Why Chemical Vapor Deposition Grown MoS 2 Samples Outperform Physical Vapor Deposition Samples: Time-Domain ab Initio Analysis Linqiu Li, † Run Long, § and Oleg V. Prezhdo* ,†,‡ † Department of Chemistry and ‡ Department of Physics and Astronomy, University of Southern California, Los Angeles, California 90089, United States § College of Chemistry, Key Laboratory of Theoretical & Computational Photochemistry of Ministry of Education, Beijing Normal University, Beijing 100875, PR China * S Supporting Information ABSTRACT: Two-dimensional transition metal dichalcogenides (TMDs) have drawn strong attention due to their unique properties and diverse applications. However, TMD performance depends strongly on material quality and defect morphology. Experiments show that samples grown by chemical vapor deposition (CVD) outperform those obtained by physical vapor deposition (PVD). Experiments also show that CVD samples exhibit vacancy defects, while antisite defects are frequently observed in PVD samples. Our time-domain ab initio study demonstrates that both antisites and vacancies accelerate trapping and nonradiative recombination of charge carriers, but antisites are much more detrimental than vacancies. Antisites create deep traps for both electrons and holes, reducing energy gaps for recombination, while vacancies trap primarily holes. Antisites also perturb band-edge states, creating significant overlap with the trap states. In comparison, vacancy defects overlap much less with the band-edge states. Finally, antisites can create pairs of electron and hole traps close to the Fermi energy, allowing trapping by thermal activation from the ground state and strongly contributing to charge scattering. As a result, antisites accelerate charge recombination by more than a factor of 8, while vacancies enhance the recombination by less than a factor of 2. Our simulations demonstrate a general principle that missing atoms are significantly more benign than misplaced atoms, such as antisites and adatoms. The study rationalizes the existing experimental data, provides theoretical insights into the diverse behavior of different classes of defects, and generates guidelines for defect engineering to achieve high-performance electronic, optoelectronic, and solar-cell devices. KEYWORDS: Transition-metal dichalcogenides, electron-hole recombination, antisite and vacancy defects, time-dependent density functional theory, nonadiabatic molecular dynamics T he success of single-layer graphene has opened up exploration and research into the physics of two- dimensional materials. 1-4 Due to the zero bandgap, charge carriers rapidly recombine in graphene, limiting its optoelec- tronic and solar energy applications. 5,6 Two-dimensional transition metal dichalcogenides (TMDs) of the general formula MX 2 , where M = Mo or W and X = S, Se, or Te, have drawn strong attention as possible substitutes of graphene. 7-12 The unique chemical, electrical, mechanical, and optical properties of TMDs, such as strong catalytic activity, high current-carrying capacity, moderate flexibility, large charge-carrier mobility, and high photoluminescence efficiency, are stimulating growing research efforts. 13-18 MoS 2 is the one of the most extensively studied TMDs. 11 Its monolayer is composed of a plane of hexagonally arranged molybdenum atoms sandwiched between two planes of hexagonally arranged sulfur atoms. 19 The properties of single- layer MoS 2 are superior to those of bulk MoS 2 in many ways. Single-layer MoS 2 is a direct bandgap semiconductor with higher photoluminescence efficiency. 20 It is marginally stronger than the bulk crystal. 21 Because of the true two-dimensional nature, monolayer MoS 2 outperforms three-dimensional materials in transistor applications. The electronic transport of MoS 2 field-effect transistors shows a steeper subthreshold swing and a higher on/off ratio. 7 Furthermore, MoS 2 has strong spin-orbit coupling and extra valley degrees of freedom, which can be exploited for the development of novel valleytronics. 22 Owing to its excellent optical and electric properties, MoS 2 is a promising building block for a new generation of electronic and optoelectronic materials. Devices based on mechanically exfoliated MoS 2 exhibit good electric performance. However, the thickness, shape, and number of layers of mechanically exfoliated MoS 2 are not controllable. 23 For large-scale applications, though, large area and continuous thin films of MoS 2 are a must, limiting applicability of mechanically exfoliated MoS 2 . Physical vapor deposition (PVD) and chemical vapor deposition (CVD) enable the controlled growth of large area TMD films with precise atomic-scale thick- Received: April 14, 2018 Revised: May 17, 2018 Published: May 18, 2018 Letter pubs.acs.org/NanoLett Cite This: Nano Lett. 2018, 18, 4008-4014 © 2018 American Chemical Society 4008 DOI: 10.1021/acs.nanolett.8b01501 Nano Lett. 2018, 18, 4008-4014 Downloaded via UNIV OF SOUTHERN CALIFORNIA on November 7, 2019 at 16:47:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.