First-principles molecular dynamics simulations are used to gain an atomistic-level insight into how the molecular behavior of interfacial water is influenced by specific surface adsorbates. Although the overall hydrophobic versus hydrophilic character of a given surface is widely recognized to be important in determining the behavior of interfacial water molecules, we show that subtle molecular details may also play a role in determining the dynamical behavior of water. By comparing water diffusivity at three different nonpolar surfaces, we find that specific surface features can lead to a suppression of hydrogen bond network ring structures by enhancing hexagonal spatial distributions of water molecules near the surface. Such a distinct molecule-dependent behavior of the interfacial water was found to persist well into the liquid, while most structural properties are noticeably influenced in only the first water layer.

Excited electron dynamics at semiconductor–molecule interfaces is ubiquitous in various energy conversion technologies. However, a quantitative understanding of how molecular details influence the quantum dynamics of excited electrons remains a great scientific challenge because of the complex interplay of different processes with various time scales. Here, we employ first-principles electron dynamics simulations to investigate how molecular features govern the dynamics in a representative interface between the hydrogen-terminated Si(111) surface and a cyanidin molecule. Hot electron transfer to the chemisorbed molecule was observed but was short-lived on the molecule. Interfacial electron transfer to the chemisorbed molecule was found to be largely decoupled from hot electron relaxation within the semiconductor surface. While the hot electron relaxation was found to take place on a time scale of several hundred femtoseconds, the subsequent interfacial electron transfer was slower by an order of magnitude. At the same time, this secondary process of picosecond electron transfer is comparable in time scale to typical electron trapping into defect states in the energy gap.

The role of surface termination on phonon-mediated relaxation of an excited electron in quantum dots was investigated using first-principles simulations. The surface terminations of a silicon quantum dot with hydrogen and fluorine atoms lead to distinctively different relaxation behaviors, and the fluorine termination shows a nontrivial relaxation process. The quantum confined electronic states are significantly affected by the surface of the quantum dot, and we find that a particular electronic state dictates the relaxation behavior through its infrequent coupling to neighboring electronic states. Dynamical fluctuation of this electronic state results in a slow shuttling behavior within the manifold of unoccupied electronic states, controlling the overall dynamics of the excited electron with its characteristic frequency of this shuttling behavior. The present work revealed a unique role of surface termination, dictating the hot electron relaxation process in quantum-confined systems in the way that has not been considered previously.

•Quantum Monte Carlo (QMC) was employed for describing charge transfer in NaCl dimer.•Fixed node approximation was examined by obtaining density via reputation QMC.•Slater determinants of single-particle orbitals were used for the fermion nodes.•Dependence of charge transfer was noticeable although energy was found insensitive.The phenomenon of ion pairing in aqueous solutions is of widespread importance in chemistry and physics, and charge transfer between the ions plays a significant role. We examine the performance of quantum Monte Carlo (QMC) calculations for describing the charge transfer behavior in a NaCl dimer. The influence of the fermion nodes is investigated by obtaining the electron density using the reptation Monte Carlo approach. The fermion nodes are given by single-particle orbitals in Slater–Jastrow trial wavefunctions. We consider the single-particle orbitals from Hartree–Fock and density functional theory calculations with several exchange-correlation approximations. Appreciable dependence of the charge transfer on the fixed-node approximation was found although the total energy was found to be rather insensitive. Our work shows that a careful examination of the fixed-node approximation is necessary for quantifying charge transfer in QMC calculations even when other properties such as reaction energetics are insensitive to the approximation.

The staggered alignment of quasiparticle energy levels is widely regarded to be the key criterion necessary for electron–hole charge separation to occur at heterogeneous material interfaces. However, staggered energy levels at nanoscale interfaces, such as those between organic molecules and inorganic quantum dots, do not necessarily imply charge separation across the interface because the excitonic effect is often significant. Using quantum Monte Carlo calculations, we perform a detailed study of the role of the excitonic effects on charge separation across a representative set of interfaces between organic molecules and quantum dots. We find that the exciton binding energy of charge transfer excitons is significantly larger than would be estimated from a simple Coulombic analysis and, at these nanoscale interfaces, can be as significant as that of Frenkel excitons. This implies that charge transfer excitons can act as trap states and facilitate electron−hole recombination instead of charge separation. We conclude that in general, for nanoscale interfaces, high-fidelity quantum many-body calculations are essential for an accurate evaluation of the detailed energetic balance between localized and delocalized excitons and, thus, are crucial for the predictive treatment of interfacial charge separation processes.

We performed molecular dynamics simulations on four types of systems containing ion and solvating water. Two systems contained a cation (Na+ or K+), and two other systems an anion (Cl– or I–). Classical molecular dynamics simulations were performed using three different force fields: a fixed charge force field, a polarizable force field that includes explicit polarization, and also a recently developed force field that includes polarization and charge transfer. These simulations were then compared to first-principles molecular dynamics simulations. While the first-principles simulations showed that the anions accelerated water translational diffusion, the cations slowed it down. In simulations with the classical force fields, only the force field that incorporates explicit charge transfer reproduced this ion-specific behavior. Additional simulations performed to understand the effect of charge transfer demonstrated that two competitive factors determine the behavior of water translational diffusion: the ions diminished charge accelerates water, while the net charge acquired by water either accelerates or slows down its dynamics. Our results show that charge transfer plays a crucial role in governing the water dynamics in aqueous ionic solutions.Keywords: charge redistribution; first-principle molecular dynamics; molecular dynamics; polarizable force field; water diffusion;

Photocatalytic reduction of carbon dioxide (CO2) into hydrocarbons is an attractive approach for mitigating CO2 emission and generating useful fuels at the same time. Titania (TiO2) is one of the most promising photocatalysts for this purpose, and nanostructured TiO2 materials often lead to an increased efficiency for the photocatalytic reactions. However, what aspects of and how such nanomaterials play the important role in the improved efficiency are yet to be understood. Using first-principles calculations, reaction mechanisms on the surface of bulk anatase TiO2(101) and of a small TiO2 nanocluster were investigated to elucidate the role of four-fold coordinated titanium atoms and quantum confinement (QC) in the CO2 reduction. Significant barrier reduction observed on the nanocluster surface is discussed in terms of how the under-coordinated titanium atoms and QC influence CO2 reduction kinetics at surface. It is shown that the reduction to CO can be greatly facilitated by the under-coordinated titanium atoms, and they also make CO2 anion formation favorable at surfaces.

First-principles molecular dynamics simulations are used to gain an atomistic-level insight into how the molecular behavior of interfacial water is influenced by specific surface adsorbates. Although the overall hydrophobic versus hydrophilic character of a given surface is widely recognized to be important in determining the behavior of interfacial water molecules, we show that subtle molecular details may also play a role in determining the dynamical behavior of water. By comparing water diffusivity at three different nonpolar surfaces, we find that specific surface features can lead to a suppression of hydrogen bond network ring structures by enhancing hexagonal spatial distributions of water molecules near the surface. Such a distinct molecule-dependent behavior of the interfacial water was found to persist well into the liquid, while most structural properties are noticeably influenced in only the first water layer.