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PubMed2006

Morphology-controlled synthesis of ZnO nanostructures by a simple round-to-round metal vapor deposition route.

One-dimensional ZnO nanostructures with different morphologies have been successfully synthesized through a simple round-to-round metal vapor deposition route at 550 degrees C with a zinc powder covered indium film as the source material. The structures and morphologies of the products were characterized in detail by using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Studies found that the morphology of the products can be easily tuned from one experimental round to another. Possible growth mechanisms for the formation of one-dimensional ZnO nanostructures with different morphologies are discussed. Photoluminescence studies show that there are sharp UV emission and broad defect-related green emissions for the products obtained in all experimental rounds. Relative intensity of the UV emission to defect-related emissions gradually increased from one experimental round to another.

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PubMed2006

Fine-tuning the synthesis of ZnO nanostructures by an alcohol thermal process.

A simple and efficient alcohol thermal technique was applied to control the growth of the dimensions and morphology of ZnO nanostructures under mild conditions, where surfactant was not necessary. The size of ZnO nanocrystals increased with growth temperature and they transformed into nanorods with different aspect ratios through tuning the reaction time. The length of nanorods increased significantly with the reaction time, but their thickness only slightly increased. The as-prepared ZnO nanocrystals were monocrystalline and the growth orientation of ZnO nanorods was [001]. Photoluminescence measurements showed a blue shift in violet emission with a reduction in crystal size and revealed the quantum confinement effect. Electron irradiation induced structural damage was observed in the ZnO nanorods synthesized at 120 degrees C. Experimental results proved that the possible growth mechanism of ZnO nanostructures was oriented attachment.

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PubMed2006

Nanostructured Pt functionlized multiwalled carbon nanotube based hydrogen sensor.

Nanostructured Pt functionalized multiwalled carbon nanotubes (MWNTs) produced by catalytic chemical vapor deposition are good room-temperature hydrogen sensors. MWNTs have been synthesized by catalytic chemical vapor deposition of acetylene using a fixed-bed catalytic reactor over hydrides of Mm(0.2)Tb(0.8)CO2 obtained through hydrogen decrepitation technique. Purified and chemically treated MWNTs have been functionalized by Pt resulting in nanostructured dispersion of Pt on CNTs. Structural, morphological, and vibrational characterizations have been carried out using XRD, SEM, TEM, HRTEM, Raman spectroscopy, and FTIR spectroscopy, respectively. Dispersion of Pt on MWNTs was confirmed by elemental analysis using EDX. Systematic investigations of hydrogen sensing properties of Pt-MWNT ensembles have been carried out. The Pt-MWNTs thin films are stable after several cycles of adsorption and desorption. The change in electrical resistance due to hydrogen adsorption is reversible, with increases to saturation on exposure to hydrogen gas. The result demonstrates that the Pt-MWNTs are p-type semiconductor materials, and chemically treated MWNTs functionalized with Pt show the better hydrogen sensing response at room temperature.

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PubMed2006

Facile synthesis and catalytic property of porous tin dioxide nanostructures.

Porous tin dioxide (SnO(2)) nanostructures consisting of nanoplates are prepared through thermal decomposition of the mixed solution composed of dibutyltin dilaurate and acetic acid. The aggregations of the nanoplates give rise to large macropores with the size of about 100-300 nm. These nanoplates have a wormhole-like porous structure with the size of about 4 nm and possess high surface area. X-ray powder diffraction, transmission electron microscopy, scanning electron microscopy, infrared spectroscopy, and nitrogen sorption have been employed to characterize the obtained porous structures. It is found that the obtained nanostructures exhibit excellent catalytic activity toward methanol decomposition. Such porous structures with high surface area have promising industrial applications as catalysts.

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PubMed2006

Surfactantless synthesis of multiple shapes of gold nanostructures and their shape-dependent SERS spectroscopy.

A useful method for the synthesis of various gold nanostructures is presented. The results demonstrated that flowerlike nanoparticle arrays, nanowire networks, nanosheets, and nanoflowers were obtained on the solid substrate under different experimental conditions. In addition, surface-enhanced Raman scattering (SERS) spectra of 4-aminothiophenol (4-ATP) on the as-prepared gold nanostructures of various shapes were measured, and their shape-dependent properties were evaluated. The intensity of the SERS signal was the smallest for the gold nanosheets, and the flowerlike nanoparticle arrays gave the strongest SERS signals.

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PubMed2006

Growth of ZnO nanostructures with different morphologies by using hydrothermal technique.

ZnO nanostructures, including nanotowers, nanovolcanoes, nanorods, nanotubes, and nanoflowers, have been grown by using the hydrothermal technique. Most of the ZnO nanostructures show the perfect hexagonal cross section and well-faceted top and side surfaces. The basic chemistry and growth mechanism are discussed. By increasing the reaction time, the volcano-like and tube-like ZnO structures were formed due to the Ostwald ripening process and the selective adsorption of the complexes. By using the seed layer, the dense-arrayed, regular oriented ZnO nanorods were obtained due to the decreased nucleation barrier and the abundant interfaces as well as the increasing surface roughness.

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PubMed2006

Shape control of single crystalline bismuth nanostructures.

A synthesis approach for shape control of single crystalline Bi nanostructures has been developed. By controlling the molar ratio of PVP and Bi in a polyol process, Bi nanocubes with an edge length of approximately 60-80 nm, triangular nanoplates with an edge length of 200-500 nm, and nanospheres with an average diameter of 75 nm have been successfully synthesized. In the same synthetic process, Bi nanobelts with lengths of up to 80 microm and widths of up to 0.6 microm were synthesized in large quantities by introducing a trace amount of Fe3+ species into the reaction system. These single crystalline nanostructure Bi materials are expected to find potential applications in a variety of areas including high efficiency thermoelectric devices.

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PubMed2006

Additivity in the optical kerr effect spectra of binary ionic liquid mixtures: implications for nanostructural organization.

Low-frequency spectra of binary room-temperature ionic liquid (RTIL) mixtures of 1-pentyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and 1-pentyl-3-methylimidazolium bromide in the 0-250 cm(-1) region were studied as a function of mole fraction at 295 K. The spectra were obtained by use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The spectra of these binary mixtures are well described by the weighted sums of the spectra for the neat RTILs. This surprising result implies that the intermolecular modes giving rise to the spectra of the neat liquids must also produce the spectra of the mixtures. Additivity of the OKE spectra can be explained by a model in which locally ordered domains are assumed to exist in the neat liquid with the structures of these locally ordered domains preserved upon mixing. Recently published molecular dynamics simulations show that RTILs are nanostructurally organized with ionic networks and nonpolar regions. If ionic networks also exist in the mixture, the additivity of the OKE spectra implies that there are "blocks" along the network of the mixture that are ordered in the same way as in the neat liquids. These "block co-networks" would have a nanostructural organization resembling that of a block copolymer.

Letter↗
PubMed2006

Controllable assembly of WO3 nanorods/nanowires into hierarchical nanostructures.

The controlled synthesis of two novel h-WO3 hierarchical structures made of nanorods/nanowires has been successfully realized in a large scale via a simple hydrothermal method. It is demonstrated that the morphology of the final products is significantly influenced by adding different sulfates. The urchinlike and ribbonlike structures of WO3 can be selectively prepared by adding Rb2SO4 and K2SO4, respectively. The morphology evolvement and the growth mechanism were studied carefully. The sulfate-induced oriented attachment growth mechanism has been proposed for the possible formation mechanism of the ribbonlike sample. For urchinlike products, two growing stages are believed to be involved in the growth process. The current understanding of the growth mechanism of these nanostructures may be potentially applied for designing other oriented or hierarchical nanostructures based on 1D nanoscale building blocks through the direct solution-growth.

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PubMed2004

A massively parallel electrochemical approach to the miniaturization of organic micro- and nanostructures on surfaces.

This paper describes a simple and convenient strategy for reducing the dimensions of organic micro-and nanostructures on metal surfaces. By varying electrochemical desorption conditions, features patterned by dip-pen nanolithography or micro contact printing and made of linear alkanethiols or selenols can be gradually desorbed in a controlled fashion. The process is referred to as electrochemical whittling because the adsorbate desorption is initiated at the exterior of the feature and moves inward as a function of time. The whittling process and adsorbate desorption were studied as a function of substrate morphology, adsorbate head and tail groups, and electrolyte solvent and salt. Importantly, one can independently address different nanostructures made of different adsorbates and effect their miniaturization based upon ajudicious selection of adsorbate, applied potential, and supporting electrolyte. Some of the physical and chemical origins of these observations have been elucidated.

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PubMed2004

Room temperature solution synthesis of monodispersed single-crystalline ZnO nanorods and derived hierarchical nanostructures.

In this work, we report a room temperature wet-chemical approach to synthesize highly regulated, monodispersed ZnO nanorods and derived hierarchical nanostructures. In particular, ZnO has been prepared into single-crystalline conical or prismatic nanorods, and various hierarchical structures such as hexagonally branched, reversed umbrella-type, and cactus-like ZnO nanostructures comprising individual c-oriented nanorods. Depending on the synthetic conditions used, the diameter of nanorods can be controlled with a size down to 10-30 nm, while the aspect ratio can be controlled up to 50-100. Various preparative parameters, such as initial reactant concentrations, solvents, ligands, surfactants, precursor salts, and reaction time, have been systematically examined. Due to slow reactions at room temperature, excellent crystallinity and high morphological yield (100% in most cases) have been achieved via tuning the synthetic parameters. Our photoluminescence and UV measurements also confirm the attained crystal perfection and size uniformity.

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PubMed2004

From rolling ball to complete wetting: the dynamic tuning of liquids on nanostructured surfaces.

In this work, for the first time, a dynamic electrical control of the wetting behavior of liquids on nanostructured surfaces, which spans the entire possible range from the superhydrophobic behavior to nearly complete wetting, has been demonstrated. Moreover, this kind of dynamic control was obtained at voltages as low as 22 V. We have demonstrated that the liquid droplet on a nanostructured surface exhibits sharp transitions between three possible wetting states as a function of applied voltage and liquid surface tension. We have examined experimentally and theoretically the nature of these transitions. The reported results provide novel methods of manipulating liquids at the microscale.

Letter↗
PubMed2004

Nanostructure of open water-channel reversed micelles. I. 1H NMR spectroscopy and molecular modeling.

A recently proposed model for the rodlike reversed micelles of nickel(II) bis(2-ethylhexyl)phosphate is examined in greater detail using 1H NMR spectroscopy and molecular modeling. 1H NMR spectra show that the solubilized water molecules are situated in a different environment compared with the water molecules in classical (AOT) reversed micelles. Geometry optimization and molecular dynamics simulation clearly indicate that the water molecules are not located in the interior core of the reversed micelles, but instead the water molecules exist in compartments or channels in the surface of these rodlike reversed micelles, thereby confirming the open water-channel model of reversed micelles. Molecular modeling was also employed to examine the effects of surfactant molecular structure, cosurfactant, solvent aromaticity, and temperature on the nanostructure of the reversed micellar aggregates. It is significant that molecular modeling provides an interpretation of the nanostructure of reversed micellar aggregates that is consistent with a variety of known experimental observations reported in the liquid/liquid extraction literature. These findings show that the structure of reversed micelles is much richer at the nanoscale level than previously recognized.

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PubMed2005

Probing the local structure and mechanical response of nanostructures using force modulation and nanofabrication.

Nanostructures of self-assembled monolayers (SAMs) are designed and produced using coadsorption and nanografting techniques. Because the structures of these artificially engineered domains are predesigned and well-characterized, a systematic investigation is possible to study the mechanical responses to force modulation under atomic force microscope tips. Force modulation imaging reveals characteristic contrast sensitivity to changes in molecular-level packing, molecule chain lengths, domain boundaries, and surface chemical functionalities in SAMs. By means of actively tuning the driving frequency, the resonances at the tip-surface contact are selectively activated. Therefore, specific surface features, such as the edges of the domains and nanostructures or desired chemical functionalities, can be selectively enhanced in the amplitude images. These observations provide a new and active approach in materials characterization and the study of nanotribology using atomic force microscopy.

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PubMed2004

Synthesis and nanostructure of strong polyelectrolyte brushes in amphiphilic diblock copolymer monolayers on a water surface.

We synthesized an ionic amphiphilic diblock copolymer, poly(hydrogenated isoprene)-b-poly(styrenesulfonic acid) (PIp-h2-b-PSS), by living anionic polymerization, and the nanostructure of its monolayer spread on a water surface was directly investigated by the in situ X-ray reflectivity technique. The monolayer of the diblock copolymer on a water surface had a smooth hydrophobic PIp-h2 layer on water and a "carpet"/polymer brush double layer in a hydrophilic sodium polystyrene sulfonate (PSSNa) layer under the water. The surface pressure dependence and PSSNa chain length dependence of the PIp-h2 layer thickness and the brush nanostructure were quantitatively studied. The effect of salt concentration in the subphase was also investigated in aqueous solutions containing 0-2 M NaCl. The salt effect on monolayer structure occurred at around 0.2 M. The thickness of the PSS brush layer decreased at salt concentrations above 0.2 M, while no structural change was observed below 0.2 M. This critical salt concentration is thought to be related to the balance of ionic concentrations inside the brush and in bulk solution.

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PubMed2004

Nanostructuring of a polymeric substrate with well-defined nanometer-scale topography and tailored surface wettability.

This study demonstrates a simple and highly reproducible method for fabricating well-defined nanostructured polymeric surfaces with aligned nanoembosses or nanofibers of controllable aspect ratios, showing remarkable structural similarity with interesting natural biostructures such as the wing surface of Cicada orni and the leaf surface of Lotus. Our studies on the present biomimetic surfaces revealed that the wetting property of the nanostructured surface of a given chemical composition could be systematically controlled by rendering nanometer-scale roughness. The nanofabricating method we developed can be readily extended to other thermoplastic polymeric materials (e.g., light-emitting polymers, conducting polymers, block copolymers, liquid crystalline polymers), and it could be applied to developing a new generation of optical and electronic devices.

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PubMed2004

Molecular-level control of feature separation in one-dimensional nanostructure assemblies formed by biomolecular nanolithography.

In this paper, we present a convenient and reliable method to organize small gold nanoparticles (d(CORE) = 1.5 nm) into linear chains with precisely controlled interparticle spacing over a range of 1.5-2.8 nm through biomolecular nanolithography. Controlling the feature separations of 1 to a few nanometers with angstrom-level precision is a key requirement in electronic and optical applications of nanostructures to tune the properties of the nanostructures and manipulate the interactions between neighboring structures. Here, chains are formed in solution by utilizing functional-group-directed self-assembly to organize ligand-stabilized gold nanoparticles onto DNA templates. The spacing between neighboring nanoparticles can be controlled chemically and tuned at the molecular level by utilizing nanoparticles possessing ligand shells of varying thickness to achieve angstrom-level resolution at spacings of 1.5, 2.1, and 2.8 nm. The small standard deviation (< or = 20%) in the values for the interparticle spacing illustrates the reproducibility of the approach. Because the interparticle spacing is enforced by the ligand shell rather than the scaffold, the spacing is uniform even in nonlinear sections of the chain. We further show that the assembly process is robust and produces extended linear nanoparticle chains of up to 1 microm in length and a total coverage of > 90%. All structures and interparticle spacings were analyzed using transmission electron microscopy. Our results demonstrate the potential of scaffold-assisted assembly approaches for patterning features with tunable dimensions on a length scale that is important for future applications of these materials in nanoscale electronics and optics.

DNA↗
PubMed2005

Microemulsion-mediated solvothermal synthesis of SrCO3 nanostructures.

The controlled synthesis of SrCO3 nanostructures with different morphologies, such as rodlike, whiskerlike, ellipsoidlike, and spherelike, were successfully achieved by the cationic surfactant-CTAB-microemulsion-mediated solvothermal method. Various comparison experiments showed that fundamental experimental parameters, such as the molar ratio of H2O to CTAB and the concentration of reactants, played important roles in the morphological control of SrCO3 nanostructures. A lower molar ratio of H2O to CTAB favors rodlike products, whereas a higher molar ratio generally leads to the formation of spherical products. A rational mechanism based on fusion, aggregate, and coalescence of microemulsion droplets is proposed for the selective formation of various morphologies. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and field-emission scanning electron microscope (FE-SEM) were used to characterize the products.

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