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Sample records for bilayer graphene gap

  1. Effect of impurity doping in gapped bilayer graphene

    SciTech Connect

    Han, Qi; Yan, Baoming; Jia, Zhenzhao; Niu, Jingjing; Yu, Dapeng; Wu, Xiaosong

    2015-10-19

    Impurity doping plays a pivotal role in semiconductor electronics. We study the doping effect in a two-dimensional semiconductor, gapped bilayer graphene. By employing in situ deposition of calcium on the bilayer graphene, dopants are controllably introduced. Low temperature transport results show a variable range hopping conduction near the charge neutrality point persisting up to 50 K, providing evidence for the impurity levels inside the gap. Our experiment confirms a predicted peculiar effect in the gapped bilayer graphene, i.e., formation of in-gap states even if the bare impurity level lies in the conduction band. The result provides perspective on the effect of doping and impurity levels in semiconducting bilayer graphene.

  2. Anomalous conductivity noise in gapped bilayer graphene heterostructure

    NASA Astrophysics Data System (ADS)

    Aamir, Mohammed Ali; Karnatak, Paritosh; Sai, T. Phanindra; Ghosh, Arindam

    Bilayer graphene has unique electronic properties - it has a tunable band gap and also, valley symmetry and pseudospin degree of freedom like its single layer counterpart. In this work, we present a study of conductance fluctuations in dual gated bilayer graphene heterostructures by varying the Fermi energy and the band gap independently. At a fixed band gap, we find that the conductance fluctuations obtained by Fermi energy ensemble sampling increase rapidly as the Fermi energy is tuned to charge neutrality point (CNP) whereas the time-dependent conductance fluctuations diminish rapidly. This discrepancy is completely absent at higher number densities, where the transport is expected to be through the 2D bulk of the bilayer system. This observation indicates that near the CNP, electrical transport is highly sensitive to Fermi energy, but becomes progressively immune to time-varying disorder. A possible explanation may involve transport via edge states which becomes the dominant conduction mechanism when the bilayer graphene is gapped and Fermi energy is situated close to the CNP, thereby causing a dimensional crossover from 2D to 1D transport. Our experiment outlines a possible experimental protocol to probe intrinsic topological states in gapped bilayer graphene.

  3. Molecular doping and band-gap opening of bilayer graphene.

    PubMed

    Samuels, Alexander J; Carey, J David

    2013-03-26

    The ability to induce an energy band gap in bilayer graphene is an important development in graphene science and opens up potential applications in electronics and photonics. Here we report the emergence of permanent electronic and optical band gaps in bilayer graphene upon adsorption of π electron containing molecules. Adsorption of n- or p-type dopant molecules on one layer results in an asymmetric charge distribution between the top and bottom layers and in the formation of an energy gap. The resultant band gap scales linearly with induced carrier density though a slight asymmetry is found between n-type dopants, where the band gap varies as 47 meV/10(13) cm(-2), and p-type dopants where it varies as 40 meV/10(13) cm(-2). Decamethylcobaltocene (DMC, n-type) and 3,6-difluoro-2,5,7,7,8,8-hexacyano-quinodimethane (F2-HCNQ, p-type) are found to be the best molecules at inducing the largest electronic band gaps up to 0.15 eV. Optical adsorption transitions in the 2.8-4 μm region of the spectrum can result between states that are not Pauli blocked. Comparison is made between the band gaps calculated from adsorbate-induced electric fields and from average displacement fields found in dual gate bilayer graphene devices. A key advantage of using molecular adsorption with π electron containing molecules is that the high binding energy can induce a permanent band gap and open up possible uses of bilayer graphene in mid-infrared photonic or electronic device applications.

  4. Excitonic gap formation and condensation in the bilayer graphene structure

    NASA Astrophysics Data System (ADS)

    Apinyan, V.; Kopeć, T. K.

    2016-09-01

    We have studied the excitonic gap formation in the Bernal Stacked, bilayer graphene (BLG) structures at half-filling. Considering the local Coulomb interaction between the layers, we calculate the excitonic gap parameter and we discuss the role of the interlayer and intralayer Coulomb interactions and the interlayer hopping on the excitonic pair formation in the BLG. Particularly, we predict the origin of excitonic gap formation and condensation, in relation to the furthermost interband optical transition spectrum. The general diagram of excitonic phase transition is given, explaining different interlayer correlation regimes. The temperature dependence of the excitonic gap parameter is shown and the role of the chemical potential, in the BLG, is discussed in details.

  5. Gap state analysis in electric-field-induced band gap for bilayer graphene

    PubMed Central

    Kanayama, Kaoru; Nagashio, Kosuke

    2015-01-01

    The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~250 meV is obtained at the maximum displacement field of ~3.1 V/nm, where the current on/off ratio of ~3 × 103 is demonstrated at 20 K. The density of states for the gap states are in the range from the latter half of 1012 to 1013 eV−1cm−2. Although the large amount of gap states at the interface of high-k oxide/bilayer graphene limits the current on/off ratio at present, our results suggest that the reduction of gap states below ~1011 eV−1cm−2 by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications. PMID:26511395

  6. Gapped phase in AA-stacked bilayer graphene

    NASA Astrophysics Data System (ADS)

    Brey, L.; Fertig, H. A.

    2013-03-01

    AA-stacked bilayer graphene supports Fermi circles in its bonding and antibonding bands, which coincide exactly, leading to symmetry breaking in the presence of electron-electron interactions. We analyze a continuum model of this system in the Hartree-Fock approximation, using a self-consistently screened interaction that accounts for the gap in the spectrum in the broken symmetry state. The order parameter in the ground state is shown to be of the Ising type, involving transfer of charge between the layers in opposite directions for different sublattices. We analyze the Ising phase transition for the system and argue that it continuously evolves into a Kosterlitz-Thouless transition in the limit of vanishing interlayer separation d. The transition temperature is shown to depend only on the effective spin stiffness of the system even for d>0, and an estimate of its value suggests that the transition temperature is of the order of a few degrees Kelvin.

  7. Quantum multicriticality in bilayer graphene with a tunable energy gap

    NASA Astrophysics Data System (ADS)

    Throckmorton, Robert E.; Das Sarma, S.

    2014-11-01

    We develop a theory for quantum phases and quantum multicriticality in bilayer graphene in the presence of an explicit energy gap in the noninteracting spectrum by extending previous renormalization group (RG) analyses of electron-electron interactions in gapless bilayer graphene at finite temperature to include the effect of an electric field applied perpendicular to the sample, which produces an energy gap in the single-particle electron-hole dispersion. We determine the possible outcomes of the resulting RG equations, represented by "fixed rays" along which ratios of the coupling constants remain constant and map out the leading instabilities of the system for an interaction of the form of a Coulomb interaction that is screened by two parallel conducting plates placed equidistant from the electron. We find that some of the fixed rays on the "target plane" found in the zero-field case are no longer valid fixed rays, but that all four of the isolated rays are still valid. We also find five additional fixed rays that are not present in the zero-field case. We then construct maps of the leading instability (or instabilities) of the system for the screened Coulomb-like interaction as a function of the overall interaction strength and interaction range for four values of the applied electric field. We find that the pattern of leading instabilities is the same as that found in the zero-field case, namely, that the system is unstable to a layer antiferromagnetic state for short-ranged interactions, to a nematic state for long-ranged interactions, and to both for intermediate-ranged interactions. However, if the interaction becomes too long ranged or too weak, then the system will exhibit no instabilities. The ranges at which the nematic instability first appears, the antiferromagnetic instability disappears, and the nematic instability disappears all decrease with increasing applied electric field. Our main qualitative finding, that the applied electric field opposes

  8. Band-gap engineering with a twist: Formation of intercalant superlattices in twisted graphene bilayers

    NASA Astrophysics Data System (ADS)

    Symalla, Franz; Shallcross, Sam; Beljakov, Igor; Fink, Karin; Wenzel, Wolfgang; Meded, Velimir

    2015-05-01

    Graphene-based materials have long been considered as promising building blocks for a new generation of high-frequency (terahertz) electronic devices, but their use is complicated by the lack of an intrinsic band gap in graphene itself. Here we exploit synthetically controllable incommensuration of twisted graphene bilayers as a scaffold for intercalation of alkali metal ions with the periodicity of the bilayer supercell. Systematic exploration of the energy profiles of the ions as a function of position suggests that the alkali metal ions aggregate commensurately with the symmetry of the twisted bilayer. The intercalated alkali metal ions act as a source of a periodic perturbation on the level of the bilayer supercell, which permits opening and engineering of a band gap between graphene's π bands. The twist angle between the graphene layers determines the structure and disorder of the intercalant sublattice and, consequently, the magnitude of the band gap. Appropriate choices of the intercalant and twist angle thus permit band-gap engineering in graphene. We offer arguments that the impact of intercalation on the all important charge mobility of graphene will be rather small.

  9. Two-photon absorption in gapped bilayer graphene with a tunable chemical potential.

    PubMed

    Brinkley, M K; Abergel, D S L; Clader, B D

    2016-09-14

    Despite the now vast body of two-dimensional materials under study, bilayer graphene remains unique in two ways: it hosts a simultaneously tunable band gap and electron density; and stems from simple fabrication methods. These two advantages underscore why bilayer graphene is critical as a material for optoelectronic applications. In the work that follows, we calculate the one- and two-photon absorption coefficients for degenerate interband absorption in a graphene bilayer hosting an asymmetry gap and adjustable chemical potential-all at finite temperature. Our analysis is comprehensive, characterizing one- and two-photon absorptive behavior over wide ranges of photon energy, gap, chemical potential, and thermal broadening. The two-photon absorption coefficient for bilayer graphene displays a rich structure as a function of photon energy and band gap due to the existence of multiple absorption pathways and the nontrivial dispersion of the low energy bands. This systematic work will prove integral to the design of bilayer-graphene-based nonlinear optical devices.

  10. Two-photon absorption in gapped bilayer graphene with a tunable chemical potential

    NASA Astrophysics Data System (ADS)

    Brinkley, M. K.; Abergel, D. S. L.; Clader, B. D.

    2016-09-01

    Despite the now vast body of two-dimensional materials under study, bilayer graphene remains unique in two ways: it hosts a simultaneously tunable band gap and electron density; and stems from simple fabrication methods. These two advantages underscore why bilayer graphene is critical as a material for optoelectronic applications. In the work that follows, we calculate the one- and two-photon absorption coefficients for degenerate interband absorption in a graphene bilayer hosting an asymmetry gap and adjustable chemical potential—all at finite temperature. Our analysis is comprehensive, characterizing one- and two-photon absorptive behavior over wide ranges of photon energy, gap, chemical potential, and thermal broadening. The two-photon absorption coefficient for bilayer graphene displays a rich structure as a function of photon energy and band gap due to the existence of multiple absorption pathways and the nontrivial dispersion of the low energy bands. This systematic work will prove integral to the design of bilayer-graphene-based nonlinear optical devices.

  11. Band-gap engineering in chemically conjugated bilayer graphene: Ab initio calculations

    NASA Astrophysics Data System (ADS)

    Duong, Dinh Loc; Lee, Seung Mi; Chae, Sang Hul; Ta, Quang Huy; Lee, Si Young; Han, Gang Hee; Bae, Jung Jun; Lee, Young Hee

    2012-05-01

    One-side chemical conjugation of bilayer graphene has limitations not only on opening a band gap of less than 0.2 eV due to a small electric field across bilayer graphene but also on generating highly degenerate semiconducting properties by shifting the Fermi level into either a valence band or a conduction band due to the requirement of heavy doping concentration. Here, we proposed a new strategy of band-gap engineering of bilayer graphene by chemically conjugating double sides of bilayer graphene, one side with an electron-donating group and another side with an electron-withdrawing group. The compensated charges not only created a large band gap of 0.3 eV by invoking an internally strong local dipole field in bilayer graphene but also removed degeneracy by shifting the Fermi level within the band gap. Our approach is easy and straightforward, environmentally stable, and scalable for integration, which is in good contrast with the previous reports of fabricating nanoribbons and high electric field operation.

  12. Quantum multicriticality in bilayer graphene with a tunable energy gap

    NASA Astrophysics Data System (ADS)

    Throckmorton, Robert

    2015-03-01

    We extend previous renormalization group (RG) analyses of electron-electron interactions in gapless bilayer graphene at finite temperature to include the effect of an electric field applied perpendicular to the sample. We determine the possible outcomes of the resulting RG equations, represented by ``fixed rays'' along which ratios of the coupling constants remain constant and map out the leading instabilities of the system for an interaction of the form of a Coulomb interaction that is screened by two parallel conducting plates placed equidistant from the electron. We then construct maps of the leading instability (or instabilities) of the system for the screened Coulomb-like interaction as a function of the overall interaction strength and interaction range for four values of the applied electric field. We find that the pattern of leading instabilities is the same as that found in the zero-field case, namely that the system is unstable to a layer antiferromagnetic state for short-ranged interactions, to a nematic state for long-ranged interactions, and to both for intermediate-ranged interactions. However, if the interaction becomes too long-ranged or too weak, then the system will exhibit no instabilities. Supported by LPS-CMTC and ARO-MURI.

  13. Tunneling Plasmonics in Bilayer Graphene.

    PubMed

    Fei, Z; Iwinski, E G; Ni, G X; Zhang, L M; Bao, W; Rodin, A S; Lee, Y; Wagner, M; Liu, M K; Dai, S; Goldflam, M D; Thiemens, M; Keilmann, F; Lau, C N; Castro-Neto, A H; Fogler, M M; Basov, D N

    2015-08-12

    We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At subnanometer separation, such layers can form either a strongly coupled bilayer graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nanoimaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single- and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene, yet another implication of interlayer tunneling. Our work uncovers essential plasmonic properties in bilayer graphene and suggests a possibility to achieve novel plasmonic functionalities in graphene few-layers.

  14. Fabry-Pérot interference in gapped bilayer graphene with broken anti-Klein tunneling.

    PubMed

    Varlet, Anastasia; Liu, Ming-Hao; Krueckl, Viktor; Bischoff, Dominik; Simonet, Pauline; Watanabe, Kenji; Taniguchi, Takashi; Richter, Klaus; Ensslin, Klaus; Ihn, Thomas

    2014-09-12

    We report the experimental observation of Fabry-Pérot interference in the conductance of a gate-defined cavity in a dual-gated bilayer graphene device. The high quality of the bilayer graphene flake, combined with the device's electrical robustness provided by the encapsulation between two hexagonal boron nitride layers, allows us to observe ballistic phase-coherent transport through a 1-μm-long cavity. We confirm the origin of the observed interference pattern by comparing to tight-binding calculations accounting for the gate-tunable band gap. The good agreement between experiment and theory, free of tuning parameters, further verifies that a gap opens in our device. The gap is shown to destroy the perfect reflection for electrons traversing the barrier with normal incidence (anti-Klein tunneling). The broken anti-Klein tunneling implies that the Berry phase, which is found to vary with the gate voltages, is always involved in the Fabry-Pérot oscillations regardless of the magnetic field, in sharp contrast with single-layer graphene.

  15. Microscopic theoretical model study of band gap opening in AA-stacked bi-layer graphene

    NASA Astrophysics Data System (ADS)

    Sahu, Sivabrata; Parashar, S. K. S.; Rout, G. C.

    2016-05-01

    We address here a tight-binding theoretical model calculation for AA-stacked bi-layer graphene taking into account of a biased potential between two layers to study the density of states and the band dispersion within the total Brillouin zone. We have calculated the electronic Green's function for electron operator corresponding to A and B sub lattices by Zubarev's Green's function technique from which the electronic density of states and the electron band energy dispersion are calculated. The numerically computed density of states and band energy dispersions are investigated by tuning the biased potential to exhibit the band gap by varying the different physical parameters.

  16. Twisting bilayer graphene superlattices.

    PubMed

    Lu, Chun-Chieh; Lin, Yung-Chang; Liu, Zheng; Yeh, Chao-Hui; Suenaga, Kazu; Chiu, Po-Wen

    2013-03-26

    Bilayer graphene is an intriguing material in that its electronic structure can be altered by changing the stacking order or the relative twist angle, yielding a new class of low-dimensional carbon system. Twisted bilayer graphene can be obtained by (i) thermal decomposition of SiC; (ii) chemical vapor deposition (CVD) on metal catalysts; (iii) folding graphene; or (iv) stacking graphene layers one atop the other, the latter of which suffers from interlayer contamination. Existing synthesis protocols, however, usually result in graphene with polycrystalline structures. The present study investigates bilayer graphene grown by ambient pressure CVD on polycrystalline Cu. Controlling the nucleation in early stage growth allows the constituent layers to form single hexagonal crystals. New Raman active modes are shown to result from the twist, with the angle determined by transmission electron microscopy. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.

  17. Bilayer Graphene Electromechanical Systems

    NASA Astrophysics Data System (ADS)

    Champagne, Alexandre; Storms, Matthew; Yigen, Serap; Reulet, Bertrand

    Bilayer graphene is an outstanding electromechanical system, and its electronic and mechanical properties, as well as their coupling, are widely tunable. To the best of our knowledge, simultaneous charge transport and mechanical spectroscopy (via RF mixing) has not been realized in bilayer graphene. We present data showing clear electromechanical resonances in three suspended bilayer devices whose length range from 1 to 2 microns. We first describe the low-temperature current annealing of the devices which is crucial to achieve the transconductance, I -VG , necessary to implement a RF mixing detection method. We describe our RF mixing circuit and data. We measure clear mechanical resonances ranging in frequency from 50 to 140 MHz. We show that we can smoothly tune the resonance frequencies of our bilayer resonators with mechanical strain applied via a backgate voltage. We measure quality factors up to 4000. We briefly discuss the effects of the RF driving power on the dispersion of the mechanical resonance. We aim to use these high quality mechanical resonance as a mechanical sensor of the bilayer quantum Hall phase transitions. We show initial data of a bilayer mechanical resonance as a function of magnetic field and quantum Hall phase transitions.

  18. Topological valley transport at bilayer graphene domain walls

    NASA Astrophysics Data System (ADS)

    Ju, Long; Shi, Zhiwen; Nair, Nityan; Lv, Yinchuan; Jin, Chenhao; Velasco, Jairo; Ojeda-Aristizabal, Claudia; Bechtel, Hans A.; Martin, Michael C.; Zettl, Alex; Analytis, James; Wang, Feng

    2015-04-01

    Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. A tunable bandgap can be induced in bilayer graphene by an external electric field, and such gapped bilayer graphene is predicted to be a topological insulating phase protected by no-valley mixing symmetry, featuring quantum valley Hall effects and chiral edge states. Observation of such chiral edge states, however, is challenging because inter-valley scattering is induced by atomic-scale defects at real bilayer graphene edges. Recent theoretical work has shown that domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators. Here we report an experimental observation of ballistic (that is, with no scattering of electrons) conducting channels at bilayer graphene domain walls. We employ near-field infrared nanometre-scale microscopy (nanoscopy) to image in situ bilayer graphene layer-stacking domain walls on device substrates, and we fabricate dual-gated field effect transistors based on the domain walls. Unlike single-domain bilayer graphene, which shows gapped insulating behaviour under a vertical electrical field, bilayer graphene domain walls feature one-dimensional valley-polarized conducting channels with a ballistic length of about 400 nanometres at 4 kelvin. Such topologically protected one-dimensional chiral states at bilayer graphene domain walls open up opportunities for exploring unique topological phases and valley physics in graphene.

  19. Bilayer Graphene: An Electrically Tunable Semiconductor

    NASA Astrophysics Data System (ADS)

    Min, Hongki; Sahu, Bhagawan; Banerjee, Sanjay; MacDonald, Allan

    2007-03-01

    Using ab initio density functional theory calculations, we verify [1,2] that the energy band structure of bilayer graphene can be tuned by applying an external electric field. As the strength of the external electric field increases, the electronic spectrum of bilayer graphene changes from a that of a zero-gap semiconductor to that of a gapped semiconductor. From the ab initio calculations the external field dependence of the screened interlayer potential difference and tunneling amplitudes are extracted by fitting to a tight-binding model. We discuss the role of interlayer correlations in determining the size of the gap and the accuracy of local density approximation. [1] Edward McCann and Vladimir I. Fal'ko, Phys. Rev. Lett. 96, 086805 (2006). [2] Taisuke Ohta, Aaron Bostwick,, Thomas Seyller, Karsten Horn, and Eli Rotenberg, Science 313, 951 (2006).

  20. Influence of defects on the electronic structures of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kishimoto, Ken; Okada, Susumu

    2016-02-01

    Based on first-principles total-energy calculation, we investigate the electronic structures of bilayer graphene, one of which layers possesses atomic or topological defects, to explore the possibility of band gap engineering of graphene by means of physisorption of defective graphene. Our calculations show that the pristine graphene layer possesses a finite energy gap between bonding and antibonding π states because of the potential undulation caused by the other graphene layers with defects. We also found that the gap values strongly depend on the defect species and their mutual arrangement with respect to the pristine layer.

  1. Visualizing Atomic-Scale Negative Differential Resistance in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Kim, Keun Su; Kim, Tae-Hwan; Walter, Andrew L.; Seyller, Thomas; Yeom, Han Woong; Rotenberg, Eli; Bostwick, Aaron

    2013-01-01

    We investigate the atomic-scale tunneling characteristics of bilayer graphene on silicon carbide using the scanning tunneling microscopy. The high-resolution tunneling spectroscopy reveals an unexpected negative differential resistance (NDR) at the Dirac energy, which spatially varies within the single unit cell of bilayer graphene. The origin of NDR is explained by two near-gap van Hove singularities emerging in the electronic spectrum of bilayer graphene under a transverse electric field, which are strongly localized on two sublattices in different layers. Furthermore, defects near the tunneling contact are found to strongly impact on NDR through the electron interference. Our result provides an atomic-level understanding of quantum tunneling in bilayer graphene, and constitutes a useful step towards graphene-based tunneling devices.

  2. Visualizing atomic-scale negative differential resistance in bilayer graphene.

    PubMed

    Kim, Keun Su; Kim, Tae-Hwan; Walter, Andrew L; Seyller, Thomas; Yeom, Han Woong; Rotenberg, Eli; Bostwick, Aaron

    2013-01-18

    We investigate the atomic-scale tunneling characteristics of bilayer graphene on silicon carbide using the scanning tunneling microscopy. The high-resolution tunneling spectroscopy reveals an unexpected negative differential resistance (NDR) at the Dirac energy, which spatially varies within the single unit cell of bilayer graphene. The origin of NDR is explained by two near-gap van Hove singularities emerging in the electronic spectrum of bilayer graphene under a transverse electric field, which are strongly localized on two sublattices in different layers. Furthermore, defects near the tunneling contact are found to strongly impact on NDR through the electron interference. Our result provides an atomic-level understanding of quantum tunneling in bilayer graphene, and constitutes a useful step towards graphene-based tunneling devices. PMID:23373943

  3. Electronic properties of a biased graphene bilayer.

    PubMed

    Castro, Eduardo V; Novoselov, K S; Morozov, S V; Peres, N M R; Lopes dos Santos, J M B; Nilsson, Johan; Guinea, F; Geim, A K; Castro Neto, A H

    2010-05-01

    We study, within the tight-binding approximation, the electronic properties of a graphene bilayer in the presence of an external electric field applied perpendicular to the system-a biased bilayer. The effect of the perpendicular electric field is included through a parallel plate capacitor model, with screening correction at the Hartree level. The full tight-binding description is compared with its four-band and two-band continuum approximations, and the four-band model is shown to always be a suitable approximation for the conditions realized in experiments. The model is applied to real biased bilayer devices, made out of either SiC or exfoliated graphene, and good agreement with experimental results is found, indicating that the model is capturing the key ingredients, and that a finite gap is effectively being controlled externally. Analysis of experimental results regarding the electrical noise and cyclotron resonance further suggests that the model can be seen as a good starting point for understanding the electronic properties of graphene bilayer. Also, we study the effect of electron-hole asymmetry terms, such as the second-nearest-neighbour hopping energies t' (in-plane) and γ(4) (inter-layer), and the on-site energy Δ.

  4. Transfer matrix theory of monolayer graphene/bilayer graphene heterostructure superlattice

    SciTech Connect

    Wang, Yu

    2014-10-28

    We have formulated a transfer matrix method to investigate electronic properties of graphene heterostructure consisting of monolayer graphene and bilayer counterpart. By evaluating transmission, conductance, and band dispersion, we show that, irrespective of the different carrier chiralities in monolayer graphene and bilayer graphene, superlattice consisting of biased bilayer graphene barrier and monolayer graphene well can mimic the electronic properties of conventional semiconductor superlattice, displaying the extended subbands in the quantum tunneling regime and producing anisotropic minigaps for the classically allowed transport. Due to the lateral confinement, the lowest mode has shifted away from the charge neutral point of monolayer graphene component, opening a sizeable gap in concerned structure. Following the gate-field and geometry modulation, all electronic states and gaps between them can be externally engineered in an electric-controllable strategy.

  5. Controlling the Electronic Structure of Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Ohta, Taisuke; Bostwick, Aaron; McChesney, Jessica; Seyller, Thomas; Horn, Karsten; Rotenberg, Eli

    2007-03-01

    Carbon-based materials such as carbon nanotubes, graphite intercalation compounds, fullerenes, and ultrathin graphite films exhibit many exotic phenomena such as superconductivity and an anomalous quantum Hall effect. These findings have caused renewed interest in the electronic structure of ultrathin layers of graphene: a single honeycomb carbon layer that is the building block for these materials. There is a strong motivation to incorporate graphene multilayers into atomic-scale devices, spurred on by rapid progress in their fabrication and manipulation. We have synthesized bilayer graphene thin films deposited on insulating silicon carbide and characterized their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands [1]. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic scale electronic devices. [1] T. Ohta, A. Bostwick, T. Seyller, K. Horn, E. Rotenberg, Science, 313, 951 (2006).

  6. Self healing nature of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Debroy, Sanghamitra; Pavan Kumar Miriyala, V.; Vijaya Sekhar, K.; Acharyya, Swati Ghosh; Acharyya, Amit

    2016-08-01

    The phenomenon of self healing of cracks in bilayer graphene sheet has been studied using molecular dynamics simulations. The bilayer graphene sheet was subjected to uniaxial tensile load resulting in initiation and propagation of cracks on exceeding the ultimate tensile strength. Subsequently, all forces acting on the sheet were removed and sheet was relaxed. The cracks formed in the graphene sheet healed without any external aid within 0.4 ps The phenomenon of self healing of the cracks in graphene sheet was found to be independent of the length of the crack, but occurred for critical crack opening distance less than 5 Å for AA stacked sheet and 13 Å for AB stacked bilayer graphene sheet. Self healing was observed for both AB (mixed stacking of armchair and zigzag graphene sheet) and AA (both sheets of similar orientation i.e. either armchair-armchair or zigzag-zigzag) stacking of bilayer graphene sheet.

  7. 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

  8. Negative Coulomb Drag in Double Bilayer Graphene.

    PubMed

    Li, J I A; Taniguchi, T; Watanabe, K; Hone, J; Levchenko, A; Dean, C R

    2016-07-22

    We report on an experimental measurement of Coulomb drag in a double quantum well structure consisting of bilayer-bilayer graphene, separated by few layer hexagonal boron nitride. At low temperatures and intermediate densities, a novel negative drag response with an inverse sign is observed, distinct from the momentum and energy drag mechanisms previously reported in double monolayer graphene. By varying the device aspect ratio, the negative drag component is suppressed and a response consistent with pure momentum drag is recovered. In the momentum drag dominated regime, excellent quantitative agreement with the density and temperature dependence predicted for double bilayer graphene is found. PMID:27494491

  9. Negative Coulomb Drag in Double Bilayer Graphene.

    PubMed

    Li, J I A; Taniguchi, T; Watanabe, K; Hone, J; Levchenko, A; Dean, C R

    2016-07-22

    We report on an experimental measurement of Coulomb drag in a double quantum well structure consisting of bilayer-bilayer graphene, separated by few layer hexagonal boron nitride. At low temperatures and intermediate densities, a novel negative drag response with an inverse sign is observed, distinct from the momentum and energy drag mechanisms previously reported in double monolayer graphene. By varying the device aspect ratio, the negative drag component is suppressed and a response consistent with pure momentum drag is recovered. In the momentum drag dominated regime, excellent quantitative agreement with the density and temperature dependence predicted for double bilayer graphene is found.

  10. Negative Coulomb Drag in Double Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Li, J. I. A.; Taniguchi, T.; Watanabe, K.; Hone, J.; Levchenko, A.; Dean, C. R.

    2016-07-01

    We report on an experimental measurement of Coulomb drag in a double quantum well structure consisting of bilayer-bilayer graphene, separated by few layer hexagonal boron nitride. At low temperatures and intermediate densities, a novel negative drag response with an inverse sign is observed, distinct from the momentum and energy drag mechanisms previously reported in double monolayer graphene. By varying the device aspect ratio, the negative drag component is suppressed and a response consistent with pure momentum drag is recovered. In the momentum drag dominated regime, excellent quantitative agreement with the density and temperature dependence predicted for double bilayer graphene is found.

  11. Temperature effect on plasmons in bilayer graphene

    SciTech Connect

    Patel, Digish K. Sharma, A. C.; Ashraf, S. S. Z.; Ambavale, S. K.

    2015-06-24

    We have theoretically investigated the plasmon dispersion and damping rate of doped bilayer graphene (BLG) at finite temperatures within the random phase approximation. Our computed results on plasmon dispersion show that plasmon frequency enhances with increasing temperatures in contrast to single layer graphene where it is suppressed. This can be attributed to the fact that the dynamic response of the electron gas or screening in bilayer graphene is different from that of single layer graphene. Further the temperature effect on damping rate is also discussed.

  12. Piezoelectricity in asymmetrically strained bilayer graphene

    NASA Astrophysics Data System (ADS)

    Van der Donck, M.; De Beule, C.; Partoens, B.; Peeters, F. M.; Van Duppen, B.

    2016-09-01

    We study the electronic properties of commensurate faulted bilayer graphene by diagonalizing the one-particle Hamiltonian of the bilayer system in a complete basis of Bloch states of the individual graphene layers. Our novel approach is very general and can be easily extended to any commensurate graphene-based heterostructure. Here, we consider three cases: (i) twisted bilayer graphene, (ii) bilayer graphene where triaxial stress is applied to one layer and (iii) bilayer graphene where uniaxial stress is applied to one layer. We show that the resulting superstructures can be divided into distinct classes, depending on the twist angle or the magnitude of the induced strain. The different classes are distinguished from each other by the interlayer coupling mechanism, resulting in fundamentally different low-energy physics. For the cases of triaxial and uniaxial stress, the individual graphene layers tend to decouple and we find significant charge transfer between the layers. In addition, this piezoelectric effect can be tuned by applying a perpendicular electric field. Finally, we show how our approach can be generalized to multilayer systems.

  13. Graphene: Mind the gap

    NASA Astrophysics Data System (ADS)

    Novoselov, Kostya

    2007-10-01

    Research now shows that interaction with silicon carbide substrate leads to the opening of a semiconductor gap in epitaxial graphene. This is an important first step towards bandgap engineering in this two-dimensional crystal, and its incorporation in electronic devices.

  14. Resonant Tunneling in Double Bilayer Graphene Heterostructures

    NASA Astrophysics Data System (ADS)

    Fallahazad, Babak; Lee, Kayoung; Kang, Sangwoo; Xue, Jiamin; Larentis, Stefano; Corbet, Christopher; Kim, Kyounghwan; Movva, Hema; Taniguchi, Takashi; Watanabe, Kenji; Register, Leonard; Banerjee, Sanjay; Tutuc, Emanuel

    2015-03-01

    We present the realization and characterization of independently contacted and rotationally aligned double bilayer graphene heterostructures, that show gate-tunable tunneling resonances and negative differential resistance in their interlayer current-voltage characteristics. Our devices are fabricated by successively stacking mechanically exfoliated bilayer graphene and hexagonal boron nitride dielectric using a layer-by-layer transfer technique. The bilayers are rotationally aligned during the device fabrication by selecting flakes with straight edges, and using them as a reference for alignment. We determine the heterostructure energy band alignment at the tunneling resonance using the individual layer carrier densities, and including the chemical potential dependence on the carrier density. Our analysis show that the tunneling resonances occur when the charge neutrality points of the two bilayer graphene are energetically aligned, which suggests the resonances stem from the momentum conserving tunneling. This work has been supported by NRI-SWAN, ONR, and Intel.

  15. How Bilayer Graphene Got a Bandgap

    SciTech Connect

    Feng Wang

    2009-06-02

    Graphene is the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. But theres a catch: graphene has no bandgap. Now Feng Wang and his colleagues at Berkeley Lab and UC Berkeley have engineered a bandgap in bilayer graphene that can be precisely controlled from 0 to 250 milli-electron volts, which is the energy of infrared radiation.

  16. How Bilayer Graphene Got a Bandgap

    ScienceCinema

    Wang, Feng

    2016-07-12

    Graphene is the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. But theres a catch: graphene has no bandgap. Now Feng Wang and his colleagues at Berkeley Lab and UC Berkeley have engineered a bandgap in bilayer graphene that can be precisely controlled from 0 to 250 milli-electron volts, which is the energy of infrared radiation.

  17. How Bilayer Graphene Got a Bandgap

    ScienceCinema

    Feng Wang

    2010-01-08

    Graphene is the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. But theres a catch: graphene has no bandgap. Now Feng Wang and his colleagues at Berkeley Lab and UC Berkeley have engineered a bandgap in bilayer graphene that can be precisely controlled from 0 to 250 milli-electron volts, which is the energy of infrared radiation.

  18. Magnetic ratchet effect in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kheirabadi, Narjes; McCann, Edward; Fal'ko, Vladimir I.

    2016-10-01

    We consider the orbital effect of an in-plane magnetic field on electrons in bilayer graphene, deriving linear-in-field contributions to the low-energy Hamiltonian arising from the presence of either skew interlayer coupling or interlayer potential asymmetry, the latter being tunable by an external metallic gate. To illustrate the relevance of such terms, we consider the ratchet effect in which a dc current results from the application of an alternating electric field in the presence of an in-plane magnetic field and inversion-symmetry breaking. By comparison with recent experimental observations in monolayer graphene [C. Drexler et al., Nat. Nanotechnol. 8, 104 (2013), 10.1038/nnano.2012.231], we estimate that the effect in bilayer graphene can be two orders of magnitude greater than that in monolayer graphene, illustrating that the bilayer is an ideal material for the realization of optoelectronic effects that rely on inversion-symmetry breaking.

  19. Thermoelectric Power in Bilayer Graphene Device with Ionic Liquid Gating.

    PubMed

    Chien, Yung-Yu; Yuan, Hongtao; Wang, Chang-Ran; Lee, Wei-Li

    2016-01-01

    The quest for materials showing large thermoelectric power has long been one of the important subjects in material science and technology. Such materials have great potential for thermoelectric cooling and also high figure of merit ZT thermoelectric applications. We have fabricated bilayer graphene devices with ionic-liquid gating in order to tune its band gap via application of a perpendicular electric field on a bilayer graphene. By keeping the Fermi level at charge neutral point during the cool-down, we found that the charge puddles effect can be greatly reduced and thus largely improve the transport properties at low T in graphene-based devices using ionic liquid gating. At (Vig, Vbg) = (-1 V, +23 V), a band gap of about 36.6 ± 3 meV forms, and a nearly 40% enhancement of thermoelectric power at T = 120 K is clearly observed. Our works demonstrate the feasibility of band gap tuning in a bilayer graphene using ionic liquid gating. We also remark on the significant influence of the charge puddles effect in ionic-liquid-based devices.

  20. Bilayer graphene: physics and application outlook in photonics

    NASA Astrophysics Data System (ADS)

    Yan, Hugen

    2015-05-01

    Layered materials, such as graphene, transition metal dichacogenides and black phosphorus have attracted lots of attention recently. They are emerging novel materials in electronics and photonics, with tremendous potential in revolutionizing the traditional electronics and photonics industry. Marrying layered material to the nanophotonics is being proved fruitful. With the recent emphasis and development of metasurfaces in nanophotonics, atomically thin materials can find their unique position and strength in this field. In this article, I will focus on one specific two dimensional material: bilayer graphene. Basic physics will be reviewed, such as band-gap opening, electron-phonon interaction, phonon-plasmon interaction and Fano resonances in the optical response. Moreover, I will review the application of bilayer graphene as a sensitive and fast photodetector. An outlook will be given in the final part of the paper.

  1. π Berry phase and Veselago lens in a bilayer graphene np junction

    NASA Astrophysics Data System (ADS)

    Park, Sunghun; Sim, H.-S.

    2011-12-01

    Klein tunneling in gapless bilayer graphene, perfect reflection of electrons injecting normal to a pn junction, is expected to disappear in the presence of energy band gap induced by external gates. We theoretically show that the Klein effect still exists in gapped bilayer graphene, provided that the gaps in the n and p regions are balanced such that the polarization of electron pseudospin has the same normal component to the bilayer plane in the regions. We attribute the Klein effect to π Berry phase (rather than the conventional value 2π of bilayer graphene) and to electron-hole and time-reversal symmetries. The Klein effect and the π Berry phase can be identified in an electronic Veselago lens, an important component of graphene-based electron optics.

  2. Bilayer graphene quantum dot defined by topgates

    SciTech Connect

    Müller, André; Kaestner, Bernd; Hohls, Frank; Weimann, Thomas; Pierz, Klaus; Schumacher, Hans W.

    2014-06-21

    We investigate the application of nanoscale topgates on exfoliated bilayer graphene to define quantum dot devices. At temperatures below 500 mK, the conductance underneath the grounded gates is suppressed, which we attribute to nearest neighbour hopping and strain-induced piezoelectric fields. The gate-layout can thus be used to define resistive regions by tuning into the corresponding temperature range. We use this method to define a quantum dot structure in bilayer graphene showing Coulomb blockade oscillations consistent with the gate layout.

  3. Berry phase transition in twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Rode, Johannes C.; Smirnov, Dmitri; Schmidt, Hennrik; Haug, Rolf J.

    2016-09-01

    The electronic dispersion of a graphene bilayer is highly dependent on rotational mismatch between layers and can be further manipulated by electrical gating. This allows for an unprecedented control over electronic properties and opens up the possibility of flexible band structure engineering. Here we present novel magnetotransport data in a twisted bilayer, crossing the energetic border between decoupled monolayers and coupled bilayer. In addition a transition in Berry phase between π and 2π is observed at intermediate magnetic fields. Analysis of Fermi velocities and gate induced charge carrier densities suggests an important role of strong layer asymmetry for the observed phenomena.

  4. Graphene Monoxide Bilayer As a High-Performance on/off Switching Media for Nanoelectronics.

    PubMed

    Woo, Jungwook; Yun, Kyung-Han; Chung, Yong-Chae

    2016-04-27

    The geometries and electronic characteristics of the graphene monoxide (GMO) bilayer are predicted via density functional theory (DFT) calculations. All the possible sequences of the GMO bilayer show the typical interlayer bonding characteristics of two-dimensional bilayer systems with a weak van der Waals interaction. The band gap energies of the GMO bilayers are predicted to be adequate for electronic device application, indicating slightly smaller energy gaps (0.418-0.448 eV) compared to the energy gap of the monolayer (0.536 eV). Above all, in light of the band gap engineering, the band gap of the GMO bilayer responds to the external electric field sensitively. As a result, a semiconductor-metal transition occurs at a small critical electric field (EC = 0.22-0.30 V/Å). It is therefore confirmed that the GMO bilayer is a strong candidate for nanoelectronics. PMID:27046262

  5. Graphene Monoxide Bilayer As a High-Performance on/off Switching Media for Nanoelectronics.

    PubMed

    Woo, Jungwook; Yun, Kyung-Han; Chung, Yong-Chae

    2016-04-27

    The geometries and electronic characteristics of the graphene monoxide (GMO) bilayer are predicted via density functional theory (DFT) calculations. All the possible sequences of the GMO bilayer show the typical interlayer bonding characteristics of two-dimensional bilayer systems with a weak van der Waals interaction. The band gap energies of the GMO bilayers are predicted to be adequate for electronic device application, indicating slightly smaller energy gaps (0.418-0.448 eV) compared to the energy gap of the monolayer (0.536 eV). Above all, in light of the band gap engineering, the band gap of the GMO bilayer responds to the external electric field sensitively. As a result, a semiconductor-metal transition occurs at a small critical electric field (EC = 0.22-0.30 V/Å). It is therefore confirmed that the GMO bilayer is a strong candidate for nanoelectronics.

  6. Hexagonal-shaped monolayer-bilayer quantum disks in graphene: A tight-binding approach

    NASA Astrophysics Data System (ADS)

    da Costa, D. R.; Zarenia, M.; Chaves, Andrey; Pereira, J. M.; Farias, G. A.; Peeters, F. M.

    2016-07-01

    Using the tight-binding approach, we investigate confined states in two different hybrid monolayer-bilayer systems: (i) a hexagonal monolayer area surrounded by bilayer graphene in the presence of a perpendicularly applied electric field and (ii) a hexagonal bilayer graphene dot surrounded by monolayer graphene. The dependence of the energy levels on dot size and external magnetic field is calculated. We find that the energy spectrum for quantum dots with zigzag edges consists of states inside the gap which range from dot-localized states, edge states, to mixed states coexisting together, whereas for dots with armchair edges, only dot-localized states are observed.

  7. Graphene-Templated Supported Lipid Bilayer Nanochannels.

    PubMed

    Li, Wan; Chung, Jean K; Lee, Young Kwang; Groves, Jay T

    2016-08-10

    The use of patterned substrates to impose geometrical restriction on the lateral mobility of molecules in supported lipid membranes has found widespread utility in studies of cell membranes. Here, we template-pattern supported lipid membranes with nanopatterned graphene. We utilize focused ion beam milling to pattern graphene on its growth substrate, then transfer the patterned graphene to fresh glass substrates for subsequent supported membrane formation. We observe that graphene functions as an excellent lateral diffusion barrier for supported lipid bilayers. Additionally, the observed diffusion dynamics of lipids in nanoscale graphene channels reveal extremely low boundary effects, a common problem with other materials. We suggest this is attributable to the ultimate thinness of graphene. PMID:27362914

  8. Layer resolved capacitive probing of graphene bilayers

    NASA Astrophysics Data System (ADS)

    Zibrov, Alexander; Parmentier, François; Li, Jia; Wang, Lei; Hunt, Benjamin; Dean, Cory; Hone, James; Taniguchi, Takashi; Watanabe, Kenji; Young, Andrea

    Compared to single layer graphene, graphene bilayers have an additional ``which-layer'' degree of freedom that can be controlled by an external electric field in a dual-gated device geometry. We describe capacitance measurements capable of directly probing this degree of freedom. By performing top gate, bottom gate, and penetration field capacitance measurements, we directly extract layer polarization of both Bernal and twisted bilayers. We will present measurements of hBN encapsulated bilayers at both zero and high magnetic field, focusing on the physics of the highly degenerate zero-energy Landau level in the high magnetic field limit where spin, valley, and layer degeneracy are all lifted by electronic interactions.

  9. Self-folding graphene-polymer bilayers

    NASA Astrophysics Data System (ADS)

    Deng, Tao; Yoon, ChangKyu; Jin, Qianru; Li, Mingen; Liu, Zewen; Gracias, David H.

    2015-05-01

    In order to incorporate the extraordinary intrinsic thermal, electrical, mechanical, and optical properties of graphene with three dimensional (3D) flexible substrates, we introduce a solvent-driven self-folding approach using graphene-polymer bilayers. A polymer (SU-8) film was spin coated atop chemically vapor deposited graphene films on wafer substrates and graphene-polymer bilayers were patterned with or without metal electrodes using photolithography, thin film deposition, and etching. After patterning, the bilayers were released from the substrates and they self-folded to form fully integrated, curved, and folded structures. In contrast to planar graphene sensors on rigid substrates, we assembled curved and folded sensors that are flexible and they feature smaller form factors due to their 3D geometry and large surface areas due to their multiple rolled architectures. We believe that this approach could be used to assemble a range of high performance 3D electronic and optical devices of relevance to sensing, diagnostics, wearables, and energy harvesting.

  10. Self-folding graphene-polymer bilayers

    SciTech Connect

    Deng, Tao; Yoon, ChangKyu; Jin, Qianru; Li, Mingen; Liu, Zewen; Gracias, David H.

    2015-05-18

    In order to incorporate the extraordinary intrinsic thermal, electrical, mechanical, and optical properties of graphene with three dimensional (3D) flexible substrates, we introduce a solvent-driven self-folding approach using graphene-polymer bilayers. A polymer (SU-8) film was spin coated atop chemically vapor deposited graphene films on wafer substrates and graphene-polymer bilayers were patterned with or without metal electrodes using photolithography, thin film deposition, and etching. After patterning, the bilayers were released from the substrates and they self-folded to form fully integrated, curved, and folded structures. In contrast to planar graphene sensors on rigid substrates, we assembled curved and folded sensors that are flexible and they feature smaller form factors due to their 3D geometry and large surface areas due to their multiple rolled architectures. We believe that this approach could be used to assemble a range of high performance 3D electronic and optical devices of relevance to sensing, diagnostics, wearables, and energy harvesting.

  11. Theoretical investigation of structural and optical properties of semi-fluorinated bilayer graphene

    NASA Astrophysics Data System (ADS)

    Xiao-Jiao, San; Bai, Han; Jing-Geng, Zhao

    2016-03-01

    We have studied the structural and optical properties of semi-fluorinated bilayer graphene using density functional theory. When the interlayer distance is 1.62 Å, the two graphene layers in AA stacking can form strong chemical bonds. Under an in-plane stress of 6.8 GPa, this semi-fluorinated bilayer graphene becomes the energy minimum. Our calculations indicate that the semi-fluorinated bilayer graphene with the AA stacking sequence and rectangular fluorinated configuration is a nonmagnetic semiconductor (direct gap of 3.46 eV). The electronic behavior at the vicinity of the Fermi level is mainly contributed by the p electrons of carbon atoms forming C=C double bonds. We compare the optical properties of the semi-fluorinated bilayer graphene with those of bilayer graphene stacked in the AA sequence and find that the semi-fluorinated bilayer graphene is anisotropic for the polarization vector on the basal plane of graphene and a red shift occurs in the [010] polarization, which makes the peak at the low-frequency region located within visible light. This investigation is useful to design polarization-dependence optoelectronic devices. Project supported by the Program of Educational Commission of Heilongjiang Province, China (Grant No. 12541131).

  12. Electronic properties of graphene-based bilayer systems

    NASA Astrophysics Data System (ADS)

    Rozhkov, A. V.; Sboychakov, A. O.; Rakhmanov, A. L.; Nori, Franco

    2016-08-01

    This article reviews the theoretical and experimental work related to the electronic properties of bilayer graphene systems. Three types of bilayer stackings are discussed: the AA, AB, and twisted bilayer graphene. This review covers single-electron properties, effects of static electric and magnetic fields, bilayer-based mesoscopic systems, spin-orbit coupling, dc transport and optical response, as well as spontaneous symmetry violation and other interaction effects. The selection of the material aims to introduce the reader to the most commonly studied topics of theoretical and experimental research in bilayer graphene.

  13. Giant magnetoresistance in bilayer graphene nanoflakes

    NASA Astrophysics Data System (ADS)

    Farghadan, Rouhollah; Farekiyan, Marzieh

    2016-09-01

    Coherent spin transport through bilayer graphene (BLG) nanoflakes sandwiched between two electrodes made of single-layer zigzag graphene nanoribbon was investigated by means of Landauer-Buttiker formalism. Application of a magnetic field only on BLG structure as a channel produces a perfect spin polarization in a large energy region. Moreover, the conductance could be strongly modulated by magnetization of the zigzag edge of AB-stacked BLG, and the junction, entirely made of carbon, produces a giant magnetoresistance (GMR) up to 100%. Intestinally, GMR and spin polarization could be tuned by varying BLG width and length. Generally, MR in a AB-stacked BLG strongly increases (decreases) with length (width).

  14. Modulation of band gap by an applied electric field in silicene-based hetero-bilayers.

    PubMed

    Nigam, Sandeep; Gupta, Sanjeev K; Majumder, C; Pandey, Ravindra

    2015-05-01

    Electronic properties of the hetero-structures consisting of silicene, graphene and BN monolayers under the influence of an electric field were investigated using density functional theory. With no electric field, both silicene/graphene and silicene/BN were shown to have a finite gap of about ∼50 meV, though silicene is a zero-gap two-dimensional material. Application of the field perpendicular to the bilayer system was found to facilitate modulation of the band gap, exhibiting an approximately linear relationship with the gap energy, in contrast to what was seen for the constituent monolayers. Also, the degree of the modulation was mainly determined by the Si-pz electronic states at the interface of the silicene/graphene and silicene/BN bilayers.

  15. Quantum many body physics in single and bilayer graphene

    NASA Astrophysics Data System (ADS)

    Nandkishore, Rahul

    Two dimensional electron systems (2DES) provide a uniquely promising avenue for investigation of many body physics. Graphene constitutes a new and unusual 2DES, which may give rise to unexpected collective phenomena. However, the vanishing density of states in charge neutral single layer graphene suppresses many body effects, and one has to alter the system to observe strongly ordered states. We consider three ways of accessing quantum many body physics using graphene. First, we consider doping single layer graphene to a Van Hove singularity in the density of states. We show that there are strong instabilities to several strongly ordered states, with the leading instability being to a d-wave superconducting state. The superconducting state realizes chiral superconductivity, an exotic form of superconductivity wherein the phase of the order parameter winds by 47r as we go around the Fermi surface. We also discuss the nature of the spin density wave state which is the principal competitor to superconductivity in doped graphene. Next, we study bilayer graphene (BLG), which has a non-vanishing density of states even at charge neutrality. We show that Coulomb interactions give rise to a zero bias anomaly in the tunneling density of states for BLG, which manifests itself at high energy scales. We also show that the quadratic band crossing in BLG is unstable to arbitrarily weak interactions, and estimate the energy scale for formation of strongly ordered states. We show that gapped states in BLG have topological properties, and we classify the various possible gapped and gapless states in terms of symmetries. We study the competition between various ordered states, and discuss how the nature of the ground state may be deduced experimentally. We also discuss recent experimental observations of strongly ordered states in bilayer graphene. Finally, we study bilayer graphene in a transverse magnetic field, focusing on the properties of the quantum Hall ferromagnet (QHF) state

  16. Valley filtering using electrostatic potentials in bilayer graphene

    NASA Astrophysics Data System (ADS)

    da Costa, D. R.; Chaves, Andrey; Sena, S. H. R.; Farias, G. A.; Peeters, F. M.

    2015-07-01

    Propagation of an electron wave packet through a quantum point contact (QPC) defined by electrostatic gates in bilayer graphene is investigated. The gates provide a bias between the layers, in order to produce an energy gap. If the gates on both sides of the contact produce the same bias, steps in the electron transmission probability are observed, as in the usual QPC. However, if the bias is inverted on one of the sides of the QPC, only electrons belonging to one of the Dirac valleys are allowed to pass, which provides a very efficient valley filtering.

  17. The Coulomb excitations of Bernal bilayer graphene under external fields

    SciTech Connect

    Wu, Jhao-Ying; Lin, Ming-Fa

    2014-03-31

    We study the field effects on the Coulomb excitation spectrum of Bernal bilayer graphene by using the tight-binding model and the random-phase approximation. The electric field opens the band gap and creates the saddle points, the latter brings about a prominent interband plasmon. On the other hand, the magnetic field induces the dispersionless Landau levels (LLs) that causes the inter-LL plasmons. The two kinds of field-induced plasmon modes can be further tuned by the magnitude of momentum transfer and the field strength. The predicted results may be further validated by the inelastic light-scattering or high-resolution electron-energy-loss spectroscopy (HREELLS)

  18. Atomistic mechanisms for bilayer growth of graphene on metal substrates

    SciTech Connect

    Chen, Wei; Cui, Ping; Zhu, Wenguang; Kaxiras, Efthimios; Gao, Yanfei; Zhang, Zhenyu

    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, 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.

  19. Atomistic mechanisms for bilayer growth of graphene on metal substrates

    DOE PAGESBeta

    Chen, Wei; Cui, Ping; Zhu, Wenguang; Kaxiras, Efthimios; Gao, Yanfei; Zhang, Zhenyu

    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

  20. Space charge and screening in bilayer graphene.

    PubMed

    Kolomeisky, Eugene B; Straley, Joseph P; Abrams, Daniel L

    2016-11-30

    Undoped bilayer graphene is a two-dimensional semimetal with a low-energy excitation spectrum that is parabolic in the momentum. As a result, the screening of an arbitrary external charge Ze is accompanied by a reconstruction of the ground state: valence band electrons (for Z  >  0) are promoted to form a space charge around the charge while the holes leave the physical picture. The outcome is a flat neutral object resembling the regular atom except that for [Formula: see text] it is described by a strictly linear Thomas-Fermi theory. This theory also predicts that the bilayer's static dielectric constant is the same as that of a two-dimensional electron gas in the long-wavelength limit. PMID:27636158

  1. Twisted bi-layer graphene: microscopic rainbows.

    PubMed

    Campos-Delgado, J; Algara-Siller, G; Santos, C N; Kaiser, U; Raskin, J-P

    2013-10-11

    Blue, pink, and yellow colorations appear from twisted bi-layer graphene (tBLG) when transferred to a SiO2 /Si substrate (SiO2 = 100 nm-thick). Raman and electron microscope studies reveal that these colorations appear for twist angles in the 9-15° range. Optical contrast simulations confirm that the observed colorations are related to the angle-dependent electronic properties of tBLG combined with the reflection that results from the layered structure tBLG/100 nm-thick SiO2 /Si. PMID:23606323

  2. Terahertz conductivity of twisted bilayer graphene.

    PubMed

    Zou, Xingquan; Shang, Jingzhi; Leaw, Jianing; Luo, Zhiqiang; Luo, Liyan; La-o-Vorakiat, Chan; Cheng, Liang; Cheong, S A; Su, Haibin; Zhu, Jian-Xin; Liu, Yanpeng; Loh, Kian Ping; Castro Neto, A H; Yu, Ting; Chia, Elbert E M

    2013-02-01

    Using terahertz time-domain spectroscopy, the real part of optical conductivity [σ(1)(ω)] of twisted bilayer graphene was obtained at different temperatures (10-300 K) in the frequency range 0.3-3 THz. On top of a Drude-like response, we see a strong peak in σ(1)(ω) at ~2.7 THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7 THz to an enhanced density of states at that energy, which is caused by the presence of a van Hove singularity arising from a commensurate twisting of the two graphene layers. PMID:23432306

  3. Interaction driven quantum Hall effect in artificially stacked graphene bilayers.

    PubMed

    Iqbal, Muhammad Zahir; Iqbal, Muhammad Waqas; Siddique, Salma; Khan, Muhammad Farooq; Ramay, Shahid Mahmood; Nam, Jungtae; Kim, Keun Soo; Eom, Jonghwa

    2016-01-01

    The honeycomb lattice structure of graphene gives rise to its exceptional electronic properties of linear dispersion relation and its chiral nature of charge carriers. The exceptional electronic properties of graphene stem from linear dispersion relation and chiral nature of charge carries, originating from its honeycomb lattice structure. Here, we address the quantum Hall effect in artificially stacked graphene bilayers and single layer graphene grown by chemical vapor deposition. The quantum Hall plateaus started to appear more than 3 T and became clearer at higher magnetic fields up to 9 T. Shubnikov-de Hass oscillations were manifestly observed in graphene bilayers texture. These unusual plateaus may have been due to the layers interaction in artificially stacked graphene bilayers. Our study initiates the understanding of interactions between artificially stacked graphene layers. PMID:27098387

  4. Interaction driven quantum Hall effect in artificially stacked graphene bilayers

    PubMed Central

    Iqbal, Muhammad Zahir; Iqbal, Muhammad Waqas; Siddique, Salma; Khan, Muhammad Farooq; Ramay, Shahid Mahmood; Nam, Jungtae; Kim, Keun Soo; Eom, Jonghwa

    2016-01-01

    The honeycomb lattice structure of graphene gives rise to its exceptional electronic properties of linear dispersion relation and its chiral nature of charge carriers. The exceptional electronic properties of graphene stem from linear dispersion relation and chiral nature of charge carries, originating from its honeycomb lattice structure. Here, we address the quantum Hall effect in artificially stacked graphene bilayers and single layer graphene grown by chemical vapor deposition. The quantum Hall plateaus started to appear more than 3 T and became clearer at higher magnetic fields up to 9 T. Shubnikov-de Hass oscillations were manifestly observed in graphene bilayers texture. These unusual plateaus may have been due to the layers interaction in artificially stacked graphene bilayers. Our study initiates the understanding of interactions between artificially stacked graphene layers. PMID:27098387

  5. Electron dynamics of the buffer layer and bilayer graphene on SiC

    NASA Astrophysics Data System (ADS)

    Shearer, Alex J.; Johns, James E.; Caplins, Benjamin W.; Suich, David E.; Hersam, Mark C.; Harris, Charles B.

    2014-06-01

    Angle- and time-resolved two-photon photoemission (TPPE) was used to investigate electronic states in the buffer layer of 4H-SiC(0001). An image potential state (IPS) series was observed on this strongly surface-bound buffer layer, and dispersion measurements indicated free-electron-like behavior for all states in this series. These results were compared with TPPE taken on bilayer graphene, which also show the existence of a free-electron-like IPS series. Lifetimes for the n = 2, and n = 3 states were obtained from time-resolved TPPE; slightly increased lifetimes were observed in the bilayer graphene sample for the n = 2 the n = 3 states. Despite the large band gap of graphene at the center of the Brillouin zone, the lifetime results demonstrate that the graphene layers do not behave as a simple tunneling barrier, suggesting that the buffer layer and graphene overlayers play a direct role in the decay of IPS electrons.

  6. Large-signal model of the bilayer graphene field-effect transistor targeting radio-frequency applications: Theory versus experiment

    SciTech Connect

    Pasadas, Francisco Jiménez, David

    2015-12-28

    Bilayer graphene is a promising material for radio-frequency transistors because its energy gap might result in a better current saturation than the monolayer graphene. Because the great deal of interest in this technology, especially for flexible radio-frequency applications, gaining control of it requires the formulation of appropriate models for the drain current, charge, and capacitance. In this work, we have developed them for a dual-gated bilayer graphene field-effect transistor. A drift-diffusion mechanism for the carrier transport has been considered coupled with an appropriate field-effect model taking into account the electronic properties of the bilayer graphene. Extrinsic resistances have been included considering the formation of a Schottky barrier at the metal-bilayer graphene interface. The proposed model has been benchmarked against experimental prototype transistors, discussing the main figures of merit targeting radio-frequency applications.

  7. Graphene homojunction: closed-edge bilayer graphene by pseudospin interaction

    NASA Astrophysics Data System (ADS)

    Yan, Jiaxu; Li, Chao; Zhan, Da; Liu, Lei; Shen, Dezhen; Kuo, Jer-Lai; Chen, Shoushun; Shen, Zexiang

    2016-04-01

    Depending on the sublattices they are propagated in, low-energy electrons or holes are labeled with pseudospin. By engineering pseudospin interactions, we propose that two critical features of a junction, i.e., band gap opening and spatial charge separation, can be realized in graphene layers with proper stacking. We also demonstrate theoretically that such a graphene diode may play a role in future pseudospin electronics such as for harvesting solar energy.

  8. Wavevector filtering through single-layer and bilayer graphene with magnetic barrier structures

    NASA Astrophysics Data System (ADS)

    Masir, M. Ramezani; Vasilopoulos, P.; Peeters, F. M.

    2008-12-01

    We show that the angular range of the transmission through magnetic barrier structures can be efficiently controlled in single-layer and bilayer graphenes and this renders the structure's efficient wavevector filters. As the number of magnetic barriers increases, this range shrinks, the gaps in the transmission versus energy become wider, and the conductance oscillates with the Fermi energy.

  9. The effect of spin-orbit coupling in band structure and edge states of bilayer graphene

    SciTech Connect

    Sahdan, Muhammad Fauzi; Darma, Yudi

    2015-04-16

    Topological insulators are predicted to be useful ranging from spintronics to quantum computation. Graphene was first predicted to be the precursor of topological insulator by Kane-Mele. They developed a Hamiltonian model to describe the gap opening in graphene. In this work, we investigate the band structure of bilayer grapheme and also its edge states by using this model with analytical approach. The results of our calculation show that the gap opening occurs at K and K’ point in bilayer graphene.In addition, a pair of gapless edge modes occurs both in the zigzag and arm-chair configurations are no longer exist. There are gap created at the edge even though thery are very small.

  10. A circuit model for defective bilayer graphene transistors

    NASA Astrophysics Data System (ADS)

    Umoh, Ime J.; Moktadir, Zakaria; Hang, Shuojin; Kazmierski, Tom J.; Mizuta, Hiroshi

    2016-05-01

    This paper investigates the behaviour of a defective single-gate bilayer graphene transistor. Point defects were introduced into pristine graphene crystal structure using a tightly focused helium ion beam. The transfer characteristics of the exposed transistors were measured ex-situ for different defect concentrations. The channel peak resistance increased with increasing defect concentration whilst the on-off ratio showed a decreasing trend for both electrons and holes. To understand the electrical behaviour of the transistors, a circuit model for bilayer graphene is developed which shows a very good agreement when validated against experimental data. The model allowed parameter extraction of bilayer transistor and can be implemented in circuit level simulators.

  11. Topological valley transport at bilayer graphene domain walls

    NASA Astrophysics Data System (ADS)

    Ju, Long; Shi, Zhiwen; Nair, Nityan; Lv, Yinchuan; Jin, Chenhao; Velasco, Jairo, Jr.; Ojeda-Aristizabal, Claudia; Bechtel, Hans; Martin, Michael; Zettl, Alex; Analytis, James; McEuen, Paul; Wang, Feng

    Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. An external electric field can induce a tunable bandgap in bilayer graphene, and domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators. In this talk, I will present our efforts on revealing the topologically protected edge states at AB-BA domain walls by combining near field infrared nanoscopy with electrical transport measurement. These one-dimensional valley-polarized conducting channels feature a ballistic length of about 400 nanometres at 4 kelvin.

  12. Detection of bilayer lipid with graphene nanoribbon

    NASA Astrophysics Data System (ADS)

    Akbari, Elnaz; Buntat, Zolkafle; Afroozeh, Abdolkarim; Zeinalinezhad, Alireza; Nilashi, Mehrbakhsh

    2015-09-01

    Single-layer graphene consists of sp 2-bonded carbon atoms arranged in a two-dimensional (2D) hexagonal lattice comprising a thin layer of single carbon atoms. Owing to its special characteristics including electrical, physical, and optical properties, graphene is considered more suitable for sensor applications than other materials. Moreover, it is possible to produce biosensors using electrolyte-gated field-effect transistors based on graphene (GFETs) to identify the alterations in charged lipid membrane properties. This paper illustrates how membrane thickness and electrical charge can result in a monolayer GFET, with emphasis on conductance variation. It is proposed that the thickness and electrical charge of the lipid bilayer are functions of carrier density, and equations relating these suitable control parameters were derived. Adaptive neuro fuzzy inference system (ANFIS) has been incorporated to obtain other model for conductance characteristic. The comparison between the analytical models and ANFIS with the experimental data extracted from previous work show an acceptable agreement. [Figure not available: see fulltext.

  13. Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kim, Keun Su; Walter, Andrew L.; Moreschini, Luca; Seyller, Thomas; Horn, Karsten; Rotenberg, Eli; Bostwick, Aaron

    2013-10-01

    Charge carriers in bilayer graphene are widely believed to be massive Dirac fermions that have a bandgap tunable by a transverse electric field. However, a full transport gap, despite its importance for device applications, has not been clearly observed in gated bilayer graphene, a long-standing puzzle. Moreover, the low-energy electronic structure of bilayer graphene is widely held to be unstable towards symmetry breaking either by structural distortions, such as twist, strain, or electronic interactions that can lead to various ground states. Which effect dominates the physics at low energies is hotly debated. Here we show both by direct band-structure measurements and by calculations that a native imperfection of bilayer graphene, a distribution of twists whose size is as small as ~0.1°, is sufficient to generate a completely new electronic spectrum consisting of massive and massless Dirac fermions. The massless spectrum is robust against strong electric fields, and has a unusual topology in momentum space consisting of closed arcs having an exotic chiral pseudospin texture, which can be tuned by varying the charge density. The discovery of this unusual Dirac spectrum not only complements the framework of massive Dirac fermions, widely relevant to charge transport in bilayer graphene, but also supports the possibility of valley Hall transport.

  14. Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene.

    PubMed

    Kim, Keun Su; Walter, Andrew L; Moreschini, Luca; Seyller, Thomas; Horn, Karsten; Rotenberg, Eli; Bostwick, Aaron

    2013-10-01

    Charge carriers in bilayer graphene are widely believed to be massive Dirac fermions that have a bandgap tunable by a transverse electric field. However, a full transport gap, despite its importance for device applications, has not been clearly observed in gated bilayer graphene, a long-standing puzzle. Moreover, the low-energy electronic structure of bilayer graphene is widely held to be unstable towards symmetry breaking either by structural distortions, such as twist, strain, or electronic interactions that can lead to various ground states. Which effect dominates the physics at low energies is hotly debated. Here we show both by direct band-structure measurements and by calculations that a native imperfection of bilayer graphene, a distribution of twists whose size is as small as ~0.1°, is sufficient to generate a completely new electronic spectrum consisting of massive and massless Dirac fermions. The massless spectrum is robust against strong electric fields, and has a unusual topology in momentum space consisting of closed arcs having an exotic chiral pseudospin texture, which can be tuned by varying the charge density. The discovery of this unusual Dirac spectrum not only complements the framework of massive Dirac fermions, widely relevant to charge transport in bilayer graphene, but also supports the possibility of valley Hall transport. PMID:23892785

  15. Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene

    NASA Astrophysics Data System (ADS)

    Bostwick, Aaron

    2014-03-01

    Charge carriers in bilayer graphene are widely believed to be massive Dirac fermions that have a bandgap tunable by a transverse electric field. However, a full transport gap, despite its importance for device applications, has not been clearly observed in gated bilayer graphene, a long-standing puzzle. Moreover, the low-energy electronic structure of bilayer graphene is widely held to be unstable towards symmetry breaking either by structural distortions, such as twist, strain, or electronic interactions that can lead to various ground states. Which effect dominates the physics at low energies is hotly debated. We find by direct band-structure measurements and by calculations that a native imperfection of bilayer graphene, a distribution of twists whose size is as small as ~ 0.1°, is sufficient to generate a completely new electronic spectrum consisting of massive and massless Dirac fermions. The massless spectrum is robust against strong electric fields, and has a unusual topology in momentum space consisting of closed arcs having an exotic chiral pseudospin texture, which can be tuned by varying the charge density. The discovery of this unusual Dirac spectrum may be widely relevant to charge transport in bilayer graphene.

  16. Electronic properties of bilayer graphenes strongly coupled to interlayer stacking and an external field

    DOE PAGESBeta

    Park, Changwon; Ryou, Junga; Hong, Suklyun; Sumpter, Bobby G.; Kim, Gunn; Yoon, Mina

    2015-07-02

    Bilayer graphene (BLG) with a tunable band gap appears interesting as an alternative to graphene for practical applications; thus, its transport properties are being actively pursued. Using density functional theory and perturbation analysis, we investigated, under an external electric field, the electronic properties of BLG in various stackings relevant to recently observed complex structures. We established the first phase diagram summarizing the stacking-dependent gap openings of BLG for a given field. Lastly, we further identified high-density midgap states, localized on grain boundaries, even under a strong field, which can considerably reduce the overall transport gap.

  17. Periodic barrier structure in AA-stacked bilayer graphene

    NASA Astrophysics Data System (ADS)

    Redouani, Ilham; Jellal, Ahmed

    2016-06-01

    We study the charge carriers transport in an AA-stacked bilayer graphene modulated by a lateral one-dimensional multibarrier structure. We investigate the band structures of our system, that is made up of two shifted Dirac cones, for finite and zero gap. We use the boundary conditions to explicitly determine the transmission probability of each individual cone (τ =+/- 1) for single, double and finite periodic barrier structure. We find that the Klein tunneling is only possible when the band structure is gapless and can occur at normal incidence as a result of the Dirac nature of the quasiparticles. We observe that the band structure of the barriers can have more than one Dirac points for finite periodic barrier. The resonance peaks appear in the transmission probability, which correspond to the positions of new cones index like associated with τ =+/- 1. Two conductance channels through different cones (τ =+/- 1) are found where the total conductance has been studied and compared to the cases of single layer and AB-stacked bilayer graphene.

  18. Robustly Engineering Thermal Conductivity of Bilayer Graphene by Interlayer Bonding.

    PubMed

    Zhang, Xiaoliang; Gao, Yufei; Chen, Yuli; Hu, Ming

    2016-02-25

    Graphene and its bilayer structure are the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. Their realistic applications in emerging nanoelectronics usually call for thermal transport manipulation in a controllable and precise manner. In this paper we systematically studied the effect of interlayer covalent bonding, in particular different interlay bonding arrangement, on the thermal conductivity of bilayer graphene using equilibrium molecular dynamics simulations. It is revealed that, the thermal conductivity of randomly bonded bilayer graphene decreases monotonically with the increase of interlayer bonding density, however, for the regularly bonded bilayer graphene structure the thermal conductivity possesses unexpectedly non-monotonic dependence on the interlayer bonding density. The results suggest that the thermal conductivity of bilayer graphene depends not only on the interlayer bonding density, but also on the detailed topological configuration of the interlayer bonding. The underlying mechanism for this abnormal phenomenon is identified by means of phonon spectral energy density, participation ratio and mode weight factor analysis. The large tunability of thermal conductivity of bilayer graphene through rational interlayer bonding arrangement paves the way to achieve other desired properties for potential nanoelectronics applications involving graphene layers.

  19. Robustly Engineering Thermal Conductivity of Bilayer Graphene by Interlayer Bonding

    PubMed Central

    Zhang, Xiaoliang; Gao, Yufei; Chen, Yuli; Hu, Ming

    2016-01-01

    Graphene and its bilayer structure are the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. Their realistic applications in emerging nanoelectronics usually call for thermal transport manipulation in a controllable and precise manner. In this paper we systematically studied the effect of interlayer covalent bonding, in particular different interlay bonding arrangement, on the thermal conductivity of bilayer graphene using equilibrium molecular dynamics simulations. It is revealed that, the thermal conductivity of randomly bonded bilayer graphene decreases monotonically with the increase of interlayer bonding density, however, for the regularly bonded bilayer graphene structure the thermal conductivity possesses unexpectedly non-monotonic dependence on the interlayer bonding density. The results suggest that the thermal conductivity of bilayer graphene depends not only on the interlayer bonding density, but also on the detailed topological configuration of the interlayer bonding. The underlying mechanism for this abnormal phenomenon is identified by means of phonon spectral energy density, participation ratio and mode weight factor analysis. The large tunability of thermal conductivity of bilayer graphene through rational interlayer bonding arrangement paves the way to achieve other desired properties for potential nanoelectronics applications involving graphene layers. PMID:26911859

  20. Sub-wavelength antenna enhanced bilayer graphene tunable photodetector

    DOEpatents

    Beechem, III, Thomas Edwin; Howell, Stephen W.; Peters, David W.; Davids, Paul; Ohta, Taisuke

    2016-03-22

    The integration of bilayer graphene with an absorption enhancing sub-wavelength antenna provides an infrared photodetector capable of real-time spectral tuning without filters at nanosecond timescales.

  1. Space charge and screening in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kolomeisky, Eugene B.; Straley, Joseph P.; Abrams, Daniel L.

    2016-11-01

    Undoped bilayer graphene is a two-dimensional semimetal with a low-energy excitation spectrum that is parabolic in the momentum. As a result, the screening of an arbitrary external charge Ze is accompanied by a reconstruction of the ground state: valence band electrons (for Z  >  0) are promoted to form a space charge around the charge while the holes leave the physical picture. The outcome is a flat neutral object resembling the regular atom except that for Z\\gg 1 it is described by a strictly linear Thomas-Fermi theory. This theory also predicts that the bilayer’s static dielectric constant is the same as that of a two-dimensional electron gas in the long-wavelength limit.

  2. Electronic band structure effects in monolayer, bilayer, and hybrid graphene structures

    NASA Astrophysics Data System (ADS)

    Puls, Conor

    Since its discovery in 2005, graphene has been the focus of intense theoretical and experimental study owing to its unique two-dimensional band structure and related electronic properties. In this thesis, we explore the electronic properties of graphene structures from several perspectives including the magnetoelectrical transport properties of monolayer graphene, gap engineering and measurements in bilayer graphene, and anomalous quantum oscillation in the monolayer-bilayer graphene hybrids. We also explored the device implications of our findings, and the application of some experimental techniques developed for the graphene work to the study of a complex oxide, Ca3Ru2O7, exhibiting properties of strongly correlated electrons. Graphene's high mobility and ballistic transport over device length scales, make it suitable for numerous applications. However, two big challenges remain in the way: maintaining high mobility in fabricated devices, and engineering a band gap to make graphene compatible with logical electronics and various optical devices. We address the first challenge by experimentally evaluating mobilities in scalable monolayer graphene-based field effect transistors (FETs) and dielectric-covered Hall bars. We find that the mobility is limited in these devices, and is roughly inversely proportional to doping. By considering interaction of graphene's Dirac fermions with local charged impurities at the interface between graphene and the top-gate dielectric, we find that Coulomb scattering is responsible for degraded mobility. Even in the cleanest devices, a band gap is still desirable for electronic applications of graphene. We address this challenge by probing the band structure of bilayer graphene, in which a field-tunable energy band gap has been theoretically proposed. We use planar tunneling spectroscopy of exfoliated bilayer graphene flakes demonstrate both measurement and control of the energy band gap. We find that both the Fermi level and

  3. Dual doped monolayer and bilayer graphene: The case of 4p and 2p elements

    NASA Astrophysics Data System (ADS)

    Denis, Pablo A.; Iribarne, Federico

    2016-08-01

    4p/2p dual-doped monolayer and bilayer graphene were studied via first principle calculations. Generally, dopants prefer to be agglomerated. A second dopant significantly reduces formation energies. Thus, partially reduced graphene oxide would favor substitutional doping by facilitating the introduction of the 4p dopants. Dual-doping can tune the band gap from 0.1 to 0.8 eV. For bilayer graphene, large atomic radii elements (Gallium and Germanium) form interlayer bonds with the undoped sheet. For some dual-doped graphenes, interlayer GaC and GeC bonds were formed, increasing the chemical reactivity of the undoped layer and affecting its electronic structure, with metallic or semiconducting characters observed.

  4. Rotational disorder in twisted bilayer graphene.

    PubMed

    Beechem, Thomas E; Ohta, Taisuke; Diaconescu, Bogdan; Robinson, Jeremy T

    2014-02-25

    Conventional means of stacking two-dimensional (2D) crystals inevitably leads to imperfections. To examine the ramifications of these imperfections, rotational disorder and strain are quantified in twisted bilayer graphene (TBG) using a combination of Raman spectroscopic and low-energy electron diffraction imaging. The twist angle between TBG layers varies on the order of 2° within large (50-100 μm) single-crystalline grains, resulting in changes of the emergent Raman response by over an order of magnitude. Rotational disorder does not evolve continuously across the large grains but rather comes about by variations in the local twist angles between differing contiguous subgrains, ∼ 1 μm in size, that themselves exhibit virtually no twist angle variation (ΔΘ ∼ 0.1°). Owing to weak out-of-plane van der Waals bonding between azimuthally rotated graphene layers, these subgrains evolve in conjunction with the 0.3% strain variation observed both within and between the atomic layers. Importantly, the emergent Raman response is altered, but not removed, by these extrinsic perturbations. Interlayer interactions are therefore resilient to strain and rotational disorder, a fact that gives promise to the prospect of designer 2D solid heterostructures created via transfer processes.

  5. Terahertz conductivity of twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Chia, Elbert E. M.; Zou, Xingquan; Shang, Jingzhi; Leaw, Jianing; Luo, Zhiqiang; Luo, Liyan; Cheong, Siew Ann; Su, Haibin; Zhu, Jian-Xin; Castro Neto, A. H.; Yu, Ting

    2013-03-01

    Using terahertz time-domain spectroscopy, the real part of optical conductivity [σ1 (ω) ] of twisted bilayer graphene was obtained at different temperatures (10 - 300 K) in the frequency range 0.3 - 3 THz. On top of a Drude-like response, we see a strong and narrow peak in σ1 (ω) at ~2.7 THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7 THz to an enhanced density of states at that energy, that is caused by the presence of van Hove singularities arising from a commensurate twisting of the two graphene layers. Singapore MOE AcRF Tier 2 (ARC 23/08), NRF-CRP (NRF-CRP4-2008-04), NNSA of the U.S. DOE at LANL (DE-AC52-06NA25396), LANL LDRD Program, NRF-CRP (R-144-000-295-281), DOE DE-FG02-08ER46512, ONR MURI N00014-09-1-1063.

  6. Compressibility of bilayer graphene: the role of disorder

    NASA Astrophysics Data System (ADS)

    Abergel, David

    2011-03-01

    We discuss the role of disorder caused by charged impurities on the compressibility of bilayer graphene. In doing so, we take into account the full hyperbolic dispersion relation and the presence of a gap between the valence and conduction bands to produce an exact calculation of dμ/dn for the non-disordered case. We then introduce two methods for including the disorder in a statistical way and evaluate the effectiveness of each by comparing their predictions with recent experiments. We find that averaging is best done at the level of the observable quantity: in this case the compressibility. This work is done in collaboration with Sankar Das Sarma and Euyheon Hwang, and supported by US-ONR, NRI-SWAN, and UMD-CNAM.

  7. Band gap opening in bilayer silicene by alkali metal intercalation.

    PubMed

    Liu, Hongsheng; Han, Nannan; Zhao, Jijun

    2014-11-26

    Recently, bilayer and multilayer silicene have attracted increased attention following the boom of silicene, which holds great promise for future applications in microelectronic devices. Herein we systematically investigate all stacking configurations of bilayer silicene and the corresponding electronic properties. Strong coupling is found between two silicene layers, which destroys the Dirac cones in the band structures of pristine silicene and makes bilayer silicene sheets metallic. However, intercalation of alkali metal (especially potassium) can effectively decouple the interaction between two silicene layers. In the K-intercalated bilayer silicene (KSi4), the Dirac cones are recovered with a small band gap of 0.27 eV located about 0.55 eV below the Fermi level. Furthermore, intercalation of K(+) cations in bilayer silicene (K(+)Si4) results in a semiconductor with a moderate band gap of 0.43 eV, making it ideal for microelectronic applications.

  8. Dynamical conductivity of AA-stacked bilayer graphene

    NASA Astrophysics Data System (ADS)

    Tabert, C. J.; Nicol, E. J.

    2012-08-01

    We calculate the dynamical conductivity of AA-stacked bilayer graphene as a function of frequency and in the presence of a finite chemical potential due to charging. Unlike the monolayer, we find a Drude absorption at charge neutrality in addition to an interband absorption with onset of twice the interlayer hopping energy. At finite doping, the interband absorption exhibits two edges, which depend on both chemical potential and interlayer hopping energy. We study the behavior as a function of varying chemical potential relative to the interlayer hopping energy scale and compute the partial optical sum. The results are contrasted with the previously published case of AB stacking. While we focus on in-plane conductivity, we also provide the perpendicular conductivity for both AB and AA stacking. We also examine conductivity for other variations with AA stacking, such as AAA-stacked trilayer. Based on proposed models for topological insulators discussed in the literature, we also consider the effect of spin-orbit coupling on the optical properties of an AA-stacked bilayer, which illustrates the effect of an energy gap opening at points in the band structure.

  9. Tuning the energy gap of bilayer α-graphyne by applying strain and electric field

    NASA Astrophysics Data System (ADS)

    Yang, Hang; Wu, Wen-Zhi; Jin, Yu; Wan-Lin, Guo

    2016-02-01

    Our density functional theory calculations show that the energy gap of bilayer α-graphyne can be modulated by a vertically applied electric field and interlayer strain. Like bilayer graphene, the bilayer α-graphyne has electronic properties that are hardly changed under purely mechanical strain, while an external electric field can open the gap up to 120 meV. It is of special interest that compressive strain can further enlarge the field induced gap up to 160 meV, while tensile strain reduces the gap. We attribute the gap variation to the novel interlayer charge redistribution between bilayer α-graphynes. These findings shed light on the modulation of Dirac cone structures and potential applications of graphyne in mechanical-electric devices. Project supported by the National Key Basic Research Program of China (Grant Nos. 2013CB932604 and 2012CB933403), the National Natural Science Foundation of China (Grant Nos. 51472117 and 51535005), the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures, China (Grant No. 0414K01), the Nanjing University of Aeronautics and Astronautics (NUAA) Fundamental Research Funds, China (Grant No. NP2015203), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

  10. The ground state construction of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Giuliani, Alessandro; Jauslin, Ian

    2016-09-01

    We consider a model of half-filled bilayer graphene, in which the three dominant Slonczewski-Weiss-McClure hopping parameters are retained, in the presence of short-range interactions. Under a smallness assumption on the interaction strength U as well as on the inter-layer hopping ɛ, we construct the ground state in the thermodynamic limit, and prove that the pressure and two-point Schwinger function, away from its singularities, are analytic in U, uniformly in ɛ. The interacting Fermi surface is degenerate, and consists of eight Fermi points, two of which are protected by symmetries, while the locations of the other six are renormalized by the interaction, and the effective dispersion relation at the Fermi points is conical. The construction reveals the presence of different energy regimes, where the effective behavior of correlation functions changes qualitatively. The analysis of the crossover between regimes plays an important role in the proof of analyticity and in the uniform control of the radius of convergence. The proof is based on a rigorous implementation of fermionic renormalization group methods, including determinant estimates for the renormalized expansion.

  11. Direct imaging of topological edge states at a bilayer graphene domain wall

    NASA Astrophysics Data System (ADS)

    Yin, Long-Jing; Jiang, Hua; Qiao, Jia-Bin; He, Lin

    2016-06-01

    The AB-BA domain wall in gapped graphene bilayers is a rare naked structure hosting topological electronic states. Although it has been extensively studied in theory, a direct imaging of its topological edge states is still missing. Here we image the topological edge states at the graphene bilayer domain wall by using scanning tunnelling microscope. The simultaneously obtained atomic-resolution images of the domain wall provide us unprecedented opportunities to measure the spatially varying edge states within it. The one-dimensional conducting channels are observed to be mainly located around the two edges of the domain wall, which is reproduced quite well by our theoretical calculations. Our experiment further demonstrates that the one-dimensional topological states are quite robust even in the presence of high magnetic fields. The result reported here may raise hopes of graphene-based electronics with ultra-low dissipation.

  12. Direct imaging of topological edge states at a bilayer graphene domain wall.

    PubMed

    Yin, Long-Jing; Jiang, Hua; Qiao, Jia-Bin; He, Lin

    2016-01-01

    The AB-BA domain wall in gapped graphene bilayers is a rare naked structure hosting topological electronic states. Although it has been extensively studied in theory, a direct imaging of its topological edge states is still missing. Here we image the topological edge states at the graphene bilayer domain wall by using scanning tunnelling microscope. The simultaneously obtained atomic-resolution images of the domain wall provide us unprecedented opportunities to measure the spatially varying edge states within it. The one-dimensional conducting channels are observed to be mainly located around the two edges of the domain wall, which is reproduced quite well by our theoretical calculations. Our experiment further demonstrates that the one-dimensional topological states are quite robust even in the presence of high magnetic fields. The result reported here may raise hopes of graphene-based electronics with ultra-low dissipation. PMID:27312315

  13. Direct imaging of topological edge states at a bilayer graphene domain wall

    PubMed Central

    Yin, Long-Jing; Jiang, Hua; Qiao, Jia-Bin; He, Lin

    2016-01-01

    The AB–BA domain wall in gapped graphene bilayers is a rare naked structure hosting topological electronic states. Although it has been extensively studied in theory, a direct imaging of its topological edge states is still missing. Here we image the topological edge states at the graphene bilayer domain wall by using scanning tunnelling microscope. The simultaneously obtained atomic-resolution images of the domain wall provide us unprecedented opportunities to measure the spatially varying edge states within it. The one-dimensional conducting channels are observed to be mainly located around the two edges of the domain wall, which is reproduced quite well by our theoretical calculations. Our experiment further demonstrates that the one-dimensional topological states are quite robust even in the presence of high magnetic fields. The result reported here may raise hopes of graphene-based electronics with ultra-low dissipation. PMID:27312315

  14. Crossover from retro to specular Andreev reflections in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Efetov, Dmitri K.; Efetov, Konstantin B.

    2016-08-01

    Ongoing experimental progress in the preparation of ultraclean graphene/superconductor (SC) interfaces enabled the recent observation of specular interband Andreev reflections (ARs) at bilayer graphene (BLG )/NbSe2 van der Waals interfaces [Efetov et al., Nat. Phys. 12, 328 (2016), 10.1038/nphys3583]. Motivated by this experiment we theoretically study the differential conductance across a BLG/SC interface at the continuous transition from high to ultralow Fermi energies EF in BLG. Using the Bogoliubov-de Gennes equations and the Blonder-Tinkham-Klapwijk formalism we derive analytical expressions for the differential conductance across the BLG/SC interface. We find a characteristic signature of the crossover from intraband retro (high EF) to interband specular (low EF) ARs that manifests itself in a strongly suppressed interfacial conductance when the excitation energy |ɛ |=| EF|<Δ (the SC gap). The sharpness of these conductance dips is strongly dependent on the size of the potential step at the BLG/SC interface U0.

  15. Negative terahertz conductivity in disordered graphene bilayers with population inversion

    SciTech Connect

    Svintsov, D.; Otsuji, T.; Ryzhii, V.; Mitin, V.; Shur, M. S.

    2015-03-16

    The gapless energy band spectra make the structures based on graphene and graphene bilayer with the population inversion to be promising media for the interband terahertz (THz) lasing. However, a strong intraband absorption at THz frequencies still poses a challenge for efficient THz lasing. In this paper, we show that in the pumped graphene bilayer, the indirect interband radiative transitions accompanied by scattering of carriers by disorder can provide a substantial negative contribution to the THz conductivity (together with the direct interband transitions). In the graphene bilayer on high-κ substrates with point charged defects, these transitions substantially compensate the losses due to the intraband (Drude) absorption. We also demonstrate that the indirect interband contribution to the THz conductivity in a graphene bilayer with the extended defects (such as the charged impurity clusters) can surpass by several times the fundamental limit associated with the direct interband transitions, and the Drude conductivity as well. These predictions can affect the strategy of the graphene-based THz laser implementation.

  16. Electronic properties of asymmetrically doped twisted graphene bilayers

    NASA Astrophysics Data System (ADS)

    Trambly de Laissardière, Guy; Namarvar, Omid Faizy; Mayou, Didier; Magaud, Laurence

    2016-06-01

    Rotated graphene bilayers form an exotic class of nanomaterials with fascinating electronic properties governed by the rotation angle θ . For large rotation angles, the electron eigenstates are restricted to one layer and the bilayer behaves like two decoupled graphene layers. At intermediate angles, Dirac cones are preserved but with a lower velocity and van Hove singularities are induced at energies where the two Dirac cones intersect. At very small angles, eigenstates become localized in peculiar moiré zones. We analyze here the effect of an asymmetric doping for a series of commensurate rotated bilayers on the basis of tight-binding calculations of their band dispersions, density of states, participation ratio, and diffusive properties. While a small doping level preserves the θ dependence of the rotated bilayer electronic structure, larger doping induces a further reduction of the band velocity in the same way as a further reduction of the rotation angle.

  17. Anomalous Spectral Features of a Neutral Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Cheng, C.-M.; Xie, L. F.; Pachoud, A.; Moser, H. O.; Chen, W.; Wee, A. T. S.; Castro Neto, A. H.; Tsuei, K.-D.; Özyilmaz, B.

    2015-05-01

    Graphene and its bilayer are two-dimensional systems predicted to show exciting many-body effects near the neutrality point. The ideal tool to investigate spectrum reconstruction effects is angle-resolved photoemission spectroscopy (ARPES) as it probes directly the band structure with information about both energy and momentum. Here we reveal, by studying undoped exfoliated bilayer graphene with ARPES, two essential aspects of its many-body physics: the electron-phonon scattering rate has an anisotropic k-dependence and the type of electronic liquid is non-Fermi liquid. The latter behavior is evident from an observed electron-electron scattering rate that scales linearly with energy from 100 meV to 600 meV and that is associated with the proximity of bilayer graphene to a two-dimensional quantum critical point of competing orders.

  18. Electro-absorption of silicene and bilayer graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Abdelsalam, Hazem; Talaat, Mohamed H.; Lukyanchuk, Igor; Portnoi, M. E.; Saroka, V. A.

    2016-07-01

    We study numerically the optical properties of low-buckled silicene and AB-stacked bilayer graphene quantum dots subjected to an external electric field, which is normal to their surface. Within the tight-binding model, the optical absorption is calculated for quantum dots, of triangular and hexagonal shapes, with zigzag and armchair edge terminations. We show that in triangular silicene clusters with zigzag edges a rich and widely tunable infrared absorption peak structure originates from transitions involving zero energy states. The edge of absorption in silicene quantum dots undergoes red shift in the external electric field for triangular clusters, whereas blue shift takes place for hexagonal ones. In small clusters of bilayer graphene with zigzag edges the edge of absorption undergoes blue/red shift for triangular/hexagonal geometry. In armchair clusters of silicene blue shift of the absorption edge takes place for both cluster shapes, while red shift is inherent for both shapes of the bilayer graphene quantum dots.

  19. Anomalous spectral features of a neutral bilayer graphene.

    PubMed

    Cheng, C-M; Xie, L F; Pachoud, A; Moser, H O; Chen, W; Wee, A T S; Castro Neto, A H; Tsuei, K-D; Özyilmaz, B

    2015-01-01

    Graphene and its bilayer are two-dimensional systems predicted to show exciting many-body effects near the neutrality point. The ideal tool to investigate spectrum reconstruction effects is angle-resolved photoemission spectroscopy (ARPES) as it probes directly the band structure with information about both energy and momentum. Here we reveal, by studying undoped exfoliated bilayer graphene with ARPES, two essential aspects of its many-body physics: the electron-phonon scattering rate has an anisotropic k-dependence and the type of electronic liquid is non-Fermi liquid. The latter behavior is evident from an observed electron-electron scattering rate that scales linearly with energy from 100 meV to 600 meV and that is associated with the proximity of bilayer graphene to a two-dimensional quantum critical point of competing orders. PMID:25985064

  20. Gate-Tunable Resonant Tunneling in Double Bilayer Graphene Heterostructures

    NASA Astrophysics Data System (ADS)

    Fallahazad, Babak; Lee, Kayoung; Kang, Sangwoo; Xue, Jiamin; Larentis, Stefano; Corbet, Christopher; Kim, Kyounghwan; Movva, Hema C. P.; Taniguchi, Takashi; Watanabe, Kenji; Register, Leonard F.; Banerjee, Sanjay K.; Tutuc, Emanuel

    2015-01-01

    We demonstrate gate-tunable resonant tunneling and negative differential resistance in the interlayer current-voltage characteristics of rotationally aligned double bilayer graphene heterostructures separated by hexagonal boron-nitride (hBN) dielectric. An analysis of the heterostructure band alignment using individual layer densities, along with experimentally determined layer chemical potentials indicates that the resonance occurs when the energy bands of the two bilayer graphene are aligned. We discuss the tunneling resistance dependence on the interlayer hBN thickness, as well as the resonance width dependence on mobility and rotational alignment.

  1. Wrinkled bilayer graphene with wafer scale mechanical strain

    NASA Astrophysics Data System (ADS)

    Mikael, Solomon; Seo, Jung-Hun; Javadi, Alireza; Gong, Shaoqin; Ma, Zhenqiang

    2016-05-01

    Wafer-scale strained bilayer graphene is demonstrated by employing a silicon nitride (Si3N4) stressor layer. Different magnitudes of compressive stress up to 840 MPa were engineered by adjusting the Si3N4 deposition recipes, and different strain conditions were analyzed using Raman spectroscopy. The strained graphene displayed significant G peak shifts and G peak splitting with 16.2 cm-1 and 23.0 cm-1 of the G band and two-dimensional band shift, which corresponds to 0.26% of strain. Raman mapping of large regions of the graphene films found that the largest shifts/splitting occurred near the bilayer regions of the graphene films. The significance of our approach lies in the fact that it can be performed in a conventional microfabrication process, i.e., the plasma enhanced chemical vapor deposition system, and thus easily implemented for large scale production.

  2. Theory of resonant tunneling in bilayer-graphene/hexagonal-boron-nitride heterostructures

    SciTech Connect

    Barrera, Sergio C. de la; Feenstra, Randall M.

    2015-03-02

    A theory is developed for calculating vertical tunneling current between two sheets of bilayer graphene separated by a thin, insulating layer of hexagonal boron nitride, neglecting many-body effects. Results are presented using physical parameters that enable comparison of the theory with recently reported experimental results. Observed resonant tunneling and negative differential resistance in the current–voltage characteristics are explained in terms of the electrostatically-induced band gap, gate voltage modulation, density of states near the band edge, and resonances with the upper sub-band. These observations are compared to ones from similar heterostructures formed with monolayer graphene.

  3. Soliton-dependent plasmon reflection at bilayer graphene domain walls

    NASA Astrophysics Data System (ADS)

    Jiang, Lili; Shi, Zhiwen; Zeng, Bo; Wang, Sheng; Kang, Ji-Hun; Joshi, Trinity; Jin, Chenhao; Ju, Long; Kim, Jonghwan; Lyu, Tairu; Shen, Yuen-Ron; Crommie, Michael; Gao, Hong-Jun; Wang, Feng

    2016-08-01

    Layer-stacking domain walls in bilayer graphene are emerging as a fascinating one-dimensional system that features stacking solitons structurally and quantum valley Hall boundary states electronically. The interactions between electrons in the 2D graphene domains and the one-dimensional domain-wall solitons can lead to further new quantum phenomena. Domain-wall solitons of varied local structures exist along different crystallographic orientations, which can exhibit distinct electrical, mechanical and optical properties. Here we report soliton-dependent 2D graphene plasmon reflection at different 1D domain-wall solitons in bilayer graphene using near-field infrared nanoscopy. We observe various domain-wall structures in mechanically exfoliated graphene bilayers, including network-forming triangular lattices, individual straight or bent lines, and even closed circles. The near-field infrared contrast of domain-wall solitons arises from plasmon reflection at domain walls, and exhibits markedly different behaviours at the tensile- and shear-type domain-wall solitons. In addition, the plasmon reflection at domain walls exhibits a peculiar dependence on electrostatic gating. Our study demonstrates the unusual and tunable coupling between 2D graphene plasmons and domain-wall solitons.

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

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

    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 Ids of 4.2 A/mm, and a peak transconductance gm of 2880 mS/mm. Intrinsic current-gain cutoff frequency fT of 407 GHz is obtained. The exciting DC and RF performances obtained in the quasi-free-standing bilayer epitaxial graphene transistor show the great application potential of this material system.

  5. Theoretical Study of All-Electrical Quantum Wire Valley Filters in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Wu, Yu-Shu; Lue, Ning-Yuan; Chen, Yen-Chun; Jiang, Jia-Huei; Chou, Mei-Yin

    Graphene electrons carry valley pseudospin, due to the double valley degeneracy in graphene band structure. In gapped graphene, the pseudospin is coupled to an in-plane electric field, through the mechanism of valley-orbit interaction (VOI), Based on the VOI, a family of electrically-controlled valleytronic devices have been proposed. Here, we report the theoretical study of a recently proposed valley filter consisting of a Q1D channel in bilayer graphene defined and controlled by electrical gates. We discuss two types of calculations - those of energy subband structure in the channel and electron transmission through a valley valve consisting of two proposed filters. For the former, we have developed a tight binding formulation in the continuum limit. For the latter, we employ the recursive Green's function method. Results from the calculations will be presented. Financial support by MoST, Taiwan, ROC is acknowledged.

  6. Addressing Raman features of individual layers in isotopically labeled Bernal stacked bilayer graphene

    NASA Astrophysics Data System (ADS)

    Costa, Sara D.; Weis, Johan Ek; Frank, Otakar; Fridrichová, Michaela; Kalbac, Martin

    2016-06-01

    In this report important Raman modes for the evaluation of strain in graphene (the 2D and 2D‧) are analyzed. The isotope labeling is used to disentangle contribution of individual graphene layers of graphene bilayer to the studied Raman modes. It is shown that for Bernal-stacked bilayers, the 2D and the 2D‧ Raman modes have three distinct components that can be assigned to processes originating solely from the top graphene layer, bottom graphene layer, and from a combination of processes originating both from the top and bottom layers. The reported results thus enable addressing the properties of individual graphene layers in graphene bilayer by Raman spectroscopy.

  7. NMR parameters in gapped graphene systems

    NASA Astrophysics Data System (ADS)

    Crisan, Mircea; Grosu, Ioan; Ţifrea, Ionel

    2016-06-01

    We calculate the nuclear spin-lattice relaxation time and the Knight shift for the case of gapped graphene systems. Our calculations consider both the massive and massless gap scenarios. Both the spin-lattice relaxation time and the Knight shift depend on temperature, chemical potential, and the value of the electronic energy gap. In particular, at the Dirac point, the electronic energy gap has stronger effects on the system nuclear magnetic resonance parameters in the case of the massless gap scenario. Differently, at large values of the chemical potential, both gap scenarios behave in a similar way and the gapped graphene system approaches a Fermi gas from the nuclear magnetic resonance parameters point of view. Our results are important for nuclear magnetic resonance measurements that target the 13C active nuclei in graphene samples.

  8. Existence of nontrivial topologically protected states at grain boundaries in bilayer graphene: signatures and electrical switching.

    PubMed

    Jaskólski, W; Pelc, M; Chico, Leonor; Ayuela, A

    2016-03-21

    Recent experiments [L. Ju, et al., Nature, 2015, 520, 650] confirm the existence of gapless states at domain walls created in gated bilayer graphene, when the sublattice stacking is changed from AB to BA. These states are significant because they are topologically protected, valley-polarized and give rise to conductance along the domain wall. Current theoretical models predict the appearance of such states only at domain walls, which preserve the sublattice order. Here we show that the appearance of the topologically protected states in stacking domain walls can be much more common in bilayer graphene, since they can also emerge in unexpected geometries, e.g., at grain boundaries with atomic-scale topological defects. We focus on a bilayer system in which one of the layers contains a line of octagon-double pentagon defects that mix graphene sublattices. We demonstrate that gap states are preserved even with pentagonal defects. Remarkably, unlike previous predictions, the number of gap states changes by inverting the gate polarization, yielding an asymmetric conductance along the grain boundary under gate reversal. This effect, linked to defect states, should be detectable in transport measurements and could be exploited in electrical switches.

  9. Existence of nontrivial topologically protected states at grain boundaries in bilayer graphene: signatures and electrical switching

    NASA Astrophysics Data System (ADS)

    Jaskólski, W.; Pelc, M.; Chico, Leonor; Ayuela, A.

    2016-03-01

    Recent experiments [L. Ju, et al., Nature, 2015, 520, 650] confirm the existence of gapless states at domain walls created in gated bilayer graphene, when the sublattice stacking is changed from AB to BA. These states are significant because they are topologically protected, valley-polarized and give rise to conductance along the domain wall. Current theoretical models predict the appearance of such states only at domain walls, which preserve the sublattice order. Here we show that the appearance of the topologically protected states in stacking domain walls can be much more common in bilayer graphene, since they can also emerge in unexpected geometries, e.g., at grain boundaries with atomic-scale topological defects. We focus on a bilayer system in which one of the layers contains a line of octagon-double pentagon defects that mix graphene sublattices. We demonstrate that gap states are preserved even with pentagonal defects. Remarkably, unlike previous predictions, the number of gap states changes by inverting the gate polarization, yielding an asymmetric conductance along the grain boundary under gate reversal. This effect, linked to defect states, should be detectable in transport measurements and could be exploited in electrical switches.

  10. Existence of nontrivial topologically protected states at grain boundaries in bilayer graphene: signatures and electrical switching.

    PubMed

    Jaskólski, W; Pelc, M; Chico, Leonor; Ayuela, A

    2016-03-21

    Recent experiments [L. Ju, et al., Nature, 2015, 520, 650] confirm the existence of gapless states at domain walls created in gated bilayer graphene, when the sublattice stacking is changed from AB to BA. These states are significant because they are topologically protected, valley-polarized and give rise to conductance along the domain wall. Current theoretical models predict the appearance of such states only at domain walls, which preserve the sublattice order. Here we show that the appearance of the topologically protected states in stacking domain walls can be much more common in bilayer graphene, since they can also emerge in unexpected geometries, e.g., at grain boundaries with atomic-scale topological defects. We focus on a bilayer system in which one of the layers contains a line of octagon-double pentagon defects that mix graphene sublattices. We demonstrate that gap states are preserved even with pentagonal defects. Remarkably, unlike previous predictions, the number of gap states changes by inverting the gate polarization, yielding an asymmetric conductance along the grain boundary under gate reversal. This effect, linked to defect states, should be detectable in transport measurements and could be exploited in electrical switches. PMID:26931739

  11. Growth from below: graphene bilayers on Ir(111).

    PubMed

    Nie, Shu; Walter, Andrew L; Bartelt, Norman C; Starodub, Elena; Bostwick, Aaron; Rotenberg, Eli; McCarty, Kevin F

    2011-03-22

    We elucidate how graphene bilayers form on Ir(111). Low-energy electron diffraction (LEED) reveals that the two graphene layers are not always rotationally aligned. Monitoring this misalignment during growth shows that second-layer islands nucleate between the existing layer and the substrate. This mechanism occurs both when C segregates from the Ir and when elemental C is deposited from above. Low-energy electron microscopy (LEEM) and angle-resolved photoemission spectroscopy (ARPES) show that second-layer nucleation occurs preferentially under the first-layer rotational variants that are more weakly bound to the substrate. New-layer nucleation tends to occur inhomogeneously at substrate defects. Thus new-layer nucleation should be rapid on substrates that weakly bind graphene, making growth unstable toward mound formation initiated at substrate defects. In contrast, stronger binding permits layer-by-layer growth, as for Ru(0001). ARPES shows that bilayer graphene has two slightly p-doped π-bands. The work function of bilayer graphene is dominated by the orientation of the bottom layer. PMID:21322532

  12. Transfer-free grown bilayer graphene transistors for digital applications

    NASA Astrophysics Data System (ADS)

    Wessely, Pia Juliane; Wessely, Frank; Birinci, Emrah; Riedinger, Bernadette; Schwalke, Udo

    2013-03-01

    We invented a novel method to fabricate graphene transistors on oxidized silicon wafers without the need to transfer graphene layers. By means of catalytic chemical vapor deposition (CCVD) the in situ grown bilayer graphene transistors (BiLGFETs) are realized directly on oxidized silicon substrate, whereby the number of stacked graphene layers is determined by the selected CCVD process parameters, e.g. temperature and gas mixture. BiLGFETs exhibit ultra-high on/off-current ratios of 107 at room temperature, exceeding previously reported values by several orders of magnitude. This will allow a simple and low-cost integration of graphene devices for digital nanoelectronic applications in a hybrid silicon CMOS environment for the first time.

  13. Asymmetric bilayer graphene nanoribbon MOSFETs for analog and digital electronics

    NASA Astrophysics Data System (ADS)

    Dinarvand, A.; Ahmadi, V.; Darvish, Gh.

    2016-05-01

    In this paper, a new structure was proposed for bilayer graphene nanoribbon field-effect transistor (BGNFET) mainly to enhance the electrical characteristics in analog and digital applications. The proposed device uses two metallic gates on the top and bottom of a bilayer graphene nanoribbon, which is surrounded by SiO2 and connected to heavily doped source/drain contacts. Electrical properties of the proposed device were explored using fully self-consistent solution of Poisson and Schrödinger equations based on the nonequilibrium Green's function (NEGF) formalism. Significant improvements in the electrical behavior was seen in the simulation results for gates asymmetrically biased. The comparison with graphene nanoribbon FET showed that the proposed structure benefited from higher intrinsic voltage gain and cut-off frequency and improved switching characteristics such as delay and Ion/Ioff ratio.

  14. Superlattice structures in twisted bilayers of folded graphene.

    PubMed

    Schmidt, Hennrik; Rode, Johannes C; Smirnov, Dmitri; Haug, Rolf J

    2014-01-01

    The electronic properties of bilayer graphene strongly depend on relative orientation of the two atomic lattices. Whereas Bernal-stacked graphene is most commonly studied, a rotational mismatch between layers opens up a whole new field of rich physics, especially at small interlayer twist. Here we report on magnetotransport measurements on twisted graphene bilayers, prepared by folding of single layers. These reveal a strong dependence on the twist angle, which can be estimated by means of sample geometry. At small rotation, superlattices with a wavelength in the order of 10 nm arise and are observed by friction atomic force microscopy. Magnetotransport measurements in this small-angle regime show the formation of satellite Landau fans. These are attributed to additional Dirac singularities in the band structure and discussed with respect to the wide range of interlayer coupling models.

  15. Electron dynamics of the buffer layer and bilayer graphene on SiC

    SciTech Connect

    Shearer, Alex J.; Caplins, Benjamin W.; Suich, David E.; Harris, Charles B.; Johns, James E.; Hersam, Mark C.

    2014-06-09

    Angle- and time-resolved two-photon photoemission (TPPE) was used to investigate electronic states in the buffer layer of 4H-SiC(0001). An image potential state (IPS) series was observed on this strongly surface-bound buffer layer, and dispersion measurements indicated free-electron-like behavior for all states in this series. These results were compared with TPPE taken on bilayer graphene, which also show the existence of a free-electron-like IPS series. Lifetimes for the n = 2, and n = 3 states were obtained from time-resolved TPPE; slightly increased lifetimes were observed in the bilayer graphene sample for the n = 2 the n = 3 states. Despite the large band gap of graphene at the center of the Brillouin zone, the lifetime results demonstrate that the graphene layers do not behave as a simple tunneling barrier, suggesting that the buffer layer and graphene overlayers play a direct role in the decay of IPS electrons.

  16. Raman spectroscopy of suspended mono and bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kitt, Alexander; Feldman, Benjamin; Remi, Sebastian; Martin, Jens; Swan, Anna; Yacoby, Amir; Goldberg, Bennett

    2010-03-01

    Suspended mono and bilayer graphene flakes have been shown to have higher mobility and lower disorder than their supported counterparts^1. The geometry which decouples the flake from the substrate also causes an as yet uncharacterized backgate specific strain due to an electrostatic attraction between the graphene and the back gated substrate. We study this strain using spatially resolved Raman spectroscopy with a diffraction limited spot size. Upon application of uni-axial strain the unit cell is stretched reducing the symmetry of the system and breaking the double degeneracy of the G band causing a split in the peak. Additionally the Raman modes show a linear softening as a function of strain in the case of supported graphene. Suspended flakes provide an ideal system to study back gate tunable strain while avoiding complications due to substrates including the determination of the Poisson ratio and sample slippage^2. Here we present preliminary results of our observations. 1: B Feldman, J Martin, A Yacoby, ``Broken-symmetry states and divergent resistance in suspended bilayer graphene'', Nature Physics, doi:10.1038/nphys1406 2: C Metzger et al, ``Biaxial strain in graphene adhered to shallow depressions'', Accepted for publication in Nano Letters

  17. Directional photoelectric current across the bilayer graphene junction.

    PubMed

    Shafranjuk, S E

    2009-01-01

    A directional photon-assisted resonant chiral tunneling through a bilayer graphene barrier is considered. An external electromagnetic field applied to the barrier switches the transparency T in the longitudinal direction from its steady state value T = 0 to the ideal T = 1 at no energy costs. The switch happens because the ac field affects the phase correlation between the electrons and holes inside the graphene barrier, changing the whole angular dependence of the chiral tunneling (directional photoelectric effect). The suggested phenomena can be implemented in relevant experiments and in various sub-millimeter and far-infrared optical electronic devices.

  18. Band Structure Asymmetry of Bilayer Graphene Revealed by Infrared Spectroscopy

    SciTech Connect

    Li, Z.Q.; Henriksen, E.A.; Jiang, Z.; Hao, Zhao; Martin, Michael C.; Kim, P.; Stormer, H.L.; Basov, Dimitri N.

    2008-12-10

    We report on infrared spectroscopy of bilayer graphene integrated in gated structures. We observe a significant asymmetry in the optical conductivity upon electrostatic doping of electrons and holes. We show that this finding arises from a marked asymmetry between the valence and conduction bands, which is mainly due to the inequivalence of the two sublattices within the graphene layer and the next-nearest-neighbor interlayer coupling. From the conductivity data, the energy difference of the two sublattices and the interlayer coupling energy are directly determined.

  19. AA-stacked bilayer square ice between graphene layers

    NASA Astrophysics Data System (ADS)

    Sobrino Fernandez Mario, M.; Neek-Amal, M.; Peeters, F. M.

    2015-12-01

    Water confined between two graphene layers with a separation of a few Å forms a layered two-dimensional ice structure. Using large scale molecular dynamics simulations with the adoptable ReaxFF interatomic potential we found that flat monolayer ice with a rhombic-square structure nucleates between the graphene layers which is nonpolar and nonferroelectric. We provide different energetic considerations and H-bonding results that explain the interlayer and intralayer properties of two-dimensional ice. The controversial AA stacking found experimentally [Algara-Siller et al., Nature (London) 519, 443 (2015), 10.1038/nature14295] is consistent with our minimum-energy crystal structure of bilayer ice. Furthermore, we predict that an odd number of layers of ice has the same lattice structure as monolayer ice, while an even number of ice layers exhibits the square ice AA stacking of bilayer ice.

  20. Insitu CCVD grown bilayer graphene transistors for applications in nanoelectronics

    NASA Astrophysics Data System (ADS)

    Wessely, Pia Juliane; Schwalke, Udo

    2014-02-01

    We invented a method to fabricate graphene field effect transistors (GFETs) on oxidized silicon wafers in a Silicon CMOS compatible process. The graphene layers needed are grown in situ by means of a transfer-free catalytic chemical vapor deposition (CCVD) process directly on silicon dioxide. Depending on the process parameters the fabrication of single, double or multi-layer graphene FETs (GFETs) is possible. The produced graphene layers have been characterized by SEM, TEM, TEM-lattice analysis as well as Raman-Spectroscopy. Directly after growth, the fabricated GFETs are electrically functional and can be electrically characterized via the catalyst metals which are used as contact electrodes. In contrast to monolayer graphene FETs, the fabricated bilayer graphene FETs (BiLGFETs) exhibit unipolar p-type MOSFET behavior. Furthermore, the on/off current-ratio of 104 up to several 107 at room temperature of the fabricated BiLGFETs allows their use in digital logic applications [1]. In addition, a stable hysteresis of the GFETs enables their use as memory devices without the need of storage capacitors and therefore very high memory device-densities are possible. The whole fabrication process is fully Si-CMOS compatible, enabling the use of hybrid silicon/graphene electronics.

  1. Twisted bilayer blue phosphorene: A direct band gap semiconductor

    NASA Astrophysics Data System (ADS)

    Ospina, D. A.; Duque, C. A.; Correa, J. D.; Suárez Morell, Eric

    2016-09-01

    We report that two rotated layers of blue phosphorene behave as a direct band gap semiconductor. The optical spectrum shows absorption peaks in the visible region of the spectrum and in addition the energy of these peaks can be tuned with the rotational angle. These findings makes twisted bilayer blue phosphorene a strong candidate as a solar cell or photodetection device. Our results are based on ab initio calculations of several rotated blue phosphorene layers.

  2. Perfect spin filtering controlled by an electric field in a bilayer graphene junction: Effect of layer-dependent exchange energy

    NASA Astrophysics Data System (ADS)

    Kitakorn, Jatiyanon; I-Ming, Tang; Bumned, Soodchomshom

    2016-07-01

    Magneto transport of carriers with a spin-dependent gap in a ferromagnetic-gated bilayer of graphene is investigated. We focus on the effect of an energy gap induced by the mismatch of the exchange fields in the top and bottom layers of an AB-stacked graphene bilayer. The interplay of the electric and exchange fields causes the electron to acquire a spin-dependent energy gap. We find that, only in the case of the anti-parallel configuration, the effect of a magnetic-induced gap will give rise to perfect spin filtering controlled by the electric field. The resolution of the spin filter may be enhanced by varying the bias voltage. Perfect switching of the spin polarization from + 100% to -100% by reversing the direction of electric field is predicted. Giant magnetoresistance is predicted to be easily realized when the applied electric field is smaller than the magnetic energy gap. It should be pointed out that the perfect spin filter is due to the layer-dependent exchange energy. This work points to the potential application of bilayer graphene in spintronics. Project supported by the Kasetsart University Research and Development Institute (KURDI) and Thailand Research Fund (TRF) (Grant No. TRG5780274).

  3. Perfect spin filtering controlled by an electric field in a bilayer graphene junction: Effect of layer-dependent exchange energy

    NASA Astrophysics Data System (ADS)

    Kitakorn, Jatiyanon; I-Ming, Tang; Bumned, Soodchomshom

    2016-07-01

    Magneto transport of carriers with a spin-dependent gap in a ferromagnetic-gated bilayer of graphene is investigated. We focus on the effect of an energy gap induced by the mismatch of the exchange fields in the top and bottom layers of an AB-stacked graphene bilayer. The interplay of the electric and exchange fields causes the electron to acquire a spin-dependent energy gap. We find that, only in the case of the anti-parallel configuration, the effect of a magnetic-induced gap will give rise to perfect spin filtering controlled by the electric field. The resolution of the spin filter may be enhanced by varying the bias voltage. Perfect switching of the spin polarization from + 100% to ‑100% by reversing the direction of electric field is predicted. Giant magnetoresistance is predicted to be easily realized when the applied electric field is smaller than the magnetic energy gap. It should be pointed out that the perfect spin filter is due to the layer-dependent exchange energy. This work points to the potential application of bilayer graphene in spintronics. Project supported by the Kasetsart University Research and Development Institute (KURDI) and Thailand Research Fund (TRF) (Grant No. TRG5780274).

  4. Superconducting properties of lithium-decorated bilayer graphene

    NASA Astrophysics Data System (ADS)

    Szczȩśniak, Dominik

    2015-07-01

    The present study provides a comprehensive theoretical analysis of the superconducting phase in selected lithium-decorated bilayer graphene nanostructures. The numerical calculations, conducted within the Eliashberg formalism, give quantitative estimations of the most important thermodynamic properties such as the critical temperature, specific heat, critical field and others. It is shown that discussed lithium-graphene systems present enhancement of their thermodynamic properties comparing to the monolayer case, e.g., the critical temperature can be raised to ∼15 \\text{K} . Furthermore, estimated characteristic thermodynamic ratios exceed predictions of the Bardeen-Cooper-Schrieffer theory suggesting that the considered lithium-graphene systems can be properly analyzed only within the strong-coupling regime.

  5. Realization of free-standing silicene using bilayer graphene

    SciTech Connect

    Neek-Amal, M.; Sadeghi, A.; Berdiyorov, G. R.; Peeters, F. M.

    2013-12-23

    The available synthesized silicene-like structures have been only realized on metallic substrates which are very different from the standalone buckled silicene, e.g., the Dirac cone of silicene is destroyed due to lattice distortion and the interaction with the substrate. Using graphene bilayer as a scaffold, a route is proposed to synthesize silicene with electronic properties decoupled from the substrate. The buckled hexagonal arrangement of silicene between the graphene layers is found to be very similar to the theoretically predicted standalone buckled silicene which is only very weakly van der Waals coupled to the graphene layers with a graphite-like interlayer distance of 3.42 Å and without any lattice distortion. We found that these stacked layers are stable well above room temperature.

  6. Electron-phonon interaction and pairing mechanism in superconducting Ca-intercalated bilayer graphene.

    PubMed

    Margine, E R; Lambert, Henry; Giustino, Feliciano

    2016-01-01

    Using the ab initio anisotropic Eliashberg theory including Coulomb interactions, we investigate the electron-phonon interaction and the pairing mechanism in the recently-reported superconducting Ca-intercalated bilayer graphene. We find that C6CaC6 can support phonon-mediated superconductivity with a critical temperature Tc = 6.8-8.1 K, in good agreement with experimental data. Our calculations indicate that the low-energy Caxy vibrations are critical to the pairing, and that it should be possible to resolve two distinct superconducting gaps on the electron and hole Fermi surface pockets. PMID:26892805

  7. Electron-phonon interaction and pairing mechanism in superconducting Ca-intercalated bilayer graphene

    NASA Astrophysics Data System (ADS)

    Margine, E. R.; Lambert, Henry; Giustino, Feliciano

    2016-02-01

    Using the ab initio anisotropic Eliashberg theory including Coulomb interactions, we investigate the electron-phonon interaction and the pairing mechanism in the recently-reported superconducting Ca-intercalated bilayer graphene. We find that C6CaC6 can support phonon-mediated superconductivity with a critical temperature Tc = 6.8-8.1 K, in good agreement with experimental data. Our calculations indicate that the low-energy Caxy vibrations are critical to the pairing, and that it should be possible to resolve two distinct superconducting gaps on the electron and hole Fermi surface pockets.

  8. Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene.

    PubMed

    Cao, Y; Luo, J Y; Fatemi, V; Fang, S; Sanchez-Yamagishi, J D; Watanabe, K; Taniguchi, T; Kaxiras, E; Jarillo-Herrero, P

    2016-09-01

    Twisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moiré physics and interlayer hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theoretically predicted. Moreover, Shubnikov-de Haas oscillations and tight binding calculations reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moiré wave vector.

  9. Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Cao, Y.; Luo, J. Y.; Fatemi, V.; Fang, S.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P.

    2016-09-01

    Twisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moiré physics and interlayer hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theoretically predicted. Moreover, Shubnikov-de Haas oscillations and tight binding calculations reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moiré wave vector.

  10. Electron-phonon interaction and pairing mechanism in superconducting Ca-intercalated bilayer graphene

    PubMed Central

    Margine, E. R.; Lambert, Henry; Giustino, Feliciano

    2016-01-01

    Using the ab initio anisotropic Eliashberg theory including Coulomb interactions, we investigate the electron-phonon interaction and the pairing mechanism in the recently-reported superconducting Ca-intercalated bilayer graphene. We find that C6CaC6 can support phonon-mediated superconductivity with a critical temperature Tc = 6.8–8.1 K, in good agreement with experimental data. Our calculations indicate that the low-energy Caxy vibrations are critical to the pairing, and that it should be possible to resolve two distinct superconducting gaps on the electron and hole Fermi surface pockets. PMID:26892805

  11. Optical rectification at visible frequency in biased bilayer graphene

    NASA Astrophysics Data System (ADS)

    Hipolito, F.; Pereira, Vitor M.

    2015-03-01

    The second order response of the electrical current to an electromagnetic field is analyzed within the framework of non-equilibrium many-body perturbation theory for the case of a two-dimensional electronic system such as graphene and its bilayer. The absence of inversion symmetry in a biased graphene bilayer allows a finite DC response in second order to an AC electromagnetic wave. The induced DC current is evaluated for biased bilayer at finite temperature, and its tunability is analyzed as a function of electron density, which can be experimentally varied by means of a global gate voltage applied to the sample. Both intrinsic and photon drag microscopic processes are considered, as they contribute on similar footing to the photocurrent in general. However, the dependencies of these two contributions on the polarization state of the incident light are different, which allows the manipulation of the relative contribution of intrinsic versus photon drag contributions by tuning the experimental parameters. For example, the photocurrent emerging from circularly polarized light stems entirely from photon drag, as the circular photogalvanic effect is forbidden by the C3 rotation symmetry of the honeycomb lattice.

  12. Specific heat of twisted bilayer graphene: Engineering phonons by atomic plane rotations

    SciTech Connect

    Nika, Denis L.; Cocemasov, Alexandr I.; Balandin, Alexander A.

    2014-07-21

    We have studied the phonon specific heat in single-layer, bilayer, and twisted bilayer graphene. The calculations were performed using the Born-von Karman model of lattice dynamics for intralayer atomic interactions and spherically symmetric interatomic potential for interlayer interactions. We found that at temperature T < 15 K, specific heat varies with temperature as T{sup n}, where n = 1 for graphene, n = 1.6 for bilayer graphene, and n = 1.3 for the twisted bilayer graphene. The phonon specific heat reveals an intriguing dependence on the twist angle in bilayer graphene, which is particularly pronounced at low temperature. The results suggest a possibility of phonon engineering of thermal properties of layered materials by twisting the atomic planes.

  13. van Hove Singularity Enhanced Photochemical Reactivity of Twisted Bilayer Graphene.

    PubMed

    Liao, Lei; Wang, Huan; Peng, Han; Yin, Jianbo; Koh, Ai Leen; Chen, Yulin; Xie, Qin; Peng, Hailin; Liu, Zhongfan

    2015-08-12

    Twisted bilayer graphene (tBLG) exhibits van Hove singularities (VHSs) in the density of states that can be tuned by changing the twist angle (θ), sparking various novel physical phenomena. Much effort has been devoted to investigate the θ-dependent physical properties of tBLG. Yet, the chemical properties of tBLG with VHSs, especially the chemical reactivity, remain unexplored. Here we report the first systematic study on the chemistry of tBLG through the photochemical reaction between graphene and benzoyl peroxide. Twisted bilayer graphene exhibits θ-dependent reactivity, and remarkably enhanced reactivity is obtained when the energy of incident laser matches with the energy interval of the VHSs of tBLG. This work provides an insight on the chemistry of tBLG, and the successful enhancement of chemical reactivity derived from VHS is highly beneficial for the controllable chemical modification of tBLG as well as the development of tBLG based devices.

  14. Electromagnetic coupling of spins and pseudospins in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Winkler, R.; Zülicke, U.

    2015-03-01

    We present a theoretical study of bilayer-graphene's electronic properties in the presence of electric and magnetic fields. In contrast to known materials, including single-layer graphene, any possible coupling of physical quantities to components of the electric field has a counterpart where the analogous component of the magnetic field couples to exactly the same quantities. For example, a purely electric spin splitting appears as the magneto-electric analogue of the magnetic Zeeman spin splitting. The measurable thermodynamic response induced by magnetic and electric fields is thus completely symmetric. The Pauli magnetization induced by a magnetic field takes exactly the same functional form as the polarization induced by an electric field. Although they seem counterintuitive, our findings are consistent with fundamental principles such as time reversal symmetry. For example, only a magnetic field can give rise to a macroscopic spin polarization, whereas only a perpendicular electric field can induce a macroscopic polarization of the sublattice-related pseudospin in bilayer graphene. These rules enforced by symmetry for the matter-field interactions clarify the nature of spins versus pseudospins. We have obtained numerical values of prefactors for relevant terms. NSF Grant DMR-1310199 and Marsden Fund Contract No. VUW0719.

  15. Dynamic Screening and Spectral Functions in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Sensarma, Rajdeep; Hwang, Euyheon; Das Sarma, Sankar

    2011-03-01

    We study the dynamic screening of Coulomb interactions in a bilayer graphene system within Random phase approximation. We derive an analytic expression for the dielectric function of the system and study the dispersion and damping of low energy plasmon modes. The quadratic dispersion and chirality of bilayer graphene systems lead to a plasmon dispersion which is distinct both from 2D electron gas and monolayer graphene plasmons. We also look at the effects of dynamic screening on the single particle spectral function of the system. We determine the quasiparticle weight, the effective mass and the damping of quasiparticles, which give a complete description of the low energy spectral function of the system.The compressibility of the system is also obtained from the self-energy renormalization of the chemical potential. We find that the presence of the second band leads to a well screened effective interaction, leading to much smaller renormalization of the Fermi liquid parameters in comparison to a 2D electron gas. However, the dynamic nature of the screening is very important in obtaining the single particle properties of this system. The authors acknowledge support from CNAM, Univ. of Maryland, and US-ONR MURI.

  16. Piezoelectric enhancement by surface effect in hydrofluorinated graphene bilayer

    SciTech Connect

    Kim, Hye Jung; Noor-A-Alam, Mohammad; Shin, Young-Han

    2015-04-14

    We investigated the piezoelectricity of dipolar hydrofluorinated graphene (C{sub 2}HF){sub n} multilayers with first-principles calculations. Our results reveal that the dipole moment decreases as the number of layers increases, because electron and hole carriers are induced at the top and bottom layers due to the depolarization field. These carriers make (C{sub 2}HF){sub n} multilayers more stable by decreasing the depolarization field in the material. Through the calculation of the average layer piezoelectric stress constant e{sub 31}/ℓ in ℓ-layer chair (C{sub 2}HF){sub n} multilayers, we confirmed that the piezoelectricity of the bilayer is about three times larger than that of the monolayer and bulk material. Moreover, we found that the electron and hole carriers on the top and bottom layers played a significant role in the piezoelectric enhancement of the bilayer.

  17. Giant Frictional Drag in Double Bilayer Graphene Heterostructures

    NASA Astrophysics Data System (ADS)

    Lee, Kayoung; Xue, Jiamin; Dillen, David C.; Watanabe, Kenji; Taniguchi, Takashi; Tutuc, Emanuel

    2016-07-01

    We study the frictional drag between carriers in two bilayer graphene flakes separated by a 2-5 nm thick hexagonal boron nitride dielectric. At temperatures (T ) lower than ˜10 K , we observe a large anomalous negative drag emerging predominantly near the drag layer charge neutrality. The anomalous drag resistivity increases dramatically with reducing T , and becomes comparable to the layer resistivity at the lowest T =1.5 K . At low T the drag resistivity exhibits a breakdown of layer reciprocity. A comparison of the drag resistivity and the drag layer Peltier coefficient suggests a thermoelectric origin of this anomalous drag.

  18. Giant Frictional Drag in Double Bilayer Graphene Heterostructures.

    PubMed

    Lee, Kayoung; Xue, Jiamin; Dillen, David C; Watanabe, Kenji; Taniguchi, Takashi; Tutuc, Emanuel

    2016-07-22

    We study the frictional drag between carriers in two bilayer graphene flakes separated by a 2-5 nm thick hexagonal boron nitride dielectric. At temperatures (T) lower than ∼10  K, we observe a large anomalous negative drag emerging predominantly near the drag layer charge neutrality. The anomalous drag resistivity increases dramatically with reducing T, and becomes comparable to the layer resistivity at the lowest T=1.5  K. At low T the drag resistivity exhibits a breakdown of layer reciprocity. A comparison of the drag resistivity and the drag layer Peltier coefficient suggests a thermoelectric origin of this anomalous drag. PMID:27494492

  19. Giant Frictional Drag in Double Bilayer Graphene Heterostructures.

    PubMed

    Lee, Kayoung; Xue, Jiamin; Dillen, David C; Watanabe, Kenji; Taniguchi, Takashi; Tutuc, Emanuel

    2016-07-22

    We study the frictional drag between carriers in two bilayer graphene flakes separated by a 2-5 nm thick hexagonal boron nitride dielectric. At temperatures (T) lower than ∼10  K, we observe a large anomalous negative drag emerging predominantly near the drag layer charge neutrality. The anomalous drag resistivity increases dramatically with reducing T, and becomes comparable to the layer resistivity at the lowest T=1.5  K. At low T the drag resistivity exhibits a breakdown of layer reciprocity. A comparison of the drag resistivity and the drag layer Peltier coefficient suggests a thermoelectric origin of this anomalous drag.

  20. Negative terahertz conductivity in remotely doped graphene bilayer heterostructures

    SciTech Connect

    Ryzhii, V.; Ryzhii, M.; Mitin, V.; Shur, M. S.; Otsuji, T.

    2015-11-14

    Injection or optical generation of electrons and holes in graphene bilayers (GBLs) can result in the interband population inversion enabling the terahertz (THz) radiation lasing. The intraband radiative processes compete with the interband transitions. We demonstrate that remote doping enhances the indirect interband generation of photons in the proposed GBL heterostructures. Therefore, such remote doping helps to surpass the intraband (Drude) absorption, and results in large absolute values of the negative dynamic THz conductivity in a wide range of frequencies at elevated (including room) temperatures. The remotely doped GBL heterostructure THz lasers are expected to achieve higher THz gain compared with previously proposed GBL-based THz lasers.

  1. Ultra low 1/f noise in suspended bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kumar, Manohar; Laitinen, Antti; Cox, Daniel; Hakonen, Pertti J.

    2015-06-01

    We have studied 1/f noise power SI in suspended bilayer graphene devices. Around the Dirac point, we observe ultra low noise amplitude on the order of f *SI/Ib2=10-9 . The low frequency noise level is barely sensitive to intrinsic carrier density, but temperature and external doping are found to influence the noise power. In our current-annealed samples, the 1/f noise is dominated by resistance fluctuations at the contacts. Temperature dependence of the 1/f noise suggests the presence of trap states in the contact regions, with a nearly exponential distribution function displaying a characteristic energy of 0.12 eV. At 80 K, the noise displays an air pressure sensitivity that corresponds to ˜0.3 ppm gas detection sensitivity; this indicates the potential of suspended graphene as a platform for gas sensing applications.

  2. Analytical investigation of bilayer lipid biosensor based on graphene.

    PubMed

    Akbari, Elnaz; Buntat, Zolkafle; Shahraki, Elmira; Parvaz, Ramtin; Kiani, Mohammad Javad

    2016-01-01

    Graphene is another allotrope of carbon with two-dimensional monolayer honeycomb. Owing to its special characteristics including electrical, physical and optical properties, graphene is known as a more suitable candidate compared to other materials to be used in the sensor application. It is possible, moreover, to use biosensor by using electrolyte-gated field effect transistor based on graphene (GFET) to identify the alterations in charged lipid membrane properties. The current article aims to show how thickness and charges of a membrane electric can result in a monolayer graphene-based GFET while the emphasis is on the conductance variation. It is proposed that the thickness and electric charge of the lipid bilayer (LLP and QLP) are functions of carrier density, and to find the equation relating these suitable control parameters are introduced. Artificial neural network algorithm as well as support vector regression has also been incorporated to obtain other models for conductance characteristic. The results comparison between analytical models, artificial neural network and support vector regression with the experimental data extracted from previous work show an acceptable agreement.

  3. Opening a band gap without breaking lattice symmetry: a new route toward robust graphene-based nanoelectronics

    NASA Astrophysics Data System (ADS)

    Kou, Liangzhi; Hu, Feiming; Yan, Binghai; Frauenheim, Thomas; Chen, Changfeng

    2014-06-01

    Developing graphene-based nanoelectronics hinges on opening a band gap in the electronic structure of graphene, which is commonly achieved by breaking the inversion symmetry of the graphene lattice via an electric field (gate bias) or asymmetric doping of graphene layers. Here we introduce a new design strategy that places a bilayer graphene sheet sandwiched between two cladding layers of materials that possess strong spin-orbit coupling (e.g., Bi2Te3). Our ab initio and tight-binding calculations show that a proximity enhanced spin-orbit coupling effect opens a large (44 meV) band gap in bilayer graphene without breaking its lattice symmetry, and the band gap can be effectively tuned by an interlayer stacking pattern and significantly enhanced by interlayer compression. The feasibility of this quantum-well structure is demonstrated by recent experimental realization of high-quality heterojunctions between graphene and Bi2Te3, and this design also conforms to existing fabrication techniques in the semiconductor industry. The proposed quantum-well structure is expected to be especially robust since it does not require an external power supply to open and maintain a band gap, and the cladding layers provide protection against environmental degradation of the graphene layer in its device applications.

  4. Graphene-silicene bilayer: A nanocapacitor with permanent dipole and piezoelectricity effect

    NASA Astrophysics Data System (ADS)

    Peymanirad, F.; Neek-Amal, M.; Beheshtian, J.; Peeters, F. M.

    2015-10-01

    Using density functional theory, we study the electronic properties of a graphene-silicene bilayer (GSB). A single layer of silicene binds to the graphene layer with adhesion energy of about 25 meV/atom. This adhesion energy between the two layers follows accurately the well-known -1 /z2 dispersion energy as found between two infinite parallel plates. In small flakes of GSB with hydrogenated edges, negative charge is transferred from the graphene layer to the silicene layer, producing a permanent and a switchable polar bilayer, while in an infinite GSB, the negative charge is transferred from the silicene layer to the graphene layer. The graphene-silicene bilayer is a good candidate for a nanocapacitor with piezoelectric capabilities. We found that the permanent dipole of the bilayer can be tuned by an external perpendicular electric field.

  5. Helium isotope enrichment by resonant tunneling through nanoporous graphene bilayers.

    PubMed

    Mandrà, Salvatore; Schrier, Joshua; Ceotto, Michele

    2014-08-21

    Graphene is impermeable to gases, but introducing subnanometer pores can allow for selective gas separation. Because graphene is only one atom thick, tunneling can play an important role, especially for low-mass gases such as helium, and this has been proposed as a means of separating (3)He from (4)He. In this paper, we consider the possibility of utilizing resonant tunneling of helium isotopes through nanoporous graphene bilayers. Using a model potential fit to previously reported DFT potential energy surfaces, we calculate the thermal rate constant as a function of interlayer separation using a recently described time-independent method for arbitrary multibarrier potentials. Resonant transmission allows for the total flux rate of (3)He to remain the same as the best-known single-barrier pores but doubles the selectivity with respect to (4)He when the optimal interlayer spacing of 4.6 Å is used. The high flux rate and selectivity are robust against variations of the interlayer spacing and asymmetries in the potential that may occur in experiment. PMID:24854987

  6. AA-stacked bilayer graphene quantum dots in magnetic field

    NASA Astrophysics Data System (ADS)

    Belouad, Abdelhadi; Zahidi, Youness; Jellal, Ahmed

    2016-05-01

    By applying the infinite-mass boundary condition, we analytically calculate the confined states and the corresponding wave functions of AA-stacked bilayer graphene (BLG) quantum dots (QDs) in the presence of an uniform magnetic field B. It is found that the energy spectrum shows two set of levels, which are the double copies of the energy spectrum for single layer graphene, shifted up–down by +γ and -γ , respectively. However, the obtained spectrum exhibits different symmetries between the electron and hole states as well as the intervalley symmetries. It is noticed that, the applied magnetic field breaks all symmetries, except one related to the intervalley electron–hole symmetry, i.e. {E}{{e}}(τ ,m)=-{E}{{h}}(τ ,m). Two different regimes of confinement are found: the first one is due to the infinite-mass barrier at weak B and the second is dominated by the magnetic field as long as B is large. We numerically investigated the basics features of the energy spectrum to show the main similarities and differences with respect to monolayer graphene, AB-stacked BLG and semiconductor QDs. Dedicated to Professor Dr Hachim A Yamani on the occasion of his 70th birthday.

  7. Continuous and reversible tuning of the disorder-driven superconductor-insulator transition in bilayer graphene.

    PubMed

    Lee, Gil-Ho; Jeong, Dongchan; Park, Kee-Su; Meir, Yigal; Cha, Min-Chul; Lee, Hu-Jong

    2015-01-01

    The influence of static disorder on a quantum phase transition (QPT) is a fundamental issue in condensed matter physics. As a prototypical example of a disorder-tuned QPT, the superconductor-insulator transition (SIT) has been investigated intensively over the past three decades, but as yet without a general consensus on its nature. A key element is good control of disorder. Here, we present an experimental study of the SIT based on precise in-situ tuning of disorder in dual-gated bilayer graphene proximity-coupled to two superconducting electrodes through electrical and reversible control of the band gap and the charge carrier density. In the presence of a static disorder potential, Andreev-paired carriers formed close to the Fermi level in bilayer graphene constitute a randomly distributed network of proximity-induced superconducting puddles. The landscape of the network was easily tuned by electrical gating to induce percolative clusters at the onset of superconductivity. This is evidenced by scaling behavior consistent with the classical percolation in transport measurements. At lower temperatures, the solely electrical tuning of the disorder-induced landscape enables us to observe, for the first time, a crossover from classical to quantum percolation in a single device, which elucidates how thermal dephasing engages in separating the two regimes.

  8. Semiconductor-halfmetal-metal transition and magnetism of bilayer graphene nanoribbons/hexagonal boron nitride heterostructure

    NASA Astrophysics Data System (ADS)

    Ilyasov, V. V.; Meshi, B. C.; Nguyen, V. C.; Ershov, I. V.; Nguyen, D. C.

    2014-12-01

    The paper presents the results of ab initio study of electronic structure modulation and edge magnetism in the antiferromagnetic (AF) bilayer zigzag graphene nanoribbons (AF-BZGNR)/hexagonal boron nitride (h-BN(0001)) semiconductor heterostructure induced with transverse external electric field (Eext) and nanomechanical compression (extension), performed within the framework of the density functional theory using Grimme's DFT(PBE)-D2 scheme. For the first time we established critical values of Eext and interlayer distance in the bilayer for the BZGNR/h-BN(0001) heterostructure providing for semiconductor-halfmetal-metal phase transition for one of the electron spin configurations. We discovered the effect of preserved local magnetic moment (0.3μB) of edge carbon atoms of the lower (buffer) graphene nanoribbon during nanomechanical uniaxial compression (or extension) of the BZGNR/h-BN(0001) semiconductor heterostructure. It has been demonstrated that magnetic properties of the AF-BZGNR/h-BN(0001) semiconductor heterostructure can be controlled using Eext. In particular, the local magnetic moment of edge carbon atoms decreases by 10% at a critical value of the positive potential. We have established that local magnetic moments and band gaps can be altered in a wide range using nanomechanical uniaxial compression and Eext, thus making the AF-BZGNR/h-BN(0001) semiconductor heterostructure potentially promising for nanosensors, spin filters, and spintronics applications.

  9. Continuous and reversible tuning of the disorder-driven superconductor-insulator transition in bilayer graphene.

    PubMed

    Lee, Gil-Ho; Jeong, Dongchan; Park, Kee-Su; Meir, Yigal; Cha, Min-Chul; Lee, Hu-Jong

    2015-01-01

    The influence of static disorder on a quantum phase transition (QPT) is a fundamental issue in condensed matter physics. As a prototypical example of a disorder-tuned QPT, the superconductor-insulator transition (SIT) has been investigated intensively over the past three decades, but as yet without a general consensus on its nature. A key element is good control of disorder. Here, we present an experimental study of the SIT based on precise in-situ tuning of disorder in dual-gated bilayer graphene proximity-coupled to two superconducting electrodes through electrical and reversible control of the band gap and the charge carrier density. In the presence of a static disorder potential, Andreev-paired carriers formed close to the Fermi level in bilayer graphene constitute a randomly distributed network of proximity-induced superconducting puddles. The landscape of the network was easily tuned by electrical gating to induce percolative clusters at the onset of superconductivity. This is evidenced by scaling behavior consistent with the classical percolation in transport measurements. At lower temperatures, the solely electrical tuning of the disorder-induced landscape enables us to observe, for the first time, a crossover from classical to quantum percolation in a single device, which elucidates how thermal dephasing engages in separating the two regimes. PMID:26310774

  10. Continuous and reversible tuning of the disorder-driven superconductor-insulator transition in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Lee, Gil-Ho; Jeong, Dongchan; Park, Kee-Su; Meir, Yigal; Cha, Min-Chul; Lee, Hu-Jong

    2015-08-01

    The influence of static disorder on a quantum phase transition (QPT) is a fundamental issue in condensed matter physics. As a prototypical example of a disorder-tuned QPT, the superconductor-insulator transition (SIT) has been investigated intensively over the past three decades, but as yet without a general consensus on its nature. A key element is good control of disorder. Here, we present an experimental study of the SIT based on precise in-situ tuning of disorder in dual-gated bilayer graphene proximity-coupled to two superconducting electrodes through electrical and reversible control of the band gap and the charge carrier density. In the presence of a static disorder potential, Andreev-paired carriers formed close to the Fermi level in bilayer graphene constitute a randomly distributed network of proximity-induced superconducting puddles. The landscape of the network was easily tuned by electrical gating to induce percolative clusters at the onset of superconductivity. This is evidenced by scaling behavior consistent with the classical percolation in transport measurements. At lower temperatures, the solely electrical tuning of the disorder-induced landscape enables us to observe, for the first time, a crossover from classical to quantum percolation in a single device, which elucidates how thermal dephasing engages in separating the two regimes.

  11. Ruderman-Kittel-Kasuya-Yosida interaction at finite temperature: Graphene and bilayer graphene

    NASA Astrophysics Data System (ADS)

    Klier, N.; Shallcross, S.; Sharma, S.; Pankratov, O.

    2015-11-01

    We investigate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities in both single layer and Bernal stacked bilayer graphene, finding a number of striking anomalies in the temperature dependence of this interaction. In undoped single layer graphene the strength of the RKKY interaction for substitutional impurities anomalously increases upon increasing temperature, an effect that persists up to and beyond room temperature. For impurities intercalated in the Bernal stacked bilayer and a doping that places the chemical potential near the antibonding band edge, a qualitative change of the RKKY interaction with temperature occurs: a low-temperature oscillatory interaction develops into a high-temperature antiferromagnetic coupling, accompanied by an overall increase of the interaction strength. The origin of the temperature anomalies can be traced back to specific features of the density of states: the vanishing density of states at the apex of the Dirac cone in single layer graphene, and the "kink" in the density of states at the antibonding band edge in the case of the Bernal bilayer.

  12. Coexisting massive and massless Dirac fermions in quasi-freestanding bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kim, Keun Su; Walter, Andrew L.; Moreschini, Luca; Seyller, Thomas; Horn, Karsten; Rotenberg, Eli; Bostwick, Aaron

    2013-03-01

    The most widely accepted theoretical model to describe charge carriers in bilayer graphene is ``massive Dirac fermions'', characterized by a nearly parabolic band pair touching each other at the Dirac energy. This electronic structure of bilayer graphene is widely believed to be unstable towards symmetry breaking either by structural distortions, such as twist and strain, or electronic interactions. In this work, we investigate quasi-freestanding bilayer graphene by angle-resolved photoemission spectroscopy, which shows an unexpected electronic spectrum, consisting of both massive and massless Dirac fermions. The latter has a unique band topology with a chiral pseudospin texture, and its origin will be discussed in terms of symmetry breaking induced by a native imperfection of bilayer graphene.

  13. Electric gating induced bandgaps and enhanced Seebeck effect in zigzag bilayer graphene ribbons

    NASA Astrophysics Data System (ADS)

    Vu, Thanh-Tra; Tran, Van-Truong

    2016-08-01

    We theoretically investigate the effect of a transverse electric field generated by side gates and a vertical electric field generated by top/back gates on energy bands and transport properties of zigzag bilayer graphene ribbons (Bernal stacking). Using atomistic tight binding calculations and Green’s function formalism we demonstrate that a bandgap is opened when either field is applied and even enlarged under simultaneous influence of the two fields. Interestingly, although vertical electric fields are widely used to control the bandgap in bilayer graphene, here we show that transverse fields exhibit a more positive effect in terms of modulating a larger range of bandgap and retaining good electrical conductance. The Seebeck effect is also demonstrated to be enhanced strongly—by about 13 times for a zigzag bilayer graphene ribbon with 16 chain lines. These results may motivate new designs of devices made of bilayer graphene ribbons using electric gates.

  14. Enhanced sensitivity of a microfabricated resonator using a graphene-polystyrene bilayer membrane

    SciTech Connect

    Yun, Minhyuk; Lee, Eunho; Cho, Kilwon; Jeon, Sangmin

    2014-08-18

    A graphene layer was synthesized using chemical vapor deposition methods and a polystyrene solution was spin-cast onto the graphene film. The graphene-polystyrene bilayer membrane was attached between the two tines of a microfabricated quartz tuning fork (QTF). The modulus of the graphene-polystyrene bilayer was measured to be twice that of a pristine polystyrene membrane. Exposure of the membrane-coated QTF to ethanol vapor decreased the resonance frequency of the microresonator. The bilayer membrane-coated QTF produced a frequency change that was three times the change obtained using a polystyrene membrane-coated QTF, with a lower degree of degradation in the Q factor. The limit of detection of the bilayer membrane-coated QTF to ethanol vapor was determined to be 20 ppm.

  15. Growth of ultra-uniform graphene using a Ni/W bilayer metal catalyst

    SciTech Connect

    Yang, Jae Hoon; Hwang, Jae Seok; Yang, Hyoung Woo; Kang, Dae Joon; Jang, A-Rang; Shin, Hyeon Suk; Jang, Jae-Eun

    2015-01-26

    We investigated a bilayer catalyst system consisting of polycrystalline Ni and W films for growing mono-layer graphene over large areas. Highly uniform graphene was grown on Ni/W bilayer film with 100% coverage. The graphene grown on Ni/W bilayer film and transferred onto an insulating substrate exhibited average hole and electron mobilities of 727 and 340 cm{sup 2}V{sup −1}s{sup −1}, respectively. A probable growth mechanism is proposed based on X-ray diffractometry and transmission electron microscopy, which suggests that the reaction between diffused carbon and tungsten atoms results in formation of tungsten carbides. This reaction allows the control of carbon precipitation and prevents the growth of non-uniform multilayer graphene on the Ni surface; this has not been straightforwardly achieved before. These results could be of importance in better understanding mono-layer graphene growth, and suggest a facile fabrication route for electronic applications.

  16. Realizing 1-D conducting channel between oppositely gated regions in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Lee, Janghee; Watanabe, Kenji; Taniguchi, Takashi; Lee, Hu-Jong

    The band gap of bilayer graphene (BLG) can be tuned by applying an external electric field perpendicular to the plane of a BLG sheet. If direction of the electric fields in two adjacent regions in BLG are opposite, one-dimensional (1-D) conducting channel emerges at the boundary between two regions with chiral nature. In this presentation, we introduce a method for fabricating two pairs of split-gates attached to BLG, which is sandwiched between two atomically clean hexagonal boron nitride (h-BN) single crystals and thus allows ballistic transport of carriers at least within the device size. Current-voltage characteristics show a large transport gap, which is comparable to the results obtained from optical measurements and numerical calculations. Opening the band gap in two adjacent regions of the BLG flake by oppositely gated electric fields, we observed metallic behavior in transport characteristics along the boundary between the two regions although the resistance of two gapped regions are a few hundreds of k Ω. These results indicate that a 1-D conducting channel formed between the two regions where the induced band gaps were inverted to each other. The formation of this 1-D conducting channel mimics the topological edge conducting channels emerging at the boundary of a two-dimensional topological insulator and may be utilized for applying BLG to valleytronics

  17. In Situ Observations of Free-Standing Graphene-like Mono- and Bilayer ZnO Membranes.

    PubMed

    Quang, Huy T; Bachmatiuk, Alicja; Dianat, Arezoo; Ortmann, Frank; Zhao, Jiong; Warner, Jamie H; Eckert, Jürgen; Cunniberti, Gianaurelio; Rümmeli, Mark H

    2015-11-24

    ZnO in its many forms, such as bulk, thin films, nanorods, nanobelts, and quantum dots, attracts significant attention because of its exciting optical, electronic, and magnetic properties. For very thin ZnO films, predictions were made that the bulk wurtzite ZnO structure would transit to a layered graphene-like structure. Graphene-like ZnO layers were later confirmed when supported over a metal substrate. However, the existence of free-standing graphene-like ZnO has, to the best of our knowledge, not been demonstrated. In this work, we show experimental evidence for the in situ formation of free-standing graphene-like ZnO mono- and bilayer ZnO membranes suspended in graphene pores. Local electron energy loss spectroscopy confirms the membranes comprise only Zn and O. Image simulations and supporting analysis confirm that the membranes are graphene-like ZnO. Graphene-like ZnO layers are predicted to have a wide band gap and different and exciting properties as compared to other ZnO structures.

  18. Conductance fluctuations in chaotic bilayer graphene quantum dots.

    PubMed

    Bao, Rui; Huang, Liang; Lai, Ying-Cheng; Grebogi, Celso

    2015-07-01

    Previous studies of quantum chaotic scattering established a connection between classical dynamics and quantum transport properties: Integrable or mixed classical dynamics can lead to sharp conductance fluctuations but chaos is capable of smoothing out the conductance variations. Relativistic quantum transport through single-layer graphene systems, for which the quasiparticles are massless Dirac fermions, exhibits, due to scarring, this classical-quantum correspondence, but sharp conductance fluctuations persist to a certain extent even when the classical system is fully chaotic. There is an open issue regarding the effect of finite mass on relativistic quantum transport. To address this issue, we study quantum transport in chaotic bilayer graphene quantum dots for which the quasiparticles have a finite mass. An interesting phenomenon is that, when traveling along the classical ballistic orbit, the quasiparticle tends to hop back and forth between the two layers, exhibiting a Zitterbewegung-like effect. We find signatures of abrupt conductance variations, indicating that the mass has little effect on relativistic quantum transport. In solid-state electronic devices based on Dirac materials, sharp conductance fluctuations are thus expected, regardless of whether the quasiparticle is massless or massive and whether there is chaos in the classical limit.

  19. Conductance fluctuations in chaotic bilayer graphene quantum dots.

    PubMed

    Bao, Rui; Huang, Liang; Lai, Ying-Cheng; Grebogi, Celso

    2015-07-01

    Previous studies of quantum chaotic scattering established a connection between classical dynamics and quantum transport properties: Integrable or mixed classical dynamics can lead to sharp conductance fluctuations but chaos is capable of smoothing out the conductance variations. Relativistic quantum transport through single-layer graphene systems, for which the quasiparticles are massless Dirac fermions, exhibits, due to scarring, this classical-quantum correspondence, but sharp conductance fluctuations persist to a certain extent even when the classical system is fully chaotic. There is an open issue regarding the effect of finite mass on relativistic quantum transport. To address this issue, we study quantum transport in chaotic bilayer graphene quantum dots for which the quasiparticles have a finite mass. An interesting phenomenon is that, when traveling along the classical ballistic orbit, the quasiparticle tends to hop back and forth between the two layers, exhibiting a Zitterbewegung-like effect. We find signatures of abrupt conductance variations, indicating that the mass has little effect on relativistic quantum transport. In solid-state electronic devices based on Dirac materials, sharp conductance fluctuations are thus expected, regardless of whether the quasiparticle is massless or massive and whether there is chaos in the classical limit. PMID:26274258

  20. Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene.

    PubMed

    Cao, Y; Luo, J Y; Fatemi, V; Fang, S; Sanchez-Yamagishi, J D; Watanabe, K; Taniguchi, T; Kaxiras, E; Jarillo-Herrero, P

    2016-09-01

    Twisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moiré physics and interlayer hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theoretically predicted. Moreover, Shubnikov-de Haas oscillations and tight binding calculations reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moiré wave vector. PMID:27661712

  1. Chirality-assisted electronic cloaking of confined States in bilayer graphene.

    PubMed

    Gu, Nan; Rudner, Mark; Levitov, Leonid

    2011-10-01

    We show that the strong coupling of pseudospin orientation and charge carrier motion in bilayer graphene has a drastic effect on transport properties of ballistic p-n-p junctions. Electronic states with zero momentum parallel to the barrier are confined under it for one pseudospin orientation, whereas states with the opposite pseudospin tunnel through the junction totally uninfluenced by the presence of confined states. We demonstrate that the junction acts as a cloak for confined states, making them nearly invisible to electrons in the outer regions over a range of incidence angles. This behavior is manifested in the two-terminal conductance as transmission resonances with non-Lorentzian, singular peak shapes. The response of these phenomena to a weak magnetic field or electric-field-induced interlayer gap can serve as an experimental fingerprint of electronic cloaking. PMID:22107310

  2. Topological phase transition induced by spin-orbit coupling in bilayer graphene.

    PubMed

    Xu, Lei; Zhou, Yuan; Gong, Chang-De

    2013-08-21

    We study the topological phase transition in biased bilayer graphene in the presence of intrinsic and Rashba spin-orbit couplings. The system exhibits a complicated topological phase transition depending on the given parameters. The topological phase transition between these phases is always accompanied by the bulk gap closing and reopening, and can be realized by tuning the bias voltage. The stability of these topological phases are also investigated. We find that the weak (strong) topological insulator phase remains stable under a finite exchange field provided that the effect of intrinsic (Rashba) spin-orbit coupling is dominant, and this also holds for the quantum valley Hall phase if the spatial inversion symmetry breaking overcomes the time-reversal symmetry breaking.

  3. Tuning the electronic band gap of graphene by oxidation

    SciTech Connect

    Dabhi, Shweta D.; Jha, Prafulla K.

    2015-06-24

    Using plane wave pseudo potential density functional theory, we studied the electronic properties of graphene with different C:O ratio. In this work, we discussed the changes that occur in electronic band structure of graphene functionalized with different amount of epoxy group. Electronic band gap depends on C:O ratio in graphene oxide containing epoxy group. The present work will have its implication for making devices with tunable electronic properties by oxidizing graphene.

  4. The effect of the dopant nature on the reactivity, interlayer bonding and electronic properties of dual doped bilayer graphene.

    PubMed

    Denis, Pablo A; Iribarne, Federico

    2016-09-21

    Herein, we report on the structural, chemical reactivity and electronic properties of dual-doped bilayer graphene (DDBG). Only one of the layers was doped with a pair of 3p-2p elements. Aluminum was the only dopant which prefers to interact with the undoped layer. The interlayer interaction energies of DDBG are smaller than those determined for bilayer graphene, except for AlN and AlO DDBG. This effect is due to the presence of weak Al-C interlayer bonds. The dopants increase the reactivity of both the doped and undoped layers. Interestingly, we found that hydrogenation is a method that can be used to switch on/off the interlayer bonding, as it controlled the X-C interlayer distance (X = Al, Si, P, S). The magnetic moment of the systems can be adjusted by the position of the 3p dopant. In effect, when X interacts with the doped layer, the magnetic moment is reduced, while it is maintained when X fails to interact. Finally, we found that the doped layer is able to break the symmetry of the undoped sheet and small gaps can be opened in the band structure of the undoped layer. As observed for single doped monolayer graphene, the most effective element for such purposes is P, which opened gaps close to 0.2 eV. For SiN DDBG, the spin filtering properties are enhanced with respect to the monolayered structure. PMID:27545204

  5. Effects of alloying 30 at. % Ni using a Cu catalyst on the growth of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Lee, Wan-Gyu; Kim, Eunho; Jung, Jongwan

    2012-12-01

    A small percentage graphene bilayer where the first layer was fully covered with a graphene sheet was grown on alloy foils via an inductively coupled plasma-chemical vapor deposition chamber. Compared with Cu foils, the alloy foils led to faster growth of the graphene film, while maintaining the same quality, homogeneity, and thickness uniformity over the whole area synthesized as the growth characteristics of a monolayer graphene grown on Cu. Moreover, the combined catalyst had a graphene film simultaneously grown with a metallic compound of magnesium silicate in some regions and of 200 nm Cu2.4S in other regions. Nevertheless, graphene was grown continuously and highly homogenously over the entire large area synthesized without boundaries between regions. Thus the resulting graphene growth is affected primarily by the Cu catalyst and partly by the Ni and that the quality of the graphene is dependent on the Cu catalyst.

  6. Planar electromagnetic band-gap structure based on graphene

    NASA Astrophysics Data System (ADS)

    Dong, Yanfei; Liu, Peiguo; Yin, Wen-Yan; Li, Gaosheng; Yi, Bo

    2015-06-01

    Electromagnetic band-gap structure with slow-wave effect is instrumental in effectively controlling electromagnetic wave propagation. In this paper, we theoretically analyze equivalent circuit model of electromagnetic band-gap structure based on graphene and evaluate its potential applications. Graphene electromagnetic band-gap based on parallel planar waveguide is investigated, which display good characteristics in dynamically adjusting the electromagnetic wave propagation in terahertz range. The same characteristics are retrieved in a spiral shape electromagnetic band-gap based on coplanar waveguide due to tunable conductivity of graphene. Various potential terahertz planar devices are expected to derive from the prototype structures.

  7. Ultrafast electron crystallography of heterogeneous structures: Gold-graphene bilayer and ligand-encapsulated nanogold on graphene

    NASA Astrophysics Data System (ADS)

    Liang, Wenxi; Schäfer, Sascha; Zewail, Ahmed H.

    2012-07-01

    Here, we report studies of structures comprising a nanoscale gold-graphene bilayer and ligand-encapsulated nanogold on graphene multilayers. The observed time scale for the heating dynamics of the gold layer is significantly slower, when compared to previous results on free-standing gold films, and is independent of the level of carrier excitation. A model is proposed which incorporates the local carrier excitation in the gold layer, carrier relaxation in the graphene layer and heating of the gold layer by thermal conduction. When gold is isolated from graphene by ligand encapsulation, the carriers become again localized, consistent with the two phase description.

  8. Energy levels of exciton in a gapped graphene sheet

    NASA Astrophysics Data System (ADS)

    Fallah, Farhang; Esmaeilzadeh, Mahdi

    2013-08-01

    A theory is presented for exciton formation in a graphene sheet using the center-of-mass approximation. The energy levels and wavefunctions of exciton are calculated analytically which show that the exciton can form if the band gap of graphene is not zero. We show that the energy gap of graphene plays the role of the mass which if not zero, leads to formation of the excitons. It is shown that the main quantum number of the exciton ground state changes with the graphene dielectric constant. Also, all of the states are found to be four-fold degenerate. The binding energy of exciton can reach as high as 1/4 of the energy gap of graphene which is notable among the conventional quasi-2D systems. This result can play an important rule in the photonics of graphene.

  9. Thinning of large-area graphene film from multilayer to bilayer with a low-power CO2 laser.

    PubMed

    Lin, Zhe; Huang, Ting; Ye, Xiaohui; Zhong, Minlin; Li, Lin; Jiang, Juan; Zhang, Wen; Fan, Lili; Zhu, Hongwei

    2013-07-12

    Bilayer graphene has attracted a great deal of attention for many electronic and optical applications. Although large-area bilayer graphene can be synthesized by chemical vapor deposition (CVD), multilayer growth often occurs and subsequent processes are required to obtain uniform bilayer films. We report an efficient way of thinning multilayer graphene film by low-power CO2 laser irradiation in vacuum. With a laser power density of ~10(2) W cm(-2), pristine graphene film of 4-5 layers can be thinned to a bilayer free of defects in 30 s. Contrary to previous laser-assisted graphene thinning processes, which reduced graphene layers precisely and locally with a high power density and a small beam diameter, our approach enables high-efficiency thinning of large-area graphene film whilst using a significantly reduced power density and an increased laser beam diameter.

  10. The possibility of superconductivity in twisted bilayer graphene

    SciTech Connect

    Manaf, Muhamad Nasruddin Santoso, Iman Hermanto, Arief

    2015-09-30

    We discuss the possibility of superconductivity in Twisted Bilayer Graphene (TBG). In this study we use TBG model with commensurate rotation θ=1.16° in which the van-Hove singularities (VHS) arise at 6 meV from the Fermi level. We use BCS standard formula that include Density of States (DOS) to calculate the critical temperature (T{sub C}). Based on our calculation we predict that superconductivity will not arise in Pristine TBG because pairing potential has infinity value. In this situation, Dirac Fermions do not interact with each other since they do not form the bound states. Superconductvity may arise when the Fermi level is shifted towards the VHS. Based on this calculation, we predict that T{sub C} has value between 0.04 K and 0.12 K. The low value of T{sub C} is due to highly energetic of in plane phonon vibration which reduce the effective electron-phonon coupling. We conclude that doped TBG is candidate for Dirac Fermion superconductor.

  11. Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures

    PubMed Central

    Hu, Xiaohui; Kou, Liangzhi; Sun, Litao

    2016-01-01

    The direct band gap of monolayer semiconducting transition-metal dichalcogenides (STMDs) enables a host of new optical and electrical properties. However, bilayer STMDs are indirect band gap semiconductors, which limits its applicability for high-efficiency optoelectronic devices. Here, we report that the direct band gap can be achieved in bilayer MoSe2-WSe2 lateral heterostructures by alternating stacking orders. Specifically, when Se atoms from opposite layers are stacked directly on top of each other, AA and A’B stacked heterostructures show weaker interlayer coupling, larger interlayer distance and direct band gap. Whereas, when Se atoms from opposite layers are staggered, AA’, AB and AB’ stacked heterostructures exhibit stronger interlayer coupling, shorter interlayer distance and indirect band gap. Thus, the direct/indirect band gap can be controllable in bilayer MoSe2-WSe2 lateral heterostructures. In addition, the calculated sliding barriers indicate that the stacking orders of bilayer MoSe2-WSe2 lateral heterostructures can be easily formed by sliding one layer with respect to the other. The novel direct band gap in bilayer MoSe2-WSe2 lateral heterostructures provides possible application for high-efficiency optoelectronic devices. The results also show that the stacking order is an effective strategy to induce and tune the band gap of layered STMDs. PMID:27528196

  12. Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures.

    PubMed

    Hu, Xiaohui; Kou, Liangzhi; Sun, Litao

    2016-08-16

    The direct band gap of monolayer semiconducting transition-metal dichalcogenides (STMDs) enables a host of new optical and electrical properties. However, bilayer STMDs are indirect band gap semiconductors, which limits its applicability for high-efficiency optoelectronic devices. Here, we report that the direct band gap can be achieved in bilayer MoSe2-WSe2 lateral heterostructures by alternating stacking orders. Specifically, when Se atoms from opposite layers are stacked directly on top of each other, AA and A'B stacked heterostructures show weaker interlayer coupling, larger interlayer distance and direct band gap. Whereas, when Se atoms from opposite layers are staggered, AA', AB and AB' stacked heterostructures exhibit stronger interlayer coupling, shorter interlayer distance and indirect band gap. Thus, the direct/indirect band gap can be controllable in bilayer MoSe2-WSe2 lateral heterostructures. In addition, the calculated sliding barriers indicate that the stacking orders of bilayer MoSe2-WSe2 lateral heterostructures can be easily formed by sliding one layer with respect to the other. The novel direct band gap in bilayer MoSe2-WSe2 lateral heterostructures provides possible application for high-efficiency optoelectronic devices. The results also show that the stacking order is an effective strategy to induce and tune the band gap of layered STMDs.

  13. Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures

    NASA Astrophysics Data System (ADS)

    Hu, Xiaohui; Kou, Liangzhi; Sun, Litao

    2016-08-01

    The direct band gap of monolayer semiconducting transition-metal dichalcogenides (STMDs) enables a host of new optical and electrical properties. However, bilayer STMDs are indirect band gap semiconductors, which limits its applicability for high-efficiency optoelectronic devices. Here, we report that the direct band gap can be achieved in bilayer MoSe2-WSe2 lateral heterostructures by alternating stacking orders. Specifically, when Se atoms from opposite layers are stacked directly on top of each other, AA and A’B stacked heterostructures show weaker interlayer coupling, larger interlayer distance and direct band gap. Whereas, when Se atoms from opposite layers are staggered, AA’, AB and AB’ stacked heterostructures exhibit stronger interlayer coupling, shorter interlayer distance and indirect band gap. Thus, the direct/indirect band gap can be controllable in bilayer MoSe2-WSe2 lateral heterostructures. In addition, the calculated sliding barriers indicate that the stacking orders of bilayer MoSe2-WSe2 lateral heterostructures can be easily formed by sliding one layer with respect to the other. The novel direct band gap in bilayer MoSe2-WSe2 lateral heterostructures provides possible application for high-efficiency optoelectronic devices. The results also show that the stacking order is an effective strategy to induce and tune the band gap of layered STMDs.

  14. Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures.

    PubMed

    Hu, Xiaohui; Kou, Liangzhi; Sun, Litao

    2016-01-01

    The direct band gap of monolayer semiconducting transition-metal dichalcogenides (STMDs) enables a host of new optical and electrical properties. However, bilayer STMDs are indirect band gap semiconductors, which limits its applicability for high-efficiency optoelectronic devices. Here, we report that the direct band gap can be achieved in bilayer MoSe2-WSe2 lateral heterostructures by alternating stacking orders. Specifically, when Se atoms from opposite layers are stacked directly on top of each other, AA and A'B stacked heterostructures show weaker interlayer coupling, larger interlayer distance and direct band gap. Whereas, when Se atoms from opposite layers are staggered, AA', AB and AB' stacked heterostructures exhibit stronger interlayer coupling, shorter interlayer distance and indirect band gap. Thus, the direct/indirect band gap can be controllable in bilayer MoSe2-WSe2 lateral heterostructures. In addition, the calculated sliding barriers indicate that the stacking orders of bilayer MoSe2-WSe2 lateral heterostructures can be easily formed by sliding one layer with respect to the other. The novel direct band gap in bilayer MoSe2-WSe2 lateral heterostructures provides possible application for high-efficiency optoelectronic devices. The results also show that the stacking order is an effective strategy to induce and tune the band gap of layered STMDs. PMID:27528196

  15. Energy gap of novel edge-defected graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Yuan, Weiqing; Wen, Zhongquan; Li, Min; Chen, Li; Chen, Gang; Ruan, Desheng; Gao, Yang

    2016-08-01

    Herein, the effects of width and boundary defects on the energy gap of graphene nanoribbons (GNRs) have been explored and theoretically investigated by means of semi-empirical atomic basis Extended Hückel method. Due to the existence of boundary defects, the energy gap of GNRs is mainly determined by the width of graphene nanoribbons for armchair graphene nanoribbons (AGNRs) or zigzag graphene nanoribbons (ZGNRs). Interestingly, the energy gap of AGNRs with a 120° V-type defect displays the monotone decreasing tendency when the width reaches to 2 nm, while the energy gap of intrinsic AGNRs is oscillatory. At the same time, the energy gap of U-type defected ZGNRs is opened, which differs from the zero energy gap characteristics of the intrinsic zigzag graphene. Furthermore, the size of energy gap of the defected AGNRs and ZGNRs with the same width is proved to be very close. Calculation results demonstrate that the energy gap of GNRs is just inversely proportional to the width and has little to do with the crystallographic direction. All the findings above provide a basis for energy gap engineering with different edge defects in GNRs and signify promising prospects in graphene-based semiconductor electronic devices.

  16. High-temperature quantum anomalous Hall effect in honeycomb bilayer consisting of Au atoms and single-vacancy graphene

    PubMed Central

    Han, Yan; Wan, Jian-Guo; Ge, Gui-Xian; Song, Feng-Qi; Wang, Guang-Hou

    2015-01-01

    The quantum anomalous Hall effect (QAHE) is predicted to be realized at high temperature in a honeycomb bilayer consisting of Au atoms and single-vacancy graphene (Au2-SVG) based on the first-principles calculations. We demonstrate that the ferromagnetic state in the Au2-SVG can be maintained up to 380 K. The combination of spatial inversion symmetry and the strong SOC introduced by the Au atoms causes a topologically nontrivial band gap as large as 36 meV and a QAHE state with Chern number C = −2. The analysis of the binding energy proved that the honeycomb bilayer is stable and feasible to be fabricated in experiment. The QAHEs in Ta2-SVG and other TM2-SVGs are also discussed. PMID:26574924

  17. Josephson coupling between superconducting islands on single- and bi-layer graphene

    NASA Astrophysics Data System (ADS)

    Mancarella, Francesco; Fransson, Jonas; Balatsky, Alexander

    2016-05-01

    We study the Josephson coupling of superconducting (SC) islands through the surface of single-layer graphene (SLG) and bilayer graphene (BLG) in the long-junction regime, as a function of the distance between the grains, temperature, chemical potential and external (transverse) gate-voltage. For SLG, we provide a comparison with existing literature. The proximity effect is analyzed through a Matsubara Green’s function approach. This represents the first step in a discussion of the conditions for the onset of a granular superconductivity within the film, made possible by Josephson currents flowing between superconductors. To ensure phase coherence over the 2D sample, a random spatial distribution can be assumed for the SC islands on the SLG sheet (or intercalating the BLG sheets). The tunable gate-voltage-induced band gap of BLG affects the asymptotic decay of the Josephson coupling-distance characteristic for each pair of SC islands in the sample, which results in a qualitatively strong field dependence of the relation between Berezinskii–Kosterlitz–Thouless transition critical temperature and gate voltage.

  18. Goos-Hänchen shifts in AA-stacked bilayer graphene superlattices

    NASA Astrophysics Data System (ADS)

    Zahidi, Youness; Redouani, Ilham; Jellal, Ahmed

    2016-07-01

    The quantum Goos-Hänchen shifts of the transmitted electron beam through an AA-stacked bilayer graphene superlattices are investigated. We found that the band structures of graphene superlattices can have more than one Dirac point, their locations do not depend on the number of barriers. It was revealed that any n-barrier structure is perfectly transparent at normal incidence around the Dirac points created in the superlattices. We showed that the Goos-Hänchen shifts display sharp peaks inside the transmission gap around two Dirac points (E =VB + τ, E =VW + τ), which are equal to those of transmission resonances. The obtained Goos-Hänchen shifts are exhibiting negative as well as positive behaviors and strongly depending on the location of Dirac points. It is observed that the maximum absolute values of the shifts increase as long as the number of barriers is increased. Our analysis is done by considering four cases: single, double barriers, superlattices without and with defect.

  19. Josephson coupling between superconducting islands on single- and bi-layer graphene

    NASA Astrophysics Data System (ADS)

    Mancarella, Francesco; Fransson, Jonas; Balatsky, Alexander

    2016-05-01

    We study the Josephson coupling of superconducting (SC) islands through the surface of single-layer graphene (SLG) and bilayer graphene (BLG) in the long-junction regime, as a function of the distance between the grains, temperature, chemical potential and external (transverse) gate-voltage. For SLG, we provide a comparison with existing literature. The proximity effect is analyzed through a Matsubara Green’s function approach. This represents the first step in a discussion of the conditions for the onset of a granular superconductivity within the film, made possible by Josephson currents flowing between superconductors. To ensure phase coherence over the 2D sample, a random spatial distribution can be assumed for the SC islands on the SLG sheet (or intercalating the BLG sheets). The tunable gate-voltage-induced band gap of BLG affects the asymptotic decay of the Josephson coupling-distance characteristic for each pair of SC islands in the sample, which results in a qualitatively strong field dependence of the relation between Berezinskii-Kosterlitz-Thouless transition critical temperature and gate voltage.

  20. Adsorption of beryllium atoms and clusters both on graphene and in a bilayer of graphite investigated by DFT.

    PubMed

    Ferro, Yves; Fernandez, Nicolas; Allouche, Alain; Linsmeier, Christian

    2013-01-01

    We herein investigate the interaction of beryllium with a graphene sheet and in a bilayer of graphite by means of periodic DFT calculations. In all cases, we find the beryllium atoms to be more weakly bonded on graphene than in the bilayer. Be(2) forms both magnetic and non-magnetic structures on graphene depending on the geometrical configuration of adsorption. We find that the stability of the Be/bilayer system increases with the size of the beryllium clusters inserted into the bilayer of graphite. We also find a charge transfer from beryllium to the graphite layers. All these results are analysed in terms of electronic structure.

  1. Spin Hall and spin Nernst effects due to intrinsic spin-orbit coupling in monolayer and bilayer graphene.

    PubMed

    Dyrdał, Anna; Barnaś, Józef

    2012-12-01

    We consider intrinsic contributions to the spin Hall and spin Nernst effects in monolayer and bilayer graphene. The spin Hall (Nernst) effect consists in the generation of transverse spin current by longitudinal electric field (temperature gradient). The relevant electronic spectrum for monolayer and bilayer graphene has been obtained from the corresponding effective Hamiltonians. Both spin Hall and spin Nernst conductivities have been determined within the linear response theory and Green function formalism. The influence of an external vertical voltage between the two atomic sheets in the case of a bilayer graphene is also analyzed and discussed. This voltage can generally lead to a phase transition between the topological insulator phase and conventional insulator. In the case of bilayer graphene, the main focuss is on an asymmetrical case, with different spin-orbit parameters in the two atomic sheets. Such a difference may be generated by different atomic planes adjacent to bilayer graphene on its both sides.

  2. Doping efficiency of single and randomly stacked bilayer graphene by iodine adsorption

    SciTech Connect

    Kim, HoKwon; Renault, Olivier; Rouchon, Denis; Mariolle, Denis; Chevalier, Nicolas; Tyurnina, Anastasia; Simonato, Jean-Pierre; Dijon, Jean

    2014-07-07

    We report on the efficiency and thermal stability of p-doping by iodine on single and randomly stacked, weakly coupled bilayer polycrystalline graphene, as directly measured by photoelectron emission microscopy. The doping results in work function value increase of 0.4–0.5 eV, with a higher degree of iodine uptake by the bilayer (2%) as compared to the single layer (1%) suggesting iodine intercalation in the bilayer. The chemistry of iodine is identified accordingly as I{sub 3}{sup −} and I{sub 5}{sup −} poly iodide anionic complexes with slightly higher concentration of I{sub 5}{sup −} in bilayer than monolayer graphene, likely attributed to differences in doping mechanisms. Temperature dependent in-situ annealing of the doped films demonstrated that the doping remains efficient up to 200 °C.

  3. Dynamical gap generation in graphene nanoribbons: An effective relativistic field theoretical model

    SciTech Connect

    Chaves, A. J.; Paula, W. de; Frederico, T.; Lima, G. D.; Cordeiro, C. E.; Delfino, A.

    2011-04-15

    We show that the assumption of a nontrivial zero band gap for a graphene sheet within an effective relativistic field theoretical model description of interacting Dirac electrons on the surface of graphene describes the experimental band gap of graphene nanoribbons for a wide range of widths. The graphene band gap is dynamically generated, corresponding to a nontrivial gapless solution, found in the limit of an infinitely wide graphene ribbon. The nanoribbon band gap is determined by the experimental graphene work function.

  4. Bound and free self-interstitial defects in graphite and bilayer graphene: A computational study

    SciTech Connect

    Gulans, Andris; Puska, Martti J.; Nieminen, Risto M.

    2011-07-01

    The role of self-interstitials in the response of layered carbon materials such as graphite, bilayer graphene and multiwalled carbon nanotubes to irradiation has long remained a puzzle. Using density-functional-theory methods with an exchange and correlation functional which takes into account the interlayer van der Waals interaction in these systems without any material-specific empirical parameters, we study the energetics and migration of single- and di-interstitials in graphite and bilayer graphene. We show that two classes of interstitials, ''bound'' and ''free,'' can coexist. The latter are mobile at room and lower temperatures, which explains the experimental data and reconciles them with the results of atomistic simulations. Our results shed light on the behavior of graphite and carbon nanotubes under irradiation and have implications for irradiation-mediated processing of bilayer graphene.

  5. Nonlinear optical conductivity of bilayer graphene with Rashba spin-orbit interaction in the terahertz regime

    SciTech Connect

    Liu, Zheng; Cao, J. C.; Sanderson, Matthew; Zhang, Chao

    2015-07-28

    The effect of Rashba spin-orbit coupling on the nonlinear optical conductivity in a bilayer graphene is investigated. We demonstrate the very different role played by the Rashba term and interlayer hopping; in some cases, the two roles can be quite opposite. It is found that the Rashba term can either enhance or suppress the nonlinear effect in a bilayer graphene, depending on the strength of the interlayer hopping. For a weak interlayer hopping, the Rashba term can significantly enhance the nonlinear effect. An analytical result was derived, showing the interplay of the Rashba effect and the interlayer hopping effect.

  6. Controlling dynamical thermal transport of biased bilayer graphene by impurity atoms

    NASA Astrophysics Data System (ADS)

    Rezania, Hamed; Yarmohammadi, Mohsen

    2016-07-01

    We address the dynamical thermal conductivity of biased bilayer graphene doped with acceptor impurity atoms for AA-stacking in the context of tight binding model Hamiltonian. The effect of scattering by dilute charged impurities is discussed in terms of the self-consistent Born approximation. Green's function approach has been exploited to find the behavior of thermal conductivity of bilayer graphene within the linear response theory. We have found the frequency dependence of thermal conductivity for different values of concentration and scattering strength of dopant impurity. Also the dependence of thermal conductivity on the impurity concentration and bias voltage has been investigated in details.

  7. Phase boundary of spin-polarized-current state of electrons in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Yan, Xin-Zhong; Ma, Yinfeng; Ting, C. S.

    2016-06-01

    Using a four-band Hamiltonian, we study the phase boundary of spin-polarized-current state (SPCS) of interacting electrons in bilayer graphene. The model of spin-polarized-current state has previously been shown to resolve a number of experimental puzzles in bilayer graphene. The phase boundaries of the SPCS with and without the external voltage between the two layers are obtained in this work. An unusual phase boundary where there are two transition temperatures for a given carrier concentration is found at finite external voltage. The physics of this phenomenon is explained.

  8. Phase transitions in two tunnel-coupled HgTe quantum wells: Bilayer graphene analogy and beyond

    PubMed Central

    Krishtopenko, S. S.; Knap, W.; Teppe, F.

    2016-01-01

    HgTe quantum wells possess remarkable physical properties as for instance the quantum spin Hall state and the “single-valley” analog of graphene, depending on their layer thicknesses and barrier composition. However, double HgTe quantum wells yet contain more fascinating and still unrevealed features. Here we report on the study of the quantum phase transitions in tunnel-coupled HgTe layers separated by CdTe barrier. We demonstrate that this system has a 3/2 pseudo spin degree of freedom, which features a number of particular properties associated with the spin-dependent coupling between HgTe layers. We discover a specific metal phase arising in a wide range of HgTe and CdTe layer thicknesses, in which a gapless bulk and a pair of helical edge states coexist. This phase holds some properties of bilayer graphene such as an unconventional quantum Hall effect and an electrically-tunable band gap. In this “bilayer graphene” phase, electric field opens the band gap and drives the system into the quantum spin Hall state. Furthermore, we discover a new type of quantum phase transition arising from a mutual inversion between second electron- and hole-like subbands. This work paves the way towards novel materials based on multi-layered topological insulators. PMID:27476745

  9. Phase transitions in two tunnel-coupled HgTe quantum wells: Bilayer graphene analogy and beyond

    NASA Astrophysics Data System (ADS)

    Krishtopenko, S. S.; Knap, W.; Teppe, F.

    2016-08-01

    HgTe quantum wells possess remarkable physical properties as for instance the quantum spin Hall state and the “single-valley” analog of graphene, depending on their layer thicknesses and barrier composition. However, double HgTe quantum wells yet contain more fascinating and still unrevealed features. Here we report on the study of the quantum phase transitions in tunnel-coupled HgTe layers separated by CdTe barrier. We demonstrate that this system has a 3/2 pseudo spin degree of freedom, which features a number of particular properties associated with the spin-dependent coupling between HgTe layers. We discover a specific metal phase arising in a wide range of HgTe and CdTe layer thicknesses, in which a gapless bulk and a pair of helical edge states coexist. This phase holds some properties of bilayer graphene such as an unconventional quantum Hall effect and an electrically-tunable band gap. In this “bilayer graphene” phase, electric field opens the band gap and drives the system into the quantum spin Hall state. Furthermore, we discover a new type of quantum phase transition arising from a mutual inversion between second electron- and hole-like subbands. This work paves the way towards novel materials based on multi-layered topological insulators.

  10. Anomalous carrier dynamics in bilayer graphene in presence of mechanical strain: A theoretical study

    NASA Astrophysics Data System (ADS)

    Enamullah

    2016-05-01

    One of the optical response of charge carriers in bilayer graphene, anomalous Rabi oscillation is investigated theoretically in presence of mechanical strain. Rabi oscillation in extreme non-resonance regime is known as anomalous Rabi oscillation, has been predicted theoretically in single layer graphene by new technique known as asymptotic rotating wave approximation. In this article, we have shown a strong dependence of anomalous Rabi oscillations of charge carriers on the mechanical strain near the vanishing point of conduction and valance band.

  11. Raman spectra of bilayer graphene covered with Poly(methyl methacrylate) thin film

    SciTech Connect

    Xia Minggang; Su Zhidan; Zhang Shengli

    2012-09-15

    The Raman spectra of bilayer graphene covered with poly(methyl methacrylate) (PMMA) were investigated. Both the G and 2D peaks of PMMA-coated graphene were stiff and broad compared with those of uncovered graphene. This could be attributed to the residual strain induced by high-temperature baking during fabrication of the nanodevice. Furthermore, the two 2D peaks stiffened and broadened with increasing laser power, which is just the reverse to uncovered graphene. The stiffness is likely caused by graphene compression induced by the circular bubble of the thin PMMA film generated by laser irradiation. Our findings may contribute to the application of PMMA in the strain engineering of graphene nanodevices.

  12. Oxygen-Activated Growth and Bandgap Tunability of Large Single-Crystal Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Hao, Yufeng; Hone, James; Ruoff, Rodney; Colombo, Luigi; the Hone Group Team

    Distinct from zero-bandgap single-layer graphene, Bernal-stacked bilayer graphene (BLG) is a semiconductor whose bandgap can be tuned by a transverse electric field, making it a unique material for a number of electronic and photonic devices. In this presentation, we will focus on the most recent progress in the identification of new growth mechanisms towards large-area single-layer BLG on Copper: multiple control experiments and first-principles calculations are used to support the proposed mechanisms. We emphasize that trace amount of impurities on metal surface are critical to initiate graphene growth and affect the growth kinetics. Furthermore, contrary to the traditional viewpoint that graphene growth is always surface-limited process, our new observations strongly suggest that metal bulk plays a role to feed carbon species for graphene growth. State-of-the-art structural characterizations and electrical transport measurements of the CVD graphene layers will be presented as well.

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

    SciTech Connect

    Yu, Cui; Liu, Qingbin; Li, Jia; Lu, Weili; He, Zezhao; Cai, Shujun; Feng, Zhihong

    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 and more impurities, which limit the carrier mobility of the quasi-free standing bilayer graphene samples.

  14. Strain-induced gap transition and anisotropic Dirac-like cones in monolayer and bilayer phosphorene

    SciTech Connect

    Wang, Can; Xia, Qinglin Nie, Yaozhuang; Guo, Guanghua

    2015-03-28

    The electronic properties of two-dimensional monolayer and bilayer phosphorene subjected to uniaxial and biaxial strains have been investigated using first-principles calculations based on density functional theory. Strain engineering has obvious influence on the electronic properties of monolayer and bilayer phosphorene. By comparison, we find that biaxial strain is more effective in tuning the band gap than uniaxial strain. Interestingly, we observe the emergence of Dirac-like cones by the application of zigzag tensile strain in the monolayer and bilayer systems. For bilayer phosphorene, we induce the anisotropic Dirac-like dispersion by the application of appropriate armchair or biaxial compressive strain. Our results present very interesting possibilities for engineering the electronic properties of phosphorene and pave a way for tuning the band gap of future electronic and optoelectronic devices.

  15. Coupled Dirac fermions and neutrino-like oscillations in twisted bilayer graphene.

    PubMed

    Xian, Lede; Wang, Z F; Chou, M Y

    2013-11-13

    The low-energy quasiparticles in graphene can be described by a Dirac-Weyl Hamiltonian for massless fermions, hence graphene has been proposed to be an effective medium to study exotic phenomena originally predicted for relativistic particle physics, such as Klein tunneling and Zitterbewegung. In this work, we show that another important particle-physics phenomenon, the neutrino oscillation, can be studied and observed in a particular graphene system, namely, twisted bilayer graphene. It has been found that graphene layers grown epitaxially on SiC or by the chemical vapor deposition method on metal substrates display a stacking pattern with adjacent layers rotated by an angle with respect to each other. The quasiparticle states in two distinct graphene layers act as neutrinos with two flavors, and the interlayer interaction between them induces an appreciable coupling between these two "flavors" of massless fermions, leading to neutrino-like oscillations. In addition, our calculation shows that anisotropic transport properties manifest in a specific energy window, which is accessible experimentally in twisted bilayer graphene. Combining two graphene layers enables us to probe the rich physics involving multiple interacting Dirac fermions. PMID:24079848

  16. Collapsed armchair single-walled carbon nanotubes as an analog of closed-edged bilayer graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Hasegawa, Masayuki; Nishidate, Kazume; Yoshimoto, Noriyuki

    2015-12-01

    Recently, radially collapsed single-walled carbon nanotubes (SWCNTs) have been recognized as an analog of closed-edged bilayer graphene nanoribbons (GNRs). To explore this analogy, we first make detailed analyses of the radial deformation and collapse of armchair SWCNTs using the density functional theory with van der Waals corrections. The traditional threshold diameters dividing SWCNTs into the three regimes are obtained as Dmeta=2.2 nm and Dabs=5.1 nm , where deformed configurations can be stabilized (metastable) for tubes with diameter D >Dmeta and are energetically more favorable than the cylindrical tube for D >Dabs (absolute stability). We find that the present result for Dabs is marginally in excellent agreement with the most plausible experimental result. We also identify, for the first time, other threshold diameters given by Dflat 1=3.3 nm and Dflat 2=4.4 nm in between Dmeta and Dabs, where the cross-sectional shape of a collapsed SWCNT is peanutlike for D Dflat 2. This bistability for tubes with D between Dflat 1 and Dflat 2 implies that the most stable configuration of these collapsed tubes cannot necessarily be achieved by molecular dynamics simulation. Electronic structures of collapsed armchair SWCNTs are also investigated to find that substantial band gaps develop in the flattened armchair tubes with Bernal stacked opposing faces. These band gap openings of flattened SWCNTs are explored by exploiting their analogies to bilayer graphene and bilayer GNRs. We find in particular that band gaps of flattened armchair SWCNTs with dumbbell-like cross sections, whose width is denoted W , show a scale behavior, ˜1 /Wflat , with Wflat=W -3.38 (nm ) , where Wflat is found to be the width of the flat region, consistent with recent theoretical analyses for a model of collapsed SWCNTs [T. Nakanishi and T. Ando, Phys. Rev. B 91, 155420 (2015), 10.1103/PhysRevB.91

  17. Graphene-diamond interface: Gap opening and electronic spin injection

    NASA Astrophysics Data System (ADS)

    Ma, Yandong; Dai, Ying; Guo, Meng; Huang, Baibiao

    2012-06-01

    Creating a finite band gap, injecting electronic spin, and finding a suitable substrate are the three important challenges for building graphene-based devices. Here, first-principles calculations are performed to investigate the electronic and magnetic properties of graphene adsorbed on the (111) surface of diamond, which is synthesized experimentally [Nature10.1038/nature09979 472, 74 (2011); J. Appl. Phys.10.1063/1.3627370 110, 044324 (2011); Nano Lett.10.1021/nl204545q 12, 1603 (2012); ACS Nano10.1021/nn204362p 6, 1018 (2012)]. Our results reveal that the graphene adsorbed on the diamond surface is a semiconductor with a finite gap depending on the adsorption arrangements due to the variation of on-site energy induced by the diamond surface, with the extra advantage of maintaining main characters of the linear band dispersion of graphene. More interestingly, different from typical graphene/semiconductor hybrid systems, we find that electronic spin can arise ``intrinsically'' in graphene owing to the exchange proximity interaction between electrons in graphene and localized electrons in the diamond surface rather than the characteristic graphene states. These predications strongly revive this new synthesized system as a viable candidate to overcome all the aforementioned challenges, providing an ideal platform for future graphene-based electronics.

  18. 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

  19. Determination of the electronic structure of bilayer graphene from infrared spectroscopy results

    SciTech Connect

    Zhang, L. M.; Li, Z. Q.; Basov, D. N.; Fogler, M. M.; Hao, Z.; Martin, Michael C.

    2008-11-12

    We present an experimental study of the infrared conductivity, transmission, and reflection of a gated bilayer graphene and their theoretical analysis within the Slonczewski-Weiss-McClure (SWMc) model. The infrared response is shown to be governed by the interplay of the interband and the intraband transitions among the four bands of the bilayer. The position of the main conductivity peak at thecharge-neutrality point is determined by the interlayer tunneling frequency. The shift of this peak as a function of the gate voltage gives information about less known parameters of the SWMc model such as those responsible for the electron-hole and sublattice asymmetries. These parameter values are shown to be consistent with recent electronic structure calculations for the bilayer graphene and the SWMc parameters commonly used for the bulk graphite.

  20. Confining and repulsive potentials from effective non-Abelian gauge fields in graphene bilayers

    NASA Astrophysics Data System (ADS)

    González, J.

    2016-10-01

    We investigate the effect of shear and strain in graphene bilayers, under conditions where the distortion of the lattice gives rise to a smooth one-dimensional modulation in the stacking sequence of the bilayer. We show that strain and shear produce characteristic Moiré patterns which can have the same visual appearance on a large scale, but representing graphene bilayers with quite different electronic properties. The different features in the low-energy electronic bands can be ascribed to the effect of a fictitious non-Abelian gauge field mimicking the smooth modulation of the stacking order. Strained and sheared bilayers show a complementary behavior, which can be understood from the fact that the non-Abelian gauge field acts as a repulsive interaction in the former, expelling the electron density away from the stacking domain walls, while behaving as a confining interaction leading to localization of the electronic states in the sheared bilayers. In this latter case, the presence of the effective gauge field explains the development of almost flat low-energy bands, resembling the form of the zeroth Landau level characteristic of a Dirac fermion field. The estimate of the gauge field strength in those systems gives a magnitude of the order of several tens of tesla, implying a robust phenomenology that should be susceptible of being observed in suitably distorted bilayer samples.

  1. Single-layer and bilayer graphene superlattices: collimation, additional Dirac points and Dirac lines.

    PubMed

    Barbier, Michaël; Vasilopoulos, Panagiotis; Peeters, François M

    2010-12-13

    We review the energy spectrum and transport properties of several types of one-dimensional superlattices (SLs) on single-layer and bilayer graphene. In single-layer graphene, for certain SL parameters an electron beam incident on an SL is highly collimated. On the other hand, there are extra Dirac points generated for other SL parameters. Using rectangular barriers allows us to find analytical expressions for the location of new Dirac points in the spectrum and for the renormalization of the electron velocities. The influence of these extra Dirac points on the conductivity is investigated. In the limit of δ-function barriers, the transmission T through and conductance G of a finite number of barriers as well as the energy spectra of SLs are periodic functions of the dimensionless strength P of the barriers, Pδ(x) = V(x)/ħv(F), with v(F) the Fermi velocity. For a Kronig-Penney SL with alternating sign of the height of the barriers, the Dirac point becomes a Dirac line for P = π/2+nπ with n an integer. In bilayer graphene, with an appropriate bias applied to the barriers and wells, we show that several new types of SLs are produced and two of them are similar to type I and type II semiconductor SLs. Similar to single-layer graphene SLs, extra 'Dirac' points are found in bilayer graphene SLs. Non-ballistic transport is also considered. PMID:21041227

  2. Selectively enhanced photocurrent generation in twisted bilayer graphene with van Hove singularity.

    PubMed

    Yin, Jianbo; Wang, Huan; Peng, Han; Tan, Zhenjun; Liao, Lei; Lin, Li; Sun, Xiao; Koh, Ai Leen; Chen, Yulin; Peng, Hailin; Liu, Zhongfan

    2016-01-01

    Graphene with ultra-high carrier mobility and ultra-short photoresponse time has shown remarkable potential in ultrafast photodetection. However, the broad and weak optical absorption (∼ 2.3%) of monolayer graphene hinders its practical application in photodetectors with high responsivity and selectivity. Here we demonstrate that twisted bilayer graphene, a stack of two graphene monolayers with an interlayer twist angle, exhibits a strong light-matter interaction and selectively enhanced photocurrent generation. Such enhancement is attributed to the emergence of unique twist-angle-dependent van Hove singularities, which are directly revealed by spatially resolved angle-resolved photoemission spectroscopy. When the energy interval between the van Hove singularities of the conduction and valance bands matches the energy of incident photons, the photocurrent generated can be significantly enhanced (up to ∼ 80 times with the integration of plasmonic structures in our devices). These results provide valuable insight for designing graphene photodetectors with enhanced sensitivity for variable wavelength.

  3. Uniformity of large-area bilayer graphene grown by chemical vapor deposition

    NASA Astrophysics Data System (ADS)

    Sheng, Yuewen; Rong, Youmin; He, Zhengyu; Fan, Ye; Warner, Jamie H.

    2015-10-01

    Graphene grown by chemical vapor deposition (CVD) on copper foils is a viable method for large area films for transparent conducting electrode (TCE) applications. We examine the spatial uniformity of large area films on the centimeter scale when transferred onto both Si substrates with 300 nm oxide and flexible transparent polyethylene terephthalate substrates. A difference in the quality of graphene, as measured by the sheet resistance and transparency, is found for the areas at the edges of large sheets that depends on the supporting boat used for the CVD growth. Bilayer graphene is grown with uniform properties on the centimeter scale when a flat support is used for CVD growth. The flat support provides consistent delivery of precursor to the copper catalyst for graphene growth. These results provide important insights into the upscaling of CVD methods for growing high quality graphene and its transfer onto flexible substrates for potential applications as a TCE.

  4. Uniformity of large-area bilayer graphene grown by chemical vapor deposition.

    PubMed

    Sheng, Yuewen; Rong, Youmin; He, Zhengyu; Fan, Ye; Warner, Jamie H

    2015-10-01

    Graphene grown by chemical vapor deposition (CVD) on copper foils is a viable method for large area films for transparent conducting electrode (TCE) applications. We examine the spatial uniformity of large area films on the centimeter scale when transferred onto both Si substrates with 300 nm oxide and flexible transparent polyethylene terephthalate substrates. A difference in the quality of graphene, as measured by the sheet resistance and transparency, is found for the areas at the edges of large sheets that depends on the supporting boat used for the CVD growth. Bilayer graphene is grown with uniform properties on the centimeter scale when a flat support is used for CVD growth. The flat support provides consistent delivery of precursor to the copper catalyst for graphene growth. These results provide important insights into the upscaling of CVD methods for growing high quality graphene and its transfer onto flexible substrates for potential applications as a TCE. PMID:26349521

  5. Selectively enhanced photocurrent generation in twisted bilayer graphene with van Hove singularity

    PubMed Central

    Yin, Jianbo; Wang, Huan; Peng, Han; Tan, Zhenjun; Liao, Lei; Lin, Li; Sun, Xiao; Koh, Ai Leen; Chen, Yulin; Peng, Hailin; Liu, Zhongfan

    2016-01-01

    Graphene with ultra-high carrier mobility and ultra-short photoresponse time has shown remarkable potential in ultrafast photodetection. However, the broad and weak optical absorption (∼2.3%) of monolayer graphene hinders its practical application in photodetectors with high responsivity and selectivity. Here we demonstrate that twisted bilayer graphene, a stack of two graphene monolayers with an interlayer twist angle, exhibits a strong light–matter interaction and selectively enhanced photocurrent generation. Such enhancement is attributed to the emergence of unique twist-angle-dependent van Hove singularities, which are directly revealed by spatially resolved angle-resolved photoemission spectroscopy. When the energy interval between the van Hove singularities of the conduction and valance bands matches the energy of incident photons, the photocurrent generated can be significantly enhanced (up to ∼80 times with the integration of plasmonic structures in our devices). These results provide valuable insight for designing graphene photodetectors with enhanced sensitivity for variable wavelength. PMID:26948537

  6. Theoretical study of electronic transport properties of a graphene-silicene bilayer

    NASA Astrophysics Data System (ADS)

    Berdiyorov, G. R.; Bahlouli, H.; Peeters, F. M.

    2015-06-01

    Electronic transport properties of a graphene-silicene bilayer system are studied using density-functional theory in combination with the nonequilibrium Green's function formalism. Depending on the energy of the electrons, the transmission can be larger in this system as compared to the sum of the transmissions of separated graphene and silicene monolayers. This effect is related to the increased electron density of states in the bilayer sample. At some energies, the electronic states become localized in one of the layers, resulting in the suppression of the electron transmission. The effect of an applied voltage on the transmission becomes more pronounced in the layered sample as compared to graphene due to the larger variation of the electrostatic potential profile. Our findings will be useful when creating hybrid nanoscale devices where enhanced transport properties will be desirable.

  7. Two-dimensional topological insulator state and topological phase transition in bilayer graphene.

    PubMed

    Qiao, Zhenhua; Tse, Wang-Kong; Jiang, Hua; Yao, Yugui; Niu, Qian

    2011-12-16

    We show that gated bilayer graphene hosts a strong topological insulator (TI) phase in the presence of Rashba spin-orbit (SO) coupling. We find that gated bilayer graphene under preserved time-reversal symmetry is a quantum valley Hall insulator for small Rashba SO coupling λ(R), and transitions to a strong TI when λ(R)>√[U(2)+t(⊥)(2)], where U and t(⊥) are, respectively, the interlayer potential and tunneling energy. Different from a conventional quantum spin Hall state, the edge modes of our strong TI phase exhibit both spin and valley filtering, and thus share the properties of both quantum spin Hall and quantum valley Hall insulators. The strong TI phase remains robust in the presence of weak graphene intrinsic SO coupling.

  8. Theoretical study of electronic transport properties of a graphene-silicene bilayer

    SciTech Connect

    Berdiyorov, G. R.; Bahlouli, H.; Peeters, F. M.

    2015-06-14

    Electronic transport properties of a graphene-silicene bilayer system are studied using density-functional theory in combination with the nonequilibrium Green's function formalism. Depending on the energy of the electrons, the transmission can be larger in this system as compared to the sum of the transmissions of separated graphene and silicene monolayers. This effect is related to the increased electron density of states in the bilayer sample. At some energies, the electronic states become localized in one of the layers, resulting in the suppression of the electron transmission. The effect of an applied voltage on the transmission becomes more pronounced in the layered sample as compared to graphene due to the larger variation of the electrostatic potential profile. Our findings will be useful when creating hybrid nanoscale devices where enhanced transport properties will be desirable.

  9. Abrupt current switching in graphene bilayer tunnel transistors enabled by van Hove singularities

    PubMed Central

    Alymov, Georgy; Vyurkov, Vladimir; Ryzhii, Victor; Svintsov, Dmitry

    2016-01-01

    In a continuous search for the energy-efficient electronic switches, a great attention is focused on tunnel field-effect transistors (TFETs) demonstrating an abrupt dependence of the source-drain current on the gate voltage. Among all TFETs, those based on one-dimensional (1D) semiconductors exhibit the steepest current switching due to the singular density of states near the band edges, though the current in 1D structures is pretty low. In this paper, we propose a TFET based on 2D graphene bilayer which demonstrates a record steep subthreshold slope enabled by van Hove singularities in the density of states near the edges of conduction and valence bands. Our simulations show the accessibility of 3.5 × 104 ON/OFF current ratio with 150 mV gate voltage swing, and a maximum subthreshold slope of (20 μV/dec)−1 just above the threshold. The high ON-state current of 0.8 mA/μm is enabled by a narrow (~0.3 eV) extrinsic band gap, while the smallness of the leakage current is due to an all-electrical doping of the source and drain contacts which suppresses the band tailing and trap-assisted tunneling. PMID:27098051

  10. Electromagnetic absorption and Kerr effect in quantum Hall ferromagnetic states of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Côté, R.; Barrette, Manuel; Bouffard, Élie

    2015-09-01

    In a quantizing magnetic field, the chiral two-dimensional electron gas in Landau level N =0 of bilayer graphene goes through a series of phase transitions at integer filling factors ν ∈[-3 ,3 ] when the strength of an electric field applied perpendicularly to the layers is increased. At filling factor ν =3 , the electron gas can be described by a simple two-level system where layer and spin degrees of freedom are frozen. The gas then behaves as an orbital quantum Hall ferromagnet. A Coulomb-induced Dzyaloshinskii-Moriya term in the orbital pseudospin Hamiltonian is responsible for a series of transitions first to a Wigner crystal state and then to a spiral state as the electric field is increased. Both states have a nontrivial orbital pseudospin texture. In this work, we study how the phase diagram at ν =3 is modified by an electric field applied in the plane of the layers and then derive several experimental signatures of the uniform and nonuniform states in the phase diagram. In addition to the transport gap, we study the electromagnetic absorption and the Kerr rotation due to the excitations of the orbital pseudospin-wave modes in the broken-symmetry states.

  11. Abrupt current switching in graphene bilayer tunnel transistors enabled by van Hove singularities

    NASA Astrophysics Data System (ADS)

    Alymov, Georgy; Vyurkov, Vladimir; Ryzhii, Victor; Svintsov, Dmitry

    2016-04-01

    In a continuous search for the energy-efficient electronic switches, a great attention is focused on tunnel field-effect transistors (TFETs) demonstrating an abrupt dependence of the source-drain current on the gate voltage. Among all TFETs, those based on one-dimensional (1D) semiconductors exhibit the steepest current switching due to the singular density of states near the band edges, though the current in 1D structures is pretty low. In this paper, we propose a TFET based on 2D graphene bilayer which demonstrates a record steep subthreshold slope enabled by van Hove singularities in the density of states near the edges of conduction and valence bands. Our simulations show the accessibility of 3.5 × 104 ON/OFF current ratio with 150 mV gate voltage swing, and a maximum subthreshold slope of (20 μV/dec)‑1 just above the threshold. The high ON-state current of 0.8 mA/μm is enabled by a narrow (~0.3 eV) extrinsic band gap, while the smallness of the leakage current is due to an all-electrical doping of the source and drain contacts which suppresses the band tailing and trap-assisted tunneling.

  12. Abrupt current switching in graphene bilayer tunnel transistors enabled by van Hove singularities.

    PubMed

    Alymov, Georgy; Vyurkov, Vladimir; Ryzhii, Victor; Svintsov, Dmitry

    2016-01-01

    In a continuous search for the energy-efficient electronic switches, a great attention is focused on tunnel field-effect transistors (TFETs) demonstrating an abrupt dependence of the source-drain current on the gate voltage. Among all TFETs, those based on one-dimensional (1D) semiconductors exhibit the steepest current switching due to the singular density of states near the band edges, though the current in 1D structures is pretty low. In this paper, we propose a TFET based on 2D graphene bilayer which demonstrates a record steep subthreshold slope enabled by van Hove singularities in the density of states near the edges of conduction and valence bands. Our simulations show the accessibility of 3.5 × 10(4) ON/OFF current ratio with 150 mV gate voltage swing, and a maximum subthreshold slope of (20 μV/dec)(-1) just above the threshold. The high ON-state current of 0.8 mA/μm is enabled by a narrow (~0.3 eV) extrinsic band gap, while the smallness of the leakage current is due to an all-electrical doping of the source and drain contacts which suppresses the band tailing and trap-assisted tunneling. PMID:27098051

  13. Chemically enhanced double-gate bilayer graphene field-effect transistor with neutral channel for logic applications.

    PubMed

    Nourbakhsh, Amirhasan; Agarwal, Tarun K; Klekachev, Alexander; Asselberghs, Inge; Cantoro, Mirco; Huyghebaert, Cedric; Heyns, Marc; Verhelst, Marian; Thean, Aaron; De Gendt, Stefan

    2014-08-29

    In this article, we present the simulation, fabrication, and characterization of a novel bilayer graphene field-effect transistor exhibiting electron mobility up to ~1600 cm(2) V(-1) s(-1), a room temperature I on/I off ≈ 60, and the lowest total charge (~10(11) cm(-2)) reported to date. This is achieved by combined electrostatic and chemical doping of bilayer graphene, which enables one to switch off the device at zero top-gate voltage. Using density functional theory and atomistic simulations, we obtain physical insight into the impact of chemical and electrostatic doping on bandgap opening of bilayer graphene and the effect of metal contacts on the operation of the device. Our results represent a step forward in the use of bilayer graphene for high-performance logic devices in the beyond-complementary metal-oxide-semiconductor (CMOS) technology paradigm. PMID:25101635

  14. Chemically enhanced double-gate bilayer graphene field-effect transistor with neutral channel for logic applications

    NASA Astrophysics Data System (ADS)

    Nourbakhsh, Amirhasan; Agarwal, Tarun K.; Klekachev, Alexander; Asselberghs, Inge; Cantoro, Mirco; Huyghebaert, Cedric; Heyns, Marc; Verhelst, Marian; Thean, Aaron; De Gendt, Stefan

    2014-08-01

    In this article, we present the simulation, fabrication, and characterization of a novel bilayer graphene field-effect transistor exhibiting electron mobility up to ˜1600 cm2 V-1 s-1, a room temperature I on/I off ≈ 60, and the lowest total charge (˜1011 cm-2) reported to date. This is achieved by combined electrostatic and chemical doping of bilayer graphene, which enables one to switch off the device at zero top-gate voltage. Using density functional theory and atomistic simulations, we obtain physical insight into the impact of chemical and electrostatic doping on bandgap opening of bilayer graphene and the effect of metal contacts on the operation of the device. Our results represent a step forward in the use of bilayer graphene for high-performance logic devices in the beyond-complementary metal-oxide-semiconductor (CMOS) technology paradigm.

  15. Li Storage Properties of Disordered Single- and Bi-Layer Graphene

    NASA Astrophysics Data System (ADS)

    Yildirim, H.; Kinaci, Alper; Zhao, Zhi-Jian; Chan, Maria; Greeley, Jeffrey P.

    2015-03-01

    Due to the limited capacity of the traditional intercalation-type graphite materials (373 mAh/g, LiC6) , much effort has been made to explore new anode materials to meet the increasing demand for batteries of high energy density. Among them, graphene has much attracted attention as an ideal platform for higher Li storage capacity, and for obtaining fundamental understanding of Li-C interaction. In this respect, we performed extensive first-principles calculations to model Li adsorption and intercalation in single- and bi-layer graphene, which are activated by defects for Li adsorption. For a wide range of Li coverages, the calculations predict that defect-free single layer graphene is not thermodynamically favorable compared to bulk metallic Li. However, graphene activated by defects are generally found to bind Li more strongly, and the interaction strength is sensitive to both the nature of defects and their densities. A rigorous thermodynamic analysis establishes the theoretical Li storage capacities of the defected graphene, and in some cases, these capacities are found to approach, although not exceed, those of bulk graphite. We will provide a performance comparison between defected single- and bi-layer graphene and bulk-graphite for Li storage capacities. A detailed analysis of the effect of the van der Walls (vdW) interactions will also be presented.

  16. Strain-induced band-gap engineering of graphene monoxide and its effect on graphene

    NASA Astrophysics Data System (ADS)

    Pu, H. H.; Rhim, S. H.; Hirschmugl, C. J.; Gajdardziska-Josifovska, M.; Weinert, M.; Chen, J. H.

    2013-02-01

    Using first-principles calculations we demonstrate the feasibility of band-gap engineering in two-dimensional crystalline graphene monoxide (GMO), a recently reported graphene-based material with a 1:1 carbon/oxygen ratio. The band gap of GMO, which can be switched between direct and indirect, is tunable over a large range (0-1.35 eV) for accessible strains. Electron and hole transport occurs predominantly along the zigzag and armchair directions (armchair for both) when GMO is a direct- (indirect-) gap semiconductor. A band gap of ˜0.5 eV is also induced in graphene at the K' points for GMO/graphene hybrid systems.

  17. Ca intercalated bilayer graphene as a thinnest limit of superconducting C6Ca.

    PubMed

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

    2012-11-27

    Success in isolating a 2D graphene sheet from bulky graphite has triggered intensive studies of its physical properties as well as its application in devices. Graphite intercalation compounds (GICs) have provided a platform of exotic quantum phenomena such as superconductivity, but it is unclear whether such intercalation is feasible in the thinnest 2D limit (i.e., bilayer graphene). Here we report a unique experimental realization of 2D GIC, by fabricating calcium-intercalated bilayer graphene C(6)CaC(6) on silicon carbide. We have investigated the structure and electronic states by scanning tunneling microscopy and angle-resolved photoemission spectroscopy. We observed a free-electron-like interlayer band at the Brillouin-zone center, which is thought to be responsible for the superconductivity in 3D GICs, in addition to a large π* Fermi surface at the zone boundary. The present success in fabricating Ca-intercalated bilayer graphene would open a promising route to search for other 2D superconductors as well as to explore its application in devices.

  18. Switchable magnetic moment in cobalt-doped graphene bilayer on Cu(111): An ab initio study

    NASA Astrophysics Data System (ADS)

    Souza, Everson S.; Scopel, Wanderlã L.; Miwa, R. H.

    2016-06-01

    In this work, we have performed an ab initio theoretical investigation of substitutional cobalt atoms in the graphene bilayer supported on the Cu(111) surface (Co/GBL/Cu). Initially, we examined the separated systems, namely, graphene bilayer adsorbed on Cu(111) (GBL/Cu) and a free standing Co-doped GBL (Co/GBL). In the former system, the GBL becomes n -type doped, where we map the net electronic charge density distribution along the GBL-Cu(111) interface. The substitutional Co atom in Co/GBL lies between the graphene layers, and present a net magnetic moment mostly due to the unpaired Co-3 dz2 electrons. In Co/GBL/Cu, we found that the Cu(111) substrate rules (i) the energetic stability, and (ii) the magnetic properties of substitutional Co atoms in the graphene bilayer. In (i), the substitutional Co atom becomes energetically more stable lying on the GBL surface, and in (ii), the magnetic moment of Co/GBL has been quenched due to the Cu(111) → Co/GBL electronic charge transfer. We verify that such a charge transfer can be tuned upon the application of an external electric field, and thus mediated by a suitable change on the electronic occupation of the Co-dz2 orbitals, we found a way to switch-on and -off the magnetization of the Co-doped GBL adsorbed on the Cu(111) surface.

  19. Stacking order dependent mechanical properties of graphene/MoS{sub 2} bilayer and trilayer heterostructures

    SciTech Connect

    Elder, Robert M. E-mail: mahesh.neupane.ctr@mail.mil; Neupane, Mahesh R. E-mail: mahesh.neupane.ctr@mail.mil; Chantawansri, Tanya L.

    2015-08-17

    Transition metal dichalcogenides (TMDC) such as molybdenum disulfide (MoS{sub 2}) are two-dimensional materials that show promise for flexible electronics and piezoelectric applications, but their weak mechanical strength is a barrier to practical use. In this work, we perform nanoindentation simulations using atomistic molecular dynamics to study the mechanical properties of heterostructures formed by combining MoS{sub 2} with graphene. We consider both bi- and tri-layer heterostructures formed with MoS{sub 2} either supported or encapsulated by graphene. Mechanical properties, such as Young's modulus, bending modulus, ultimate tensile strength, and fracture strain, are extracted from nanoindentation simulations and compared to the monolayer and homogeneous bilayer systems. We observed that the heterostructures, regardless of the stacking order, are mechanically more robust than the mono- and bi-layer MoS{sub 2}, mainly due to the mechanical reinforcement provided by the graphene layer. The magnitudes of ultimate strength and fracture strain are similar for both the bi- and tri-layer heterostructures, but substantially larger than either the mono- and bi-layer MoS{sub 2}. Our results demonstrate the potential of graphene-based heterostructures to improve the mechanical properties of TMDC materials.

  20. Effect of radical fluorination on mono- and bi-layer graphene in Ar/F{sub 2} plasma

    SciTech Connect

    Tahara, K.; Iwasaki, T.; Hatano, M.; Matsutani, A.

    2012-10-15

    Fluorinated graphene has the possibility to achieve unique properties and functions in graphene. We propose a highly controlled fluorination method utilizing fluorine radicals in Ar/F{sub 2} plasma. To suppress ion bombardments and improve the reaction with fluorine radicals on graphene, the substrate was placed 'face down' in the plasma chamber. Although monolayer graphene was more reactive than bilayer, fluorination of bilayer reached the level of I{sub D}/I{sub G} {approx} 0.5 in Raman D peak intensity at 532 nm excitation. Annealing fluorinated samples proved reversibility of radical fluorination for both mono- and bi-layer graphenes. X-ray photoelectron spectroscopy showed the existence of carbon-fluorine bonding.

  1. Atomic structure of epitaxial graphene sidewall nanoribbons: flat graphene, miniribbons, and the confinement gap.

    PubMed

    Palacio, Irene; Celis, Arlensiú; Nair, Maya N; Gloter, Alexandre; Zobelli, Alberto; Sicot, Muriel; Malterre, Daniel; Nevius, Meredith S; de Heer, Walt A; Berger, Claire; Conrad, Edward H; Taleb-Ibrahimi, Amina; Tejeda, Antonio

    2015-01-14

    Graphene nanoribbons grown on sidewall facets of SiC have demonstrated exceptional quantized ballistic transport up to 15 μm at room temperature. Angular-resolved photoemission spectroscopy (ARPES) has shown that the ribbons have the band structure of charge neutral graphene, while bent regions of the ribbon develop a bandgap. We present scanning tunneling microscopy and transmission electron microscopy of armchair nanoribbons grown on recrystallized sidewall trenches etched in SiC. We show that the nanoribbons consist of a single graphene layer essentially decoupled from the facet surface. The nanoribbons are bordered by 1-2 nm wide bent miniribbons at both the top and bottom edges of the nanoribbons. We establish that nanoscale confinement in the graphene miniribbons is the origin of the local large band gap observed in ARPES. The structural results presented here show how this gap is formed and provide a framework to help understand ballistic transport in sidewall graphene.

  2. A theoretical study of symmetry-breaking organic overlayers on single- and bi-layer graphene

    NASA Astrophysics Data System (ADS)

    Morales-Cifuentes, Josue; Einstein, T. L.

    2013-03-01

    An ``overlayer'' of molecules that breaks the AB symmetry of graphene can produce (modify) a band gap in single- (bi-) layer graphene.[2] Since the triangular shaped trimesic acid (TMA) molecule forms two familiar symmetry breaking configurations, we are motivated to model TMA physisorption on graphene surfaces in conjunction with experiments by Groce et al. at UMD. Using VASP, with ab initio van der Waals density functionals (vdW-DF), we simulate adsorption of TMA onto a graphene surface in several symmetry-breaking arrangements in order to predict/understand the effect of TMA adsorption on experimental observables. Supported by NSF-MRSEC Grant DMR 05-20471.

  3. Enhanced current-rectification in bilayer graphene with an electrically tuned sloped bandgap.

    PubMed

    Aparecido-Ferreira, Alex; Miyazaki, Hisao; Li, Song-Lin; Komatsu, Katsuyoshi; Nakaharai, Shu; Tsukagoshi, Kazuhito

    2012-12-21

    We propose a novel sloped dielectric geometry in graphene as a band engineering method for widening the depletion region and increasing the electrical rectification effect in graphene pn junctions. Enhanced current-rectification was achieved in a bilayer graphene with a sloped dielectric top gate and a normal back gate. A bias was applied to the top gate to induce a spatially modulated and sloped band configuration, while a back-gate bias was applied to open a bandgap. The sloped band can be tuned to separate n- and p-type regions in the bilayer graphene, depending on a suitable choice of gate voltage. The effective depletion region between the n- and p-type regions can be spatially enlarged due to the proposed top-gate structure. As a result, a strong non-linear electric current was observed during drain bias sweeping, demonstrating the expected rectification behavior with an on/off ratio higher than all previously reported values for graphene pn junctions. The observed rectification was modified to a linear current-voltage relationship by adjusting the biases of both gates to form an nn- or pp-type junction configuration. These results demonstrate that an external voltage can control the current flow in atomic film diodes.

  4. 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).

  5. Strain-modulated excitonic gaps in mono- and bi-layer MoSe2

    NASA Astrophysics Data System (ADS)

    Jianting, Ji; Anmin, Zhang; Tianlong, Xia; Po, Gao; Yinghao, Jie; Qian, Zhang; Qingming, Zhang

    2016-07-01

    Photoluminescence (PL) and Raman spectra under uniaxial strain were measured in mono- and bi-layer MoSe2 to comparatively investigate the evolution of excitonic gaps and Raman phonons with strain. We observed that the strain dependence of excitonic gaps shows a nearly linear behavior in both flakes. One percent of strain increase gives a reduction of ∼ 42 meV (∼ 35 meV) in A-exciton gap in monolayer (bilayer) MoSe2. The PL width remains little changed in monolayer MoSe2 while it increases rapidly with strain in the bilayer case. We have made detailed discussions on the observed strain-modulated results and compared the difference between monolayer and bilayer cases. The hybridization between 4d orbits of Mo and 4p orbits of Se, which is controlled by the Se–Mo–Se bond angle under strain, can be employed to consistently explain the observations. The study may shed light into exciton physics in few-layer MoSe2 and provides a basis for their applications. Project supported by the National Basic Research Program of China (Grant No. 2012CB921701) and the National Natural Science Foundation of China (Grant Nos. 11474357 and 11004245). Qingming Zhang and Tianlong Xia were supported by the Fundamental Research Funds for the Central Universities of China and the Research Funds of Renmin University of China.

  6. Electronic properties of graphene nano-flakes: Energy gap, permanent dipole, termination effect, and Raman spectroscopy

    SciTech Connect

    Singh, Sandeep Kumar Peeters, F. M.; Neek-Amal, M.

    2014-02-21

    The electronic properties of graphene nano-flakes (GNFs) with different edge passivation are investigated by using density functional theory. Passivation with F and H atoms is considered: C{sub N{sub c}} X{sub N{sub x}} (X = F or H). We studied GNFs with 10 < N{sub c} < 56 and limit ourselves to the lowest energy configurations. We found that: (i) the energy difference Δ between the highest occupied molecular orbital and the lowest unoccupied molecular orbital decreases with N{sub c}, (ii) topological defects (pentagon and heptagon) break the symmetry of the GNFs and enhance the electric polarization, (iii) the mutual interaction of bilayer GNFs can be understood by dipole-dipole interaction which were found sensitive to the relative orientation of the GNFs, (iv) the permanent dipoles depend on the edge terminated atom, while the energy gap is independent of it, and (v) the presence of heptagon and pentagon defects in the GNFs results in the largest difference between the energy of the spin-up and spin-down electrons which is larger for the H-passivated GNFs as compared to F-passivated GNFs. Our study shows clearly the effect of geometry, size, termination, and bilayer on the electronic properties of small GNFs. This study reveals important features of graphene nano-flakes which can be detected using Raman spectroscopy.

  7. Perfect Spin-filtering in graphene monolayer-bilayer superlattice with zigzag boundaries

    PubMed Central

    Yu, Hang; Liu, Jun-Feng

    2016-01-01

    We show that the spontaneous magnetization is formed at the zigzag boundary between monolayer and bilayer graphene by the self-consistent calculation based on Hubbard model. In a monolayer- bilayer graphene superlattice with zigzag boundaries, it is surprising that nearly 100% spin polarization is achieved in the energy window around the Dirac point, no matter the magnetization configuration at two boundaries is parallel or antiparallel. The reason is that the low-energy transport is only influenced by the magnetization at one edge, but not by that at the other. The underlying physics is unveiled by the spin-split band structure and the distribution of the wave-function pertaining to the lowest (highest) subband of electron (hole). PMID:27140666

  8. Perfect Spin-filtering in graphene monolayer-bilayer superlattice with zigzag boundaries

    NASA Astrophysics Data System (ADS)

    Yu, Hang; Liu, Jun-Feng

    2016-05-01

    We show that the spontaneous magnetization is formed at the zigzag boundary between monolayer and bilayer graphene by the self-consistent calculation based on Hubbard model. In a monolayer- bilayer graphene superlattice with zigzag boundaries, it is surprising that nearly 100% spin polarization is achieved in the energy window around the Dirac point, no matter the magnetization configuration at two boundaries is parallel or antiparallel. The reason is that the low-energy transport is only influenced by the magnetization at one edge, but not by that at the other. The underlying physics is unveiled by the spin-split band structure and the distribution of the wave-function pertaining to the lowest (highest) subband of electron (hole).

  9. High temperature characteristics of bilayer epitaxial graphene field-effect transistors on SiC Substrate

    NASA Astrophysics Data System (ADS)

    Ze-Zhao, He; Ke-Wu, Yang; Cui, Yu; Qing-Bin, Liu; Jing-Jing, Wang; Jia, Li; Wei-Li, Lu; Zhi-Hong, Feng; Shu-Jun, Cai

    2016-06-01

    In this paper, high temperature direct current (DC) performance of bilayer epitaxial graphene device on SiC substrate is studied in a temperature range from 25 °C to 200 °C. At a gate voltage of -8 V (far from Dirac point), the drain-source current decreases obviously with increasing temperature, but it has little change at a gate bias of +8 V (near Dirac point). The competing interactions between scattering and thermal activation are responsible for the different reduction tendencies. Four different kinds of scatterings are taken into account to qualitatively analyze the carrier mobility under different temperatures. The devices exhibit almost unchanged DC performances after high temperature measurements at 200 °C for 5 hours in air ambience, demonstrating the high thermal stabilities of the bilayer epitaxial graphene devices. Project supported by the National Natural Science Foundation of China (Grant No. 61306006).

  10. Conductivity and scattering in graphene bilayers: Numerically exact results versus Boltzmann approach

    NASA Astrophysics Data System (ADS)

    Xu, Hengyi; Heinzel, T.; Zozoulenko, I. V.

    2011-09-01

    We derive analytical expressions for the conductivity of bilayer graphene (BLG) using the Boltzmann approach within the the Born approximation for a model of Gaussian disorders describing both short- and long-range impurity scattering. The range of validity of the Born approximation is established by comparing the analytical results to exact tight-binding numerical calculations. A comparison of the obtained density dependencies of the conductivity with experimental data shows that the BLG samples investigated experimentally so far are in the quantum scattering regime where the Fermi wavelength exceeds the effective impurity range. In this regime both short- and long-range scattering lead to the same linear density dependence of the conductivity. Our calculations imply that bilayer and single-layer graphene have the same scattering mechanisms. We also provide an upper limit for the effective, density-dependent spatial extension of the scatterers present in the experiments.

  11. Raman spectroscopy measurement of bilayer graphene's twist angle to boron nitride

    SciTech Connect

    Cheng, Bin; Wang, Peng; Pan, Cheng; Miao, Tengfei; Wu, Yong; Lau, C. N.; Bockrath, M.; Taniguchi, T.; Watanabe, K.

    2015-07-20

    When graphene is placed on hexagonal boron nitride with a twist angle, new properties develop due to the resulting moiré superlattice. Here, we report a method using Raman spectroscopy to make rapid, non-destructive measurements of the twist angle between bilayer graphene and hexagonal boron nitride. The lattice orientation is determined by using flakes with both bilayer and monolayer regions, and using the known Raman signature for the monolayer to measure the twist angle of the entire flake. The widths of the second order Raman peaks are found to vary linearly in the superlattice period and are used to determine the twist angle. The results are confirmed by using transport measurements to infer the superlattice period by the charge density required to reach the secondary resistance peaks. Small twist angles are also found to produce a significant modification of the first order Raman G band peak.

  12. The effects of impurity doping on the optical properties of biased bilayer graphene

    NASA Astrophysics Data System (ADS)

    Rezania, Hamed; Yarmohammadi, Mohsen

    2016-07-01

    We address the optical conductivity of doped AA-stacked bilayer graphene in the presence of a finite bias voltage at finite temperature. The effect of scattering by dilute charged impurities is discussed in terms of the self-consistent Born approximation. Green's function approach has been implemented to find the behavior of optical conductivity of bilayer graphene within linear response theory. We have found the frequency dependence of optical conductivity for different values of concentration and scattering strength of dopant impurity. Also the dependence of optical conductivity on the impurity concentration and bias voltage has been investigated in details. A peak appears in the plot of optical conductivity versus impurity concentration for different values of chemical potential. Furthermore we find optical conductivity reduces with frequency for any impurity concentration and scattering strength.

  13. Graphene field effect transistor without an energy gap.

    PubMed

    Jang, Min Seok; Kim, Hyungjun; Son, Young-Woo; Atwater, Harry A; Goddard, William A

    2013-05-28

    Graphene is a room temperature ballistic electron conductor and also a very good thermal conductor. Thus, it has been regarded as an ideal material for postsilicon electronic applications. A major complication is that the relativistic massless electrons in pristine graphene exhibit unimpeded Klein tunneling penetration through gate potential barriers. Thus, previous efforts to realize a field effect transistor for logic applications have assumed that introduction of a band gap in graphene is a prerequisite. Unfortunately, extrinsic treatments designed to open a band gap seriously degrade device quality, yielding very low mobility and uncontrolled on/off current ratios. To solve this dilemma, we propose a gating mechanism that leads to a hundredfold enhancement in on/off transmittance ratio for normally incident electrons without any band gap engineering. Thus, our saw-shaped geometry gate potential (in place of the conventional bar-shaped geometry) leads to switching to an off state while retaining the ultrahigh electron mobility in the on state. In particular, we report that an on/off transmittance ratio of 130 is achievable for a sawtooth gate with a gate length of 80 nm. Our switching mechanism demonstrates that intrinsic graphene can be used in designing logic devices without serious alteration of the conventional field effect transistor architecture. This suggests a new variable for the optimization of the graphene-based device--geometry of the gate electrode.

  14. The role of roughness-induced damping in the oscillatory motion of bilayer graphene.

    PubMed

    Ye, Zhijiang; Otero-de-la-Roza, Alberto; Johnson, Erin R; Martini, Ashlie

    2014-10-24

    A multi-scale theoretical model is presented that is the first to offer quantitative agreement with experimental measurements of self-retraction and oscillation of bilayer graphene. The model integrates density-functional theory calculations of the energetics driving flake retraction and molecular-dynamics simulations capturing the dynamic response of laterally-offset rough surfaces. We demonstrate that nanoscale roughness explains self-retraction motion and propose a recipe for tuning that motion by controlling friction. PMID:25274535

  15. Effect of impurity doping on tunneling conductance in AB-stacked bi-layer graphene: A tight-binding study

    NASA Astrophysics Data System (ADS)

    Rout, G. C.; Sahu, Sivabrata; Panda, S. K.

    2016-04-01

    We report here a microscopic tight-binding model calculation for AB-stacked bilayer graphene in presence of biasing potential between the two layers and the impurity effects to study the evolution of the total density of states with special emphasis on opening of band gap near Dirac point. We have calculated the electron Green's functions for both the A and B sub-lattices by Zubarev technique. The imaginary part of the Green's function gives the partial and total density of states of electrons. The density of states are computed numerically for 1000 × 1000 grid points of the electron momentum. The evolution of the opening of band gap near van-Hove singularities as well as near Dirac point is investigated by varying the different interlayer hoppings and the biasing potentials. The inter layer hopping splits the density of states at van-Hove singularities and produces a V-shaped gap near Dirac point. Further the biasing potential introduces a U shaped gap near Dirac point with a density minimum at the applied potential(i.e. at V/2).

  16. AB-stacked square-like bilayer ice in graphene nanocapillaries.

    PubMed

    Zhu, YinBo; Wang, FengChao; Bai, Jaeil; Zeng, Xiao Cheng; Wu, HengAn

    2016-08-10

    Water, when constrained between two graphene sheets and under ultrahigh pressure, can manifest dramatic differences from its bulk counterparts such as the van der Waals pressure induced water-to-ice transformation, known as the metastability limit of two-dimensional (2D) liquid. Here, we present result of a new crystalline structure of bilayer ice with the AB-stacking order, observed from molecular dynamics simulations of constrained water. This AB-stacked bilayer ice (BL-ABI) is transformed from the puckered monolayer square-like ice (pMSI) under higher lateral pressure in the graphene nanocapillary at ambient temperature. BL-ABI is a proton-ordered ice with square-like pattern. The transition from pMSI to BL-ABI is through crystal-to-amorphous-to-crystal pathway with notable hysteresis-loop in the potential energy during the compression/decompression process, reflecting the compression/tensile limit of the 2D monolayer/bilayer ice. In a superheating process, the BL-ABI transforms into the AB-stacked bilayer amorphous ice with the square-like pattern. PMID:27468430

  17. Electron-hole asymmetric fractional quantum Hall effect in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Feldman, Ben; Kou, Angela; Levin, Andrei; Halperin, Bertrand; Watanabe, Kenji; Taniguchi, Takashi; Yacoby, Amir

    2014-03-01

    At zero magnetic field, the electronic spectrum of bilayer graphene is electron-hole symmetric to first order. In a magnetic field, the lowest two orbital states occur at zero energy, and they combine with the spin and valley degrees of freedom to yield an eightfold degenerate lowest Landau level. Both external fields and electron-electron interactions can break these symmetries, leading to a uniquely rich and tunable phase diagram of many-body states. In this talk, I will present local electronic compressibility measurements of high quality bilayer graphene performed using a scanning single-electron transistor. We observe clear fractional quantum Hall states at filling factors ν = -10/3, -4/3, 2/3 and 8/3, with additional states appearing at ν = -17/5, -7/5, 3/5 and 13/5. Remarkably, this sequence breaks electron-hole symmetry and instead follows an even-odd pattern between integer quantum Hall states. Our results highlight the key role played by the orbital degree of freedom in the many-body physics of bilayer graphene.

  18. Observation of even denominator fractional quantum Hall effect in suspended bilayer graphene.

    PubMed

    Ki, Dong-Keun; Fal'ko, Vladimir I; Abanin, Dmitry A; Morpurgo, Alberto F

    2014-01-01

    We investigate low-temperature magneto-transport in recently developed, high-quality multiterminal suspended bilayer graphene devices, enabling the independent measurement of the longitudinal and transverse resistance. We observe clear signatures of the fractional quantum Hall effect with different states that are either fully developed, and exhibit a clear plateau in the transverse resistance with a concomitant dip in longitudinal resistance or incipient, and exhibit only a longitudinal resistance minimum. All observed states scale as a function of filling factor ν, as expected. An unprecedented even-denominator fractional state is observed at ν = -1/2 on the hole side, exhibiting a clear plateau in Rxy quantized at the expected value of 2h/e(2) with a precision of ∼0.5%. Many of our observations, together with a recent electronic compressibility measurement performed in graphene bilayers on hexagonal boron-nitride (hBN) substrates, are consistent with a recent theory that accounts for the effect of the degeneracy between the N = 0 and N = 1 Landau levels in the fractional quantum Hall effect and predicts the occurrence of a Moore-Read type ν = -1/2 state. Owing to the experimental flexibility of bilayer graphene, which has a gate-dependent band structure, can be easily accessed by scanning probes, and can be contacted with materials such as superconductors, our findings offer new possibilities to explore the microscopic nature of even-denominator fractional quantum Hall effect.

  19. Giant Faraday rotation induced by the Berry phase in bilayer graphene under strong terahertz fields

    NASA Astrophysics Data System (ADS)

    Yang, Fan; Xu, Xiaodong; Liu, Ren-Bao

    2014-04-01

    High-order terahertz (THz) sideband generation in semiconductors is a phenomenon with physics similar to that of high-order harmonic generation but in a regime of much lower frequency. Our previous paper [1] found that the electron-hole pair excited by a weak optical laser can accumulate a Berry phase along a cyclic trajectory under the driving of a strong elliptically polarized THz field. Furthermore, the Berry phase appears as the Faraday rotation angle of the emission signal under short-pulse excitation in monolayer MoS_{2}. In this paper, the theory of the Berry phase in THz extreme nonlinear optics is applied to biased bilayer graphene with Bernal stacking, which has similar Bloch band features and optical properties to monolayer MoS_{2}, such as the time-reversal related valleys and the valley contrasting optical selection rule. However, the biased bilayer graphene has much larger Berry curvature than monolayer MoS_{2}, which leads to a large Berry phase of the quantum trajectory and in turn a giant Faraday rotation of the optical emission (˜1 rad for a THz field with frequency 1 THz and strength 8 kV cm-1). This surprisingly big angle shows that the Faraday rotation can be induced more efficiently by the Berry curvature in momentum space than by the magnetic field in real space. It provides opportunities to use bilayer graphene and THz lasers for ultrafast electro-optical devices.

  20. Copper-Vapor-Assisted Rapid Synthesis of Large AB-Stacked Bilayer Graphene Domains on Cu-Ni Alloy.

    PubMed

    Yang, Chao; Wu, Tianru; Wang, Haomin; Zhang, Guanhua; Sun, Julong; Lu, Guangyuan; Niu, Tianchao; Li, Ang; Xie, Xiaoming; Jiang, Mianheng

    2016-04-01

    The synergic effects of Cu85Ni15 and the copper vapor evaporated from copper foil enabled the fast growth of a ≈300 μm bilayer graphene in ≈10 minutes. The copper vapor reduces the growth rate of the first graphene layer while the carbon dissolved in the alloy boosts the growth of the subsequently developed second graphene layer with an AB-stacking order. PMID:26915342

  1. Raman mapping investigation of chemical vapor deposition-fabricated twisted bilayer graphene with irregular grains.

    PubMed

    Chen, Yuming; Meng, Lijuan; Zhao, Weiwei; Liang, Zheng; Wu, Xing; Nan, Haiyan; Wu, Zhangting; Huang, Shan; Sun, Litao; Wang, Jinlan; Ni, Zhenhua

    2014-10-21

    Bilayer graphene as a prototype of two-dimensional stacked material has recently attracted great attention. The twist angle between graphene layers adds another dimension to control its properties. In this study, we used Raman mapping to investigate the twist angle dependence of properties of twisted bilayer graphene (TBG) with irregular grains that was fabricated by chemical vapor deposition (CVD). Different Raman parameters including intensity, width, and position of G and 2D peaks were used to distinguish TBG with different twist angles. The statistical results from Raman imaging on the distribution of twist angle are consistent with the results from selected area election diffraction (SAED). Finally, the Raman peak at approximately 1347 cm(-1) for TBG with a large twist angle was assigned to the D-like peak, although it has similar excitation energy dependence of frequency as the defect-induced D peak. Theoretical calculation further confirmed that vacancy-like defect is not favored in the formation energy for TBG with a large twist angle as compared to monolayer graphene or TBG with other twist angles. These results will help to advance the understanding of TBG properties, especially for CVD samples with irregular grains.

  2. Spin dynamics in bilayer graphene: Role of electron-hole puddles and Dyakonov-Perel mechanism

    NASA Astrophysics Data System (ADS)

    Van Tuan, Dinh; Adam, Shaffique; Roche, Stephan

    2016-07-01

    We report on spin transport features which are unique to high quality bilayer graphene, in the absence of magnetic contaminants and strong intervalley mixing. The time-dependent spin polarization of a propagating wave packet is computed using an efficient quantum transport method. In the limit of vanishing effects of substrate and disorder, the energy dependence of the spin lifetime is similar to monolayer graphene with an M -shaped profile and minimum value at the charge neutrality point, but with an electron-hole asymmetry fingerprint. In sharp contrast, the incorporation of substrate-induced electron-hole puddles (characteristics of supported graphene either on SiO2 or hBN ) surprisingly results in a large enhancement of the low-energy spin lifetime and a lowering of its high-energy values. Such a feature, unique to the bilayer, is explained in terms of a reinforced Dyakonov-Perel mechanism at the Dirac point, whereas spin relaxation at higher energies is driven by pure dephasing effects. This suggests further electrostatic control of the spin transport length scales in graphene devices.

  3. Evidence for Interlayer Coupling and Moiré Periodic Potentials in Twisted Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Ohta, Taisuke; Robinson, Jeremy T.; Feibelman, Peter J.; Bostwick, Aaron; Rotenberg, Eli; Beechem, Thomas E.

    2012-11-01

    We report a study of the valence band dispersion of twisted bilayer graphene using angle-resolved photoemission spectroscopy and ab initio calculations. We observe two noninteracting cones near the Dirac crossing energy and the emergence of van Hove singularities where the cones overlap for large twist angles (>5°). Besides the expected interaction between the Dirac cones, minigaps appeared at the Brillouin zone boundaries of the moiré superlattice formed by the misorientation of the two graphene layers. We attribute the emergence of these minigaps to a periodic potential induced by the moiré. These anticrossing features point to coupling between the two graphene sheets, mediated by moiré periodic potentials.

  4. Evidence for interlayer coupling and moiré periodic potentials in twisted bilayer graphene.

    PubMed

    Ohta, Taisuke; Robinson, Jeremy T; Feibelman, Peter J; Bostwick, Aaron; Rotenberg, Eli; Beechem, Thomas E

    2012-11-01

    We report a study of the valence band dispersion of twisted bilayer graphene using angle-resolved photoemission spectroscopy and ab initio calculations. We observe two noninteracting cones near the Dirac crossing energy and the emergence of van Hove singularities where the cones overlap for large twist angles (>5°). Besides the expected interaction between the Dirac cones, minigaps appeared at the Brillouin zone boundaries of the moiré superlattice formed by the misorientation of the two graphene layers. We attribute the emergence of these minigaps to a periodic potential induced by the moiré. These anticrossing features point to coupling between the two graphene sheets, mediated by moiré periodic potentials. PMID:23215315

  5. Electron-phonon coupling in bilayer and single-layer graphene at sub-Kelvin temperatures

    NASA Astrophysics Data System (ADS)

    McKitterick, Chris; Vora, Heli; Du, Xu; Rooks, Michael; Prober, Daniel

    2014-03-01

    Graphene has been proposed by many groups as a detector of terahertz photons1 , 2 , 3, due to its very small heat capacity and predicted low thermal conductance. We present Johnson noise thermometry measurements of single and bilayer graphene samples fabricated at Stony Brook University and at Yale University. These measurements probe the graphene electron-phonon coupling at sub-Kelvin temperatures. The devices are fabricated with superconducting contacts (NbN at Stony Brook, Al and Nb at Yale) to confine the hot electrons in the graphene device, diminishing the contribution of electron out-diffusion in cooling the electron system. By using commercially-available CVD-grown graphene for some samples, we can define large area sections, allowing us to emphasize the thermal conductance due to electron-phonon coupling. These measurements allow for performance estimates for using similar graphene devices to detect terahertz photons. 1C. B. McKitterick, D. E. Prober, B. S. Karasik, Journal of Applied Physics 113, 044512 (2013). 2H. Vora, P. Kumaravadivel, B. Nielsen, X. Du, Applied Physics Letters 100, 153507 (2012). 3K. Fong, K. Schwab, Physical Review X 2, 1 (2012). This work supported by NSF-DMR 0907082.

  6. Enhanced Piezoelectric Energy Harvesting Performance of Flexible PVDF-TrFE Bilayer Films with Graphene Oxide.

    PubMed

    Bhavanasi, Venkateswarlu; Kumar, Vipin; Parida, Kaushik; Wang, Jiangxin; Lee, Pooi See

    2016-01-13

    Ferroelectric materials have attracted interest in recent years due to their application in energy harvesting owing to its piezoelectric nature. Ferroelectric polymers are flexible and can sustain larger strains compared to inorganic counterparts, making them attractive for harvesting energy from mechanical vibrations. Herein, we report, for the first time, the enhanced piezoelectric energy harvesting performance of the bilayer films of poled poly(vinylidene fluoride-trifluoroethylene) [PVDF-TrFE] and graphene oxide (GO). The bilayer film exhibits superior energy harvesting performance with a voltage output of 4 V and power output of 4.41 μWcm(-2) compared to poled PVDF-TrFE films alone (voltage output of 1.9 V and power output of 1.77 μWcm(-2)). The enhanced voltage and power output in the presence of GO film is due to the combined effect of electrostatic contribution from graphene oxide, residual tensile stress, enhanced Young's modulus of the bilayer films, and the presence of space charge at the interface of the PVDF-TrFE and GO films, arising from the uncompensated polarization of PVDF-TrFE. Higher Young's modulus and dielectric constant of GO led to the efficient transfer of mechanical and electrical energy.

  7. Optical properties of Bernal-stacked bilayer graphene: A theoretical study

    NASA Astrophysics Data System (ADS)

    Kadi, Faris; Malic, Ermin

    2014-01-01

    We present a microscopic study of optical properties of Bernal-stacked bilayer graphene. Our focus lies on the absorption spectrum explicitly including the impact of fully momentum-dependent optical and Coulomb matrix elements. Our approach is based on the density matrix formalism combined with tight-binding wave functions. The energy dispersion of bilayer graphene exhibits four parabolic bands resulting in interesting optical features: We find a pronounced low-energy peak, which can be clearly ascribed to cross transitions at the Dirac point. Furthermore, the bilayer spectrum shows two energetically close absorption peaks in the ultraviolet region resulting from interband transitions at the saddle point of the Brillouin zone. We discuss the impact of the carrier-light coupling element containing the optical selection rules and determining the oscillator strength of all possible optical transitions. Furthermore, we show the influence of the Coulomb interaction accounting for a considerable overall energy renormalization and the formation of electron-hole pairs at the saddle point.

  8. Enhanced Piezoelectric Energy Harvesting Performance of Flexible PVDF-TrFE Bilayer Films with Graphene Oxide.

    PubMed

    Bhavanasi, Venkateswarlu; Kumar, Vipin; Parida, Kaushik; Wang, Jiangxin; Lee, Pooi See

    2016-01-13

    Ferroelectric materials have attracted interest in recent years due to their application in energy harvesting owing to its piezoelectric nature. Ferroelectric polymers are flexible and can sustain larger strains compared to inorganic counterparts, making them attractive for harvesting energy from mechanical vibrations. Herein, we report, for the first time, the enhanced piezoelectric energy harvesting performance of the bilayer films of poled poly(vinylidene fluoride-trifluoroethylene) [PVDF-TrFE] and graphene oxide (GO). The bilayer film exhibits superior energy harvesting performance with a voltage output of 4 V and power output of 4.41 μWcm(-2) compared to poled PVDF-TrFE films alone (voltage output of 1.9 V and power output of 1.77 μWcm(-2)). The enhanced voltage and power output in the presence of GO film is due to the combined effect of electrostatic contribution from graphene oxide, residual tensile stress, enhanced Young's modulus of the bilayer films, and the presence of space charge at the interface of the PVDF-TrFE and GO films, arising from the uncompensated polarization of PVDF-TrFE. Higher Young's modulus and dielectric constant of GO led to the efficient transfer of mechanical and electrical energy. PMID:26693844

  9. Theoretical study of polyiodide formation and stability on monolayer and bilayer graphene.

    PubMed

    Tristant, Damien; Puech, Pascal; Gerber, Iann C

    2015-11-28

    The presence of polyiodide complexes have been reported several times when carbon-based materials were doped by iodine molecules, but their formation mechanism remains unclear. By using first-principles calculations that include nonlocal correlation effects by means of a van der Waals density functional approach, we propose that the formation of triiodide (I3(-)) and pentaiodide (I5(-)) is due to a large density of iodine molecules (I2) in interaction with a carbonaceous substrate. As soon as the concentration of surface iodine reaches a threshold value of 12.5% for a graphene monolayer and 6.25% for a bilayer, these complexes spontaneously appear. The corresponding structural and energetic aspects, electronic structures and vibrational frequencies support this statement. An upshift of the Dirac point from the Fermi level with values of 0.45 and 0.52 eV is observed for adsorbed complexes on graphene and intercalated complexes between two layers, respectively. For doped-graphene, it corresponds to a graphene hole density of around 1.1 × 10(13) cm(-2), in quantitative agreement with experiments. Additionally, we have studied the thermal stability at room temperature of these adsorbed ions on graphene by means of ab initio molecular dynamics, which also shows successful p-doping with polyiodide complexes.

  10. Electromechanically generating electricity with a gapped-graphene electric generator

    NASA Astrophysics Data System (ADS)

    Dressen, Donald; Golovchenko, Jene

    2015-03-01

    We demonstrate the fabrication and operation of a gapped-graphene electric generator (G-GEG) device. The G-GEG generates electricity from the mechanical oscillation of droplets of electrolytes and ionic liquids. The spontaneous adsorption of ionic species on graphene charges opposing electric double-layer capacitors (EDLCs) on each half of the device. Modulating the area of contact between the droplet and graphene leads to adsorption/desorption of ions, effectively charging/discharging each EDLC and generating a current. The flow of current supports a potential difference across the G-GEG due to the device's internal impedance. Both the magnitude and polarity of the induced current and voltage show a strong dependence on the type of ionic species used, suggesting that certain ions interact more strongly with graphene than others. We find that a simple model circuit consisting of an AC current source in series with a resistor and a time-varying capacitor accurately predicts the device's dynamic behavior. Additionally, we discuss the effect of graphene's intrinsic quantum capacitance on the G-GEG's performance and speculate on the utility of the device in the context of energy harvesting.

  11. Graphene Supported Graphone/Graphane Bilayer Nanostructure Material for Spintronics

    PubMed Central

    Ray, Sekhar C.; Soin, Navneet; Makgato, Thuto; Chuang, C. H.; Pong, W. F.; Roy, Susanta S.; Ghosh, Sarit K.; Strydom, André M.; McLaughlin, J. A.

    2014-01-01

    We report an investigation into the magnetic and electronic properties of partially hydrogenated vertically aligned few layers graphene (FLG) synthesized by microwave plasma enhanced chemical vapor deposition. The FLG samples are hydrogenated at different substrate temperatures to alter the degree of hydrogenation and their depth profile. The unique morphology of the structure gives rise to a unique geometry in which graphane/graphone is supported by graphene layers in the bulk, which is very different from other widely studied structures such as one-dimensional nanoribbons. Synchrotron based x-ray absorption fine structure spectroscopy measurements have been used to investigate the electronic structure and the underlying hydrogenation mechanism responsible for the magnetic properties. While ferromagnetic interactions seem to be predominant, the presence of antiferromagnetic interaction was also observed. Free spins available via the conversion of sp2 to sp3 hybridized structures, and the possibility of unpaired electrons from defects induced upon hydrogenation are thought to be likely mechanisms for the observed ferromagnetic orders. PMID:24457465

  12. Graphene supported graphone/graphane bilayer nanostructure material for spintronics.

    PubMed

    Ray, Sekhar C; Soin, Navneet; Makgato, Thuto; Chuang, C H; Pong, W F; Roy, Susanta S; Ghosh, Sarit K; Strydom, André M; McLaughlin, J A

    2014-01-24

    We report an investigation into the magnetic and electronic properties of partially hydrogenated vertically aligned few layers graphene (FLG) synthesized by microwave plasma enhanced chemical vapor deposition. The FLG samples are hydrogenated at different substrate temperatures to alter the degree of hydrogenation and their depth profile. The unique morphology of the structure gives rise to a unique geometry in which graphane/graphone is supported by graphene layers in the bulk, which is very different from other widely studied structures such as one-dimensional nanoribbons. Synchrotron based x-ray absorption fine structure spectroscopy measurements have been used to investigate the electronic structure and the underlying hydrogenation mechanism responsible for the magnetic properties. While ferromagnetic interactions seem to be predominant, the presence of antiferromagnetic interaction was also observed. Free spins available via the conversion of sp(2) to sp(3) hybridized structures, and the possibility of unpaired electrons from defects induced upon hydrogenation are thought to be likely mechanisms for the observed ferromagnetic orders.

  13. About the linewidth of cyclotron resonance in band-gap graphene

    NASA Astrophysics Data System (ADS)

    Kryuchkov, S. V.; Kukhar', E. I.

    2015-01-01

    The critical amplitude of circularly polarized electromagnetic wave when the hysteresis of cyclotron absorption takes place, was found for band-gap graphene. The dependence of critical amplitude on the gap value and on the relaxation time was investigated. The conditions of applicability of linear theory describing the electromagnetic response of band-gap graphene in a non-zero magnetic field were found. The power of the circularly polarized electromagnetic radiation absorbed by band-gap graphene in the presence of a magnetic field was calculated. The linewidth of cyclotron absorption was shown to be not zero even for pure band-gap graphene.

  14. Band-gap scaling of graphene nanohole superlattices

    NASA Astrophysics Data System (ADS)

    Liu, Wei; Wang, Z. F.; Shi, Q. W.; Yang, Jinlong; Liu, Feng

    2009-12-01

    Based on the tight-binding model, we investigate band structures of graphene nanohole (GNH) superlattices as a function of NH size and density. One common origin of band gaps for GNH superlattices with NHs of either armchair or zigzag edges is the quantum-confinement effect due to the periodic potential introduced by the NHs, which turns the semimetallic sheet into a direct-gap semiconductor. Additional band gaps also open for GNH superlattices with NHs of zigzag edges in a ferromagnetic ground state, arising from the staggered sublattice potential on the zigzag edges due to edge magnetization. Our calculations reveal a generic scaling relation that both types of band gaps increase linearly with the product of NH size and density.

  15. First-principles study of carrier-induced ferromagnetism in bilayer and multilayer zigzag graphene nanoribbons

    SciTech Connect

    Sawada, Keisuke; Ishii, Fumiyuki; Saito, Mineo

    2014-04-07

    We studied magnetism in bilayer and multilayer zigzag graphene nanoribbons (ZGNRs) through first-principles density functional theory calculations. We found that the magnetic ground state of bilayer ZGNRs is the C-type antiferromagnetic (AFM) state, which is the AFM order between intraplane-edge carbon atoms and ferromagnetic (FM) order between interplane edge carbon atoms. In the cases of infinitely stacked multilayer ZGNRs, i.e., zigzag graphite nanoribbons, the C-type AFM state is also the most stable. By carrier doping, we found that the magnetic ground state changed from the C-AFM state to the FM state and, thus, realized two-dimensional FM surface (edge) states of graphite with a metallic conductivity.

  16. Defects in bilayer silica and graphene: common trends in diverse hexagonal two-dimensional systems

    PubMed Central

    Björkman, Torbjörn; Kurasch, Simon; Lehtinen, Ossi; Kotakoski, Jani; Yazyev, Oleg V.; Srivastava, Anchal; Skakalova, Viera; Smet, Jurgen H.; Kaiser, Ute; Krasheninnikov, Arkady V.

    2013-01-01

    By combining first-principles and classical force field calculations with aberration-corrected high-resolution transmission electron microscopy experiments, we study the morphology and energetics of point and extended defects in hexagonal bilayer silica and make comparison to graphene, another two-dimensional (2D) system with hexagonal symmetry. We show that the motifs of isolated point defects in these 2D structures with otherwise very different properties are similar, and include Stone-Wales-type defects formed by structural unit rotations, flower defects and reconstructed double vacancies. The morphology and energetics of extended defects, such as grain boundaries have much in common as well. As both sp2-hybridised carbon and bilayer silica can also form amorphous structures, our results indicate that the morphology of imperfect 2D honeycomb lattices is largely governed by the underlying symmetry of the lattice. PMID:24336488

  17. Transport properties of bilayer graphene in a strong in-plane magnetic field

    NASA Astrophysics Data System (ADS)

    Van der Donck, M.; Peeters, F. M.; Van Duppen, B.

    2016-03-01

    A strong in-plane magnetic field drastically alters the low-energy spectrum of bilayer graphene by separating the parabolic energy dispersion into two linear Dirac cones. The effect of this dramatic change on the transport properties strongly depends on the orientation of the in-plane magnetic field with respect to the propagation direction of the charge carriers and the angle at which they impinge on the electrostatic potentials. For magnetic fields oriented parallel to the potential boundaries an additional propagating mode that results from the splitting into Dirac cones enhances the transmission probability for charge carriers tunneling through the potentials and increases the corresponding conductance. Our results show that the chiral suppression of transmission at normal incidence, reminiscent of bilayer graphene's 2 π Berry phase, is turned into a chiral enhancement when the magnetic field increases, thus indicating a transition from a bilayer to a monolayer-like system at normal incidence. Further, we find that the typical transmission resonances stemming from confinement in a potential barrier are shifted to higher energy and are eventually transformed into antiresonances with increasing magnetic field. For magnetic fields oriented perpendicular to the potential boundaries we find a very pronounced transition from a bilayer system to two separated monolayer-like systems with Klein tunneling emerging at certain incident angles symmetric around 0, which also leaves a signature in the conductance. For both orientations of the magnetic field, the transmission probability is still correctly described by pseudospin conservation. Finally, to motivate the large in-plane magnetic field, we show that its energy spectrum can be mimicked by specific lattice deformations such as a relative shift of one of the layers. With this equivalence we introduce the notion of an in-plane pseudomagnetic field.

  18. In-Plane Electric Polarization of Bilayer Graphene Nanoribbons Induced by an Interlayer Bias Voltage.

    PubMed

    Okugawa, Ryo; Tanaka, Junya; Koretsune, Takashi; Saito, Susumu; Murakami, Shuichi

    2015-10-01

    We theoretically show that an interlayer bias voltage in the AB-stacked bilayer graphene nanoribbons with armchair edges induces an electric polarization along the ribbon. Both tight-binding and ab initio calculations consistently indicate that when the bias voltage is weak, the polarization shows opposite signs depending on the ribbon width modulo three. This nontrivial dependence is explained using a two-band effective model. A strong limit of the bias voltage in the tight-binding model shows either one-third or zero polarization, which agrees with the topological argument. PMID:26550741

  19. In-Plane Electric Polarization of Bilayer Graphene Nanoribbons Induced by an Interlayer Bias Voltage.

    PubMed

    Okugawa, Ryo; Tanaka, Junya; Koretsune, Takashi; Saito, Susumu; Murakami, Shuichi

    2015-10-01

    We theoretically show that an interlayer bias voltage in the AB-stacked bilayer graphene nanoribbons with armchair edges induces an electric polarization along the ribbon. Both tight-binding and ab initio calculations consistently indicate that when the bias voltage is weak, the polarization shows opposite signs depending on the ribbon width modulo three. This nontrivial dependence is explained using a two-band effective model. A strong limit of the bias voltage in the tight-binding model shows either one-third or zero polarization, which agrees with the topological argument.

  20. Hybrid matrix method for stable numerical analysis of the propagation of Dirac electrons in gapless bilayer graphene superlattices

    NASA Astrophysics Data System (ADS)

    Briones-Torres, J. A.; Pernas-Salomón, R.; Pérez-Álvarez, R.; Rodríguez-Vargas, I.

    2016-05-01

    Gapless bilayer graphene (GBG), like monolayer graphene, is a material system with unique properties, such as anti-Klein tunneling and intrinsic Fano resonances. These properties rely on the gapless parabolic dispersion relation and the chiral nature of bilayer graphene electrons. In addition, propagating and evanescent electron states coexist inherently in this material, giving rise to these exotic properties. In this sense, bilayer graphene is unique, since in most material systems in which Fano resonance phenomena are manifested an external source that provides extended states is required. However, from a numerical standpoint, the presence of evanescent-divergent states in the eigenfunctions linear superposition representing the Dirac spinors, leads to a numerical degradation (the so called Ωd problem) in the practical applications of the standard Coefficient Transfer Matrix (K) method used to study charge transport properties in Bilayer Graphene based multi-barrier systems. We present here a straightforward procedure based in the hybrid compliance-stiffness matrix method (H) that can overcome this numerical degradation. Our results show that in contrast to standard matrix method, the proposed H method is suitable to study the transmission and transport properties of electrons in GBG superlattice since it remains numerically stable regardless the size of the superlattice and the range of values taken by the input parameters: the energy and angle of the incident electrons, the barrier height and the thickness and number of barriers. We show that the matrix determinant can be used as a test of the numerical accuracy in real calculations.

  1. First-principles study of hydrogen adsorption on titanium-decorated single-layer and bilayer graphenes

    NASA Astrophysics Data System (ADS)

    Pan, Hong-Zhe; Wang, Yong-Long; He, Kai-Hua; Wei, Ming-Zhen; Ouyang, Yu; Chen, Li

    2013-06-01

    The adsorption of hydrogen molecules on titanium-decorated (Ti-decorated) single-layer and bilayer graphenes is studied using density functional theory (DFT) with the relativistic effect. Both the local density approximation (LDA) and the generalized gradient approximation (GGA) are used for obtaining the region of the adsorption energy of H2 molecules on Ti-decorated graphene. We find that a graphene layer with titanium (Ti) atoms adsorbed on both sides can store hydrogen up to 9.51 wt% with average adsorption energy in a range from -0.170 eV to -0.518 eV. Based on the adsorption energy criterion, we find that chemisorption is predominant for H2 molecules when the concentration of H2 molecules absorbed is low while physisorption is predominant when the concentration is high. The computation results for the bilayer graphene decorated with Ti atoms show that the lower carbon layer makes no contribution to hydrogen adsorption.

  2. Landau-level dispersion and the quantum Hall plateaus in bilayer graphene

    SciTech Connect

    Zarenia, M.; Peeters, F. M.; Vasilopoulos, P.; Pourtolami, N.

    2013-12-04

    We study the quantum Hall effect (QHE) in bilayer graphene using the Kubo-Greenwood formula. At zero temperature the Hall conductivity σ{sub yx} is given by σ{sub yx} = 4(N+1)e{sup 2}/h with N the index of the highest occupied Landau level (LL). Including the dispersion of the LLs and their width, due to e.g. scattering by impurities, produces the plateau of the n = 0 LL in agreement with experimental results on doped samples and similar theoretical results on single-layer graphene plateaus widen with impurity concentration. Further, the evaluated resistivity ρ{sub xx} exhibits a strong, oscillatory dependence on the electron concentration. Explicit results are obtained for δ-function impurities.

  3. Unraveling the interlayer-related phonon self-energy renormalization in bilayer graphene

    PubMed Central

    Araujo, Paulo T.; Mafra, Daniela L.; Sato, Kentaro; Saito, Riichiro; Kong, Jing; Dresselhaus, Mildred S.

    2012-01-01

    In this letter, we present a step towards understanding the bilayer graphene (2LG) interlayer (IL)-related phonon combination modes and overtones as well as their phonon self-energy renormalizations by using both gate-modulated and laser-energy dependent inelastic scattering spectroscopy. We show that although the IL interactions are weak, their respective phonon renormalization response is significant. Particularly special, the IL interactions are mediated by Van der Waals forces and are fundamental for understanding low-energy phenomena such as transport and infrared optics. Our approach opens up a new route to understanding fundamental properties of IL interactions which can be extended to any graphene-like material, such as MoS2, WSe2, oxides and hydroxides. Furthermore, we report a previously elusive crossing between IL-related phonon combination modes in 2LG, which might have important technological applications. PMID:23264879

  4. Transport properties of AB stacked (Bernal) bilayer graphene on and without substrate within 2- and 4-band approximations

    SciTech Connect

    Gusynin, V. P. Sharapov, S. G.; Reshetnyak, A. A.

    2015-10-27

    We present the results of the calculations of longitudinal and Hall conductivities of AB-stacked bilayer graphene as a function of frequency, finite chemical potential, temperature both with and without magnetic fields on a base of 2- and 4-band effective models. The limited cases of the conductivities for direct current are derived. The relations being important for optoelectronic among Hall conductivities and Faraday, Kerr angles in the AB-bilayers samples in the electric and magnetic fields when the radiation passes across bilayer sheets on different kinds a substrate are obtained.

  5. Band Gap Modulation of Bilayer MoS2 Under Strain Engineering and Electric Field: A Density Functional Theory

    NASA Astrophysics Data System (ADS)

    Nguyen, Chuong V.; Hieu, Nguyen N.; Ilyasov, Victor V.

    2016-08-01

    In this work, we investigate band-gap tuning in bilayer MoS2 by an external electric field and by applied biaxial strain. Our calculations show that the band gaps of bilayer MoS2 can be tuned by the perpendicular electric field or biaxial strain. The band gaps of bilayer MoS2 decrease with increasing applied electric field or biaxial strain. When the electric field was introduced, electronic levels are split due to the separation of the valence sub-band and the conduction sub-band states. Our calculations also show that the change in the band gap of bilayer MoS2 is due to the separation of electronic levels by electric field via the Stark effect. At the electric field E_{Field} = 5.5 V/nm or biaxial strain ɛ = 15%, bilayer MoS2 becomes metallic. The semiconductor-metal phase transition in bilayer MoS2 plays an important role in its application for nanodevices.

  6. 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.

  7. Oxygen-activated growth and bandgap tunability of large single-crystal bilayer graphene.

    PubMed

    Hao, Yufeng; Wang, Lei; Liu, Yuanyue; Chen, Hua; Wang, Xiaohan; Tan, Cheng; Nie, Shu; Suk, Ji Won; Jiang, Tengfei; Liang, Tengfei; Xiao, Junfeng; Ye, Wenjing; Dean, Cory R; Yakobson, Boris I; McCarty, Kevin F; Kim, Philip; Hone, James; Colombo, Luigi; Ruoff, Rodney S

    2016-05-01

    Bernal (AB)-stacked bilayer graphene (BLG) is a semiconductor whose bandgap can be tuned by a transverse electric field, making it a unique material for a number of electronic and photonic devices. A scalable approach to synthesize high-quality BLG is therefore critical, which requires minimal crystalline defects in both graphene layers and maximal area of Bernal stacking, which is necessary for bandgap tunability. Here we demonstrate that in an oxygen-activated chemical vapour deposition (CVD) process, half-millimetre size, Bernal-stacked BLG single crystals can be synthesized on Cu. Besides the traditional 'surface-limited' growth mechanism for SLG (1st layer), we discovered new microscopic steps governing the growth of the 2nd graphene layer below the 1st layer as the diffusion of carbon atoms through the Cu bulk after complete dehydrogenation of hydrocarbon molecules on the Cu surface, which does not occur in the absence of oxygen. Moreover, we found that the efficient diffusion of the carbon atoms present at the interface between Cu and the 1st graphene layer further facilitates growth of large domains of the 2nd layer. The CVD BLG has superior electrical quality, with a device on/off ratio greater than 10(4), and a tunable bandgap up to ∼100 meV at a displacement field of 0.9 V nm(-1). PMID:26828845

  8. 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. PMID:25363605

  9. Sandwich beam model for free vibration analysis of bilayer graphene nanoribbons with interlayer shear effect

    SciTech Connect

    Nazemnezhad, Reza E-mail: rnazemnezhad@du.ac.ir; Shokrollahi, Hassan; Hosseini-Hashemi, Shahrokh

    2014-05-07

    In this study, sandwich beam model (SM) is proposed for free vibration analysis of bilayer graphene nanoribbons (BLGNRs) with interlayer shear effect. This model also takes into account the intralayer (in-plane) stretch of graphene nanoribbons. The molecular dynamics (MD) simulations using the software LAMMPS and Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential are done to validate the accuracy of the sandwich model results. The MD simulation results include the two first frequencies of cantilever BLGNRs with different lengths and two interlayer shear moduli, i.e., 0.25 and 4.6 GPa. These two interlayer shear moduli, 0.25 and 4.6 GPa, can be obtained by sliding a small flake of graphene on a large graphene substrate when the parameter of E-LJ term in AIREBO potential, epsilon-CC, is set to be 2.84 and 45.44 meV, respectively. The SM results for a wide range of bending rigidity values show that the proposed model, i.e., the SM, predicts much better than the previous beam model in which the intralayer stretch is ignored. In addition, it is observed that the model can properly predict the natural frequencies of BLGNRs for various values of the bending rigidity and the interlayer shear modulus.

  10. Oxygen-activated growth and bandgap tunability of large single-crystal bilayer graphene.

    PubMed

    Hao, Yufeng; Wang, Lei; Liu, Yuanyue; Chen, Hua; Wang, Xiaohan; Tan, Cheng; Nie, Shu; Suk, Ji Won; Jiang, Tengfei; Liang, Tengfei; Xiao, Junfeng; Ye, Wenjing; Dean, Cory R; Yakobson, Boris I; McCarty, Kevin F; Kim, Philip; Hone, James; Colombo, Luigi; Ruoff, Rodney S

    2016-05-01

    Bernal (AB)-stacked bilayer graphene (BLG) is a semiconductor whose bandgap can be tuned by a transverse electric field, making it a unique material for a number of electronic and photonic devices. A scalable approach to synthesize high-quality BLG is therefore critical, which requires minimal crystalline defects in both graphene layers and maximal area of Bernal stacking, which is necessary for bandgap tunability. Here we demonstrate that in an oxygen-activated chemical vapour deposition (CVD) process, half-millimetre size, Bernal-stacked BLG single crystals can be synthesized on Cu. Besides the traditional 'surface-limited' growth mechanism for SLG (1st layer), we discovered new microscopic steps governing the growth of the 2nd graphene layer below the 1st layer as the diffusion of carbon atoms through the Cu bulk after complete dehydrogenation of hydrocarbon molecules on the Cu surface, which does not occur in the absence of oxygen. Moreover, we found that the efficient diffusion of the carbon atoms present at the interface between Cu and the 1st graphene layer further facilitates growth of large domains of the 2nd layer. The CVD BLG has superior electrical quality, with a device on/off ratio greater than 10(4), and a tunable bandgap up to ∼100 meV at a displacement field of 0.9 V nm(-1).

  11. Band gap formation in graphene by in-situ doping

    SciTech Connect

    Park, Jeongho; Mitchel, W. C.; Brown, Gail J.; Grazulis, Lawrence; Smith, Howard E.; Pacley, Shanee D.; Boeckl, John J.; Eyink, Kurt G.; Mou, Shin; Tomich, David H.; Hoelscher, John E.; Elhamri, Said

    2011-05-16

    We report the formation of band gaps in as-grown stacks of epitaxial graphene with opposite doping. Control of in-situ doping during carbon source molecular beam epitaxy growth on SiC was achieved by using different carbon sources. Doping heterostructures were grown by stacking n-type material from a C{sub 60} source on p-type material from a graphite filament source. Activation energies for the resistivity and carrier concentration indicated band gaps up to 200 meV. A photoconductivity threshold was observed in the range of the electrical activation energies. Band gap formation is attributed to electric fields induced by spatially separated ionized dopants of opposite charge.

  12. Vortex and gap generation in gauge models of graphene

    SciTech Connect

    Oliveira, O.; Cordeiro, C.E.; Delfino, A.; Paula, W. de; Frederico, T.

    2011-04-15

    Effective quantum field theoretical continuum models for graphene are investigated. The models include a complex scalar field and a vector gauge field. Different gauge theories are considered and their gap patterns for the scalar, vector, and fermion excitations are investigated. Different gauge groups lead to different relations between the gaps, which can be used to experimentally distinguish the gauge theories. In this class of models the fermionic gap is a dynamic quantity. The finite-energy vortex solutions of the gauge models have the flux of the ''magnetic field'' quantized, making the Bohm-Aharonov effect active even when external electromagnetic fields are absent. The flux comes proportional to the scalar field angular momentum quantum number. The zero modes of the Dirac equation show that the gauge models considered here are compatible with fractionalization.

  13. Nanoscale, electric field-driven water bridges in vacuum gaps and lipid bilayers.

    PubMed

    Ho, Ming-Chak; Levine, Zachary A; Vernier, P Thomas

    2013-11-01

    Formation of a water bridge across the lipid bilayer is the first stage of pore formation in molecular dynamic (MD) simulations of electroporation, suggesting that the intrusion of individual water molecules into the membrane interior is the initiation event in a sequence that leads to the formation of a conductive membrane pore. To delineate more clearly the role of water in membrane permeabilization, we conducted extensive MD simulations of water bridge formation, stabilization, and collapse in palmitoyloleoylphosphatidylcholine bilayers and in water-vacuum-water systems, in which two groups of water molecules are separated by a 2.8 nm vacuum gap, a simple analog of a phospholipid bilayer. Certain features, such as the exponential decrease in water bridge initiation time with increased external electric field, are similar in both systems. Other features, such as the relationship between water bridge lifetime and the diameter of the water bridge, are quite different between the two systems. Data such as these contribute to a better and more quantitative understanding of the relative roles of water and lipid in membrane electropore creation and annihilation, facilitating a mechanism-driven development of electroporation protocols. These methods can be extended to more complex, heterogeneous systems that include membrane proteins and intracellular and extracellular membrane attachments, leading to more accurate models of living cells in electric fields. PMID:23644990

  14. Nanoscale, electric field-driven water bridges in vacuum gaps and lipid bilayers.

    PubMed

    Ho, Ming-Chak; Levine, Zachary A; Vernier, P Thomas

    2013-11-01

    Formation of a water bridge across the lipid bilayer is the first stage of pore formation in molecular dynamic (MD) simulations of electroporation, suggesting that the intrusion of individual water molecules into the membrane interior is the initiation event in a sequence that leads to the formation of a conductive membrane pore. To delineate more clearly the role of water in membrane permeabilization, we conducted extensive MD simulations of water bridge formation, stabilization, and collapse in palmitoyloleoylphosphatidylcholine bilayers and in water-vacuum-water systems, in which two groups of water molecules are separated by a 2.8 nm vacuum gap, a simple analog of a phospholipid bilayer. Certain features, such as the exponential decrease in water bridge initiation time with increased external electric field, are similar in both systems. Other features, such as the relationship between water bridge lifetime and the diameter of the water bridge, are quite different between the two systems. Data such as these contribute to a better and more quantitative understanding of the relative roles of water and lipid in membrane electropore creation and annihilation, facilitating a mechanism-driven development of electroporation protocols. These methods can be extended to more complex, heterogeneous systems that include membrane proteins and intracellular and extracellular membrane attachments, leading to more accurate models of living cells in electric fields.

  15. Toward Cove-Edged Low Band Gap Graphene Nanoribbons

    PubMed Central

    2015-01-01

    Graphene nanoribbons (GNRs), defined as nanometer-wide strips of graphene, have attracted increasing attention as promising candidates for next-generation semiconductors. Here, we demonstrate a bottom-up strategy toward novel low band gap GNRs (Eg = 1.70 eV) with a well-defined cove-type periphery both in solution and on a solid substrate surface with chrysene as the key monomer. Corresponding cyclized chrysene-based oligomers consisting of the dimer and tetramer are obtained via an Ullmann coupling followed by oxidative intramolecular cyclodehydrogenation in solution, and much higher GNR homologues via on-surface synthesis. These oligomers adopt nonplanar structures due to the steric repulsion between the two C–H bonds at the inner cove position. Characterizations by single crystal X-ray analysis, UV–vis absorption spectroscopy, NMR spectroscopy, and scanning tunneling microscopy (STM) are described. The interpretation is assisted by density functional theory (DFT) calculations. PMID:25909566

  16. Bilayer Graphene-Hexagonal Boron Nitride Heterostructure Negative Differential Resistance Interlayer Tunnel FETs

    NASA Astrophysics Data System (ADS)

    Kang, Sangwoo; Fallahazad, Babak; Lee, Kayoung; Movva, Hema; Kim, Kyounghwan; Corbet, Chris; Taniguchi, Takashi; Watanabe, Kenji; Colombo, Luigi; Register, Leonard; Tutuc, Emanuel; Banerjee, Sanjay

    2015-03-01

    We present the operation of a vertical tunneling field effect transistor using a stacked double bilayer graphene (BLG) and hexagonal boron nitride (hBN) heterostructure. The device is fabricated with the so-called Van der Waals transfer method with the edges of the top and bottom BLG flakes being rotationally aligned to roughly 60°. The device shows multiple negative differential resistance (NDR) peaks which can be adjusted through the gate bias. Temperature dependent measurements show that the peak width of the differential conductance broadens and the height lowered when the temperature is increased, which is indicative of resonant tunneling. Through electrostatic calculations, it is shown that the multiple peaks occur when the two conduction bands at the K-point of the top and bottom bilayer graphene become aligned at certain bias conditions. It is also shown that by adjusting the rotational alignment of the bands of the top and bottom BLG through an in-plane magnetic field, the conductance peaks can be broadened. In addition, utilizing the NDR characteristic of the device, one-transistor latch or SRAM operation is demonstrated.

  17. Canted antiferromagnetic phase of the ν=0 quantum Hall state in bilayer graphene.

    PubMed

    Kharitonov, Maxim

    2012-07-27

    Motivated to understand the nature of the strongly insulating ν=0 quantum Hall state in bilayer graphene, we develop the theory of the state in the framework of quantum Hall ferromagnetism. The generic phase diagram, obtained in the presence of the isospin anisotropy, perpendicular electric field, and Zeeman effect, consists of the spin-polarized ferromagnetic (F), canted antiferromagnetic (CAF), and partially (PLP) and fully (FLP) layer-polarized phases. We address the edge transport properties of the phases. Comparing our findings with the recent data on suspended dual-gated devices, we conclude that the insulating ν=0 state realized in bilayer graphene at lower electric field is the CAF phase. We also predict a continuous and a sharp insulator-metal phase transition upon tilting the magnetic field from the insulating CAF and FLP phases, respectively, to the F phase with metallic edge conductance 2e(2)/h, which could be within the reach of available fields and could allow one to identify and distinguish the phases experimentally.

  18. Observation of helical edge states and fractional quantum Hall effect in a graphene electron-hole bilayer

    NASA Astrophysics Data System (ADS)

    Luo, Jason Yuanhong; Sanchez-Yamagishi, Javier; Watanabe, Kenji; Taniguchi, Takashi; Jarillo-Herrero, Pablo

    1D electronic systems are common theoretical building blocks for constructing quantum circuits, motivating a search for new experimental systems where 1D edge states of different quantum numbers can be coupled to each other by design. Twisted bilayer graphene allows for the stacking of two separate 1D quantum hall edge states, thus providing a natural sandbox for studying different types of edge state interactions. Via doping to form an electron-hole bilayer at moderate magnetic fields, we can induce edge modes of opposite chiralities and spin polarizations on different layers, thereby creating helical edge states reminiscent of a two-dimensional quantum spin Hall system. We report magnetotransport measurements of high-quality twisted bilayer graphene, showing how non-local measurements allow us to elucidate the nature and robustness of the helical edge states, as well as hints of fractional edge state interactions that are observable at higher magnetic fields.

  19. Theoretical assessment of feasibility to sequence DNA through interlayer electronic tunneling transport at aligned nanopores in bilayer graphene

    PubMed Central

    Prasongkit, Jariyanee; Feliciano, Gustavo T.; Rocha, Alexandre R.; He, Yuhui; Osotchan, Tanakorn; Ahuja, Rajeev; Scheicher, Ralph H.

    2015-01-01

    Fast, cost effective, single-shot DNA sequencing could be the prelude of a new era in genetics. As DNA encodes the information for the production of proteins in all known living beings on Earth, determining the nucleobase sequences is the first and necessary step in that direction. Graphene-based nanopore devices hold great promise for next-generation DNA sequencing. In this work, we develop a novel approach for sequencing DNA using bilayer graphene to read the interlayer conductance through the layers in the presence of target nucleobases. Classical molecular dynamics simulations of DNA translocation through the pore were performed to trace the nucleobase trajectories and evaluate the interaction between the nucleobases and the nanopore. This interaction stabilizes the bases in different orientations, resulting in smaller fluctuations of the nucleobases inside the pore. We assessed the performance of a bilayer graphene nanopore setup for the purpose of DNA sequencing by employing density functional theory and non-equilibrium Green’s function method to investigate the interlayer conductance of nucleobases coupling simultaneously to the top and bottom graphene layers. The obtained conductance is significantly affected by the presence of DNA in the bilayer graphene nanopore, allowing us to analyze DNA sequences. PMID:26634811

  20. Theoretical assessment of feasibility to sequence DNA through interlayer electronic tunneling transport at aligned nanopores in bilayer graphene.

    PubMed

    Prasongkit, Jariyanee; Feliciano, Gustavo T; Rocha, Alexandre R; He, Yuhui; Osotchan, Tanakorn; Ahuja, Rajeev; Scheicher, Ralph H

    2015-12-04

    Fast, cost effective, single-shot DNA sequencing could be the prelude of a new era in genetics. As DNA encodes the information for the production of proteins in all known living beings on Earth, determining the nucleobase sequences is the first and necessary step in that direction. Graphene-based nanopore devices hold great promise for next-generation DNA sequencing. In this work, we develop a novel approach for sequencing DNA using bilayer graphene to read the interlayer conductance through the layers in the presence of target nucleobases. Classical molecular dynamics simulations of DNA translocation through the pore were performed to trace the nucleobase trajectories and evaluate the interaction between the nucleobases and the nanopore. This interaction stabilizes the bases in different orientations, resulting in smaller fluctuations of the nucleobases inside the pore. We assessed the performance of a bilayer graphene nanopore setup for the purpose of DNA sequencing by employing density functional theory and non-equilibrium Green's function method to investigate the interlayer conductance of nucleobases coupling simultaneously to the top and bottom graphene layers. The obtained conductance is significantly affected by the presence of DNA in the bilayer graphene nanopore, allowing us to analyze DNA sequences.

  1. Theoretical assessment of feasibility to sequence DNA through interlayer electronic tunneling transport at aligned nanopores in bilayer graphene.

    PubMed

    Prasongkit, Jariyanee; Feliciano, Gustavo T; Rocha, Alexandre R; He, Yuhui; Osotchan, Tanakorn; Ahuja, Rajeev; Scheicher, Ralph H

    2015-01-01

    Fast, cost effective, single-shot DNA sequencing could be the prelude of a new era in genetics. As DNA encodes the information for the production of proteins in all known living beings on Earth, determining the nucleobase sequences is the first and necessary step in that direction. Graphene-based nanopore devices hold great promise for next-generation DNA sequencing. In this work, we develop a novel approach for sequencing DNA using bilayer graphene to read the interlayer conductance through the layers in the presence of target nucleobases. Classical molecular dynamics simulations of DNA translocation through the pore were performed to trace the nucleobase trajectories and evaluate the interaction between the nucleobases and the nanopore. This interaction stabilizes the bases in different orientations, resulting in smaller fluctuations of the nucleobases inside the pore. We assessed the performance of a bilayer graphene nanopore setup for the purpose of DNA sequencing by employing density functional theory and non-equilibrium Green's function method to investigate the interlayer conductance of nucleobases coupling simultaneously to the top and bottom graphene layers. The obtained conductance is significantly affected by the presence of DNA in the bilayer graphene nanopore, allowing us to analyze DNA sequences. PMID:26634811

  2. Generation of full polarization in ferromagnetic graphene with spin energy gap

    SciTech Connect

    Wu, Qing-Ping; Liu, Zheng-Fang E-mail: aixichen@ecjtu.edu.cn; Liu, Zhi-Min; Chen, Ai-Xi E-mail: aixichen@ecjtu.edu.cn; Xiao, Xian-Bo

    2014-12-22

    We propose a workable scheme for the generation of full spin polarization in ferromagnetic graphene system with strain or Rashba spin-orbit interaction. A spin energy gap can be opened in ferromagnetic graphene system in the presence of strain or Rashba spin-orbit interaction, leading to the full polarization in the spin energy gap. In addition, under the combined modulation of strain and Rashba spin-orbit interaction, the ferromagnetic graphene system can generate significantly large spin-polarized current with a full polarization in the spin energy gap. It is anticipated to apply such a phenomenon to design the electron spin devices based on the graphene.

  3. Edge effects on band gap energy in bilayer 2H-MoS{sub 2} under uniaxial strain

    SciTech Connect

    Dong, Liang; Wang, Jin; Dongare, Avinash M.; Namburu, Raju; O'Regan, Terrance P.; Dubey, Madan

    2015-06-28

    The potential of ultrathin MoS{sub 2} nanostructures for applications in electronic and optoelectronic devices requires a fundamental understanding in their electronic structure as a function of strain. Previous experimental and theoretical studies assume that an identical strain and/or stress state is always maintained in the top and bottom layers of a bilayer MoS{sub 2} film. In this study, a bilayer MoS{sub 2} supercell is constructed differently from the prototypical unit cell in order to investigate the layer-dependent electronic band gap energy in a bilayer MoS{sub 2} film under uniaxial mechanical deformations. The supercell contains an MoS{sub 2} bottom layer and a relatively narrower top layer (nanoribbon with free edges) as a simplified model to simulate the as-grown bilayer MoS{sub 2} flakes with free edges observed experimentally. Our results show that the two layers have different band gap energies under a tensile uniaxial strain, although they remain mutually interacting by van der Waals interactions. The deviation in their band gap energies grows from 0 to 0.42 eV as the uniaxial strain increases from 0% to 6% under both uniaxial strain and stress conditions. The deviation, however, disappears if a compressive uniaxial strain is applied. These results demonstrate that tensile uniaxial strains applied to bilayer MoS{sub 2} films can result in distinct band gap energies in the bilayer structures. Such variations need to be accounted for when analyzing strain effects on electronic properties of bilayer or multilayered 2D materials using experimental methods or in continuum models.

  4. Thin film transistors gas sensors based on reduced graphene oxide poly(3-hexylthiophene) bilayer film for nitrogen dioxide detection

    NASA Astrophysics Data System (ADS)

    Xie, Tao; Xie, Guangzhong; Zhou, Yong; Huang, Junlong; Wu, Mei; Jiang, Yadong; Tai, Huiling

    2014-10-01

    Reduced graphene oxide (RGO)/poly(3-hexylthiophene) (P3HT) bilayer films were firstly utilized as active layers in OTFT gas sensors for nitrogen dioxide (NO2) detection. The OTFT with RGO/P3HT bilayer film exhibited the typical transistor characteristics and better gas sensing properties at room temperature. The electrical parameters of OTFTs based on pure P3HT film and RGO/P3HT bilayer film were calculated. The threshold voltage of OTFT was positively shifted due to the high concentration carriers in RGO. The sensing properties of the sensor with RGO/P3HT bilayer film were also investigated. Moreover, the sensing mechanism was analyzed as well.

  5. Strain-Induced Energy Band Gap Opening in Two-Dimensional Bilayered Silicon Film

    NASA Astrophysics Data System (ADS)

    Ji, Z.; Zhou, R.; Lew Yan Voon, L. C.; Zhuang, Y.

    2016-10-01

    This work presents a theoretical study of the structural and electronic properties of bilayered silicon film (BiSF) under in-plane biaxial strain/stress using density functional theory (DFT). Atomic structures of the two-dimensional (2-D) silicon films are optimized by using both the local-density approximation (LDA) and generalized gradient approximation (GGA). In the absence of strain/stress, five buckled hexagonal honeycomb structures of the BiSF with triangular lattice have been obtained as local energy minima, and their structural stability has been verified. These structures present a Dirac-cone shaped energy band diagram with zero energy band gaps. Applying a tensile biaxial strain leads to a reduction of the buckling height. Atomically flat structures with zero buckling height have been observed when the AA-stacking structures are under a critical biaxial strain. Increase of the strain between 10.7% and 15.4% results in a band-gap opening with a maximum energy band gap opening of ˜0.17 eV, obtained when a 14.3% strain is applied. Energy band diagrams, electron transmission efficiency, and the charge transport property are calculated. Additionally, an asymmetric energetically favorable atomic structure of BiSF shows a non-zero band gap in the absence of strain/stress and a maximum band gap of 0.15 eV as a -1.71% compressive strain is applied. Both tensile and compressive strain/stress can lead to a band gap opening in the asymmetric structure.

  6. Nanometric Gap Structure with a Fluid Lipid Bilayer for the Selective Transport and Detection of Biological Molecules.

    PubMed

    Ando, Koji; Tanabe, Masashi; Morigaki, Kenichi

    2016-08-01

    The biological membrane is a natural biosensing platform that can detect specific molecules with extremely high sensitivity. We developed a biosensing methodology by combining a model biological membrane and a nanometer-sized gap structure on a glass substrate. The model membrane comprised lithographically patterned polymeric and fluid lipid bilayers. The polymeric bilayer was bonded to a poly(dimethylsiloxane) (PDMS) sheet by using an adhesion layer with a defined thickness (lipid vesicles). Extruded lipid vesicles having a biotin moiety on the surface were used as the adhesion layer in conjunction with the biotin-streptavidin linkage. A gap structure was formed between the fluid bilayer and PDMS (nanogap junction). The thickness of the gap structure was several tens of nanometers, as determined by the thickness of the adhesion layer. The nanogap junction acted as a sensitive biosensing platform. From a mixture of proteins (cholera toxin and albumin), the target protein (cholera toxin) was selectively transported into the gap by the specific binding to a glycolipid (GM1) in the fluid bilayer and lateral diffusion. The target protein molecules were then detected with an elevated signal-to-noise ratio due to the reduced background noise in the nanometric gap. The combination of selective transport and reduced background noise drastically enhanced the sensitivity toward the target protein. The nanogap junction should have broad biomedical applications by realizing highly selective and sensitive biosensing in samples having diverse coexisting molecules. PMID:27427950

  7. Topological gaps without masses in driven graphene-like systems

    NASA Astrophysics Data System (ADS)

    Iadecola, Thomas; Neupert, Titus; Chamon, Claudio

    2014-03-01

    We illustrate the possibility of realizing band gaps in graphene-like systems that fall outside the existing classification of gapped Dirac Hamiltonians in terms of masses. As our primary example we consider a band gap arising due to time-dependent distortions of the honeycomb lattice. By means of an exact, invertible, and transport-preserving mapping to a time-independent Hamiltonian, we show that the system exhibits Chern-insulating phases with quantized Hall conductivities +/-e2 / h . The chirality of the corresponding gapless edge modes is controllable by both the frequency of the driving and the manner in which sublattice symmetry is broken by the dynamical lattice modulations. We demonstrate that, while these phases are in the same topological sector as the Haldane model, they are nevertheless separated from the latter by a gap-closing transition unless an extra parameter is added to the Hamiltonian. Finally, we discuss a promising possible realization of this physics in photonic lattices. This work is supported in part by DOE Grant DEF-06ER46316 (T.I. and C.C.).

  8. Understanding the origin of band gap formation in graphene on metals: graphene on Cu/Ir(111).

    PubMed

    Vita, H; Böttcher, S; Horn, K; Voloshina, E N; Ovcharenko, R E; Kampen, Th; Thissen, A; Dedkov, Yu S

    2014-01-01

    Understanding the nature of the interaction at the graphene/metal interfaces is the basis for graphene-based electron- and spin-transport devices. Here we investigate the hybridization between graphene- and metal-derived electronic states by studying the changes induced through intercalation of a pseudomorphic monolayer of Cu in between graphene and Ir(111), using scanning tunnelling microscopy and photoelectron spectroscopy in combination with density functional theory calculations. We observe the modifications in the band structure by the intercalation process and its concomitant changes in the charge distribution at the interface. Through a state-selective analysis of band hybridization, we are able to determine their contributions to the valence band of graphene giving rise to the gap opening. Our methodology reveals the mechanisms that are responsible for the modification of the electronic structure of graphene at the Dirac point, and permits to predict the electronic structure of other graphene-metal interfaces. PMID:25027748

  9. Point defects in twisted bilayer graphene: A density functional theory study

    NASA Astrophysics Data System (ADS)

    Ulman, Kanchan; Narasimhan, Shobhana

    2014-06-01

    We have used ab initio density functional theory, incorporating van der Waals corrections, to study twisted bilayer graphene (TBLG) where Stone-Wales defects or monovacancies are introduced in one of the layers. We compare these results to those for defects in single-layer graphene or Bernal stacked graphene. The energetics of defect formation is not very sensitive to the stacking of the layers or the specific site at which the defect is created, suggesting a weak interlayer coupling. However, signatures of the interlayer coupling are manifested clearly in the electronic band structure. For the "γγ" Stone-Wales defect in TBLG, we observe two Dirac cones that are shifted in both momentum space and energy. This up/down shift in energy results from the combined effect of a charge transfer between the two graphene layers and a chemical interaction between the layers, which mimics the effects of a transverse electric field. Charge density plots show that states near the Dirac points have significant admixture between the two layers. For Stone-Wales defects at other sites in TBLG, this basic structure is modified by the creation of minigaps at energy crossings. For a monovacancy, the Dirac cone of the pristine layer is shifted up in energy by ˜0.25 eV due to a combination of the requirement of the equilibration of Fermi energy between the two layers with different numbers of electrons, charge transfer, and chemical interactions. Both kinds of defects increase the density of states at the Fermi level. The monovacancy also results in spin polarization, with magnetic moments on the defect of 1.2-1.8 μB.

  10. Highly bendable bilayer-type photo-actuators comprising of reduced graphene oxide dispersed in hydrogels

    PubMed Central

    Kim, Dowan; Lee, Heon Sang; Yoon, Jinhwan

    2016-01-01

    To avoid the problem of reduced graphene oxide (rGO) restacking in aqueous solution, the preparation of light-responsive poly(N-isopropylacrylamide) incorporating rGO (PNIPAm/rGO) was achieved by the chemical reduction of GO dispersed in the hydrogel matrix. Due to the enhanced photothermal efficiency of the rGO, the prepared PNIPAm/rGO underwent large volume reductions in response to irradiation by visible light of modest intensity. With respect to potential applications, bilayer-type photo-actuators comprising a PNIPAm/rGO active layer and poly(acrylamide) passive layer were fabricated; these achieved a full bending motion upon visible-light exposure. Adjusting the swelling ratio of each layer in the initial state yielded bidirectional photo-actuators that showed the active motion of turning inside out. Furthermore, we demonstrated that the fabricated actuation system would exhibit controlled bending motion in response to solar radiation. PMID:26865239

  11. Bilayer graphene under pressure: Electron-hole symmetry breaking, valley Hall effect, and Landau levels

    NASA Astrophysics Data System (ADS)

    Munoz, F.; Collado, H. P. Ojeda; Usaj, Gonzalo; Sofo, Jorge O.; Balseiro, C. A.

    2016-06-01

    The electronic structure of bilayer graphene under pressure develops very interesting features with an enhancement of the trigonal warping and a splitting of the parabolic touching bands at the K point of the reciprocal space into four Dirac cones, one at K and three along the T symmetry lines. As pressure is increased, these cones separate in reciprocal space and in energy, breaking the electron-hole symmetry. Due to their energy separation, their opposite Berry curvature can be observed in valley Hall effect experiments and in the structure of the Landau levels. Based on the electronic structure obtained by density functional theory, we develop a low energy Hamiltonian that describes the effects of pressure on measurable quantities such as the Hall conductivity and the Landau levels of the system.

  12. Bilayer graphene formed by passage of current through graphite: evidence for a three-dimensional structure

    NASA Astrophysics Data System (ADS)

    Harris, Peter J. F.; Slater, Thomas J. A.; Haigh, Sarah J.; Hage, Fredrik S.; Kepaptsoglou, Despoina M.; Ramasse, Quentin M.; Brydson, Rik

    2014-11-01

    The passage of an electric current through graphite or few-layer graphene can result in a striking structural transformation, but there is disagreement about the precise nature of this process. Some workers have interpreted the phenomenon in terms of the sublimation and edge reconstruction of essentially flat graphitic structures. An alternative explanation is that the transformation actually involves a change from a flat to a three-dimensional structure. Here we describe detailed studies of carbon produced by the passage of a current through graphite which provide strong evidence that the transformed carbon is indeed three-dimensional. The evidence comes primarily from images obtained in the scanning transmission electron microscope using the technique of high-angle annular dark-field imaging, and from a detailed analysis of electron energy loss spectra. We discuss the possible mechanism of the transformation, and consider potential applications of ‘three-dimensional bilayer graphene’.

  13. Impurity effects on electrical conductivity of doped bilayer graphene in the presence of a bias voltage

    NASA Astrophysics Data System (ADS)

    E, Lotfi; H, Rezania; B, Arghavaninia; M, Yarmohammadi

    2016-07-01

    We address the electrical conductivity of bilayer graphene as a function of temperature, impurity concentration, and scattering strength in the presence of a finite bias voltage at finite doping, beginning with a description of the tight-binding model using the linear response theory and Green’s function approach. Our results show a linear behavior at high doping for the case of high bias voltage. The effects of electron doping on the electrical conductivity have been studied via changing the electronic chemical potential. We also discuss and analyze how the bias voltage affects the temperature behavior of the electrical conductivity. Finally, we study the behavior of the electrical conductivity as a function of the impurity concentration and scattering strength for different bias voltages and chemical potentials respectively. The electrical conductivity is found to be monotonically decreasing with impurity scattering strength due to the increased scattering among electrons at higher impurity scattering strength.

  14. Highly bendable bilayer-type photo-actuators comprising of reduced graphene oxide dispersed in hydrogels

    NASA Astrophysics Data System (ADS)

    Kim, Dowan; Lee, Heon Sang; Yoon, Jinhwan

    2016-02-01

    To avoid the problem of reduced graphene oxide (rGO) restacking in aqueous solution, the preparation of light-responsive poly(N-isopropylacrylamide) incorporating rGO (PNIPAm/rGO) was achieved by the chemical reduction of GO dispersed in the hydrogel matrix. Due to the enhanced photothermal efficiency of the rGO, the prepared PNIPAm/rGO underwent large volume reductions in response to irradiation by visible light of modest intensity. With respect to potential applications, bilayer-type photo-actuators comprising a PNIPAm/rGO active layer and poly(acrylamide) passive layer were fabricated; these achieved a full bending motion upon visible-light exposure. Adjusting the swelling ratio of each layer in the initial state yielded bidirectional photo-actuators that showed the active motion of turning inside out. Furthermore, we demonstrated that the fabricated actuation system would exhibit controlled bending motion in response to solar radiation.

  15. Reply to "Comment on `Creating in-plane pseudomagnetic fields in excess of 1000 T by misoriented stacking in a graphene bilayer' "

    NASA Astrophysics Data System (ADS)

    He, Wen-Yu; Su, Ying; Yang, Mudan; He, Lin

    2016-06-01

    In the Comment, the authors argue that due to the different symmetry properties, the comparison between the twisting and in-plane magnetic field in bilayer graphene fails. However, we do not think it addresses the main result of our work. Here from the perspective of tunable chirality and quantum transmission signature, we review the feature shared by twisting and in-plane magnetic field in bilayer graphene and show that the Comment does not contradict our main results.

  16. Indirect-direct band gap transition through electric tuning in bilayer MoS{sub 2}

    SciTech Connect

    Zhang, Z. Y.; Si, M. S. Wang, Y. H.; Gao, X. P.; Sung, Dongchul; Hong, Suklyun; He, Junjie

    2014-05-07

    We investigate the electronic properties of bilayer MoS{sub 2} exposed to an external electric field by using first-principles calculations. It is found that a larger interlayer distance, referring to that by standard density functional theory (DFT) with respect to that by DFT with empirical dispersion corrections, makes indirect-direct band gap transition possible by electric control. We show that external electric field effectively manipulates the valence band contrast between the K- and Γ-valleys by forming built-in electric dipole fields, which realizes an indirect-direct transition before a semiconductor-metal transition happens. Our results provide a novel efficient access to tune the electronic properties of two-dimensional layered materials.

  17. Interaction-Induced Dirac Fermions from Quadratic Band Touching in Bilayer Graphene.

    PubMed

    Pujari, Sumiran; Lang, Thomas C; Murthy, Ganpathy; Kaul, Ribhu K

    2016-08-19

    We revisit the effect of local interactions on the quadratic band touching (QBT) of the Bernal honeycomb bilayer model using renormalization group (RG) arguments and quantum Monte Carlo (QMC) simulations. We present a RG argument which predicts, contrary to previous studies, that weak interactions do not flow to strong coupling even if the free dispersion has a QBT. Instead, they generate a linear term in the dispersion, which causes the interactions to flow back to weak coupling. Consistent with this RG scenario, in unbiased QMC simulations of the Hubbard model we find compelling evidence that antiferromagnetism turns on at a finite U/t despite the U=0 hopping problem having a QBT. The onset of antiferromagnetism takes place at a continuous transition which is consistent with (2+1)D Gross-Neveu criticality. We conclude that generically in models of bilayer graphene, even if the free dispersion has a QBT, small local interactions generate a Dirac phase with no symmetry breaking and that there is a finite-coupling transition out of this phase to a symmetry-broken state. PMID:27588872

  18. Interactions of Graphene Oxide with Model Cell Membranes: Probing Nanoparticle Attachment and Lipid Bilayer Disruption.

    PubMed

    Liu, Xitong; Chen, Kai Loon

    2015-11-10

    With the rapid growth in the application of graphene oxide (GO) in diverse fields, the toxicity of GO toward bacterial and mammalian cells has recently attracted extensive research attention. While several mechanisms have been proposed for the cytotoxicity of GO, the attachment of GO to cell membranes is expected to be the key initial process that precedes these mechanisms. In this study, we investigate the propensity for GO to attach to and disrupt model cell membranes using supported lipid bilayers (SLBs) and supported vesicular layers (SVLs) that are composed of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The deposition kinetics of GO on SLBs were determined using quartz crystal microbalance with dissipation monitoring and were observed to increase with increasing electrolyte (NaCl and CaCl2) concentrations, indicating that GO attachment to SLBs was controlled by electrostatic interactions. The GO deposition kinetics measured at elevated electrolyte concentrations were lower than mass-transfer-limited kinetics, likely due to the presence of hydration forces between GO and SLBs. Upon the attachment of GO to supported vesicles that were encapsulated with a fluorescent dye, dye leakage was detected, thus indicating that the lipid vesicles were disrupted. When the exposure of the SVL to the GO suspension was terminated, the leakage of dye decreased significantly, demonstrating that the pores on the lipid bilayers have a self-healing ability. PMID:26466194

  19. Interaction-Induced Dirac Fermions from Quadratic Band Touching in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Pujari, Sumiran; Lang, Thomas C.; Murthy, Ganpathy; Kaul, Ribhu K.

    2016-08-01

    We revisit the effect of local interactions on the quadratic band touching (QBT) of the Bernal honeycomb bilayer model using renormalization group (RG) arguments and quantum Monte Carlo (QMC) simulations. We present a RG argument which predicts, contrary to previous studies, that weak interactions do not flow to strong coupling even if the free dispersion has a QBT. Instead, they generate a linear term in the dispersion, which causes the interactions to flow back to weak coupling. Consistent with this RG scenario, in unbiased QMC simulations of the Hubbard model we find compelling evidence that antiferromagnetism turns on at a finite U /t despite the U =0 hopping problem having a QBT. The onset of antiferromagnetism takes place at a continuous transition which is consistent with (2 +1 )D Gross-Neveu criticality. We conclude that generically in models of bilayer graphene, even if the free dispersion has a QBT, small local interactions generate a Dirac phase with no symmetry breaking and that there is a finite-coupling transition out of this phase to a symmetry-broken state.

  20. Energy gaps of atomically precise armchair graphene sidewall nanoribbons

    NASA Astrophysics Data System (ADS)

    Wang, Wen-Xiao; Zhou, Mei; Li, Xinqi; Li, Si-Yu; Wu, Xiaosong; Duan, Wenhui; He, Lin

    2016-06-01

    Theoretically, it has been demonstrated that armchair Graphene nanoribbons (GNRs) can be divided into three families, i.e., Na=3 p ,Na=3 p +1 , and Na=3 p +2 (here Na is the number of dimer lines across the ribbon width and p is an integer), according to their electronic structures, and the energy gaps for the three families are quite different even with the same p . However, a systematic experimental verification of this fundamental prediction is still lacking, owing to very limited atomic-level control of the width of the armchair GNRs investigated. Here, we studied electronic structures of the armchair GNRs with atomically well-defined widths ranging from Na=6 to Na=26 by using a scanning tunneling microscope. Our result demonstrated explicitly that all the studied armchair GNRs exhibit semiconducting gaps and, more importantly, the observed gaps as a function of Na are well grouped into the three categories, as predicted by density-functional theory calculations. Such a result indicated that the electronic properties of the armchair GNRs can be tuned dramatically by simply adding or cutting one carbon dimer line along the ribbon width.

  1. Building Large-Domain Twisted Bilayer Graphene with van Hove Singularity.

    PubMed

    Tan, Zhenjun; Yin, Jianbo; Chen, Cheng; Wang, Huan; Lin, Li; Sun, Luzhao; Wu, Jinxiong; Sun, Xiao; Yang, Haifeng; Chen, Yulin; Peng, Hailin; Liu, Zhongfan

    2016-07-26

    Twisted bilayer graphene (tBLG) with van Hove Singularity (VHS) has exhibited novel twist-angle-dependent chemical and physical phenomena. However, scalable production of high-quality tBLG is still in its infancy, especially lacking the angle controlled preparation methods. Here, we report a facile approach to prepare tBLG with large domain sizes (>100 μm) and controlled twist angles by a clean layer-by-layer transfer of two constituent graphene monolayers. The whole process without interfacial polymer contamination in two monolayers guarantees the interlayer interaction of the π-bond electrons, which gives rise to the existence of minigaps in electronic structures and the consequent formation of VHSs in density of state. Such perturbation on band structure was directly observed by angle-resolved photoemission spectroscopy with submicrometer spatial resolution (micro-ARPES). The VHSs lead to a strong light-matter interaction and thus introduce ∼20-fold enhanced intensity of Raman G-band, which is a characteristic of high-quality tBLG. The as-prepared tBLG with strong light-matter interaction was further fabricated into high-performance photodetectors with selectively enhanced photocurrent generation (up to ∼6 times compared with monolayer in our device).

  2. Enhancement of electron–phonon coupling in Cs-overlayered intercalated bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kleeman, J.; Sugawara, K.; Sato, T.; Takahashi, T.

    2016-05-01

    We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on cesium (Cs) intercalated bilayer graphene with a Cs overlayer (Cs-C8CsC8). Low-energy electron diffraction shows a (2  ×  2) pattern consistent with intercalation of a Cs layer similar to bulk C8Cs, in addition to the signature of a nearly commensurate superstructure created by the Cs overlayer. ARPES results reveal folding of the π bands due to the periodic (2  ×  2) potential of the intercalated Cs atoms, together with a free-electron-like state at the Γ point. Significant mass renormalization is observed in the band dispersion near the Fermi level, indicative of strong electron–phonon coupling. Based on analysis of the self-energy, we find anisotropic electron–phonon coupling with an estimated strength of λ =0.38   ±  0.02 in the K-Γ direction, and λ =0.60+/- 0.02 in the K-M direction. This coupling is much larger than that of other doped graphenes, and comparable to superconducting bulk GICs. We attribute this large electron–phonon coupling constant to the presence of the Cs overlayer, which highly dopes {π\\ast} bands, and creates a structure similar to stage-I graphite intercalation compounds.

  3. Enhancement of electron-phonon coupling in Cs-overlayered intercalated bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kleeman, J.; Sugawara, K.; Sato, T.; Takahashi, T.

    2016-05-01

    We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on cesium (Cs) intercalated bilayer graphene with a Cs overlayer (Cs-C8CsC8). Low-energy electron diffraction shows a (2  ×  2) pattern consistent with intercalation of a Cs layer similar to bulk C8Cs, in addition to the signature of a nearly commensurate superstructure created by the Cs overlayer. ARPES results reveal folding of the π bands due to the periodic (2  ×  2) potential of the intercalated Cs atoms, together with a free-electron-like state at the Γ point. Significant mass renormalization is observed in the band dispersion near the Fermi level, indicative of strong electron-phonon coupling. Based on analysis of the self-energy, we find anisotropic electron-phonon coupling with an estimated strength of λ =0.38   ±  0.02 in the K-Γ direction, and λ =0.60+/- 0.02 in the K-M direction. This coupling is much larger than that of other doped graphenes, and comparable to superconducting bulk GICs. We attribute this large electron-phonon coupling constant to the presence of the Cs overlayer, which highly dopes {π\\ast} bands, and creates a structure similar to stage-I graphite intercalation compounds.

  4. Nonlinear dynamics of bi-layered graphene sheet, double-walled carbon nanotube and nanotube bundle

    NASA Astrophysics Data System (ADS)

    Gajbhiye, Sachin O.; Singh, S. P.

    2016-05-01

    Due to strong van der Waals (vdW) interactions, the graphene sheets and nanotubes stick to each other and form clusters of these corresponding nanostructures, viz. bi-layered graphene sheet (BLGS), double-walled carbon nanotube (DWCNT) and nanotube bundle (NB) or ropes. This research work is concerned with the study of nonlinear dynamics of BLGS, DWCNT and NB due to nonlinear interlayer vdW forces using multiscale atomistic finite element method. The energy between two adjacent carbon atoms is represented by the multibody interatomic Tersoff-Brenner potential, whereas the nonlinear interlayer vdW forces are represented by Lennard-Jones 6-12 potential function. The equivalent nonlinear material model of carbon-carbon bond is used to model it based on its force-deflection relation. Newmark's algorithm is used to solve the nonlinear matrix equation governing the motion of the BLGS, DWCNT and NB. An impulse and harmonic excitations are used to excite these nanostructures under cantilevered, bridged and clamped boundary conditions. The frequency responses of these nanostructures are computed, and the dominant resonant frequencies are identified. Along with the forced vibration of these structures, the eigenvalue extraction problem of armchair and zigzag NB is also considered. The natural frequencies and corresponding mode shapes are extracted for the different length and boundary conditions of the nanotube bundle.

  5. Gap opening due to topological defects in graphene

    NASA Astrophysics Data System (ADS)

    Nunes, Ricardo; Araújo, Joice; Chacham, Helio

    2009-03-01

    Stone-Wales defects (SWD = two adjacent pentagon-heptagon pairs) are common low-energy defects in carbon nanotubes. Previously, Crespi et al.[PRB, 53, 1996] have proposed a purely-carbon covalent metal sheet called ``pentaheptite,'' consisting entirely of SWDs, with a relatively low formation energy of 0.32 eV/atom, with respect to graphene. In this work, we consider three different families of periodic carbon sheets containing topological defects (TDs = pentagons and heptagons). The families differ by the density of TDs in a seed structure. In each family, we generate periodic structures in which isolated pentagons and heptagons are surrounded only by hexagons. By means of ab initio calculations, we propose that, depending on the density and distribution of TDs, these carbon sheets may behave as a semiconductor, a metal or a semimetal. In the range of TD concentrations we examine, the sheets are stable in a planar form, but, allowing for the corrugation generated by the curvature fields associated with the isolated TDs, leads to lower formation energies and to either a reduction of the density of states or to gap opening at the Fermi level. Formation energies can be very small: in particular, we obtain a semiconducting structure with a formation energy of only 92 meV/atom with respect to graphene.

  6. A valley and spin filter based on gapped graphene

    NASA Astrophysics Data System (ADS)

    Wang, Jing; Long, Mengqiu; Zhao, Wen-Sheng; Hu, Yue; Wang, Gaofeng; Chan, K. S.

    2016-07-01

    We study highly valley- and spin-polarized current in single-layer gapped graphene without spin–orbit coupling. The structure considered is a three-barrier structure with one spin-splitting barrier and two electrical potential barriers with vector potentials. The electrons in the two valleys transmit differently because of the valley-dependent reflection between two adjacent barriers, while the spin-up and spin-down electrons transmit differently because of the spin splitting. The structure is different from other structures in which spin–orbit coupling plays an important role in the observation of valley- and spin-polarized current. We can control the spin and valley polarization by changing the width of the barrier or the strength of the spin splitting. The structure proposed in this paper can be used to make valley and spin devices.

  7. Excitation gap of fractal quantum hall states in graphene

    NASA Astrophysics Data System (ADS)

    Luo, Wenchen; Chakraborty, Tapash

    2016-01-01

    In the presence of a magnetic field and an external periodic potential the Landau level spectrum of a two-dimensional electron gas exhibits a fractal pattern in the energy spectrum which is described as the Hofstadter’s butterfly. In this work, we develop a Hartree-Fock theory to deal with the electron-electron interaction in the Hofstadter’s butterfly state in a finite-size graphene with periodic boundary conditions, where we include both spin and valley degrees of freedom. We then treat the butterfly state as an electron crystal so that we could obtain the order parameters of the crystal in the momentum space and also in an infinite sample. A phase transition between the liquid phase and the fractal crystal phase can be observed. The excitation gaps obtained in the infinite sample is comparable to those in the finite-size study, and agree with a recent experimental observation.

  8. Giant topological insulator gap in graphene with 5d adatoms.

    PubMed

    Hu, Jun; Alicea, Jason; Wu, Ruqian; Franz, Marcel

    2012-12-28

    Two-dimensional topological insulators (2D TIs) have been proposed as platforms for many intriguing applications, ranging from spintronics to topological quantum information processing. Realizing this potential will likely be facilitated by the discovery of new, easily manufactured materials in this class. With this goal in mind, we introduce a new framework for engineering a 2D TI by hybridizing graphene with impurity bands arising from heavy adatoms possessing partially filled d shells, in particular, osmium and iridium. First-principles calculations predict that the gaps generated by this means exceed 0.2 eV over a broad range of adatom coverage; moreover, tuning of the Fermi level is not required to enter the TI state. The mechanism at work is expected to be rather general and may open the door to designing new TI phases in many materials.

  9. Zero-energy modes and valley asymmetry in the Hofstadter spectrum of bilayer graphene van der Waals heterostructures with hBN

    NASA Astrophysics Data System (ADS)

    Chen, Xi; Wallbank, J. R.; Mucha-Kruczyński, M.; McCann, E.; Fal'ko, V. I.

    2016-07-01

    We investigate the magnetic minibands of a heterostructure consisting of bilayer graphene (BLG) and hexagonal boron nitride (hBN) by numerically diagonalizing a two-band Hamiltonian that describes electrons in BLG in the presence of a moiré potential. Due to inversion-symmetry breaking characteristic for the moiré potential, the valley symmetry of the spectrum is broken, but despite this, the zero-energy Landau level in BLG survives, albeit with reduced degeneracy. In addition, we derive effective models for the low-energy features in the magnetic minibands and demonstrate the appearance of secondary Dirac points in the valence band, which we confirm by numerical simulations. Then, we analyze how single-particle gaps in the fractal energy spectrum produce a sequence of incompressible states observable under a variation of carrier density and magnetic field.

  10. NanoARPES of twisted bilayer graphene on SiC: absence of velocity renormalization for small angles.

    PubMed

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

    2016-01-01

    The structural and electronic properties of twisted bilayer graphene (TBG) on SiC(000) grown by Si flux-assisted molecular beam epitaxy were investigated using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy with nanometric spatial resolution. STM images revealed a wide distribution of twist angles between the two graphene layers. The electronic structure recorded in single TBG grains showed two closely-spaced Dirac π bands associated to the two stacked layers with respective twist angles in the range 1-3°. The renormalization of velocity predicted in previous theoretical calculations for small twist angles was not observed. PMID:27264791

  11. NanoARPES of twisted bilayer graphene on SiC: absence of velocity renormalization for small angles

    PubMed Central

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

    2016-01-01

    The structural and electronic properties of twisted bilayer graphene (TBG) on SiC(000) grown by Si flux-assisted molecular beam epitaxy were investigated using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy with nanometric spatial resolution. STM images revealed a wide distribution of twist angles between the two graphene layers. The electronic structure recorded in single TBG grains showed two closely-spaced Dirac π bands associated to the two stacked layers with respective twist angles in the range 1–3°. The renormalization of velocity predicted in previous theoretical calculations for small twist angles was not observed. PMID:27264791

  12. Monte Carlo simulation of magnetic properties of a nano-graphene bilayer in a longitudinal magnetic field

    NASA Astrophysics Data System (ADS)

    Wang, Wei; Liu, Ruijia; Lv, Dan; Luo, Xiaohong

    2016-10-01

    Monte Carlo simulation has been used to study the magnetic properties of a nano-graphene bilayer which consists of the upper layer A with spin-3/2 and the bottom layer B with spin-5/2. The effects of the single-ion anisotropy, the intralayer exchange coupling and the longitudinal magnetic field on the magnetization, the susceptibility, the blocking temperature and hysteresis loops of the mixed-spin nano-graphene bilayer system have been examined detailedly. In particular, the variations of the blocking temperature with different intralayer exchange couplings, single-ion anisotropies, and the longitudinal magnetic field are obtained for the present system. Many multiple hysteresis loop behaviors have also been found, depending on the combinations of both the upper and bottom layer magnetizations in the longitudinal magnetic field. Through a comparison, our results obtained are according well with other theoretical researches and experimental results.

  13. Persistent valley currents and topological transport in gapped graphene

    NASA Astrophysics Data System (ADS)

    Levitov, Leonid

    2015-03-01

    The anomalous Hall effect (AHE), arising due to Berry curvature in materials with broken inversion symmetry, results in topological currents flowing in system bulk transversely to the applied electric field. We will discuss recent work on AHE in materials with several valleys, such as e.g. graphene and transition metal dichalcogenide monolayers, where these currents have been observed [Mak et al., Science 344, 1489 (2014); Gorbachev et al., Science 346, 448 (2014)]. Interestingly, these materials do not fit the paradigm of topological materials with Chern bands and associated topologically protected edge modes dominating (quantized) Hall conductivity. Here, in contrast, gapless edge states may be absent since they are not enforced by topology or symmetry. Further, even when present, these states are not protected against backscattering due to roughness on the atomic scale. Naively, this would lead one to conclude that topological currents cease to exist. If true, this would imply that the key manifestations, such as the valley Hall conductivity and orbital magnetization, vanish in the gapped state. We will argue that the opposite is true: the absence of conducting edge modes does not present an obstacle since valley currents can be transmitted by the bulk states in the filled Fermi sea beneath the gap. This leads to an interesting behavior: rather than being vanishingly small, valley currents reach maximum value in the gapped state. Such undergap currents can also occur as persistent currents in the thermodynamic ground state and dominate orbital magnetization in valley-polarized gapped systems. We will conclude with discussing requirements for dissipationless valley transport and argue that they can be met under realistic conditions. Based on the work done in collaboration with Yuri Lensky, Polnop Samutpraphoot and Justin Song.

  14. Band gap engineering of chemical vapor deposited graphene by in situ BN doping.

    PubMed

    Chang, Cheng-Kai; Kataria, Satender; Kuo, Chun-Chiang; Ganguly, Abhijit; Wang, Bo-Yao; Hwang, Jeong-Yuan; Huang, Kay-Jay; Yang, Wei-Hsun; Wang, Sheng-Bo; Chuang, Cheng-Hao; Chen, Mi; Huang, Ching-I; Pong, Way-Faung; Song, Ker-Jar; Chang, Shoou-Jinn; Guo, Jing-Hua; Tai, Yian; Tsujimoto, Masahiko; Isoda, Seiji; Chen, Chun-Wei; Chen, Li-Chyong; Chen, Kuei-Hsien

    2013-02-26

    Band gap opening and engineering is one of the high priority goals in the development of graphene electronics. Here, we report on the opening and scaling of band gap in BN doped graphene (BNG) films grown by low-pressure chemical vapor deposition method. High resolution transmission electron microscopy is employed to resolve the graphene and h-BN domain formation in great detail. X-ray photoelectron, micro-Raman, and UV-vis spectroscopy studies revealed a distinct structural and phase evolution in BNG films at low BN concentration. Synchrotron radiation based XAS-XES measurements concluded a gap opening in BNG films, which is also confirmed by field effect transistor measurements. For the first time, a significant band gap as high as 600 meV is observed for low BN concentrations and is attributed to the opening of the π-π* band gap of graphene due to isoelectronic BN doping. As-grown films exhibit structural evolution from homogeneously dispersed small BN clusters to large sized BN domains with embedded diminutive graphene domains. The evolution is described in terms of competitive growth among h-BN and graphene domains with increasing BN concentration. The present results pave way for the development of band gap engineered BN doped graphene-based devices.

  15. Local strain effect on the band gap engineering of graphene by a first-principles study

    SciTech Connect

    Gui, Gui; Booske, John; Ma, Zhenqiang E-mail: mazq@engr.wisc.edu; Morgan, Dane; Zhong, Jianxin E-mail: mazq@engr.wisc.edu

    2015-02-02

    We have systematically investigated the effect of local strain on electronic properties of graphene by first-principles calculations. Two major types of local strain, oriented along the zigzag and the armchair directions, have been studied. We find that local strain with a proper range and strength along the zigzag direction results in opening of significant band gaps in graphene, on the order of 10{sup −1 }eV; whereas, local strain along the armchair direction cannot open a significant band gap in graphene. Our results show that appropriate local strain can effectively open and tune the band gap in graphene; therefore, the electronic and transport properties of graphene can also be modified.

  16. Large-Scale Synthesis of a Uniform Film of Bilayer MoS2 on Graphene for 2D Heterostructure Phototransistors.

    PubMed

    Chen, Chuanmeng; Feng, Zhihong; Feng, Yiyu; Yue, Yuchen; Qin, Chengqun; Zhang, Daihua; Feng, Wei

    2016-07-27

    The large-scale synthesis of atomically thin, layered MoS2/graphene heterostructures is of great interest in optoelectronic devices because of their unique properties. Herein, we present a scalable synthesis method to prepare centimeter-scale, continuous, and uniform films of bilayer MoS2 using low-pressure chemical vapor deposition. This growth process was utilized to assemble a heterostructure by growing large-scale uniform films of bilayer MoS2 on graphene (G-MoS2/graphene). Atomic force microscopy, Raman spectra, and transmission electron microscopy characterization demonstrated that the large-scale bilayer MoS2 film on graphene exhibited good thickness uniformity and a polycrystalline nature. A centimeter-scale phototransistor prepared using the G-MoS2/graphene heterostructure exhibited a high responsivity of 32 mA/W with good cycling stability; this value is 1 order of magnitude higher than that of transferred MoS2 on graphene (2.5 mA/W). This feature results from efficient charge transfer at the interface enabled by intimate contact between the grown bilayer MoS2 (G-MoS2) and graphene. The ability to integrate multilayer materials into atomically thin heterostructures paves the way for fabricating multifunctional devices by controlling their layer structure.

  17. Large-Scale Synthesis of a Uniform Film of Bilayer MoS2 on Graphene for 2D Heterostructure Phototransistors.

    PubMed

    Chen, Chuanmeng; Feng, Zhihong; Feng, Yiyu; Yue, Yuchen; Qin, Chengqun; Zhang, Daihua; Feng, Wei

    2016-07-27

    The large-scale synthesis of atomically thin, layered MoS2/graphene heterostructures is of great interest in optoelectronic devices because of their unique properties. Herein, we present a scalable synthesis method to prepare centimeter-scale, continuous, and uniform films of bilayer MoS2 using low-pressure chemical vapor deposition. This growth process was utilized to assemble a heterostructure by growing large-scale uniform films of bilayer MoS2 on graphene (G-MoS2/graphene). Atomic force microscopy, Raman spectra, and transmission electron microscopy characterization demonstrated that the large-scale bilayer MoS2 film on graphene exhibited good thickness uniformity and a polycrystalline nature. A centimeter-scale phototransistor prepared using the G-MoS2/graphene heterostructure exhibited a high responsivity of 32 mA/W with good cycling stability; this value is 1 order of magnitude higher than that of transferred MoS2 on graphene (2.5 mA/W). This feature results from efficient charge transfer at the interface enabled by intimate contact between the grown bilayer MoS2 (G-MoS2) and graphene. The ability to integrate multilayer materials into atomically thin heterostructures paves the way for fabricating multifunctional devices by controlling their layer structure. PMID:27381011

  18. Band gap engineering for graphene by using Na{sup +} ions

    SciTech Connect

    Sung, S. J.; Lee, P. R.; Kim, J. G.; Ryu, M. T.; Park, H. M.; Chung, J. W.

    2014-08-25

    Despite the noble electronic properties of graphene, its industrial application has been hindered mainly by the absence of a stable means of producing a band gap at the Dirac point (DP). We report a new route to open a band gap (E{sub g}) at DP in a controlled way by depositing positively charged Na{sup +} ions on single layer graphene formed on 6H-SiC(0001) surface. The doping of low energy Na{sup +} ions is found to deplete the π* band of graphene above the DP, and simultaneously shift the DP downward away from Fermi energy indicating the opening of E{sub g}. The band gap increases with increasing Na{sup +} coverage with a maximum E{sub g}≥0.70 eV. Our core-level data, C 1s, Na 2p, and Si 2p, consistently suggest that Na{sup +} ions do not intercalate through graphene, but produce a significant charge asymmetry among the carbon atoms of graphene to cause the opening of a band gap. We thus provide a reliable way of producing and tuning the band gap of graphene by using Na{sup +} ions, which may play a vital role in utilizing graphene in future nano-electronic devices.

  19. Reflectivity properties of graphene with a nonzero mass-gap parameter

    NASA Astrophysics Data System (ADS)

    Klimchitskaya, G. L.; Mostepanenko, V. M.

    2016-05-01

    The reflectivity properties of graphene with a nonzero mass-gap parameter are investigated in the framework of a Dirac model using the polarization tensor in (2 +1 ) -dimensional space-time. For this purpose, a more simple explicit representation for the polarization tensor along the real frequency axis is found. The approximate analytic expressions for the polarization tensor and for the reflectivities of graphene are obtained in different frequency regions at any temperature. We show that the nonzero mass-gap parameter has a profound effect on the reflectivity of graphene. Specifically, at zero temperature the reflectivity of gapped graphene goes to zero with vanishing frequency. At nonzero temperature the same reflectivities are equal to unity at zero frequency. We also find the resonance behavior of the reflectivities of gapped graphene at both zero and nonzero temperature at the border frequency determined by the width of the gap. At nonzero temperature the reflectivities of graphene drop to zero in the vicinity of some frequency smaller than the border frequency. Our analytic results are accompanied with numerical computations performed over a wide frequency region. The developed formalism can be used in devising nanoscale optical detectors and optoelectronic switches and in other optical applications of graphene.

  20. Wide-band-gap wrinkled nanoribbon-like structures in a continuous metallic graphene sheet

    NASA Astrophysics Data System (ADS)

    Li, Si-Yu; Zhou, Mei; Qiao, Jia-Bin; Duan, Wenhui; He, Lin

    2016-08-01

    To generate a moderate band gap in a graphene monolayer is a very important but rather difficult task. A rare working solution of this problem is to cut it into one-dimensional (1D) nanometer-wide ribbons. Here we show that, instead of cutting the graphene monolayer, a wide band gap can be created in a unique 1D strained structure, i.e., a wrinkled graphene-nanoribbon-like (GNR-like) structure, of a continuous graphene sheet via strong hybridization between the graphene and the metal substrate. The wrinkled GNR-like structures with widths of only a few nanometers are observed in a continuous graphene sheet grown on a Rh foil by using thermal strain engineering. Spatially resolved scanning tunneling spectroscopy revealed a band-gap opening of a few hundred meV in the GNR-like structure in an otherwise continuous metallic graphene sheet, directly demonstrating the realization of a metallic-semiconducting-metallic junction entirely in a graphene monolayer.

  1. Strong plasmon reflection at nanometer-size gaps in monolayer graphene on SiC.

    PubMed

    Chen, Jianing; Nesterov, Maxim L; Nikitin, Alexey Yu; Thongrattanasiri, Sukosin; Alonso-González, Pablo; Slipchenko, Tetiana M; Speck, Florian; Ostler, Markus; Seyller, Thomas; Crassee, Iris; Koppens, Frank H L; Martin-Moreno, Luis; García de Abajo, F Javier; Kuzmenko, Alexey B; Hillenbrand, Rainer

    2013-01-01

    We employ tip-enhanced infrared near-field microscopy to study the plasmonic properties of epitaxial quasi-free-standing monolayer graphene on silicon carbide. The near-field images reveal propagating graphene plasmons, as well as a strong plasmon reflection at gaps in the graphene layer, which appear at the steps between the SiC terraces. When the step height is around 1.5 nm, which is two orders of magnitude smaller than the plasmon wavelength, the reflection signal reaches 20% of its value at graphene edges, and it approaches 50% for step heights as small as 5 nm. This intriguing observation is corroborated by numerical simulations and explained by the accumulation of a line charge at the graphene termination. The associated electromagnetic fields at the graphene termination decay within a few nanometers, thus preventing efficient plasmon transmission across nanoscale gaps. Our work suggests that plasmon propagation in graphene-based circuits can be tailored using extremely compact nanostructures, such as ultranarrow gaps. It also demonstrates that tip-enhanced near-field microscopy is a powerful contactless tool to examine nanoscale defects in graphene.

  2. Tunable and Sizable Band Gap of Single Layer Graphene Sandwiched between Hexagonal Boron Nitride

    NASA Astrophysics Data System (ADS)

    Zheng, Jiaxin; Qu, Heruge; Liu, Qihang; Qin, Rui; Zhou, Jing; Yu, Dapeng; Gao, Zhengxiang; Lu, Jing; Luo, Guangfu; Nagase, Shigeru; Mei, Wai-Ning

    2012-02-01

    It is a big challenge to open a tunable and sizable band gap of single layer graphene without big loss in structural integrity and carrier mobility. By using density functional theory calculations, we show that the band gap of single layer graphene can be opened to 0.16 (without electrical field) and 0.34 eV (with a strong electrical field) when sandwiched between two hexagonal boron nitride single layers in a proper way. The zero-field band gaps are increased by about 50% when many-body effects are included. Ab initio quantum transport simulation of a dual-gated FET out of such a sandwich structure further confirms an electrical field-enhanced transport gap. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance single layer graphene field effect transistors.

  3. Room temperature detection of individual molecular physisorption using suspended bilayer graphene.

    PubMed

    Sun, Jian; Muruganathan, Manoharan; Mizuta, Hiroshi

    2016-04-01

    Detection of individual molecular adsorption, which represents the ultimate resolution of gas sensing, has rarely been realized with solid-state devices. So far, only a few studies have reported detection of individual adsorption by measuring the variation of electronic transport stemming from the charge transfer of adsorbate. We report room-temperature detection of the individual physisorption of carbon dioxide molecules with suspended bilayer graphene (BLG) based on a different mechanism. An electric field introduced by applying back-gate voltage is used to effectively enhance the adsorption rate. A unique device architecture is designed to induce tensile strain in the BLG to prevent its mechanical deflection onto the substrate by electrostatic force. Despite the negligible charge transfer from a single physisorbed molecule, it strongly affects the electronic transport in suspended BLG by inducing charged impurity, which can shut down part of the conduction of the BLG with Coulomb impurity scattering. Accordingly, we can detect each individual physisorption as a step-like resistance change with a quantized value in the BLG. We use density functional theory simulation to theoretically estimate the possible resistance response caused by Coulomb scattering of one adsorbed CO2 molecule, which is in agreement with our measurement. PMID:27152344

  4. Quantum-enhanced tunable second-order optical nonlinearity in bilayer graphene.

    PubMed

    Wu, Sanfeng; Mao, Li; Jones, Aaron M; Yao, Wang; Zhang, Chuanwei; Xu, Xiaodong

    2012-04-11

    Second order optical nonlinear processes involve the coherent mixing of two electromagnetic waves to generate a new optical frequency, which plays a central role in a variety of applications, such as ultrafast laser systems, rectifiers, modulators, and optical imaging. However, progress is limited in the mid-infrared (MIR) region due to the lack of suitable nonlinear materials. It is desirable to develop a robust system with a strong, electrically tunable second order optical nonlinearity. Here, we demonstrate theoretically that AB-stacked bilayer graphene (BLG) can exhibit a giant and tunable second order nonlinear susceptibility χ((2)) once an in-plane electric field is applied. χ((2)) can be electrically tuned from 0 to ~10(5) pm/V, 3 orders of magnitude larger than the widely used nonlinear crystal AgGaSe(2). We show that the unusually large χ((2)) arise from two different quantum enhanced two-photon processes thanks to the unique electronic spectrum of BLG. The tunable electronic bandgap of BLG adds additional tunability on the resonance of χ((2)), which corresponds to a tunable wavelength ranging from ~2.6 to ~3.1 μm for the up-converted photon. Combined with the high electron mobility and optical transparency of the atomically thin BLG, our scheme suggests a new regime of nonlinear photonics based on BLG. PMID:22369519

  5. Monte Carlo study of strongly interacting degenerate fermions: A model for voltage-biased bilayer graphene

    NASA Astrophysics Data System (ADS)

    Armour, Wes; Hands, Simon; Strouthos, Costas

    2013-03-01

    We formulate a model of Nf=4 flavors of relativistic fermion in 2+1d in the presence of a chemical potential μ coupled to two flavor doublets with opposite sign, akin to isospin chemical potential in QCD. This is argued to be an effective theory for low energy electronic excitations in bilayer graphene, in which an applied voltage between the layers ensures equal populations of particles on one layer and holes on the other. The model is then reformulated on a spacetime lattice using staggered fermions, and in the absence of a sign problem, simulated using an orthodox hybrid Monte Carlo algorithm. With the coupling strength chosen to be close to a quantum critical point believed to exist for Nf

  6. Room temperature detection of individual molecular physisorption using suspended bilayer graphene

    PubMed Central

    Sun, Jian; Muruganathan, Manoharan; Mizuta, Hiroshi

    2016-01-01

    Detection of individual molecular adsorption, which represents the ultimate resolution of gas sensing, has rarely been realized with solid-state devices. So far, only a few studies have reported detection of individual adsorption by measuring the variation of electronic transport stemming from the charge transfer of adsorbate. We report room-temperature detection of the individual physisorption of carbon dioxide molecules with suspended bilayer graphene (BLG) based on a different mechanism. An electric field introduced by applying back-gate voltage is used to effectively enhance the adsorption rate. A unique device architecture is designed to induce tensile strain in the BLG to prevent its mechanical deflection onto the substrate by electrostatic force. Despite the negligible charge transfer from a single physisorbed molecule, it strongly affects the electronic transport in suspended BLG by inducing charged impurity, which can shut down part of the conduction of the BLG with Coulomb impurity scattering. Accordingly, we can detect each individual physisorption as a step-like resistance change with a quantized value in the BLG. We use density functional theory simulation to theoretically estimate the possible resistance response caused by Coulomb scattering of one adsorbed CO2 molecule, which is in agreement with our measurement. PMID:27152344

  7. Generation and detection of pure valley current by electrically induced Berry curvature in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Shimazaki, Y.; Yamamoto, M.; Borzenets, I. V.; Watanabe, K.; Taniguchi, T.; Tarucha, S.

    2015-12-01

    The field of `Valleytronics’ has recently been attracting growing interest as a promising concept for the next generation electronics, because non-dissipative pure valley currents with no accompanying net charge flow can be manipulated for computational use, akin to pure spin currents. Valley is a quantum number defined in an electronic system whose energy bands contain energetically degenerate but non-equivalent local minima (conduction band) or maxima (valence band) due to a certain crystal structure. Specifically, spatial inversion symmetry broken two-dimensional honeycomb lattice systems exhibiting Berry curvature is a subset of possible systems that enable optical, magnetic and electrical control of the valley degree of freedom. Here we use dual-gated bilayer graphene to electrically induce and control broken inversion symmetry (or Berry curvature) as well as the carrier density for generating and detecting the pure valley current. In the insulating regime, at zero-magnetic field, we observe a large nonlocal resistance that scales cubically with the local resistivity, which is evidence of pure valley current.

  8. Tuning electronic properties of bilayer Bernal graphene nanoribbon by magnetic modulation

    NASA Astrophysics Data System (ADS)

    Li, T. S.; Hsieh, C. T.; Chang, S. C.

    2014-12-01

    This study shows that the electronic properties of bilayer Bernal graphene nanoribbon can be effectively tuned by a spatially modulated magnetic field. The electronic structures are significantly influenced by varying the field strength or the period of the modulated field. In addition, there exists competition between the magnetic confinement effect and the quantum confinement effect. The density of states (DOS) exhibits many asymmetric square-root divergent peaks due to the oscillatory parabolic subbands. These peaks can be classified into primary and secondary ones. The number, height, and energy of the peaks depend sensitively on the strength and the period of the modulated field. The evolution of the DOS peak energy with the field strength and the period is explored. The energies of primary peaks first exhibit linear, then square-root dependence on the field strength. On the other hand, the field strength dependence of the secondary peaks is purely square root. These theoretical predictions can be validated by STS or magneto-transmission measurements.

  9. Band gap engineering for single-layer graphene by using slow Li(+) ions.

    PubMed

    Ryu, Mintae; Lee, Paengro; Kim, Jingul; Park, Heemin; Chung, Jinwook

    2016-08-01

    In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We report on the ion-induced modification of the electronic properties of single-layer graphene (SLG) grown on a SiC(0001) substrate by doping low-energy (5 eV) Li(+) ions. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li(+) ion dose as well as the following thermal treatment, and is the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band suggest that Li(+) ions do not intercalate below the topmost graphene layer, but cause a significant charge asymmetry between the carbon sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li(+) ions, which may play a pivotal role in the utilization of graphene in future graphene-based electronic nano-devices. PMID:27345294

  10. Band gap engineering for single-layer graphene by using slow Li+ ions

    NASA Astrophysics Data System (ADS)

    Ryu, Mintae; Lee, Paengro; Kim, Jingul; Park, Heemin; Chung, Jinwook

    2016-08-01

    In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We report on the ion-induced modification of the electronic properties of single-layer graphene (SLG) grown on a SiC(0001) substrate by doping low-energy (5 eV) Li+ ions. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li+ ion dose as well as the following thermal treatment, and is the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band suggest that Li+ ions do not intercalate below the topmost graphene layer, but cause a significant charge asymmetry between the carbon sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li+ ions, which may play a pivotal role in the utilization of graphene in future graphene-based electronic nano-devices.

  11. Characterization and physical modeling of MOS capacitors in epitaxial graphene monolayers and bilayers on 6H-SiC

    NASA Astrophysics Data System (ADS)

    Winters, M.; Sveinbjörnsson, E. Ö.; Melios, C.; Kazakova, O.; Strupiński, W.; Rorsman, N.

    2016-08-01

    Capacitance voltage (CV) measurements are performed on planar MOS capacitors with an Al2O3 dielectric fabricated in hydrogen intercalated monolayer and bilayer graphene grown on 6H-SiC as a function of frequency and temperature. Quantitative models of the CV data are presented in conjunction with the measurements in order to facilitate a physical understanding of graphene MOS systems. An interface state density of order 2 ṡ 1012 eV-1 cm-2 is found in both material systems. Surface potential fluctuations of order 80-90meV are also assessed in the context of measured data. In bilayer material, a narrow bandgap of 260meV is observed consequent to the spontaneous polarization in the substrate. Supporting measurements of material anisotropy and temperature dependent hysteresis are also presented in the context of the CV data and provide valuable insight into measured and modeled data. The methods outlined in this work should be applicable to most graphene MOS systems.

  12. Precise milling of nano-gap chains in graphene with a focused helium ion beam

    NASA Astrophysics Data System (ADS)

    Zhou, Yangbo; Maguire, Pierce; Jadwiszczak, Jakub; Muruganathan, Manoharan; Mizuta, Hiroshi; Zhang, Hongzhou

    2016-08-01

    A focused helium ion beam was used to introduce nano-sized gap chains in graphene. The effect of beam scanning strategies in the fabrication of the nano-gap chains was investigated. The tuning of graphene conductivity has been achieved by modulating the magnitude and uniformity of the ion dose and hence the morphology of the nano-gap chains. A model based on the site-specific and dose-dependent conductivity was built to understand the tuning of the conductivity, taking into account the nanoscale non-uniformity of irradiation.

  13. Gap opening in the zeroth Landau level in gapped graphene: pseudo-Zeeman splitting in an angular magnetic field.

    PubMed

    Tahir, M; Sabeeh, K

    2012-04-01

    We present a theoretical study of gap opening in the zeroth Landau level in gapped graphene as a result of pseudo-Zeeman interaction. The applied magnetic field couples with the valley pseudospin degree of freedom of the charge carriers leading to the pseudo-Zeeman interaction. To investigate its role in transport at the charge neutrality point (CNP), we study the integer quantum Hall effect in gapped graphene in an angular magnetic field in the presence of pseudo-Zeeman interaction. Analytical expressions are derived for the Hall conductivity using the Kubo-Greenwood formula. We also determine the longitudinal conductivity for elastic impurity scattering in the first Born approximation. We show that pseudo-Zeeman splitting leads to a minimum in the collisional conductivity at high magnetic fields and a zero plateau in the Hall conductivity. Evidence for activated transport at CNP is found from the temperature dependence of the collisional conductivity.

  14. Charge Inversion and Topological Phase Transition at a Twist Angle Induced van Hove Singularity of Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Kim, Youngwook; Herlinger, Patrick; Moon, Pilkyung; Koshino, Mikito; Taniguchi, Takashi; Watanabe, Kenji; Smet, Jurgen H.

    2016-08-01

    Van Hove singularities (VHS's) in the density of states play an outstanding and diverse role for the electronic and thermodynamic properties of crystalline solids. At the critical point the Fermi surface connectivity changes and topological properties undergo a transition. Opportunities to systematically pass a VHS at the turn of a voltage knob and study its diverse impact are however rare. With the advent of van der Waals heterostructures, control over the atomic registry of neigbouring graphene layers offers an unprecedented tool to generate a low energy VHS easily accessible with conventional gating. Here we have addressed magnetotransport when the chemical potential crosses the twist angle induced VHS in twisted bilayer graphene. A topological phase transition is experimentally disclosed in the abrupt conversion of electrons to holes or vice versa, a loss of a non-zero Berry phase and distinct sequences of integer quantum Hall states above and below the singularity.

  15. Resonant tunneling diode based on band gap engineered graphene antidot structures

    NASA Astrophysics Data System (ADS)

    Palla, Penchalaiah; Ethiraj, Anita S.; Raina, J. P.

    2016-04-01

    The present work demonstrates the operation and performance of double barrier Graphene Antidot Resonant Tunnel Diode (DBGA-RTD). Non-Equilibrium Green's Function (NEGF) frame work with tight-binding Hamiltonian and 2-D Poisson equations were solved self-consistently for device study. The interesting feature in this device is that it is an all graphene RTD with band gap engineered graphene antidot tunnel barriers. Another interesting new finding is that it shows negative differential resistance (NDR), which involves the resonant tunneling in the graphene quantum well through both the electron and hole bound states. The Graphene Antidot Lattice (GAL) barriers in this device efficiently improved the Peak to Valley Ratio to approximately 20 even at room temperature. A new fitting model is developed for the number of antidots and their corresponding effective barrier width, which will help in determining effective barrier width of any size of actual antidot geometry.

  16. Graphene on a metal surface with an h-BN buffer layer: gap opening and N-doping

    NASA Astrophysics Data System (ADS)

    Wang, Tao; Lu, Yunhao; Feng, Y. P.

    2016-04-01

    Graphene grown on a metal surface, Cu(111), with a boron-nitride (h-BN) buffer layer is studied. Our first-principles calculations reveal that charge is transferred from the copper substrate to graphene through the h-BN buffer layer which results in n-doped graphene in the absence of a gate voltage. More importantly, a gap of 0.2 eV, which is comparable to that of a typical narrow gap semiconductor, opens just 0.5 eV below the Fermi level at the Dirac point. The Fermi level can be easily shifted inside this gap to make graphene a semiconductor, which is crucial for graphene-based electronic devices. A graphene-based p-n junction can be realized with graphene eptaxially grown on a metal surface.

  17. ZnO Nanoparticles/Reduced Graphene Oxide Bilayer Thin Films for Improved NH3-Sensing Performances at Room Temperature

    NASA Astrophysics Data System (ADS)

    Tai, Huiling; Yuan, Zhen; Zheng, Weijian; Ye, Zongbiao; Liu, Chunhua; Du, Xiaosong

    2016-03-01

    ZnO nanoparticles and graphene oxide (GO) thin film were deposited on gold interdigital electrodes (IDEs) in sequence via simple spraying process, which was further restored to ZnO/reduced graphene oxide (rGO) bilayer thin film by the thermal reduction treatment and employed for ammonia (NH3) detection at room temperature. rGO was identified by UV-vis absorption spectra and X-ray photoelectron spectroscope (XPS) analyses, and the adhesion between ZnO nanoparticles and rGO nanosheets might also be formed. The NH3-sensing performances of pure rGO film and ZnO/rGO bilayer films with different sprayed GO amounts were compared. The results showed that ZnO/rGO film sensors exhibited enhanced response properties, and the optimal GO amount of 1.5 ml was achieved. Furthermore, the optimal ZnO/rGO film sensor showed an excellent reversibility and fast response/recovery rate within the detection range of 10-50 ppm. Meanwhile, the sensor also displayed good repeatability and selectivity to NH3. However, the interference of water molecules on the prepared sensor is non-ignorable; some techniques should be researched to eliminate the effect of moisture in the further work. The remarkably enhanced NH3-sensing characteristics were speculated to be attributed to both the supporting role of ZnO nanoparticles film and accumulation heterojunction at the interface between ZnO and rGO. Thus, the proposed ZnO/rGO bilayer thin film sensor might give a promise for high-performance NH3-sensing applications.

  18. Photonic band gap characteristics of one-dimensional graphene-dielectric periodic structures

    NASA Astrophysics Data System (ADS)

    Al-sheqefi, F. U. Y.; Belhadj, W.

    2015-12-01

    In this paper, we study theoretically, the transmission properties of a one-dimensional graphene-dielectric periodic structure by using the transfer matrix method. Within the framework of this method, we confirm earlier finding that a periodic structure composed of a stack of monolayer graphene sheets separated by dielectric slabs, possesses photonic band-gap (PBG) properties and supports a series of bandpass and band-stop regions at low-terahertz frequencies. Our calculations showed that the suggested structure possesses in addition to the structural Bragg gaps, a new type of band gap that exhibits a rather versatile behavior with varying angle of incidence. We find this type of band gap is omnidirectional (omni-gap) for both transverse electric (TE) and transverse magnetic (TM) polarizations. Our results show that 1D graphene-dielectric periodic structures are very good candidates for band gap engineering. Specifically, we demonstrate the existence of a band gap region for both polarizations which survives for incident angles as high as 80°. Moreover, we show how our proposed structure can also function as a highly efficient polarization splitter. It is also found that the band gaps can be tuned by tuning the properties of the graphene via a gate voltage. In order to investigate difference between the omni-gap and Bragg PBG, we plot the electromagnetic field profiles for some critical frequencies. The proposed structure is promising and can work as a gate tunable perfect stop filter which completely blocks both polarizations, and may have many other potential applications.

  19. Enlarged band gap and electron switch in graphene-based step-barrier structure

    SciTech Connect

    Lu, Wei-Tao Ye, Cheng-Zhi; Li, Wen

    2013-11-04

    We study the transmission through a step-barrier in gapped graphene and propose a method to enlarge the band gap. The step-barrier structure consists of two or more barriers with different strengths. It is found that the band gap could be effectively enlarged and controlled by adjusting the barrier strengths in the light of the mass term. Klein tunneling at oblique incidence is suppressed due to the asymmetry of step-barrier, contrary to the cases in single-barrier and superlattices. Furthermore, a tunable conductance channel could be opened up in the conductance gap, suggesting an application of the structure as an electron switch.

  20. Superfluidity and collective properties of excitonic polaritons in gapped graphene in a microcavity

    NASA Astrophysics Data System (ADS)

    Berman, Oleg L.; Kezerashvili, Roman Ya.; Ziegler, Klaus

    2012-12-01

    We predict the formation and superfluidity of polaritons in an optical microcavity formed by excitons in gapped graphene embedded there and microcavity photons. The Rabi splitting related to the creation of an exciton in a graphene layer in the presence of the band gap is obtained. It is demonstrated that the Rabi splitting decreases when the energy gap increases, while the larger value of the dielectric constant of the microcavity gives a smaller value for the Rabi splitting. The analysis of collective excitations as well as the sound velocity is presented. We show that the superfluid density ns and temperature of the Kosterlitz-Thouless phase transition Tc are decreasing functions of the energy gap.

  1. Strong Plasmon Reflection at Nanometer-Size Gaps in Monolayer Graphene on SiC

    NASA Astrophysics Data System (ADS)

    Kuzmenko, Alexey B.; Chen, Jiaining; Nesterov, Maxim L.; Nikitin, Alexey Yu.; Thongrattanasiri, Sukosin; Alonso-Gonzalez, Pablo; Slipchenko, Tetiana M.; Speck, Florian; Ostler, Markus; Seyller, Thomas; Crassee, Iris; Koppens, Frank H. L.; Martin-Moreno, Luis; Garcia de Abajo, F. Javier; Hillenbrand, Rainer

    2014-03-01

    Tip-enhanced infrared near-field microscopy is used to study propagating plasmons in epitaxial quasi-free-standing monolayer graphene on silicon carbide. We observe that plasmons are strongly reflected at graphene gaps at the steps between the substrate terraces. For the step height of only 1.5 nm, which is two orders of magnitude smaller than the plasmon wavelength, the reflection signal reaches 20 percent of its value at graphene edges, and it approaches 0.5 for steps of 5 nm. We support this observation with extensive numerical simulations and give physical rationale for this intriguing phenomenon. Our work suggests that plasmon propagation in graphene-based circuits can be controlled using ultracompact nanostructures. J. Chen et al., Nano Lett., DOI: 10.1021/nl403622t (2013).

  2. Using the thickness of graphene to template lateral subnanometer gaps between gold nanostructures.

    PubMed

    Zaretski, Aliaksandr V; Marin, Brandon C; Moetazedi, Herad; Dill, Tyler J; Jibril, Liban; Kong, Casey; Tao, Andrea R; Lipomi, Darren J

    2015-01-14

    This work demonstrates the use of single-layer graphene as a template for the formation of subnanometer plasmonic gaps using a scalable fabrication process called "nanoskiving." These gaps are formed between parallel gold nanowires in a process that first produces three-layer thin films with the architecture gold/single-layer graphene/gold, and then sections the composite films with an ultramicrotome. The structures produced can be treated as two gold nanowires separated along their entire lengths by an atomically thin graphene nanoribbon. Oxygen plasma etches the sandwiched graphene to a finite depth; this action produces a subnanometer gap near the top surface of the junction between the wires that is capable of supporting highly confined optical fields. The confinement of light is confirmed by surface-enhanced Raman spectroscopy measurements, which indicate that the enhancement of the electric field arises from the junction between the gold nanowires. These experiments demonstrate nanoskiving as a unique and easy-to-implement fabrication technique that is capable of forming subnanometer plasmonic gaps between parallel metallic nanostructures over long, macroscopic distances. These structures could be valuable for fundamental investigations as well as applications in plasmonics and molecular electronics.

  3. Using the thickness of graphene to template lateral subnanometer gaps between gold nanostructures.

    PubMed

    Zaretski, Aliaksandr V; Marin, Brandon C; Moetazedi, Herad; Dill, Tyler J; Jibril, Liban; Kong, Casey; Tao, Andrea R; Lipomi, Darren J

    2015-01-14

    This work demonstrates the use of single-layer graphene as a template for the formation of subnanometer plasmonic gaps using a scalable fabrication process called "nanoskiving." These gaps are formed between parallel gold nanowires in a process that first produces three-layer thin films with the architecture gold/single-layer graphene/gold, and then sections the composite films with an ultramicrotome. The structures produced can be treated as two gold nanowires separated along their entire lengths by an atomically thin graphene nanoribbon. Oxygen plasma etches the sandwiched graphene to a finite depth; this action produces a subnanometer gap near the top surface of the junction between the wires that is capable of supporting highly confined optical fields. The confinement of light is confirmed by surface-enhanced Raman spectroscopy measurements, which indicate that the enhancement of the electric field arises from the junction between the gold nanowires. These experiments demonstrate nanoskiving as a unique and easy-to-implement fabrication technique that is capable of forming subnanometer plasmonic gaps between parallel metallic nanostructures over long, macroscopic distances. These structures could be valuable for fundamental investigations as well as applications in plasmonics and molecular electronics. PMID:25555061

  4. Sandwich nanoarchitecture of Si/reduced graphene oxide bilayer nanomembranes for Li-ion batteries with long cycle life.

    PubMed

    Liu, Xianghong; Zhang, Jun; Si, Wenping; Xi, Lixia; Eichler, Barbara; Yan, Chenglin; Schmidt, Oliver G

    2015-02-24

    The large capacity loss and huge volume change of silicon anodes severely restricts their practical applications in lithium ion batteries. In this contribution, the sandwich nanoarchitecture of rolled-up Si/reduced graphene oxide bilayer nanomembranes was designed via a strain released strategy. Within this nanoarchitecture, the inner void space and the mechanical feature of nanomembranes can help to buffer the strain during lithiation/delithiation; the alternately stacked conductive rGO layers can protect the Si layers from excessive formation of SEI layers. As anodes for lithium-ion batteries, the sandwiched Si/rGO nanoarchitecture demonstrates long cycling life of 2000 cycles at 3 A g(-1) with a capacity degradation of only 3.3% per 100 cycles.

  5. Band gap engineered nano perforated graphene microstructures for field effect transistor

    NASA Astrophysics Data System (ADS)

    Palla, Penchalaiah; Tiwari, Durgesh Laxman; Ansari, Hasan Raza; Babu, Taraprasanna Saha; Ethiraj, Anita Sagadevan; Raina, J. P.

    2016-05-01

    To make use of exceptional properties of graphene in Field effect Transistor (FETs) for switching devices a band gap must be introduced in order to switch -off the device. Through periodic nano perforations a semi-metallic graphene is converted into semiconducting graphene. To understand the device physics behind the reported experiments theoretical simulations has been carried out. The present paper illustrates nano perforated semiconducting graphene Field effect Transistor (FETs) with micron scale dimensions. The simulation has been performed using drift-diffusion semi-classical and tight-binding based non-equilibrium green's function (NEGF) methods. The obtained simulation results are compared with previously reported experimental work. The device dimensions considered for simulations and the experiment are similar with neck width, hole periodicity and channel length of 6.3 nm, 16.3 nm and 1 µm respectively. The interesting and new finding in this work is the p-type I-V characteristics for small band gap devices and n-type behavior for large band gap devices.

  6. Trigonal warping, pseudodiffusive transport, and finite-system version of the Lifshitz transition in magnetoconductance of bilayer graphene Corbino disks

    NASA Astrophysics Data System (ADS)

    Rut, Grzegorz; Rycerz, Adam

    2016-02-01

    Using the transfer matrix in the angular-momentum space we investigate the impact of trigonal warping on magnetotransport and scaling properties of a ballistic bilayer graphene in the Corbino geometry. Although the conductivity at the charge-neutrality point and zero magnetic field exhibits a one-parameter scaling, the shot-noise characteristics, quantified by the Fano factor F and the third charge-transfer cumulant R , remain pseudodiffusive. This shows that the pseudodiffusive transport regime in bilayer graphene is not related to the universal value of the conductivity but can be identified by higher charge-transfer cumulants. For Corbino disks with larger radii ratios, the conductivity is suppressed by the trigonal warping, mainly because the symmetry reduction amplifies backscattering for normal modes corresponding to angular-momentum eigenvalues ± 2 ℏ . Weak magnetic fields enhance the conductivity, reaching the maximal value near the crossover field BL=4/3 √{3 }(ℏ /e ) t't⊥[t02a (Ro-Ri) ]-1 , where t0 (t⊥) is the nearest-neighbor intralayer (interlayer) hopping integral, t' is the skew-interlayer hopping integral, and Ro (Ri) is the outer (inner) disk radius. For magnetic fields B ≳ BL we observe quasiperiodic conductance oscillations characterized by the decreasing mean value <σ > -σ0∝ BL/B , where σ0=(8 /π ) e2/h . The conductivity, as well as higher charge-transfer cumulants, show beating patterns with an envelope period proportional to √{B /BL }. This constitutes a qualitative difference between the high-field (B ≫ BL ) magnetotransport in the t'=0 case [earlier discussed in Rut and Rycerz, J. Phys.: Condens. Matter 26, 485301 (2014), 10.1088/0953-8984/26/48/485301] and in the t'≠ 0 case, providing a finite-system analog of the Lifshitz transition.

  7. Boundary conditions at closed edge of bilayer graphene and energy bands of collapsed nanotubes

    NASA Astrophysics Data System (ADS)

    Nakanishi, Takeshi; Ando, Tsuneya

    2016-10-01

    Band structure is systematically studied in an effective-mass scheme in collapsed armchair and zigzag nanotubes based on the model in which collapsed tubes are regarded as bilayer ribbons with closed edges. Boundary conditions at closed edges, describing the connection of the envelope wave functions between the bottom and top layers, are derived. Among electronic states in bilayers, which change sensitively depending on the relative displacement of two layers, those having wave functions matching well with the obtained boundary conditions, i.e., unaffected by the presence of closed edges, constitute important states near the Fermi level in collapsed nanotubes.

  8. Electron Optics with Dirac Fermions: Electron Transport in Monolayer and Bilayer Graphene Through Magnetic Barrier and Their Superlattices

    NASA Astrophysics Data System (ADS)

    Agrawal (Garg), Neetu; Ghosh, Sankalpa; Sharma, Manish

    2013-04-01

    In this review article we discuss the recent progress in studying ballistic transport for charge carriers in graphene through highly inhomogeneous magnetic field known as magnetic barrier in combination with gate voltage induced electrostatic potential. Starting with cases for a single or double magnetic barrier we also review the progress in understanding electron transport through the superlattices created out of such electromagnetic potential barriers and discuss the possibility of experimental realization of such systems. The emphasis is particularly on the analogy of such transport with propagation of light wave through medium with alternating dielectric constant. In that direction we discuss electron analogue of optical phenomena like Fabry-Perot resonances, negative refraction, Goos-Hänchen effect, beam collimation in such systems and explain how such analogy is going to be useful for device generation. The resulting modification of band structure of Dirac fermions, the emergence of additional Dirac points was also discussed accompanied by brief section on the interconvertibility of electric and magnetic field for relativistic Dirac fermions. We also discuss the effect of such electromagnetic potential barrier on bilayer graphene (BLG) in a similar framework.

  9. Molecular dynamics simulations on deformation and fracture of bi-layer graphene with different stacking pattern under tension

    NASA Astrophysics Data System (ADS)

    Jiao, M. D.; Wang, L.; Wang, C. Y.; Zhang, Q.; Ye, S. Y.; Wang, F. Y.

    2016-02-01

    Based on AIREBO (Adaptive Intermolecular Reactive Empirical Bond Order) potential, molecular dynamics simulations (MDs) are performed to study the mechanical behavior of AB- and AA-stacked bi-layer graphene films (BGFs) under tension. Stress-strain relationship is established and deformation mechanism is investigated via morphology analysis. It is found that AA-stacked BGFs show wavy folds, i.e. the structural instability, and the local structure of AB-stacked BGFs transforms into AA-stacked ones during free relaxation. The values of the Young's modulus obtained for AA-stacked zigzag and armchair BGFs are 797.2 GPa and 727.4 GPa, and those of their AB-stacked counterparts are 646.7 GPa and 603.5 GPa, respectively. In comparison with single-layer graphene, low anisotropy is observed for BGFs, especially AB-stacked ones. During the tensile deformation, hexagonal cells at the edge of BGFs are found to transform into pentagonal rings and the number of such defects increases with the rise of tensile strain.

  10. Schottky barrier contrasts in single and bi-layer graphene contacts for MoS{sub 2} field-effect transistors

    SciTech Connect

    Du, Hyewon; Kim, Taekwang; Shin, Somyeong; Kim, Dahye; Seo, Sunae; Kim, Hakseong; Lee, Sang Wook; Sung, Ji Ho; Jo, Moon-Ho; Lee, Myoung Jae; Seo, David H.

    2015-12-07

    We have investigated single- and bi-layer graphene as source-drain electrodes for n-type MoS{sub 2} transistors. Ti-MoS{sub 2}-graphene heterojunction transistors using both single-layer MoS{sub 2} (1M) and 4-layer MoS{sub 2} (4M) were fabricated in order to compare graphene electrodes with commonly used Ti electrodes. MoS{sub 2}-graphene Schottky barrier provided electron injection efficiency up to 130 times higher in the subthreshold regime when compared with MoS{sub 2}-Ti, which resulted in V{sub DS} polarity dependence of device parameters such as threshold voltage (V{sub TH}) and subthreshold swing (SS). Comparing single-layer graphene (SG) with bi-layer graphene (BG) in 4M devices, SG electrodes exhibited enhanced device performance with higher on/off ratio and increased field-effect mobility (μ{sub FE}) due to more sensitive Fermi level shift by gate voltage. Meanwhile, in the strongly accumulated regime, we observed opposing behavior depending on MoS{sub 2} thickness for both SG and BG contacts. Differential conductance (σ{sub d}) of 1M increases with V{sub DS} irrespective of V{sub DS} polarity, while σ{sub d} of 4M ceases monotonic growth at positive V{sub DS} values transitioning to ohmic-like contact formation. Nevertheless, the low absolute value of σ{sub d} saturation of the 4M-graphene junction demonstrates that graphene electrode could be unfavorable for high current carrying transistors.

  11. Robust band gap and half-metallicity in graphene with triangular perforations

    NASA Astrophysics Data System (ADS)

    Gregersen, Søren Schou; Power, Stephen R.; Jauho, Antti-Pekka

    2016-06-01

    Ideal graphene antidot lattices are predicted to show promising band gap behavior (i.e., EG≃500 meV) under carefully specified conditions. However, for the structures studied so far this behavior is critically dependent on superlattice geometry and is not robust against experimentally realistic disorders. Here we study a rectangular array of triangular antidots with zigzag edge geometries and show that their band gap behavior qualitatively differs from the standard behavior which is exhibited, e.g., by rectangular arrays of armchair-edged triangles. In the spin unpolarized case, zigzag-edged antidots give rise to large band gaps compared to armchair-edged antidots, irrespective of the rules which govern the existence of gaps in armchair-edged antidot lattices. In addition the zigzag-edged antidots appear more robust than armchair-edged antidots in the presence of geometrical disorder. The inclusion of spin polarization within a mean-field Hubbard approach gives rise to a large overall magnetic moment at each antidot due to the sublattice imbalance imposed by the triangular geometry. Half-metallic behavior arises from the formation of spin-split dispersive states near the Fermi energy, reducing the band gaps compared to the unpolarized case. This behavior is also found to be robust in the presence of disorder. Our results highlight the possibilities of using triangular perforations in graphene to open electronic band gaps in systems with experimentally realistic levels of disorder, and furthermore, of exploiting the strong spin dependence of the system for spintronic applications.

  12. Label-Free Electrochemiluminescence Aptasensor for 2,4,6-Trinitrotoluene Based on Bilayer Structure of Luminescence Functionalized Graphene Hybrids.

    PubMed

    Li, Guixin; Yu, Xiuxia; Liu, Danqing; Liu, Xiaoying; Li, Fang; Cui, Hua

    2015-11-01

    The electrochemiluminescence (ECL) behavior of N-(aminobutyl)-N-(ethylisoluminol)/hemin dual-functionalized graphene hybrids (A-H-GNs) and luminol-functionalized silver/graphene oxide composite (luminol-AgNPs-GO) was investigated under cyclic voltammetry and pulse potential. It was found that A-H-GNs and luminol-AgNPs-GO exhibited excellent ECL activity. On this basis, a label-free ECL aptasensor for 2,4,6-trinitrotoluene (TNT) detection was developed based on bilayer structure of luminescence functionalized graphene hybrids consisting of A-H-GNs and luminol-AgNPs-GO. First, positively charged chitosan-coated A-H-GNs were modified on the surface of indium-doped tin oxide electrode by simple dripping and drying in the air; after that, the modified electrode was immersed in negatively charged luminol-AgNPs-GO modified with aptamer (apta-biotin-SA-luminol-AgNPs-GO) to form apta-biotin-SA-luminol-AgNPs-GO/CS-A-H-GNs/ITO electrode (i.e., aptasensor) by electrostatic interaction. In the presence of TNT, a remarkable decrease in ECL signals was observed due to the formation of aptamer-TNT complex. TNT could be detected based on the inhibition effect. The aptasensor exhibits a wide dynamic range from 1.0 × 10(-12) to 1.0 × 10(-9) g/mL, with a low detection limit of 6.3 × 10(-13) g/mL for the determination of TNT, which is superior to most previously reported bioassays for TNT. Moreover, the proposed aptasensor has been successfully applied to the detection of TNT in environmental water. It is sensitive, selective, and simple, avoiding complicated labeling and purification procedures. Due to the wide target recognition range of aptamer, this strategy provides a promising way to develop new aptasensor for other analytes.

  13. Modulation of Dirac points and band-gaps in graphene via periodic fullerene adsorption

    NASA Astrophysics Data System (ADS)

    Liu, Xiao; Wen, Yanwei; Chen, Zhengzheng; Lin, Hao; Chen, Rong; Cho, Kyeongjae; Shan, Bin

    2013-05-01

    The structural, energetic and electronic properties of periodic graphene nanobud (PGNB) with small-diameter fullerenes (C20, C34, C42, and C60) adsorbed have been investigated by first-principles plane wave method. The bond-to-ring cycloaddition is found to be energetically most stable among various configurations and the minimum energy paths of different-sized fullerenes attaching to graphene indicate that smaller fullerene shows lower energy barriers due to its larger surface curvature. For perfectly ordered adsorption, band structures analyses by both density functional theory (DFT) and tight binding (TB) methods show that the Dirac cone of graphene can be generally preserved despite the sp2 to sp3 bond hybridization change for selected carbon atoms in graphene sheet. However, the position of the Dirac points inside the Brillouin zone has a shift from the hexagonal corner and can be effectively modulated by changing the fullerenes' concentration. For practical applications, we show that a considerable band gap (˜0.35 eV) can be opened by inducing randomness in the orientation of the fullerene adsorption and an effective order parameter is identified that correlates well with the magnitude of the band gap opening.

  14. Stacking nature and band gap opening of graphene: Perspective for optoelectronic applications

    NASA Astrophysics Data System (ADS)

    Ullah, Naeem; Zhang, R. Q.; Murtaza, G.; Yar, Abdullah; Mahmood, Asif

    2016-11-01

    Using first principles density functional theory calculations, we have performed geometrical and electronic structure calculations of two-dimensional graphene(G) sheet on the hexagonal boron nitride (h-BN) with different stacking orders. We found that AB stacking appears as the ground state while AA-stacking is a local minima. Band gap opening in the hybrid G/h-BN is sensitive to the interlayer distance and stacking arrangement. Charge redistribution in the graphene sheet determined the band gap opening where the onsite energy difference between carbon lattice atoms of G-sheet takes place. Similar behavior can be observed for the proposed h-BN/G/h-BN tri-layer system. Stacking resolved calculations of the absorptive part of complex dielectric function and optical conductivity revealed the importance of the proposed hybrid systems in the optoelectronics.

  15. Bound states of Dirac fermions in monolayer gapped graphene in the presence of local perturbations

    NASA Astrophysics Data System (ADS)

    Mohsen, Yarmohammadi; Malek, Zareyan

    2016-06-01

    In graphene, conductance electrons behave as massless relativistic particles and obey an analogue of the Dirac equation in two dimensions with a chiral nature. For this reason, the bounding of electrons in graphene in the form of geometries of quantum dots is impossible. In gapless graphene, due to its unique electronic band structure, there is a minimal conductivity at Dirac points, that is, in the limit of zero doping. This creates a problem for using such a highly motivated new material in electronic devices. One of the ways to overcome this problem is the creation of a band gap in the graphene band structure, which is made by inversion symmetry breaking (symmetry of sublattices). We investigate the confined states of the massless Dirac fermions in an impured graphene by the short-range perturbations for “local chemical potential” and “local gap”. The calculated energy spectrum exhibits quite different features with and without the perturbations. A characteristic equation for bound states (BSs) has been obtained. It is surprisingly found that the relation between the radial functions of sublattices wave functions, i.e., , , and , , can be established by SO(2) group.

  16. Transport gap in vertical devices made of incommensurately misoriented graphene layers

    NASA Astrophysics Data System (ADS)

    Nguyen, V. Hung; Dollfus, P.

    2016-02-01

    By means of atomistic tight-binding calculations, we investigate the transport properties of vertical devices made of two incommensurately misoriented graphene layers. For a given transport direction (Ox-axis), we define two classes of rotated graphene lattice distinguished by difference in lattice symmetry and, hence, in Brillouin zone. In particular, these two classes correspond to two different cases where the position of their Dirac cones in the k y -axis is determined differently, i.e. Ky\\prime={{K}y}=0 or Ky\\prime=-{{K}y}=2π /3{{L}y} (L y is the periodic length along the Oy axis). As a consequence, in devices made of two layers of different lattice classes, the misalignment of Dirac cones between the left and right graphene sections opens a finite energy-gap of conductance that can reach a few hundreds of meV. We also show that strain engineering can be used to further enlarge the transport gap and to diminish the sensitivity of the gap on the twist angle and on the commensurateness of the layer stack.

  17. EDITORIAL: Focus on Graphene

    NASA Astrophysics Data System (ADS)

    Peres, N. M. R.; Ribeiro, Ricardo M.

    2009-09-01

    Graphene physics is currently one of the most active research areas in condensed matter physics. Countless theoretical and experimental studies have already been performed, targeting electronic, magnetic, thermal, optical, structural and vibrational properties. Also, studies that modify pristine graphene, aiming at finding new physics and possible new applications, have been considered. These include patterning nanoribbons and quantum dots, exposing graphene's surface to different chemical species, studying multilayer systems, and inducing strain and curvature (modifying in this way graphene's electronic properties). This focus issue includes many of the latest developments on graphene research. Focus on Graphene Contents Electronic properties of graphene and graphene nanoribbons with 'pseudo-Rashba' spin-orbit coupling Tobias Stauber and John Schliemann Strained graphene: tight-binding and density functional calculations R M Ribeiro, Vitor M Pereira, N M R Peres, P R Briddon and A H Castro Neto The effect of sublattice symmetry breaking on the electronic properties of doped graphene A Qaiumzadeh and R Asgari Interfaces within graphene nanoribbons J Wurm, M Wimmer, I Adagideli, K Richter and H U Baranger Weak localization and transport gap in graphene antidot lattices J Eroms and D Weiss Electronic properties of graphene antidot lattices J A Fürst, J G Pedersen, C Flindt, N A Mortensen, M Brandbyge, T G Pedersen and A-P Jauho Splitting of critical energies in the n=0 Landau level of graphene Ana L C Pereira Double-gated graphene-based devices S Russo, M F Craciun, M Yamamoto, S Tarucha and A F Morpurgo Pinning and switching of magnetic moments in bilayer graphene Eduardo V Castro, M P López-Sancho and M A H Vozmediano Electronic transport properties of graphene nanoribbons Katsunori Wakabayashi, Yositake Takane, Masayuki Yamamoto and Manfred Sigrist Many-body effects on out-of-plane phonons in graphene J González and E Perfetto Graphene zigzag ribbons, square

  18. Liquid-assisted tip manipulation: fabrication of twisted bilayer graphene superlattices on HOPG

    NASA Astrophysics Data System (ADS)

    Yin, Long Jing; Wang, Wen Xiao; Feng, Ke Ke; Nie, Jia-Cai; Xiong, Chang Min; Dou, Rui-Fen; Naugle, Donald G.

    2015-09-01

    We use the tip of a scanning tunneling microscope (STM) to manipulate single weakly bound nanometer-sized sheets on a highly oriented pyrolytic graphite (HOPG) surface through artificially increasing the tip and sample interaction by pretreatment of the surface using a liquid thiol molecule. By this means it is possible to tear apart a graphite sheet against a step and fold this part onto the HOPG surface and thus generate graphene superlattices with hexagonal symmetry. The tip and sample surface interactions, including the van der Waals force, electrostatic force and capillary attraction force originating from the Laplace pressure due to the formation of a highly curved fluid meniscus connecting the tip and sample, are discussed quantitatively to understand the formation mechanism of a graphene superlattice induced by the STM tip. The capillary force plays a key role in manipulating the graphite surface sheet under humid conditions. Our approach provides a simple and feasible route to prepare controllable superlattices and graphene nanoribbons and also to better understand the process of generation of a graphene superlattice on the surface of HOPG with the tip.We use the tip of a scanning tunneling microscope (STM) to manipulate single weakly bound nanometer-sized sheets on a highly oriented pyrolytic graphite (HOPG) surface through artificially increasing the tip and sample interaction by pretreatment of the surface using a liquid thiol molecule. By this means it is possible to tear apart a graphite sheet against a step and fold this part onto the HOPG surface and thus generate graphene superlattices with hexagonal symmetry. The tip and sample surface interactions, including the van der Waals force, electrostatic force and capillary attraction force originating from the Laplace pressure due to the formation of a highly curved fluid meniscus connecting the tip and sample, are discussed quantitatively to understand the formation mechanism of a graphene superlattice

  19. Semiconducting Graphene on Silicon from First-Principles Calculations.

    PubMed

    Dang, Xuejie; Dong, Huilong; Wang, Lu; Zhao, Yanfei; Guo, Zhenyu; Hou, Tingjun; Li, Youyong; Lee, Shuit-Tong

    2015-08-25

    Graphene is a semimetal with zero band gap, which makes it impossible to turn electric conduction off below a certain limit. Transformation of graphene into a semiconductor has attracted wide attention. Owing to compatibility with Si technology, graphene adsorbed on a Si substrate is particularly attractive for future applications. However, to date there is little theoretical work on band gap engineering in graphene and its integration with Si technology. Employing first-principles calculations, we study the electronic properties of monolayer and bilayer graphene adsorbed on clean and hydrogen (H)-passivated Si (111)/Si (100) surfaces. Our calculation shows that the interaction between monolayer graphene and a H-passivated Si surface is weak, with the band gap remaining negligible. For bilayer graphene adsorbed onto a H-passivated Si surface, the band gap opens up to 108 meV owing to asymmetry introduction. In contrast, the interaction between graphene and a clean Si surface is strong, leading to formation of chemical bonds and a large band gap of 272 meV. Our results provide guidance for device designs based on integrating graphene with Si technology.

  20. Polarization field gradient effects in inhomogeneous metal-ferroelectric bilayers: Optical response and band gap tunability

    NASA Astrophysics Data System (ADS)

    Vivas C., H.; Vargas-Hernández, C.

    2012-06-01

    Optical constants, reflectivity response and direct band gap energy (Egd) were calculated and simulated by developing an electrodynamic-based model for a three medium system, namely vacuum/ferroelectric film/metallic substrate. Depolarization effects due to the contact between the metallic substrate and the FE film, as well as the spatially dependent profile of the dielectric susceptibility ε(z) enter into the formalism by adapting the phenomenological Landau-Ginzburg-Devonshire theory (LGD). Absorption coefficient is obtained from the Lambert-Beer-Bouguer (LBB) approximation and the direct band gap energy as a function of the characteristic length is calculated by using the general Tauc power law. Numerical simulations lead to range of values for tunable Egd from 2.6 to 2.8 eV for characteristic lengths up to 30% the thickness of the film, in concordance with recent reports.

  1. Liquid-assisted tip manipulation: fabrication of twisted bilayer graphene superlattices on HOPG.

    PubMed

    Yin, Long Jing; Wang, Wen Xiao; Feng, Ke Ke; Nie, Jia-Cai; Xiong, Chang Min; Dou, Rui-Fen; Naugle, Donald G

    2015-09-28

    We use the tip of a scanning tunneling microscope (STM) to manipulate single weakly bound nanometer-sized sheets on a highly oriented pyrolytic graphite (HOPG) surface through artificially increasing the tip and sample interaction by pretreatment of the surface using a liquid thiol molecule. By this means it is possible to tear apart a graphite sheet against a step and fold this part onto the HOPG surface and thus generate graphene superlattices with hexagonal symmetry. The tip and sample surface interactions, including the van der Waals force, electrostatic force and capillary attraction force originating from the Laplace pressure due to the formation of a highly curved fluid meniscus connecting the tip and sample, are discussed quantitatively to understand the formation mechanism of a graphene superlattice induced by the STM tip. The capillary force plays a key role in manipulating the graphite surface sheet under humid conditions. Our approach provides a simple and feasible route to prepare controllable superlattices and graphene nanoribbons and also to better understand the process of generation of a graphene superlattice on the surface of HOPG with the tip.

  2. Role of substrate induced electron-phonon interactions in biased graphitic bilayers

    NASA Astrophysics Data System (ADS)

    Davenport, A. R.; Hague, J. P.

    2016-08-01

    Bilayers of graphitic materials have potential applications in field effect transistors (FETs). A potential difference applied between certain ionic bilayers made from insulating graphitic materials such as BN, ZnO and AlN could reduce gap sizes, turning them into useful semiconductors. On the other hand, opening of a small semiconducting gap occurs in graphene bilayers under applied field. The aim here is to investigate to what extent substrate induced electron-phonon interactions (EPIs) modify this gap change. We examine EPIs in several lattice configurations of graphitic bilayers, using a perturbative approach. The typical effect of EPIs on the ionic bilayers is an undesirable gap widening. The size of this gap change varies considerably with lattice structure and the magnitude of the bias. When bias is larger than the non-interacting gap size, EPIs have the smallest effect on the bandgap, especially in configurations with A{{A}\\prime} and AB structures. Thus careful selection of substrate, lattice configuration and bias strength to minimise the effects of EPIs could be important for optimising the properties of electronic devices. We use parameters related to BN in this article. In practice, the results presented here are broadly applicable to other graphitic bilayers, and are likely to be qualitatively similar in metal dichalcogenide bilayers such as MoS2, which are already of high interest for their use in FETs.

  3. Role of substrate induced electron–phonon interactions in biased graphitic bilayers

    NASA Astrophysics Data System (ADS)

    Davenport, A. R.; Hague, J. P.

    2016-08-01

    Bilayers of graphitic materials have potential applications in field effect transistors (FETs). A potential difference applied between certain ionic bilayers made from insulating graphitic materials such as BN, ZnO and AlN could reduce gap sizes, turning them into useful semiconductors. On the other hand, opening of a small semiconducting gap occurs in graphene bilayers under applied field. The aim here is to investigate to what extent substrate induced electron–phonon interactions (EPIs) modify this gap change. We examine EPIs in several lattice configurations of graphitic bilayers, using a perturbative approach. The typical effect of EPIs on the ionic bilayers is an undesirable gap widening. The size of this gap change varies considerably with lattice structure and the magnitude of the bias. When bias is larger than the non-interacting gap size, EPIs have the smallest effect on the bandgap, especially in configurations with A{{A}\\prime} and AB structures. Thus careful selection of substrate, lattice configuration and bias strength to minimise the effects of EPIs could be important for optimising the properties of electronic devices. We use parameters related to BN in this article. In practice, the results presented here are broadly applicable to other graphitic bilayers, and are likely to be qualitatively similar in metal dichalcogenide bilayers such as MoS2, which are already of high interest for their use in FETs.

  4. Gap opening in graphene by 1D and 2D periodic corrugations

    NASA Astrophysics Data System (ADS)

    Naumov, Ivan; Bratkovsky, Alexander

    2012-02-01

    Using first-principles methods and symmetry arguments, we show that a graphene monolayer, which is periodically corrugated in one or two direction(s), can be either semimetal or semiconductor, depending on how strong corrugation is or how the initial symmetry is broken. In the case of 1D periodic ripples, a gap at the Dirac points opens up only due to (i) breaking of the inversion symmetry or equivalence between A and B sublattices and/or (ii) merging of two inequivalent Dirac points, D and -D. Since breaking the inversion symmetry has only relatively modest effect, a tangible gap can be mainly induced by mutual annihilation of the Dirac points, which requires large corrugations, close to mechanical breaking point. In contrast to 1D, the 2D ripples can additionally induce a semiconducting gap via mixing of electronic states belonging to two different K, K' valleys. In this case, a gap on the order of 0.5 eV can be opened up at strains safely lower than the graphene failure strain [1]. [4pt] [1] I.I. Naumov, A.M. Bratkovsky, arXiv:1104.0314v1.

  5. Three-wave collinear difference-frequency mixing and terahertz coherent emission from gapped graphene

    NASA Astrophysics Data System (ADS)

    Margulis, Vl. A.; Muryumin, E. E.; Gaiduk, E. A.

    2015-12-01

    The second-order nonlinear optical susceptibility χ (2)(-ω3 ;ω1 ,ω2) corresponding to three-wave mixing of coherent radiation of the form ω3 =ω1 -ω2 is calculated for epitaxial graphene on a SiC substrate inducing the sublattice (inversion) asymmetry of the graphene and opening up a gap of about 0.26 eV in its π-electron-energy spectrum. The analytical treatment of the χ (2) is based on the tight-binding approximation for π electrons and the original Genkin-Mednis nonlinear-conductivity-theory formalism including mixed intra- and interband terms. It is found that throughout the transparency region of the graphene, the absolute magnitude of the χ (2) may be as large as 10-5 esu, which opens up new opportunities to generate terahertz (THz) coherent output from the graphene excited by two collinear mid-infrared ω1 and ω2 laser beams normally incident on its surface. The output power density produced at the difference frequency ω1 -ω2 of 10 THz is estimated to be 0.1 μW/cm2 for 10 MW/cm2 pump peak intensities, and conditions are discussed under which a few orders of magnitude enhancement of the output power could be achieved in future experiments.

  6. Charge screening and carrier transport in AA-stacked bilayer graphene: Tuning via a perpendicular electric field

    NASA Astrophysics Data System (ADS)

    Mohammadi, Yawar

    2015-01-01

    The static dielectric function in AA-stacked bilayer graphene (BLG), subjected to an electric field applied perpendicular to layers, is calculated analytically within the random phase approximation (RPA). This result is used to calculate the screened Coulomb interaction and the electrical conductivity. The screened Coulomb interaction, which here can be tuned by the perpendicular electric field, shows a power-law decay as 1 /(γ2 +V2) at long-distance limit where V and γ are the electrical potential and the inter-layer hopping energy respectively, indicating that the Coulomb interaction is suppressed at high perpendicular electric fields. Furthermore, our results for the effect of the short-range and the long-range (Coulomb) scattering on the electrical conductivity show that the short-range scattering yields a constant electrical conductivity which is not affected by the perpendicular electric filed. While the electrical conductivity limited by the Coulomb scattering is enhanced by the perpendicular electric field and increases linearly in V2 at small V with a finite value at V=0, indicating that we can tune the electrical conductivity in AA-stacked BLG by applying a perpendicular electric field.

  7. Large gap electron-hole superfluidity and shape resonances in coupled graphene nanoribbons

    PubMed Central

    Zarenia, M.; Perali, A.; Peeters, F. M.; Neilson, D.

    2016-01-01

    We predict enhanced electron-hole superfluidity in two coupled electron-hole armchair-edge terminated graphene nanoribbons separated by a thin insulating barrier. In contrast to graphene monolayers, the multiple subbands of the nanoribbons are parabolic at low energy with a gap between the conduction and valence bands, and with lifted valley degeneracy. These properties make screening of the electron-hole interaction much weaker than for coupled electron-hole monolayers, thus boosting the pairing strength and enhancing the superfluid properties. The pairing strength is further boosted by the quasi one-dimensional quantum confinement of the carriers, as well as by the large density of states near the bottom of each subband. The latter magnifies superfluid shape resonances caused by the quantum confinement. Several superfluid partial condensates are present for finite-width nanoribbons with multiple subbands. We find that superfluidity is predominately in the strongly-coupled BEC and BCS-BEC crossover regimes, with large superfluid gaps up to 100 meV and beyond. When the gaps exceed the subband spacing, there is significant mixing of the subbands, a rounding of the shape resonances, and a resulting reduction in the one-dimensional nature of the system. PMID:27108968

  8. Moiré scaling of the sliding force in twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Koren, E.; Duerig, U.

    2016-07-01

    The weak interlayer binding in two-dimensional layered materials such as graphite gives rise to distinguished low-friction properties if the atomic lattices at the interface are rotated with respect to one another. The lack of crystal symmetry leads to poorly understood correlations and cancelations of the interlayer atomic forces. Here we report on a powerful tiling method based on the moiré superstructure which allows us to study the intricate interplay of the interlayer forces in a systematic manner. Based on numerical simulation data for a circular graphene flake on an infinite graphene substrate, it is shown that the sliding force is dominated by a rim area consisting of incomplete moiré tiles. This rim force, which scales with the number of atoms in a moiré tile and as the radius to the power of 0.5, is minimal whenever the sliding structure can be approximated by a hexagon composed of an integer number Nt of moiré tiles. Intriguingly, the corresponding area force scales as Nt to the power of 0.25, i.e., it increases with size, whereas it has been often argued that interlayer forces should add up to a zero value for large twisted systems. However, at specific twist angles the moiré structure is commensurate with the graphene lattice, leading to a perfect force correlation in the moiré tiles. Correspondingly, the area force becomes dominant and scales as Nt, i.e., as the radius to the power of 2.

  9. Bose-Einstein condensation and superfluidity of magnetoexcitons in bilayer graphene

    SciTech Connect

    Berman, Oleg L.; Lozovik, Yurii E.; Gumbs, Godfrey

    2008-04-15

    We propose experiments to observe Bose-Einstein condensation and superfluidity of quasi-two-dimensional spatially indirect magnetoexcitons in two-layer graphene. The energy spectrum of collective excitations, the sound spectrum, and the effective magnetic mass of magnetoexcitons are presented in the strong magnetic field regime. The superfluid density n{sub S} and the temperature of the Kosterlitz-Thouless phase transition T{sub c} are shown to be increasing functions of the excitonic density n but decreasing functions of B and the interlayer separation D.

  10. Dynamical gap generation in graphene with frequency-dependent renormalization effects

    NASA Astrophysics Data System (ADS)

    Carrington, M. E.; Fischer, C. S.; von Smekal, L.; Thoma, M. H.

    2016-09-01

    We study the frequency dependencies in the renormalization of the fermion Green's function for the π -band electrons in graphene and their influence on the dynamical gap generation at sufficiently strong interaction. Adopting the effective QED-like description for the low-energy excitations within the Dirac-cone region, we self-consistently solve the fermion Dyson-Schwinger equation in various approximations for the photon propagator and the vertex function with special emphasis on frequency-dependent Lindhard screening and retardation effects.

  11. A theoretical study of fluorographene as substrates for mono-/Bi-layer graphene

    NASA Astrophysics Data System (ADS)

    Guo, Zhendong; Fan, Lei; Mei, Lingqi; Xu, Yang; Yu, Bin

    2013-12-01

    Using density functional theory, we discover fluorographene (CF) could help significantly preserve the superb electronic properties of graphene, depending on lattice stacking-order. Compared with h-BN, CF produces a much weaker screening effect for bilayer graphene when external electric field is applied, revealing a huge advantage in gap engineering. The studies suggest that fluorographene could be a promising route towards implementing highly functional substrate or gate dielectric materials for graphene-inspired device applications.

  12. Quantum Monte Carlo study of magnetic impurity in bilayer grephene

    NASA Astrophysics Data System (ADS)

    Sun, J. H.; Hu, F. M.; Tang, H. K.; Lin, Hai-Qing

    2012-02-01

    It is expected to observe many different properties in bilayer graphene when compared with single layer graphene due to the differences in crystal structure. Additionally, bilayer system offers a freedom of inducing a gap in the energy band by applying a shift in the electrochemical potential to two graphene layers. In this work, we study the magnetic properties of an Anderson magnetic adatom in Bernal stacking bilayer graphene and compare the results with those of single layer counterpart. Several different cases such as different adatom position and different potential bias of two layers are studied using the quantum Monte Carlo method. In all the cases, we find that the impurity local magnetic moment can be switched between relatively large and small values by tuning the chemical potential. We apply MaxEnT method to compute impurity spectral density and find its behavior to differ from that of an impurity in a single layer graphene. We also calculate various correlation functions and make comparisons.

  13. Breaking inversion symmetry in a state-dependent honeycomb lattice: artificial graphene with tunable band gap

    NASA Astrophysics Data System (ADS)

    Weinberg, M.; Staarmann, C.; Ölschläger, C.; Simonet, J.; Sengstock, K.

    2016-06-01

    Here, we present the application of a novel method for controlling the geometry of a state-dependent honeycomb lattice: the energy offset between the two sublattices of the honeycomb structure can be adjusted by rotating the atomic quantization axis. This enables us to continuously tune between a homogeneous graphene-like honeycomb lattice and a triangular lattice and to open an energy gap at the characteristic Dirac points. We probe the symmetry of the lattice with microwave spectroscopy techniques and investigate the behavior of atoms excited to the second energy band. We find a striking influence of the energy gap at the Dirac cones onto the lifetimes of bosonic atoms in the excited band.

  14. Protein-Based Graphene Biosensors: Optimizing Artificial Chemoreception in Bilayer Lipid Membranes.

    PubMed

    Siontorou, Christina G; Georgopoulos, Konstantinos N; Nikoleli, Georgia-Paraskevi; Nikolelis, Dimitrios P; Karapetis, Stefanos K; Bratakou, Spyridoula

    2016-09-07

    Proteinaceous moieties are critical elements in most detection systems, including biosensing platforms. Their potential is undoubtedly vast, yet many issues regarding their full exploitation remain unsolved. On the other hand, the biosensor formats with the higher marketability probabilities are enzyme in nature and electrochemical in concept. To no surprise, alternative materials for hosting catalysis within an electrode casing have received much attention lately to demonstrate a catalysis-coated device. Graphene and ZnO are presented as ideal materials to modify electrodes and biosensor platforms, especially in protein-based detection. Our group developed electrochemical sensors based on these nanomaterials for the sensitive detection of cholesterol using cholesterol oxidase incorporated in stabilized lipid films. A comparison between the two platforms is provided and discussed. In a broader sense, the not-so-remote prospect of quickly assembling a protein-based flexible biosensing detector to fulfill site-specific requirements is appealing to both university researchers and industry developers.

  15. Protein-Based Graphene Biosensors: Optimizing Artificial Chemoreception in Bilayer Lipid Membranes.

    PubMed

    Siontorou, Christina G; Georgopoulos, Konstantinos N; Nikoleli, Georgia-Paraskevi; Nikolelis, Dimitrios P; Karapetis, Stefanos K; Bratakou, Spyridoula

    2016-01-01

    Proteinaceous moieties are critical elements in most detection systems, including biosensing platforms. Their potential is undoubtedly vast, yet many issues regarding their full exploitation remain unsolved. On the other hand, the biosensor formats with the higher marketability probabilities are enzyme in nature and electrochemical in concept. To no surprise, alternative materials for hosting catalysis within an electrode casing have received much attention lately to demonstrate a catalysis-coated device. Graphene and ZnO are presented as ideal materials to modify electrodes and biosensor platforms, especially in protein-based detection. Our group developed electrochemical sensors based on these nanomaterials for the sensitive detection of cholesterol using cholesterol oxidase incorporated in stabilized lipid films. A comparison between the two platforms is provided and discussed. In a broader sense, the not-so-remote prospect of quickly assembling a protein-based flexible biosensing detector to fulfill site-specific requirements is appealing to both university researchers and industry developers. PMID:27618113

  16. Protein-Based Graphene Biosensors: Optimizing Artificial Chemoreception in Bilayer Lipid Membranes

    PubMed Central

    Siontorou, Christina G.; Georgopoulos, Konstantinos N.; Nikoleli, Georgia-Paraskevi; Nikolelis, Dimitrios P.; Karapetis, Stefanos K.; Bratakou, Spyridoula

    2016-01-01

    Proteinaceous moieties are critical elements in most detection systems, including biosensing platforms. Their potential is undoubtedly vast, yet many issues regarding their full exploitation remain unsolved. On the other hand, the biosensor formats with the higher marketability probabilities are enzyme in nature and electrochemical in concept. To no surprise, alternative materials for hosting catalysis within an electrode casing have received much attention lately to demonstrate a catalysis-coated device. Graphene and ZnO are presented as ideal materials to modify electrodes and biosensor platforms, especially in protein-based detection. Our group developed electrochemical sensors based on these nanomaterials for the sensitive detection of cholesterol using cholesterol oxidase incorporated in stabilized lipid films. A comparison between the two platforms is provided and discussed. In a broader sense, the not-so-remote prospect of quickly assembling a protein-based flexible biosensing detector to fulfill site-specific requirements is appealing to both university researchers and industry developers. PMID:27618113

  17. Positional dependence of energy gap on line defect in armchair graphene nanoribbons: Two-terminal transport and related issues

    NASA Astrophysics Data System (ADS)

    Dutta, Paramita; Maiti, Santanu K.; Karmakar, S. N.

    2013-07-01

    The characteristics of energy band spectrum of armchair graphene nanoribbons in the presence of line defect are analyzed within a simple non-interacting tight-binding framework. In metallic nanoribbons, an energy gap may or may not appear in the band spectrum depending on the location of the defect line, while in semiconducting ribbons, the gaps are customized, yielding the potential applicabilities of graphene nanoribbons in nanoscale electronic devices. With a more general model, we also investigate two-terminal electron transport using Green's function formalism.

  18. Image potential states in monolayer, bilayer, and trilayer epitaxial graphene studied with time- and angle-resolved two-photon photoemission spectroscopy

    NASA Astrophysics Data System (ADS)

    Takahashi, Kazutoshi; Imamura, Masaki; Yamamoto, Isamu; Azuma, Junpei; Kamada, Masao

    2014-04-01

    Image potential states (IPSs) on monolayer, bilayer, and trilayer graphene epitaxially grown on SiC(0001) have been studied by time- and angle-resolved two-photon photoemission (2PPE) spectroscopy. The free-electron-like dispersions of even and odd symmetry IPSs with a quantum number of n = 1+, 1-, 2, 3 were observed. All observed IPSs showed the dispersions with effective masses of m*=1.0±0.1me. The 2PPE intensity of the lowest IPS (n = 1+) was attenuated with an increasing number of graphene layers. The time-resolved 2PPE measurements revealed that these IPSs have significantly shorter lifetimes, suggesting a coupling of IPSs with electronic states in the buffer layer and the SiC substrate.

  19. Band gap engineering in penta-graphene by substitutional doping: first-principles calculations

    NASA Astrophysics Data System (ADS)

    Berdiyorov, G. R.; Dixit, G.; Madjet, M. E.

    2016-11-01

    Using density functional theory, we study the structure, electronic properties and partial charges of a new carbon allotrope—penta-graphene (PG)—substitutionally doped by Si, B and N. We found that the electronic bandgap of PG can be tuned down to 0.2 eV due to carbon substitutions. However, the value of the band gap depends on the type and location of the dopants. For example, the strongest reduction of the band gap is obtained for Si substitutions on the top (bottom) plane of PG, whereas the substitution in the middle plane of PG has a smaller effect on the band gap of the material. Surface termination with fluorine and hydroxyl groups results in an increase of the band gap together with considerable changes in electronic and atomic partial charge distribution in the system. Our findings, which are robust against the use of different exchange-correlation functionals, indicate the possibility of tuning the bandgap of the material to make it suitable for optoelectronic and photovoltaic applications.

  20. Band gap engineering in penta-graphene by substitutional doping: first-principles calculations.

    PubMed

    Berdiyorov, G R; Dixit, G; Madjet, M E

    2016-11-30

    Using density functional theory, we study the structure, electronic properties and partial charges of a new carbon allotrope-penta-graphene (PG)-substitutionally doped by Si, B and N. We found that the electronic bandgap of PG can be tuned down to 0.2 eV due to carbon substitutions. However, the value of the band gap depends on the type and location of the dopants. For example, the strongest reduction of the band gap is obtained for Si substitutions on the top (bottom) plane of PG, whereas the substitution in the middle plane of PG has a smaller effect on the band gap of the material. Surface termination with fluorine and hydroxyl groups results in an increase of the band gap together with considerable changes in electronic and atomic partial charge distribution in the system. Our findings, which are robust against the use of different exchange-correlation functionals, indicate the possibility of tuning the bandgap of the material to make it suitable for optoelectronic and photovoltaic applications. PMID:27633017

  1. Band gap engineering in penta-graphene by substitutional doping: first-principles calculations.

    PubMed

    Berdiyorov, G R; Dixit, G; Madjet, M E

    2016-11-30

    Using density functional theory, we study the structure, electronic properties and partial charges of a new carbon allotrope-penta-graphene (PG)-substitutionally doped by Si, B and N. We found that the electronic bandgap of PG can be tuned down to 0.2 eV due to carbon substitutions. However, the value of the band gap depends on the type and location of the dopants. For example, the strongest reduction of the band gap is obtained for Si substitutions on the top (bottom) plane of PG, whereas the substitution in the middle plane of PG has a smaller effect on the band gap of the material. Surface termination with fluorine and hydroxyl groups results in an increase of the band gap together with considerable changes in electronic and atomic partial charge distribution in the system. Our findings, which are robust against the use of different exchange-correlation functionals, indicate the possibility of tuning the bandgap of the material to make it suitable for optoelectronic and photovoltaic applications.

  2. Electronic Structures, Bonding Configurations, and Band-Gap-Opening Properties of Graphene Binding with Low-Concentration Fluorine

    SciTech Connect

    Duan, Yuhua; Stinespring, Charter D.; Chorpening, Benjamin

    2015-06-18

    To better understand the effects of low-level fluorine in graphene-based sensors, first-principles density functional theory (DFT) with van der Waals dispersion interactions has been employed to investigate the structure and impact of fluorine defects on the electrical properties of single-layer graphene films. The results show that both graphite-2H and graphene have zero band gaps. When fluorine bonds to a carbon atom, the carbon atom is pulled slightly above the graphene plane, creating what is referred to as a CF defect. The lowest-binding energy state is found to correspond to two CF defects on nearest neighbor sites, with one fluorine above the carbon plane and the other below the plane. Overall this has the effect of buckling the graphene. The results further show that the addition of fluorine to graphene leads to the formation of an energy band (BF) near the Fermi level, contributed mainly from the 2p orbitals of fluorine with a small contribution from the porbitals of the carbon. Among the 11 binding configurations studied, our results show that only in two cases does the BF serve as a conduction band and open a band gap of 0.37 eV and 0.24 eV respectively. The binding energy decreases with decreasing fluorine concentration due to the interaction between neighboring fluorine atoms. The obtained results are useful for sensor development and nanoelectronics.

  3. Electronic Structures, Bonding Configurations, and Band-Gap-Opening Properties of Graphene Binding with Low-Concentration Fluorine

    DOE PAGESBeta

    Duan, Yuhua; Stinespring, Charter D.; Chorpening, Benjamin

    2015-06-18

    To better understand the effects of low-level fluorine in graphene-based sensors, first-principles density functional theory (DFT) with van der Waals dispersion interactions has been employed to investigate the structure and impact of fluorine defects on the electrical properties of single-layer graphene films. The results show that both graphite-2H and graphene have zero band gaps. When fluorine bonds to a carbon atom, the carbon atom is pulled slightly above the graphene plane, creating what is referred to as a CF defect. The lowest-binding energy state is found to correspond to two CF defects on nearest neighbor sites, with one fluorine abovemore » the carbon plane and the other below the plane. Overall this has the effect of buckling the graphene. The results further show that the addition of fluorine to graphene leads to the formation of an energy band (BF) near the Fermi level, contributed mainly from the 2p orbitals of fluorine with a small contribution from the porbitals of the carbon. Among the 11 binding configurations studied, our results show that only in two cases does the BF serve as a conduction band and open a band gap of 0.37 eV and 0.24 eV respectively. The binding energy decreases with decreasing fluorine concentration due to the interaction between neighboring fluorine atoms. The obtained results are useful for sensor development and nanoelectronics.« less

  4. Penetration of Oxidized Carbon Nanospheres through Lipid Bilayer Membrane: Comparison to Graphene Oxide and Oxidized Carbon Nanotubes, and Effects of pH and Membrane Composition.

    PubMed

    Seemork, Jiraporn; Sansureerungsikul, Titiporn; Sathornsantikun, Kamonluck; Sinthusake, Tarit; Shigyou, Kazuki; Tree-Udom, Thapakorn; Jiangchareon, Banphot; Chiablaem, Khajeelak; Lirdprapamongkol, Kriengsak; Svasti, Jisnuson; Hamada, Tsutomu; Palaga, Tanapat; Wanichwecharungruang, Supason

    2016-09-14

    Here we show that the ability of oxidized carbon particles to penetrate phospholipid bilayer membrane varies with the particle shapes, chemical functionalities on the particle surface, lipid compositions of the membrane and pH conditions. Among the similar surface charged oxidized carbon particles of spherical (oxidized carbon nanosphere, OCS), tubular (oxidized carbon nanotube, OCT), and sheet (oxidized graphene sheet, OGSh) morphologies, OCS possesses the highest levels of adhesion to lipid bilayer membrane and penetration into the cell-sized liposome. OCS preferably binds better to the disordered lipid bilayer membrane (consisting of 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine) as compared to the ordered membrane (consisting of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine and cholesterol). The process of OCS-induced leak on the membrane is pH responsive and most pronounced under an acidic condition. Covalently decorating the OCS's surface with poly(ethylene oxide) or (2-aminoethyl)trimethylammonium moieties decreases its ability to interact with the membrane. When used as carriers, OCSs can deliver curcumin into nucleus of A549 human lung cancer and human embryonic kidney cells, in contrast, curcumin molecules delivered by OCTs remain in the cytoplasm. OGShs cannot significantly enter cells and cannot induce noticeable cellular uptake of curcumin. PMID:27404585

  5. Penetration of Oxidized Carbon Nanospheres through Lipid Bilayer Membrane: Comparison to Graphene Oxide and Oxidized Carbon Nanotubes, and Effects of pH and Membrane Composition.

    PubMed

    Seemork, Jiraporn; Sansureerungsikul, Titiporn; Sathornsantikun, Kamonluck; Sinthusake, Tarit; Shigyou, Kazuki; Tree-Udom, Thapakorn; Jiangchareon, Banphot; Chiablaem, Khajeelak; Lirdprapamongkol, Kriengsak; Svasti, Jisnuson; Hamada, Tsutomu; Palaga, Tanapat; Wanichwecharungruang, Supason

    2016-09-14

    Here we show that the ability of oxidized carbon particles to penetrate phospholipid bilayer membrane varies with the particle shapes, chemical functionalities on the particle surface, lipid compositions of the membrane and pH conditions. Among the similar surface charged oxidized carbon particles of spherical (oxidized carbon nanosphere, OCS), tubular (oxidized carbon nanotube, OCT), and sheet (oxidized graphene sheet, OGSh) morphologies, OCS possesses the highest levels of adhesion to lipid bilayer membrane and penetration into the cell-sized liposome. OCS preferably binds better to the disordered lipid bilayer membrane (consisting of 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine) as compared to the ordered membrane (consisting of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine and cholesterol). The process of OCS-induced leak on the membrane is pH responsive and most pronounced under an acidic condition. Covalently decorating the OCS's surface with poly(ethylene oxide) or (2-aminoethyl)trimethylammonium moieties decreases its ability to interact with the membrane. When used as carriers, OCSs can deliver curcumin into nucleus of A549 human lung cancer and human embryonic kidney cells, in contrast, curcumin molecules delivered by OCTs remain in the cytoplasm. OGShs cannot significantly enter cells and cannot induce noticeable cellular uptake of curcumin.

  6. Competition between spontaneous symmetry breaking and single-particle gaps in trilayer graphene

    NASA Astrophysics Data System (ADS)

    Lee, Y.; Tran, D.; Myhro, K.; Velasco, J.; Gillgren, N.; Lau, C. N.; Barlas, Y.; Poumirol, J. M.; Smirnov, D.; Guinea, F.

    2014-12-01

    Many physical phenomena can be understood by single-particle physics; that is, treating particles as non-interacting entities. When this fails, many-body interactions lead to spontaneous symmetry breaking and phenomena such as fundamental particles’ mass generation, superconductivity and magnetism. Competition between single-particle and many-body physics leads to rich phase diagrams. Here we show that rhombohedral-stacked trilayer graphene offers an exciting platform for studying such interplay, in which we observe a giant intrinsic gap ~42 meV that can be partially suppressed by an interlayer potential, a parallel magnetic field or a critical temperature ~36 K. Among the proposed correlated phases with spatial uniformity, our results are most consistent with a layer antiferromagnetic state with broken time reversal symmetry. These results reflect the interplay between externally induced and spontaneous symmetry breaking whose relative strengths are tunable by external fields, and provide insight into other low-dimensional systems.

  7. Third-order terahertz response of gapped, nearly-metallic armchair graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Wang, Yichao; Andersen, David R.

    2016-11-01

    We use time dependent perturbation theory to study the terahertz nonlinear response of gapped intrinsic and extrinsic nearly-metallic armchair graphene nanoribbons of finite length under an applied electric field. Generally, the nonlinear conductances exhibit contributions due to single-photon, two-photon, and three-photon processes. The interference between each of these processes results in remarkably complex behavior for the third-order conductances, including quantum dot signatures that should be measurable with a relatively simple experimental configuration. Notably, we observe sharp resonances in the isotropic third-order response due to the Van Hove singularities in the density of states at one-, two-, and three-photon resonances. However, these resonances are absent in the anisotropic third-order response; a result of the overall symmetry of the system.

  8. Third-order terahertz response of gapped, nearly-metallic armchair graphene nanoribbons.

    PubMed

    Wang, Yichao; Andersen, David R

    2016-11-30

    We use time dependent perturbation theory to study the terahertz nonlinear response of gapped intrinsic and extrinsic nearly-metallic armchair graphene nanoribbons of finite length under an applied electric field. Generally, the nonlinear conductances exhibit contributions due to single-photon, two-photon, and three-photon processes. The interference between each of these processes results in remarkably complex behavior for the third-order conductances, including quantum dot signatures that should be measurable with a relatively simple experimental configuration. Notably, we observe sharp resonances in the isotropic third-order response due to the Van Hove singularities in the density of states at one-, two-, and three-photon resonances. However, these resonances are absent in the anisotropic third-order response; a result of the overall symmetry of the system. PMID:27633050

  9. Theoretical study of electronic and tribological properties of h-BNC2/graphene, h-BNC2/h-BN and h-BNC2/h-BNC2 bilayers.

    PubMed

    Ansari, Narjes; Nazari, Fariba; Illas, Francesc

    2015-05-21

    Density functional theory based methods are used to investigate the interlayer sliding energy landscape (ISEL), binding energy and interlayer spacing between h-BNC2/graphene (I), h-BNC2/h-BN (II) and h-BNC2/h-BNC2 (III) bilayer structures for three, six and fourteen different stacking patterns, respectively. Our results show that, in the studied cases, increasing the atomic variety of the ingredient monolayers leads to an ISEL corrugation increase as well. For the studied bilayers the ISEL is obtained by means of the registry index. For sufficiently large flakes of h-BNC2 on graphene sheets with the largest incommensurability and the least monolayer anisotropy, a robust superlubricity occurs regardless of the relative interlayer orientation. On the other hand, for the h-BNC2/h-BNC2 bilayer exhibiting the least incommensurability and the most monolayer anisotropy, the occurrence of robust superlubricity depends on the relative interlayer orientation.

  10. Atomic resolution imaging of the two-component Dirac-Landau levels in a gapped graphene monolayer

    NASA Astrophysics Data System (ADS)

    Wang, Wen-Xiao; Yin, Long-Jing; Qiao, Jia-Bin; Cai, Tuocheng; Li, Si-Yu; Dou, Rui-Fen; Nie, Jia-Cai; Wu, Xiaosong; He, Lin

    2015-10-01

    The wave function of Dirac fermions is a two-component spinor. In graphene, a one-atom-thick film showing two-dimensional Dirac-like electronic excitations, the two-component representation, reflects the amplitude of the electron wave function on the A and B sublattices. This unique property provides unprecedented opportunities to image the two components of Dirac fermions spatially. Here, we report atomic resolution imaging of two-component Dirac-Landau levels in gapped graphene monolayers by scanning tunneling microscopy and spectroscopy. A gap of about 20 meV, driven by inversion symmetry breaking by the substrate potential, is observed in the graphene sheets on both SiC and graphite substrates. Such a gap splits the n =0 Landau level (LL) into two levels, 0+ and 0-. We demonstrate that the amplitude of the wave function of the 0+ LL is mainly on the A sites and that of the 0- LL is mainly on the B sites of graphene, characterizing the internal structure of the spinor of the n =0 LL. This provides direct evidence of the two-component nature of Dirac fermions.

  11. Band gap engineering in finite elongated graphene nanoribbon heterojunctions: Tight-binding model

    SciTech Connect

    Tayo, Benjamin O.

    2015-08-15

    A simple model based on the divide and conquer rule and tight-binding (TB) approximation is employed for studying the role of finite size effect on the electronic properties of elongated graphene nanoribbon (GNR) heterojunctions. In our model, the GNR heterojunction is divided into three parts: a left (L) part, middle (M) part, and right (R) part. The left part is a GNR of width W{sub L}, the middle part is a GNR of width W{sub M}, and the right part is a GNR of width W{sub R}. We assume that the left and right parts of the GNR heterojunction interact with the middle part only. Under this approximation, the Hamiltonian of the system can be expressed as a block tridiagonal matrix. The matrix elements of the tridiagonal matrix are computed using real space nearest neighbor orthogonal TB approximation. The electronic structure of the GNR heterojunction is analyzed by computing the density of states. We demonstrate that for heterojunctions for which W{sub L} = W{sub R}, the band gap of the system can be tuned continuously by varying the length of the middle part, thus providing a new approach to band gap engineering in GNRs. Our TB results were compared with calculations employing divide and conquer rule in combination with density functional theory (DFT) and were found to agree nicely.

  12. Hybrid graphene and graphitic carbon nitride nanocomposite: gap opening, electron-hole puddle, interfacial charge transfer, and enhanced visible light response.

    PubMed

    Du, Aijun; Sanvito, Stefano; Li, Zhen; Wang, Dawei; Jiao, Yan; Liao, Ting; Sun, Qiao; Ng, Yun Hau; Zhu, Zhonghua; Amal, Rose; Smith, Sean C

    2012-03-01

    Opening up a band gap and finding a suitable substrate material are two big challenges for building graphene-based nanodevices. Using state-of-the-art hybrid density functional theory incorporating long-range dispersion corrections, we investigate the interface between optically active graphitic carbon nitride (g-C(3)N(4)) and electronically active graphene. We find an inhomogeneous planar substrate (g-C(3)N(4)) promotes electron-rich and hole-rich regions, i.e., forming a well-defined electron-hole puddle, on the supported graphene layer. The composite displays significant charge transfer from graphene to the g-C(3)N(4) substrate, which alters the electronic properties of both components. In particular, the strong electronic coupling at the graphene/g-C(3)N(4) interface opens a 70 meV gap in g-C(3)N(4)-supported graphene, a feature that can potentially allow overcoming the graphene's band gap hurdle in constructing field effect transistors. Additionally, the 2-D planar structure of g-C(3)N(4) is free of dangling bonds, providing an ideal substrate for graphene to sit on. Furthermore, when compared to a pure g-C(3)N(4) monolayer, the hybrid graphene/g-C(3)N(4) complex displays an enhanced optical absorption in the visible region, a promising feature for novel photovoltaic and photocatalytic applications. PMID:22339061

  13. Tuning band gap of monolayer and bilayer SnS2 by strain effect and external electric field: A first principles calculations

    NASA Astrophysics Data System (ADS)

    Rahman, Abeera; Shin, Young-Han

    Recently many efforts have been paid to two-dimensional layered metal dichalcogenides (LMDs). Among them MoS2 has become a prototype LMD, and recent studies show surprising and rich new physics emerging in other van der Waals materials such as layered SnS2 [1-4]. SnS2 is a semiconducting earth-abundant material and Sn is a group IV element replacing the transition metal in MoS2. SnS2 shows new possibilities in various potential applications. However, the knowledge on basic properties of layered SnS2 is still not well understood. In this study, we consider two types of structures; 1T with P 3 m 1 (164) space group and 1H with P63 / mmc (194) space group. Our first principles calculations show that the 1T structure for SnS2 is more stable than the 1H structure whereas latter is more stable for MoS2. Moreover,in contrast to MoS2,SnS2 shows an indirect band gap both for 1T and 1H structures while 1T MoS2 is metallic and 1H has a direct band gap. We also study strain effect in the range of 0-10% on the band structure for monolayer and bilayer SnS2 (both for 1T and 1H structures).We find significant change in their band gaps. We also investigate the bilayer SnS2 with and without out-of-plane stress. This research was supported by Brain Korea 21 Plus Program and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (NRF-2014M3A7B4049367, NRF-2014R1A2A1A1105089).

  14. Theoretical study on stability of hybrid bilayers

    NASA Astrophysics Data System (ADS)

    Silva, Thiago S.; de Lima Bernardo, Bertúlio; Azevedo, Sèrgio

    2015-04-01

    Motivated by the recent experimental realization of the hybrid nanostructure of graphene and boron nitride (h-BN) sheet, and studies of gap modulation by strain, we use first principles calculations based on density functional theory to investigate the effects of strain in hybrid bilayers composed of two monolayers of graphene with a nanodomain of {{B}3}{{N}3}. The calculations were made with two different approximations for the functional exchange-correlation, GGA and VDW-DF. We investigate the modification in the electronic structure and structural properties of various configurations of the hybrid bilayers. Among the configurations, those with Bernal stacking are found to be more stable when compared to the others. Studies of the compressive strain influence were made only in the structure that has been shown to be the most stable. We have found that the two approximations used in the calculations exhibit the same results for the electronic properties of all structures. The opening of the energy gap due to strain was possible in the calculations by using the GGA approximation, but the same does not happen in the calculations using the VDW-DF approximation. Our analysis shows that the VDW-DF approximation is better suited for studies involving surfaces.

  15. Enhanced spin polarization in graphene with spin energy gap induced by spin-orbit coupling and strain

    SciTech Connect

    Liu, Zheng-Fang; Wu, Qing-Ping E-mail: aixichen@ecjtu.jx.cn; Chen, Ai-Xi E-mail: aixichen@ecjtu.jx.cn; Xiao, Xian-Bo; Liu, Nian-Hua

    2014-05-28

    We investigate the possibility of spin polarization in graphene. The result shows that a spin energy gap can be opened in the presence of both spin-orbit coupling and strain. We find that high spin polarization with large spin-polarized current is achieved in the spin energy gap. However, only one of the two modulations is present, no spin polarization can be generated. So the combination of the two modulations provides a way to design tunable spin polarization without need for a magnetic element or an external magnetic field.

  16. Layered reduced graphene oxide with nanoscale interlayer gaps as a stable host for lithium metal anodes

    NASA Astrophysics Data System (ADS)

    Lin, Dingchang; Liu, Yayuan; Liang, Zheng; Lee, Hyun-Wook; Sun, Jie; Wang, Haotian; Yan, Kai; Xie, Jin; Cui, Yi

    2016-07-01

    Metallic lithium is a promising anode candidate for future high-energy-density lithium batteries. It is a light-weight material, and has the highest theoretical capacity (3,860 mAh g–1) and the lowest electrochemical potential of all candidates. There are, however, at least three major hurdles before lithium metal anodes can become a viable technology: uneven and dendritic lithium deposition, unstable solid electrolyte interphase and almost infinite relative dimension change during cycling. Previous research has tackled the first two issues, but the last is still mostly unsolved. Here we report a composite lithium metal anode that exhibits low dimension variation (∼20%) during cycling and good mechanical flexibility. The anode is composed of 7 wt% ‘lithiophilic’ layered reduced graphene oxide with nanoscale gaps that can host metallic lithium. The anode retains up to ∼3,390 mAh g–1 of capacity, exhibits low overpotential (∼80 mV at 3 mA cm–2) and a flat voltage profile in a carbonate electrolyte. A full-cell battery with a LiCoO2 cathode shows good rate capability and flat voltage profiles.

  17. Layered reduced graphene oxide with nanoscale interlayer gaps as a stable host for lithium metal anodes.

    PubMed

    Lin, Dingchang; Liu, Yayuan; Liang, Zheng; Lee, Hyun-Wook; Sun, Jie; Wang, Haotian; Yan, Kai; Xie, Jin; Cui, Yi

    2016-07-01

    Metallic lithium is a promising anode candidate for future high-energy-density lithium batteries. It is a light-weight material, and has the highest theoretical capacity (3,860 mAh g(-1)) and the lowest electrochemical potential of all candidates. There are, however, at least three major hurdles before lithium metal anodes can become a viable technology: uneven and dendritic lithium deposition, unstable solid electrolyte interphase and almost infinite relative dimension change during cycling. Previous research has tackled the first two issues, but the last is still mostly unsolved. Here we report a composite lithium metal anode that exhibits low dimension variation (∼20%) during cycling and good mechanical flexibility. The anode is composed of 7 wt% 'lithiophilic' layered reduced graphene oxide with nanoscale gaps that can host metallic lithium. The anode retains up to ∼3,390 mAh g(-1) of capacity, exhibits low overpotential (∼80 mV at 3 mA cm(-2)) and a flat voltage profile in a carbonate electrolyte. A full-cell battery with a LiCoO2 cathode shows good rate capability and flat voltage profiles.

  18. Layered reduced graphene oxide with nanoscale interlayer gaps as a stable host for lithium metal anodes

    NASA Astrophysics Data System (ADS)

    Lin, Dingchang; Liu, Yayuan; Liang, Zheng; Lee, Hyun-Wook; Sun, Jie; Wang, Haotian; Yan, Kai; Xie, Jin; Cui, Yi

    2016-07-01

    Metallic lithium is a promising anode candidate for future high-energy-density lithium batteries. It is a light-weight material, and has the highest theoretical capacity (3,860 mAh g-1) and the lowest electrochemical potential of all candidates. There are, however, at least three major hurdles before lithium metal anodes can become a viable technology: uneven and dendritic lithium deposition, unstable solid electrolyte interphase and almost infinite relative dimension change during cycling. Previous research has tackled the first two issues, but the last is still mostly unsolved. Here we report a composite lithium metal anode that exhibits low dimension variation (˜20%) during cycling and good mechanical flexibility. The anode is composed of 7 wt% ‘lithiophilic’ layered reduced graphene oxide with nanoscale gaps that can host metallic lithium. The anode retains up to ˜3,390 mAh g-1 of capacity, exhibits low overpotential (˜80 mV at 3 mA cm-2) and a flat voltage profile in a carbonate electrolyte. A full-cell battery with a LiCoO2 cathode shows good rate capability and flat voltage profiles.

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

    SciTech Connect

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

    2014-11-03

    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 mA level. Our study reveals that the barrier height at the Schottky junction is low (∼0.58 eV) due to the Fermi-level pinning above the Dirac point.

  20. Electrical and Mechanical Properties of Graphene

    NASA Astrophysics Data System (ADS)

    Bao, Wenzhong

    devices. A surprising finding in these systems is the observation of insulating states in both suspended bilayer and trilayer graphene devices, which arises from electronic interactions. In bilayer graphene, we observe a phase transition between the single-particle metallic state and the interaction-induced insulating state in ultra-clean BLG, which can be tuned by temperature, disorder, charge density n and perpendicular electric field E ⊥. In trilayer graphene we demonstrate dramatically different transport properties arising from the different stacking orders, and an unexpected spontaneous gap opening in charge neutral ABC-stacked trilayer graphene. One of graphene's unique properties is that it is nature's thinnest elastic membrane with exceptional mechanical properties. In chapter 7 I will describe the first direct observation and controlled creation of one- and two-dimensional periodic ripples in suspended graphene sheets, using both spontaneously and thermally generated strains. We are able to control ripple orientation, wavelength and amplitude by controlling boundary conditions and exploiting graphene's negative thermal expansion coefficient, which we measure to be much larger than that of graphite. In addition, we also study the morphological change of suspended graphene sheets by apply gate voltages, which is a simple and direct method to strain and buckle graphene. Our experimental results contribute to the fundamental understanding of electrical and mechanical properties of graphene, and may have important implications for future graphene based applications.

  1. Orbital-specific Tunability of Many-Body Effects in Bilayer Graphene by Gate Bias and Metal Contact

    NASA Astrophysics Data System (ADS)

    Fukidome, Hirokazu; Kotsugi, Masato; Nagashio, Kosuke; Sato, Ryo; Ohkochi, Takuo; Itoh, Takashi; Toriumi, Akira; Suemitsu, Maki; Kinoshita, Toyohiko

    2014-01-01

    Graphene, a 2D crystal bonded by π and σ orbitals, possesses excellent electronic properties that are promising for next-generation optoelectronic device applications. For these a precise understanding of quasiparticle behaviour near the Dirac point (DP) is indispensable because the vanishing density of states (DOS) near the DP enhances many-body effects, such as excitonic effects and the Anderson orthogonality catastrophe (AOC) which occur through the interactions of many conduction electrons with holes. These effects renormalize band dispersion and DOS, and therefore affect device performance. For this reason, we have studied the impact of the excitonic effects and the AOC on graphene device performance by using X-ray absorption spectromicroscopy on an actual graphene transistor in operation. Our work shows that the excitonic effect and the AOC are tunable by gate bias or metal contacts, both of which alter the Fermi energy, and are orbital-specific.

  2. Topological phase transition in hexagonal boron-nitride bilayers modulated by gate voltage

    NASA Astrophysics Data System (ADS)

    Jin, Guojun; Zhai, Xuechao

    2013-03-01

    We study the gate-voltage modulated electronic properties of hexagonal boron-nitride bilayers with two different stacking structures in the presence of intrinsic and Rashba spin-orbit interactions. Our analytical results show that there are striking cooperation effects arising from the spin-orbit interactions and the interlayer bias voltage. For realizing topological phase transition, in contrast to a gated graphene bilayer for increasing its energy gap, the energy gap of a boron-nitride bilayer is significantly reduced by an applied gate voltage. For the AA stacking-bilayer which has the inversion symmetry, a strong topological phase is found, and there is an interesting reentrant behavior from a normal phase to a topological phase and then to a normal phase again, characterized by the topological index. Therefore, the gate voltage modulated AA-boron nitride bilayer can be taken as a newcomer of the topological insulator family. For the AB stacking-bilayer which is lack of the inversion symmetry, it is always topologically trivial, but exhibits an unusual quantum Hall phase with four degenerate low-energy states localized at a single edge. It is suggested that these theoretical findings could be verified experimentally in the transport properties of boron-nitride bylayers. This research was supported by the NSFC (Nos. 60876065, 11074108), PAPD, and NBRPC (Nos. 2009CB929504, 2011CB922102).

  3. Synthesis of sub-millimeter Bi-/multi-layer graphene by designing a sandwiched structure using copper foils

    NASA Astrophysics Data System (ADS)

    Zhao, Zhijuan; Jia, Kunpeng; Shaw, Jonathan C.; Zhu, Zhenwei; Wan, Wen; Zhan, Linjie; Li, Mengping; Wang, Haosen; Chen, Xiangping; Li, Zhancheng; Chen, Shanshan; Zhou, Yinghui; Kaner, Richard B.; Cai, Weiwei

    2016-09-01

    Bernal-stacked (AB-stacked) bilayer graphene has been receiving significant attention because it has a tunable band-gap under an applied vertical electric field. Herein, we designed a sandwiched structure simply by embedding one piece of Cu sheet into a Cu pocket to establish an environment that suppresses Cu evaporation and ensures that both surfaces of Cu sheet are smooth to grow large-size bilayer graphene (BLG) and multilayer graphene (MLG). Single-diffusion and double-diffusion mechanisms help explain graphene growth on both the Cu pocket and the Cu sheet, respectively. On the basis of the double-diffusion mechanism, we prepared AB-stacked sub-millimeter BLG and MLG with diameters up to 603 μm and 793 μm, respectively. Our work regarding the improvement of the quality and single-crystal size of graphene domains helps broaden the potential applications in materials chemistry and microelectronic devices.

  4. First principles study of trilayers of graphene-BN-graphene

    NASA Astrophysics Data System (ADS)

    Zhong, Xiaoliang; Amorim, Rodrigo; Pandey, Ravindra; Karna, Shashi

    2012-02-01

    The stability, electronic structure and electronic transport properties of graphene-BN-graphene (C-BN-C) trilayers are studied in the framework of density functional theory. Different stacking formats, i.e., AAA, ABA and ABC stackings are considered. The ABA stacking is found to be most energetically favorable, followed by ABC and AAA stackings. The interlayer spacing of trilayers are close to those of corresponding C-BN bilayers, while the intralayer bond length can be regarded as the weighted mean of constituent layers. All considered configurations are found to be metallic, independent of stacking formats. When an external electric field is applied perpendicularly, electronic band structures undergo stacking-dependent variations. While both AAA and ABA stackings show good tunability of energy gap, ABC stacking shows less flexibility of gap tuning. We will also present the results of the electronic transport properties which are modeled by sandwiching trilayers between gold contact electrodes.

  5. Exploring the relative bending of a CVD graphene monolayer with gap-plasmons.

    PubMed

    Min, Young Hwan; Park, Won-Hwa

    2014-08-21

    We report a spectroscopic indicator showing the bending of a chemical vapor deposition (CVD) graphene monolayer on Cu foil or an arbitrary substrate after transfer. Using a Au nanoparticle (NP)-graphene monolayer-Au thin film (TF) junction system, the Radial Breathing-Like Mode (RBLM) Raman signal from the sandwiched graphene monolayer is evidently observed by employing a local z-polarized incident field formed at the Au NP-Au TF junction. We also utilized the RBLM intensity as a quantitative tool with a wide dynamic range (∼300%) compared to the 2D peak width (∼35%) for determining the relative degree of bending on the Au TF substrate. The RBLM signal from the CVD graphene monolayer is anticipated to be used as a valuable marker in exploring out-of-plane directional properties.

  6. Exploring the relative bending of a CVD graphene monolayer with gap-plasmons

    NASA Astrophysics Data System (ADS)

    Min, Young Hwan; Park, Won-Hwa

    2014-07-01

    We report a spectroscopic indicator showing the bending of a chemical vapor deposition (CVD) graphene monolayer on Cu foil or an arbitrary substrate after transfer. Using a Au nanoparticle (NP)-graphene monolayer-Au thin film (TF) junction system, the Radial Breathing-Like Mode (RBLM) Raman signal from the sandwiched graphene monolayer is evidently observed by employing a local z-polarized incident field formed at the Au NP-Au TF junction. We also utilized the RBLM intensity as a quantitative tool with a wide dynamic range (~300%) compared to the 2D peak width (~35%) for determining the relative degree of bending on the Au TF substrate. The RBLM signal from the CVD graphene monolayer is anticipated to be used as a valuable marker in exploring out-of-plane directional properties.We report a spectroscopic indicator showing the bending of a chemical vapor deposition (CVD) graphene monolayer on Cu foil or an arbitrary substrate after transfer. Using a Au nanoparticle (NP)-graphene monolayer-Au thin film (TF) junction system, the Radial Breathing-Like Mode (RBLM) Raman signal from the sandwiched graphene monolayer is evidently observed by employing a local z-polarized incident field formed at the Au NP-Au TF junction. We also utilized the RBLM intensity as a quantitative tool with a wide dynamic range (~300%) compared to the 2D peak width (~35%) for determining the relative degree of bending on the Au TF substrate. The RBLM signal from the CVD graphene monolayer is anticipated to be used as a valuable marker in exploring out-of-plane directional properties. Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr01586j

  7. Metal-assisted exfoliation (MAE): green process for transferring graphene to flexible substrates and templating of sub-nanometer plasmonic gaps (Presentation Recording)

    NASA Astrophysics Data System (ADS)

    Zaretski, Aliaksandr V.; Marin, Brandon C.; Moetazedi, Herad; Dill, Tyler J.; Jibril, Liban; Kong, Casey; Tao, Andrea R.; Lipomi, Darren J.

    2015-09-01

    This paper describes a new technique, termed "metal-assisted exfoliation," for the scalable transfer of graphene from catalytic copper foils to flexible polymeric supports. The process is amenable to roll-to-roll manufacturing, and the copper substrate can be recycled. We then demonstrate the use of single-layer graphene as a template for the formation of sub-nanometer plasmonic gaps using a scalable fabrication process called "nanoskiving." These gaps are formed between parallel gold nanowires in a process that first produces three-layer thin films with the architecture gold/single-layer graphene/gold, and then sections the composite films with an ultramicrotome. The structures produced can be treated as two gold nanowires separated along their entire lengths by an atomically thin graphene nanoribbon. Oxygen plasma etches the sandwiched graphene to a finite depth; this action produces a sub-nanometer gap near the top surface of the junction between the wires that is capable of supporting highly confined optical fields. The confinement of light is confirmed by surface-enhanced Raman spectroscopy measurements, which indicate that the enhancement of the electric field arises from the junction between the gold nanowires. These experiments demonstrate nanoskiving as a unique and easy-to-implement fabrication technique that is capable of forming sub-nanometer plasmonic gaps between parallel metallic nanostructures over long, macroscopic distances. These structures could be valuable for fundamental investigations as well as applications in plasmonics and molecular electronics.

  8. 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.

  9. Giant gap quantum spin Hall effect and valley-polarized quantum anomalous Hall effect in cyanided bismuth bilayers

    NASA Astrophysics Data System (ADS)

    Ji, Wei-xiao; Zhang, Chang-wen; Ding, Meng; Zhang, Bao-min; Li, Ping; Li, Feng; Ren, Miao-juan; Wang, Pei-ji; Zhang, Run-wu; Hu, Shu-jun; Yan, Shi-shen

    2016-08-01

    Bismuth (Bi) has attracted a great deal of attention for its strongest spin–orbit coupling (SOC) strength among main group elements. Although quantum anomalous Hall (QAH) state is predicted in half-hydrogenated Bi honeycomb monolayers Bi2H, the experimental results are still missing. Halogen atoms (X = F, Cl and Br) were also frequently used as modifications, but Bi2X films show a frustrating metallic character that masks the QAH effects. Here, first-principle calculations are performed to predict the full-cyanided bismuthene (Bi2(CN)2) as 2D topological insulator supporting quantum spin Hall state with a record large gap up to 1.10 eV, and more importantly, half-cyanogen saturated bismuthene (Bi2(CN)) as a Chern insulator supporting a valley-polarized QAH state, with a Curie temperature to be 164 K, as well as a large gap reaching 0.348 eV which could be further tuned by bi-axial strain and SOC strength. Our findings provide an appropriate and flexible material family candidate for spintronic and valleytronic devices.

  10. Giant gap quantum spin Hall effect and valley-polarized quantum anomalous Hall effect in cyanided bismuth bilayers

    NASA Astrophysics Data System (ADS)

    Ji, Wei-xiao; Zhang, Chang-wen; Ding, Meng; Zhang, Bao-min; Li, Ping; Li, Feng; Ren, Miao-juan; Wang, Pei-ji; Zhang, Run-wu; Hu, Shu-jun; Yan, Shi-shen

    2016-08-01

    Bismuth (Bi) has attracted a great deal of attention for its strongest spin-orbit coupling (SOC) strength among main group elements. Although quantum anomalous Hall (QAH) state is predicted in half-hydrogenated Bi honeycomb monolayers Bi2H, the experimental results are still missing. Halogen atoms (X = F, Cl and Br) were also frequently used as modifications, but Bi2X films show a frustrating metallic character that masks the QAH effects. Here, first-principle calculations are performed to predict the full-cyanided bismuthene (Bi2(CN)2) as 2D topological insulator supporting quantum spin Hall state with a record large gap up to 1.10 eV, and more importantly, half-cyanogen saturated bismuthene (Bi2(CN)) as a Chern insulator supporting a valley-polarized QAH state, with a Curie temperature to be 164 K, as well as a large gap reaching 0.348 eV which could be further tuned by bi-axial strain and SOC strength. Our findings provide an appropriate and flexible material family candidate for spintronic and valleytronic devices.

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

    SciTech Connect

    Sahdan, Muhammad Fauzi Darma, Yudi

    2014-03-24

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

  12. 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

  13. Monolayer-to-bilayer transformation of silicenes and their structural analysis

    NASA Astrophysics Data System (ADS)

    Yaokawa, Ritsuko; Ohsuna, Tetsu; Morishita, Tetsuya; Hayasaka, Yuichiro; Spencer, Michelle J. S.; Nakano, Hideyuki

    2016-02-01

    Silicene, a two-dimensional honeycomb network of silicon atoms like graphene, holds great potential as a key material in the next generation of electronics; however, its use in more demanding applications is prevented because of its instability under ambient conditions. Here we report three types of bilayer silicenes that form after treating calcium-intercalated monolayer silicene (CaSi2) with a BF4- -based ionic liquid. The bilayer silicenes that are obtained are sandwiched between planar crystals of CaF2 and/or CaSi2, with one of the bilayer silicenes being a new allotrope of silicon, containing four-, five- and six-membered sp3 silicon rings. The number of unsaturated silicon bonds in the structure is reduced compared with monolayer silicene. Additionally, the bandgap opens to 1.08 eV and is indirect; this is in contrast to monolayer silicene which is a zero-gap semiconductor.

  14. Monolayer-to-bilayer transformation of silicenes and their structural analysis

    PubMed Central

    Yaokawa, Ritsuko; Ohsuna, Tetsu; Morishita, Tetsuya; Hayasaka, Yuichiro; Spencer, Michelle J. S.; Nakano, Hideyuki

    2016-01-01

    Silicene, a two-dimensional honeycomb network of silicon atoms like graphene, holds great potential as a key material in the next generation of electronics; however, its use in more demanding applications is prevented because of its instability under ambient conditions. Here we report three types of bilayer silicenes that form after treating calcium-intercalated monolayer silicene (CaSi2) with a BF4− -based ionic liquid. The bilayer silicenes that are obtained are sandwiched between planar crystals of CaF2 and/or CaSi2, with one of the bilayer silicenes being a new allotrope of silicon, containing four-, five- and six-membered sp3 silicon rings. The number of unsaturated silicon bonds in the structure is reduced compared with monolayer silicene. Additionally, the bandgap opens to 1.08 eV and is indirect; this is in contrast to monolayer silicene which is a zero-gap semiconductor. PMID:26847858

  15. Engineering topological states in graphene systems

    NASA Astrophysics Data System (ADS)

    Qiao, Zhenhua

    2013-03-01

    In this talk, I will introduce our recent progress on engineering various topological states in graphene systems. The presentation includes two parts: (i) We show that in monolayer graphene, Rashba spin-orbit coupling (SOC) together with Zeeman field can open a nontrivial bulk gap to host the quantum anomalous-Hall effect. We further show that this can be realized via doping magnetic metal atoms on graphene. In Bernal stacking bilayer graphene, an interlayer potential difference breaks the inversion symmetry and opens a bulk gap to support the quantum valley-Hall effect. We find that Rashba SOC can induce a topological phase transition from the quantum valley-Hall effect to a Z2 topological insulator. When the Zeeman field is further considered, a rich variety of topological phases emerge. (ii) When the mass term (e.g., sublattice potential in monolayer graphene, or interlayer potential difference in bilayer graphene) varies spatially, topologically protected 1D kink states arise along zero lines. We demonstrate that such 1D kink state exhibits zero bend resistance for arbitrary turns in its propagating path. We further point out that similar kink states can be tailored in graphene nanoroads in boron nitride sheets. When the kink current experiences a crossing junction composed of four zero lines, we find the splitting of the 1D kink state at the bifurcation point obeys an explicit law of current partition. Collaborators: Shengyuan A. Yang, Wanxiang Feng, Jun Ding, Jeil Jung, Wang-Kong Tse, Chungwei Lin, Hua Jiang, Haiwen Liu, Junren Shi, Jian Wang, Yugui Yao, Allan H. MacDonald, and Qian Niu

  16. All-graphene-battery: bridging the gap between supercapacitors and lithium ion batteries.

    PubMed

    Kim, Haegyeom; Park, Kyu-Young; Hong, Jihyun; Kang, Kisuk

    2014-01-01

    Herein, we propose an advanced energy-storage system: all-graphene-battery. It operates based on fast surface-reactions in both electrodes, thus delivering a remarkably high power density of 6,450 W kg(-1)(total electrode) while also retaining a high energy density of 225 Wh kg(-1)(total electrode), which is comparable to that of conventional lithium ion battery. The performance and operating mechanism of all-graphene-battery resemble those of both supercapacitors and batteries, thereby blurring the conventional distinction between supercapacitors and batteries. This work demonstrates that the energy storage system made with carbonaceous materials in both the anode and cathode are promising alternative energy-storage devices.

  17. All-graphene-battery: bridging the gap between supercapacitors and lithium ion batteries

    PubMed Central

    Kim, Haegyeom; Park, Kyu-Young; Hong, Jihyun; Kang, Kisuk

    2014-01-01

    Herein, we propose an advanced energy-storage system: all-graphene-battery. It operates based on fast surface-reactions in both electrodes, thus delivering a remarkably high power density of 6,450 W kg−1total electrode while also retaining a high energy density of 225 Wh kg−1total electrode, which is comparable to that of conventional lithium ion battery. The performance and operating mechanism of all-graphene-battery resemble those of both supercapacitors and batteries, thereby blurring the conventional distinction between supercapacitors and batteries. This work demonstrates that the energy storage system made with carbonaceous materials in both the anode and cathode are promising alternative energy-storage devices. PMID:24923290

  18. Bonding between graphene and MoS{sub 2} monolayers without and with Li intercalation

    SciTech Connect

    Ahmed, Towfiq; Modine, N. A.; Zhu, Jian-Xin

    2015-07-27

    We performed density functional theory (DFT) calculations for a bi-layered heterostructure combining a graphene layer with a MoS{sub 2} layer with and without intercalated Li atoms. Our calculations demonstrate the importance of the van der Waals (vdW) interaction, which is crucial for forming stable bonding between the layers. Our DFT calculation correctly reproduces the linear dispersion, or Dirac cone, feature at the Fermi energy for the isolated graphene monolayer and the band gap for the MoS{sub 2} monolayer. For the combined graphene/MoS{sub 2} bi-layer, we observe interesting electronic structure and density of states (DOS) characteristics near the Fermi energy, showing both the gap like features of the MoS{sub 2} layer and in-gap states with linear dispersion contributed mostly by the graphene layer. Our calculated total DOS in this vdW heterostructure reveals that the graphene layer significantly contributes to pinning the Fermi energy at the center of the band gap of MoS{sub 2}. We also find that intercalating Li ions in between the layers of the graphene/MoS{sub 2} heterostructure enhances the binding energy through orbital hybridizations between cations (Li adatoms) and anions (graphene and MoS{sub 2} monolayers). Moreover, we calculate the dielectric function of the Li intercalated graphene/MoS{sub 2} heterostructure, the imaginary component of which can be directly compared with experimental measurements of optical conductivity in order to validate our theoretical prediction. We observe sharp features in the imaginary component of the dielectric function, which shows the presence of a Drude peak in the optical conductivity, and therefore metallicity in the lithiated graphene/MoS{sub 2} heterostructure.

  19. Dimers and trimers of polycyclic aromatic hydrocarbons as models of graphene bilayers and trilayers: enhanced electron density at the edges

    NASA Astrophysics Data System (ADS)

    Yadav, Amarjeet; Mishra, P. C.

    2014-01-01

    Structures of dimers and trimers of polycyclic aromatic hydrocarbons (PAHs) having zig-zag edges, and continuous electron density and molecular electrostatic potential (MEP) distributions in these systems were studied in gas phase. Dimers of benzene and naphthalene for which high-accuracy results are available were used to test the reliability of four different functionals of density functional theory in combination with the 6-31G(d,p) basis set. The dispersion-corrected WB97XD functional was found to be distinctly superior to the other three functionals used and was employed to study PAH dimers and trimers. Electronic structures and geometries of dimers of a four benzene ring and a nine benzene ring systems and trimers of the four benzene ring system were investigated. The dimers and trimers of PAHs were found to be of parallel-displaced type, as observed experimentally for graphene. The enhanced electron density edge effect found in the PAH monomers earlier is found to exist in the dimers and trimers also.

  20. Strain effects on the band gap and work function of zigzag single-walled carbon nanotubes and graphene: A comparative study

    NASA Astrophysics Data System (ADS)

    Lin, Ken-Ming; Huang, Yu-Hui; Su, W. S.; Leung, T. C.

    2014-05-01

    First-principles local density functional calculation of the electronic structure and work function of zigzag single-wall carbon nanotubes (SWCNTs) and graphene under strain are presented. We found that there is a small circumference strain even for unstrained zigzag SWCNTs that release the curvature energy of the tube. Therefore, we propose that there are two effects contributing to the opening of the band gap of unstrained (3q, 0) SWCNTs: the electron transfer enhancement effect and the circumference strain effect. We show that the band gap and work function of strained zigzag SWCNTs can be successfully estimated from the band structure and work function of strained graphene based on the zone folding method with circumference strain effect. The circumference strain effect is crucial to obtain a correct result. The rate of change of the band gap with respect to the strain for zigzag SWCNTs is almost independent of the value of the strain and the radius of the tube.

  1. Surface enhanced Raman scattering by graphene-nanosheet-gapped plasmonic nanoparticle arrays for multiplexed DNA detection

    NASA Astrophysics Data System (ADS)

    Duan, Bo; Zhou, Jiajing; Fang, Zheng; Wang, Chenxu; Wang, Xiujuan; Hemond, Harold F.; Chan-Park, Mary B.; Duan, Hongwei

    2015-07-01

    We have developed a new type of surface enhanced Raman scattering (SERS) substrate with thiolated graphene oxide (tGO) nanosheets sandwiched between two layers of closely packed plasmonic nanoparticles. The trilayered substrate is built up through alternative loading of interfacially assembled plasmonic nanoparticle arrays and tGO nanosheets, followed by coating the nanoparticle surfaces with poly(ethylene glycol) (PEG). Here tGO plays multifunctional roles as a 2D scaffold to immobilized interfacially assembled plasmonic nanoparticles, a nanospacer to create SERS-active nanogaps between two layers of nanoparticle arrays, and a molecule harvester to enrich molecules of interest via π-π interaction. In particular, the molecule harvesting capability of the tGO nanospacer and the stealth properties of PEG coating on the plasmonic nanoparticles collectively lead to preferential positioning of selective targets such as aromatic molecules and single-stranded DNA at the SERS-active nanogap hotspots. We have demonstrated that an SERS assay based on the PEGylated trilayered substrate, in combination with magnetic separation, allows for sensitive, multiplexed ``signal-off'' detection of DNA sequences of bacterial pathogens.We have developed a new type of surface enhanced Raman scattering (SERS) substrate with thiolated graphene oxide (tGO) nanosheets sandwiched between two layers of closely packed plasmonic nanoparticles. The trilayered substrate is built up through alternative loading of interfacially assembled plasmonic nanoparticle arrays and tGO nanosheets, followed by coating the nanoparticle surfaces with poly(ethylene glycol) (PEG). Here tGO plays multifunctional roles as a 2D scaffold to immobilized interfacially assembled plasmonic nanoparticles, a nanospacer to create SERS-active nanogaps between two layers of nanoparticle arrays, and a molecule harvester to enrich molecules of interest via π-π interaction. In particular, the molecule harvesting capability of

  2. Strain effects on the optical conductivity of gapped graphene in the presence of Holstein phonons beyond the Dirac cone approximation

    NASA Astrophysics Data System (ADS)

    Yarmohammadi, Mohsen

    2016-08-01

    In this paper we study the optical conductivity and density of states (DOS) of doped gapped graphene beyond the Dirac cone approximation in the presence of electron-phonon (e-ph) interaction under strain, i.e., within the framework of a full π-band Holstein model, by using the Kubo linear response formalism that is established upon the retarded self-energy. A new peak in the optical conductivity for a large enough e-ph interaction strength is found which is associated to transitions between the midgap states and the Van Hove singularities of the main π-band. Optical conductivity decreases with strain and at large strains, the system has a zero optical conductivity at low energies due to optically inter-band excitations through the limit of zero doping. As a result, the Drude weight changes with e-ph interaction, temperature and strain. Consequently, DOS and optical conductivity remains stable with temperature at low e-ph coupling strengths.

  3. Nitrogen Doping Enables Covalent-Like π–π Bonding between Graphenes

    DOE PAGESBeta

    Tian, Yong-Hui; Huang, Jingsong; Sheng, Xiaolan; Sumpter, Bobby G.; Yoon, Mina; Kertesz, Miklos

    2015-07-07

    In neighboring layers of bilayer (and few-layer) graphenes, both AA and AB stacking motifs are known to be separated at a distance corresponding to van der Waals (vdW) interactions. In this Letter, we present for the first time a new aspect of graphene chemistry in terms of a special chemical bonding between the giant graphene "molecules". Through rigorous theoretical calculations, we demonstrate that the N-doped graphenes (NGPs) with various doping levels can form an unusual two-dimensional (2D) pi-pi bonding in bilayer NGPs bringing the neighboring NGPs to significantly reduced interlayer separations. The interlayer binding energies can be enhanced by upmore » to 50% compared to the pristine graphene bilayers that are characterized by only vdW interactions. Such an unusual chemical bonding arises from the pi-pi overlap across the vdW gap while the individual layers maintain their in-plane pi-conjugation and are accordingly planar. Moreover, the existence of the resulting interlayer covalent-like bonding is corroborated by electronic structure calculations and crystal orbital overlap population (COOP) analyses. In NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked and the 3D bulk exhibits metallic characteristics both in the in-plane and along the stacking directions.« less

  4. Nitrogen Doping Enables Covalent-Like π-π Bonding between Graphenes.

    PubMed

    Tian, Yong-Hui; Huang, Jingsong; Sheng, Xiaolan; Sumpter, Bobby G; Yoon, Mina; Kertesz, Miklos

    2015-08-12

    The neighboring layers in bilayer (and few-layer) graphenes of both AA and AB stacking motifs are known to be separated at a distance corresponding to van der Waals (vdW) interactions. In this Letter, we present for the first time a new aspect of graphene chemistry in terms of a special chemical bonding between the giant graphene "molecules". Through rigorous theoretical calculations, we demonstrate that the N-doped graphenes (NGPs) with various doping levels can form an unusual two-dimensional (2D) π-π bonding in bilayer NGPs bringing the neighboring NGPs to significantly reduced interlayer separations. The interlayer binding energies can be enhanced by up to 50% compared to the pristine graphene bilayers that are characterized by only vdW interactions. Such an unusual chemical bonding arises from the π-π overlap across the vdW gap while the individual layers maintain their in-plane π-conjugation and are accordingly planar. The existence of the resulting interlayer covalent-like bonding is corroborated by electronic structure calculations and crystal orbital overlap population (COOP) analyses. In NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked and the 3D bulk exhibits metallic characteristics both in the in-plane and along the stacking directions. PMID:26151153

  5. Nitrogen Doping Enables Covalent-Like π-π Bonding between Graphenes.

    PubMed

    Tian, Yong-Hui; Huang, Jingsong; Sheng, Xiaolan; Sumpter, Bobby G; Yoon, Mina; Kertesz, Miklos

    2015-08-12

    The neighboring layers in bilayer (and few-layer) graphenes of both AA and AB stacking motifs are known to be separated at a distance corresponding to van der Waals (vdW) interactions. In this Letter, we present for the first time a new aspect of graphene chemistry in terms of a special chemical bonding between the giant graphene "molecules". Through rigorous theoretical calculations, we demonstrate that the N-doped graphenes (NGPs) with various doping levels can form an unusual two-dimensional (2D) π-π bonding in bilayer NGPs bringing the neighboring NGPs to significantly reduced interlayer separations. The interlayer binding energies can be enhanced by up to 50% compared to the pristine graphene bilayers that are characterized by only vdW interactions. Such an unusual chemical bonding arises from the π-π overlap across the vdW gap while the individual layers maintain their in-plane π-conjugation and are accordingly planar. The existence of the resulting interlayer covalent-like bonding is corroborated by electronic structure calculations and crystal orbital overlap population (COOP) analyses. In NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked and the 3D bulk exhibits metallic characteristics both in the in-plane and along the stacking directions.

  6. Nitrogen-Doping Enables Covalent-Like pi-pi Bonding between Graphenes

    SciTech Connect

    Tian, Yong-Hui; Huang, Jingsong; Sumpter, Bobby G; Kertesz, Prof. Miklos

    2015-01-01

    The neighboring layers in bi-layer (and few-layer) graphenes of both AA and AB stacking motifs are known to be separated at a distance corresponding to van der Waals (vdW) interactions. In this Letter, we present for the first time a new aspect of graphene chemistry in terms of a special chemical bonding between the giant graphene molecules . Through rigorous theoretical calculations, we demonstrate that the N-doped graphenes (NGPs) with various doping levels can form an unusual two-dimensional (2D) pi pi bonding in bi-layer NGPs bringing the neighboring NGPs to significantly reduced interlayer separations. The interlayer binding energies can be enhanced by up to 50% compared to the pristine graphene bi-layers that are characterized by only vdW interactions. Such an unusual chemical bonding arises from the pi pi overlap across the vdW gap while the individual layers maintain their in-plane pi-conjugation and are accordingly planar. The existence of the resulting interlayer covalent-like bonding is corroborated by electronic structure calculations and crystal orbital overlap population (COOP) analyses. In NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked and the 3D bulk exhibits metallic characteristics both in the in-plane and along the stacking directions.

  7. Nitrogen Doping Enables Covalent-Like π–π Bonding between Graphenes

    SciTech Connect

    Tian, Yong-Hui; Huang, Jingsong; Sheng, Xiaolan; Sumpter, Bobby G.; Yoon, Mina; Kertesz, Miklos

    2015-07-07

    In neighboring layers of bilayer (and few-layer) graphenes, both AA and AB stacking motifs are known to be separated at a distance corresponding to van der Waals (vdW) interactions. In this Letter, we present for the first time a new aspect of graphene chemistry in terms of a special chemical bonding between the giant graphene "molecules". Through rigorous theoretical calculations, we demonstrate that the N-doped graphenes (NGPs) with various doping levels can form an unusual two-dimensional (2D) pi-pi bonding in bilayer NGPs bringing the neighboring NGPs to significantly reduced interlayer separations. The interlayer binding energies can be enhanced by up to 50% compared to the pristine graphene bilayers that are characterized by only vdW interactions. Such an unusual chemical bonding arises from the pi-pi overlap across the vdW gap while the individual layers maintain their in-plane pi-conjugation and are accordingly planar. Moreover, the existence of the resulting interlayer covalent-like bonding is corroborated by electronic structure calculations and crystal orbital overlap population (COOP) analyses. In NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked and the 3D bulk exhibits metallic characteristics both in the in-plane and along the stacking directions.

  8. Graphene-based structure, method of suspending graphene membrane, and method of depositing material onto graphene membrane

    DOEpatents

    Zettl, Alexander K.; Meyer, Jannik Christian

    2013-04-02

    An embodiment of a method of suspending a graphene membrane across a gap in a support structure includes attaching graphene to a substrate. A pre-fabricated support structure having the gap is attached to the graphene. The graphene and the pre-fabricated support structure are then separated from the substrate which leaves the graphene membrane suspended across the gap in the pre-fabricated support structure. An embodiment of a method of depositing material includes placing a support structure having a graphene membrane suspended across a gap under vacuum. A precursor is adsorbed to a surface of the graphene membrane. A portion of the graphene membrane is exposed to a focused electron beam which deposits a material from the precursor onto the graphene membrane. An embodiment of a graphene-based structure includes a support structure having a gap, a graphene membrane suspended across the gap, and a material deposited in a pattern on the graphene membrane.

  9. Effects of the electric field on the properties of ZnO-graphene composites: a density functional theory study.

    PubMed

    Geng, Wei; Zhao, Xuefei; Zan, Wenyan; Liu, Huanxiang; Yao, Xiaojun

    2014-02-28

    In this work, the effects of the electric field on the properties of ZnO-graphene composites were theoretically studied using density functional theory calculations. Three types of ZnO-graphene composites including composites of pristine graphene, graphene with defects as well as graphene oxide and a ZnO bilayer were studied. We calculated and analyzed the binding energies, charge transfer, band structures and work functions of the above composites under the external electric fields. The DFT calculation results demonstrate that the binding energies are sensitive to the electric field, and increasing the external electric field gives rise to stronger binding energies. The extent of charge transfer is correlated with the magnitude of the external electric field, but the band gaps are hardly affected by the external electric field. The work functions vary depending on the different structures of the composites and surface sides, and they are also tunable by the external electric field. PMID:24285715

  10. Effects of the electric field on the properties of ZnO-graphene composites: a density functional theory study.

    PubMed

    Geng, Wei; Zhao, Xuefei; Zan, Wenyan; Liu, Huanxiang; Yao, Xiaojun

    2014-02-28

    In this work, the effects of the electric field on the properties of ZnO-graphene composites were theoretically studied using density functional theory calculations. Three types of ZnO-graphene composites including composites of pristine graphene, graphene with defects as well as graphene oxide and a ZnO bilayer were studied. We calculated and analyzed the binding energies, charge transfer, band structures and work functions of the above composites under the external electric fields. The DFT calculation results demonstrate that the binding energies are sensitive to the electric field, and increasing the external electric field gives rise to stronger binding energies. The extent of charge transfer is correlated with the magnitude of the external electric field, but the band gaps are hardly affected by the external electric field. The work functions vary depending on the different structures of the composites and surface sides, and they are also tunable by the external electric field.

  11. Monolayer graphene from a green solid precursor

    NASA Astrophysics Data System (ADS)

    Kalita, Golap; Wakita, Koichi; Umeno, Masayoshi

    2011-06-01

    Monolayer and bilayer graphene sheets are synthesized by simple control pyrolysis of solid botanical derivative camphor (C 10H 16O), a green and renewable carbon source. Raman studies show much intense 2D peak than that of G peak, signifying presence of monolayer graphene. Transmission electron microscopic study shows predominately monolayer or bilayer graphene sheets, while trilayer graphene sheet were also observed. Synthesized graphene film on copper foil is transferred to poly(ethylene terephthalate) substrate to fabricate transparent electrode. Electrical and optical measurement shows a sheet resistance of 860 Ω/sq with a transmittance of 91% at 550 nm wavelength of the graphene film. The technique to fabricate monolayer or bilayer graphene based film from camphor is both viable and scalable for potential large area electronic applications.

  12. Electronic structures and elastic properties of monolayer and bilayer transition metal dichalcogenides MX2 (M = Mo, W; X = O, S, Se, Te): A comparative first-principles study

    NASA Astrophysics Data System (ADS)

    Zeng, Fan; Zhang, Wei-Bing; Tang, Bi-Yu

    2015-09-01

    First-principle calculations with different exchange-correlation functionals, including LDA, PBE, and vdW-DF functional in the form of optB88-vdW, have been performed to investigate the electronic and elastic properties of two-dimensional transition metal dichalcogenides (TMDCs) with the formula of MX2(M = Mo, W; X = O, S, Se, Te) in both monolayer and bilayer structures. The calculated band structures show a direct band gap for monolayer TMDCs at the K point except for MoO2 and WO2. When the monolayers are stacked into a bilayer, the reduced indirect band gaps are found except for bilayer WTe2, in which the direct gap is still present at the K point. The calculated in-plane Young moduli are comparable to that of graphene, which promises possible application of TMDCs in future flexible and stretchable electronic devices. We also evaluated the performance of different functionals including LDA, PBE, and optB88-vdW in describing elastic moduli of TMDCs and found that LDA seems to be the most qualified method. Moreover, our calculations suggest that the Young moduli for bilayers are insensitive to stacking orders and the mechanical coupling between monolayers seems to be negligible. Project supported by the Construct Program of the Key Discipline in Hunan Province, China and Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, China.

  13. Tunneling conductance in a gapped graphene-based normal metal-insulator-d-wave superconductor junction: Case of massive Dirac electrons

    NASA Astrophysics Data System (ADS)

    Goudarzi, H.; Khezerlou, M.

    2010-12-01

    Using the extended Blonder-Tinkham-Klapwijk formalism, the normal conductance spectra in a normal/insulator/d-wave superconductor gapped graphene junction, considering effect of asymmetric pairing potential (anisotropic d-wave) is investigated in the limit of a thin barrier. The charged Dirac carriers in this structure are treated as massive relativistic particles. The exact solutions of Dirac-Bogoliubov-de Gennes Hamiltonain for three normal gapped graphene, insulator and superconductor region of junction and related normal and Andreev reflection coefficients are obtained. In this work, we focus to study in detail the effect of rotated angle α caused by d-wave order parameter of superconductor in tunneling conductance behavior in our system. In particular, the conductance spectra in terms of the bias voltage eV, the d-wave superconducting orientation angular α and also the electrostatic potentials U0 and V0 is plotted. It is shown that by increasing rotated angle α in that case of U0→∞ and mv2F/ EF=0.99, the maximum of resonant peak of normal conductance decreases and also the position of peak shifts from eV/Δ=1 to progressively lower values. We also observe an oscillatory behavior of conductance versus insulator potential V0.

  14. Electromagnetic metamaterial-inspired band gap and perfect transmission in semiconductor and graphene-based electronic and photonic structures

    NASA Astrophysics Data System (ADS)

    Mahdy, M. R. C.; Al Sayem, Ayed; Shahriar, Arif; Shawon, Jubayer; Al-Quaderi, Golam Dastegir; Jahangir, Ifat; Matin, M. A.

    2016-04-01

    In this article, at first we propose a unified and compact classification of single negative electromagnetic metamaterial-based perfect transmission unit cells. The classes are named as: type-A, -B and -C unit cells. Then based on the classification, we have extended these ideas in semiconductor and graphene regimes. For type-A: Based on the idea of electromagnetic Spatial Average Single Negative bandgap, novel bandgap structures have been proposed for electron transmission in semiconductor heterostructures. For type-B: with dielectric-graphene-dielectric structure, almost all angle transparency is achieved for both polarizations of electromagnetic wave in the terahertz frequency range instead of the conventional transparency in the microwave frequency range. Finally the application of the gated dielectric-graphene-dielectric has been demonstrated for the modulation and switching purpose.

  15. Theory of valley-dependent transport in graphene-based lateral quantum structures

    NASA Astrophysics Data System (ADS)

    Chen, Feng-Wu; Chou, Mei-Yin; Chen, Yiing-Rei; Wu, Yu-Shu

    2016-08-01

    Modulation of electronic states in two-dimensional materials can be achieved by using in-plane variations of the band gap or the average potential in lateral quantum structures. In the atomic configurations with hexagonal symmetry, this approach makes it possible to tailor the valleytronic properties for potential device applications. In this work, we present a multiband theory to calculate the valley-dependent electron transport in graphene-based lateral quantum structures. As an example, we consider the structures with a single interface that exhibits an energy gap or potential discontinuity. The theoretical formalism proceeds within the tight-binding description, by first deriving the local bulk complex band structures in the regions of a constant gap or potential and, next, joining the local wave functions across the interface via a cell-averaged current operator to ensure the current continuity. The theory is applied to the study of electron reflection off and transmission through an interface. Both reflection and transmission are found to exhibit valley-contrast behavior that can be used to generate valley-polarized electron sources. The results vary with the type of interfaces, as well as between monolayer and bilayer graphene-based structures. In the monolayer case, the valley contrast originates from the band warping and only becomes sizable for incident carriers of high energy, whereas in AB-stacked bilayer graphene, the vertical interlayer coupling emerges as an additional important cause for valley contrast, and the favorable carrier energy is also found to be drastically lower. Our numerical results clearly demonstrate the propitious valleytronic properties of bilayer graphene structures.

  16. Intriguing electronic properties of two-dimensional MoS2/TM2CO2 (TM = Ti, Zr, or Hf) hetero-bilayers: type-II semiconductors with tunable band gaps.

    PubMed

    Li, Xinru; Dai, Ying; Ma, Yandong; Liu, Qunqun; Huang, Baibiao

    2015-03-27

    Two-dimensional (2D) transition metal compound (TMC) monolayers, as well as their van der Waals heterostructures with unique properties, are fundamentally and technologically intriguing. Here, heterostructures consisting of a MoS2 monolayer and TM2CO2 (TM = Ti, Zr or Hf) monolayers are systematically researched by means of the density functional theory (DFT). Different from semiconductor/metal contacts, MoS2 and TM2CO2 monolayers are all semiconductors with band gaps ranging from 0.25-1.67 eV. According to rigorous screening of stacking patterns, MoS2/Zr2CO2 is shown to be an indirect type-II semiconductor with the maximum valence and minimum conduction bands spatially separated on opposite monolayers. Simultaneously, the interface charges transfer from Zr2CO2 to MoS2 results in a built-in field that separates the electrons and holes efficiently. Also, the smaller effective masses of electrons and the holes of band edges indicate the higher carrier mobility. Moreover, strain regulation can make the hetero-bilayer's character a semiconductor-semimetal-metal transition. The physical insights pave the way for the good performance of MoS2/TM2CO2 in next-generation electronic devices and photocatalysts.

  17. Spontaneous Exciton Condensate in Transition Metal Dichalcogenides electron-hole bilayer System

    NASA Astrophysics Data System (ADS)

    Debnath, Bishwajit; Barlas, Yafis; Wickramaratne, Darshana; Neupane, Mahesh; Lake, Roger

    Spontaneous Bose-Einstein Exciton condensation (BEC) in spatially separated graphene layers has received intense theoretical and experimental attention, due to its promise for low-dissipation electronic devices. We have investigated BN-separated monolayers of transition metal dichalcogenides (TMDs) to explore the possibility of achieving exciton superfluidity in this class of 2D materials. The top and bottom monolayers can consist of either same TMD (homo-bilayer) or a combination of different TMDs (hetero-bilayer). The particle density in each monolayer is tuned by independent gate biasing. In TMDs, the almost equivalent particle-hole symmetry is an assisting factor towards achieving condensation. The calculated exciton binding energies are found to be as large as 50 - 250 meV, which is a result of the large effective masses in the TMDs. For BN thicknesses of around 3nm, the interaction strength is large and no longer in the weak coupling regime. Therefore, to calculate the excitonic gap, we use a modified Eliashberg formalism in which the phonon-mediated interaction is replaced with the inter-layer screened coulomb interaction between TMD bilayers. This work was supported in part by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA and NSF EFRI-2DARE 143395.

  18. Tuning near-gap electronic structure, interface charge transfer and visible light response of hybrid doped graphene and Ag3PO4 composite: Dopant effects

    PubMed Central

    He, Chao-Ni; Huang, Wei-Qing; Xu, Liang; Yang, Yin-Cai; Zhou, Bing-Xin; Huang, Gui-Fang; Peng, P.; Liu, Wu-Ming

    2016-01-01

    The enhanced photocatalytic performance of doped graphene (GR)/semiconductor nanocomposites have recently been widely observed, but an understanding of the underlying mechanisms behind it is still out of reach. As a model system to study the dopant effects, we investigate the electronic structures and optical properties of doped GR/Ag3PO4 nanocomposites using the first-principles calculations, demonstrating that the band gap, near-gap electronic structure and interface charge transfer of the doped GR/Ag3PO4(100) composite can be tuned by the dopants. Interestingly, the doping atom and C atoms bonded to dopant become active sites for photocatalysis because they are positively or negatively charged due to the charge redistribution caused by interaction. The dopants can enhance the visible light absorption and photoinduced electron transfer. We propose that the N atom may be one of the most appropriate dopants for the GR/Ag3PO4 photocatalyst. This work can rationalize the available experimental results about N-doped GR-semiconductor composites, and enriches our understanding on the dopant effects in the doped GR-based composites for developing high-performance photocatalysts. PMID:26923338

  19. Observation of the Hofstadter butterfly in graphene on boron nitride

    NASA Astrophysics Data System (ADS)

    Maher, Patrick; Dean, Cory; Forsythe, Carlos; Wang, Lei; Ghahari, Fereshte; Moon, Pilkyung; Koshino, Mikito; Watanabe, Kenji; Taniguchi, Takashi; Shepard, Ken; Hone, James; Kim, Philip

    2013-03-01

    In 1976, Douglas Hofstadter considered the general problem of 2D electrons subject to both a magnetic field and a periodic potential. His solution predicted a remarkably complex energy spectrum exhibiting self-similar fractal structure, termed the Hofstadter Butterfly. Experimental exploration of this problem has been limited by the difficulty of fabricating a system with a lattice constant on the order of the magnetic length. It has recently been shown that single layer graphene on hexagonal-BN develops a Moiré pattern with a length of up to 15 nm when the rotational angle between the two lattices approaches zero. We present data demonstrating that for bilayer graphene on hexagonal boron nitride, the effect of the modulation potential associated with the Moiré pattern is large enough to be observable by standard transport. Under large magnetic fields, additional gaps appear within the usual bilayer quantum Hall spectrum, consistent with calculations of the Hofstadter spectrum. We present the first direct experimental evidence of the longstanding theoretical prediction that the gaps arising from the superlattice are characterized by two integer quantum numbers.

  20. 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

  1. Electrostatically Confined Monolayer Graphene Quantum Dots with Orbital and Valley Splittings.

    PubMed

    Freitag, Nils M; Chizhova, Larisa A; Nemes-Incze, Peter; Woods, Colin R; Gorbachev, Roman V; Cao, Yang; Geim, Andre K; Novoselov, Kostya S; Burgdörfer, Joachim; Libisch, Florian; Morgenstern, Markus

    2016-09-14

    The electrostatic confinement of massless charge carriers is hampered by Klein tunneling. Circumventing this problem in graphene mainly relies on carving out nanostructures or applying electric displacement fields to open a band gap in bilayer graphene. So far, these approaches suffer from edge disorder or insufficiently controlled localization of electrons. Here we realize an alternative strategy in monolayer graphene, by combining a homogeneous magnetic field and electrostatic confinement. Using the tip of a scanning tunneling microscope, we induce a confining potential in the Landau gaps of bulk graphene without the need for physical edges. Gating the localized states toward the Fermi energy leads to regular charging sequences with more than 40 Coulomb peaks exhibiting typical addition energies of 7-20 meV. Orbital splittings of 4-10 meV and a valley splitting of about 3 meV for the first orbital state can be deduced. These experimental observations are quantitatively reproduced by tight binding calculations, which include the interactions of the graphene with the aligned hexagonal boron nitride substrate. The demonstrated confinement approach appears suitable to create quantum dots with well-defined wave function properties beyond the reach of traditional techniques. PMID:27466881

  2. Electrostatically Confined Monolayer Graphene Quantum Dots with Orbital and Valley Splittings

    PubMed Central

    2016-01-01

    The electrostatic confinement of massless charge carriers is hampered by Klein tunneling. Circumventing this problem in graphene mainly relies on carving out nanostructures or applying electric displacement fields to open a band gap in bilayer graphene. So far, these approaches suffer from edge disorder or insufficiently controlled localization of electrons. Here we realize an alternative strategy in monolayer graphene, by combining a homogeneous magnetic field and electrostatic confinement. Using the tip of a scanning tunneling microscope, we induce a confining potential in the Landau gaps of bulk graphene without the need for physical edges. Gating the localized states toward the Fermi energy leads to regular charging sequences with more than 40 Coulomb peaks exhibiting typical addition energies of 7–20 meV. Orbital splittings of 4–10 meV and a valley splitting of about 3 meV for the first orbital state can be deduced. These experimental observations are quantitatively reproduced by tight binding calculations, which include the interactions of the graphene with the aligned hexagonal boron nitride substrate. The demonstrated confinement approach appears suitable to create quantum dots with well-defined wave function properties beyond the reach of traditional techniques. PMID:27466881

  3. Tunable spin-orbit coupling and symmetry-protected edge states in graphene/WS2

    NASA Astrophysics Data System (ADS)

    Yang, Bowen; Tu, Min-Feng; Kim, Jeongwoo; Wu, Yong; Wang, Hui; Alicea, Jason; Wu, Ruqian; Bockrath, Marc; Shi, Jing

    2016-09-01

    We demonstrate clear weak anti-localization (WAL) effect arising from induced Rashba spin-orbit coupling (SOC) in WS2-covered single-layer and bilayer graphene devices. Contrary to the uncovered region of a shared single-layer graphene flake, WAL in WS2-covered graphene occurs over a wide range of carrier densities on both electron and hole sides. At high carrier densities, we estimate the Rashba SOC relaxation rate to be ˜ 0.2 {{ps}}-1 and show that it can be tuned by transverse electric fields. In addition to the Rashba SOC, we also predict the existence of a‘valley-Zeeman’ SOC from first-principles calculations. The interplay between these two SOC’s can open a non-topological but interesting gap in graphene; in particular, zigzag boundaries host four sub-gap edge states protected by time-reversal and crystalline symmetries. The graphene/WS2 system provides a possible platform for these novel edge states.

  4. Electrostatically Confined Monolayer Graphene Quantum Dots with Orbital and Valley Splittings.

    PubMed

    Freitag, Nils M; Chizhova, Larisa A; Nemes-Incze, Peter; Woods, Colin R; Gorbachev, Roman V; Cao, Yang; Geim, Andre K; Novoselov, Kostya S; Burgdörfer, Joachim; Libisch, Florian; Morgenstern, Markus

    2016-09-14

    The electrostatic confinement of massless charge carriers is hampered by Klein tunneling. Circumventing this problem in graphene mainly relies on carving out nanostructures or applying electric displacement fields to open a band gap in bilayer graphene. So far, these approaches suffer from edge disorder or insufficiently controlled localization of electrons. Here we realize an alternative strategy in monolayer graphene, by combining a homogeneous magnetic field and electrostatic confinement. Using the tip of a scanning tunneling microscope, we induce a confining potential in the Landau gaps of bulk graphene without the need for physical edges. Gating the localized states toward the Fermi energy leads to regular charging sequences with more than 40 Coulomb peaks exhibiting typical addition energies of 7-20 meV. Orbital splittings of 4-10 meV and a valley splitting of about 3 meV for the first orbital state can be deduced. These experimental observations are quantitatively reproduced by tight binding calculations, which include the interactions of the graphene with the aligned hexagonal boron nitride substrate. The demonstrated confinement approach appears suitable to create quantum dots with well-defined wave function properties beyond the reach of traditional techniques.

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

    SciTech Connect

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

    2014-12-15

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

  6. Twisted Graphene Nanostructures

    NASA Astrophysics Data System (ADS)

    Gani, Satrio; Virgus, Yudistira; Rossi, Enrico

    2015-03-01

    Recent advances in fabrication techniques have made possible the realization of graphene nanostructures with atomic precision. Some of the nanostructures realized are completely novel. We study the electronic properties of such novel graphene nanostructures when deposited on two dimensional crystals. In particular we study the case when the two dimensional crystal is graphene, or bilayer graphene. We obtain results for the nanostructure electronic spectrum and find how the spectrum is affected by the coupling between the nanostructure and the two-dimensional substrate. In particular we study how the ``twist'' angle between the graphene nanostructure and the two-dimensional crystal affects the spectrum of the nanostructure. Work supported by ONR-N00014-13-1-0321 and ACS-PRF # 53581-DNI5.

  7. Graphene/ferroelectrics/graphene hybrid structure: Asymmetric doping of graphene layers

    SciTech Connect

    Duong, Dinh Loc; Lee, Si Young; Kim, Seong Kyu; Lee, Young Hee

    2015-06-15

    We report graphene/ferroelectric/graphene hybrid structure to demonstrate an asymmetrical doping in two graphene layers, one side with electrons and another side with holes. Two ferroelectrics, a poly(vinylidenefluoride) (PVDF) and a hydrofluorinated graphene, were used to demonstrate the concept with density functional calculations, revealing the Fermi level shift of 0.35 and 0.75 eV, respectively. This concept was confirmed by Raman spectroscopy using graphene/poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE))/graphene hybrid, which can easily form β-phase close to our simulation model. G-band peak position was downshifted for electron doping and upshifted for hole doping. This hybrid structure opens an opportunity to study bilayer graphene system with a controllable thickness for a wide range of high carrier concentration.

  8. Structural and electronic properties of multilayer graphene on monolayer hexagonal boron nitride/nickel (111) interface system: A van der Waals density functional study

    NASA Astrophysics Data System (ADS)

    Yelgel, Celal

    2016-02-01

    The structural and electronic properties of multilayer graphene adsorbed on monolayer hexagonal boron nitride (h-BN)/Ni(111) interface system are investigated using the density functional theory with a recently developed non-local van der Waals density functional (rvv10). The most energetically favourable configuration for a monolayer h-BN/Ni(111) interface is found to be N atom atop the Ni atoms and B atom in fcc site with the interlayer distance of 2.04 Å and adsorption energy of 302 meV/BN. Our results show that increasing graphene layers on a monolayer h-BN/Ni(111) interface leads to a weakening of the interfacial interaction between the monolayer h-BN and Ni(111) surface. The adsorption energy of graphene layers on the h-BN/Ni(111) interface is found to be in the range of the 50-120 meV/C atom as the vertical distance from h-BN to the bottommost graphene layers decreases. With the adsorption of a multilayer graphene on the monolayer h-BN/Ni(111) interface system, the band gap of 0.12 eV and 0.25 eV opening in monolayer graphene and bilayer graphene near the K point is found with an upward shifting of the Fermi level. However, a stacking-sensitive band gap is opened in trilayer graphene. We obtain the band gap of 0.35 eV close to the K point with forming a Mexican hat band structure for ABC-stacked trilayer graphene.

  9. Nonlinear photocurrents in two-dimensional systems based on graphene and boron nitride

    NASA Astrophysics Data System (ADS)

    Hipolito, F.; Pedersen, Thomas G.; Pereira, Vitor M.

    2016-07-01

    The dc photoelectrical currents can be generated purely as a nonlinear effect in uniform media lacking inversion symmetry without the need for a material junction or bias voltages to drive it, in what is termed photogalvanic effect. These currents are strongly dependent on the polarization state of the radiation, as well as on topological properties of the underlying Fermi surface such as its Berry curvature. In order to study the intrinsic photogalvanic response of gapped graphene, biased bilayer graphene (BBG), and hexagonal boron nitride (hBN), we compute the nonlinear current using a perturbative expansion of the density matrix. This allows a microscopic description of the quadratic response to an electromagnetic field in these materials, which we analyze as a function of temperature and electron density. We find that the intrinsic response is robust across these systems and allows for currents in the range of pA cm/W to nA cm/W. At the independent-particle level, the response of hBN-based structures is significant only in the ultraviolet due to their sizable band gap. However, when Coulomb interactions are accounted for by explicit solution of the Bethe-Salpeter equation, we find that the photoconductivity is strongly modified by transitions involving exciton levels in the gap region, whose spectral weight dominates in the overall frequency range. Biased bilayers and gapped monolayers of graphene have a strong photoconductivity in the visible and infrared window, allowing for photocurrent densities of several nA cm/W. We further show that the richer electronic dispersion of BBG at low energies and the ability to change its band gap on demand allows a higher tunability of the photocurrent, including not only its magnitude but also, and significantly, its polarity.

  10. The effect of electron induced hydrogenation of graphene on its electrical transport properties

    NASA Astrophysics Data System (ADS)

    Woo, Sung Oh; Teizer, Winfried

    2013-07-01

    We report a deterioration of the electrical transport properties of a graphene field effect transistor due to energetic electron irradiation on a stack of Poly Methyl Methacrylate (PMMA) on graphene (PMMA/graphene bilayer). Prior to electron irradiation, we observed that the PMMA layer on graphene does not deteriorate the carrier transport of graphene but improves its electrical properties instead. As a result of the electron irradiation on the PMMA/graphene bilayer, the Raman "D" band appears after removal of PMMA. We argue that the degradation of the transport behavior originates from the binding of hydrogen generated during the PMMA backbone secession process.

  11. Evolution of interlayer coupling in twisted molybdenum disulfide bilayers

    NASA Astrophysics Data System (ADS)

    Liu, Kaihui; Zhang, Liming; Cao, Ting; Jin, Chenhao; Qiu, Diana; Zhou, Qin; Zettl, Alex; Yang, Peidong; Louie, Steve G.; Wang, Feng

    2014-09-01

    Van der Waals coupling is emerging as a powerful method to engineer physical properties of atomically thin two-dimensional materials. In coupled graphene-graphene and graphene-boron nitride layers, interesting physical phenomena ranging from Fermi velocity renormalization to Hofstadter’s butterfly pattern have been demonstrated. Atomically thin transition metal dichalcogenides, another family of two-dimensional-layered semiconductors, can show distinct coupling phenomena. Here we demonstrate the evolution of interlayer coupling with twist angles in as-grown molybdenum disulfide bilayers. We find that the indirect bandgap size varies appreciably with the stacking configuration: it shows the largest redshift for AA- and AB-stacked bilayers, and a significantly smaller but constant redshift for all other twist angles. Our observations, together with ab initio calculations, reveal that this evolution of interlayer coupling originates from the repulsive steric effects that leads to different interlayer separations between the two molybdenum disulfide layers in different stacking configurations.

  12. Orbital-dependent Electron-Hole Interaction in Graphene and Associated Multi-Layer Structures.

    PubMed

    Deng, Tianqi; Su, Haibin

    2015-01-01

    We develop an orbital-dependent potential to describe electron-hole interaction in materials with structural 2D character, i.e. quasi-2D materials. The modulated orbital-dependent potentials are also constructed with non-local screening, multi-layer screening, and finite gap due to the coupling with substrates. We apply the excitonic Hamiltonian in coordinate-space with developed effective electron-hole interacting potentials to compute excitons' binding strength at M (π band) and Γ (σ band) points in graphene and its associated multi-layer forms. The orbital-dependent potential provides a range-separated property for regulating both long- and short-range interactions. This accounts for the existence of the resonant π exciton in single- and bi-layer graphenes. The remarkable strong electron-hole interaction in σ orbitals plays a decisive role in the existence of σ exciton in graphene stack at room temperature. The interplay between gap-opening and screening from substrates shed a light on the weak dependence of σ exciton binding energy on the thickness of graphene stacks. Moreover, the analysis of non-hydrogenic exciton spectrum in quasi-2D systems clearly demonstrates the remarkable comparable contribution of orbital dependent potential with respect to non-local screening process. The understanding of orbital-dependent potential developed in this work is potentially applicable for a wide range of materials with low dimension. PMID:26610715

  13. Orbital-dependent Electron-Hole Interaction in Graphene and Associated Multi-Layer Structures

    PubMed Central

    Deng, Tianqi; Su, Haibin

    2015-01-01

    We develop an orbital-dependent potential to describe electron-hole interaction in materials with structural 2D character, i.e. quasi-2D materials. The modulated orbital-dependent potentials are also constructed with non-local screening, multi-layer screening, and finite gap due to the coupling with substrates. We apply the excitonic Hamiltonian in coordinate-space with developed effective electron-hole interacting potentials to compute excitons’ binding strength at M (π band) and Γ (σ band) points in graphene and its associated multi-layer forms. The orbital-dependent potential provides a range-separated property for regulating both long- and short-range interactions. This accounts for the existence of the resonant π exciton in single- and bi-layer graphenes. The remarkable strong electron-hole interaction in σ orbitals plays a decisive role in the existence of σ exciton in graphene stack at room temperature. The interplay between gap-opening and screening from substrates shed a light on the weak dependence of σ exciton binding energy on the thickness of graphene stacks. Moreover, the analysis of non-hydrogenic exciton spectrum in quasi-2D systems clearly demonstrates the remarkable comparable contribution of orbital dependent potential with respect to non-local screening process. The understanding of orbital-dependent potential developed in this work is potentially applicable for a wide range of materials with low dimension. PMID:26610715

  14. The magneto-optical properties of non-uniform graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Chung, Hsien-Ching; Lin, Ming-Fa

    2015-03-01

    When synthesizing few-layer graphene nanoribbons (GNRs), non-uniform GNRs would be made simultaneously. Recently, the non-uniform GNRs, which is a stack of two GNRs with unequal widths, have been fabricated by mechanically exfoliated from bulk graphite. Some theoretical predictions have been reported, such as gap opening and transport properties. Under the influence of magnetic fields, magnetic quantization takes place and drastically changes the electronic properties. By tuning the geometric configuration, four categories of magneto-electronic spectra are exhibited. (1) The spectrum is mostly contributed by quasi-Landau levels (QLLs) of monolayer GNRs. (2) The spectrum displays two groups of QLLs, and the non-uniform GNR behaves like a bilayer one. (3) An intermediate category, the spectrum is composite disordered. (4) The spectrum presents the coexistence of monolayer and bilayer spectra. In this work, the magneto-electronic and optical properties for different geometric configurations are given, such as energy dispersions, density of states, wave functions, and magneto-absorption spectra are presented. Furthermore, the transformation between monolayer and bilayer spectra as well as the coexistence of monolayer and bilayer spectra are discussed in detail. One of us (Hsien-Ching Chung) thanks Ming-Hui Chung and Su-Ming Chen for financial support. This work was supported in part by the National Science Council of Taiwan under Grant Number 98-2112-M-006-013-MY4.

  15. Trifluoromethylation of graphene

    NASA Astrophysics Data System (ADS)

    Zhou, Lin; Zhou, Lushan; Wang, Xi; Yu, Jingwen; Yang, Mingmei; Wang, Jianbo; Peng, Hailin; Liu, Zhongfan

    2014-09-01

    We demonstrate trifluoromethylation of graphene by copper-catalyzed free radical reaction. The covalent addition of CF3 to graphene, which changes the carbon atom hybridization from sp2 to sp3, and modifies graphene in a homogeneous and nondestructive manner, was verified with Raman spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy reveals that CF3 groups are grafted to the basal plane of graphene, with about 4 at. % CF3 coverage. After trifluoromethylation, the average resistance increases by nearly one order of magnitude, and an energy gap of about 98 meV appears. The noninvasive and mild reaction to synthesize trifluoromethylated graphene paves the way for graphene's applications in electronics and biomedical areas.

  16. Large-scale real-space density-functional calculations: Moiré-induced electron localization in graphene

    SciTech Connect

    Oshiyama, Atsushi Iwata, Jun-Ichi; Uchida, Kazuyuki; Matsushita, Yu-Ichiro

    2015-03-21

    We show that our real-space finite-difference scheme allows us to perform density-functional calculations for nanometer-scale targets containing more than 100 000 atoms. This real-space scheme is applied to twisted bilayer graphene, clarifying that Moiré pattern induced in the slightly twisted bilayer graphene drastically modifies the atomic and electronic structures.

  17. 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

  18. Theoretical study on strain induced variations in electronic properties of 2H-MoS{sub 2} bilayer sheets

    SciTech Connect

    Dong, Liang; Dongare, Avinash M.; Namburu, Raju R.; O'Regan, Terrance P.; Dubey, Madan

    2014-02-03

    The strain dependence of the electronic properties of bilayer sheets of 2H-MoS{sub 2} is studied using ab initio simulations based on density functional theory. An indirect band gap for bilayer MoS{sub 2} is observed for all variations of strain along the basal plane. Several transitions for the indirect band gap are observed for various strains for the bilayer structure. The variation of the band gap and the carrier effective masses for the holes and the electrons for the bilayer MoS{sub 2} structure under conditions of uniaxial strain, biaxial strain, as well as uniaxial stress is investigated.

  19. Weak-coupling instabilities of SU(N) fermions on the Bernal-stacked honeycomb bilayer in presence of on-site Hubbard Interactions

    NASA Astrophysics Data System (ADS)

    Pujari, Sumiran; Lang, Thomas C.; Kaul, Ribhu K.

    Bernal-stacked bilayer graphene hosts an interesting 'non-relativistic' semi-metallic dispersion different from monolayer graphene. At this quadratic band touching, short-range interactions are marginal and hence cause instabilities to a variety of ground states. In this work we consider the instabilities of even N species of fermions on the Bernal bilayer with an SU (N) -symmetric contact interaction. For SU (2) fermions with an on-site Hubbard interaction the ground state has been found to be to a magnetic Néel state for all strengths of the interaction. In contrast, the leading weak coupling instability for N > 2 is a non-magnetic ground state, which is gapped and odd under time reversal. On the other hand, at strong coupling we expect Néel or VBS ground states of the effective self-conjugate SU (N) spin models. Motivated by this observation, we investigate the phase diagram for even N > 2 using determinantal quantum Monte Carlo computations. Support from NSF Grant DMR-1056536 and XSEDE Grant DMR-150037.

  20. Electrical Transport and Thermal Expansion in van der Waals Materials: Graphene and Topological Insulator

    NASA Astrophysics Data System (ADS)

    Jing, Lei

    Novel two-dimensional materials with weak interlayer Van der Waals interaction are fantastic platforms to study novel physical phenomena. This thesis describes our investigation on two different Van der Waals materials: graphene and bismuth selenide with calcium doping (CaxBi 2-xSe3, x as the doping level) in the topological insulator family. Firstly, we characterize the electrical transport behaviors of high-quality substrate-supported bilayer graphene devices with suspended metal gates. The device exhibits a transport gap induced by external electric field with an on/off ratio of 20,000, which could be explained by variable range hoping between localized states or disordered charge puddles. At large magnetic field, the device presents quantum Hall plateau at fractional values of conductance quantum, which arises from the equilibration of edge states between differentially doped regions. Secondly, we present our study on the electronic transport of CaxBi 2-xSe3 thin films, which are three-dimensional topological insulators and coupled with superconducting leads. In these novel Josephson transistors, we observe different characteristic features by energy dispersion spectrum (EDS) and Raman spectroscopy, and the weak suppression in the critical current Ic. Thirdly, we explore the thermal expansion of suspended graphene. By in-situ scanning electron microscope (SEM), we measure the thickness-dependence of graphene's negative thermal expansion coefficient (TEC). We propose that there is a competitive relation between the intrinsic TEC and the friction from the substrate and the graphene. Lastly, in collaboration with Dr. Nikolai Kalugin from New Mexico Tech., we explore the graphene's application as a quantum Hall effect infrared photodetector. This graphene-based detector can be operated at higher temperature (liquid nitrogen) and wider frequency than the previous implementations of quantum Hall detector.

  1. Theoretical studies of structure-property relations in graphene-based carbon nanostructures

    NASA Astrophysics Data System (ADS)

    Maroudas, Dimitrios

    2014-03-01

    This presentation focuses on establishing relations between atomic structure, electronic structure, and properties in graphene-based carbon nanostructures through first-principles density functional theory calculations and molecular-dynamics simulations. We have analyzed carbon nanostructure formation from twisted bilayer graphene, upon creation of interlayer covalent C-C bonds due to patterned hydrogenation or fluorination. For small twist angles and twist angles near 30 degrees, interlayer covalent bonding generates superlattices of diamond-like nanocrystals and of fullerene-like configurations, respectively, embedded within the graphene layers. The electronic band gaps of these superlattices can be tuned through selective chemical functionalization and creation of interlayer bonds, and range from a few meV to over 1.2 eV. The mechanical properties of these superstructures also can be precisely tuned by controlling the extent of chemical functionalization. Importantly, the shear modulus is shown to increase monotonically with the fraction of sp3-hybridized C-C bonds. We have also studied collective interactions of multiple defects such as random distributions of vacancies in single-layer graphene (SLG). We find that a crystalline-to-amorphous structural transition occurs at vacancy concentrations of 5-10% over a broad temperature range. The structure of our defect-induced amorphized graphene is in excellent agreement with experimental observations of SLG exposed to a high electron irradiation dose. Simulations of tensile tests on these irradiated graphene sheets identify trends for the ultimate tensile strength, failure strain, and toughness as a function of vacancy concentration. The vacancy-induced amorphization transition is accompanied by a brittle-to-ductile transition in the failure response of irradiated graphene sheets and even heavily damaged samples exhibit tensile strengths near 30 GPa, in significant excess of those typical of engineering materials.

  2. Optical pure spin current injection in graphene

    NASA Astrophysics Data System (ADS)

    Rioux, Julien; Burkard, Guido

    2013-03-01

    Pure spin current injection by optical methods is investigated in single-layer and bilayer graphene within the tight-binding model, including bias and interlayer coupling effects. Interlayer coupling in bilayer graphene has a distinct qualitative effect on the polarization dependence of the spin current injection. In combination with interlayer coupling, which induces trigonal warping of the electronic bands, the bias voltage allows to control the warping at the Fermi surface. The resulting implications for the spin current injection are presented. Unlike the previously presented charge current injection [J. Rioux et al., PRB 83, 195406 (2011)], the effect presented here relies on a single monochromatic beam.

  3. Equilibrium Configurations of Lipid Bilayer Membranes and Carbon Nanostructures

    NASA Astrophysics Data System (ADS)

    Ivaïlo, M. Mladenov; Peter, A. Djondjorov; Mariana, Ts. Hadzhilazova; Vassil, M. Vassilev

    2013-02-01

    The present article concerns the continuum modelling of the mechanical behaviour and equilibrium shapes of two types of nano-scale objects: fluid lipid bilayer membranes and carbon nanostructures. A unified continuum model is used to handle four different case studies. Two of them consist in representing in analytic form cylindrical and axisymmetric equilibrium configurations of single-wall carbon nanotubes and fluid lipid bilayer membranes subjected to uniform hydrostatic pressure. The third one is concerned with determination of possible shapes of junctions between a single-wall carbon nanotube and a fiat graphene sheet or another single-wall carbon nanotube. The last one deals with the mechanical behaviour of closed fluid lipid bilayer membranes (vesicles) adhering onto a fiat homogeneous rigid substrate subjected to micro-injection and uniform hydrostatic pressure.

  4. Gate tunable spin transport in graphene with Rashba spin-orbit coupling

    NASA Astrophysics Data System (ADS)

    Tan, Xiao-Dong; Liao, Xiao-Ping; Sun, Litao

    2016-10-01

    Recently, it attracts much attention to study spin-resolved transport properties in graphene with Rashba spin-orbit coupling (RSOC). One remarkable finding is that Klein tunneling in single layer graphene (SLG) with RSOC (SLG + R for short below) behaves as in bi-layer graphene (BLG). Based on the effective Dirac theory, we reconsider this tunneling problem and derive the analytical solution for the transmission coefficients. Our result shows that Klein tunneling in SLG + R and BLG exhibits completely different behaviors. More importantly, we find two new transmission selection rules in SLG + R, i.e., the single band to single band (S → S) and the single band to multiple bands (S → M) transmission regimes, which strongly depend on the relative height among Fermi level, RSOC, and potential barrier. Interestingly, in the S → S transmission regime, only normally incident electrons have capacity to pass through the barrier, while in the S → M transmission regime the angle-dependent tunneling becomes very prominent. Using the transmission coefficients, we also derive spin-resolved conductance analytically, and conductance oscillation with the increasing barrier height and zero conductance gap are found in SLG + R. The present study offers new insights and opportunities for developing graphene-based spin devices.

  5. Structural and electronic properties of Li-intercalated graphene on SiC(0001)

    NASA Astrophysics Data System (ADS)

    Caffrey, Nuala M.; Johansson, Leif I.; Xia, Chao; Armiento, Rickard; Abrikosov, Igor A.; Jacobi, Chariya

    2016-05-01

    We investigate the structural and electronic properties of Li-intercalated monolayer graphene on SiC(0001) using combined angle-resolved photoemission spectroscopy and first-principles density functional theory. Li intercalates at room temperature both at the interface between the buffer layer and SiC and between the two carbon layers. The graphene is strongly n -doped due to charge transfer from the Li atoms and two π bands are visible at the K ¯ point. After heating the sample to 300 ∘C , these π bands become sharp and have a distinctly different dispersion to that of Bernal-stacked bilayer graphene. We suggest that the Li atoms intercalate between the two carbon layers with an ordered structure, similar to that of bulk LiC6. An AA stacking of these two layers becomes energetically favourable. The π bands around the K ¯ point closely resemble the calculated band structure of a C6LiC6 system, where the intercalated Li atoms impose a superpotential on the graphene electronic structure that opens gaps at the Dirac points of the two π cones.

  6. Tetragonal bismuth bilayer: a stable and robust quantum spin hall insulator

    NASA Astrophysics Data System (ADS)

    Kou, Liangzhi; Tan, Xin; Ma, Yandong; Tahini, Hassan; Zhou, Liujiang; Sun, Ziqi; Aijun, Du; Chen, Changfeng; Smith, Sean C.

    2015-12-01

    Topological insulators (TIs) exhibit novel physics with great promise for new devices, but considerable challenges remain to identify TIs with high structural stability and large nontrivial band gap suitable for practical applications. Here we predict by first-principles calculations a two-dimensional (2D) TI, also known as a quantum spin Hall (QSH) insulator, in a tetragonal bismuth bilayer (TB-Bi) structure that is dynamically and thermally stable based on phonon calculations and finite-temperature molecular dynamics simulations. Density functional theory and tight-binding calculations reveal a band inversion among the Bi-p orbits driven by the strong intrinsic spin-orbit coupling, producing a large nontrivial band gap, which can be effectively tuned by moderate strains. The helical gapless edge states exhibit a linear dispersion with a high Fermi velocity comparable to that of graphene, and the QSH phase remains robust on a NaCl substrate. These remarkable properties place TB-Bi among the most promising 2D TIs for high-speed spintronic devices, and the present results provide insights into the intriguing QSH phenomenon in this new Bi structure and offer guidance for its implementation in potential applications.

  7. Properties and applications of chemically functionalized graphene

    NASA Astrophysics Data System (ADS)

    Craciun, M. F.; Khrapach, I.; Barnes, M. D.; Russo, S.

    2013-10-01

    The vast and yet largely unexplored family of graphene materials has great potential for future electronic devices with novel functionalities. The ability to engineer the electrical and optical properties in graphene by chemically functionalizing it with a molecule or adatom is widening considerably the potential applications targeted by graphene. Indeed, functionalized graphene has been found to be the best known transparent conductor or a wide gap semiconductor. At the same time, understanding the mechanisms driving the functionalization of graphene with hydrogen is proving to be of fundamental interest for energy storage devices. Here we discuss recent advances on the properties and applications of chemically functionalized graphene.

  8. A Transparent and Flexible Graphene-Piezoelectric Fiber Generator.

    PubMed

    Fuh, Yiin Kuen; Kuo, Chien Cheng; Huang, Zih Ming; Li, Shan Chien; Liu, En Rui

    2016-04-13

    Piezoelectric fiber-based generators are prepared by combining two distinctive materials - poly(vinlyidene fluoride) fibers and monolayer/bilayer graphene. Novelty lies in the replacement of opaque metal electrodes with transparent graphene electrodes which enable the graphene-piezoelectric fiber generator to exhibit high flexibility and transparency as well as a great performance with an achievable output of voltage/current about 2 V/200 nA.

  9. A Transparent and Flexible Graphene-Piezoelectric Fiber Generator.

    PubMed

    Fuh, Yiin Kuen; Kuo, Chien Cheng; Huang, Zih Ming; Li, Shan Chien; Liu, En Rui

    2016-04-13

    Piezoelectric fiber-based generators are prepared by combining two distinctive materials - poly(vinlyidene fluoride) fibers and monolayer/bilayer graphene. Novelty lies in the replacement of opaque metal electrodes with transparent graphene electrodes which enable the graphene-piezoelectric fiber generator to exhibit high flexibility and transparency as well as a great performance with an achievable output of voltage/current about 2 V/200 nA. PMID:26929015

  10. Mode manipulation and near-THz absorptions in binary grating-graphene layer structures

    PubMed Central

    2014-01-01

    The excitation and absorption properties of grating coupled graphene surface plasmons were studied. It was found that whether a mode can be excited is mainly determined by the frequency of incident light and the duty ratio of gratings. In the structure consisting graphene bilayer, a blueshift of the excitation frequency existed when the distance between neighbor graphene layer were decreased gradually. In graphene-grating multilayer structures, a strong absorption (approximately 90% at maximum) was found in near-THz range. PMID:24559407

  11. Edge State and Intrinsic Hole Doping in Bilayer Phosphorene

    NASA Astrophysics Data System (ADS)

    Osada, Toshihito

    2015-01-01

    Using a simple LCAO model by Harrison, we have qualitatively studied the edge state of bilayer phosphorene, which is a unit structure of the layered crystal of black phosphorus. This model successfully reproduces the isolated edge state in the bulk gap in monolayer phosphorene. In bilayer phosphorene, however, it shows that edge states are almost buried in the valence band and there is no isolated midgap edge state at the zigzag edge. Since the buried edge state works as acceptor, holes are doped from the edge state into the bulk. This gives a possible explanation for p-type conduction in undoped black phosphorus. Under the vertical electric field, the intrinsic hole doping is reduced because a part of edge states move into the gap. These features of bilayer phosphorene might be better suited for device application.

  12. THz detection in graphene nanotransistors

    NASA Astrophysics Data System (ADS)

    Tredicucci, Alessandro; Vitiello, Miriam S.; Polini, Marco; Pellegrini, Vittorio

    2014-03-01

    Nanotransistors offer great prospect for the development of innovative THz detectors based on the non-linearity of transport characteristics. Semiconductor nanowires are appealing for their one-dimensional nature and intrinsically low capacitance of the devices, while graphene, with its record-high room-temperature mobility, has the potential to exploit plasma wave resonances in the transistor channel to achieve high-responsivity and tuneable detection. First graphene detectors have been recently demonstrated in both monolayer and bilayer field effect devices performances already suitable for first imaging application. Here will discuss the physics and technology of these devices, their operation, as well as first examples of imaging applications.

  13. The physics of epitaxial graphene on SiC(0001).

    PubMed

    Kageshima, H; Hibino, H; Tanabe, S

    2012-08-01

    Various physical properties of epitaxial graphene grown on SiC(0001) are studied. First, the electronic transport in epitaxial bilayer graphene on SiC(0001) and quasi-free-standing bilayer graphene on SiC(0001) is investigated. The dependences of the resistance and the polarity of the Hall resistance at zero gate voltage on the top-gate voltage show that the carrier types are electron and hole, respectively. The mobility evaluated at various carrier densities indicates that the quasi-free-standing bilayer graphene shows higher mobility than the epitaxial bilayer graphene when they are compared at the same carrier density. The difference in mobility is thought to come from the domain size of the graphene sheet formed. To clarify a guiding principle for controlling graphene quality, the mechanism of epitaxial graphene growth is also studied theoretically. It is found that a new graphene sheet grows from the interface between the old graphene sheets and the SiC substrate. Further studies on the energetics reveal the importance of the role of the step on the SiC surface. A first-principles calculation unequivocally shows that the C prefers to release from the step edge and to aggregate as graphene nuclei along the step edge rather than be left on the terrace. It is also shown that the edges of the existing graphene more preferentially absorb the isolated C atoms. For some annealing conditions, experiments can also provide graphene islands on SiC(0001) surfaces. The atomic structures are studied theoretically together with their growth mechanism. The proposed embedded island structures actually act as a graphene island electronically, and those with zigzag edges have a magnetoelectric effect. Finally, the thermoelectric properties of graphene are theoretically examined. The results indicate that reducing the carrier scattering suppresses the thermoelectric power and enhances the thermoelectric figure of merit. The fine control of the Fermi energy position is thought to

  14. Immobilized lipid-bilayer materials

    DOEpatents

    Sasaki, Darryl Y.; Loy, Douglas A.; Yamanaka, Stacey A.

    2000-01-01

    A method for preparing encapsulated lipid-bilayer materials in a silica matrix comprising preparing a silica sol, mixing a lipid-bilayer material in the silica sol and allowing the mixture to gel to form the encapsulated lipid-bilayer material. The mild processing conditions allow quantitative entrapment of pre-formed lipid-bilayer materials without modification to the material's spectral characteristics. The method allows for the immobilization of lipid membranes to surfaces. The encapsulated lipid-bilayer materials perform as sensitive optical sensors for the detection of analytes such as heavy metal ions and can be used as drug delivery systems and as separation devices.

  15. Efficient H{sub 2} production over Au/graphene/TiO{sub 2} induced by surface plasmon resonance of Au and band-gap excitation of TiO{sub 2}

    SciTech Connect

    Liu, Yang; Yu, Hongtao; Wang, Hua; Chen, Shuo; Quan, Xie

    2014-11-15

    Highlights: • Both surface plasmon resonance and band-gap excitation were used for H{sub 2} production. • Au/Gr/TiO{sub 2} composite photocatalyst was synthesized. • Au/Gr/TiO{sub 2} exhibited enhancement of light absorption and charge separation. • H{sub 2} production rate of Au/Gr/TiO{sub 2} was about 2 times as high as that of Au/TiO{sub 2}. - Abstract: H{sub 2} production over Au/Gr/TiO{sub 2} composite photocatalyst induced by surface plasmon resonance of Au and band-gap excitation of TiO{sub 2} using graphene (Gr) as an electron acceptor has been investigated. Electron paramagnetic resonance study indicated that, in this composite, Gr collected electrons not only from Au with surface plasmon resonance but also from TiO{sub 2} with band-gap excitation. Surface photovoltage and UV–vis absorption measurements revealed that compared with Au/TiO{sub 2}, Au/Gr/TiO{sub 2} displayed more effective photogenerated charge separation and higher optical absorption. Benefiting from these advantages, the H{sub 2} production rate of Au/Gr/TiO{sub 2} composite with Gr content of 1.0 wt% and Au content of 2.0 wt% was about 2 times as high as that of Au/TiO{sub 2}. This work represents an important step toward the efficient application of both surface plasmon resonance and band-gap excitation on the way to converting solar light into chemical energy.

  16. Controlling graphene plasmons with resonant metal antennas and spatial conductivity patterns.

    PubMed

    Alonso-González, P; Nikitin, A Y; Golmar, F; Centeno, A; Pesquera, A; Vélez, S; Chen, J; Navickaite, G; Koppens, F; Zurutuza, A; Casanova, F; Hueso, L E; Hillenbrand, R

    2014-06-20

    Graphene plasmons promise unique possibilities for controlling light in nanoscale devices and for merging optics with electronics. We developed a versatile platform technology based on resonant optical antennas and conductivity patterns for launching and control of propagating graphene plasmons, an essential step for the development of graphene plasmonic circuits. We launched and focused infrared graphene plasmons with geometrically tailored antennas and observed how they refracted when passing through a two-dimensional conductivity pattern, here a prism-shaped bilayer. To that end, we directly mapped the graphene plasmon wavefronts by means of an imaging method that will be useful in testing future design concepts for nanoscale graphene plasmonic circuits and devices.

  17. Hyperfine interaction in hydrogenated graphene

    NASA Astrophysics Data System (ADS)

    Garcia, Noel; Melle, Manuel; Fernandez-Rossier, Joaquin

    We study the hyperfine interaction of Hydrogen chemisorbed in graphene nanostructures with a gap in their spectrum, such as islands and ribbons. Chemisorption of Hydrogen on graphene results in a bound in-gap state that hosts a single electron localized around the adatom. Using both density functional theory and a four-orbital tight-binding model we study the hyperfine interaction between the hydrogen nuclear spin and the conduction electrons in graphene. We find that the strength of the hyperfine interaction decreases for larger nanostructures for which the energy gap is smaller. We then compare the results of the hyperfine interaction for large nanostructures with those of graphene 2D crystal with a periodic arrangement of chemisorbed Hydrogen atoms, obtaining very similar results. The magnitude of the hyperfine interaction is about 150 MHz, in line with that of Si:P. We acknowledge financial support by Marie-Curie-ITN 607904-SPINOGRAPH.

  18. Few-layer and symmetry-breaking effects on the electrical properties of ordered CF3Cl phases on graphene

    NASA Astrophysics Data System (ADS)

    Morales-Cifuentes, Josue; Wang, Yilin; Reutt-Robey, Janice; Einstein, T. L.

    2014-03-01

    An effective pseudopotential mechanism for breaking the inherent sub-lattice symmetry of graphene has been studied using DFT calculations on hexagonal boron nitride. Electrical detection of CF3Cl phase transitions on graphene shows the existence of a commensurate ordered phase in which this can be tested. We study the electronic properties of this phase using VASP ver 5.3.3, with ab initio van der Waals density functionals (vdW-DF1 and vdW-DF2). Consistent with a physisorbed phase, binding energies and charge transfer per CF3Cl molecule are calculated to be on the order of 280meV and 0.01e, respectively. By exploring different coverages and orientations of this ordered phase we are able to open a band gap in some configurations; said gap is in the range of 8 to 80meV depending on the strength of the effective pseudopotential. Furthermore, we calculate the screening of these effects in bi-layer and tri-layer graphene. Work supported by NSF-MRSEC at UMD, grant DMR 05-20471 and NSF-CHE 13-05892.

  19. Collective Excitations in Electron-Hole Bilayers

    SciTech Connect

    Kalman, G. J.; Hartmann, P.; Donko, Z.; Golden, K. I.

    2007-06-08

    We report a combined analytic and molecular dynamics analysis of the collective mode spectrum of a bipolar (electron-hole) bilayer in the strong coupling classical limit. A robust, isotropic energy gap is identified in the out-of-phase spectra, generated by the combined effect of correlations and of the excitation of the bound dipoles. In the in-phase spectra we identify longitudinal and transverse acoustic modes wholly maintained by correlations. Strong nonlinear generation of higher harmonics of the fundamental dipole oscillation frequency and the transfer of harmonics between different modes is observed.

  20. Magnetoresistance effect in Fe20Ni80/graphene/Fe20Ni80 vertical spin valves

    NASA Astrophysics Data System (ADS)

    Entani, Shiro; Seki, Takeshi; Sakuraba, Yuya; Yamamoto, Tatsuya; Takahashi, Saburo; Naramoto, Hiroshi; Takanashi, Koki; Sakai, Seiji

    2016-08-01

    Vertical spin valve devices with junctions of single- and bi-layer graphene interlayers sandwiched with Fe20Ni80 (Permalloy) electrodes were fabricated by exploiting the direct growth of graphene on the Permalloy. The linear current-voltage characteristics indicated that ohmic contacts were realized at the interfaces. The systematic characterization revealed the significant modification of the electronic state of the interfacial graphene layer on the Permalloy surface, which indicates the strong interactions at the interface. The ohmic transport was attributable to the strong interface-interaction. The vertical resistivity of the graphene interlayer and the spin asymmetry coefficient at the graphene/Permalloy interface were obtained to be 0.13 Ω cm and 0.06, respectively. It was found that the strong interface interaction modifies the electronic structure and metallic properties in the vertical spin valve devices with bi-layer graphene as well as single-layer graphene.

  1. Graphene-nickel interfaces: a review

    NASA Astrophysics Data System (ADS)

    Dahal, Arjun; Batzill, Matthias

    2014-02-01

    Graphene on nickel is a prototypical example of an interface between graphene and a strongly interacting metal, as well as a special case of a lattice matched system. The chemical interaction between graphene and nickel is due to hybridization of the metal d-electrons with the π-orbitals of graphene. This interaction causes a smaller separation between the nickel surface and graphene (0.21 nm) than the typical van der Waals gap-distance between graphitic layers (0.33 nm). Furthermore, the physical properties of graphene are significantly altered. Main differences are the opening of a band gap in the electronic structure and a shifting of the π-band by ~2 eV below the Fermi-level. Experimental evidence suggests that the ferromagnetic nickel induces a magnetic moment in the carbon. Substrate induced geometric and electronic changes alter the phonon dispersion. As a consequence, monolayer graphene on nickel does not exhibit a Raman spectrum. In addition to reviewing these fundamental physical properties of graphene on Ni(111), we also discuss the formation and thermal stability of graphene and a surface-confined nickel-carbide. The fundamental growth mechanisms of graphene by chemical vapor deposition are also described. Different growth modes depending on the sample temperature have been identified in ultra high vacuum surface science studies. Finally, we give a brief summary for the synthesis of more complex graphene and graphitic structures using nickel as catalyst and point out some potential applications for graphene-nickel interfaces.

  2. Spin caloritronics in graphene

    SciTech Connect

    Ghosh, Angsula; Frota, H. O.

    2015-06-14

    Spin caloritronics, the combination of spintronics with thermoelectrics, exploiting both the intrinsic spin of the electron and its associated magnetic moment in addition to its fundamental electronic charge and temperature, is an emerging technology mainly in the development of low-power-consumption technology. In this work, we study the thermoelectric properties of a Rashba dot attached to two single layer/bilayer graphene sheets as leads. The temperature difference on the two graphene leads induces a spin current, which depends on the temperature and chemical potential. We demonstrate that the Rashba dot behaves as a spin filter for selected values of the chemical potential and is able to filter electrons by their spin orientation. The spin thermopower has also been studied where the effects of the chemical potential, temperature, and also the Rashba term have been observed.

  3. Characterizing Edge and Stacking Structures of Exfoliated Graphene by Photoelectron Diffraction

    NASA Astrophysics Data System (ADS)

    Matsui, Fumihiko; Ishii, Ryo; Matsuda, Hiroyuki; Morita, Makoto; Kitagawa, Satoshi; Matsushita, Tomohiro; Koh, Shinji; Daimon, Hiroshi

    2013-11-01

    The two-dimensional C 1s photoelectron intensity angular distributions (PIADs) and spectra of exfoliated graphene flakes and crystalline graphite were measured using a focused soft X-ray beam. Suitable graphene samples were selected by thickness characterization using Raman spectromicroscopy after transferring mechanically exfoliated graphene flakes onto a 90-nm-thick SiO2 film. In every PIAD, a Kagomé interference pattern was observed, particularly clearly in the monolayer graphene PIAD. Its origin is the overlap of the diffraction rings formed by an in-plane C-C bond honeycomb lattice. Thus, the crystal orientation of each sample can be determined. In the case of bilayer graphene, PIAD was threefold-symmetric, while those of monolayer graphene and crystalline graphite were sixfold-symmetric. This is due to the stacking structure of bilayer graphene. From comparisons with the multiple scattering PIAD simulation results, the way of layer stacking as well as the termination types in the edge regions of bilayer graphene flakes were determined. Furthermore, two different C 1s core levels corresponding to the top and bottom layers of bilayer graphene were identified. A chemical shift to a higher binding energy by 0.25 eV for the bottom layer was attributed to interfacial interactions.

  4. Doping of graphene during chemical exfoliation

    NASA Astrophysics Data System (ADS)

    Srivastava, Pawan Kumar; Yadav, Premlata; Ghosh, Subhasis

    2013-02-01

    Graphene provides a perfect platform to explore the unique electronic properties in two-dimensions. However, most electronic applications are handicapped by the absence of a semiconducting gap in pristine graphene. To control the semiconducting properties of graphene, doping is regarded as one of the most feasible methods. Here we demonstrate that graphene can be effectively doped during chemical exfoliation of highly ordered pyrolitic graphite in organic solvents. Layered structure of graphene sheets was confirmed by confocal Raman spectroscopy and doping was probed by analyzing shift in Raman peak positions and transistor transfer (IDS-VGS) characteristics.

  5. Stationary waves in a superfluid exciton gas in quantum Hall bilayers.

    PubMed

    Pikalov, A A; Fil, D V

    2011-07-01

    Stationary waves in a superfluid magnetoexciton gas in ν = 1 quantum Hall bilayers are considered. The waves are induced by counterpropagating electrical currents that flow in a system with a point obstacle. It is shown that stationary waves can emerge only in imbalanced bilayers in a certain diapason of currents. It is found that the stationary wave pattern is modified qualitatively under a variation of the ratio of the interlayer distance to the magnetic length [Formula: see text]. The advantages of using graphene-dielectric-graphene sandwiches for the observation of stationary waves are discussed. We determine the range of parameters (the dielectric constant of the layer that separates two graphene layers and the ratio d/l) for which the state with superfluid magnetoexcitons can be realized in such sandwiches. Typical stationary wave patterns are presented as density plots.

  6. Stationary waves in a superfluid exciton gas in quantum Hall bilayers.

    PubMed

    Pikalov, A A; Fil, D V

    2011-07-01

    Stationary waves in a superfluid magnetoexciton gas in ν = 1 quantum Hall bilayers are considered. The waves are induced by counterpropagating electrical currents that flow in a system with a point obstacle. It is shown that stationary waves can emerge only in imbalanced bilayers in a certain diapason of currents. It is found that the stationary wave pattern is modified qualitatively under a variation of the ratio of the interlayer distance to the magnetic length [Formula: see text]. The advantages of using graphene-dielectric-graphene sandwiches for the observation of stationary waves are discussed. We determine the range of parameters (the dielectric constant of the layer that separates two graphene layers and the ratio d/l) for which the state with superfluid magnetoexcitons can be realized in such sandwiches. Typical stationary wave patterns are presented as density plots. PMID:21666305

  7. Nanoparticle-lipid bilayer interactions studied with lipid bilayer arrays.

    PubMed

    Lu, Bin; Smith, Tyler; Schmidt, Jacob J

    2015-05-01

    The widespread environmental presence and commercial use of nanoparticles have raised significant health concerns as a result of many in vitro and in vivo assays indicating toxicity of a wide range of nanoparticle species. Many of these assays have identified the ability of nanoparticles to damage cell membranes. These interactions can be studied in detail using artificial lipid bilayers, which can provide insight into the nature of the particle-membrane interaction through variation of membrane and solution properties not possible with cell-based assays. However, the scope of these studies can be limited because of the low throughput characteristic of lipid bilayer platforms. We have recently described an easy to use, parallel lipid bilayer platform which we have used to electrically investigate the activity of 60 nm diameter amine and carboxyl modified polystyrene nanoparticles (NH2-NP and COOH-NP) with over 1000 lipid bilayers while varying lipid composition, bilayer charge, ionic strength, pH, voltage, serum, particle concentration, and particle charge. Our results confirm recent studies finding activity of NH2-NP but not COOH-NP. Detailed analysis shows that NH2-NP formed pores 0.3-2.3 nm in radius, dependent on bilayer and solution composition. These interactions appear to be electrostatic, as they are regulated by NH2-NP surface charge, solution ionic strength, and bilayer charge. The ability to rapidly measure a large number of nanoparticle and membrane parameters indicates strong potential of this bilayer array platform for additional nanoparticle bilayer studies.

  8. Prediction of superconductivity in Li-intercalated bilayer phosphorene

    SciTech Connect

    Huang, G. Q.; Xing, Z. W.; Xing, D. Y.

    2015-03-16

    It is shown that bilayer phosphorene can be transformed from a direct-gap semiconductor to a BCS superconductor by intercalating Li atoms. For the Li-intercalated bilayer phosphorene, we find that the electron occupation of Li-derived band is small and superconductivity is intrinsic. With increasing the intercalation of Li atoms, both increased metallicity and strong electron-phonon coupling are favorable for the enhancement of superconductivity. The obtained electron-phonon coupling λ can be larger than 1 and the superconducting temperature T{sub c} can be increased up to 16.5 K, suggesting that phosphorene may be a good candidate for a nanoscale superconductor.

  9. Coexistence of metallic and insulating-like states in graphene.

    PubMed

    Wu, Fang; Huang, Jing; Li, Qunxiang; Deng, Kaiming; Kan, Erjun

    2015-03-10

    Since graphene has been taken as the potential host material for next-generation electric devices, coexistence of high carrier mobility and an energy gap has the determining role in its real applications. However, in conventional methods of band-gap engineering, the energy gap and carrier mobility in graphene are seemed to be the two terminals of a seesaw, which limit its rapid development in electronic devices. Here we demonstrated the realization of insulating-like state in graphene without breaking Dirac cone. Using first-principles calculations, we found that ferroelectric substrate not only well reserves the Dirac fermions, but also induces pseudo-gap states in graphene. Calculated transport results clearly revealed that electrons cannot move along the ferroelectric direction. Thus, our work established a new concept of opening an energy gap in graphene without reducing the high mobility of carriers, which is a step towards manufacturing graphene-based devices.

  10. Coexistence of metallic and insulating-like states in graphene.

    PubMed

    Wu, Fang; Huang, Jing; Li, Qunxiang; Deng, Kaiming; Kan, Erjun

    2015-01-01

    Since graphene has been taken as the potential host material for next-generation electric devices, coexistence of high carrier mobility and an energy gap has the determining role in its real applications. However, in conventional methods of band-gap engineering, the energy gap and carrier mobility in graphene are seemed to be the two terminals of a seesaw, which limit its rapid development in electronic devices. Here we demonstrated the realization of insulating-like state in graphene without breaking Dirac cone. Using first-principles calculations, we found that ferroelectric substrate not only well reserves the Dirac fermions, but also induces pseudo-gap states in graphene. Calculated transport results clearly revealed that electrons cannot move along the ferroelectric direction. Thus, our work established a new concept of opening an energy gap in graphene without reducing the high mobility of carriers, which is a step towards manufacturing graphene-based devices. PMID:25754862

  11. Penetration and lateral diffusion characteristics of polycrystalline graphene barriers.

    PubMed

    Yoon, Taeshik; Mun, Jeong Hun; Cho, Byung Jin; Kim, Taek-Soo

    2014-01-01

    We report penetration and lateral diffusion behavior of environmental molecules on synthesized polycrystalline graphene. Penetration occurs through graphene grain boundaries resulting in local oxidation. However, when the penetrated molecules diffuse laterally, the oxidation region will expand. Therefore, we measured the lateral diffusion rate along the graphene-copper interface for the first time by the environment-assisted crack growth test. It is clearly shown that the lateral diffusion is suppressed due to the high van der Waals interaction. Finally, we employed bilayer graphene for a perfect diffusion barrier facilitated by decreased defect density and increased lateral diffusion path.

  12. Electronic dispersion from long-range atomic ordering and periodic potentials in two overlapping graphene sheets

    NASA Astrophysics Data System (ADS)

    Ohta, Taisuke; Robinson, Jeremy; Feibelman, Peter; Beechem, Thomas; Diaconescu, Bogdan; Bostwick, Aaron; Rotenberg, Eli; Kellogg, Gary

    2013-03-01

    A worldwide effort is underway to learn how to build devices that take advantage of the remarkable electronic properties of graphene and other two-dimensional crystals. An outstanding question is how stacking two or a few such crystals affects their joint electronic behavior. Our talk concerns ``twisted bilayer graphene (TBG),'' that is, two graphene layers azimuthally misoriented. Applying angle-resolved photoemission spectroscopy and density functional theory, we have found van Hove singularities (vHs) and associated mini-gaps in the TBG electronic spectrum, which represent unambiguous proof that the layers interact. Of particular interest is that the measured and calculated electronic dispersion manifests the periodicity of the moiré superlattice formed by the twist. Thus, there are vHs not just where the Dirac cones of the two layers overlap, but also at the boundaries of the moiré superlattice Brillouin zone. Moirés, ubiquitous in hybrid solids based on two-dimensional crystals, accordingly present themselves as tools for manipulating the electronic behavior. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. DOE's National Nuclear Security Administration under contract DE-AC04-94AL85000.

  13. Phase diagram and edge excitations of the ν = 0 quantum Hall state in graphene

    NASA Astrophysics Data System (ADS)

    Kharitonov, Maxim

    2013-03-01

    The interaction-induced broken-symmetry incompressible quantum Hall states in graphene at integer and fractional filling factors have by now been firmly established in transport and compressibility measurements. However, identifying their precise nature (e.g., how the symmetry is broken) still remains a tough challenge: on the experimental side, transport and compressibility probes do not provide direct information about the physical order; on the theoretical side, the presence of additional to spin discrete degrees of freedom, valleys, results in a variety of competing phases in this multicomponent system. As the prime example of this rich behavior, I will present a generic phase diagram for the intriguing ν = 0 state, obtained within the framework of quantum Hall ``ferromagnetism.'' The diagram consists of the canted antiferromagnetic, ferromagnetic, charge-density-wave (charge-layer-polarized), and Kekulé (interlayer-coherent) phases in monolayer (bilayer). I will then discuss the edge excitations of the ν = 0 state. Remarkably, the edge excitations are nonuniversal (e.g., can be gapped or gapless) and crucially depend on which phase is realized in the bulk of the system. Besides being of considerable theoretical interest, these unprecedented properties simultaneously allow one to infer about the nature of the phases from the transport experiments. I will present arguments based on this analysis and existing data why the insulating ν = 0 state realized in real bilayer (and possibly, monolayer) graphene is likely to be canted antiferromagnetic. Finally, I will mention how this theoretical framework can be generalized to fractional quantum Hall states in graphene, which could shed light on some of the puzzling features of the recent experiments. This research was supported by the U.S. DOE under contracts No. DE-FG02-99ER45790 and No. DE- AC02-06CH11357.

  14. Graphene synthesis and characterization on copper

    NASA Astrophysics Data System (ADS)

    Mohsin, Ali

    Graphene, two dimensional sheet of carbon atoms has recently gained attention as some of its properties are promising for electronics applications e.g. higher mobility that translates to higher operating frequency for devices geared towards radio frequency applications. Excellent optical transmittance combined with its semi metallic behavior makes it an important material for transparent contacts in solar cells. To bring graphene to the production level, synthesis methods are needed for its growth on wafer scale. It has been shown that chemical vapor deposition (CVD) is one of the techniques that can potentially synthesize wafer scale graphene. Recently copper has gained popularity as an important substrate material for graphene growth due to its lower carbon solubility, which allows better control over number of graphene layers. Here we report optimization of graphene growth on copper foils with our home made atmospheric pressure chemical vapor deposition (APCVD) setup. Graphene growth on copper under APCVD was non self-limiting similar to other reports. It was found that apart from growth parameters surface texture plays a very important role in graphene growth. In fact, few layer and bilayer graphene were obtained on the regions where copper surface was not uniform, confirmed by Raman spectroscopy. To improve copper surface texture thin layer of copper film was evaporated by electron beam evaporation before the graphene growth process. After this modification, monolayer graphene was obtained on areas as large as 300 microm x 300 microm confirmed by Raman area maps. Graphene transfer procedure was also optimized so that graphene on metal surface could be transferred to insulating substrate.

  15. Nanoparticle-lipid bilayer interactions studied with lipid bilayer arrays

    NASA Astrophysics Data System (ADS)

    Lu, Bin; Smith, Tyler; Schmidt, Jacob J.

    2015-04-01

    The widespread environmental presence and commercial use of nanoparticles have raised significant health concerns as a result of many in vitro and in vivo assays indicating toxicity of a wide range of nanoparticle species. Many of these assays have identified the ability of nanoparticles to damage cell membranes. These interactions can be studied in detail using artificial lipid bilayers, which can provide insight into the nature of the particle-membrane interaction through variation of membrane and solution properties not possible with cell-based assays. However, the scope of these studies can be limited because of the low throughput characteristic of lipid bilayer platforms. We have recently described an easy to use, parallel lipid bilayer platform which we have used to electrically investigate the activity of 60 nm diameter amine and carboxyl modified polystyrene nanoparticles (NH2-NP and COOH-NP) with over 1000 lipid bilayers while varying lipid composition, bilayer charge, ionic strength, pH, voltage, serum, particle concentration, and particle charge. Our results confirm recent studies finding activity of NH2-NP but not COOH-NP. Detailed analysis shows that NH2-NP formed pores 0.3-2.3 nm in radius, dependent on bilayer and solution composition. These interactions appear to be electrostatic, as they are regulated by NH2-NP surface charge, solution ionic strength, and bilayer charge. The ability to rapidly measure a large number of nanoparticle and membrane parameters indicates strong potential of this bilayer array platform for additional nanoparticle bilayer studies.The widespread environmental presence and commercial use of nanoparticles have raised significant health concerns as a result of many in vitro and in vivo assays indicating toxicity of a wide range of nanoparticle species. Many of these assays have identified the ability of nanoparticles to damage cell membranes. These interactions can be studied in detail using artificial lipid bilayers, which

  16. Molecular dynamics simulation of temperature profile in partially hydrogenated graphene and graphene with grain boundary.

    PubMed

    Lotfi, Erfan; Neek-Amal, M; Elahi, M

    2015-11-01

    Temperature profile in graphene, graphene with grain boundary and vacancy defects and hydrogenated graphene with different percentage of H-atoms are determined using molecular dynamics simulation. We also obtained the temperature profile in a graphene nanoribbon containing two types of grain boundaries with different misorientation angles, θ=21.8° and θ=32.2°. We found that a temperature gap appears in the temperature profile of a graphene nanoribbon with a grain boundary at the middle. Moreover, we found that the temperature profile in the partially hydrogenated graphene varies with the percentage of hydrogens, i.e. the C:H ratio. Our results show that a grain boundary line in the graphene sheet can change the thermal transport through the system which might be useful for controlling thermal flow in nanostructured graphene.

  17. The formation and electronic properties of hydrogenated bilayer silicene from first-principles

    SciTech Connect

    Rui, Wang; Shaofeng, Wang; Xiaozhi, Wu

    2014-07-14

    Density-functional calculations of the formation and electronic properties of bilayer silicene and hydrogenated bilayer silicene are present. The structure optimization and phonon calculations are carried out to demonstrate that bilayer silicene has stable structure with AB stacking and preserves the linear energy dispersions near the K and K′ points in the hexagonal Brillouin zone, and our results agree well with the previous calculations. Based on the symmetry of bilayer silicene, we study the stability of four hydrogenated bilayer silicene by the phonon-mode analysis. The configurations of semihydrogenated one-layer and semihydrogenated double-layer are confirmed as dynamically stable with no imaginary phonon frequencies. The case of semihydrogenated one-layer shows ferromagnetic ground state due to the unpaired 3p electrons, while that of semihydrogenated double-layer is predicted to be a indirect band gap semiconductor with nonmagnetic ground state.

  18. Single-crystalline monolayer and multilayer graphene nano switches

    SciTech Connect

    Li, Peng; Cui, Tianhong; Jing, Gaoshan; Zhang, Bo; Sando, Shota

    2014-03-17

    Growth of monolayer, bi-layer, and tri-layer single-crystalline graphene (SCG) using chemical vapor deposition method is reported. SCG's mechanical properties and single-crystalline nature were characterized and verified by atomic force microscope and Raman spectroscopy. Electro-mechanical switches based on mono- and bi-layer SCG were fabricated, and the superb properties of SCG enable the switches to operate at pull-in voltage as low as 1 V, and high switching speed about 100 ns. These devices exhibit lifetime without a breakdown of over 5000 cycles, far more durable than any other graphene nanoelectromechanical system switches reported.

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

    PubMed Central

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

    2016-01-01

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

  20. Intriguing electronic properties of two-dimensional MoS2/TM2CO2 (TM = Ti, Zr, or Hf) hetero-bilayers: type-II semiconductors with tunable band gaps

    NASA Astrophysics Data System (ADS)

    Li, Xinru; Dai, Ying; Ma, Yandong; Liu, Qunqun; Huang, Baibiao

    2015-03-01

    Two-dimensional (2D) transition metal compound (TMC) monolayers, as well as their van der Waals heterostructures with unique properties, are fundamentally and technologically intriguing. Here, heterostructures consisting of a MoS2 monolayer and TM2CO2 (TM = Ti, Zr or Hf) monolayers are systematically researched by means of the density functional theory (DFT). Different from semiconductor/metal contacts, MoS2 and TM2CO2 monolayers are all semiconductors with band gaps ranging from 0.25-1.67 eV. According to rigorous screening of stacking patterns, MoS2/Zr2CO2 is shown to be an indirect type-II semiconductor with the maximum valence and minimum conduction bands spatially separated on opposite monolayers. Simultaneously, the interface charges transfer from Zr2CO2 to MoS2 results in a built-in field that separates the electrons and holes efficiently. Also, the smaller effective masses of electrons and the holes of band edges indicate the higher carrier mobility. Moreover, strain regulation can make the hetero-bilayer’s character a semiconductor-semimetal-metal transition. The physical insights pave the way for the good performance of MoS2/TM2CO2 in next-generation electronic devices and photocatalysts.