The energy levels and local structures of Eu3+ incorporated in the lattice and surface sites of ZnO nanocrystals were investigated based on the high-resolution fluorescence spectra at 10 K. Radiative emissions from 5D1 were first observed for Eu3+ at the lattice site of ZnO. It is shown that the site symmetry of Eu3+ at the lattice site descends from C3v to Cs or C1, whereas Eu3+ ions at the surface occupy more disordered sites of the lowest symmetry C1. The luminescence decay of 5D0 at the lattice site, showing a rise time and longer lifetime, behaves distinctly from that of the surface sites. Because of a small filling factor (52%) of nanoparticles, the 5D0 lifetime of Eu3+ is significantly affected by the surrounding medium, which can be well interpreted with the virtual-cavity model. The Judd−Ofelt intensity parameters of Eu3+ in ZnO nanocrystals were determined, with Ω2,4,6 values of (9.59, 8.11, <0.25) and (21.51, 2.30, <0.25) in units of 10-20 cm2 for Eu3+ at the surface and lattice sites, respecti...

Optical Spectroscopy of Eu3+ Doped ZnO Nanocrystals

https://cyberleninka.org/article/n/152508.pdf

Recent advances in sol–gel synthesis of monolithic silica and silica-based glasses

Trivalent europium ions have been successfully incorporated in anatase TiO2 nanocrystals via a sol−gel solvothermal synthesis, in spite of a large mismatch in ionic radius between Eu3+ and Ti4+. The photoluminescence intensity of Eu3+:TiO2 nanocrystals has been significantly improved, which is comparable to that of common red phosphors. Multiple sites of Eu3+ in TiO2 nanocrystals have been identified by the technique of site-selective spectroscopy. Eu3+ ions at two lattice sites exhibit sharp emission and excitation peaks with site symmetries descending from D2d to approximate C2v and D2 as a result of the lattice distortion, whereas Eu3+ ions at disordered site near the surface are analogous to Eu3+ ions located in a glasslike environment. Extended X-ray absorption fine structure is utilized for identifying the local structure and Eu3+ coordination. The luminescence dynamics and crystal-field levels of Eu3+ at different sites have been analyzed. A growth mechanism for the incorporation of Eu3+ in the ana...

Evidence of Trivalent Europium Incorporated in Anatase TiO2 Nanocrystals with Multiple Sites

A new series of layered magnets with the formula [M(L-tartrate)] (M = Mn(II), Co(II), Fe(II), Ni(II); L-tartrate = (2R,3R)-(+)-tartrate) has been prepared. All of these compounds are isostructural and crystallize in the chiral orthorhombic space group I222, as found by X-ray structure analysis. Their structure consists of a three-dimensional polymeric network in which each metal shows distorted octahedral coordination bound to four L-tartrate ligands, two of which chelate through an alcohol and a carboxylate group and the other two bind terminally through a monodentate carboxylate group. The chirality of the ligand imposes a Delta conformation on all metal centers. Magnetically, the paramagnetic metal centers form pseudotetragonal layers in which each metal is surrounded by four other metals, with syn,anti carboxylate bridges. These salts show intralayer antiferromagnetic or ferromagnetic interactions, depending on the electronic configuration of the metal, and weak interlayer antiferromagnetic interaction. In all cases the magnetic properties are strongly affected by the anisotropy of the system, and the presence of magnetic canting has been found. The Mn derivative behaves as a weak ferromagnet with a critical temperature of 3.3 K. The Ni derivative shows very unusual magnetic behavior in that it exhibits antiferromagnetic ordering below 6 K, the onset of spontaneous magnetization arising from spin reorientation into a canted phase below 4.5 K, and a field-induced ferromagnetic state above 0.3 T at 2 K, behavior typical of metamagnets. The Fe and Co derivatives show antiferromagnetic interactions between spin carriers, but do not order above 2 K.

Chiral Molecular Magnets: Synthesis, Structure, and Magnetic Behavior of the Series [M(L‐tart)] (M = MnII, FeII, CoII, NiII; L‐tart = (2R,3R)‐(+)‐tartrate)

A comprehensive survey of electronic structure and optical properties of rare-earth ions embedded in semiconductor nanocrystals (NCs) is of vital importance for their potential applications in areas as diverse as luminescent bioprobes, lighting, and displays. Er3+ -doped anatase TiO2 NCs, synthesized via a facile sol-gel solvothermal method, exhibit intense and well-resolved intra-4f emissions of Er3+ . Crystal-field (CF) spectra of Er3+ in TiO2 NCs are systematically studied by means of high-resolution emission and excitation spectra at 10-300 K. The CF analysis of Er3+ assuming a site symmetry of C(2v) yields a small root-mean-square deviation of 25.1 cm(-1) and reveals the relatively large CF strength (549 cm(-1) ) of Er3+, thus verifying the rationality of the C(2v) symmetry assignment of Er3+ in anatase TiO2 NCs. Based on a simplified thermalization model for the temperature-dependent photoluminescence (PL) dynamics from (4) S(3/2) , the intrinsic radiative luminescence lifetimes of (4) S(3/2) and (2) H(11/2) are experimentally determined to be 3.70 and 1.73 μs, respectively. Green and red upconversion (UC) luminescence of Er3+ can be achieved upon laser excitation at 974.5 nm. The UC intensity of Er3+ in Yb/Er-codoped NCs is found to be about five times higher than that of Er-singly-doped counterparts as a result of efficient Yb3+ sensitization and energy transfer upconversion (ETU) evidenced by its distinct UC luminescence dynamics. Furthermore, the origin of defect luminescence is revealed based on the temperature-dependent PL spectra upon excitation above the TiO2 bandgap at 325 nm.

Er3+‐Doped Anatase TiO2 Nanocrystals: Crystal‐Field Levels, Excited‐State Dynamics, Upconversion, and Defect Luminescence

Glass ceramics of composition 95SiO2-5SnO2 doped with 0.4mol% Eu3+ have been prepared by thermal treatment of sol-gel glasses. The segregated SnO2 nanocrystals present a mean size comparable to the bulk exciton Bohr radius (about 2.4nm), corresponding to a wide band-gap quantum-dot system in an insulator SiO2 glass. A fraction of the Eu3+ ions is incorporated to the SnO2 nanocrystals in the process. In these strong confinement conditions, the energy gap presents a high dependence on the nanocrystal size. Taking advantage of this effect, it has been possible to excite selectively the Eu3+ ions located in the SnO2 nanocrystals, by energy transfer from the host, obtaining emission spectra that depend on the nanocrystal size. The Eu3+ ions environment in small nanocrystals (radius under 2nm) are very distorted, meanwhile they are like crystalline for nanocrystals with a radius of some nanometers.

Nanocrystal-size selective spectroscopy in SnO2:Eu3+ semiconductor quantum dots

Polycrystalline samples of dimeric cadmium tartrate, [(Cd,C4H4O6)2H2O)]3H2O [labeled CdT(I)], were studied using impedance measurements and x-ray powder diffraction. The dependence of the real part of the dielectric constant on temperature showed a sharp peak at about 65 °C, revealing a structural phase transition, while the other broad peak in the temperature range (70<T<85 °C) was due to the loss of water molecules. The x-ray powder diffraction patterns at three temperatures (25, 60, and 70 °C) are consistent with three nonequivalent space groups. According to these results, it seems that this compound undergoes two successive phase transitions: P212121→P21→Pnmn, suggesting an intermediate ferroelectric behavior, labeled CdT(II) between a paraelectric CdT(I) and an anhydrous phase, labeled CdT(III).

J. Peraza

Papers

Nanocrystal-size selective spectroscopy in SnO2:Eu3+ semiconductor quantum dots

Structural and dielectric characterization of cadmium tartrate

Luminescence of Er3+-doped nanostructured SiO2–LaF3 glass-ceramics prepared by the sol–gel method

Characterization and thermal and electromagnetic behaviour of gadolinium-doped calcium tartrate crystals grown by the solution technique

Electrical conductivity of doped and undoped calcium tartrate