Novel quantum dots: Water-based CdTeSe/ZnS and YAG hybrid phosphor for white light-emitting diodes

Novel water-based core/shell CdTeSe/ZnS quantum dots (QDs) were synthesized by aqueous method. The CdTeSe/ZnS QDs were investigated by high resolution transmission electron microscopy, energy dispersive spectrometry, UV–vis absorption spectra, and photoluminescence spectrum. The as-prepared QDs capped with ZnS shell were spherical in shape with an excellent quantum yield of 16% and emitted bright yellow light. In addition, the CdTeSe/ZnS QDs can be excited by blue or near-UV region, which is an advantage over wavelength converters for white light-emitting diodes (LEDs). White LEDs based on CdTeSe/ZnS QDs, commercially known as Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce), and hybrid phosphor of CdTeSe/ZnS QDs and YAG:Ce, were fabricated. The luminescent properties of the resultant white LEDs were evaluated. The higher red-component in the emission spectrum from CdTeSe/ZnS QDs increased the color rendering index (CRI) value of the commercial YAG:Ce-based white LEDs, and the hybrid phosphor-based white LED had CIE-1993 color coordinate, color temperature, and CRI values of (0.3125, 0.2806), 7108 K and 83.3, respectively.     

Pure red emission hybrid light-emitting devices based on the blend of CdSe/ZnS quantum dots and an n-type polymer

We report on hybrid light-emitting devices (HLEDs) based on CdSe/ZnS quantum dots in polymer matrices. Color purity of the HLED with a mixed matrix consisting of polyvinylcarbazole (PVK) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) improves with the increasing concentration of PBD. We attribute this improvement to the increased electron mobility of the matrix. By using an n-type cyano-containing polymer derived from 6-(3,6-dibromo-9H-carbazol-9-yl)hexanenitrile and tetraphenylsilane to replace PVK:PBD as the matrix, further pure red emission with Commission Internationale De L'Eclairage coordinates of (0.67, 0.33) and maximum external quantum efficiency of 2.51% has been achieved.     

Light-controlled bioelectrochemical sensor based on CdSe/ZnS quantum dots

This study reports on the oxygen sensitivity of quantum dot electrodes modified with CdSe/ZnS nanocrystals. The photocurrent behavior is analyzed for dependence on pH and applied potential by potentiostatic and potentiodynamic measurements. On the basis of the influence of the oxygen content in solution on the photocurrent generation, the enzymatic activity of glucose oxidase is evaluated in solution. In order to construct a photobioelectrochemical sensor which can be read out by illuminating the respective electrode area, two different immobilization methods for the fixation of the biocatalyst have been investigated. Both covalent cross-linking and layer-by-layer deposition of GOD by means of the polyelectrolyte polyallylamine hydrochloride show that a sensor construction is possible. The sensing properties of this type of electrode are drastically influenced by the amount and density of the enzyme on top of the quantum dot layer, which can be advantageously adjusted by the layer-by-layer technique. By depositing four bilayers [GOD/PAH](4) on the CdSe/ZnS electrode, a fast-responding sensor for the concentration range of 0.1-5 mM glucose can be prepared. This study opens the door to multianalyte detection with a nonstructured sensing electrode, localized enzymes, and spatial read-out by light.     

Traps and performance of MEH-PPV/CdSe(ZnS) nanocomposite-based organic light-emitting diodes

We report the fabrication and investigations of organic light-emitting diodes (OLEDs) using a composite made by mixing poly[2-methoxy-5(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with CdSe/ZnS core/shell quantum dots (QDs). The electroluminescence efficiency of the studied devices was found to be improved as compared to devices using MEH-PPV. Moreover, the current density decreased with increasing QD concentration. We checked the effects of QDs on the electrical transport by determining the trap states, making use of the charge-based deep level transient spectroscopy (Q-DLTS) technique. The most striking result obtained is the decrease in trap density when adding QDs to the MEH-PPV polymer film. These results suggest that QDs would heal defects in nanocomposite-based devices and that CdSe/ZnS QDs prevent the trap center formation.     

Shell distribution on colloidal CdSe/ZnS quantum dots

Scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) was used to determine the chemical distribution of semiconductor shell material around colloidal core-shell CdSe/ZnS quantum dots (QDs). EELS signals from positions around the QD indicate a well-defined shell of ZnS surrounding the CdSe core, but the distribution of the shell material is highly anisotropic. This nonuniformity may reflect the differences in chemical activity of the crystal faces of the core QD and implies a nonoptimal QD surface passivation.     

High color rendering index white LED based on nano-YAG:Ce3+ phosphor hybrid with CdSe/CdS/ZnS core/shell/shell quantum dots

To improve the poor color rendering index (CRI) of YAG:Ce-based white light-emitting diode (LED) due to the lack of red spectral component, core/shell/shell CdSe/CdS/ZnS quantum dots (QDs) were synthesized and blended into nano-YAG:Ce³⁺ phosphors. Prominent spectral evolution has been achieved by increasing the content of QDs. A white LED combining a blue LED with the blends of nano-YAG phosphors and orange- and red-emission QDs with a weight ratio of 1:1:1 was obtained. This kind of white LED showed excellent white light with luminescent efficiency, color coordinates, CRI and correlated color temperature (CCT) of 82.5?lm/W, (0.3264, 0.3255), 91 and 4580?K, respectively.     

Organic light-emitting devices fabricated utilizing core/shell CdSe/ZnS quantum dots embedded in a polyvinylcarbazole

Electrical and the optical properties of organic light-emitting devices (OLEDs) fabricated utilizing core/shell CdSe/ZnS quantum dots (QDs) embedded in a polyvinylcarbazole (PVK) layer were investigated. An abrupt increase of the current density above an applied voltage of 12 V for OLEDs consisting of Al/LiF/4,7-diphenyl-1,10-phenanthroline/bis-(2-methyl-8-quinolinolate)-4-(phenylphenolato) aluminium/[CdSe/ZnS QDs embedded in PVK]/poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate)/ITO/glass substrate was attributed to the existence of the QDs. Photoluminescence spectra showed that the peaks at 390 and 636 nm corresponding to the PVK layer and the CdSe/ZnS QDs were observed. While the electroluminescence (EL) peak of the OLEDs at low voltage range was related to the PVK layer, the EL peak of the OLEDs above 12 V was dominantly attributed to the CdSe/ZnS QDs. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of the OLEDs at high voltages were (0.581, 0.380) indicative of a red color. When the holes existing in the PVK layer above 12 V were tunneled into the CdSe/ZnS QDs, the holes occupied by the CdSe/ZnS QDs combined with the electrons in the PVK layer to emit a red color related to the CdSe/ZnS QDs.     

CdSe and CdSe/ZnS quantum dots for the detection of C-reactive protein

In this study, a method for the detection of C-reactive protein (CRP) using CdSe and CdSe/ZnS quantum dots (QDs) is proposed. CdSe and CdSe/ZnS core-shell QDs are synthesised by using 2-mercaptosuccinic acid (MSA) as a capping agent. These QDs were then subjected to various characterisation studies, namely X-ray diffraction and transmission electron microscope for size and structure, Fourier transform infrared spectroscopy for the confirmation of functional groups, ultraviolet–visible absorption and fluorescence spectroscopy for optical characteristics and dynamic light scattering for hydrodynamic changes of QDs. Two biochemical mixtures were developed: one by mixing blood serum containing CRP and CdSe-phosphorylethanolamine (PEA) and the other by mixing blood serum with CdSe/ZnS-PEA. When these mixtures are observed for fluorescence due to interaction of QDs with CRP, a correlation between changes in fluorescence for different concentrations of CRP is noted. The result demonstrates that CRP can be detected with the help of QDs without using any antibodies.     

Nanocrystalline Y3Al5O12:Ce phosphor-based white light-emitting diodes embedded with CdS:Mn/ZnS core/shell quantum dots

120 nm-sized yellow emitting Y₃Al₅O₁₂:Ce (YAG:Ce) nanocrystalline phosphors and 4.5 nm-sized orange emitting CdS:Mn/ZnS core/shell structured quantum dots (QDs) were prepared by a modified polyol and a reverse micelle chemistry, respectively. To compensate a poor color rendering index (CRI) of YAG:Ce nanophosphor-based white light-emitting diodes (LEDs) due to the lack of red spectral component, CdS:Mn/ZnS QDs were blended into YAG:Ce nanophosphors. Blends of nanophosphors and QDs exhibited the prominent spectral evolution with an increasing content of QDs. A hybrid white LED, which combines a blue LED with the blend of nanophosphor and QDs with a weight ratio of 1:1, was demonstrated with an improved CRI value of 85.     

Synthesis of Zn(1-x)Cd(x)S:Mn/ZnS quantum dots and their application to light-emitting diodes

3.6?nm sized Mn-doped Zn(1-x)Cd(x)S quantum dots (QDs) with the composition (x) of 1, 0.5, 0.2 and 0 were synthesized by a reverse micelle approach. The bandgap energy of Zn(1-x)Cd(x)S:Mn QDs was tuned to a higher energy by increasing the Zn content, and the actual composition of alloyed Zn(1-x)Cd(x)S:Mn QDs was found to be different from the solution composition. Consecutive overcoating of the Zn(1-x)Cd(x)S:Mn QD surface by a ZnS shell was done, and the core/shell structured QDs exhibited quantum yields of 14-30%, depending on the composition of the core QDs. Using CdS:Mn/ZnS QDs, orange and white light-emitting diodes (LEDs) pumped by a near-UV and blue LED chips, respectively, were fabricated and their optical properties are described.     

Photoluminescence of colloidal CdSe/ZnS quantum dots under oxygen atmosphere

The effects of oxygen versus vacuum ambients on colloidal CdSe/ZnS quantum dots (QDs) were studied using both continuous and time-resolved photoluminescence (PL) measurements. The PL intensities were found to be an order of magnitude higher in an oxygen atmosphere, which is explained by the passivation of surface defects by oxygen absorption. The decay of PL intensities can be best fitted by a biexponential function with lifetimes of approximately 1 ns for the fast decay and approximately 10 ns for the slow decay. Based on the emission-energy dependence of carrier lifetimes and of the amplitude ratio of the fast-decay component to the slow-decay component, we suggest that the fast and slow PL decay of colloidal CdSe/ZnS QDs is caused by the recombination of delocalized carriers in the internal core states and the localized carriers in the surface states, respectively.     

Photoactivation of CdSe/ZnS quantum dots embedded in silica colloids

A study of the influence of the local environment on the light-induced luminescence enhancement of CdSe/ZnS quantum dots (QD) embedded in silica colloids that are dispersed in various solvents is presented. The photoluminescence of the embedded QD is enhanced up to a factor of ten upon photoactivation by ultraviolet or visible light. This enhancement is strongly dependent on the local environment. The thickness-dependent permeability of the silica shell covering the QD controls the influence of the solvent on the QD. If foreign ions are present the activation state is stabilized after termination of the activation, whereas in their absence the process is partially reversible. A new qualitative model for the photoactivation of QD in various environments is developed. It comprises light-induced passivation and subsequent oxidation processes. The embedded QD also retain their fluorescence quantum yield inside living cells. Moreover, they can be activated for many hours in living cells by laser radiation in the visible regime.     

Synthesis and photoluminescence of bright water-soluble CdSe/ZnS quantum dots overcoated by hybrid organic shell

Photoluminescence properties from water soluble CdSe/ZnS QDs encapsulated with hybrid trioctylphosphine-poly(acrylamide-co-acrylic acid)-ethanolamine (TOPO-PSMA-EA) shell have been investigated. It was found that PL efficiency of CdSe/ZnS QDs in water was increased 5–30% after introducing PSMA-EA polymers to encapsulate CdSe/ZnS-TOPO QDs. Higher PSMA concentrations were found to enhance the PL efficiency of QDs up to 1.8 folds, which is ascribed to a better packing and passivation of the TOPO-PSMA-EA shell over the QDs. Time-resolved photoluminescence suggested that the mean lifetime of photoexcited carriers in the water-soluble CdSe/ZnS-TOPO-PSMA-EA QDs elongated 2–17 ns compared with that of uncoated samples, indicating that PL quenching defects were effectively removed for CdSe/ZnS QDs with hybrid TOPO-PSMA-EA shell.     

Photo-induced suppression of plasmonic emission enhancement of CdSe/ZnS quantum dots

Emission of semiconductor quantum dots can be increased via two fundamentally different processes: (i) surface plasmon resonances (plasmonic emission enhancement) and (ii) irradiation with light (photo-induced fluorescence enhancement). In this paper we theoretically and experimentally study the mutual impacts of these processes on each other in quantum dot solids. We show that when thin films of colloidal quantum dots are placed in the vicinity of Au nano-islands, the plasmonic enhancement of the radiative decay rates of quantum dots and Forster energy transfer can hinder the photo-induced fluorescence enhancement of these films. This in turn leads to significant suppression of their plasmonic emission enhancement when they are irradiated with a laser beam. We investigate the impact of the sizes and shapes of the metallic nanoparticles in this process and theoretically analyze how plasmons and energy transfer can hinder the electrostatic barrier responsible for photo-induced fluorescence enhancement.     

Continuous distribution of emission states from single CdSe/ZnS quantum dots

The photoluminescence dynamics of colloidal CdSe/ZnS/streptavidin quantum dots were studied using time-resolved single-molecule spectroscopy. Statistical tests of the photon-counting data suggested that the simple "on/off" discrete state model is inconsistent with experimental results. Instead, a continuous emission state distribution model was found to be more appropriate. Autocorrelation analysis of lifetime and intensity fluctuations showed a nonlinear correlation between them. These results were consistent with the model that charged quantum dots were also emissive, and that time-dependent charge migration gave rise to the observed photoluminescence dynamics.     

Calix[8]arene coated CdSe/ZnS quantum dots as C60-nanosensor

Here, we report an optical sensor for fullerene C(60) in water using CdSe/ZnS quantum dots coated by p-tertbutylcalix[8]arene. This C(60)-nanosensor is based on the selective host-guest interaction between fullerene C(60) and p-tertbutylcalix[8]arene. The procedure for the synthesis of p-tertbutylcalix[8]arene-CdSe/ZnS complex is described, and its fluorescent characteristics are also reported. We found that the interaction between C(60) fullerene and p-tertbutylcalix[8]arene-CdSe/ZnS complex quenches the original fluorescence of calix-QDs according to the Stern-Volmer equation. The mechanism of interaction is discussed. Finally, the potential application of the proposed method using the designed nanosensor for determination of C(60) in spiked environmental river water samples is demonstrated. For the analysis of river samples, a liquid-liquid extraction multistep preconcentration procedure is proposed. The method, which is simple and rapid, allows the detection of 5 μg L(-1) of fullerene. This sensor could be a useful tool for environmental and toxicological studies.     

Preparation of a ZnS shell on CdSe quantum dots using a single-molecular ZnS precursor

A simple synthetic route to the preparation of a thin ZnS shell on CdSe quantum dot cores from the air-stable, single-molecular precursor zinc diethyldithiocarbamate, Zn(S(2)CNEt(2))(2), in the three-component solvent system octadec-1-ene/oleylamine/tri-n-octylphosphine (ODE/OLA/TOP) is presented. The one-pot synthesis proceeds through heating of the solution of CdSe cores and the amount of crystalline Zn(S(2)CNEt(2))(2) corresponding to a shell thickness of two monolayers of ZnS to 110-120 °C for 1-2 h. The role of the surfactants OLA and TOP and the significance of the temperature and the amount of Zn(S(2)CNEt(2))(2) have been investigated with optical absorption and luminescence spectroscopy. We show that the presence of both OLA and TOP is crucial for the low-temperature growth and that the amount of precursor corresponding to two monolayers of ZnS results in the highest quality of core/shell CdSe/ZnS quantum dots.     

White light emitting diodes realized by using an active packaging method with CdSe/ZnS quantum dots dispersed in photosensitive epoxy resins

White light emitting diodes (LEDs) have been realized using the active packaging (AP) method. The starting materials were bare InGaN LED chips and CdSe/ZnS core-shell quantum dots (QDs) dispersed in photosensitive epoxy resins. Such hybrid LED devices were fabricated using QD mixtures with one ('single'), two ('dual') or four ('multi') emission wavelengths. The?AP method allows for convenient adjustment of multiple parameters such as the CIE-1931 coordinate (x, y), color temperature, and color rending index (CRI). All samples show good white balance, and under a 20?mA working current the luminous efficacies of the single, dual, and multi hybrid devices were 8.1?lm?W(-1), 5.1?lm?W(-1), and 6.4?lm?W(-1), respectively. The corresponding quantum efficiencies were 4.1%, 3.1%, and 3.1%; the CRIs were 21.46, 43.76, and 66.20; and the color temperatures were 12?000, 8190, and 7740?K. This shows that the CRI of the samples can be enhanced by broadening the QD emission band, as is exemplified by the 21.46 CRI of the single hybrid LED compared to the 66.20 value for the multi hybrid LED. In addition, we were able to increase the CRI of the single hybrid LED from 15.31 to 32.50 by increasing the working currents from 1 to 50?mA.     

Cyto-/genotoxic effect of CdSe/ZnS quantum dots in human lung adenocarcinoma cells for potential photodynamic UV therapy applications

Quantum dots (QDs) are luminescent nanoparticles (NPs) with promising potential in numerous medical applications, but there remain persistent human health and safety concerns. Although the cytotoxic effects of QDs have been extensively investigated, their genotoxic effects remain under-explored. This study scrutinized the cyto- and genotoxic effects of QDs with a Cadmium selenide/Zinc sulfide (CdSe/ZnS) core/shell, and suggests comprehensive guidelines for the application of QDs in cancer therapy. QDs were used to treat A549 cells in the presence and absence of ultraviolet A/B (UVA/UVB) irradiation. QD-induced cell death was evaluated by 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), apoptosis, and lactate dehydrogenase (LDH) assays, as well as by real-time PCR analysis of differential mRNA levels of genes, such as ataxia telangiectasia mutated (ATM), p53, and caspase-9, involved in apoptosis. The genotoxic effect of CdSe/ZnS QDs was measured in human cancer cells, for the first time, by comet and micronucleus assays. Treatment with CdSe/ZnS QDs and UVB irradiation resulted in the most severe extent of cell death, indicating strong induction of phototoxicity by CdSe/ZnS QDs in the presence of UVB. Both apoptotic and necrotic cell death were observed upon QDs and UVB combined treatment. The induction of Olive tail moments and micronuclei formation was also most significant when CdSe/ZnS QDs and UVB irradiation were combined. Our results on the genotoxic effect and mechanistic details of CdSe/ZnS QD-induced cell death suggest that UVB irradiation is the most effective method for increasing the potency of QDs during photodynamic cancer therapy.