The influence of sodium nanoparticles formation on luminescent properties of fluorophosphate glasses containing molecular clusters and quantum dots of lead selenide

The influence of sodium nanoparticles and secondary heat treatment conditions on the spectralluminescent characteristics of fluorophosphate glasses with PbSe molecular clusters and quantum dots is studied. Experiments with glasses containing no sodium nanoparticles show that their thermal treatment leads to the formation of molecular clusters with subsequent formation of quantum dots having an intense luminescence. The results of numerical simulation for glasses with sodium nanoparticles shows that heat treatment leads to formation of a sodium fluoride shell on the nanoparticles surface. It is shown that quenching of the luminescence of PbSe molecular clusters and quantum dots takes place in these glasses.     

Spectral-luminescent properties of silver molecular clusters and nanoparticles formed by ion exchange in antimony-doped photo-thermo-refractive glasses

Optics and Spectroscopy - The formation of silver molecular clusters and nanoparticles in photo-thermo-refractive (PTR) glasses with different antimony contents has been investigated using ion...     

The formation of acetylcholine receptor clusters visualized with quantum dots

Background  

Motor innervation of skeletal muscle leads to the assembly of acetylcholine receptor (AChR) clusters in the postsynaptic membrane at the vertebrate neuromuscular junction (NMJ). Synaptic AChR aggregation, according to the diffusion-mediated trapping hypothesis, involves the establishment of a postsynaptic scaffold that "traps" freely diffusing receptors into forming high-density clusters. Although this hypothesis is widely cited to explain the formation of postsynaptic AChR clusters, direct evidence at molecular level is lacking.     

Preparation and photoelectric properties of cadmium sulfide quantum dots

Cadmium sulfide quantum dots(CdS QDs) are widely used in solar cells, light emitting diodes, photocatalysis, and biological imaging because of their unique optical and electrical properties. However, there are some drawbacks in existing preparation techniques for CdS QDs, such as protection of inert gas, lengthy reaction time, high reaction temperature, poor crystallinity, and non-uniform particle size distribution. In this study, we prepared CdS QDs by liquid phase synthesis under ambient room temperature and atmospheric pressure using sodium alkyl sulfonate, CdCl_2, and Na_2S as capping agent, cadmium, and sulfur sources respectively. This technique offers facile preparation, efficient reaction, low-cost, and controllable particle size. The as-prepared CdS QDs exhibited good crystallinity, excellent monodispersity, and uniform particle size. The responsivity of CdS QDs-based photodetector is greater than 0.3 μA/W, which makes them suitable for use as ultra-violet(UV) detectors.     

Influence of stoichiometry on the luminescent properties of InAs quantum dots grown on a heterostructure

We report photoluminescence (PL) spectra of InP/In_xGa_1-xAs/InAs/InP dot-in-a-well structures grown by MOVPE, with different compositions of the ternary layer. Measurements with atomic force microscopy showed that the largest quantum dot (QD) height is obtained when the InAs QDs are grown on the In_xGa_1-xAs layer with a mismatch of 1000 ppm, and the height decreases as the mismatch departs from this value. PL spectra of the QDs showed an asymmetric band, which involves transitions between dot energy levels and can be deconvoluted into two peaks. The highest energy PL peak of this band was observed for the sample with the QDs grown on top of the lattice-matched In_xGa_1-xAs layer and it shifted to lower energies for strained samples as the degree of mismatch increased. Theoretical calculations of the energy levels of the entire structure were used to interpret the obtained PL spectra and determine the possible detection tunability range.     

The influence of the polymer-stabilizer molecular weight on the spectral luminescence properties of composite sols and coatings containing PbS quantum dots

The influence of the polyvinylpyrrolidone (PVP) molecular weight on the stability and spectral luminescence properties of sols of lead sulfide nanocrystals and the related composite coatings has been studied. It is shown that the spectral properties of PbS sols stabilized with low-molecular (PVP) and the related coatings are determined to a great extent by the formation of large particle aggregates in these materials and, accordingly, high level of light scattering. It is effective to use low-molecular PVP for preparing powder materials containing PbS quantum dots (QDs), because it allows one to perform fast powder precipitation and form small semiconductor particles. High-molecular PVP provides high aggregative and sedimentation stabilities of semiconductor nanocrystal sols. This polymer is effective for use in preparing stable QD sols and homogeneous coatings transparent in the visible spectral range.     

Optical properties of silver clusters formed by ion irradiation

Precipitation of silver clusters in silica is achieved by different methods: ion implantation, ion beam mixing of superimposed layers and ion irradiation of films deposited by means of co-sputtering or sol-gel technique. Main features of the nanoparticles depending on the preparation method are investigated by TEM. The optical extinction resonance of these clusters is analysed in terms of sizes and interaction between the clusters on the basis of calculations. We found that resonances in sputtered and gel films with low metal concentrations are well described by plasmon polaritons in isolated clusters and calculations based on Mie theory allow the study of their growth under irradiation. This theory is not appropriate to describe the optical response of silver clusters in silica implanted with Ag concentrations higher than 5 at.% or in ion beam mixed films, because of the interaction between clusters. Using an effective medium model, it is demonstrated that the random dispersion of clusters in implantation films causes fluctuations and, on average, an increase of the clusters polarization. On the contrary, the particular arrangement of the clusters with a bimodal size distribution in ion beam mixed films induces a screening effect between the clusters of largest size. Received 10 January 2002 / Received in final form 26 April 2002 Published online 19 July 2002     

The influence of ligands on the preparation and optical properties of water-soluble CdTe quantum dots

In a systematic investigation has been found that ligands play an important role in both the water-phase preparation and optical properties of CdTe quantum dots. Experiments were performed using three typical thioalkyl acids as ligands, namely mercaptoacetic acid (MAA), l-cysteine (Cys) and reduced glutathione (GSH). The growth rate and size-distribution of CdTe quantum dots (QDs) are shown to depend on the type of ligands. A proper choice of ligand enables to make lager nanocrystals with narrower size-distribution. The effects of pH (buffer solution), illumination, heating and cations on the spectroscopic properties of CdTe QDs for the three ligands are reported. In addition, three same-size CdTe QDs were individually characterized by micellar electrokinetic capillary electrophoresis with laser-induced fluorescent detection, which proved their monodisperse size-distribution and different electric charge distribution on the surface for each of the three different type of QDs.     

Mechanism for the formation of molecular clusters under ion sputtering

The mechanism for the formation of molecular Si _n O _2n+1 ^? clusters above the surface during the recombination of ions, atoms, and molecules independently sputtered in individual cascades is presented. Clusters form when sputtering products Si, O, SiO and SiO₂ (monomers) join the active anion O^? successively as a result of pair collisions between them. The joining of monomers involves different combinations of them; in this case, the (Si _n O_2n+1)^? group is formed and consists of monomers with the same masses, but with different monomer compositions in the chain (combinatorial synthesis).     

The influence of cysteamine and counterion concentrations on the properties of CdSe/ZnS quantum dots

The influence of the cysteamine surfactant concentration on the stability of CdSe/ZnS nanoparticles (NPs) solubilized by this compound at the phase interface between two immiscible liquids is considered. The steady-state and time-resolved fluorescence spectroscopy data show that the fluorescence quantum yield of cysteamine-coated NPs and their stability to aggregation in a potassium phosphate buffer are determined by the balance between the concentrations of surfactant in the aqueous phase and hydrophobic NPs in the nonpolar phase (chloroform, toluene, etc.). It is found that the brightest and most stable hydrophilic NPs can be obtained by completely coating them by cysteamine molecules without a surfactant deficit or excess in the reaction at the phase interface.     

Sizes and fluorescence of cadmium sulfide quantum dots

Cadmium sulfide quantum dots have been synthesized by wet chemical deposition from an aqueous solution. The sizes of the quantum dots determined by dynamic light scattering directly in the colloidal solution and by intermittent-contact atomic force microscopy in the dry sediment agree with each other. It has been found that splitting of the fluorescence peaks of the quantum dots can be affected by the disorder of the atomic structure of cadmium sulfide quantum dots.     

The influence of ligand type on self-organization and optical properties of cadmium selenide quantum dots

A method for successive replacement of organic shells of colloidal cadmium selenide quantum dots (QDs) of different sizes is proposed. It is found that the spectral parameters of QD samples depend on the type of organic shells. It is shown that the structural morphology is independent of the QD size and is determined by the chemical composition of the organic shell. Spectral analysis of the luminescence of QD-based superstructures shows that the luminescence wavelength and intensity strongly depend on the degree of QD surface passivation.     

The irradiation influence on the properties of silver sulfide (Ag2S) colloidal nanoparticles

The aqueous solutions of different stability containing silver sulfide (Ag₂S) nanoparticles are studied. The stable, transparent, and turbid solutions have been subjected to daylight for 7 months, to ultraviolet and laser irradiation, as well as to an electron beam. Solar radiation is found to favor the Ag₂S reduction to Ag and/or the formation of Ag₂S/Ag hybrid nanoparticles in the solution. At a high amount of hybrid nanoparticles, the exciton–plasmon interaction causes asymmetry in the absorption spectra. The exposure of Ag₂S particles precipitated from the solution with the electron beam leads to the reversible growth of Ag threads. The possible exciton–plasmon interplay mechanisms in Ag₂S/Ag hybrid nanoparticles are considered. The physical mechanisms of the changing Ag₂S stoichiometry, the formation of metallic Ag and Ag₂S/Ag hybrid nanoparticles are the generation of hot carriers and the energy transfer (exciton–plasmon interaction) in a metal–semiconductor hybrid nanosystem are elucidated, as well.

     

Carrier gas effects on the SiGe quantum dots formation

SiGe quantum dots (QDs) grown by ultra-high vacuum chemical vapor deposition using H₂ and He carrier gases are investigated and compared. SiGe QDs using He carrier gas have smaller dot size with a better uniformity in terms of dot height and dot base as compared to the H₂ carrier gas. There is a higher Ge composition and less compressive strain in the SiGe QDs grown in He than in H₂ as measured by Raman spectroscopy. The Ge content is higher for He growth than H₂ growth due to hydrogen induced Si segregation and the lower interdiffusivity caused by the more strain relaxation in the He-grown SiGe dots. The photoluminescence also confirms more compressive strain for H₂ growth than He growth. Hydrogen passivation and Ge-H cluster formation play an important role in the QDs growth.     

Ultrasensitive fluorescent probe for copper ion based on cadmium selenide/cadmium sulfide quantum dots capped with dimercaprol

A new ultrasensitive copper ion fluorescent probe based on cadmium selenide/cadmium sulfide quantum dots capped with dimercaprol is described. Dimercaprol was bonded to the quantum dots through a surface ligand exchange to form dimercaprol-capped quantum dots whose fluorescence could be quenched by the coordination of dimercaprol on quantum dots’ surface with copper ion. The fluorescent probe based on dimercaprol-capped quantum dots showed a very good linear response range to copper ion from 0.1 to 50?µg?L^?1 with the detection limit of 0.087?µg?L^?1. The proposed method exhibited excellent sensitivity and selectivity due to the specific and strong affinity of dimercaprol with copper ion and the unique photoluminescence properties of quantum dots. The possible quenching mechanism was discussed and the probe was successfully applied to the determination of ultratrace copper in real samples.     

Electronic and luminescent properties of Cr-doped cadmium sulfide nanowires

Cr-doped CdS nanowires were synthesized in large scale through thermal co-evaporation of CdS and metal Cr powders. General morphology, detailed microstructure and optical properties were characterized using various techniques. Devices consisting of individual Cr-doped CdS nanowire were fabricated and they exhibited remarkable rectifying characteristics. I-V curves of individual Cr-doped CdS nanowire devices demonstrate that the present nanowires are n-type doped and have high conductivity (10.96 \Omega ^-1cm^-1, indicating great potential applications in nanoscale electronic and optoelectronic devices.     

Optical and luminescent properties of the lead and barium molybdates

Time-resolved luminescence as well as excitation and reflectivity spectra of the oriented lead and barium molybdate single crystals were studied using synchrotron radiation. Features in reflectivity spectra in the fundamental absorption region were analyzed. The contribution of electronic states of lead cation to the formation of the bandgap in PbMoO₄ is supposed. The role of lead states in the intrinsic luminescence of PbMoO₄ is discussed.