Fast-grown CdS quantum dots: Single-source precursor approach vs microwave route

A cross-disciplinary protocol of characterization by joint techniques enables one to closely compare chemical and physical properties of CdS quantum dots (QDs) grown by single source precursor methodology (SSPM) or by microwave synthetic route (MWSR). The results are discussed in relation with the synthesis protocols. The QD average sizes, reproducible as a function of the temperatures involved in the growth processes, range complementarily in 2.8–4.5 nm and 4.5–5.2 nm for SSPM and MWSR, respectively. Hexagonal and cubic structures after X-ray diffraction on SSPM and MWSR grown CdS QDs, respectively, are tentatively correlated to a better crystalline quality of the latter with respect to the further ones, suggested by (i) a remarkable stability of the MWSR grown QDs after exposure to air during several days and (ii) no evidence of their fragmentation during mass spectrometry (MS) analyses, after a fair agreement between size dispersities obtained by transmission electron microscopy (TEM) and MS, in contrast with the discrepancy found for the SSPM grown QDs. Correlatively, a better optical quality is suggested for the MWSR grown QDs by the resolution of n > 1 excitonic transitions in their absorption spectra. The QD average sizes obtained by TEM and deduced from MS are in overall agreement. This agreement is improved for the MWSR grown QDs, taking into account a prolate shape of the QDs also observed in the TEM images. For both series of samples, the excitonic responses vs the average sizes are consistent with the commonly admitted empirical energy-size correspondence. A low energy PL band is observed in the case of the SSPM grown QDs. Its decrease in intensity with QD size increase suggests a surface origin tentatively attributed to S vacancies. In the case of the MWSR grown QDs, the absence of this PL is tentatively correlated to an absence of S vacancies and therefore to the stable behavior observed when the QDs are exposed to air.     

Luminescence characterization of (Ca1−xSrx)(S1−ySey):Eu,M (M = Sc and Y) for high color rendering white LED

The highly efficient red phosphors (Ca_1−xSr_x)(S_1−ySe_y):Eu²⁺,M³⁺ (M = Sc and Y) were prepared, starting from CaCO₃, SrCO₃, Eu₂O₃, Sc₂O₃, Y₂O₃, S, and SeO₂ with a flux, by a conventional solid-state reaction. The optimized red phosphors converted 11.8% (Sc³⁺) and 11.7% (Y³⁺) of the absorbed blue light into luminescence. These quantum values are much higher than Q = 3.0% of CaS:Eu²⁺. For the fabrication of light-emitting diodes (LEDs), the prepared phosphors were coated with MgO from non-aqueous solution to overcome their weakness against moisture. White LEDs were fabricated by pasting the prepared red phosphors and the yellow YAG:Ce³⁺ phosphor on an InGaN blue chip (λ_ems = 446.5 nm). The incorporation of the red phosphor to the YAG:Ce³⁺ phosphor resulted in an improved color rendering index (Ra) from 70 to 80.     

Optical switches based on CdS single nanowire

CdS nanowires have been synthesized by a composite-hydroxide-mediated approach. The characterization of the nanowire with X-ray diffraction, scanning electron microscopy, and transmission electron microscopy indicated a single-crystalline hexagonal structure growing along direction with length up to 100 μm. The UV-visible reflection spectrum demonstrated a band gap of 2.36 eV. A strong light emission centered at 543 nm was observed under different excitation wavelengths of 300, 320, 360 and 400 nm, which was further confirmed by a bright fluorescent imaging of a single CdS nanowire. The photocurrent response based on a single CdS nanowire showed distinct optical switch under the intermittent illumination of white light. The rise and decay time were less than 1.0 and 0.2 s, respectively, indicating high crystallization with fewer trap centers in the CdS nanowires. It is possible that the undesirable trapping effects on grain-boundaries for photoconductors could be avoided thanks to the single-crystalline nature of the CdS nanowires.     

Investigation of size dependent structural and optical properties of thin films of CdSe quantum dots

Cadmium selenide (CdSe) quantum dots were grown on indium tin oxide substrate using wet chemical technique for possible application as light emitting devices. The structural, morphological and luminescence properties of the as deposited thin films of CdSe Q-dot have been investigated, using X-ray diffraction, transmission electron microscopy, atomic force microscopy and optical and luminescence spectroscopy. The quantum dots have been shown to deposit in an organized array on ITO/glass substrate. The as grown Q-dots exhibited size dependent blue shift in the absorption edge. The effect of quantum confinement also manifested as a blue shift of photoluminescence emission. It is shown that the nanocrystalline CdSe exhibits intense photoluminescence as compared to the large grained polycrystalline CdSe films.     

Size-dependent blue luminescent CdS nanocrystals synthesized through a single-source molecular precursor route

In this paper, we report on the synthesis of size-dependent blue luminescent CdS nanocrystals by using a new nonhydrolytic single-source molecular method. The size of the synthesized CdS nanocrystals could be easily controlled by adjusting the ratio of reaction sources under inert atmosphere. The studies on the optical properties reveal an obvious size-dependent photoluminescence characteristic of the synthesized nanocrystals.     

Two-step synthesis of witherite and tuning of morphology

Compared to conventional methods, a simple route is offered for synthesis of BaCO₃ with CO₂ and least energy consumption. And the morphology of BaCO₃ was tuned by controlling reaction conditions. The X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were employed to characterize the products, the results showed that the products with various morphologies had the same crystal phase and growth direction. However, the distinct difference of the prepared products were determined by Raman and UV-vis spectra, showing shape-dependent optical properties. The results provide a useful model system for investigating the shape-dependent optical properties of carbonate nanocrystals.     

Photoluminescence-enhanced biocompatible quantum dots by phospholipid functionalization

A simple two-step strategy using phospholipid (PPL) to functionalize core/shell CdSe/ZnS quantum dots (QDs) has been described. The experimental data show that the use of S-H terminated PPL results not only in the high colloidal stability of core/shell CdSe/ZnS QDs in the aqueous phase, but also in the significant enhancement of photoluminescence. The degree of the enhancement is a function of the PPL-CdSe/ZnS QDs sample concentration. These results might be promising for future biological platform in new devices ranging from photovoltaic cells to biosensors and other devices.     

Photoluminescence study of PVP capped CdS nanoparticles embedded in PVA matrix

Photoluminescence properties of polyvinyl pyrrolidone (PVP) capped cadmium sulphide (CdS) nanoparticles embedded in polyvinyl alcohol matrix (PVA) are reported. The PVP-CdS nanoparticles are prepared by non-aqueous method wherein cadmium nitrate is used as the cadmium source and hydrogen sulphide as the sulphur source. The synthesized nanoparticles are dispersed in polyvinyl alcohol (PVA) matrix and cast as self-standing flexible (PVP-CdS)-PVA films. The nanocomposites are characterized by optical absorption spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. XRD and TEM studies show the formation of cubic CdS particles with average size ∼3-5 nm. Thermal studies, carried out to observe the changes in PVA matrix due to the incorporation of PVP-CdS nanoparticles show strong interaction between the polymer matrix and nanoparticles. The photoluminescence emission spectra of the nanocomposites show two peaks, at 502 and 636 nm, which are attributed to the band edge and surface defects respectively, of CdS nanoparticles. Effective surface capping with optimum concentration of polyvinyl pyrrolidone leads to the quenching of surface defect-related emission.     

Synthesis, characterization and fluorescence property of CdS/P(N-iPAAm) nanocomposites

A novel nanocomposite in which CdS nanoparticles were embedded in poly(N-isopropylacrylamide) (P(N-iPAAm)) matrix have been fabricated. The particle size of CdS nanoparticles ranged from 10 nm to 40 nm could be adjusted with the varying of the inorganic contents. The nanocomposites have been characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), thermo-gravimetric analysis (TGA), high resolution transmission electron microscope (HRTEM), UV-vis absorption and fluorescence spectra (FLS) measurements. The cell volume of CdS nanoparticles embedded in polymer matrix was smaller than the standard value and the nanocomposites with 12.0% inorganic content showed a good fluorescence property.     

Studies of interaction of amines with TOPO/TOP capped CdSe quantum dots: Role of crystallite size and oxidation potential

This work reports the interaction of aliphatic (triethyl amine, butyl amine) and aromatic amines (PPD, aniline) with CdSe quantum dots of varied sizes. The emission properties and lifetime values of CdSe quantum dots were found to be dependent on the oxidation potential of amines and crystallite sizes. Smaller CdSe quantum dots (size 5 nm) ensure better surface coverage of amines and hence higher quenching efficiency of amines could be realized as compared to larger CdSe quantum dots (size 14 nm). Heterogeneous quenching of amines due to the presence of accessible and inaccessible set of CdSe fluorophores is indicated. PPD owing to its lowest oxidation potential (0.26 V) has been found to have higher quenching efficiency as compared to other amines TEA and aniline having oxidation potentials 0.66 and >1.0 V, respectively. Butyl amine on the other hand, plays a dual role: its post-addition acts as a quencher for smaller and enhances emission for larger CdSe quantum dots, respectively. The beneficial effect of butyl amine in enhancing emission intensity could be attributed to enhance capping effect and better passivation of surface-traps.     

Role of additives; sodium dodecyl sulphate and manganese chloride on morphology of Zn1−xMnxO nanoparticles and their photoluminescence properties

In the present study Zn_1−xMn_xO (x = 0, 0.05 and 0.1) nanoparticles (NPs) have been synthesised in aqueous solution phase at mild reaction temperature 100 °C in moderate alkaline medium (pH = 9.5), and the role of external additives; like sodium dodecyl sulphate and manganese chloride on the morphology and size of the products has been explored on the basis of transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectral analyses data. ZnO hexagonal nano-plates, core–shell like spherical/ellipsoidal Zn_0.95Mn_0.05O structures and thin sheets, thorn/needle mixed shaped Zn_0.9Mn_0.1O structures have been observed in TEM and SEM images. Zn(OH)₂ formed in moderate alkaline medium, converted to Zn(II) hydroxo complex ions on dissolution, which further recrystallizes to produce wurtzite ZnO at 100 °C. From XRD and EDX analysis, successful doping of Mn²⁺ ions at the Zn²⁺ sites in ZnO host has been proved. In the photoluminescence spectra, the observed blue shifts in NBE peaks and decrease of emissions intensity on Mn doping have thoroughly been discussed in the present investigation.     

Large-scale synthesis of hexagonal cone-shaped ZnO nanoparticles with a simple route and their application to photocatalytic degradation

We report the large-scale synthesis of hexagonal cone-shaped ZnO nanoparticles by the esterification between zinc acetate and alcohol. The morphology of the ZnO nanoparticles was investigated by transmission electron microscopy, selected area electron diffraction and scanning electron microscopy measurements. The synthesized ZnO nanoparticles are single-crystalline with hexagonal phase and show a strong UV emission at −378 nm due to the excellent crystallinity of particles. A possible formation mechanism of the hexagonal cone-shape structure is proposed. Furthermore, the as-prepared ZnO particles exhibit high photocatalytic activity for the photocatalytic degradation of Rhodamine B, indicating that the ZnO nanostructure is promising as a semiconductor photocatalyst.     

Growth of quantum-confined CdS nanoparticles inside Ti-MCM-41 as a visible light photocatalyst

CdS nanocrystallites have been successfully incorporated into the mesopores of Ti-MCM-41 by a two-step method involving ion-exchange and sulfidation. The X-ray diffraction patterns (XRD), UV-vis absorption spectra (UV-vis), photoluminescence spectra (PL), Raman spectra and N₂ adsorption-desorption isotherms were used to characterize the structure of the composite materials. It is found that most of the CdS nanocrystallites are about 2.6 nm, less than the pore diameter of Ti-MCM-41. The CdS nanocrystallites inside the mesopores of Ti-MCM-41 host show a significant blue shift in the UV-vis absorption spectrum. Under irradiation of visible light (λ > 430 nm), the composite material has greater and more stable photocatalytic activity for hydrogen evolution than bulk CdS, which can be explained by the effective charge separation between the CdS nanocrystallites and mesoporous Ti-MCM-41.     

Annealing effects on the chemical deposited CdS films and the electrical properties of CdS/CdTe solar cells

CdS layers grown by chemical bath deposition (CBD) are annealed in the oxygen and argon-hydrogen atmosphere respectively. It has been found that the open circuit voltage of the CdS/CdTe solar cell increases when the CBD CdS is annealed with oxygen before the deposition of CdTe by close spaced sublimation (CSS), while the performance of the solar cell decreases when the CBD CdS is annealed with argon-hydrogen. Electronic properties of the CdS films are investigated using X-ray photo-electron spectroscopy (XPS), which indicates that the Fermi level is shifting closer to the conduction band after annealing in the oxygen and consequently a higher open circuit voltage of the solar cell can be obtained.     

Synthesis of silver nanoparticles in a polyvinylpyrrolidone (PVP) paste, and their optical properties in a film and in ethylene glycol

Silver nanoparticles were synthesized in a paste of polyvinylpyrrolidone formed after mixing PVP with acetone and a small volume of aqueous silver nitrate under magnetic stirring. A film made with the material was characterized by UV-vis spectroscopy. The obtained spectrum shows a single peak at 438 nm, arising from the surface plasmon absorption of silver colloids. This result clearly indicates that silver nanoparticles are embedded in PVP. When the pre-treated PVP-Ag colloid is dissolved in ethylene glycol, the UV-vis spectrum of the resulting dispersion shows an absorption peak at 433 nm, whose maximum absorption blue shifts to 416 nm after 18 days of agitation. The silver nanoparticles have an average particle size of 4.12 nm. Because the IR band assigned to the carbonyl group of the PVP shifts to longer wavelengths, the interaction of this polymer with silver nanoparticles seems to take place through the carbonyl oxygen.     

Synthesis and surface plasmon resonance properties of carbon-coated Cu and Co nanoparticles

Carbon-coated Cu and Co nanoparticles were synthesized by the carbonization of PVA-metal hydroxide complexes. The possible reaction process and surface plasmon resonance (SPR) properties of the Cu and Co nanoparticles enwrapped in carbon layer were explored. The XRD results showed that no byproducts, such as oxides and carbides, were formed in the products, and the Cu and Co nanoparticles were effectively protected against oxidation by the carbon layer. The size ranges of the metal nanoparticles were 20-50 nm for Cu and 15-65 nm for Co, respectively. UV-vis absorption spectra showed that the SPR bands of the Cu and Co nanoparticles coated with carbon were red-shifted mainly due to the increase of the effective dielectric constant of the surrounding medium induced by the carbon layer.     

An approach to suppress the blue-shift of photoluminescence peaks in coupled multilayer InAs/GaAs quantum dots by high temperature post-growth annealing

Effect of post-growth annealing on 10 layer stacked InAs/GaAs quantum dots (QDs) with InAlGaAs/GaAs combination capping layer grown by molecular beam epitaxy has been investigated. The QD heterostructure shows a low temperature (8 K) photoluminescence (PL) emission peak at 1267 nm. No frequency shift in the peak emission wavelength is seen even for annealing up to 700 °C which is desirable for laser devices requiring strict tolerances on operating wavelength. This is attributed to the simultaneous effect of the strain field, propagating from the seed layer to the active layer of the multilayer QD (MQD) and the indium atom gradient in the capping layer due to the presence of a quaternary InAlGaAs layer. Higher activation energy (of the order of ∼250 meV) even at 650 °C annealing temperature also signifies the stronger carrier confinement potential of the QDs. All these results demonstrate higher thermal stability of the emission peak of the devices using this QD structure.     

Hybrid nanostructures of Au nanocrystals and ZnO nanorods: Layer-by-layer assembly and tunable blue-shift band gap emission

A layer-by-layer assembly technique was developed to synthesize the hybrid nanostructures of Au nanocrystals with diameter of about 5 nm and ZnO nanorods via the electrostatic interaction. In comparison with ZnO nanorods, the Au-ZnO hybrid nanostructures exhibited the broadened and red-shifted surface plasmon band, enhanced band gap emission, and suppressed defect emission due to the strong interfacial coupling between Au and ZnO. Moreover, the band gap emission of the Au-ZnO hybrid nanostructures is controllably blue-shifted with decreasing distance between the Au nanocrystals and ZnO nanorods tuned by the amount of the polyelectrolyte layers due to the exciton and plasmon interactions.     

Self-assembled Cu2O flowerlike architecture: Polyol synthesis, photocatalytic activity and stability under simulated solar light

Self-assembled Cu₂O flowerlike architecture has been synthesized by polyol process with the presence of acetamide. The synthesized flowerlike architecture is composed of the petals assembled by small crystalline with size of 5-6 nm. The growth mechanism is proposed that the dissolution of the small particles in microsphere center and then the recrystallization on the surface of the microspheres with the increase of reaction time results in the formation of the final flowerlike architecture. The novel architecture shows a blue shift of absorption edge compared to Cu₂O nanocubes and good photocatalytic activity for the degradation of dye brilliant red X-3B under simulated solar light. Moreover, the flowerlike architecture is more stable than Cu₂O nanocubes during photocatalytic process since the photocatalytic activity of the second reused architecture sample is still twice as high as that of the original nanocubes. The Cu₂O flowerlike architecture may be a good photocatalyst candidate for wastewater treatment.     

The preparation,characterization and optical properties of Cd2V2O7 and CdCO3 compounds

Cadmium vanadium oxides (Cd₂V₂O₇) and Cadmium carbonates (CdCO₃) were synthesized via a facile hydrothermal method. X-ray diffraction (XRD), Raman spectroscopy, infrared spectrometer (IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the structure, morphology and chemical state of the samples, respectively. The photoluminescence (PL) properties of the as-synthesized Cd₂V₂O₇ and CdCO₃ were measured at room temperature using an excitation wavelength of 325 nm. The Cd₂V₂O₇ shows two visible light emission centers located at 589 and 637 nm, which are supposed to be relevant to local defects in Cd₂V₂O₇. The CdCO₃ shows three emission centers located at 408, 530 and 708 nm, which are supposed to be relevant to the electron transition from the conduction band to valence band and defect related energy level.