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Processes at Plasma-Matter Interfaces: An Overview and Future Trends
Igor Levchenko, Kateryna Bazak
File 1 : Vol30_No3_Review And Research-37~48.pdf (0 byte)

DOI: 10.22661/AAPPSBL.2020.30.3.37

Processes at Plasma-Matter Interfaces:
An Overview and Future Trends

IGOR LEVCHENKO1,2, KATERYNA BAZAKA3,1,2,7, OLEG BARANOV4,5,
OLEKSII CHERKUN1,7, MICHAEL KEIDAR6 and SHUYAN XU1,7
1 PLASMA SOURCES AND APPLICATION CENTER/SPACE PROPULSION CENTRE SINGAPORE, NIE,
NANYANG TECHNOLOGICAL UNIVERSITY, 637616, SINGAPORE
2 SCIENCE AND ENGINEERING FACULTY, QUEENSLAND UNIVERSITY OF TECHNOLOGY, BRISBANE, AUSTRALIA
3 RESEARCH SCHOOL OF ELECTRICAL, ENERGY AND MATERIALS ENGINEERING,
THE AUSTRALIAN NATIONAL UNIVERSITY, ACT 2601, AUSTRALIA
4 NATIONAL AEROSPACE UNIVERSITY, KHARKIV, UKRAINE
5 JOŽEF STEFAN INSTITUTE, LJUBLJANA, SLOVENIA, EU
6 MECHANICAL AND AEROSPACE ENGINEERING, SCHOOL OF ENGINEERING AND APPLIED SCIENCE,
THE GEORGE WASHINGTON UNIVERSITY, SCIENCE & ENGINEERING HALL 3550, 800 22ND STREET, NORTHWEST WASHINGTON, DC 20052
7 CENTER FOR SPACE PROPULSION AND GRAVITATIONAL UNIVERSE (SPaGU), HANGZHOU INSTITUTE FOR ADVANCED STUDY, THE UNIVERSITY OF CHINESE ACADEMY OF SCIENCES,
1 XIANG SHAN ZHI NONG, HANG ZHOU, CHINA 310024

Communicated by Won Namkung

ABSTRACT

Plasma-based technologies for material treatment represent a rapidly developing field that promises significant advantages over many other technological approaches. Due to the use of electrically charged high-energy fluxes of matter, plasma-based technologies can be used for fast, energy efficient treatment of various materials, including metals, ceramics, plastics and complex compounds. In this review, we briefly outline the processes that take place at various plasma-materials interfaces, and discuss future trends and possible novel applications for plasma-based technologies in the treatment and processing of materials.

INTRODUCTION

Plasma is the fourth state of matter, and is generally a mixture of ions, electrons, neutral particles and radicals [1, 2]. Currently, plasma-based systems and techniques are used across a very broad spectrum of applications ranging from space propulsion [3-9] and plasma thrusters [10-12], medicine [13-15], biology [16-18], materials treatment [19, 20], agriculture [21, 22], to many other areas [23]. For technological applications, the processes at the interface between plasma and matter are of key importance, since they determine material and energy fluxes to the surface from the plasma bulk.

Apart from technological systems, application of cold plasmas for treatment of biological tissues and living cells may be extremely important, as demonstrated by the coronavirus COVID-19 pandemic 2020 [24, 25].

In general, separation of electric charges at the interface results in the formation of a plasma-surface sheath, with a width that could be controlled by changing plasma parameters and properties of the surface. As such, fluxes of material and energy to the surface could be efficiently controlled by modulating plasma and surface parameters, with the possibility of attaining sophisticated control over fluxes when the surface is micro- and nano-structured [26-29].

Since electrons in plasma feature much higher velocities than heavier ions and radicals, a surface immersed in plasma acquires a negative electric charge (the so-called floating potential), even when no external potential is applied. As a result, ions are accelerated by the electric field towards the surface, bringing additional energy to the surface. These phenomena allow for the efficient growth of small particles, such as nanotubes and graphene nanoflakes, in plasma (Fig. 1).

 

Fig. 1: Plasma-based systems may offer a simple yet efficient means for nanofabrication. A schematic of the process for graphene flake fabrication in an arc discharge setup. (a) The apparatus for the graphene growth consists of an anode, a cathode, a graphene collecting surface and (optionally) a focusing magnet. A DC discharge is sustained in He gas. A carbon material is supplied via anode evaporation. (b) The carbonaceous deposit is removed from the surface and undergoes ultrasonication, and eventually (c) a graphene flake suspension is produced which can be used, e.g., for printing (d). Processes at the plasma-solid interface are important (e): here, we show an influx of carbon atoms from the plasma to the growing graphene flake, the evaporation of carbon atoms from the graphene surface, the surface diffusion of carbon atoms around the graphene surface, and the incorporation of carbon adatoms into the edges of flakes. Reprinted from [30] under the Creative Commons Attribution International License (CC BY).

PLASMA SHEATHES AND MATERIAL FLUXES

Bulk plasma is usually separated from electrically charged surfaces, and a sheath is formed between them. In this sheath, a strong electric field is sustained [30, 31]. It is usually assumed that the ions from plasma enter the sheath with the Bohm velocity:

(1)

where Te is the electron temperature, and mi is the mass of ions in plasma. The plasma behavior near the surface depends on the relation of electron temperature to surface potential US. When the electron temperature is low, i.e. TeUS, the sheath is thick and its thickness may be calculated as:

(2)

where λD is the Debye length. The Debye length may be expressed as:

(3)

where ε0 is the dielectric constant, n is the plasma density, and e is the elemental electron charge.

When the sheath is thin, i.e. when a low bias has been applied to the surface, the sheath width may be estimated as:

(4)

i.e., as several Debye lengths; here kλ is a constant which could be typically assumed in the range of 1 to 5 [32].

During the growth of nanostructures in the presence of plasma, a very complex electric field between the plasma bulk and nano-structured surface is formed. As a result, the fluxes of ions from plasma depend on the morphology of the surface, as well as the plasma parameters. Importantly, the nature of the material from which these nanostructures are made (conductive or insulating) also influence the structure of the electric field, and hence the distribution of ion fluxes over the surface [33-36].

In the electric field, the ions move along complex trajectories, and eventually deposit onto the nanostructures. The structure of the electric field acting onto an ion located in a point described by vector r (ion position vector) can be simulated using the expression:

(5)

where N is the number of nanostructures growing on the surface, ρi is the density of the electric charge on the surface, z is the distance from the surface to the ion position vector, r is the ion position vector relative to the ith nanostructure, and Si is the surface area of the nanostructure. Using this expression, the equation of ion motion can be directly integrated and the distribution of ion fluxes over the growing nanostructures can be calculated. Next, the growth of the nanostructure can be simulated using the equations for surface diffusion over the growing nanostructures and the surface of the substrate [26, 30, 33]. Apparently the electric field does not affect the trajectories of neutral particles, thus the uniform deposition of neutral species should be added to the electron field-modulated flux.

The two-dimensional flux of atoms to the edges of the growing nanostructures, along with the direct ion fluxes from plasma, are the most important parameters that determine the growth kinetics and morphology of plasma-grown structures. The equation for surface diffusion may be used in the form [36]:

(6)

where DS is the diffusion coefficient, η is the density of adatoms on the surface, ψin is the flux of atoms and ions from the plasma sheath to the substrate, and ψvp is the evaporation flux from the surface.

 

Fig. 2: Solid, liquid, gas? Unique benefits of using different types of media for material fabrication and processing. A transition between these states within the same technological run affords a wide diversity of possible processes at different time, energy, density and temperature scales arising from the interactions between the species: from femtoseconds for collisions to milliseconds for adsorption events; with the highest range of achieved energy (0.1 to 104 eV), and density of particles (1014 to 1022 m-3). The environment presents a large number of possible means of process control, through electrical and magnetic fields, pressure gradients, and friction forces. There is the possibility to drive the process as a non-equilibrium ballistic or as a thermal treatment. There is a large variety of physical and chemical processes in the volume and on the surface. There is the possibility for selective processing of objects from the nano- to micro scale, and there is a possibility to develop processes that meet environmental sustainability requirements. Reprinted with permission from [35]. Copyright RSC, 2018.

The processes at the interface between plasma and liquids are much more complex than the processes at the interface between plasmas and solid surfaces, and usually involve multiple plasma-initiated chemical reactions. Nevertheless, these plasma-liquid interfacial processes are very important in both material processing and medicine [44]. Below we will briefly outline some important applications of plasma-surface interfacial systems, and the relevant processes at the interfaces.

Plasmas feature a very rich set of control parameters and control knobs that allow for sophisticated adjustment of the nucleation and growth of nanostructures in plasma-based reactive environments. In Fig. 2, various aggregate states, along with the control means and technological schemes, are shown in their interaction with the processes and various plasma-generated particles. Plasma itself can be generated and sustained in various aggregate states.

 

Fig. 3: The synergistic relationship between plasmas, nanomaterials and their biomedical applications. Reprinted from [37] under the Creative Commons Attribution International License (CC BY).

Electric and magnetic fields are broadly used to control the plasma generation and fluxes of matter and energy from the plasma to the nanostructured surfaces. The thin green ring in Fig. 2 also lists the set of the most popular plasma-based techniques that are currently in use for technological and other tasks, namely:

DC - Direct current discharge,
Arc - Arc discharges,
CCP - Capacitively coupled plasma,
ICP - Inductively coupled plasma,
MW - Microwave plasma, and
ECR - Electron cyclotron resonance.

When plasmas interact with a surface, a large number of processes are initiated and sustained through the electric energy applied to the discharge. Among the most important processes are:

Ionization: Generation of ions from neutral atoms, usually via impact by ions or/and electrons, or via ultraviolet radiation in plasma;
Excitation: Transition of electron bounds in a neutral particle shell to another state with a higher energy; and
Dissociation: Splitting of molecules or complexes to smaller fragment, also under the impact of ions or/and electrons, or via ultraviolet radiation in plasma.

 

Fig. 4: (a) An atmospheric pressure glow micro-discharge setup. A small (several mm) gap between the cathode and the surface of the liquid accommodates a bunch of plasma. (b) Photographs of the discharge patterns above the therapeutic media during the activation process. The self-organized patterns have complex structures that are strongly dependent on the voltage-current conditions. (c) Optical emission spectra generated by the atmospheric pressure glow discharge above water.
(d) Current-voltage dependency of the system with optical photographs of the self-organized stratified interface patterns. It is possible to distinguish four discharge stages. (e) A table summarizing the relationships between discharge stages, self-organized patterns and optical emissions. Reprinted from [17] under the Creative Commons Attribution International License (CC BY).

Importantly, some of the above processes occur in plasma and solid/liquid matter, causing, e.g., evaporation or penetration of atoms into the growing structures. Plasmas can be generated and sustained in various media, and their parameters can be varied over a very wide range, typically in:

Ion and neutrals temperatures: from 100 °C to several thousand °C;
Electron temperatures: from fractions of eV to several tens of eV;
Electron density: up to 1022 m-3 (for high current arc discharges); and
Interaction time: from 10-15 to 10-3 s for thermally - driven processes.

PLASMA-LIQUID INTERFACES

Plasma-liquid interactions in biomedical applications

The application of plasma in medicine is a very promising field, in particular for the treatment of tumours and cancers of various origin. Many different types of atmospheric pressure plasmas have been tested for their reactivity toward cancer cells, and significant progress has been achieved in using plasmas in oncotherapy [17, 38]. It is currently believed that reactive oxygen and nitrogen species (ROS and RNS), along with plasma-generated ultra-violet radiation, are the most active components capable of selective killing of various types of cancer cells.

Importantly, in biomedical applications, plasma is not simply a source of reactive species, but it utilizes several additional phenomena that give rise to desirable synergistic effects (Fig. 3). For example, when applied to tissues directly, plasmas can induce cellular responses through ion and electron fluxes, ultraviolet radiation, and plasma-synthesized chemicals in the liquid, as well as via plasma-synthesized nanoparticles, which could perform many useful functions - e.g., they could kill cancer cells, or transport and release therapeutic agents. At present, reactive oxygen and nitrogen species (ROS and RNS) are the primary plasma-synthesized reagents used in the treatment of cancer cells; however, other promising species emerge as the field continues to develop.

Currently it is assumed that the following mechanisms are responsible for the formation of H2O2, NO2- and other RONS in plasma-treated liquids [16, 38, 39]:

O2 + e- → O2- ,
O2- + H+ → HO2,
HO2 + e- → HO2-,
HO2- + H+ → H2O2,
HO2 + e → H2O* + e,
H2O* → *OH + H*,
H* + O2 → *OH + O,
O2 + e → O + O + e,
O + H2O → 2*OH,
*OH + O → *HO2,
*HO2 → H + O2*,
O + O2 → O3,

(1) 
(2) 
(3) 
(4) 
(5) 
(6) 
(7) 
(8) 
(9) 
(10) 
(11) 
(12) 

and many others [16, 40].

The self-organization of plasmas at plasma-liquid interfaces was recently demonstrated as a tool to boost the efficiency of plasma-based anti-cancer therapy. In this case, the plasma forms ordered structures [41] over the surface of the liquid, a process that involves an interplay between a large set of physical and chemical parameters. The self-organized patterns of plasmas over the surface of liquids was found to be more effective in killing cancer cells when compared to typical non-organized plasma jets. Specifically, plasmas with self-organized patterns have better overall effectiveness in killing cancer cells than non-self-organized plasmas [17, 38]. The precise mechanisms that underpin the nature of the observed enhanced biological activity are yet to be fully elucidated, and consequently are subjects for intense research efforts [42, 43]. Moreover, the RON and ROS densities are also higher in the self-organized stratified plasmas (Fig. 4).

 

Fig. 5: The use of the European Cooperation for Science and Technology (COST) jet to study the reactions leading to SiO2-based thin films deposited by cold atmospheric pressure plasma (CAP). Reprinted from [47] under the Creative Commons Attribution International License (CC BY).

Plasma-liquid interactions in nanotechnology

Interfacial processes that define plasma interactions with liquid surfaces also play an important role in nanotechnology and synthesis of nanoscale materials [44]. Similar to applications in biology and medicine, reactive oxygen species (ROS) may be useful in driving desirable plasma-based interfacial reactions in nanotechnology. Figure 5 is an example of a set of important reactions induced by plasma in liquids for SiO2-based thin film deposition. Plasma-generated ROS species play a key role in this chain of processes. In the experiments made under the umbrella of the European Cooperation for Science and Technology (COST) funding organization, [45] the He gas with hexamethyldisiloxane (HMDSO) was used to study the effect of ROS on film formation. A reaction pathway has been proposed to form the films enriched with Si-O [46]. Specifically, addition of oxygen to the mixture resulted in the formation of ROS, which in turn provided additional Si-O bonds. Schematics of the reactions are shown in Fig. 5 [47].

As we have mentioned in the previous sub-section, synergistic effects can arise when plasmas are applied directly to living tissues through the plasma-driven synthesis of nanomaterials directly in the living tissues or in the ambient liquid media affected by plasma. For these applications, metallic and composite nanoparticles are of principal importance [14].

 

Fig. 6: Examples of important reactions at the plasma-liquid interface: reduction of metal ions, diffusion and nucleation of nanoparticles. Reduction is initiated by solvated electrons, or plasma-generated reactive species. Reprinted from [37] under the Creative Commons Attribution International License (CC BY).

Figure 6 illustrates a typical example of the processes at the plasma-liquid interface during atmospheric plasma-based synthesis of nanoparticles [37]. This process was conducted in an open-air setting, thus resulting in a very high density of ROS. No additional catalysts or reducing agents are needed in this process. This is an essentially green technology, capable of producing nanomaterials for various applications without complex and harmful chemical processes and reagents.

Another important method for nanoparticle production is by pulsing discharge in water. While in this case, no plasma is present above the liquid to form an interface, the plasma-liquid interfaces are still formed during the discharge in the evaporated liquid media. In this case, a similar chain of reactions could be produced, resulting in an efficient, fast formation of nanoparticles [48]. In this process, metal electrodes (cathodes and anodes) were submerged into the liquid, and the pulsing power was supplied to the electrodes. As a result, plasmas were generated directly in the liquid, in the gap of approx. 0.2 mm. The pulse duration was about 20 μs.

 

Fig. 7: Schematic of pulsed-plasma-in-liquid method. Reprinted from [48] under the Creative Commons Attribution International License (CC BY).

 

Fig. 8: Interfacial processes involved in the formation of Pd-Fe alloy nanoparticles by pulsed plasma in water. Reprinted from [48] under the Creative Commons Attribution International License (CC BY).

A vibrator was installed over one of the electrodes, to ensure continuous generation of the plasma. In this system, plasmas with extremely high temperatures reaching 2500 °C were sustained. Not surprisingly, the metals were efficiently ionized and the ions penetrated into the liquid, where they formed complex nanoparticles upon cooling (Figs. 7 and 8). Since the pulses were very short and the liquid efficiently cooled the dispersed metal, very small nanoparticles were produced.

This example illustrates how very high plasma temperatures, together with efficient cooling in liquids, allow for the efficient control of nanosynthesis.

PLASMA-SOLID INTERFACES

Two major types of plasma-solid interactions

We will distinguish the two major types of the plasma-solid interfaces: (i) plasma interactions with a relatively flat surface, e.g. during the formation of a thin film or multilayered structure; and (ii) plasma interactions with nanostructured surfaces or arrays of vertically aligned nanostructures on the surfaces immersed in the active plasma. In the first case, plasma chemistry and surface diffusion (partially influenced by plasma [49]) will largely determine the behavior of the system. In the second case, due to the strong influence of the nanostructure-generated electric fields, focusing of the ion fluxes at the nanostructures will take place. Importantly, when plasma interacts with nanostructures, electric fields influence significantly the shapes and distribution of nanostructure sizes, including the morphology of the surface and sharpness of the nanostructures [21, 27].

Plasma-solid interactions in surface treatment

Let us examine a set of typical processes at the interface between the plasma and a flat surface with the formation of hydrogenated amorphous silicon film as an example. The process starts from nucleation of small particles that then grow into hydrogenated amorphous silicon. There are three relatively independent processes that proceed at various levels:

(i) Processes in the plasma sheath that determine the plasma-matter interactions at the interface;
(ii) Interactions of ions and radicals with the surface, determining the material influx to the growth zone; and
(iii) Growth on the surface, resulting in the formation of the hydrogenated amorphous silicon film.

 

Fig. 9: Schematic representation of the basic surface reactions of SiH3 and H radicals during the α-Si:H film growth used in the model. Left panel: processes in plasma. Right panel: processes on the surface and in the plasma-surface sheath: (1) adsorption of SiH3; (2) desorption of SiH3; (3) H abstraction by a H radical; (4) H abstraction by a SiH3 radical; (5) SiH3 abstraction by a SiH3 radical; (6) direct chemisorption of SiH3 into a dangling bond; (7) direct chemisorption of H into a dangling bond; (8) hopping of the adsorbed SiH3 on hydride sites; (9) chemisorption of adsorbed SiH3 into a dangling bond; and (10) ion sputtering of the mono-hydride sites. Reprinted from [49] under the Creative Commons Attribution International License (CC BY).

Figure 9 shows several important processes in this system. While some processes (such as the formation of radicals, atom ionization and others) occur in plasma, many other key processes take place in the plasma-surface sheath and directly on the surface. These processes are, e.g., desorption and adsorption of SiH3 molecules; abstraction of H by H and SiH3 radicals; abstraction of SiH3 with the use of SiH3 radicals; attachment of H and SiH3 to dangling bonds; diffusion of SiH3 across the surface; and sputtering of hydrogen from the surface by the direct impact of ions from the plasma [49].

This process was conducted in the reactor with a highly reactive inductively-coupled plasma [8], which is a common medium for the synthesis of hydrogenated silicon layers. The key specific energies are represented in Table 1.

Table 1. Typical plasma parameters at the surface. Reprinted from [49] under the Creative Commons Attribution International License (CC BY).

Parameter

Value

Electron temperature
Ion temperature
Plasma density
Neutral gas pressure
Substrate potential
Substrate temperature
Percentage of SiH3 gas
Percentage of H gas
Percentage of Ar gas

1-3 eV
0.01-0.15 eV
1010 to 1012 cm-3
20-100 mTorr
0 to -300 V
300-700 K
10-40 %
5-35 %
55-85 %

 

Fig. 10: Schematic of the processes during nucleation and the first stage of the growth of vertical nanoflakes in plasma. Dissociation of CH4 molecules and surface diffusion result in the formation of hexagonal carbon structures. The presence of H atoms in the plasma system assists in the nucleation of the carbon radical and simultaneously acts as the etchant gas for a-C [50, 51].

Plasma-solid interactions for growing vertical nanostructures

Figure 10 shows a set of chemical and physical phenomena that occur during the growth of vertically-aligned carbon nanostructures, such as vertical surface-attached nanotubes and graphene flakes. Importantly, hydrogen atoms are more reactive towards α-carbon, as compared to the sp2 and sp3 hybridized carbons.

This results in the removal of α-carbon and then ensures nucleation and growth of crystalline carbon nanoflakes, forming sharp, highly reactive edges. Some publications also demonstrate the important role of oxygen and nitrogen, which help to remove α-carbon from the growing nanostructures.

Oxygen radicals O* significantly reduce the density of surface defects, and suppress the nucleation of carbon and formation of interfacial layers. As a result, the carbon nanostructures are formed without any interfacial layers. Moreover, oxygen ensures the growth of carbon nanowalls in a vertical direction via the removal of any small nuclei that could form horizontal layers. Next, OH radicals also help to remove the α-carbon. Admixture of Ar atoms in the reactive environment helps to form C2 molecules through direct impact dissociation. Importantly, the high density of argon atoms in the reactive environment helps to increase the density of carbon dimers, thus ensuring higher graphitization [53]. Furthermore, Ar atoms in plasma enhance the growth of carbon nanowalls due to the higher stability of plasma.

 

Fig. 11: A schematic explanation of the processes during the carbon nanowall growth. E: The direction of an electric field; CHX(g): HC growth species; C(G): Graphene sheets; H: Atomic hydrogen used as an etchant. CHX(α): α-C etched along with H atoms in the form of hydrocarbon (HC); VG edge: Edges of the vertically-oriented carbon nanowalls.

Figure 11 shows the interfacial plasma-solid processes during the growth of well-shaped carbon nanowalls in the process, where CH4 and H2 molecules are involved. Vertical graphene flakes bend when the network of sp2 hybridized carbons overcomes the relevant energy threshold. Due to the presence of ions and electrons in plasma, an electric field orthogonal to the surface is formed, which contributes to overcoming the energy threshold on sharp edges of the growing nanostructures. As a result, the vertical graphene flakes bend in the electric field. Plasma also etches the dangling bonds at sharp edges, thus promoting the formation of the network of isolated, stand-alone nanostructures.

A strong electric field at the sharp edges of growing nanostructures ensures focusing of the ion current toward the edges, thus resulting in carbon nanowalls with greater height [54]. In addition, the electric field ensures the formation of sp2 hybridized carbons, which serve as nucleation centers. Moreover, the lateral electric field at the sharp edges of nanowalls interacts with surface plasmons and could change the growth mode of the vertical carbon nanostructures [55].

 

Fig. 12: SEM images of MoO3 nanostructures. (a-e) Deposited in the pin-to-pin electrode configuration, (f-h) deposited in the pin-to-plate electrode configuration; (i) porous networks of MoO3 deposited in the pin- to-plate configuration. More details on the processes, plasma parameters and characterization of nanostructures may be found elsewhere [56]. Reprinted under the Creative Commons Attribution International License (CC BY).

To demonstrate the potential of plasma-based processes for materials treatment and activation, let us examine several examples (more examples of the processes of various types may be found in numerous references provided). Figure 12 illustrates many types of nanostructures synthesized by pulsing plasmas. Using a short pulsing plasma discharge, quite different structures may be obtained in an energy efficient process. As compared to other techniques, such as thermal furnace-based methods, the use of plasma-based process environments has the potential to significantly lower the energy cost of the final product [56]. While the whole set of processes involved in the growth of vertical graphenes in plasma is very complex, it offers a multitude of opportunities to control the process (Fig. 13).

 

Fig. 13: Summary of time-temperature growth regimes for the initial growth of different carbon nanostructures. Reprinted from [51] under the Creative Commons Attribution International License (CC BY).

 

Fig. 14: (a) Schematic diagram for the atmospheric pressure plasma reactor (RF is an abbreviation for radio frequency). (b) Side-view photo showing the ignited plasma with gas flowing from the top. (c) Typical transmission electron microscopic image of the nanoparticles produced in argon plasma. Reprinted from [57] with permission. Copyright RSC 2016, under the Creative Commons Attribution International License (CC BY).

Another example of highly controllable, energy efficient synthesis of nanostructures in plasma-based techniques, with plasma-surface interfacial processes taking the leading role, is shown in Fig. 14. Free-standing perfectly crystalline silicon carbide nanocrystals were fabricated in atmospheric pressure plasmas using the low-cost, ligand-free technique.

This process resulted in the synthesis of ultra-small nanocrystals that were highly crystalline and perfectly controllable in sizes. Moreover, this plasma-based technology ensures very low levels of surface contamination for the fabricated nanostructures. The use of atmospheric pressure significantly reduces the cost of the product.

FUTURE TRENDS

A deep understanding of physical and chemical processes occurring at the plasma-surface interfaces is critically important for many plasma applications, spanning medicine and biology, plasma-based space propulsion [58-63], synthesis of novel materials [64-66] and sophisticated control of material activation and functionalization [67]. Despite the great progress already made in this field, further studies are needed to satisfy the growing demand from the materials sciences and plasma physics communities. In our opinion, the major directions should be as follows:

✓ More attention should be paid to the self-organized behavior of interfacial systems;

Further progress should be made in designing novel, robotic, and artificial intelligence-enabled methods for rapid testing and measurement of the interfacial processes, both on surfaces and in plasmas. This would ensure fast progress in understanding the complex processes and the interference between the processes. On the other hand, plasma-based technologies are complex and certainly, their automatization is also a complex problem. An approach for such automatization was developed by the authors at the plasma and schematics level [8].

Next, further progress should be made in numerical modeling and simulation of the interfacial processes with the help of super-fast supercomputers and distributed computer networks. This would help to achieve faster progress without the involvement of expensive, time consuming experiments and extremely expensive equipment.

Taking into account the key role that the interfacial processes play in the aforementioned systems, further studies in this area should be considered as a matter of priority.

Acknowledgements: This work was supported in part by the following funds and organizations: the Office for Space Technology and Industry - Space Research Program (OSTIn-SRP/EDB) through the National Research Foundation, Singapore, and in part by the Ministry of Education Academic Research Fund (MoE AcRF, grant No. Rp6/16 Xs), Singapore; I. Levchenko acknowledges the support from the Science and Engineering Faculty, Queensland University of Technology; O. Baranov acknowledges the support from the PEGASUS (Plasma Enabled and Graphene Allowed Synthesis of Unique Nano-Structures) project, funded by the European Union's research and innovation program, Horizon, under grant agreement No. 766894; and the authors would like to express special thanks to L. Xu, S. Huang and the entire group at the Plasma Sources and Applications Centre / Space Propulsion Centre, Singapore (PSAC/SPCS) for their help.

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