Tiny gems on steps find future in films

Diamond dazzles not only by its brilliance. It boasts a long list of other stellar qualities, such as being the world’s hardest material and best heat conductor. Such traits give diamond tremendous, yet largely untapped, promise for uses in the electronics, optics, and other industries.

Now, Hong Kong scientists report an experiment that they say may help unleash more of diamond’s potential.

Manufacturers already use numerous methods to coat ordinary materials with diamond films. Many drill bits and other cutting tools, for instance, come with a thin diamond veneer. No process today can make those films of a single crystal, whose uniformity would offer the greatest advantages. Such crystals would be especially desirable for high-voltage, high-temperature electronics, where diamond shows particular promise.

Despite diamond’s clarity, scientists studying its formation have had perpetual difficulty peering into the extremely small-scale, early steps of the process. In these first stages, carbon atoms adhere to an underlying substance in small clusters, which take on diamond’s characteristic tetrahedral structure—a process known as nucleation.

In the new research, “we have observed for the first time ever a diamond nucleation site on silicon,” says Shuit-Tong Lee of the City University of Hong Kong. He and his coworkers found two diamond seed crystals that had assembled in nicks or steps on an irregular silicon surface.

Other crystals appeared in a more disorganized carbon coating. Using transmission electron microscopy and other imaging techniques, the team saw seeds as small as 2 nanometers in diameter—containing only about 2,000 atoms.

At least one of the seeds from a nick site grew in perfect alignment to the underlying silicon crystal pattern, the researchers claim in the Jan. 7 Science. By controlling growth conditions to favor such well-aligned crystals and eliminate competing seeds, the scientists hope to grow single-crystal coatings. The team expects results in a year or two, Lee says.

“Nucleation is a black art and a very difficult thing to study,” comments James E. Butler, who experiments with diamond coatings at the Naval Research Laboratory in Washington, D.C. Though he describes the Hong Kong work as an excellent attempt at studying nucleation, Butler says the report needs confirmation because it provides only two examples of crystal seeds on steps among the many crystals observed.

Most of today’s diamond-on-nondiamond coatings are carpets of extremely small, aligned crystals, known as polycrystalline diamond. They work very well for most applications except electronics, says Butler. Researchers are also experimenting with more tightly aligned multiple-crystal, or textured, film for electronic devices. Lee says, “We believe our method will produce film better than textured film.”

His group discovered the nucleation sites by studying diamond grown with a common technique known as chemical vapor deposition. It takes place in a reactor containing a mixture of hydrogen and hydrocarbon gases at less than atmospheric pressure and heated to 700º to 900ºC.

Interactions of the gases with the surface of an object to be coated cause carbon molecules to stick. Some form diamond seeds, which grow into films when the hydrocarbon-gas concentration is lowered.

In their experiments, Lee and his colleagues applied a voltage between the silicon and a heated filament that splits hydrogen molecules into their constituent atoms and breaks down hydrocarbon molecules. This well-established technique, which is known to assist nucleation, propels charged, reactive hydrocarbons into the surface. There, the energetic particles may gouge out the steps, several atomic planes deep, that the scientists observed serving as nucleation sites, Butler says.