Defect induced exciton states in transition metal dichalcogenide WSe2 monolayer from first principles

Yifeng Chen 1, Su Ying Quek 1,2
1.Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117542, Singapore
2.Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore
E-mail: phyqsy@nus.edu.sg

Recent intriguing optical phenomena such as single photon emitters, defect-bound excitons, etc are observed in two dimensional transition metal dichalcogenide WSe2 monolayers. The nature of these exciton states are believed to be from either point defects or strain. However, a clear understanding of the exact origin and the light-matter interaction there is still lacking. The mostly abundant points defects in WSe2 were believed to be Se vacancies. However, recent investigations from our group have shown that oxygen molecules can dissociate easily at Se vacancy sites at room temperature, largely passivating these vacancies with oxygen atoms and eliminating the in-gap defect bands. Furthermore, the oxygen atom can diffuse into the WSe2 monolayer, forming an interstitial point defect. The simulated scanning tunnelling microscopy (STM) images for these defects agree well with recent experimental STM images at different bias voltages. The next most favourable point defects are predicted to be Se/W antisites, under typical Se-rich growth conditions. Here, using first principles density functional theory (DFT) method and many body perturbation theory based GW-BSE (Bethe-Salpeter equation) method, we investigate closely the implications of these three types of point defects (O/Se substitution, O interstitials, Se/W antisites) on the optical response of monolayer WSe2. Based on our findings, we propose that oxygen interstitial defects are the most likely candidates for single photon emitters in WSe2 monolayer, while O/Se substitution defect does not induce additional exciton states below the pristine WSe2 A peak. Furthermore, spin-orbit splitting effects are considered to explain the experimentally observed multiple peaks of single photon emitters with energies ranging 40~80 meV below the pristine A peak. For Se/W antisites, a number of exciton states are found below the pristine A peak, with their emission properties determined by the relaxation dynamics. Overall, our first principles results here provide crucial insights and understandings about these defect induced exciton states in monolayer WSe2, and paves the way for further optoelectronic and nanophotonic manipulation and device integration of these features in future studies.