Electrochromic (EC) materials change their optical properties (darken/lighten) in the presence of a small electric potential difference, and are suitable for application in energy-efficient windows, antiglare automobile rear-view mirrors, sunroofs, displays, and hydrogen sensors. [1][2][3][4] There are two important criteria for selecting an EC material. The first is the time constant of the ion-intercalation reaction, which is limited both by the diffusion coefficient and by the length of the diffusion path. While the former depends on the chemical structure and crystal structure of the metal oxide, the latter is determined by the material's microstructure. [11] In the case of a nanoparticle, the smallest dimension is represented by the diffusion path length. Thus, designing a nanostructure with a small radius, while maintaining the proper crystal structure, is key to obtaining a material with fast insertion kinetics, enhanced durability, and superior performance. The second important criterion is coloration efficiency (CE), the change in optical density (OD) per unit inserted charge (Q), that is, CE = D(OD)/DQ. [12] A high CE provides large optical modulation with a small charge insertion or extraction. This is a crucial parameter for EC devices, since a lower charge-insertion or -extraction rate enhances the longterm cycling stability. Among inorganic materials, tungsten oxides have been most extensively studied. Up until now, amorphous WO 3 films have exhibited the highest CE in the visible region of the electromagnetic spectrum. However, because of their high dissolution rate in acidic electrolyte solutions, these films can only be used in lithium-based electrolytes, resulting in slower response times. Furthermore, extended durability, even in Li + systems, has not yet been demonstrated. Inexpensive conducting and redox polymers have attracted increased attention for use as EC materials because of their fast response times and high contrast ratios. [13][14][15] However, disadvantages include multiple coloration in the visible spectral range and poor UV stability. By fabricating EC films from crystalline WO 3 nanoparticles, the state-of-the-art technology of producing EC materials has been profoundly advanced. Crystalline WO 3 nanoparticles have been grown by an economical hot-wire chemical-vapordeposition (HWCVD) process, and a unique electrophoresis technique is employed for the fabrication of porous nanoparticle films. The porosity of the films not only increases the surface area and ion-insertion kinetics, but also reduces the overall material cost, leading to an inexpensive, large-area EC material. Compared to conventional amorphous WO 3 films prepared by vacuum deposition, nanoparticle films deposited by electrophoresis exhibit vastly superior electrochemical-cycling stability in acidic electrolytes, a higher charge density, and comparable CE. This greatly enhanced stability and charge capacity are attributed to the crystalline nanoparticles employed in this work. These initial results will ultimately ...

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to 35 nm long that were similar to those produced previously 6 . Above 550 °C, only crystalline folded sheets of MoS 2 were produced.TEM stage-tilting experiments on 30 two-and three-layered structures showed that the small rhomboids, rectangles and hexagons were different projections of the same three-dimensional structure: an octahedron (Fig. 1a). The TEM image for a closed three-layer structure changes with tilts of 10°and 20° (Fig. 1b). The image at 0°is the projection expected for an octahedron orientated such that only two triangular faces are seen. When it is tilted, the projection changes, resulting in a nearly rectangular projection at 20°. Stick models depicting how an octahedron's projection changes with tilting are also shown in Fig. 1b. The model octahedron was orientated to project a match to the 0°image, and the model was then tilted with the same axis used in the TEM. Many other TEM tilt sequences could also be generated with projections of a model octahedron. In some cases, slight movements of the particles on the TEM grid ruined the correlation, but individual images could still be represented by the projection of an octahedron.The edge length of the octahedron may be calculated from TEM projections, assuming a regular octahedral structure. A histogram of edge lengths for 30 different structures is shown in Fig. 1c. Two pronounced peaks are seen at 12-13 and 16 times the a lattice constant (the Mo-Mo distance, 3.16 Å) of MoS 2 for two-and three-layer species, respectively. The spacing between the layers is about 0.6 nm, in good agreement with the interlayer spacing in bulk MoS 2 . The edge of the three-layer species is about four a lattice constants larger than that of the underlying two-layer structure, exactly the size required to maintain the bulk interlayer spacing.Although the reasons for these specific sizes are not clear, a preference for two-and three-layer structures may be associated with the two-and three-layer polytypes 7 . The octahedral shape might be anticipated for a closed MoS 2 structure as the triangular faces share the symmetry of the trigonal Mo and S sublattices. Rounded corners and edges are also expected for MoS 2 sheets, which cannot be severely bent without strain. Energy-dispersive spectroscopy indicated a Mo:S ratio of about 1:2 with no detectable impurities. Satisfying such a ratio exactly is impossible in an octahedron, but several arrangements come close. For example, the Mo-S coordination could remain trigonal prismatic, as in the bulk form, with a given face being slightly rich or poor in sulphur. The structure at the vertices is unclear, but either a four-membered Mo ring 3 or a single Mo atom might be stable (B. Parkinson, personal communication).TEM measurements could be performed only on nano-octahedra that were separated from the agglomerates formed on the TEM grid. Consequently, we cannot yet estimate the density of nano-octahedra in the laser-generated soots. We are purifying these inorganic fullerenes so that we can ascertain their properties, and are al...

We deposited epitaxial Ba0.4Sr0.6TiO3 (BST) films via laser ablation on MgO and LaAlO3 (LAO) substrates for tunable microwave devices. Postdeposition anneals (∼1100 °C in O2) improved the morphology and overall dielectric properties of films on both substrates, but shifted the temperature of maximum dielectric constant (Tmax) up for BST/LAO and down for BST/MgO. These substrate-dependent Tmax shifts had opposite effects on the room-temperature dielectric properties. Overall, BST films on MgO had the larger maximum dielectric constant (ε/ε0⩾6000) and tunability (Δε/ε⩾65%), but these maxima occurred at 227 K. 30 GHz phase shifters made from similar films had figures of merit (ratio of maximum phase shift to insertion loss) of ∼45°/dB and phase shifts of ∼400° under 500 V (∼13 V/μm) bias, illustrating their utility for many frequency-agile microwave devices.

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