Pressure-induced two-color photoluminescence in MnF(2) at room temperature

A novel two-color photoluminescence (PL) is found in MnF(2) at room temperature under high pressure. Contrary to low-temperature PL, PL at room temperature is unusual in transition-metal concentrated materials like MnF(2), since the deexcitation process at room temperature is fully governed by energy transfer to nonradiative centers. We show that room-temperature PL in MnF(2) originates from two distinct Mn(2+) emissions in the high-pressure cotunnite phase. The electronic structure and the excited-state dynamics are investigated by time-resolved emission and excitation spectroscopy at high pressure.     

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⁺.     

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.     

Characterization of Cu1.4Mn1.6O4/PPy composite electrodes

In this work, we have studied the multilayered polypyrrole(PPy)/oxide composite electrode on glassy carbon (GC) having the structure GC/PPy/PPy(Cu_1.4Mn_1.6O₄)/PPy using X-ray Photoelectron Spectroscopy and Mn K-edge and Cu K-edge XANES and EXAFS. The mixed oxide particles have been incorporated into the PPy matrix simultaneously to the electropolymerization of Py from a solution containing 0.1 M Py + 0.15 M KCl + Cu_1.4Mn_1.6O₄. The XPS data have shown that, prior to the incorporation of the oxide into the PPy matrix, it contains Cu⁺, Cu²⁺, Mn³⁺ and Mn⁴⁺. The XPS, XANES and EXAFS results have shown that when the oxide is incorporated into the PPy matrix, the Cu⁺ present in the original oxide suffers dismutation to give Cu²⁺ and metallic Cu. The metallic Cu is segregated out of the spinel structure. The Mn K-edge XANES and EXAFS data show that, after the incorporation into the PPy matrix, Mn is present as Mn³⁺ and Mn⁴⁺ occupying octahedral sites in a spinel-related structure while the Cu K-edge XANES and EXAFS data indicate that copper occupies tetrahedral sites predominantly in that structure but having a large degree of disorder in the second and higher coordination shells.     

Photoluminescence studies of ZnO thin films on R-plane sapphire substrates grown by sol–gel method

Zinc oxide (ZnO) thin films on R-plane sapphire substrates were grown by the sol–gel spin-coating method. The optical properties of the ZnO thin films were investigated using photoluminescence. In the UV range, the asymmetric near-band-edge emission was observed at 300 K, which consisted of two emissions at 3.338 and 3.279 eV. Eight peaks at 3.418, 3.402, 3.360, 3.288, 3.216, 3.145, 3.074, and 3.004 eV, which respectively correspond to the free exciton (FX), bound exciton, transverse optical (TO) phonon replica of FX recombination, and first-order longitudinal optical phonon replica of FX and the TO (1LO+TO), 2LO+TO, 3LO+TO, 4LO+TO, and 5LO+TO, were obtained at 12 K. From the temperature-dependent PL, it was found that the emission peaks at 3.338 and 3.279 eV corresponded to the FX and TO, respectively. The activation energy of the FX and TO emission peaks was found to be about 39.3 and 28.9 meV, respectively. The values of the fitting parameters of Varshni's empirical equation were α=4×10^?3 eV/K and β=4.9×10³ K, and the S factor of the ZnO thin films was 0.658. With increasing temperature, the exciton radiative lifetime of the FX and TO emissions increased. The temperature-dependent variation of the exciton radiative lifetime for the TO emission was slightly higher than that for the FX emission.     

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.     

Structural and magnetic ordering in La0.6Ca0.4MnO3

The structural and magnetic ordering in La_0.6Ca_0.4MnO₃ has been studied by neutron powder diffraction as a function of temperature between 15 and 300 K. The para-ferromagnetic transition at T∼250 K is accompanied by significant structural distortions in the form of octahedral Mn–O₆ rotations. At 15 K, the total refined ferromagnetic moment on the Mn site was obtained as 3.1 μ_B, in reasonable agreement with the total expected average moment of mixed Mn³⁺/Mn⁴⁺ matrix.     

Yellow-orange light emission from Mn2+-doped ZnS nanoparticles

ZnS nanoparticles with Mn²⁺ doping (1–2.5%) have been prepared through a simple soft chemical route, namely the chemical precipitation method. The nanostructures of the prepared undoped ZnS and Mn²⁺-doped ZnS:Mn nanoparticles have been analyzed using X-ray diffraction (XRD), Scanning electron microscope (SEM), transmission electron microscope (TEM) and UV–vis spectrophotometer. The size of the particles is found to be in 2–3 nm range. Room-temperature photoluminescence (PL) spectrum of the undoped sample only exhibits a blue-light emission peaked at ∼365 nm under UV excitation. However, from the Mn²⁺-doped samples, a yellow-orange emission from the Mn^{2+ 4}T₁–⁶A₁ transition is observed along with the blue emission. The prepared 2.5% Mn²⁺-doped sample shows efficient emission of yellow-orange light with the peak emission at ∼580 nm with the blue emission suppressed.     

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr³⁺ centers in submicron and nanostructured Al₂O₃ crystals of different phase compositions at temperatures of 300 and 7.5 K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of ²E, ⁴T₂, and ⁴T₁ excited states of Cr³⁺ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al₂O₃ nanoscale crystals is formed by intracenter transitions (2.5–5.5 eV region), by charge transfer band (6.9 eV) and by effective formation of impurity-bound excitons (9.0 eV region). Such impurity-bound excitons correspond to O₂p→Al₃s electron transition in surroundings of an impurity Cr³⁺ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100 nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al₂O₃ crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al₂O₃ at an excitation energy above 19 eV (more than 2E_g).     

Luminescence properties of pHEMA-TiO2 gels based hybrids materials

Photoluminescence (PL) of photochromic pHEMA-TiO₂ gels-based hybrids was studied by means of time- and energy-resolved spectroscopy at temperatures between 300 K and 10 K. The PL band at 485 nm is assigned to S0←T1 transition of methoxyphenol (organic molecule added to the commercial monomer hydroxyethyl methacrylate, HEMA and used as an inhibitor of spontaneous polymerisation) in the polymer environment, while the PL band at 600 nm is assigned to the self-trapped exciton onto octahedral TiO₆ site of the inorganic component. The mechanisms of the excited states population are discussed. In particular it is shown that both singlet-triplet energy transfer in methoxyphenol and methoxyphenol–TiO₂ charge transfer are strongly affected by the material composition and temperature. The hypothesis about the photoexcited holes annihilation with the trapped electrons is confirmed to be one of main mechanisms limiting the Ti³⁺ centres concentration.     

Optical properties,luminescence quenching mechanism and radiation hardness of Eu-doped GaN red powder phosphor

We report on the luminescence quenching mechanism of Eu-doped GaN powder phosphor produced with a low-cost, high yield rapid-ammonothermal method. We have studied as-synthesized and acid rinsed Eu-doped GaN powders with the Eu concentration of ～0.5 at.%. The Eu-doped GaN photoluminescence (PL) was investigated with 325 nm excitation wavelength at hydrostatic pressures up to 7.7 GPa in temperature range between 12 K and 300 K. The room temperature integrated Eu³⁺ ion PL intensity from acid rinsed material is a few times stronger than from the as-synthesized material. The temperature dependent PL studies revealed that the thermal quenching of the dominant Eu³⁺ ion transition (⁵D₀ → ⁷F₂) at 622 nm is stronger in the chemically modified phosphor indicating more efficient coupling between the Eu³⁺ ion and passivated GaN powder grains. Furthermore, it was found that thermal quenching of Eu³⁺ ion emission intensity can be completely suppressed in studied materials by applied pressure. This is due to stronger localization of bound exciton on Eu³⁺ ion trap induced by hydrostatic pressure. Furthermore, the effect of 2 MeV oxygen irradiation on the PL properties has been investigated for highly efficient Eu-doped GaN phosphor embedded in KBr–GaN:Eu³⁺ composite. Fairly good radiation damage resistance was obtained for 1.7 × 10¹² to 5 × 10¹³ cm^?2 oxygen fluence. Preliminary data indicate that Eu-doped GaN powder phosphor can be considered for devices in a radiation environment.     

Antiferromagnetic phase transition in garnet-type AgCa2Mn2V3O12 and NaPb2Mn2V3O12

Ferrimagnetism has been extensively studied in garnets, whereas it is rare to find the antiferromagnet. Present work will demonstrate antiferromagnetism in the two Mn–V-garnets. Antiferromagnetic phase transition in AgCa₂Mn₂V₃O₁₂ and NaPb₂Mn₂V₃O₁₂ has been found, where the magnetic Mn²⁺ ions locate only on octahedral A site. The heat capacity shows sharp peak due to antiferromagnetic order with the Néel temperature T_N=23.8 K for AgCa₂Mn₂V₃O₁₂ and T_N=14.2 K for NaPb₂Mn₂V₃O₁₂. The magnetic entropy change over a temperature range 0–50 K is 13.9 J K^?1 mol-Mn²⁺-ions^?1 for AgCa₂Mn₂V₃O₁₂ and 13.6 J K^?1 mol-Mn²⁺-ions^?1 for NaPb₂Mn₂V₃O₁₂, which are in good agreement with calculated value of Mn²⁺ ion with spin S=5/2. The magnetic susceptibility shows the Curie–Weiss behavior over the range 29–350 K. The effective magnetic moment μ_eff and the Weiss constant θ are μ_eff=6.20 μ_B Mn²⁺-ion^?1 and θ=?34.1 K (antiferromagnetic sign) for AgCa₂Mn₂V₃O₁₂ and μ_eff=6.02 μ_B Mn²⁺-ion^?1 and θ=?20.8 K for NaPb₂Mn₂V₃O₁₂.     

Photoluminescence properties of Eu2+–Mn2+ codoped Ca-α-SiAlON phosphors

Eu²⁺–Mn²⁺ codoped Ca-α-SiAlON phosphors, Ca_0.736?ySi_9.6Al_2.4O_0.8N_15.2:0.064 Eu²⁺, yMn²⁺, were firstly synthesized by the high temperature solid state reaction method. The effects of doped Eu²⁺ and Eu²⁺–Mn²⁺ concentrations on the photoluminescence properties of the as-prepared phosphors were investigated systematically. Powder X-ray diffraction shows that pure Ca-α-SiAlON phase is synthesized after sintering at 1700 °C for 2 h under 0.5 MPa N₂ atmosphere. The excitation spectra of Eu²⁺-doped Ca-α-SiAlON phosphors are characterized by two dominant bands centered at 286 nm and 395 nm, respectively. The photoluminescent spectrum of Eu²⁺-doped Ca-α-SiAlON phosphor exhibits an intense emission band centered at 580 nm due to the allowed 4f ⁶5d→4f ⁷ transition of Eu²⁺, showing that the phosphor is a good candidate for creating white light when coupled to a blue LED chip. The intensities of both excitation and emission spectra monotonously decrease with the increment of codoped Mn²⁺ content (i.e. y value), indicating that energy transfer between Eu²⁺ and Mn²⁺ is inefficient in the case of Eu²⁺–Mn²⁺ codoped Ca-α-SiAlON phosphors.     

Excitonic magnetic polaron dynamics of MBE grown CdTe/Cd1 − xMnxTe,Cd1 − xMnxTe/Cd1 − vMgyTe single quantum wells in magneti fields

Excitonic lifetimes in Cd_1 − xMn_Ue2Te, Cd_1 − xMg_xTe epilayers and CdTe/Cd_1 − xMn_xTe, Cd_1 − xMn_xTe/Cd_1 − vMg_yTe single quantum wells with different well widths and Mn, Mg compositions are investigated. The excitonic lifetimes are found to reduce drastically by applying external magnetic fields to samples with giant Zeeman splittings. The observed phenomenon is interpreted in terms of the PL decay time contribution from the long-life dark excitons which can convert to excitons for recombinations by a spin-flip process. We attribute the lifetime reduction to the depletion of dark excitons due to their crossing over the exciton energies for dipole allowed transitions in magnetic fields.     

Mid gap photoluminescence from GaN:Mn,a magnetic semiconductor

The defect and morphology of GaN monocrystals with Mn content 10¹⁹ cm⁻³ were examined by fluorescence confocal microscopy and spectroscopy. The fluorescence spectral investigation was carried out in a region very close to the defect centers. Contrary to earlier results, we did observe a characteristic fluorescence line of Mn corresponding to the ⁴T₁→⁶A₁ and ⁴T₂→⁶A₁ transitions, suggesting the predominant presence of Mn²⁺ (d⁵). In addition, strong emission lines were observed at 1.60 and at 1.85 eV when the sample was excited with light of 436 and 365 nm, respectively. An energy scheme is proposed to explain the observed data coherently.     

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.     

Phase dependent photoluminescence and energy transfer in Ca2P2O7: Eu2+, Mn2+ phosphors for white LEDs

α- and β-Ca₂P₂O₇: Eu²⁺, Mn²⁺ phosphors were prepared by solid-state reaction. Phase transition from tetragonal (β-phase) to monoclinic (α-phase) is performed. A strong orange emission of Mn²⁺ is observed in both α-and β-Ca₂P₂O₇: Eu²⁺, Mn²⁺ upon near ultraviolet (UV) excitation through energy transfer from Eu²⁺ to Mn²⁺. The transfer efficiencies for various Mn²⁺ concentrations are estimated based on lifetime measurements of the fluorescence of Eu²⁺ in the two phases. The photoluminescence excitation spectra of α-Ca₂P₂O₇: Eu²⁺, Mn²⁺ can cover 400 nm of the near-UV range, denoting its potential use as a phosphor with intense orange component for white light emitting diodes (LEDs).     

Photoluminescence properties of Er-doped β-FeSi2 grown by ion implantation

Photoluminescence (PL) properties of Er-doped β-FeSi₂ (β-FeSi₂:Er) and Er-doped Si (Si:Er) grown by ion implantation were investigated. In PL measurements at 4.2 K, the β-FeSi₂:Er showed the 1.54 μm PL due to the intra-4f shell transition of ⁴I_13/2→⁴I_15/2 in Er³⁺ ions without a defect-related PL observed in Si:Er. In the dependence of the PL intensity on excitation photon flux density, the obtained optical excitation cross-section σ in β-FeSi₂:Er (σ=7×10⁻¹⁷ cm²) is smaller than that in Si:Er (σ=1×10^-15 cm²). In the time-resolved PL and the temperature dependence of the PL intensity, the 1.54 μm PL in β-FeSi₂:Er showed a longer lifetime and larger activation energies for non-radiative recombination (NR) processes than Si:Er. These results revealed that NR centers induced by ion implantation damage were suppressed in β-FeSi₂:Er, but the energy back transfer from Er³⁺ to β-FeSi₂ was larger than Si:Er.     

Electron spin resonance and cation distribution studies of the Co0.6Zn0.4MnxFe2−xO4 ferrite system

The ESR spectra of the ferrite system Co_0.6Zn_0.4Mn_xFe_2−xO₄ (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) were obtained at room temperature. The experimental values of the magnetic moment (μ_exp) were estimated from the ESR spectra and the cation distribution was consequently established from the values of μ_exp. The systematic decrease in ESR line width observed in our present study was attributed to the decrease of Fe²⁺ concentration with increasing Mn content. The resonance field decreases and reaches a minimum at high values of Mn content whereas the magnetic moment reaches a maximum at these values. The IR spectra were recorded in the range 200–1200 cm⁻¹. The bands at 569 (ν₁) and 389 cm⁻¹ were assigned to the tetrahedral and octahedral complexes, respectively. The band at 441 cm⁻¹ is due to the Mn–O bond vibration. The theoretical lattice parameter was calculated and was found to be larger than the experimental one a_exp due to the presence of Mn⁴⁺ ions.