Synthesis and Optical Properties of Gold Nanodecahedra with Size Control

Abstract

The synthesis of nanoparticles with a tight control on their size and shape is a requirement for the achievement of many nanotechnology goals, since these are the main parameters determining the material properties at the nanoscale. Among the many different materials currently under investigation, semiconductor and metal nanoparticles are extremely attractive because of the possibility of controlling their electronic and optical properties through tailoring size and shape during synthesis. For instance, the presence of sharp edges or tips has been shown to increase electric-field enhancement, [1] which is important for applications involving metal nanoparticles as sensors. Additionally, nanoparticle morphology will ultimately determine the way in which nanoparticles can be assembled. While spheres, rods, cubes, and flat prisms are the most thoroughly studied shapes, there have been several reports on the synthesis and detailed characterization of other geometries, and it is believed that metal nanoparticles tend to grow with the structure of one of the Platonic solids, that is, with all faces made of the same regular polygon and the same number of polygons meeting at each corner. [2–4] Non-Platonic structures that have been reported for metal nanocrystals include nanorods, either with the geometry of single-crystal or pentatwinned prisms, [5–9] as well as flat platelets, usually with triangular or hexagonal shape, [10–13] in which faces are made of more than one polygon. Recently, other elaborate geometries have also been reported, such as nanocages [14] or nanostars. [15] In this paper we report the synthesis with tight size control and high monodispersity of nanometer-sized, uniform gold nanoparticles with decahedral morphology, in which all ten faces are equilateral triangles, but not the same number of triangles meet at every corner (four or five are possible), and for this reason they do not qualify as Platonic nanocrystals, but can rather be classified as Johnson structures. These decahedral particles have a lower symmetry (D5h) than Platonic solids, but still display a striking beauty and attractive optical properties that are strongly influenced by particle size. Although the geometry (and thus the aspect ratio) is precisely the same for particles with different sizes, clear changes in the optical responses have been observed and modeled through a boundary element method (BEM) for bicones. The synthesis is based on our previous work where N,N-dimethylformamide (DMF) was used as both solvent and reducing agent [16] to generate silver nanoparticles of various shapes, including spheres, [17] nanoprisms [18] and nanowires. [19] The