Ultrafast carrier dynamics in colloidal WS2 nanosheets

Transition metal dichalcogenides (TMDs) have attracted much research interest in recent years due to their intriguing optical and electrical properties. Insights in the photophysics of TMDs are necessary to advance their use in optoelectronic applications. Much of the work to date mainly concentrated on the TMDs nanosheets obtained by exfoliation or chemical vapor deposition. Considering colloidal synthesis can be used to produce TMDs nanosheets in large quantities with controllable dimensions and surface properties. Unravelling the fundamental phenomena of colloidal TMDs nanosheets upon photoexcitation is thus important to advance their further application in optoelectrics and photonics. Here, we report the carrier dynamics in WS2 nanosheets obtained through colloidal synthesis by use of broadband ultrafast transient absorption (TA) spectroscopy. We show that state filling by electrons and holes contributes to the bleach of A and B exciton features, whereas a strong band gap reduction leads to an overall red-shift of the spectra, as previously reported for TMDs nanosheets. By means of a detailed deconvolution of the complete absorption spectrum, we find a common ultrafast timescale for the A and B-exciton bleach recovery, indicating fast electron capture by defects. Given that similar carrier dynamics have been demonstrated in CVD-grown WS2 nanosheets, we conclude that a bottom-up synthesis can produce nanosheets of comparable quality as more commonly used techniques, yet has the potential to be used in solution-based processes with associated merits of cost-effectiveness and increased compatibility with different substrates