Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y5: Opening the Gap: Chemical Functionalization and Substitution in Graphene |
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Sponsoring Units: DCOMP DCP Chair: Shaffique Adam, National Institute of Standards and Technology Room: Ballroom C1 |
Friday, March 25, 2011 8:00AM - 8:36AM |
Y5.00001: Magnetic Moment and Electronic Correlations in Chemically Functionalized Graphene Invited Speaker: Magnetic moments in extended systems are the result of local electronic correlations. In the case of graphene functionalized with chemisorbed atoms such as hydrogen, fluorine, or oxygen, the Anderson Model picture, where correlations in a localized state are responsible for the formation of a magnetic moment, has to be modified to properly describe the magnetic moment formation and their interactions. We use a tight-binding model with local correlations to analyze the results obtained with Density Functional Theory calculations for these systems. The model allows the treatment of local correlations beyond the mean field level and the investigations of a possible Kondo effect. We find that the Coulomb repulsion at the carbon atoms near the impurity play a crucial role in the magnetic moment formation. External doping with a gate voltage can control the nature of the binding and the formation of the magnetic moment. This effect could be observed in transport experiments as the scattering of the graphene electrons at the Fermi energy strongly depends on the structure of the defect. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 9:12AM |
Y5.00002: A theoretical study of chemical functionalisation of graphene: graphane and graphXene Invited Speaker: Chemical functionalisation of graphene is reported from a first principles, theoretical study [1]. The electronic structure, including band gap, of H adsorbed on graphene (i.e. graphane) is discussed in this presentation [2]. In addition, adsorption of Group VII elements on graphane (named graphXene) is also reported [3]. Similarities and differences in the chemical binding and electronic structure of graphane and graphXene are analyzed. The adsorption on graphene is found to, depending on adatoms, result in sp2 or sp3 binding, where in general the sp3 bonded systems show a bandgap. The theoretical calculations make use of both GGA functionals as well as the GW approximation. In addition to large graphene layers, theoretical analysis of functionalised graphene nano-ribbons will also be presented [4]. References: \\[4pt] [1] V. A. Coleman, et al.,J. Phys. D: Appl. Phys. 41,062001 (2008); S. H. M. Jafri, et al, J. Phys. D 43, 45404 (2010). \\[0pt] [2] S.Lebegue, et al., Phys. Rev. B 79, 245117 (2009). \\[0pt] [3] M.Klintenberg, et al., Phys. Rev. B 81, 85433 (2010). \\[0pt] [4] S. Bhandary, et al., Phys. Rev. B 82, 165405 (2010). [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:48AM |
Y5.00003: Gap control via graphene solid-state reactions Invited Speaker: While a gapless dispersion law of Dirac fermions in graphene does warrant admiration, to serve as useful semiconductor graphene needs a gap. Relatively inert, it can nevertheless be induced to react. A generic outcome of a reaction, C + A -$>$ C$_{1-x}$A$_{x}$ is a transition of some C-atoms from their sp$^ {2}$- into sp$^{3}$-state, corresponding to a situation of the insulating, ultimate (mono- or few-layer) diamond slab [1]. Computations support a concept that the product of such reactions (A = H, F, O, Cl, etc.) forms a well-defined phase [2], permitting a patterning of 2D-geometries with useful properties: interconnects-nanoroads [3], quantum isles-dots [4], etc. Comparison of hydrogenation (A = H) into graphAne with fluorination (A = F) into 2D-teflon, shows the former as hindered by nucleation barrier and reversible (H-storage), in contrast to barrier-less reaction into a stable CF in the latter. *** In collaboration with F. Ding, E. Penev, M.A. Ribas, and A.K. Singh. ***\\[4pt] [1] E. Munoz, et al., Diamond \& Related Mater., 19, 368, 2010.\\[0pt] [2] Y. Lin, et al., Phys. Rev. B, 78, 041402(R), 2008.\\[0pt] [3] A.K. Singh and BIY, Nano Lett., 9, 1540, 2009.\\[0pt] [4] A.K. Singh, et al., ACS Nano, 4, 3510, 2010.\\[0pt] [5] ``Patterning on fluorinated graphene,'' M. Ribas, et al., Nano Res. (2010). [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:24AM |
Y5.00004: Spin-polarized semiconductor induced by magnetic impurities in graphene Invited Speaker: Magnetic impurities adsorbed on graphene sheets are coupled antiferromangetically via the itinerant electrons in the graphene. We study this interaction and its impact on the electrons' spectral density by use of unbiased Monte-Carlo simulations. The antiferromagnetic order breaks the symmetry between the sublattices, and a gap for the itinerant electrons opens. Our simulations show that the itinerant states below and above the gap are not dispersionless states trapped by the impurities, but are instead mobile states with a large dispersion. We compare various scenarios for the impurity distribution and find that random doping produces a standard semiconductor. If, on the other hand, all or most of the impurities are localized in the same sublattice, the spin degeneracy is lifted and the conduction band becomes spin-polarized. We also discuss the properties of edge states at edges or magnetic domain boundaries.\\[4pt] M.~Daghofer, N.~Zheng, A.~Moreo; Phys.~Rev.~B {\bf 82}, 121405(R) (2010) [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 11:00AM |
Y5.00005: Graphene monofluoride: a wide bandgap material derived from graphene Invited Speaker: Fluorination provides an effective way of controlling the properties of carbon materials. In this talk, I will describe our experimental and theoretical work on the synthesis, structural, electrical and optical properties of fully fluorinated graphene and graphite, i. e., graphene monofluoride CF and graphite monofluoride (CF)$_n$. (CF)$_n$ is synthesized by reacting HOPG graphite with F$_2$ gas at high temperature. Transmission electron microscopy and electron diffraction measurements show crystalline few-layer CF with a lattice constant 4\% larger than that of graphene, in good agreement with first principle calculations. We observe the E$_g$ and A$_{1g}$ Raman modes of graphene monofluoride using UV Raman spectroscopy. Photoluminescence measurements of (CF)$_n$ using variable excitation wavelength (244-514 nm) and temperature (5-295 K) show several emission modes in the visible spectrum, which likely originate from mid-gap defect states. The absence of the band edge emission suggests a large band gap of greater than 5 eV. Partially fluorinated graphene fluoride exhibits non-linear, strongly insulating transport with variable-range hopping temperature dependence, consistent with the presence of localized states due to missing fluorine atoms. Highly conductive graphene can be recovered by annealing CF in Ar/H$_2$ at high temperature, resulting in a conductance improvement of five orders of magnitude. As a transparent and atomically thin insulator, graphene monofluoride may find its use in graphene electronics and photonics. In collaboration with: Bei Wang, Shih-Ho Cheng, Justin Sparks, Humberto Gutierrez, Ke Zou, Ning Shen, Youjian Tang, Qingzhen Hao, Awnish Gupta, Peter Eklund, Vincent Crespi, Jorge Sofo and Fujio Okino (Shinshu University, Japan). References: Cheng et al, ``Reversible fluorination of graphene: towards a two-dimensional wide band gap semiconductor,'' Phys. Rev. B 81, 205435 (2010) Wang et al, ``Photoluminescence from nanocrystalline graphite monofluoride,'' Appl. Phys. Lett. 97, 141915 (2010) [Preview Abstract] |
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