Kim Hyeon‐Deuk

Multiple exciton generation and recombination (MEG and MER) dynamics in semiconductor quantum dots (QDs) are simulated using ab initio time-dependent density functional theory in combination with nonadiabatic molecular dynamics. The approach differs from other MEG and MER theories because it provides atomistic description, employs time-domain representation, allows for various dynamical regimes, and includes electron-phonon interactions. MEG rapidly accelerates with energy, reflecting strong ene

We developed time-domain ab initio simulation of Auger phenomena, including multiple exciton generation (MEG) and recombination (MER). It is the first approach describing phonon-assisted processes and early dynamics. MEG starts below the electronic threshold, strongly accelerating with energy. Ligands are particularly important to phonon-assisted MEG, which therefore can be probed with infrared spectroscopy. Short-time gaussian component gives 5-10% of MEG, justifying rate theories that assume e

This paper reports on durable and nearly temperature-independent (at 298-328 K) T-type photochromism of colloidal Cu-doped ZnS nanocrystals (NCs). The color of Cu-doped ZnS NC powder changes from pale yellow to dark gray by UV light irradiation, and the color changes back to pale yellow on a time scale of several tens of seconds to minutes after stopping the light irradiation, while the decoloration reaction is accelerated to submillisecond in solutions. This decoloration reaction is much faster

State-of-the-art time domain density functional theory and non-adiabatic (NA) molecular dynamic simulations are used to study phonon-induced relaxation of photoexcited electrons and holes in Ge and Si quantum dots (QDs). The relaxation competes with productive processes and causes energy and voltage losses in QD solar cells. The ab initio calculations show that quantum confinement makes the electron and hole density of states (DOS) more symmetric in Si and Ge QDs compared to bulk. Surprisingly,

Photoexcited dynamics of electrons and holes in semiconductor quantum dots (QD), including phonon-induced relaxation, multiple exciton generation, fission and recombination (MEG, MEF and MER), were simulated by combining ab initio time-dependent density functional theory and non-adiabatic molecular dynamics. These nonequilibrium phenomena govern the optical properties and photoexcited dynamics of QDs, determining the branching between electronic processes and thermal energy losses. Our approach

Quantum confinement in nanoscale materials allows Auger-type electron-hole energy exchange. We show by direct time-domain atomistic simulation and analytic theory that Auger processes give rise to a new mechanism of charge transfer (CT) on the nanoscale. Auger-assisted CT eliminates the renown Marcus inverted regime, rationalizing recent experiments on CT from quantum dots to molecular adsorbates. The ab initio simulation reveals a complex interplay of the electron-hole and charge-phonon channel

Auger processes, multiple exciton generation, and Auger recombination, provide and disturb a potential route to increase solar cell efficiencies by creating multiple charge carriers, respectively. Physical mechanisms of the Auger processes can be deduced from the temperature dependence. Our real-time ab initio simulation found logarithmic temperature dependence of the Auger rates in semiconductor quantum dots (QDs), which agrees well with the recent experimental observations. This anomalous temp

Effects of the exciton-exciton coherence transfer (EECT) in strongly coupled molecular aggregates are investigated from the reduced time-evolution equation which we have developed to describe EECT. Starting with the nonlinear response function, we obtained explicit contributions from EECT to four-wave-mixing spectrum such as photon echo, taking into account double exciton states, static disorder, and heat-bath coupling represented by arbitrary spectral densities. By using the doorway-window pict

Liquid para-hydrogen (p-H2) is a typical quantum liquid which exhibits strong nuclear quantum effects (NQEs) and thus anomalous static and dynamic properties. We propose a real-time simulation method of wave packet (WP) molecular dynamics (MD) based on non-empirical intra- and inter-molecular interactions of non-spherical hydrogen molecules, and apply it to condensed-phase p-H2. The NQEs, such as WP delocalization and zero-point energy, are taken into account without perturbative expansion of pr

Semiquantum liquid water molecular dynamics simulation was developed using the time-dependent Hartree approach. The classical intra- and intermolecular potential functions of water were extended to describe the wave packet (WP) hydrogen atoms. The equations of motion with an extended phase space including auxiliary coordinates and momenta representing the hydrogen WP widths were derived and solved. The molecular dynamics simulation of semiquantum water demonstrated that the semiquantum hydrogen

Quantum effects such as zero-point energy and delocalization of wave packets (WPs) representing water hydrogen atoms are essential to understand anomalous energetics and dynamics in water. Since quantum calculations of many-body dynamics are highly complicated, no one has yet directly viewed the quantum WP dynamics of hydrogen atoms in liquid water. Our semiquantum molecular dynamics simulation made it possible to observe the hydrogen WP dynamics in liquid water. We demonstrate that the microsco

Nuclear quantum effects play a dominant role in determining the phase diagram of H2. With a recently developed quantum molecular dynamics simulation method, we examine dynamical and structural characters of solid H2 under vapor pressure, demonstrating the difference from liquid and high-pressure solid H2. While stable hexagonal close-packed lattice structures are reproduced with reasonable lattice phonon frequencies, the most stable adjacent configuration exhibits a zigzag structure, in contrast

Supercooled hydrogen liquid as well as superfluid have continued to elude experimental observation due to rapid crystallization. We computationally realized and investigated supercooled hydrogen liquid by a recently developed non-empirical real-time molecular dynamics method, which describes non-spherical hydrogen molecules with the nuclear quantum effects. We demonstrated that the hydrogen supercooled liquid is not a simply cooled liquid but rather exhibits intrinsic structural and dynamical ch

We have calculated the nonlinear response function of a DNA duplex helix including the contributions from the exciton population and coherence transfers by developing an appropriate exciton theory as well as by utilizing a projector operator technique. As a representative example of DNA double helices, the B-form (dA)10-(dT)10 is considered in detail. The Green functions of the exciton population and coherence transfer processes were obtained by developing the DNA exciton Hamiltonian. This enabl