Size dependence of optical properties and internal structure of plasma grown carbonaceous nanoparticles studied by in situ Rayleigh-Mie scattering ellipsometry

We have analyzed the growth process of a-C:H nanoparticles in Ar–C₂H₂ and Ar–CH₄ plasmas by means of in situ Rayleigh-Mie scattering ellipsometry. Complex refractive index (m=n-ki), mean particle radius (r), and particle size distribution (σ) of a-C:H nanoparticles are carefully determined and compared with those of a-C:H thin films deposited at similar conditions. It is found that, in both plasmas, small particles (r≪25 nm) have graphitelike properties whereas large particles have polymerlike characteristics. These results indicate that the particles have internal structures of a hard core and soft mantles on it. The size distribution of the nanoparticles in both Ar–CH₄ and Ar–C₂H₂ plasmas is essentially monodisperse with a narrow Gaussian size distribution. A systematic comparison between the growth and atomic structure models for a-C:H materials and criteria for interstellar dust (ISD) grains known from astrophysical observations is made. Significant agreement is found in a wide range of radii, supporting earlier findings that plasma-polymerized a-C:H nanoparticles might be a good candidate as an ISD analog.