Main Research Publications Members Useful links Group Pictures Rice University Chemistry Department |
Absorption and Scattering Spectroscopy of Single Metallic Nanoparticles and NanowiresA surface plasmon is created when the mobile electrons at a metal surface oscillate in phase with an interacting light wave. For isolated metallic nanoparticles, the plasmon oscillation is localized at the particle and causes a strong absorption and scattering of visible and infrared light. The spectral position of the plasmon resonance is determined by the particle size and shape, the refractive index of the local environment, and the metal. To eliminate inhomogeneous broadening due to distributions in particle size and shape, we use single particle spectroscopic techniques to study the detailed optical properties of individual metallic nanoparticles. Single particle spectra give the homogeneous linewidth of the plasmon resonance and allow physical insight into the decay mechanisms of the plasmon oscillation as a function of nanoparticle size, shape, and composition.
Single particle scattering is the most common tool to visualize individual metallic nanoparticles, which takes
advantage of the strong scattering cross section of the plasmon resonance, which scales as R6. However, as the
particle size decreases, scattering becomes too weak to detect. In contrast, plasmon absorption depends on the
volume, i.e. R3 scaling, and allows the detection of metallic nanoparticles with diameters as small as a few
nanometers. We have constructed a setup that combines dark-field scattering with photothermal imaging
(Berciaud et al., PRL 2004, 93, 257402). In photothermal imaging, heating of an individual
nanoparticle due to plasmon absorption caused by a modulated excitation beam induces a temperature dependent
refractive index modulation in the nanoparticle and its surrounding medium, which is probed optically with a
second laser beam. The figure below shows a photothermal image of 12 nm spherical gold nanoparticles taken on
our setup (left). Our goal is to compare plasmon absorption and scattering properties of individual
nanoparticles and nanowires. The right part of the figure shows correlated photothermal (top) and scattering
images (bottom) of a 20x2800 nm nanowire.
The strong absorption of light followed by heating of the nanoparticles and their environment has a potential application as a photothermal cancer treatment method. By selective labeling of cancerous cells with metallic nanoparticles followed by laser irradiation, a localized increase in temperature is created, which induces cell death. Understanding the absorption of light and hence heat in single particles of different sizes and shapes is therefore of great interest. Group members involved: Publications: |
