji Hye; Park, Young-Jae; Ko, Kang Bok; Kim, Hee Yun; Kim, Hyun Kyu; Ryu, Jae Hyoung; Katharria, Y S; Choi, Chel-Jong; Hong, Chang-Hee

2013-01-01

The future of solid-state lighting relies on how the performance parameters will be improved further for developing high-brightness light-emitting diodes. Eventually, heat removal is becoming a crucial issue because the requirement of high brightness necessitates high-operating current densities that would trigger more joule heating. Here we demonstrate that the embedded graphene oxide in a gallium nitride light-emitting diode alleviates the self-heating issues by virtue of its heat-spreading ability and reducing the thermal boundary resistance. The fabrication process involves the generation of scalable graphene oxide microscale patterns on a sapphire substrate, followed by its thermal reduction and epitaxial lateral overgrowth of gallium nitride in a metal-organic chemical vapour deposition system under one-step process. The device with embedded graphene oxide outperforms its conventional counterpart by emitting bright light with relatively low-junction temperature and thermal resistance. This facile strategy may enable integration of large-scale graphene into practical devices for effective heat removal.

Localized in situ polymerization on graphene surfaces for stabilized graphene dispersions.

PubMed

Das, Sriya; Wajid, Ahmed S; Shelburne, John L; Liao, Yen-Chih; Green, Micah J

2011-06-01

We demonstrate a novel in situ polymerization technique to develop localized polymer coatings on the surface of dispersed pristine graphene sheets. Graphene sheets show great promise as strong, conductive fillers in polymer nanocomposites; however, difficulties in dispersion quality and interfacial strength between filler and matrix have been a persistent problem for graphene-based nanocomposites, particularly for pristine graphene. With this in mind, a physisorbed polymer layer is used to stabilize graphene sheets in solution. To create this protective layer, we formed an organic microenvironment around dispersed graphene sheets in surfactant solutions, and created a nylon 6, 10 or nylon 6, 6 coating via interfacial polymerization. Technique lies at the intersection of emulsion and admicellar polymerization; a similar technique was originally developed to protect luminescent properties of carbon nanotubes in solution. These coated graphene dispersions are aggregation-resistant and may be reversibly redispersed in water even after freeze-drying. The coated graphene holds promise for a number of applications, including multifunctional graphene-polymer nanocomposites. © 2011 American Chemical Society

Graphene Growth on Low Carbon Solubility Metals

NASA Astrophysics Data System (ADS)

Wofford, Joseph Monroe

Advances in synthesis are imperative if graphene is to fulfill its scientific and technological potential. Single crystal graphene of is currently available only in the small flakes generated by mechanical exfoliation. Layers of larger size may be grown either by the thermal decomposition of SiC or by chemical vapor deposition on metals. However, as they are currently implemented, these methods yield graphene films of inferior quality. Thus the requirement for wafer-scale, high-quality graphene films remains unmet. This dissertation addresses this issue by examining graphene growth on metal surfaces. Through a survey of the fundamental underlying processes, it provides guidance for improving the quality of the resulting graphene films. Graphene was grown on Cu(100), Cu(111), and Au(111) by physical vapor deposition of elemental C. The nucleation and growth behaviors of graphene were evaluated by low-energy electron microscopy. Graphene tends to nucleate heterogeneously at surface imperfections although it also does so homogeneously on Cu(111) and Au(111). Graphene growing on Cu(100) is governed by the attachment kinetics of C at the propagating crystal front. The resulting angularly dependent growth rate sculpts individual crystals into elongated lobes. In contrast, graphene growth on both Cu(111) and Au(111) is surface diffusion limited. This yields ramified, dendritic graphene islands. Graphene films grown on Cu(100) contain significant rotational disorder. This disorder is partially attributable to the symmetry mismatch between film and substrate. The common symmetry between graphene and Cu(111) contributes to a significant reduction in disorder in films grown on this surface. Most graphene domains occupy a ˜6º arc of orientations. On Au(111) the vast majority of graphene domains are locked into alignment with the substrate surface. The extraordinary extent of their orientational homogeneity is such that the resulting graphene film is a quasi-single crystal

Large area planar stanene epitaxially grown on Ag(1 1 1)

NASA Astrophysics Data System (ADS)

Yuhara, Junji; Fujii, Yuya; Nishino, Kazuki; Isobe, Naoki; Nakatake, Masashi; Xian, Lede; Rubio, Angel; Le Lay, Guy

2018-04-01

Artificial post-graphene elemental 2D materials have received much attention recently. Especially, stanene, the tin analogue of graphene, is expected to be a robust 2D topological insulator, even above room temperature. We have grown epitaxial 2D stanene on a Ag(1 1 1) single crystal template and determined its crystalline structure synergetically by scanning tunneling microscopy, high-resolution synchrotron radiation photoemission spectroscopy, and advanced first principles calculations. From the STM images, we show that stanene forms a nearly planar structure in large domains. A detailed core-level spectroscopy analysis as well as DFT calculations reveal that the stanene sheet lays over an ordered 2D Ag2Sn surface alloy, but not directly on a bulk-terminated Ag(1 1 1) surface. The electronic structure exhibits a characteristic 2D band with parabolic dispersion due to the non-negligible interaction with the underlying surface alloy.

Growth of strontium ruthenate films by hybrid molecular beam epitaxy

DOE PAGES

Marshall, Patrick B.; Kim, Honggyu; Ahadi, Kaveh; ...

2017-09-01

We report on the growth of epitaxial Sr 2RuO 4 films using a hybrid molecular beam epitaxy approach in which a volatile precursor containing RuO 4 is used to supply ruthenium and oxygen. The use of the precursor overcomes a number of issues encountered in traditional molecular beam epitaxy that uses elemental metal sources. Phase-pure, epitaxial thin films of Sr 2RuO 4 are obtained. At high substrate temperatures, growth proceeds in a layer-by-layer mode with intensity oscillations observed in reflection high-energy electron diffraction. Films are of high structural quality, as documented by x-ray diffraction, atomic force microscopy, and transmission electronmore » microscopy. In conclusion, the method should be suitable for the growth of other complex oxides containing ruthenium, opening up opportunities to investigate thin films that host rich exotic ground states.« less

Graphene-based smart materials

NASA Astrophysics Data System (ADS)

Yu, Xiaowen; Cheng, Huhu; Zhang, Miao; Zhao, Yang; Qu, Liangti; Shi, Gaoquan

2017-09-01

The high specific surface area and the excellent mechanical, electrical, optical and thermal properties of graphene make it an attractive component for high-performance stimuli-responsive or 'smart' materials. Complementary to these inherent properties, functionalization or hybridization can substantially improve the performance of these materials. Typical graphene-based smart materials include mechanically exfoliated perfect graphene, chemical vapour deposited high-quality graphene, chemically modified graphene (for example, graphene oxide and reduced graphene oxide) and their macroscopic assemblies or composites. These materials are sensitive to a range of stimuli, including gas molecules or biomolecules, pH value, mechanical strain, electrical field, and thermal or optical excitation. In this Review, we outline different graphene-based smart materials and their potential applications in actuators, chemical or strain sensors, self-healing materials, photothermal therapy and controlled drug delivery. We also introduce the working mechanisms of graphene-based smart materials and discuss the challenges facing the realization of their practical applications.

Large-area growth of multi-layer hexagonal boron nitride on polished cobalt foils by plasma-assisted molecular beam epitaxy

PubMed Central

Xu, Zhongguang; Tian, Hao; Khanaki, Alireza; Zheng, Renjing; Suja, Mohammad; Liu, Jianlin

2017-01-01

Two-dimensional (2D) hexagonal boron nitride (h-BN), which has a similar honeycomb lattice structure to graphene, is promising as a dielectric material for a wide variety of potential applications based on 2D materials. Synthesis of high-quality, large-size and single-crystalline h-BN domains is of vital importance for fundamental research as well as practical applications. In this work, we report the growth of h-BN films on mechanically polished cobalt (Co) foils using plasma-assisted molecular beam epitaxy. Under appropriate growth conditions, the coverage of h-BN layers can be readily controlled by growth time. A large-area, multi-layer h-BN film with a thickness of 5~6 nm is confirmed by Raman spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. In addition, the size of h-BN single domains is 20~100 μm. Dielectric property of as-grown h-BN film is evaluated by characterization of Co(foil)/h-BN/Co(contact) capacitor devices. Breakdown electric field is in the range of 3.0~3.3 MV/cm, which indicates that the epitaxial h-BN film has good insulating characteristics. In addition, the effect of substrate morphology on h-BN growth is discussed regarding different domain density, lateral size, and thickness of the h-BN films grown on unpolished and polished Co foils. PMID:28230178

Process for growing epitaxial gallium nitride and composite wafers

DOEpatents

Weber, Eicke R.; Subramanya, Sudhir G.; Kim, Yihwan; Kruger, Joachim

2003-05-13

A novel growth procedure to grow epitaxial Group III metal nitride thin films on lattice-mismatched substrates is proposed. Demonstrated are the quality improvement of epitaxial GaN layers using a pure metallic Ga buffer layer on c-plane sapphire substrate. X-ray rocking curve results indicate that the layers had excellent structural properties. The electron Hall mobility increases to an outstandingly high value of .mu.>400 cm.sup.2 /Vs for an electron background concentration of 4.times.10.sup.17 cm.sup.-3.

Crystal-phase intergradation in InAs nanostructures grown by van der Waals heteroepitaxy on graphene

NASA Astrophysics Data System (ADS)

Choi, Ji Eun; Yoo, Jinkyoung; Lee, Donghwa; Hong, Young Joon; Fukui, Takashi

2018-04-01

This study demonstrates the crystal-phase intergradation of InAs nanostructures grown on graphene via van der Waals epitaxy. InAs nanostructures with diverse diameters are yielded on graphene. High-resolution transmission electron microscopy (HR-TEM) reveals two crystallographic features of (i) wurtzite (WZ)-to-zinc blende (ZB) intergradation along the growth direction of InAs nanostructures and (ii) an increased mean fraction of ZB according to diameter increment. Based on the HR-TEM observations, a crystal-phase intergradation diagram is depicted. We discuss how the formation of a WZ-rich phase during the initial growth stage is an effective way of releasing heterointerfacial stress endowed by the lattice mismatch of InAs/graphene for energy minimization in terms of less in-plane lattice mismatching between WZ-InAs and graphene. The WZ-to-ZB evolution is responsible for the attenuation of the bottom-to-top surface charge interaction as growth proceeds.

A new direct growth method of graphene on Si-face of 6H-SiC by synergy of the inner and external carbon sources

NASA Astrophysics Data System (ADS)

Yang, Zhiyuan; Xu, Shicai; Zhao, Lili; Zhang, Jing; Wang, Zhengping; Chen, Xiufang; Cheng, Xiufeng; Yu, Fapeng; Zhao, Xian

2018-04-01

Graphene is a promising two-dimensional material that has possible application in various disciplines, due to its super properties, including high carrier mobility, chemical stability, and optical transparency etc. In this paper, we report an inner and external carbon synergy (IECS) method to grow graphene on Si-face of 6H-SiC. This method combined the advantages of chemical vapor deposition (CVD) and traditional epitaxial growth (EG) based on silicon carbide, which providing a feasible approach for growing graphene on the SiC substrates. The graphene was synthesized within just 3 min, which was more than one order of magnitude faster than the graphene grown on 6H-SiC substrates by the traditional EG method. The growth temperature was ∼200 °C lower than the EG process. The directly grown graphene maintained the compatibility with the semiconductor technique, which is benefit for use in graphene-based microelectronic devices.

Interface engineering in epitaxial growth of layered oxides via a conducting layer insertion

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yun, Yu; Meng, Dechao; Wang, Jianlin

2015-07-06

There is a long-standing challenge in the fabrication of layered oxide epitaxial films due to their thermodynamic phase-instability and the large stacking layer number. Recently, the demand for high-quality thin films is strongly pushed by their promising room-temperature multiferroic properties. Here, we find that by inserting a conducting and lattice matched LaNiO{sub 3} buffer layer, high quality m = 5 Bi{sub 6}FeCoTi{sub 3}O{sub 18} epitaxial films can be fabricated using the laser molecular beam epitaxy, in which the atomic-scale sharp interface between the film and the metallic buffer layer explains the enhanced quality. The magnetic and ferroelectric properties of the high qualitymore » Bi{sub 6}FeCoTi{sub 3}O{sub 18} films are studied. This study demonstrates that insertion of the conducting layer is a powerful method in achieving high quality layered oxide thin films, which opens the door to further understand the underline physics and to develop new devices.« less

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Que, Yande; Xiao, Wende, E-mail: wdxiao@iphy.ac.cn, E-mail: hjgao@iphy.ac.cn; Chen, Hui

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- andmore » 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.« less

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

NASA Astrophysics Data System (ADS)

Que, Yande; Xiao, Wende; Chen, Hui; Wang, Dongfei; Du, Shixuan; Gao, Hong-Jun

2015-12-01

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.

Van der Waals Epitaxy of Functional Oxide Heterostructures

NASA Astrophysics Data System (ADS)

Chu, Ying-Hao

In the diligent pursuit of low-power consumption, multifunctional, and environmentally friendly electronics, more sophisticated requirements on functional materials are on demand. Recently, the discovery of 2D layered materials has created a revolution to this field. Pioneered by graphene, these new 2D materials exhibit abundant unusual physical phenomena that is undiscovered in bulk forms. These materials are characterized with their layer form and almost pure 2D electronic behavior. The confinement of charge and heat transport at such ultrathin planes offers possibilities to overcome the bottleneck of present device development in thickness limitation, and thus push the technologies into next generation. Van der Waals epitaxy, an epitaxial growth method to combine 2D and 3D materials, is one of current reliable manufacturing processes to fabricate 2D materials by growing these 2D materials epitaxially on 3D materials. Then, transferring the 2D materials to the substrates for practical applications. In the mean time, van der Waals epitaxy has also been used to create free-standing 3D materials by growing 3D materials on 2D materials and then removing them from 2D materials since the interfacial boding between 2D and 3D materials should be weak van der Waals bonds. In this study, we intend to take the same concept, but to integrate a family of functional materials in order to open new avenue to flexible electronics. Due to the interplay of lattice, charge, orbital, and spin degrees of freedom, correlated electrons in oxides generate a rich spectrum of competing phases and physical properties. Recently, lots of studies have suggested that oxide heterostructures provide a powerful route to create and manipulate the degrees of freedom and offer new possibilities for next generation devices, thus create a new playground for researchers to investigate novel physics and the emergence of fascinating states of condensed matter. In this talk, we use a 2D layered material as

Magneto-optical fingerprints of distinct graphene multilayers using the giant infrared Kerr effect

NASA Astrophysics Data System (ADS)

Ellis, Chase T.; Stier, Andreas V.; Kim, Myoung-Hwan; Tischler, Joseph G.; Glaser, Evan R.; Myers-Ward, Rachael L.; Tedesco, Joseph L.; Eddy, Charles R.; Gaskill, D. Kurt; Cerne, John

2013-11-01

The remarkable electronic properties of graphene strongly depend on the thickness and geometry of graphene stacks. This wide range of electronic tunability is of fundamental interest and has many applications in newly proposed devices. Using the mid-infrared, magneto-optical Kerr effect, we detect and identify over 18 interband cyclotron resonances (CR) that are associated with ABA and ABC stacked multilayers as well as monolayers that coexist in graphene that is epitaxially grown on 4H-SiC. Moreover, the magnetic field and photon energy dependence of these features enable us to explore the band structure, electron-hole band asymmetries, and mechanisms that activate a CR response in the Kerr effect for various multilayers that coexist in a single sample. Surprisingly, we find that the magnitude of monolayer Kerr effect CRs is not temperature dependent. This unexpected result reveals new questions about the underlying physics that makes such an effect possible.

Molecular beam epitaxy of quasi-freestanding transition metal disulphide monolayers on van der Waals substrates: a growth study

NASA Astrophysics Data System (ADS)

Hall, Joshua; Pielić, Borna; Murray, Clifford; Jolie, Wouter; Wekking, Tobias; Busse, Carsten; Kralj, Marko; Michely, Thomas

2018-04-01

Based on an ultra-high vacuum compatible two-step molecular beam epitaxy synthesis with elemental sulphur, we grow clean, well-oriented, and almost defect-free monolayer islands and layers of the transition metal disulphides MoS2, TaS2 and WS2. Using scanning tunneling microscopy and low energy electron diffraction we investigate systematically how to optimise the growth process, and provide insight into the growth and annealing mechanisms. A large band gap of 2.55 eV and the ability to move flakes with the scanning tunneling microscope tip both document the weak interaction of MoS2 with its substrate consisting of graphene grown on Ir(1 1 1). As the method works for the synthesis of a variety of transition metal disulphides on different substrates, we speculate that it could be of great use for providing hitherto unattainable high quality monolayers of transition metal disulphides for fundamental spectroscopic investigations.

Revealing the Crystalline Integrity of Wafer-Scale Graphene on SiO2/Si: An Azimuthal RHEED Approach.

PubMed

Lu, Zonghuan; Sun, Xin; Xiang, Yu; Washington, Morris A; Wang, Gwo-Ching; Lu, Toh-Ming

2017-07-12

The symmetry of graphene is usually determined by a low-energy electron diffraction (LEED) method when the graphene is on the conductive substrates, but LEED cannot handle graphene transferred to SiO 2 /Si substrates due to the charging effect. While transmission electron microscopy can generate electron diffraction on post-transferred graphene, this method is too localized. Herein, we employed an azimuthal reflection high-energy electron diffraction (RHEED) method to construct the reciprocal space mapping and determine the symmetry of wafer-size graphene both pre- and post-transfer. In this work, single-crystalline Cu(111) films were prepared on sapphire(0001) and spinel(111) substrates with sputtering. Then the graphene was epitaxially grown on single-crystalline Cu(111) films with a low pressure chemical vapor deposition. The reciprocal space mapping using azimuthal RHEED confirmed that the graphene grown on Cu(111) films was single-crystalline, no matter the form of the monolayer or multilayer structure. While the Cu(111) film grown on sapphire(0001) may occasionally consist of 60° in-plane rotational twinning, the reciprocal space mapping revealed that the in-plane orientation of graphene grown atop was not affected. The proposed method for checking the crystalline integrity of the post-transferred graphene sheets is an important step in the realization of the graphene as a platform to fabricate electronic and optoelectronic devices.

Commercial aspects of epitaxial thin film growth in outer space

NASA Technical Reports Server (NTRS)

Ignatiev, Alex; Chu, C. W.

1988-01-01

A new concept for materials processing in space exploits the ultra vacuum component of space for thin film epitaxial growth. The unique low earth orbit space environment is expected to yield 10 to the -14th torr or better pressures, semiinfinite pumping speeds and large ultra vacuum volume (about 100 cu m) without walls. These space ultra vacuum properties promise major improvement in the quality, unique nature, and the throughput of epitaxially grown materials especially in the area of semiconductors for microelectronics use. For such thin film materials there is expected a very large value added from space ultra vacuum processing, and as a result the application of the epitaxial thin film growth technology to space could lead to major commercial efforts in space.

Combining graphene with silicon carbide: synthesis and properties - a review

NASA Astrophysics Data System (ADS)

Shtepliuk, Ivan; Khranovskyy, Volodymyr; Yakimova, Rositsa

2016-11-01

Being a true two-dimensional crystal, graphene possesses a lot of exotic properties that would enable unique applications. Integration of graphene with inorganic semiconductors, e.g. silicon carbide (SiC) promotes the birth of a class of hybrid materials which are highly promising for development of novel operations, since they combine the best properties of two counterparts in the frame of one hybrid platform. As a specific heterostructure, graphene on SiC performs strongly, dependent on the synthesis method and the growth modes. In this article, a comprehensive review of the most relevant studies of graphene growth methods and mechanisms on SiC substrates has been carried out. The aim is to elucidate the basic physical processes that are responsible for the formation of graphene on SiC. First, an introduction is made covering some intriguing and not so often discussed properties of graphene. Then, we focus on integration of graphene with SiC, which is facilitated by the nature of SiC to assume graphitization. Concerning the synthesis methods, we discuss thermal decomposition of SiC, chemical vapor deposition and molecular beam epitaxy, stressing that the first technique is the most common one when SiC substrates are used. In addition, we briefly appraise graphene synthesis via metal mediated carbon segregation. We address in detail the main aspects of the substrate effect, such as substrate face polarity, off-cut, kind of polytype and nonpolar surfaces on the growth of graphene layers. A comparison of graphene grown on the polar faces is made. In particular, growth of graphene on Si-face SiC is critically analyzed concerning growth kinetics and growth mechanisms taking into account the specific characteristics of SiC (0001) surfaces, such as the step-terrace structure and the unavoidable surface reconstruction upon heating. In all subtopics obstacles and solutions are featured. We complete the review with a short summary and concluding remarks.

Frictional behavior of atomically thin sheets: hexagonal-shaped graphene islands grown on copper by chemical vapor deposition.

PubMed

Egberts, Philip; Han, Gang Hee; Liu, Xin Z; Johnson, A T Charlie; Carpick, Robert W

2014-05-27

Single asperity friction experiments using atomic force microscopy (AFM) have been conducted on chemical vapor deposited (CVD) graphene grown on polycrystalline copper foils. Graphene substantially lowers the friction force experienced by the sliding asperity of a silicon AFM tip compared to the surrounding oxidized copper surface by a factor ranging from 1.5 to 7 over loads from the adhesive minimum up to 80 nN. No damage to the graphene was observed over this range, showing that friction force microscopy serves as a facile, high contrast probe for identifying the presence of graphene on Cu. Consistent with studies of epitaxially grown, thermally grown, and mechanically exfoliated graphene films, the friction force measured between the tip and these CVD-prepared films depends on the number of layers of graphene present on the surface and reduces friction in comparison to the substrate. Friction results on graphene indicate that the layer-dependent friction properties result from puckering of the graphene sheet around the sliding tip. Substantial hysteresis in the normal force dependence of friction is observed with repeated scanning without breaking contact with a graphene-covered region. Because of the hysteresis, friction measured on graphene changes with time and maximum applied force, unless the tip slides over the edge of the graphene island or contact with the surface is broken. These results also indicate that relatively weak binding forces exist between the copper foil and these CVD-grown graphene sheets.

Catalyst-Free Growth of Three-Dimensional Graphene Flakes and Graphene/g-C₃N₄ Composite for Hydrocarbon Oxidation.

PubMed

Chen, Ke; Chai, Zhigang; Li, Cong; Shi, Liurong; Liu, Mengxi; Xie, Qin; Zhang, Yanfeng; Xu, Dongsheng; Manivannan, Ayyakkannu; Liu, Zhongfan

2016-03-22

Mass production of high-quality graphene flakes is important for commercial applications. Graphene microsheets have been produced on an industrial scale by chemical and liquid-phase exfoliation of graphite. However, strong-interaction-induced interlayer aggregation usually leads to the degradation of their intrinsic properties. Moreover, the crystallinity or layer-thickness controllability is not so perfect to fulfill the requirement for advanced technologies. Herein, we report a quartz-powder-derived chemical vapor deposition growth of three-dimensional (3D) high-quality graphene flakes and demonstrate the fabrication and application of graphene/g-C3N4 composites. The graphene flakes obtained after the removal of growth substrates exhibit the 3D curved microstructure, controllable layer thickness, good crystallinity, as well as weak interlayer interactions suitable for preventing the interlayer stacking. Benefiting from this, we achieved the direct synthesis of g-C3N4 on purified graphene flakes to form the uniform graphene/g-C3N4 composite, which provides efficient electron transfer interfaces to boost its catalytic oxidation activity of cycloalkane with relatively high yield, good selectivity, and reliable stability.

Investigation of tip sonication effects on structural quality of graphene nanoplatelets (GNPs) for superior solvent dispersion.

PubMed

Baig, Zeeshan; Mamat, Othman; Mustapha, Mazli; Mumtaz, Asad; Munir, Khurram S; Sarfraz, Mansoor

2018-07-01

The exceptional properties of graphene and its structural uniqueness can improve the performance of nanocomposites if it can attain the uniform dispersion. Tip sonication assisted graphene solvent dispersion has been emerged as an efficient approach but it can cause significant degradation of graphene structure. This study aimed to evaluate the parametric influence of tip sonication on the characteristics of sp 2 carbon structure in graphene nanoplatelets by varying the sonication time and respective energy at three different amplitudes (60%, 80% and 100%). The study is essential to identify appropriate parameters so as to achieve high-quality and defect-free graphene with a highly desirable aspect ratio after solvent dispersion for composite reinforcement. Quantitative approach via Raman spectroscopy is used to find the defect ratio and lateral size of graphene evolved under the effect of tip sonication parameters. Results imply that the defect ratio is steady and increases continually with GNPs, along with the transformation to the nano-crystalline stage I up to 60 min sonication at all amplitudes. Exfoliation was clearly observed at all amplitudes together with sheet re-stacking due to considerable size reduction of sheets with large quantity. Finally, considerable GNPs fragmentation occurred during sonication with increased amplitude and time as confirmed by the reduction of sp 2 domain (La) and flake size. This also validates the formation of edge-type defect in graphene. Convincingly, lower amplitude and time (up to 60 min) produce better results for a low defect content and larger particle size as quantified by Raman analysis. Copyright © 2018 Elsevier B.V. All rights reserved.

Graphene as a Buffer Layer for Silicon Carbide-on-Insulator Structures

PubMed Central

Astuti, Budi; Tanikawa, Masahiro; Rahman, Shaharin Fadzli Abd; Yasui, Kanji; Hashim, Abdul Manaf

2012-01-01

We report an innovative technique for growing the silicon carbide-on-insulator (SiCOI) structure by utilizing polycrystalline single layer graphene (SLG) as a buffer layer. The epitaxial growth was carried out using a hot-mesh chemical vapor deposition (HM-CVD) technique. Cubic SiC (3C-SiC) thin film in (111) domain was realized at relatively low substrate temperature of 750 °C. 3C-SiC energy bandgap of 2.2 eV was confirmed. The Si-O absorption band observed in the grown film can be caused by the out-diffusion of the oxygen atom from SiO2 substrate or oxygen doping during the cleaning process. Further experimental works by optimizing the cleaning process, growth parameters of the present growth method, or by using other growth methods, as well, are expected to realize a high quality SiCOI structure, thereby opening up the way for a breakthrough in the development of advanced ULSIs with multifunctionalities.

Enhanced magnetic and thermoelectric properties in epitaxial polycrystalline SrRuO3 thin films.

PubMed

Woo, Sungmin; Lee, Sang A; Mun, Hyeona; Choi, Young Gwan; Zhung, Chan June; Shin, Soohyeon; Lacotte, Morgane; David, Adrian; Prellier, Wilfrid; Park, Tuson; Kang, Won Nam; Lee, Jong Seok; Kim, Sung Wng; Choi, Woo Seok

2018-03-01

Transition metal oxide thin films show versatile electric, magnetic, and thermal properties which can be tailored by deliberately introducing macroscopic grain boundaries via polycrystalline solids. In this study, we focus on the modification of magnetic and thermal transport properties by fabricating single- and polycrystalline epitaxial SrRuO 3 thin films using pulsed laser epitaxy. Using the epitaxial stabilization technique with an atomically flat polycrystalline SrTiO 3 substrate, an epitaxial polycrystalline SrRuO 3 thin film with the crystalline quality of each grain comparable to that of its single-crystalline counterpart is realized. In particular, alleviated compressive strain near the grain boundaries due to coalescence is evidenced structurally, which induced the enhancement of ferromagnetic ordering of the polycrystalline epitaxial thin film. The structural variations associated with the grain boundaries further reduce the thermal conductivity without deteriorating the electronic transport, and lead to an enhanced thermoelectric efficiency in the epitaxial polycrystalline thin films, compared with their single-crystalline counterpart.

Electronic Structure, Surface Doping, and Optical Response in Epitaxial WSe2 Thin Films.

PubMed

Zhang, Yi; Ugeda, Miguel M; Jin, Chenhao; Shi, Su-Fei; Bradley, Aaron J; Martín-Recio, Ana; Ryu, Hyejin; Kim, Jonghwan; Tang, Shujie; Kim, Yeongkwan; Zhou, Bo; Hwang, Choongyu; Chen, Yulin; Wang, Feng; Crommie, Michael F; Hussain, Zahid; Shen, Zhi-Xun; Mo, Sung-Kwan

2016-04-13

High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct-indirect band gap crossover to trilayer WSe2. In the monolayer limit, WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Finally, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.

Epitaxial growth and characterization of CuGa2O4 films by laser molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Wei, Hongling; Chen, Zhengwei; Wu, Zhenping; Cui, Wei; Huang, Yuanqi; Tang, Weihua

2017-11-01

Ga2O3 with a wide bandgap of ˜ 4.9 eV can crystalize in five crystalline phases. Among those phases, the most stable monoclinic β-Ga2O3 has been studied most, however, it is hard to find materials lattice matching with β-Ga2O3 to grown epitaxial thin films for optoelectronic applications. In this work, CuGa2O4 bulk were prepared by solid state reaction as target, and the films were deposited on sapphire substrates by laser molecular beam epitaxy (L-MBE) at different substrate temperatures. The influences of substrate temperature on structural and optical properties have been systematically investigated by means of X-ray diffraction, Transmission electron microscope and UV-vis absorption spectra. High quality cubic structure and [111] oriented CuGa2O4 film can be obtained at substrate temperature of 750 °C. It's also demonstrated that the CuGa2O4 film has a bandgap of ˜ 4.4 eV and a best crystal quality at 750 °C, suggesting that CuGa2O4 film is a promising candidate for applications in ultraviolet optoelectronic devices.

Promising applications of graphene and graphene-based nanostructures

NASA Astrophysics Data System (ADS)

Nguyen, Bich Ha; Hieu Nguyen, Van

2016-06-01

The present article is a review of research works on promising applications of graphene and graphene-based nanostructures. It contains five main scientific subjects. The first one is the research on graphene-based transparent and flexible conductive films for displays and electrodes: efficient method ensuring uniform and controllable deposition of reduced graphene oxide thin films over large areas, large-scale pattern growth of graphene films for stretchble transparent electrodes, utilization of graphene-based transparent conducting films and graphene oxide-based ones in many photonic and optoelectronic devices and equipments such as the window electrodes of inorganic, organic and dye-sensitized solar cells, organic light-emitting diodes, light-emitting electrochemical cells, touch screens, flexible smart windows, graphene-based saturated absorbers in laser cavities for ultrafast generations, graphene-based flexible, transparent heaters in automobile defogging/deicing systems, heatable smart windows, graphene electrodes for high-performance organic field-effect transistors, flexible and transparent acoustic actuators and nanogenerators etc. The second scientific subject is the research on conductive inks for printed electronics to revolutionize the electronic industry by producing cost-effective electronic circuits and sensors in very large quantities: preparing high mobility printable semiconductors, low sintering temperature conducting inks, graphene-based ink by liquid phase exfoliation of graphite in organic solutions, and developing inkjet printing technique for mass production of high-quality graphene patterns with high resolution and for fabricating a variety of good-performance electronic devices, including transparent conductors, embedded resistors, thin-film transistors and micro supercapacitors. The third scientific subject is the research on graphene-based separation membranes: molecular dynamics simulation study on the mechanisms of the transport of

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

NASA Astrophysics Data System (ADS)

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

2017-02-01

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.

Electron-beam induced nano-etching of suspended graphene

PubMed Central

Sommer, Benedikt; Sonntag, Jens; Ganczarczyk, Arkadius; Braam, Daniel; Prinz, Günther; Lorke, Axel; Geller, Martin

2015-01-01

Besides its interesting physical properties, graphene as a two-dimensional lattice of carbon atoms promises to realize devices with exceptional electronic properties, where freely suspended graphene without contact to any substrate is the ultimate, truly two-dimensional system. The practical realization of nano-devices from suspended graphene, however, relies heavily on finding a structuring method which is minimally invasive. Here, we report on the first electron beam-induced nano-etching of suspended graphene and demonstrate high-resolution etching down to ~7 nm for line-cuts into the monolayer graphene. We investigate the structural quality of the etched graphene layer using two-dimensional (2D) Raman maps and demonstrate its high electronic quality in a nano-device: A 25 nm-wide suspended graphene nanoribbon (GNR) that shows a transport gap with a corresponding energy of ~60 meV. This is an important step towards fast and reliable patterning of suspended graphene for future ballistic transport, nano-electronic and nano-mechanical devices. PMID:25586495

Exploring Ag(111) Substrate for Epitaxially Growing Monolayer Stanene: A First-Principles Study

PubMed Central

Gao, Junfeng; Zhang, Gang; Zhang, Yong-Wei

2016-01-01

Stanene, a two-dimensional topological insulator composed of Sn atoms in a hexagonal lattice, is a promising contender to Si in nanoelectronics. Currently it is still a significant challenge to achieve large-area, high-quality monolayer stanene. We explore the potential of Ag(111) surface as an ideal substrate for the epitaxial growth of monolayer stanene. Using first-principles calculations, we study the stability of the structure of stanene in different epitaxial relations with respect to Ag(111) surface, and also the diffusion behavior of Sn adatom on Ag(111) surface. Our study reveals that: (1) the hexagonal structure of stanene monolayer is well reserved on Ag(111) surface; (2) the height of epitaxial stanene monolayer is comparable to the step height of the substrate, enabling the growth to cross the surface step and achieve a large-area stanene; (3) the perfect lattice structure of free-standing stanene can be achieved once the epitaxial stanene monolayer is detached from Ag(111) surface; and finally (4) the diffusion barrier of Sn adatom on Ag(111) surface is found to be only 0.041 eV, allowing the epitaxial growth of stanene monolayer even at low temperatures. Our above revelations strongly suggest that Ag(111) surface is an ideal candidate for growing large-area, high-quality monolayer stanene. PMID:27373464

The Assembling of Poly (3-Octyl-Thiophene) on CVD Grown Single Layer Graphene

NASA Astrophysics Data System (ADS)

Jiang, Yanqiu; Yang, Ling; Guo, Zongxia; Lei, Shengbin

2015-12-01

The interface between organic semiconductor and graphene electrode, especially the structure of the first few molecular layers at the interface, is crucial for the device properties such as the charge transport in organic field effect transistors. In this work, we have used scanning tunneling microscopy to investigate the poly (3-octyl-thiophene) (P3OT)-graphene interface. Our results reveal the dynamic assembling of P3OT on single layer graphene. As on other substrates the epitaxial effect plays a role in determining the orientation of the P3OT assembling, however, the inter-thiophene distance along the backbone is consistent with that optimized in vaccum, no compression was observed. Adsorption of P3OT on ripples is weaker due to local curvature, which has been verified both by scanning tunneling microscopy and density functional theory simulation. Scanning tunneling microscopy also reveals that P3OT tends to form hairpin folds when meets a ripple.

Nucleation of graphene layers on magnetic oxides: Co 3O 4(111) and Cr 2O 3(0001) from theory and experiment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Beatty, John; Cheng, Tao; Cao, Yuan

We report directly grown strongly adherent graphene on Co 3O 4(111) by carbon molecular beam epitaxy (C MBE) at 850 K and density functional theory (DFT) findings that the first graphene layer is reconstructed to fit the Co 3O 4 surface, while subsequent layers retain normal graphene structure. This adherence to the Co 3O 4 structure results from partial bonding of half the carbons to top oxygens of the substrate. This structure is validated by X-ray photoelectron spectroscopy and low-energy electron diffraction studies, showing layer-by-layer graphene growth with ~0.08 electrons/carbon atom transferred to the oxide from the first graphene layer,more » in agreement with DFT. In contrast, for Cr 2O 3 DFT finds no strong bonding to the surface and C MBE on Cr 2O 3(0001) yields only graphite formation at 700 K, with C desorption above 800 K. As a result, strong graphene-to-oxide charge transfer aids nucleation of graphene on incommensurate oxide substrates and may have implications for spintronics.« less

Nucleation of graphene layers on magnetic oxides: Co 3O 4(111) and Cr 2O 3(0001) from theory and experiment

DOE PAGES

Beatty, John; Cheng, Tao; Cao, Yuan; ...

2016-12-14

We report directly grown strongly adherent graphene on Co 3O 4(111) by carbon molecular beam epitaxy (C MBE) at 850 K and density functional theory (DFT) findings that the first graphene layer is reconstructed to fit the Co 3O 4 surface, while subsequent layers retain normal graphene structure. This adherence to the Co 3O 4 structure results from partial bonding of half the carbons to top oxygens of the substrate. This structure is validated by X-ray photoelectron spectroscopy and low-energy electron diffraction studies, showing layer-by-layer graphene growth with ~0.08 electrons/carbon atom transferred to the oxide from the first graphene layer,more » in agreement with DFT. In contrast, for Cr 2O 3 DFT finds no strong bonding to the surface and C MBE on Cr 2O 3(0001) yields only graphite formation at 700 K, with C desorption above 800 K. As a result, strong graphene-to-oxide charge transfer aids nucleation of graphene on incommensurate oxide substrates and may have implications for spintronics.« less

A platform for large-scale graphene electronics--CVD growth of single-layer graphene on CVD-grown hexagonal boron nitride.

PubMed

Wang, Min; Jang, Sung Kyu; Jang, Won-Jun; Kim, Minwoo; Park, Seong-Yong; Kim, Sang-Woo; Kahng, Se-Jong; Choi, Jae-Young; Ruoff, Rodney S; Song, Young Jae; Lee, Sungjoo

2013-05-21

Direct chemical vapor deposition (CVD) growth of single-layer graphene on CVD-grown hexagonal boron nitride (h-BN) film can suggest a large-scale and high-quality graphene/h-BN film hybrid structure with a defect-free interface. This sequentially grown graphene/h-BN film shows better electronic properties than that of graphene/SiO2 or graphene transferred on h-BN film, and suggests a new promising template for graphene device fabrication. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

One-step electrochemical synthesis of nitrogen and sulfur co-doped, high-quality graphene oxide.

PubMed

Parvez, Khaled; Rincón, Rosalba A; Weber, Nils-Eike; Cha, Kitty C; Venkataraman, Shyam S

2016-04-28

High-quality graphene oxide (GO) with high crystallinity and electrical conductivity as well as in situ doped with nitrogen and sulfur is obtained via the electrochemical exfoliation of graphite. Furthermore, iron incorporated GO sheets show promising catalytic activity and stable methanol tolerance durability when used as electrocatalysts for the oxygen reduction reaction.

In vacancies in InN grown by plasma-assisted molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Reurings, Floris; Tuomisto, Filip; Gallinat, Chad S.; Koblmüller, Gregor; Speck, James S.

2010-12-01

The authors have applied positron annihilation spectroscopy to study the effect of different growth conditions on vacancy formation in In- and N-polar InN grown by plasma-assisted molecular beam epitaxy. The results suggest that the structural quality of the material and limited diffusion of surface adatoms during growth dictate the In vacancy formation in low electron-density undoped epitaxial InN, while growth conditions and thermodynamics have a less important role, contrary to what is observed in, e.g., GaN. Furthermore, the results imply that in high quality InN, the electron mobility is likely limited not by ionized point defect scattering, but rather by threading dislocations.

Graphene-Molybdenum Disulfide-Graphene Tunneling Junctions with Large-Area Synthesized Materials.

PubMed

Joiner, Corey A; Campbell, Philip M; Tarasov, Alexey A; Beatty, Brian R; Perini, Chris J; Tsai, Meng-Yen; Ready, William J; Vogel, Eric M

2016-04-06

Tunneling devices based on vertical heterostructures of graphene and other 2D materials can overcome the low on-off ratios typically observed in planar graphene field-effect transistors. This study addresses the impact of processing conditions on two-dimensional materials in a fully integrated heterostructure device fabrication process. In this paper, graphene-molybdenum disulfide-graphene tunneling heterostructures were fabricated using only large-area synthesized materials, unlike previous studies that used small exfoliated flakes. The MoS2 tunneling barrier is either synthesized on a sacrificial substrate and transferred to the bottom-layer graphene or synthesized directly on CVD graphene. The presence of graphene was shown to have no impact on the quality of the grown MoS2. The thickness uniformity of MoS2 grown on graphene and SiO2 was found to be 1.8 ± 0.22 nm. XPS and Raman spectroscopy are used to show how the MoS2 synthesis process introduces defects into the graphene structure by incorporating sulfur into the graphene. The incorporation of sulfur was shown to be greatly reduced in the absence of molybdenum suggesting molybdenum acts as a catalyst for sulfur incorporation. Tunneling simulations based on the Bardeen transfer Hamiltonian were performed and compared to the experimental tunneling results. The simulations show the use of MoS2 as a tunneling barrier suppresses contributions to the tunneling current from the conduction band. This is a result of the observed reduction of electron conduction within the graphene sheets.

Microstructure fabrication process induced modulations in CVD graphene

NASA Astrophysics Data System (ADS)

Matsubayashi, Akitomo; Zhang, Zhenjun; Lee, Ji Ung; LaBella, Vincent P.

2014-12-01

The systematic Raman spectroscopic study of a "mimicked" graphene device fabrication is presented. Upon photoresist baking, compressive stress is induced in the graphene which disappears after it is removed. The indirect irradiation from the electron beam (through the photoresist) does not significantly alter graphene characteristic Raman peaks indicating that graphene quality is preserved upon the exposure. The 2D peak shifts and the intensity ratio of 2D and G band, I(2D)/I(G), decreases upon direct metal deposition (Co and Py) suggesting that the electronic modulation occurs due to sp2 C-C bond weakening. In contrast, a thin metal oxide film deposited graphene does not show either the significant 2D and G peaks shift or I(2D)/I(G) decrease upon the metal deposition suggesting the oxide protect the graphene quality in the fabrication process.

Graphene-Si heterogeneous nanotechnology

NASA Astrophysics Data System (ADS)

Akinwande, Deji; Tao, Li

2013-05-01

It is widely envisioned that graphene, an atomic sheet of carbon that has generated very broad interest has the largest prospects for flexible smart systems and for integrated graphene-silicon (G-Si) heterogeneous very large-scale integrated (VLSI) nanoelectronics. In this work, we focus on the latter and elucidate the research progress that has been achieved for integration of graphene with Si-CMOS including: wafer-scale graphene growth by chemical vapor deposition on Cu/SiO2/Si substrates, wafer-scale graphene transfer that afforded the fabrication of over 10,000 devices, wafer-scalable mitigation strategies to restore graphene's device characteristics via fluoropolymer interaction, and demonstrations of graphene integrated with commercial Si- CMOS chips for hybrid nanoelectronics and sensors. Metrology at the wafer-scale has led to the development of custom Raman processing software (GRISP) now available on the nanohub portal. The metrology reveals that graphene grown on 4-in substrates have monolayer quality comparable to exfoliated flakes. At room temperature, the high-performance passivated graphene devices on SiO2/Si can afford average mobilities 3000cm2/V-s and gate modulation that exceeds an order of magnitude. The latest growth research has yielded graphene with high mobilities greater than 10,000cm2/V-s on oxidized silicon. Further progress requires track compatible graphene-Si integration via wafer bonding in order to translate graphene research from basic to applied research in commercial R and D laboratories to ultimately yield a viable nanotechnology.

Chemical Vapor Deposition Synthesis of Graphene-Based Materials and Chemical Modulation of Graphene Electronics

NASA Astrophysics Data System (ADS)

Yan, Zheng

Graphene, a two-dimensional sp2-bonded carbon material, has attracted enormous attention due to its excellent electrical, optical and mechanical properties. Recently developed chemical vapor deposition (CVD) methods could produce large-size and uniform polycrystalline graphene films, limited to gas carbon sources, metal catalyst substrates and degraded properties induced by grain boundaries. Meanwhile, pristine monolayer graphene exhibits a standard ambipolar behavior with a zero neutrality point in field-effect transistors (FETs), limiting its future electronic applications. This thesis starts with the investigation of CVD synthesis of pristine and N-doped graphene with controlled thickness using solid carbon sources on metal catalyst substrates (chapter 1), and then discusses the direct growth of bilayer graphene on insulating substrates, including SiO2, h-BN, Si3N4 and Al2O3, without needing further transfer-process (chapter 2). Chapter 3 discusses the synthesis of high-quality graphene single crystals and hexagonal onion-ring-like graphene domains, and also explores the basic growth mechanism of graphene on Cu substrates. To extend graphene's potential applications, both vertical and planar graphene-carbon nanotube hybrids are fabricated using CVD method and their interesting properties are investigated (chapter 4). Chapter 5 discusses how to use chemical methods to modulate graphene's electronic behaviors.

Electronic Structure and Morphology of Graphene Layers on SiC

NASA Astrophysics Data System (ADS)

Ohta, Taisuke

2008-03-01

Recent years have witnessed the discovery and the unique electronic properties of graphene, a sheet of carbon atoms arranged in a honeycomb lattice. The unique linear dispersion relation of charge carriers near the Fermi level (``Dirac Fermions'') lead to exciting transport properties, such as an unusual quantum Hall effect, and have aroused scientific and technological interests. On the way towards graphene-based electronics, a knowledge of the electronic band structure and the morphology of epitaxial graphene films on silicon carbide substrates is imperative. We have studied the evolution of the occupied band structure and the morphology of graphene layers on silicon carbide by systematically increasing the layer thickness. Using angle-resolved photoemission spectroscopy (ARPES), we examine this unique 2D system in its development from single layer to multilayers, by characteristic changes in the π band, the highest occupied state, and the dispersion relation in the out-of-plane electron wave vector in particular. The evolution of the film morphology is evaluated by the combination of low-energy electron microscopy and ARPES. By exploiting the sensitivity of graphene's electronic states to the charge carrier concentration, changes in the on-site Coulomb potential leading to a change of π and π* bands can be examined using ARPES. We demonstrate that, in a graphene bilayer, the gap between π and π* bands can be controlled by selectively adjusting relative carrier concentrations, which suggests a possible application of the graphene bilayer for switching functions in electronic devices. This work was done in collaboration with A. Bostwick, J. L. McChesney, and E. Rotenberg at Advanced Light Source, Lawrence Berkeley National Laboratory, K. Horn at Fritz-Haber-Institut, K. V. Emtsev and Th. Seyller at Lehrstuhl für Technische Physik, Universität Erlangen-Nürnberg, and F. El Gabaly and A. K. Schmid at National Center for Electron Microscopy, Lawrence Berkeley

Self-assembly of ordered graphene nanodot arrays

DOE PAGES

Camilli, Luca; Jørgensen, Jakob H.; Tersoff, Jerry; ...

2017-06-29

Our ability to fabricate nanoscale domains of uniform size in two-dimensional materials could potentially enable new applications in nanoelectronics and the development of innovative metamaterials. But, achieving even minimal control over the growth of two-dimensional lateral heterostructures at such extreme dimensions has proven exceptionally challenging. Here we show the spontaneous formation of ordered arrays of graphene nano-domains (dots), epitaxially embedded in a two-dimensional boron–carbon–nitrogen alloy. These dots exhibit a strikingly uniform size of 1.6 ± 0.2 nm and strong ordering, and the array periodicity can be tuned by adjusting the growth conditions. Furthemore, we explain this behaviour with a modelmore » incorporating dot-boundary energy, a moiré-modulated substrate interaction and a long-range repulsion between dots. This new two-dimensional material, which theory predicts to be an ordered composite of uniform-size semiconducting graphene quantum dots laterally integrated within a larger-bandgap matrix, holds promise for novel electronic and optoelectronic properties, with a variety of potential device applications.« less

Epitaxial growth of hybrid nanostructures

NASA Astrophysics Data System (ADS)

Tan, Chaoliang; Chen, Junze; Wu, Xue-Jun; Zhang, Hua

2018-02-01

Hybrid nanostructures are a class of materials that are typically composed of two or more different components, in which each component has at least one dimension on the nanoscale. The rational design and controlled synthesis of hybrid nanostructures are of great importance in enabling the fine tuning of their properties and functions. Epitaxial growth is a promising approach to the controlled synthesis of hybrid nanostructures with desired structures, crystal phases, exposed facets and/or interfaces. This Review provides a critical summary of the state of the art in the field of epitaxial growth of hybrid nanostructures. We discuss the historical development, architectures and compositions, epitaxy methods, characterization techniques and advantages of epitaxial hybrid nanostructures. Finally, we provide insight into future research directions in this area, which include the epitaxial growth of hybrid nanostructures from a wider range of materials, the study of the underlying mechanism and determining the role of epitaxial growth in influencing the properties and application performance of hybrid nanostructures.

High quality factor GaAs-based photonic crystal microcavities by epitaxial re-growth.

PubMed

Prieto, Ivan; Herranz, Jesús; Wewior, Lukasz; González, Yolanda; Alén, Benito; González, Luisa; Postigo, Pablo A

2013-12-16

We investigate L7 photonic crystal microcavities (PCMs) fabricated by epitaxial re-growth of GaAs pre-patterned substrates, containing InAs quantum dots. The resulting PCMs show hexagonal shaped nano-holes due to the development of preferential crystallographic facets during the re-growth step. Through a careful control of the fabrication processes, we demonstrate that the photonic modes are preserved throughout the process. The quality factor (Q) of the photonic modes in the re-grown PCMs strongly depends on the relative orientation between photonic lattice and crystallographic directions. The optical modes of the re-grown PCMs preserve the linear polarization and, for the most favorable orientation, a 36% of the Q measured in PCMs fabricated by the conventional procedure is observed, exhibiting values up to ~6000. The results aim to the future integration of site-controlled QDs with high-Q PCMs for quantum photonics and quantum integrated circuits.

Controlled assembly of graphene-capped nickel, cobalt and iron silicides

PubMed Central

Vilkov, O.; Fedorov, A.; Usachov, D.; Yashina, L. V.; Generalov, A. V.; Borygina, K.; Verbitskiy, N. I.; Grüneis, A.; Vyalikh, D. V.

2013-01-01

The unique properties of graphene have raised high expectations regarding its application in carbon-based nanoscale devices that could complement or replace traditional silicon technology. This gave rise to the vast amount of researches on how to fabricate high-quality graphene and graphene nanocomposites that is currently going on. Here we show that graphene can be successfully integrated with the established metal-silicide technology. Starting from thin monocrystalline films of nickel, cobalt and iron, we were able to form metal silicides of high quality with a variety of stoichiometries under a Chemical Vapor Deposition grown graphene layer. These graphene-capped silicides are reliably protected against oxidation and can cover a wide range of electronic materials/device applications. Most importantly, the coupling between the graphene layer and the silicides is rather weak and the properties of quasi-freestanding graphene are widely preserved. PMID:23835625

Photosensitive graphene transistors.

PubMed

Li, Jinhua; Niu, Liyong; Zheng, Zijian; Yan, Feng

2014-08-20

High performance photodetectors play important roles in the development of innovative technologies in many fields, including medicine, display and imaging, military, optical communication, environment monitoring, security check, scientific research and industrial processing control. Graphene, the most fascinating two-dimensional material, has demonstrated promising applications in various types of photodetectors from terahertz to ultraviolet, due to its ultrahigh carrier mobility and light absorption in broad wavelength range. Graphene field effect transistors are recognized as a type of excellent transducers for photodetection thanks to the inherent amplification function of the transistors, the feasibility of miniaturization and the unique properties of graphene. In this review, we will introduce the applications of graphene transistors as photodetectors in different wavelength ranges including terahertz, infrared, visible, and ultraviolet, focusing on the device design, physics and photosensitive performance. Since the device properties are closely related to the quality of graphene, the devices based on graphene prepared with different methods will be addressed separately with a view to demonstrating more clearly their advantages and shortcomings in practical applications. It is expected that highly sensitive photodetectors based on graphene transistors will find important applications in many emerging areas especially flexible, wearable, printable or transparent electronics and high frequency communications. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

High-quality AlN epitaxy on nano-patterned sapphire substrates prepared by nano-imprint lithography.

PubMed

Zhang, Lisheng; Xu, Fujun; Wang, Jiaming; He, Chenguang; Guo, Weiwei; Wang, Mingxing; Sheng, Bowen; Lu, Lin; Qin, Zhixin; Wang, Xinqiang; Shen, Bo

2016-11-04

We report epitaxial growth of AlN films with atomically flat surface on nano-patterned sapphire substrates (NPSS) prepared by nano-imprint lithography. The crystalline quality can be greatly improved by using the optimized 1-μm-period NPSS. The X-ray diffraction ω-scan full width at half maximum values for (0002) and (102) reflections are 171 and 205 arcsec, respectively. The optimized NPSS contribute to eliminating almost entirely the threading dislocations (TDs) originating from the AlN/sapphire interface via bending the dislocations by image force from the void sidewalls before coalescence. In addition, reducing the misorientations of the adjacent regions during coalescence adopting the low lateral growth rate is also essential for decreasing TDs in the upper AlN epilayer.

High-quality AlN epitaxy on nano-patterned sapphire substrates prepared by nano-imprint lithography

NASA Astrophysics Data System (ADS)

Zhang, Lisheng; Xu, Fujun; Wang, Jiaming; He, Chenguang; Guo, Weiwei; Wang, Mingxing; Sheng, Bowen; Lu, Lin; Qin, Zhixin; Wang, Xinqiang; Shen, Bo

2016-11-01

We report epitaxial growth of AlN films with atomically flat surface on nano-patterned sapphire substrates (NPSS) prepared by nano-imprint lithography. The crystalline quality can be greatly improved by using the optimized 1-μm-period NPSS. The X-ray diffraction ω-scan full width at half maximum values for (0002) and (102) reflections are 171 and 205 arcsec, respectively. The optimized NPSS contribute to eliminating almost entirely the threading dislocations (TDs) originating from the AlN/sapphire interface via bending the dislocations by image force from the void sidewalls before coalescence. In addition, reducing the misorientations of the adjacent regions during coalescence adopting the low lateral growth rate is also essential for decreasing TDs in the upper AlN epilayer.

High-quality AlN epitaxy on nano-patterned sapphire substrates prepared by nano-imprint lithography

PubMed Central

Zhang, Lisheng; Xu, Fujun; Wang, Jiaming; He, Chenguang; Guo, Weiwei; Wang, Mingxing; Sheng, Bowen; Lu, Lin; Qin, Zhixin; Wang, Xinqiang; Shen, Bo

2016-01-01

We report epitaxial growth of AlN films with atomically flat surface on nano-patterned sapphire substrates (NPSS) prepared by nano-imprint lithography. The crystalline quality can be greatly improved by using the optimized 1-μm-period NPSS. The X-ray diffraction ω-scan full width at half maximum values for (0002) and (102) reflections are 171 and 205 arcsec, respectively. The optimized NPSS contribute to eliminating almost entirely the threading dislocations (TDs) originating from the AlN/sapphire interface via bending the dislocations by image force from the void sidewalls before coalescence. In addition, reducing the misorientations of the adjacent regions during coalescence adopting the low lateral growth rate is also essential for decreasing TDs in the upper AlN epilayer. PMID:27812006

Long-Term Stability of Photovoltaic Hybrid Perovskites achieved by Graphene Passivation via a Water- and Polymer-Free Graphene Transfer Method

NASA Astrophysics Data System (ADS)

Tseng, W.-S.; Jao, M.-H.; Hsu, C.-C.; Wu, C.-I.; Yeh, N.-C.

Organic-inorganic hybrid perovskites such as CH3NH3PbX3 (X = I, Br) have been intensively studied in recent years because of their rapidly improving photovoltaic power conversion efficiency. However, severe instability of these materials in ambient environment has been a primary challenge for practical applications. To address this issue, we employ high-quality PECVD-grown graphene to passivate the hybrid perovskites. In contrast to existing processes for transferring graphene from the growth substrates to other surfaces that involve either polymer or water, which are incompatible with photovoltaic applications of these water-sensitive hybrid perovskites, we report here a new water- and polymer-free graphene transferring method. Studies of the Raman, x-ray and ultraviolet photoemission spectroscopy (XPS and UPS) demonstrated excellent quality of monolayer PECVD-grown graphene samples after their transfer onto different substrates with the water- and polymer-free processing method. In particular, graphene was successfully transferred onto the surface of CH3NH3PbI3 thin films with sample quality intact. Moreover, XPS and UPS studies indicated that even after 3 months, the fully graphene-covered perovskite films remained spectroscopically invariant, which was in sharp contrast to the drastic changes, after merely one week, in both the XPS and UPS of a control CH3NH3PbI3 sample without graphene protection. Beckman Inst. in Caltech. Dragon Gate Program in Taiwan.

Enhanced van der Waals epitaxy via electron transfer enabled interfacial dative bond formation

DOE PAGES

Xie, Weiyu; Lu, Toh -Ming; Wang, Gwo -Ching; ...

2017-11-14

Enhanced van der Waals (vdW) epitaxy of semiconductors on a layered vdW substrate is identified as the formation of dative bonds. For example, despite that NbSe 2 is a vdW layeredmaterial, first-principles calculations reveal that the bond strength at a CdTe-NbSe 2 interface is five times as large as that of vdW interactions at a CdTe-graphene interface. Finally, the unconventional chemistry here is enabled by an effective net electron transfer from Cd dangling-bond states at a CdTe surface to metallic nonbonding NbSe 2 states, which is a necessary condition to activate the Cd for enhanced binding with Se.

Enhanced van der Waals epitaxy via electron transfer enabled interfacial dative bond formation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xie, Weiyu; Lu, Toh -Ming; Wang, Gwo -Ching

Enhanced van der Waals (vdW) epitaxy of semiconductors on a layered vdW substrate is identified as the formation of dative bonds. For example, despite that NbSe 2 is a vdW layeredmaterial, first-principles calculations reveal that the bond strength at a CdTe-NbSe 2 interface is five times as large as that of vdW interactions at a CdTe-graphene interface. Finally, the unconventional chemistry here is enabled by an effective net electron transfer from Cd dangling-bond states at a CdTe surface to metallic nonbonding NbSe 2 states, which is a necessary condition to activate the Cd for enhanced binding with Se.

Aqueous solution epitaxy of CdS layers on CuInSe 2

NASA Astrophysics Data System (ADS)

Furlong, M. J.; Froment, M.; Bernard, M. C.; Cortès, R.; Tiwari, A. N.; Krejci, M.; Zogg, H.; Lincot, D.

1998-09-01

Epitaxial CdS thin films have been deposited from an aqueous ammonia solution containing cadmium ions and thiourea as precursors on single crystalline CuInSe 2 films prepared by MBE on Si(1 1 1) and GaAs(1 0 0) substrates. The structure and quality of the films were investigated by RHEED, glancing angle XRD and HRTEM in cross-section. The films are cubic on (1 0 0) substrates, and mixed cubic and hexagonal on (1 1 1) substrates due to the presence of stacking faults parallel to the substrate. The growth is under surface kinetic control with an activation energy of 85 kJ mol -1. Epitaxy improves with increasing temperature and an epitaxial transition temperature at approx. 60°C is demonstrated in the selected experimental conditions. The epitaxy is very sensitive to the preparation of the surface. Beneficial effects of in situ or ex situ chemical etching are found. Similarities between aqueous solution and vapor-phase chemical depositions are pointed out.

Thermal Exfoliation of Natural Cellulosic Material for Graphene Synthesis

NASA Astrophysics Data System (ADS)

Ray, Ajoy Kumar; Chatterjee, Somenath; Singh, Jitendra Kumar; Bapari, Himangshu

2015-01-01

Hibiscus flower petals have been used as a cheap natural resource precursor for cost-effective synthesis of high quality graphene by thermal exfoliation process. In order to compare the quality of graphene obtained from the flower petals directly with the flower petals pretreated with nickel(II) chloride, Raman spectroscopic technique has been used as the structural probe. The role of temperature and the effect of nickel on thermal exfoliation process have been examined. It has been observed that graphene obtained via nickel incorporation is of better quality because NI2+ ions that get dispersed in the layered-structured cellulose at elevated temperatures get reduced to the metallic state, which in turn push the graphitic layers during thermal exfoliation to produce good quality graphene. In contrast, no such driving force is present in cellulose and hemi-cellulose of flower petals that contain lignin.

Graphene growth on Ge(100)/Si(100) substrates by CVD method.

PubMed

Pasternak, Iwona; Wesolowski, Marek; Jozwik, Iwona; Lukosius, Mindaugas; Lupina, Grzegorz; Dabrowski, Pawel; Baranowski, Jacek M; Strupinski, Wlodek

2016-02-22

The successful integration of graphene into microelectronic devices is strongly dependent on the availability of direct deposition processes, which can provide uniform, large area and high quality graphene on nonmetallic substrates. As of today the dominant technology is based on Si and obtaining graphene with Si is treated as the most advantageous solution. However, the formation of carbide during the growth process makes manufacturing graphene on Si wafers extremely challenging. To overcome these difficulties and reach the set goals, we proposed growth of high quality graphene layers by the CVD method on Ge(100)/Si(100) wafers. In addition, a stochastic model was applied in order to describe the graphene growth process on the Ge(100)/Si(100) substrate and to determine the direction of further processes. As a result, high quality graphene was grown, which was proved by Raman spectroscopy results, showing uniform monolayer films with FWHM of the 2D band of 32 cm(-1).

Stabilizing Ir(001) Epitaxy on Yttria-Stabilized Zirconia Using a Thin Ir Seed Layer Grown by Pulsed Laser Deposition

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fan, Lisha; Jacobs, Christopher B.; Rouleau, Christopher M.

In this paper, we demonstrate the reproducible epitaxial growth of 100 nm thick Ir(001) films on a heteroepitaxial stack consisting of 5 nm Ir and 100 nm yttria-stabilized zirconia (YSZ) grown on Si(001) substrates. It is shown that a 5 nm thick Ir layer grown by pulsed laser deposition in the same chamber as the YSZ film without breaking the vacuum is the key to stabilizing Ir(001) epitaxial growth. Growth of the Ir seed layer with pure (001) orientation occurs only in a narrow growth temperature window from 550 to 750 °C, and the fraction of Ir(111) increases at substratemore » temperatures outside of this window. The Ir seed layer prevents exposure of the YSZ film to air during sample transfer and enables highly reproducible Ir(001) heteroepitaxy on YSZ buffered Si(001). In contrast, if Ir is grown directly on a bare YSZ layer that was exposed to ambient conditions, the films are prone to change orientation to (111). These results reveal that preserving the chemical and structural purity of the YSZ surface is imperative for achieving Ir(001) epitaxy. The narrow range of the mosaic spread values from eight experiments demonstrates the high yield and high reproducibility of Ir(001) heteroepitaxy by this approach. Lastly, the improved Ir(001) epitaxial growth method is of great significance for integrating a variety of technologically important materials such as diamond, graphene, and functional oxides on a Si platform.« less

Stabilizing Ir(001) Epitaxy on Yttria-Stabilized Zirconia Using a Thin Ir Seed Layer Grown by Pulsed Laser Deposition

DOE PAGES

Fan, Lisha; Jacobs, Christopher B.; Rouleau, Christopher M.; ...

2016-11-18

In this paper, we demonstrate the reproducible epitaxial growth of 100 nm thick Ir(001) films on a heteroepitaxial stack consisting of 5 nm Ir and 100 nm yttria-stabilized zirconia (YSZ) grown on Si(001) substrates. It is shown that a 5 nm thick Ir layer grown by pulsed laser deposition in the same chamber as the YSZ film without breaking the vacuum is the key to stabilizing Ir(001) epitaxial growth. Growth of the Ir seed layer with pure (001) orientation occurs only in a narrow growth temperature window from 550 to 750 °C, and the fraction of Ir(111) increases at substratemore » temperatures outside of this window. The Ir seed layer prevents exposure of the YSZ film to air during sample transfer and enables highly reproducible Ir(001) heteroepitaxy on YSZ buffered Si(001). In contrast, if Ir is grown directly on a bare YSZ layer that was exposed to ambient conditions, the films are prone to change orientation to (111). These results reveal that preserving the chemical and structural purity of the YSZ surface is imperative for achieving Ir(001) epitaxy. The narrow range of the mosaic spread values from eight experiments demonstrates the high yield and high reproducibility of Ir(001) heteroepitaxy by this approach. Lastly, the improved Ir(001) epitaxial growth method is of great significance for integrating a variety of technologically important materials such as diamond, graphene, and functional oxides on a Si platform.« less

Epitaxial graphene on SiC(0001): functional electrical microscopy studies and effect of atmosphere.

PubMed

Kazakova, O; Burnett, T L; Patten, J; Yang, L; Yakimova, R

2013-05-31

Surface potential distribution, V(CPD), and evolution of atmospheric adsorbates on few and multiple layers (FLG and MLG) of graphene grown on SiC(0001) substrate have been investigated by electrostatic and Kelvin force microscopy techniques at T = 20-120 °C. The change of the surface potential distribution, ΔV(CPD), between FLG and MLG is shown to be temperature dependent. The enhanced ΔV(CPD) value at 120 °C is associated with desorption of adsorbates at high temperatures and the corresponding change of the carrier balance. The nature of the adsorbates and their evolution with temperature are considered to be related to the process of adsorption and desorption of the atmospheric water on MLG domains. We demonstrate that both the nano- and microscale wettability of the material are strongly dependent on the number of graphene layers.

Epitaxial growth of CZT(S,Se) on silicon

DOEpatents

Bojarczuk, Nestor A.; Gershon, Talia S.; Guha, Supratik; Shin, Byungha; Zhu, Yu

2016-03-15

Techniques for epitaxial growth of CZT(S,Se) materials on Si are provided. In one aspect, a method of forming an epitaxial kesterite material is provided which includes the steps of: selecting a Si substrate based on a crystallographic orientation of the Si substrate; forming an epitaxial oxide interlayer on the Si substrate to enhance wettability of the epitaxial kesterite material on the Si substrate, wherein the epitaxial oxide interlayer is formed from a material that is lattice-matched to Si; and forming the epitaxial kesterite material on a side of the epitaxial oxide interlayer opposite the Si substrate, wherein the epitaxial kesterite material includes Cu, Zn, Sn, and at least one of S and Se, and wherein a crystallographic orientation of the epitaxial kesterite material is based on the crystallographic orientation of the Si substrate. A method of forming an epitaxial kesterite-based photovoltaic device and an epitaxial kesterite-based device are also provided.

BioGraphene: Direct Exfoliation of Graphite in a Kitchen Blender for Enzymology Applications.

PubMed

Kumar, C V; Pattammattel, A

2016-01-01

A high yielding method for the aqueous exfoliation of graphite crystals to produce high quality graphene nanosheets in a kitchen blender is described here. Bovine serum albumin (BSA), β-lactoglobulin, ovalbumin, lysozyme, and hemoglobin as well as calf serum were used for the exfoliation of graphene. Among these, BSA gave the maximum exfoliation efficiency, exceeding 4mgmL(-1)h(-1) of graphene. Quality of graphene produced was examined by Raman spectroscopy, which indicated 3-5 layer graphene of very high quality and very low levels of defects. Transmission electron microscopy indicated an average size of ~0.5μm flakes. The graphene/BSA dispersions were stable over pH 3.0-11, and at 5°C or 50°C, for more than 2 months. Current approach gave higher rates of BSA/graphene (BioGraphene) in better yields than other methods. Calf serum, when used in place of BSA, also gave high yields of good quality BioGraphene and these preparations may be of direct use for cell culture studies. A simple example of BioGraphene preparation is described that can be adapted in most laboratories, and graphene-adsorbed glucose oxidase is nearly as active as the free enzyme. Current approach may facilitate large-scale production of graphene in most laboratories around the world and it may open new opportunities for biological applications of graphene. © 2016 Elsevier Inc. All rights reserved.

Epitaxial growth of three dimensionally structured III-V photonic crystal via hydride vapor phase epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zheng, Qiye; Kim, Honggyu; Zhang, Runyu

2015-12-14

Three-dimensional (3D) photonic crystals are one class of materials where epitaxy, and the resultant attractive electronic properties, would enable new functionalities for optoelectronic devices. Here we utilize self-assembled colloidal templates to fabricate epitaxially grown single crystal 3D mesostructured GaxIn1-xP (GaInP) semiconductor photonic crystals using hydride vapor phase epitaxy (HVPE). The epitaxial relationship between the 3D GaInP and the substrate is preserved during the growth through the complex geometry of the template as confirmed by X-ray diffraction (XRD) and high resolution transmission electron microscopy. XRD reciprocal space mapping of the 3D epitaxial layer further demonstrates the film to be nearly fullymore » relaxed with a negligible strain gradient. Fourier transform infrared spectroscopy reflection measurement indicates the optical properties of the photonic crystal which agree with finite difference time domain simulations. This work extends the scope of the very few known methods for the fabrication of epitaxial III-V 3D mesostructured materials to the well-developed HVPE technique.« less

Epitaxial growth of three dimensionally structured III-V photonic crystal via hydride vapor phase epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zheng, Qiye; Kim, Honggyu; Zhang, Runyu

2015-12-14

Three-dimensional (3D) photonic crystals are one class of materials where epitaxy, and the resultant attractive electronic properties, would enable new functionalities for optoelectronic devices. Here we utilize self-assembled colloidal templates to fabricate epitaxially grown single crystal 3D mesostructured Ga{sub x}In{sub 1−x}P (GaInP) semiconductor photonic crystals using hydride vapor phase epitaxy (HVPE). The epitaxial relationship between the 3D GaInP and the substrate is preserved during the growth through the complex geometry of the template as confirmed by X-ray diffraction (XRD) and high resolution transmission electron microscopy. XRD reciprocal space mapping of the 3D epitaxial layer further demonstrates the film to bemore » nearly fully relaxed with a negligible strain gradient. Fourier transform infrared spectroscopy reflection measurement indicates the optical properties of the photonic crystal which agree with finite difference time domain simulations. This work extends the scope of the very few known methods for the fabrication of epitaxial III-V 3D mesostructured materials to the well-developed HVPE technique.« less

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

DTIC Science & Technology

2014-10-23

in barrier height at the graphene –silicon carbide Schottky junction,” Nat. Commun. 4, 2752 (2013). 31H. Yang, J. Heo, S. Park, H. J. Song, D. H. Seo, K...displacement. The shift of the Dirac point defines the Schottky barrier height and will determine the practicality of employing the wide-bandgap...are thought to critically influence technologi- cally relevant properties such as Dirac point shift and Schottky barrier height . Furthermore, this

Gapped electronic structure of epitaxial stanene on InSb(111)

DOE PAGES

Xu, Cai-Zhi; Chan, Yang-Hao; Chen, Peng; ...

2018-01-11

We report that stanene (single-layer gray tin), with an electronic structure akin to that of graphene but exhibiting a much larger spin-orbit gap, offers a promising platform for room-temperature electronics based on the quantum spin Hall (QSH) effect. This material has received much theoretical attention, but a suitable substrate for stanene growth that results in an overall gapped electronic structure has been elusive; a sizable gap is necessary for room-temperature applications. Here, we report a study of stanene, epitaxially grown on the (111)B-face of indium antimonide (InSb). Angle-resolved photoemission spectroscopy measurements reveal a gap of 0.44 eV, in agreement withmore » our first-principles calculations. Lastly, the results indicate that stanene on InSb(111) is a strong contender for electronic QSH applications.« less

Gapped electronic structure of epitaxial stanene on InSb(111)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Cai-Zhi; Chan, Yang-Hao; Chen, Peng

We report that stanene (single-layer gray tin), with an electronic structure akin to that of graphene but exhibiting a much larger spin-orbit gap, offers a promising platform for room-temperature electronics based on the quantum spin Hall (QSH) effect. This material has received much theoretical attention, but a suitable substrate for stanene growth that results in an overall gapped electronic structure has been elusive; a sizable gap is necessary for room-temperature applications. Here, we report a study of stanene, epitaxially grown on the (111)B-face of indium antimonide (InSb). Angle-resolved photoemission spectroscopy measurements reveal a gap of 0.44 eV, in agreement withmore » our first-principles calculations. Lastly, the results indicate that stanene on InSb(111) is a strong contender for electronic QSH applications.« less

Synthesis of Graphene Based Membranes: Effect of Substrate Surface Properties on Monolayer Graphene Transfer.

PubMed

Kafiah, Feras; Khan, Zafarullah; Ibrahim, Ahmed; Atieh, Muataz; Laoui, Tahar

2017-01-21

In this work, we report the transfer of graphene onto eight commercial microfiltration substrates having different pore sizes and surface characteristics. Monolayer graphene grown on copper by the chemical vapor deposition (CVD) process was transferred by the pressing method over the target substrates, followed by wet etching of copper to obtain monolayer graphene/polymer membranes. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and contact angle (CA) measurements were carried out to explore the graphene layer transferability. Three factors, namely, the substrate roughness, its pore size, and its surface wetting (degree of hydrophobicity) are found to affect the conformality and coverage of the transferred graphene monolayer on the substrate surface. A good quality graphene transfer is achieved on the substrate with the following characteristics; being hydrophobic (CA > 90°), having small pore size, and low surface roughness, with a CA to RMS (root mean square) ratio higher than 2.7°/nm.

Electronic structure, surface doping, and optical response in epitaxial WSe 2 thin films

DOE PAGES

Zhang, Yi; Ugeda, Miguel M.; Jin, Chenhao; ...

2016-03-14

High quality WSe 2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe 2/BLG. We observe that a bilayer of WSe 2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct–indirect band gap crossover to trilayer WSe 2. In the monolayer limit, WSe 2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observedmore » among all the MX 2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe 2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Lastly, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.« less

Efficient preparation of graphene liquid cell utilizing direct transfer with large-area well-stitched graphene

NASA Astrophysics Data System (ADS)

Sasaki, Yuki; Kitaura, Ryo; Yuk, Jong Min; Zettl, Alex; Shinohara, Hisanori

2016-04-01

By utilizing graphene-sandwiched structures recently developed in this laboratory, we are able to visualize small droplets of liquids in nanometer scale. We have found that small water droplets as small as several tens of nanometers sandwiched by two single-layer graphene are frequently observed by TEM. Due to the electron beam irradiation during the TEM observation, these sandwiched droplets are frequently moving from one place to another and are subjected to create small bubbles inside. The synthesis of a large area single-domain graphene of high-quality is essential to prepare the graphene sandwiched cell which safely encapsulates the droplets in nanometer size.

Automatic graphene transfer system for improved material quality and efficiency

PubMed Central

Boscá, Alberto; Pedrós, Jorge; Martínez, Javier; Palacios, Tomás; Calle, Fernando

2016-01-01

In most applications based on chemical vapor deposition (CVD) graphene, the transfer from the growth to the target substrate is a critical step for the final device performance. Manual procedures are time consuming and depend on handling skills, whereas existing automatic roll-to-roll methods work well for flexible substrates but tend to induce mechanical damage in rigid ones. A new system that automatically transfers CVD graphene to an arbitrary target substrate has been developed. The process is based on the all-fluidic manipulation of the graphene to avoid mechanical damage, strain and contamination, and on the combination of capillary action and electrostatic repulsion between the graphene and its container to ensure a centered sample on top of the target substrate. The improved carrier mobility and yield of the automatically transferred graphene, as compared to that manually transferred, is demonstrated by the optical and electrical characterization of field-effect transistors fabricated on both materials. In particular, 70% higher mobility values, with a 30% decrease in the unintentional doping and a 10% strain reduction are achieved. The system has been developed for lab-scale transfer and proved to be scalable for industrial applications. PMID:26860260

Scalable graphene production from ethanol decomposition by microwave argon plasma torch

NASA Astrophysics Data System (ADS)

Melero, C.; Rincón, R.; Muñoz, J.; Zhang, G.; Sun, S.; Perez, A.; Royuela, O.; González-Gago, C.; Calzada, M. D.

2018-01-01

A fast, efficient and simple method is presented for the production of high quality graphene on a large scale by using an atmospheric pressure plasma-based technique. This technique allows to obtain high quality graphene in powder in just one step, without the use of neither metal catalysts and nor specific substrate during the process. Moreover, the cost for graphene production is significantly reduced since the ethanol used as carbon source can be obtained from the fermentation of agricultural industries. The process provides an additional benefit contributing to the revalorization of waste in the production of a high-value added product like graphene. Thus, this work demonstrates the features of plasma technology as a low cost, efficient, clean and environmentally friendly route for production of high-quality graphene.

Powder free PECVD epitaxial silicon by plasma pulsing or increasing the growth temperature

NASA Astrophysics Data System (ADS)

Chen, Wanghua; Maurice, Jean-Luc; Vanel, Jean-Charles; Cabarrocas, Pere Roca i.

2018-06-01

Crystalline silicon thin films are promising candidates for low cost and flexible photovoltaics. Among various synthesis techniques, epitaxial growth via low temperature plasma-enhanced chemical vapor deposition is an interesting choice because of two low temperature related benefits: low thermal budget and better doping profile control. However, increasing the growth rate is a tricky issue because the agglomeration of clusters required for epitaxy leads to powder formation in the plasma. In this work, we have measured precisely the time evolution of the self-bias voltage in silane/hydrogen plasmas at millisecond time scale, for different values of the direct-current bias voltage applied to the radio frequency (RF) electrode and growth temperatures. We demonstrate that the decisive factor to increase the epitaxial growth rate, i.e. the inhibition of the agglomeration of plasma-born clusters, can be obtained by decreasing the RF OFF time or increasing the growth temperature. The influence of these two parameters on the growth rate and epitaxial film quality is also presented.

Columnar epitaxy of hexagonal and orthorhombic silicides on Si(111)

NASA Technical Reports Server (NTRS)

Fathauer, R. W.; Nieh, C. W.; Xiao, Q. F.; Hashimoto, Shin

1990-01-01

Columnar grains of PtSi and CrSi2 surrounded by high-quality epitaxial silicon are obtained by ultrahigh vacuum codeposition of Si and metal in an approximately 10:1 ratio on Si(111) substrates heated to 610-840 C. This result is similar to that found previously for CoSi2 (a nearly-lattice-matched cubic-fluorite crystal) on Si(111), in spite of the respective orthorhombic and hexagonal structures of PtSi and CrSi2. The PtSi grains are epitaxial and have one of three variants of the relation defined by PtSi(010)/Si(111), with PtSi 001 line/Si 110 line type.

Epitaxial Garnets and Hexagonal Ferrites.

DTIC Science & Technology

1982-04-20

goenv.o -,y la)ers were YIG (yttrium iron garnet ) films grown by liquid phase epitaxy w:* ( LPE ) on gadolinium gallium garnet (GGG) substrates. Magnetic...containing three epitaxial layers. In addition to the MSW work oil garnets , LPE of lithium ferrite and hexagonal fertites was studied. A substituted lead...of a stripline. The other layers are epitaxial films , generally YIG (yttrium iron garnet ) with magnetic properties adjusted by suitable modifications

The effect of copper substrate’s roughness on graphene growth process via PECVD

NASA Astrophysics Data System (ADS)

Fan, Tengfei; Yan, Cuixia; Lu, Jianchen; Zhang, Lianchang; Cai, Jinming

2018-04-01

Despite many excellent properties, the synthesis of high quality graphene with low-cost way is still a challenge, thus many different factors have been researched. In this work, the effect of surface roughness to the graphene quality was studied. Graphene was synthesized by plasma enhanced chemical vapor deposition (PECVD) method on copper substrates with different roughness from 0.074 μm to 0.339 μm, which were prepared via annealing, corrosion or polishing, respectively. Ar+ plasma cleaning was applied before graphene growth in order to accommodate similar surface chemical reactivity to each other. Scanning electron microscope and Raman spectroscope were employed to investigate the effect of surface roughness, which reveals that the graphene quality decrease first and then increase again according to the ratio of ID/IG in Raman spectroscopy. When the ratio of ID/IG reaches the largest number, the substrate roughness is 0.127 μm, where is the graphene quality changing point. First principle calculation was applied to explain the phenomenon and revealed that it is strongly affected by the graphene grain size and quantity which can induce defects. This strategy is expected to guide the industrial production of graphene.

Growth and Characterization of Epitaxial Piezoelectric and Semiconductor Films.

DTIC Science & Technology

1980-07-01

quality epitaxial films at low growth rates. This process is limited to films up to a few microns thickness. The aluminum chloride/ ammonia CVD process has... scrubber through a rotary Vacuum pump maintaining Reactions.-DEZ is an electron deficient compound a pressure of about 400 Torr inside the reaction chain

Microstructure fabrication process induced modulations in CVD graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matsubayashi, Akitomo, E-mail: amatsubayashi@albany.edu; Zhang, Zhenjun; Lee, Ji Ung

The systematic Raman spectroscopic study of a “mimicked” graphene device fabrication is presented. Upon photoresist baking, compressive stress is induced in the graphene which disappears after it is removed. The indirect irradiation from the electron beam (through the photoresist) does not significantly alter graphene characteristic Raman peaks indicating that graphene quality is preserved upon the exposure. The 2D peak shifts and the intensity ratio of 2D and G band, I(2D)/I(G), decreases upon direct metal deposition (Co and Py) suggesting that the electronic modulation occurs due to sp{sup 2} C-C bond weakening. In contrast, a thin metal oxide film deposited graphenemore » does not show either the significant 2D and G peaks shift or I(2D)/I(G) decrease upon the metal deposition suggesting the oxide protect the graphene quality in the fabrication process.« less

Industrial graphene metrology.

PubMed

Kyle, Jennifer Reiber; Ozkan, Cengiz S; Ozkan, Mihrimah

2012-07-07

Graphene is an allotrope of carbon whose structure is based on one-atom-thick planar sheets of carbon atoms that are densely packed in a honeycomb crystal lattice. Its unique electrical and optical properties raised worldwide interest towards the design and fabrication of future electronic and optical devices with unmatched performance. At the moment, extensive efforts are underway to evaluate the reliability and performance of a number of such devices. With the recent advances in synthesizing large-area graphene sheets, engineers have begun investigating viable methodologies for conducting graphene metrology and quality control at industrial scales to understand a variety of reliability issues including defects, patternability, electrical, and physical properties. This review summarizes the current state of industrial graphene metrology and provides an overview of graphene metrology techniques. In addition, a recently developed large-area graphene metrology technique based on fluorescence quenching is introduced. For each metrology technique, the industrial metrics it measures are identified--layer thickness, edge structure, defects, Fermi level, and thermal conductivity--and a detailed description is provided as to how the measurements are performed. Additionally, the potential advantages of each technique for industrial use are identified, including throughput, scalability, sensitivity to substrate/environment, and on their demonstrated ability to achieve quantified results. The recently developed fluorescence-quenching metrology technique is shown to meet all the necessary criteria for industrial applications, rendering it the first industry-ready graphene metrology technique.

Method of depositing epitaxial layers on a substrate

DOEpatents

Goyal, Amit

2003-12-30

An epitaxial article and method for forming the same includes a substrate having a textured surface, and an electrochemically deposited substantially single orientation epitaxial layer disposed on and in contact with the textured surface. The epitaxial article can include an electromagnetically active layer and an epitaxial buffer layer. The electromagnetically active layer and epitaxial buffer layer can also be deposited electrochemically.

Super-narrow, extremely high quality collective plasmon resonances at telecom wavelengths and their application in a hybrid graphene-plasmonic modulator.

PubMed

Thackray, Benjamin D; Thomas, Philip A; Auton, Gregory H; Rodriguez, Francisco J; Marshall, Owen P; Kravets, Vasyl G; Grigorenko, Alexander N

2015-05-13

We present extremely narrow collective plasmon resonances observed in gold nanostripe arrays fabricated on a thin gold film, with the spectral line full width at half-maximum (fwhm) as low as 5 nm and quality factors Q reaching 300, at important fiber-optic telecommunication wavelengths around 1.5 μm. Using these resonances, we demonstrate a hybrid graphene-plasmonic modulator with the modulation depth of 20% in reflection operated by gating of a single layer graphene, the largest measured so far.

Boosting Photon Harvesting in Organic Solar Cells with Highly Oriented Molecular Crystals via Graphene-Organic Heterointerface.

PubMed

Jo, Sae Byeok; Kim, Hyun Ho; Lee, Hansol; Kang, Boseok; Lee, Seongkyu; Sim, Myungsun; Kim, Min; Lee, Wi Hyoung; Cho, Kilwon

2015-08-25

Photon harvesting in organic solar cells is highly dependent on the anisotropic nature of the optoelectronic properties of photoactive materials. Here, we demonstrate an efficient approach to dramatically enhance photon harvesting in planar heterojunction solar cells by using a graphene-organic heterointerface. A large area, residue-free monolayer graphene is inserted at anode interface to serve as an atomically thin epitaxial template for growing highly orientated pentacene crystals with lying-down orientation. This anisotropic orientation enhances the overall optoelectronic properties, including light absorption, charge carrier lifetime, interfacial energetics, and especially the exciton diffusion length. Spectroscopic and crystallographic analysis reveal that the lying-down orientation persists until a thickness of 110 nm, which, along with increased exciton diffusion length up to nearly 100 nm, allows the device optimum thickness to be doubled to yield significantly enhanced light absorption within the photoactive layers. The resultant photovoltaic performance shows simultaneous increment in Voc, Jsc, and FF, and consequently a 5 times increment in the maximum power conversion efficiency than the equivalent devices without a graphene layer. The present findings indicate that controlling organic-graphene heterointerface could provide a design strategy of organic solar cell architecture for boosting photon harvesting.

Graphene FETs Based on High Resolution Nanoribbons for HF Low Power Applications

NASA Astrophysics Data System (ADS)

Mele, David; Mehdhbi, Sarah; Fadil, Dalal; Wei, Wei; Ouerghi, Abdelkarim; Lepilliet, Sylvie; Happy, Henri; Pallecchi, Emiliano

2018-03-01

In this paper we present high frequency field effect transistors based on graphene nanoribbons arrays (GNRFETs). The nanoribbons serve as a channel for the transistors and are fabricated with a process based on e-beam lithography and dry etching of high mobility hydrogen intercalated epitaxial graphene. The widths of the nanoribbons vary from 50 to 20 nm, less than half those measured in previous reports for GNRFETs. Hall measurements reveal that the devices are p-doped, with mobility on the order of 2300 cm2/Vs. From DC characteristics, we find that the maximum ratio IMAX/IMIN is 5 obtained at 50 nm ribbons width. The IV characteristics of the GNRFETs are slightly non-linear at high bias without a full saturation. Therefore, despite the aggressive scaling of the graphene nanoribbon width, a bandgap is still not observed in our measurements. The high frequency performances of our GNRFETs are already significant at low bias. At 300 mV drain source voltage, the highest intrinsic (extrinsic) cut-off frequency ft reaches 82 (18) GHz and the extrinsic maximum oscillation frequency fmax is 20 GHz, which is promising for low power applications.

Selective epitaxy using the gild process

DOEpatents

Weiner, Kurt H.

1992-01-01

The present invention comprises a method of selective epitaxy on a semiconductor substrate. The present invention provides a method of selectively forming high quality, thin GeSi layers in a silicon circuit, and a method for fabricating smaller semiconductor chips with a greater yield (more error free chips) at a lower cost. The method comprises forming an upper layer over a substrate, and depositing a reflectivity mask which is then removed over selected sections. Using a laser to melt the unmasked sections of the upper layer, the semiconductor material in the upper layer is heated and diffused into the substrate semiconductor material. By varying the amount of laser radiation, the epitaxial layer is formed to a controlled depth which may be very thin. When cooled, a single crystal epitaxial layer is formed over the patterned substrate. The present invention provides the ability to selectively grow layers of mixed semiconductors over patterned substrates such as a layer of Ge.sub.x Si.sub.1-x grown over silicon. Such a process may be used to manufacture small transistors that have a narrow base, heavy doping, and high gain. The narrowness allows a faster transistor, and the heavy doping reduces the resistance of the narrow layer. The process does not require high temperature annealing; therefore materials such as aluminum can be used. Furthermore, the process may be used to fabricate diodes that have a high reverse breakdown voltage and a low reverse leakage current.

Noise in Graphene Superlattices Grown on Hexagonal Boron Nitride.

PubMed

Li, Xuefei; Lu, Xiaobo; Li, Tiaoyang; Yang, Wei; Fang, Jianming; Zhang, Guangyu; Wu, Yanqing

2015-11-24

Existing in almost all electronic systems, the current noise spectral density, originated from the fluctuation of current, is by nature far more sensitive than the mean value of current, the most common characteristic parameter in electronic devices. Existing models on its origin of either carrier number or mobility are adopted in practically all electronic devices. For the past few decades, there has been no experimental evidence for direct association between 1/f noise and any other kinetic phenomena in solid state devices. Here, in the study of a van der Waals heterostructure of graphene on hexagonal BN superlattice, satellite Dirac points have been characterized through 1/f noise spectral density with pronounced local minima and asymmetric magnitude associated with its unique energy dispersion spectrum, which can only be revealed by scanning tunneling microscopy and low temperature magneto-transport measurement. More importantly, these features even emerge in the noise spectra of devices showing no minima in electric current, and are robust at all temperatures down to 4.3 K. In addition, graphene on h-BN exhibits a record low noise level of 1.6 × 10(-9) μm(2) Hz(-1) at 10 Hz, more than 1 order of magnitude lower than previous results for graphene on SiO2. Such an epitaxial van der Waals material system not only enables an unprecedented characterization of fundamentals in solids by 1/f noise, but its superior interface also provides a key and feasible solution for further improvement of the noise level for graphene devices.

Urea-assisted liquid-phase exfoliation of natural graphite into few-layer graphene

NASA Astrophysics Data System (ADS)

Hou, Dandan; Liu, Qinfu; Wang, Xianshuai; Qiao, Zhichuan; Wu, Yingke; Xu, Bohui; Ding, Shuli

2018-05-01

The mass production of graphene with high quality is desirable for its wide applications. Here, we demonstrated a facile method to exfoliate natural graphite into graphene in organic solvent by assisting of urea. The exfoliation of graphite may originate from the "molecular wedge" effect of urea, which can intercalate into the edge of natural graphite, thus facilitating the production of graphene dispersion with a high concentration up to 1.2 mg/mL. The obtained graphene is non-oxidized with negligible defects. Therefore, this approach has great promise in bulk production of graphene with superior quality for a variety of applications.

Crystal Growth of Graphene Films and Graphene Nanoribbons via Chemical Vapor Deposition

NASA Astrophysics Data System (ADS)

Jacobberger, Robert Michael

Graphene is a two-dimensional carbon allotrope that has exceptional properties, including high charge carrier mobility, thermal conductivity, mechanical strength, and flexibility. Graphene is a semimetal, prohibiting its use in semiconductor applications in which a bandgap is required. However, graphene can be transformed from a semimetal into a semiconductor if it is confined into one-dimensional nanoribbons narrower than 10 nm with well-defined armchair edges. In this work, we study the crystal growth of graphene via chemical vapor deposition (CVD), which is the most promising method to produce graphene films on the industrial scale. We explore the growth of isolated graphene crystals, continuous graphene films, and narrow graphene nanoribbons with armchair edges. We gain key insight into the critical growth parameters and mechanisms that influence the crystal morphology, orientation, defect density, and evolution, providing an empirical understanding of the diverse growth behaviors observed in literature. Using this knowledge, we synthesize graphene with remarkably low pinhole density and achieve high-quality graphene at 750 °C on Cu(111), which is over 250 °C lower than the temperature typically used to grow graphene on copper from methane. We also describe our breakthrough in graphene nanoribbon synthesis. Highly anisotropic nanoribbons are formed on Ge(001) if an exceptionally slow growth rate is used. The nanoribbons are self-defining with predominantly smooth armchair edges, are self-aligning, and have tunable width to < 10 nm. High-performance field-effect transistors incorporating these nanoribbons as channels display high conductance modulation > 10,000 and high conductance > 5 muS. This directional and anisotropic growth enables the fabrication of semiconducting nanoribbons directly on conventional semiconductor wafers and, thus, promises to allow the integration of nanoribbons into future hybrid integrated circuits. We additionally report our

Graphene radio frequency receiver integrated circuit.

PubMed

Han, Shu-Jen; Garcia, Alberto Valdes; Oida, Satoshi; Jenkins, Keith A; Haensch, Wilfried

2014-01-01

Graphene has attracted much interest as a future channel material in radio frequency electronics because of its superior electrical properties. Fabrication of a graphene integrated circuit without significantly degrading transistor performance has proven to be challenging, posing one of the major bottlenecks to compete with existing technologies. Here we present a fabrication method fully preserving graphene transistor quality, demonstrated with the implementation of a high-performance three-stage graphene integrated circuit. The circuit operates as a radio frequency receiver performing signal amplification, filtering and downconversion mixing. All circuit components are integrated into 0.6 mm(2) area and fabricated on 200 mm silicon wafers, showing the unprecedented graphene circuit complexity and silicon complementary metal-oxide-semiconductor process compatibility. The demonstrated circuit performance allow us to use graphene integrated circuit to perform practical wireless communication functions, receiving and restoring digital text transmitted on a 4.3-GHz carrier signal.

Graphene radio frequency receiver integrated circuit

NASA Astrophysics Data System (ADS)

Han, Shu-Jen; Garcia, Alberto Valdes; Oida, Satoshi; Jenkins, Keith A.; Haensch, Wilfried

2014-01-01

Graphene has attracted much interest as a future channel material in radio frequency electronics because of its superior electrical properties. Fabrication of a graphene integrated circuit without significantly degrading transistor performance has proven to be challenging, posing one of the major bottlenecks to compete with existing technologies. Here we present a fabrication method fully preserving graphene transistor quality, demonstrated with the implementation of a high-performance three-stage graphene integrated circuit. The circuit operates as a radio frequency receiver performing signal amplification, filtering and downconversion mixing. All circuit components are integrated into 0.6 mm2 area and fabricated on 200 mm silicon wafers, showing the unprecedented graphene circuit complexity and silicon complementary metal-oxide-semiconductor process compatibility. The demonstrated circuit performance allow us to use graphene integrated circuit to perform practical wireless communication functions, receiving and restoring digital text transmitted on a 4.3-GHz carrier signal.

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

PubMed

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

2016-09-15

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.

Low-temperature atomic layer epitaxy of AlN ultrathin films by layer-by-layer, in-situ atomic layer annealing.

PubMed

Shih, Huan-Yu; Lee, Wei-Hao; Kao, Wei-Chung; Chuang, Yung-Chuan; Lin, Ray-Ming; Lin, Hsin-Chih; Shiojiri, Makoto; Chen, Miin-Jang

2017-01-03

Low-temperature epitaxial growth of AlN ultrathin films was realized by atomic layer deposition (ALD) together with the layer-by-layer, in-situ atomic layer annealing (ALA), instead of a high growth temperature which is needed in conventional epitaxial growth techniques. By applying the ALA with the Ar plasma treatment in each ALD cycle, the AlN thin film was converted dramatically from the amorphous phase to a single-crystalline epitaxial layer, at a low deposition temperature of 300 °C. The energy transferred from plasma not only provides the crystallization energy but also enhances the migration of adatoms and the removal of ligands, which significantly improve the crystallinity of the epitaxial layer. The X-ray diffraction reveals that the full width at half-maximum of the AlN (0002) rocking curve is only 144 arcsec in the AlN ultrathin epilayer with a thickness of only a few tens of nm. The high-resolution transmission electron microscopy also indicates the high-quality single-crystal hexagonal phase of the AlN epitaxial layer on the sapphire substrate. The result opens a window for further extension of the ALD applications from amorphous thin films to the high-quality low-temperature atomic layer epitaxy, which can be exploited in a variety of fields and applications in the near future.

Low-temperature atomic layer epitaxy of AlN ultrathin films by layer-by-layer, in-situ atomic layer annealing

PubMed Central

Shih, Huan-Yu; Lee, Wei-Hao; Kao, Wei-Chung; Chuang, Yung-Chuan; Lin, Ray-Ming; Lin, Hsin-Chih; Shiojiri, Makoto; Chen, Miin-Jang

2017-01-01

Low-temperature epitaxial growth of AlN ultrathin films was realized by atomic layer deposition (ALD) together with the layer-by-layer, in-situ atomic layer annealing (ALA), instead of a high growth temperature which is needed in conventional epitaxial growth techniques. By applying the ALA with the Ar plasma treatment in each ALD cycle, the AlN thin film was converted dramatically from the amorphous phase to a single-crystalline epitaxial layer, at a low deposition temperature of 300 °C. The energy transferred from plasma not only provides the crystallization energy but also enhances the migration of adatoms and the removal of ligands, which significantly improve the crystallinity of the epitaxial layer. The X-ray diffraction reveals that the full width at half-maximum of the AlN (0002) rocking curve is only 144 arcsec in the AlN ultrathin epilayer with a thickness of only a few tens of nm. The high-resolution transmission electron microscopy also indicates the high-quality single-crystal hexagonal phase of the AlN epitaxial layer on the sapphire substrate. The result opens a window for further extension of the ALD applications from amorphous thin films to the high-quality low-temperature atomic layer epitaxy, which can be exploited in a variety of fields and applications in the near future. PMID:28045075

Basal-plane dislocations in bilayer graphene - Peculiarities in a quasi-2D material

NASA Astrophysics Data System (ADS)

Butz, Benjamin

2015-03-01

Dislocations represent one of the most fascinating and fundamental concepts in materials science. First and foremost, they are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly alter the local electronic or optical properties of semiconductors and ionic crystals. In layered crystals like graphite dislocation movement is restricted to the basal plane. Thus, those basal-plane dislocations cannot escape enabling their confinement in between only two atomic layers of the material. So-called bilayer graphene is the thinnest imaginable quasi-2D crystal to explore the nature and behavior of dislocations under such extreme boundary conditions. Robust graphene membranes derived from epitaxial graphene on SiC provide an ideal platform for their investigation. The presentation will give an insight in the direct observation of basal-plane partial dislocations by transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. The investigation reveals striking size effects. First, the absence of stacking fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern, which corresponds to an alternating AB <--> BA change of the stacking order. Most importantly, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane, which directly results from accommodation of strain. In fact, the buckling completely changes the strain state of the bilayer graphene and is of key importance for its electronic/spin transport properties. Due to the high degree of disorder in our quasi-2D material it is one of the very few examples for a perfect linear magnetoresistance, i.e. the linear dependency of the in-plane electrical resistance on a magnetic field applied perpendicular to the graphene sheet up to field strengths of more than 60 T. This research is financed by the German Research

Delaminated graphene at silicon carbide facets: atomic scale imaging and spectroscopy.

PubMed

Nicotra, Giuseppe; Ramasse, Quentin M; Deretzis, Ioannis; La Magna, Antonino; Spinella, Corrado; Giannazzo, Filippo

2013-04-23

Atomic-resolution structural and spectroscopic characterization techniques (scanning transmission electron microscopy and electron energy loss spectroscopy) are combined with nanoscale electrical measurements (conductive atomic force microscopy) to study at the atomic scale the properties of graphene grown epitaxially through the controlled graphitization of a hexagonal SiC(0001) substrate by high temperature annealing. This growth technique is known to result in a pronounced electron-doping (∼10(13) cm(-2)) of graphene, which is thought to originate from an interface carbon buffer layer strongly bound to the substrate. The scanning transmission electron microscopy analysis, carried out at an energy below the knock-on threshold for carbon to ensure no damage is imparted to the film by the electron beam, demonstrates that the buffer layer present on the planar SiC(0001) face delaminates from it on the (112n) facets of SiC surface steps. In addition, electron energy loss spectroscopy reveals that the delaminated layer has a similar electronic configuration to purely sp2-hybridized graphene. These observations are used to explain the local increase of the graphene sheet resistance measured around the surface steps by conductive atomic force microscopy, which we suggest is due to significantly lower substrate-induced doping and a resonant scattering mechanism at the step regions. A first-principles-calibrated theoretical model is proposed to explain the structural instability of the buffer layer on the SiC facets and the resulting delamination.

Magnetite nano-islands on Graphene

NASA Astrophysics Data System (ADS)

Anderson, Nathaniel; Zhang, Qiang; Rosenberg, Richard; Vaknin, David

X-ray magnetic circular dichroism (XMCD) of ex-situ iron nano-islands grown on graphene reveals that iron oxidation spontaneously leads to the formation of magnetite nano-particles - i.e, the formation of the inverse spinel Fe3O4. Fe islands have been grown with two different heights (20 and 75 MLs) on epitaxial graphene and we have determined their magnetic behavior both as function of temperature and applied external field. Our XAS and XMCD at an applied magnetic field of B = 5 T show that the thin film (20 MLs) is totally converted to magnetite whereas the thicker film (75 MLs) exhibits magnetite properties but also those of pure metal iron. For both samples, temperature dependence of the XMCD shows clear transitions at ~120 K consistent with the Verwey transition of bulk magnetite. XMCD at low temperatures shows a weak hysteresis and provide the average spin and angular-momentum moments, the dipolar term, and the total moment . In addition, manipulation and comparison of the XMCD data from both samples allows us to extract information about the pure iron nano-islands from the thicker sample. Ames Laboratory is supported by the U.S. DOE, BES, MSE Contract No. DE-AC02-07CH11358. APS is supported by U.S. DOE Contract No. DE-AC02-06CH11357.

Electrical properties of graphene-metal contacts.

PubMed

Cusati, Teresa; Fiori, Gianluca; Gahoi, Amit; Passi, Vikram; Lemme, Max C; Fortunelli, Alessandro; Iannaccone, Giuseppe

2017-07-11

The performance of devices and systems based on two-dimensional material systems depends critically on the quality of the contacts between 2D material and metal. A low contact resistance is an imperative requirement to consider graphene as a candidate material for electronic and optoelectronic devices. Unfortunately, measurements of contact resistance in the literature do not provide a consistent picture, due to limitations of current graphene technology, and to incomplete understanding of influencing factors. Here we show that the contact resistance is intrinsically dependent on graphene sheet resistance and on the chemistry of the graphene-metal interface. We present a physical model of the contacts based on ab-initio simulations and extensive experiments carried out on a large variety of samples with different graphene-metal contacts. Our model explains the spread in experimental results as due to uncontrolled graphene doping and suggests ways to engineer contact resistance. We also predict an achievable contact resistance of 30 Ω·μm for nickel electrodes, extremely promising for applications.

Electronic transport in graphene-based heterostructures

NASA Astrophysics Data System (ADS)

Tan, J. Y.; Avsar, A.; Balakrishnan, J.; Koon, G. K. W.; Taychatanapat, T.; O'Farrell, E. C. T.; Watanabe, K.; Taniguchi, T.; Eda, G.; Castro Neto, A. H.; Özyilmaz, B.

2014-05-01

While boron nitride (BN) substrates have been utilized to achieve high electronic mobilities in graphene field effect transistors, it is unclear how other layered two dimensional (2D) crystals influence the electronic performance of graphene. In this Letter, we study the surface morphology of 2D BN, gallium selenide (GaSe), and transition metal dichalcogenides (tungsten disulfide (WS2) and molybdenum disulfide (MoS2)) crystals and their influence on graphene's electronic quality. Atomic force microscopy analysis shows that these crystals have improved surface roughness (root mean square value of only ˜0.1 nm) compared to conventional SiO2 substrate. While our results confirm that graphene devices exhibit very high electronic mobility (μ) on BN substrates, graphene devices on WS2 substrates (G/WS2) are equally promising for high quality electronic transport (μ ˜ 38 000 cm2/V s at room temperature), followed by G/MoS2 (μ ˜ 10 000 cm2/V s) and G/GaSe (μ ˜ 2200 cm2/V s). However, we observe a significant asymmetry in electron and hole conduction in G/WS2 and G/MoS2 heterostructures, most likely due to the presence of sulphur vacancies in the substrate crystals. GaSe crystals are observed to degrade over time even under ambient conditions, leading to a large hysteresis in graphene transport making it a less suitable substrate.

Engineering the Charge Transfer in all 2D Graphene-Nanoplatelets Heterostructure Photodetectors

PubMed Central

Robin, A.; Lhuillier, E.; Xu, X. Z.; Ithurria, S.; Aubin, H.; Ouerghi, A.; Dubertret, B.

2016-01-01

Two dimensional layered (i.e. van der Waals) heterostructures open up great prospects, especially in photodetector applications. In this context, the control of the charge transfer between the constituting layers is of crucial importance. Compared to bulk or 0D system, 2D materials are characterized by a large exciton binding energy (0.1–1 eV) which considerably affects the magnitude of the charge transfer. Here we investigate a model system made from colloidal 2D CdSe nanoplatelets and epitaxial graphene in a phototransistor configuration. We demonstrate that using a heterostructured layered material, we can tune the magnitude and the direction (i.e. electron or hole) of the charge transfer. We further evidence that graphene functionalization by nanocrystals only leads to a limited change in the magnitude of the 1/f noise. These results draw some new directions to design van der Waals heterostructures with enhanced optoelectronic properties. PMID:27143413

Performance-Enhanced Activated Carbon Electrodes for Supercapacitors Combining Both Graphene-Modified Current Collectors and Graphene Conductive Additive

PubMed Central

Wang, Rubing; Qian, Yuting; Li, Weiwei; Zhu, Shoupu; Liu, Fengkui; Guo, Yufen; Chen, Mingliang; Li, Qi; Liu, Liwei

2018-01-01

Graphene has been widely used in the active material, conductive agent, binder or current collector for supercapacitors, due to its large specific surface area, high conductivity, and electron mobility. However, works simultaneously employing graphene as conductive agent and current collector were rarely reported. Here, we report improved activated carbon (AC) electrodes (AC@G@NiF/G) simultaneously combining chemical vapor deposition (CVD) graphene-modified nickel foams (NiF/Gs) current collectors and high quality few-layer graphene conductive additive instead of carbon black (CB). The synergistic effect of NiF/Gs and graphene additive makes the performances of AC@G@NiF/G electrodes superior to those of electrodes with CB or with nickel foam current collectors. The performances of AC@G@NiF/G electrodes show that for the few-layer graphene addition exists an optimum value around 5 wt %, rather than a larger addition of graphene, works out better. A symmetric supercapacitor assembled by AC@G@NiF/G electrodes exhibits excellent cycling stability. We attribute improved performances to graphene-enhanced conductivity of electrode materials and NiF/Gs with 3D graphene conductive network and lower oxidation, largely improving the electrical contact between active materials and current collectors. PMID:29762528

Performance-Enhanced Activated Carbon Electrodes for Supercapacitors Combining Both Graphene-Modified Current Collectors and Graphene Conductive Additive.

PubMed

Wang, Rubing; Qian, Yuting; Li, Weiwei; Zhu, Shoupu; Liu, Fengkui; Guo, Yufen; Chen, Mingliang; Li, Qi; Liu, Liwei

2018-05-15

Graphene has been widely used in the active material, conductive agent, binder or current collector for supercapacitors, due to its large specific surface area, high conductivity, and electron mobility. However, works simultaneously employing graphene as conductive agent and current collector were rarely reported. Here, we report improved activated carbon (AC) electrodes (AC@G@NiF/G) simultaneously combining chemical vapor deposition (CVD) graphene-modified nickel foams (NiF/Gs) current collectors and high quality few-layer graphene conductive additive instead of carbon black (CB). The synergistic effect of NiF/Gs and graphene additive makes the performances of AC@G@NiF/G electrodes superior to those of electrodes with CB or with nickel foam current collectors. The performances of AC@G@NiF/G electrodes show that for the few-layer graphene addition exists an optimum value around 5 wt %, rather than a larger addition of graphene, works out better. A symmetric supercapacitor assembled by AC@G@NiF/G electrodes exhibits excellent cycling stability. We attribute improved performances to graphene-enhanced conductivity of electrode materials and NiF/Gs with 3D graphene conductive network and lower oxidation, largely improving the electrical contact between active materials and current collectors.

Epitaxy of mercury-based high temperature superconducting films on oxide and metal substrates

NASA Astrophysics Data System (ADS)

Xie, Yi-Yuan

High-Tc superconducting (HTS) cuprates are highly anisotropic thus epitaxy along certain crystalline directions is essential to realize high-current-carrying capability at temperatures above 77 K. Hg-based HTS (Hg-HTS) cuprates have the record-high Tc up to 135 K, therefore are of great interest for fundamental research and practical applications. However, growth Of epitaxial Hg-HTS films is extremely difficult in conventional thermal-reaction process since Hg is highly volatile. Motivated by this, we first developed a cation-exchange process for growing epitaxial Hg-HTS films, which involves two steps: selection of precursor matrices with predesigned structure and composition followed by cation-exchange processing. New materials are formed via "atomic surgery" on an existing structure rather than thermal reaction among amorphous oxides in conventional process, thus the structural features of the precursor are inherited by the new material. Using epitaxial Tl-based HTS films as precursor and annealing them in Hg-vapor, epitaxial Hg-HTS films with superior quality have been obtained. This success encouraged us to develop epitaxy on metal tapes for coated conductors and On large-area wafers for electronic devices. For coated conductors, we addressed three critical issues: epitaxy on metal substrates, enhancement of in-field Jcs and scale-up in thickness and length. First, using a fabrication scheme that combines two processes: cation-exchange and fast-temperature-ramping-annealing, epitaxial HgBa2CaCu2O6+delta films were grown on rolling-assisted-biaxially-textured Ni substrates buffered with CeO 2/YSZ/CeO2 for the first time. We fabricated HgBa2CaCu 2O6+delta coated conductors with Tc = 122--124 K and self-field Jc > 1 x 106A/cm2 at 92 K which are record-high for HTS coated conductors. Second, we demonstrated improved in-field J cs via overdoping HgBa2CaCu 2O6+delta films (by means Of charge "overdoped"), heavy-ion-irradiation and substrate engineering. Finally

Epitaxial Stabilization of a-PbO2 Structure in MnF2 Layers on Si and GaP

DTIC Science & Technology

2001-06-01

Before the epitaxy, the substrates were dipped in a HF solution and fixed on Si platelets with InGa eutectic . The crystalline quality of the substrates...15 keV. We used a recrystallization annealing (RA) in the 550-700’C range to improve the MnF2 film quality of some epitaxial structures grown at... recrystallization annealing. The inset in Fig. 1(a) shows the RHEED pattern of a 30 nm MnF2 film grown at 100°C and annealed at 550’C. Well-pronounced

Facet-Selective Epitaxy of Compound Semiconductors on Faceted Silicon Nanowires.

PubMed

Mankin, Max N; Day, Robert W; Gao, Ruixuan; No, You-Shin; Kim, Sun-Kyung; McClelland, Arthur A; Bell, David C; Park, Hong-Gyu; Lieber, Charles M

2015-07-08

Integration of compound semiconductors with silicon (Si) has been a long-standing goal for the semiconductor industry, as direct band gap compound semiconductors offer, for example, attractive photonic properties not possible with Si devices. However, mismatches in lattice constant, thermal expansion coefficient, and polarity between Si and compound semiconductors render growth of epitaxial heterostructures challenging. Nanowires (NWs) are a promising platform for the integration of Si and compound semiconductors since their limited surface area can alleviate such material mismatch issues. Here, we demonstrate facet-selective growth of cadmium sulfide (CdS) on Si NWs. Aberration-corrected transmission electron microscopy analysis shows that crystalline CdS is grown epitaxially on the {111} and {110} surface facets of the Si NWs but that the Si{113} facets remain bare. Further analysis of CdS on Si NWs grown at higher deposition rates to yield a conformal shell reveals a thin oxide layer on the Si{113} facet. This observation and control experiments suggest that facet-selective growth is enabled by the formation of an oxide, which prevents subsequent shell growth on the Si{113} NW facets. Further studies of facet-selective epitaxial growth of CdS shells on micro-to-mesoscale wires, which allows tuning of the lateral width of the compound semiconductor layer without lithographic patterning, and InP shell growth on Si NWs demonstrate the generality of our growth technique. In addition, photoluminescence imaging and spectroscopy show that the epitaxial shells display strong and clean band edge emission, confirming their high photonic quality, and thus suggesting that facet-selective epitaxy on NW substrates represents a promising route to integration of compound semiconductors on Si.

Mechanical Properties of Graphene-Rubber Nanocomposites

NASA Astrophysics Data System (ADS)

Anhar, N. A. M.; Ramli, M. M.; Hambali, N. A. M. A.; Aziz, A. A.; Mat Isa, S. S.; Danial, N. S.; Abdullah, M. M. A. B.

2017-11-01

This research focused on development of wearable sensor device by using Prevulcanized Natural Rubber (PV) and Epoxidized Natural Rubber (ENR 50) latex incorporated with graphene oxide (GO), graphene paste, graphene powder and reduced graphene oxide (rGO) powder. The compounding formulation and calculation were based on phr (parts per hundred rubber) and all the samples were then tested for mechanical properties using Instron 5565 machine. It was found that the sonication effects on tensile strength may have better quality of tensile strength compared to non-sonicated GO. For PV incorporate GO, the optimum loading was best determined at loading 1.5 phr with or without sonication and similar result was recorded for PV/G. For ENR 50 incorporate graphene paste and rGO powder nanocomposite shows the best optimum was at 3.0 phr with 24 hours’ sonication.

Wafer-scale layer transfer of GaAs and Ge onto Si wafers using patterned epitaxial lift-off

NASA Astrophysics Data System (ADS)

Mieda, Eiko; Maeda, Tatsuro; Miyata, Noriyuki; Yasuda, Tetsuji; Kurashima, Yuichi; Maeda, Atsuhiko; Takagi, Hideki; Aoki, Takeshi; Yamamoto, Taketsugu; Ichikawa, Osamu; Osada, Takenori; Hata, Masahiko; Ogawa, Arito; Kikuchi, Toshiyuki; Kunii, Yasuo

2015-03-01

We have developed a wafer-scale layer-transfer technique for transferring GaAs and Ge onto Si wafers of up to 300 mm in diameter. Lattice-matched GaAs or Ge layers were epitaxially grown on GaAs wafers using an AlAs release layer, which can subsequently be transferred onto a Si handle wafer via direct wafer bonding and patterned epitaxial lift-off (ELO). The crystal properties of the transferred GaAs layers were characterized by X-ray diffraction (XRD), photoluminescence, and the quality of the transferred Ge layers was characterized using Raman spectroscopy. We find that, after bonding and the wet ELO processes, the quality of the transferred GaAs and Ge layers remained the same compared to that of the as-grown epitaxial layers. Furthermore, we realized Ge-on-insulator and GaAs-on-insulator wafers by wafer-scale pattern ELO technique.

Graphene: A Comprehensive Review.

PubMed

Ghuge, Aditya D; Shirode, Abhay R; Kadam, Vilasrao J

2017-01-01

Graphene, a one-atom thick, two-dimensional sheets of sp2hybridized carbon atoms packed in a hexagonal lattice with a Caron-Carbon distance of about 0.142 nm. Its extended honeycomb network forms the basic building block of other important allotropes; it can be stacked to form 3-Dgraphite, rolled to form 1-D-nanotubes and wrapped to form 0-D-fullerenes. Long-range π conjugation in graphene results in its extraordinary thermal, mechanical and electrical properties, which have been the interest of many theoretical studies and recently became an exciting area for scientists. Graphene is impermeable to gas and liquids, has excellent thermal conductivity and higher current density in comparison to other most effective materials. All of its exceptional properties have opened up new avenues for the use of graphene in nano-devices and nano-systems, which initiated its prominent use as a material for drug targeting. In addition, several fabrication techniques are outlined, starting from the mechanical exfoliation of high-quality graphene to the direct growth on silicon carbide or metal substrates and from the chemical routes utilizing graphene oxide to the newly developed approach at the molecular level. By this article reviewers intend to emphasize on unique properties, fabrication techniques and updated applications of graphene. In addition, we discuss about the potential of graphene in drug targeting in fields of nanotechnology, biomedical engineering and technology and its use for innovations in various fields such as electronics and photonics. Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.org.

CVD growth of graphene at low temperature

NASA Astrophysics Data System (ADS)

Zeng, Changgan

2012-02-01

Graphene has attracted a lot of research interest owing to its exotic properties and a wide spectrum of potential applications. Chemical vapor deposition (CVD) from gaseous hydrocarbon sources has shown great promises for large-scale graphene growth. However, high growth temperature, typically 1000^oC, is required for such growth. In this talk, I will show a revised CVD route to grow graphene on Cu foils at low temperature, adopting solid and liquid hydrocarbon feedstocks. For solid PMMA and polystyrene precursors, centimeter-scale monolayer graphene films are synthesized at a growth temperature down to 400^oC. When benzene is used as the hydrocarbon source, monolayer graphene flakes with excellent quality are achieved at a growth temperature as low as 300^oC. I will also talk about our recent progress on low-temperature graphene growth using paraterphenyl as precursor. The successful low-temperature growth can be qualitatively understood from the first principles calculations. Our work might pave a way to economical and convenient growth route of graphene, as well as better control of the growth pattern of graphene at low temperature.

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.

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.

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

Repeated growth and bubbling transfer of graphene with millimetre-size single-crystal grains using platinum.

PubMed

Gao, Libo; Ren, Wencai; Xu, Huilong; Jin, Li; Wang, Zhenxing; Ma, Teng; Ma, Lai-Peng; Zhang, Zhiyong; Fu, Qiang; Peng, Lian-Mao; Bao, Xinhe; Cheng, Hui-Ming

2012-02-28

Large single-crystal graphene is highly desired and important for the applications of graphene in electronics, as grain boundaries between graphene grains markedly degrade its quality and properties. Here we report the growth of millimetre-sized hexagonal single-crystal graphene and graphene films joined from such grains on Pt by ambient-pressure chemical vapour deposition. We report a bubbling method to transfer these single graphene grains and graphene films to arbitrary substrate, which is nondestructive not only to graphene, but also to the Pt substrates. The Pt substrates can be repeatedly used for graphene growth. The graphene shows high crystal quality with the reported lowest wrinkle height of 0.8 nm and a carrier mobility of greater than 7,100 cm(2) V(-1) s(-1) under ambient conditions. The repeatable growth of graphene with large single-crystal grains on Pt and its nondestructive transfer may enable various applications.

Repeated growth and bubbling transfer of graphene with millimetre-size single-crystal grains using platinum

PubMed Central

Gao, Libo; Ren, Wencai; Xu, Huilong; Jin, Li; Wang, Zhenxing; Ma, Teng; Ma, Lai-Peng; Zhang, Zhiyong; Fu, Qiang; Peng, Lian-Mao; Bao, Xinhe; Cheng, Hui-Ming

2012-01-01

Large single-crystal graphene is highly desired and important for the applications of graphene in electronics, as grain boundaries between graphene grains markedly degrade its quality and properties. Here we report the growth of millimetre-sized hexagonal single-crystal graphene and graphene films joined from such grains on Pt by ambient-pressure chemical vapour deposition. We report a bubbling method to transfer these single graphene grains and graphene films to arbitrary substrate, which is nondestructive not only to graphene, but also to the Pt substrates. The Pt substrates can be repeatedly used for graphene growth. The graphene shows high crystal quality with the reported lowest wrinkle height of 0.8 nm and a carrier mobility of greater than 7,100 cm2 V−1 s−1 under ambient conditions. The repeatable growth of graphene with large single-crystal grains on Pt and its nondestructive transfer may enable various applications. PMID:22426220

Cyan laser diode grown by plasma-assisted molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Turski, H., E-mail: henryk@unipress.waw.pl; Muziol, G.; Wolny, P.

We demonstrate AlGaN-cladding-free laser diodes (LDs), operating in continuous wave (CW) mode at 482 nm grown by plasma-assisted molecular beam epitaxy (PAMBE). The maximum CW output power was 230 mW. LDs were grown on c-plane GaN substrates obtained by hydride vapor phase epitaxy. The PAMBE process was carried out in metal-rich conditions, supplying high nitrogen flux (Φ{sub N}) during quantum wells (QWs) growth. We found that high Φ{sub N} improves quality of high In content InGaN QWs. The role of nitrogen in the growth of InGaN on (0001) GaN surface as well as the influence of LDs design on threshold currentmore » density are discussed.« less

Switching of Photonic Crystal Lasers by Graphene.

PubMed

Hwang, Min-Soo; Kim, Ha-Reem; Kim, Kyoung-Ho; Jeong, Kwang-Yong; Park, Jin-Sung; Choi, Jae-Hyuck; Kang, Ju-Hyung; Lee, Jung Min; Park, Won Il; Song, Jung-Hwan; Seo, Min-Kyo; Park, Hong-Gyu

2017-03-08

Unique features of graphene have motivated the development of graphene-integrated photonic devices. In particular, the electrical tunability of graphene loss enables high-speed modulation of light and tuning of cavity resonances in graphene-integrated waveguides and cavities. However, efficient control of light emission such as lasing, using graphene, remains a challenge. In this work, we demonstrate on/off switching of single- and double-cavity photonic crystal lasers by electrical gating of a monolayer graphene sheet on top of photonic crystal cavities. The optical loss of graphene was controlled by varying the gate voltage V g , with the ion gel atop the graphene sheet. First, the fundamental properties of graphene were investigated through the transmittance measurement and numerical simulations. Next, optically pumped lasing was demonstrated for a graphene-integrated single photonic crystal cavity at V g below -0.6 V, exhibiting a low lasing threshold of ∼480 μW, whereas lasing was not observed at V g above -0.6 V owing to the intrinsic optical loss of graphene. Changing quality factor of the graphene-integrated photonic crystal cavity enables or disables the lasing operation. Moreover, in the double-cavity photonic crystal lasers with graphene, switching of individual cavities with separate graphene sheets was achieved, and these two lasing actions were controlled independently despite the close distance of ∼2.2 μm between adjacent cavities. We believe that our simple and practical approach for switching in graphene-integrated active photonic devices will pave the way toward designing high-contrast and ultracompact photonic integrated circuits.

The effect of copper pre-cleaning on graphene synthesis.

PubMed

Kim, Soo Min; Hsu, Allen; Lee, Yi-Hsien; Dresselhaus, Mildred; Palacios, Tomás; Kim, Ki Kang; Kong, Jing

2013-09-13

Copper foil is the most common substrate to synthesize monolayer graphene by chemical vapor deposition (CVD). The surface morphology and conditions of the copper foil can be very different depending on the various suppliers or different batches. These surface properties of copper strongly affect the growth behavior of graphene, thus rendering the growth conditions irreproducible when different batches of Cu foil are used. Furthermore, the quality of the graphene is severely affected as well. In this work, we report a facile method of copper pre-cleaning to improve the graphene quality and the reproducibility of the growth process. We found that the commercial Ni etchant (based on nitric acid) or nitric acid is the most effective cleaning agent among various acidic or basic solutions. The graphene grown on thus-treated copper surfaces is very clean and mostly monolayer when observed under scanning electron microscopy (SEM) and optical imaging, as compared to the graphene grown on untreated copper foil. Different batches (but with the same catalog number) of copper foil from Alfa Aesar Company were examined to explore the effect of copper pre-cleaning; consistent growth results were obtained when pre-cleaning was used. This method overcomes a commonly encountered problem in graphene growth and could become one of the standard protocols for preparing the copper foil substrate for growing graphene or other 2D materials.

Self-organization of dislocation-free, high-density, vertically aligned GaN nanocolumns involving InGaN quantum wells on graphene/SiO2 covered with a thin AlN buffer layer.

PubMed

Hayashi, Hiroaki; Konno, Yuta; Kishino, Katsumi

2016-02-05

We demonstrated the self-organization of high-density GaN nanocolumns on multilayer graphene (MLG)/SiO2 covered with a thin AlN buffer layer by RF-plasma-assisted molecular beam epitaxy. MLG/SiO2 substrates were prepared by the transfer of CVD graphene onto thermally oxidized SiO2/Si [100] substrates. Employing the MLG with an AlN buffer layer enabled the self-organization of high-density and vertically aligned nanocolumns. Transmission electron microscopy observation revealed that no threading dislocations, stacking faults, or twinning defects were included in the self-organized nanocolumns. The photoluminescence (PL) peak intensities of the self-organized GaN nanocolumns were 2.0-2.6 times higher than those of a GaN substrate grown by hydride vapor phase epitaxy. Moreover, no yellow luminescence or ZB-phase GaN emission was observed from the nanocolumns. An InGaN/GaN MQW and p-type GaN were integrated into GaN nanocolumns grown on MLG, displaying a single-peak PL emission at a wavelength of 533 nm. Thus, high-density nitride p-i-n nanocolumns were fabricated on SiO2/Si using the transferred MLG interlayer, indicating the possibility of developing visible nanocolumn LEDs on graphene/SiO2.

Multilayer graphene growth on polar dielectric substrates using chemical vapour deposition

NASA Astrophysics Data System (ADS)

Karamat, S.; Çelik, K.; Shah Zaman, S.; Oral, A.

2018-06-01

High quality of graphene is necessary for its applications at industrial scale production. The most convenient way is its direct growth on dielectrics which avoid the transfer route of graphene from metal to dielectric substrate usually followed by graphene community. The choice of a suitable dielectric for the gate material which can replace silicon dioxide (SiO2) is in high demand. Various properties like permittivity, thermodynamic stability, film morphology, interface quality, bandgap and band alignment of other dielectrics with graphene needs more exploration. A potential dielectric material is required which could be used to grow graphene with all these qualities. Direct growth of graphene on magnesium oxide (MgO) substrates is an interesting idea and will be a new addition in the library of 2D materials. The present work is about the direct growth of graphene on MgO substrates by an ambient pressure chemical vapour deposition (CVD) method. We address the surface instability issue of the polar oxides which is the most challenging factor in MgO. Atomic force microscopy (AFM) measurements showed the topographical features of the graphene coated on MgO. X-ray photoelectron spectroscopy (XPS) study is carried out to extract information regarding the presence of necessary elements, their bonding with substrates and to confirm the sp-2 hybridization of carbon, which is a characteristic feature of graphene film. The chemical shift is due to the surface reconstruction of MgO in the prepared samples. For graphene-MgO interface, valence band offset (VBO) and conduction band offset (CBO) extracted from valence band spectra reported. Further, we predicted the energy band diagram for single layer and thin film of graphene. By using the room-temperature energy band gap values of MgO and graphene, the CBO is calculated to be 6.85 eV for single layer and 5.66 eV for few layer (1-3) of graphene layers.

Strain-free bulk-like GaN grown by hydride-vapor-phase-epitaxy on two-step epitaxial lateral overgrown GaN template

NASA Astrophysics Data System (ADS)

Gogova, D.; Kasic, A.; Larsson, H.; Hemmingsson, C.; Monemar, B.; Tuomisto, F.; Saarinen, K.; Dobos, L.; Pécz, B.; Gibart, P.; Beaumont, B.

2004-07-01

Crack-free bulk-like GaN with high crystalline quality has been obtained by hydride-vapor-phase-epitaxy (HVPE) growth on a two-step epitaxial lateral overgrown GaN template on sapphire. During the cooling down stage, the as-grown 270-μm-thick GaN layer was self-separated from the sapphire substrate. Plan-view transmission electron microscopy images show the dislocation density of the free-standing HVPE-GaN to be ˜2.5×107 cm-2 on the Ga-polar face. A low Ga vacancy related defect concentration of about 8×1015 cm-3 is extracted from positron annihilation spectroscopy data. The residual stress and the crystalline quality of the material are studied by two complementary techniques. Low-temperature photoluminescence spectra show the main neutral donor bound exciton line to be composed of a doublet structure at 3.4715 (3.4712) eV and 3.4721 (3.4718) eV for the Ga- (N-) polar face with the higher-energy component dominating. These line positions suggest virtually strain-free material on both surfaces with high crystalline quality as indicated by the small full width at half maximum values of the donor bound exciton lines. The E1(TO) phonon mode position measured at 558.52 cm-1 (Ga face) by infrared spectroscopic ellipsometry confirms the small residual stress in the material, which is hence well suited to act as a lattice-constant and thermal-expansion-coefficient matched substrate for further homoepitaxy, as needed for high-quality III-nitride device applications.

Screening effect of graphite and bilayer graphene on excitons in MoSe2 monolayer

NASA Astrophysics Data System (ADS)

Wang, Yuan; Zhang, Shuai; Huang, Di; Cheng, Jingxin; Li, Yingguo; Wu, Shiwei

2017-03-01

Excitons in transition metal dichalcogenide monolayer have recently attracted great interest due to their extremely large binding energy, causing giant bandgap renormalization. In this work, we examined the screening effect of graphite and bilayer graphene on the excitons in molybdenum diselenide (MoSe2) monolayer grown by molecular beam epitaxy (MBE). Through the combinational study of scanning tunneling spectroscopy (STS) and photoluminescence (PL) measurements, we determined the binding energy of ~0.58 eV for MoSe2 monolayer on both substrates at 16 K, and no obvious difference between them. Our result is consistent with a previous report [Zhang et al 2015 Nano Letters 15, 6494], but is contradictory to another one [Ugeda 2014 Nature Materials 13, 1091]. Physical picture for no noticeable difference on screening effect between bilayer graphene and graphite substrate is discussed. Possible reasons for causing the discrepancy are also mentioned.

Functional Epitaxial Oxide Devices

DTIC Science & Technology

2010-04-12

complex oxides , epitaxial growth, antennas, varactors 16. SECURITY CLASSIFICATION OF: REPORT U b. ABSTRACT u c. THIS PAGE u 17. LIMITATION OF...Technical Report DATES COVERED (From - To) 17-06-2008-31-12-2009 4. TITLE AND SUBTITLE Functional Epitaxial Oxide Devices 5a. CONTRACT NUMBER NA...This research effort addresses the need for high performance radio frequency (RF) components, specifically varactors and miniaturized, high gain

The Selective Epitaxy of Silicon at Low Temperatures.

NASA Astrophysics Data System (ADS)

Lou, Jen-Chung

1991-01-01

This dissertation has developed a process for the selective epitaxial growth (SEG) of silicon at low temperatures using a dichlorosilane-hydrogen mixture in a hot-wall low pressure chemical vapor deposition (LPCVD) reactor. Some basic issues concerning the quality of epilayers --substrate preparation, ex-situ and in-situ cleaning, and deposition cycle, have been studied. We find it necessary to use a plasma etch to open epitaxial windows for the SEG of Si. A cycled plasma etch, a thin sacrificial oxide growth, and an oxide etching step can completely remove plasma-etch-induced surface damage and contaminants, which result in high quality epilayers. A practical wafer cleaning step is developed for low temperature Si epitaxial growth. An ex-situ HF vapor treatment can completely remove chemical oxide from the silicon surface and retard the reoxidation of the silicon surface. An in-situ low-concentration DCS cycle can aid in decomposition of surface oxide during a 900 ^circC H_2 prebake step. An HF vapor treatment combined with a low-concentration of DCS cycle consistently achieves defect-free epilayers at 850^circC and lower temperatures. We also show that a BF_sp{2}{+ } or F^+ ion implantation is a potential ex-situ wafer cleaning process for SEG of Si at low temperatures. The mechanism for the formation of surface features on Si epilayers is also discussed. Based on O ^+ ion implantation, we showed that the oxygen incorporation in silicon epilayers suppresses the Si growth rate. Therefore, we attribute the formation of surface features to the local reduction of the Si growth rate due to the dissolution of oxide islands at the epi/substrate interface. Finally, with this developed process for the SEG of silicon, defect-free overgrown epilayers are also obtained. This achievement demonstrates the feasibility for the future silicon-on-oxide (SOI) manufacturing technology.

Ultrathin IBAD MgO films for epitaxial growth on amorphous substrates and sub-50 nm membranes

DOE PAGES

Wang, Siming; Antonakos, C.; Bordel, C.; ...

2016-11-07

Here, a fabrication process has been developed for high energy ion beam assisted deposition (IBAD) biaxial texturing of ultrathin (~1 nm) MgO films, using a high ion-to-atom ratio and post-deposition annealing instead of a homoepitaxial MgO layer. These films serve as the seed layer for epitaxial growth of materials on amorphous substrates such as electron/X-ray transparent membranes or nanocalorimetry devices. Stress measurements and atomic force microscopy of the MgO films reveal decreased stress and surface roughness, while X-ray diffraction of epitaxial overlayers demonstrates the improved crystal quality of films grown epitaxially on IBAD MgO. The process simplifies the synthesis ofmore » IBAD MgO, fundamentally solves the “wrinkle” issue induced by the homoepitaxial layer on sub-50 nm membranes, and enables studies of epitaxial materials in electron/X-ray transmission and nanocalorimetry.« less

Superconducting Ga/GaSe layers grown by van der Waals epitaxy

NASA Astrophysics Data System (ADS)

Desrat, W.; Moret, M.; Briot, O.; Ngo, T.-H.; Piot, B. A.; Jabakhanji, B.; Gil, B.

2018-04-01

We report on the growth of GaSe films by molecular beam epitaxy on both (111)B GaAs and sapphire substrates. X-ray diffraction reveals the perfect crystallinity of GaSe with the c-axis normal to the substrate surface. The samples grown under Ga rich conditions possess an additional gallium film on top of the monochalcogenide layer. This metallic film shows two normal-to-superconducting transitions which are detected at T c ≈ 1.1 K and 6.0 K. They correspond likely to the β and α-phases of gallium in the form of bulk and droplets respectively. Our results demonstrate that van der Waals epitaxy can lead to future high quality hybrid superconductor/monochalcogenide heterostructures.

Epitaxial lift-off of electrodeposited single-crystal gold foils for flexible electronics

NASA Astrophysics Data System (ADS)

Mahenderkar, Naveen K.; Chen, Qingzhi; Liu, Ying-Chau; Duchild, Alexander R.; Hofheins, Seth; Chason, Eric; Switzer, Jay A.

2017-03-01

We introduce a simple and inexpensive procedure for epitaxial lift-off of wafer-size flexible and transparent foils of single-crystal gold using silicon as a template. Lateral electrochemical undergrowth of a sacrificial SiOx layer was achieved by photoelectrochemically oxidizing silicon under light irradiation. A 28-nanometer-thick gold foil with a sheet resistance of 7 ohms per square showed only a 4% increase in resistance after 4000 bending cycles. A flexible organic light-emitting diode based on tris(bipyridyl)ruthenium(II) that was spin-coated on a foil exploited the transmittance and flexibility of the gold foil. Cuprous oxide as an inorganic semiconductor that was epitaxially electrodeposited onto the gold foils exhibited a diode quality factor n of 1.6 (where n = 1.0 for an ideal diode), compared with a value of 3.1 for a polycrystalline deposit. Zinc oxide nanowires electrodeposited epitaxially on a gold foil also showed flexibility, with the nanowires intact up to 500 bending cycles.

Remote catalyzation for direct formation of graphene layers on oxides.