Fluorescence Line Narrowing Research Articles

Eu3+ Site Distribution and Local Distortion of Photoluminescent Ca3 WO6 :(Eu3+ , K+ ) Double Perovskites as High-Color-Purity Red Phosphors.

In this study, Eu3+ and K+ co-doped Ca3 WO6 double perovskite, a high-color-purity red phosphor, is quantitatively investigated using synchrotron X-ray diffraction Rietveld refinement, energy dispersive X-ray spectroscopy, and high-resolution PL spectroscopy. The Eu3+ fluorescence line-narrowing (FLN) results, used to estimate the Eu3+ occupation at given sites (so-called A and B sites) in the host crystal (A2 BMO6 ; A, B=Ca; M=W), reveal that the Eu3+ ions have a twin distribution in both the A and B sites with high asymmetry ratios of ΛA =9.7 and ΛB =10.7, respectively. More interestingly, at lower Eu3+ doping levels, the ions are predominantly located at the B sites (≈75%), indicating that the high color purity of Ca3 WO6 :(Eu3+ , K+ ) is mainly caused by the high asymmetry ratios of the Eu3+ sites. Rietveld refinement combined with the FLN technique provides more reliable results for increasing the Eu3+ substitution at the A site with Eu3+ and K+ doping concentrations, which lower the lattice energy of Ca3 WO6 :(Eu3+ , K+ ). The structural distortions owing to K+ co-doping in terms of the quadratic elongation and bond-angle variance exhibit good correlations with the Ca(Eu)O12 A-site cuboctahedra and Ca(Eu)O6 B-site octahedra, partially accounting for the higher Λvalues.

A site-selective fluorescence spectroscopy study of the crystal phases of KY3F10: Leveraging the optical response of Eu3+ ions

The present work deals with the investigation of the site-selective symmetries in Eu3+-doped KY3F10 with different crystal phases. Employing time-resolved fluorescence line-narrowing (TRFLN), a study of the luminescent response of the dopant is reported in samples presenting a single δ-phase, a single α-phase, and a mixture of both δ and α. A new, simple, very low time-consuming and high-yield method was developed to obtain nanospheres of the δ-phase. For both crystal phases, the main site symmetry is spectrally corroborated with that expected by the crystallographic substitution position (C2v in δ, and C4v in α). In addition, the accuracy of the measurements also unveils the presence of significantly distorted Eu3+ ions in the surface of the nanoparticles in all the samples studied, a spectral behavior commonly found in glassy systems. All these findings contribute to the proper understanding of the optical response of luminescent dopants in this and related types of complex fluorides. It is worth highlighting the fact that the huge potential of the unexplored δ-phase is demonstrated, since the site-selective emission of the Eu3+-doped δ-phase is up to 20 times more intense than that of the α-phase.

A Comparative Study of the Band-Edge Exciton Fine Structure in Zinc Blende and Wurtzite CdSe Nanocrystals

In this paper, we studied the role of the crystal structure in spheroidal CdSe nanocrystals on the band-edge exciton fine structure. Ensembles of zinc blende and wurtzite CdSe nanocrystals are investigated experimentally by two optical techniques: fluorescence line narrowing (FLN) and time-resolved photoluminescence. We argue that the zero-phonon line evaluated by the FLN technique gives the ensemble-averaged energy splitting between the lowest bright and dark exciton states, while the activation energy from the temperature-dependent photoluminescence decay is smaller and corresponds to the energy of an acoustic phonon. The energy splittings between the bright and dark exciton states determined using the FLN technique are found to be the same for zinc blende and wurtzite CdSe nanocrystals. Within the effective mass approximation, we develop a theoretical model considering the following factors: (i) influence of the nanocrystal shape on the bright-dark exciton splitting and the oscillator strength of the bright exciton, and (ii) shape dispersion in the ensemble of the nanocrystals. We show that these two factors result in similar calculated zero-phonon lines in zinc blende and wurtzite CdSe nanocrystals. The account of the nanocrystals shape dispersion allows us to evaluate the linewidth of the zero-phonon line.

(Invited) Design and Synthesis of Colloidal QDs for Quantum Emitters

On-demand generation of quantum photons is one of the four major targets of Quantum Information Science (QIS), identified by the NSF as one of the five “industries of the future.” Colloidal semiconductor quantum dots (QDs) are viable candidates for quantum materials, owing to their tunable, bright, and stable emission. Identifying a suitable semiconductor material and engineering long exciton coherence times is key to producing single coherent photons on demand. We show that colloidal compositionally-graded CdSe/ZnS QDs designed with a high degree of asymmetric internal strain demonstrate superior quantum emission properties compared to conventional sharp-interface CdSe/ZnS QDs. In comparison to conventional colloidal CdSe/ZnS core/shell QDs, we find that, in asymmetrically strained CdSe QDs, over six times more light is emitted directly by the bright exciton. Temperature-dependent PL dynamics and fluorescence line narrowing (FLN) spectroscopy reveals the radiative recombination rates of bright and dark excitons, the relaxation rate between the two, and the energy spectra of the quantized acoustic phonons in the QDs that can contribute to relaxation processes. We attribute the six-fold improvement to the four-fold deceleration of the bright-dark relaxation and the doubled radiative recombination rate in the strained QDs.The next step in this research is to bring these improvement from the visible to the near-IR emission, where most communication bandwidths lie. This area has been traditionally dominated by III-V materials, which are chemically challenging in the colloidal state. We design, synthesize, and characterize alternative strong near-IR materials and assess them as potential quantum emitters. Last but not least, we demonstrate applications of electrochemistry in programmable hot syntheses of colloidal QDs.

Ultra-slow electron spin-lattice relaxation in a low magnetic field enables massive optical spin polarization in the 4A2 ground state of Cr3+ in ruby* *This paper is dedicated to my late friend and mentor Dr Alex Szabo, the originator of transient spectral hole-burning and fluorescence line narrowing in the solid state, whose knowledge of the spectroscopy of ruby is unsurpassed.

Ruby (α-Al2O3 doped with Cr3+) has been an archetypal material in the development of optical spectroscopy of the solid state for the last 150 years and was the first material that was demonstrated to lase. Notwithstanding the vast literature on ruby, one effect was somehow missed: in a magnetic field B ∥c ∼ 235 mT, the spin-lattice relaxation time T 1 for the |+3/2⟩ level in the 4A2 ground state is massively lengthened to ∼12 s at 1.4 K as demonstrated in this study. This very long relaxation time enables optical pumping of the |+3/2⟩ level via the R 1(±1/2) lines and a considerable +3/2 spin polarization of ∼95% is readily achieved. The observed magnetic field dependence can be quantitatively described using the one-phonon relaxation process.

Optical Property Tuning of Single-Wall Carbon Nanotubes by Endohedral Encapsulation of a Wide Variety of Dielectric Molecules.

Specific and tunable modification to the optical properties of single-wall carbon nanotubes (SWCNTs) is demonstrated through direct encapsulation into the nanotube interior of guest molecules with widely varying static dielectric constants. Filled through simple ingestion of the guest molecule, each SWCNT population is demonstrated to display a robust modification to absorbance, fluorescence, and Raman spectra. Over 30 distinct compounds, covering static dielectric constants from 1.8 to 109, are inserted in large diameter SWCNTs (d = 1.104-1.524 nm) and more than 10 compounds in small diameter SWCNTs (d = 0.747-1.153 nm), demonstrating that the general effect of filler dielectric on the nanotube optical properties is a monotonic energy reduction (red-shifting) of the optical transitions with increased magnitude of the dielectric constant. Systematic fitting of the two-dimensional fluorescence-excitation and Raman spectra additionally enables determination of the critical filling diameter for each molecule and distinguishing of overall trends from specific guest-host interactions. Comparisons to predictions from existing theory are presented, and specific guest molecule/SWCNT chirality combinations that disobey the general trend and theory are identified. A general increase of the fluorescence intensity and line narrowing is observed for low dielectric constants, with long linear alkane filled SWCNTs exhibiting emission intensities approaching those of empty SWCNTs. These results demonstrate an exploitable modulation in the optical properties of SWCNTs and provide a foundation for examining higher-order effects, such as due to nonbulk-like molecule stacking, in host-guest interactions in well-controlled nanopore size materials.

Delocalization Effects in Chlorophyll Fluorescence: Nonperturbative Line Shape Analysis of a Vibronically Coupled Dimer.

Non-adiabatic vibrational/electronic (vibronic) interactions in photosynthetic pigment/protein complexes (PPCs) have recently attracted considerable interest as a potential source for long-lived dynamic coherence and optimized light harvesting. The analysis of such effects is limited, however, by the complexity of the vibrational spectrum of biological pigments such as chlorophyll (Chl) molecules, which often makes numerical calculations prohibitively expensive and complicates the interpretation of experimental spectroscopic data. This work contributes to both challenges by using numerically exact computational methods to systematically examine vibronic mixing effects in the low-temperature fluorescence spectra of a Chl dimer possessing a full complement of local vibrations, using parameters extracted from experimental data. The results highlight the varying roles local vibrations can play in energy-transfer dynamics, both enhancing delocalization through vibronic resonance and, conversely, inducing dynamic localization by acting as a "self-bath" for local electronic transitions. In the specific context of line-narrowed fluorescence, the results indicate that, while low-frequency features are strongly suppressed by delocalization, high-frequency modes are likely to be dynamically localized in the parameter regime relevant to most photosynthetic complexes.

Fluorescence studies of porphycene in various cryogenic environments

Low temperature fluorescence of porphycene, a structural isomer of porphyrin, has been studied using polymer samples, matrix isolation, and fluorescence line narrowing (FLN) techniques. Contrary to the case of the chromophore embedded in a nitrogen matrix, the emission from polymer samples at temperatures above 10 K exhibits strong dependence on the wavelength of excitation: increasing the excitation energy leads to gradual broadening and, finally, loss of vibrational structure. A rather unusual observation is the similarity of the structured fluorescence spectra obtained for excitations into S1 and S2 states. This finding indicates a correlation between the site distributions in S1 and S2. A similar idea has been put forward earlier for tetraphenylporphyrin [I. Lee, G. J. Small, and J. M. Hayes, J. Phys. Chem. 94, 3376 (1990)]. We propose that the correlation is due to isotropic polarizability in the molecular plane; calculations confirm such hypothesis. For porphycene, an additional factor that can contribute to the effect is a rapid trans-trans tautomerization that leads to the rotation of x and y in-plane axes of the molecule. FLN spectra reveal significant band broadening for excitation into S2. This suggests that the site correlation is not of 1:1 type or that at 4.2 K the site exchange dynamics is frozen in comparison with the situation at higher temperatures.

Wavelength-Dependent Exciton-Vibrational Coupling in the Water-Soluble Chlorophyll Binding Protein Revealed by Multilevel Theory of Difference Fluorescence Line-Narrowing.

One of the most powerful line-narrowing techniques used to unravel the homogeneous lineshapes of inhomogeneously broadened systems is difference fluorescence line-narrowing spectroscopy. When this spectroscopy was applied to multichromophoric systems so far, the spectra were analyzed by an effective two-level system approach, composed of the electronic ground state and the lowest exciton state. An effective Huang-Rhys factor was assigned for the coupling of this state to the vibrations. Here, we extend this approach by including a multilevel line shape theory, which takes into account the excitonic coupling between pigments and thereby the effect of the delocalization of the excited states explicitly. In this way, it becomes possible to extract the spectral density of the local exciton-vibrational coupling. The theory is applied to the recombinant water-soluble chlorophyll binding protein reconstituted with chlorophyll a or b and reveals a significant decrease of the Huang-Rhys factor of the local exciton-vibrational coupling with decreasing transition energy of the chlorophylls. This decrease could be due to the increase in steric interactions reducing the flexibility of the environment and red-shifting the site energy of the pigments.

Evaluation of Electron-Phonon Coupling and Spectral Densities of Pigment-Protein Complexes by Line-Narrowed Optical Spectroscopy.

Spectral hole burning (SHB) and difference fluorescence line narrowing (ΔFLN) are routinely used for investigations of electron-phonon coupling in photosynthetic pigment-protein complexes as well as in other amorphous systems at cryogenic temperatures. Nevertheless, the Huang-Rhys factors S, an integral measure of electron-phonon coupling strength, and the phonon spectral densities obtained by SHB and ΔFLN over the past years have differed significantly in the case of certain photosynthetic pigment-protein complexes. In this work, the specific properties of both types of line-narrowing spectroscopic techniques that may lead to these discrepancies are critically analyzed by a combined experimental and computational approach, using the CP29 antenna complex of green plants as a suitable model system. We confirm that only ΔFLN at low fluence, by providing access to the homogeneously broadened spectrum, is able to deliver correct S values, while SHB may significantly under- or overestimate them, depending on the burn fluence. We also discuss possible other sources of discrepancies in the literature data, e.g., in the case of LHC II aggregates and correct numerical errors found in some previous records.

Exciton Fine Structure and Lattice Dynamics in InP/ZnSe Core/Shell Quantum Dots.

Nanocrystalline InP quantum dots (QDs) hold promise for heavy-metal-free optoelectronic applications due to their bright and size-tunable emission in the visible range. Photochemical stability and high photoluminescence (PL) quantum yield are obtained by a diversity of epitaxial shells around the InP core. To understand and optimize the emission line shapes, the exciton fine structure of InP core/shell QD systems needs be investigated. Here, we study the exciton fine structure of InP/ZnSe core/shell QDs with core diameters ranging from 2.9 to 3.6 nm (PL peak from 2.3 to 1.95 eV at 4 K). PL decay measurements as a function of temperature in the 10 mK to 300 K range show that the lowest exciton fine structure state is a dark state, from which radiative recombination is assisted by coupling to confined acoustic phonons with energies ranging from 4 to 7 meV, depending on the core diameter. Circularly polarized fluorescence line-narrowing (FLN) spectroscopy at 4 K under high magnetic fields (up to 30 T) demonstrates that radiative recombination from the dark F = ±2 state involves acoustic and optical phonons, from both the InP core and the ZnSe shell. Our data indicate that the highest intensity FLN peak is an acoustic phonon replica rather than a zero-phonon line, implying that the energy separation observed between the F = ±1 state and the highest intensity peak in the FLN spectra (6 to 16 meV, depending on the InP core size) is larger than the splitting between the dark and bright fine structure exciton states.

Fluorescence line narrowing and Judd-Ofelt theory analyses of [formula omitted]-doped low-silica calcium aluminosilicate glass and glass-ceramic

Eu-doped calcium aluminosilicate glass and glass-ceramic were synthesized and studied by site-selective spectroscopy. Using both broadband excitation and fluorescence line narrowing techniques, the D05→F17 emission transition was studied under resonant excitation at different wavelengths within the F07→D05 band. In order to explore the local structure of the systems, crystal-field analysis was applied assuming C2v site symmetry. Moreover, from the emission spectra of glass and glass-ceramic systems, Judd-Ofelt parameters Ωt (t = 2, 4) were considered. Transition probability (A), stimulated emission cross-section (σ), radiative lifetime (τR) and branching ratios (β) due to the transition D05→FJ7 (J = 1, 2, 3 and 4) of Eu3+ ions were evaluated in different local fields and their relative variations were discussed. The crystallization process, which converted the glass into glass-ceramic, promoted an important enhancement in the quantum efficiency, which was improved from 45% to 60%.

Addressing the exciton fine structure in colloidal nanocrystals: the case of CdSe nanoplatelets.

We study the band-edge exciton fine structure and in particular its bright-dark splitting in colloidal semiconductor nanocrystals by four different optical methods based on fluorescence line narrowing and time-resolved measurements at various temperatures down to 2 K. We demonstrate that all these methods provide consistent splitting values and discuss their advances and limitations. Colloidal CdSe nanoplatelets with thicknesses of 3, 4 and 5 monolayers are chosen for experimental demonstrations. The bright-dark splitting of excitons varies from 3.2 to 6.0 meV and is inversely proportional to the nanoplatelet thickness. Good agreement between experimental and theoretically calculated size dependence of the bright-dark exciton splitting is achieved. The recombination rates of the bright and dark excitons and the bright to dark relaxation rate are measured by time-resolved techniques.

Investigation of the local environment of Eu3+ in a silicophosphate glass using site-selective spectroscopy and Molecular Dynamics simulations

Silicophosphate glasses (SiO2-P2O5) doped with Eu3+ ions were synthesized by the sol-gel process. Optical properties of these glasses were investigated by means of emission spectra and lifetime measurements. The Fluorescence Line Narrowing (FLN) technique was also used to explore the local structure around the Eu3+ ions in this host and to understand the role of phosphate as a codopant. As it is the case for aluminum, the ability of phosphate to avoid the rare earth clustering was investigated, and the role of this codopant in modifying the local order around the rare earth ion was evidenced. The analysis of the FLN spectra and lifetime measurements is consistent with this interpretation. Molecular Dynamics simulations were performed to evaluate and confirm these structural features. Two classes of europium sites were distinguished in agreement with the experimental characterization.

Conformational Changes in Pigment-Protein Complexes at Low Temperatures-Spectral Memory and a Possibility of Cooperative Effects.

We employed nonphotochemical hole burning (NPHB) and fluorescence line narrowing (FLN) spectroscopies to explore protein energy landscapes and energy transfer processes in dimeric Cytochrome b6f, containing one chlorophyll molecule per protein monomer. The parameters of the energy landscape barrier distributions quantitatively agree with those reported for other pigment-protein complexes involved in photosynthesis. Qualitatively, the distributions of barriers between protein substates involved in the light-induced conformational changes (i.e., -NPHB) are close to glass-like ∼1/√V (V is the barrier height) and not to Gaussian. There is a high degree of correlation between the heights of the barriers in the ground and excited states in individual pigment-protein systems, as well as nearly perfect spectral memory. Both NPHB and hole recovery are due to phonon-assisted tunneling associated with the increase of the energy of a scattered phonon. As the latter is unlikely for simultaneously both the hole burning and the hole recovery, proteins must exhibit a NPHB mechanism involving diffusion of the free volume toward the pigment. Entities involved in the light-induced conformational changes are characterized by md(2) value of about 1.0 × 10(-46) kg·m(2). Thus, these entities are protons or, alternatively, small groups of atoms experiencing sub-Å shifts. However, explaining all spectral hole burning and recovery data simultaneously, employing just one barrier distribution, requires a drastic decrease in the attempt frequency to about 100 MHz. This decrease may occur due to cooperative effects. Evidence is presented for excitation energy transfer between the chlorophyll molecules of the adjacent monomers. The magnitude of the dipole-dipole coupling deduced from the Δ-FLN spectra is in good agreement with the structural data, indicating that the explored protein was intact.

Comments on the optical lineshape function: application to transient hole-burned spectra of bacterial reaction centers.

The vibrational spectral density is an important physical parameter needed to describe both linear and non-linear spectra of multi-chromophore systems such as photosynthetic complexes. Low-temperature techniques such as hole burning (HB) and fluorescence line narrowing are commonly used to extract the spectral density for a given electronic transition from experimental data. We report here that the lineshape function formula reported by Hayes et al. [J. Phys. Chem. 98, 7337 (1994)] in the mean-phonon approximation and frequently applied to analyzing HB data contains inconsistencies in notation, leading to essentially incorrect expressions in cases of moderate and strong electron-phonon (el-ph) coupling strengths. A corrected lineshape function L(ω) is given that retains the computational and intuitive advantages of the expression of Hayes et al. [J. Phys. Chem. 98, 7337 (1994)]. Although the corrected lineshape function could be used in modeling studies of various optical spectra, we suggest that it is better to calculate the lineshape function numerically, without introducing the mean-phonon approximation. New theoretical fits of the P870 and P960 absorption bands and frequency-dependent resonant HB spectra of Rb. sphaeroides and Rps. viridis reaction centers are provided as examples to demonstrate the importance of correct lineshape expressions. Comparison with the previously determined el-ph coupling parameters [Johnson et al., J. Phys. Chem. 94, 5849 (1990); Lyle et al., ibid. 97, 6924 (1993); Reddy et al., ibid. 97, 6934 (1993)] is also provided. The new fits lead to modified el-ph coupling strengths and different frequencies of the special pair marker mode, ωsp, for Rb. sphaeroides that could be used in the future for more advanced calculations of absorption and HB spectra obtained for various bacterial reaction centers.

Fluorescence line narrowing and Δ-FLN spectra in the presence of excitation energy transfer between weakly coupled chromophores.

The results of simulations of fluorescence line narrowed (FLN) and Δ-FLN spectra of weakly coupled chromophore dimers and trimers, connected by slow (hundreds of picoseconds) excitation energy transfer and embedded in amorphous solids such as proteins, are presented. The model includes wavelength-dependent excitation energy transfer (EET) rate distributions converted to nonidentical sub- site distribution function (sub-SDF) (one for each EET rate), as well as spectral hole burning in pigments excited via EET. Emission of the donor molecules and realistic EET probability dependence on the donor-acceptor energy gap are also included. The shapes of the FLN and Δ-FLN spectra are very sensitive to even small variations of interpigment coupling in this weak-coupling regime, and proper modeling is essential for extracting correct parameters from the experimental spectra. Applications of the model to the trimeric FMO complex and the dimeric cytochrome b6f are demonstrated.

Modeling of fluorescence line-narrowed spectra in weakly coupled dimers in the presence of excitation energy transfer.

This work describes simple analytical formulas to describe the fluorescence line-narrowed (FLN) spectra of weakly coupled chromophores in the presence of excitation energy transfer (EET). Modeling studies for dimer systems (assuming low fluence and weak coupling) show that the FLN spectra (including absorption and emission spectra) calculated for various dimers using our model are in good agreement with spectra calculated by: (i) the simple convolution method and (ii) the more rigorous treatment using the Redfield approach [T. Renger and R. A. Marcus, J. Chem. Phys. 116, 9997 (2002)]. The calculated FLN spectra in the presence of EET of all three approaches are very similar. We argue that our approach provides a simplified and computationally more efficient description of FLN spectra in the presence of EET. This method also has been applied to FLN spectra obtained for the CP47 antenna complex of Photosystem II reported by Neupane et al. [J. Am. Chem. Soc. 132, 4214 (2010)], which indicated the presence of uncorrelated EET between pigments contributing to the two lowest energy (overlapping) exciton states, each mostly localized on a single chromophore. Calculated and experimental FLN spectra for CP47 complex show very good qualitative agreement.

Fluorescence spectroscopy study of mesoporous SiO2 particles containing Gd2O3:Eu3+

Site-selective and fluorescence line narrowing (FLN) laser spectroscopy was used to study the structure of Eu3+-doped Gd2O3-based material embedded in the mesopores of submicron-sized monodisperse spherical amorphous SiO2 particles that are of interest for biomedical applications. The phase composition of the material in the pores was determined and the transformations that occur under heat treatment were studied. The room-temperature homogeneous linewidth of 5D0 − 7F0 transition of Eu3+ ions in different Gd2O3/SiO2 phases was measured with FLN technique. The radiative lifetime of the 5D0 excited state of Eu3+ ions was measured with time-resolved fluorescence technique; the local field effects on radiative transitions in the composite material are discussed.

ChemInform Abstract: Low‐Temperature Spectroscopy of Organic Molecules in Solid Matrices: From the Shpol′skii Effect to Laser Luminescent Spectromicroscopy for All Effectively Emitting Single Molecules

AbstractReview: 175 refs.