Evolution of $s{p}^{\mathit{2}}$ networks with substrate temperature in amorphous carbon films: Experiment and theory

The evolution of $s{p}^2$ hybrids in amorphous carbon (a-C) films deposited at different substrate temperatures was studied experimentally and theoretically. The bonding structure of a-C films prepared by filtered cathodic vacuum arc was assessed by the combination of visible Raman spectroscopy, x-ray absorption, and spectroscopic ellipsometry, while a-C structures were generated by molecular-dynamics deposition simulations with the Brenner interatomic potential to determine theoretical $s{p}^2$ site distributions. The experimental results show a transition from tetrahedral a-C (ta-C) to $s{p}^2$-rich structures at $\sim 500\mathrm{K}$. The $s{p}^2$ hybrids are mainly arranged in chains or pairs whereas graphitic structures are only promoted for $s{p}^2$ fractions above 80%. The theoretical analysis confirms the preferred pairing of isolated $s{p}^2$ sites in ta-C, the coalescence of $s{p}^2$ clusters for medium $s{p}^2$ fractions, and the pronounced formation of rings for $s{p}^2$ fractions $>80\%$. However, the dominance of sixfold rings is not reproduced theoretically, probably related to the functional form of the interatomic potential used.

Figure 1

Raman spectra (open dots) and the corresponding fitting curves (solid lines) for a-C films grown by FCVA at different substrate temperatures.

Figure 2

Fitting results from the Raman spectra of Fig. 1.

Figure 3

XANES spectra of a-C films deposited by FCVA at different substrate temperatures together with the reference spectra of diamond and graphite. A sample grown by electron-beam evaporation of graphite (e-C) is also shown as representation of a $\sim 100\%s{p}^2$ disordered material.

Figure 4

Detailed view of the ${\pi }^{*}$ region of the XANES spectra displayed in Fig. 3.

Figure 5

Calculated $s{p}^2$ content from the XANES spectra (●) and total intensity of the $\mathrm{C}\left(1s\right)$ edge (엯). The lines are a guide to the eye to evidence the transition from diamond-like to graphite-like structures starting at $\sim 500\mathrm{K}$.

Figure 6

Complex dielectric function derived from SE measurements.

Figure 7

Comparison of the SE (●) and Raman (엯) results with the $s{p}^2$ content calculated by XANES.

Figure 8

SP ring frequencies from MD simulated films as a function of the $s{p}^2$ fraction. The deposition energy of all these films was 40 eV. The lines are plotted as “guide to the eye.”

Figure 9

Size distributions of SP rings of $s{p}^2$ atoms in the MD simulated films which contain one spacious $s{p}^2$ network throughout the whole film.