Atomic bonding in amorphous carbon alloys: A thermodynamic approach

Abstract

The free energy model previously developed for the prediction of the bonding in amorphous Si-based alloys is extended here to amorphous carbon alloys, a-CxH1−x, containing carbon atoms with sp3 and sp2 hybridization. Predictions have been made for the bonds present in the alloys, with the case of ‘‘chemical’’ ordering at T=0 K corresponding to phase separation into separate C (sp3) and C(sp2) regions. For T≳0 K phase separation is eliminated and there is no evidence for the clustering of graphitic carbon, indicating the importance of the configurational entropy in influencing the bonding in the alloys. Hydrogen atoms are predicted to bond preferentially to C (sp3) atoms for all T. The sp3/sp2 ratio is predicted to increase with increasing H content, as observed experimentally, and also with increasing T due to entropy effects. Predictions have been made for the distribution of bonds in tetrahedral C(sp3)- and planar C(sp2)=C(sp2)-centered units. It is found that essentially no aromatic or graphitic structures are present in typical alloys. The a-CxH1−x alloys have been proposed to consist of five amorphous components: diamondlike, graphitic, polymeric, olefinic, and mixed diamond–graphitic (d–g) components. It is predicted that the polymeric and mixed d–g components dominate in typical plasma-deposited alloy films while the mixed d–g component dominates in hydrogen-free a-C films.

Thermal stability of diamondlike carbon films

“…Thermodynamic modeling of such networks taking into account the different energetics of bonding for sp 3 and sp 2 carbon atoms has been performed recently. 13 The probabilities of all possible configurations of local ordering of sp 3 and sp 2 carbon atoms and hydrogen in the alloys have been predicted. The main difference in the local arrangement of a particular C͑sp 2 ͒ϭC͑sp 2 ͒ double bond is the number of other C͑sp 2 ͒ atoms connected to the four available bonds, and for a particular C͑sp 2 ͒-H bond whether the remaining single bond of the C͑sp 2 ͒ atom is formed with a C͑sp 3 ͒ or C͑sp 2 ͒ atom ͓one C͑sp 2 ͒ atom is always bonded to a C͑sp 2 ͒ atom by a double bond͔.…”

“…The graphitelike structure is considered to be a planar unit consisting of a double bond surrounded by four C͑sp 2 ͒ atoms. 13 Even in the case of mixed bonding of carbon atoms, i.e., bonding between C͑sp 3 ͒ and C͑sp 2 ͒ atoms, hydrogen is predicted to be bonded to the C͑sp 3 ͒ atoms.…”

Structural characterization of amorphous SiCxNy chemical vapor deposited coatings

“…This C-C signature, already observed in Si-C alloys with carbon excess, 42,50-53 corresponds certainly to a specific structure which could be described like a random covalent network of tetrahedral-trigonal bonding carbons with distorted bond angles and bond lengths. [54][55][56][57][58] As a matter of fact, at first, it has been shown 57 that in mixed sp 2 Ϫsp 3 bonded carbon layers, the overall Raman spectrum is dominated by the G component, because the cross section of the graphite stretching mode is much higher than that of the 1332 cm Ϫ1 diamond mode. Second, Raman spectrum calculations 54 have shown that the incorporation of a small amount of tetrahedral bonding atoms ͑5%-10%͒ into a sp 2 network produced a downward shift in the G peak.…”

Continuum theory of carbon phases

“…First, these materials are always produced in the form of thin films, wires, or small particles that is, nanostructures. Second, although theoretical calculations and numerical simulations of such materials produced energies of formation significantly higher than those of graphite and diamond at the same temperatures [13,[21][22][23], these materials possess certain degree of thermodynamic stability. The MD/MC cluster simulation methods are an excellent tool of materials study when equilibrium properties of phases-graphite, diamond, liquid, and amorphous-are considered [5][6][7][8][9][10][11][12][13].…”