Main Emission Bands Research Articles

Multimodal optical detection, identification and reduction of Cr(VI) with carbon dots and its PVA/polyNIPAM spheres based 2D luminescent photonic structures

Multiple emissive carbon dots (CDs) were synthesized through a one-step solvothermal acid-assisted method using citric acid and urea. Three main emission bands were observed at ∼431 nm, ∼501 nm, and ∼622 nm, corresponding to excitation wavelengths of 360 nm, 430 nm, and 550 nm, respectively. Simultaneous detection through salient discernible features of three metal ions has been achieved. Emission of CDs with Fe(III) shows reduction in intensity at 360 nm and 550 nm wavelengths and an enhancement of photoluminescence (PL) intensity at an excitation wavelength of 430 nm. Al(III) revealed turn-on PL signals, along with a redshift when excited at 360 and 430 nm, and turn-off intensity at excitation of 550 nm. Under all the excitation wavelengths, Cr(VI) exhibits PL quenching accompanied with an additional red shift. This sensor enhances the accuracy and reliability of the probe through multiple distinctive and characteristic responses for all the three ions for which the probes were selective. Apart from the optical detection at a certain wavelength, the Cr(VI) reduction to the less toxic Cr(III) through as-prepared CDs could be monitored at three different excitation wavelengths. The Fe(III) and Cr(VI) detection in solid state has also been demonstrated through 2D luminescent photonic structures formed with CDs-PVA (poly-vinyl alcohol)-polyNIPAM (poly (N-isopropylacrylamide)) composites that can possibly be used for anticounterfeiting applications.

Thulium-doped titanate-germanate glasses for infrared photonics

In this paper, we performed a comparative spectroscopic analysis of Tm3+-doped glasses based on GeO2-BaO-Ga2O3 (GBG) and TiO2-GeO2-BaO-Ga2O3 (TGBG) to achieve broadband luminescence at 1.8 μm. Glasses were prepared at different doping concentrations (0.05 mol% and 1mol%) in order to investigate the effect of Tm3+ ions as well as host composition (GeO2:TiO2 molar ratios) on near-IR luminescence properties. We report on the hypersensitivity of the transition 3H6 → 3F4 of Tm3+ in a series of titanate-germanate glasses, which was shifted to longer wavelengths with increasing TiO2 concentration. The relative intensity of the main near-IR emission band corresponding to the 3F4 → 3H6 (1.8 μm) transition of Tm3+ was 4.8-fold enhanced for glass with molar ratio GeO2:TiO2 = 1:5 compared to GBG glass system. We observed the luminescence quenching phenomena for samples containing more than 0.5 mol% of Tm3+ ions. The calculated spectroscopic parameters, such as measured luminescence lifetime (τm) and the quantum efficiency (η), were discussed and compared to the different laser glasses. Importantly, luminescence decay curves have been examined using the Inokuti-Hirayama model. Additionally, the correlation between optical properties and local structure in glasses was well evidenced by Raman spectroscopy. The proposed strategy, in combination with various chemical composition of glass host activated by various Tm2O3 concentration, might be beneficial for modern near-infrared photonics.

Photoluminescent Bi-doped CsPbX3 (X: Br, I) perovskite quantum dots for optoelectronic devices

Perovskite quantum dots (PQDs) became a hot spot in recent years due to their amazing properties, such as the high photoluminescence quantum yield, tunable emission, and narrow bandwidth being important for their application in different optoelectronic devices. In this work, Bi-doped CsPbBr3 and Bi-doped CsPbI3 PQDs were synthesized through the hot-injection method and compared with pristine CsPbBr3 and CsPbI3 to analyze the effect of Bi and the halogen on their properties. In addition, all the samples were synthesized at 130°C, 150°C, and 170°C with the aim of analyzing the effect of the temperature. The results showed a wide range of the emission wavelength from around 500 nm (Bi-doped CsPbBr3) to 630 nm (Bi-doped CsPbI3) as a consequence of the effect of the halogen in “X” position and a slight blueshift in the main photoluminescence emission band after doping the pristine quantum dots with Bi.Graphical abstractImpact statementWe believe that the work in this article represents an important advance in the application of perovskite quantum dots in optoelectronics applications, such as in LEDs or lasers. We report here the synthesis and characterization of Bi-doped CsPbX3 perovskite quantum dots (PQDs), being X: Br and I. These Bi-doped PQDs show a wide range of the emission wavelength from around 500 nm (Bi-doped CsPbBr3) to 630 nm (Bi-doped CsPbI3) as a consequence of the effect of the halogen in “X” position and a slight blueshift in the main photoluminescence emission band after doping the pristine quantum dots with Bi. Therefore, they are good candidates to fabricate optoelectronic devices such as LEDs and lasers thanks to their high photoluminescence emission and their tunable emission.

Effect of mixed TeO2–B2O3 glass former in Ho3+/Yb3+-doped glass system: Raman spectroscopy, Judd-Ofelt and luminescence investigations

In this work, glass with composition (75-x)B2O3-xTeO2-11Bi2O3–10Li2O-1Ho2O3-3Yb2O3 (x = 10–60 mol%) were synthesized using the melt quenching technique to investigate the effect of mixed glass former between TeO2 and B2O3 on the spectroscopic properties of glass. The Raman Spectroscopy has revealed a structural depolymerization undergone by the glasses with a sudden increment at x = 30 mol% that might arise from structural competition between TeO2 and B2O3. Optical absorption spectra have revealed 9 absorption peaks corresponding to Ho3+ ion transitions while one peak belongs to Yb3+ ion transition. The bonding parameter signifies the glasses are ionic in nature. The oscillator strength and Judd-Ofelt parameter Ω2,4,6 have reduced against increment of TeO2 concentration indicated a low asymmetry and low rigidity except for x = 30 mol% that may be due to strong interaction between TeO2 and B2O3. The radiative properties such as spontaneous emission probability, branching ratio, radiative lifetime and stimulated emission cross-section have showcased the glass potential for laser application. The luminescence spectra of studied glasses upon 450 nm excitation have disclosed two main emission bands which are 530 nm and 642 nm that corresponding to 5F4–5I8 and 5F5–5I8, of Ho3+ respectively whilst an extra emission recorded at 738 nm was ascribed to 5F4–5I7 of Ho3+ due to energy transfer process from Yb3+ to Ho3+. The emission intensity of the samples has enhanced with the TeO2 addition due to reduction of phonon energy. CIE 1931 diagram has shown a greener emission colour of glass samples over TeO2 addition.

Temperature and power-dependent photoluminescence spectroscopy in suspended WSe2 monolayer

This work, the photoluminescence (PL) of a suspended monolayer of WSe2 was investigated to avoid the substrate effect, and its evolution with temperature and excitation energy. Raman spectroscopy and PL demonstrated the monolayer characteristic of the sample due to the absence of the B1 2g peak along with a symmetric PL peak centered at approximately 1.65 eV. The PL spectrum exhibited two main emission bands attributed to a neutral exciton (X0) and a trion (X T ), where X0 showed a redshift with temperature variation from 10 K to 310 K, and the intensity ratio between the bands varied from 2.68 to 3.96 eV. The energy evolution of the bands as a function of temperature was analyzed using the modified Varshni equation, yielding an electron–phonon coupling constant with a value of 1.54 (2.11) for X0 (X T ). The intensity behavior was studied using the multi-level model, which provided activation energies of 58.0 meV for X0 and 94.6 meV for X T . The redshift of the mentioned bands is explained by the Urbach formalism, attributing this shift to an increase in phonon density in the material. Computational calculations demonstrated an increase in the material’s lattice parameter with rising temperature, resulting in an increase in the W–Se bond length and a decrease in the distance between Se atoms from different sublattices, causing the direct transition to decrease with temperature. In conclusion, this study showed that the observed evolution in the PL spectrum of the suspended WSe2 monolayer could be related to the increase of phonon density in the material which is important for the future applications of 2D materials in optoelectronics.

Organic LEDs Based on Bis(8-hydroxyquinoline) Zinc Derivatives with a Styryl Group.

For the first time, organic light-emitting diodes (OLEDs) based on bis(8-hydroxyquinoline) zinc with a styryl group (ZnStq) dispersed in poly(N-vinylcarbazole) matrix (ZnStq_R:PVK, where R = H, Cl, OCH3) were fabricated. The ZnStq_R:PVK films made via the spin-coating method were used as the active layer in these devices. The produced OLEDs showed strong electroluminescence with yellow emissions at 590, 587 and 578 nm for the ZnStq_H:PVK, ZnStq_Cl:PVK and ZnStq_OCH3:PVK, respectively. For all the studied thin films, the main photoluminescence emission bands were observed between 565 and 571 nm. The OLED with the ZnStq_OCH3:PVK layer with a narrow electroluminescence spectrum was found to have sufficient color purity to produce ultra-high-resolution displays with reduced power consumption (full width at half maximum of 59 nm, maximum brightness of 2244 cd/m2 and maximum current efficiency of 1.24 cd/A, with a turn-on voltage of 6.94 V and a threshold voltage of 7.35 V). To characterize the photophysical properties of the active layer, the ZnStq_R:PVK layers samples were additionally deposited on glass and silicon substrates. We found that the obtained results predestine ZnStq_R:PVK layers for use in the lighting industry in the future.

Regularities of Manganese Charge State Formation and Luminescent Properties of Mn-Doped Al2O3, YAlO3, and Y3Al5O12 Single Crystalline Films

In this work, three sets of single crystalline films (SCF) of Al2O3:Mn sapphire, YAlO3:Mn perovskite (YAP:Mn), and Y3Al5O12:Mn garnet (YAG:Mn), with a nominal Mn content of 0.1%, 1%, and 10 atomic percent (at.%) in the melt-solutions, were crystallized by the liquid phase epitaxy (LPE) method onto sapphire, YAP and YAG substrates, respectively. We have also calculated the average segregation coefficient of Mn ions for Al2O3:Mn, YAP:Mn and YAG:Mn SCFs with Mn content in the melt-solution in the 0.1–10% concentration range, which was equal to 0.1, 0.14 and 0.2, respectively. The main goal of the conducted research was the spectroscopic determination of the preferable valence states of manganese ions which were realized in the SCFs of sapphire, perovskite and garnet depending on the Mn content. For this purpose, the absorption, cathodoluminescence (CL), photoluminescence (PL) emission/excitation spectra and PL decay kinetics of Al2O3:Mn, YAP:Mn and YAG:Mn SCFs with different Mn concentrations were studied. Based on the CL and PL spectra, we showed that Mn ions, depending on the Mn content in the melt-solution, are incorporated in Al2O3:Mn, YAP:Mn and YAG:Mn SCFs in the different charged states and are located in the different crystallographic positions of the mentioned oxide lattices. We have observed the presence of the luminescence of Mn4+, Mn3+ and Mn2+ valence states of manganese ions in CL spectra in all SCFs under study with 0.1 and 1% Mn concentrations. Namely, the Mn4+ ion valence state with the main sharp emission bands peaked at 642 and 672 nm, related to the 2E → 4A2 transitions, was found in the luminescence spectra of the all studied Al2O3:Mn SCFs. The luminescence of the Mn2+ valence state was found only in YAP:Mn and YAG:Mn SCFs, grown from melt solution with 1% Mn content, in the emission bands peaked at 525 and 560 nm, respectively, related to the 4T1 → 6A1 transitions. The PL and CL spectra of YAP:Mn and YAG:Mn SCFs with the Mn content in the 0.1–1% range show that the main valence state of manganese ions in these films is Mn3+ with the main emission bands peaking at 655 and 608 nm, respectively, related to the 1T2 → 5E transitions. Meanwhile, higher than 1% Mn content in the melt solution causes a strong concentration quenching of luminescence of all Mn centers in Al2O3:Mn, YAP:Mn and YAG:Mn SCFs.

Understanding the Optical Properties of Doped and Undoped 9-Armchair Graphene Nanoribbons in Dispersion.

Graphene nanoribbons are one-dimensional stripes of graphene with width- and edge-structure-dependent electronic properties. They can be synthesized bottom-up in solution to obtain precise ribbon geometries. Here we investigate the optical properties of solution-synthesized 9-armchair graphene nanoribbons (9-aGNRs) that are stabilized as dispersions in organic solvents and further fractionated by liquid cascade centrifugation (LCC). Absorption and photoluminescence spectroscopy reveal two near-infrared absorption and emission peaks whose ratios depend on the LCC fraction. Low-temperature single-nanoribbon photoluminescence spectra suggest the presence of two different nanoribbon species. Based on density functional theory (DFT) and time-dependent DFT calculations, the lowest energy transition can be assigned to pristine 9-aGNRs, while 9-aGNRs with edge-defects, caused by incomplete graphitization, result in more blue-shifted transitions and higher Raman D/G-mode ratios. Hole doping of 9-aGNR dispersions with the electron acceptor F4TCNQ leads to concentration dependent bleaching and quenching of the main absorption and emission bands and the appearance of red-shifted, charge-induced absorption features but no additional emission peaks, thus indicating the formation of polarons instead of the predicted trions (charged excitons) in doped 9-aGNRs.

Photoluminescence and electronic structure of α'-Ag8GeS6 crystals

The Ag8GeS6 crystal was synthesized by the direct melting of the elementary Ag, Ge, and S high-purity grade stoichiometric mixture in a sealed quartz ampoule. This argyrodite crystallizes in orthorhombic structure [Pna21 space group (No. 33)] below ≈483 K. The theoretical first-principle calculations of the electronic band structure and density of states of an α'-Ag8GeS6 crystal are estimated by the generalized gradient approximation. The Perdew–Burke–Ernzerhof functional was utilized. All of the calculated parameters correlate well with the known experimental data. Based on the electronic band structure, the effective mass of the electrons and holes was calculated. The anisotropic behavior of electronic band structure is discussed. Temperature and carrier concentration dependence of the Seebeck coefficient is calculated. The temperature behavior of α'-Ag8GeS6 photoluminescence spectra is presented. The emission spectra are studied in the temperature range between 17 and 130 K and laser power excitation between 4 and 512 mW and in the spectral range of 750–1650 nm. Two main emission bands at ≈1.35 and ≈1.57 eV are observed and discussed.

One-dimensional LaF3:RE3+ (RE=Eu, Nd) nanostructured phosphors: Controllable synthesis and luminescence properties

A novel and universal method combining electrospinning with a twi-crucible fluorinating technology was utilized to fabricate one-dimensional LaF3:RE3+ (RE = Eu, Nd) nanostructured phosphors. A series of nanofibrous/nanoribbon-shaped LaF3:RE3+ (RE = Eu, Nd) phosphors were devised and facilely fabricated through the calcination of the relevant nanofibrous/nanoribbon-shaped La2O3:RE3+ (RE = Eu, Nd) precursors in air by applying a twi-crucible fluorinating method utilizing NH4HF2 as a fluorine source without additional usage of reducing gas and protective gas. Nanofibrous/nanoribbon-shaped La2O3:RE3+ (RE = Eu, Nd) precursors were synthesized by calcinating the electrospun nanofibrous/nanoribbon-shaped [RE(NO3)3+La(NO3)3]/polyvinyl pyrrolidone (PVP) composites. Nanofibrous/nanoribbon-shaped LaF3:RE3+ (RE = Eu, Nd) phosphors possess hexagonal structures with a space group of P63/mmc. Nanofibrous LaF3:RE3+ (RE = Eu, Nd) phosphors present excellent fibrous morphology with a uniform diameter of 131–143 nm. Nanoribbon-shaped LaF3:Eu3+ phosphors with 94.5 nm in thickness and 5.1 ± 0.4 μm in width were acquired. Nanoribbon-shaped LaF3:Nd3+ phosphors have a thickness of 179 nm and a width of 4.2 ± 0.4 μm. Upon excitation by ultraviolet light at 396 nm, nanofibrous/nanoribbon-shaped LaF3:Eu3+ phosphors emit the main emission band at approximately 591 nm, corresponding to the 5D0→7F1 magnetic dipole transition of Eu3+. With an increment of Eu3+ concentration, the fluorescent strength of nanofibrous LaF3:Eu3+ phosphors is improved greatly and achieves a maximum when the Eu3+ concentration is approximately 5 mol%. Moreover, the nanofiber/nanoribbon-shaped LaF3:Eu3+ phosphors emit a yellow color. For Nd3+-doped phosphors, nanofibrous/nanoribbon-shaped LaF3:Nd3+ phosphors possess near-infrared optical performances, and the main emission peak is at 854 nm, corresponding to the 4F3/2 → 4I9/2 transition of Nd3+. The luminescence intensity of LaF3:Nd3+ phosphors decreases significantly with increasing Nd3+ content and reaches a maximum at 5 mol% Nd3+. The corresponding formative mechanisms of nanofibrous/nanoribbon-shaped RE3+ (RE = Eu, Nd) phosphors are expounded, and the corresponding construction technique is established. The new findings in this work lay a foundation for wide applications of rare earth fluoride one-dimensional nanostructures.

Temperature-Dependent and Time-Resolved Luminescence Characterization of γ-Ga2O3 Nanoparticles.

The temperature-dependent luminescence properties of γ-Ga2O3 nanoparticles prepared by a precipitation method are investigated under steady-state and pulsed-light excitation. The main photoluminescence (PL) emission at room temperature consists of a single blue band centered around 2.76 eV, which hardly undergoes a blueshift of 0.03 eV when temperature goes down to 4 K. The emission behaves with a positive thermal quenching following an Arrhenius-type curve. The data fitting yields two non-radiative levels affecting the emission band with activation energies of 7 meV and 40 meV. On the other hand, time-resolved PL measurements have also been taken and studied as a function of the temperature. The data analysis has resulted in two lifetimes: one of 3.4 ns and the other of 32 ns at room temperature, which undergo an increase up to 4.5 ns and 65 ns at T = 4 K, respectively. Based on both stationary and dynamic PL results, a model of radiative and non-radiative levels associated with the main emission bands of γ-Ga2O3 is suggested. Finally, by using PL excitation measurements, an estimation of the bandgap and its variation with temperature between 4 K and room temperature were obtained and assessed against O'Donnell-Chen's law. With this variation it has been possible to calculate the average of the phonon energy, resulting in ⟨ħω⟩ = 10 ± 1 meV.

Spectroscopic characterization of Er3+-doped CaF2 nanoparticles: Luminescence concentration quenching, radiation trapping and transition probabilities

Er3+-doped CaF2 nanoparticles (NPs) with variable dopant concentration were synthesized by a direct precipitation method. X-Ray Powder Diffraction, SEM and TEM were used to analize the crystalline structure and morphology. The spectroscopic characterization, as function of the Er3+ content, has been performed under CW and pulsed excitation. Under steady state conditions, it has been found that the intensity of the main emission bands is affected by luminescence quenching processes. The population dynamics, recorded under pulsed excitation, confirms not only the existence of quenching processes but also the occurrence of radiation trapping. The intrinsic transition probabilities of the main Er3+ emitting manifolds, in absence of quenching and radiation trapping, have been estimated through a procedure commonly used in bulk doped materials. A modified Judd-Ofelt analysis has been performed to determine the radiative transition probabilities, radiative lifetimes and branching ratios of the Er3+ levels. Finally, an estimation of the gap law in these NPs is given.

Investigation of Solvent Effect and H-Bonding on Spectroscopic Properties of 1-(3-Amino-6-(2,5-dichlorothiophen-3-yl)-4-phenylfuro[2,3-b]Pyridin-2-yl) Ethenone: Experimental and Computational Study.

The furo[2,3-b]pyridine moiety is an important scaffold for many biologically active compounds, therefore, the spectral data of the derivative 1-(3-Amino-6-(2,5-dichlorothiophen-3-yl)-4-phenylfuro[2,3-b]pyridin-2-yl) ethenone (FP1) were investigated. Analysis of absorption-pH profile and Förster cycle of FP1 revealed that its excited state is more acidic than its ground state ([Formula: see text] < [Formula: see text]). The main fluorescence emission band of FP1 at 480nm (in hexane) is shifted to longer wavelengths with increasing polarities of solvents. Linear Lippert's plot and linear correlation between bands maxima and Camlet-Taft parameter, α, of the protic solvents indicated efficient intramolecular charge transfer and noticeable H-bonding. Moreover, the disappearance of the absorption band of FP1 at 385nm in water, along with the noticeable red shift and quenching of the emission band, and the lower lifetime, relative to nonaqueous solvents, indicate the interruption of the furo[2,3-b]pyridine aromatic moiety. In addition, results from the Time Dependent Density Functional Theory (TDDFT) and Molecular Mechanic (MM) calculations were in agreement with experimentally determined spectra of FP1.

Abnormal Eu3+ → Eu2+ Reduction in Ca9−xMnxEu(PO4)7 Phosphors: Structure and Luminescent Properties

β-Ca3(PO4)2-type phosphors Ca9-xMnxEu(PO4)7 have been synthesized by high-temperature solid-phase reactions. The crystal structure of Ca8MnEu(PO4)7 was characterized by synchrotron X-ray diffraction. The phase transitions, magnetic and photoluminescence (PL) properties were studied. The abnormal reduction Eu3+ → Eu2+ in air was observed in Ca9-xMnxEu(PO4)7 according to PL spectra study and confirmed by X-ray photoelectron spectroscopy (XPS). Eu3+ shows partial reduction and coexistence of Eu2+/3+ states. It reflects in combination of a broad band from the Eu2+ 4f65d1 → 4f7 transition and a series of sharp lines attributed to 5D0 → 7FJ transitions of Eu3+. Eu2+/Eu3+ ions are redistributed among two crystal sites, M1 and M3, while Mn2+ fully occupies octahedral site M5 in Ca8MnEu(PO4)7. The main emission band was attributed to the 5D0 → 7F2 electric dipole transition of Eu3+ at 395 nm excitation. The abnormal quenching of Eu3+ emission was observed in Ca9-xMnxEu(PO4)7 phosphors with doping of the host by Mn2+ ions. The phenomena of abnormal reduction and quenching were discussed in detail.

Toward Near-Infrared Emission in Pt(II)-Cyclometallated Compounds: From Excimers’ Formation to Aggregation-Induced Emission

Two series of Pt(II)-cyclometallated compounds containing N^C^N tridentate and alkynyl-chromophore ligands have been synthesized and structurally characterized. The N^C^N ligands differ on the presence of R1 = H or F in the central aromatic ring, while six different chromophores have been introduced to the alkynyl moiety. Single-crystal X-ray structures for some of the compounds reveal the presence of weak intermolecular contacts responsible for the formation of some dimers or aggregates. The photophysical characterization shows the presence of two emission bands in solution assigned to the 3π-π* transition from the N^C^N ligands mixed with 3MLCT/3ILCT transitions (higher energy band) in deaerated samples. The formation of excimers has also been identified as a broad band at longer wavelengths [near-infrared (NIR) emission] that becomes the main emission band for compounds containing phenanthrene as the chromophore. NIR emission behavior has also been explored using acetonitrile/water mixtures, and the presence of aggregates that emit at ca. 650 nm has also been detected.

Spectroscopic properties and microchip laser performance of Yb:LaCa4O(BO3)3 crystal

In this work, the polarized absorption and emission spectra of the Yb:LaCa4O(BO3)3 (Yb:LaCOB) crystal were studied, and the microchip laser performance was investigated with 1.0 mm thick crystals cut along its three principal optical axes. The strongest absorption is found to occur at 977.2 nm for E//X, with the cross-section amounting to 2.21 × 10−20 cm2, the largest for all of the Yb-doped oxyborates developed so far; whereas the maximum cross-section of the main emission band, 0.30 × 10−20 cm2 for E//Z, proves to be the smallest among all the Yb-doped oxyborates. The X-cut microchip crystal laser could be operated simultaneously in the main emission band and in the long-wavelength sideband, producing an output power of 2.15 W with optical conversion efficiency of 48.1 %. While operated in the emission sideband, the Z-cut microchip laser could generate 2.14 W of output power at 1068.6–1077.8 nm with an optical conversion efficiency of 45.1 %. Output power of 2-W level could also be produced from the X-cut microchip laser operating at 1028.1–1033.6 nm, or from the Z-cut microchip laser operating at 1042.7–1050.7 nm, with optical conversion efficiencies higher than 40 %.

Особенности изменения интенсивностей основных полос фотолюминесценции ионов Tb-=SUP=-3+-=/SUP=- и их сателлитов в поликристаллическом люминофоре Gd-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-:Tb(3 mol&amp;#37;)

Samples of Gd2O3:Tb(3 mol%) phosphor were obtained by sol-gel method followed by annealing at 800oC and 1200oC in air. At high annealing temperature, the intensities of the main emission bands 484 and 541 nm increase, but the ratio of the intensities of these emission bands to their satellites 493 and 549 nm respectively decreases. Based on the analysis of X-ray diffractometry, radiation spectra, far-infrared and Raman spectroscopy, as well as diffuse reflectance spectroscopy, we established: the increase in the crystallinity of the samples with a significant reduction of the lattice strain stress at elevated annealing temperatures, changes in the structure of the bandgap with degenerated acceptor and donor zones of impurities Tb4+ and Tb3+ respectively. The diffuse reflection spectra of the sample after annealing at 800oC under optical excitation showed a direct charge transition through the bandgap with Eg = 2.56 eV. After elevated annealing temperature the concentration of Tb4+ ions decreases due to reduction to Tb3+. As a result, at low excitation energies the degeneracy of the acceptor zone is still preserved and there is a direct transition of charges through the bandgap with Eg = 2.55 eV. At high excitation energies the degeneracy of the acceptor zone is removed and there is a direct transition through the bandgap with Eg = 3.39 eV. These effects are accompanied by a relatively large increase in the emission intensity of the satellites, especially at the 549 nm.

Peculiarities of changes in the intensities of the main photoluminescence bands of Tb-=SUP=-3+-=/SUP=- ions and their satellites in polycrystalline Gd-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- : Tb(3 mol%)

Samples of Gd2O3 : Tb(3 mol%) phosphor were obtained by sol-gel method followed by annealing at 800oC and 1200oC in air. At high annealing temperature, the intensities of the main emission bands 484 and 541 nm increase, but the ratio of the intensities of these emission bands to their satellites 493 and 549 nm respectively decreases. Based on the analysis of X-ray diffractometry, emission spectra, far-infrared and Raman spectroscopy, as well as diffuse reflectance spectroscopy, we established: the increase in the crystallinity of the samples with a significant reduction of the lattice strain stress at elevated annealing temperatures, changes in the structure of the bandgap with degenerated acceptor and donor zones of impurities Tb4+ and Tb3+ respectively. The diffuse reflection spectra of the sample after annealing at 800oC under optical excitation showed a direct charge transition through the bandgap with Eg=2.56 eV. After elevated annealing temperature the concentration of Tb4+ ions decreases due to reduction to Tb3+. As a result, at low excitation energies the degeneracy of the acceptor zone is still preserved and there is a direct transition of charges through the bandgap with Eg=2.55 eV. At high excitation energies the degeneracy of the acceptor zone is removed and there is a direct transition through the bandgap with Eg=3.39 eV. These effects are accompanied by a relatively large increase in the emission intensity of the satellites, especially at the 549 nm. Keywords: Gd oxide, Tb3++ photoluminescence spectra, far infrared and Raman spectroscopy spectra, structure of the bandgape, distribution of Tb3+ and Tb4+ in cation sublattice.

Quantification of the resonant energy transfer processes in Er3+/Yb3+ co-doped Ca3Al2Si3O12 glasses

The resonant cross relaxation processes between Yb3+ and Er3+ ions in calcium alumino-silicate glasses have been quantified under selective Er3+ excitation. The infrared emission spectra, measured under steady state conditions (CW excitation to the 4I9/2 erbium level), have allowed to obtain an experimental relationship linking the transfer (Yb3+ → Er3+) and back transfer (Er3+ → Yb3+) parameters. These measurements combined with the dynamics of the main emitting levels, measured under pulsed excitation to the 2H11/2 erbium level, have allowed the fully quantification of the energy transfer parameters. The obtained values, C25=5.5×10−18cm3s−1 (Yb3+ → Er3+), C52=1.5×10−18cm3s−1 (Er3+ → Yb3+) and C27=7.6×10−18cm3s−1 (up-conversion mechanism, estimated from the Judd-Ofelt analysis previously reported), can be used to predict the temporal evolution of the main luminescent emission bands.

Synthesis and characterization of red-emitting Yb/Ho-CaSiO3 upconversion phosphors

Host material plays an important role in obtaining efficient photon upconversion and downshifting luminescence. Generally, fluoride and oxyfluoride glasses-based materials are used for high-efficiency photon upconversion. However, the poor thermal stability of fluoride glasses and the toxicity of fluorine ions limit their applications. In this report, Yb/Ho-doped CaSiO3 wollastonite phosphors have been demonstrated as efficient red-emitting upconversion phosphors. The phosphors have been synthesized by microwave hydrothermal process followed by heat treatment at 1050 ​°C in the air environment. 2M phase of the β-wollastonite has been confirmed by X-ray diffraction and Raman spectroscopy while the existence of the Yb and Ho dopants in the CaSiO3 lattice has been confirmed by X-ray photoelectron spectroscopy. The synthesized samples showed strong red upconversion emission centered at 662 ​nm and near-infrared downshifting emissions at 2016 ​nm upon 980 ​nm excitation. The emissions were found to depend significantly on the Ho concentration. Temporal evolution of the main emission bands was investigated to show that the energy transfer upconversion from Yb to Ho ions was responsible for the efficient upconversion and downshifting phenomenon.