Constructing nanocrystal-in-glass composites for smart windows

The integration of inorganic nanocrystals as building units into mesoscale architectures yields materials wherein the components and their interfaces are both essential in defining structure and function. Randomly mesostructured nanocrystal-in-amorphous niobia composites can be formed by chemically linking niobium polyoxometalate (POM) clusters to colloidal nanocrystals in the solution phase. When films of these assemblies are thermally annealed, the clusters undergo condensation. They cross-link to form a continuous amorphous niobia matrix surrounding, and covalently linked to, the embedded nanocrystals. The resulting composite materials combine intrinsic characteristics of each component and exhibit unique functionality that we ascribe to reconstruction at the nanocrystal-glass interface. An architected nanocomposite can instead be formed when the arrangement of the nanocrystals into a mesoporous framework is accomplished first, using a block copolymer template. In this case, POMs are in-filled in a second step, then annealed to form the nanocrystal-niobia glass composite. Our composite metal oxide thin films exhibit a unique optical switching response to electrochemical reduction. Namely, they independently control the transmittance of visible and near infrared light as a function of voltage. These results highlight the tremendous opportunity to tune structure at both the atomic and nanometer length scales to realize new functionality

Dynamics of equilibrium linked colloidal gels

Colloids that attractively bond to only a few neighbors (e.g., patchy particles) can form equilibrium gels with distinctive dynamic properties that are stable in time. Here, we use a coarse-grained model to explore the dynamics of linked networks of patchy colloids whose average valence is macroscopically, rather than microscopically, constrained. Simulation results for the model show dynamic hallmarks of equilibrium gel formation and establish that the colloid-colloid bond persistence time controls the characteristic slow relaxation of the self-intermediate scattering function. The model features re-entrant network formation without phase separation as a function of linker concentration, centered at the stoichiometric ratio of linker ends to nanoparticle surface bonding sites. Departures from stoichiometry result in linker-starved or site-starved networks with reduced connectivity and shorter characteristic relaxation times with lower activation energies. Underlying the re-entrant trends, dynamic properties vary monotonically with the number of effective network bonds per colloid, a quantity that can be predicted using Wertheim's thermodynamic perturbation theory. These behaviors suggest macroscopic in situ strategies for tuning the dynamical response of colloidal networks.Comment: 25 pages, 9 figure

Intrinsic Optical and Electronic Properties from Quantitative Analysis of Plasmonic Semiconductor Nanocrystal Ensemble Optical Extinction

The optical extinction spectra arising from localized surface plasmon resonance in doped semiconductor nanocrystals (NCs) have intensities and lineshapes determined by free charge carrier concentrations and the various mechanisms for damping the oscillation of those free carriers. However, these intrinsic properties are convoluted by heterogeneous broadening when measuring spectra of ensembles. We reveal that the traditional Drude approximation is not equipped to fit spectra from a heterogeneous ensemble of doped semiconductor NCs and produces fit results that violate Mie scattering theory. The heterogeneous ensemble Drude approximation (HEDA) model rectifies this issue by accounting for ensemble heterogeneity and near-surface depletion. The HEDA model is applied to tin-doped indium oxide NCs for a range of sizes and doping levels but we expect it can be employed for any isotropic plasmonic particles in the quasistatic regime. It captures individual NC optical properties and their contributions to the ensemble spectra thereby enabling the analysis of intrinsic NC properties from an ensemble measurement. Quality factors for the average NC in each ensemble are quantified and found to be notably higher than those of the ensemble. Carrier mobility and conductivity derived from HEDA fits matches that measured in the bulk thin film literature

Tuning Nanocrystal Surface Depletion by Controlling Dopant Distribution as a Route Toward Enhanced Film Conductivity

Electron conduction through bare metal oxide nanocrystal (NC) films is hindered by surface depletion regions resulting from the presence of surface states. We control the radial dopant distribution in tin-doped indium oxide (ITO) NCs as a means to manipulate the NC depletion width. We find in films of ITO NCs of equal overall dopant concentration that those with dopant-enriched surfaces show decreased depletion width and increased conductivity. Variable temperature conductivity data shows electron localization length increases and associated depletion width decreases monotonically with increased density of dopants near the NC surface. We calculate band profiles for NCs of differing radial dopant distributions and, in agreement with variable temperature conductivity fits, find NCs with dopant-enriched surfaces have narrower depletion widths and longer localization lengths than those with dopant-enriched cores. Following amelioration of NC surface depletion by atomic layer deposition of alumina, all films of equal overall dopant concentration have similar conductivity. Variable temperature conductivity measurements on alumina-capped films indicate all films behave as granular metals. Herein, we conclude that dopant-enriched surfaces decrease the near-surface depletion region, which directly increases the electron localization length and conductivity of NC films

Assembly of colloidal nanocrystals into open networks

Inorganic nanocrystals exhibit a wide variety of optical, electronic, chemical, and electrochemical functionality that is synthetically tunable based on their size and composition. Their properties and emerging methods for functionalizing their surfaces with specific chemical agents pose the exciting prospect to program the assembly of nanostructured materials whose properties depend intimately on both the characteristics of the building blocks and the mesoscale organization of these in the assembly. In this presentation, I describe novel strategies for assembling optically active nanocrystals into organized gel networks. In particular, theoretical frameworks predict open gel architectures when the extent of inter-particle bonding (i.e. valence) is constrained.[1] To achieve a chemically tunable valence, we functionalized semiconductor nanocrystals with highly charged chalcogenidometallates clusters that induce long range repulsive interactions.[2] The addition of controlled amounts of a cationic crosslinking agent determines the assembly of the nanocrystals into a low volume fraction gel. In another assembly strategy, short range attractive forces are induced between charge-stabilized nanocrystal colloids by the introduction of oligomeric polyethylene glycol (PEG). At low PEG concentrations, it can crosslink nanocrystals into a gel. At higher concentrations, PEG effectively passivates the nanocrystal surfaces, yet excess PEG can induce gel network assembly through the depletion attraction. The organization of the gel networks is characterized by small angle X-ray scattering, from which the fractal dimension that describes the gel topology is determined. We present an integrated approach leveraging theory, synthesis, characterization, and simulation to predict, realize, and analyze the formation of low volume fraction gels from colloidal nanocrystals with unusual optical properties in the visible and infrared spectral ranges. References: [1] BA Lindquist, RB Jadrich, DJ Milliron, TM Truskett, “On the Formation of Equilibrium Gels via a Macroscopic Bond Limitation,” J. Chem. Phys. 145 (2016), 074906. [2] A Singh, BA Lindquist, GK Ong, RB Jadrich, A Singh, H Ha, CJ Ellison, TM Truskett, DJ Milliron, “Linking Semiconductor Nanocrystals into Gel Networks through All-Inorganic Bridges,” Angew. Chem. Int. Ed. 54 (2015), 14840-14844

Wertheim’s thermodynamic perturbation theory with double- bond association and its application to colloid–linker mixtures

We extend Wertheim’s thermodynamic perturbation theory to derive the association free energy of a multicomponent mixture for which double bonds can form between any two pairs of the molecules’ arbitrary number of bonding sites. This generalization reduces in limiting cases to prior theories that restrict double bonding to at most one pair of sites per molecule. We apply the new theory to an associating mixture of colloidal particles (“colloids”) and flexible chain molecules (“linkers”). The linkers have two functional end groups, each of which may bond to one of several sites on the colloids. Due to their flexibility, a significant fraction of linkers can “loop” with both ends bonding to sites on the same colloid instead of bridging sites on different colloids. We use the theory to show that the fraction of linkers in loops depends sensitively on the linker end-to-end distance relative to the colloid bonding-site distance, which suggests strategies for mitigating the loop formation that may otherwise hinder linker-mediated colloidal assembly.This research was primarily supported by the National Science Foundation through the Center for Dynamics and Control of Materials: an NSF MRSEC under Cooperative Agree- ment No. DMR-1720595, with additional support from an Arnold O. Beckman Postdoctoral Fellowship (Z.M.S.) and the Welch Foun- dation (Grant Nos. F-1696 and F-1848).Center for Dynamics and Control of Material