Prominent exciton emission dynamics and charged exciton (trion)-phonon coupling aspects in γ- irradiated WSe2 nanosheets

We report on the nature of charge-neutral exciton emission decay, exciton-trion conversion as well as their phonon coupling in exfoliated WSe2 nanosheets subjected to similar to 1.3 MeV energetic gamma-photons. Steady-state photoluminescence (PL) spectra, upon deconvolution, showed prominent emission peaks at similar to 680 nm, similar to 712 nm, and similar to 730 nm and ascribed to 2s, and 1s excitons and trions respectively, while the chalcogen vacancy (V-Se) mediated defect emission peak found at similar to 790 nm. From the X-ray photoelectron spectroscopy (XPS) analysis, we noticed a clear shifting of the fundamental peaks due to W and Se toward higher binding energy values and were attributed to the radiation-induced alteration in the oxidation states locally, taking advantage of the manifestation of point defects/vacancies. Moreover, the time-resolved PL spectra signify an anomalous variation in the fast parameter (0.4-0.86 ns), and with slow parameter (2.46-3.82 ns) getting slower with increasing gamma-dose. A slight variation in the exciton-phonon coupling factor is noted with Se-ph < 1 indicating a weaker interaction and featuring maximal envelope over all the phonon replica near the zero-phonon line. In contrast, trion-phonon coupling factor, St-ph > 1, or < 1 depending on its spin degeneracy (zeta = 2 s + 1; with 2, for s = 1/2 and 4 for s = 3/2). The highest St-ph value determined was similar to 1.82 & PLUSMN; 0.2 which declines monotonically with increasing dose but remained greater than unity up to a gamma-dose as high as 96 kGy. The implication of trion-phonon coupling has immense scope in nano-photovoltaics, and nano-electronics as well as heat sinks in nanogenerators.

Automated summary

We investigated the properties of charge-neutral exciton emission decay, exciton-trion conversion, and their phonon coupling in exfoliated WSe2 nanosheets. The study found prominent emission peaks corresponding to 2s excitons, 1s excitons, and trions, as well as a defect emission peak mediated by chalcogen vacancy. The variation in exciton-phonon and trion-phonon coupling factors have potential significance in nano-photovoltaics, nano-electronics, and heat sinks in nanogenerators.