Nonlinear optical characteristics of CdS and ZnS nanoparticles implanted into zirconium oxide thin films

The nonlinear refractive indices γ and nonlinear absorption coefficients of ZrO₂ films doped with CdS or ZnS nanoparticles, as well as with various metals, are measured. The effects of semiconductor and metal nanoparticles and annealing on the nonlinear optical properties of films are studied. The structural parameters of films, determined by electron microscopy and x-ray dispersion spectroscopy, are compared to the optical and nonlinear optical characteristics of these media. The high magnitude of γ of the films ((3±0.6)×10^?11 cm² W^{? 1}) is attributed to the surface enhancement effect in semiconductor nanoparticles. On the basis of Z-scan data obtained at different intensities of radiation, it is shown that the variations in γ of the ZrO₂:CdS(Cr) and ZrO₂:ZnS(Mn) films are related to the generation of free carriers.     

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.     

Structural, optical and photoelectrochemical characterization of CdS nanowire synthesized by chemical bath deposition and wet chemical etching

Nanocrystalline thin films of CdS have been grown onto flexible plastic and titanium substrates by a simple and environmentally benign chemical bath deposition (CBD) method at room temperature. The films consist of clusters of CdS nanoparticles. The clusters of CdS nanoparticles in the films were successfully converted into nanowire (NW) networks using chemical etching process. The possible mechanism of the etching phenomenon is discussed. These films were examined for their structural, surface morphological and optical properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV-vis spectrophotometry techniques, respectively. Photoelectrochemical (PEC) investigations were carried out using cell configuration as n-CdS/(1 M NaOH + 1 M Na₂S + 1 M S)/C. The film of nanowires was found to be hexagonal in structure with the preferential orientation along the (0 0 2) plane. The nanowires have widths in the range of 50-150 nm and have lengths of the order of a few micrometers. Optical studies reveal that the CdS nanowires have value of band gap 2.48 eV, whereas it is 2.58 eV for nanoparticles of CdS. Finally, we report on the ideality of junction improvement of PEC cells when CdS nanoparticles photoelectrode converted into nanowires photoelectrode.     

Synthesis and investigation of nonlinear optical properties of semiconductor ZnS nanoparticles

ZnS nanoparticles were prepared by a simple chemical method and using PVP (poly vinylpyrrolidone) as capping agent. The sample was characterized by UV-vis spectrophotometer, X-ray diffraction (XRD) and Z-scan technique. XRD pattern showed that the ZnS nanoparticles had zinc blende structure with an average size of about 2.18 nm. The value of band gap of these nanoparticles was measured to be 4.20 eV. The nonlinear optical properties of ZnS nanoparticles in aqueous solution were studied by Z-scan technique using CW He-Ne laser at 632.8 nm. The nonlinear absorption coefficient (β) was estimated to be as high as 3.2×10⁻³ cm/W and the nonlinear refractive index (n₂) was in order of 10⁻⁸ cm²/W. The sign of the nonlinear refractive index obtained negative that indicated this material exhibits self-defocusing optical nonlinearity.     

Preparation and characterization of ZnS/CdS bi-layer for CdTe solar cell application

In this work, bilayer ZnS/CdS film was prepared as an improved window layer of CdTe solar cell. TEM was used to observe the cross section of the bilayer structure. The total thickness of ZnS/CdS film was about 60 nm, which could allow more photons to pass through it and contribute to the photocurrent. Optical properties of the bilayers were investigated using UV–vis spectroscopy. Compared with poor transmission of standard CdS film in the short wavelength range of 350–550 nm, the transmission of ZnS/CdS was improved and reached above 50%. The ZnS/CdS was annealed with CdCl₂. X-ray photoelectron spectroscopy (XPS) was used to investigate its chemical properties. A possible diffusion between CdS and ZnS was observed after annealing. The efficiency of standard CdS/CdTe solar cell was 9.53%. The device based on ZnS/CdS window layer had a poor 6% efficiency. With annealing treatment on ZnS/CdS layer, the performance was improved and reached 10.3%. In addition, the homogeneity of solar cell performance was improved using ZnS/CdS window layer. A thin ZnS layer was quite effective to reduce the possible shunt paths and short parts of window layer and consequently contributed to fabrication of a homogeneous CdTe solar cell.     

Synthesis and characterization of CdS/PVA nanocomposite films

A series CdS/PVA nanocomposite films with different amount of Cd salt have been prepared by means of the in situ synthesis method via the reaction of Cd²⁺-dispersed poly vinyl-alcohol (PVA) with H₂S. The as-prepared films were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) absorption, photoluminescence (PL) spectra, Fourier transform infrared spectroscope (FTIR) and thermogravimetric analysis (TGA). The XRD results indicated the formation of CdS nanoparticles with hexagonal phase in the PVA matrix. The primary FTIR spectra of CdS/PVA nanocomposite in different processing stages have been discussed. The vibrational absorption peak of CdS bond at 405 cm⁻¹ was observed, which further testified the generation of CdS nanoparticles. The TGA results showed incorporation of CdS nanoparticles significantly altered the thermal properties of PVA matrix. The photoluminescence and UV-vis spectroscopy revealed that the CdS/PVA films showed quantum confinement effect.     

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.     

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.     

Strong green luminescence of Ni2+-doped ZnS nanocrystals

ZnS nanoparticles doped with Ni²⁺ have been obtained by chemical co-precipitation from homogeneous solutions of zinc and nickel salt compounds, with S^2- as precipitating anion, formed by decomposition of thioacetamide (TAA). The average size of particles doped with different mole ratios, estimated from the Debye–Scherrer formula, is about 2–2.5 nm. The nanoparticles could be doped with nickel during synthesis without altering the X-ray diffraction pattern. A Hitachi M-850 fluorescence spectrophotometer reveals the emission spectra of samples. The absorption spectra show that the excitation spectra of Ni-doped ZnS nanocrystallites are almost the same as those of pure ZnS nanocrystallites (λ_ex=308–310 nm). Because a Ni²⁺ luminescent center is formed in ZnS nanocrystallites, the photoluminescence intensity increases with the amount of ZnS nanoparticles doped with Ni²⁺. Stronger and stable green-light emission (520 nm) (its intensity is about two times that of pure ZnS nanoparticles) has been observed from ZnS nanoparticles doped with Ni²⁺. Received: 18 December 2000 / Accepted: 17 March 2001 / Published online: 20 June 2001     

Surface modification and fluorescent properties of Mn2+-doped and undoped ZnS nanoparticles

Abstract

ZnS nanoparticles and Mn²⁺-doped ZnS nanoparticles were prepared by a reverse micelle reaction system. In addition, ZnS and Mn²⁺-doped ZnS nanoparticles were modified with poly(vinyl alcohol) (PVA) and 1-dodecanethiol (C₁₂H₂₅SH). The average particle size of the ZnS sample is determined around 2.3 nm by using the well-known Scherrer equation, which is in accordance with the results obtained from UV–vis and TEM analysis. Fluorescence intensity of the Mn²⁺-doped ZnS nanoparticles increases with increasing Mn²⁺ content compared with undoped ZnS nanoparticles, and coating PVA can also make fluorescence intensity increase. Different Zn²⁺/S^2- or C₁₂H₂₅SH/Zn²⁺ can affect intensity of PL emission peak and its position, which is discussed in this paper.     

ZnS overlayer on in situ chemical bath deposited CdS quantum dot-assembled TiO2 films for quantum dot-sensitized solar cells

ZnS overlayers were deposited on the CdS quantum dot (QD)-assembled TiO₂ films, where the CdS QDs were grown on the TiO₂ by repeated cycles of the in situ chemical bath deposition (CBD). With increasing the CdS CBD cycles, the CdS QD-assembled TiO₂ films were transformed from the TiO₂ film partially covered by small CdS QDs (Type I) to that fully covered by large CdS QDs (Type II). The ZnS overlayers significantly improved the overall energy conversion efficiency of both Types I and II. The ZnS overlayers can act as the intermediate layer and energy barrier at the interfaces. However, the dominant effects of the ZnS overlayers were different for the Types I and II. For Type I, ZnS overlayer dominantly acted as the intermediate layer between the exposed TiO₂ surface and the electrolyte, leading to the suppressed recombination rate for the TiO₂/electrolyte and the significantly enhanced charge-collection efficiency. On the contrary, for Type II, it dominantly acted as the efficient energy barrier at the interface between the CdS QDs and the electrolyte, leading to the hindered recombination rate from the large CdS QDs to the electrolyte and thus enhanced electron injection efficiency.     

Effects of doping on the surface energies of nanocrystals and evidence from studies at high pressure

Zn_1−XMn_XS (X=0.85% and 1.26%) nanoparticles have been synthesized using a specially designed equipment and we have studied the influence of doping Mn²⁺ on the surface energy of ZnS. The high pressure behaviors of ZnS nanocrystals with different dopant contents have been investigated using angle-dispersive synchrotron X-ray powder diffraction up to 45.1 GPa. Theoretical calculations show that doping with Mn²⁺ increases the surface energy of the nanocrystals. The theoretical result has been further corroborated by our experimental observation of an increase in the phase transition pressure of Mn²⁺ doped ZnS nanocrystals in diamond-anvil-cell studies.     

Synthesis and photoluminescence characteristics of doped ZnS nanoparticles

Free-standing powders of doped ZnS nanoparticles have been synthesized by using a chemical co-precipitation of Zn²⁺, Mn²⁺, Cu²⁺ and Cd²⁺ with sulfur ions in aqueous solution. X-ray diffraction analysis shows that the diameter of the particles is ∼2–3 nm. The unique luminescence properties, such as the strength (its intensity is about 12 times that of ZnS nanoparticles) and stability of the visible-light emission, were observed from ZnS nanoparticles co-doped with Cu²⁺ and Mn²⁺. The nanoparticles could be doped with copper and manganese during the synthesis without altering the X-ray diffraction pattern. However, doping shifts the luminescence to 520–540 nm in the case of co-doping with Cu²⁺ and Mn²⁺. Doping also results in a blue shift on the excitation wavelength. In Cd²⁺-doped ZnS nanometer-scale particles, the fluorescence spectra show a red shift in the emission wavelength (ranging from 450 nm to 620 nm). Also a relatively broad emission (ranging from blue to yellow) has been observed. The results strongly suggest that doped ZnS nanocrystals, especially two kinds of transition metal-activated ZnS nanoparticles, form a new class of luminescent materials. Received: 16 October 2000 / Accepted: 17 October 2000 / Published online: 23 May 2001     

Synthesis and photoluminescent and nonlinear optical properties of manganese doped ZnS nanoparticles

In this work we synthesized ZnS:Mn²⁺ nanoparticles by chemical method using PVP (polyvinylpyrrolidone) as a capping agent in aqueous solution. The structure and optical properties of the resultant product were characterized using UV-vis optical spectroscopy, X-ray diffraction (XRD), photoluminescence (PL) and z-scan techniques. UV-vis spectra for all samples showed an excitonic peak at around 292 nm, indicating that concentration of Mn²⁺ ions does not alter the band gap of nanoparticles. XRD patterns showed that the ZnS:Mn²⁺ nanoparticles have zinc blende structure with the average crystalline sizes of about 2 nm. The room temperature photoluminescence (PL) spectrum of ZnS:Mn²⁺ exhibited an orange-red emission at 594 nm due to the ⁴T₁-⁶A₁ transition in Mn²⁺. The PL intensity increased with increase in the Mn²⁺ ion concentration. The second-order nonlinear optical properties of nanoparticles were studied using a continuous-wave (CW) He-Ne laser by z-scan technique. The nonlinear refractive indices of nanoparticles were in the order of 10⁻⁸ cm²/W with negative sign and the nonlinear absorption indices of these nanoparticles were obtained to be about 10⁻³ cm/W with positive sign.     

Photoconductivity of poly-<Emphasis Type="Italic">p</Emphasis>-xylylene + CdS nanocomposite films over a wide temperature range

For poly-p-xylylene + CdS (PPX + CdS) nanocomposite films, the dependences of the photo-conductivity σ _ph (T) on the concentration C of CdS nanoparticles, intensity and wavelength of exciting light, and temperature T within 15–300 K are examined. An appreciable photocurrent appears at C ≥ 10 vol %, when a large percolation cluster of CdS nanoparticles is formed. The photocurrent spectrum is compared to the absorption spectrum of the film. The photocurrent I _ph (P) increases with the intensity of light flux P in a wavelength range near 435 nm according to the I _ph (P) ∼ P ⁿ power law, where n < 1. At 15 K, the photoconductivity of films with C ≈ 11.5 and 13.5 vol % is higher than that of a pure CdS film (C = 100 vol %) by factors of ≈100 and ≈30, respectively. For films with C > 11.5 vol %, the σ _ph (T) dependence at low T exhibits a metal-like character (σ _ph (T) decreases with increasing temperature). Atomic force microscopy is used to examine the surface topography of PPX + CdS films, which is found to be strongly dependent on the concentration of nanoparticles. The dark conductivity and photoconductivity of nanocomposite films arise due to the thermo- and photoexcitation transfer of electrons from the CdS nanoparticles to the PPX matrix with the formation of an electronic double layer at the PPX matrix-large percolation CdS cluster interface, a process that populates the phenyl rings of the adjacent PPX layer with excess electrons. As a result, various mechanisms of electron transfer in the polymer matrix can be realized: Mott’s hopping conduction mechanism with variable-range hopping in the matrix between CdS clusters and the metal-like behavior of the conductivity in the polymer shell of the large cluster at low temperatures. The polymer shell contains excess electrons on the phenyl rings -C₆H₄- in the composition of anion-resonances -C₆H₄⁻-.     

Preparation and photoluminescent properties of doped nanoparticles of ZnS by solid-state reaction

Nanometer-sized Eu³⁺-doped ZnS and Mn²⁺-doped ZnS particles were prepared by solid-state method at low temperature. The structures and properties of those materials were characterized by X-ray diffraction (XRD) and photoluminescent spectroscopy techniques. The XRD patterns reveal that the doped ZnS nanoparticles belong to zinc-blende structure. The concentration of doping ions has little effect on the sizes of the doped ZnS nanoparticles, which mainly depends on the temperature of preparation. The emission peaks from the ⁵D₀→⁷F_J (J=1, 2, and 4) electronic energy transitions of Eu³⁺ were observed in the emission spectra of the ZnS:Eu³⁺ nanoparticles. The intensity ratio of the two peaks from the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions indicates that more Eu³⁺ ions occupy the sites with no inversion symmetry. For the ZnS:Mn²⁺ nanoparticles, an orange emission from the ⁴T₁→⁶A₁ transition of Mn²⁺ is present, indicating that the doping ions occupy the positions of the ZnS lattices. Meanwhile, UV-induced luminescence enhancement was observed for the ZnS:Mn²⁺ nanoparticles, the possible reason of which is discussed primarily.     

Effect of ambient hydrogen sulfide on the physical properties of vacuum evaporated thin films of zinc sulfide

Evaporated thin films of zinc sulfide (ZnS) have been deposited in a low ambient atmosphere of hydrogen sulfide (H₂S ∼10⁻⁴ Torr). The H₂S atmosphere was obtained by a controlled thermal decomposition of thiourea [CS(NH₂)₂] inside the vacuum chamber. It has been observed that at elevated substrates temperature of about 200 °C helps eject any sulfur atoms deposited due to thermal decomposition of ZnS during evaporation. The zinc ions promptly recombine with H₂S to give better stoichiometry of the deposited films. Optical spectroscopy, X-ray diffraction patterns and scanning electron micrographs depict the better crystallites and uniformity of films deposited by this technique. These deposited films were found to be more adherent to the substrates and are pinhole free, which is a very vital factor in device fabrication.     

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.     

Adsorption and photocatalytic degradation of methylene blue on Zn1−xCuxS nanoparticles prepared by a simple green method

Nanoparticles of Zn_1−xCu_xS with various dopant contents (0 ≤ x ≤ 0.15) were prepared in water by refluxing for 90 min at about 95 °C. Powder X-ray diffraction (XRD) patterns of the nanoparticles demonstrate that loading of Cu²⁺ ions does not change the crystal structure of ZnS. Scanning electron microscopy (SEM) images demonstrate that size of the nanoparticles decreases with increasing Cu²⁺ ions. UV-Vis diffuse reflectance spectra (DRS) of the nanoparticles show significant absorption in visible light region. Adsorption capacity of the nanoparticles for methylene blue (MB) increases with mole fraction of copper ions. Photocatalytic activity of the nanoparticles toward photodegradation of MB was evaluated under visible light irradiation. The results indicate that Zn_0.85Cu_0.15S nanoparticles exhibit highest photocatalytic activity among the prepared samples. Moreover, effects of refluxing time applied for preparation of the nanoparticles and calcination temperature were investigated.     

Microstructural, optical and photocatalytic properties of CdS doped TiO2 thin films

CdS doped TiO₂ thin films (with CdS content=0, 3, 6, 9 and 12 at%) were grown on glass substrates. The X-ray diffraction analysis revealed that the films are polycrystalline of monoclinic TiO₂ structure. The microstructure parameters of the films such as crystallite size (D_ν) and microstrain (e) are calculated. Both the crystallites size and the microstrain are decreased with increasing CdS content. The optical constants have been determined in terms of Murmann's exact equations. The refractive index and extinction coefficient are increased with increasing CdS content. The optical band gap is calculated in the strong absorption region. The possible optical transition in these films is found to be an allowed direct transition. The values of E_g^opt are found to decrease as the CdS content increased. The films with 3 at% CdS content have better decomposition efficiency than undoped TiO₂. The films with 6 at% and 9 at% CdS content have decomposition efficiency comparable to that of undoped TiO₂, although they have lower band gap. The CdS doped TiO₂ could have a better impact on the decomposing of organic wastes.