Microscopic and Spectroscopic Characteristics of CVD Synthetic Blue Diamond

Natural blue diamonds are produced in very small quantities and command extremely high prices, mostly appearing in the auction market. With the increasing demand for blue diamonds, some synthetic or colour modified blue diamonds came into people's view. At present, common boron-doped blue diamond synthesized by high temperature and high pressure (HPHT) method tend to have a cross-shaped colour distribution characteristic of growth zoning. Irradiated type Ia blue diamonds display blue-green hues controlled by the relative intensities of N3 and GR1 colour centers. Irradiated type Ⅱa diamonds primarily show GR1-dominated colouration with GR2-8 colour centers as auxiliary contributors, so we can't control the intensity of the colour center in an artificial and quantitative way, which results in the colour being unstable. Diamond synthesized by chemical vapor deposition (CVD) has distinct advantages in controlling doped boron element concentration and maintaining uniformity and consistency. However, due to the difficulty of its growth, there are few reports of blue diamonds synthesized by CVD. In this study, a blue diamond synthesized by CVD was performed a series of optical non-destructive characterization tests by magnification examination, ultraviolet-visible spectrophotometer, infrared spectrometer, Raman spectrometer, and photoluminescence spectrometer. Comparing with irradiation modified and HPHT blue diamonds, this boron doped blue diamond synthesized by CVD was similar to natural blue diamonds in colour, with a more uniform colour distribution, clean inclusion, and a few feather and dark substances. UV-Vis spectra showed that the absorption features of the CVD synthetic blue diamonds are similar to the natural blue diamonds but distinct from the GR1 center absorption band (741nm) in irradiation-treated diamonds. The infrared spectra revealed the sample to be a type Ⅱ b diamond. The boron-related absorption peaks at 1 290 cm^-1 and 2 458 cm^-1 differed from the 2 455 cm^-1 and 2 802 cm^-1 peaks observed in natural blue diamonds, as well as the 1 450 cm^-1 peak associated with the H1a defect center in radiation-treated blue diamonds. Additionally, fluctuations in the three-phonon region (>3 000 cm^-1 ) suggested a high boron content. Raman spectra exhibited 1 332 cm^-1 diamond intrinsic asymmetry peak resulting from boron doping. Photoluminescence spectroscopy tests revealed the presence of NV⁰ defects (575 nm), NV^-defects (637 nm), and SiV^-defects (737 nm).