We demonstrate switchable polarized thermal emission from VO2 nanofin stacks fabricated by co-deposition, etching, and oxidation. We find that reverse switching of the thermal emission is enabled by a reflecting underlayer and induced by either short oxidation time or additional deposition of a reflecting underlayer. Observed thermal emission is well explained by a biaxial Bruggeman effective medium model, which predicts the strong polarization change for aligned fin layers in the micron thickness range. The dominant polarization of the emission is modulated by the presence of a reflector, oxidation of the fins, fin fill-factor, and structural anisotropy. Normal incidence polarized emittance change of up to 0.6 is theoretically possible, and we were able to demonstrate a change of 0.34, similar to that predicted by the model.
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.
There has been continued recent interest in radiative sky cooling of coated flat surfaces that are able to passively attain sub-ambient temperatures. As the lowest incoming infrared radiation from a clear sky occurs at the zenith, a surface which sees mainly this region of the sky will receive much lower levels of sky radiation than one which views the whole sky, since the near-horizon contains significantly more incoming radiation. Two approaches to extra cooling are thus angular selectivity, which limits oblique outgoing as well as incoming radiation, and macroscopic reflectors which block oblique incoming sky radiation, while directing most outgoing emitted radiation towards the near zenith. This work focuses on the second of these techniques. We maximise cooling potential via coated 3D printed structures which can passively maintain a thermal reservoir below ambient temperature throughout the night and day. Novel design methods are used to fabricate and test structures which maximise outgoing thermal radiation from a surface, while minimising its illumination by incoming radiation from the sky and sun. Preliminary results gave 10°C below ambient both day and night during a Sydney spring. 3D printing allows the production of complex designed mirror cones with relatively low thermal conductivity. Post processing of the 3D printed structures allows the desired surface textures and optical properties to be created.
The scope for maximizing the albedo of a painted surface to produce low cost new and retro-fitted super-cool roofing is explored systematically. The aim is easy to apply, low cost paint formulations yielding albedos in the range 0.90 to 0.95. This requires raising the near-infrared (NIR) spectral reflectance into this range, while not reducing the more easily obtained high visible reflectance values. Our modified version of the four-flux method has enabled results on more complex composites. Key parameters to be optimized include; fill factors, particle size and material, using more than one mean size, thickness, substrate and binder materials. The model used is a variation of the classical four-flux method that solves the energy transfer problem through four balance differential equations. We use a different approach to the characteristic parameters to define the absorptance and scattering of the complete composite. This generalization allows extension to inclusion of size dispersion of the pigment particle and various binder resins, including those most commonly in use based on acrylics. Thus, the pigment scattering model has to take account of the matrix having loss in the NIR. A paint ranking index aimed specifically at separating paints with albedo above 0.80 is introduced representing the fraction of time at a sub-ambient temperature.
Angled columnar structures produced by oblique angle deposition possess useful optical polarization effects. It is well known that this is due to structural anisotropy but the relative contributions of factors affecting this anisotropy are not fully understood in all cases. Serial bideposited films where the azimuth is changed during deposition can have greater birefringence if the azimuths are directly opposed. In contrast, in this article the properties of perpendicular azimuth films are studied: silicon films at tilt angles 50-80° were deposited and analyzed. Electron microscopy confirmed that the silicon nanostructures were formed off-axis, meaning they did not develop along the deposition axes but followed the averaged azimuth. Optical measurements confirm that the maximum birefringence occurs closer to glancing angles, and optical modelling demonstrates that in contrast to fixed azimuth films the birefringence in these perpendicular azimuth films is primarily modulated by depolarization factors.
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.
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 resonances that occur in nanostructured metal layers are modulated in thin film stacks if the nanostructured layer is separated from a reflecting conducting layer by various thicknesses of thin dielectric. We have measured and modeled the optical response of interacting silver layers, with alumina spacer thickness ranging from a few nm to 50 nm, for s- and p-polarized incident light, and a range of incident angles. Standard thin film models, including standard effective medium models for the nanostructured layer, will break down for spacer thickness below a critical threshold. For example, with polarisation in the film plane and some nano-islands, it may occur at around 10 nm depending on spacer refractive index. Of particular interest here are novel effects observed with the onset of percolation in the nanolayer. Hot spot effects can be modified by nearby mirrors. Other modes to consider include (a) a two-particle mode involving a particle and its mirror image (b) A Fano resonance from hybridisation of localized and de-localised plasmon modes (c) a Babinet’s core-(partial) shell particle with metal core-dielectric shell in metal (d) spacing dependent phase modulation (e) the impact of field gradients induced by the mirror at the nano-layer.
The preferred surface spectral response for sub-ambient sky cooling varies according to the amount of water vapor in
the atmosphere and the operating difference (Ta-Ts) between ambient and emitter surface temperatures. While all
good candidates average high emittance from 7.9 μm to 13 μm, where the atmosphere is most transparent (the IR "sky
window"), the preferred spectral response in the remainder of the Planck spectrum depends on a number of factors.
Emittances E in studies to date have been near the two extremes of a high E ~ 0.85 to 0.95, and an E value between 0.3
to 0.4 for surfaces which emit strongly only in the sky window. Cooling rates and ideal spectral properties vary with
operating conditions. The reasons behind this will be explained for select different coatings, using spectral densities for
emitted outgoing radiation, which is Ts dependent, and the incoming radiation that is absorbed, which is fixed unless
the atmosphere changes. Higher E surfaces always work best above and just below ambient but external factors that
reduce incoming radiation from the atmosphere, including very low humidity or heat mirror apertures, extend this
preference down to lower surface temperatures. Sky window spectrally selective coatings do not benefit as much
because they already absorb little incoming radiation, but always have the potential to achieve very much colder
temperatures if non-radiative heat gains are kept low.
To achieve strong net thermal radiation emission from surfaces whose temperature is at or below ambient it is
important to have high absorptance between 7.9 μm to 13 μm where the atmosphere is most transparent. Outside
of this band the atmosphere behaves like a black body emitter and hence at these wavelengths net radiant heat
loss is normally not possible at sub-ambient temperatures. It becomes possible using two types of angular
selectivity, which also improve emission between 7.9 μm to 13 μm. One is coating based, and one uses external
heat mirrors. In the latter low emittance mirrors replace the higher emitting segments of the atmosphere. The
coating's net gain is a result of its reflectance rise countering the atmosphere's drop in transparency as ray angles
to the zenith approach the horizontal. These ideas are examined in the context of experimental data on coatings
which rely on nanostructure to largely limit their spectral absorption to the atmosphere's transparent band. The
angular selective coating becomes possible in two multilayer types (a) one nano-layer is strongly reflective (b)
one layer has much higher index than the other. Type (a) materials as nanoparticles provide surface phonon
resonance in the desired absorption band.
The response to applied electric fields of vanadium dioxide thin films above and below the phase transition depends on the size of grains if below ~200nm across, and on aluminum doping above a critical concentration. Tc drops as doping level increases, but does not depend on grain size. The observed phase transition undergoes a remarkable qualitative shift as the applied field goes from optical to low frequencies. The expected insulator to metal transition is found at optical frequencies, but at low frequencies an insulator-to-insulator transition occurs. Optical switching at both T < Tc and T > Tc is nearly independent of doping level and grain size. In contrast dc properties in both phases are sensitive to both factors. The band gaps from optical and dc data differ, and densities of states change with doping level. Such behaviour can arise if there is a transient phase change. The way doping and grain size can support such a phase is discussed. Only individual nanograins need to switch phases coherently to explain data, not the whole sample. Resistance as a function of composition across the transition was derived using effective medium compositional analysis of optical data in the hysteresis zone. The percolation thresholds are not at the usual Tc values.
The response to applied electric fields of vanadium dioxide thin films above and below the phase transition is shown
experimentally to depend on the size of grains if below ~200nm across, and on aluminum doping above a critical
concentration. Tc drops as doping level increases, but does not depend on grain size. The observed phase transition
undergoes a remarkable qualitative shift as the applied field goes from optical to low frequencies. The expected insulator
to metal transition is found at optical frequencies, but at low frequencies an insulator-to-insulator transition occurs.
Optical switching at both T < Tc and T > Tc is nearly independent of doping level and grain size. In contrast dc properties
in both phases are quite sensitive to both factors. The band gaps predicted by optical and dc data differ, and densities of
states change with doping level. Lattice or electron dynamics alone cannot yield such behaviour, but it can arise if there
is a transient phase change. The way doping and grain size can support such a phase is discussed. Only individual
nanograins need to switch phases coherently to explain data, not the whole sample. Resistance as a function of
composition across the transition was derived using effective medium compositional analysis of optical data at
temperatures in the hysteresis zone. Expected percolation behaviour does not arise in such an analysis, with the observed
thresholds different when heating and cooling, and they occur at temperatures which differ from the usual Tc values.
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.
Abstract. Conductor-insulator nanocomposites in which the conductor percolates can have optical responses at longer wavelengths like dense conductors with an effective plasma frequency wp*. This applies at wavelengths where the Bergman spectral function F for permittivity varies sufficiently slowly with wavelength. wp* can be engineered by varying the components, the nanostructure's topology, or the dielectric volume fraction f. The homogenized conductor acts like a dense conductor whose charge carriers have effective mass meff*. Results are presented for wp*(f) and meff*(f) using the Maxwell Garnett (MG) and Bruggemann (BR) models for spheres and aligned ellipsoids. In the BR case meff*(f) at the percolation concentration is singular. Example wp* data for spheres and ellipsoids are given which match predictions. Anisotropy in effective mass is considered, such that effective plasma frequency can depend strongly on polarisation direction of incident light.
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%.
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.
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