When we deposit amorphous carbon using evaporation or sputtering, it generally contains a small amount of sp3 bonding. Even in tetrahedral amorphous carbon, the fraction of sp3 bonds exceeds 80%.
By examining the phase diagram of carbon, we can observe that diamond (sp3) is situated well above the graphite phase, with high pressure required to transition to it.
Can someone explain the reason behind the presence of sp3-bonded carbons in amorphous carbon?
Whether the most stable phase is forming or not, depends on the growth conditions and subsequently the microscopic growth behaviour. Upon landing on the surface, a carbon atom will attach at some random place wherever it has the possibility for partial or even full saturation and it won't be picky about it whether that involves just sigma bonds or also a pi system. At higher temperatures, it may detach again and diffuse to an energetically more favorable position thus forming a pi bond while at lower temperatures it will just relax locally or do nothing at all.
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