Refractive index of ZnO ultrathin films alternated with Al2O3 in multilayer heterostructures.

The design of optoelectronic devices made with ZnO superlattices requires the knowledge of the refractive index, which currently can be done only for films thicker than 30 nm. In this work, we present an effective medium approach to determine the refractive index of ZnO layers as thin as 2 nm. The approach was implemented by determining the refractive index of ZnO layers ranging from 2 nm to 20 nm using spectroscopic ellipsometry measurements in multilayers. For a precise control of morphology and thickness, the superlattices were fabricated with atomic layer deposition (ALD) with alternating layers of 2 nm thick Al2O3 and ZnO, labeled as N ZnO-Al2O3, where N = 10, 20, 30, 50, 75 and 100. The total thickness of all superlattices was kept at 100 nm. The approach was validated by applying it to similar superlattices reported in the literature and fitting the transmittance spectra of the superlattices.

Synthesis and characterization of (3-Aminopropyl)trimethoxy-silane (APTMS) functionalized Gd2O3:Eu(3+) red phosphor with enhanced quantum yield.

We report the surface modification of nanocrystalline Gd2O3:Eu(3+) phosphor by (3-Aminopropyl)trimethoxysilane (APTMS). The nanoparticles were first coated with silica using the Stöber process, and then annealed at 650 °C for 2 h. Afterwards, APTMS was functionalized onto the silica layer to obtain Gd2O3:Eu(3+) nanoparticles bearing amine groups on the surface. The effect of silica coating, and the subsequent annealing process on the crystallization of the nanophosphor were analyzed by x-ray diffraction (XRD). High-resolution transmission electron microscopy (HR-TEM) confirmed the presence of a silica layer of ∼45 nm thickness. X-ray photoelectron (XPS) and Fourier transform infrared (FTIR) spectroscopy confirmed the presence of silica and the amine groups. Photoluminescence (PL) analysis demonstrated an increased emission after functionalization of nanoparticles. Absolute quantum yield (QY) measurements revealed an 18% enhancement in QY in functionalized nanoparticles compared with unmodified nanoparticles, which is of great importance for their biomedical applications.

Density functional theory simulation of the adsorption of sulphur multilayers on Au(100).

The adsorption of sulphur multilayers on Au(100) has been studied using density functional theory (DFT) within the generalized gradient approximation (GGA). The first sulphur layer was adsorbed on the four-fold sites of the unreconstructed Au(100) surface forming a lattice. The experimental parameters of the lattice were reproduced taking into account the surface expansion of the topmost Au(100) layer. This expansion should occur when gold islands are formed after the lifting of hex-reconstruction, which allows the lateral movement of the gold atoms. The second sulphur layer, on top of the lattice, consisted of eight S atoms (octomer phase) in a quasi-rectangular arrangement. The structural optimization of the octomer phase was achieved in a specific spatial orientation with respect to the lattice. The analysis of Bader atomic charges and the projected density of states (PDOS) demonstrated that charge transfer from the Au(100) surface to the sulphur layers, sulphur chemisorption and sulphur-sulphur inter-layer mixing of electronic states control the formation of sulphur multilayers.

Nano and micro reoriented domains and their relation with the crystal structure in the New Fe1.5Zn1.5B7O13Cl boracite.

New iron-zinc chlorine single crystals of Fe1.5Zn1.5B7O13Cl boracite were grown by chemical transport reactions in closed quartz ampoules, at a temperature of 1173 K. The crystal structure was characterized by X-ray powder diffraction (XRD) using the Rietveld refinement method and belongs to the trigonal/rombohedral system with space group R3c (No. 161). The cell parameters were a = 8.5726(1) angstroms, c = 21.0116(4) angstroms, V = 1337.26(3) angstroms3 and Z = 6. The refinement successfully proceeded and ended with sound merit figure values chi2 = 2.25, R(B) = 6.12%. Chemical analysis was performed with X-ray energy dispersive spectroscopy (EDS) and X-ray fluorescence (XRF). Ferroelectric nano and micro reoriented domains were found in this material using polarizing optical microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The examination by TEM showed that in the trigonal/rombohedral system of Fe1.5Zn1.5B7O13Cl nanodomain structures exist. Thin (50-100 nm) mostly planar domains parallel to (100) were frequently observed in Fe1.5Zn1.5B7O13Cl boracite.

Catalytic activity of gold nanoparticles incorporated into modified zeolites.

Gold catalysts modified by Fe and Ni and supported on different zeolite matrixes have been studied by TEM, TPR, and catalytic testing. The presence of a metal oxide additive allows stabilizing small gold particles, particularly in the case of Fe. The shape of light-off curves shows two temperature regions of the catalyst activity, a low-temperature range below 250 degrees C and a high-temperature range above 300 degrees C. This situation is explained considering the existence of at least two types of catalytically active sites of gold assigned to gold clusters and gold nanoparticles, respectively, while the ionic state of gold (Au3+) remains inactive. It is shown that interaction of gold with Fe promoter leads to activation of catalysts at low temperature due to a change of electronic state and redox properties of gold. NiO additive cause a similar, but less pronounced effect.

EDC and sulfo-NHS functionalized on PVC-g-PEGMA for streptokinase immobilization.

In this work, streptokinase (SK) coenzyme was coupled onto functionalized graft copolymer poly(vinyl chloride)-g-poly(ethylene glycol)methacrylate (PEGMA) using the water soluble carbodiimide 1-ethyl-3-(3-dimethyl aminopropyl carbodiimide hydrochloride) and sulfo-N-Hydroxysulfo succinimide. Different graft values of PEGMA copolymers were prepared via preirradiation method in air by using ultraviolet light. SK was coupled onto graft copolymer and the enzyme activity was determined before and after immobilization. Surface morphology, grafting, and enzymatic activity were studied by using infrared spectroscopy, atomic force microscopy, and fluorescence microscopy. Results revealed that graft value of 0.15 provide optimum conditions to retain original SK activity, the enzyme distribution was 2.8 µg cm-2 of copolymer surface and kinetic parameters, K m and V max were 0.14 µM and 29.8 µM min-1, respectively.

Biocatalytic synthesis of polypyrrole powder, colloids, and films using horseradish peroxidase.

Polypyrrole was synthesized in high yield by a biocatalytic method in mild aqueous media using hydrogen peroxide as oxidizer. A redox mediator, 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) diammonium salt, was used to oxidize the pyrrole. ABTS is a very effective peroxidase substrate, which was enzymatically oxidized to generate a radical cation that in turn was able to chemically oxidize pyrrole. This indirect biocatalytic method was implemented because pyrrole is not a substrate of horseradish peroxidase, however, the polymerization process was successfully optimized and later adapted to prepare also polypyrrole thin films and water dispersible polypyrrole colloids. The polypyrrole powder and colloids were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, electrical conductivity, and thermogravimetric analysis. In addition, the deposition of the polypyrrole thin film was monitored using a quartz-crystal microbalance and its morphology studied by optical and scanning electron microscopy. The biocatalytic polymerization of pyrrole results in a polymer spectroscopically very similar to chemically synthesized polypyrrole.