Researchers developing nanoscale devices already have tools for moving and stacking molecules and for scratching channels in surfaces. But a report in the 8 August Physical Review Letters could lead to a new atom-moving tool: light. The researchers have found that--for a brief moment at least--ordinary light can turn solid graphite into a structure as malleable as putty, and possibly turn it into diamond. This research could lead to new nanoscale techniques wherein a laser builds structures of diamond and graphite on a surface of a carbon thin film.

While chunks of industrial diamonds can be made through chemical reactions, there is currently no way to convert the graphite form of carbon to the diamond structure in thin films. But imagine if a focused laser beam could do such a conversion. The beam could "write" a nanoscale electronic circuit in a thin graphite layer, exploiting the strength and insulating properties of diamond in some areas and the semiconductive nature of graphite in other areas, says David Tománek of Michigan State University in East Lansing.

Tománek and his colleagues have taken a step toward this dream by showing a specific structural change in graphite resulting from laser light. The team illuminated a graphite target with 45-femtosecond pulses from a near-infrared laser. Synchronized with the light pulses were short bursts of an electron beam that allowed the team to probe the carbon atoms' positions on the picosecond timescale, using the technique of electron diffraction.

Atoms of graphite normally bond within two-dimensional layers that are 3.4 Angstroms apart, with no bonds between layers. But the team saw a large fraction of the atoms in the top few layers briefly form into a layer just 1.9 Angstroms from its neighbor. Combining this observation with other diffraction analyses and computer simulations, they concluded that within 14 picoseconds following the laser pulse, many atoms form inter-layer bonds similar in some ways to those in diamond, but these bonds disappear by 45 picoseconds.

With electrons forming different bonds in this transient state, the graphite structure becomes softer, says Tománek. He hopes ultimately to push this state into the full diamond structure, rather than letting it fall back to graphite. But the work suggests that every pencil tip exposed to white light has a tiny number of atoms that gain enough extra energy from random fluctuations to snap to the diamond structure after being excited to the soft intermediate state.

Tomas Weller of the ISIS Neutron Facility in England has shown that the right kind of laser pulse can convert industrial diamonds into graphite, and he, too, hopes to someday make nanostructures using light. Regarding the Michigan State team's work, Weller is excited by the answers it brings to the emerging field of photoexcitation in solids. "It kind of draws together a number of different questions that have been bubbling along in these fields, and it links them together." he says.

--Mike Wofsey