Oxygen-terminated diamond: insights into the correlation between surface oxygen configurations and work function values

The surface modification of diamond holds crucial significance in various applications, such as electron emission devices, Schottky diodes and field-effect transistors. Oxygen termination is one of the most common termination types, where three distinct carbon-oxygen bonded configurations (i.e. ether, ketone and hydroxyl) are typically found concurrently on the surface. An unanswered question in this context is whether each oxygen-containing functional group influences the tuning of the surface electronic properties of diamond. Here, we systematically explore the surface oxidation of (100)-oriented single-crystal diamond using four different oxidation methods: oxygen plasma exposure, UV-ozone irradiation, acid treatment and oxygen-gas thermal cracking. The relative concentration of the oxygen functional groups varies significantly depending upon the oxidation technique used. Specifically, O2-plasma and thermal-cracking methods result in higher percentages of ether configurations (52.1% and 71.5%, respectively), whereas UV-ozone and acid-oxidation treatments preferentially yield ketone (47.9%) and hydroxyl (51.3%) groups. Additionally, employing state-of-the-art surface-science techniques, the surface electronic structure of each oxygenated sample is revealed. These experimental findings show that specific oxidation methods significantly alter the work function of all four oxidised surfaces, being measured in the range of 4.65 eV and 5.79 eV. This study paves the way towards selective tuning of the oxygen surface configurations suitable for fabricating diamond-based energy materials.