Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R36: Synthesis and Properties of 2D Materials and HeterostructuresFocus
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Sponsoring Units: DMP Chair: Jong-Hyun Ahn, Yonsei University Room: LACC 410 |
Thursday, March 8, 2018 8:00AM - 8:12AM |
R36.00001: Quaternery two dimensional transiton metal dichalcogenide alloys Sandhya Susarla, Alex Kutana, Jordan Hachtel, Vidya Kochat, Amey Apte, Robert Vajtai, Juan Carlos Idrobo, Boris Yakobson, Chandra Sekhar Tiwary, Pulickel Ajayan Monolayers of 2D TMDs, due to their unique band gap are useful in various optoelectronic devices. Alloying in 2D TMDs has gained importance due to its band gap tunability. However this tunability could futher be improved by increasing number of components, which provides more flexibility and degress of freedom. In the present work, quaternary alloys (MoxW1-xS2ySe2(1-y)) (0< x,y <1) have been synthesised using CVD. The composition of the alloy has been tuned by changing the growth temperature, resulting in wide band gap tunability from 1.6 to 1.85 eV. Detailed DFT calcaulations support the experimental data. This wide range of band gap tunability would open the doors for 2D TMDs to be used in applications like laser diodes and LEDs. |
Thursday, March 8, 2018 8:12AM - 8:24AM |
R36.00002: Layer-by-layer epitaxial growth of transition-metal dichalcogenide thin films and heterostructures on sapphire by molecular-beam epitaxy. Masaki Nakano, Yue Wang, Yuta Kashiwabara, Hideki Matsuoka, Yoshihiro Iwasa Bottom up synthesis of transition-metal dichalcogenide (TMDC) thin film is of significant importance in particular for future advanced researches on TMDC-based heterostructures. The most-frequently-used method is chemical-vapor deposition enabling wafer-scale growth of monolayer TMDCs, but growing epitaxial thin films of TMDCs with in-plane orientation in millimeter scale is still challenging. We have recently succeeded in establishing a versatile route to layer-by-layer epitaxial growth of millimeter-scale TMDC thin films on insulating sapphire substrates by molecular-beam epitaxy (MBE)[1]. The growth recipe is broadly applicable to various TMDCs, opening a door to fabrication of van der Waals heterostructures with various types of TMDCs toward novel properties and functionalities. In the presentation, we will introduce our growth recipe and discuss epitaxial relationships between TMDC thin films and sapphire substrates revealed by in-plane x-ray diffraction measurements. We will also show transport properties of those MBE-grown TMDC epitaxial thin films and heterostructures. [1] M. Nakano et al., Nano Lett. 17, 5595 (2017). |
Thursday, March 8, 2018 8:24AM - 8:36AM |
R36.00003: Controlling WSe2 Synthesis Parameters for Improved Device Performance Ankit Sharma, Wenjuan Zhu Tungsten diselenide (WSe2), a two-dimensional transition metal dichalcogenide, has attracted widespread research interest due to its direct bandgap and tunable electronic properties. While WSe2 films can be obtained through mechanical exfoliation of a bulk crystal, other routes of synthesis must be explored to make WSe2 suitable for industrial use. This work is a systematic exploration of the ambient pressure chemical vapor deposition (CVD) process of WSe2 synthesis, as it promises higher scalability and comparable electronic performance versus exfoliated samples. We report on the role of seeding promoters and hydrogen in the CVD process, as well as the effects of high-temperature annealing and precursor ratio on overall WSe2 film quality. The result of our work is high-quality WSe2 samples suitable for device fabrication or transfer. This work improves the viability for WSe2 and other transition metal dichalcogenides to be integrated into future nanoelectronics. |
Thursday, March 8, 2018 8:36AM - 9:12AM |
R36.00004: Janus monolayers of transition metal dichalcogenides Invited Speaker: Lain-Jong Li Our recent demonstration in vapor phase growth of TMD monolayers and their lateral heterojunctions has stimulated the research in growth and applications (1,2). In addition to the commonly discussed symmetric 2D materials, we have also developed a method that can precisely manipulate arrangement of chalcogenide atoms (S and Se) along the vertical direction of TMD. This new strategy allows us to fabricate a MoSSe Janus structure, where the transition metals are sandwiched by selenium at upmost and sulfur at bottom. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements. (3). References: (1) Adv. Mater. 24, 2320 (2012). (2) Science 349, 524 (2015). (3) Nature Nanotech. (2017) doi:10.1038/nnano.2017.100 |
Thursday, March 8, 2018 9:12AM - 9:24AM |
R36.00005: Electron beam-induced synthesis of hexagonal 1H-MoSe2 from square β-FeSe decorated with Mo adatoms John Brehm, Junhao Lin, Jiadong Zhou, Hunter Sims, Zheng Liu, Sokrates Pantelides, Kazu Suenaga Two-dimensional (2D) materials have generated interest in the scientific community because of the advanced electronic applications they might offer. Powerful electron beam microscopes have been used not only to evaluate the structures of these materials, but to manipulate them as well, by forming vacancies and nano-fragments or joining nano-islands together. In this presentation I will show a movie in which the electron beam in a scanning transmission electron microscope (STEM) can be used in yet another way: to mediate the synthesis of 2D 1H-MoSe2 from Mo-decorated 2D β-FeSe and simultaneously image the process on the atomic scale. Prior to committing microscope and materials resources to this project, the choice of reactants was first evaluated as a reasonable system for this proof of concept synthesis via quantum mechanical calculations. These calculations will be described in detail and include a methodology for finding a reaction path to forming a stable 1H-MoSe2 nucleation kernel within pure β-FeSe in which the pertinent energy barriers are smaller than the energy that can be supplied by a STEM electron beam. |
Thursday, March 8, 2018 9:24AM - 9:36AM |
R36.00006: Elucidating the Role of Seeding Promoters on MoSe2 Synthesis and its Impact on Optoelectronic Properties Saujan Sivaram, Berry Jonker Monolayer transition metal dichalcogenides (TMDs) are attractive for future optoelectronic and valleytronic applications due to their direct band gap and preferential valley filling in response to circularly polarized light. Despite this promise, few processes exist to synthesize single crystals of primarily monolayers over a large area. Chemical vapor deposition (CVD) is an attractive option to produce TMDs at the research scale; however, selenide-based TMDs (e.g., MoSe2 and WSe2) are particularly difficult to synthesize. To promote lateral growth, large organic molecules are frequently deposited onto the substrate prior to growth. The role of these promoters during synthesis and their subsequent effect on TMD properties is unclear. In this work, we synthesize monolayer MoSe2 on SiO2 with and without the seeding agent, PTAS, and find significant differences in their growth behavior. Our results suggest that PTAS acts as a reservoir to increase the local concentration of Mo and Se precursors, thereby promoting MoSe2 growth. We explain the diminished PL in the PTAS-assisted MoSe2 as a consequence of the shorter nucleation time, which results in exposure to the effects of high temperature. These findings serve as important steps towards the scalable growth of monolayer TMDs. |
Thursday, March 8, 2018 9:36AM - 9:48AM |
R36.00007: Synthesis of large area and high quality MoS2 on Au(111) monolayers with single domain orientation Harsh Bana, Elisabetta Travaglia, Luca Bignardi, Paolo Lacovig, Charlotte Sanders, Maciej Dendzik, Matteo Michiardi, Marco Bianchi, Daniel Lizzit, Francesco Presel, Dario De Angelis, Nicoleta Apostol, Pranab Das, jun fujii, Ivana Vobornik, Rosanna Larciprete, Alessandro Baraldi, Philip Hofmann, Silvano Lizzit The employment of transition metal dichalcogenides, and in particular single layer (SL) MoS2 for electronic devices, demands for a controllable growth of highly crystalline layers featuring large area with a low concentration of defects to preserve their outstanding electronic properties. Moreover, thanks to the peculiar electronic structure, new degrees of freedom are accessible allowing for spin- and valley-dependent phenomena, that can be retained in devices only through singly-oriented domains. Current chemical vapor deposition methods have not been able to achieve this and have produced mirror twin domains leading to the formation of domain boundaries and dislocations in the layer. |
Thursday, March 8, 2018 9:48AM - 10:00AM |
R36.00008: Direct Growth of WS2 on h-BN by Chemical Vapor Deposition Jesse Thompson, Christopher Chen, Brandon Blue, Hechin Chen, Jairo Velasco Jr., Masa Ishigami, Shaul Aloni Transition metal dichalcogenides (TMDCs) exhibit electronic properties that drastically change in the monolayer limit, sensitive phase transitions, and other unique features, spurring interest in device-scale integration of these materials. However, scalable and reproducible synthesis of these materials has proven challenging. Previous works have stressed the impact of synthesis parameters, including relative precursor concentrations, carrier gas species, pressure, temperature, and humidity, on the type, quality, and quantity of material synthesized. Here, we employ a chemical vapor deposition (CVD) conversion process to synthesize tungsten disulfide (WS2) crystals atop exfoliated hexagonal boron nitride (h-BN) crystals. The products of these syntheses were characterized with atomic force microscopy, scanning electron microscopy, and Raman microscopy. These results yield insights into CVD conversion as an avenue for next-generation electronics and direct growth of vertical van der Waals heterostructures. |
Thursday, March 8, 2018 10:00AM - 10:12AM |
R36.00009: Epitaxial Growth of MoS2 on h-BN without Mirror Grain Boundaries Yuanxi Wang, Fu Zhang, Nasim Alem, Vincent Crespi Recent efforts in perfecting the epitaxial growth of polar 2D materials appear to stop short at one last hurdle: the interaction between the grown 2D sheets and the carefully chosen, nearly-commensurate substrates is at best strong enough to energetically favor only two crystal orientations related by in-plane inversion, yet too weak to lift this remaining near-degeneracy, allowing formation of mirror grain boundaries that degrade carrier mobility. Using first-principles calculations, we examine the prospect of defects in a hexagonal boron nitride substrate facilitating the nucleation and epitaxial attachment of transition metal dichalcogenide monolayers on top, with full orientation control. |
Thursday, March 8, 2018 10:12AM - 10:24AM |
R36.00010: Healing a Topological Scar Benjamin Katz, Vincent Crespi A novel defect type in two-dimensional systems is presented, which changes the local coordination number of an atom in an otherwise regular structure. While point-like by itself, such a 'coordination defect' has an unusually large number of involved atoms and a potentially dramatic influence on the growth of the system following its formation, due to its introduction of a mismatch between bond network topology and physical ring size. The potential growth pathways after the occurrence of such a defect in graphene are followed using molecular dynamics and first-principles calculations; the inherent conflict between the topological requirements and the actual chemical/physical structure that occurs as the system heals the defect can result in varied morphologies, including a runaway feedback that spawns one or more semi-infinite grain boundaries. Energy comparisons from first principles are used to evaluate the likelihood of this result under various conditions. The appearance of this defect type is predicted to have similar ramifications across a broad array of two-dimensional systems, potentially providing a new method of controlling grain boundary behavior and location. |
Thursday, March 8, 2018 10:24AM - 10:36AM |
R36.00011: Ultra-thin NbS2 nanoflakes grown by sulfurization of niobium oxide films Zhen Li, Wencao Yang, Jun Chen, Enzhi Xu, Haoming Liu, Yaroslav Losovyj, Madilynn Werbianskyj, Xingchen Ye, Herbert Fertig, Shixiong Zhang Niobium disulfide (NbS2) is one of the few layered transition metal dichalcogenides (TMDs) that exhibit metallic behavior and superconductivity. Atomically thin nanoflakes allow for the study of novel electrical transport properties induced by low dimensionality. Chemical vapor deposition growth of NbS2 nanoflakes so far has mostly involved corrosive precursors such as NbCl5 and H2S which may complicate the synthesis process. In this work, we report on a sulfurization method to synthesize ultra-thin NbS2 in which the above harsh precursors were avoided. Niobium oxide films were first grown on sapphire substrates by pulsed laser deposition and were subsequently annealed in sulfur atmosphere, yielding NbS2 nanoflakes with thickness down to bilayer. We will discuss how the electrical properties and Raman vibrational modes change as a function of thickness, stoichiometry and aging. |
Thursday, March 8, 2018 10:36AM - 10:48AM |
R36.00012: Computational Synthesis of MoS2 Layer by the Direct Sulfidation of MoO3 Surfaces: A Reactive Molecular Dynamics Study Sungwook Hong, Chunyang Sheng, Rajiv Kalia, Aiichiro Nakano, Priya Vashishta Monolayer MoS2 is a promising candidate for next-generation electric devices due to its outstanding electronic, optical, and chemical properties. Chemical vapor deposition (CVD) is the most effective method to bring this layered material into mass production for a wide range of nanoscale applications. During CVD synthesis, a direct sulfidation of MoO3 surfaces is a critical reaction event, leading to mono/few MoS2 layers on substrates. However, an atomic-level understanding of the sulfidation process remains elusive. In this work, we present first-principles informed and validated reactive molecular dynamics simulations of the direct sulfidation of MoO3 surface. We find that reduction and sulfidation events locally occur on the MoO3 surface, resulting in surface defects whereas the healing of the defects could be controlled by further introduction of sulfur precursors. Our work clarifies an origin of surface defects on MoS2 layers as well as optimized conditions for the defect healing process, allowing us to refine scalable CVD techniques for synthesis of defect-free MoS2 layers. |
Thursday, March 8, 2018 10:48AM - 11:00AM |
R36.00013: Solid-Solid Interfacial Synthesis of Atomically Thin Graphitic Carbon Nitride Film Tokio Katoh, Koichiro Saiki Two-dimensional (2D) graphitic carbon nitride (g-C3N4): crystalline 2D conjugated polymer an analogy to graphene is an attractive candidate for a metal-free and visible light-responsive photocatalyst. Due to its high specific surface area, various kinds of attempts have been made to fabricate 2D g-C3N4. Most of them are based on a top-down method which separates monolayer g-C3N4 via chemical or physical exfoliation of layered bulk g-C3N4. However, the delamination process causes structural damage which limits the grain size and doping of impurities to exfoliated 2D g-C3N4. Thus, bottom-up synthesis methods are desperately demanded for a large scale and impurity free 2D g-C3N4 production. Here, we present an entirely new approach using solid/solid interface act as a non-equilibrium reaction field which exclusively promotes 2D polymer conjugation. Melamine (C3N6H6) sandwiched between two solids: a sapphire substrate and Au thin-film was annealed and polymerized to form an atomically thin film of g-C3N4. We characterized the stoichiometric composition and morphology of confined conjugated films on a sapphire substrate by XPS and AFM. Ultrathin g-C3N4 films have been found resulting from constrained 2D polymer crystallization. |
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