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The intention of this chapter is to review and summarize the many approaches to fabricate aligned arrays of metal oxide nanowires (NWs), broadly separated into three classes (i) the growth of catalyst particles, seed particles, or NWs directly, utilizing a template (which is removed after filling); (ii) lithographic methods to fabricate arrays of either catalyst/seed particles or the NWs themselves directly; and (iii) post-synthetic methods of aligning freestanding NWs from powder or solution. Some examples of creative ways in which multiple approaches have been used in combination as well as future avenues for research will be discussed. Our focus in this review is primarily on the alignment of NWs, and for that reason, our descriptions of synthetic methods, properties, and applications of the resulting arrays will be limited.

Nanowires grown using the vapor–liquid–solid (VLS) mechanism are highly attractive components for functional nanomaterials since they grow along unique crystallographic axes to form defect-free single crystals with well-controlled dimensions. To date, however, these free-standing wires have been put to little use, since their ordered arrangement or placement is highly challenging. Here, we report an approach to create ordered arrays of nanoscale interfaces, in which we utilize the reverse of the VLS mechanism (the solid–liquid–vapor (SLV) mechanism) to etch the inverse of a nanowire, a “negative nanowire”, into a single crystal. In this way, we achieve essentially the same array of crystallographic surfaces as would be achieved by growing a large array of nanowires but in a way that creates a single object which is easy to handle. The SLV mechanism is a unique approach in that it is governed by the same crystallography which makes the VLS mechanism attractive but, additionally, poses several key advantage...

High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics., The high-temperature tetragonal phase of HfO2 is technologically useful but difficult to stabilize at room temperature. Here, the authors observe in real-time the transformation of a HfO2 nanorod from its room temperature to tetragonal phase, at 1000° less than its bulk temperature, suggesting that size confinement may kinetically trap this phase.

Transitioning energy-intensive and environmentally intensive processes toward sustainable conditions is necessary in light of the current global condition. To this end, photocatalytic processes represent new approaches for H2 generation; however, their application toward tandem catalytic reactivity remains challenging. Here, we demonstrate that metal oxide materials decorated with noble metal nanoparticles advance visible light photocatalytic activity toward new reactions not typically driven by light. For this, Pd nanoparticles were deposited onto Cu2O cubes to generate a composite structure. Once characterized, their hydrodehalogenation activity was studied via the reductive dechlorination of polychlorinated biphenyls. To this end, tandem catalytic reactivity was observed with H2 generation via H2O reduction at the Cu2O surface, followed by dehalogenation at the Pd using the in situ generated H2. Such results present methods to achieve sustainable catalytic technologies by advancing photocatalytic approaches toward new reaction systems.

Since the observation of single-molecule surface-enhanced Raman scattering (SMSERS) in 1997, questions regarding the nature of the electromagnetic hot spots responsible for such observations still persist. For the first time, we employ electron-energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) to obtain maps of the localized surface plasmon modes of SMSERS-active nanostructures, which are resolved in both space and energy. Single-molecule character is confirmed by the bianalyte approach using two isotopologues of Rhodamine 6G. Surprisingly, the STEM/EELS plasmon maps do not show any direct signature of an electromagnetic hot spot in the gaps between the nanoparticles. The origins of this observation are explored using a fully three-dimensional electrodynamics simulation of both the electron-energy-loss probability and the near-electric field enhancements. The calculations suggest that electron beam excitation of the hot spot is possible, but only when the electron beam is located outside of the junction region.

The use of bottom-up fabrication of nanostructures for nanotechnology inherently requires two-dimensional control of the nanostructures at a particular surface. This could in theory be achieved crystallographically with a structure whose three-dimensional unit cell has two or more—tuneable—dimensions on the nanometre scale. Here, we present what is to our knowledge the first example of a truly periodic two-dimensional nanometre-scale phase separation in any inorganic material, and demonstrate our ability to tune the unit-cell dimensions. As such, it represents great potential for the use of standard ceramic processing methods for nanotechnology. The phase separation occurs spontaneously in the homologous series of the perovskite-based Li-ion conductor, (Nd2/3−xLi3x)TiO3, to give two phases whose dimensions both extend into the nanometre scale. This unique feature could lead to its application as a template for the assembly of nanostructures or molecular monolayers.

The well-known vapor-liquid-solid (VLS) mechanism results in high-purity, single-crystalline wires with few defects and controllable diameters, and is the method of choice for the growth of nanowires for a vast array of nanoelectronic devices. It is of utmost importance, therefore, to understand how such wires interact with metallic interconnects-an understanding which relies on comprehensive knowledge of the initial growth process, in which a crystalline wire is ejected from a metallic particle. Though ubiquitous, even in the case of single elemental nanowires the VLS mechanism is complicated by competing processes at multiple heterogeneous interfaces, and despite decades of study, there are still aspects of the mechanism which are not well understood. Recent breakthroughs in studying the mechanism and kinetics of VLS growth have been strongly aided by the use of in situ techniques, and would have been impossible through other means. As well as several systematic studies of nanowire growth, reports which focus on the role and the nature of the catalyst tip reveal that the stability of the droplet is a crucial factor in determining nanowire morphology and crystallinity. Additionally, a reverse of the VLS process dubbed solid-liquid-vapor (SLV) has been found to result in the formation of cavities, or "negative nanowires". Here, we present a series of heating studies conducted in situ in the transmission electron microscope (TEM), in which we observe the complete dissolution of metal oxide nanowires into the metal catalyst particles at their tips. We are able to consistently explain our observations using a solid-liquid-vapor (SLV) type mechanism in which both evaporation at the liquid-vapor interface and adhesion of the catalyst droplet to the substrate surface contribute to the overall rate.

The direct observation of Li diffusion in Li-doped zinc oxide nanowires (NWs) was realized by using in situ heating in the scanning transmission electron microscope (STEM). A continuous increase of low atomic mass regions within a single NW was observed between 200 °C and 600 °C when heated in vacuum, which was explained by the conversion of interstitial to substitutional Li in the ZnO NW host lattice. A kick-out mechanism is introduced to explain the migration and conversion of the interstitial Li (Lii) to Zn-site substitutional Li (LiZn), and this mechanism is verified with low-temperature (11 K) photoluminescence measurements on as-grown and annealed Li-doped zinc oxide NWs, as well as the observation of an increase of NW surface roughing with applied bias.

A structure designed to show functionality on the nanometer length scale ideally would spontaneously form periodic nanometer-scale patterns comprising regions with contrasting properties. Here we report the synthesis of three new oxides that spontaneously form a variety of nanostructures with a periodic arrangement of phases with compositional and functional contrast. This is achieved through the partial substitution of Ti by Al, Cr, and Mn in periodically phase-separated (Nd2/3-xLi3x)TiO3 nanochessboard structures. The generality of this spontaneous compositional nanopatterning is promising for an array of exotic bulk and nanostructural properties.

We present real-time transmission electron microscopy of nanogap formation by feedback controlled electromigration that reveals a remarkable degree of crystalline order. Crystal facets appear during feedback controlled electromigration indicating a layer-by-layer, highly reproducible electromigration process avoiding thermal runaway and melting. These measurements provide insight into the electromigration induced failure mechanism in sub-20 nm size interconnects, indicating that the current density at failure increases as the width decreases to approximately 1 nm.

Electromigrated nanogaps have shown great promise for use in molecular scale electronics. We have fabricated nanogaps on free-standing transparent SiN(x) membranes which permit the use of transmission electron microscopy (TEM) to image the gaps. The electrodes are formed by extending a recently developed controlled electromigration procedure and yield a nanogap with approximately 5 nm separation clear of any apparent debris. The gaps are stable, on the order of hours as measured by TEM, but over time (months) relax to about 20 nm separation determined by the surface energy of the Au electrodes. A major benefit of electromigrated nanogaps on SiN(x) membranes is that the junction pinches in away from residual metal left from the Au deposition which could act as a parasitic conductance path. This work has implications to the design of clean metallic electrodes for use in nanoscale devices where the precise geometry of the electrode is important.

We illustrate a simple method to synthesize highly ordered ZnO axial p-n homojunction-containing nanowires using a low temperature method, and on a variety of substrates. X-ray diffraction, scanning transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy are used to reveal high quality single-crystalline wires with a [001] growth direction. The study of electrical transport through a single nanowire based device and cathodoluminescence via scanning transmission electron microscopy demonstrates that an axial p-n junction exists within each ZnO nanowire. This represents the first low temperature synthesis of axial p-n homojunction-containing ZnO nanowires with uniform and controllable diameters.