A robust method for epitaxial deposition of Au onto the surface of Ag nanostructures is demonstrated, which allows effective conversion of Ag nano­structures of various morphologies into Ag@Au counterparts, with the anisotropic ones showing excellent plasmonic properties comparable to the original Ag nanostructures while significantly enhanced stability. Sulfite plays a determining role in the success of this epitaxial deposition as it strongly complexes with gold cations to completely prevent galvanic replacement while it also remains benign to the Ag surface to avoid any ligand‐assisted oxidative etching. By using Ag nanoplates as an example, it is shown that the corresponding Ag@Au nanoplates possess remarkable plasmonic properties that are virtually Ag‐like, in clear contrast to Ag@Au nanospheres that exhibit much lower plasmonic activities than their Ag counterparts. As a result, they display high durability and activities in surface‐enhanced Raman scattering applications. This strategy may represent a general platform for depositing a noble metal on less stable metal nanostructures, thus opening up new opportunities in rational design of functional metal nanomaterials for a broad range of applications.

We propose a design and numerical study of an optically tunable metamaterial based on an electric-fieldcoupled inductor-capacitor resonator variant in the terahertz regime. In contrast to earlier proposed structures, we demonstrate that a blueshift of the resonance frequency under illumination can be accomplished with realistic material parameters and a broadband tuning range on the order of 40% has been demonstrated, which is found to be based on a photoconductivity-induced mode-switching effect. We also present a variant of this structure, which simultaneously possesses two resonance frequencies and can be used as an optically switchable dual-band resonator. Our all-optical modulators and switches may offer a step forward in filling the "THz gap."

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Mesoporous non-siliceous materials, in particular mesoporous transition metal oxides (m-TMOs), are of interest due to their fascinating electronic, redox, and magnetic properties for a wide range of applications in catalysis and energy storage. Control of the porosity (e.g., pore size, wall thickness, and surface area) and the crystalline degree (e.g., phase composition, crystallinity, and crystal grain size) of m-TMOs are critical for those applications. To crystallize TMOs, high temperature annealing is often needed to remove the amorphous defects and/or tune the compositions of different crystalline phases. This has brought many challenges to surfactant or block copolymer templates used in the process of evaporation-induced-self-assembly to prepare m-TMOs. In this review, we summarize the most recent achievements including the findings in our own laboratory on the use of organosilicate-containing colloids for the templated growth of mesoporous materials. We review a few key examples of preparing crystalline mesoporous oxides using different templating methods. The colloidal templating method by which mesoporous nanostructures can be stabilized up to 1,000°C is highlighted. The applications of m-TMOs and meso metal-oxide hybrids synthesized using organosilicate-containing colloidal templates in photocatalysis and high-temperature catalysis are also discussed.

The catalytic activity of MnO 2 nanosheets towards oxygen evolution depends highly on their interlayer environment. We present a systematic investigation on fine-tuning of the interlayer environment of MnO 2 nanosheets by intercalation through a facile cation exchange with inexpensive first-row transition metal cations, including Ni 2 + , Co 2 + , Cu 2 + , Zn 2 + , and Fe 3 + ions. Among them, the Ni-intercalated MnO 2 nanosheets show remarkably enhanced OER activity and long-term stability, compared to pristine MnO 2 nanosheets. The overpotential of 330 mV at a current density of 10 mA cm À 2 is observed for the Ni-intercalated MnO 2 nanosheets. The ehancement mechanism of OER is studied by comparing physiochemical properties, such as the oxidation state of Mn, the interlayer distance, the increase in the disorder/twisting of MnO 6 octahedra, and the interlayer cooperative binding of water molecules. The Ni intercalation, different from other metal cations, strengthens the MnÀ O bond perpendicularly to the layer chains to facilitate the interlayer catalysis possibly between two Mn sites, and thus promotes the efficiency of oxygen evolution.

Photorefractive materials can form "instant" holograms without time-consuming development steps. Their potential applications include image processing, optical data storage, and correction of image distortion, but the cost of crystal growth and preparation has been a primary impediment to commercial application. Polymers, on the other hand, are low in cost and readily fabricated in a variety of forms. Photorefractive polymers were constructed with performance that matched or exceeded the performance of available photorefractive crystals. The largest observed two-beam energy coupling gain coefficient for the polymers was 56 per centimeter.

In this work, we numerically demonstrate an all-optical tunable Fano resonance in a fishnet metamaterial(MM) based on a metal/phase-change material(PCM)/metal multilayer. We show that the displacement of the elliptical nanoholes from their centers can split the single Fano resonance (FR) into a double FR, exhibiting higher quality factors. The tri-layer fishnet MMs with broken symmetry accomplishes a wide tuning range in the mid-infrared(M-IR) regime by switching between the amorphous and crystalline states of the PCM (Ge2Sb2Te5). A photothermal model is used to study the temporal variation of the temperature of the Ge2Sb2Te5 film to show the potential for switching the phase of Ge2Sb2Te5 by optical heating. Generation of the tunable double FR in this asymmetric structure presents clear advantages as it possesses a fast tuning time of 0.36 ns, a low pump light intensity of 9.6 μW/μm2, and a large tunable wavelength range between 2124 nm and 3028 nm. The optically fast tuning of double FRs using phase change metamaterials(PCMMs) may have potential applications in active multiple-wavelength nanodevices in the M-IR region.

On one hand, the capping ligand adsorbs on the surface of noble metal nanocrystals, which minimizes their surface energy and provides interparticle repulsion forces to afford a stable colloid of the noble metal nanocrystals, rather than their aggregates. [9] On the other hand, conventionally, the control of the morphology of noble metal nanocrystals heavily relies on the selective adsorption of capping ligands on specific facets of the noble metal nanocrystals. [2c] Therefore, colloidal nanocrystals are synthesized exclusively as a hybrid with an inorganic core that is enclosed by a capping ligand.Although the capping ligands are essential in controlling the crystal growth of the noble metal nanocrystals and maintaining their colloidal property, they are usually detrimental to their activities in many applications, such as surfaceenhanced Raman scattering (SERS) and catalysis. [10] Au and Ag based noble metal nanocrystals show strong localized surface plasmon resonance (LSPR) in the visible range of the spectrum, which produces intense electromagnetic fields at their close vicinity and makes them outstanding substrates for SERS-based chemical and biosensing applications. [11] The SERS activity is highly dependent on the formation of "hotspots," where the electromagnetic field is extremely strong. These hotspots, however, are usually occupied by the capping ligands, which makes the hotspots inaccessible to the analytes, leading to weak Raman signals and thus low detecting sensitivity of the analytes. [10a,b] On the other hand, noble metal nanocrystals represent an emerging family of heterogeneous catalysts for many organic, electrocatalytic, and gas-phase reactions, with the metal surface or the metal/ support interface being the catalytically active site. [1c,12] The ligands strongly bound to the surface usually prevent the reactants from approaching the active sites, leading to partially or fully poisoning of the catalyst. [10b] Therefore, it is highly desirable to remove the capping ligands after the colloidal synthesis of the noble metal nanocrystals, which promises significantly enhanced optical and catalytic properties.Up to date, harsh physical and chemical processes are usually employed to remove the capping ligands from the noble metal nanocrystals. These processes include plasma cleaning, [13] UV-ozone cleaning, [14] cleaning by concentrated acetic acid, [12f ] and thermal annealing in air. [15] The capping Noble metal nanocrystals that are free of capping ligands promise significantly enhanced activities in surface-enhanced Raman scattering (SERS) and catalytic applications. Conventional physical and chemical processes to remove the capping ligands are usually too harsh to retain the morphology and surface structure of the noble metal nanocrystals. In this work, a mild, effective, and robust strategy is presented to remove the capping ligands from the surface of noble metal nanocrystals. Polyvinylpyrrolidone and oleylamine, which are generally known to adsorb strongly on the metal surface, have b...