Experimental and Theoretical Studies of Electron Transfer Dynamics at Semiconductor-Liquid Interfaces

A. Nozik, National Renewable Energy Laboratory

Research Objectives

Our ER/Chemical Sciences Solar Photochemistry Program involves a coupled experimental and theoretical study of photoinduced and dark electron transfer (ET) between a semiconductor and a molecular species solvated in a liquid next to the semiconductor. The experimental program for study of this semiconductor-liquid interface (SLI) ET involves measurements of its dynamics, using femtosecond and picosecond time-resolved spectroscopy. To interpret the experimental data, and to discover new experimental research directions, we use computationally intensive theoretical methodologies.

Computational Approach

Our computational methods include first-principles molecular-dynamics (FPMD) simulations of the SLI. We have also developed new computational methods of wave packet propagation to study the ET scattering event at the SLI, and new model Hamiltonian approaches for these systems. A second class of methodologies use the semiclassical Monte Carlo, drift-diffusion, and balance-equation approaches to model very-large-scale systems.

Accomplishments

In past years we have developed a number of new theoretical methodologies to address SLI ET. In our FPMD, both electron and nuclear dynamics are realistically described, allowing the full dynamical quantum mechanical structure to be explored. Our new computational methods of wave-packet propagation allow detailed study of the ET scattering event at the SLI. New model Hamiltonian approaches for these systems allow physically transparent interpretation of the simulations. Our FPMD simulations represent among the first applications of FPMD to solid-liquid interface ET.

Key outcomes of these studies are a detailed description of how electrons become relocalized from the redox species to the semiconductor and vice versa; trends in overall electron transfer rate constants (as a function of electric fields, temperature, crystal face, and solvent and redox species); vibrational spectra of molecules at surfaces before and after electron transfer; a large catalogue of local densities of states that is (in principle) explorable by scanning tunneling microscopy (STM); and detailed studies of the time-scale and configurational dependencies of SLI electron transfer.

Significance

Our FPMD simulations have opened up a new window for the understanding of SLI systems, which up to now have been primarily addressed by archaic analytical models. Pronounced shortcomings of conventional models have been revealed, which call into question even their qualitative foundations.

Electron density trapped in a redox species (solvated) next to a semiconductor-liquid interface. The redox species density appears at the bottom of the picture (four atoms have significant electron density). The rest of the electron wave packet is localized around semiconductor atoms (top two-thirds of the picture). This is a snapshot of a dynamic electron transfer simulation, and is a cross section through the 3D system.

Publications

B. B. Smith and A. J. Nozik, "Study of electron transfer at semiconductor-liquid interfaces addressing the full system electronic structure," Chem. Phys. 205, 47-72 (1996).

B. B. Smith and A. J. Nozik, "Theoretical studies of electron transfer and electron localization at the semiconductor-liquid interface," J. Phys. Chem. B 101, 2459-2475 (1997).

A. Meier, D. C. Selmarten, B. B. Smith, and A. J. Nozik, "Ultrafast electron transfer across semiconductor-molecule interfaces: GaAs/Co(Cp)2+/0," J. Phys. Chem. B (in press, 1998).