Intraionic Transitions Research Articles

Novel high-brightness yellow light-emitting phosphor Ba2GdGaO5: Bi3+ for NUV-pumped WLED application

Developing high-brightness luminescence materials is always a significant work, especially for bismuth-based phosphors, due to their incompletely spin-allowed intra-ionic electron transition of 1S0–3P1. In this work, high-brightness light emitting of Bi3+ ion was achieved in Ba2GdGaO5, and its internal quantum efficiency (IQE) reaches up to 96%. Under the near-ultraviolet (NUV) light excitation, Bi3+-activated Ba2GdGaO5 emits a broad-band yellow light peaked at 566 nm with full width at half maximum (FWHM) of 126 nm. Compared with commercial yellow phosphor Y3Al5O12: Ce3+, the as-prepared sample shows a similar emission band, but preforms a better thermal stability. The significant photoluminescence characteristic of Bi3+ ion in Ba2GdGaO5 is demonstrated to be derived from the extra-ionic electronic transition of Bi3+ ion, which is identified as metal to metal charge transfer band (MMCT). This mechanism was discussed in detailed combined with the crystal and electronic structure of Ba2GdGaO5, which is expected to pave an effective way for developing high-brightness bismuth-based phosphors.

Gemological Characteristics of Lvwen Stone and Its Color Genesis

“Lvwen stone” is a yellow-green carbonate jade gemstone. In this study, the gemological characteristics and color genesis of Lvwen stone were investigated using conventional gemological testing methods and analytical techniques, such as X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV–VIS), laser ablation plasma mass spectrometry (LA-ICP-MS), and scanning electron microscopy (SEM). The chemical composition of Lvwen stone is mainly Ca, with lesser amounts of Mg, Mn, Cu, Zn, Fe, and other trace elements. The rare earth element distribution pattern indicates that Lvwen stone is characterized by MREE depletion and a positive Ce anomaly. The mineralogic composition of Lvwen stone is calcite, and trace-element- and crystal-size-induced colors result in its characteristic banded appearance. The white (or light green) bands consist of comparatively coarse calcite crystals (~100 μm) that are oriented perpendicularly to the band plane, accounting for their poor light transmittance. In contrast, the dark green matrix is composed of cryptocrystalline calcite crystals that are uniform in size (~10 μm) and tightly packed, resulting in superior light transmittance. Lvwen stone has a 6A1→4E(4D)d-d intra-ion electronic transition absorption band of Fe3+ at ~380–450 nm and a 2E→2T2(2D)d-d intra-ion electronic transition absorption band of Cu2+ at ~580–780 nm. This indicates that both the intra-ion electronic transitions of Fe3+ and Cu2+ give rise to the unique yellow-green color of the material. Lvwen stone is produced by ultra-high-pressure tectonic fluids in a relatively closed, reducing environment, and the green matrix was formed earlier than the white bands.

Optical characteristics of Tl+ centers in CsCaCl3, KCaCl3, and CsCl crystals

We have investigated the absorption, luminescence, and excitation spectra of the Tl+ centers in CsCaCl3:Tl+, KCaCl3:Tl+, and CsCl:Tl+ crystals in the energy region up to the vacuum ultraviolet region. The A, B, and C absorption bands attributed to the intra-ionic transitions were observed in the transparent energy region below their respective fundamental absorption edge. The parameters characterizing the Tl+ centers were estimated by the energy positions of the A, B, and C absorption bands and the absorption intensity ratio of the C to A bands. These values are consistent with those of the Tl+ centers in other ionic chloride crystals. The luminescence bands attributed to the radiative transitions from several relaxed excited states were observed even under excitation of the A absorption band. Although the intensities of the luminescence bands change with an increase in temperature from low temperature, the total integrated luminescence intensity remained almost constant. The temperature dependence of the luminescence intensity can be explained by the thermally activated non-radiative transitions among the relaxed excited states and the radiative transitions from them. By including the Tl+ centers in alkali and ammonium chloride crystals, the features of the Tl+ centers in the resulting chloride compound crystals have been comprehensively discussed regardless of the different chloride-ionic configurations around the Tl+ ions.

Structural characterization &, optical and electrical properties of LiGaO2 & LiGa5O8 micro-nanoparticles-based photodetectors

Lithium gallium oxide (LiGaO2 & LiGa5O8) nanoparticles were successfully prepared through thermal evaporation method under the different atmospheric-conditions. The experimental results show that the LiGa5O8 nanoparticles are easier to synthesize in the air, while the LiGaO2 nanoparticles are more suitable for argon environment. The luminescence properties of LiGaO2 & LiGa5O8 nanoparticle-based photodetector show that the near-infrared emission is mainly owing to the defect level caused by Li ions in the lattice and the 2S–2P0 intra-ion transition of Li ions and the green-yellow emission is caused by intrinsic defects. The electrical properties of the LiGaO2 & LiGa5O8 nanoparticle-based photodetector indicate that the resistance value increases significantly in the case of illumination compared with dark conditions. The experimental results show that the nanoparticle-based photodetector is very sensitive to temperature change (24–40 °C).

Tuning Green to Red Color in Erbium Niobate Micro- and Nanoparticles.

Tetragonal Er0.5Nb0.5O2 and monoclinic ErNbO4 micro- and nanoparticles were prepared by the citrate sol–gel method and heat-treated at temperatures between 700 and 1600 °C. ErNbO4 revealed a spherical-shaped crystallite, whose size increased with heat treatment temperatures. To assess their optical properties at room temperature (RT), a thorough spectroscopic study was conducted. RT photoluminescence (PL) spectroscopy revealed that Er3+ optical activation was achieved in all samples. The photoluminescence spectra show the green/yellow 2H11/2, 4S3/2→4I15/2 and red 4F9/2→4I15/2 intraionic transitions as the main visible recombination, with the number of the crystal field splitting Er3+ multiplets reflecting the ion site symmetry in the crystalline phases. PL excitation allows the identification of Er3+ high-energy excited multiplets as the preferential population paths of the emitting levels. Independently of the crystalline structure, the intensity ratio between the green/yellow and red intraionic transitions was found to be strongly sensitive to the excitation energy. After pumping the samples with a resonant excitation into the 4G11/2 excited multiplet, a green/yellow transition stronger than the red one was observed, whereas the reverse occurred for higher excitation photon energies. Thus, a controllable selective excited tunable green to red color was achieved, which endows new opportunities for photonic and optoelectronic applications.

Light converting Yb3+/Er3+ doped YVO4 nanoparticles for biological applications

We have synthesized luminescent YVO4: Yb, Er nanoparticles (NPs) in the size range of 10–700 nm. A bright green luminescence of the dry NPs powder due to Er3+ ions emission centered at 526 and 550 nm was compared for upconverting and downconverting laser excitations at 980 nm and 257 nm, respectively. Moreover, the NPs exhibited bright luminescence in colloidal water solution upon 980 nm laser irradiation. All experiments showed very weak radiative 4F9/2→4I15/2 emission from Er3+ ions at 660 nm that indirectly indicates low efficiency of multiphonon intraionic transitions and insensitivity of the NPs to the luminescence quenchers in the water solution. A broad red band in the luminescence spectrum appearing under intense 257 nm laser excitation can be attributed to emission from crystal lattice defects in the NPs. Based on these facts we propose that YVO4: Yb, Er NPs are promising as efficient upconversion fluorescent nanoprobes for certain biological applications, like optogenetics.

Observation of spin-orbit excitations and Hund's multiplets in Ca2RuO4

We use Ru $L_3$-edge (2838.5 eV) resonant inelastic x-ray scattering (RIXS) to quantify the electronic structure of Ca$_2$RuO$_4$, a layered $4d$-electron compound that exhibits a correlation-driven metal-insulator transition and unconventional antiferromagnetism. We observe a series of Ru intra-ionic transitions whose energies and intensities are well described by model calculations. In particular, we find a $\rm{J}=0\rightarrow 2$ spin-orbit excitation at 320 meV, as well as Hund's-rule driven $\rm{S}=1\rightarrow 0$ spin-state transitions at 750 and 1000 meV. The energy of these three features uniquely determines the spin-orbit coupling, tetragonal crystal-field energy, and Hund's rule interaction. The parameters inferred from the RIXS spectra are in excellent agreement with the picture of excitonic magnetism that has been devised to explain the collective modes of the antiferromagnetic state. $L_3$-edge RIXS of Ru compounds and other $4d$-electron materials thus enables direct measurements of interactions parameters that are essential for realistic model calculations.

Thermal growth, structural and optical characterization of hierarchical Bi2O3 - MoO3 nanostructures

Branched α-Bi2O3 nanowires decorated with MoO3 nanoplates have been grown by a two–step sequential evaporation–deposition method. Their morphology, structural, compositional and luminescence properties were investigated by a set of spatially-resolved characterization techniques including scanning and high-resolution transmission electron microscopy (SEM, HRTEM), energy dispersive X-ray microanalysis (EDX), micro-Raman spectroscopy and cathodoluminescence (CL) in a SEM. Different zones can be distinguished in these complex structures. Close to the tips, the α-Bi2O3 nanowires appear covered by a high density of α–MoO3 nanoplates, as revealed by the SEM, HRTEM and micro-Raman observations. The Bi2O3 trunks of these hierarchical arrangements appear covered by a lower density of nanoplates, which decreases towards the base of the structures. Besides α–MoO3, Raman measurements reveal the existence of crystals of other MoO3 phases, as well as different Mo oxides, in these zones. CL spectra from the different regions show emission bands related to oxygen vacancies and Bi3+ intraionic transitions in α-Bi2O3, while luminescence emission of the MoO3 nanoplates is mainly related to complex defects involving Mo5+ ions and oxygen vacancies.

Effect of Eu2+ doping on structural, optical, magnetic and photovoltaic properties of ZnS quantum dots

ZnS and Eu2+: ZnS QDs with different doping concentrations were prepared by co-precipitation method at room temperature. It was observed that all samples form in cubic structure. The Eu2+ (0.1%): ZnS, Eu2+ (0.3%): ZnS and Eu2+ (0.5%): ZnS QDs display broad photoluminescence emission peaks at 513.5 nm, 511. 7 nm and 510.6 nm, respectively, are related to the Eu2+ intra-ion transition of 4f6d1 – 4f7. The M-H measurements indicated that unlike ZnS QDs, Eu2+: ZnS QDs with different doping concentrations indicates ferromagnetic behavior at room temperature. The incident photon to electron conversion efficiency (IPCE) is improved with the Eu2+ doping, which proposes the QD solar cell efficiency is able to be boosted by Eu2+ doping by reason of broadened absorption spectra. Accordingly, the data point out that Eu2+: ZnS QDs with different doping concentrations are able to be appropriate materials for photovoltaic and spintronic applications.

Doping β-Ga2O3 with europium: influence of the implantation and annealing temperature

β-Ga2O3 bulk single crystals were doped by ion implantation at temperatures from room temperature to 1000 °C, using a 300 keV Europium beam with a fluence of 1 × 1015 at cm−2. Rising the implantation temperature from room temperature to 400–600 °C resulted in a significant increase of the substitutional Eu fraction and of the number of Eu ions in the 3+ charge state as well as in a considerable decrease of implantation damage. Eu is found in both charge states 2+ and 3+ and their relative fractions are critically dependent on the implantation and annealing temperature, suggesting that defects play an important role in stabilizing one of the charge states. The damage recovery during post-implant annealing is a complex process and typically defect levels first increase for intermediate annealing temperatures and a significant recovery of the crystal only starts around 1000 °C. Cathodoluminescence spectra are dominated by the sharp Eu3+ related intra-ionic 4f transition lines in the red spectral region. They show a strong increase of the emission intensity with increasing annealing temperature, in particular for samples implanted at elevated temperature, indicating the optical activation of Eu3+ ions. However, no direct correlation of emission intensity and Eu3+ fraction was found, again pointing to the important role of defects on the physical properties of these luminescent materials.

Raman and cathodoluminescence analysis of transition metal ion implanted Ga2O3 nanowires

The structural and luminescence properties of gallium oxide nanowires doped with chromium or manganese have been investigated. Undoped Ga2O3 nanostructures have been fabricated by a thermal evaporation method, while doping was subsequently achieved by ion implantation followed by thermal annealing. Scanning electron microscopy (SEM) analysis has shown that this doping process does not alter the morphology of the nanostructures. Ion implantation results in partial amorphization of the crystal lattice, as deduced from Raman spectroscopy studies. Thermal annealing at different temperatures was carried out in order to restore the crystallinity of the nanowires. Raman spectroscopy analysis demonstrates that recrystallization starts at about 700°C and a complete recrystallization is achieved at about 1000°C. Cathodoluminescence (CL) analysis has been used to study the emissions in the 300–900nm range. As-implanted nanowires virtually do not emit any light, which is related to their poor crystal quality and the implantation induced defects. Thermal annealing results in effective CL emission. In particular, a clear correlation between crystallinity of the nanowires doped with Cr and the emission from the 2E-4A2 and 4T2-4A2 intraionic transitions has been observed. On the other hand, emissions directly related to intraionic transitions of Mn have not been found in the nanowires implanted with this ion. The influence of the implantation process and annealing temperature on the observed changes in the donor-acceptor pairs (DAP) band of Ga2O3 is discussed.

Comparative study of Y0.9Er0.1V1−xPxO4 nanophosphors with x = 0, 0.1, 0.5, 0.9 and 1 prepared by sol-gel and hydrothermal processes

Y0.9Er0.1V1−xPxO4 nanophosphors with x = 0, 0.1, 0.5, 0.9 and 1 were synthesized by a sol-gel process and a low-temperature hydrothermal synthesis, in both cases under basic reaction conditions. The structural characteristics, particle morphology and luminescence properties of synthetized samples were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), micro-Raman and photoluminescence (PL) spectroscopies. XRD patterns of the samples obtained as single phases show diffraction maxima which can be indexed on the basis of a tetragonal symmetry of space group I41/amd with Z = 4, compatible with a zircon-type structure. TEM micrographs show spherical particles for samples prepared by sol-gel and more elongated particles for samples synthesized via hydrothermal. Phosphorus incorporation increases the average particle size and their elongation. A broadening of the Raman bands was observed when vanadium is substituted by phosphorus into the Y0.9Er0.1VO4 host, evidencing a structural distortion or non-equivalency vibrations of different vanadate groups. PL emission lines related to Er3+ intraionic transitions were observed in all investigated samples. The structural changes caused by phosphorus addition are responsible for a reduced energy transfer from the vanadate groups to the Er3+ ions, leading to a PL intensity decrease and a broadening of the rare earth luminescence emission bands.

Eu2+ -induced enhancement of defect luminescence of ZnS.

The Eu2+ -induced enhancement of defect luminescence of ZnS was studied in this work. While photoluminescence (PL) spectra exhibited 460nm and 520nm emissions in both ZnS and ZnS:Eu nanophosphors, different excitation characteristics were shown in their photoluminescence excitation (PLE) spectra. In ZnS nanophosphors, there was no excitation signal in the PLE spectra at the excitation wavelength λex >337nm (the bandgap energy 3.68eV of ZnS); while in ZnS:Eu nanophosphors, two excitation bands appeared that were centered at 365nm and 410nm. Compared with ZnS nanophosphors, the 520nm emission in the PL spectra was relatively enhanced in ZnS:Eu nanophosphors and, furthermore, in ZnS:Eu nanophosphors the 460nm and 520nm emissions increased more than 10 times in intensity. The reasons for these differences were analyzed. It is believed that the absorption of Eu2+ intra-ion transition and subsequent energy transfer to sulfur vacancy, led to the relative enhancement of the 520nm emission in ZnS:Eu nanophosphors. In addition, more importantly, Eu2+ acceptor-bound excitons are formed in ZnS:Eu nanophosphors and their excited levels serve as the intermediate state of electronic relaxation, which decreases non-radiative electronic relaxation and thus increases the intensity of the 460nm and 520nm emission dramatically. In summary, the results in this work indicate a new mechanism for the enhancement of defect luminescence of ZnS in Eu2+ -doped ZnS nanophosphors. Copyright © 2016 John Wiley & Sons, Ltd.

Controlled synthesis of Eu2+ and Eu3+ doped ZnS quantum dots and their photovoltaic and magnetic properties

Eu-doped ZnS quantum dots (QDs) have been synthesized by wet-chemical method and found to form in zinc blende (cubic) structure. Both Eu2+ and Eu3+ doped ZnS can be controllably synthesized. The Eu2+ doped ZnS QDs show broad photoluminescence emission peak around 512 nm, which is from the Eu2+ intra-ion transition of 4f6d1 – 4f7, while the Eu3+ doped samples exhibit narrow emission lines characteristic of transitions between the 4f levels. The investigation of the magnetic properties shows that the Eu3+ doped samples exhibit signs of ferromagnetism, on the other hand, Eu2+ doped samples are paramagnetic of Curie-Weiss type. The incident photon to electron conversion efficiency is increased with the Eu doping, which suggests the QD solar cell efficiency can be enhanced by Eu doping due to widened absorption windows. This is an attractive approach to utilize benign and environmentally friendly wide band gap ZnS QDs in solar cell technology.

Effects of preparation method and pH variation on the structural characteristics and luminescence properties of Y0.9Er0.1VO4 and Y0.9Er0.1V0.9Cr0.1O4 nanopowders

Y0.9Er0.1VO4 and Y0.9Er0.1V0.9Cr0.1O4 nanopowders were synthesized by both sol–gel and hydrothermal synthesis. The effects of the preparation method, pH variation and replacement of V5+ by Cr5+ on the structure, particle shape and luminescence properties were investigated. A higher crystallinity is observed in the samples prepared in acid conditions by the sol–gel process, as compared with those synthesized in basic conditions. The opposite trend is found in those samples prepared by hydrothermal synthesis. Broadening of the Raman peaks was observed in spectra of the nanophosphors incorporating Cr, evidencing a certain distortion or non-equivalency of vibrations of (VO4)3− groups. TEM micrographs show changes in the particle shape related to the synthesis conditions. Photoluminescence (PL) related to Er3+ intraionic transitions was observed in all the samples investigated. An enhanced PL emission was observed in samples with a high degree of crystallinity. Magnetization measurements confirm the +5 oxidation state of Cr in the samples.

Effect of magnetic field on intraionic photoluminescence of (Zn,Co)Se

We report on optical properties of epitaxially grown Zn1−xCoxSe diluted magnetic semiconductor. Excitonic photoluminescence reveals efficient energy transfer from band carriers to Co2+ ions. Magneto-photoluminescence of 2T1–4A2 intraionic transition of Co2+ in ZnSe is evidenced and discussed based on reported magneto-absorption results. Determined values of g factors are gA=0.62± 0.09 and gB=3.71± 0.05 for optical transitions, and ge=−0.8±0.2 for the 2T1 excited state of Co2+. Cobalt ion concentration in the sample is determined using both reflection measurements of excitonic giant Zeeman effect and direct magnetometry.

Photoluminescence studies of giant Zeeman effect in MBE-grown cobalt-based dilute magnetic semiconductors

This work is devoted to epitaxy and magnetooptical study of (Zn,Co)Te and (Cd,Co)Te dilute magnetic semiconductors. Giant Zeeman effect is observed in both reflectivity and photoluminescence measurements. Magnetoreflectivity is used to determine cobalt composition. Magnetophotoluminescence reveals quenching due to intra-ionic cobalt transitions. The quenching is more effective at zero magnetic field than at high magnetic field, and in (Cd,Co)Te comparing to (Zn,Co)Te.

Effect of synthesis conditions on the structural characteristics and luminescence properties of Y0.9Eu0.1V1−xCrxO4 (0 ≤ x ≤ 0.5) nanopowders

Y0.9Eu0.1V1−xCrxO4 (0 ≤ x ≤ 0.5) nanophosphors were synthesized under acid conditions (pH = 2) by a one-step sol–gel process followed by a thermal treatment at 843 K in oxygen flow. The obtained nanopowders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and by Raman and photoluminescence (PL) spectroscopies in a confocal microscope. The influence of pH on the structural characteristics and luminescence properties of the nanophospors was also assessed. XRD measurements reveal a progressive loss of crystallinity as chromium substitutes for vanadium in the samples. Such replacement induces a progressive broadening of the Raman peaks as the Cr content increases, evidencing a certain structural distortion involving different (VO4)3− groups due to the insertion of Cr5+ ions into the host. TEM images of the obtained nanopowders show changes in the particle shape and size of samples grown a different pH values. The addition of chromium hampers an effective energy transfer from the vanadate groups to the Eu3+ ions. This results in a strong decrease of the PL emission intensity corresponding to the intraionic transitions of this rare earth, irrespective of the synthesis conditions of the nanopowders.

Designing quantum dots for solotronics

Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. It has already been shown that optical control of a magnetic ion spin is feasible using the carriers confined in a quantum dot. However, a serious obstacle was the quenching of the exciton luminescence by magnetic impurities. Here we show, by photoluminescence studies on thus-far-unexplored individual CdTe dots with a single cobalt ion and CdSe dots with a single manganese ion, that even if energetically allowed, nonradiative exciton recombination through single-magnetic-ion intra-ionic transitions is negligible in such zero-dimensional structures. This opens solotronics for a wide range of as yet unconsidered systems. On the basis of results of our single-spin relaxation experiments and on the material trends, we identify optimal magnetic-ion quantum dot systems for implementation of a single-ion-based spin memory.

Synthesis of europium-doped zinc oxide micro- and nanowires

Single crystalline Eu3+-doped wurtzite ZnO micro- and nanowires were synthesized by a chemical vapor deposition method (CVD). The nanostructures were grown by autocatalytic mechanism at walls of an alumina boat. The structure and properties of the doped ZnO is fully characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectrometry (EDX), scanning and transmission electron microscopy (SEM and TEM), and photoluminescence (PL) methods. The synthesis was carried out for 10 min giving vertically aligned nanowires with mean diameter of 50–400 nm and with length of up to several microns. The nanowires were grown along ±[0001] direction. The concentration of Eu3+ dopant in the synthesized nanowires was varied from 0.7 to 0.9 at %. The crystal structure and microstructures of the doped nanomaterials were discussed and compared with undoped ZnO. The photoluminescence spectra show that emission of doped samples were shifted towards orange-red region (2.02 eV) relative to undoped zinc oxide nanostructures (2.37 eV) due to Eu3+ intraionic transitions from ZnO/Eu.