Photoluminescence studies on energy transfer processes in cerium-doped Gd3Al2Ga3O12 crystals

We have measured photoluminescence properties of cerium-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG) crystals at low temperatures with use of synchrotron radiation. Excitation spectra for the Ce³⁺ 5d–4f emission exhibit prominent peaks at Gd³⁺ intra-4f absorption bands. The Gd³⁺ intra-4f emission band is observed at 3.91 eV, but is not in resonance with the lowest energy Gd³⁺ intra-4f absorption band at 3.95 eV. The temperature dependence of the Gd³⁺ emission intensity is not correlated with that of the Ce³⁺ emission intensity. Decay curves of the Ce³⁺ emission were also measured at 9 K under excitation at various photon energies. The decay curve is remarkably changed, depending on the excitation photon energies. The present results give us hints to understand the whole of energy transfer processes in Ce:GAGG crystals.     

Photoluminescence of TiO2 films co-doped with Tb/ Gdand energy transfer from TiO2/Gd to Tb ions

Nanometer TiO₂ thin films doped with different concentration of Tb were prepared by sol-gel method and characterized by X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. XRD results show preferentially oriented (101) anatase films. TEM image indicates that the TiO₂ films consist of TiO₂ grains with diameter about 15 nm. Under room temperature, strong visible luminescence of Tb³⁺ ions due to intra-4f shell transitions are obtained and the PL intensity is found to have a well matching relation with the doping concentration of Tb³⁺ ions. Concentration quenching of PL occurs when Tb³⁺ concentration exceeds a certain value (9.2 mol%). Furthermore, the luminescence intensity is improved obviously after co-doping with Gd³⁺ ions because of the sensitization effects of Gd³⁺ ions to Tb³⁺ ions in TiO₂ system. The energy transfer mechanism from TiO₂ and Gd³⁺ ions to Tb³⁺ ions was proposed.     

Photoluminescence mechanisms of color-tunable Sr2CeO4: Eu3+, Dy3+ phosphors based on experimental and first-principles investigation

We report single-phased color-tunable phosphors (Sr₂CeO₄: Eu³⁺, Dy³⁺) synthesized by a polymer-network gel method for UV–LED. The photoluminescence properties and possible energy transfer mechanisms of Eu³⁺ and Dy³⁺ in Sr₂CeO₄ were investigated by experiments and first principles calculations. The results show that the ⁵D₀ → ⁷F₂ emission of Eu³⁺ is enhanced by the increase in the amount of Eu³⁺ ions and Eu³⁺ substitution makes more stable defect than Dy³⁺ substitution does. The photoluminescence mechanism of Sr_1.994−xEu_xDy_0.006CeO₄ can be explained by the energy transfer model with the consideration of the defect conditions in the crystals.     

Photoluminescence and energy transfer investigation from SiO2: Tb,Au inverse opals to rhodamine-B dyes

Energy transfer has attracted extensive attention due to its widespread applications in medical diagnostics, DNA analysis and lighting devices. There are few reports on the energy transfer from rare earth ions to dyes. In the present work, the SiO₂:Tb inverse opals with and without Au nanoparticles were prepared, and the organic rhodamine-B (RhB) dyes were filled into the voids of SiO₂:Tb inverse opals. Non-radiative and radiative energy transfer processes from the SiO₂:Tb inverse opals to the RhB were observed. The influence of Au nanoparticles and photonic band gap on the energy transfer from SiO₂:Tb inverse opals to the RhB was investigated. The Au nanoparticles enhanced energy transfer was observed due to the surface plasmon resonance effects of the Au nanoparticles. When the emission peaks from the SiO₂:Tb inverse opal is overlapped with the photonic band gap, the emission suppression of the SiO₂:Tb inverse opal as well the emission enhancement of the RhB dyes were obtained, which is attributed to improved energy transfer caused by the photonic band gap. The steady state rate equations were used to explain enhancement of energy transfer caused by the photonic band gap.     

Optical properties of the Tm and energy transfer between Tm→Pr ions in P2O5-CaO-SrO-BaO phosphate glass

A P₂O₅-CaO-SrO-BaO phosphate glass doped with Tm³⁺ and glasses doped with (Tm³⁺, Pr³⁺) were used for this study. The photo-luminescence behaviors of Tm³⁺ and Pr³⁺ in phosphate glass were investigated by absorption, excitation and emission spectroscopy. The energy transfer between Tm³⁺ and Pr³⁺ in phosphate glasses (which exhibit a variety of transfer efficiencies) was studied. The experimental quantum efficiencies of the luminescence of Tm³⁺ η₀ and (Tm³⁺, Pr³⁺) doped phosphate glasses were measured to give η/η₀ = 0.447, 0.305, and 0.179 for (0.4 mol% Pr³⁺, 1.0 mol% Tm³⁺), (0.8%Pr³⁺, 1.0%Tm³⁺) and (1.6 mol% Pr³⁺, 1.0 mol% Tm³⁺), respectively. In order to verify the nature of the ion coupling in our phosphate glass system, we applied the Inokuti-Hirayama model. The non-radiative energy transfer rate from Tm³⁺ to Pr³⁺, transfer efficiencies, and the donor-acceptor distance have been calculated and compared with obtained experimental data. As usual, the efficiency and the probability of energy transfer increase with the concentration of the acceptor.     

Energy transfer and morphology study of a new iridium based cyclometalated phosphorescent complex

This paper reports synthesis and characterization of a new phosphorescent iridium complex namely iridium (III) bis (2-phenylpyridinato–N, C^2′) picolinate [(ppy)₂Irpic]. For its future use in organic light emitting diodes (OLED), we doped the iridium complex into various host materials and carried out a study of energy transfer process through photoluminescence (PL) and photoluminescence excitation (PLE) measurements onto the blended films with different concentrations of (ppy)₂Irpic. The blended films were casted by the spin coating method, as to make the device fabrication process cheaper. The selected host materials were poly(N-vinylcarbazole) (PVK), poly(9,9-dihexylfluorene) (PDHF) and N,N′-diphenyl-bis(3-methylphenyl) [1,1′-biphenyl]-4,4′-diamine (TPD). The blended films with TPD as a host were made in poly(methyl-methacrylate) (PMMA) matrix. The emission from the host molecule disappears at a particular concentration of (ppy)₂Irpic, which is determined as 5 wt%, 12 wt% and 20 wt% of the host materials for blended PVK, TPD (20% by weight of PMMA) and TPD (10% by weight of PMMA) films, respectively. No energy transfer could be observed for PDHF for the concentration range of 0.1–20 wt%. The energy transfer efficiency and the morphology of the various blends were also studied in order to find out the most appropriate host material for the device applications.     

Spatial and temporal observation of energy transfer processes in Pr-doped phosphate glasses

A spatially resolved microluminescence technique was used to measure the spatial distribution of emitted light and photon propagation into two different Pr³⁺ doped phosphate glasses. The photon diffusion length for specific wavelengths was measured. It was found that excited state re-absorption plays a crucial role in the propagation. Our results suggest that this microluminescence technique can be applied in the investigation of energy transfer processes in rare-earth doped systems.     

Hydrogen-related effects in GaInP/GaAs HBTs: incorporation, removal and influence on device reliability

We report the result of investigation on hydrogen effects on GaInP/GaAs HBT structures originating from different MOCVD and CBE suppliers. It is demonstrated that hydrogen gives rise to initial unstable electrical behaviour by cross-examination of samples with and without hydrogen either intrinsically or by thermal-assisted removal. Annealing conditions to remove hydrogen have been optimized on the basis of SIMS analyses and Gummel plot characteristics to control eventual degradation of the junctions. It has been found that under particular doping and growth conditions, C₂---H complexes can be formed. These defects appear more stable than C---H complexes which may explain the difficulty to remove hydrogen from some epitaxial layers.     

Migration and transfer of excitation energy in homogeneously dispersed porphyrin monolayers prepared from amphiphilic copolymers

Photophysical processes of tetraphenylporphyrin (TPP) units incorporated into a polymer monolayer have been studied by fluorescence spectroscopy with emphasis on energy migration and aggregate formation of TPPs in the two-dimensional sheet. Although UV–VIS absorption spectra showed few of TPP aggregates, the fluorescence was largely quenched in the high-TPP samples due to efficient energy migration and following energy transfer to the aggregates. With the aid of computer simulation, time-resolving decay analysis revealed the large diffusion coefficient of excitation energy and a low concentration of aggregate, suggesting that the TPP moieties were placed in the plane not with a random distribution, but with an ordered arrangement, probably due to some mutual repulsive forces which prevented overlap of the TPP units at the air–water interface. These results suggest that the polymer monolayer containing TPP units can be utilized as a component of molecular assemblies for designing efficient energy harvesting systems.     

Down-shifting by energy transfer in Dy-Tb co-doped YF3-based sol-gel nano-glass-ceramics for photovoltaic applications

Highly transparent sol-gel derived nano-glass-ceramics containing YF₃ nanocrystals doped with Dy³⁺ or Tb³⁺ and co-doped with Dy³⁺-Tb³⁺ have been successfully developed. A structural analysis and luminescence spectra features confirmed the incorporation of rare-earth ions into precipitated YF₃ nanocrystals, where efficient energy transfer from Dy³⁺ to Tb³⁺ takes place. Higher efficiency is obtained with increasing Tb³⁺ concentration, and no quenching effects are observed. Observed luminescence features leads the way to enhance solar cell spectral response by down-shifting of the incident solar spectrum, with an extended absorption range from 300 to 500 nm, yielding to a predominant green emission by co-doping with Dy³⁺ and Tb³⁺ ions.     

(La,Dy)2W2O9 tungstates: Selected synthesis,enhanced luminescence through Gd3+ co-doping,and favorable quantum efficiency

In this study, a series of well dispersed (La_1-xDy_x)₂W₂O₉ and (La_0.98-yGd_yDy_0.02)₂W₂O₉ phosphors were selected synthesized by co-precipitation and subsequent calcination, where the effects of RE³⁺:WO₄^2- molar ratio and solution pH were investigated. The triclinic (La_1-xDy_x)₂W₂O₉ exhibited a series of sharp emission peaks assigned to ⁴F_9/2→⁶H_15/2, ⁶H_13/2, ⁶H_11/2, ⁶H_9/2 transitions of Dy³⁺, with the 575 nm (⁴F_9/2→⁶H_13/2) emission being the strongest under the excitation of 350 nm. The optimal Dy³⁺ concentration was determined to be 2 at%. Gd³⁺ co-doping apparently affect the phase and luminescence of the final product. The product undergoes a phase transformation from triclinic to monoclinic when Gd³⁺ doping level is over 25 at%. The emission intensity, quantum yield, color purity, and thermal stability of the (La_0.98Dy_0.02)₂W₂O₉ phosphor were comprehensively enhanced by Gd³⁺ co-doping and favorable quantum efficiency of 51.23% was obtained.     

Stretchable semiconductor technologies with high areal coverages and strain-limiting behavior: demonstration in high-efficiency dual-junction GaInP/GaAs photovoltaics

Notched islands on a thin elastomeric substrate serve as a platform for dual-junction GaInP/GaAs solar cells with microscale dimensions and ultrathin forms for stretchable photovoltaic modules. These designs allow for a high degree of stretchability and areal coverage, and they provide a natural form of strain-limiting behavior, helping to avoid destructive effects of extreme deformations.     

Optical properties of Eu-activated calcium aluminosilicates synthesized in different atmospheres

The optical properties of Eu-activated CaAl₂Si₂O₈ appear to be totally dependent on the atmosphere during the synthesis processing. Compared with oxidizing/reducing-sintered samples, a significant enhancement in both the photoluminescence and afterglow for the vacuum-sintered sample is observed. The deficiency of energy transfer among the different emission centers is the prevailing contribution to the highest photoluminescence intensity of the vacuum-sintered sample. A distinction in afterglow characteristics of CaAl₂Si₂O₈:Eu prepared in different atmosphere is suggested to be mainly due to the distribution of different trap depths at three types of emission centers.     

Photoluminescence and energy transfer of terbium doped titania film

Strong visible luminescence of Tb³⁺ions due to intra-4f shell transitions are obtained from Tb³⁺-doped titania (TiO₂) films fabricated by sol-gel method. Based on the overlap of excitation band of Tb³⁺-doped TiO₂ and absorption band of undoped TiO₂, we propose an energy transfer mechanism from TiO₂ host to Tb³⁺ions. Photoluminescence (PL) intensity is found to have a well matching relation with the doping concentration of Tb³⁺ ions. Concentration quenching of PL occurs when Tb³⁺ concentration exceeds a certain value (9.5 mol%). Luminescence intensity is improved obviously after co-doped with Ce³⁺ ions because of the sensitization effect of Ce³⁺ ions and the dispersion of Tb³⁺ ions in TiO₂ system.     

Strong luminescence and efficient energy transfer in Eu3+/Tb3+-codoped ZnO nanocrystals

Single crystalline Eu³⁺/Tb³⁺-codoped ZnO nanocrystals have been synthesized by using a simple co-precipitation method. Successful doping is realized so that strong green and red luminescence can be efficiently excited by ultraviolet and near ultraviolet radiation, demonstrating an efficient energy transfer from ZnO host to rare earth ions. The energy transfer from the ZnO host to Tb³⁺ in ZnO: Tb³⁺ samples and ZnO host to Eu³⁺ in the ZnO: Eu³⁺ samples under UV excitation are investigated. It is found that the red ⁵D₀ → ⁷F₂ emission of Eu³⁺ ions decreases with increasing temperature but the green ⁵D₄ → ⁷F₅ emission of Tb³⁺ ions increases with increasing temperature, implying a different energy transfer processes in the two samples. Moreover, energy transfer from Tb³⁺ ions to Eu³⁺ ions in ZnO nanocrystals is also observed by analyzing luminescence spectra and the decay curves. By adjusting the doping concentration, the Eu³⁺/Tb³⁺-codoped ZnO phosphors emit green and red luminescence with chromaticity coordinates near white light region, high color purity and high intensity, indicating that they are promising light-conversion materials and have potential in field emission display devices and liquid crystal display backlights.     

Sensitized luminescence and energy transfer in Ce and Eu codoped calcium magnesium chlorosilicate

Ce³⁺ and Eu²⁺ ions codoped calcium magnesium chlorosilicate [Ca₈Mg(SiO₄)₄Cl₂] phosphors have been synthesized and characterized. Intense bluish-green light was observed under ultraviolet and blue light excitations. The diffuse reflection, excitation and emission spectra of Ca₈Mg(SiO₄)₄Cl₂:Ce³⁺,Eu²⁺ were measured at room temperature, and enhancement of Eu²⁺ emission was confirmed. The fluorescence lifetimes of Ce³⁺ were measured, and the mechanism of nonradiative energy transfer from Ce³⁺ to Eu²⁺ is found to be electric dipole–dipole interaction. The probabilities and efficiencies of nonradiative energy transfer were also calculated, and the results indicate that the efficiency can be as high as 86% in this material system.     

Luminescence properties and energy transfer studies of a color tunable BaY2Si3O10:Tm3+,Dy3+ phosphor

A series of color-tunable and white light emitting phosphors BaY₂Si₃O₁₀:Tm³⁺,Dy³⁺ were synthesized by a high temperature solid-state reaction, and their phase structure, photoluminescence properties, and energy transfer processes between rare-earth ions were investigated in detail. Upon UV excitation, white light emission depending on dopant concentrations could be achieved by integrating a blue emission band located at 458 nm and an orange one located at 576 nm attributed to Tm³⁺ and Dy³⁺ ions, respectively. In addition, the energy transfer process between Tm³⁺ and Dy³⁺ ions was demonstrated to be a resonant type via a dipole–quadrupole mechanism. Preliminary studies showed that the phosphor might be promising as a single-phased white-light-emitting phosphor for UV chip pumped white-light LEDs.     

Single-component and white light-emitting phosphor BaAl2Si2O8: Dy3+, Eu3+ synthesis,luminescence, energy transfer,and tunable color

A series of Dy³⁺ - Eu³⁺ co-doped BaAl₂Si₂O₈ phosphors were prepared via the conventional solid-state reaction method. Their crystal structure, luminescent characteristic and lifetime were investigated. The optimum doping concentrations of Dy³⁺and Eu³⁺ are both 0.05 for Dy³⁺ or Eu³⁺ singly doped BaAl₂Si₂O₈. Furthermore, BaAl₂Si₂O₈: 0.05Dy³⁺ and BaAl₂Si₂O₈: 0.05Eu³⁺ emits yellow and red light. The emission color of BaAl₂Si₂O₈: Dy³⁺, Eu³⁺ could be tuned from yellow to white due to the energy transfer. This energy transfer from Dy³⁺ to Eu³⁺ was confirmed and investigated by photoluminescence spectra and the decay time of energy donor Dy³⁺ ions. With constantly increasing Eu³⁺ concentration, the energy transfer efficiency from Dy³⁺ to Eu³⁺ in BaAl₂Si₂O₈ host increased gradually and reached as high as 81%, the quantum yield was about 47.43%. BaAl₂Si₂O₈: Dy³⁺, Eu³⁺ phosphors can be effectively excited by UV (about 348 nm) light and emit visible light from yellow to white by altering the concentration ratio of Dy³⁺ and Eu³⁺, indicating that the phosphors have potential applications as a white light-emitting phosphor for display and lighting.     

Luminescent properties and energy transfer processes of co-doped Yb–Er poly-crystalline YAG matrix

A poly-crystalline YAG matrix was obtained through the precipitation method; this matrix was single and co-doped with ytterbium and erbium ions, i.e., Yb:YAG, Er:YAG and Yb,Er:YAG. It is found that the measured luminescent properties are similar to those reported for a mono-crystal YAG matrix. In addition, by studying the energy transfer processes in co-doped samples, it is shown that at high erbium concentrations the red emission is enhanced through an up-conversion process that takes place from the ⁴I_13/2 to the ⁴F_9/2 state of erbium ions. This enhanced red emission becomes comparable in intensity to the observed green emission and occurs by pumping at 800 nm through a back energy transfer process.