Incorporation and thermal evolution of rhodamine 6G dye molecules adsorbed in porous columnar optical SiO2 thin films

Conformal TiO2 Aerogel-Like Films by Plasma Deposition: from Omniphobic Antireflective Coatings to Perovskite Solar Cell Photoelectrodes

The ability to control the porosity of thin oxide films is a key factor determining their properties. Despite the abundance of dry processes for synthesizing oxide porous layers, a high porosity range is typically achieved by spin-coating-based wet chemical methods. Besides, special techniques such as supercritical drying are required to replace the pore liquid with air while maintaining the porous network. In this study, we propose a new method for the fabrication of ultraporous titanium dioxide thin films at room or mild temperatures (T ≤ 120 °C) by a sequential process involving plasma deposition and etching. These films are conformal to the substrate topography even for high-aspect-ratio substrates and show percolated porosity values above 85% that are comparable to those of advanced aerogels. The films deposited at room temperature are amorphous. However, they become partly crystalline at slightly higher temperatures, presenting a distribution of anatase clusters embedded in the sponge-like open porous structure. Surprisingly, the porous structure remains after annealing the films at 450 °C in air, which increases the fraction of embedded anatase nanocrystals. The films are antireflective, omniphobic, and photoactive, becoming superhydrophilic when subjected to ultraviolet light irradiation. The supported, percolated, and nanoporous structure can be used as an electron-conducting electrode in perovskite solar cells. The properties of the cells depend on the aerogel-like film thickness, which reaches efficiencies close to those of commercial mesoporous anatase electrodes. This generic solvent-free synthesis is scalable and applicable to ultrahigh porous conformal oxides of different compositions, with potential applications in photonics, optoelectronics, energy storage, and controlled wetting.

Concentration-Dependent Emission of Annealed Sol-Gel Layers Incorporated with Rhodamine 19 and 6G as the Route to Tunable High-Temperature Luminescent Materials

The sol-gel technology allows for the development of materials for nonlinear optics and photonics through the synthesis of multifunctional ceramic materials. Although the nature of the amorphous matrix allows the material to be doped with a large amount of the active components without quenching, it may affect the spectroscopic characteristics of the dye (e.g., result in a shift of absorption and emission peaks with drying time, presumably with a change of concentration). This study presents the material (SiO₂ impregnated with organic dyes—Rhodamine 6G and 19) with tunable emissions obtained by the authors upon annealing at different temperatures within the range of 100–300 °C. Possible observed effects were discussed based on spectroscopic properties and thermal studies of the synthesized material. Concerning annealing at different temperatures, an effect on concentration was observed. At the same time, a longer heating process at 300 °C revealed a protective function of sol-gel-derived silica for the organic dye; the longer heating did not cause any further significant changes in the dye’s emission, which indicates the preservative role of the sol-gel layers. Furthermore, etching tests of thin layers were conducted, resulting in smooth side edges of the waveguide. The tests have shown that it is possible to use dye-doped sol-gel layers as active components in photonics platforms.

Biocatalyst and Colorimetric/Fluorescent Dual Biosensors of H2O2 Constructed via Hemoglobin-Cu3(PO4)2 Organic/Inorganic Hybrid Nanoflowers

In this article, the three-dimensional hemoglobin (Hb)-Cu₃(PO₄)₂ organic/inorganic hybrid nanoflowers (Hb-Cu₃(PO₄)₂ HNFs) self-assembled by nanopetals were synthesized via a facile one-pot green synthetic method. The compositions and microstructure of the Hb-Cu₃(PO₄)₂ HNFs were well-characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and UV-vis spectrometry, respectively. The as-prepared Hb-Cu₃(PO₄)₂ HNFs were to be used as a biocatalyst to construct colorimetric/fluorescent dual biosensors. The experimental results show that the colorimetric/fluorescent dual biosensors exhibited two linear responses in the range of 2-10 ppb and 20-100 ppb for H₂O₂. The colorimetric and fluorescent detection limits were 0.1 and 0.01 ppb, respectively. Compared with the free Hb, the biocatalytic activity of the Hb-Cu₃(PO₄)₂ HNFs can be improved for 3-4 times under optimal conditions. The sensing performance of these Hb-Cu₃(PO₄)₂ HNF-based dual biosensors can be contributed such that the active sites of Hb molecules were more exposed on the surface of the Cu₃(PO₄)₂ nanopetals. Second, the unique nanopetal-assembled hybrid flowerlike structure was favorable to contact the detected substance with the biosensors. The dual biosensors were successfully applied for the determination of H₂O₂ in rainwater, tap water, and waste water samples. These results show that the dual biosensors had a potential application in the field of medical analysis, environmental monitoring, and food engineering.

S-Nitrosothiols (SNO) as light-responsive molecular activators for post-synthesis fluorescence augmentation in fluorophore-loaded nanospheres

For the first time S-nitrosothiol is engineered into fluorophore-loaded silica nanospheres for post-synthesis, light-triggered fluorescence augmentation.

Preparation and Optimization of Fluorescent Thin Films of Rosamine-SiO2/TiO2 Composites for NO2 Sensing

The incorporation of a prototypical rosamine fluorescent dye from organic solutions into transparent and microstructured columnar TiO2 and SiO2 (MO2) thin films, prepared by evaporation at glancing angles (GAPVD), was evaluated. The aggregation of the adsorbed molecules, the infiltration efficiency and the adsorption kinetics were studied by means of UV-Vis absorption and fluorescence spectroscopies. Specifically, the infiltration equilibrium as well as the kinetic of adsorption of the emitting dye has been described by a Langmuir type adsorption isotherm and a pseudosecond order kinetic model, respectively. The anchoring mechanism of the rosamine to the MO2 matrix has been revealed by specular reflectance Fourier transform infrared spectroscopy and infiltration from aqueous solutions at different pH values. Finally, the sensing performance towards NO2 gas of optimized films has been assessed by following the changes of its fluorescence intensity revealing that the so-selected device exhibited improved sensing response compared to similar hybrid films reported in the literature.

Growth Assisted by Glancing Angle Deposition: A New Technique to Fabricate Highly Porous Anisotropic Thin Films

We report a new methodology based on glancing angle deposition (GLAD) of an organic molecule in combination with perpendicular growth of a second inorganic material. The resulting thin films retain a very well-defined tilted columnar microstructure characteristic of GLAD with the inorganic material embedded inside the columns. We refer to this new methodology as growth assisted by glancing angle deposition or GAGLAD, since the material of interest (here, the inorganic) grows in the form of tilted columns, though it is deposited under a nonglancing configuration. As a "proof of concept", we have used silver and zinc oxide as the perpendicularly deposited material since they usually form ill-defined columnar microstructures at room temperature by GLAD. By means of our GAGLAD methodology, the typical tilted columnar microstructure can be developed for materials that otherwise do not form ordered structures under conventional GLAD. This simple methodology broadens significantly the range of materials where control of the microstructure can be achieved by tuning the geometrical deposition parameters. The two examples presented here, Ag/Alq3 and ZnO/Alq3, have been deposited by physical vapor deposition (PVD) and plasma enhanced chemical vapor deposition (PECVD), respectively: two different vacuum techniques that illustrate the generality of the proposed technique. The two type of hybrid samples present very interesting properties that demonstrate the potentiality of GAGLAD. On one hand, the Ag/Alq3 samples present highly optical anisotropic properties when they are analyzed with linearly polarized light. To our knowledge, these Ag/Alq3 samples present the highest angular selectivity reported in the visible range. On the other hand, ZnO/Alq3 samples are used to develop highly porous ZnO thin films by using Alq3 as sacrificial material. In this way, antireflective ZnO samples with very low refractive index and extinction coefficient have been obtained.

Super-aperture metrology: overcoming a fundamental limit in imaging smooth highly curved surfaces

The imaging of smooth, highly curved or tilted surfaces is widely recognized as one of the most challenging and unsolved problems in optical imaging and metrology today. The reason is that even when such surfaces are imaged using high aperture microscope objectives the steepness of the features causes the light to be reflected in such a way that it is not captured by the lens. This is true even in the limiting case of unity numerical aperture since the illuminating light may also be reflected in the forward direction. In order to overcome this fundamental problem we have developed a method whereby such specimens are covered with a readily removable organic fluorescent film thereby creating an isotropic scattering surface. We show that we are readily able to detect slopes with angles close 90° using a 0.75 NA objective--an 82% improvement over the theoretical aperture limit. Issues of variation in film thickness deposition are shown to be readily accommodated. This approach may be used with other fluorophore materials, organic or inorganic, since there is no need for biocompatibility in this application.

Anisotropic In-Plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma-Assisted ITO Thin Films e-Beam-Evaporated at Oblique Angles

ITO thin films have been prepared by electron beam evaporation at oblique angles (OA), directly and while assisting their growth with a downstream plasma. The films microstructure, characterized by scanning electron microscopy, atomic force microscopy, and glancing incidence small-angle X-ray scattering, consisted of tilted and separated nanostructures. In the plasma assisted films, the tilting angle decreased and the nanocolumns became associated in the form of bundles along the direction perpendicular to the flux of evaporated material. The annealed films presented different in-depth and sheet resistivity as confirmed by scanning conductivity measurements taken for the individual nanocolumns. In addition, for the plasma-assisted thin films, two different sheet resistance values were determined by measuring along the nanocolumn bundles or the perpendicular to it. This in-plane anisotropy induces the electrochemical deposition of elongated gold nanostructures. The obtained Au-ITO composite thin films were characterized by anisotropic plasmon resonance absorption and a dichroic behavior when examined with linearly polarized light.

Bending induced self-organized switchable gratings on polymeric substrates

We present a straightforward procedure of self-surface patterning with potential applications as large area gratings, invisible labeling, optomechanical transducers, or smart windows. The methodology is based in the formation of parallel micrometric crack patterns when polydimethylsiloxane foils coated with tilted nanocolumnar SiO2 thin films are manually bent. The SiO2 thin films are grown by glancing angle deposition at room temperature. The results indicate that crack spacing is controlled by the film nanostructure independently of the film thickness and bending curvature. They also show that the in-plane microstructural anisotropy of the SiO2 films due to column association perpendicular to the growth direction determines the anisotropic formation of parallel cracks along two main axes. These self-organized patterned foils are completely transparent and work as customized reversible diffraction gratings under mechanical activation.

Laser induced enhancement of dichroism in supported silver nanoparticles deposited by evaporation at glancing angles

Silver nanoparticles (NPs) depicting well defined surface plasmon resonance (SPR) absorption were deposited on flat substrates by physical vapor deposition in a glancing angle configuration. The particles were characterized by scanning electron microscopy and atomic force microscopy and their optical properties examined by UV-vis absorption spectroscopy using linearly polarized light. It was found that, depending on the amount of deposited silver and the evaporation angle, part of the 'as-prepared' samples present NPs characterized by an anisotropic shape and a polarization dependent SPR absorption and different colors when using polarized white light at 0° and 90°. Low-power irradiation of these materials with an infrared Nd-YAG nanosecond laser in ambient conditions produced an enhancement in such dichroism. At higher powers, the dichroism was lost and the SPR bands shifted to lower wavelengths as a result of the reshaping of the silver NPs in the form of spheres. The possible factors contributing to the observed changes in dichroism are discussed.

Complexes of polydopamine-modified clay and ferric ions as the framework for pollutant-absorbing supramolecular hydrogels

Clay-based functional hydrogels were facilely prepared via a bioinspired approach. Montmorillonite (clay) was exfoliated into single layers in water and then coated with a thin layer of polydopamine (PDOPA) via in situ polymerization of dopamine under basic aqueous conditions. When a small amount of ferric salt was added into aqueous suspensions of the polydopamine-coated clay (D-clay), D-clay and Fe(3+) ions could rapidly self-assemble into three-dimensional networks through the formation of coordination bonds. Consequently, supramolecular hydrogels were formed at very low D-clay contents. Rheological measurements show that the D-clay/Fe(3+) hydrogels exhibit fairly elastic response in low stain range, and have self-healing capability upon removal of applied large stress. More importantly, the hydrogels can be used as adsorbents to effectively remove Rhodamine 6G (Rh6G), an organic pollutant, from water. UV-vis absorption spectra of the Rh6G-loaded hydrogels show bands related to π-π stacking interactions between the aromatic moieties of PDOPA and Rh6G, confirming the formation of PDOPA/Rh6G complex on the surface of D-clay.

Novel guests for porous columnar thin films: the switchable perchlorinated trityl radical derivatives

TiO(2) and SiO(2) porous thin films consisting of tilted nanocolumns prepared by glancing angle evaporation (GLAD) have been infiltrated with guest derivatives belonging to the family of perchlorinated trityl radicals, novel guest molecules presenting an open-shell electronic configuration associated with paramagnetism, fluorescence, and electroactivity. The main driving forces for infiltration from aqueous solutions of the carboxylate-substituted radical derivatives are the electrostatic interactions between their negative charge and the net positive charges induced on the film pores. Positive charges on the internal surface of the films were induced by either adjusting the radical solution pH at values lower than the point of zero charge (PZC) of the oxide or passivating the nanocolumns oxide surface with a positively charged aminosilane. The infiltrated composite thin films are robust and easy to handle thanks to the physical protection exerted by the film columns. They also keep the multifunctionality of the used guests, as confirmed by electron paramagnetic resonance (EPR), UV-vis spectroscopy, and fluorescence spectroscopy. To prove the electroactivity of the infiltrated porous films, a porous TiO(2) host layer was supported onto conductive indium tin oxide (ITO). By application of an appropriate redox potential, the guest radical molecules have been reversibly switched from their open-shell electronic configuration to their diamagnetic state and hence changed their optical properties. On the basis of these results, it is herein proposed that the appropriate surface functionalization of the pore internal surface of GLAD thin films can be used to prepare novel radical-oxide composite thin films usable for the development of robust switchable electrically driven photonic and magnetic devices.

Active and optically transparent tetracationic porphyrin/TiO(2) composite thin films

Fluorescent tetracationic porphyrin (TMPyP) molecules have been incorporated into optically transparent TiO(2) thin films acting as a host material. The films, with a columnar structure and open pores, were prepared by electron evaporation at glancing angles (GAPVD). The open porosity of the films has been estimated by measuring a water adsorption isotherm with a quartz crystal monitor. TMPyP molecules were infiltrated in the host thin films by their immersion into water solutions at controlled values of pH. The state of the adsorbed molecules, the infiltration efficiency, and the adsorption kinetics were assessed by analyzing the optical response of the films by UV-vis absorption and fluorescence techniques. The infiltration efficiency was directly correlated with the acidity of the medium, increasing at basic pHs as expected from simple considerations based on the concepts of the point of zero charge (PZC) developed for colloidal oxides. By a quantitative evaluation based on the analysis of the UV spectra, the infiltration process has been described by a Langmuir type adsorption isotherm and an Elovich-like kinetics. The accessibility of the infiltrated molecules in the TMPyP/TiO(2) composite films is assessed by following the changes of their optical properties when exposed to the acid vapors and their subsequent recovery with time.