Inside Back Cover In article number 2201260, Zhu and co‐workers demonstrate an innovative strategy to directly track the evolution of hot electron temperature distribution with simultaneously high temporal and spatial resolution. An anomalous contraction of hot electron distribution unravels the competition between transport and cooling. This work enriches the tool kit of ultrafast techniques and provides guidance for rational design of micro/nanodevices.

Elucidation of hot carrier transport and cooling mechanisms at the micro‐/nanoscale is critical for optoelectronics, thermal management, and photocatalysis. Spatiotemporal evolution of hot electrons is usually convoluted with their ultrafast dynamics. Herein, an ultrafast microscopy is employed to directly track the spatiotemporal distribution of photoexcited hot electrons, providing a transformative approach to unravel the competitive relationship of transport and cooling. In the temporal evolution profiles of hot electron distribution, an anomalous contracting stage showing obvious thickness and fluence dependency is observed, with a characteristic end time indicating the completion of electron–phonon (e‐ph) thermalization. Hot electron transport plays a prominent role in the competition with e‐ph coupling, while interfacial heat dissipation dominates nonequilibrium state evolution with thickness below ballistic length. This work significantly enriches the tool kit of ultrafast techniques and provides guidance for rational design and optimization of micro‐/nanodevices.

MXenes have garnered significant interest in energy conversion and catalysis due to their exceptional optical and electronic properties. The extended lifetime of photocarriers and high carrier mobility can be attributed to large polarons in MXenes. Therefore, it is crucial to comprehend the fundamental dynamics of polarons in MXenes. Here, we report a comprehensive analysis of the small polarons using ultrafast spectroscopy. Small polarons are found to form efficiently under transversal surface plasmon (SP) excitation. Under interband transition (IBT) and longitudinal SP excitation, small polaron formation is less efficient. In the thermal migration stage, multipolar surface plasmon excitation measurements reveal that heat flows both vertically and laterally across the flakes under transversal SP and IBT excitation and predominantly migrates laterally in plane under longitudinal SP excitation. Our findings offer critical insights into the polaron dynamics of MXenes and are valuable in the advancement of MXene-based devices.

No abstract