The roles of hydrogen in the diamond/amorphous carbon phase transitions of oxygen ion implanted ultrananocrystalline diamond films at different annealing temperatures

We report that the diffusion and desorption of hydrogen (H) play a key role in the diamond/amorphous carbon phase transitions of O+-implanted UNCD films at different annealing temperatures (Ta) by using high resolution transmission electronic microscopy (HRTEM), vis-uv Raman, and Fourier transform infrared (FTIR) spectroscopy measurements. The results of HRTEM and uv Raman spectroscopy measurements show that with Ta increasing from 500 to 900 °C, the amorphous carbon in grain boundaries (GBs) transits to diamond phase. Visible Raman spectroscopy measurements show that the amount of H bonded to trans-polyacetylene (TPA) chains in GBs reduces with Ta increasing to 900 °C, while that of H terminating to the surfaces of diamond grains increases confirmed by FTIR measurements. It reveals that H diffuses from GBs to the surfaces of diamond grains. In this process, the active H extracts H which terminates the diamond surface, leaving a reactive surface site. This gives a chance for the neighbored amorphous carbon clusters to attach to the surface site, so that diamond grains become larger. After 1000 °C annealing, the amount of diamond phase dramatically decreases and diamond transits to amorphous carbon by HRTEM and uv Raman spectroscopy. It is observed that the amount of H bonded to TPA chains in GBs and that of H terminating to the surfaces of diamond grains dramatically decreases from visible Raman spectroscopy and FTIR measurements. It is revealed that H is desorbed from both surfaces of diamond grains and GBs, which forces diamond grains to collapse to amorphous carbon.