ABSTRACT
Noble-metal nanoparticles are ideal as nanoscale building blocks for new materials because their optical responses are dominated by localized surface plasmon resonance (LSPR) excitation, providing efficient and tunable absorption that is controlled by their size, shape, and composition [1–7]. The precise placement of such nanoparticle 1340building blocks enables one to dictate the structural properties of a material including 3D crystal habit [5, 6], metal-semiconductor “nanoparticle doping” [8], nanoparticle packing density [9, 10], and programmability over multiple length scales [2, 11]. Indeed, such architectural control provides greater opportunities for designing optical properties than is available for most other materials. Here, we use DNA-programmable assembly to study metallurgical control over arrangement, metal composition, and hierarchical order in assemblies of plasmonic nanoparticles, leading to “alloy” and “bimetallic” assemblies that have diverse optical properties in which homogenized dielectric properties are not always applicable even though metal volume fractions are just a few percent. In “alloy” nanoparticle assemblies (Fig. 73.1, left), where the distribution of metallic composition is random, absorption and color can be linearly and precisely controlled by changing the Ag:Au ratio. This applies to both assemblies comprised of atomic alloy (AgAu) nanoparticles and also to nanoscopic alloy assemblies comprised of homogeneous nanoparticles (Ag or Au) that are randomly distributed within the superstructure. A third possibility involves “bimetallic” assemblies, here defined as structures where the metallic composition is homogeneous within an individual (subwavelength) layer (Fig. 73.1, right), but where interlayer interactions lead to asymmetric reflectivity in the collective optical response. In these systems, the optical properties are asymmetric in that they differ depending on the material orientation to incident light.
