Excited state dynamics play key roles in numerous condensed phase and molecular materials designed for solar energy, opto-electronics, spintronics and other applications. Controlling these far-from-equilibrium processes and steering them in desired directions require understanding of material’s response on the nanometer scale and with fine time resolution. We couple, in a unique way, real-time time-dependent density functional theory for the evolution of electrons with non-adiabatic molecular dynamics for atomic motions to model such non-equilibrium response in the time-domain and at the atomistic level. The talk will describe the basics of the simulation methodology and will discuss several recent applications, such as metal halide perovskites, metallic and semiconducting quantum dots, and transition metal dichalcogenides, among the broad variety of systems studied in our group.
Time-domain non-adiabatic ab initio simulations are performed to study the phonon-assisted hot electron relaxation
dynamics in CdSe spherical quantum dots (QDs) and elongated quantum dots (EQDs). EQDs have a narrower band gap
and denser electron and hole energy states than QDs. As temperature increases, band gap values will become smaller due
to thermal expansion effect. Also more phonons are excited to scatter with electrons and thus result in a higher relaxation
rate for hot electrons. Besides, it is also found in our simulation that hot electron relaxation rate in EQDs has a weaker
temperature dependence than in QDs, which could be attributed to the larger thermal expansion in EQDs.
Temperature dependent dynamics of phonon-assisted relaxation of hot carriers, both electrons and holes, is
studied in a PbSe nanocrystal using ab initio time-domain density functional theory. The electronic structure
is first calculated, showing that the hole states are denser than the electron states. Fourier transforms of the
time resolved energy levels show that the hot carriers couple to both acoustic and optical phonons. At higher
temperature, more phonon modes in the high frequency range participate in the relaxation process due to their
increased occupation number. The phonon-assisted hot carrier relaxation time is predicted using non-adiabatic
molecular dynamics, and the results clearly show a temperature-activation behavior. The complex temperature
dependence is attributed to the combined effects of the phonon occupation number and the thermal expansion.
Comparing the simulation results with experiments, we suggest that the multiphonon relaxation channel is
efficient at high temperature, while the Auger-like process may dominate the relaxation at low temperature.
This combined mechanism can explain the weak temperature dependence at low temperature and stronger
temperature dependence at higher temperature.
The photoinduced electron transfer (ET) from a molecular electron donor to the TiO2 semiconductor acceptor triggering Gratzel solar cells and other photochemical applications is investigated. The reported simulations reproduce the experimentally observed ET time-scale, establish the reaction mechanism, and provide a detailed picture of the ET process. The electronic structure of the chromophore-semiconductor system is simulated by density functional theory (DFT). Ab initio molecular dynamics (MD), including non-adiabatic (NA)transitions between electronic states, NAMD, is used to follow the ET reaction in real-time and at the molecular level. The simulation indicates that thermally driven adiabatic ET s dominant at room temperature. Vibrational motions of the chromophores induce oscillations of the photoexcited state energy that drives the photoexcited state in and out of the TiO2 conduction band. Two distinct types of ET events are observed depending on the initial
conditions. At low initial energies the photoexcited state is well localized on the chromophore, and an activation is required for ET, with comparable contributions from both the adiabatic
and NA mechanisms. At high initial energies the photoexcited state is already substantially delocalized into the TiO2 substrate. The remaining fraction of the ET process occurs rapidly and by the adiabatic mechanism.
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