Looking for the sparkle in carbon films.

Looking for the sparkle in carbon films

Diamond has a remarkable combination of properties that don't often go together in a single material. Not only hard and transparent, it's also both an electrical insulator and an excellent conductor of heat. Those qualities led physicist Carl B. Collins and his colleagues at the University of Texas at Dallas to consider diamond as a suitable lattice in which to embed atomic nuclei--the working ingredients for a proposed gamma-ray laser (SN: 11/1/86 p.276). That meant finding an efficient way to lay down thin, uniform diamond films on top of silicon, glass and other surfaces.

The diamond-growing method chosen by Collins and his team involves shining intense, brief pulses of laser light on the surface of a block of very pure graphite. The laser vaporizes the surface material, cutting a crater while explosively sending out a high-temperature plume containing ionized carbon atoms. An electric field guides the charged particles to the surface to be coated. Each laser pulse, lasting only 10 nanoseconds, lays down a single layer of atoms over an area as large as 10 square centimeters. The resulting transparent film, about 200 angstroms thick, has a mirror-smooth finish and excellent optical quality, Collins says. The researchers describe their technique and preliminary results in the July 18 APPLIED PHYSICS LETTERS.

What isn't clear is the nature of the carbon film produced. Measurements of the film's index of refraction -- how much it bends light -- give values less than but close to diamond's refractive index. Unlike graphite, which is a good electrical conductor, but like diamond, the film has a high resistance to electrical current. Those observations suggest the laser-produced film is at least diamond-like, if not truly diamond.

Alternative methods for growing diamond or diamond-like films tend to be slow and require high operating temperatures (SN: 8/23/86, p.118). Moreover, surfaces to be coated usually must be specially treated to receive such films. Surfaces coated by the laser technique need no such treatment. And because much milder operating conditions are used, it's possible to lay down carbon coatings on a variety of materials that would be damaged at higher temperatures. The laser-produced diamond-like coating itself may turn out to be a good seedbed on which to grow crystalline diamond.

Collins and his group have not yet tested their carbon films to see if the films have the right properties for use i n a gamma-ray laser." We don't really know all the details of how the laser process works yet," Collins says. "We have about 20 variables that we aren't used to working with all at the same time. We're spending all our time just trying to master the process variables."