Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L40: Substrate Effects on Monolayers |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Enrique Cobas, Naval Research Lab Room: LACC 501C |
Wednesday, March 7, 2018 11:15AM - 11:27AM |
L40.00001: Zeeman Splitting of MoS2 Monolayers on Ferromagnetic EuS Substrates Tenzin Norden, Peiyao Zhang, Chuan Zhao, Arman Najafi, Changliang Zhu, Guoxing Miao, George Kioseoglou, Renat Sabirianov, Hao Zeng, Athos Petrou We have compared the Zeeman splitting ΔE of the A exciton in MoS2 monolayers deposited on SiO2/Si with those from MoS deposited on ferromagnetic EuS substrates. The ΔE is defined as the energy difference between the σ- and the σ+ exciton components with the magnetic field perpendicular to the sample plane. The Zeeman splitting for the SiO2 substrate samples is linear in magnetic field with a slope of 0.2 meV/T. The samples on EuS substrates behave in a different way: For |B|< 1 T the slope of the ΔE vs B plotbecomes ~ -2 meV/T. Here 1 T is the field at which the EuS magnetization saturates. For |B|>1 T the slope changes sign and becomes ~ 0.8 meV/T. In the related WSe2 /EuS system, the slope for |B|>1T returns to the 0.2 meV/T1, i.e. the exchange enhanced valley splitting saturates as the magnetization of EuS saturates. The enhanced valley splitting with opposite signs at different field ranges for MoS2 on EuS suggests competing mechanisms for valley splitting. |
Wednesday, March 7, 2018 11:27AM - 11:39AM |
L40.00002: Interactions between TMDCs and ferromagnetic EuS Chuan Zhao, Peiyao Zhang, Tenzin Norden, Xiao Wang, Andy Clark, Baiyi Kong, Yongseong Choi, George Kioseoglou, Guoxing Miao, Renat Sabirianov, Xuemei Cheng, Athos Petrou, Hao Zeng Monolayer transition metal dichalcogenides are of great interest in valleytronics application. The broken inversion symmetry leads to two degenerate but inequivalent valleys K and K’, which provide unique opportunities for valley control through helicity of light. Lifting the valley degeneracy would enable the control of valley degree of freedom by an electric field. Recently we and others demonstrated that, by exploiting the exchange field from a ferromagnetic substrate, the valley splitting of TMDs such as WSe2 can be strongly enhanced. In this work, we explore the different effects of exchange field on two types of TMDs: WSe2 vs WS2. For both materials, we observed enhanced valley splitting; however, the signs of the valley splitting are opposite for WS2 and WSe2. The slope of the valley splitting for the A exciton of WSe2 is about 3meV/T, while WS2 show a value of -5meV/T. We further used X-ray magnetic circular dichroism to probe the induced magnetic moment in TMDs. Finally we report our effort to tune the sign and magnitude of valley splitting by anion alloying of S and Se. The interactions between TMDCs and ferromagnetic substrates provide attractive tuning knobs for valley control. |
Wednesday, March 7, 2018 11:39AM - 11:51AM |
L40.00003: Bandgap renormalization and band offset in 2D materials caused by dielectric screening Lutz Waldecker, Archana Raja, Roland Koch, Aaron Bostwick, Chris Jozwiak, Eli Rotenberg, Tony Heinz Modifying the external dielectric environment of a 2D semiconductor monolayers offers a new approach to bandgap engineering [1]. While Rydberg spectroscopy using optical absorption can provide access to the quasiparticle bandgap, and its variation with the surrounding dielectric environment, the distribution of the energy offset of valence and conduction bands across the lateral junction induced in a 2D semiconductor by a change in the external dielectric has not yet been examined experimentally. Here, we present measurements of the influence of dielectric screening on the band structure of WS2 monolayers by employing angle-resolved photoemission spectroscopy with micrometer resolution (μ-ARPES). We visualize the valence bands of the semiconductor WS2 on different substrates and determine their energy with respect to atomic core levels. In addition, we determine the bandgap of the samples by measuring ground and excited exciton transitions with optical spectroscopy, which are extrapolated to yield the quasiparticle bandgap and, thus, the energy of the conduction band. We discuss the influence of dielectric screening on the valence bands in different parts of the Brillouin zone. |
Wednesday, March 7, 2018 11:51AM - 12:03PM |
L40.00004: Electrical Transport in hybrid graphene/CrI3 junctions Dmitry Shcherbakov, Petr Stepanov, Kenji Watanabe, Takashi Taniguchi, Jin Hu, Yanglin Zhu, Zhiqiang Mao, Chun Ning Lau Two-dimensional materials have gained world-wide interest in recent decade, and many phenomena that were found in bulk, such as superconductivity and magnetism, have been observed in single atomic layers. Here we experimentally study heterostructures consisting of chromium iodide, which has recently shown to display monolayer ferromagnetism, and graphene. We have fabricated high-mobility microscopic devices for low-temperature charge transport measurements. Latest data as a function of charge density, magnetic field and temperature will be discussed. |
Wednesday, March 7, 2018 12:03PM - 12:15PM |
L40.00005: Density Functional Theory Calculations of Graphene-Substrate Systems Stuart Shepard, Manuel Smeu Since graphene is supported by a material in most applications, understanding the interactions between graphene and different substrates is crucial for determining their effects on graphene’s electronic properties. For accurate modeling of such systems, density functional theory (DFT) using functionals that include dispersion, or van der Waals (vdW), interactions is necessary to gain insights into these effects. In this work, we use the SCAN functional as well as the PBE functional with various vdW corrections to study graphene on Ni and Au, among other substrates. Also, to study the sensitivity of the Fermi level, an external electric field will be applied to the system. By determining the graphene adsorption type, distance, and energy for known graphene-substrate geometries, we evaluate the different substrate effects. Also considered will be the adsorption of molecules on various graphene-substrate combinations. These results are compared to experiments and theoretical calculations with different vdW functionals to assess the validity of using the SCAN functional in graphene-substrate systems. |
Wednesday, March 7, 2018 12:15PM - 12:27PM |
L40.00006: Epitaxial Graphene Buffer Layer Electronic Devices Jean-Philippe Turmaud, Dogukan Deniz, James Gigliotti, Yiran Hu, Yue Hu, Vladimir Prudkovskiy, Claire Berger, Walt de Heer The first graphene layer grown by thermal decomposition of the (0001) surface of hexagonal silicon |
Wednesday, March 7, 2018 12:27PM - 12:39PM |
L40.00007: Probing the Dielectric Response of the Interfacial Buffer Layer in Epitaxial Graphene via Optical Spectroscopy Albert Rigosi, Heather Hill, Sugata Chowdhury, Yanfei Yang, Nhan Nguyen, Francesca Tavazza, Randolph Elmquist, David Newell, Angela Hight Walker Monolayer epitaxial graphene (EG) is a suitable candidate for a variety of electronic applications. One advantage of EG growth on the Si face of SiC is that it develops as a single crystal, as does the layer below, referred to as the interfacial buffer layer (IBL), whose properties include an electronic band gap. Though various electrical and non-optical probing experiments have been conducted on this buffer layer, studies pertaining to its optical properties have not yet been rigorously explored. In this work, we combine measurements from Mueller matrix ellipsometry, differential reflectance contrast, atomic force microscopy, and Raman spectroscopy, as well as calculations from Kramers-Kronig analyses and density functional theory (DFT), to determine the dielectric function of the IBL within the energy range of 1 eV to 8.5 eV. |
Wednesday, March 7, 2018 12:39PM - 12:51PM |
L40.00008: Effect of the Metal Substrate on Interlayer Interactions in Bilayer Graphene Matthew Christian, Erin Johnson Bilayer graphene (BLG) has been shown to have advantageous electronic |
Wednesday, March 7, 2018 12:51PM - 1:03PM |
L40.00009: Scanning Tunneling Microscopy Study of Graphene Nanoribbons on Semiconducting Substrates Minghao Cheng, Drew Elderberg, Samson Jenekhe, Selvam Subramaniyan, Abhay Narayan Recently, several solution-based synthetic techniques have been developed to synthesize Graphene Nanoribbons (GNR), one-dimensional analogs to graphene and close relatives to carbon nanotubes. These large molecules have interesting spectroscopic properties and have been considered as building blocks of nanoelectronic circuits. To this end, it is interesting to isolate single GNRs on surfaces and examine their electronic properties by spectroscopic or transport techniques. A chief barrier to doing this is finding ways to deposit them cleanly on the appropriate substrate so they can be well isolated from their environment. In this work, use a novel dry transfer technique to deposit various GNR molecules on to metallic HOPG and semiconducting WSe2 substrates. We study their atomic structure with scanning tunneling microscopy (STM) and electronic structure with scanning tunneling spectroscopy (STS). Apart from the spectroscopic properties of the GNRs themselves, we will describe interactions between GNR molecules and the substrate they are placed on, as well as molecule-molecule interactions that can promote chain formation on certain substrates. |
Wednesday, March 7, 2018 1:03PM - 1:15PM |
L40.00010: An Electronic Structure Perspective of Sliding of Graphene on SiC Ceren Sayin, Oguz Gulseren We investigate sliding of the top-most layer of few-layer graphene (FLG) systems with layer thicknesses from n=1 to n=4 on 4H-SiC{0001} surfaces using density functional theory. We examine the frictional properties through energy profiles and changes in barrier heights with respect to sliding directions, number of layers and surface termination both for sliding at a fixed height and sliding under external normal force. The relation between electronic structures and friction mechanisms; as well as the dependence of the electronic properties of FLG on SiC on stacking order, substrate charge transfer and applied load, are discussed via a thorough examination of the changes in the band structures and charge densities during sliding. |
Wednesday, March 7, 2018 1:15PM - 1:27PM |
L40.00011: Temperature dependence of carrier density in monolayer graphene on SiC Chi-Te Liang, Chieh-Wen Liu, Chiashain Chuang, Yanfei Yang, Randolph Elmquist, Yi-Ju Ho, Hsin-Yen Lee The Hall resistance, which shows a linear magnetic field dependence at low fields, has been widely used to calculate the carrier density n of a two-dimensional system. Nevertheless, it has been pointed out that electron-electron (e-e) interactions enhanced by disorder can result in a logarithmic temperature (lnT) dependence of the Hall slope RH=1/(ne) without changing the carrier density. Therefore the T dependence of the Hall slope, which may lead to misleading n(T), should be treated with great care. In contrast, Shubnikov-de Haas (SdH) oscillations, which can also be used to calculate n, are not affected by e-e interactions. This is the famous Kohn’s theorem. In this work, we present both Hall and SdH measurements on monolayer graphene grown on SiC. In the low T regime, the minima of the observed SdH oscillations in B do not change with increasing T. Such results clearly demonstrate that n is T-independent over this measurements range. In contrast, the observed a lnT dependence of the Hall slope, which can be ascribed to e-e interactions enhanced by disorder, leads to a wrong interpretation of n(T). |
Wednesday, March 7, 2018 1:27PM - 1:39PM |
L40.00012: Extremely flat band in bilayer graphene on silicon carbide Dmitry Marchenko, Daniil Evtushinsky, Evangelos Golias, Andrei Varykhalov, Thomas Seyller, Oliver Rader Frequent reports of superconductivity in graphite at elevated temperatures even above 300 K demand an explanation. It has been suggested that this is an effect of flat band formation at graphite surfaces and/or interfaces. In the present work we discover by angle-resolved photoemission that bilayer graphene on SiC shows an extremely flat band. We demonstrate that the band extends two-dimensionally around the K point and present a general model for flat band formation in bilayer graphene and related systems. |
Wednesday, March 7, 2018 1:39PM - 1:51PM |
L40.00013: Role of surface optical phonons in the resistivity of graphene YOUNGGYU YOU, Jeonghwan Ahn, Bae Ho Park, Yongkyung Kwon, Jhang Ho We have experimentally investigated the effect of surface optical phonons on the resistivity of graphene in the presence of three different substrates; HfO2, SiO2, and h-BN. Surface optical phonon energies of HfO2, SiO2 and h-BN are 22, 59, and 101 meV, respectively, and we find the temperature (T) dependence of resistivity is significantly depending on the substrate materials. The resistivity shows a linear T-dependence at low T due to acoustic phonon scattering, and becomes superlinear as surface optical phonons play an important role at higher temperatures. The transition occurs below 100 K for the graphene on HfO2 substrates whereas the linear T-dependence deviates only after 200 K for the graphene on h-BN. Our experimental data are well fitted with the model including the remote interfacial phonon scattering by the surface optical phonons of the substrates. |
Wednesday, March 7, 2018 1:51PM - 2:03PM |
L40.00014: Observation of spontaneous high p-type doping on graphene-Talc heterostructure Edrian Mania, Ananias Alencar, Alisson Cadore, Bruno Carvalho, Kenji Watanabe, Takashi Taniguchi, Bernardo Neves, Helio Chacham, Leonardo Campos Graphene, an atomically thick layer of carbon atoms arranged in a hexagonal lattice, has attracted a lot of interest in basic and in applied physics. With the advent of h-BN crystals, it is possible to improve graphene device quality and uncover many interesting quantum effects of Dirac fermions. On the other hand, heterostructures prepared using other 2D materials do not lead to considerable improvements of graphene devices quality, but they allow the development of other non-linear electronics elements. Here we report that an atomically flat graphene-talc heterostructure is spontaneously p-type doped up to n ~ 2.2 x 1013 cm-2 and show excellent charge mobility (~ 25,000 cm2V−1s−1). Raman investigation confirmed a preferential charge accumulation on graphene-talc rather than graphene on SiO2. In addition to potentially improving solar cell efficiency, graphene doping via van der Waals stacking is also a promising route towards controlling the band gap opening in bilayer graphene, promoting a steady n or p type doping in graphene and, eventually, providing a new path to access superconducting states in graphene, predicted to exist only at very high doping. |
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