Colloidal CdS Quantum Dots Research Articles

Modeling Temperature-Dependent Photoluminescence Dynamics of Colloidal CdS Quantum Dots Using Long Short-Term Memory (LSTM) Networks

This study addresses the challenge of modeling temperature-dependent photoluminescence (PL) in CdS colloidal quantum dots (QD), where PL properties fluctuate with temperature, complicating traditional modeling approaches. The objective is to develop a predictive model capable of accurately capturing these variations using Long Short-Term Memory (LSTM) networks, which are well suited for managing temporal dependencies in time-series data. The methodology involved training the LSTM model on experimental time-series data of PL intensity and temperature. Through numerical simulation, the model’s performance was assessed. Results demonstrated that the LSTM-based model effectively predicted PL trends under different temperature conditions. This approach could be applied in optoelectronics and quantum dot-based sensors for enhanced forecasting capabilities.

Methods for Obtaining One Single Larmor Frequency, Either v1 or v2, in the Coherent Spin Dynamics of Colloidal Quantum Dots.

The coexistence of two spin components with different Larmor frequencies in colloidal CdSe and CdS quantum dots (QDs) leads to the entanglement of spin signals, complicating the analysis of dynamic processes and hampering practical applications. Here, we explored several methods, including varying the types of hole acceptors, air or anaerobic atmosphere and laser repetition rates, in order to facilitate the obtention of one single Larmor frequency in the coherent spin dynamics using time-resolved ellipticity spectroscopy at room temperature. In an air or nitrogen atmosphere, manipulating the photocharging processes by applying different types of hole acceptors, e.g., Li[Et3BH] and 1-octanethiol (OT), can lead to pure spin components with one single Larmor frequency. For as-grown QDs, low laser repetition rates favor the generation of the higher Larmor frequency spin component individually, while the lower Larmor frequency spin component can be enhanced by increasing the laser repetition rates. We hope that the explored methods can inspire further investigations of spin dynamics and related photophysical processes in colloidal nanostructures.

Charge trapped CdS quantum dot embedded polymer matrix for a high speed and low power memristor.

The data storage requirement in the digital world is increasing day by day with the advancement of the internet of things. In this respect, nonvolatile resistive random-access memory is an option that provides high density and low power data storage capabilities. In this work, zero-dimensional colloidal CdS quantum dots and a polymer composite at an appropriate ratio were used to fabricate a memristive device. Comparison with a pristine CdS quantum dot-based device reveals that a surrounding matrix around the quantum dots is needed for observing memristive behavior. The quantum dots embedded in the polymer matrix device showed extremely stable electrical switching behavior that can be operated for more than 300 cycles and 60 000 seconds. Moreover, the device needs extremely low power to operate at a very high speed. The smooth surface morphology dictates a charge trapping mechanism for the switching phenomenon; however, an interplay between different charge transport mechanisms leads to the fast switching and high on-off ratio of the device.

Dopant mediated augmentation of nanotwinning and anomalous emission behaviour

The influence of copper as a metallic dopant on the properties of CdS colloidal quantum dots (CQDs) prepared by chemical co-precipitation method is investigated here. Dopant induced change in microstructural defects type, density, twinning angle, stress generation and some unusual emission behaviour are noticed. Besides, the substitution of Cu in mixed oxidation states i.e. Cu+ and Cu2+ with dominating nature of Cu+ is confirmed by x-ray photoelectron spectroscopy (XPS) studies. The crystallite sizes observed by transmission electron microscopy (TEM) are well supported by the sizes estimated using Scherer formula and are found to be either below or comparable to Bohr excitation radius of bulk CdS. A competitive intensity variation between band edge emission and intrinsic defect assisted emission without generation/contribution of extrinsic defect sponsored emission is observed. The microstructural defects (nanotwinning angle) and related stress may be the possible sources of variation in crystallite size, crystallinity and uncommon recombination process in the CQDs.

Exciton Dynamics in Colloidal CdS Quantum Dots with Intense and Stokes Shifted Photoluminescence in a Single Decay Channel.

CdS quantum dots (QDs), synthesized by a sol-gel method, exhibit significantly Stokes shifted bright photoluminescence (PL), predominantly from the trap states. Surprisingly, the PL decay at the emission maximum is single-exponential. This is an unusual observation for as-prepared QDs and indicates a narrow distribution in the nature of trap states. A closer look reveals an additional fast component for the decays at shorter emission wavelengths, presumably due to the band edge emission, which remains elusive in the steady-state spectra. Indeed, a significantly narrower and blue-shifted emission band is observed in the decay-associated spectra. The contribution of this component to the steady-state PL intensity is shown to be overwhelmed by that of the significantly stronger trap emission. Exciton dynamics in the quantum dots is elucidated using transient absorption spectra, in which the stimulated emission is observed even at low pump power.

Plasmon-exciton nanostructures, based on CdS quantum dots with exciton and trap state luminescence

The study is devoted to the establishment of plexciton coupling manifestations in the luminescence of colloidal CdS quantum dots (3.7 nm), passivated with Oleic acid (OA) in mixtures with spherical silver nanoparticles (Ag NPs), coated with Oleylamine (OAm) with size of 10–15 nm. Mixtures of CdS QDs and Ag NPs in toluene, in which the associates formation occurs as a result of the π-staking and dipole-dipole interaction have been realized. The case of the simultaneous effect of plasmonic nanoparticles on exciton and trap state luminescence is considered. Under conditions of maximum overlap of Ag NPs extinction spectrum (415 nm) and exciton luminescence band, it was found that the exciton luminescence quenching (430 nm), accompanied by an decrease in its lifetime from 7.4 ns to 2.5 nm due to resonance energy transfer with an efficiency of 0.66 is dominant effect. At the same time, under conditions of weak plexciton coupling, caused by an insignificant overlap of Ag NPs plasmon band, an increase in the intensity of trap state luminescence (603 nm) by a factor of 1.8 with simultaneous decrease in its luminescence lifetime from 75 ns to 25 ns was observed due to the manifestation of the Purcell effect. The obtained results demonstrate the possibility of increasing the quantum yield of QDs trap state luminescence due to the Purcell effect.

Synthesis and application of colloidal CdS quantum dots as interface modification material in perovskite solar cells.

In this study, colloidal CdS quantum dots were synthesized, structurally characterized, and their effect on performance of perovskite solar cells was observed by using them as interface modification agent between TiO2/perovskite. Colloidal CdS quantum dots were synthesized based on two-phase method and characterized by X-ray diffraction and Transmission Electron Microscopy techniques. The average particle size of CdS quantum dots have found to be around 5 nm. Oleic acid was used as capping agent during synthesis to lead solubility in organic solvents. Obtained quantum dots are coated on compact TiO2 layer for surface modification. A decrease was observed when oleic acid capped CdS quantum dots were used at interface, while significant improvement was observed when ligand exchange was carried out by pyridine before perovskite layer. Reference solar cells showed 11.6% efficiency, while pyridine capped CdS modified solar cells' efficiency was 13.2%. Besides the improvement in efficiency, reproducibility of solar cells also was increased by using pyridine capped CdS as interface material.

Hyperfine-Induced Electron-Spin Dephasing in Negatively Charged Colloidal Quantum Dots: A Survey of Size Dependence.

The electron spin relaxation processes are complicated in semiconductor quantum dots. Different spin relaxation mechanisms may result in an increased or decreased spin relaxation rate with the size. The information on size-dependent spin dynamics helps to clarify and better understand the underlying spin relaxation processes. We investigate the size dependence of the electron spin dynamics in negatively photocharged CdSe and CdS colloidal quantum dots by time-resolved ellipticity spectroscopy. It is revealed that the electron spin dephasings of photodoped electron in zero or weak magnetic fields are dominated by the electron-nuclear hyperfine interaction for all measured samples. The hyperfine-induced electron spin dephasing time is ∼1-2 ns at room temperature and decreases with decreasing the size D. In addition to a size-dependent dephasing time that is directly proportional to D3/2, our measurements also show a size-independent time component, likely due to the laser-induced nuclear spin ordering.

Transient absorption spectral dynamics of hybrid associates of CdS quantum dots and methylene blue dye

The transient absorption spectra dynamics for hybrid associates of colloidal CdS quantum dots, passivated by thioglycolic acid (CdS/TGA QDs), and methylene blue cationic dye molecules (MB+) were studied by femtosecond spectroscopy. It was established that one of the main channels of relaxation of the CdS/TGA QDs excitation energy in hybrid associates based on CdS/TGA QDs and MB+ is a fast energy transfer to the reduced forms of MB+ (MBOH•, MB•) that are formed still at the synthesis stage of samples. This channel strongly competes with the resonant energy transfer from the centers of radiative recombination in quantum dots to MB+. Therefore, for hybrid associates based on QDs/TGA and methylene blue, there is no any noticeable signal of the transient absorption corresponding to the triplet state of MB+ dye, which is then known to be actively involved in charge transfer.

Photocatalytic Properties of Mn:CdS Colloidal Quantum Dots, Stabilized by Mercaptoacetic Acid

Photocatalytic Properties of Mn:CdS Colloidal Quantum Dots, Stabilized by Mercaptoacetic Acid

Photoinduced charge transfer interaction between citrus limon extract passivated colloidal CdS with Methyl Violet & Beta Vulgaris dyes

Recently, use of different plant extracts for the green synthesis of CdS nanoparticles has appealed significant attention throughout the world. The components contained in antioxidant rich citrus fruits e.g. orange and lemon have attracted significant attention due to their chemical and biological properties. To date, the the synthesis of CdS nanoparticles using citrus limon extract as a surface functionalizing agent has not been explored. In the present study a reliable green synthesis method was developed by introducing Citrus Limon Extract (CLE) as a new surface functionalizing agent for the development of colloidal CdS quantum dots (CCQDs). Photo induced interaction between CLE functionalized colloidal CdS quantum dots with Methyl Violet (MV) & Beta Vulgaris (BV) dyes have been explored. Dyes adsorption on CdS surface results in quenching of dye molecules emission intensity. UV–Visible spectroscopic studies evidently reveal idiosyncratic photo-induced charge transfer interaction of CdS QDs with Methyl Violet (MV) dye noticeably differs from its interaction with Beta Vulgaris (BV) dye. Beta vulgaris dye shows H-aggregation on CLE capped CdS QD surface. Photo-induced quenching of both the dyes attributed to electron (e-) transfer to the conduction band (CB) of CdS from the excited state of the dyes.

Temperature features of non-radiative energy transfer in hybrid associates of CdS/TGA quantum dots with methylene blue molecules

The paper presents the study results of non-radiative resonance energy transfer of electronic excitation from colloidal CdS quantum dots passivated with thioglycolic acid (CdS/TGA QDs) with an average size of 2.9 nm to methylene blue (MB+) molecules in temperature ranging from 300 to 80 K. In an aqueous MB+ and colloidal CdS/TGA QDs mixture, the increase in (MB+)2 dimers fraction and shift of the absorption band peaks of monomers (MB+) from 662 to 676 nm and dimers (MB+)2 from 606 to 590 nm were found. It is concluded that the hybrid associates of CdS/TGA+MB+ and CdS/TGA+(MB+)2 are formed. Luminescence quenching at CdS/TGA QDs band and MB+ luminescence enhancement under increase in MB+ molecules concentration were observed. A decrease in average lifetime of CdS/TGA QDs luminescence from 25 to 4 ns was found for MB+ concentration of 42 μ M. The Stern–Volmer constant, determined from the data on luminescence quenching, is 0.36 ∗ 106 M− 1. It is concluded that there is non-radiative resonance energy transfer (FRET) from the luminescence center of CdS/TGA quantum dots to MB+ molecule. Decreasing in temperature value to 80 K led to an increase in the luminescence intensity by 12 times. At the same time, the average lifetime of luminescence for pure CdS/TGA QDs increases from 25 to 72 ns. The Stern–Volmer constant increased to 0.64 ∗ 106 M− 1. It is concluded that its increase is due to an increase in average lifetime of CdS/TGA QD luminescence. In the framework of FRET theory, the rate constant of non-radiative energy transfer was estimated for an even distribution of CdS/TGA quantum dots and MB molecules in solution. Its is much smaller (6.7 ∗ 104 s− 1) than the experimentally determined value (2.5 ∗ 108 s− 1). This fact confirms the conclusion about the formation of CdS/TGA+MB hybrid system with effective FRET.

Enhancement of nonlinear optical response of methylene blue and azure a during association with colloidal CdS quantum dots

The nonlinear optical response of thiazine dyes during association with colloidal CdS quantum dots (QDs) applying the z-scan technique at wavelength of 532 nm with use 10 ns pulses of Nd:YAG laser second harmonic were studied. Increasing in nonlinear absorption intensity in hybrid associates in comparison with the initial components were found. Moreover, UV–vis absorption spectrum shows the effective formation of dyes H-aggregates during association with colloidal QDs. Two-photon mechanism of observed nonlinear absorption in hybrid associates is proposed, based on a sharper dependence of the normalized transmittance on the incident intensity (distance between the sample and focal plane of focusing lens). And for pure solutions, the reverse saturated absorption is preferred nonlinear absorption mechanism. The nonlinear absorption coefficients for MB and AzA in the hybrid associates were found. They are β ≈ 8.4 × 10−9 cm W-1 и ≈ 1.34 × 10-8 cm W-1, respectively. It was shown that it is possible to control the parameters of nonlinear absorption of thiazine dyes via the hybrid association.

Fragmentation of Magic-Size Cluster Precursor Compounds into Ultrasmall CdS Quantum Dots with Enhanced Particle Yield at Low Temperatures.

Colloidal small-size CdS quantum dots (QDs) are produced usually with low particle yield, together with side products such as the particular precursor compounds (PCs) of magic-size clusters (MSC). Here, we report our synthesis of small-size CdS QDs without the coexistence of the PC and thus with enhanced particle yield. For a conventional reaction of cadmium oleate (Cd(OA)2 ) and sulfur (S) in 1-octadecene (ODE), we show that after the formation of the PC in the pre-nucleation stage, the addition of tri-n-octylphosphine oxide (TOPO) facilitates the production of small-size QDs. We demonstrate that TOPO fragmentizes the PC that have formed, which enables the nucleation and growth of small-size QDs even at room temperature. Our findings introduce a new approach to making small-size QDs without the coexistence of the PC and with improved particle yield. Providing experimental evidence for the two-pathway model proposed for the pre-nucleation stage of colloidal binary QDs, the present study aids in the advance of non-classical nucleation theory.

Femtosecond dynamics of photoexcitation in hybrid systems of CdS quantum dots with methylene blue

Abstract The dynamics of electronic excitation in hybrid associates of colloidal CdS quantum dots passivated by thioglycolic acid (CdS/TGA) with methylene blue cationic dye molecules (MB + ) was studied by femtosecond transient absorption spectroscopy and nanosecond time-resolved spectroscopy. From the luminescence decays it was found that non-radiative resonance energy transfer (FRET) from the recombination luminescence centers accompanied by acceptor fluorescence is observed in the CdS/TGA-MB + systems with MB + monomers. It was found that in the exciton decay dynamics of the hybrid systems based on CdS/TGA QDs and methylene blue, there is substantial competition between the resonance energy transfer in MB + and the energy transfer to MB • and MBOH • forms.

Particle size enlargement and 6-fold fluorescence enhancement of colloidal CdS quantum dots induced by selenious acid

Herein, colloidal CdS QDs have been synthesized by using cysteine as a stabilizing agent. The interaction between the CdS QDs and selenious acid was monitored by using UV-visible, fluorescence and Fourier transform infrared (FTIR) spectroscopy. The onset of absorption of the CdS QDs (430 nm) was progressively red-shifted upon increase in the concentration of selenious acid at pH 10. It indicated enlargement of the particle size which was confirmed by the dynamic light scattering (DLS) measurements. Interestingly, the addition of 100 μM selenious acid at pH 6 resulted in a 6-fold enhancement of the red emission (λmax = 617 nm) of the CdS QDs. The particle size enlargement of CdS was due to an electrostatic interaction between selenious acid and QD stabilizer cysteine. The 6-fold fluorescence enhancement was of the CdS QDs was explained on the basis of hydrogen-bonding interaction between selenious acid and cysteine. The fluorescence-based method was applied for the sensing of selenious acid at pH 6.

Selective Photocatalytic Oxidation of Benzyl Alcohol to Benzaldehyde or C–C Coupled Products by Visible-Light-Absorbing Quantum Dots

This Forum Article describes the photocatalytic oxidation of benzyl alcohol by visible-light-absorbing colloidal CdS quantum dots (QDs), with 99% selectivity for benzaldehyde or 91% selectivity for...

Size-Dependent Optical Properties of Colloidal CdS Quantum Dots Passivated by Thioglycolic Acid

The results of studies of the optical properties of ensembles of colloidal CdS quantum dots passivated by thioglycolic acid are reported. The average dimensions of the quantum dots lie in the range from 1.7 to 5.8 nm. By X-ray diffraction studies and Raman spectroscopy, it is established that quantum dots crystallize with the formation of a cubic crystal lattice. The systematic features of the quantum-confinement effect in the optical absorption and luminescence spectra, as well as in the luminescence excitation spectra, are established. The features manifest themselves as a blue shift of the corresponding bands with decreasing quantum-dot dimensions. From analysis of the nanosecond luminescence kinetics, it is concluded that experimentally observed radiative recombination proceeds by the donor–acceptor mechanism.

A facile synthesis route for preparing aqueous colloidal CdS quantum dots with size-tunable optical properties

A simple one-pot route for the chemical synthesis of CdS colloidal quantum dots in aqueous medium was developed. In this wet chemical preparation, the anionic surfactant SDS (sodium dodecylsulfate) was used as the organic stabilizer added to the solution of chemical precursors containing the appropriate sources of the metallic cation (Cd2+) and the chalcogenide anion (S2-). The optical characterization was performed by means of ultraviolet-visible absorption and photoluminescence spectroscopy. Particle size distributions were obtained from the statistical analysis of atomic force microscopy (AFM) images generated for several quantum dot samples. The maximum of each AFM histogram was correlated to the size determined from absorption onset measurements combined with a well-established effective mass model for the size-dependent bandgap of spherical semiconductor quantum dots. An alternative theoretical model capable of providing improved size estimates was also proposed and implemented. From a simple experimental protocol based on the variation of the chalcogenide precursor concentration in the reaction medium, CdS quantum dots with size-tunable optical absorption and emission were synthesized.

Photoinduced Degradation of the Optical Properties of Colloidal Ag2S and CdS Quantum Dots Passivated by Thioglycolic Acid

The results of studying degradation of the optical properties of colloidal Ag2S and CdS quantum dots (QDs) 2.6–3.2 nm in size passivated by thioglycolic acid (TGA) are presented. The photoluminescence intensity of colloidal Ag2S QDs has been found to decrease under laser irradiation at a wavelength of 445 nm, beginning with the effective power of 10 mW. The observed effect is interpreted as a photochemical reaction of formation of new nonradiative-recombination channels in Ag2S QDs upon excitation. It is established for colloidal CdS QDs passivated by TGA that a decrease in the optical density in the entire absorption spectrum and the luminescence intensity is accompanied by precipitation of the colloidal particles in a cell and related to photodegradation of the passivating shell.