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Why CVD grown MoS2 samples outperform PVD samples: Time-domain ab initio analysis Linqiu Li, Run Long, and Oleg V. Prezhdo Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01501 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018
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Nano Letters
Why CVD grown MoS2 samples outperform PVD samples: Time-domain ab initio analysis
Linqiu Li,1 Run Long,2 Oleg V. Prezhdo1,3,*
1
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States
2
College of Chemistry, Key Laboratory of Theoretical & Computational Photochemistry of Ministry of Education,
Beijing Normal University, Beijing, 100875, P. R. China
3
Department of Physics and Astronomy, University of Southern California, Los Angeles, California 90089, United
States
*Corresponding author. E-mail:
[email protected]. Phone: +1 213 821-3116
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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
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Nano Letters
The success of single-layer graphene has opened up the 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 opto-electronic and solar energy applications.5,
6
Two-
dimensional transition metal dichalcogenides (TMDs) of the general formula MX2, where M =Mo, W and X =S, Se 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 MoS2 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 MoS2 are superior to those of bulk MoS2 in many ways. Single-layer MoS2 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 MoS2 outperforms three dimensional materials in transistor applications. The electronic transport of MoS2 field effect transistors shows a steeper sub-threshold swing and a higher on/off ratio.7 Further, MoS2 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, MoS2 is a promising building block for a new generation of electronic and optoelectronic materials. Devices based on mechanically exfoliated MoS2 exhibit good electric performance. However, the thickness, shape and number of layers of mechanically exfoliated MoS2 are not controllable.23 For large-scale applications though, large area and continuous thin films of MoS2 are a must, limiting applicability of
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mechanically exfoliated MoS2. On the other hand, physical vapor deposition (PVD) and chemical vapor deposition (CVD) enable controlled growth of large area TMD films with precise atomic scale thickness.24, 25 At the same time, the charge carrier mobility are much lower in PVD and CVD grown samples than in mechanically exfoliated samples. The highest reported mobility reaches 81 cm2V-1s-1 for mechanically exfoliated samples, 45 cm2V-1s-1 for CVD grown samples and
