Enrico Masetti

Ultraviolet optical and microstructural properties of MgF2 and LaF3 coatings deposited by ion-beam sputtering and boat and electron-beam evaporation

Single layers of MgF2 and LaF3 were deposited upon superpolished fused-silica and CaF2 substrates by ion-beam sputtering (IBS) as well as by boat and electron beam (e-beam) evaporation and were characterized by a variety of complementary analytical techniques. Besides undergoing photometric and ellipsometric inspection, the samples were investigated at 193 and 633 nm by an optical scatter measurement facility. The structural properties were assessed with atomic-force microscopy, x-ray diffraction, TEM techniques that involved conventional thinning methods for the layers. For measurement of mechanical stress in the coatings, special silicon substrates were coated and analyzed. The dispersion behavior of both deposition materials, which was determined on the basis of various independent photometric measurements and data reduction techniques, is in good agreement with that published in the literature and with the bulk properties of the materials. The refractive indices of the MgF2 coatings ranged from 1.415 to 1.440 for the wavelength of the ArF excimer laser (193 nm) and from 1.435 to 1.465 for the wavelength of the F2 excimer laser (157 nm). For single layers of LaF3 the refractive indices extended from 1.67 to 1.70 at 193 nm to approximately 1.80 at 157 nm. The IBS process achieves the best homogeneity and the lowest surface roughness values (close to 1 nm(rms)) of the processes compared in the joint experiment. In contrast to MgF2 boat and e-beam evaporated coatings, which exhibit tensile mechanical stress ranging from 300 to 400 MPa, IBS coatings exhibit high compressive stress of as much as 910 MPa. A similar tendency was found for coating stress in LaF3 single layers. Experimental results are discussed with respect to the microstructural and compositional properties as well as to the surface topography of the coatings.

A comparison of the electrochromic properties of WO3 films intercalated with H+, Li+ and Na+

The electrochromic response of W03 thin films under electrochemical insertion from nonaqueous electrolytes of one of the three different ions: protons, lithium and sodium cations is reported. In spite of the common belief that the nature of the ion does not change the electrochromic effect, we show that the sample colouring and bleaching are dependent on the intercalant ion and on the insertion rate. The facile insertion of protons is responsible for the highest optical contrast and the quickest response time of this intercalation electrode. Lithium and sodium-intercalated electrodes do not show large optical difference from the proton-intercalated tungsten trioxide unless large charging currents are used. Subtle changes in electrochromic efficiency and in the optical contrast in the red part of the spectrum can be detected; these are larger for the sodium than for the lithium intercalant. According to our analysis, the slow insertion kinetics of Na+ and the formation of a new MxWO3 (M=Li, Na) compound is responsible for most of the observed differences.

Optical and electrochemical properties of cerium-zirconium mixed oxide thin films deposited by sol-gel and r.f. sputtering

Films of Ce‐Zr mixed oxide were produced by sol‐gel and r.f. sputtering. These films can be used as ‘passive’ counter-electrodes in electrochromic smart windows because they retain their full transparency in both the oxidised and reduced state. Li intercalation was accomplished electrochemically using a liquid electrolyte. Electrochemical behaviour of the samples was found to be dependent on the heat treatment (sol‐gel deposited film) and crystallite orientation (sputter deposited films). XRD analysis on sputter deposited films showed that the films are crystalline and grow following the orientation of the underlying tin doped indium oxide (ITO) film. Films of Ce‐Zr mixed oxide lacking in (111) crystallite orientation show continuous evolution of the voltammograms and reach a maximum value for the cycled charge only after a large number of cycles. The lithium diAusion coeAcient, calculated from GITT measurements, is in the range 10 ˇ12 ‐10 ˇ14 cm 2 s ˇ1 for sputter deposited films and becomes as low as 10 ˇ15 cm 2 s ˇ1 for sol‐gel deposited films. Optical constants of the thin films were calculated from reflectance and transmittance spectra. Refractive index values are in the range of 2.15‐2.30 at l=633 mn depending on the deposition method. A sharp absorption edge at about 320 nm is seen in accordance with CeO2 optical properties. # 1999 Elsevier Science Ltd. All rights reserved.

Electrochromic windows based on polyaniline, tungsten oxide and gel electrolytes

Abstract The characteristics and the performance of all-plastic, flexible electrochromic devices made using polyaniline and tungsten oxide as electrodes and a gel electrolyte are presented and discussed. Both electrochromic materials were deposited on indium-tin-oxide plastic substrates. Besides a very promising behaviour in term of light modulation and cyclability, this particular design allows flexible and not-fragile windows to be realized.

Sputter deposited cerium–vanadium oxide: optical characterization and electrochromic behavior

Vanadium pentoxide has been the subject of many works owing to its capacity to host large amounts of lithium. Many studies where V2O5 has been mixed with different transparent oxides, such as CeO2 ,Z rO 2 ,T iO 2, have been performed in order to preserve the ion-storage capacity and compensate the unwanted multicolor cathodic electrochromism. In the present work, we report on the optical properties of cerium and vanadium mixed oxides obtained by radio frequency sputtering in reactive ArO2 atmosphere. Since the electrochromic properties of thin films strongly depend on the oxidation state of the transition metal in the oxide, the role of oxygen partial pressure during the deposition is of fundamental importance. A set of samples was deposited by decreasing the oxygen partial pressure in the chamber from 1: 6t o 1:1000 of the total pressure and their optical properties were compared to those of the samples deposited in pure argon. The majority of deposited films changes coloration from yellowish to transparent upon lithium insertion, showing an anodic-like electrochromic behavior. Optical constants (refractive index and extinction coefficient) and film thickness were evaluated in the visible range from photometric measurements of both as-deposited and intercalated films.

Study of lithium diffusion in RF sputtered Nickel–Vanadium mixed oxides thin films

Abstract The kinetics of electrochemical lithium insertion inside RF sputtered Ni/V mixed oxides thin films have been investigated employing different electrochemical techniques. The AC electrochemical impedance spectra, recorded after 10 cycles, showed three steps clearly involved in the intercalation mechanism of lithium in the oxide films: (i) a charge transfer process to the electrolyte/electrode interface; (ii) a solid-state diffusion of Li; and (iii) a space limited diffusion due to the finite volume of the film. This latter portion of the impedance spectra was used to calculate the Li chemical diffusion coefficients ( D Li ). D Li values show an initial increase up to an injected charge of 30 mC cm −2 and then, upon further intercalation, a decay probably due to the limited Li motion through a decreased number of available sites. These results are in good agreement with the ones obtained by potentiostatic intermittent titration technique applied to the same electrodes. Both techniques agree in giving the same trend for D Li upon Li intercalation. The same sets of measurements were carried out after 100 and 1000 Li insertion/deinsertion cycles in order to analyze the effect of prolonged cycling on the mechanism of lithium diffusion.

Development of a novel ellipsometer based on a four-detector photopolarimeter

Abstract A novel ellipsometer was developed for real time analysis of the growth of thin films; its construction and testing is reported. It is based on a four-detector photopolarimeter, a polarization detection system where the light is successively reflected from the first to the fourth detector in such a way that the incidence plane is changed at each reflection. The four measured currents are linearly related to the Stokes polarization vector of the incombing light through a matrix that can be worked out at each wavelength by calibration. As UV enhanced silicon detectors are employed, the instrument is able to operate in the 300–1100 nm optical range and the time resolution depends only on the specific detector and electronic speeds. Besides the photopolarimeter, only a linear polarizer is required to set up the ellipsometer, therefore it is a very compact and inexpensive instrument. The main drawbacks are related to the amplitude division and to the very small optical aperture. A laser is required as light source and at present the effective wavelength range is limited. The ellipsometer was calibrated and tested at 632.8 nm (HeNe laser). The averaged error for the measurement of Ψ and Δ were respectively 0.13° and 0.20°. The ability to characterize the surface was checked by comparing results with those obtained using a null ellipsometer.

Sputter-deposited cerium vanadium mixed oxide as counter-electrode for electrochromic devices

Abstract Oxide films deposited by reactive radio frequency sputtering of target made of mixed CeO 2 and V 2 O 5 (1:1) powders show an anodic electrochromic character upon lithium insertion. As-deposited films look yellowish and become fully transparent for an inserted charge of 30 mC/cm 2 of Li ions. A total charge of 60 mC/cm 2 has been reversible cycled in the 1 to 5 V versus Li range in an electrochemical cell without any significant degradation. Following these preliminary results, films of Ce–V mixed oxide made by r.f. sputtering are suitable for working in tandem with tungsten oxide in a practical electrochromic device.

Analysis of the influence of the gas pressure during the deposition of electrochromic WO3 films by reactive r.f. sputtering of W and WO3 target

Abstract Tungsten trioxide is the most accepted material for electrochromic devices. In the work thin films of WO 3 were deposited by reactive r.f. sputtering of both metallic (W) and ceramic (WO 3 ) targets to study the correlation between the electrochromic properties and the structures of the films. Samples were grown at different pressures of Ar+O 2 in order that the energy regimes of the sputtered particles on the condensing surface could be set either below or above the thermalisation diffusion limit. Lithium ions were intercalated in the films in an aprotic electrolyte and the colouring/bleaching behaviour as a function of the intercalated amount of lithium was detected in the 1st and 10th cycle. From these measurements, the electrochromic properties of the films were worked out. The optical and morphological characteristics were analysed respectively, by spectrophotometric and X-TEM measurements. The amount of water present in the films, detected by IR spectroscopy, turned out to be well correlated to the film morphology and also to the porosity.

Electrochemical and optical characterization of RF-sputtered thin films of vanadium-nickel mixed oxides

The optical and electrochemical properties of thin films of a nickel vanadium mixed oxide have been investigated as a new Li + intercalation compound, to be used as an optically passive counterelectrode in electrochromic switching devices. The thin film samples were prepared by radio frequency (RF) sputtering under different preparation conditions, using targets with different Ni V ratios and a reactive (20% O 2 ) or, alternatively, non-reactive (pure Ar) sputtering atmosphere. The optical properties of as-grown and Li + inserted extracted electrodes were determined by computer fitting of the UV Vis NIR transmittance and specular reflectance spectra. The electrochemical measurements of Li + insertion and extraction consisted of cyclic voltammetry, chronopotentiometry and potentiostatic intermittent titration technique. The samples showed interesting electrochemical and optical properties, a good electrochemical reversibility and ion-storage capacity exceeding 40 mC cm -2 , lasting for several hundreds cycles at a current of 50 μA cm - 2 . The electrodes displayed an almost passive electrochromic behavior during the electrochemical experiments.