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Structure of amorphous carbon

From measurements and theoretical calculations done in the works presented above, common structural characteristics appear: The ta-C is a hard and dense material, mostly made of distorted sp3bonds. A considerable amount of strain exists, due to localized melting and rapid quenching during its formation, that leaves the lattice in a stressed state. Part of the internal strain energy is relieved by the presence of sp2 bonded atoms. These threefold atoms tend to form small clusters, owing to the delocalization of the $\pi$states [53], and perhaps $\pi$-bonded pairs [48]. and are thought to control the band gap [54]. Moreover, an annealing process at high temperature may relieve stresses and bring the sample to a still dense but sp2-rich phase [48]. This is explained by the fact that threefold sites are the energetically favorable geometry. Temperature will supply the necessary energy to remove weakly bonded sp3 atoms from their sites. The bonds are then transformed to sp2 bonds, relieving the local strain and lowering the energy.

The a-C lattice has a less dense structure and is mainly sp2bonded. In a sample made of $\approx$ 80% of sp2 bonds, threefold coordinated atoms tend to form large clusters of diameter $\approx$ 15 $\AA$ embedded in a matrix of fourfold atoms [54]. These sp2 bonds are distorted and non planar, and the threefold atoms form thick layers of average thickness of 1.0-1.2 $\AA$, which are spaced more closely than the sheets of graphite [50]. These planes are composed of mainly fivefold and sevenfold rings, with a small amount of sixfold rings [35,48,16,50], leading to an insignificant medium range order. These observations are not supported by the work of J. Robertson and E.P. O'Reilly [54] who found a much more ordered configuration of sheets with mostly sixfold rings.


next up previous contents
Next: Stability of amorphous carbon Up: Structural modifications induced by Previous: Amorphous carbon
David Saada
2000-06-22