Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A1: Focus Session: Graphene, Adsorbates, and Electronic Structure |
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Sponsoring Units: DMP Chair: Jeremy Robinson, Naval Research Laboratory Room: 001A |
Monday, March 2, 2015 8:00AM - 8:12AM |
A1.00001: Electronic and Structural Properties of Vacancies and Hydrogen Adsorbates on Trilayer Graphene Marcos Menezes, Rodrigo Capaz Using ab initio calculations, we study the electronic and structural properties of vacancies and hydrogen adsorbates on trilayer graphene. Those defects are found to share similar low-energy electronic features, since they both remove a $p_z$ electron from the honeycomb lattice and induce a defect level near the Fermi energy. However, a vacancy also leaves unpaired $\sigma$ electrons on the lattice, which lead to important structural differences and also contribute to magnetism. We explore both ABA and ABC stackings and compare properties such as formation energies, magnetic moments, spin density and the local density of states (LDOS) of the defect levels. These properties show a strong sensitivity to the layer in which the defect is placed and smaller sensitivities to sublattice placing and stacking type. Finally, for the ABC trilayer, we also study how these states behave in the presence of an external electrical field, which opens a tunable gap in the band structure of the non-defective system. The $p_z$ defect states show a strong hybridization with band states as the field increases, with reduction and eventually loss of magnetization, and a non-magnetic, midgap-like state is found when the defect is at the middle layer. [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A1.00002: Hidden Kekule Order of Ghost Atoms on Monolayer Graphene Christopher Gutierrez, Lola Brown, Edward Lochocki, Ethan Rosenthal, Cheol-Joo Kim, Yui Ogawa, Kyle Shen, Jiwoong Park, Abhay Pasupathy Various charge and spin ordered phases have been predicted to exist in graphene when the lattice symmetry is broken on the atomic scale. One such phase is the Kekule distortion, whereby the C-C bond symmetry is broken and the graphene unit cell is tripled. It has been proposed that when certain adatoms are placed on monolayer graphene, strong interactions can exist between them mediated by the graphene lattice. The graphene-adatom interaction can induce Kekule order in the graphene itself, and move the adatoms to produce a hidden Kekule ordering. In this talk I will discuss evidence from scanning tunneling microscopy, electron diffraction and angle resolved photoemission spectroscopy that shows the existence of this unique ordering in epitaxial graphene on copper. Interestingly, we find in this case that the Kekule order is induced by a dilute number of ``ghost atoms'' -- unidentified atomic features -- in the otherwise perfect copper lattice underneath monolayer graphene. [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A1.00003: Ytterbiun-driven strong enhancement of electron-phonon coupling in graphene Choongyu Hwang, Duck Young Kim, David A. Siegel, Kevin T. Chan, Jesse Noffsinger, Alexei V. Fedorov, Marvin L. Cohen, Borje Johansson, Jeffrey B. Neaton, Alessandra Lanzara The interactions between electrons and phonons attract practical and fundamental interests in graphene, as they can not only govern transport properties, but also realize novel phenomena, such as superconductivity. By using angle-resolved photoemission spectroscopy in conjunction with first principles calculations, we provide an experimental evidence for the strong enhancement of electron-phonon coupling in graphene. Ytterbiun adsorption leads to the enhancement of electron-phonon coupling as much as a factor of 10 with respect to as-grown graphene, resulting in the highest strength ever measured for graphene and suggesting a viable route to the realization of superconducting graphene. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A1.00004: Molecular Bound States of Supercritical Charged Impurities on Graphene Kirill Velizhanin, Lyudmyla Adamska, Dmitry Solenov Functionalization of graphene by chemical groups/atoms allows one to tune its electronic, chemical and mechanical properties. For example, metallic adatoms (e.g., Li, Ca, Y) can be important in applications ranging from hydrogen storage to superconductivity. Such adatoms bind ionically to graphene and the resulting positive ions move along graphene relatively freely, so understanding the energetics of their interaction with graphene and between each other becomes critical for assessing stability of resulting materials in practical applications. It has recently been demonstrated that ions with charge greater than $Z\sim 1$ induce a very peculiar non-linear electronic polarization of graphene, which is reminiscent to the Dirac vacuum reconstruction around superheavy nuclei. In our work we demonstrate that such non-linear polarization qualitatively changes not only graphene electronic structure but also the energetics of the effective graphene-mediated interaction between such ions. In my talk, I will discuss the properties of such effective interaction and its dependence on various parameters of the system. In particular, I will report on our finding that molecular bound states of supercritically charged ions can be formed on graphene at certain conditions. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A1.00005: Doping, adsorption, and polarity of atomic-layer materials: A predictive theory from systematic first-principles study Susumu Saito, Yoshitaka Fujimoto, Takashi Koretsune Based on the extensive first-principles electronic-structure study of various doped hexagonal boron-nitride (h-BN) atomic layers as well as that of various doped graphene and carbon nanotubes, we propose a simple but predictive theory of polarity in doped atomic-layer materials. We first report the electronic structure of the pristine h-BN, h-BN layers with B and B$_3$N vacancies which have been experimentally produced and observed frequently, and doped h-BN layers, and show that both p-type and n-type h-BN layers can be produced in a variety of ways. We next review the electronic structure of doped graphene and carbon nanotubes and the effect of the H adsorption which can even change the polarity of the system. Finally we propose a simple but predictive theory which is based on the number of valence electrons of each system, and can explain the polarities of all the h-BN, graphene, and nanotube-based systems studied so far. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A1.00006: Screening properties of graphene layers studied by Kelvin Probe Force Microscopy and Landau Level Spectroscopy John Vetick, Chih-Pin Lu, Michael Altvater, Junxi Duan, Guohong Li, Eva Y. Andrei Graphene is one of the best conductors known, but due to its two dimensional structure and the need to support it on insulating substrates, its electronic properties are often masked by substrate-induced random potential fluctuations. In order to realize graphene's full potential for electronic application it is therefore important to understand its screening properties and to find ways to minimize substrate invasiveness. We employed Kelvin Probe Force microscopy (KPFM) to investigate the screening properties of CVD grown graphene crystals as a function of layer number and substrate material using a gated device geometry. The KPFM study was complemented by low temperature scanning tunneling microscopy and Landau level spectroscopy in similar samples and device configurations. Measurements were carried out on single layer, bilayer, trilayer and twisted bilayer samples deposited on SiO$_{2}$ and hBN substrates. Our findings show that twisted graphene layers provide superior screening of charged impurities and random potentials while at the same time preserving the unique electronic band structure of single layer graphene. [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:24AM |
A1.00007: Adsorption of monovalent aluminum halides on graphene defects Sufian Alnemrat, Joseph Hooper Density functional theory is used to study the adsorption of the monovalent aluminum halide AlCl on pristine and defective graphene (GR). Recent experimental efforts have shown that monovalent aluminum halide solutions can nucleate and grow small Al nanoparticles on a graphene surface. This nucleation and growth process may also shed light on how similar monovalent AlCl and AlBr solutions assemble into ligand-stabilized metalloid clusters. In this study we examined several point-type defects on GR to determine favorable sites for cluster nucleation. We found that the AlCl weakly physisorbs on pure GR, Stone-Wales point defects, and N-, B- doped-GR. On the other hand, the AlCl monomer chemisorbs on monovacancies, divacancies, and pyridine-like N, B, and O doped monovacancies. The binding energy is on the order of 5.0 eV and a strong covalent bond between the Al and the dangling bonds on the C-atoms near the defect sites is observed. In the pyridinelike N- and O-~ monovacancy defects, additional bonding between Al and the dopant-atoms results in the strongest binding energy of any graphene variant considered. [Preview Abstract] |
Monday, March 2, 2015 9:24AM - 9:36AM |
A1.00008: Absence of a transp ort signature of spin-orbit coupling in graphene with indium adatoms Jia Zhenzhao, Yan Baoming, Niu Jingjing, Han Qi, Zhu Rui, Wu Xiaosong, Yu Dapeng Enhancement of the spin-orbit coupling in graphene may lead to various topological phenomena and also find applications in spintronics. Adatom absorption has been proposed as an effective way to achieve the goal. In particular, great hope has been held for indium in strengthening the spin-orbit coupling and realizing the quantum spin Hall effect. To search for evidence of the spin-orbit coupling in graphene absorbed with indium adatoms, we carry out extensive transport measurements, \textit{i.e.}, weak localization magnetoresistance, quantum Hall effect and non-local spin Hall effect. No signature of the spin-orbit coupling is found. Possible explanations are discussed. [Preview Abstract] |
Monday, March 2, 2015 9:36AM - 9:48AM |
A1.00009: Direct observation of ordered configurations of hydrogen adatoms on graphene Chenfang Lin, Yexin Feng, Yingdong Xiao, Michael Duerr, Xiangqian Huang, Xiaozhi Xu, Ruguang Zhao, Enge Wang, Xin-Zheng Li, Zonghai Hu Ordered configurations of hydrogen adatoms on graphene have received great attention because they are closely tied to tuning of graphene properties including large band gap opening and formation of specific magnetic orders, both of which are highly desirable in potential applications. Many ordered structures of hydrogenated graphene have been proposed, including double sided and single sided ones, with the calculated band gap width depending on the respective H coverage. However, none of these ordered structures has been observed directly. Here we report direct imaging of several ordered configurations of H adatoms on graphene by scanning tunneling microscopy. The H atoms in the configurations exhibit apparent sublattice selectivity and tiny deviations from the exact atop-of-carbon positions. Scanning tunneling spectroscopy measurements of the configurations showed a larger than 0.6 eV gap in the local density of states. These findings can be well explained by our density functional theory simulations based on models of double sided H configurations.\\[4pt] Z. H. thanks the NBRP of China (Grant 2012CB921300) and the Chinese Ministry of Education for financial supports. E. W., X. Z. L. and Z. H. thank the National Natural Science Foundation of China (Grant 11074005, 91021007 and 11275008) for financial supports. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A1.00010: Disorder induced topological transition in graphene with random adatoms Eduardo Castro, Mar\'{i}a L\'{o}pez-Sancho, Mar\'{I}a Vozmediano Abstract One of the first proposals for a two-dimensional topological insulator was made for graphene, the so called Kane-Mele model, but the very low spin-orbit coupling makes this phase undetectable. It has been suggested that randomly depositing certain heavy adatoms can amplify the effect by many orders, and that a dilute concentration should be enough to open a detectable topological gap. Still lacking, however, is a precise determination of the critical density of random adatoms based in the evolution of the topological index. Based in a finite size analysis of the topological index as a function of the density of randomly distributed adatoms, and also on the localization properties of the system accessed through the Lyapunov exponent, we not only determine the critical density but also establish the nature of this peculiar topological transition. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A1.00011: Electronic Structure of Iridium Clusters on Graphene Bradford A. Barker, Aaron J. Bradley, Miguel M. Ugeda, Sinisa Coh, Alex Zettl, Michael F. Crommie, Marvin L. Cohen, Steven G. Louie Graphene was predicted to exhibit non-trivial Z2 topology, but its exceedingly weak spin-orbit coupling prevented this from being observed. Previous theoretical work has proposed enhancing the spin-orbit coupling strength by depositing individual adatoms adsorbed onto the surface of graphene. We show experimental evidence that the iridium adatoms cluster, with a cluster size of at least two atoms. We investigate through theoretical calculations the orientation of the iridium dimers on graphene, contrast the electronic structure of iridium dimers with iridium monomers, and compare the theoretical iridium dimer electronic structure calculations with the experimental results determined via scanning tunneling spectroscopy. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A1.00012: Platinum Nanoparticles Strongly Bonded to Freestanding Graphene Paul Thibado, J.K. Schoelz, P.K. Ghosh, J. Thompson, L. Dong, M. Neek-Amal, F.M. Peeters Freestanding graphene membranes were successfully functionalized with platinum nanoparticles (Pt NPs). The membranes were imaged using high-resolution transmission electron microscopy, revealing a homogeneous distribution of uniformly sized, single-crystal Pt NPs that exhibit a preferred orientation and nearest-neighbor distance. The Pt NPs were also found to be partially elevated by the graphene substrate, as deduced from atomic-resolution scanning tunneling microscopy (STM) images. Furthermore, the electrostatic force between the STM tip and sample was utilized to estimate the binding energy of the Pt NPs to the suspended graphene. Local strain accumulation due to strong sp$^{3}$ bond formation is thought to be the origin of the Pt NP self-organization. Such detailed insight into the atomic nature of this functionalized system was only possible through the cooperation of dual microscopic techniques combined with molecular dynamics simulations. The findings are expected to shape future approaches to develop high-performance electronics based on nanoparticle-functionalized graphene as well as fuel cells using Pt NP catalysts. [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A1.00013: Local Property Change of Graphene Induced by a Cu Nanoparticle Li-Wei Huang, Horng-Tay Jeng, Chia-Seng Chang Investigating the detailed impact from an individual nanoparticle on graphene membrane is a great challenge. We employed ultra-high vacuum electron microscopy and first-principles calculations to reveal the changes on the graphene's morphology and electronic structures upon adsorption of a Cu nanoparticle. Our findings show that the significant amount of charge transfer from the individual Cu nanoparticle to graphene causes local electronic redistributions at the interface and a consequent recess of the graphene with prominent tilt angles. [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A1.00014: Adsorption of molecular hydrogen on Pd(Pt) decorated graphene Narayan Adhikari, Asim Khaniya, Saran Lamichhane, Nurapati Pantha We have performed the first-principles based Density Functional Theory (DFT) calculations to study the stability, geometrical structures, and electronic properties of a Pd(Pt) atom adsorbed graphene to investigate the possibility of using Pd(Pt) decorated graphene as energy storage materials with reference to pristine graphene. The London dispersion forces have been incorporated by the DFT-D2 levels of calculations implemented in Quantum Espresso packages. Our findings show that Pd and Pt both adsorb on graphene at Bridge site. The electronic structures of Pd(Pt) adsorbed graphene possesses band gap opening due to breaking of the symmetry of graphene. Further we have studied the adsorption of moelcular hydrogen ((H 2 ) n , n = 1-7) on the Pd(Pt)-graphene system. The adatom Pd(Pt) enhances the binding energy per hydrogen molecule in Pd(Pt)-graphene system in comparison to that in the pristine graphene. The binding energy per hydrogen molecule of the adatom-graphene system decreases as the number of H 2 molecules increases and finally it saturates to 0.15 eV (0.16 eV) per hydrogen molecule for Pd-graphene (Pt-graphene) systems respectively. [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A1.00015: Graphene Nanoislands on NI(111): Structural and Scattering Properties Aran Garcia-Lekue, Marc Olle, Gustavo Ceballos, Aitor Mugarza, Pietro Gambardella, Andres Arnau, Daniel Sanchez-Portal The graphene-metal interaction can be exploited to engineer hybrid structures with novel electronic and magnetic properties. The graphene-Ni interface is an interesting case where the interaction with the ferromagnetic substrate opens hybridization gaps and induces magnetic moments.We investigate the electronic properties of graphene nanoislands grown on Ni(111),using local tunneling spectroscopy measurements combined with spin-polarized \textit{ab initio }calculations.We show that the electron scattering at the graphene edges is found to be spin- and edge-dependent. This behavior is attributed to the strong distortion of the electronic structure at the interface, which opens a gap and spin-polarizes the Dirac bands of graphene. We also demonstrate that edge scattering is strongly structure dependent, with asymmetries in the reflection amplitude of up to 30{\%} for reconstructed and unreconstructed zig-zag edges.These results suggest a lateral 2D spin filtering for graphene layers, similar to that occurring across the interface. \\[4pt] [1] M. Olle et al. Nano Lett. 12, 4431 (2012)\\[0pt] [2] A. Garcia-Lekue et al., Phys. Rev. Lett. 112, 066802 (2014)\\[0pt] [3] A. Garcia-Lekue et al., submitted [Preview Abstract] |
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