Chromogenic materials and devices

Resume : Cholesteric liquid crystal polymers display selective reflection properties arising from their helical ordering. The temperature response of these polymers, comprising dynamic reflection color changes upon variation of temperature, can be exploited in energy-saving smart windows. In my lecture, the design, fabrication, and application of temperature responsive infrared reflective polymer coatings will be discussed.

Resume : Thermochromic materials are used in a wide range of applications such as temperature memory sensors (cold chain break sensor, threshold temperature sensor for aeronautic application?), temperature sensors for safety (kitchen tools?), smart materials for specified application (smart windows or concrete for building energy efficiency?) or in day-to-day life as smart pigment (textile, ink, packaging?). NiTiO3 is a yellow pigment which adopts a crystalline structure (ilmenite) very close to corundum and thus to hematite. Ilmenite structure is based on a hexagonal close-packed anion lattice with cations occupying two-thirds of the available octahedral interstices (space group R ). Nickel titanate is actively studied for its photocatalytic and magnetic applications but not for thermochromic ones. As chromium-doped corundum and hematite present thermochromic behaviour, the compound was studied to detect any potential ability. The powders were synthesized via a modified Pechini method followed by calcination step (700 °C). For the first time, thermochromic properties have been evidenced on this compound in the temperature range (Room Temperature (RT) ? 400 °C), passing from dark yellow colour at RT to darker brown at 400 °C. The optical properties of the powders have been evaluated using visible spectrometry vs temperature and ?E colour difference measurements. L* and b* parameters both decrease whereas a* increases with temperature. The colour change, i.e. the thermochromism, is continuous and totally reversible as L*a*b* parameters come back to their original values after returning to RT. The cycling has been also checked. Since metal doping of this compound is known to change its colour, we also investigate the effect of doping on the final colour and thermochromism. In order to better explain the thermochromism mechanisms, a deep study of the material was made using High-Temperature X-Ray Diffraction combined with Rietveld refinements, Raman spectroscopy.

Resume : The building sector consumes approximately one third of the total energy worldwide. A large part of this energy is used for heating and cooling of buildings, which can be drastically reduced through application of energy-efficient windows. Novel smart window technologies can provide optimal use of solar heat in regions with changing requirements from summer to winter. Here, we studied vanadium dioxide (VO2), which is a promising optical material for this purpose because of its thermochromic property. Based on its structural phase transition from the semiconductive monoclinic to the conductive rutile structure at a defined switching temperature of 68°C, it switches from a solar infrared transmissive to blocking state. This switch is reversible, the switching temperature can be lowered via doping, and VO2 comprising materials are transparent in the visible, which makes them interesting for application in thermochromic windows that regulate solar heat gain. Very recently, we have reported the preparation and characterization of thermochromic coatings consisting of VO2 and SiO2, the switching kinetics of thermochromic powders, and the impact of thermochromic windows on energy savings in intermediate climates. Here, we report the development of thermochromic coatings and laminates comprising W-doped VO2 nanoparticles as functional pigments. We obtained these pigments via wet bead milling of a W-doped VO2 powder obtained by high-temperature conversion of organometallic precursors in a tube furnace. By DSC and XRD analyses, we demonstrated that high purity W-doped VO2 powders were formed, which were successfully milled to functional thermochromic nanopigments with a switching temperature of 22°C. We applied the nanopigments in both polyvinyl butyral (PVB) polymer films and in silica sol-gel coatings with tunable optical properties for smart window applications. For laminated glass using VO2-pigmented PVB films, a visible transmission of more than 60% with a solar modulation of 8% was reached. Similar optical properties were reached for VO2-pigmented silica coatings with a layer thickness of 300 nm.

Resume : VO2 is a broadly studied thermochromic material, which undergoes a metal-insulator transition approx. at 68°C, accompanied by a crystallographic transition from a low-temperature monoclinic phase to a high-temperature tetragonal rutile structure. Optical and electrical properties change drastically with the phase transition and offer a path to various applications, such as thermal solar collectors, ultrafast electronic devices, smart windows... Nevertheless, the elaboration of pure VO2 films on large surfaces remains a challenge due to the difficult task of avoiding other phases belonging to the vanadium-oxygen binary system [1]. In previous studies, we have demonstrated that the oxidation of VN thin films is a new method to form high-quality thermochromic VO2 [2,3]. The present work aims to achieve thermochromic VO2 from the controlled oxidation of another new precursor: sputter-deposited V2N films. Thermochromic VO2 films have been obtained by air oxidation of V2N samples performed at two temperatures (450 and 550°C). X-ray diffraction and Raman spectrometry of the V2N oxidized films evidence that VO2 and V2O5 are the only phases obtained throughout the oxidation process. VO2 is the first oxide formed coexisting with V2N for a long time at 450°C or swiftly vanishing at 550°C. EELS results show that the oxidation of V2N to VO2 occurs in two stages. First, the V2N turns into VN before the oxidation to VO2. Further oxidation leads to the formation of V2O5, diminishing the thermochromic performance of the films. As the thickness of the initial V2N layer increases from about 100 nm to 445 nm, the optical modulation properties of the obtained VO2 films change from a negative value of the emissivity switch to a positive value. These results may pave the way for future research on the oxidation of V2N as a new precursor to form high-quality thermochromic VO2. [1] Yang et al., Annu. Rev. Mater. Res. 41 (2011) 337?367. [2] García-Wong et al., Sol. Energy Mater. Sol. Cells. 210 (2020) 110474. [3] García-Wong et al., J. Materiomics. 7 (2021) 657?664.

Resume : VO2 is a well-known reversible material for its Insulator-to-Metal Transition (IMT) that takes place around 70°C. The closeness between the room temperature and the IMT temperature makes VO2 a suitable candidate for resistive switching systems and thermochromic materials1. The growth by Atomic Layer Deposition (ALD) allows running deposition at temperatures lower than 250°C, enabling deposition on all kinds of substrates, with a low thermal budget. Vanadium oxide films were deposited at 240°C on silicon (100) and glass substrates from water as an oxidizing agent and vanadium tri-isopropoxide (VTIP) as vanadium precursor. Annealing was held at a temperature ranging from 400°C to 550°C by 50°C steps, under forming gas (95% Ar, 5% H2) for 60 minutes. A structural analysis was performed by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), and Atomic Force Microscopy (AFM), showing that as-deposited films are amorphous, whereas crystallization in VO2 and V2O5 phase happens around 500°C. Crystallized films are polycrystalline with crystallites reaching 0.3 µm. For a better understanding, XPS and RBS studies will also be presented. Optical and electrical analyzes were also conducted by Raman spectroscopy, Fourier Transformed IR Spectroscopy (FTIR), ellipsometric spectroscopy, and electrical resistivity measurements. According to these spectroscopic techniques, films annealed at 500°C present a repeatable IMT around 70°C. For films on silicon substrate, FTIR spectra show a specially interesting result with the increase of the optical absorbance from 0.1 OD at room temperature up to 0.9 OD above IMT temperature on a wavenumber range extending from 4000 to 1000 cm-1, alongside the disappearance of the low-temperature VO2 peaks in FTIR and Raman spectroscopy. The IMT effect is also observable in the UV-visible range on the glass substrate. For electrical measurements by 4-probe resistivity, the resistivity was found to decrease down to 10-1 ?.cm upon IMT temperature. On the whole, vanadium oxide films were characterized between RT and 100°C and present encouraging properties adjustable with temperature, especially in the IR range, and paves the way for future applications in thermochromic materials. 1. Prasadam, V. P. et al. Atomic layer deposition of vanadium oxides: process and application review. Mater. Today Chem. 12, 396?423 (2019).

Resume : Tungsten oxide (WO3) is the earliest discovered, and most widely used inorganic electrochromic material, which can change its optical properties (transmittance, reflectance, or absorption) when induced by an electrical current or voltage, resulting in variable colors. Owing to its high stability, strong weatherability, good adhesion to glass substrates and ease for large-scale production, the WO3 material has a high commercial prospect. However, in its pristine form, WO3 only displays a one-fold color modulation from transparent to blue, making it very difficult to achieve the utmost goal of full-color tunability for future electrochromic technology. Here, we introduced a Fabry-Perot cavity structure, tuning the thickness, refractive index, repetition period of the dielectric material in the cavities and photonic bandgap, realizing muti-color inorganic electrochromic devices. Such a simply structured device can significantly change to other more brilliant colours by changing the optical indices (n, k) of the tungsten oxide layer through ion insertion under applied potentials. Meanwhile, we emphasized the integration of multicolor electrochromic devices with other advanced energy, environment and electronic technologies to obtain new concept and multi-functional devices of new intelligent energy storage systems, high color purity devices and ?Janus? devices.

Resume : In space, the thermal equilibrium of a satellite versus its environment is governed by radiative exchanges. In order to save on-board weight and electrical power, it is strongly interesting to develop new IR-switchable coatings that are able to operate variable infrared emissivity. Indeed such technology can hence offer a high infrared emissivity in ?hot cases? when it is needed to reject heat, and a low emissivity during ?cold cases? when it is suitable to keep the heat within the satellite orbiting around the Earth. Indeed the installed heating power sized on the coldest thermal case can be drastically reduced.[1] In this context and to mimic the satellite temperature conditions, a small radiator has been built: it consists of an assembly of a matrix made of 4x4 electroemissive tiles on an aluminum plate preliminarily equipped with thermistors[2]. A frame cover ensures electrical contact with the 16 tiles; the latter consists in a PEO/NBR semi-interpenetrating polymer network with two PEDOT layers on each side.[3] Hence, adding an electrolyte, the tiles can be operated as a two-electrodes system in which one PEDOT layer exhibits IR-optical switches. The radiator total active surface is 82cm². Once embedded with Kapton®, the radiator is placed in a vacuum chamber and tested under thermal conditions close to the space environment (-25°C up to 80°C) . A thermal sequence has been setup to follow the behavior of the radiator in both ?hot? and ?cold? cases (without taking into account the solar flux) and, at the same time, compared to optical solar reflectors (OSR) taken as a reference. When switched between high and low emissivities according to the thermal load, the radiator was quite more effective than the OSR in preserving the heat inside the satellite during cold environments while in a "hot" environment, the thermal rejection was close to that of the OSR proving the interest of this technology in the thermal regulation of satellites. To this efficiency in the thermal regulation is associated a significant saving in on-board installed electric power dedicated to the heating in cold phases of the mission and thus a reduced mass of the satellite (smaller solar panels and/or batteries). Moreover this energy saving allows a more efficient use of electrical propulsion during Electrical Orbit Raising (EOR) phase and should allow then to reduce drastically the duration of this non-operational phase. In a second part, we will discuss some important aspects especially the reduction of solar absorption and the possibility to prepare flexible tiles to reduce again the on-board weight of the radiator. References [1] F. Vidal, G. Petroffe, L. Beouch, S. Cantin, C. Chevrot, P.-H. Aubert, J.-P. Dudon, Active Thermal Control of Satellites with Electroactive Materials. In: Rasmussen L. (Eds) Smart Materials. Springer, Cham., 2022, pp. 221?254. [2] G. Petroffe, L. Beouch, S. Cantin, C. Chevrot, P.-H. Aubert, J.-P. Dudon, F. Vidal, Sol. Energy Mater. Sol. Cells 2019, 200, 110035. [3] L.J. Goujon, P.-H. Aubert, L. Sauques, F. Vidal, D. Teyssié, C. Chevrot, Sol. Energy Mater. Sol. Cells 2014, 127, 33?42.

Resume : Electrolytes play an indispensable role in electrochromic devices as it facilitates redox switching process by providing essential ions and balancing residual charges. However, many insurmountable problems exist with typical electrolytes which generally made up of salts in aqueous or organic medium. Aqueous electrolytes, though having good ionic conductivities and relatively safe, are widely limited by their narrow operating temperature and potential. While organic electrolytes can offer better thermal and electrochemical stabilities, their flammability nature is a serious drawback that could restrict widespread applications. Ionic liquids (ILs), which are basically room-temperature liquid salts, are promising and safer alternatives to conventional electrolytes, owing to their outstanding stability and safety features. Nevertheless, high-fluidity nature of ILs could induce a serious risk of leakage and complicates device fabrication, eventually leading to device failure. In this regard, a rationally designed vinyl-functionalized IL monomer has been prepared with the aim of realizing free-standing poly(ionic liquids) (PILs) film for electrochromic applications. The electrolyte ink formulation exhibited a fast-curing process ( < 30 s) under ambient condition, rendering a film with high transparency (> 90 %), excellent ionic conductivity (ca. 4 mS/cm), and good thermal characteristics (>300 ?C). Taking the advantage of in-situ curing process, printing of the ink has also been realized, providing a new strategy for high throughput coating of solid electrolyte. For the electrode materials, a coordination polymer (CP) has been prepared via a highly reproducible electropolymerization process. The growth of this CP on the electrode can be facilely manipulated by electrochemical parameters, resulting in different surface morphology and controllable thickness. Detailed analysis is also established, showing that the CP film growth has taken place in an anisotropic one-dimensional modality. By integrating this CP-modified electrode with the PIL, electrochromic devices have been demonstrated with high optical contrast, fast response time, and good cyclic endurance

Resume : Luminescent mechanochromic materials displaying switchable luminescence properties in response to external mechanical stimuli[1] are attracting wide interest for potential applications in strain detection, optical recording and storage, and anti-counterfeiting.[2,3] Regarding security issues, mechanically induced damages need to be detected at the early stage of stress formation[4] and luminescent mechanochromic materials enable the straightforward visible detection of strain with emission changes easily detectable by naked eyes or by simple non-invasive spectroscopic means.[5] Efficient mechanochromic luminescent materials should exhibit high-contrast luminescent change in emission color or intensity for an easy detection and this effect should be preferentially reversible. For practical applications of mechanical sensing, the design of thin films is suitable compared with powdered luminescent mechanochromic compound with no mechanical strength. In parallel, the in-depth understanding of the underlying mechanochromism mechanism is naturally of utmost importance for guiding the development of such stimuli-responsive devices. In this context, we report our investigations regarding the study of luminescent mechanochromic copper iodide complexes and their development as active component for the synthesis of mechanically responsive materials.[6] The luminescent mechanochromic properties of these compounds formulated [Cu4I4L4] (L = organic ligand) are characterized by a contrasted emission change upon mechanical solicitations. Moreover, these compounds exhibit luminescence thermochromism with temperature-dependent luminescence properties.[7] Structural characterizations and spectroscopic analyses supported by DFT (Density Functional Theory) calculations permit to get insights into the mechanochromism mechanism. The synthesis of mechanically-responsive films is also presented. [1] Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu and J. Xu, Chem. Soc. Rev., 2012, 41, 3878?3896. [2] H. Yu, X. Song, N. Xie, J. Wang, C. Li and Y. Wang, Adv. Funct. Mater., 2021, 31, 2007511. [3] C. Wang, D. Wang, V. Kozhevnikov, X. Dai, G. Turnbull, X. Chen, J. Kong, B. Z. Tang, Y. Li and B. B. Xu, Nat. Commun., 2020, 11, 1448. [4] M. E. McFadden and M. J. Robb, J. Am. Chem. Soc., 2019, 141, 11388?11392. [5] S. Kato, S. Furukawa, D. Aoki, R. Goseki, K. Oikawa, K. Tsuchiya, N. Shimada, A. Maruyama, K. Numata and H. Otsuka, Nat. Commun., 2021, 12, 126. [6] R. Utrera-Melero, B. Huitorel, M. Cordier, F. Massuyeau, J.-Y. Mevellec, N. Stephant, P. Deniard, C. Latouche, C. Martineau-Corcos and S. Perruchas, J. Mater. Chem. C, 2021, 9, 7991?8001. [7] S. Perruchas, Dalton Transactions, 2021, 50, 12031?12044.

Resume : In this research, an inorganic electrochromic device was fabricated to achieve quadruple modes using unique electrolyte chemistry inducing localized surface plasmon resonance(LSPR): 1) transparent, 2) visible light block, 3) near-infrared block and 4) Vis-NIR block. This ECD can control selectively visible light and near-infrared lights to reduce the consumption of heating and cooling energy as well as protect the privacy of the building. Tungsten oxide(WO3) thin films, which are conventionally used as inorganic electrochromic materials, and antimony Tin-doped Oxide(ATO) thin films, which are ion storage materials for charge balance within the devices, were fabricated by nanoparticle deposition system(NPDS), one of the dry deposition methods. ATL electrolyte which stands for mixing silver nitrate(AgNO3), tetrabutylammonium bromide(TBABr), and lithium perchlorate(LiClO4), has been developed in this study. The ECD was successfully produced by injecting ATL electrolytes including lithium and silver ions between WO3 and ATO thin films. Despite the simple structure, controlling the ion movements as well as the surface roughness of thin films overcomes the limitation of the single coloration of WO3 thin films. Indeed, NPDS can fabricate a much rougher surface than other methods such as sputtering. NPDS has supersonically sprayed ceramic or metal powders to the substrate through the pressure difference using a vacuum pump and air compressor. NPDS has the advantage of being cheap due to the low-temperature and low-vacuum process. Moreover, NPDS is an eco-friendly method since it does not require any toxic binder by directly spraying powders. In the case of WO3, it can only change from transparent to a blue color when lithium ions are intercalated to the atomic structure of WO3. At the blue state, WO3 thin films selectively block near-infrared wavelength. ?T which means the difference in light transmittance was measured to be 57 % at 900 nm. To achieve multiple modes, silver ions were controlled by the applied voltage. The optical properties of the ECD depend on the roughness of the surface which silver ions could deposit. When silver ions were deposited on the rough WO3 thin films, the scattering of incident light was maximized within the ECD. As a result, the ECD can block visible light wavelength and 0 % transmittance at all wavelength ranges was achieved due to the synergistic effect of WO¬¬3 coloration due to intercalation of lithium ions. Finally, the LSPR effect was used to selectively block visible light. LSPR is the phenomenon in which metal nanoparticles absorb a specific wavelength through an amplified electric field within the particles. Two-step voltages were applied to rough ATO thin films and then spherical silver nanoparticles were formed on ATO thin film and observed by SEM. Moreover, ?T was measured to be 51 % at 600 nm. In conclusion, the inorganic ECD successfully demonstrated four states using the rough surface of thin films as well as unique ATL electrolytes inducing the LSPR effect.

Resume : A decade ago, photochromic properties were first reported for yttrium-hydride thin films oxidized by air-exposure [1]. Since then, the material systems have been subject to thorough investigation by several research groups, in order to explain the physical origin of the effect, understand the details of the material?s nature as well as to tailor the material?s properties towards application. The group at Uppsala University has initially contributed to the field by comprehensive investigations of the chemical composition of ex-situ deposited thin films using in particular ion beam analysis. These studies included a correlation of the optical performance of the thin films with chemical composition, and helped to establish a phase diagram [2]. Specifically, we observe photochromism for an Y/O ratio of 0.5 to 1.5, with the largest photochromic effect close to 0.5 and monotonically decaying to 1.5. Later, these efforts were broadened to include a wider number of techniques for a comprehensive characterization of the systems, indicating a dual-phase nature of the investigated photochromic systems [3]. In parallel, we started to perform in-situ characterization correlating optical properties with the materials chemistry [4]. Subsequently, full in-situ synthesis and characterization of the systems was conducted, revealing details of the oxygen uptake and illustrating the complete synthesis pathway [5]. Very recently, the interaction of the material system with the environment and its necessity for the photochromic effect were investigated in detail [6][7]. These results showed a pronounced interaction of the system with different ambients but also proved the photochromic effect to be not dependent on material exchange with the environment. Additionally, we have recently started experiments using isotopic labelling to study diffusion in the material systems, revealing that long-distance migration of hydrogen does not play a role in the photochromic effect and that the oxidation of the films progresses along the grain boundaries of the columnar structure [8][9]. Finally, we currently perform complementary characterization of other properties such as photoresistivity during illumination and bleaching cycles. [1] T. Mongstad et al., Sol. Ener. Mat. Sol. Cells 95 (2011) 3596 [2] D. Moldarev et al., Phys. Rev. Mat. 2 (2019) 115203 [3] M. Hans et al., Adv. Opt. Mat. (2020) 2000822 [4] M.V. Moro et al., Sol. Ener. Mat. Sol. Cells 201 (2019) 110119 [5] K. Kantre et al., Scr. Mat. 186 (2020) 352 [6] D. Moldarev et al., J. Appl. Phys. 129 (2021) 153101 [7] M.V. Moro et al., Phys. Stat. Sol. RLL (2021) 2000608 [8] M.V. Moro et al., Phys. Stat. Sol. RLL (2021) 2100508 [9] D. Moldarev et al., Eur. J. Phys. Conf. Ser. (accepted)

Resume : The oxyhydride thin film research is a spin-off from the intensive research on the metal-to-insulator transition in yttrium and related rare earth metal hydrides. This transition can be induced by 1)hydrogenation from the dihydride to the trihydride phase [1], by 2)a reduction in free carrier density [2], and by 3)applying GPa pressure on a trihydride film [3]. Photochromism was first observed under 5GPa pressure and co-existing fcc dihydride and hcp trihydide phases[4]. No photochromism is observed in the transparent trihydride state. Only when adding a substantial amount of oxygen to the lattice do the transparent semiconducting films become photochromic. In most cases, these photochromic films are derived from reactively sputtered dihydride films. The metallic films oxidize on exposure to air, provided that they have a sufficient porosity. The oxidation is self-limiting and results in semiconducting films with a bandgap of about 2.6 to 3.5eV.[] The mechanism of photochromism in rare-earth oxyhydrides is still unclear. In my talk I will show recent characterization experiments using EXAFS, NMR, muSR and positron annihilation spectroscopy. While no final conclusion regarding the mechanism can yet be drawn, these experiments show that the point defect structure is important for the effect to occur. Moreover, indications for a reversible segregation into a small fraction of a metallic state is observed. Acknowledgements: This work was partially supported by the Mat4Sus Research Program with Project Number680.M4SF.034 and by the Open Technology research program with Project Number 13282; both financed by The Netherlands Organisation for Scientific Research (NWO). [1] Huiberts, J. N. et al. Yttrium and lanthanum hydride films with switchable optical properties. Nature 380, 231 (1996). [2]Hoekstra, A.F.Th. Light-induced metal-to-insulator transition in a switchable mirror, Phys. Rev. Lett. 86 (2001) 5349 [3] Palasyuk T., Tkacz M., Pressure-induced structural phase transition in rare-earth trihydrides. Part III. Systematics, Solid State Communications 141, (2007) Pages 354-358 [4] Ohmura,A. et al., Photochromism in yttrium hydride Appl. Phys. Lett. 91 (2007) p151904 [5] Mongstad, T. et al., A new thin film photochromic material: Oxygen-containing yttrium hydride, Solar Energy Materials & Solar Cells 95 (2011) 3596?3599

Resume : The reversibility and fatigability of the light triggered commutation of photochromic materials are crucial specifications for concrete applications. The family of dithienylethenes (DTE) is known for its excellent photochromic behavior in solution [1], but has shown limitation in the solid state in terms of complete, fast and reversible photoswitching. In particular, some of DTE crystals demonstrate breaking with high photoconversion yield, which makes structural analysis complicated. Polyoxometalates (POMs) are metal-oxide clusters that can efficiently tune or exalt the optical properties of photoactive molecules in the solid state [2-4]. Very recently, we have elaborated the first supramolecular assembly of a normal DTE cation with an octamolybdate unit [5]. This coupling modifies the photoresponse of the organic switch via both steric and electronic effects, and gives rise to an increased cycloreversion rate, resulting in a fast and complete re-opening process. The hybrid material also exhibits high light-driven ?recording?erasing? potentialities. The crystal structure after UV-light irradiation contains a mixture of open- and closed-ring DTE isomers with a conversion rate of about 28% which represents one of the highest conversions for a DTE-based material. Geometry optimization and electronic structures have been performed by ab initio DFT calculations to improve the structural model and nicely model the solid-state optical properties. In addition, five other supramolecular assemblies have been obtained by associating normal or mixed cationic DTEs with different POM units [6]. Combined X-ray crystallographic and spectroscopic analyses have highlighted the precise impact of DTEs and POMs on the photoswitching ability of the assemblies, and gave fine understanding on synergistic effects between both organic and inorganic components. References [1] M. Irie, T. Fukaminato, K. Matsuda, S. Kobatake, Chem. Rev. 2014, 114, 12174?12277. [2] H. Dridi, A. Boulmier, P. Bolle, A. Dolbecq, J.-N. Rebilly, F. Banse, L. Ruhlmann, H. Serier-Brault, R. Dessapt, P. Mialane, O. Oms, J. Mater. Chem. C 2020, 8, 637-649. [3] P. Bolle, T. Benali, C. Menet, M. Puget, E. Faulques, J. Marrot, P. Mialane, A. Dolbecq, H. Serier-Brault, O. Oms, R. Dessapt, Inorg. Chem. 2021, 60, 12602-12609. [4] P. Bolle, Y. Chéret, C. Roiland, L. Sanguinet, E. Faulques, H. Serier-Brault, P.-A. Bouit, M. Hissler, R. Dessapt, Chem. Asian J. 2019, 14, 1642-1646. [5] P. Bolle, C. Menet, M. Puget, H. Serier-Brault, S. Katao, V. Guerchais, F. Boucher, T. Kawai, J. Boixel, R. Dessapt, J. Mater. Chem. C 2021, 9, 13072-13076. [6] O. Stetsiuk, P. Bolle, M. Cordier, J. Boixel, R. Dessapt, J. Mater. Chem. C 2022, DOI : 10.1039/D1TC04561J.

Resume : Silver chloride (AgCl), as a compound among the photochromic class of silver halides, exhibits a reversible alteration in transparency and colour when exposed to sunlight and in particular, under intense ultraviolet (UV) radiation. Based on this it poses an excellent candidate for the development of advanced photochromic materials targeting a variety of applications. Aerospace applications in low earth orbit devices is among others a typical example in which the remarkable photochromic switching features of the AgCl salt could be explored. In principle, AgCl crystal can reversibly become non-transparent to the sunlight?s harmful ionizing rays that radiate onto the structure and electronics, while being transparent to higher-wavelength light (IR) that induces useful solar heat flux. Despite these benefits, a continuous scientific and technological challenge remains how to practically employ AgCl coatings and exploit its reversible photochromic features in real life applications. On this basis, we herein present a simple, low-temperature, post-glass melting encapsulation fabrication protocol in which AgCl thin layers are incorporated within silver metaphosphate glass (AgPO3). The selection of AgPO3 glass is mainly based on its relative ?soft? nature (Tg=192 oC) that allows the feasible embedment of the AgCl layer, while being transparent in most of the visible range, and thus suitable for smart photochromic windows applications. The described synthesis procedure allows the controlled positioning of a AgCl layer within the host glass matrix, while the properties of the layer itself could be modified accordingly. Namely, preliminary results show a direct dependence of the composite AgCl-AgPO3 glass photochromic response with the morphological features of the encapsulated AgCl layer, i.e. thickness and position. The photochromic response time upon varying UV irradiation dose is also considered. Ongoing work involves efforts on further enhancing the photochromic performance upon exploiting the presence of silver nanoparticles within the glass, as well as, by introducing periodic patterns on the glass surface by means of laser processing. As a final part of this study, theoretical studies of the developed composite glasses photochromic performance are conducted by means of 3U thermal nanosatellite structure model, targeting the optimization of the photochromic performance, upon varying the composition and structure of the proposed AgCl-AgPO3 glass architectures.

Resume : Rare-earth metal oxyhydrides represent a very promising class of new materials that exhibit a powerful synergy of oxide and hydride properties due to the strong coupling of charge, lattice, and composition degrees of freedom. This opens up the most interesting possibilities for the development of highly functional materials capable of working in different environments. Our studies were motivated by the experimental work performed on the synthesis and various properties of oxyhydride chemical systems [1-3]. However, numerous studies of these systems have raised a number of fundamental questions concerning the key factors and underlying mechanisms responsible for the formation pathways, stability, structural, electronic, and electrical features of these materials [4]. For example, the synthesis of a single-phase oxyhydride material is complicated by the fact that very little is known about how to control the formation of a given structural configuration and composition during oxidation. In the context of first-principles materials engineering, our main focus has been the systematic study of the structure and mechanisms of phase formation to understand how material properties can be controlled, manipulated, and altered. Based on the results of high-performance computational and crystal chemical modeling, we performed a structure-oriented analysis of the relevant structure-property-function relationships to explore how partial oxidation, stoichiometry effects, atomic packing, and compositional differences can be used to systematically govern the physical properties of rare earth metal oxyhydrides [5,6]. In the context of multifunctionality, we describe the technological perspectives of various applications of materials derived from rare earth metal oxyhydrides. Acknowledgments: A.P. was supported by the Estonian Research Council grant PRG347, S.Z.K. received funding from the Research Council of Norway through project 309827. References: [1] V.N. Fokin et al., Russ. J. Gen. Chem. 2004, 74(4), 489. [2] T. Mongstad et al., Sol. Energy Mater. Sol. Cells 2011, 95, 3596. [3] J. P. Maehlen et al., J. Alloys Compd. 2013, 580(1), 119. [4] A. Pishtshev, S. Z. Karazhanov, Solid State Commun. 2014, 194, 39. [5] A. Pishtshev et al., Cryst. Growth Des. 2019, 19, 2574. [6] E. Strugovshchikov et al., Pure Appl. Chem. 2021, 93(11), 1293.

Resume : Some butterfly species such as the orange oakleaf (Kallima inachus) have strikingly different colors on the dorsal (front) sides of their wings compared to those on the ventral (back) sides of their wings, which helps camouflage the butterflies from predators and attract potential mates. However, few human-made materials, devices and technologies can mimic such differential coloring for a long time. Here, we develop a new type of Janus-structured two-sided electrochromic devices that, upon application of different voltages, exhibits a coloration state on one side that is distinctly different from that on the other side. This is achieved by inserting an optically thin (4-8 nm) metallic layer with a complex refractive index, such as a layer composed of tungsten, titanium, copper or silver, into typical electrochromic structures. As an example, the resulting structure can appear green, mid-aquamarine, turquoise, ultramarine, deep blue-violet, and dark purple when viewed from one side, and red lilac, plum, purplish red, sandy brown, olive yellow or dark yellow when viewed from the other side. Based on this, an artificial butterfly with color variations between the dorsal and ventral wing surfaces is created, which closely resembles a dry leaf when its wings are close and exhibits various bright colors when its wings are open.

Resume : Transmissive-to-black chromism is a result of continuous pursuit of complete and null transmissive optical material for full control of the intensity and spectrum of incoming solar light in smart windows. While most transmissive-to-black devices are electrically activated, an alternative approach based on dual stimulation of a novel dual responsive molecule has been developed by anchoring asymmetrically alkylated viologen onto an engineered poly(2-isopropyl-2-oxazoline)-based polymer. Its thermochromic and electrochromic capability when triggered concurrently displays absorption of the entire visible light resulting to a black state that is ideal for building facades requiring absolute privacy. Autonomous chromic switchability is likewise conceivable by independently stimulating the material electrically or thermally where it shows unique colored state of vibrant magenta and a hazy state, respectively. Tunability of the material?s overall performance in terms of optical contrast, switching kinetics, coloration efficiency and cyclic stability based on the lattice water concentration was also elucidated. Additionally, the switchable device exhibits self-bleaching ability and short-term ?memory? effect that may expand its potential in many other applications.

Resume : This study demonstrates the synthesis of switchable single-molecule dual responsive (electrochromic and thermochromic) devices generated by tethering a viologen onto thermo-responsive poly(ionic liquid)s via an alkyl linker. With the combined properties of an electrochromic viologen and thermochromic PNIPAM and ion conductive PIL in a single molecule, this novel dual responsive (electro- and thermo-responsive) material exhibits multiple functionalities such that it acts both as an electrochrome, thermochrome, and electrolyte. One single molecule (all-in-one) single-layer device consisting of ITO/smart PIL/ITO was assembled on a glass substrate that can enhance its optical efficiency and characteristics of phase shift. The proposed smart device exhibits tunable transparency and electrochromic properties due to the thermo-responsive property of PNIPAM and the electrochromic property of a viologen and PIL as electroactive, respectively. This study?s results present a promising tool for the material design of high-performance stable devices for long-term operation in smart windows. Keywords: Dual response; Thermochromic; Electrochromic; Smart window

Resume : Considering the fact that energy usage for heating and cooling of buildings is a major contributor to the energy consumption of the built environment, there is a clear need for energy-efficient windows. For intermediate climates with hot summers and cold winters, smart windows with switchable solar control properties are ideal.[1] Thermochromic materials are of interest for application in smart windows since their optical properties change based on changing outdoor temperature. In our study, we used VO2 as thermochromic material because of its reversible structural phase transition from a semiconductive monoclinic to a conductive rutile structure at a defined switching temperature of 68°C. Dopants, such as W, can reduce the switching temperature to values between 15°C and 30°C, which is required for smart windows.[1] Here, we report a nanocomposite single-layer coating comprising VO2 and SiO2, and yielding unrivalled optical properties.[2] We prepared these coatings by dip coating of a SiO2-coated float glass in an alcoholic solution of a vanadium(IV) oxalate complex and pre-oligomerized tetraethoxysilane, and thermally annealed the dried xerocoat in a two-step process. During thermal anneal of the xerocoat, phase separation occurred which resulted in Si and V rich domains. This process was studied in detail for a series of coatings with varying VO2/SiO2 ratio and varying coating thickness using high resolution transmission electron microscopy. We obtained non-scattering coatings with low surface roughness and randomly distributed VO2 nanodomains, incorporated in a SiO2 matrix. They displayed unrivalled optical properties combining Tvis > 60% with ?Tsol ? 10%. Additionally, we demonstrate that with a pencil hardness ? 4H and a stability in ambient environment of at least 11 months, the coatings are suitable for industrial processing into insulating glass units. With building energy simulations on a typical Dutch building, we demonstrated that the use of smart windows with our thermochromic coatings can lead to energy savings of 24%, which equals annual cost savings of up to 470 ? per household.[2] [1] Mann D, Yeung C, Habets R, Vroon Z, Buskens P, Comparative building energy simulation study of static and thermochromically adaptive energy-efficient glazing in various climate regions, Energies 2020, 13, 2842. [2] Yeung CPK, Habets R, Leufkens L, Colberts F, Stout K, Verheijen MA, Vroon Z, Mann D, Buskens P, Phase separation of VO2 and SiO2 on SiO2-Coated float glass yields robust thermochromic coating with unrivalled optical properties, Solar Energy Materials and Solar Cells 2021, 230, 111238.

Resume : In recent decade(s) there has been an increasing interest in the chromic materials because of the remarkable optoelectronic features they possess. Being the fact that the structural evaluation stays still uncertain, yttrium based oxyhydride thin films are the one of the most promising structures with photochromic behaviour. In contemplation of the understanding/constructing, as regards the scope of this research, the relation between crystal arrangements and spectroscopic and electrical characteristics; yttrium, yttrium oxide and yttrium oxyhydride thin films were deposited by e-beam evaporator. The deposition, around 200 nm in thickness, were carried out from metal pieces (% 99.99, Purity) at 298 K (± 5) onto Si (001) and soda-lime glass and kapton substrates. And the following approaches were performed with the intention of creating this correlation: X-ray diffraction and Transmission Electron microscopy (structural investigation), Vacuum Fourier transform infrared spectrometer (vibrational spectrum analysis), Spectroscopic ellipsometer (optical characterization), X-ray photoelectron spectroscopy (oxidation state determination), and X-ray absorption spectroscopy (local structure analysis and chemical state identification). The findings of the absorption edge (K) of yttrium reveal that the oxidation state of Y in oxyhdride structure stays in the between the metallic yttrium and fully oxidized yttrium structure that corresponds to an unusual oxidation state. This result is in correspondence with the outcomes obtained from XPS, where the binding energy (Y 3d) shifts to the higher values depending on the chemical state of yttrium. The fully oxidized yttrium lattice vibrations at around 300, 380 and 550 cm-1 were detected by IR spectroscopy for yttrium oxide and yttrium oxyhydride thin films. The differences in the intensities and the widths of the vibration peaks indicates either the H atoms disturb the original cubic structure of sesquioxides and /or formation of new phases. This effect is visible either in the TEM images or on the X-ray diffractograms. The findings from this study might enhance our understanding on the arrangements of H atoms in the host crystal structure which is directly related to the photochromic demeanour. This research is partially supported by The Latvian Council of Science (LZP FLPP) Nr. lzp-2020/1-0345

Resume : Thermochromic materials based on VO2 thin films and multilayers are highly promising candidates for applications in smart windows, due to their ability of solar modulation and luminous transmittance. Prevention or minimization of energy losses and enhancement of the thermal performance of the glass will reduce energy consumption in buildings and prevent excess CO2 emissions. Their efficiency is governed by precise control and minimization of the critical temperature Tc by various ways, buffer layers or imposed strain being some of them. In this study, a detailed structural characterization of VO2 thin films has been performed, using predominately electron microscopy methods, post-experimental image analysis and processing. The VO2 thin films have been grown by Pulsed Lased Deposition (PLD) on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT) or Si substrates, with variable thickness. The general morphology, film thickness, interfacial and surface roughness, crystallographic orientation and epitaxial relationship of films and substrates were deduced. Microscopy results revealed that thin films adopt a columnar morphology and predominately crystallize in the rutile VO2 polymorph, although regions of the monoclinic VO2 phase have been also detected. These findings are consequently compared with complementary physical property measurements of the VO2 thin films and discussed in detail.

Resume : The first-order insulator-to-metal transition of VO2 at 68°C keeps VO2?based coatings at the top amid promising thermochromic materials for energy-saving windows with smart control of solar heat transmission. However, for actual applications, the thermochromic performance of VO2?based coatings must be optimized by lowering the transition temperature, increasing the luminous transmittance, and improving the switching parameters. To address all the above challenges in polycrystalline films, an alternative strategy is highly demanded. Here we show an improvement of all issues together by combining a nanothermochromic approach for enhancing the luminous transmittance with a strain-driven one for reducing both the transition temperature and hysteresis loop width. The laminar VO2-TiO2 nanocomposite films were fabricated by a temperature-driven spinodal decomposition of VxTi1xO2 solid solution. The latter was grown on fused quartz substrates by using a cost-effective technique of metalorganic aerosol deposition. The optical properties of the nanocomposites were compared to those of single-phase VO2 by measuring the transmittance in the wavelength range of 400÷2500 nm at temperatures 25÷100°C. The nanocomposite film reveals a 15-degrees reduced transition temperature and a 7-degrees narrowed hysteresis loop width, pointing to a strained state, which emerges as a result of the elastic coupling between V- and Ti-rich layers. The nanocomposite films exhibit a 5% integral luminous transmittance enhancement, demonstrating the nanothermochromic approach principle in action. In our case, this approach was realized by the integration of functional VO2?rich layers into a more transparent matrix of TiO-rich ones. As a result of both advances, the thermochromic performance, which was calculated as the difference between integral solar transmittances at 25°C and 100°C, increases up to 14% for the nanocomposite compared to 8.5% for VO2. We expect that subsequent process optimizations may further enhance the thermochromic performance, opening thus the way to the application of strained VO2-TiO2 nanocomposites as thermochromic coatings for smart windows. Financial support from Deutsche Forschungsgemeinschaft via SFB 1073 (TP Z02 and B02) and Moldavian State Program 20.80009.5007.12 is acknowledged.

Resume : In this paper, we report the fabrication of a chromogenic sensor for fast identification of Candida spp. The responsible mechanism is chemo-recognition: functionalized materials with targeted sensitivity towards VOCs produced by Candida species are immobilized onto a disposable, low cost and simple SiO2 support. The color changes when the sensitive layer is exposed to a medium containing Candida species. This color variation can be clearly seen with the naked eye. This sensor can be used by physicians as a new instrument for real-time identification of pathological cases involving candidiasis. This is extremely important in obstetrics because infections can be transmitted to the newborn. Keywords: chromogenic sensor, Candida spp., medicine sensor. Acknowledgements: 87PD/2020, 393PED/2020, 18/N/2019, 18PFE/30.12.2021