Andrea L. Rodarte

Quantum dot/liquid crystal composite materials: self-assembly driven by liquid crystal phase transition templating

The isotropic to nematic liquid crystal (LC) phase transition is used to create organized assemblies of CdSe/ZnS core/shell quantum dots (QDs). Under controlled conditions, well dispersed QDs are expelled from the ordered domains of nematic LC into the remaining isotropic domains. The final LC phase produces three dimensional QD assemblies that are situated at the defect points in the LC volume. Through the luminescence of the QDs we are able to track the movement of the nanoparticles as the phase is formed as well as spectrally probe the resulting QD assemblies. Forster resonance energy transfer (FRET) measurements, combined with small angle X-ray scattering (SAXS) data reveal that the QD assemblies have a consistent inter-particle spacing of approximately 7.6 nm. Additionally, the location of the assemblies is shown to be controllable by utilizing beads as defect nucleation points.

Tuning Quantum-Dot Organization in Liquid Crystals for Robust Photonic Applications

Mesogenic ligands have the potential to provide control over the dispersion and stabilization of nanoparticles in liquid crystal (LC) phases. The creation of such hybrid materials is an important goal for the creation of soft tunable photonic devices, such as the LC laser. Herein, we present a comparison of isotropic and mesogenic ligands attached to the surface of CdSe (core-only) and CdSe/ZnS (core/shell) quantum dots (QDs). The mesogenic ligands flexible arm structure enhances ligand alignment, with the local LC director promoting QD dispersion in the isotropic and nematic phases. To characterize QD dispersion on different length scales, we apply fluorescence microscopy, X-ray scattering, and scanning confocal photoluminescent imaging. These combined techniques demonstrate that the LC-modified QDs do not aggregate into the dense clusters observed for dots with simple isotropic ligands when dispersed in liquid crystal, but loosely associate in a fluid-like droplet with an average interparticle spacing >10 nm. Embedding the QDs in a cholesteric cavity, we observe comparable coupling effects to those reported for more closely packed isotropic ligands.

Dynamics of spontaneous emission of quantum dots in a one-dimensional cholesteric liquid crystal photonic cavity

We investigate the modulation of recombination lifetimes of CdSe/ZnS quantum dots (QDs) dispersed in a cholesteric liquid crystal (CLC) photonic cavity. Using ultrafast spectroscopic techniques we focus on the time-resolved emission from QD ensembles in CLC matrices with either planar or homeotropic alignment. In the case of planar alignment and a well-defined spectral stop-band (reflection band) we observe the emergence of a second, faster decay time of less than 2 ns. This short recombination pathway is observed only in samples where the QD emission spectrum partially overlaps the CLC stop-band by 50% or more. Samples prepared with homeotropic alignment do not have a stop-band and, consequently, do not lead to spectral or dynamical modulation of the QD emission. Our observations indicate that coupling between the excitonic and the photonic cavity modes results in an enhancement and modulation of spontaneous emission in the liquid crystal medium.

Dye-integrated cholesteric photonic luminescent solar concentrator

We have developed organic dye-integrated thin-film liquid crystalline photonic luminescent solar concentrators (LSCs), where the chirality of the liquid crystal (LC) results in the formation of a one-dimensional photonic cavity. By varying the different LSC parameters, including dye concentration, spectral position of the photonic band-gap and the LC phase, and by using spectroscopic and electrical characterisation, we have systematically studied the effects of self-absorption, incident absorption and confinement of down-converted emission on optical efficiency. Our results demonstrate that the efficiency of our LSCs is significantly enhanced in the LC phase when the photonic band-gap is at long wavelengths (>600 nm), overcoming associated low incident absorption and higher self-absorption. We reach the significant conclusion that focusing on improving the confinement of dye-emitted photons, rather than on increasing incident absorption, is a more promising route to enhancing thin-film LC-based LSC performance.

Directed assembly and in situ manipulation of semiconductor quantum dots in liquid crystal matrices

The ability to control and direct self-assembly of nanostructures into specific geometries with new functionalities, while preserving their original optical and electronic properties, is an attractive research endeavor. We have fabricated liquid crystal (LC) based matrices into which chemically synthesized nanostructures of varied morphologies and compositions are uniformly dispersed. Using high resolution spatially- and time-resolved scanning photoluminescence (PL) measurements, we have demonstrated directed nanoparticle assembly and manipulation in situ. This includes (a) directional assembly and electric field modulated re-orientation of disk-shaped gallium selenide nanoparticles using a nematic LC matrix, and (b) spectral modulation of chemically synthesized core shell CdSe/ZnS quantum dots (QDs) embedded in a cholesteric liquid crystal (CLC) matrix. Our work opens up the possibility of designing new QD based optical devices where spatial control of orientation, wavelength and polarization of the embedded QDs would allow great flexibility and added functionalities.