Processing of amorphous carbon films by ultrafast temperature treatment in a confined geometry

Abstract

A pressure cell with an anvil made of sapphire and the other made of tungsten carbide, was constructed to process thin film samples using a high power Nd:YAG pulsed laser, in a regime of ultrafast quenching rate and confined geometry. The sapphire anvil worked as the optical window to the laser beam and also as a good thermal conductor substrate. Thin films of amorphous carbon deposited over copper substrate were processed under pressure by Nd:YAG laser pulses. This process induces the formation of a high temperature region at the sample surface during a very short time interval of the order of the 8 ns laser pulse duration. To avoid the complete evaporation of the film, an external pressure of about 0.5 to 1.0 GPa was applied, confining the sample. With the aid of the nanosecond pulsed laser, absorbed on a very thin film sample, this specially designed apparatus provides the means to produce ultrafast quenching as the formation of a plume is suppressed and heat dissipation is accelerated by the high thermal conductivity of the copper substrate and sapphire anvil. The processed samples were analyzed by microRaman spectroscopy and the results indicated the formation of polyynic carbyne structures, as revealed by the presence of a characteristic Raman peak at about 2150 cm-1. Another set of Raman peaks observed at 996, 1116, and 1498 cm-1, also appeared when the amorphous carbon film was processed with a sequence of more than three consecutive laser pulses. These peaks, whose general aspect is very similar to that of polyacethylene (CnHn), could be ascribed to the cumulenic carbyne structure, stabilized by some dispersed copper atoms.