Here we present our recent developments in temperature dependent ellipsometry, FTIR and emittance measurements of flat and structured vanadium dioxide (VO2) surfaces allowing significant control of switchable radiative cooling beyond that attainable via traditional VO2 surfaces. VO2 undergoes a metal-insulator transition at a critical temperature of ~ 68°C; previous work has investigated tuning of this critical temperature over a wide range of temperatures. Here we exploit the shift in optical properties to produce surfaces with various emittance temperature profiles that modulate the thermal radiative transfer to/from a surface.
Designing surfaces with different temperature emittance profiles requires accurate optical/thermal characterisation of materials. VO2 is produced by sputtering of vanadium followed by post deposition annealing in a 0.1Torr to 0.3Torr Air atmosphere at 450°C to 550°C, in-situ optical monitoring allows for accurate termination of the annealing process once the desired optical response is achieved.
Cermet coatings based on nanoparticles of Au or Ag in a stable dielectric matrix provide a combination of spectral-selectivity and microstructural stability at elevated temperatures. The nanoparticles provide an absorption peak due to their localized surface plasmon resonance and the dielectric matrix provides red-shifting and intrinsic absorption from defects. The matrix and two separated cermet layers combined add mechanical support, greater thermal stability and extra absorptance. The coatings may be prepared by magnetron sputtering. They have solar absorptance ranging between 91% and 97% with low thermal emittance making them suitable for application in solar thermal conversion installations.
Silver is considered as one of the most desirable materials for plasmonic devices due to it having low loss, low epsilon2, across the visible spectrum. In addition, silver nanotriangles can self-assemble into complex structures that can include tip-totip or base-to-base arrangements. While the optical properties of tip-to-tip dimers of nanotriangles have been quite intensively studied, the geometric inverse, the base-to-base configuration, has received much less attention. Here we report the results of a computational study of the optical response of this latter configuration. Calculations were performed using the discrete dipole approximation. The effect of gap size and substrate are considered. The results indicate that the base-to-base configuration can sustain a strong coupled dipole and various multimode resonances. The pairing of the parallel triangle edges produces a strongly capacitive configuration and very intense electric fields over an extended volume of space. Therefore, the base-to-base configuration could be suitable for a range of plasmonic applications that require a strong and uniform concentration of electric field. Examples include refractometeric sensing or metal-enhanced fluorescence.
Titanium nitride is a golden-colored semiconductor with metallic optical properties. It is already widely used in room temperature spectrally-selective coatings. In contrast, aluminum nitride is a relatively wide-band gap, non-metallic material. Both nitrides have exceptional thermal stability, to over 1000 °C, but are susceptible to oxidation. We will show here that composite coatings consisting of these materials and their complex oxides have considerable potential for spectrally-selective applications, including at elevated temperatures. In particular, we examine the metastable materials produced by magnetron sputtering. The effective dielectric functions of these materials can be tuned over a wide range by manipulation of their microstructure. This provides a strategy to assemble materials with tunable dielectric functions using a 'bottom-up' approach. The results are compared to those achievable by conventional, 'top-down', planar optical stacks comprised of alternating layers of TiNx and AlN.
The optical and electrical responses of open, nanoscale, metal networks are of interest in a variety of technologies including transparent conducting electrodes, charge storage, and surfaces with controlled spectral selectivity. The properties of such nanoporous structures depend on the shape and extent of individual voids and the associated hyper-dimensional connectivity and density of the metal filaments. Unfortunately, a quantitative understanding of this dependence is at present only poorly developed. Here we address this problem using numerical simulations applied to a systematically designed series of prototypical sponges. The sponges are produced by a Monte Carlo simulation of the dealloying of Ag-Al alloys containing from 60% to 85% Al. The result is a series of Ag sponges of realistic morphology. The optical properties of the sponges are then calculated by the discrete dipole approximation and the results used to construct an 'effective medium' model for each sponge. We show how the resulting effective medium can be correlated with the associated morphological characteristics of each target and how the optical properties are primarily controlled by the density of the sponge and its state of percolation.
The optical properties of nanostructures control the performance of applications that are based on localized surface plasmon resonances. Here we use finite-difference time-domain calculations to explore the effect of geometry and material-of-construction on the transmission and near-field optical intensity of arrays of closely-spaced five-pointed nano-star shapes. We compared isolated solid star shapes to star-shaped nano-gaps set within a surrounding square metal shape. The materials investigated were silver, gold, copper and aluminum. The study showed that both the geometry and material chosen had a significant effect on the resulting transmittance spectra. Transmittance spectra of arrays of solid five-pointed nano-stars did not show any strong absorption peaks in the visible region whereas, in contrast, the arrays of star-shaped nano-gaps set within the metal squares did show strong absorption peaks. However, on closer examination it became obvious that the enhanced electric field of the latter was mostly on the corners of the square metal domains and not actually in or on the star-shaped nano-gaps. Therefore we deduce that arrays of simple metal squares will be more suitable as substrates for surface enhanced Raman spectroscopy than arrays of stars or star-shaped nano-gaps. Gold, silver and copper were suitable choices for the latter type of array. Aluminum was unsuitable, at least for applications in the visible part of the spectrum, because it was associated with relatively weak electric fields.
The electrical and optical properties of mesoporous gold are compared to those of thin porous gold films and a simulated
thin film made by randomly distributing voids in gold, until the voids fill 76% of film volume. All layers are electrically
conducting but in some cases the critical percolation thresholds are close to zero, so conduction is possible at very high
void content. Significant qualitative differences are apparent between the properties of mesoporous gold, and very thin
sputtered gold containing voids, in plasmonic responses at optical frequencies and in dc resistance, both as a function of
fill factor. The mesoporous films have an effective plasma frequency determined by void fill factor and structure, but do
not support surface plasmons. In contrast thin porous gold layers display optical features associated with localized and
de-localized surface plasmons. Sputtered porous gold is 2-dimensional and its percolation threshold requires a "Swiss-cheese"
rather than particle cluster model. Thicker mesoporous layers have critical parameters consistent with very high
connectivity, or equivalently large hyper-dimensionality. Our meso-gold samples display various hyper-dimensionalities
from 3 to above 10.
Vanadium dioxide undergoes a reversible metal-insulator phase transition at about 68°C. Coatings of this compound are reflective in the infrared above this temperature, and transmissive or absorptive below it, while resistivity changes by several orders of magnitude. We present a convenient method for depositing films with nano-size grains, which are then optically and electrically characterised. Emphasis in this study is the impact of aluminum doping and grain structure. The optical hysteresis is presented and its switching range is not altered at different doping levels but the value of transition temperature Tc does shift. In contrast hysteresis in dc resistance does change with a strong correlation between the fall in resistance in the semiconductor state with doping, the drop in Tc and the electrical properties in the metal state. For grain sizes under about 180 nm the conductivity in the metal phase is not linear in temperature but is thermally activated, with activation energies ΔE dependent on both grain size G and doping level. Simple mathematical relationships are found connecting ΔE with G and with carrier density in the semiconductor state. ΔE ranges in our samples from 0.15 eV in the smallest grain sizes to around 0.06 eV. This anomalous low frequency metal response is linked to excitations that arise in the metal phase associated with transient singlet pairing on neighbouring sites. Such pairing is weakened by doping, and in large grains appears to be present but incoherent.
Coatings that can self-modulate their optical properties as a function of an external stimulus are of significant technological interest. In this regard, the possibilities for thermo- or electrochromic materials such as VO2 and WO3 are already comparatively well-known. Here, however, we explore a new kind of 'smart' coating, based on the active control of a plasmon resonance in nanoparticles. One possible system is based on the modulation of the plasmon resonance of a precious metal nanorod or nanosphere by an active dielectric shell. The active dielectric undergoes an insulator-to-metal transition on increase of temperature which modulates the plasmon resonance of the underlying precious metal nanoparticle, thereby changing the wavelength at which its optical extinction is maximum. In the case of nanorods, the absorption maximum of the longitudinal plasmon is particularly sensitive to the aspect ratio of the nanoparticle and the dielectric properties of the environment, and may be readily tuned across the visible and near-infrared portions of the spectrum. In addition, nanoparticles of certain thermochromic dielectrics, such as VO2, are expected to have a plasmon resonance of their own which can be switched on or off by control of the temperature. We consider some of the possibilities, using both the discrete dipole approximation and the exact analytical solution due to Mie to calculate the optical properties.
The recent development of new types of light-activated, conformation-changing polymers has stimulated much interest. These 'smart' materials offer new functionalities and may enable diverse novel devices. One possible application of these substances may be in optically-driven nanoscale actuators, especially in the case of devices in which a plasmon resonance in a precious-metal nanostructure is actively modulated. A one-way or two-way shape memory effect is possible, however, application at the nanoscale will necessitate certain design changes. Nanoscale devices based on these materials could conceivably be used for drug-release or to switch the spectral selectivity of a coating.
The role of the plasma frequency ωp of conductors in their use for various solar energy and energy efficiency tasks, especially in transparent solar control window coatings, is analysed for a range of materials including noble and other metals, transparent compound conductors and the metallic phase of VO2. Ways of adjusting ωp for improved functionality are considered, including novel mesoporous metals and composites that can have an "apparent" or effective plasma frequency. While high ωp is needed for high thermal infra-red (IR) reflectance and strong surface plasmon resonant absorption, it is not the only requirement. The location of inter-band terms relative to ωp and the solar infra-red, effective bandwidth, and a high relaxation frequency can each alter these responses substantially. Two materials with elevated carrier relaxation rates, in one case when intrinsic, and in another due to mesostructure, are used to demonstrate this impact. Solar control and visible performance of a mesoporous gold film is analysed.
KEYWORDS: Molecules, Scanning tunneling microscopy, Data modeling, Electrodes, Monte Carlo methods, Electron transport, Reliability, Molecular electronics, Metals, Nanotechnology
Scanning tunneling microscopy measurements of tunneling through molecules adsorbed on a surface have been simulated using a standard empirical model based upon the Wentzel-Kramer-Brillouin method applied to tunneling through a barrier. The Gaussian noise inherent in these experiments has been added to the model data using a Monte Carlo technique. By generating multiple sets of current-voltage curves and fitting these to the model we have evaluated how reliably barrier height can be determined as a function of noise level. The results suggest that for constant percentage standard deviation in the noise greater than 5% the barrier height cannot be determined reliably. At this level, the standard deviation in the estimate of the barrier height is about 10%. Weighted fits give more reliable estimates of the barrier height. If the height of the tip above the molecule is known, so that the fit is only a single parameter the barrier height can be determined reliably even at percentage noise levels as high as 20%. However, in this case unweighted fits must be used otherwise the estimated value deviates by up to 15% from the true value. Data with constant absolute noise give similar results. The effects of experimental resolution have been evaluated in a similar manner and are shown to have a significant influence on the reliability. At a resolution of about 0.1% of full scale the standard deviation in the estimate of barrier height is only about 2% but increases rapidly to 10% for a resolution of about 1%.
We report on first principles calculations of the tunneling current across n-alkanedithiol molecules (n = 4,6,8,10,12) sandwiched between two Au {111} electrodes. The conductance drops exponentially with increased chain length with decay parameter βn = 0.9. The results are compared with scanning tunneling microscopy measurements on decanedithiol and with other n-alkanedithiol (n = 6,8,10) results in the literature. The theoretical results are found to be an order of magnitude larger than experimental values but follow the same trend. However, two additional, more realistic, geometries are modeled by changing the bond type and by combining the first-principles results with a Wentzel-Kramer-Brillouin (WKB) expression for tunneling across the air gap that is invariably present during scanning tunneling microscopy (STM) measurements. These results are more compatible with the experimental data.
In this paper we report on new techniques for making self-ordered porous layers of alumina of varying aspect ratios on glass, without the use of lithographic or masking techniques. Use of RF etching in one of the hole forming steps and also when filling the holes with sputtered metal is shown to be advantageous over additional anodisation. These hole arrays have intrinsically interesting optical responses which will be reported, but their main use is for nano-patterning of subsequent deposited layers either as templates or as masks. High resolution images demonstrate the uniformity in nanohole diameter and in the spacing between holes, which can be achieved when care is used in production. While many nanostructured materials can be deposited using these Porous Anodic Alumina (PAA) templates we focus here on filling the vertical cylindrical holes with silver. Etching during hole filling leads to better-controlled structures and more efficient processes. Novel optical data on the resultant conducting columnar rings will be presented. Spectrally much sharper plasmon resonant features are found than those reported for classical and more random silver column and island arrays. The optical properties are analysed from an effective medium perspective using data from spectrophotometry and ellipsometry. Fitting this data gives modelled layer thickness and the vertical profile in close agreement with direct SEM imaging. The effective refractive indices of the silver columnar layer have interesting and potentially useful dispersion characteristics.
The possible role of self-assembled monolayers (SAMs) as the dielectric component of nanoscale capacitors is considered. SAMs of two rather different molecules, α,α’-p-xylyldithiol ('XYL’) and dodecanedithiol ('C12’) were produced on a gold {111} substrate, and characterized with respect to their conductivity. The data were fitted with a double tunnel barrier model, in which the two SAMs are primarily differentiated by barrier height and thickness with that of XYL having a thickness of 1.0 nm and a barrier height of 0.78 eV compared to 1.69 nm and 1.39 eV for C12. The remaining parameters of the model were determined by Monte Carlo optimization. Assuming perfect connection of top and bottom electrodes, the leakage current through the XYL at 1 volt is calculated to be 1.4x105 A/cm2, compared to 2.7x104 A/cm2 through C12. These values are not as low as can be obtained with SiO2 of the same thickness. However, SAMs are readily and precisely produced by simple, low temperature processes, a factor which may allow them a niche in the future.
Nanoscale particles of metals such as gold can interact with light by means of a plasmon resonance, even though they are much smaller than the wavelengths of visible light. The proportions of light that are absorbed and scattered vary with wavelength. Any light that is absorbed will cause heating of the particles, and this effect may potentially be exploited for solar glazing coatings, nanoscale lithography or medical treatments. The position of maximum absorption of an isolated spherical nanoparticle is 518 nm, but this may be significantly red-shifted by means of decreasing the symmetry to an prolate spheroid or 'nanorod’, or by producing a metal 'nanoshell’ on a dielectric core, or by aggregating insulated spherical particles. Absorption peaks in the vicinity of 655 nm for aggregated particles and 780 nm for prolate spheroids are demonstrated here. Absorbed energy is released as heat into the environment of the particles, and will cause a temperature rise within the particle the magnitude of which depends upon the value of the effective heat transfer coefficient between particle and environment. The latter is not known, but we show how highly localized temperature rises of some tens of Celsius might be conceivable in systems illuminated by sunlight.
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.
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