Detecting the Photoexcited Carrier Distribution Across GaAs/Transition Metal Oxide Interfaces by Coherent Longitudinal Acoustic Phonons.

A prominent architecture for solar energy conversion layers diverse materials, such as traditional semiconductors (Si, III-V) and transition metal oxides (TMOs), into a monolithic device. The efficiency with which photoexcited carriers cross each layer is critical to device performance and dependent on the electronic properties of a heterojunction. Here, by time-resolved changes in the reflectivity after excitation of an n-GaAs/p-GaAs/TMO (Co3O4, IrO2) device, we detect a photoexcited carrier distribution specific to the p-GaAs/TMO interface through its coupling to phonons in both materials. The photoexcited carriers generate two coherent longitudinal acoustic phonons (CLAPs) traveling in opposite directions, one into the TMO and the other into the p-GaAs. This is the first time a CLAP is reported to originate at a semiconductor/TMO heterojunction. Therefore, these experiments seed future modeling of the built-in electric fields, the internal Fermi level, and the photoexcited carrier density of semiconductor/TMO interfaces within multilayered heterostructures.

In situ X-ray absorption spectroscopy of transition metal based water oxidation catalysts.

X-ray absorption studies of the geometric and electronic structure of primarily heterogeneous Co, Ni, and Mn based water oxidation catalysts are reviewed. The X-ray absorption near edge and extended X-ray absorption fine structure studies of the metal K-edge, characterize the metal oxidation state, metal-oxygen bond distance, metal-metal distance, and degree of disorder of the catalysts. These properties guide the coordination environment of the transition metal oxide radical that localizes surface holes and is required to oxidize water. The catalysts are investigated both as-prepared, in their native state, and under reaction conditions, while transition metal oxide radicals are generated. The findings of many experiments are summarized in tables. The advantages of future X-ray experiments on water oxidation catalysts, which include the limited data available of the oxygen K-edge, metal L-edge, and resonant inelastic X-ray scattering, are discussed.

Primary steps in the reaction of OH radicals with peptide systems: perspective from a study of model amides.

This study reports on an ab initio investigation into the effects of radical attack on peptide backbones using eight model amides. The eight model amides are divided into two subcategories, formamide and acetamide. Within each subcategory, there are four unique systems which include the parent, cis and trans methyl, and the dimethyl structures. The mechanism for hydrogen abstraction shows alpha-C abstraction is kinetically and thermodynamically favorable for all formamide systems. The addition of a methyl group on the nitrogen results in a secondary competitive pathway at the gamma-C site. In the parent acetamide, beta-abstraction is preferred. The addition of a methyl group on the amino group of acetamide results in the introduction of a secondary pathway that will outcompete beta-abstraction, both thermodynamically and kinetically. For all systems, H-abstraction off the nitrogen is the least preferred path. With the addition of a methyl group on the nitrogen, H-abstraction off the nitrogen becomes more favorable. To explain site preference, geometry structures and stabilizations are examined. The results of this study have implications for future studies on radical attack of peptide and protein systems.