**3. Plasma deposition of new materials**

Since the first literature reports describing – nearly 140 years ago – the formation of solid products during electrical discharge in a tube filled with acetylene, many researches working in the field of plasma chemistry have observed the presence of high molecular weight materials as reaction by-products. These products were usually considered disadvantageous – they were deposited on the reactor walls and, due to their good adhesion to glass and insolubility in organic solvents, were not easily removable. They began to stimulate a scientific interest as late as in the sixties of the 20th century only, after Goodman (Goodman, 1960) had reported a successful application of plasma polymerized styrene films as an insulating layer in nuclear batteries. A vast amount of literature concerning plasma deposited films (plasma polymers), their properties, structure and mechanism of formation as well as potential application in various technologies has been published since then. The application ability of plasma polymers originates from both their often unique properties and relative simplicity of their production (Biederman, 2004; and references therein).

At the beginning, the interest was mainly limited to classical monomers, i.e. substances known to be able to polymerize in the conventional way (e.g. ethylene, styrene, butadiene). Hence, at that time the term "plasma polymer" was introduced as the result of the supposed analogy to conventional polymerized materials. In soon turned out, however, that many other low molecular weight organic, organometallic and inorganic compounds undergo plasma polymerization as well. Plasma polymers and conventional polymers, even though

Cold Plasma – A Promising Tool for the Development of Electrochemical Cells 111

thus to decrease the density of localized states in the mid-gap.2 Thermally evaporated a-Si has about 1026 eV−1m−3 states in the mid-gap, whereas the density of states for typical a-Si:H films is about 1021 eV−1m−3, i.e. five orders of magnitude lower. This just explains why one can control the electronic properties of a-Si:H by doping with donor and acceptor centers,

The most frequent method used to prepare doped films is plasma copolymerization from a mixture of the film precursor and the dopant agent. Diborane (B2H6) and phosphine (PH3) are often used as sources of acceptor centers (boron atoms) and donor centers (phosphorus atoms), respectively. Recently, liquid compounds instead of these gases, such as triethylboron (B(C2H5)3) and trimethylphosphine (P(CH3)3) have become more and more popular dopant agents. They are less toxic, more stable and their low vapor pressure offers facilities for precise controlling of the doping process. In turn, as a-Si:H film precursor, one can use not only SiH4, but also, for example, a mixture of SiCl4 and H2, disilane (Si2H6), trisilane (Si3H8), cyclohexasilane (Si6H12), etc. (Pokhodnya et al., 2009; Searle, 1998; Tyczkowski, 2004). So, as one can see, the possibilities of designing and controlling the

molecular and electronic structure of plasma deposited films are indeed enormous.

*p-*type material (LeComber & Spear, 1979; Tyczkowski, 2004).

gaseous precursor ratio (Tyczkowski, 2004).

films, see, for example, (Tyczkowski, 2004).

As an example of designing the electronic structure of a-Si:H films, the electrical conductivity of these films doped with acceptors (boron) and donors (phosphorus) is shown in Fig. 5. The room-temperature conductivity *σ* of the films is plotted against the ratio of the number of dopant agent molecules to the number of silane molecules in the gaseous mixture. In the center of the graph, the conductivity around 10−6 S/m is representative of undoped a-Si:H films, which typically are *n*-type material. Thus, even a small quantity of P atoms (donors) increases *σ* rapidly. In the case of B atoms (acceptors), however, we see that initially *σ* decreases to about 10−10 S/m. This is connected with the transition from *n*-type to

Fig. 5. Room temperature conductivity *σ* of *n*- and *p*-type a-Si:H, plotted as a function of the

2 For more detailed description of the electronic structure models for plasma deposited amorphous

contrary to a-Si (LeComber & Spear, 1979).

they should be produced from the same precursor (monomer), have practically nothing in common. The fundamental difference is that mer units cannot be defined in the case of plasma polymers. A large variety of chemical species created in the plasma, statistical combination them into high molecular structures and generally a high degree of their crosslinking cause that the structure of such a material is very often much closer to that of covalent glasses than that of conventional polymers.

Frequently plasma polymers are not classified in respect of the type of monomer but from a point of view of their chemical composition and morphology. For example, amorphous (a-) covalent material obtained by plasma polymerization of silane (SiH4), which is composed of silicon and hydrogen, can be termed as a-Si:H. In turn, amorphous plasma polymer deposited from acrylonitrile (C3H3N) can be called as plasma-polymerized (pp-) acrylonitrile or a-CXNY:H. It is usually met, but it is not a rule, that if the plasma polymer structure is close to a covalent glass structure, the latter notation is used. If plasma polymer reveals nano- or microcrystalline structures, prefixes nc- or µc- are put in the place of a-.

The structure and properties of plasma polymers are closely connected with a thin-film form, in which they are produced. In general, the thickness of the films is between a few nanometers and a few micrometers. Appropriate choice of precursors and plasma process parameters allow for the preparation of such thin films with a huge variety of structure and properties. Hence, there is a wide and diverse range of their current and anticipated applications, such as electronic and photoelectronic materials, insulating coatings, catalytic films, semi-permeable and electrolyte membranes, protecting layers, and many others. Some of these uses are also related to the electrochemical systems.
