Photoluminescence quenching of CdTe/CdS core-shell quantum dots in aqueous solution by ZnO nanocrystals

Photoluminescence (PL) properties of 3-mercaptopropionic acid (MPA) coated CdTe/CdS core-shell quantum dots (QDs) in aqueous solution in the presence of ZnO colloidal nanocrystals were studied by steady-state and time-resolved PL spectroscopy. The PL quenching of CdTe/CdS core-shell QDs with addition of purified ZnO nanocrystals resulted in a decrease in PL lifetime and a small red shift of the PL band. It was found that CdTe(1.5 nm)/CdS type II core-shell QDs exhibited higher efficiency of PL quenching than the CdTe(3.0 nm)/CdS type I core-shell QDs, indicating an electron transfer process from CdTe/CdS core-shell QDs to ZnO nanocrystals. The experimental results indicated that the efficient electron transfer process from CdTe/CdS core-shell QDs to ZnO nanocrystals could be controlled by changing the CdTe core size on the basis of the quantum confinement effect.     

Synthesis,biocompatibility and luminescence properties of quantum dots conjugated with amino acid-functionalized β-cyclodextrin

A series of CdSe and CdSe/CdS quantum dots (QDs) labeled with amino acid-modified β-cyclodextrin (β-CD) was prepared by a simple ultrasonic method. These amino acid-modified β-CD-coated QDs are very soluble and stable in biological buffer. They also have high colloidal stability and strong optical emission properties that are similar to those of untreated tri-n-octylphosphine oxide (TOPO)-coated QDs. The quantum yields (QYs) of these amino acid-modified β-CD-coated CdSe and CdSe/CdS QDs in biological buffer were found to be very high. In particular, the QYs of the positively charged l-His-β-CD-coated CdSe/CdS QDs were as high as 33.5±1.8%. In addition, the fluorescence lifetime of these QDs was also very long in PBS solutions as determined by frequency domain spectroscopy. For example, the lifetime of l-His-β-CD-coated CdSe/CdS QDs was 8.6 ns. The in vitro cytotoxicity of these QDs in ECV-304, SH-SY5Y and HeLa cells was found to be lower. l-His-β-CD-coated CdSe/CdS QDs were the least cytotoxic (IC₅₀ 95.6±3.2 mg mL^?1 in ECV-304 cells after 48 h). The flow cytometry results show that the positively charged amino acid led to a considerable increase in biocompatibility of QDs. This may be attributed to the presence of an amino acid-modified β-CD outer layer, which enhanced the biocompatibility.     

Two-step annealed CdS/CdSe co-sensitizers for quantum dot-sensitized solar cells

CdS/CdSe co-sensitizers on TiO₂ films were annealed using a two-step procedure; high temperature (300 °C) annealing of TiO₂/CdS quantum dots (QDs), followed by low temperature (150 °C) annealing after the deposition of CdSe QDs on the TiO₂/CdS. For comparison, two types of films were prepared; CdS/CdSe-assembled TiO₂ films conventionally annealed at a single temperature (150 or 300 °C) and non-annealed films. The 300 °C-annealed TiO₂/CdS/CdSe showed severe coalescence of CdSe QDs, leading to the blocked pores and hindered ion transport. The QD-sensitized solar cell (QD-SSC) with the 150 °C-annealed TiO₂/CdS/CdSe exhibited better overall energy conversion efficiency than that with the non-annealed TiO₂/CdS/CdSe because the CdSe QDs annealed at a suitable temperature (150 °C) provided better light absorption over long wavelengths without the hindered ion transport. The QD-SSC using the two-step annealed TiO₂/CdS/CdSe increased the cell efficiency further, compared to the QD-SSC with the 150 °C-annealed TiO₂/CdS/CdSe. This is because the 300 °C-annealed, highly crystalline CdS in the two-step annealed TiO₂/CdS/CdSe improved electron transport through CdS, leading to a significantly hindered recombination rate.     

Synthesis of CdSe/CdS with a simple non-TOP-based route

A convenient and non-TOP-based route for the synthesis of core-shell CdSe/CdS quantum dots (QDs) is developed for the first time. Simple reagents, such as cadmium oxide, selenium powder, sodium sulfide, paraffin and oleic acid with obvious advantages are used to replace organometallics. This simple route allows the preparation of a series of core-shell CdSe/CdS QDs emitting in a wide wavelength range (from 510 to 615 nm). After passivation of CdSe by CdS shell using sodium sulfide as the source of sulfur at 80 °C, the quantum yields (QYs) are improved from 15-30% to 35-50% and remained stable at least for 4 months. A narrow bandwidth (FWHM<50 nm) indicates that the as-prepared QDs have uniform size distribution, desirable dispersibility and good fluorescence properties. The whole procedure can be carried out either open to air or under nitrogen atmosphere, which is simpler, greener and cheaper as compared with TOP-based route.     

Fabrication of highly luminescent InP/Cd and InP/CdS quantum dots

Highly luminescent InP/Cd and InP/CdS core-shell QDs were fabricated by sequential addition of cadmium acetylacetonate and dodecanethiol to InP core solutions, which showed a red-shift in absorption and emission. ICP measurement revealed the existence of cadmium and TEM images showed the increased size of InP/CdS QDs. PXRD data identified zinc blend structures of InP and InP/CdS QDs, which indexed to the (1 1 1), (2 2 0) and (3 1 1) planes. The slight shift of peaks between InP and InP/CdS QDs can demonstrate the existence of CdS shell structures.     

Interactions between CdSe/CdS quantum dots and DNA through spectroscopic and electrochemical methods

The interaction of CdSe/CdS quantum dots (QDs) with Herring sperm-DNA (hs-DNA) has been studied by UV-vis spectroscopy and electrochemical method. Cu(phen)₂^2+/1+ (phen = 1, 10-phenanthroline) was used as an indicator for electroactive dsDNA or ssDNA. The apparent association constant has been deduced (4.94 × 10³ M⁻¹ and 2.39 × 10² M⁻¹) from the absorption spectral changes of the dsDNA-QDs and ssDNA-QDs. The results of dissociation method suggest that Cu(phen)₂^2+/1+ is more easily dissociated from dsDNA or ssDNA modified gold electrode (dsDNA/Au or dsDNA/Au) in presence of QDs. The dissociation rate constant (k) of Cu(phen)₂^2+/1+ on dsDNA/Au is 4.48 times higher than that in absence of QDs, while k is 2.34 times higher than that in absence of QDs on ssDNA/Au in Tris buffer with low ionic strength (pH 7.0, 0.5 mM NaCl). The results illuminate that hs-DNA has high affinity for QDs due to electrostatic force, hydrogen bonds, and van der Waals interactions, and the binding force of QDs with dsDNA is stronger than ssDNA.     

Shell-dependent hole transport in highly luminescent CdSe-core CdS/ZnCdS/ZnS multi-shell nanocrystals

The photoinduced hole transfer dynamics from CdSe quantum dots (QDs), shelled with ZnS or CdS/CdZnS/ZnS layers, to organic hole transporting materials (HTMs) is investigated by absorption, steady-state and time-resolved photoluminescence (PL) spectroscopy. The PL intensity and lifetime of the QDs are dramatically quenched when HTMs are added into the dilute QD solution. The quenching efficiency of the QDs significantly decreases with increasing the shell thickness and increases with decreasing the oxidation potential of the HTMs. These facts are correlated with the photoinduced hole transfer from the QDs to the HTMs. The above results are helpful in understanding the photoexcitation dynamics-related phenomena of organic molecule conjugated nano-object.     

Nanotwinning in CdS quantum dots

High resolution transmission electron microscopy, X-ray diffraction and photoluminescence measurements are carried out in order to study the defects in CdS quantum dots (QDs), synthesized in cubic phase by chemical co-precipitation method. The nanotwinning structures in CdS quantum dots (∼2.7 nm) are reported for the first time. Mostly CdS QDs are characterized by existence of nanotwin structures. The twinning structures are present together with stacking faults in some QDs while others exist with grain boundaries. Raman spectroscopy analysis shows intense and broad peaks corresponding to fundamental optical phonon mode (LO) and the first over tone mode (2LO) of CdS at 302 cm⁻¹ and 605 cm⁻¹ respectively. A noticeable shift is observed in Raman lines indicating the effect of phonon confinement. Fourier transform infrared spectroscopy analysis confirms the presence of Cd–S stretching bands at 661 cm⁻¹ and 706 cm⁻¹. The photoluminescence spectrum shows emission in yellow and red regions of visible spectrum. The presence of stacking faults and other defects are explained on the basis of X-rays diffraction patterns and are correlated with photoluminescence spectrum. These nanotwinning and microstructural defects are responsible for different emissions from CdS QDs.     

CdSe quantum dots decorated by mercaptosuccinic acid as fluorescence probe for Cu

Water-soluble CdSe quantum dots (QDs) were synthesized using mercaptosuccinic acid (MSA) as a stabilizer. The growth process and characterization of CdSe quantum dots were determined by transmission electron microscopy (TEM), X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, Ultraviolet-visible (UV-vis) spectroscopy, and Fourier transform infrared (FT-IR) spectroscopy. Results demonstrated the MSA-capped CdSe QDs were highly crystalline and possessed good optical properties. Further, the resulting products could be used as fluorescent probes to detect Cu²⁺ ions in physiological buffer solution. The response was linearly proportional to the concentration of Cu²⁺ ion in the range 2×10⁻⁸- 3.5×10⁻⁷ mol L⁻¹ with a detection limit of 3.4 nmol L⁻¹.     

Transition behavior from uncoupled to coupled multiple stacked CdSe/ZnSe self-assembled quantum-dot arrays

Transition behavior from uncoupled to coupled multiple stacked CdSe/ZnSe quantum-dot (QD) arrays grown by molecular beam epitaxy were investigated. Transmission electron microscopy showed that vertically stacked self-assembled CdSe QD arrays were embedded in the ZnSe barriers. The results for the photoluminescence (PL) data at 18 K demonstrated clearly that the transition behavior from uncoupled to coupled peaks depended on the ZnSe barrier thickness. The temperature-dependent PL measurements showed that the activation energy of the electrons confined in the CdSe QDs increased dramatically with decreasing ZnSe spacer layer thickness due to the strong coupling between CdSe/ZnSe QD arrays. The present observations can help improve understanding of the dependence of the coupling behavior and activation energy in CdSe/ZnSe QDs on the spacer layer thickness.     

Static quenching of bovine serum albumin conjugated with small size CdS nanocrystalline quantum dots

The author reports on a strong fluorescence quenching of a model transport protein, bovine serum albumin BSA, when bioconjugated with CdS quantum dots QDs. The 4.4 nm size CdS QDs were synthesized using wet chemistry method and were characterized using UV-vis spectroscopy, scanning electron microscopy SEM and X-ray diffraction XRD techniques. It was found that the BSA fluorescence quenching increases linearly with increasing the CdS QDs concentrations in the range of 3×10⁻⁷-2.0×10⁻⁶ mol L⁻¹. This quenching is explained in terms of Stern-Volmer equation and is ascribed to static quenching with quenching constant 1.321×10⁴ L mol⁻¹ at 300 K.     

Chemical role of amines in the colloidal synthesis of CdSe quantum dots and their luminescence properties

The role of organic amines in the colloidal synthesis of CdSe quantum dots (QDs) has been studied. CdSe QDs were synthesized from the source solutions containing 5 vol% of amines having various alkyl chain lengths, stereochemical sizes and electron donation abilities. The role of the additional amines was evaluated on the basis of the photoluminescence (PL) properties such as PL wavelength and intensity of the obtained CdSe QDs. The observed PL spectra were explained by the fact that the amines behaved as capping ligands on the surface of the QDs in the product colloidal solution and complex ligands for cadmium in the source solutions. It was shown that the particle size was controlled by the diffusion process depending on the mass and stereochemical shape of the amines, and the luminescence intensity increased with the increasing electron donation ability and capping density of the amines.     

Mechanism of fluorescence energy transfer in aqueous solution cadmium sulfide quantum dots

Cadmium sulfide (CdS) quantum dots (QDs) prepared by a convenient chemical method have been characterized using absorption, fluorescence, and photoluminescence excitation techniques. The photoluminescence excitation studies show that there is an electron transfer from the surface adsorbate (thiourea) to CdS QDs in aqueous solution. The excitation band with peak maximum at 5.8 eV is assigned to the electronic transitions in the chemisorbed thiourea, whereas the excitation band between 3.45 and 3.7 eV corresponds to the band-to-band transition within the nanocrystalline CdS host. The absorption spectroscopy of the CdS QD solutions shows a strong absorption peak which is generated from thiourea. The band-edge fluorescence of the CdS QDs has also been investigated. It is shown that the fluorescence property of the CdS QDs can be enhanced by adding cadmium chloride (CdCl₂) solution.     

Temperature induced size selective synthesis of hybrid Cds/pepsin nanocrystals and their photoluminescence investigation

There is growing interest in materials chemistry for taking advantage of the physical and chemical properties of biomolecules in the development of next generation nanoscale materials for opto-electronic applications. A biomimetic approach to materials synthesis offers the possibility of controlling size, shape, crystal structure, orientation, and organization. The great progress has been made in the control that can be exerted over optical materials synthesis using biomolecules (protein, nucleic acid)/mineral interfaces as templates for directed synthesis. We have synthesized the CdS nanocrystals using pepsin by biomimetic technique at four different set temperatures. X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) results showed that we are able to tune the size and distribution profile just by tuning the reaction (Rx) temperature and goes towards excitonic Bhor radius (2.5 nm) at low temperature (4 °C). The narrow absorption peak at 260 nm from Cd²⁺-pepsin complex dominates and indicates the size dispersion of the modified CdS nanoparticles are fairly monodisperse. Effective mass approximation (EMA) shows large blue-shift (~1 eV) in the band gap for the cubic phase from bulk hexagonal CdS. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra are dominated by a strong and narrow band-edge emission tunable in the blue region indicating a narrow size distribution. The reduction in PL efficiency is observed when the Rx temperature increases however no change in PLE spectra and temporal profiles of the band-edge PL is observed. At 4 °C, high emission efficiency with shift of PL spectrum in the violet region is observed for 1.7 nm size CdS quantum dots (QDs). Presence of pepsin has slowed the PL decay which is of the order of 100 μs.     

The Dynamic Resistance of CdS/CdSe/ZnS Co-Sensitized TiO2 Solar Cells

Quantum dots' sensitized solar cells (QDSSCs) can create the high-performance and low-cost photovoltaic in the future. In this study, we synthesized the film of TiO₂/CdS/CdSe/ZnS photoanodes by successive ionic layer adsorption reaction (SILAR) method. The absorption spectra, photoluminescent spectra and electrochemical impedance spectra (EIS) of the film TiO₂/CdS/CdSe/ZnS photoanodes show that the structure of energy levels in the conduction band (CB) of photoanode materials CdS, CdSe, and ZnS quantum dots (QDs) can absorb a great number of photons in each region and inject stimulated electrons quickly into the conduction band (CB) of TiO₂. Furthermore, we also studied the influence of the SILAR cycles on the dynamic resistance, the lifetime of electrons in QDSSCs through Nyquist and Bode.     

Effect of temperature and pumping power on the photoluminescence properties of type-II CdTe/CdSe core-shell QDs

CdTe/CdSe core-shell QDs type II was prepared by two steps synthetic process and embedded in polymer host. Emission in the NIR region was recorded. The effect of Ar laser excitation at different powers on the optical properties of the prepared QDs is studied at different temperatures (300-10 K). Amplified spontaneous emission is observed at room temperature and at 10 K. Also, lifetime measurement of prepared QDs was documented using N₂-laser.     

Preparation of Ru-loaded CdS/Al-HMS nanocomposites and production of hydrogen by photocatalytic degradation of formic acid

The composite of aluminum-substituted mesoporous silica (Al-HMS) molecular sieve coupled with CdS (CdS/Al-HMS) was prepared by template, ion exchange and sulfurization reactions. The result of low angle XRD patterns showed that the low content of 2.5 wt% CdS is incorporated inside Al-HMS channels. The results of diffuse reflectance UV-visible spectra and fluorescence emission spectra exhibited that the absorption edge and photoluminescence peak for CdS/Al-HMS are blue-shifted about 75 nm and 40 nm in comparison to bulk CdS, respectively. The activities of hydrogen production by photocatalytic degradation of formic acid were evaluated under visible light irradiation (λ ≥ 420 nm) and the CdS/Al-HMS loaded 0.07 wt% Ru showed the highest H₂ evolution at a rate of 3.7 mL h⁻¹ with an apparent quantum yield of 1.2% at 420 nm.     

Time-Resolved Fluorescence Spectroscopy Study on the Photophysical Behavior of Quantum Dots

Room-temperature time-resolved luminescence measurements on single CdSe/ZnS quantum dots (QDs) are presented. Fluorescence emission spectra were recorded over periods of up to 30 minutes with a time resolution as small as 6 ms. For QDs in ambient air, a clear 30–40 nm blue shift in the emission wavelength is observed, before the luminescence stops after about 2–3 minutes because of photobleaching. In a nitrogen atmosphere, the blue shift is absent while photobleaching occurs after much longer times (i.e., 10–15 minutes). These observations are explained by photoinduced oxidation. The CdSe surface is oxidized during illumination in the presence of oxygen. This effectively results in shrinkage of the CdSe core diameter by almost 1 nm and consequently in a blue shift. The faster fading of the luminescence in air suggests that photoinduced oxidation results in the formation of non-radiative recombination centers at the CdSe/CdSeO_x interface. In a nitrogen atmosphere, photoinduced oxidation is prevented by the absence of oxygen. Additionally, a higher initial light output for CdSe/ZnS QDs in air is observed. This can be explained by a fast reduction of the lifetime of the long-lived defect states of CdSe QDs by oxygen.     

A sol-gel method for preparing ZnO quantum dots with strong blue emission

ZnO quantum dots (QDs) with strong blue emission have been successfully synthesized by sol-gel method, and their crystal structures, sizes, and photoluminescence properties were characterized by X-ray diffractometer, scanning electron microscope, and ultraviolet-visible spectroscopy. It has been found that ZnO QDs had a hexagonal wurtzite crystal structure, and their average diameter was about 16.0-32.2 nm. Both the reaction time and temperature were found to have a strong influence on the average size and photoluminescence properties of ZnO QDs. Longer reaction time and higher reaction temperature resulted in larger average size for ZnO QDs. It has been shown that at reaction temperature 60 °C the emission intensity for ZnO QDs increased first with reaction time before 7 h and then decreased after 7 h. For the same reaction time 7 h, ZnO QDs synthesized at 60 °C showed the strongest emission intensity. It was found that annealing in nitrogen, vacuum, and air all resulted in an increase of the size of ZnO QDs and a reduction in their photoluminescence. The dependence of the size and properties of ZnO QDs on the reaction parameters as well as the annealing conditions has been discussed.     

Luminescent Properties of Hybrid Nanostructures Based on Quantum Dots of CdS,Europium 1,3-Diketonate,and Methylene Blue Molecules

The luminescent properties of hybrid nanostructures constructed from colloidal quantum dots (QDs) of CdS passivated with thioglycolic acid, europium(III) tris(tenoyltrifluoroacetonate), and methylene blue dye molecules are studied. Spectral features typical for the formation of core/shell QDs of the CdS/CdS:Eu³⁺ type are found. It is noted that the adsorption of the europium complex at the QD interfaces and the formation of QDs of the CdS/TGA/Eu³⁺ are probable. Spectral patterns that reveal nonradiative energy transfer from the recombination luminescence centers of CdS QDs to the Eu³⁺ ions in the CdS/CdS:Eu³⁺ and CdS/TGA/Eu³⁺ structures are obtained. This is manifested in quenching the recombination luminescence of QDs and in the ignition of the intracentric luminescence of Eu³⁺, which enhance with an increase in the concentration of the europium complex. When such structures are combined with methylene blue molecules, the half-width of the absorption spectra is found to increase by 10–15% with an unchanged position of the absorption band maximum. With an increase in the concentration of methylene blue molecules, decreases in the intensity of the recombination luminescence band of CdS QDs at a wavelength of 530 nm and in the luminescence intensity of Eu³⁺ ions and simultaneously the rise up of the fluorescence of methylene blue at a wavelength of about 675 nm are observed. At the same time, a decrease in the luminescence lifetime of the bands of QDs and europium ions are observed. It is concluded that the nonradiative excitation energy transfer from both the recombination luminescence centers and Eu³⁺ ions to methylene blue molecules takes place.