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 ...
Using two different photoacoustic techniques for a two-layer system of variable thickness, we show that the thermal diffusivity and the thermal conductivity are completely determined, based upon the efFective-sample model widely used in heat-transfer problems. A procedure to establish a standard photothermal technique for measuring both the thermal diffusivity and the thermal conductivity is also discussed.
An absolute thermal lens method to determine fluorescence quantum efficiency and concentration quenching of solids is described in this work. The quantum efficiency of low silica calcium aluminate glasses doped with different concentrations of neodymium dioxide and melted under vacuum conditions to remove water has been measured by using mode-mismatched thermal lens spectrometry. It has been shown that the thermal lens signal amplitude is linearly dependent on neodymium concentrations up to 4.0 wt %, changing significantly from 4.5 to 5.0 wt %, indicating that there was quenching of the fluorescence only above 4.0 wt % neodymium dioxide. The quantitative treatment for the thermal lens effect provided the absolute value of the sample's fluorescence quantum efficiency. The technique is simple to perform and can be applied for a wide range of fluorescent materials. ͓S0163-1829͑98͒00714-0͔
We have compared the myocardial alterations in rats made hypertensive by the chronic inhibition of nitric oxide biosynthesis with those having renal hypertension (two kidney-one clip model). Male Wistar rats were chronically administered the nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) for 2, 4 and 8 weeks. Both groups initially developed a similar increase in blood pressure but only the 2K-1C rats developed myocardial hypertrophy after 2-4 weeks. L-NAME-treated animals developed a similar degree of hypertrophy following 8 weeks of treatment. As observed by light microscopy, the myocardial alterations in the latter animals consisted of extensive areas of fibrosis and myocardial necrosis, especially in regions of the subendocardium. The histological alterations induced by L-NAME were not caused by the accompanying hypertension, since the 2K-1C animals had a similar increase in arterial blood pressure without any significant alterations in the heart morphology. 2K-1C rats treated chronically with L-NAME behaved in a manner similar to the L-NAME-treated animals with regard to both the blood pressure increases and cardiac morphological alterations. Animals which received the inactive enantiomer D-NAME did not develop hypertension nor did they have any morphological abnormalities. Both the coronary flow and the contractile capacity of hearts isolated from rats treated with L-NAME for 8 weeks were impaired compared to control animals. These results indicate that the chronic inhibition of NO biosynthesis causes cardiac ischemia associated with a mechanical dysfunction that is unrelated to cardiac hypertrophy which is similar to those seen in some patients suffering from chronic arterial hypertension.
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