Interplay of Electrostatic Dipoles and Monopoles with Elastic Interactions in Nematic Liquid Crystal Nanocolloids.

Doping of nematic liquid crystals with colloidal nanoparticles presents a rich soft matter platform for controlling material properties and discovering diverse condensed matter phases. We describe nematic nanocolloids that simultaneously exhibit strong electrostatic monopole and dipole moments and yield competing long-range anisotropic interactions. Combined with interactions due to orientational elasticity and order parameter gradients of the nematic host medium, they lead to diverse forms of self-assembly both in the bulk of an aligned liquid crystal and when one-dimensionally confined by singular topological defect lines. Such nanocolloids exhibit facile responses to electric fields. We demonstrate electric reconfigurations of nanocolloidal pair-interactions and discuss how our findings may lead to realizing ferroelectric and dielectric molecular-colloidal fluids with different point group symmetries.

Aerogel from Sustainably Grown Bacterial Cellulose Pellicles as a Thermally Insulative Film for Building Envelopes.

Improving building energy performance requires the development of new highly insulative materials. An affordable retrofitting solution comprising a thin film could improve the resistance to heat flow in both residential and commercial buildings and reduce overall energy consumption. Here, we propose cellulose aerogel films formed from pellicles produced by the bacteria Gluconacetobacter hansenii as insulation materials. We studied the impact of the density and nanostructure on the aerogels' thermal properties. A thermal conductivity as low as 13 mW/(K·m) was measured for native pellicle-based aerogels that were dried as-is with minimal post-treatment. The use of waste from the beer brewing industry as a solution to grow the pellicle maintained the cellulose yield obtained with standard Hestrin-Schramm media, making our product more affordable and sustainable. In the future, our work can be extended through further diversification of food wastes as the substrate sources, facilitating higher potential production and larger applications.

Plasmonic Metamaterial Gels with Spatially Patterned Orientational Order via 3D Printing.

Optical properties can be programmed on mesoscopic scales by patterning host materials while ordering their nanoparticle inclusions. While liquid crystals are often used to define the ordering of nanoparticles dispersed within them, this approach is typically limited to liquid crystals confined in classic geometries. In this work, the orientational order that liquid crystalline colloidal hosts impose on anisotropic nanoparticle inclusions is combined with an additive manufacturing method that enables engineered, macroscopic three-dimensional (3D) patterns of co-aligned gold nanorods and cellulose nanocrystals. These gels exhibit polarization-dependent plasmonic properties that emerge from the unique interaction between the host medium's anisotropic optical properties defined by orientationally ordered cellulose nanocrystals, from the liquid crystal's gold nanorod inclusions, and from the complexity of spatial patterns accessed with 3D printing. The gels' optical properties that are defined by the interplay of these effects are tuned by controlling the gels' order, which is tuned by adjusting the gels' cellulose nanocrystal concentrations. Lithe optical responsiveness of these composite gels to polarized radiation may enable unique technological applications like polarization-sensitive optical elements.

Electrostatically controlled surface boundary conditions in nematic liquid crystals and colloids.

Differing from isotropic fluids, liquid crystals exhibit highly anisotropic interactions with surfaces, which define boundary conditions for the alignment of constituent rod-like molecules at interfaces with colloidal inclusions and confining substrates. We show that surface alignment of the nematic molecules can be controlled by harnessing the competing aligning effects of surface functionalization and electric field arising from surface charging and bulk counterions. The control of ionic content in the bulk and at surfaces allows for tuning orientations of shape-anisotropic particles like platelets within an aligned nematic host and for changing the orientation of director relative to confining substrates. The ensuing anisotropic elastic and electrostatic interactions enable colloidal crystals with reconfigurable symmetries and orientations of inclusions.

Charge Transport Modulation in PbSe Nanocrystal Solids by Au xAg1- x Nanoparticle Doping.

Nanocrystal (NC) solids are an exciting class of materials, whose physical properties are tunable by choice of the NCs as well as the strength of the interparticle coupling. One can consider these NCs as "artificial atoms" in analogy to the formation of condensed matter from atoms. Akin to atomic doping, the doping of a semiconducting NC solid with impurity NCs can drastically alter its electronic properties. A high degree of complexity is possible in these artificial structures by adjusting the size, shape, and composition of the building blocks, which enables "designer" materials with targeted properties. Here, we present the doping of the PbSe NC solids with a series of Au xAg1- x alloy nanoparticles (NPs). A combination of temperature-dependent electrical conductance and Seebeck coefficient measurements and room-temperature Hall effect measurements demonstrates that the incorporation of metal NPs both modifies the charge carrier density of the NC solids and introduces energy barriers for charge transport. These studies point to charge carrier injection from the metal NPs into the PbSe NC matrix. The charge carrier density and charge transport dynamics in the doped NC solids are adjustable in a wide range by employing the Au xAg1- x NP with different Au:Ag ratio as dopants. This doping strategy could be of great interest for thermoelectric applications taking advantage of the energy filtering effect introduced by the metal NPs.

Electric switching of visible and infrared transmission using liquid crystals co-doped with plasmonic gold nanorods and dichroic dyes.

Smart windows and many other applications require synchronous or alternating facile electric switching of transmitted light intensity in visible and near infrared spectral ranges, but most electrochromic devices suffer from slow, nonuniform switching, high power consumption and limited options for designing spectral characteristics. Here we develop a guest-host mesostructured composite with rod-like dye molecules and plasmonic nanorods spontaneously aligned either parallel or orthogonally to the director of the liquid crystal host. This composite material enables fast, low-voltage electric switching of electromagnetic radiation in visible and infrared ranges, which can be customized depending on the needs of applications, like climate-dependent optimal solar gain control in smart windows.

Hierarchical Materials Design by Pattern Transfer Printing of Self-Assembled Binary Nanocrystal Superlattices.

We demonstrate the fabrication of hierarchical materials by controlling the structure of highly ordered binary nanocrystal superlattices (BNSLs) on multiple length scales. Combinations of magnetic, plasmonic, semiconducting, and insulating colloidal nanocrystal (NC) building blocks are self-assembled into BNSL membranes via the liquid-interfacial assembly technique. Free-standing BNSL membranes are transferred onto topographically structured poly(dimethylsiloxane) molds via the Langmuir-Schaefer technique and then deposited in patterns onto substrates via transfer printing. BNSLs with different structural motifs are successfully patterned into various meso- and microstructures such as lines, circles, and even three-dimensional grids across large-area substrates. A combination of electron microscopy and grazing incidence small-angle X-ray scattering (GISAXS) measurements confirm the ordering of NC building blocks in meso- and micropatterned BNSLs. This technique demonstrates structural diversity in the design of hierarchical materials by assembling BNSLs from NC building blocks of different composition and size by patterning BNSLs into various size and shape superstructures of interest for a broad range of applications.

Gold Nanoparticle Internal Structure and Symmetry Probed by Unified Small-Angle X-ray Scattering and X-ray Diffraction Coupled with Molecular Dynamics Analysis.

Shape and size are known to determine a nanoparticle's properties. Hardly ever studied in synthesis, the internal crystal structure (i.e., particle defects, crystallinity, and symmetry) is just as critical as shape and size since it directly impacts catalytic efficiency, plasmon resonance, and orients anisotropic growth of metallic nanoparticles. Hence, its control cannot be ignored any longer in today's research and applications in nanotechnology. This study implemented an unprecedented reliable measurement combining these three structural aspects. The unified small-angle X-ray scattering and diffraction measurement (SAXS/XRD) was coupled with molecular dynamics to allow simultaneous determination of nanoparticles' shape, size, and crystallinity at the atomic scale. Symmetry distribution (icosahedra-Ih, decahedra-Dh, and truncated octahedra-TOh) of 2-6 nm colloidal gold nanoparticles synthesized in organic solvents was quantified. Nanoparticle number density showed the predominance of Ih, followed by Dh, and little, if any, TOh. This result contradicts some theoretical predictions and highlights the strong effect of the synthesis environment on structure stability. We foresee that this unified SAXS/XRD analysis, yielding both statistical and quantitative counts of nanoparticles' symmetry distribution, will provide new insights into nanoparticle formation, growth, and assembly.

Amorphous to crystal conversion as a mechanism governing the structure of luminescent YVO4:Eu nanoparticles.

The development of functional materials by taking advantage of the physical properties of nanoparticles needs an optimal control over their size and crystal quality. In this context, the synthesis of crystalline oxide nanoparticles in water at room temperature is a versatile and industrially appealing process but lacks control especially for "large" nanoparticles (>30 nm), which commonly consist of agglomerates of smaller crystalline primary grains. Improvement of these syntheses is hampered by the lack of knowledge on possible intermediate, noncrystalline stages, although their critical importance has already been outlined in crystallization processes. Here, we show that during the synthesis of luminescent Eu-doped YVO4 nanoparticles a transient amorphous network forms with a two-level structuration. These two prestructuration scales constrain topologically the nucleation of the nanometer-sized crystalline primary grains and their aggregation in nanoparticles, respectively. This template effect not only clarifies why the crystal size is found independent of the nucleation rate, in contradiction with the classical nucleation models, but also supports the possibility to control the final nanostructure with the amorphous phase.

How to prepare the brightest luminescent coatings?

We address here the question of studying the parameters affecting the brightness of luminescent nanoparticulate coatings, among which are the absorption rate, the internal quantum yield of the phosphor nanoparticles, and the extraction factor of the emitted light in a solid angle perpendicular to the substrate. Experimental investigations are achieved on spray-deposited YVO4:Eu particles, a system whose synthesis and properties are well documented so that particles of different sizes and microstructure can be considered. This allows a quantitative evaluation of the factors affecting film brightness. Considering a film made from raw colloidal particles, this work shows that its brightness is limited by a factor of 5 due to altered quantum yield of nanoparticles, a factor of 1.75 by dielectric effects and a factor of 2.4 by light extraction issues. This investigation, through providing quantitative evaluations of these different parameters, opens the way toward a possible rational design of inorganic luminescent coatings, with a possible improvement of brightness that could reach a factor of 30 as compared to simple films made directly from colloidal suspensions.