Gold/carbon superfine particles produced in pulsed CO2 laser stimulated plasmas

Abstract

A new technique for the production of composite nano materials was used to produce carbon coated gold superfine particles in the 100 nm size range. Particles were produced in pulsed CO2 laser (2 J/pulse at 3×109 W/cm2 ) stimulated plasmas using solid Au targets and 1:3 mixtures of CH4, CO, or CO2 with He or H2 gases at 1.3 kPa pressure. The particle production rate was dependent on the presence and type of carbonaceous gas species, and the partial and total gas pressures. Thin films of the black superfine particles were deposited on glass, ceramic and metal substrates. X-ray photoelectron spectroscopic analyses showed that Au was in the metallic state in the particles, both before and after thermal annealing, at an average concentration of 95% by mass. Fourier transform ion cyclotron resonance mass spectrometry of gaseous products emitted during thermal and laser anneals identified the major constituents as carbon dioxide and hydrocarbons. Scanning electron microscopy images of thermally annealed films showed that the particles retained their relatively uniform 100 nm diam size, despite significant changes in optical and electrical properties. This is attributed to loss of the carbon coating on the exterior of the Au particles. While some of the carbon is catalytically oxidized during thermal annealing, the majority remains in the bulk of the film, presumably concentrated at subsurface grain boundaries. Annealing the films in air produced sharp, permanent color changes to purple at 470 K and red/yellow at 493 K due to the loss of the carbon coating from the basic Au particle. Films deposited at 373 K on glass substrates were ∼5× more reflective in the 450–750 nm portion of the spectrum than films deposited at 300 K due to increased particle packing density. Particle films were also annealed using an unfocused CO2 laser beam and a radio-frequency field. A focused argon ion laser beam was used to anneal lines as narrow as 0.5 μm in particle films on glass substrates. The lowest sheet resistance value measured for an unannealed particle film deposited on glass at 298 K was 5600 Ω/square. Resistance versus temperature (in air) measurements across a 2 mm wide gap of a coated ceramic feedthrough exhibited minima of 9 mΩ at 450 K and 4.47 MΩ at 655 K, and a maximum of >20 MΩ from 493–600 K. The temperature dependent electrical and optical properties of the Au/C particle films, especially their high temperature stability relative to traditional gold blacks, make them potential candidates for applications in infrared optical systems.