Quantum Coherence Facilitates Efficient Charge Separation at a MoS 2 /MoSe 2 van der Waals Junction Run Long* ,†,‡ and Oleg V. Prezhdo* ,§ † College of Chemistry, Key Laboratory of Theoretical and Computational Photochemistry of Ministry of Education, Beijing Normal University, Beijing, 100875, People’s Republic of China ‡ School of Physics and Complex and Adaptive Systems Lab, University College Dublin, Dublin 4, Ireland § Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States * S Supporting Information ABSTRACT: Two-dimensional transition metal dichalcoge- nides (MX 2 , M = Mo, W; X = S, Se) hold great potential in optoelectronics and photovoltaics. To achieve efficient light- to-electricity conversion, electron-hole pairs must dissociate into free charges. Coulomb interaction in MX 2 often exceeds the charge transfer driving force, leading one to expect inefficient charge separation at a MX 2 heterojunction. Experiments defy the expectation. Using time-domain density functional theory and nonadiabatic (NA) molecular dynamics, we show that quantum coherence and donor-acceptor delocalization facilitate rapid charge transfer at a MoS 2 / MoSe 2 interface. The delocalization is larger for electron than hole, resulting in longer coherence and faster transfer. Stronger NA coupling and higher acceptor state density accelerate electron transfer further. Both electron and hole transfers are subpicosecond, which is in agreement with experiments. The transfers are promoted primarily by the out-of-plane Mo-X modes of the acceptors. Lighter S atoms, compared to Se, create larger NA coupling for electrons than holes. The relatively slow relaxation of the “hot” hole suggests long-distance bandlike transport, observed in organic photovoltaics. The electron-hole recombination is notably longer across the MoS 2 /MoSe 2 interface than in isolated MoS 2 and MoSe 2 , favoring long-lived charge separation. The atomistic, time-domain studies provide valuable insights into excitation dynamics in two-dimensional transition metal dichalcogenides. KEYWORDS: MoS 2 /MoSe 2 van der Waals heterojunction, nonadiabatic molecular dynamics, time-domain density functional theory, quantum coherence, charge separation and recombination, nonradiative relaxation V an der Waals heterojunctions constructed with two- dimensional (2D) transition metal dichalcogenides (MX 2 , M = Mo, W; X = S, Se) have received broad interest in optoelectronic and photovolatic applications. 1,2 Many MX 2 monolayers are direct band gap semiconductors 3,4 capable of strong light-matter interactions. 5-7 It is particularly important that MX 2 monolayers maintain their direct band structure in a heterojunction. 8,9 This is possible because the layers couple by weak van der Waals interaction. Single-layer MX 2 is often advantageous over few-layer MX 2 , which undergoes direct-to- indirect band gap transition with increasing number of layers. The optical absorption of a MX 2 heterojunction is expected to be a sum of the absorptions of the individual components. 10 Further, MX 2 materials constitute promising candidate for Li-S batteries and efficient hydrogen production. 11 First-principles calculations predict that most MX 2 hetero- junctions have type-II band alignment, where the conduction band minimum (CBM) and the valence band maximum (VBM) reside in different monolayers. 8,12,13 Such alignment can facilitate efficient separation of photoexcited electrons and holes, 9,13 result in long photogenerated charge carrier lifetimes, and reduce interfacial electron-hole recombination. Because of low dielectric constants, the Coulomb interaction is poorly screened in the 2D MX 2 materials. Theoretical studies have predicted exciton binding energies ranging from 0.5 to 1.1 eV in MX 2 monolayers. 14-16 For most MX 2 heterojunctions, such values are consistently larger than the charge separation driving force, determined by the offset between the donor and acceptor CBM for electron transfer, and the offset between the VBM for hole transfer. In particular, the CBM and VBM offsets in the MoS 2 /MoSe 2 heterojunction are 0.37 and 0.63 eV, respec- tively. 13,17 The corresponding offsets are 0.31 and 0.36 eV for the MoS 2 /WS 2 heterojunction. 17 The CBM and VBM offsets in the MoS 2 /WSe 2 heterojunction are 0.76 and 0.83 eV, as determined using the X-ray photoelectron spectroscopy and scanning tunneling spectroscopy. 18 Comparison between the exciton binding energies and the charge transfer driving forces Received: December 24, 2015 Revised: February 13, 2016 Published: February 16, 2016 Letter pubs.acs.org/NanoLett © 2016 American Chemical Society 1996 DOI: 10.1021/acs.nanolett.5b05264 Nano Lett. 2016, 16, 1996-2003 Downloaded via UNIV OF SOUTHERN CALIFORNIA on November 8, 2019 at 00:14:45 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.