In situ formation and assembly of CdS nanocrystallites into polyhedrons on Eggshell membrane at room temperature

A room-temperature soakage procedure was carried out to successfully form and assemble CdS nanocrystallites into polyhedrons on the eggshell membrane (ESM). Based on the biomaterial ESM served as the reactive substrate and some ESM biomacromolecules acted as the surfactants, CdS nanocrystallites were in situ formed, further assembled into well-distributed polyhedrons, and finally performed CdS-ESM hybrid nanocomposites. This moderate bioinspired strategy would also be of great value to prepare novel functional nanocomposites.     

Patterning and photoluminescence of CdS nanocrystallites on silk fibroin fiber

CdS nanocrystallites could be formed and assembled into nanoparticle strings and hexagons on natural silk fibroin fiber (SFF) through a room-temperature bio-inspired process. Herein, the biomaterial SFF served as reactive substrate, not only provides the in situ formation sites for CdS nanocrystallites, but also directs the arrangement of nanocrystalline CdS simultaneously. The photoluminescence (PL) of the resulting nanocomposites CdS/SFF is investigated extensively. The PL peaks observed from CdS nanoparticle strings are similar to those of separate CdS nanoparticles, corresponding to the band-edge emission of their individual building blocks (QD-CdS). Moreover, CdS nanoparticle hexagons perform a red-shifted and broadened emission peak.     

Computational study of the formation of aluminum-graphene nanocrystallites

The molecular dynamics method is used to study the formation of the Al/graphene nanocomposite in the structural grains of different size under the action of internal stresses. The behavior of graphene sheets inside an individual structural grain as well as in the process of two Al grains containing graphene are joined is investigated. The motion of graphene films, starting from the middle of the aluminum matrix, ends with their location at the crystallite boundaries. Graphene moves in the Al matrix along closely packed planes. In this case, graphene sheets acquire curvature. An intergrowth of graphene sheets is also observed. A contact between two Al-C nanocrystallites through a graphene interlayer is created. The self-diffusion coefficients of atoms and the partial potential energies increased with decreasing nanocrystallite size. The angular distribution of the nearest geometric neighbors and the distribution of distances to the nearest neighbors are determined using the construction of Voronoi polyhedra.     

A novel preparation method and investigation of sprayed CdS films

The main reason for the low photovoltaic efficiency of Cu₂S/CdS cells, in which the CdS film is deposited by solution spraying, is the very small average dimensions of the crystalline grains. The difficulty in depositing films with transverse dimensions of crystalline grains larger than 1 m is due to the fast decrease of substrate temperature during the spraying deposition of the CdS film. We have since arranged an experimental set-up able to maintain the surface temperature of the substrate constant. In this way it is possible to control the parameters which affect the deposition and to obtain a high degree of reproducibility of morphological properties. Average grain sizes of more than 1 m are obtained. We have investigated the crystalline grain sizes and the film thickness as a function of the deposition temperature and the spraying rate.Work supported by Ministero Pubblica Istruzione and Centro Regionale Ricerche Nucleari e Struttura della materia     

Irradiation-induced formation of nanocrystallites with C15 Laves phase structure in bcc iron

A three-dimensional periodic structure is proposed for self-interstitial clusters in body-centered-cubic metals, as opposed to the conventional two-dimensional loop morphology. The underlying crystal structure corresponds to the C15 Laves phase. Using density functional theory and interatomic potential calculations, we demonstrate that in α-iron these C15 aggregates are highly stable and immobile and that they exhibit large antiferromagnetic moments. They form directly in displacement cascades, and they can grow by capturing self-interstitials. They thus constitute an important new element to account for when predicting the microstructural evolution of iron base materials under irradiation.     

A novel ethanol templating synthesis of ordered lamellar superstructured crystalline zirconia

Soft template technique has attracted great interest, because it is a facile, inexpensive and efficient synthesis strategy for ordered superstructural systems. Here, a novel ethanol template was used to synthesize the ordered lamellar superstructured crystalline zirconia (Lα-ZrO₂) without post-treatments and surfactants. ZrOCl₂ and NaOH were served as Zr source and precipitant, respectively. XRD analysis showed that Lα-ZrO₂ is crystalline. XPS spectra indicated the physical adsorption of ethanol molecules in Lα-ZrO₂. TEM further observed and proved the 1.36-nm period of superstructure detected and calculated by SAXRD (1.35 nm), which is composed of 0.68-nm thick ZrO₂ and pore alternatively. In contrast, the template-free ZrO₂ (TF-ZrO₂) presents no superstructure and is poorly crystallized. As a soft template, ethanol presents the roles of (i) inducing the growth of zirconia layers, (ii) directing the self-assembly of ordered lamellar superstructure, and (iii) decreasing the crystallization temperature. The possible mechanism of ethanol serving as a soft template was proposed and discussed in thermodynamics.     

Excitation of Er atoms by energy transfer from Si nanocrystallites embedded in SiO2 matrices fabricated by laser ablation

We have investigated excitation of Er³⁺ ions via energy transfer from Si nanocrystallites embedded in SiO₂ films. The Er-doped films were fabricated using a laser ablation technique. We found that a photoluminescence (PL) excitation spectra of Er³⁺ ions coincides with that of Si nanocrystallites. Thus, it is evident that Er³⁺ ions are excited via the luminescent singlet state in Si nanocrystallites. Furthermore, we obtained the results that support the energy transfer mechanism. PL intensity of Er³⁺ ions increases with Er concentration while that of Si nanocrystallites decrease inversely. PL intensity of Er³⁺ ions increases with temperature from cryogenic to room temperature under photo-excitation at power density higher than 110 mW/cm². The increase is characteristic of the luminescent state in Si nanocrystallites but not any state in Er³⁺ ions. PACS 61.72.Ww; 61.46.+w; 81.15Fg     

Phase transition of CdS in the presence of ethylenediamine and formation of hollow CdS submicron particles with needle-like structure

Solid submicron particles, composed of amorphous, hexagonal and cubic phase CdS, were synthesized in water–ethylenediamine solution by precipitating Cd²⁺ ions with S^2? ions, which were generated from the radiolytic reduction of Na₂S₂O₃. In addition, the effects of the absorbed dose on the crystallization process of CdS were investigated in detail. Moreover, hollow CdS submicron particles with needle-like structure could be obtained from the solid CdS submicron particles probably through a hollowing process based on Ostwald ripening and the phase transition of CdS from amorphous to crystalline. In the above processes, ethylenediamine played an important role.     

Silica-scavenging effect in zirconia electrolytes: assessment of lanthanum silicate formation

Yttria-stabilized zirconia (YSZ)-based composite electrolytes were produced by mixture and firing of the base electrolyte, namely with silica, and silica and lanthanum oxide, to study the impact of presence and formation of new phases on the electrical performance of the composite materials. Combined information obtained from structural, microstructural, and electrical characterization confirmed the formation of an apatite-type solid solution based on La_9.33Si₆O₂₆. The effectiveness of lanthanum oxide addition to remove silica by reaction was demonstrated. However, the conductivity of the composite electrolytes is lower than that of YSZ, probably due to lanthanum zirconate formation in-between the ceramic grains and/or relatively poor transport properties of the formed lanthanum silicate phase. The adopted procedure can be extended to other systems and combinations of properties based on predictable phase interactions.     

Spatio-temporal structure formation and self-diffraction in CdS using laser-induced thermal gratings

The semiconductor CdS is well known to show various kinds of photo-thermal and photo-electric optical nonlinearities. We present here to our knowledge first results of spatiotemporal structure formation using laser-induced thermal gratings in CdS. By the means of an optical multi-channel analyzer the spatial resolution of transverse dynamic switching processes was observed directly. Spatially and time resolved self-diffracted signals were measured in the far-field. The experimental results agree very well with calculations obtained by solving the heat-flow equation and using a fast Fourier transformation. The calculations verify the experimental parameters, in particular the thermal diffusivity D0.1 cm²/s at room temperature. To obtain a better transverse structuring, CdS samples were also investigated which were fixed on a sapphire substrate for longitudinal heat sinking.     

A novel two-phase thermal approach for synthesizing CdSe/CdS core/shell nanostructure

CdSe/CdS core/shell nanocrystals have been synthesized through a low cost and simple two-phase thermal route. The optical spectroscopy and structural characterization evidenced the core/shell structure of the CdSe/CdS nanoparticles. The X-ray diffraction patterns of CdSe and CdSe/CdS nanoparticles exhibited peak positions corresponding to those of their bulk cubic crystal structures. The X-ray photoelectron spectroscopy data confirmed the elemental composition of the CdSe/CdS nanoparticles. The absorption spectra of core/shell nanoparticles showed red shift with respect to the core CdSe nanoparticles. The photoluminescence study indicates that the intensity of the emission maximum is considerably increased in the core/shell structure as compared with the parent material, and the capping of CdS nanoparticles with CdSe material exhibit a near band-edge emission, indicating a successful passivation by removing surface defects. The high-resolution transmission microscope images of the bare and core/shell nanoparticles ascertained the monodispersed and well-defined spherical particles. The average particle sizes for CdSe and CdSe/CdS nanoparticles are 2.5 and 5 nm, respectively, thus confirming, the larger diameter of CdSe/CdS core/shell nanostructure than the core CdSe nanoparticles.     

Raman characterization of localized CdS nanostructures synthesized by UV irradiation in sol–gel silica matrices

A new technique of UV irradiation has been used for the first time to create microstructures of CdS nanoparticles in bulk xerogels. Porous silica matrices, which were first soaked in a solution containing CdS precursors, were subjected to irradiation using a nanopulsed ArF laser with a wavelength of 193 nm and a repetition rate of 10 Hz. Resonant micro‐Raman spectra, recorded using the 325‐nm line of a He‐Cd laser (with a continuous power less than 0.5 mW so as to avoid the thermal formation of nanoparticles) made it possible to identify CdS nanoparticles within the inscribed yellow zones and also to estimate the average particle size (3.6 to 8.0 nm, depending on the number of UV pulses used). The emission of CdS particles embedded in the silica matrix under excitation at 351.1 nm was studied by photoluminescence spectroscopy. It was then possible to show the effect of the number of pulses on the electronic structure of the nanocrystals. Finally, Raman spectra were used to monitor the structural changes in the silica matrix caused by the irradiation. It was found that the pulsed UVirradiation resulted in a local densification of the matrix, which was compensated for by a depolymerization process of the Si O Si network. In spite of this pulsed irradiation and the resulting structural depolymerization, no apparent ablation or cracking of the samples was observed. Copyright © 2010 John Wiley & Sons, Ltd.     

Fabrication of microchannels in single-crystal GaN by wet-chemical-assisted femtosecond-laser ablation

We investigated micro- and nano-fabrication of wide band-gap semiconductor gallium nitride (GaN) using a femtosecond (fs) laser. Nanoscale craters were successfully formed by wet-chemical-assisted fs-laser ablation, in which the laser beam is focused onto a single-crystal GaN substrate in a hydrochloric acid (HCl) solution. This allows efficient removal of ablation debris produced by chemical reactions during ablation, resulting in high-quality ablation. However, a two-step processing method involving irradiation by a fs-laser beam in air followed by wet etching, distorts the shape of the crater because of residual debris. The threshold fluence for wet-chemical-assisted fs-laser ablation is lower than that for fs-laser ablation in air, which is advantageous for improving fabrication resolution since it reduces thermal effects. We have fabricated craters as small as 510 nm by using a high numerical aperture (NA) objective lens with an NA of 0.73. Furthermore, we have formed three-dimensional hollow microchannels in GaN by fs-laser direct-writing in HCl solution.     

Model for UV induced formation of gold nanoparticles in solid polymeric matrices

UV irradiation of polymeric PMMA films containing HAuCl₄ followed by annealing at 60-80 °C forms gold nanoparticles directly within the bulk material. The kinetics of nanoparticle formation was traced by extinction spectra of nanocomposite film changes vs annealing time. We propose that UV irradiation causes HAuCl₄ dissociation and thus provides a polymeric matrix with atomic gold. The presence of an oversaturated solid solution of atomic gold in the polymeric matrix leads to Au nanoparticle formation during annealing. This process can be understood as a phase transition of the first order. In this paper we apply several common kinetic models of the phase transition for describing Au nanoparticle formation inside the solid polymer matrix. We compare predictions of these models with the experimental data and show that these models cannot describe the process. We propose that the stabilization effect of the matrix on the growing gold nanoparticles is important. The simplest model introducing some probability for the transition from growing nanoparticle to the non-growing, stabilized form is suggested. It is shown that this model satisfactorily describes the experimentally observed evolution of the extinction spectrum of Au nanoparticles forming in a polymer matrix.     

A novel miniature zirconia gas sensor with pseudo-reference: Amperometric operation providing unambiguous determination of air-to-fuel ratio

The problem of ambiguity of the output of a single-chamber amperometric zirconia oxygen sensor with the possible confusion of lean and rich operation of a combustion system is well known. The solution previously proposed entails the addition of a second chamber containing a piped reference gas such as air. In this paper the novel solution proposed is to generate a pseudo-reference gas, which always contains excess oxygen, in the second chamber so eliminating the need for a piped reference gas. The detailed theory of the device in one particular mode is developed and confirmed by experimental tests in O₂-N₂ and CO-CO₂-N₂ mixtures. The device was also operated in the flue of a gas-burning system over a wide range of air-to-fuel ratios from rich to lean. The output changed monotonically over the whole range switching sign as stoichiometry was traversed. An amperometric zirconia gas sensor with such a characteristic has previously only been described for a sensor with supplied reference gas of fixed composition.Nomenclature D _X Diffusion coefficient of gas X - E Nernst EMF between cell (B) and (A) - F The Faraday - I _A(B) Current applied to cell A(B) - I _AS Critical current corresponding to P _CO,B=P _{O 2},B=0. - K Equilibrium constant for the reaction 2CO₂2CO+O₂ - P _COS Critical partial pressure of CO in sample gas corresponding to P _CO,B =P _{O 2 B}=0 - P _X Partial pressure of X in the sample gas - P _X,A(B) Partial pressure of X in cell A(B) - PRE Pseudo-reference electrode - R Gas constant - T Temperature of operation (K) - V _r Reference voltage (feedback loop aims to maintain E=V _r) - X Represents O₂, CO or CO₂ - _X,A(B) Leak conductance of diffusion barrier in cell A(B) in relation to gas X. For a hole of uniform cross-sectional area S and length L, _X =D _XS/RTL - Normalized air-to-fuel ratio     

Optical spectroscopy of opal matrices with CdS embedded in its pores: Quantum confinement and photonic band gap effects

Il Nuovo Cimento D - The spectra of transmission and reflection of synthetic opal which has 3-dimensional periodic structure were measured at different orientations of incident beam relative to the...     

Stability of luminescent 3C-SiC nanocrystallites in aqueous solution

We study the long-term stability of luminescent suspended 3C-SiC nanocrystallites. Polycrystalline 3C-SiC wafers were electrochemically etched in HF-ethanol electrolyte and then ultrasonically treated in water to achieve spherical crystallites with diameters of 1–7 nm. The suspended 3C-SiC nanocrystallites exhibit tunable intense emissions which follow well the quantum size effect. After storage in air for over 7 months, partial larger 3C-SiC crystallites aggregate and form precipitates, while smaller ones remain uniformly dispersed in water and retain well luminescent properties. This characteristics makes small 3C-SiC crystallites excellent micro-emitters which are especially useful in biological and medical applications.