Energy Migration Research Articles

A Generalized Approach to Photon Avalanche Upconversion in Luminescent Nanocrystals.

Photon avalanching nanoparticles (ANPs) exhibit extremely nonlinear upconverted emission valuable for subdiffraction imaging, nanoscale sensing, and optical computing. Avalanching has been demonstrated with Tm3+-, Pr3+-, or Nd3+-doped nanocrystals, but their emission is limited to a few wavelengths and materials. Here, we utilize Gd3+-assisted energy migration to tune the emission wavelengths of Tm3+-sensitized ANPs and generate highly nonlinear emission from Eu3+, Tb3+, Ho3+, and Er3+ ions. The upconversion intensities of these spectrally discrete ANPs scale with nonlinearity factor s = 10-17 under 1064 nm excitation at power densities as low as 7 kW cm-2. This strategy for imprinting avalanche behavior on remote emitters can be extended to fluorophores adjacent to ANPs, as we demonstrate with CdS/CdSe/CdS core/shell/shell quantum dots. ANPs with rationally designed energy transfer networks provide the means to transform conventional linear emitters into a highly nonlinear ones, expanding the use of photon avalanching in biological, chemical, and photonic applications.

Realization of broadband ∼1.8 μm luminescence by level population inversion in Tm3+ doped tellurite glass for laser applications

This study aims to investigate the characteristics of Tm3+ doped TeO2–B2O3–Al2O3Na2O glass. The structures that exist in host glass were obtained from the Raman spectrum. Due to the introduction of sodium and aluminum, the TeOTe bond is broken, and the amount of bridge oxygen is reduced, causing the transformation of [TeO4] structural units to [TeO3+1] and [TeO3] structural units. The spectrum characteristics of Tm3+ in the samples were predicted by theoretical calculations. Prepared Tm3+ doped glass has large Ω2 and small Ω6, which is connected to the ligand field's asymmetry and the stiffness of the network. Furthermore, under the excitation of 808 nm laser diode, broadband ∼1.8 μm fluorescence was observed and the effective fluorescent bandwidth Δλeff of 1830 nm emission in the glass is about 234 nm. For the sample with 0.5 mol% Tm2O3, the large values of emission cross-section product (9.08 × 10−21 cm2) and σemi × Δλeff product (21.25 × 10−26 cm3) indicate a good gain property as well as a good gain bandwidth. Eventually, the energy migration process and cross-relaxation process between thulium ions were analyzed. The micro-parameter of the CR process is 4.049 × 10−40 cm6/s. Hence, the results indicate that the prepared Tm3+ doped glass could be a potential candidate for ∼2 μm mid-infrared laser devices.

Exploring the persistence and transience of energy poverty: evidence from a Greek household survey

The present study uses four rounds of household panel data to investigate consensual-based energy poverty in Greece. Employing dynamic Probit random effects and Wooldridge conditional maximum likelihood (WCML) estimators, we find evidence of genuine state dependence effects in consensual-based energy poverty among Greek households. Poverty persistence (10-12%) effects are also evident in our data. Socioeconomic, demographic, market, household, and climatic characteristics are essential predictors of energy poverty. Around 9–10% of the households seem chronically energy poor, while education, income level, dwelling characteristics, migration background, and employment status affect the chances of suffering and exiting from energy poverty. Empirical results have significant policy implications that could mitigate residential energy poverty.

Stability and Degradation in Lead Halide Perovskite Nanocrystals via Regulation of Lattice Strain.

It is still quite challenging to achieve high-performance and stable blue perovskite materials due to their instability and degradation. The lattice strain provides an important pathway to investigate the degradation process. In this article, the lattice strain in perovskite nanocrystals was regulated by the ratio of Cs+, EA+, and Rb+ cations with different sizes. Their electrical structure, formation energy, and ion migration activation energy were calculated with the density functional theory (DFT) method. The luminescence properties and stability of blue lead bromide perovskite nanocrystals were analyzed with spectra regulation from 516 to 472 nm. It was demonstrated that the lattice strain plays an important role in the luminescence performance and degradation process of perovskite materials. The study provides the positive correlation between lattice strain and degradation as well as luminescence properties in lead halide perovskite materials, which is of great importance in uncovering their degradation mechanism and developing stable and high-performance blue perovskite materials.

Modeling and optical characterization of γ-irradiated Cu2+Doped borate glasses as bandpass optical filters

Novel systems of alkali borate glasses with various CuO concentrations from 0.2 up to 1.5 mol% have been prepared by the conventional melt-quenching technique. Several optical, physical, and structural characteristics of the prepared glasses have been examined before and after incremental γ-irradiation doses. The intensity of the blue color of the glasses is controlled by CuO concentrations, with Cu+/Cu2+ in ionic equilibrium. UV/Vis absorption peaks centered at 740 nm can be correlated to a tetragonal twist of the octahedral structure due to 2B1g-2B2g electronic transition. Photoluminescence (PL) extinction spectra can be attributed to Cu+-Cu+ energy migration after Cu+→ Cu2+ energy transfer. Electron spin resonance (ESR) measurements show sharp signals at magnetic fields ranging from 3344 to 3326 Gauss (g = 2.08–2.09), corresponding to the divalent copper state (Cu2+) in octahedral units. FTIR spectra revealed stable structures towards irradiation. Either an increase in CuO content or irradiation led to greater UV absorption, lower PL intensity, lower bandgap (Eopt), lower density, larger molar volume, and slightly more intense ESR signals. Copper ions can be considered as modifiers since they increase the degree of disorder in the glassy network. UV/NIR band stop, FWHM, and center of peaks confirmed the high efficiency of the prepared Cu-doped glasses as colored optical bandpass filters in the green and near-blue regions for UV/IR light laser protection.

Theoretical and experimental design for promoting the oxygen reduction activity of SrCoO3 perovskite-based cathode for PCFCs by bimetal-ions doping

Low temperatures ceramic fuel cells (LT-CFCs) including solid oxide and proton conducting fuel cells (SOFCs/PCFCs), hold a great promise for abundant large-and small scaled real-application. Sluggish oxygen reduction reaction (ORR) activity at lower operating temperatures hindered the development of LT-CFCs. Strontium cobalt-based perovskite-oxides (SrCoO3−δ) materials have been frequently considered as a promising next-generation cathode for CFCs. Different doping stratigy are used to improve the mixed ionic (H+/O2-) and electronic conductivity of SrCoO3−δ, which help to extend the electrochemical reaction zone for oxygen reduction. Herein, we introduced bi-metal doping of La and Ce at B-site of SrCoO3−δ to produce SrCo0.8Ce0.1La0.1O3-δ (SCCL) pervoskite structure for PCFCs cathode. The developed SLCC cathode exhibits ultra-low-area-specific polarization resistance of 0.11 Ω ∙ cm2 and large oxygen reduction reaction (ORR) response at intermediate operating temperatures. We have demonstrated power density of 510 mWcm−2 using SLCC cathode over proton conducting BaCe0.7Zr0.2Y0.1O3-δ electrolyte at 600 °C and even with possible operation at 450 °C. The ORR activity of SLCC perovskites is found to strongly rely on the two proposed descriptors, where La and Ce doping at Co-site-in SrCoO3−δ located in the auspicious zone for predicting the moderate value of vacancy formation energy and ions migration barrier. The obtained results can further help to develop more functional SrCoO3−δ-based electro-catalysts for LT-CFCs and other related applications.

Luminescence mechanism modeling in Eu3+ doped polymethyl methacrylate by microscopic and macroscopic approaches

In this work, red emission is obtained from polymethylmethacrylate (PMMA) doped with europium ions (Eu3+) under ultraviolet excitation. PMMA:Eu3+ samples are successfully synthesized by free radical polymerization and characterized by X-ray diffraction (DRX) and photoluminescence spectroscopy. DRX patterns show the amorphous nature of PMMA and the crystalline phase of Eu3+. Optical bands related to Eu3+ in the polymer composites are observed in excitation and emission spectra and are attributed to the corresponding electronic transitions. The interaction between Eu3+ ions is studied through the emission spectra of PMMA:Eu3+ samples with different Eu3+ concentrations under an excitation of 395 nm. Luminescent emission response is analyzed theoretically using rate equation models with macroscopic and microscopic approaches. The numerical solution of the model agrees with the experimental results. The rate equation model considers that the interaction between Eu3+ ions is driven by the direct energy migration (Eu3+ → Eu3+). The Förster distance and the ratio of the pumping rate and the radiative and nonradiative relaxation constant of the PMMA:Eu3+ system are calculated, and it is demonstrated that the energy transfer rate and the quantum efficiency for both approaches agree. The results allow to predict the luminescent behavior of the PMMA:Eu3+ system with different concentrations of Eu3+ ions, and the method described here can be applied to other similar systems.

Femtosecond exciton relaxation in chlorosomes of the photosynthetic green bacterium <i>Chloroflexus aurantiacus</i>

In the green bacteria Chloroflexus (Cfx.) aurantiacus, the process of photosynthesis begins with the absorption of light by chlorosomes, peripheral antennas consisting of thousands of bacteriochlorophyll c (BChl c) molecules combined into oligomeric structures. In this case, excited states are formed in BChl c, the energy of which migrates along the chlorosome towards the baseplate and further to the reaction center, where the primary charge separation occurs. Energy migration is accompanied by nonradiative electronic transitions between numerous exciton states, that is exciton relaxation. In this work, we studied the dynamics of exciton relaxation in Cfx. aurantiacus chlorosomes using difference femtosecond spectroscopy at cryogenic temperature (80 K). Chlorosomes were excited by 20 fs light pulses at wavelengths from 660 to 750 nm, and the difference (light-dark) kinetics were measured at a wavelength of 755 nm. Mathematical analysis of the data obtained revealed the kinetic components with characteristic time constants of 140, 220, and 320 fs responsible for the exciton relaxation. As the excitation wavelength decreased, the number of components and their relative contribution increased. Theoretical modeling of the results obtained was carried out on the base of the cylindrical model of BChl c aggregates. Nonradiative transitions between groups of exciton bands were ascribed by the system of kinetic equations. The model in which the energetic and structural disorder was taken into account turned out to be the most adequate.

Effects of Eu3+ concentration on the thermoluminescence properties of narrow-band red-emitting Eu3+: SrLaGaO4 phosphor

A series of Eu3+ doped SrLaGaO4 ceramic phosphors were successfully synthesized by the solid-state reaction method. The optical properties of (x at%) Eu: SrLaGaO4 samples with x = (0.1, 1, 3, 5, 10, 15, 20, 30) were investigated and analyzed. The room temperature emission spectra of xEu: SrLaGaO4 ceramic samples, under blue and UV excitation wavelengths, showed a maximum relative emission intensity at 620 nm, corresponding to the hypersensitive 5D0→7F2 transition, with a full width at half maximum (FWHM) of ∼7 nm, for a doping concentration with Eu3+ ions of 10 at% (x = 10). The asymmetry parameter (the ratio between the 5D0→7F2 and 5D0→7F1 transitions) and the critical distance between Eu3+ ions allowed to obtain a very good color purity of the red emission (88 %) for the 10Eu: SrLaGaO4 phosphor. The cross-relaxation and energy migration processes between Eu3+ ions in the 10Eu: SrLaGaO4 sample are negligible and do not contribute to the quenching of the 5D0 emitting level. From the investigation of the temperature-dependent luminescence intensity ratio (LIR) of the 5D0→7F2 and 5D0→7F1 transitions of the 10Eu: SrLaGaO4 sample in the range of 10–575 K, the relative sensitivity parameter (Sr) was found to be 0.4 % K−1 at 10 K and 0.45 % K−1 at 450 K. These results suggest that 10Eu: SLO phosphor is a versatile material that could be operated as a temperature sensor over a wide range of temperatures, as well as a narrow-band red-emitting phosphor for phosphor-converted white light-emitting diodes (pc-WLEDs).

Fe−O−P bond in MIL-88A(Fe)/BOHP heterojunctions as a highway for rapid electron transfer to enhance photo-Fenton abatement of enrofloxacin

MIL-88A(Fe)/BOHP (MxBy) heterojunctions were firstly fabricated via a ball-milling method, which were adopted as catalysts to activate hydrogen peroxide (H2O2) with the aid of visible light illumination to build a heterogeneous photocatalysis-Fenton-like synergetic system. The catalytic degradation efficiency of enrofloxacin (ENR) (10 mg/L) in the optimal M3B7/Vis/H2O2 system reached nearly 100 % with appreciable mineralization ability. Experimental and theoretical results co-unraveled that the formation of interfacial Fe−O−P chemical bonds between MIL-88A(Fe) and BOHP effectively reduced the charge transfer energy barrier (59.41–26.69 eV) and migration distance (3.104–1.917 Å), allowing the photo-generated carriers to be efficiently separated in space and improving the activation efficiency of H2O2 for producing more active species. Impressively, the constructed M3B7/Vis/H2O2 system was a cost-optimal and green technology for antibiotic wastewater treatment based on Electrical Energy per Order (EE/O) concept, life cycle assessment (LCA) and quantitative structure–activity relationship (QSAR) analysis.

Photoluminescence of Tb3+/Sm3+ ions co-activated in SrO - Na2O - WO3 - tellurite glasses for multicolor laser applications

A novel series of Tb3+, Sm3+ single doped and Tb3+/Sm3+ co-incorporated tungsten tellurite glasses were synthesized by melt quenching technique and corresponding structural as well as luminescence features of the prepared glasses have been reported here. Spectral overlapping between the luminescence spectra of Tb3+ ions and the excitation spectra of Sm3+ ions manifests that the energy transfer process takes place from Tb3+ ions to Sm3+ ions. By using the dual excitations at 377 and 484 nm, the titled co-doped glasses emit green light of wavelength 542 nm along with reddish – orange colour light at 599 nm. In addition to this, there is no possibility of reverse energy transfer which is validated with the help of excitation at 403 nm (Sm3+ ions) as major evidence. The lifetimes of all co-doped glasses decline with increasing Tb3+ doping level in the ligand matrices, indicating the energy migration process takes place from Tb3+→ Sm3+. The chromaticity coordinates of all synthesized co-doped glasses lie in yellowish-orange region of CIE1931 diagram and it shifts to deep yellow region when Tb3+ ion concentration varies. Our findings propose that the titled glasses can be used as visible laser materials for multicolor laser applications.

Effect of thermal treatment on ZnSe quantum dots and energy transfer in borosilicate glasses doped with ZnSe and Er3+/ZnSe

This study assessed the effect of heat treatment on the spectroscopic properties of borosilicate glasses doped with ZnSe quantum dots (QDs) and co-doped with Er3+/ZnSe. The gradual growth of ZnSe QDs upon heat treatment was probed by TEM. The emission intensity of ZnSe-doped glasses rapidly increased up to 20 h of heat treatment because of the formation and development of ZnSe QDs, then gradually decreased due to the rise in non-radiative loss between the enlarged particles. With increasing number of ZnSe QDs, the energy transfer from QDs to Er3+ ions became prominent in Er3+/ZnSe-co-doped glasses, showing enhanced emission up to 20 h of heat treatment. The excited state 4S3/2 lifetime values, increased with heat treatment, whereas an opposite trend was observed for the lifetime values of state 4I13/2, indicating a possible participation of the ZnSe QDs in facilitating the energy migration mechanism through the 4I13/2 level, followed by non-radiative decay.

Characteristic stacked structures and luminescent properties of dinuclear lanthanide complexes with pyrene units.

A novel design strategy of stacked organic fluorophores using dinuclear lanthanide (Ln(III)) complexes is demonstrated for the formation of excimer. The dinuclear Ln(III) complexes are composed of two Ln(III) (Eu(III) or Gd(III)) ions, six hexafluoroacetylacetonate (hfa), and two pyrene-based phosphine oxide ligands. Single-crystal analysis revealed a rigid pyrene-stacked structure via CH-F (pyrene/hfa) intramolecular interactions. The rigid aggregation structures of the two-typed organic ligands around Ln(III) resulted in high thermal stability (decomposition temperature: 340°C). The aggregated ligands exhibited excimer-type green emission from the stacked pyrene-center. The change in the Ln(III) ion promotes effective shifts of excimer emissions (Gd(III):500nm, Eu(III):490nm). The organic aggregation system using red-luminescent Eu(III) also provides temperature-sensitive ratiometric emission composed of π-π* and 4f-4f transitions by energy migration between aggregated ligands and Eu(III).

Comparison of spectral and temporal fluorescence parameters of aqueous tryptophan solutions frozen in the light and in the dark

Comparative investigation of temperature dependences of the fast and slow fluorescence decay components in an aqueous solution of tryptophan molecules, frozen in the light and in the dark, was made in the range of –170 °C to + 20 °C. A quantitative analysis of the model of direct and reverse electronic transitions in tryptophan molecule from an excited state to the ground state and to the charge transfer state (CTS) was carried out. Three main spectral regions of tryptophan fluorescence are distinguished that differ in behavior of temperature dependences of the transition rates from the excited state of tryptophan to CTS. The key role of the hydrogen bond system dynamics determining the nonlinear behavior of tryptophan fluorescence parameters in the selected spectral regions is shown. The nonlinear behavior of the fluorescence lifetime, position and shape of fluorescence spectra with changing temperature is determined by tryptophan interaction with an aqueous medium. Besides, the rate of solvation shift of fluorescence spectra is observed. The important role of temperature rearrangements in the system of hydrogen bonds of the water solution surrounding the tryptophan molecule in the excited state is shown.The obtained results can be used to explain the key role of hydrogen bonds in the processes of charge transfer and energy migration in biological systems.

A Model of the Temperature Dependence of Concentration Quenching of Luminescence in Doped Insulators

We have developed a model for the temperature dependence of the concentration quenching of luminescent materials, and have applied this model to YAG:Ce. This model of concentration quenching is based on an assumption in which, following excitation of an activator ion, there is energy migration via non-radiative energy transfer among similar ions, finally ending at a “killer” site. The temperature dependence of a single ion-ion energy transfer process is assumed to be determined solely by the overlap of the emission and absorption bands of the donor and acceptor ions. Using our previous results to calculate the temperature dependence of this overlap function, we propose a model for calculating the effect of concentration quenching on the lifetime of the luminescent ion. This model has two parameters, one of which is the average number of energy transfer jumps that occur before reaching a killer site. In the case of YAG:Ce, we show that the observed temperature dependence of the Ce lifetime at various concentrations can be explained satisfactorily using this model.

Probing free and bound excitons in Eu-doped CsPbBr3 by temperature-dependent photoluminescence and time-resolved photoluminescence

This report presents the recombination mechanisms of free and bound excitons in Eu-doped CsPbBr3. Through temperature-dependent photoluminescence (PL), the emission intensity of free excitons decreased as the temperature decreased from 300 to 10 K. The emission intensity of bound excitons appeared at T = 100 K and increased rapidly as the temperature decreased to 10 K. These observations are attributable to the dynamic migration of thermal energy between free and bound excitons, which is caused by the richer Eu3+ ion. The exciton-like emission can be demonstrated through power-dependent PL. Furthermore, the radiative and nonradiative recombinations of the bound exciton were determined through temperature-dependent time resolved photoluminescence (TR-PL). As a function of emission energies, the PL lifetimes of the bound exciton were investigated to corroborate that the bound exciton has a larger localization depth with decreasing temperature, which is another reason for the increased emission intensity of the bound exciton.

Stimuli-responsive triplet-triplet annihilation upconversion with guanidyl functionalized annihilators for enhanced ratiometric sensing of trace water in MeOH

Adding stimuli-responsiveness to TTA-UC is appealing as it provides a unique platform for biosensing and chemosensing. Here, we synthesized three guanidyl functionalized annihilators A-1∼A-3 for stimuli-responsive TTA-UC. The annihilator A-1 self-assembled into microparticles in aprotic solvent like THF, CHCl3 and toluene with the DPA units arranged in an ordered manner via the strong hydrogen bonding interactions between the guanidyl groups, which guaranteed effective energy migration of triplet excitons, thus, efficient TTA-UC with ΦUC of 10.2% was observed in THF. While MeOH degraded the well-aligned aggregates, accompanied by the decrease of ΦUC to 2.9%. For A-2 and A-3, weak UC emissions in all the investigated solvents were observed due to π-π stacking of anthracene core. Interestingly, the destroyed hydrogen-bonding network of A-1 was successfully reconstructed by the addition of trace water with ΦUC recovered from 2.9% to 9.2% when 3% water was added. We also found that the well-aligned aggregates significantly accelerated the TTA process but have no impact on the TTET process, thus, enhanced ratiometric sensing of trace H2O in methanol was achieved with the unchanged phosphorescence as the internal standard and a detection limit as low as 0.084% (v/v) was achieved.

Strongly reduced hysteresis scintillation luminescence in lanthanide doped fluoride nanoscintillators

X-ray excited optical luminescence (XEOL) imaging shows great potential applications in security inspection and computed tomography. Lanthanide doped fluoride nanoscintillators (NSs) do not only exhibit advantages of simple synthetic procedures, tunable emissions and high environmental stability, but also show many new properties through constructing core/shell architectures. However, the strong hysteresis scintillation luminescence greatly restrict their practical applications. Herein, we verify that this hysteresis can be fully restricted by employing core/shell structure to inhibit the generation of Frenkel defects and incorporating Gd3+ ions to introduce extra energy migration process. As a result, upon continuous X-ray irradiation, the XEOL intensity of the LiYF4: 15Tb/25Gd@LiYF4 core/shell NSs almost keeps unchanged. Our results provide important information for designing new NSs with greatly reduced hysteresis scintillation luminescence, which show promising applications in the field of stable X-ray imaging.

Shell Engineering on Thermal Sensitivity of Lifetime-Based NIR Nanothermometers for Accurate Temperature Measurement in Murine Internal Liver Organ.

Lifetime-based NIR luminescent nanothermometry is ideally suited for temperature detection in living cells and in vivo, but the thermal sensitivity (Sr) modulation remains elusive. Herein, a thorough investigation is performed to unveil the shell effect on lifetime-based Sr by finely controlling the shell thickness of lanthanide-doped core-shell-shell nanoparticles. Owing to the space-dependent energy transfer and back energy transfer between Nd3+ and Yb3+ as well as the energy migration to surface quenchers, both active and inert shells can regulate the thermal-dependent nonradiative decays and NIR luminescence lifetime of Yb3+, which in turn modulates the Sr from 0.56% to 1.54% °C-1. After poly(acrylic acid) modification of the optimal architecture, the tiny nanoprobes possess robust stability to fluctuations in the microenvironment, which enables accurate temperature mapping of inflammation in the internal liver organ of living mouse. This work will provide new insights for optimizing Sr and guidance for precise temperature measurements in vivo.

Visible luminescence and energy migration mechanism of Sm3+ in phospho-tellurite glasses by co-activating with Tb3+ ions for solid state lighting device applications

Luminescence characteristics including the energy migration mechanism of Sm3+/Tb3+ co-activated BaO–PbO–Na2O-phospho-tellurite glasses have been reported. The XRD, FTIR, UV–VIS-NIR absorption, luminescence and CIE are examined to expose their structural and optical properties. The luminescence spectra of Sm3+/Tb3+ co-doped glasses show the green emission (Tb3+ ions) and reddish-orange colour emission (Sm3+ ions) upon dual excitation at 377 nm and 484 nm. The reduction of emission intensities (Tb3+ions) and self-emission quenching effect of Sm3+ ions have been observed when concentration of Sm3+ ions increases into the glass matrix which reveals the evidence of non-radiative energy transfer from Tb3+ (sensitizer) to Sm3+ (activator) ions. The phenomenon of energy migration is also verified with the help of Forster-Dexter theory. The chromaticity coordinates of all Sm3+/Tb3+ co-activated glasses shift from greenish to yellowish region with dependence of activator's concentration. The paper highlights that the present glasses can be utilized in light emitting devices.