Electronic structure and chemical bonding of nanocrystalline-TiC/amorphous-C nanocomposites

The electronic structure of nanocrystalline (nc-) TiC/amorphous C nanocomposites has been investigated by soft x-ray absorption and emission spectroscopy. The measured spectra at the $\text{Ti}2p$ and $\text{C}1s$ thresholds of the nanocomposites are compared to those of Ti metal and amorphous C. The corresponding intensities of the electronic states for the valence and conduction bands in the nanocomposites are shown to strongly depend on the TiC carbide grain size. An increased charge transfer between the $\text{Ti}3d\text{−}{e}_g$ states and the $\text{C}2p$ states has been identified as the grain size decreases, causing an increased ionicity of the TiC nanocrystallites. It is suggested that the charge transfer occurs at the interface between the nanocrystalline-TiC and the amorphous-C matrix and represents an interface bonding which may be essential for the understanding of the properties of nc-TiC/amorphous C and similar nanocomposites.

Figure 1

(Color online) XRD from the nc-TiC/a-C films with different grain size (in parenthesis).

Figure 2

(Color online) A series of $\text{C}1s$ XPS spectra of the nc-TiC/a-C films with different grain size (in parenthesis), plotted on a binding-energy scale. The spectra were measured after sputter etching.

Figure 3

(Color online) Top and middle: $\text{Ti}2p$ TEY- and TFY-SXA spectra of nc-TiC/a-C films with different grain size (in parenthesis) in comparison to bulk Ti metal. Bottom: nonresonant $\text{Ti}{L}_{2,3}$ SXE spectra of nc-TiC/a-C and Ti metal excited at 490 eV.

Figure 4

(Color online) $\mathrm{C}1s$ TEY- and TFY-SXA spectra of nc-TiC/a-C films with different grain size (in parenthesis) and amorphous C with the characteristic ${\pi }^{\ast }$ and ${\sigma }^{\ast }$ peak regions indicated by the horizontal arrows at the bottom. The dotted vertical lines indicate the excitation energies (1), $\left(2^{\prime }\right)$ and (2) for the SXE measurements shown in Fig. 5.

Figure 5

(Color online) Resonant and nonresonant $\text{C}K$ SXE spectra of nc-TiC/a-C films with different grain size (in parenthesis) and amorphous C excited at peak (1) at 284.9, the shoulder $\left(2^{\prime }\right)$ at 287.2 eV and peak (2) at 288.2 eV in the SXA spectra and nonresonant at 310 eV. The occupied $\sigma$ and $\pi$ bands of a-C are indicated at the bottom.

Figure 6

(Color online) Difference plots of resonant and nonresonant $\text{C}K$ SXE spectra from Fig. 5 of three nc-TiC/a-C films with different grain size (in parenthesis) obtained by subtraction from a superposition of the spectra with the 15 nm grain size and a-C excited at peak (1) at 284.9, at the shoulder $\left(2^{\prime }\right)$ at 287.2 and peak (2) at 288.2 eV in the SXA spectra and nonresonant at 310 eV.