Atomistic tight-binding computations of the structural and optical properties of CdTe/CdX (X=S and Se)/ZnS core/shell/shell nanocrystals

A study of CdTe/CdX (X=S and Se)/ZnS core/shell/shell nanocrystals is carried out using atomistic tight-binding theory and the configuration interaction method to provide information for applications in bioimaging, biolabeling, display devices and near-infrared electronic instruments. The calculations yield the dependences of the internal and external passivated shells on the natural behaviours of CdTe/CdX (X=S and Se)/ZnS core/shell/shell nanocrystals. The reduction of the optical band gaps is observed with increasing numbers of monolayers in the external ZnS shell due to quantum confinement. Interestingly, the optical band gaps of CdTe/CdS/ZnS core/shell/shell nanocrystals are greater than those of CdTe/CdSe/ZnS core/shell/shell nanocrystals. In the presence of an external ZnS-coated shell, electron–hole wave function overlaps, oscillation strengths, ground-state exchange energies and Stokes shift are improved, whereas ground-state coulomb energies and fine-structure splitting are reduced. The oscillation strengths, Stokes shift and fine-structure splitting are reduced with the increase in external ZnS shell thickness. The oscillation strengths, Stokes shift and fine-structure splitting of CdTe/CdS/ZnS core/shell/shell nanocrystals are larger than those of CdTe/CdSe/ZnS core/shell/shell nanocrystals. Reduction of the atomistic electron–hole interactions is observed with increasing external ZnS shell size. The strong electron–hole interactions are more probed in CdTe/CdS/ZnS core/shell/shell nanocrystals than in CdTe/CdSe/ZnS core/shell/shell nanocrystals.     

CuInS2纳米晶的制备和发光性质

以十二硫醇为溶剂,通过选择合适的金属源制备了各种尺寸的CuInS₂量子点。观察到随着粒子的尺寸减小,其吸收和发光光谱明显蓝移,存在明显的量子尺寸效应。通过在CuInS₂纳米晶表面包覆ZnS壳层,发现随着壳层厚度增加,其发光量子效率明显提高,最大达到了48%;继续增加壳层厚度,其发光量子效率反而降低。进一步测量它们的荧光寿命,发现包覆ZnS壳层后的CuInS₂纳米晶的荧光寿命明显增加,证实表面包覆明显减少其表面的无辐射复合中心,提高了其发光效率。进一步制备了CuInS₂/ZnS核壳量子点发光二极管,并对其电致发光性质进行了研究。     

Microemulsion-mediated synthesis of cadmium zinc sulfide nanocrystals with composition-modulated optical properties

Cadmium zinc sulfide nanocrystals were synthesized by a microemulsion-mediated process, which involving two steps: the preparation of CdS (or ZnS) seeds and the succedent hydrothermal growth of ZnS (or CdS) component. The XRD results show that the cadmium zinc sulfide nanocrystals with CdS seeds present a hexagonally homogeneous alloyed structure, while the ones with ZnS seeds mainly take on the characteristic of hexagonal CdS nanocrystals. The intrinsic factors influencing the crystal structures were discussed. The UV-vis and photoluminescence (PL) spectra indicate that the optical properties of the obtained nanocrystals with CdS seeds can be continuously modulated by tuning their compositions, although their sizes and size distributions are not under a strict control. The composition-modulated strategy, along with the hydrothermal microemulsion process, will be an effective route to achieve semiconductor nanocrystals with tunable optical properties under more manageable conditions.     

Highly luminescent (ZnSe)ZnS core-shell quantum dots for blue to UV emission: synthesis and characterization

We synthesized nearly monodisperse bare ZnSe nanocrystallites having luminescence which ranges in wavelength from 340 to 430 nm via nucleation due to supersaturation and growth followed by size selective precipitation. Bare ZnSe dots' outermost surface is passivated with organic HDA/TOP. In order to enhance the radiative emission from the semiconductor nanocrystals, we capped the bare ZnSe quantum dots with ZnS semiconductor materials of a wider band gap and 5% of lattice mismatch and produced highly luminescent core-shell (ZnSe)ZnS quantum dots. The core-shell (ZnSe)ZnS nanocrystals show 20 times or more as greatly enhanced luminescence quantum yields as those of bare ZnSe nanocrystals. The ZnSe bare dots and the (ZnSe)ZnS core-shell dots have cubic zinc blende structures as expected from the bulk structure. The observed shapes of bare ZnSe and core-shell (ZnSe)ZnS dots are nearly spherical or ellipsoidal with the aspect ratios of 1.2 and 1.4, respectively. They are not faceted.     

过渡金属Co2+离子掺杂ZnS硫化物纳米晶制备与中红外发光特性研究

利用不含有机相的简单水热法制备了Co^2+∶ZnS纳米晶,纳米晶具有立方闪锌矿结构,平均晶粒尺寸约为8.3 nm,在808 nm激光泵浦下具有2~5μm波段的中红外荧光发射,中心波长位于3400 nm和4700 nm,分别对应Co^2+离子的4T2(F)→4 A 1(F)和4T1(F)→4T2(F)的能级跃迁.进一步将制备的纳米晶在还原气氛下进行800℃热处理,获得立方闪锌矿和纤锌矿混合晶型的纳米晶,平均晶粒尺寸增大到22.5 nm左右,热处理后的纳米晶表面羟基含量更低,中红外荧光发射强度显著提高.该Co^2+∶ZnS纳米晶的制备方法简单、在制备过程中不引入有机相等荧光淬灭中心,同时证明通过后热处理过程可以进一步减少表面缺陷及羟基含量,使荧光强度得到大幅提升.     

Luminescence enhancement of Mn doped ZnS nanocrystals passivated with zinc hydroxide

Mn-doped ZnS nanocrystals prepared by solvothermal method have been successfully coated with different thicknesses of Zn(OH)₂ shells through precipitation reaction. The impact of Zn(OH)₂ shells on luminescent properties of the ZnS:Mn nanocrystals was investigated. X-ray diffraction (XRD) measurements showed that the ZnS:Mn nanocrystals have cubic zinc blende structure. The morphology of nanocrystals is spherical shape measured by transmission electron microscopy (TEM). ZnS:Mn/Zn(OH)₂ core/shell nanocrystals exhibited much improved luminescent properties than those of unpassivated ZnS:Mn nanocrystals. The luminescence enhancement was observed with the Zn(OH)₂ shell thickening by photoluminescence (PL) spectra at room temperature and the luminescence lifetime of transition from ⁴T₁ to ⁶A₁ of Mn²⁺ ions was also prolonged. This result was led by the effective, robust passivation of ZnS surface states by the Zn(OH)₂ shells, which consequently suppressed nonradiative recombination transitions.     

Brighter glow in ZnS nanocrystals with polarized light

No known reports exist on luminescence enhancement under polarized light excitation. In this study, ZnS nanocrystals have been observed to produce brighter luminescence when excited by polarized light. ZnS:Mn bulk and nanocrystals have shown fivefold to tenfold increase in photoluminescence (PL) intensity when excited with linearly polarized light at 305 nm and 340 nm. Luminescence enhancement to a lesser degree was observed with linearly polarized light excitation for ZnS:Cu, Al and ZnS:Ag, Al nanocrystals. The observations suggest emission intensity dependence on the degree of anisotropy, which could be correlated mainly with the symmetry of the luminescence center and also to a lesser extent with nanoparticle shape asymmetry.     

Synthesis and characterisation: Zinc oxide–sulfide nanocomposites

A novel synthesis method is presented for the preparation of nanosized-semiconductor zinc oxide–sulphide (ZnO/ZnS) core–shell nanocomposites, both formed sequentially from a single-source solid precursor. ZnO nanocrystals were synthesized by a simple co-precipitation method and ZnO/ZnS core–shell nanocomposites were successfully fabricated by sulfidation of ZnO nanocrystals via a facile chemical synthesis at room temperature. The as-obtained samples were characterized by X-ray diffraction and transmission electron microscopy. The results showed that the pure ZnO nanocrystals were hexagonal wurtzite crystal structures and the ZnS nanoparticles were sphalerite structure with the size of about 10 nm grown on the surface of the ZnO nanocrystals. Optical properties measured reveal that ZnO/ZnS core–shell nanocomposites have integrated the photoluminescent effect of ZnO and ZnS. Based on the results of the experiments, a possible formation mechanism of ZnO/ZnS core–shell nanocomposites was also suggested. This treatment is suggested to improve various properties of optoelectronically valuable ZnO/ZnS nanocomposites. These nanosized semiconductor nanocomposites can form a new class of luminescent materials for various applications.     

Influences of capping molecules on optical properties of nanocrystalline ZnS:Cu

ZnS:Cu nanocrystals capped with different capping molecules have been successfully synthesized by a simple aqueous method. The prepared nanocrystals were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive analysis by X-rays (EDAX). The surface characterization of the nanocrystals was done by FTIR spectroscopy. The effect of capping agents on absorption and photoluminescence (PL) spectra of the ZnS:Cu nanocrystals was studied. A blue shift of the absorption peaks was observed and attributed to a quantum confinement effect, which increases the band gap energy. The photoluminescence spectra of the capped ZnS:Cu nanocrystals showed a broad peak in the range of 460–480 nm. The intensity of the PL spectra strongly depended on the capping agents.     

Improved photoluminescence of ZnS:Mn nanocrystals by microwave assisted growth of ZnS shell

The photoluminescence (PL) of ZnS:Mn nanocrystals was improved greatly by microwave assisted growth of ZnS shell. Under optimized conditions, the luminescence quantum yield of ZnS:Mn nanocrystals increased from 2.8% to 12.1% after the growth of the ZnS shell. Time-resolved fluorescence spectroscopic and electron paramagnetic resonance measurements indicate that the improvement of the dispersivity of the doped Mn ions is responsible for the PL enhancement. Growth of the ZnS shell not only facilitated the diffusion of Mn ions during microwave irradiation but also prohibited the segregation of Mn ions on the particle surface. As a result, more isolated Mn²⁺ ions were produced after the growth of the ZnS shell, and thus the orange luminescence of ZnS:Mn nanocrystals was enhanced greatly.     

Raman scattering study of molecules adsorbed on ZnS nanocrystals

The adsorption of 4‐mercaptopyridine (4‐Mpy) molecules on ZnS nanocrystals was investigated by means of Raman spectroscopy. We compared the Raman signals of 4‐Mpy molecules adsorbed on ZnS nanocrystals and Ag substrate. The differences in the adsorption of 4‐Mpy molecules on the semiconductor and the metal substrate were noted. The results demonstrated that adsorbed species on the semiconductor ZnS nanocrystals can be detected by Raman spectroscopy. Copyright © 2006 John Wiley & Sons, Ltd.     

Energy transfer from organic surface adsorbate-polyvinyl pyrrolidone molecules to luminescent centers in ZnS nanocrystals

Multi-colour emitting doped ZnS nanocrystals surface capped with pyridine (P-ZnS) or polyvinyl pyrrolidone (PVP-ZnS) have been synthesized by wet chemical methods. The photoluminescence studies show that the dopant related emission from P-ZnS nanocrystals are caused by the energy transfer from band-to-band excitation of the host lattice. However, in the case of PVP capped ZnS, considerable enhancement in the emission intensity was observed and the corresponding excitation spectra appeared dramatically broadened due to the presence of multiple excitation bands with peak maxima at 235, 253, 260, 275, and 310 nm. The bands from 235 to 275 nm are assigned to the electronic transitions in the chemisorbed PVP molecules whereas the excitation maximum around 310 nm corresponds to the band-to-band transition within the nanocrystalline ZnS host. The presence of PVP related energy bands in the excitation spectrum indicates the process of energy transfer from the surface adsorbed PVP molecules to dopant centers in ZnS nanocrystals. This study brings out a heterogeneous sensitizer-activator relation between organic surface adsorbate and doped semiconducting nanocrystals.     

单核/双壳结构CdSe/CdS/ZnS纳米晶的合成与发光性质

以巯基乙酸为稳定剂,在水溶液中合成了单核/双壳结构的CdSe/CdS/ZnS纳米晶。在内核CdSe和外壳ZnS之间的内壳CdS作为晶格匹配调节层,能够很好的改善核/壳界面处的性能,而且,最外层ZnS能够最大程度地使激子受限。用TEM和XPS对纳米晶进行了表征,并且用光致发光光谱和吸收光谱对不同核壳结构的纳米晶的发光性能进行了比较,结果表明单核/双壳结构的纳米晶具有更加优异的发光特性。     

Photostability of luminescent water-soluble cadmium selenide nanocrystals with chemical surface modification

We have developed a method for solubilization of hydrophobic CdSe/ZnS nanocrystals of the core/shell type, obtained by high-temperature synthesis in coordinating organic solvents. The method is based on chemical modification of the surface of the nanocrystals with hydrophilic organic mercapto compounds. We have observed that long-chain mercaptoundecanoic acid molecules effectively protect the surface of CdSe/ZnS nanocrystals in water, increasing (compared with short-chain molecules) the photostability of the nanocrystals. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 506–509, July–August, 2006.     

Tb ions in presence of ZnS:Mn nanocrystals immobilized on silica: Energy transfer ZnS→Tb and coordination state of Mn ions

Three-component luminescent material consisting of silica xerogel as a support with immobilized ZnS:Mn²⁺ nanocrystals and Tb³⁺ ions was compared with such two-component materials as the silica support with ZnS:Mn²⁺ as well as the support with Tb³⁺. In each case the nanocrystals and the lanthanide ions were immobilized at silica surface by impregnation procedure. Size of the ZnS quantum dots doped with Mn²⁺ were estimated by Scherrer method from the X-ray diffraction (XRD) pattern. The materials have been characterized by EPR and optical spectroscopy techniques. EPR spectra allow to distinguish two different Mn²⁺ sites: the first is assigned to isolated Mn²⁺ substitutionally and incorporated into cubic ZnS lattice and the second is ascribed to the Mn²⁺ situated near the nanocrystal surface. From the optical spectra we have found that in the three-component material, energy transfer from excited ZnS:Mn²⁺ nanocrystals to Tb³⁺ ions takes place. The different mechanisms of such transfer are discussed.     

CdSe/ZnS纳晶中相干电声耦合效应的二维电子光谱研究

Coherent exciton-phonon coupling in CdSe/ZnS nanocrystals have been investigated by temperature-dependent two-dimensional electronic spectroscopy (2DES) measurements. Benefiting from the ability of 2DES to dissect assembles in nanocrystal films, we have clearly identified experimental evidences of coherent coupling between exciton and phonon in CdSe/ZnS nanocrystals. In time domain, 2DES signals of excitonic transitions beat at a frequency resonant to a longitudinal optical phonon mode; in energy domain, phonon side bands are distinct at both Stokes and anti-Stokes sides. When temperature increases, phonon-induced exciton dephasing is observed with dramatic broadening of homogeneous linewidth. The results suggest exciton-phonon coupling is essential in elucidating the quantum dynamics of excitonic transitions in semiconductor nanocrystals.     

Zinc sulfide nanoparticles: Spectral properties and photocatalytic activity in metals reduction reactions

Formation of zinc sulfide nanocrystals in aqueous solutions of various polymers has been studied. Spectral properties of ZnS nanoparticles have been investigated, the structure of the long-wave edge of the fundamental absorption band of ZnS nanocrystals has been analyzed. It has been shown that the variation of the synthesis conditions (stabilizer nature and concentration, solution viscosity, ZnS concentration, etc.) allows tailoring of the ZnS nanocrystals size in the range of 3–10 nm. Photochemical processes in colloidal ZnS solutions, containing zinc chloride and sodium sulfite, have been investigated. It has been found that the irradiation of such solutions results in the reduction of Zn(II), the rate of this reaction growing at a decrease in the size of ZnS nanoparticles. Kinetics of photocatalytic Zn(II) reduction has been studied. It has been concluded that two-electron reduction of adsorbed Zn(II) species is the rate-determining stage of this reaction. Photocatalytic activity of ZnS nanoparticles in KAu(CN)₂ reduction in aqueous solutions has been discovered. Spectral characteristics and kinetics of ZnS/Au⁰ nanocomposite formation have been studied. It has been shown that the photoreduction of gold(I) complex is the equilibrium reaction due to the reverse oxidation of gold nanoparticles by ZnS valence band holes.     

玻璃基质中ZnS纳米晶Mn2+的EPR谱

用融熔法制备分散有ZnS:Mn²⁺纳米晶的纳硼硅(Na₂O-B₂O₃-SiO₂)玻璃在不同温度、时间对样品进行退火处理,得到不同尺寸的纳米晶.研究了玻璃基质中ZnS:Mn²⁺纳米晶的EPR谱、微波功率饱和EPR谱及发射光谱,发现Mn²⁺有二种组态,即替位组态和间隙位组态.分析结果表明替位组态Mn²⁺及周围立方晶场的畸变程度直接影响光学发光特性.     

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.     

聚氧化乙烯介质中的掺杂硫化锌纳米晶的制备及光学性质

简述了聚氧化乙烯介质中的铽、铕、铥、钆、钇、镓、锰掺杂的硫化锌纳米晶的制备方法以及紫外吸收光谱、激发光谱和光激发发射光谱.制成的硫化锌纳米晶直径为3.0～3.5nm.