High-quality CdSe Research Articles

Fabrication of high-quality CdSe quantum dots for green laser diodes by molecular beam epitaxy

Fabrication of high quality CdSe quantum dots is described in this paper. The source material was CdSe compound and the amount of the source supply was fixed to 2.2 monolayers throughout the study. The orientation of GaAs substrates, the source supply rate and the growth interruption time were changed as growth parameters. Photoluminescence (PL) was measured at 14 K. The high PL intensity and the narrow line width were used as measures of high quality. Optimum growth conditions were established.

Facile and Reproducible Synthesis of Red-Emitting CdSe Nanocrystals in Amine with Long-Term Fixation of Particle Size and Size Distribution

Numerous approaches have been reported for the synthesis of CdSe nanocrystals, but there is no effective facile method for the synthesis of high-quality large, red-emitting CdSe nanocrystals. Herein we report a convenient and reproducible approach for the preparation of high-quality (high luminescent quantum yield, nearly uniform size and shape distribution) large, red-emitting CdSe nanocrystals via injection of cadmium oleate solution into oleylamine media containing Se precursor at high temperature. The obtained red-emitting CdSe nanocrystals possess an unusually long-term (∼15 h) fixation of particle size, size distribution, and high luminescent quantum yields in the high temperature annealing process. In addition, the final size of the CdSe nanocrystals obtained is heavily dependent on the amount of trioctylphosphine used, but it is insensitive to other parameters such as reaction temperature, precursor concentration, and reaction time. Through the variation of the amount of trioctylphosphine used, a large range of particle sizes and a wide variety of luminescent colors were obtained. The reported approach is suitable for large-scale production of high-quality CdSe nanocrystals, especially for large, red-emitting ones, due to its easy scale-up, high reproducibility, and low cost.

Single-Crystalline CdSe Nanostructures: from Primary Grains to Oriented Nanowires

Single-crystalline CdSe nanostructures with adjustable aspect ratios: starting from a cadmium chalcogenolate single-source precursor, we obtained high-quality CdSe nanostructures with aspect ratios in the range of 2−100. The application of subsequent wetting−crystallization−cooling cycles allowed a stepwise alteration of the aspect ratio of the single-crystalline products.

Synthesis of Deep-Red-Emitting CdSe Quantum Dots and General Non-Inverse-Square Behavior of Quantum Confinement in CdSe Quantum Dots

High-quality red-emitting CdSe quantum dots were synthesized by a microwave-assisted heating method in which the particle size was controlled mainly by changing reaction temperatures from 190 to 27...

The optimization of CdSe nanoparticles synthesis in the apoferritin cavity

AbstractCadmium selenide nanoparticles (CdSe NPs) were synthesized in the cavity of the cage‐shaped protein, apoferritin, by designing a slow chemical reaction, which employs tetraamminecadmium ion and selenourea. The ammonia water concentration and solution pH were optimized and the suppression of the formation of selenium oxide was also studied by deoxydation of the reaction solution. At the concentration of 5 mM ammonia water and at pH 9.5, with N2 gas bubbling, the core formation ratio (CFR) of CdSe NPs reached 75% and yield of protein ratio (YPR) was as high as 98%. N2 gas bubbling of reaction solution was found to be critical for the high CFR and high YPR of the CdSe NPs synthesis in apoferritin cavity. Using this optimized conditions, high quality CdSe NPs were obtained by the bio‐template, apoferritin protein method. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-Yield Fabrication and Electrochemical Characterization of Tetrapodal CdSe, CdTe, and CdSexTe1–x Nanocrystals

The high-yield fabrication of tetrapodal CdSe, CdTe, and CdSexTe1–x nanocrystals is systematically studied. CdSe nanocrystals are prepared by first controlling the synthesis of high-quality wurtzite CdSe and zinc blende CdSe nanocrystals at a relatively high temperature (260 °C) by selecting different ligands. Then, based on the phase control of the CdSe nanocrystals, two nanoparticle-tailoring routes (i.e., a seed-epitaxial route and ligand-dependent multi-injecting route) are used, and a high yield of CdSe tetrapods is obtained. CdTe nanocrystals are prepared by adjusting the ligand composition and the ratio of Cd to Te; CdTe tetrapods are synthesized in high yield using a mixed ligand that does not contain alkylphosphonic acids. Moreover, the nanoscale Te powder (Te nanowires/nanorods), which is highly soluble in the ligand solvent, is first used as a Te source to synthesize CdTe nanocrystals, which remarkably enhanced the output of the CdTe nanocrystals in one reaction. Furthermore, composition-tunable ternary CdSexTe1–x alloyed tetrapods are synthesized on a large scale, for the first time, by thermolyzing the mixture of the organometallic Cd precursor and the mixed (Se + Te) source in a mixed-ligand solution. The CdSe, CdTe, and CdSexTe1–x nanocrystals are characterized by transmission electron microscopy (TEM), high-resolution TEM, selected-area electron diffraction, X-ray diffraction, and UV-vis and photoluminescence (PL) spectroscopy. Interesting nonlinear, composition-dependent absorption and PL spectra are observed for the ternary CdSexTe1–x alloyed nanocrystals. The band-edge positions of the nanocrystals of CdSe, CdSexTe1–x, and CdTe are systematically studied by cyclic voltammetry.

Highly luminescent CdSe nanocrystalline films prepared by chemical bath deposition

We report on the preparation of high-quality CdSe nanocrystalline films with a strongly enhanced photoluminescence efficiency and carrier lifetime. This substantial improvement of the properties of the prepared films was accomplished by a modification of the chemical bath deposition preparation procedure. The characteristics of the prepared films were investigated in detail by measuring their optical properties, structural morphology and chemical composition. The changes induced by the modification of the preparation were interpreted in terms of a passivation of nonradiative decay channels in individual nanocrystals forming the films.

Colloidal CdSe Quantum Wires by Oriented Attachment

We report here a relatively low temperature (100-180 degrees C) synthetic route to high-quality and single-crystalline CdSe nanowires using air-stable and generic chemicals. The diameter of nanowires was controlled and varied in an exceptionally small size regime, between 1.5 and 6 nm. This was achieved by using alkylamines, a single type or a mixture of two different types of amines, with different chain lengths and varying the reaction temperature. The experimental results suggest the coexistence of two types of fragments in the prewire aggregates, known as "pearl-necklace" or "string-of-pearls" in the literature, which are loosely associated and chemically fused sections.

Effect of ligands and growth temperature on the growth kinetics and crystal size of colloidal CdSe nanocrystals

Using trioctylphosphine (TOP) as the ligand of the Se source and oleic acid (OA) as the ligand of the Cd precursor, high-quality CdSe nanocrystals were synthesized in the noncoordinating solvent octadecene (ODE) with the standard airless techniques. The initial TOP:Se, OA:Cd and Cd:Se molar ratios are determinative factors for the growth kinetics, crystal size and size distribution of the as-synthesized CdSe nanocrystals. A large excess of OA, TOP and Se, were found necessary to achieve a narrow absorption profile. By simply changing the amount of the two ligands or the growth temperature, we are able to not only control the balance of the initial nucleation and the subsequent growth of CdSe nanocrystals, but also tune the crystal size and size distribution.

Water-soluble CdSe and CdSe/CdS nanocrystals: A greener synthetic route

We report a new green synthetic route of CdSe and core–shell CdSe/CdS nanoparticles (NPs) in aqueous solutions. This route is performed under water-bath temperature, using Se powder as a selenium source to prepare CdSe NPs, and H 2S generated by the reaction of Na 2S H 2SO 4 as a sulfur source to synthesize core–shell CdSe/CdS NPs at 25–35 °C. The synthesis time of every step is only 20 min. After illumination with ambient natural light, photoluminescence (PL) intensities of CdSe NPs enhanced up to 100 times. The core–shell CdSe/CdS NPs have stronger photoactive luminescence with quantum yields over 20%. The obtained CdSe NPs exhibit a favorable narrow PL band (FWHM: 50–37 nm) with increasing molar ratio of Cd/Se from 4:1 to 10:1 at pH 9.1 in the crude solution, whereas PL band of corresponding CdSe/CdS NPs is slightly narrower. The emission maxima of nanocrystals can be tuned in a wider range from 492 to 592 nm in water by changing synthesis temperature of CdSe core than those reported previously. The resulting new route is of particular interest as it uses readily-available reagents and simple equipment to synthesize high-quality water-soluble CdSe and CdSe/CdS nanocrystals.

Growth, structural and optical studies of CdSe/ZnSe nanostructures grown by MBE on GaAs and Si substrates

Structural (XRD and HRTEM) and optical properties of self-assembled CdSe quantum dots with ZnSe barriers have been grown by molecular beam epitaxy on GaAs and Si/Ge substrates. The results of XRD and HRTEM measurements showed that high-quality CdSe QD arrays embedded in ZnSe barriers could be grown on Si/Ge virtual substrates. It has been shown, that QD structures grown on different substrates exhibit comparable high luminescence efficiency at low temperatures. Two peaks were observed in cathodoluminescence (CL) spectra of some samples grown on Si/Ge and GaAs substrates. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A New Class of Capping Ligands for CdSe Nanocrystal Synthesis

A new method is described for the synthesis of high-quality CdSe nanocrystals (NCs) that eliminates the need for tri-octylphosphine oxide (TOPO). This new method is an adaptation of previously reported methods that use Cd salts as precursors for the CdSe NCs. High-boiling esters and ketones such as hexadecyl hexadecanoate (HH) and benzophenone (BP) appear to be excellent replacements for TOPO in this reaction. Compared with TOPO, HH offers a significant synthetic advantage as it slows the initial growth of the NC nuclei formed after injection of the Se precursor. This allows for superior control over particle size in the initial stages of the reaction, and thus permits facile production of high-quality CdSe NCs with very small diameters (<4 nm). Based upon the results presented here, a possible role is identified for TOPO in the CdSe NC synthesis. Understanding of the role of TOPO and other reaction components is crucial for continued progress in the synthesis of high-quality NCs.

Phosphine-Free Synthesis of CdSe Nanocrystals

High quality CdSe nanocrystals have been prepared using elemental selenium as the chalcogenide precursor dispersed in 1-octadecene (ODE). The conditions used to prepare the Se precursor were found to be critical for successful nanocrystal synthesis. Systematic titration of the Se precursor solution with tri-n-octylphosphine (TOP) allowed the Se reactivity to be tuned and the final particle size to be controlled. Band-edge and surface related emission were observed for samples prepared in the presence and absence of added TOP. In the absence of a selenium passivant, the crystal structure of CdSe nanocrystals could be altered from zinc blende to wurtzite by the addition of bis(2,2,4-trimethylpentyl)phosphinic acid (TMPPA).

High-Temperature Microfluidic Synthesis of CdSe Nanocrystals in Nanoliter Droplets

The high-temperature synthesis of CdSe nanocrystals in nanoliter-volume droplets flowing in a perfluorinated carrier fluid through a microfabricated reactor is presented. A flow-focusing nanojet structure with a step increase in channel height reproducibly generated octadecene droplets in Fomblin Y 06/6 perfluorinated polyether at capillary numbers up to 0.81 and with a droplet:carrier fluid viscosity ratio of 0.035. Cadmium and selenium precursors flowing in octadecene droplets through a high-temperature (240-300 degrees C) glass microreactor produced high-quality CdSe nanocrystals, as verified by optical spectroscopy and transmission electron microscopy. Isolating the reaction solution in droplets prevented particle deposition and hydrodynamic dispersion, allowing the reproducible synthesis of nanocrystals at three different temperatures and four different residence times in the span of 4 h. Our synthesis of a wide range of nanocrystals at high temperatures, high capillary numbers, and low viscosity ratio illustrates the general utility of droplet-based microfluidic reactors to encapsulate nanoliter volumes of organic or aqueous solutions and to precisely control chemical or biochemical reactions.

Synthesis of size-controlled CdSe quantum dots and characterization of CdSe–conjugated polymer blends for hybrid solar cells

High-quality CdSe quantum dots (Qdots) were produced using the wet chemical synthetic method. These size-controlled CdSe Qdots were characterized by UV–vis absorption spectroscopy, photoluminescence (PL) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The XRD patterns and TEM images showed that the CdSe Qdots had a wurtzite crystalline structure. Bandgap-tuning CdSe Qdots were used to fabricate solar cells. We report the extensive experimental results of the CdSe Qdots–conjugated polymer composites used for the fabrication of the hybrid solar cells, by blending them between CdSe Qdots–poly (3-hexylthiophene) (P3HT) and CdSe Qdots/[poly(1-methoxy-4-(2-ethylhexyloxy-2,5-phenylenevinylene))] (MEH-PPV) polymer.

New Organometallic Approach to Synthesize High-quality CdSe Quantum Dots

Abstract High-quality CdSe quantum dots have been synthesized through an organometallic method with cadmium methoxide as a cadmium precursor. Without any size sorting, the FWHM (full width at half-maximum) in PL spectra almost for all CdSe samples is less than 30 nm and only 22 nm from the best sample. The emission peaks with different particle size in toluene can be tuned from 478 to 654 nm corresponding to a color change from blue-green to red.

A New Route to Zinc-Blende CdSe Nanocrystals: Mechanism and Synthesis

We report the possible mechanism of forming of CdSe nanocrystals in the high boiling point solvents with long alkane chains and a novel Non-TOP-Based route to zinc-blende CdSe nanocrystals. A new mechanism shows that there exits a redox reaction in the long alkane chain solvents: Se is reduced to H2Se gas; at the same time, the long alkane chains are oxidated to alkene chains; then, the Cd complex reacts with H2Se to form CdSe nanocrystals. Possible chemical reaction equations involved in the process of forming the CdSe nanocrystals have been discussed. The alkene chain and H2Se were detected respectively by a series of experiments to support the new mechanism. Under the guidance of this mechanism, we have developed a much cheaper and greener Non-TOP-Based route for the synthesis of a size series of high-quality zinc-blende (cubic) CdSe nanocrystals. Low-cost, green, and environmentally friendlier reagents are used, without use of expensive solvents such as trioctylphosphine (TOP) or tributylphosphine (TBP). The new route enables us to achieve high-quality CdSe nanocrystals with sharp ultraviolet and visible (UV-vis) absorption peaks, controllable size (2.0-5.0 nm), bright photoluminescence (PL), narrow PL full width of half-maximum (fwhm) (29-48 nm), and high PL quantum yield (up to 60%) without any size sorting.

The use of heat transfer fluids in the synthesis of high-quality CdSe quantum dots,core/shell quantum dots, and quantum rods

Fluorescent semiconductor nanoparticles, or quantum dots, have potential uses as anoptical material, in which the optoelectronic properties can be tuned precisely by particlesize. Advances in chemical synthesis have led to improvements in size and shape control,cost, and safety. A limiting step in large-scale production is identified to be the rawmaterials cost, in which a common synthesis solvent, octadecene, accounts for most of thematerials cost for a batch of CdSe quantum dots. Thus, less expensive solvents are needed.In this paper, we identify heat transfer fluids, a class of organic liquids commonlyused in chemical process industries to transport heat between unit operations, asalternative solvents for quantum dot synthesis. We specifically show that two heattransfer fluids can be used successfully in the synthesis of CdSe quantum dotswith uniform particle sizes. We show that the synthesis chemistry for CdSe/CdScore/shell quantum dots and CdSe quantum rods can also be performed in heattransfer fluids. With the aid of a population balance model, we interpret theeffect of different HT fluids on QD growth kinetics in terms of solvent effects, i.e.,solvent viscosity, CdSe bulk solubility in the solvent, and surface free energy.

Size-tuning and Optical Properties of High-quality CdSe Nanoparticles Synthesized from Cadmium Stearate

Abstract High-quality CdSe nanoparticles were synthesized with a hot-injection method from cadmium stearate and selenium powder in the size range of 2.0 ± 0.2 to 8.6 ± 2.3 nm, corresponding to blue to red emission. The pod-shaped nanoparticles were formed at low reaction temperature, and exhibited sharp emission peak.

Colloidal CdSe Nanocrystals from Tri-n-Octylphosphine: Part I: Growth and Optical Properties from Polar and Non-Polar Solvents

This manuscript reports the ready synthesis of high-quality colloidal CdSe nanocrystals from tri-n-octylphosphine (TOP) and reports the optical properties of the resulting CdSe dispersed in nonpolar and polar environments. With cadmium oxide (CdO) as the Cd precursor and TOPSe as the Se source, the CdSe nanocrystals were synthesized in a reaction medium consisting of TOP. The synthetic approach is as simple as: the swift injection of a TOPSe/TOP solution into a CdO/TOP solution at one particular (high) temperature, with subsequent growth at a lower temperature. The temporal evolution of the optical properties, including absorption and luminescence, were monitored in detail. The photoluminescence (PL) properties of the TOP-capped CdSe investigated comprise the intensity, emission peak wavelength, full width at half maximum of the PL spectra, and stability; these PL properties are sensitive to the history of growth as well as the environment of dispersion. An appropriate parameter, termed "Sensitivity Index (SI)," is proposed to define the percent difference in the PL intensity that arises when the dots are dispersed in non-polar hexane (Hex) as compared to polar tetrahydrofuran (THF). The decrease in the PL efficiency on going from Hex to THF is attributed to changes in surface states rather than in the internal core as more ligands are removed in THF. Dilution experiments further confirm the suggestion of the ligand removal mechanism and provide a conceptual estimate of surface quality via the SI parameter. Due to a slow growth in size (the size of the nanocrystals is almost constant during early 20 min) and good PL stability (tested as PL changes after storage for days both in dispersed or solid states, as well as after UV-irradiation), we suggest that our synthetic approach is likely to be practical for large-scale CdSe nanocrystals with good PL stability.