Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F8: Functionalization and Decoration of Graphene |
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Sponsoring Units: DCMP Chair: Jun Zhu, Pennsylvania State University Room: 307 |
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F8.00001: Hydroxyl-decorated Graphene Systems: Organic metal-free Ferroelectrics, Multiferroics, and Proton battery Cathode Materials Menghao Wu, J.D. Burton, Evgeny Tsymbal, Xiao Cheng Zeng, Puru Jena Through density-functional-theory calculations we show that hydroxylized graphene systems are ideal candidates for light-weight organic ferroelectric materials with giant polarizations. For example, the polarization of semi-hydroxylized graphane and graphone as well as fully hydroxylized graphane are, respectively, 41.1, 43.7, 67.7 $\mu$C/cm$^{2}$, much higher than any organic ferroelectric materials known to date. In addition, hydroxylized graphone is multiferroic due to the coexistence of ferroeletricity and ferromagnetism. Zigzag graphene nanoribbons decorated by hydroxyl groups also exhibit ferroelectric properties with a large polarization of 27.0 $\mu$C/cm$^{2}$. Moreover, proton vacancies at the end of ribbons can induce large dipole moments that can be reversed by both hopping of protons and rotation of O-H bonds under an electric field. These materials have the potential as high-capacity cathode materials with specific capacity six times larger than lead-acid batteries and five times that of lithium-ion batteries. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F8.00002: Quantum Sticking of Atomic Hydrogen to Graphene Yanting Zhang, Adam Doherty, Andrew Geragotelis, Dennis Clougherty We consider the low-energy behavior of the sticking probability of atomic hydrogen to suspended graphene. For energy transfer through the flexural modes of graphene, we find that the inelastic coupling falls in the subOhmic regime. Thus the effects of low-frequency fluctuations of the graphene sheet are crucially important for quantum sticking. We analytically solve for the low-energy asymptotic behavior of the sticking coefficient using a variational mean-field method [D.P. Clougherty and Y. Zhang, Phys. Rev. Lett. 109, 120401 (2012)]. We find that as a result of strong coupling to the low-frequency flexural modes of graphene, a new scaling law results. For suspended graphene at finite temperature, we find that at a critical incident energy, the sticking probability drops discontinuously; below this critical energy, the sticking probability is suppressed by the orthogonality catastrophe. We compare our nonperturbative variational results to those obtained by using Fermi's golden rule. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F8.00003: Electronic structure of oxygen functionalized graphene nanoribbons Adam Simbeck, Deyang Gu, Neerav Kharche, Saroj Nayak We investigate the electronic and magnetic properties of armchair graphene nanoribbons whose edges are passivated by oxygen. Using a first-principles density functional approach and the many-body GW method we find that oxygen-passivation results in a rich geometrical environment which in turn determines the electronic and magnetic properties of the ribbon. For planar systems we report magnetic ground states whose electronic structure depends upon the magnetic coupling between edges. For non-planar ribbons we report a nonmagnetic ground state with a band gap that decreases as a function of increasing ribbon width. Our results will be discussed in light of previous experimental and computational studies. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F8.00004: Optical properties of functionalized monolayer and bilayer graphene JinLuo Cheng, Cuauht\'emoc Salazar, John E. Sipe We use {\it ab initio} calculations to investigate the structures, band structures, and optical properties of functionalized monolayer and bilayer graphene, where a hydrogen atom is attached to only one carbon atom site periodically every few unit cells. The hydrogen atom distorts the carbon atoms vertically, but the inplane structure is approximately unchanged. The ground state acquires a bandgap due to adsorption depending on the supercell size, and shows magnetic order, which is in agreement with a recent experiment [1]. The calculated optical absorption spectra displays detailed structures at lower photon frequencies than that of the pristine graphene.\\[4pt] J. Hong {\it et~al.}, Sci. Rep. {\bf 2}, 624 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F8.00005: Ferromagnetism in hydrogenated epitaxial graphene on 6H-SiC A.J.M. Giesbers, K. Uhlirova, M. Konecny, J. Aarts, C.F.J. Flipse Graphene remains a material of interest in both fundamental and applied physics due to its unique combination of properties [1] such as its mechanical strength, surface sensitivity, relativistic bandstructure and large spin relaxation length. Functionalizing graphene leads to a whole new range of properties [2] varying from photoluminescence in graphene oxide [3] to ferromagnetism in hydrogenated graphene [4]. Here we will show a detailed investigation of the (ferro-)magnetic properties of hydrogenated epitaxial graphene on SiC (HeG). The magnetization of the of the HeG shows a clear hysteresis loop, which remains visible up to room temperature with a saturation magnetization of 0.5 $\mu_{\mathrm{B}}$/hexagon. The saturation magnetization depends on the hydrogen coverage and shows a strong anisotropy to the sample orientation with respect to the magnetic field. [1] N. M. R. Peres, Rev. Mod. Phys. 82, 2673 (2010) [2] W. Wei and X. Qu, Small 8, 2138 (2012). [3] Z. Luo et al., Appl. Phys. Lett. 94, 111909 (2009). [4] L. Xie et al., Appl. Phys. Lett. 98, 193113 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F8.00006: Electronic structures and magnetism of hydrogenated and fluorinated graphene with vacancies Bi-Ru Wu, Chih-Kai Yang Graphene is a gapless semiconductor. As graphene is covered with one layer of hydrogen or fluorine, it becomes a wide band gap insulator. However, vacancies are easily found during the hydrogenated or fluorinated processes. We investigate the electronic structure and magnetism of the hydrogenated and fluorinated graphene with a variety of configuration of vacancies. We found that a continuous zigzag chain distribution of vacancies will result linear energy dispersion both in the hydrogenated and fluorinated graphene. This finding should be very useful for the design of graphene based electronic devices. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F8.00007: First-principles study of the spin-orbit interaction in graphene induced by hydrogen adatoms Martin Gmitra, Denis Kochan, Jaroslav Fabian We have performed first principles calculations of the spin-orbit coupling effects in hydrogenated graphene structures, for varying hydrogen coverage densities, using the linearized augmented plane wave method as implemented in the FLEUR code. The covalent bonding between the hydrogen and carbon atoms leads to a local structural puckering of graphene sheets, giving rise to an overlap between the Dirac and sigma electrons and a giant enhancement (from roughly 0.01 to 1 meV) of the local spin-orbit interaction. The calculated effects on the band structure and the emerging spin patterns of the electronic states can be well explained by effective Hamiltonian models derived from group theoretical principles. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F8.00008: A Model for the Origin of Spin half Para-magnetism in Fluorinated Graphene Piali Aditya, Alejandro Suarez, Tyler Maunu, Diego B. Carrasco, Jorge Sofo It came as a surprise when the Manchester group reported a paramagnetic response in fluorinated graphene [\textit{Nair et al., Nature Physics 8, 199-202 (2012)}]. The response is characteristic of non-interacting spin 1/2 with a concentration that is almost zero up to 60{\%} fluorination and peaks at 80{\%} fluorination. The density is never larger than a few spins per 1000 carbon atoms. Prior DFT calculations show an absence of magnetism for dilute fluorinated graphene samples [\textit{Sofo et al., Phys. Rev. B Rapid Comm. 83, 081411 (2011)}]. We propose that the magnetic response originates from regions with a small number of non-fluorinated carbon atoms surrounded by fluorinated ones. In support of this model we combine the exact response of the non-fluorinated regions with a stochastic model to account for the fluorination process. Our calculation reproduces the magnetic response of the samples and tracks the origin of this magnetic phenomenon to the grain boundary between fluorinated patches. If our model is correct, the number of spins in this sample is not an intrinsic quantity but is determined by the fluorination process. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F8.00009: Analytic Local and Total Density of States for Hydrogen Adatoms on Graphene Nicholas Pike, David Stroud Spin transport through graphene is strongly influenced by the presence of adatoms with unpaired spins, such as hydrogen adatoms. In this work, we calculate the local density of states (LDOS) for a simple model of hydrogen on graphene using a tight binding model. The model includes nearest neighbor hopping between carbon atoms, the value of the hydrogen energy level, hopping between the carbon and hydrogen atoms, and a Hubbard U-term to account for the on-site Coulomb interaction. When U = 0, we develop an exact analytic equation for the LDOS on the adatom site, and for the total density of states (DOS). When U $\neq 0$, we carry out the same calculation but treat the Hubbard term using mean-field theory. We find that the hydrogen adatom has a net non-integer spin polarization, and that some of the electronic density is transferred from the hydrogen adatom to the graphene host. Possible implications of these results for spin transport through graphene will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F8.00010: Electrical detection of phase changes in adsorbed neutral dipolar molecules on graphene Yilin Wang, Wenzhong Bao, Shudong Xiao, Michael Fuhrer, Janice Reutt-Robey Graphene is a very promising material for sensing application because its transport properties are highly sensitive to adsorbates on its surface. Here, we study the carrier-density-dependent resistance of bilayer graphene to neutral dipolar adsorbates under ultra-high vacuum condition. Halocarbon molecules with known dipole moment are deposited on graphene at $\sim$ 20 K. After deposition of a few monolayers of molecules, the resistance of graphene near the Dirac point is measured as a function of carrier density (tuned by gate voltage) and temperature, from 20 K to room temperature. We observe negligible shifts of the gate voltage of maximum resistance, indicating negligible charge transfer from adsorbate to graphene. In the temperature-dependent-resistance curve, a sharp step-like increase and decrease in resistance occur at $\sim$ 45 K and $\sim$ 65 K, respectively. We relate these abrupt changes in resistance to phase transitions in the adsorbate overlayer. The same molecules adsorbed on graphite are known to exhibit a complex temperature - coverage phase diagram. We will discuss the relationship between graphene resistance and the phases of molecules on graphite. This work was supported by the NSF-MRSEC at the University of Maryland, DMR 0520471 [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F8.00011: Optical properties of hydrogenated graphene from first principles Sebastian Putz, Martin Gmitra, Jaroslav Fabian We investigate the effect of hydrogen coverage on the optical properties of single-side hydrogenated graphene from first principles. To account for different degrees of uniform hydrogen coverage we calculate the complex dielectric function for graphene supercells of various size, each containing a single additional H atom. We use the Linearized Augmented Planewave (LAPW) method, as implemented in WIEN2k, to show that the hydrogen coverage strongly influences the complex dielectric function and thus the optical properties of hydrogenated graphene. The absorption coefficient in the visible range, for example, has different characteristic features depending on the hydrogen coverage. This opens up new possibilities of determining the hydrogen coverage of hydrogenated graphene samples in the experiment by contact-free optical absorption measurements. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F8.00012: A Theoretical Analysis of the Effect of the Hydrogenation of Graphene to Graphane on Its Mechanical Properties Q. Peng, Chao Liang, Wei Ji, Suvranu De We investigated the mechanical properties of graphene and graphane using first-principles calculations based on density-functional theory. A conventional unitcell containing a hexagonal ring made of carbon atoms was chosen to capture the finite wave vector ``soft modes", which affect the the fourth and fifth elastic constants considerably. Graphane has about 2/3 ultimate strengths in all three tested deformation modes -- {\em armchair}, {\em zigzag}, and {\em biaxial}-- compared to graphene. However, graphane has larger ultimate strains in {\em zigzag} deformation, and smaller in {\em armchair} deformation. We obtained the second, third, fourth, and fifth order elastic constants for a rigorous continuum description of the elastic response. Graphane has a relatively low in-plane stiffness of 240 N/m which is about 2/3 of that of graphene, and a very small Poisson ratio of 0.078, 44\% of that of graphene. The pressure dependence of the second order elastic constants were predicted from the third order elastic constants. The Poisson's ratio monotonically decreases with increasing pressure. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F8.00013: Fluorination of CVD graphene: the role of wrinkles, folds, multi-layer islands and grain boundaries Bei Wang, Junjie Wang, J. Zhu Chemical functionalization, such as fluorination, can modify the gapless band structure of graphene and turn it into an insulator. Fluorinated graphene (FG) can potentially be integrated into graphene electronics and serve as ultrathin gate dielectrics or tunnel barriers. Here we present our effort in synthesizing and understanding the properties of FG. Graphene sheets synthesized by chemical vapor deposition (CVD) are fluorinated using CF$_{4}$ plasma under varying conditions. The resulting FG is systematically examined using a wide range of spectroscopic and microscopic tools including XPS, Raman, FTIR, electrical transport and conductive AFM. We obtain high F:C ratio of 0.1-1. Our results show that 1. Morphological features of CVD graphene (wrinkles, folds, multi-layer islands) are less fluorinated and charge transport in FG occurs through the conductive network formed by these features. 2. Lattice defects and grain boundaries play a significant role in the chemical reactivity of CVD graphene. XPS studies indicate the formation and evolution of CF$_{x}$ (x=1,2,3) bonds, as well as oxygen-passivated defect sites in FG. These studies highlight current challenges in realizing electronics-grade FG and point to the possible pathways forward. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F8.00014: Theory of the hydrogen adatoms induced spin-orbit coupling in graphene Denis Kochan, Martin Gmitra, Jaroslav Fabian We have analyzed the first-principles data of the electronic structure of hydrogenation in graphene by means of group theory derived effective Hamiltonians. We propose effective models for semihydrogenated graphene as well as for graphene with a single hydrogen adatom. The chemisorption of hydrogen modifies the structural symmetry of the plane graphene in two essential ways---it breaks the pseudospin (sublattice) symmetry and induces rippling. We show that in addition to the Rashba spin-orbit interaction there emerges another spin-orbit field which is induced by the pseudospin inversion asymmetry due to the adatoms. Our realistic effective Hamiltonians should be useful for spin transport and spin relaxation investigations. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F8.00015: Adsorption Configurations of Carbon Monoxide on Gold Monolayer Supported by Graphene or Hexagonal Boron Nitride Film: A First-Principles Study Lu Wang, Wai-Ning Mei, Jiaxin Zheng, Jing Lu, Peter Dowben Using density functional theory with a semiempirical van der Waals approach proposed by Grimme, the adsorption behavior of carbon monoxide on a gold monolayer supported by graphene or monolayer hexagonal boron nitride has been investigated. Based on the changes in the Dirac cone of graphene and a Bader charge analysis, we observe that the Au(111) monolayer gains a small electron charge from graphene and monolayer $h$-BN. The adsorbed CO molecule adopts similar adsorption configurations on Au(111)/graphene and Au(111)/$h$-BN with Au-C distance 2.17$-$2.50 {\AA} and Au-C-O angle of $123.9^{\circ} - 139.6^{\circ}$. Moreover, we found that for low CO coverages, bonding to the gold surface is surprisingly energy-favorable. Yet the CO adsorption binding energy diminishes at high coverage due to the repulsive van der Waals interactions between CO molecules. [Preview Abstract] |
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