CdTe,核 壳型CdTe/CdS及CdTe/ZnS量子点的合成及表征

在水相中制备了半导体CdTe纳米晶,核 壳型CdTe/CdS和CdTe/ZnS纳米晶（即量子点;QDs）.利用扫描隧道显微镜（STM）和荧光光谱(FS)对合成的纳米晶量子点进行了研究,并且根据FS的数据进行了量子效率的计算.STM的结果表明合成的量子点直径约为3 nm并且分布良好.为了提高量子效率,对Cd2+浓度和Cd2+∶S2－比例等反应条件进行了研究,结果表明随着回流时间的增加,核 壳型量子点CdTe/CdS的量子效率总体上呈下降趋势.CdTe/CdS在pH8.5,Cd2+∶S2－=10∶1（摩尔比）时可获得80.0%的最大量子效率.同时制备了核 壳型量子点CdTe/ZnS,其最大发射波长由551 nm（CdTe）红移到635 nm（CdTe/ZnS）表明量子点的尺寸在增长,但是量子效率下降到14.4%. 当前研究的量子点可适用于生物标记,生物成像,以及基于共振能量转移的生物传感研究.     

Co-sensitized quantum dot solar cell based on ZnO nanowire

An efficient photoelectrode is fabricated by sequentially assembled CdS and CdSe quantum dots (QDs) onto a ZnO-nanowire film. As revealed by UV-vis absorption spectrum and scanning electron microscopy (SEM), CdS and CdSe QDs can be effectively adsorbed on ZnO-nanowire array. Electrochemical impedance spectroscopy (EIS) measured demonstrates that the electron lifetime for ZnO/CdS/CdSe (13.8 ms) is calculated longer than that of ZnO/CdS device (6.2 ms), which indicates that interface charge recombination rate is reduced by sensitizing CdSe QDs. With broader light absorption range and longer electron lifetime, a power conversion efficiency of 1.42% is achieved for ZnO based CdS/CdSe co-sensitized solar cell under the illumination of one Sun (AM 1.5G, 100 mW cm⁻²).     

Strong violet luminescence from ZnO nanocrystals grown by the low-temperature chemical solution deposition

The luminescence properties of zinc oxide (ZnO) nanocrystals grown from solution are reported. The ZnO nanocrystals were characterized by scanning electron microscopy, X-ray diffraction, cathodo- and photoluminescence (PL) spectroscopy. The ZnO nanocrystals have the same regular cone form with the average sizes of 100-500 nm. Apart from the near-band-edge emission around 381 nm and a weak yellow-orange band around 560-580 nm at 300 K, the PL spectra of the as-prepared ZnO nanocrystals under high-power laser excitation also showed a strong defect-induced violet emission peak in the range of 400 nm. The violet band intensity exhibits superlinear excitation power dependence while the UV emission intensity is saturated at high excitation laser power. With temperature raising the violet peak redshifts and its intensity increases displaying unconventional negative thermal quenching behavior, whereas intensity of the UV and yellow-orange bands decreases. The origin of the observed emission bands is discussed.     

Synthesis of CdSe/CdS with a simple non-TOP-based route

A convenient and non-TOP-based route for the synthesis of core-shell CdSe/CdS quantum dots (QDs) is developed for the first time. Simple reagents, such as cadmium oxide, selenium powder, sodium sulfide, paraffin and oleic acid with obvious advantages are used to replace organometallics. This simple route allows the preparation of a series of core-shell CdSe/CdS QDs emitting in a wide wavelength range (from 510 to 615 nm). After passivation of CdSe by CdS shell using sodium sulfide as the source of sulfur at 80 °C, the quantum yields (QYs) are improved from 15-30% to 35-50% and remained stable at least for 4 months. A narrow bandwidth (FWHM<50 nm) indicates that the as-prepared QDs have uniform size distribution, desirable dispersibility and good fluorescence properties. The whole procedure can be carried out either open to air or under nitrogen atmosphere, which is simpler, greener and cheaper as compared with TOP-based route.     

水溶性CdTe/CdS纳米晶表面SiO2壳层的反相胶束法包覆

"提出了一种水相中制备CdTe/CdS核壳结构纳米粒子的方法.用Te粉作为碲源,用Na2S作为硫源,在50 ℃下制备了CdTe/CdS核壳结构纳米粒子. 用紫外可见吸收光谱和荧光光谱分析了CdS壳层对CdTe核的影响. 随CdS壳层厚度的增加,紫外可见吸收光谱和荧光光谱均发生了红移. CdS壳层厚度较薄时,CdTe/CdS纳米晶的荧光强度较CdTe纳米粒子有显著提高;而CdS壳层厚度较厚时,CdTe/CdS纳米晶的荧光强度会逐渐降低. 用反相胶束法在CdTe/CdS核壳结构纳米粒子的表面包被一层SiO2,     

Enhanced Oxidation Stability of Quasi Core-Shell Alloyed CdSeS Quantum Dots Prepared Through Aqueous Microwave Synthesis Technique

Quasi core shell alloyed CdSeS quantum dots (QDs) have been prepared through a facile aqueous-phase route employing microwave irradiation technique. The optical spectroscopy and structure characterization evidenced the quasi core shell alloyed structures of CdSeS QDs. The X-ray diffraction patterns of the obtained CdSeS QDs displayed peak positions very close to those of bulk cubic CdS crystal structures and the result of X-ray photoelectron spectroscopy data re-confirmed the thick CdS shell on the CdSe core. The TEM images and HRTEM images of the CdSeS QDs ascertained the well-defined spherical particles and a relatively narrow size distribution. On the basis, the stability of the obtained QDs in an oxidative environment was also discussed using etching reaction by H₂O₂. The experiments result showed the as-prepared QDs present high tolerance towards H₂O₂, obviously superior to the commonly used CdTe QDs and core-shell CdTe/CdS QDs, which was attributed to the unique quasi core-shell CdSeS crystal structure and the small lattice mismatch between CdSe and CdS semiconductor materials. This assay provided insight to obtain high stable crystal structured semiconductor nanocrystals in the design and synthesis process.     

Temperature induced size selective synthesis of hybrid Cds/pepsin nanocrystals and their photoluminescence investigation

There is growing interest in materials chemistry for taking advantage of the physical and chemical properties of biomolecules in the development of next generation nanoscale materials for opto-electronic applications. A biomimetic approach to materials synthesis offers the possibility of controlling size, shape, crystal structure, orientation, and organization. The great progress has been made in the control that can be exerted over optical materials synthesis using biomolecules (protein, nucleic acid)/mineral interfaces as templates for directed synthesis. We have synthesized the CdS nanocrystals using pepsin by biomimetic technique at four different set temperatures. X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) results showed that we are able to tune the size and distribution profile just by tuning the reaction (Rx) temperature and goes towards excitonic Bhor radius (2.5 nm) at low temperature (4 °C). The narrow absorption peak at 260 nm from Cd²⁺-pepsin complex dominates and indicates the size dispersion of the modified CdS nanoparticles are fairly monodisperse. Effective mass approximation (EMA) shows large blue-shift (~1 eV) in the band gap for the cubic phase from bulk hexagonal CdS. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra are dominated by a strong and narrow band-edge emission tunable in the blue region indicating a narrow size distribution. The reduction in PL efficiency is observed when the Rx temperature increases however no change in PLE spectra and temporal profiles of the band-edge PL is observed. At 4 °C, high emission efficiency with shift of PL spectrum in the violet region is observed for 1.7 nm size CdS quantum dots (QDs). Presence of pepsin has slowed the PL decay which is of the order of 100 μs.     

Facile synthesis of mercaptosuccinic acid-capped CdTe/CdS/ZnS core/double shell quantum dots with improved cell viability on different cancer cells and normal cells

Water-soluble, mercaptosuccinic acid (MSA)-capped CdTe/CdS/ZnS core/double shell quantum dots (QDs) were prepared by successive growth of CdS and ZnS shells on the as-synthesized CdTe/CdS_thin core/shell quantum dots. The formation of core/double shell structured QDs was investigated by ultraviolet-visible (UV–Vis) absorption and photoluminescence (PL) spectroscopy, PL decay studies, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The core/double shell QDs exhibited good photoluminescence quantum yield (PLQY) which is 70% higher than that of the parent core/shell QDs, and they are stable for months. The average particle size of the core/double shell QDs was ～3 nm as calculated from the transmission electron microscope (TEM) images. The cytotoxicity of the QDs was evaluated on a variety of cancer cells such as HeLa, MCF-7, A549, and normal Vero cells by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) cell viability assay. The results showed that core/double shell QDs were less toxic to the cells when compared to the parent core/shell QDs. MCF-7 cells showed proliferation on incubation with QDs, and this is attributed to the metalloestrogenic activity of cadmium ions released from QDs. The core/double shell CdTe/CdS/ZnS (CSS) QDs were conjugated with transferrin and successfully employed for the biolabeling and fluorescent imaging of HeLa cells. These core/double shell QDs are highly promising fluorescent probe for cancer cell labeling and imaging applications.     

Static quenching of bovine serum albumin conjugated with small size CdS nanocrystalline quantum dots

The author reports on a strong fluorescence quenching of a model transport protein, bovine serum albumin BSA, when bioconjugated with CdS quantum dots QDs. The 4.4 nm size CdS QDs were synthesized using wet chemistry method and were characterized using UV-vis spectroscopy, scanning electron microscopy SEM and X-ray diffraction XRD techniques. It was found that the BSA fluorescence quenching increases linearly with increasing the CdS QDs concentrations in the range of 3×10⁻⁷-2.0×10⁻⁶ mol L⁻¹. This quenching is explained in terms of Stern-Volmer equation and is ascribed to static quenching with quenching constant 1.321×10⁴ L mol⁻¹ at 300 K.     

Functionalized CdS nanospheres and nanorods

Functionalized nanoparticles are discussed. Surfaces of CdS:Mn/ZnS core/shell nanospheres (Qdots) were converted from hydrophobic to hydrophilic by growth of a SiO₂ shell. The colloidal dispersion was stabilize by adding a surfactant with a negative surface charge, and a cell-penetrating-peptide, TAT, was attached through a primary amine group. The TAT functionalized Qdots were shown to pass the blood-brain-barrier and luminescence in the infused half of the brain.In addition, nanorods of S²⁻ rich CdS were synthesized by reaction of excess S with Cd precursors in the presence of ethylene diamine. The photoluminescence (PL) peak from the S²⁻ rich CdS nanorods was broad with a maximum at ∼710 nm, which was 40 nm longer in wavelength than the PL peak from Cd²⁺ rich CdS (∼670 nm) nanorods. The influence of surface electron or hole trap states on the luminescent pathway of CdS nanorods were used to explain these shifts in wavelength. Nanocrystals of Au with ∼2 nm diameters were grown on S²⁻ rich surfaces of CdS nanorods. Significant quenching of photoluminescence was observed from Au nanocrystals on CdS nanorods due to interfacial charge separation. Charge separation by Au nanocrystals on CdS resulted in enhanced UV photocatalytic degradation of Procion red mix-5B (PRB) dye in aqueous solution.     

Microwave-Assisted Aqueous Synthesis of Highly Luminescent Carboxymethyl Chitosan-Coated CdTe/CdS Quantum Dots as Fluorescent Probe for Live Cell Imaging

This paper describes the development of a simplified and rapid method for the aqueous synthesis of quantum dots (QDs) with CdTe cores and gradient CdS external shells (CdTe/CdS QDs) aided by microwave irradiation. Several synthesis parameters, such as molar ratio of reagents, pH, reaction temperature, and reaction time, were studied in details. Under the optimized conditions, highly effective CdTe/CdS QDs could be synthesized in aqueous phase in only 15 min. In order to improve the biocompatibility of the CdTe/CdS QDs, these QDs were then interacted with carboxymethyl chitosan (CMC) so as they could be used as fluorescent probes in the aqueous phase. With the incorporation of CMC, the stability of modified QDs was found to have improved significantly (from 4 months to more than 10 months at room temperature). The photoluminescence quantum yield (PLQY) of the modified QDs could reach 75%, other parameters include a full width at half maximum of the emission (FWHM) spectrum as 40 ~ 60 nm, and an average size, estimated from electron microscopic images, as 3.5 nm. As fluorescent probes, these modified QDs were successfully used for imaging live Madin–Darby canine kidney (MDCK) cells, in which the preliminary results indicated that these modified QDs demonstrated good biocompatibility and showed promising applications for bio-labeling and imaging.     

A simple fluorescence quenching method for roxithromycin determination using CdTe quantum dots as probes

A new method for the determination of roxithromycin based on the fluorescence quenching of 3-mercaptopropionic acid-capped CdTe quantum dots (MPA-CdTe QDs) was developed. In ethanol medium, the fluorescence of CdTe quantum dots at 552 nm was quenched in the presence of roxithromycin. Based on this a simple, sensitive, and selective method for rapid determination of roxithromycin was described. Reaction time, interfering substances on the fluorescence quenching, and mechanism of the interaction of CdTe QDs with roxithromycin were investigated. After optimization, the proposed method allows the determination of roxithromycin over the range 25.0-350.0 μg ml⁻¹. The detection limit is 4.6 μg ml⁻¹. The proposed method was successfully applied to commercial capsules and tablets with satisfactory results. The recovery of the method was in the range of 96.8-102.5%.     

Synthesis and performance of core-shell structured ZnO/In2O3 composites in situ growth

Core-shell structured ZnO/In₂O₃ composites were successfully synthesized via situ growth method. Phase structure, morphology, microstructure and property of the products were investigated by X-ray diffraction (XRD), TG-DTA, field emission scanning electron microscopy (FESEM), energy-dispersive spectrometry (EDS), transmission electron microscope (TEM) and photoluminescence (PL). Results show that the core-shell structures consist of spindle-like ZnO with about 800 nm in length and 200 nm in diameter, and In₂O₃ particles with a diameter of 50 nm coated on the surface of ZnO uniformly. HMTA plays an important role in the formation of core-shell structures and the addition of In₂O₃ has a great effect on PL spectrum. Possible mechanism for the formation of core-shell structures is also proposed in this paper.     

pH-dependent aggregation and photoluminescence behavior of thiol-capped CdTe quantum dots in aqueous solutions

pH-dependent aggregation of thiol-capped CdTe quantum dots (QDs) in solutions was observed with a confocal microscope. The average size of the QD aggregates increased from 28 nm to 1.4 μm as the pH decreased from 12 to 3. The basic condition improved the dispersion of QDs while the acidic condition caused the detachment of surface ligands, leading to the aggregation of QDs. A PL lifetime of 80 ns was detected for QDs at pH from 12 to 7, while it was shortened to 57 and 34 ns at pH 5 and 3, respectively, due to the formation of surface defects.     

Synthesis and characterization of water-soluble, stable and highly luminescent itaconic acid/methacrylic acid copolymer-coated CdSe/CdS quantum dots

The itaconic acid/methacrylic acid (PIA-MAA) copolymer ligand (PIA-MAA) with carboxylate anchoring groups was found to efficiently convert CdSe/CdS core-shell quantum dots (QDs) in chloroform to water-soluble PIA-MAA-ligand stabilized nanocrystals (PIA-MAA-QDs). The quantum yield (QY) of the resulting PIA-MAA-QDs was 24%. In addition, the carboxylate-based PIA-MAA-QDs survived UV irradiation in air for at least 16 days. Upon UV irradiation, the PIA-MAA-QDs became about 2 times brighter than the original CdSe/CdS QDs in chloroform, and the UV-brightened photoluminescence (PL) can retain the brightness for at least several months. Experimental results further confirmed that the PIA-MAA-QDs were more stable than the original CdSe/CdS QDs against strong acid, strong oxidant, photochemical and thermal treatments. The PIA-MAA-QDs were soluble and stable in water, some polar solvents and buffer solutions. In addition to good performance of PIA-MAA-QDs, the synthesis of PIA-MAA ligands and the corresponding water-soluble QDs is relatively simple.     

Fabrication of highly luminescent InP/Cd and InP/CdS quantum dots

Highly luminescent InP/Cd and InP/CdS core-shell QDs were fabricated by sequential addition of cadmium acetylacetonate and dodecanethiol to InP core solutions, which showed a red-shift in absorption and emission. ICP measurement revealed the existence of cadmium and TEM images showed the increased size of InP/CdS QDs. PXRD data identified zinc blend structures of InP and InP/CdS QDs, which indexed to the (1 1 1), (2 2 0) and (3 1 1) planes. The slight shift of peaks between InP and InP/CdS QDs can demonstrate the existence of CdS shell structures.     

Synthesis of stable dispersion of ZnO quantum dots in aqueous medium showing visible emission from bluish green to yellow

Aqueous dispersion of 4-8 nm size stable ZnO quantum dots (QDs) exhibiting luminescence in the visible region have been synthesized by a simple solution growth technique at room temperature. Silica has been used as capping agent to control the particle size as well as to achieve uniform dispersion of QDs in aqueous medium. X-ray diffractometer (XRD) analysis reveals formation phase pure ZnO particles having wurzite (hexagonal) structure. Atomic force microscope (AFM) images show that the particles are spherical in shape, having average crystalline sizes ∼4, 5.5 and 8 nm for samples prepared at pH values of 10, 12 and 14, respectively. From the optical absorption studies, the band gap energy of QDs is found to be blue shifted as compared to bulk ZnO (3.36 eV) due to the quantum confinement effect and is consistent with the band gap calculated by using effective-mass approximation model. The photoluminescence (PL) observed in these QDs has been attributed to the presence of defect centers.     

Surface-defect-states photoluminescence in CdS nanocrystals prepared by one-step aqueous synthesis method

CdS nanocrystals were prepared by a simple one-step aqueous synthesis method using thioglycolic acid (TGA) as the capping molecule, and characterized by X-ray powder diffraction (XRD), UV-vis absorption spectra and photoluminescence (PL) emission spectroscopy. The effects of both TGA/Cd and Cd/S molar ratio on the surface-defect-state PL intensity of CdS nanocrystals have been investigated. It was found that all of the as-prepared CdS nanocrystals showed a strong broad emission in the range of 450-700 nm centered at 560 nm, which was attributed to the recombination of an electron trapped in a sulfur vacancy with a hole in the valance band of CdS. The surface-defect-states emission intensity of CdS nanocrystals significantly increased with the increase of Cd/S molar ratio, and showed a maximum when Cd/S molar ratio was 2.0. If Cd/S molar ratio continued to increase, namely more than 2.0, the surface-defect-states emission intensity would decrease. It was found that the surface-defect-states emission intensity increased with the increase the TGA/Cd molar ratio, and showed a maximum when the TGA/Cd molar ratio was equal to 1.8, and a further increase of the TGA/Cd molar ratio would lead to the decrease of the surface-defect-states emission intensity.     

Photoluminescence from ZnO-SiO2 opals with different sphere diameters and thicknesses

We systematically investigated the photoluminescence (PL) and transmittance characteristics of ZnO-SiO₂ opals with varied positions of the stop-band and film thicknesses. An improved ultraviolet (UV) luminescence was observed from ZnO-SiO₂ composites over pure ZnO nanocrystals under 325 nm He-Cd laser excitation at room temperature. The UV PL of ZnO nanocrystals in SiO₂ opals with stop-bands center of 410 nm is sensitive to the thickness of opal films, and the UV PL intensity increases with the film thickness increasing. The PL spectra of ZnO nanocrystals in SiO₂ opals with stop-bands center of 570 nm show a suppression of the weak visible band. The experimental results are discussed based on the scattering and/or absorbance in opal crystals.     

Photoluminescence Characteristics of ZnCuInS-ZnS Core-Shell Semiconductor Nanocrystals

The photoluminescence(PL) characteristics of ZnCuInS quantum dots(QDs) with varying ZnS shell thicknesses of 0, 0.5, and 1.5 layers are investigated systemically by time-correlated single-photon counting measurements and temperature-dependent PL measurements. The results show that a ZnS shell thickness of 1.5 layers can effectively improve the PL quantum yield in one order of magnitude by depressing the surface trapping states of the core ZnCuInS QDs at room temperature. However, the PL measurements at the elevated temperature reveal that the core-shell nanocrystals remain temperature-sensitive with respect to their relatively thin shells.The temperature sensitivity of these small-sized single-layered core-shell nanocrystals may find applications as effective thermometers for the in vivo detection of biological reactions within cells.