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

Abstract

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.

Quantum Dot/Liquid Crystal Nanocomposites in Photonic Devices

“…One of the most apparent differences here seen in the optical microscopy is that at this concentration (0.1 wt%), the ODA-QD mixture exhibits much larger aggregates in the cholesteric liquid crystal (CLC) whereas the LC-QDs produce small but also distinct aggregates. This result is consistent with prior results [13,14] in which the nematic phase transition acts to assemble the included nanoparticles into templated structures at the phase transition. At 0.1 wt% and above, we expect aggregate formation for both of these particle types [18].…”

“…No significant effects on the emission spectrum were observed between the two particle types, despite the fact that QDs in the ODA-QD clusters are closer together (~7.6 nm [13]) compared with LC-QDs (~11 nm [18]). Localizing the QD film outside the cavity produced detrimental effects on performance, as the QD emission detected in leg 1 was reflected and did not travel through the CLC.…”

“…A particle with uniform planar or homeotropic anchoring will induce a topological defect in the liquid crystal director field due to the frustration between local molecular alignment and anchoring conditions at the particle surface. We observe splay bend deformation of the local liquid crystal director as a result of nanoparticle inclusion in the nematic phase [13]. The free energy cost of these deformations is described by the Frank elastic constants.…”

“…The mesogenic ligands may act to stabilize these structures (Figure 4) providing a mechanism to create hierarchical composite materials. Recently, our group developed a new methodology to assemble large-scale QD structures using the particles' spatial distribution across the nematic to isotopic phase transition [13,14]. Figure 4a,b shows a fluorescence image of QD "vesicles" or micro-shells stabilized by this process in which the particles are densely packed with an average separation of ~11 nm [14].…”

“…We will discuss our approaches to producing liquid crystal-based materials with defined nanoparticle distributions, focusing specifically on quantum dots owing to their unique size-tunable luminescent properties. Nanoparticle ligands can be designed to respond to the ordered molecular environment of a liquid crystal phase, inducing dispersion [12], clustering [13] or assembly at the interface of a liquid crystal phase transition [14]. These techniques can provide easily scalable methods for producing a rich variety of quantum dot and nanoparticles assemblies.…”