Influence of Shape on the Surface Plasmon Resonance of Tungsten Bronze Nanocrystals

Abstract

Localized surface plasmon resonance phenomena have recently been investigated in unconventional plasmonic materials such as metal oxide and chalcogenide semiconductors doped with high concentrations of free carriers. We synthesize colloidal nanocrystals of Cs x WO 3 , a tungsten bronze in which electronic charge carriers are introduced by interstitial doping. By using varying ratios of oleylamine to oleic acid, we synthesize three distinct shapes of these nanocrystalshexagonal prisms, truncated cubes, and pseudosphereswhich exhibit strongly shape-dependent absorption features in the near-infrared region. We rationalize these differences by noting that lower symmetry shapes correlate with sharper plasmon resonance features and more distinct resonance peaks. The plasmon peak positions also shift systematically with size and with the dielectric constant of the surrounding media, reminiscent of typical properties of plasmonic metal nanoparticles.

Chemical Control of Plasmons in Metal Chalcogenide and Metal Oxide Nanostructures

“…As the number of faces on the nanoparticle surface increases, three important trends are apparent: i) the main/largest resonance peak blueshifts, ii) smaller peaks move closer to the main peak and may be hidden, and 3) the width of the main resonance peak increases. [ 15,17 ] The same trends are observed when decreasing the aspect ratio (length over width) of plasmonic nanoparticles, where the plasmon peaks shift closer together. In rod-shaped noble metals (e.g., Au [ 21 ] and Ag [ 22 ] ), the plasmon mode of the longer axis is typically well defi ned and easy to observe due to strong coupling to the electromagnetic fi eld.…”

“…[ 15 ] An in-depth treatment of the fundamental physical considerations that govern plasmon location, breadth, and decay in these chalcogenide and metal-oxide materials is covered in a recent review by Comin and Manna. [ 16 ] Particle morphology is another important avenue for tuning the LSPR, and it is established for noble metals that changing the size and shape will infl uence the resonant frequency.…”

“…[ 16 ] Particle morphology is another important avenue for tuning the LSPR, and it is established for noble metals that changing the size and shape will infl uence the resonant frequency. [ 17 ] Only recently have metal chalcogenides of varying shapes been developed, such as doped tungsten oxide (prisms, cubes and spheres) [ 15 ] and copper telluride (plates, cubes and rods). [ 18 ] Recent work by Agrawal et al [ 19 ] highlights the importance of morphology by calculating the infl uence of n on the LSPR of an indium doped cadmium oxide (ICO) sphere, cube, rounded cube, and octahedron.…”

“…This is believed to be a consequence of strong electric fi elds being so sensitive to the shape of the nanoparticle and boundary conditions. [ 15 ] is plasmonic as a result of interstitial doping, where Cs + ions are positioned in the defect-induced (oxygen-vacancy) channels of tungsten oxide. Mattox et al [ 15 ] report the synthesis of Cs x WO 3 as phase-pure, monodisperse nanoparticles; hexagonal prisms, truncated cubes, and pseudo-spheres.…”

“…[ 15 ] is plasmonic as a result of interstitial doping, where Cs + ions are positioned in the defect-induced (oxygen-vacancy) channels of tungsten oxide. Mattox et al [ 15 ] report the synthesis of Cs x WO 3 as phase-pure, monodisperse nanoparticles; hexagonal prisms, truncated cubes, and pseudo-spheres. These nanocrystals exhibit strongly shape-dependent LSPR features in the near-infrared (NIR) region.…”