The effect of sp2 fraction and bonding disorder on micro-mechanical and electronic properties of a-C:H films

There is a notion that in carbonaceous materials the sp3 hybridised fraction determines the mechanical properties, while the sp2 fraction controls the electro-optical properties. We investigated a narrow range of a-C:H films synthesised in an open hydrocarbon plasma source. The a-C:H films displayed unambiguous variation in mechanical properties as measured by nanoindentation measurements. However the proportion of sp3 constituent, determined using X-ray C1s core-level photoelectron spectroscopy and Fourier transform infrared spectroscopy, was found to be essentially the same for all studied samples. The Tauc gap was found to be controlled by the perturbation of the π tail states of sp2 phase and its magnitude varied inversely with a surface conduction gap determined by scanning tunnelling microscopy measurements. These results indicate that mechanical properties of a-C:H films are also influenced by the degree of disorder of the sp2 fraction, as characterised by bond saturation and symmetry.