Photoluminescence of Gold Nanorods Probed
Daniel S. Burgess
Gold nanorods show potential as tiny light sources for labeling and sensing applications. To better understand how the emissions of these structures depend on their size and shape, scientists at Argonne National Laboratory’s Chemistry Div. and Center for Nanoscale Materials in Illinois and at the Laboratoire de Nanotechnologie et d’Instrumentation Optique at Université de Technologie de Troyes in France have investigated two-photon-induced photoluminescence from individual gold nanorods. Their results indicate that the decay of localized surface plasmons — collective electron oscillations bound to the interface of a metal and a dielectric medium — is responsible for the emissions.
The peak wavelength of the two-photon-induced photoluminescence from individual gold nanorods exhibited a blueshift with decreasing rod length (a). The aspect ratio r represents the ratio of the length to width, and the solid lines correspond to the scattering spectra from nanorods with aspect ratios of 1.6 and 2.3. The emission virtually disappeared as the polarization of the excitation was aligned with the narrow width of the rods (b and c). The arrows indicate the orientation of the polarization. Courtesy of Argonne National Laboratory. ©2006, American Physical Society.
Gary P. Wiederrecht of Argonne National Laboratory explained that the finding suggests potential applications of such nanoscale optical antennae in minute sensors. Because the plasmon resonance and damping of the radiative decay vary with environment, he said, changes in the photoluminescence could be used to indicate the presence of molecular adsorbates or a modification of the local dielectric constant.
In the work, the researchers excited 50- to 300-nm-long, 30-nm wide nanorods grown by electron-beam lithography with 120-fs pulses of linearly polarized 785-nm radiation from a Coherent Inc. Ti:sapphire laser. Pulse energies of 60 µW yielded peak intensities of approximately 4 GW/cm2 on the samples.
They used an Olympus Corp. inverted microscope equipped with a 1.4-NA objective to focus the excitation and to collect the resulting photoluminescence, which was directed to a PerkinElmer Inc. single-photon avalanche photodiode or to an Andor Technology plc spectrograph.
They found that the peak wavelength of two-photon-induced photoluminescence exhibited a blueshift with decreasing rod length. The emission virtually disappeared as the polarization of the excitation was aligned with the narrow width of the rods, further suggesting the influence of localized surface plasmons.
To confirm this, the scientists monitored the effect on the photoluminescence as the intensity of the laser was increased to melt the nanorods into nanodots. They again noted a blueshift in the peak wavelength, consistent with the spectral dependence of surface plasmon resonances on the shape of the structure supporting them.
Wiederrecht said that additional applications of the nanorods might involve novel plasmonic light sources and integrated components. Of particular interest to the investigators is their use in the launching of plasmons into all-metal planarized waveguides, he said.
Physical Review Letters, Dec. 31, 2005, 267405.
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