This summer Kyle Kabasaras presented his original research project at the UC Merced undergrad research symposium.
Investigating quantum dot assembly in a cholesteric liquid crystal
An ongoing goal in condensed matter physics is directly controlling the self-assembly of quantum dots (QDs) into specific structures while maintaining their original electronic and optical properties. One method of controlling the self-assembly of QDs is to disperse them within a liquid crystal (LC) medium and apply a variety of thermal stimulations. Recently, our lab developed a method of creating spherical, vesicle-shaped QDs within a nematic LC. Vesicle formation depends on the QD concentration in the LC as well as the LC’s intermolecular dispersion forces and thermal properties. In this project, we investigate the dispersion of CdSe/ZnS (core/shell) QDs in a cholesteric LC (CLC) mediumand predict the QD aggregations to cluster near the LC defects. By varying parameters such as QD concentration and temperature, we exploit the CLC’s sensitive optical and thermal properties. To observe these effects, we apply spectrophotometry, polarized optical microscopy, and fluorescence microscopy. These techniques highlight the aggregation of QDs within the host CLC and identify how LC phase transitions determine where QDaggregates form. This work illustrates the possibility of new LC-based QD devices, and we will continue by exploring the lasing potential of our sample.
“All optical switching of nematic liquid crystal films driven by localized surface plasmons” M.T. Quint, S. Delgado, Z.S. Nuno, L.S. Hirst and S. Ghosh, OPTICS EXPRESS, 23,5, 6888 (2015) Link
We have demonstrated an all-optical technique for reversible in-plane and out-of-plane switching of nematic liquid crystal molecules in few micron thick films. Our method leverages the highly localized electric fields (“hot spots”) and plasmonic heating that are generated in the near-field region of densely packed gold nanoparticle layers optically excited on-resonance with the localized surface plasmon absorption. Using polarized microscopy and transmission measurements, we observe this switching from homeotropic to planar over a temperature range starting at room temperature to just below the isotropic transition, and at on-resonance excitation intensity less than 0.03 W/cm2. In addition, we controllably vary the in-plane directionality of the liquid crystal molecules in the planar state by altering the linear polarization of the incident excitation. Using discrete dipole simulations and control measurements, we establish spectral selectivity in this new and interesting perspective for photonic application using low light power.
“Self-assembled nanoparticle micro-shells templated by liquid crystal sorting”A. R. Rodarte, B.H. Cao, H. Panesar, R.J. Pandolfi, M. Quint, L. Edwards, S. Ghosh, J.E. Hein and L.S. Hirst, Soft Matter, 10.1039/C4SM02326A (2015) Link
A current goal in nanotechnology focuses on the assembly of different nanoparticle types into 3D organized structures. In this paper we report the use of a liquid crystal host phase in a new process for the generation of micron-scale vesicle-like nanoparticle shells stabilized by ligand–ligand interactions. The constructs formed consist of a robust, thin spherical layer, composed of closely packed quantum dots (QDs) and stabilized by local crystallization of the mesogenic ligands. Ligand structure can be tuned to vary QD packing within the shell and made UV cross-linkable to allow for intact shell extraction into toluene. The assembly method we describe could be extended to other nanoparticle types (metallic, magnetic etc.), where hollow shell formation is controlled by thermally sorting mesogen-functionalized nanoparticles in a liquid crystalline host material at the isotropic to nematic transition. This process represents a versatile method for making non-planar 3D nano-assemblies.
Soft Matter and biophysics at the University of California, Merced