Sol-gel synthesis of Eu3+ doped silica-gold nanorod composites with tunable optical properties.

Gold nanorods (AuNRs) suspension at various concentrations was added into the sol-gel process to engineer nanostructured europium-doped silica host matrices as light-emitting composites. For this purpose, the samples were prepared following two different routes depending on the chemicals used as dopant and catalyst: (a) Eu(NO3)3·5H2O and HNO3, and (b) EuCl·6H2O and HCl. In any case, samples adding various concentrations of AuNRs suspension were prepared. The structural characterization of the samples was through STEM, backscattered electrons (BSE), and EDS analysis. Additionally, their optical properties were evaluated by PL spectroscopy and CIE colorimetry. The results confirmed that (a) methodology produced samples with AuNRs embedded and randomly distributed in the samples. However, these features were not observed in the samples obtained through (b) due to AuNRs dissolution in HCl media. Regarding the optical properties, the analysis of the relative intensity ratio 5D0 → 7F2/5D0 → 7F1 suggested that Eu3+ ions occupy non-centrosymmetric sites in (a) host matrices and centrosymmetric sites in (b). Hence, the increase of AuNRs suspension when fabricating (a) host matrices produced remarkable color changes in the luminescence of the samples towards the reddish-orange region. Meanwhile, the dissolution of AuNRs in (b) minimized the localized surface plasmon resonance (LSPR) effects on the Eu3+ luminescence. These findings revealed that the evaluation and selection of chemicals are critical factors when engineering these materials for more efficient coupling between the LSPR and Eu3+ luminescence.

Synthesis of gold decorated silica nanoparticles and their photothermal properties.

Silica-Gold Nanostructures (SGNs), composed of a silica core decorated by gold nanoparticles, have the photothermal capacity to transform near-infrared (NIR) wavelengths into heat. This work presents a simple, efficient, and replicable method of synthesis of SGNs and their characterization by: (1) transmission electron microscopy to obtain micrographs of the particles and their corresponding diameter distribution; (2) diffraction patterns showing the amorphous atomic arraignment of the silica and the crystalline atomic arrangement of the gold nanoparticles; (3) zeta potential confirming the stability of the SGNs in a colloidal solution; and (4) thermal images displaying the capacity of SGNs to convert NIR irradiation into heat and their respective increment in temperature. SGNs were synthesized over silica cores with diameters of 63, 83, and 132 nm and decorated with a partial gold shell. They were heated with a coherent light intensity of 340 mW/cm2 with a wavelength of 852 nm. This wavelength is within the range of the optical window of the human body; therefore, SGNs may be used for the photothermal ablation of tumors with no damage to the tissue. The heating of different dimensions of SGNs took 6-8 min of NIR radiation, and their cooling, once the laser was turned off, was in the order of 2-3 min. It was found that SGNs, with a core diameter of 132 nm, have a notable photothermal capacity. That enables them to increase the temperature of their surroundings by 4.4 ºC. This increment in temperature is sufficient to induce cellular necrosis, which makes SGNs a good option for photothermal treatments.

In-Situ Preparation of CdTe Quantum Dots Capped with a ß-Cyclodextrin-Epichlorohydrin Polymer: Polymer Influence on the Nanocrystal's Optical Properties.

ß-Cyclodextrin (ßCD), the less water soluble of the cyclodextrins, has been used as a capping agent in the preparation of semiconductor nanocrystals or quantum dots (QDs). Nevertheless, no reports have been found in the use of the highly water-soluble polymer of this, prepared by the crosslinking of the ßCD units with epichlorohydrin in basic medium (ßCDP). This polymer, besides to overcome the low solubility of the ßCD, increases the inclusion constant of the guest; two parameters that deserve its use as capping agent, instead of the native cyclodextrin. In the present manuscript, we afforded the in-situ aqueous preparation of cadmium telluride (CdTe) QDs capped with ßCDP. The polymer influence on the photoluminescent properties of the nanocrystals was analyzed. The ßCDP controls the nanocrystals growth during the Oswald ripening stage. Consequently, the CdTe capped ßCDP QDs showed lower Stokes-shift values, higher photoluminescent efficiency, and narrower size distribution than for nanocrystals obtained in the absence of polymer. Transmission electron microscopy (TEM) micrographs and energy dispersive X-ray spectroscopy (EDS) analysis revealed the composition and crystallinity of the CdTe QDs. This ßCDP capped CdTe QDs is a potential scaffold for the supramolecular modification of QDs surface.

Long- and Short-Range Structure of Ferrimagnetic Iron-Chromium Maghemites.

Maghemite-like materials containing Fe(3+) and Cr(3+) in comparable amounts have been prepared by solution-combustion synthesis. The conditions of synthesis and the magnetic properties are described. These materials are ferrimagnetic and are much more stable than pure iron maghemite since their maghemite-hematite transformation takes place at about ∼ 700 °C instead of ∼ 300 °C, as usually reported. These materials were studied by synchrotron radiation X-ray diffraction (XRD) and by X-ray absorption fine structure (XAFS) of the K-absorption edge of two elements. High-resolution XRD patterns were processed by means of the Rietveld method. Thus, maghemites were studied by XAFS in both Fe and Cr K-edges to clarify the short-range structure of the investigated systems. Pre-edge decomposition and theoretical modeling of X-ray absorption near edge structure transitions were performed. The extended X-ray absorption fine structure (EXAFS) spectra were fitted considering the facts that the central atom of Fe is able to occupy octahedral and tetrahedral sites, each with a weight adjustment, while Cr occupies only octahedral sites. Interatomic distances were determined for x = 1, by fitting simultaneously both Fe and Cr K-edges average EXAFS spectra. The results showed that the cation vacancies tend to follow a regular pattern within the structure of the iron-chromium maghemite (FeCrO3).

The effects of porosity on optical properties of semiconductor chalcogenide films obtained by the chemical bath deposition.

This paper is dedicated to study the thin polycrystalline films of semiconductor chalcogenide materials (CdS, CdSe, and PbS) obtained by ammonia-free chemical bath deposition. The obtained material is of polycrystalline nature with crystallite of a size that, from a general point of view, should not result in any noticeable quantum confinement. Nevertheless, we were able to observe blueshift of the fundamental absorption edge and reduced refractive index in comparison with the corresponding bulk materials. Both effects are attributed to the material porosity which is a typical feature of chemical bath deposition technique. The blueshift is caused by quantum confinement in pores, whereas the refractive index variation is the evident result of the density reduction. Quantum mechanical description of the nanopores in semiconductor is given based on the application of even mirror boundary conditions for the solution of the Schrödinger equation; the results of calculations give a reasonable explanation of the experimental data.