Fig. 4: Exciton diffusion in different energy landscapes.

a The energy landscape generated by strain is characterized by measuring the emission spectrum of the monolayer WS₂ in different positions. Excitation and detection are done at the same location as described in the top panel. The color map in the bottom panel shows the spectral emission normalized to the maximum at different positions. The energy corresponding to the exciton complexes is highlighted by circular markers of different colors: red (X⁰, XX⁰, \({X}_{T}^{-}\), \({X}_{S}^{-}\), \({{XX}}^{-}\)), green (I⁰), yellow (D⁰), blue (T₁), and rose (\({D}_{K3}^{-}\) and \({D}_{\varGamma 5}^{-}\)). All exciton species show an energy shift up to 20 meV corresponding to a linear tensile strain gradient from 0 to 0.4%. b Downhill diffusion of excitons created at the top of the potential gradient (top panel) and measured at increasing distances from the generation location. The diffusion of the dark excitons appears clearly from the spectral response as a function of the distance from the excitation (bottom panel). c Uphill diffusion of excitons created at the bottom of the potential gradient (top panel) and measured at increasing distances from the generation location. Differently from the downhill case, only dark excitons propagate far away from the excitation spot. After propagation due to repulsive interactions, dark excitons relax towards the bottom of their dispersion and recombine radiatively via different paths, generating I⁰, T₁, and \({D}_{P}^{-}\).