巯基包覆CdSe和CdSe/CdS核壳纳米晶的水相合成与表征

利用水相合成的方法制备了巯基包覆的具有较高荧光量子产率的CdSe和CdSe／CdS纳米晶．水相合成方法的优点是原料低廉、安全可靠和重复性高，缺点是纳米晶的尺寸分布较宽，发光效率不是很高．采用X-射线粉末衍射、吸收和荧光等光谱手段对纳米晶的平均尺度、粒径分布、晶体结构及发光特性进行了表征。在77K到300K的温度范围内，随着温度降低，CdSe纳米晶的发光峰逐渐蓝移，而CdSe／Cds纳米晶发光峰位基本不随温度变化而变化．此外，在325nm激光辐照下，CdSe／CdS纳米晶的荧光寿命比CdSe纳米晶延长了6倍左右，稳定性大幅度提高．以上结果表明，核壳结构的CdSe／CdS纳米晶具有较高的发光效率和良好的稳定性，具有广阔的应用前景．     

Structural,optical and morphological studies of undoped and Zn-doped CdSe QDs via aqueous route synthesis

Undoped and Zn-doped CdSe quantum dots (QDs) were successfully synthesized by the chemical precipitation method. The structural, optical and morphological properties of the synthesized undoped and Zn-doped CdSe QDs were studied by X-ray diffraction (XRD), UV–visible absorption spectroscopy, photoluminescence (PL) spectroscopy, fluorescence lifetime spectroscopy, scanning electron microscopy (SEM), field emission transmission electron microscopy (FE-TEM) and FTIR. The synthesized undoped and Zn-doped CdSe QDs were in cubic crystalline phase, which was confirmed by the XRD technique. From the UV–visible absorption spectral analysis, the absorption wavelengths of both undoped and Zn-doped CdSe QDs show blue-shift with respect to their bulk counterpart as a result of quantum confinement effect. The highest luminescence intensity was observed for CdSe QDs doped with 4% Zn by PL studies. TEM analysis shows that the prepared QDs are spherical in shape.     

Localized surface plasmon resonance enhanced photoluminescence of CdSe QDs in PMMA matrix on silver colloids with different shapes

Localized surface plasmon resonance (LSPR) enhanced photoluminescences (PL) from CdSe quantum dots (QDs) on worm-like or quasi-spherical silver colloids have been investigated. The shape of silver colloid film is controlled by annealing temperature (200 °C∼350 °C). Strong PL enhancements of CdSe QDs on both as-grown and annealed silver colloid films are observed. The results show that the PL enhancement factor of CdSe QDs on worm-like silver colloid film reaches as high as 15-fold. Moreover, the enhancement factor is 5 times larger than that obtained from the quasi-spherical silver colloids. The superiority of worm-like silver nanostructure on LSPR enhanced photoluminescence is attributed to its larger size, hot spots and multiple dipole resonance modes coupling, which are induced by aggregation effect.     

Preparation and characterization of novel CdSe quantum dots modified with poly (d,l-lactide) nanoparticles

In this paper, novel CdSe quantum dots (QDs) modified with poly (d, l-lactide) (PLA) nanoparticles by nanoprecipitation method was reported. CdSe QDs modified with PLA nanoparticles were characterized by Photon correlation spectroscope (PCS), transmission electron microscope (TEM), flluorescence spectrophotometer and fluorescence microscope. The modified CdSe QDs were spherical and relatively uniform. The modified CdSe QDs were water soluble and their strong yellow fluorescence emission was observed both in vitro and in vivo. The fluorescence of the modified CdSe QDs was stable in aqueous solution for more than 30 d. These modified CdSe QDs are expected to have much potential for biological labeling and diagnostics based on above properties.     

Synthesis and photoluminescence of bright water-soluble CdSe/ZnS quantum dots overcoated by hybrid organic shell

Photoluminescence properties from water soluble CdSe/ZnS QDs encapsulated with hybrid trioctylphosphine-poly(acrylamide-co-acrylic acid)-ethanolamine (TOPO-PSMA-EA) shell have been investigated. It was found that PL efficiency of CdSe/ZnS QDs in water was increased 5–30% after introducing PSMA-EA polymers to encapsulate CdSe/ZnS-TOPO QDs. Higher PSMA concentrations were found to enhance the PL efficiency of QDs up to 1.8 folds, which is ascribed to a better packing and passivation of the TOPO-PSMA-EA shell over the QDs. Time-resolved photoluminescence suggested that the mean lifetime of photoexcited carriers in the water-soluble CdSe/ZnS-TOPO-PSMA-EA QDs elongated 2–17 ns compared with that of uncoated samples, indicating that PL quenching defects were effectively removed for CdSe/ZnS QDs with hybrid TOPO-PSMA-EA shell.     

Temperature-controlled synthesis of CdSe nanocrystals with narrow size distribution

CdSe quantum dots (QDs) with narrow size distribution and fine crystallinity were synthesized in paraffin liquid through temperature-control method. TEM, HRTEM, SEAD, XRD, PL and UV-VIS spectra were used to characterize the size, crystal structure and photoluminescence (PL) properties of CdSe nanocrystals. The PL spectra and TEM results revealed that the monodispersed and uniformed CdSe QDs with narrow size distribution were synthesized at a certain reaction temperature. HRTEM images combined with selected area electron diffraction (SAED) and XRD patterns illustrated that CdSe QDs showed near-perfect zinc-blende and wurtzite crystallinity at different temperatures. The Gibbs-Thomson calculation provided a thermodynamic explanation for obtaining the CdSe nanocrystals with narrow size distribution by temperature-control method.     

Aqueous synthesis of luminescent magic sized CdSe nanoclusters

Highly stable water-soluble CdSe nanoclusters (NCs) with magic size were successfully synthesized using homocysteine (HCY) as capping ligands. Their sizes were tunable between 1.2 and 2.0 nm depending on reflux time. The final products were characterized by UV-vis absorption, steady and time-resolved photoluminescence (PL) spectra, X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). XRD analysis showed that the HCY-capped CdSe NCs were of the cubic structure, UV-vis absorption spectra and HRTEM micrograph exhibited that the NCs were nearly monodisperse and relatively uniform. The as-prepared CdSe NCs had a PL quantum yield of up to 1.4%, almost comparable to the CdSe magic sized clusters prepared by an organometallic route.     

水相中CdSe与核/壳CdSe/CdS量子点的制备与发光特性研究

以巯基乙酸为稳定剂在水相中制备了CdSe与核/壳型CdSe/CdS量子点水溶胶, 用紫外-可见吸收光谱和发射光谱研究了它们的发光特性, 并且用X射线粉末衍射(XRD)、透射电镜(TEM)和X射线光电子能谱(XPS)表征了它们的结构、形貌和化学组成, 结果表明使用该方法制备的量子点分散性良好, 而且用CdS对CdSe进行表面修饰以后的发光强度明显提高, 发射光谱和吸收光谱都有红移现象, 不同粒径颗粒的吸收峰的位置也有所不同.     

pH-sensitive photoluminescence for aqueous thiol-capped CdTe nanocrystals

pH-sensitive photoluminescence (PL) is an important property of aqueous nanocrystals (NCs) towards NCs-based intelligent applications. Previous works mainly focused on the effect of pH during NC growth process on PL of the aqueous NCs. The effect of pH during application process on PL of as-prepared NCs is still not fully understood. In this work, we brought out a general mechanism for the pH-sensitive PL behaviors of as-prepared aqueous CdTe NCs capped by aqueous thiol ligands, such as carboxylic-acid-terminated 3-mercaptopropionic acid (MPA) and thioglycolic acid (TGA) ligands, hydroxyl-terminated 1-thioglycerol (TG) ligands and amine-terminated 2-mercaptoethylamine (MA) ligands. A major contribution of this work is finding the key role of ligand terminal groups in the diffuse process of free Cd-ligand complexes toward NCs. This terminal group effect is the main reason for PL alteration of NCs during pH adjustment process. Besides the terminal group effect, PL of aqueous NCs is also affected by the aggregation effect, the thiol group effect and the counter ion effect. These effects make different contributions to PL of NCs at different pH ranges. By using this mechanism, we successfully explained the complex pH-sensitive PL behaviors of MPA, TGA, TG and MA-capped CdTe NCs.     

The effect of annealing and photoactivation on the optical transitions of band-band and surface trap states of colloidal quantum dots in PMMA

For optoelectronic applications, colloidal CdSe quantum dots (QDs) have been integrated into solid devices by using optically transparent polymer matrices that embedded the colloidal QDs. We systematically studied the effect of annealing and photoactivation on the band-band (BB) and surface trap state (STS) transitions of colloidal CdSe QDs embedded in polymethylmethacrylate (QDs-PMMA). The QDs-PMMA composites demonstrate enhancement of the STS emissions while their annealing leads to an intensity quench of both BB and STS emissions. The annealing process also causes the red shift of the BB emission. By contrast, photoactivation of QDs-PMMA composites results in the remarkable recovery of luminescence intensity accompanied by blue shift of the emissions. Furthermore, it is found in the photoactivation process that the STS emission can be saturated earlier than the BB emission, which renders it possible to tune the light color of the emissions. The combination of annealing and photoactivation could undoubtedly provide an effective way to precisely tune the colors of light-emitting devices that use colloidal CdSe QDs.     

Synthesis and size dependent optical studies in CdSe quantum dots via inverse micelle technique

Cadmium selenide quantum dots (CdSe QDs) were successfully synthesized without using trioctylphosphine (TOP). The XRD pattern showed zinc-blend phase of the CdSe QDs. The absorption and PL spectra exhibit a strong blue shift as the QDs size decreases due to the quantum confinement effect. In addition, the quantum efficiency of CdSe QDs with TOP capping is higher than CdSe QDs with oleic acid capping. TEM image shows a spherical shape, compact and dense structure of CdSe QDs. A good agreement between the Tauc's model and experimentally measured absorption spectra of CdSe QDs is achieved. The FTIR peak at ～1712 cm^?1 spectra confirms the influence of oleic acid as a capping agent.     

Self-surface passivation of CdX (X = Se, Te) quantum dots

A small portion of a reaction mixture including unpurified CdX (X = Se or Te) quantum dots (QDs), in which unreacted Cd and Se ions were left together with coordinating solvents, was dropped into an organic solvent. The CdX QDs in this organic solution showed enhancement of photoluminescence (PL) efficiency, growth of particles, and focusing of size distribution for more than 10 h at room temperature (RT, -23 degrees C). These effects were attributed to passivation of QDs' surface by Cd and X ions present in the solution. No external energy source was used for these achievements; therefore, the process is termed as self-surface passivation. The self-surface passivation was reproduced using purified CdX QDs with additional Cd and X ions in an organic solvent. The self-surface passivation method was applied to RT-synthesized CdSe QDs, which is characterized by a broad PL spectrum (fwhm - 150 nm) for monodispersed QDs, to modify their emission characteristics. On self-surface passivation, the broad PL spectrum was narrowed (fwhm - 35 nm) and the QDs were grown. The X-ray diffraction measurements of RT-synthesized CdSe QDs and that subsequently aged in 1-butanol showed that crystallinity of the samples was improved on aging.     

在多相体系中制备核壳型CdSe/ZnS量子点

在多相体系中制得了CdSe/ZnS半导体量子点,采用X射线粉末衍射(XRD)、电子透射显微镜(TEM)等测试手段对产物进行了表征,结果表明CaSe/CdS量子点尺寸均一、形貌规则,具有立方晶体结构.初步研究了此体系的反应机理,并通过紫外-可见吸收光谱和荧光光谱对产物的光学性质进行了分析,结果表明在CASe量子点外面包覆一定厚度的ZnS壳层后,其激子发射强度和量子效率显著提高.     

Photoluminescence-enhanced biocompatible quantum dots by phospholipid functionalization

A simple two-step strategy using phospholipid (PPL) to functionalize core/shell CdSe/ZnS quantum dots (QDs) has been described. The experimental data show that the use of S-H terminated PPL results not only in the high colloidal stability of core/shell CdSe/ZnS QDs in the aqueous phase, but also in the significant enhancement of photoluminescence. The degree of the enhancement is a function of the PPL-CdSe/ZnS QDs sample concentration. These results might be promising for future biological platform in new devices ranging from photovoltaic cells to biosensors and other devices.     

Synthesis of water-soluble CdSe quantum dots by ligand exchange with p-sulfonatocalix(n)arene (n = 4, 6) as fluorescent probes for amino acids

A simple, rapid ligand exchange route to prepare highly fluorescent, stable and water-soluble CdSe quantum dots (QDs) is reported by using p-sulfonatocalix(n)arene (SFCA(n), n = 4 or 6) instead of original trioctylphosphine oxide (TOPO) ligands. The ligands were found to have a profound effect on the luminescence response of QDs to amino acids. The SFCA(4) coated CdSe QDs were sensitive to methionine. On the other hand, SFCA(6) coated CdSe QDs turned out to be sensitive to phenylalanine. To demonstrate the detection capability of these new probes, SFCA(n) (n = 4, 6) coated CdSe QDs were used to detect methionine and phenylalanine in physiological buffer solution. Under optimal conditions, the relative fluorescence intensities of the SFCA(n) coated QDs (n = 4, 6) increased linearly with increasing concentration of amino acids. Methionine and phenylalanine enhanced the luminescence of SFCA(n) coated QDs with a concentration dependence that was best described by a Langmuir-type equation. The possible mechanism is also discussed.     

Highly photoluminescent and photostable CdSe quantum dot-nylon hybrid composites for efficient light conversion applications

Highly photoluminescent hexadecylamine (HDA) capped core CdSe quantum dots (QDs) with fluorescent quantum yields (QYs) up to 60% were synthesized using a hot injection method and directly incorporated into nylon polymer. For the incorporation of crude CdSe QDs into nylon a simple reproducible and upscalable one pot approach was developed without the need of further purification steps. The photoluminescence (PL) properties of the core QDs and the resulting QD-polymer hybrid composites were investigated and compared. Red emitting hybrid materials exhibit a QY of 60% with a high potential for applications in direct light and energy conversion. The hybrid materials could be successfully utilized as LED conversion layers. By avoiding exposure to oxygen the hybrid films can be kept for a month without detecting a significant decrease in luminescence. Various three dimensional structures are easily available opening doors for further applications such as novel materials for fluorescence standard development in laser scanning microscopy (LSM).     

Effect of shell layers on luminescence of colloidal CdSe/Zn(0.5)Cd(0.5)Se/ZnSe/ZnS core/multishell quantum dots

Colloidal CdSe/Zn(0.5)Cd(0.5)Se/ZnSe/ZnS core/multishell quantum dots (QDs) were synthesized by using the well developed successive ion layer adsorption and reaction (SILAR) technique. The UV-vis and PL spectra, TEM, X-ray diffraction and Raman measurement were performed to investigate the structure and optical properties of prepared QDs during the growth of shell layers, which indicated that the stress in CdSe core became stronger with the increasing shell thickness. Due to the gradual adjustment of the lattice parameters in the radial direction and the radial increase of the respective valence- and conduction-band offsets, the optical measurements show a significant enhancement in the photoluminescence quantum yield (QY) and an expedited radiative decay in QDs overcoated with thicker shell. The temperature-dependent optical spectra were measured, and the relation between the microstructure and the optical properties of these core/multishell quantum dots was discussed.     

Efficient quasi-stationary charge transfer from quantum dots to acceptors physically-adsorbed in the ligand monolayer

Alkanoate-coated CdSe/CdS core/shell quantum dots(QDs)with near-unity photoluminescence(PL)quantum yield and monoexponential PL decay dynamics are applied for studying quasi-stationary charge transfer from photo-excited QDs to quinone derivatives physically-adsorbed within the ligand monolayer of a QD.Though PL quenching efficiency due to electron transfer can be up to>80%,transient PL and transient absorption spectra reveal that the charge transfer rate ranges from single-digit nanoseconds to sub-nanoseconds,which is~3 orders of magnitude slower than that of static charge transfer and?2 orders of magnitude faster than that of collisional charge transfer.The physically-adsorbed acceptors can slowly(500-1,000 min dependent on the size of the quinone derivatives)desorb from the ligand monolayer after removal of the free acceptors.Contrary to collisional charge transfer,the efficiency of quasi-stationary charge transfer increases as the ligand length increases by providing additional adsorption compartments in the elongated hydrocarbon chain region.Because ligand monolayer commonly exists for a typical colloidal nanocrystal,the quasi-stationary charge transfer uncovered here would likely play an important role when colloidal nanocrystals are involved in photocatalysis,photovoltaic devices,and other applications related to photo-excitation.     

Synthesis of highly luminescent CdTe/ZnO core/shell quantum dots in aqueous solution

The synthesis and photoluminescence (PL) properties of aqueous CdTe/ZnO core/shell quantum dots (QDs) have been investigated by using thiolglycolic acid as a capping reagent. The highlighted contribution of the present study was CdTe QDs coated with a ZnO shell by controlling the hydrolysis process of Zn(OAc)₂. The QDs benefitted from overcoming the high lattice mismatch between CdTe and ZnO. The PL peak wavelength of the CdTe/ZnO QDs with high PL quantum yields up to 88% was located in a range between 547 and 596 nm by adjusting the size of CdTe cores and the thickness of ZnO shells. The results of X-ray diffraction analysis and transmission electron microscopy observation indicate that the dot-shaped CdTe/ZnO QDs (566 nm) with an average size of 2.2 nm in diameter belong to the cubic CdTe crystal structure. Due to the passivation of surface defects, it is found that the luminescence decay curves accord with a biexponential decay model of exciton and trap radiation behavior. The average PL lifetimes of CdTe (571 nm) and CdTe/ZnO (596 nm) QDs at room temperature are 27.3 and 35.1 ns, respectively.     

CdSe胶质量子点的电致发光特性研究

采用胶体化学法合成硒化镉(CdSe)胶质量子点, 在此基础上制成了以CdSe胶质量子点为有源层, 结构为ITO/ZnS/CdSe/ZnS/Al的电致发光(EL)器件. 透射电镜测量表明量子点的尺寸为4.3 nm, 扫描电子显微镜测量ZnS薄膜和Al薄膜结果显示表面均较为平整, 由器件结构的X射线衍射分析观察到了CdSe(111)、ZnS(111)等晶面的衍射, 表明器件中包含了CdSe量子点和ZnS绝缘层材料. 光致发光谱表征胶质量子点的室温发光峰位于614 nm, 电致发光测量得到器件在室温下的发光波长位于450 ~ 850 nm, 峰值在800 nm附近. 本文对电致发光机制及其与光致发光谱的区别进行了讨论.