A highly ordered metal nanohole array (platinum and gold) was fabricated by a two-step replication of the honeycomb structure of anodic porous alumina. Preparation of the negative porous structure of porous alumina followed by the formation of the positive structure with metal resulted in a honeycomb metallic structure. The metal hole array of the film has a uniform, closely packed honeycomb structure approximately 70 nanometers in diameter and from 1 to 3 micrometers thick. Because of its textured surface, the metal hole array of gold showed a notable color change compared with bulk gold.

https://science.sciencemag.org/content/sci/268/5216/1466.full.pdf

Ordered Metal Nanohole Arrays Made by a Two-Step Replication of Honeycomb Structures of Anodic Alumina

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.

https://science.sciencemag.org/content/sci/266/5193/1961.full.pdf

Nanomaterials: a membrane-based synthetic approach.

This paper reviews a relatively new method for preparing nanomaterials:  membrane-based synthesis. This method entails the synthesis of the desired material within the pores of a nanoporous membrane. Because the membranes employed contain cylindrical pores of uniform diameter, monodisperse nanocylinders of the desired material, whose dimensions can be carefully controlled, are obtained. These nanocylinders may be either hollow (a tubule) or solid (a fibril or nanowire). We call this approach the “template” method because the pores in the nanoporous membranes are used as templates for forming the desired material. This template method is a very general approach; it has been used to prepare nanotubules and fibrils of polymers, metals, semiconductors, carbons, and other materials.

Membrane-Based Synthesis of Nanomaterials

The self-assembly of monodisperse gold and silver colloid particles into monolayers on polymer-coated substrates yields macroscopic surfaces that are highly active for surface-enhanced Raman scattering (SERS). Particles are bound to the substrate through multiple bonds between the colloidal metal and functional groups on the polymer such as cyanide (CN), amine (NH(2)), and thiol (SH). Surface evolution, which can be followed in real time by ultraviolet-visible spectroscopy and SERS, can be controlled to yield high reproducibility on both the nanometer and the centimeter scales. On conducting substrates, colloid monolayers are electrochemically addressable and behave like a collection of closely spaced microelectrodes. These favorable properties and the ease of monolayer construction suggest a widespread use for metal colloid-based substrates.

https://science.sciencemag.org/content/sci/267/5204/1629.full.pdf

Self-Assembled Metal Colloid Monolayers: An Approach to SERS Substrates.

A highly ordered gold nanodot array was fabricated by vacuum evaporation using an anodic porous alumina membrane with through-holes of nanometer scale as a mask. This technique resulted in an orderly arrangement of Au dots with a diameter of approximately 40 nm over a large area on a Si substrate.

https://iopscience.iop.org/article/10.1143/JJAP.35.L126/pdf

Fabrication of gold nanodot array using anodic porous alumina as an evaporation mask

Long, nanometer-size metallic wires can be synthesized by injection of the conducting melt into nanochannel insulating plates. Large-area arrays of parallel wires 200 nanometers in diameter and 50 micrometers long with a packing density of 5 x 108 per square centimeter have been fabricated in this way. When charged, the ends of the wires generate strong, short-range electric fields. The nanowire electric fields have been imaged at high spatial resolution with a scanning force microscope.

https://science.sciencemag.org/content/sci/263/5148/800.full.pdf

Nanowire Array Composites

: Metallic nanostructures can be prepared from nanochemical dielectric hosts by injection of the conducting melt. Nanowire arrays and random networks have been fabricated in this way. The role of the microstructure in the bulk electronic properties of the composites as well as applications of these materials are discussed. (jg)

Microengineered conducting composites from nanochannel templates

Densely packed arrays (76% volume fraction) of 10 μm diameter parallel indium wires exhibit an enhanced transmission, of ∼103, relative to an indium foil of equal thickness for far-infrared (k<80 cm−1) propagating along the wire length. The absorption increases as k0.45±0.07 and is explained by the dynamic Maxwell–Garnett model, which includes eddy current dissipation. The effective surface conductivity is depressed fiftyfold with respect to the bulk. The implications for plasmons in metal wire microstructures and for developing simultaneously transmissive and conductive composites are discussed.

Far-infrared propagation in metal wire microstructures

Adsorption isotherms of ${\mathrm{H}}_{2}$, ${\mathrm{D}}_{2}$, and Ne have been measured in the temperature range from 15 K to the corresponding critical points in samples of porous Vycor glass. From the Brunauer-Emmett-Teller theory the surface layer coverages are determined. These are found to be temperature dependent. A model-independent approach allows us to fit the data for coverages ranging from submonolayer to thin film, below capillary condensation, for each adsorbate at all temperatures with a temperature-independent curve. This characteristic curve represents the distribution of adsorption sites versus the adsorption potential. In the intermediate coverage range, the isotherms exhibit the modified Frenkel-Halsey-Hill (FHH) behavior. The adsorption saturates for low-adsorption potentials. The characteristic curve is a useful universal curve since it is roughly the same for the three species investigated. We examine the relative strengths of the surface potentials and densities of the two isotopic modifications of hydrogen and of the more classical Ne adsorbed on porous Vycor glass. The characteristic adsorption curve is compared with results from two models for the adsorbate: Dubinin's isotherm for microporous solids and its extension to rough surfaces which places importance on the porosity of the surface, and Halsey's model, which is an extension of the FHH isotherm that takes into account the long-range variations of substrate adsorption potential.

Temperature-dependent adsorption of hydrogen, deuterium, and neon on porous Vycor glass

We consider the propagation of electromagnetic fields (EMF) through a composite dielectric medium supporting an array of parallel metal wires. The Maxwell-Garnett approximation has been previously used to describe the dielectric properties of such media. We show that, in addition to an effective dielectric constant, the medium also has an effective diamagnetic permeability associated with electric currents induced in the wires. We present a self-consistent model that allows for evaluation of the properties of EMF propagating along the wire length. Results from this model are in agreement with measurements of the infrared absorption of an array of 10-\ensuremath{\mu}m diameter, parallel indium wires in a glass matrix. We obtain a ${k}^{1/2}$ absorption dependence in the far-infrared $(kl60{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}),$ and a ${k}^{2}$ dependence due to losses in the glass matrix at higher frequencies. We also find that the reflectance is low in comparison to the bulk metal. If interwire effects are neglected, the EMF's velocity of propagation is given by ${c/n}_{i},$ where ${n}_{i}$ is the insulator refractive index, regardless of the metal volume fraction. For self-supported arrays ${(n}_{i}=1),$ we have verified experimentally that the velocity of propagation in the array is equal to the velocity of light in free space.

T. E. Huber

Papers

Nanowire Array Composites

Microengineered conducting composites from nanochannel templates

Far-infrared propagation in metal wire microstructures

Temperature-dependent adsorption of hydrogen, deuterium, and neon on porous Vycor glass

Electromagnetic wave propagation through a wire array composite