Luminescence study of cerium-doped La2Hf2O7: Effects due to trivalent and tetravalent cerium and oxygen vacancies

X-ray excited emission spectra, photoluminescence excitation and emission spectra, optical reflectivity spectra, and pulsed X-ray and optical excited luminescence decay measurements are reported for cerium-doped La₂Hf₂O₇ powders prepared by solid state synthesis. A broad luminescence associated with oxygen vacancies is observed in the region 350–750 nm with a peak around 460 nm. The photoluminescence spectra and the number of oxygen vacancies vary for samples annealed in oxidizing or reducing atmospheres and with the temperature of the synthesis process. Increasing the cerium concentration reduces the oxygen-vacancy-related emission due to the presence of Ce⁴⁺. First principles calculations predict that Ce⁴⁺ can substitute in Hf sites; this is confirmed from the optical reflectivity spectrum of cerium-doped La₂Hf₂O₇. Photoluminescence excitation and emission spectra characteristic of Ce⁴⁺ charge transfer transitions and possibly Ce³⁺ are also observed. Although trivalent cerium may be present, no emission observed from cerium-doped La₂Hf₂O₇ can be attributed to Ce³⁺ in La sites.     

Photoluminescence and photocatalytic activity of zinc tungstate powders

ZnWO₄ powders with grain size in range 20 nm–10 μm have been synthesized by a simple combustion method and subsequent calcinations. The photocatalytic activities of powders were tested by degradation of methylene blue solution under UV light. The luminescence spectra and luminescence decay kinetics were studied and luminescence decay time dependence on average powder-grain size was obtained. The correlation between self-trapped exciton luminescence decay time and photocatalytic activity of ZnWO₄ powders was shown. A model explaining the excitonic luminescence decay time correlation with photocatalytic activity was proposed.     

A new self-activated yellow-emitting phosphor Zn2V2O7 for white LED

A new self-activated yellow-emitting Zn₂V₂O₇ phosphor was synthesized by high temperature solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the sample with monoclinic formation of Zn₂V₂O₇. The excitation and emission spectra indicated the phosphor can be efficiently excited by near ultraviolet (NUV) light in 220–400 nm range and exhibit a bright broad yellow emission with the highest emission intensity at 531 nm. The broad emission band from 400 to 650 nm can be attributed to the charge transfer transition in the VO₄ tetrahedra, which suggests that the phosphor is a promising yellow phosphor applied for white light-emitting diodes (WLED).     

New sign of vacuum ultraviolet driven crystal growth in ternary oxide Zn 3 V 2 O 8 films

Ternary oxide Zn₃V₂O₈ films was successfully formed on glass substrates only by means of vacuum ultraviolet irradiation to the spin-coated metal–organic solutions. From Fourier transform infrared spectroscopy measurements, we confirmed that the crystal growth proceeded at metal–organic bond cleavage under VUV irradiation. The crystalline size of the obtained materials was estimated to be 1.5–2.5 nm. The prepared Zn₃V₂O₈ films clearly showed yellow luminescence, corresponding to a charge transfer transition in the VO₄ tetrahedra.     

The influence of Eu3+ doping on photoluminescent properties of red emitting phosphor YInGe2O7:Eu3+ by microwave assisted and conventional sintering method

This paper describes an investigation of the crystalline morphology and photoluminescent properties of YInGe₂O₇ powders doped with different Eu³⁺ concentrations using microwave assisted sintering and conventional sintering. X-ray powder diffraction analysis confirmed the formation of monoclinic YInGe₂O₇ structure as YInGe₂O₇:Eu³⁺ powders were sintered at 1200 °C in microwave furnace for 1 h, and the raw material phase of Y₂O₃ was observed when Eu³⁺ concentration was below 30 mol%. Scanning electron microscopy showed microwave assisted sintering results in smaller particle size and more uniform grain size distribution. In the photoluminescent (PL) studies, the concentration quenching effect was observed under the excitation at 393 nm, but not under the excitation at CTS band. The ⁵D₀→⁷F₂ transition (620 nm), exhibits a non-exponential decay behavior as YInGe₂O₇:Eu³⁺ powders were sintered by microwave with the Eu³⁺ concentration higher than 50 mol%.     

Structural and optical properties of nanoparticulate Y2O3:Eu2O3 made by microwave plasma synthesis

Nanoparticulate Y₂O₃:Eu₂O₃ with a small, uniform particle size and a well-defined composition was synthesized using a low temperature microwave plasma process. The structural evolution and the luminescence properties were studied in different states of annealing and Eu₂O₃ addition using X-ray diffraction, transmission electron microscopy, and UV-photoluminescence spectroscopy. As synthesized, the samples were amorphous and showed only weak luminescence. Subsequent annealing steps from 500°C to 800°C lead to the formation and growth of cubic Y₂O₃:Eu₂O₃ nanocrystals (5–20 nm) and a concomitant strong increase of the luminescence yield already at small grain sizes in the range of 10 nm. No self-quenching effects were observed up to 11 mol% Eu₂O ₃.     

Scintillation and optical stimulated luminescence of Ce-doped CaF2

Scintillation and optical stimulated luminescence of Ce 0.1–20% doped CaF₂ crystals prepared by Tokuyama Corp. were investigated. In X-ray induced scintillation spectra, luminescence due to Ce³⁺ 5d–4f transition appeared around 320 nm with typically 40 ns decay time. By ²⁴¹Am 5.5 MeV α-ray irradiation, 0.1% doped one showed the highest scintillation light yield and the light yield monotonically decreased with Ce concentrations. Optically stimulated luminescence after X-ray irradiation was observed around 320 nm under 550 or 830 nm stimulation in all samples. As a result, intensities of optically stimulated luminescence were proportional to Ce concentrations. Consequently, scintillation and optically stimulated luminescence resulted to have a complementary relation in Ce-doped CaF₂ system.     

Finite size effects on the structural and magnetic properties of sol–gel synthesized NiFe2O4 powders

Nanoparticles of nickel ferrite have been synthesized by the sol–gel method and the effect of grain size on its structural and magnetic properties have been studied in detail. X-ray diffraction (XRD) studies revealed that all the samples are single phasic possessing the inverse spinel structure. Grain size of the sol–gel synthesized powders has been determined from the XRD data and the strain graph. A grain size of 9 nm was observed for the as prepared powders of NiFe₂O₄ obtained through the sol–gel method. It was also observed that strain was induced during the firing process. Magnetization measurements have been carried out on all the samples prepared in the present series. It was found that the specific magnetization of the nanosized NiFe₂O₄ powders was lower than that of the corresponding coarse-grained counterparts and decreased with a decrease in grain size. The coercivity of the sol–gel synthesized NiFe₂O₄ nanoparticles attained a maximum value when the grain size was 15 nm and then decreased as the grain size was increased further.     

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

Photoluminescence quenching in zinc oxide modified by Al2O3 and Al2O3 · CeO2 Nanopowders under proton irradiations

The photoluminescence spectra of initial ZnO powder and that modified by Al₂O₃, Al₂O₃ · CeO₂ nanopowders are investigated in the range 360–660 nm before and after 100-keV proton irradiation. It is found that the introduction of nanoparticles causes a decrease in the UV-band intensity and a change in the luminescence bands in the visual spectrum due to V _O ⁺ oxygen vacancies, O _int ^- interstitial oxygen, and V _Zn ^- zinc vacancies. Luminescence quenching in the UV and visible spectra occurs under the effect of protons. The decomposition of the spectra into elementary defects and analysis of changes in their integrated intensity during modification and irradiation of the powders are carried out.     

Optical spectroscopy of Er-doped LaVO4 crystal

Transparent single crystals of erbium-doped LaVO₄ in form of platelets having an average size 0.5×2×2 mm³ were obtained by the flux method using Pb₂V₂O₇ as the solvent. Inductively coupled plasma (ICP) measurement revealed that the actual Er/La=0.3% molar ratio in the crystals is lower than Er/La=1% nominal molar ratio due to significant difference of ionic radii of La and Er. Luminescence spectra and luminescence decay curves for Er³⁺ transitions in the visible and near infrared region were recorded. Unpolarized Raman spectrum of undoped LaVO₄ crystal was acquired, too. Up-converted green emission following excitation at 808 and 970 nm was observed and dependence of its intensity on incident excitation power was determined. Based on experimental data gathered the relaxation dynamics of excited states of Er³⁺ was analyzed and mechanisms involved in the up-conversion phenomena were discussed.     

Oxidation state of vanadium in amorphous MnV2O6 formed during discharge–charge cycle and the improvement of its synthesis condition

Crystalline powders of anhydrous MnV₂O₆ were successfully synthesized at a temperature below 200 °C under autogenous hydrothermal condition. MnV₂O₆ powders, which were synthesized using high concentration solutions, more than 0.1 mol/L, gave a relatively high reversible capacity of 600 mA h/g and interesting cyclic performance, reversible capacity increasing to more than an initial charge capacity after the 3rd or 4th cycle of charge–discharge. The change in oxidation state of V ion on the course of the 1st discharge–charge process was investigated by different techniques, such as XAFS and XPS. During discharging, V⁵⁺ was found to change gradually to V⁴⁺, but it returned back completely to V⁵⁺ on charging process. The thinner particles of MnV₂O₆ crystals with rod-like morphology were synthesized at 135 °C, and much better anodic performance was achieved, much smaller irreversible capacity as about 300 mA h/g, stable reversible capacity as 600 mA h/g, and 100% Coulombic efficiency.     

Critical view on TL/OSL properties of Li2B4O7 nanoparticles doped with Cu,Ag and co-doping Cu,Ag: Dose response study

Small size (25 nm) Li₂B₄O₇ nanoparticles doped with different concentrations of Cu, Ag and co-doped with Cu, Ag were prepared by solid state sintering at 700 °C. The crystalline phase and particle sizes analysis were carried out by XRD and TEM. FTIR study reveals the formation of vibrational bonds at 1600–1200 cm⁻¹, 1500–700 cm⁻¹, 950–870 cm⁻¹ and 870–415 cm⁻¹. The kinetic parameters of the TL glow curves were evaluated using CGCD procedure in R-software. The CW-OSL decay curves were fitted with third order exponential decay curves and photoionization cross sections of each component were evaluated. The lifetime of the main TL dosimetric peak were also calculated to check the stability of the signal. Dose responses of the synthesized Li₂B₄O₇ nanoparticles for both the TL and CW-OSL were studied in the range of 0.02 mGy to50 Gy and found to be linear upto this range. Fading of the CW-OSL decay curves were also studied. The MDD of the synthesized samples were also calculated and observed to be 15 μGy.     

Characteristics of high sensitivity ethanol gas sensors based on nanostructured spinel Zn1−xCoxAl2O4

Nanocrystalline powders of Zn_1−xCo_xAl₂O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) mixed oxides, with cubic spinel structure were successfully prepared by the ethylene glycol mediated citrate sol-gel method. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and microstructure by transmission electron microscopy (TEM). X-ray diffraction results showed that the samples were in single phase with the space group Fd-3m. TEM analysis showed that the powders with spherical shape were uniform in particle size of about 17-24 nm with mesoporous in nature. Further investigations were carried out by FT-IR. Thick films of as-prepared Zn_1−xCo_xAl₂O₄ powders were fabricated using screen-printing technique. The response of Zn_1−xCo_xAl₂O₄ based thick films towards different reducing gases (liquefied petroleum gas, hydrogen, hydrogen sulfide, ethanol gas and ammonia) was investigated. The sensor response largely depends on the composition, temperature and the test gas species. The Co (cobalt) content has a considerable influence on the gas-sensing properties of Zn_1−xCo_xAl₂O₄. Especially, Zn_0.4Co_0.6Al₂O₄ composition exhibited high response with better selectivity to 100 ppm C₂H₅OH gas at 150 °C. The instant response (∼7 s) and fast recovery (∼16 s) are the main features of this sensor.     

Synthesis,structural and optical properties of Eu3+–doped Ca2V2O7 nanophosphors

Europium doped calcium pyrovanadate nanoparticles Ca₂V₂O₇:Eu³⁺, having a size of 57–63 nm, were synthesized using combustion process. Structure, morphological and optical properties of nanophosphors have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), fluorescence spectrometry (PL) and Fourier transform infra-red (FT-IR) spectroscopy. X-ray studies shows that a pure triclinic Ca₂V₂O₇ phase was obtained at 900 °C temperature. The red emission observed at 620 nm upon excitation at 305 nm is due to hypersensitive transition ⁵D₀ → ⁷F₂ of luminescent activator Eu³⁺ location at a site with no inversion symmetry in Ca₂V₂O₇ crystal lattice. High luminescent intensity and easy synthesis technique make this red phosphor a promising candidate for application as luminescent materials.     

Syntheses and X-ray excited luminescence properties of Ba3BP3O12 , BaBPO5 and Ba3BPO7

Ba₃BP₃O₁₂, BaBPO₅ and Ba₃BPO₇ have been synthesized by the solid-state reaction method. The X-ray-excited luminescent properties of these compounds were investigated, all of these compounds show broad emission band with the peak center at about 400 nm. The room temperature fluorescent decay of these compound powders was measured. Compared to the other two compounds, Ba₃BPO₇ has the maximum emission intensity while Ba₃BP₃O₁₂ has the fastest decay time with 56.3 ns(99.7%). Combining with the X-ray excited luminescent spectra and fluorescent decay profiles of these compounds, it is supposed that all of these compounds should correspond to two different luminescent mechanisms; the luminescent mechanisms of these compounds were discussed. The potential applications of these compounds are pointed out.     

Effect of Yb3+ concentration on photoluminescence properties of cubic Gd2O3 phosphor

Yb³⁺ doped phosphor of Gd₂O₃ (Gd₂O₃:Yb³⁺) have been prepared by solid state reaction method. The structure and the particle size have been determined by X-ray powder diffraction measurements. The average particle size of the phosphor is in between 35 and 50 nm. The particle size and structure of the phosphor was further confirmed by TEM analysis. The visible and NIR luminescence spectra were recorded under the 980 nm laser excitation. The visible upconversion luminescence of Yb³⁺ ion was due to cooperative luminescence and the presence of rare earth impurity ions. The cooperative upconversion and NIR luminescence spectra as a function of Yb³⁺ ion concentration were measured and the emission intensity variation with Yb³⁺ ion concentration was discussed. Yb³⁺ energy migration quenched the cooperative luminescence of Gd₂O₃:Yb³⁺ phosphor with doping level over 5%, while the NIR emission luminescence continuously increases with increasing Yb³⁺ ion concentration.     

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 magnetic properties of sol-gel Co2xNi0.5−x Zn0.5−xFe2O4 thin films

Nanocrystalline Co_2xNi_0.5−xZn_0.5−xFe₂O₄ (x=0−0.5) thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology as well as magnetic and microwave absorption properties of the films calcined at 1073 K were studied using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometry. All films were uniform without microcracks. The Co content in the Co-Ni-Zn films resulted in a grain size ranging from 15 to 32 nm while it ranged from 33 to 49 nm in the corresponding powders. Saturation and remnant magnetization increased with increase in grain size, while coercivity demonstrated a drop due to multidomain behavior of crystallites for a given value of x. Saturation magnetization increased and remnant magnetization had a maximum as a function of grain size independent of x. In turn, coercivity increased with x independent of grain size. Complex permittivity of the Co-Ni-Zn ferrite films was measured in the frequency range 2-15 GHz. The highest hysteretic heating rate in the temperature range 315-355 K was observed in CoFe₂O₄. The maximum absorption band shifted from 13 to 11 GHz as cobalt content increased from x=0.1 to 0.2.     

Europium(III) Concentration Effect on the Spectroscopic and Photoluminescent Properties of BaMoO4:Eu

BaMoO₄:Eu (BEMO) powders were synthesized by the polymeric precursor method (PPM), heat treated at 800 °C for 2 h in a heating rate of 5 °C/min and characterized by powder X-ray diffraction patterns (XRD), Fourier Transform Infra-Red (FTIR) and Raman spectroscopy, besides room temperature Photoluminescence (PL) measurements. The emission spectra of BEMO samples under excitation of 394 nm present the characteristic Eu³⁺ transitions. The relative intensities of the Eu³⁺ emissions increase as the concentration of this ion increases from 0.01 to 0.075 mol, but the luminescence is drastically quenched for the Ba_0.855Eu_0.145MoO₄ sample. The one exponential decay curves of the Eu^{3+ 5}D₀→⁷F₂ transition, λ _exc = 394 nm and λ _em = 614 nm, provided the decay times of around 0.54 ms for all samples. It was observed a broadening of the Bragg reflections and Raman bands when the Eu⁺³ concentration increases as a consequence of a more disordered material. The presence of MoO₃ and Eu₂Mo₂O₇ as additional phases in the BEMO samples where observed when the Eu³⁺ concentration was 14.5 mol%.