Van der Waals Colloidal Crystals.

A general guiding principle for colloidal crystallization is to tame the attractive enthalpy such that it slightly overwhelms the repulsive interaction. As-synthesized colloids are generally designed to retain a strong repulsive potential for the high stability of suspensions, encoding appropriate attractive potentials into colloids has been key to their crystallization. Despite the myriad of interparticle attractions for colloidal crystallization, the van der Waals (vdW) force remains unexplored. Here, it is shown that the implementation of gold cores into silica colloids and the resulting vdW force can reconfigure the pair potential well depth to the optimal range between -1 and -4 kBT at tens of nanometer-scale colloidal distances. As such, colloidal crystals with a distinct liquid gap can be formed, which is evidenced by photonic bandgap-based diffractive colorization.

Strain and crystallographic identification of the helically concaved gap surfaces of chiral nanoparticles.

Identifying the three-dimensional (3D) crystal plane and strain-field distributions of nanocrystals is essential for optical, catalytic, and electronic applications. However, it remains a challenge to image concave surfaces of nanoparticles. Here, we develop a methodology for visualizing the 3D information of chiral gold nanoparticles ≈ 200 nm in size with concave gap structures by Bragg coherent X-ray diffraction imaging. The distribution of the high-Miller-index planes constituting the concave chiral gap is precisely determined. The highly strained region adjacent to the chiral gaps is resolved, which was correlated to the 432-symmetric morphology of the nanoparticles and its corresponding plasmonic properties are numerically predicted from the atomically defined structures. This approach can serve as a comprehensive characterization platform for visualizing the 3D crystallographic and strain distributions of nanoparticles with a few hundred nanometers, especially for applications where structural complexity and local heterogeneity are major determinants, as exemplified in plasmonics.

Ultralow-Loss Substrate for Nanophotonic Dark-Field Microscopy.

For the colloidal nanophotonic structures, a transmission electron microscope (TEM) grid has been widely used as a substrate of dark-field microscopy because a nanometer-scale feature can be effectively determined by TEM imaging following dark-field microscopic studies. However, an optically lossy carbon layer has been implemented in conventional TEM grids. A broadband scattering from the edges of the TEM grid further restricted an accessible signal-to-noise ratio. Herein, we demonstrate that the freely suspended, ultrathin, and wide-scale transparent nanomembrane can address such challenges. We developed a 1 mm by 600 µm scale and 20 nm thick poly(vinyl formal) nanomembrane, whose area is around 180 times wider than a conventional TEM grid, so that the possible broadband scattering at the edges of the grid was effectively excluded. Also, such nanomembranes can be formed without the assistance of carbon support; allowing us to achieve the highest signal-to-background ratio of scattering among other substrates.

Enantioselective sensing by collective circular dichroism.

Quantitative determination and in situ monitoring of molecular chirality at extremely low concentrations is still challenging with simple optics because of the molecular-scale mismatch with the incident light wavelength. Advances in spectroscopy1-4 and nanophotonics have successfully lowered the detection limit in enantioselective sensing, as it can bring the microscopic chiral characteristics of molecules into the macroscopic scale5-7 or squeeze the chiral light into the subwavelength scale8-17. Conventional nanophotonic approaches depend mainly on the optical helicity density8,9 by localized resonances within an individual structure, such as localized surface plasmon resonances (LSPRs)10-16 or dielectric Mie resonances17. These approaches use the local chiral hotspots in the immediate vicinity of the structure, whereas the handedness of these hotspots varies spatially. As such, these localized resonance modes tend to be error-prone to the stochasticity of the target molecular orientations, vibrations and local concentrations18,19. Here we identified enantioselective characteristics of collective resonances (CRs)20 arising from assembled 2D crystals of isotropic, 432-symmetric chiral gold nanoparticles (helicoids)21,22. The CRs exhibit a strong and uniform chiral near field over a large volume above the 2D crystal plane, resulting from the collectively spinning, optically induced dipoles at each helicoid. Thus, energy redistribution by molecular back action on the chiral near field shifts the CRs in opposite directions, depending on the handedness of the analyte, maximizing the modulation of the collective circular dichroism (CD).

DNA Origami Guided Self-Assembly of Plasmonic Polymers with Robust Long-Range Plasmonic Resonance.

Plasmonic polymers consisting of metallic nanoparticles (NPs) are able to squeeze light into the deep-subwavelength space and transfer along a highly confined nanoscale path in long range. DNA nanotechnology, particularly benefiting from the molecular programmability of DNA origami, has provided otherwise nearly impossible platforms for constructing plasmonic nanoparticle polymers with designer configurations and nanoscale gaps. Here, we design and assemble a DNA origami hashtag tile that is able to polymerize into one-dimensional chains with high rigidity. The DNA origami hashtag chains are used as frames to enable robust, versatile, and precise arrangement of metallic NPs into micrometer-long chiral and magnetic plasmonic polymers, which are capable of efficiently transporting plasmonic angular momentum and magnetic surface plasmonic polaritons at the deep-subwavelength scale. Our work provides a molecular platform for the fabrication of long, straight, and structurally complex nanoparticle polymers with emerging plasmonic properties that are appealing to a variety of fields.

Exploiting Colloidal Metamaterials for Achieving Unnatural Optical Refractions.

The scaling down of meta-atoms or metamolecules (collectively denoted as metaunits) is a long-lasting issue from the time when the concept of metamaterials was first suggested. According to the effective medium theory, which is the foundational concept of metamaterials, the structural sizes of meta-units should be much smaller than the working wavelengths (e.g., << 1/5 wavelength). At relatively low frequency regimes (e.g., microwave and terahertz), the conventional monolithic lithography can readily address the materialization of metamaterials. However, it is still challenging to fabricate optical metamaterials (metamaterials working at optical frequencies such as the visible and near-infrared regimes) through the lithographic approaches. This serves as the rationale for using colloidal self-assembly as a strategy for the realization of optical metamaterials. Colloidal self-assembly can address various critical issues associated with the materialization of optical metamaterials, such as achieving nanogaps over a large area, increasing true 3D structural complexities, and cost-effective processing, which all are difficult to attain through monolithic lithography. Nevertheless, colloidal self-assembly is still a toolset underutilized by optical engineers. Here, the design principle of the colloidally self-assembled optical metamaterials exhibiting unnatural refractions, the practical challenge of relevant experiments, and the future opportunities are critically reviewed.

DNA Base Pair Stacking Crystallization of Gold Colloids.

We describe a DNA base pair (bp) stacking driven 3D crystallization of 70-80 nm gold nanospheres (Au NSs) into a large-area, face-centered-cubic (FCC) lattice. Although great advances have been achieved over the past decade, DNA nanoparticle (NP) crystallization has relied solely on the base complementary binding. This limits the accessible crystal size particularly for the larger and heavier Au NPs (>50 nm). In this work, we argue that the use of DNA bp-stacking (so-called blunt-end stacking) instead of complementary binding can widen the scope of controlled interparticle interactions used to assemble larger Au colloids into a larger-area crystal. Through the optimization of the melting transition, relatively large Au NSs (e.g., 75 nm) with nearly ideal roundness can be crystallized into FCC crystals with the area of up to approximately 1400 µm2. A strong metallodielectric stopband is experimentally observed in the visible range, confirming the high quality of our self-assembled Au colloidal crystals.

Soft Plasmonic Assemblies Exhibiting Unnaturally High Refractive Index.

The increases in refractive indices (n) of materials are crucial for transformative optical technologies. With the progress of monolithic lithography, large advances have been achieved with several semiconductors, including silicon, germanium, and gallium arsenide, which generally provide higher n of ∼4.0 compared to those of other elements. Nevertheless, above this upper limit of naturally available n, the range of light-matter interactions could be unprecedentedly expanded, which in turn enriches the possible applications. Here, we present a soft self-assembly of polyhedral Au colloids as a promising method to achieve unnaturally high n values. The interfacial assembly of Au nanocubes provides n of 6.4 at the resonant wavelength (near-infrared) and 4.5 in the off-resonant regimes (mid-infrared), which have not been previously reached. The soft self-assembly of polyhedral Au colloids can be a versatile and highly effective route for the fabrication of optical metamaterials with unnaturally high n values.

Bioinspired Toolkit Based on Intermolecular Encoder toward Evolutionary 4D Chiral Plasmonic Materials.

Over the last two decades, nanophotonics, including plasmonics and metamaterials, have promised compelling opportunities for exotic control over light-matter interactions. The strong chiral light-matter interaction is a representative example. Three-dimensional (3D) chirality has existed naturally only in organic molecules and bio-organisms, but a negligible chiroptic effect was attained with these naturally occurring materials because of their small absorption cross sections. However, inspired by biological chirality, nanophotonic chiral materials have greatly expanded the design space of accessible chiroptic effects (e.g., pushing the chiral light-matter interaction to an exceptional regime, such as a broad-band circular polarizer, negative refractive index, and sensitive chiral sensing). Nevertheless, it is still a challenge to achieve precisely defined and dynamically reconfigurable chiral morphologies that further increase the chiroptic effect. Biological systems continue to inspire approaches to the design and synthesis of precisely defined 3D nanostructures. In particular, a living organism can program the evolutionary pathway of highly complexed 3D chiral morphology precisely from the molecular scale to the macroscopic scale while simultaneously enabling dynamic reconfiguration of their chirality. What if we could harness the power of biological selectivity and evolutionary capability in synthesizing chiral plasmonic materials? We envisioned that platform technology mimicking biological principles would enable control of 3D chiral structures for effective plasmonic interactions with polarized light and further impart the concept of time-dependent evolution (3D + 1D = 4D) to bring about responsive and dynamic changes in chiral plasmonics. In this Account, we review our efforts to develop the biomolecule-based synthesis of 3D chiral plasmonic materials and share the vision that as in biological systems, chirality can be programmed at the molecular level and hierarchically transferred at multiple scales to develop macroscopic chirality. Accompanied by a biomimetic time-dependent chirality of singular plasmonic nanometals, we also summarize recent achievements in the chemistry and nanophotonics communities pursuing 4D plasmonics that are closely related to our research. The biomimetic and bioinspired approaches discussed in this Account will provide new synthetic insights into implementing chiral nanomaterials and extend the range of accessible nanophotonic design. We hope that the molecular encoding approach will be useful to achieve dynamic light-matter interactions at unprecedented dimensions, time scales, and chirality.

Magnetic Plasmon Networks Programmed by Molecular Self-Assembly.

Nanoscale manipulation of magnetic fields has been a long-term pursuit in plasmonics and metamaterials, as it can enable a range of appealing optical properties, such as high-sensitivity circular dichroism, directional scattering, and low-refractive-index materials. Inspired by the natural magnetism of aromatic molecules, the cyclic ring cluster of plasmonic nanoparticles (NPs) has been suggested as a promising architecture with induced unnatural magnetism, especially at visible frequencies. However, it remains challenging to assemble plasmonic NPs into complex networks exhibiting strong visible magnetism. Here, a DNA-origami-based strategy is introduced to realize molecular self-assembly of NPs forming complex magnetic architectures, exhibiting emergent properties including anti-ferromagnetism, purely magnetic-based Fano resonances, and magnetic surface plasmon polaritons. The basic building block, a gold NP (AuNP) ring consisting of six AuNP seeds, is arranged on a DNA origami frame with nanometer precision. The subsequent hierarchical assembly of the AuNP rings leads to the formation of higher-order networks of clusters and polymeric chains. Strong emergent plasmonic properties are induced by in situ growth of silver upon the AuNP seeds. This work may facilitate the development of a tunable and scalable DNA-based strategy for the assembly of optical magnetic circuitry, as well as plasmonic metamaterials with high fidelity.

Using highly uniform and smooth selenium colloids as low-loss magnetodielectric building blocks of optical metafluids.

We systematically analyzed the magnetodielectric resonances of Se colloids for the first time in an attempt to utilize them as building blocks for all-dielectric optical metafluids. By taking advantages of the synergistic properties of Se colloids, including their (i) high-refractive-index at optical frequencies, (ii) unprecedented structural uniformity, and (iii) ready availability, we were able to observe Kerker-type directional light scattering, resulting from the efficient coupling between strong electric and magnetic resonances, directly from Se colloidal suspensions. Thus, the use of Se colloids as a generic magnetodielectric building block suggests the opportunity for the production of fluidic low-loss optical antennas, which can be processed via spin-coating and painting.

Photofluidic Near-Field Mapping of Electric-Field Resonance in Plasmonic Metasurface Assembled with Gold Nanoparticles.

We present a near-field mapping of electric fields from the individual superspherical and ultrasmooth gold nanoparticles (AuNPs) and artificially assembled AuNP nanostructures by measuring the reconfiguration of an azobenzene-containing polymer(azo-polymer) film. Various configurations of AuNPs and the azo-polymer were studied with atomic force microscopy measurements and calculations. The interference was systematically studied with AuNP dimers of various gap distances and different embedding depth in the polymer film. Finally, we successfully demonstrated the interference of standing waves in artificially assembled plasmonic metasurface.

Assembly of "3D" plasmonic clusters by "2D" AFM nanomanipulation of highly uniform and smooth gold nanospheres.

Atomic force microscopy (AFM) nanomanipulation has been viewed as a deterministic method for the assembly of plasmonic metamolecules because it enables unprecedented engineering of clusters with exquisite control over particle number and geometry. Nevertheless, the dimensionality of plasmonic metamolecules via AFM nanomanipulation is limited to 2D, so as to restrict the design space of available artificial electromagnetisms. Here, we show that "2D" nanomanipulation of the AFM tip can be used to assemble "3D" plasmonic metamolecules in a versatile and deterministic way by dribbling highly spherical and smooth gold nanospheres (NSs) on a nanohole template rather than on a flat surface. Various 3D plasmonic clusters with controlled symmetry were successfully assembled with nanometer precision; the relevant 3D plasmonic modes (i.e., artificial magnetism and magnetic-based Fano resonance) were fully rationalized by both numerical calculation and dark-field spectroscopy. This templating strategy for advancing AFM nanomanipulation can be generalized to exploit the fundamental understanding of various electromagnetic 3D couplings and can serve as the basis for the design of metamolecules, metafluids, and metamaterials.

Petal-Inspired Diffractive Grating on a Wavy Surface: Deterministic Fabrications and Applications to Colorizations and LED Devices.

Interestingly, the petals of flowering plants display unique hierarchical structures, in which surface relief gratings (SRGs) are conformably coated on a curved surface with a large radius of curvature (hereafter referred to as wavy surface). However, systematic studies on the interplay between the diffractive modes and the wavy surface have not yet been reported, due to the absence of deterministic nanofabrication methods capable of generating combinatorially diverse SRGs on a wavy surface. Here, by taking advantage of the recently developed nanofabrication composed of evaporative assembly and photofluidic holography inscription, we were able to achieve (i) combinatorially diverse petal-inspired SRGs with controlled curvatures, periodicities, and dimensionalities, and (ii) systematic optical studies of the relevant diffraction modes. Furthermore, the unique diffraction modes of the petal-inspired SRGs were found to be useful for the enhancement of the outcoupling efficiency of an organic light emitting diode (OLED). Thus, our systematic analysis of the interplay between the diffractive modes and the petal-inspired SRGs provides a basis for making more informed decisions in the design of petal-inspired diffractive grating and its applications to optoelectronics.