Assembly of gold nanoparticle and CdSe/CdS quantum dot dimers for applications in sensing, optics, and plasmonics

Abstract

The assembly of inorganic nanoparticles provides the opportunity to modify the properties of the individual component particles. The coupling of nanoparticles has been used in the past to enhance scattering, fluorescence, and absorption, leading to applications in sensing, photocatalysis, and light harvesting. However, the exact properties that emerge are highly dependent on the precise orientation of the coupled nanoparticles, and creating uniform, discrete assemblies remains a challenge. In particular, the coupling of plasmonic and excitonic nanoparticles has untapped potential if strong coupling between their respective plasmons and excitons can be reliably achieved. Novel properties, for example Fano interference and Rabi splitting, are expected to arise when these particles are strongly coupled. But coupling strength is especially dependent on the precise arrangement of the component particles. Nevertheless, a system composed of a single emitter quantum dot strongly coupled to plasmonic nanoparticles would have widespread applications in photonics, optics, and information processing, making such a system desirable. To this end, the assembly of three different nanoparticle systems was investigated. Homodimers consisting of two gold nanospheres were synthesized and used to model a system for the study of protein-protein interactions based on the small-angle X-ray detection of proteins tagged with gold nanospheres. Next, dimers of gold nanospheres and nanorods were prepared to study the coupling of plasmonic particles on a sub-nanometer scale. Finally, excitonic nanoparticles (CdSe/CdS quantum dots) were attached to gold nanoparticles. Strong coupling was achieved between a single CdSe/CdS quantum dot and the gap plasmon formed between a gold nanosphere and a silver film, providing the first definitive evidence of strong coupling between a single emitter and a gap plasmon. It was observed that the yield of strongly coupled particles was limited by the geometry of nanospheres. To address this, a method was developed to sharpen gold nanorods post-synthesis by etching them with cysteamine. The sharpened tips of these nanorods provide the local geometry that was observed to be necessary to obtain strong coupling. The work described here demonstrates the wide-ranging applications of inorganic nanoparticle assemblies, from bio-detection to photonics, and represents a major step towards the development of usable materials using strongly coupled plasmonic and excitonic nanoparticles.