Synthesis and photoluminescence of ZnS quantum dots

Single-phase zinc sulphide (ZnS) quantum dots were synthesized by a chemical method. The influence of the pH value of the Zn(CH3COO)2 solution on the size and photoluminescence properties of the ZnS quantum dots was evaluated. X-ray power diffraction, transmission electron microscopy, and ultraviolet-visible spectroscopy were used to characterize the structure, size, surface states, and photoluminescence properties of ZnS quantum dots. The results showed that the crystal structure of ZnS quantum dots was a cubic zinc blende structure, and their average diameter was about 3.0 nm. ZnS quantum dots with good distribution and high purity were obtained. A strong broad band centered at about 320 nm was observed in the excitation spectrum of ZnS quantum dots. Their emission spectrum peaking at about 408 nm, was due mostly to the trap-state emission. The relative integrated emission intensity of ZnS quantum dots decreased as the pH value of the Zn(CH3COO)2 solution increased, which could be ascribed to the increase in average diameter of the ZnS quantum dots as the pH value of Zn(CH3COO)2 solution increased.     

Effects of multiple organic ligands on size uniformity and optical properties of ZnSe quantum dots

The effects of multi-ligands on the formation and optical transitions of ZnSe quantum dots have been investigated. The dots are synthesized using 3-mercapto-1,2-propanediol and polyvinylpyrrolidone ligands, and have been characterized by X-ray diffraction, transmission electron microscopy (TEM), UV–visible absorption spectroscopy, photoluminescence spectroscopy, and Fourier transform infrared spectroscopy. TEM reveals high monodispersion with an average size of 4 nm. Polymer-stabilized, organic ligand-passivated ZnSe quantum dots exhibit strong UV emission at 326 nm and strong quantum confinement in the UV–visible absorption spectrum. Uniform size and suppressed surface trap emission are observed when the polymer ligand is used. The possible growth mechanism is discussed.     

Synthesis of ordered ZnS nanotubes by MOCVD-template method

Semiconductor ZnS nanotubes arrays were synthesized in the pores of the porous anodic alumina (PAA) membranes by using metal organic chemical vapor deposition (MOCVD) template methods. The morphology and structure of the ZnS nanotubes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and X-ray diffraction (XRD). It is found that the ZnS nanotubes with diameters in range of 140–250 nm and the length up to tens of microns are polycrystalline. Energy-dispersion spectroscopy (EDS) and X-ray photoelectron spectroscopy analysis (XPS) indicate that the stoichiometric ZnS was formed. A green-blue emission band centered at 510 nm was observed in the photoluminescence spectrum of the ZnS nanotubes.     

Photoluminescent properties of ZnS:Mn nanoparticles with in-built surfactant

Mn-doped ZnS nanoparticles, having average diameter 3–5 nm, have been synthesized using chemical precipitation technique without using any external capping agent. Zinc blende crystal structure has been confirmed using the X-ray diffraction studies. The effect of various concentrations of Mn doping on the photoluminescent properties of ZnS nanoparticles has been studied. The time-resolved photoluminescence spectra of the ZnS:Mn quantum dots have been recorded and various parameters like lifetimes, trap depths, and decay constant have been calculated from the decay curves at room temperature. The band gap was calculated using UV–Visible absorption spectra.     

Size dependent structural, optical and morphological properties of ZnS:Cu thin films

ZnS:Cu thin films have been deposited on glass substrate by a simple neutral pH solution synthesis route and chemical bath deposition technique. The copper concentration was varied between 0 and 0.1 M%. The X-ray diffraction and scanning electron microscope studies show the average size of the nanoparticles are below 4 nm (Bohr diameter). The effect of film thickness on the optical and structural properties has been studied. The optical absorption studies show the band gap energy of ZnS:Cu films decreases from 3.68 to 3.43 eV as thickness varied from 318.3 to 334.1 nm. The structural estimation shows the variation in particle size from 2.67 to 3.14 nm with thickness. The insignificant change in band gap may be due to the increase in particle size and quantum size effect.     

微乳液法制备ZnS∶Cu纳米微粒及其光学性能的表征

利用微乳液法制备出ZnS∶Cu纳米微粒。XRD表明微粒粒径为3～5nm,且为立方晶型结构。在紫外吸收光谱中,ZnS∶Cu纳米微粒吸收峰蓝移。发射光谱表明ZnS∶Cu纳米微粒产生一个位于482nm的绿色发射带。     

CdSe胶质量子点的电致发光特性研究

采用胶体化学法合成硒化镉(CdSe)胶质量子点, 在此基础上制成了以CdSe胶质量子点为有源层, 结构为ITO/ZnS/CdSe/ZnS/Al的电致发光(EL)器件. 透射电镜测量表明量子点的尺寸为4.3 nm, 扫描电子显微镜测量ZnS薄膜和Al薄膜结果显示表面均较为平整, 由器件结构的X射线衍射分析观察到了CdSe(111)、ZnS(111)等晶面的衍射, 表明器件中包含了CdSe量子点和ZnS绝缘层材料. 光致发光谱表征胶质量子点的室温发光峰位于614 nm, 电致发光测量得到器件在室温下的发光波长位于450 ~ 850 nm, 峰值在800 nm附近. 本文对电致发光机制及其与光致发光谱的区别进行了讨论.     

Co2+掺杂水溶性ZnS量子点的制备及其光致发光性能

采用共沉淀法,以3-巯基丙酸为表面修饰剂,成功制备出Co2+掺杂水溶性ZnS量子点。采用X射线衍射仪、透射电子显微镜、原子发射光谱仪、紫外-可见吸收光谱仪和荧光分光光度计等,研究了Co2+掺杂剂及掺杂量对ZnS量子点的晶体结构、形貌和发光性能等的影响。结果表明:所得产物均为ZnS立方型闪锌矿结构,量子点呈不规则球形,粒径主要集中在5.2 nm左右;掺杂样品发红色荧光,发光性能明显增强,属于Co2+形成的杂质能级(4A1—4T1)与缺陷的复合发光。同时,利用红外吸收光谱对Co2+掺杂水溶性ZnS量子点的形成机理进行了初步探讨。     

Studies on optical absorption and photoluminescence of thioglycerol-stabilized ZnS nanoparticles

ZnS quantum dots of size 3 nm are prepared at 303 K using ZnSO₄ and Na₂S₂O₃ precursors with thioglycerol as stabilizing agent. Cd²⁺ doped ZnS were prepared by varying doping concentration from 1 to 8 wt.%. ZnS quantum dots were mixed with CdS quantum dots of size 4 nm in the 3:1, 2:1, 1:1, 1:2, 1:3 and 1:4 M ratio. The nanoparticles were characterized by UV–vis, photoluminescence (PL), XRD and high-resolution TEM measurements. The XRD pattern, high-resolution TEM image and SAED pattern reveal that the nanoparticles are in well-crystallized cubic phase. The band gap of ZnS has increased from the bulk value 3.7 to 4.11 eV showing quantum size effect. Excitonic transition is observed at 274 nm in UV absorption and PL emission at 411 nm. Doping with Cd²⁺ red-shifts both UV and PL spectral bands and enhances the PL band of ZnS nanoparticles. Mixing CdS and ZnS quantum dots in different molar ratios shows red-shift of the band edge in the CdS/ZnS hybrid system. In the 1:1 hybrid system of CdS/ZnS nanoparticles, PL band is red-shifted and the intensity is almost doubled with respect to that of CdS nanoparticles.     

Effect of swift heavy ion irradiation on bare and coated ZnS quantum dots

The present study compares structural and optical modifications of bare and silica (SiO₂) coated ZnS quantum dots under swift heavy ion (SHI) irradiation. Bare and silica coated ZnS quantum dots were prepared following an inexpensive chemical route using polyvinyl alcohol (PVA) as the dielectric host matrix. X-ray diffraction (XRD) and transmission electron microscopy (TEM) study of the samples show the formation of almost spherical ZnS quantum dots. The UV-Vis absorption spectra reveal blue shift relative to bulk material in absorption energy while photoluminescence (PL) spectra suggests that surface state and near band edge emissions are dominating in case of bare and coated samples, respectively. Swift heavy ion irradiation of the samples was carried out with 160 MeV Ni¹²⁺ ion beam with fluences 10¹² to 10¹³ ions/cm². Size enhancement of bare quantum dots after irradiation has been indicated in XRD and TEM analysis of the samples which has also been supported by optical absorption spectra. However similar investigations on irradiated coated quantum dots revealed little change in quantum dot size and emission. The present study thus shows that the coated ZnS quantum dots are stable upon SHI irradiation compared to the bare one.     

Growth of mane-like ZnS nanostructures by using a two-heating zone-tube furnace: structural characterization and photoluminescence

We have synthesized ZnS nanowires with mane-like branches by thermal evaporation of ZnS powder on the Au-coated Si(100) substrate using a two-heating zone tube furnace. The ZnS powder and the Si substrate were kept at 1,000 and 850 °C, respectively, in a nitrogen atmosphere during synthesis of the ZnS nanostructures. Field emission scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and photoluminescence (PL) spectroscopy analyses were performed to investigate the structure, morphology and photoluminescence properties of the products. The axial nanowires grow along the [002] direction and have diameters of 100–200 nm, while on the other hand the branch nanowires grow along the [101] direction and their diameters and lengths are 30–50 and 800–100 nm, respectively. The room temperature PL spectrum with a Gaussian fitting exhibits two visible light emission bands centered at around 397 and 458 nm.     

Enhancement of photoluminescence in ZnS/ZnO quantum dots interfacial heterostructures

ZnS/ZnO quantum dots (QDs) were synthesized by controlled oxidation of ZnS nanoparticles. HRTEM image showed small nanocrystals of size 4 nm and the magnified image of single quantum dot shows interfacial heterostructure formation. The optical absorption spectrum shows a blue shift of 0.19 and 0.23 eV for ZnO and ZnS QDs, respectively. This is due to the confinement of charge carries within the nanostructures. Enormous enhancement in UV emission (10 times) is reported which is attributed to interfacial heterostructure formation. Raman spectrum shows phonons of wurtzite ZnS and ZnO. Phonon confinement effect is seen in the Raman spectrum wherein LO phonon peaks of ZnS and ZnO are shifted towards lower wavenumber side and are broadened.     

CdSe(ZnS) nanocomposite luminescent high temperature sensor

High temperature luminescence-based sensing is demonstrated by embedding colloidal CdSe(ZnS) quantum dots into a high temperature SiO(2) dielectric matrix. The nanocomposite was fabricated by a solution process method. As-prepared CdSe(ZnS) quantum dots in the nanocomposite sensor show an absorption band at a wavelength of 600 nm (2.06 eV). Photoluminescence (PL) measurements show a room temperature emission peak at 606 nm (2.04 eV). The temperature-dependent emission spectra study shows for the first time a CdSe(ZnS)-SiO(2) nanocomposite-based high temperature sensor. The temperature-dependent spectral and intensity modes of the nanocomposite thin film photoluminescence were investigated from 295-525 K. The sensor shows a variation of the emission wavelength as a function of temperature with a sensitivity of ～ 0.11 nm °C( - 1). The film morphology and roughness are characterized using AFM.     

水溶性ZnS量子点的共沉淀法制备、 表征及其光学性能

以硫化钠和乙酸锌为反应物,3-巯基丙酸为表面包覆剂,利用共沉淀法制备了水溶性ZnS量子点。并采用X射线衍射仪、透射电子显微镜和荧光分光光度计等对样品的结构、形貌、粒径和光学性能进行了表征。结果表明:所得样品为ZnS立方型闪锌矿结构,量子点的形状呈不规则球形,粒径主要集中在4.8nm左右;样品在585～590nm之间出现了黄色荧光发射波峰。同时,利用红外光谱对ZnS量子点的合成机理进行了初步分析。     

Luminescence of er doped ZnS quantum dots excited by infrared lasers

ZnS:Er quantum dots were prepared in aqueous medium from readily available precursors. The construction, morphology and luminescence properties of the ZnS:Er quantum dots were evaluated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence spectra. The average particle size was calculated using the Scherrer formula to be 4 nm, which is also observed from high resolution transmission electron microscopy (HRTEM) image. Different laser wavelengths at 976 +/- 2 nm and 1480 nm were utilized as the excitation source. ZnS:Er quantum dots had a fluorescence spectrum in 1550 nm region through the 4I13/2 --> 4I15/2 transition. Furthermore, intensity increased with increasing excitation intensity and dopant concentration. The reason for the photoluminescence spectra broadening is discussed. It is because the energy levels of Er3+ are split by a coulombic interaction between electrons, including spin correction and spin-orbit coupling, and eventually by the Stark effect due to ZnS QDs crystal field and local coordination.     

Strong quantum confinement effect in nanocrystalline CdS

CdS quantum dots have been prepared by chemical method. The X-ray diffraction results indicated the formation of CdS nanoparticles with hexagonal phase and grain size 2.5 nm. The HRTEM analysis reveals the formation of CdS quantum dots with an average grain size of ~2.5 nm. The X-ray photoelectron spectroscopy spectra exhibit the 3d _5/2 and 3d _3/2 peaks corresponding to cadmium and the S2p _3/2 peak corresponding to sulphur. Optical studies by UV–vis spectroscopy show a blue shifted absorption at 471 nm because of the quantum confined excitonic absorption. The photoluminescence spectra of CdS exhibited a broad green emission band centred at around 494 nm.     

Synthesis and characterization of water-soluble <Emphasis Type="SmallCaps">l</Emphasis>-cysteine-modified ZnS nanocrystals doped with silver

The water-soluble Ag⁺-doped ZnS nanocrystals surface capped with cysteine (expressed as ZnS:Ag/Cys) were synthesized in aqueous solution by using l-cysteine as surface modifier. The crystal structure, size, shape, component, and spectral properties of ZnS:Ag/Cys nanocrystals were characterized by X-ray power diffraction, transmission electron microscope, energy dispersive X-ray analysis, inductively coupled plasma atomic emission spectrometry, infrared spectrum, UV–Vis absorption spectrum, and photoluminescence spectrum. The results show that the spherical ZnS:Ag/Cys nanocrystals with an average diameter of 2.6 nm have good fluorescent characteristics, their fluorescence intensity is enhanced greatly after doped with Ag⁺. And the sulfur atoms in cysteine molecules are coordinated with metal ions on the surface of the nanocrystals, the cysteine modified on the surface of ZnS:Ag/Cys nanocrystals renders the nanocrystals water soluble and biocompatible. The ZnS:Ag/Cys nanocrystals have potential applications in molecular assembly and biological fluorescence analysis.     

Poly (vinyl alcohol) thin film filled with CdSe–ZnS quantum dots: Fabrication,characterization and optical properties

A transparent poly (vinyl alcohol) (PVA) nanocomposite thin film (30–50 nm) reinforced with core/shell cadmium selenide (CdSe)/zinc sulfide (ZnS) quantum dots (QDs) was fabricated by a drop-casting method. A narrow peak at ～556 nm observed in the UV–vis spectrum indicates the uniformly dispersed QDs in the PVA matrix. FT-IR analysis indicates the interaction between the QDs and the polymer matrix. Both PVA and PVA-QDs nanocomposite thin films show polarized light dependent absorption properties with several different absorption peaks. As compared to the only fluorescent emission peak at 574 nm of QDs, the pure PVA and PVA-DDs nanocomposites show an excitation wavelength dependent fluorescent emission property.     

Low-temperature synthesis and optical properties of wurtzite ZnS nanowires

Single-crystal wurtzite ZnS nanowires have been synthesized via a facile solution route with polyethylene glycol-400 as inducing template at low temperature (170 °C). The as-prepared products have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and energy dispersive X-ray analysis (EDX). Raman and photoluminescence spectrum (PL) were used to investigate the optical properties of ZnS nanowires. The strong emission peak centered at 322 nm in PL spectrum could be attributed to the band to band transitions.     

Structural and optical properties of Co-doped ZnS nanoparticles synthesized by a capping agent

Cobalt-doped Zinc sulfide (ZnS) nanoparticles were prepared by a simple chemical method using alkyl hydroxyl ethyl dimethyl ammonium chloride (YH) as capping agent. The structural and optical properties of prepared cobalt-doped ZnS nanoparticles have been characterized. X-ray diffraction patterns and transmission electron microscope images reveal pure cubic ZnS phase with size of about 5–2 nm for all cobalt-doped ZnS nanoparticles. The lattice constant of the samples decreases slightly by the introduction of Co²⁺ The absorption edge of the ZnS:Co²⁺ nanoparticles is blue-shifted as compared with that of bulk ZnS, indicating the quantum confinement effect. The photoluminescence emission band exhibits a blue shift for Co-doped ZnS nanoparticles as compared to the ZnS nanoparticles.