Leandro P. Ravaro

Influence of pH of colloidal suspension on the electrical conductivity of SnO2 thin films deposited via Sol-Gel-Dip-Coating

Tin dioxide (SnO2) thin films are deposited by the dip-coating technique from colloidal suspensions prepared with distinct pH through the sol-gel method. The decrease of the pH contributes to the destruction of an electrical layer adjacent to particles in solution, leading to a high degree of aggregation among these particles due to the generation of cross-linked bonds (Sn-O-Sn) between them. The aggregation affects the electrical properties of films, because the pH variation produces particle with distinct sizes in the film. Undoped samples prepared from pH 6 leads to the highest conductivity among the investigated undoped samples, in agreement with X-ray diffractograms, which indicate higher crystallinity for lower pH. Arrhenius plot evaluated from temperature dependent conductivity data leads to activation energies of the deepest level between 67 to 140 meV, for the films prepared from suspensions with pH 6 to 11. The most probable explanation for this variation in the conductivity and activation energy is related to distinct potential barriers between grains, due to distinct packing caused by cross-linked bonds formed during suspension phase. Characterization of samples lightly doped with Er3+ confirms that acid pH leads to higher conductivity, but the highest conduction takes place at even lower pH when compared to undoped thin films.

The polynuclear complex Cu4I4py4 loaded in mesoporous silica: photophysics, theoretical investigation, and highly sensitive oxygen sensing application

The polynuclear complex Cu4I4py4 has been largely studied in solution and in the powder form due to its interesting luminescent properties, which are largely dependent on temperature and pressure. In this work, we present the synthesis of the complex and its wet impregnation in a mesoporous silica host obtained by sol-gel methodology. For optimized loadings, the well-dispersed guest molecules exhibit strong interaction with molecular oxygen, resulting in a significant quenching of the luminescence. The process is highly reversible with a Stern-Volmer constant of Ksv = 33.8, which is the largest value found in the literature for similar complexes in the solid state, suggesting that the new material is a promising candidate for high sensitivity oxygen sensing. Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations reveal a weak intermolecular interaction between the two guest complexes in the excited state, suggesting the formation of an excited state complex (excimer). The assumption of a triplet excimer formation is confirmed by temperature- and concentration-dependent experiments, which provides a new way to explain the giant Stokes shift observed for the guest complex in different media.

Nanoparticle characterization of Er-doped SnO2 pellets obtained with different pH of colloidal suspension

SnO2:2 at. %Er xerogel samples were obtained by sol-gel technique from colloidal suspensions with distinct pHs. The evaluation of critical regions inside the nanocrystallite is fundamental for the interpretation of the influence of pH on the emission data. In this way, the nanocrystal depletion layer thickness was obtained with the help of photoluminescence, Raman, X-ray diffraction, and field-emission gun scanning electron microscopy measurements. It was observed that acid suspensions (pH < 7) lead to high surface disorder in which a larger number of cross-linked bonds Sn-O-Sn among nanoparticles are present. For these samples, the nanoparticle depletion layer is larger as compared to samples obtained from other pH. Photoluminescence measurement in the near infrared region indicates that the emission intensity of the transition 4I13/2 → 4I15/2 is also influenced by the pH of the starting colloidal suspension, generating peaks more or less broadened, depending on location of Er3+ ions in the SnO2 lattice ...

Eco-friendly luminescent hybrid materials based on EuIII and LiI co-doped chitosan

Biopolymer-based materials have been of particular interest as alternatives to synthetic polymers due to their low toxicity, biodegradability and biocompatibility. Among them, chitosan is one of the most studied ones and has recently been investigated for the application as solid state polymer electrolytes. Furthermore, it can serve as a host for luminescent species such as rare earth ions, opening up the possibility of combined electro-optical functionality, of particular interest for electroluminescent devices. In this study, we perform a fundamental, initial, investigation of chitosan based luminescent materials doped with EuIII and LiI triflate salts, from the structural, photophysical and conducting points of view. Because the host presents a broad emission band in the blue to green, while EuIII emits in the red, fine-tuning of emission colour and/or generation of white light is proven possible, by proper combination of optimized composition and excitation scheme. Europium lifetimes (5D0) are in the range 270-350 µs and quantum yields are up to 2%. Although LiI does not interfere with the luminescent properties, it grants ion-conducting properties to the material suggesting that a combination of both properties could be useful in the development of electro-luminescent devices.

Optical materials based on copper (I) complexes and CdTe quantum dots loaded in solid matrices

This work reports the photophysical properties of copper (I) complexes and CdTe QDs loaded in a mesoporous silica matrix obtained by sol-gel method with an average pore diameter of 10 nm. The colloidal suspension of CdTe, synthesized in aqueous phase, presents maximum emission at λem = 520 nm with full width at half maximum (FWHM) of 45 nm. While loaded into the solid SiO2 matrix, the QD emission had small shift, according to the type of incorporation strategy. The complexes Cu4I4py4 (1) and C30H28N2O2PCuI (2) incorporated on mesoporous silica showed a shift in emission of λem = 580 nm to 645 nm and λem = 660 nm to 575 nm, respectively. This shift of the emission is caused by distortion of the molecular complex, in the MLCT excited state, inside the matrix, known as rigidochromic effect. Also, a sensitivity of these hybrids materials (1@SiO2 and 2@SiO2) was observed to oxygen. The mesoporous matrix dispersed the molecules of the complex, so as to enable a reversible interaction with oxygen in the center of the metal that quenches of luminescence and provides a new functionality to the complex. The new functional materials proposed in this study have potential for applications in optical materials devices.

CCDC 1827902: Experimental Crystal Structure Determination

Related Article: Leandro Piaggi Ravaro, Ana Carolina Mafud, Zhi Li, Eric W. Reinheimer, Carlos A. Simone, Yvonne Primerano Mascarenhas, Peter C. Ford, Andrea Simone Stuchi de Camargo|2018|Dyes Pigm.|159|464|doi:10.1016/j.dyepig.2018.07.020