Control of cluster synthesis in nano-glassy carbon films

Carbon films were prepared by pulsed laser deposition (PLD), changing buffer gas nature and pressure and laser power density. Nanometer-sized cluster assembled (CA) films, resulting from direct aggregation of carbon clusters in the ablation plume, were obtained. Visible Raman spectroscopy shows that all films are trigonally co-ordinated and structurally disordered, with a dependence of the degree of disorder on the deposition parameters. The microstructure and morphology of the films were studied in a complementary way by scanning electron microscopy (SEM) both in plane and in cross-section, and by atomic force microscopy (AFM). Different growth modes are found in the deposited CA films, depending on the interplay of laser fluence and nature-pressure of the buffer gas. Threshold fluences of increasing value separate dense columnar growth from sponge like morphology, from an open dendritic structure. AFM pictures show that our glass-like carbon films consist of agglomerates of nanometer-sized clusters. Cluster formation in the plume is modeled, allowing to estimate the average number of carbon atoms per cluster. The calculated size of the clusters depends mainly on ambient gas pressure. Cluster sizes obtained by model predictions agree with those directly observed by transmission electron microscopy (TEM) imaging and with the deduced film coherence length from Raman spectroscopy conducted herein.