Prolonged Exciton Survival with Enhanced Spatial Contraction in Layered WSSe Alloys

The spatial extent of an optically detected excitation population is determined by both particle motion and spatially selective survival, so a more persistent signal need not imply greater spreading. Here, we combine broadband transient transmission spectroscopy, fluence-dependent kinetics, and transient absorption microscopy to follow the A-exciton bleach in monolayer, bilayer, and trilayer nominal WS0.9Se1.1 alloys. Increasing layer thickness produces a pronounced temporal–spatial contrast. The bleach becomes more persistent, whereas its spatial profile undergoes stronger transient contraction. Fluence-dependent kinetics further reveal that enhanced survival after the initial decay can coexist with faster depletion at later times. Joint analysis using a trapping–diffusion model with shared reaction parameters supports a picture in which traps fill more rapidly near the excitation center, suppressing further capture there, while continued capture at the periphery narrows the bleach-active population. Diffusion and density-dependent loss oppose this narrowing and contribute to the subsequent width recovery. These results show that temporal persistence and spatial spreading need not evolve together when relaxation is spatially selective.