G. Bader

Tunable electrochromic photonic crystals

Photonic crystals based on the electrochromic phenomenon have been fabricated and proposed for band gap tuning. Electrochromic tungsten trioxide (WO3) inverse opals have been fabricated by polystyrene colloidal crystal templating. The WO3 matrix was obtained through a dip-infiltrating sol-gel process, with subsequent removal of the polymer microspheres by calcination. Scanning electron micrographs confirm the ordering of the hexagonal macroporous structure. The reflection spectra show two pronounced Bragg diffraction peaks. By inserting lithium into the crystals, the first reflection peak shifts gradually toward shorter wavelength for 36 nm, while the second reflection peak shifts toward longer wavelength for about 28 nm. This should be of great interest for photonic device applications.

Study of the phase transition and the thermal nitridation of nanocrystalline sol–gel titania films

Abstract Nanocrystalline titania films were prepared by a complexing agent-assisted sol–gel method and converted to titanium nitride by a thermal nitridation process. The effect of acetylacetone (AcAc), diethanolamine (DEA) and acid catalysts (HCl and HNO 3 ) on the structure and morphology of the heat-treated titania films and on their nitridation products was examined by FTIR spectroscopy, X-ray diffraction (XRD) and atomic force microscopy (AFM). The carbothermal reduction of titania during the nitridation process with the formation of carboxynitrides has been considered. The results showed that the oxide to nitride transition strongly depends on the complexing agent used to prepare the titania films. The XRD results indicated the dependence of the lattice parameter of the nitridation product on the complexing agent or acid catalyst: AcAc and DEA lead to TiN x with a lattice parameter α close to the theoretical value, while with HCl the lattice parameter was found sensibly lower showing the presence of an oxynitride. These results are accounted for by the effect of complexing agents and acid catalysts on the size of both TiO 2 and TiN grains and the different reactivity of the anatase and rutile phases. The possibility of tailoring the composition and morphology of TiN films by using complexing agents is envisaged.

Lithiation studies on some transition metal oxides for an all-solid thin film electrochromic system

A study of the lithiation behavior of some transition metal oxides has been carried out. Using a dry method of lithiation changes in optical and electrical properties of WO3, V2O5 and Nb2O5 films prepared under different conditions are studied. The optimum results and techniques are used for the fabrication of an all-solid electrochromic (EC) system based on the utilization of high coloration efficiency WO3 as the base electrochromic layer and the low coloration efficiency V2O5 as the counter electrode. LiBO2 has been used as the ion conducting layer. Such system has been operated over several hundred optical switching cycles without any degradation so far. All results indicate the suitability of the system for “smart” window application.

Reflection–transmission photoellipsometry: theory and experiments

We propose a method that uses reflection and transmission photoellipsometry to analyze samples consisting of thin films combined with semitransparent thick layers or substrates in the form of multilayer structures. Athick film or substrate is defined as a layer for which no interference effects can be observed for a given wavelength resolution, and contributions from multiple reflections in the substrate are taken into account in the theoretical treatment. An automatic reflection-transmission spectroscopic ellipsometer was built to test the theory, and satisfactory results have been obtained. Examples corresponding to a strongly absorbing film deposited on a glass substrate and a highly transmitting film also deposited on glass are shown. In both cases a good fit between theory and experiment is found. The photoellipsometric method presented is particularly suited to the analysis of actual samples of energy-efficient coatings for windows.

Transmission and reflection ellipsometry of thin films and multilayer systems

Ellipsometric studies are generally carried out in the reflection mode rather than in the transmission mode, requiring invariably opaque substrates or substrates in which the backreflection is minimized or suppressed by different methods. In the present research we used a transmission and reflection photoellipsometry method to study electrochromic materials and their multilayer systems deposited on thick substrates. The role of the substrate is examined carefully, and the contributions from multiple reflections in the substrate are taken into account in the theoretical treatment. This procedure not only allows the study of thin films deposited on quasi-transparent substrates, but when carried out in conjunction with reflection measurements it greatly improves the accuracy in the determination of the optical constants. Optical measurements are carried out on an automatic reflection transmission spectroscopic ellipsometer. Solid-state ionic materials used in electrochromic systems such as indium tin oxide, tungsten oxide, and their multilayer structures deposited on glass substrates are used as examples. A software based on the above theory, optikan, was developed to model and analyze such systems. It is demonstrated that the photoellipsometry method proposed is especially suited to analyzing electrochromic materials and transmitting devices in a nondestructive way.

Low-loss one-dimensional metallodielectric photonic crystals fabricated by metallic insertions in a multilayer dielectric structure

Low-loss one-dimensional metallodielectric photonic crystals were obtained by inserting Ag layers into ZnS/MgF2 quarter-wave multilayers. They were fabricated by thermal evaporation of ZnS, MgF2, Ag, and MgF2 layers alternately onto a glass substrate. For a sample of 3.5 periods of ZnS (61.7 nm)/MgF2 (52.5 nm)–Ag (19 nm)–MgF2 (52.5 nm), the transmittance spectrum shows a wide and deep band gap between 420 and 790 nm with rejection level of 10 dB per lattice. Above 790 nm, there is a 210 nm wide high passband. The transmittance at the bandedge is about 60%. Its reflectance spectra over an incident angle range of 10°–70° show a 240 nm wide low-loss omnidirectional high reflection band for both p and s polarizations. All the experimental measurements match well with the theoretical results calculated by the transfer matrix method.

Optical and electrochemical properties of vanadium pentoxide gel thin films

Optical and electrochemical properties of vanadium pentoxide (V2O5) films prepared by sol‐gel method from organic and inorganic precursors were studied. The organic gel was obtained by hydrolysis and polycondensation of vanadium isopropoxide dissolved in isopropanol. The inorganic gel was prepared from aqueous solution of sodium vanadate. Lithium insertion in LiyV2O5 films (0<y≤2.0) was investigated by cyclic voltammetry and electrochemical potential spectroscopy, showing good reversibility up to y∼2.0 and structure transitions from α phase to e phase and δ phase in the range 0<y≤1.0. Highest values for chemical diffusion coefficient were about 3×10−10 cm2/s at y∼0.8, as determined from complex impedance measurements.

Structure and infrared spectra of sol—gel derived tungsten oxide thin films

Abstract FT-IR/ATR spectroscopy and transmission/reflection ellipsometry were used to study the structure and the optical properties of hydrated tungsten oxide thin films prepared by a sol—gel synthesis. The formation of the film on an internal reflection element was followed spectroscopically. Bands were assigned to WO bonds belonging to different condensed species. The close relationship between the spectral pattern, and the type and hydration state of the substrate is evidenced. The thickness and the optical properties of the hydrated tungsten oxide film were determined by transmission/reflection ellipsometry.

p‐polarized optical properties of aggregated Au films

Transmittance with p‐polarized light at oblique incidence has been measured for aggregated Au films in the region from 0.40 to 0.65 μm. At large values of the incidence angle, two minima associated with the perpendicular and parallel plasma resonances in the particles were observed. The dependence of these absorption features on film characteristics was accounted for by a simple effective medium model assuming a spheroidal shape for the particles. In this model, the substrate effect has been incorporated by mirror‐image contributions of the induced dipoles. A log‐normal distribution for the axial ratio was assumed in order to take into account the broadening of the absorption characteristics. Satisfactory agreement between theory and experiment has been obtained using only the optical thickness as a fitting parameter, and assuming a certain standard deviation for the log‐normal distribution.

Study of lithium intercalation into tungsten oxide films prepared by different methods

Abstract The interaction of lithium atoms into amorphous tungsten oxide films deposited by thermal evaporation, sputtering and sol-gel synthesis is studied spectroscopically. The mechanism of insertion of lithium into the film depends on the amount and the bonding state of water to the tungsten oxide network. Thermally evaporated tungsten oxide film contains water bonded chemically in the form of a hydrogen tungsten bronze which facilitates the accommodation of a high number of lithium atoms. In the sputtered film, water is physisorbed and the lithiation is incomplete. Tungsten oxide films prepared by sol-gel synthesis have structural water molecules directly involved in the lithiation process. The formation of lithium tungstate independent of the preparation method is demonstrated.