We investigate the transport of excitons and trions in monolayer semiconductor WS 2 subjected to controlled non-uniform mechanical strain. We actively control and tune the strain profiles with an AFM-based setup in which the monolayer is indented by an AFM tip. Optical spectroscopy is used to reveal the dynamics of the excited carriers. The non-uniform strain configuration locally changes the valence and conduction bands of WS 2 , giving rise to effective forces attracting excitons and trions towards the point of maximum strain underneath the AFM tip. We observe large changes in the photoluminescence spectra of WS 2 under strain, which we interpret using a drift-diffusion model. We show that the transport of neutral excitons, a process that was previously thought to be efficient in non-uniformly strained 2D semiconductors and termed as funneling, is negligible at room temperature in contrast to previous observations. Conversely, we discover that redistribution of free carriers under non-uniform strain profiles leads to highly efficient conversion of excitons to trions. Conversion efficiency reaches 100% even without electrical gating. Our results explain inconsistencies in previous experiments and pave the way towards new types of optoelectronic devices.

We present for the first time an experimental setup that strains 2D materials with an AFM and with optical access. We find that the main mechanism is the efficient conversion of excitons to charged trions.