Preparation and characterization of gold-coated silver triangular platelets in nanometer scale

Colloids of triangular silver platelets and the platelets coated with a layer of gold have been prepared and analyzed by dynamic light scattering (DLS), transmission electron microscopy (TEM), high-resolution electron microscopy (HRTEM), ultraviolet-visible spectroscopy (UV–vis). The particle size distribution measured by DLS indicates the Ostwald ripening process in evolution of triangles is fed by small spherical particles during light illumination. The shape of most particles is a truncated equilateral triangle and the degree of truncation decreases as the size increases. Triangular platelets are of edge length 50–60 nm and thickness 6–10 nm. The edge length increases to 60–90 nm after depositing a gold layer. To preserve the triangular geometry, a gold layer of thickness less than 5 nm is preferred. The epitaxial nature of gold grown on the {1 1 1}{image} planes of the silver crystal is clearly illustrated in HRTEM images when the thickness of gold is less than 1 or 2 nm. A moiré fringe is usually observed when the Au thickness is higher than 5 nm. The moiré fringe is simulated by rotating two overlapped lattices of the same spacing. The peak position of in-plane dipole resonance at 680 nm is sensitive to the degree of truncation of silver triangle. It shifts towards 615 nm owing to corner rounding by the heat. The gold coating broadens and shifts the peak of in-plane resonance toward shorter wavelengths, hence the cool blue color changes into a warmer color.