Viviane Pilla

Quantum yield excitation spectrum "UV-visible… of CdSe/ZnS core-shell quantum dots by thermal lens spectrometry

A recently developed thermal lens spectrometry configuration has been used to study CdSe/ZnS core-shell quantum dots (QDs) suspended in toluene and tetrahydrofuran (THF) solvents. The special features of this configuration make it very attractive to measure fluorescence quantum yield (η) excitation spectrum since it simplifies the measurement procedure and consequently improve the accuracy. Furthermore, the precision reached is much higher than in conventional photoluminescence (PL) technique. Two methods, called reference sample and multiwavelength have been applied to determine η, varying excitation wavelength in the UV-visible region (between 335–543 nm). The η and PL spectra are practically independent of the excitation wavelength. For CdSe/ZnS QDs suspended in toluene we have obtained η=76±2%. In addition, the aging effect on η and PL has been studied over a 200 h period for QDs suspended in THF.

Fluorescence quantum efficiency measurements in the presence of Auger upconversion by the thermal lens method

Mode-mismatched thermal lens (TL) measurements in Cr3+-doped fluoride crystals (LiSrAIF6 and LiSrGaF6) are reported. A nonlinear increase of the TL signal, and decrease of quantum efficiency, with increasing excitation power was observed and attributed to energy-transfer upconversion (ETU). Assuming an upconversion rate that is proportional to the excited-state population, Wup = gammaN(e), the theoretical model developed fits the experimental data well. The ETU parameters (gamma) were determined with unprecedented accuracy for low Cr concentrations. Thermo-optical parameters (K, ds/dT) were also obtained.

Measurement of the thermo-optic coefficient in liquids by laser-induced conical diffraction and thermal lens techniques

We report on a method to determine the temperature coefficient of the refractive index (dn/dT) for non-fluorescent liquids which is based on the experimental observation of conical diffraction induced by self-phase modulation (SPM). Light absorbing impurities were added to allow for heat deposition by the excitation laser beam. We studied samples of tetrahydrofuran (THF, C4H8O) and toluene (C6H5CH3) with suspended cadmium selenide/zinc sulfide (CdSe/ZnS) core–shell quantum dots as absorbing impurities and samples of water (H2O) and THF with a small addition of blue-black commercial ink. The well-known thermal lens (TL) technique was also employed in this work for the sake of comparison with the proposed conical diffraction pattern. Thermo-optical parameters obtained by the mode-mismatched thermal lens (TL) technique validated the SPM measurements of dn/dT. In addition, we applied in this work the Fresnel–Kirchhoff diffraction formalism for the theoretical prediction of the rings that are observed in the far-field region. The technique proposed herein to determine dn/dT may serve as a fast, cost-effective optical method for evaluating adulterated or impure chemicals.

Core–shell quantum dots tailor the fluorescence of dental resin composites

OBJECTIVES We characterized the optical properties, such as absorbance and fluorescence, of dental resins containing quantum dots (QD). We also determined the doping level needed to obtain a broad and nearly flat emission spectrum that provides the perception of white color. METHODS The samples studied were resin composites from Charisma (Heraeus Kulzer) prepared with CdSe/ZnS core-shell QD (0.05-0.77 mass%). RESULTS The results showed that the fluorescence of dental resin composites can be tailored by using CdSe/ZnS core-shell quantum dots. CONCLUSION QD core incorporation into dental resins allows the fabrication of restorative materials with fluorescence properties that closely match those of natural human teeth.

Optical Characterization in Annatto and Commercial Colorific

Nowadays, the synthetic dyes (as erythrosine, ponceau and tartrazina) and natural colourants (as annatto, paprika, curcuma and anthocyanin) are indispensable in the food, pharmaceutical and cosmetics applications. However, the use of natural colourant has been considered safer for human consumption then synthetic dye. For practical applications of the coloring, optical properties are important for the understanding of the characteristics of them. In this work, we presented the absorption and fluorescence spectroscopic characterizations of annatto extracts obtained from the seeds of the tropical shrub Bixa orellana L. solutions and commercial colourant. The measurements were performed in annatto extracts with acetone and chloroform in different concentrations range (3.5–52.5) μg/mL. The main carotenoids detected in annatto seeds is bixin. The numerical calculus of the absorbance spectra for cis- and trans-bixin conformation is presented. In addition, for commercial colourant, the measurements were performed for six different brands and five lots each one. Modifications in the shape of the colorific fluorescence spectra were observed and it can be an indicative of differences in the industrial methods applied for obtaining annatto pigments powders and/or the possibility of the presence of other impurities added in the commercial powders.

Thermal quenching of the fluorescence quantum efficiency in colquiriite crystals measured by thermal lens spectrometry

The dual-beam mode-mismatched thermal lens technique was used to study the temperature dependence of the absolute fluorescence quantum efficiency (η) of a thermal-quenching fluorescence (TQF) process in Cr3+-doped colquiriite crystals (LiSAF and LiSGaF), from 300 to 450 K. The research was developed at a low excitation-power level in order to eliminate the energy-transfer upconversion effect. The results showed that TQF is the main loss mechanism involved. The thermal diffusivity, the thermal conductivity, and the specific heat of the samples were also measured in the same temperature range.

Fluorescence quantum efficiency measurements using the thermal lens technique

The single wavelength thermal lens (TL) method jointly with the multiwavelength TL method are used to determine the fluorescence quantum efficiency η of Nd3+-doped glasses and crystals. The concentration quenching and the upconversion parameter for Nd3+-doped solids is also described in this article. In addition, the TL method is used to study the temperature behavior of η for Cr3+-doped fluoride crystal.

Fluorescence quantum efficiency of CdSe/ZnS quantum dots embedded in biofluids: pH dependence

The radiative quantum efficiency (η) of CdSe/ZnS core-shell quantum dots (QDs) embedded in synthetic oral fluid was measured using a thermal lens (TL) technique. TL transient measurements were performed using the mode-mismatched dual-beam (excitation and probe) configuration. Thermal optical characterization of CdSe/ZnS QDs was performed for two different core sizes (3.9 and 5.1 nm) incorporated into synthetic saliva with different potential of hydrogen (pH 4–8) values. The thermal diffusivity (D) and average emission wavelength (⟨λem⟩) results are approximately independent of the pH of the solutions evaluated. The fractions of absorbed energy converted into heat (φ) and η are dependent on both the fluid pH and core size of the CdSe/ZnS core-shell QDs. The dependence on pH was also evidenced by fluorescence measurements, which corroborate the results obtained by the thermal lens technique.

Thermal-lens study of thermo-optical and spectroscopic properties of polyaniline

Thermal-lens measurements were performed in PVA/PAni solution in function of PAni concentrations which varied from 0.008 to 0.03 (wt %). We observed a very small increase of the thermal diffusivity with PAni concentration from 1.05 to 1.1×10−3 cm2/s. The fluorescence quantum efficiency behavior from the TL results shows that η decreases from 0.48 to 0.09 with the increase of PAni. These results are in agreement with the behavior of fluorescence spectra that decrease with concentration increase. The experiments were carried out with the protonation (doping) state of the polyaniline contained in the solution corresponding to the conducting emeraldine salt.

Self-Induced Phase Modulation for Thermo-Optical Characterization of Annatto Extracted Using Different Solvents:

Thermo-optical parameters, including the refractive index temperature coefficient (dn/dT) and the absolute non-radiative quantum efficiency (φ), were determined in solutions of a natural dye using a method based on thermally driven self-phase modulation (SPM) effects. The ring-pattern response to SPM effects was determined for annatto extracts obtained from the seeds of the tropical shrub Bixa orellana L. This approach determined the linear behavior of the ring numbers (N) observed in the far-field region as a function of excitation laser power (Pe). Thermooptical characterizations of bixin (C25H30O4) solutions extracted using three solvents (toluene, chloroform, and acetone) were performed. The φ results are in good experimental agreement with those obtained using the thermal lens (TL) technique. Values obtained for thermal diffusivity parameters were typical for pure solvents.