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⁺-Ag⁰ pairs, and nonluminescent Ag nanoparticles (NPs), where Ag⁺-Ag⁰→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 Ag₂O and SnO, and 4 mol% of Ag₂O 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⁺-Ag⁰ 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⁺-Ag⁰ plasmon resonance energy transfer. The data thus indicates that energy transfer between Ag⁺-Ag⁰ pairs and NPs is bi-directional.     

Structural and luminescence studies of trivalent europium complexes with pentaethylene glycol and 18-crown-6 ligands in the presence of picrate anion

Two new isostructural complexes of europium picrate (Eu-Pic) with pentaethylene glycol (EO5) and 18-crown-6 (18C6) ligands formed complexes of molecular formula [Eu(Pic)₂(18C6)]⁺(Pic)⁻I and [Eu(Pic)₂(EO5)]⁺(Pic)⁻II have been isolated and characterised. Compound I showed 10-coordination number through six oxygen atoms from the 18C6 ligand and two bidentate picrate anions. Meanwhile, compound II exhibited 9-coordination number via six oxygen atoms from EO5 ligand, two oxygen atoms from a bidentate and one oxygen atom from monodentate picrate anions. Photoluminescence (PL) spectra of the solid-state europium complexes display sharp lines which are assigned to ⁵D₀→⁷F_0-4 and ⁵D₁→⁷F_1,2,4 transitions. No emission of polyether ligands is observed, indicating that the energy transfer from the polyether ligands to the Eu³⁺ ion is quite efficient. The PL spectra of [Eu(Pic)₂(OH₂)₆]⁺(Pic)⁻·6H₂O III, [Eu(NO₃)₃(OH₂)₃]·(18C6) IV, [Eu(NO₃)₃·6H₂O] V and Eu₂O₃VI are also observed. Compounds I-IV exhibited high Ω₂ intensity parameter values, namely 16.93, 10.23, 17.10 and 12.35 (in units of 10⁻²⁰ cm²), respectively. These relatively high values reflect the hypersensitive behaviour of the ⁵D₀→⁷F₂ transition and indicate that the Eu³⁺ ion is located in a highly polarisable chemical environment.     

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 Eu³⁺ 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 ⁵D₀ emitting state in Eu³⁺. After HT at 843 K, a quenching effect is observed on Eu³⁺ 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.     

Influences of compositions and ligands on photoluminescent properties of Eu(III) ions in composite europium complex/PMMA systems

Three kinds of europium complexes; Eu(phen)₂Cl₃(H₂O)₂, Eu(DN-bpy)phenCl₃(H₂O)₂ and Eu(DB-bpy)phenCl₃(H₂O)₂ (phen: 1,10-phenanthroline, DN-bpy: 4,4′-Dinonyl-2,2′-dipyridyl, DB-bpy: 4,4′-Di-tert-butyl-2,2′-dipyridyl) were prepared and then incorporated into polymethyl methacrylate (PMMA) matrix with different molar ratios of CO groups/Eu³⁺ ions. The final solid composites were formed by a self-assembly process among Eu³⁺ ion, the ligands and PMMA during the solvent evaporation process, and then the ligands re-coordinate to Eu(III). It was found that the ligands affect not only the emission properties of the pure complexes, but also the miscibility of the complexes and PMMA. More than one kind of symmetric sites of Eu³⁺ ions were formed in the composites due to the coordination of CO in PMMA to Eu³⁺ ions. The micro-environments of Eu(III) in the composites were changed with the compositions and the ligands, leading to the change in the crystalline structure, and consequently, the emission characteristics.     

Use of tin as a surfactant material for the growth of thin silver films on silicon oxide

In this letter, we report on the use of tin as an effective surfactant material for silver growing on silicon oxide. We observed that submonolayers of Sn pre-deposited on SiO₂ result in earlier film coalescence and formation of smoother Ag layers. We suggest that Sn atoms reduce the Ag-adatom mobility resulting in experimentally observed increased island density and decreased film roughness. Angle-resolved X-ray photoelectron spectroscopy reveals that Sn remains under the Ag layer giving circumstantial evidence that at later stages of Ag film growth Sn does not influence the interlayer transport.     

Synergistic effect of silver seeds and organic modifiers on the morphology evolution mechanism of silver nanoparticles

Triangular, truncated triangular, quadrangular, hexagonal, and net-structured silver nanoplates as well as decahedral silver nanoparticles were manipulatively prepared starting from silver nitrate and silver seeds in the presence of poly(ethylene glycol) (PEG), poly(N-vinyl pyrrolidone) (PVP), and Tween 80 at room temperature, respectively. UV-vis spectroscopy, XRD, HRTEM, SAED, and FTIR were used to illustrate the crystal growth process and to characterize the resultant silver nanoparticles. It was found that the silver seeds and organic modifiers synergistically affected the morphology evolution of the silver nanoparticles. The co-presence of silver seeds and PEG was beneficial to the formation of triangular and truncated triangular silver nanoplates; the silver seeds and PVP favored the formation of polygonal silver nanoplates; the silver seeds and Tween 80 preferred to the formation of net-structured silver plates. The morphology evolution of the resultant silver nanoparticles was correlated with the crystallinity of the silver seeds and the adsorption ability of the organic modifiers on the crystal surfaces.     

Preparation of silver nanoparticles by laser ablation in polyvinylpyrrolidone solutions

We performed laser ablation of a silver plate in polyvinylpyrrolidone (PVP) aqueous solutions to prepare silver nanoparticles. Secondary laser irradiation onto the prepared colloidal solutions was also carried out. It was revealed that the formation efficiency was increased by addition of PVP as well as the stability of nanoparticles. The result of shadowgraph measurements suggested that the increased ablation efficiency by PVP is attributable to increased secondary etching efficiency by the solvent-confined plasma toward the silver plate. On the other hand, the size decrease of the nanoparticles by addition of PVP was more remarkable during the secondary irradiation process than in the laser ablation (nanoparticle preparation) process. This result indicates that emitted materials interact less sufficiently with PVP molecules in the laser ablation process than in the secondary laser irradiation process.     

Temperature dependence of the luminescence lifetime of a europium complex immobilized in different polymer matrices

The temperature dependence of the luminescence lifetime of temperature sensor films based on europium (III) thenoyltrifluoroacetonate (EuTTA) as sensor dye in various polymer matrices such as polystyrene (PS), polymethylmethacrylate (PMMA), polyurethane (PU) and model airplane dope was studied and compared. The luminescence lifetime of EuTTA was found to depend on the polymer matrix. The temperature sensitivity of lifetime was maximum for EuTTA-PS coating in the temperature range of 10-60 °C. The effect of concentration of the sensor dye in the polymer on the lifetime and temperature sensitivity was also studied.     

Evolution of the optical properties of a silver-doped phosphate glass during thermal treatment

The optical properties of a silver-doped phosphate glass have been monitored during thermal processing at several fixed temperatures by in situ optical microspectroscopy. Silver nanoparticle (NP) formation and growth processes were assessed by analysis of surface plasmon resonance spectral features. Nucleation and growth processes were distinguished, which appeared temperature and time dependent. While nucleation was favored at low temperatures, relatively high temperatures promoted NP growth by silver diffusion. Photoluminescence spectra acquired along with optical absorption data indicated a continuous reduction of Ag⁺-Ag⁰ pairs concomitant with NP precipitation, suggesting their role as nucleation centers. The work of Ag NP formation and the activation energy for silver diffusion were estimated.     

Surface and catalytic properties of doped tin oxide nanoparticles

Mixed oxides composed of Zn-Sn, Ti-Sn and V-Sn were prepared by a co-precipitation method and evaluated as catalysts for methanol oxidation in an ambient fixed-bed reactor. Surface analysis by X-ray photoelectron spectroscopy (XPS) revealed an electronic interaction between dopant and Sn atoms in the oxide structure and showed the formation of surface states associated with the dopants. Oxygen vacancies were present on the Zn-doped oxide, and the oxidation of methanol to carbon oxides was favored. The Ti-doped oxide exhibited a favorable selectivity to dimethyl ether, related to the oxygen anions near Ti centers. Vanadium dopants not only dramatically increased the catalytic activity but also promoted the partial oxidation of methanol to formaldehyde. Results demonstrate that the bridging dopant-O-Sn bond acts as active sites and influences product distribution.     

Synthesis of NaYF4 nanocrystals doped with Yb/Er and influence of citric acid on the green and red luminescence

In this paper, NaYF₄ nanocrystals doped with Yb³⁺ and Er³⁺ were synthesized in a medium containing polyethylene glycol and citric acid. This nanocrystal presents up-converting green and red emission bands which were simultaneously observed under the excitation of a 980 infrared diode laser. Mainly, the green to red ratio (GRR) of the up-conversion emission of the hexagonal NaYF₄ nanocrystals doped with Yb³⁺ and Er³⁺ can be finely tuned by changing the content of citric acid to be nearly an arithmetic progression, i.e. 6/4, 5/4, 4/4, and 3/4. The further analysis revealed that citric acid plays a key role in improving the surface crystallinity of NaYF₄ nanocrystals doped with Yb³⁺ and Er³⁺, to which the achievement of fine controlling on GRR is ascribed.     

Adhesion of silver nanoparticles on the clay substrates; modeling and experiment

Adhesion of silver nanoparticles on the montmorillonite and kaolinite substrates has been investigated using molecular modeling (force field calculations) that enabled the estimation and comparison of adhesion energies for Ag/montmorillonite and Ag/kaolinite nanocomposites and revealed the preferred orientation of Ag nanoparticles on the silicate substrates. Results of the modeling have been confronted with experiment (X-ray fluorescence, high-resolution transmission electron microscopy). This confrontation has shown that the results of the modeling are consistent with the experimental data and illustrated the capability of the molecular modeling for prediction of the nanoparticles orientation, structure and stability of the nanoparticle/substrate nanocomposite.     

Luminescence properties of the Eu-doped porous glass and spontaneous reduction of Eu to Eu

The luminescent properties of Eu³⁺-doped porous glass were investigated and reduction of Eu³⁺ to Eu²⁺ was observed when the Eu³⁺-doped porous glasses were heat treated in air. The absorption, emission and excitation spectra of Eu³⁺-doped porous glasses were measured and the results significantly depended on the treating temperature. The integral intensity of Eu³⁺ and Eu²⁺ ions at different temperatures was also investigated to investigate this reduction process. Electron spin resonance (ESR) spectra of the samples were also measured, which also confirmed the reduction effect. A possible mechanism was discussed to explain this process.     

Surface states and its influence on luminescence in ZnS nanocrystallite

In this pape,r the influence of surface effects on the self-activated (SA) luminescence in ZnS nanoparticles prepared by the wet-chemical method is presented. It is observed that the luminescence of SA decreases dramatically by rinsing with methanol. In the rinsed sample, the luminescence of SA increases more by ultraviolet (UV) light irradiation. To clarify its origin, the Raman spectra and electron paramagnetic resonance (EPR) are studied. The results demonstrate that the vibrational modes assigned to organic functional groups of -OH and -COO and -CH₃ decreases remarkably by rinsing, while the EPR signal originated from the unpaired electrons of some transition metal impurity ions including Mn²⁺ increases. It is suggested that the SA centers prefer to occupy the sites near the surface and that the donor of SA emission may be partly related to the organic functional groups of -OH and -COO adsorbed on the surface. The surface-dangling bonds caused by unpaired electrons of some transition metal impurity ions play a role of surface states, leading to the quenching of the SA emissions. The organic functional groups chemically combine with these surface-dangling bonds leading to the decrease in surface states and surface nonradiative relaxation channels and to the increase in the SA emissions.     

Middle-infrared luminescence of Nd ions in silver halide crystals

The optical absorption and the luminescence emission in the middle infrared (mid-IR) were investigated in AgCl_xBr_1−x crystals doped with Nd³⁺ ions. Strong luminescence emission, in the spectral range 4.5-5.8 μm, in mid-IR was observed for the first time in Nd³⁺-doped silver halide crystals. Various optical parameters were calculated for the Nd³⁺-doped crystals, using the Judd-Ofelt approximation. The measured results and the calculated parameters indicate that these doped crystals could be used for the development of mid-IR solid-state lasers or mid-IR fiber lasers.     

Enhanced photoluminescence of conjugated polymer thin films on nanostructured silver

Photoluminescence of a conjugated polymer in the presence of surface plasmons on metallic nanoparticles is studied. A layered device structure was constructed that enabled control over nanoparticle diameter and separation between the polymer and nanoparticles layers. The dependence of the surface plasmon evanescent field and energy transfer has been investigated with the largest enhancement in photoluminescence observed at a 40 nm distance separation between the fluorophore and the surface plasmon. A spectrum of surface plasmon resonances ranging from the emission to the absorption energies of the conjugated polymer revealed largest enhancements when the resonance was tuned to the conjugated polymer emission energy. At peak photoluminescence the maximum photoluminescent enhancement was found to be 5.6 times of the photoluminescence of the control structure and the total integrated enhancement was 5.9 times.     

The colouration by 60 Co-gamma rays of NaCl and KCl crystals doped with europium ions

Pure and europium doped NaCl and KCl crystals have been irradiated with 60 Co gamma rays and with ultraviolet light. The dose was 10 v kGy and temperatures during irradiation were ambient and liquid nitrogen. The effects of irradiation are determined using optical absorption and thermoluminescence. The role of the europium dopant is compared for the two materials. Factors investigated include the temperature of irradiation, the concentration of the dopant and the state of impurity aggregation and precipitation. A link is indicated between impurity precipitates and UV stimulated thermoluminescence.     

Understanding the influence of nanoenvironment on luminescence of rare-earth ions

This paper presents an overview of the recent results on upconversion and photoluminescence of rare-earth ions in nanoenvironments. The role of the rare-earth ion concentration, crystal size and crystal phase on the up and downconversion emission of rare-earth ions in oxide nanocrystals and their underlying mechanisms are discussed. It is also found that the luminescence lifetime of the excited state rare-earth ions is sensitive to the particle crystalline phase and size. The analysis suggests that the modifications of radiative and nonradiative relaxation mechanisms are due to local symmetry structure of the host lattice and crystal size respectively     

Middle-infrared luminescence of praseodymium ions in silver halide crystals and fibers

The luminescence of AgBr, AgCl, and AgClBr crystals and fibers doped with Pr3+ ions was investigated in the middle-infrared spectral range. We measured the absorption, emission, and kinetic parameters over a broad temperature range. Strong luminescence in the spectral range 4-5.5 microm was observed for the first time to our knowledge in silver halide crystals and fibers at room temperature. No noticeable differences were observed between the crystals and the fibers. We calculated various optical parameters for Pr:AgBr and Pr:AgCl crystals, using the Judd-Ofelt approximation. Both the measured results and the calculated parameters indicate that these doped crystals and fibers would be good candidates for the fabrication of mid-IR solid-state lasers or fiber lasers.     

Modeling of X-ray excited luminescence intensity dependence on the nanoparticle size

The thermalization length distribution of electrons over their kinetic energy in a conduction band is calculated on the basis of the data on the electron effective mass, density of states in conduction band, dielectric permittivity and energy of longitudinal optical phonons. The method of modeling of a recombinational luminescence intensity dependence on the nanoparticle size is proposed on the basis of the assumption that the contribution to a recombinational luminescence gives only those charge carriers which in the result of thermalization did not reach a near-surface layer of nanoparticles. Using such the approach the theoretical dependence of recombinational luminescence intensity on the nanoparticle size for LaPO₄ and LuPO₄ are calculated. The revealed correlation of experimental and theoretical dependences confirms that the commensurability of electron thermalization length with nanoparticle size is the main reason of the sharp decrease of X-ray excited luminescence intensity when the nanoparticle size decreases.