Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T8: Carbon Nanostructures: Transport and Optical Phenomena |
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Sponsoring Units: DMP Chair: Lilia Woods, University of South Florida Room: 307 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T8.00001: Dynamic Negative Compressibility of Few-Layer Graphene, h-BN, and MoS$_2$ Bernardo Neves, Ana Paula Barboza, Helio Chacham, Camilla Oliveira, Thales Fernandes, Erlon Martins Ferreira, Braulio Archanjo, Ronaldo Batista, Alan Oliveira We report a novel mechanical response of few-layer graphene, h-BN, and MoS2 to the simultaneous compression and shear by an atomic force microscope (AFM) tip. The response is characterized by the vertical expansion of these two-dimensional (2D) layered materials upon compression. Such effect is proportional to the applied load, leading to vertical strain values (opposite to the applied force) of up to 150{\%}. The effect is null in the absence of shear, increases with tip velocity, and is anisotropic. It also has similar magnitudes in these solid lubricant materials (few-layer graphene, h-BN, and MoS2), but it is absent in single-layer graphene and in few-layer mica and Bi2Se3. We propose a physical mechanism for the effect where the combined compressive and shear stresses from the tip induce dynamical wrinkling on the upper material layers, leading to the observed flake thickening. The new effect (and, therefore, the proposed wrinkling) is reversible in the three materials where it is observed.\footnote{A. P. M. Barboza, H. Chacham, C. K. Oliveira, T. F. D. Fernandes, E. H. Martins Ferreira, B. S. Archanjo, R. J. C. Batista, A. B. de Oliveira and B. R. A. Neves, \textit{Nano Lett. }\textbf{12}, 2313$-$2317 (2012).} [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T8.00002: Scaling of Non-Saturating Magnetoresistance in HOPG Nicholas Cornell, Myron Salamon, Anvar Zakhidov There have been many various resistive and field dependent behaviors observed in Highly Oriented Pyrolytic Graphite (HOPG). We found HOPG samples to vary significatly in their temperature dependent resistances, even between portions of the same sample. All samples exhibit non-saturating magnetoresistance (MR) and, at low temperatures, Shubnikov-de Haas (SdH) oscillations. These oscillations give rise to a mobility $\mu =1.2$ T$^{-1}$ at $5$ K. The MR follows a scaling behavior that is predicted by a model based on the Hall effect in granular materials and that predicts a crossover to linear behavior with a characteristic field $H_{0}$ on the order of $\mu ^{-1}$, or $0.8$ T, in agreement with experiment. Data at higher temperatures can be collapsed to a single curve if $H_{0}(T)$ increases linearly with temperature. Analysis of the SdH data gives a 2D carrier density in agreement with previous results, and a large mean-free path relative to crystallite size. [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 8:36AM |
T8.00003: Mechanical and Electrostatic Properties of Freestanding Graphene Functionalized With Tin Oxide (SnO$_2$) Matthew Ackerman, Peng Xu, Steven Barber, Kevin Schoelz, Dejun Qi, Paul Thibado, Lifeng Dong, James Hansen Polymer/graphene blends have shown promise for building inexpensive and efficient heterojunction solar cells. It has been shown that efficiencies can be enhanced if the graphene membrane is functionalized by n-type inorganic nanocrystals, but it has proved difficult to directly chemically modify graphene. In this talk we present for the first time a two-step solution based technique which directly and uniformly deposits SnO$_2$ nanoparticles onto a graphene membrane. Films are characterized using X-ray energy dispersive spectrometry (EDS) and field emission scanning electron microscopy (FESEM) to determine elemental composition and density of coverage. A novel technique known as electrostatic manipulation scanning tunneling microscopy (EM-STM) is employed to characterize the affect of the nanoparticles on the mechanical and electrostatic properties of the functionalized graphene relative to pristine membranes. Evidence is presented that during the deposition stage graphene wraps around and encapsulates the nanoparticles. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T8.00004: Can graphene allotropes surpass the high thermal conductivity of graphene? Zacharias Fthenakis, Zhen Zhu, David Tom\'anek Searching for materials with very high thermal conductivity, we explore the possibility that specific carbon allotropes may even surpass the high thermal conductivity of graphene and carbon nanotubes. We focus our study on graphene allotropes including 5-7 or 5-5-8 haeckelites with planar structure and $sp^2$ graphitic bonding. In contrast to graphene, these anisotropic systems should also conduct heat differently in different directions. Our computational studies use non-equilibrium molecular dynamics simulations based on the valence-bond force field parameterized by Tersoff and a Nose-Hoover thermostat to regulate the temperature. Whereas thermal conductivity of most haeckelite systems is reduced by an order of magnitude in comparison to graphene due to a lower phonon mean-free path, there is a distinct possibility that the isotropic thermal conductivity of graphene may be surpassed at least along particular directions in specific artificial haeckelite superstructures. [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T8.00005: Radiative heat transfer in low-dimensional systems -- microscopic mode Lilia Woods, Anh Phan, David Drosdoff Radiative heat transfer between objects can increase dramatically at sub-wavelength scales. Exploring ways to modulate such transport between nano-systems is a key issue from fundamental and applied points of view. We advance the theoretical understanding of radiative heat transfer between nano-objects by introducing a microscopic model, which takes into account the individual atoms and their atomic polarizabilities. This approach is especially useful to investigate nano-objects with various geometries and give a detailed description of the heat transfer distribution. We employ this model to study the heat exchange in graphene nanoribbon/substrate systems. Our results for the distance separations, substrates, and presence of extended or localized defects enable making predictions for tailoring the radiative heat transfer at the nanoscale. [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T8.00006: Van der Waals/Casimir interactions in graphene nanoribbons David Drosdoff, Lilia Woods The isolation of graphitic nanostructures and their potential applications for novel devices have spurred new interest in the properties of low dimensionality systems. One important interaction in the sub-micron scale is the van der Waals/Casimir force. The general Casimir force between two planes in terms of the dielectric response of the materials was originally formulated by Lifshitz, which was subsequently generalized to two dimensional systems. In this talk, the formulation of the non-retarded dispersive force in terms of the dielectric response functions for quasi-one dimensional systems will be discussed. Results from the application of the developed theory to the interaction between graphene nanoribbons will be presented. [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T8.00007: Prediction of ultra-high ON/OFF ratio nanoelectromechanical switches from covalently bound C60 chains: An ab initio study Han Seul Kim, Yong-Hoon Kim Applying a first-principles computational approach combining density-functional theory and matrix Green's function calculations, we analyze the microscopic origin of the switching behavior experimentally observed for the fullerene C$_{\mathrm{60}}$ chains oligomerized via [2$+$2] cycloaddition and propose a scheme to significantly improve the device performance. Considering infinite C$_{\mathrm{60}}$ chains, we first confirm that bound C$_{\mathrm{60}}$ chains with significant orbital hybridizations and band formation should in principle induce a higher conductance state. However, we find that large metal-C$_{\mathrm{60}}$ distances adopted in the scanning tunneling microscope (STM) setup can result in the experimentally observed opposite switching state assignment. The switching ordering and ratio is in fact found to sensitively depend on the STM tip metal species and the associated band bending direction in the C$_{\mathrm{60}}$--STM tip vacuum gap. We demonstrate that a junction configuration in which the C$_{\mathrm{60}}$--STM tip distance is maintained at short distances via nanoelectromechanical tip movement can achieve a metal-independent and drastically improved switching performance based on the intrinsically better electronic connectivity in the oligomerized C$_{\mathrm{60}}$ chains. [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T8.00008: Infrared magneto-optical Kerr and Faraday measurements of carbon nano-onions M. Murat Arik, C.T. Ellis, Alok Mukherjee, John Cerne, A. Molina Ontoria, L. Echegoyen, M.N. Chaur Carbon nano-onions (CNOs) are multilayer fullerenes in the form of concentric spherical graphene shells with diameters on the order of 10 nm. Angular resolved photoemission spectroscopy [1] has shown that the electronic structure of CNOs is more similar to graphite nanocrystals than fullerene molecules. Previously, we have observed rich Landau level structure in planar multilayer graphene using infrared Kerr and Faraday measurements [2], and now apply these techniques to CNOs. We report infrared (100-1000 meV) Faraday and Kerr measurements on CNOs at temperatures down to 10K and magnetic fields up to 7T. These infrared polarization-sensitive magneto-optical measurements allows us to study confinement effects in Dirac and bilayer quasiparticles, interlayer coupling among neighboring graphene shells, as well as inter-CNO coupling between neighboring CNOs. This work is supported by NSF-DMR1006078. \\[4pt] [1] M. Montalti, et al., Phys. Rev. B 67, 113401 (2003) \\[0pt] [2] C.T. Ellis, et al., Proc. 37th Intl. Conf. on Infrared, Millimeter and Terahertz Waves, 2012, Wollongong, Australia (2012) [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T8.00009: The Aharonov-Bohm effect in M\"obius rings Zehao Li, L. Ramdas Ram-Mohan Electron transmission through finite-width 2D ring structures is calculated for cylindrical, flat (Aharonov-Bohm), and M\"obius rings. In the presence of an external magnetic field, curves of constructive transmission display a pattern similar to that for a 1D ring. The periodicity in the magnetic flux, in units of $h/e$, is weakly broken on 2D rings of finite width, so that a description with a 1D-path is very acceptable. The unusual states with half-integer values of $\langle L_z \rangle$ observed on M\"obius rings, display a different characteristic in transmission. Such resonant states are in constructive interference for transmission at magnetic fields where the contribution from ordinary states with integer $\langle L_z \rangle$ is in destructive interference, and vice versa. This leads to an alternating dominance of the set of half-integer $\langle L_z \rangle$ states and the set of integer $\langle L_z \rangle$ states in transport with increasing magnetic fields. We anticipate that M\"obius rings would be synthesized with graphene ribbons in the near future. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T8.00010: Electron Transport in Solvated Porous Nanocarbons Artem Baskin, Petr Kral We study electron transport in porous nanocarbons (PNCs) in vacuum, gases, and ionic solutions. Using state of the art electronic structure methods and nonequilibrium Green's functions techniques, we explore the band structures [1] and the current-voltage characteristics of PNCs with different sizes, shapes, positioning and functionalization of pores, edges, and types of electrodes. We find that the presence of ions and molecules around PNCs can largely influence their electron transmissivity. Therefore, PNCs could be used for highly sensitive detection of ions and polar molecules passing around them. [1] A. Baskin and P. Kral, Electronic Structures of Porous Nanocarbons, Sci. Rep. 1, 36 (2011). [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T8.00011: Exciton Spectra of Two-Dimensional Semiconducting Carbon Structures Shouting Huang, Yufeng Liang, Li Yang We employ the first-principles GW-Bethe-Salpeter Equation (BSE) approach to study excitonic effects on optical absorption spectra of several newly discovered two-dimensional (2D) semiconducting carbon structures. Unique exciton spectra are observed, in which the order of exciton energies and degeneracies are qualitatively different from those of bulk semiconductors. We propose a modified hydrogen-like model that clearly explains their exciton spectra. Our modeling effort gives rise to a convenient way to understand excitonic spectra and estimate the exciton binding energy of 2D semiconductors. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T8.00012: First-Principles Studies of the Vibrational Stark Effect in C60 Peter Doak, Yajing Li, Douglas Natelson, Leeor Kronik, Jeffrey Neaton C60 has played a central role in molecular and organic electronics, where coupling between charge and vibrational degrees of freedom is of paramount importance. Recent surface-enhanced Raman scattering (SERS) studies of C60-Au junctions have reported significant shifts in vibrational mode frequencies with applied bias. Here we compute the magnitude of the vibrational Stark effect in gas-phase C60 and seek to understand and simulate the shifts in Raman mode frequencies observed in these electromigration junction-SERS experiments. Using density functional theory and a finite-difference approach, we calculate trends in the vibrational Stark effect for different modes of gas-phase C60, comparing directly to experiment and assessing the role of substrate-induced charging and external electric fields. This work supported by DOE and computational resources provided by NERSC. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T8.00013: Anomalous response of supported few-layer hexagonal boron nitride to DC electric fields: a confined water effect? Camilla Oliveira, Matheus Matos, M\'ario Mazzoni, H\'elio Chacham, Bernardo Neves Hexagonal boron nitride (h-BN) is a two-dimensional compound from III-V family, with the atoms of boron and nitrogen arranged in a honeycomb lattice, similar to graphene. Unlike graphene though, h-BN is an insulator material, with a gap larger than 5 eV. Here, we use Electric Force Microscopy (EFM) to study the electrical response of mono and few-layers of h-BN to an electric field applied by the EFM tip. Our results show an anomalous behavior in the dielectric response for h-BN for different bias orientation: for a positive bias applied to the tip, h-BN layers respond with a larger dielectric constant than the dielectric constant of the silicon dioxide substrate; while for a negative bias, the h-BN dielectric constant is smaller than the dielectric constant of the substrate. Based on first-principles calculations, we showed that this anomalous response may be interpreted as a macroscopic consequence of confinement of a thin water layer between h-BN and substrate. These results were confirmed by sample annealing and also also by a comparative analysis with h-BN on a non-polar substrate. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T8.00014: Electronic band structure and phonons in V2O5 Churna Bhandari, Walter R.L. Lambrecht Among the vanadium oxides, V$_2$O$_5$ presents special interest as a layered material. As for other layered materials, it is of interest to search for changes in its electronic structure and phonon spectrum in the monolayer modification of this material. For example, reduced screening may modify phonon modes affected by long-range Coulomb interactions. As a preliminary we here present a first-principles study of the bulk electronic band structure and the phonons at the $\Gamma$-point. Density functional calculations in the local density approximation were carried out for the electronic band structure and the density functional perturbation method was used for the phonon calculations. We used LDA and norm-conserving pseudopotentials in the abinit code. A group theoretical analysis is used to label the phonon modes. Non-analyticity is included for the LO modes. The band structures are in good agreement with previous work and yield an indirect band gap. Relaxed structural properties are also in good agreement with experiment. Simulated infrared and Raman spectra will be presented. Our results will be compared with experimental and previous theoretical work. [Preview Abstract] |
Thursday, March 21, 2013 10:48AM - 11:00AM |
T8.00015: Calculation of the optical properties of the nitrogen-vacancy center in diamond Denis Antonov, J\"{o}rg Wrachtrup, Gabriel Bester We calculate the optical properties of extended and nanoscale diamond structures with embedded nitrogen-vacancy centers (NV). In particular, the negatively charged NV$^-$ center is a promising candidate for the manipulation of quantum states, quantum processing [1] and high resolution magnetometry [2]. For these applications a precise prediction and understanding of the optical properties of NV$^-$ centers and of coupled NV$^-$ centers, which are less than 10 nm apart, is required. For this task, we derive spin-polarized atomic effective pseudopotentials (AEPs [3]), which deliver results with DFT quality, but allow us to treat the large number of atoms required for the calculation of coupled NV centers. The ensuing wave functions are used in a configuration interaction approach to obtain the correlated excitonic spectra. Our results for the single defect centers are in good agreement with earlier theoretical reports [4]. The experimental zero phonon line (ZPL) and the band gap of the diamond system were reproduced with an error of 0.5\%. [1] Bermudez et al., Phys. Rev. Lett. {\bf 107}, 150503 (2011) [2] Zhao et al., Nature Nanotechnology {\bf 7},657-662 (2012) [3] J. R. C\'{a}rdenas and G. Bester, Phys. Rev. B {\bf 86}, 115332 (2012) [4] Gali et al., Phys. Rev. B {\bf} [Preview Abstract] |
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