Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties VII |
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Sponsoring Units: DMP Chair: Eugenio Cinquanta, CNR-IMM MDM Laboratory Room: Mile High Ballroom 1E |
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q51.00001: Germananes: Germanium Graphane Analogues Invited Speaker: Joshua Goldberger Graphene's success has shown that it is not only possible to create stable, single-atom thick sheets from a crystalline solid, but that these materials have fundamentally different properties than the parent material. Our interest focuses on the synthesis and properties of Group IV graphane analogues. We have synthesized for the first time, mm-scale crystals of a hydrogen-terminated germanium multilayered graphane analogue (germanane, GeH) from the topochemical deintercalation of CaGe$_{2}$. This layered van der Waals solid is analogous to multilayered graphane. The surface layer of GeH only slowly oxidizes in air over the span of five months, while the underlying layers are resilient to oxidation. We demonstrate that it is possible to covalently terminate the external surface with organic substituents to tune the electronic structure, and enhance the stability. These materials represent a new class of covalently terminated graphane analogues having great potential for a wide range of optoelectronic and sensing applications, especially since theory predicts a direct band gap of 1.53 eV and an electron mobility of 18,000 cm2/Vs which is five times higher than that of bulk Ge. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q51.00002: Unveiling the origin of the linear dispersion in silicene/Ag(111) Mingxing Chen, Michael Weinert The band structure of the recently synthesized (3$\times$3) silicene monolayer on (4$\times$4) Ag(111) is investigated using density functional theory. To compare to recent angle-resolved photoemission spectroscopy (ARPES) experiments [Phys.\ Rev.\ Lett.\ {\bf 108}, 155501 (2012)], including the photon energy ($k_\perp$) dependence of the spectra, we use a $k$-projection technique to unfold the supercell bands of both silicene and the substrate onto the corresponding primitive cells. Our calculations reproduce the observed linear dispersion across the K point of (1$\times$1) silicene observed, but demonstrate that this is not a Dirac state, but rather originates from the Ag(111) substrate, thus resolving the controversy concerning the origin of the linear dispersion in silicene/Ag(111). [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q51.00003: Hexagonal Warping and Quasiparticle Chirality in Silicene Probed by Scanning Tunneling Microscopy Baojie Feng, Lan Chen, Kehui Wu We performed low temperature scanning tunneling microscopy (STM) and spectroscopy (STS) studies on the electronic properties of ($\surd 3 \times \surd 3$)R30 phase of silicene on Ag(111) surface. We found the existence of Dirac Fermion chirality through the observation of 1.5 and 1.0 power law decay of quasiparticle interference (QPI) patterns. Moreover, in contrast to the trigonal warping of Dirac cone in graphene, we found that the Dirac cone of silicene is hexagonally warped, which is further confirmed by density functional calculations and explained by the unique superstructure of silicene. Our results demonstrate that the ($\surd 3 \times \surd 3$)R30 phase is an ideal system to investigate the unique Dirac Fermion properties of silicene. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q51.00004: Microscopic origin of the $\pi$ states in epitaxial silicene on ZrB$_2$(0001) Antoine Fleurence, Yasuo Yodhida, Chi-Cheng Lee, Taisuke Ozaki, Yukio Hasegawa, Yukiko Yamada-Takamura Silicene, the graphene-like allotrope of silicon is the object of a recently raised enthusiasm, due to the perspectives opened by the novelty of its electronic, physical and chemical properties deriving from its $\pi$ electronic system. So far, silicene only exists in epitaxial forms on metallic substrates. In particular, the spontaneous and self-terminating segregation of silicon atoms on the (0001) surface of zirconium diboride (ZrB$_2$) thin films epitaxied on Si(111) gives rise to a wide-scale uniform $(\sqrt{3}\times\sqrt{3})$-reconstructed two-dimensional silicene sheet $[1]$. By means of low-temperature scanning tunneling spectroscopy and density functional theory calculations, we investigated the impact of the buckling of epitaxial silicene at atomic scale on the electronic properties. The microscopic origin of the valence and conduction states was determined and the strong contribution of the p$_z$ orbitals of specific Si atoms to those states demonstrate their $\pi$ character. A clear correlation between the estimated orbital hybridization of the Si atoms and the buckling was also found. $[1]$: A. Fleurence et al., Phys. Rev. Lett., 108, 245501 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q51.00005: Growth and Oxidation of Silicene Nano-Ribbons on Ag(110) Mohamed Rachid Tchalala, H. Enriquez, A. Mayne, G. Dujardin, H. Oughaddou, M. Ait Ali Scanning tunneling microscopy (STM) and high resolution photoemission electron spectroscopy (HR-PES) are used to study the growth and the oxidation of silicene nano-ribbons (NRs) on Ag(110) substrate. Deposition of silicon on Ag(110) induces a self-assembled silicene NRs having a (2x5) superstructure. We find out that the NRs are not reactive to molecular oxygen. However for a certain bias, the STM tip can dissociate the molecular oxygen which reacts then immediately with the NRs. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q51.00006: Unusual Optoelectronic Properties of Hydrogenated Bilayer Silicene: From Solar Absorber to Light-emitting Diode Applications Bing Huang, Hui-Xiong Deng, Hoonkyung Lee, Changwon Park, Mina Yoon, Bobby Sumpter, Feng Liu, Sean Smith, Su-Huai Wei Silicon is arguably the greatest electronic material, but not so good an optoelectronic material. By employing first-principles calculations and cluster-expansion approach, we discover that hydrogenated bilayer silicene (BS) shows promising potential as new optoelectronic materials. Most significantly, hydrogenation will covert the intrinsic BS, a strongly indirect semiconductor, into a direct-gap semiconductor with a widely tunable band gap. At low hydrogen concentrations, four ground states of single- and double-side hydrogenated BS are characterized with dipole-allowed direct (or quasidirect) band gaps in the desirable range from 1 to 1.5 eV, suitable for solar applications. At high hydrogen concentrations, three well-ordered double-side hydrogenated BS structures exhibit direct (or quasidirect) band gaps in the range of red, green, and blue colors, respectively, affording white light emitting diodes. Our findings open a door to the search of new silicon-based light-absorption and light-emitting materials for earth-abundant high-efficiency optoelectronic applications. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q51.00007: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q51.00008: Photo-induced Modulation Doping in Graphene/Boron nitride Heterostructures Jairo Velasco Jr., Long Ju, Edwin Hwang, Salman Kahn, Casey Nosiglia, Hsin-Zon Tsai, Wei Yang, Guangyu Zhang, Takashi Taniguchi, Kenji Watanabe, Yuanbo Zhang, Michael Crommie, Alex Zettl, Feng Wang Van der Waals heterostructures (VDH) provide an exciting new platform for materials engineering, where a variety of layered materials with different electrical, optical and mechanical responses can be stacked together to enable new physics and novel functionalities. We report an emerging optoelectronic phenomenon (i.e. photo-induced modulation doping) in the graphene-boron nitride VDH (G/BN heterostructure). We find it enables flexible and repeatable writing and erasing of charge doping in graphene with optical light. We show that the photo-induced modulation doping maintains the remarkable carrier mobility of the G/BN heterostructure, and it can be used to generate spatially varying doping profiles like pn junctions. Our work contributes towards understanding light matter interactions in VDHs, and introduces a simple technique for creating inhomogeneous doping in high mobility graphene devices. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q51.00009: Sub-nanometer milling of layered materials by a focused Helium Ion Beam Hongzhou Zhang, Daniel Fox, Yangbo Zhou, Robert O'Connell The modification of the structure and geometry of materials at the nanoscale can be used to tailor their properties. A controllable process which can achieve this is required for the development of next generation nano-devices. We used the highly focused beam of helium ions in a helium ion microscope (HIM) to fabricate nanostructures within various layered materials such as graphene, MoS$_{2}$, TiO$_{2}$ and Mn$_{2}$O$_{3}$. Arbitrary patterns can be defined in order to produce structures such as nanoribbons. The edge configuration of atoms in such structures plays a large role in defining their properties. High resolution transmission electron microscopy (TEM) and scanning-TEM (STEM) were used to analyse the structure of the materials after milling. The direct milling of the materials by the helium ions means this approach is suitable for a wide range of nanomaterials. Complex structures can be realized via sophisticated beam control. This also results in the ability to mill along different directions in a crystal, producing edges with different configurations. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q51.00010: Universal method for creating optically active nanostructures on layered materials Tim Kidd, Rui He, Andrew Stollenwerk, Aaron Oshea, Ben Beck, Kyle Spurgeon, Genda Gu We report a new method for the creating of nanostructures using a scanning electron microscope. Residual organic molecules on the surface of layered materials can be excited by electron beam radiation to burrow into the open spaces between the layers of these materials, and then are broken down further to form photoluminescent carbon nanoclusters. Surface characterization by atomic force microscopy shows the surface is nearly undamaged at the molecular level by this process, and a lack of nanostructure formation in non-layered materials confirms that the structures are created by sub-surface incorporation. The presence of carbon nanoclusters was determined by Raman Spectroscopy and photoluminescence in the visible light range. The nanostructures are react strongly to visible light, making them readily apparent using an optical microscope even for features measuring only a few nanometers tall. This technique can be used on apparently any layered material, with successful results on dichalcogenides, topological insulators, graphite, and high temperature copper oxide superconductors. This technique can create patterned nanostructures with vertical resolution at the nanometer scale and lateral resolution of tens of nanometers depending on beam spot size. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q51.00011: Topological Zero-mode States in Graphene/hBN Superlattices Justin Song, Leonid Levitov Lateral heterostructures comprised of layered Van der Waal's materials, such as graphene on hexagonal boron nitride (hBN) are a new playground in which electronic Hamiltonians can be engineered, and new electronic states can be found. G/hBN heterostructures provide an instructive example wherein the lateral superlattice periodicity can match typical electron wavelengths. As a result, G-hBN coupling, even if weak, strongly affects electronic states. We will describe how novel electronic states can arise in G/hBN superlattices. Importantly, G/hBN exhibits a potential landscape which has a spatially alternating mass (sub-lattice asymmetric) allowing topologically protected 1-D states to form along its nodal lines. These analogs of Topological Insulator surface states have clear manifestations which allow them to be observed. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q51.00012: Novel Electronic Properties of Si-Doped Boron Nitride Monolayers Sanjeev Gupta, Haiying He, Douglas Banyai, Mingsu Si, Ravindra Pandey, Shashi Karna In this work, we address the most critical fundamental question on the effect of doping, especially by Si, on the stability, electronic, magnetic and electron transport properties of the two-dimensional BN monolayers using density functional theory. Si substitutions at B, N and a divacancy site created by the removal of a B-N pair are investigated. The non-equal valence state of the Si dopant leads to a non-zero magnetic moment for the single site substitution. Si-induced gap states in BN monolayer energy spectrum are also observed, which demonstrate profound impact on the electron transport properties of the BN monolayer. Unique features in the device characteristics of Si-doped BN monolayers are predicted including a significant enhancement of current at the Si site, diode-like asymmetric current-voltage response and negative differential resistance. The calculated STM images clearly discern the site-dependence of Si dopants in the monolayer. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q51.00013: Flakes of artificial graphene in magnetic fields Esa Rasanen, Michael Aichinger, Stefan Janecek Artificial graphene [1] (AG) is a man-made nanomaterial that can be constructed by arranging molecules on a metal surface [2] or by fabricating a quantum-dot lattice in a semiconductor heterostructure [3]. In both cases, AG resembles graphene in many ways [1-4], but it also has additional appealing features such as tunability with respect to the lattice constant, system size and geometry, and edge configuration. Here we solve numerically the electronic states of various hexagonal AG flakes similar to those in Ref. [2]. In particular, we demonstrate the formation of the Dirac point as a function of the lattice size and its response to an external, perpendicular magnetic field. Secondly, we examine the complex behavior of the energy levels as functions of both the system size and magnetic field. Eventually, we find the formation of ``Hofstadter's butterfly''-type patterns in the energy spectrum.\\[4pt] [1] For a recent review, see M. Polini et al., Nature Nanotech. 8, 625 (2013).\\[0pt] [2] K. K. Gomes et al., Nature 483, 306 (2012).\\[0pt] [3] M. Gibertini et al., Phys. Rev. B 79, 241406(R) (2009).\\[0pt] [4] E. Rasanen et al., Phys. Rev. Lett. 108, 246803 (2012). [Preview Abstract] |
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