Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z25: Focus Session: Graphene XIX: Electronic Properties |
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Sponsoring Units: DMP Chair: Silvia Viola, Boston University Room: 327 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z25.00001: Chiral structure and mixed parity anomalies in graphene-related systems Akihiro Tanaka We reanalyze the chiral symmetry structure of graphene and its variants (boron-nitride sheets, bond alternated graphene, etc) using a representation of Dirac fermions previously employed by the author in a search for topological effects in the pi- flux state of a square lattice electron system (PRL ${\bf 95}$ 036402 (2005)). We find that the electromagnetic responses of nontopological insulators to curvature-induced gauge fields mimics in an interesting way the responses of topological insulators of the Haldane/Kane-Mele category to conventional (Maxwellian) gauge fields. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z25.00002: Many-electron Effects on the Electronic Structure and Optical Spectrum of Few-layer Graphene Li Yang, Jack Deslippe, Cheol-Hwan Park, Marvin Cohen, Steven Louie We present a first-principles calculation of the optical properties of single- and few-layer graphene with many-electron effects included, employing the GW-Bethe Salpeter equation (GW- BSE) approach. We have found enhanced excitonic effects that result in significant changes in the optical absorption of few- layer graphene as compared to the independent-particle picture. Our calculated absorption spectrum is in good agreement with recent experiments. This study is of importance for understanding excitonic effects in two-dimensional semimetal systems and expected to be useful for possible optoelectronics applications of graphene. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z25.00003: Geometrical constraint on alkali and halogen adsorption on graphene ChengIng Chia A seamless sp2 graphene sheet prevents the penetration of atoms through the sheet, yet allows the penetration of electrons. Thus, a suspended single sheet graphene forms a geometrical constrained background by separating the surrounding vacuum into upper half and lower half spaces. Alkali and halogen atoms, each constrained to one of the spaces, are forced to interact electrostatically via charge transfer through the sheet. A new type of chemical interaction is formed under this constraint, which we call topologically frustration bonding. We have calculated the interaction of a K atom on the upper surface with a halogen atom on the lower surface of a pure-carbon graphene sheet using density functional theory. The system becomes ferroelectric under this new geometrical constraint. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z25.00004: Pivotal role of buffer layer in tuning electronic properties of epitaxial graphene Yufeng Guo, Wanlin Guo, Changfeng Chen We explore the response of epitaxial bilayer graphene on SiC and Ru to electric field and mechanical tuning using first- principles calculations. Our calculations reveal that, in contrast to prevailing view, the buffer layer plays an active role in the distribution of charge transfer within the epitaxial graphene layers and with the substrate. The charge distribution and electronic structure are also sensitive to the type of substrate. These results provide new insights for fundamental understanding and practical application of these fascinating materials. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z25.00005: Graphene on Silicon Dioxide: Band gap modulation via substrate surface chemistry Philip Shemella, Saroj K. Nayak We have studied the electronic structure of graphene deposited on a SiO$_2$ surface using density functional methods. The band structure of the graphene monolayer strongly depends on surface characteristics of the underlying SiO$_2$ surface: for an oxygen-terminated surface, the monolayer exhibits a finite energy band gap while the band gap is closed when the oxygen atoms on the substrate are passivated with hydrogen atoms. We find that at least a graphene bilayer is required for a near zero energy gap when deposited on a substrate without H-passivation. Our results are discussed in the light of recent experiments. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z25.00006: Current polarization in B-doped graphene nanoribbons: ab initio simulations Alexandre Rocha, Thiago Martins, Adalberto Fazzio, Ant\^onio J. R. da Silva Single layer graphene has been recently isolated and can pave the way to a number of nanoscale technologies. One interesting possibility is to use the spin of the electron - instead of its charge - as information carrier in carbon-based systems where the spin coherence length can reach hundreds of nanometers. Up until now, spintronics devices have been assembled using magnetic electrodes as a source of spin polarized electrons. In this work we use a combination of density functional theory and non-equilibrium Green's functions techniques to study the electronic transport properties of graphene nanoribbons (GNRB) up to 500 nm long containing substitutional Boron atoms. We demonstrate that in realistic systems where the B atoms are randomly distributed along the GNRB, the polarization of the current can reach up to 100\% and is independent of impurity concentration. These effects can be explained in terms of different scattering probabilities for majority and minority spins from a single B atom. This consequently leads to different Anderson localization lengths for each spin population. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z25.00007: Impact of the electron-electron correlation on phonon dispersion:Failure of LDA and GGA DFT functionals in graphene and graphite. Michele Lazzeri, Claudio Attaccalite, Ludger Wirtz, Angel Rubio, Francesco Mauri GW is nowadays the most accurate ab-initio method to determine electronic bands. So far GW has never been used to determine neither the electron-phonon coupling (EPC) nor phonon dispersions. We show that GW approach [1] can be used to compute the EPC and the phonon dispersion. In particular, in graphene and graphite, standard DFT (LDA and GGA) underestimates, by a factor of 2, the slope of the highest optical branch at the zone boundary (K) and the square of its EPC by almost 80\%. On the contrary, GW reproduces the experimental phonon dispersion near K, the value of the EPC, and the electronic band dispersion, in agreement with phonon dispersions from inelastic x-ray scattering and Raman spectroscopy. Comparing these results with other computational methods, the B3LYP hybrid functional gives phonons close to GW but overestimates the EPC at K by about 30\%. Within Hartree-Fock, the graphene structure displays an instability under a distortion following the A'1 phonon at K. [1] M. Lazzeri et al., Phys. Rev. B 78, 081406(R) (2008). [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z25.00008: First-Principles Studies of Oxidation Functional Groups on Graphene Jia-An Yan, Mei-Yin Chou Opening a band gap in monolayer graphene is of special interest for the graphene-based electronics applications. Inspired by the potential applications of graphene oxide, we have systematically investigated the effects of the oxidation functional groups (epoxy and hydroxyl) on the structural, energetics, and electronic properties of graphene by first-principles calculations. Our energetics calculations show that the OH group tends to aggregate to the neighboring carbon sites of an epoxy group, resulting in the formation of several possible building units. We find that the epoxy group strongly hybridizes with the extended $\pi$ ($\pi^*$) bands, giving rise to a shift of the Dirac point in the momentum space and a decrease in the Fermi velocity. In contrast, the adsorption of a single hydroxyl group leads to the formation of a localized state and a gap opening near the Fermi level. The oxidation concentration dependence of the energy gap is investigated. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z25.00009: Phyiscal adsorption induced band gap openning in graphene Youjian Tang, VIncent Crespi Gapping graphene is crucial for enabling its use in next-generation electronic devices. Here we show that physical adsorption of suitable aromatic molecules onto graphene can generatate a moderate band gap of approximately 0.125 eV, with an adsorption energy ~0.67 eV. The reason for such a band gap is that the Lowest unoccupied molecule orbit of adsobate is right across the fermi level of graphene and thus gennerate a big perturbation on graphene dirac point energy level. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z25.00010: First-Principles Studies of Covalent Functionalization of Graphene by Carboxyl Groups Nabil Al-Aqtash, Igor Vasiliev We study the mechanism of covalent functionalization of graphene by the carboxyl (COOH) group in the framework of density functional theory combined with the generalized gradient approximation. The structures and binding energies of the COOH group attached to the surface of graphene are examined in cases of graphene containing no defects, containing a Stone-Wales defect, and containing a vacancy. Our calculations confirm that the binding of the COOH group with graphene is significantly stronger in the presence of surface defects. We also observe substantial changes in the structure of defective graphene after the attachment of the COOH group. These results suggest that surface defects play an important role in the carboxylation of graphene. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z25.00011: First-principles calculations of electronic transport through graphene with realistic metallic leads Salvador Barraza-Lopez, M. Y. Chou We present transmission characteristics for electrons through graphene with realistic metallic contacts. The methodology relies on an in-house version of the electronic transport SMEAGOL code [1], in which the memory required to allocate for the matrices of contact leads and the graphene sheet in the Green's function solver is distributed into more than one processor, for a given electron energy. We are able to accommodate for commensurate graphene-metal supercells which have the correct atomic structure (namely, stress caused by contracting/extending the metal contacts to match the periodicity of graphene is avoided). In addition, and despite of the large size of the leads, the electronic properties and transport are computed at the density-functional theory level [2] within a double-zeta plus polarization basis[3], ensuring the accuracy of the atomic forces in the system, as well as on the final transmission characteristics. [1] A. R. Rocha et al, PRB. \textbf{73}, 085414 (2006); [2] J. M. Soler et al, J. Phys.: Condens. Matter \textbf{14}, 2745-2779 (2002); [3] J. Junquera et al, PRB \textbf{64}, 235111 (2001). [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z25.00012: Exfoliation of graphene flake from SiC substrate using hydrogen injection; a first-principle study Bora Lee, Seungwu Han, Yong-Sung Kim Recently there is an immense interest in studying graphene for investigating its unique electronic properties as well as practical applications to nanoscale devices. Up to now there are two methods to obtain graphene layers. The first one is a mechanical method in which the single graphene sheet is split off the bulk graphite crystals using adhesives. The other method is graphitization of SiC surfaces by annealing at elevated temperatures. Even though the latter approach can provide a graphene layer in a more controlled way, the exfoliation of the graphene layer still poses a big challenge. In this presentation, based on the first-principles results, we propose a novel exfoliation method using hydrogen. As a model system, the 6H-SiC(0001) 4$\times $4 cell is used, which corresponds to the 3$\times $3 graphene cell. We calculate the binding energy of single hydrogen atom in various places; above and below graphene surface and inside the first SiC layer. The binding energies of hydrogen are calculated for different coverages. It is found that at high coverages, the hydrogen atoms prefer to bind below the graphene surface, cutting the graphene-SiC bonds. This means that the graphene can be exfoliated in the hydrogen-rich environment. The detailed analysis including the electronic structures will be presented. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z25.00013: Electronic Properties of Graphene Oxide Geunsik Lee, Kyeongjae Cho Graphene has shown promising electronic properties as future device applications beyond the current CMOS (complimentary metal-oxide-semiconductor) technology based on silicon microelectronics. As a critical insulating component in all-carbon nanoelectronic devices, graphene oxides (GOs) are shown to have insulating behavior, but their electronic and atomic structures are poorly understood. We investigated electrical property of GO using density functional theory (DFT) and non-equilibrium Green's function (NEGF) method with tight binding (TB) scheme. We model the basal plane oxidation with top site (OH) and bridge site (epoxide) chemisorptions. By varying the chemisorption ratio of the hydroxyls and epoxides as well as their coverage, the conductance of GO is calculated and quantitatively compared with experimental reports. We have investigated the electronic structure of graphene and GO multilayers for pseudospin device application. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z25.00014: Lattice Monte Carlo studies of quantum critical phenomena in graphene Timo Lahde, Joaquin Drut The Lattice Monte Carlo approach is well suited to the study of strongly interacting fermionic systems, such as the quasi-relativistic charge carriers in graphene, as it is non-perturbative and takes full account of quantum fluctuations. Recent simulational results on the semimetal-insulator critical point in graphene are presented, with emphasis on the question whether the transition to an insulating phase is of second order or of infinite order. This critical point is likely to be relevant for the physics of suspended graphene, as its location determined in arXiv:0807.0834 (see abstract by J.~E.~Drut) suggests that suspended graphene should be an insulator rather than a semimetal. An observable of particular interest is the DC conductivity of graphene, as most analytical studies underpredict this by a factor $\sim 3$. It has been pointed out that a complete description of the DC conductivity of graphene should account for non-perturbative effects due to the long-range Coulomb interaction between the fermionic quasiparticles. A possible method for determining the DC conductivity of graphene using the Lattice Monte Carlo technique is presented. [Preview Abstract] |
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