Nanowire Array Composites

Zinc Oxide Nanostructure Deposition into Sub-5 nm Vertical Mesopores in Silica Hard Templates by Atomic Layer Deposition

Nanostructures synthesised by hard-templating assisted methods are advantageous as they retain the size and morphology of the host templates which are vital characteristics for their intended applications. A number of techniques have been employed to deposit materials inside porous templates, such as electrodeposition, vapour deposition, lithography, melt and solution filling, but most of these efforts have been applied with pore sizes higher in the mesoporous regime or even larger. Here, we explore atomic layer deposition (ALD) as a method for nanostructure deposition into mesoporous hard templates consisting of mesoporous silica films with sub-5 nm pore diameters. The zinc oxide deposited into the films was characterised by small-angle X-ray scattering, X-ray diffraction and energy-dispersive X-ray analysis.

Preparation of Alumina Membrane Filters with Framework Structures by Al Anodization

Alumina membrane filters consisting of a thin filter layer (200-700 nm in thickness) supported by a framework (30-100 μm in thickness) were fabricated on the basis of the anodization of an Al substrate. The solution permeability of alumina membrane filters with tubular pores of uniform size and shape varies with membrane thickness; therefore, membrane filters with a filter layer thickness of less than 1 μm showed high solution permeability. Although the filter layer is thin, it is reinforced by a thick framework, enabling the fabrication of large freestanding membrane filters (2.5 × 2 cm2 in size). The pore size of the anodic porous alumina formed by the anodization of Al can be controlled in the range from 10 nm to ca. 2 μm by varying the preparation conditions. The alumina membrane obtained by this method is therefore useful as a membrane filter that can efficiently separate particles of various sizes, including microplastics, viruses, and bacteria. This method enables the fabrication of membrane filters with uniform pore size, high solution permeability, and sufficient mechanical strength for handling as freestanding membranes, which have been difficult to achieve with the reported preparation processes for membrane filters. The obtained membrane filters with frameworks can be applied to efficient microfiltration.

Preparation of ordered nanohole array structures by anodization of prepatterned Cu, Zn, and Ni

Ordered nanohole array structures prepared by anodization of prepatterned metals.

Efficient fabrication of ordered alumina through-hole membranes using a TiO2 protective layer prepared by atomic layer deposition

Ordered alumina through-hole membrane with an interhole distance of 1 μm.

Preparation of Ordered Anodic Porous Alumina with Single-Nanometer-Order-Size Holes by Atomic Layer Deposition

Ordered anodic porous alumina with controlled-size holes on the order of a single-nanometer scale was obtained by the atomic layer deposition (ALD) of Al₂O₃ or TiO₂. The thin metal oxide layers of uniform thickness were formed successfully on the inner wall of the hole of the ordered anodic porous alumina with high aspect ratios by ALD. The hole diameter of the ordered anodic porous alumina could be controlled precisely by adjusting the number of cycles of ALD. The obtained anodic porous alumina with an ordered hole arrangement of reduced holes will be applied to various application fields requiring uniform-sized holes on the order of a single-nanometer scale, such as the starting material for preparing various types of quantum effect devices.

Self-ordered anodic porous alumina with inter-hole spacing over 1.5 μm

Self-ordered anodic porous alumina with inter-hole spacing of 1.8 μm.

Unconventional Defects in a Quasi-One-Dimensional KMn6Bi5

Quasi-one-dimensional (Q1D) structures comprising a compact array of indefinitely long 1D nanowires (NWs) are scarce, especially in a bulk device-scale showing metallic and semiconducting behaviors along different axes. Unlike plentiful observations of nature of defects in three-/two-dimensional materials, there is a notable paucity of such reports in Q1D. Herein we present unconventional motific defects and their properties in a bulk Q1D KMn₆Bi₅ crystal, in which an individual NW motif acts as one body. We discovered motific inter- and intra-NW defects, such that a linear set of 1D motifs are displaced. Stress generates two domains with altered inter-NW spacings and a Bi-Mn solid solution grain, leading to a local bulk plasmon shift due to NW array reconfiguration as well as atomic rearrangement. The observation of such exotic defects and associated phenomena in this Q1D may provide guidance on overall defect mechanism in other Q1D systems and their collective anisotropic properties.

Fabrication of Silica Nanoparticle Monolayer Arrays Using an Anodic Aluminum Oxide Template

Non-close-packed (NCP) silica nanoparticle monolayer arrays (SNMA) on ordered porous anodic aluminum oxide (AAO) templates were fabricated for the first time by a novel two-step spin-coating technique. The obtained NCP-SNMA-AAO was composed of silica nanoparticles (average primary particle size of 440 nm) and well-organized nanopores on the AAO substrates. NCP-SNMA-AAO with a supporting ratio of 87% silica nanoparticles showed a hydrophilic surface (water contact angle of 51.0°), while the original AAO substrate shows a hydrophobic surface (water contact angle of 107.9°). The maximum coefficient of static friction was decreased by 29% (0.327 → 0.233). The coefficient of dynamic friction was also decreased by 20% (0.281 → 0.226). We found that controlling the silica supporting ratio using the two-step spin-coating technique is an effective approach for surface modification of an AAO substrate.

Thermoelectrics of Nanowires

The field of thermoelectric research has undergone a renaissance and boom in the past two and a half decades, largely fueled by the prospect of engineering electronic and phononic properties in nanostructures, among which semiconductor nanowires (NWs) have served both as an important platform to investigate fundamental thermoelectric transport phenomena and as a promising route for high thermoelectric performance for diverse applications. In this Review, we provide a comprehensive look at various aspects of thermoelectrics of NWs. We start with a brief introduction of basic thermoelectric phenomena, followed by synthetic methods for thermoelectric NWs and a summary of their thermoelectric figures of merit (ZT). We then focus our discussion on charge and heat transport, which dictate thermoelectric power factor and thermal conductivity, respectively. For charge transport, we cover the basic principles governing the power factor and then review several strategies using NWs to enhance it, including earlier theoretical and experimental work on quantum confinement effects and semimetal-to-semiconductor transition, surface engineering and complex heterostructures to enhance the carrier mobility and power factor, and the recent emergence of topological insulator NWs. For phonon transport, we broadly categorize the work on thermal conductivity of NWs into five different effects: classic size effect, acoustic softening, surface roughness, complex NW morphology, and dimensional crossover. Finally, we discuss the integration of NWs for device applications for thermoelectric power generation and cooling. We conclude our review with some outlooks for future research.

One-Step, Catalyst-Free, Scalable in Situ Synthesis of Single-Crystal Aluminum Nanowires in Confined Graphene Space

Nanowires have a wide range of applications, such as transparent electrodes, Li-ion battery anodes, light-emitting diodes, solar cells, and electronic devices. Currently, aluminum (Al) nanowires can be synthesized by thermally induced substitution of germanium (Ge) nanowires, chemical vapor deposition on other metal substrates, and template-assisted growth methods. However, there are still challenges in fabricating extremely high-purity nanowires, large-scale manufacturing, and simplifying the synthesis process and conditions. Here, we report for the first time that single-crystal Al nanowires can be one-step, in situ synthesized on a reduced graphene oxide (RGO) substrate on a large scale without using any catalysts. Through a simple high temperature treatment process, commercial micro-sized Al powders in RGO film were transformed into a single-crystal Al nanowire with an average length of 1.2 μm and an average diameter of 18 nm. The possible formation mechanism of the single-crystal Al nanowires is proposed as follows: hot aluminum atoms eject from the pristine aluminum/alumina core/shell structure of Al powders when they build up enough energy from the thermal stress under high temperature and confined space conditions, which is supported by both experimental and computational results. The method introduced here can be extended to allow the synthesis of one-dimensional highly reactive materials, like alkali metal nanowires, in confined spaces.

Bifunctional Tellurium Nanodots for Photo-Induced Synergistic Cancer Therapy

Elemental tellurium (Te) nanoparticles are increasingly important in a variety of applications such as thermoelectricity, photoconductivity, and piezoelectricity. However, they have been explored with limited success in their biomedical use, and thus a tremendous challenge still exists in the exploration of Te nanoparticles that can treat tumors as an effective anticancer agent. Here, we introduce bifunctional Te nanodots with well-defined nanostructure as an effective anticancer agent for photo-induced synergistic cancer therapy with tumor ablation, which is accomplished using hollow albumin nanocages as a nanoreactor. Under near-infrared light irradiation, Te nanodots can produce effective photothermal conversion, as well as highly reactive oxygen species such as •O₂^- and dismutated •OH via a type-I mechanism through direct electron transfer, thereby triggering the potent in vivo hyperthermia and simultaneous intracellular reactive oxygen species at tumors. Moreover, Te nanodots possess perfect resistance to photobleaching, effective cytoplasmic translocation, preferable tumor accumulation, as well as in vivo renal elimination, promoting severe photo-induced cell damage and subsequent synergy between photothermal and photodynamic treatments for tumor ablation. These findings provide the insight of elemental Te nanodots for biomedical research.

Tunable Magneto-Optical Kerr Effects of Nanoporous Thin Films

Magnetoplasmonics, combining magnetic and plasmonic functions, has attracted increasing attention owing to its unique magnetic and optical properties in various nano-architectures. In this work, Ag, CoFeB and ITO layers are fabricated on anodic aluminum oxide (AAO) porous films to form hybrid multi-layered nanoporous thin films by magnetron sputtering deposition process. The designed nanostructure supports localized surface plasmon resonance (LSPR) and tunable magneto-optical (MO) activity, namely, the sign inversion, which can be controlled by AAO porous film geometry (pore diameter and inter-pore spacing) flexibly. The physical mechanism of this special MO phenomena is further analyzed and discussed by the correlation of Kerr rotation and electronic oscillations controlled by the surface plasmon resonance that is related to the nanoporous structure.

Template-based syntheses for shape controlled nanostructures

A variety of nanostructured materials are produced through template-based synthesis methods, including zero-dimensional, one-dimensional, and two-dimensional structures. These span different forms such as nanoparticles, nanowires, nanotubes, nanoflakes, and nanosheets. Many physical characteristics of these materials such as the shape and size can be finely controlled through template selection and as a result, their properties as well. Reviewed here are several examples of these nanomaterials, with emphasis specifically on the templates and synthesis routes used to produce the final nanostructures. In the first section, the templates have been discussed while in the second section, their corresponding synthesis methods have been briefly reviewed, and lastly in the third section, applications of the materials themselves are highlighted. Some examples of the templates frequently encountered are organic structure directing agents, surfactants, polymers, carbon frameworks, colloidal sol-gels, inorganic frameworks, and nanoporous membranes. Synthesis methods that adopt these templates include emulsion-based routes and template-filling approaches, such as self-assembly, electrodeposition, electroless deposition, vapor deposition, and other methods including layer-by-layer and lithography. Template-based synthesized nanomaterials are frequently encountered in select fields such as solar energy, thermoelectric materials, catalysis, biomedical applications, and magnetowetting of surfaces.

Nonequilibrium microstructures for Ag-Ni nanowires

This work illustrates that a variety of nanowire microstructures can be obtained either by controlling the nanowire formation kinetics or by suitable thermal processing of as-deposited nanowires with nonequilibrium metastable microstructure. In the present work, 200-nm diameter Ag-Ni nanowires with similar compositions, but with significantly different microstructures, were electrodeposited. A 15 mA deposition current produced nanowires in which Ag-rich crystalline nanoparticles were embedded in a Ni-rich amorphous matrix. A 3 mA deposition current produced nanowires in which an Ag-rich crystalline phase formed a backbone-like configuration in the axial region of the nanowire, whereas the peripheral region contained Ni-rich nanocrystalline and amorphous phases. Isothermal annealing of the nanowires illustrated a phase evolution pathway that was extremely sensitive to the initial nanowire microstructure.

Hard template synthesis of metal nanowires

Metal nanowires (NWs) have attracted much attention because of their high electron conductivity, optical transmittance, and tunable magnetic properties. Metal NWs have been synthesized using soft templates such as surface stabilizing molecules and polymers, and hard templates such as anodic aluminum oxide, mesoporous oxide, carbon nanotubes. NWs prepared from hard templates are composites of metals and the oxide/carbon matrix. Thus, selecting appropriate elements can simplify the production of composite devices. The resulting NWs are immobilized and spatially arranged, as dictated by the ordered porous structure of the template. This avoids the NWs from aggregating, which is common for NWs prepared with soft templates in solution. Herein, the hard template synthesis of metal NWs is reviewed, and the resulting structures, properties and potential applications are discussed.

Anemone-like nanostructures for non-lithographic, reproducible, large-area, and ultra-sensitive SERS substrates

The melt-infiltration technique enables the fabrication of complex nanostructures for a wide range of applications in optics, electronics, biomaterials, and catalysis. Here, anemone-like nanostructures are produced for the first time under the surface/interface principles of melt-infiltration as a non-lithographic method. Functionalized anodized aluminum oxide (AAO) membranes are used as templates to provide large-area production of nanostructures, and polycarbonate (PC) films are used as active phase materials. In order to understand formation dynamics of anemone-like structures finite element method (FEM) simulations are performed and it is found that wetting behaviour of the polymer is responsible for the formation of cavities at the caps of the structures. These nanostructures are examined in the surface-enhanced-Raman-spectroscopy (SERS) experiment and they exhibit great potential in this field. Reproducible SERS signals are detected with relative standard deviations (RSDs) of 7.2-12.6% for about 10,000 individual spots. SERS measurements are demonstrated at low concentrations of Rhodamine 6G (R6G), even at the picomolar level, with an enhancement factor of ∼10(11). This high enhancement factor is ascribed to the significant electric field enhancement at the cavities of nanostructures and nanogaps between them, which is supported by finite difference time-domain (FDTD) simulations. These novel nanostructured films can be further optimized to be used in chemical and plasmonic sensors and as a single molecule SERS detection platform.

Diamond nanowires: fabrication, structure, properties, and applications

C(sp(3) )C-bonded diamond nanowires are wide band gap semiconductors that exhibit a combination of superior properties such as negative electron affinity, chemical inertness, high Young's modulus, the highest hardness, and room-temperature thermal conductivity. The creation of 1D diamond nanowires with their giant surface-to-volume ratio enhancements makes it possible to control and enhance the fundamental properties of diamond. Although theoretical comparisons with carbon nanotubes have shown that diamond nanowires are energetically and mechanically viable structures, reproducibly synthesizing the crystalline diamond nanowires has remained challenging. We present a comprehensive, up-to-date review of diamond nanowires, including a discussion of their synthesis along with their structures, properties, and applications.

Isoporous micro/nanoengineered membranes

Isoporous membranes are versatile structures with numerous potential and realized applications in various fields of science such as micro/nanofiltration, cell separation and harvesting, controlled drug delivery, optics, gas separation, and chromatography. Recent advances in micro/nanofabrication techniques and material synthesis provide novel methods toward controlling the detailed microstructure of membrane materials, allowing fabrication of membranes with well-defined pore size and shape. This review summarizes the current state-of-the-art for isoporous membrane fabrication using different techniques, including microfabrication, anodization, and advanced material synthesis. Various applications of isoporous membranes, such as protein filtration, pathogen isolation, cell harvesting, biosensing, and drug delivery, are also presented.

Nanomaterials: a membrane-based synthetic approach

Materials with nanoscopic dimensions not only have potential technological applications in areas such as device technology and drug delivery but also are of fundamental interest in that the properties of a material can change in this regime of transition between the bulk and molecular scales. In this article, a relatively new method for preparing nanomaterials, membrane-based synthesis, is reviewed. This method entails synthesis of the desired material within the pores of a nanoporous membrane. Because the membranes used contain cylindrical pores of uniform diameter, monodisperse nanocylinders of the desired material, whose dimensions can be carefully controlled, are obtained. This "template" method has been used to prepare polymers, metals, semiconductors, and other materials on a nanoscopic scale.

In situ synthesis and alignment of Au nanoparticles within hexagonally packed cylindrical domains of diblock copolymers in bulk

We present a simple method to prepare hexagonally packed metallic nanocylinders based on gold nanoparticles embedded in a copolymeric matrix. The gold nanoparticles are generated selectively within the P4VP-rich cylindrical domains of a polystyrene-b-poly-4-vinylpyridine (PS-b-P4VP) diblock copolymer. In order to achieve this selectivity, a gold precursor (HAuCl4) is coupled to the pyridine blocks of a spherical PS327-b-P4VP27 block copolymer. In consequence, the hybrid block copolymer is able to self-assemble in a hexagonally packed cylinders morphology. The application of mechanical oscillatory shear improved markedly the alignment of these nanocylinders, while simultaneously the gold precursor was reduced in situ into gold nanoparticles. Following rheological characterization in the linear viscoelastic regime, a set of alignment parameters were comprehensively selected and checked with a series of transmission electron microscopy (TEM) micrographs. An optimal temperature of alignment was found after systematic evaluation of samples sheared at different temperatures. The block copolymer exhibited an increase in the domain period as a consequence of chain rearrangements around the newly formed gold nanoparticles. The hexagonally packed morphology was preserved, and under the optimal conditions single grain sizes showed significant improvement to macroscale order in comparison to nonaligned samples. In contrast to current multistep lithographic techniques, the present method constitutes a simple path to produce three-dimensional organic-inorganic conductive nanowires with periodicities at the macroscopic level.

Fast electrochemistry of conductive polymer nanotubes: synthesis, mechanism, and application

Conductive polymers exhibit several interesting and important properties, such as metallic conductivity and reversible convertibility between redox states. When the redox states have very different electrochemical and electronic properties, their interconversion gives rise to changes in the polymers' conformations, doping levels, conductivities, and colors, useful attributes if they are to be applied in displays, energy storage devices, actuators, and sensors. Unfortunately, the rate of interconversion is usually slow, at best on the order a few hundred milliseconds, because of the slow transport of counterions into the polymer layer to balance charge. This phenomenon is one of the greatest obstacles toward building rapidly responsive electrochemical devices featuring conductive polymers. One approach to enhancing the switching speed is decreasing the diffusion distance for the counterions in the polymer. We have found that nanotubular structures are good candidates for realizing rapid switching between redox states because the counterions can be readily doped throughout the thin nanotube walls. Although the synthesis of conductive polymer nanotubes can be performed using electrochemical template synthesis, the synthetic techniques and underlying mechanisms controlling the nanotube morphologies are currently not well established. We begin this Account by discussing the mechanisms for controlling the structures from hollow nanotubes to solid nanowires. The applied potential, monomer concentration, and base electrode shape all play important roles in determining the nanotubes' morphologies. A mechanism based on the rates of monomer diffusion and reaction allows the synthesis of nanotubes at high oxidation potentials; a mechanism dictated by the base-electrode shape dominates at very low oxidation potentials. The structures of the resulting conductive polymer nanotubes, such as those of poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole, can be characterized using scanning electron microscopy and transmission electron microscopy. We also discuss these materials in terms of their prospective use in nanotube-based electrochemical devices. For example, we describe an electrochromic device incorporating PEDOT nanotubes that exhibits an ultrafast color switching rate (<10 ms) and strong coloration. In addition, we report a supercapacitor based on PEDOT nanotubes that can provide more than 80% of its own energy density, even at power demands as high as 25 kW/kg.

Using cylindrical domains of block copolymers to self-assemble and align metallic nanowires

Block copolymer thin films can be used as soft templates for a wide range of surfaces where large area patterns of nanoscale features are desired. The cylindrical domains of acid-sensitive, self-assembled monolayers of polystyrene-poly(2-vinylpyridine) block copolymers on silicon surfaces were utilized as structural elements for the production of parallel metal nanowires. Metal ion loading of the P2VP block with simple aqueous solutions of anionic metal complexes is accomplished via protonation of this basic block, rendering it cationic; electrostatic attraction leads to a high local concentration of metal complexes within the protonated P2VP domain. A subsequent brief plasma treatment simultaneously removes the polymer and produces metallic nanowires. The morphology of the patterns can modulated by controlling solution concentration, deposition time, and molecular weight of the block copolymers, as well as other factors. Horizontal metallic nanoarrays can be aligned on e-beam lithographically defined silicon substrates within different shapes, via graphoepitaxy. This method is highly versatile as the procedures to manipulate nanowire composition, dimension, spacing, and orientation are straightforward and based upon efficient aqueous inorganic chemistry.

Template-based synthesis of nanorod, nanowire, and nanotube arrays

This review introduces and summarizes the fundamentals and various technical approaches developed for the template-based synthesis of nanorod, nanowire and nanotube arrays. After a brief introduction to various concepts for the growth of nanorods, nanowires and nanobelts, attention will be focused mainly on the most widely used and well established techniques for the template-based growth of nanorod arrays: electrochemical deposition, electrophoretic deposition, filling of templates by capillary force and centrifugation, and chemical conversion. In each section, relevant fundamentals will be first introduced, followed with examples to illustrate the specific details of each technique.

Highly ordered uniform single-crystal Bi nanowires: fabrication and characterization

A mechanical pressure injection technique has been used to fabricate uniform bismuth (Bi) nanowires in the pores of an anodic aluminum oxide (AAO) template. The AAO template was prepared from general purity aluminum by a two-step anodization followed by heat treatment to achieve highly ordered nanochannels. The nanowires were then fabricated by an injection technique whereby the molten Bi was injected into the AAO template using a hydraulic pressure method. The Bi nanowires prepared by this method were found to be dense and continuous with uniform diameter throughout the length. Electron diffraction experiments using the transmission electron microscope on cross-sectional and free-standing longitudinal Bi nanowires showed that the majority of the individual nanowires were single crystalline, with preferred orientation of growth along the [011] zone axis of the pseudo-cubic structure. The work presented here provides an inexpensive and effective way of fabricating highly ordered single-crystalline Bi nanowires, with uniform size distributions.

Crystal-Face Dependence and Photoetching-Induced Increases of Dye-Sensitized Photocurrents at Single-Crystal Rutile TiO2 Surfaces

Dye-sensitized photocurrents at (100)-, (001)-, and (110)-cut TiO(2) rutile surfaces were increased by photoetching of TiO(2), but the increasing ratio strongly depended on the cut crystal faces and the illumination intensity for the photoetching. For the (110)-cut surface, the photocurrent increase was moderately large and in proportion to the increase in the surface area of TiO(2) induced by the photoetching, irrespective of the illumination intensity for the photoetching. On the other hand, the photocurrent increases for the (001)- and (100)-cut surfaces, especially that for the (001)-cut surface, were prominent and largely exceeded the increases in the surface area. The results were explained by taking into account the following factors: (1) The (001)- and (100)-cut surfaces were thermodynamically unstable in contrast to the (110)-cut surface and had thicker inactive surface layers (or higher densities of surface defects), produced by surface reconstruction during heat treatment of TiO(2) at 550 degrees C in a hydrogen atmosphere for getting n-type semiconductivity. (2) Photoetching not only increased the surface area through formation of nanoholes and grooves at the surface but also effectively removed the thin inactive surface layers (or surface defects).