The Synthesis Of High Purity Diamonds And Study On The Nitrogen-vacancy Center

According to the concentration of nitrogen impurity,diamonds are divided into two categories: type I and type II.For further classification,the type II diamonds are divided into two types: type IIa(neither nitrogen nor boron are detected by IR)and type IIb(only the defects related to boron are detected by IR).Type IIa crystals contain very little nitrogen(< 1 ppm),ever no nitrogen at all,and there is no boron impurity in IIa crystals.It is called “pure” diamonds because of its few impurities.At present,the temperature gradient method(TGM)is an important method to synthesize large single crystals of diamond.However,during diamond crystal growth,the reaction chamber inevitably lead to impurities from the surrounding environment entering into the diamond crystals.Recent studies have shown that many colorless synthetic diamonds prepared by high temperature and high pressure(HPHT)often appear peak located at 2802 cm-1 in infrared spectra attributed to boron-related impurity.It indicates that the colorless crystal synthesized by HPHT method contains a small amount of boron.Because the raw materials inevitably contain boron which is easy to enter into the diamond crystals due to the similiar atomic radius with carbon.Thus,how to synthesize high purity IIa type diamond without boron has become an important research direction.In this paper,colorless diamond crystals were prepared by solvent with different Fe content and different nitrogen getters,respectively.How to synthesize high purity diamond crystal by high temperature and high pressure method was studied.We explored how to reduce and eliminate boron impurities containing in colorlessdiamond crystals.In addition,the NV centers in diamond have unique properties.In particular,the NV centers in high purity IIa type diamonds have attracted more attention.So we have also carried out a series of study on NV centers.Firstly,the NV centers in diamonds with different nitrogen content(0-1537 ppm)were studied in detail.It is found that the nitrogen impurities have a great influence on the formation and intensity of NV centers in crystals.Secondly,we carried out annealing treatment on the high purity IIa type diamond and high nitrogen content diamonds at 3.5 GPa and 1500-1960 °C.The NV centers in the two types of diamonds were studied after annealing treatment.The transformation of nitrogen and the color of high nitrogen diamonds after annealing were also studied.The NV centers in the two types of diamonds were compared after annealing treatment.Research contents and innovative achievements are as follows:1.First,we synthesized colorless diamond crystals used Fe-based solvent.High-purity graphite was used as carbon source.The effects of Fe content(high-Fe-type and low-Fe-type)on boron concentration were specially investigated.The colorless diamonds was investigated by FTIR,Raman spectroscopy.All the colorless diamonds exhibit the typical boron-related absorption peaks of 2802 cm-1 in infrared(IR)absorption spectra,but the boron concentrations are different.Compared with the low-Fe-type alloy solvent,it is more difficult for boron atoms to incorporate into diamond in high-Fe-type alloy solvent system.The results show that the high-Fe-type solvent is more beneficial to the synthesis of high purity diamond.2.We synthesized colorless diamond crystals using Ti and Al powders as nitrogen getters,respectively.The characteristics of the synthesized diamonds and the content of boron were analyzed by infrared spectroscopy(IR)and Raman spectroscopy.When Ti was used as nitrogen getters,the infrared spectra results show that the colorless synthetic diamonds have the typical boron-related absorption peaks of 2802 cm-1,it indicates that the boron exists in the colorless diamond crystal.When Al was used as nitrogen getters,there was no boron related peaks in infrared spectra,it means no boron in the colorless diamond crystals.The results show that Al can suppress the boron entry into the diamonds.For comparison,we added 20% Ti+8% Band 20% Al+8% B in carbon source to synthesize crystals,respectively.The results show that the boron-related peaks of IIb diamonds synthesized in 20% Al+8% B system are weaker than those synthesized in 20% Ti+8% B system.It is proved again that Al can prevent boron entering into the diamonds.3.The NV centers were investigated in diamonds with different nitrogen concentrations.The Raman measurements indicated that the nitrogen atoms caused the Raman peaks to shift to lower a frequency.The photoluminescence(PL)results showed that the intensity of the NV-centers increased with the increase in the nitrogen content when the diamonds had low nitrogen content(< 80 ppm),With a further increase in the nitrogen content in the diamonds(> 372 ppm),the intensity of the NV-centers gradually decreased until it became undetectable.Additionally,we found that the NV0 centers were more common in diamonds with low nitrogen content(< 80 ppm).We believe that this study provide a better understanding of the relationship between the nitrogen and the NV centers in HPHT synthetic diamonds.4.The high purity IIa type and high nitrogen content diamonds were annealed at pressure 3.5 GPa and temperature at 1600-1960 °C.The formations and changes of NV centers in the two types of diamonds were investigated.The Raman spectra indicated that annealing temperature has little effect on the Raman peaks of the two types of diamonds.In addition,we find that the high nitrogen content diamonds show different colours at different annealing temperatures.Its illustrate that the different nitrogen polymerization degree will lead to obvious difference in colours.For high purity IIa type diamonds,the PL results showed that when annealing temperature increases gradually,the NV0 centers appears first,then the NV-and H3 centers appears,and the intensity of NV0 centers is much higher than that of NV-and H3 centers.For high nitrogen content diamonds,only NV-centers were detected,and the intensity of NV-centers increased with the increase of annealing temperature.