Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T1: Focus Session: Beyond Graphene - Silicene and Germanene |
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Sponsoring Units: DMP Chair: Roland Kawakami, Ohio State University Room: 001A |
Thursday, March 5, 2015 11:15AM - 11:27AM |
T1.00001: Electron Energy Loss Spectroscopy Studies of Silicene and Graphene Oleksiy Roslyak, Antonious Balassis, Godfrey Gumbs Silicene is the silicon counterpart of graphene. However, the spin-orbit interaction in silicene opens up a substantial band gap. Consequently, there is an unambiguous low-frequency plasmon excitation originating from intra- and inter-band transitions. Both the plasmon intensity and lifetime depend on the width of the excitation gap separating electron-hole pairs, We present a formalism based on self-consistent field theory to investigates the rate of loss of energy of a beam of charged particles moving parallel and perpendicular to a silicene layer. The energy loss spectrum is presented as a function of the speed of the charged particles. We further study and compare the part of the energy absorbed by collective plasma excitation as well as by single particle excitations. We report clear spectral signatures of semi-metal to either band insulator or topological insulator. Plasmonic features of silicone flakes are also studied and compared to those of pristine graphene. [Preview Abstract] |
Thursday, March 5, 2015 11:27AM - 11:39AM |
T1.00002: Large Area Transfer and Optoelectronic Properties of Multilayer Epitaxial Germanane Walid Amamou, Patrick Odenthal, Beth Bushong, Dante O'hara, Yunqiu Luo, Jeremiah Van Baren, Igor Pinchuk, Yi Wu, Marc Bockrath, Harry Tom, Joshua Goldberger, Roland Kawakami Germanane (GeH), the germanium-based analog of graphane (CH), is of particular interest due to its direct band gap and surface covalent functionalization. Furthermore, its large spin orbit coupling makes it possible to explore novel physical phenomena such as quantum spin hall effect at room temperature. Currently, large area GeH films are synthesized on Ge(111) wafers using substrate reaction or molecular beam epitaxy combined with chemical processing. This results in a high quality GeH film that is left on top of the germanium substrate. In order to perform the electrical characterization of GeH, it is required to transfer the film to an insulating substrate. Here, we demonstrate a highly efficient, nondestructive electrochemical route for the transfer of molecular beam epitaxy (MBE) GeH film from Ge(111) surfaces. This technique enables us to characterize the optoelectronic properties of epitaxial GeH after transfer such as I-V characteristics and photoconductivity wavelength dependence. [Preview Abstract] |
Thursday, March 5, 2015 11:39AM - 11:51AM |
T1.00003: Band Parameters of Two-Dimensional Materials: Phosphorene and Silicene Lok Lew Yan Voon, Alejandro Lopez Bezanilla, Jianwei Wang, Yong Zhang, Morten Willatzen The method of invariant is used to derive effective Hamiltonians in the presence of strain and external fields for phosphorene and silicene as examples of two-dimensional materials with and without a band gap. The band structure parameters have been obtained by fitting to density-functional theory calculations. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:03PM |
T1.00004: Temperature-dependent phase transitions in epitaxial silicene on ZrB$_2$(0001) Antoine Fleurence, Yukiko Yamada-Takamura Silicene differs from graphene, its carbon counterpart, by a mixed sp$^2$/sp$^3$ hybridization of the Si atoms that gives it particularly interesting mechanical and electronic properties. Silicene can form by the spontaneous and self-terminating segregation of Si atoms on the (0001) surface of zirconium diboride (ZrB$_2$) thin films grown on Si(111) $[1]$. This stable form of silicene is particularly suitable for the investigation of the temperature dependence of its mechanical properties. Whereas, the amount of Si atoms does not vary, scanning tunneling microscopy and low-energy electron diffraction clearly indicate that two reversible phase transitions occur when the temperature is raised. At room-temperature, the silicene sheet is textured into one-dimensional arrays of interconnected 2.7 nm-wide ribbon-shaped stress domains $[1]$. Around 870 K, this ordered surface evolves into a surface made of wider domains with no ordering of the domain boundaries. At 930 K, the silicene sheet loses its structure and turns into a two-dimensional gas of Si atoms. While cooling down, silicene crystallizes reversibly. The origin of the phase transitions of epitaxial silicene will be discussed. $[1]$ A. Fleurence et al. Phys. Rev. Lett. 108 245501 (2012). [Preview Abstract] |
Thursday, March 5, 2015 12:03PM - 12:15PM |
T1.00005: Electron confinement at the Si-MoS2 heterosheet junction Alessandro Molle, Daniele Chiappe, Davide Rotta, Alessio Lamperti, Carlo Grazianetti, Eugenio Cinquanta, Marco Fanciulli Two dimensional (2D) elementary materials such as silicene, germanene or phosphorene are emerging alternatives to graphene which adds to the consolidated class of layered metal dichalchogenides. In particular, 2D silicon nanosheets would benefit from the potential integration with the quite ubiquitous Si technology thus opening new scaling perspectives of the conventional electronic devices. In this framework we report here on the 2D epitaxy of a Si monolayer onto a MoS$_{2}$ template with a locally hexagonal registry [1]. The experimental data are consistent with an ab initio calculated highly stretched silicene lattice [2]. High resolution photoemission spectroscopy investigations evidence a nearly metallic character of the Si nanosheet and a significant band bending on the MoS$_{2}$ side which claims for a Si-induced electron accumulation. Integration into a bottom gated field effect transistor results in the effective transport at the Si/MoS$_{2}$ heterosheet interface which is rationalized in terms of an electronic confinement. \\[4pt] [1] Chiappe et al, Adv. Mater. 26, 2096 (2014);\\[0pt] [2] Scalise et al, 2D Materials 1, 011010 (2014). [Preview Abstract] |
Thursday, March 5, 2015 12:15PM - 12:27PM |
T1.00006: Poisson's Ratio of Layered Two-dimensional Crystals Sungjong Woo, Hee Chul Park, Young-Woo Son We have investigated the elastic properties of multilayered graphene as well as h-BN and MoS$_2$ using a first-principles approach with up-to-date nonlocal exchange-correlation energy functional. Our analysis shows that the Poisson's ratios of multilayered graphene, h-BN and MoS$_2$ along out-of-plane direction are negative, near zero and positive, respectively, spanning all possibilities for sign of the ratios. While the in-plane Poisson's ratios are positive regardless of their disparate electronic and structural properties, the characteristic interlayer interactions as well as atomic stacking structures are shown to determine the sign of their out-of-plane ratios, highlighting their intertwined nature between elastic and electronic properties. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T1.00007: Observation of Strong Intermode Coupling in a MoS2 Mechanical Resonator Chang-Hua Liu, In Soo Kim, Spencer Park, KunHo Yoon, Sarah Howell, Lincoln Lauhon Achieving the strong coupling regime in downscaled oscillators facilitates quantum control of macroscopic mechanics. Many approaches, including the use of dynamical backaction between mechanical resonators and photonic or superconducting cavities, require sophisticated device structures. We will describe strong mode coupling in a simple system: an ultrathin MoS2 mechanical resonator. Thermal fluctuations are exploited to study different vibrational modes of the resonator and observe avoided crossings in the vibrational spectra, indicating the entanglement of mechanical motion. Furthermore, when parametrically pumping the resonator with light, the dynamic optically induced strain leads to Stokes and anti-Stokes sidebands as well as normal-mode splitting. These signatures provide strong evidence that phonon populations can be redistributed between different vibrational modes, and also confirm that an MoS2 resonator can be operated into the strong coupling regime. These observations further suggest that 2D materials could offer a platform for developing full quantum control of nanomechanics. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T1.00008: Measuring nanoscale friction between 2D materials and SiO$_2$: hBN, MoS$_2$ and Phosphorene Jason Christopher, Steven Koenig, Angelo Ziletti, Bo Wen, Zheng Han, Alex Kitt, Xuanye Wang, Logan Kageorge, Cory Dean, Barbaros Ozyilmaz, Anna Swan, Bennett Goldberg Unlike their 3D counterparts, 2D materials can be tuned with strain, leading to ``strain engineering'' of electrical, optical and thermal properties. Control of the precise location, magnitude and direction of a strain field depends critically on characterizing and understanding any motion or sliding between the 2D material and its anchor points. With this goal in mind, we determine the friction between three different atomically thin materials and a SiO$_2$ substrate. Our experimental setup consists of cylindrical, micro-chambers we etch into a SiO$_2$ substrate and seal with a 2D material membrane. We pressurize the outside of the micro-chamber, causing the 2D material to deform and slide on the substrate. The resulting strain distribution and amount of sliding is mapped using high spatial resolution Raman spectroscopy from which we determine the friction, Gr\"uneisen and shear deformation potentials. MoS$_2$, hBN, and Phosphorene have been chosen because they are good candidates for strain engineering applications. Additionaly, in the case of MoS$_2$ and Phosphorene we measure the photoluminescence spectrum from which we determine the strain dependence of the band gap. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T1.00009: Dirac Electrons in Silicene on Ag(111): Do they exist? Chi-Ruei Pan, Mei-Yin Chou There have been quite a few experimental attempts to grow silicene on Ag(111) in the past two years. However, there are still controversies about whether a silicene layer with massless Dirac fermions actually exists on Ag(111). Chen \textit{et al.} [1] measured the interference patterns in the differential conductance map by scanning tunneling spectroscopy and found a linear dispersion relation as the evidence for the existence of massless Dirac fermions. On the other hand, Lin \textit{et al.} [2] found no Landau level sequences appearing in the tunneling spectra under a magnetic field, concluding that the Si-Ag interaction is strong enough to break the symmetry of silicene. In order to resolve these conflicting experimental findings, we have studied various Si/Ag configurations on the surface and their related electronic structures using first-principles density-functional calculations. Comparisons with experimental results will be discussed. [1] Chen et al. Phys. Rev. Lett. 109, 056804 (2012). [2] Lin et al., Phys. Rev. Lett. 110, 076801 (2013). [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T1.00010: Structural and electronic properties of freestanding bilayer silicene Yuki Sakai, Atsushi Oshiyama We systematically study the structural and electronic properties of freestanding bilayer silicene within the framework of the density functional theory. Starting from 11 different initial atomic configurations, we find 19 total-energy minimized structures with respect to the in-plane lattice parameter. Six of these 19 structures are found to be thermodynamically stable since they do not possess negative phonon frequency modes in their phonon spectra obtained with the density functional perturbation theory. The interlayer bond lengths of these structures are shorter than 2.5 {\AA}, indicating their covalent-like interlayer interactions. Four of those six structures are described by the $1\times1$ unit cell (4 silicon atoms in the unit cell) while the other two structures are described by a $2\times2$ supercell. The structures with a $2\times2$ supercell have relatively lower total energies than the $1\times1$ structures. These low-energy structures have local atomic configurations whose bond angles and bond lengths are similar to those of the ideal diamond structure by making protrusion of silicon atoms. The six stable structures are found to exhibit semimetallic or semiconducting electronic properties depending on their stacking geometries. [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T1.00011: Heterostructures by inserting Oxygen Monolayers in Si: 2D Nanolattice Growth, Electronic properties and MOSFET Device Characteristics Suseendran Jayachandran, Koen Martens, Augustin Lu, Kengo Nishio, Geoffrey Pourtois, Annelies Delabie, Matty Caymax, Marc Heyns We discuss how heterostructures can be created in silicon by inserting oxygen monolayers, as well as what Density Functional Theory simulations predict in terms of electronic properties of these 2D nanolattices. We also discuss the experimental electrical characteristics of Metal-Oxide-Semiconductor Field Effect Transistors with 2D nanolattice channels. By using short (up to 500ms) low temperature O$_{3}$ reaction on H-terminated Si, the deposited O content can be controlled near the monolayer level, as demonstrated by SIMS measurements. Epitaxial deposition of Si on an O layer and 2D nanolattices with up to 5 periods have been achieved by CVD using SiH$_{4}$ at 500C. We discuss the structural and electronic properties calculated with density functional theory and give an overview of the most promising Si superlattices in terms of anticipated mobility enhancement. We report on the electrical device characteristics of 2D nanolattice Schottky diodes, MOS capacitors and MOSFETs. We discuss the impact of defectivity on electrical characteristics and the impact of the 2D nanolattices on MOSFET carrier mobility. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T1.00012: Intrinsic magnetism, band gap opening and optical absorption in bilayer silicene Xinquan Wang, Zhigang Wu It has been long sought to create magnetism out of simple non-magnetic materials like silicon. Here we show that intrinsic magnetism exists in bilayer silicene with no need to cut, etch, or dope. Unlike bilayer graphene, strong covalent interlayer bonds formed in bilayer silicene breaks the original $\pi$-bonding network of each layer, leaving the un-bonded electrons unpaired and localized to carry magnetic moments. These magnetic moments then couple ferromagnetically within each layer while antiferromagnetically across layers, giving rise to an infinite magnetic sheet with structural integrity and magnetic homogeneity. Our \textit{ab initio} many-body calculations using the GW approach reveals that the unique magnetic ordering results in a fundamental band gap of 0.55 eV. Furthermore, we computed absorption spectrum by solving the Bethe-Salpeter equation, and our results suggest very strong absorption near the absorption edge. The integration of intrinsic magnetism and spontaneous band gap opening makes bilayer silicene attractive to future nanoelectronics as well as spin-based computation and data storage. This material could also be used as excellent light absorber, and its small band gap and one-dimensional confinement might be employed for efficient multi-exciton generation. [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T1.00013: Structural and electronic properties of 2D Si layers formed by deposition of Si on silicene / ZrB$_2$ Cyrus F. Hirjibehedin, Tobias G. Gill, Ben Warner, Henning Prueser, Kohei Aoyagi, Rainer Friedlein, Antoine Fleurence, Jerzy Sadowski, Yukiko Yamada-Takamura Silicene has been predicted to share many interesting properties with graphene. These are significantly modified by interactions with the metallic substrates in existing epitaxial silicene systems [1]. For many 2D layered materials, like graphene and MoS$_{2}$, these properties can change dramatically when going from the mono- to multi-layer regime. Recent experiments studying multiple Si layers on Ag(111) have shown that, remarkably, the multilayers are more metallic than the monolayer [2]. Here we report studies of 2D layers formed by depositing Si on silicene/ZrB$_{2}$. Using LEED, STM, and ARPES, we find that the deposition of very small amounts of Si cause structural changes to the silicene monolayer without strongly modifying its electronic properties. Additional deposition of Si, however, results in the formation of a second Si layer that has substantially different structural domains and is more metallic. This trend continues for 3 ML up to the deposition of 10 ML. These result illustrate the rich array of properties that can be manifested in novel 2D Si nanostructures and highlight the dramatic variation in Si phases that can be seen on different substrates. [1] A. Fleurence et al., PRL 108, 245501 (2012) [2] P. Vogt et al., APL 104, 021602 (2014) [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T1.00014: Silicene on Silver: fundamental physical properties and integration in Field-Effect Transistors Eugenio Cinquanta, Li Tao, Guido Fratesi, Carlo Grazianetti, Marco Fanciulli, Giovanni Onida, Deji Akinwande, Alessandro Molle To date, Silicene encountered different bottlenecks as reliable option in the framework of two-dimensional materials beyond graphene. Its physical properties are not completely unveiled and this, combined with its environmental instability, limits its possible integration into devices. Here we show a comprehensive characterization of Silicene by combining ab-initio calculations with optical spectroscopies. We elucidate the role of Ag in determining the electronic band structure and the optical response of differently oriented Silicene superstructures. We also show how these fundamental properties reflect in devices by presenting the experimental evidence of the ambipolar electrical transport in Silicene based field effect transistor. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T1.00015: New Phases of Germanene V. Ongun Ozcelik, Engin Durgun, Salim Ciraci Germanene, a graphene-like single-layer structure of Ge, has been shown to be stable and recently grown on Pt and Au substrates. We show that a Ge adatom adsorbed on germanene pushes down the host Ge atom underneath and forms a dumbbell structure. This exothermic process occurs spontaneously. The attractive dumbbell-dumbbell interaction favors high coverage of dumbbells which contain crucial information about multilayer germanene and silicene. This Letter heralds stable new phases of germanene, which are constructed from periodically repeating coverage of dumbbell structures and display diversity of electronic and magnetic properties. Formation of multilayer silicene from the dumbbell units is also presented.\\[4pt] [1] V. O. Ozcelik, E. Durgun, and S. Ciraci, J. Phys. Chem. Lett. 5 (15), 2694-2699 (2014).\\[4pt] [2] S. Cahangirov, V. O. Ozcelik, A. Rubio, and S. Ciraci, Phys. Rev. B 90, 085426 (2014). [Preview Abstract] |
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