Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W51: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization VII |
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Sponsoring Units: DMP Chair: David Cobden, University of Washington Room: Mile High Ballroom 1E |
Thursday, March 6, 2014 2:30PM - 3:06PM |
W51.00001: Spin-valley physics and field effect on transition metal dichalcogenides Invited Speaker: Yoshihiro Iwasa Transition metal dichalcogenide (TMD) is attracting growing interest as two dimensional (2D) crystals beyond graphene. Of particular importance is an optoelectronic functionality based on valleytronics, which is strongly coupled with spintronics through the spin-orbit interactions, making TMD a quite unique system. Field effect transistor (FET) plays crucial roles, not only because of its ambipolar operations [1] and electric field induced superconductivity [2], but also because of its inherent broken inversion symmetry causing electric field induced Zeeman splitting [3]. In this presentation, we review our latest achievements on spin-valley physics and FET functionalities in TMD materials. We demonstrated for the first time the spin/valley polarization using spin- and angle resolved-photoemission spectroscopy. This became possible by choosing noncentrosymmetric bulk crystals. Photoluminescence circular dichroism proved that the noncentrosymmetric stacking enhances the valley polarization in bilayer, indicating that the noncentrosymmetric MoS$_{2}$ crystals are useful materials for the future valleytronics. As for the field effect, we performed systematic investigations of ambipolar FETs in MoX$_{2}$ (X $=$ S, Se, and Te), and found new field induced superconductivity in MoSe$_{2}$. 2D nature of electric field induced superconductivity was unambiguously demonstrated by the anisotropic $H_{\mathrm{c2}}$. We also demonstrated electroluminescence using the field effect geometry. \\[4pt] [1] Y. J. Zhang et al., \textit{Nano Lett.} 12, 1136 (2012), \textit{ibid.} 13, 3023 (2013).\\[0pt] [2] J. T. Ye et al., \textit{Science} 338, 1193 (2013).\\[0pt] [3] H. T. Yuan et al., \textit{Nat. Phys.} 9, 563 (2013). [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W51.00002: Electric control of spin in monolayer WSe2 field effect transistors Lei Zhang, Kui Gong, Dongping Liu, Lei Liu, Yu Zhu, Yonghong Zhao, Hong Guo We report a first principles theoretical investigation of quantum transport in monolayer WSe2 field effect transistor (FET). Due to a strong spin-orbit interaction (SOI) and the atomic structure of the two-dimensional (2D) lattice, monolayer WSe2 has an interesting electronic structure that exhibits Zeeman-like up-down spin texture near the K and K' points of the Brillouin zone. In a FET, the gate electric field induces an extra, externally tunable SOI that re-orients the spins into a Rashba-like texture thereby realizing electric control of the spin. Quantum transport is modulated by the spin texture, namely by if the spin orientation of the carrier after the gated channel region, matches or miss-matches that of the FET drain electrode. The carrier current in the FET is labelled both the spin index and the valley index, realizing spintronics and valleytronics in the same device. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W51.00003: Carrier distribution and negative compressibility in graphene-MoS$_{2}$ heterostructures Stefano Larentis, John R. Tolsma, Babak Fallahazad, David C. Dillen, Kyoung Kim, Allan H. MacDonald, Emanuel Tutuc We report the investigation of electrical properties and magnetotransport in monolayer graphene - multilayered MoS$_{2}$ heterostructures. The devices are fabricated by dry transfer of graphene layers onto exfoliated MoS$_{2}$. The conductivity dependence on the back-gate bias shows the ambipolar behavior characteristic of graphene, along with a marked saturation of the conductivity on the electron branch. Magnetotransport measurements reveal that the conductivity saturation is the result of electrons populating the lower mobility MoS$_{2}$ layer at a positive, threshold back-gate bias. Experimental data from heterostructures with different thicknesses allow the extraction of the band offset between the MoS$_{2}$ conduction band and the graphene charge neutrality point. Surprisingly, the carrier density in graphene reveals a marked decrease as a function of gate bias near the MoS$_{2}$ population threshold, an observation which implies that electrons in MoS$_{2}$ have negative compressibility at low carrier density. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W51.00004: Negative Compressibility and Charge Partitioning Between Graphene and MoS$_{2}$ Two-Dimensional Electron Gases John Tolsma, Stefano Larentis, Emanuel Tutuc, Allan MacDonald Electron-electron interactions often have opposite influences on thermodynamic properties of electrons in graphene compared to conventional two-dimensional electron gases (2DEGs), for example by lowering charge and spin-susceptibilities in the graphene case and enhancing them in the ordinary 2DEG case [1]. In ordinary 2DEGs the charge susceptibility diverges at a finite carrier density, below which the compressibility becomes negative. We theoretically explore the influence of this qualitative difference on how charge is partitioned between a MoS$_{2}$ and a graphene sheet 2DEG when they act as a compound capacitor electrode. Our theory is based on a random phase approximation for charge fluctuations in the 2DEGS and the coupling constant formulation for the ground state energy. We find that in the ideal case the MoS$_{2}$ 2DEG carrier density jumps immediately to a finite value when it is initially populated and discuss how this effect is moderated by disorder. \\[4pt] [1] Yafis Barlas, T. Pereg-Barnea, Marco Polini, Reza Asgari, and A.H. MacDonald, \textit{PRL} \textbf{98, }236601 (2007). [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W51.00005: Optoelectronics with electrically tunable PN diodes in monolayer WSe2 Hugh Churchill, Britton Baugher, Yafang Yang, Pablo Jarillo-Herrero We describe the transport and optoelectronic behavior of ambipolar monolayer WSe$_2$ devices in which two local gates are used to define a PN junction exclusively within the sheet of WSe$_2$. With these electrically tunable PN junctions, we demonstrate both PN and NP diodes with ideality factors better than 2. Under excitation with light, the diodes show photodetection responsivity of 210 mA/W and photovoltaic power generation with a peak external quantum efficiency of 0.2\%, promising numbers for a nearly transparent monolayer sheet in a lateral device geometry. Finally, we demonstrate a light-emitting diode based on monolayer WSe$_2$. These devices provide a fundamental building block for ultra-thin, flexible, and nearly transparent optoelectronic and electronic applications based on ambipolar dichalcogenide materials. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W51.00006: Ambipolar light emitting transistors on transition-metal dichalcogenides Yijin Zhang, Ryuji Suzuki, Yoshihiro Iwasa Transition-metal dichalcogenides (TMDs) are known to show ambipolar transistor operation, in which both electron and hole can transport through TMD channel materials [1]. As widely investigated in organic transistors, ambipolar transistor has additional functionality of the efficient light emitting source by simultaneously introducing electron and hole in the channel, forming a bias tunable p-n junction [2]. Recently, tunable yet stable p-n junction has been realized in MoS2 using a device structure of electric double layer transistor (EDLT), taking advantage of liquid gate dielectric [3]. We fabricated EDLT devices with tungsten diselenide (WSe2), molybdenum diselenide (MoSe2), and molybdenum disulfide (MoS2) as channel materials, and observed electroluminescence (EL) from both monolayers and multilayers. The peak energy suggests that EL occurs at K point in the momentum space even in multilayer samples, in contrast with band modulation from monolayer to multilayers [4]. Such a light emitting device will be a fundamental device in opto-valleytronics application. [1] Y. J. Zhang et al. Nano Lett. 12, 1136 (2012) [2] J. Zaumseil et al. Nat. Mater. 5, 69 (2006) [3] Y. J. Zhang et al. Nano Lett. 13, 3023 (2013) [4] A. Splendiani, et al. Nano Lett. 10, 1271 (2010) [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W51.00007: RKKY interaction in MoS$_2$ Diego Mastrogiuseppe, Nancy Sandler, Sergio Ulloa MoS$_2$ belongs to a family of layered compounds --the transition metal dichalcogenides-- that are attracting increasing attention in the solid state community due to their very rich phase diagram. In particular, the semiconducting ones in their 2D form, are of particular interest in the search for a new generation of devices in nanoelectronics and nanophotonics. The hexagonal lattice allows one to describe the low-energy physics with a massive Dirac equation around the $K$ and $K'$ points. Moreover, the presence of a large intrinsic spin-orbit interaction due to the presence of transition metal atoms, leads to a valley-dependent splitting of the states of an otherwise spin-degenerate valence spectrum. We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities in the direct band gap semiconducting single-layer MoS$_2$, focusing in the p-doped case. Going beyond a recent study [1], we include the effects of the spin-degenerate valence bands at the center of the Brillouin zone, relevant for energies close to the valence band maximum. The easy experimental tunability of the carrier concentration by electrical or chemical means, makes possible the study of the carrier-mediated spin-spin interaction at different fillings.\\[4pt] [1] PRB 87, 125401. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W51.00008: Transport Properties and Devices of Molybdenum Disulfide Fenglin Wang, Petr Stepanov, Jeanie Lau Molybdenum Disulfide (MoS2) is a very promising material especially the monolayer MoS2 with a direct bandgap; however, the low mobility is the major obstacle currently. We have combined multiple methods to improve the mobility, also investigate into the possible mechanism of the mobility bottleneck. With the help of additional gates, we are able to achieve ambipolar transport in MoS2 devices.We will present the latest experimental results. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W51.00009: Single Layer Transition Metal Dichalcogenides Transistors Yuhang Jiang, Jinhai Mao, Eva Andrei Single layer Transition Metal Dichalcogenides (TMDs), such as MoS$_{\mathrm{2}}$, WS$_{\mathrm{2}}$ and TaS$_{\mathrm{2}}$, are atomically thin two-dimensional materials with unique electronic properties different from their bulk counterparts. The lack of inversion symmetry, high mass of the components and the 2D geometry lead to strong spin-orbit coupling and to a metal insulator transition (MIT). Recent progressing ultrathin sample preparation and nanodevice fabrication has opened new opportunities to explore the transport properties of these layers for potential applications in nanoelectronics. In particular the ability to gate these samples across the MIT, carries the promise of sharp switching characteristics that defeat the thermodynamically imposed limiton the sub-threshold slope in standard field effect transistors. We will report on the electronic properties of field effect transistors fabricated with monolayer in the TMD family under conditions of extreme doping achieved by ionic liquid gating. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W51.00010: Effects of the dielectric environment on the electron transport properties of single-layer MoS$_{2}$ Simone Bertolazzi, Adrien Allain, Dominik Lembke, Andras Kis Two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, offer a wide range of electronic, optical and mechanical properties that can be advantageous for several applications in nanotechnology. Among these materials, single-layer molybdenum disulfide (MoS$_{2})$ shows great potential for scaling field-effect transistor devices, due to an optimal electrostatic control of the 2D semiconducting sheet, large energy bandgap and minimal leakage currents. However, to fully exploit the potential of this atomically thin semiconductor, additional experimental efforts need to be undertaken to boost the device performance and access the theoretical intrinsic electron mobilities. To pursue this objective, it is mandatory to reduce the density of charged impurities, both in the semiconducting sheet and in its surrounding environment, and to limit carrier scattering induced by polar optical phonons in the dielectric surface. Here we present the results of our recent experimental investigation of the electron transport properties of single-layer MoS$_{2}$ mechanically exfoliated/transferred onto different substrates, with varying surface chemistry, surface roughness and dielectric permittivity. We will show temperature-dependent four-terminal measurements of the electrical conductivity of single-layer MoS$_{2}$ in contact with various insulating materials, including 2D sheets of hexagonal boron nitride, organic polymers and metal oxides. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W51.00011: Time-Dependent Structural Phase Transitions of Two-dimensional Intercalated Layered Oxides Kristie Koski, Philip Zucker, Bryan Reed We demonstrate time-dependent phase transitions in metal-intercalated 2D layered MoO$_{3}$. Copper metal atoms are chemically intercalated into ultrathin 2D nanocrystalline MoO$_{3}$ using a novel method we developed to intercalate high densities of zero-valent atomic species. In-situ transmission electron microscopy (TEM), operating on a timescale of seconds, and Dynamic TEM, operating on nanosecond time scales show that unique, time-dependent phase transitions can be driven in these two-dimensional layered oxide nanoribbons. Very different structures arise on different time scales, indicating a competition between kinetics and thermodynamics in determining the resulting structure. Control experiments in pure MoO$_{3}$ show no such transitions, thus it appears that the copper intercalant is an essential part of the process. Measurements of the nanosecond-scale transformation are consistent with a local reordering of material within the original unit cell, while the slower transition is characterized by an incommensurate superlattice possibly associated with a charge density wave. This work opens new ground for accessing novel phases of matter in two-dimensional layered nanomaterials. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W51.00012: Characterization of atomically thin layers of 1T-TaS$_{2}$ Adina Luican-Mayer, Jeffrey R. Guest, Saw Wai Hla 1T-TaS$_{2}$ is a transition metal dichalcogenide that shows a wealth of correlated phenomena: it is metallic at higher temperatures, it has four temperature-dependent charge density wave phases with distinct structures [1]; at low temperatures it shows Mott insulator behavior and it becomes superconducting under pressure [2,3]. Due to the weak van der Waals bonding between its layers we show that it is possible, by mechanical exfoliation, to obtain atomically thin 1T-TaS$_{2}$ crystals. In this talk we address the question of how the transition from bulk to few layers affects the different phases of this material. Specifically, we discuss resistivity measurements for flakes of 1T-TaS$_{2}$ exfoliated onto the surface of Si/SiO$_{2}$ complemented by temperature-dependent Raman spectroscopy characterization. \\[4pt] [1] Thomson, R. E. et al. Phys. Rev. B 49,16899-16916 (1994).\\[0pt] [2] Fazekas, P. and Tosatti, E. Phil. Mag. B 39, 229-244 (1979).\\[0pt] [3] Sipos, B. et al. Nature Materials 7,960-965 (2008). [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W51.00013: Tunable CDW and Superconductivity Phase Transitions in 1T-TaS$_2$ Thin Films through Gate-controlled Intercalation Yijun Yu, Fangyuan Yang, Yajun Yan, YoungJai Choi, Sejoong Kim, Young-Woo Son, Sang-Wook Cheong, Xianhui Chen, Yuanbo Zhang 1T-TaS$_2$ has a rich set of complex phases as a result of competition among multiple electronic orders in this layered material. The delicate balance among the various phases makes 1T-TaS$_2$ very sensitive to external modulations. Using PEO/LiClO$_4$ solid electrolyte as a medium between a gate electrode and the sample, we have successfully intercalated Li ions into 1T-TaS$_2$ thin flakes in a continuous and reversible way. This allows us to probe the interplay between CDW phases, Mott phase, and superconducting phase as the concentration of Lithium is varied. Apart from 1T-TaS$_2$, our method of controlled intercalation is a promising new technique which could be applied to other layered materials. [Preview Abstract] |
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