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Zhang W, Matsuda K, Miyauchi Y. Photostability of Monolayer Transition-Metal Dichalcogenides in Ambient Air and Acidic/Basic Aqueous Solutions. ACS OMEGA 2019; 4:10322-10327. [PMID: 31460125 PMCID: PMC6648714 DOI: 10.1021/acsomega.9b01067] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/15/2019] [Accepted: 05/31/2019] [Indexed: 06/01/2023]
Abstract
We report on the photostability of monolayer (1L) transition-metal dichalcogenides (TMDCs) in air and in aqueous solutions, as probed using photoluminescence spectroscopy. 1L-WSe2 was readily degraded under continuous irradiation of visible light in aqueous solutions, whereas 1L-MoS2 was relatively stable in both ambient air and aqueous solutions. The stability difference between these two materials was mainly ascribed to the oxidization reaction at the interface of 1L-TMDCs and the O2/H2O redox system induced by both band alignment and photogenerated holes. This interpretation was strongly supported by the observation of the lower degradation rate of 1L-WSe2 in the dark and in degassed water with a lower concentration of dissolved oxygen compared with the degradation rate of 1L-WSe2 in distilled water. Furthermore, the degradation rate was also nearly proportional to the number of photogenerated carriers. The degradation rate under acidic conditions was smaller than that under the basic conditions. The results are attributed to the oxidation/reduction potential of 1L-WSe2 and to the dissolution reaction of degraded species, both of which are strongly pH-dependent.
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Kumar A, Sebastian A, Das S, Ringe E. In Situ Optical Tracking of Electroablation in Two-Dimensional Transition-Metal Dichalcogenides. ACS APPLIED MATERIALS & INTERFACES 2018; 10:40773-40780. [PMID: 30387342 DOI: 10.1021/acsami.8b14585] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are a unique class of 2D materials possessing unique optoelectronic properties when exfoliated into mono- and few-layer sheets. Recently, electroablation (EA) has become of interest as a promising synthesis method for single-layer sheets of TMDs. Here, we introduce spectroelectrochemical micro-extinction spectroscopy (SE-MExS) as a high-throughput technique to study electrochemical thinning of TMDs as it occurs. This approach enables the parallel use of spectroscopy and imaging to nondestructively characterize 2D materials in situ. We unravel optoelectronics of the TMDs by observing changes in optical properties during EA. We find that the EA process for MoS2, WS2, MoSe2, and WSe2 occurs edge first, generating high density of edge sites. Our results show that stable monolayers of MoS2, WS2, and MoSe2 can be synthesized from bulk precursors by the EA process, while conversely, no WSe2 remains postablation.
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Affiliation(s)
| | - Amritanand Sebastian
- Department of Engineering Science and Mechanics , Pennsylvania State University , University Park , Pennsylvania 16802 , United States
| | - Saptarshi Das
- Department of Engineering Science and Mechanics , Pennsylvania State University , University Park , Pennsylvania 16802 , United States
- Materials Research Institute , Pennsylvania State University , University Park , Pennsylvania 16802 , United States
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Patra TK, Zhang F, Schulman DS, Chan H, Cherukara MJ, Terrones M, Das S, Narayanan B, Sankaranarayanan SKRS. Defect Dynamics in 2-D MoS 2 Probed by Using Machine Learning, Atomistic Simulations, and High-Resolution Microscopy. ACS NANO 2018; 12:8006-8016. [PMID: 30074765 DOI: 10.1021/acsnano.8b02844] [Citation(s) in RCA: 36] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Structural defects govern various physical, chemical, and optoelectronic properties of two-dimensional transition-metal dichalcogenides (TMDs). A fundamental understanding of the spatial distribution and dynamics of defects in these low-dimensional systems is critical for advances in nanotechnology. However, such understanding has remained elusive primarily due to the inaccessibility of (a) necessary time scales via standard atomistic simulations and (b) required spatiotemporal resolution in experiments. Here, we take advantage of supervised machine learning, in situ high-resolution transmission electron microscopy (HRTEM) and molecular dynamics (MD) simulations to overcome these limitations. We combine genetic algorithms (GA) with MD to investigate the extended structure of point defects, their dynamical evolution, and their role in inducing the phase transition between the semiconducting (2H) and metallic (1T) phase in monolayer MoS2. GA-based structural optimization is used to identify the long-range structure of randomly distributed point defects (sulfur vacancies) for various defect densities. Regardless of the density, we find that organization of sulfur vacancies into extended lines is the most energetically favorable. HRTEM validates these findings and suggests a phase transformation from the 2H-to-1T phase that is localized near these extended defects when exposed to high electron beam doses. MD simulations elucidate the molecular mechanism driving the onset of the 2H to 1T transformation and indicate that finite amounts of 1T phase can be retained by increasing the defect concentration and temperature. This work significantly advances the current understanding of defect structure/evolution and structural transitions in 2D TMDs, which is crucial for designing nanoscale devices with desired functionality.
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Schulman DS, May-Rawding D, Zhang F, Buzzell D, Alem N, Das S. Superior Electro-Oxidation and Corrosion Resistance of Monolayer Transition Metal Disulfides. ACS APPLIED MATERIALS & INTERFACES 2018; 10:4285-4294. [PMID: 29278319 DOI: 10.1021/acsami.7b17660] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Physics of monolayer and few-layer transition metal dichalcogenides (TMDs) and chemistry of few-layer TMDs have been well studied in recent years in the context of future electronic, optoelectronic, and energy harvesting applications. However, what has escaped the attention of the scientific community is the unique chemistry of monolayer TMDs. It has been demonstrated that the basal plane of multilayer TMDs is chemically inert, whereas edge sites are chemically active. In this article, we experimentally demonstrate that the edge reactivity of the TMDs can be significantly impeded at the monolayer limit through monolayer/substrate interaction, thus making the monolayers highly resistant to electrooxidation and corrosion. In particular, we found that few-layer flakes of MoS2 and WS2 exfoliated on conductive TiN substrates are readily corroded beyond a certain positive electrode potential, while monolayer remnants are left behind unscathed. The electrooxidation resistance of monolayers was confirmed using a plethora of characterization techniques including atomic force microscope (AFM) imaging, Raman spectroscopy, photoluminescence (PL) mapping, scanning/transmission electron microscope (S/TEM) imaging, and selected area electron diffraction (SAED). It is believed that strong substrate monolayer interaction compared to the relatively weak interlayer van der Waals interaction is responsible for the superior monolayers chemical stability in highly corrosive oxidizing environments. Our findings could pave the way for the implementation of monolayer transition metal disulfides as superior anticorrosion coating which can have a significant socioeconomic impact.
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Affiliation(s)
- Daniel S Schulman
- Materials Science and Engineering, ‡Energy Engineering, and §Engineering Science and Mechanics, Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Dan May-Rawding
- Materials Science and Engineering, ‡Energy Engineering, and §Engineering Science and Mechanics, Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Fu Zhang
- Materials Science and Engineering, ‡Energy Engineering, and §Engineering Science and Mechanics, Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Drew Buzzell
- Materials Science and Engineering, ‡Energy Engineering, and §Engineering Science and Mechanics, Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Nasim Alem
- Materials Science and Engineering, ‡Energy Engineering, and §Engineering Science and Mechanics, Pennsylvania State University , University Park, Pennsylvania 16802, United States
| | - Saptarshi Das
- Materials Science and Engineering, ‡Energy Engineering, and §Engineering Science and Mechanics, Pennsylvania State University , University Park, Pennsylvania 16802, United States
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Schulman DS, Sebastian A, Buzzell D, Huang YT, Arnold AJ, Das S. Facile Electrochemical Synthesis of 2D Monolayers for High-Performance Thin-Film Transistors. ACS APPLIED MATERIALS & INTERFACES 2017; 9:44617-44624. [PMID: 29210272 DOI: 10.1021/acsami.7b14711] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
In this paper, we report high-performance monolayer thin-film transistors (TFTs) based on a variety of two-dimensional layered semiconductors such as MoS2, WS2, and MoSe2 which were obtained from their corresponding bulk counterparts via an anomalous but high-yield and low-cost electrochemical corrosion process, also referred to as electro-ablation (EA), at room temperature. These monolayer TFTs demonstrated current ON-OFF ratios in excess of 107 along with ON currents of 120 μA/μm for MoS2, 40 μA/μm for WS2, and 40 μA/μm for MoSe2 which clearly outperform the existing TFT technologies. We found that these monolayers have larger Schottky barriers for electron injection compared to their multilayer counterparts, which is partially compensated by their superior electrostatics and ultra-thin tunnel barriers. We observed an Anderson type semiconductor-to-metal transition in these monolayers and also discussed possible scattering mechanisms that manifest in the temperature dependence of the electron mobility. Finally, our study suggests superior chemical stability and electronic integrity of monolayers even after being exposed to extreme electro-oxidation and corrosion processes which is promising for the implementation of such TFTs in harsh environment sensing. Overall, the EA process proves to be a facile synthesis route offering higher monolayer yields than mechanical exfoliation and lower cost and complexity than chemical vapor deposition methods.
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Affiliation(s)
| | | | | | - Yu-Ting Huang
- Department of Mechanical Engineering, University of Hong Kong , Pokfulam, Hong Kong
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