Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W2: Focus Session: Beyond Graphene - Devices I |
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Sponsoring Units: DMP Chair: Xia Hong, University of Nebraska-Lincoln Room: 001B |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W2.00001: Optically Induced PN Junction Diode and Photovoltaic Response on Ambipolar MoSe2 Field-effect Transistor Nihar Pradhan, Zhengguang Lu, Daniel Rhodes, Mauricio Terrones, Dmitry Smirnov, Luis Balicas Transition metal dichalcogenides (TMDs) have emerged as an attractive material for electronic and optoelectronic devices due to their sizable band gap, flexibility and reduced dimensionality, which makes them promising candidates for applications in translucent optoelectronics components, such as solar cells and light emitting diodes. Here, we present an optically induced diode like response and concomitant photovoltaic effect in few-atomic layers molybdenum diselenide (MoSe$_{2})$ field-effect transistors. Compared to recently reported PN junctions based on TMDs, ambipolar MoSe$_{2}$ shows nearly ideal diode rectification under illumination, with a sizable photovoltaic efficiency. The observed light induced diode response under fixed gate voltage, yields a maximum open circuit voltage 0.28V and short circuit current 230nA at 30uW incident laser power. The sense of current rectification can be altered by changing the polarity of the applied gate voltage ($V_{\mathrm{bg}})$. At $V_{\mathrm{bg}}=$0V the highest electrical power obtained is 175pW corresponding to a maximum photovoltaic efficiency of 0.01{\%}. These values increased to 11nW and 0.05{\%} under a $V_{\mathrm{bg}} = $-7.5V. At an excitation voltage 1V we observed maximum photocurrent responsivity surpassing 100mA/W with corresponding external quantum efficiency $\sim$ 30{\%}. [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W2.00002: Electrical energy harvesting from single-atomic-layer MoS$_{2}$ Lei Wang, Wenzhuo Wu, Yilei Li, Tony Heinz, Zhong Lin Wang, James Hone Monolayer MoS$_{2}$ is predicted to be strongly piezoelectric, an effect that disappears in the bulk due to the opposite orientations of adjacent atomic layers. We observe the first experimental study of the piezoelectric properties of two-dimensional (2D) MoS$_{2}$. We find that cyclic stretching and releasing of thin MoS$_{2}$ flakes with an odd number of atomic layers produces oscillating piezoelectric voltage and current outputs, while no output is observed for flakes with an even number of layers. In agreement with theoretical predictions, the output increases with decreasing thickness and reverses sign when the strain direction is rotated by 90 degrees. Transport measurements show a strong piezotronic effect in single layer MoS$_{2}$, but not in bilayer and bulk MoS$_{2}$. The coupling between piezoelectricity and semiconducting properties in 2D nanomaterials may enable applications in powering nanodevices, adaptive bio-probes and tunable/stretchable electronics/optoelectronics. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W2.00003: Dimensionality effects on electronic properties of lateral two-dimensional junctions Henry Yu, Alex Kutana, Boris I. Yakobson We study lateral junctions of two-dimensional materials, including graphene, 2D BN, and transition metal dichalcogenides. A common feature of low-dimensional contacts is that unlike bulk devices, local charge transfer near the contact cannot equilibrate Fermi levels throughout the material, necessitating nonlocal charge redistribution. These nonlocal charges will affect the physical properties of the junction. We obtain the solution for carrier and potential distributions in symmetric and asymmetric 2D junctions with different densities of states and doping levels, using a macroscopic model and DFT calculations. The scaling of the depletion length with doping level and built-in voltage is determined by the dimensionality of the junction. The implications for operation of low-dimensional devices are discussed. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:42PM |
W2.00004: Control and characterization of the metallic surface state of bulk insulating Bi$_2$Se$_3$ Invited Speaker: Michael Fuhrer Bi$_2$Se$_3$ is a three dimensional strong topological insulator with a conducting two-dimensional surface state whose existence is guaranteed by topology. The bulk Bi$_2$Se$_3$ has a 300 meV bandgap, but is often a degenerately \textit{n}-doped metal in as-grown material. I will discuss our efforts to remove this doping in thin crystals and films to achieve surface-dominated conduction. Electrochemical gating (using PEO+LiClO4 electrolyte) or molecular doping (using F4-TCNQ) is shown to effectively bring the Fermi energy of thin (3-20 nm) exfoliated Bi$_2$Se$_3$ crystals to the conduction band edge, where it can be further modulated at low temperature using field-effect gating. These techniques allowed us to reveal the gapless ambipolar transport in the topological surface, and measure the minimum conductivity,\footnote{D. Kim et al., \textit{Nature Physics} \textbf{8}, 460 (2012)} electron-acoustic phonon scattering,\footnote{D. Kim et al., \textit{Phys. Rev. Lett.} \textbf{109}, 166801 (2012)} thermopower,\footnote{D. Kim et al., \textit{Nano Lett.} \textbf{14}, 1701 (2014)} and inter-surface coupling of the topological surfaces.\footnote{S. Cho et al., \textit{Nano Letters} \textbf{11}, 1925 (2011); S. Cho et al., \textit{Nano Letters} \textbf{12}, 469 (2012); D. Kim et al., \textit{Nature Comm.} \textbf{4}, 2040 (2013)} Recently we have developed techniques to measure the transport properties of Bi$_2$Se$_3$ in situ during growth in ultra-high vacuum, enabling better understanding of the doping mechanisms.\footnote{J. Hellerstedt et al., \textit{APL} \textbf{105}, 173506 (2014)} We have also studied vacuum-deposited MoO$_3$ as a highly effective acceptor dopant which remains stable on air exposure for time scales of days.\footnote{M.T. Edmonds et al., \textit{ACS Nano} \textbf{8}, 6400 (2014)} [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W2.00005: Charge Transport of MoS$_2$ Supported by Thiol-Decorated Self-Assembled Monolayer Doron Naveh, vlada Artel, Moshe Kirshner Intrinsic charge transport in MoS$_2$ supported by thiols was recently reported [1] and was attributed to passivation of sulfur vacancies and suppression of charged impurities from the dielectric substrate. In this talk we will present the transport characteristics of single layer and few-layer MoS$_2$ on thiol-decorated self-assembled alkyl-siloxane monolayer. \\[4pt] [1] Z. Yu \textit{et al.}, \textit{Towards intrinsic charge transport in monolayer ?molybdenum disulfide by defect and interface engineering} Nature Commun. \textbf{5}(2014). [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W2.00006: Generation and electric control of spin--valley-coupled circular photogalvanic current in WSe$_{2}$ Hongtao Yuan, Harold Y. Hwang, Yi Cui Compared to the weak spin-orbit-interaction (SOI) in graphene, layered transitionmetal chalcogenides MX$_{2}$ have heavy 4d/5d elements with strong atomic SOI, providing a unique way to extend functionalities of novel spintronics and valleytronics devices. Such a valley polarization achieved via valley-selective circular dichroism has been predicted theoretically and demonstrated with optical experiments in MX$_{2}$ systems. Despite the exciting progresses, the generation of a valley/spin current by valley polarization in MX$_{2}$ remains elusive and a great challenge. A spin/valley current in MX$_{2}$ compounds caused by such a valley polarization has never been observed, nor its electric-field control. In this talk, we demonstrated, within an electric-double-layer transistor based on WSe$_{2}$, the manipulation of a spin-coupled valley photocurrent whose direction and magnitude depend on the degree of circular polarization of the incident radiation and can be further greatly modulated with an external electric field. Such room temperature generation and electric control of valley/spin photocurrent provides a new property of electrons in MX$_{2}$ systems, thereby enabling new degrees of control for quantum-confined spintronics devices. (In collaboration with S.C. Zhang, Y.L. Chen, Z.X. Shen, B Lian, H.J. Zhang, G Xu, Y Xu, B Zhou, X.Q. Wang, B Shen X.F. Fang) Acknowledge the support from DoE, BES, Division of MSE under contract DE-AC02-76SF00515. [Preview Abstract] |
(Author Not Attending)
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W2.00007: Electrical Transport of Field-Effect Transistors Based on CVD Grown Two-Dimensional Layered Materials Ming-Wei Lin, Xufan Li, Kai Wang, Alexander Puretzky, Christopher Rouleau, David Geohegan, Kai Xiao By changing the layer numbers, the electrical transport of field effect transistors based on CVD grown two-dimensional (2D) layered materials of transition metal dichalcongenides (TMDCs) such as MoSe$_{\mathrm{2}}$, WSe$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ shown the different characteristics will be demonstrated. The transport measurements show that the altered semiconductor characteristics of these 2D materials can be possibly attributed to the shift of Fermi level when changing the number of layers. Besides, the transport characteristics can be tuned by adjusting the W/Mo doping level and mobility is also increased with increasing the layer numbers. In addition, the annealing effect on these CVD grown 2D layered materials will be discussed. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W2.00008: Coulomb blockade in few-layer MoS2 based single electron transistor Kyunghoon Lee, Zhaohui Zhong Transition metal dichalcogenides (TMDCs) based two dimensional materials are attracting much attention for their interesting electronic and optical properties, including high on/off current ratio, indirect to direct band gap transition, and valley polarized carrier transport. Nevertheless, study of the low temperature electron transport in atomic thin layered TMDCs is still in its infancy. One of the major hurdles for electron transport study lies in the large metal/semiconductor junction barrier for carrier injection, which leads to the contact resistance dominated charge transport in short channel nanoscale devices. Here, we report on the fabrication of few-layer MoS2 single electron transistor using low work function metal for the contact electrodes. We observed Coulomb blockade phenomena attributed to single electron charging on a fairly clean quantum dot. The details of the bias, gate and temperature dependence will be discussed.. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W2.00009: Annealing and ionic liquid gating on suspended molybdenum disulfide devices Fenglin Wang, Petr Stepanov, Mason Gray, Mikhail Itkis, Robert Haddon, Chun Ning Lau We fabricate suspended molybdenum disulfide (MoS$_{\mathrm{2}})$ field effect transistors (FET) devices and develop an effective gas annealing technique that significantly improves device quality and increases conductance by 3-4 orders of magnitude.~Temperature dependence measurements reveal two transport mechanisms: electron-phonon scattering at high temperatures and thermal activation over a gate-tunable barrier height at low temperatures. Our results suggest that transport in these devices is not limited by the substrates. Moreover,~this suspended MoS$_{\mathrm{2}}$~device structure provides double surface access for ionic liquid gating. We are able to extract the dielectric constant of the ionic liquid, and the~latest experimental results will be presented. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W2.00010: Influence of the Metal-MoS$_{2}$ interface on MoS$_{2}$ Transistor Performance Hui Yuan, Guangjun Cheng, Angela Hight Walker, Lin You, Joseph J. Kopanski, Qiliang Li, Curt A. Richter We compare the electrical characteristics of MoS$_{2}$ field-effect transistors (FETS) with Ag source/drain contacts with transistors with Ti contacts, and we demonstrate that the metal-MoS$_{2}$ interface is crucial to the final device performance. The topography of 5nm Au/5nm Ag (contact layer) and 5nm Au/5nm Ti metal films deposited onto mono- and few-layer MoS$_{2}$ was characterized by using scanning electron microscopy and atomic force microscopy. The surface morphology of the Au/Ti films on MoS$_{2}$ shows a rough, dewetting pattern while Au/Ag forms smooth, dense films. These smoother and denser Au/Ag contacts lead to improved carrier transport efficiency. FETs with Ag contacts show more than 60 times higher on-state current and a steeper subthreshold slope. Raman spectroscopy of MoS$_{2}$ covered with Au/Ag or Au/Ti films revealed that the contact layer is Ag or Ti, respectively. In addition, there is a dramatic difference in the heat transfer between the MoS$_{2}$ and the two metals: while laser heating is observed in Au/Ti covered MoS$_{2}$, no heating effects are seen in Au/Ag covered MoS$_{2}$. It is reasonable to conclude that the smoother and denser Ag contact leads to higher carrier transport efficiency and contributes to the improved thermal properties. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W2.00011: Negative Differential Transconductance in a MoS2/WSe2 Heterojunction Field Effect Transistor Ahmad Zubair, Amirhasan Nourbakhsh, Mildred Dresselhaus, Stefan De Gendt, Tomas Palacios In this work, we demonstrate the negative transconductance in heterojunction transistors made of two-dimensional materials for the first time. Negative transconductance plays a key role in multi-valued logic/memory and frequency multiplication circuits. The simpler fabrication method of stacked van der Waals heterostructures compared to the conventional bulk semiconductors and large area CVD growth of the layered 2D materials systems makes it a prime candidate for scalable novel applications of their heterostructures. Vertically stacked MoS$_{\mathrm{2}}$/WSe$_{\mathrm{2}}$ heterostructures are fabricated by mechanical exfoliation and an in-house dry transfer process. A two-step process of e-beam lithography and metal deposition (Au on MoS$_{\mathrm{2}}$, and Pd on WSe$_{\mathrm{2}})$ were performed to fabricate n-type MoS$_{\mathrm{2}}$ and ambipolar WSe$_{\mathrm{2}}$ FET. The transfer characteristics on the non-overlapping regions shows the expected characteristics of the n-type, MoS$_{\mathrm{2}}$ FET and ambipolar WSe2 FET. At the same time, the transfer characteristics of the overlapping region between MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ show negative differential transconductance. With proper scaling and careful optimization this negative differential transconductance will lead to novel applications. [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W2.00012: Quantum transport measurement of few-layer WTe2 field effect devices Jianhao Chen, Xin Liu, Shibing Tian, Chenglong Zhang, Shuang Jia We have performed systematic quantum transport measurement on field effect devices fabricated from few-layer WTe2 single crystals. We found that the magnetoresistance of few-layer WTe2 could be very different from that of bulk samples, which may arise from the imbalance of electron and hole carriers in the samples. We shall discuss our findings in more details in light of recent progress in our experiment. [Preview Abstract] |
Thursday, March 5, 2015 5:18PM - 5:30PM |
W2.00013: Electronic Transport of Encapsulated WSe2 Fabricated by Pick-up of Pre-patterned hBN Yafang Yang, Hugh Churchill, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero We report high quality WSe2 devices encapsulated between two hexagonal boron nitride (hBN) flakes using a pick-up method with etched hBN flakes. Previous work on graphene has shown that sample disorder can be greatly reduced via isolation from charge impurities in the substrate by means of encapsulation. However, the effect of encapsulation still remains unknown for dichalcogenides devices. Besides, the quality of contact to TMDs is also a critical factor limiting the transport performance of such devices. To measure the transport properties of dichalcogenide devices as a function of temperature, low resistance electrical contacts must be made to the material. To achieve this, we encapsulate few-layer WSe2 in hexagonal boron nitride that has been patterned to allow ionic liquid doping of the contact region. This technique simultaneously protects the WSe2 surface above and below, resulting in the highest mobility few-layer WSe2 devices reported to date. [Preview Abstract] |
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