Fig. 4.

Self-trapping process. a Scheme showing two-step formation process of STEs in Sb₂S₃: hole is self-trapped first and then electron is captured by trapped hole to form STE. b Carrier-trapping kinetics (open symbols) under different photoexcited carrier densities N₀ (5 × 10¹⁸ cm⁻³ to 1.2 × 10²⁰ cm⁻³) for Sb₂S₃ single-crystal flake and their fits to two-step formation mechanism (solid lines). c Qualitative adiabatic potential energy curve showing photoexcitation from ground state (GS) to excited state (ES), which further evolves into STE state by deforming lattice and losing substantial energy