Nanograin VO<inf>2</inf> in the metal phase: A plasmonic system with falling dc resistivity as temperature rises

Thin films of vanadium dioxide with grain sizes smaller than 60nm have a metallic phase with excellent plasmonic response, but their dc resistivity falls as temperature rises to values well above the metal-insulator transition. At the transition optical switching is complete, but the switch in dc resistance is incomplete. In the metallic phase, nanograin and large grain samples have similar values of both plasma frequency and relaxation rate. However, plasmonic response in nanograins is stronger due to the absence of a low energy interband transition found in large grain films. Conductivity rises with thermal activation energy of 108meV, which is well below that in the semiconductor phase. Possible mechanisms for 'non-metal-like' dc behaviour in this plasmonic system are briefly discussed. They include fluctuations, which are coherent in nanograins but incoherent for larger grains. Nanoscale systems seem preferable for optical switching applications and large grain structures for dc switching work. © IOP Publishing Ltd

The apparent optical indices of spongy nanoporous gold

Very thin spongy nanoporous gold films have a unique nanostructure and hence unusual properties. Our interest in these materials is also due to their wide range of potential application (1,2). An optical study for such nanostructured films is of fundamental interest for understanding how light interacts with such a spongy nanoporous structure. In general the gold either percolates or is very closely packed. This surface plasmons, and surface plasmon resonant effects, are expected to play a key role given the large surface area of metal and the metal backbone of the nanostructure. The ropological complexity of the nano-void network is also expected to be a major influence. The optical response has, for a metal system, quite unusual dispersion relations for the effective complex refractive index components n*, k*. Once these are better understood new optical engineering possibilities arise. We are not aware of any optical studies for spongy metal film nanostructures apart from a brief preliminary report of our own on one such film 93) whose nanstructure was different to the spongy nanoporous films presented here. We check the internal consistency and physical accpetability of the results with a Kramers-Kronig analysis of the spectrumn of n*, k* values, because of their unusual spectral character

Bulk and surface plasmons in highly nanoporous gold films

The far field plasmonic behaviour of nanoporous gold films with void densities ranging from 60% to 90% has been investigated and modelled. These layers have good dc conductivity and quite different nanostructure to traditional porous layers in which the metal percolates. Our gold films with void density f above 70% have high thermal emittance for a conductor at their thicknesses and their flat spectral response at visible and near infrared wavelengths is not metal like. We derive effective optical constants which become plasmonic at wavelengths between 1.8 and 4 νm for f from 72 to 87%. This onset is much longer than that in bulk gold. For void densities below 70% the onset of plasmonic behaviour is much closer to the dense material. A simple test is implemented to test for surface plasmon polaritons (SPPs) under illumination. The more porous films show no evidence of SPP, while the less porous films display weak evidence. Thus by tailoring void content in these nanostructures we can tailor the onset of effective plasmonic response across a wide range from 0.8 to 4 νm and emittance from around 0.9 down to low values. An effective uniform metal response is thus found in the presence of surface nanostructure without the interface absorption found in dense gold layers with structured surfaces. © 2007 IOP Publishing Ltd

Mie and bragg plasmons in subwavelength silver semi-shells

2D arrays of silver semi-shells of 100 and 200 nm diameter display complex reflection and transmission spectra in the visible and near-IR. Here these spectral features are deconstructed and it is demonstrated that they result from the coupling of incident light into a delocalized Bragg plasmon, and the latter's induction of localized Mie plasmons in the arrays. These phenomena permit the excitation of transverse dipolar plasmon resonances in the semi-shells despite an ostensibly unfavorable orientation with respect to normally incident light. The resulting spectral feature in the mid-visible is strong and tunable. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA

Mesoporous gold sponge

Mesoporous gold sponge may be prepared by the removal of aluminium from AuAl2 by an alkaline leach. The resulting material has nanoscale pores and channels, with a high specific surface area that can be exploited in electrochemical applications. For example, the material may serve as the basis of a more sensitive capacitive sensor or biosensor, as an electrode material for a high efficiency ultracapacitor, as the semi-transparent current collector in a dye sensitized photovoltaic cell, or as the lithium storage electrode in a lithium ion cell. The properties of the sponge may be controlled by varying its density, pore size, and pore size distribution, factors which are in turn controlled by the microstructure of the precursor compound and the conditions of deposition. © CSIRO 2007

Nanocapacitive circuit elements

"Natural" lithography was used to prepare arrays of nanoscale capacitors on silicon. The capacitance was verified by a novel technique based on the interaction of a charged substrate with the electron beam of a scanning electron microscope. The "nanocapacitors" possessed a capacitance of ∼1 × 10-16F and were observed to hold charge for over an hour. Our results indicate that fabricating nanostructures using natural lithography may provide a viable alternative for future nanoelectronic devices. © 2008 American Chemical Society

Optical response of nanostructured metal/dielectric composites and multilayers

The homogeneous optical response in conducting nanostructured layers, and in insulating layers containing dense arrays of self assembled conducting nanoparticles separated by organic linkers, is examined experimentally through their effective complex indices (n*, k*). Classical effective medium models, modified to account for the 3-phase nanostructure, are shown to explain (n*, k*) in dense particulate systems but not inhomogeneous layers with macroscopic conductance for which a different approach to homogenisation is discussed, (n*, k*) data on thin granular metal films, thin mesoporous gold, and on thin metal layers containing ordered arrays of voids, is linked to properties of the surface plasmon states which span the nanostructured film. Coupling between evanescent waves at either surface counterbalanced by electron scattering losses must be considered. Virtual bound states for resonant photons result, with the associated transit delay leading to a large rise in n* in many nanostructures. Overcoating n-Ag with alumina is shown to alter (n*, k*) through its impact on the SP coupling. In contrast to classical optical homogenisation, effective indices depend on film thickness. Supporting high resolution SEM images are presented

the fire assay reloaded

The fire assay process is still the most accurate and precise method for measuring the gold content in gold alloys. Scanning electron microscopy and transmission electron microscopy have been applied to observe the change in microstructure of the samples undergoing the fire assay process. The performed observations reveal that the microstructure of the specimen is more complex than expected. Before the parting stage, the specimen is not a perfect gold–silver binary alloy but contains also copper–silver oxides and other residual compounds. The parting stage appears to be a dealloying process leading to a nanoporous gold nanostructure. What observed after partition explains the evolution of the shape and colour of the specimen and may allow for a better comprehension of the procedure and an improvement in the method

Electrochemical capacitance of mesoporous gold

The surfaces of nanoscale gold particles and components are oxide-free under normal ambient conditions. This unusual attribute permits the exploration of microstructures and functionalities that would not be feasible for less noble metals. Here we consider the electrochemical properties of mesoporous gold sponges, prepared by de-alloying an AuAl2 precursor. The sponges have a high specific surface area, with an average pore diameter of 12 nm, but are prone to sinter. They may be prepared in bulk, or, more usefully, as coatings. Their electrochemical capacitance divided by their nominal surface area is high and, at a cell voltage of 0.6 V, reaches 100 mF/cm2 for bulk samples and 2 mF/cm2 for coatings. This is up to a thousand times greater than the 50 to 100 μF/cm2 exhibited by a planar gold surface

Optical properties of mesoporous gold films

Mesoporous gold thin films on glass substrates were fabricated by sputtering of AuAl2 precursor films followed by a de-alloying etch. The resulting sponge-like Au films have very high internal surface area due to nanoscale pores and channels. Scattering is not significant and the optical properties for such nanostructured films were examined using ellipsometry and spectrophotometry. The complex refractive indices of the optically equivalent uniform smooth layer satisfy Kramers-Kronig (KK) self-consistency but have unusual dispersion relations and magnitudes for a film containing the amount of noble metal present. The reflectance at infrared wavelengths is neither metal-nor insulator-like, and the indices have unique dispersion curves. © 2005 IOP Publishing Ltd