Impurity-Induced Plasmon Damping in Individual Cobalt-Doped Hollow Au Nanoshells Christyn A. Thibodeaux, † Vikram Kulkarni, ‡ Wei-Shun Chang, § Oara Neumann, † Yang Cao, ∥ Bruce Brinson, § Ciceron Ayala-Orozco, § Chih-Wei Chen, ∥ Emilia Morosan, §,∥,⊥ Stephan Link, †,§,⊥,# Peter Nordlander, †,∥,⊥,# and Naomi J. Halas* ,†,‡,§,∥,⊥,# † Department of Electrical and Computer Engineering, ‡ Applied Physics Graduate Program, § Department of Chemistry, ∥ Department of Physics and Astronomy, ⊥ Rice Quantum Institute, and # the Laboratory for Nanophotonics, Rice University, MS-378, 6100 Main Street, Houston, Texas 77005, United States * S Supporting Information ABSTRACT: The optical properties of plasmonic nanoparticles in the size range corresponding to the electrostatic, or dipole, limit have the potential to reveal effects otherwise masked by phase retardation. Here we examine the optical properties of individual, sub-50 nm hollow Au nanoshells (Co-HGNS), where Co is the initial sacrificial core nanoparticle, using single particle total internal reflection scattering (TIRS) spectroscopy. The residual Co present in the metallic shell induces a substantial broadening of the homogeneous plasmon resonance line width of the Co- HGNS, where the full width at half-maximum (fwhm) broadens proportionately with increasing Co content. This doping-induced line broadening provides a strategy for controlling plasmon line width independent of nanoparticle size, and has the potential to substantially modify the relative decay channels for localized nanoparticle surface plasmons. ■ INTRODUCTION Plasmonic nanoparticles are currently of widespread interest as wavelength tunable nanoscale optical components, where both size and geometry provide important mechanisms for coupling to light. For spherical nanoparticles, an increase in nanoparticle size results in a redshifting of the dipole resonance and the appearance of additional plasmon resonances at higher frequencies. For nanoparticles of more complex geometries, plasmon resonances become a function of both size and shape. Conversely, nanoparticle size and geometry can be adjusted to maintain a constant plasmon resonance frequency in some plasmonic nanoparticles over a relatively large particle size range. 1 While much attention has been paid to the tuning of plasmon resonance energies, far fewer studies have examined mechanisms for controlling nanoparticle plasmon line widths and line shapes. For the dipolar plasmon mode of a metallic nanosphere, the line width changes with increasing nanoparticle size, a characteristic signature of a bright plasmon mode. A direct comparison of the plasmon line shapes of individual nanorods and nanospheres showed the inherently narrower line shape characteristic of nanorods due to the absence of overlapping interband transitions. 2 For complex plasmonic clusters, the line shape broadening of superradiant plasmon modes can result in the energetic overlap of bright and dark plasmon modes, resulting in Fano resonant line shapes and the plasmonic analogue of electromagnetically induced trans- parency. 3−6 For individual nanoparticles in the electrostatic limit, the homogeneous line width of a plasmon is inversely proportional to the plasmon lifetime. Plasmons have both radiative 7 and nonradiative 8−10 decay channels. For radiative decay, the plasmon decays into a photon. 11 For nonradiative decay, the plasmon decays by generating phonons and energetic electron−hole pairs. 12 Hot electron−hole pair generation due to plasmon decay can be used for optically generated carrier injection in photodetector devices, 13 has been directly shown to facilitate photocatalytic reactions on plasmonic substrates, 14,15 and may facilitate DNA release at nanoparticle surfaces for light-triggered gene delivery. 16 Alteration of the plasmon line width of a nanoparticle is an indirect indication of a modification of its plasmon decay channels and may, for example, provide a route to more efficient hot carrier generation in specific plasmonic nanosystems. Here we examine the effect of Co inpurities on the plasmon line shape of individual hollow gold nanoshells (Co-HGNS) with particle diameters of nominally 50 nm. When HGNS are synthesized by a galvanic replacement reaction where a sacrificial core nanoparticle is oxidized simultaneously with the reduction of a metallic layer on the nanoparticle exterior, the resulting nanoparticle retains some of the residual metal of Special Issue: Spectroscopy of Nano- and Biomaterials Symposium Received: May 6, 2014 Revised: June 10, 2014 Published: June 12, 2014 Article pubs.acs.org/JPCB © 2014 American Chemical Society 14056 dx.doi.org/10.1021/jp504467j | J. Phys. Chem. B 2014, 118, 14056−14061