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.     

Colloidal CdSe nanocrystals from tri-n-octylphosphine: Part II: control of growth rate for high quality and large-scale production by tuning Cd-to-Se stoichiometry

Colloidal CdSe nanocrystals were synthesized in reaction media consisting of tri-n-octylphosphine (TOP) without addition of other species; the single-step approach used cadmium oxide (CdO) and TOPSe as Cd and Se sources, respectively. The temporal evolution of the optical properties of the growing TOP-capped CdSe nanocrystals was monitored for a couple of hours, showing that there are two distinguishable stages of growth: an early stage (less than 5 minutes) and a later stage; the growth kinetics of the two stages is a function of the Cd-to-Se precursor molar ratios. A rational choice of 2-6Cd-to-1Se molar ratio was found, based on the temporal evolution of the photoluminescent (PL) efficiency (studied as PL intensity and sensitivity to the media of dispersion, and non-resonant Stokes shifts). For a 2Cd-to-1Se synthesis, the growth in size was slow in the early stages and became fast in the later stages; this fast-later-stage feature could be suppressed by going to a synthesis with a 4-6Cd-to-1Se mole ratio: the nanocrystals between 0.5-60 min growth time exhibit very much similar optical properties, with less than 19 nm redshift of bandgap absorption and emission occurring. Thus, the synthetic route developed here, with a rational 4-6Cd-to-1Se molar ratio, enables us to produce high-quality CdSe nanocrystals on a large-scale with a high degree of synthetic reproducibility. The insights gained facilitate a deeper understanding of the concept of what constitutes high-quality nano-crystals: high PL efficiency resulting from a low growth rate, which can be thoroughly and readily investigated by the red-shift rate of the band-gap peak positions; in addition, the insights gained help us to define a proper synthetic approach for large-scale production with high-quality product.     

不同温度下硒化镉(CdSe)量子点的生长及荧光性研究

研究了以氧化镉（CdO）和硒（Se）粉为前驱体,在三辛基膦（TOP）和油酸中合成无机半导体量子点（quantum dots, QDs）CdSe.研究了在不同的反应温度下粒子的生长,通过紫外吸收光谱（UV-Vis）、荧光发射光谱（PL）、透射电子显微镜（TEM）等手段跟踪反应过程并对样品性能进行了表征.实验结果表明,反应温度和反应时间对量子点的生长和荧光性能有很大的影响.     

Preparation and characterization of CdSe nanoparticles in the presence of Triocytlphosphine as solvent and capping agent

Triocytlphosphine (TOP)-capped CdSe nanoparticles (NPs) have been successfully prepared by the one-pot solution growth method at 240 °C under argon atmosphere. In particular, The TOP used in this process as the single coordinating solvent is favorable for probing capping mechanism of CdSe NPs surface. The growth process and characterization of CdSe NPs were determined by photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), Transmission electron microscopy (TEM), Ultraviolet-visible (UV-Vis) spectroscopy and Fourier transform infrared spectroscopy (FTIR). Results demonstrated the TOP-capped CdSe NPs to be well dispersed and uniform in shape and the diameter of the particle was confined within 8 nm. PL measurement showed the near band-edge luminescence of the final product.     

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.     

In-situ observation of nucleation and growth of PbSe magic-sized nanoclusters and regular nanocrystals

In-situ observation of the temporal evolution of the absorption of PbSe nanocrystals (NCs) via a low-temperature noninjection approach is presented. Based on a model reaction of lead oleate (Pb(OA)(2) ) and n-trioctylphosphine selenide (TOPSe) in 1-octadecene at 35-80 °C, the use of commercially available TOP (90 or 97%) in affecting the formation of the NCs is explored. TOPSe solutions made from TOP 90% exhibited higher reactivity than those made from TOP 97%. (31)P NMR spectroscopy detected no dioctylphosphine selenide (DOPSe) but some DOP in ≈1.0 M TOPSe/TOP solution (made from TOP 90%), as well as no diphenylphosphine selenide (DPPSe) when DPP was added to the ≈1.0 M solution. Hence, it is proposed that, for the formation of PbSe monomers, an indirect pathway dominates with the formation of a Pb-P complex/intermediate, which results from the activation of Pb(OA)(2) by a phosphine compound (such as DPP, DOP, or TOP) and in turn reacts with TOPSe. With the use of TOP 90% and the addition of secondary phosphine DPP, the formation of PbSe magic-sized nanoclusters (MSNCs) and regular NCs (RNCs) is investigated. With proper tuning of the synthesis conditions, the formation of various PbSe MSNCs versus RNCs is monitored in situ with versus without the addition of DPP, or at different reaction temperatures but otherwise identical synthetic formulation and reaction parameters. Accordingly, the degree of supersaturation (DS) of the PbSe monomer affecting the development of these PbSe MSNCs versus RNCs is proposed; the higher the DS, the more the MSNCs are favored. Also, surface-determined cluster-cluster aggregation is proposed to be the growth mechanism for both the RNCs and MSNCs. For the former, quantized growth is followed by continuous growth. For the latter, the sizes of the magic-sized families are calculated.     

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.     

Colloidal synthesis and air-annealing of CdSe nanorods for the applications in hybrid bulk hetero-junction solar cells

TOP/TOPO-capped CdSe nanorods were synthesized by colloidal route process using hexylphosphonic acid as a ligand and CdO as a Cd precursor, and the influence of thermal annealing in air on the morphology, crystal structure and optical properties of CdSe nanorods has been elucidated by XRD, TEM and PL analysis. It was observed that as-synthesized CdSe nanorods of ∼ 4-5 nm diameters and ∼ 20 nm lengths go through solid state morphology transformation upon annealing at 350 °C in air atmosphere, forming the nanorods with bullet shape with one side pointed and the other side having a blunt hexagon with the average diameter of ∼ 12 nm and length of ∼ 25 nm. The annealed CdSe nanorods showed improved PL emissions without the shift of peak positions, indicating significant improvement of the crystallinity and optoelectronic properties of nanorods caused by reduction of defects.     

High quality zinc-blende CdSe nanocrystals synthesized in a hexadecylamine–oleic acid–paraffin liquid mixture

Safe, common, and low-cost compounds were used as solvents for the non-tri-n-octylphosphine (TOP) synthesis of high quality CdSe nanocrystals (NCs) in open air. In oleic acid–paraffin liquid system, CdSe nanocrystals in the less common zinc-blende (ZB, cubic) crystal structure have been obtained. The effects of adding n-hexadecylamine (HDA) to different solutions were discussed. Stable, highly homogeneous and luminescent CdSe nanocrystals were formed by adding n-hexadecylamine to Cd solution rather than to Se solution. Without any size sorting, the size distribution of the nanocrystals can be readily controlled and the highest photoluminescence (PL) quantum efficiency (QE) of the nanocrystals was up to 20–30%.     

Synthesis and optical properties of CdS nanorods and CdSe nanocrystals using oleylamine as both solvent and stabilizer

CdS nanorods and CdSe nanocrystals were prepared via the one-pot synthesis approach in oleylamine (OLA) system. The OLA used in this process as both the solvent and stabilizer is favorable for probing capping mechanism and simplifying experimental steps. The growth process and characterization of cadmium chalcogenide nanocrystals were determined by X-ray diffraction (XRD), transmission electron microscopy (TEM), Ultraviolet-visible (UV-vis) spectroscopy and photoluminescence (PL) spectroscopy. Results demonstrated OLA-capped CdS nanorods and CdSe nanocrystals were highly crystalline and had good optical properties.     

Synthesis of CdSe nanocrystals in a noncoordinating solvent: effect of reaction temperature on size and optical properties

Colloidal synthesis of high quality CdSe nanocrystals with controllable size and tunable properties have been one of the most important topics of research over the last decade, in view of its huge technological potentials. CdSe is one of the most studied nanocrystals of this category because of its photoluminescence tunability across the visible spectrum. We have synthesized CdSe nanocrystals using CdO precursor in a noncoordinating solvent and studied the effect of the reaction temperature on the size and optical properties of the nanocrystals. The size of the nanocrystals could be varied systematically in the range of 3.5 to 6.6 nm diameter with a remarkably narrow size distribution by controlling only the reaction temperature, without any need for a post-synthesis processing. The band gap and the corresponding band edge emission could be tuned across the entire visible range by tuning the size of the nanocrystals. The narrow width of the photoluminescence emissions of different colours (blue to red) make these nanocrystals a potential candidate for different optical and optoelectronic devices.     

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.     

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．     

CdSe/Cd(x)Zn1-xS core/shell nanocrystals: core morphology and luminescent property

CdSe cores with rod (an aspect ratio of 1.8, d-5 nm) and spherical (an aspect ratio of 1, d-5 nm) morphologies were fabricated by two kinds of organic approaches through adjusting growth processes. Because of large difference of size and morphology, two kinds of cores revealed different absorption spectra. However, these cores exhibited almost same photoluminescence (PL) spectra with a red-emitting PL peak of around 625 nm. This is ascribed that they have a similar size in diameter. A graded Cd(x)Zn1-xS shell of larger band gap was grown around CdSe rods and spheres using oleic acid as a capping agent. Based on the growth kinetics of CdS and ZnS, interfacial segregation was created to preferentially deposit CdS near the core, providing relaxation of the strain at the core/shell interface. For spherical CdSe cores, the homogeneous deposition of the Cd(x)Zn1-xS shell created spherical core/shell nanocrystals (NCs) with a size of 7.1 nm in diameter. In the case of using CdSe cores with rod morphology, the anisotropic aggregation behaviors of CdS monomers on CdSe rods led to the size (approximately 10 nm in diameter) of spherical CdSe/Cd(x)Zn1-xS core/shell NCs with a small difference to the length of the CdSe rod (approximately 8.9 nm). The resulting spherical core/shell NCs created by the rod and spherical cores exhibited almost same PL peak wavelength (652 and 653 nm for using rod and spherical cores, respectively), high PL efficiency up to 50%, and narrow PL spectra (36 and 28 nm of full with at half maximum of PL spectra for the core/shell NCs with CdSe spheres and rods, respectively). These core/shell NCs provide an opportunity for the study of the evolution of PL properties as the shape of semiconductor NCs.     

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.     

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.     

Detection of pathogens using luminescent CdSe/ZnS dendron nanocrystals and a porous membrane immunofilter

A biosensor system for detection of pathogens was developed by using CdSe/ZnS core/shell dendron nanocrystals with high efficiency and stability as fluorescence labels and a flowing chamber with a microporous immunofilter. The antibody-immobilized immunofilter captured the targeted pathogens, Escherichia coli O157:H7 as an example for bacteria and hepatitis B being a model system for viruses. The CdSe/ZnS core/shell dendron nanocrystals were conjugated with the corresponding antibodies and then passed through the microporous membrane where they attached to the membrane-antigen-antibody. The efficient and stable photoluminescence (PL) of the CdSe/ZnS nanocrystals on the formed "sandwich" structure complexes (membrane-antigen-antibody conjugated with the nanocrystals) was used as the detection means. The effects of the pore size of the membranes, buffer pH, and assay time on the detection of E. coli O157:H7 were investigated and optimized. The detectable level of this new system was as low as 2.3 CFU/mL for E. coli O157:H7 and 5 ng/mL for hepatitis B surface Ag (HBsAg). The assay time was shortened to 30 min without any enrichment and incubation.     

Solvothermal synthesis of well-dispersed MF(2) (M = Ca,Sr,Ba) nanocrystals and their optical properties

MF(2)?(M = Ca,Sr,Ba) nanocrystals (NCs) were synthesized via a solvothermal process in the presence of oleic acid and characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectra, UV/vis absorption spectra, photoluminescence (PL) excitation and emission spectra, and lifetimes, respectively. In the synthetic process, oleic acid as a surfactant played a crucial role in confining the growth and solubility of the MF(2) NCs. The as-prepared CaF(2), SrF(2) and BaF(2) NCs present morphologies of truncated octahedron, cube and sheet in a narrow distribution, respectively. Possible growth mechanisms were proposed to explain these results. The as-prepared NCs are highly crystalline and can be well dispersed in cyclohexane to form stable and clear colloidal solutions, which demonstrate strong emission bands centred at 400?nm in photoluminescence (PL) spectra compared with the cyclohexane solvent. The PL properties of the colloidal solutions of the as-prepared NCs can be ascribed to the trap states of surface defects.     

魔力尺寸硒化镉纳米晶的压力稳定性研究

在极端条件下应用的耐压材料必须具有极高的结构和性能稳定性.魔力尺寸纳米晶体具有良好的结构、超小的粒子尺寸和精确的原子组成,逐渐成为研究的热点.采用胶体化学方法合成了硒化镉(CdSe)魔力尺寸纳米晶,其第一激子吸收峰位于463 nm处,相应的光致发光光谱表现出窄的半高宽,仅为13 nm.进一步利用金刚石对顶砧压机研究了所...     

Preparation and characteristics of nanotetrapods CdSe-polymer hybrid solar cells

Tetrapod-shaped CdSe nanocrystals were obtained using a simple method. HRTEM shows that the average size of the tetrapod core are about 4 nm, widths of the tetrapod arms are about 4 nm and lengths of the arms are about 20 nm. XRD patterns reveal that the OA-capped CdSe tetrapod nanocrystals have a hexagonal wurtzite structure. A hybrid solar cell fabricated based on an 8:1 (w/w) blend of CdSe tetrapod nanocrystals and MEH–PPV showed a maximum power conversion efficiency of 0·46% under an air mass 1·5 global condition. The effects of nanocrystal composition on the photovoltaic properties of hybrid solar cells based on nanotetrapods CdSe/MEH–PPV were investigated. The power conversion efficiency values initially increased and then decreased, but the V _OC values linearly decreased from 1·1 to 0·25 V with increased CdSe nanotetrapod in the blend film. The significant quenching of PL with increased nanotetrapod concentration indicated photo-induced charge transfer between MEH–PPV and CdSe.