Unlocking ultrafast hot hole transport in transition metal oxides governed by the nature of optical transitions Keming Li 1 , Mischa Bonn 2 , Hai I. Wang 2,3 , Tong Zhu 1 1 Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing, China, 2 Max Planck Institute for Polymer Research, Mainz, Germany, 3 Nanophotonics, Debye Institute for Nanomaterials Science, Utrecht University, Utrecht, the Netherlands The low carrier mobility in transition metal oxides (TMOs) within the polaron transport framework is widely recognized as a key limitation to their optoelectronic performance. While optical transitions have been recently proposed to profoundly impact TMOs' charge generation and transport dynamics, the underlying mechanisms remain poorly understood. Here, we combine ultrafast optical nanoscopy with terahertz (THz) spectroscopy to investigate the nature and transport capabilities of non-equilibrium hot carriers by varying the pump photon energy hʋ . We report two distinct hole transport regimes in the typical TMOs, Co 3 O 4 and α-Fe 2 O 3 : band-like transport of energetic hot holes in the first several picoseconds (ps) with ultrahigh diffusivity (on the order of 100 cm 2 /s) and slower polaron-dominated hopping transport (down to 10 -3 cm 2 /s) thereafter. More importantly, we demonstrate an effective modulation of hot hole diffusion constant by selective optical excitations: contrary to the intuition that increasing pump photon energy hʋ can always enhance hot carrier diffusion, our result demonstrates that both the TMO composition and the nature of the optical excitations can play a fundamentally important role in tailoring sub- ps hot carrier transport dynamics. In Co 3 O 4 , hot holes generated by metal-to-metal charge transfer under 1.55 eV excitation exhibit an ultrahigh diffusion constant of 290 cm 2 /s, seven times higher than that of hot holes generated by more energetic photons, e.g. 2.58 eV excitations via ligand-to-metal charge transfer. We further confirm the highly delocalized nature of hot holes in Co 3 O 4 by ultrafast THz photoconductivity measurements. Our findings underscore the pivotal role of transient hot carrier dynamics and suggest new pathways for optimizing energy management in optoelectronic and photocatalytic applications by controlling photoexcited states.
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