Excitation Migration Research Articles

Insights into the Structural, Thermal/Dilatometric, and Optical Properties of Dy3+-Doped Phosphate Glasses for Lighting Applications.

Dysprosium-doped glasses are of interest for applications in light-emitting devices, yet the full range of effects of Dy3+ ions on glass properties is not fully understood. In this work, phosphate glasses with 50P2O5-(50 - x)BaO-xDy2O3 (0 ≤ x ≤ 4.0 mol %) nominal compositions were prepared by melting and the impact of Dy3+ ions on glass physical, structural, thermo-mechanical, and optical properties was evaluated. Following refractive index, density, and X-ray diffraction characterizations, the glasses were studied comprehensively through Raman spectroscopy, X-ray photoelectron spectroscopy, dilatometry, optical absorption, and photoluminescence (PL) spectroscopy. The thorough investigation and data analyses shed light on the Dy3+-driven structural and thermal properties reported here for the first time. The thermal expansion behavior was put in context with the reported data for other lanthanides and analyzed in the framework of the high ionic field strengths, leading to tighter glass networks. Further, a detailed analysis of the absorption, PL, and emission decay curves was carried out, providing insights into the origin of the optical behavior. Supported is the hypothesis that the cross-relaxation channels between Dy3+ ions taking place at low concentrations are responsible for the decrease in the decay times while the PL attractive for lighting applications still improves. Conversely, high Dy3+ concentrations facilitate the emission quenching proceeding via an electric dipole-dipole interaction likely incorporating the resonant excitation migration pathway for Dy3+-Dy3+ mean distances shorter than ∼15 Å.

Preparation of 2D/2D CoAl-LDH/BiO(OH)XI1−X Heterojunction Catalyst with Enhanced Visible–Light Photocatalytic Activity for Organic Pollutants Degradation in Water

Hydrotalcite/bismuth solid solution (2D/2D CoAl-LDH/BiO(OH)XI1−X) heterojunction photocatalysts were fabricated through a hydrothermal route. Because of their identical layered structure and interlayer hydroxides, CoAl-LDH(2D) and BiO(OH)XI1−X(2D) form a tightly bonded heterojunction, resulting in efficient light absorption, excitation, and carrier migration conversion. At the same time, the large specific surface area and abundant hydroxyl groups of the layered structure make the heterojunction catalyst exhibit excellent performance in the photocatalytic degradation of organic pollutants. Under visible light irradiation and in the presence of 1 g/L of the catalyst, 10 mg/L of methyl orange (MO) in water could be completely degraded within 20 min, and the degradation rate of tetracycline (TC) reached 99.23% within 5 min. CoAl-LDH/BiO(OH)XI1−X still maintained good photocatalytic degradation activity of tetracycline after five cycles, and the structure of the catalyst did not change. The reaction mechanism related to the degradation of TC by photocatalytic reactions was explored in detail, and the photoexcitation of the semiconductor heterojunction, as well as the subsequent free radical reaction process and the degradation pathway of TC were clarified. This work provides a promising strategy for the preparation of efficient photocatalytic materials and the development of water purification technology.

Photoluminescence Study of Undoped and Eu-Doped Alkali-Niobate Aluminosilicate Glasses and Glass-Ceramics.

In this study, the photoluminescence (PL) behavior of two aluminosilicate glass series containing alkali-niobates ranging from 0.4 to 20 mol% was investigated. The glasses exhibit an intense visible emission centered at ~18,400 cm-1 for the peralkaline series and at higher energies (~19,300 cm-1) for the metaluminous glasses. However, the photoluminescence emission intensity varies significantly with the niobate content and the bulk chemistry. PL and fluorescence lifetime measurements indicate that the broad emission bands result from the overlap of different niobate populations, whose distribution changes with niobate content. The distinct PL behavior in the two glass series was related to the structural evolution of the niobate units upon niobium addition. An enhancement of the visible emission was observed for a higher fraction of distorted [NbO6] units. Eu-doping was carried out as a structural probe of the glass network, and also to determine if these glasses could be used as potential rare earth element (REE) activators. The crystal field strength around Eu ions is strongly dependent on the bulk chemistry and the niobate content. Furthermore, the peralkaline series showed energy transfer from the host [NbO6] to Eu3+, confirming the feasibility of exploring niobate glasses and glass-ceramics as lanthanide ion-activated luminescent materials. In addition, glass-ceramics (GCs) containing alkali-niobate phases with a perovskite-like structure were developed and studied to verify the optical performance of these materials. It was verified that the bulk chemistry influences crystallization behavior, and also the photoluminescence response. The transparent GC from the metaluminous series exhibits a quenching of the Eu3+ emission, whereas an enhanced emission intensity is observed for the peralkaline GC. The latter shows a strong excitation-dependent PL emission, suggesting energy transfer and migration of electronic excitation from one Eu population to another. Additionally, Eu3+ emissions arising from the D15 and D25 excited states were observed, highlighting the low phonon energy achievable in niobo-aluminosilicate hosts.

Boosting photocatalytic overall water splitting performance by dual-metallic single Ni and Pd atoms decoration of MoS2: A DFT study

Herein, we propose a novel MoS2 supported single Ni and Pb atoms catalyst (Ni/Pd@MoS2), on which the enhanced efficiency of water splitting is confirmed by quantitative density functional theory (DFT) calculations. The Bader charge, electron density difference (EDD) and projected density of state (PDOS) analysis demonstrates that the moderate electron transfer is reached by the synergy of the Ni atom and the Pd atom. As extra channel for excited carrier migration, the present of defect states obviously prolongs the carrier lifetime (143.45 ps). The unique electronic environment contributes to the electronic synergy and spatially separated of the effective active sites, leading the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) happen smoothly on the Pd and S site, respectively. Our research presents a novel idea for the development of highly efficient diatomic catalysts towards overall water splitting.

Influence of donor or acceptor presence on excitation states in molecular chains: Nonadiabatic polaron approach.

In this paper, we considered a molecular structure that consists of a molecular chain and an additional molecule (donor or acceptor) that can inject (or remove) single excitation (vibron, electron, etc.) onto the molecular chain. We assumed that the excitation forms a self-trapped state due to the interaction with mechanical oscillations of the chain structure elements. We analyzed the energy spectra of the excitation and showed that its state (when it migrates to the molecular chain) has the properties of the nonadiabatic polaron state. The conditions under which the excitation can migrate from one subsystem to another one were considered. It was shown that the presence of a "donor" molecule cannot significantly change the properties of the excitation located on the molecular chain. At the same time, the molecular chain can affect the position of the energy level of the excitation localized on the donor subsystem. Indirectly, this can influence the process of excitation migration from one subsystem to another one. The influence of the basic energy parameters of the system and the environment temperature on this process are discussed. The entire system was assumed to be in thermal equilibrium with the environment.

On the mechanism of luminescent YVO4:Eu3+ as turn-off luminescent probe for selective detection of Cu2+ ions

Poly(acrylic acid) (PAA) funtionalized luminescent YVO4:Eu3+ nanocrystals could be successfully prepared by polyol method. The surface functionalization of YVO4:Eu3+ nanocrystal is ascertained from the Fourier transform infrared spectroscopy. Photoluminescence studies confirm the occurrence of resonant energy transfer from vanadate (VO43−) to the excited states of Eu3+ resulting in strong red emission. Luminescent YVO4:Eu3+ nanocrystals shows the selective detection of Cu2+ among various metal ions. The intensity of Eu3+ emission could be reduced to ∼92 % with 9 μM of Cu2+. The behavior of quenching is dominant by dynamic nature. The detection limit could be achieved ∼17 nM. With the calculated decay rate constants, the excited photon migration from Eu3+ to Cu2+ ions is ascertained which leads in quenching of Eu3+ emission. While, from VO43− to Cu2+ is insignificant. Alkaline medium is more sensitive towards Cu2+ detection. This nanocrystal has a response time of ∼7 min.

Photophysical analysis of dual fluorescence and phosphorescence emissions observed for semi-aliphatic polyimides at lower temperatures.

The photoluminescence (PL) properties of four types of blue fluorescent semi-aliphatic polyimides (PIs) derived from aromatic dianhydrides (ODPA, BPDA, HQDEA, and BPADA) and an alicyclic diamine (DCHM) were investigated at temperatures ranging from room temperature (RT, 298 K) to 30 K to analyse the origins of their non-radiative relaxation (NR) processes. These PIs exhibited significant increases in fluorescence (FL) intensity and lifetimes when lowering the temperature, stabilising below 100 K. The PIs containing ether (-O-) linkages showed a shoulder peak at around 500 nm below 150 K, which is attributable to phosphorescence (PH). These results show that the NR deactivation at RT includes three processes: intersystem crossing (ISC) from the excited singlet (S1) to the triplet (T1) state, temperature-dependent NR from the S1 state, which becomes suppressed below around 100 K, and temperature-independent NR. Based on the analyses of the temperature dependences, polymer structures, and quantum chemical analysis of molecular orbitals, we contemplate that the temperature-dependent NR is attributable to the excitation quenching by defect states mediated by excitation migration, and the temperature-independent NR may be caused by the deactivation of the excited state induced by molecular vibrations.

In situ surface-enhanced infrared absorption analysis of the excited carrier transfer from n-type Si photoelectrode to Pt oxygen evolution cocatalyst by probing adsorbed CO molecules

BackgroundThe development of efficient photoelectrode material for water-splitting reactions has received considerable attention as it produces hydrogen gas from water using solar energy. Photoexcited holes migrate to the surface of an n-type photoelectrode oxidizing water to oxygen, and excited electrons migrate to the counter electrode reducing protons to hydrogen. However, there is limited knowledge of excited carrier transfer. MethodsTherefore, in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) has been used to investigate the excited hole migration from n-type Si photoelectrode to Pt oxygen evolution cocatalysts by monitoring the frequency of CO adsorbed on the Pt cocatalysts. Significant findingsAt the positive potential of Si photoelectrode under UV irradiation, the SEIRA spectrum of the CO adsorbed on the Pt cocatalyst shifted to higher wavenumbers owing to the excited hole transfer from n-type Si photoelectrode to the Pt cocatalyst attributed to band bending at the interface between the Si photoelectrode and Pt cocatalysts. It was successfully demonstrated that the regulation of photoexcited carriers was crucial for enhancing the photoelectrochemical activity of water splitting.

Non-Covalent Assembly of Multiple Fluorophores in Edible Protein/Lipid Hydrogels for Applications in Multi-Step Light Harvesting and White-Light Emission.

The design and production of biodegradable and sustainable non-toxic materials for solar-energy harvesting and conversion is a significant challenge. Here, our goal was to report the preparation of novel protein/lipid hydrogels and demonstrate their utility in two orthogonal fundamental studies-light harvesting and white-light emission. Our hydrogels contained up to 90% water, while also being self-standing and injectable with a syringe. In one application, we loaded these hydrogels with suitable organic donor-acceptor dyes and demonstrated the energy-transfer cascade among four different dyes, with the most red-emitting dye as the energy destination. We hypothesized that the dyes were embedded in the protein/lipid phase away from the water pools as monomeric entities and that the excitation of any of the four dyes resulted in intense emission from the lowest-energy acceptor. In contrast to the energy-transfer cascade, we demonstrate the use of these gels to form a white-light-emitting hydrogel dye assembly, in which excitation migration is severely constrained. By restricting the dye-to-dye energy transfer, the blue, green, and red dyes emit at their respective wavelengths, thereby producing the composite white-light emission. The CIE color coordinates of the emission were 0.336 and 0.339-nearly pure white-light emission. Thus, two related studies with opposite requirements could be accommodated in the same hydrogel, which was made from edible ingredients by a simple method. These gels are biodegradable when released into the environment, sustainable, and may be of interest for energy applications.

Modeling of the entangled states transfer processes in microtubule tryptophan system

The paper simulates the process of the migration of a single energy excitation along a chain of tryptophans in cell microtubules connected by dipole–dipole interaction. The paper shows that the excited states propagation rate falls within the range of nerve impulse velocity. It was shown that such a process also causes a transfer of quantum entanglement between tryptophans, so that microtubules can be considered as signaling system, the basis for transmitting information via the quantum channel. The conditions under which the migration of entangled states in the microtubule is possible are obtained. In a certain sense, it allows us to argue that the signal function of tryptophans works as an analogue of a quantum repeater that transmits entangled states over microtubule by relaying through intermediate tryptophans. Thus, the paper shows that the tryptophan system can be considered as an environment that supports the existence of entangled states during the time comparable with the time of the processes in biosystems.

BaF2:Eu2+/3+,Tb3+nanofibres achieve enhanced multicolor luminescence and white-light emissionviamulti-channel excitation and energy migration procedure

BaF2:Eu2+/3+,Tb3+nanofibres with multicolor luminescence and white-light emission are constructedviaelectrospinning and di-crucible fluorination technology.

Deciphering modes of long-range energy transfer in perovskite crystals using confocal excitation and wide-field fluorescence spectral imaging

Excitation energy migration beyond mesoscale is of contemporary interest for both solar photovoltaic and light-emissive devices, especially in context of organometal halide perovskites (OMHPs) which have been shown to have very long (charge carrier) diffusion lengths. While understanding the energy propagation pathways in OMHPs is crucial for further advancement of material design and improvement of opto-electronic features, the simultaneous existence of multiple processes like carrier diffusion, photon recycling, and photon transport makes it often complex to differentiate them. In this study, we unravel the diverse yet dominant excitation energy transfer mode(s) in crystalline MAPbBr3 micron-sized 1D rods and plates by localized (confocal) laser excitation coupled with spectrally-resolved wide-field fluorescence imaging. While rarely used, this technique can efficiently probe excitation migration beyond the diffraction limit and can be realized by simple modification of existing epifluorescence microscopy setups. We find that in rods of length below ∼2 microns, carrier diffusion dominates amongst various energy transfer processes. However, the transient non-radiative defects severely inhibit the extent of carrier migration and also temporarily affect the radiative recombination dynamics of the photo-carriers. For MAPbBr3 plates of several tens of micrometers, we find that the photoluminescence (PL) spectral characteristics remain unaltered at short distances (< ∼3 μm) while at a larger distance, the spectral profile is gradually red-shifted. This implies that carrier diffusion dominates over small distances, while photon recycling, i.e., repeated re-absorption and re-emission of photons, propagates excitation energy transfer over extended length scales with assistance from wave-guided photon transport. Our findings can potentially be used for future studies on the characterization of energy transport mechanisms in semiconductor solids as well as for organic (molecular) self-assembled microstructures.

Energy Transport in SiO2 Crystals: Luminescence Excitation Spectra of Stishovite and α-Quartz

Abstract The migration of elementary electronic excitations was studied in a single crystal of stishovite and compared with migration in a crystal of α-quartz and polycrystalline stishovite powder. The research method is based on comparing the transfer of absorbed energy to luminescence centers, used as detectors of quasiparticles, and the near-surface nonradiative annihilation of electronic excitations. A sign of migration is the appearance of some minima in the photoluminescence (PLE) excitation spectrum in the region of maxima in the intrinsic absorption spectrum. The PLE spectrum of stishovite contains the first minimum at 9.8 eV, indicating the migration of electronic excitations and the existence of an intrinsic absorption band in stishovite at 9.8 eV. In α-quartz, the first minimum in the PLE spectrum is located at 10.5 eV and corresponds well to the intrinsic absorption band of the exciton.

Molecular polaritonics in dense mesoscopic disordered ensembles

We study the dependence of the vacuum Rabi splitting (VRS) on frequency disorder, vibrations, near-field effects, and density in molecular polaritonics. In the mesoscopic limit, static frequency disorder alone can already introduce a loss mechanism from polaritonic states into a dark state reservoir, which we quantitatively describe, providing an analytical scaling of the VRS with the level of disorder. Disorder additionally can split a molecular ensemble into donor-type and acceptor-type molecules and the combination of vibronic coupling, dipole-dipole interactions, and vibrational relaxation induces an incoherent FRET (F\"orster resonance energy transfer) migration of excitations within the collective molecular state. This is equivalent to a dissipative disorder and has the effect of saturating and even reducing the VRS in the mesoscopic, high-density limit. Overall, this analysis allows to quantify the crucial role played by dark states in cavity quantum electrodynamics with mesoscopic, disordered ensembles.

Mechanisms of the energy transfer between thulium ions in tungstate and molybdate crystals

In this work, we investigated mechanisms of the energy transfer in Tm : KY(WO4)2, Tm : KLu(WO4)2 and Tm:NaBi(MoO4)2 crystals. Room-temperature absorption and emission spectra were used to determine microparameters of energy migration among thulium ions in the 3H4 and 3F4 excited states in the frames of Förster – Dexter theory. Parameters of cross-relaxation 3H4 + 3H6 → 3F4 + 3F4 and energy migration were obtained via analysis of luminescence decay 3H4 → 3F4 with a hopping model. The parameters describing excitation migration between thulium ions in 3H4 state obtained by two methods were in good agreement. It has been shown that the dipole-dipole mechanism of interaction is responsible for the efficient cross-relaxation process in the crystals under study. The results indicate that the energy migration between 3H4 enhances the cross-relaxation at thulium content more than ∼1.3–1.5 at. % in these laser materials. The obtained values of the migration parameters CDD exceed the values of the cross-relaxation parameters CDA, and the energy transfer in these materials can be described with the hopping model. An efficient cross-relaxation process leads to the relatively high efficiencies of the systems based on these crystals under pumping at 0.8 µm. The dominant process of energy migration between thulium ions in 3F4 excited state makes tungstate and molybdate crystals good candidates for the Ho3+ co-activation for laser generation at 2.1 µm. Parameters obtained in this study can be used for mathematical modeling of laser characteristics.

New NIR emission from Sm3+ in Yb3+-Sm3+ co-doped tellurite glass

Yb3+, Sm3+ and Yb3+-Sm3+ doped tellurium oxide glasses were prepared. Their spectroscopic properties in the near-infrared region were studied. In the singly doped Sm3+ glass, excited by 403 nm, emission transitions occurred from the usual level 4G5/2 leading to strong fluorescence mainly in the visible range. In the co-doped Yb3+-Sm3+ glass Yb3+ donor ions were excited by 976 nm and transferred their energy by non-radiative processes to Sm3+ acceptor ions, promoting them to 6F11/2 and 6F9/2. Using an amplified spontaneous emission experiment, new emissions in Sm3+ from 6F11/2 and 6F9/2 were observed at 1080, 1240, 1435 and 1457 nm. These are the first reported emissions from these levels in Sm3+. The Yb3+ to Sm3+ energy transfer process was studied by measurement of the fluorescence decay curve of the Yb3+ donor. A diffusion model taking into account the migration of excitation amongst the Yb3+ donor ions was used to fit the decay curve and subsequently determine the donor-acceptor energy transfer microparameter CDA and the transfer parameter γ. The experimental values of these parameters were higher than those calculated by Dexter's model for resonant non-radiative transfer between donor and acceptor ions. These results demonstrate the importance of the non-resonant phonon-assisted Yb3+ to Sm3+ energy transfer processes in the tellurite glass. Energy transfer was efficient as evidenced by a large reduction in the Yb3+ lifetime in the Yb3+-Sm3+ glass compared to in the singly doped Yb3+ glass. The new emissions from Sm3+ were though of low fluorescent intensity. This was due to the high non-radiative decay rates of levels 6F11/2 and 6F9/2 by multi-phonon relaxation.

3D spectral fluorescence signature of cerium(III)-melamine coordination polymer: A novel sensing material for explosive detection.

Hidden or buried explosives are the most common scenario by terrorist attacks; therefore explosive vapour detection is a vital demand. Explosives are electron deficient materials; the vicinity of explosives to fluorescent material can encounter electron migration. This study reports on facile synthesis of cerium (III)-melamine coordination polymer (CeM-CP) with exclusive optical properties. CeM-CP demonstrated novel spectral fluorescence properties over visible and infrared bands when stimulated with UVA LED source at 385nm of 100mW power. Stimulated CeM-CP demonstrated unique spectral fluorescence signal at 400, 700, and 785nm. These fluorescent signals were correlated to cerium coordination with four nitrogen atoms; vacant orbital will be available for electron excitation migration. Spectral fluorescent signals were quenched as CeM-CP was subjected to TNT vapours. Hyperspectral imaging offered 3D plot of fluorescence signature. The main outcome is that complete fluorescence signal attenuation was achieved at 785nm. CeM-CP could act as as a novel sensing element for explosive vapour detection.

Unraveling the Excited-State Dynamics and Light-Harvesting Functions of Xanthophylls in Light-Harvesting Complex II Using Femtosecond Stimulated Raman Spectroscopy.

Photosynthesis in plants starts with the capture of photons by light-harvesting complexes (LHCs). Structural biology and spectroscopy approaches have led to a map of the architecture and energy transfer pathways between LHC pigments. Still, controversies remain regarding the role of specific carotenoids in light-harvesting and photoprotection, obligating the need for high-resolution techniques capable of identifying excited-state signatures and molecular identities of the various pigments in photosynthetic systems. Here we demonstrate the successful application of femtosecond stimulated Raman spectroscopy (FSRS) to a multichromophoric biological complex, trimers of LHCII. We demonstrate the application of global and target analysis (GTA) to FSRS data and utilize it to quantify excitation migration in LHCII trimers. This powerful combination of techniques allows us to obtain valuable insights into structural, electronic, and dynamic information from the carotenoids of LHCII trimers. We report spectral and dynamical information on ground- and excited-state vibrational modes of the different pigments, resolving the vibrational relaxation of the carotenoids and the pathways of energy transfer to chlorophylls. The lifetimes and spectral characteristics obtained for the S1 state confirm that lutein 2 has a distorted conformation in LHCII and that the lutein 2 S1 state does not transfer to chlorophylls, while lutein 1 is the only carotenoid whose S1 state plays a significant energy-harvesting role. No appreciable energy transfer takes place from lutein 1 to lutein 2, contradicting recent proposals regarding the functions of the various carotenoids (Son et al. Chem.2019, 5 (3), 575–584). Also, our results demonstrate that FSRS can be used in combination with GTA to simultaneously study the electronic and vibrational landscapes in LHCs and pave the way for in-depth studies of photoprotective conformations in photosynthetic systems.

Novel laser induced fluorescence with hyperspectral imaging of amplifying fluorescent melamine resin for TNT vapor detection

Terrorism by means of explosive is an imminent threat; instant detection of explosive vapors is a vital demand. Explosives are electron deficient molecules; the vicinity of explosives to fluorescent material could hinder excitation migration resulting in fluorescence quenching. This study reports on the facile synthesis of porous melamine–formaldehyde–diethylenetriaminepentaacetic acid (MF–DTPA) resin with surface area of 162 m2/g, and average pore diameter is 94.7 (Å). MF–DTPA resin demonstrated unique ability to capture TNT vapors with complete change in its color from white to reddish. Furthermore, MF–DTPA resin demonstrated novel fluorescence properties over visible band (400–560 nm) and infrared band (700–760 nm) when stimulated with UVA LED source at 385 nm of 100 mW power. These two emission bands were completely quenched when MF–DTPA was subjected to TNT vapors. Amplified chemosensory response was archived via wired receptors in a series. Hyperspectral imaging associated with 2D moving average filter was employed to develop enhanced color intensity maps.

Multiple dye interactions in plastic scintillators: Effects on pulse shape discrimination

The scintillation process in plastic scintillators has been studied with mixed systems containing high concentrations of multiple fluorescence dyes. It has been shown that the triplet–triplet interaction phenomena leading to the production of delayed light and pulse shape discrimination (PSD) can vary depending on the triplet energies of the interacting dyes. At small differences between the first excited triplet state energies, found to be below 0.1 eV, a pair of interacting dyes of different molecular species can be involved in heterogeneous excitation migration and triplet–triplet annihilation that result in the enhancement of delayed light and PSD, due to the engagement of both dyes in the process. At differences above ∼0.27 eV, excitation trapping on the lower energy triplet molecules leads to conditions when triplet–triplet migration and annihilation can proceed only homogeneously between molecules of the same species. The results explain the different effects produced by the selection of fluorescent dyes in previously reported PSD systems containing one primary and one secondary dyes. They also demonstrate a non-traditional approach to the design of new plastic scintillators with multiple dyes that, at improved scintillation and PSD performance, may simplify the production process.