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PubMed2001

Probing single quantum dots by micro-photoluminescence.

Using micro-photoluminescence, emissions from single CdSe quantum dots were observed from the cleaved (110) facets of ZnSe/CdSe/ZnSe heterostructures grown on GaAs (001) substrates. The emission intensity of a single quantum dot was linearly proportional to the excitation intensity, demonstrating excitonic features. Emissions from these single quantum dots were found to polarize within the (001) plane, providing information on the shapes of the quantum dots.

Cadmium Compounds↗
PubMed2001

CdTe nanocrystals: synthesis, optical characterization, and pseudopotential calculation of the band gap.

CdTe nanocrystals were synthesized in aqueous solution using 1-thioglycerol and 2-mercaptoethanol as surface stabilizers. The nanocrystals were characterized by means of X-ray powder diffraction and UV-vis absorption measurements. The UV-vis absorption spectra exhibit two distinct transition lines. Comparison of the experimental measurements with the results of the empirical pseudopotential calculations of the CdTe nanocrystals showed that the lower energy absorption line can be assigned to the heavy-hole exciton transition, whereas the higher energy absorption line can be attributed to the light-hole exciton transition.

Cadmium Compounds↗
PubMed2002

Voltage tunable electroluminescence of CdTe nanoparticle light-emitting diodes.

Strong electroluminescence was observed in a CdTe nanoparticle light-emitting diode that was fabricated by layer-by-layer assembly of poly(p-phenylenevinylene) and CdTe nanoparticles. The turn-on voltage for the electroluminescence is 4.0 V. The electroluminescence is exclusively from CdTe nanoparticles, with a 30 nm red shift in emission wavelength from the photoluminescence and shifts to longer wavelengths at higher applied voltages. Possible mechanisms for this red shift are discussed, and it is concluded that the de-trapping of surface states and trap centers due to electric excitation, the involvement of traps in the electroluminescence, and the quantum confined Stark effect are the major reasons for the red shift.

Cadmium Compounds↗
PubMed2002

Light-emitting devices based on ruthenium (II) bipyridine complexes coupled with cadmium sulfide nanoparticles.

The fabrication of a light-emitting device based on a composite consisting of cadmium sulfide nanoparticle, poly(vinylalcohol), and ruthenium 2,2'-bipyridyl complex is reported. The electroluminescence spectrum of this composite is similar to the combination of the emission from ruthenium complexes and cadmium sulfide. A stable electroluminescence at low threshold voltage (4 V), with a brightness of 680 cd/m2 and an external quantum efficiency of approximately 0.2% was achieved. it was demonstrated that the cadmium sulfide nanoparticles were able to improve the performance of the device and enhance the electron carrier mobility as compared to the device based on pure ruthenium complex.

2,2'-Dipyridyl↗
PubMed2002

Optical properties of the synthetic nanocomposites SiO2/CdS/poly(styrene-co-maleic anhydride) and SiO2/CdS/poly(styrene-co-maleimide).

Hybrid materials consisting of SiO2/CdS particles dispersed in poly(styrene-co-maleic anhydride) and poly(styrene-co-maleimide) have been synthesized and characterized. The polymer nanocomposites were synthesised in situ in the presence of previously prepared inorganic fillers (SiO2/CdS). The nanocomposites were synthesized with the use of as-prepared or surface-modified SiO2/CdS fillers. For both types of nanocomposites, the optical properties were evaluated and the observation of size quantization effects in the optical spectra is discussed. In this context, the influence of the inorganic fillers and polymer matrices on the optical properties of the final nanocomposites was investigated.

Cadmium Compounds↗
PubMed2003

Visualizing the self-assembly of tubulin with luminescent nanorods.

We succeeded in labeling tubulin with luminescent CdSe nanorods. The labeled tubulin was still able to self-assemble into microtubules, demonstrating that the protein has remained functional. The comparison with rhodamine-labeled tubulin revealed that whereas the rhodamine bleached completely within the observed time frame, the nanorods did not show any bleaching, which makes it possible to follow the key events of self-assembly at high time resolution. Preparation and derivatization of nanorods are described. Water-soluble, protein-reactive group-containing particles were covalently coupled to tubulin. The conjugate was purified by one cycle of assembly and disassembly and used to induce the formation of fluorescent microtubules. These results suggest that the use of luminescent nanorods should allow continuous confocal monitoring of dynamic biological processes.

Biocompatible Materials↗
PubMedDate not supplied

Ultrasound-assisted fabrication of nanoporous CdS films.

A new method for fabricating nanoporous CdS films is reported. It involves exposing the CdS solution with ultrasound waves during the process of dip coating. Indium tin oxide (ITO)-coated glass and plastic (commercial transparency) were used as substrates. In each case three different precursors were used for dip coating. The precursors used were CdCl2 and thiourea in one case and CdS nanoparticles prepared by sonochemical and microwave-assisted methods in the other two cases. X-ray diffraction studies performed on these powders show a phase corresponding to cubic CdS. The Field Emission Scanning Electron Microscopy (FE-SEM) images of the films on plastic showed uniform pores with a diameter of 80 nm for all three methods. Optical absorption measurements indicated a blue shift and multiple peaks in the absorption curve. The FE-SEM observations of the films on an ITO/glass substrate indicated a crystalline film with voids. The UV-vis absorption results indicated a blue shift in the absorption with an absorption edge at 435, 380, and 365 nm for CdS films made by solution growth, sonochemical, and microwave routes, respectively. The magnitude of the absorption is dependent on film thickness, and the observed blue shift in the absorption can be explained on the basis of quantum confinement effects.

Adsorption↗
PubMed2004

Cadmium sulfide and cadmium selenide/cadmium sulfide nanoparticles stabilized in water with poly(cysteine acrylamide).

Cysteine acrylamide (N-acryloyl L-cysteine) stabilizes CdS nanoparticles as the particles form in aqueous dispersions. Cysteine acrylamide also exchanges for citrate on the surfaces of CdSe and core/shell CdSe/CdS nanoparticles to provide greater stability. Heating of the nanoparticle dispersions polymerizes the cysteine acrylamide on the surface to form a more efficient polydentate stabilizer. The polymer-coated nanoparticle dispersions are colloidally stable even after removal of low molecular weight solutes by dialysis. Emission quantum yields of the polymer-coated CdSe and CdSe/CdS samples were 0.9% and 2.6%, respectively, after aging of the samples in light. CdSe/CdS coated with poly(cysteine acrylamide) is colloidally stable for at least two years in the dark at 5 degrees C.

Acrylic Resins↗
PubMed2004

Synthesis and characterization of nanostructural hydrogel and template for CdS nanofibers.

Hydrogels based on beta-D-glucopyranoside-substituent thiosemicarbazide derivatives were synthesized and used as template to fabricate netlike CdS nanofibers. The self-assembling properties of the hydrogels and their effect on the synthesis of CdS nanostructure were studied. Fourier transform infrared spectra, UV-visible absorption spectra and X-ray diffraction revealed that pi-pi stacking, hydrogen bonds and interdigitated interactions between hydrophobic chains had influence on the formation of self-assembled lamellar hydrogel. Transmission electron microscopic and electron diffraction observation showed the formation of hexagonal stacking nanofiber-based CdS semiconductor materials. The transmission electron microscopic pictures of the CdS nanostructure in difference growth stages revealed the growth mechanism of CdS nanofibers templated by hydrogel.

Cadmium Compounds↗
PubMedDate not supplied

Formation of nanoparticle arrays on S-layer protein lattices.

Crystalline bacterial cell surface layers (S-layers) composed of identical protein units have been used as binding templates for well-organized arrangements of nanoparticles. Isolated S-layer proteins were recrystallized into monomolecular arrays on solid substrates (such as silicon wafers and SiO2-coated grids) and in suspension forming so-called self-assembly products. These S-layer assemblies were studied by atomic force microscopy and transmission electron microscopy (TEM). The orientation of the S-layer lattice, exhibiting anisotropic surface properties, on the solid surface and on the self-assembly products, was compared with the orientation on the bacterial cell. On both bacterial cells and SiO2 surfaces the outer face of the S-layer protein was exposed. On the self-assembly products occasionally the inner face was also visible. Metal- and semiconductor nanoparticles 2 to 10 nm in mean diameter were covalently or electrostatically bound to the solid-supported S-layers and self-assembly products. TEM studies reveal that upon activation of carboxyl groups in the S-layer lattice with 1-ethyl-3,3'(dimethylaminopropyl)carbodiimide (EDC), a close-packed monolayer of 4-nm amino-functionalized CdSe nanoparticles could be covalently established on the S-layer lattice. Because of electrostatic interactions, anionic citrate-stabilized Au nanoparticles (5 nm in diameter) formed a superlattice at those sites where the inner face of the S-layer lattice was exposed. In contrast, cationic semiconductor nanoparticles (such as amino-functionalized CdSe particles) formed arrays on the outer face of the solid-supported S-layer lattices.

Adsorption↗
PubMed2005

CdS nanoparticles grown in a polymer matrix by chemical bath deposition.

Nanocrystallites of CdS have been grown by chemical bath deposition within the pores of poly(vinyl alcohol) (PVA) on glass and Si substrates. The CdS-PVA composite films are transparent in the visible region. XRD and TEM diffraction patterns confirmed the nanocrystalline CdS phase formation. TEM study of the film revealed the manifestation of nano CdS phase formation and the average particles size was varied in the range 5-12 nm. UV-vis spectrophotometric measurement showed high transparency (nearly 80% in the wavelength range 550-900 nm) of the films with a direct allowed band gap lying in the range 2.64-3.25 eV. Particle sizes have also been calculated from the shift of band gap with respect to that of bulk value and were found to be in the range 3.3-6.44 nm. The high dielectric constant (lies in the range 120-250 at high frequency) of PVA/CdS nanocomposite compared to that of pure PVA (-28) has been observed. The dielectric constant decreases with increase of dispersibility of the CdS nanoparticles within PVA. The nanocrystalline PVA/CdS thin films have also showed field emission properties with a turn-on field of approximately 6.6 V/microm, whereas only PVA thin film and bulk CdS on PVA have shown no field emission.

Cadmium Compounds↗
PubMed2005

Photoluminescence and raman study of CdS-Al2O3 nanocomposite films prepared by sol-gel techniques.

The optical and microstructural properties of CdS-Al2O3 nanocomposite (CdS-Al2O3 = 20:80 to 50:50) thin films synthesized by sol-gel techniques were studied. Optical transmission spectra indicated a marked blue shift of the absorption edge due to quantum confinement. Band gaps of CdS-Al2O3 nanocomposites were found to vary in the range 3.69-2.61 eV. The sizes of the nanocrystals, estimated from the blue shift (0.2-1.2 eV) of the absorption edges and transmission electron microscopy, were found to vary in the range 2.8-7.0 nm. X-ray diffraction studies showed reflections from (111), (200), (220), and (311) planes of CdS in the cubic phase. Microstructural characterization by high-resolution transmission electron microscope (HRTEM) indicated well crystallinity of the nanoparticles and lattice fringes supported the cubic phase of CdS. Raman spectroscopy was carried out for CdS-Al2O3 nanocomposites, which indicated a prominent peak at approximately 299 cm(-1). Significant changes in the peak position and intensity of the Raman peak were observed with varying the annealing temperature (373-573 K). Photoluminescence measurements indicated a prominent broad peak at approximately 1.81 eV due to the surface defects in the CdS nanocrystallites. The present study revealed Al2O3 to be a good capping material for CdS nanoparticles.

Aluminum Oxide↗
PubMed2005

Polymer encapsulation of CdE (E = S, se) quantum dot ensembles via in-situ radical polymerization in miniemulsion.

Cadmium sulfide and cadmium selenide/polymer nanocomposites were prepared via in-situ radical polymerization in miniemulsion. Organically capped CdE (E = S, Se) quantum dots (QDs) were used as the starting materials and ensembles of these dots were encapsulated with no need of further surface treatment. The use of two polymer matrices was investigated: polystyrene (PS) and poly(n-butyl acrylate) (PBA). In both cases, homogenous nanocomposites were obtained and their optical properties were investigated by visible absorption and photoluminescence spectroscopy. Quantum size effects were assigned to the nanocomposites, indicating the integrity of the individual QDs upon polymer encapsulation using the miniemulsion process.

Acrylates↗
PubMed2005

Switching time of nanowire memory.

Cadmium sulfide nanowires of 10-nm diameter, electrodeposited in porous anodic alumina films, exhibit an electronic bistability that can be harnessed for non-volatile memory. The current-voltage characteristics of the wires show two stable conductance states that are well-separated (conductances differ by more than 4 orders of magnitude) and long lived (longevity > 1 year at room temperature). These two states can encode binary bits 0 and 1. Here we report measurements of the switching time to switch from the high- to the low-conductance state. These measurements also shed some light on the physical mechanisms underlying the bistability.

Cadmium Compounds↗
PubMed2005

Role of free cadmium and selenium ions in the potential mechanism for the enhancement of photoluminescence of CdSe quantum dots under ultraviolet irradiation.

The present study describes an enhancement of the photoluminescence of CdSe quantum dots under long-term ultraviolet irradiation in organic solvents. The photoenhancement effect followed multiexponential kinetics and was found to depend on several factors: intensity of ultraviolet light, polarity of the solvent, presence of capping agents on the nanocrystal surface, and presence of free Cd and Se ions in the solution. High intensity ultraviolet irradiation provoked a rapid enhancement of the photoluminescence of CdSe nanocrystals, reaching the maximum with subsequent photoluminescence decay. Low-intensity ultraviolet irradiation provoked a comparatively slow enhancement of the photoluminescence of CdSe nanocrystals, reaching saturation after 5-6 hours of irradiation in organic solvents (butanol and chloroform). The photoenhancement effect was reversible or irreversible depending on the additional ingredients. The role of free Cd and Se in these processes was clarified. The results are discussed in the context of ultraviolet induced liberation of free Cd and Se ions from the nanocrystal surface and their hypothetical reversible deposition with trapping of the surface holes and influencing the efficiency of radiative versus nonradiative exciton decay during the enhancement of photoluminescence.

Cadmium↗
PubMed2005

Preparation and characterization of overcoated II-VI quantum dots.

A convenient route for the synthesis of high-quality overcoated II-VI quantum dots (QDs) is reported in this paper. Simple salts, such as Cd(Ac)2 and Zn(Ac)2 were used to replace organometallics, whose disadvantage is obvious. Size-tunable core/shell structured QDs (CdSe/ZnS, CdSe/CdS, etc.) were synthesized. They were of narrow size distribution and had good monodispersivity and photoluminescence (PL) properties. The spectrum was symmetrical and sharp-pointed (with the full width at half-maximum (fwhm) of about 20-30 nm). The quantum yield (QY) was improved to 60-80% from 20-30% for bare QDs and remained stable at least for 6 months. The primary overcoated QDs were modified with biomacromolecules by a direct mechanical rubbing strategy, which is very simple and fast. The results obtained by UV-vis, PL, atomic force microscopy (AFM), and fluorescence microscopy imaging showed that the modified QDs were of good fluorescent and monodisperse characteristics. They are likely to be used further for biological labels.

Cadmium Compounds↗
PubMed2005

Colloidal CdSe nanocrystals from tri-n-octylphosphine: Part 1: growth and optical properties from polar and non-polar solvents.

This manuscript reports the ready synthesis of high-quality colloidal CdSe nanocrystals from tri-n-octylphosphine (TOP) and reports the optical properties of the resulting CdSe dispersed in nonpolar and polar environments. With cadmium oxide (CdO) as the Cd precursor and TOPSe as the Se source, the CdSe nanocrystals were synthesized in a reaction medium consisting of TOP. The synthetic approach is as simple as: the swift injection of a TOPSe/TOP solution into a CdO/TOP solution at one particular (high) temperature, with subsequent growth at a lower temperature. The temporal evolution of the optical properties, including absorption and luminescence, were monitored in detail. The photoluminescence (PL) properties of the TOP-capped CdSe investigated comprise the intensity, emission peak wavelength, full width at half maximum of the PL spectra, and stability; these PL properties are sensitive to the history of growth as well as the environment of dispersion. An appropriate parameter, termed "Sensitivity Index (SI)," is proposed to define the percent difference in the PL intensity that arises when the dots are dispersed in non-polar hexane (Hex) as compared to polar tetrahydrofuran (THF). The decrease in the PL efficiency on going from Hex to THF is attributed to changes in surface states rather than in the internal core as more ligands are removed in THF. Dilution experiments further confirm the suggestion of the ligand removal mechanism and provide a conceptual estimate of surface quality via the SI parameter. Due to a slow growth in size (the size of the nanocrystals is almost constant during early 20 min) and good PL stability (tested as PL changes after storage for days both in dispersed or solid states, as well as after UV-irradiation), we suggest that our synthetic approach is likely to be practical for large-scale CdSe nanocrystals with good PL stability.

Cadmium↗
PubMed2005

Colloidal CdSe nanocrystals from tri-n-octylphosphine: Part II: control of growth rate for high quality and large-scale production by tuning Cd-to-Se stoichiometry.

Colloidal CdSe nanocrystals were synthesized in reaction media consisting of tri-n-octylphosphine (TOP) without addition of other species; the single-step approach used cadmium oxide (CdO) and TOPSe as Cd and Se sources, respectively. The temporal evolution of the optical properties of the growing TOP-capped CdSe nanocrystals was monitored for a couple of hours, showing that there are two distinguishable stages of growth: an early stage (less than 5 minutes) and a later stage; the growth kinetics of the two stages is a function of the Cd-to-Se precursor molar ratios. A rational choice of 2-6Cd-to-1Se molar ratio was found, based on the temporal evolution of the photoluminescent (PL) efficiency (studied as PL intensity and sensitivity to the media of dispersion, and non-resonant Stokes shifts). For a 2Cd-to-1Se synthesis, the growth in size was slow in the early stages and became fast in the later stages; this fast-later-stage feature could be suppressed by going to a synthesis with a 4-6Cd-to-1Se mole ratio: the nanocrystals between 0.5-60 min growth time exhibit very much similar optical properties, with less than 19 nm redshift of bandgap absorption and emission occurring. Thus, the synthetic route developed here, with a rational 4-6Cd-to-1Se molar ratio, enables us to produce high-quality CdSe nanocrystals on a large-scale with a high degree of synthetic reproducibility. The insights gained facilitate a deeper understanding of the concept of what constitutes high-quality nano-crystals: high PL efficiency resulting from a low growth rate, which can be thoroughly and readily investigated by the red-shift rate of the band-gap peak positions; in addition, the insights gained help us to define a proper synthetic approach for large-scale production with high-quality product.

Cadmium↗