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.     

Thermal neutron flux monitoring using new-type detectors

The applicability of new-type unshielded detectors based on zinc sulfide ZnS(Ag) and lithium fluoride enriched with ⁶Li isotope to 90% to thermal neutron measurements is demonstrated. The results of measurements of the thermal neutron counting rate and flux near the Earth’s surface are presented. The existence of the concentration gradient of thermal neutrons near the Earth’s surface at placing the counter at the various levels (from −4 to 10.5 m) in an experimental building is shown. The effect of meteoparameters on the counting rate of thermal neutrons for a long time is shown.     

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.     

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.     

Photoluminescence of Cu:ZnS, Ag:ZnS, and Au:ZnS nanoparticles applied in Bio-LED

In this work, transition elements, including Cu²⁺, Ag⁺, and Au³⁺, were used to dope in zinc sulfide (ZnS) by chemical solution synthesis to prepare Cu:ZnS, Ag:ZnS, and Au:ZnS nanoparticles, respectively. Transition elements doping ZnS nanoparticles form the electronic energy level between the conduction band and valance band, which will result in the green light emission. There is a zinc sulfide emission shift from blue (~3.01 eV) to green light (~2.15 eV). We also found that Au:ZnS nanoparticles will emit a green light (~2.3 eV) and a blue light (~2.92 eV) at the same time because the mechanism of blue light emission was not broken after Au element had been doped. Furthermore, we used sodium chlorophyllin copper salt to simulate chlorophyll in biological light emission devices (Bio-LED). We combined copper chlorophyll with Cu:ZnS, Ag:ZnS, and Au:ZnS nanoparticles by a self-assembly method. Then, we measured its photoluminescence spectroscopy and X-ray photoelectron spectroscopy to study its emission spectrum and bonding mode. We found that Au:ZnS nanoparticles are able to emit green and blue light to excite the red light emission of copper chlorophyll, which is a potential application of Bio-LED.     

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.     

Thermal neutron flux detection near the Earth’s surface

The thermal neutron flux near the Earth’s surface has been measured using large unshielded neutron scintillator detectors of a new type, based on ZnS(Ag) and lithium fluoride, enriched with ⁶Li to 90%. The existence of a gradient of the thermal neutron concentration near the Earth’s surface is shown.     

Low-background EN-detector for the investigation of the neutron component of EASs

Thermal neutrons are detected using a scintillator compound based on ZnS(Ag) with B₂O₃ added to it. The pulse height generated by single charged particles in a scintillator of small thickness (50 mg/cm²) is not larger than that of a noise signal due to a low sensitivity to single charged particles. As a result, the detector satisfies the requirements of a low-background one in the context of cosmic-ray experiments. The detector is to be used for neutron detection in extensive air showers.     

Polarized emission of doped ZnS crystals

Polarization of light emitted in various spectral ranges was studied in two single crystals ZnS:Cu, Cl and ZnS:Ag, Cu, Al. The G-Cu, B-Cu and a small amount of S-A centers have been identified in the ZnS:Cu, Cl crystal by the spectral and polarization methods. The B-Ag band was found in the spectrum of the ZnS:Ag, Cu, Al crystal and its polarization properties investigated. This emission appears to be always polarized perpendicular to the [111]_c axis of the stacking faults independently of the polarization of the exciting light. The symmetry of the B-Ag center is not lower than that of the host lattice. Analogy with G-Cu centers suggests a model for the B-Ag center in which the polarization comes from the symmetry properties of the Ag²⁺ orbitals in the trigonal field of stacking faults.     

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.     

ZnS(Ag)/<Superscript>6</Superscript>LiF and LiI(Eu) scintillators and silicon photomultipliers for thermal neutron detectors with high space and time resolution

ZnS(Ag)/⁶LiF and LiI(Eu) scintillators for thermal neutron detectors have been investigated and neutron detectors based on these scintillators and photomultipliers have been tested. The efficiencies of these detectors are 5 and 66%, respectively. The possibility of developing position-sensitive detectors of thermal neutrons with high space and time resolution is discussed.     

A study of ZnS(Ag)/6LiF with different mass ratios

The properties of ZnS(Ag)/⁶LiF samples with three different mass ratios were studied. The study showed that the EJ426 sample with the mass ratio of 3:1 had the highest detection efficiency of thermal neutrons, which was 32.4% in the experiment. Furthermore, this sample had the largest charge spectrum. The light yield of its surface at the average value of the charge spectrum was approximately 8.01 × 10³ photons/neutron. The gamma sensitivity of the sample was better than 10⁻⁶ at the threshold of 350 photoelectrons. Therefore, EJ426 is a good candidate for a position-sensitive thermal neutron detector.     

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.     

Optically stimulated luminescence in Ag doped Li2B4O7 single crystal and its sensitivity to neutron detection and dosimetry in OSL mode

Optically stimulated luminescence (OSL) measurements have been carried out on single crystals of Ag doped Li₂B₄O₇ (LTB:Ag) after exposure to various nuclear radiations. The time integrated OSL intensity is found to be linear in the range from 0.1 Gy to 500 Gy. Fading of the OSL signal was found to be around 36% in 48 h. The presence of ⁶Li and ¹⁰B has been gainfully utilized to measure doses of thermal neutrons. Further, the large difference between the wavelength of the stimulation source (∼460 nm) and emission from the LTB:Ag at 270 nm has enhanced the signal-to-noise ratio in a simple OSL set-up with suitable filters. The high sensitivity of the LTB:Ag to thermal neutrons will be useful in variety of applications including personal dosimetry in mixed-fields and imaging devices for neutron radiography.     

The Monte-Carlo simulation on a scintillator neutron detector

A simulation of the properties of the shifting scintillator neutron detector using 6LiF/ZnS(Ag) scintillation screens is performed.The simulation results show that the light attenuation length of standard BC704 scintillator is about 0.65 mm.Its thermal neutron detection efficiency,gamma sensitivity and intrinsic spatial resolution can achieve around 50.0%,10 5and 0.18 mm(along X-axis) respectively.For the detector,air coupling position resolution is better than the silicone oil coupling.Some of the simulation results are compared with experimental results.They are in agreement.This work will be helpful for constructing neutron detector for high intensity powder diffractometer at Chinese spallation neutron source.     

Blue-emitting LaAlO3:Tm3+, In3+ phosphors for field emission displays

Tm³⁺ and In³⁺ co-doped LaAlO₃ phosphors were prepared by a Pechini sol–gel method and characterized by X-ray diffraction, scanning electron microscope, and cathodoluminescence spectrum. The phosphor is composed of slightly aggregated particles with approximately spherical shape and a narrow size range of 1.0–1.5 μm. Under voltage electron beam excitation, the phosphor shows the characteristic emissions of Tm³⁺. All the color purity, radiant efficiency, luminous efficiency, and stability of the optimum LaAlO₃:0.01Tm³⁺, 0.04In³⁺ phosphor are superior to these of commercial ZnS:Ag,Cl phosphor. These tests suggest that it could be a potential candidate as a blue phosphor for field emission displays.     

Luminescence of nanocrystalline ZnS:Cu

Temperature dependent luminescence and luminescence lifetime measurements are reported for nanocrystalline ZnS:Cu²⁺ particles. Based on the variation of the emission wavelength as a function of particle size (between 3.1 and 7.4 nm) and the low quenching temperature (T_q=135 K), the green emission band is assigned to recombination of an electron in a shallow trap and Cu²⁺. The reduction in lifetime of the green emission (from 20 μs at 4 K to 0.5 μs at 300 K) follows the temperature quenching of the emission. In addition to the green luminescence, a red emission band, previously only reported for bulk ZnS:Cu²⁺, is observed. The red emission is assigned to recombination of a deeply trapped electron and Cu²⁺. The lifetime of the red emission is longer (about 40 μs at 4 K) and the quenching temperature is higher.     

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.     

Neutron-interferometric measurement of the coherent scattering length of the isotopes of silver

Employing a new measuring scheme with the D 18 Neutron Interferometer at the ILL, Grenoble, the thermal neutron coherent scattering lengthb _c for bound atoms of natural Ag,¹⁰⁷Ag and¹⁰⁹Ag have been measured at the wavelengthλ=1.8742 Å. The results, corrected for composition and density of the actual samples, are nat Ag:b _c =5.922 (7) fm¹⁰⁷Ag:¹⁰⁷ b _c =7.555 (11) fm¹⁰⁹Ag:¹⁰⁹ b _c =4.165 (11) fm The result is in agreement with an earlier value obtained with the interferometer for nat Ag (5.932 (6)) using a different measuring scheme and a totally different sample. For nat Ag and¹⁰⁷Ag the values determined interferometrically are ?0.05 and ?0.08 fm smaller, repectively, than those measured with the Christiansen filter method.     

Photoluminescence properties of impurity-doped ZnS nanocrystals fabricated by sequential ion implantation

We have studied photoluminescence (PL) spectrum and dynamics of Cu- and Al-doped ZnS (ZnS:Cu,Al) nanocrystals fabricated by sequential implantation of Zn⁺, S⁺, Cu⁺, and Al⁺ ions into Al₂O₃ matrices. These samples exhibit intense green PL under UV light excitation. The space- and time-resolved PL measurements show that the broad green PL is due to the donor–acceptor (DA) pair luminescence of single ZnS:Cu,Al nanocrystals.