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.     

Synthesis and photoluminescence of water-soluble Mn-doped ZnS quantum dots

The water-soluble Mn²⁺-doped ZnS quantum dots (Mn:ZnS d-dots) were synthesized by using thioglycolic acid (TGA) as stabilizer in aqueous solutions in air, and characterized by X-ray powder diffraction (XRD), UV-vis absorption spectra and photoluminescence (PL) emission spectroscopy. The sizes of Mn:ZnS d-dots were determined to be about 2 nm using XRD measurements and the UV-vis absorption spectra. It was found that the Mn²⁺⁴T₁ → ⁶A₁ emission intensity of Mn:ZnS d-dots significantly increased with the increase of Mn²⁺ concentration, and showed a maximum when Mn²⁺ doping content was 1.5%. If Mn²⁺ concentration continued to increase, namely more than 1.5%, the Mn²⁺⁴T₁ → ⁶A₁ emission intensity would decrease. In addition, the effects of TGA/(Zn + Mn) molar ratio on PL were investigated. It was found that the peak intensity ratio of Mn²⁺⁴T₁ → ⁶A₁ emission to defect-states emission showed a maximum when the TGA/(Zn + Mn) molar ratio was equal to 1.8.     

Synthesis of CdS nanocrystals by a microwave activated method and investigation of the photoluminescence and electroluminescence properties

We have synthesized CdS nanocrystals (NCs) by a microwave activated method. CdSO₄ and Na₂S₂O₃ were used as the precursors and thioglycolic acid (TGA) was used as capping agent molecule. The aqueous synthesis was based on the heat sensitivity of Na₂S₂O₃. In this method, microwave irradiation creates the activation energy for dissociation of Na₂S₂O₃ and leads to the CdS NCs formation. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses demonstrated hexagonal phase CdS NCs with an average size around 3 nm for sample prepared at 5 min irradiation time. A band gap range of 3.38-2.89 eV was possible only by increasing the microwave irradiation time, corresponding to a 2.7-3.7 nm size. Photoluminescence (PL) spectra showed a white emission between 400 and 750 nm. The best attained PL quantum yield (QY) of the NCs was about 10%. Synthesized NCs were used as emissive layer in a light emitting device (LED) with ITO/PEDOT:PSS/PVK/CdS-NCs/AL structure. Turn on voltage of fabricated device was about 7 V. The CIE color coordinate of the LED at (0.34, 0.43) demonstrated a near white light LED with an emission on green-yellow boundary of white.     

Mechanoluminescence and thermoluminescence of Mn doped ZnS nanocrystals

This paper reports the synthesis of ZnS:Mn nanocrystals by the chemical route in which mercaptoethanol was used as the capping agent. The particle size of such nanocrystals was measured using XRD and TEM patterns and was found to be in between 3and 5 nm. It was found that the peak position of TL glow curve and the TL intensity of ZnS:Mn nanoparticles increases as the particle size is decreased. The isothermal decay technique is used to determine the trap-depth. The stability of the charge carriers in the traps increase with the decrease in size of the nanoparticles. The higher stability may be attributed to the higher surface/volume ratio and also to the increase in the trap-depth with decreasing particle size. When a ZnS:Mn nanocrystal is deformed the peak intensity I_m increases linearly with the increasing height of the load. After I_m, initially the ML intensity decreases at a fast rate, and later on it decreases at a slow rate. The ML in ZnS:Mn nanocrystals can be understood on the basis of the piezoelectrically induced electron detrapping model.     

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.     

SrS:Ce/ZnS:Mn-A di-band phosphor for near-UV and blue LED-converted white-light emitting diodes

The SrS:Ce/ZnS:Mn phosphor blends with various combination viz 75:25, 50:50 and 25:75 were assign to generate the white-light emission using near-UV and blue-light emitting diodes (LED) as an excitation source. The SrS:Ce exhibits strong absorption at 427 nm and the corresponding intense emission occurs at 480 and 540 nm due to electron transition from 5d(²D)−4f(²F_5/2, _7/2) of Ce³⁺ ion as a result of spin-orbit coupling. The ZnS:Mn excited under same wavelength shows broad emission band with λ_max=582 nm originates due to 3d (⁴G−⁶S) level of Mn²⁺. Photoluminescence studies of phosphor blend excited using near-UV to blue light confirms the emitted radiation varies from cool to warm white light in the range 430-600 nm, applicable to LED lightings. The CIE chromaticity coordinate values measured using SrS:Ce/ZnS:Mn phosphor blend-coated 430 nm LED pumped phosphors in the ratio 75:25, 50:50 and 25:75 are found to be (0.235, 0.125), (0.280, 0.190) and (0.285, 0.250), respectively.     

Radiative quantum efficiency of CdSe/ZnS core-shell colloidal solutions: Size-dependence

Thermo-optical parameters of CdSe/ZnS core-shell nanoparticles suspended in toluene were measured using a thermal lens (TL) technique. TL transient measurements were performed using the mode-mismatched dual-beam (excitation and probe) configuration. A He-Ne laser at λ_p = 632.8 nm was used as the probe beam and an Ar⁺ laser (at λ_e = 514.5 nm) was used as the excitation beam for studies as a function of both core size and concentration of CdSe/ZnS nanocrystals. The fraction thermal load (φ) and radiative quantum efficiencies (η) of the CdSe/ZnS were determined. Dependence on core size (∼2-5 nm) and concentration (∼0.01-0.62 mg/ml) was observed for both φ and η parameters.     

Synthesis and fluorescence properties of pure and metal-doped spherical ZnS particles from EDTA-metal complexes

Monodispersed spherical ZnS particles as well as doped with Cu, Mn ions were synthesized from metal-chelate solutions of ethylenediamine tetraacetate (EDTA) and thioacetamide (TAA). The characterizations of the ZnS-based particles were investigated via TEM, SEM, XRD, TG/DTA and PL measurements. The sphere size was controlled from 50 nm to 1 μm by adjusting the nucleation temperatures and molar ratio of Zn-EDTA to TAA. The emission intensity continuously increased with the increase of the particle size. When the ZnS microspheres were annealed at 550-800 °C, there were two specific emission bands with the centers at 454 nm and 510 nm, which were associated with the trapped luminescence arising from the surface states and the stoichiometric vacancies, respectively. When Cu²⁺ was introduced into ZnS microspheres, the dominant emission was red-shifted from 454 to 508 nm, fluorescence intensity also sharply increased. However, for the Mn²⁺-doped ZnS, the emission intensity was significantly enhanced without the shift of emission site.     

Photophysical and photoluminescence properties of co-activated ZnS:Cu,Mn phosphors

ZnS:Cu,Mn phosphors were prepared by conventional solid state reaction with the aid of NaCl-MgCl₂ flux at 900 °C. The samples were characterized by X-ray powder diffraction, UV-vis absorbance spectra and photoluminescence spectra. All samples possess cubic structure. Cu has a much stronger effect on the absorption property of ZnS than Mn. Incorporation of Mn into ZnS host only slightly enhances the light absorption, while addition of Cu remarkably increases the ability of absorption due to ground state Cu⁺ absorption. The emission spectra of the ZnS:Cu,Mn phosphors consist of three bands centered at about 452, 520 and 580 nm, respectively. Introduction of Mn significantly quenches the green luminescence of ZnS:Cu. The excitation energy absorbed by Cu is efficiently transferred to Mn activators non-radiatively and the Mn luminescence can be sensitized by Cu behaving as a sensitizer (energy donor).     

Functionalized CdS nanospheres and nanorods

Functionalized nanoparticles are discussed. Surfaces of CdS:Mn/ZnS core/shell nanospheres (Qdots) were converted from hydrophobic to hydrophilic by growth of a SiO₂ shell. The colloidal dispersion was stabilize by adding a surfactant with a negative surface charge, and a cell-penetrating-peptide, TAT, was attached through a primary amine group. The TAT functionalized Qdots were shown to pass the blood-brain-barrier and luminescence in the infused half of the brain.In addition, nanorods of S²⁻ rich CdS were synthesized by reaction of excess S with Cd precursors in the presence of ethylene diamine. The photoluminescence (PL) peak from the S²⁻ rich CdS nanorods was broad with a maximum at ∼710 nm, which was 40 nm longer in wavelength than the PL peak from Cd²⁺ rich CdS (∼670 nm) nanorods. The influence of surface electron or hole trap states on the luminescent pathway of CdS nanorods were used to explain these shifts in wavelength. Nanocrystals of Au with ∼2 nm diameters were grown on S²⁻ rich surfaces of CdS nanorods. Significant quenching of photoluminescence was observed from Au nanocrystals on CdS nanorods due to interfacial charge separation. Charge separation by Au nanocrystals on CdS resulted in enhanced UV photocatalytic degradation of Procion red mix-5B (PRB) dye in aqueous solution.     

Enhancement of photoluminescence of ZnS:Mn nanocrystals modified by surfactants with phosphate or carboxyl groups via a reverse micelle method

A colloidal suspension of ZnS:Mn nanocrystals was prepared in sodium bis(2-ethylhexyl)suflosuccinate reverse micelles, and then modified by surfactants with phosphate or carboxyl groups. The photoluminescent intensity at 580 nm due to d-d transition of Mn²⁺ ions increases up to a factor of 6.3 and its quantum efficiency increases from 1.7% to 8.1% after modification. According to ³¹P nuclear magnetic resonance spectra, surfactants with phosphate groups adsorb on the surface of ZnS nanocrystal and ³¹P nucleus spins are relaxed rapidly by interaction with five unpaired 3d electrons of Mn²⁺, showing that phosphate groups are located in the vicinity of Mn²⁺. The excitation spectra for the emission due to phosphate or carboxyl groups are similar to those for the emission at 580 nm corresponding to the excitation of ZnS. Both excitation spectra shift in parallel with increasing the amount of surfactant to show the linear relationship. We, therefore, attribute the increase in quantum efficiency at 580 nm to additional energy transfer of ZnS→functional groups→Mn²⁺ as well as to the reduction of energy loss due to non-radiative transition by surface modification.     

First-order hyperpolarizability of ZnS nanocrystal quantum dots studied by hyper-Rayleigh scattering

Hyper-Rayleigh scattering technique was used to measure for the first time the first-order hyperpolarizability (β) of ZnS nanocrystals with 2.5 nm mean diameter. Results show that the ‘per ZnS particle’ β value is 2.34×10⁻²⁷ esu and the ‘per ZnS formula unit’ β value is 1.63×10⁻²⁸ esu. An increase by at least two orders of magnitude in the β value per ZnS formula unit is found when compared with bulk ZnS. Two possible contributions originating from nanocrystal surface electric field and solvent field were experimentally excluded. Other two contributions, bulk-like contribution and surface contribution, are considered. Especially, the latter is emphasized due to the special surface structure of nanocrystals.     

Optical properties of annealed Mn-doped ZnS nanoparticles

Cysteine stabilized ZnS and Mn²⁺-doped ZnS nanoparticles were synthesized by a wet chemical route. Using the ZnS:Mn²⁺ nanoparticles as seeds, silica-coated ZnS (ZnS@Si) and ZnS:Mn²⁺ (ZnS:Mn²⁺@Si) nanocomposites were formed in water by hydrolysis and condensation of tetramethoxyorthosilicate (TMOS). The influence of annealing in air, formier gas, and argon at 200-1000 °C on the chemical stability of ZnS@Si and ZnS:Mn²⁺@Si nanoparticles with and without silica shell was examined. Silica-coated nanoparticles showed an improved thermal stability over uncoated particles, which underwent a thermal combustion at 400 °C. The emission of the ZnS@Si and ZnS:Mn²⁺@Si passed through a minimum in photoluminescence intensity when annealed at 600 °C. Upon annealing at higher temperatures, ZnS@Si conserved the typical emission centered at 450 nm (blue). ZnS:Mn²⁺@Si yielded different high intensity emissions when heated to 800 °C depending on the gas employed. Emissions due to the Mn²⁺ at 530 nm (green; Zn₂SiO₄:Mn²⁺), 580 nm (orange; ZnS:Mn²⁺@Si), and 630 nm (red; ZnS:Mn²⁺@Si) were obtained. Therefore, with a single starting product a set of different colors was produced by adjusting the atmosphere wherein the powder is heated.     

Synthesis of high quality and monodisperse CdS:Mn/ZnS and CdS:Mn/CdS core–shell nanoparticles

CdS:Mn²⁺/ZnS and CdS:Mn²⁺/CdS core–shell nanoparticles were synthesized in aqueous medium via chemical precipitation method in an ambient atmosphere. Polyvinylpyrrolidone (PVP) was used as a capping agent. The effect of the shell (ZnS and CdS) thickness on CdS:Mn²⁺ nanoparticles was investigated. Inorganically passivated core/shell nanocrystals having a core (CdS:Mn²⁺) diameter of 4 nm and a ZnS-shell thickness of ∼0.5 nm exhibited improved PL intensity. Optimum concentration of doping ions (Mn²⁺) was selected through optical study. For all the core–shell samples two emission peaks were observed, the first one is band edge emission in the lower wavelength side due to energy transfer to the Mn²⁺ ions in the crystal lattice; the second emission is characteristic peak of Mn²⁺ ions (⁴T₁ → ⁶A₁). The XRD, TEM and PL results showed that the synthesized core–shell particles were of high quality and monodisperse.     

Synthesis of robust water-soluble ZnS:Mn/SiO2 core/shell nanoparticles

Water-soluble Mn doped ZnS (ZnS:Mn) nanocrystals synthesized by using 3-mercaptopropionic acid (MPA) as stabilizer were homogeneously coated with a dense silica shell through a multi-step procedure. First, 3-mercaptopropyl triethoxy silane (MPS) was used to replace MPA on the particle surface to form a vitreophilic layer for further silica deposition under optimal experimental conditions. Then a two-step silica deposition was performed to form the final water-soluble ZnS:Mn/SiO₂ core/shell nanoparticles. The as-prepared core/shell nanoparticles show little change in fluorescence intensity in a wide range of pH value.     

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.     

Barium molybdate and barium tungstate nanocrystals synthesized by a cyclic microwave irradiation

BaMoO₄ and BaWO₄ nanocrystals were synthesized from Ba(NO₃)₂ and Na₂MeO₄ (Me=Mo and W) solutions using 50% of 600 W microwave irradiation for 20 min. The products were characterized using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and photoluminescence (PL) spectrophotometry. They show that the products are good dispersed nanocrystals (poly-nanocrystals) of single-phase scheelite tetragonal structure with the vibration modes corresponding to the molybdate and tungstate compounds. Their photoluminescence was detected at 415 and 392 nm for BaMoO₄ and BaWO₄, respectively.     

Modification of Cd1−xMnxTe crystal surface layers by nanopulsed laser irradiation

The surface modification of Cd_1−xMn_xTe (x = 0-0.3) crystal wafers under pulsed laser irradiation has been studied. The samples were irradiated by a Q-switched ruby laser with pulse duration of 80 ns. Optical diagnostics of laser-induced thermal processes were carried out by means of time-resolved reflectivity measurements at wavelengths 0.53 and 1.06 μm. Laser irradiation energy density, E varied in the range of 0.1-0.6 J/cm². Morphology of irradiated surface was studied using scanning electron microscopy. The energy density whereby the sample surface starts to melt, depends on Mn content and is equal to 0.12-0.14 J/cm² for x ≤ 0.2, in the case of x = 0.3 this value is about 0.35 J/cm². The higher Mn content leads to higher melt duration. The morphology of laser irradiated surface changes from a weakly modified surface to a single crystal strained one, with an increase in E. Under irradiation with E in the range of 0.21-0.25 J/cm², the oriented filamentary crystallization is observed. The Te inclusions on the surface are revealed after the irradiation of samples with small content of Mn.     

Synthesis of nanocrystalline and mesoporous zinc sulphide from a single precursor Zn(SOCCH3)2Lut2 complex

We report the formation of mesoporous zinc sulphide, composed by the fine network of nanoparticles, which was formed via a single precursor Zn(SOCCH₃)₂Lut₂ complex. The complex was chemically synthesized using zinc carbonate basic, 3,5-lutidine and thioacetic acid, in air. The metal precursor complex was characterized using different conventional techniques. Thermogravimetric analysis (TGA) result indicates that the decomposition of the complex starts at 100 °C and continues up to 450 °C, finally yielding ZnS. ZnS nanocrystals were characterized by powder X-ray diffraction (XRD) technique, field emission scanning electron microscopy (FESEM), N₂-sorption isotherm, UV-vis spectroscopy and photoluminescence (PL) spectroscopy. The grain diameter of nanocrystals was found to be 4-5 nm. The material followed Type-IV N₂-sorption isotherm, which is the characteristic of mesoporous materials. The band gap energy, as obtained from optical measurements was around 3.8 eV.     

Characterization and luminescent properties of SiO2:ZnS:Mn and ZnS:Mn nanophosphors synthesized by a sol-gel method

ZnS and SiO₂-ZnS nanophosphors, with or without different concentration of Mn²⁺ activator ions, were synthesized by using a sol-gel method. Dried gels were annealed at 600 °C for 2 h. Structure, morphology and particle sizes of the samples were determined by using X-ray diffraction (XRD), highresolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM). The diffraction peaks associated with the zincblende and the wurtzite structures of ZnS were detected from as prepared ZnS powders and additional diffraction peaks associated with ZnO were detected from the annealed powders. The particle sizes of the ZnS powders were shown to increase from 3 to 50 nm when the powders were annealed at 600 °C. An UV-Vis spectrophotometer and a 325 nm He-Cd laser were used to investigate luminescent properties of the samples in air at room temperature. The bandgap of ZnS nanoparticles estimated from the UV-Vis data was 4.1 eV. Enhanced orange photoluminescence (PL) associated with ⁴T₁→⁶A₁ transitions of Mn²⁺ was observed from as prepared ZnS:Mn²⁺and SiO₂-ZnS:Mn²⁺ powders at 600 nm when the concentration of Mn²⁺ was varied from 2-20 mol%. This emission was suppressed when the powders were annealed at 600 °C resulting in two emission peaks at 450 and 560 nm, which can be ascribed to defects emission in SiO₂ and ZnO respectively. The mechanism of light emission from Mn²⁺, the effect of varying the concentration on the PL intensity, and the effect of annealing are discussed.