Universal Theory of Light Scattering of Randomly Oriented Particles: A Fluctuational-Electrodynamics Approach for Light Transport Modeling in Disordered Nanostructures.

Disordered nanostructures are commonly encountered in many nanophotonic systems, from colloid dispersions for sensing to heterostructured photocatalysts. Randomness, however, imposes severe challenges for nanophotonics modeling, often constrained by the irregular geometry of the scatterers involved or the stochastic nature of the problem itself. In this Article, we resolve this conundrum by presenting a universal theory of averaged light scattering of randomly oriented objects. Specifically, we derive expansion-basis-independent formulas of the orientation-and-polarization-averaged absorption cross section, scattering cross section, and asymmetry parameter, for single or a collection of objects of arbitrary shape. These three parameters can be directly integrated into traditional unpolarized radiative energy transfer modeling, enabling a practical tool to predict multiple scattering and light transport in disordered nanostructured materials. Notably, the formulas of average light scattering can be derived under the principles of fluctuational electrodynamics, allowing the analogous mathematical treatment to the methods used in thermal radiation, nonequilibrium electromagnetic forces, and other associated phenomena. The proposed modeling framework is validated against optical measurements of polymer composite films with metal-oxide microcrystals. Our work may contribute to a better understanding of light-matter interactions in disordered systems, such as plasmonics for sensing and photothermal therapy, photocatalysts for water splitting and CO2 dissociation, photonic glasses for artificial structural colors, and diffuse reflectors for radiative cooling, to name just a few.

High-Performance Planar Thin Film Thermochromic Window via Dynamic Optical Impedance Matching.

Window coatings with dynamic solar transmittance represent an excellent opportunity to reduce building heating and cooling loads, which account for >40% of energy consumed by the built environment. In particular, inorganic vanadium dioxide-based thermochromic coatings offer long lifetimes (>30 years) and can be passively integrated into a window system without additional electronics or power requirements. However, their limited solar modulation depth and wide phase-change hysteresis have traditionally restricted their ability to adapt to changing weather conditions. Here, we derive an optical performance limit for thin film vanadium dioxide coatings, which we find to be far beyond the current literature. Furthermore, we experimentally demonstrate a solution-processed multilayer thin film coating that uses temperature-dependent optical impedance matching to approach the optical performance limit. The thin film coating demonstrated has a record solar transmittance modulation of 21.8% while maintaining a high level of visible transparency (∼50%) and minimal hysteresis (∼10 °C). This work represents a step-change in thin film thermochromic window coatings and, as a result, establishes planar thin film vanadium dioxide as the most viable morphology for high-performance thermochromic windows.

Directed evolution of artificial repeat proteins as habit modifiers for the morphosynthesis of (111)-terminated gold nanocrystals.

Natural biocomposites are shaped by proteins that have evolved to interact with inorganic materials. Protein directed evolution methods which mimic Darwinian evolution have proven highly successful to generate improved enzymes or therapeutic antibodies but have rarely been used to evolve protein-material interactions. Indeed, most reported studies have focused on short peptides and a wide range of oligopeptides with chemical binding affinity for inorganic materials have been uncovered by phage display methods. However, their small size and flexible unfolded structure prevent them from dictating the shape and crystallinity of the growing material. In the present work, a specific set of artificial repeat proteins (αRep), which exhibit highly stable 3D folding with a well-defined hypervariable interacting surface, is selected by directed evolution of a very efficient home-built protein library for their high and selective affinity for the Au(111) surface. The proteins are built from the extendable concatenation of self-compatible repeated motifs idealized from natural HEAT proteins. The high-yield synthesis of Au(111)-faceted nanostructures mediated by these αRep proteins demonstrates their chemical affinity and structural selectivity that endow them with high crystal habit modification performances. Importantly, we further exploit the protein shell spontaneously assembled on the nanocrystal facets to drive protein-mediated colloidal self-assembly and on-surface enzymatic catalysis. Our method constitutes a generic tool for producing nanocrystals with determined faceting, superior biocompatibility and versatile bio-functionalization towards plasmon-based devices and (bio)molecular sensors.

Low-Power Optical Trapping of Nanoparticles and Proteins with Resonant Coaxial Nanoaperture Using 10 nm Gap.

We present optical trapping with a 10 nm gap resonant coaxial nanoaperture in a gold film. Large arrays of 600 resonant plasmonic coaxial nanoaperture traps are produced on a single chip via atomic layer lithography with each aperture tuned to match a 785 nm laser source. We show that these single coaxial apertures can act as efficient nanotweezers with a sharp potential well, capable of trapping 30 nm polystyrene nanoparticles and streptavidin molecules with a laser power as low as 4.7 mW. Furthermore, the resonant coaxial nanoaperture enables real-time label-free detection of the trapping events via simple transmission measurements. Our fabrication technique is scalable and reproducible, since the critical nanogap dimension is defined by atomic layer deposition. Thus our platform shows significant potential to push the limit of optical trapping technologies.

Nanoparticles Self-Assembly Driven by High Affinity Repeat Protein Pairing.

Proteins are the most specific yet versatile biological self-assembling agents with a rich chemistry. Nevertheless, the design of new proteins with recognition capacities is still in its infancy and has seldom been exploited for the self-assembly of functional inorganic nanoparticles. Here, we report on the protein-directed assembly of gold nanoparticles using purpose-designed artificial repeat proteins having a rigid but modular 3D architecture. αRep protein pairs are selected for their high mutual affinity from a library of 10(9) variants. Their conjugation onto gold nanoparticles drives the massive colloidal assembly of free-standing, one-particle thick films. When the average number of proteins per nanoparticle is lowered, the extent of self-assembly is limited to oligomeric particle clusters. Finally, we demonstrate that the aggregates are reversibly disassembled by an excess of one free protein. Our approach could be optimized for applications in biosensing, cell targeting, or functional nanomaterials engineering.

Computationally Guided Assembly of Oriented Nanocubes by Modulating Grafted Polymer-Surface Interactions.

The bottom-up fabrication of ordered and oriented colloidal nanoparticle assemblies is critical for engineering functional nanomaterials beyond conventional polymer-particle composites. Here, we probe the influence of polymer surface ligands on the self-orientation of shaped metal nanoparticles for the formation of nanojunctions. We examine how polymer graft-surface interactions dictate Ag nanocube orientation into either edge-edge or face-face nanojunctions. Specifically, we investigate the effect of end-functionalized polymer grafts on nanocube assembly outcomes, such as interparticle angle and interparticle distance. Our assembly results can be directly mapped onto our theoretical phase diagrams for nanocube orientation, enabling correlation of experimental variables (such as graft length and metal binding strength) with computational parameters. These results represent an important step toward unifying modeling and experimental approaches to understanding nanoparticle-polymer self-assembly.

Multimodal plasmonics in fused colloidal networks.

Harnessing the optical properties of noble metals down to the nanometre scale is a key step towards fast and low-dissipative information processing. At the 10-nm length scale, metal crystallinity and patterning as well as probing of surface plasmon properties must be controlled with a challenging high level of precision. Here, we demonstrate that ultimate lateral confinement and delocalization of surface plasmon modes are simultaneously achieved in extended self-assembled networks comprising linear chains of partially fused gold nanoparticles. The spectral and spatial distributions of the surface plasmon modes associated with the colloidal superstructures are evidenced by performing monochromated electron energy-loss spectroscopy with a nanometre-sized electron probe. We prepare the metallic bead strings by electron-beam-induced interparticle fusion of nanoparticle networks. The fused superstructures retain the native morphology and crystallinity but develop very low-energy surface plasmon modes that are capable of supporting long-range and spectrally tunable propagation in nanoscale waveguides.