Plasma is considered as the fourth state of matter besides solids, liquids, and gases, filled with atoms, molecules, ions, and radicals, in which the number of positive ions and negative ions is equal. Plasma can be generated by ionizing gas, providing sufficient energy for molecules to collide with electrons. Methods for generating plasma typically include flame, microwave, discharge, and shock, but the most popular method in laboratories and industries is glow discharge, which involves applying high voltage to two electrodes in a vacuum chamber containing low-pressure gas, inducing discharge or ionization between the electrodes. 

Currently, plasma equipment is considered as an efficient tool for material preparation or modification. Through the physical and chemical interactions between its highly reactive particles and the material surface, plasma equipment achieves eight efficient and environmentally friendly application scenarios: it can delaminate layered materials, precisely create vacancies in the crystal lattice, or dope multiple elements into materials to change their properties; at the same time, it is also widely used for depositing various thin film coatings, fine etching of surfaces, and surface functionalization through oxidation, nitridation, sulfuration, phosphating, etc.; in addition, plasma can reduce oxides at low temperatures and form functional polymer films on the material surface by inducing polymerization reactions. These applications collectively reflect the great value of plasma technology as a multifunctional, highly controllable, and environmentally friendly tool for advanced material processing and modification.

(1) Layered peeling

When high-energy particles of plasma interact with material surfaces through momentum transfer, activated surface atoms/molecules can overcome weak intermolecular forces, such as van der Waals forces or hydrogen bonds. Therefore, two-dimensional layered materials, including graphene and layered double hydroxides, can be delaminated by plasma [33]. Compared to traditional mechanical or liquid delamination methods, plasma delamination is more effective and does not involve toxic or environmentally unfriendly chemicals or gases. Furthermore, plasma delaminated products typically do not contain contaminants such as surfactants, maintaining their inherent properties.

(2) vacancy

During the momentum transfer process of incident active particles in plasma, surface atoms or ions are sputtered out, leaving vacancies in the crystal lattice. The degree of sputtering can be adjusted by the type, energy, and direction of the incident particles. So far, various vacancies [including cation vacancies (such as Co and Fe vacancies), anion vacancies (such as O, S, and N vacancies), and multiple vacancies (such as cation and O vacancies)] have been reported due to their low formation energies [34], and they play an attractive role in tuning material properties. Compared to many other methods (such as thermal treatment and chemical methods), plasma technology has advantages such as high efficiency, good controllability, and environmental friendliness.

(3) Doping

Doping with non-metallic elements (such as N, O, S, P, B, F, etc.), metallic elements (Mg, Ti, Fe, Al, Ni, Cu, etc.), and dual/triple elements (such as N-S, N-P, N-S-P) has been widely applied in material preparation. As mentioned earlier, momentum transfer can endow particles in plasma with high energy, and these particles can be implanted into the framework of the matrix material to achieve doping effects. Compared to in-situ growth, plasma doping can render the doped receptors in a highly activated state, resulting in high efficiency.

(4) deposition

Plasma has been widely used for depositing coatings and thin films. Compared to deposition driven by thermal deposition (such as thermal evaporation, thermal chemical vapor deposition), plasma deposition can significantly reduce the nucleation barrier due to the high reactivity of substances, allowing for a more flexible selection of processing conditions (such as temperature, time, atmosphere, and substrate). Therefore, various materials can be deposited through plasma technology, such as carbon, metals, oxides, and nitrides.

(5) etching

In addition to physical sputtering behavior, charged particles in plasma also exhibit high chemical activity, facilitating chemical reactions (such as reactive ion etching) to form volatile products. These physical and chemical reactions consequently lead to surface etching effects. Compared to other techniques (such as wet chemical etching), plasma etching exhibits high repeatability and excellent control. Furthermore, by controlling the composition and velocity of the discharge gas, etching selectivity and higher etching rates can be achieved.

(6) Oxidation/Nitriding/Sulfurization/Phosphating

Plasma is often used to functionalize materials through oxidation, nitriding, sulfurization, and phosphating, in order to prepare multifunctional surface compounds and improve the corrosion resistance, conductivity, and electrochemical properties of materials. For example, oxygen-containing plasma (O-plasma) can generate reactive oxygen species (O, O+, O −), which are easily reactive with metals to form oxides or peroxides. Similarly, nitrides, sulfides, and phosphides can be formed in nitrogen plasma (N-plasma), sulfur plasma (S-plasma), and phosphorus plasma (P-plasma), respectively, using different raw materials (N2, NH3, H2S, PH3, etc.).

(7) Restore

Plasma (H-plasma) formed from reducing gases such as hydrogen has been widely used for reducing or removing oxides (such as CuO and graphene oxide) due to its rich content of atomic hydrogen, ionic hydrogen, excited hydrogen atoms, and hydrogen molecules. Compared with thermal reduction, plasma induced reduction can be carried out at lower temperatures, with good controllability, expanding the application range of materials, especially temperature sensitive materials. In addition, plasma method has also attracted much attention due to its environmental friendliness and safety.

(8) Aggregation

When exposed to plasma, monomers can undergo addition polymerization or condensation reactions. Plasma polymerization can significantly alter surface properties such as surface energy, hydrophilicity, and adhesion. Meanwhile, the polymerization process is controlled by factors such as atmosphere, monomer type, substrate, and processing time. However, the mechanism of plasma polymerization remains controversial. The currently proposed mechanisms include free radical chain growth polymerization, ion chain growth polymerization, ion molecule reaction, monomer cleavage polymerization, free radical chain growth copolymerization, and chemical linkage at free radical sites or functional groups.