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Highly porous nanocrystalline TiO2 films were sensitized with ZnS semiconductor nanocrystals by using a three-step facile strategy. The TiO2 matrices were firstly impregnated with the zinc bis(benzyl)thiol precursor that, unlike many other metal chalcogenides, is easily dispersible in the most common organic solvents. In the second step of the adopted synthetic strategy, the TiO2 impregnated with zinc thiolate was annealed at 250 °C in order to induce nucleation and growth of ZnS nanocrystals inside the porous matrices as well as on the film surface. A final cleavage in dichloromethane allowed us to remove all residual organic reagents and products. The size of the ZnS nanocrystals can be tuned by variation of the annealing time as shown by X-ray diffraction measurements, without changing the crystallographic phase structure (zincblende structure). Scanning electron microscopy analysis showed a random spatial dispersion of the ZnS nanocrystals and no remarkable aggregation was observed. The sensor device based on TiO2-ZnS nanocomposite was able to detect NO2 gas at 270 °C down to 1 ppm with negligible response towards reducing gases (CO, CH4). Compared to bare TiO2 film, the TiO2-ZnS nanocomposite shows higher sensitivity and lower recovery time towards an oxidant NO2 gas.

Highly porous nanocrystalline films of TiO 2 sensitized with semiconductor nanocrystals (NCs) are very promising for photovoltaic applications, NCs being more versatile and efficient than organic dyes as sensitizer. In this framework, the visible light absorption of TiO 2 can be improved by the superposition of CdS NC absorption band, that is tunable between 2.5 and 5.2 eV. Here, we report on a novel in situ approach for the synthesis of CdS NCs in mesoporous sol-gel TiO 2 anatase films by using as starting reagent for CdS, [Cd(SBz) 2 ]MI, where MI represents 1-methylimidazole. The precursor has been adsorbed into the TiO 2 pores and then decomposed at 480 °C, the procedure being studied for TiO 2 films with different porosity. The matrices have been investigated by atomic force microscopy. X-ray diffraction measurements on TiO 2 matrix and on the corresponding CdS/TiO 2 nanocomposite material revealed a variation of the NCs size of both TiO 2 and CdS as a function of the matrix porosity. The optical properties of the TiO 2 and CdS/TiO 2 have been studied by UV-Vis transmission spectroscopy. CdS/TiO 2 nanocomposites, having higher absorption efficiency with respect to TiO 2 , are suitable as building blocks for 3rd generation solar cell design.

The investigation of the surface and interface of a multilayer coating is a key factor for producing high quality optical device. In this work, we present a non-destructive technique as X-ray reflectivity to study deeply the chemical and structural quality of the multilayer. XPS depth profile analysis and AFM images have been used to evaluate the chemical intermixing and the surface roughness in order to verify the X-ray results.

In this work, we propose a new method for the determination of the optical parameters of an inhomogeneous thin optical film deposited on a substrate of finite thickness. The method can be applied successfully, if the following hyphotheses are satisfied: the film exhibits small optical inhomogeneities throughout the thickness (less than 5%), smooth refractive index profiles, weak absorption (extinction coefficient ≤ 0.02) and negligible diffuse light scattering. Useful formulae are derived describing the effects of refractive-index inhomogeneity and absorption not only on the transmittance and reflectance extrema but also for any other wavelength value. This allows us to calculate the refractive index inhomogeneity and to subtract its contribution from the measured transmittance and reflectance curves starting from approximate refraction index and film thickness values. In this way, it is possible to obtain the transmittance and reflectance spectra of a homogeneous film whose optical parameters are the mean values of those of the inhomogeneous film. These new transmittance and reflectance curves can be easily processed with any computer program for the determination of the optical parameters and film thickness based on the homogeneous thin film optical model.

SiO2 films have been deposited by ion-beam sputtering techniques either without assistance or with Ar-ion assistance and the role of the substrate temperature during the deposition process on the film damage threshold by laser light at the wavelength of 308 nm (XeCl) has been investigated. The photoacoustic probe beam deflection technique has been used to determine damage thresholds and it is shown that this technique provides valuable information on the mechanisms leading to laser damage. It has been found that the samples deposited by the Ar-ion beam sputtering technique are characterized by higher damage thresholds at lower temperatures of the substrate than the SiO2 films deposited without ion assistance.

Electrically conducting organic polymers change their conductivity and relative permittivity when exposed to volatile chemicals [1.2]. The changes depend on the frequency, the concentration and the type of the chemical. In particular the frequency dependence could be used for making more selective sensors. A polypyrrole (PPY) film gas sensor was designed and constructed in order to probe it. The low frequency (20 Hz−10 KHz) a.c. response was studied when it was exposed to 200 ppm of four different vapours: methanol, acetone, ethyl acetate and ethanol. Response patterns to each vapour were obtained by varying the measurement frequencies. The pattern for each vapour investigated was very different from the others, and so it has been possible to discriminate between them. The results demonstrate that it is possible to increase the sensor selectivity using the low frequency a.c. response rather than d.c. resistance change. Ageing and temperature dependence of resistance and capacitance are also reported.

Nanostructured titania thin films were prepared under controlled atmospheric conditions by the sol–gel dip-coating technique on glass, fused silica and (1 0 0)-silicon substrates. Two different sol–gel routes were employed by using different precursor solutions, a highly acid solution and a polymer-like solution. The influence of sol composition and of the substrate type on the morphology, coating porosity, surface roughness, crystalline phases and grain size of the titania films were investigated in detail. In addition, the relationship between microstructural/morphological properties and the optical properties (energy gap, refractive index and extinction coefficient) and the hydrophilic performance of the coatings were evaluated. Our experimental results clearly indicate that the sol composition and substrate type remarkably influence the films' morphology and microstructure; moreover, they consequently modify the optical response and hydrophilic performances of the samples, showing that superhydrophilic titania coatings can be obtained opportunely by choosing the composition of the precursor sol–gel solution. Blue shift of the band gap energy and a band structure mutation from indirect to direct were also revealed. The hydrophilic properties and the change in the band gaps transition can be attributed to oxygen vacancies on the surface of the titania nanocrystallites that gives rise to Ti3+ sites and, consequently, to structural changes/defects of the anatase nanoarchitecture. These findings allow us to design and tailor the optical and hydrophilic properties of the titania coatings.

Nanostructured gadolinia thin layers with small sized crystalline grains (∅ ≈ 3–13 nm) were successfully prepared by the sol–gel process on fused silica and silicon substrates. The film growth, the chemical composition, the morphology and the microstructure of the films were investigated in detail by thermo-gravimetric analysis, differential thermal analysis, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and x-ray diffraction. The experimental results clearly show that the morphological and structural properties of the films are not influenced by the substrate properties and the Gd2O3 cubic crystalline phase (symmetry group ) is enhanced when the films are free from organic residuals and decompose to Gd-oxides. FTIR analysis of gadolinia layers grown on silicon suggest that the formation of Gd2O3 grains may favour the diffusion of oxygen through the coatings. The optical spectroscopy measurements evidence a blue shift of the energy gap and suggest a quantum confinement effect in nano-sized Gd2O3. In addition, two absorption features are observed, which can be attributed to a simultaneous presence of direct and indirect electronic transition indicating a modification of the band structure when the grain size changes from very small to larger Gd2O3 grains.