Formation of cluster-assembled carbon nano-foam by high-repetition-rate laser ablation

High-repetition-rate laser ablation and deposition of carbon vapours results in the formation of quite different carbonaceous structures depending on the pressure of the ambient Ar gas in the chamber. Diamond-like carbon films form at a pressure below ≈ 0.1 Torr whereas a diamond-like carbon nano-foam is created above 0.1 Torr. Although laser-deposited amorphous carbon films have been extensively investigated in the past, here we present what, to our knowledge, is the first report of the production of a granular low-density carbon nano-foam with rich fraction of sp³ bonding. The bulk density of various foam samples was in the range (2-10)×10^-3 g/cm³, and the specific surface area was 300-400 m²/g. The resistivity of the foam measured at low-voltage (±30 V) is (1-3)×10⁹ Ohm cm at room temperature and (1-10)×10¹³ Ohm cm at 80 K. The dc conductivity of this low-density carbon foam and its temperature dependence appears to be very close to that of RF-sputtered solid amorphous diamond-like carbon films. The presented kinetic analysis of the carbon vapour in the Ar ambient demonstrates qualitative agreement between the predicted laser plume parameters and those measured in the experiments. Theoretical predictions of the parameters and the process of carbon vapour diffusion through the ambient gas, allow us to propose a self-consisted periodic model leading to the formation of the carbon clusters in the experimental chamber.