The Zhengzhou Research Institute of Harbin Institute of Technology has made important progress in the field of preparing pure and dispersed nanodiamonds by CVD method.

Nanodiamond (ND) is mainly obtained through detonation or high pressure and high temperature (HPHT) methods. They are often surrounded by a non-diamond shell, resulting in emission quenching and color center instability. Furthermore, functional groups on the particle surface containing oxygen and nitrogen lead to hard agglomeration. ND can be obtained after high temperature and high pressure diamond crushing. They have sharp edges that will scratch the surface and contain metal impurities in the working environment. Both techniques are time-consuming. Other methods for preparing ND include high-energy ball milling, laser shock wave, etc

Generally, CVD-prepared NDs are nucleated directly on the substrate or seed substrate, and the nucleation and growth processes occur continuously in the growth environment, resulting in easy formation of interconnections between particles. Considering the independent particles, growth must be stopped before they come into contact, which will result in low yields. Additionally, the particles are difficult to collect due to their strong adhesion to the substrate. Due to the limitations of these methods, new preparation methods must be developed. In a microwave plasma-assisted chemical vapor deposition (MPCVD) growth environment, diamond may be able to nucleate in the gas phase. However, the nucleation environment and growth mechanisms have not been further studied.

Recently, the infrared thin film and crystal team of Harbin Institute of Technology used microwave plasma-assisted chemical vapor deposition (MPCVD) technology to perform gas-phase nucleation by regulating the plasma state. The prepared NDs have high dispersion, high purity, good shape controllability, and defect-free properties. The advantages of low density have greatly promoted the application competitiveness of NDs in high-precision application fields. Relevant research results were published in "Powder Technology" under the title "Vapor phase nucleation and sedimentation of dispersed nanodiamonds by MPCVD" and obtained a Chinese invention patent.

Figure 1. (a) Schematic diagram of the OES and filter observation method for measuring plasma; (b) Schematic diagram of the molybdenum tray; (c) Relative position of the molybdenum tray and the molybdenum column.

Figure 2. (a) Spatial distribution of Hα when MoC is not used (b) Spatial distribution of Hα when MoC is used

Figure 3. (a) SEM of NDs prepared without MoC; (b-d) SEM&TEM of NDs prepared with MoC

Figure 4. (a) Raman spectrum of nanodiamond before and after using MoC; (b) XRD pattern of nanodiamond before and after using MoC; (c) Nanodiamond dispersion

At present, nanodiamond is used in precision polishing, electrochemistry (drug detection, sewage treatment, environmental monitoring) due to its excellent properties such as high specific surface area, stable properties, wide electrochemical window, modifiable surface groups, and stable color center. etc.), biomedicine (medical skin care, bioluminescence imaging, drug transportation, gene therapy, cancer diagnosis and treatment, etc.), quantum optics (single photon light source) and other cutting-edge fields.

Spherical nanodiamonds not only have extremely high specific surface area, stability and biocompatibility, but also will not scratch the surface of application objects, such as biological skin, blood vessels and precision devices. They can be chemically modified to carry drugs. It has extremely high application value in the fields of beauty and skin care, drug transportation, targeted therapy, and electrochemical monitoring and sensing.

By designing the structure of the molybdenum tray, the researchers controlled the distribution of groups in the plasma to create a gas-phase nucleation space for nanodiamonds. The nanodiamond particles prepared by this method are purer than the detonation method and HPHT method. It breaks through the limitations of the output and film morphology of nanodiamonds prepared by the traditional CVD method. The particles not only have high yield, but also remain dispersed and have high crystallinity. They are widely used in medicines. It has strong application competitiveness in fields such as transportation, biological imaging, and quantum light sources.