Effects of oxidation on silicon vacancy photoluminescence and microstructure of separated domain formed nanodiamond films

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In order to increase the oxidation sites for enhancing the Si-V photoluminescence intensity of nanocrytalline diamond films, we prepare nanocrystalline diamond films; these films each are comprised of separated domains and oxidized for different times. Each single domain consists of nanodiamond grains with a size of larger than 100 nm and amorphous carbon. In the gaps between domains of separated domain there is formed a film that allows more sites to contact air to ensure the efficient oxidation of the film. As a result, silicon vacancy photoluminescence intensity of the separated domain forming the film is largely enhanced by about 22.7 times after oxidation. The SEM images and Raman spectra of oxidized samples show that the film contains flower-shaped diamond aggregates, each of which is comprised of radially arranged diamond grains. The mixture of nanodiamond grains and amorphous carbon fills the gaps between diamond petals. These fillers disappear after long-term oxidation, but the diamond petals stay stable. Raman spectra show that the amount of amorphous carbon largely decreases after oxidation, while diamond content apparently rises. Hydrogen is desorbed from the film after short-time oxidation according to Raman spectra, thus the quenching effect on silicon vacancy photoluminescence caused by hydrogen termination of diamond surface state is removed. Diamond petals of large size and nanodiamond grains in the fillers are both silicon vacancy photoluminescence sources of the film; the exposed diamond flats on the surface of unoxidized domains provide limited silicon vacancy photoluminescence for the film. The sufficient exposure of diamond grains after the removal of amorphous carbon leads to the significant enhancement of film’s silicon vacancy photoluminescence. With longer-time oxidation, the photoluminescence of film will slightly decrease due to the disappearance of small-sized nanodiamond grains, but the film photoluminescence almost remains stable in both intensity and property due to the stability of large-sized diamond grains. The film after 140-min oxidation remains photoluminescence enhancement, 8.3 times the photoluminescence of the unoxidized sample. The full width at half maximum of photoluminescence peak declines to merely 5.6－6.0 nm because of diamond petals’ high degree of order, which is advantageous for diamond silicon vacancy photoluminescence.