Nanosize induced effects in luminescent ZnS:Mn2+ quantum dots

Nanocrystals (NCs) of II–VI semiconductors of few nanometers average size, called quantum dots (QDs), are now intensely investigated as radiation detectors. Besides the expected quantum confinement and influence of surface states, our electron paramagnetic resonance investigations of cZnS QDs doped with Mn²⁺ ions, correlated with structural data, underline that other properties should be also taken into consideration in developing the II–VI semiconductor QDs as radiation detectors. Thus, the preferential localization of Mn²⁺ in the core of the cubic ZnS QDs at substitutional Zn²⁺ cation sites next to a stacking lattice defect is expected to lead, besides changes in the impurity energy levels, to specific aggregation properties. An outer shell of different composition can also influence the structural properties of the QDs core with effects on the optical properties as well.     

Green and red photoluminescence from ZnAl2O4:Mn phosphors prepared by sol–gel method

Luminescence investigations of Mn-activated ZnAl₂O₄ phosphors prepared by using sol–gel method were described. The phosphor was characterized by X-ray diffraction (XRD) and electronic paramagnetic resonance (EPR). The EPR spectra of the samples suggested that Mn ions possessed homogeneous distribution in ZnAl₂O₄ phosphors. Photoluminescence studies of the prepared phosphors showed green and red emissions. The red emission became weaker, and vanished at last with sintering temperature increasing from 600 to 900 °C in reducing atmosphere, while the intensity of green emission peak increased. Furthermore, when the phosphor was sintered at 900 °C in air, the intensity of red and green emissions decreased, but the value of intensity ratio increased. It suggested that the green emission resulted from Mn²⁺ and the red emission resulted from Mn⁴⁺.     

Thermoluminescence,photoluminescence and EPR studies on Mn2+ activated yttrium aluminate (YAlO3) perovskite

Thermoluminescence (TL) measurements were carried out on undoped and Mn²⁺ doped (0.1 mol%) yttrium aluminate (YAlO₃) nanopowders using gamma irradiation in the dose range 1–5 kGy. These phosphors have been prepared at furnace temperatures as low as 400 °C by using the combustion route. Powder X-ray diffraction confirms the orthorhombic phase. SEM micrographs show that the powders are spherical in shape, porous with fused state and the size of the particles appeared to be in the range 50–150 nm. Electron Paramagnetic Resonance (EPR) studies reveal that Mn ions occupy the yttrium site and the valency of manganese remains as Mn²⁺. The photoluminescence spectrum shows a typical orange-to-red emission at 595 nm and suggests that Mn²⁺ ions are in strong crystalline environment. It is observed that TL intensity increases with gamma dose in both undoped and Mn doped samples. Four shouldered TL peaks at 126, 240, 288 and 350 °C along with relatively resolved glow peak at 180 °C were observed in undoped sample. However, the Mn doped samples show a shouldered peak at 115 °C along with two well defined peaks at ～215 and 275 °C. It is observed that TL glow peaks were shifted in Mn doped samples. The kinetic parameters namely activation energy (E), order of kinetics (b), frequency factor (s) of undoped, and Mn doped samples were determined at different gamma doses using the Chens glow peak shape method and the results are discussed in detail.     

An EPR Study on the Chemical form of Mn2+ in the Chinese Loess Samples

Electron paramagnetic resonance (EPR) spectroscopy in combination with thermal methods were used to identify and characterize Mn²⁺ in the Chinese loess that is a multimineral system. EPR spectra of the loess samples from the classic loess-paleosol section in central China show the presence of trace amounts of Mn²⁺; whereas paleosol samples present no Mn²⁺ EPR signal. The spectral changes upon step heating from room temperature to 1000 °C suggest that this EPR signal in the loess arises from Mn substituted into CaCO₃. This study provides a direct evidence that the loess-paleosol profiles were formed under the changing redox conditions caused by a past climatic change.     

EPR/FMR Investigation of Mn-Doped SiCN Ceramics

SiCN magnetic ceramics doped with Mn²⁺ ions were synthesized at the pyrolysis temperature of 1,100° C, using CERASET™ as liquid polymer precursor and polymer manganese(II) acetylacetonate as dopant, and investigated by electron paramagnetic resonance (EPR)/ferromagnetic resonance (FMR) technique. The predominant source of ferromagnetism in SiCN samples doped with Mn ions, as synthesized here, is the ensemble of ferromagnetic nanoparticles of Mn₅Si₃C _x incorporated into the amorphous SiC/Mn structure. The fluctuation of magnetization due to ferromagnetic Mn₅Si₃C_x particles significantly broadens the EPR lines at the phase-transition temperature (363 K). This is the first fabrication of a SiCN/Mn ceramic, which exhibits room-temperature ferromagnetism.     

Effects of sintering temperature and atmosphere on magnetism in Mn-doped TiO2 bulk samples

Magnetic properties have been investigated on Mn doped TiO₂(Ti_0.98Mn_0.02O₂) bulk samples prepared by solid state reaction, which were sintered at different temperature ranging from 450 ^°C to 900 ^°C in air and argon atmosphere, respectively. The results show that the magnetic properties were strongly dependent on the sintering temperature and atmosphere. For samples sintered in air, the magnetization initially increase with the increase of sintering temperature up to 600 ^°C and thereafter it decrease. While the magnetization of samples sintered in argon atmosphere decreases monotonically with the increase of sintering temperature. Furthermore, for samples sintered at 600 ^°C in air, the magnetic susceptibility exhibits a dominant Curie-Weiss behaviour and no magnetic transition is observed over the temperature range from 10 to 300 K. In contrast, for samples sintered in argon atmosphere, besides the magnetic transition near 45 K perhaps caused by Mn₃O₄, another magnetic transition appears near room temperature.     

Electronic transport in the novel SOFC anode material La1−xSrxCr0.5Mn0.5O3±δ

Electronic conductivity in the potential SOFC anode material La_1−xSr_xCr_0.5Mn_0.5O_3±δ has been investigated in the range 0.2 < x < 0.3. log(σT) vs. 1/T plots indicate conduction via thermally activated polaron hopping. At 900 °C, conductivity in air increases with Sr²⁺ via an increase in [B_B] holes (B—transition metal). X-ray absorption spectroscopy (XAS) studies indicate that compensation for A-site Sr substitution and oxygen vacancy formation is via the Mn cation only; Cr maintains a 3+ oxidation state and 6-fold oxygen coordination. Electronic transport occurs by percolation between Mn cations in a disordered B-site sub-lattice. Conductivity decreases with p(O₂), which is indicative of p-type conduction behaviour, but the relationship cannot be explained by a simple redox equilibrium involving Mn³⁺, Mn⁴⁺ and oxygen, possibly due to co-existence of Mn²⁺, Mn³⁺ and Mn⁴⁺ via disproportionation as with La_1−xSr_xMnO_3±δ.     

ZnO:Mn as a member of II-VI:Mn family

ZnO:Mn thin films are grown by the metal organic chemical vapor deposition technique. Mn (x) varies in the 0<x<0.44 range. Vegard’s law has been verified for the lattice parameters. Electron paramagnetic resonance (EPR) measurements prove the substitutional incorporation of Mn²⁺ on zinc sites. The behavior of the EPR line width regarding temperature is discussed. All ZnO:Mn layers show antiferromagnetic interaction and a J₁/k_B=-15 K effective exchange constant. The optical band gap of ZnO:Mn increases with the manganese concentration. Raman spectroscopy reveals a Mn-related scattering band. PACS 71.55.Gs; 75.50.Pp; 61.10.Nz; 76.30.Fc; 75.30.Et; 78.40.-q     

EPR study of Fe3+ and Mn2+ in CeO2 and ThO2

Attempts were made to grow CeO₂ and ThO₂ single crystals doped with transition metal ions. Only Fe³⁺ and Mn²⁺ could be detected by the EPR technique. The EPR spectrum of Fe³⁺ in CeO₂ exhibits the well-known fine structure in cubic fields. The parameters areg=2.0044(1) anda=15.6(1)·10^?4 cm^?1. The hyperfine constantA for⁵⁷Fe in hexahedral coordination was found to be 8.9(1)·10^?4 cm^?1. The EPR spectrum of Mn²⁺ in CeO₂ reveals two cubic Mn²⁺ centers. The parameters for center 1 areg=1.9999(1) andA=86.9(1)·10^?4 cm^?1 and for center 2g=1.9984(1) andA=87.0(1)·10^?4 cm^?1. Heating the Mn doped CeO₂ samples in hydrogen, the Mn²⁺ centers transform from cubic into trigonal centers with approximate values ofg=1.9988(2),A=84.5(6)·10^?4 cm^?1 andD=203(1)·10^?4 cm^?1. The two observed Mn²⁺ centers in ThO₂ exhibita priori axial symmetry with approximate values ofg=2.0006(2),A=88.9(4)·10^?4 cm^?1 andD=33(3)·10^?4 cm^?1.     

Zn1−xMnxO : A typical member of the II-VI:Mn DMS family

ZnO:Mn thin films are grown by MOCVD technique. Mn(x) varies in 0<x<0.44 range. Vegrad’s law has been verified for the lattice parameters. EPR measurements prove the substitution incorporation of Mn²⁺ on zinc sites. The behavior of EPR line-width regarding temperature is discussed. All ZnO:Mn layers show antiferromagnetic interaction and a effective exchange constant. Observation of excitons, giant Zeeman effect, and trace of the Brillouin function is evidence for high quality of the crystal lattice and of substitutional incorporation of manganese ions in place of Zn, as Mn²⁺.     

Thermoluminescence,infrared reflectivity and electron paramagnetic resonance properties of hemimorphite

Silicate mineral hemimorphite has been investigated concerning its TL, IR and EPR properties. A broad TL peak around 180 °C and a weaker and narrower peak around 360 °C were found in a sample annealed at 600 °C for 1 h and then irradiated. The deconvolution using the CGCD method revealed peaks around 132, 169, 222 and 367 °C. The reflectivity measurements showed several bands in the NIR region due to H₂O, OH and Al–OH complexes. No band was observed in the visible region. The thermal treatments were carried out from ∼110 to 940 °C and dehydration was observed, first causing a diminishing optical absorption in general and the disappearance of water and hydroxyl absorption bands. The EPR spectrum of natural hemimorphite, presented Cu²⁺ signals at g = 2.4 and g = 2.1 plus E₁′ signal superposed to Fe³⁺ signal around g = 2.0.     

Preparation and characterization of manganese and cobalt based semiconducting ceramics

The spinel phases Mn_3-x Co _x O₄ are prepared directly and at low temperature (600-700°C) by means of thermal decomposition of mixed oxalate salt precursors in air or controlled atmosphere. The powders so obtained when sintered at 1200°C behave as semiconducting or insulating ceramics, depending on their cobalt content. The electrical properties of these ceramics have been found to be related to the distribution and valencies of the cations in the spinel structure. The study of their crystalline structure and stability, complemented by electrical measurements have permitted us to distinguish two types of compounds. The first type includes those given by Mn_3-x Co _x O₄ with 0 < x < 1. These are stable at high temperature, have a tetragonal structure and are insulators. The cation distributions for them can be deduced from that of hausmannite as given by Mn²⁺ [Mn³⁺ ₂]O₄, by substituting Mn²⁺ ions with Co²⁺ ions. The second type of compounds correspond to the cubic phases and occur for 1 < x < 3. These are unstable at high temperature and show a lower resistivity (e.g. 360 ° cm) which is of particular interest for their application in the design of negative temperature coefficient (N.T.C.) thermistor components.     

Luminescence properties of green emission of SiO2/Zn2SiO4:Mn nanocomposite prepared by sol–gel method

Green light emitting Mn²⁺ doped Zn₂SiO₄ particles embedded in SiO₂ host matrix were synthesized by a sol–gel method. After the incorporation of ZnO:Mn nanoparticles in a silica monolith using sol–gel method with supercritical drying of ethyl alcohol in two steps, it was heat treated in air at 1200 °C for 2 h in order to obtain the SiO₂/α-Zn₂SiO₄:Mn nanocomposites. The microstructure of phosphor crystals was characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). XRD results indicate that the pure phase α-Zn₂SiO₄ with rhombohedral structure was obtained after thermal treatment at 1200 °C. The SiO₂/α-Zn₂SiO₄:Mn nanocomposites with a Mn doping concentration of 1.5 at% exhibit two broadband emissions in the visible range: a strong green emission at around 525 nm and a second one in the range between 560 and 608 nm. This nanocomposite with a Mn doping concentration of 0.05 shows the highest relative emission intensity. Upon 255 nm excitation, the luminescence decay time of the green emission of Zn₂SiO₄:Mn around 525 nm is 11 ms. The luminescence spectra at 525 nm (⁴T₁–⁶A₁) and lifetime of the excited state of Mn²⁺ ions-doped Zn₂SiO₄ nanocrystals are investigated.     

Structural and optical characterization of sol–gel derived Tm-doped BaTiO3 nanopowders and ceramics

Nanocrystalline Tm³⁺(5%)-doped BaTiO₃ (BT-Tm) has been synthesized by the sol–gel method. The morphology, structure, and optical properties of powders and ceramics were characterized. The average grain size of the gel precursor annealed at 700 and 900 °C was 20 nm and 30 nm, respectively. These powders were single phase and crystallized with a cubic structure while the BT-Tm sintered ceramics were crystallized with the tetragonal BaTiO₃ structure. The photoluminescence spectra showed typical transitions of Tm³⁺ ions and a structure consistent with the Tm³⁺ ions incorporation in the BaTiO₃ crystalline lattice. Thermoluminescence peaks recorded at 300 °C (for annealed samples) or at 230 °C for the ceramic sample were assigned to the recombination of the Tm²⁺-electron traps located mainly at the surface of the nano-crystals or inside the microcrystals, respectively.     

Thermally evaporated CdS/CdTe thin film solar cells: Optimization of CdCl2 evaporation treatment on absorber layer

CdCl₂ treatment is crucial in the fabrication of highly efficient CdS/CdTe thin-film solar cells. This study reports a comprehensive analysis of thermal evaporated CdS/CdTe thin-film solar cells when the CdTe absorber layer is CdCl₂ annealed at temperatures from 340 to 440 °C. Samples were characterized for structural, optical, morphological and electrical properties. The films annealed at 400 °C showed better crystallinity with a cubic zinc blende structure having large grains. Higher refractive index, optical conductivity, and absorption coefficient were recorded for the CdTe films annealed at 400 °C with CdCl₂. Optimum photoactive properties for CdS/CdTe thin-film solar cells were also obtained when samples were annealed at 400 °C for 20 min with CdCl₂, and the best device exhibited V_OC of 668.4 mV, J_SC of 13.6 mA cm⁻², FF of 53.9% and an efficiency of 4.9%.     

Thermoluminescence study of aluminum oxide doped with therbium and thulium

In this work, α-Al₂O₃ doped either with Tb³⁺ or Tm³⁺ was prepared by combustion synthesis techniques for thermoluminescent (TL) ionizing radiation dosimetry applications. In this method, the reactants (aluminum nitrate, urea and therbium or thulium nitrate) are ignited in a muffle furnace at temperatures as low as 500 °C. This synthesis route is an alternative technique to the conventional fabrication methods of materials based on α-Al₂O₃ (Czochralsky, Vernuil), where high melting temperatures and reducing atmospheres are required. After combustion, the samples were annealed at temperatures ranging from 1000 to 1400 °C for 4 h in order to obtain the pure α-phase structure and were then irradiated with a Co-60 gamma radiation source. The annealed samples present a well defined TL glow peak with a maximum at approximately 200 °C and linear TL response in the dose range 0.5–5 Gy. It was observed that a 0.1 mol% concentration of Tb³⁺ or Tm³⁺ and annealing at 1400 °C optimize the TL sensitivity. The highest sensitivity was found for Tm³⁺ doped samples which were approximately 25 times more sensitive than Tb³⁺ doped samples. These results strongly suggest that combustion synthesis is a suitable technique to prepare doped aluminum oxide material and that Tm³⁺ doped aluminum oxide is a potential material for TL radiation dosimetry.     

Effect of heat treatments on the luminescence properties of Zn2SiO4:Mn2+ phosphors prepared by glycothermal methods

Manganese-doped Zn₂SiO₄ phosphors with different crystal structures and morphologies were synthesized by glycothermal reactions of zinc acetate dihydrate and manganese(II) acetate tetrahydrate with tetraethyl orthosilicate in various glycols at 315 °C. The reactions in 1,3-propanediol and 1,4-butanediol yielded α-Zn₂SiO₄:Mn²⁺, whereas the reactions in ethylene glycol and 1,5-pentanediol yielded β-Zn₂SiO₄:Mn²⁺ and ZnO, respectively. The samples obtained in 1,4-butanediol and 1,3-propanediol emitted green light (522 nm), and the sample prepared in 1,4-butanediol showed a higher emission intensity. The photoluminescence intensity of the Zn_1.96Mn_0.04SiO₄ phosphor prepared by a glycothermal reaction in 1,4-butanediol and subsequently calcined at 1100 °C was twice as high as that of the sample synthesized by a conventional solid-state reaction. The high emission efficiency was obtained because the highly homogeneous distribution of Mn²⁺ in the α-Zn₂SiO₄ host synthesized by the glycothermal reaction was maintained during calcination treatment in air.     

EPR and luminescence of Mn2+ in MgF2 single crystals

EPR spectra of Mn²⁺ in MgF₂ crystals grown by the Bridgman technique were measured at 300 and 20 K. Application of the superposition model to the zero field splitting (ZFS) patterns indicates complete absence of dynamic contributions to the ZFS. The luminescence spectra are also in accord with substitutional incorporation of Mn²⁺.     

EPR study on the La2/3Ca1/3Mn1−x Cu x O y system

The interaction between Mn and Cu ions is studied by measuring the resistance and electron paramagnetic resonance (EPR) at different temperatures of the Cu-doped compound La_2/3Ca_1/3Mn_1−xCu_xO_y. A new transition inR-T curve and substantial enhancement in magnetoresistance are induced by the substitution of Mn ions by Cu ions. The EPR measurement shows that two resonance signals appear at temperature lower than the spin-ordering temperature of Mn ions. A tentative interpretation for the observed phenomena is proposed by considering the interaction between the Cu/Mn ions besides the Mn³⁺/Mn⁴⁺ ions.     

Effect of annealing on natural calcitic crystals—A thermostimulated luminescence (TSL) study

The quality crystals (Calcitic limestone) were selected using the UV–visible methylene blue adsorption method. The thermostimulated luminescence (TSL) glow curve characteristics of six well crystallized limestone samples were analyzed. The glow curves of unannealed sample show only one peak in the range 320–330 °C. The sample irradiated with a gamma dose of 100 Gy shows two additional peaks in the range of 113–125 °C and 242–260 °C when recorded with linear heating rate of 10 °C/s. The annealed sample also shows the same trend as that of irradiated sample. Annealing treatment above 250 °C increases the sensitivity of all TSL peaks except 320 °C. On the other hand, annealing at 750 °C caused a collapse in the TSL sensitivity. The enhancement in TSL sensitivity was found to depend on the annealing temperature and time. Annealing treatment at 650 °C for 4 h followed by quenching in air is the optimum condition for TSL sensitization. The response to gamma irradiation is linear in the range from 0.5 Gy to 10⁴ Gy. The emission spectra of all the samples show an emission at around 610 nm but with different intensities for each TSL peak. With reference to earlier work, it may be assumed that the recombination site always involves Mn²⁺ ions. The observation made through infra-red (IR) and X-ray diffraction (XRD) studies with thermal treatment shows the structural changes of calcite from D_3h to C_s symmetry at 750 °C. The Thermogravimetric-Differential Thermal Analysis (TG-DTA) analysis shows the calcite gets disordered at 760 °C. Hence, the collapse in the TSL sensitivity at 750 °C is due to structural change or structural disorderedness.