Mapping the structural and optical properties of anisotropic gold nanoparticles

The morphology and optical properties of gold nanorods are obtained using a strategic combination of theoretical and numerical techniques. Shape-dependent phase diagrams are developed, and the structural and color properties of anisotropic gold nanoparticles are predicted for two different sizes and colloidal concentrations. Calculated maps reveal an intimate relationship between size, morphology, temperature, environment, and optical properties, and confirm that the aspect ratio of the nanorod is an important factor influencing the color of the colloidal samples. When the aspect ratio value is between 1 and 5, the color of the sample changes from pink to blue and green, but if the aspect ratio is larger than 5 the colloid shows slight color variations in green-yellow hues. It is also found that although the solution is heated, at temperatures below melting point, there can be a transition in morphology but the color remains the same for a given aspect ratio, so morphological variations are unlikely to degrade the optical performance, provided the aspect ratio is preserved. We also conclude that the nanorod size plays a secondary role, in contrast to common assumption, since the color palette is quite independent of size. Color variations of samples with different refraction indices and the effect of different 'end truncations' for square nanorods are also examined in detail, both of which can modify the relationship between aspect ratio and color transition, although the overall color palette remains the same. As we will show, the theoretical results discussed here are in excellent agreement with experimental observations.