Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X37: Devices from 2D Materials VII - Scalable devicesFocus
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Sponsoring Units: DMP Chair: Mahesh Neupane, Army Research Lab Room: LACC 411 |
Friday, March 9, 2018 8:00AM - 8:12AM |
X37.00001: Deterministic Folding of 2D Materials for Electronic Device Application Huan Zhao, Han Wang Atomically-thin two dimensional materials such as graphene have exceptional thermal and mechanical properties, therefore two-dimensional (2D) sheets can be deterministically stretched, strained and folded into various origami structures. The deterministic folding of 2D materials can induce novel optical, electronic, and magnetic properties. In this talk, a deterministic self-folding technique for creating folded 2D material structures will be reported. This new folding approach can realize micrometer and sub-micrometer scale folding of 2D sheets such as graphene, MoS2, h-BN and their heterostructures at well-defined angles and positions, without external polymers attached to the sheets. The technique is efficient and scalable. Utilizing this folding technique, well-defined vertical 2D homostructures and heterostructures with interlayer interactions can be achieved. All-2D material tri-gate transistors can be efficiently constructed using this folding technique, which demonstrate excellent electrical performance and potential for circuit level applications. This technique could be promising for realizing advanced foldable and adaptive 2D electronics, nano-actuating, and bio-nano interfaces. |
Friday, March 9, 2018 8:12AM - 8:24AM |
X37.00002: Characterization of Monolayer Transition Metal Dichalcogenides Obtained by Metal Deposition Assisted Mechanical Exfoliation Eshaan Patheria, Andrew Joe, Luis Jauregui, Kateryna Pistunova, Philip Kim Transition metal dichalcogenides (TMDs) are atomically thin two-dimensional semiconductors that exhibit rich optoelectronic phenomena, making them strong candidates for next-generation semiconductor devices. Many prototypical devices have been demonstrated in van der Waals heterostructures of various TMDs. However, the scalability of these devices and the ability to study their properties is severely limited by the small size of atomically thin monolayers of TMDs obtained using standard mechanical exfoliation (~ 4 by 4 µm). Recently, gold deposition assisted exfoliation has been demonstrated for obtaining large TMD monolayers (~ 100 by 100 µm) from bulk crystals1. Here we report the optoelectronic characterization of TMD monolayers obtained using gold deposition assisted exfoliation. We report key optoelectronic figures of merit of these samples that are relevant to both fundamental research and device applications, such as Hall mobility, Raman spectroscopy and low-temperature photoluminescence (T = 4 K). We will also discuss several modifications to the gold-mediated exfoliation technique and their impact on these figures of merit. |
Friday, March 9, 2018 8:24AM - 8:36AM |
X37.00003: Esaki Diodes based on MoS2/p-Si Heterostructures Kai Xu, Yuhang Cai, Zijing Zhao, Wenjuan Zhu Recently, two-dimensional (2D) materials have been intensively studied for next-generation ultrathin and flexible electronic and optoelectronic devices. Compared with one-dimensional (1D) nanostructures, 2D materials are more suitable for 3D monolithic integration and more compatible with traditional CMOS microfabrication. In this work, we demonstrate Esaki diodes based on MoS2/p-Si heterostructures for the first time. Large-scale and high-quality molybdenum disulfide (MoS2) was grown on a highly doped silicon substrate in a chemical vapor deposition (CVD) system. The domain size of monolayer MoS2 is about 50 µm. Raman and AFM further confirmed that the synthesized MoS2 is monolayer. The electrical properties of MoS2/p-Si heterostructures were also systematically investigated by conductive AFM. Interestingly, the IV curve exhibits prominent negative differential resistance (NDR) effect at room temperature. This NDR effect is due to the band-to-band tunneling in the MoS2 and highly doped p-Si heterostructures. This work provides the experimental groundworks for Esaki diodes based on TMDC/Si heterostructures and opens up new opportunities for novel devices based on 2D materials and 3D semiconductors. |
Friday, March 9, 2018 8:36AM - 8:48AM |
X37.00004: Building Flexible, Microscopic Sensor Nodes with 2D Materials Marek Hempel, Elaine McVay, Jing Kong, Tomas Palacios Most sensors in use today are of macroscopic dimensions. However, making sensors smaller than the diameter of a human hair could enable a new paradigm of sensing. For example, having sensors that are small and light enough to travel with the wind could be deployed by the millions in the atmosphere and would enable a new way of environmental monitoring. In this project, we work towards this vision by developing a microscopic sensor nodes, called synthetic cell or SynCell that can sense analytes in liquids or air. Our SynCells consist of 100-um-wide flexible polymer disks that have 3 chemical sensors and ID numbers as identifier in the form of ROM transistors. The transistor channels and sensors are made of a single atomic layer of molybdenum disulfide (MoS2). This material is flexible and useful to build digital electronics as well as very sensitive sensors. So far, we have demonstrated good MoS2 transistor performance and evaluated the response of the chemical sensors to analytes such as triethylamine. We also showed the ability to manipulate the SynCells by external magnetic fields. Furthermore, we developed a simple process to release the SynCells from the substrate by peeling them off with tape. |
Friday, March 9, 2018 8:48AM - 9:00AM |
X37.00005: Patterning and stacking for batch fabrication of transistor arrays using two dimensional materials. Preeti Poddar, Saien Xie, Andrew Mannix, Kibum Kang, Jiwoong Park Building integrated circuits is the basis of modern electronics and optoelectronics. Conventional device fabrication processes involve patterning with polymer based lithography, deposition, and etching of thin film materials. They introduce surface contamination or damage and are not ideal for atomically-thin, all-surface materials such as graphene and transition metal dichalcogenides (TMDs). Here, we report a new approach for building integrated circuits using TMDs under ambient condition without using conventional lithography, metal deposition or etching. A 532-nm pulsed laser beam is used to achieve wafer-scale, resist-free patterns of TMDs. The scalability and reliability of such process is demonstrated by fabricating an array of field effect transistors (FETs). The devices are generated by patterning semiconducting TMDs (e.g. MoS2, MoSe2) for channels and aligned transfer of metal structures, also patterned using our laser beam, for electrodes and interconnects. Our devices show improved hysteresis compared to conventional lithography-based FETs. Our work provides a general method for wafer-scale device fabrication using surface sensitive materials while maintaining their intrinsic properties and could be useful for realization of high performance atomically-thin circuitry. |
Friday, March 9, 2018 9:00AM - 9:12AM |
X37.00006: Defect Design and Functionalization of Two Dimensional MoS2 Structures via Defected Graphene Stamping Dundar Yilmaz, Roghayyeh Lotfi, Chowdhury M. Ashraf, Adri Van Duin We propose a novel method to create vacancy defects on MoS2 structures. The method is similar to a potato stamp process consists of creating vacancy defects on graphene layer and stamping to the MoS2 surface. Based on nudged elastic band calculations we predict that sulfur atoms on the surface will diffuse into vacancy sites on the graphene. Separation of graphene layer will carry away diffused sulfur atoms leaving MoS2 surface with sulfur vacancies. We carried molecular dynamics simulations with the ReaxFF reactive force field to test the potato stamp concept – and then functionalized the MoS2 surface defects with epoxy molecules. We observed dissociation of epoxy molecules at the vacancy site as exposed metal atoms performed catalytic activity. |
Friday, March 9, 2018 9:12AM - 9:48AM |
X37.00007: Large scale ultrathin opto-electronics using 2D materials grown by chemical vapour deposition Invited Speaker: Jamie Warner 2D monolayers of transition metal dichalcogenides (TMDs), such as MoS2 and WS2, are direct band gap semiconductors and offer new approaches to creating ultrathin, transparent and flexible electronics by integrating with graphene. Graphene's semi-metal band structure gives rise to new behavior as an electrode in contact with the 2D TMDs that modulate the transistor and photodetector behavior. In this talk I will discuss our latest results on how to create arrays of all 2D devices using only CVD grown 2D materials, in both lateral geometry and vertical stacked configurations. I will show how simple layer by layer assembly can create large scale arrays of cross bar graphene electrodes with MoS2 or WS2 in between and the unique photoresponse these systems have. I will then compare the photo-physics to those in lateral form and explain the mechanisms of charge transfer and doping. The layer dependent response of devices will be reported along with the enhancement of photodetectors by using MoS2/WS2 stacks with type II band offset. |
Friday, March 9, 2018 9:48AM - 10:00AM |
X37.00008: Measuring and Manipulating the Adhesion of Graphene Marc Miskin, Chao Sun, Itai Cohen, William Dichtel, Paul McEuen We present a technique to measure the surface energies between two dimensional materials and substrates. As a specific example, we characterize the delamination of single-layer graphene from monolayers of pyrene tethered to glass in water. We maximize the graphene adhesion energy by varying the density of pyrene in the monolayer, enabling high-fidelity graphene-transfer protocols that can resist failure under sonication. Additionally, we find that the energies of graphene peeling and re-adhesion exhibit a dramatic rate-independent hysteresis, differing by a factor of 100. This work establishes a rational means to control the adhesion of 2D materials, and enables a systematic approach to engineer stimuli-responsive adhesives and mechanical technologies at the nanoscale. |
Friday, March 9, 2018 10:00AM - 10:12AM |
X37.00009: Wet processing changes stacking order in multilayer graphene flakes Ralf Weitz, Fabian Geisenhof, Felix Winterer Quantum transport in multilayer graphene is interesting in many aspects. For example, it was shown that in ultraclean samples of graphene bilayers [1] and recently also multilayers[2], the exchange interaction leads to a novel phase, who’s nature is currently still under debate. At the heart of answering this question is knowledge of the local stacking order during a charge transport experiment. Here, we show that the structuring of metal contacts can induce a non-local transition from ABC to ABA stacking. This stacking has been identified by spatially resolved Raman and scattering SNOM measurements. We discuss possible reasons for this transformation. |
Friday, March 9, 2018 10:12AM - 10:24AM |
X37.00010: Novel CVD Technique for Producing As-grown 2D Materials-Based Devices with Naturally Formed Contacts Sudiksha Khadka, Shrouq Aleithan, Thushan Wickramasinghe, Ruhi Thorat, Eric Stinaff We have developed a new process for creating as-grown, naturally contacted, 2D materials-based devices which is scalable, reproducible, and potentially CMOS compatible with broad implications for basic research and industrial applications. Monolayer films are controllably grown on and around patterned regions of transition metals. Measurements of first generation metal-semiconductor-metal photodiodes, show responsivities on the order of 1 to 10 A/W and a time response on the order of 2 µs, an order of magnitude faster than the best reported result. We will also present the latest results from electrical characterization techniques including 2 and 4-terminal measurements and mobility measurements such as Hall and field-effect. Early results show contact resistances on the order of 0.1 to 1 MΩ●µm and 2D material resistivities on the order of kΩ/square, which are competitive with traditionally made devices. Complex device structures and wafer scale circuits can be envisioned which would be potentially compatible with existing CMOS technology whereby 2D materials-based devices could be grown directly onto silicon wafers and incorporated into the circuitry during either front-end or back-end processing. |
Friday, March 9, 2018 10:24AM - 10:36AM |
X37.00011: High-Mobility CVD-Grown Graphene Device Fabrication with Perfluoropolymers Jianan Li, Jen-Feng Hsu, Hyungwoo Lee, Shivendra Tripathi, Qing Guo, Lu Chen, Mengchen Huang, Shonali Dhingra, Jung-Woo Lee, Chang-Beom Eom, Patrick Irvin, Jeremy Levy, Brian D'Urso The transfer of graphene grown by chemical vapor deposition (CVD) using amorphous polymers represents a widely implemented method for graphene-based electronic device fabrication. However, the most commonly used polymer, poly(methyl methacrylate) (PMMA), leaves a residue on the graphene that limits the mobility. Here we report a method for graphene transfer and patterning that employs a perfluoropolymer—Hyflon—as a transfer handle and to protect the graphene against contamination from photoresists or other polymers. CVD-grown graphene transferred this way onto LaAlO3/SrTiO3 heterostructures is atomically clean, with high mobility (~30,000 cm2V−1s−1) near the Dirac point at 2 K and clear, quantized Hall and magnetoresistance. Local control of the LaAlO3/SrTiO3 interfacial metal-insulator transition—through the graphene—is preserved with this transfer method. The use of perfluoropolymers, such as Hyflon, with CVD-grown graphene and other 2D materials can readily be implemented with other polymers or photoresists. |
Friday, March 9, 2018 10:36AM - 10:48AM |
X37.00012: High Mobility from Wet-Transferred Encapsulated CVD Graphene Domenico De Fazio, David Purdie, Anna Ott, Philipp Braeuninger-Weimer, Stephan Hofmann, Ilya Goykhman, Andrea Ferrari, Antonio Lombardo High room temperature mobility (μRT) in graphene grown on metals by chemical vapour deposition (CVD) and transferred to arbitrary substrates is necessary to prepare state of the art photonic and optoelectronic devices. We show that encapsulation in hBN, following wet transfer of CVD graphene on SiO2 enables devices with μRT of 60000cm2V-1s-1, at least twice that shown in previous reports using wet transfer [1, 2]. This approach can be extended for the fabrication of heterostructures formed from any CVD layered material. |
Friday, March 9, 2018 10:48AM - 11:00AM |
X37.00013: Super Nernstian pH sensing using ionic liquid gated 2D transition metal dichalcogenide transistors Son Le, Nick Guros, Siyuan Zhang, Robert Bruce, Jeff Klauda, Arvind Balijepalli, Curt Richter Dual-gated field-effect transistor pH sensors (dualFETs) with asymmetric gates were developed that surpassed the Nernst sensitivity of 59 mV/pH by orders of magnitude. We also demonstrated an improved signal-to-noise ratio (SNR) and thereby a lower limit of detection relative to previously reported dualFETs [1]. We present an experimental pH sensing study by using 2D transition metal dichalcogenide transistors, fabricated on oxide substrates, with ionic liquid top gates. Our devices operate at low voltages and feature asymmetric front-gate (ionic liquid) and back-gate (substrate oxide) capacitances which allow a signal amplification ranging from 35 to 200 depending on the substrate oxide thickness (70 nm and 300 nm). We demonstrate that in the dual-gate configuration, the dualFETs response to a change in pH increases proportionally to the ratio of the asymmetric gate capacitances, and exceeds the Nernst limit. This higher sensitivity, combined with lower limit of detection makes dualFETs a powerful tool for field monitoring of pH with wide ranging applications in healthcare, environmental monitoring and agriculture. |
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