Monolayer transition metal dichalcogenides, MX2 (M = Mo, W and X = S, Se), are direct-gap semiconductors with many interesting properties capable of producing an all-surface material applicable to sensing, single-atom storage and other quantum-based technologies. Here we report on the optical control of single layers of MX2 such that the photoluminescence (PL) is solely from the trion state. After trion isolation, changes in the Raman spectra were observed: there is a decrease in the intensity of the out of plane mode and an enhancement of the 2LA mode. The effect is reversible, and our results suggest that the changes of the strength of a particular excitonic state are due to surface interactions with ambient environment. In addition, spatial non-uniformity is probed by studying variations of strain and the PL emission as a function of position on our sample. The boundaries of mechanically exfoliated MX2 as well as boundaries intentionally created via fs laser ablation were investigated. The edges exhibit significant Raman shifts as well as remarkably enhanced PL emission compared to their respective central area. Finally, we probe the degree of circular polarization of the emitted PL as a function of the photo-excitation energy and temperature to elucidate spin-dependent inter- and intra-valley relaxation mechanisms. This work was supported by the FP7-REGPOT-2012-2013-1, under grant agreement 316165.
George Kioseoglou, Marc Currie, Aubrey T. Hanbicki, Ioannis Paradisanos, Emmanuel Stratakis, Costas Fotakis, and Berrend T. Jonker, "Optical control of exciton states and enhanced valley Zeeman splitting in WS2
(Conference Presentation)," Proc. SPIE 9931, Spintronics IX, 99313F (Presented at SPIE Nanoscience + Engineering: August 31, 2016; Published: 4 November 2016); https://doi.org/10.1117/12.2236911.5167075286001.
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