Red, Hot, and Gold

Top: Gold particles are laid down on the substrate.
Middle: A pinpoint laser illuminates some of the particles, heating them, while a precursor molecule (the crablike thing) drifts by.
Bottom: The hot particles break down the precursor molecule on contact, causing deposits to form on top of themselves.

The ancient Greeks used finely ground gold to color glass, which paradoxically turned it a rich ruby red. They didn’t know it, says Caltech staff scientist David Boyd, but they were using nanoparticles. Since then, many people have exploited the odd optical properties of nanoparticles. Now Boyd and his colleagues are taking advantage of their equally odd thermal ones in a technique called “plasmon-assisted chemical vapor deposition” that adds a powerful new tool to the methods available for making microdevices.

In the November issue of Nano Letters, Boyd and colleagues report that the process can be used to create a variety of nanostructures. The underlying material, or substrate, as it is called, is coated with gold nanoparticles and placed in a vacuum chamber that is then filled with a carrier gas containing a precursor of the material to be deposited. A low-power laser whose wavelength matches a natural resonance in the gold particles is focused onto a small spot about one micron in diameter, or less than a hundredth the diameter of a human hair, which quickly heats up by several hundred degrees—hot enough so that the particles decompose the precursor molecules in the vapor, forming microscopic deposits. Since this does not happen at nearby cool particles outside the laser spot, structures form only where the laser shines, allowing one to “draw” patterns by moving the laser across the substrate.

The key is the surprisingly low thermal conductivity at the tiny scales involved, explains Boyd. The gold nanoparticles absorb energy from the laser very efficiently, but do not conduct the heat away to their surroundings very well. They thus can be heated to much higher temperatures than one would expect.

The process requires a laser about as powerful as a green laser pointer, says David Goodwin, professor of mechanical engineering and applied physics and a coauthor of the paper. The ability to write micron-scale or smaller structures directly, without the need for conventional lithographic patterning and etching, while also keeping the substrate cool outside the laser spot, opens up new possibilities for the types of structures that could easily be fabricated.

The researchers grew lead oxide “wires” as small as a few tens of nanometers in diameter on a glass substrate, and predict that even smaller structures are possible. The team has also deposited titanium oxide and cerium oxide. “Anything that can be deposited as a film by conventional means can probably be deposited with this technique,” Boyd says.

The paper’s other authors are Leslie Greengard, of New York University’s Courant Institute of Mathematical Sciences; Mark Brongersma of Stanford University, a former Caltech postdoc; and Mohamed Y. El-Naggar (MS ’02), who has completed all the requirements for his Caltech PhD and is now a postdoc at the University of Southern California. —RT