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Optical properties of nano-polycrystalline diamond (NPD) synthesized by direct conversion from high-purity graphite under high pressure and temperature were investigated by infrared (IR), ultraviolet (UV), and photoluminescence (PL) spectroscopy. IR spectroscopy revealed that NPD has no large lattice strain and very few impurities, but possesses an absorption band attributed to a trace of hexagonal diamond. UV spectra displayed a continuous absorption wing due to structural defects and secondary absorption edge attributed to electron transitions from aggregated nitrogen. PL spectra exhibited peaks of N3 centers and broad bands related to dislocations.
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Sumiya et al. (2009) studied this question.
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