Properties Of CdS Quantum Dots Research Articles

Improvement of the luminescent properties of cadmium sulfide quantum dots by a post-synthesis modification

Here the improvement of the luminescent properties of CdS quantum dots by a post-synthesis modification with aqueous solutions of NaOH at different concentrations is presented. The CdS quantum dots were synthesized by a microwave-assisted method using citrate ions as stabilizer. The addition of the hydroxide ions increased the intensity of the orange-red emission by about 80%. Besides, a violet-blue emission was achieved by means of this post-synthesis modification. The hydroxide ions control the precipitation equilibria of the CdS and Cd(OH)2, dissolving and precipitating the surface of the quantum dots. The NaOH treatment increases the number of traps, which produces less band-edge and more deep-trap emission, which explains the decrease and increase in the intensity of the violet-blue and orange-red emissions, respectively.

Application of Solanum lycopersicum (tomato) hairy roots for production of passivated CdS nanocrystals with quantum dot properties

Semiconductor quantum dot particles have a wide range of applications in medicine, bioassays, computing and photovoltaics. Biological synthesis is an attractive approach for mass production of quantum dots as cells have the capacity to passivate the particles with organic ligands. In this work, hairy roots of Solanum lycopersicum (tomato) were used to produce CdS nanoparticles with quantum dot properties. Treatment of the roots with 100μM Cd during the mid-growth phase of batch culture elicited cellular responses for Cd detoxification without affecting root growth. A combination of freeze-drying and freeze-thawing of the roots was used to extract Cd from the biomass; anion-exchange chromatography was then applied to selectively remove metal–phytochelatin complexes. Size-fractionation using gel filtration allowed the recovery of phytochelatin-capped Cd- and inorganic sulphide-containing nanoparticles displaying the size and size-dependent optical/electronic properties of CdS quantum dots. At 4–10nm in diameter, these particles fluoresced at wavelengths corresponding to blue-violet on the colour spectrum and exhibited a high level of photostability with prolonged excitation. Whereas 69% of the Cd extracted from the roots was associated with phytochelatin peptides, the maximum yield of CdS nanocrystals with quantum dot properties was 1.4% of the total Cd taken up into the biomass. This work demonstrates a new culture-based approach for the biosynthesis of metallo-organic semiconductor quantum dots using hairy roots.

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.

Effect of gamma-ray irradiation on the size and properties of CdS quantum dots in reverse micelles

Cadmium sulfide quantum dots 1.3–5.6nm in size have been synthesized in sodium bis(2-ethylhexy1)sulfosuccinate (AOT)–water–isooctane micellar solutions with various [H2O]/[AOT] molar ratios (w=2.5, 5.0 or 10). Gamma irradiation method has been used to change the size and optical properties of quantum dots. It has been found that γ-irradiation reduces the size polydispersity of quantum dots in the micellar system and alters their fluorescent properties. Fluorescence intensity is enhanced after γ-irradiation. The average fluorescence lifetime of single quantum dots sized 5.2±0.4nm increases from 5.14 to 6.39ns after γ-irradiation at a dose of 7.9kGy. To the best of our knowledge, this is the first report on fluorescence lifetime of single CdS quantum dots in micellar solution.

Temperature-sensitive growth kinetics and photoluminescence properties of CdS quantum dots

As a critical issue in “green” synthesis, the mechanism by which the growth temperature influences the photoluminescence (PL) properties of quantum dots (QDs) remains unclear. In this work, temperature sensitive growth kinetics and PL evolution of thioglycolic acid (TGA) capped CdS QDs were systematically investigated. This revealed that TGA capped CdS QDs coarsened below and above 90 °C experienced completely different growth kinetics, i.e. a two-stage growth from oriented attachment to Ostwald ripening (OA–OR) and a hybrid growth of the two mechanisms (OA + OR), respectively. With the evolution of the OA–OR growth, the corresponding PL shows characteristics of a gradual weakness of the defect-related emissions centered at ∼650 nm and a steadily enhanced band-edge emission peaking at ∼450–470 nm. For the OA + OR hybrid growth, the defect-related emission band steadily coexists with the band-edge emission after an initial decrease. In-depth kinetics and high-resolution transmission electron microscopy (HRTEM) investigation indicate that a low temperature favors a long-term OA self-integration stage, by which internal defects could be relaxed or even eliminated. For a high temperature, this tendency could be greatly inhibited by the concurrent OR growth just around these defects. The FTIR analysis further revealed that the adsorption–desorption equilibrium of capping ligands was temperature-sensitive, resulting in different growth kinetics and PL characteristics of CdS QDs.

Effect of Cadmium Sulfide Quantum Dots on Physical Properties of R-Phycoerythrin as a Protein Matrix

The synthesis and analysis of nanostructures in the cavities of protein molecules is a promising research field in the industry of nanosystems. In this study, atomic force microscopy (AFM) has been used to evaluate the properties of CdS quantum dots synthesized in the tunnel cavities of R-phycoerythrin, a 290 kDa water-soluble pigment protein responsible for light harvesting in red algae. It has been shown that R-phycoerythrin dissolved in deionized water to a concentration of 50 μg/ml is prone to self-organization into regular spatial structures upon adsorption on the surface of mica, but no such structuring takes place in films prepared from R-phycoerythrin solutions diluted tenfold. In the latter case, protein molecules are deformed, as judged from the analysis of the surface profile. R-phycoerythrin with CdS quantum dots in protein cavities (the concentration of the preparation was (48 μg/ml) loses the self-organization ability and is not deformed upon adsorption on the mica surface. Analysis of AFM images by flicker-noise spectroscopy has shown that incorporation of CdS quantum dots into R-phycoerythrin molecules provides for "smoothing" of the protein surface, with various irregularities being leveled off. Conversely, the irregularity of the protein surface increases when R-phycoerythrin molecules are arranged into three-dimensional branching structures. It is concluded that CdS quantum dots interfere with protein-protein interactions and restrain the conformational mobility of the protein. The anomalously rigid structure of R-phycoerythrin in the presence of CdS is due to its conformational rearrangements during the synthesis of quantum dot.

Effect of Cadmium Sulfide Quantum Dots on Physical Properties of R-Phycoerythrin as a Protein Matrix

The synthesis and analysis of nanostructures in the cavities of protein molecules is a promising research field in the industry of nanosystems. In this study, atomic force microscopy (AFM) has been used to evaluate the properties of CdS quantum dots synthesized in the tunnel cavities of R-phycoerythrin, a 290 kDa water-soluble pigment protein responsible for light harvesting in red algae. It has been shown that R-phycoerythrin dissolved in deionized water to a concentration of 50 μg/ml is prone to self-organization into regular spatial structures upon adsorption on the surface of mica, but no such structuring takes place in films prepared from R-phycoerythrin solutions diluted tenfold. In the latter case, protein molecules are deformed, as judged from the analysis of the surface profile. R-phycoerythrin with CdS quantum dots in protein cavities (the concentration of the preparation was (48 μg/ml) loses the self-organization ability and is not deformed upon adsorption on the mica surface. Analysis of AFM images by flicker-noise spectroscopy has shown that incorporation of CdS quantum dots into R-phycoerythrin molecules provides for “smoothing” of the protein surface, with various irregularities being leveled off. Conversely, the irregularity of the protein surface increases when R-phycoerythrin molecules are arranged into three-dimensional branching structures. It is concluded that CdS quantum dots interfere with protein–protein interactions and restrain the conformational mobility of the protein. The anomalously rigid structure of Rphycoerythrin in the presence of CdS is due to its conformational rearrangements during the synthesis of quantum dot. Keywords: Quantum dots, CdS nanoparticles, R-phycoerythrin, atomic force microscopy, flicker-noise spectroscopy, nanostructures, protein molecules, protein–protein interactions, conformational mobility, biomacromolecules

Effect of Cadmium Sulfide Quantum Dots on Physical Properties of R-Phycoerythrin as a Protein Matrix

The synthesis and analysis of nanostructures in the cavities of protein molecules is a promising research field in the industry of nanosystems. In this study, atomic force microscopy (AFM) has been used to evaluate the properties of CdS quantum dots synthesized in the tunnel cavities of R-phycoerythrin, a 290 kDa water-soluble pigment protein responsible for light harvesting in red algae. It has been shown that R-phycoerythrin dissolved in deionized water to a concentration of 50 μg/ml is prone to self-organization into regular spatial structures upon adsorption on the surface of mica, but no such structuring takes place in films prepared from R-phycoerythrin solutions diluted tenfold. In the latter case, protein molecules are deformed, as judged from the analysis of the surface profile. R-phycoerythrin with CdS quantum dots in protein cavities (the concentration of the preparation was (48 μg/ml) loses the self-organization ability and is not deformed upon adsorption on the mica surface. Analysis of AFM images by flicker-noise spectroscopy has shown that incorporation of CdS quantum dots into R-phycoerythrin molecules provides for “smoothing” of the protein surface, with various irregularities being leveled off. Conversely, the irregularity of the protein surface increases when R-phycoerythrin molecules are arranged into three-dimensional branching structures. It is concluded that CdS quantum dots interfere with protein–protein interactions and restrain the conformational mobility of the protein. The anomalously rigid structure of Rphycoerythrin in the presence of CdS is due to its conformational rearrangements during the synthesis of quantum dot.

The role of the surface ligand in the optical properties of CdS quantum dots in poly(vinyl alcohol) matrix

The synthesis and characterization of CdS nanoparticles prepared in poly(vinyl alcohol) (PVA) in situ, to produce a series of CdS/PVA nanocomposite films, is described in this paper. The role of 2-mercaptoethanol as the surface ligand for the nanoparticles has been investigated. Different molar concentrations of the cadmium precursor have also been evaluated, aiming at the preparation of stable aqueous colloidal systems and polymeric films. UV–visible (UV–Vis) and photoluminescence spectroscopies together with scanning electron microscopy and transmission electron microscopy have been used for characterization of the growth kinetics and the relative stability of CdS nanocrystals in the polymeric matrix. The results clearly indicate the formation of hexagonal CdS nanoparticles embedded in the PVA matrix. PVA was not effective in stabilizing colloidal CdS nanoparticles against aggregation. However, it leads to a displacement of the first optical transition of CdS due to compressive deformation effect. The combination of PVA with 2-mercaptoethanol as surface ligands had a strong effect on the optical properties of the resulting embedded CdS nanoparticles.

Dependences of the optical absorption and photovoltaic properties of CdS quantum dot-sensitized solar cells on the CdS quantum dot adsorption time

We have studied the optical absorption properties of CdS quantum dots (QDs) adsorbed on TiO2 electrodes and the photovoltaic properties of CdS quantum dot-sensitized solar cells (QDSSCs). CdS QDs were grown directly on a nanostructured TiO2 surface by a chemical bath deposition method for different adsorption times from 30 min to 8 h. The growing size of CdS QDs on the TiO2 electrodes with increasing adsorption time was confirmed by a redshift in the optical spectra, measured using photoacoustic (PA) spectroscopy. The steepness factor, which reflects the amount of disorder and the number of defects, was also estimated from the PA spectrum. The steepness factor decreased sharply at 1 h adsorption time when the CdS QDs commenced aggregating. Subsequently, the steepness factor increased for longer adsorption times, indicating that the density of surface states of the CdS QDs had decreased due to a decrease in the ratio of surface area to volume with the growth in average size. The photovoltaic properties of sandwich structure solar cells were studied under an illumination of air mass (AM) 1.5 using Cu2S on brass as a counter electrode and a polysulfide (S/S2−) redox system as the electrolyte. The maximum photovoltaic conversion efficiency obtained was 1.22%, which was for the device that had 2 h CdS adsorption. The variations in series resistance and shunt resistance were estimated from the slopes of photocurrent density-photovoltage curves near the short circuit and open circuit conditions, respectively. With increasing adsorption time, both resistances decreased until 2 h adsorption when the maximum photoconversion efficiency was observed. These results indicate that the CdS adsorption time is closely correlated with the photovoltaic properties of CdS QDSSCs.

Procedures for Controlling the Size, Structure and Optical Properties of CdS Quantum Dots during Synthesis in Aqueous Solution

We report an easy approach for the synthesis of CdS Quantum Dots (CdS QDs) with high luminescence and temporal stability through the reaction of Cd(2+) and S(2-) in the presence of mercaptoacetic acid (MAA) as a capping reagent in aqueous medium, under normal pressure and room temperature. The influence of several experimental variables, including temperature, pH, the Cd/S ratio and the Cd/MAA ratio, on the optical properties of the QDs obtained was studied systematically. The experimental results indicate that these variables play an important role in determining the size and state of the surface of the nanoparticles, and hence their luminescent properties and temporal stability. The general aspects of nanocrystal nucleation and growth in the synthesis of nanocrystals were studied. The best conditions for the synthesis of nanoparticles of high quality are also reported. The CdS nanocrystals obtained exhibited a narrow PL band, with reproducible room-temperature quantum yields.

Study on the Physical Properties of CdS Quantum Dots Synthesized by Ligand Exchange in a Cd2+-mercaptopropionic Acid Aqueous Solution at Room Temperature

Study on the Physical Properties of CdS Quantum Dots Synthesized by Ligand Exchange in a Cd2+-mercaptopropionic Acid Aqueous Solution at Room Temperature

Preparation and Characterization of Biocompatible Quaternized Chitosan Nanoparticles Encapsulating CdS Quantum Dots

Nanoparticles were produced by encapsulating CdS quantum dots (QDs) with quaternized chitosan (N-(2- hydroxyl) propyl-3-trimethyl ammonium chitosan chloride, HTCC), in order to improve general biocompatibility and stability of pure QDs. The properties of CdS QDs encapsulated HTCC nanoparticles (HTCC/CdS QDs) can be controlled by changing the mass ratios of QD to HTCC (16:1, 8:1, 4:1, 2:1, 1:1, 1:2, 1:4, 1:8). Characterizations of HTCC/CdS QDs nanoparticles were performed using ultraviolet-visible, fluorescence spectrometry, and sizezeta analysis. As compared with nonencapsulated QDs, these HTCC/CdS QDs nanoparticles would keep their original optical properties, and greatly improve the quantum yield and stability in room temperature. The quantum yield can be improved from 9% to 23%. When the mass ratio of QD and HTCC was 1.0, the nanoparticles had the highest quantum yield (23%). After being stored for a week, the nanoparticles could still keep stable and high fluorescence intensity, while that of non-encapsulated QDs almost disappeared. In vitro 3-(4,5-dimethylthiazol-2-yl)- 2,5-diphenyltetrazolium bromide (MTT) cytotoxicity tests on primary myoblast cells suggested that the cytotoxicity of the QDs was greatly reduced after HTCC encapsulation. Therefore, due to the increase of biocompability, HTCC/ CdS QDs nanoparticles can be potentially used in biological applications and labeling of biomolecules.

Effect of solvent-induced structural modifications on optical properties of CdS nanoparticles

We have investigated the effects of solvent used during synthesis on structural and optical properties of CdS quantum dots. Different methods of synthesis for the production of CdS quantum dots are presented. These are: (a) wet chemical co-precipitation in non-aqueous medium (i.e. methanol); (b) wet chemical co-precipitation in aqueous medium (deionized water) and (c) solid state reaction. It is demonstrated that the use of methanol as solvent leads to a strong enhancement of PL intensity of CdS quantum dots for use in optoelectronic devices. These products were characterized by X-ray powder diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). The change in bandgap with size-quantization was investigated by UV-VIS absorption spectroscopy. CdS nanocrystals prepared in non-aqueous medium have narrow size distribution than those prepared in aqueous medium and solid state reaction. Phase transformation of CdS nanocrystals from a cubic to hexagonal structure was observed in methanol solution. The formation of CdS/Cd(OH)2 nanostructure was also confirmed using X-ray diffraction pattern. This suggests that the strong enhancement of the PL intensity may have originated from the remarkable reduction of non-radiative recombination process, due to surface defects of quantum dots. The red shift of the Raman peaks compared to that for bulk CdS may be attributed to optical phonon confinement.

CdS quantum dots for measurement of the size-dependent optical properties of thiol capping

The optical- and size-dependent properties of CdS quantum dots (QDs) were analyzed in the presence and absence of different capping agents in aqueous medium. The QDs have been characterized by UV–Vis, Photoluminescence, Fourier-transform infrared spectroscopy, X-ray diffraction, and Fluorescence lifetime measurements. QDs with the presence of thiol group in cubic phase with small grain size were observed in XRD and decrease in particle size of the same with increase in band gap is deduced through UV–Vis and XRD studies. The FT-IR spectrum confirms the interaction of thiol group with CdS. Fluorescence lifetime of capped QDs was higher compared to uncapped CdS QDs. The surface passivation of thiol group on CdS is shown in photoluminescence studies.

Influence of SPP co-stabilizer on the optical properties of CdS quantum dots grown in PVA

Quantum dots of cadmium sulfide have been synthesized in a series of aqueous solutions of two stabilizers: a fixed concentration of polyvinyl alcohol polymer with a variable amount of sodium polyphosphate. Optical absorption and photoluminescence measurements have been performed in order to assess the optical properties of the colloidal solutions. Transmission electron microscopy images have been recorded to obtain structural information of the nanocrystals. The results obtained from the spectroscopic techniques reveal two different behaviors as the amount of sodium polyphosphate is increased in the samples. The photoluminescence of quantum dots in a polyvinyl alcohol solution without sodium polyphosphate is quenched. The addition of this co-stabilizer activates the luminescence, being more intense and blue-shifted as the amount of sodium polyphosphate is increased. However, further addition of this product beyond a organic polymer-co-stabilizer weight ratio of 1:6.25⋅10−3 leads to the inverse behavior: a decreasing luminescence intensity and a red shift of the emission peaks are found. Transmission electron microscopy measurements indicate that a deagglomeration of the particles is produced as the amount of co-stabilizer is increased.

Synthesis and optical properties of CdS quantum dots embedded in silica matrix thin films and their applications as luminescent solar concentrators

CdS quantum dot (QD) solar concentrators were prepared by a sol–gel spin coating method. Thin films were prepared at different annealing temperatures and characterized by X-ray diffraction and spectroscopic techniques. The effect of temperature on the optical properties of CdS QDs embedded in silica matrix was assessed before and after exposure of the samples to sunlight for up to 4 weeks. The results show that as the annealing temperature increases, the fluorescent intensity and Stokes shift decrease. Therefore lower temperatures are preferable for the preparation of highly efficient QD solar concentrator systems.

Strong enhancement of band-edge photoluminescence in CdS quantum dots prepared by a reverse-micelle method

We have investigated the effects of surface modification on photoluminescence (PL) properties of CdS quantum dots (QDs) prepared by a reverse-micelle method. It is demonstrated that the modification of QD surface with a Cd(OH)2 layer leads to a strong enhancement of the band-edge PL intensity of CdS QDs in reverse micelles. The PL-decay profile before the surface modification exhibits a fast decay component less than 50ps. After the modification, the fast-decay component disappears. The drastic change of the PL-decay profiles before and after the surface modification corresponds to the marked increase of the PL intensity. This suggests that the strong enhancement of the band-edge PL intensity originates from the remarkable reduction of nonradiative recombination processes, which usually result in a very short decay time, due to surface defects of QDs.

Improved optical properties of CdS quantum dots by ligand exchange

High-quality CdS nanocrystals with highly efficient and narrow band edge emission have been prepared by means of decomposition of suitable precursors in non-coordinating solvents at high temperature. The role played by capping ligands, solvents and their interactions has been investigated by optical measurements. Capping layer exchange treatments with different amines have been carried out suggesting a possible influence of alkylic chain length in modifying the optical properties of CdS nanocrystals. Enhancement of band edge emission has been demonstrated with octylamine ligand.

Modified spontaneous emission properties of CdS quantum dots embedded in novel three-dimensional microcavities

Modified spontaneous emission properties in the presence of confined photon modes inside the three-dimensional (3-D) optical microcavities are demonstrated. Self-formed pyramidal-shaped semiconductor structures fabricated by selective-area growth technique are utilized as an optical microcavity in which discrete photon modes are generated. Noticeable modification of spontaneous emission from active layers embedded in microcavity structures is clearly observed in μ-PL spectra at room temperature. Almost perfect coincidence between enhanced photoluminescence peak wavelengths and the resonance modes inside microcavity is observed. Furthermore, Purcell factors calculated from the obtained Q values reach ∼9, which is inaccessible in the planar microcavities with only one-dimensional photon confinement normal to the layers. These results indicate that the effective coupling between electronic system and 3-D confined optical fields is realized by the achievement of present low-loss 3-D microcavities with small cavity volume V c whose dimension is comparable with λ 3.