Tuning the Optical Properties of Nanoscale Materials on Surfaces Through Controlled Exchange Reactions on Cadmium Selenide Quantum Dots and Patterning of Gold and QD Nanoparticle Arrays

Abstract

This work focused on the integration of CdSe quantum dots (QDs) and Au nanoparticles (NPs) as building blocks for the development of quantum dot and plasmonic based optical and sensing devices. The manipulation of nanomaterials was performed to tune their surface and optical properties to study their fundamental chemical and physical processes as well as their applications.

First, the cation exchange reactions between CdSe QDs and Ag+ cations were explored as the surface properties of the QDs were altered. The surface properties of CdSe QDs were tuned by local exposure of laser through photopatterning. The CdSe QDs with different surface properties altered by photopatterning were then exposed to a solution of increasing Ag+ concentrations. The decrease in the photoluminescence intensity of CdSe QDs were monitored and used as an indication for the cation exchange reactions taking place. Photoluminescence data confirmed that the cation exchange reactions exhibited different efficiencies for QDs that were photo-altered in comparison to pristine QDs.

The second area of the study employed the use of directed assembly to fabricate pattern arrays of Au NPs. Fabrication of metallic nanostructure arrays using polystyrene (PS) microspheres often required the use of photo-resist, chemical etching and metal evaporation processes, here they were use to direct the assembly of colloidal Au nanoparticles (NPs) by simple evaporative processes. This created an inexpensive and simple bench-top method for bottom-up fabrication of metallic nanostructure arrays. Optimum parameter for the formation of uniform Au nanoring arrays was achieved by controlling the relative humidity (RH) and the relative concentration of Au NPs to PS microspheres. The fabricated Au nanostructure arrays serve as great platforms not only for future applications in plasmonic based sensing and optical devices but also for the fundamental studies of their localized surface plasmon resonance (LSPR). An alternative method of patterning, a combination between the use of a self-assembled monolayer (SAM) molecular linker and PS mask fabricated via polydimethylsiloxane (PDMS) micro-stamp was also discussed briefly.

Particle lithography is a versatile method and can be used to fabricate pattern array of different nanomaterials with similar surface affinities. It was then modified to reduce the non-specific binding of CdSe QDs forming uniform nanoring arrays. The photoluminescence of the fabricated CdSe nanoring arrays was then altered to locally tune their surface and optical properties. This allowed for the post-fabrication surface modification of each nanoring for the creation of CdSe nanoring arrays of different surface and optical properties.