Controllable Aspect Ratio Research Articles

Colloidal synthesis of homogeneously alloyed CdSexS1−x nanorods with compositionally tunable photoluminescence

Homogenously alloyed CdSe(x)S(1-x) nanorods with controlled aspect ratios are synthesised by a hot-injection colloidal route. The optical absorption and photoluminescence emission are compositionally tunable with chalcogen ratios. The synthetic protocol is sufficiently robust to allow good control of rod aspect ratios, with low polydispersities, suited for their rational assembly into superstructures.

Polymer nanofibers by controllable infiltration of vapour swollen polymers into cylindrical nanopores

We report on a new method for the fabrication of polymer nanofibers based on wetting of cylindrical alumina nanopores with solvent vapour swollen polymers. Solvent annealing is used to swell and soften polymers at room temperature, so that they are allowed to infiltrate into cylindrical nanopores by capillary force. The method of swelling assisted infiltration is capable of eliminating the problem of thermal degradation from the widely used “melt wetting”; in addition, the thus formed nanofibers display well aligned one-dimensional structures and controllable aspect ratios, in contrast to those from “solution wetting”, the morphologies of which are difficult to be controlled. By using the proposed method, several homopolymers including thermo-sensitive polyvinyl chloride (PVC) and conducting poly(3-hexylthiophene) (P3HT) were found to form nanofibers with high aspect ratios. In particular, we have studied the microphase separation behaviour of vapour swollen block copolymers (BCPs) under cylindrical confinement, and found that the intrinsic ordered pattern of BCPs (i.e., in the melt state) would transform into a different structure due to solvent swelling under cylindrical confinement.

Synergistic effects of the aspect ratio of TiO2 nanowires and multi-walled carbon nanotube embedment for enhancing photovoltaic performance of dye-sensitized solar cells

The existence of numerous interfacial boundaries among TiO2 nanoparticles (NPs) accumulated in the photoelectrode layer of dye-sensitized solar cells (DSSCs) hinders the effective transport of photogenerated electrons to an electrode. Therefore, as a replacement for TiO2 NPs, one-dimensional TiO2 nanowires (NWs) can be suggested to provide pathways for fast electron transport by significantly reducing the number of interfacial boundaries. In order to provide direct evidence for the better performance of such longer TiO2 NWs than shorter TiO2 NWs, we examine the effect of the controlled aspect ratio of the TiO2 NWs randomly accumulated in the photoelectrode layer on the photovoltaic performance of DSSCs. It is clearly found that longer TiO2 NWs significantly improve the electron transport by reducing the TiO2/dye/electrolyte interfacial contact resistance. Furthermore, the embedment of multi-walled carbon nanotubes (MWCNTs) as an effective charge transfer medium in longer TiO2 NWs is proposed in this study to promote more synergistic effects, which lead to significant improvements in the photovoltaic properties of DSSCs.

Seed-mediated growth and manipulation of Au nanorods via size-controlled synthesis of Au seeds

Seed-mediated growth of gold (Au) nanorods with highly controllable length, width, and aspect ratio was accomplished via carefully size-controlled synthesis of the original Au seeds. A slow dynamic growth of Au nanoparticle seeds was observed after reduction of the Au salt (i.e., hydrogen tetrachloroaurate (III) hydrate) by sodium borohydride (NaBH4) in the presence of cetyltrimethyl ammonium bromide (CTAB). As such, the size of the Au nanoparticle seeds can therefore be manipulated through control over the duration of the reaction period (i.e., aging times of 2, 8, 48, 72, and 144 h were used in this study). These differently sized Au nanoparticles were subsequently used as seeds for the growth of Au nanorods, where the additions of Au salt, CTAB, AgNO3, and ascorbic acid were employed. Smaller Au nanoparticle seeds obtained via short growth/aging time resulted in Au nanorods with higher aspect ratio and thus longer longitudinal surface plasmon wavelength (LSPW). The larger Au nanoparticle seeds obtained via longer growth/aging time resulted in Au nanorods with lower aspect ratio and shorter LSPW.

Controllable growth of ZnO mesocrystals using a facile electrochemical approach

We developed an electrochemical approach for the synthesis of ZnO mesocrystals with controlled aspect ratios. The ZnO mesocrystals performed 3 stages including nucleation, growth and subsequently self-assembly into a 3D architecture during the growth process. A nonclassical mechanism ‘oriented attachment (OA)’ is responsible for the formation of wurtzite ZnO mesocrystals. Based on the understanding for the growth mechanism, various ZnO mesocrystals with different aspect ratios was synthesized by controlling the current density of deposition. Their cathodoluminescent (CL) spectra showed an enhanced increase with the decrease of aspect ratios (length to diameter). This Letter is potentially helpful for researchers to understand the aggregation-driven formation of complex high-order structured materials.

Note: Axially pull-up electrochemical etching method for fabricating tungsten nanoprobes with controllable aspect ratio

A mathematical model representing the relation between pulling up speed, time and aspect ratio is reported, accordingly the axially pull-up electrochemical etching method for fabricating nanoprobes is proposed. The tungsten probes with predetermined shape and aspect ratio according to the model were successfully produced with this method. Then the probes were installed inside a micromanipulation system to manipulate the carbon nanotubes and measure their current-voltage (I-V) characteristics. The probe fabrication and application experiments demonstrated the reasonability and reliability of the model and method developed in this note.

Design of Anisotropic Co3O4 Nanostructures: Control of Particle Size, Assembly, and Aspect Ratio

Alignment of Co3O4 nanoparticles to yield a range of anisotropic nanostructures with a controlled aspect ratio (1:5 to 1:13) was possible from a single nanostructured precursor (cobalt oxalate) that was obtained by a microemulsion method under controlled kinetic parameters and the temperature of decomposition. The role of the cationic surfactant is critical for obtaining the anisotropic nanostructures of Co3O4 comprising an assembly of Co3O4 nanoparticles (diameter 5–50 nm). The shape (from spherical to highly elongated) and size of individual nanoparticles could be varied by the temperature of decomposition. Higher temperature led to elongated and larger particles. All of the nanorods showed antiferromagnetic behavior with a decrease in Neel temperature (TN) with decrease in the average size of the individual oxide nanoparticles. The variation of TN with the particle diameter follows the finite-size scaling relation. These nanostructures were found to be excellent electrocatalysts for oxygen evolution reaction (OER), with a high current density of 104 mA/cm2.

Mixed Lanthana/Ceria Nanorod-Supported Gold Catalysts for Water–Gas-Shift

We report on the preparation of LaxCe1−xOy nanorods (x = 0−0.5) with uniform aspect ratio, and their evaluation as active supports for Au in the water–gas shift (WGS) reaction. By independently controlling aspect ratio and composition, we demonstrate that the reducibility of the nanorods is determined by the preferential exposure of highly active {110} planes and the La:Ce ratio. After deposition of Au onto these nanorods, the WGS activity is closely correlated with the reducibility of the oxides, indicating that the strong relationship between activity and reducibility previously reported for nanoparticle-based catalysts holds independent of morphology.

Two-step controllable electrochemical etching of tungsten scanning probe microscopy tips

Dynamic electrochemical etching technique is optimized to produce tungsten tips with controllable shape and radius of curvature of less than 10 nm. Nascent features such as "dynamic electrochemical etching" and reverse biasing after "drop-off" are utilized, and "two-step dynamic electrochemical etching" is introduced to produce extremely sharp tips with controllable aspect ratio. Electronic current shut-off time for conventional dc "drop-off" technique is reduced to ∼36 ns using high speed analog electronics. Undesirable variability in tip shape, which is innate to static dc electrochemical etching, is mitigated with novel "dynamic electrochemical etching." Overall, we present a facile and robust approach, whereby using a novel etchant level adjustment mechanism, 30° variability in cone angle and 1.5 mm controllability in cone length were achieved, while routinely producing ultra-sharp probes.

Self-assembled ferromagnetic cobalt/yttria-stabilized zirconia nanocomposites for ultrahigh density storage applications

We report on a low-cost, innovative approach for synthesizing prepatterned, magnetic nanostructures, the shapes and dimensions of which can be easily tuned to meet requirements for next-generation data storage technology. The magnetic nanostructures consist of self-assembled Co nanodots and nanowires embedded in yttria-stabilized zirconia (YSZ) matrices. The controllable size and aspect ratio of the nanostructures allows the selection of morphologies ranging from nanodots to nanowires. Co nanowires show strong shape anisotropy and large remanence at 300 K. In contrast, Co nanodots display minimal effects of magnetocrystalline anisotropy and superparamagnetic relaxation above the blocking temperature. These prepatterned magnetic nanostructures are very promising candidates for data storage technology with an ultrahigh density of 1 terabit in−2 or higher.

Experimental characterization of electrochemical synthesized Fe nanowires for biomedical applications

Fe based nanomaterials have shown extensive application promises in medical diagnosis and treatment due to their biocompatibility. Using template assisted electrodeposition, iron based nanowires with controllable size, aspect ratio, and magnetic anisotropy have been fabricated. In situ synchrotron diffraction technique has been used to reveal the nanowire growth mechanism and provide real time compositional and crystallographic information. Biocompatibility of the nanowires with Rat-2 fibroblast cells has been evaluated and compared with magnetite nanoparticles. Using an external magnetic field, cell manipulation through the use of these magnetic nanowires has been demonstrated.

Catalyst and its diameter dependent growth kinetics of CVD grown GaN nanowires

GaN nanowires were grown using chemical vapor deposition with controlled aspect ratio. The catalyst and catalyst-diameter dependent growth kinetics is investigated in detail. We first discuss gold catalyst diameter dependent growth kinetics and subsequently compare with nickel and palladium catalyst. For different diameters of gold catalyst there was hardly any variation in the length of the nanowires but for other catalysts with different diameter a strong length variation of the nanowires was observed. We calculated the critical diameter dependence on adatoms pressure inside the reactor and inside the catalytic particle. This gives an increasing trend in critical diameter as per the order gold, nickel and palladium for the current set of experimental conditions. Based on the critical diameter, with gold and nickel catalyst the nanowire growth was understood to be governed by limited surface diffusion of adatoms and by Gibbs–Thomson effect for the palladium catalyst.

Rapid controllable high-concentration synthesis and mutual attachment of silver nanowires

A new well-designed two-step injection solution-based method has been proposed to rapidly prepare uniform silver nanowires on a gram-scale. Uniform silver nanowires with different and controllable aspect ratios can be synthesized in high yields. Due to a high chemical reaction rate arising from high AgNO3 concentration, the whole synthetic process of the silver nanowires is rapid and can be controlled within 20 min. The influences of different experimental parameters on the morphologies of silver nanoproducts in such a high-concentration synthesis were investigated. It was found that the addition of an appropriate amount of Cl− ions had significant impact on the formation and the aspect ratios of the silver nanowires. Solution volume effect was also studied. It was shown that our synthetic method strongly favours the synthesis of silver nanowires in large solution volumes. A reasonable formation mechanism of the silver nanowires was put forward based on novel experimental phenomena. The mutual attachment of silver nanowires was observed and was found to play an important role in such high-concentration synthesis of silver nanowires. This facile scalable preparation method of silver nanowires can decrease the synthetic cost greatly and broaden the research and application areas of silver nanowires.

Controlling bubble density in MWNT/polymer nanocomposite foams by MWNT surface modification

Polymer nanocomposite foams are promising substitutes for polymeric foams. Carbon nanotube/polymer nanocomposite foams possess high strength, low density, and can be made conductive. Creating polymer foams with controlled foam morphology is of great importance for controlling foam properties. The foam morphology is influenced by the foaming conditions and filler properties. For carbon nanotube/polymer composite foams, dispersion state and aspect ratio of the carbon nanotubes have been shown to influence the bubble density and bubble size. In the current study, the influence of carbon nanotube surface chemistry on the bubble density of multi-walled carbon nanotube/poly(methyl methacrylate), MWNT/PMMA, nanocomposite foams was investigated. The surface of the MWNTs with controlled aspect ratio was covalently modified with glycidyl phenyl ether (GPE). Surface modified MWNT/PMMA nanocomposite foams were produced using a supercritical carbon dioxide foaming process. At constant MWNT concentration, the bubble density of polymer nanocomposite foams filled with GPE surface modified MWNT was found to be several times higher than that of polymer nanocomposite foams filled with nitric acid treated MWNT. After the MWNTs were modified with GPE, the surface chemistry of the MWNT became the dominant factor in determining the bubble density while the MWNT aspect ratio became less influential.

Controlling aspect ratio of colloidal silver nanorods using response surface methodology

The properties of metallic nanorods vary due to changes in their composition, size and shape, which all depend on the aspect ratio of the nanorods. This work focuses on the optimization of the aspect ratio of silver nanorods using response surface methodology (RSM). Seed-mediated approach, which is the newest method with less difficulty, has been used for the synthesis of silver nanorods. First, silver ions were reduced with sodium borohydride in the presence of sodium citrate dehydrate, as stabilizer. Then, the prepared seeds were added to a solution containing more metal salts, a weak reducing agent (ascorbic acid) and a rod-like micellar template (cetyltrimethylammonium bromide, CTAB). Three factors affecting the aspect ratio were optimized. These factors were ratio of metal seed to metal salt, amount of ascorbic acid and the concentration of CTAB. Mathematical models were proposed for the prediction of transverse and longitudinal plasmon band wavelengths and the optimized condition was obtained for the synthesis of the silver nanorods with highest aspect ratio. It is found that the ratio of metal seeds to metal salt plays a major role in controlling the aspect ratio of the silver nanorods. The optimum values for the factors were found to be 0.06 mL of seed, 7.2 × 10 −3 M of ascorbic acid and 0.05 M of the CTAB in the final solution. The optimum predicted value for the longitudinal plasmon band was 653.9 nm, which is in agreement with the experimental value of 655.0 nm.

The Effect of Nanoseed Concentration on the Aspect Ratio of Gold Nanorod

This paper reports the synthesis of gold nanorod with controlled aspect ratio prepared by varying the concentration of nanoseed addition into the growth solution via seed mediated growth method. In typical process, the gold nanorod with aspect ratio from ca. 1.7 to 3.1 can be successfully obtained. The nanorod seems to be perfect rod morphology instead of bone-like or tear drop structure. Owing to the simplicity in preparing the gold nanorod, the present approach could be used as potential alternative for producing gold nanorod with special properties for used in the currently existing application.

Synthesis and Characterization of Wurtzite ZnTe Nanorods with Controllable Aspect Ratios

ZnTe nanorods with controllable aspect ratios were synthesized using polytellurides a tellurium precursor. The use of polytellurides which allow nucleation and growth at relatively low temperature is the key to formation of wurtzite phase and controlled anisotropic growth along c-axis. The aspect ratio of the resulting ZnTe nanorods was controlled by tuning the temperature that in turn controls the kinetics of the nanocrystal growth. A diameter dependent quantum confinement effect in ZnTe nanorods was observed by UV-vis absorption spectroscopy. Transient absorption measurements show ultrafast charge injection dynamics from ZnTe nanorods, suggesting their strong potential for applications in photocatalysis.

Optical Property of a Colloidal Solution of Platinum and Palladium Nanorods: Localized Surface Plasmon Resonance

The optical properties of Pt and Pd nanorods as a function of aspect ratio were investigated and systematically compared with Au nanorods of similar physical dimensions. The overall optical properties of Pt and Pd nanorods were tailored by controlling aspect ratio and could be tuned from 400 nm up to near-IR spectral window. Their longitudinal mode red shifts as the length increases, whereas the transverse mode blue shifts very slightly. The comparison of observed LSPR modes in Pt and Pd NRs with Au NRs as a function of aspect ratio exhibited that Pd NRs are most sensitive to variations in rod length. The magnitude of longitudinal mode shift per unit change of NR length could be obtained from the slope of a linear fit of each plot. Pd NRs show the largest dependence with a value ca. 6.72 (Δλ/ΔL). For Pt and Au NRs, the slope is ca. 4.23 and 3.11, respectively.

Gram-Scale Synthesis of Silica Nanotubes with Controlled Aspect Ratios by Templating of Nickel-Hydrazine Complex Nanorods

We have developed a robust method for the synthesis of silica nanotubes with controlled aspect ratios on a large scale by templating against rod-like nanocrystals. Crystalline nanorods of a nickel-hydrazine complex are first formed in reverse micelles by surfactant capping on side facets, and subsequent silica coating and selective etching give rise to silica nanotubes of high uniformity and yield. The length of the silica nanotubes is tunable in the range 37-340 nm and can reach as long as micrometers. Control of the length is conveniently achieved by tuning the hydrazine/nickel ratio, which affects the growth kinetics of the nanocrystal templates. The inner diameter of the silica nanotubes can be adjusted in the range 10-20 nm by choosing different surfactants. This method is unique in utilizing reverse micelles as discrete nanoscale reactors for the growth of nanocrystals, allowing for precise control of the features of the nanotubes and opening up new opportunities in the synthesis of novel anisotropic nanomaterials, construction of nanodevices, and potential drug delivery applications.

Controlling Aspect Ratio of Copper Group Nanowire Arrays by Electrochemical Deposition in the Nanopores of AAO

Highly ordered Cu, Ag and Au nanowire arrays with high aspect ratio and highly dense self-supporting nanowire patterns of copper group were successfully prepared using cyclic voltammetry with the assistance of anodic aluminum oxide (AAO) template. The X-ray diffraction (XRD) patterns of the metal nanowries were indexed to the face-centered cubic structure. The field emission scanning electron microscope (FE-SEM) results demonstrated that the length of nanowire could be controlled by changing the electrodepositon conditions. The aspect ratio of nanowire arrays can be tuned.