Interaction of Thermally Activated and Molecular Oxygen with Hydrogenated and Hydrogen-Free Diamond Film Surfaces: Chemical Reactivity and Electron Emission Properties

Abstract

In the present work we study the processes involved in the interaction of molecular and thermally activated oxygen with hydrogen-terminated and hydrogen-free polycrystalline diamond surfaces. As revealed by H+ photodesorption, near edge X-ray absorption fine structure and X-ray photoelectron spectroscopies, molecular oxygen does not adsorb onto the hydrogenated diamond surface; however, thermally activated oxygen does adsorb onto it. The dominant reaction pass leads to the formation of surface C–O–H bonds, although some abstraction of chemisorbed hydrogen happens as well. On the surface regions, where such an abstraction takes place, C=O (ads) bonds are mostly produced. The hydrogen-free diamond surface is considerably more reactive: the interaction of thermally activated oxygen with it produces C=O and C–O–C bonds, and unactivated oxygen molecules are adsorbed predominantly as C=O. The surface concentration of oxygen, however, is lower in the case of the exposure to unactivated molecules. As established by photon induced secondary electron emission (SEE) measurements, the hydrogen-terminated, oxygen-free diamond surface exhibits negative electron affinity and high SEE intensity. The adsorption of thermally activated oxygen results in a slightly positive electron affinity (PEA) (∼0.4 eV) and in a reduction in the intensity of SEE, whereas the electron emission properties of this sample exposed to molecular oxygen seem to be unaffected. The clean diamond surface has already a low SEE intensity along with a PEA of ∼1.2 eV. This is why the adsorption of molecular oxygen has little effect on its electron emission properties. A treatment of this surface with activated oxygen results in a further deterioration of the electron emission properties as revealed by a PEA of ∼1.7 eV and a poor SEE intensity.