Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D30: Graphene: Hydrogenation and Defects |
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Sponsoring Units: DCMP Chair: Taisuke Ohta, Sandia National Laboratories Room: C147/154 |
Monday, March 21, 2011 2:30PM - 2:42PM |
D30.00001: Thermal conductivity of partially hydrogenated graphene Jeong Yun Kim, Jeffrey Grossman Graphene superlattices made with partial hydrogenation are of great interest and have been explored recently due to the enhanced tunability of electronic properties as a function of the hydrogenation pattern. However, the thermal transport properties of such materials have received little attention. In this work, we investigate the effects of 2D periodic patterns of hydrogen atoms on the thermal conductivity of partially hydrogenated graphene using classical molecular dynamics simulations and an Einstein relation. Our calculations show that the thermal conductivity of partially hydrogenated graphene varies substantially as a function of hydrogen coverage, periodicity, edge shape, and width of hydrogenated region compared to the bare graphene region. In addition, we show that the use of patterned 2D shapes of hydrogenation on graphene could lead substantially lower thermal conductivities that may be of interest for thermoelectric applications. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D30.00002: Thermoelectric Properties of Hydrogenated Graphene Ruwantha Jayasingha, Kasun Fernando, Christof Keebaugh, Robert Stallard, Gamini Sumanasekera We have studied the temperature dependence of thermopower (S) and 4-probe resistance (R) of large area Graphene subjected to various degree of hydrogenation. Graphene samples with electrical contacts mounted within a quartz reactor was placed inside a custom made inductively coupled plasma coil and hydrogen gas was introduced to a pressure of $\sim $ 10 Torr. Samples were placed well away from the plasma and both S and R were monitored \textit{in --situ} during the hydrogenation. At desired level of hydrogenation the plasma was turned off and the sample was cooled down to $\sim $140 K by lowering the reactor into a liquid nitrogen dewar and both R(T) and S(T) were measured. Both S(T) and R(T) show metal to insulator transition characteristics during the progressive hydrogenation. Both epitaxially grown Graphene on Si-terminated face of SiC and Graphene grown by chemical vapor deposition and transferred on to quartz substrate were studied. The CVD grown sample was found to be p-type under ambient condition but could be tuned to n-type after high temperature annealing at 550 K in a vacuum of 2x10$^{-7 }$Torr. In contrast, epitaxial sample was n-type under ambient conditions. However, the hydrogenation was performed on both samples under degassed conditions. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D30.00003: The Influence of Hydrogenation on the Hall Effect in Exfoliated Mono- and Multi-layer Graphene Y. Mo, J.D. Jones, P.E. Ecton, M. Maneshian, W.D. Hoffman, A.V. Jesseph, N. Shepherd, G.F. Verbeck, J.M. Perez, Z. Ye, G. Zhao Graphene samples exfoliated from highly ordered pyrolytic graphite are deposited using the standard scotch-tape method on 300nm thick SiO$_{2}$ covered and slightly conductive Si substrates. Devices with 4 silver electrode pads on the graphene samples for Hall effect measurements are made with simple evaporation procedures by using transmission electron microscopy grids as masks. At room temperature, we measure the Hall effect of mono- and multi-layer graphene before and after plasma hydrogenation. During plasma hydrogenation, the sample substrates are biased at +150 V to attract electrons in the plasma for hydrogenation and push away ions in the plasma avoiding possible damage to the graphene. We also measure the Hall effect after annealing the samples at 200 $^{\circ}$C and vacuum of 10$^{-6}$ torr for an hour. Micro-Raman is employed to monitor the quality and change of the graphene at each process step. We compare the Hall effect results for pristine, hydrogenated, and annealed mono- and multi- layer graphene samples. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D30.00004: Density of states of a graphene in the presence of strong point defects Bor-Luen Huang, Ming-Che Chang, Chung-Yu Mou The density of states near zero energy in a graphene due to strong point defects with random positions are computed. Instead of focusing on density of states directly, we analyze eigenfunctions of inverse T-matrix in the unitary limit. Based on numerical simulations, we find that the squared magnitudes of eigenfunctions for the inverse T-matrix show random-walk behavior on defect positions. As a result, squared magnitudes of eigenfunctions have equal a priori probabilities, which further implies that the density of states is characterized by the well-known Thomas-Porter type distribution. The numerical findings of Thomas-Porter type distribution is further derived in the saddle-point limit of the corresponding replica field theory of inverse T-matrix. Furthermore, the influences of the Thomas-Porter distribution on magnetic and transport properties of a graphene, due to its divergence near zero energy, are also examined. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D30.00005: Tuning graphene's electronic structure via unbalanced disordered sublattices and defect superlattices Francois Varchon, Aur\'elien Lherbier, Jean-Christophe Charlier Graphene, a single carbon plane arranged on a honeycomb lattice, has received a lot of attention in the last few years due to its very appealing physical properties as the room temperature quantum hall effect, a large coherence length or a high electronic mobility. These basic properties hold a high application potential for graphene in nanoelectronics. Nevertheless the future of this field strongly depends on the possibility to control the electronic properties of this material. On the basis of extensive tight-binding and ab initio calculations, we demonstrate the possibility to tune graphene's electronic structure via realistic atomic defects (epoxide and hydroxyl groups chemisorbed on graphene). For example we report on the bandgap opening in graphene monolayer induced by unbalanced disordered sublattices. Our findings show that the bandgap width depends on the nature, the concentration and the distribution (random, semi-random, periodic) of the impurities. We also perform an indepth study about the special case of periodic distribution of atomic defects. We demonstrate the existence of three different families of defect superlattices which conduct to specific band structures and therefore could lead to different electronic and transport properties [1]. \\[4pt] [1] A. Lherbier, F. Varchon, J.-C. Charlier (in preparation) [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D30.00006: Numerical study of impurity effects in Graphene Zhou Li, Frank Marsiglio, Stepan Grinek, Jie Chen It is known that long-range Coulomb impurities could induce a novel supercritical regime in gapped graphene [1]. For short range impurities, the electron wave function is less localized near the band edge and thus numerical results may depend on the size and boundary conditions of the simulated graphene. For six attractive impurities forming a quantum well with radius=a(a is the distance between two nearest neighbor atoms in graphene), we found that the bound states will not merge into the continuum. The results from a finite size exact diagonalization with open boundary conditions agree well with that from an infinite size study based on Green's functions. Also an efficient numerical approach based on kernel polynomial methods [2] will be adopted to evaluate the Green's function accurately in the regime with strong interference effects and compared to T-matrix results. \\[4pt] [1] V.M. Pereira et.al, Phys. Rev. B, 78, 8, 2008, pp. 085101. \\[0pt] [2] L.Covaci et.al, Phys. Rev. Lett. 105, 167006 (2010) [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D30.00007: Plasma Hydrogenation of n-Layer Graphene J.D. Jones, W.D. Hoffman, A.V. Jesseph, C.J. Morris, G.F. Verbeck, J.M. Perez We propose a new mechanism for the hydrogenation of mono-, bi-, and tri-layer graphene samples using an H$_{2}$ plasma. We find that hydrogenation occurs as a result of electron irradiation of H$_{2}$O adsorbates on the sample rather than H species from within the plasma. We propose that the mechanism is electron-impact fragmentation of the H$_{2}$O adsorbates occurring naturally above and below the sample. The stability of the hydrogenation increases with the incident electron energy, allowing for hydrogenated samples that are stable at temperatures $>$ 200 $^{\circ}$C. We also observe fully hydrogenated bi- and tri-layer graphene, which may be evidence for new materials, diamane and \textit{triamane}. Diamane, a two atom thick layer of hydrogenated diamond, is predicted to have a band gap of 3.12 eV and be stronger than graphane, hydrogenated graphene. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D30.00008: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 4:06PM - 4:18PM |
D30.00009: Oxygen reduction activity of BN decorated bulk defects in graphene Shyam Kattel, Boris Kiefer, Plamen Atanassov We use Density-Functional-Theory to investigate the interaction between O$_{2}$ and H$_{2}$O$_{2}$ with co-doped bulk BN defects in graphene. The results show that the mixed defects are thermodynamically stable in contrast to the nitrogen only defects that need a transition metal for stabilization. The interaction between O$_{2}$ and H$_{2}$O$_{2}$ and the BN defects are found to be very different: O$_{2}$ is adsorbed as a molecule on boron with a bond length increase of $\sim $20{\%}. H$_{2}$O$_{2}$, on the other hand, is predicted to adsorb dissociatively to form B(OH)$_{2}$. The predicted binding energy (BE) of O$_{2}$ is similar to the N only defects. This observation suggests that BN defects promote the reduction of O$_{2}$ to H$_{2}$O$_{2}$. However, we also found that the binding energy per OH is $\sim $75{\%} higher than the corresponding BE for the N only defect. Thus, restoring the catalytic site through OH removal is more difficult as compared to the N only defect. This implies that bulk BN defects are most likely less active than N only defects and edge BN defects which enhance ORR. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D30.00010: Effect of Laser Irradiation on Structural and Electronic Properties of Single-layer and Bi-layer Graphene Pubudu Galwaduge, Joseph Lambert, Roberto Ramos Graphene is a two-dimensional crystal with remarkable electronic properties which have made it a component of interest in fabricating chemical sensors, superconducting devices and room-temperature transistors. Fabrication and metrology techniques typically use energetic beams such as lasers which are likely to induce unintentional changes in graphene. We report results of Time-Resolved Raman Spectroscopy experiments that investigate the effect of low,medium, and high power laser irradiation on the structural and electronic properties of single-layer and bi-layer graphene. We have irradiated graphene using a 514.5nm laser at power levels of 1.8mW, 9mW and 18mW. Changes in electronic and structural properties were observed by observing the time evolution of the Raman D and G bands. Under irradiation at 1.8mW and 9mW, single layer graphene flakes show changes in charge carrier concentration. Under irradiation at 18mW,single layer graphene shows signs of defect formation and breakdown into nano-crystalline graphene. Bi-layer graphene shows no measurable changes in the Raman D and G bands under irradiation at 9mW. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D30.00011: Effect of irradiation by electron-beam and oxygen plasma on graphene studied with Raman spectroscopy and electronic transport Romaneh Jalilian, Isaac Childres, Luis A. Jauregui, Michael Foxe, Jifa Tian, Igor Jovanovic, Yong P. Chen We report a study of the effects of electron-beam irradiation and oxygen plasma etching on graphene and graphene field-effect transistors (GFET). For both types of exposure, Raman spectra show a characteristic evolution with increasing irradiation-induced disorder. Electron-beam exposure causes a down-shifting in the charge-neutral point (CNP), interpreted as due to a hole-doping in the substrate. Oxygen plasma etching causes an up-shifting of the CNP, interpreted as due to hole-doping molecules adsorbed on the plasma-induced defects. Both types of exposure decrease the carrier mobilities and minimum conductivity of graphene. Additionally, weak localization and the quantum Hall effect are characterized in exposed devices. Our findings are valuable for understanding the effects of irradiation damage on graphene and the physics of disordered graphene through artificially generated defects. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D30.00012: Stability and Mobility of Vacancy Defects in Monolayer Graphene Wei Chen, Haiping Lan, Ping Cui, Jinlong Yang, Zhenyu Zhang Using DFTB and first-principle calculations, we study the stability and mobility of vacancy defects in graphene. First, we calculate the formation energy of vacancy defects of varying sizes in different supercells, including its dependence on the Brillouin zone sampling. We find a large difference, of ~1eV, in the formation energy between the value with only Gamma-point sampling and that with more symmetrical k-point sampling in the 3N*3N (N=2,3,4) supercells. This variance is attributed to significant contributions of the electronic states around the Dirac points. We then explore the mobility of the vacancy defects, including single atom vacancy, trivacancy, and tetravacancy. We find that both trivacancy and tetravacancy have relatively small activation energies for migration via a Stone-Wales transformation of the edge atoms. These results will be compared with recent experimental observations. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D30.00013: Controllable defect healing and N-doping of graphene by CO and NO molecules Bin Wang, Sokrates Pantelides Controllable defect healing and N-doping in graphene would be very valuable for potential device applications. Here we report first-principles molecular dynamic simulations that suggest a procedure with fast dynamics and low thermal budget. Vacancies can be healed by sequential exposure to CO and NO molecules. A CO molecule gets adsorbed at a vacancy site and a NO molecule subsequently removes the extra O by forming NO$_{2}$. Controllable N-doping can be achieved by sequential vacancy creation (e.g. by electron beam) and subsequent exposure to NO molecules at room temperature. A combination of CO and NO molecules can potentially provide simultaneous healing and doping at a desirable ratio. The proposed strategy introduces no extra defects and is promising for graphene-based materials in radiation environments. [Preview Abstract] |
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