| Due to its unique property of negative electron affinity, diamond photoemission has been studied extensively in vacuum but only recent studies have examined its ability to photoemit electrons into aqueous systems. These studies showed that the photoemission of electrons into aqueous systems leads to the creation of solvated electrons which are capable of catalyzing the selective reduction of CO2 to CO as well as the reduction of N2 to NH 4, reactions that cannot be realized with typical semiconducting catalysts. While this work shows promise, little is understood of how the electron is emitted from the surface and how this process differs from emission into vacuum. In addition, due to diamond's wide indirect band-gap of 5.5 eV, diamond is a relatively weak absorber of light even for photon's whose energy exceeds the band-gap of light.;In the second chapter, I attempted to overcome problems associated with diamond's band-gap by increasing the optical absorption of multicrystalline diamond through the use of nitrogen as a substitional n-type dopant, which adds deep-lying mid-gap defect states allowing for the excitation of electrons to the conduction band using lower energy photons. In principle, nitrogen donor states could have allowed for the sequential 2-photon absorption of photons using ~4 eV light, but photophysical and photochemical measurements revealed that my nitrogen-doped diamond films were less effective at photoemission when compared to nominally undoped diamond films. Through characterization of the films and measurements using time-resolved photocurrent measurements, I discovered nitrogen doping decreases the efficiency of electron emission into water. The introduction of nitrogen gas during my diamond growth process leads to smaller grain sizes and greater incorporation of graphitic carbon into the films, reducing the electron diffusion coefficient and decreasing the lifetime of conduction-band electrons.;In the third chapter, I instead investigated the effect the substrate diamond is grown on has on the photoemission from the diamond's surface rather than relying on the absorption of photons within the diamond film. By growing a series of diamond thin films with varying thickness on Mo, Ti, and Nb, I was able to determine for every substrate there is a maximum in the photocatalytic activity that occurs just when the diamond coalesces into a contiguous film, then decreases as the film becomes thicker. The observed maximum in photocatalytic activity suggests the origin of the photoemission from the diamond thin films partly arises from excitation within the substrate. Based on Mott-Schottky analysis, I was also able to establish band-alignment at the diamond-water interface for the diamond films in that study.;This investigation is continued in chapter 4 where I demonstrated photoemission from diamond thin films grown on 100 nm layers of Nd (&phis;M = 3.2 eV) for the first time and showed that a low work function substrate leads to a lower photon energy threshold for photoemission from diamond thin films than traditional diamond growth substrates such as Mo, Ti, or Nb. Through experiments done with diamond thin films grown on Si, I also show increases in minority charge carrier lifetimes within the substrate significantly enhance photoemission from diamond thin films.;Lastly, the work in chapter 5 focused on understanding photoemission from diamond into gaseous environments. In that investigation, I showed that photoemission from diamond into gases results in atmospheric-pressure photoemission whose photocurrent depends on the relevant mobilities based on the gases present. |
