Site-selective Excitation Research Articles

Luminescence spectra and site-occupation of Eu3+ centres in lithium antimonate LiSbO3 lattice

It is a challenge to accurately determine the exact location of the rare earth ion (RE) in the lattice when there are significant differences in radius, electronegativity and valence between the lattice cation and the corresponding RE activator in a host material. This study presents the findings on the luminescence and site occupancy of Eu3+ emission centers in the lithium antimonate LiSbO3 lattice. The phase-formation, structures, elemental composition, band transition characteristics, luminescence properties, and lifetimes of the Eu3+-doped phosphors were investigated. Eu3+-doped LiSbO3 exhibits a typical 5D0→7F2 red luminescence transition at 613 nm. Site-selective excitation, luminescence, and decay properties at the 7F0→5D0 transition wavelength were measured using a tunable pulsed narrow-band dye laser. Two Eu3+ luminescence centers in LiSbO3 were identified across the temperature range from 10 K to 300 K. The doping of RE(Eu3+) in LiSbO3 consistently occupies two crystallographic positions of Li+ and Sb5+. Furthermore, the probability of RE(Eu3+) ions preferentially occupying the Li+ (M) sites is notably high. The decay curves and luminescence lifetimes of the two Eu3+ centers were determined. Drawing from the experimental data, we discuss the potential defects induced by Eu3+ doping in LiSbO3 and the mechanism of charge compensation. These results could be instrumental in the advancement of new RE-activated luminescent materials or serve as a reference for investigating the specific positioning of RE ions within a phosphor lattice.

Site-Selective Excitation of Ti3+ Ions in Rutile TiO2 via Anisotropic Intra-Atomic 3d → 3d Transition.

Site-selective excitation (SSE), which is usually realized by tuning the wavelength of absorbed light, is an ideal way to study bond-selective chemistry, analyze the crystal structure, investigate protein conformation, etc., eventually leading to active manipulation of desired processes. Herein, SSE has been explored in (110)-, (100)-, and (011)-faced rutile TiO2, a prototypical material in both surface science and photocatalysis fields. Using ultraviolet photoelectron spectroscopy and photon energy-, substrate orientation-, and laser polarization-dependent two-photon photoemission spectroscopy (2PPE), intra-atomic 3d → 3d transition from the split Ti3+ 3d orbitals, i.e., band gap states and excited states at ∼1.00 eV below and ∼2.40 eV above the Fermi level, respectively, has been proven for all of the samples, suggesting that it is a common property of this material. The distinct structure of rutile TiO2 results in the anisotropic 3d → 3d transitions with the transition dipole moment along the long axes ([110] and [11̅0]) of TiO6 blocking units. This anisotropy facilitates the selective excitation of Ti3+ ions in the two types of TiO6, which cannot be realized by conventional wavelength tuning, via polarization alignment of the excitation source. Discovery in this work builds the foundation for future investigation of site-selective photophysical and photochemical processes and eventually possible active manipulation in this material at the atomic level.

Cooperative luminescence of Yb3+ ions in multisite YAM crystal

A series of Yb3+-doped yttrium aluminum monoclinic Y4Al2O9 (Yb:YAM) single crystals with Yb3+ ion concentrations of 0.1, 1, 5 and 10 at.% was grown by means of micro pulling down method. Blue emission was observed in Yb3+ activated YAM crystals under infrared site-selective laser excitation. Through the excitation and emission spectra, decay time measurements and excitation intensity dependence the process responsible for this anti-Stokes emission was assigned to the cooperative deexcitation of two Yb3+ ions. The results show that at 10% Yb3+ dopant concentration investigated system could be considered as single-site system in which cooperative luminescence is dominated by the energetically lowest Yb3+ site. Theoretical cooperative emission spectrum and the cooperative luminescence rate of Yb3+:YAM were calculated.

Intrinsic- and Rare Earth Ion-Activated Luminescence in NbAlO4 with Wide Structure Channels.

Compounds with ordered and interconnected channels have versatile multifunctional applications in technological fields. In this work, we report the intrinsic- and Eu3+-activated luminescence in NbAlO4 with a wide channel structure. NbAlO4 is an n-type semiconductor with an indirect allowed transition and a band-gap energy of 3.26 eV. The conduction band and valence band are composed of Nb 3d and O 2p states, respectively. Unlike the common niobate oxide Nb2O5, NbAlO4 exhibits efficient self-activated luminescence with good thermal stability even at room temperature. The AlO4 tetrahedron effectively blocks the transfer/dispersion of excitation energy between NbO6 chains in NbAlO4, allowing for effective self-activated luminescence from NbO6 activation centers. Moreover, Eu3+-doped NbAlO4 displayed a bright red luminescence of 5D0 → 7F2 transition at 610 nm. The site-selective excitation and luminescence of Eu3+ ions in a spectroscopic probe were utilized to investigate the doping mechanism. It is evidenced that Eu3+ is doped in the structure channel in NbAlO4 lattices, not in the normal cation sites of Nb5+ or Al3+. The experimental findings are valuable in developing new luminescent materials and improving the understanding of the material's channel structure.

Probing of DNA Photochemistry with C-Nucleosides of Xanthones and Triphenylene as Photosensitizers To Study the Formation of Cyclobutane Pyrimidine Dimers.

The direct and sequence-dependent investigation of photochemical processes in DNA on the way to cyclobutane pyrimidine dimers (CPDs) as DNA damage requires the probing by photochemically different photosensitizers. The C-nucleosides of xanthone, thioxanthone, 3-methoxyxanthone, and triphenylene as photosensitizers were synthesized by Heck couplings and incorporated into ternary photoactive DNA architectures. This structural approach allows the site-selective excitation of the DNA by UV light. Together with a single defined site for T-T dimerization, not only the direct CPD formation but also the distance-dependent CPD formation in DNA as well as the possibility for energy transport processes could be investigated. Direct CPD formation was observed with xanthone, 3-methoxyxanthone, and triphenylene as sensitizers but not with thioxanthone. Only xanthone was able to induce CPDs remotely by a triplet energy transfer over up to six intervening A-T base pairs. Taken together, more precise information on the sequence dependence of the DNA triplet photochemistry was obtained.

Multipartite-entanglement generation in coupled microcavity arrays

We consider photonic arrays made from quantum emitters in optically coupled microcavities as a platform for entanglement generation. These offer a large degree of tunability with the possibility of site-selective optical excitation. Coherent pumping is considered to drive transitions between vacuum and entangled target states both in a time-dependent manner, and in a quantum bath engineering approach to create entanglement in the steady-state. We demonstrate a numerical scheme that allows to generalize the determination of excitation parameters to larger array sizes and different classes of entangled states. This study is a step towards using coupled cavity arrays as a hardware platform in novel quantum-photonic applications in quantum computing and quantum machine learning.

Reveal luminescence differences via comparative studies of dynamic spectra in Eu3+-activated BaLa2WO7 and SrLa2WO7 phosphors

Eu3+-doped BaLa2WO7 and SrLa2WO7 phosphors were prepared using the conventional solid-state reaction synthesis. The experiments have confirmed that the luminescence properties are completely different although they the isostructural monoclinic structure with a monoclinic space group of P21/b: compared with SrLa2WO7:Eu3+, BaLa2WO7:Eu3+ has stronger luminescence and higher thermal stability. The dynamic characteristics were studied via the laser site-selective excitation and luminescence technique. The two distinct Eu3+ centers Eu3+(A) and Eu3+(B) were confirmed corresponding to the substitution for La1 and La2, respectively. Eu3+(A) could transfer energy to Eu3+(B) in BaLa2WO7:Eu3+, but Eu3+(B) cannot realize back transfer to Eu3+(A). However, a mutual energy transfer could be achieved between Eu3+(A) and Eu3+(B) in SrLa2WO7:Eu3+ resulting in excitation energy diffusion. The energy dispersion may cause the excited electrons to be trapped in some defects and impurities, thereby reducing the luminescence effect. The results could help develop new reddish-orange emitting materials by introducing Eu3+ as a structural probe for doping sites and microstructure.

Zeeman and laser site selective spectroscopy of C1 point group symmetry Sm3+ centres in Y2SiO5: a parametrized crystal-field analysis for the 4f 5 configuration

Parametrized crystal-field analyses are presented for both the six and seven fold coordinated, C1 symmetry Sm3+ centres in Y2SiO5, based on extensive spectroscopic data spanning the infrared to optical regions. Laser site-selective excitation and fluorescence spectroscopy as well as Zeeman absorption spectroscopy performed along multiple crystallographic directions has been utilized, in addition to previously determined g tensors for the 6H5/2Z1 and 4G5/2A1 states. The resultant analyses give good approximation to the experimental energy levels and magnetic splittings, yielding crystal-field parameters consistent with the few other lanthanide ions for which such analyses are available.

(INVITED)High-resolution spectroscopic studies of random strains in ferroelastic domains in the LaAlO3:Tm3+ single crystal

Comprehensive spectroscopic, magnetization, and theoretical studies of a LaAlO3:Tm3+ single crystal in the ferroelastic R3‾c phase are reported. The Tm3+ ions substitute for the La3+ ions at sites with the D3 symmetry. High-resolution absorption, photoluminescence, and site-selective emission and excitation spectra were measured in the broad spectral range from 4000 to 28000 cm−1 at temperatures 4.2–5 K. The two-fold degeneracy of the ground state of Tm3+ was uncovered by magnetization measurements. Energies and symmetry properties of wave functions of crystal-field levels of the Tm3+ ions were determined and successfully reproduced by crystal-field calculations. Specific profiles with a dip at the center of spectral lines corresponding to transitions involving non-Kramers doublets give evidence for random strains in the studied multidomain sample. The value of 0.1–0.8 cm−1 of deformational splitting of non-Kramers doublets exceeds hyperfine splittings by more than an order of magnitude. The observed line shapes were successfully modeled assuming the interaction of Tm3+ ions with random deformations described by the generalized two-dimensional Lorentzian distribution with the width of (7 ± 0.5)⋅ 10−4. The simulation was performed using the electron-deformation coupling constants calculated in the framework of the exchange-charge model.

Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)

Zn1+xGa2−2xGexO4 (ZGGO:Cr)-persistent phosphor, with a molar fraction, x, of x = 0.1, doped with a 0.5% molar of chromium, was synthesised via solid-state reaction at 1350 °C for 36 h. X-ray diffraction measurements and Raman spectroscopy evidence a single crystalline phase corresponding to the cubic spinel structure. Room temperature (RT) photoluminescence (PL) and afterglow decay profiles were investigated using above and below bandgap excitation. In both cases, persistent PL was observed for almost 8 h, mainly originating from a Cr3+ defect, the so-called N2 optical centre. RT PL excitation and diffuse reflectance allow identification of the best pathways of Cr3+ red/NIR emission, as well as estimation of the ZGGO bandgap energy at 4.82 eV. An in-depth investigation of the observed luminescence at 15 K and temperature-dependent PL under site-selective excitation reveals the spectral complexity of the presence of several optically active Cr3+ centres in the ZGGO host that emit in almost the same spectral region. Furthermore, the temperature dependence of the R-lines’ intensity indicates the existence of thermal populating processes between the different optical centres. Such observations well account for a wide distribution of defect trap levels available for carrier capture/release, as measured by the persistent luminescence decay, from which the carriers are released preferentially to the N2 Cr3+-related optical centre.

Intramolecular hydrogen transfer in DNA induced by site-selective resonant core excitation.

We present experimental evidence for soft X-ray induced intramolecular hydrogen transfer in the protonated synthetic tri-oligonucleotide d(FUAG) in the gas-phase (FU: fluorouracil). The trinucleotide cations were stored in a cryogenic ion trap and exposed to monochromatic synchrotron radiation. Photoionization and photofragmentation product ion yields were recorded as a function of photon energy. Predominanly glycosidic bond cleavage leading to formation of nucleobase-related fragments is observed. In most cases, glycosidic bond cleavage is accompanied by single or double hydrogen transfer. The combination of absorption-site-sensitive soft X-ray spectroscopy with fragment specific mass spectrometry allows to directly relate X-ray absorption site and fragmentation site. We observe pronounced resonant features in the competition between single and double hydrogen transfer towards nucleobases. A direct comparison of experimental data with time-dependent density functional theory calculations, using short range corrected hybrid functionals, reveal that these hydrogen transfer processes are universal and not limited to population of particular excited states localized at the nucleobases. Instead, hydrogen transfer can occur upon X-ray absorption in any nucleobase and in the DNA backbone. Resonances seem to occur because of site-selective suppression of hydrogen transfer channels. Furthermore, non-covalent interactions of the optimized ground state geometries were investigated to identify intramolecular hydrogen bonds along which hydrogen transfer is most likely.

KLaF4:Nd3+doped transparent glass-ceramics processed by spark plasma sintering

Transparent oxyfluoride glass-ceramics (GCs) containing KLaF4 nanocrystals (NCs) doped with Nd3+were prepared by Spark Plasma Sintering (SPS). Glass powder pellets were sintered under a vacuum atmosphere, optimizing the processing parameters such as particle size, pressure, temperature, and holding time to obtain full densification. Transparency decreased when decreasing the particle size due to higher carbon contamination from the die. The α-KLaF4 crystalline phase was identified by X-ray diffraction (XRD) and its average crystal size was 10–20 nm. High-resolution transmission electron microscopy (HR-TEM) confirmed the presence of KLaF4 nanocrystals with incorporated Nd3+ions. Low-temperature site-selective emission and excitation spectra of Nd3+ions confirmed that α- KLaF4 was the predominant polymorph, although a minor presence of β- KLaF4 was also demonstrated.

Light Harvesting from Oxygen Vacancies and A- and B-Site Dopants in CaSnO3 Perovskite through Efficient Photon Utilization and Local Site Engineering

Significant research has been carried out in ABO3 perovskites during last decades in tailoring their luminescence properties for advanced optoelectronics. However, research related to exploring contribution of defects, both cation and anion vacancies in photoluminescence for multicolor photon emission has never been explored. Here, in this work, we have harnessed the full gamut of light emission from violet-blue to deep red originating from oxygen vacancies (OVs) in undoped CaSnO3 (CSO) and Eu@CaO8 and Eu@SnO6 sites in the doped CaSnO3:Eu3+ (CSOE) perovskite. Any contribution of Sn2+ and Eu2+, respectively, in photoluminescence of CSO and CSOE has been completely ruled out through X-ray absorption near-edge structure. Synchrotron radiation-based extended X-ray absorption fine structure and positron annihilation lifetime spectroscopy showed simultaneous distribution of Eu3+ ions at both asymmetric CaO8 and symmetric SnO6 sites owing to a very high negative formation energy value of −6.56 eV for the same. By efficient photon utilization arising from two different charge transfer bands, axial oxygen of SnO6 octahedra O1 → Eu3+ CTB (∼250 nm) preferentially transfers energy to Eu@CaO8 leading to intense electric dipole transition, whereas equatorial oxygen of SnO6 octahedra O2 → Eu3+ CTB (∼300 nm) excites Eu@SnO6 leading to intense magnetic dipole transition. This analogy is confirmed by DFT, wherein it was found that the calculated energy difference between the valence band maxima and conduction band minima is 4.77 eV (∼250 nm) and 4.3 eV (∼300 nm) for Eu@Ca and Eu@Sn sites, respectively. This concept of site-selective excitation and local site engineering can be applied to a variety of optical materials, which provides a unique strategy for tuning other properties of multifunctional crystals that are highly sensitive to the dopant’s local environment.

Combined spectroscopic and DFT studies of local defect structures in beta-tricalcium phosphate doped with Nd(III)

We investigated the distribution of the Nd3+ impurity centers over five Ca2+ positions in the β-Ca3(PO4)2 lattice (space group R3c) using low-temperature site-selective laser spectroscopy and fluorescence kinetics measured by double spectral selection in combination with the results of periodic density functional theory (DFT) calculations. Site-selective fluorescence excitation spectra of the Nd3+ ion were measured by fine tuning the laser wavelength in the spectral band of the 4I9/2(1) → 4G5/2(1,2) transitions and by fluorescence detection in the spectral band of the 4F3/2(1) → 4I9/2(1,2) transitions. The kinetics of nonradiatiative energy transfer from the 4F3/2(1) crystal field (CF) level was used to probe the local structure of Nd3+ sites. Theoretical modelling of the replacement of Ca2+ ions by Nd3+ ions with different charge compensation schemes was carried out. In the context of heterovalent substitution, hydrothermal synthesis conditions, and charge-balance mechanisms, two structural models of substitution have been proposed and thoroughly studied: substitution by a single Nd3+ ion accompanied by a trapped hydroxyl group, and substitution with a pair of Nd3+ ions. Numerical modeling of possible substitution scenarios calculated for a large number of combinations of cation-exchange sites in the lattice was carried out. The results obtained were classified according to the parameters of the accommodation of impurities and the energies of defect formation. The most favorable configurations of the distribution of impurity Nd3+ cations in the β-Ca3(PO4)2 lattice were predicted. By combining the “energy transfer probe” analysis and the results of the structural modeling, a relationship was found between the local geometry and the spectral and kinetic properties of luminescence of the Nd3+ optical centers. This allowed us to accurately determine the local structural geometries of eight experimentally detected the Nd3+ optical centers among the various possibilities in the distribution of the dopant ions over five cationic positions in β-Ca3(PO4)2. Six optical centers represent different types of Nd3+ - Nd3+ pairs, the formation of which is energetically favorable for charge compensation according to the 3Ca2+ → 2Nd3+ + ⎕ scheme, when one calcium position remains vacant. Two optical centers are single centers of the Nd3+ ion in which the charge is compensated by negatively charged hydroxyl ions intercalated during hydrothermal synthesis according to the scheme Ca2+→ Nd3+ + OH−. It was assumed that these pairs are located in several structural cavities free of PO4 tetrahedra.

Crystallization Process and Site-Selective Excitation of Nd3+ in LaF3/NaLaF4 Sol–Gel-Synthesized Transparent Glass-Ceramics

In this study, transparent oxyfluoride glass-ceramics (GCs) with NaLaF4 nanocrystals (NCs) were prepared by the sol–gel method for the first time. Three different molar ratios of La(CH3COO)3/Na(CH3COO) were used to obtain the GCs, which were sintered at 450, 550 and 650 °C for 1 min. X-ray diffraction (XRD) was employed to follow the evolution of the xerogel during the heat treatments and to study crystal growth for the three temperatures. In all cases, the LaF3 crystalline phase was present, but crystallization of NaLaF4 was only promoted at 650 °C. Thermogravimetric and thermodifferential analysis (TGA-DTA) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the crystallization process. High-resolution transmission electron microscopy (HRTEM) was employed to confirm NaLaF4 crystallization and determine the size distribution. The incorporation of Nd3+ ion into NaLaF4 and LaF3 nanocrystals was confirmed by site-selective emission and excitation spectra. The Nd3+ emission intensities in both phases depend not only on the NaLaF4/LaF3 ratio but also on their emission efficiencies.

Nd3+doped- SiO2–KLaF4 oxyfluoride glass-ceramics prepared by sol-gel

Neodymium (Nd3+) doped SiO2–KLaF4 oxyfluoride glass-ceramic (GCs) containing 10 mol% and 20 mol% of KLaF4 nanocrystals (NCs) were prepared for the first time by sol-gel and sintered at 550 °C. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were used to analyze the evolution of the xerogel during the heat treatment, high-resolution transmission electron microscopy (HRTEM) was employed to study the formation and distribution of NCs in the silica matrix. The decomposition of trifluoroacetic acid (TFA) and the crystallization of fluoride nanocrystals were also confirmed. The presence of α-KLaF4 and β-KLaF4 crystalline phases with maximum crystal sizes of 24 nm and 17 nm respectively were clearly observed after heat treatment at 550 °C for 1 min. Site-selective emission and excitation spectra confirmed the incorporation of Nd3+ ions in both crystalline phaseswith emission of Nd3+ most predominant in the β-KLaF4 NCs.

Site selective substitution of Eu3+ and its effects on structural and luminescent properties of monoclinic YP5O14 pentaphosphate

Y1-xEuxP5O14 (0 < x ≤ 1) pentaphosphate polycrystallines were synthesized by solid state reaction and the effects of Eu3+ content on the structure and luminescent properties were investigated. X-ray diffraction and Infrared spectroscopy studies revealed a sequence of the composition-driven structural phase transitions C2/c → P21/c; occur at the lower Eu3+ concentration (below 40 mol% of Eu3+ doping). The spectroscopic properties in UV-excitable range of the doped compounds were investigated. Site-selective laser excitation in the 5D2 level of Eu3+ ion was performed and luminescence decay times were measured at 30 K for 5 and 10%Eu3+ -doped YP5O14 samples. The obtained results clearly show that the Eu3+ ions are distributed over two distinct crystallographic sites. Careful analysis of the main 5D2→7FJ (J = 0, 2) transitions revealed a good agreement between the observed and expected components numbers for the local symmetry predicated upon the crystal structure of the monoclinic YP5O14. The spectroscopic results are discussed and correlated with the structural data.

Curium(III) and europium(III) as luminescence probes for plant cell (Brassica napus) interactions with potentially toxic metals

We have investigated the interaction of the actinide Cm(III) and its lanthanide homologue Eu(III) with cells of Brassica napus in suspension. This study combines biochemical techniques (plant cell response) with spectroscopic experiments to determine the chemical speciation of hazardous metals in contact with the plant cells. Experiments conducted over a period of 7 d showed that B. napus cells were able to bioassociate both potentially toxic metals in significant amounts up to 0.58 µmol Eu/gfresh cells and 1.82 µmol Cm/gfresh cells at 30 µM Eu(III) and 0.68 µM Cm(III), respectively. For Cm(III), a biosorption process could be identified as soon as 5 h post-exposure with 73 ± 4% of the Cm(III) bioassociated. Luminescence spectroscopy results based on UV and site-selective excitation confirmed the existence of three Cm(III)/Eu(III) [M(III)] species in both the supernatants and cells. The findings detailed herein support that M(III) coordinates to two kinds of carboxyl groups and phosphate groups.

Site-selective excitation and photoluminescence properties of a cyan-emitting NaGd9(SiO4)6O2: Bi3+ phosphor for potential application in white light-emitting diodes

Bismuth ions, as excellent non-rare-earth activators for luminescent materials, possess characteristic excitation in the near ultraviolet (n-UV) region rather than in the visible region and could avoid reabsorption as compared to traditional rare-earth (e.g., Eu2+, Ce3+) phosphors. In this work, a series of apatite-type NaGd9(SiO4)6O2: xBi3+ (NGSO) phosphors with a hexagonal crystal system were synthesized through high-temperature solid-solid reactions. They showed intense n-UV absorption and cyan emission of Bi3+ ions. The optimal Bi3+ doping concentration was 0.02. Further, there were two types of Bi3+ ions that provided seven- and nine-coordination in NGSO: xBi3+ phosphors, resulting in two luminescence centers at 462 and 558 nm, respectively. Bi3+ (Ⅰ) ions were dominant for the emission at 558 nm excited at a wavelength of 340 nm, and Bi3+ (Ⅱ) ions governed the luminescence at 462 nm excitation at 370 nm; energy transfer occurred from Bi3+ (Ⅰ) to Bi3+ (Ⅱ) ions. Finally, good thermal stability with activation energy (Ea) of 0.296 eV was demonstrated. These findings exhibit the potential application of NGSO: 0.02Bi3+ phosphors as cyan phosphors in n-UV white light-emitting diodes.

Inner shell photofragmentation of 2Cl-pyrimidine studied by mass spectrometry and electron–ion coincidence experiments

Photoelectron spectroscopy, mass spectrometry and electron–ion coincidence experiments combined with tunable synchrotron radiation have been used to study the decay and fragmentation of 2Cl-pyrimidine after Cl(2p), C(1s) and N(1s) excitations. The goal is to investigate how the state- and site-selected excitation and the chemical environment affect the fragmentation paths of the molecule and to make a comparison with fragmentation induced by direct valence ionization. It has been found that the site-selective inner shell excitation affects the branching ratio of the fragments, while the particular fragmentation channels of the cation are determined by the final state populated in the resonant decay of the core excited states. Effects of nuclear motion in the core excited states and the possible ultrafast molecular dissociation following the Cl(2p → σ *) core excitation are discussed.