Xiaoshuang Shen

Three-Dimensional Plasmonic Nanoclusters

Assembling nanoparticles into well-defined structures is an important way to create and tailor the optical properties of materials. Most advances in metamaterials research to date have been based on structures fabricated in two-dimensional planar geometries. Here, we show an efficient method for assembling noble metal nanoparticles into stable, three-dimensional (3-D) clusters, whose optical properties can be highly sensitive or remarkably independent of cluster orientation, depending on particle number and cluster geometry. Some of the clusters, such as tetrahedra and icosahedra, could serve as the optical kernels for metafluids, imparting metamaterial optical properties into disordered media such as liquids, glasses, or plastics, free from the requirement of nanostructure orientation.

Unconventional Chain‐Growth Mode in the Assembly of Colloidal Gold Nanoparticles

The assembly of nano-objects is a fundamental step toward the development of functional nanodevices. For the optimization of properties and functions, it is important to achieve structural precision. Electronic and plasmonic coupling between metallic nanoparticles (NPs) has been known to give novel electronic and optical properties. Such coupling is critically dependent on structural parameters, such as interparticle spacing and the spatial organization of individual NPs. In comparison to higher-order nanostructures, onedimensional (1D) NP chains are more convenient building blocks for circuits in nanoelectronics, optoelectronics, and biosensors. Minimizing structural irregularity is essential: a large gap may break the coupling along a chain, and branching may cause a short circuit. A popular approach for assembling NPs is to utilize templates, such as biomolecules, polymers, and lithographically created patterns. A general problem of these methods is the lack of means to achieve close packing of the NPs in a chain and avoid irregular gaps. On the other hand, simple colloidal aggregation can bring NPs closely together, with short and regular gaps dictated by their ligand layers. However, colloidal-assembly methods based on random aggregation are generally difficult to control. Thus there is great interest to devise controlling methods. Linear assembly of NPs has been achieved by exploiting the anisotropic ligand organization on Au nanorods, or the phase segregation of immiscible ligands on nanospheres. In addition, intrinsic interactions of NPs, such as dipole–dipole interactions or electrostatic charge repulsion, have been proposed to explain the 1D aggregation of NPs. Despite these advances, it remains a great challenge to program the behavior of NPs for a longer-range order, for example, in terms of preventing branching and controlling cross-sectional width. The aggregation of colloidal NPs in a 3D space can be compared to the polymerization of organic monomers. The general characteristic of step-growth polymerization is that all monomers and oligomers are reactive. Their random combination quickly consumes monomers, so that the laterstage reaction mostly involves the coupling of oligomers (i.e., m-to-n coupling; m and n are the numbers of structural units in the chain). If this reaction is inefficient, few long chains will be obtained. In contrast, in chain-growth polymerization the active species (e.g., radicals or initiators) are limited in concentration, thus making the coupling between monomers (1-to-1 coupling) unfavorable. The excess monomers can continuously react with the active species and supply the chain growth through 1-to-n coupling. Because all NPs in a colloid are capable of aggregating, their “polymerization” tends to follow the step-growth mode. Once monomers are depleted, the m-to-n coupling of larger clusters becomes more difficult and less selective, leading to branches and irregularities. Hence, obtaining long chains (> 30 NPs in length) is very difficult. Here, we report the use of polymer shells to assist the 1D assembly of colloidal AuNPs (Figure 1). The system is unique in that the aggregation of

Assembly of colloidal nanoparticles directed by the microstructures of polycrystalline ice.

We show that the microstructures of polycrystalline ice can serve as a confining template for one-dimensional assembly of colloidal nanoparticles. Upon simply freezing an aqueous colloid, the nanoparticles are excluded from ice grains and form chains in the ice veins. The nanoparticle chains are transferable and can be strengthened by polymer encapsulation. After coating with polyaniline shells, simple sedimentation is used to remove large aggregates, enriching single-line chains of 40 nm gold nanoparticles with a total length of several micrometers. When gold nanorods were used, they formed one-dimensional aggregates with specific end-to-end conformation, indicating the confining effects of the nanoscale ice veins at the final stage of freezing. The unbranched and ultralong plasmonic chains are of importance for future study of plasmonic coupling and development of plasmonic waveguides.

Chiral Transformation: From Single Nanowire to Double Helix

We report a new type of water-soluble ultrathin Au-Ag alloy nanowire (NW), which exhibits unprecedented behavior in a colloidal solution. Upon growth of a thin metal (Pd, Pt, or Au) layer, the NW winds around itself to give a metallic double helix. We propose that the winding originates from the chirality within the as-synthesized Au-Ag NWs, which were induced to untwist upon metal deposition.

Controlling Reversible Elastic Deformation of Carbon Nanotube Rings

We show that bundles of carbon nanotubes can be coiled into ring structures by controlling the contraction of their polymer shells. With the robust carbon nanotubes, we demonstrate their reversible transformation between circular and compressed rings in a colloid.

Measuring the unusually slow ionic diffusion in polyaniline via study of yolk-shell nanostructures.

We demonstrate a unique capability in partially oxidizing the oligoaniline shell on gold nanoparticles to polyaniline. Because of the solubility difference, the unreacted inner shell section can be selectively dissolved by 2-propanol, giving yolk-shell nanostructures and, thus, making it possible for assessing the oxidized section. The ionic diffusion through the polymer shell is found to be the rate-determining step in the overall process. Conservative estimates show that the diffusion coefficient of AuCl(4)(-) is at least 700 times slower than that of the typical rate values in traditional studies. It is most likely caused by the lack of micropores in the polymer structures. Such mircopores are hard to avoid in preparing polymer membranes by casting or drying of polymers dissolved in organic solvents. We can rule out the presence of irregular pores on the basis of the uniformly oxidized shell section. With the nanoscale shells, the system is sensitive enough to detect minute changes in the shell or small differences among the individual nanoparticles. Even with a small increase in porosity, for example, when the polyaniline shell is swollen using small amounts of DMF (3%, 5%, or 10% in aqueous solutions), the diffusion coefficient of AuCl(4)(-) increases to 4, 11, and 17 times, respectively. Thus, our study demonstrates a new methodology for studying the diffusion of ions in hydrophobic polymers.

Induced coiling action : exploring the intrinsic defects in five-fold twinned silver nanowires

Growth of polythiophene (PTh) on five-fold twinned Ag nanowires (NWs) is not symmetrical due to preferred etching of their intrinsic defects. This imbalance of polymer formation leads to consistent bending action along the etched NWs, coiling the resulting Ag-PTh nanocomposites into planar spirals. We studied the etching intermediates and also the effects of the surface ligands in order to understand the symmetry-breaking action. The defect-dependent etching chemistry offers a new means to induce motion and a novel perspective in the ordered occurrence of certain defects. We demonstrate that Ag can be deposited back onto the coiled Ag-PTh composite to form metallic spirals.

Developing Mutually Encapsulating Materials for Versatile Syntheses of Multilayer Metal–Silica–Polymer Hybrid Nanostructures

New methods to self-assemble polystyrene-block-poly(acrylic acid) (PSPAA) on silica via electrostatic interaction and to deposit silica on PSPAA shells are developed. The mutual encapsulation of silica and PSPAA allows versatile syntheses of well-controlled nanohybrids.