UV photocathodes based on nanodiamond particles: Effect of carbon hybridization on the efficiency

Photocathodes working in reflection-mode are made of rich-diamond (R-D) and rich-graphite (R-G) nanodiamond (ND) layers, deposited on different conductive substrates by means of the pulsed spray technique at low deposition temperatures (120 and 150 °C) and starting from two types of ND particles. The two powders with an average grain size of 250 nm have variable sp2(graphite phase) and sp3(diamond phase) hybridized carbon contents, as assessed by Raman spectroscopy and transmission electron microscopy. The ND particles are employed as-received or treated in H2microwave plasmas. The principal aim of the paper is the comparative study of R-D and R-G photocathodes in the vacuum ultraviolet spectral range from 140 to 210 nm, where they exhibit a high quantum efficiency and a good stability over time upon exposure to air. Specifically, the quantum efficiency values at 140 nm of photocathodes based on hydrogenated R-D and R-G layers are 26.8 and 47%, respectively, proving that the more defective ND particles are, the more efficiently they emit. Moreover, these QE values are higher than those derived by photocathodes based on microwave plasma enhanced chemical vapor deposition diamond films (14% at 140 nm) and the highest recorded in the state of international art.