Experimental Excitation Spectra Research Articles

Synthesis, Luminescence and Crystal Field Calculations of Mn4+- Doped Li2TiO3 Deep Red Phosphor

Red emission of the Mn4+ ions incorporated in different materials is used in various applications, such as lighting, holographic recording, thermoluminescence dosimetry, thermometry, etc. Mn4+ emission can improve color-rendering-index of phosphor-converted white-light-emitting diodes, and it is a promising candidate to replace the well-established rare-earth Eu activator in red phosphors. Here, synthesis, structure, detailed spectroscopic and crystal field analysis of Mn4+ doped Li2TiO3 are presented. We have successfully synthesized Li2Ti0.995Mn0.005O3 powder by a solid state synthesis. All precursors were mixed with stoichiometric ratio Li : Ti = 2 : 1 and calcined at 800⁰C for 5 h to produce pure-phase. Structural analysis was done by X-ray diffraction. It confirmed monoclinic Li2TiO3 structure with space group C2/2, in which the octahedral sites are occupied by Ti4+ and Mn4+. Optical characterization included measurements of photoluminescence excitation and emission spectra, emission decay analyses and diffuse reflection measurements. Optical spectra showed emission and absorption bands originating from d–d electronic transitions (d3 configuration). Kubelka-Munk function of the measured diffuse reflectance spectrum showed characteristic absorption of Mn4+. Deep red emission at 680 nm under λexc= 490 nm is a result of the spin-forbidden 2Eg→4A2 and 4T2→ 4A2 electron transition of the tetravalent manganese ions. The energy levels of the Mn4+ ions in Li2TiO3 were calculated using the crystal field exchange-charge model. Three parameters are needed to describe completely each energy level: the Racah parameters B and C, which determine the positions of the free ion energy levels, and the crystal field strength denoted by Dq, which describes the splitting of the free ion terms. These parameters were calculated and showed good agreement with the experimental photoluminescence excitation and emission spectra. Figure 1

Microscopic description of triaxiality in Ru isotopes with covariant energy density functional theory

The triaxiality in nuclear low-lying states has attracted great interests for many years. Recently, the reduced transition probabilities for levels near the ground state in $^{110}$Ru have been measured and provided strong evidences for a triaxial shape of this nucleus. The aim of this work is to provide a microscopic study of low-lying states for the Ru isotopes with $A\sim100$ and to examine in detail the role of triaxiality, and the evolution of quadrupole shapes with the isospin and spin degrees of freedom. The low-lying excitation spectra and transition probabilities of even-even Ru isotopes are described at the beyond mean-field level by solving a five-dimensional collective Hamiltonian with parameters determined by constrained self-consistent mean-field calculations based on the relativistic energy density functional PC-PK1. The calculated energy surfaces, low-energy spectra, intraband and interband transition rates, as well as some characteristic collective observables, such as $E(4_{\rm g.s.}^+)/E(2^+_{\rm g.s.})$, $E(2^+_\gamma)/E(4^+_{\rm g.s.})$, $B(E2; 2^+_{\rm g.s.}\to 0^+_{\rm g.s.})$, and $\gamma$ band staggerings are in a good agreement with the available experimental data. The main features of the experimental low-lying excitation spectra and electric transition rates are well reproduced, and thus strongly support the onset of triaxiality in the low-lying excited states of the Ru isotopes around $^{110}$Ru.

Investigation of the spectral properties and magnetism of BiFeO3 by dynamical mean-field theory

Using the local density approximation plus dynamical mean-field theory (LDA+DMFT), we have computed the valence-band photoelectron spectra and magnetic excitation spectra of ${\mathrm{BiFeO}}_{3}$, one of the most studied multiferroics. Within the DMFT approach, the local impurity problem is tackled by the exact diagonalization solver. The solution of the impurity problem within the LDA+DMFT method for the paramagnetic and magnetically ordered phases produces result in agreement with the experimental data on electronic and magnetic structures. For comparison, we also present results obtained by the $\mathrm{LDA}+U$ approach which is commonly used to compute the physical properties of this compound. Our LDA+DMFT derived electronic spectra match adequately with the experimental hard x-ray photoelectron spectroscopy and resonant photoelectron spectroscopy for Fe $3d$ states, whereas the $\mathrm{LDA}+U$ method fails to capture the general features of the measured spectra. This indicates the importance of accurately incorporating the dynamical aspect of electronic correlation among Fe $3d$ orbitals to reproduce the experimental excitation spectra. Specifically, the LDA+DMFT derived density of states exhibits a significant amount of Fe $3d$ states at the position of Bi lone pairs, implying that the latter are not alone in the spectral scenario. This fact might modify our interpretation about the origin of ferroelectric polarization in this material. Our study demonstrates that the combination of orbital cross sections for the constituent elements and broadening schemes for the spectral functions are crucial to explain the detailed structures of the experimental electronic spectra. Our magnetic excitation spectra computed from the LDA+DMFT result conform well with the inelastic neutron scattering data.

First‐principles and crystal‐field calculations of the electronic and optical properties of two novel red phosphors Rb2HfF6:Mn4+ and Cs2HfF6:Mn4+

AbstractThe electronic, structural, and optical properties of 2 red phosphors, Rb2HfF6:Mn4+ and Cs2HfF6:Mn4+, are evaluated using the first‐principles and crystal field theory methods. The calculated trigonal splitting of the Mn4+ orbital triplets perfectly matches the experimental excitation spectra. The structural and electronic properties of the mixed compound RbCsHfF6 are also studied theoretically. In the mixed compound, the inversion center symmetry around the Hf site is removed. This symmetry lowering may result in an increase in the Mn4+ 2E→4A2 zero phonon line (ZPL) intensity, which is very weak in the 2 end members. This finding may be of interest for increasing the phosphor luminosity. It is believed that such a mechanism of local site symmetry lowering by preparing solid solutions may be used for other systems as well, to gain ZPL intensity and perhaps to minimize thermal losses, eventually leading to improved phosphor materials.

First-principles calculation on the electronic structure and optical properties of Eu2+ doped γ-AlON phosphor

The crystal structure, electronic structure, and optical properties of Eu-doped γ-AlON at various Eu concentrations were obtained from density functional theory. Based on the calculated results, the luminescence properties and mechanism of Eu-doped γ-AlON are discussed. The calculated results demonstrate that AlON:Eu2+ phosphor exhibits a direct band gap, which is advantageous for luminescence. The absorption spectrum of AlON:Eu2+ phosphor has a single intense broad absorption band from 275 to 425nm with a peak at 355nm, which is consistent with corresponding experimental excitation spectra. The existence of Eu‒N bonds enhanced the local covalence of Eu2+, hence the optical stability of AlON:Eu2+ phosphor.

Shape-phase transitions in odd-mass γ -soft nuclei with mass A≈130

Quantum phase transitions between competing equilibrium shapes of nuclei with an odd number of nucleons are explored using a microscopic framework of nuclear energy density functionals and a particle-boson core coupling model. The boson Hamiltonian for the even-even core nucleus, as well as the spherical single-particle energies and occupation probabilities of unpaired nucleons, are completely determined by a constrained self-consistent mean-field calculation for a specific choice of the energy density functional and pairing interaction. Only the strength parameters of the particle-core coupling have to be adjusted to reproduce a few empirical low-energy spectroscopic properties of the corresponding odd-mass system. The model is applied to the odd-A Ba, Xe, La and Cs isotopes with mass $A\approx 130$, for which the corresponding even-even Ba and Xe nuclei present a typical case of $\gamma$-soft nuclear potential. The theoretical results reproduce the experimental low-energy excitation spectra and electromagnetic properties, and confirm that a phase transition between nearly spherical and $\gamma$-soft nuclear shapes occurs also in the odd-A systems.

Luminescence of Mn4+ ions in CaTiO3 and MgTiO3 perovskites: Relationship of experimental spectroscopic data and crystal field calculations

Herein, the synthesis, structural and crystal field analysis and optical spectroscopy of Mn4+ doped metal titanates ATiO3 (A = Ca, Mg) are presented. Materials of desired phase were prepared by molten salt assisted sol-gel method in the powder form. Crystallographic data of samples were obtained by refinement of X-ray diffraction measurements. From experimental excitation and emission spectra and structural data, crystal field parameters and energy levels of Mn4+ in CaTiO3 and MgTiO3 were calculated by the exchange charge model of crystal-field theory. It is found that crystalline field strength is lower (Dq = 1831 cm−1) in the rhombohedral Ilmenite MgTiO3 structure due to the relatively longer average Mn4+O2− bond distance (2.059 Å), and higher (Dq = 2017 cm−1) in orthorhombic CaTiO3 which possess shorter average Mn4+O2− bond distance (1.956 Å). Spectral positions of the Mn4+2Eg → 4A2g transition maxima is 709 nm in MgTiO3 and 717 nm in CaTiO3 respectively in good agreement with calculated values.

Long-range interactions between Rydberg atoms

We present an overview over theoretical models to describe adiabatic potential-energy curves, experimental excitation spectra, and electronic and nuclear dynamics in interacting Rydberg-atom pairs at large internuclear separations. The potential-energy curves and molecular wavefunctions are determined from the multipole expansion of the static Coulomb interaction which is evaluated numerically in a product basis of atomic orbitals. The convergence of this approach both in the truncation of the multipole expansion as well as in the size of the product basis is discussed, and the comparison of simulated excitation spectra is established as a useful criterium to test the convergence of the calculation. We finally discuss the dynamics of electronic and nuclear motions of pairs of Rydberg atoms, focusing on the stability of ultralong range Rydberg molecules with respect to autoionization.

Synthesis, Structure and Luminescence of Mn4+- Doped ATiO3 (A=Ca, Mg, Zn) Deep Red Phosphors

Red emission of the Mn4+ ions incorporated in different materials is used in various applications, such as lighting and holographic recording. Mn4+ emission can improve color-rendering-index of phosphor-converted white-light-emitting diodes, and it is a promising candidate to replace the well-established rare-earth Eu activator in red phosphors. Here, synthesis, structure, morphology, and detailed spectroscopic and crystal field analysis of Mn4+ doped metal titanates ATiO3 (A = Ca, Mg, Zn) are presented. Materials were prepared by combining sol-gel and molten salt methods. In the first step a typical sol-gel procedure is applied in which acetic acid, absolute ethanol, tetrabutyl titanate and metal compound were mixed at the molar ratio of 3:16:1:1 with appropriate amount of manganeze nitrate hexahydrate. In the second step, obtained dry gels were mixed with eutectic salt NaCl/KCl = 1:1 at molar ratio of 1:10, and then calcined at 750oC. Structural analysis was done by X-ray diffraction for all samples. Optical characterization included measurements of photoluminescence excitation and emission spectra, emission decay analyses and diffuse reflection measurements. Optical spectra showed emission and absorption bands originating from d–d electronic transitions (a d3 configuration). The energy levels of the Mn4+ ions in the crystal field of ATiO3 are calculated using the exchange charge model and showed nice matching with the experimental photoluminescence excitation and emission spectra.

Structure of fluorescent metal clusters on a DNA template.

Luminescent metal clusters are a subject of growing interest in recent years due to their bright emission from visible to near infrared range. Detailed structure of the fluorescent complexes of Ag and other metal clusters with ligands still remains a challenging task. In this joint experimental and theoretical study we synthesized Ag-DNA complexes on a DNA oligonucleotide emitting in violet- green spectral range. The structure of DNA template was determined by means of various spectral measurements (CD, MS, XPS). Comparison of the experimental fluorescent excitation spectra and calculated absorption spectra for different QM/MM optimized structures allowed us to determine the detailed structure of the green cluster containing three silver atoms in the stem of the DNA hairpin structure stabilized by cytosine-Ag+-cytosine bonds.

Order by virtual crystal field fluctuations in pyrochlore XY antiferromagnets

Conclusive evidence of order by disorder is scarce in real materials. Perhaps one of the strongest cases presented has been for the pyrochlore XY antiferromagnet Er2Ti2O7, with the ground state selection proceeding by order by disorder induced through the effects of quantum fluctuations. This identification assumes the smallness of the effect of virtual crystal field fluctuations that could provide an alternative route to picking the ground state. Here we show that this order by virtual crystal field fluctuations is not only significant, but competitive with the effects of quantum fluctuations. Further, we argue that higher-multipolar interactions that are generically present in rare-earth magnets can dramatically enhance this effect. From a simplified bilinear-biquadratic model of these multipolar interactions, we show how the virtual crystal field fluctuations manifest in Er2Ti2O7 using a combination of strong coupling perturbation theory and the random phase approximation. We find that the experimentally observed psi2 state is indeed selected and the experimentally measured excitation gap can be reproduced when the bilinear and biquadratic couplings are comparable while maintaining agreement with the entire experimental spin-wave excitation spectrum. Finally, we comments on possible tests of this scenario and discuss implications for other order-by-disorder candidates in rare-earth magnets.

In Depth Analysis on the Excitation Spectrum of SrS: Eu<sup>2+</sup>, Dy<sup>3+</sup>

The long persistent phosphor SrS: Eu2+, Dy3+ was prepared by hydrothermal method. The crystalline structure was characterized by using X-ray diffraction (XRD). The experimental excitation spectrum was carried out with Flurolog-3 fluorescent spectrometer. The results show that there are seven sharp sub-peaks on the wide band spectrum. This paper focuses on the origin of the seven peaks. Rhodamine B standard solution was used to confirmed that these peaks correspond to the light source (Xe lamp) emission in Flurolog-3 fluorescent spectrometer. The excitation spectrum of Eu2+ in SrS: Eu2+, Dy3+ was corrected with Rhodamine B. It was found that the corrected spectrum was quite different from the experimental spectrum.

Deep-Red Emitting Mn4+ Doped Mg2TiO4 Nanoparticles

Synthesis, structure, morphology, and detailed spectroscopic and crystal-field analysis of Mn4+ doped Mg2TiO4 nanoparticles (NPs) are presented. These Mg2TiO4:Mn4+ NPs are obtained through a Pechini-type polymerized complex route and calcination at 600 °C, and are approximately 10 nm in diameter and loosely agglomerated into 1-μm particles, as evidenced from transmission electron microscopy. These NPs exhibit strong, sharp red emission at 658 nm (with a 1.2 ms emission decay) as a result of the spin-forbidden 2Eg → 4A2g electron transition of the tetravalent manganese ions. No signatures of the presence of manganese ions in divalent or trivalent valence states are observed in the NPs with either photoluminescence or diffuse reflection spectroscopy. The energy levels of the Mn4+ ions in a trigonal crystal field of Mg2TiO4 are calculated using the exchange-charge model and are well matched with the experimental photoluminescence excitation and emission spectra. The absolute values of the calculated crystal-...

Ab initio Study on Spectral Properties of Charge-Compensated Ce3+ in NaF

We report an ab initio study of spectral properties of Ce3+ doped at Na+ site of the NaF crystal, with the charge imbalance compensated by two oxygen substitutions for fluoride (OF′) in the first coordination shell or two sodium vacancies (VNa′) in the second coordination sphere. Density functional theory calculations within the supercell model are first performed to optimize the local structures of the charge-compensated Ce3+, based on which Ce-centered embedded clusters are constructed and wave function-based CASSCF/CASPT2/RASSI-SO calculations are carried out to obtain the energies of 4f1 and 5d1 levels. By comparing the calculated 4f→5d transition energies with experimental excitation spectra at low temperatures, the lowest 4f→5d transition band peaked at 390 nm is assigned to the Ce3+ with charge compensation by two coordinating OF′ substitutions, rather than to the Ce3+ with compensation by two VNa′ vacancies, as proposed earlier. The electronic reason for the large redshift (by ∼8000 cm−1) of the lowest 4f→d transition as induced by the two nearby OF′ substitutions is analyzed in terms of the changes in the centroid shift and crystal-field splitting.

Hg-Xe exciplex formation in mixed Xe/Ar matrices: molecular dynamics and luminescence study.

Luminescence of Hg((3)P1) atoms trapped in mixed Ar/Xe matrices containing a small amount of Xe is reported. Broad emission bands, strongly red-shifted from absorption are recorded which are assigned to strong complexes formed between the excited mercury Hg* and xenon atoms. Molecular dynamics calculations are performed on simulated Xe/Ar samples doped with Hg to follow the behavior of Hg* in the mixed rare gas matrices leading to exciplex formation. The role of Xe atoms in the first solvation shell (SS1) around Hg was investigated in detail, revealing the formation of two kinds of triatomic exciplexes; namely, Xe-Hg*-Xe and Ar-Hg*-Xe. The first species exists only when two xenon atoms are present in SS1 with specific geometries allowing the formation of a linear or quasi-linear exciplex. In the other geometries, or in the presence of only one Xe in SS1, a linear Ar-Hg*-Xe exciplex is formed. The two kinds of exciplexes have different emission bands, the most red-shifted being that involving two Xe atoms, whose emission is very close to that observed in pure Xe matrices. Simulations give a direct access to the analysis of the experimental absorption, emission, and excitation spectra, together with the dynamics of exciplexes formation.

Quasi-bound states and continuum absorption background of polar Al0.5Ga0.5N/GaN quantum dots

A theoretical interpretation of the photoluminescence excitation spectra of self-organized polar GaN/(Al,Ga)N quantum dots is presented. A numerical method assuming a realistic shape of the dots and including the built-in electric field effects is developed to calculate their energy structure and hence their optical absorption. The electron and hole spectra show the existence of a set of quasi-bound states that does not originate from the wetting layer and plays a crucial role in the observed absorption spectrum of the GaN/(Al,Ga)N dots. Transitions involving these quasi-bound states and wetting layer states give a sufficient explanation for the observed continuum absorption background. The properties of this absorption band, especially its extension, depend strongly on the dot's size. Our simulation provides a natural explanation of the experimental luminescence excitation spectra of ensembles of dots of different heights. Our theoretical model can be convenient for future optical studies including systems with more complicated potentials.

Heterogeneity of Threadlike Shape of DNA-Stabilized Silver Fluorescent Clusters

DNA is widely used as a stabilizing matrix in the synthesis of fluorescent silver nanoclusters (AgNCs) possessing good biocompatibility and high brightness. We studied a group of threadlike shaped clusters up to 8 atoms having an elongated shape, so that each atom is connected with only one or two neighbors. Such the choice have been proved by recent calculations of electronic excitation spectra of threadlike shaped silver clusters showing intense the lowest transition, energy of which depends strongly on the chain bend [1]. These properties are in a good agreement with a special feature of the experimental luminescence excitation spectra of Ag clusters stabilized by polymer matrix. The formation of elongated forms of clusters necessarily involves several binding sites regularly arranged on DNA. Following this approach we calculate geometry and electronic excitation spectra of threadlike silver clusters bound to the minor groove of polyC oligomer containing an ordered arrangement of the carbonyl oxygens as binding sites using density functional theory method. We also performed MD calculations for polyC oligomer in water-ionic solution. Conformational dynamics of DNA backbone causes the different equilibrium states of the clusters chain bend, which provides a large variety of emitting species.1. Ruslan R. Ramazanov, Alexei I. Kononov. Excitation Spectra Argue for Threadlike Shape of DNA-Stabilized Silver Fluorescent Clusters. J. Phys. Chem. C, 2013, 117 (36), pp 18681-18687.

Structural, optical and crystal field analyses of undoped and Mn2+-doped ZnS nanoparticles synthesized via reverse micelle route

Zinc sulfide, both as a bulk material and in nanocrystalline form, is a valuable luminescent material with important applications. Doped ZnS nanoparticles of around 5nm are the material of choice for optoelectronic applications running in the UV region owing to their significant quantum size effect. This paper concerns detailed structural, spectroscopic and crystal field studies of ZnS nanoparticles, both pure and doped with Mn2+ ions, successfully synthesized at room temperature using a simple reverse micelle technique in the Triton X-100/cyclohexane medium. The resulting ZnS sphalerite phase small-size nanoparticles (3–5nm) have a much larger energy band gap (~4.7eV) than that reported for the bulk ZnS (3.6eV), thus confirming a pronounced quantum confinement effect. The electron paramagnetic resonance data provided evidence for the existence of two distinct environments for Mn2+ ions: the interior (core) and near the surface of the nanoparticles. The presence of an Mn2+-characteristic orange emission centered at 600nm confirmed that our samples were properly doped with Mn2+ ions, as the 4T1→6A1 radiation transition could arise only on the basis of Mn2+ ions incorporated into the ZnS nanoparticles. To the best of our knowledge, our finding include the longest decay time component for the orange emission ever observed, timed at about 3.3ms. The experimental excitation spectra were analyzed and the transitions assigned using the exchange charge model of theory of crystal field, which allowed to calculate the energy level scheme of the Mn2+ ions. The results presented in this paper provide us with detailed information about the ZnS sphalerite nanocrystals studied and can be readily applied to other similar systems.

Model of a GaAs Quantum Dot Embedded in a Polymorph AlGaAs Nanowire

We report on a numerical model of quasi-1-D and quasi-0-dimensional semiconductor heterostructures. This model is strictly based on experimental structures of cylindrical nanocolumns of AlGaAs grown by molecular-beam epitaxy in the (1 1 1) direction. The nanocolumns are of 20-50 nm in diameter and 0.5-1 μm in length and contain a single GaAs quantum dot of 2 nm in thickness and 15-45 nm in diameter. Since the crystal phase of these nanowires spontaneously switches during the growth from zincblende (Zb) to wurzite (Wz) structures, we implement a continuum elastic model and an eight-band k→ · p→ model for polymorph crystal structures. The model is used to compute electromechanical fields, wave-function energies of the confined states and optical transitions. The model compares a pure Zb structure with a polymorph in which the Zb disk of GaAs is surrounded by Wz barriers and results are compared to experimental photoluminescence excitation spectra. The good agreement found between theory and features in the spectra supports the polyphorm model.

Conformational Effects on the Lowest Excited States of Benzoyl-Pyrrolopyridazine: Insights from PCM Time-Dependent DFT

Time-dependent density functional theory (TD-DFT) computations and steady-state electronic spectroscopy measurements are performed on two recently synthesized pyrrolopyridazines to account for the detrimental effect of benzoyl substitution on the blue fluorescence emission. In the case of the highly fluorescent ester derivative, planar in ground state, we show that TD-DFT using the PBE0 and B3LYP hybrid functionals in the state-specific solvation approach provides an accurate description of absorption and emission properties. In benzoyl-pyrrolopyridazine, the (pretwisted) orientations of the benzoyl group and the solvent polarity are both found to modulate the nature of the lowest excited states. The first excited state has nπ* character at ground-state geometry of the main conformer (carbonyl group facing the diazine ring) in nonpolar solvents and become nearly degenerate with a ππ* state in polar solvents. The latter, lower than the nπ* state at the ground state geometry of a minor conformer, relaxes into a twisted intramolecular charge transfer. Experimental absorption and excitation spectra are consistent with the conformational-dependent picture of the lowest excited state (as derived from TD-DFT). A rather qualitative agreement in predicting the fluorescence emission wavelength is achieved in computations employing the CAM-B3LYP and BH&HLYP functionals, whereas global hybrids with low or moderate amounts of exact exchange exhibit the expected TD-DFT failure with up to 1 eV underestimated transition energies.