Localized coherent phonon generation in monolayer MoSe from ultrafast exciton trapping at shallow traps

• Published in: Nanoscale horizons Vol. 8; no. 9; pp. 1282 - 1287
• Main Authors: Bae, Soungmin, Jeong, Tae Young, Raebiger, Hannes, Yee, Ki-Ju, Kim, Yong-Hoon
• Format: Journal Article
• Published: 21.08.2023
• ISSN: 2055-6756; 2055-6764
• DOI: 10.1039/d3nh00194f

We report spectroscopic evidence for the ultrafast trapping of band edge excitons at defects and the subsequent generation of defect-localized coherent phonons (CPs) in monolayer MoSe 2 . While the photoluminescence measurement provides signals of exciton recombination at both shallow and deep traps, our time-resolved pump-probe spectroscopy on the sub-picosecond time scale detects localized CPs only from the ultrafast exciton trapping at shallow traps. Based on occupation-constrained density functional calculations, we identify the Se vacancy and the oxygen molecule adsorbed on a Se vacancy as the atomistic origins of deep and shallow traps, respectively. Establishing the correlations between the defect-induced ultrafast exciton trapping and the generation of defect-localized CPs, our work could open up new avenues to engineer photoexcited carriers through lattice defects in two-dimensional materials. Spectroscopic evidence for the conversion of defect-trapped excitons into localized coherent phonons in monolayer MoSe 2 is provided, and the V Se + O 2 complex (an oxygen molecule adsorbed on a Se vacancy) is identified as the shallow trap center.