![]()
Chapter 1
Introduction
Since the 1980s, gases made of ionized particles have been used to synthesize materials with a specific desired surface chemistry or modify the chemical surface properties of an object. Such an ionized gas is called plasma and is usually referred to as the fourth state of matter in addition to the conventional solid, liquid, and gaseous forms. There is a rather broad variety of plasmas and they can be classified on the basis of different criteria, as summarized in Table 1.1.
TABLE 1.1 Classification of Plasmas Following Different Criteria
| Criteria | Plasma |
| Operating pressure | low-pressure plasma |
| | atmospheric-pressure plasma |
| Thermodynamic equilibrium | thermal or equilibrium plasma (Telectron ≈ Tion ≈ Tgas) |
| | non-thermal plasma or non-equilibrium plasma (Telectron >> Tion ≈ Tgas) |
| | low-temperature plasma (Tgas < 2000 K) |
| | high-temperature plasma (Tgas > 2000 K) |
| Plasma generation | microwave frequency discharge (300 MHz–300 GHz) |
| | radio frequency discharge (450 kHz–3.0 MHz; 13.56 MHz) |
| | DC discharge |
| | dielectric barrier discharge |
| | corona discharge |
| | electric arc |
| | hollow cathode discharge |
| | electron beam |
| | plasma torch |
| | alternating current |
| Type of coupling | inductive coupling |
| | capacitive coupling |
|
Low pressure and atmospheric plasmas can be distinguished on the basis of the operative pressure. In the first case, plasmas are generated in vacuum chambers where the precursor pressure typically ranges between a fraction of a millibar and 10−4 millibar. Atmospheric pressure plasma jets operate at ambient pressure without the need for complex equipment to reach low pressures. Due to the combination of simplicity, low cost, and wide possibilities of material treatment and modification, at present they are one of the most promising technologies. Plasmas can be classified also on the basis of temperature, that is, thermal equilibrium. In non-thermal plasmas, the temperatures of the electrons and the ions are in a thermal non-equilibrium. Due to the different mass, the temperature of the electrons (i.e., the kinetic energy) ranges between several electron volt (eV), whereas the temperature of the positively charged ions and neutral species is around room temperature. 1 , 2 This corresponds to a quite low overall plasma temperature ranging from 300 K to 1000 K. For this reason, the plasma is also called “cold plasma” and is favorable for the synthesis of materials at low temperature. In high-temperature plasmas, the temperatures are about 107 K, which is typical of fusion plasmas 3 used to produce energy. We will focus on low-temperature, plasmas, which are used to modify the properties of materials.
The common feature of all the plasmas is that they use vaporized chemical compounds or gaseous precursors. In a typical plasma process, the precursor molecules are introduced and ionized in a chamber (the plasma reactor) with selected concentrations. Then, another kind of classification may be done on the basis of the energy source used to generate and sustain the plasma. The source of energy can be thermal energy, a flame, a laser, a microwave or radio frequency (RF), ...
