利用离子型Cd源制备ZnSe/CdSe核-壳结构纳米晶

在有机相体系中利用ZnSe前驱体纳米晶制备过程中的富Se环境,以引入Cd2+的方式在相对温和的环境下通过控制Cd2+离子的加入量及调节反应时间,成功制备了ZnSe/CdSe核-壳复合结构纳米晶.利用X射线衍射(XRD)、透射电镜(TEM)、紫外-可见吸收光谱(UV-vis)和荧光光谱(FL)对其结构形貌以及光学性质进行表征和分析的结果表明,CdSe以外延生长的方式包覆在ZnSe纳米晶表面从而形成具有良好结晶性的核-壳复合结构,其荧光发射始终保持良好单色性,同时实现了在500～620nm可见光范围内的连续可调.     

Fabrication and characterization of mesoporous Co3O4 core/mesoporous silica shell nanocomposites

A mesoporous Co(3)O(4) core/mesoporous silica shell composite with a variable shell thickness of 10-35 nm was fabricated by depositing silica on Co(3)O(4) superlatticed particles. The Brunauer-Emmett-Teller (BET) surface area of the composite with a shell thickness of ca. 2.0 nm was 238.6 m(2)/g, which varied with the shell thickness, and the most frequent pore size of the shell was ca. 2.0 nm. After the shell was eroded with hydrofluoric acid, mesoporous Co(3)O(4) particles with a pore size of ca. 8.7 nm could be obtained, whose BET surface area was 86.4 m(2)/g. It is proposed that in the formation of the composite the electropositive cetyltrimethylammonium bromide (CTAB) micelles were first adsorbed on the electronegative Co(3)O(4) particle surface, which directed the formation of the mesoporous silica on the Co(3)O(4) particle surface. Electrochemical measurements showed that the core/shell composites exhibited a higher discharge capacity compared with that of the bare Co(3)O(4) particles.     

2D self-organization of core/shell Co(hcp)/Co nanocrystals

In this paper we report the preparation of ordered hexagonal 2D arrays of core/shell Cohcp/CoO nanocrystals. A full structural investigation has been carried out using high-resolution transmission electron microscopy, electron diffraction, and electron energy-loss spectroscopy.     

Synthesis of CdSe, ZnSe, and ZnxCd1-xSe nanocrystals and their silica sheathed core/shell structures

Uniform ZnxCd1-xSe nanocrystals have been prepared at the artificially designed water-oil interface using Na2SeO3, Cd(NO3)2, and Zn(NO3)2 as precursors. The chemical composition and band gap of the ZnxCd1-xSe nanocrystals can be adjusted via different combinations of source material. The coating of a SiO2 shell could transform the hydrophobic particles into hydrophilic particles. An advantage of this method is that a water phase could be added to the oleic acid (OLEA) synthesis system, which could be extended to make the synthesis of various nanocrystals more simple and flexible.     

Type-II quantum dots: CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures

Type-II band engineered quantum dots (CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures) are described. The optical properties of these type-II quantum dots are studied in parallel with their type-I counterparts. We demonstrate that the spatial distribution of carriers can be controlled within the type-II quantum dots, which makes their properties strongly governed by the band offset of the comprising materials. This allows access to optical transition energies that are not restricted to band gap energies. The type-II quantum dots reported here can emit at lower energies than the band gaps of comprising materials. The type-II emission can be tailored by the shell thickness as well as the core size. The enhanced control over carrier distribution afforded by these type-II materials may prove useful for many applications, such as photovoltaics and photoconduction devices.     

Luminescent CdSe/CdS core/shell nanocrystals in dendron boxes: superior chemical,photochemical and thermal stability

The surface ligands, generation-3 (G3) dendrons, on each semiconductor nanocrystal were globally cross-linked through ring-closing metathesis (RCM). The global cross-linking of the dendron ligands sealed each nanocrystal in a dendron box, which yielded box-nanocrystals. Although the dendron ligands coated CdSe nanocrystals (CdSe dendron-nanocrystals) were already quite stable, the stability of CdSe box-nanocrystals against chemical, photochemical, and thermal treatments were dramatically improved in comparison to that of the original dendron-nanocrystals. Furthermore, the box structure of the ligands monolayer coupled with the stable inorganic CdSe/CdS core/shell nanocrystals resulted in a class of extremely stable nanocrystal/ligands complexes. The band edge photoluminescence of the core/shell dendron-nanocrystals and box-nanocrystals were partially remained, and could be further brightened through controlled chemical oxidation or photooxidation. Practically, the stability of the box-nanocrystals is sufficient for most fundamental studies and technical applications. The box-nanocrystals may represent a general solution for the commonly encountered instability for many types of colloidal nanocrystals. The size distribution of the empty dendron boxes formed by the dissolution of the inorganic nanocrystals in concentrated HCl was very narrow. The empty boxes as new types of polymer capsules are soluble in solution, mesoporous, and with a very thin but stable peripheral. Those nanometer-sized cavities should be of interest for many purposes in the field of solution host-guest chemistry.     

Synthesis of core/shell nanoparticles of Au/CdSe via Au-Cd bialloy precursor

We present a novel method for the preparation of ultrasmall Au/CdSe core/shell particles. Au-Cd bialloy particles of 4.7 nm diameter were prepared as the precursor. The Cd component in the precursor reacted with the Se source at a temperature of 205 degrees C and was heated to 250 degrees C, leading to formation of a Au/CdSe core/shell structure. The sizes of Au/CdSe nanoparticles have a narrow distribution with an average size of 6.0 nm and Au core of 2.2 nm diameter. The X-ray diffraction pattern and the images of the high-resolution electron transmission microscopy show that the Au cores and the CdSe shells of Au/CdSe core/shell nanoparticles are both well crystallized, and the CdSe shells are in a cubic phase. The absorption spectrum of the Au/CdSe nanoparticles combines the absorption behaviors of the Au cores and the CdSe shells.     

Fluorescence blinking statistics from CdSe core and core/shell nanorods

We report fluorescence blinking statistics measured from single CdSe nanorods (NRs) of seven different sizes with aspect ratios ranging from 3 to 11. This study also included core/shell CdSe/ZnSe NRs and core NRs with two different surface ligands producing different degrees of surface passivation. We compare the findings for NRs to our measurements of blinking statistics from spherical CdSe core and CdSe/ZnS core/shell nanocrystals (NCs). We find that, for both NRs and spherical NCs, the off-time probability distributions are well described by a power law, while the on-time probability distributions are best described by a truncated power law, P(tau(on)) approximately tau(on)(-alpha)e((-tau)(on)/(tau)(c)). The measured crossover time, tau(c), is indistinguishable within experimental uncertainty for core and core/shell NRs, as well as for core NRs with different ligands, for the same core size, indicating that surface passivation does not affect the blinking statistics significantly. We find that, at fixed excitation intensity, 1/tau(c) increases approximately linearly with increasing NR aspect ratio; for a given sample, 1/tau(c) increases very gradually with increasing excitation intensity. Examining 1/tau(c)versus the single-particle photon absorption rate for all samples indicates that the change in NR absorption cross section with sample size can account for some but not all of the differences in crossover time. This suggests that the degree of quantum confinement may be partially responsible for the aspect ratio dependence of the crossover time.     

Synthesis and characterization of mondispersed CdSe nanocrystals at lower temperature

Nearly monodispersed CdSe quantum dots have been prepared by a soft solution approach using air-stable reagents at lower temperature. The temporal evolution of the absorption and room temperature photoluminescence spectra were used to follow the reaction process and to characterize the optical properties of as-prepared CdSe quantum dots. The results exhibited clear exciton peaks in absorption and bright band-edge luminescence. The structures of the CdSe nanocrystals were determined by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The influence of the temperature on the properties of the resultant CdSe nanocrystals was investigated. The distribution of properties within ensembles of CdSe nanocrystals was also studied. A drastic difference in the photoluminescence efficiencies of size-selected fractions was observed.     

Synthesis of InAs/CdSe/ZnSe core/shell1/shell2 structures with bright and stable near-infrared fluorescence

A complex InAs/CdSe/ZnSe core/shell1/shell2 (CSS) structure is synthesized, where the intermediate CdSe buffer layer decreases strain between the InAs core and the ZnSe outer shell. This structure leads to significantly improved fluorescence quantum yield as compared to previously prepared core/shell structures and enables growth of much thicker shells. The shell growth is done using a layer-by-layer method in which the shell cation and anion precursors are added sequentially allowing for excellent control, and a good size distribution is maintained throughout the entire growth process. The CSS structure is characterized using transmission electron microscopy, as well as by X-ray diffraction and X-ray photoelectron spectroscopy which provide evidence for shell growth. The quantum yield for CSS with small InAs cores reaches over 70%-exceptional photoluminescence intensity for III-V semiconductor nanocrystals. In larger InAs cores there is a systematic decrease in the quantum yield, with a yield of approximately 40% for intermediate size cores down to a few percent in large cores. The CSS structures also exhibit very good photostability, vastly improved over those of organically coated cores, and transformation into water environment via ligand exchange is performed without significant decrease of the quantum yield. These new InAs/CdSe/ZnSe CSS nanocrystals are therefore promising near-IR chromophores for biological fluorescence tagging and optoelectronic devices.     

水溶性CdSe/CdS纳米晶合成条件的优化研究

以巯基乙醇为修饰剂,在水溶液中合成了稳定的CdSe/CdS纳米晶,应用单因素法和多目标单纯形法探索合成条件。通过透射电镜观察所合成的纳米晶的形貌和大小,用紫外-可见吸收光谱和荧光光谱对其光学特性进行了表征。并且以L-色氨酸荧光量子产率0.14为标准,测量了合成的CdSe/CdS纳米晶的荧光量子产率为0.37。     

Ultra-broadband near-infrared excitable upconversion core/shell nanocrystals

Monodisperse Er(3+):NaGdF(4)@Ho(3+):NaGdF(4)@NaGdF(4) active-core/active-shell/inert-shell nanocrystals, which can extend the near-infrared wavelength excitable range for upconversion (UC) emissions, were successfully fabricated for the first time. Importantly, doping of Er(3+) and Ho(3+) into the core and shell respectively suppresses adverse energy transfers between them, resulting in intense UC emissions for both Er(3+) and Ho(3+) dopants.     

Radial-position-controlled doping in CdS/ZnS core/shell nanocrystals

In this paper, we report a new doping approach using a three-step synthesis to make high-quality Mn-doped CdS/ZnS core/shell nanocrystals. This approach allows precise control of the Mn radial position and doping level in the core/shell nanocrystals. On the basis of this synthetic advance, we have demonstrated the first example in which optical properties of Mn-doped nanocrystals strongly depend on Mn radial positions inside the nanocrystals. In addition, we have synthesized nanocrystals with a room-temperature Mn-emission quantum yield of 56%, which is nearly twice as high as that of the best Mn-doped nanocrystals reported previously. Nanocrystals with such a high-emission quantum yield are very important to applications such as nanocrystal-based biomedical diagnosis.     

Preparation and application of L-cysteine-modified CdSe/CdS core/shell nanocrystals as a novel fluorescence probe for detection of nucleic acid

The water-soluble L-cysteine-modified CdSe/CdS core/shell nanocrystals (expressed as CdSe/CdS/Cys nanocrystals) have been synthesized in aqueous by using L-cysteine as stabilizer. The size, shape, component and spectral property of CdSe/CdS/Cys nanocrystals were characterized by high-resolution transmission electron microscope (HRTEM), energy dispersive X-ray fluorescence (EDX), infrared spectrum (IR) and photoluminescence (PL). The results showed that the spherical CdSe/CdS/Cys nanocrystals with an average diameter of 2.3 nm have favorable fluorescent property, theirs photostability and fluorescence intensity are enhanced greatly after overcoating with CdS. The cysteine modified on the surface of core/shell CdSe/CdS nanocrystals renders the nanocrystals water-soluble and biocompatible. Based on the fluorescence quenching of the nanocrystals in the presence of calf thymus deoxyribonucleic acid (ct-DNA), a fluorescence quenching method has been developed for the determination of ct-DNA by using the nanocrystals as a novel fluorescence probe. The pH value of the system was selected at pH 7.4, with excitation and emission wavelength at 380 and 522 nm, respectively. Under the optimal conditions, the fluorescence quenching intensity of the system is linear with the concentration of ct-DNA in the range of 0.1-3.5 microg/mL (r=0.9987). The detection limit is 0.06 microg/mL. And two synthetic samples were analyzed satisfactorily.     

Large-scale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction

Successive ion layer adsorption and reaction (SILAR) originally developed for the deposition of thin films on solid substrates from solution baths is introduced as a technique for the growth of high-quality core/shell nanocrystals of compound semiconductors. The growth of the shell was designed to grow one monolayer at a time by alternating injections of air-stable and inexpensive cationic and anionic precursors into the reaction mixture with core nanocrystals. The principles of SILAR were demonstrated by the CdSe/CdS core/shell model system using its shell-thickness-dependent optical spectra as the probes with CdO and elemental S as the precursors. For this reaction system, a relatively high temperature, about 220-240 degrees C, was found to be essential for SILAR to fully occur. The synthesis can be readily performed on a multigram scale. The size distribution of the core/shell nanocrystals was maintained even after five monolayers of CdS shell (equivalent to about 10 times volume increase for a 3.5 nm CdSe nanocrystal) were grown onto the core nanocrystals. The epitaxial growth of the core/shell structures was verified by optical spectroscopy, TEM, XRD, and XPS. The photoluminescence quantum yield (PL QY) of the as-prepared CdSe/CdS core/shell nanocrystals ranged from 20% to 40%, and the PL full-width at half-maximum (fwhm) was maintained between 23 and 26 nm, even for those nanocrystals for which the UV-vis and PL peaks red-shifted by about 50 nm from that of the core nanocrystals. Several types of brightening phenomena were observed, some of which can further boost the PL QY of the core/shell nanocrystals. The CdSe/CdS core/shell nanocrystals were found to be superior in comparison to the highly luminescent CdSe plain core nanocrystals. The SILAR technique reported here can also be used for the growth of complex colloidal semiconductor nanostructures, such as quantum shells and colloidal quantum wells.     

Synthesis of multifunctional Fe3O4 core/hydroxyapatite shell nanocomposites by biomineralization

Superparamagnetic nanoparticles with surface functional groups (-OH, -COOH and -NH(2)) were modified by in situ deposition of hydroxyapatite (HA) on the materials' surface through the biomineralization process to form Fe(3)O(4) core/hydroxyapatite shell nanocomposites. They possess potential applications as targeted carriers for antitumor drugs and as bone tissue engineering scaffolds by integrating multiple functions into a single nanosystem.     

Photorefractive performance of a CdSe/ZnS core/shell nanoparticle-sensitized polymer

We report the photorefractive performance of a polymer composite sensitized by CdSe/ZnS core/shell nanoparticles, and also comprising poly(N-vinylcarbazole) and an electro-optic chromophore. The nanoparticles are characterized by absorption and photoluminescence spectroscopy, elemental analysis, transmission electron microscopy, and powder x-ray diffraction. The electro-optic response of the composite is measured independently of the photorefractive effect by transmission ellipsometry. An asymmetric two-beam coupling gain of 30.6+/-0.4 cm(-1) is obtained, confirming photorefractivity. Degenerate four-wave mixing is used to assess photorefractive performance and, at a poling field of 70 V microm(-1), yields a diffraction efficiency of 4.21%+/-0.03%, a holographic contrast of 3.05 x 10(-4)+/-1 x 10(-6), a space-charge rise time of 25+/-2 s, and a sensitivity of 4.7 x 10(-5)+/-4 x 10(-6) cm3 J(-1). These results constitute a significant improvement on the performance of previous nanoparticle-sensitized photorefractive polymer composites.     

Fe3O4/Polypyrrole/Au nanocomposites with core/shell/shell structure: synthesis, characterization, and their electrochemical properties

Uniform Fe3O4 nanospheres with a diameter of 100 nm were rapidly prepared using a microwave solvothermal method. Then Fe304/polypyrrole (PPy) composite nanospheres with well-defined core/shell structures were obtained through chemical oxidative polymerization of pyrrole in the presence of Fe3O4; the average thickness of the coating shell was about 25 nm. Furthermore, by means of electrostatic interactions, plentiful gold nanoparticles with a diameter of 15 nm were assembled on the surface of Fe3O4/PPy to get Fe3O4/PPy/Au core/shell/shell structure. The morphology, structure, and composition of the products were characterized by transmission electronic microscopy (TEM), scanning electronic microscopy (SEM), X-ray powder diffraction (XRD), and Fourier transform infrared (FT-IR) spectroscopy. The resultant nanocomposites not only have the magnetism of Fe3O4 nanoparticles that make the nanocomposites easily controlled by an external magnetic field but also have the good conductivity and excellent electrochemical and catalytic properties of PPy and Au nanoparticles. Furthermore, the nanocomposites showed excellent electrocatalytic activities to biospecies such as ascorbic acid (AA).     

On doping CdS/ZnS core/shell nanocrystals with Mn

This paper presents a mechanistic study on the doping of CdS/ZnS core/shell semiconductor nanocrystals with Mn based on a three-step synthesis, which includes host-particle synthesis, Mn-dopant growth, and ZnS-shell growth. We used a combination of electron paramagnetic resonance spectroscopy (EPR) and inductively coupled plasma atomic emission spectroscopy (ICP) to monitor Mn-doping level and growth yield during doping synthesis at both the dopant-growth and ZnS-shell-growth steps. First, our kinetic study shows that Mn adsorption onto the nanocrystal surface includes the formation of weakly and strongly bound Mn. The formation of weakly bound Mn is associated with a chemical equilibrium between adsorbed Mn species on the nanocrystal surface and free Mn species in growth solution, while the formation of strongly bound Mn exhibits first-order kinetics with an activation-energy barrier of 211 +/- 13 kJ/mol. Second, our results demonstrate that both weakly and strongly bound Mn can be removed from the surface of nanocrystals during ZnS-shell growth. The replacement of strongly bound Mn requires a higher temperature than that of weakly bound Mn. The yield of the replacement of strongly bound Mn is strongly dependent on the temperature of ZnS-shell growth. Third, our results show that the Mn-growth yield is not dependent on the size and crystal structure of nanocrystals. All together, these results suggest a mechanism in which nanocrystal doping is determined by the chemical kinetics of three activation-controlled processes: dopant adsorption, replacement, and ZnS-shell growth.     

Mn:ZnSe/ZnS核-壳结构纳米晶的无膦法制备和光学性质

以溶于十八烯的Se作为Se前驱体,在无膦条件下制备得到了具有较高量子产率的Mn:ZnSe纳米晶.为了进一步提高纳米晶的稳定性和发光强度,运用外延生长的方法进行ZnS壳层包覆并得到了具有核-壳结构的Mn:ZnSe/ZnS纳米晶.X射线衍射、透射电子显微镜及吸收和荧光光谱测试结果表明,该方法合成的Mn:ZnSe纳米晶以及核-壳结构Mn:ZnSe/ZnS纳米晶均为闪锌矿结构,具有良好的单分散性,包覆ZnS外壳层后量子产率可达到60%以上.此外,对ZnS壳层厚度和Mn2+的掺杂量对Mn:ZnSe/ZnS纳米晶发光强度的影响及发光机制也进行了初步讨论.