Nonradiative Relaxation Channels Research Articles

Phonon-Driven Femtosecond Dynamics of Excitons in Crystalline Pentacene from First Principles.

Nonradiative exciton relaxation processes are critical for energy transduction and transport in optoelectronic materials, but how these processes are connected to the underlying crystal structure and the associated electron, exciton, and phonon band structures, as well as the interactions of all these particles, is challenging to understand. Here, we present a first-principles study of exciton-phonon relaxation pathways in pentacene, a paradigmatic molecular crystal and optoelectronic semiconductor. We compute the momentum- and band-resolved exciton-phonon interactions, and use them to analyze key scattering channels. We find that both exciton intraband scattering and interband scattering to parity-forbidden dark states occur on the same ∼100 fs timescale as a direct consequence of the longitudinal-transverse splitting of the bright exciton band. Consequently, exciton-phonon scattering exists as a dominant nonradiative relaxation channel in pentacene. We further show how the propagation of an exciton wave packet is connected with crystal anisotropy, which gives rise to the longitudinal-transverse exciton splitting and concomitant anisotropic exciton and phonon dispersions. Our results provide a framework for understanding the role of exciton-phonon interactions in exciton nonradiative lifetimes in molecular crystals and beyond.

Molecular Engineering and Confinement Effect Powered Ultrabright Nanoparticles for Improving Sensitivity of Lateral Flow Immunoassay.

The application of traditional lateral flow immunoassay (LFIA)-based gold nanoparticles (AuNPs) to measure traces of target chemicals is usually challenging. In this study, we developed an integrated strategy based on molecular engineering and the spatial confinement of nanoparticles (NPs) to obtain ultrahigh quantum yields (QYs) of aggregation-induced emission (AIE) fluorescence NPs and employed them for the highly sensitive detection of T-2 toxin on the LFIA platform. Tetraethyl-4,4',4″,4‴-(ethene-1,1,2,2-tetrayl)tetrabenzoate (TCPEME), an AIE luminogen, was designed using molecular engineering to lower the energy gap, achieving higher QYs (26.26%) than previous AIEgens (13.02%). Subsequently, TCPEME-doped fluorescence NPs (TFNPs) achieved ultrahigh QYs, up to 84.55%, which were generated from the strong restriction of the NP state, efficiently suppressing nonradiative relaxation channels verified by ultrafast electron dynamics. On the LFIA platform, the sensitivity of the designed TFNP-based LFIA (TFNP-LFIA) was 10.4-fold and 4.3-fold more sensitive than that of the AuNP-LFIA and TPENP-LFIA for detecting the T-2 toxin, respectively. In addition, TFNP-LFIA was used for detecting T-2 toxin in samples and showed satisfactory recoveries (79.5 to 122.0%) with CV (1.49 to 11.75%), which implied excellent application potential for TFNP-LFIA. Overall, dual improvement of the molecule in fluorescence performance originating from the molecular engineering and spatial confinement of NPs could be an efficient tool for promoting the development of high-performance reporters in LFIA.

Donor-acceptor pair transitions in MgAl2O4 spinel

The luminescence of the well-known spinel-structured functional compound MgAl2O4 was studied at cryogenic temperatures below 10 K in a wide spectral range from near-IR to VUV under synchrotron radiation and electron beam excitation. The most intense luminescence band at ∼5 eV, which origin remained largely unknown up to now, showed characteristic behaviour of the donor-acceptor pair (DAP) centres and was assigned to the so-called antisite defects. The antisite DAPs are formed by position-swapped cations AlMg● and MgAl′, which define the inversion degree of the spinel structure and, to the best of our knowledge, always exceed 10% (>1.5 × 1021 cm−3) in the synthetic solids. Full data set allowed confirming that extremely high concentration of these intrinsic defects is the main reason for the strong 5-eV luminescence, which mechanism includes the electron transfer from MgAl′ to AlMg● (excitation) following by that from VO● to MgAl× (emission). Irradiation with He + ions up to fluences of 1017 cm−2 randomly displaced cations to empty sites of the spinel lattice resulting in more intragap defect levels, which induce a non-radiative energy relaxation channel quenching closely spaced while unaffecting distant DAP luminescence.

Excitation Density Effects in the Luminescence Yield and Kinetics of MAPbBr3 Single Crystals

The luminescent Z-scan technique with time resolution is applied to the study of the luminescence properties of CH3NH3PbBr3 single crystals representative of the family of hybrid organic–inorganic lead perovskites successfully applied recently in photovoltaics and currently investigated as potential nanosecond scintillators. The third harmonic of Ti-sapphire laser (λ = 266 nm) with a pulse duration of 26 fs and 1 kHz frequency was applied for the luminescence excitation creating the charge carriers with the estimated density from 1017 to 1021 cm−3 in the temperature range from 13 to 300 K. Temperature and excitation density dependence of the luminescence yield and kinetics is interpreted with the consideration of the temperature-dependent binding of electrons and holes into excitons, a saturation of defects responsible for the non-radiative relaxation channel competing with exciton creation; absorption saturation resulting in the increased penetration depth of the excitation radiation and hence the increased contribution of the re-absorption.

Nd3+/Sm3+ codoped NaLa(WO4)2 glass ceramic: A novel optical heater

Transparent Nd3+/Sm3+ codoped tungstate silicate glass ceramics were prepared and used for the photothermal conversion process. XRD patterns, TEM image and the enhanced Raman signals confirm the appearance of the tetragonal scheelite NaLa(WO4)2 nanocrystals in the vitreous phase. In comparison to the precursor glass, the enhancement of photoluminescence of Nd3+ ions in the glass ceramics attributes to the enrichment of Nd3+ ion in the precipitated low phonon-energy tetragonal scheelite NaLa(WO4)2 nanocrystals. The rapid reduction of photoluminescence of Nd3+ ions in the Nd3+/Sm3+ codoped glass ceramics demonstrates that a strong energy transfer from Nd3+ to Sm3+ takes place, which provides more non-radiative relaxation channels and is beneficial for the improving the photothermal conversion efficiency. Under the irradiation of 808 nm laser diode, a significant temperature rise is observed in the Nd3+/Sm3+ codoped glass ceramics and may be used as a good optical heater.

Angular shaped AIE generator based luminophores for mechanochromism: An explosive sensor

We have synthesized (E)-N,N-diphenyl-4-(2-(2-(4′-(1,2,2-triphenylvinyl)-[1,1′-biphenyl]− 4-yl)vinyl)− 1H-phenanthro[9,10-d]imidazol-1-yl)aniline (TPE-DBPPI) and (E)− 2-(4′-(2-(1-(4-(diphenylamino)phenyl)− 1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)-[1,1′-biphenyl]− 4-yl)− 3,3-diphenylacrylonitrile (TPA-PPICN) consist of triphenylamine (donor), phenanthroimidazole (acceptor) and tetraphenylethylene (AIE generator). The twisted TPE-DBPPI and TPA-PPICN depict aggregation induced emission enhancement (AIEE) and demonstrated mechanochromic (MC) behavior between blue and green emission. The restricted-intramolecular-rotation (RIR) of aryl rings of TPE (TPE-DBPPI) and TPA (TPA-PPICN) in the aggregated phase reduced the non-radiative relaxation channel hence, increase of emission intensity with high photoluminescence quantum yield (PLQY) in film state was observed. The PLQY of TPE-DBPPI / TPA-PPICN in various phases are of 58/76 % (pristine), 62/83 % (grinding) and 65/88 % (annealing). The film optical studies and powder XRD (PXRD) analyses implies that the change of twisted crystalline phase of TPE-DBPPI and TPA-PPICN to planar amorphous state results in mechanochromism. Also, these materials detect picric acid in aqueous and film owing to the proton-binding ability of azomethine nitrogen of imidazole which was confirmed by both NMR and DFT studies. The high negative ∆Get [− 3.01 eV (TPE-DBPPI) and − 3.06 eV (TPA-PPICN)] indicates the thermodynamic feasibility of photo electron transfer (PET) process.

Solid-state carbon dots with orange phosphorescence and tunable fluorescence via in-situ growth in phthalimide crystal matrix

Solid-state fluorescent and phosphorescent carbon dots (CDs) have widespread applications, with difficulties in obtainment. In this study, solid-state carbon dots (SCDs) emitting green, yellow, and orange fluorescence were fabricated using urea and phthalic acid as precursors. The band gap size was systematically controlled by varying the ratio of raw material nitrogen. By changing the temperature during synthesis, the phosphorescent properties of SCDs were adjusted, and the longest lifetime possessed 156.8 ms of orange phosphorescence. CDs were attached to the crystals of phthalimide, allowing a certain spacing between the particles and suppressing the phenomenon of aggregation-induced quenching (ACQ), thus obtaining solid-state fluorescence. Detailed characterization and density flooding theory (DFT) calculations further indicate that the reduction of the conjugation domain leads to the redshift of the solid-state fluorescence. The phosphorescence appearance is attributed to the presence of crystals that inhibit the nonradiative relaxation channels, and N elements that promote spin-orbit coupling interactions. Based on their special luminescent properties, they have been successfully applied in the fabrication of (0.38,0.54), (0.44,0.54) and (0.51, 0.47) light-emitting diodes, as well as in the production of secure anti-counterfeit cryptographic inks and their successful application in digital encryption.

First-Principles Study on Optoelectronic Properties of Fe-Doped Montmorillonite Clay.

A theoretical investigation is conducted to describe optoelectronic properties of Fe-doped montmorillonite nanoclay under spin states of low spin (LS), intermediate spin (IS), and high spin (HS). Ground state electronic properties are studied using spin-polarized density functional theory calculations. The nonradiative and radiative relaxation channels of charge carriers are studied by computing nonadiabatic couplings (NACs) using an "on-the-fly" approach from adiabatic molecular dynamics trajectories. The NACs are further processed using a reduced density matrix approach with the Redfield formalism. The computational results are presented for electronic density of states, absorption spectra, charge carrier dynamics, and photoluminescence (PL) by comparing various spin multiplicities. Results on spin α and spin β components are independent and quite different because of the partial occupation of Fe 3d states. Overall, HS is the most stable with the largest Fe-O distances. One finds different nonradiative relaxation pathways in space and on the time scale for electrons and holes. The Redfield PL reveals obvious Fe 3d-3d transitions for LS and IS.

Long-Lived Organic Room-Temperature Phosphorescence from Amorphous Polymer Systems.

Long-lived organic room-temperature phosphorescence (RTP) materials have recently drawn extensive attention because of their promising applications in information security, biological imaging, optoelectronic devices, and intelligent sensors. In contrast to conventional fluorescence, the RTP phenomenon originates from the slow radiative transition of triplet excitons. Thus, enhancing the intersystem crossing (ISC) rate from the lowest excited singlet state (S1) to the excited triplet state and suppressing the nonradiative relaxation channels of the lowest excited triplet state (T1) are reasonable methods for realizing highly efficient RTP in purely organic materials. Over the past few decades, many strategies have been designed on the basis of the above two crucial factors. The introduction of heavy atoms, aromatic carbonyl groups, and other heteroatoms with abundant lone-pair electrons has been demonstrated to strengthen the spin-orbit coupling, thereby successfully facilitating the ISC process. Furthermore, the rigid environment is commonly constructed through crystal engineering to restrict intramolecular motions and intermolecular collisions to decrease excited-state energy dissipation. However, most crystal-based organic RTP materials suffer from poor processability, flexibility, and reproducibility, becoming a thorny obstacle to their practical application.Amorphous organic polymers with long-lived RTP characteristics are more competitive in materials science. The intertwined structures and long chains of polymers not only ensure the rigid environment with multiple interactions but also protect triplet excitons from the surroundings, which are conducive to realizing ultralong and bright RTP emission. Exploring the fabrication strategies, intrinsic mechanisms, and practical applications of organic long-lived RTP polymers is highly desirable but remains a formidable challenge. In particular, intelligent organic RTP polymer systems that are capable of dynamically responding to external stimuli (e.g., light, temperature, oxygen, and humidity) have been rarely reported. To develop multifunctional RTP materials and expand their potential applications, a great amount of effort has been expended.This Account gives a summary of the significant advances in amorphous organic RTP polymer systems, especially smart stimulus-responsive ones, focusing on the construction of a rigid environment to suppress nonradiative deactivation by abundant inter/intramolecular interactions. The typical interactions in RTP polymer systems mainly include hydrogen bonding, ionic bonding, and covalent bonding, which can change the molecular electronic structures and affect the energy dissipation channels of the excited states. An in-depth understanding of intrinsic mechanisms and an extensive exploration of potential applications for excitation-dependent color-tunable, ultraviolet (UV) irradiation-activated, temperature-dependent, water-responsive, and circularly polarized RTP polymer systems are distinctly illustrated in this Account. Furthermore, we propose some detailed perspectives in terms of materials design, mechanism exploration, and promising application potential with the hope to provide helpful guidance for the future development of amorphous organic RTP polymers.

Aggregation Turns BODIPY Fluorophores into Photosensitizers: Reversibly Switching Intersystem Crossing On and Off for Smart Photodynamic Therapy

Aggregation Turns BODIPY Fluorophores into Photosensitizers: Reversibly Switching Intersystem Crossing On and Off for Smart Photodynamic Therapy

Fluoroindate glasses co-doped with Pr3+/Er3+ for near-infrared luminescence applications

Fluoroindate glasses co-doped with Pr3+/Er3+ ions were synthesized and their near-infrared luminescence properties have been examined under selective excitation wavelengths. For the Pr3+/Er3+ co-doped glass samples several radiative and nonradiative relaxation channels and their mechanisms are proposed under direct excitation of Pr3+ and/or Er3+. The energy transfer processes between Pr3+ and Er3+ ions in fluoroindate glasses were identified. In particular, broadband near-infrared luminescence (FWHM = 278 nm) associated to the 1G4 → 3H5 (Pr3+), 1D2 → 1G4 (Pr3+) and 4I13/2 → 4I15/2 (Er3+) transitions of rare earth ions in fluoroindate glass is successfully observed under direct excitation at 483 nm. Near-infrared luminescence spectra and their decays for glass samples co-doped with Pr3+/Er3+ are compared to the experimental results obtained for fluoroindate glasses singly doped with rare earth ions.

Dark-state impact on the exciton recombination of WS2 monolayers as revealed by multi-timescale pump-probe spectroscopy

The luminescence yield of transition metal dichalcogenide monolayers frequently suffers from the formation of long-lived dark states, which include excitons with intervalley charge carriers, spin-forbidden transitions, and a large center-of-mass momentum located outside the light cone of dispersion relations. Efficient relaxation from bright exciton states to dark states suppresses the quantum yield of photon emission. In addition, the radiative recombination of excitons is heavily influenced by Auger-type exciton-exciton scattering, which yields another nonradiative relaxation channel at room temperature. Here, we show that Auger-type scattering is promoted not only between (bright) excitons but also between excitons and long-lived dark states. We studied the luminescence dynamics of monolayer WS2 capped with hexagonal BN over broad time ranges of picoseconds to milliseconds using carefully designed pump-and-probe techniques. We observed that luminescence quenching associated with Auger-type scattering occurs on 1-100 microsecond time scales, which thus correspond to the lifetimes of the relevant dark states. The broad distribution of the measured lifetimes implies the impact of various types of long-lived states on the exciton annihilation process.

Toward Understanding Optical Properties of Amyloids: A Reaction Path and Nonadiabatic Dynamics Study.

Amyloids have unique structural, chemical, and optical properties. Although much theoretical effort has been directed toward understanding amyloid nucleation, the understanding of their optical properties has remained rather limited. In particular, the photophysical mechanisms leading to near-UV excitation and characteristic blue-green luminescence in amyloid systems devoid of aromatic amino acids have not been resolved. We use ab initio static calculations and nonadiabatic dynamics simulations to study the excited electronic states of model amyloid-like peptides. We show that their photophysics is essentially governed by the multitude of nπ* states with excitation localized on the amide groups. The strong stabilization of the nπ* states with respect to the amide group deplanarization and the concomitant increase of the oscillator strength make excitation in the near-UV possible. With respect to emission, our dynamics simulations revealed that the amyloid cross β arrangement effectively hinders the nonradiative relaxation channels usually operative in proteins. Finally, we show that after relaxation of the photoexcited peptides toward the minimum of the different nπ* states, fluorescence takes place in the visible (green) part of the electromagnetic spectrum.

Investigation of Morphology-Controlled Ultrafast Relaxation Processes of Aggregated Porphyrin.

Here, we have synthesized rod and flake shaped morphology of porphyrin aggregates from 5, 10, 15, 20-tetra (4-n-octyloxyphenyl) porphyrin (4-opTPP) molecule which are evident from scanning electron microscopy (SEM). The formation of J-type aggregation is evident from steady state and time-resolved fluorescence spectroscopic studies. Ultrafast transient absorption spectroscopic studies reveal that the excited state lifetime is controlled by the morphology and the time constant for S1→S0 relaxation changes from 3.05 ps to 744 ps with changing the shape from rod to flake, respectively. In spite of similar exciton coupling energy in both the aggregates, the flake shaped aggregates undergo a faster exciton relaxation process and the non-radiative relaxation channels are found to depend on the shape of aggregates. The fundamental understanding of morphology controlled ultrafast relaxation processes of aggregated porphyrin is important for designing efficient light harvesting devices.

Structural characterization and photoluminescence of (Gd1-xErx)2O3 nanophosphors synthesized by co-precipitation of layered precursors

In this paper, the layered (Gd1-xErx)2(OH)5NO3·nH2O (LGEH) with a variable content of erbium were synthesized by controlled double-jet precipitation (CDJP) at a constant pH value. The effect of pH co-precipitation on the morphology, crystallinity, porosity of the LGEH, as well as on the spectral-luminescent properties of oxide nanophosphors obtained by their heat treatment is significant. We have found that the samples with the lowest disordering and the highest of photoluminescence (PL) intensity of Er3+ ions are obtained from amorphous sponge-like precursor co-precipitated at pH 9.5. The PL of nanostructural (Gd1-xErx)2O3 depends on both structural-morphology properties and samples composition. It was also found that efficiency of radiative and non-radiative relaxation channels for excited Er3+ ions mainly depends on three factors: nanocrystals size, specific surface area and erbium concentration. For explanation of the PL behavior in the green and red spectral ranges, the energy diagram showing the intracenter energy transitions and intercenter energy migration in synthesized nanophosphors was proposed.

Dibenzo[a,c]phenazine-phenothiazine dyad: AIEE, polymorphism, distinctive mechanochromism, high sensitivity to pressure

Herein, we report a new dibenzo[a,c]phenazine-phenothiazine dyad (BNPTZ) bridged by a C–N single bond, which realizes AIEE, polymorphism, mechanochromsim and high sensitivity to hydrostatic pressure, due to the twisted D-A structure. BNPTZ exhibits AIEE activity in THF/H2O system, owing to aggregation-inhibited non-radiative relaxation channels of the excited states. Fortunately, two types of single crystals of BNPTZ with distinctive luminescence were cultivated and the effect of different conformation and complicated intermolecular interactions on luminescent property was analysed. BNPTZ demonstrates responses to both shearing force and hydrostatic pressure. Interestingly, two different crystalline phases are involved in the color-changing process upon shearing and fuming. Upon shearing, the crystal powder suffers the luminescence color change from orange-yellow (565 nm) to redness (604 nm), because of the phase transition from OY-phase to amorphous triggered by external force. When fuming, the amorphous phase would recover to YG-phase unexpectedly. When subjecting two single crystals to hydrostatic pressure, the OY-phase single crystal underwent a clear color switch from yellow to orange and eventually to red. The YG-phase single crystal displayed a different color alteration from yellow-green to orange and further to red. Both of the two single crystals exhibited relatively high sensitivity to pressure with a wide spectral shift over 100 nm. Up to now, BNPTZ is a rare dyad that achieves polymorphs with high sensitivity to pressure.

Non-radiative relaxation processes in luminescence of InP/ZnS quantum dots

Photoluminescence (PL) thermal quenching of InP/ZnS core/shell nanocrystals of 4.2 (QD-1) and 4.6 (QD-2) nm in diameter is investigated in the temperature range of 6.5-296 K. Observed PL spectra are found to originate from exciton and defect-related transitions. Temperature behavior of the emission bands indicates effects of inhomogeneous broadening due to distributions of quantum dots parameters in the ensembles. PL thermal quenching analysis unveils two non-radiative relaxation channels for each band in both samples. Activation energies of 130 and 119 meV in QD-1 and QD-2, correspondingly, for the exciton band can be assigned to the bright-bright splitting of the exciton fine structure.

Temperature dependence of photoluminescence lifetime of atomically-thin WSe2 layer

At cryogenic temperatures, the photoluminescence (PL) spectrum of monolayer WSe2 features a number of lines related to the recombination of so-called localized excitons (LEs). The intensity of these lines strongly decreases with increasing temperature. In order to understand the mechanism behind this phenomenon we carried out a time-resolved experiment, which revealed a similar trend in the PL decay time. Our results identify the opening of additional non-radiative relaxation channels as a primary cause of the observed temperature quenching of the LEs’ PL.

Direct Visualization of Exciton Transport in Defective Few-Layer WS2 by Ultrafast Microscopy.

As defects usually limit the exciton diffusion in 2D transition metal dichalcogenides (TMDCs), the interaction knowledge of defects and exciton transport is crucial for achieving efficient TMDC-based devices. A direct visualization of defect-modulated exciton transport is developed in few-layer WS2 by ultrafast transient absorption microscopy. Atomic-scale defects are introduced by argon plasma treatment and identified by aberration-corrected scanning transmission electron microscopy. Neutral excitons can be captured by defects to form bound excitons in 7.75-17.88 ps, which provide a nonradiative relaxation channel, leading to decreased exciton lifetime and diffusion coefficient. The exciton diffusion length of defective sample has a drastic reduction from 349.44 to 107.40 nm. These spatially and temporally resolved measurements reveal the interaction mechanism between defects and exciton transport dynamics in 2D TMDCs, giving a guideline for designing high-performance TMDC-based devices.

Effect of Auger recombination in ensemble of CdSe nanocrystals on their luminescence

The temperature dependence of the photoluminescence intensity of colloidal CdSe nanocrystals having 5 nm in diameter was studied. It was demonstrated that the photoluminescence intensity exhibits a hysteresis at temperatures below 220 K. This effect was explained by a model taking into account formation of an additional nonradiative relaxation channel associated with Auger recombination promoted by hole trapping on the surface of the nanocrystals. The activation energy of the hole release process obtained in the framework of the model was 0.7 eV.