High-throughput Toroidal Grating Beamline for Photoelectron Spectroscopy at CAMD

Modifying Metastable Sr1-xBO3-δ (B = Nb, Ta, and Mo) Perovskites for Electrode Materials

The presence of surface/deep defects in 4d- and 5d-perovskite oxide (ABO3, B = Nb, Ta, Mo, etc.) nanoparticles (NPs), originating from multivalent B-site cations, contributes to suppressing their metallic properties. These defect states can be removed using a H2/Ar thermal treatment, enabling the recovery of their electronic properties (i.e., low electrical resistivity, high carrier concentration, etc.) as expected from their electronic structure. Therefore, to engineer the electronic properties of these metastable perovskites, an oxygen-controlled crystallization approach coupled with a subsequent H2/Ar treatment was utilized. A comprehensive study of the effect of the post-treatment time on the electronic properties of these perovskite NPs was performed using a combination of scattering, spectroscopic, and computational techniques. These measurements revealed that a metallic-like state is stabilized in these oxygen-reduced NPs due to the suppression of deep rather than surface defects. Ultimately, this synthetic approach can be employed to synthesize ABO3 perovskite NPs with tunable electronic properties for application into electrochemical devices.

Adsorption of Polarized Molecules for Interfacial Band Engineering of Doped TiO2 Thin Films

Owing to their chemical and mechanical stability, metal-oxides have emerged as potential alternatives for conventional pure-metal and organic molecule-based solid-state electronic devices. Traditionally, band engineering of these metal-oxides has been performed to improve the efficiency of solar cells and transistors. However, recent advancements in the field of oxide-based electronic devices demand reversible band structure engineering for applications in next-generation adaptive electronics and memory devices. Therefore, this work aims to reversibly engineer the surface band structure of doped metal-oxides using stable organic ligands with weak dipoles. Para-substituted benzoic acid (BZA) ligands with positive and negative dipole moments were adsorbed in situ on the surface of TiO₂:Ni²⁺ thin film to modify the interfacial dipole moment, and the valence band structure was probed using surface-sensitive ultraviolet photoelectron spectroscopy (UPS). UPS, paired with density functional theory (DFT) simulations, demonstrate the ability to selectively tune interfacial electronic/chemical landscapes with ligand-dependent dipole moment. The unique ability to reversibly tune the band bending at the organic-inorganic interface of doped metal-oxide semiconductors using molecular dipoles is expected to play a key role in the development of metal-oxide-based adaptive electronics that outperform the conventional polymer-based and Si-based devices.

Probing environmentally significant surface radicals: Crystallographic and temperature dependent adsorption of phenol on ZnO

Environmentally persistent free radicals (EPFRs) are toxic organic/metal oxide composite particles that have been discovered to form from substituted benzenes chemisorbed to metal oxides. Here, we perform photoelectron spectroscopy, electron energy loss spectroscopy, and low energy electron diffraction of phenol chemisorbed to ZnO(1 0 1̱ 0) and (0 0 0 1̱)-Zn to observe electronic structure changes and charge transfer as a function adsorption temperature. We show direct evidence of charge transfer from the ZnO surfaces to the phenol. This evidence can help gain a better understanding of EPFRs and be used to develop possible future remediation strategies.

Electronic signatures of a model pollutant-particle system: chemisorbed phenol on TiO₂(110)

Environmentally persistent free radicals (EPFRs) are a class of composite organic/metal oxide pollutants that have recently been discovered to form from a wide variety of substituted benzenes chemisorbed to commonly encountered oxides. Although a qualitative understanding of EPFR formation on particulate metal oxides has been achieved, a detailed understanding of the charge transfer mechanism that must accompany the creation of an unpaired radical electron is lacking. In this study, we perform photoelectron spectroscopy and electron energy loss spectroscopy on a well-defined model system-phenol chemisorbed on TiO2(110) to directly observe changes in the electronic structure of the oxide and chemisorbed phenol as a function of adsorption temperature. We show strong evidence that, upon exposure at high temperature, empty states in the TiO2 are filled and the phenol HOMO is depopulated, as has been proposed in a conceptual model of EPFR formation. This experimental evidence of charge transfer provides a deeper understanding of the EPFR formation mechanism to guide future experimental and computational studies as well as potential environmental remediation strategies.