Lattice Expansion Enables Large Surface Carrier Diffusion in WS₂ Monolayer

Two-dimensional (2D) materials hold great promise for next-generation optoelectronic devices, with photogenerated charge carrier transport being critical to their performance. However, the influence of photoexcitation-induced commensurate lattice thermal effects on surface charge carrier dynamics is poorly understood. Traditional photon-pump/photon-probe methods have constraints in capturing the subtle yet critical surface dynamics, especially for these ultrathin materials due to challenges in spatial resolution and penetration depth. In this study, we utilized scanning ultrafast electron microscopy (SUEM), a technique that offers unparalleled sensitivity to surface phenomena that are entirely inaccessible through other methods. Our findings reveal a ∼1.4% negative thermal expansion at elevated temperatures, inducing internal strain that modifies the electronic structure and significantly enhances surface carrier transport, resulting in an order-of-magnitude improvement in photodetection performance. Moreover, we demonstrate that photoinduced charge carrier diffusion occurs predominantly within the first tens of picoseconds after photoexcitation, a regime characterized by thermal excitation resulting from carrier-phonon interactions. These results establish a direct link among lattice thermal expansion, carrier dynamics, and optoelectronic performance.