First principles study of the hot electron relaxation dynamics in rutile TiO₂

Abstract

<p indent="0mm">In the process of solar energy conversion, the dynamics of excited hot carriers are crucial in determining the efficiency of energy transfer. We investigated hot electron relaxation in rutile TiO₂ using the non-adiabatic molecular dynamics in momentum space (NAMD_k) method. In this study, we choose hot electrons with two different initial energies (<italic>E</italic>_{<italic>ini</italic>} = 0.64 eV and <italic>E</italic>_{<italic>ini</italic>} = <sc>2.75 eV),</sc> each with momentum located at the Γ point, to study their relaxation dynamics. The results show that the hot electron relaxation process mainly involves two stages: (1) First, the hot electrons are scattered to the iso-energy surface within the first Brillouin zone, forming the hot electron ensemble, which can be understood as the momentum relaxation process. For <italic>E</italic>_{<italic>ini</italic>} = <sc>0.64 eV</sc> and <italic>E</italic>_{<italic>ini</italic>} = <sc>2.75 eV,</sc> the momentum relaxation ranges from 15 to <sc>25 fs.</sc> (2) Subsequently, the hot electron ensemble as a whole gradually relaxes to the conduction band minimum which can be understood as the energy relaxation process. Here, the energy relaxation time of <italic>E</italic>_{<italic>ini</italic>} = <sc>0.64 eV</sc> and <italic>E</italic>_{<italic>ini</italic>} = <sc>2.75 eV</sc> are 39 and <sc>97 fs,</sc> respectively. During the hot electron relaxation, energy is transferred to the TiO₂ lattice through electron-phonon coupling. As an ionic crystal, Fröhlich type electron-phonon coupling plays a key role in the hot electron relaxation, causing most of the energy of the hot electrons to be transferred to the A_1g phonons, which corresponds to the Ti‒O stretching longitudinal optical phonon. Our study provides new physical insights for understanding the hot electron relaxation and energy conversion processes in rutile TiO₂.</p>

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