Synthesis Of CdSe Quantum Dots Research Articles

Synthesis of CdSe quantum dots in PVA matrix by radiolytic methods

Polymer films of polyvinyl alcohol (PVA) containing CdSe quantum dots of various sizes from 3 nm to 5 nm have been synthesised by the radiation-chemical route using CdSO4 and Na2SeSO3 as the starting materials under a reducing condition. 2 MeV electron beam from an electron accelerator and the gamma radiation from a cobalt-60 gamma chamber were used as the irradiation sources for the syntheses. The particle sizes were small in the case of electron beam irradiation as compared to those in the case of the gamma irradiation. The sizes of the quantum dots were strongly dependent on the absorbed dose rate and the initial precursor concentrations. The CdSe quantum dots exhibit excitonic absorption at room temperature. The films containing these quantum dots were quite stable for months.

Synthesis of CdSe quantum dots with luminescence in the violet region of the solar spectrum

We have designed a simple, one-step synthesis of CdSe quantum dots with photoluminescence frequencies ranging from the red through to the violet region of the solar spectrum. The photoluminescence peaks have FWHM of 30 nm indicating absorption over a narrow range of wavelengths. The effect of solvent type and solvent boiling point on the physical and photoluminescence properties of the quantum dots has been studied. High boiling point, non-polar solvents shift the photoluminescence peak to longer wavelengths and low boiling point, polar solvents shift the photoluminescence peak to shorter wavelengths.

Exploiting Nucleation and Growth in the Synthesis and Electrical Passivation of CdSe Quantum Dots

The synthesis of CdSe quantum dots (QDs) is usually achieved through reaction of the appropriate precursors in a solution containing weakly adsorbing capping ligands. Here we compare nucleation and growth of CdSe QDs synthesized in hexadecylamine and trioctylphosphine oxide with stearic acid, hexylphosphonic phosphonic acid, or tetradecylphosphonic acid. We show that the nature of the capping ligand dramatically influences nucleation and growth, and can be used to control QD diameter and hence optical properties. We also show how electrical passivation with either ZnS or (Cd, Zn)S influences the size and optical properties of the QDs.

The synthesis of CdSe quantum dots with carboxyl group and study on their optical characteristics

AbstractQuantum dots are nanocrystal semiconductors which attract lots of research interests due to their peculiar optical properties. CdSe/ZnS quantum dots have been synthesized via pyrolysis of organometallic reagents. The color of the quantum dot changes from yellow‐green to red as their size increases with reaction time. Photoluminescence quantum efficiency of CdSe quantum dots have been enhanced by passivating the surface of CdSe quantum dots with ZnS layers. Quantum dots are nanocrystal semiconductors which attract lots of research interests due to their peculiar optical properties. CdSe/ZnS quantum dots have been synthesized via pyrolysis of organometallic reagents. The color of the quantum dot changes from yellow‐green to red as their size increases with reaction time. Photoluminescence quantum efficiency of CdSe quantum dots have been enhanced by passivating the surface of CdSe quantum dots with ZnS layers. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis of CdSe Quantum Dots by Evaporation of Bulk CdSe using SMAD and Digestive Ripening Processes

Here we report synthesis of CdSe quantum dots (QDs) by evaporation of bulk CdSe using the solvated metal atom dispersion method (SMAD), followed by reflux in toluene and in t-butyltoluene (TBT). The as-prepared SMAD product exhibits broad photoluminescence spectra (PL), but upon reflux in toluene or t-butyltoluene, interestingly, the PL spectra become narrow with an increase in fluorescence intensity. The temporal evolution of quantum dots was monitored by in situ UV−vis spectroscopy. The XRD data reveal that the formed CdSe QDs retain the wurtzite structure of the starting bulk CdSe material. The merits of this synthesis procedure are the use of bulk CdSe as starting material, possibility of scale up, elimination of high-temperature injection and size-selective precipitation processes.

CdSe Quantum Dot-Sensitized TiO2 Electrodes: Effect of Quantum Dot Coverage and Mode of Attachment

We have investigated the sensitization of nanoporous titanium dioxide by previously synthesized CdSe quantum dots (QDs) protected with trioctylphosphine. Covering the nanoporous TiO2 films with QDs has been achieved using two strategies: (i) direct adsorption from dichoromethane dispersions and (ii) anchoring the QDs through a molecular linker, concretely, mercaptopropionic acid (MPA). In contrast with MPA-mediated adsorption, direct adsorption leads to a high degree of QD aggregation, as revealed by atomic force microscopy (AFM) images obtained with both TiO2 nanoporous films and monocrystalline surfaces. Importantly, at saturation, only 14% of the real surface area of a 5-μm thick P25 TiO2 layer is covered for both attachment modes. For MPA attachment, the incident photon-to-current efficiency (IPCE) increases with the loading, whereas a maximum (close to 40% at the QD excitonic peak) is defined for intermediate coverages in the case of QD direct adsorption. In addition, for equivalent QD loading, IPCE ...

Synthesis of Cadmium Selenide Quantum Dots from a Non-Coordinating Solvent: Growth Kinetics and Particle Size Distribution

Here we report on the synthesis of CdSe quantum dots from a noncoordinating solvent. We show that nucleation and growth is very fast and is completed within 100 s. The subsequent increase in average particle size is due to diffusion limited coarsening. Growth and coarsening can be quenched by injection of dodecanethiol. Finally, we compare the size distribution obtained from analysis of the absorption edge with the size distribution obtained from analysis of transmission electron microscope images.

Optical Characteristics of CdSe Quantum Dots Depending on Growth Conditions and Surface Passivation

We investigated the effects of different growth conditions and surface passivation on the growth of CdSe quantum dots (QDs). The synthesis of CdSe QDs by pyrolysis of organometallic reagents was performed by using the hot-matrix method. In order to modify the size and quality of CdSe QDs, we controlled the growth temperature from 250 degrees C to 350 degrees C and the relative amount of trioctylphosphin (as the ligand of the Cd and Se precursors) to be injected into the coordinating solvent trioctylphosphineoxide. Moreover, an effective surface passivation of mono-disperse CdSe QDs was achieved by overcoating them with a larger band gap material, such as ZnS.

Preparation of CdSe Quantum Dots in Ionic Liquids

Quantum dots are nanocrystalline semiconductors that have potential applications across many fields ranging from electronics (LEDs, photovoltaics) to the life sciences (fluorescent tags for biological molecules). At the nano level, the band gap is tuneable because it is a function of the size of the nanoparticle so that the optical and electronic properties can be quite easily adjusted. Traditional semiconductors lack this versatility. To date, synthesis of semiconductor quantum dots such as CdSe has been reported in volatile organic solvents at high temperatures, which are costly and could be difficult to use if significant scale up operations are required in the near future for these materials. We have investigated the use of a new class of solvent, namely ionic liquids, for the synthesis of CdSe quantum dots. Ionic liquids offer a number of significant advantages over volatile organic solvents including low vapour pressure, low flammability and exceptional chemical and thermal stability. A series of phosphonium ionic liquids with different anions were examined in this work and it was found that, with non co-ordinating anions, CdSe quantum dots with identical spectroscopic properties to those produced in organic solvents could be easily synthesized.

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.

Efficient vacuum ultraviolet light frequency downconversion by thin films of CdSe quantum dots

Frequency downconversion is an effective method for the detection of ionizing radiation. We demonstrate the utilization of thin films of chemically synthesized CdSe quantum dots for this purpose. A high quantum yield of ∼0.2 is obtained at room temperature, which does not depend on the excitation wavelength in the range from 32 to 114nm. The measured decay time is short, in the nanosecond regime, and the visible light output is stable. A significant advantage of such semiconductor quantum dots is the potential to tune the emission wavelength via selection of the nanoparticle size.

Synthesis and Properties of Water-Soluble Core–Shell–Shell Silica–CdSe/CdS–Silica Nanoparticles

This paper describes the synthesis of highly water-soluble and fluorescent core-shell-shell silica-CdSe/CdS-silica nanoparticles (CSS silica-QDs-silica NPs). We used cadmium nitrate and 1,1-dimethyl-2-selenourea precursors to synthesize CdSe quantum dots (QDs) in aqueous solution under simultaneous illumination with a diode-pumped solid state green laser and a Xe-Hg lamp. After passivation of the CdSe QDs with CdS, the CdSe/CdS QDs were then conjugated covalently to (3-mercaptopropyl)trimethoxysilane (MPS); we call these nanoparticles "MPS-QDs". We mixed the MPS-QDs with tetraethoxysilane (TEOS), ethanol, and NH3. By controlling the concentrations of the reagents, the stirring speed, and the reaction time, we synthesized CSS silica-QDs-silica NPs having sizes ranging from 75 to 190 nm. The incubation time for preparing the MPS-QDs and their concentrations are important parameters in determining the morphologies of the CSS silica-QDs-silica NPs. When we mixed 50 nM MPS-QDs, 1.1 mM TEOS, and 78 mM NH3 and reacted them at a stirring speed of 750 rpm, we obtained 85-nm-diameter CSS silica-QDs-silica NPs having a QD shell thickness of about 20 nm. The CSS silica-QDs-silica NPs provide a strong photoluminescence intensity (quantum yield 88%) and exhibit enhanced stability both photochemically and in high-conductivity media (e.g., 1.0 M NaCl).

Facile Sonochemical Synthesis of Highly Luminescent ZnS−Shelled CdSe Quantum Dots

The sonochemically driven synthesis of CdSe quantum dots (QDs) and their subsequent sonochemical ZnS shelling is described. By adapting the use of Cd(OAc)2, TOPO, and hexadecylamine to an ultrasound-driven synthesis and by applying a subsequent sonochemical ZnS shelling procedure to CdSe QD cores using Zn-ethylxanthate, highly luminescent QDs with quantum yields of 50% to 60%, narrow emission spectra (fwhm ∼25 nm), and size distributions of ∼10% were obtained. Furthermore, QD synthesis via ultrasound has several attractive features, such as a straightforward process control in the absence of any outside heat source and the ability to achieve nucleation and growth at bulk temperatures notably lower than those required for thermal techniques. The approach presented herein is accessible to laboratories with limited synthetic expertise to create CdSe/ZnS QDs.

Confined Synthesis of CdSe Quantum Dots and Characterization

Nanoclusters of II‐VI compound semiconductor CdSe were synthesized within the mesopores of MCM‐41 sodium aluminium silicate framework material following an ion exchange route after suitably protecting the outer walls of the pore. The integrity of the framework after formation of the nanocomposite was confirmed by XRD. XRD and elemental analysis revealed the formation of the clusters. The absorption and photoluminescence spectroscopy of the composite yields results consistent with the blue shift expected as a consequence of quantum confinement in these nanoclusters. The Raman spectrum of the CdSe‐MCM composite shows evidences of phonon confinement.

Biomimetic synthesis of CdSe quantum dots through emulsion liquid membrane system of gas‐liquid transport

AbstractThe cadmium selenide quantum dots (QD) have been synthesized by template‐control in an emulsion liquid membrane system. The system consisted of kerosene as solvent, L152 (dialkylene succinimide) as surfactant, N7301 (trialiphatic amine, Ra, R=C8‐C10) as carrier, 0.1 mol/L CdCl2 solution as internal‐aqueous phase and H2Se gas as external phase. Additive organic template agent in internal‐aqueous phase was necessary to fom CdSe QD. The influence of the nature of template and its concentration on sizes of the formed CdSe QD has also been studied. Transmission electron microscopy showed that the sizes of the products could be controlled down to 3‐4 nm. X‐ray diffraction analysis revealed that the crystals had cubic structure. The formation process and the optical properties of CdSe QD have also been presented.

Spin spectroscopy of dark excitons in CdSe quantum dots to 60 T

Polarization-resolved magnetophotoluminescence spectroscopy is used to study exciton spin states in 40--80 \AA{} diameter chemically synthesized CdSe quantum dots (QDs) at temperatures $T=1.2--50\mathrm{K}.$ The spin polarization is found not to saturate in magnetic fields to 60 T and time-resolved studies indicate a thermal population of exciton states. A simple model incorporating the angle-dependent Zeeman splitting and bright-dark level mixing in these randomly oriented quantum dots is constructed in quantitative agreement with the data. Fits using this model yield a dark exciton g factor of \ensuremath{\sim}0.9 at $T=1.45\mathrm{K},$ which is independent of QD diameter and exhibits a surprising increase with increasing temperature.

Spin coherence in semiconductor quantum dots

Femtosecond-resolved Faraday rotation is used to probe spin dynamics in chemically synthesized CdSe quantum dots 22--80 \AA{} in diameter from T=6--282 K. The precession of optically injected spins in a transverse magnetic field indicates that the measured relaxation lifetime of the spin polarization is dominated by inhomogeneous dephasing, ranging from \ensuremath{\sim}3 ns at zero field to 100 ps at 4 T. Fourier analysis reveals a multiperiodic Larmor precession, with several distinct g factors ranging from \ensuremath{\sim}1.1 to 1.7.

Synthesis and analysis CdSe Quantum dot with a Microfluidic Reactor Using a Combinatorial Synthesis System

<p>A microfluidic reactor with computer-controlled programmable isocratic pumps and online detectors is employed as a combinatorial synthesis system to synthesize and analyze materials for fabricating CdSe quantum dots for various applications. Four reaction condition parameters, namely, the reaction temperature, reaction time, Cd/Se compositional ratio, and precursor concentration, are combined in synthesis condition sets, and the size of the synthesized CdSe quantum dots is determined for each condition. The average time corresponding to each reaction condition for obtaining the ultraviolet–visible absorbance and photoluminescence spectra is approximately 10 min. Using the data from the combinatorial synthesis system, the effects of the reaction conditions on the synthesized CdSe quantum dots are determined. Further, the data is used to determine the relationships between the reaction conditions and the CdSe particle size. This method should aid in determining and selecting the optimal conditions for synthesizing nanoparticles for diverse applications.</p>