Green nanotechnology

Nanotechnology, in particular nanophotonics, is proving essential to achieving green outcomes of sustainability and renewable energy at the scales needed. Coatings, composites and polymeric structures used in windows, roof and wall coatings, energy storage, insulation and other components in energy efficient buildings will increasingly involve nanostructure, as will solar cells. Nanostructures have the potential to revolutionize thermoelectric power and may one day provide efficient refrigerant free cooling. Nanomaterials enable optimization of optical, opto-electrical and thermal responses to this urgent task. Optical harmonization of material responses to environmental energy flows involves (i) large changes in spectral response over limited wavelength bands (ii) tailoring to environmental dynamics. The latter includes engineering angle of incidence dependencies and switchable (or chromogenic) responses. Nanomaterials can be made at sufficient scale and low enough cost to be both economic and to have a high impact on a short time scale. Issues to be addressed include human safety and property changes induced during manufacture, handling and outdoor use. Unexpected bonuses have arisen in this work, for example the savings and environmental benefits of cool roofs extend beyond the more obvious benefit of reduced heat flows from the roof into the building. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)

Amplified radiative cooling via optimised combinations of aperture geometry and spectral emittance profiles of surfaces and the atmosphere

Net thermal radiation cooling, from surfaces at sub-ambient temperatures, to the night sky is amplified if the aperture to the sky is partially blocked with heat mirrors. The temperature at which radiation loss stagnates (the effective sky temperature) falls continuously as the aperture closes and is derived in terms of the aperture size and the spectral properties and temperatures of the atmosphere and of the emitting surface. Cooling surfaces must have high absorptance between 7.9 μm and 13 μm where the atmosphere is most transparent. The best response for the remainder of the Planck radiation spectrum surprisingly switches between two spectral extremes at a temperature which falls as the aperture gets smaller. A perfect absorber is best above this switch, while surfaces which reflect all of this radiation are best below it. A simple formula is presented for the cross-over temperature as a function of aperture size. With known material properties plus representative non-radiative heat gains a high emittance surface is generally better except when heat mirrors are not used. A known high emittance roof paint at 10° C below ambient, under a 45° aperture lined with shiny aluminium, can achieve a net output power near 135 W m-2 under a clear sky. © 2009 Elsevier B.V. All rights reserved

Metal nanoparticle plasmonics inside reflecting metal films

Oxide coated metal nanoparticles buried within a thin metal layer support a surface plasmon resonance. A local dip occurs in spectral reflectance along with a switching off of the film's plasmonic response. Models are introduced in which these resonances are tunable by altering the ratio of oxide thickness to core particle radius. The optical response of two experimental examples is presented and modeled using effective medium theory. Beyond the resonance zone the doped layer switches back to the plasmonic response of a nanoporous version of the host metal whose effective plasma frequency arises only from the percolating component. © 2010 American Institute of Physics

A Subambient Open Roof Surface under the Mid-Summer Sun

A novel material open to warm air stays below ambient temperature under maximum solar intensities of mid-summer. It is found to be 11 °C cooler than a commercial white cool roof nearby. A combination of specially chosen polymers and a silver thin film yields values near 100% for both solar reflectance, and thermal emittance at infrared wavelengths from 7.9 to 13 μm

Evolution of plasmonic response in growing silver thin films with pre-percolation non-local conduction and emittance drop

The optical response of growing silver thin films undergoes a transition dominated by three distinct plasmonic modes, two localized and one delocalized. Their admix as a function of added mass is analysed. The onset of the delocalized or Drude mode occurs before the sharp electrical percolation transition so optically the full insulator-metal transition is broadened. A scaling explanation supported by images shows Ag islands only have to link up over 200-300 nm to yield partial delocalization. The localized modes are (i) from silver nano-islands and (ii) a transitional anomalous mode, peaking near the dc critical percolation point, from islands surrounded by network. Growing silver within a multilayer oxide stack is compared with that on glass. The transition in thermal emittance matches that in the delocalized mode. Its broadening enables practical tuning of intermediate emittance by varying mass. © 2011 IOP Publishing Ltd

Field profiles for spherical conductive nanoparticles and metallic-shell/dielectric-core nano-composites

Profiles of the electric field strength |E|2/|E 0|2 for spherical metallic shells on a dielectric core are presented both inside the particle and outside. The dependence of the near-field strength and extent on shell thickness and total particle size is discussed qualitatively. Although the internal fields inside the shell and in the core are larger than for homogeneous particles, for not too thick shells, this does not translate into a stronger near-field away from the surface of the shell. The fields inside the shell, at the low energy resonance and close to it, are rotated by π/2 with respect to fields inside homogeneous particles, which means that the maximum field strengths in the shell are perpendicular to the incident polarisation. This follows from the fact that the low energy resonance for a shell is for the largest dipole moment of the whole system, which compensates the incident field. The largest moment is created when the same charges are collected at both interfaces (shell/medium and core/shell) along the incident polarisation. This creates regions of low field densities at the poles along the incident polarisation, because same charge fields repel each other. Following from that, the field lines are bunched up at the perpendicular poles, creating large field line densities and hence large fields at these points. The case for opposite charges across the interfaces creates the high energy, antisymmetric resonance

Five layer narrow band position variable filters for sharp colours and ultra low emittance

A simplified approach to creating narrow visible and near IR transmission bands with thin films is outlined utilising just five layers on glass, three of which are thin silver. These films have very high reflection at most wavelengths except for a narrow anti-reflection band where reflectance can be very close to zero and transmittance is close to 70%. In addition these properties are combined with IR reflectance approaching 99% for a very small thermal signature. Spatial variation of narrow band colour is easily achieved and is demonstrated with production of a simple wedge filter covering the full visible spectrum. Measured CIE colour contours in transmittance and reflectance are presented and spectral data on experimental films is compared with ideal models. Potential sources of small departures from ideal models are examined to assess the scope for future improvements. © 2008 Springer-Verlag

Dual metal-insulator and insulator-insulator switching in nanoscale and Al doped VO<inf>2</inf>

Thin films of VO2 doped with aluminium, or with nanoscale grain sizes, have been produced. They display semiconductor resistive behaviour above the transition temperature Tc, but a metallic and plasmonic optical response. All samples optically switch over almost identical large ranges at the transition, but have quite variable resistive switching. At fixed grain size a rigorous new quantitative correlation is found between semiconductor resistivity below Tc and the activation energy above Tc as Al doping level varies. Large crystals doped with Al also display this dual behaviour. A possible mechanism is discussed involving fast local fluctuations on neighbouring V4+ ions involving transient dimers with no net spin. Such fluctuations would then need to interact and correlate their motion over the scale of a nanograin within the lifetime of the dimer excitation. © 2008 IOP Publishing Ltd