The influence of the environment on monolayer tungsten diselenide photoluminescence

Abstract

We report on a comprehensive experimental analysis of the influence of the environment on the optical properties of monolayer WSe2. Different dielectric-environmental configurations with hBN as buffer and encapsulant were studied at room temperature and 10 K using time-integrated and time-resolved photoluminescence spectroscopy. Furthermore, Raman signal was evaluated to highlight the influence of the surroundings on the monolayer system. Here, linewidth and intensity improvement for 2D excitons as a consequence of hBN-encapsulation is demonstrated, and a change of strain and doping levels in our produced structures are observed. The increase of the exciton radius for hBN-buffered samples in comparison to WSe2 on bare substrates is indicated via an exciton–exciton annihilation study, while hBN as buffer is found to generally shorten the effective radiative lifetime. Furthermore, we report that trions experience a weaker interaction with the WSe2 phonons than excitons, while the coupling to phonons is consistently decreased when WSe2 is capped by hBN. Ultimately, our energy-resolved analysis of the dynamics at 10 K shows that the individual excitonic modes exhibit different photoluminescence decay times. A comparison of the different hBN-WSe2 configurations shows that the shortest measurable time constants, which are on the ps-scale, generally increase when buffering or encapsulating the 2D semiconductor, while sandwiching caused the strongest lifetime increase on these short time scales. In contrast, our monolayer–monolayer heterostructures indicate fast charge/energy transfer (<1 ps) from MoSe2 to WSe2 and a comparably slow radiative recombination of the type-I A excitons depending on the tilt angle between the two lattices.