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We report a simple and effective method for fabricating and patterning high-quality ZnO nanowire arrays using carbonized photoresists to control the nucleation site, density, and growth direction of the nanowires. The ZnO nanowires fabricated using this method show excellent alignment, crystal quality, and optical properties that are independent of the substrates. The carbonized photoresists provide perfect nucleation sites for the growth of aligned ZnO nanowires and they also perfectly connect to the nanowires to form ideal electrodes that can be used in many applications of ZnO nanomaterials.
ACS Nano 2009 Jan 27
PMID:High-quality ZnO nanowire arrays directly fabricated from photoresists. 1920 48

Core-shell ZnO:MgO nanocrystals have been synthesized by a sequential preparative procedure and capped with carboxymethyl beta-cyclodextrin (CMCD) cavities, thereby rendering the surface of the nanocrystals hydrophilic and the particles water-soluble. The water-soluble CMCD-capped ZnO:MgO nanocrystals emit strongly in the visible region (450-680 nm) on excitation by UV radiation and are stable over extended periods and over a range of pH values. The integrity of the cyclodextrin cavities is preserved on capping and retains their capability for complexation of hydrophobic species in aqueous solutions. Here we report the use of the water-soluble cyclodextrin-capped ZnO:MgO nanocrystals as energy donors for fluorescence resonance energy transfer studies. The organic dye Nile Red has been included within the anchored cyclodextrin cavities to form a noncovalent CMCD ZnO:MgO-Nile Red assembly in aqueous solution. Significant Nile Red fluorescence at 640 nm is observed on band gap excitation of the ZnO:MgO in the UV, indicating efficient resonance energy transfer (RET) from the nanocrystals to the included dye. The number of acceptor molecules interacting with a single donor in the CMCD ZnO:MgO-Nile Red assembly may be altered by controlling the filling up of the anchored cavities by Nile Red, leading to a variation in the efficiency of resonance energy transfer. The donor-acceptor distance was estimated from the efficiency measurements. The Nile Red emission following RET shows a pronounced thermochromic shift, suggesting the possible use of the CMCD ZnO:MgO-Nile Red assembly as thermometers in aqueous solutions.
ACS Nano 2008 Jul
PMID:Resonance energy transfer from beta-cyclodextrin-capped ZnO:MgO nanocrystals to included Nile Red guest molecules in aqueous media. 1920 17

Nanomaterials synthesized from nanobuilding blocks promise size-dependent properties, associated with individual nanoparticles, together with collective properties of ordered arrays. However, one cannot position nanoparticles at specific locations; rather innovative ways of coaxing these particles to self-assemble must be devised. Conversely, model nanoparticles can be placed in any desired position, which enables a systematic enumeration of nanostructure from model nanobuilding blocks. This is desirable because a list of chemically feasible hypothetical structures will help guide the design of strategies leading to their synthesis. Moreover, the models can help characterize nanostructure, calculate (predict) properties, or simulate processes. Here, we start to formulate and use a simulation strategy to generate atomistic models of nanomaterials, which can, potentially, be synthesized from nanobuilding block precursors. Clearly, this represents a formidable task because the number of ways nanoparticles can be arranged into a superlattice is infinite. Nevertheless, numerical tools are available to help build nanoparticle arrays in a systematic way. Here, we exploit the "rules of crystallography" and position nanoparticles, rather than atoms, at crystallographic sites. Specifically, we explore nanoparticle arrays with cubic, tetragonal, and hexagonal symmetries together with primitive, face centered cubic and body centered cubic nanoparticle "packing". We also explore binary nanoparticle superlattices. The resulting nanomaterials, spanning CeO(2), Ti-doped CeO(2), ZnO, ZnS, MgO, CaO, SrO, and BaO, comprise framework architectures, with cavities interconnected by channels traversing (zero), one, two and three dimensions. The final, fully atomistic models comprise three hierarchical levels of structural complexity: crystal structure, microstructure (i.e., grain boundaries, dislocations), and superlattice structure.
ACS Nano 2008 Jun
PMID:Mapping nanostructure: a systematic enumeration of nanomaterials by assembling nanobuilding blocks at crystallographic positions. 1920 42

A weak acid selective etching strategy was put forward to fabricate oxide-based hollow nanoparticles (HNPs) using core/shell nanostructures of active metal/oxide nanoparticles as sacrificial templates. ZnO-based HNPs, including pure ZnO, Au/ZnO, Pt/ZnO, and Au/Pt/ZnO HNPs with diameter below 50 nm and shell thickness below 6 nm has been first achieved at low temperature. The diameter, thickness, and even sizes of ZnO and noble metal ultrafine crystals of HNPs can be well adjusted by the etching process. Synchronous with the formation of HNPs, the internal metal-semiconductor interfaces can be controllably eliminated (Zn-ZnO) and reconstructed (noble metal-ZnO). Excitingly, such microstructure manipulation has endued them with giant improvements in related performances, including the very strong blue luminescence with enhancement over 3 orders of magnitude for the pure ZnO HNPs and the greatly improved photocatalytic activity for the noble metal/ZnO HNPs. These give them strong potentials in relevant applications, such as blue light emitting devices, environment remediation, drug delivery and release, energy storage and conversion, and sensors. The designed fabrication procedure is simple, feasible, and universal for a series of oxide and noble metal/oxide HNPs with controlled microstructure and improved performances.
ACS Nano 2008 Aug
PMID:ZnO-based hollow nanoparticles by selective etching: elimination and reconstruction of metal-semiconductor interface, improvement of blue emission and photocatalysis. 1920 70

The electronic properties of semiconductor (SiC, GaN, BN, ZnO, ZnS, and CdS) nanowires and nanotubes were investigated using first-principles calculations based on density functional theory and generalized gradient approximation. Different size or surface-to-volume ratio dependences were found for the II-VI (ZnO, ZnS, and CdS) and IV-IV (SiC) and III-V (GaN and BN) nanostructures. For SiC, GaN, and BN nanostructures, the band gap decreases with the increase of the surface-to-volume ratio or the reduction of the diameter, while for ZnO, ZnS, and CdS nanostructures, the band gap increases with the increase of surface-to-volume ratio or the reduction of the diameter. The mechanism is attributed to the competition between the interaction from dangling p-like and sigma states and the quantum confinement effect.
ACS Nano 2008 Nov 25
PMID:Semiconductor nanowires and nanotubes: effects of size and surface-to-volume ratio. 1920 9

Zinc oxide is a unique material that exhibits exceptional semiconducting, piezoelectric, and pyroelectric properties. Nanostructures of ZnO are equally as important as carbon nanotubes and silicon nanowires for nanotechnology and have great potential applications in nanoelectronics, optoelectronics, sensors, field emission, light-emitting diodes, photocatalysis, nanogenerators, and nanopiezotronics. Fundamental understanding about the growth of ZnO nanowires is of critical importance for controlling their size, composition, structure, and corresponding physical and chemical properties. The papers by She et al. and Ito et al. in this issue describe the controlled growth and field-emission properties of individual nanostructures, respectively. These studies provide new approaches and insight into the controlled growth and electrical properties of ZnO nanostructures.
ACS Nano 2008 Oct 28
PMID:Splendid one-dimensional nanostructures of zinc oxide: a new nanomaterial family for nanotechnology. 1920 46

A selective growth method for ZnO vertical nanowire arrays is demonstrated using self-assembled gold nanoparticles as the growth catalyst. Gold nanoparticles functionalized with bifunctional (thiol-phosphonic acid) ligands assemble rapidly and selectively onto a patterned ZnO seed layer. Vertical ZnO nanowire arrays are grown by the vapor-liquid-solid (VLS) deposition method from the ZnO seed layer through the catalytic effect of the bound gold nanoparticles. This synthesis method offers a number of advantages for producing ZnO nanowires because it permits selective placement through directed self-assembly of gold nanoparticles, enables rapid growth, eliminates vacuum deposition processing, and minimizes the amount of gold waste when compared to traditional methods that require vapor deposition of gold films.
ACS Nano 2008 Oct 28
PMID:Selective growth of vertical ZnO nanowire arrays using chemically anchored gold nanoparticles. 1920 44

Both electrical and field emission measurements were carried out to study the correlation between resistance and field emission performance of individual one-dimensional (1D) ZnO nanostructures. Three types of 1D ZnO nanostructures were investigated (i.e., agave-like shape, pencil-like shape, and hierarchical structure) and were prepared by thermal chemical vapor transport and condensation without using any catalyst. The 1D ZnO nanostructures have obvious differences in resistance and thus conductivity from type to type. In addition, in the same type of 1D ZnO nanostructure, each individual emitter may also have variation in resistance and thus in conductivity. The field emission performance of the ZnO emitters was found to be strongly correlated with the resistance of each individual ZnO nanostructure: (i) a ZnO emitter with low resistance will have better emission; (ii) a high resistance region in a ZnO nanostructure is liable to the initiation of a vacuum breakdown event. The results indicate that, besides the uniformity in the geometrical structure, the uniformity in conductivity of the emitters in an array should be ensured, in order to meet the requirement of device application.
ACS Nano 2008 Oct 28
PMID:Correlation between resistance and field emission performance of individual ZnO one-dimensional nanostructures. 1920 42

Nanomaterials (NM) exhibit novel physicochemical properties that determine their interaction with biological substrates and processes. Three metal oxide nanoparticles that are currently being produced in high tonnage, TiO(2), ZnO, and CeO(2), were synthesized by flame spray pyrolysis process and compared in a mechanistic study to elucidate the physicochemical characteristics that determine cellular uptake, subcellular localization, and toxic effects based on a test paradigm that was originally developed for oxidative stress and cytotoxicity in RAW 264.7 and BEAS-2B cell lines. ZnO induced toxicity in both cells, leading to the generation of reactive oxygen species (ROS), oxidant injury, excitation of inflammation, and cell death. Using ICP-MS and fluorescent-labeled ZnO, it is found that ZnO dissolution could happen in culture medium and endosomes. Nondissolved ZnO nanoparticles enter caveolae in BEAS-2B but enter lysosomes in RAW 264.7 cells in which smaller particle remnants dissolve. In contrast, fluorescent-labeled CeO(2) nanoparticles were taken up intact into caveolin-1 and LAMP-1 positive endosomal compartments, respectively, in BEAS-2B and RAW 264.7 cells, without inflammation or cytotoxicity. Instead, CeO(2) suppressed ROS production and induced cellular resistance to an exogenous source of oxidative stress. Fluorescent-labeled TiO(2) was processed by the same uptake pathways as CeO(2) but did not elicit any adverse or protective effects. These results demonstrate that metal oxide nanoparticles induce a range of biological responses that vary from cytotoxic to cytoprotective and can only be properly understood by using a tiered test strategy such as we developed for oxidative stress and adapted to study other aspects of nanoparticle toxicity.
ACS Nano 2008 Oct 28
PMID:Comparison of the mechanism of toxicity of zinc oxide and cerium oxide nanoparticles based on dissolution and oxidative stress properties. 1920 59

ZnO nanowires, CuO nanowires, and alpha-Fe(2)O(3) nanotubes as well as their corresponding arrays have been successfully synthesized via a low cost, generalizable, and simplistic template method. Diameters of one-dimensional (1-D) metal oxide nanostructures ( approximately 60-260 nm), measuring micrometers in length, can be reliably and reproducibly controlled by the template pore channel dimensions. Associated vertically aligned arrays have been attached to the surfaces of a number of geometrically significant substrates, such as curved plastic and glass rod motifs. The methodology reported herein relies on the initial formation of an insoluble metal hydroxide precursor, initially resulting from the reaction of the corresponding metal solution and sodium hydroxide, and its subsequent transformation under mild conditions into the desired metal oxide nanostructures. Size- and shape-dependent optical, magnetic, and catalytic properties of as-prepared 1-D metal oxides were investigated and noted to be mainly comparable to or better than the associated properties of the corresponding bulk oxides. A plausible mechanism for as-observed wire and tube-like motifs is also discussed.
ACS Nano 2008 May
PMID:A facile and mild synthesis of 1-D ZnO, CuO, and alpha-Fe(2)O(3) nanostructures and nanostructured arrays. 1920 92


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