Besides nanostructured materials, individual particles are key elements for nanosciences. The structuring properties of liquid crystals (LCs) are appealing to assemble them, to organize them on substrates or to design functional composites. We present here an overview on particles/LC systems, the size of the dispersed particles being larger than the typical LC length. We first summarize the large number of advances made these last 10 years concerning microparticles assemblies. We then discuss the evolution of the relevant interactions between nanoparticles (NPs) dispersed in LCs when their size decreases from micrometers to nanometers. Various NPs assemblies obtained, either in LC bulk, at interfaces or within LC distorted areas or topological defects are then reported and discussed. Finally, we consider the recent possibilities to use NPs as building elements of complex fluids. We discuss accordingly the LC phases, which can be obtained with pure inorganic NPs in concentrated solution, as well as the sel...

Ordering nano- and microparticles assemblies with liquid crystals

Liquid crystal nanoscience, a field exploring the mutually beneficial combination of the unique properties of nanoscale materials and fluid, yet ordered liquid crystalline phases, is increasingly focusing on semiconductor quantum dots. In one major research thrust, the anisotropic properties of the liquid crystal host are sought to facilitate the assembly of quantum dots into arrays, in another, both size- and shape-dependent optical and electronic properties of quantum dots are used to manipulate optical, electro-optical and alignment properties of liquid crystalline materials. This feature article reviews recent accomplishments and new insights in this fascinating area of soft matter nanocomposites including work from our laboratory on a series of CdSe and CdTe quantum dots as additives in nematic liquid crystal hosts.

Quantum dots as liquid crystal dopants

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.

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

Using transmittance electron microscopy, fluorescence and polarizing optical microscopy, optical spectroscopy, and fluorescent correlation spectroscopy, it was shown that CdSe/ZnS quantum dots coated with a specifically designed surfactant were readily dispersed in nematic liquid crystal (LC) to form stable colloids. The mixture of an alkyl phosphonate and a dendritic surfactant, where the constituent molecules contain promesogenic units, enabled the formation of thermodynamically stable colloids that were stable for at least 1 year. Stable colloids are formed due to minimization of the distortion of the LC ordering around the quantum dots.

https://erc.ncat.edu/?docid=2E224D5439474954293D4D23575F5E3E2D4A3B4820A

Thermodynamically Stable Dispersions of Quantum Dots in a Nematic Liquid Crystal

Investigations of the mixtures of semiconducting quantum scale particles in anisotropic liquid crystal (LC) medium have become a vibrant area of research primarily due to their very interesting phenomenology. The results of these investigations fall into four groups: (i) Photoluminescent emissive properties of the quantum particles ordinarily depend on the size, shape, and chemical nature of the particles. These undergo important changes in their spectrum, polarization, and isotropy of emission when dissolved in an anisotropic LC phase. Moreover, their response to external stimuli such as mechanical, optical, or electric fields is altered in important ways; (ii) physical properties of LCs such as viscosity, dielectric relaxation, etc are modified by the addition of quantum particles. Their presence in ferroelectric smectic LC is known to give rise to an antiferro- to ferri-electric phase transition and suppresses the paraelectric phase; (iii) switching characteristics of LC devices are altered in important ways by the addition of quantum particles. Their threshold voltage is usually lowered, contrast ratio, and switching speed of nematic, ferroelectric, and cholesteric devices may increase or decrease depending on the concentration, applied field, and particle anisotropy; and (iv) controlled aggregation of quantum particles at the interface between isotropic and LC domains, near added polystyrene beads, and in the vicinity of point defects gives rise to interesting photonic structures, enables studies of photon antibunching and single photon sources. Clearly, there is a need to understand the basic and applied aspects of these systems and find routes to their technological applications including sensors, electrooptical devices, and solar energy harvesting. This review provides an overview of recent work involving liquid crystals and a variety of quantum particles.

https://iopscience.iop.org/article/10.1088/0034-4885/79/5/056502/pdf

Emissivity and electrooptical properties of semiconducting quantum dots/rods and liquid crystal composites: a review.

We demonstrate the formation and spatial modulation of strongly coupled gallium selenide quantum dot (QD) nanoassemblies suspended in a nematic liquid-crystal (NLC) matrix at room temperature. Using static and dynamic optical techniques we show that the coupled QDs aggregate with a well-defined directionality commensurate with the NLC director axis. This results in highly anisotropic spectral properties of the QD assembly. The spatial orientation of the aggregates is selectively controlled in situ by the application of in-plane electric fields. The strong interdot coupling further increases the excitonic recombination rate which is both direction and electric field dependent. This electrical modulation, a noninvasive process, could potentially be an important functionality for the design and creation of building blocks for novel optoelectronic devices.

https://faculty.ucmerced.edu/sites/default/files/sghosh/files/docs/Electrical%20modulation%20of%20static%20and%20dynamic%20spectroscopic%20properties%20of%20coupled%20nanoscale%20GaSe%20quantum%20dot%20assemblies_.pdf

W. C. Chin

Papers

Electrical modulation of static and dynamic spectroscopic properties of coupled nanoscale GaSe quantum dot assemblies