Alternative Fabrication Process for Nanostructured Alloys Promises Dramatic Cost Reduction
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RESEARCH/RESEARCHERS Bandgap Fluorescence of Carbon Nanotubes Observed Members of the fullerene family of materials have been the subject of intense research due to their novel physical and chemical properties. Optical studies of carbon nanotubes have been hampered by the tendency of these materials to aggregate into bundles. Richard Smalley, R. Bruce Weisman, and their co-workers in the Department of Chemistry at Rice University have reported a separation methodology that produced aqueous suspensions of single nanotubes encased in a nonperturbing layer of sodium dodecyl sulfate (SDS) surfactant, allowing the optical properties of single nanotubes to be studied (see figure). These isolated singlewalled carbon nanotubes were found to exhibit structured absorption and photoluminescence (PL) in the near-IR region. Both the absorbance and PL were affected by reversible protonation of the nanotubes in solution. According to Smalley, “optical activity in the near-IR [spectrum] implies potential applications in the fiberoptic communications and bioimaging technologies.” The synthesis and purification of the individual nanotubes were described in the July 26 issue of Science. The researchers synthesized the nanotubes in a highpressure CO reactor. The raw nanotube product was sonicated in an aqueous dispersion of SDS to break up the nanotube bundles. The single nanotubes were then separated from the remaining bundles by centrifugation and dispersed in deuterium oxide for spectroscopic studies. Atomic force microscopy showed that the sonicated nanotubes varied in length from 80 nm to 200 nm with an average length of 130 nm, an expected distribution for heavily sonicated nanotubes. The absence of damage to the nanotube walls was confirmed by Raman spectroscopy. The absorption spectra of the isolated single nanotubes showed spectral structure that was enhanced and blueshifted compared with those of nanotube bundles. The nanotubes were also found to display bright, structured photoluminescence with a quantum yield of 0.1%. The PL peaks closely matched the absorbance peaks, indicating that the PL comes from the nanotubes. It was found that protonation of the nanotube side walls at pH < 5 resulted in the broadening of absorption spectral structure and the loss of PL. Absorption spectral structure and PL intensity were restored following removal of the adsorbed species by adjusting the pH to >7, or irradiating the suspension with UV light to induce photo734
Figure. Molecular dynamics simulation of a single nanotube encased in a micelle of sodium dodecyl sulfate in water. Such nanotubes were found to exhibit photoluminescence at near-IR wavelengths.
desorption. As part of a continuing effort to understand fullerene-based materials, the Smalley and Weisman groups are currently investigating the electronic structure of isolated single-walled nanotubes through detailed spectroscopic analysis. GREG KHITROV
Alternative Fabrication Process for Nanostructured Alloys Promises Dramatic Cost Reduction The improvement in mechanical propert
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