Huang-Rhys Factor Research Articles

Enhancement of the lighting quality for white light-emitting diodes with CaSc2O4:Ce3+ phosphor

<span>For the task of realizing greater progress for the light output in white light-emitting diodes (WLEDs), t</span><span lang="EN-GB">his study focuses on the luminescence temperature subordination feature of CaSc<sub>2</sub>O<sub>4</sub>:Ce<sup>3+</sup> phosphor (abbreviated to CaS for this study). Some other aspects of the phosphor were also included in this piece of paper: Huang-Rhys coupling factor, Stokes shift, triggering power, abatement temperature and especially, abatement behavior in CaSc<sub>2</sub>O<sub>4</sub>:Ce<sup>3+</sup>. Creating the bluish-green LEDs by the combination of blue InGaN chip and CaSc<sub>2</sub>O<sub>4</sub>:Ce<sup>3+</sup> is the primary purpose. CaSc<sub>2</sub>O<sub>4</sub>:Ce<sup>3+</sup> appears to be a decent green phosphor that can be used in WLEDs made of blue InGaN chip. </span><span>Production tasks may be based on our investigation for the task of making desirable WLED devices that meet the production demands.</span>

Electron-phonon coupling and vibrational properties of size-selected linear carbon chains by resonance Raman scattering

UV resonance Raman spectroscopy of size-selected linear sp-carbon chains unveils vibrational overtones and combinations up to the fifth order. Thanks to the tunability of the synchrotron source, we excited each H-terminated polyyne (HCnH with n = 8,10,12) to the maxima of its vibronic absorption spectrum allowing us to precisely determine the electronic and vibrational structure of the ground and excited states for the main observed vibrational mode. Selected transitions are shown to enhance specific overtone orders in the Raman spectrum in a specific way that can be explained by a simple analytical model based on Albrecht’s theory of resonance Raman scattering. The determined Huang–Rhys factors indicate a strong and size-dependent electron-phonon coupling increasing with the sp-carbon chain length.

Effect of temperature on Electron-Phonon coupling of carotenoids by Two-Dimensional correlation resonance Raman spectroscopy

Carotenoids, the most prevalent light-absorbing pigments in nature, perform many functions in photosynthetic systems. The Raman and absorption spectra of carotenoids (including lycopene and β-carotene) were measured in the temperature range between 303 K and 83 K. As the temperature decreased, the Huang-Rhys factor and the electron–phonon coupling constant also decreased. Nevertheless, the Raman scattering cross-section increased. The results suggested that lycopene was more sensitive to changes in temperature due to the longer conjugated length and the different open-chain structure as compared to β-carotene. Two-dimensional correlation resonance Raman spectroscopy (2DCRRS) showed a correlation between electron–phonon coupling and changes in temperature. Moreover, the complex mode evolution of lycopene rendered the molecule more sensitive to temperature. Density functional theory (DFT) simulation was used to optimize the geometrical structure, and bond length alternation (BLA) calculation was performed. The result can be applied to investigate the temperature dependence of electron-coupling in π-conjugated molecules.

Modulation strategy of manganese valence state in yttrium gallium garnet

Different manganese ions (Mn2+, Mn3+, and Mn4+) can be stabilized in garnet hosts, while the relevant stabilization problem of Mn2+ ions remains elusive. In this study, combined with co-doping Si4+ ions and reduction processing, Mn2+ ions are successfully stabilized in the Y3Ga5O12 (YGG) gallium garnet host, and the relevant stabilization mechanism is analyzed in detail. The obtained result indicates that the introduction of Si4+ can speed up the conversion from Mn4+ and Mn3+ ions to Mn2+ ones effectively, and the introduced Mn2+ ions can occupy two kinds of doping sites in the YGG lattice, corresponding to eight-coordinated Y3+ sites and six-coordinated Ga3+ sites. The dodecahedral Mn2+ ions manifest a stronger fluorescence intensity than that of the octahedral Mn2+ ions, and the increase of Si4+-doped content is more beneficial to enhance the photoluminescence performance of the octahedral Mn2+ ones. Additionally, octahedral Mn2+ ions exhibit a larger Huang-Rhys factor (S) value (S = 3.41) than that of dodecahedral Mn2+ ions (S = 1.34), meaning that the six-coordinated Mn2+ sites demonstrate a much stronger electric-phonon coupling strength. Finally, the temperature sensing strategy based on the temperature-dependent fluorescence distinctions between the two kinds of Mn2+ sites in the YGG host is also proposed, further proving the application potential of Mn2+ ions in the field of optical thermometry.

Manipulating Spectral Width and Emission Wavelength towards Highly Efficient Blue Asymmetric Carbazole Fused Multi-Resonance Emitters.

The novel carbazole-based multiresonance types of thermally activated delayed fluorescence (MR-TADF) emitters of mICz-DABNA and BFCz-DABNA are reported, and their spectroscopic properties are investigated with the inductive effect on the central nitrogen atom for pure and deep blue emission. With the introduction of electron-donating/-withdrawing properties of substituents, emitters exhibited the bathochromic/hypsochromic shifted emission, respectively, compared to simple carbazole-based MR-TADF. Moreover, their spectral bandwidths became narrower. Theoretical calculation indicated that the meta-positioned bulky moiety restricts the molecular geometry discrepancy and reduces the Huang-Rhys factors. Particularly, the organic light-emitting diode (OLED) with 3% BFCz-DABNA exhibited the maximum external quantum efficiency of 28.0% with the Commission International de l'Éclairage (CIE) of (0.13, 0.09), which is the best record value among single-boron MR-TADF devices of CIE y < 0.10.

(INVITED)Synthesis and luminescence properties of M(AlCl4)2:Eu2+ (M = Ca, Sr, Ba) a promising new Class of Eu2+ containing phosphors

In this work, the luminescence properties of Eu2+ doped in the host lattices M(AlCl4)2 (M = Ca, Sr, Ba) have been studied for the first time. The host compounds crystallize in a tetragonal (Ca), an orthorhombic (Sr) and a monoclinic (Ba) crystal structure where the alkaline earth site is 8-fold coordinated by Cl− ions in all cases. Therefore, these materials are ideal candidates to investigate structure related luminescence properties of Eu2+ ions. The emission spectra of the Eu2+ doped compounds are characterized by broad 4f65d1 → 4f7 transition bands located in the UV range between 388 nm and 400 nm (25773 - 25000 cm−1). Their maxima are blue shifted compared to the binary alkaline earth chloride hosts which can be explained by the influence of the [AlCl4]- ion on the crystal field splitting in addition to increasing metal-ligand distances. This shifts the lowest excited 5d level of Eu2+ to much higher energies. Furthermore, Huang-Rhys factors S and coupled phonon energies ħω were determined by temperature dependent emission measurements. In addition, radiative decay times at 10 K were determined.

Exciton interaction with acoustic phonons in PbS nanocrystals

The interaction between an exciton and acoustic phonons via the deformation potential in PbS nanocrystals is calculated in the $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model. It is shown that the anisotropy of the deformation potential leads to nonzero interaction of an exciton with spheroidal phonons with the total angular momentum $j=2$. The size dependence of the Huang-Rhys factors for the fundamental breathing mode and its overtones as well as for spheroidal vibrations with the total angular momentum $j=2$ is evaluated.

Preparation and characterization of an efficient and ultra-narrow fluorescence of Eu(III) complex with ultrasensitive sensing to gallic acid

A microcrystalline Eu(III) complex is constructed by dehydroabietic acid (DDA) and 1,10-phenanthroline (Phen) by a one-step precipitation method. FTIR, UV, TG and elemental analysis reveal the successful coordination of Eu(III) ions with dehydroabietic acid (DDA) and 1,10-phenanthroline (Phen) through Eu–O and Eu–N bonds, respectively. Comparing with the Eu(III) complex built up by a single ligand of Phen, the Eu(III) complex with DDA and Phen exhibits an enhanced absolute quantum yield with the maximum of 95.76%, and a decreased full width at half maxima of 5D0 → 7F2 transition with the 2.56 nm at 308 K. After introducing DDA into coordinate structure, not only the C–H bending vibrations of the heterocyclic rings in Phen is suppressed, but also the Huang-Rhys factor and the reorganization energy of the Eu(III) complex decrease. The sensing performance of the Eu(III) complexes based on DDA and Phen can detect gallic acid effectively in ethanol solution between 9.48 × 10−3 mM and 6.78 × 10−2 mM with a good linear relationship.

Unveiling the phonon effect on the narrow-band deep-red emission from solution-combustion synthesized Mn4+ doped CaYAlO4 microcrystals

In this study, we report a unique solution-combustion synthesis of Mn4+ doped CaYAlO4 microcrystals and demonstrate a narrow-band deep-red emission. Through the simulation of low-temperature photoluminescence (PL) spectroscopy and theoretical fitting on temperature-dependent PL intensity at T < 400 K, we obtained a dominant phonon mode with energy of 40 meV during the vibronic coupling process, with a small Huang-Rhys factor of S= 0.68. From the analysis of the measured PL lifetime evolution of Mn4 + doped CaYAlO4, it is found that luminescence thermal quenching effect was mainly attributed to four-dominant phonon interaction with Mn4+ electronic states. More interestingly, the deviation of luminescence decay curve from the single-exponential mode at T > 400 K suggests that the occurrence of two radiative transitions is a consequence of ionically thermal excitation to the second excited state of 4T2. These findings provide not only a general approach to identify the predominant phonon vibration, but also deep insights into the effect of weak electron-phonon coupling on PL properties of solid-state luminescent.

Self‐Trapped Excitons in 2D SnP2S6 Crystal with Intrinsic Structural Distortion

AbstractExploring and utilizing novel materials with self‐trapped excitons (STEs) is highly desired due to their unique physical properties and multiple optoelectronic applications. Here, a 2D SnP2S6 crystal is reported with obvious STEs emission caused by distorted [SnS6]8‐ and [P2S6]4‐ octahedral units and strong electron‐phonon coupling. The STEs feature wide photoluminescence (PL) spectra range from 600 to 850 nm and a large Stokes redshift of ≈0.6 eV. Carrier dynamics measurements including temperature‐dependent PL and transient absorption reveal a large Huang–Rhys factor of ≈18.3 and a self‐trapping process of ≈101–103 ps in the 2D SnP2S6 crystal. Such self‐trapped states enable 2D SnP2S6 crystal a wide spectral response range and excellent photodetection performance. As a result, high responsivity (22.8 A W‐1) and detectivity (3.98 × 1010 Jones) are achieved under 365 nm light illumination. These results provide a deep insight into the photophysical process of STEs, which lays the foundation for developing novel STEs‐based materials and optoelectronic devices.

Theoretical insights into room temperature phosphorescence emission with anti-Kasha behavior in aggregate

Organic room temperature phosphorescence (RTP) materials with long lifetimes have shown promising applications in organic light emitting diodes and bioimaging fields. However, the inner luminescent mechanisms for RTP especially for high lying triplet state emission, have not been unveiled. Herein, based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT), the photophysical properties of four RTP systems with triphenylethylene derivatives as skeletons are studied. Excited state dynamic process in aggregate is systematically studied by combining quantum mechanics with molecular mechanics (QM/MM) method coupled with thermal vibration correlation function (TVCF) method. Moreover, the intermolecular interactions are measured by the independent gradient model based on Hirshfeld partition (IGMH) method, the Huang-Rhys factor, reorganization energy and spin-orbit coupling (SOC) constant are calculated, radiative and non-radiative decay as well as intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes are all investigated. Results indicate that the radiative decay process from the lowest triplet excited state (T1) to ground state (S0) is low and the non-radiative decay process is high, various calculations are also performed to exclude the emission process from T1 to S0, thus non-luminescence is determined for T1. Furthermore, a large energy gap between T2 and T1 is observed, and high radiative decay and low non-radiative decay processes from T2 to S0 are confirmed. Thus, the mechanisms of RTP from high lying triplet state T2 are revealed. Furthermore, through wise molecular design and detailed calculations, the mechanical insights are detected that the RTP emissions with anti-Kasha behavior are largely related to the unique molecular configurations with triphenylethylene derivatives as skeletons. Our studies give reasonable explanations for the previous experimental measurements and provide underlying perspectives for RTP mechanisms from high lying triplet state T2, which could promote the development of new efficient RTP emitters.

B800-to-B850 relaxation of excitation energy in bacterial light harvesting: All-state, all-mode path integral simulations.

We report fully quantum mechanical simulations of excitation energy transfer within the peripheral light harvesting complex (LH2) of Rhodopseudomonas molischianum at room temperature. The exciton-vibration Hamiltonian comprises the 16 singly excited bacteriochlorophyll states of the B850 (inner) ring and the 8 states of the B800 (outer) ring with all available electronic couplings. The electronic states of each chromophore couple to 50 intramolecular vibrational modes with spectroscopically determined Huang-Rhys factors and to a weakly dissipative bath that models the biomolecular environment. Simulations of the excitation energy transfer following photoexcitation of various electronic eigenstates are performed using the numerically exact small matrix decomposition of the quasiadiabatic propagator path integral. We find that the energy relaxation process in the 24-state system is highly nontrivial. When the photoexcited state comprises primarily B800 pigments, a rapid intra-band redistribution of the energy sharply transitions to a significantly slower relaxation component that transfers 90% of the excitation energy to the B850 ring. The mixed character B850* state lacks the slow component and equilibrates very rapidly, providing an alternative energy transfer channel. This (and also another partially mixed) state has an anomalously large equilibrium population, suggesting a shift to lower energy by virtue of exciton-vibration coupling. The spread of the vibrationally dressed states is smaller than that of the eigenstates of the bare electronic Hamiltonian. The total population of the B800 band is found to decay exponentially with a 1/e time of 0.5ps, which is in good agreement with experimental results.

Thermally Activated Delayed Fluorescence of a Pyromellitic Diimide Derivative in the Film Environment Investigated by Combined QM/MM and MS-CASPT2 Methods.

Arylene diimide compounds exhibit thermally activated delayed fluorescence (TADF), but its mechanism remains elusive. Herein we studied the TADF mechanism of a carbazole-substituted pyromellitic diimide derivative (CzPhPmDI) in poly(methyl methacrylate) (PMMA) film by using DFT, TD-DFT, and MS-CASPT2 methods within the QM/MM framework. We found that the TADF mechanism involves three electronic states (i.e., S0, S1, and T1), but the T2 state is not involved because its energy is higher than the S1 state by 6.9 kcal/mol. By contrast, the T1 state is only 3.2 kcal/mol lower than the S1 state and such small energy difference benefits the reverse intersystem crossing (rISC) process from T1 to S1 thereto TADF. This point is seconded by relevant radiative and nonradiative rates calculated. At room temperature, the ISC rate from S1 to T1 is calculated to be 6.1 × 106 s-1, which is larger than the fluorescence emission rate, 2.2 × 105 s-1; thus, the dominant S1 population converts to the T1 state. However, in the T1 state, the rISC process (1.8 × 104 s-1) becomes the most important channel because of the negligible phosphorescence emission rate (3.5 × 10-2 s-1). So, the T1 population is still converted back to the S1 state to fluoresce enabling TADF. Unfortunately, the rISC process is blocked in low temperature. Besides, we found that relevant Huang-Rhys factors have dominant contribution from low-frequency vibrational motion related to the torsional motion of functional groups. These gained insights could provide useful information for the design of organic TADF materials with excellent luminescence efficiency.

Modulated excited state geometry and Electron-Phonon coupling of lutein by temperature and solvent polarizability

Resonance Raman spectroscopy is one of the spectroscopic methods often chosen for studying linear polyene molecules because the Raman intensities of their υ1 (C = C) and υ2 (C-C) stretching vibrations are sensitive to electron–phonon coupling and the π-electron energy gap. Here, the resonance Raman and absorption spectra of lutein were studied as a function of solvent polarizabilities and of temperature in the CS2 solvent. For lutein in CS2, as the temperature decreased and CS solidified, the Raman scattering cross-section (RSCS) and the electron–phonon coupling constant had opposite dependence trends on temperature. The wavenumber of the lutein 0–0 electronic transition showed a marked shift to lower wavenumbers when the polarizability of the solvents decreased, and the Huang-Rhys (HR) factors and electron–phonon coupling also decreased. This work helps explore the influence of the external environment (e.g., temperature and solvent) on the excited state geometry of linear polyene molecules.

Characterizing Mode Anharmonicity and Huang-Rhys Factors Using Models of Femtosecond Coherence Spectra.

Femtosecond laser pulses readily produce coherent quantum beats in transient–absorption spectra. These oscillatory signals often arise from molecular vibrations and therefore may contain information about the excited-state potential energy surface near the Franck–Condon region. Here, by fitting the measured spectra of two laser dyes to microscopic models of femtosecond coherence spectra (FCS) arising from molecular vibrations, we classify coherent quantum-beat signals as fundamentals or overtones and quantify their Huang–Rhys factors and anharmonicity values. We discuss the extracted Huang–Rhys factors in the context of quantum-chemical computations. This work solidifies the use of FCS for analysis of coherent quantum beats arising from molecular vibrations, which will aid studies of molecular aggregates and photosynthetic proteins.

Theoretical study on aggregation-induced emission of new multi-layer 3D chiral molecules

ABSTRACT Multilayer three-dimensional (3D) chiral molecules have potential application in luminescent material fields, but the mechanism of their AIE effect is not clear. In this work, the theoretical mechanism of the aggregation-induced emission (AIE) effect has been studied for three 3D molecules of 2-X-1,3-bis(8-(4-Y-phenyl)naphthalen-1-yl)-1,3-dihydrobenzo[d][1,3,2] diazapho-sphole-2-oxie (A. X = NH2, Y = H; B. X = i-Pr, Y = H; C. X = NH2, Y = OMe). The geometrical structures and spectral properties in solution, crystal and aggregation are discussed using the combined approach of molecular dynamics (MD), DFT/TD-DFT and ONIOM QM/MM calculations. The S0 and S1 structural changes of 3D molecules show that the variation of the naphthalene and benzene rings is reduced in aggregation. Huang-Rhys (HR) factor and reorganisation energy are smaller in aggregation than in solution, especially in low-frequency vibration modes. It suggests that aggregation restricts the rotation of benzene and naphthalene rings and constrains the low-frequency vibration modes, which allows the radiated channel of the excited state molecule to be opened and glow when aggregated. This study provides an excellent explanation for the AIE phenomenon of 3D chiral molecules in experiments.

TDDFT Investigation of the Raman and Resonant Raman Spectra of Fluorescent Protein Chromophore Models.

The off-resonance and resonant Raman spectra have been simulated for models of fluorescent protein chromophores, those of the green fluorescent protein (GFP, called FP1) and of DsRed (called FP2), which presents a longer π-conjugated path, with the aim of providing a systematic investigation of structural but also computational aspects. These were performed at the (time-dependent) density functional theory [(TD)DFT] level. The off-resonance intensities have been calculated from the derivatives of the frequency-dependent polarizability with respect to the normal coordinates while the resonant ones have been evaluated using Huang-Rhys factors determined from the gradients of the excitation energies with respect to the normal coordinates. When applied with the M05 meta-GGA exchange-correlation functional, this simple computational scheme can reproduce most of the experimental Raman signatures of FP1 in its protonated and deprotonated forms, the differences of vibrational signatures of the cis (Z) and trans (E) isomers, as well as their changes as a function of the excitation wavelength. On the other hand, testing the predictions made for FP2 would require new experimental work. It was also observed that simulations with methods that inadequately predict the resonant Raman spectra could nevertheless produce a UV-vis absorption spectrum that is quite similar to the one obtained with better methods, once realistic peak broadening has been applied.

Carbon trimer as a 2 eV single-photon emitter candidate in hexagonal boron nitride: A first-principles study

Identifying the atomic origin of the well-recognized 2 eV single-photon emitters in hexagonal boron nitride (hBN) has remained a long debate and important for controllable single-photon emission generation. Here, the authors investigate the static and dynamic properties of carbon trimers from first-principles with many-body interactions. Based on the calculated results, they propose C${}_{2}$C${}_{N}$ to be a promising candidate, which has Zero-Phonon Line, Huang-Rhys factor, photoluminescence lifetime and lineshape all in agreement with the current experimental observations. The near-infrared emission from C${}_{2}$C${}_{N}$ is suggested to be another identifier for experimental verification of the nature of single-photon emitters in hBN.

Simulation of Emission Spectra of Transition Metal Dichalcogenide Monolayers with the Multimode Brownian Oscillator Model.

The multimode Brownian oscillator model is employed to simulate the emission spectra of transition metal dichalcogenide (TMD) monolayers. Good agreement is obtained between measured and simulated photoluminescence spectra of WSe2, WS2, MoSe2, and MoS2 at various temperatures. The Huang-Rhys factor extracted from the model can be associated with that from the modified semiempirical Varshni equation at high temperatures. Individual mechanisms leading to the unique temperature-dependent emission spectra of those TMDs are validated by the multimode Brownian oscillator (MBO) fitting, while it is, in turn, confirmed that the MBO analysis is an effective method for studying the optical properties of TMD monolayers. Parameters extracted from the MBO fitting can be used to explore exciton-photon-phonon dynamics of TMDs in a more comprehensive model.

Thermal quenching of self-trapped exciton luminescence in nanostructured hafnia

The intrinsic optical properties and peculiarities of the energy structure of hafnium dioxide largely determine the prospects for applying the latter in new generation devices of optoelectronics and nanoelectronics. In this work, we have studied the diffuse reflectance spectra at room temperature for a nanostructured powder of nominally pure HfO2 with a monoclinic crystal structure and, as well its photoluminescence in the temperature range of 40–300 K. We have also estimated the bandgap Eg under the assumption made for indirect (5.31 eV) and direct (5.61 eV) allowed transitions. We have detected emission with a 4.2 eV maximum at T < 200 K and conducted an analysis of the experimental dependencies to evaluate the activation energies of thermal quenching (140 meV) and enhancement (3 meV) processes. Accounting for both the temperature behavior of the spectral characteristics and the estimation of the Huang-Rhys factor S » 1 has shown that radiative decay of self-trapped excitons forms the mechanism of the indicated emission. In this case, the localization is mainly due to the interaction of holes with active vibrational modes of oxygen atoms in non-equivalent (O3f and O4f) crystal positions. Thorough study of the discussed excitonic effects can advance development of hafnia-based structures with a controlled optical response.