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PubMed2005

Growth kinetics of one-dimensional KNbO3 nanostructures by hydrothermal processing routes.

The growth kinetics of one-dimensional single-crystalline KNbO(3) nanostructures (nanowires and nanofingers, the latter understood as defective nanowires) prepared by hydrothermal processing routes has been theoretically studied. A model taking into account the cube-based morphology of the nanostructures, their defects as the KOH proportion in the starting solution increases, and the partial depletion of species in the solution at the kink regions is proposed. Such a model allows the morphological evolution of the nanostructures to be successfully reproduced, shedding light on the origin of their highly anisotropic growth.

Journal Article↗
PubMed2005

Synthesis and electrochemical properties of nanostructured LiCoO2 fibers as cathode materials for lithium-ion batteries.

Nanostructured LiCoO2 fibers were prepared by the sol-gel related electrospinning technique using metal acetate and citric acid as starting materials. The transformation from the xerogel fibers to the LiCoO2 fibers and the nanostructure of LiCoO2 fibers have been investigated in detail. The LiCoO2 fibers with 500 nm to 2 mum in diameter were composed of polycrystalline nanoparticles in sizes of 20-35 nm. Cyclic voltammetry and charge-discharge experiments were applied to characterize the electrochemical properties of the fibers as cathode materials for lithium-ion batteries. The cyclic voltammogram curves indicated faster diffusion and migration of Li+ cations in the nanostructured LiCoO2 fiber electrode. In the first charge-discharge process, the LiCoO2 fibers showed the initial charge and discharge capacities of 216 and 182 (mA.h)/g, respectively. After the 20th cycle, the discharge capacity decreased to 123 (mA.h)/g. The X-ray diffraction and high-resolution transmission electron microscopy analyses indicated that the large loss of capacity of fiber electrode during the charge-discharge process might mainly result from the dissolution of cobalt and lithium cations escaping from LiCoO2 to form the crystalline Li2CO3 and CoF2 impurities.

Journal Article↗
PubMed2005

Controlling the morphology of ZnO nanostructures in a low-temperature hydrothermal process.

ZnO nanostructures of different morphologies were grown in a controlled manner using a simple low-temperature hydrothermal technique. Controlling the content of ethylenediamine (soft surfactant) and the pH of the reaction mixture, nanoparticles, nanorods, and flowerlike ZnO structures could be synthesized at temperatures 80-100 degrees C with excellent reproducibility. High-resolution electron microscopy revealed the well crystalline nature of all the nanostructures with preferential growth along the [002] direction for linear structures. Photoluminescence spectra of the as-grown nanostructures revealed oxygen-vacancy-related defects in them, which could be reduced by air annealing at 250 degrees C. Possible mechanisms for the variation of morphology with synthesis parameters are discussed.

Journal Article↗
PubMed2005

Hydrogen bonding effects on the surface structure and photoelectrochemical properties of nanostructured SnO2 electrodes modified with porphyrin and fullerene composites.

Hydrogen bonding effects on surface structure, photophysical properties, and photoelectrochemistry have been examined in a mixed film of porphyrin and fullerene composites with and without hydrogen bonding on indium tin oxide and nanostructured SnO2 electrodes. The nanostructured SnO2 electrodes modified with the mixed films of porphyrin and fullerene composites with hydrogen bonding exhibited efficient photocurrent generation compared to the reference systems without hydrogen bonding. Atomic force microscopy, infrared reflection absorption, and ultraviolet-visible absorption spectroscopies and time-resolved fluorescence lifetime and transient absorption spectroscopic measurements disclosed the relationship between the surface structure and photophysical and photoelectrochemical properties relating to the formation of hydrogen bonding between the porphyrins and/or the C60 moieties in the films on the electrode surface. These results show that hydrogen bonding is a highly promising methodology for the fabrication of donor and acceptor composites on nanostructured semiconducting electrodes, which exhibit high photoelectrochemical properties.

Electrochemistry↗
PubMed2005

Hydrothermal synthesis and pseudocapacitance properties of MnO2 nanostructures.

The effect of varying the hydrothermal time to synthesize manganese oxide (MnO(2)) nanostructures was investigated along with their influence on structural, morphological, compositional, and electrochemical properties in supercapacitor electrode materials. XRD and TEM studies showed that the MnO(2) prepared in shorter hydrothermal dwell time was a mixture of amorphous and nanocrystalline particles, and there was an evolution of crystallinity of the nanostructures as the dwell time increased from 1 to 18 h. Interestingly, SEM, TEM, and HRTEM revealed a variety of structures ranging from nanostructured surface with a distinct platelike morphology to nanorods depending upon the hydrothermal reaction time employed during the preparation of the manganese oxide: increasing the amount of individual nanorods in the materials prepared with longer hydrothermal reaction time. The surface area of the synthesized nanomaterials varied from 100 to 150 m(2)/g. Electrochemical properties were evaluated using cyclic voltammetry (CV) and galvanostatic charge-discharge studies, and the capacitance values were in the range 72-168 F/g depending upon synthesis conditions. The formation mechanism of the nanorods and their impact on the specific capacitance were discussed in detail.

Journal Article↗
PubMed2006

Morphology of nanostructured platinum in mesoporous materials-effect of solvent and intrachannel surface.

An intrachannel surface of host silica was functionalized through the reaction of surface silanol groups with silanes to generate a monolayer of positively charged groups, and together with the strongly adsorbed and negatively charged PtCl6(2-), resulting in nanostructured platinum-mesoporous silica composites. The highly dispersed Pt nanoparticles and nanonetworks are fabricated from (CH3O)3Si(CH2)3N(CH3)3+Cl- functionalized mesoporous silica MCM-48 with H2PtCl6 in ethanol and water solvent, and characterized by PXRD, XAS, TEM, and N2 adsorption. The solvent of H2PtCl6 solution is found to affect the mobility of Pt precursors and the resulting morphology of nanostructured metallic Pt. The effect of the intrachannel surface properties on the incorporation and the morphology of nanostructured Pt on the deposition of Pt(NH3)4Cl2 and H2PtCl6 on Al-doped or C-coated mesoporous silica MCM-41 is also studied relative to that on pure silica MCM-41.

Journal Article↗
PubMed2006

Linear nanostructure formation of aldehydes by self-directed growth on hydrogen-terminated silicon(100).

The self-directed growth of organic molecules on silicon surfaces allows for the rapid, parallel production of hybrid organic-silicon nanostructures. In this work, the formation of benzaldehyde- and acetaldehyde-derived nanostructures on hydrogen-terminated H-Si(100)-2x1 surface is studied by scanning tunneling microscopy in ultrahigh vacuum and by quantum mechanical methods. The reaction is a radical-mediated process that binds the aldehydes, through a strong Si-O covalent bond, to the surface. The aldehyde nanostructures are generally composed of double lines of molecules. Two mechanisms that lead to double line growth are elucidated.

Journal Article↗
PubMed2006

Gold nanostructures from cube-shaped crystalline intermediates.

Conventional bottom-up approaches for building nanostructures rely on the ability to synthesize nanoparticles of different shapes and sizes in a controlled manner that are then assembled to produce useful structures. Here, we present an alternate approach for producing nanostructures based on the formation of a crystalline intermediate in which the metal ion can be reduced in a controlled manner. Partial reduction of HAuCl4 by a long-chain amine results in the formation of a cube-shaped crystalline intermediate in which Au is present in a +1 oxidation state. By control of the nucleation of the metal in the intermediate, a variety of nanostructures can be synthesized. Here, we present results on the formation of superlattices, hollow cubes, nanotubes, and extended hollow structures starting from the intermediate. Direct evidence for the formation of metal within the intermediate by in situ electron-beam-induced reduction in the transmission electron microscope is presented.

Journal Article↗
PubMed2006

Aqueous solution route to high-aspect-ratio zinc oxide nanostructures on indium tin oxide substrates.

High-aspect-ratio ZnO nanowires and nanotubes are formed on indium tin oxide (ITO) substrates using a three-step route at low temperatures. The three steps, including successive ionic layer absorption and reaction (SILAR) deposition of the ZnO seed layer, hydrothermal annealing of the seed layer, and chemical bath deposition (CBD) of the one-dimensional (1D) ZnO nanostructures, are all conducted in aqueous solutions at temperatures below 120 degrees C. Both the hydrothermal annealing of the SILAR seed layer and the low-concentration precursor solution employed in the CBD process are crucial in order to synthesize the uniform and high-aspect-ratio ZnO nanostructures on the ITO substrate. TEM analyses reveal that both the nanowire and the nanotube possess the single-crystal structure and are grown along [001] direction. Room-temperature cathodoluminescence spectrum of the 1D ZnO nanostructures shows a sharp ultraviolet emission at 375 nm and a broad green-band emission.

Journal Article↗
PubMed2006

Dimensionality dependence of optical properties and quantum confinement effects of hydrogenated silicon nanostructures.

The excited state properties of linear, planar, and spherical hydrogenated silicon nanostructures are studied systematically with use of a time-dependent Hartree-Fock (TDHF) approach with a semiempirical Hamiltonian. The calculated optical gaps decrease significantly from linear, planar, to spherical silicon structures, showing that the optical gap is dimensionality dependent and hence it can be varied by solely managing the shape of the nanostructures. Remarkably, the calculated exciton sizes of the lowest dipole-allowed excited states for both silicon chains and planes are approximately 26 A, revealing that the quantum confinement effect should be significantly enhanced when the sizes of silicon nanostructures are smaller than this value but not dependent on the dimensionality. A similar trend is also observed for hydrogenated silicon spherical clusters.

Journal Article↗
PubMed2006

Phase transformations in bulk nanostructured potassium niobiosilicate glasses.

In potassium niobiosilicate (KNS) glasses, nanostructuring can be driven and controlled by thermal treatments at the glass transition temperature and/or by modulation of the chemical composition. The tight relationship between nanostructure and nonlinear optical properties suggests these bulk nanomaterials as an appealing route to nanophotonics. The focus of this paper is placed on assessing the phase transformations which occur in these materials upon annealing at the glass transition temperature and subsequent heating. High-temperature resolved X-ray diffraction (HTXRD) and high-resolution transmission electron microscopy (HRTEM) experiments are integrated with previously published results for in-depth insight. It will be shown that nanostructuring evolves from nucleation of niobium-rich nanocrystals, which are up to 20 nm large, uniformly distributed in the matrix bulk, and metastable. Formation kinetics as well as phase transformation of the nanocrystals are determined by the glass composition. Depending on it, nanocrystal nucleation can be preceded or not by phase separation, and the nanocrystals' phase transition can be of first or second order.

Journal Article↗
PubMed2006

Molybdenum trioxide nanostructures: the evolution from helical nanosheets to crosslike nanoflowers to nanobelts.

MoO(3) nanostructures with different morphologies, such as helical nanosheets, crosslike nanoflowers, and nanobelts, have been synthesized on a large scale by an environmentally friendly chemical route. The evolution process from helical nanosheets to crosslike nanoflowers to nanobelts is observed for the first time. The influences of reaction time and the molar ratio of molybdenum and H(2)O(2) on the morphologies of MoO(3) nanostructures have been investigated. The synthetic process is environmentally friendly and may be extended to synthesize nanostructures of other metal (W, Ti, and Cr) oxides.

Journal Article↗
PubMed2004

Anomalous IR properties of nanostructured films created by square wave potential on an array of Pt microelectrodes: an in situ microscope FTIRS study of CO adsorption.

Thin films of different nanostructures on an array of nine Pt microelectrodes were prepared by applying a square wave potential treatment for different times (tau). It has been measured from the cyclic voltammetric studies that the relative surface roughness of the films was increased slightly and reached a maximal value of about 2.5. SEM studies demonstrated that with the increase of tau, the growth of island-shaped Pt crystallites on the films led to the formation of plumelike crystallites that can reach about 2-3.5 microm in length when tau exceeded 70 min. In situ microscope FTIR reflection spectroscopic studies illuminated that CO adsorbed on the array yielded different anomalous IR features. With the increase of tau, the direction of the CO L band (linearly bonded CO) was transformed from the negative-going direction (normal IR adsorption) to bipolar (Fano-like spectral line shape) and finally to the positive-going direction (abnormal IR adsorption). The intensity of the CO L band was enhanced significantly and a maximal enhancement factor of about 33 was measured when tau was 40 min; the center of the CO L band and the Stark tuning rate also showed regular changes. This study demonstrated that specific nanostructures of Pt thin films can be prepared through a square wave potential treatment for different times and revealed the intrinsic relationship between anomalous IR properties and surface nanostructures of the thin films.

Journal Article↗
PubMed2006

Nanolithographic write, read, and erase via reversible nanotemplated nanostructure electrodeposition on alkanethiol-modified Au(111) in an aqueous solution.

A write, read, and erase nanolithographic method, combining in situ electrodeposition of metal nanostructures with atomic force microscopy (AFM) nanoshaving of a 1-hexadecanethiol (HDT) self-assembled monolayer (SAM) on Au(111) in an aqueous solution, is reported. The AFM tip defines the local positioning of nanotemplates via the irreversible removal of HDT molecules. Nanotemplates with lateral dimensions as narrow as 25 nm are created. The electroactive nanotemplates determine the size, shape, and position of the metal nanostructures. The potential applied to the substrate controls the amount of metal deposited and the kinetics of the deposition. Metal nanostructures can be reversibly and repeatedly electrodeposited and stripped out of the nanotemplates by applying appropriate potentials.

Journal Article↗
PubMed2006

Fabrication and investigation of nanostructures on transition metal dichalcogenide surfaces using a scanning tunneling microscope.

Nanometer-scale holes have been fabricated on the surfaces of the semiconducting transition metal dichalcogenides (TMDCs) molybdenum ditelluride (MoTe2) and molybdenum disulfide (MoS2) by applying voltage pulses from the tip of a scanning tunneling microscope (STM) operating in ultrahigh vacuum (UHV). It was found that the tip geometry (tip shape and sharpness) influences the formation and structure of the atomic-scale nanostructures. Threshold voltage ranges for the surface modification of MoTe2 (3.0 +/- 0.3 V) and MoS2 (3.4 +/- 0.3 V) were determined. Negative sample voltage pulses applied to a p-type MoTe2 surface produced much larger and deeper nanometer-scale holes when compared with those produced by positive voltage pulses. The existence of threshold voltages and the pulse polarity dependence of nanostructure fabrication suggests that an electric field evaporation mechanism is applicable. Support for this mechanism was obtained by nanostructuring metallic TMDC NbSe2, where both the produced features and the threshold voltages (3.0 +/- 0.3 V) were similar for both positive and negative voltage pulses.

Journal Article↗
PubMed2006

Simultaneous removal of thiolated membrane proteins resulting in nanostructured lipid layers.

Self-organization of membrane-embedded peptides and proteins causes the formation of lipid mesostructures in the membranes. One example is purple membranes (PM), which consist of lipids and bacteriorhodopsin (BR) as the only protein component. The BRs form a hexagonal crystalline lattice. A complementary structure is formed by the lipids. Employing BR and PM as an example, we report a method where major parts of the mesoscopic self-assembled protein structures can be extracted from the lipid bilayer membrane. A complementary lipid nanostructure remains on the substrate. To remove such a large number of thiolated proteins simultaneously by applying a mechanical force, they are first reacted at physiological conditions with gold nanoparticles, and then a thin gold film is sputtered onto them that fuses with the gold nanoparticles forming a uniform layer, which finally can be lifted off. In this step, all of the previously gold-labeled proteins are pulled out of the membrane simultaneously. A stable lipid nanostructure is obtained on the mica substrate. Its stability is due to either binding of the lipids to the substrate through ionic bonds or to enough residual proteins to stabilize the lipid nanostructure against reorganization. This method may be applied easily and efficiently wherever thiolated proteins or peptides are employed as self-assembling and structure-inducing units in lipid membranes.

Bacteriorhodopsins↗
PubMed2006

Thermoresponsive behavior of poly(n-isopropylacrylamide) hydrogels containing gold nanostructures.

We report the changes in the structure and thermoresponsive behavior of poly(N-isopropylacrylamide) (PNIPAm) hydrogels when gold nanostructures are synthesized in situ within the hydrogel matrix. Cross-linked PNIPAm hydrogels were synthesized using NIPAm and 0.00-3.50% (w/w versus NIPAm) of N,N'-methylenebisacrylamide (MBAm) and/or N,N'-cystaminebisacrylamide (CBAm) as cross-linking agents. The hydrogels were soaked in potassium tetrachloroaurate to introduce gold ions. The hydrogels containing Au3+ were then immersed in a sodium borohydride solution to reduce the gold ions. Infrared spectroscopy, UV-visible spectroscopy, and equilibrium swelling were used to examine the structural/physical differences between gels of different compositions; UV-visible spectroscopy and mass measurements were used to observe the kinetics and thermodynamics of the hydrogel volume phase transition. These studies revealed several differences in the physical characteristics and thermoresponsive behavior of hydrogels based on cross-linker identity and the presence or absence of gold nanostructures. Hydrogels with gold nanostructures and high CBAm and low MBAm content have equilibrium swelling masses 3-20 times their native analogues. In comparison, gold-containing hydrogels with high MBAm and low CBAm content have swelling masses that are equal to their native analogues. Additionally, the gold-containing PNIPAm hydrogels cross-linked with only CBAm have a deswelling temperature of approximately 40 degrees C, approximately 8 degrees C above the samples cross-linked with only MBAm. Varying the CBAm content and introducing gold enables tuning of the deswelling temperature.

Journal Article↗
PubMed2005

Facile scheme for fabricating solid-state nanostructures using e-beam lithography and solution precursors.

We demonstrate a facile approach for site-specific fabrication of organic, inorganic, and hybrid solid-state nanostructures through a novel combination of electron-beam lithography (eBL) and spin coating of liquid and sol-gel precursors, termed soft eBL. By using eBL patterned resists as masks in combination with a low cost process such as spin coating, directed growth of nanostructures with controlled dimensions is achieved without the need for the costly and difficult process step of etching ceramics. The highly versatile nature of the scheme is highlighted through the fabrication of nanostructures of a variety of materials such as ferroelectric, optoelectronic, and conducting polymeric materials at different length scales and spatial densities on a multitude of substrates.

Journal Article↗