Synthesis And Characterization Of Large Diamond Single Crystal In Oxygen-enriched Environment

Diamond has gained widespread attention for its excellent physical and chemical properties,and now it has been applied in industrial fields on a larger scale.In recent years,diamond has shown great potential for applications in quantum dots,semiconductors,optoelectronics,etc.Expanding new applications of diamond materials is an important frontier hotspot in the field.It is well known that there are many impurities inside diamond,and these impurities not only affect the growth of diamond,but also directly influence various properties of diamond.Oxygen is one of the most common impurities,but we know little about it.Recent studies have found that oxygen-doped diamond has n-type properties,which has further stimulated researchers’interest in oxygen and its related impurities in diamond.Currently,the effect of oxygen on diamond growth and properties is not well understood,and the synthesis of high-quality,large-size oxygen-containing diamond single crystals remains a challenge.Therefore,the research related to oxygen-doped diamond is not only beneficial to expand the functional properties of diamond materials,but also has important scientific significance to understand the formation mechanism of natural diamond.Based on this,the present study was carried out to investigate the effect of oxygen on diamond growth by introducing oxygen-containing additives（Ni O and Ni₂O₃）into the synthesis of large single crystals of diamond under high temperature and pressure in an oxygen-rich environment on a domestic six-sided top hydraulic press with Fe-Ni as the catalyst.The main studies are as follows.（1）The effect of Ni O on Ib diamondThe effect of Ni O on the growth of diamond was investigated in the synthesis of diamond by adding Ni O to the Fe-Ni-C system.The analysis revealed that the crystal quality gradually deteriorated with the increase of Ni O addition,and the breakage appeared until there was no crystal shape.The content and form of nitrogen also changed,with the addition of Ni O,the nitrogen content was higher than that of the diamond synthesized without Ni O,and diamond containing 872 ppm of nitrogen was obtained at 4.0 wt%Ni O addition.And the A-centered nitrogen appeared in the diamond at Ni O addition of 2.0 wt%and above.The Raman spectra showed that there were different degrees of redshift in the Raman peak positions after the addition of Ni O,and the higher the nitrogen content,the greater the redshift.The half-height width represents the change of crystal stress related to the crystal defects caused by Ni O.When the crystalline form is intact,the stress increases with the increase of defects,and when the defects increase to a certain extent,it causes the breakage of the crystalline form,but the stress inside the crystal is released and the half-height width decreases.xps shows that oxygen enters into the diamond lattice in the form of C=O,N-O bonding.（2）Effect of Ni2O3 on Ib diamondHigh-quality oxygen-containing diamonds were synthesized by adding Ni₂O₃ to the Fe-Ni-C system,and it was found that,with the increase of Ni₂O₃ addition,the diamond growth rate decreased,the V-shaped interval of crystal growth shifted upward,the growth pressure and temperature conditions increased,the crystal quality deteriorated,and the morphology of the crystals changed from polycrystalline facets at low oxygen content to octahedral morphology dominated by{111}facets.The presence of oxygen affects the nitrogen content and the form of diamond present,and the nitrogen content of the sample increases after Ni₂O₃ addition and the A-center nitrogen starts to appear when the content of Ni₂O₃ reaches 3.5 wt%.The XPS results confirm the successful entry of O into the diamond lattice in the form of C-O,C=O,O-C=O and N=O.（3）Effect of Ni2O3 on IIa diamondThe effect of Ni₂O₃ on the growth of type IIa diamond was investigated by introducing Ni₂O₃ into the Fe-Ni-Ti-C system capable of synthesizing type IIa diamond,and it was found that:along with the addition of Ni₂O₃,the crystallinity of the synthesized diamond decreased,the growth rate decreased,inclusions appeared in the crystal,and the color of the crystal changed.the addition of Ni₂O₃ may have oxidized the nitrogen remover titanium,the nitrogen content of the diamond increased,impurities entered the diamond lattice more easily,and the boron content increased.XPS results showed that the oxygen was present in the synthesized diamond samples in the form of O-C and C=O bonding.