Influence of Cumulenic Chains on the Vibrational and Electronic Properties of $sp\mathrm{\text{−}}s{p}^2$ Amorphous Carbon

We report the production and characterization of a form of amorphous carbon with $sp\mathrm{\text{−}}s{p}^2$ hybridization (atomic fraction of $sp$ hybridized species $\ge 20\%$) where the predominant $sp$ bonding appears to be $(=\mathrm{C}=\mathrm{C}={)}_n$ cumulene. Vibrational and electronic properties have been studied by in situ Raman spectroscopy and electrical conductivity measurements. Cumulenic chains are substantially stable in high vacuum conditions for temperatures lower than 250 K and they influence the electrical transport properties of the $sp\mathrm{\text{−}}s{p}^2$ carbon through a self-doping mechanism by pinning the Fermi level closer to one of the mobility gap edges. Upon heating above 250 K the cumulenic species decay to form graphitic nanodomains embedded in the $s{p}^2$ amorphous matrix thus reducing the activation energy of the material. This is the first example of a pure carbon system where the $sp$ hybridization influences bulk properties.

Figure 1

(a) Raman spectra of a cluster-assembled carbon film deposited in UHV conditions on a substrate cooled to 150 K and then heated to 250, 275, 300, and 325 K, respectively. The two Gaussian fits ($C1$ and $C2$) of the $C$ band are indicated. A linear background was subtracted from the spectra. (b) Plot of the relative intensity of the $C$ band (circles) as a function of the substrate temperature. The evolution of the relative intensity of the $C1$ (open triangles) and $C2$ (open squares) peaks is also shown.

Figure 2

(a) Arrhenius plots of the current flowing in a cluster-assembled film (voltage applied 100 V). Each plot corresponds to one of the Raman spectra shown in Fig. 1 and it is then labeled by the relative intensity of the corresponding $C2$ peak. The linear fits are indicated. (b) Plot of the intercept value of the linear fits ($I_0$) as a function of the relative intensity of the $C2$ peak ($I_{C2}^{\mathrm{rel}}$). (c) Plot of the slope value of the linear fits ($E_a$) as a function of $I_{C2}^{\mathrm{rel}}$.

Figure 3

(a) Comparison between Raman spectra of a cluster-assembled film taken at 150 and 325 K normalized to their effective intensity $I_{\mathrm{eff}}$ (see text). The corresponding percentage of $sp$ hybridized atoms is indicated. The difference spectrum, expanded by a factor of 2, is indicated at the bottom with the Gaussian fit of $D$ and $G$ bands. (b) Molecular dynamics simulated VDOS of a $sp\mathrm{\text{−}}s{p}^2$ system. The two VDOS are calculated for systems containing 48% and 27% of $sp$ hybridized atoms. Their difference is reported at the bottom expanded by a factor of 2.