Synthesized CdSe Nanocrystals Research Articles

Growth Kinetics and Optical Proprties of CdSe Nanocrystals

We were synthesized CdSe nanocrystals (NC) at a temperature of 260 oC and controlling their growth kinetics follows different reaction times from 1 minute to 180 minutes. Base on their optical properties and estimated size to track their growth kinetics follow the Lamer model. Structure and morphology characterized were investigated by XRD pattern and transmission electron microscopy (TEM). All samples show cubic zinc blende type structures. The average NCs size can be calculated by absorption spectra, XRD and TEM, these results lie in close vicinity with each other. The energies of photoluminescence (PL) peaks and band gap of CdSe NCs can be tuned within the regions of 1.99 eV to 2.13 eV and 1.92 eV to 2.12 eV with increasing reaction time. The Stokes shifts and PL emission peaks are narrow, thus confirming the formation of uniformly distributed NCs.

On the size-dependence of CdSe nanocrystals for photon upconversion with anthracene.

In triplet-triplet annihilation based photon upconversion, controlling triplet energy transfer (TET) through the system is key to unlocking higher efficiencies. In this work, we vary the size of colloidally synthesized CdSe nanocrystals (NCs) to examine the effects on TET during photon upconversion, using steady-state measurements and transient absorption spectroscopy. As the CdSe NC size increases, the photon upconversion quantum yield (QY) decreases due to the decrease in the rate of TET from CdSe to the surface bound anthracene transmitter ligand, as expected for the Marcus description of energy transfer from the transmitter to the NC. Long microsecond transmitter lifetimes are critical to high photon upconversion QYs.

Opto-electronics properties of starch capped CdSe nanoparticles

The nanoparticles of a prominent luminescent material, cadmium selenide (CdSe) have been prepared by simple wet chemical route using starch as capping agent. The X-ray diffraction of the synthesized CdSe nanocrystals exhibits hexagonal phase with average crystallite size decreasing with capping agent concentration. Surface morphology of CdSe nanoparticles has been studied using scanning electron microscopy. The UV–Vis absorption shows blue shift in absorption edge indicating increased band gap with decreasing the particle size due to quantum confinement. The photoluminescence and electroluminescence (EL) measurement show single peak at about 650 nm with increasing intensity for smaller nanocrystals, however emission peak is quite broad in case of EL indicating involvement of impurity states. The luminescence intensity increases because of increased oscillator strength and the better passivation of the surface defects responsible for non-radiative relaxation process. The light emission starts at lower threshold voltage when the concentration of capping agent is increased.

(Invited) Controlling Optical Properties of Semiconductor Nanocrystals: Chiral Quantum Dots and Luminescent Solar Concentrators

This talk will discuss two areas of research in utilizing semiconductor nanocrystals in nanophotonics. The first part of the talk will describe our recent work on chiral semiconductor nanocrystals. Chiral optical materials exhibit differential optical properties under right handed and left handed circularly polarized light, and may find application in metaphotonic devices, in spin-based systems, or as luminescent sensors. Chirality can be introduced into semiconductor nanocrystals via different routes, including from chiral ligands on the surface of the nanocrystal. However, the application of these materials in photonics is limited by the relatively low dissymmetry factors. Using post-synthetic ligand exchange, we synthesized a class of chiral CdSe nanocrystals with a variety of different chiral ligands bound to the surface. We first synthesized CdSe nanocrystals via non-hot injection methods, with native, achiral carboxylic acids on the surface. As synthesized, these nanocrystals do not exhibit any circular dichroism. Post-synthetic ligand exchange of the native ligands to chiral carboxylic acids results in circular dichroism spectra associated with the excitonic transitions of the CdSe nanocrystal. We examine a family of related carboxylic acid ligands, which reveal a 30-fold difference in dissymmetry factors, reaching as high as 7 x 10-4, and show how the structure of the ligands influences the resulting circular dichroism spectra. The second part of the talk will discuss the use of luminescent semiconductor nanocrystals in diffuse light concentrators for solar energy conversion. Luminescent solar concentrators consist of embedded luminescent materials inside a polymer waveguide, such that high energy light is absorbed by the luminophore and the emitted light is trapped within the waveguide and concentrated onto a solar cell. Efficient operation of these concentrators requires not only high quality luminescent materials, but also a photonic mirror to trap luminescent light within the concentrator. We have recently designed a series of mirrors that operate cooperatively with luminescent materials inside these concentrators, showing how the design of the mirrors changes with different luminophore properties. The top-surface mirror serves to trap luminescent light inside the concentrator that would otherwise couple to the escape cone. This light propagates at a steep angle, and is especially vulnerable to reabsorption losses before reaching the edge of the concentrator. By incorporating a metamirror into the LSC, this light can be steered toward the edge for improved collection. We show that this metamirror in turn lowers the required quantum yield; equivalent performance is achieved from a luminophore with 76% quantum yield as one in a standard design with 99% quantum yield. We then designed a series of different top mirrors for different specific properties of candidate luminophores, including Si and CdSe/CdS of varying shell thickness, showing how the luminophore and mirror can be designed cooperatively.

Optimization of CdSe nanocrystals synthesis with a microfluidic reactor and development of combinatorial synthesis process for industrial production

A lab scale microfluidic reactor with computer-controlled programmable isocratic pumps and online detectors was employed to syntheses CdSe nanocrystal and operational parameters for lab scale microfluidic reactor were optimized. Three reaction parameters, viz. the reaction time, the [Cd]/[Se] molar concentration ratio, and the precursor concentrations, were optimized for optical characteristics, the particle diameter, and product yield. The average time corresponding to each reaction condition for obtaining the ultraviolet–visible (UV–vis) absorbance and photoluminescence spectra was approximately 10min. Based on lab scale microfluidic reactor a combinatorial reactor for synthesis CdSe on commercial interest has been redesigned and reproducibility of the CdSe synthesis, diameter size, and product yield were verified at the obtained condition from the lab scale synthesis. Using the data from the combinatorial synthesis system, the effects of the reaction conditions on the synthesized CdSe nanocrystals and their yield were elucidated. Further, the data were used to determine the relationships between the reaction conditions, the CdSe particle diameter, and the product yield. Advantages of reported synthesis are, unlikely, other reported processes instead of batch scale operation, the combinatorial synthesis system can be used for continuous production, if required. Through the process, controlling the precursor concentration, flow rate, and other physical and instrumental parameter, desired sized NCs can be synthesized precisely, which is a remarkable aspect of the combinatorial synthesis reported here. Depending upon the requirement, automation can be done for preciseness, reproducibility and mass production of the NCs.

Synthesis and Characterization of Cadmium Selenide Nanoparticles for Optoelectronic Application

In this communication, we illustrate an effortless, convenient and less hazardous route for the synthesis of good quality cadmium selenide nanoparticles by thermolysis of a novel single source precursor, CdCl2[Se{(CH2)2NH2}2] under nitrogen atmosphere at 330°C. The synthesized nanomaterials have been characterised by X-ray powder diffractometer (XRD), ultraviolet–visible (UV–Vis) spectrophotometer, transmission electron microscope (TEM) and scanning electron microscope (SEM). The several X-ray diffracted peaks of synthesized CdSe nanocrystals revealed the uniqueness of Hexagonal (Wurtzite) CdSe with an average particle sizeof 76nm. Electronicspectrum showswide energyband gap of2.94eV.

Pyrolytic synthesis of densely packed grown CdSe nanoparticles using a dual function octylamine for hybrid solar cells

We report here a well crystallized wurtzite phased CdSe nanocrystals by utilizing only octylamine ligand without the use of standard surfactants (TOPO, Fatty acids, Long chain alkyalmines, ODE) by a pyrolytic synthesis at 270°C. The synthesized CdSe nanocrystals possess a diameter of ~6nm with a band gap of 1.85eV. Hybrid P3HT:CdSe heterojunction solar cells have been fabricated from the blends of the synthesized CdSe nanocrystals and poly (3-hexylthiophene) (P3HT) in a chloroform solvent which shows a maximum power conversion efficiency of 1.41%. However, standard CdSe quantum dot nanocrystals show a low conversion efficiency of 0.08% due to presence of a thick ligand shell. The enhanced efficiency can be attributed to the surface narrow size distribution (thin shell) and better optical properties; additionally it does not require post synthetic pyridine treatment. The details of morphological and optical properties of samples have been analyzed by TEM, XRD, UV–vis, PL, J–V and EQE techniques.

Comparative study on coating CdSe nanocrystals with surfactants

We have synthesized CdSe nanocrystals (NCs) in sizes from 2.2 to 5.1 nm passivated with hydrophobic trioctylphosphine oxide (TOPO) in combination trioctylphosphine (TOP) or tributylphosphine (TBP) to obtain particles of the type CdSe/TOPO/TOP or CdSe/TOPO/TBP. These NCs were then dispersed in aqueous solution of ionic or non-ionic surfactants (such as stearate, oleic acid, Tween) using a biphase (water and chloroform or hexane) transfer method. It is found that both the structure of the surfactant and the native surface of the ligand govern the coating of the NCs with surfactants. More specifically, the hydrophobicity-hydrophilicity balance of the surfactant regulates the coating efficacy, thereby transferring the NC from the organic to the aqueous phase. The type of ligand on the NCs and the kind of coating surfactant also affect photoluminescence (PL). The ratio of PL and absorbance unit (defined as PL per 0.1 AU) was implemented as a tool to monitor changes in PL intensity and wavelength as a function of size, coatings and surface defects. Finally, the distribution of CdSe nanocrystals between pseudophases in cloud point extraction was discussed based on experimental results. It was concluded that the size of CdSe nanocrystal present in an appropriate pseudophase is correlated with the way in which the non-ionic surfactant coats CdSe nanocrystals. FigureCoating of CdSe semiconductor nanocrystals with surfactants impacts nanocrystals’ spectral features. Absorbance of first exciton absorption band was used to estimate ability of surfactant to disperse CdSe nanocrystals. Photoluminescence (PL) intensity and position of PL band were analysed in terms of nanocrystal’s surface phenomena via surfactants applied for coating. Electronic supplementary materialThe online version of this article (doi:10.1007/s00604-013-1062-z) contains supplementary material, which is available to authorized users.

Synthesis and characterization of CdSe nanocrystals in the presence of butylamine as a capping agent

TOPO-capped cadmium selenide (CdSe) nanocrystals of sizes between 3 and 8 nm have been synthesized, and the surface-capping molecule, trioctylphosphine oxide, was replaced by butylamine. The effects of changing the surface ligands of the synthesized CdSe nanocrystals on the structural, optical, and electrical properties were investi- gated. The shift toward shorter wavelength (higher energy) in the visible range of the optical absorption band edge was observed by UV-Vis spectroscopy, and a blue-shift of the photoluminescence peaks was observed with luminescent quenching. Surface modification was found to cause an increase in the surface energy of nanocrystals, resulting in the improvement in charge carrier separation and cell performance in applications towards bulk hetero-junction solar cells.

Highly luminescent polystyrene embedded CdSe quantum dots obtained through a modified colloidal synthesis route

The present work describes a modified colloidal synthesis route for the preparation of CdSe nanocrystals, which were further embedded in a polystyrene matrix. Used as a reaction medium, glycerin makes it possible to obtain nanocrystals at lower temperatures in all process stages and an easier separation from the reaction mass. The influence of two different cadmium compounds used as primary cadmium sources over the quality of synthesized CdSe nanocrystals is established. Having a double role, namely in the formation of a cadmium precursor and that of a surface capping agent, three saturated monocarboxylic fatty acids are tested. The synthesis path is refined by evaluating the luminescent properties of CdSe nanocrystals resulting from each test. The obtained CdSe nanocrystals were further embedded in a polystyrene matrix, resulting in a highly luminescent composite. The solution-processed highly luminescent composite might be used as a conversion layer in various optoelectronic devices.

Characterization of CdSe nanocrystals coated with amphiphiles. A capillary electrophoresis study

We have synthesized CdSe nanocrystals (NCs) possessing a trioctylphosphine surface passivation layer and modified with amphiphilic molecules to form a surface bilayer. The NCs covered with single amphiphiles are not stable in aqueous solution, but a mixed amphiphilic system is shown to provide stability in solution over several months. The solutions of the modified NCs were characterized by UV-Vis absorbance, photoluminescence, and transmission electron microscopy. An electrophoretic study revealed two operational modes. The first relies on the enrichment of NCs using a micellar plug as a tool. The accumulation of NCs at the plug-electrolyte buffer interface results in a sharp peak. By controlling the electrophoretic conditions, nanocrystals were forced to exit a micellar plug into an electrolyte buffer. We conclude that a system consisting of modified nanocrystals and a micellar plug can act as a mixed pseudomicellar system, where modified nanocrystals play the role of pseudomicelles. FigureElectrophoretic focusing of amphiphile coated CdSe nanocrystals using a micellar plug. Electronic supplementary materialThe online version of this article (doi:10.1007/s00604-011-0727-8) contains supplementary material, which is available to authorized users.

Investigation on the phosphine-free synthesis of CdSe nanocrystals by cadmium precursor injection

A green route for the synthesis of high quality zinc blende CdSe nanocrystals has been successfully established by using phosphine-free Se precursors combined with a cadmium precursor injection method. The effects of using different Se precursors, and adjustment of the molar ratios between the Cd and Se precursors have been investigated in detail to optimize the synthesis conditions. Absorption and photoluminescence (PL) spectra, X-ray diffraction (XRD), and transmission electron microscopy (TEM) were used for the characterization of synthesized CdSe nanocrystals. High brightness with the photoluminescence quantum yields up to 50% and narrow size-distribution with the full width at half-maximum (FWHM) below 24 nm were obtained under the optimized conditions. More than 5 g of high quality CdSe nanocrystals have been synthesized in one large scale reaction.

High quality synthesis of monodisperse zinc-blende CdSe and CdSe/ZnS nanocrystals with a phosphine-free method

Highly monodisperse zinc blende CdSe nanocrystals have been synthesized by using different phosphine-free Se precursors successfully. To understand the reaction mechanism and obtain high quality CdSe nanocrystals, the effects on the use of different Se and/or Cd precursors, the adjustment of the molar ratios between Cd and Se precursors, and the selection of suitable reaction and growth temperatures have been studied in details. Absorption spectrum, fluorescence spectrum, X-ray diffraction (XRD), and transmission electron microscopy (TEM) were used for the characterization of synthesized CdSe nanocrystals. The quality of as-prepared CdSe nanocrystals was reached the same high level compared with the method using phosphine selenium precursors, its quantum yields were among 30 to 60% and photoluminescence (PL) full width at half-maximum (FWHM) was well controlled between 22 and 28 nm. As core, such zinc blende CdSe nanocrystals were also used to synthesize CdSe/ZnS, CdSe/CdS, and CdSe/CdS/ZnS core-shell nanocrystals. The quantum yields of as-prepared core-shell nanocrystals were among 50 to 80%. Large-scale syntheses of such core-shell nanocrystals have been successfully demonstrated and as many as 3 g of high quality CdSe/ZnS nanocrystals were easily synthesized with the use of only low-cost, green, and environmentally friendlier reagents.

High Temperature Preparation of Core and Core/Shell Composite Nanocrystals in a Multiphase Microreactor

Microreactors can be used for continuous operation and precise control of heating conditions in chemical synthesis. Therefore, microreactors are expected to offer a more efficient strategy for obtaining homogeneous nanocrystals and easier of size control than the general batch reaction system. CdSe nanocrystals can be turned into emission color by controlling the size of nanocrystals. In this study, we synthesized CdSe nanocrystals by a multiphase microreactor. The CdSe nanocrystals were coated with ZnSe and ZnS by using the multiphase microreactor. The nanocrystals are coated on their surface with a shell of a larger band gap semiconductor, which can improve luminescence efficiency. Furthermore, we investigated the residence time in relation to size of nanocrystals and optimized the coating layer thickness.

Synthesis of CdSe Nanocrystals in a Noncoordinating Solvent: Effect of Reaction Temperature on Size and Optical Properties

Colloidal synthesis of high quality CdSe nanocrystals with controllable size and tunable properties have been one of the most important topics of research over the last decade, in view of its huge technological potentials. CdSe is one of the most studied nanocrystals of this category because of its photoluminescence tunability across the visible spectrum. We have synthesized CdSe nanocrystals using CdO precursor in a noncoordinating solvent and studied the effect of the reaction temperature on the size and optical properties of the nanocrystals. The size of the nanocrystals could be varied systematically in the range of 3.5 to 6.6 nm diameter with a remarkably narrow size distribution by controlling only the reaction temperature, without any need for a post-synthesis processing. The band gap and the corresponding band edge emission could be tuned across the entire visible range by tuning the size of the nanocrystals. The narrow width of the photoluminescence emissions of different colours (blue to red) make these nanocrystals a potential candidate for different optical and optoelectronic devices.

Enhancement of the photoluminescence of CdSe quantum dots during long-term UV-irradiation: privilege or fault in life science research?

The present study describes an impressive enhancement of the photoluminescence (PL) intensity of low-temperature synthesized CdSe nanocrystals (75 °C) during long-term UV-irradiation. The integrated PL-intensity of CdSe core and CdSe/ZnS core/shell nanocrystals, dispersed in chloroform, enhanced about 3 and 6 times, respectively, during 9 h exposure to UV-light, without any significant changes in the characteristic absorbance spectra and shifting of PL-spectra. After termination of the irradiation a comparatively slow photobleaching was detected with τ 1/2=6 h for CdSe core and τ 1/2=14 h for CdSe/ZnS core/shell nanocrystals. The most impressive was the effect of UV-irradiation on the photoluminescence of water-soluble CdSe nanocrystals. The integrated PL-intensity enhanced about 10 times during 11 h exposure to UV-light and the improved PL-intensity was preserved during 3 days after termination of the irradiation without any significant photobleaching. The results are discussed in the context of application of CdSe nanocrystals as novel fluorophores in life science experiments.

Multiparticle interactions and stimulated emission in chemically synthesized quantum dots

We study the effect of multiparticle interactions on optical gain and stimulated emission in close-packed solids of chemically synthesized CdSe nanocrystals (nanocrystal quantum dots). An analysis of pump-dependent nonlinear absorption signals indicates that the band-edge optical gain is due to multiparticle states with a dominant contribution from doubly excited nanocrystals (quantum-confined biexcitons). We observe that optical gain dynamics are due to the competition between ultrafast hole surface trapping and multiparticle Auger decay. We analyze the effect of intrinsic Auger recombination on optical gain lifetimes and gain pump intensity thresholds.