The design and development of multifunctional composite materials from artificial nano-constituents is one of the most compelling current research areas. This drive to improve over nature and produce ‘meta-materials’ has met with some success, but results have proven limited with regards to both the demonstration of synergistic functionalities and in the ability to manipulate the material properties post-fabrication and in situ. Here, magnetic nanoparticles (MNPs) and semiconducting quantum dots (QDs) are co-assembled in a nematic liquid crystalline (LC) matrix, forming composite structures in which the emission intensity of the quantum dots is systematically and reversibly controlled with a small applied magnetic field (<100 mT). This magnetic field-driven brightening, ranging between a two- to three-fold peak intensity increase, is a truly cooperative effect: the LC phase transition creates the co-assemblies, the clustering of the MNPs produces LC re-orientation at atypical low external field, and this re-arrangement produces compaction of the clusters, resulting in the detection of increased QD emission. These results demonstrate a synergistic, reversible, and an all-optical process to detect magnetic fields and additionally, as the clusters are self-assembled in a fluid medium, they offer the possibility for these sensors to be used in broad ranging fluid-based applications.

“Tuning quantum dot organization in liquid crystal for robust photonics applications”

A.L. Rodarte, Z.S. Nuno, B.H. Cao, R. J. Pandolfi, M. Quint, S. Ghosh, J. Hein and L.S. Hirst, CHEM PHYS CHEM, Volume 15, Issue 7,  pages 1413–1421, (2014) Link

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 ligand′s 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.

“Dye sensitized cholesteric liquid crystalline photonic luminescent solar concentrator”

A.L. Rodarte, F. Cisneros, L.S. Hirst and S. Ghosh, LIQUID CRYSTALS, DOI:10.1080/02678292.2014.924163, (2014) Link

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.