About the author: Yan, senior consultant of the 3rd insurance institute of china Artificial Products Professional Committee, former dean and professor of Jewelry College of China Geo University.

Chen Meihua, a member of the 3rd Artificial Products Professional Committee of insurance association of china, is a professor at the Jewelry College of China Geo University.

There are several methods to synthesize diamond by chemical vapor deposition, such as hot filament method, flame method, plasma spraying method and microwave plasma method, but the most commonly used method is microwave plasma method. This is a synthesis method at high temperature (800 ~ 1000℃) and low pressure (104Pa). Carbon-containing gas, methane (CH4) and hydrogen are transported to the evacuated reaction chamber by a pump through a pipeline, and the gas is heated by microwaves, and the substrate in the chamber is also heated. Microwave generates plasma, and carbon decomposes from the state of gaseous compounds into a single free atom state. After diffusion and convection, it finally precipitates on the heated substrate in the form of diamond. Hydrogen atoms play an important role in inhibiting graphite formation (Figure 1, Figure 2).

The so-called plasma is simply that the gas is ionized into positive ions and negative ions under the action of electric field, which usually appear in pairs and remain electrically neutral. This state is called the fourth state of outgassing, liquid and solid foreign matter. For example, CH compounds are ionized into C and H plasmas.

Figure 1 synthesis of CVD diamond by microwave plasma method

(According to Martineau et al., 2004)

Fig. 2 schematic diagram of plasma and carbon crystallization

When the substrate is silicon or metal material instead of diamond, the diamond film is polycrystalline due to the different orientations of diamond grains. If the substrate is a diamond single crystal, single crystal diamond can be grown in the same crystallization direction based on it. The substrate acts as a seed crystal. The diamond used as the substrate can be natural diamond, high pressure and high temperature synthetic diamond or CVD synthetic diamond. The substrate is cut into a thin plate shape, and its top and bottom surfaces are roughly parallel to the vertical plane of diamond ({100} plane).

1. History and present situation of diamond synthesis by chemical vapor deposition.

1952 William Ever-sole of American Federal Silicon Carbide Company successfully grew diamond by in-phase epitaxy with carbon-containing gas at low pressure. This is earlier than the time when Swiss ASEA Company 1953 and American General Electric Company (GE) 1954 announced the synthesis of diamonds by high pressure and high temperature method, so Eversole is considered as the first person to synthesize diamonds. However, at that time, the speed of CVD diamond growth was very slow, and few people believed that its speed could be increased to be suitable for commercial growth.

Since 1956, Soviet scientists have significantly improved the speed of diamond synthesis by CVD, when diamond films were grown on non-diamond substrates. In the early 1980s, this synthetic technology made a major breakthrough in Japan. 1982, Matsumoto of Japan's National Institute of Inorganic Materials (NIRIM) announced that the growth rate of diamonds had exceeded 1 micron/hour ... which aroused the world's interest in using this technology for various industrial purposes.

At the end of 1980s, the industrial diamond department of De Beers Company (now Element Six Company) began to study CVD diamond synthesis, and quickly gained a leading position in this field, providing many industrial products for CVD diamond synthesis.

This technology has also been applied in the jewelry industry, that is, using polycrystalline diamond film (DF) and diamond-like carbon (DLC) as coating (coating) to optimize some natural gems, including diamonds.

Although the growth rate of CVD diamond was greatly improved at that time, which made it possible to grow thinner diamond layers for some industrial uses and gem coatings, it was still impossible to produce jewelry materials for cutting and grinding facets because of the need for thicker single crystal diamond. The depth of the 0.5-carat round drill is greater than 3mm. If the speed is 0.00 1 mm/hour, the required drill blank must grow at least 18 cycles. It can be seen that the slow speed is still the main factor hindering the synthesis of thick single crystal diamond by CVD.

In 1990s, great progress was made in the research and development of single crystal diamond synthesized by chemical vapor deposition. First, in 1990, researchers from Nijmegen University in the Netherlands grew a CVD single crystal with a thickness of 0.5 mm by flame and hot filament method. Later, the American company Crystallume also reported that 1993 used microwave CVD to grow single crystal diamond with similar thickness. Bazian equals 1993. It is reported that single crystal diamond with a thickness of 65438 0.2 mm has been grown. DTC and Element Six have produced a large number of single crystal diamonds for research purposes, including nitrogen-doped brown diamonds and pure colorless diamonds, as well as boron-doped blue diamonds and diamonds that have been treated at high pressure and high temperature after synthesis.

2 1 century, a breakthrough was made in the research and development of single crystal diamond synthesized by CVD for jewelry.

Apollo Diamond Company has been engaged in the research and development of single crystal diamond synthesized by CVD for many years. In the autumn of 2003, the single crystal diamond for jewelry synthesized by CVD began commercial production, mainly Ⅱ A brown to nearly colorless diamond single crystal, weighing more than 1 carat. At the same time, the experimental production of Ⅱ A colorless diamonds and Ⅱ B blue diamonds also began. Apollo Diamond Company estimated that its total faceted diamond output in 2005 was 5000 ~ 1 10,000 carats, mostly 0.25 ~ 0.33 carats, but it could also produce 1 carat (Figures 3 and 4).

Fig. 3 colorless brown CVD diamond

(According to Martineau et al., 2004)

Fig. 4 CVD diamond equipment and synthesis process

(According to DTC, 2005)

At the International Diamond Conference held in Japan in May, 2005, American Yan He Hamley (Carnegie Laboratory) revealed that due to the improvement of technical methods, they have been able to grow 5 ~ 65,438+000 μ m/h single crystals at high speed (65,438+000 carats), which is about five times faster than the diamond commercially produced by high pressure and high temperature method and other CVD methods. They also predicted that colorless single crystal diamonds of inches (about 300 carats) could be grown.

It can be seen that the prospect of CVD diamond synthesis for jewelry is very gratifying, and its influence on diamond industry should not be underestimated.

2. Characteristics and identification of single crystal diamond synthesized by chemical vapor deposition.

In recent years, some research and appraisal institutions have been devoted to studying the characteristics and appraisal of synthetic single crystal diamond. The information to be introduced here comes from three papers in Gems&Gemology magazine of American Gemology Institute.

1) Wang Wuyi et al. (2003) summarized the properties and identification characteristics of 13 samples previously produced by Apollo Diamond Company.

2)Martineau et al. (2004) summarized the research results of thousands of experimental samples produced by DTC and Element Six in recent 15 years (including those cut into facets after synthesis). In addition to the nitrogen-containing brown diamonds and pure near-colorless diamonds like Apollo Diamonds, there are also blue diamonds doped with boron and diamonds treated at high pressure and high temperature after synthesis.

3) Wang Wuyi et al. (2005) summarized the properties and identification characteristics of six experimental samples from the French University of Paris 13 laboratory. Three of them are nitrogen-doped, and the other three are high-purity diamonds grown under the condition of minimizing impurity content.

The samples involved in the above-mentioned papers are all grown by microwave method in chemical vapor deposition, so there are many similarities in the characteristics and identification methods summarized in the paper, but their characteristics are also different due to the different synthesis processes and methods (including experimental purposes and conditions, doping types and concentrations, substrate types, etc.). ).

1. transparent

Because natural diamond, high-pressure high-temperature synthetic diamond or CVD synthetic diamond are cut into parallel {100} crystal planes (vertical) or small-angle slices with {100} as the substrate, most single crystals grown by CVD are plate-shaped, with a large top surface in the {100} direction and occasionally visible edges. The parts where octahedral surface {11} and dodecahedral surface {1 10} are distributed usually contain more inclusions, which are parts with poor growth quality and are not easy to polish (Figures 5 and 6).

Fig. 5 crystal morphology of natural diamond, HTHP synthetic diamond and CVD synthetic diamond.

Fig. 6 Morphological differences between natural diamonds and CVD synthetic diamonds

By magnifying the growth surface of nitrogen-doped diamond with differential interference microscope or gem microscope, a "growth step" can be observed, which consists of a "growth step" and an inclined "vertical plate" separating them (Figures 7 and 8).

Fig. 7 Surface growth characteristics of CVD diamond with {100} plane (according to Wang Wuyi et al., 2005).

Fig. 8 "growth step" phenomenon on the surface of nitrogen-doped diamond

(According to Martineau et al., 2004)

2. Diamond type and color

Martineau et al. (2004) divided the experimental samples of DTC and element six into four categories.

(1) nitrogen-doped CVD synthetic diamond

Because a small amount of air will inevitably enter the reaction chamber during the synthesis process, and the air contains nitrogen, and the added raw material gas will also contain impurity nitrogen, so it is difficult to completely eliminate the nitrogen in synthetic diamonds. When the nitrogen content is low, it belongs to Ⅱ A type, and when the nitrogen content is high, it belongs to Ⅰ B type. Except for a few that are almost colorless, most of them have earthy tones (some samples of Paris University 13 have gray tones), which is obviously different from natural and yellow-tone diamonds synthesized at high pressure and high temperature. Most of the existing products of Apollo Diamond Company belong to this category, most of which are IIA and a few are I b. The existing experiments show that nitrogen can obviously improve the growth rate of synthetic diamonds, so nitrogen can sometimes be controlled artificially (Figure 9).

(2) Nitrogen-doped CVD synthetic diamond treated at high pressure and high temperature.

Experiments show that high pressure and high temperature heat treatment can weaken the brown tone of diamond synthesized by nitrogen-doped CVD. Because the earthy tones of diamond synthesized by nitrogen-doped CVD is related to factors such as N-V (nitrogen hole) center and has nothing to do with plastic deformation, the color reduction at high pressure and high temperature is also related to reforming N-V (nitrogen hole) center and has nothing to do with repairing plastic deformation.

(3) synthesizing boron-doped diamond by chemical vapor deposition.

When B2H6 is added to the feed gas during the synthesis process, the synthetic diamond will contain a small amount of boron, belonging to Ⅱ B type, and its color is light blue to dark blue (Figure 10).

(4) High-purity CVD synthetic diamond containing no other impurities except hydrogen.

It belongs to nearly colorless to colorless IIA diamond. As hydrogen is a component of raw gas, impurity hydrogen is inevitable, so it is very difficult to strictly control nitrogen and boron, and the growth rate is much slower than that of nitrogen-doped gas (Figure 1 1).

Fig. 9 nitrogen-doped brown CVD diamond

Figure 10 boron-doped blue CVD diamond

(Figure 9 ~ 1 1, according to Martineau et al., 2004)

Figure 1 1 high purity CVD diamond

3. Ribbon

Magnified observation in the direction perpendicular to the crystal growth (that is, the direction parallel to the {100} plane) shows that the layered distribution of colors can be seen in the experimental samples of Element Six Company. Brown stripes can be seen in nitrogen-doped brown diamonds, while blue stripes can be seen in boron-doped blue diamonds (figure 12).

Brown stripes can also be seen in Apollo Diamond products.

Figure 12 Brown stripes in Apollo diamond products.

(According to Wang Wuyi et al., 2003)

Step 4 include

There are few inclusions, and not all samples can be observed. There are mainly some needle-like inclusions and some black irregular particles, which are called non-diamond carbon (Figure 13). Because these can also be seen in natural and high-pressure and high-temperature synthetic diamonds, the identification is of little significance. However, the diamond synthesized by microwave CVD will not have the metal inclusions common in diamond synthesized at high pressure and high temperature, and will not have magnetism.

The purity grades of several nitrogen-doped finished diamonds in Apollo diamond samples are VS 1 to SI2.

Fig. 13 acicular inclusions (left) and non-diamond carbon inclusions (right)

(According to Wang Wuyi et al., 2003)

5. Abnormal birefringence (figure 14, figure 15)

Figure/abnormal extinction of kloc-0/4 CVD diamond (left) and natural diamond (right)

(According to Wang Wuyi et al., 2003)

Figure 15 Observation in parallel growth direction (top) and vertical direction (bottom)

(According to Martineau et al., 2004)

Under the orthogonal polarizing microscope, we can usually see abnormal lattice birefringence caused by residual internal strain, showing low interference color, but we can see high interference color around some defects. On the whole, its abnormal birefringence is weaker than that of natural diamond, but in the distribution parts of octahedron {11} and dodecahedron {1 10} at the edge, the abnormal birefringence is strong and the interference color is high.

6. Ultraviolet fluorescence

Of Apollo's 13 samples, 8 were inert under long-wave ultraviolet, and the rest were light orange, orange yellow or yellow. Under SW LV, except for 1 sample, all samples show from weak to medium orange to orange yellow. No phosphorescence was observed.

The samples of Paris 13 University, including nitrogen-doped samples and high-purity samples, are inert under LW ultraviolet and SW ultraviolet, and only the substrate of 1 sample has not been determined.

Element Six's 14 nitrogen-doped faceted diamond appears light orange to orange under LW UV and SW UV. Eight-sided high-purity CVD synthetic diamonds are inert under long-wave ultraviolet and short-wave ultraviolet. Five faceted boron-doped diamonds are inert under LM UV and blue-green with blue phosphorescence under SW UV.

To sum up, except for boron-doped diamonds, most CVD synthetic diamonds have great changes under LW ultraviolet and SW ultraviolet, which can be inert to orange and difficult to be used as identification basis.

7. Observe luminescence with DiamondView.

The luminescence characteristics of CVD diamond under short-wave ultraviolet light were observed by DiamondView of De Beers. It is found that nitrogen-doped diamond shows strong orange to orange-red fluorescence (Figure 16, Figure 17, Figure 18), which is related to the N-V center. Nitrogen-doped diamonds treated at high pressure and high temperature are mainly green. High-purity CVD synthetic diamonds do not show orange fluorescence under DiamondView, but some samples have weak blue luminescence, which is related to dislocations in the crystal lattice. This blue light will also appear in the four corners of nitrogen-doped diamonds. Boron-doped diamond synthesized by CVD shows bright blue fluorescence, some of which are green-blue (figure 19), which has phosphorescence effect and can last for several seconds to dozens of seconds. CVD diamond does not show octahedral luminescence mode of natural diamond and cubic-octahedral luminescence mode of diamond synthesized at high pressure and high temperature in diamond view. Interestingly, CVD diamond was grown on the substrate of high-pressure and high-temperature synthetic diamond, and the cubic-octahedral luminescence pattern of high-pressure and high-temperature synthetic diamond could be seen without removing the substrate (Figure 20).

Figure 16 DiamondView for observing the luminescence phenomenon of CVD diamond

(According to Martineau et al., 2004)

Photo 17 diamond view to observe the luminescence phenomenon of Apollo diamonds

(According to Wang Wuyi et al., 2003)

Dense oblique stripes can be seen on the vertical {100} section of CVD nitrogen-doped diamond (the stripe spacing is quite stable, ranging from 0.00 1mm to 0.2mm for different samples). This is an important distinguishing feature of nitrogen-doped diamond synthesized by CVD. Although natural IIA diamonds occasionally glow orange, they do not have such stripes. After high pressure and high temperature treatment, the luminescence of nitrogen-doped diamond changed from green to blue-green, but dense stripes were still visible (Figure 2 1).

Figure 18 CVD diamond grown on the diamond substrate synthesized at high pressure and high temperature shows different colors from the substrate in DiamondView.

(According to Wang Wuyi et al., 2003)

Figure19 Fluorescence of Boron Doped Diamond Synthesized by CVD

(According to Wang Wuyi et al., 2003)

Fig. 20 Fluorescence of CVD nitrogen-doped (left) and CVD high purity diamond (right)

(According to Wang Wuyi et al., 2005)

Fig. 2 1 comparison of fluorescence after untreated and high temperature and high pressure treatment.

(According to Martineau et al., 2004)

CVD boron-doped diamonds also show stripes, pits or both under DiamondView, which is not found in natural Ⅱ B blue diamonds (Figure 22).

Fig. 22 stripes and pits of CVD boron-doped diamond

(According to Martineau et al., 2004)

8. Cathodoluminescence image

It has the same luminescent characteristics as DiamondView mentioned above.

9. Photoluminescence spectrum and cathodoluminescence spectrum (Figures 23 and 24)

The samples of Six Element Company were irradiated with laser beams of 325nm(HeCd), 488nm (argon ion), 5 14nm (argon ion), 633nm(HeNe, He-Ne) and 785nm (near infrared diode) and their luminescence spectra were studied. The samples of Six Element Company were irradiated with cathode rays and their luminescence spectra were studied.

Table 1 Luminescent spectrum characteristics of various diamonds

Martineau and others agree with Zaitsev (200 1) that 467nm and 533nm only appear in CVD synthetic diamond, but point out that they will no longer exist after high pressure and high temperature treatment. I also agree with Wang Wuyi et al. (2003) that 596nm and 597nm have distinguishing significance for CVD nitrogen-doped diamonds, but point out that not all samples have 596/597 peaks.

10. Ultraviolet-visible-near infrared absorption spectrum and infrared absorption spectrum (Figure 25, Figure 26 and Figure 27)

Fig. 23 luminescence spectrum of nitrogen-doped CVD diamond irradiated by 5 14 argon ion laser beam.

(According to Martineau et al., 2004)

Fig. 24 Luminescence spectrum (b) of nitrogen-containing CVD diamond (A) and the same sample irradiated by 325nm He-Cd laser beam at high pressure and high temperature.

(According to Martineau et al., 2004)

Fig. 25 UV-Vis absorption spectra of nitrogen-doped CVD diamond (A) and the same diamond (B) after high pressure and high temperature treatment.

(According to Martineau et al., 2004)

Martineau et al. (2004) studied various types of CVD diamond synthesized by Element Six Company with several types of spectrometers, and obtained the results in Table 2.

Table 2 Spectral characteristics of various diamonds

Martineau et al. (2004) thought that the absorption at 365nm, 520nm, 596 nm and 625nm in UV-Vis-NIR spectrum was the characteristic of nitrogen-doped diamond synthesized by CVD, which was not found in nitrogen-doped diamond treated at high pressure and high temperature, nor in natural diamond and diamond synthesized at high pressure and high temperature.

Fig. 26 infrared spectrum of nitrogen-doped CVD diamond from Apollo company.

(According to Wang Wuyi et al., 2003)

Martineau et al. (2004) also agreed with Wang Wuyi et al. (2003), that is, 8753 cm- 1, 7354 cm- 1, 6856 cm- 1 and 6425 cm- 1 related to hydrogen in infrared spectrum. 3323 cm- 1 and 3 123 cm- 1 are the characteristics of nitrogen-doped diamonds synthesized by CVD, which are not found in nitrogen-doped diamonds treated at high pressure and high temperature, nor in natural diamonds and high pressure and high temperature synthetic diamonds. After high pressure and high temperature treatment, 3 107cm- 1 was absorbed, which was also found in some natural diamonds.

Fig. 27 infrared absorption spectrum of Apollo nitrogen-doped CVD diamond

(According to Wang Wuyi, 2005)

X-ray morphology analysis of 1 1.

X-ray morphology analysis shows obvious columnar structure in the cross section parallel to the growth direction, while many dark spots or fuzzy lattices are seen in the cross section perpendicular to the growth direction. It is believed that this columnar structure is the result of some dislocations that appear at or near the matrix interface and begin to extend upward during the growth of diamond crystals.

Three. Concluding remarks

For a small number of finished nitrogen-doped diamonds entering the market today, the brownish tone, thin thickness and abnormal extinction characteristics of the finished products can provide some clues for identification, but the final identification depends on DiamondView and cathodoluminescence image analysis and spectral data in large laboratories, including emission spectrum and absorption spectrum data. Due to the continuous improvement of CVD process for synthesizing single crystal diamond, especially the appearance of high-purity CVD diamond and the high-pressure and high-temperature heat treatment of nitrogen-doped CVD diamond, the luminescent image characteristics and spectral characteristics of nitrogen-doped CVD diamond can not be effectively identified, which further increases the difficulty of identification. However, we believe that the gemological community will continue to analyze and summarize the situation and find a way to identify it.

Main references

Identification of synthetic diamond grown by chemical vapor deposition. Gemmology and Gemmology, 40( 1):2~25.

Wang Wuyi, Thomas Moses, Robert C. Rinales, 2003, growing gem-grade synthetic diamonds by chemical vapor deposition. Gems&Gemolo-gy，39(4):206~283。

Wang Wuyi, Alexandre Tallaire, Matthew S.Hall, 2005, experimental CVD synthetic diamond from Limp -CNRS. Gem and gemmology, 4 1(3):234~244.