Tunable photonic crystals offer an interesting possibility to adjust the photonic band gap (PBG) as per requirement.
Various methods of achieving this have been tried that include polarization of liquid crystals, thermal effects and more.
Electrochromic (EC) materials in which a reversible optical property change can be induced with the application of a
small electric field offer a novel possibility to tune the PBG in a controlled and reversible way. The reversible chemical
change and the ensuing change of optical constants in these periodically arranged EC materials make the PBG tunability
possible. In a recent work we have demonstrated for the first time the PBG tunability of EC materials deposited in the
form of periodic inverse opals. This earlier work was carried out with the well known Tungsten Trioxide (WO3) EC thin
films into which lithium intercalation was done by a dry lithiation method. In the present work we report on the
fabrication of a simple tunable photonic crystal device based on electrochemical insertion/extraction of lithium based on
WO3 inverse opals.
Porous tungsten oxide films have been prepared by a nonhydrolitic sol-gel method using poly(ethylene glycol) (PEG) as a structure directing agent. The method entails the hydrolysis of an ethanolic solution of tungsten ethoxide (formed by the reaction of WCl6 with ethanol) followed by condensation and polymerization at the PEG-tungsten oxide oligometers interface. A highly porous WO3 framework was obtained after PEG was burned off by calcination at a relativley low temperature. AFM images of the films treated thermally show an ordered material rather than microscopic particulates. Both fibrilar nanostructures and striped phase can be obtained via this approach, depending on the concentration of PEG in the coating solution. XRD data from the fibrils indicate that they are crystalline with very small crystals, whereas the striped phases obtained with 20% PEG correspond to two crystalline phases, one, the stoichiometric WO3 and the other one an oxygen deficient phase, containing larger crystals (~28 nm). The results show that PEG promotes the formation of oxygen deficient phases and delays crystallization. Compared to WO3 with no PEG, the optical and electrochromic properties of the macroporous tungsten oxide films appear to be significantly improved. The formation of organized nanostructures is tentatively accounted for by the strong hydrogen bonding interactions between PEG and the tungsten oxide oligomers.
Nanocrystalline titania films were prepared by a complexing agent-assisted sol-gel dip-coating process. The effect of acetylacetone, diethanolamine and polyethylene glycol on the structure and morphology of the heat-treated titania films was examined by Raman and FTIR spectroscopy, x-ray diffraction and atomic force microscopy (AFM). The effect the complexing agents have on the anatase to rutile phase transition during the heat treatment process is studied. The understanding of this effect is expected to enhance our capacity to tailor the composition and morphology of films and thus their properties. The Raman and the infrared spectra of nanocrystalline titania films and the changes induced by the heat treatment were also investigated. We have studied the size of the crystallites in TiO2 films and its dependence on the type of complexing agent used.
The electrochromic (EC) property, reversible coloration of certain materials under double injection of ions and electrons, of transition metal oxides (TMO) such as tungsten trioxide is known to depend strongly on the nature and structure of these materials. Possibility exists to tailor the EC behavior of the different TMOs as per the optical modulation needed. In this work the electrochromic performance of three types of tungsten trioxide films deposited under different conditions leading to nanocrystalline, polycrystalline and amorphous films has been studied by dry lithiation method. A comparative study of the EC coloration of these three types of films has been carried out, with a special emphasis on the nanocrystalline films. The techniques of spectrophotometry and atomic force microscopy have been employed for this study. Each type of tungsten trioxide (WO3) film exhibits a special nature of coloration indicating the potential for its specific application. The nanocrystalline films seem to exhibit a higher overall coloration efficiency and a selective optical modulation compared to the polycrystalline or amorphous films. These films exhibit a very high degree of transmission in the clear state and a high degree of optical modulation concentrated in the infrared region. Hence, the NC films may be of more interest for smart windows from the point of view of energy efficiency. The amorphous films, and even the polycrystalline films under high degree of lithiation, may be more suited for large area display device application due to their efficiency coloration in the visible region of the spectra.
Two sets of WO3 thin films were prepared by rf sputtering from tungsten oxide and tungsten metal targets, respectively. Lithium insertion studies were carried out in these two types of films. It has been found that the films sputtered from the metal targets showed an irreversible uptake of lithium without any coloration. The aim of the present work is to study this specific behavior of such films in comparison to those deposited from tungsten oxide targets. These films have been studied by thermogravimetry, UV-Vis spectroscopy, electron probe, ellipsometry, and ATR-FTIR spectroscopy. The stoichiometry of both types of films is found to be identical. The ATR-FTIR spectra show a difference in the W-O stretching vibration pattern of the two types of films, clearly indicating structural differences in the as- deposited WO3 films. However, both types of films show the presence of little or no water in the as-deposited state. Upon the insertion of lithium it is found that the films deposited from the oxide target show the formation of 'bronze' right from the very early stage of insertion as indicated by the blue coloration. This formation is reflected in the ATR-FTIR spectra by the increasing slope of the absorption in the mid-infrared range. Contrary to this behavior, the films deposited from the metal target do not show any coloration in the initial stages of lithium insertion. The infrared spectra corresponding to this behavior show little change in the mid-infrared range. However, a widening of the W-O stretching band is observed in these spectra reflecting a structural reorganization in the films. With continued lithium insertion, however, the behavior in the two types of films becomes similar. Hence, in the initial stages of coloration the quantity of lithium participating in the electrochromic coloration of the films deposited from metal target seems to be lower than in the other types of film. The optical constants of these films have also been determined using reflection- transmission ellipsometry showing important differences in the films. Hence, the electrochromic behavior of sputtered tungsten oxide films depends strongly on the type of deposition.
Ellipsometric studies are generally carried out in reflection mode rather than in transmission mode, requiring invariably opaque substrates or substrates in which the back reflection is minimized or suppressed by different methods. In the present work, we have used a transmission and reflection photo-ellipsometry 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 carried out in conjunction with reflection measurements improves greatly the accuracy in the determination of the optical constants. Optical measurements have been carried out on an automatic reflection-transmission spectrophoto- ellipsometer. Solid state ionics 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, has been developed to model and analyze such systems. It is demonstrated that the photo-ellipsometry method proposed is especially suited to analyze in a non-destructive way electrochromic materials and transmitting devices.
In the present work we have studied in detail the lithiation behavior of reactively sputtered crystalline tungsten oxide (WO3) films. These films, depending on their thickness and other deposition conditions, exhibit a wide range of changes under lithium insertion from deep coloration to no coloration at all. This gives the possibility of using the WO3 films either as electrochromic (EC) layer or as counter electrode (CE) layer for lithium ion storage, respectively. From these results a quasi- symmetric electrochromic system encompassing a- WO3 as EC layer and c-WO3 as the CE layer is fabricated and studied for its electrochromic behavior.
Electrochromic systems with their reversible optical switching ability offer important and interesting application possibilities. Work was carried out in our laboratory towards the fabrication of an all-solid electrochromic system. Lithiation studies of two transition metal oxides (W03 and V205) were done for their use as base electrochromic layer and counter electrode layer in the solid system. Also a thermally evaporated layer of LiBO2 was studied for its application as ion conducting layer. The optimum results from these studies were used for the fabrication of an all-solid electrochromic prototype. The results on the electrochromic performance of this system are presented. Several hundred coloration and bleaching cycles of such systems have been carried out without any change in its performance. All results indicate the high suitability of such a system for "smart" window application.
Design considerations and fabrication method of an all-solid-state five layer electrochromic (EC) system are discussed. A dry method of lithiation of the system is proposed and characterized completely for its use in the fabrication of EC systems. Some preliminary data on the EC switching of the complete system are presented.