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Spectroscopy Bares Nanoparticle Structure

Photonics Spectra
Sep 2002
Daniel C. McCarthy

Sometimes you need a bigger laser to study smaller particles. Specifically, a group of Dutch researchers applied a free-electron laser to study nanoparticles representing matter between the atomic and bulk states.

Infrared spectroscopy demonstrated a marked change in the vibrational spectra of nanoclusters, whose cubic layers measured less than three atoms.

Conventional materials research has focused either on single atoms or on the bulk limit. But material properties vary dramatically between these two extremes, where a few atoms bond together to form clusters. The particles within this range are of increasing technological importance as semiconductors, biotechnology and other fields employ nanotechnology. However, clusters are not made easily and often dissolve quickly, making them difficult to study by conventional methods.

Researchers from the FOM Institute for Plasma Physics Rijnhuizen in Nieuwegein and the University of Nijmegen presented vibrational spectra for binary compounds containing between eight and 100 atoms, specifically niobium or tantalum carbide. They derived these spectra from resonance-enhanced multiphoton ionization, a technique that earlier yielded the first direct infrared spectra of gas-phase metal carbide clusters.

Although this is a well-established spectroscopic method, what sets the Dutch study apart is its application of a free-electron laser, which was tuned from 370 to 1650 cm21 to correspond to the vibrational frequency of the clusters. The spectrum this produced revealed a lot about the particles' structure.

The researchers found, for instance, that vibrational spectra of particles comprising more than 20 atoms corresponded to spectra of the bulk compound. This suggested that these nanocrystal structures resemble those of the bulk material and are just a cubic alternating arrangement of atoms. However, the spectra of species that comprised fewer than 20 atoms varied dramatically from the spectra of bulk material.

The scientists speculated that this discrepancy could be a finite size effect that occurs in systems whose cubic dimensions include three or more atoms. Whatever the cause, the work demonstrates the transition of vibrational spectra from the atomic to the bulk level.

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