Correlation between bonding structure and microstructure in fullerenelike carbon nitride thin films

The bonding structure of highly ordered fullerenelike (FL) carbon nitride $\left(\mathrm{C}{\mathrm{N}}_x\right)$ thin films has been assessed by x-ray absorption near-edge spectroscopy (XANES). Samples with different degrees of FL character have been analyzed to discern spectral signatures related to the FL microstructure. The XANES spectra of $\mathrm{FL}\text{−}\mathrm{C}{\mathrm{N}}_x$ films resemble that of graphitic $\mathrm{C}{\mathrm{N}}_x$, evidencing the $s{p}^2$ hybridization of both C and N atoms. The FL structure is achieved with the promotion of N in threefold positions over pyridinelike and cyanidelike bonding environments. In addition, the relative ${\pi }^{*}∕{\sigma }^{*}$ XANES intensity ratio at the $\mathrm{C}\left(1s\right)$ edge is independent of the FL character, while it decreases $\sim 40\%$ at the $\mathrm{N}\left(1s\right)$ edge with the formation of FL arrangements. This result indicates that there is no appreciable introduction of $\mathrm{C}\text{−}s{p}^3$ hybrids with the development of FL structures and, additionally, that a different spatial localization of $\pi$ electrons at C and N sites takes place in curved graphitic structures. The latter has implications for the elastic properties of graphene sheets and could, as such, explain the outstanding elastic properties of $\mathrm{FL}\text{−}\mathrm{C}{\mathrm{N}}_x$.

Figure 1

Diagram showing the different structures obtained in $\mathrm{C}{\mathrm{N}}_x$ films grown by dc-MS and the parameter window for the synthesis of $\mathrm{FL}\text{−}\mathrm{C}{\mathrm{N}}_x$ (shadowed region). The circles refer to the growth conditions of the samples considered in this work (see Table ), with the arrows marking the direction of increasing FL character.

Figure 2

$\mathrm{C}\left(1s\right)$ (a) and $\mathrm{N}\left(1s\right)$ (b) XANES spectra for $\mathrm{C}{\mathrm{N}}_x$ films grown with 16% ${\mathrm{N}}_2$ in the gas mixture and different substrate temperatures. (See set $T\#$ in Table and Fig. 1.) The spectra of graphite, diamond, cBN, and hBN are shown as reference for $s{p}^2$ and $s{p}^3$ hybridizations.

Figure 3

$\mathrm{C}\left(1s\right)$ (a) and $\mathrm{N}\left(1s\right)$ (b) XANES spectra for $\mathrm{C}{\mathrm{N}}_x$ films grown with different $\mathrm{Ar}∕{\mathrm{N}}_2$ gas mixtures at a substrate temperature of $723\mathrm{K}$. (See set N# in Table and Fig. 1.)

Figure 4

Relative ${\pi }^{*}∕{\sigma }^{*}$ XANES intensity at the $\mathrm{C}\left(1s\right)$ and $\mathrm{N}\left(1s\right)$ absorption edges for the spectra shown in Fig. 2. The evolution towards the FL structure implies a decrease in the contribution of $\pi$ electrons from N atoms, while it is kept constant for C atoms.