In recent years, increasing numbers of pink-coloured CVD synthetic diamonds have appeared on the market. One of the major sources is Huzhou SinoC Semiconductor Science and Technology Co., Ltd., Zhejiang province of China. In this article, seven pink-coloured CVD-grown diamonds produced in the last two years by Huzhou have been investigated and identified, including their gemological and spectroscopic characteristics. In DiamondView, they fluoresced orange–red, with an obscure striated growth structure, which is common for CVD synthetics. The mid-IR absorption spectra of these samples showed some single nitrogen and hydrogen-related features (1130, 1344, 3123, 3323 cm−1), which indicated that the diamonds were type Ib and were CVD-grown diamonds. The H1a defect annealed out at approximately 1400 °C, whereas the 3107 cm−1 defect was produced by annealing above 1700 or 1800 °C. This implied that the samples had undergone two separate heat treatments: first, a high-temperature anneal (possibly an HPHT treatment to reduce any brown colour), which would have produced the 3107 cm−1 defects and a small number of A centres, followed by irradiation, followed by annealing above 800 °C to make the vacancies mobile. The UV–Vis–NIR absorption spectra showed distinct NV-related features (575 and 637 nm), the main reason for the pink colour. Photoluminescence spectra obtained at liquid nitrogen temperature recorded radiation-related emissions (388.9, 503.5 nm), a strong N-V centre, H3 and H2 defects, and many unassigned emissions. These pink CVD products can be separated from natural and treated pink-coloured diamonds by a combination of optical spectroscopic properties, such as fluorescence colour, and absorption features in the infrared and UV–Vis regions.
Keywords: pink CVD-grown synthetic diamonds; post treated; Chinese company
Most as-grown and gem-quality CVD-grown diamonds are colourless, near colourless, or brown, with varying degrees of saturation. Other colours can be produced by introducing other colour centres or defects either during the growth process or with post-growth treatment. Pink CVD synthetic diamonds were reported first in 2007 and appeared on the gem market around 2010 [[
In the past several years, significant progress has been made with synthetic diamonds produced by the chemical vapour deposition (CVD) method in China [[
For this study, HuZhou Sino-C Semiconductor Technology Company provided seven pink or pinkish CVD synthetic diamonds to the National Gemstone Testing Center (China) for examination. The samples ranged from 0.71 ct to 1.38 ct and showed varying saturation of pink colouration. These diamonds are representative of the current pink-coloured production grown at HuZhou Sino-C Semiconductor Technology Company. See Table 1 for a full description of the individual samples.
The following testing and analysis procedures were performed on all samples. A gemological microscope was used to observe the inclusions and anomalous birefringence. Reactions to ultraviolet (UV) radiation were checked in a darkened room with a long-wave (365 nm) and shortwave (253 nm) lamp. Fluorescence, phosphorescence, and growth characteristics were observed with a DiamondView™ instrument developed by the De Beers Group in England. Fourier-transform infrared (FTIR) spectroscopy was performed using a Thermo Nicolet 6700 spectrometer in the range of 6000–400 cm
All the samples had a distinct pink hue (Figure 1). Some were brownish purple; some were orange pink, and those with the most attractive colour were highly saturated bright pink.
In general, all the CVD synthetic diamonds showed an even colour distribution with no visible colour concentrations.
The samples had relatively high clarity; all of them had a VS clarity grade.
Black pinpoints and irregular inclusions were visible with magnification in all samples (Figure 2) and appeared to consist of non-diamond carbon, as described for CVD synthetic diamond by Martineau et al. [[
With cross-polarised light, strong birefringence was another important feature of these CVD samples. They all showed low- to moderate-order interference colours (Figure 3). The patterns were linear or grid-like, but not the Tatami structure typical of type IIa natural diamonds [[
All of samples showed moderate to strong orange red fluorescence to shortwave UV radiation. No phosphorescence to long or shortwave UV radiation was seen.
As observed with DiamondView™, all samples showed strong red or orangey red fluorescence (Figure 4). As seen in the table, the fluorescence was evenly distributed; from the pavilion, we can observe the growth striation and the multi-run growth lines. In addition, all samples showed very weak red phosphorescence. The body of the pink CVD synthetic diamonds also changed in colour to brown pink after being observed in DiamondView™ (strong UV irradiation).
The typical absorptions were observed in three regions of 1500–1000, 3500–2700, and 9000–5000 cm
In the 3500–2700 cm
Most of the samples (Figure 6) displayed absorption features including 575, 595, 637, and 737 nm peaks and a broad band at ~515 nm. Two of them displayed an obvious 850 nm absorption peak. The broad ~515 nm absorption resulted in the ultimate pink colour.
Many PL emission lines were recorded using five laser excitations in the UV–Vis–NIR region.
With the 325 nm laser, all samples showed a sharp 388.9 nm peak, but there were no 415 nm lines. The sharp line at 388.9 nm and the related broad bands at ~400 and ~410 nm (Figure 7a) were attributed to the 389 nm centre, which previously has been associated with radiation damage in all types of diamond and is stronger in those containing single nitrogen [[
With blue laser excitation (473 nm; Figure 7b,c), all samples displayed distinct 503.2 (H3) and 503.5 nm (3H) peaks. The intensity of 3H varied significantly between samples and did not show a clear correlation with other absorption or emission features. The 3H peak was clearly separated from H3 in all samples. In five samples, the H3 was stronger than 3H; in two samples, H3 was weaker than 3H. Most samples displayed a group of sharp peaks at 480.8, 487.5, 488, 488.2, 488.7, 492, 494.1, 495.3, 498.2, 498.9, 500.4, and 501.6 nm (Figure 7c), whose structures are still unknown.
Green laser (532 nm) excitation revealed strong NV centre emission systems in all samples, with ZPLs at 575 and 637 nm (Figure 7d). The 575 nm line was generally stronger. The 575/637 intensity ratio ranged from 2 to 15.
Doublet emission at 596.5 and 597.0 nm has been documented as a common feature of colourless, near-colourless, and brown CVD synthetic diamonds [[
With the 785 or 830 nm laser excitation, many luminescence lines could be detected, including 796/797, 806.6, 819.6, 825.0, 850.5, 853.8, 869.7, 843.5, 853.8, 869.7, 876.1, 884.7, 885.8, 909.2, 945.5, 962.3, 970.7, 982.6, and 986.3 nm (Figure 7e). All the samples displayed 986.3 nm (H2), 850.5, and 853.8 nm lines. All but one displayed the distinct 945.5 nm line.
The pink CVD lab-grown diamonds examined here are notably different from previous gem-quality pink CVD products: they display a stronger pink or orange–pink colour and are more attractive than those reported before [[
The pink colouration of these CVD samples is caused mainly by NV centres, which absorb most yellow, green, and orange wavelengths. Two transmission "windows" are created in two main regions: one at a slightly higher wavelength than 637 nm, introducing a pink-to-red hue component to the body colour; and the other centred at ~450 nm, passing blue light and thus producing a blue component.
The pink CVD synthetic diamonds in this study displayed weak H-related absorptions that occurred predominantly at 3107 and 3030 cm
We detected irradiation-related defects both in near-infrared and mid-infrared regions. Weak absorption caused by H1a at 1450 cm
We could observe the 388.9, 503.5, and 595 nm defects related to irradiation in the UV–Vis and/or PL spectra (Figure 7a–c), but we could not observe the ND1 defect documented by Wang et al. [[
The 388.9 nm centre is the radiation damage product characteristic of all types of diamonds [[
The 3H centre (503.5 nm), observed in all types of diamond after electron irradiation, is believed to involve the self-interstitial [[
The 595 nm line can be seen both in the UV–Vis spectrum and negatively in the PL spectrum when excited with 473 and 532 nm laser excitation. The 595 nm line is often found in irradiated and annealed (T < 1000 °C) type Ia or Ib diamonds [[
We also could observe the H3, H2, 575, and 637 nm defects related to high temperatures in the UV–Vis and/or PL spectra. A typical feature of the PL spectrum of the pink CVD-grown diamonds is the relatively strong H3 centre (Figure 7b). The intensity of the H3 peaks were around 3.3–6.5 times that of the diamond Raman line. The H3 defect ((NVN)
All the samples displayed the H2 (986.2 nm) peaks in the PL spectra. The H2 centre is often found in diamonds containing aggregated and single nitrogen simultaneously and those subjected to a higher-temperature process [[
When single-nitrogen-containing diamonds are irradiated and annealed at a medium temperature, the vacancy will be captured by nitrogen and form a new centre, the NV centre, which produces 575 and 637 nm peaks. In this study, the NV centres were dominant in the PL spectra, and the intensity of 575 nm was stronger than that of 637 nm. The NV emission was the main cause of the pink colour and the obvious orange–red fluorescence.
Highly saturated natural pink diamonds are rare. Separation of these treated pink CVD lab-grown diamonds from other pink diamonds can be challenging, as some samples may be mistaken for other types of diamonds when using various gemological and spectroscopic features. The concentration of nitrogen is less than 1 ppm, which means that samples can easily be mistaken for type IIa diamonds without detected nitrogen from the IR spectrum. Otherwise, the typical CVD absorption peaks at 3123 and 3323 cm
Proper identification of this gem material can be achieved through a combination of standard gemological properties and characteristic spectroscopic features. There is no doubt that the CVD growth technique will continue to improve, and CVD synthetic diamonds of increasingly superior quality will be produced.
Graph: Figure 1 The pink or pinkish type Ib CVD synthetic diamonds examined for this study were treated by irradiation and annealing. The sample numbers are ZX11, 09, 13, 12, 04, 03, 10, respectively, from left to right and top to bottom. Photos by Z. Song.
Graph: Figure 2 Black pinpoint and irregular inclusions (probably non-diamond carbon) are present in the most of the CVD synthetic diamonds, as shown here in the ZX03 sample grown by Huzhou SinoC Semiconductor Science and Technology Co., Ltd. In China. Photomicrograph by Z. Song; magnified 50×.
Graph: Figure 3 Strong birefringence is displayed in the ZX04 sample when observed with crossed polarisers. (a) Linear pattern magnified 16×. (b) Grid-like pattern magnified 10×. Photomicrograph by Z. Song.
Graph: Figure 4 In DiamondView, these pink CVD diamonds display strong orange or orange red fluorescence. The fluorescence pattern of the ZX03 shown here is indistinct from the table (a), and the characteristic striated growth features are clearly seen from the pavilion (b). Photos by Z. Song.
Graph: Figure 5 In the 1000–1500 cm−1 region (a), the mid-infrared spectrum of these pink CVD synthetic diamonds revealed that they are type Ib with a very small concentration of isolated nitrogen. In the 2800–3400 cm−1 region (b), these samples revealed 3123, 3107, and 3030 cm−1 peaks due to hydrogen. In the near-infrared region of 8000–10,000 cm−1 (c), a broad absorption band at the centre of ~9288 cm−1 could be detected.
Graph: Figure 6 UV-Vis-NIR spectra of these CVD synthetic diamonds show strong absorption due to the NV centre. In addition, defects such as 595 nm were detected.
Graph: Figure 7 Numerous emission lines are present in the PL spectra of these synthetic diamonds. Distinct 388.9 nm peak could be detected in all the samples with the 325 nm laser excitation (a). Several emissions and relatively strong H3 and 3H peaks in the 480–510 nm regions were observed with the 473 nm laser excitation (b , c). Extremely strong NV emissions and negative 594.6 nm reported with the 532 nm laser excitation (d), and many emissions including 986.2 and 945.5 nm when excited by the 785 and 830 nm lasers (e).
Graph: crystals-11-00872-g007b.tif
Table 1 The gemological information about the pink CVD-grown synthetic diamonds studied in this research.
Sample No. Weight (ct) Cut Colour Clarity LW SW ZX03 0.71 Round brilliant Brownish purple VS medium orange red medium orange red ZX04 1.38 Cushion purple VS strong orange red strong orange red ZX09 1.04 Emerald Rare pink orange VS strong orange red strong orange red ZX10 0.80 Rectangular Rare vivid pink VS strong orange red strong orange red ZX11 1.03 Rectangular Rare orangey pink VS strong orange red strong orange red ZX12 1.13 Rectangular Rare vivid pink VS strong orange red strong orange red ZX13 1.16 Round brilliant Rare purplish pink VS strong orange red strong orange red
Conceptualization, Z.S.; methodology, Z.S.; investigation, B.G. and H.D.; writing—original draft preparation, Z.S.; writing—review and editing, Z.S.; visualization, W.Z.; project administration, Z.S. All authors have read and agreed to the published version of the manuscript.
This research was funded by the NGTC Research Foundation, grant number NGTC20200300, and partially funded by the China National Science Foundation, grant numbers 41473030, 41272086, and 42073008.
Not applicable.
Not applicable.
The authors declare no conflict of interest.
CVD chemical vapor deposition HPHT high-pressure high-temperature GR general radiation ZPL zero phonon line NV nitrogen vacancy LW longwave SW shortwave
We thank Alan Collins for supplying pers. comms. mentioned above and Hongming Liu of HuZhou Sino-C Semiconductor Co., who supplied all the samples for study.
By Zhonghua Song; Huiru Dai; Bo Gao and Wenfang Zhu
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