Neutral and charged excitons interplay in non-uniformly strain-engineered WS₂

We investigate the response of excitons in two-dimensional semiconductors to nonuniformity of mechanical strain. In our approach to non-uniform strain-engineering, a WS2 monolayer is suspended over a triangular hole. Large (>2%), strongly non-uniform (>0.28% µm-1), and in-situ tunable strain is induced in WS2 by pressurizing it with inert gas. We observe a pronounced shift of the spectral weight from neutral to charged excitons at the center of the membrane, in addition to well-known strain-dependent bandgap modification. We show that the former phenomenon is a signature of a new effect unique for non-uniform strain: funneling of free carriers towards the region of high strain followed by neutral to charged exciton conversion. Our result establishes non-uniform strain engineering as a novel and useful experimental 'knob' for tuning optoelectronic properties of 2D semiconductors.