Preparation and nonlinear optical properties of Au nanoparticles doped TiO2 thin films

Nano-sized noble metal nanoparticles doped dielectric composite films with large third-order nonlinear susceptibility due to the confinement and the enhancement of local field were considered to be applied for optical information processing devices, such as optical switch or all optical logical gates. In this paper, sol–gel titania thin films doped with gold nanoparticles (AuNPs, ~10 nm in average size) were prepared. AuNPs were firstly synthesized from HAuCl₄ in aqueous solution at ~60 °C, using trisodium citrate as the reducing agent, polyvinylpyrrolidone as the stable agent; then the particle size and optical absorption spectra of the AuNPs in aqueous solutions were characterized by transmitting electron microscopy and UV–Vis–NIR spectrometry. Sol–gel 2AuNPs–100TiO₂ (in %mol) thin films (5 layers, ~1 μm in thickness) were deposited on silica glass slides by multilayer dip-coating. After heat-treated at 300–1,000 °C in air, the AuNPs–TiO₂ thin films were investigated by X-ray diffraction, scanning electron microscopy and atomic force microscopy. The nonlinear optical properties of the AuNPs–TiO₂ thin films were measured with the Z-scan technique, using a femtosecond laser (200 fs) at the wavelength of 800 nm. The third-order nonlinear refractive index and nonlinear absorption coefficient of 2AuNPs–100TiO₂ films were at the order of 10^?12 cm²/W, and the order of 10^?6 cm/W, respectively, and the third-order optical nonlinear susceptibility χ⁽³⁾ was ~6.88 × 10^?10 esu.     

Dual fluorescence of Methylene Blue encapsulated in silica matrix

Dynamics related to the optical effect of Methylene Blue (MB) dye molecules in solid host has been investigated. MB doped silica samples of varying concentrations are prepared by the sol–gel technique in acidic environment. Absorption and laser induced fluorescence spectra have been recorded for the samples. Interesting result of dual fluorescence at lower concentrations is explained. Attempt is done to study the dynamics of the dual fluorescence. These results may be useful for designing and developing solid-state optoelectronic devices.     

Mesostructured Dye‐Doped Titanium Dioxide for Micro‐Optoelectronic Applications

Optically transparent, mesostructured titanium dioxide thin films were fabricated using an amphiphilic poly(alkylene oxide) block copolymer template in combination with retarded hydrolysis of a titanium isopropoxide precursor. Prior to calcination, the films displayed a stable hexagonal mesophase and high refractive indices (1.5 to 1.6) relative to mesostructured silica (1.43). After calcination, the hexagonal mesophase was retained with surface areas >300 m² g^?1. The dye Rhodamine 6G (commonly used as a laser dye) was incorporated into the copolymer micelle during the templating process. In this way, novel dye‐doped mesostructured titanium dioxide films were synthesised. The copolymer not only directs the film structure, but also provides a solubilizing environment suitable for sustaining a high monomer‐to‐aggregate ratio at elevated dye concentrations. The dye‐doped films displayed optical thresholdlike behaviour characteristic of amplified spontaneous emission. Soft lithography was successfully applied to micropattern the dye‐doped films. These results pave the way for the fabrication and demonstration of novel microlaser structures and other active optical structures. This new, high‐refractive index, mesostructured, dye‐doped material could also find applications in areas such as optical coatings, displays and integrated photonic devices.     

Fluorescent nanocrystals grown in sol–gel thin films for generic stable and sensitive sensors

Through the sol–gel route, we have well-controlled the preparation of fluorescent organic nanocrystals grown in silicate thin films. This process is based on the confined nucleation and growth of dyes in the pores of wet gels. The resulting nanocomposite sol–gel thin films, coated onto low-cost substrates, exhibit coupled properties: transparency, stability, easy shaping of sol–gel thin films and high fluorescence intensity coming from organic nanocrystals. The sensitivity of the fluorescence intensity of nanocrystals to their environments can be exploited for the development of optical sensors. Indeed, Förster Resonance Energy Transfer can inhibit nanocrystal fluorescence when probe molecules are adsorbed or grafted on the nanocrystal surface after their diffusion through the pores of the sol–gel matrix. We investigated by time-resolved fluorescence spectroscopy the effect of nanocrystal size and probe concentration on the fluorescence quenching in presence of Methylene Blue used in this study as molecular probe. As strong fluorescence quenchings can be achieved, even for low probe concentrations, these hybrid organic–inorganic nanocoposites are promising for the development of sensor devices by increasing their fluorescence contrasts under specific chemical or biological environments.     

Zn2+ doped ormosil–phosphotungstate hybrid films with enhanced photochromic response

Owing to the wide use of photochromic materials in UV sensors and dosimeters, considerable efforts have been made to increase the UV-response and sensitivity of the existing classes of photochromic materials. In this study, we report a simple sol–gel route for the preparation of highly photochromic transparent films based on ormosil–phosphotungstate hybrid materials. The effect of addition of Zn²⁺ ions on the photochromic response of these hybrid films and the possible mechanism involved is discussed. Compared to the undoped samples, the photochromic response of the Zn²⁺-doped hybrid films increases by 59–237 % depending on the concentration of Zn²⁺ ions added to the sol formulation. No structural or electronic change in the phosphotungstate dye was observed by vibrational spectroscopy or UV spectroscopy, though micro X-ray fluorescence (μ-XRF) analysis showed that the addition of Zn²⁺ in the sol–gel preparations leads to an increase in the amount of phosphotungstate (HPW) incorporated in the dip-coated films. Furthermore, TEM and nano-energy-dispersive X-ray showed formation of nano-agglomerates consisting of Zn and HPW in the Zn²⁺-doped samples. Zn K-edge X-ray absorption near edge structure analysis also confirmed the formation of the salt H_x[Zn(OH₂)₆] _2?xPW₁₂O₄₀. It is suggested that these Zn-phosphotungstate nano-agglomerates get trapped into the ormosil network during films preparation leading to increased concentration of the phosphotungstate anions in the films, in accordance with results from μ-XRF analysis. Raman spectroscopy confirmed that the Keggin structure of HPW is preserved in the hybrid films. FTIR spectra of the matrix part of the samples are identical before and after UV-irradiation, which suggests that the photochromic process does not involve oxidation of the organic functionalities. These highly photochromic hybrid films are promising candidates for the design of practical UV-sensing devices and dosimeters.     

Preparation of stable second‐order nonlinear optical films by sol–gel technique

A stable nonlinear optical (NLO) film containing “T” type alkoxysilane dye was prepared by sol–gel technology. This crosslinked “T” type alkoxysilane dye was synthesized and fully characterized by FTIR, UV–Vis spectra, and ¹H‐NMR. Followed by hydrolysis and copolymerization processes of the alkoxysilane with γ‐glycidoxypropyl trimethoxysilane (KH560) and tetraethoxysilane (TEOS), high quality inorganic–organic hybrid second‐order NLO films were obtained by spin coating. The “T” type structure of the alkoxysilane was found to be effective for improving the temporal stability of the optical nonlinearity due to the reduction in the relaxation of the chromophore in the film materials. Copyright © 2009 John Wiley & Sons, Ltd.     

Optical properties of a polyethylene dispersion with a luminescent silica prepared by surface grafting of a perylene derivative

A micrometric silica modified with perylene derivative (SiO₂–TES) has been prepared by hydrolysis–condensation reactions between silica and N,N’-bis-(3-triethoxysilylpropyl)-perylene-tetracarboxyldiimide (P-TES) and utilized as filler in LLDPE films together with a compatibilizer. Spectroscopic analyses on SiO₂–TES confirmed the grafting of P-TES on silica, while its amount was determined by thermogravimetric analysis. Solid state NMR provided information about the structure of silicon atoms involved in the condensation of SiO₂ and P-TES that resulted mainly in silicon atoms grafted with bi-dentate anchorages.UV–Vis and fluorescence analyses carried out on P-TES showed the ability of the dye to generate J-type aggregates in apolar solvents. The same analyses on SiO₂–TES revealed the presence of both isolated and aggregated dye molecules grafted on silica surface, while on polymeric dispersions of SiO₂–TES, they have shown only the presence of P-TES aggregates grafted to silica.Thanks to the direct correlation between optical properties (obtained both by UV–Vis and by fluorescence analyses) and the chemical environment of composites, the luminescent silica SiO₂–TES could be used to estimate the filler dispersion extent in different microcomposite polymer materials.     

High dispersion and fluorescence of anthracene doped in polyphenylsiloxane films

Phenylsilsesquioxane-diphenylsiloxane glass thick films doped with anthracene were prepared from homogeneous coating sols obtained from two different approaches. One approach consisted in incorporating the dye during the glass preparation (which implies the incorporation of the dye in an aqueous media). The doped-glass was further dissolved in the solvents mixture composed of cyclohexane and acetone. The other approach which is non-aqueous consisted simply in dissolving directly the preformed non-doped polyphenylsiloxane glass in the dye solutions. The stability of the organic-inorganic hybrid glass matrix in most organic solvents makes possible the incorporation of the dye without problems of miscibility and dispersion in the hybrid matrix. The coating was performed at room temperature using spin-coating technique prior to further heat-treatment. Crack-free and homogeneous films of high optical quality were obtained. The optical properties of the doped films based on their absorption and emission spectra were discussed owing the incorporation route of the dye. The results showed that the non-aqueous approach used to incorporate the dye minimizes the dye aggregation. This property associated to the preparation route permits to obtain optically active hybrid films loaded with high concentrations of anthracene (in the order of 10⁻² M) which enhance the fluorescence of the doped films. The hybrid doped-films obtained represent therefore a tremendous potential for applications in the field of optics and photonics including the development of new nonlinear optical materials. Bouzid Menaa and Masahide Takahashi both the author contributed equally.     

Exciton energy transfer between optically forbidden states of molecular aggregates

3-ethyl-2-[3-(3-ethyl-2(3H)-benzoxazolylidene)-1-propenyl]benzoxazolium iodide (dye I) and pseudoisocyanine bromide are employed to form H aggregates as donors and J aggregates as acceptors. The energy of an H band of the H aggregates is higher than that of a J band of the J aggregates. It was confirmed that excitation of the H band does not emit fluorescence by comparison of excitation spectra of dye I H aggregates with that of dye I monomer. Absorption, fluorescence, and excitation spectra of spin-coated films of H aggregates mixed with various quantities of J aggregates have been observed. Excitation spectra probed at the J band are found to have a component of the H band. Fluorescence spectra originated from excitation of the H band are extracted and qualitatively analyzed. It is confirmed that excitation of the H band causes to emit fluorescence of a J band of the J aggregates. These phenomena show that exciton energy can transfer from the lowest energy in electronic states of the H aggregate, which state is optically forbidden, to electronic state of the J aggregate.     

Controlling the π‐Stacking Behavior of Pyrene Derivatives: Influence of H‐Bonding and Steric Effects in Different States of Aggregation

The performance of opto‐electronic devices built from low‐molecular‐weight dye molecules depends crucially on the stacking properties and the resulting coupling of the chromophoric systems. Herein we investigate the influence of H‐bonding amide and bulky substituents on the π‐stacking of pyrene‐containing small molecules in dilute solution, as supramolecular aggregates, and in the solid state. A set of four pyrene derivatives was synthesized in which benzene or 4‐tert‐butyl benzene was linked to the pyrene unit either through an ester or an amide. All four molecules form supramolecular H‐aggregates in THF solution at concentrations above 1×10^?4 mol L^?1. These aggregates were transferred on a solid support and crystallized. We investigate: the excimer formation rates within supramolecular aggregates; the formation of H‐bonds as well as the optical changes during the transition from the amorphous to the crystalline state; and the excimer to monomer fluorescence ratio in crystalline films at low temperatures. We reveal that in solution supramolecular aggregation depends predominantly on the pyrene chromophores. In the crystalline state, however, the pyrene stacking can be controlled gradually by H‐bonding and steric effects. These results are further confirmed by molecular modeling. This work bears fundamental information for tailoring the solid state of functional optoelectronic materials.     

Azo dye doped polymer films for nonlinear optical applications

Azo dye doped polymer films were prepared on glass substrates using spin-coating technique. FTIR, UV-Vis spectra and PL measurements were recorded to characterize the structure of the metanil yellow doped PVA films. Surface morphology and thickness of the films were studied using AFM and FESEM. The magnitude of both real and imaginary parts of third-order nonlinear susceptibility χ3 of metanil yellow were determined by the Z-scan technique. The nonlinear refractive index n2 and the nonlinear absorption coefficient β of the azo dye doped polymer films were calculated respectively. The real part of the third-order susceptibility χ3 is much larger than its imaginary part indicating that the third-order optical response of the metanil yellow doped PVA films is dominated by the optical nonlinear refractive behavior.     

Controlling the growth of particle size and size distribution of silica nanoparticles by the thin film structure

Nanostructured silicondioxide thin films were prepared by sol–gel spin coating technique. The SiO₂ films were made using a conventional mixture of tetraethoxysilane (TEOS), deionized water and ethanol with various NH₃/TEOS ratios. The nanostructured silica films were made using a mixture of the SiO₂ sol and regular SiO₂ sol to control the enlargement of the particles inside the films. The structural, morphological and optical characterizations of the as-deposited and annealed films were carried out using X-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy, NKD spectrophotometer and ultraviolet–visible (UV–vis) spectroscopy. The transmittance data of the infrared spectra of the films were recorded using an FT-IR Spectrometer. The XRD studies showed that as-deposited films were amorphous and the formation of the alfa-cristobalite phase of the silica film was investigated at annealing temperature close to 1,100 °C. Optical properties of the transmittance spectra in the s and p-polarization modes were collected. Refractive indices and extinction coefficients were determined with respect to the NH₃/TEOS ratios in the compositions of the films. Optical cut-off wavelength values were investigated from the extrapolation of the absorbance spectra which was estimated from the UV–vis spectroscopy measurements. A red shift in the absorption threshold indicated that the size of silica nanoparticles was increased by an increase in the NH₃/TEOS volume ratio from 1:64 to 1:8.     

In situ detection of birefringent mesoscopic H and J aggregates of thiacarbocyanine dye in solution

Polarized-light microscopy, fluorescence microscopy, atomic force microscopy as well as absorption and fluorescence spectroscopy were used to characterize mesoscopic structures of both supramolecular H and J aggregates of 3,3'-disulfopropyl-5,5'-dichloro-9-methyl thiacarbocyanine dye in aqueous solution. Polarized-light microscopy visualizes in situ the mesoscopic morphology of the H and J aggregates and distinguishes between them by their own colors. The H aggregate having a fibrous structure showed negative birefringence, namely, the refractive index along the fiber short axis was higher than that of the long axis, so that pi-electron chromophores of the dye molecule are likely to orient along the short axis of the elongated fibers. The degree of birefringence of the H aggregate fiber was approximately -0.3. Investigations on the concentration dependence of the absorption spectra showed that the amount of J aggregates increased at the expense of a decrease in the amount of H aggregates. With respect to the J aggregates, a small dot morphology was observed at a relatively low dye concentration of 3.0 mM. With an increase of the dye concentration up to 10 mM, the morphology changed into mesoscopic fibers. In contrast, fluorescence microscopy for the fibrous J aggregates reveals that the constituent molecules are approximately aligned along the long axis of the fibers.     

The synthesis and characteristic study of 6FDA-6FHP-NLO polyimide/SiO2 nanohybrid materials

Hybrid nanocomposite films of silica (SiO₂) in polyimide (PI) from 4,4^′-(hexafluoroisopropylidene) diphthalic arhydride (6FDA), 2,2-Bis (3-amino-4-hydroxyphenyl) hexafluoropropane (6FHP) and nonlinear optical (NLO) molecule have been successfully fabricated by an in situ sol-gel process. The silica content in the hybrid films was varied from 0 to 22.5 wt%. These nanocomposite films exhibit fair good optical transparency. Fourier transform infrared (FTIR) spectroscopy results confirm the formation of SiO₂ particles in PI matrix. Scanning electron microscope (SEM) images show that the SiO₂ phase is well dispersed in the polymer matrix. Their glass transition behavior and thermal stability were investigated by differential scanning calorimeter (DSC) and thermal gravimetric analysis (TG).     

Organically modified silica nanoparticles co-encapsulating photosensitizing drug and aggregation-enhanced two-photon absorbing fluorescent dye aggregates for two-photon photodynamic therapy

We report energy-transferring organically modified silica nanoparticles for two-photon photodynamic therapy. These nanoparticles co-encapsulate two-photon fluorescent dye nanoaggregates as an energy up-converting donor and a photosensitizing PDT drug as an acceptor. They combine two features: (i) aggregation-enhanced two-photon absorption and emission properties of a novel two-photon dye and (ii) nanoscopic fluorescence resonance energy transfer between this nanoaggregate and a photosensitizer, 2-devinyl-2-(1-hexyloxyethyl)pyropheophorbide. Stable aqueous dispersions of the co-encapsulating nanoparticles (diameter < or = 30 nm) have been prepared in the nonpolar interior of micelles by coprecipitating an organically modified silica sol with the photosensitizer and an excess amount of the two-photon dye which forms fluorescent aggregates by phase separation from the particle matrix. Using a multidisciplinary nanophotonic approach, we show: (i) indirect excitation of the photosensitizer through efficient two-photon excited intraparticle energy transfer from the dye aggregates in the intracellular environment of tumor cells and (ii) generation of singlet oxygen and in vitro cytotoxic effect in tumor cells by photosensitization under two-photon irradiation.     

Near-field fluorescence spectroscopy and photochemistry of organic mesoscopic materials

In this Review, an emerging research field of near-field fluorescence and photochemistry studies on molecular materials is introduced and relevant background, instrumentation, attractive topics, and future perspectives are discussed. Principles of near-field scanning optical microscope and technically important points are described, and our picosecond near-field fluorescence microspectroscopic system is explained. Its high performance of 100 nm spatial, a few ps temporal, and 1 nm spectral resolutions makes it possible to correlate topography, fluorescence image, fluorescence spectrum, and fluorescence rise and decay curve with each other. Near-field fluorescence spectroscopy reveals thickness-dependent fluorescence spectra of tetracene microcrystals, relations between photophysics and morphology of charge transfer microcrystals, and inhomogeneous inner structure of single microcrystals in anthracene-tetracene films. Similar fluorescence and morphology studies are described for polymer films, Langmuir-Blodgett films, and J aggregates. Some anthracene solids show interesting photothermal and photochemical nanometer morphological changes, while photoisomerization in organic crystals, and photolithography and ablation of polymer films upon near-field excitation are introduced. Future perspectives near-field excitation with shorter wavelength and/or higher intensity and various kinds of pump-probe measurement are discussed in view of photochemistry studies.     

Early aggregation in prion peptide nanostructures investigated by nonlinear and ultrafast time-resolved fluorescence spectroscopy

We report the characterization of early aggregates in the self-assembly of prion peptides using nonlinear and ultrafast time-resolved fluorescence spectroscopy. The dye-labeled peptide and dye/peptide guest-host systems were used to demonstrate the feasibility of the new approach. By measuring the two-photon absorption cross-section, small aggregates of the dye labeled peptide were characterized. Ultrafast time-resolved fluorescence anisotropy spectroscopy reveals the packing state (microenvironment) of the probes to be tightly associated with aggregates and associated with aggregation progression of the peptides. Fluorescence intensity decay shows a correlation with growth of aggregates having a high level of structured beta-sheet content. A new binding ligand Cascade Yellow shows promise for beta-sheet recognition of prion peptide nanostructures. These findings may have implications for in vivo studies of neurotoxic aggregates targeting with fluorescence markers. Also, these results may provide insight into molecular design of peptide-based nanomaterials.     

Pore size and channel structure study of mesoporous SiO2 films on sensing trace explosive

In this work, a new type of naphthalene dye was synthesized for detecting nitro explosives in solution and mesoporous film system. The fluorescence quenching mechanism between dye and explosive was determined. For detecting nitro explosives in gas system, mesoporous silica films doped with this fluorescent dye were successfully fabricated using the sol–gel technology with different templates. Two-dimensional hexagonal structure mesoporous silica film was found sensitive to traces of nitroaromatic trinitrotoluene. When enlarging the pore size, to improve the sensitivity, a lower sensitivity was actually obtained. This can be explained by the small molecule diameter of the explosive and the relatively large surface area of the mesoporous films. The Brunauer–Emmett–Teller surface area can be enlarged by a suitable 3D pore channel structure so that the fluorescence quenching sensitivity is improved for the nitro aromatic compounds vapors. Optimizing the pore size and channel structure can therefore well improve the sensing efficiency in this system.     

Synthesis and nonlinear optical properties of side chain liquid crystalline polymers containing azobenzene push–pull chromophores

Azobenzene monomeric precursors bearing piperazine as donor moiety with different withdrawing groups and derived side chain polymethacrylates have been prepared and characterized. Monomers having terminal cyano or nitro groups, and the corresponding polymers, exhibited smectic A phases. Linear and nonlinear optical properties of every monomer and thin films of the cyano polymer ( pol‐PZ‐CN ) have been also studied. UV‐vis spectroscopy revealed out‐of‐plane orientation in the as prepared films, as confirmed by waveguide refractive index measurements. Moreover, absorption spectra indicated the presence of azo aggregates in these films. The initial molecular arrangement has been modified by applying thermal annealing within the mesophase range and UV‐blue irradiation. Although thermal annealing resulted in a significant amplification of the out‐of‐plane optical anisotropy due to thermotropic self‐organization of side chain azo moieties, irradiation with 440 nm light induced some disruption of aggregates. The nonlinear optical response of Corona poled films has been studied by second harmonic generation measurements, and the influence of the molecular arrangement on the nonlinear d_ij coefficients has been analyzed. The more efficient poling corresponded to preirradiated films. In any case, a noticeable degree of polar order (70% of the initial d₃₃ value) remained for several months after the poling in films kept at RT. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 232–242, 2010     

Self-assembly of thiacyanine dyes in water for the synthesis of active hybrid nanofibres

Water-soluble self-assembled nanostructures were synthesised by simple counter ion exchange of thiacyanine dyes which helps in the formation of nematic (N) and hexagonal (M) chromonic liquid-crystalline (CLCs) phases. Conjugated double bonds as central spacers connected between two benzothiazole segments affect water solubility and liquid crystal formation. Aggregation-dependent properties characterised by ultraviolet–visible, fluorescence and ¹H nuclear magnetic resonance spectroscopy. Sol–gel reaction of dye aggregates with silica species furnishes entangled nanotubular fibres with constant diameter and their length in excess of micrometres, having templates of pore sizes below the mesoporous range. The π–π stacked chromonic aggregate dyes are also of importance in shape selective catalysis, adsorption, desorption micro-patterned materials, and provide a significant step towards biosensor medical applications because of their water-soluble nature.