Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session C26: 2D Devices: Low-dimensional Properties and ContactsFocus
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Sponsoring Units: DMP Chair: Doug Strachan, University of Kentucky Room: 325 |
Monday, March 14, 2016 2:30PM - 2:42PM |
C26.00001: Electron Transport Simulations of 4-Terminal Crossed Graphene Nanoribbons Devices Pedro Brandimarte, Nick R. Papior, Mads Engelund, Aran Garcia-Lekue, Thomas Frederiksen, Daniel S\'anchez-Portal Recently, it has been reported theoretically a current switching mechanism by voltage control in a system made by two perpendicular 14-armchair graphene nanoribbons (GNRs) [1]. In order to investigate the possibilities of using crossed GNRs as ON/OFF devices, we have studied their electronic and transport properties as function structural parameters determining the crossing. Our calculations were performed with TranSIESTA code [2], which has been recently generalized to consider $N\ge1$ arbitrarily distributed electrodes at finite bias. We find that the transmission along each individual GNR and among them strongly depends on the stacking. For a 60$^\circ$ rotation angle, the lattice matching in the crossing region provokes a strong scattering effect that translates into an increased interlayer transmission. [1] K. Masum Habib and R. Lake, Phys. Rev. B 86, 045418 (2012); [2] M. Brandbyge et al, Phys. Rev. B 65, 165401 (2002). [Preview Abstract] |
Monday, March 14, 2016 2:42PM - 2:54PM |
C26.00002: Spatially Mapping Dirac Fermions in a Graphene Quantum Dot Juwon Lee, Dillon Wong, Jairo Velasco Jr, Joaquin Rodriguez-Nieva, Salman Kahn, Phong Vo, Hsin-Zon Tsai, Takashi Taniguchi, Kenji Watanabe, Alex Zettl, Feng Wang, Leonid Levitov, Michael Crommie Quantum confinement in graphene is important for tuning and exploiting graphene's electronic, spin, and optical properties. Here we present a novel technique for creating gate-tunable graphene quantum dots that are fully exposed and compatible with surface characterization tools. Using scanning tunneling microscopy (STM), we are able to spatially visualize and characterize the electronic structure of these gate-tunable graphene quantum dots. The quantum interference patterns observed in this way can be compared to the predictions of the Dirac equation, thus providing new insight into the behavior of confined ultra-relativistic Dirac fermions. [Preview Abstract] |
Monday, March 14, 2016 2:54PM - 3:06PM |
C26.00003: Imaging Quantum Confinement in Multiple Graphene Quantum Dots Dillon Wong, Jairo Velasco, Juwon Lee, Joaquin Rodriguez-Nieva, Salman Kahn, Phong Vo, Hsinzon Tsai, Takashi Taniguchi, Kenji Watanabe, Alex Zettl, Feng Wang, Leonid Levitov, Michael Crommie Quantum dots provide a useful means for controlling the electronic and spin degrees of freedom of mesoscale and nanoscale materials. Here we demonstrate a new method for fabricating interacting graphene quantum dots that is compatible with electrostatic gating and visualization by way of scanning tunneling microscopy (STM). Using this new technique we have created and spatially characterized systems of two or more interacting quantum dots. Our results show that it is possible to engineer electronic wave functions in graphene with a high degree of spatial control. [Preview Abstract] |
Monday, March 14, 2016 3:06PM - 3:42PM |
C26.00004: Probing electric properties at the boundary of planar 2D heterostructure Invited Speaker: Jewook Park The quest for novel two-dimensional (2D) materials has led to the discovery of hybridized 2D atomic crystals. Especially, planar 2D heterostructure provides opportunities to explore fascinating electric properties at abrupt one-dimensional (1D) boundaries reminiscent to those seen in the 2D interfaces of complex oxides. By implementing the concept of epitaxy to 2D space, we developed a new growth technique to epitaxially grow hexagonal boron nitride (hBN) from the edges of graphene, forming a coherent planar heterostructure [1]. At the interface of hBN and graphene, a polar-on-nonpolar 1D boundary can be formed which is expected to possess peculiar electronic states associated with the polarity of hBN and edge states of graphene Scanning tunneling microscopy and spectroscopy (STM/S) measurements revealed an abrupt 1D zigzag oriented boundary, with boundary states about 0.6 eV below or above the Fermi level depending on the termination of the hBN at the boundary [2]. The boundary states are extended along the boundary and exponentially decay into the bulk of graphene and hBN. Combined STM/S and first-principles theory study not only disclose spatial and energetic distribution of interfacial state but also reveal the origin of boundary states and the effect of the polarity discontinuity at the interface By probing electric properties at the boundary in the atomic scale, planar 2D heterostructure is demonstrated as a promising platform for discovering emergent phenomena at the 1D interface in 2D materials. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. 1 L. Liu and J.Park et al., \textit{Science }\textbf{343}, 163 (2014). 2 J. Park and JK Lee et al., \textit{Nature Commun}. \textbf{5}, 5403 (2014). [Preview Abstract] |
Monday, March 14, 2016 3:42PM - 3:54PM |
C26.00005: Electronic Transport of Encapsulated WSe2 Fabricated by Pick-up of Pre-patterned hBN Yafang Yang, 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] |
Monday, March 14, 2016 3:54PM - 4:06PM |
C26.00006: Influence of Metal Contacts on Graphene Transport Characteristics and Its Removal with Nano-carbon Interfacial Layer Akinobu Kanda, Yu Ito, Kenta Katakura, Hiroki Sonoda, Shoma Higuchi, Hikari Tomori, Youiti Ootuka Graphene is a promising candidate for the next-generation electronic material. While considerable effort has been devoted to achieve higher mobility in graphene films, relatively little attention has been paid to the effect of metal contacts, which are indispensable to the electric devices. At a graphene/metal interface, mainly due to the difference in work functions, carriers are injected from the metal to graphene. The resulting shift of local Dirac point is not limited at the graphene/metal interface but extends into the graphene channel. This carrier doping affects more significantly the performance of graphene field effect devices with shorter channel, as well as may conceal Dirac physics at the graphene/metal interface. Here, we experimentally investigate the channel length dependence of graphene transport properties in a wide gate-voltage range and extract the effect of metal contact. Several metal species are investigated. We reveal the origin of electron-hole asymmetry and the effect of the chemical interaction between graphene and metal, and derive the effective work function of graphene (4.93 eV). Furthermore, we succeed in reducing the influence of metal contact by inserting a thin nano-carbon layer (amorphous carbon or multilayer graphene (MLG)) at the interface. [Preview Abstract] |
Monday, March 14, 2016 4:06PM - 4:18PM |
C26.00007: Atomically Thin One-Dimensional Contacts to Two-Dimensional Semiconductors Marcos Guimaraes, Hui Gao, Kibum Kang, Daniel Ralph, Jiwoong Park Two dimensional van der Waals materials, including graphene and transition metal dichalcogenides (TMDs), are promising candidates for atomically thin circuitry. However, electrical contacts to semiconducting TMDs made using metal electrodes (e.g. Ti, Au) show high contact resistances ($\geq$ 50 k$\Omega$.$\mu$m). This makes it difficult to study and utilize the intrinsic properties of TMD materials, and the 3D metal contacts add significant thickness to the final devices. Here we report one-dimensional, atomically-thin, lateral contacts between graphene and monolayer TMDs with low contact resistance. The graphene/TMD lateral heterostructures are mechanically strong, and the structural and electronic properties of each individual material are well preserved after the growth processes. The interface between graphene and MoS$_{2}$ shows a much lower resistance (below 20 k$\Omega$.$\mu$m) than conventional metal contacts despite its atomic thickness and one dimensionality. Our devices exhibit linear I-V characteristics and very weak temperature dependence down to 77 K, confirming the ohmic properties of our contacts. By studying graphene/WS$_{2}$ devices fabricated in a similar way we show that our method is universal and can be expanded to other two-dimensional semiconducting TMDs. [Preview Abstract] |
Monday, March 14, 2016 4:18PM - 4:30PM |
C26.00008: High-Performance WSe$_{\mathrm{2}}$, MoS$_{\mathrm{2}}$, and MoSe$_{\mathrm{2}}$ Transistors Enabled by a New Contact Strategy Hsun Jen Chuang, Bhim Chamlagain, Michael Koehler, Meeghage Madusanka Perera1, Jiaqiang Yan, David Mandrus, David Tománek, Zhixian Zhou Fabrication of high-performance transistors of transition metal dichalcogenides (TMDs) including WSe$_{\mathrm{2}}$, MoS$_{\mathrm{2}}$, and MoSe$_{\mathrm{2\thinspace }}$has been a major challenge in 2D electronics. The performance of current metal-contacted TMDs is limited by the presence of a significant Schottky barrier in most cases. Here we introduce a new strategy for fabricating low-resistance ohmic contacts to a variety of TMDs. We demonstrate low contact resistance $\approx $ 0.3 k$\Omega \mu $m, high on/off ratios up to \textgreater 10$^{\mathrm{9}}$, and high drive currents exceeding 320 $\mu $A $\mu $m$^{\mathrm{-1\thinspace }}$in few-layer WSe$_{\mathrm{2}}$ field-effect transistors (FETs). These favorable characteristics are combined with a two-terminal field-effect hole mobility $\mu_{\mathrm{FE}} \quad \approx $ 2x10$^{\mathrm{2}}$ cm$^{\mathrm{2\thinspace }}$V$^{\mathrm{-1}}$s$^{\mathrm{-1}}$ at room temperature, which increases to \textgreater 2x 10$^{\mathrm{3}}$ cm$^{\mathrm{2\thinspace }}$V$^{\mathrm{-1\thinspace }}$s$^{\mathrm{-1}}$ at cryogenic temperatures. We observe a similar performance also in MoS$_{\mathrm{2}}$ and MoSe$_{\mathrm{2}}$ FETs. *We acknowledge the partial support by NSF grant number DMR-1308436 and the WSU Presidential Research Enhancement Award. [Preview Abstract] |
Monday, March 14, 2016 4:30PM - 4:42PM |
C26.00009: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 4:42PM - 4:54PM |
C26.00010: One-Dimensional Electrical Contact to Molybdenum Disulfide Zheng Yang, Changho Ra, Faisal Ahmed, Daeyeong Lee, Minsup Choi, Xiaochi Liu, Deshun Qu, Won Jong Yoo Molybdenum disulfide (MoS$_{\mathrm{2}})$ is one of the promising two-dimensional materials for future application in nano electronics, which has high carrier mobility, very good stability under atmosphere, proper band gap, etc. However, its application to electronic switching devices is hindered by Fermi level pinning at metal-MoS$_{\mathrm{2}}$ interfaces. Here, we experimentally demonstrate one-dimensional electrical contact to MoS$_{\mathrm{2\thinspace }}$formed via controllable plasma etching. We fabricated Al/MoS$_{\mathrm{2\thinspace }}$FET (n-type), Mo/MoS$_{\mathrm{2\thinspace }}$FET (n-type), and Pd/MoS$_{\mathrm{2}}$ FET (ambipolar). For Mo/MoS$_{\mathrm{2}}$ FET (n-type), on/off current ratio is around 10$^{\mathrm{8}}$ and mobility is around 104 cm$^{\mathrm{2}}$/(Vs). By contrast, for Pd/MoS$_{\mathrm{2}}$ FET (ambipolar), on/off current ratio is around 10$^{\mathrm{8}}$, hole mobility is ranged from 350 to 650 cm$^{\mathrm{2}}$/(Vs), and the mean free path of holes at 9K is around 23 nm. All the measured mobilities are evaluated by using two-terminal field-effect configuration. We can also achieve complementary logic gates with intrinsic MoS$_{\mathrm{2}}$/metal one-dimensional electrical contact. [Preview Abstract] |
Monday, March 14, 2016 4:54PM - 5:06PM |
C26.00011: Engineering MoS2 contact with graphene electrodes under electrostatic doping En-Min Shih, Rebeca Ribeiro-Palau, Ghidewon Arefe, Young-Duck Kim, Jia Li, James Hone, Cory Dean Semiconductor transition metal dichalcogenides (TMDs) are 2D semiconductors that host attractive transport properties such as unconventional quantum Hall effect and spin-valley physics. However, metal contacts typically result in a Schottky barrier, making it difficult to access fundamental properties of the intrinsic charge transport. In this report, we utilize graphene electrodes to achieve ohmic contact to MoS$_2$ monolayer and bilayer. Our devices are fully capsulated by boron-nitride, which reduces the disorders from Si/SiO$_2$ substrate, and benefit from a dual-gate geometry, which allow us to independently dope the MoS$_2$ channel and graphene contact regions. The transition from non-ohmic to ohmic contacts is studied as a function of graphene doping and the MoS$_2$ carrier density. Our results reveal the operational range of these new devices, and provide new insight into future device design. [Preview Abstract] |
Monday, March 14, 2016 5:06PM - 5:18PM |
C26.00012: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 5:18PM - 5:30PM |
C26.00013: Doping, strain engineering, and interlayer interaction in bilayer hexagonal boron nitride sheets Susumu Saito, Yoshitaka Fujimoto We study electronic properties of bilayer hexagonal boron nitride (h-BN) sheets with different stacking sequences in the framework of the density-functional theory. The bulk h-BN material usually takes the so-called AA (or AA') stacking, corresponding to the "non-polar" bilayer h-BN sheet. On the other hand, the rhombohedral BN takes the ABC stacking, and the corresponding bilayer sheet has "upper" and "lower" layers which are not equivalent with each other. Interestingly, the energetics of stacking sequences for bilayer h-BN sheets is found to be different from that for bulk h-BN materials. We report that strain engineering for bilayer h-BN sheets can possess much wider possibilities than that for monolayer h-BN due to the modification of the interlayer interaction. We also study the substitutional C doping into bilayer h-BN sheets, and report the energetics and the strain effect for these C-doped sheets. Finally we discuss the similarities and differences between bilayer h-BN sheets and double-wall h-BN nanotubes. [Preview Abstract] |
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