CVD-Grown Diamond with Unusual Fluorescence Colour Appearance
Dr Ramchandra Patil (ramchandrap@gemscience.net)
Sr. Research Associate
Gemological Science International
Mumbai, India
Rapidly improving chemical vapour deposition (CVD) technology can now produce large, high-quality, near-colourless, and colourless laboratory-grown diamonds. Manufacturers can control growth rate, morphology and purity to obtain products of desirable quality. For example, the addition of oxygen favours single-crystal over polycrystalline growth, and the presence of nitrogen enhances the growth rate by eight-fold (Harris and Weiner 1989; Jin and Moustakas 1994; Liang et al. 2009; Sally Eaton-Magaña and D’Haenens-Johansson 2012). However, the presence of nitrogen during the growth process turns CVD-grown diamonds yellowish or light brown; this colouration is associated with nitrogen-vacancy-hydrogen (NVH−) complexes (Liang et al. 2009). The concentration of NVH− centres can be reduced by either HPHT or LPHT annealing, so most near-colourless or colourless CVD-grown diamonds undergo post-growth treatment.
Recently a CVD-grown diamond was submitted to the Gemological Science International (GSI) Mumbai laboratory for post-growth treatment identification that showed anomalous fluorescence colouration when viewed in the DiamondView instrument. As it was rotated in the face-down position, the 1.27 ct round brilliant (7.00 × 6.93 × 4.26 mm; G colour and VS1 clarity), its fluorescence colour appeared to change from greenish blue to violet. As shown in Figure 1, at the initial orientation (0°) it luminesced greenish blue, while at 180° rotation, it fluoresced both violet and greenish blue. This is shown more distinctly in Figure 2, which at 180° reveals a growth plane dividing the pavilion plane into two parts—an upper part that fluoresces greenish blue and a lower part that luminesces violet. The sample also phosphoresced a weak greenish blue, which was stronger in the upper part of the sample. The growth plane is oriented approximately at an angle of 45° to 50 ° relative to the table facet. Due to this, there is variation in the amount of violet luminescence zone seen as the sample was rotated. This results in a change of apparent fluorescence colour with rotation. We infer that the abrupt change in fluorescence and phosphorescence zones is due to intentional or unintentional variations in deposition conditions or interruptions during CVD growth that modified the impurity content of the synthetic diamond.
The diamond was further tested with Fourier-transform infrared (FTIR), Raman and photoluminescence (PL) spectroscopy (at liquid-nitrogen temperature). FTIR spectroscopy revealed that the specimen was type IIa, with no hydrogen-related absorptions at either 3107 or 3123 cm–1. PL spectra with 532 nm laser excitation showed [Si-V]– emissions at 737 nm and 766 nm, a characteristic feature of CVD-grown diamonds. Both Raman and PL spectra showed weak emissions from NV centres at 575.0 and 637.0 nm. The absence of a 596/597 nm doublet in the Raman spectrum is evidence of post-growth treatment for colour enhancement (Eaton-Magaña and Shigley 2016; Philip M. Martineau et al. 2004).
The luminescence colour of the diamond is related to various defect centres, and the distribution and concentration of defects create distinctive patterns during diamond growth naturally or in a laboratory. Nearly all diamonds luminesce in the DiamondView due to the instrument’s ultra-short-wave UV excitation, with energy greater than the diamond’s band gap (i.e. wavelengths 225 nm and energies 5.5 eV). As-grown colourless, near-colourless, or brown CVD synthetic diamonds usually luminesce orange, red or pink, or show a mottled distribution of purple, red and blue due to the NV centre (Lu et al. 2021; Eaton-Magaña and Shigley 2016; D’Haenens-Johansson, Butler, and Katrusha 2022). Post-growth treatment (i.e. HPHT annealing) for decolourisation alters the pre-existing NV centres or creates new defects, as evidenced by various emission peaks that can be observed with PL spectroscopy. Consequently, the fluorescence colour of CVD-grown diamonds changes from as-grown red, pink, orange, and purple to green or greenish blue. Furthermore, HPHT-annealed CVD-grown diamonds show strong green-blue phosphorescence that can be associated with N–B donor-acceptor pairs (D’Haenens-Johansson, Butler, and Katrusha 2022; Eaton-Magaña and Shigley 2016). The addition of traces of nitrogen and boron influences the growth rate of CVD-grown diamonds, and the presence of boron significantly reduces the concentration of defects (D’Haenens-Johansson, Butler, and Katrusha 2022). The non-uniform distribution of N and B impurities is inferred to be responsible for the unusual luminescence pattern observed for this synthetic diamond.
Figure 1: DiamondView fluorescence images of the 1.27 ct CVD-grown diamond show an apparent change in fluorescence colour from greenish blue to a combination of violet and greenish blue as the diamond is rotated 360°. Composite photo by Ramchandra G Patil, GSI
Figure 2: DiamondView fluorescence and phosphorescence images of the 1.27 ct CVD-grown diamond at rotations of 0° and 180° reveal a growth plane in the latter orientation that divides the pavilion plane into two parts: the upper part fluoresces greenish blue and the lower luminesces violet. Composite photo by Ramchandra G Patil, GSI
Figure 3: PL (a) and Raman (b) spectroscopy of the 1.27 ct synthetic diamond with 532 nm laser excitation show features typical of CVD growth, including strong emissions from NV centres at 575 and 637 nm, and moderate CVD-specific emissions at 737 and 766 nm from the [Si-V]– centre.
Researchers and scientists continuously experiment with growth technology to increase the growth rate and reduce the overall cost of producing laboratory-grown diamonds. The diamond described here could be part of such experimentation. More such unusual features in laboratory-grown diamonds are likely to be encountered in the future.
References
D’Haenens-Johansson, Ulrika F. S., James E. Butler, and Andrey N. Katrusha. 2022. “Synthesis of Diamonds and Their Identification.” Reviews in Mineralogy and Geochemistry 88 (1): 689–753. https://doi.org/10.2138/rmg.2022.88.13.
Eaton-Magaña, Sally, and James E. Shigley. 2016. “Observations on Cvd-Grown Synthetic Diamonds: A Review.” Gems and Gemology. Gemological Institute of America (GIA). https://doi.org/10.5741/GemS.52.3.222.
Harris, Stephen J., and Anita M. Weiner. 1989. “Effects of Oxygen on Diamond Growth.” Applied Physics Letters 55 (21): 2179–81. https://doi.org/10.1063/1.102350.
Jin, S., and T. D. Moustakas. 1994. “Effect of Nitrogen on the Growth of Diamond Films.” Applied Physics Letters 65 (4): 403–5. https://doi.org/10.1063/1.112315.
Liang, Qi, Chih shiue Yan, Yufei Meng, Joseph Lai, Szczesny Krasnicki, Ho kwang Mao, and Russell J. Hemley. 2009. “Recent Advances in High-Growth Rate Single-Crystal CVD Diamond.” Diamond and Related Materials 18 (5–8): 698–703. https://doi.org/10.1016/j.diamond.2008.12.002.
Lu, Qi, Huaiyu Gong, Qingfeng Guo, Xuren Huang, and Jiayi Cai. 2021. “Gemological Characteristic Difference between Colorless Cvd Synthetic Diamonds and Natural Diamonds.” Materials 14 (20). https://doi.org/10.3390/ma14206225.
Philip M. Martineau, Simon C. Lawson, Andy J. Taylor, Samantha J. Quinn, and David J. F. Evans, and Michael J. Crowder. 2004. “Identification of Synthetic Diamond Grown Using Chemical Vapor Deposition (Cvd).” Gems & Gemology 40 (1): 2–25.
Sally Eaton-Magaña, and Ulrika F. S. D’Haenens-Johansson. 2012. “Recent Advances in CVD Synthetic Diamond Quality.” Gem & Gemology 48 (2): 124–27.