Continuous white luminescence of ZnS:Pb2+ and core/shell ZnS:Pb2+/ZnS nanocrystals

High-quality water-soluble ZnS:Pb2+ nanocrystals were synthesized via a simple chemical codepositing method. The as-synthesized ZnS:Pb2+ nanocrystals show high monodispersity and crystallinity with a narrow size distribution (3.2 +/- 0.4 nm). ZnS:Pb2+/ZnS core/shell structures were also obtained by coating a ZnS shell displaying significantly enhanced photoluminescence (PL) intensity and photostability. For the ZnS:Pb2+/ZnS samples the position of emission spectrum shows a red-shift of approximately 10 nm, which produces a fairly pure white emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.31, 0.33). These phenomena are explained by a model of multiple Pb2+ luminescent centers.     

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.     

Luminescent properties of ZnS:Mn2+/ZnS core/shell nanocrystals

High-quality ZnS:Mn2+/ZnS core/shell nanocrystals (NCs) with a core crystal diameter of 6.1 nm and 1.15 nm thick shells were synthesized via a high-boiling solvent process. The energy levels of the conduction band and valance band are estimated to be -3.2 eV and -6.8 eV by cyclic voltammetry and ultraviolet-visible (UV-vis) absorption spectra. The ZnS:Mn2+/ZnS NC emission peak is primarily located at 580 nm under 310 nm light excitation, originating from the charge transition from 4T1 to 6A1 within the 3d5 configuration of the Mn2+ ion. Based on ZnS:Mn2+/ZnS NCs as the active layer electroluminescent devices, the emission peak mainly locates at 460 nm with one shoulder emission peaking at 580 nm. The photoluminescence and electroluminescence properties of ZnS:Mn2+/ZnS NCs are investigated in the view of charge carrier injection and energy level alignment.     

Synthesis and luminescence properties of ZnS:Mn/ZnS core/shell nanorod structures

Mn-doped ZnS nanorods synthesized by solvothermal method were successfully coated with ZnS shells of various thicknesses. The powder X-ray diffraction (XRD) measurements showed the ZnS:Mn nanorods were wurtzite structure with preferential orientation along c-axis. Transmission electron microscopy images (TEM) revealed that the ZnS shells formed from small particles, growing along a-axis orientation, which was proved by the XRD measurements. Room temperature photoluminescence (PL) spectra showed that the intensity of Mn emission first increased and then decreased with the thickening of the ZnS shells. The effects of ZnS shells on the luminescence properties of ZnS:Mn nanorods is discussed.     

Phase and morphology controlled synthesis of high-quality ZnS nanocrystals

Through a facile solvothermal method, the controlled preparation of ZnS nanocrystals with different phases and morphologies was achieved only by changing the organic additives. By adding the surfactant of sodium dodecyl benzene sulfonate (SDBS) into the reaction, the cubic heart-like ZnS nanoparticles with uniform size were obtained in a large scale. While, with the assistance of the biomolecule of alginic acid, the pure phase of hexagonal ZnS nanospheres assembled from small ZnS nanoparticles were synthesized. The optical properties of the obtained ZnS nanocrystals were investigated by ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) spectra. The quantum confinement effect could be observed clearly in ZnS nanoparticles.     

核壳结构CdS/ZnS荧光量子点的制备与荧光性能研究

以氧化镉为镉源、硫单质为硫源、油酸为配体、在十八烯体系中合成单分散的CdS纳米颗粒,研究了配体浓度对纳米微粒的生长动力学、颗粒尺寸分布的影响.采用乙基黄原酸锌作为Zn、S源的反应前体,采用逐滴滴加的方法制备了具有核壳结构的CdS/ZnS量子点,吸收光谱和荧光光谱表明CdS/ZnS纳米粒子比单一的CdS纳米粒子具有更优异的发光特性.透射电子显微镜、X射线粉末衍射、X射线光电子能谱、选区电子衍射证明ZnS在CdS表面进行了有效包覆.所制备核壳结构纳米粒子具有较好的尺寸分布,荧光发射峰半高峰宽为18～20nm,荧光量子产率达40%.     

Solvothermal synthesis,characterization and photoluminescence studies of ZnS:Eu nanocrystals

Europium doped zinc sulfide nanocrystals (ZnS:Eu) are prepared by solvothermal method. Crystallite size and lattice constant of the prepared samples are calculated from the X-ray diffraction patterns. The as-prepared samples are found to be a mixture of complex chemical groups. Heat treatment of the samples at 300 °C resulted in ZnS:Eu state. The crystal structure is not affected by the increase in the concentration of Eu from 1 mol% to 5 mol%. Fourier Transform Infrared Spectroscopy (FTIR) studies showed that characteristic absorption bands of hydroxyl groups and the acetate bands increased with increase in Eu concentration. The morphological results studied using Scanning Electron Microscopy (SEM) indicate agglomeration of nanoparticles and a marginal increase in the particle size. Photoluminescence (PL) spectra of the samples showed a prominent emission band peaked at ∼400 nm besides three weak ones at ∼422, 485 and 530 nm. The PL intensity increased with increase in Eu concentration.     

Pump-intensity- and shell-thickness-dependent evolution of photoluminescence blinking in individual core/shell CdSe/CdS nanocrystals

We report a systematic study of photoluminescence (PL) intensity and lifetime fluctuations in individual CdSe/CdS core/shell nanocrystal quantum dots (NQDs) as a function of shell thickness. We show that while at low pump intensities PL blinking in thin-shell (4-7 monolayers, MLs) NQDs can be described by random switching between two states of high (ON) and low (OFF) emissivities, it changes to the regime with a continuous distribution of ON intensity levels at high pump powers. A similar behavior is observed in samples with a medium shell thickness (10-12 MLs) without, however, the PL intensity ever switching to a complete "OFF" state and maintaining ca. 30% emissivity ("gray" state). Further, our data indicate that highly stable, blinking-free PL of thick-shell (15-19 MLs) NQDs ("giant" or g-NQDs) is characterized by nearly perfect Poisson statistics, corresponding to a narrow, shot-noise limited PL intensity distribution. Interestingly, in this case the PL lifetime shortens with increasing pump power and the PL decay may deviate from monoexponential. However, the PL intensity distribution remains shot-noise limited, indicating the absence of significant quantum yield fluctuations at a given pump power intensity during the experimental time window.     

Highly luminescent blue emitting CdS/ZnS core/shell quantum dots via a single-molecular precursor for shell growth

Highly luminescent blue-emitting CdS/ZnS core/shell quantum dots (QDs) were synthesized in N-oleoylmorpholine by two facile steps: first, the CdS core QDs were prepared via a simple one-pot method involving a direct reaction of Cd precursor cadmium stearate and S precursor S powder in solvent N-oleoylmorpholine; second, ZnS shells were successively overcoated on CdS core through the decomposition of single molecular precursor zinc diethyldithiocarbamate. The thickness of shell was precisely tuned by controlling drip feed speed and amount of shell precursor. The obtained CdS/ZnS core/shell QDs showed the maximum photoluminescent quantum yield of 54.8% and narrow spectra bandwidth, exhibiting high monodispersity, good color purity and long fluorescent lifetimes. The CdS/ZnS core/shell QDs with tunable emission wavelength of 424–470 nm were obtained by controlling the thickness of ZnS shell overgrown on different-sized CdS QDs, which are promising materials for blue light-emitting devices.     

ZnS:Cu纳米颗粒的制备及发光性质

采用水热法制备了Cu离子掺杂的ZnS(ZnS:Cu)纳米颗粒,研究了锌硫比和反应时间对ZnS:Cu纳米颗粒光致发光性质的影响.通过X射线衍射(XRD)和透射电子显微镜(TEM)对样品的物相和形貌进行分析表征,发现该方法得到立方闪锌矿结构的球形ZnS: Cu纳米晶,粒径在1～6nm之间.室温下,用350nm波长的紫外光激发ZnS:Cu纳米粒子,可以得到归属于浅施主能级与铜￡z能级之间的跃迁产生的绿色发光,发光强度随锌硫比的增大和反应时间的延长先增强后减弱,发射峰位随锌硫比和反应时间的变化有一定移动.认为浅施主能级为与硫空位有关的能级,锌硫比和反应时间对硫空往的数量和能级位置有一定影响.     

Growth of semiconducting nanocrystals of CdS and ZnS

In this work, we report the study of growth of CdS as well as ZnS nanocrystals using in-situ small angle X-ray scattering (SAXS) technique, in presence of thio-glycerol as capping agent. We observe that the diameter of the nanocrystal is controlled between 1 and 3 nm by varying the temperature of the reaction. Further, the self-focusing of the size distribution can be observed and is more pronounced at higher temperatures reducing its relative width from 25% to 10%.     

Epitaxial growth of colloidal hydrophilic lattice matched HgTe/CdTe core/shell nanocrystals for UV luminescence

The present work reports a bottom up approach to synthesize water dispersed and highly luminescent hydrophilic l-cysteine ethyl ester hydrochloride (LEEH) capped and lattice matched HgTe/CdTe core/shell nanoparticles (NPs). The LEEH concentration was varied to optimize the luminescent properties of the core/shell NPs by controlling the particle size and its size distribution. FTIR measurements confirm the LEEH capping to the core/shell nanostructures. Transmission electron microscopy (TEM) measurements identify the HgTe, core size as 2.0 nm and that of the shell, epitaxially grown over the core has the thickness as 0.8 nm which corresponds to 2.1 monolayers of CdTe for an optimum LEEH concentration of 4.8 mM. The absorption studies showed excitonic absorption at 293 nm for the optimized core/shell NPs. More than three fold increase of fluorescence (FL) yield in case of the core/shell structure compared to the core has been observed in this study. X-ray diffraction and micro-Raman studies do not show an alloying effect. The crystalline sizes estimated both from X-ray diffraction and HRTEM are found to be comparable.     

High color rendering index white LED based on nano-YAG:Ce3+ phosphor hybrid with CdSe/CdS/ZnS core/shell/shell quantum dots

To improve the poor color rendering index (CRI) of YAG:Ce-based white light-emitting diode (LED) due to the lack of red spectral component, core/shell/shell CdSe/CdS/ZnS quantum dots (QDs) were synthesized and blended into nano-YAG:Ce³⁺ phosphors. Prominent spectral evolution has been achieved by increasing the content of QDs. A white LED combining a blue LED with the blends of nano-YAG phosphors and orange- and red-emission QDs with a weight ratio of 1:1:1 was obtained. This kind of white LED showed excellent white light with luminescent efficiency, color coordinates, CRI and correlated color temperature (CCT) of 82.5?lm/W, (0.3264, 0.3255), 91 and 4580?K, respectively.     

Phosphine-free synthesis of high-quality reverse type-I ZnSe/CdSe core with CdS/Cd(x)Zn(1 - x)S/ZnS multishell nanocrystals and their application for detection of human hepatitis B surface antigen

Highly photoluminescent (PL) reverse type-I ZnSe/CdSe nanocrystals (NCs) and ZnSe/CdSe/CdS/Cd(x)Zn(1 - x)S/ZnS core/multishell NCs were successfully synthesized by a phosphine-free method. By this low-cost, 'green' synthesis route, more than 10 g of high-quality ZnSe/CdSe/CdS/Cd(x)Zn(1 - x)S/ZnS NCs were synthesized in a large scale synthesis. After the overgrowth of a CdS/Cd(x)Zn(1 - x)S/ZnS multishell on ZnSe/CdSe cores, the PL quantum yields (QYs) increased from 28% to 75% along with the stability improvement. An amphiphilic oligomer was used as a surface coating agent to conduct a phase transfer experiment, core/multishell NCs were dissolved in water by such surface modification and the QYs were still kept above 70%. The as-prepared water dispersible ZnSe/CdSe/CdS/Cd(x)Zn(1 - x)S/ZnS core/multishell NCs not only have high fluorescence QYs but also are extremely stable in various physiological conditions. Furthermore, a biosensor system (lateral flow immunoassay system, LFIA) for the detection of human hepatitis B surface antigen (HBsAg) was developed by using this water-soluble core/multishell NCs as a fluorescent label and a nitrocellulose filter membrane for lateral flow. The result showed that such ZnSe/CdSe/CdS/Cd(x)Zn(1 - x)S/ZnS core/multishell NCs were excellent fluorescent labels to detect HBsAg. The sensitivity of HBsAg detection could reach as high as 0.05 ng ml( - 1).     

Reduced photo-instability of luminescence spectrum of core-shell CdSe/CdS nanocrystals

Coated CdSe/CdS nanocrystals have been synthesized through a new reaction routine in micelle solution. High resolution transmission electron microscope image demonstrated the monodispersity and core-shell structure of CdSe/CdS nanocrystals. Photooxidation experiments shown that by coating with a wider bandgap material CdS, the surface states of CdSe cores could be decreased and the photo-instability of their luminescence spectrum could be reduced. This phenomena were temporary interpreted that lack of carriers at the outlayer surface reduced photooxidation degradation of CdSe/CdS nanocrystals.     

Effects of synthesis temperature on particle size/shape and photoluminescence characteristics of ZnS:Cu nanocrystals

ZnS:Cu nanocrystals are prepared by solution synthesis technique, and the optimum range of synthesis temperature and the particle size/shape for the high photoluminescence (PL) properties of ZnS:Cu nanocrystals are investigated in this study. With an increase of synthesis temperature, the crystal structure is not changed. However, the particle size increases slightly. PL intensity is maximum for the sample prepared at 85 °C for the temperature range used in this study (70–95 °C). This is the first observation for the ZnS:Cu nanocrystals prepared by solution synthesis technique. The X-ray photoelectron spectroscopy (Cu2p_3/2), performed to explain this phenomenon, demonstrates that with lower synthesis temperature (<85 °C), Cu acts as the normal accepter dopant, leading to the high PL intensity. With higher temperature (>90 °C), however, Cu is transformed into CuO, and CuO acts as the nonradiative recombination center, resulting in the substantial decrease of PL intensity observed in this study.