1A1 Transition Research Articles

Thermally stable and highly efficient Ta-doped CsV1-xO3 green-light-emitting phosphors for WLED

Owing to their energy-saving and environmentally friendly characteristics, rare-earth-free ultraviolet-excited green-luminescent materials have garnered significant attention. To improve the luminescence efficiencies of green-luminescent materials, this study synthesized Ta-doped CsV1-xO3 green-light-emitting phosphors through a solid-state-reaction method, adopting an optimal Ta doping concentration of 10%. Theoretical calculations and reflection spectra revealed an intrinsic indirect band gap of 2.86 eV for CsV0.9Ta0.1O3, lower than that of CsVO3 (3.63 eV). Under 365 nm excitation, the as-prepared phosphors exhibited broadband emissions ranging from 400 nm to 700 nm, attributed to the 3T2→1A1 and 3T1→1A1 transitions within VO4 tetrahedra. Notably, adding Ta5+ ions to CsVO3 increased its emission intensity by 1.39 times and induced a redshift in its excitation spectrum from 369 to 375 nm. Furthermore, Ta5+ doping significantly distorted the VO4 tetrahedra and influenced the optical properties of the synthesized phosphors. Specifically, the nearest of Cs–V bond length increased, and weak interactions between V5+ ions and Cs + ions led to a 1.5-fold higher internal quantum efficiency (60.49%) of CsV0.9Ta0.1O3 compared to that of CsVO3. Furthermore, CsV0.9Ta0.1O3 demonstrated high thermal stability, retaining 75.57% of its initial intensity even at 140 °C. Overall, these results indicate the potential applicability of CsV0.9Ta0.1O3 as a green-emitting phosphor for white-light-emitting diode applications.

Thermochromic Pr3+ doped CaMoO4 phosphor with diverse thermal responses for temperature sensing

CaMoO4:Pr3+ thermochromic phosphors with diverse thermal responses for temperature sensing were prepared by the traditional solid-phase reaction method. The typical CaMoO4:Pr3+ had scheelite structure belonging to tetragonal crystal system and space group of I41/a (88). Pr3+ ions can be easily substituted for Ca2+ ions of host CaMoO4 because of similar ionic radius. CaMoO4: 1.5% Pr3+ have the block structure with mean size of 6.84 μm. The Eg (∼3.93 eV) value of pure CaMoO4 is bigger than that (∼3.65 eV) of CaMoO4: 1.5%Pr3+, attributing to the existence of intermediate defect energy levels. Appropriate Pr3+ doping concentration is 1.5%, and the concentration quenching phenomenon can be explained by the concrete electric multipole type of d-d interaction. The emission peak at ∼605 nm from 1D2→3H4 transition have a good thermal stability of 99.452%@423 K, while the wide band centered at ∼490 nm from 3T1,2 → 1A1 transition in the MoO42− complex and 3P0→3H4 transition in Pr3+ have a poor thermal stability of 27.572%@423 K. Calculated activation energy is 0.239 eV. Temperature-dependent FIR for optical thermometry was constructed due to their diverse thermal responses. CaMoO4: Pr3+ phosphor had good relative sensitivities of 2.216%, 0.969% and 0.932% based on FIR of I605 nm/I490 nm with Boltzmann distribution, modified Boltzmann distribution and exponential equation fitting. Thermochromic behavior and thermal quenching mechanism are investigated. The obtained relative sensitivity is better than that of most phosphors, implying that CaMoO4: Pr3+ has a potential for application in optical thermometry.

Highly efficient and self-activating Zn3V2O8 phosphor for the fabrication of cool-white light emitting devices

Achieving an ideal white light with eco-friendly phosphor material with a high quantum efficiency (QE) together with an appreciable correlated color temperature (CCT) and color rendering index (CRI) is considered as a global challenge. To address the issue, we have fabricated a self-activated Zn3V2O8 phosphor-based white light emitting device (WLED) as single emitting compound with desired CCT and color purity values of ∼5074 K and 10.47 × 10−2 respectively, using a moderately low temperature assisted solid-state reaction process carefully ruled out the presence of rare-earth ions. The prepared Zn3V2O8 phosphor exhibited a highly visible yellow (575 nm) photoluminescence (PL) induced by the 3T2 (t152e) →1A1 and 3T1(t152e) →1A1 (t16) transitions of vanadate [VO4]3-, which unveil a significant QE of 67%. The temperature-dependent PL spectra showed a high thermal stability and CIE color coordinates (0.45, 0.47), falling in the exact yellow region. The combination of Zn3V2O8 phosphor and deep blue (405 nm, 40 mW) emission from LED resulted in white light emission with CIE color coordinates and CRI of (0.33, 0.36) and 93, respectively. This work shows that the newly exposed broadband yellow-emitting Zn3V2O8 phosphors as a potential candidate for phosphor-converted WLEDs with high color quality via a non-rare-earth activation approach.

Threshold Photoelectron Spectrum of m-Benzyne.

Due to their unusual electronic structure, the biradical m-benzyne, C6H4, and its cation are of considerable interest in chemistry. Here, the photoion mass-selected threshold photoelectron spectrum of the m-benzyne biradical is presented. An adiabatic ionization energy of 8.65 ± 0.015 eV is derived, while a vibrational progression of 0.10 eV is assigned to the ν9+ ring breathing mode, in excellent agreement with computations. The experimental spectrum was reproduced well by Franck-Condon spectral modeling of the 2A1 ← X 1A1 transition, in which the cation retains a monocyclic C6 framework. The energetically close-lying bicyclic 2A2 cation state exhibits low Franck-Condon factors, due to the large change in geometry, and thus cannot be observed.

Red color Sr2NaMg2V3O12:Eu3+ phosphor with high thermal stability for w-LEDs

Sr2NaMg2V3O12:xEu phosphors with uniform particle size/morphology and good dispersion can be synthesized via solid state reaction. Under the optimum excitation wavelength of ∼340 nm, the Sr2NaMg2V3O12:xEu phosphor displays the broad emission band at ∼500 nm (blue-green emission, 3T1,2 → 1A1 transition of [VO4]3– and sharp emission peak at ∼610 nm (red emission, 5D0→7F2 transition of Eu3+), respectively. With the Eu3+ content increasing, the emission intensity of [VO4]3– decreases gradually while the emission intensity of Eu3+ increases, which is ascribed to the energy transfer from [VO4]3– to Eu3+ ions. The decay time of [VO4]3– (∼500 nm) increases gradually with the Eu3+ content increasing. The energy transfer efficiency and mechanism of [VO4]3–→Eu3+ ions were analyzed, respectively. The emission color of Sr2NaMg2V3O12:xEu phosphor was regulated from green to red via altering the Eu3+ ions concentration. It is delightful for us that the white color emission can be just achieved when the Eu3+ content is 5 at% (x = 0.05), and the w-LED can be assembled. By studying the luminescence temperature dependence and activation energy of Sr2NaMg2V3O12:xEu phosphor, it is shown that the phosphor has good thermal stability. The Sr2NaMg2V3O12:xEu powder is expected to be a new type of phosphor widely used in light and display areas.

Ammonia Borane, NH3 BH3 : A Threshold Photoelectron-Photoion Coincidence Study of a Potential Hydrogen-Storage Material.

We have investigated the photoionization of ammonia borane (AB) and determined adiabatic ionization energy to be 9.26±0.03 eV for the X+ 2E←X 1A1 transition. Although the threshold photoelectron spectrum appears at first glance to be similar to the one of the isosteric ethane, the electronic situation differs markedly, due to different orbital energies. In addition, an appearance energy AE0K(NH3BH3, NH3BH2 +)= 10.00±0.03 eV has been determined, corresponding to the loss of a hydrogen atom at the BH3‐site. From the data, a 0 K bond dissociation energy for the B−H bond in the cation of 71.5±3 kJ mol−1 was derived, whereas the one in the neutral compound has been estimated to be 419±10 kJ mol−1.

High-Resolution Double Velocity Map Imaging Photoelectron Photoion Coincidence Spectrometer for Gas-Phase Reaction Kinetics.

We present a new photoelectron photoion coincidence (PEPICO) spectrometer that combines high mass resolution of cations with independently adjustable velocity map imaging of both cations and electrons. We photoionize atoms and molecules using fixed-frequency vacuum ultraviolet radiation. Mass-resolved photoelectron spectra associated with each cation's mass-to-charge ratio can be obtained by inversion of the photoelectron image. The mass-resolved photoelectron spectra enable kinetic time-resolved probing of chemical reactions with isomeric resolution using fixed-frequency radiation sources amenable to small laboratory settings. The instrument accommodates a variety of sample delivery sources to explore a broad range of physical chemistry. To demonstrate the time-resolved capabilities of the instrument, we study the 193 nm photodissociation of SO2 via the C̃(1B2) ← X̃(1A1) transition. In addition to the well-documented O(3Pj) + SO(3Σ-) channel, we observe direct evidence for a small yield of S(3Pj) + O2(3Σg-) as a primary photodissociation product channel, which may impact sulfur mass-independent fractionation chemistry.

Facile synthesis of BaMoO4 and BaMoO4/BaWO4 heterostructures with type -I band arrangement and enhanced photoluminescence properties

A series of BaMoO4 and BaMoO4/BaWO4 phosphors were successfully prepared via a polyacrylamide gel method and low temperature calcination technology. The effects of sintering temperature and mass percentage of BaMoO4/BaWO4 on the phase purity, functional group, surface morphology, charge state, photoluminescence properties and photocatalytic activity of the prepared products were studied in detail. The results indicate that the BaMoO4 phosphor is a scheelite tetragonal structure with high crystallinity. The photoluminescence spectra indicates that the phosphors have a strong blue emission peak at 440 nm with excitation wavelength of 282 nm for the BaMoO4 phosphor, and three emission peaks at 400, 440 and 460 nm with excitation wavelength of 284 nm for the BaMoO4/BaWO4 phosphors. These photoluminescence behaviors can be ascribed to the 1T2→1A1 transition, Jahn–Teller distorted tetrahedral symmetry of [MoO4]2- and surface defect. Photocatalytic experiments further confirmed that the BaMoO4/BaWO4 phosphors exhibit a high recombination rate of electron hole pairs. The result further indicates that the type-I band arrangement structure of BaMoO4/BaWO4 phosphors is beneficial to enhance the photoluminescent properties of single-phase phosphors. This study provides a novel route for preparing the type-I band arrangement structure composite phosphors with high photoluminescent properties and potential applications in light emitting devices, optoelectronic devices, laser devices and white pigments.

Luminescent properties of Na2GdMg2(VO4)3:Eu3+ red phosphors for NUV excited pc-WLEDs

Herein, a series of novel Na2GdMg2(VO4)3:Eu3+ (NGMVO:Eu3+) red phosphors were elaborated by conventional solid-state reaction process. Their structural features, luminescent properties, energy transfer were researched at length. XRD patterns indicate that NGMVO:Eu3+ crystallized in single cubic garnet structure. Under the excitation of near ultraviolet light at 356 nm, the emission spectra of NGMVO host could be divided in two parts that resulted from 3T2 → 1A1 and 3T1 → 1A1 transitions of VO43−. While NGMVO:Eu3+ phosphors show intense sharp red emission peaks including 590, 610, 652 and 706 nm that originated from 5D0 → 7FJ (J = 1–4) transitions of Eu3+, respectively. The optimal concentration of Eu3+ is 0.7. Importantly, NGMVO:0.7Eu3+ sample presents high energy transfer efficiency (89 %) and high external quantum efficiency (48.3 %). Besides, its emission intensity remains 79 % at 420 K compared with that at 300 K, proving the good thermal stability of phosphors. All above results suggest that NGMVO:Eu3+ red phosphors have latent applications in white light emitting diodes.

KCa2Mg2V3O12: A novel efficient rare-earth-free self-activated yellow-emitting phosphor

The development of efficient near-ultraviolet (near-UV) excitable inorganic phosphors is highly important for application in near-UV-pumped white light-emitting diodes (LEDs). Herein, we reported on a novel efficient rare-earth-free yellow-emitting KCa2Mg2V3O12 vanadate phosphor. These phosphors were prepared by using a conventional high-temperature solid-state reaction method. Photoluminescence study revealed that KCa2Mg2V3O12 phosphor exhibited a broad excitation band in the 250−420 nm wavelength range with a peak at 346 nm, owing to the absorption of [VO4]3− groups. Upon 346 nm excitation, KCa2Mg2V3O12 phosphor showed bright yellow emission in the 400−800 nm range with a maximum at 540 nm and a full width at half maximum of 161 nm. Such self-activated broadband yellow luminescence was attributed to the 3T2→1A1 and 3T1→1A1 transitions of [VO4]3− groups. The Commission Internationale de l´Eclairage (CIE) color coordinates were calculated to be (0.3741, 0.4679), which were located in the yellow region in the CIE chromaticity diagram. Impressively, the internal quantum efficiency of KCa2Mg2V3O12 phosphor was measured to be as great as 41 %. In addition, the temperature-dependent emission spectra were recorded, and the activation energy Ea was determined to be 0.366 eV. The newly developed KCa2Mg2V3O12 phosphor might have application potential as color converter in white LEDs.

High-resolution spectroscopy of the [formula omitted] and [formula omitted] bands in the [formula omitted] transition of [formula omitted

We present a high-resolution spectroscopic study of the A~1B2-X~1A1 transition of SiC2 by laser-induced fluorescence. The gas-phase SiC2 molecules are produced in a discharge plasma and cooled in a supersonic jet expansion. By employing a home-made single longitudinal mode optical parametric oscillator as the narrow bandwidth laser source, laser excitation spectra have been recorded with a transition frequency accuracy of ~0.004 cm-1 for the 000,201,301 and 311 bands. Rotational analyses of the high-resolution data have yielded accurate spectroscopic constants, including the centrifugal distortion constants, for the upper vibronic states studied here.

Luminescence Behavior of GdVO4: Tb Nanocrystals in Silica Glass-Ceramics

Glass ceramics with GdVO4: Tb nanocrystals impregnated in the highly transparent silica glass were prepared by the porous glass and sintering process and confirmed by XRD, Raman spectrum, and TEM. Spectral analysis shows that there are multifarious energy transfer processes in GdVO4: Tb nanocrystals, such as VO43− → Tb3+, Gd3+ → VO43−, Gd3+ → VO43− → Tb3+, and Gd3+ → Tb3+, and the main one is VO43− → Tb3+. In this process, 3T1,2 → 1A1 transition of VO43− transfers the energy to Tb3+ and generates 5D3 → 7F6,5,4,3,2 and 5D4 → 7F5 transitions of Tb3+. The energy transfer efficiency is 29.5%, and the excitation wavelength range of Tb3+ ions can be shifted from 230–260 nm to 280–365 nm. This shows that GdVO4 can effectively change the excitation wavelength of Tb3+, which is conducive to the application of Tb3+ ions excited by LED light sources.

A Novel Highly Efficient Host‐Sensitized Red Emitting (Ba2YV3O11 : Eu3+) Phosphor for Hybrid White LEDs

AbstractThe present investigation focused on the sequence of Eu3+ activated red‐emitting vanadate phosphor for the high performance white LEDs. The blue emitting host lattice (Ba2YV3O11) and red emitting Ba2YV3O11 : Eu3+ phosphors were synthesized by the conventional solid‐state method and studied their optical properties in details. The host lattice and Eu3+ activated red phosphors were crystallized in the monoclinic crystal system. The Ba2YV3O11 emission was found in the bluish‐green region (444 and 512 nm, which corresponds to the 3T2→1A1 and 3T1→1A1 transitions), and the Eu3+ activated phosphors (Ba2Y1‐xEuxV3O11, x=0‐1, insteps of 0.1) display intense red emission at 618 nm due to the 5D0→7F2 electric dipole (ED) transition. The concentration quenching was observed at x=0.6, the detailed energy transfer mechanism was also explored. Thermal stability of the Ba2Y0.4Eu0.6V3O11 red phosphor retains the ∼60 % emission intensity at 423 K, and the internal quantum efficiency (IQE) was found to be 30 %. The red and white LEDs were fabricated by using the synthesized red phosphor with the combination of near UV LED (n‐UV), the white LED showed white emission with CIE is x=0.359; y=0.395; higher CRI (89) and low CCT (4693 K) values. These results prescribe that the presently synthesized phosphor can act as a suitable red‐emitting component in hybrid white LEDs.

Electronic excitation of H2O by positron impact

The electronic excitation of H2O molecule by positron impact for the (1b1→4a1)1B1 and (3a1→4a1)1A1 transitions has been calculated from threshold to 60 eV. The calculations were performed with the Schwinger multichannel method considering two and three channel-coupling schemes. We have found that the electronic excitation cross sections induced by positron impact show negligible dependence with the number of collision channels taken into account and seem to be always smaller or at most of the same magnitude than the corresponding ones for electrons using similar representations for the target excited states. The calculated cross sections exhibit lower magnitude when compared to the experimental data of Tattersall et al. [W. Tattersall, L. Chiari, J.R. Machacek, E. Anderson, R.D.White, M.J. Brunger, S.J. Buckman, G. Garcia, F. Blancoand J.P. Sullivan, J. Chem. Phys. 140, 044320 (2014)], while showing good agreement with the excitation cross sections recommended by Blanco et al. [F. Blanco, A.M. Roldan, K. Krupa, R.P. McEachran, R.D. White, S. Marjanovic, Z. Lj. Petrovic, M.J. Brunger, J.R. Machacek, S.J. Buckman, J.P. Sullivan, L. Chiari, P. Limao-Vieira, G. Garcia, J. Phys. B: At. Mol. Opt. Phys. 49, 145001 (2016)] for energies above 30 eV. Perspectives to go beyond this prelusive investigation on positron-H2O electronic excitation, such as inclusion of target polarization effects, are discussed.

Characterization of the Ground States of BeC2 and BeC2- via Photoelectron Velocity Map Imaging Spectroscopy.

Due to their potentially unique properties, beryllium carbide materials have been the subject of many theoretical studies. However, experimental validation has been lacking due to the difficulties of working with Be. Neutral beryllium dicarbide has been predicted to have a T-shaped equilibrium structure (C2v), while previous quantum chemistry calculations for the structure of the anion had not yielded consistent results. In this study, we report photoelectron velocity map imaging spectra for the BeC2- X 2A1 → BeC2 X 1A1 transition. These data provide vibrational frequencies and the electron affinity of BeC2. Ab initio electronic structure calculations, validated against the experimental data, show that both the anion and the neutral form have C2v equilibrium geometries with polar covalent bonding between Be and the C2 subunit. Computed vibrational frequencies and the electron affinity, obtained at the CCSD(T) level of theory, were found to be in good agreement with the measurements.

Two-layer Gaussian-based MCTDH study of the S1 ← S vibronic absorption spectrum of formaldehyde using multiplicative neural network potentials

The absorption spectrum of the vibronically allowed S1(1A2) ← S0(1A1) transition of formaldehyde is computed by combining multiplicative neural network (NN) potential surface fits, based on multireference electronic structure data, with the two-layer Gaussian-based multiconfiguration time-dependent Hartree (2L-GMCTDH) method. The NN potential surface fit avoids the local harmonic approximation for the evaluation of the potential energy matrix elements. Importantly, the NN surface can be constructed so as to be physically well-behaved outside the domain spanned by the ab initio data points. A comparison with experimental results shows spectroscopic accuracy of the converged surface and 2L-GMCTDH quantum dynamics.

The excited-state structure and photophysics of isolated acenaphthylene

We report a picosecond time-resolved photoionization and photoelectron spectroscopic study of acenaphthylene, C12H8 in a free jet. The origin for the A 1B2 (S1) ← X 1A1 (S0) transition is found at 21,757 ± 15 cm−1, while the one for the B 2A1 ← X 1A1 transition is observed at 30,076 ± 27 cm−1. For both transitions vibrational bands are resolved and assigned. For the origin of the S1 state a lifetime of 380 ps is measured, which decreases with excess energy. Internal conversion to the electronic ground state seems to be the dominant deactivation pathway. For the S2 state an additional component with a very short lifetime of 1 ps or less is observed. This indicates a two-step deactivation via S1 to the electronic ground state. A small and unstructured signal in the photoelectron spectra at long delay times suggests the presence of an additional deactivation channel, most likely intersystem crossing.

Ro-vibronic transition intensities for triatomic molecules from the exact kinetic energy operator; electronic spectrum for the C̃ 1B2 ← X̃ 1A1 transition in SO2.

A procedure for calculating ro-vibronic transition intensities for triatomic molecules within the Born-Oppenheimer approximation is reported. Ro-vibrational energy levels and wavefunctions are obtained with the DVR3D suite, which solves the nuclear motion problem with an exact kinetic energy operator. Absolute transition intensities are calculated both with the Franck-Condon approximation and with a full transition dipole moment surface. The theoretical scheme is tested on C̃ 1B2 ← X̃ 1A1 ro-vibronic transitions of SO2. Ab initio potential energy and dipole moment surfaces are generated for this purpose. The calculated ro-vibronic transition intensities and cross sections are compared with the available experimental and theoretical data.

Structural and luminescence properties of self-yellow emitting undoped and (Ca, Ba, Sr)-doped Zn2V2O7 phosphors synthesized by combustion method

A self-activated yellow emitting Zn2V2O7 was synthesized by combustion method. The influence of the processing parameters such as synthesis temperature and dopants concentration on the structure, morphology and luminescence properties was investigated. The X-ray diffraction (XRD) analysis confirmed that the samples have a tetragonal structure and no significant structural change was observed in varying both the synthesis temperature and the dopants concentration. The estimated average crystallite size was 78nm for the undoped samples synthesized at different temperatures and 77nm for the doped samples. Scanning electron microscope (SEM) images showed agglomerated hexagonal-shaped particles with straight edges at low temperatures and the shape of the particles changed to cylindrical structures at moderate temperatures. At higher temperatures, the morphology changed completely. However, the morphologies of the doped samples looked alike. The photoluminescence (PL) of the product exhibited broad emission bands ranging from 400 to 800nm. The best luminescence intensity was observed for the undoped Zn2V2O7 samples and those synthesized at 600 ℃. Any further increase in synthesis temperature, type and concentration of dopants led to a decrease in the luminescence intensity. The broad band emission peak of Zn2V2O7 consisted of two broad bands corresponding to emissions from the Em1 (3T2→1A1) and Em2 (3T1→1A1) transitions.

Dopant Concentration induced optical changes in Ba1−xEuxMoO4: A green and facile approach towards tunable photoluminescent material

Ba1−xEuxMoO4 phosphors were synthesized by complex polymerization method. The prepared phosphor material was extensively characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetry (TG) and differential thermal analysis (DTA). The introduction of europium upto 2mol% doesn’t change the crystal structure. The influence of Eu concentration on the luminescent properties of phosphors was analyzed. It was seen that, under UV irradiation, the pure BaMoO4 sample shows multicolor emission. The blue and green emission was attributed to 1T2 → 1A1 transition within MoO42− moeity (shallow defects), whereas the red- orange emission to the defect MoO3 group (deep defects). The intensity of magnetic dipole transition (MDT) was found to be lower than that of electric dipole transition (EDT) which indicates the low -⁠ symmetry acentric environment of Eu3+ ions. Based on Stark splitting, the actual site symmetry of Eu3+ in BaMoO4 was clarified as C6v; reduced from actual S4 for Ba2+ ion in BaMoO4. The decay curve shows two different lifetime value 11.2 µs (τ1) (host contribution) and 693 µs (τ2) (europium sitting at Ba2+ site) with magnitudes 14% and 86% respectively. The value of host dopant energy transfer efficiency calculated under forbidden f-f transition excitation was found to be lower at 12.6% as compared to 76% in case of excitation by allowed charge transfer band. To find out the concentration quenching mechanism, the critical energy-transfer distance for the Eu3+ ions was evaluated to be 33.81Å, which was found to be greater than 10Å and signify electric multipolar interactions as the only mode of energy transfer The intensity of peaks corresponds to EDT was found to be high in comparison to the peak due to MDT in all the Eu- doped samples: indicating the asymmetric local site symmetry around Eu3+. This is also reflected in the JO (Judo Ofelt) parameter trend; at all concentrations, Ω2 value is greater than Ω4 indicating the high covalency of the Eu3+ ligand bonding and asymmetric environment around europium ion in BaMoO4, which is highly probable because it occupies the asymmetric Ba2+ position.