Photoelectron Emission at Semiconductor-Liquid Interfaces

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

PART 1: NON-TECHNICAL SUMMARY This research project, supported by the Solid State and Materials Chemistry program at NSF, focuses on new approaches to achieving light-induced electron emission into water and other non-vacuum environments using solid-state materials. Electrons in water and other liquids have many unusual and important properties, including the ability to initiate very difficult chemical reactions. While nearly all other electron-emitting materials are unstable in water, diamond thin films are chemically stable and able to emit electrons into water, air, and other non-vacuum environments, but only when illuminated with ultraviolet light. In this project, researchers are conducting fundamental research on diamond films and other solid-state materials with the goal of enhancing efficiency and stability of light-induced electron emission. Research includes investigating the fundamental mechanisms involved in light-induced electron emission and exploring new approaches to enhancing the efficiency of this process by manipulating the optical properties of laboratory-grown diamond films. This work could lead to a new generation of highly stable, versatile and efficient electron emitters that could be used to initiate high-energy chemical reactions and would have broader use in other technologies such as optical detectors. This project incorporates advanced training and professional development opportunities for students and postdoctoral scholars and includes efforts to increase the diversity of the scientific workforce by providing summer research opportunities for students from under-represented groups. PART 2: TECHNICAL SUMMARY This research, supported by the Solid State and Materials Chemistry program at NSF, is aimed at understanding the atomic-scale factors that control the ability of diamond and related wide-bandgap semiconductors to emit electrons into water and other non-vacuum environments. Hydrogen-terminated surfaces of diamond are chemically stable and exhibit negative electron affinity, thereby yielding barrier-free emission of conduction-band electrons into water and other non-vacuum environments. However, excitation of electrons across the diamond bandgap requires deep ultraviolet light with wavelengths less than 220 nm. This research explores the formation, optical properties, and photoelectrochemical properties of heterostructures coupling diamond with optically active materials that can inject electrons into its conduction band more efficiently using longer-wavelength light. One approach involves incorporating nanoparticles that have low workfunctions or plasmonic resonances into diamond films. Detailed measurements of photoelectrochemical response and electron emission properties as a function of wavelength and other parameters are being used to extract fundamental insights into the mechanisms of electron excitation and emission. Exploratory work is being performed using other wide-bandgap materials with high-lying conduction bands. Graduate students and postdoctoral scholars supported on this project receive extensive mentoring and professional development opportunities. This project also supports summer research for students from under-represented groups and broader efforts to enhance diversity of the scientific work force. Ultimately this research provides fundamental new insights into the nature of internal photoemission processes and may lead to new materials and structures that can act as stable, energy-efficient electron emitters into water and other non-vacuum environments. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.