具有狭窄荧光光谱的红色荧光CdSe纳米晶合成研究

采用胶体化学方法合成了红色荧光的CdSe纳米晶，具有较大的粒径和狭窄的荧光峰。其制备方法是以粒径较小的CdSe纳米晶（3-4nm）作为晶种，再通过相应前驱体的缓慢加入来得到晶种各向同性的生长，最终生成较大粒径的CdSe纳米晶，颗粒直径可达8-9nm，且荧光峰值在红光波长范围。所得的材料具有很好的单分散性和均匀的粒径分布，表现出半峰宽≤30nm的狭窄而对称的荧光光谱。     

Effect of different aqueous synthesis parameters on the size of CdSe nanocrystals

The variation of CdSe nanoparticle size as a function of synthesis conditions is presented. Cadmium sulphate (CdSO₄), cadmium chloride (CdCl₂) and sodium selenosulphate (Na₂SeSO₃) solutions were used as precursors. Nanoparticles were synthesized by aqueous chemical methods. The synthesis parameters studied were pH, Cd:Se ratio and the type of stabilizing agent. Three different stabilizing agents were used, thioglycolic acid, mercaptoethanol and poly(vinyl pyrrolidone). Fourier transform infrared spectroscopy results confirmed the presence of the stabilizing agent on the surface of the nanoparticles. Ultraviolet visible and X-ray powder diffraction measurements were used to estimate the trend of size variations of the particles with different synthesis parameters, which agreed fairly by both techniques and the crystal structure. Additionally, the size of the nanoparticles was obtained by transmission electron microscopy measurements. Whilst the effect of pH was different for each of the different stabilizing agents due to the different chemical groups in the thiol compounds and the size of the nanoparticles varied with the used stabilizing agents, the effect of Cd:Se ratio in the size of nanoparticles showed the same tendency for the several stabilizing agents.     

A mild phosphine-free synthesis of alkylamine-capped CdSe nanocrystals

A new phosphine-free approach has been developed to synthesize high-quality cadmium selenide (CdSe) nanocrystals with cubic zinc-blende structure, by using the highly reactive selenium (Se) precursor at milder temperature than that used in the traditional phosphine route. This Se precursor was obtained from the reduction of Se powder by sodium borohydride in N,N-dimetbylformamide, in the absence of phosphine. Without the addition of other long-chain coordinating substances in this approach, the alkylamines such as dodecylamine (DDA) and octylamine (OA) were used as reaction solvents, and they also acted as surface capping reagents to produce DDA-capped and OA-capped CdSe NCs, respectively. The rapid nucleation and slow growth were observed by ultraviolet-visible absorption spectrum. The resulting OA-capped CdSe NCs grew faster compared with DDA-capped CdSe NCs under the same other conditions. These as-synthesized CdSe nanocrystals showed relatively narrow size distribution and high photoluminescence quantum efficiency (up to 9.4% for OA-capped CdSe NCs). This mild approach is low cost, relatively low danger and high production yield (approximately 80%), indicating that it is very effective for the phosphine-free synthesis of alkylamine-capped CdSe nanocrystals.     

Facile synthesis of CdSe nanocrystals for bulk-heterojunction solar cells

Oleic acid (OA)-capped cadmium selenide (CdSe) nanocrystals (NCs) have been synthesized via a new high-scale route. X-ray diffraction and transmission electron microscopy confirmed that rod-like hexagonal (wurtzite) CdSe NCs with an average size of 10 nm were obtained via this new route. The obtained CdSe NCs were treated individually with pyridine and tert-butylamine (t-BA) for ligand exchange. Fourier transform infrared spectra of the as-synthesized and treated CdSe NCs confirmed the removal of OA ligands from the surface of CdSe NCs after treatments with pyridine and t-BA. Bulk-heterojunction (BHJ) solar cell devices were prepared using untreated and surface treated CdSe NCs blended with poly (3-hexylthiophene-2,5-diyl) (P3HT) polymer. BHJ solar cell devices made from P3HT:(surface treated CdSe NCs) blends showed greater improvement in photovoltaic performances compared to P3HT:(untreated CdSe NCs) blend. The improvement in photovoltaic performances was due to the increase of electron mobility in P3HT:(CdSe NCs) blends after surface treatment of CdSe NCs.     

A surfactant-free recipe for shape-controlled synthesis of CdSe nanocrystals

We described surfactant-free recipes for the synthesis of CdSe nanocrystals (NCs) with well-controlled morphologies at a relatively low temperature. Dot-, rod-, tetrapod-and sphere-shaped CdSe NCs were prepared with trioctylphosphine oxide (TOPO) as a non-equilibrium solvent and trioctylphosphine selenide (TOPSe) and cadmium carboxylates as Se and Cd precursors, respectively. It was found that the morphology and stacking pattern of the CdSe NCs were related to the preparation conditions such as the concentration of the injected TOPSe(monomer concentration), reaction temperature and chain length of the cadmium carboxylate precursors. At a reaction temperature of 240?°C, CdSe NCs with a tetrapod selectivity of up to 85% were obtained in the presence of cadmium myristate under high concentrated TOPSe injection, and the in situ-formed myristic acid supplied the best acidic ligand with optimal amount to stabilize the anisotropic growth of the tetrapods. The intentional addition of more myristic acid in the reaction system would block the growth pathway of the tetrapods. Using cadmium laurate, cadmium palmitate and cadmium stearate as the cadmium precursors would reduce the formation of the tetrapods, showing the very low selectivity of the tetrapods.     

Effects of reaction temperature on size and optical properties of CdSe nanocrystals

We report experimental results on the reaction temperature dependence of luminescence properties in size-controlled CdSe nanocrystals. Such reaction temperature dependent property is also sizedependent. The diameter of the CdSe nanocrystals is tuned from 4–11.0 nm by varying the reaction temperatures. The growth process and characterization of CdSe nanocrystals are determined by photoluminescence (PL) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, X-ray photoelectron spectrometry (XPS), X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The influence of reaction conditions on the growth of CdSe nanocrystals demonstrates that low reaction temperature is favourable for the formation of high quality CdSe nanocrystals.     

Rapid synthesis of CdSe nanocrystals in aqueous solution at room temperature

Water-soluble thioglycolic acid-capped CdSe nanocrystals (NCs) were prepared in aqueous solution at room temperature. We investigated the effects of pH values on the fluorescence intensity of the as-prepared CdSe NCs, and discussed the influence of the initial pH values on the fluorescence property. Their mean diameter was estimated to be 1.9 nm depending on the initial pH values in the preparation, the photoluminescence quantum yield could reach as high as 1.9%, almost comparable to the CdSe NCs prepared by an organometallic route. Finally, the products were characterized by Fourier transform infrared spectrometry (FTIR), atomic force microscope (AFM) and X-ray powder diffraction (XRD). AFM image showed that the NCs were ball-shaped with good dispersibility. XRD analysis disclosed that the CdSe NCs were of cubic zinc-blended structure.     

Raman scattering for the size of CdSe and CdS nanocrystals and comparison with other techniques

The sizes of the ensembles of CdSe regular nanocrystals (RNCs), CdSe magic-sized nanocrystals (MSNCs), and CdS RNCs were investigated by Raman scattering. The nanocrystal ensembles were synthesized via wet-chemistry approaches. The size distribution increases from CdSe MSNCs (2.26 nm), to CdSe RNCs (3.52 nm), and to CdS RNCs (3-8 nm and 4-10 nm). The sizes derived from Raman spectra are compared with those from other characterization tools such as UV/vis spectroscopy and transmission electron microscopy (TEM). The present study suggests that Raman scattering is an alternative and reliable technique for the determination of nanocrystal size and size distribution.     

Sequential synthesis of type II colloidal CdTe/CdSe core-shell nanocrystals

Colloidal type II CdTe/CdSe nanocrystals were synthesized by sequential addition of a tri-n-octylphosphine telluride (TOPTe)/TOP solution and several shell-precursor solutions to a CdO/TOP solution; the shell-precursor solutions consisted of CdO and TOPSe in TOP. For the growth of the CdTe core, the TOPTe/TOP solution was swiftly added to the CdO/TOP solution at a higher temperature (300 degrees C) than the growth temperature (250 degrees C). For the growth of the CdSe shell, in contrast, the CdO/TOPSe/TOP solution was slowly added to the CdTe/TOP solution at a lower temperature than the growth temperature (200-240 degrees C). The temporal evolution of the optical properties of the growing core-shell nanocrystals was monitored in detail. During the growth of the CdSe shell, the core-shell nanocrystals exhibited interesting changes in photoluminescence (PL) properties. The highest PL efficiency (approximately 38 %) was detected from core-shell nanocrystals with a CdSe shell thickness of 0.4-0.5 nm (indicated by TEM); the formation of the first monolayer is proposed. Our synthetic approach is well suited to a practical realization of engineering materials with bandgaps in the near-IR and IR spectral ranges.     

Preparation of copper-doped CdSe nanocrystals

Sols of stabilized copper-doped CdSe nanocrystals in a nonpolar high-boiling solvent have been synthesized using cadmium oleate, copper stearate, and trioctylphosphine selenide as starting reagents. The average size of the nanocrystals is 2.8–2.9 nm, with a 10% variance, as evaluated from their absorption spectra. The samples show excitonic luminescence and bright near-IR (700–900 nm) luminescence with a lifetime on the order of 0.5–1 μs. The luminescence spectroscopy data are consistent with the assumption that the copper distribution over the nanocrystals follows Poisson’s law. The average copper content of the samples is 0.1–2 atoms per nanocrystal.     

Development of flurbiprofen-loaded nanoparticles with a narrow size distribution using sucrose

Objective: A novel flurbiprofen-loaded nanoemulsion which gave uniform emulsion droplets with a narrow size distribution was previously reported to be prepared using membrane emulsification method. The purpose of this study is to develop a novel flurbiprofen-loaded nanoparticle with a narrow size distribution and improved bioavailability.

Method: The nanoparticle was prepared by solidifying nanoemulsion using sucrose as a carrier via spray drying method. Its physicochemical properties were investigated using SEM, DSC and PXRD. Furthermore, dissolution and bioavailability in rats were evaluated compared to a flurbiprofen-loaded commercial product.

Results: The flurbiprofen-loaded nanoparticles with flurbiprofen/sucrose/surfactant mixture (1/20/2, weight ratio) gave good solidification and no stickiness. They associated with about 70?000-fold improved drug solubility and had a mean size of about 300 nm with a narrow size distribution. Flurbiprofen was present in a changed amorphous state in these nanoparticles. Moreover, the nanoparticles gave significantly shorter T_max, and higher AUC and C_max of the drug compared to the commercial product (p?0.05). In particular, they showed about nine-fold higher AUC of the drug than did the commercial product

Conclusion: These flurbiprofen-loaded nanoparticles prepared with sucrose by the membrane emulsification and spray drying method would be a potential candidate for orally delivering poorly water-soluble flurbiprofen with enhanced bioavailability.     

Sonochemical synthesis and shape change of colloidal CdSe nanocrystals by high-intensity ultrasound

In this article, we have developed the synthesis of CdSe nanocrystals by the introduction of high-intensity ultrasound combined with an anionic surfactant (sodium dodecyl sulfate, SDS). TEM, XRD, and SEM EDS confirmed the successful synthesis of CdSe nanoparticles with zinc blende crystal phase. UV, PL, and TEM revealed that large particles settled to the bottom of the reaction flask. In the lower part precipitate, nanorods of different aspect ratios were also observed. The CdSe nanorods were formed by self-assembly due to the SDS surfactant and high-intensity ultrasound. A three-stage mechanism for the synthesis of CdSe nanorods was proposed. The effect of SDS concentration on the shape of nanorods was also investigated. At medium concentrations of SDS (0.2 M), one-dimensional CdSe nanorods with different aspect ratios were obtained. When using low concentrations of SDS (0.1 M), two-dimensional square-like crystals were observed due to all growth crystal faces having roughly the same surface energy.     

Colloidal PbS nanocrystals with narrow photoluminescence spectra

Luminescent PbS nanocrystals have been synthesized by the colloidal method. PVA has been employed to modify the surface of prepared PbS nanocrystals and improve their optical properties. Optical and morphological characteristics of lead sulfide nanocrystals have been studied by high resolution electron microscopy and spectrophotometer. Optical absorption and photoluminescence (PL) studies of the PbS nanocrystals have shown the strong quantum confinement effects. For the first time, the prepared lead sulfide nanocrystals have emitted at 608 nm wavelength with narrow band width (21 nm) and high Stokes shift. Experimental results have shown that the surface charge traps have higher contribution to the optical properties of colloidal PbS nanocrystals and in our sample photoluminescence was due to hole relaxation. These properties made them a material with potential application in nanophotonics.     

Symmetry considerations in CdSe nanocrystals

Semiconductor nanocrystals or quantum dots show a wide range of physical properties depending on their size or shape. In this paper, we show that symmetry is also an important characteristic that can lead to different electronic and optical properties. We use pseudopotential density-functional theory, within a real space approach, and address the sensitivity of electronic and optical properties with respect to the symmetry point groups associated to CdSe nanocrystals.     

Reversible and hierarchical composite gels with CdSe nanocrystals

To synthesize composite solid materials of metal salt and CdSe nanocrystals by a simple one-step method has been described. These solids can form stable gel in some organic solvent, such as benzene, cyclohexane and 1-butanol, especial in n-decane even below 0.1 wt/vol.%. Furthermore, these gels appear strong fluorescence which can be easily adjusted by the gel concentration. Temperature-dependent fluorescence spectra of composite gels suggested that the CdSe NCs aggregate together in gel state which would induce the energy transfer between nanocrystals and these aggregates could be reversibly disintegrated when gel was heated to form sol. TEM observations provided the further evidence of the energy transfer and suggested that the CdSe NCs were enchased regularly not only on the surface of self assembly of metal salt, but also embedded inside of self assembly in composite gel with small size nanocrystals. In contrast, in composite organogel with large nanocrystals they were only enchased on the edge of self assembly.     

One-pot/three-step synthesis of zinc-blende CdSe/CdS core/shell nanocrystals with thick shells

A one-pot/three-step synthetic scheme was developed for phase-pure epitaxy of CdS shells on zinc-blende CdSe nanocrystals to yield shells with up to sixteen monolayers.The key parameters for the epitaxy were identified,including the core nanocrystal concentration,solvent type/composition,quality of the core nanocrystals,epitaxial growth temperature,type/concentration of ligands,and composition of the precursors.Most of these key parameters were not influential when the synthetic goal was thin-shell CdSe/CdS core/shell nanocrystals.The finalized synthetic scheme was reproducible at an almost quantitative level in terms of the crystal structure,shell thickness,and optical properties.     

Porous carbon nanofibers with narrow pore size distribution from electrospun phenolic resins

Phenolic resin-based porous carbon nanofibers (PCNFs) with large surface area and narrow pore size distribution have been successfully prepared using novolac-type phenolic resin as precursor. The high molecular weight precursor was first synthesized in this study, then was dissolved in methanol. The PCNFs were finally obtained through electrospinning the phenolic resin polymer solution followed by successive curing and carbonization without activation. The N₂ adsorption/desorption isotherms reveal that the PCNFs have high specific surface area about 812 m²/g, the pore size falls in the range of 0.4-0.7 nm and the pore volume is 0.91 cm³/g. The vapor adsorption testing demonstrated that PCNFs exhibited different adsorption performance for ethanol and water.     

Electronic impurity doping in CdSe nanocrystals

We dope CdSe nanocrystals with Ag impurities and investigate their optical and electrical properties. Doping leads not only to dramatic changes but surprising complexity. The addition of just a few Ag atoms per nanocrystal causes a large enhancement in the fluorescence, reaching efficiencies comparable to core-shell nanocrystals. While Ag was expected to be a substitutional acceptor, nonmonotonic trends in the fluorescence and Fermi level suggest that Ag changes from an interstitial (n-type) to a substitutional (p-type) impurity with increased doping.