Paramagnetic defects in hydrothermally grown few-layered MoS 2 nanocrystals
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Chinnathambi Karthik Micron School of Materials Science and Engineering, Boise State University, Boise, Idaho 83725, USA
Madhu Kongara Micron School of Materials Science and Engineering, Boise State University, Boise, Idaho 83725, USA; and Department of Physics, Boise State University, Boise, Idaho 83725, USA
Srinivasa Rao Singamanenia) Department of Physics, The University of Texas at El Paso, El Paso, Texas 79968, USA (Received 22 January 2018; accepted 30 April 2018)
In the recent past, two-dimensional (2D) nanocrystalline (NC) transition metal dichalcogenides such as MoS2 received a great deal of attention due to their extraordinary physical properties. There has been a great interest to study the defects present in MoS2 NCs, which alter the material’s catalytic, electrical, and magnetic properties. This work reports paramagnetic point defects present in the hydrothermally grown 2H–MoS2 NCs. X-band electron spin resonance (ESR) spectroscopy has been used to identify the defects which contain unpaired electron spins in the as-prepared and Ar-annealed MoS2 NCs. At least seven ESR signals were detected originating from four inequivalent paramagnetic defect sites such as adsorbed oxygen species, sulfur vacancies, thio-, and oxo-Mo51. Upon Ar-annealing, most of these defects did not survive, instead conduction ESR signal was observed. This work signifies the importance of employing ESR spectroscopy and broadens the knowledge in identifying the atomic defects in MoS2 NCs.
I. INTRODUCTION
Among the two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have attracted great attention due to their unique physical properties that render them numerous potential applications in the fields of catalysis, energy storage, and optoelectronics.1–10 These are a class of materials with the formula MX2, where M is a transition metal element (Mo, W, etc.) and X is a chalcogen (S, Se, or Te). These materials form layered structures of the form X– M–X, with the chalcogen atoms in two hexagonal planes separated by a plane of metal atoms through van der Waals interactions. For instance, MoS2 is a semiconductor with an indirect band gap of ;1.3 eV in the bulk, turning to a direct gap of 1.85 eV in monolayer form. Furthermore, fewlayered nanocrystalline (NC) MoS2 is being widely studied1–6 for its promising and attractive electrocatalytic properties in the hydrogen evolution reaction due to increasing number of active sites and enhanced intrinsic conductivity, compared to MoS2 bulk counterparts. However, the performance of the devices, which incorporate these materials, is strongly determined by a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.149
the presence of intrinsic and extrinsic defects. For instance, Addou and colleagues have extensively studied the extrinsic defects in natural MoS2 bulk crystals.11 Most interestingly, intrinsic defects such as sulfur vacancies, molybdenum antisites, and grain boundaries can actually enhance catalytic properties12–14 and introduce mag
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