π-bonded clusters in amorphous carbon materials

Abstract

Total energies favour a segregation of the sp2 sites in amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C : H) into sp2-bonded clusters embedded in a sp3-bonded matrix. The size of the clusters determines the local bandgap. The presence of clusters forces the mobility edges further into the bands. Clustering also introduces strong symmetrical fluctuations of the band edges. This causes intracluster transitions between localized and localized states to become allowed. The effect of such clustering on the luminescence properties, the resonant Raman spectra, the Urbach tail and the subgap defect optical absorption of a-C: H is discussed. The sp2 sites form mainly sixfold rings within the clusters. The presence of a small fraction of adjacent fivefold and sevenfold rings washes out the thirdneighbour peak at 2%4A in the radial distribution function of sputtered a-C.Amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C : H) contain both sp3 and sp2 carbon sites, with 1-5% sp3 sites in evaporated a-C (Green, McKenzie and Lukins 1990) and sputtered a-C (Cho et al. , Pan et al. 1991, and 40-70 sp3 sites in plasma-deposited a-C : H (Koidl et al. 1990, Tamor, Vassell andCarduner 1991). The sp3 sites and sp2 sites are connected together by (T bonds which form the skeleton of the random network. The sp2 sites also possess x states which form x bonds with neighbouring sp2 sites. The x states of n bonds lie closer to the Fermi level E , than do the c states; so the n: states form both the valence-and the conduction-band-edge states in a-C and a-C : H and consequently they control the electronic properties of both materials (Robertson 1986, Robertson and OReilly 1987). Analysis of the energetics of x bonding suggests that the sp2 sites will segregate into graphitic clusters embedded in a sp3-bonded matrix. The difference between the bandgaps of the sp2-and sp3-bonded regions produces very large fluctuations in the local bandgap (Robertson 1988). This paper describes the consequences of such fluctuations and clustering and discusses the evidence that the clustering occurs.The clustering of sp2 sites arises from the non-local energetics of their x bonding.The energetics of c and n bonds are separable, to first order. The stronger (T bonds form the skeleton of the network. The two-centre quality of the c bonds allows their total energy to be written as the sum of the energies of the individual bonds; that is it is a local function. The x bonds are more complex. They can form two-centre bonds such as the C=C bond in ethylene. They can also form multicentre or resonant bonds as in polyacetylene, benzene or graphite. This resonant character prevents the total x bond energy being expressed as a sum of two-centre terms as it adds non-local (off-site or longer-range) terms. The energetics of x bonding favours sp2 sites forming sixfold ring aromatic species such as benzene, rather than chain (olefinic) species such as ethylene or polyacetylene. Their energetics also favours the fusing of sixfold rings into polyaromati...

Nitrogen-related dopant and defect states in CVD diamond

“…Nesládek et al 15 attribute this to Ϫ* band-type transitions in amorphous carbon. 20,21 Close similarity to a-C is obvious only in the spectral range 1-2 eV ͑see Fig. 3͒.…”

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“…There is a supposition for diamondlike films that the reduction in band gap with post-growth annealing can be attributed to the encroachment on the gap by aromatic π -π * states. It is argued that the π -π * gap in DLC decreases as the degree of sp 2 clustering increases, in the same way as it does for polyaromatic molecules [4][5][6][7][8][9]. It seems that such clustering need not necessarily be aromatic [4].…”

Evolution under annealing and nitrogen implantation of the mechanical properties of amorphous carbon films

“…The spectra, in the 800-1800 cm À1 range, are the result of two contributions, one at about 1400 and the other one at 1570 cm À1 . They correspond to sp 2 C-C disordered carbon (D peak) [16] and sp 2 C-C graphitelike carbon (G peak) [17], respectively. When the annealing temperature increases from 250 8C up to 800 8C, the spectra clearly exhibit two concomitant evolutions.…”

“…doi:10.1016/j.tsf.2004.11.170 mechanical properties of the film ( [7], and references therein). In this paper, we report the evolution of residual stresses in a-C films studied in situ during annealing cycles up to 800 8C as well as the evolution of hardness and indentation modulus after nitrogen implantation up to 5Â10 16 ions/cm 2 .…”