In 1928, Irving Langmuir first coined the term plasma to describe regions containing ions and electrons with balanced charges, where the electron density () is approximately equal to the ion density (). Although plasma is quasi-neutral, its free charge carriers such as electrons and ions endow it with higher electrical conductivity than metals, alongside abundant excited-state particles, granting it unique properties absent in solids, liquids and gases. More importantly, the particle temperature and energy density inside plasma exceed those of other states of matter, enabling the generation of high-energy species even at low temperatures. This triggers physical and chemical reactions that are difficult or impossible to achieve via conventional approaches. When plasma interacts with materials, plasma modification can be categorized into seven types based on modification mechanisms: exfoliation, vacancy construction, element doping, film deposition, etching, reduction and polymerization.

1. Exfoliation

High-energy plasma particles act on material surfaces through momentum transfer, allowing activatable surface atoms and molecules to overcome weak intermolecular forces including van der Waals forces and hydrogen bonds, thereby achieving material exfoliation. Accordingly, two-dimensional layered materials such as graphene and layered double hydroxides (LDHs) can be exfoliated via plasma treatment.
Unlike traditional mechanical exfoliation or solvent-assisted exfoliation, plasma exfoliation uses no toxic or environmentally hazardous chemicals or gases. Moreover, products from plasma exfoliation are generally free of contaminants like surfactants, preserving their intrinsic properties.

2. Vacancy Construction

During momentum transfer between the material and incident active plasma species, atoms or ions in the material lattice are sputtered away to form vacancies. The degree of sputtering can be regulated by adjusting the type, energy and incident direction of incoming particles. Plasma can introduce cation vacancies (e.g., Co, Fe vacancies) and anion vacancies (e.g., O, S, N vacancies).

3. Doping

Doping with non-metallic elements (N, O, S, etc.), metallic elements (Mg, Ti, Fe, etc.), and binary/ternary element combinations (N–S, N–P, N–S–P, etc.) is widely adopted for material modification of electrochemical energy storage devices. The high-energy particles within plasma embed heteroatoms into the framework of the host material to realize doping. Compared with in-situ growth, plasma doping yields highly activated doped acceptors with higher efficiency.

4. Deposition

Plasma is extensively applied for coating and thin-film deposition. In contrast to thermally driven deposition techniques (thermal evaporation, thermal chemical vapor deposition, etc.), plasma-enhanced deposition features highly reactive precursors that lower the nucleation barrier, allowing a wider selection of processing conditions including temperature, duration, air pressure and substrate types. As such, plasma technology can be used to deposit carbon, metal oxides, metal nitrides and other substances.

5. Etching

Apart from physical sputtering effects, excited plasma species possess strong chemical reactivity and can react with materials to form volatile products. Combined physical and chemical reactions create the etching effect. Plasma etching delivers excellent repeatability compared with alternative techniques. Furthermore, abundant etching recipes and high etching rates can be obtained by tuning the composition and flow rate of discharge gas.

6. Reduction

Plasma generated from reducing gases such as hydrogen is rich in atomic hydrogen, hydrogen ions, excited hydrogen atoms and hydrogen molecules, and is widely used to reduce materials or remove oxides from material surfaces. Unlike thermal reduction, plasma reduction proceeds at relatively low temperatures, making it suitable for materials requiring low-temperature processing.

7. Polymerization

Plasma breaks chemical bonds in monomers to produce polymerization-favorable intermediates, initiating polymerization or polycondensation reactions. Plasma polymerization can be controlled by adjusting atmosphere composition, monomer type, precursors, treatment duration and other parameters. Nevertheless, the mechanism of plasma polymerization remains controversial. Proposed mechanisms include free-radical chain-growth polymerization, ionic chain-growth polymerization, ion–molecule reactions, monomer cleavage-recombination polymerization, free-radical chain-growth copolymerization, as well as chemical grafting onto radical sites or functional groups for polymerization.