Polymer-Like C:H Thin Film Coating of Nanopowders in Capacitively Coupled RF Discharge

Abstract

Nanopowders of amorphous SiO₂, with typical particle sizes of 30–80 nm, were treated under the non-equilibrium plasma conditions, created by a capacitively coupled (CC) RF discharge formed in pure methane or ethane. The plasma gas flow rate varied between 0.02 and 0.06 slpm, with reactor pressures between 1000 and 5000 Pa, and applied RF power inputs between 700 and 1500 W. The plasma properties were monitored through measurements of the rotational temperature, as derived from the C₂ 5160 Å Swan band and N₂ second positive 3577 Å band, and the atomic hydrogen excitation temperature, from the H_β H_γ and H_δ lines during the powder treatment process. The compositions of the gases that passed through the plasma were analyzed by mass spectrometry. In spite of the evidence for the presence of C_nH_2n+2 and C_nH_2n (n=1–3) species and acetylene in the discharge, the homogeneous formation of soot was not observed. At the same time, the introduced nanoparticles were observed to act as centers for the inception and growth of C:H thin coatings in the form of a polymer-like hydrocarbon layers, of thickness between <5 and 30 nm. The results of TEM, IR spectroscopy, thermo-gravimetric and precision calorimetric analysis performed on the treated powders provide evidence to the formation of an amorphous, high density C:H matrix on the particles' surfaces.