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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. 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.

  7. Planar tunneling measurements of the energy gap in biased bilayer graphene

    NASA Astrophysics Data System (ADS)

    Puls, Conor P.; Liu, Ying

    2012-11-01

    We present an analysis on the determination of the energy gap in biased bilayer graphene using tunneling measurements, report our experimental results obtained from planar tunneling spectroscopy, and compare them with those from electrical transport measurements. Bilayer graphene flakes were prepared by exfoliating from bulk graphite onto SiO2 thermally grown on a doped Si substrate. Due to the low carrier density of bilayer graphene, the Fermi level and electronic structure are expected to be highly sensitive to tunnel bias-induced charging, which is neglected in traditional tunnel junctions. We found that the tunneling signal generally exhibited a "V"-shaped tunneling conductance background that did not shift with back gate voltage, possibly due to a two-step tunneling process. We observed a tunable suppression in the tunneling conductance that follows theoretical predictions for a band gap in biased bilayer graphene. We explore the evolution of the band gap by tuning the electric field and charge carrier density produced by the tunneling bias and back gate, and compare experimental results with numerical simulations. Finally, we compare these findings with transport measurements of top- and bottom-gated bilayer graphene field effect transistors featuring similar gate dielectrics.

  8. A promising way to open an energy gap in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Hao, Jialei; Huang, Chengxi; Wu, Haiping; Qiu, Yuhao; Gao, Qian; Hu, Zhenpeng; Kan, Erjun; Zhang, Lixin

    2015-10-01

    There has been huge research interest in the energy gap problem of monolayer and bilayer graphene due to their great potential in practical applications. Herein, based on first-principles calculations, we report a promising way to open a large band gap in bilayer graphene (BLG) by sandwiching it between two substrates, although this is not usually expected to occur due to the weak interlayer interactions dominated by van der Waals forces. Taking surface-functionalized boron-nitrides as substrates, we predict from first-principles calculations that BLG can have energy gaps ranging from 0.35 eV to 0.55 eV, depending on the substrates and stacking order. Compared to other methods of band-gap manipulation in BLG, the structural integrity of BLG is well-preserved in our study, and the predicted energy gap is suitable for electric devices. Since the proposed method is easily realized in experiments, our results will hopefully accelerate the application of graphene in semiconductor devices and promote the development of graphene technology.There has been huge research interest in the energy gap problem of monolayer and bilayer graphene due to their great potential in practical applications. Herein, based on first-principles calculations, we report a promising way to open a large band gap in bilayer graphene (BLG) by sandwiching it between two substrates, although this is not usually expected to occur due to the weak interlayer interactions dominated by van der Waals forces. Taking surface-functionalized boron-nitrides as substrates, we predict from first-principles calculations that BLG can have energy gaps ranging from 0.35 eV to 0.55 eV, depending on the substrates and stacking order. Compared to other methods of band-gap manipulation in BLG, the structural integrity of BLG is well-preserved in our study, and the predicted energy gap is suitable for electric devices. Since the proposed method is easily realized in experiments, our results will hopefully accelerate the

  9. Energy Gaps and Layer Polarization of Integer and Fractional Quantum Hall States in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Shi, Yanmeng; Lee, Yongjin; Che, Shi; Pi, Ziqi; Espiritu, Timothy; Stepanov, Petr; Smirnov, Dmitry; Lau, Chun Ning; Zhang, Fan

    2016-02-01

    Owing to the spin, valley, and orbital symmetries, the lowest Landau level in bilayer graphene exhibits multicomponent quantum Hall ferromagnetism. Using transport spectroscopy, we investigate the energy gaps of integer and fractional quantum Hall (QH) states in bilayer graphene with controlled layer polarization. The state at filling factor ν =1 has two distinct phases: a layer polarized state that has a larger energy gap and is stabilized by high electric field, and a hitherto unobserved interlayer coherent state with a smaller gap that is stabilized by large magnetic field. In contrast, the ν =2 /3 quantum Hall state and a feature at ν =1 /2 are only resolved at finite electric field and large magnetic field. These results underscore the importance of controlling layer polarization in understanding the competing symmetries in the unusual QH system of BLG.

  10. Single-electron gap in the spectrum of twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

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

    2017-01-01

    We investigate the gap in the single-electron spectrum of twisted bilayer graphene. In a perfect infinite lattice of a twisted bilayer, the gap varies exponentially in response to weak changes of the twist angle. Such a large sensitivity makes theoretical predictions of the gap nearly impossible, since experimentally the twist angle is always known with finite accuracy. To address this issue, we numerically study finite clusters of twisted bilayer graphene. For finite systems, changing the twist angle causes a gradual crossover between gapless and gapped regimes. The crossover occurs when the finite-size quantization energy becomes comparable to the matrix elements responsible for the generation of the gap. We further argue that disorder scattering can induce similar crossover, in which the mean-free path plays the same role as the system size for the finite clusters. It is demonstrated that to observe the gap experimentally, it is necessary to have a sample of suitable purity and to possess the ability to tune the twist angle accurately.

  11. 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.

  12. Tunneling Plasmonics in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

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

  13. Transport in bilayer and trilayer graphene: band gap engineering and band structure tuning

    NASA Astrophysics Data System (ADS)

    Zhu, Jun

    2014-03-01

    Controlling the stacking order of atomically thin 2D materials offers a powerful tool to control their properties. Linearly dispersed bands become hyperbolic in Bernal (AB) stacked bilayer graphene (BLG). Both Bernal (ABA) and rhombohedral (ABC) stacking occur in trilayer graphene (TLG), producing distinct band structures and electronic properties. A symmetry-breaking electric field perpendicular to the sample plane can further modify the band structures of BLG and TLG. In this talk, I will describe our experimental effort in these directions using dual-gated devices. Using thin HfO2 film deposited by ALD as gate dielectric, we are able to apply large displacement fields D > 6 V/nm and observe the opening and saturation of the field-induced band gap Eg in bilayer and ABC-stacked trilayer graphene, where the conduction in the mid gap changes by more than six decades. Its field and temperature dependence highlights the crucial role played by Coulomb disorder in facilitating hopping conduction and suppressing the effect of Eg in the tens of meV regime. In contrast, mid-gap conduction decreases with increasing D much more rapidly in clean h-BN dual-gated devices. Our studies also show the evolution of the band structure in ABA-stacked TLG, in particular the splitting of the Dirac-like bands in large D field and the signatures of two-band transport at high carrier densities. Comparison to theory reveals the need for more sophisticated treatment of electronic screening beyond self-consistent Hartree calculations to accurately predict the band structures of trilayer graphene and graphenic materials in general.

  14. Irradiated bilayer graphene

    NASA Astrophysics Data System (ADS)

    Abergel, D. S. L.; Chakraborty, Tapash

    2011-01-01

    We describe the gated bilayer graphene system when it is subjected to intense terahertz frequency electromagnetic radiation. We examine the electron band structure and density of states via exact diagonalization methods within Floquet theory. We find that dynamical states are induced which lead to modification of the band structure. We first examine the situation where there is no external magnetic field. In the unbiased case, dynamical gaps appear in the spectrum which manifest as dips in the density of states. For finite inter-layer bias (where a static gap is present in the band structure of unirradiated bilayer graphene), dynamical states may be induced in the static gap. These states can show a high degree of valley polarization. When the system is placed in a strong magnetic field, the radiation induces coupling between the Landau levels which allows dynamical levels to exist. For strong fields, this means the Landau levels are smeared to form a near-continuum of states.

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

    NASA Astrophysics Data System (ADS)

    Li, Yuanchang

    2016-12-01

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

  16. Extreme sensitivity of the electric-field-induced band gap to the electronic topological transition in sliding bilayer graphene

    NASA Astrophysics Data System (ADS)

    Lee, Kyu Won; Lee, Cheol Eui

    2015-12-01

    We have investigated the effect of electronic topological transition on the electric field-induced band gap in sliding bilayer graphene by using the density functional theory calculations. The electric field-induced band gap was found to be extremely sensitive to the electronic topological transition. At the electronic topological transition induced by layer sliding, four Dirac cones in the Bernal-stacked bilayer graphene reduces to two Dirac cones with equal or unequal Dirac energies depending on the sliding direction. While the critical electric field required for the band gap opening increases with increasing lateral shift for the two Dirac cones with unequal Dirac energies, the critical field is essentially zero with or without a lateral shift for the two Dirac cones with equal Dirac energies. The critical field is determined by the Dirac energy difference and the electronic screening effect. The electronic screening effect was also found to be enhanced with increasing lateral shift, apparently indicating that the massless helical and massive chiral fermions are responsible for the perfect and imperfect electronic screening, respectively.

  17. Controlling the Electronic Structure of Bilayer Graphene

    NASA Astrophysics Data System (ADS)

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

    2006-08-01

    We describe the synthesis of bilayer graphene thin films deposited on insulating silicon carbide and report the characterization of 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. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic-scale electronic devices.

  18. 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.

  19. Transport gap and hysteretic behavior of the Ising quantum Hall ferromagnets in |N |>0 Landau levels of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Luo, Wenchen; Côté, R.

    2014-12-01

    The chiral two-dimensional electron gas in Landau levels |N |>0 of a Bernal-stacked graphene bilayer has a valley-pseudospin Ising quantum Hall ferromagnetic behavior at odd filling factors νN=1 ,3 of these fourfold degenerate states. At zero interlayer electrical bias, the ground state at these fillings is spin polarized and electrons occupy one valley or the other while a finite electrical bias produces a series of valley pseudospin-flip transitions. In this work, we discuss the hysteretic behavior of the Ising quantum Hall ferromagnets. We compute the transport gap due to different excitations: bulk electron-hole pairs, electron-hole pairs confined to the coherent region of a valley-pseudospin domain wall, and spin or valley-pseudospin skyrmion-antiskyrmion pairs. We determine which of these excitations has the lowest energy at a given value of the Zeeman coupling, bias, and magnetic field.

  20. Band-unfolding approach to moiré-induced band-gap opening and Fermi level velocity reduction in twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Nishi, Hirofumi; Matsushita, Yu-ichiro; Oshiyama, Atsushi

    2017-02-01

    We report on the energy spectrum of electrons in twisted bilayer graphene (tBLG) obtained by the band-unfolding method in the tight-binding model. We find the band-gap opening at particular points in the reciprocal space, that elucidates the drastic reduction of the Fermi-level velocity with the tiny twisted angles in tBLGs. We find that moiré pattern caused by the twist of the two graphene layers generates interactions among Dirac cones, otherwise absent, and the resultant cone-cone interactions peculiar to each point in the reciprocal space causes the energy gap and thus reduces the Fermi-level velocity.

  1. Antiferromagnetic state in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kharitonov, Maxim

    2012-11-01

    Motivated by the recent experiment of Velasco Jr. [J. Velasco Jr. , Nat. Nanotechnology1748-338710.1038/nnano.2011.251 7, 156 (2012)], we develop a mean-field theory of the interaction-induced antiferromagnetic (AF) state in bilayer graphene at charge neutrality point at arbitrary perpendicular magnetic field B. We demonstrate that the AF state can persist at all B. At higher B, the state continuously crosses over to the AF phase of the ν=0 quantum Hall ferromagnet, recently argued to be realized in the insulating ν=0 state. The mean-field quasiparticle gap is finite at B=0 and grows with increasing B, becoming quasilinear in the quantum Hall regime, in accord with the reported behavior of the transport gap. By adjusting the two free parameters of the model, we obtain a simultaneous quantitative agreement between the experimental and theoretical values of the key parameters of the gap dependence—its zero-field value and slope at higher fields. Our findings suggest that the insulating state observed in bilayer graphene in Ref. is antiferromagnetic (canted, once the Zeeman effect is taken into account) at all magnetic fields.

  2. Electromechanical oscillations in bilayer graphene.

    PubMed

    Benameur, Muhammed M; Gargiulo, Fernando; Manzeli, Sajedeh; Autès, Gabriel; Tosun, Mahmut; Yazyev, Oleg V; Kis, Andras

    2015-10-20

    Nanoelectromechanical systems constitute a class of devices lying at the interface between fundamental research and technological applications. Realizing nanoelectromechanical devices based on novel materials such as graphene allows studying their mechanical and electromechanical characteristics at the nanoscale and addressing fundamental questions such as electron-phonon interaction and bandgap engineering. In this work, we realize electromechanical devices using single and bilayer graphene and probe the interplay between their mechanical and electrical properties. We show that the deflection of monolayer graphene nanoribbons results in a linear increase in their electrical resistance. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. The proposed theoretical model suggests that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of individual graphene layers with respect to each other. Our work shows that bilayer graphene conceals unexpectedly rich and novel physics with promising potential in applications based on nanoelectromechanical systems.

  3. Electromechanical oscillations in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Benameur, Muhammed M.; Gargiulo, Fernando; Manzeli, Sajedeh; Autès, Gabriel; Tosun, Mahmut; Yazyev, Oleg V.; Kis, Andras

    2015-10-01

    Nanoelectromechanical systems constitute a class of devices lying at the interface between fundamental research and technological applications. Realizing nanoelectromechanical devices based on novel materials such as graphene allows studying their mechanical and electromechanical characteristics at the nanoscale and addressing fundamental questions such as electron-phonon interaction and bandgap engineering. In this work, we realize electromechanical devices using single and bilayer graphene and probe the interplay between their mechanical and electrical properties. We show that the deflection of monolayer graphene nanoribbons results in a linear increase in their electrical resistance. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. The proposed theoretical model suggests that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of individual graphene layers with respect to each other. Our work shows that bilayer graphene conceals unexpectedly rich and novel physics with promising potential in applications based on nanoelectromechanical systems.

  4. Energy levels of hybrid monolayer-bilayer graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Mirzakhani, M.; Zarenia, M.; Ketabi, S. A.; da Costa, D. R.; Peeters, F. M.

    2016-04-01

    Often real samples of graphene consist of islands of both monolayer and bilayer graphene. Bound states in such hybrid quantum dots are investigated for (i) a circular single-layer graphene quantum dot surrounded by an infinite bilayer graphene sheet and (ii) a circular bilayer graphene quantum dot surrounded by an infinite single-layer graphene. Using the continuum model and applying zigzag boundary conditions at the single-layer-bilayer graphene interface, we obtain analytical results for the energy levels and the corresponding wave spinors. Their dependence on perpendicular magnetic and electric fields are studied for both types of quantum dots. The energy levels exhibit characteristics of interface states, and we find anticrossings and closing of the energy gap in the presence of a bias potential.

  5. Phonons in twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

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

    2013-07-01

    We theoretically investigate phonon dispersion in AA-stacked, AB-stacked, and twisted bilayer graphene with various rotation angles. The calculations are performed using the Born-von Karman model for the intralayer atomic interactions and the Lennard-Jones potential for the interlayer interactions. It is found that the stacking order affects the out-of-plane acoustic phonon modes the most. The difference in the phonon densities of states in the twisted bilayer graphene and in AA- or AB-stacked bilayer graphene appears in the phonon frequency range 90-110 cm-1. Twisting bilayer graphene leads to the emergence of different phonon branches—termed hybrid folded phonons—which originate from the mixing of phonon modes from different high-symmetry directions in the Brillouin zone. The frequencies of the hybrid folded phonons depend strongly on the rotation angle and can be used for noncontact identification of the twist angles in graphene samples. The obtained results and the tabulated frequencies of phonons in twisted bilayer graphene are important for the interpretation of experimental Raman data and in determining the thermal conductivity of these material systems.

  6. Band structure mapping of bilayer graphene via quasiparticle scattering

    NASA Astrophysics Data System (ADS)

    Yankowitz, Matthew; Wang, Joel I.-Jan; Li, Suchun; Birdwell, A. Glen; Chen, Yu-An; Watanabe, Kenji; Taniguchi, Takashi; Quek, Su Ying; Jarillo-Herrero, Pablo; LeRoy, Brian J.

    2014-09-01

    A perpendicular electric field breaks the layer symmetry of Bernal-stacked bilayer graphene, resulting in the opening of a band gap and a modification of the effective mass of the charge carriers. Using scanning tunneling microscopy and spectroscopy, we examine standing waves in the local density of states of bilayer graphene formed by scattering from a bilayer/trilayer boundary. The quasiparticle interference properties are controlled by the bilayer graphene band structure, allowing a direct local probe of the evolution of the band structure of bilayer graphene as a function of electric field. We extract the Slonczewski-Weiss-McClure model tight binding parameters as γ0 = 3.1 eV, γ1 = 0.39 eV, and γ4 = 0.22 eV.

  7. Bias induced modulation of electrical and thermal conductivity and heat capacity of BN and BN/graphene bilayers

    NASA Astrophysics Data System (ADS)

    Chegel, Raad

    2017-04-01

    By using the tight binding approximation and Green function method, the electronic structure, density of state, electrical conductivity, heat capacity of BN and BN/graphene bilayers are investigated. The AA-, AB1- and AB2- BN/graphene bilayers have small gap unlike to BN bilayers which are wide band gap semiconductors. Unlike to BN bilayer, the energy gap of graphene/BN bilayers increases with external field. The magnitude of the change in the band gap of BN bilayers is much higher than the graphene/BN bilayers. Near absolute zero, the σ(T) is zero for BN bilayers and it increases with temperature until reaches maximum value then decreases. The BN/graphene bilayers have larger electrical conductivity larger than BN bilayers. For both bilayers, the specific heat capacity has a Schottky anomaly.

  8. 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.

  9. Observation of Anomalous Resistance Behavior in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Liu, Yanping; Lew, Wen Siang; Liu, Zongwen

    2017-01-01

    Our measurement results have shown that bilayer graphene exhibits an unexpected sharp transition of the resistance value in the temperature region 200 250 K. We argue that this behavior originates from the interlayer ripple scattering effect between the top and bottom ripple graphene layer. The inter-scattering can mimic the Coulomb scattering but is strongly dependent on temperature. The observed behavior is consistent with the theoretical prediction that charged impurities are the dominant scatters in bilayer graphene. The resistance increase with increasing perpendicular magnetic field strongly supports the postulate that magnetic field induces an excitonic gap in bilayer graphene. Our results reveal that the relative change of resistance induced by magnetic field in the bilayer graphene shows an anomalous thermally activated property.

  10. Band gap opening in methane intercalated graphene.

    PubMed

    Hargrove, Jasmine; Shashikala, H B Mihiri; Guerrido, Lauren; Ravi, Natarajan; Wang, Xiao-Qian

    2012-08-07

    Recent experimental work has demonstrated production of quasi-free-standing graphene by methane intercalation. The intercalation weakens the coupling of adjacent graphene layers and yields Dirac fermion behaviour of monolayer graphene. We have investigated the electronic characteristics of a methane intercepted graphene bilayer under a perpendicularly applied electric field. Evolution of the band structure of intercalated graphene as a function of the bias is studied by means of density-functional theory including interlayer van der Waals interactions. The implications of controllable band gap opening in methane-intercalated graphene for future device applications are discussed.

  11. 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.

  12. Thermal conductivity of twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Li, Hongyang; Ying, Hao; Chen, Xiangping; Nika, Denis L.; Cocemasov, Alexandr I.; Cai, Weiwei; Balandin, Alexander A.; Chen, Shanshan

    2014-10-01

    We have investigated experimentally the thermal conductivity of suspended twisted bilayer graphene. The measurements were performed using an optothermal Raman technique. It was found that the thermal conductivity of twisted bilayer graphene is lower than that of monolayer graphene and the reference, Bernal stacked bilayer graphene in the entire temperature range examined (~300-700 K). This finding indicates that the heat carriers - phonons - in twisted bilayer graphene do not behave in the same manner as that observed in individual graphene layers. The decrease in the thermal conductivity found in twisted bilayer graphene was explained by the modification of the Brillouin zone due to plane rotation and the emergence of numerous folded phonon branches that enhance the phonon Umklapp and normal scattering. The results obtained are important for understanding thermal transport in two-dimensional systems.

  13. Electronic structure of bilayer graphene physisorbed on metal substrates

    NASA Astrophysics Data System (ADS)

    Khan, Emroz; Rahman, Tahmid Sami; Subrina, Samia

    2016-11-01

    Graphene-metal interfaces have recently become popular for graphene growth and for making contacts in numerous thermal and photo-electronic devices. A number of studies have already been made to investigate the interfacial properties when single layer graphene is grown on metal substrates. In this study, we consider the physisorption of bilayer graphene on metals and find a significant bandgap opening which is otherwise absent in the single layer case. This gap arises from the asymmetry in the bilayer due to the charge transfer process at the interface. This charge transfer also causes doping in the bilayer graphene and a corresponding shift in the Fermi level. In this work, we present a thorough investigation into the induced bandgap and Fermi level shift when bilayer graphene is adsorbed on Cu, Al, Ag, Pt, and Au(111) surfaces first by reporting their values from Density Functional Theory (DFT) studies with Local Density Approximation functional used for exchange-correlation energy. Next, to obtain an enhanced picture of the surface physics at play (which is usually obscured by the complexities of DFT), we provide an analytical model to relate the induced bandgap and Fermi level shift to the metal work function and interface separation distance. The values predicted from the model shows a high degree of correlation with the values obtained from the DFT simulation. The results are expected to greatly facilitate the understanding of bilayer graphene adsorption on metals, which in turn may aid the study of graphene electronic devices.

  14. 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).

  15. Impurities and electronic localization in graphene bilayers

    NASA Astrophysics Data System (ADS)

    Ojeda Collado, H. P.; Usaj, Gonzalo; Balseiro, C. A.

    2015-01-01

    We analyze the electronic properties of bilayer graphene with Bernal stacking and a low concentration of adatoms. Assuming that the host bilayer lies on top of a substrate, we consider the case where impurities are adsorbed only on the upper layer. We describe nonmagnetic impurities as a single orbital hybridized with carbon's pz states. The effect of impurity doping on the local density of states with and without a gated electric field perpendicular to the layers is analyzed. We look for Anderson localization in the different regimes and estimate the localization length. In the biased system, the field-induced gap is partially filled by strongly localized impurity states. Interestingly, the structure, distribution, and localization length of these states depend on the field polarization.

  16. 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

  17. Energy levels of bilayer graphene quantum dots

    NASA Astrophysics Data System (ADS)

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

    2015-09-01

    Within a tight binding approach we investigate the energy levels of hexagonal and triangular bilayer graphene (BLG) quantum dots (QDs) with zigzag and armchair edges. We study AA- and AB- (Bernal) stacked BLG QDs and obtain the energy levels in both the absence and the presence of a perpendicular electric field (i.e., biased BLG QDs). Our results show that the size dependence of the energy levels is different from that of monolayer graphene QDs. The energy spectrum of AB-stacked BLG QDs with zigzag edges exhibits edge states which spread out into the opened energy gap in the presence of a perpendicular electric field. We found that the behavior of these edges states is different for the hexagonal and triangular geometries. In the case of AA-stacked BLG QDs, the electron and hole energy levels cross each other in both cases of armchair and zigzag edges as the dot size or the applied bias increases.

  18. Graphene monolayer rotation on Ni(111) facilitates bilayer graphene growth

    NASA Astrophysics Data System (ADS)

    Dahal, Arjun; Addou, Rafik; Sutter, Peter; Batzill, Matthias

    2012-06-01

    Synthesis of bilayer graphene by chemical vapor deposition is of importance for graphene-based field effect devices. Here, we demonstrate that bilayer graphene preferentially grows by carbon-segregation under graphene sheets that are rotated relative to a Ni(111) substrate. Rotated graphene monolayer films can be synthesized at growth temperatures above 650 °C on a Ni(111) thin-film. The segregated second graphene layer is in registry with the Ni(111) substrate and this suppresses further C-segregation, effectively self-limiting graphene formation to two layers.

  19. Graphene Monolayer Rotation on Ni(111) Facilities Bilayer Graphene Growth

    SciTech Connect

    Batzill M.; Sutter P.; Dahal, A.; Addou, R.

    2012-06-11

    Synthesis of bilayer graphene by chemical vapor deposition is of importance for graphene-based field effect devices. Here, we demonstrate that bilayer graphene preferentially grows by carbon-segregation under graphene sheets that are rotated relative to a Ni(111) substrate. Rotated graphene monolayer films can be synthesized at growth temperatures above 650 C on a Ni(111) thin-film. The segregated second graphene layer is in registry with the Ni(111) substrate and this suppresses further C-segregation, effectively self-limiting graphene formation to two layers.

  20. Broken-Symmetry Quantum Hall States in Twisted Bilayer Graphene

    PubMed Central

    Kim, Youngwook; Park, Jaesung; Song, Intek; Ok, Jong Mok; Jo, Younjung; Watanabe, Kenji; Taniquchi, Takashi; Choi, Hee Cheul; Lee, Dong Su; Jung, Suyong; Kim, Jun Sung

    2016-01-01

    Twisted bilayer graphene offers a unique bilayer two-dimensional-electron system where the layer separation is only in sub-nanometer scale. Unlike Bernal-stacked bilayer, the layer degree of freedom is disentangled from spin and valley, providing eight-fold degeneracy in the low energy states. We have investigated broken-symmetry quantum Hall (QH) states and their transitions due to the interplay of the relative strength of valley, spin and layer polarizations in twisted bilayer graphene. The energy gaps of the broken-symmetry QH states show an electron-hole asymmetric behaviour, and their dependence on the induced displacement field are opposite between even and odd filling factor states. These results strongly suggest that the QH states with broken valley and spin symmetries for individual layer become hybridized via interlayer tunnelling, and the hierarchy of the QH states is sensitive to both magnetic field and displacement field due to charge imbalance between layers. PMID:27905496

  1. Broken-Symmetry Quantum Hall States in Twisted Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Kim, Youngwook; Park, Jaesung; Song, Intek; Ok, Jong Mok; Jo, Younjung; Watanabe, Kenji; Taniquchi, Takashi; Choi, Hee Cheul; Lee, Dong Su; Jung, Suyong; Kim, Jun Sung

    2016-12-01

    Twisted bilayer graphene offers a unique bilayer two-dimensional-electron system where the layer separation is only in sub-nanometer scale. Unlike Bernal-stacked bilayer, the layer degree of freedom is disentangled from spin and valley, providing eight-fold degeneracy in the low energy states. We have investigated broken-symmetry quantum Hall (QH) states and their transitions due to the interplay of the relative strength of valley, spin and layer polarizations in twisted bilayer graphene. The energy gaps of the broken-symmetry QH states show an electron-hole asymmetric behaviour, and their dependence on the induced displacement field are opposite between even and odd filling factor states. These results strongly suggest that the QH states with broken valley and spin symmetries for individual layer become hybridized via interlayer tunnelling, and the hierarchy of the QH states is sensitive to both magnetic field and displacement field due to charge imbalance between layers.

  2. Temperature effect on plasmons in bilayer graphene

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

    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.

  3. RKKY interaction in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Mohammadi, Yawar; Moradian, Rostam

    2015-12-01

    We study the RKKY interaction between two magnetic impurities located on the same layer (intralayer case) or on different layers (interlayer case) in undoped bilayer graphene (BLG) in the four-bands model, by directly calculating the Green functions in the eigenvalues and eigenvectors representation. Our results show that both intra- and interlayer RKKY interactions between two magnetic impurities located on the same (opposite) sublattice are always ferromagnetic (antiferromagnetic). Furthermore we find unusual long-distance decay of the RKKY interaction in BLG. The intralyer RKKY interactions between two magnetic impurities located on the same sublattice, J AnAn(R) and J BnBn(R), decay closely as 1 /R6 and 1 /R2 at large impurity distances respectively, but when they are located on opposite sublattices the RKKY interactions exhibit 1 /R4 decays approximately. In the interlayer case, the RKKY interactions between two magnetic impurities located on the same sublattice show a decay close to 1 /R4 at large impurity distances, but if two magnetic impurities be on opposite sublattices the RKKY interactions, J A1B2(R) and J B1A2(R), decay closely as 1 /R6 and 1 /R2 respectively. Both intra- and interlayer RKKY interactions have anisotropic oscillatory factors which for intralayer case is equal to that for single layer graphene (SLG). Our results at weak and strong interlayer coupling limits reduce to the RKKY interaction of SLG and that of BLG in the two-bands approximation respectively.

  4. Emergent Electromagnetism in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Winkler, Roland; Zülicke, Ulrich

    2013-03-01

    Recently atomically flat layers of carbon known as graphene have become the rising star in spintronics as their electrons carry not only the ordinary spin degree of freedom, but they also have a pseudospin degree of freedom tied to the electrons' orbital motion which could enable new routes for spintronics. Here we focus on bilayer graphene (BLG). Using group theory we have established a complete description of how electrons in BLG interact with electric and magnetic fields. We show that electrons in BLG experience an unusual type of matter-field interactions where magnetic and electric fields are virtually equivalent: every coupling of an electron's degrees of freedom to a magnetic field is matched by an analogous coupling of the same degrees of freedom to an electric field. This counter-intuitive duality of matter-field interactions allows novel ways to create and manipulate spin and pseudo-spin polarizations via external fields that are not available in other materials. See arXiv:1206.4761. This work was supported by Marsden Fund contract no. VUW0719, administered by the Royal Society of New Zealand. Work at Argonne was supported by DOE BES under Contract No. DE-AC02-06CH11357.

  5. Thermoelectric power in a bilayer graphene device

    NASA Astrophysics Data System (ADS)

    Yung-Yu, Chien; Hongtao, Yuan; Chang-Ran, Wang; Chun-Hsuan, Lin; Wei-Li, Lee; Geballe LaboratoryAdvanced Materials Collaboration; Institute of Physics, Academia Sinica, Taipei, Taiwan Team

    2014-03-01

    There have been great interests on band gap engineering in a bilayer graphene (BLG) device, where inversion symmetry breaking by a perpendicular electric field can give rise to a sizable band gap. In our previous works, we have demonstrated a large enhancement in the thermoelectric power (TEP) associated with the band gap opening in a dual-gated BLG device. It is, therefore, an interesting question to ask whether even larger TEP can be achieved with a larger perpendicular electric field applied. We explored such possibility by utilizing the ionic liquid gating technique in BLG devices. By controlling the side gate voltage of ionic liquid and the bottom gate voltage via SiO2/Si substrate, large increase of the sheet resistance at charge neutral point was observed suggesting the opening of a band gap. At T = 120 K, TEP increases by more than 44% with a side gate voltage of ~ 1V. The influence of charge puddles to TEP using ionic liquid gating will be discussed.

  6. 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.

  7. 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.

  8. 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.

  9. Strain solitons and topological defects in bilayer graphene.

    PubMed

    Alden, Jonathan S; Tsen, Adam W; Huang, Pinshane Y; Hovden, Robert; Brown, Lola; Park, Jiwoong; Muller, David A; McEuen, Paul L

    2013-07-09

    Bilayer graphene has been a subject of intense study in recent years. The interlayer registry between the layers can have dramatic effects on the electronic properties: for example, in the presence of a perpendicular electric field, a band gap appears in the electronic spectrum of so-called Bernal-stacked graphene [Oostinga JB, et al. (2007) Nature Materials 7:151-157]. This band gap is intimately tied to a structural spontaneous symmetry breaking in bilayer graphene, where one of the graphene layers shifts by an atomic spacing with respect to the other. This shift can happen in multiple directions, resulting in multiple stacking domains with soliton-like structural boundaries between them. Theorists have recently proposed that novel electronic states exist at these boundaries [Vaezi A, et al. (2013) arXiv:1301.1690; Zhang F, et al. (2013) arXiv:1301.4205], but very little is known about their structural properties. Here we use electron microscopy to measure with nanoscale and atomic resolution the widths, motion, and topological structure of soliton boundaries and related topological defects in bilayer graphene. We find that each soliton consists of an atomic-scale registry shift between the two graphene layers occurring over 6-11 nm. We infer the minimal energy barrier to interlayer translation and observe soliton motion during in situ heating above 1,000 °C. The abundance of these structures across a variety of samples, as well as their unusual properties, suggests that they will have substantial effects on the electronic and mechanical properties of bilayer graphene.

  10. Strain solitons and topological defects in bilayer graphene

    PubMed Central

    Alden, Jonathan S.; Tsen, Adam W.; Huang, Pinshane Y.; Hovden, Robert; Brown, Lola; Park, Jiwoong; Muller, David A.; McEuen, Paul L.

    2013-01-01

    Bilayer graphene has been a subject of intense study in recent years. The interlayer registry between the layers can have dramatic effects on the electronic properties: for example, in the presence of a perpendicular electric field, a band gap appears in the electronic spectrum of so-called Bernal-stacked graphene [Oostinga JB, et al. (2007) Nature Materials 7:151–157]. This band gap is intimately tied to a structural spontaneous symmetry breaking in bilayer graphene, where one of the graphene layers shifts by an atomic spacing with respect to the other. This shift can happen in multiple directions, resulting in multiple stacking domains with soliton-like structural boundaries between them. Theorists have recently proposed that novel electronic states exist at these boundaries [Vaezi A, et al. (2013) arXiv:1301.1690; Zhang F, et al. (2013) arXiv:1301.4205], but very little is known about their structural properties. Here we use electron microscopy to measure with nanoscale and atomic resolution the widths, motion, and topological structure of soliton boundaries and related topological defects in bilayer graphene. We find that each soliton consists of an atomic-scale registry shift between the two graphene layers occurring over 6–11 nm. We infer the minimal energy barrier to interlayer translation and observe soliton motion during in situ heating above 1,000 °C. The abundance of these structures across a variety of samples, as well as their unusual properties, suggests that they will have substantial effects on the electronic and mechanical properties of bilayer graphene. PMID:23798395

  11. Chemical potential and tunneling in bilayer graphene using double bilayer graphene heterostructures

    NASA Astrophysics Data System (ADS)

    Tutuc, Emanuel

    2015-03-01

    Vertical heterostructures consisting of atomic layers separated by insulators can open a window to explore the role of electron interaction in these materials, otherwise not accessible in single layer devices. We describe here one such heterostructure, consisting of two bilayer graphene flakes separated by a hexagonal boron-nitride dielectric. Using the top layer as a resistively detected Kelvin probe we map the chemical potential of the bottom bilayer graphene as a function of electron density, perpendicular magnetic field, and transverse electric field. At zero magnetic field the chemical potential reveals a strongly non-linear dependence on density, with an electric field induced energy gap at charge neutrality. The data allow a direct measurement of the electric field-induced bandgap at zero magnetic field, the orbital Landau level energies, and the broken symmetry quantum Hall state gaps in high magnetic fields. In samples where the two layers are rotationally aligned the interlayer tunneling current measured as a function of interlayer bias reveals a gate-tunable negative differential resistance thanks to momentum conserving tunneling. Remarkably, the resonance width has a weak temperature dependence in the range 1.5 K to 300 K. Work done in collaboration with K. Lee, B. Fallahazad, S. Kang, J. Xue, D. C. Dillen, K. Kim, L. F. Register, S. K. Banerjee, T. Taniguchi, and K. Watanabe. This work supported by the Office of Naval Research, the Nanoelectronics Research Initiative SWAN center, and Intel Corp.

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

    PubMed

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

    2016-02-08

    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.

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

    PubMed Central

    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. PMID:26852799

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

    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.

  15. Enhanced Thermoelectric Power in Dual-Gated Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Wang, Chang-Ran; Lu, Wen-Sen; Hao, Lei; Lee, Wei-Li; Lee, Ting-Kuo; Lin, Feng; Cheng, I.-Chun; Chen, Jian-Zhang

    2011-10-01

    The thermoelectric power of a material, typically governed by its band structure and carrier density, can be varied by chemical doping that is often restricted by solubility of the dopant. Materials showing large thermoelectric power are useful for many industrial applications, such as the heat-to-electricity conversion and the thermoelectric cooling device. Here we show a full electric-field tuning of thermoelectric power in a dual-gated bilayer graphene device resulting from the opening of a band gap by applying a perpendicular electric field on bilayer graphene. We uncover a large enhancement in thermoelectric power at a low temperature, which may open up a new possibility in low temperature thermoelectric application using graphene-based device.

  16. Enhanced thermoelectric power in dual-gated bilayer graphene.

    PubMed

    Wang, Chang-Ran; Lu, Wen-Sen; Hao, Lei; Lee, Wei-Li; Lee, Ting-Kuo; Lin, Feng; Cheng, I-Chun; Chen, Jian-Zhang

    2011-10-28

    The thermoelectric power of a material, typically governed by its band structure and carrier density, can be varied by chemical doping that is often restricted by solubility of the dopant. Materials showing large thermoelectric power are useful for many industrial applications, such as the heat-to-electricity conversion and the thermoelectric cooling device. Here we show a full electric-field tuning of thermoelectric power in a dual-gated bilayer graphene device resulting from the opening of a band gap by applying a perpendicular electric field on bilayer graphene. We uncover a large enhancement in thermoelectric power at a low temperature, which may open up a new possibility in low temperature thermoelectric application using graphene-based device.

  17. High-yield chemical vapor deposition growth of high-quality large-area AB-stacked bilayer graphene.

    PubMed

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

    2012-09-25

    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 electric 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 H(2)/CH(4) 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 demonstrate that devices made of the as-grown bilayer graphene exhibit typical characteristics of AB-stacked bilayer graphene with the highest carrier mobility exceeding 4000 cm(2)/V · s at room temperature, comparable to that of the exfoliated bilayer graphene.

  18. Interfacial Properties of Bilayer and Trilayer Graphene on Metal Substrates

    PubMed Central

    Zheng, Jiaxin; Wang, Yangyang; Wang, Lu; Quhe, Ruge; Ni, Zeyuan; Mei, Wai-Ning; Gao, Zhengxiang; Yu, Dapeng; Shi, Junjie; Lu, Jing

    2013-01-01

    One popular approach to prepare graphene is to grow them on transition metal substrates via chemical vapor deposition. By using the density functional theory with dispersion correction, we systematically investigate for the first time the interfacial properties of bilayer (BLG) and trilayer graphene (TLG) on metal substrates. Three categories of interfacial structures are revealed. The adsorption of B(T)LG on Al, Ag, Cu, Au, and Pt substrates is a weak physisorption, but a band gap can be opened. The adsorption of B(T)LG on Ti, Ni, and Co substrates is a strong chemisorption, and a stacking-insensitive band gap is opened for the two uncontacted layers of TLG. The adsorption of B(T)LG on Pd substrate is a weaker chemisorption, with a band gap opened for the uncontacted layers. This fundamental study also helps for B(T)LG device study due to inevitable graphene/metal contact. PMID:23803738

  19. Ultrafast Dynamics of Massive Dirac Fermions in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Ulstrup, Søren; Johannsen, Jens Christian; Cilento, Federico; Miwa, Jill A.; Crepaldi, Alberto; Zacchigna, Michele; Cacho, Cephise; Chapman, Richard; Springate, Emma; Mammadov, Samir; Fromm, Felix; Raidel, Christian; Seyller, Thomas; Parmigiani, Fulvio; Grioni, Marco; King, Phil D. C.; Hofmann, Philip

    2014-06-01

    Bilayer graphene is a highly promising material for electronic and optoelectronic applications since it is supporting massive Dirac fermions with a tunable band gap. However, no consistent picture of the gap's effect on the optical and transport behavior has emerged so far, and it has been proposed that the insulating nature of the gap could be compromised by unavoidable structural defects, by topological in-gap states, or that the electronic structure could be altogether changed by many-body effects. Here, we directly follow the excited carriers in bilayer graphene on a femtosecond time scale, using ultrafast time- and angle-resolved photoemission. We find a behavior consistent with a single-particle band gap. Compared to monolayer graphene, the existence of this band gap leads to an increased carrier lifetime in the minimum of the lowest conduction band. This is in sharp contrast to the second substate of the conduction band, in which the excited electrons decay through fast, phonon-assisted interband transitions.

  20. Resonance Raman spectroscopy in twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Righi, A.; Venezuela, P.; Chacham, H.; Costa, S. D.; Fantini, C.; Ruoff, R. S.; Colombo, L.; Bacsa, W. S.; Pimenta, M. A.

    2013-12-01

    In this work we study the Raman spectra of twisted bilayer graphene samples, with different twisting angles, by changing the incident laser energy between 2.54 and 4.14 eV. The spectra exhibit a number of extra peaks, classified in different families, each one associated with bilayer graphenes with different twisting rotational angles. We theoretically analyze the laser energy dependence of these extra peaks considering a set of discrete wavevectors within the interior of the Brillouin zone of graphene, which activate special double-resonance Raman processes. Our result show a nice qualitative agreement between the experimental and simulated spectra, demonstrating that these extra peaks are indeed ascribed to an umklapp double-resonance process in graphene systems.

  1. New Dirac fermions in periodically modulated bilayer graphene.

    PubMed

    Tan, Liang Z; Park, Cheol-Hwan; Louie, Steven G

    2011-07-13

    We investigate the effect of periodic potentials on the electronic structure of bilayer graphene and show that there is a critical value of the external potential below which new Dirac fermions are generated in the low-energy band structure, and above which a band gap is opened in the system. Our results, obtained from a self-consistent tight-binding calculation, can be simply explained by a two-band continuum model as a consequence of the pseudospin physics in graphene. The findings are robust against changes in the form of the potential, as well as bias voltages between the layers.

  2. 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.

  3. Coexistence of antiferromagnetism and d+id superconducting correlations in the graphene bilayer

    NASA Astrophysics Data System (ADS)

    Milovanović, M. V.; Predin, S.

    2012-11-01

    We discuss the t-J-U model on a honeycomb monolayer that has the same low-energy description of the kinetic term as the graphene bilayer, and in particular study coexistence of antiferromagnetism and superconducting correlations that originate from Cooper pairs without phase coherence. We show that the model is relevant for the description of the graphene bilayer and that the presence of the d+id superconducting correlations with antiferromagnetism can lead to quadratic dependence in small magnetic fields of the gap of the effective monolayer consistent with the transport measurements of Velasco on the graphene bilayer.

  4. 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.

  5. 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.

  6. Faraday rotation in bilayer graphene-based integrated microcavity.

    PubMed

    Da, Hai-Xia; Yan, Xiao-Hong

    2016-01-01

    Bernal-stacked bilayer graphene has rich ground states with various broken symmetries, allowing the existence of magneto-optical (MO) effects even in the absence of an external magnetic field. Here we report controllable Faraday rotation (FR) of bilayer graphene induced by electrostatic gate voltage, whose value is 10 times smaller than the case of single layer graphene with a magnetic field. A proposed bilayer graphene-based microcavity configuration enables the enhanced FR angle due to the large localized electromagnetic field. Our results offer unique opportunities to apply bilayer graphene for MO devices.

  7. Quantum anomalous Hall state in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Nandkishore, Rahul; Levitov, Leonid

    2010-09-01

    We present a symmetry-based analysis of competition between different gapped states that have been proposed in bilayer graphene (BLG), which are all degenerate on a mean-field level. We classify the states in terms of a hidden SU(4) symmetry, and distinguish symmetry-protected degeneracies from accidental degeneracies. One of the states, which spontaneously breaks discrete time-reversal symmetry but no continuous symmetry, is identified as a quantum anomalous Hall (QAH) state, which exhibits quantum Hall effect at zero magnetic field. We investigate the lifting of the accidental degeneracies by thermal and zero-point fluctuations, taking account of the modes softened under renormalization group (RG). Working in a “saddle point plus quadratic fluctuations” approximation, we identify two types of RG-soft modes which have competing effects. Zero-point fluctuations, dominated by “transverse” modes which are unique to BLG, favor the QAH state. Thermal fluctuations, dominated by “longitudinal” modes, favor a SU(4) symmetry-breaking multiplet of states. We discuss the phenomenology and experimental signatures of the QAH state in BLG, and also propose a way to induce the QAH state using weak external magnetic fields.

  8. Electric field response in bilayer graphene: Ab initio investigation

    NASA Astrophysics Data System (ADS)

    Mori, Yutaro; Minamitani, Emi; Ando, Yasunobu; Kasamatsu, Shusuke; Watanabe, Satoshi

    2016-11-01

    Stimulated by quantum capacitance measurements, we have investigated the electric properties of bilayer graphene (BLG) with carrier doping under an external electric field using ab initio calculations. We found that the relative permittivity of BLG depends weakly on the applied electric field, and that the BLG can be regarded as a dielectric material rather than a pair of metallic films. We also found that carrier doping affects the band gap of BLG under electric fields, although carrier doping has a much smaller effect on the band gap and density of states than the application of electric fields.

  9. 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).

  10. Structure of twisted and buckled bilayer graphene

    NASA Astrophysics Data System (ADS)

    Jain, Sandeep K.; Juričić, Vladimir; Barkema, Gerard T.

    2017-03-01

    We study the atomic structure of twisted bilayer graphene, with very small mismatch angles (θ ∼ {0.28}0), a topic of intense recent interest. We use simulations, in which we combine a recently presented semi-empirical potential for single-layer graphene, with a new term for out-of-plane deformations, (Jain et al 2015 J. Phys. Chem. C 119 9646) and an often-used interlayer potential (Kolmogorov et al 2005 Phys. Rev. B 71 235415). This combination of potentials is computationally cheap but accurate and precise at the same time, allowing us to study very large samples, which is necessary to reach very small mismatch angles in periodic samples. By performing large scale atomistic simulations, we show that the vortices appearing in the Moiré pattern in the twisted bilayer graphene samples converge to a constant size in the thermodynamic limit. Furthermore, the well known sinusoidal behavior of energy no longer persists once the misorientation angle becomes very small (θ \\lt {1}0). We also show that there is a significant buckling after the relaxation in the samples, with the buckling height proportional to the system size. These structural properties have direct consequences on the electronic and optical properties of bilayer graphene.

  11. 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).

  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. Performance projection of bilayer graphene nanoribbon FET through quantum mechanical simulation

    NASA Astrophysics Data System (ADS)

    Rawat, Brajesh; Paily, Roy

    2016-12-01

    A quantum transport simulator based on a self-consistent solution of the Schrödinger equation within non-equilibrium Green’s function formalism and 2D Poisson equation for a bilayer graphene nanoribbon (bilayer GNR) field-effect transistor (FET) has been developed to examine the ballistic performance of a device. It is found that the lateral confinement employed in bilayer graphene to form the bilayer GNR largely increases the ON/OFF current ({I}{{ON}}/{I}{{OFF}}) ratios of FET without significantly degrading its ON current ({I}{{ON}}). On the other hand, the interlayer coupling considerably decreases the confinement-induced energy gap of the bilayer GNR and largely increases the {I}{{ON}} of the narrow bilayer GNR FET at the cost of lower {I}{{ON}}/{I}{{OFF}} ratios in comparison with the GNR FET.

  14. Unconventional fractional quantum Hall effect in monolayer and bilayer graphene

    PubMed Central

    Jacak, Janusz; Jacak, Lucjan

    2016-01-01

    The commensurability condition is applied to determine the hierarchy of fractional fillings of Landau levels in monolayer and in bilayer graphene. The filling rates for fractional quantum Hall effect (FQHE) in graphene are found in the first three Landau levels in one-to-one agreement with the experimental data. The presence of even denominator filling fractions in the hierarchy for FQHE in bilayer graphene is explained. Experimentally observed hierarchy of FQHE in the first and second Landau levels in monolayer graphene and in the zeroth Landau level in bilayer graphene is beyond the conventional composite fermion interpretation but fits to the presented nonlocal topology commensurability condition. PMID:27877866

  15. Asymptotic discontinuities in the RKKY interaction in the graphene Bernal bilayer

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    We derive the asymptotics of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities in the graphene Bernal bilayer using a four-band model of the bilayer spectrum. We find three distinct regimes depending on the position of the Fermi energy in the bilayer spectrum: in the bonding-antibonding gap, at the gap edge, and outside the gap. In particular, for impurities on the bilayer bonding sublattice (the "A " sublattice) and Fermi energies close to the bonding-antibonding gap edge Eg, we identify a (integrable) logarithmic divergence of the integrand of the RKKY exchange integral. This divergence drives a number of novel RKKY effects for impurities on the A sublattice: (i) an asymptotic R-5 /2 term at the gap edge and (ii) a derivative discontinuity in RKKY interaction as a function of the Fermi energy at the gap edge. In the case of intercalated impurities (impurities between the two layers of the bilayer), we find a remarkable discontinuity in the period of the RKKY oscillation at the gap edge. The period of the oscillation limits to λ =√{2 }π ℏ vF/t⊥ as EF→Eg from below the gap edge, while it limits to λ →∞ if EF→Eg from above the gap edge (t⊥ is the interlayer coupling, vF the Fermi velocity of graphene). The origin of this discontinuity we attribute to (i) the A sublattice divergence and (ii) interference effects driven by the intrinsic valley degree of freedom of graphene. On this basis, we predict that the magnetic response of intercalated bilayer graphene will show a profound sensitivity to doping for Fermi energies near the bonding-antibonding gap edge.

  16. Intrinsic magnetism and spontaneous band gap opening in bilayer silicene and germanene.

    PubMed

    Wang, Xinquan; Wu, Zhigang

    2017-01-18

    It has been long sought to create magnetism out of simple non-magnetic materials, such as silicon and germanium. Here we show that intrinsic magnetism exists in bilayer silicene and germanene with no need to cut, etch, or dope. Unlike bilayer graphene, strong covalent interlayer bonding formed in bilayer silicene and germanene breaks the original π-bonding network of each layer, leaving the unbonded electrons unpaired and localized to carry magnetic moments. These magnetic moments then couple ferromagnetically within each layer while antiferromagnetically across two layers, giving rise to an infinite magnetic sheet with structural integrity and magnetic homogeneity. Furthermore, this unique magnetic ordering results in fundamental band gaps of 0.55 eV and 0.32 eV for bilayer silicene and germanene, respectively. The integration of intrinsic magnetism and spontaneous band gap opening makes bilayer silicene and germanene attractive for future nanoelectronics as well as spin-based computation and data storage.

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

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    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. These findings allow us to propose a novel alternating growth scheme to realize mass production of bilayer graphene.

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

    DOE PAGES

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

    2015-01-08

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

  19. 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.

  20. Excitonic instabilities and insulating states in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Song, Kok Wee; Liang, Yung-Ching; Haas, Stephan

    2012-11-01

    The competing ground states of bilayer graphene are studied by applying renormalization group techniques to a bilayer honeycomb lattice with nearest neighbor hopping. In the absence of interactions, the Fermi surface of this model at half-filling consists of two nodal points with momenta K, K', where the conduction band and valence band touch each other, yielding a semimetal. Since near these two points the energy dispersion is quadratic with perfect particle-hole symmetry, excitonic instabilities are inevitable if interband interactions are present. Using a perturbative renormalization group analysis up to the one-loop level, we find different competing ordered ground states, including ferromagnetism, superconductivity, spin and charge density wave states with ordering vector Q=K-K', and excitonic insulator states. In addition, two states with valley symmetry breaking are found in the excitonic insulating and ferromagnetic phases. This analysis strongly suggests that the ground state of bilayer graphene should be gapped, and with the exception of superconductivity, all other possible ground states are insulating.

  1. Edge currents shunt the insulating bulk in gapped graphene

    NASA Astrophysics Data System (ADS)

    Zhu, M. J.; Kretinin, A. V.; Thompson, M. D.; Bandurin, D. A.; Hu, S.; Yu, G. L.; Birkbeck, J.; Mishchenko, A.; Vera-Marun, I. J.; Watanabe, K.; Taniguchi, T.; Polini, M.; Prance, J. R.; Novoselov, K. S.; Geim, A. K.; Ben Shalom, M.

    2017-02-01

    An energy gap can be opened in the spectrum of graphene reaching values as large as 0.2 eV in the case of bilayers. However, such gaps rarely lead to the highly insulating state expected at low temperatures. This long-standing puzzle is usually explained by charge inhomogeneity. Here we revisit the issue by investigating proximity-induced superconductivity in gapped graphene and comparing normal-state measurements in the Hall bar and Corbino geometries. We find that the supercurrent at the charge neutrality point in gapped graphene propagates along narrow channels near the edges. This observation is corroborated by using the edgeless Corbino geometry in which case resistivity at the neutrality point increases exponentially with increasing the gap, as expected for an ordinary semiconductor. In contrast, resistivity in the Hall bar geometry saturates to values of about a few resistance quanta. We attribute the metallic-like edge conductance to a nontrivial topology of gapped Dirac spectra.

  2. Superlubricity in quasicrystalline twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Koren, Elad; Duerig, Urs

    2016-05-01

    The unique atomic positions in quasicrystals lead to peculiar self-similarity and fractal-like structural morphology. Accordingly, many of the material properties are supposed to manifest exceptional characteristics. In this Rapid Communication, we explain through numerical simulations the fundamental and peculiar aspects of quasicrystals wearless friction manifested in a 30° twisted bilayer graphene system. In particular, the sliding force exhibits a fractal structure with distinct area correlations due to the natural mixture between both periodic and aperiodic lateral modulations. In addition, zero power scaling of the sliding force with respect to the contact area is demonstrated for a geometric sequence of dodecagonal elements.

  3. Spontaneous symmetry breaking in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Kharitonov, Maxim

    2012-02-01

    Recent experiments [1-4] provided compelling evidence for the correlated electron behavior in undoped bilayer graphene at both zero and finite magnetic field. The key question concerns the nature of the broken-symmetry phases realized experimentally. I will present the phase diagram for the zero-density state in the quantum Hall regime (ν=0 state) obtained within the framerwork of quantum Hall ferromagnetism. Comparing these results with the experimental data of Refs. [1,4], I will argue that the ν=0 insulating state realized in bilayer graphene is the canted antiferromagnetic phase. I will also show that the (canted) antiferromagnetic phase can persist at all magnetic fields down to zero and argue that this is the most likely scenario for the insulating state observed in Ref. [4]. [4pt] [1] R. T. Weitz et al., Science 330, 812 (2010). [0pt] [2] F. Freitag et al., arXiv:1104.3816 (2011). [0pt] [3] A. S. Mayorov, et al., Science 333, 860 (2011). [0pt] [4] J. Velasco Jr. et al., arXiv:1108.1609 (2011). [0pt] [5] M. Kharitonov, arXiv:1103.6285, arXiv:1105.5386, arxiv:1109.1553 (2011).

  4. 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.

  5. 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.

  6. Structural and elastic properties of hybrid bilayer graphene/h-BN with different interlayer distances using DFT

    NASA Astrophysics Data System (ADS)

    Ansari, R.; Malakpour, S.; Ajori, S.

    2014-08-01

    Importance of synthesizing graphene-substrate hybrid structure to open a band gap in graphene and apply them in novel nanoelectronic devices is undeniable. Graphene/hexagonal boron-nitride (h-BN) hybrid bilayer is an important type of these structures. The synthesized h-BN/graphene is found to have interesting electrical properties which is very sensitive to the change of the interlayer distance. This has encourages researchers to tune the energy and band gap of such structures. A change in the interlayer distance can also alter the mechanical properties, considerably, due to the variation of interaction energies. The current study is aimed to characterize the mechanical properties variation with interlayer distance change for h-BN/graphene hybrid bilayer structure. To this end, density functional theory calculations are employed within the generalized gradient approximation (GGA) framework. The results demonstrate that there are different possible equilibrium interlayer distances between layers related to two types of layer configuration, i.e. AA and AB. It is found that increasing the interlayer distance causes reduction of Young's modulus. Also, Young's modulus of hybrid structure is approximately between those of graphene/graphene and h-BN/h-BN bilayer structures and also lower than pristine monolayer graphene and graphite. Unlike the pure bilayer structures, Poisson's ratio of hybrid bilayer structure is found to be higher than those of pristine monolayer graphene and h-BN nanosheets.

  7. Ghost Fano Resonance of Excitons in Twisted Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Liang, Yufeng

    2014-03-01

    Metallic systems are generally considered to be unable to harbor tightly bound excitons because of the strong screening effect as well as the absence of a finite band gap. Previously, exception has only been found in one-dimensional metallic carbon nanotubes due to the depressed screening effects and the symmetry gap. We explore the exciton spectra of twisted bilayer graphene (tBLG) and predict the existence of even more strongly bound exciton (with binding energy as large as 0.5eV) in this system despite of its higher dimensionality. Based on our results from first-principles simulations and effective model calculations, a mechanism known as the ghost Fano resonance is proposed for the bound exciton formation in metallic systems beyond the dimensonality-related argument. Our results shed light on engineering the e-h excitations in the few-layer van der Waals heterojunction. NSF Grant No. DMR-1207141.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

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

    SciTech Connect

    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.

  13. Electronic Properties of Bilayer Graphene Strongly Coupled to Interlayer Stacking and an External Electric Field

    NASA Astrophysics Data System (ADS)

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

    2015-07-01

    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. We further identified high-density midgap states, localized on grain boundaries, even under a strong field, which can considerably reduce the overall transport gap.

  14. 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

  15. 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.

  16. 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.

  17. Controlled synthesis of bilayer graphene on nickel

    PubMed Central

    2012-01-01

    We report a uniform and low-defect synthesis of bilayer graphene on evaporated polycrystalline nickel films. We used atmospheric pressure chemical vapor deposition with ultra-fast substrate cooling after exposure to methane at 1,000°C. The optimized process parameters, i.e., growth time, annealing profile and flow rates of various gases, are reported. By using Raman spectroscopy mapping, the ratio of 2D to G peak intensities (I2D/IG) is in the range of 0.9 to 1.6 over 96% of the 200 μm × 200 μm area. Moreover, the average ratio of D to G peak intensities (ID/IG) is about 0.1. PMID:22863171

  18. 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.

  19. Analytical Study of Tunable Bilayered-Graphene Dipole Antenna

    DTIC Science & Technology

    2011-03-03

    can be opened up in a bilayer graphene ( BLG ) using an external bias [3]. Recently, theoretical models and experiments have shown that bilayer graphene...of a MOSFET transistor, this high impedance layer and ground plane will provide a vertical electric field to create a bandgap in the BLG layer...This study will use bandgap tuning in the BLG to provide theoretical data on tuning a dipole antenna in different sequences along the antenna length

  20. 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

  1. Band gap opening in graphene: a short theoretical study

    NASA Astrophysics Data System (ADS)

    Sahu, Sivabrata; Rout, G. C.

    2017-03-01

    Graphene, being a gapless semiconductor, cannot be used in pristine form for nano-electronic applications. Therefore, it is essential to generate a finite gap in the energy dispersion at Dirac point. We present here the tight-binding model Hamiltonian taking into account of various interactions for tuning band gap in graphene. The model Hamiltonian describes the hopping of the π-electrons up to third nearest-neighbours, substrate effects, Coulomb interaction at two sub-lattices, electron-phonon interaction in graphene-on-substrates and high phonon frequency vibrations, besides the bi-layer graphene. We have solved the Hamiltonian using Zubarev's double time single particle Green's function technique. The quasi-particle energies, electron band dispersions, the expression for effective band gap and the density of states (DOS) are calculated numerically. The results are discussed by varying different model parameters of the system. It is observed that the electron DOS and band dispersion exhibit linear energy dependence near Dirac point for nearest-neighbour hopping integral. However, the second and third nearest-neighbour hoppings provide asymmetry in DOS. The band dispersions exhibit wider band gaps with stronger substrate effect. The modified gap in graphene-on-substrate attains its maximum value for Coulomb interaction energy U_{C} = 1.7 t1 . The critical Coulomb interaction is enhanced to U_{C} = 2.5 t1 to produce maximum band gap in the presence of electron-phonon interaction and phonon vibration. The bi-layer graphene exhibits Mexican hat type band gap near Dirac point for transverse gating potential. The other conclusions for the present work are described in the text.

  2. Scanning tunneling spectroscopy of inhomogeneous electronic structure in monolayer and bilayer graphene on SiC

    NASA Astrophysics Data System (ADS)

    Brar, Victor W.; Zhang, Yuanbo; Yayon, Yossi; Ohta, Taisuke; McChesney, Jessica L.; Bostwick, Aaron; Rotenberg, Eli; Horn, Karsten; Crommie, Michael F.

    2007-09-01

    The authors present a scanning tunneling spectroscopy (STS) study of the local electronic structure of single and bilayer graphene grown epitaxially on a SiC(0001) surface. Low voltage topographic images reveal fine, atomic-scale carbon networks, whereas higher bias images are dominated by emergent spatially inhomogeneous large-scale structure similar to a carbon-rich reconstruction of SiC(0001). STS spectroscopy shows an ˜100meV gaplike feature around zero bias for both monolayer and bilayer graphene/SiC, as well as significant spatial inhomogeneity in electronic structure above the gap edge. Nanoscale structure at the SiC/graphene interface is seen to correlate with observed electronic spatial inhomogeneity. These results are relevant for potential devices involving electronic transport or tunneling in graphene/SiC.

  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. Disorder-tuned selection of ordered state in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Zhang, Junhua; Nandkishore, Rahul; Rossi, Enrico

    2014-03-01

    The nature of the symmetry-broken state driven by interaction in bilayer graphene (BLG) has attracted a lot of interest. Theoretical studies predict various possible ordered phases as the candidate for the ground state of BLG. To identify what instability is the most favorable in BLG, a number of experiments have been performed by several groups. However, there is no consensus: some experiments show evidence for a fully gapped state while others seem more consistent with a nematic state. By exploring the influence of disorder on a variety of competing ordered states, we find that the pair breaking effect due to disorder varies among the candidate phases, giving rise to different amount of suppression on the mean-field transition temperatures. This suggests a simple and natural scenario to resolve the discrepancy between experimental observations. Work supported by ONR grant number ONR-N00014-13-1-0321.

  5. Edge states in twisted bilayer graphene: quantum spin Hall and electron-hole bilayers

    NASA Astrophysics Data System (ADS)

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

    2015-03-01

    Twisted bilayer graphene offers a unique platform for studying 1d edge states in a bilayer 2-dimensional electron gas. Despite being spaced by only 0.34 nm, a large interlayer twist decouples the layers in the bulk, while opening the door for interesting interactions at the edges. To probe this physics, we study the electronic transport through quantum Hall edge modes in twisted bilayer graphene devices. Using dual electrostatic gates, we independently control the filling factor of each layer to form different combinations of bilayer edge states while measuring their conductance. The most dramatic transport effects are observed when the layers are doped to have edge states of opposite chiralities, resulting in coexisting electron- and hole-like states. We will present evidence that, in this regime, the twisted bilayer graphene can form a quantum spin Hall state where edge states in each layer counter-propagate in opposite directions with opposite spin polarizations. This bilayer realization offers a flexible system to study quantum spin Hall edge transport as well as to build more complex 1d circuits. We will also discuss the possibility for fractional generalizations of this edge physics and our measurements of the fractional QHE in twisted bilayer graphene.

  6. Fractal butterflies of chiral fermions in bilayer graphene: Phase transitions and emergent properties

    NASA Astrophysics Data System (ADS)

    Ghazaryan, Areg; Chakraborty, Tapash

    2015-12-01

    We have studied the influence of electron-electron interaction on the fractal butterfly spectrum of Dirac fermions in biased bilayer graphene in the fractional quantum Hall effect (FQHE) regime. We demonstrate that the butterfly spectrum exhibits remarkable phase transitions between the FQHE gap and the butterfly gap for chiral electrons in bilayer graphene, when the periodic potential strength or the bias voltage is varied. We also find that, in addition to those phase transitions, by varying the bias voltage one can effectively control the periodic potential strength experienced by the electrons. The electron-electron interaction causes the butterfly spectrum to exhibit new gaps inside the Bloch sub-bands not found in the single-particle case. We expect that both the observed phase transition and other new features in the butterfly spectrum of interacting Dirac fermions will be of great interest to researchers from diverse fields.

  7. Homogeneous bilayer graphene film based flexible transparent conductor.

    PubMed

    Lee, Seunghyun; Lee, Kyunghoon; Liu, Chang-Hua; Zhong, Zhaohui

    2012-01-21

    Graphene is considered as a promising candidate to replace conventional transparent conductors due to its low opacity, high carrier mobility and flexible structure. Multi-layer graphene or stacked single layer graphenes have been investigated in the past but both have their drawbacks. The uniformity of multi-layer graphene is still questionable, and single layer graphene stacks require many transfer processes to achieve sufficiently low sheet resistance. In this work, bilayer graphene film grown with low pressure chemical vapor deposition was used as a transparent conductor for the first time. The technique was demonstrated to be highly efficient in fabricating a conductive and uniform transparent conductor compared to multi-layer or single layer graphene. Four transfers of bilayer graphene yielded a transparent conducting film with a sheet resistance of 180 Ω(□) at a transmittance of 83%. In addition, bilayer graphene films transferred onto the plastic substrate showed remarkable robustness against bending, with sheet resistance change less than 15% at 2.14% strain, a 20-fold improvement over commercial indium oxide films.

  8. A Full electric-field tuning of thermoelectric power in a dual-gated Bi-layer graphene device

    NASA Astrophysics Data System (ADS)

    Lee, Wei-Li; Wang, Chang-Ran; Lu, Wen-Sen; Hao, Lei; Lee, Ting-Kuo; Lin, Feng; Cheng, I.-Chun; Chen, Jiang-Zhang

    2012-02-01

    By using high quality microcrystals of hexagonal boron nitride as top gate dielectric, we fabricated dual-gated bilayer graphene devices. We demonstrate a full electric field tuning of thermoelectric power resulting from the opening of a band-gap by applying a perpendicular electric field on bilayer graphene. We uncover a large enhancement in thermoelectric power at low temperature. At 15 K, the thermoelectric power can be amplified by more than four-fold attaining a value of ˜ 50μV/K at a displacement field of 0.8 V/nm. Our result may open up a new possibility in thermoelectric application using graphene-based device.

  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. Localized plasmons in bilayer graphene nanodisks

    NASA Astrophysics Data System (ADS)

    Wang, Weihua; Xiao, Sanshui; Mortensen, N. Asger

    2016-04-01

    We study localized plasmonic excitations in bilayer graphene (BLG) nanodisks, comparing AA-stacked and AB-stacked BLG and contrasting the results to the case of two monolayers without electronic hybridization. The electrodynamic response of the BLG electron gas is described in terms of a spatially homogeneous surface conductivity, and an efficient alternative two-dimensional electrostatic approach is employed to carry out all the numerical calculations of plasmon resonances. Due to unique electronic band structures, the resonance frequency of the traditional dipolar plasmonic mode in the AA-stacked BLG nanodisk is roughly doping independent in the low-doping regime, while the mode is highly damped as the Fermi level approaches the interlayer hopping energy γ associated with tunneling of electrons between the two layers. In addition to the traditional dipolar mode, we find that the AB-stacked BLG nanodisk also hosts a new plasmonic mode with energy larger than γ . This mode can be tuned by either the doping level or structural size, and, furthermore, this mode can dominate the plasmonic response for realistic structural conditions.

  11. Twinning and twisting of tri- and bilayer graphene.

    PubMed

    Brown, Lola; Hovden, Robert; Huang, Pinshane; Wojcik, Michal; Muller, David A; Park, Jiwoong

    2012-03-14

    The electronic, optical, and mechanical properties of bilayer and trilayer graphene vary with their structure, including the stacking order and relative twist, providing novel ways to realize useful characteristics not available to single layer graphene. However, developing controlled growth of bilayer and trilayer graphene requires efficient large-scale characterization of multilayer graphene structures. Here, we use dark-field transmission electron microscopy for rapid and accurate determination of key structural parameters (twist angle, stacking order, and interlayer spacing) of few-layer CVD graphene. We image the long-range atomic registry for oriented bilayer and trilayer graphene, find that it conforms exclusively to either Bernal or rhombohedral stacking, and determine their relative abundances. In contrast, our data on twisted multilayers suggest the absence of such long-range atomic registry. The atomic registry and its absence are consistent with the two different strain-induced deformations we observe; by tilting the samples to break mirror symmetry, we find a high density of twinned domains in oriented multilayer graphene, where multiple domains of two different stacking configurations coexist, connected by discrete twin boundaries. In contrast, individual layers in twisted regions continuously stretch and shear independently, forming elaborate Moiré patterns. These results, and the twist angle distribution in our CVD graphene, can be understood in terms of an angle-dependent interlayer potential model.

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

    PubMed

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

    2016-06-17

    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.

  13. 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.

  14. 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.

  15. Synthesis of quasi-free-standing bilayer graphene nanoribbons on SiC surfaces

    NASA Astrophysics Data System (ADS)

    Oliveira, Myriano H., Jr.; Lopes, Joao Marcelo J.; Schumann, Timo; Galves, Lauren A.; Ramsteiner, Manfred; Berlin, Katja; Trampert, Achim; Riechert, Henning

    2015-07-01

    Scaling graphene down to nanoribbons is a promising route for the implementation of this material into devices. Quantum confinement of charge carriers in such nanostructures, combined with the electric field-induced break of symmetry in AB-stacked bilayer graphene, leads to a band gap wider than that obtained solely by this symmetry breaking. Consequently, the possibility of fabricating AB-stacked bilayer graphene nanoribbons with high precision is very attractive for the purposes of applied and basic science. Here we show a method, which includes a straightforward air annealing, for the preparation of quasi-free-standing AB-bilayer nanoribbons with different widths on SiC(0001). Furthermore, the experiments reveal that the degree of disorder at the edges increases with the width, indicating that the narrower nanoribbons are more ordered in their edge termination. In general, the reported approach is a viable route towards the large-scale fabrication of bilayer graphene nanostructures with tailored dimensions and properties for specific applications.

  16. Synthesis of quasi-free-standing bilayer graphene nanoribbons on SiC surfaces

    PubMed Central

    Oliveira, Jr., Myriano H.; Lopes, Joao Marcelo J.; Schumann, Timo; Galves, Lauren A.; Ramsteiner, Manfred; Berlin, Katja; Trampert, Achim; Riechert, Henning

    2015-01-01

    Scaling graphene down to nanoribbons is a promising route for the implementation of this material into devices. Quantum confinement of charge carriers in such nanostructures, combined with the electric field-induced break of symmetry in AB-stacked bilayer graphene, leads to a band gap wider than that obtained solely by this symmetry breaking. Consequently, the possibility of fabricating AB-stacked bilayer graphene nanoribbons with high precision is very attractive for the purposes of applied and basic science. Here we show a method, which includes a straightforward air annealing, for the preparation of quasi-free-standing AB-bilayer nanoribbons with different widths on SiC(0001). Furthermore, the experiments reveal that the degree of disorder at the edges increases with the width, indicating that the narrower nanoribbons are more ordered in their edge termination. In general, the reported approach is a viable route towards the large-scale fabrication of bilayer graphene nanostructures with tailored dimensions and properties for specific applications. PMID:26158645

  17. Stacking transition in bilayer graphene caused by thermally activated rotation

    NASA Astrophysics Data System (ADS)

    Zhu, Mengjian; Ghazaryan, Davit; Son, Seok-Kyun; Woods, Colin R.; Misra, Abhishek; He, Lin; Taniguchi, Takashi; Watanabe, Kenji; Novoselov, Kostya S.; Cao, Yang; Mishchenko, Artem

    2017-03-01

    Crystallographic alignment between two-dimensional crystals in van der Waals heterostructures brought a number of profound physical phenomena, including observation of Hofstadter butterfly and topological currents, and promising novel applications, such as resonant tunnelling transistors. Here, by probing the electronic density of states in graphene using graphene-hexagonal boron nitride-graphene tunnelling transistors, we demonstrate a structural transition of bilayer graphene from incommensurate twisted stacking state into a commensurate AB stacking due to a macroscopic graphene self-rotation. This structural transition is accompanied by a topological transition in the reciprocal space and by pseudospin texturing. The stacking transition is driven by van der Waals interaction energy of the two graphene layers and is thermally activated by unpinning the microscopic chemical adsorbents which are then removed by the self-cleaning of graphene.

  18. 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.

  19. Nonlocal transport in dual-gated bilayer graphene

    NASA Astrophysics Data System (ADS)

    Shimazaki, Yuya; Yamamoto, Michihisa; Watanabe, Kenji; Taniguchi, Takashi; Tarucha, Seigo

    2014-03-01

    We report nonlocal transport measurement of biased bilayer graphene. Dual gated bilayer graphene Hall bars sandwiched between two h-BN insulating layers were prepared using the transfer technique with PMMA thin flims. We measured both local and non-local transport at temperatures between 1.5 K and 200 K. We found enhancement of the nonlocal resistance near the charge neutrality point when we increase the perpendicular electric field. Observed nonlocal resistance at 70K is much larger than what is expected as the Ohmic contribution from van der Pauw formula with measured local resistivity. This observation indicates additional contribution to the nonlocal transport in biased bilayer graphene. We present temperature and displacement field dependence of the nonlocal resistance and discuss its origin in terms of valley Hall effect and transport through disordered edge states.

  20. Anomalous Coulomb drag in bilayer graphene double layers

    NASA Astrophysics Data System (ADS)

    Liu, Xiaomeng; Taniguchi, Takashi; Watanabe, Kenji; Kim, Philip

    Bilayer graphene double-layer structure consists of two layers of bilayer graphene separated by atomically thin hexagonal boron nitride (hBN). With a perfect Fermi surface nesting and strong electron-electron interaction (ECoulomb > Ekinetic), such systems offer exciting platforms to study interaction driven phenomena, such as Coulomb drag and exciton condensation. We fabricate ultra-clean encapsulated bilayer graphene double layers with dry pick-up method. Room temperature drag measurement on our devices shows the sign of drag agree with the typical Fermi liquid behavior. However, at lower temperatures, the sign of drag reversed, indicating a new drag mechanism emerges and dominates. We measure this with different geometry, temperature, bias and gating to investigate the origin of such effect and discuss the implication of the drag sign changes.

  1. Spatially indirect exciton condensate phases in double bilayer graphene

    NASA Astrophysics Data System (ADS)

    Su, Jung-Jung; MacDonald, Allan H.

    2017-01-01

    We present a theory of spatially indirect exciton condensate states in systems composed of a pair of electrically isolated Bernal graphene bilayers. The ground-state phase diagram in a two-dimensional displacement-field/inter-bilayer-bias space includes layer-polarized semiconductors, spin-density-wave states, exciton condensates, and states with mixed excitonic and spin order. We find that two different condensate states, distinguished by a chirality index, are stable under different electrical control conditions.

  2. Spin susceptibility of disordered gapped graphene systems

    NASA Astrophysics Data System (ADS)

    Grosu, I.; Biter, T. L.

    2017-02-01

    We calculate the spin susceptibility for the case of gapped graphene systems in the presence of disorder. The average single-particle density of states in gapped graphene with disorder was calculated, using the Born and the T-matrix approximations. The temperature dependence of the static spin susceptibility was analyzed. The influence of the chemical potential position and disorder is also discussed.

  3. Disorder-tuned selection of order in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Zhang, Junhua; Nandkishore, Rahul; Rossi, E.

    2015-05-01

    The nature of the interaction-driven spontaneously broken-symmetry state in charge-neutral bilayer graphene (BLG) has attracted a lot of interest. Theoretical studies predict various ordered states as the candidates for the ground state of BLG in the absence of external fields. Several experiments have been performed by different groups to identify the nature of the collective ground state in BLG. However, so far, there is no consensus: some experiments show evidence that suggests the establishment of a nematic gapless state, while others present results that are more consistent with the establishment of a fully gapped state. Moreover, even among the experiments that appear to see a bulk gap, some of the samples are found to be conducting (suggesting the existence of gapless edge states), while others are insulating. Here we explore the hypothesis that disorder might explain the discrepancy between experiments. We find that the pair-breaking effect due to nonmagnetic short-range disorder varies among the candidate ground states, giving rise to different amounts of suppression of their mean-field transition temperatures. Our results indicate that BLG can undergo a transition between different ordered states as a function of the disorder strength, providing a possible scenario to resolve the discrepancy between experimental observations.

  4. Edge currents shunt the insulating bulk in gapped graphene

    PubMed Central

    Zhu, M. J.; Kretinin, A. V.; Thompson, M. D.; Bandurin, D. A.; Hu, S.; Yu, G. L.; Birkbeck, J.; Mishchenko, A.; Vera-Marun, I. J.; Watanabe, K.; Taniguchi, T.; Polini, M.; Prance, J. R.; Novoselov, K. S.; Geim, A. K.; Ben Shalom, M.

    2017-01-01

    An energy gap can be opened in the spectrum of graphene reaching values as large as 0.2 eV in the case of bilayers. However, such gaps rarely lead to the highly insulating state expected at low temperatures. This long-standing puzzle is usually explained by charge inhomogeneity. Here we revisit the issue by investigating proximity-induced superconductivity in gapped graphene and comparing normal-state measurements in the Hall bar and Corbino geometries. We find that the supercurrent at the charge neutrality point in gapped graphene propagates along narrow channels near the edges. This observation is corroborated by using the edgeless Corbino geometry in which case resistivity at the neutrality point increases exponentially with increasing the gap, as expected for an ordinary semiconductor. In contrast, resistivity in the Hall bar geometry saturates to values of about a few resistance quanta. We attribute the metallic-like edge conductance to a nontrivial topology of gapped Dirac spectra. PMID:28211517

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

    SciTech Connect

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

    2016-01-04

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

  6. 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.

  7. Electronic transport of bilayer graphene with asymmetry line defects

    NASA Astrophysics Data System (ADS)

    Zhao, Xiao-Ming; Wu, Ya-Jie; Chen, Chan; Liang, Ying; Kou, Su-Peng

    2016-11-01

    In this paper, we study the quantum properties of a bilayer graphene with (asymmetry) line defects. The localized states are found around the line defects. Thus, the line defects on one certain layer of the bilayer graphene can lead to an electric transport channel. By adding a bias potential along the direction of the line defects, we calculate the electric conductivity of bilayer graphene with line defects using the Landauer-Büttiker theory, and show that the channel affects the electric conductivity remarkably by comparing the results with those in a perfect bilayer graphene. This one-dimensional line electric channel has the potential to be applied in nanotechnology engineering. Project supported by the National Basic Research Program of China (Grant Nos. 2011CB921803 and 2012CB921704), the National Natural Science Foundation of China (Grant Nos. 11174035, 11474025, 11504285, and 11404090), the Specialized Research Fund for the Doctoral Program of Higher Education, China, the Fundamental Research Funds for the Central Universities, China, the Scientific Research Program Fund of the Shaanxi Provincial Education Department, China (Grant No. 15JK1363), and the Young Talent Fund of University Association for Science and Technology in Shaanxi Province, China.

  8. Landau quantization and Fermi velocity renormalization in twisted graphene bilayers

    NASA Astrophysics Data System (ADS)

    Yin, Long-Jing; Qiao, Jia-Bin; Wang, Wen-Xiao; Zuo, Wei-Jie; Yan, Wei; Xu, Rui; Dou, Rui-Fen; Nie, Jia-Cai; He, Lin

    2015-11-01

    Currently there is a lively discussion concerning Fermi velocity renormalization in twisted bilayers and several contradicted experimental results are reported. Here we study electronic structures of the twisted bilayers by scanning tunneling microscopy (STM) and spectroscopy (STS). The interlayer coupling strengths between the adjacent bilayers are measured according to energy separations of two pronounced low-energy van Hove singularities (VHSs) in the STS spectra. We demonstrate that there is a large range of values for the interlayer interaction not only in different twisted bilayers, but also in twisted bilayers with the same rotation angle. Below the VHSs, the observed Landau quantization in the twisted bilayers is identical to that of massless Dirac fermions in graphene monolayer, which allows us to measure the Fermi velocity directly. Our result indicates that the Fermi velocity of the twisted bilayers depends remarkably on both the twisted angles and the interlayer coupling strengths. This removes the discrepancy about the Fermi velocity renormalization in the twisted bilayers and provides a consistent interpretation of all current data.

  9. 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.

  10. Electrical control of the RKKY interaction in bilayer graphene

    NASA Astrophysics Data System (ADS)

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

    2016-11-01

    The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between impurity spins is calculated for bilayer graphene in the presence of a layer symmetry-breaking external electric field. We find that for intercalated impurities (i.e., impurity atoms between the two constituent layers of the bilayer) the interaction is extraordinarily sensitive to such a field. In particular, (i) the form of the RKKY interaction may be tuned between oscillatory, ferromagnetic, and antiferromagnetic simply by varying the external field, and (ii) the strength of the RKKY interaction may be increased by an order of magnitude by application of an external field. This sensitivity arises directly from the "Mexican hat" form that the low-energy spectrum takes in an applied field. These finding suggest that heterostructures of intercalated magnetic atoms in bilayer graphene may represent a possible system for electrical control over magnetic structure.

  11. Manipulating interface states in monolayer-bilayer graphene planar junctions.

    PubMed

    Zhao, Fang; Xu, Lei; Zhang, Jun

    2016-05-11

    We report on transport properties of monolayer-bilayer graphene planar junctions in a magnetic field. Due to its unique geometry, the edge and interface states can be independently manipulated by either interlayer potential or Zeeman field, and the conductance exhibits interesting quantized behaviors. In the hybrid graphene junction, the quantum Hall (QH) conductance is no longer antisymmetric with respect to the charge neutrality point. When the Zeeman field is considered, a quantum spin Hall (QSH) phase is found in the monolayer region while the weak-QSH phase stays in the bilayer region. In the presence of both interlayer potential and Zeeman field, the bilayer region hosts a QSH phase, whereas the monolayer region is still in a QH phase, leading to a spin-polarized current in the interface. In particular, the QSH phase remains robust against the disorder.

  12. Manipulating interface states in monolayer-bilayer graphene planar junctions

    NASA Astrophysics Data System (ADS)

    Zhao, Fang; Xu, Lei; Zhang, Jun

    2016-05-01

    We report on transport properties of monolayer-bilayer graphene planar junctions in a magnetic field. Due to its unique geometry, the edge and interface states can be independently manipulated by either interlayer potential or Zeeman field, and the conductance exhibits interesting quantized behaviors. In the hybrid graphene junction, the quantum Hall (QH) conductance is no longer antisymmetric with respect to the charge neutrality point. When the Zeeman field is considered, a quantum spin Hall (QSH) phase is found in the monolayer region while the weak-QSH phase stays in the bilayer region. In the presence of both interlayer potential and Zeeman field, the bilayer region hosts a QSH phase, whereas the monolayer region is still in a QH phase, leading to a spin-polarized current in the interface. In particular, the QSH phase remains robust against the disorder.

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

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

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

  14. 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.

  15. 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.

  16. Symmetry breaking in the zero-energy Landau level in bilayer graphene.

    PubMed

    Zhao, Y; Cadden-Zimansky, P; Jiang, Z; Kim, P

    2010-02-12

    The quantum Hall effect near the charge neutrality point in bilayer graphene is investigated in high magnetic fields of up to 35 T using electronic transport measurements. In the high-field regime, the eightfold degeneracy in the zero-energy Landau level is completely lifted, exhibiting new quantum Hall states corresponding to filling factors nu=0, 1, 2, and 3. Measurements of the activation energy gaps for the nu=2 and 3 filling factors in tilted magnetic fields exhibit no appreciable dependence on the in-plane magnetic field, suggesting that these Landau level splittings are independent of spin. In addition, measurements taken at the nu=0 charge neutral point show that, similar to single layer graphene, the bilayer becomes insulating at high fields.

  17. Trigonal warping in bilayer graphene: Energy versus entanglement spectrum

    NASA Astrophysics Data System (ADS)

    Predin, Sonja; Wenk, Paul; Schliemann, John

    2016-03-01

    We present a mainly analytical study of the entanglement spectrum of Bernal-stacked graphene bilayers in the presence of trigonal warping in the energy spectrum. Upon tracing out one layer, the entanglement spectrum shows qualitative geometric differences to the energy spectrum of a graphene monolayer. However, topological quantities such as Berry-phase-type contributions to Chern numbers agree. The latter analysis involves not only the eigenvalues of the entanglement Hamiltonian but also its eigenvectors. We also discuss the entanglement spectra resulting from tracing out other sublattices. As a technical basis of our analysis, we provide closed analytical expressions for the full eigensystem of bilayer graphene in the entire Brillouin zone with a trigonally warped spectrum.

  18. Superlattice structures in twisted bilayers of folded graphene

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    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.

  19. 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.

  20. Electronic transport properties and first-principles study of graphene/h-BN and h-BN bilayers

    NASA Astrophysics Data System (ADS)

    Ashhadi, M.; Hadavi, M. S.; Sarri, Z.

    2017-03-01

    We use a tight binding approach to study of electron transport properties of bilayers of zig-zag graphene/h-BN nanoribbon (GBNNR) and h-BN nanoribbon (BNNR) embedded between two bilayer of zig-zag graphene nanoribbons (GNR), which are considered as electrodes. In this study, the parameters of tight biding hopping and on-site energies are obtained by comparing the tight binding band structure graphene/h-BN and h-BN bilayers with density functional theory (DFT) calculations. We numerically compute the transport properties in terms of transmission and current-voltage characteristic. Our calculations show that the electron transport can open a conduction gap in the GNR/BNNR/GNR structure.

  1. Control over band structure and tunneling in bilayer graphene induced by velocity engineering.

    PubMed

    Cheraghchi, Hosein; Adinehvand, Fatemeh

    2014-01-08

    The band structure and transport properties of massive Dirac fermions in bilayer graphene with velocity modulation in space are investigated in the presence of a previously created band gap. It is pointed out that velocity engineering may be considered as a factor to control the band gap of symmetry-broken bilayer graphene. The band gap is direct and independent of velocity value if the velocity modulated in two layers is set up equally. Otherwise, in the case of interlayer asymmetric velocity, not only is the band gap indirect, but also the electron-hole symmetry fails. This band gap is controllable by the ratio of the velocity modulated in the upper layer to the velocity modulated in the lower layer. In more detail, the shift of momentum from the conduction band edge to the valence band edge can be engineered by the gate bias and velocity ratio. A transfer matrix method is also elaborated to calculate the four-band coherent conductance through a velocity barrier possibly subjected to a gate bias. Electronic transport depends on the ratio of velocity modulated inside the barrier to that for surrounding regions. As a result, a quantum version of total internal reflection is observed for thick enough velocity barriers. Moreover, a transport gap originating from the applied gate bias is engineered by modulating the velocities of the carriers in the upper and lower layers.

  2. 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.

  3. Charge inhomogeneity in a single and bilayer graphene

    NASA Astrophysics Data System (ADS)

    Dahal, Hari; Wehling, Tim; Bedell, Kevin; Zhu, Jian-Xin; Balatsky, Alexander

    2008-03-01

    We study the possibility of charge ordered state in both single and bilayer graphene using a real space tight binding model. We find that the single layer graphene always remains in a liquid phase; the reason being the higher kinetic energy compared to the potential energy. The bilayer graphene on the other hand can have an inhomogeneous distribution of the charge, namely the charge density wave (CDW) state. The CDW state is commensurate with the lattice. The charge ordered state is stabilized by the Coulomb interaction of the carriers of two layers. We also predicted a kinetic energy driven (KID) inhomogeneous phase. This phase can be stabilized by the inter layer hopping energy. The KID phase and the CDW phase compete with each other below the half filling whereas they cooperate above half filling. For the physical parameter of bilayer graphene CDW phase always wins over the KID phase. Hari P. Dahal, Tim O. Wehling, Kevin S. Bedell, Jian-Xin Zhu, Alexander V. Balatsky

  4. Band gap tunning in BN-doped graphene systems with high carrier mobility

    SciTech Connect

    Kaloni, T. P.; Schwingenschlögl, U.; Joshi, R. P.; Adhikari, N. P.

    2014-02-17

    Using density functional theory, we present a comparative study of the electronic properties of BN-doped graphene monolayer, bilayer, trilayer, and multilayer systems. In addition, we address a superlattice of pristine and BN-doped graphene. Five doping levels between 12.5% and 75% are considered, for which we obtain band gaps from 0.02 eV to 2.43 eV. We demonstrate a low effective mass of the charge carriers.

  5. Model of an exotic chiral superconducting phase in a graphene bilayer.

    PubMed

    Hosseini, Mir Vahid; Zareyan, Malek

    2012-04-06

    We theoretically demonstrate the formation of a new type of unconventional superconductivity in graphene materials, which exhibits a gapless property. The studied superconductivity is based on an interlayer pairing of chiral electrons in bilayer graphene, which results in an exotic s-wave spin-triplet condensate order with anomalous thermodynamic properties. These include the possibility of a temperature-induced condensation causing an increase of the pairing gap with increasing temperature and an entropy of the stable superconducting state which can be higher than its value in the normal state. Our study reveals the analogy of the interlayer superconductivity in graphene materials to the color superconductivity in dense quark matter and the gapless pairing states in nuclear matter and ultracold atomic gases.

  6. Landau quantization in graphene monolayer, Bernal bilayer, and Bernal trilayer on graphite surface

    NASA Astrophysics Data System (ADS)

    Yin, Long-Jing; Li, Si-Yu; Qiao, Jia-Bin; Nie, Jia-Cai; He, Lin

    2015-03-01

    Electronic properties of surface areas decoupled from graphite are studied using scanning tunneling microscopy and spectroscopy. We show that it is possible to identify the decoupled graphene monolayer, the Bernal bilayer, and the Bernal trilayer on a graphite substrate according to their tunneling spectra in a high magnetic field. The decoupled monolayer and bilayer exhibit Landau quantization of massless and massive Dirac fermions, respectively. The substrate generates a sizable band gap ˜35 meV in the Bernal bilayer, therefore, the eightfold degenerate Landau level at the charge neutrality point is split into two valley-polarized quartets polarized on each layer. In the decoupled Bernal trilayer, we find that both massless and massive Dirac fermions coexist and its low-energy band structure can be described quite well by taking into account only the nearest-neighbor intra- and interlayer hopping parameters. A strong correlation between the Fermi velocity of the massless Dirac fermions and the effective mass of the massive Dirac fermions is observed in the graphene trilayer. Our result demonstrates that the surface of graphite provides a natural ideal platform to probe the electronic spectra of graphene layers.

  7. Anomalous Sequence of Quantum Hall Liquids Revealing a Tunable Lifshitz Transition in Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Varlet, Anastasia; Bischoff, Dominik; Simonet, Pauline; Watanabe, Kenji; Taniguchi, Takashi; Ihn, Thomas; Ensslin, Klaus; Mucha-Kruczyński, Marcin; Fal'ko, Vladimir I.

    2014-09-01

    Bilayer graphene is a unique system where both the Fermi energy and the low-energy electron dispersion can be tuned. This is brought about by an interplay between trigonal warping and the band gap opened by a transverse electric field. Here, we drive the Lifshitz transition in bilayer graphene to experimentally controllable carrier densities by applying a large transverse electric field to a h-BN-encapsulated bilayer graphene structure. We perform magnetotransport measurements and investigate the different degeneracies in the Landau level spectrum. At low magnetic fields, the observation of filling factors -3 and -6 quantum Hall states reflects the existence of three maxima at the top of the valence-band dispersion. At high magnetic fields, all integer quantum Hall states are observed, indicating that deeper in the valence band the constant energy contours are singly connected. The fact that we observe ferromagnetic quantum Hall states at odd-integer filling factors testifies to the high quality of our sample. This enables us to identify several phase transitions between correlated quantum Hall states at intermediate magnetic fields, in agreement with the calculated evolution of the Landau level spectrum. The observed evolution of the degeneracies, therefore, reveals the presence of a Lifshitz transition in our system.

  8. 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

  9. Interlayer thermal conductance within a phosphorene and graphene bilayer.

    PubMed

    Hong, Yang; Zhang, Jingchao; Zeng, Xiao Cheng

    2016-11-24

    Monolayer graphene possesses unusual thermal properties, and is often considered as a prototype system for the study of thermal physics of low-dimensional electronic/thermal materials, despite the absence of a direct bandgap. Another two-dimensional (2D) atomic layered material, phosphorene, is a natural p-type semiconductor and it has attracted growing interest in recent years. When a graphene monolayer is overlaid on phosphorene, the hybrid van der Waals (vdW) bilayer becomes a potential candidate for high-performance thermal/electronic applications, owing to the combination of the direct-bandgap properties of phosphorene with the exceptional thermal properties of graphene. In this work, the interlayer thermal conductance at the phosphorene/graphene interface is systematically investigated using classical molecular dynamics (MD) simulation. The transient pump-probe heating method is employed to compute the interfacial thermal resistance (R) of the bilayer. The predicted R value at the phosphorene/graphene interface is 8.41 × 10(-8) K m(2) W(-1) at room temperature. Different external and internal conditions, i.e., temperature, contact pressure, vacancy defect, and chemical functionalization, can all effectively reduce R at the interface. Numerical results of R reduction as a function of temperature, interfacial coupling strength, defect ratio, or hydrogen coverage are reported with the most R reduction amounting to 56.5%, 70.4%, 34.8% and 84.5%, respectively.

  10. 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.

  11. Directional photoelectric current across the bilayer graphene junction.

    PubMed

    Shafranjuk, S E

    2009-01-07

    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.

  12. 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.

  13. Broken symmetry states in bilayer graphene in electric and in-plane magnetic fields

    NASA Astrophysics Data System (ADS)

    Jia, Junji; Pyatkovskiy, P. K.; Gorbar, E. V.; Gusynin, V. P.

    2017-01-01

    Broken symmetry states in bilayer graphene in perpendicular electric E⊥ and in-plane magnetic B∥ fields are studied in the presence of the dynamically screened long-range Coulomb interaction and the symmetry-breaking contact four-fermion interactions. The integral gap equations are solved numerically, and it is shown that the momentum dependence of gaps is essential: It diminishes by an order of magnitude the gaps compared to the case of momentum-independent approximation, and the obtained gap magnitudes are found to agree well with existing experimental values. We derived a phase diagram of bilayer graphene at the neutrality point in the plane (B∥,E⊥) showing that the (canted) layer antiferromagnetic (LAF) state remains a stable ground state of the system at large B∥. On the other hand, while the LAF phase is realized at small values of E⊥, the quantum valley Hall (QVH) phase is the ground state of the system at values E⊥>Ec r(B∥) , where a critical value Ec r(B∥) increases with in-plane magnetic field B||.

  14. The effect of h-BN buffer layers in bilayer graphene on Co (111)

    NASA Astrophysics Data System (ADS)

    Li, Can; Liu, Yan; Zhang, Bin; Wang, Tao; Guo, Qing; Sheng, Kuang; Yin, You

    2015-05-01

    Understanding of the interface of Co/graphene is essential for applications of graphene-based devices, as well as in the process of graphene synthesis. In this paper, the Co/graphene interface, including five structures of bilayer graphene (BLG) on Co (111) surface with bilayer or monolayer BN buffer sheets, is investigated by using density functional theory calculations. The corresponding atomic and electronic structures and Mulliken charge populations are also analyzed. The bilayer BN sheets are found to be the thinnest insulator for the backside Co metal gate, which shields BLG from Co substrate pining, decreases the charges influenced by the substrate, and improves BLG transport mobility.

  15. 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).

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

    SciTech Connect

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

    2014-01-01

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

  17. 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.

  18. Dual-gated bilayer graphene hot-electron bolometer.

    PubMed

    Yan, Jun; Kim, M-H; Elle, J A; Sushkov, A B; Jenkins, G S; Milchberg, H M; Fuhrer, M S; Drew, H D

    2012-06-03

    Graphene is an attractive material for use in optical detectors because it absorbs light from mid-infrared to ultraviolet wavelengths with nearly equal strength. Graphene is particularly well suited for bolometers-devices that detect temperature-induced changes in electrical conductivity caused by the absorption of light-because its small electron heat capacity and weak electron-phonon coupling lead to large light-induced changes in electron temperature. Here, we demonstrate a hot-electron bolometer made of bilayer graphene that is dual-gated to create a tunable bandgap and electron-temperature-dependent conductivity. The bolometer exhibits a noise-equivalent power (33 fW Hz(-1/2) at 5 K) that is several times lower, and intrinsic speed (>1 GHz at 10 K) three to five orders of magnitude higher than commercial silicon bolometers and superconducting transition-edge sensors at similar temperatures.

  19. Direct transfer and Raman characterization of twisted graphene bilayer

    SciTech Connect

    Othmen, R.; Ajlani, H.; Oueslati, M.; Cavanna, A.; Madouri, A.

    2015-03-09

    Twisted bilayer graphene (tBLG) is constituted of a two-graphene layer with a mismatch angle θ between the two hexagonal structures. It has recently attracted much attention—thanks to its diverse electronic and optical properties. Here, we study the tBLG fabricated by the direct transfer of graphene monolayer prepared by chemical vapor deposition (CVD) onto another CVD graphene layer remaining attached to the copper foil. We show that high quality and homogeneous tBLG can be obtained by the direct transfer which prevents interface contamination. In this situation, the top graphene layer plays a supporting mechanical role to the bottom graphene layer as confirmed by optical microscopy, scanning electron microscopy, and Raman spectroscopy measurements. The effect of annealing tBLG was also investigated using micro-Raman spectroscopy. The Raman spectra exhibit a splitting of the G peak as well as a change in the 2D band shape indicating a possible decoupling of the two monolayers. We attribute these changes to the different interactions of the top and bottom layers with the substrate.

  20. Valley-filtered edge states and quantum valley Hall effect in gated bilayer graphene.

    PubMed

    Zhang, Xu-Long; Xu, Lei; Zhang, Jun

    2017-03-06

    Electron edge states in gated bilayer graphene in the quantum valley Hall (QVH) effect regime can carry both charge and valley. We show that an interlayer potential splits the zero-energy level and opens a bulk gap, yielding counterpropagating edge modes with different valleys. A rich variety of valley current states can be obtained by tuning the applied boundary potential and lead to the QVH effect as well as unbalanced QVH effect. A method to individually manipulate the edge states by the boundary potentials is proposed.

  1. 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

  2. Study on temperature-dependent carrier transport for bilayer graphene

    NASA Astrophysics Data System (ADS)

    Liu, Yali; Li, Weilong; Qi, Mei; Li, Xiaojun; Zhou, Yixuan; Ren, Zhaoyu

    2015-05-01

    In order to investigate the temperature-dependent carrier transport property of the bilayer graphene, graphene films were synthesized on Cu foils by a home-built chemical vapor deposition (CVD) with C2H2. Samples regularity, transmittance (T) and layer number were analyzed by transmission electron microscope (TEM) images, transmittance spectra and Raman spectra. Van Der Pauw method was used for resistivity measurements and Hall measurements at different temperatures. The results indicated that the sheet resistance (Rs), carrier density (n), and mobility (μ) were 1096.20 Ω/sq, 0.75×1012 cm-2, and 7579.66 cm2 V-1 s-1 at room temperature, respectively. When the temperature increased from 0 °C to 240 °C, carrier density (n) increased from 0.66×1012 cm-2 to 1.55×1012 cm-2, sheet resistance (Rs) decreased from 1215.55 Ω/sq to 560.77 Ω/sq, and mobility (μ) oscillated around a constant value 7773.99 cm2 V-1 s-1. The decrease of the sheet resistance (Rs) indicated that the conductive capability of the bilayer graphene film increased with the temperature. The significant cause of the increase of carrier density (n) was the thermal activation of carriers from defects and unconscious doping states. Because the main influence on the carrier mobility (μ) was the lattice defect scattering and a small amount of impurity scattering, the carrier mobility (μ) was temperature-independent for the bilayer graphene.

  3. Explanation of ν=−12 fractional quantum Hall state in bilayer graphene

    PubMed Central

    Jacak, L.

    2016-01-01

    The commensurability condition is applied to determine the hierarchy of fractional filling of Landau levels for fractional quantum Hall effect (FQHE) in monolayer and bilayer graphene. Good agreement with experimental data is achieved. The presence of even-denominator filling fractions in the hierarchy of the FQHE in bilayer graphene is explained, including the state at ν=−12. PMID:27118883

  4. Magnetoabsorption spectra of bilayer graphene ribbons with Bernal stacking

    NASA Astrophysics Data System (ADS)

    Huang, Y. C.; Chang, C. P.; Lin, M. F.

    2008-09-01

    Magnetoabsorption spectra of bilayer graphene ribbons with Bernal stacking are studied by the Peierls-coupling tight-binding method. When the magnetic confinement prevails over the quantum confinement, low-energy spectra chiefly exhibit many Landau peaks, which are strongly modified by the inter-ribbon interactions and the magnetic-field magnitude (B) . The spectra show denser Landau peaks in bilayer graphene ribbon than in a monolayer ribbon with the same ribbon width. The absorption frequencies of Landau peaks of a wide monolayer ribbon show the B dependence, while those of a bilayer ribbon exhibit a varying B -field dependence. In the spectra region ω≤100meV , the absorption frequencies of Landau peaks are linearly dependent on the magnetic-field magnitude. At ω≥100meV , they evolve from the B dependence to the B dependence with the increase in the field strength. The absorption frequencies of Landau peaks exhibit B dependence at B≥20T . The relationship between the magneto-optical properties and electronic structures (the state energies and wave functions) are explored. The Landau wave functions are illustrated and used to identify the optical selection rule.

  5. Photon-assisted transport in bilayer graphene flakes

    NASA Astrophysics Data System (ADS)

    Zambrano, D.; Rosales, L.; Latgé, A.; Pacheco, M.; Orellana, P. A.

    2017-01-01

    The electronic conductance of graphene-based bilayer flake systems reveals different quantum interference effects, such as Fabry-Pérot resonances and sharp Fano antiresonances on account of competing electronic paths through the device. These properties may be exploited to obtain spin-polarized currents when the same nanostructure is deposited above a ferromagnetic insulator. Here, we study how the spin-dependent conductance is affected when a time-dependent gate potential is applied to the bilayer flake. Following a Tien-Gordon formalism, we explore how to modulate the transport properties of such systems via appropriate choices of the ac-field gate parameters. The presence of an oscillating field opens the possibility of tuning the original antiresonances for a large set of field parameters. We show that interference patterns can be partially or fully removed by the time-dependent gate voltage. The results are reflected in the corresponding weighted spin polarization, which can reach maximum values for a given spin component. We found that differential conductance maps as functions of bias and gate potentials show interference patterns for different ac-field parameter configurations. The proposed bilayer graphene flake systems may be used as a frequency detector in the THz range.

  6. 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.

  7. 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.

  8. 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.

  9. Current flow in biased bilayer graphene: the role of sublattices

    NASA Astrophysics Data System (ADS)

    Paez, Carlos; Bahamon, Dario; Pereira, Ana

    2015-03-01

    We investigate here how the current flows over a bilayer graphene in the presence of an external electric field perpendicularly applied (biased bilayer). Charge density polarization between layers in these systems is known to create a layer pseudospin, which can be manipulated by the electric field. Our results show that current does not necessarily flow over regions of the system with higher charge density. Charge can be predominantly concentrated over one layer, while current flows over the other layer. We find that this phenomenon occurs when the charge density becomes highly concentrated over only one of the sublattices, as the electric field breaks layer and sublattice symmetries for a Bernal-stacked bilayer. For bilayer nanoribbons, the situation is even more complex, with a competition between edge and bulk effects for the definition of the current flow. We show that, in spite of not flowing trough the layer where charge is polarized to, the current in these systems also defines a controllable layer pseudospin.

  10. Current flow in biased bilayer graphene: Role of sublattices

    NASA Astrophysics Data System (ADS)

    Páez, C. J.; Bahamon, D. A.; Pereira, Ana L. C.

    2014-09-01

    We investigate here how the current flows over a bilayer graphene in the presence of an external electric field perpendicularly applied (biased bilayer). Charge density polarization between layers in these systems is known to create a layer pseudospin, which can be manipulated by the electric field. Our results show that current does not necessarily flow over regions of the system with higher charge density. Charge can be predominantly concentrated over one layer, while current flows over the other layer. We find that this phenomenon occurs when the charge density becomes highly concentrated over only one of the sublattices, as the electric field breaks layer and sublattice symmetries for a Bernal-stacked bilayer. For bilayer nanoribbons, the situation is even more complex, with a competition between edge and bulk effects for the definition of the current flow. We show that, in spite of not flowing trough the layer where charge is polarized to, the current in these systems also defines a controllable layer pseudospin.

  11. 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.

  12. Quasi-Flat Plasmonic Bands in Twisted Bilayer Graphene.

    PubMed

    Stauber, Tobias; Kohler, Heinerich

    2016-11-09

    The charge susceptibility of twisted bilayer graphene is investigated in the Dirac cone, respectively, random-phase approximation. For small enough twist angles θ ≲ 2°, we find weakly Landau damped interband plasmons, that is, collective excitonic modes that exist in the undoped material with an almost constant energy dispersion. In this regime, the loss function can be described as a Fano resonance, and we argue that these excitations arise from the interaction of quasi-localized states with the incident light field. These predictions can be tested by nanoinfrared imaging and possible applications include a "perfect" lens without the need of left-handed materials.

  13. 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.

  14. 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.

  15. Observation of variable hybridized-band gaps in Eu-intercalated graphene.

    PubMed

    Sung, Sijin; Kim, Sooran; Lee, Paengro; Kim, Jingul; Ryu, Min-Tae; Park, Heemin; Kim, Kyoo; Min, Byung; Chung, Jinwook

    2017-03-27

    We report europium (Eu)-induced changes in the π-band of graphene (G) formed on 6H-SiC(0001) surface by a combined study of photoemission measurements and density functional theory (DFT) calculations. Our photoemission data reveal that Eu intercalates upon annealing at 120 °C into the region between graphene and buffer layer (BL) to form a G/Eu/BL system, where a band gap of 0.29 eV opens at room temperature. This band gap is found to increase further to 0.48 eV upon cooling down to 60 K. Our DFT calculations suggest that the increased band gap originates from the enhanced hybridization between graphene π-Eu 4f band due to the increased magnetic ordering upon cooling. These Eu atoms continue to intercalate further down below the BL to produce a bilayer graphene (G/BL/Eu) upon annealing at 300 °C. The π-band stemming from the BL then exhibits another band gap of 0.37 eV, which appears to be a gap due to the strong hybridization between the π-band of the BL and the Eu 4f band. The Eu-intercalated graphene thus illustrates an example of versatile band gaps formed under different thermal treatments, which may play a critical role for future applications in graphene-based electronics.

  16. 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.

  17. First-principles study of the electrical and lattice thermal transport in monolayer and bilayer graphene

    NASA Astrophysics Data System (ADS)

    D'Souza, Ransell; Mukherjee, Sugata

    2017-02-01

    We report the transport properties of monolayer and bilayer graphene from first-principles calculations and Boltzmann transport theory (BTE). Our resistivity studies on monolayer graphene show Bloch-Grüneisen behavior in a certain range of chemical potentials. By substituting boron nitride in place of a carbon dimer of graphene, we predict a twofold increase in the Seebeck coefficient. A similar increase in the Seebeck coefficient for bilayer graphene under the influence of a small electric field ˜0.3 eV has been observed in our calculations. Graphene with impurities shows a systematic decrease of electrical conductivity and mobility. We have also calculated the lattice thermal conductivities of monolayer graphene and bilayer graphene using phonon BTE which show excellent agreement with experimental data available in the temperature range 300-700 K.

  18. Quantum-Hall Activation Gaps in Graphene

    NASA Astrophysics Data System (ADS)

    Giesbers, A. J. M.; Zeitler, U.; Katsnelson, M. I.; Ponomarenko, L. A.; Mohiuddin, T. M.; Maan, J. C.

    2007-11-01

    We have measured the quantum-Hall activation gaps in graphene at filling factors ν=2 and ν=6 for magnetic fields up to 32 T and temperatures from 4 to 300 K. The ν=6 gap can be described by thermal excitation to broadened Landau levels with a width of 400 K. In contrast, the gap measured at ν=2 is strongly temperature and field dependent and approaches the expected value for sharp Landau levels for fields B>20T and temperatures T>100K. We explain this surprising behavior by a narrowing of the lowest Landau level.

  19. Tunable phonon-induced transparency in bilayer graphene nanoribbons.

    PubMed

    Yan, Hugen; Low, Tony; Guinea, Francisco; Xia, Fengnian; Avouris, Phaedon

    2014-08-13

    In the phenomenon of plasmon-induced transparency, which is a classical analogue of electromagnetically induced transparency (EIT) in atomic gases, the coherent interference between two plasmon modes results in an optical transparency window in a broad absorption spectrum. With the requirement of contrasting lifetimes, typically one of the plasmon modes involved is a dark mode that has limited coupling to the electromagnetic radiation and possesses relatively longer lifetime. Plasmon-induced transparency not only leads to light transmission at otherwise opaque frequency regions but also results in the slowing of light group velocity and enhanced optical nonlinearity. In this article, we report an analogous behavior, denoted as phonon-induced transparency (PIT), in AB-stacked bilayer graphene nanoribbons. Here, light absorption due to the plasmon excitation is suppressed in a narrow window due to the coupling with the infrared active Γ-point optical phonon, whose function here is similar to that of the dark plasmon mode in the plasmon-induced transparency. We further show that PIT in bilayer graphene is actively tunable by electrostatic gating and estimate a maximum slow light factor of around 500 at the phonon frequency of 1580 cm(-1), based on the measured spectra. Our demonstration opens an avenue for the exploration of few-photon nonlinear optics and slow light in this novel two-dimensional material.

  20. Low-energy theory for the graphene twist bilayer

    NASA Astrophysics Data System (ADS)

    Weckbecker, D.; Shallcross, S.; Fleischmann, M.; Ray, N.; Sharma, S.; Pankratov, O.

    2016-01-01

    The graphene twist bilayer represents the prototypical system for investigating the stacking degree of freedom in few-layer graphenes. The electronic structure of this system changes qualitatively as a function of angle, from a large-angle limit in which the two layers are essentially decoupled—with the exception of a 28-atom commensuration unit cell for which the layers are coupled on an energy scale of ≈8 meV —to a small-angle strong-coupling limit. Despite sustained investigation, a fully satisfactory theory of the twist bilayer remains elusive. The outstanding problems are (i) to find a theoretically unified description of the large- and small-angle limits, and (ii) to demonstrate agreement between the low-energy effective Hamiltonian and, for instance, ab initio or tight-binding calculations. In this article, we develop a low-energy theory that in the large-angle limit reproduces the symmetry-derived Hamiltonians of Mele [Phys. Rev. B 81, 161405 (2010), 10.1103/PhysRevB.81.161405], and in the small-angle limit shows almost perfect agreement with tight-binding calculations. The small-angle effective Hamiltonian is that of Bistritzer and MacDonald [Proc. Natl. Acad. Sci. (U.S.A.) 108, 12233 (2011), 10.1073/pnas.1108174108], but with the momentum scale Δ K , the difference of the momenta of the unrotated and rotated special points, replaced by a coupling momentum scale g(c )=8/π √{3 }a sinθ/2 . Using this small-angle Hamiltonian, we are able to determine the complete behavior as a function of angle, finding a complex small-angle clustering of van Hove singularities in the density of states (DOS) that after a "zero-mode" peak regime between 0 .90°<θ <0 .15° limits θ <0 .05° to a DOS that is essentially that of a superposition DOS of all bilayer stacking possibilities. In this regime, the Dirac spectrum is entirely destroyed by hybridization for -0.25 graphene

  1. DFT Calculations of the Electronic Structure and Interlayer Interaction in the Li-INTERCALATED Graphene Bilayer

    NASA Astrophysics Data System (ADS)

    Petrova, N. V.; Yakovkin, I. N.

    The electronic band structure, density of states (DOS) and interlayer interaction in Li-intercalated graphene bilayers are studied by means of density functional theory (DFT) calculations. It has been found that for a pristine bilayer, the relative shift of graphene layers from AB stacking configuration, pertinent to a bulk graphite, to AA configuration results in the opening of the bandgap at Fermi level, so that the bilayer becomes a semiconductor. The Li intercalation of the graphene bilayer significantly increases the density of states at Fermi level, which can be considered as an increased metallicity. The electronic density in the space between graphene layers also substantially increases and leads to related increase of the interlayer interaction. We hope that the obtained results of calculations will be useful for various applications of Li-intercalated graphene layers in nanoelectronics.

  2. Spin- and valley-coupled electronic states in monolayer WSe{sub 2} on bilayer graphene

    SciTech Connect

    Sugawara, K.; Souma, S.; Sato, T.; Tanaka, Y.; Takahashi, T.

    2015-08-17

    We have fabricated a high-quality monolayer WSe{sub 2} film on bilayer graphene by epitaxial growth and revealed the electronic states by spin- and angle-resolved photoemission spectroscopy. We observed a direct energy gap at the Brillouin-zone corner in contrast to the indirect nature of gap in bulk WSe{sub 2}, which is attributed to the lack of interlayer interaction and the breaking of space-inversion symmetry in monolayer film. A giant spin splitting of ∼0.5 eV, which is the largest among known monolayer transition-metal dichalcogenides, is observed in the energy band around the zone corner. The present results suggest a high potential applicability of WSe{sub 2} to develop advanced devices based with the coupling of spin- and valley-degrees of freedom.

  3. Grain size control for CVD-grown single crystal mono- and bi-layer graphene

    NASA Astrophysics Data System (ADS)

    Luo, Zhengtang

    2015-03-01

    By suppressing the nucleation density during Chemical Vapor Deposition (CVD) growth, we demonstrate that the large-size single crystal monolayer and bilayer graphene can be synthesized by this method. For single layer, single crystals with diameter up to 5.9 mm, have been successfully obtained by adjusting degree of oxidation during surface treatment step and hydrogen annealing duration during growth, thereby allow us to control nucleation density and consequently to control graphene grains sizes. For bilayer growth, our main strategy is to maximize the duration that is controlled by the absorption-diffusion mechanism. With this method, sub-millimeter size single crystal bilayer graphene is also obtained. Electron transport measurement on those produced graphene has shown carrier mobility that is comparable with that of mechanical exfoliated graphene, indicating the high quality of our graphene sample. This project is supported by the Research Grant Council of Hong Kong SAR (Project Number 623512 and DAG12EG05).

  4. Bilayer graphene with parallel magnetic field and twisting: Phases and phase transitions in a highly tunable Dirac system

    NASA Astrophysics Data System (ADS)

    Yang, Kun; Roy, Bitan

    2014-03-01

    The effective theory for bi-layer graphene, subject to parallel/in-plane magnetic fields is discussed. We show that with a sizable in-plane magnetic field the trigonal warping becomes irrelevant, and one ends up with two Dirac points in the vicinity of each valleys in the low-energy limit, similar to the twisted bi-layer graphene. Combining twisting and parallel field thus gives rise to a Dirac system with tunable Fermi velocity and ultra violet cutoff. If the interactions are sufficiently strong, several fully gapped states can be realized in these systems, in addition to the ones in pristine setup. Symmetry based classification of the order parameters will be discussed. We also present the quantum critical behavior of various phase transitions driven by the twisting and the magnetic field. Effects of an additional perpendicular fields, and possible ways to realize the some of the new massive phases will be highlighted.

  5. Channel morphology effect on water transport through graphene bilayers

    NASA Astrophysics Data System (ADS)

    Liu, Bo; Wu, Renbing; Law, Adrian Wing-Keung; Feng, Xi-Qiao; Bai, Lichun; Zhou, Kun

    2016-12-01

    The application of few-layered graphene-derived functional thin films for molecular filtration and separation has recently attracted intensive interests. In practice, the morphology of the nanochannel formed by the graphene (GE) layers is not ideally flat and can be affected by various factors. This work investigates the effect of channel morphology on the water transport behaviors through the GE bilayers via molecular dynamics simulations. The simulation results show that the water flow velocity and transport resistance highly depend on the curvature of the graphene layers, particularly when they are curved in non-synergic patterns. To understand the channel morphology effect, the distributions of water density, dipole moment orientation and hydrogen bonds inside the channel are investigated, and the potential energy surface with different distances to the basal GE layer is analyzed. It shows that the channel morphology significantly changes the distribution of the water molecules and their orientation and interaction inside the channel. The energy barrier for water molecules transport through the channel also significantly depends on the channel morphology.

  6. Epitaxial nucleation of CVD bilayer graphene on copper.

    PubMed

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

    2016-12-08

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

  7. 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.

  8. Bandgap opening in bilayer graphene at metal contacts

    NASA Astrophysics Data System (ADS)

    Nouchi, Ryo

    A bandgap is opened in bilayer graphene (BLG) by introducing a potential difference between the two graphene layers, raising expectations for its application to a transistor channel. The potential difference can be introduced, for example by charge transfer from surface adsorbates. Thus, a finite bandgap is expected to be opened also at a metal contact, an inevitable component of transistors, where interfacial charge transfer occurs to align the Fermi levels of the metallic electrode and the underlying BLG. The bandgap at the metal-BLG interface can be detected by the superlinear current-voltage characteristics in back-gate field-effect transistors, caused by carriers propagating through the bandgap, i.e., by the band-to-band transport. The superlinearity was higher in the positively-gated region, attributed to hole doping from the Cr/Au electrodes. The control experiments using single-layer graphene (SLG) did not have a superlinearity, which is consistent with the fact that a sizeable bandgap is not expected at the metal-SLG interface. The current transport through the bandgap should be an additional source of electrode-contact resistance.

  9. Channel morphology effect on water transport through graphene bilayers

    PubMed Central

    Liu, Bo; Wu, Renbing; Law, Adrian Wing-Keung; Feng, Xi-Qiao; Bai, Lichun; Zhou, Kun

    2016-01-01

    The application of few-layered graphene-derived functional thin films for molecular filtration and separation has recently attracted intensive interests. In practice, the morphology of the nanochannel formed by the graphene (GE) layers is not ideally flat and can be affected by various factors. This work investigates the effect of channel morphology on the water transport behaviors through the GE bilayers via molecular dynamics simulations. The simulation results show that the water flow velocity and transport resistance highly depend on the curvature of the graphene layers, particularly when they are curved in non-synergic patterns. To understand the channel morphology effect, the distributions of water density, dipole moment orientation and hydrogen bonds inside the channel are investigated, and the potential energy surface with different distances to the basal GE layer is analyzed. It shows that the channel morphology significantly changes the distribution of the water molecules and their orientation and interaction inside the channel. The energy barrier for water molecules transport through the channel also significantly depends on the channel morphology. PMID:27929106

  10. Physicochemical insight into gap openings in graphene

    PubMed Central

    Zhu, Y. F.; Dai, Q. Q.; Zhao, M.; Jiang, Q.

    2013-01-01

    Based on a newly developed size-dependent cohesive energy formula for two-dimensional materials, a unified theoretical model was established to illustrate the gap openings in disordered graphene flakes, involving quantum dots, nanoribbons and nanoporous sheets. It tells us that the openings are essentially dominated by the variation in cohesive energy of C atoms, associated to the edge physicochemical nature regarding the coordination imperfection or the chemical bonding. In contrast to those ideal flakes, consequently, the gaps can be opened monotonously for disordered flakes on changing their sizes, affected by the dimension, geometric shape and the edge saturation. Using the density functional theory, accordingly, the electronic structures of disordered flakes differ to the ideal case because of the edge disorder. Our theoretical predictions have been validated by available experimental results, and provide us a distinct way for the quantitative modulation of bandgap in graphene for nanoelectronics. PMID:23524635

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

    PubMed Central

    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

  12. 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.

  13. Extracting the complex optical conductivity of mono- and bilayer graphene by ellipsometry

    NASA Astrophysics Data System (ADS)

    Chang, You-Chia; Liu, Chang-Hua; Liu, Che-Hung; Zhong, Zhaohui; Norris, Theodore B.

    2014-06-01

    A method for analysis of spectroscopic ellipsometry data is demonstrated to extract the optical conductivity of mono- and bilayer chemical-vapor-deposited graphene. We model graphene as a truly two-dimensional (2D) material with a sheet conductivity, rather than a phenomenological effective refractive index as has been used in the literature. This technique measures both the real and imaginary part of the optical conductivity, which is important for graphene optoelectronics and metamaterials. Using this method, we obtain broadband measurements of the complex optical conductivity for mono- and bilayer graphene from ultraviolet to mid-infrared wavelengths. We also study how chemical doping with nitric acid modifies the complex optical conductivity.

  14. Semiconducting properties of bilayer graphene modulated by an electric field for next-generation atomic-film electronics

    NASA Astrophysics Data System (ADS)

    Tsukagoshi, K.; Li, S.-L.; Miyazaki, H.; Aparecido-Ferreira, A.; Nakaharai, S.

    2014-03-01

    A practical wide bandgap was induced in bilayer graphene using a perpendicular electric field. A self-assembled gate insulator was used to apply a large electric field. The wide bandgap allows the operation of fundamental logic gates composed of bilayer graphene transistors. The results reviewed here indicate the potential for graphene electronics to be realized as emerging transistors with an atomically thin semiconductor.

  15. 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.

  16. 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.

  17. 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.

  18. Magnetic properties of bilayer graphene armchair nanoribbons: A Monte Carlo study

    NASA Astrophysics Data System (ADS)

    Masrour, R.; Jabar, A.

    2017-03-01

    In this work, the lattice structure of bilayer graphene armchair nanoribbons atoms within the same layer are studied by Monte Carlo simulations. The ground state phase diagrams of mixed spin-3 and spin-7/2 Ising model on a of bilayer graphene armchair nanoribbons are studied using the Monte Carlo simulations. The reduced transition temperatures with the exchange interactions have been given. The total magnetization and magnetic susceptibility with the crystal field have been established for different plane exchange interactions. Finally, we have given the magnetic hysteresis cycle for different plane exchange interactions, different temperatures, and different crystal field in bilayer graphene armchair nanoribbons. The bilayer graphene armchair nanoribbons exhibit the superparamagnetic behavior at the reduced transition temperature and for a fixed of crystal field.

  19. Generation of valley-polarized electron beam in bilayer graphene

    SciTech Connect

    Park, Changsoo

    2015-12-28

    We propose a method to produce valley-polarized electron beams using a bilayer graphene npn junction. By analyzing the transmission properties of electrons through the junction with zigzag interface in the presence of trigonal warping, we observe that there exist a range of incident energies and barrier heights in which transmitted electrons are well polarized and collimated. From this observation and by performing numerical simulations, it is demonstrated that valley-dependent electronic currents with nearly perfect polarization can be generated. We also show that the peak-to-peak separation angle between the polarized currents is tunable either by incident energy or by barrier height each of which is controlled by using top and back gate voltages. The results can be used for constructing an electron beam splitter to produce valley-polarized currents.

  20. Band tunneling through double barrier in biased graphene bilayer

    NASA Astrophysics Data System (ADS)

    Abdullah, Hasan M.; El Mouhafid, Abderrahim; Bahlouli, H.; Jellal, Ahmed

    2017-02-01

    We calculate the transport properties of charge carriers in graphene bilayers across symmetric and asymmetric double potential barrier considering energies exceeding the inter-layer coupling where two transport modes exist. Evaluating the transmission and reflection probabilities and corresponding conductances, we show that the transport is sensitive to the distance between the two barriers. Moreover, we explain the characteristic features observed in the numerical calculations, such as resonance tunneling at normal incidence, based on the Febry–Pèrot oscillations and ballistic transmission carried out by the evanescent waves. Finally, we compute the conductance of each mode separately and investigate contributions from inter-mode scattering and show that some geometric potential parameters can be used to control the total conductance of the system.

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

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

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

  2. 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.

  3. 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

  4. Tunable doping and band gap of graphene on functionalized hexagonal boron nitride with hydrogen and fluorine.

    PubMed

    Tang, Shaobin; Yu, Jianping; Liu, Liangxian

    2013-04-14

    First-principles calculations have been used to investigate the structural and electronic properties of graphene supported on functionalized hexagonal boron nitride (h-BN) with hydrogen and fluorine atoms. Our results show that the hydrogenation and fluorination of the h-BN substrate modify the electronic properties of graphene. Interactions of graphene with fully hydrogenated or fully fluorinated h-BN and half-hydrogenated and half-fluorinated h-BN with H at N sites and F at the B sites can lead to n- or p-type doping of graphene. The different doping effect may be attributed to the significant charge transfer from graphene to the substrate. Interestingly, when graphene is supported on the functionalized h-BN with H at B sites and F at N sites (G/HBNF), a finite band gap of 79 meV in graphene is opened due to the equivalence breaking of two sublattices of graphene, and can be effectively modulated by changing the interlayer spacing, increasing the number of functionalized BN layers, and applying an external electric field. More importantly, the modification of the band gap in G/HBNF with a functionalized BN bilayer by the electric field is more pronounced than that of the single-layer h-BN, which is increased to 408 meV with 0.8 V Å(-1). Thus, graphene on chemically modified h-BN with a tunable and sizeable band gap may provide a novel way for fabricating high-performance graphene-based nanodevices.

  5. Substrate-induced band gap opening in epitaxial graphene

    SciTech Connect

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

    2007-09-08

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

  6. 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.

  7. Layer-selective half-metallicity in bilayer graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Jeon, Gi Wan; Lee, Kyu Won; Lee, Cheol Eui

    2015-05-01

    Half-metallicity recently predicted in the zigzag-edge graphene nanoribbons (ZGNRs) and the hydrogenated carbon nanotubes (CNTs) enables fully spin-polarized electric currents, providing a basis for carbon-based spintronics. In both carbon systems, the half-metallicity arises from the edge-localized electron states under an electric field, lowering the critical electric field Dc for the half-metallicity being an issue in recent works on ZGNRs. A properly chosen direction of the electric field alone has been predicted to significantly reduce Dc in the hydrogenated CNTs, which in this work turned out to be the case in narrow bilayer ZGNRs (biZGNRs). Here, our simple model based on the electrostatic potential difference between the edges predicts that for wide biZGNRs of width greater than ~2.0 nm (10 zigzag carbon chains), only one layer of the biZGNRs becomes half-metallic leaving the other layer insulating as confirmed by our density functional theory (DFT) calculations. The electric field-induced switching of the spin-polarized current path is believed to open a new route to graphene-based spintronics applications.

  8. Magnetoelectric coupling, Berry phase, and Landau level dispersion in a biased bilayer graphene

    NASA Astrophysics Data System (ADS)

    Zhang, Lingfeng M.; Fogler, Michael M.; Arovas, Daniel P.

    2011-08-01

    We study the energy spectrum of a graphene bilayer in the presence of transverse electric and magnetic fields. We find that the resulting Landau levels exhibit a nonmonotonic dependence on the electric field, as well as numerous level crossings. This behavior is explained using quasiclassical quantization rules that properly take into account the pseudospin of the quasiparticles. The pseudospin generates the Berry phase, which leads to a shift in energy quantization and results in a pseudo-Zeeman effect. The latter depends on the electric field, alternates in sign among the two valleys, and also reduces the band gap. Analytic formulas for other pseudospin-related quantities, such as the anomalous Hall conductivity, are derived and compared with prior theoretical work.

  9. Role of interlayer spacing in electrical transport of bilayer graphene nanoribbon: Perpendicular and armchair direction

    NASA Astrophysics Data System (ADS)

    Jamaati, Maryam; Namiranian, Afshin

    2017-01-01

    The electrical conductance of bilayer zigzag graphene nanoribbon is numerically investigated taking advantage of Green's function. The calculations are performed within the tight binding model, which describes the interaction between carbon atoms within a layer via nearest neighbor and carbon atoms of different layers using continuum model. Our findings reveal sensitivity of the bilayer graphene conductance to changes in its relative displacement of two layers in perpendicular and armchair directions. We find that the conductance oscillates as a function of system width, and finally reduces rapidly as the relative distance of two layers becomes larger than half the system width. The results show that the conductance of the bilayer graphene could be tuned via displacement of two layers. Moreover, we obtain different results for conductance of narrow and wide bilayer nanoribbon.

  10. Modulation of bandgap in bilayer armchair graphene ribbons by tuning vertical and transverse electric fields

    NASA Astrophysics Data System (ADS)

    Vu, Thanh-Tra; Nguyen, Thi-Kim-Quyen; Huynh, Anh-Huy; Phan, Thi-Kim-Loan; Tran, Van-Truong

    2017-02-01

    We investigate the effects of external electric fields on the electronic properties of bilayer armchair graphene nano-ribbons. Using atomistic simulations with Tight Binding calculations and the Non-equilibrium Green's function formalism, we demonstrate that (i) in semi-metallic structures, vertical fields impact more effectively than transverse fields in terms of opening larger bandgap, showing a contrary phenomenon compared to that demonstrated in previous studies in bilayer zigzag graphene nano-ribbons; (ii) in some semiconducting structures, if transverse fields just show usual effects as in single layer armchair graphene nano-ribbons where the bandgap is suppressed when varying the applied potential, vertical fields exhibit an anomalous phenomenon that the bandgap can be enlarged, i.e., for a structure of width of 16 dimer lines, the bandgap increases from 0.255 eV to the maximum value of 0.40 eV when a vertical bias equates 0.96 V applied. Although the combined effect of two fields does not enlarge the bandgap as found in bilayer zigzag graphene nano-ribbons, it shows that the mutual effect can be useful to reduce faster the bandgap in semiconducting bilayer armchair graphene nano-ribbons. These results are important to fully understand the effects of electric fields on bilayer graphene nano-ribbons (AB stacking) and also suggest appropriate uses of electric gates with different edge orientations.

  11. Raman spectroscopy of graphene and bilayer under biaxial strain: bubbles and balloons.

    PubMed

    Zabel, Jakob; Nair, Rahul R; Ott, Anna; Georgiou, Thanasis; Geim, Andre K; Novoselov, Kostya S; Casiraghi, Cinzia

    2012-02-08

    We use graphene bubbles to study the Raman spectrum of graphene under biaxial (e.g., isotropic) strain. Our Gruneisen parameters are in excellent agreement with the theoretical values. Discrepancy in the previously reported values is attributed to the interaction of graphene with the substrate. Bilayer balloons (intentionally pressurized membranes) have been used to avoid the effect of the substrate and to study the dependence of strain on the interlayer interactions.

  12. Experimental observation of surface states and Landau levels bending in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Yin, Long-Jing; Zhang, Yu; Qiao, Jia-Bin; Li, Si-Yu; He, Lin

    2016-03-01

    We report on microscopic measurements of the low-energy electronic structures both at the zigzag and armchair edges of bilayer graphene using scanning tunneling microscopy and spectroscopy (STM and STS). We have found that, both in the absence and in the presence of a magnetic field, an almost zero-energy peak in the density of states was localized at the zigzag edges, as expected for the surface states at the zigzag edges of bilayer graphene. In the quantum Hall regime, we have clearly observed Landau levels bending away from the charge neutrality point near both the zigzag and armchair edges. Such a result is direct evidence for the evolution of Landau levels into quantum Hall edge states in graphene bilayers. Our experiment indicates that it is possible to explore rich quantum Hall physics in graphene systems using STM and STS.

  13. 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.

  14. Optical properties of dielectric plates coated with gapped graphene

    NASA Astrophysics Data System (ADS)

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

    2017-01-01

    The optical properties of dielectric plates coated with gapped graphene are investigated on the basis of first principles of quantum electrodynamics. The reflection coefficients and reflectivities of graphene-coated plates are expressed in terms of the polarization tensor of gapped graphene and the dielectric permittivity of plate material. Simple approximate expressions for the required combinations of components of the polarization tensor applicable in the wide frequency region, where the presence of a gap influences the optical properties, are found. Numerical computations of the reflectivities of graphene-coated SiO 2 plates are performed for different values of the mass-gap parameter at different temperatures. It is shown that with an increasing gap width the reflectivity of a graphene-coated plate at the normal incidence decreases by up to a factor of 8 depending on the values of frequency and mass-gap parameter. The angle dependences of reflectivities for both polarizations of the incident electromagnetic waves have been computed for Si and SiO 2 plates coated with gapped graphene. We demonstrate that the TM reflectivity has a minimum value at some angle of incidence depending on the mass-gap parameter, frequency and temperature, whereas the TE reflectivity depends on the angle of incidence monotonously. However, for the graphene coatings with a nonzero mass-gap parameter the reflected light cannot be fully polarized. Possible applications of the obtained results are discussed.

  15. 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.

  16. Study of the optical phonons on gated twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Chung, Ting Fung; He, Rui; Wu, Tai-Lung; Chen, Yong P.

    2015-03-01

    In twisted bilayer graphene (tBLG), the low-energy van-Hove singularities (vHs) in the density of states (DOS) can be continuously tuned by twisting the two layers, leading to distinct electronic and optical properties compared to Bernal-stacked BLG (AB-BLG). This effect has been explored using resonance Raman scattering, showing enhanced Raman G and ZO' (low frequency, layer breathing vibration) bands when the vHs energy resonates with excitation laser energy. We have studied the influence on vHs and Raman bands in gated tBLG devices (at resonant twist angle ~13° under a 532 nm laser light). We observed that the G band splits with increasing doping, attributed to asymmetric doping of charge carriers in the two layers. The strongly quenched G band intensity at high doping level is ascribed to the suppression of resonant interband transitions between the two saddle points (in conduction and valence bands) which are displaced in the momentum space by gate-tuning. We have also measured the doping dependence of ZO' band and R band in tBLG. Our results demonstrate that electric-field can be used to tune the optoelectronic and vibrational properties in tBLG devices.

  17. 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

  18. 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

  19. 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.

  20. 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.

  1. Gas adsorption, energetics and electronic properties of boron- and nitrogen-doped bilayer graphenes

    NASA Astrophysics Data System (ADS)

    Fujimoto, Yoshitaka; Saito, Susumu

    2016-10-01

    We study stabilities and electronic properties of several environmental polluting or toxic gas molecules (CO, CO2, NO, and NO2) adsorbed on B and N atoms in bilayer graphene using first-principles electronic-structure calculations. We find that NO and NO2 molecules can be bound chemically on B-doped bilayer graphene with large adsorption energies, while CO and CO2 molecules are not adsorbed chemically on B-doped one. In the case of the N-doped graphene, all four gases do not bind with chemical bonds but adsorb rather physically with small adsorption energies at long distances between gases and graphene. The adsorptions of NO and NO2 molecules on B-doped bilayer graphene induce the acceptor states above the Fermi energy, and we also find that the charge transfer takes place when the NO and the NO2 molecules are adsorbed. Thereby, the B-doped bilayer graphene is expected to be useful for NO and NO2 gas sensor materials.

  2. 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

    2013-01-09

    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.

  3. Electronic structure of twisted bilayer graphene with doping and under electric fields

    NASA Astrophysics Data System (ADS)

    Xian, Lede; Barraza-Lopez, Salvador; Chou, Mei-Yin

    2011-03-01

    Rotational stacking faults of graphene layers in epitaxial graphene are believed to electronically decouple adjacent layers, thus single-layer graphene-like behavior can be observed. In addition, the layers close to the SiC substrate are known to be electron doped. Using density functional theory and a pi-electron, highly tuned tight-binding model, we study the modifications of the band structure in rotational stack-faulted bilayer graphene induced by doping and by external electric fields. In particular, the interlayer coupling, the magnitude of the Fermi velocity, and the possible impact on charge transport will be discussed.

  4. 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.

  5. Tunneling magnetoresistance tuned by a vertical electric field in an AA-stacked graphene bilayer with double magnetic barriers

    SciTech Connect

    Wang, Dali; Jin, Guojun

    2013-12-21

    We investigate the effect of a vertical electric field on the electron tunneling and magnetoresistance in an AA-stacked graphene bilayer modulated by the double magnetic barriers with parallel or antiparallel configuration. The results show that the electronic transmission properties in the system are sensitive to the magnetic-barrier configuration and the bias voltage between the graphene layers. In particular, it is found that for the antiparallel configuration, within the low energy region, the blocking effect is more obvious compared with the case for the parallel configuration, and even there may exist a transmission spectrum gap which can be arbitrarily tuned by the field-induced interlayer bias voltage. We also demonstrate that the significant discrepancy between the conductance for both parallel and antiparallel configurations would result in a giant tunneling magnetoresistance ratio, and further the maximal magnetoresistance ratio can be strongly modified by the interlayer bias voltage. This leads to the possible realization of high-quality magnetic sensors controlled by a vertical electric field in the AA-stacked graphene bilayer.

  6. Pauli magnetic susceptibility of bilayer graphene and hexagonal boron-nitride

    NASA Astrophysics Data System (ADS)

    Mousavi, Hamze; Jalilvand, Samira; Kurdestany, Jamshid Moradi

    2016-12-01

    We study the contribution of s and p orbitals on the Pauli magnetic susceptibility (PMS) and density of state (DOS) of the following three structures (1) bilayer graphene (2) bilayer boron-nitride (BN) and (3) bilayer graphene-BN within a two-band tight-binding Harrison Hamiltonian and the Green's function technique. It is shown that in all three cases, the contribution of s and px or py orbitals have no states around the Fermi level, while for bilayer graphene and graphene-BN the total DOS and DOS of pz orbital appear to be a linear function around this level. We show explicitly that for bilayer BN the contribution of pz orbital does not have states around the Fermi level, because of ionization energy difference between the boron (B) and nitrogen (N) atoms. We find that the bandwidth of s, px or py is more extension than case of pz orbital as a result of the Van-Hove singularities in the DOS. This leads to consideration of the PMS in two, low and high temperature, regions.

  7. 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.

  8. 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.

  9. Gate-controlled topological conducting channels in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Li, Jing; Wang, Ke; McFaul, Kenton J.; Zern, Zachary; Ren, Yafei; Watanabe, Kenji; Taniguchi, Takashi; Qiao, Zhenhua; Zhu, Jun

    2016-12-01

    The existence of inequivalent valleys K and K‧ in the momentum space of 2D hexagonal lattices provides a new electronic degree of freedom, the manipulation of which can potentially lead to new types of electronics, analogous to the role played by electron spin. In materials with broken inversion symmetry, such as an electrically gated bilayer graphene (BLG), the momentum-space Berry curvature Ω carries opposite sign in the K and K‧ valleys. A sign reversal of Ω along an internal boundary of the sheet gives rise to counterpropagating 1D conducting modes encoded with opposite-valley indices. These metallic states are topologically protected against backscattering in the absence of valley-mixing scattering, and thus can carry current ballistically. In BLG, the reversal of Ω can occur at the domain wall of AB- and BA-stacked domains, or at the line junction of two oppositely gated regions. The latter approach can provide a scalable platform to implement valleytronic operations, such as valves and waveguides, but it is technically challenging to realize. Here, we fabricate a dual-split-gate structure in BLG and present evidence of the predicted metallic states in electrical transport. The metallic states possess a mean free path (MFP) of up to a few hundred nanometres in the absence of a magnetic field. The application of a perpendicular magnetic field suppresses the backscattering significantly and enables a junction 400 nm in length to exhibit conductance close to the ballistic limit of 4e2/h at 8 T. Our experiment paves the way to the realization of gate-controlled ballistic valley transport and the development of valleytronic applications in atomically thin materials.

  10. 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.

  11. 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

  12. Bilayer Graphene as a Platform for Bosonic Symmetry-Protected Topological States

    NASA Astrophysics Data System (ADS)

    Bi, Zhen; Zhang, Ruixing; You, Yi-Zhuang; Young, Andrea; Balents, Leon; Liu, Chao-Xing; Xu, Cenke

    2017-03-01

    Bosonic symmetry protected topological (BSPT) states, the bosonic analogue of topological insulators, have attracted enormous theoretical interest in the last few years. Although BSPT states have been classified by various approaches, there is so far no successful experimental realization of any BSPT state in two or higher dimensions. In this paper, we propose that a two-dimensional BSPT state with U (1 )×U (1 ) symmetry can be realized in bilayer graphene in a magnetic field. Here the two U (1 ) symmetries represent total spin Sz and total charge conservation, respectively. The Coulomb interaction plays a central role in this proposal—it gaps out all the fermions at the boundary, so that only bosonic charge and spin degrees of freedom are gapless and protected at the edge. Based on the above conclusion, we propose that the bulk quantum phase transition between the BSPT and trivial phase, which can be driven by applying both magnetic and electric fields, can become a "bosonic phase transition" with interactions. That is, only bosonic modes close their gap at the transition, which is fundamentally different from all the well-known topological insulator to trivial insulator transitions that occur for free fermion systems. We discuss various experimental consequences of this proposal.

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

    NASA Astrophysics Data System (ADS)

    Park, Changwon; Ryu, Junga; Hong, Suklyun; Sumpter, Bobby; Kim, Gunn; Yoon, Mina

    2015-03-01

    In the design of bilayer graphene (BLG)-based switching devices, it is critical to understand the complex stacking structures observed experimentally and their impact on the overall electronic properties. Using a maximally localized Wannier function, a highly accurate tight-binding Hamiltonian based on density functional theory was constructed and the stacking-dependent evolution of BLGs electronic band structures and their response to an external electric field were systematically investigated. Although the crossing band structures remain at any stacking configurations (i.e., no energy gap opens), the wavefunction characteristics around the Fermi level can differ qualitatively for different stackings. This difference is conveyed to energy gap opening properties in the presence of an external electric field. We, for the first time, established a phase diagram summarizing the stacking-dependent electronic structures of BLG, separating metallic and semiconducting characteristics for a given external field. The research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

  14. Electron transmission through bilayer graphene: A time-dependent first-principles study

    NASA Astrophysics Data System (ADS)

    Miyauchi, Hironari; Ueda, Yoshihiro; Suzuki, Yasumitsu; Watanabe, Kazuyuki

    2017-03-01

    Incident-energy-dependent electron transmittances through single-layer graphene (SLG) and bilayer graphene (BLG) were investigated using time-dependent density functional theory. The transmittances of BLG with two kinds of stacking exhibit an unexpected crossing at a certain incident electron energy. The behavior is preserved for the BLG with reduced or increased layer distances compared to that of typical BLG. We determined the origin of the crossing by investigating transmission electron diffraction patterns for SLG.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-12-01

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

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

    PubMed

    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.

  1. 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.

  2. 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

  3. A new approach to study the effect of generation rate on drain-source current of bilayer graphene transistors

    NASA Astrophysics Data System (ADS)

    Ahmad, H.; Ghadiry, M.; AbdManaf, A.

    2016-10-01

    This paper presents a new approach to study the effect of impact ionization on the current of bilayer graphene field effect transistors. Analytical models for surface potential and current together with a Monte Carlo approach which include the edge effect scattering are used to calculate the current and generation rate in bilayer graphene transistors due to ionization. FlexPDE simulation is also employed for verification of surface potential modeling. Using the approach, the profile of generation rate, surface potential and current are plotted with respect to several structural parameters. We have shown that ignoring this effect in the modeling will result in an error of up to 10 % for a typical 30 nm bilayer graphene field effect transistor. As a result, we conclude that any analytical study ignoring the ionization is incomplete for bilayer graphene field effect transistors. The model presented here can be applied in optimization of photo detectors based on graphene.

  4. Electronic transport and shot noise in a Thue-Morse bilayer graphene superlattice with interlayer potential bias

    NASA Astrophysics Data System (ADS)

    Li, Yuanqiao; Zhang, Hongmei; Liu, De

    2016-06-01

    In this paper, we evaluate the transport properties of a Thue-Morse AB-stacked bilayer graphene superlattice with different interlayer potential biases. Based on the transfer matrix method, the transmission coefficient, the conductance, and the Fano factor are numerically calculated and discussed. We find that the symmetry of the transmission coefficient with respect to normal incidence depends on the structural symmetry of the system and the new transmission peak appears in the energy band gap opening region. The conductance and the Fano factor can be greatly modulated not only by the Fermi energy and the interlayer potential bias but also by the generation number. Interestingly, the conductance exhibits the plateau of almost zero conductance and the Fano factor plateaus with Poisson value occur in the energy band gap opening region for large interlayer potential bias.

  5. 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

  6. 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.

  7. 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.

  8. Magnetism of an adatom on bilayer graphene and its control: A first-principles perspective

    NASA Astrophysics Data System (ADS)

    Nafday, Dhani; Saha-Dasgupta, T.

    2013-11-01

    We present a first-principles investigation of the electronic and magnetic properties of an adatom on bilayer graphene within the framework of density functional theory. In particular, we study the influence of an applied gate voltage which modifies the electronic states of the bilayer graphene as well as shifts the adatom energy states relative to that of the graphene energy states. Our study carried out for a choice of three different adatoms, Na, Cu, and Fe, shows that the nature of adatom-graphene bonding evolves from ionic to covalent, in moving from an alkali metal, Na, to a transition metal, Cu or Fe. This leads to the formation of magnetic moments in the latter cases (Cu, Fe) and the absence of a magnetic moment in the former (Na). Application of an external electric field to bilayer graphene completely changes the scenario, switching on a magnetic moment for the Na adatom, and switching off the magnetic moments for Cu and Fe adatoms. Our results have important implications for fundamental studies of controlled adatom magnetism and spintronics applications in nanotechnology.

  9. Magnetism of Adatom on Bilayer Graphene and its Control: A First-principles Perspective

    NASA Astrophysics Data System (ADS)

    Saha-Dasgupta, Tanusri; Nafday, Dhani

    2014-03-01

    We present first-principles investigation of the electronic and magnetic properties of adatom on bilayer graphene within the framework of density functional theory. In particular, we study the influence of an applied gate-voltage which modifies the electronic states of the bilayer graphene as well as shifts the adatom energy states relative to that of the graphene energy states. Our study carried out for a choice of three different adatoms, Na, Cu and Fe, shows that the nature of adatom-graphene bonding evolves from ionic to covalent, in moving from alkali metal, Na to transition metal, Cu or Fe. This leads to the formation of magnetic moments in the latter cases (Cu, Fe) and its absence in the former (Na). Application of an external electric field to bilayer graphene, completely changes the scenario, switching on a magnetic moment for Na adatom, and switching off the magnetic moments for Cu, and Fe adatoms. Our results have important implications for fundamental studies of controlled adatom magnetism and spintronics application in nanotechnology. The authors thank Ministry of Earth Science and Department of Science and Technology, India for financial support.

  10. 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.

  11. Spatially resolving unconventional interface Landau quantization in a graphene monolayer-bilayer planar junction

    NASA Astrophysics Data System (ADS)

    Yan, Wei; Li, Si-Yu; Yin, Long-Jing; Qiao, Jia-Bin; Nie, Jia-Cai; He, Lin

    2016-05-01

    Hybrid quantum Hall (QH) junctions have been extensively studied by transport measurements due to their exciting physics and device applications. Here we report on spatially resolving electronic properties of such a junction on the nanoscale. We present a subnanometer-resolved scanning tunneling microscopy (STM) and scanning tunneling spectroscopy study of a monolayer-bilayer graphene planar junction in the QH regime. The atomically well-defined interface of such a junction allows us to spatially resolve the interface electronic properties. Around the interface, we detect Landau quantization of massless Dirac fermions as expected in the graphene monolayer for filled states of the junction, whereas unexpectedly, only Landau quantization of massive Dirac fermions as expected in the graphene bilayer is observed for empty states. The observed unconventional interface Landau quantization arises from the fact that the quantum conductance across the interface is solely determined by the minimum filling factors (number of edge modes) in the graphene monolayer and bilayer regions of the junction. Our finding opens the way to spatially explore the QH effect of different graphene hybrid structures only using a STM.

  12. 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

    2016-03-07

    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.

  13. 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

  14. Shape-alterable and -recoverable graphene/polyurethane bi-layered composite film for supercapacitor electrode

    NASA Astrophysics Data System (ADS)

    Tai, Zhixin; Yan, Xingbin; Xue, Qunji

    2012-09-01

    In this paper, a graphene/shape-memory polyurethane (PU) composite film, used for a supercapacitor electrode, is fabricated by a simple bonding method. In the composite, formerly prepared graphene paper is closely bonded on the surface of the PU slice, forming a bi-layered composite film. Based on the good flexibility of graphene paper and the outstanding shape holding capacity of PU phase, the resulting composite film can be changed into various shapes. Also, the composite film shows excellent shape recovery ability. The graphene/PU composite film used as the electrode maintains a satisfactory electrochemical capacitance of graphene material and there is no decay in the specific capacitance after long-cycle testing, making it attractive for novel supercapacitors with special shapes and shape-memory ability.

  15. Electrical properties of bilayer graphene synthesized using surface wave microwave plasma techniques at low temperature

    NASA Astrophysics Data System (ADS)

    Yamada, Takatoshi; Kato, Hiromitsu; Okigawa, Yuki; Ishihara, Masatou; Hasegawa, Masataka

    2017-01-01

    Bilayer graphene was synthesized at low temperature using surface wave microwave plasma techniques where poly(methyl metacrylate) (PMMA) and methane (CH4) were used as carbon sources. Temperature-dependent Hall effect measurements were carried out in a helium atmosphere. Sheet resistance, sheet carrier density and mobility showed weak temperature dependence for graphene from PMMA, and the highest carrier mobility is 740 cm2 V-1 s-1. For graphene from CH4, tunneling of the domain boundary limited carrier transport. The difference in average domain size was determined by Raman signal maps. In addition, residuals of PMMA were detected on graphene from PMMA. The low sheet resistances of graphene synthesized at a temperature of 280 °C using plasma techniques were explained by the PMMA related residuals rather than the domain sizes.

  16. 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.

  17. 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

  18. Ising quantum Hall ferromagnetism in Landau levels |N|≥1 of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Luo, Wenchen; Côté, R.; Bédard-Vallée, Alexandre

    2014-08-01

    A magnetic field applied perpendicularly to the chiral two-dimensional electron gas (C2DEG) in a Bernal-stacked bilayer graphene quantizes the kinetic energy into a discrete set of Landau levels N =0,±1,±2,.... While Landau level N =0 is eightfold degenerate, higher Landau levels (|N|≥1) are fourfold degenerate when spin and valley degrees of freedom are counted. In this work, the Hartree-Fock approximation is used to study the phase diagram of the C2DEG at integer fillings ν˜=1,2,3 of these higher Landau levels. At these filling factors, the C2DEG is a valley or spin Ising quantum Hall ferromagnet. At odd fillings, the C2DEG is spin polarized and has all its electrons in one valley or the other. There is no intervalley coherence, in contrast with most of the ground states in Landau level N =0. At even filling ν˜=2, the C2DEG is either fully spin polarized with electrons occupying both valleys or spin unpolarized with electrons occupying one of the two valleys. A finite electric field (or bias) applied perpendicularly to the plane of the C2DEG induces a series of first-order phase transitions between these different ground states. The transport gap or its slope is discontinuous at the bias where a transition occurs. Such discontinuity may result in a change in the transport properties of the C2DEG at that bias.

  19. Contact resistance at planar metal contacts on bilayer graphene and effects of molecular insertion layers.

    PubMed

    Nouchi, Ryo

    2017-03-01

    The possible origins of metal-bilayer graphene (BLG) contact resistance are investigated by taking into consideration the bandgap formed by interfacial charge transfer at the metal contacts. Our results show that a charge injection barrier (Schottky barrier) does not contribute to the contact resistance because the BLG under the contacts is always degenerately doped. We also showed that the contact-doping-induced increase in the density of states (DOS) of BLG under the metal contacts decreases the contact resistance owing to enhanced charge carrier tunnelling at the contacts. The contact doping can be enhanced by inserting molecular dopant layers into the metal contacts. However, carrier tunnelling through the insertion layer increases the contact resistance, and thus, alternative device structures should be employed. Finally, we showed that the inter-band transport by variable range hopping via in-gap states is the largest contributor to contact resistance when the carrier type of the gated channel is opposite to the contact doping carrier type. This indicates that the strategy of contact resistance reduction by the contact-doping-induced increase in the DOS is effective only for a single channel transport branch (n- or p-type) depending on the contact doping carrier type.

  20. Contact resistance at planar metal contacts on bilayer graphene and effects of molecular insertion layers

    NASA Astrophysics Data System (ADS)

    Nouchi, Ryo

    2017-03-01

    The possible origins of metal–bilayer graphene (BLG) contact resistance are investigated by taking into consideration the bandgap formed by interfacial charge transfer at the metal contacts. Our results show that a charge injection barrier (Schottky barrier) does not contribute to the contact resistance because the BLG under the contacts is always degenerately doped. We also showed that the contact-doping-induced increase in the density of states (DOS) of BLG under the metal contacts decreases the contact resistance owing to enhanced charge carrier tunnelling at the contacts. The contact doping can be enhanced by inserting molecular dopant layers into the metal contacts. However, carrier tunnelling through the insertion layer increases the contact resistance, and thus, alternative device structures should be employed. Finally, we showed that the inter-band transport by variable range hopping via in-gap states is the largest contributor to contact resistance when the carrier type of the gated channel is opposite to the contact doping carrier type. This indicates that the strategy of contact resistance reduction by the contact-doping-induced increase in the DOS is effective only for a single channel transport branch (n- or p-type) depending on the contact doping carrier type.

  1. 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.

  2. Chemical vapour deposition growth of large single crystals of monolayer and bilayer graphene

    NASA Astrophysics Data System (ADS)

    Zhou, Hailong; Yu, Woo Jong; Liu, Lixin; Cheng, Rui; Chen, Yu; Huang, Xiaoqing; Liu, Yuan; Wang, Yang; Huang, Yu; Duan, Xiangfeng

    2013-06-01

    The growth of large-domain single crystalline graphene with the controllable number of layers is of central importance for large-scale integration of graphene devices. Here we report a new pathway to greatly reduce the graphene nucleation density from ~106 to 4 nuclei cm-2, enabling the growth of giant single crystals of monolayer graphene with a lateral size up to 5 mm and Bernal-stacked bilayer graphene with the lateral size up to 300 μm, both the largest reported to date. The formation of the giant graphene single crystals eliminates the grain boundary scattering to ensure excellent device-to-device uniformity and remarkable electronic properties with the expected quantum Hall effect and the highest carrier mobility up to 16,000 cm2 V-1 s-1. The availability of the ultra large graphene single crystals can allow for high-yield fabrication of integrated graphene devices, paving a pathway to scalable electronic and photonic devices based on graphene materials.

  3. Chemical vapour deposition growth of large single crystals of monolayer and bilayer graphene.

    PubMed

    Zhou, Hailong; Yu, Woo Jong; Liu, Lixin; Cheng, Rui; Chen, Yu; Huang, Xiaoqing; Liu, Yuan; Wang, Yang; Huang, Yu; Duan, Xiangfeng

    2013-01-01

    The growth of large-domain single crystalline graphene with the controllable number of layers is of central importance for large-scale integration of graphene devices. Here we report a new pathway to greatly reduce the graphene nucleation density from ~10(6) to 4 nuclei cm(-2), enabling the growth of giant single crystals of monolayer graphene with a lateral size up to 5 mm and Bernal-stacked bilayer graphene with the lateral size up to 300 μm, both the largest reported to date. The formation of the giant graphene single crystals eliminates the grain boundary scattering to ensure excellent device-to-device uniformity and remarkable electronic properties with the expected quantum Hall effect and the highest carrier mobility up to 16,000 cm(2) V(-1) s(-1). The availability of the ultra large graphene single crystals can allow for high-yield fabrication of integrated graphene devices, paving a pathway to scalable electronic and photonic devices based on graphene materials.

  4. MoS2-WSe2 Hetero Bilayer: Possibility of Mechanical Strain Induced Band Gap Engineering

    NASA Astrophysics Data System (ADS)

    Sharma, Munish; Kumar, Ashok; Ahluwalia, P. K.

    2014-03-01

    The tunability of band gap in two-dimensional (2D) hetero-bilayers of MoS2-WSe2 with applied mechanical strains (in-plane and out-of-plane) in two different types of stackings (AA and AB) have been investigated in the framework of density functional theory (DFT). The in-plane biaxial tensile strain is found to reduce electronic band gap monotonically and rendered considered bilayer into metal at 6% of applied strain. The transition pressure required for complete semiconductor-to-metal transition is found to be of 7.89 GPa while tensile strength of the reported hetero-bilayer has been calculated 10 GPa at 25% strain. In case of vertical compression strain, 16 GPa pressure has been calculated for complete semiconductor-to-metal transition. The band-gap deformation potentials and effective masses (electron and hole) have been found to posses strong dependence on the type of applied strain. Such band gap engineering in controlled manner (internal control by composition and external control by applied strain) makes the considered hetero-bilayer as a strong candidate for the application in variety of nano scale devices.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. Quantum electrodynamic approach to the conductivity of gapped graphene

    NASA Astrophysics Data System (ADS)

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

    2016-11-01

    The electrical conductivity of graphene with a nonzero mass-gap parameter is investigated starting from the first principles of quantum electrodynamics in (2+1)-dimensional space time at any temperature. The formalism of the polarization tensor defined over the entire plane of complex frequency is used. At zero temperature we reproduce the results for both real and imaginary parts of the conductivity, obtained previously in the local approximation, and generalize them taking into account the effects of nonlocality. At nonzero temperature the exact analytic expressions for real and imaginary parts of the longitudinal and transverse conductivities of gapped graphene are derived, as well as their local limits and approximate expressions in several asymptotic regimes. Specifically, a simple local result for the real part of conductivity of gapped graphene valid at any temperature is obtained. According to our results, the real part of the conductivity is not equal to zero for frequencies exceeding the width of the gap and goes to the universal conductivity with increasing frequency. The imaginary part of conductivity of gapped graphene varies from infinity at zero frequency to minus infinity at the frequency defined by the gap parameter and then goes to zero with further increase of frequency. The analytic expressions are accompanied by the results of numerical computations. Possible future generalization of the used formalism is discussed.

  10. Local spectroscopy of moiré-induced electronic structure in gate-tunable twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Wong, Dillon; Wang, Yang; Jung, Jeil; Pezzini, Sergio; DaSilva, Ashley M.; Tsai, Hsin-Zon; Jung, Han Sae; Khajeh, Ramin; Kim, Youngkyou; Lee, Juwon; Kahn, Salman; Tollabimazraehno, Sajjad; Rasool, Haider; Watanabe, Kenji; Taniguchi, Takashi; Zettl, Alex; Adam, Shaffique; MacDonald, Allan H.; Crommie, Michael F.

    2015-10-01

    Twisted bilayer graphene (tBLG) forms a quasicrystal whose structural and electronic properties depend on the angle of rotation between its layers. Here, we present a scanning tunneling microscopy study of gate-tunable tBLG devices supported by atomically smooth and chemically inert hexagonal boron nitride (BN). The high quality of these tBLG devices allows identification of coexisting moiré patterns and moiré super-superlattices produced by graphene-graphene and graphene-BN interlayer interactions. Furthermore, we examine additional tBLG spectroscopic features in the local density of states beyond the first van Hove singularity. Our experimental data are explained by a theory of moiré bands that incorporates ab initio calculations and confirms the strongly nonperturbative character of tBLG interlayer coupling in the small twist-angle regime.

  11. Bilayer graphene with parallel magnetic field and twisting: Phases and phase transitions in a highly tunable Dirac system

    NASA Astrophysics Data System (ADS)

    Roy, Bitan; Yang, Kun

    2013-12-01

    The effective theory for bilayer graphene (BLG), subject to parallel/in-plane magnetic fields, is derived. With a sizable magnetic field the trigonal warping becomes irrelevant, and one ends up with two Dirac points in the vicinity of each valley in the low-energy limit, similar to the twisted BLG. Combining twisting and parallel field thus gives rise to a Dirac system with tunable Fermi velocity and cutoff. If the interactions are sufficiently strong, several fully gapped states can be realized in these systems, in addition to the ones in a pristine setup. Transformations of the order parameters under various symmetry operations are analyzed. The quantum critical behavior of various phase transitions driven by the twisting and the magnetic field is reported. The effects of an additional perpendicular field and possible ways to realize the new massive phases are highlighted.

  12. Robust fractional quantum Hall effect in the N=2 Landau level in bilayer graphene.

    PubMed

    Diankov, Georgi; Liang, Chi-Te; Amet, François; Gallagher, Patrick; Lee, Menyoung; Bestwick, Andrew J; Tharratt, Kevin; Coniglio, William; Jaroszynski, Jan; Watanabe, Kenji; Taniguchi, Takashi; Goldhaber-Gordon, David

    2016-12-21

    The fractional quantum Hall effect is a canonical example of electron-electron interactions producing new ground states in many-body systems. Most fractional quantum Hall studies have focussed on the lowest Landau level, whose fractional states are successfully explained by the composite fermion model. In the widely studied GaAs-based system, the composite fermion picture is thought to become unstable for the N≥2 Landau level, where competing many-body phases have been observed. Here we report magneto-resistance measurements of fractional quantum Hall states in the N=2 Landau level (filling factors 4<|ν|<8) in bilayer graphene. In contrast with recent observations of particle-hole asymmetry in the N=0/N=1 Landau levels of bilayer graphene, the fractional quantum Hall states we observe in the N=2 Landau level obey particle-hole symmetry within the fully symmetry-broken Landau level. Possible alternative ground states other than the composite fermions are discussed.

  13. 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

  14. Screening effect of graphite and bilayer graphene on excitons in MoSe2 monolayer

    NASA Astrophysics Data System (ADS)

    Wang, Yuan; Zhang, Shuai; Huang, Di; Cheng, Jingxin; Li, Yingguo; Wu, Shiwei

    2017-03-01

    Excitons in transition metal dichalcogenide monolayer have recently attracted great interest due to their extremely large binding energy, causing giant bandgap renormalization. In this work, we examined the screening effect of graphite and bilayer graphene on the excitons in molybdenum diselenide (MoSe2) monolayer grown by molecular beam epitaxy (MBE). Through the combinational study of scanning tunneling spectroscopy (STS) and photoluminescence (PL) measurements, we determined the binding energy of ~0.58 eV for MoSe2 monolayer on both substrates at 16 K, and no obvious difference between them. Our result is consistent with a previous report [Zhang et al 2015 Nano Letters 15, 6494], but is contradictory to another one [Ugeda 2014 Nature Materials 13, 1091]. Physical picture for no noticeable difference on screening effect between bilayer graphene and graphite substrate is discussed. Possible reasons for causing the discrepancy are also mentioned.

  15. Effects of annealing on the ripple texture and mechanical properties of suspended bilayer graphene

    NASA Astrophysics Data System (ADS)

    Annamalai, M.; Mathew, S.; Jamali, M.; Zhan, D.; Palaniapan, M.

    2013-04-01

    Periodic ripples of amplitude ˜15 nm were formed in suspended bilayer graphene after nanoindentation with incremental forces up to 600 nN. The structure was annealed at ˜620 K in high vacuum and the corresponding modifications in the mechanical properties and surface morphology were investigated. The pre-tension of the pristine sample was found to be 1.46 N m-1 and after annealing it was reduced to 0.72 N m-1. The nanometre-sized ripples induced by mechanical excitation were found to be flattened after annealing. Tailoring surface corrugations in bilayer graphene through nanoindentation and thermal engineering of these ripples thus provides an innovative fabrication route for flexible electronic devices and strain sensors.

  16. 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).

  17. 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.

  18. Robust fractional quantum Hall effect in the N=2 Landau level in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Diankov, Georgi; Liang, Chi-Te; Amet, François; Gallagher, Patrick; Lee, Menyoung; Bestwick, Andrew J.; Tharratt, Kevin; Coniglio, William; Jaroszynski, Jan; Watanabe, Kenji; Taniguchi, Takashi; Goldhaber-Gordon, David

    2016-12-01

    The fractional quantum Hall effect is a canonical example of electron-electron interactions producing new ground states in many-body systems. Most fractional quantum Hall studies have focussed on the lowest Landau level, whose fractional states are successfully explained by the composite fermion model. In the widely studied GaAs-based system, the composite fermion picture is thought to become unstable for the N≥2 Landau level, where competing many-body phases have been observed. Here we report magneto-resistance measurements of fractional quantum Hall states in the N=2 Landau level (filling factors 4<|ν|<8) in bilayer graphene. In contrast with recent observations of particle-hole asymmetry in the N=0/N=1 Landau levels of bilayer graphene, the fractional quantum Hall states we observe in the N=2 Landau level obey particle-hole symmetry within the fully symmetry-broken Landau level. Possible alternative ground states other than the composite fermions are discussed.

  19. Electron retroreflection and spin beam splitting in a twisted graphene bilayer

    NASA Astrophysics Data System (ADS)

    Xu, Yafang; Jin, Guojun

    2016-12-01

    We theoretically investigate the various reflection processes in a twisted graphene bilayer-based normal conductor/superconductor junction. It is found that the special spinor wave functions in strongly doped superconductor region lead the Andreev reflection to be suppressed completely. For the 100% electron → electron reflection, except the traditional specular reflection, electrons can undergo retroreflection, which is sensitive to the band contour and can be used to confirm the van Hove singularities in the twisted graphene bilayer. By depositing a ferromagnetic insulator on the N region, we further find that electrons can be spatially separated with spin-down electrons specular reflected and spin-up electrons retroreflected, or vice versa. It provides a new mechanism to realize a spin beam splitter.

  20. Fine tuning of optical transition energy of twisted bilayer graphene via interlayer distance modulation

    NASA Astrophysics Data System (ADS)

    del Corro, Elena; Peña-Alvarez, Miriam; Sato, Kentaro; Morales-Garcia, Angel; Bousa, Milan; Mračko, Michal; Kolman, Radek; Pacakova, Barbara; Kavan, Ladislav; Kalbac, Martin; Frank, Otakar

    2017-02-01

    Twisted bilayer graphene (tBLG) represents a family of unique materials with optoelectronic properties tuned by the rotation angle between the two layers. The presented work shows an additional way of tweaking the electronic structure of tBLG by modifying the interlayer distance, for example by a small uniaxial out-of-plane compression. We have focused on the optical transition energy, which shows a clear dependence on the interlayer distance, both experimentally and theoretically.

  1. 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.

  2. Graphene field effect transistor without an energy gap

    PubMed Central

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

    2013-01-01

    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. PMID:23671093

  3. 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.

  4. 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.

  5. 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.

  6. Transport properties of monolayer and bilayer graphene supported by hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Li, Jing; Zou, Ke; Seiwell, Donald; Zhu, Jun

    2013-03-01

    We present transport studies on hexagonal boron nitride (h-BN) supported monolayer and bilayer graphene. Following the method introduced by Dean et al, we first exfoliate thin sheets of h-BN (15-20 nm) to SiO2/Si substrate then align and transfer exfoliated graphene flakes onto the h-BN sheets. E-beam lithography is used to process the samples into Hall bar devices. We find that current annealing at low temperature can increase the mobility of as-fabricated devices but often introduces large density inhomogeneity at the same time. AFM images of annealed devices reveal the limitations of this technique. In comparison, thermal annealing is much more reliable in improving the sample quality. Bilayer devices annealed in a flow of Ar/H2 at 450C for 5 hours show high mobility of 30,000 cm2/Vs at low temperature. We observe high-quality Shubnikov-de Hass (SdH) oscillations and degeneracy-lifted Landau levels in these samples. We extend existing measurements of the electron and hole effective mass in bilayer graphene to lower carrier density regimes and discuss the implications of the results. Department of Applied Physics, Yale University

  7. Fractional Quantum Hall Effect in the Second Landau Level of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Diankov, Georgi; Amet, Francois; Lee, Menyoung; Bestwick, Andrew; Tharratt, Kevin; Liang, Chi-Te; Goldhaber-Gordon, David

    2015-03-01

    Bilayer graphene exhibits rich Quantum Hall physics due to valley, spin and orbital degrees of freedom that lead to a variety of polarization states. We study the Fractional Quantum Hall Effect (FQHE) in ultra-clean multiterminal bilayer graphene devices on boron nitride with a local graphite gate at magnetic fields of up to 45 T. We measure mobility of up to 1 million cm2/V.s and very low disorder. In addition to the broken-symmetry integer states, we unambiguously resolve a variety of fractions and focus on a series of fractions in the Second Landau Level, which do not follow particle-hole asymmetry. From the magnetic field dependence of the fractions, we find that some of these fractions have spin-polarized ground states while others are unpolarized, and we present a possible explanation for this difference. This work provides insights into how the symmetry-breaking electron-electron interactions and Zeeman splitting interact to produce a rich landscape of composite fermions in the Second Landau Level of bilayer graphene.

  8. Tunable gap graphene micro-ribbons for terahertz plasmonics

    NASA Astrophysics Data System (ADS)

    Huang, Danhong; Gumbs, Godfrey; Roslyak, Oleksiy

    2012-02-01

    Maxwell's equations are solved for an array of graphene micro-ribbons located at the interface between a vacuum half-space and a half-space of a dielectric substrate. Our calculations are include mode-mixing in the optical-response function. A closed-form analytic expression is obtained for the nonlocal optical-response function of a graphene layer with an induced energy gap which is then employed in our calculations beyond the long-wavelength approximation. Both the reflectivity and transmissivity spectral functions are calculated. Specifically, we obtain their dependences on the period of the array, the ribbon width, chemical potential of doped graphene, energy gap between the valence and conduction bands, substrate refractive index, and incident angle of a plane-wave electromagnetic field. Additionally, a qualitative comparison is made between our calculated results in this paper and the recent experimental data given by Ju, et al./, [Nature Nanotechnology, 6, 630 (2011)].

  9. New insights into the opening band gap of graphene oxides

    NASA Astrophysics Data System (ADS)

    Tran, Ngoc Thanh Thuy; Lin, Shih-Yang; Lin, Ming-Fa

    Electronic properties of oxygen absorbed few-layer graphenes are investigated using first-principle calculations. They are very sensitive to the changes in the oxygen concentration, number of graphene layer, and stacking configuration. The feature-rich band structures exhibit the destruction or distortion of the Dirac cone, opening of band gap, anisotropic energy dispersions, O- and (C,O)-dominated energy dispersions, and extra critical points. The band decomposed charge distributions reveal the π-bonding dominated energy gap. The orbital-projected density of states (DOS) have many special structures mainly coming from a composite energy band, the parabolic and partially flat ones. The DOS and spatial charge distributions clearly indicate the critical orbital hybridizations in O-O, C-O and C-C bonds, being responsible for the diversified properties. All of the few-layer graphene oxides are semi-metals except for the semiconducting monolayer ones.

  10. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  12. Pseudomagnetoexcitons in strained graphene bilayers without external magnetic fields

    NASA Astrophysics Data System (ADS)

    Wang, Zhigang; Fu, Zhen-Guo; Zheng, Fawei; Zhang, Ping

    2013-03-01

    We propose a strained graphene double-layer (SGDL) system for detecting pseudomagnetoexcitons (PME) in the absence of external magnetic fields. The carriers in each graphene layer experience different strong pseudomagnetic fields (PMFs) due to strain engineering, which give rise to Landau quantization. The pseudo-Landau levels of electron-hole pairs under inhomogeneous PMFs in the SGDL are obtained analytically in the absence of Coulomb interactions. Based on the derived optical absorption selection rule for PMEs, we interpret the optical absorption spectra as indicating the formation of Dirac-type PMEs. We also predict that in the presence of inhomogeneous PMFs, the superfluidity-normal phase-transition temperature of PMEs is greater than that under homogeneous PMFs.

  13. Helical edge states and fractional quantum Hall effect in a graphene electron-hole bilayer

    NASA Astrophysics Data System (ADS)

    Sanchez-Yamagishi, Javier D.; Luo, Jason Y.; Young, Andrea F.; Hunt, Benjamin M.; Watanabe, Kenji; Taniguchi, Takashi; Ashoori, Raymond C.; Jarillo-Herrero, Pablo

    2016-10-01

    Helical 1D electronic systems are a promising route towards realizing circuits of topological quantum states that exhibit non-Abelian statistics. Here, we demonstrate a versatile platform to realize 1D systems made by combining quantum Hall (QH) edge states of opposite chiralities in a graphene electron-hole bilayer at moderate magnetic fields. Using this approach, we engineer helical 1D edge conductors where the counterpropagating modes are localized in separate electron and hole layers by a tunable electric field. These helical conductors exhibit strong non-local transport signals and suppressed backscattering due to the opposite spin polarizations of the counterpropagating modes. Unlike other approaches used for realizing helical states, the graphene electron-hole bilayer can be used to build new 1D systems incorporating fractional edge states. Indeed, we are able to tune the bilayer devices into a regime hosting fractional and integer edge states of opposite chiralities, paving the way towards 1D helical conductors with fractional quantum statistics.

  14. Edge configurational effect on band gaps in graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Deepika, Kumar, T. J. Dhilip; Shukla, Alok; Kumar, Rakesh

    2015-03-01

    In this article, we put forward a resolution to the prolonged ambiguity in energy band gaps between theory and experiments of fabricated graphene nanoribbons (GNRs). Band structure calculations using density functional theory are performed on oxygen-passivated GNR supercells of customized edge configurations without disturbing the inherent s p2 hybridization of carbon atoms. Direct band gaps are observed for both zigzag and armchair GNRs, consistent with the experimental reports. In addition, we provide an explanation of the experimentally observed scattered band gap values of GNRs as a function of width in a crystallographic orientation on the basis of edge configurations. We conclude that edge configurations of GNRs significantly contribute to band gap formation in addition to its width for a given crystallographic orientation and will play a crucial role in band gap engineering of GNRs for future research on fabrication of nanoelectronic devices.

  15. Electron-Phonon Coupling in Alkali Doped Bilayer Graphene Studied by ARPES

    NASA Astrophysics Data System (ADS)

    Kleeman, James; Sugawara, Katsuaki; Sato, Takafumi; Takahashi, Takashi

    2014-03-01

    Graphene intercalation compounds are a class of materials consisting of stacked graphene sheets, with dopant adatoms ordered in-between them. These materials exhibit an unusual superconducting state, for which characteristic electron-phonon coupling has been suggested. Recent advances in angle-resolved photoemission spectroscopy (ARPES) have enabled high precision measurement of electron-phonon coupling in GICs. Coupling at the graphite-derived π bands was found to be highly anisotropic in the GIC KC8, being much stronger in the K-M than K- Γ directions. This unusual anisotropy is not predicted by previous superconducting theories. A much smaller anisotropy has also been seen in recent studies of K-doped graphene monolayers. In order to examine the presence of anisotropic coupling in the graphene-metal system, we have performed ARPES on the bilayer graphene GIC. We have found that C8RbC8 exhibits strong, anisotropic coupling, similar to that in GICs. The origin of this coupling, as well as its relation to possible superconductivity in ultrathin GICs is discussed.

  16. Formation and structural analysis of twisted bilayer graphene on Ni(111) thin films

    NASA Astrophysics Data System (ADS)

    Iwasaki, Takayuki; Zakharov, Alexei A.; Eelbo, Thomas; Waśniowska, Marta; Wiesendanger, Roland; Smet, Jurgen H.; Starke, Ulrich

    2014-07-01

    We synthesized twisted bilayer graphene on single crystalline Ni(111) thin films to analyze the statistical twist angle distribution on a large scale. The twisted bilayer graphene was formed by combining two growth methods, namely the catalytic surface reaction of hydrocarbons and carbon segregation from Ni. Low energy electron diffraction (LEED) investigations directly revealed dominant twist angles of 13°, 22°, 38°, and 47°. We show that the angle distribution is closely related to the sizes of Moiré superlattices which form at commensurate rotation angles. In addition to the commensurate angles, quasi-periodic Moiré structures were also formed in the vicinity of the dominant angles, confirmed by microscopic observations with low energy electron microscopy and scanning tunneling microscopy (STM). The quasi-periodic Moiré patterns are presumably caused by insufficient mobility of carbon atoms during the segregation growth while cooling. Micro-LEED studies reveal that the size of single twisted domains is below 400 nm. Atomic-scale characterization by STM indicates that the twisted layer grown by segregation is located underneath the layer grown by surface reaction, i.e. between the Ni surface and the top single-crystal graphene layer.

  17. Fractional Quantum Hall States in Bilayer Graphene Probed by Transconductance Fluctuations.

    PubMed

    Kim, Youngwook; Lee, Dong Su; Jung, Suyong; Skákalová, Viera; Taniguchi, T; Watanabe, K; Kim, Jun Sung; Smet, Jurgen H

    2015-11-11

    We have investigated fractional quantum Hall (QH) states in Bernal-stacked bilayer graphene using transconductance fluctuation measurements. A variety of odd-denominator fractional QH states with νQH → νQH + 2 symmetry, as previously reported, are observed. However, surprising is that also particle-hole symmetric states are clearly resolved in the same measurement set. We attribute their emergence to the reversal of orbital states in the octet level scheme induced by a strong local charge imbalance, which can be captured by the transconductance fluctuations. Also the even-denominator fractional QH state at filling -1/2 is observed. However, contrary to a previous study on a suspended graphene layer [ Ki et al. Nano Lett. 2014, 14 , 2135 ], the particle-hole symmetric state at filling 1/2 is detected as well. These observations suggest that the stability of both odd and even denominator fractional QH states is very sensitive to local transverse electric fields in bilayer graphene.

  18. 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.

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

    NASA Astrophysics Data System (ADS)

    Sawada, Keisuke; Ishii, Fumiyuki; Saito, Mineo

    2014-04-01

    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.

  20. Anomalous ballistic transport in disordered bilayer graphene: A Dirac semimetal induced by dimer vacancies

    NASA Astrophysics Data System (ADS)

    Van Tuan, Dinh; Roche, Stephan

    2016-01-01

    We report anomalous quantum transport features in bilayer graphene in the presence of a random distribution of structural vacancies. By using an efficient real-space Kubo-Greenwood transport methodology, the impact of a varying density of dimer versus nondimer vacancies is investigated in very large scale disordered models. While nondimer vacancies are shown to induce localization regimes, dimer vacancies result in an unexpected ballistic regime whose energy window surprisingly enlarges with increasing impurity density. Such counterintuitive phenomenon is explained by the formation of an effective linear dispersion in the bilayer band structure, which roots in the symmetry breaking effects driven by dimer vacancies, and provides a realization of Dirac semimetals in high dimension.

  1. Tunable Fermi level and hedgehog spin texture in gapped graphene

    NASA Astrophysics Data System (ADS)

    Varykhalov, A.; Sánchez-Barriga, J.; Marchenko, D.; Hlawenka, P.; Mandal, P. S.; Rader, O.

    2015-07-01

    Spin and pseudospin in graphene are known to interact under enhanced spin-orbit interaction giving rise to an in-plane Rashba spin texture. Here we show that Au-intercalated graphene on Fe(110) displays a large (~230 meV) bandgap with out-of-plane hedgehog-type spin reorientation around the gapped Dirac point. We identify two causes responsible. First, a giant Rashba effect (~70 meV splitting) away from the Dirac point and, second, the breaking of the six-fold graphene symmetry at the interface. This is demonstrated by a strong one-dimensional anisotropy of the graphene dispersion imposed by the two-fold-symmetric (110) substrate. Surprisingly, the graphene Fermi level is systematically tuned by the Au concentration and can be moved into the bandgap. We conclude that the out-of-plane spin texture is not only of fundamental interest but can be tuned at the Fermi level as a model for electrical gating of spin in a spintronic device.

  2. 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.

  3. 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.

  4. Plasmon-phonon coupling in graphene-hyperbolic bilayer heterostructures

    NASA Astrophysics Data System (ADS)

    Yin, Ge; Yuan, Jun; Jiang, Wei; Zhu, Jianfei; Ma, Yungui

    2016-11-01

    Polar dielectrics are important optical materials enabling the subwavelength manipulation of light in infrared due to their capability to excite phonon polaritons. In practice, it is highly desired to actively modify these hyperbolic phonon polaritons (HPPs) to optimize or tune the response of the device. In this work, we investigate the plasmonic material, a monolayer graphene, and study its hybrid structure with three kinds of hyperbolic thin films grown on SiO2 substrate. The inter-mode hybridization and their tunability have been thoroughly clarified from both the band dispersions and the mode patterns numerically calculated through a transfer matrix method. Our results show that these hybrid multilayer structures are of strong potentials for applications in plasmonic waveguides, modulators and detectors in infrared. Project supported by the National Natural Science Foundation of China (Grant No. 61271085) and the Natural Science Foundation of Zhejiang Province, China (Grant No. LR15F050001).

  5. Spin relaxation in bilayer graphene: the role of electron-electron scattering

    NASA Astrophysics Data System (ADS)

    Katiyar, Saurabh; Ghosh, Bahniman; Salimath, Akshay Kumar

    2016-02-01

    This paper investigates the influence of electron-electron scattering on spin relaxation length in bilayer graphene using semiclassical Monte Carlo simulation. Both D'yakonov-P'erel and Elliot-Yafet mechanisms are considered for spin relaxation. It is shown that spin relaxation length decreases by 17 % at 300 K on including electron-electron scattering. The reason of this variation in spin relaxation length is that the ensemble spin is modified upon an e-e collision, and also e-e scattering rate is greater than phonon scattering rate which causes change in spin transport profile.

  6. Observation of Distinct Electron-Phonon Couplings in Gated Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Malard, L. M.; Elias, D. C.; Alves, E. S.; Pimenta, M. A.

    2008-12-01

    A Raman study of a back gated bilayer graphene sample is presented. The changes in the Fermi level induced by charge transfer splits the Raman G band, hardening its higher component and softening the lower one. These two components are associated with the symmetric (S) and antisymmetric vibration (AS) of the atoms in the two layers, the later one becoming Raman active due to inversion symmetry breaking. The phonon hardening and softening are explained by considering the selective coupling of the S and AS phonons with interband and intraband electron-hole pairs.

  7. Intense and tunable second-harmonic generation in biased bilayer graphene

    NASA Astrophysics Data System (ADS)

    Brun, Søren J.; Pedersen, Thomas G.

    2015-05-01

    The centrosymmetric two-dimensional material bilayer graphene (BLG) does not show dipole-allowed second-harmonic generation (SHG) in its pristine form. However, the symmetry can be broken by applying an electric field perpendicular to the layer. Here, we present a theoretical study of SHG from biased BLG. We show that the sheet second-harmonic susceptibility reaches very large values of several hundred nm2/V in the midinfrared region. The SHG is tunable depending on the strength of the electric field. Furthermore, a strong, tunable double resonance appears in the spectrum. We believe that this study could spark interest in the nonlinear optical properties of biased BLG.

  8. Evidence for Interlayer Coupling and Moire Periodic Potentials in Twisted Bilayer Graphene

    DTIC Science & Technology

    2012-11-02

    published 2 November 2012) We report a study of the valence band dispersion of twisted bilayer graphene using angle-resolved photoemission spectroscopy...2012) P HY S I CA L R EV I EW LE T T E R S week ending 2 NOVEMBER 2012 0031-9007=12=109(18)=186807(6) 186807-1 2012 American Physical Society Report ...Documentation Page Form ApprovedOMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per

  9. Quasi-free-standing bilayer graphene nanoribbons probed by electronic transport

    NASA Astrophysics Data System (ADS)

    Miccoli, Ilio; Aprojanz, Johannes; Baringhaus, Jens; Lichtenstein, Timo; Galves, Lauren A.; Lopes, Joao Marcelo J.; Tegenkamp, Christoph

    2017-01-01

    Direct growth of graphene nanostructures by using concepts of self-assembly and intercalation without further lithography and transfer processes is beneficial for their integration into device applications. In this letter, we report on bilayer graphene nanoribbons, typically 100 nm in width, grown along step edges of SiC(0001) substrates. The ribbons are electrically decoupled from the substrate by an oxygen treatment. By means of a 4-tip STM system, the microscopic structure and transport properties were comprehensively studied. The ribbons reveal a robust hole concentration of around 1 × 1013 cm-2 and mobilities up to 700 cm2/Vs at room temperature. The comparably high mobilities are a consequence of interlayer hopping of the charge carriers. The transport is not limited by the step roughness; thus, this scalable process can be easily extended to arbitrarily shaped structures.

  10. Optical study of nonuniform quantum-Hall ferromagnetic states in bilayer and trilayer graphene

    NASA Astrophysics Data System (ADS)

    Barrette, Manuel; Côté, René

    2015-03-01

    The chiral two-dimensional electron gas in the N = 0 Landau level of a Bernal-stacked bilayer graphene is host to a variety of broken-symmetry ground states that can be described as layer, spin, or orbital quantum Hall ferromagnets (QHFs). At filling factors ν = 1 , 3 , an externally applied electric field between the two layers can induce a transition from uniform to nonuniform orbital QHF states with an helical or skyrmionic texture of electric dipoles. A similar skyrmionic texture can also arise in the N = 0 Landau level of an ABC-stacked trilayer graphene. In this talk, we discuss the optical properties of these textured ground states. We compute their electromagnetic absorption as well as the Kerr and Faraday rotations induced by their collective excitations and show that each textured phase has a distinct optical signature.

  11. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

    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.

  13. 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.

  14. Wide-gap semiconducting graphene from nitrogen-seeded SiC.

    PubMed

    Wang, F; Liu, G; Rothwell, S; Nevius, M; Tejeda, A; Taleb-Ibrahimi, A; Feldman, L C; Cohen, P I; Conrad, E H

    2013-10-09

    All carbon electronics based on graphene have been an elusive goal. For more than a decade, the inability to produce significant band-gaps in this material has prevented the development of graphene electronics. We demonstrate a new approach to produce semiconducting graphene that uses a submonolayer concentration of nitrogen on SiC sufficient to pin epitaxial graphene to the SiC interface as it grows. The resulting buckled graphene opens a band gap greater than 0.7 eV in the otherwise continuous metallic graphene sheet.

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

    NASA Astrophysics Data System (ADS)

    Nazemnezhad, Reza; Shokrollahi, Hassan; Hosseini-Hashemi, Shahrokh

    2014-05-01

    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.

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

    SciTech Connect

    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.

    2016-02-01

    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 104, and a tunable bandgap up to -100 meV at a displacement field of 0.9 V nm-1.

  17. 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.

  18. 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.

  19. Modification of thermal and electronic properties of bilayer graphene by using slow Na+ ions

    NASA Astrophysics Data System (ADS)

    Ryu, Mintae; Lee, Paengro; Kim, Jingul; Park, Heemin; Chung, Jinwook

    2016-12-01

    Bilayer graphene (BLG) has an extensive list of industrial applications in graphene-based nanodevices such as energy storage devices, flexible displays, and thermoelectric devices. By doping slow Na+ ions on Li-intercalated BLG, we find significantly improved thermal and electronic properties of BLG by using angle-resolved photoemission and high-resolution core level spectroscopy (HRCLS) with synchrotron photons. Our HRCLS data reveal that the adsorbed Na+ ions on a BLG produced by Li-intercalation through single layer graphene (SLG) spontaneously intercalate below the BLG, and substitute Li atoms to form Na-Si bonds at the SiC interface while preserving the same phase of BLG. This is in sharp contrast with no intercalation of Na+ ions on SLG though neutral Na atoms intercalate. The Na+-induced BLG is found to be stable upon heating up to T = 400 °C, but returns to SLG when heated at T d = 500 °C. The evolution of the π-bands upon doping the Na+ ions followed by thermal annealing shows that the carrier concentration of the π-band may be artificially controlled without damaging the Dirac nature of the π-electrons. The doubled desorption temperature from that (T d = 250 °C) of the Na-intercalated SLG together with the electronic stability of the Na+-intercalated BLG may find more practical and effective applications in advancing graphene-based thermoelectric devices and anode materials for rechargeable batteries.

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

    NASA Astrophysics Data System (ADS)

    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 104, and a tunable bandgap up to ˜100 meV at a displacement field of 0.9 V nm-1.

  1. Diastase induced green synthesis of bilayered reduced graphene oxide and its decoration with gold nanoparticles.

    PubMed

    Maddinedi, Sireesh Babu; Mandal, Badal Kumar; Patil, Sagar Hindurao; Andhalkar, Vaibhav Vilas; Ranjan, Shivendu; Dasgupta, Nandita

    2017-01-01

    In this paper, we report an enzyme dependent, green one-pot deoxygenation cum decoration method to synthesize diastase-conjugated reduced graphene oxide (DRG) nanosheets, DRG/gold nanoparticles (DRG/Au) composite. The DRG synthesis was completed in 7h under heating at 90°C on water bath. Selected area electron diffraction (SAED) and Atomic force microscopy (AFM) study has revealed the formation of bilayered reduced graphene oxide sheets. Transmission electron microscopy (TEM) images of DRG/Au composite have shown the uniform decoration of gold nanoparticles (AuNPs) onto the DRG nanosheet surface. Fourier transform infrared spectroscopy (FTIR) and Raman results additionally have shown the functionalization of enzyme molecules onto the DRG nanosheet surface after reduction making it as an effective platform towards the efficient binding of gold nanoparticles. In vitro cytotoxicity studies by MTT assay on A549 and HCT116 cell lines exhibited that the cytotoxicity of the prepared graphene oxide (GO), DRG and DRG/Au is dose dependant. These results have shown that this synthetic method is effective for the production of large scale graphene in a low cost, simple and green method. Since this process avoids the use of hazardous and toxic substances, the produced DRG/Au composites are likely to offer various potential applications in biology and medicine.

  2. Magneto-thermodynamic properties of gapped graphene-like structures

    NASA Astrophysics Data System (ADS)

    Yarmohammadi, Mohsen; Beig-Mohammadi, Maryam; Shirzadi, Bahram

    2017-02-01

    By applying the Green's function technique and using the tight-binding Hamiltonian model, thermodynamic properties of gapped graphene-like structures, including silicon carbide (SiC), boron nitride (BN) and beryllium monooxide (BeO) in the presence of a transverse magnetic field are investigated. In fact, we have studied electronic density of states (DOS), electronic heat capacity (EHC) and magnetic susceptibility (MS) in order to investigate the dynamics of Dirac fermions. At an applied certain value of magnetic field, the band gap width increases for SiC, BN and BeO structures with respect to the gapless graphene and a double peak appears in DOS with increasing of quantum states. On the other hand, the band gap size decreases with magnetic field. We have found that EHC and MS increase slightly at low temperatures with gap and magnetic field. Also, EHC and MS reach to their maximum value at a critical temperature point while an increase behavior has been observed for high temperatures significantly.

  3. 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.

  4. Spin asymmetric band gap opening in graphene by Fe adsorption

    NASA Astrophysics Data System (ADS)

    del Castillo, E.; Cargnoni, F.; Achilli, S.; Tantardini, G. F.; Trioni, M. I.

    2015-04-01

    The adsorption of Fe atom on graphene is studied by first-principles Density Functional Theory. The structural, electronic, and magnetic properties are analyzed at different coverages, all preserving C6v symmetry for the Fe adatom. We observed that binding energies, magnetic moments, and adsorption distances rapidly converge as the size of the supercell increases. Among the considered supercells, those constituted by 3n graphene unit cells show a very peculiar behavior: the adsorption of a Fe atom induces the opening of a spin-dependent gap in the band structure. In particular, the gap amounts to tenths of eV in the majority spin component, while in the minority one it has a width of about 1 eV for the 3 × 3 supercell and remains significant even at very low coverages (0.25 eV for θ ≃ 2%). The charge redistribution upon Fe adsorption has also been analyzed according to state of the art formalisms indicating an appreciable charge transfer from Fe to the graphene layer.

  5. Van Hove singularities in doped twisted graphene bilayers studied by scanning tunneling spectroscopy

    NASA Astrophysics Data System (ADS)

    Cherkez, V.; de Laissardière, G. Trambly; Mallet, P.; Veuillen, J.-Y.

    2015-04-01

    The effect of electron doping on the van Hove singularities (vHs) which develop in twisted graphene bilayers (tBLs) is studied for a broad range of rotation angles θ (1 .5∘<θ <15∘) by means of scanning tunneling microscopy and spectroscopy. Bilayer and trilayer graphene islands were grown on the 6H-SiC(000-1) (3 ×3 ) surface, which results in tBLs doped in the 1012cm-2 range by charge transfer from the substrate. For large angles, doping manifests in a strong asymmetry of the positions of the upper (in empty states) and lower (in occupied states) vHs with respect to the Fermi level. The splitting of these vHs energies is found essentially independent of doping for the whole range of θ values, but the center of theses vHs shifts towards negative energies with increasing electron doping. Consequently, the upper vHs crosses the Fermi level for smaller angles (around 3∘ ). The analysis of the data performed using tight-binding calculations and simple electrostatic considerations shows that the interlayer bias remains small (<100 mV ) for the doping level resulting from the interfacial charge transfer (≃5 ×1012cm-2) .

  6. Comparative study on the nonlinear properties of bilayer graphene and silicene under tension

    NASA Astrophysics Data System (ADS)

    Xu, Peng; Yu, Zhongyuan; Yang, Chuanghua; Lu, Pengfei; Liu, Yumin; Ye, Han; Gao, Tao

    2014-11-01

    Atomic structures and nonlinear properties of single layer graphene (SLG), bilayer graphene (BLG), single layer silicene (SLS), and bilayer silicene (BLS) under equiaxial tension and uniaxial tensions along armchair and zigzag directions have been investigated comparatively using first-principles calculations. First, we have calculated the dependences of atomic structures (bond length, interlayer distance, and buckling height) of BLG and BLS on strain under three types of tensions. There exists the weak Van der Waals interaction between two layers of BLG and the interlayer distance is not variable with strain for three types of tensions. However, the interlayer of BLS is the covalent bond interaction, and the distance decreases with the increasing strain for three types of tensions. The continuum description of elastic response is formulated by expanding the elastic strain energy density in a Taylor series in strain truncated after the third-order term. The in-plane second- and third-order elastic constants of BLG and BLS have been obtained by fitting to the strain energy density versus Lagrangian strain relationships. The results show the in-plane stiffnesses of BLG and BLS become slightly larger than those of their single layer counterparts. In spite of the interlayer Si-Si covalent bond between two layers of BLS, its stiffness is still much less than BLG and SLG. Poisson's ratios of BLG and BLS basically maintain unchanged compared to their single layer counterparts.

  7. Interlayer vacancy defects in AA-stacked bilayer graphene: Density functional theory predictions.

    PubMed

    Vuong, Amanda; Trevethan, Tom; Latham, Christopher; Ewels, Chris; Erbahar, Doğan; Briddon, Patrick; Rayson, Mark; Heggie, Malcolm

    2017-02-09

    AA-stacked graphite and closely related structures, where carbon atoms are located in registry in adjacent graphene layers, are a feature of graphitic systems including twisted and folded bilayer graphene, and turbostratic graphite. We present the results of ab initio density functional theory calculations performed to investigate the complexes that are formed from the binding of vacancy defects across neighbouring layers in AA-stacked bilayers. As with AB stacking, the carbon atoms surrounding lattice vacancies can form interlayer structures with sp2 bonding that are lower in energy than in-plane reconstructions. The sp2 interlayer bonding of adjacent multivacancy defects in registry creates a type of stable sp2 bonded 'wormhole' or tunnel defect between the layers. We also identify a new class of 'mezzanine' structure characterised by sp3 interlayer bonding, resembling a prismatic vacancy loop. The V6 hexavacancy variant, where six sp3 carbon atoms sit midway between two carbon layers and bond to both, is substantially more stable than any other vacancy aggregate in AA-stacked layers. Our focus is on vacancy generation and aggregation in the absence of extreme temperatures or intense beams.

  8. Interlayer vacancy defects in AA-stacked bilayer graphene: density functional theory predictions

    NASA Astrophysics Data System (ADS)

    Vuong, A.; Trevethan, T.; Latham, C. D.; Ewels, C. P.; Erbahar, D.; Briddon, P. R.; Rayson, M. J.; Heggie, M. I.

    2017-04-01

    AA-stacked graphite and closely related structures, where carbon atoms are located in registry in adjacent graphene layers, are a feature of graphitic systems including twisted and folded bilayer graphene, and turbostratic graphite. We present the results of ab initio density functional theory calculations performed to investigate the complexes that are formed from the binding of vacancy defects across neighbouring layers in AA-stacked bilayers. As with AB stacking, the carbon atoms surrounding lattice vacancies can form interlayer structures with sp 2 bonding that are lower in energy than in-plane reconstructions. The sp 2 interlayer bonding of adjacent multivacancy defects in registry creates a type of stable sp 2 bonded ‘wormhole’ or tunnel defect between the layers. We also identify a new class of ‘mezzanine’ structure characterised by sp 3 interlayer bonding, resembling a prismatic vacancy loop. The V 6 hexavacancy variant, where six sp 3 carbon atoms sit midway between two carbon layers and bond to both, is substantially more stable than any other vacancy aggregate in AA-stacked layers. Our focus is on vacancy generation and aggregation in the absence of extreme temperatures or intense beams.

  9. Robust fractional quantum Hall effect in the N=2 Landau level in bilayer graphene

    PubMed Central

    Diankov, Georgi; Liang, Chi-Te; Amet, François; Gallagher, Patrick; Lee, Menyoung; Bestwick, Andrew J.; Tharratt, Kevin; Coniglio, William; Jaroszynski, Jan; Watanabe, Kenji; Taniguchi, Takashi; Goldhaber-Gordon, David

    2016-01-01

    The fractional quantum Hall effect is a canonical example of electron–electron interactions producing new ground states in many-body systems. Most fractional quantum Hall studies have focussed on the lowest Landau level, whose fractional states are successfully explained by the composite fermion model. In the widely studied GaAs-based system, the composite fermion picture is thought to become unstable for the N≥2 Landau level, where competing many-body phases have been observed. Here we report magneto-resistance measurements of fractional quantum Hall states in the N=2 Landau level (filling factors 4<|ν|<8) in bilayer graphene. In contrast with recent observations of particle–hole asymmetry in the N=0/N=1 Landau levels of bilayer graphene, the fractional quantum Hall states we observe in the N=2 Landau level obey particle–hole symmetry within the fully symmetry-broken Landau level. Possible alternative ground states other than the composite fermions are discussed. PMID:28000663

  10. 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.

  11. Energy band gaps in graphene nanoribbons with corners

    NASA Astrophysics Data System (ADS)

    Szczȩśniak, Dominik; Durajski, Artur P.; Khater, Antoine; Ghader, Doried

    2016-05-01

    In the present paper, we study the relation between the band gap size and the corner-corner length in representative chevron-shaped graphene nanoribbons (CGNRs) with 120° and 150° corner edges. The direct physical insight into the electronic properties of CGNRs is provided within the tight-binding model with phenomenological edge parameters, developed against recent first-principle results. We show that the analyzed CGNRs exhibit inverse relation between their band gaps and corner-corner lengths, and that they do not present a metal-insulator transition when the chemical edge modifications are introduced. Our results also suggest that the band gap width for the CGNRs is predominantly governed by the armchair edge effects, and is tunable through edge modifications with foreign atoms dressing.

  12. Gap engineering in strained fold-like armchair graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Torres, V.; León, C.; Faria, D.; Latgé, A.

    2017-01-01

    Strained fold-like deformations on armchair graphene nanoribbons (AGNRs) can be properly engineered in experimental setups, and could lead to a controlling tool for gaps and transport properties. Here, we analyze the electronic properties of folded AGNRs relating to the electronic responses and the mechanical deformation. An important and universal parameter for the gap engineering is the ribbon percent-width variation, i.e., the difference between the deformed and undeformed ribbon widths. AGNRs band gap can be tuned mechanically in a well-defined bounded range of energy values, eventually leading to a metallic system. This characteristic provides a controllable degree of freedom that allows manipulation of electronic currents. We show that the numerical results are analytically predicted by solving the Dirac equation for the strained system.

  13. 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.

  14. Electron-phonon scattering and in-plane electric conductivity in twisted bilayer graphene

    NASA Astrophysics Data System (ADS)

    Ray, N.; Fleischmann, M.; Weckbecker, D.; Sharma, S.; Pankratov, O.; Shallcross, S.

    2016-12-01

    We have surveyed the in-plane transport properties of the graphene twist bilayer using (i) a low-energy effective Hamiltonian for the underlying electronic structure, (ii) an isotropic elastic phonon model, and (iii) the linear Boltzmann equation for elastic electron-phonon scattering. We find that transport in the twist bilayer is profoundly sensitive to the rotation angle of the constituent layers. Similar to the electronic structure of the twist bilayer, the transport is qualitatively different in three distinct angle regimes. At large angles (θ >≈10∘ ) and at temperatures below an interlayer Bloch-Grüneisen temperature of ≈10 K, the conductivity is independent of the twist angle, i.e., the layers are fully decoupled. Above this temperature the layers, even though decoupled in the ground state, are recoupled by electron-phonon scattering and the transport is different both from single-layer graphene as well as the Bernal bilayer. In the small-angle regime θ <≈2∘ , the conductivity drops by two orders of magnitude and develops a rich energy dependence, reflecting the complexity of the underlying topological changes (Lifshitz transitions) of the Fermi surface. At intermediate angles, the conductivity decreases continuously as the twist angle is reduced, while the energy dependence of the conductivity presents two sharp transitions, that occur at specific angle-dependent energies, and that may be related to (i) the well-studied van Hove singularity of the twist bilayer and (ii) a Lifshitz transition that occurs when trigonally placed electron pockets decorate the strongly warped Dirac cone. Interestingly, we find that, while the electron-phonon scattering is dominated by layer symmetric flexural phonons in the small-angle limit, at large angles, in contrast, it is the layer antisymmetric flexural mode that is most important. We examine the role of a layer perpendicular electric field finding that it affects the conductivity strongly at low temperatures

  15. Determining the nature of the gap in semiconducting graphene

    NASA Astrophysics Data System (ADS)

    Prestigiacomo, J. C.; Nath, A.; Osofsky, M. S.; Hernández, S. C.; Wheeler, V. D.; Walton, S. G.; Gaskill, D. K.

    2017-02-01

    Since its discovery, graphene has held great promise as a two-dimensional (2D) metal with massless carriers and, thus, extremely high-mobility that is due to the character of the band structure that results in the so-called Dirac cone for the ideal, perfectly ordered crystal structure. This promise has led to only limited electronic device applications due to the lack of an energy gap which prevents the formation of conventional device geometries. Thus, several schemes for inducing a semiconductor band gap in graphene have been explored. These methods do result in samples whose resistivity increases with decreasing temperature, similar to the temperature dependence of a semiconductor. However, this temperature dependence can also be caused by highly diffusive transport that, in highly disordered materials, is caused by Anderson-Mott localization and which is not desirable for conventional device applications. In this letter, we demonstrate that in the diffusive case, the conventional description of the insulating state is inadequate and demonstrate a method for determining whether such transport behavior is due to a conventional semiconductor band gap.

  16. Determining the nature of the gap in semiconducting graphene

    PubMed Central

    Prestigiacomo, J. C.; Nath, A.; Osofsky, M. S.; Hernández, S. C.; Wheeler, V. D.; Walton, S. G.; Gaskill, D. K.

    2017-01-01

    Since its discovery, graphene has held great promise as a two-dimensional (2D) metal with massless carriers and, thus, extremely high-mobility that is due to the character of the band structure that results in the so-called Dirac cone for the ideal, perfectly ordered crystal structure. This promise has led to only limited electronic device applications due to the lack of an energy gap which prevents the formation of conventional device geometries. Thus, several schemes for inducing a semiconductor band gap in graphene have been explored. These methods do result in samples whose resistivity increases with decreasing temperature, similar to the temperature dependence of a semiconductor. However, this temperature dependence can also be caused by highly diffusive transport that, in highly disordered materials, is caused by Anderson-Mott localization and which is not desirable for conventional device applications. In this letter, we demonstrate that in the diffusive case, the conventional description of the insulating state is inadequate and demonstrate a method for determining whether such transport behavior is due to a conventional semiconductor band gap. PMID:28181521

  17. Determining the nature of the gap in semiconducting graphene.

    PubMed

    Prestigiacomo, J C; Nath, A; Osofsky, M S; Hernández, S C; Wheeler, V D; Walton, S G; Gaskill, D K

    2017-02-09

    Since its discovery, graphene has held great promise as a two-dimensional (2D) metal with massless carriers and, thus, extremely high-mobility that is due to the character of the band structure that results in the so-called Dirac cone for the ideal, perfectly ordered crystal structure. This promise has led to only limited electronic device applications due to the lack of an energy gap which prevents the formation of conventional device geometries. Thus, several schemes for inducing a semiconductor band gap in graphene have been explored. These methods do result in samples whose resistivity increases with decreasing temperature, similar to the temperature dependence of a semiconductor. However, this temperature dependence can also be caused by highly diffusive transport that, in highly disordered materials, is caused by Anderson-Mott localization and which is not desirable for conventional device applications. In this letter, we demonstrate that in the diffusive case, the conventional description of the insulating state is inadequate and demonstrate a method for determining whether such transport behavior is due to a conventional semiconductor band gap.

  18. Step-edge-induced resistance anisotropy in quasi-free-standing bilayer chemical vapor deposition graphene on SiC

    SciTech Connect

    Ciuk, Tymoteusz; Cakmakyapan, Semih; Ozbay, Ekmel; Caban, Piotr; Grodecki, Kacper; Pasternak, Iwona; Strupinski, Wlodek; Krajewska, Aleksandra; Szmidt, Jan

    2014-09-28

    The transport properties of quasi-free-standing (QFS) bilayer graphene on SiC depend on a range of scattering mechanisms. Most of them are isotropic in nature. However, the SiC substrate morphology marked by a distinctive pattern of the terraces gives rise to an anisotropy in graphene's sheet resistance, which may be considered an additional scattering mechanism. At a technological level, the growth-preceding in situ etching of the SiC surface promotes step bunching which results in macro steps ~10 nm in height. In this report, we study the qualitative and quantitative effects of SiC steps edges on the resistance of epitaxial graphene grown by chemical vapor deposition. We experimentally determine the value of step edge resistivity in hydrogen-intercalated QFS-bilayer graphene to be ~190 Ωμm for step height hS = 10 nm and provide proof that it cannot originate from mechanical deformation of graphene but is likely to arise from lowered carrier concentration in the step area. Our results are confronted with the previously reported values of the step edge resistivity in monolayer graphene over SiC atomic steps. In our analysis, we focus on large-scale, statistical properties to foster the scalable technology of industrial graphene for electronics and sensor applications.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    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.

  20. 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

  1. Adsorption, intercalation and diffusion of Na on defective bilayer graphene: a computational study

    NASA Astrophysics Data System (ADS)

    Yang, Shaobin; Li, Sinan; Tang, Shuwei; Shen, Ding; Dong, Wei; Sun, Wen

    2017-04-01

    The interaction between inserted/adsorbed Na and the structures of pristine and defective bilayer graphene (BLG) with Stone-Wales (SW), mono-vacancy (MV) and divacancy (DV) defects have been investigated by the first-principles calculations. The computational results show that the intercalation of Na in the interlayer of BLG with DV defects is more energetically favorable than Na adsorption on the surface. The lower formation energies of Na adsorption/intercalation on/in the BLG with DV defect reflect a stronger attraction between Na and DV defects compared with MV and SW defects. A significant charge transfer occurs from Na to near graphene layer(s) of BLG. The adsorption and intercalation of Na not only induce more significant structural distortion into the upper layer graphene with SW defect but also spin polarization for MV and DV defects. The results of migration energy barriers show that Na prefers to diffuse toward the DV site, and the diffusion outward the DV site is more difficult in comparison with the SW and MV defects. As a consequence, more Na atoms would be trapped in the region of the DV defect, leading to larger capacity than SW and MV defects.

  2. Raman identification of edge alignment of bilayer graphene down to the nanometer scale.

    PubMed

    Zhang, Xin; Li, Qiao-Qiao; Han, Wen-Peng; Lu, Yan; Shi, Wei; Wu, Jiang-Bin; Mikhaylushkin, Arkady S; Tan, Ping-Heng

    2014-07-07

    The ideal edges of bilayer graphene (BLG) are that the edges of the top and bottom graphene layers (GLs) of BLG are well-aligned. Actually, the alignment distance between the edges of the top and bottom GLs of a real BLG can be as large as the submicrometer scale or as small as zero, which cannot be distinguished using an optical microscope. Here, we present a detailed Raman study on BLG at its edges. If the alignment distance of the top and bottom GLs of BLG is larger than the laser spot, the measured D mode at the edge of the top GL of BLG shows a similar spectral profile to that of disordered BLG. If the alignment distance is smaller than the laser spot, the D mode at a real BLG edge shows three typical spectral profiles similar to that at the edge of SLG, that of the well-aligned edge of BLG, or a combination of both. We show the sensitivity and ability of Raman spectroscopy to acquire the alignment distance between two edges of top and bottom GLs of BLG as small as several nanometers, which is far beyond the diffraction limit of a laser spot. This work opens the possibility to probe the edge alignment of multi-layer graphene.

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

    NASA Astrophysics Data System (ADS)

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

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

  4. Infrared nano-imaging and nano-spectroscopy of surface plasmons in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Fei, Zhe; Iwinski, Eric G.; Rodin, Alesandr S.; Wagner, Martin; Liu, Mengkun; Dai, Siyuan; Goldflam, Michael D.; Bao, Wenzhong; Lee, Yongjin; Lau, Chun Ning; Kailmann, Fritz; Castro-Neto, Antonio H.; Zhang, Lingfeng M.; Fogler, Michael M.; Basov, Dimitri N.

    2014-03-01

    Bernal stacking bilayer graphene (BLG) has demonstrated its capability for application in a wide range of fields including electronics, photonics and energy engineering. So far, plasmonic properties of BLG have not been fully explored experimentally despite broad interests. Here, we report infrared nano-imaging and nano-spectroscopy of surface plasmons (SPs) in BLG. We found that BLG also supported gate-tunable SPs in the mid-infrared range with nevertheless smaller wavelength compared to equally doped single-layer graphene (SLG) and randomly stacked double-layer graphene (DLG). In addition, the coupling between BLG plasmons and SiO2 phonons appeared much weaker compared to SLG plasmons. Further analysis indicated that these observations about SPs in BLG were attributed to interlayer coupling that affects strongly the electronic structure. Our work uncovered all the essential characteristics of BLG plasmons, and suggested the possibility of developing carbon-based plasmonic circuits where SLG, BLG and DLG are all functioning building blocks.

  5. Edge effects on band gap energy in bilayer 2H-MoS2 under uniaxial strain

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

    The potential of ultrathin MoS2 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 MoS2 film. In this study, a bilayer MoS2 supercell is constructed differently from the prototypical unit cell in order to investigate the layer-dependent electronic band gap energy in a bilayer MoS2 film under uniaxial mechanical deformations. The supercell contains an MoS2 bottom layer and a relatively narrower top layer (nanoribbon with free edges) as a simplified model to simulate the as-grown bilayer MoS2 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 MoS2 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.

  6. Band Gap Tuning of Armchair Graphene Nanoribbons by Using Antidotes

    NASA Astrophysics Data System (ADS)

    Zoghi, Milad; Goharrizi, Arash Yazdanpanah; Saremi, Mehdi

    2017-01-01

    The electronic properties of armchair graphene nanoribbons (AGNRs) can be changed by creating antidotes within the pristine ribbons and producing antidote super lattice AGNRs (ASL-AGNRs). In the present work, band gap tuning of ASL-AGNRs is investigated by varying the width of ribbons ( d W) and the distance between antidotes ( d L) for five different antidote topologies. Numerical tight-binding model is applied to obtain the band structure of the ribbons. Based on our results, it is found that the band gap of ASL-AGNRs can be increased or decreased in different cases. Furthermore, changing the width of ribbons generally results in more predictable␣band gap profiles compared to the variation of distance between antidotes. Consequently, by opting appropriate antidote topologies and dimensional parameters ( d W and d L), it is possible to gain a desired band gap size. This can be considered as an alternative solution in design of electronic and optoelectronic devices where tunable band gap values are needed.

  7. Origin of band gaps in graphene on hexagonal boron nitride

    PubMed Central

    Jung, Jeil; DaSilva, Ashley M.; MacDonald, Allan H.; Adam, Shaffique

    2015-01-01

    Recent progress in preparing well-controlled two-dimensional van der Waals heterojunctions has opened up a new frontier in materials physics. Here we address the intriguing energy gaps that are sometimes observed when a graphene sheet is placed on a hexagonal boron nitride substrate, demonstrating that they are produced by an interesting interplay between structural and electronic properties, including electronic many-body exchange interactions. Our theory is able to explain the observed gap behaviour by accounting first for the structural relaxation of graphene’s carbon atoms when placed on a boron nitride substrate, and then for the influence of the substrate on low-energy π-electrons located at relaxed carbon atom sites. The methods we employ can be applied to many other van der Waals heterojunctions. PMID:25695638

  8. Hydrogenated monolayer graphene with reversible and tunable wide band gap and its field-effect transistor

    PubMed Central

    Son, Jangyup; Lee, Soogil; Kim, Sang Jin; Park, Byung Cheol; Lee, Han-Koo; Kim, Sanghoon; Kim, Jae Hoon; Hong, Byung Hee; Hong, Jongill

    2016-01-01

    Graphene is currently at the forefront of cutting-edge science and technology due to exceptional electronic, optical, mechanical, and thermal properties. However, the absence of a sizeable band gap in graphene has been a major obstacle for application. To open and control a band gap in functionalized graphene, several gapping strategies have been developed. In particular, hydrogen plasma treatment has triggered a great scientific interest, because it has been known to be an efficient way to modify the surface of single-layered graphene and to apply for standard wafer-scale fabrication. Here we show a monolayer chemical-vapour-deposited graphene hydrogenated by indirect hydrogen plasma without structural defect and we demonstrate that a band gap can be tuned as wide as 3.9 eV by varying hydrogen coverage. We also show a hydrogenated graphene field-effect transistor, showing that on/off ratio changes over three orders of magnitude at room temperature. PMID:27830748

  9. Hydrogenated monolayer graphene with reversible and tunable wide band gap and its field-effect transistor

    NASA Astrophysics Data System (ADS)

    Son, Jangyup; Lee, Soogil; Kim, Sang Jin; Park, Byung Cheol; Lee, Han-Koo; Kim, Sanghoon; Kim, Jae Hoon; Hong, Byung Hee; Hong, Jongill

    2016-11-01

    Graphene is currently at the forefront of cutting-edge science and technology due to exceptional electronic, optical, mechanical, and thermal properties. However, the absence of a sizeable band gap in graphene has been a major obstacle for application. To open and control a band gap in functionalized graphene, several gapping strategies have been developed. In particular, hydrogen plasma treatment has triggered a great scientific interest, because it has been known to be an efficient way to modify the surface of single-layered graphene and to apply for standard wafer-scale fabrication. Here we show a monolayer chemical-vapour-deposited graphene hydrogenated by indirect hydrogen plasma without structural defect and we demonstrate that a band gap can be tuned as wide as 3.9 eV by varying hydrogen coverage. We also show a hydrogenated graphene field-effect transistor, showing that on/off ratio changes over three orders of magnitude at room temperature.

  10. 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.

  11. 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.

  12. Modification of thermal and electronic properties of bilayer graphene by using slow Na(+) ions.

    PubMed

    Ryu, Mintae; Lee, Paengro; Kim, Jingul; Park, Heemin; Chung, Jinwook

    2016-12-02

    Bilayer graphene (BLG) has an extensive list of industrial applications in graphene-based nanodevices such as energy storage devices, flexible displays, and thermoelectric devices. By doping slow Na(+) ions on Li-intercalated BLG, we find significantly improved thermal and electronic properties of BLG by using angle-resolved photoemission and high-resolution core level spectroscopy (HRCLS) with synchrotron photons. Our HRCLS data reveal that the adsorbed Na(+) ions on a BLG produced by Li-intercalation through single layer graphene (SLG) spontaneously intercalate below the BLG, and substitute Li atoms to form Na-Si bonds at the SiC interface while preserving the same phase of BLG. This is in sharp contrast with no intercalation of Na(+) ions on SLG though neutral Na atoms intercalate. The Na(+)-induced BLG is found to be stable upon heating up to T = 400 °C, but returns to SLG when heated at T d = 500 °C. The evolution of the π-bands upon doping the Na(+) ions followed by thermal annealing shows that the carrier concentration of the π-band may be artificially controlled without damaging the Dirac nature of the π-electrons. The doubled desorption temperature from that (T d = 250 °C) of the Na-intercalated SLG together with the electronic stability of the Na(+)-intercalated BLG may find more practical and effective applications in advancing graphene-based thermoelectric devices and anode materials for rechargeable batteries.

  13. Low-dimensional gap plasmons for enhanced light-graphene interactions

    PubMed Central

    Kim, Yunjung; Yu, Sunkyu; Park, Namkyoo

    2017-01-01

    Graphene plasmonics has become a highlighted research area due to the outstanding properties of deep-subwavelength plasmon excitation, long relaxation time, and electro-optical tunability. Although the giant conductivity of a graphene layer enables the low-dimensional confinement of light, the atomic scale of the layer thickness is severely mismatched with optical mode sizes, which impedes the efficient tuning of graphene plasmon modes from the degraded light-graphene overlap. Inspired by gap plasmon modes in noble metals, here we propose low-dimensional hybrid graphene gap plasmon waves for large light-graphene overlap factor. We show that gap plasmon waves exhibit improved in-plane and out-of-plane field concentrations on graphene compared to those of edge or wire-like graphene plasmons. By adjusting the chemical property of the graphene layer, efficient and linear modulation of hybrid graphene gap plasmon modes is also achieved. Our results provide potential opportunities to low-dimensional graphene plasmonic devices with strong tunability. PMID:28240230

  14. Low-dimensional gap plasmons for enhanced light-graphene interactions.

    PubMed

    Kim, Yunjung; Yu, Sunkyu; Park, Namkyoo

    2017-02-27

    Graphene plasmonics has become a highlighted research area due to the outstanding properties of deep-subwavelength plasmon excitation, long relaxation time, and electro-optical tunability. Although the giant conductivity of a graphene layer enables the low-dimensional confinement of light, the atomic scale of the layer thickness is severely mismatched with optical mode sizes, which impedes the efficient tuning of graphene plasmon modes from the degraded light-graphene overlap. Inspired by gap plasmon modes in noble metals, here we propose low-dimensional hybrid graphene gap plasmon waves for large light-graphene overlap factor. We show that gap plasmon waves exhibit improved in-plane and out-of-plane field concentrations on graphene compared to those of edge or wire-like graphene plasmons. By adjusting the chemical property of the graphene layer, efficient and linear modulation of hybrid graphene gap plasmon modes is also achieved. Our results provide potential opportunities to low-dimensional graphene plasmonic devices with strong tunability.

  15. Low-dimensional gap plasmons for enhanced light-graphene interactions

    NASA Astrophysics Data System (ADS)

    Kim, Yunjung; Yu, Sunkyu; Park, Namkyoo

    2017-02-01

    Graphene plasmonics has become a highlighted research area due to the outstanding properties of deep-subwavelength plasmon excitation, long relaxation time, and electro-optical tunability. Although the giant conductivity of a graphene layer enables the low-dimensional confinement of light, the atomic scale of the layer thickness is severely mismatched with optical mode sizes, which impedes the efficient tuning of graphene plasmon modes from the degraded light-graphene overlap. Inspired by gap plasmon modes in noble metals, here we propose low-dimensional hybrid graphene gap plasmon waves for large light-graphene overlap factor. We show that gap plasmon waves exhibit improved in-plane and out-of-plane field concentrations on graphene compared to those of edge or wire-like graphene plasmons. By adjusting the chemical property of the graphene layer, efficient and linear modulation of hybrid graphene gap plasmon modes is also achieved. Our results provide potential opportunities to low-dimensional graphene plasmonic devices with strong tunability.

  16. 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.

  17. Charge-Density Wave in Ca-Intercalated Bilayer Graphene Induced by Commensurate Lattice Matching

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    We report the emergence of a charge-density wave (CDW) in Ca-intercalated bilayer graphene (C6Ca C6 ), the thinnest limit of superconducting C6Ca , observed by low-temperature, high-magnetic-field scanning tunneling microscopy or spectroscopy, and angle-resolved photoemission spectroscopy. While the possible superconductivity was not observed in epitaxially grown C6Ca C6 on a SiC substrate, a CDW order different from that observed on the surface of bulk C6Ca was observed. It is inferred that the CDW state is induced by the potential modulation due to the commensurate lattice matching between the C6Ca C6 film and the SiC substrate.

  18. Mapping of Bernal and non-Bernal stacking domains in bilayer graphene using infrared nanoscopy.

    PubMed

    Jeong, Gyouil; Choi, Boogeon; Kim, Deok-Soo; Ahn, Seongjin; Park, Baekwon; Kang, Jin Hyoun; Min, Hongki; Hong, Byung Hee; Kim, Zee Hwan

    2017-03-23

    Bilayer graphene (BLG) shows great potential as a new material for opto-electronic devices because its bandgap can be controlled by varying the stacking orders, as well as by applying an external electric field. An imaging technique that can visualize and characterize various stacking domains in BLG may greatly help in fully utilizing such properties of BLG. Here we demonstrate that infrared (IR) scattering-type scanning near-field optical microscopy (sSNOM) can visualize Bernal and non-Bernal stacking domains of BLG, based on the stacking-specific inter- and intra-band optical conductivities. The method enables nanometric mapping of stacking domains in BLG on dielectric substrates, augmenting current limitations of Raman spectroscopy and electron microscopy techniques for the structural characterization of BLG.

  19. Threshold voltage roll-off modelling of bilayer graphene field-effect transistors

    NASA Astrophysics Data System (ADS)

    Saeidmanesh, M.; Ismail, Razali; Khaledian, M.; Karimi, H.; Akbari, E.

    2013-12-01

    An analytical model is presented for threshold voltage roll-off of double gate bilayer graphene field-effect transistors. To this end, threshold voltage models of short- and long-channel states have been developed. In the short-channel case, front and back gate potential distributions have been modelled and used. In addition, the tunnelling probability is modelled and its effect is taken into consideration in the potential distribution model. To evaluate the accuracy of the potential model, FlexPDE software is employed with proper boundary conditions and a good agreement is observed. Using the proposed models, the effect of several structural parameters on the threshold voltage and its roll-off are studied at room temperature.

  20. Analytical study of subthreshold behaviour of double gate bilayer graphene field effect transistors

    NASA Astrophysics Data System (ADS)

    Saeidmanesh, M.; Khaledian, M.; Ghadiry, M.; Ismail, Razali

    2014-11-01

    In this paper, several analytical models have been developed for 2-D potential distribution, subthreshold current, drain induced barrier lowering (DIBL), and subthreshold-slope (SS) to study the subthreshold behaviour of bilayer graphene filed effect transistors (BLG-FETs). The models are grounded on the basis of the exact solution of the two-dimensional Poisson’s equation while the quantum capacitance effect has been considered throughout the models. The accuracy of the potential distribution model is verified by its analytical results that agree well with those of the FlexPDE Poisson's equation solver program. In addition, the effects of the channel length, the oxide thickness, quantum capacitance, and gate biases on subthreshold parameters of BLG-FETs have been explored and the results are compared with those of the silicon FETs.

  1. 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.

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

    PubMed

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

    2016-02-11

    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.

  3. 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.

  4. Phase diagram of a graphene bilayer in the zero-energy Landau level

    NASA Astrophysics Data System (ADS)

    Knothe, Angelika; Jolicoeur, Thierry

    2016-12-01

    Bilayer graphene under a magnetic field has an octet of quasidegenerate levels due to spin, valley, and orbital degeneracies. This zero-energy Landau level is resolved into several incompressible states whose nature is still elusive. We use a Hartree-Fock treatment of a realistic tight-binding four-band model to understand the quantum ferromagnetism phenomena expected for integer fillings of the octet levels. We include the exchange interaction with filled Landau levels below the octet states. This Lamb-shift-like effect contributes to the orbital splitting of the octet. We give phase diagrams as a function of applied bias and magnetic field. Some of our findings are in agreement with experiments. We discuss the possible appearance of phases with orbital coherence.

  5. Magnetic field dependence of energy levels in biased bilayer graphene quantum dots

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

    Using the tight-binding approach, we study the influence of a perpendicular magnetic field on the energy levels of hexagonal, triangular, and circular bilayer graphene (BLG) quantum dots (QDs) with zigzag and armchair edges. We obtain the energy levels for AB (Bernal)-stacked BLG QDs in both the absence and the presence of a perpendicular electric field (i.e., biased BLG QDs). We find different regions in the spectrum of biased QDs with respect to the crossing point between the lowest-electron and -hole Landau levels of a biased BLG sheet. Those different regions correspond to electron states that are localized at the center, edge, or corner of the BLG QD. Quantum Hall corner states are found to be absent in circular BLG QDs. The spatial symmetry of the carrier density distribution is related to the symmetry of the confinement potential, the position of zigzag edges, and the presence or absence of interlayer inversion symmetry.

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

    NASA Astrophysics Data System (ADS)

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

    2014-07-01

    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.

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

    PubMed Central

    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

  8. 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.

  9. A grand canonical Monte Carlo study of SO2 capture using functionalized bilayer graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Maurya, Manish; Singh, Jayant K.

    2017-01-01

    Grand canonical Monte Carlo (GCMC) simulation is used to study the adsorption of pure SO2 using a functionalized bilayer graphene nanoribbon (GNR) at 303 K. The functional groups considered in this work are OH, COOH, NH2, NO2, and CH3. The mole percent of functionalization considered in this work is in the range of 3.125%-6.25%. GCMC simulation is further used to study the selective adsorption of SO2 from binary and ternary mixtures of SO2, CO2, and N2, of variable composition using the functionalized bilayer graphene nanoribbon at 303 K. This study shows that the adsorption and selectivity of SO2 increase after the functionalization of the nanoribbon compared to the hydrogen terminated nanoribbon. The order of adsorption capacity and selectivity of the functionalized nanoribbon is found to follow the order COOH > NO2 > NH2 > CH3 > OH > H. The selectivity of SO2 is found to be maximum at a pressure less than 0.2 bar. Furthermore, SO2 selectivity and adsorption capacity decrease with increase in the molar ratio of SO2/N2 mixture from 1:1 to 1:9. In the case of ternary mixture of SO2, CO2, N2, having compositions of 0.05, 0.15, 0.8, the selectivity of SO2 over N2 is higher than that of CO2 over N2. The maximum selectivity of SO2 over CO2 is observed for the COOH functionalized GNR followed by NO2 and other functionalized GNRs.

  10. Electric-field-induced destruction of quasi-Landau levels in bilayer graphene nanoribbons.

    PubMed

    Chung, Hsien-Ching; Su, Wu-Pei; Lin, Ming-Fa

    2013-01-21

    The magneto-electronic properties of bilayer zigzag graphene nanoribbons are investigated by the Peierls tight-binding method. In the presence of magnetic fields, Landau quantization leads to the formation of Landau subbands. For the bilayer nanoribbons, these subbands are partially dispersionless in k-space and are called quasi-Landau levels (QLLs). Perpendicular electric fields, serving as the top gate, push the QLLs to higher state energy and split the flat subbands. From the evidence of band structure and density of states, the QLLs remain dispersionless and the corresponding peaks are still the main structure of density of states, which means that the material properties related to the QLLs are unchanged. However, the wave functions present a totally different evidence that the Landau wave functions are severely mixed, and the corresponding material properties would be strongly affected or destroyed. The wave functions provide an effective way to comprehend the characteristics of the flat subbands and Landau subbands. The energy spectra, density of states, and wave functions are discussed in detail.

  11. 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.

  12. Spin transport in graphene

    NASA Astrophysics Data System (ADS)

    Özyilmaz, Barbaros

    2012-02-01

    Conventional electronic transistors involve the control of electronic charge at the nanoscale to realize memory, logic and communication functions. All these electronic charges, however, also carry a spin that remains unutilized in present commercial devices. This has motivated the search for new materials that propagate spin-polarized currents over large distances. Among the most promising materials for spintronics has been graphene. Micron-scale spin relaxation lengths have been previously demonstrated in single-layer graphene. Recently, we showed that bilayer graphene is a far more interesting candidate for spintronics. By fabricating spin valves on bilayer graphene we have achieved at room temperature spin relaxation times up to 2 nanoseconds, which are an order of magnitude higher than for single layer graphene [1]. Furthermore, the spin-relaxation time scales inversely with the mobility of BLG sample. This indicates the importance of D'yakonov-Perel' spin scattering in BLG. Last not but least, the presence of an electric field tunable band gap in bilayer graphene makes it particularly appealing. Our work provides fundamental insight into the unique properties of bilayer graphene for spintronic applications. Remarkably, a similar difference between single layer and bilayer graphene is also observed in large area graphene grown by the CVD method on copper. These results demonstrate the potential of CVD graphene in realistic spintronics devices [2]. [4pt] [1] T - Y. Yang et al., Observation of Long Spin-Relaxation Times in Bilayer Graphene at Room Temperature, PRL (2011). [0pt] [2] A. Avsar et al., Towards Wafer Scale Fabrication of Graphene Based Spin Valve Devices, Nano Lett. (2011).

  13. 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).

  14. 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.

  15. Orbital diamagnetism of weakly doped bilayer graphene in a magnetic field.

    PubMed

    Lv, Min; Wan, Shaolong

    2011-06-01

    We investigate the orbital diamagnetism of weakly doped bilayer graphene (BLG) in a spatially smoothly varying magnetic field and obtain the general analytic expression for the orbital susceptibility of BLG, with finite wavenumber and Fermi energy, at zero temperature. We find that the magnetic field screening factor of BLG is dependent on the wavenumber, which results in a more complicated screening behavior compared with that of monolayer graphene (MLG). We also study the induced magnetization and electric current in BLG, under a nonuniform magnetic field, and find that they are qualitatively different from those for MLG and the two-dimensional electron gas (2DEG). However, as for MLG, a magnetic object placed above BLG is repelled by a diamagnetic force from the BLG, which is approximately equivalent to the force produced by its mirror image on the other side of the BLG with a reduced amplitude dependent on the typical length of the systems. BLG shows crossover behaviors in the responses to the external magnetic field, intermediate between those of MLG and 2DEG.

  16. Competing nematic, antiferromagnetic, and spin-flux orders in the ground state of bilayer graphene

    NASA Astrophysics Data System (ADS)

    Lemonik, Y.; Aleiner, I.; Fal'ko, V. I.

    2012-06-01

    We analyze the phase diagram of bilayer graphene (BLG) at zero temperature and zero doping. Assuming that at high energies the electronic system of BLG can be described within a weak-coupling theory (consistent with the experimental evidence), we systematically study the evolution of the couplings with going from high to low energies. The divergences of the couplings at some energies indicate the tendency towards certain symmetry breakings. Carrying out this program, we found that the phase diagram is determined by microscopic couplings defined on the short distances (initial conditions). We explored all plausible space of these initial conditions and found that the three states have the largest phase volume of the initial couplings: nematic, antiferromagnetic, and spin flux (a.k.a. quantum spin Hall). In addition, ferroelectric and two superconducting phases appear only near the very limits of the applicability of the weak-coupling approach. The paper also contains the derivation and analysis of the renormalization group equations and the group theory classification of all the possible phases which might arise from the symmetry breakings of the lattice, spin rotation, and gauge symmetries of graphene.

  17. Gate-tunable nanoplasmonic effects in single- and bi-layer graphene

    NASA Astrophysics Data System (ADS)

    Fei, Zhe; Andreev, Gregory; Bao, Wenzhong; Rodin, Aleksandr; McLeod, Alexander; Zhang, Lingfeng; Zhao, Zeng; Dominguez, Gerardo; Thiemens, Mark; Fogler, Michael; Castro-Neto, Antonio; Lau, Chunning; Keilmann, Fritz; Basov, Dimitri

    2012-02-01

    We employed near-field infrared (IR) nanoscopy and nanoimaging to study mid-IR nanoplasmonic effects of both single-layer graphene (SLG) and bilayer graphene (BLG) on SiO2/Si substrate. In our previous study, we found that SLG enhanced and blueshifted the surface phonon resonance of SiO2 due to plasmon-phonon coupling [Z. Fei et al. Nano. Lett. 2011]. Here we report that both these effects are also observed in BLG. Using back-gate we were able to systematically change the carrier density in both SLG and BLG while monitoring the evolution of the hybrid plasmon-phonon resonance. New data are in accord with our point-dipole modeling results. IR imaging with nanoscale resolution revealed fringe patterns extending along the edges of both SLG and BLG. We ascribe these patterns to the interference of plasmon waves launched by the near-field probe with those reflected from the edges. Detailed analysis allowed us to observe gate-induced changes in the plasmon dispersion of both SLG and BLG, which are consistent with the notion of massless Dirac fermions in SLG and massive carriers in BLG.

  18. 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.

  19. Covalent Carbene Functionalization of Graphene: Toward Chemical Band-Gap Manipulation.

    PubMed

    Sainsbury, Toby; Passarelli, Melissa; Naftaly, Mira; Gnaniah, Sam; Spencer, Steve J; Pollard, Andrew J

    2016-02-01

    In this work, we employ dibromocarbene (DBC) radicals to covalently functionalize solution exfoliated graphene via the formation of dibromocyclopropyl adducts. This is achieved using a basic aqueous/organic biphasic reaction mixture to decompose the DBC precursor, bromoform, in conjunction with a phase-transfer catalyst to facilitate ylide formation and carbene migration to graphene substrates. DBC-functionalized graphene (DBC-graphene) was characterized using a range of spectroscopic and analytical techniques to confirm the covalent nature of functionalization. Modified optical and electronic properties of DBC-graphene were investigated using UV-vis spectroscopy, analysis of electrical I-V transport properties, and noncontact terahertz time-domain spectroscopy. The implications of carbene functionalization of graphene are considered in the context of scalable radical functionalization methodologies for bulk-scale graphene processing and controlled band-gap manipulation of graphene.

  20. When noncovalent interactions are stronger than covalent bonds: Bilayer graphene doped with second row atoms, aluminum, silicon, phosphorus and sulfur

    NASA Astrophysics Data System (ADS)

    Denis, Pablo A.

    2011-05-01

    Herein, we investigate bilayer graphene doped with second-row atoms using the LDA, M06-L, and VDW-DF functionals. For 2-3 at.% or lower Al and P dopings the structure with a short interlayer distance and without a covalent bond between the heteroatoms is more stable than that with a covalent bond and longer interlayer separation. However, for Si the linked structure is more stable. Doped bilayer-graphene is prone to the attachment of more heteroatoms, as reflected by the large adsorption energies, which decrease in the following order Al > Si > P. We show that it is feasible to tune the electronic properties, and the interlayer-interaction energy varying the type or amount of second row atoms.

  1. Magnetic Field Studies Near Superconducting Transition in MBE Grown Monolayer NbSe2 on Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Onishi, Seita; Ugeda, Miguel M.; Zhang, Yi; Chen, Yi; Ojeda-Aristizabal, Claudia; Ryu, Hyejin; Mo, Sung-Kwan; Hussain, Zahid; Shen, Zhi-Xun; Crommie, Michael F.; Zettl, Alex

    Following the work by Frindt on the superconductivity of NbSe2 at reduced thicknesses, recent breakthroughs have enabled the study of bilayers and monolayers. Staley et. al., Tsen et. al., Cao et. al. and Xi et. al. have studied superconductivity in bilayers and monolayers of NbSe2 after mechanical exfoliation and encapsulation with another layered material to protect from air. In this work, we have investigated the superconductivity in monolayer NbSe2 prepared by molecular beam epitaxy growth (MBE) on bilayer graphene (BLG). The superconducting transition has an onset temperature of 1.9K, midpoint temperature of 0.65K and reaches zero resistance at 0.46K. The upper critical field perpendicular to the NbSe2 monolayer is 0.5T at 100mK. We will show the effect of magnetic fields near the superconducting transition and compare with existing theories

  2. Bilayered graphene/h-BN with folded holes as new nanoelectronic materials: modeling of structures and electronic properties

    NASA Astrophysics Data System (ADS)

    Chernozatonskii, Leonid A.; Demin сtor A., Vi; Bellucci, Stefano

    2016-11-01

    The latest achievements in 2-dimensional (2D) material research have shown the perspective use of sandwich structures in nanodevices. We demonstrate the following generation of bilayer materials for electronics and optoelectronics. The atomic structures, the stability and electronic properties of Moiré graphene (G)/h-BN bilayers with folded nanoholes have been investigated theoretically by ab-initio DFT method. These perforated bilayers with folded hole edges may present electronic properties different from the properties of both graphene and monolayer nanomesh structures. The closing of the edges is realized by C-B(N) bonds that form after folding the borders of the holes. Stable ≪round≫ and ≪triangle≫ holes organization are studied and compared with similar hole forms in single layer graphene. The electronic band structures of the considered G/BN nanomeshes reveal semiconducting or metallic characteristics depending on the sizes and edge terminations of the created holes. This investigation of the new types of G/BN nanostructures with folded edges might provide a directional guide for the future of this emerging area.

  3. Bilayered graphene/h-BN with folded holes as new nanoelectronic materials: modeling of structures and electronic properties

    PubMed Central

    Chernozatonskii, Leonid A.; Demin, Viсtor A.; Bellucci, Stefano

    2016-01-01

    The latest achievements in 2-dimensional (2D) material research have shown the perspective use of sandwich structures in nanodevices. We demonstrate the following generation of bilayer materials for electronics and optoelectronics. The atomic structures, the stability and electronic properties of Moiré graphene (G)/h-BN bilayers with folded nanoholes have been investigated theoretically by ab-initio DFT method. These perforated bilayers with folded hole edges may present electronic properties different from the properties of both graphene and monolayer nanomesh structures. The closing of the edges is realized by C-B(N) bonds that form after folding the borders of the holes. Stable ≪round≫ and ≪triangle≫ holes organization are studied and compared with similar hole forms in single layer graphene. The electronic band structures of the considered G/BN nanomeshes reveal semiconducting or metallic characteristics depending on the sizes and edge terminations of the created holes. This investigation of the new types of G/BN nanostructures with folded edges might provide a directional guide for the future of this emerging area. PMID:27897237

  4. Significant band-gap opening in graphene and Pd-doped graphene via the adsorption of ionized methane

    NASA Astrophysics Data System (ADS)

    Wang, Su-Fang; Chen, Li-Yong; Zhang, Jian-Min

    2017-04-01

    First-principles calculations are performed to study the adsorptions of ionized methane (i.e., CHn+ (n = 3,4) fragments) on pristine graphene (G-CHn+) and Pd-doped graphene (G/Pd-CHn+). Remarkably, CH3+ adsorption induces significant band-gap for both systems, while it is absent in the cases of both CH4+ and CH3 adsorptions. The charge-induced gaps are found to be about 665 meV and 401 meV for G-CH3+ and G/Pd-CH3+ systems, respectively. Promisingly, the Pd-doped graphene with CH3+ adsorption not only achieves a significant band-gap at Dirac point, but also retains nearly linear dispersion near the Fermi level. Both hole effect and localized electron hybridization mediate the band-gap opening. Within DFT + U scheme, coulomb-correction dependences of band-gap, Fermi velocity and effective mass of carriers are handled for the Pd-doped graphene with CH3+ adsorption. These results may be interesting for exploring the applications of graphene in band-gap engineering and gaseous ionization detectors.

  5. Single-bilayer graphene oxide sheet tolerance and glutathione redox system significance assessment in faba bean ( Vicia faba L.)

    NASA Astrophysics Data System (ADS)

    Anjum, Naser A.; Singh, Neetu; Singh, Manoj K.; Shah, Zahoor A.; Duarte, Armando C.; Pereira, Eduarda; Ahmad, Iqbal

    2013-07-01

    Adsorbents based on single-bilayer graphene oxide sheet (hereafter termed "graphene oxide") are widely used in contaminated environments cleanup which may easily open the avenues for their entry to different environmental compartments, exposure to organisms and their subsequent transfer to human/animal food chain. Considering a common food crop—faba bean ( Vicia faba L.) germinating seedlings as a model plant system, this study assesses the V. faba-tolerance to different concentrations (0, 100, 200, 400, 800, and 1600 mg L-1) of graphene oxide (0.5-5 μm) and evaluates glutathione (γ-glutamyl-cysteinyl-glycine) redox system significance in this context. The results showed significantly increased V. faba sensitivity under three graphene oxide concentrations (in order of impact: 1,600 > 200 > 100 mg graphene oxide L-1), which was accompanied by decreased glutathione redox (reduced glutathione-to-oxidized glutathione) ratio, reduced glutathione pool, as well as significant and equally elevated activities of glutathione-regenerating (glutathione reductase) and glutathione-metabolizing (glutathione peroxidase; glutathione sulfo-transferase) enzymes. Contrarily, the two graphene oxide concentrations (in order of impact: 800 > 400 graphene oxide mg L-1) yielded promising results; where, significant improvements in V. faba health status (measured as increased graphene oxide tolerance) were clearly perceptible with increased ratio of the reduced glutathione-to-oxidized glutathione, reduced glutathione pool and glutathione reductase activity but decreased activities of glutathione-metabolizing enzymes. It is inferred that V. faba seedlings-sensitivity and/or tolerance to graphene oxide concentrations depends on both the cellular redox state (reduced glutathione-to-oxidized glutathione ratio) and the reduced glutathione pool which in turn are controlled by a finely tuned modulation of the coordination between glutathione-regenerating and glutathione-metabolizing enzymes.

  6. Dirac gap-induced graphene quantum dot in an electrostatic potential

    NASA Astrophysics Data System (ADS)

    Giavaras, G.; Nori, Franco

    2011-04-01

    A spatially modulated Dirac gap in a graphene sheet leads to charge confinement, thus enabling a graphene quantum dot to be formed without the application of external electric and magnetic fields [G. Giavaras and F. Nori, Appl. Phys. Lett. 97, 243106 (2010)]. This can be achieved provided the Dirac gap has a local minimum in which the states become localized. In this work, the physics of such a gap-induced dot is investigated in the continuum limit by solving the Dirac equation. It is shown that gap-induced confined states couple to the states introduced by an electrostatic quantum well potential. Hence the region in which the resulting hybridized states are localized can be tuned with the potential strength, an effect which involves Klein tunneling. The proposed quantum dot may be used to probe quasirelativistic effects in graphene, while the induced confined states may be useful for graphene-based nanostructures.

  7. 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.

  8. Theory of low-power ultra-broadband terahertz sideband generation in bi-layer graphene

    NASA Astrophysics Data System (ADS)

    Crosse, J. A.; Xu, Xiaodong; Sherwin, Mark S.; Liu, R. B.

    2014-09-01

    In a semiconductor illuminated by a strong terahertz (THz) field, optically excited electron-hole pairs can recombine to emit light in a broad frequency comb evenly spaced by twice the THz frequency. Such high-order THz sideband generation is of interest both as an example of extreme nonlinear optics and also as a method for ultrafast electro-optical modulation. So far, this phenomenon has only been observed with large field strengths (~10 kV cm-1), an obstacle for technological applications. Here we predict that bi-layer graphene generates high-order sidebands at much weaker THz fields. We find that a THz field of strength 1 kV cm-1 can produce a high-sideband spectrum of about 30 THz, 100 times broader than in GaAs. The sidebands are generated despite the absence of classical collisions, with the quantum coherence of the electron-hole pairs enabling recombination. These remarkable features lower the barrier to desktop electro-optical modulation at THz frequencies, facilitating ultrafast optical communications.

  9. Direct observation of asymmetric band structure of bilayer graphene through quantum capacitance measurements

    NASA Astrophysics Data System (ADS)

    Kanayama, Kaoru; Nagashio, Kosuke; Nishimura, Tomonori; Toriumi, Akira

    2014-03-01

    Although upper conduction and valence sub-bands in bilayer graphene are known to be asymmetric, a detailed analysis based on the electrical measurements is very limited due to the infirm quality of gate insulator. In this study, the electrical quality of the top-gate Y2O3 insulator is drastically improved by the high-pressure O2 post-deposition annealing at 100 atm and the carrier density of ~8*1013 cm-2 is achieved. In quantum capacitance measurements, the drastic increase of the density of states is observed in addition to the van Hove singularity, suggesting that the Fermi energy reaches upper sub-band. At the same carrier density, the sudden reduction of the conductivity is observed, indicating that the inter-band scattering occurs. The estimated carrier density required to fill the upper sub-bands is different between electron and hole sides, i.e., asymmetric band structure between upper conduction and valence bands is revealed by the electrical measurements.

  10. Retro reflection of electrons at the interface of bilayer graphene and superconductor

    PubMed Central

    Ang, Yee Sin; Ma, Zhongshui; Zhang, C.

    2012-01-01

    Electron reflection at an interface is a fundamental quantum transport phenomenon. The most famous electron reflection is the electron→hole Andreev reflection (AR) at a metal/superconductor interface. While AR can be either specular or retro-type, electron→electron reflection is limited to only the specular type. Here we show that electrons can undergo retro-reflection in bilayer graphene (BLG). The underlying mechanism for this previously unknown process is the anisotropic constant energy band contour of BLG. The electron group velocity is fully reversed upon reflection, causing electrons to be retro-reflected. Utilizing a BLG/superconductor junction (BLG/S) as a model structure, we show that the unique low energy quasiparticle nature of BLG results in two striking features: (1) AR is completely absent, making BLG/S 100% electron reflective; (2) electrons are valley-selectively focused upon retro-reflection. Our results suggest that BLG/S is a valley-selective Veselago electron focusing mirror which can be useful in valleytronic applications. PMID:23264876

  11. 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.

  12. Retro reflection of electrons at the interface of bilayer graphene and superconductor.

    PubMed

    Ang, Yee Sin; Ma, Zhongshui; Zhang, C

    2012-01-01

    Electron reflection at an interface is a fundamental quantum transport phenomenon. The most famous electron reflection is the electron→hole Andreev reflection (AR) at a metal/superconductor interface. While AR can be either specular or retro-type, electron→electron reflection is limited to only the specular type. Here we show that electrons can undergo retro-reflection in bilayer graphene (BLG). The underlying mechanism for this previously unknown process is the anisotropic constant energy band contour of BLG. The electron group velocity is fully reversed upon reflection, causing electrons to be retro-reflected. Utilizing a BLG/superconductor junction (BLG/S) as a model structure, we show that the unique low energy quasiparticle nature of BLG results in two striking features: (1) AR is completely absent, making BLG/S 100% electron reflective; (2) electrons are valley-selectively focused upon retro-reflection. Our results suggest that BLG/S is a valley-selective Veselago electron focusing mirror which can be useful in valleytronic applications.

  13. First Principle DFT Study of Electric Field Effects on the Characteristics of Bilayer Graphene

    NASA Astrophysics Data System (ADS)

    Sabzyan, Hassan; Sadeghpour, Narges

    2017-01-01

    First principle density functional theory methods, local density and Perdew-Burke-Ernzerhof generalized gradient approximations with Goedecker pseudopotential (LDA-G & PBE-G), are used to study the electric field effects on the binding energy and atomic charges of bilayer graphene (BLG) at the Γ point of the Brillouin zone based on two types of unit cells (α and β) containing nC=8-32 carbon atoms. Results show that application of electric fields of 4-24 V/nm strengths reduces the binding energies and induces charge transfer between the two layers. The transferred charge increases almost linearly with the strength of the electric field for all sizes of the two types of unit cells. Furthermore, the charge transfer calculated with the α-type unit cells is more sensitive to the electric field strength. The calculated field-dependent contour plots of the differential charge densities of the two layers show details of charge density redistribution under the influence of the electric field.

  14. 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.

  15. Retro reflection of electrons at the interface of bilayer graphene and superconductor

    NASA Astrophysics Data System (ADS)

    Ang, Yee Sin; Ma, Zhongshui; Zhang, C.

    2012-12-01

    Electron reflection at an interface is a fundamental quantum transport phenomenon. The most famous electron reflection is the electron-->hole Andreev reflection (AR) at a metal/superconductor interface. While AR can be either specular or retro-type, electron-->electron reflection is limited to only the specular type. Here we show that electrons can undergo retro-reflection in bilayer graphene (BLG). The underlying mechanism for this previously unknown process is the anisotropic constant energy band contour of BLG. The electron group velocity is fully reversed upon reflection, causing electrons to be retro-reflected. Utilizing a BLG/superconductor junction (BLG/S) as a model structure, we show that the unique low energy quasiparticle nature of BLG results in two striking features: (1) AR is completely absent, making BLG/S 100% electron reflective; (2) electrons are valley-selectively focused upon retro-reflection. Our results suggest that BLG/S is a valley-selective Veselago electron focusing mirror which can be useful in valleytronic applications.

  16. Electron interactions in bilayer graphene: Marginal Fermi liquid and zero-bias anomaly

    NASA Astrophysics Data System (ADS)

    Nandkishore, Rahul; Levitov, Leonid

    2010-09-01

    We analyze the many-body properties of bilayer graphene (BLG) at charge neutrality, governed by long-range interactions between electrons. Perturbation theory in a large number of flavors is used in which the interactions are described within a random phase approximation, taking account of dynamical screening effect. Crucially, the dynamically screened interaction retains some long-range character, resulting in log2 renormalization of key quantities. We carry out the perturbative renormalization group calculations to one loop order and find that BLG behaves to leading order as a marginal Fermi liquid. Interactions produce a log squared renormalization of the quasiparticle residue and the interaction vertex function while all other quantities renormalize only logarithmically. We solve the RG flow equations for the Green’s function with logarithmic accuracy and find that the quasiparticle residue flows to zero under RG. At the same time, the gauge-invariant quantities, such as the compressibility, remain finite to log2 order, with subleading logarithmic corrections. The key experimental signature of this marginal Fermi liquid behavior is a strong suppression of the tunneling density of states, which manifests itself as a zero bias anomaly in tunneling experiments in a regime where the compressibility is essentially unchanged from the noninteracting value.

  17. Band gap engineering for single-layer graphene by using slow Li(+) ions.

    PubMed

    Ryu, Mintae; Lee, Paengro; Kim, Jingul; Park, Heemin; Chung, Jinwook

    2016-08-05

    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.

  18. Spin-Orbit Coupling Induced Gap in Graphene on Pt(111) with Intercalated Pb Monolayer.

    PubMed

    Klimovskikh, Ilya I; Otrokov, Mikhail M; Voroshnin, Vladimir Yu; Sostina, Daria; Petaccia, Luca; Di Santo, Giovanni; Thakur, Sangeeta; Chulkov, Evgueni V; Shikin, Alexander M

    2017-01-24

    Graphene is one of the most promising materials for nanoelectronics owing to its unique Dirac cone-like dispersion of the electronic state and high mobility of the charge carriers. However, to facilitate the implementation of the graphene-based devices, an essential change of its electronic structure, a creation of the band gap should controllably be done. Brought about by two fundamentally different mechanisms, a sublattice symmetry breaking or an induced strong spin-orbit interaction, the band gap appearance can drive graphene into a narrow-gap semiconductor or a 2D topological insulator phase, respectively, with both cases being technologically relevant. The later case, characterized by a spin-orbit gap between the valence and conduction bands, can give rise to the spin-polarized topologically protected edge states. Here, we study the effect of the spin-orbit interaction enhancement in graphene placed in contact with a lead monolayer. By means of angle-resolved photoemission spectroscopy, we show that intercalation of the Pb interlayer between the graphene sheet and the Pt(111) surface leads to formation of a gap of ∼200 meV at the Dirac point of graphene. Spin-resolved measurements confirm the splitting to be of a spin-orbit nature, and the measured near-gap spin structure resembles that of the quantum spin Hall state in graphene, proposed by Kane and Mele [ Phys. Rev. Lett. 2005 , 95 , 226801 ]. With a bandstructure tuned in this way, graphene acquires a functionality going beyond its intrinsic properties and becomes more attractive for possible spintronic applications.

  19. A dilute Cu(Ni) alloy for synthesis of large-area Bernal stacked bilayer graphene using atmospheric pressure chemical vapour deposition

    SciTech Connect

    Madito, M. J.; Bello, A.; Dangbegnon, J. K.; Momodu, D. Y.; Masikhwa, T. M.; Barzegar, F.; Manyala, N.; Oliphant, C. J.; Jordaan, W. A.; Fabiane, M.

    2016-01-07

    A bilayer graphene film obtained on copper (Cu) foil is known to have a significant fraction of non-Bernal (AB) stacking and on copper/nickel (Cu/Ni) thin films is known to grow over a large-area with AB stacking. In this study, annealed Cu foils for graphene growth were doped with small concentrations of Ni to obtain dilute Cu(Ni) alloys in which the hydrocarbon decomposition rate of Cu will be enhanced by Ni during synthesis of large-area AB-stacked bilayer graphene using atmospheric pressure chemical vapour deposition. The Ni doped concentration and the Ni homogeneous distribution in Cu foil were confirmed with inductively coupled plasma optical emission spectrometry and proton-induced X-ray emission. An electron backscatter diffraction map showed that Cu foils have a single (001) surface orientation which leads to a uniform growth rate on Cu surface in early stages of graphene growth and also leads to a uniform Ni surface concentration distribution through segregation kinetics. The increase in Ni surface concentration in foils was investigated with time-of-flight secondary ion mass spectrometry. The quality of graphene, the number of graphene layers, and the layers stacking order in synthesized bilayer graphene films were confirmed by Raman and electron diffraction measurements. A four point probe station was used to measure the sheet resistance of graphene films. As compared to Cu foil, the prepared dilute Cu(Ni) alloy demonstrated the good capability of growing large-area AB-stacked bilayer graphene film by increasing Ni content in Cu surface layer.

  20. A dilute Cu(Ni) alloy for synthesis of large-area Bernal stacked bilayer graphene using atmospheric pressure chemical vapour deposition

    NASA Astrophysics Data System (ADS)

    Madito, M. J.; Bello, A.; Dangbegnon, J. K.; Oliphant, C. J.; Jordaan, W. A.; Momodu, D. Y.; Masikhwa, T. M.; Barzegar, F.; Fabiane, M.; Manyala, N.

    2016-01-01

    A bilayer graphene film obtained on copper (Cu) foil is known to have a significant fraction of non-Bernal (AB) stacking and on copper/nickel (Cu/Ni) thin films is known to grow over a large-area with AB stacking. In this study, annealed Cu foils for graphene growth were doped with small concentrations of Ni to obtain dilute Cu(Ni) alloys in which the hydrocarbon decomposition rate of Cu will be enhanced by Ni during synthesis of large-area AB-stacked bilayer graphene using atmospheric pressure chemical vapour deposition. The Ni doped concentration and the Ni homogeneous distribution in Cu foil were confirmed with inductively coupled plasma optical emission spectrometry and proton-induced X-ray emission. An electron backscatter diffraction map showed that Cu foils have a single (001) surface orientation which leads to a uniform growth rate on Cu surface in early stages of graphene growth and also leads to a uniform Ni surface concentration distribution through segregation kinetics. The increase in Ni surface concentration in foils was investigated with time-of-flight secondary ion mass spectrometry. The quality of graphene, the number of graphene layers, and the layers stacking order in synthesized bilayer graphene films were confirmed by Raman and electron diffraction measurements. A four point probe station was used to measure the sheet resistance of graphene films. As compared to Cu foil, the prepared dilute Cu(Ni) alloy demonstrated the good capability of growing large-area AB-stacked bilayer graphene film by increasing Ni content in Cu surface layer.

  1. Experimental and theoretical investigation of relative optical band gaps in graphene generations

    NASA Astrophysics Data System (ADS)

    Bhatnagar, Deepika; Singh, Sukhbir; Yadav, Sriniwas; Kumar, Ashok; Kaur, Inderpreet

    2017-01-01

    Size and chemical functionalization dependant optical band gaps in graphene family nanomaterials were investigated by experimental and theoretical study using Tauc plot and density functional theory (DFT). We have synthesized graphene oxide through a modified Hummer’s method using graphene nanoplatelets and sequentially graphene quantum dots through hydrothermal reduction. The experimental results indicate that the optical band gap in graphene generations was altered by reducing the size of graphene sheets and attachment of chemical functionalities like epoxy, hydroxyl and carboxyl groups plays a crucial role in varying optical band gaps. It is further confirmed by DFT calculations that the π orbitals were more dominatingly participating in transitions shown by projected density of states and the molecular energy spectrum represented the effect of attached functional groups along with discreteness in energy levels. Theoretical results were found to be in good agreement with experimental results. All of the above different variants of graphene can be used in native or modified form for sensor design and optoelectronic applications.

  2. Gap opening in the zeroth Landau level in gapped graphene: pseudo-Zeeman splitting in an angular magnetic field.

    PubMed

    Tahir, M; Sabeeh, K

    2012-04-04

    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.

  3. Effect of Zeeman splitting and interlayer bias potential on electron transport in bilayer graphene

    NASA Astrophysics Data System (ADS)

    Zhang, Ying-Tao; Xie, X. C.; Sun, Qing-feng

    2012-07-01

    Motivated by the recent experiments [Weitz , ScienceSCIEAS0036-807510.1126/science.1194988 330, 812 (2010) and Kim , Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.107.016803 107, 016803 (2011)], we present here a theoretical analysis for the Hall resistance, the longitudinal resistance, and the Hall conductance of a six-terminal bilayer graphene Hall bar under a perpendicular magnetic field. The Landauer-Büttiker formalism combined with the nonequilibrium Green function method is applied. In the presence of both the Zeeman splitting and interlayer bias potential U, the spin and valley degeneracies of Landau levels are lifted, which leads to the result that the filling factor ν can be an arbitrary integer number and the Hall resistance exhibits a quantum plateau at 1/ν(h/e2). While ν=0, the system can be in both the spin-polarized and valley-polarized regions. For the valley-polarized ν=0 region, the longitudinal resistance is predicted to have a very large value which means that the system is a quantum Hall insulator even though there are four states in the Fermi surface. However, in the spin-polarized ν=0 region, the system is predicted to display a quantum spin Hall effect, in which the spin-up and spin-down edge states are counterpropagating. In both the spin-polarized and valley-polarized ν=0 regions, the Hall conductances show zero quantum plateaus, although the Hall resistances are very different. In addition, due to the counterpropagating edge states, the longitudinal resistance exhibits some fractional quantum plateaus with values 2/9(h/e2), 1/4(h/e2), 1/2(h/e2), etc.

  4. Charge Inversion and Topological Phase Transition at a Twist Angle Induced van Hove Singularity of Bilayer Graphene.

    PubMed

    Kim, Youngwook; Herlinger, Patrick; Moon, Pilkyung; Koshino, Mikito; Taniguchi, Takashi; Watanabe, Kenji; Smet, Jurgen H

    2016-08-10

    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 neighboring 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 nonzero Berry phase and distinct sequences of integer quantum Hall states above and below the singularity.

  5. 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.

  6. Graphene nanoribbons as low band gap donor materials for organic photovoltaics: quantum chemical aided design.

    PubMed

    Osella, Silvio; Narita, Akimitsu; Schwab, Matthias Georg; Hernandez, Yenny; Feng, Xinliang; Müllen, Klaus; Beljonne, David

    2012-06-26

    Graphene nanoribbons (GNRs) are strips of graphene cut along a specific direction that feature peculiar electronic and optical properties owing to lateral confinement effects. We show here by means of (time-dependent) density functional theory calculations that GNRs with properly designed edge structures fulfill the requirements in terms of electronic level alignment with common acceptors (namely, C(60)), solar light harvesting, and singlet-triplet exchange energy to be used as low band gap semiconductors for organic photovoltaics.

  7. Wavelength-Tunable IR Detector based on Suspended Bilayer Graphene Micro Ribbons

    DTIC Science & Technology

    2013-11-05

    photoelectric effect , which is promising towards CVD-grown graphene photodetectors approaching THz cut-off frequencies. chemical vapor deposition, strain...tuning, bandgap-tunability, suspended graphene, microribbons, photodetector, THz cut-off frequencies, photophysics, photoelectric effect , photo...devices are dominated by the photoelectric effect , which is promising towards CVD-grown graphene photodetectors approaching THz cut-off frequencies

  8. Analytical modeling of photon absorption coefficient in mono and bilayer circular graphene quantum dots for light absorber applications

    NASA Astrophysics Data System (ADS)

    Tamandani, Shahryar; Darvish, Ghafar

    2017-02-01

    We present an analytical method to calculate photon absorption coefficient in mono and bilayer circular graphene quantum dots (CGQDs). We use kobo equation to extract new closed relation as the main goal. First, we calculate real and imaginary part of optical conductance separately. Then, joint density of states is obtained using a new relation that was extracted for the energy levels of mono and bilayer circular grapheme quantum dots. In this work we use closed equations to calculate energy levels in CGQDs. Next we obtain a new closed formula to calculate the photon absorption coefficient. The results show that the absorption coefficient is related to the size of CGQDs and number of layers. The photon absorption coefficient becomes lower with larger size of CGQDs. It is seen that the results of our method is compatible with the results of practical works. We also compare photon absorption in biased and unbiased bilayer CGQDs and investigate the effect of external magnetic field on photon absorption. rights reserved

  9. The size and shape dependence of graphene domain on the band gap of h-BN

    NASA Astrophysics Data System (ADS)

    Kah, Cherno B.; Kirigeehanage, Saliya; Smith, Lyle; Yu, Ming; Jayanthi, Chakram; Wu, Shiyu

    2015-03-01

    This talk will report the structure and electronic characteristics of graphene domains embedded in a hexagonal boron-nitride sheet (h-BN) with the goal of band gap tuning in mind. Different shapes (triangular, circular, rectangular, and irregular structures) and sizes of graphene domains will be studied. The structural stability of these hybrid materials will be studied using a new generation of the semi-empirical Hamiltonian (SCED-LCAO) developed recently [arXiv:1408.4931]. It is found that the lattice mismatch between graphene domains and the h-BN generates large strain, leading to a reduction or a symmetry breaking of the hexagonal lattice of h-BN. The extent of the strain depends on the shape and the size of the domain, as well as on the distribution of B atoms around the graphene domains. This effect also creates impurity states in the band gap of h-BN and changes the band gap. The interplay between the shape and size of graphene domains, the local strain around the domains and the nature of the impurity states on the band gap of h-BN will be discussed.

  10. ZnO Nanoparticles/Reduced Graphene Oxide Bilayer Thin Films for Improved NH3-Sensing Performances at Room Temperature.

    PubMed

    Tai, Huiling; Yuan, Zhen; Zheng, Weijian; Ye, Zongbiao; Liu, Chunhua; Du, Xiaosong

    2016-12-01

    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.

  11. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2017-02-01

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

  13. 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.

  14. Massive Dirac fermion transport in a gapped graphene-based magnetic tunnel junction

    NASA Astrophysics Data System (ADS)

    Soodchomshom, Bumned; Tang, I.-Ming; Hoonsawat, Rassmidara

    2009-08-01

    The spin transport in a graphene-based magnetic (NG/ferromagnetic barrier (FB)/NG) tunnel junction with the graphene sheet being grown on a SiC substrate is investigated. Zhou et al. [Nat. Mater. 6 (2007) 770] has shown that in these epitaxial grown graphene sheets, the electrons behave like massive relativistic particles with an energy gap of 2 Δ∼260 meV opening up in the energy spectrum of the massive relativistic electron. Basing on assumption that gap in graphene can occur under the influence of the magnetic field, we find that in the case of thick ferromagnetic graphene barriers, the electronic gap causes the barrier to behave as a strong insulator when the gate potential is in the range 400-130 meV< V G<400+130 meV (and the energy level Ef∼400 meV above Dirac point). For these values of V G, the spin polarization of the junction is P(%)∼100% except for V G= E f, where it is P(%)=0%. The current of the junction, for thick FB, can be rapidly switched from a 100% spin up current to a 100% spin down current by small variation of V G from V G< E f to V G> E f , the features of a perfect spin filtering electronic junction.

  15. 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).

  16. Mid-gap states and Kondo effect in disordered graphene

    NASA Astrophysics Data System (ADS)

    Lewenkopf, Caio; Miranda, Vladimir; Dias da Silva, Luis

    2012-02-01

    Recent experiments on graphene flakes with short range scattering defects have stengthen the interest on Kondo physics in graphene systems. The experimental data show a temperature dependence of the resistivity consistent with the low-temperature Kondo screening of local magnetic moments. While the linear dispersion in the density of states in graphene justify a pseudogap Kondo model showing a rich variety of quantum critical behavior as a function of the gate-controlled chemical potential, the presence of disorder can alter this effect in favor of the ``standard'' Kondo model, with a Fermi-liquid ground state. We study these effects with different numerical methods. Tight-binding calculations for diluted scattering defects show the appearance of quasi-localized midgap states in the local density of states at the vicinity of the charge neutrality point. This leads to the formulation a Anderson-like model of localized states within the graphene matrix, which may lead to a Kondo screening consistent with the experiments. To verify this hypothesis, we perform numerical renormalization group (NRG) calculations to study the gate-dependence of the Kondo temperature and the transport properties of this model.

  17. Confinement, transport gap, and valley polarization from a double barrier structure in graphene

    NASA Astrophysics Data System (ADS)

    Gunlycke, Daniel; White, Carter

    2013-03-01

    Engineering a gap in graphene without degrading its exceptional transport properties is arguably the main obstacle preventing a breakthrough in graphene-based nanoelectronics. To create such a gap, a lot of effort has been devoted to making graphene nanoribbons. Unlike ordinary nanoribbons, we propose a structure formed between two thin parallel transport barriers that is penetrable by electrons in surrounding graphene states. The transport across this railroad track structure is governed by resonant tunneling through quasi-bound states within the confinement. The transport barriers, modeled by chemically decorated line defects, are highly reflective, causing the resonances to form continuous bands closely matching the band structure of a zigzag ribbon. Because boundary-localized states cannot carry any transport, the resonance bands must terminate at the dimensional crossover between extended and boundary-localized states. As the confined region contains no states near the Fermi level extending across the railroad track structure, electrons approaching it experience a transport gap Eg = 2 ℏvF / W , where W is the separation between the barriers. In addition to offering confinement and a transport gap, the structure allows for nearly perfect valley polarization. This work was supported by the Office of Naval Research, directly and through the Naval Research Laboratory.

  18. Edge functionalised & Li-intercalated 555-777 defective bilayer graphene for the adsorption of CO2 and H2O

    NASA Astrophysics Data System (ADS)

    Lalitha, Murugan; Lakshmipathi, Senthilkumar; Bhatia, Suresh K.

    2017-04-01

    The adsorption of CO2 and H2O on divacanacy (DV) defected graphene cluster, and its bilayer counterpart is investigated using first-principles calculations. Both single and bilayer DV graphene cluster, are functionalised with H and F atoms. On these sheets the gas molecules are physisorbed, and the divacancy defect effectively improves the adsorption of CO2, while fluorination enhances the hydrophobicity of the graphene cluster. Among the convex and concave curvature regions induced due to the DV defect, the adsorption of the gas molecules on the concave meniscus is more favourable. Fluorine termination induces 73% reduction in Henry law constants for H2O, while for the CO2 molecule it increases by 8%, which indicates the DV defective sheet is a better candidate for CO2 capture compared to the STW defective sheet. Besides, both AA and AB divacant defect bilayer sheets are equally stable, wherein AA stacking results in a cavity between the sheets, while in AB stacking, the layers slide one over the other. Nevertheless, both these bilayer sheets are comparatively stabler than the monolayer. However, intercalation of lithium decreases the interlayer separation, particularly in AA stacking, which enhances the CO2 adsorption, but in the Bernal stacking enhances it hydrophobicity.

  19. Large area single and bilayer graphene with controlled orientation for each layer

    NASA Astrophysics Data System (ADS)

    Brown, Lola; Lochocki, Edward; Gutiérrez, Christopher; Pasupathy, Abhay; Shen, Kyle; Park, Jiwoong; Cornell Collaboration; Cornell-Columbia Collaboration

    2014-03-01

    The creation and exploration of artificial graphene structures has recently become the focus of great interest. In particular, controlling the interlayer twist angles in multilayer graphene stacks allows modulation of the overall band structure. However, producing such a structure remains difficult due to the random distribution of twist angles in as-grown samples. Here we report a novel way for creating large area graphene stacks with a pre-determined twist angle. We first grow single layer graphene whose orientation is aligned over a few cm length scale on copper foil. The overall angle alignment of the graphene is confirmed using low energy electron microscopy (LEED) and transmission electron microscopy techniques. Since the graphene is well aligned over a few centimeters, we can create large area graphene stacks with known twist angle by transferring these graphene layers while controlling the orientation of each layer during transfer. We confirm that the layers are coupled by probing the resulting band structure using angle resolved photoemission spectroscopy (ARPES), and examining their interlayer optical resonance features using spatially resolved hyperspectral (DUV-Vis-NIR wavelengths). This new method is scalable, and controllable and thus paves the way to explore and exploit the novel properties of two-dimensional crystals in artificial stacks with controlled interlayer structures. Second affiliation: Kavli Institute at Cornell for Nanoscale Science.

  20. Interactions between graphene oxide and wide band gap semiconductors

    NASA Astrophysics Data System (ADS)

    Kawa, M.; Podborska, A.; Szaciłowski, K.

    2016-09-01

    The graphene oxide (GO) and GO@TiO2 nanocomposite have been synthesised by using modified Hummers method and ultrasonics respectively. The materials were characterized by using X-ray diffraction, Fourier transform infrared spectroscopy and UV-Vis absorption spectroscopy. It was found that the interaction between GO and TiO2 affects the average interlayer spacing in carbonaceous material. The formation of bonds between various oxygen-containing functional groups and surface of titanium dioxide was investigated. One of them formed between the quinone structures (occur in graphene oxide) and titanium atoms exhibited 1.5 bond order. Furthermore the charge-transfer processes in GO@TiO2 composite were observed.

  1. 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.

  2. Determination of gap solution and critical temperature in doped graphene superconductivity

    NASA Astrophysics Data System (ADS)

    Xu, Chenmei; Yang, Yisong

    2017-04-01

    It is shown that the gap solution and critical transition temperature are significantly enhanced by doping in a recently developed BCS formalism for graphene superconductivity in such a way that positive gap and transition temperature both occur in arbitrary pairing coupling as far as doping is present. The analytic construction of the BCS gap and transition temperature offers highly effective globally convergent iterative methods for the computation of these quantities. A series of numerical examples are presented as illustrations which are in agreement with the theoretical and experimental results obtained in the physics literature and consolidate the analytic understanding achieved.

  3. Energy gap of extended states in SiC-doped graphene nanoribbon: Ab initio calculations

    NASA Astrophysics Data System (ADS)

    Liu, Xiaoshi; Wu, Yong; Li, Zhongyao; Gao, Yong

    2017-04-01

    The energy gap of extended states in zigzag graphene nanoribbons (ZGNRs) was examined on the basis of density-functional theory. In isolated ZGNRs, the energy gap is inversely proportional to the width of ribbon. It agrees well with the results from the Dirac equation in spin-unpolarized ZGNRs, although the considered ZGNRs have spin-polarized edges. However, the energy gap in SiC-doped ZGNRs cannot be modeled by effective width approximation. The doping also lifts the spin-degenerate of edge states and results in a metallic-like band structure near the Fermi level in SiC-doped ZGNRs. Our calculations may be helpful for understanding the origin of the reported single-channel ballistic transport in epitaxial graphene nanoribbons.

  4. The effects of gap parameter and spin polarization on electronic Hartree and correlation energies of doped graphene nanoribbon

    NASA Astrophysics Data System (ADS)

    Rezania, Hamed; Abdi, Ameneh

    2017-04-01

    We study the behaviors of both Hartree and correlation energies of undoped gapped armchair graphene nanoribbon using random phase approximation in the context of Hubbard model Hamiltonian. Specially, the effects of spin polarization and gap parameter on electron density dependence of Hartree and correlation energies of armchair graphene nanoribbon has been addressed. Our results show the variation of gap parameter leads to considerable effect on correlation and Hartree energy behavior of spin unpolarized gapped graphene in the middle electron density region. However local Hubbard interaction parameter affects the behaviors of Hartree and correlation energy on the whole range of electron density in zero magnetization case. We also show that a considerable reduction has been observed for density dependence of Hartree and correlation energies of spin polarized gapped graphene nanoribbon.

  5. Dynamical Energy Gap Engineering in Graphene via Oscillating Out-of-Plane Deformations

    NASA Astrophysics Data System (ADS)

    Sandler, Nancy; Zhai, Dawei

    The close relation between electronic properties and mechanical deformations in graphene has been the topic of active research in recent years. Interestingly, the effect of deformations on electronic properties can be understood in terms of pseudo-magnetic fields, whose spatial distribution and intensity are controllable via the deformation geometry. Previous results showed that electromagnetic fields (light) have the potential to induce dynamical gaps in graphene's energy bands, transforming graphene from a semimetal to a semiconductor. However, laser frequencies required to achieve these regimes are in the THz regime, which imposes challenges for practical purposes. In this talk we report a novel method to create dynamical gaps using oscillating mechanical deformations, i.e., via time-dependent pseudo-magnetic fields. Using the Floquet formalism we show the existence of a dynamical gap in the band structure at energies set by the frequency of the oscillation, and with a magnitude tuned by the geometry of the deformation. This dynamical-mechanical manipulation strategy appears as a promising venue to engineer electronic properties of suspended graphene devices. Work supported by NSF-DMR 1508325.

  6. 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.

  7. Two-electron bound states near a Coulomb impurity in gapped graphene

    NASA Astrophysics Data System (ADS)

    De Martino, Alessandro; Egger, Reinhold

    2017-02-01

    We formulate and solve the perhaps simplest two-body bound-state problem for interacting Dirac fermions in two spatial dimensions. A two-body bound state is predicted for gapped graphene monolayers in the presence of weakly repulsive electron-electron interactions and a Coulomb impurity with charge Z e >0 , where the most interesting case corresponds to Z =1 . We introduce a variational Chandrasekhar-Dirac spinor wave function and show the existence of at least one bound state. This state leaves clear signatures accessible by scanning tunneling microscopy. One may thereby obtain direct information about the strength of electron-electron interactions in graphene.

  8. Single-bilayer graphene oxide sheet impacts and underlying potential mechanism assessment in germinating faba bean (Vicia faba L.).

    PubMed

    Anjum, Naser A; Singh, Neetu; Singh, Manoj K; Sayeed, Iqbal; Duarte, Armando C; Pereira, Eduarda; Ahmad, Iqbal

    2014-02-15

    This study investigates the impact of different single-bilayer graphene oxide sheet (hereafter 'graphene oxide', GO; size: 0.5-5 μm) concentrations (0, 100, 200, 400, 800 and 1,600 mg L(-1)) and underlying potential mechanisms in germinating faba bean (Vicia faba L.) seedlings. The study revealed both positive and negative concentration-dependent GO-effects on V. faba. Significant negative impacts of GO concentrations (ordered by magnitude of effect: 1600>200>100 mg GO L(-1)) were indicated by decreases in growth parameters and the activity of H2O2-decomposing enzymes (ascorbate peroxidase, APX; catalase, CAT), and by increases in the levels of electrolyte leakage (EL), H2O2, and lipid and protein oxidation. The positive impacts of 400 and 800 mg GO L(-1) included significant improvements in V. faba health status indicated by decreased levels of EL, H2O2, and lipid and protein oxidation, and by enhanced H2O2-decomposing APX and CAT activity, and increased proline and seed-relative water content. V. faba seedlings-polypeptide patterns strongly substantiated these GO-concentration effects. Overall, the positive effects of these two GO concentrations (800>400 mg L(-1)) on V. faba seedlings indicate their safe nature and allow to suggest further studies.

  9. 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.

  10. 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.

  11. 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.

  12. Hofstadter butterflies and magnetically induced band-gap quenching in graphene antidot lattices

    NASA Astrophysics Data System (ADS)

    Pedersen, Jesper Goor; Pedersen, Thomas Garm

    2013-06-01

    We study graphene antidot lattices (GALs) in magnetic fields. Using a tight-binding model and a recursive Green's function technique that we extend to deal with periodic structures, we calculate Hofstadter butterflies of GALs. We compare the results to those obtained in a simpler gapped graphene model. A crucial difference emerges in the behavior of the lowest Landau level, which in a gapped graphene model is independent of magnetic field. In stark contrast to this picture, we find that in GALs the band gap can be completely closed by applying a magnetic field. While our numerical simulations can only be performed on structures much smaller than can be experimentally realized, we find that the critical magnetic field for which the gap closes can be directly related to the ratio between the cyclotron radius and the neck width of the GAL. In this way, we obtain a simple scaling law for extrapolation of our results to more realistically sized structures and find resulting quenching magnetic fields that should be well within reach of experiments.

  13. Opening the band gap of graphene through silicon doping for the improved performance of graphene/GaAs heterojunction solar cells.

    PubMed

    Zhang, S J; Lin, S S; Li, X Q; Liu, X Y; Wu, H A; Xu, W L; Wang, P; Wu, Z Q; Zhong, H K; Xu, Z J

    2016-01-07

    Graphene has attracted increasing interest due to its remarkable properties. However, the zero band gap of monolayered graphene limits it's further electronic and optoelectronic applications. Herein, we have synthesized monolayered silicon-doped graphene (SiG) with large surface area using a chemical vapor deposition method. Raman and X-ray photoelectron spectroscopy measurements demonstrate that the silicon atoms are doped into graphene lattice at a doping level of 2.7-4.5 at%. Electrical measurements based on a field effect transistor indicate that the band gap of graphene has been opened via silicon doping without a clear degradation in carrier mobility, and the work function of SiG, deduced from ultraviolet photoelectron spectroscopy, was 0.13-0.25 eV larger than that of graphene. Moreover, when compared with the graphene/GaAs heterostructure, SiG/GaAs exhibits an enhanced performance. The performance of 3.4% silicon doped SiG/GaAs solar cell has been improved by 33.7% on average, which was attributed to the increased barrier height and improved interface quality. Our results suggest that silicon doping can effectively engineer the band gap of monolayered graphene and SiG has great potential in optoelectronic device applications.

  14. Tuning the transport gap of functionalized graphene via electron beam irradiation

    NASA Astrophysics Data System (ADS)

    Martins, Steven E.; Withers, Freddie; Dubois, Marc; Craciun, Monica F.; Russo, Saverio

    2013-03-01

    We demonstrate a novel method to tune the energy gap ɛ1 between the localized states and the mobility edge of the valence band in chemically functionalized graphene by changing the coverage of fluorine adatoms via electron-beam irradiation. From the temperature dependence of the electrical transport properties we show that ɛ1 in partially fluorinated graphene CF0.28 decreases upon electron irradiation up to a dose of 0.08 C cm-2. For low irradiation doses (<0.1 C cm-2) partially fluorinated graphene behaves as a lightly doped semiconductor with impurity bands close to the conduction and valence band edges, whereas for high irradiation doses (>0.2 C cm-2) the electrical conduction takes place via Mott variable range hopping.

  15. 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.

  16. Growth of single and bilayer graphene by filtered cathodic vacuum arc technique

    SciTech Connect

    Kesarwani, A. K.; Panwar, O. S. Bisht, Atul; Dhakate, S. R.; Rakshit, R. K.; Singh, V. N.; Kumar, Ashish

    2016-03-15

    The authors present a viable process to grow the high quality graphene films with control over number of layers by the filtered cathodic vacuum arc (FCVA) technique. In the FCVA process, the different carbon concentrations can be controlled by precisely tuning the arc time (1–4 s). The arc generated carbon was deposited on the nickel catalyst at 800 °C, annealed for 10 min, and cooled down to room temperature in the presence of hydrogen gas, resulting in the graphene films with control over number of layers. Prior to arcing, hydrogen etching of nickel was carried out to clean the surface of the substrate. A growth model to prepare the high quality graphene has also been proposed. The as-grown graphene films were transferred to different substrates and are characterized by Raman spectroscopy, optical microscopy, high resolution transmission electron microscopy, and atomic force microscopy to determine the number of layers present in these films. Raman spectra of the prepared graphene films exhibit change in the G peak position from 1582.4 to 1578.1 cm{sup −1}, two-dimensional (2D) peak shifts from 2688.5 to 2703.8 cm{sup −1}, the value of I{sub 2D}/I{sub G} increased from 0.38 to 3.82, and the full width at half maxima of 2D peak changed from 41 to 70 cm{sup −1}, for different layers of graphene films. The high resolution transmission electron microscopy image revealed that the graphene films prepared for 1 and 2 s arc times have single and bi- or trilayered structures, respectively.

  17. 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.

  18. Structural and electronic properties of AB- and AA-stacking bilayer-graphene intercalated by Li, Na, Ca, B, Al, Si, Ge, Ag, and Au atoms

    NASA Astrophysics Data System (ADS)

    Tayran, Ceren; Aydin, Sezgin; Çakmak, Mehmet; Ellialtıoğlu, Şinasi

    2016-04-01

    The structural and electronic properties of X (=Li, Na, Ca, B, Al, Si, Ge, Ag, and Au)-intercalated AB- and AA-stacking bilayer-graphene have been investigated by using ab initio density functional theory. It is shown that Boron (Lithium)-intercalated system is energetically more stable than the others for the AB (AA) stacking bilayer-graphene systems. The structural parameters, electronic band structures, and orbital nature of actual interactions are studied for the relaxed stable geometries. It is seen that the higher the binding energy, the smaller is the distance between the layers, in these systems. The electronic band structures for these systems show that different intercalated atoms can change the properties of bilayer-graphene differently. For qualitative description of the electronic properties, the metallicities of the systems are also calculated and compared with each other. The Mulliken analysis and electron density maps clearly indicate that the interactions inside a single layer (intralayer interactions) are strong and highly covalent, while the interactions between the two layers (interlayer interactions) are much weaker.

  19. 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.

  20. Wigner localization in a graphene quantum dot with a mass gap

    NASA Astrophysics Data System (ADS)

    Guerrero Becerra, Karina Andrea; Rontani, Massimo

    2014-03-01

    The role of electron-electron interactions in graphene is an open issue that impacts on the operation of quantum dots (QDs) and other graphene-based devices. Whereas electrons in bulk graphene allegedly behave as noninteracting particles except for subtle effects, there is strong evidence that electrons in carbon-based nanostructures-nanotubes-form Wigner molecules [Nat. Phys. 9, 576 (2013)]. Besides, a significant effort is presently devoted to minimize the role of disorder in next-generation graphene QDs. Here we show theoretically that Dirac electrons in a clean, circular graphene QD with a mass gap induced by the breaking of sublattice symmetry form a Wigner molecule for realistic values of device parameters. The evidence is the combined analysis of many-body energies, one-body densities, and pair correlation functions obtained through the exact diagonalization of the interacting Dirac-Weyl Hamiltonian. This method, which uses two different sublattice envelopes and includes both inequivalent Dirac cones, allows us to take all many-body correlations into account. The experimental signature of Wigner localization is the suppression of the fourfold periodicity of the filling sequence and the quenching of excitation energies, accessible through Coulomb blockade spectroscopy.

  1. High-order harmonic generation from gapped graphene: Perturbative response and transition to nonperturbative regime

    NASA Astrophysics Data System (ADS)

    Dimitrovski, Darko; Madsen, Lars Bojer; Pedersen, Thomas Garm

    2017-01-01

    We consider the interaction of gapped graphene in the two-band approximation using an explicit time-dependent approach. In addition to the full high-order harmonic generation (HHG) spectrum, we also obtain the perturbative harmonic response using the time-dependent method at photon energies covering all the significant features in the responses. The transition from the perturbative to the fully nonperturbative regime of HHG at these photon energies is studied in detail.

  2. Preparation of Copper (Cu)-Nickel (Ni) Alloy Thin Films for Bilayer Graphene Growth

    DTIC Science & Technology

    2016-02-01

    1, 4:1, and 3:1 were developed for use as a catalyst for the growth of graphene. A design of experiments was initiated with 3 tasks to complete: 1...Experimental 2 2.1 Metal Catalyst Preparation 2 2.2 Alloy and Grain Growth Conditions 3 3. Results and Discussion 4 3.1 Metal Catalyst ...surface is fully covered by 1 layer of graphene due to the self- limiting effect.4 Recently, Cu-nickel (Ni) alloy foils have been used as a catalyst to

  3. Opening the band gap of graphene through silicon doping for the improved performance of graphene/GaAs heterojunction solar cells

    NASA Astrophysics Data System (ADS)

    Zhang, S. J.; Lin, S. S.; Li, X. Q.; Liu, X. Y.; Wu, H. A.; Xu, W. L.; Wang, P.; Wu, Z. Q.; Zhong, H. K.; Xu, Z. J.

    2015-12-01

    Graphene has attracted increasing interest due to its remarkable properties. However, the zero band gap of monolayered graphene limits it's further electronic and optoelectronic applications. Herein, we have synthesized monolayered silicon-doped graphene (SiG) with large surface area using a chemical vapor deposition method. Raman and X-ray photoelectron spectroscopy measurements demonstrate that the silicon atoms are doped into graphene lattice at a doping level of 2.7-4.5 at%. Electrical measurements based on a field effect transistor indicate that the band gap of graphene has been opened via silicon doping without a clear degradation in carrier mobility, and the work function of SiG, deduced from ultraviolet photoelectron spectroscopy, was 0.13-0.25 eV larger than that of graphene. Moreover, when compared with the graphene/GaAs heterostructure, SiG/GaAs exhibits an enhanced performance. The performance of 3.4% silicon doped SiG/GaAs solar cell has been improved by 33.7% on average, which was attributed to the increased barrier height and improved interface quality. Our results suggest that silicon doping can effectively engineer the band gap of monolayered graphene and SiG has great potential in optoelectronic device applications.Graphene has attracted increasing interest due to its remarkable properties. However, the zero band gap of monolayered graphene limits it's further electronic and optoelectronic applications. Herein, we have synthesized monolayered silicon-doped graphene (SiG) with large surface area using a chemical vapor deposition method. Raman and X-ray photoelectron spectroscopy measurements demonstrate that the silicon atoms are doped into graphene lattice at a doping level of 2.7-4.5 at%. Electrical measurements based on a field effect transistor indicate that the band gap of graphene has been opened via silicon doping without a clear degradation in carrier mobility, and the work function of SiG, deduced from ultraviolet photoelectron

  4. Spin- and valley-dependent Goos-Hänchen effect in silicene and gapped graphene structures

    NASA Astrophysics Data System (ADS)

    Azarova, E. S.; Maksimova, G. M.

    2017-01-01

    We investigate the Goos-Hänchen shift for ballistic electrons (i) reflected from a step-like inhomogeneity of the potential energy and (or) effective mass, and (ii) transmitted through a ferromagnetic barrier region in monolayer silicene or gapped graphene. For the electrons reflected from a single interface we found that the Goos-Hänchen shift is valley-polarized for gapped graphene structure, and valley- and spin-polarized for silicene due to the spin-valley coupling. In contrast, for example, to gapless graphene the lateral beam shift in gapped structures occurs not only in the case of total, but also of partial, reflection, i.e. at the angles smaller than the critical angle of total reflection. We have also demonstrated that the valley- and spin-polarized displacement of the electron beam, transmitted through a ferromagnetic silicene barrier, resonantly depends on the barrier width. The resonant values of the displacement can be controlled by adjusting the electric potential, the external perpendicular electric field, and the exchange field induced by an insulating ferromagnetic substrate.

  5. 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

  6. 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.

  7. Bilayer graphene moire pattern amplitude vs. twist angle identified using scanning tunneling microscopy

    NASA Astrophysics Data System (ADS)

    Thompson, Joshua; Ghosh, Pijush; Thibado, Paul; Neek-Amal, Mehdi; Peeters, Francois; Wheeler, V. D.; Myers-Ward, R. L.; Eddy, C. R., Jr.; Gaskill, D. K.

    2015-03-01

    Twisted stacked layers of graphene have unique electronic properties. The layers produce a moire pattern with a wavelength determined by the twist angle. In addition, however, the surface of the moire pattern also has an increased corrugation amplitude when compared to untwisted AB-stacked graphene. The deformation strains the top-layer graphene lattice and realizes, in a natural way, triaxial stress creation as proposed by Guinea et al. Consequently, very large pseudo-magnetic fields can be created depending on the amplitude of the corrugations. Until now, no relation has been found between the moire twist angle and its corrugation amplitude. We found, using scanning tunneling microscopy, as the wavelength of the moire pattern increases so does its amplitude. Our experiments are supported by first-principles directed elasticity theory. The membrane corrugation amplitude at arbitrary twist angle is found to be a consequence of a competition between the van der Waals bonding energy and the energy required to bend the graphene. Financial support was provided, in part, by the Office of Naval Research under Grant N00014-10-1-0181, the National Science Foundation under Grant DMR-0855358.

  8. 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.

  9. Membrane amplitude and triaxial stress in twisted bilayer graphene deciphered using first-principles directed elasticity theory and scanning tunneling microscopy

    NASA Astrophysics Data System (ADS)

    Neek-Amal, M.; Xu, P.; Qi, D.; Thibado, P. M.; Nyakiti, L. O.; Wheeler, V. D.; Myers-Ward, R. L.; Eddy, C. R.; Gaskill, D. K.; Peeters, F. M.

    2014-08-01

    Twisted graphene layers produce a moiré pattern (MP) structure with a predetermined wavelength for a given twist angle. However, predicting the membrane corrugation amplitude for any angle other than pure AB-stacked or AA-stacked graphene is impossible using first-principles density functional theory (DFT) due to the large supercell. Here, within elasticity theory, we define the MP structure as the minimum-energy configuration, thereby leaving the height amplitude as the only unknown parameter. The latter is determined from DFT calculations for AB- and AA-stacked bilayer graphene in order to eliminate all fitting parameters. Excellent agreement with scanning tunneling microscopy results across multiple substrates is reported as a function of twist angle.

  10. Surface enhanced Raman scattering of gold nanoparticles supported on copper foil with graphene as a nanometer gap.

    PubMed

    Xiang, Quan; Zhu, Xupeng; Chen, Yiqin; Duan, Huigao

    2016-02-19

    Gaps with single-nanometer dimensions (<10 nm) between metallic nanostructures enable giant local field enhancements for surface enhanced Raman scattering (SERS). Monolayer graphene is an ideal candidate to obtain a sub-nanometer gap between plasmonic nanostructures. In this work, we demonstrate a simple method to achieve a sub-nanometer gap by dewetting a gold film supported on monolayer graphene grown on copper foil. The Cu foil can serve as a low-loss plasmonically active metallic film that supports the imaginary charge oscillations, while the graphene can not only create a stable sub-nanometer gap for massive plasmonic field enhancements but also serve as a chemical enhancer. We obtained higher SERS enhancements in this graphene-gapped configuration compared to those in Au nanoparticles on Cu film or on graphene-SiO2-Si. Also, the Raman signals measured maintained their fine features and intensities over a long time period, indicating the stability of this Au-graphene-Cu hybrid configuration as an SERS substrate.

  11. Field emission properties of chemical vapor deposited individual graphene

    SciTech Connect

    Zamri Yusop, Mohd; Kalita, Golap; Yaakob, Yazid; Takahashi, Chisato; Tanemura, Masaki

    2014-03-03

    Here, we report field emission (FE) properties of a chemical vapor deposited individual graphene investigated by in-situ transmission electron microscopy. Free-standing bilayer graphene is mounted on a cathode microprobe and FE processes are investigated varying the vacuum gap of cathode and anode. The threshold field for 10 nA current were found to be 515, 610, and 870 V/μm for vacuum gap of 400, 300, and 200 nm, respectively. It is observed that the structural stability of a high quality bilayer graphene is considerably stable during emission process. By contacting the nanoprobe with graphene and applying a bias voltage, structural deformation and buckling are observed with significant rise in temperature owing to Joule heating effect. The finding can be significant for practical application of graphene related materials in emitter based devices as well as understanding the contact resistance influence and heating effect.

  12. 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.

  13. Band-gap engineering by Bi intercalation of graphene on Ir(111)

    NASA Astrophysics Data System (ADS)

    Warmuth, Jonas; Bruix, Albert; Michiardi, Matteo; Hänke, Torben; Bianchi, Marco; Wiebe, Jens; Wiesendanger, Roland; Hammer, Bjørk; Hofmann, Philip; Khajetoorians, Alexander A.

    2016-04-01

    We report on the structural and electronic properties of a single bismuth layer intercalated underneath a graphene layer grown on an Ir(111) single crystal. Scanning tunneling microscopy (STM) reveals a hexagonal surface structure and a dislocation network upon Bi intercalation, which we attribute to a √{3 }×√{3 }R 30∘ Bi structure on the underlying Ir(111) surface. Ab initio calculations show that this Bi structure is the most energetically favorable and illustrate that STM measurements are most sensitive to C atoms in close proximity to intercalated Bi atoms. Additionally, Bi intercalation induces a band gap (Eg=0.42 eV) at the Dirac point of graphene and an overall n doping (˜0.39 eV ) as seen in angular-resolved photoemission spectroscopy. We attribute the emergence of the band gap to the dislocation network which forms favorably along certain parts of the moiré structure induced by the graphene/Ir(111) interface.

  14. Effective mass in bilayer graphene at low carrier densities: The role of potential disorder and electron-electron interaction

    NASA Astrophysics Data System (ADS)

    Li, J.; Tan, L. Z.; Zou, K.; Stabile, A. A.; Seiwell, D. J.; Watanabe, K.; Taniguchi, T.; Louie, Steven G.; Zhu, J.

    2016-10-01

    In a two-dimensional electron gas, the electron-electron interaction generally becomes stronger at lower carrier densities and renormalizes the Fermi-liquid parameters, such as the effective mass of carriers. We combine experiment and theory to study the effective masses of electrons and holes me* and mh* in bilayer graphene in the low carrier density regime on the order of 1 ×1011c m-2 . Measurements use temperature-dependent low-field Shubnikov-de Haas oscillations observed in high-mobility hexagonal boron nitride supported samples. We find that while me* follows a tight-binding description in the whole density range, mh* starts to drop rapidly below the tight-binding description at a carrier density of n =6 ×1011c m-2 and exhibits a strong suppression of 30% when n reaches 2 ×1011c m-2 . Contributions from the electron-electron interaction alone, evaluated using several different approximations, cannot explain the experimental trend. Instead, the effect of the potential fluctuation and the resulting electron-hole puddles play a crucial role. Calculations including both the electron-electron interaction and disorder effects explain the experimental data qualitatively and quantitatively. This Rapid Communication reveals an unusual disorder effect unique to two-dimensional semimetallic systems.

  15. Gap-fill type HSQ/ZEP520A bilayer resist process-(II): HSQ island and spacer formation

    NASA Astrophysics Data System (ADS)

    Chen, Wei-Su; Gu, Pei-Yi; Kao, Ming-Jer; Tsai, Ming-Jinn

    2008-03-01

    Hydrogen silsesquioxane (HSQ) bilayer resist (BLR) processes are attractive to obtain nano-sized features with high aspect ratio by dry-transferring thin e-beam pattern to thick underlayer to strengthen the etch resistance. However, there are drawbacks of high e-beam dosage for HSQ patterning and difficulty in controlling the underlayer resist profile by O2 plasma with anisotropic etching. In this study gap-fill type HSQ/ZEP520A BLR processes were studied to overcome these problems. The advantage of gap-fill type BLR processes is that the dosage for patterning on thick ZEP520A e-beam positive resist is not as high as that for HSQ and the resist profile can be tuned by exposure and development processes without depending on O2 plasma. By gap-filling of HSQ in ZEP520A trench patterns and then stripping ZEP520A by O2 plasma the tone is conversed from trench to line. The gap filling quality attributes include (1) the void size and number of HSQ lines and (2) spacer adhesion on HSQ line edge. Only the non-diluted HSQ solution could completely fill the trench and the HSQ line formed after stripping of ZEP520A. The spacer formed by diluted HSQ is found to be composed of oxide without any ZEP520A-related elements by FTIR analysis. The ZEP520A trench CD monotonically increases with decrease of W/L ratio. The HSQ line CD also follows the same trend. The extension of HSQ in ZEP520A, i.e. HSQ line CD minus ZEP520A trench CD, basically follows the reverse trend. It is therefore concluded that extension of HSQ lines in ZEP520A and HSQ spacers are formed from the diffused HSQ in trench sidewall without any reaction with ZEP520A. Voids were generally observed at the bottom of the HSQ line. Size and quantity of voids are larger for lower W/L ratios, indicating that the voids were formed due to insufficient HSQ volume for gap-filling. Increasing e-beam dose, baking or reflow temperature, and reflow of ZEP520A before HSQ coating could reduce the void formation. Multiple gap

  16. Critical behavior of reduced QED4 ,3 and dynamical fermion gap generation in graphene

    NASA Astrophysics Data System (ADS)

    Kotikov, A. V.; Teber, S.

    2016-12-01

    The dynamical generation of a fermion gap in graphene is studied at the infra-red Lorentz-invariant fixed point where the system is described by an effective relativistic-like field theory: reduced QED4 ,3 with N four-component fermions (N =2 for graphene), where photons are (3 +1 ) dimensional and mediate a fully retarded interaction among (2 +1 )-dimensional fermions. A correspondence between reduced QED4 ,3 and QED3 allows us to derive an exact gap equation for QED4 ,3 up to next-to-leading order. Our results show that a dynamical gap is generated for α >αc, where 1.03 <αc<1.08 in the case N =2 or for N

  17. Subharmonic gap structure of normal-state conductance and thermoelectric effect in a graphene-based nano device.

    PubMed

    Park, K S; Park, Joonho; Yi, K S

    2008-09-01

    Using the quantum transport of chiral Dirac fermions in graphene, we investigate the normal-state conductance and thermoelectric effect of a nano device under the ballistic superconductor-graphene-superconductor (SGS) model. Because of the Josephson effect and Andreev reflections, there exists an oscillatory behavior of the normal-state conductance flowing through the successive discrete energy levels on a finite-sized graphene contacted to the superconducting leads. The normal-state conductance displays a rich structure of subharmonic gaps controlled by means of a gate voltage on the discrete energy levels near the Fermi energy. Since the Fermi energy is an essential factor in determining the nature of conduction such as n or p type, we study the thermoelectric effect over the graphene-based nano device. It is shown that the thermoelectric effect can provide information on the location of the Fermi energy with respect to the energy levels of the finite-sized graphene.

  18. 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.

  19. 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.

  20. 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

  1. Graphene-like carbon nitride layers: stability, porosity, band gaps, and magnetic ground states

    NASA Astrophysics Data System (ADS)

    Chacham, Helio; da Silva-Araujo, Joice; Brito, Walber

    In the present work, we investigate the relative stability and electronic properties of carbon nitride (CxNy) graphene-like structures using a combination of a new bond-counting method and density-functional-theory (DFT) first-principles calculations. We obtain analytical and numerical results for the energetics and the morphology of graphene-like CxNy For instance, at high N concentrations, the bond-counting method allows us to search among millions of possible structures, and we find several ones with ab initio formation energies per N atom comparable to, or even smaller than, that of the isolated graphitic N impurity. Those structures are characterized by a variety of nanoporous graphene morphologies. The low-energy C-N structures also present a variety of band gaps, from zero to 1.6 eV, which can be tuned by stoichiometry and porosity. Several structures also present ferro- and antiferromagnetic ground states. We thank support from CNPq, CAPES, and FAPEMIG.

  2. Band gap and chemically ordered domain structure of a graphene analogue BCN

    NASA Astrophysics Data System (ADS)

    Venu, K.; Kanuri, S.; Raidongia, K.; Hembram, K. P. S. S.; Waghmare, U. V.; Datta, R.

    2010-12-01

    Chemically synthesized few layer graphene analogues of B xC yN z are characterized by aberration corrected transmission electron microscopy and high resolution electron energy loss spectroscopy (HREELS) to determine the local phase, electronic structure and band gap. HREELS band gap studies of a B xC yN z composition reveal absorption edges at 2.08, 3.43 and 6.01 eV, indicating that the B xC yN z structure may consist of domains of different compositions. The K-absorption edge energy position of the individual elements in B xC yN z is determined and compared with h-BN and graphite. An understanding of these experimental findings is developed with complementary first-principles based calculations of the various ordered configurations of B xC yN z.

  3. Transport gap engineering by contact geometry in graphene nanoribbons: Experimental and theoretical studies on artificial materials

    NASA Astrophysics Data System (ADS)

    Stegmann, Thomas; Franco-Villafañe, John A.; Kuhl, Ulrich; Mortessagne, Fabrice; Seligman, Thomas H.

    2017-01-01

    Electron transport in small graphene nanoribbons is studied by microwave emulation experiments and tight-binding calculations. In particular, it is investigated under which conditions a transport gap can be observed. Our experiments provide evidence that armchair ribbons of width 3 m +2 with integer m are metallic and otherwise semiconducting, whereas zigzag ribbons are metallic independent of their width. The contact geometry, defining to which atoms at the ribbon edges the source and drain leads are attached, has strong effects on the transport. If leads are attached only to the inner atoms of zigzag edges, broad transport gaps can be observed in all armchair ribbons as well as in rhomboid-shaped zigzag ribbons. All experimental results agree qualitatively with tight-binding calculations using the nonequilibrium Green's function method.

  4. Four-fold Raman enhancement of 2D band in twisted bilayer graphene: evidence for a doubly degenerate Dirac band and quantum interference.

    PubMed

    Wang, Yanan; Su, Zhihua; Wu, Wei; Nie, Shu; Lu, Xinghua; Wang, Haiyan; McCarty, Kevin; Pei, Shin-shem; Robles-Hernandez, Francisco; Hadjiev, Viktor G; Bao, Jiming

    2014-08-22

    We report the observation of a strong 2D band Raman in twisted bilayer graphene (tBLG) with large rotation angles under 638 nm and 532 nm visible laser excitations. The 2D band Raman intensity increased four-fold as opposed to the two-fold increase observed in single-layer graphene. The same tBLG samples also exhibited rotation-dependent G-line resonances and folded phonons under 364 nm UV laser excitation. We attribute this 2D band Raman enhancement to the constructive interference between two double-resonance Raman pathways, which were enabled by a nearly degenerate Dirac band in the tBLG Moiré superlattices.

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

    NASA Astrophysics Data System (ADS)

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

    2014-11-01

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

  6. 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.

  7. 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.

  8. DMRG study of fractional quantum Hall effect and valley skyrmions in graphene

    NASA Astrophysics Data System (ADS)

    Shibata, Naokazu

    2011-12-01

    The ground state and low-energy excitations of graphene and its bilayer are investigated by the density matrix renormalization group (DMRG) method. We analyze the effect of Coulomb interaction between the electrons including valley degrees of freedoms. The obtained results show finite charge excitation gap at various fractional fillings νn = 1/3, 2/5, 2/3 in the n = 0 and 1 Landau levels of single-layer graphene (SLG) and n = 2 Landau level of bilayer graphene (BLG). The lowest charge excitations at ν = 1/3, and 1 in SLG are valley skyrmions.

  9. 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.

  10. Electronic Structures, Bonding Configurations, and Band-Gap-Opening Properties of Graphene Binding with Low-Concentration Fluorine

    DOE PAGES

    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

  11. Electronic Structures, Bonding Configurations, and Band-Gap-Opening Properties of Graphene Binding with Low-Concentration Fluorine.

    PubMed

    Duan, Yuhua; Stinespring, Charter D; Chorpening, Benjamin

    2015-10-01

    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-2 H 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 p orbitals 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.

  12. 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.

  13. 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.

  14. Gaps induced by inversion symmetry breaking and second-generation Dirac cones in graphene/hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Wang, Eryin; Lu, Xiaobo; Ding, Shijie; Yao, Wei; Yan, Mingzhe; Wan, Guoliang; Deng, Ke; Wang, Shuopei; Chen, Guorui; Ma, Liguo; Jung, Jeil; Fedorov, Alexei V.; Zhang, Yuanbo; Zhang, Guangyu; Zhou, Shuyun

    2016-12-01

    Graphene/hexagonal boron nitride (h-BN) has emerged as a model van der Waals heterostructure as the superlattice potential, which is induced by lattice mismatch and crystal orientation, gives rise to various novel quantum phenomena, such as the self-similar Hofstadter butterfly states. Although the newly generated second-generation Dirac cones (SDCs) are believed to be crucial for understanding such intriguing phenomena, fundamental knowledge of SDCs, such as locations and dispersion, and the effect of inversion symmetry breaking on the gap opening, still remains highly debated due to the lack of direct experimental results. Here we report direct experimental results on the dispersion of SDCs in 0°-aligned graphene/h-BN heterostructures using angle-resolved photoemission spectroscopy. Our data unambiguously reveal SDCs at the corners of the superlattice Brillouin zone, and at only one of the two superlattice valleys. Moreover, gaps of approximately 100 meV and approximately 160 meV are observed at the SDCs and the original graphene Dirac cone, respectively. Our work highlights the important role of a strong inversion-symmetry-breaking perturbation potential in the physics of graphene/h-BN, and fills critical knowledge gaps in the band structure engineering of Dirac fermions by a superlattice potential.

  15. Chemical Vapor Deposition Synthesis and Terahertz Photoconductivity of Low-Band-Gap N = 9 Armchair Graphene Nanoribbons.

    PubMed

    Chen, Zongping; Wang, Hai I; Teyssandier, Joan; Mali, Kunal S; Dumslaff, Tim; Ivanov, Ivan; Zhang, Wen; Ruffieux, Pascal; Fasel, Roman; Räder, Hans Joachim; Turchinovich, Dmitry; De Feyter, Steven; Feng, Xinliang; Kläui, Mathias; Narita, Akimitsu; Bonn, Mischa; Müllen, Klaus

    2017-03-15

    Recent advances in bottom-up synthesis of atomically defined graphene nanoribbons (GNRs) with various microstructures and properties have demonstrated their promise in electronic and optoelectronic devices. Here we synthesized N = 9 armchair graphene nanoribbons (9-AGNRs) with a low optical band gap of ∼1.0 eV and extended absorption into the infrared range by an efficient chemical vapor deposition process. Time-resolved terahertz spectroscopy was employed to characterize the photoconductivity in 9-AGNRs and revealed their high intrinsic charge-carrier mobility of approximately 350 cm(2)·V(-1)·s(-1).

  16. 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.

  17. 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).

  18. 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.

  19. Eighty-Eight Percent Directional Guiding of Spin Currents with 90 μm Relaxation Length in Bilayer Graphene Using Carrier Drift.

    PubMed

    Ingla-Aynés, Josep; Meijerink, Rick J; Wees, Bart J van

    2016-08-10

    Electrical control of spin signals and long distance spin transport are major requirements in the field of spin electronics. Here, we report the efficient guiding of spin currents at room temperature in high mobility hexagonal boron nitride encapsulated bilayer graphene using carrier drift. Our experiments, together with modeling, show that the spin relaxation length, that is 7.7 μm at zero bias, can be tuned from 0.6 to 90 μm when applying a DC current of ∓90 μA, respectively. Our results also show that we are able to direct spin currents to either side of a spin injection contact. Eighty-eight percent of the injected spins flows to the left when Idc = -90 μA and eighty-two percent flows to the right when the drift current is reversed. These results show the potential of carrier drift for spin-based logic operations and devices.

  20. 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

    2012-03-07

    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.

  1. Mass inversion in graphene by proximity to dichalcogenide monolayer

    NASA Astrophysics Data System (ADS)

    Alsharari, Abdulrhman M.; Asmar, Mahmoud M.; Ulloa, Sergio E.

    2016-12-01

    Proximity effects resulting from depositing a graphene layer on a transition-metal dichalcogenide substrate layer change the dynamics of the electronic states in graphene, inducing spin-orbit coupling and staggered potential effects. An effective Hamiltonian that describes different symmetry-breaking terms in graphene, while preserving time-reversal invariance, shows that an inverted mass band-gap regime is possible. The competition of different perturbation terms causes a transition from an inverted mass phase to a staggered gap in the bilayer heterostructure as seen in its phase diagram. A tight-binding calculation of the bilayer validates the effective model parameters. A relative gate voltage between the layers may produce such a phase transition in experimentally accessible systems. The phases are characterized in terms of Berry curvature and valley Chern numbers, demonstrating that the system may exhibit quantum spin Hall and valley Hall effects.

  2. 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.

  3. Finite-size effects on electronic structure and local properties in passivated AA-stacked bilayer armchair-edge graphene nanoribbons.

    PubMed

    Chen, Xiongwen; Shi, Zhengang; Xiang, Shaohua; Song, Kehui; Zhou, Guanghui

    2017-03-01

    Based on the tight-binding model and dual-probe scanning tunneling microscopy technology, we theoretically investigate the electronic structure and local property in the passivated AA-stacked bilayer armchair-edge graphene nanoribbons (AABLAGNRs). We show that they are highly sensitive to the size of the ribbons, which is evidently different from the single-layer armchair-edge graphene nanoribbons. The '3p' rule only applies to the narrow AABLGNRs. Namely, in the passivated 3p- and (3p  +  1)-AABLGNRs, the narrow ribbons are semiconducting while the medium and wide ribbons are metallic. Although the passivated (3p  +  2)-AABLGNRs are metallic, the '3j' rule only applies to the narrow and medium ribbons. Namely, electrons are in the semiconducting states at sites of line 3j while they are in the metallic states at other sites. This induces a series of parallel and discrete metallic channels, consisting of lines 3j  -  1 and 3j  -  2, for the low-energy electronic transports. In the passivated wide (3p  +  2)-AABLGNRs, all electrons are in the metallic states. Additionally, the '3p' and '3j' rules are controllable to disappear and reappear by applying an external perpendicular electric field. Resultantly, an electric filed-driven current switch can be realized in the passivated narrow and medium (3p  +  2)-AABLGNRs.

  4. Finite-size effects on electronic structure and local properties in passivated AA-stacked bilayer armchair-edge graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Chen, Xiongwen; Shi, Zhengang; Xiang, Shaohua; Song, Kehui; Zhou, Guanghui

    2017-03-01

    Based on the tight-binding model and dual-probe scanning tunneling microscopy technology, we theoretically investigate the electronic structure and local property in the passivated AA-stacked bilayer armchair-edge graphene nanoribbons (AABLAGNRs). We show that they are highly sensitive to the size of the ribbons, which is evidently different from the single-layer armchair-edge graphene nanoribbons. The ‘3p’ rule only applies to the narrow AABLGNRs. Namely, in the passivated 3p- and (3p  +  1)-AABLGNRs, the narrow ribbons are semiconducting while the medium and wide ribbons are metallic. Although the passivated (3p  +  2)-AABLGNRs are metallic, the ‘3j’ rule only applies to the narrow and medium ribbons. Namely, electrons are in the semiconducting states at sites of line 3j while they are in the metallic states at other sites. This induces a series of parallel and discrete metallic channels, consisting of lines 3j  -  1 and 3j  -  2, for the low-energy electronic transports. In the passivated wide (3p  +  2)-AABLGNRs, all electrons are in the metallic states. Additionally, the ‘3p’ and ‘3j’ rules are controllable to disappear and reappear by applying an external perpendicular electric field. Resultantly, an electric filed-driven current switch can be realized in the passivated narrow and medium (3p  +  2)-AABLGNRs.

  5. Conversion of Langmuir–Blodgett monolayers and bilayers of poly(amic acid) through polyimide to graphene

    NASA Astrophysics Data System (ADS)

    Jo, Hye Jin; Lyu, Ji Hong; Ruoff, Rodney S.; Lim, Hyunseob; In Yoon, Seong; Jeong, Hu Young; Shin, Tae Joo; Bielawski, Christopher W.; Shin, Hyeon Suk

    2017-03-01

    Various solid carbon sources, particularly poly(methyl methacrylate), have been used as precursors to graphene. The corresponding growth process generally involves the decomposition of the solids to hydrocarbon gases followed by their adsorption on metallic substrates (e.g., Cu). We report a different approach that uses a thermally-resistant polyimide (PI) as a carbon precursor. Langmuir–Blodgett films of poly(amic acid) (PAA) were transferred to copper foils and then converted to graphene via a PI intermediate. The Cu foil substrate was also discovered to facilitate the orientation of aromatic moieties upon carbonization process of the PI. As approximately 50% of the initial quantity of the PAA was found to remain at 1000 °C, thermally-stable polymers may reduce the quantity of starting material required to prepare high quality films of graphene. Graphene grown using this method featured a relatively large domain size and an absence of adventitious adlayers.

  6. 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.

  7. Orbital-specific Tunability of Many-Body Effects in Bilayer Graphene by Gate Bias and Metal Contact

    PubMed Central

    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. PMID:24429879

  8. The possibility of chemically inert, graphene-based all-carbon electronic devices with 0.8 eV gap.

    PubMed

    Qi, Jing Shan; Huang, Jian Yu; Feng, Ji; Shi, Da Ning; Li, Ju

    2011-05-24

    Graphene is an interesting electronic material. However, flat monolayer graphene does not have significant gap in the electronic density of states, required for a large on-off ratio in logic applications. We propose here a novel device architecture, composed of self-folded carbon nanotube-graphene hybrids, which have been recently observed experimentally in Joule-heated graphene. These experiments demonstrated the feasibility of cutting, folding, and welding few-layer graphene in situ to form all-carbon nanostructures with complex topologies. The electronic gap of self-folded nanotubes can be combined with the semimetallicity of graphene electrodes to form a "metal-semiconductor-metal" junction. By ab initio calculations we demonstrate large energy gaps in the transmission spectra of such junctions, which preserve the intrinsic transport characteristics of the semiconducting nanotubes despite topologically necessary disinclinations at the flat graphene-curved nanotube interface. These all-carbon devices are proposed to be constructed by contact probe cutting and high-temperature annealing and, if produced, would be chemically stable at room temperature under normal gas environments.

  9. 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).

  10. First Principles Study of Band Structure and Band Gap Engineering in Graphene for Device Applications

    DTIC Science & Technology

    2015-03-20

    functionalized graphene. 15. SUBJECT TERMS NDE, Finite Difference Methods, Carbon Fiber composites 16...results. Introduction Graphene, the super carbon , is now accepted as wonder material with new physics and it has caused major...theoretical work [19,21-24] are reported on substitutional covalent bonding in carbon /nitrogen, carbon /boron, and carbon /boron/nitrogen systems. Some of the

  11. Tunability of 1/f Noise at Multiple Dirac Cones in hBN Encapsulated Graphene Devices.

    PubMed

    Kumar, Chandan; Kuiri, Manabendra; Jung, Jeil; Das, Tanmoy; Das, Anindya

    2016-02-10

    The emergence of multiple Dirac cones in hexagonal boron nitride (hBN)-graphene heterostructures is particularly attractive because it offers potentially better landscape for higher and versatile transport properties than the primary Dirac cone. However, the transport coefficients of the cloned Dirac cones is yet not fully characterized and many open questions, including the evolution of charge dynamics and impurity scattering responsible for them, have remained unexplored. Noise measurements, having the potential to address these questions, have not been performed to date in dual-gated hBN-graphene-hBN devices. Here, we present the low-frequency 1/f noise measurements at multiple Dirac cones in hBN encapsulated single and bilayer graphene in dual-gated geometry. Our results reveal that the low-frequency noise in graphene can be tuned by more than two-orders of magnitude by changing carrier concentration as well as by modifying the band structure in bilayer graphene. We find that the noise is surprisingly suppressed at the cloned Dirac cone compared to the primary Dirac cone in single layer graphene device, while it is strongly enhanced for the bilayer graphene with band gap opening. The results are explained with the calculation of dielectric function using tight-binding model. Our results also indicate that the 1/f noise indeed follows the Hooge's empirical formula in hBN-protected devices in dual-gated geometry. We also present for the first time the noise data in bipolar regime of a graphene device.

  12. 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.

  13. Two-dimensional wide-band-gap II-V semiconductors with a dilated graphene-like structure

    NASA Astrophysics Data System (ADS)

    Zhang, Xue-Jing; Liu, Bang-Gui

    2016-12-01

    Since the advent of graphene, two-dimensional (2D) materials have become very attractive and there is growing interest in exploring new 2D materials beyond graphene. Here, through density-functional theory (DFT) calculations, we predict 2D wide-band-gap II-V semiconductor materials of M3X2 (M = Zn, Cd and X = N, P, As) with a dilated graphene-like honeycomb structure. In this structure the group-V X atoms form two X-atomic planes symmetrically astride the centering group-IIB M atomic plane. Our DFT calculation shows that 2D Zn3N2, Zn3P2 and Zn3As2 have direct band gaps of 2.87, 3.81 and 3.55 eV, respectively, and 2D Cd3N2, Cd3P2 and Cd3As2 exhibit indirect band gaps of 2.74, 3.51 and 3.29 eV, respectively. Each of the six 2D materials is shown to have effective carrier (either hole or electron) masses down to 0.03m 0-0.05m 0. The structural stability and feasibility of experimental realization of these 2D materials has been shown in terms of DFT phonon spectra and total energy comparison with related existing bulk materials. On the experimental side, there already are many similar two-coordinate structures of Zn and other transition metals in various organic materials. Therefore, these 2D semiconductors can enrich the family of 2D electronic materials and may have promising potential for achieving novel transistors and optoelectronic devices.

  14. 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.

  15. Topological superconductor with a large Chern number and a large bulk excitation gap in single-layer graphene

    NASA Astrophysics Data System (ADS)

    Wang, L.; Wu, M. W.

    2016-02-01

    We show that a two-dimensional topological superconductor (TSC) can be realized in a hybrid system with a conventional s -wave superconductor proximity coupled to a quantum anomalous Hall (QAH) state from the Rashba and exchange effects in single-layer graphene. With very low or even zero doping near the Dirac points, i.e., two inequivalent valleys, this TSC has a Chern number as large as 4, which supports four Majorana edge modes. More importantly, we show that this TSC has a robust topologically nontrivial bulk excitation gap, which can be larger or even 1 order of magnitude larger than the proximity-induced superconducting gap. This unique property paves a way for the application of QAH insulators as seed materials to realize robust TSCs and Majorana modes.

  16. 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.

  17. Thermodynamic Properties of Gapped Graphene in the Presence of a Transverse Magnetic Field by Considering Holstein Phonons

    NASA Astrophysics Data System (ADS)

    Yarmohammadi, Mohsen

    2017-02-01

    Using the Holstein model, the thermodynamic properties of gapped graphene in the presence of electron-phonon (e-ph) coupling and a transverse magnetic field are investigated. In particular, we have obtained density of states (DOS), electronic heat capacity (EHC) and magnetic susceptibility (MS) of graphene, for which carbon atoms are substituted by boron and nitride atoms in the presence of Holstein phonons and a transverse magnetic field within the Green's function approach in order to investigate the dynamic of Dirac fermions. To find the electronic self-energy due to e-ph coupling and the substituted foreign atoms, the self-consistent second order perturbation theory has been implemented. The band gap decreases with magnetic field and e-ph coupling. Also splitting of the quantum states (energy levels) due to the magnetic field is observed as double peaks in DOS (Van Hove singularities). As a remarkable result, EHC and MS are decreased due to the increase of scattering rate between electrons, an applied magnetic field, and e-ph coupling.

  18. 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

  19. 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.

  20. 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.