Energy transfer from the host to Er<Superscript>3+</Superscript> dopants in semiconductor SnO<Subscript>2</Subscript> nanocrystals segregated in sol–gel silica glasses

Undoped and Er³⁺-doped glass–ceramics of composition (100−x)SiO₂–xSnO₂, with x = 5 or 10 and with 0.4 or 0.8 mol% of Er³⁺ ions, were synthesised by thermal treatment of precursor sol–gel glasses. Structural studies were developed by X-Ray Diffraction. Wide band gap SnO₂ semiconductor quantum-dots embedded in the insulator SiO₂ glass are obtained. The mean radius of the SnO₂ nanocrystals, ranging from 2 to 3.2 nm, is comparable to the exciton Bohr radius. The luminescence properties have been analysed as a function of sample composition and thermal treatment. The results show that Er³⁺ ions are partially partitioned into the nanocrystalline phase. An efficient UV excitation of the Er³⁺ ions by energy transfer from the SnO₂ nanocrystal host is observed. The Er³⁺ ions located in the SnO₂ nanocrystals are selectively excited by this energy transfer mechanism. On the other hand, emission from the Er³⁺ ions remaining in the silica glassy phase is obtained by direct excitation of these ions.     

Influence of nanoenvironment on luminescence of Euactivated SnO2 nanocrystals

The emission intensity of the peak at 612 nm (⁵D₀→⁷F₂) of the Eu³⁺ ions activated SnO₂ nanocrystals (doped and coated) is found to be sensitive to the nanoenvironment. We have compared the luminescence efficiencies of the nanocrystals of SnO₂ doped by Eu₂O₃ with those of SnO₂ coated by Eu₂O₃ and we found that the intensities are significantly higher in coated nanocrystals. Furthermore, it is clear from luminescence intensity measurements that Eu³⁺ ions occupy low symmetry sites in the Eu₂O₃ coated SnO₂ nanocrystal. The analysis suggests that the radiative relaxation rate is higher in Eu₂O₃ coated SnO₂ nanocrystals than Eu₂O₃ doped SnO₂ nanocrystals due to the asymmetric environment of Eu³⁺ ions in coated samples.     

Quenching of Si nanocrystal photoluminescence by doping with gold or phosphorous

Si nanocrystals embedded in SiO₂ doped with P and Au at concentrations in the range of 1×10¹⁸-3×10²⁰ cm⁻³ exhibit photoluminescence quenching. Upon increasing the Au concentration, a gradual decrease in nanocrystal photoluminescence intensity is observed. Using a statistical model for luminescence quenching, we derive a typical radius of ∼3 nm for nanocrystals luminescing around 800 nm. Au doping also leads to a luminescence lifetime reduction, which is attributed to energy transfer between adjacent Si nanocrystals, possibly mediated by the presence of Au in the form of ions or nanocrystals. Doping with P at concentrations up to 3×10¹⁹ cm⁻³ leads to a luminescence enhancement, most likely due to passivation of the nanocrystal-SiO₂ interfaces. Upon further P doping the nanocrystal luminescence gradually decreases, with little change in luminescence lifetime.     

Photon down-shifting by energy transfer from Sm3+ to Eu3+ ions in sol-gel SiO2-LaF3 nano-glass-ceramics for photovoltaics

95SiO₂?C5LaF₃ sol-gel derived nano-glass-ceramics single doped with Eu³⁺ or Sm³⁺ and codoped with both of them were successfully obtained. XRD measurements confirm the precipitation of LaF₃ nanocrystals after the ceramming process, with mean size ranging from 10 to 20?nm which increases with the thermal treatment temperature. The incorporation of rare-earth ions into precipitated LaF₃ nanocrystals was confirmed from luminescence spectra. Intense yellow-red emissions were detected under UV and blue light excitation in single and codoped samples. The effect of codoping with Eu³⁺ and Sm³⁺ ions and the energy transfer mechanism between them have been analyzed in order to increase the yellow-red emissions.     

Fundamental aspects of activated nanocrystal luminescence and possible applications

Presented results of complex study of relaxation processes and interionic interaction in Y₂SiO₅:Pr³⁺ and Lu₂SiO₅:Ce³⁺ nanocrystals clearly show two fundamental aspects: the phonon quantum confinement gives rise to the new fluorescence dynamics of doped ions; the developed surface of nanocrystals stimulates the irregular distribution of doped ions within the nanocrystal volume and could be the reason of new atomic arrangement of nanocrystal. Fluorescence spectrum of isolated Y₂SiO₅:Pr³⁺nanocrystal demonstrates the intense fluorescence from the high crystal field components of split ¹D₂ manifold of Pr³⁺ as the result of a suppression of phonon-assisted relaxation under the phonon quantum confinement. The direct comparison of the data obtained for nano- and bulk Y₂SiO₅:Pr³⁺ crystals has revealed that the concentration threshold of luminescence quenching is strikingly low for nanocrystals. This effect is caused by uphill diffusion of doped ions and preferred Pr segregation at the nanocrystal surface layer that provides the relaxation of elastic tension arising due to the difference of ionic radii of Pr³⁺ and Y³⁺. Lu₂SiO₅:Ce³⁺ nanocrystals which average size is 5 nm do not demonstrate the effect of energy storage as the result of atomic packing changing that does not permit the existence of electronic traps.     

Nanoparticles size effects in thermoluminescence of oxyfluoride glass-ceramics containing Sm<Superscript>3+</Superscript>-doped CaF<Subscript>2</Subscript> nanocrystals

Oxyfluoride glass-ceramic in the system SiO₂–Al₂O₃–CaF₂–SmF₃ containing Sm³⁺-doped CaF₂ nanocrystals in the range from 15 to 150 nm size were produced by using the controlled ceramization of the precursor glass. The incorporation of the Sm³⁺-dopant ion in the glass ceramic creates new electron-trapping centers and thermoluminescence (TL) method has been used in order to trace their evolution during glass ceramization. The 370 °C TL peak observed in precursor glass has been assigned to the recombination of the electrons released from the Sm²⁺-traps in the amorphous glass network. In the glass-ceramic sample containing nanocrystals with about 15 nm size the new weak TL peaks at 270, 290, and 310 °C were attributed to the recombination of the electrons released from the Sm²⁺-traps located mainly at the surface of the CaF₂ nanocrystals. In the glass-ceramic sample containing nanocrystals with about 150 nm size, the new TL peaks at 232, 270, and 302 °C size have been assigned to the recombination of the electrons released from the Sm²⁺-traps located inside the CaF₂ nanocrystals.     

Low-temperature photoluminescence of ion-implanted SiO2:Sn+ films and glasses

Low-temperature photoluminescence spectroscopy with pulsed synchrotron excitation is applied to study the regularities of excitation and relaxation of both point defects and nanoparticles formed by tin implantation into SiO₂ films and glasses. It has been found that tin implantation followed by air and nitrogen annealing yields the formation of α-Sn nanoclusters and nonstoichiometric SnO _x nanoparticles, while a stable phase of SnO₂ does not appear. Alternative channels of luminescence excitation are revealed for nanoclusters, including energy transfer from excitons and electron-hole pairs of the host SiO₂ matrix.     

Concentration quenching anomalies of activated Y<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Pr<Superscript>3+</Superscript> nanocrystal luminescence

For the fist time in Y₂SiO₅:Pr³⁺ nanocrystals, the ordered stage in the ¹ D ₂ luminescence decay curves for Pr³⁺ ions has been observed at anomalously low doped ion concentration (0.5 at %). This effect is caused by preferred location of the activator ions in the near-surface layer of the nanocrystal that provides the relaxation of elastic tension arising due to the difference of ionic radii of Pr³⁺ and Y³⁺ ions. Concentration quenching of Pr³⁺ luminescence is caused by the cooperative cross-relaxation.     

The upconversion luminescence and magnetism in Yb<Superscript>3+</Superscript>/Ho<Superscript>3+</Superscript> co-doped LaF<Subscript>3</Subscript> nanocrystals for potential bimodal imaging

Biocompatible upconversion nanoparticles with multifunctional properties can serve as potential nanoprobes for multimodal imaging. Herein, we report an upconversion nanocrystal based on lanthanum fluoride which is developed to address the imaging modalities, upconversion luminescence imaging and magnetic resonance imaging (MRI). Lanthanide ions (Yb³⁺ and Ho³⁺) doped LaF₃ nanocrystals (LaF₃ Yb³⁺/Ho³⁺) are fabricated through a rapid microwave-assisted synthesis. The hexagonal phase LaF₃ nanocrystals exhibit nearly spherical morphology with average diameter of 9.8 nm. The inductively coupled plasma mass spectrometry (ICP-MS) analysis estimated the doping concentration of Yb³⁺ and Ho³⁺ as 3.99 and 0.41%, respectively. The nanocrystals show upconversion luminescence when irradiated with near-infrared (NIR) photons of wavelength 980 nm. The emission spectrum consists of bands centred at 542, 645 and 658 nm. The stronger green emission at 542 nm and the weak red emissions at 645 and 658 nm are assigned to ⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺, respectively. The pump power dependence of luminescence intensity confirmed the two-photon upconversion process. The nanocrystals exhibit paramagnetism due to the presence of lanthanide ion dopant Ho³⁺ and the magnetization is 19.81 emu/g at room temperature. The nanocrystals exhibit a longitudinal relaxivity (r ₁) of 0.12 s^?1 mM^?1 and transverse relaxivity (r ₂) of 28.18 s^?1 mM^?1, which makes the system suitable for developing T2 MRI contrast agents based on holmium. The LaF₃ Yb³⁺/Ho³⁺ nanocrystals are surface modified by PEGylation to improve biocompatibility and enhance further functionalisation. The PEGylated nanocrystals are found to be non-toxic up to 50 μg/mL for 48 h of incubation, which is confirmed by the MTT assay as well as morphological studies in HeLa cells. The upconversion luminescence and magnetism together with biocompatibility enables the adaptability of the present system as a nanoprobe for potential bimodal imaging.     

The effect of charge compensation by means of Na+ ions on the luminescence behavior of Sm3+-doped CaAl4O7 phosphor

Results of structural and spectroscopic measurements of Sm³⁺ doped calcium aluminates: Ca_1?xSm_xAl₄O₇ and Ca_1?2xSm_xNa_xAl₄O₇ (x=0.0005, 0.002, 0.01, 0.02, 0.03, 0.05) obtained by the modified Pechini method are presented. All samples yield intense orange–red emission under violet excitation (404.5 nm). Narrow bands corresponding to characteristic f–f intraconfigurational transition of Sm³⁺ in excitation and emission spectra were observed. The influences of the concentration of Sm³⁺ as well as charge compensation by co-doping with Na⁺ ions on the luminescent properties of the phosphor were investigated. Detailed analysis of the emission spectra of Sm³⁺ doped and Sm³⁺,Na⁺ co-doped CaAl₄O₇ powders proved that activator ions substitute Ca²⁺ in the host. Co-doping with Na⁺ ions enhanced greatly the intensity of the luminescence. Concentration dependencies of the intensity of luminescence and its decay kinetics proved the emission quenching at higher dopant contents due to cross-relaxation processes between Sm³⁺ ions. Fitting of the ⁴G_5/2 state fluorescence decay to the Inokuti–Hirayama model indicated dipole–dipole interaction as the dominant mechanism of the cross-relaxation processes.     

Strong quenching of praseodymium f-f luminescence induced by a surface of Y2SiO5:Pr nanocrystal

The direct comparison of the luminescence decay data obtained for nano- and bulk Y₂SiO₅:Pr³⁺ crystals has revealed that the concentration threshold of luminescence quenching is strikingly low for nanocrystals. Nanocrystal inhomogeneous stress field induced by a surface stimulates the segregation of the doped Pr³⁺ ions within the surface layer that provides the relaxation of elastic tension arising due to the difference of the ionic radii of Pr³⁺ and Y³⁺. The Pr³⁺ irregular distribution in the nanocrystal volume results in the Pr³⁺ local concentration increasing that facilitates the luminescence quenching.     

Luminescent features of sol–gel derived rare-earth multi-doped oxyfluoride nano-structured phosphors for white LED application

Rare-earth doped oxyfluoride 75SiO₂:25PbF₂ nano-structured phosphors for white-light-emitting diodes were synthesized by thermal treatment of precursor sol–gel derived glasses. Room temperature luminescence features of Eu³⁺, Sm³⁺, Tb³⁺, Eu³⁺/Tb³⁺, and Sm³⁺/Tb³⁺ ions incorporated into low-phonon-energy PbF₂ nanocrystals dispersed in the aluminosilicate glass matrix and excited with UV light emitting diode were investigated. The luminescence spectra exhibited strong emission signals in the red (600, 610, 625, and 646 nm), green (548 and 560 nm), and blue (485 nm) wavelength regions. White-light emission was observed in Sm/Tb and Eu/Tb double-doped activated phosphors employing UV-LED excitation at 395 nm. The dependence of the luminescence emission intensities upon annealing temperature and rare-earth concentration was also examined. The results indicated that there exist optimum annealing temperature and activator ion concentration in order to obtain intense visible emission light with high color rendering index. The study suggests that the nanocomposite phosphor based upon 75SiO₂:25PbF₂ host herein reported is a promising contender for white-light LED applications.     

The luminescence enhancement of Eu3+ ion and SnO2 nanocrystal co-doped solben gel SiO2 films

SnO₂ nanocrystal and rare-earth Eu³⁺ ion co-doped SiO₂ thin films are prepared by sol-gel and spin coating methods. The formation of tetragonal rutile structure SnO₂ nanocrystals with a uniform distribution is confirmed by X-ray diffraction and transmission electron microscopy. Fourier transform infrared spectroscopy is used to investigate the densities of the hydroxyl groups, and it is found that the emission intensity from the ⁵D₀-⁷F₂ transitions of the Eu³⁺ ions is enhanced by two orders of magnitude due to energy transfer from the oxygen-vacancy-related defects of the SnO₂ nanocrystals to nearby Eu³⁺ ions. The influences of the amounts of Sn and the post-annealing temperatures are systematically evaluated to further understand the mechanism of energy transfer. The luminescence intensity ratio of Eu³⁺ ions from electric dipole transition and magnetic dipole transition indicate the different probable locations of Eu³⁺ ions in the sol-gel thin film, which are further discussed based on temperature-dependent photoluminescence measurements.     

Strong segregation of doped ions in Y2SiO5:Pr3+ nanocrystals

Strong temperature controlled segregation of doped ions in Y₂SiO₅:Pr³⁺ nanocrystals detected by spectroscopic techniques is reported. The elastic interactions stimulate Pr³⁺ segregation thus leading to non-uniform distribution of doped ions, pair formation and, as a consequence, to abnormal low threshold of luminescence concentration quenching for Y₂SiO₅:Pr³⁺ nanocrystals.     

Concentration influence on temperature-dependent luminescence properties of samarium substituted strontium tetraborate

The series of divalent samarium substituted strontium tetraborate (Sr_1?xSm_xB₄O₇) polycrystalline samples were prepared by the conventional solid-state reaction. The phase formation of the samples was investigated by X-ray powder diffraction measurements. The luminescence spectra and decay curves of the Sm²⁺ ions were measured. Temperature dependent Sm²⁺ luminescence properties were investigated. The f–d and ⁵D₁–⁷F_J transitions appeared at 350 K and increased with increase in the temperature while the intensity of ⁵D₀–⁷F_J transitions decreased. The emission spectra pointed out that Sm²⁺ occupies of C_2v or lower symmetry site. The photoluminescence decay times of strontium tetraborate doped with different concentrations of Sm²⁺ was investigated as a function of temperature in the range of 100–500 K. However, no obvious concentration quenching was observed.     

Enhanced Conductivity and High Thermal Stability of W-Doped SnO<Subscript>2</Subscript> Based on First-Principle Calculations

Tungsten (W)-doped SnO₂ is investigated by first-principle calculations, with a view to understand the effect of doping on the lattice structure, thermal stability, conductivity, and optical transparency. Due to the slight difference in ionic radius as well as high thermal and chemical compatibility between the native element and the heterogeneous dopant, the doped system changes a little with different deviations in the lattice constant from Vegard’s law, and good thermal stability is observed as the doping level reaches x = 0.125 in Sn_1-x W _x O₂ compounds. Nevertheless, the large disparities in electron configuration and electronegativity between W and Sn atoms will dramatically modify the electronic structure and charge distribution of W-doped SnO₂, leading to a remarkable enhancement of conductivity, electron excitation in the low energy region, and the consequent optical properties, while the visible transparency of Sn_1-x W _x O₂ is still preserved. Particularly, it is found that the optimal photoelectric properties of W-doped SnO₂ may be achieved at x = 0.03. These observations are consistent with the experimental results available on the structural, thermal, electronic, and optical properties of Sn_1-x W _x O₂, thus presenting a practical way of tailoring the physical behaviors of SnO₂ through the doping technique.     

Anomalies in the concentration quenching of luminescence in doped Y2SiO5:Pr3+ nanocrystals

In Y₂SiO₅:Pr³⁺ nanocrystals, an ordered phase is observed in the ¹ D ₂ luminescence decay curves of Pr³⁺ ions at their anomalously low concentration (0.5 at %). This effect is caused by the predominant accumulation of activator ions near the nanocrystal surface, which provides relaxation of the elastic strains arising as a result of the misfit between the ionic radii of Pr³⁺ and Y³⁺. The concentration quenching of Pr³⁺ luminescence is due to cooperative cross relaxation.     

Luminescence kinetics in silica gel doped with Tb3+ ions and ZnS:Mn2+ nanocrystals

Luminescence kinetics and time-resolved luminescence spectra of SiO₂, SiO₂ doped with ZnS:Mn²⁺ nanocrystals and SiO₂ doped with ZnS:Mn²⁺, and additionally co-doped with Tb³⁺, are presented. The purposes of the paper are the analysis of the kinetics of the Tb³⁺ and Mn²⁺ intra-shell luminescence and the elucidation of the energy-transfer mechanism between the ZnS:Mn²⁺ nanocrystals and the Tb³⁺ ions. We have found a blue luminescence related to defects in the ZnS nanocrystals and an intrinsic luminescence of the SiO₂ lattice, which decays in few ns. A yellow luminescence related to the Mn^{2+ 4}T₁(G)→⁶A₁ transition and yellow sharp lines related to the ⁵D₄→⁷F₆, ⁷F₅, ⁷F₄ and ⁷F₃ transitions in Tb³⁺ are found to decay in ms. A very effective energy transfer between ZnS:Mn²⁺ nanoparticles and Tb³⁺ ions has been observed.