Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y37: Devices from 2D Materials VIII - Energy ApplicationsFocus Session
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Sponsoring Units: DMP Chair: Roland Kawakami, Ohio State University Room: LACC 411 |
Friday, March 9, 2018 11:15AM - 11:51AM |
Y37.00001: 2D Materials for Smart Life Invited Speaker: Kaustav Banerjee I will highlight the prospects of two-dimensional (2D) materials for innovating energy-efficient transistors, sensors, and passive devices targeted for next-generation electronics needed to support the emerging paradigm of the Internet of Things. More specifically, I will bring forward a few applications uniquely enabled by 2D materials and their heterostructures that have been demonstrated in my lab for realizing ultra-energy-efficient electronics. This will include the world’s first on-chip intercalated-graphene inductor that exploits a low-dimensional material property to overcome a fundamental scalability challenge in all inductors and opens up a new pathway for designing ultra-compact wireless communication systems, a 2D-channel band-to-band tunneling transistor that overcomes a fundamental power consumption challenge in all electronic devices since the invention of the first transistor, as well as a breakthrough interconnect technology based on doped-graphene-nanoribbons, which overcomes the fundamental limitations of conventional metals and provides an attractive pathway toward a low-power and highly reliable interconnect technology for next-generation integrated circuits. I will also bring forward a new class of ultra-sensitive and low-power sensors enabled by 2D materials, for ubiquitous sensing and connectivity to improve quality of life. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y37.00002: High Open Circuit Voltage in Atomically-Thin Photovoltaics Joeson Wong, Deep Jariwala, Joseph DuChene, Matthias Richter, Alexandra Welch, Wei-Hsiang Lin, Artur Davoyan, Harry Atwater In order to achieve high power conversion efficiency approaching the Shockley-Queisser limit, a photovoltaic cell must maximize its photogenerated current and voltage output. In our previous works we demonstrated strategies for light trapping and record high photocurrent generation in transition metal dichalcogenide (TMDC) photovoltaic cells. Despite having bandgaps comparable to silicon and gallium arsenide (1.1 – 1.4 eV), TMDC-based photovoltaic devices reported to date exhibit considerably lower open-circuit voltages under 1-sun illumination, typically around 300 mV. In this work, we show that using a different device geometry with carrier-selective contacts, we can obtain record open-circuit voltages (700 mV) in vertical device heterostructures composed of nickel oxide/tungsten diselenide/titanium dioxide. In particular, we show via X-ray photoemission spectroscopy that nickel oxide and titanium dioxide can act as hole- and electron-selective contacts, respectively, enabling the unprecedented open-circuit voltages in TMDC-based photovoltaics. In addition, we perform optoelectronic device modelling to explain the device transport characteristics. In summary, our results show the potential for efficient photovoltaics using atomically-thin van der Waals materials. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y37.00003: Fabrication and characterization of multi-layer WSe2 solar cells Elaine McVay, Ahmad Zubair, Amir Nourbakhsh, Tomas Palacios Single atomic layer transition metal dichalcogenides (TMDs) and their van der Waals heterostructures have been explored extensively for ultrathin optoelectronic applications. However, optoelectronic devices made of multi-layer TMD thin-films have not been as extensively studied despite their strong absorption characteristics and wide absorption frequency. In this work, we present the electronic transport and photovoltaic characteristics of multilayer (~50 nm) WSe2 devices. Multilayer WSe2 is a high mobility ambipolar semiconductor which can be tuned by either electrostatic or chemical doping. In this work, we demonstrate a schottky WSe2 solar cell using dissimilar metal contacts. The proof-of-concept dual-metal device shows an open-circuit voltage of >0.2 V, short circuit current density >4 mA/cm2 and power conversion efficiency >2% under white light illumination with input power of 300 W/m2. This study is being extended to explore methods to better optimize the WSe2 based solar cell using experimental and modeling techniques. The results suggest that the device performance can be significantly improved by engineering the WSe2 layer doping profile. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y37.00004: Abstract Withdrawn
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Friday, March 9, 2018 12:27PM - 12:39PM |
Y37.00005: Gate-tunable acoustoelectric effect in graphene Justin Lane, Liangji Zhang, Bowen Zhou, Erik Henriksen, Johannes Pollanen Piezoelectric surface acoustic waves (SAWs) are a sensitive contact-less probe for studying the electronic properties of two dimensional electron systems. We demonstrate gate-tunable acoustoelectric transport in exfoliated graphene by measuring the voltage created as SAWs dynamically drive charge carriers in the graphene. We employ a flip-chip configuration to conduct acoustoelectric measurements while simultaneously varying the graphene carrier density with a back-gate. At high carrier density we observe dependence of the acoustoelectric signal on the sign of the charge carriers, while at low densities we observe anomalous sign reversals of the acoustically generated voltage. We attribute these anomalous sign reversals to spatially heterogeneous conduction in the vicinity of charge neutrality. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y37.00006: Maxwell-Hall access resistance in graphene nanopores Subin Sahu, Michael Zwolak The resistance due to the current paths converging from bulk to a constriction – e.g., a nanopore – is a mainstay of transport phenomena. In classical electrical conduction, Maxwell, and later Hall for ionic conduction, predicted this access/convergence resistance to be independent of the bulk dimensions and inversely dependent on the pore radius. More generally, though, this resistance is contextual, it depends on the presence of functional groups/charges and fluctuations, as well as the (effective) constriction geometry/dimensions. Addressing the context generically requires all-atom simulations, but this demands enormous resources due to the algebraically decaying nature of convergence with respect to the bulk size. We develop a finite-size scaling analysis – reminiscent of the treatment of critical phenomena – that makes the access resistance accessible in such simulations. This analysis suggests that there is a “golden aspect ratio'' for the simulation cell that yields the infinite system result with a finite system. We employ this approach to resolve the experimental and theoretical discrepancies in the radius-dependence of graphene nanopore resistance. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y37.00007: Effects of plasma treatment on surface properties of 2D tungsten diselenide Inyong Moon, Sungwon Lee, Deshun Qu, Changsik Kim, Won Jong Yoo Tungsten diselenide (WSe2) can be prepared to form atomically ultrathin film and it has been studied as a next-generation semiconductor material which shows a bandgap of ~1.3eV and non-degrading performance in the air. Plasma has been frequently used for the purpose of changing surface states of semiconductor because of the advantages enabling room temperature and large scale processes. In this study, we carried out plasma treatments using various processing gases (Ar, O2, SF6, N2) and observed changes in etching rate, surface morphology and surface charge state. Atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) were used for quantitative analysis on the effects of plasma treatment. We could observe a n-type shift by introducing fluorine ions and decreasing work function to 0.7eV upon SF6 plasma treatment, while we could observe a p-type shift by introducing nitrogen ions and increasing work function to 0.6eV. The doping is further confirmed by electrical characteristics obtained by field effect transistor. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y37.00008: Gate controllable spin filteration and spin separation in transition-metal-dichalcogenide monolayers for room temperature applications Cheng-Yi Huang, Wei-Feng Tsai, Mohammad Sadi, Gaurav Gupta, Tay-Rong Chang, Chuang-Han Hsu, Horng-Tay Jeng, Geng-Chiau Liang, Hsin Lin, Arun Bansil Monolayer transition-metal dichalcogenides (TMDs) are semiconductors with direct gaps at the corners of the hexagonal Brillouin zone. Strong spin-orbit coupling in the absence of inversion symmetry yields spin-split valence bands in the TMDs around the two inequivalent valleys, which are almost fully spin-polarized without requiring magnetic fields. By exploiting these salient features of the electronic states, we propose two spintronics applications based on TMD monolayers, a spin-filter and a spin-separator. We demonstrate the high efficiency (output current with nearly 100% spin-polarization) of our proposed spin-filter/separator through quantum transport computations. Since spin-splitting energies range from 100-500 meV in the TMDs, our spin filter/separator is suitable for room temperature applications. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y37.00009: An Ultra-High Vacuum Probe Station for 2D Material Transport and in situ Functionalization Cameron Flynn, Benjamin St. Laurent, Jake Riffle, Shawna Hollen Two-dimensional materials have unique electrical and physical properties that lend themselves to specialized electronic devices. However, many of their electronic properties are sensitive to contamination and air exposure. Transport measurements typically require lithographic patterning, which is expensive, time-consuming, and contaminates the samples. Techniques that allow for four-point measurements on micron-sized samples without lithography use micro-probe stations or nano-probes installed in electron microscopes. In partnership with RHK Technology and using Kleindiek Nanotechnik's fixed-geometry four-point probes, we developed an ultra-high vacuum tool for measuring transport and functionalizing two-dimensional materials without patterning or air exposure. An inverted viewport houses an optical microscope to image the sample and align the probe. An ion gun and evaporator are aligned to the sample stage, which is temperature controlled from 90-500 K. We will present the design of this system and resulting transport measurements on flakes of black phosphorous, a material notoriously unstable in air. These results demonstrate the utility and flexibility of this method compared to more time-consuming, contaminating, and expensive approaches. |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y37.00010: Doping and Field Effect in Novel 2D Layered Oxides Kyle Crowley, Shuhao Liu, Kasun Viraj Madusanka Nilwala Gamaralalage Premasiri, Xuan Gao Layered transition metal oxides remain a relatively unexplored front in the study of 2D van der Waals materials, providing opportunities to further advance semiconductor physics and devices in a novel class of atomically thin crystals. It is usually challenging to dope these materials and achieve field effect control for applications like transistors, given their wide bandgaps. However, when the structure is altered via doping or intercalation, the bandgap and carrier concentration can also be affected, enabling gating behavior. In this talk, we will first introduce our study of doping multilayer nanoflakes of α-MoO3 by H+ ion intercalation, which creates oxygen vacancies and n-type doping. Devices were fabricated with typical electron densities in the range of 1015/cm2 and field effect mobilities of 0.1-10 cm2/Vs. Predicted anisotropic in-plane transport effects were also observed. We may then discuss doping and field effect response in additional 2D layered oxides. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y37.00011: Theory of Strain-Induced Confinement in Transition Metal Dichalcogenide Monolayers Matthew Brooks, Guido Burkard Recent experimental studies of out-of-plane straining geometries of transition metal dichalchogenide (TMD) monolayers have demonstrated sufficient band gap renormalisation for device application such as single photon emitters. Here, a simple continuum-mechanical plate-theory approach is used to estimate the topography of TMD monolayers layered atop nanopillar arrays. From such topologies, the induced conduction band potential and band gap renormalisation is given, demonstrating a potential shape that is independent of the height of deforming nanopillar. Additional, with a semi-classical WKB approximation, the expected leakage of the strain potential may be estimated as a function of the height of the deforming nanopillar. This straight forward approach is in accordance with experiment, supporting recent findings suggesting that nanopillar height improves linewidth of the single photon emitters observed at the tip of the pillar, yet has no discernible influence over the wavelengths of the emitted photons. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y37.00012: Transport Studies of Boron-Doped Diamond/Graphene Heterostructures Adrian Nosek, Robert Bogdanowicz, Mateusz Ficek, Michal Sobaszek, Lukasz Golunski, Jakub Karczewski, Andres Jaramillo-Botero, William Goddard, III, Marc Bockrath, Tadeusz Ossowski Diamond/graphene heterostructures may enable novel devices that operate at high temperatures. As a first step towards realizing such devices, boron-doped diamond thin films were grown in a microwave plasma-assisted chemical vapor deposition (CVD) system on a tantalum substrate. Mechanical transfer of the diamond films onto graphene grown by CVD on a Si/SiO2 substrate allowed us to fabricate heterostructure devices. We measured the current-voltage characteristics of the devices versus temperature, finding behavior consistent with thermally activated transport over a barrier of ~20 meV. This indicates that the contact barrier between graphene and doped diamond is relatively small. Variations in device behavior with diamond doping are possible, and our latest results will be discussed. |
Friday, March 9, 2018 2:03PM - 2:15PM |
Y37.00013: Controllable carrier doping and transport of van der Waals heterostructures Wu Shi, Salman Kahn, Sheng-Yu Wang, Hsin-Zon Tsai, Dillon Wong, Takashi Taniguchi, Kenji Watanabe, Michael Crommie, Alex Zettl Two-dimensional (2D) materials of wide-ranging properties can now be simply stacked together to form van der Waals (vdW) heterostructures, offering enormous research opportunities. To explore unprecedented physical properties and enable functional electronic devices, local gates are usually used to modulate carrier concentration and implement various doping profiles in the 2D vdW heterostructures. However, the fabrication of local gate nanostructures requires sophisticated fabrication procedures that degrade the device quality and lack flexibility. Recent work has demonstrated an alternative way to induce nanoscale rewritable doping patterns in vdW heterostructures without introducing impurities by optical illumination or applying an STM tip voltage pulse. In this work we further develop this simple but efficient local patterning technique and study the low-temperature transport properties of patterned vdW heterostructures. Our results demonstrate that this technique offers distinct advantages over conventional local gates, making it an ideal approach for designing and prototyping novel device concepts. |
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