Superentropic Material

Amorphous diamond appears to be contradictory term, like liquid crystal or glassy metal. Amorphous means non-crystalline and diamond implies crystalline. However, this terminology is meaningful because unlike silicon that forms only sp3 bonds, i.e. diamond structure, carbon may form either sp2 (graphitic) or sp3 (diamond) bond. Although there is one form of amorphous silicon, there can be at least two forms of amorphous carbon, so amorphous diamond can be distinguished from amorphous graphite, and together they are amorphous carbon.

Amorphous diamond is formally known as tetrahedral amorphous carbon (tac), it is really a diamond-like carbon (DLC) that contains no non-carbon impurities (e.g. H). Amorphous diamond is essentially a chaotic carbon mixture with distorted sp2 and sp3 bonds. As such it possesses both metallic character of conductive graphite and semiconductor character of insulating diamond. Moreover, as each carbon atom is unique in its electronic state that is determined by the degree of distortion of its bonds. Hence, amorphous diamond contains numerous discrete potential energy for electrons. In fact, amorphous diamond may have the highest density of atomic occupancy (1.8 x 1023 per cubic centimeter) that is several times higher than ordinary materials (e.g. about four times of iron atoms or silicon atoms). Thus, amorphous diamond has the highest configuration entropy for both atoms and valence electrons (Fig. 9.1).

Amorphous diamond can be conveniently deposited by PVD methods, such as by sputtering or arc depositions. Due to the low temperature (< 150°C) of deposition, amorphous diamond can be coated on most materials including metal, semiconductor, or even polymers. This flexibility makes amorphous diamond useful for many applications (Fig. 9.2).

Due to such high configuration entropy of valence electrons, amorphous diamond is capable to advance electron energy by absorbing small increments of energy, such as by converting thermal energy (lattice vibration) to potential energy (electron state). If amorphous diamond is exposed in high vacuum (e.g. 10-6 torr), the energy state may be higher than vacuum state so amorphous diamond my emit electrons simply by heating. Because amorphous diamond has the highest discrete electronic states, it is the most thermionic material known.

Figure 9.1. The high atomic density and the unique way of distorting bonds for each atom makes amorphous diamond the material with the highest configurational entropy. As a result, amorphous diamond has the densest electron states that are discrete. This is in contrast of all materials that have either overlapped electron orbitals, as in the case of metal, or few discrete electron states, as in the case of semiconductors or insulators.

Figure 9.1. The high atomic density and the unique way of distorting bonds for each atom makes amorphous diamond the material with the highest configurational entropy. As a result, amorphous diamond has the densest electron states that are discrete. This is in contrast of all materials that have either overlapped electron orbitals, as in the case of metal, or few discrete electron states, as in the case of semiconductors or insulators.

Figure 9.2. Amorphous diamond can be coated on substrates by cathodic arc process. In this case, the surface smoothness can be adjusted by the arc current; and the material properties (e.g. sp3/sp2 ratio of the carbon atoms), by the bias on the substrate.
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