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Synthesis and characterization of gem diamond single crystals in Fe-C system under high temperature and high pressure
Journal of Crystal Growth 531 (2020) 125371
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Synthesis and characterization of gem diamond single crystals in Fe-C system under high temperature and high pressure ⁎
T ⁎
Zhanke Wanga, Hongan Maa, , Shuai Fanga, Zhiqiang Yanga, Xinyuan Miaoa, Liangchao Chenb, , ⁎ Xiaopeng Jiaa, a b
State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China Key Laboratory of Material Physics of Ministry of Education, School of Physical Engineering, Zhengzhou University, Zhengzhou 450052, China
A R T I C LE I N FO
A B S T R A C T
Communicated by P. Rudolph
In this paper, the diamond large single crystals were successfully synthesized in the pure Fe-C system by temperature gradient growth (TGG) method in China-type cubic anvil high-pressure apparatus (CHPA) at 5.8 GPa and 1550 °C. The synthesized diamonds were characterized by Optical microscopy, Infrared spectroscopy, Raman spectroscopy and Photoluminescence (PL) spectroscopy. The results show that the nitrogen concentration of type Ib diamonds synthesized in pure Fe-C system is about 80 ppm, which is much lower than diamonds synthesized by Ni-based and Fe-based alloys. PL spectra result indicated that there were no peaks related to the NV centers in diamonds synthesized by Fe-C system. In addition, high-quality type IIa gem diamond can be synthesized by adding a small amount of nitrogen getter in the pure Fe-C system. We believe that this work is of great significance for the synthesis of high-purity large single type IIa diamonds without impurities such as nitrogen and metals.
Keywords: A1. Impurities A2. Growth from high temperature solutions A2. Single crystal growth B1. Diamond B1. Metals B3. Infrared devices
1. Introduction According to the concentration and presence of nitrogen and boron impurities, diamond can be divided into two types, type I and II, and further subdivided into four types of Ia, Ib, IIa and IIb [1,2]. Among them, Type IIa diamond contains no impurities such as boron, nitrogen and other metals, high crystallinity and low defect density make it exhibit high performance in diamond-based power devices. Therefore, high purity type IIa diamond is considered to be a very promising defect-free substrate. In addition, high-purity diamonds have important applications in advanced fields such as quantum fields, high-precision sensors, and high-resolution imaging technologies [3–6]. Nowadays, methods for synthesizing high-purity diamond large single crystals mainly include chemical vapor deposition (CVD) and high temperature and high pressure (HPHT) methods, but there are still problems in these two high-purity diamond synthesis methods. For example, stress and defects in diamond large single crystals prepared by CVD, metal impurities in diamond large single crystals prepared by HPHT, and so on. In order to solve the problem of metal impurities in the high-purity diamond crystals synthesized by HPHT, it is essential to select a suitable catalyst for growing a “high-purity” type IIa diamond single crystal. It is well known that metal solvent plays a vital role in the process of
⁎
synthesis of diamond by HPHT. The research of diamonds grown in different catalytic systems is required and helpful for understanding the nucleation and growth mechanism of diamond and synthesizing diamonds with specific properties [7]. Based on a lot of experimental research, it is found that conventional metal catalysts are mainly transition metals such as Fe, Co, Ni, Mn, Cr, etc. [8]. In addition, other metals (Zn, Sn, Cu, Ge, Sb, etc.) and non-metals (carbonates, silicates, hydrides, oxides, etc.) have also been used as catalysts to synthesize diamond [9–11]. These catalyst solvents require relatively high pressures and temperatures (7.0–8.0 GPa, 1600–2000 °C) in the process of synthesizing diamond, and the size of diamond is generally less than 1.0 mm. When the duration of synthesis is prolonged, spontaneous nuclei are easily formed and the quality of diamonds is difficult to control [12]. Moreover, diamonds synthesized in metal solvent systems such as Cu-C system, Ge-C system and Sb-C system are high nitrogen diamond crystals containing A-center and C-center, and the concentration is generally above 1000 ppm, which is much higher than diamond crystals growth in Ni-based, Fe-based catalyst [12–14]. According to the literature [15], the solubility of nitrogen in molten metal is related to the electronic structure of the metal atom. The outer electronic shell structures of Fe, Co and Ni are 3d64s2, 3d74s2 and 3d84s2, respectively. The smaller the number of atoms in the row
Corresponding authors. E-mail addresses: [email protected] (H. Ma), [email protected] (L. Chen), [email protected] (X. Jia).
https://doi.org/10.1016/j.jcrysgro.2019.125371 Received 14 October 2019; Received in revised form 19 November 2019; Accepted 20 November 2019 Available online 21 November 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
Journal of Crystal Growth 531 (2020) 125371
Z. Wang, et al.
elements of the periodic table, the less the number of electrons in the dshells, which means that the greater the possibility of nitrogen dissolution [16–18]. Therefore, the nitrogen atom has a greater solubility in the Fe-based catalyst than the Ni-based catalyst. That is why nitrogen impurities easily enter into diamond with the Fe-based catalyst. And because in transition metals, it is difficult for Fe atoms to enter the diamond structure, and nickel is extremely to enter the diamond, so the use of pure Fe catalyst can prevent nickel and other metal impurities from entering [19,20]. Therefore, pure Fe catalyst has an unparalleled advantage in the synthesis of high purity large single crystals of diamond. Although industrial grade diamond has been synthesized by iron powder previously, there are no reports on the synthesis of gem-quality diamond by Fe-C system. Therefore, in this work, the Ib and IIa gemquality diamond single crystals with a size of about 3 mm were successfully synthesized, and various characteristics were studied in detail.
Table 1 Experimental conditions and results. Run
P (GPa)
T (℃)
Catalyst
E-1 E-2 E-3 E-4 E-5
5.8 5.8 5.8 5.8 5.8
1300 1300 1550 1400 1550
Fe64Ni36 Fe80Ni20 Fe Fe80Ni20 Fe
Initial composition
Growth time (h)
Color
Nitrogen getter Nitrogen getter
23 23 23 23 23
Yellow Yellow Pale yellow Pale yellow Colorless
Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (Ra) and Photoluminescence (PL) spectra were used to determine the characteristics and quality of the synthesized gem-quality diamond crystals. The IR spectra was obtained on a Vertex80V Fourier transform infrared (FTIR) spectrometer with a spectral range between 1000 and 3500 cm−1 and a spectral resolution of 2 cm−1 in the transmittance mode. Raman spectroscopy was performed using a Renishaw in instrument equipped with a 532 nm excitation source. The PL spectra was measured with a 488 nm excitation and the resolution of 1.6 nm at room temperature.
2. Experimental details In this study, synthesis experiments of the gem-quality diamond was carried out by temperature gradient method (TGM) in China-type cubic anvil high-pressure apparatus (CHPA). The schematic diagram of synthesis assembly is shown in Fig. 1. High purity graphite (99.99 wt%) was used as the carbon source. Type Ib diamond was synthesized by using pure Fe (99.99 wt%) catalyst. On the basis of this, high-purity certain ratio of nitrogen getter (99.95 wt%) was utilized to synthesize type IIa diamond. A certain proportion of nitrogen getter and graphite powder were uniformly mixed and pressed into a cylindrical sample having a diameter of 16 mm as an initial carbon source. The {1 1 1} face of industrial grade diamond with a size of about 0.7 mm was selected as the seed crystal. To remove residual water from the samples, both the raw materials and sample assemblies were preserved in drying oven, and the sample assembly was dried by heating to 120 °C for 2 h before being placed in the high pressure apparatus. The synthesis pressure was 5.8 GPa and the synthesis temperature was 1550 °C. The pressure is calibrated by pressure-induced phase transition curves of bismuth, thallium and barium. The temperature was measured by the Pt-30% RH/Pt-6% RH thermocouple whose junction was placed near crystallization sample. After the HPHT experiments, the collected samples were dissolved in hot dilute nitric acid to remaining graphite and metal catalysts. And then the diamond crystals were placed in a boiling mixture of H2SO4 and HNO3 to remove the remaining graphite and metal on the diamond crystals surfaces. After treatment, the morphology of the synthesized diamond crystals were observed by optical microscope. Prior to the optical and characterizations, we repeatedly used ultrasonic equipment and deionized water for removing the residual impurities on diamond surface.
3. Results and analysis 3.1. Optical picture of the crystals The experimental conditions and the color of the obtained diamond crystals are summarized in Table 1. In order to show the nitrogen dissolving ability of pure Fe catalyst, we synthesized type Ib diamond by Fe64Ni36, Fe80Ni20 and pure Fe catalyst respectively. The type IIa diamond was synthesized by Fe80Ni20 and pure Fe with nitrogen getter. Among them, experiments E-1 to E-3 synthesized type Ib diamond with Fe64Ni36, Fe80Ni20 and pure Fe catalyst respectively, and experiments E4 and E-5 used separately Fe80Ni20 and pure Fe catalyst synthesized type IIa diamond by adding certain ratio of nitrogen getter. According to the Fe-Ni phase diagram, we can see that the melting point of pure Fe is high, When Ni is added to Fe, the melting point of the Fe-Ni Alloy decreases significantly, therefore the synthesis temperature of E-3 and E-5 is much higher than others. Fig. 2 shows optical images of the synthesized diamonds respectively. Obviously, the diamond crystal morphology synthesized by both Fe-Ni alloy and pure Fe catalyst exhibits an octahedral feature dominated by {1 1 1} sectors. The color of the diamond crystals of Fig. 2a and b did not change significantly, while the color of the diamond crystal in Fig. 2c was significantly brighter and lighter than the two diamond crystals (Fig. 2a and b). It can be seen from Fig. 2d and e that the diamond crystal synthesized by the Fe80Ni20 catalyst is yellow in color, and the diamond crystal synthesized by the pure Fe catalyst is nearly colorless. It is preliminarily judged that the nitrogen content of diamond crystals synthesized by pure Fe catalyst is less than that of Fe64Ni36 and Fe80Ni20 alloy, which proves that Fe has strong nitrogen dissolving ability. 3.2. The FTIR spectra analysis of crystals In order to further analyze the state of impurities in the diamond crystals, verify the above conclusions. In this experiment, the IR absorption spectra is used to identify the impurity kind and detect species. The IR absorption spectroscopy is usually used to measure the molecular structure, chemical composition and impurity formation, thereby determining the impurities and defect types. As shown in Fig. 3, we can clearly see that the absorption spectra at 1130 cm−1 and 1344 cm−1 in the curve a, b, and c. Generally, IR absorption bands at 1130 cm−1 and 1344 cm−1 indicate that the nitrogen atoms are isolated substitutional N impurity in the form of C-center [21–23]. It can be seen that the Ccenter nitrogen absorption peak intensity gradually decreases. The
Fig. 1. Sample assembly of diamond synthesized by HPHT: 1. Steel cap; 2. Graphite sheet; 3. Graphite heater; 4. ZrO2 + MgO sleeve; 5. Seed crystal; 6. Dolomite sleeve; 7. Copper sheet; 8. ZrO2 + MgO pillar; 9. Carbon source; 10. Metal catalyst; 11. NaCl + ZrO2 sleeve; 12. Pyrophyllite. 2
Journal of Crystal Growth 531 (2020) 125371
Z. Wang, et al.
Fig. 2. Optical images of the synthesized diamond growth in the different solvent catalysts: (a) obtained in E-1 experiment, (b) obtained in E-2 experiment, (c) obtained in E-3 experiment, (d) obtained in E-4 experiment, (e) obtained in E-5 experiment.
synthesized diamond crystals. The concentration of nitrogen impurity in the diamond crystals a, b, and c were calculated to be 230 ppm, 130 ppm, and 80 ppm, respectively. The results show that with the increase of Fe content in the metal catalyst, the nitrogen concentration in the synthesized diamond crystals decreases. The nitrogen concentration of the synthesized diamond crystals by pure Fe is only 80 ppm. Curve d and e are the infrared spectra of the diamond crystals synthesized with Fe80Ni20 and pure Fe catalyst in the same amount of nitrogen getter, respectively. Curve d shows two weak absorption peaks at 1130 cm−1 and 1344 cm−1, and the nitrogen concentration is calculated as 20 ppm. However, these two absorption peaks cannot be detected in curve e, which indicating that the concentration of C-center nitrogen of the diamond crystals synthesized by pure Fe is less than 1 ppm [27]. The above results indicate that the diamond crystals synthesized by pure Fe with certain ratio of nitrogen getter is type IIa diamond, while using Fe80Ni20 catalyst need more nitrogen getter to synthesize type IIa diamond. For the synthesis of high-purity IIa diamond crystals, it is necessary to add nitrogen getter to eliminate nitrogen in the synthetic environment, but a large amount of nitrogen getter will not only affects the characteristics of the catalyst, but also introduces more impurities in the diamond crystals. Therefore, pure Fe catalyst which need less nitrogen getter is more suitable for synthesizing high purity type IIa diamond large single crystals.
Fig. 3. The FTIR spectra of diamond crystals growth in the different solvent catalysts: (a) obtained in E-1 experiment, (b) obtained in E-2 experiment, (c) obtained in E-3 experiment, (d) obtained in E-4 experiment, (e) obtained in E-5 experiment.
nitrogen content was assessed by the absorption strength of the band peaking at about 1130 cm−1 and 1344 cm−1, the concentration of Ccenter nitrogen in the diamond can be calculated by using the absorption coefficient of the FTIR spectra according to the following formula [24–26]:
3.3. Raman spectroscopy analysis of crystals
Nc (ppm) = μ (1130 cm−1)/μ (2120 cm−1) × 5.5 × 25
As an important analytical tool for studying diamond crystals, Raman is often used to study internal structural changes of diamond crystals. Raman spectral analysis was performed on the synthesized diamond crystals in this experiment, and the results were shown in Fig. 4. It can be clearly seen from the figure that the Raman spectrum of the diamond single crystals show only a very strong and narrow peak near 1333 cm−1, which is a typical Raman characteristic peak of diamond. It proved that all synthetic diamond crystals have a single sp3 structure, which means that each diamond obtained has a high quality single crystal structure [28]. The WFHM (full widths at half maximum) of each diamond Raman peaks (a, b, c, d, and e) is 4.55 cm−1,
μ (1130 cm−1) = [A(1290 cm−1) − A(1370 cm−1)/0.31] μ (2120 cm−1) = [[40 × A(2030 cm−1) + A(2160 cm−1)]/127 − A(2120 cm−1)] where Nc (ppm) is the concentration of nitrogen, and μ and A are the corrected coefficients and the absorption intensity of the relevant peak in the FTIR spectra, respectively. As followed from the infrared absorption measurement, the calculated nitrogen concentrations of the 3
Journal of Crystal Growth 531 (2020) 125371
Z. Wang, et al.
the type Ib diamond crystals synthesized in the Fe64Ni36, Fe80Ni20 system. At the same time, we clearly see that curve d shows a strong peak at 575 nm, while curve e shows only the characteristic peak of diamond, the NV center is absent in the diamond lattice. According to the literature [31], no nitrogen-related defect center can be detected in type IIa diamond crystals, which means that the nitrogen concentration of diamond crystal obtained in E-5 experiment is less than 1 ppm. The curves in the Fig. 5 all showed a peak at 554 nm, and the peak at 554 nm was assigned to the known nitrogen-bearing centers; it is observed in some nitrogen-bearing diamonds, but the composition of the peak is currently unknown. The peak of 554 nm is not NV associated centers, therefore, no detailed explanation was given to them. 4. Conclusion In conclusion, we successfully synthesized high-quality gem-quality diamond crystals with a size of about 3.0 mm in pure Fe-C system by temperature gradient method at 5.8 GPa and 1550 °C. The IR spectrum of the diamond crystals shows that the nitrogen concentration of the pure Fe-C system synthesized diamond crystals is only 80 ppm, which is much lower than that of Fe-Ni alloy catalysts. Raman spectroscopy shows that the pure Fe catalyst does not affect the quality of the diamond crystal when it has strong ability to dissolve nitrogen. In addition, type IIa diamond crystals can be synthesized by pure Fe-C system with less nitrogen-getter. PL spectra indicate that the nitrogen-related defects in the Ib and IIa diamond crystals synthesized by pure Fe catalyst are less than those synthesized by Fe64Ni36 and Fe80Ni20 catalysts. The research results are of great value for the synthesis of high purity IIa gem-quality diamond crystals without impurities such as nitrogen and nickel.
Fig. 4. Raman spectroscopy of diamond crystals growth in the different solvent catalysts: (a) obtained in E-1 experiment, (b) obtained in E-2 experiment, (c) obtained in E-3 experiment, (d) obtained in E-4 experiment, (e) obtained in E-5 experiment.
Declaration of Competing Interest There are no conflicts of interest to declare. Acknowledgements The project was supported by the National Natural Science Foundation of China (11604246, 51772120, 51872112 and 11804305), the China Postdoctoral Science Foundation (2017M622360), the Project of Jilin Science and Technology Development Plan (Grant No. 20180201079GX).
Fig. 5. PL spectra of diamond crystals growth in the different solvent catalysts: (a) obtained in E-1 experiment, (b) obtained in E-2 experiment, (c) obtained in E-3 experiment, (d) obtained in E-4 experiment, (e) obtained in E-5 experiment.
References
4.42 cm−1, 4.36 cm−1, 4.29 cm−1, and 4.25 cm−1, respectively. It is obvious that the WFHM is gradually narrowed, indicating that the crystallinity of the diamond crystals become higher and the quality of the diamond crystals become better [29]. Combined with the results of IR absorption spectrum, it is easy to find that the pure Fe catalyst can not only reduce the entry of impurities, but also ensure the diamond very high crystallinity.
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3.4. The PL spectra analysis of crystals PL is used to study the types of impurities and defects in synthetic diamonds. In Fig. 5, curve a, b and c show the PL spectra of the type Ib diamond crystals obtained in the experiments of E-1 to E-3, respectively. We note that the spectra of curve a and b show a zero-phonon line at 637 nm which due to the negatively charged state of the center of NV− [30,31]. Furthermore, curve b exhibits a weak peak at 575 nm caused by the neutral charge states of the NV0 center. In curve c, there are no PL peaks at 575 nm and 637 nm [30,31], and no nitrogen-related defect centers are detected in diamond crystal obtained in E-3 experiment, which indicating the center of nitrogen defects (NV0, NV−) in the type Ib diamond crystals synthesized by pure Fe-C system is less than 4
Journal of Crystal Growth 531 (2020) 125371
Z. Wang, et al.
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5
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Synthesis and characterization of gem diamond single crystals in Fe-C system under high temperature and high pressure ⁎
T ⁎
Zhanke Wanga, Hongan Maa, , Shuai Fanga, Zhiqiang Yanga, Xinyuan Miaoa, Liangchao Chenb, , ⁎ Xiaopeng Jiaa, a b
State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China Key Laboratory of Material Physics of Ministry of Education, School of Physical Engineering, Zhengzhou University, Zhengzhou 450052, China
A R T I C LE I N FO
A B S T R A C T
Communicated by P. Rudolph
In this paper, the diamond large single crystals were successfully synthesized in the pure Fe-C system by temperature gradient growth (TGG) method in China-type cubic anvil high-pressure apparatus (CHPA) at 5.8 GPa and 1550 °C. The synthesized diamonds were characterized by Optical microscopy, Infrared spectroscopy, Raman spectroscopy and Photoluminescence (PL) spectroscopy. The results show that the nitrogen concentration of type Ib diamonds synthesized in pure Fe-C system is about 80 ppm, which is much lower than diamonds synthesized by Ni-based and Fe-based alloys. PL spectra result indicated that there were no peaks related to the NV centers in diamonds synthesized by Fe-C system. In addition, high-quality type IIa gem diamond can be synthesized by adding a small amount of nitrogen getter in the pure Fe-C system. We believe that this work is of great significance for the synthesis of high-purity large single type IIa diamonds without impurities such as nitrogen and metals.
Keywords: A1. Impurities A2. Growth from high temperature solutions A2. Single crystal growth B1. Diamond B1. Metals B3. Infrared devices
1. Introduction According to the concentration and presence of nitrogen and boron impurities, diamond can be divided into two types, type I and II, and further subdivided into four types of Ia, Ib, IIa and IIb [1,2]. Among them, Type IIa diamond contains no impurities such as boron, nitrogen and other metals, high crystallinity and low defect density make it exhibit high performance in diamond-based power devices. Therefore, high purity type IIa diamond is considered to be a very promising defect-free substrate. In addition, high-purity diamonds have important applications in advanced fields such as quantum fields, high-precision sensors, and high-resolution imaging technologies [3–6]. Nowadays, methods for synthesizing high-purity diamond large single crystals mainly include chemical vapor deposition (CVD) and high temperature and high pressure (HPHT) methods, but there are still problems in these two high-purity diamond synthesis methods. For example, stress and defects in diamond large single crystals prepared by CVD, metal impurities in diamond large single crystals prepared by HPHT, and so on. In order to solve the problem of metal impurities in the high-purity diamond crystals synthesized by HPHT, it is essential to select a suitable catalyst for growing a “high-purity” type IIa diamond single crystal. It is well known that metal solvent plays a vital role in the process of
⁎
synthesis of diamond by HPHT. The research of diamonds grown in different catalytic systems is required and helpful for understanding the nucleation and growth mechanism of diamond and synthesizing diamonds with specific properties [7]. Based on a lot of experimental research, it is found that conventional metal catalysts are mainly transition metals such as Fe, Co, Ni, Mn, Cr, etc. [8]. In addition, other metals (Zn, Sn, Cu, Ge, Sb, etc.) and non-metals (carbonates, silicates, hydrides, oxides, etc.) have also been used as catalysts to synthesize diamond [9–11]. These catalyst solvents require relatively high pressures and temperatures (7.0–8.0 GPa, 1600–2000 °C) in the process of synthesizing diamond, and the size of diamond is generally less than 1.0 mm. When the duration of synthesis is prolonged, spontaneous nuclei are easily formed and the quality of diamonds is difficult to control [12]. Moreover, diamonds synthesized in metal solvent systems such as Cu-C system, Ge-C system and Sb-C system are high nitrogen diamond crystals containing A-center and C-center, and the concentration is generally above 1000 ppm, which is much higher than diamond crystals growth in Ni-based, Fe-based catalyst [12–14]. According to the literature [15], the solubility of nitrogen in molten metal is related to the electronic structure of the metal atom. The outer electronic shell structures of Fe, Co and Ni are 3d64s2, 3d74s2 and 3d84s2, respectively. The smaller the number of atoms in the row
Corresponding authors. E-mail addresses: [email protected] (H. Ma), [email protected] (L. Chen), [email protected] (X. Jia).
https://doi.org/10.1016/j.jcrysgro.2019.125371 Received 14 October 2019; Received in revised form 19 November 2019; Accepted 20 November 2019 Available online 21 November 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
Journal of Crystal Growth 531 (2020) 125371
Z. Wang, et al.
elements of the periodic table, the less the number of electrons in the dshells, which means that the greater the possibility of nitrogen dissolution [16–18]. Therefore, the nitrogen atom has a greater solubility in the Fe-based catalyst than the Ni-based catalyst. That is why nitrogen impurities easily enter into diamond with the Fe-based catalyst. And because in transition metals, it is difficult for Fe atoms to enter the diamond structure, and nickel is extremely to enter the diamond, so the use of pure Fe catalyst can prevent nickel and other metal impurities from entering [19,20]. Therefore, pure Fe catalyst has an unparalleled advantage in the synthesis of high purity large single crystals of diamond. Although industrial grade diamond has been synthesized by iron powder previously, there are no reports on the synthesis of gem-quality diamond by Fe-C system. Therefore, in this work, the Ib and IIa gemquality diamond single crystals with a size of about 3 mm were successfully synthesized, and various characteristics were studied in detail.
Table 1 Experimental conditions and results. Run
P (GPa)
T (℃)
Catalyst
E-1 E-2 E-3 E-4 E-5
5.8 5.8 5.8 5.8 5.8
1300 1300 1550 1400 1550
Fe64Ni36 Fe80Ni20 Fe Fe80Ni20 Fe
Initial composition
Growth time (h)
Color
Nitrogen getter Nitrogen getter
23 23 23 23 23
Yellow Yellow Pale yellow Pale yellow Colorless
Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (Ra) and Photoluminescence (PL) spectra were used to determine the characteristics and quality of the synthesized gem-quality diamond crystals. The IR spectra was obtained on a Vertex80V Fourier transform infrared (FTIR) spectrometer with a spectral range between 1000 and 3500 cm−1 and a spectral resolution of 2 cm−1 in the transmittance mode. Raman spectroscopy was performed using a Renishaw in instrument equipped with a 532 nm excitation source. The PL spectra was measured with a 488 nm excitation and the resolution of 1.6 nm at room temperature.
2. Experimental details In this study, synthesis experiments of the gem-quality diamond was carried out by temperature gradient method (TGM) in China-type cubic anvil high-pressure apparatus (CHPA). The schematic diagram of synthesis assembly is shown in Fig. 1. High purity graphite (99.99 wt%) was used as the carbon source. Type Ib diamond was synthesized by using pure Fe (99.99 wt%) catalyst. On the basis of this, high-purity certain ratio of nitrogen getter (99.95 wt%) was utilized to synthesize type IIa diamond. A certain proportion of nitrogen getter and graphite powder were uniformly mixed and pressed into a cylindrical sample having a diameter of 16 mm as an initial carbon source. The {1 1 1} face of industrial grade diamond with a size of about 0.7 mm was selected as the seed crystal. To remove residual water from the samples, both the raw materials and sample assemblies were preserved in drying oven, and the sample assembly was dried by heating to 120 °C for 2 h before being placed in the high pressure apparatus. The synthesis pressure was 5.8 GPa and the synthesis temperature was 1550 °C. The pressure is calibrated by pressure-induced phase transition curves of bismuth, thallium and barium. The temperature was measured by the Pt-30% RH/Pt-6% RH thermocouple whose junction was placed near crystallization sample. After the HPHT experiments, the collected samples were dissolved in hot dilute nitric acid to remaining graphite and metal catalysts. And then the diamond crystals were placed in a boiling mixture of H2SO4 and HNO3 to remove the remaining graphite and metal on the diamond crystals surfaces. After treatment, the morphology of the synthesized diamond crystals were observed by optical microscope. Prior to the optical and characterizations, we repeatedly used ultrasonic equipment and deionized water for removing the residual impurities on diamond surface.
3. Results and analysis 3.1. Optical picture of the crystals The experimental conditions and the color of the obtained diamond crystals are summarized in Table 1. In order to show the nitrogen dissolving ability of pure Fe catalyst, we synthesized type Ib diamond by Fe64Ni36, Fe80Ni20 and pure Fe catalyst respectively. The type IIa diamond was synthesized by Fe80Ni20 and pure Fe with nitrogen getter. Among them, experiments E-1 to E-3 synthesized type Ib diamond with Fe64Ni36, Fe80Ni20 and pure Fe catalyst respectively, and experiments E4 and E-5 used separately Fe80Ni20 and pure Fe catalyst synthesized type IIa diamond by adding certain ratio of nitrogen getter. According to the Fe-Ni phase diagram, we can see that the melting point of pure Fe is high, When Ni is added to Fe, the melting point of the Fe-Ni Alloy decreases significantly, therefore the synthesis temperature of E-3 and E-5 is much higher than others. Fig. 2 shows optical images of the synthesized diamonds respectively. Obviously, the diamond crystal morphology synthesized by both Fe-Ni alloy and pure Fe catalyst exhibits an octahedral feature dominated by {1 1 1} sectors. The color of the diamond crystals of Fig. 2a and b did not change significantly, while the color of the diamond crystal in Fig. 2c was significantly brighter and lighter than the two diamond crystals (Fig. 2a and b). It can be seen from Fig. 2d and e that the diamond crystal synthesized by the Fe80Ni20 catalyst is yellow in color, and the diamond crystal synthesized by the pure Fe catalyst is nearly colorless. It is preliminarily judged that the nitrogen content of diamond crystals synthesized by pure Fe catalyst is less than that of Fe64Ni36 and Fe80Ni20 alloy, which proves that Fe has strong nitrogen dissolving ability. 3.2. The FTIR spectra analysis of crystals In order to further analyze the state of impurities in the diamond crystals, verify the above conclusions. In this experiment, the IR absorption spectra is used to identify the impurity kind and detect species. The IR absorption spectroscopy is usually used to measure the molecular structure, chemical composition and impurity formation, thereby determining the impurities and defect types. As shown in Fig. 3, we can clearly see that the absorption spectra at 1130 cm−1 and 1344 cm−1 in the curve a, b, and c. Generally, IR absorption bands at 1130 cm−1 and 1344 cm−1 indicate that the nitrogen atoms are isolated substitutional N impurity in the form of C-center [21–23]. It can be seen that the Ccenter nitrogen absorption peak intensity gradually decreases. The
Fig. 1. Sample assembly of diamond synthesized by HPHT: 1. Steel cap; 2. Graphite sheet; 3. Graphite heater; 4. ZrO2 + MgO sleeve; 5. Seed crystal; 6. Dolomite sleeve; 7. Copper sheet; 8. ZrO2 + MgO pillar; 9. Carbon source; 10. Metal catalyst; 11. NaCl + ZrO2 sleeve; 12. Pyrophyllite. 2
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Fig. 2. Optical images of the synthesized diamond growth in the different solvent catalysts: (a) obtained in E-1 experiment, (b) obtained in E-2 experiment, (c) obtained in E-3 experiment, (d) obtained in E-4 experiment, (e) obtained in E-5 experiment.
synthesized diamond crystals. The concentration of nitrogen impurity in the diamond crystals a, b, and c were calculated to be 230 ppm, 130 ppm, and 80 ppm, respectively. The results show that with the increase of Fe content in the metal catalyst, the nitrogen concentration in the synthesized diamond crystals decreases. The nitrogen concentration of the synthesized diamond crystals by pure Fe is only 80 ppm. Curve d and e are the infrared spectra of the diamond crystals synthesized with Fe80Ni20 and pure Fe catalyst in the same amount of nitrogen getter, respectively. Curve d shows two weak absorption peaks at 1130 cm−1 and 1344 cm−1, and the nitrogen concentration is calculated as 20 ppm. However, these two absorption peaks cannot be detected in curve e, which indicating that the concentration of C-center nitrogen of the diamond crystals synthesized by pure Fe is less than 1 ppm [27]. The above results indicate that the diamond crystals synthesized by pure Fe with certain ratio of nitrogen getter is type IIa diamond, while using Fe80Ni20 catalyst need more nitrogen getter to synthesize type IIa diamond. For the synthesis of high-purity IIa diamond crystals, it is necessary to add nitrogen getter to eliminate nitrogen in the synthetic environment, but a large amount of nitrogen getter will not only affects the characteristics of the catalyst, but also introduces more impurities in the diamond crystals. Therefore, pure Fe catalyst which need less nitrogen getter is more suitable for synthesizing high purity type IIa diamond large single crystals.
Fig. 3. The FTIR spectra of diamond crystals growth in the different solvent catalysts: (a) obtained in E-1 experiment, (b) obtained in E-2 experiment, (c) obtained in E-3 experiment, (d) obtained in E-4 experiment, (e) obtained in E-5 experiment.
nitrogen content was assessed by the absorption strength of the band peaking at about 1130 cm−1 and 1344 cm−1, the concentration of Ccenter nitrogen in the diamond can be calculated by using the absorption coefficient of the FTIR spectra according to the following formula [24–26]:
3.3. Raman spectroscopy analysis of crystals
Nc (ppm) = μ (1130 cm−1)/μ (2120 cm−1) × 5.5 × 25
As an important analytical tool for studying diamond crystals, Raman is often used to study internal structural changes of diamond crystals. Raman spectral analysis was performed on the synthesized diamond crystals in this experiment, and the results were shown in Fig. 4. It can be clearly seen from the figure that the Raman spectrum of the diamond single crystals show only a very strong and narrow peak near 1333 cm−1, which is a typical Raman characteristic peak of diamond. It proved that all synthetic diamond crystals have a single sp3 structure, which means that each diamond obtained has a high quality single crystal structure [28]. The WFHM (full widths at half maximum) of each diamond Raman peaks (a, b, c, d, and e) is 4.55 cm−1,
μ (1130 cm−1) = [A(1290 cm−1) − A(1370 cm−1)/0.31] μ (2120 cm−1) = [[40 × A(2030 cm−1) + A(2160 cm−1)]/127 − A(2120 cm−1)] where Nc (ppm) is the concentration of nitrogen, and μ and A are the corrected coefficients and the absorption intensity of the relevant peak in the FTIR spectra, respectively. As followed from the infrared absorption measurement, the calculated nitrogen concentrations of the 3
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the type Ib diamond crystals synthesized in the Fe64Ni36, Fe80Ni20 system. At the same time, we clearly see that curve d shows a strong peak at 575 nm, while curve e shows only the characteristic peak of diamond, the NV center is absent in the diamond lattice. According to the literature [31], no nitrogen-related defect center can be detected in type IIa diamond crystals, which means that the nitrogen concentration of diamond crystal obtained in E-5 experiment is less than 1 ppm. The curves in the Fig. 5 all showed a peak at 554 nm, and the peak at 554 nm was assigned to the known nitrogen-bearing centers; it is observed in some nitrogen-bearing diamonds, but the composition of the peak is currently unknown. The peak of 554 nm is not NV associated centers, therefore, no detailed explanation was given to them. 4. Conclusion In conclusion, we successfully synthesized high-quality gem-quality diamond crystals with a size of about 3.0 mm in pure Fe-C system by temperature gradient method at 5.8 GPa and 1550 °C. The IR spectrum of the diamond crystals shows that the nitrogen concentration of the pure Fe-C system synthesized diamond crystals is only 80 ppm, which is much lower than that of Fe-Ni alloy catalysts. Raman spectroscopy shows that the pure Fe catalyst does not affect the quality of the diamond crystal when it has strong ability to dissolve nitrogen. In addition, type IIa diamond crystals can be synthesized by pure Fe-C system with less nitrogen-getter. PL spectra indicate that the nitrogen-related defects in the Ib and IIa diamond crystals synthesized by pure Fe catalyst are less than those synthesized by Fe64Ni36 and Fe80Ni20 catalysts. The research results are of great value for the synthesis of high purity IIa gem-quality diamond crystals without impurities such as nitrogen and nickel.
Fig. 4. Raman spectroscopy of diamond crystals growth in the different solvent catalysts: (a) obtained in E-1 experiment, (b) obtained in E-2 experiment, (c) obtained in E-3 experiment, (d) obtained in E-4 experiment, (e) obtained in E-5 experiment.
Declaration of Competing Interest There are no conflicts of interest to declare. Acknowledgements The project was supported by the National Natural Science Foundation of China (11604246, 51772120, 51872112 and 11804305), the China Postdoctoral Science Foundation (2017M622360), the Project of Jilin Science and Technology Development Plan (Grant No. 20180201079GX).
Fig. 5. PL spectra of diamond crystals growth in the different solvent catalysts: (a) obtained in E-1 experiment, (b) obtained in E-2 experiment, (c) obtained in E-3 experiment, (d) obtained in E-4 experiment, (e) obtained in E-5 experiment.
References
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