4f Electron Transitions Research Articles

Fabrication of cerium-doped yttrium aluminum garnet thin films by a mist CVD method

We synthesized thin films, consisting of yttrium aluminum garnet doped with Ce3+ (YAG:Ce), using the mist chemical vapor deposition (CVD) method, which allows the fabrication of high-quality thin films under atmospheric conditions without the use of vacuum equipment. Under a deposition rate of approximately 1μm/h, the obtained thin films had a typical thickness of 2μm. The XRD analysis indicated that the thin films consisted of single-phase YAG:Ce. The Rutherford backscattering confirmed the stoichiometry; the composition of the film was determined to be (Y, Ce)3Al5O12, with a Ce content of Ce/(Y+Ce)=2.5%. The YAG:Ce thin films exhibited fluorescence due to the 5d–4f electronic transitions characteristic of the Ce ions occupying the eight-coordinated dodecahedral sites in the YAG lattice.

Luminescence characteristics of Lu2SiO5:Ce3+ (LSO:Ce) ceramic scintillators under VUV–UV excitation

The luminescent properties of LSO:Ce (1mol%) scintillators were characterized systematically under vacuum ultraviolet (VUV)–UV excitation. Under 185nm VUV excitation, a broad emission band was detected in the wavelength range from 350nm to 600nm at different temperatures, which is ascribed to the 5d→4f electron transitions of Ce3+. Excitation wavelength and temperature effects on luminescent properties of LSO:Ce ceramic scintillators were investigated in this paper. At 50K, the emission intensity of Ce1 (Ce3+ locating in seven-oxygen-coordinated site) was much stronger than that of Ce2 (Ce3+ locating in six-oxygen-coordinated site), and the total emission intensity excited by 355nm (Ce1) was higher than that excited by 185nm (host). The total luminescence intensity increases as the rising temperature in the temperature range of 150–300K, which is originated from thermal enhancement of transportation of free carriers dominating Ce1 emission.

First-Principles Study of Electronic, Elastic, and Optical Properties of Yb:CaF2 Crystals

Abstract The electronic structure, elastic, and optical properties of CaF2 and Yb-doped CaF2 are investigated by the first-principles calculations. The obtained lattice parameters, band structure, density of state, elastic constants, and imaginary part of dielectric function ε 2 for CaF2 agree well with the experimental and other theoretical results. A good comparison of the properties is made between CaF2 and Yb:CaF2, and it is found that: firstly, Yb doping leads to the crystal structure distortion of CaF2 and reduction of the elastic constants. Secondly, an impurity band is found at the top of valence band in the band structure of Yb:CaF2 and contributes to the bandgap narrowing. Thirdly, the peaks at about 26 eV in the optical curves disappear after Yb doping, and the remaining two peaks shift towards the longer-wavelength side, with the peak intensities drop down, indicating that Yb doping makes the ultraviolet transmittance of the system improve and the range of light in the UV region widen. Furthermore, Yb doping induces new peaks in the optical curves, which may be caused by 4f electron transitions of Yb atom. Finally, the transmittance of the doped system increases when the Yb-doped proportion decreases.

Phase formation and photoluminescence properties of Sm3+ doped Al5BO9 phosphor

A red emitting Sm3+ doped Al5BO9 novel phosphor has been synthesized by sol–gel technique using citric acid and disodium salt of EDTA as chelating agents. The synthesized polycrystalline samples were characterized using powder X-ray diffraction (XRD) pattern and photoluminescence (PL) analysis. XRD reveals that the sample synthesized using disodium salt of EDTA shows better crystallization effect when compared to citric acid. Further, the synthesized Al5BO9 sample possessing orthorhombic crystal structure with end centred lattices. The photoluminescence analysis explains that the samarium doped Al5BO9 phosphor has excitable at 400nm in the near-UV region and its emission maxima highly centred at 600nm due to 4f–4f electronic transition of Sm3+ ion. The morphology and grain sizes of the phosphor are analyzed through Scanning Electron Microscopy.

Synthesis and optical characterization of lanthanide-doped colloidal Ga2O3 nanoparticles

We demonstrate the use of thermal decomposition reaction to obtain Ga2O3 nanoparticles with average size around 5nm. The obtained nanoparticles presented a good colloidal stability and high optical transparency. We were also able to incorporate Eu3+ and Tb3+ ions inside the crystal host. The synthesized nanomaterials exhibited dual mode emission upon UV excitation, consisting of a broad band in the blue region and a characteristic series of sharp lines. The former resulted from donor–acceptor pairs recombination in Ga2O3 host, while the latter from 4f–4f electronic transitions in lanthanide ions. For fuller optical characterization of the obtained nanoparticles, we have performed wide wavelength range Z-scan studies, and calculated the values of nonlinear absorption cross-sections. Gallium (III) oxide nanoparticles showed two-photon absorption in the range between 500nm and 700nm, with molecular weight scaled nonlinear optical parameters exceeding the values for other lanthanide-doped nanoparticles of similar size.

Luminescence properties of LaF3:Ce nanoparticles encapsulated by oleic acid

Cerium ions doped lanthanum fluoride (LaF3:Ce) nanopowder as well as LaF3:Ce nanopowder whose surfaces was modified by oleic acid (OA) were synthesized by using an in-situ hydrothermal process under the various doping concentrations. Based on the XRD spectra and TEM images, it was confirmed that the crystalline structured hexagonal LaF3:Ce nanopowder was synthesized. Oleic acid was efficient for conversion of the water dispersible LaF3:Ce nanoparticles to hydrophobic ones. Surface modification was verified by FTIR absorption spectrum as well as TEM images, showing no agglomeration between 5 and 10nm scaled particles. Photoluminescence based on 5d⟶4f electronic transition of cerium ions excited at λex ∼256nm for both neat and OA encapsulated LaF3:Ce nanoparticles decreases as the cerium concentration increases, while the quantum yields of OA encapsulated nanoparticles were much lower than the neat particles due to low photon transmittance of OA at the range longer than ∼350nm.

Synthesis and determination of the structural and optical characteristics of cBN micropowder with Eu3+ ions

Cubic boron-nitride micropowder with Eu3+ ions (cBN:Eu) is synthesized under conditions of high pressures and temperatures. The structural, morphological, chemical, and optical characteristics of the cBN:Eu micropowder are studied using X-ray diffraction, energy-dispersive X-ray spectral microanalysis, photoluminescence, and optical transmission methods. It is found that the cBN:Eu lattice parameter is ∼3.615 A. The intense red luminescence of the cBN:Eu micropowder (red glow), measured in the visible region of the spectrum in the range from 550 to 750 nm, is attributed to intracenter 4f-electron transitions of the Eu3+ ions. The possible nature of the cBN:Eu micropowder luminescence is discussed.

ChemInform Abstract: Some Misconceptions Concerning the Electronic Spectra of Tri‐Positive Europium and Cerium

AbstractReview: 44 refs.

Robust magnetic order of Ce 4f-electrons coexisting with superconductivity in CeFeAsO1−x F x

The electrical resistance of CeFeAsO1−x F x (x = 0.06 and 0.08) has been measured in magnetic fields up to 40 T. At zero field, the sample with x = 0.06 shows a structural phase transition around T S ≈ 100 K, followed by a spin-density-wave (SDW) transition around T SDW ≈ 30 K. For x = 0.08, the structural phase transition is suppressed down to T S ≈ 60 K without a clear anomaly associated with the Fe-SDW transition, and superconductivity shows up at T C ≈ 25 K. At lower temperatures, both samples show a clear resistive peak around T N ≈ 4 K, which is associated with the antiferromagnetic (AFM) transition of Ce 4f-electrons. Strikingly, zero resistance is recovered upon a further lowering of the temperature to below T N for x = 0.08. Moreover, we found that the AFM transition of Ce 4f-electrons at 4 K hardly change when a magnetic field of up to 40 T was applied. This was true even in the case of x = 0.08, for which the superconductivity had been partially suppressed at such a large field.

IR, UV–vis spectroscopic and DSC investigations of europium doped tellurite glasses obtained by sol–gel synthesis

FTIR, UV–vis spectroscopy and DSC studies were utilized in order to study structural changes produced by the variation of the Eu(NO3)3 content in binary europium–tellurite glasses obtained using the sol–gel method. Our results show that the europium ions can be considered as modifiers because they will produce the transformation of [TeO4] and [TeO6] structural units into [TeO3] trigonal pyramidal units and the formation of a significant number of non-bridging oxygen ions.UV–vis studies show that the higher Eu(NO3)3 concentrations (24mol%) in the host matrix induce a growth of the number of 4f–4f electronic transitions of the Eu+3 ions and a increase of the number of non-bridging oxygen ions in the regions between 325 and 600nm.In brief, partial replacement of TeO bonds by EuO ionic bonds induces a global weakening of the network explaining the decrease of the glass transition temperature and crystallization temperature by gradually increasing of europium ions in the matrix network.

Some misconceptions concerning the electronic spectra of tri-positive europium and cerium

Tripositive europium attracts wide interest in diverse fields such as phosphors, sensors, and time-gated bioimaging agents based upon its optical emission spectra. Some inaccurate descriptions of these spectra are being amplified throughout the literature. In this tutorial review the background of electronic states, energy levels and transition intensities is provided as a pre-requisite for clarifying these misconceptions. The topics discussed encompass the electric dipole nature of intraconfigurational 4f electronic transitions, the use of europium as site symmetry and centrosymmetry probe, as well as an indicator of nephelauxetic effects. The frequently mis-used term Stokes shift is also clarified and alternative terms are given for the situations where it is incorrectly applied.

Sensitized red luminescence from Mn2+-doped Olgite-type phosphates

Bright red luminescence is observed from Mn2+-doped olgite-type Na2BaMgP2O8 with the aid of Eu2+ or Ce3+ sensitizer. Under ultraviolet (UV) light, Na2BaMgP2O8: Eu2+ and Na2BaMgP2O8: Ce3+ exhibit intense near-UV emissions derived from 5d–4f electron transition. Since their emission bands have substantial spectral overlap with the excitation bands of Mn2+ in Na2BaMgP2O8, Mn2+ emission is successfully sensitized by resonance energy transfer. Detailed spectral consideration has revealed that Eu2+ and Ce3+ occupying interlayer A-site mainly contribute to the enhancement of the Mn2+ emission.

Preparation and photocatalytic degradation of erbium doped titanium dioxide nanorods

Photocatalytic degradation of methylene blue (MB) in water was examined using Er3+-doped TiO2 (Er–TiO2) nanorods prepared by a sol–gel derived electrospinning, calcination, and subsequent mechanical grinding. Different concentrations of Er dopant in the range of 0–1.0mol% were synthesized to evaluate the effect of Er content on the photocatalytic activity of TiO2. Among Er3+–TiO2 catalysts, the 0.7mol% Er3+–TiO2 catalyst showed the highest MB degradation rate. The degradation kinetic constant (k) increased from 1.0×10−3min−1 to 5.1×10−3min−1 with the increase of Er3+ doping from 0 to 0.7mol%, but decreased down to 2.1×10−3min−1 when Er3+ content was 1.0mol%. It can be concluded that the degradation of MB under UV radiation was more efficient with Er3+–TiO2 catalyst than with pure TiO2. The higher activity might be attributed to the transition of 4f electrons of Er3+ and red shifts of the optical absorption edge of TiO2 by erbium ion doping.

Photoluminescence properties of β-SiAlON:Yb2+, a novel green-emitting phosphor for white light-emitting diodes

We have synthesized Yb2+-activated Si6−zAlzOzN8−z (0.05z2.3, 0.03 mol% Yb2+0.7 mol%) green phosphors by solid-state reaction at 1900 °C for 2 h under a nitrogen pressure of 1.0 MPa. Phase purity, photoluminescence and its thermal quenching were investigated. A single phase was obtained for all values of z and Yb2+ concentration. A distinct emission band was observed at 540 nm originating from the 5d–4f electronic transition in Yb2+ under 480 nm excitation. The photoluminescence properties mainly depended on the Yb2+ concentration and chemical composition of the matrix. The resultant phosphor showed high thermal stability, that is, the emission intensity at 150 °C was about 82% of that measured at room temperature. The experimental results indicate that β-SiAlON:Yb2+ is a potential green phosphor for white light-emitting diodes (LEDs), which use blue LEDs as the primary light source.

Optical properties of macroporous Y 3Al 5O 12 crystals doped with rare earth ions synthesized via sol–gel process from ionic precursors

We have prepared macroporous Y 3Al 5O 12 (YAG) monoliths doped with rare earth (RE) ions (RE = Ce, Eu, Sm, and Pr) via the ionic precursor-derived sol–gel reaction accompanied by phase separation and investigated their fluorescence and scattering properties. YAG monoliths with well-defined bicontinuous macropores and skeletons were synthesized from the mixture of water and ethanol containing yttrium(III), aluminum(III), and RE(III) salts using propylene oxide as a gelation initiator and poly(ethylene oxide) as a phase separation inducer. X-ray diffraction measurements reveal that the sample as-dried and that heat-treated at 700 °C are amorphous, while a single crystalline phase of YAG is precipitated in the samples heat-treated at temperatures between 800 and 1100 °C. The macroporous YAG:RE ceramics heat-treated at 800 °C and higher temperatures exhibit fluorescence due to 4f–5d (Ce 3+) and 4f–4f (Eu 3+, Sm 3+, Pr 3+) electronic transitions characteristic of the RE ions occupying the eight-coordinated dodecahedral sites in YAG lattice. Coherent backscattering experiments indicate that the scattering strength is enhanced by the densification of skeletons in macroporous YAG monoliths, depending on the heat treatment temperature.

Electrospun Er:TiO 2 nanofibrous films as efficient photocatalysts under solar simulated light

TiO 2 nanofibrous films with variable amounts of erbium were prepared via electrospinning. Analytical results demonstrated that the doping of erbium inhibited the phase transformation and the crystallite growth of TiO 2. The doped samples were more efficient for degradation of methylene blue than TiO 2, and an optimal dosage of erbium at 0.5% activated at 773 K achieved the highest degradation rate. The higher photoactivity might be attributable to the transition of 4f electrons of erbium, particle size, phase composition and the increase of the separating rate of photogenerated charges by erbium doping.

Features of the infrared reflection spectra of SmS semiconductor in the homogeneity range

Infrared reflection spectra in the range of 5200–380 cm−1 were measured for polycrystalline samarium monosulfide samples in the semiconductor phase with compositions Sm1 + x S lying within the homogeneity range (0 ≤ x ≤ 0.17) in the temperature range of 300–600 K. Five peaks with energies in the range of 1150–880 cm−1, whose positions depend weakly on the composition and temperature. It was shown that the reflection peaks are associated with 7 F 0–7 F 2 transitions of 4f electrons of Sm2+ ions.

Effect of local environment and Sm 3+-codoping on the luminescence properties in the Eu 3+-doped potassium tungstate phosphor for white LEDS

The luminescence properties of Eu 3+- and Sm 3+-doped potassium tungstate phosphors were investigated. The K 4−3( x + y ) (WO 4) 2:Eu x 3+,Sm y 3+ phosphor was produced by solid-state reactions, followed by re-firing with a flux. The phosphor showed a strong absorption in the near-UV to green region due to 4f–4f electron transitions of the Eu 3+ and Sm 3+ ions, generating a red emission. The excitation spectrum could be adjusted by Sm 3+-codoping. A small amount of Sm 3+, acting as a sensitizer, increased the energy absorption peak at 405 nm. The crystal structure and local environment around the Eu 3+ ions were determined using the Rietveld method. The crystal structure of this phosphor was determined to be monoclinic with a space group of C2/c. The small Eu–0 distance in the crystal led to high energy-level splitting at the 5D 0→ 7F 2 transition of the Eu 3+ ions, resulting in more emission peaks.

Crystal structure of BaCa 2MgSi 2O 8 and the photoluminescent properties activated by Eu 2+

Rietveld refinements of X-ray powder diffraction data have confirmed the crystal structure of BaCa 2MgSi 2O 8 prepared by a standard solid-state method. The final reliable factors were R wp=10.91%, R p=8.10%, R I=2.71%, and R F=1.14%. BaCa 2MgSi 2O 8 crystallizes in the trigonal space group P3¯m1 (no. 164) with a=5.430(3) Å and c=6.807(2) Å. The oxide has a layered structure constructed from the unit layers built up by corner-sharing MgO 6 octahedra and SiO 4 tetrahedra. Ba and Ca atoms occupy the distinct crystallographic sites; Ba atom is sited in the interlayer space and Ca atom is embedded in the layer framework. This structure was not disrupted by doping of Eu 2+ ions. The Eu 2+-doped BaCa 2MgSi 2O 8 exhibited an intense blue emission based on 5d–4f electron transitions of Eu 2+ ions under 254 nm excitation. This emission has a sufficient chromaticity as a blue phosphor. An additional analysis of the emission spectra using an empirical formula indicates that Eu 2+ is distributed into both Ba and Ca sites.

ErIII‐Cored Complexes Based on Dendritic PtII–Porphyrin Ligands: Synthesis, Near‐IR Emission Enhancement, and Photophysical Studies

AbstractA series of stable and inert complexes with ErIII cores and dendritic PtII‐porphyrin ligands exhibit strong near‐IR (NIR) emission bands via highly efficient energy transfer from the excited triplet state of the PtII‐porphyrin ligand to Er3+ ions. The NIR emission intensity of thin films of ErIII complexes at 1530 nm, originating from 4f–4f electronic transitions from the first excited state (4I13/2) to the ground state (4I15/2) of the Er3+ ion, is dramatically enhanced upon increasing the generation number (n) of the aryl ether dendrons because of site‐isolation and light‐harvesting (LH) effects. Attempts are made to distinguish the site‐isolation effect from the LH effect in these complexes. Surprisingly, the site‐isolation effect is dominant over the LH effect in the Er3+‐[Gn‐PtP]3(terpy) (terpy: 2,2′:6′,2″‐terpyridine) series of complexes, even though the present dendrimer systems with ErIII cores have a proper cascade‐type energy gradient. This might be due to the low quantum yield of the aryl ether dendrons. Thus, the NIR emission intensity of Er3+‐[G3‐PtP]3(terpy) is 30 times stronger than that of Er3+‐[G1‐PtP]3(terpy). The energy transfer efficiency between the PtII‐porphyrin moiety in the dendritic PtII‐porphyrin ligands and the Ln3+ ion increases with increasing generation number of the dendrons from 12–43 %. The time‐resolved luminescence spectra in the NIR region show monoexponential decays with a luminescence lifetime of 0.98 μs for Er3+‐[G1‐PtP]3(terpy), 1.64 μs for Er3+‐[G2‐PtP]3(terpy), and 6.85 μs for Er3+‐[G3‐PtP]3(terpy) in thin films of these complexes. All the ErIII‐cored dendrimer complexes exhibit excellent thermal stability and photostability, and possess good solubility in common organic solvents.