Effect of surface oxidation on photoluminescence of silicon vacancy color centers in the nanocrystalline diamond films

Highlights: • Three different surface oxidation treatments are performed to improve SiV photo-luminescence in nano-diamond films. • The improvement of diamond crystallinity plays an effective role in the PL enhancement of SiV centers. • Air annealing is an efficient approach to optimize the crystalline quality of nano-diamond films. The effect of three different surface oxidation approaches (air annealing, acid oxidation and oxygen plasma) on microstructural evolution and silicon vacancy (SiV) photoluminescence (PL) in the nanocrystalline diamond (NCD) films is investigated. All oxidation methods lead to the bonding of oxygen functional groups at diamond surface. The SiV centers exhibit the PL enhancement of about 105-fold in the air-annealed sample, compared with the as-deposited films. Nevertheless, the PL enhancement is about 7-fold and 2-fold in the acid- and plasma- treated samples, respectively. Combination of Raman spectra, HRTEM imaging with cross-sectional oxygen mapping confirms that such different PL behavior originates from the formed different thick oxidation layers. The air oxidation results in a thicker oxidation layer with improved crystallinity than the other two methods. In addition, when the air-annealed films are re-terminated with hydrogen, the SiV PL emission tends to drop remarkably under the same crystallinity. It indicates that hydrogen termination leads to the PL quenching of SiV centers. Therefore, our work reveals that the direct bonding of oxygen to sp³ carbon or the improvement of diamond crystalline quality plays an effective role in the PL enhancement of SiV centers during the oxidation of hydrogen-terminated NCD particles or films.