We have directly observed the Anderson localized wave functions in three dimensions in a new class of photonic band gap systems. Such systems are networks made of one-dimensional waveguides. By adopting a simple scattering geometry in a unit cell, we are able to obtain large photonic band gaps. In the presence of defects or randomness, we have systematically studied the structures of transmission and the localized wave functions inside a gap. The effects due to absorption are investigated. Excellent quantitative agreements between theory and experiments have been obtained. [S0031-9007(98) PACS numbers: 41.20.Jb In the past decade, the localization of classical waves in random media has been under intensive studies [1]. Unlike electrons, the localization of classical waves is purely a result of multiple scatterings in a random environment and free from the complications arising from interaction effects. However, due to the Rayleigh scattering at low frequencies, it is more difficult to localize classical waves than to localize electrons. In 3D, waves can be localized only in certain windows in the intermediate frequency range with a minimum dielectric contrast [2]. It has been suggested that waves are more easily localized inside a gap or pseudogap of a photonic band gap (PBG) material [3]. The PBG material in its own right is of great interest and has important implications in both fundamental science and technological applications [4]. The localization of electromagnetic waves has been observed in 1D and 2D PBG materials [5,6]. For 3D systems, efforts have been focused only on wave localization in random media. The effects arising from wave localization have been reported in microwave experiments [7]. Nevertheless, a direct interpretation of localization was complicated by the presence of large absorption. Very recently, direct evidence of light localization has been reported in strong scattering media of semiconductor powders based on the size dependence of the transmission coefficient [8]. These important developments lead us to question if one can directly observe Anderson localized wave functions in 3D. For this purpose, like earlier investigations in 2D [6], we need to investigate the strongly localized states inside the gap of a PBG system, where the localization length is short. In addition, a direct measurement of the 3D wave function should be allowed in such systems.To meet these two requirements, we propose a new class of PBG systems here. Such systems are networks connected by segments of 1D waveguides [9]. There are two important advantages in such systems. First, strong scattering can be easily introduced in a unit cell to produce large full gaps in any dimension. Thus, unlike usual PBG systems, our systems do not require a material with a large dielectric constant. Second, the wave function at each node is physically accessible so that 3D localized wave functions can be probed.In our study, the coaxial cable was adopted as the 1D waveguide. The 3D network considered was in a diamond structure...

Zinc oxide (ZnO) hierarchical structures (HSs) have recently demonstrated notable photochemical and photovoltaic performances attributed to their nano/micro combined architectures. In this study, ZnO HSs were synthesized at room temperature using ultrarapid sonochemistry. This novel approach can effectively overcome deficiencies in the synthesis via traditional direct precipitation by promoting nucleation and accelerating diffusion. Only 15 min was needed to complete the formation of highly crystallized and uniformed HSs consisting of interconnected monocrystalline nanosheets using sonochemistry. The formation of HSs through in situ observations was interpreted using a new mechanism based on oriented attachment and reconstruction. In the nonequilibrium synthesis system, thicker, porous, and coarse crystallized ZnO sheets were first constructed via oriented attachment of small-sized nanocrystals. After reconstruction, untrathin, integrated, and monocrystalline nanosheets were obtained. According to the two-dimensional nanosheets to three-dimensional HSs, the formation was much more sophisticated because repeated and parallel heterogeneous oriented attachments with reconstructions dominated the final morphologies of the HSs. The relationships between synthetic conditions and HSs structures were established. Based on the photoanodes in dye-sensitized solar cells (DSCs), the performances of these differently structured HSs were tested. HSs with densely assembled nanosheets exhibited better performances in photoelectric conversions. Systematic investigations were also carried out by selecting two representative HSs to demonstrate the critical factors governing the optical and electrical properties of photoanodes. Finally, under AM 1.5 and 100mW cm −2 light irradiation, high photoelectric conversion efficiencies of up to 6.42% were achieved. These results established a new record for quasi-solid ZnO-based DSCs.

Conventional top-down methods are only competent for construction of nanostructures at the upper end of the nanoscale, whereas bottom-up techniques reach much smaller dimensions.[1] Therefore, forming nanostructures for nanoscience and nanotechnology using monodisperse nanoparticles as building blocks has advantages over lithographic techniques. For instance, the superlattice of magnetic nanoparticles is one of the most promising candidates for high-density magnetic storage media.[2] Self-assembling of regular patterns is desired for both scientific studies and technological applications. The microscopic mechanism for self-organization of nanocrystals has been extensively investigated. [3][4][5][6][7] The interparticle attractive van der Waals force induces the self-assembly, whereas the steric interaction provides the balancing force to create stable structures. [8,9] Various macroscopic patterns have been reported, such as ordered 2D [2,10,11] and 3D superlattices. [1,12,13] In this paper, we report on a study of the superlattices of monodisperse magnetite (Fe 3 O 4 ) nanoparticles [14,15] that have a truncated octahedral shape that plays an important role in their assembly. The stable assembly is a body-centered cubic (bcc) superlattice in which all the nanoparticles are crystallographically aligned. The self-orientation of the nanoparticles in the self-assembled superlattice is critical to many applications of magnetic nanoparticles, such as information storage. As a complement to conventional lithography, the self-assembly and self-orientation have a wide range of applications. In order to determine the characteristics of individual magnetite nanoparticles, we obtained high-resolution transmission electron microscopy (HRTEM) images at the main zone axes of the nanoparticles.