(Invited) Controlling Optical Properties of Semiconductor Nanocrystals: Chiral Quantum Dots and Luminescent Solar Concentrators
Authors: Ferry Vivian E. 
Published: February 2020

This talk will discuss two areas of research in utilizing semiconductor nanocrystals in nanophotonics.

The first part of the talk will describe our recent work on chiral semiconductor nanocrystals. Chiral optical materials exhibit differential optical properties under right handed and left handed circularly polarized light, and may find application in metaphotonic devices, in spin-based systems, or as luminescent sensors. Chirality can be introduced into semiconductor nanocrystals via different routes, including from chiral ligands on the surface of the nanocrystal. However, the application of these materials in photonics is limited by the relatively low dissymmetry factors.

Using post-synthetic ligand exchange, we synthesized a class of chiral CdSe nanocrystals with a variety of different chiral ligands bound to the surface. We first synthesized CdSe nanocrystals via non-hot injection methods, with native, achiral carboxylic acids on the surface. As synthesized, these nanocrystals do not exhibit any circular dichroism. Post-synthetic ligand exchange of the native ligands to chiral carboxylic acids results in circular dichroism spectra associated with the excitonic transitions of the CdSe nanocrystal. We examine a family of related carboxylic acid ligands, which reveal a 30-fold difference in dissymmetry factors, reaching as high as 7 x 10-4, and show how the structure of the ligands influences the resulting circular dichroism spectra.

The second part of the talk will discuss the use of luminescent semiconductor nanocrystals in diffuse light concentrators for solar energy conversion. Luminescent solar concentrators consist of embedded luminescent materials inside a polymer waveguide, such that high energy light is absorbed by the luminophore and the emitted light is trapped within the waveguide and concentrated onto a solar cell. Efficient operation of these concentrators requires not only high quality luminescent materials, but also a photonic mirror to trap luminescent light within the concentrator.

We have recently designed a series of mirrors that operate cooperatively with luminescent materials inside these concentrators, showing how the design of the mirrors changes with different luminophore properties. The top-surface mirror serves to trap luminescent light inside the concentrator that would otherwise couple to the escape cone. This light propagates at a steep angle, and is especially vulnerable to reabsorption losses before reaching the edge of the concentrator. By incorporating a metamirror into the LSC, this light can be steered toward the edge for improved collection. We show that this metamirror in turn lowers the required quantum yield; equivalent performance is achieved from a luminophore with 76% quantum yield as one in a standard design with 99% quantum yield. We then designed a series of different top mirrors for different specific properties of candidate luminophores, including Si and CdSe/CdS of varying shell thickness, showing how the luminophore and mirror can be designed cooperatively.