Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y25: Focus Session: Graphene XVIII: Functionalization and Growth II |
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Sponsoring Units: DMP Chair: Yong-Jie Wang, NHMFL, Florida State University Room: 327 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y25.00001: Edge states and nitrogen substitutional doping in carbon nanoribbons Jie Jiang, Wenchang Lu, Jerry Bernholc, Piotr Boguslawski The edge states in carbon nanoribbons and the electronic and magnetic properties in N-doped carbon nanoribbons are investigated within density functional theory. While the ground state of zigzag ribbons is spin polarized, defects at the edges destroy the polarization and lead to a non-magnetic ground state. Scanning tunneling spectroscopy will thus show different features depending on edge quality. Turning to substitutionally doped carbon nanoribbons, the impurity states are elongated along the ribbon width by edge and confinement effects, which also affect their ionization energies in armchair ribbons. Formation energy calculations reveal that N atoms preferentially occupy edge sites in carbon nanoribbons. The extra electron from the donor suppresses the spin-polarization and tailor the relative magnetization at the two edges in zigzag ribbons. The interplay of impurity and edge states in zigzag ribbons leads to rich electronic effects, resulting in semiconducting or metallic behavior depending on the dopant position. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y25.00002: Counting Graphene Layers on Glass by Optical Reflection Microscopy Helgi Skulason, Peter Gaskell, Chris Rodenchuk, Thomas Szkopek Using optical reflection microscopy we can locate and count graphene layers on a bulk glass substrate. This is a reliable and low cost technique for graphene flake metrology. Optical reflection measurements are in agreement with the universal optical conductance of graphene. We present measurements of the optical conductivity of graphitic flakes showing a transition from few layer graphene to bulk graphite. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y25.00003: Molecular Physisorption on Graphene. David Carey, Thomas Connolly Ab initio calculations using both LDA and GGA functionals have been used to examine the binding energy, optimum binding intermolecular separations, molecular orientation dependence for a range of graphene lattice sites with oxygen containing molecules such as CO and NO. For all sites investigated NO has a higher binding energy than CO. For example, we find that the most stable sites are for the intermolecular axis parallel to the plane of the graphene layer with a binding energy of 195 meV for NO and 131 meV for CO using LDA VWN functional. Using the GGA PW91 functional the corresponding binding energies are 45 meV and 28 meV. When the CO or NO molecular axis is perpendicular to the graphene layer, orientation with the O atom oriented away from the graphene layer are found to be favoured than those with the O atom closer to the graphene layer. Molecular physisorption on graphene is discussed. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y25.00004: Interface structure for growth of epitaxial graphene on SiC(0001) S.H. Rhim, G. Sun, L. Li, M. Weinert In spite of the enormous effort devoted to the study of the epitaxial growth of graphene on SiC, there is not yet a consensus regarding the structure of the interface between graphene and the substrate. There have been a long standing discrepancy between low energy electron diffraction (LEED) and STM patterns regarding the periodicity of graphene on SiC(0001); the theoretical studies of the of $6\sqrt{3}\times6\sqrt{3}$ \footnote{S. Kim, J. Ihm, H. J. Choi, and Y. W. Son, Phys. Rev. Lett. {\bf 100}, 176802 (2008).} or $\sqrt{3}\times\sqrt{3}$ \footnote{F. Varchon {\em et al}, Phys. Rev. Lett. {\bf 99}, 126805 (2007); A. Mattausch and O. Pankratov, Phys. Rev. Lett. {\bf 99}, 076802 (2007} periodicity, while describing some aspects, disagree in important details with scanning tunneling microscopy (STM) images. We present a combined theoretical and experimental study, employing density functional calculations and STM, to investigate this issue. We propose the formation of a defected graphene layer at the interface, and then subsequent growth of graphene. The calculated bias-dependent STM images are in good agreement with our STM images, and provide insight into the details of the interface structure. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y25.00005: In-situ IR studies of graphene oxide reduction. Muge Acik, Laurence Goux, Yves Chabal Thermal reduction of graphene oxide (GO) synthesized by Hummer's method is studied by \textit{in-situ} infrared absorption spectroscopy in a vacuum reactor. Initially, water and hydroxyl groups are removed (100$^{o}$C), with release of CO$_{2}$. Upon reduction of epoxides and carbonyl groups, the appearance of sp2-bonded carbon (C=C bonds) is evident with detection of in-plane and out of plane vibrations. However, oxygen remains in the structure in the form of COC bonds even after 700$^{o}$C anneal. Around 290$^{o}$C, a strong increase of the absorbance associated with structure changes of GO is observed. The increase of the refractive index is attributed to an increase of electrical conductivity after reduction of GO. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y25.00006: AFM local oxidation nanolithography of graphene Lishan Weng, Liyuan Zhang, Yong P. Chen, Leonid P. Rokhinson We demonstrate the local oxidation nanopatterning of graphene films by an atomic force microscope. The technique provides a method to form insulating trenches in graphene flakes and to fabricate nanodevices with sub-nanometer precision. By utilizing this technique, a 25-nm-wide nanoribbon and submicron size nanoring were fabricated from a graphene flake. In addition we found that either trenches or bumps can be written on the graphene surface depending on the lithography conditions. It is proposed that the trenches are created by defect-associated oxidation whereas the bumps are incorporation of oxygen into the graphene lattice. Some of the bumps disappear with time as quickly as in a few minutes or as slow as in a few days. We also further investigate the possibility to remove the bumps in a controllable manner by writing trenches on top, applying opposite voltage or change the environmental conditions. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y25.00007: Hydrogen Saturation of Graphene Nanoribbons: edge states suppression and gap behavior Thiago Martins, Antonio J. R. da Silva, Adalberto Fazzio Di-hydrogenated zigzag edges Graphene Nanoribbons (2H-ZZ-GNR) are more stable than the usually studied mono-hydrogenated [1] passivation (H-ZZ-GNR). Using density functional theory, we studied a variety of 2H-ZZ-GNR configurations. We investigated how the interaction between the CH$_{2}$ units depends on their separation at the same edge as well as on the width of the ribbon. We observe, in agreement with previous studies [1], that the 2H-ZZ-GNR passivation suppresses the presence of edge states, thus eliminating the magnetic instability of H-ZZ-GNR that is responsible for the gap opening in the anti-ferromagnetic ground state configuration. Moreover, there is a reduction of coupling between edge and bulk carbon atoms, resulting in a band structure whose gap is dominated by bulk bands and confinement effects. We also studied the behavior of the gap as a function of the ribbon's width, and we observed that it quickly closes as the width is increased. \\[3pt] [1] T. Wassmann, A. P. Seitsonen, A. M. Saitta, M. Lazzeri, and F. Mauri, Phys. Rev. Lett. {\bf 101}, 096402 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y25.00008: Anomalous magnetic susceptibility and Hall effect from valley degrees of freedom Tianyi Cai, Wang Yao, Junren Shi, Qian Niu With a staggered sublattice potential, sizable gaps can occur in epitaxial graphenen films. Magnetic and transport properties of this system are studied. We predict large signal of magnetic susceptibility and relate it to the intrinsic large magnetic moments of electrons. There is also an anomalous contribution to the ordinary Hall effect, which is due to the valley dependent Berry phase. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y25.00009: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y25.00010: Si diffusion on and between graphene sheets Lede Xian, M.Y. Chou The growth of epitaxial graphene (EG) on the SiC substrate is accompanied by the evaporation of Si atoms during the growth process. The continuing loss of Si atoms takes place even after the surface graphene sheets have been formed. This atomic transport is believed to be a key element in establishing a growth mechanism to model and control the process. Using density functional theory (DFT) calculations, we have studied the diffusion of Si atoms on a single layer of graphene and between graphene sheets. The potential energy surfaces are explored. For single-layer graphene, the diffusion barrier for Si is relatively low. While for multilayers, some buckling of graphene sheets will appear and the stacking pattern also plays a role. The connection with the growth process will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y25.00011: Gauge field for the edge states in graphene Ken-ichi Sasaki, Shuichi Murakami, Riichiro Saito By considering a continuous model for graphene, we study a special gauge field for the edge state. The gauge field explains the properties of the edge state such as the existence only on the zigzag edge, the partial appearance in the k-space, and the energy position around the Fermi energy. The gauge field polarizes the pseudospin. The applications of the gauge field to the ferromagnetism of edge states and the electron-phonon interaction are reported on. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y25.00012: Collective properties of magnetobiexcitons in quantum wells' and graphene superlattices Oleg Berman, Roman Kezerashvili, Yurii Lozovik The Bose-Einstein condensation and superfluidity of quasi-two-dimensional spatially indirect magnetobiexcitons in a slab of superlattice with alternating electron and hole layers consisting from the semiconducting quantum wells (QWs) and graphene superlattice in high magnetic field are reported. The two different Hamiltonians of a dilute gas of magnetoexcitons with a dipole-dipole repulsion in superlattices consisting of both QWs and graphene layers (GLs) in the limit of high magnetic field have been reduced to one effective Hamiltonian a dilute gas of two-dimensional excitons with the renormalized effective mass of the magnetoexciton, which depends on the magntic field. The instability of the ground state of the system of interacting two-dimensional indirect magnetoexcitons in a slab of superlattice with alternating electron and hole layers in high magnetic field is found. The stable system of indirect quasi-two-dimensional magnetobiexcitons, consisting of pair of indirect excitons with opposite dipole moments is considered. The density of the superfluid component $% n_{s}(T)$ and the temperature of the Kosterlitz-Thouless phase transition to the superfluid state in the system of two- dimensional indirect magnetobiexcitons, interacting as electrical quadrupoles, are obtained for both the QW and graphene realizations. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y25.00013: The Electronic Structure of Few-Layer Graphene: Probing the Evolution from a 2-Dimensional Sheet to a 3-Dimensional Solid by Optical Spectroscopy Kin Fai Mak, Matthew Sfeir, James Misewich, Tony Heinz The evolution of the electronic structure of few-layer graphene, for $n$ = 1, 2, 3, {\ldots}, 8 atomic layers, was characterized by optical absorption spectroscopy. Each thickness of few-layer graphene exhibited well defined and distinct infrared absorption peaks associated with interband transitions. The positions of the peaks were found to obey a simple scaling relation with layer thickness. The principal features of the experimental spectra for all samples could be described consistently in terms of the electronic states of the parent graphite material through application of a specific zone-folding construct obtained when only nearest-layer interactions are considered. Both the experiment and analysis permit one to follow the convergence of the multilayer graphene response to that of graphite with increasing sample thickness. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y25.00014: Edge effects in Bilayer Graphene Nanoribbons Matheus P. Lima, Adalberto Fazzio, Antonio J. R. da Silva We investigate the geometrical and electronic structure of zigzag bilayer graphene nanoribbons (B-ZGNR), with widths that range from $w=0.6$ to $w=4.5$ $nm$. The layers are in the Bernal stacking, which means that there are two types of C atoms, those that are positioned above the center of the hexagons of the other layer, defining a B-sublattice, and those right on top of the C atoms of the other layer, forming an A-sublattice. When we cut the layer along the zigzag edge, there are two possible alignments, $\alpha$, where the outermost edge atoms belong to the A- sublattice, and $\beta$, where the outermost edge atoms belong to the B-sublattice. Thus, only the inter-layer edge interaction differs. We found that the $\alpha$ alignment is energetically favorable, with an inter-layer edges attraction, whereas for the $\beta$ there is an inter-layer edges repulsion. These edge-related forces cause a deviation from the exact Bernal stacking, resulting in a non-monotonic behavior of the energy gap with the width $w$ for the $\alpha$ B-ZGNR, with a maximum value at $w\approx 3.5nm$. This is a consequence of the competition between bulk and strongly attractive edge interactions. All results were obtained using density functional theory calculations with the inclusion of parametrized van der Waals interactions. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y25.00015: Measuring a Butterfly with Graphene Andres Concha The Hofstadter butterfly (HB) is a hierarchical structure that emerges as a consequence of the commensuration of two length scales, the magnetic length $l\sim \frac{1}{H}$ and the lattice spacing $a$ between atomic sites. We argue that by using a set of scalar potentials and an external magnetic field it is possible to measure and manipulate the HB in graphene. Our claim is based in the fact that in graphene, close to the Dirac point the theory becomes critical and as such no length scale is present in the low energy description. Thus the only relevant length scales are dictated by the magnetic length and the distance between potential barriers. It is shown that the Hall conductance in the minigaps should be directly measurable with current available technology. [Preview Abstract] |
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