Crystalline VO2.5 films were prepared by reactive dc magnetron sputtering followed by annealing posttreatment. Lithium was inserted electrochemically from an electrolyte so that LixVO2.5 (0<x<1.5) was formed. The evolution of the electromotive force (EMF) was recorded during Li intercalation. For lithiated samples, structure by x-ray diffraction, optical properties by spectrophotometry in the 0.3–2.5 μm range, infrared reflectance by spectrophotometry in the 400–1200 cm−1 interval, and the mechanical stress by beam deflectometry were studied. Changes in lattice parameters, phonon spectra, and stress levels gave a consistent picture of the structural evolution. Measurements of optical absorption and EMF were interpreted within a conceptual model of interband transitions between the O 2p band and a split V 3d band. Durability and reversibility of the Li intercalation/deintercalation were verified by potentiodynamic measurements. For comparison, the properties of highly disordered films were also measured.

The optical band-gap energy of a nanostructured tungsten trioxide film is determined using the photoacoustic spectroscopy method under continuous light excitation. The mechanism of the photoacoustic signal generation is discussed. The band-gap energy is also computed by other methods. The absorption coefficient as well as the band-gap energy of three different crystal structures of tungsten trioxide is calculated by a first-principles Green's function approach using the projector augmented wave method. The theoretical study indicates that the cubic crystal structure shows good agreement with the experimental data. © 2010 American Institute of Physics. ͓doi:10.1063/1.3313945͔Tungsten trioxide ͑WO 3 ͒ films have attracted much interest during the last decade due to their potential applications. Nanostructured WO 3 films have been used in eletrochromic ͑EC͒ devices such as displays and smart windows.1-3 For this reason, a detailed understanding of the optical processes responsible for the EC effect would greatly facilitate the optimization of EC devices.4 WO 3 is a wideband-gap semiconductor. Its band-gap energy has been mainly measured by optical absorption, varying from about 2.6 to 3.0 eV. 2,5 The band gap of WO 3 is certainly of interest for both applied and fundamental aspects. The literature is however somewhat confusing. Values below 3.0 eV have mostly been obtained assuming an indirect band gap.Taking into account that the understanding of the optical processes responsible for the EC effect is an important parameter in design and optimization of EC devices, and that the band gap energy is one of the most important parameter of semiconductors, we investigate the optical absorption in the region of the fundamental band edge by the photoacoustic spectroscopy ͑PAS͒ technique. PAS has been extensively used as a nondestructive method for measuring the optical properties of semiconductors and many other materials. 6-10The nonradiative relaxation processes-which are associated with the band structure, defect-related energy loss mechanism, etc.-can be directly and very accurately obtained from the analysis of the PAS spectra. 10The optical band-gap energy ͑E g ͒ has been determined by the PAS technique using mainly two methods. In the first, the E g value is adopted as the absorption edge obtained from a linear fitting in the plot of the square of the product between the absorption coefficient and the photon energy versus the photon energy for direct band gap, or the plot of the square root of the product between the absorption coefficient and the photon energy versus the photon energy for indirect band gap. 11 In the second, E g is estimated by the changing of the derivative near the fundamental absorption edge. 7In this letter, we analyze the PA-signal behavior of a nanostructured WO 3 film under continuous laser excitation, using an experimental procedure similar to that described in Ref. 12. The influence of the continuous excitation in the mechanisms responsible for the generation of the PA signal is discussed ...

ZnO:Al coatings were prepared by rf magnetron sputtering of ZnO together with dc magnetron sputtering of Al onto rapidly revolving unheated substrates under weakly oxidizing conditions. Optimized films had ∼1% luminous absorptance, ∼85% thermal infrared reflectance, and ∼5×10−4 Ω cm electrical resistivity at a thickness of ∼0.3 μm. The Al content was ≲2 at. %, as determined by Rutherford backscattering spectrometry. Transmission electron microscopy and electron diffraction showed ∼50-nm average crystallite size and a hexagonal wurtzite structure. Spectrophotometric transmittance and reflectance were recorded in the 0.2–50-μm wavelength interval, and the complex dielectric function was evaluated by computation. The optical data were explained from an effective mass model for n-doped semiconductors. The Al atoms are singly ionized, and the associated electrons occupy the bottom of the conduction band as free-electron gas. The Al ions act as pointlike Coulomb scatterers and are screened by the electrons according to the random phase approximation or an extension thereof. The optical properties of ZnO:Al could be understood by considering the free electrons to be damped primarily by ionized impurity scattering. ZnO:Al films can have high luminous transmittance, high solar ultraviolet absorptance, low thermal infrared emittance, and high electrical conductance; hence, they are of large interest for energy-efficient windows.

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Durable electrochromic coatings of hydrated nickel oxide were produced by reactive rf magnetron sputtering of Ni followed by treatment in KOH. Spectrophotometry was used to assess the achievable modulation of luminous and solar transmittance and to verify that the studied material is interesting for ‘‘smart window’’ applications. 15N nuclear reaction analysis suggested that coloration occurred upon hydrogen extraction.

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This paper gives a brief survey over some recent work on surface coatings for energy efficiency and solar applications. Specifically, it covers coatings for selective absorption of solar energy (for solar collectors), -selective emission of infrared radiation in the 8-13gm range (for passive cooling devices), -visible transmission combined with near-infrared reflectance (for windows with decreased inlet of solar energy), solar transmission combined with thermal-infrared reflectance (for windows with decreased outlet of thermal radiation), variable transmission of solar or visible radiation (for "smart windows" with dynamic control of radiant energy). One general conclusion from the survey is that the research field embodying these types of coatings remains, with a few exceptions, in a state of rapid progress.