Colloidal CdS Quantum Dots Research Articles

Effect of MPS concentration on ripening kinetics and structural properties of CdS quantum dots in self-assembled thin films

The colloidal CdS quantum dots (QDs) were synthesized with various molar ratios of 3-mercaptopropyltrimethoxysilane (MPS) molecules. The results show that the smaller size of CdS QDs was obtained at the higher molar concentration of MPS. Sol–gel spin coating method was used for deposition of the colloidal nanoparticles on a glass substrate. One pot production method was used to obtain self-assembled thin films of CdS QDs in SiO2 network and the grow kinetics of QDs were investigated. The samples were heat treated at various temperatures to observe the growth of the QDs. The sizes of the QDs were obtained in the range of 2–5 nm depending on the capping agent concentration and heat-treatment temperature. The activation energies for the Ostwald ripening process were found as 38.7, 56.4, 93,1 and 98.6 kJ/mol for the samples containing molar ratio of MPS:Cd = 0.25, 0.50, 1.00 and 2.00 respectively.

Plight of Mn Doping in Colloidal CdS Quantum Dots To Boost the Efficiency of Solar Cells

One of the strategies to boost the efficiency of highly promising yet struggling quantum dot solar cells (QDSCs) is to dope the QDs with optically active Mn2+ ions to create long-lived charge carriers and reduce the electron–hole recombination. The in situ deposition of QDs although offers good surface coverage and better charge separation; in “doped” QDSCs the type of doping responsible for higher efficiencies cannot be ascertained. In the electrophoretically deposited Mn:CdS QDSCs, highest lattice doping of Mn2+ ions in presynthesized colloidal CdS QDs results in the highest efficiency, whereas exchange coupled Mn2+–Mn2+ pairs increase nonradiative electron–hole recombination and decrease the efficiency of QDSCs. With Mn:Cd at. % of 1.44, the efficiency could be increased to 2.1% as compared to 1.3% for CdS QDSC. The increase in efficiency by 66.4% is due to slower charge recombination in the photoanode and the electrolyte interface and higher electron lifetime in the doped QDSCs.

Luminescence properties of hydrophilic hybrid associates of colloidal CdS quantum dots and methylene blue

Luminescence properties of hybrid associates of semiconductor colloidal CdS quantum dots (QDs) with an average diameter of 2.5nm and methylene blue (MB+) cations were investigated. Their photoluminescence spectra, excitation spectra and luminescence decay spectra recorded using the time-correlated single photon counting techniques were analyzed. The increase of efficiency of MB+ fluorescence at 405nm excitation was found. It corresponds to the absorption region of CdS QDs. When [nCdSQDs]:[nMB+] concentrations ratio changes from 100:1 to 1:3 for associates, then a decrease in the intensity of CdS QDs luminescence band (580nm) and its increase for MB+ band (674nm) were observed. At the same time it was found that the average lifetime of CdS QDs luminescence is shortened and the lifetime of MB+ luminescence becomes significantly longer. It was concluded that dynamic quenching due to nonradiative resonance energy transfer takes place. The efficiency of this process was estimated and the corresponding rate constant was calculated. It is 5.6×107to8.6×107s−1.

Spectroscopic investigation of colloidal CdS quantum dots–methylene blue hybrid associates

Spectral properties of hydrophilic associates of colloidal CdS quantum dots (QDs) with methylene blue molecules (MB) prepared by sol–gel method have been studied. The two basic types of hybrid associates were found using FTIR spectra technique. The first-type associates are characterized by planar location of MB heterocycle on QDs spherical interface. In this case MB dimerization is not observed. Both nitrogen and sulfur heteroatoms and double =N+–(CH3)2 bonds of MB are participants of association which causes to conformation of MB heterocycle and π-conjugation length decrease. In UV–Vis absorption and luminescence spectra blue shift of MB peaks for QD-MB associates in comparison with MB spectra in solutions and gelatin was found. The second type of association mostly involves MB nitrogen heteroatoms and CdS QDs interface atoms. At the same time, peaks of UV–Vis and luminescence spectra are red shifted. In this case, dimerization manifestation of MB in QD-MB associates was found. Using the time correlated single photon counting technique resonance electronic excitation energy transfer from recombination luminescence center of CdS QDs to MB molecules was found. Its efficiency is in the range of 0.36–0.51.

Decay of electronic excitations in CdS and CdS/ZnS colloidal quantum dots: spectral and kinetic investigations

Using the spectral methods of induced absorption, luminescence, and photostimulated luminescence flash, we have experimentally investigated processes of decay of electronic excitations in CdS colloidal quantum dots and in CdS/ZnS “core-shell” systems synthesized in gelatin by the sol-gel method. It has been shown that the decay of electronic excitations in colloidal quantum dots of this type is predominantly related to a fast localization of nonequilibrium charge carriers on surface defects and their subsequent recombination during times on the order of units and tens of picoseconds. The passage to core-shell systems eliminates, to a large extent, surface defects of the core, some of which are luminescence centers. However, upon using the sol-gel synthesis, a noticeable fraction of luminescence centers are formed in the interior of the CdS quantum dot, which, as well as in the case of CdS/ZnS systems, ensures localization of exciton, blocks its direct annihilation, and maintains recombination radiation.

Controllable surface‐state emission from colloidal CdS quantum dots under different growth conditions

High‐quality colloidal photoluminescent CdS quantum dots (QDs) were synthesized in non‐coordinating solvent octadecene (ODE) using oleic acid (OA) as capping molecule, and characterized by powered x‐ray diffraction (XRD), transmission electron microscope (TEM), UV‐Vis absorption spectra, photoluminescence (PL) emission spectroscopy and time‐resolved emission spectroscopy. The effects of the growth time, the molar ratio of OA/Cd and Cd/S on the PL intensity from surface‐state have been investigated. We found that the surface‐state emission could be easily controlled by tuning the growth time and the molar ratio of Cd/S.

Room-Temperature Electron Spin Generation by Femtosecond Laser Pulses in Colloidal CdS Quantum Dots.

We present an experimental investigation of optical spin orientation in colloidal CdS quantum dots (QDs) by a femtosecond laser pulse at room temperature. The spin carrier and its spin-generation process are clarified. Firstly, the observed spin signals of CdS QDs in time-resolved Faraday rotation measurements are shown to belong to electron carriers, by comparing the spin dephasing dynamics and Landé g factor between CdS QDs and bulk materials. Secondly, spin dynamics unaffected by the faster carrier recombination suggests that the spin-polarized electrons are not photoexcited but resident in the dots. Moreover, hole spins should dephase very fast compared with electron spins, otherwise the trion (two electrons with opposite spin orientations and one hole) recombination process will affect the resident electron spin signals. The electron spin is generated in a short time of which the excitation light is absorbed and the resident electron is excited to trion states, i.e., of pulse durations. Due to fast hole spin dephasing, trion recombination gives null spin signals, and the subsequent electron spin dynamics is controlled by its intrinsic mechanisms.

Spectral manifestations of hybrid association of CdS colloidal quantum dots with methylene blue molecules

We have studied the spectral properties of mixtures formed by CdS colloidal quantum dots with an average diameter of 2.5 nm and methylene blue molecules and that are dispersed into gelatin. We have revealed that, in the presence of CdS quantum dots, the luminescence intensity of methylene blue increases. We suggest a model of this effect, which is based on electronic excitation energy transfer from luminescence centers of CdS quantum dots to methylene blue molecules.

Improving efficiency of InGaN/GaN multiple quantum well solar cells using CdS quantum dots and distributed Bragg reflectors

This work demonstrates hybrid InGaN/GaN multiple quantum well (MQW) solar cells with enhanced power conversion efficiency using colloidal CdS quantum dots (QDs) and back-side distributed Bragg reflectors (DBRs). CdS QDs can absorb ultraviolet (UV) photons, which are strongly absorbed by indium tin oxide (ITO), and they emit photons with a longer wavelength. This process improves the collection of photon-generated carriers and is known as the luminescence down-shifting (LDS). Accordingly, CdS QDs can compensate for the poor utilization of UV photons in an ITO layer, enhancing the external quantum efficiency (EQE) in the UV range. The DBRs on the back of the solar cells can reflect photons of longer wavelengths back into the absorber layer, increasing the EQE (380–440nm). The combination of CdS QDs and DBRs results in broadband EQE enhancement, and yields an overall power conversion efficiency that is 20.7% better than that of a reference device without CdS QDs and DBRs.

The nature of the luminescence-flash photostimulation spectra in CdS quantum dots

The photostimulated luminescence-flash effect has been detected in colloidal CdS quantum dots synthesized by the sol–gel method. The photostimulation spectra have been studied in the quantum-energy region from 0.6 to 2.0 eV. It is shown that they are caused by optical transitions from localized states having a surface nature to quantum-size electronic states.

Optical manipulation of electron spin coherence in colloidal CdS quantum dots

We report experimental studies of optical manipulation of electron spin coherence by linearly or circularly polarized short laser pulses at room temperature, in an ensemble of colloidal CdS quantum dots. In addition to a conventional pump-probe configuration, a linearly polarized prepump pulse before the pump could significantly enhance the amplitude of pump-induced electron spin coherence, owing to the fact that prepump pulses produce more resident electrons. And a linearly or circularly polarized control pulse after the pump will efficiently suppress the spin coherence, because of the re-excitation of spin-polarized electrons to trion states by the absorption of control photons.

Formation mechanism of CdS nanoparticles with tunable luminescence via a non-ionic microemulsion route

We investigated the synthesis of CdS nanoparticles via an optimized water-in-oil microemulsion route that used the non-ionic surfactant-based system H2O–n-octane–Brij30/1-octanol. For that purpose, a microemulsion that contained Cd(II) ions (μe1) and another microemulsion that contained S2− ions (μe2) were combined. To investigate the ways in which the non-ionic microemulsion characteristics controlled the size and emission properties of colloidal CdS quantum dots, μe1 and μe2 with tunable and robust similar structure were prepared. This requirement was fulfilled by matching the water emulsification failure boundary (wefb) of the two microemulsions and carrying out synthesis along this boundary. Dynamic light scattering and fluorescence probe techniques were used to investigate the size and interfacial organization of the microemulsion water droplets, and the CdS nanoparticles were characterized by UV–Vis and static fluorescence spectrometry, TEM and HRTEM. Nanoparticles of diameter 4.5–5.5 nm exhibiting enhanced band edge emission were produced by increasing the water content of the precursor microemulsions. The experimental results were combined with a Monte Carlo simulation approach to demonstrate that growth via coagulation of seed nuclei represented the driving mechanism for the CdS nanoparticle formation in the water-in-oil microemulsion.

Growth kinetics of MPS-capped CdS quantum dots in self-assembled thin films

For this study, we prepared colloidal CdS quantum dots using 3-mercaptopropyltrimethoxysilane as capping agent. Colloidal CdS quantum dots were directly deposited on glass substrates by a spin-coating process. Coated substrates were heat-treated between 225°C and 325°C for various heat treatment time intervals to investigate the growth kinetics of the quantum dots. Results showed that sizes of the CdS quantum dots grew approximately from 2.9 to 4.6 nm, and the E1s1s energy values shifted approximately from 3.3 to 2.7 eV. Results showed that the average size of quantum dots increase by thermal treatment due to Ostwald ripening. The thermal process used to grow the size of quantum dots was examined according to the Lifshitz-Slyozov-Wagner theory. The activation energy of CdS quantum dots in thin films was calculated at approximately 44 kJ/mol.

Evidence for a Through-Space Pathway for Electron Transfer from Quantum Dots to Carboxylate-Functionalized Viologens

Ultrafast transient absorption measurements reveal that the rate constant for photoinduced electron transfer (PET) from colloidal CdS quantum dots (QDs) to alkylcarboxylate-functionalized viologens is independent of the number of methylene groups in the alkyl chain (n). The rate constant for PET is (1.2 ± 0.3) × 1010 s–1 for n = 1, 2, and 3, and for n = 0 (methylviologen). The insensitivity of the electron transfer rate constant to the length of the functional groups on the viologen suggests that a “through-space” pathway, where the electron bypasses the alkylcarboxylate and tunnels instead through only the orbitals of the QD and of the bipyridinium core, is the dominant PET pathway.

Colloidal Semiconductor Quantum Dots with Tunable Surface Composition

Colloidal CdS quantum dots (QDs) were synthesized with tunable surface composition. Surface stoichiometry was controlled by applying reactive secondary phosphine sulfide precursors in a layer-by-layer approach. The surface composition was observed to greatly affect photoluminescence properties. Band edge emission was quenched in sulfur terminated CdS QDs and fully recovered when QDs were cadmium terminated. Calculations suggest that electronic states inside the band gap arising from surface sulfur atoms could trap charges, thus inhibiting radiative recombination and facilitating nonradiative relaxation.

Long-lived, room-temperature electron spin coherence in colloidal CdS quantum dots

Time-resolved Faraday rotation spectroscopy is used to study the electron spin coherence in colloidal CdS quantum dots. Long-lived spin coherence with dephasing time T2*>3ns has been found at room temperature. Spin dynamics unaffected by the faster carrier recombination suggests the spin signal coming from the residual electrons in the dots. A small external transverse magnetic field of 50 mT can extend T2* ∼ 2 times longer compared with that in zero magnetic field. Hyperfine interaction between electron and nuclear spins limits the dephasing time in zero or low magnetic field, while for higher magnetic fields, inhomogeneous dephasing becomes to dominate the spin dynamics.

Highly efficient CdS-quantum-dot-sensitized GaAs solar cells

We demonstrate a hybrid design of traditional GaAs-based solar cell combined with colloidal CdS quantum dots. With anti-reflective feature at long wavelength and down-conversion at UV regime, the CdS quantum dot effectively enhance the overall power conversion efficiency by as high as 18.9% compared to traditional GaAs-based device. A more detailed study showed an increase of surface photoconductivity due to UV presence, and the fill factor of the solar cell can be improved accordingly.

Photoacoustic Study of CdS QDs for Application in Quantum‐Dot‐Sensitized Solar Cells

The optical properties and photovoltaic characterization of CdS quantum dots sensitized solar cells (QDSSCs) were studied. CdS QDs were prepared by the chemical solution deposition (CD) technique. Photoacoustic spectroscopy (PA) was employed to study the optical properties of the prepared samples. The sizes of the CdS QDs were estimated from transmission electron microscope (TEM) micrographs gives radii ranged from 1.57 to 1.92 nm. The current density‐voltage (J-V) characteristic curves of the assembled QDSSCs were measured. Fluorine doped Tin Oxide (FTO) substrates were coated with 20 nm‐diameter TiO2 nanoparticles (NPs). Presynthesized colloidal CdS quantum dots of different particles size were deposited on the TiO2‐coated substrates using direct adsorption (DA) method. The FTO counter electrodes were coated with platinum, while the electroelectrolyte containing redox species was sandwiched between the two electrodes. The short current density (Jsc) and efficiency (η) increases as the particle size increases. The values of Jsc increases linearly with increasing the intensities of the sun light which indicates the greater sensitivity of the assembled cells.

Simultaneous Determination of the Adsorption Constant and the Photoinduced Electron Transfer Rate for a Cds Quantum Dot–Viologen Complex

Transient absorption (TA) spectroscopy of solution-phase mixtures of colloidal CdS quantum dots (QDs) with acid-derivatized viologen molecules, N-[1-heptyl],N'-[3-carboxypropyl]-4,4'-bipyridinium dihexafluorophosphate (V(2+)), indicates electron transfer occurs from the conduction band of the QD to the LUMO of V(2+) after photoexcitation of a band-edge exciton in the QD. Analysis of the magnitude of the ground state bleach of the QD as a function of the molar ratio QD:V(2+) yields the QD-ligand adsorption constant, K(a) (4.4 × 10(4) M(-1)) for V(2+) ligands adsorbed in geometries conducive to electron transfer. The value of K(a), together with the measured rates of (i) formation of the V(+•) electron transfer product and (ii) recovery of the ground state bleach of the QD, enables determination of the intrinsic rate constant for charge separation, k(CS,int) ~ 1.7 × 10(10) s(-1), the rate for a single QD-V(2+) donor-acceptor pair. This analysis confirms previous reports that the number of ligands adsorbed to each QD is well-described by a Poisson distribution. This is the first report where the QD-ligand charge transfer and binding equilibria are quantitatively investigated simultaneously with a single technique.

Biomolecule-quantum dot systems for bioconjugation applications

In the present work, it is reported for the first time the bioconjugation of CdS quantum dots (QDs) directly with bovine serum albumin (BSA) using a one-step procedure via aqueous route at room temperature by methods of colloidal chemistry. Essentially, the bioconjugates were developed based on BSA as capping ligand for the nucleation and stabilization of CdS nanoparticles using cadmium perchlorate and thioacetamide as precursors. UV-visible spectroscopy was used to characterize the kinetics and the relative stability of CdS nanoparticles. The CdS nanocrystals were produced with the calculated average particle size below 4.0 nm, indicating they were in the so-called "quantum-size confinement range". The results have clearly indicated that BSA was effective on nucleating and stabilizing the colloidal CdS quantum dots.