A comprehensive understanding of the ultrafast electron dynamics in two-dimensional transition metal dichalcogenides (TMDs) is necessary for their applications in optoelectronic devices. In this work, we contribute a study of ultrafast electron cooling and decay dynamics in the supported and suspended monolayer WS2 by time- and energy-resolved photoemission electron microscopy (PEEM). Electron cooling in the Q valley of the conduction band is clearly resolved in energy and time, on a time scale of 0.3 ps. Electron decay is mainly via a defect trapping process on a time scale of several picoseconds. We observed that the trap states can be produced and increased by laser illumination under an ultrahigh vacuum, and the higher local optical-field intensity led to the faster increase of trap states. The enhanced defect trapping could significantly modify the carrier dynamics and should be paid attention to in photoemission experiments for two-dimensional materials.

For versatile lead-halide perovskite materials, their trap states, both in the bulk and at the surface, significantly influence optoelectronic behaviors and the performance of the materials and devices. Direct observation of the trap dynamics at the nanoscale is necessary to understand and improve the device design. In this report, we combined the femtosecond pump–probe technique and photoemission electron microscopy (PEEM) to investigate the trap states of an inorganic perovskite CsPbBr3 single-crystal microplate with spatial–temporal–energetic resolving capabilities. Several shallow trap sites were identified within the microplate, while the deep traps were resolved throughout the surface. The results revealed high-defect tolerance to the shallow traps, while the surface dynamics were dominated by the surface deep traps. The ultrafast PEEM disclosed a full landscape of fast electron transfer and accumulation of the surface trap states. These discoveries proved the excellent electronic properties of perovskite materials and the importance of surface optimization.

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Cd 3 As 2 , as a three-dimensional Dirac semimetal, has attracted considerable research interest because of its distinct optoelectronic and topological properties. However, simultaneous studies on the carrier dynamics in Cd 3 As 2 in spatial, temporal, and energy dimensions are still lacking. Here, we apply photoemission electron microscopy combined with the femtosecond pump− probe technique to reveal the dynamics of the Cd 3 As 2 nanoplate. The photoexcited electrons on the Cd 3 As 2 surface decay fast with an average decay time of 0.68 ps. The electron dynamics are independent of the spatial location by analyzing the decay curves at different positions on the Cd 3 As 2 nanoplate. The effect of boundary states on the relaxation process is negligible. The photoexcited electrons can relax in picoseconds, regardless of the pump or probe power. In the energy-resolved mode, the decay time of nonequilibrium electrons decreases from 7 to 0.25 ps as the energy of hot electrons increases. These results contribute to understanding the photoexcited electron behavior of Cd 3 As 2 .