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HPHT synthesis of N–H co-doped diamond single crystals
Journal of Crystal Growth 436 (2016) 34–39
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
HPHT synthesis of N–H co-doped diamond single crystals Chao Fang a, Xiaopeng Jia a, Ning Chen a, Yadong Li a, Longsuo Guo a, Liangchao Chen a, Hong-an Ma a,n, Xiaobing Liu b,n a b
State Key Lab of Superhard Materials, Jilin University, Changchun 130012, China Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60202, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 25 September 2015 Received in revised form 10 November 2015 Accepted 25 November 2015 Communicated by: T. Paskova Available online 10 December 2015
In this paper, a series of nitrogen and hydrogen elements (N–H) co-doped diamond single crystals were synthesized in the NiMnCo–C system with melamine (C3N6H6) additive by temperature gradient growth (TGG) method under high-pressure and high-temperature (HPHT). High nitrogen and hydrogen growth environment obviously reduces the growth rate of the diamond. The synthetic diamond crystals are octahedral shape with a green or dark green color. This results indicate that the morphology of the synthetic N–H co-doped gem-diamond can be well-controlled by TGG method. Fourier transform infrared spectroscopy (FTIR) indicate that high-quality diamond single crystals with nitrogen concentrations up to 2100 ppm have been successfully synthesized and the hydrogen related peaks at 2850 cm 1 (sp3–CH3) and 2920 cm 1 (sp3–CH2–) are found in these synthetic diamond. Meanwhile, with the increasing of C-centers the nitrogen atoms aggregate and form into A-centers in these synthetic diamonds. This is the highest quality of N–H co-doped gem-diamond single crystal synthesized in high nitrogen and high hydrogen growth environment reported so far. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Infrared spectrum A2. HPHT A2. Temperature gradient growth method B1. Diamond
1. Introduction Due to the outstanding physical and chemical properties, such as hardness, chemical inertness, and thermal conductivity, diamond as a promising material has attracted wide research attention for mechanical and electric applications [1–4]. As the most common atomic impurity in diamond, the concentration and form of nitrogen play a very important role in the behavior and physical properties of diamond. The vast majority of synthetic diamonds are dominantly type Ib (200–300 ppm). Most natural diamonds usually contain aggregated nitrogen up to thousands of ppm [5–7]. Thus the study on the effect on diamond crystallization is important to reveal the genesis of natural diamond. Recent years, diamonds with nitrogen concentration comparable with natural diamond were reported [8] to be fabricated under high pressure and high temperature (HPHT). However, the crystals have irregular shape and rough surfaces or the forms of the nitrogen in those synthetic diamond were found to present as single substitutional atoms (C-centers) [9,10] while natural diamond contains aggregated nitrogen impurities, pairs of nearest neighboring substitutional nitrogen atoms (A-centers) and groups of four atoms around n
Corresponding authors. E-mail addresses: [email protected] (H.-a. Ma), [email protected] (X. Liu).
http://dx.doi.org/10.1016/j.jcrysgro.2015.11.042 0022-0248/& 2015 Elsevier B.V. All rights reserved.
a vacancy (B-centers), leading to another important stage in solving problems of diamond genesis. Besides the nitrogen, hydrogen is one of the other common impurities in natural diamond [11–13]. Recently, some experimental researches have been focused on the study of the diamond crystalline behavior in the nitrogen and hydrogen elements (N–H) environment and some N–H co-doped diamonds have been successfully synthesized by film growth methods (FGM) and floatage method under HPHT [14,15]. However, previous work found that the synthesized diamonds usually contain low nitrogen concentration and show irregular shape with bad quality in the N–H environment. So, synthesizing high-quality N–H co-doped gem diamond in high nitrogen and high hydrogen growth environment is still a challenge. At the same time high-quality N–H co-doped gem diamond is still highly demanded for the science and industrial application. In this work, we performed a systematic study on the influence of nitrogen and hydrogen on diamond growth habit by the temperature gradient growth (TGG) method under HPHT over a wide range of temperatures from 1300 to 1470 °C and pressures from 5.4 to 6.5 GPa. High-quality N–H co-doped gem-diamonds with nitrogen concentration up to several thousands of ppm were synthesized using NiMnCo alloy. The synthesized diamonds were characterized by optical, scanning electron microscope (SEM), Fourier transform infrared micro-spectroscopy (FTIR), Singlecrystal x-ray diffraction (XRD) and Raman spectroscopy.
C. Fang et al. / Journal of Crystal Growth 436 (2016) 34–39
35
2. Experimental procedures
3. Results and discussion
Experiments on diamond crystallization were carried out using a china-type large volume Cubic High-Pressure Apparatus (CHPA) (SPD-6 1200). The sample assembly for diamond synthesis by HPHT is shown in Fig. 1. Graphite with a purity of 99.99 % was used as carbon source. The Ni70Mn25Co5 alloy (NiMnCo) was used as catalyst. Melamine (C3N6H6) (99.9% purity) powders were chose as nitrogen and hydrogen source, and Ti was used as the nitrogen getter during diamond crystalline. High-quality seed crystals with {111} facets were selected as the growth facet. Each of the sample assembly was dried by heating to 120 °C for 2 h before being placed in the high pressure apparatus to remove the influence of the residual water on the growth of diamonds. The pressure was calibrated by the pressure-induced phase transitions of bismuth, thallium, and barium. The temperature was measured by the Pt30%Rh/Pt-6%Rh thermocouple, whose junction was placed near crystallization sample [16]. After HPHT experiments, the collected samples were cracked and treated in the mixture of H2SO4 and HNO3 to remove remaining graphitic components and metal catalysts. Then the synthesized samples were observed by optical microscope and scanning electron microscope (SEM). Fourier transform infrared spectroscopy (FTIR) was used to determine the characteristics of the synthesized diamonds. The IR spectra were obtained on a Vertex80V Fourier transform infrared (FTIR) spectrometer in the spectral range between 400 and 4000 cm 1 with a spectral resolution of 2 cm 1 in the transmittance mode. Single-crystal x-ray diffraction (XRD) was used to confirm the monocrystalline structure and Raman spectroscopy was used to analyze the stress information.
3.1. The change of synthetic conditions and growth rate for synthesize diamond. The diamond growth and crystallization have been established by TGG method under the influence of N–H in the NiMnCo–C system at pressures ranging from 5.4 to 6.5 GPa and temperatures of 1300–1470 °C. The pressure and temperature conditions and the experimental results are summarized in Table 1. In the growth, we observe that both the P and T synthetic conditions were increased by the adding of C3N6H6 in the NiMnCo–C system for octahedron diamond. At 5.5 GPa and 1320 °C in NiMnCo–C system, the diamond behaves octahedron shape with high quality. When 0.03 wt% C3N6H6 is added into the system, no diamond growth was found at 5.5 GPa and 1320–1380 °C, while only regrown metastable graphite appears. With increase P–T conditions up to 5.8 GPa, the diamond is black and bad quality with irregular shape at 1320 °C. When increase temperature to 1380 °C, a high quality diamond with octahedral shape can be fabricated. Finally, the synthetic pressure has been rise up 1 GPa while the temperature for diamond synthesis increases about 150 °C in the system with 0.12 wt% C3N6H6 additive. The relationship between the minimum synthesis conditions (pressure and temperature) for growing octahedron diamond and the amount of C3N6H6 additive is shown in Fig. 2. It is found that the minimum pressure and temperature for diamond synthesis increase with adding of C3N6H6 in the system that is consistent with previous results [17]. However, when the nitrogen getter (Ti 1%) is added into the starting materials, we notice that synthetic P–T conditions for diamond synthesis were observed to decrease about 0.1 GPa and 20 °C. Thus we believe that the nitrogen
Fig. 1. The sample assembly for synthesis diamond by HPHT. 1.pyrophyllite; 2. conductive steel ring; 3.graphite heater; 4.insulator; 5.carbon source; 6.alloy catalyst; 7.as-grown diamond; 8.seed crystal.
Fig. 2. The relationships between the concentration of C3N6H6 and the minimal P and T for octahedron diamond growth.
Table 1 Experimental results of diamond synthesis in NiMnCo–C system at HPHT. Run
Initial composition
wt%
Pressure (GPa)
Temperature (°C)
Time (h)
Morphology
Color
D-1 D-2 D-3 D-4 D-5 D-6 D-7 D-8
Ti – C3N6H6 C3N6H6 C3N6H6 C3N6H6 C3N6H6 C3N6H6
1 – 0.03 0.03 0.03 0.06 0.09 0.12
5.4 5.5 5.5 5.8 5.8 6.0 6.2 6.5
1300 1320 1320–1380 1320 1380 1420 1440 1470
8 8 8 8 8 8 8 8
Octahedron Octahedron – Irregular Octahedron Octahedron Octahedron Octahedron
Colorless Yellow – Black Greenish Yellow Green Deep green Dark green
36
C. Fang et al. / Journal of Crystal Growth 436 (2016) 34–39
impurity as well as hydrogen impurity plays a key role in the increasing of P–T for diamond synthesis. Meanwhile, there is no report about the effect of high nitrogen and high hydrogen environment on gem diamond growth rate by far. We find the diamond crystal growth rate decreases with increasing the nitrogen and hydrogen content in diamondgrowing environment as shown in Fig. 3. These results indicate high nitrogen and high hydrogen environment obviously reduces the diamond grow rate.
Fig. 3. Curves of growth rate versus the added amount of C3N6H6.
3.2. Morphology, color and SEM photographs of synthetic diamond crystals The optical properties and morphology of the produced diamonds are displayed in Fig. 4. Different from the previously work that only strip or irregular diamonds with bad quality were found in N–H system by film growth method [14] and floatage method [15], the morphology of the synthetic diamond can be wellcontrolled by the TGG method. As shown, the produced diamond crystals exhibit good morphology and are mainly composed of {111} faces. It is clear that the synthesized diamond crystals change from colorless to yellow, yellowish green and dark green with the increasing of C3N6H6 in the NiMnCo–C system. The synthesized diamond with 0.03 wt% C3N6H6 additive has greenish yellow color (Fig. 4c) and the maximum size is about 2.0–2.5 mm in 8 h growth process. When the C3N6H6 additive increases up to 0.06 wt%, the diamond crystal has green color (Fig. 4d). As the C3N6H6 is increase to 0.09 wt%, the produced diamond shows deep green color (Fig. 4e). With the increase of C3N6H6 additive to 0.12 wt%, the color of the synthesized diamond crystals is almost dark green (Fig. 4f). From the optical images, the produced diamond crystals by the TGG method show good quality without obvious pits, inclusions and other defects. In order to analyze the surface characters of the synthesized diamond, we took SEM photographs of synthetic crystals, Fig. 5a–c shows the typical crystals synthesized in the NiMnCo–C system with 1 wt% Ti, 0.06 wt% and 0.12 wt% C3N6H6 additive, respectively. Fig. 5d, e, f are the partial enlarged drawing of Fig. 5a, b, c. We observe that all the surfaces are flat.
Fig. 4. Optical images of diamond obtained from the NiMnCo–C system: (a) with Ti additive; (b) conventional NiMnCo–C system; (c, d, e, f) with C3N6H6 additive: 0.03 wt%, 0.06 wt%, 0.09 wt%, 0.12 wt%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
C. Fang et al. / Journal of Crystal Growth 436 (2016) 34–39
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Fig. 5. SEM images of the diamond crystal grown from NiMnCo–C system: (a, b, c) with Ti, 0.06 wt% and 0.12 wt% C3N6H6 additive, respectively. (d, e, f) are the partial enlarged drawing of (a, b, c).
3.3. FTIR spectra and nitrogen concentrations To identify impurities and defect species in diamond, we carried IR absorption measurement on selected diamond crystals. The typical IR spectra for these diamonds are presented in Fig. 6. The synthetic diamond in the conventional NiMnCo–C system (Fig. 6b) has absorption bands at 1130 and 1344 cm 1 due to nitrogen impurities predominantly in the form of C-center (single substitutional nitrogen atoms) [18,19]. We notice that the synthetic diamond with Ti additive in the system (Fig. 6a) has no absorption bands of nitrogen impurity. FTIR spectra in Fig. 6c–f show that the diamond crystals contain high nitrogen concentration which is exist in the form of A-centers at 1282 cm 1 (pairs of nearest neighboring pairs substitutional nitrogen atoms) [18,19] and Ccenters with C3N6H6 additive in the system. The nitrogen concentration N1130 (ppm) and N1282 (ppm) were determined using the FTIR spectra and the method as follow [20–24]: uð1130cm 1 Þ ¼ A 1290cm 1 –A 1370cm 1 =0:31 uð2120cm 1 Þ ¼
40 A 2030cm 1 þ 87 A 2160cm 1 =127–A 2120cm 1
The separation formula of A-center and C-center is shown as follows: Að1282cm 1 Þ ¼ μ 1282cm 1 þ 0:35μ 1130cm 1 Að1130cm 1 Þ ¼ μ 1130cm 1 þ 0:21μ 1282cm 1 uð1282cm 1 Þ ¼ 100 A 1282cm 1 –35 A 1130cm 1 =92:65 Follow the methods of Kiflawi et.al: NC (ppm) ¼(25 72)
αC(cm 1) and Boyd et.al: NA (ppm) ¼(16.5 71) αA(cm 1), the nitrogen concentration N1130 (ppm) and N1282 (ppm) can be determined: N 1130 ðppmÞ ¼ μ 1130cm 1 =μ 2120cm 1 5:5 25 N 1282 ðppmÞ ¼ μ 1282cm 1 =μ 2120cm 1 5:5 16:5 where μ and A are absorption intensity and recorded values of absorbance, respectively. We calculated the nitrogen concentration in those diamonds using the obtained FTIR spectra. The concentration with uncertainties 10% in produced diamond crystals
is shown in Table 2. The highest nitrogen concentration up to 2100 ppm was found in the diamond synthesized in NiMnCo–C system with 0.12 wt% C3N6H6 additive. Furthermore, we notice that with the increasing C-form the nitrogen atoms aggregate and form into A-centers in these synthetic diamonds. The inset image lying at top right corner reveals major changes in hydrogen-related vibration at 2700–3500 cm 1. From the IR spectra, we notice that the hydrogen-related modes locate at around 2850 and 2920 cm 1 that is attribute to sp3–CH3 and sp3– CH2– [15,25], respectively, are found in most synthetic diamond. The intensities of the hydrogen-related peaks increase with an enhancement in the ratio of C3N6H6 additive. Furthermore, we also observe the appearance of broad peak ranging from 3100 cm 1 to 3350 cm 1 (–NH–) [26,27] in the IR spectra of N–H rich diamonds (inset image lying at top right corner: c–f). All above results indicate that the nitrogen and hydrogen have been simultaneously incorporated into the diamond. 3.4. Characterization of diamond crystal by single-crystal XRD and Raman spectra Single-crystal XRD is most commonly used for crystal structure determination. So, we confirm the monocrystalline structure of the produced diamond as shown in Fig. 7. It is found from Fig. 7 that the diffraction pattern of N–H co-doped diamond is mainly composed of the {111} lattice plane and two small weak peak at {220} and {311}. This result indicates that the crystal gives priority to {111} surface. Meanwhile, from the single-crystal diffraction pattern (inset image lying at top right corner in Fig. 7) of N–H codoped diamond, we find that a series of diffraction spots are regular arrangement, which indicate the crystal is monocrystalline structure. To further obtain information about the effect of nitrogen and hydrogen on the internal structure of diamond, Raman spectroscopy was used to analyze the stress information. The typical Raman spectra for IIa and N–H co-doped diamond crystals are recorded and shown in Fig. 8. Obviously, the Raman spectra of diamond single crystals show only a very strong and narrow peak
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C. Fang et al. / Journal of Crystal Growth 436 (2016) 34–39
Fig. 6. The FTIR spectra of diamonds synthesized from NiMnCo–C system by TGM: (a) with Ti additive; (b) conventional NiMnCo–C system; (c, d, e, f) with C3N6H6 additive: 0.03 wt%, 0.06 wt%, 0.09 wt%, 0.12 wt%.
Fig. 8. Raman spectra recorded for diamond crystals.
4. Conclusion Table 2 The nitrogen concentration of diamonds synthesized from the NiMnCo–C system. Run
D-1 D-2 D-3 D-4 D-5 D-6
C3N6H6 (wt %)
0þTi 1 wt% 0 0.03 0.06 0.09 0.12
Nitrogen content (ppm) C-form
A-form
total
0 200–260 550–650 950–1050 1050–1150 1850–1950
0 0 20–30 35–45 85–95 120–150
0 200–260 570–680 985–1095 1135–1245 1970–2100
In summary, a series of N–H co-doped gem-diamond single crystals with nitrogen concentrations up to 2100 ppm have been successfully synthesized in the NiMnCo–C system with C3N6H6 additive by TGG under HPHT conditions. Single-crystal XRD indicates that the crystal is monocrystalline structure and gives priority to {111} surface. Raman spectrum indicates that the crystal interior exhibits a low stress. This is the highest quality of N–H codoped gem-diamond single crystal reported so far. Our result indicated the growth of N–H co-doped gem-diamond can be well controlled by the TGG method.
Acknowledgments The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China under Grant no. 51172089.
References
Fig. 7. Single-crystal XRD of N–H co-doped diamond.
with a linear background, which means that each of the diamonds obtained possesses a high-quality single crystal structure. It is found that although a large amount of nitrogen and hydrogen were doped in diamond compared with IIa diamond, the Raman peak of N–H co-doped diamond only slightly shifted from 1331.80 cm 1 to 1330.90 cm 1, which indicates that the crystal interior exhibits a low stress. Therefore, it illustrates that the N–H co-doped diamond crystals synthesized in this paper have a high quality. According to the above analysis, this is the highest quality in the synthetic N–H co-doped gem-diamond single crystal reported so far.
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
HPHT synthesis of N–H co-doped diamond single crystals Chao Fang a, Xiaopeng Jia a, Ning Chen a, Yadong Li a, Longsuo Guo a, Liangchao Chen a, Hong-an Ma a,n, Xiaobing Liu b,n a b
State Key Lab of Superhard Materials, Jilin University, Changchun 130012, China Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60202, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 25 September 2015 Received in revised form 10 November 2015 Accepted 25 November 2015 Communicated by: T. Paskova Available online 10 December 2015
In this paper, a series of nitrogen and hydrogen elements (N–H) co-doped diamond single crystals were synthesized in the NiMnCo–C system with melamine (C3N6H6) additive by temperature gradient growth (TGG) method under high-pressure and high-temperature (HPHT). High nitrogen and hydrogen growth environment obviously reduces the growth rate of the diamond. The synthetic diamond crystals are octahedral shape with a green or dark green color. This results indicate that the morphology of the synthetic N–H co-doped gem-diamond can be well-controlled by TGG method. Fourier transform infrared spectroscopy (FTIR) indicate that high-quality diamond single crystals with nitrogen concentrations up to 2100 ppm have been successfully synthesized and the hydrogen related peaks at 2850 cm 1 (sp3–CH3) and 2920 cm 1 (sp3–CH2–) are found in these synthetic diamond. Meanwhile, with the increasing of C-centers the nitrogen atoms aggregate and form into A-centers in these synthetic diamonds. This is the highest quality of N–H co-doped gem-diamond single crystal synthesized in high nitrogen and high hydrogen growth environment reported so far. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Infrared spectrum A2. HPHT A2. Temperature gradient growth method B1. Diamond
1. Introduction Due to the outstanding physical and chemical properties, such as hardness, chemical inertness, and thermal conductivity, diamond as a promising material has attracted wide research attention for mechanical and electric applications [1–4]. As the most common atomic impurity in diamond, the concentration and form of nitrogen play a very important role in the behavior and physical properties of diamond. The vast majority of synthetic diamonds are dominantly type Ib (200–300 ppm). Most natural diamonds usually contain aggregated nitrogen up to thousands of ppm [5–7]. Thus the study on the effect on diamond crystallization is important to reveal the genesis of natural diamond. Recent years, diamonds with nitrogen concentration comparable with natural diamond were reported [8] to be fabricated under high pressure and high temperature (HPHT). However, the crystals have irregular shape and rough surfaces or the forms of the nitrogen in those synthetic diamond were found to present as single substitutional atoms (C-centers) [9,10] while natural diamond contains aggregated nitrogen impurities, pairs of nearest neighboring substitutional nitrogen atoms (A-centers) and groups of four atoms around n
Corresponding authors. E-mail addresses: [email protected] (H.-a. Ma), [email protected] (X. Liu).
http://dx.doi.org/10.1016/j.jcrysgro.2015.11.042 0022-0248/& 2015 Elsevier B.V. All rights reserved.
a vacancy (B-centers), leading to another important stage in solving problems of diamond genesis. Besides the nitrogen, hydrogen is one of the other common impurities in natural diamond [11–13]. Recently, some experimental researches have been focused on the study of the diamond crystalline behavior in the nitrogen and hydrogen elements (N–H) environment and some N–H co-doped diamonds have been successfully synthesized by film growth methods (FGM) and floatage method under HPHT [14,15]. However, previous work found that the synthesized diamonds usually contain low nitrogen concentration and show irregular shape with bad quality in the N–H environment. So, synthesizing high-quality N–H co-doped gem diamond in high nitrogen and high hydrogen growth environment is still a challenge. At the same time high-quality N–H co-doped gem diamond is still highly demanded for the science and industrial application. In this work, we performed a systematic study on the influence of nitrogen and hydrogen on diamond growth habit by the temperature gradient growth (TGG) method under HPHT over a wide range of temperatures from 1300 to 1470 °C and pressures from 5.4 to 6.5 GPa. High-quality N–H co-doped gem-diamonds with nitrogen concentration up to several thousands of ppm were synthesized using NiMnCo alloy. The synthesized diamonds were characterized by optical, scanning electron microscope (SEM), Fourier transform infrared micro-spectroscopy (FTIR), Singlecrystal x-ray diffraction (XRD) and Raman spectroscopy.
C. Fang et al. / Journal of Crystal Growth 436 (2016) 34–39
35
2. Experimental procedures
3. Results and discussion
Experiments on diamond crystallization were carried out using a china-type large volume Cubic High-Pressure Apparatus (CHPA) (SPD-6 1200). The sample assembly for diamond synthesis by HPHT is shown in Fig. 1. Graphite with a purity of 99.99 % was used as carbon source. The Ni70Mn25Co5 alloy (NiMnCo) was used as catalyst. Melamine (C3N6H6) (99.9% purity) powders were chose as nitrogen and hydrogen source, and Ti was used as the nitrogen getter during diamond crystalline. High-quality seed crystals with {111} facets were selected as the growth facet. Each of the sample assembly was dried by heating to 120 °C for 2 h before being placed in the high pressure apparatus to remove the influence of the residual water on the growth of diamonds. The pressure was calibrated by the pressure-induced phase transitions of bismuth, thallium, and barium. The temperature was measured by the Pt30%Rh/Pt-6%Rh thermocouple, whose junction was placed near crystallization sample [16]. After HPHT experiments, the collected samples were cracked and treated in the mixture of H2SO4 and HNO3 to remove remaining graphitic components and metal catalysts. Then the synthesized samples were observed by optical microscope and scanning electron microscope (SEM). Fourier transform infrared spectroscopy (FTIR) was used to determine the characteristics of the synthesized diamonds. The IR spectra were obtained on a Vertex80V Fourier transform infrared (FTIR) spectrometer in the spectral range between 400 and 4000 cm 1 with a spectral resolution of 2 cm 1 in the transmittance mode. Single-crystal x-ray diffraction (XRD) was used to confirm the monocrystalline structure and Raman spectroscopy was used to analyze the stress information.
3.1. The change of synthetic conditions and growth rate for synthesize diamond. The diamond growth and crystallization have been established by TGG method under the influence of N–H in the NiMnCo–C system at pressures ranging from 5.4 to 6.5 GPa and temperatures of 1300–1470 °C. The pressure and temperature conditions and the experimental results are summarized in Table 1. In the growth, we observe that both the P and T synthetic conditions were increased by the adding of C3N6H6 in the NiMnCo–C system for octahedron diamond. At 5.5 GPa and 1320 °C in NiMnCo–C system, the diamond behaves octahedron shape with high quality. When 0.03 wt% C3N6H6 is added into the system, no diamond growth was found at 5.5 GPa and 1320–1380 °C, while only regrown metastable graphite appears. With increase P–T conditions up to 5.8 GPa, the diamond is black and bad quality with irregular shape at 1320 °C. When increase temperature to 1380 °C, a high quality diamond with octahedral shape can be fabricated. Finally, the synthetic pressure has been rise up 1 GPa while the temperature for diamond synthesis increases about 150 °C in the system with 0.12 wt% C3N6H6 additive. The relationship between the minimum synthesis conditions (pressure and temperature) for growing octahedron diamond and the amount of C3N6H6 additive is shown in Fig. 2. It is found that the minimum pressure and temperature for diamond synthesis increase with adding of C3N6H6 in the system that is consistent with previous results [17]. However, when the nitrogen getter (Ti 1%) is added into the starting materials, we notice that synthetic P–T conditions for diamond synthesis were observed to decrease about 0.1 GPa and 20 °C. Thus we believe that the nitrogen
Fig. 1. The sample assembly for synthesis diamond by HPHT. 1.pyrophyllite; 2. conductive steel ring; 3.graphite heater; 4.insulator; 5.carbon source; 6.alloy catalyst; 7.as-grown diamond; 8.seed crystal.
Fig. 2. The relationships between the concentration of C3N6H6 and the minimal P and T for octahedron diamond growth.
Table 1 Experimental results of diamond synthesis in NiMnCo–C system at HPHT. Run
Initial composition
wt%
Pressure (GPa)
Temperature (°C)
Time (h)
Morphology
Color
D-1 D-2 D-3 D-4 D-5 D-6 D-7 D-8
Ti – C3N6H6 C3N6H6 C3N6H6 C3N6H6 C3N6H6 C3N6H6
1 – 0.03 0.03 0.03 0.06 0.09 0.12
5.4 5.5 5.5 5.8 5.8 6.0 6.2 6.5
1300 1320 1320–1380 1320 1380 1420 1440 1470
8 8 8 8 8 8 8 8
Octahedron Octahedron – Irregular Octahedron Octahedron Octahedron Octahedron
Colorless Yellow – Black Greenish Yellow Green Deep green Dark green
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impurity as well as hydrogen impurity plays a key role in the increasing of P–T for diamond synthesis. Meanwhile, there is no report about the effect of high nitrogen and high hydrogen environment on gem diamond growth rate by far. We find the diamond crystal growth rate decreases with increasing the nitrogen and hydrogen content in diamondgrowing environment as shown in Fig. 3. These results indicate high nitrogen and high hydrogen environment obviously reduces the diamond grow rate.
Fig. 3. Curves of growth rate versus the added amount of C3N6H6.
3.2. Morphology, color and SEM photographs of synthetic diamond crystals The optical properties and morphology of the produced diamonds are displayed in Fig. 4. Different from the previously work that only strip or irregular diamonds with bad quality were found in N–H system by film growth method [14] and floatage method [15], the morphology of the synthetic diamond can be wellcontrolled by the TGG method. As shown, the produced diamond crystals exhibit good morphology and are mainly composed of {111} faces. It is clear that the synthesized diamond crystals change from colorless to yellow, yellowish green and dark green with the increasing of C3N6H6 in the NiMnCo–C system. The synthesized diamond with 0.03 wt% C3N6H6 additive has greenish yellow color (Fig. 4c) and the maximum size is about 2.0–2.5 mm in 8 h growth process. When the C3N6H6 additive increases up to 0.06 wt%, the diamond crystal has green color (Fig. 4d). As the C3N6H6 is increase to 0.09 wt%, the produced diamond shows deep green color (Fig. 4e). With the increase of C3N6H6 additive to 0.12 wt%, the color of the synthesized diamond crystals is almost dark green (Fig. 4f). From the optical images, the produced diamond crystals by the TGG method show good quality without obvious pits, inclusions and other defects. In order to analyze the surface characters of the synthesized diamond, we took SEM photographs of synthetic crystals, Fig. 5a–c shows the typical crystals synthesized in the NiMnCo–C system with 1 wt% Ti, 0.06 wt% and 0.12 wt% C3N6H6 additive, respectively. Fig. 5d, e, f are the partial enlarged drawing of Fig. 5a, b, c. We observe that all the surfaces are flat.
Fig. 4. Optical images of diamond obtained from the NiMnCo–C system: (a) with Ti additive; (b) conventional NiMnCo–C system; (c, d, e, f) with C3N6H6 additive: 0.03 wt%, 0.06 wt%, 0.09 wt%, 0.12 wt%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
C. Fang et al. / Journal of Crystal Growth 436 (2016) 34–39
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Fig. 5. SEM images of the diamond crystal grown from NiMnCo–C system: (a, b, c) with Ti, 0.06 wt% and 0.12 wt% C3N6H6 additive, respectively. (d, e, f) are the partial enlarged drawing of (a, b, c).
3.3. FTIR spectra and nitrogen concentrations To identify impurities and defect species in diamond, we carried IR absorption measurement on selected diamond crystals. The typical IR spectra for these diamonds are presented in Fig. 6. The synthetic diamond in the conventional NiMnCo–C system (Fig. 6b) has absorption bands at 1130 and 1344 cm 1 due to nitrogen impurities predominantly in the form of C-center (single substitutional nitrogen atoms) [18,19]. We notice that the synthetic diamond with Ti additive in the system (Fig. 6a) has no absorption bands of nitrogen impurity. FTIR spectra in Fig. 6c–f show that the diamond crystals contain high nitrogen concentration which is exist in the form of A-centers at 1282 cm 1 (pairs of nearest neighboring pairs substitutional nitrogen atoms) [18,19] and Ccenters with C3N6H6 additive in the system. The nitrogen concentration N1130 (ppm) and N1282 (ppm) were determined using the FTIR spectra and the method as follow [20–24]: uð1130cm 1 Þ ¼ A 1290cm 1 –A 1370cm 1 =0:31 uð2120cm 1 Þ ¼
40 A 2030cm 1 þ 87 A 2160cm 1 =127–A 2120cm 1
The separation formula of A-center and C-center is shown as follows: Að1282cm 1 Þ ¼ μ 1282cm 1 þ 0:35μ 1130cm 1 Að1130cm 1 Þ ¼ μ 1130cm 1 þ 0:21μ 1282cm 1 uð1282cm 1 Þ ¼ 100 A 1282cm 1 –35 A 1130cm 1 =92:65 Follow the methods of Kiflawi et.al: NC (ppm) ¼(25 72)
αC(cm 1) and Boyd et.al: NA (ppm) ¼(16.5 71) αA(cm 1), the nitrogen concentration N1130 (ppm) and N1282 (ppm) can be determined: N 1130 ðppmÞ ¼ μ 1130cm 1 =μ 2120cm 1 5:5 25 N 1282 ðppmÞ ¼ μ 1282cm 1 =μ 2120cm 1 5:5 16:5 where μ and A are absorption intensity and recorded values of absorbance, respectively. We calculated the nitrogen concentration in those diamonds using the obtained FTIR spectra. The concentration with uncertainties 10% in produced diamond crystals
is shown in Table 2. The highest nitrogen concentration up to 2100 ppm was found in the diamond synthesized in NiMnCo–C system with 0.12 wt% C3N6H6 additive. Furthermore, we notice that with the increasing C-form the nitrogen atoms aggregate and form into A-centers in these synthetic diamonds. The inset image lying at top right corner reveals major changes in hydrogen-related vibration at 2700–3500 cm 1. From the IR spectra, we notice that the hydrogen-related modes locate at around 2850 and 2920 cm 1 that is attribute to sp3–CH3 and sp3– CH2– [15,25], respectively, are found in most synthetic diamond. The intensities of the hydrogen-related peaks increase with an enhancement in the ratio of C3N6H6 additive. Furthermore, we also observe the appearance of broad peak ranging from 3100 cm 1 to 3350 cm 1 (–NH–) [26,27] in the IR spectra of N–H rich diamonds (inset image lying at top right corner: c–f). All above results indicate that the nitrogen and hydrogen have been simultaneously incorporated into the diamond. 3.4. Characterization of diamond crystal by single-crystal XRD and Raman spectra Single-crystal XRD is most commonly used for crystal structure determination. So, we confirm the monocrystalline structure of the produced diamond as shown in Fig. 7. It is found from Fig. 7 that the diffraction pattern of N–H co-doped diamond is mainly composed of the {111} lattice plane and two small weak peak at {220} and {311}. This result indicates that the crystal gives priority to {111} surface. Meanwhile, from the single-crystal diffraction pattern (inset image lying at top right corner in Fig. 7) of N–H codoped diamond, we find that a series of diffraction spots are regular arrangement, which indicate the crystal is monocrystalline structure. To further obtain information about the effect of nitrogen and hydrogen on the internal structure of diamond, Raman spectroscopy was used to analyze the stress information. The typical Raman spectra for IIa and N–H co-doped diamond crystals are recorded and shown in Fig. 8. Obviously, the Raman spectra of diamond single crystals show only a very strong and narrow peak
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C. Fang et al. / Journal of Crystal Growth 436 (2016) 34–39
Fig. 6. The FTIR spectra of diamonds synthesized from NiMnCo–C system by TGM: (a) with Ti additive; (b) conventional NiMnCo–C system; (c, d, e, f) with C3N6H6 additive: 0.03 wt%, 0.06 wt%, 0.09 wt%, 0.12 wt%.
Fig. 8. Raman spectra recorded for diamond crystals.
4. Conclusion Table 2 The nitrogen concentration of diamonds synthesized from the NiMnCo–C system. Run
D-1 D-2 D-3 D-4 D-5 D-6
C3N6H6 (wt %)
0þTi 1 wt% 0 0.03 0.06 0.09 0.12
Nitrogen content (ppm) C-form
A-form
total
0 200–260 550–650 950–1050 1050–1150 1850–1950
0 0 20–30 35–45 85–95 120–150
0 200–260 570–680 985–1095 1135–1245 1970–2100
In summary, a series of N–H co-doped gem-diamond single crystals with nitrogen concentrations up to 2100 ppm have been successfully synthesized in the NiMnCo–C system with C3N6H6 additive by TGG under HPHT conditions. Single-crystal XRD indicates that the crystal is monocrystalline structure and gives priority to {111} surface. Raman spectrum indicates that the crystal interior exhibits a low stress. This is the highest quality of N–H codoped gem-diamond single crystal reported so far. Our result indicated the growth of N–H co-doped gem-diamond can be well controlled by the TGG method.
Acknowledgments The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China under Grant no. 51172089.
References
Fig. 7. Single-crystal XRD of N–H co-doped diamond.
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