Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B1: Focus Session: Graphene - Point Defects and Structural Defects |
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Sponsoring Units: DMP Chair: Benjamin Butz, Friedrich Alexander Universitaet Erlangen Room: 001A |
Monday, March 2, 2015 11:15AM - 11:27AM |
B1.00001: Spontaneous Boron-doping of Graphene at Room Temperature Lida Pan, Yande Que, Shixuan Du, Hongjun Gao, Sokrates T. Pantelides Doping graphene with boron or nitrogen is an effective way to modify its electronic properties. However, the reaction barrier for introducing these impurities is quite high, making the doping process difficult. In this work, we propose a low-energy reaction route derived from first-principles calculations and subsequently validated by experiments. The calculations show that, when graphene is placed on a ruthenium substrate and exposed to atomic boron, boron atoms can incorporate substitutionally into the graphene sheet with an energy barrier about 0.1 eV, displacing carbon atoms below the graphene sheet where they migrates away. This result suggests that spontaneous doping by boron can take place at room temperature. Following the prediction, we grew high-quality graphene on the Ru(0001) surface and then expose it to B2H6 which decomposes into atomic boron. XPS and STM results indicate that boron dopes graphene substantially without disturbing the graphene lattice, confirming the theoretical predictions. Doping by nitrogen and co-doping by B and N will also be discussed. [Preview Abstract] |
Monday, March 2, 2015 11:27AM - 11:39AM |
B1.00002: Boron Substitution in Disordered Graphene-like Carbon Joe Schaeperkoetter, Andrew Gillespie, Carlos Wexler, Peter Pfeifer X-ray photoelectron spectroscopy was used to determine both the elemental composition of boron doped carbons as well as gain insight into the arrangement of atoms in the material. The hypothesized arrangement of atoms is a direct substitution of boron for carbon into a graphene like sheet, maintaining the hexagonal honeycomb lattice of sp$^{2}$ sigma bonds. Such a boron atom would have an electronic configuration of 1s$^{2}$(sp$^{2})^{3}$. With a graphitic carbon atom, the p$_{z}$ orbitals are maintained and participate in mobile pi bonds with neighboring carbon atoms, as understood in the aromatic model. Boron, however, would require a charge donation to fill its p$_{z}$ orbital. Thus, three possible models are proposed for the out of plane electron density: (1) the orbital remains unoccupied and the boron is a free radical, (2) charge is donated from a neighboring atom and the boron atom is ionic, (3) the delocalization of charge in the aromatic system results in a partial charge transfer with an effective charge somewhere between neutral and anionic. Our results suggest that boron is not in an anionic state, and, by doing a quantitative and simultaneous analysis from multiple elemental spectra, we conclude that no more than 2 wt{\%} of boron is being substitutionally doped into the system. [Preview Abstract] |
Monday, March 2, 2015 11:39AM - 11:51AM |
B1.00003: Stabilities and electronic structures of B and N defects in bilayer graphene Yoshitaka Fujimoto, Susumu Saito Since its discovery, atomically thin monolayer of graphene has received a lot of attention. The few-layered sheets of graphene have also attracted great attention both scientifically and technologically since they exhibit different electronic structures from monolayer graphene. One of the effective ways to tune the electronic properties of carbon-based nanomaterials is to dope them with B and N atoms. Here, we study energetics and electronic properties of B and N defects in bilayer graphene, based on the first-principles density-functional theory. All kinds of dopant sites and stacking patterns (AA and AB) of bilayer sheets are studied and the site-dependent and independent behaviors of the dopants are found. We also report the electronic structures and study the STM images of doped bilayer graphene. While B-doped and N-doped defects show different STM images, the STM images are shown to be similar for AA and AB stacking patterns in both B- and N-doped defect cases. [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B1.00004: Imaging the Individual Nitrogen Dopant Atoms in Graphene with Thermoelectric Measurements Ho-Ki Lyeo, Sanghee Cho, Eui-Sub Lee, Yong-Hyun Kim, Junstin Bult, Jeffrey Blackburn Chemical doping of nitrogen has been known as a means of modifying the electronic structure of graphene grown on a Cu substrate. We used scanning thermoelectric microscope to investigate the influence of individual nitrogen dopant atoms on the local electronic structure in monolayer graphene. Nitrogen-doped graphene was grown on a single crystal Cu substrate by using the chemical vapor deposition of pyridine molecules that contain a nitrogen atom substituting a carbon atom in the carbon hexagon. Thermoelectric voltage measurements, which we have shown to be differentially sensitive to the Fermi energy state, can produce two-dimensional images of electronic signature near the individual nitrogen dopants on the atomic scale. The measurements yielded a more complicated and extended modification of electronic structure than previously measured by using tunneling spectroscopy, which can be accounted for by the theoretical simulation and experimental evidences of nitrogen inclusion obtained from Raman and XPS measurements. [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B1.00005: Manipulate the Doping of Graphene at Nanoscale with Intercalated Oxygen Xin Zhang, Hong Luo, Lei Liu, Gong Gu, Daniele Stradi, Mads Brandbyge We have created nanoscale p- and n-doped graphene regions side by side, by partially removing the oxygen between the graphene and the Cu foil growth substrate intercalated upon elongated air exposure. The Cu foil surface is almost exclusively (100) oriented, and the removal of intercalated oxygen is by thermal annealing. Scanning tunneling microscopy (STM) reveals a 0.72 $\times$ 0.72 nm square superlattice in the single layer (1L) graphene/O/Cu(100) structure, assigned to be Cu($2 \sqrt{2} \times 2 \sqrt{2}$)R45$^{\circ}$-O, which has not been reported so far. Graphene with intercalated oxygen underneath it is p-doped while the surrounding graphene areas, directly in contact with the copper surface, are n-doped. Comparing the scanning tunneling spectra (STS) of the two types of regions, we show a charge transfer-induced shift of the electronic structure. Such a shift is also observed between p- and n-doped twisted bilayer (2L) graphene regions, where the van Hove singularity (vHS) peaks are used as markers to precisely determine the energy shift. Across the boundaries between the p- and n-doped regions, the shift of the electronic structure is spatially resolved, showing the vanishing and reappearance of the vHS peaks. The experimental observations are consistent with first-principles calculations. [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:27PM |
B1.00006: Symmetry protected zero modes in graphene grain boundaries Madeleine Phillips, E.J. Mele We study electronic states in graphene grain boundaries using topological band theoretic arguments. Using bulk eigenstates, we calculate a geometric phase that counts the number of zero modes in projected bulk gaps. We argue that these localized zero modes are protected by a hidden chiral symmetry. We apply our topological theory to various grain boundary geometries and corroborate our results using numerical calculations on a tight binding lattice. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 12:39PM |
B1.00007: Conserved Bonding Sequences and Atomic Strain Fields of Graphene Grain Boundaries Haider Rasool, Colin Ophus, Ziang Zhang, Michael Crommie, Boris Yakobson, Alex Zettl Grain boundaries in polycrystalline materials have significant impact on their bulk mechanical properties. The detailed arrangement of atoms and bonding at the boundary dictate these properties and their differences from bulk single crystal behavior. In this work, we use aberration corrected high resolution transmission electron microscopy imaging to study the structure of graphene grain boundaries. By mapping the exact atomic positions of the carbon lattice, we visualize atomic scale strain organization in the material. We find that grain boundaries are comprised of conserved bonding sequences that can appear as periodic or aperiodic building blocks that give rise to similar strain behavior at the boundary. Using molecular dynamics fracture simulations, we predict that experimentally observed grain boundary structures will maintain strengths that are comparable to ideal theoretical grain boundaries. [Preview Abstract] |
Monday, March 2, 2015 12:39PM - 12:51PM |
B1.00008: Scattering properties of extended structural defects in graphene Daniel Gunlycke, Carter White A challenge preventing widespread use of graphene in nanoelectronic devices is the absence of a band gap at the Fermi level. Without a practical band gap, other ways to make electronic transport switchable are needed. One promising possibility is to use parallel graphene transport barriers to generate a transport gap. This approach requires transport barriers that are penetrable and fairly reflective. Such barriers could be formed by extended structural defects such as grain boundaries or line defects. Herein, we present scattering properties in the specular regime of a generic transport barrier described by an effective barrier coupling, an effective barrier potential, and an asymmetry parameter. We also show that these scattering properties could be probed without the need for lateral transport measurements. Instead, we suggest the use of scanning probe techniques measuring the undulations in the local density of states. That the transmissivity could be probed in equilibrium when no current flows through the barrier is a manifestation of quantum interference at the barrier. [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B1.00009: Effect of defects produced by electron irradiation on the electrical properties of graphene Adrian Balan, Julio Alejandro Rodriguez- Manzo, Matthew Puster, Marija Drndic We present a study of the effects of the defects produced by electron irradiation on the electrical and crystalline properties of graphene. We realized back or side gated electrical devices from monolayer graphene crystals suspended on a 50nm SiNx. The devices are exposed to electron irradiation inside a 200kV transmission electron microscope (TEM) and we perform in situ conductance measurements. The number of defects and the quality of the crystalline network obtained by diffraction are correlated with the observed decrease in mobility and conductivity of the devices. We observe a different behavior between type of monolayer materials, and try to associate with different conduction with defect models. [1] Towards sensitive graphene nanoribbon-nanopore devices by preventing electron beam induced damage. M. Puster, J. A. Rodriguez- Manzo, A. Balan, M. Drndic. ACS Nano,10.1021/nn405112m. [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B1.00010: Magnetic Defects in Graphene Quantum Dots Srinivasa Rao Singamaneni, Johan van Tol, Ruquan Ye, James M. Tour Spin-coherence time in graphene quantum dots (GQDs), which are highly sought-after spintronics materials is controlled by defects. To that end, exploring the nature of (spin) defect centers in these GQDs is important. Electron spin resonance (ESR) spectroscopy is an ideal local probe to investigate the spin properties of GQDs. ESR investigations are carried out on GQDs [1] as a function of temperature (6-290 K) at two distinct microwave high frequencies 239.2 and 336 GHz. The ESR signal does not show power dependence at 239.2 GHz, 6K, could be adequately described with three distinct components using Lorentzian line shape. From the experimental findings together with computer-aided simulations, we have identified them as one broad (700 Gauss) and narrow (60 Gauss) carbon-centered paramagnetic defect centers and the third one as Mn$^{2+}$ signal, which is an extrinsic impurity. The temperature dependence of carbon-derived spin centers in GQDs resembles to that of conduction electrons, in contrast to the localized spins observed by us earlier [2-5] in graphene nanoribbons.\\[4pt] [1] Ye, R. et al., \textit{Nature Commun}.~\textbf{2013},~\textit{4:2943}, 1-6;\\[0pt] [2] S. S. Rao et al Appl. Phys. Lett. \textbf{98}, 083116, (2011);\\[0pt] [3] New.Journal of Physics \textbf{13}, 113004, (2011);\\[0pt] [4] Nano Lett. \textbf{12}, 1210,(2012);\\[0pt] [5] ACS Nano \textbf{6}, 7615 (2012). [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B1.00011: A first-principles investigation on the Electronic and Magnetic properties of Hydrogen vacancy chains in Graphane Bi Ru Wu, Chih-Kai Yang We investigated a variety of configurations of hydrogen-vacancy (HV) chains in graphane with density functional theory. We found the configurations that each of zigzagged HV chains separated by one or more H chains exhibit nonmagnetic conductor or has a tiny gap. Once as the neighbored zigzag HV chains blocked by isolated H atoms, the structure transformed from a nonmagnetic conductor into a magnetic semiconductor. If the HV chains are continuously distributed, it looks like a graphene nanoribbon embedded in graphane. The zigzag edged embedded graphene nanoribbons also show antiferromagnetic. An additional H atom on the ribbon can tune the band gap and generate magnetic moment; moreover, if bare C atoms are present outside the nanoribbon also have similar effect. The results will be helpful for designing graphene-based nanoelectronic devices. [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B1.00012: Characterizing Defects Generated in Graphene by Scanning Probe Microscopy Jonathon David White, Hsiao-Mei Chien, Min-Chiang Chuang, Hung-Chieh Tsai, Hung-Wei Shui, Lo-Yueh Chang, Chia-Hao Chen, Sheng-Wei Lee, Wei-Yen Woon Graphene was prepared by chemical vapor deposition (CVD). Defects with differing topographical and tribological properties were then created by scanning probe lithography (SPL) under ambient conditions. The nature of these defect structures was then investigated by micro-Raman ($\mu $-RS) and micro-X-ray photoelectron ($\mu $-XPS) spectroscopy. Investigation of these structures suggests that, despite their physical differences, similar defects are present in both structures. In particular, $\mu $-RS indicated that the ratio of the defect Raman peaks and the effective distance between defects had a similar magnitude and dependence on the applied bias voltage during SPL for all topographies. $\mu $-XPS revealed no evidence of the generation of sp$^{3}$-type defects. The small amplitude of the C-C peak and absence of C$=$O and C-OH peaks, suggest a complete absence of graphene oxide in the defect areas. We propose that a common active mechanism - bond reconstruction - is responsible for both structures. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B1.00013: Supercriticality of charge centers in graphene probed with scanning tunneling microscopy Yuhang Jiang, Jinhai Mao, Guohong Li, D. Moldovan, M. Ramezani Masir, F. M. Peeters, Eva Y. Andrei The massless Dirac fermion carriers in graphene, with their effective fine structure constant,$\alpha_{g} $, being of order unity, provide fertile ground for exploring the physics of ultra-relativistic particles in the strong coupling limit.In particulara positive charge Z embedded in graphene is expected to exhibit supercritical behavior already for Z\textgreater Z$_{\mathrm{c}}=$0.5/$\alpha_{\mathrm{g}}$, in stark contrast to the atomic case where Z$_{\mathrm{c}}$ $\sim$ 170 is experimentally inaccessible. However due to the significant screening in graphene, attaining the supercritical regime is challenging.\footnote{Y. Wang \textit{et al}, Science, 340, 734 (2013)} We will report on a new method to create charge centerswithin the graphene layer whose charge, Z, can be tuned to exceed the critical value. Using low temperature scanning tunneling microscopy and spectroscopy we study the evolution in the local electronic structure of graphene as a function of Z, from charge neutrality to the supercritical regime, which is identified by comparing to numerical simulations. [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B1.00014: Supercriticality and screening effects in graphene Jinhai Mao, Yuhang Jiang, Guohong Li, D. Moldovan, M. Ramezani Masir, F.M. Peeters, Eva Y. Andrei The chiral nature of charge carriers in graphene prohibits backscattering and prevents confinement by electrostatic potentials, resulting in high electronic mobility and unusual phenomena such as Klein tunneling. This picture breaks down in the presence of charge impurities exceeding a critical value Z$_{\mathrm{c}}$, wherea qualitative change in behavior leads to the capture of electrons akin to atomic collapse in 3D atoms. Although in graphene Z$_{\mathrm{c}}$ is substantially lower than in 3D atoms, attaining the supercritical regime is difficult because screening can significantly reduce the effective charge of the impurity. We have devised a method of inducing a controllable amount of localized charge whose strength can be tuned by adjusting screening through a gate voltage or a magnetic field. The effect of the impurity on the local electronic structure was monitored with low temperature scanning tunneling microscopy and spectroscopy and with Landau level spectroscopy. By following the evolution of the spectra as a function of the induced charge and comparing with numerical simulations, we are able to pinpoint the onset of atomic collapse and beyond, providing new insights into the physics of supercriticality. [Preview Abstract] |
Monday, March 2, 2015 2:03PM - 2:15PM |
B1.00015: Observation of vacancy-induced suppression of electronic cooling in defected graphene Xiaosong Wu, Qi Han, Yi Chen, Gerui Liu, Dapeng Yu Previous studies of electron-phonon interaction in impure graphene have found that static disorder can give rise to an enhancement of electronic cooling. We investigate the effect of dynamic disorder and observe over an order of magnitude suppression of electronic cooling compared with clean graphene. The effect is stronger in graphene with more vacancies, confirming its vacancy-induced nature. The dependence of the coupling constant on the phonon temperature implies its link to the dynamics of disorder. Our study highlights the effect of disorder on electron-phonon interaction in graphene. In addition, the suppression of electronic cooling holds great promise for improving the performance of graphene-based bolometer and photo-detector devices. [Preview Abstract] |
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