P.J. Dereń

Power dependence of luminescence of Tb3+ -doped KYb(WO4)2 crystal

Abstract The effect of excitation intensity on luminescence properties of Tb3+-doped KYb(WO4)2 crystal is reported. In particular, emissions following the Stokes and anti-Stokes excitations were investigated. It was found that, in addition to the Tb3+ luminescence, an intense Yb3+ luminescence at room temperature was observed after a direct excitation of Tb3+ ions in spite of the fact that both ions are far off-resonance. The intensities of Tb3+ and Yb3+ luminescences were dependent on the excitation power and changed by a sublinear function (slope

Emission properties of nanostructured Eu3+ doped zinc aluminate spinels

Abstract The preparation of nanostructured ZnAl2O4 spinel powders doped with Eu3+ ions obtained by a hydrothermal method is described. XRD analyses demonstrated that the powders were single-phase spinel nanopowders with high crystallite dispersion. The averaged spinel particle size was determined to be 6 nm as prepared and it increased up to 8 nm for calcination at 500°C. The emission spectra of Eu3+ ions measured for the samples calcinated at 500°C demonstrated inhomogeneous shapes characteristic for disordered material which after heating at 1500°C changed shape into an ordered shape characteristic of the crystalline form.

Site selection spectroscopy of Cr3+ in MgAl2O4 green spinel

Abstract The optical properties of Cr3+ ions in the synthetic green spinel MgAl2O4 were re-examined by means of site selection spectroscopy. In particular, our attention was focused on the distribution of Cr3+ sites embedded in this disordered crystal. By means of site selection excitation spectroscopy, a number of Cr3+ classes associated with strong, intermediate and weak ligand field sites were identified. The fluorescence line narrowing technique was used to establish the 25 Cr3+ sites. The splitting of the 4A2 level was determined to be 1.7 cm−1. It was found that in higher concentrated samples the number of weak ligand field Cr3+ sites is increasing.

Cooperative processes in KYb(WO4)2 crystal doped with Eu3+ and Tb3+ ions

Abstract The cooperative processes occurring between Eu 3+ (Tb 3+ ) and Yb 3+ ions in the KYb(WO 4 ) 2 crystal are reported. In this crystal the excited state of Yb 3+ ( 2 F 5/2 ) is far of resonance with Eu 3+ and Tb 3+ ions. In particular, the emission processes following the Stokes excitation and the anti-Stokes excitation, e.g. down-conversion and up-conversion, respectively, were investigated. It was found that the Yb 3+ emission was fairly intense in spite of the fact that the cross-relaxation is of the non-resonant type. The up-conversion emission of Eu 3+ (red) and Tb 3+ (green) ions following a direct excitation of Yb 3+ ions was observed at room temperature. The dependence of emission intensity on incident laser power was measured. It was found that the efficiency of Stokes emissions is described by the slope of allometric curve less than 1 whereas for the anti-Stokes emission (up-conversion) the slope was around 2. The mechanisms of down- and up-conversion processes are briefly discussed.

Effect of aliovalent doping on the properties of perovskite-like multiferroic formates

We report the synthesis, as well as the thermal, dielectric, Raman, IR and luminescence studies of a chromium-doped multiferroic MOF, [(CH3)2NH2][Mn(HCOO)3] (DMMn). These studies reveal that doping with chromium(III) leads to a lowering of the ferroelectric phase transition temperature Tc. The doping also changes the character of the phase transition from strongly first-order for an undoped sample to a partially diffused one for 3.1% of chromium doping. This behavior resembles the behavior of inorganic ABO3 perovskite ferroelectrics where doping often leads to a decrease of the Tc and the diffuse character of a phase transition. We also show that the chromium-doped sample exhibits efficient luminescence. Additional studies demonstrated that the [(CH3)2NH2][MII(HCOO)3] formates (MII = Mg, Mn, or Co) may also be doped with other trivalent cations such as Al3+, In3+, Eu3+ or Er3+. Doping with these ions also leads to a decrease of the Tc and the diffuse character of the phase transition. Additional optical studies show that the europium-doped DMMn sample also exhibits luminescence properties. Thus our discovery opens up a new and simple route for the synthesis of various multifunctional amine-templated metal formate frameworks with tunable multiferroic and luminescent properties by doping these frameworks with a wide range of trivalent cations.

Preparation, X-ray analysis and spectroscopic investigation of nanostructured Lu2O3:Tb

Abstract Formation of Lu 2 O 3 through combustion of Lu(NO 3 ) 3 and CO(NH 2 ) 2 (urea) at various temperatures was investigated. From XRD patterns analysis, the optimal reaction temperature was found to be ∼600–700°C. For that range products with crystallites of ∼7–13 nm were obtained, as found from TEM pictures. With the same technique Tb-doped Lu 2 O 3 was successfully fabricated and its basic spectroscopic properties were characterized. Emission of the phosphors was found to be typical for Tb 3+ , with main components at 490 and 550 nm, resulting from radiative relaxation of 5 D 4 level. Undoped material produced broad-band luminescence peaking in blue. Raw Tb-doped samples were brownish, which was attributed to incorporation of some of the activator as Tb 4+ . Such assignment was derived from reflection, excitation and EPR spectra. The colour could be completely removed through post-fabrication vacuum heat-treatment at 1100°C.

Effect of random distribution and molecular interactions on optical properties of Er3+ dopant in KY(WO4)2 and Ho3+ in KYb(WO4)2

Abstract The spectroscopic properties of Er3+ doped KY(WO4)2 and Ho3+ doped KYb(WO4)2 single crystals are reported and related to their X-ray structures. The experimental data include a survey of electronic absorption and emission features as well as vibrational IR and Raman spectra made in the polarized light. The effect of the random distribution of the monovalent and trivalent cations as well as intermolecular interactions between the tungstate anions are discussed.

Luminescence properties and determination of optimal RE3+ (Sm3+, Tb3+ and Dy3+) doping levels in the KYP2O7 host lattice obtained by combustion synthesis

The β-KYP2O7 powders doped with Sm3+, Tb3+ and Dy3+ were obtained using a one-step urea-assisted combustion method. Their structural properties were characterized by X-ray diffraction (XRD) whereas spectroscopic properties like excitation, emission and luminescence kinetics were investigated at room temperature. β-KYP2O7 doped with Sm3+ and Dy3+ showed concentration quenching above 2 mol% of dopants whereas Tb3+ containing materials could be doped with higher concentrations. The main mechanisms of concentration quenching were identified as dipole–dipole for Sm3+ and Dy3+. Both dopants due to the abundant energy levels are strongly prone to quenching due to the non-radiative CR process whose efficiency strongly increases with the concentration of optically active ions. In the case of Tb3+ emission from both 5D3 and 5D4 levels was recorded. The 5D3 level vastly depopulates due to the CR process upon increase of the Tb3+ content whereas the slight 5D4 emission remains stable up to 5 mol%.

Laser‐excited luminescence in Ti‐doped MgAl2O4 spinel

The photoluminescence (excitation, emission, and fluorescence decay time) spectra of spinel (Ti:MgAl2O4) excited by pulsed Nd:YAG and dye lasers have been systematically investigated. Excitation into the T2g→Eg crystal‐field transition of Ti3+ produces a broad infrared emission band at 805 nm which could, in principle, be used for tunable laser applications. Two different Ti3+ centers with lifetimes of 4.2 and 41 μs contribute to this emission. At the same time this IR band can also be observed under high‐intensity excitation in the ultraviolet region (266 nm) via a two‐photon process.

Laser action in LaAlO3:Nd3+ single crystal

The spectral and laser properties of Nd3+ doped (1wt%) LaAlO3 single crystal are presented. The energy levels of the Nd3+ ion in the LaAlO3 matrix were assigned. The Judd–Ofelt parameters Ω2=1.346, Ω4=4.490, and Ω6=5.168 (all ×10−20cm−2) were evaluated. The absorption σabs and emission σemi cross sections at the respective pumping and emission wavelengths were calculated to be σabs(789.7nm)=2.42×10−20cm2, σemi(909nm)=5.02×10−20cm2, and σemi(1080nm)=7.03×10−20cm2. Continuous-wave laser action at 1080nm in LaAlO3:Nd3+ was obtained.