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Igniting a few good ideas with fullerenes

Photonics Spectra
Jun 2010
Lynn M. Savage,

A standard TV trope is the fuse made of a strand of gunpowder slowly burning while heading toward a pile of powder kegs and bundles of dynamite. This fire-starting scenario does, in fact, work, although nobody really does it that way. However, researchers at the University of Florida recently demonstrated a new laser-based technique that could take this principle to a different – and much tinier – level.

Vijay Krishna, a postdoctoral associate at the university, and his colleagues knew that one could heat carbon nanostructures to the point of ignition by irradiating them with laser light. Causing carbon nanoparticles to ignite might offer useful medical, chemical and mechanical applications, but the particles by themselves have raised questions regarding their biological and environmental safety. Krishna and his team – Nathanael Stevens, Ben Koopman and Brij Moudgil – decided to explore how carbon fullerenes combined with hydroxyl and carboxyl functional groups – which are known to be safer – would react to similar heating.

Irradiating functionalized fullerenes with a 785-nm laser produces heat and light emissions. Photos courtesy of Vijay Krishna, University of Florida.

As they report in the online edition of Nature Nanotechnology, functionalized carbon nanoparticles perform very well as ignition materials, using surprisingly low laser intensity.

Using a 785-nm laser made by B&W Tek Inc. of Newark, Del., the investigators trained a diffuse beam at several forms of the nanoparticles, including unadulterated fullerene, fullerene bromide, fullerene hydride, polyhydroxy fullerene and carboxy fullerene, as well as plain granular activated carbon. The nonfunctionalized nanoparticles and the macroscale carbon failed to ignite at laser intensities up to 90 kW/cm2. In contrast, the functionalized particles reached their ignition points with intensities in the 0.3- to 15-kW range.

Optical micrographs show the initials “UF” inscribed with a 785-nm laser on paper that has been coated with polyhydroxy-functionalized fullerenes.

Pristine fullerenes are symmetrical carbon cages that are difficult to alter using low-power irradiation. According to Krishna, adding functional carboxyl and hydroxyl groups distorts the cages, straining the structures. Irradiation by laser cleaves these functional groups and releases the stored energy inside the strained portions, he said.

The released energy comes in the form of heat and light. When irradiated in a vacuum, the fullerene bromide in particular exhibited impressive light emission for up to 10 seconds before turning into a black residue. With all of the functionalized materials, the gap between the start of the beam and the emission of light from the irradiated particles was less than 10 ms, indicating that the cleavage of the functional groups is an ultrafast process that may be exploited in the future.

Thus far, Krishna and his colleagues have tested the utility of the functionalized nanoparticles in igniting explosive materials, in treating cancer cells and in lithography.

Using the same 785-nm beam, they ignited carboxy fullerenes that were in contact with the same explosive used in blasting caps. They could instantaneously set off the traditional blasting material using a 1-W beam, which provided an intensity of less than 10 W/cm2. In total, using the low-power laser requires less energy than using a traditional electrical charge for detonations performed in tunneling or demolition.

For cancer treatments, the technique shows promise because functionalized fullerenes can be readily taken up by tumor cells. Once heated via laser, the nanoparticles explode, ripping the cells apart.

“The photothermal and photoacoustic properties of polyhydroxy fullerenes make them attractive for cancer therapy,” Krishna said. “Furthermore, these molecules are biocompatible and are rapidly eliminated from the body.”

The group also believes that the ignition process could be used in finely detailed lithography. They tested this by coating paper samples with hydroxy fullerenes, then scanned them with the laser, thereby “writing” on them with the resulting residue. Because the particles are about 1.3 nm in size, lithographical features likely would be restricted only by the beam width.

According to Krishna, his group envisions a multitude of other possible applications for functionalized fullerenes, including high-density data storage, laser-based spark plugs, room temperature hydrogen storage and release, and low-energy, catalyst-free synthesis of carbon nanoparticles.

The next step is to verify and improve the process, Krishna said. The group will expand from there.

“Further challenges are to explore the complete spectrum of electromagnetic radiation to find which bands interact with functionalized fullerenes and [to] optimize the molecular structure for various applications.”

B&W Tek Inc.Ben KoopmanBrij Moudgilcancercarbon fullerenescarbon nanoparticlescarboxy fullerenescarboxyldata storagefunctional groupshydrogen storagehydroxylignition materialsiintensityindustriallithographynanoparticlesNathanael Stevensphotoacousticphotothermalpolyhydroxyl fullerenesResearch & Technologyspark plugsTech PulsetumorUniversity of FloridaVijay Krishnalasers

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