Twofold-coordinated Sn Centers Research Articles

Origin of light emission and enhanced Eu3+ photoluminescence in tin-containing glass

A barium-phosphate glass matrix was co-doped with SnO and Eu2O3 for investigating on material luminescent properties. Optical absorption and X-ray photoelectron spectroscopy (XPS) were employed in the characterization of tin species. The prevalence of divalent tin was indicated by the XPS data in accord with a conspicuous absorption band detected around 285 nm ascribed to twofold-coordinated Sn centers (isoelectronic with Sn2+). Photoluminescence (PL) excitation spectra obtained by monitoring Eu3+ emission from the 5D0 state revealed a broad excitation band from about 250 to 340 nm, characteristic of donor/acceptor energy transfer. Under excitation of such at 290 nm, the co-doped material exhibited a bright whitish luminescence, and a four-fold enhanced Eu3+ emission relative to a purely Eu-doped reference. Time-resolved PL spectra recorded under the excitation at 290 nm exposed a broad band characteristic of the twofold-coordinated Sn centers and emission bands of Eu3+ ions, which appeared well separated in time in accord with their emission decay dynamics. The data suggested that light absorption took place at the Sn centers (donors) followed by energy transfer to Eu3+ ions (acceptors) which resulted in populating the 5D0 emitting state. Energy transfer pathways likely resulting in the enhanced Eu3+ photoluminescence and the consequential light emission were discussed.

UV-stimulated near-IR emission of Pr3+ in phosphate glass via twofold-coordinated Sn centers

The optical properties of Pr2O3 and SnO co-doped barium–phosphate glass prepared by the melt-quenching technique have been investigated. Optical absorption and X-ray photoelectron spectroscopy (XPS) are employed in the characterization of tin species. The prevalence of divalent tin is indicated by the XPS data in accord with a conspicuous absorption band detected around 287nm ascribed to twofold-coordinated Sn centers (isoelectronic with Sn2+). Upon ultraviolet (UV) photoexcitation of the tin centers, near-infrared (IR) emission from Pr3+ ions is realized. An excitation spectrum acquired by monitoring Pr3+ emission from the 1D2 state at 1.03μm revealed a broad band around 290nm consistent with a SnPr donor–acceptor energy transfer channel. The data supports a mechanism starting with the singlet-to-singlet UV excitation of Sn centers, followed by the intersystem crossing populating their triplet states emitting in the visible. From these, energy transfer occurs to 3P0, 3P1, 1I6, and 3P2 resonant states in Pr3+, from which the near-IR emitting states 1D2 and 1G4 are populated.

Enhanced photoluminescence properties of Sm3+ ions in Cu+ and Sn2+ co-doped P2O5:BaO glass

Luminescent glasses activated with Sm3+ ions are of current interest given their potential for a wide range of photonic applications. In this work, Sm3+-containing P2O5:BaO glasses are prepared by a simple melt-quench method, and the influence of CuO and SnO co-doping on Sm3+ photoluminescence (PL) is investigated. Optical absorption, solid-state 31P nuclear magnetic resonance spectroscopy, and PL spectroscopy are employed in the assessment of material optical and structural properties. The data indicates that monovalent copper ions and twofold-coordinated Sn centers are successfully stabilized in the matrix and both species can enhance the orange–red emission of Sm3+ ions. The optical properties of the material after heat treatment have been also assessed. Results indicate the chemical reduction of ionic copper via Sn2+ ultimately producing Cu nanoparticles as evidenced by the surface plasmon resonance. As a result, Sm3+ PL diminishes consistent with an excitation energy transfer to plasmonic Cu particles, i.e. the “plasmonic diluent” effect prevails.

Luminescent properties of Cu+/Sn2+-activated aluminophosphate glass

A robust aluminophosphate glass matrix containing substantial amounts of Cu2O and SnO has been prepared by the melt-quenching technique, and the luminescent properties have been assessed. The as-prepared material exhibits tunable luminescence by variation of excitation wavelength, in connection with the distinct excitation/emission properties of Cu+ ions and twofold-coordinated Sn centers in the host. Under excitation of both Cu+ and Sn species at 300nm, the co-doped material exhibits a bright whitish luminescence, whereas excitation at 360nm produces an intense orange emission arising merely from Cu+ ions. Thus, white light generation is deemed realizable by combination of cool-bluish emission intrinsic to Sn centers with the warm-orange emission from Cu+ ions. The optical properties of the material after heat treatment have been also investigated. Results indicate the chemical reduction of ionic copper via Sn2+ ultimately producing Cu nanoparticles as evidenced by the surface plasmon resonance. Consequently, the excitation-wavelength dependent photoluminescence bands appear suppressed in relation to Cu+ basically and show a dip attributed to light reabsorption by the plasmonic Cu particles.

Sn centers-mediated enhancement of 1.53 µm emission of Er3+ ions in phosphate glass

The effect of SnO co-doping on Er3+ emission in the near-infrared telecommunication window has been studied in a barium-phosphate glass matrix, obtained by melt-quenching. Photoluminescence excitation spectra acquired by monitoring Er3+4I13/2→4I15/2 emission at about 1.53µm revealed a broad UV band around 300nm, indicating a donor/acceptor energy transfer channel. The Sn co-doped glass exhibits a five-fold enhanced Er3+ emission relative to an Er-doped reference. The absorption and photoluminescence spectra support a mechanism starting with excitation of twofold-coordinated Sn centers, followed by energy transfer to erbium ions. Pathways likely resulting in the enhanced Er3+ emission are discussed.

Optical absorption, 31P NMR, and photoluminescence spectroscopy study of copper and tin co-doped barium–phosphate glasses

The optical and structural properties of 50P2O5:50BaO glasses prepared by melting have been investigated for additive concentrations of 10 and 1 mol% of CuO and SnO dopants. Absorption and photoluminescence spectroscopies were employed in the optical characterization, whereas structural properties were assessed by 31P nuclear magnetic resonance (NMR) spectroscopy. Residual Cu2+ was detectable by absorption spectroscopy for the highest concentration of CuO and SnO. More prominently, the optical data suggests contributions from both twofold-coordinated Sn centers and Cu+ ions to light absorption and emission in the glasses. The luminescence depends strongly on excitation wavelength for the highest concentration of dopants where a blue–white emission is observed under short-wavelength excitation (e.g., 260 nm) largely due to tin, while an orange luminescence is exhibited for longer excitation wavelengths (e.g., 360 nm) essentially due to Cu+ ions. On the other hand, dissimilar luminescent properties were observed in connection to Cu+ ions for the lowest concentration studied, as the copper ions were preferentially excited in a narrower range at shorter wavelengths near tin centers absorption. The structural analyses revealed the glass matrix to be composed essentially of Q2 (two bridging oxygens) and Q1 (one bridging oxygen) phosphate tetrahedra. A slight increase in the Q1/Q2 ratio reflected upon SnO doping alone suggests a major incorporation of tin into the glass network via P–O–Sn bonds, compatible with the 2-coordinated state attributed to the luminescent Sn centers. However, a significant increase in the Q1/Q2 ratio was indicated with the incorporation of copper at the highest concentration, consistent with a key role of the metal ions as network modifiers. Thus, the change in Cu+ optical properties concurs with different distributions of local environments around the ions induced by variation in metal ion concentration. Luminescence decay curve analyses were found in agreement with the presence of Cu+ ions in the glasses suggesting their existence in tetragonally-distorted octahedral sites.

Emission Properties of Sn2+ and Sm3+ Co-doped Barium Phosphate Glass

Co-doping of a P2O5:BaO glass matrix with divalent tin and trivalent samarium ions has been realized by the melt-quenching technique with the purpose of investigating the material’s luminescent properties. Photoluminescence excitation spectra obtained by monitoring Sm3+ 4G5/2 emission showed a broad excitation band around 290 nm (absent in a Sm3+-doped reference), typical of donor/acceptor energy transfer. Under such excitation, the material exhibits a reddish-white emission. Time-resolved spectra recorded under the 290-nm excitation (non-resonant with Sm3+ excitation peaks) exposed a broad blue-white band characteristic of twofold-coordinated Sn centers and orange-red emission bands of Sm3+ ions, which appeared well separated in time in accord with their emission decay dynamics. Consequently, the data indicate that light absorption occurs at Sn centers (donors) followed by energy transfer to samarium ions (acceptors) which results in populating the 4G5/2 emitting state in Sm3+. Energy transfer pathways likely resulting in the sensitization of Sm3+ photoluminescence are discussed. Results are put into context in terms of the potential of SnO and rare-earth co-doped barium phosphate glasses for use in white light-emitting devices.

Twofold-coordinated tin centers as UV sensitizers of trivalent dysprosium ions

Phosphate glasses containing Dy3+ ions were co-doped with tin (II) oxide with the purpose of investigating on the sensitizing effects of tin species on the luminescence of Dy3+ ions. The glasses were prepared by the melt-quenching technique and the spectroscopic properties were assessed by optical absorption and photoluminescence spectroscopy. The data indicated the presence of twofold-coordinated Sn centers as the tin-related luminescent species in the glass host. A remarkable Dy3+ ions emission was observed under UV excitation in the co-doped glass in connection to excitation of the Sn centers. The data shows unambiguously the role of tin as a sensitizer of Dy3+ ion luminescence. Likely energy transfer processes that result in populating the 4F9/2 emitting state in Dy3+ are discussed.

Luminescence of trivalent samarium ions in silver and tin co-doped aluminophosphate glass

This work presents the spectroscopic properties of trivalent samarium ions in a melt-quenched aluminophosphate glass containing silver and tin. Addition of 4 mol% of each Ag 2O and SnO into the glass system with 2 mol% Sm 2O 3 results in Sm 3+ ions luminescence under non-resonant UV excitation owing to energy transfer from single silver ions and/or twofold-coordinated Sn centers. Assessment of luminescence spectra and decay dynamics suggest the energy transfer mechanism to be essentially of the resonant radiative type. Moreover, a connection between the luminescent and structural properties of the rare-earth doped glass system was demonstrated. Raman spectroscopy characterization revealed that no significant variation in the glass matrix is induced by Sm 3+ doping at the concentration employed. A comparison was made with a structural study performed on the Eu 3+ doped system (containing 2 mol% Eu 2O 3 along with 4 mol% of each Ag 2O and SnO) where the radiative energy transfer mechanism was previously established. The data appears consistent regarding the lack of variation in glass structure upon the Eu 3+ and Sm 3+ doping in connection with the dominance of the radiative transfer in the matrix. Thermal treatment of the material leads to precipitation of Ag nanoparticles of a broad size range inside the dielectric as observed by transmission electron microspcopy. Assessment of 4G 5/2 excited state decay in Sm 3+ ions shows no influence from the silver particles.

Optical properties of aluminophosphate glasses containing silver, tin, and dysprosium

The spectroscopic properties of a Dy3+-doped aluminophosphate glass containing silver and tin were reported. Different oxidation and aggregation states of silver were obtained by varying silver concentration and glass thermal history. The addition of silver and tin at the lowest concentration studied results in Dy3+ ions emission under nonresonant UV excitation in connection with the appearance of an excitation band around 270 nm, which is associated to isolated Ag+ ions and twofold-coordinated Sn centers. The increase in silver and tin concentration leads to a broadening of aforementioned band and to the presence of charged silver dimers as evidenced by the appearance of an excitation band around 330 nm. The data indicated that light absorption might take place at ionic silver species and twofold-coordinated Sn centers, followed by energy transfer to Dy3+ ions. After heat treatment, ionic silver species were reduced to atomic Ag by tin with the subsequent formation of Ag nanoparticles (NPs) inside the dielectric host. A quenching effect in Dy3+ ions luminescence was shown with the presence of the Ag NPs, most notably for excitation of 6H15/2 → 4F9/2, 4I15/2, 4G11/2 transitions, which were in resonance with the dipole absorption mode of the particles. The silver NPs were believed to provide radiationless pathways for excitation energy loss in Dy3+ ions.

Silver aggregates and twofold-coordinated tin centers in phosphate glass: A photoluminescence study

The optical properties of silver species in various oxidation and aggregation states and of tin centers in melt-quenched phosphate glasses have been assessed by optical absorption and photoluminescence (PL) spectroscopy. Glasses containing silver and tin, or either dopant, were studied. Emission and excitation spectra along with time-resolved and temperature-dependent PL measurements were employed in elucidating the different emitting centers observed and investigating on their interactions. In regard to silver, the data suggests the presence of luminescent single Ag + ions, Ag +–Ag + and Ag +–Ag 0 pairs, and nonluminescent Ag nanoparticles (NPs), where Ag +–Ag 0→Ag +–Ag + energy transfer is indicated. Tin optical centers appear as twofold-coordinated Sn centers displaying PL around 400 nm ascribed to triplet-to-singlet electronic transitions. The optically active silver centers were observed in glasses where 8 mol% of both Ag 2O and SnO, and 4 mol% of Ag 2O were added. Heat treatment (HT) of the glass with the high concentration of silver and tin leads to chemical reduction of ionic silver species resulting in a large volume fraction of silver NPs and the vanishing of silver PL features. Further characterization of such heat-treated glass by transmission electron microscopy and X-ray photoelectron spectroscopy appears consistent with silver being present mainly in nonoxidized form after HT. On the other hand, HT of the glass containing only silver results in the quenching of Ag +–Ag 0 pairs emission that is ascribed to nonradiative energy transfer to Ag NPs due to the positioning of the pairs near the surface of NPs during HT. In this context, an important finding is that a faster relaxation was observed for this nanocomposite in relation to a heat-treated glass containing both silver and tin (no silver pairs) as revealed by degenerate four-wave mixing spectroscopy. Such result is attributed to Ag NP→Ag +–Ag 0 plasmon resonance energy transfer. The data thus indicates that energy transfer between Ag +–Ag 0 pairs and NPs is bi-directional.

Enhanced UV-excited luminescence of europium ions in silver/tin-doped glass

Glasses containing silver, tin and europium were prepared by the melt-quenching technique with silver nanoparticles (NPs) being embedded upon heat treatment (HT). An intensification of Eu 3+ ions emission was observed for non-resonant excitation around 270 nm, corresponding to UV absorption in the material. Optical measurements suggest that light absorption occurs at single Ag + ions and/or twofold-coordinated Sn centers followed by energy transfer to europium which results in populating the 5D 0 emitting state in Eu 3+. After HT at 843 K, a quenching effect is observed on Eu 3+ luminescence with increasing holding time in the 350–550 nm excitation range. The quenching effect shows with the presence of Ag NPs which may provide multipole radiationless pathways for excitation energy loss in europium ions.

β-Band Photoluminescence and Sn-E' Center Generation from Twofold-Coordinated Sn Centers in SiO2 Glasses Produced via Sol–Gel Method

Violet-blue photoluminescence (β-band emission) of twofold-coordinated tin (O–Sn0–O, >Sn0) atoms in silica (SiO2) glasses, synthesized via a sol–gel route, was observed with a large band tail to longer wavelength. The presence of >Sn0 centers more easily enabled us to produce Sn-associated E'-centers with X-ray irradiation, which is expected for a photosensitivity similar to that in the case of Ge-doped SiO2 glasses fiber. In the initial state of the new fabrication process, stannic oxide (SnO2) nanocrystals were precipitated in SiO2 glasses and an adequate annealing of SnO2–SiO2 glass ceramics in H2 gas led to the decomposition of SnO2 nanocrystals and concurrently the production of >Sn0 point defects. Although the quantum efficiencies of the photoluminescence were very low to be 0.95% (external) and 1.3% (internal) under near-ultraviolet excitation owing to the triplet nature of the emissive level of >Sn0 center, we could clarify the photoluminescence properties, by means of time-resolved fluorescence spectroscopy, and the >Sn0→Sn-E' conversion.