Mechanisms of Sub-Bandgap Electron Emission in Vacuum and in Water from Diamond and Diamond-Encapsulated Metal Nanoparticles

While it is well known that electron emission from H-terminated diamond can be stimulated using above-bandgap light, sub-bandgap wavelengths can also induce electron emission into vacuum. Recently there has been intense interest in understanding electron emission into water because of the unique photochemistry that can be produced. When emitted into water, electrons can solvate allowing diamond to be used as a photocatalyst for normally high energy reactions such as the reduction of nitrogen to ammonia.¹Because diamond’s wide bandgap can be bridged only photons of 5.5 eV energy, there is interest in understanding the mechanisms associated with sub-bandgap excitation. These mechanisms, especially in water, remain poorly understood.<br/>In this study, we seek to understand the mechanisms of sub-bandgap electron emission in vacuum and in water through measurements of total electron yield and the energy distribution of emitted electrons using valence-band photoemission spectroscopy stimulated by sub-bandgap light sources. Previous work showed that encapsulating metallic nanoparticles during growth of CVD films has been shown to enhance electron emission at both above and sub-bandgap wavelengths as compared to films without embedded nanoparticles.² However, the mechanism of the observed enhancements is not well understood.<br/>In this talk, we will show energy distributions of electrons emitted from thin film diamond samples with embedded metallic nanoparticles into vacuum at photon energies at 21.2, 6.2, 4.75, and 3.05 eV. By controlling the diamond growth process, we are able to vary the thickness of the diamond shell surrounding the nanoparticles. We directly detect the solvated electrons produced in water using transient absorption spectroscopy. Our work shows strong evidence for the key role played by surface states in electron emission from diamond using sub-bandgap excitation. An additional emission pathway in vacuum is observed from diamond thin films with embedded nanoparticles. These experiments provide insight into possible emission pathways i.e. surface states, defects, or plasmonic enhancements from the metallic nanoparticles.<br/> <br/>References:<br/>Zhu, D.; Zhang, L. H.; Ruther, R. E.; Hamers, R. J., Photo-illuminated diamond as a solid-state source of solvated electrons in water for nitrogen reduction. <i>Nature Materials </i><b>2013,</b> <i>12</i> (9), 836-841<br/>Li, Shuo, Jason A. Bandy, and Robert J. Hamers. Enhanced photocatalytic activity of diamond thin films using embedded Ag nanoparticles. <i>ACS Applied Materials & Interfaces</i> <b>2017</b>, 10 (6), 5395-5403.