Femtosecond Time-resolved Photoluminescence Research Articles

Excited-State Dynamics of a Bright Fluorescent Dye with Precise Control of Emission Color Using Acid-Base Equilibrium, Intramolecular Charge Transfer, and Host-Guest Chemistry.

A fluorescent dye, a dithiophene-conjugated benzothiazole derivative (DTBz), was prepared to have high fluorescence emission quantum yields (ΦF) across various organic solvents. Its emission color modulation, from bright blue to deep red, was achieved through intramolecular charge transfer (ICT), acid-base equilibrium, and host-guest chemistry. Although it exhibits a weak solvatochromic effect, DTBz exhibited a bright fluorescence emission around 480 nm upon excitation at 390 nm in most solvents. In polar solvents, such as MeOH (methanol), EtOH (ethanol), DMF (N,N-dimethylforamide), and DMSO (dimethyl sulfoxide), an additional ICT emission band emerged around 640 nm, notably intense in DMSO, resulting in a bright greenish-white emission (ΦF = 0.67). The addition of 1,8-diazabicyclo[5,4.0]undec-7-ene (DBU) altered emission characteristics, reducing emission from the local excited (LE) state and enhancing ICT state emission. The degree of emission spectral change saturation with DBU addition varied with the solvent nature. Polar solvents with high dielectric constants, like DMSO and DMF, saw a complete disappearance of LE state emission with 5 equiv of DBU, resulting in a deep red emission (ΦFs of 0.53 and 0.48, respectively). Femtosecond transient absorption spectroscopy and time-resolved photoluminescence measurements elucidated the excited-state dynamics, revealing a long-lived excited state (τ-H = 10.3 ns) at a lower energy emission (640 nm), identified as DTBz-*, supported by transient absorption spectra analysis. Further analysis, including time-resolved fluorescence decay measurements and time-dependent density-functional theory (TD-DFT) calculations, underscored the role of deprotonation of DTBz's hydroxyl group in promoting the ICT process. The CIE coordination plot demonstrated wide linear emission color changes upon successive DBU additions in all solvents, while emission color precision was achieved through host-guest chemistry. Emission changes induced by DBU were reverted to the original state upon beta-cyclodextrin (β-CD) addition, with the 1H NMR study revealing the competition between acid-base equilibrium and host-guest complex formation as the cause of emission color change.

Spectator Exciton Effects in Nanocrystals III: Unveiling the Stimulated Emission Cross Section in Quantum Confined CsPbBr3 Nanocrystals.

Quantifying stimulated emission in semiconductor nanocrystals (NCs) remains challenging due to masking of its effects on pump-probe spectra by excited state absorption and ground state bleaching signals. The absence of this defining photophysical parameter in turn impedes assignment of band edge electronic structure in many of these important fluorophores. Here we employ a generally applicable 3-pulse ultrafast spectroscopic method coined the "Spectator Exciton" (SX) approach to measure stimulated-emission efficiency in quantum confined inorganic perovskite CsPbBr3 NCs, the band edge electronic structure of which is the subject of lively ongoing debate. Our results show that in 5-6 nm CsPbBr3 NCs, a single exciton bleaches more than half of the intense band edge absorption band, while the cross section for stimulated emission from the same state is nearly 6 times weaker. Discussion of these findings in light of several recent electronic structure models for this material proves them unable to simultaneously explain both measures, proving the importance of this new input to resolving this debate. Along with femtosecond time-resolved photoluminescence measurements on the same sample, SX results also verify that biexciton interaction energy is intensely attractive with a magnitude of ∼80 meV. In light of this observation, our previous suggestion that biexciton interaction is repulsive is reassigned to hot phonon induced slowdown of carrier relaxation leading to direct Auger recombination from an excited state. The mechanism behind the extreme slowing of carrier cooling after several stages of exciton recombination remains to be determined.

Cypate-sensitized upconversion nanoprobes for intracellular and in-vivo ATP ratiometric detection

As energy currency of life, adenosine triphosphate (ATP) is highly dynamic in living organisms. It is crucial yet challenging to probe intracellular or in-vivo ATP precisely without destruction of the living biological systems. Herein, we have designed novel ATP-responsive ratiometric nanoprobes by employing upconversion nanoparticles (UCNPs) with cypate as antennas. Nuclear magnetic resonance spectroscopy was used to analyze the interfacial interactions among lanthanide nanocrystal, cypate and adenosine phosphates, disclosing the origin of detection specificity. ATP induced interruption of energy transfer pathway in cypate-sensitized UCNPs was further investigated by femtosecond transient absorption and time-resolved photoluminescence spectroscopy. The nanoprobes with dual near-infrared (NIR)-excited ratiometric UCL were successfully applied for quantification of ATP level in living cells under vegetatively growing, starving, and glycolysis-inhibiting states, respectively. Furthermore, the increase of available energy by feeding for zebrafish larvae was confirmed through monitoring the in-vivo ATP variation with the nanoprobes. Such nanoprobes showed great potential in ATP-associated biological research and clinical applications. These findings may gain deep insights into the surface science and photophysics in dye-lanthanide nanocomposites, and open new avenues for the design and bioapplications of lanthanide nanoprobes.

Ultrafast Exciton Dynamics of CH3NH3PbBr3 Perovskite Nanoclusters.

Exciton dynamics of perovskite nanoclusters has been investigated for the first time using femtosecond transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy. The TA results show two photoinduced absorption signals at 420 and 461 nm and a photoinduced bleach (PB) signal at 448 nm. The analysis of the PB recovery kinetic decay and kinetic model uncovered multiple processes contributing to electron-hole recombination. The fast component (∼8 ps) is attributed to vibrational relaxation within the initial excited state, and the medium component (∼60 ps) is attributed to shallow carrier trapping. The slow component is attributed to deep carrier trapping from the initial conduction band edge (∼666 ps) and the shallow trap state (∼40 ps). The TRPL reveals longer time dynamics, with modeled lifetimes of 6.6 and 93 ns attributed to recombination through the deep trap state and direct band edge recombination, respectively. The significant role of exciton trapping processes in the dynamics indicates that these highly confined nanoclusters have defect-rich surfaces.

Shell thickness influence on the carrier dynamics of InP/ZnS QDs

InP/ZnS core/shell quantum dots (QDs) have been widely applied in many fields due to their excellent luminescence performance. In this study, the correlation between the ZnS shell thickness and the ultrafast carrier dynamics in InP/ZnS QDs was investigated by using femtosecond time-resolved transient absorption spectroscopy and time-resolved photoluminescence techniques. We found that a thicker ZnS shell could confine more carriers to the InP core, thus effectively weakening the interactions between the carriers in the core and the interfacial defect states; this further enhanced the quantum yields and photoluminescence properties in InP/ZnS QDs.

CuBO2 : A Potential Alternative for NiO as a Hole Acceptor Layer.

P-type metal oxides, and in particular NiO, are typically used as hole accepting layers in dye-sensitized photocathodes. Delafossites (CuMO2 ) with M=B, Al, Cr or Ga have recently been proposed as attractive substitutes for NiO, with theoretically a higher hole mobility than NiO, therefore allowing a higher efficiency when the photocathode is applied in solar to fuel devices. We have experimentally validated the photoelectrochemical performance of photocathodes consisting of nanoporous CuBO2 (CBO) on Fluorine-doped Tin Oxide substrates, photosensitized with a light absorbing P1 dye. Femtosecond transient absorption and time-resolved photoluminescence studies show that light-induced hole injection occurs from the P1 dye into the CBO in a few ps, comparable to the time constant observed for NiO-based photocathodes. Importantly, the CBO-based photocathode shows significantly slower charge recombination than the NiO-based analogue. These results illustrate the promise of CBO as a p-type semiconductor in solar energy conversion devices.

Efficient Charge Transfer in the Perovskite Quantum Dot-Hemin Biocomposite: Is This Effective for Optoelectronic Applications?

Inorganic perovskite quantum dots (PQDs) have great potential for optoelectronic applications as a result of their tunable optical properties, significant absorption coefficient, and high mobility. Combining PQDs with molecular adsorbates offers exciting possibilities for future applications, making it important to study interfacial electron transfer in PQD-molecular composites. Here, we present a study of PQD and hemin composites (PQD-hemin) to understand how their interfacial electron transfer dynamics are affected by adsorbate and PQD properties. Our femtosecond ultrafast transient absorption and time-resolved photoluminescence (TRPL) studies reveal that hot carrier relaxation, charge separation, and charge recombination processes are significantly impacted in the PQD-hemin composite system under different excitations, both higher and lower energy. Additionally, our alternating current (AC)- and direct current (DC)-bias-driven electrical studies show that, despite efficient charge separation in the PQD-hemin composite system, the light-induced transient photocurrent drops. The findings on the PQD-molecular composite will give useful outlooks for designing a variety of optoelectronic devices.

Guest Editorial: Solar Energy Utilization

Guest Editorial: Solar Energy Utilization

Photoexcited state of the organic electron acceptor TCNQ molecular anion in the condensed state

The first and second excited states of 7,7,8,8-tetracyanoquinodimethane (TCNQ) anions in acetonitrile were studied employing femtosecond time-resolved photoluminescence and transient absorption spectroscopy. It was found that the second excited state, which is a superexcited state, undergoes ultrafast decay due to spontaneous electron emission with a time constant of 0.16 ps. The rationale for the fast electron emission in acetonitrile is discussed in terms of the interactions with the surrounding polar solvent molecules.

Determining the Correlation between Excited State Dynamics and Donor and Acceptor Structure in Nonfullerene Acceptors

The design of nonfullerene acceptors (NFAs), which comprise donor and acceptor moieties, has led to significant improvement in organic photovoltaic (OPV) device performance over the past few years. Although there have been many NFAs synthesized and device-optimized, there have been fewer studies on the excited-state photophysics of donor-acceptor-containing NFAs. Using a combination of femtosecond transient absorption spectroscopy and time-resolved photoluminescence (PL), we have investigated the excited-state photophysical processes in three donor-acceptor-containing NFAs, where the chemical structure of the NFAs are systematically modified to allow direct comparison (D-A1-A1-D, D-A1-D, and D-A1-A2, where D represents 9,9-di-n-propylfluorene, A1 represents benzothiadiazole, and A2 represents dicyanovinyl). A large triplet population was observed for the dimer NFA (D-A1-A1-D), whereas for D-A1-D or D-A1-A2, the nonradiative decay rate was significantly decreased, and higher PL quantum yields observed. Furthermore, a singlet exciton relaxation process between S1∗ and S1 on the picosecond timescale was observed for each of the NFAs, although the mechanism was not always the same. Torsional relaxation was found to dominate the picosecond singlet relaxation for D-A1-D and D-A1-A2 in solution and film, and for D-A1-A1-D in solution. In the film, the singlet excited state of D-A1-A1-D predominantly relaxed via resonant excitonic energy transfer. The observation of singlet relaxation on the tens of picosecond timescale also provides a plausible explanation to the reduced VOC loss commonly associated with NFA-based OPV devices.

Defects mediated charge disturbance in quantum-confined AgxS/AgInS2 random alloys – Toward slowly decaying quantum dot emitters

This works presents comprehensive studies on fundamental excitations trapping and the resulting charge destabilization in cubanite AgxS/AgInS2 quantum-confined random alloys. The observed optical activity of defect levels, related to silver alloying, is discussed basing on the femtosecond excited time-resolved photoluminescence spectrograms as well as theoretical predictions obtained via the density functional theory within the reduced cluster model. Impact of the charge trapping on the third-order optical hyper-polarizabilities has also been investigated basing on the two-photon excitation action cross sections. It has been shown that silver induced defects in the AgxS/AgInS2 random alloys are responsible for effective electron/hole trapping and strong temporal charge uncompensation. The silver on indium AgIn, silver interstitials Agi and higher-order defect clusters e.g. (AgIn2--2Agi+) formed effectively due to defect compensation mechanism have been considered in this work as providing possible pathways of defect mediated depopulation of the excited states. The electron/hole trapping mechanism is also discussed as a major mechanism responsible for fast trionic relaxations leading to strong deviation from the first-order decay law and significant temporal red-shifts of the photoluminescence band maximum. Further, it is shown that controlling the silver alloying ratio and the resulting number of defects charge states, it is possible to cause variation of the average photoluminescence decay times by an order of magnitude. The silver alloying was also found as an efficient route for significant increase of the two-photon excitation action cross section.

Recent Progress of Electron Storage Mn Center in Doped Nanocrystals

Mn doping in semiconductor nanocrystals (NCs) has been reviewed for its unequivocal relevance to solar energy harvesting in terms of both mechanistic approaches of electron storage in the Mn center and improved power conversion efficiency (PCE). Mn-doped NCs have been measured to have significantly higher PCE than the undoped counterpart, which has been reviewed for a number of different host NCs including different alloy and core–shell structures. Such increment of PCE has been attributed to electron relaxation from the hot states of the host NCs via the Mn state (that occurs at hundreds of picosecond time scale) which acts as electron-storage center. This can avoid the detrimental charge recombination significantly, and at the same time, fast electron injection from the Mn state to TiO2 leads to enhancement of PCE performance. These phenomena are clearly demonstrated by analyzing the feedback from femtosecond transient absorption, time-resolved photoluminescence, and impedance spectroscopy.

The photoluminescence, ultrafast nonlinear optical properties and carrier dynamics of 1D In-doped ZnO nanostructures: Experiment and mechanism

1D In-doped ZnO (IZO) nanostructures with different morphologies, namely nanowires, nanoseries(serial), nanorods and nanoprisms, have been successfully synthesized by chemical vapour deposition method. The structural, morphology and optical properties of IZO nanowires have been characterized. The ultrafast nonlinear optical and carrier dynamics characteristics of IZO were studied using femtosecond Z-scan, time-resolved photoluminescence (TRPL) and two-color pump-probe (TCPP) technique, respectively. The TRPL showed that the luminescence decay lifetimes of about several 10 ps and nanoseconds time response scale can be attributed to the shallow trap (ST) state, and deep trap (DT) state emissions, respectively. The mechanisms of the TRPL behavior in the different emission band were analyzed. The result of TCPP experiment showed that the bleach recovery with lifetimes on the fast (100–200 ps), and slow (>1 ns) time scale, caused by the shallow donor mixed with donor bound recombination, and donor−acceptor pair recombination (DAP), respectively. The optical transition process in the IZO nanostructures has demonstrated that the energy transfer process from the host electron-hole pairs can efficiently excite the metal ions. Comparing with the results obtained from the TRPL experiment, we found the fast decay process was resulted from the exciton migration and the slow decay to the recombination process of the relaxed excitons. Our results show that IZO nanostructures are a promising candidate in further optoelectronic device applications.

Low-threshold lasing from colloidal CdSe/CdSeTe core/alloyed-crown type-II heteronanoplatelets.

Colloidal type-II heterostructures are believed to be a promising solution-processed gain medium given their spatially separated electrons and holes for the suppression of Auger recombination and their wider emission tuning range from the visible to near-infrared region. Amplified spontaneous emission (ASE) was achieved from colloidal type-II core/shell nanocrystals several years ago. However, due to the limited charge-transfer (CT) interfacial states and minimal overlap of electron and hole wave functions, the ASE threshold has still been very high. Herein, we achieved ASE through type-II recombination at a lower threshold using CdSe/CdSeTe core/alloyed-crown nanoplatelets. Random lasing was also demonstrated in the film of these nanoplatelets under sub-ns laser-pumping. Through a detailed carrier dynamics investigation using femtosecond transient absorption, steady state, and time-resolved photoluminescence (PL) spectroscopies, we confirmed the type-II band alignment, and found that compared with normal CdSe/CdTe core/crown nanoplatelets (where no ASE/lasing was observed), CdSe/CdSeTe core/alloyed-crown nanoplatelets had a much higher PL quantum yield (75% vs. 31%), a ∼5-fold larger density of type-II charge-transfer states, a faster carrier transfer to interfaces (0.32 ps vs. 0.61 ps) and a slower Auger recombination lifetime (360 ps vs. 160 ps). Compared with CdSe/CdTe nanoplatelets, their counterparts with an alloyed crown boast a promoted charge transfer process, higher luminescence quantum yield, and smaller Auger rate, which results in their excellent application potential in solution-processed lasers and light-emitting devices.

High‐Performance CsPb1−xSnxBr3 Perovskite Quantum Dots for Light‐Emitting Diodes

All inorganic CsPbBr3 perovskite quantum dots (QDs) are potential emitters for electroluminescent displays. We have developed a facile hot-injection method to partially replace the toxic Pb2+ with highly stable Sn4+ . Meanwhile, the absolute photoluminescence quantum yield of CsPb1-x Snx Br3 increased from 45 % to 83 % with SnIV substitution. The transient absorption (TA) exciton dynamics in undoped CsPbBr3 and CsPb0.67 Sn0.33 Br3 QDs at various excitation fluences were determined by femtosecond transient absorption, time-resolved photoluminescence, and single-dot spectroscopy, providing clear evidence for the suppression of trion generation by Sn doping. These highly luminescent CsPb0.67 Sn0.33 Br3 QDs emit at 517 nm. A device based on these QDs exhibited a luminance of 12 500 cd m-2 , a current efficiency of 11.63 cd A-1 , an external quantum efficiency of 4.13 %, a power efficiency of 6.76 lm w-1 , and a low turn-on voltage of 3.6 V, which are the best values among reported tin-based perovskite quantum-dot LEDs.

Dynamics of Charged Excitons and Biexcitons in CsPbBr3 Perovskite Nanocrystals Revealed by Femtosecond Transient-Absorption and Single-Dot Luminescence Spectroscopy

Metal-halide perovskite nanocrystals (NCs) are promising photonic materials for use in solar cells, light-emitting diodes, and lasers. The optoelectronic properties of these devices are determined by the excitons and exciton complexes confined in their NCs. In this study, we determined the relaxation dynamics of charged excitons and biexcitons in CsPbBr3 NCs using femtosecond transient-absorption (TA), time-resolved photoluminescence (PL), and single-dot second-order photon correlation spectroscopy. Decay times of ∼40 and ∼200 ps were obtained from the TA and PL decay curves for biexcitons and charged excitons, respectively, in NCs with an average edge length of 7.7 nm. The existence of charged excitons even under weak photoexcitation was confirmed by the second-order photon correlation measurements. We found that charged excitons play a dominant role in luminescence processes of CsPbBr3 NCs. Combining different spectroscopic techniques enabled us to clarify the dynamical behaviors of excitons, charged excitons, and biexcitons.

DPP containing D–π–A organic dyes toward highly efficient dye-sensitized solar cells

Two diketopyrrolopyrrole (DPP) containing D–π–A organic dyes, named as TEDBn and TEDTh were prepared. Their optical, electrochemical and photophysical properties were investigated and their application in dye-sensitized solar cells (DSCs) was demonstrated. The broaden absorption and low-energy λmax of DPP containing dyes were contributed from DPP bridge and EDOT π-conjugated moiety. Compared to TEDTh, TEDBn has smaller absorption coefficient and higher optical bandgap. Nevertheless the power conversion efficiency (PCE) of TEDBn based DSC is 7.2% which is larger than that of TEDTh based DSC and 90% of N719 based cell. The higher PCE of TEDBn based device originates from both higher open-circuit voltage (VOC) and short-circuit current (JSC). Femtosecond time-resolved photoluminescence data show that the electron injection at TEDBn/TiO2 is more efficient than that at TEDTh/TiO2 interface, resulting in higher JSC (and IPCE) of TEDBn based DSC. Density function theory (DFT) calculations revealed that TEDBn is a linear structure which leads to higher dye loading on TiO2 than TEDTh which featured a bend conformation, better TiO2 surface coverage, resulting in larger VOC. Therefore, the overall efficiency of the cell sensitized by TEDBn is better than that the devices based on TEDTh.

Unraveling simultaneously enhanced open-circuit voltage and short-circuit current density in P3HT:ICBA:2,3-pyridinediol blended film based photovoltaics

A comprehensive study was carried out to understand the simultaneous enhancement of the open-circuit voltage and short-circuit current density in P3HT:ICBA:2,3-pyridinediol blended film based photovoltaics. Femtosecond time-resolved photoluminescence, steady-state absorption spectrum, atomic force microscopy and 2D grazing-incidence wide-angle x-ray scattering were used to explore the transport characteristics of excitons. The comparatively faster exciton dissociation in the P3HT:ICBA:2,3-pyridinediol blended film is attributed to the higher mobility of the excitons in P3HT. The space-charge limited-current measurements show the carrier mobility to be more balanced in the P3HT:ICBA:2,3-pyridinediol blended film than in the P3HT:PCBM:2,3-pyridinediol blended film, which is beneficial to the photo-induced current extraction. The enhanced short-circuit current density originates from the higher exciton dissociation yield in combination with the more balanced electron-hole mobility. The results should aid the design of an organic film for high-efficiency photovoltaics.

Optical nonlinearities and two-photon excited time-resolved luminescence in colloidal quantum-confined CuInS2/ZnS heterostructures

Comprehensive studies on the spectral dependencies of third-order nonlinear optical properties and one/two-photon excited time-resolved luminescence of quantum-confined ternary CuInS2/ZnS core/shell heterostructures with reference to their corresponding uncoated CuInS2 cores are presented in this work. Nonlinear refraction and nonlinear absorption in the quantum confined type-I core/shell CuInS2/ZnS and their corresponding CuInS2 cores were measured over a wide range of wavelengths (550–1500 nm) using the Z-scan technique. The results revealed the presence of size-dependent two-photon absorption bands in the red and near-infrared spectral range. The cubic nonlinearity of core/shell CuInS2/ZnS quantum heterostructures is discussed in terms of both the CuInS2 core's optical transitions and the impact of the wide-gap ZnS shell layer. Kinetics of recombination of the single/two-photon excited states was investigated with femtosecond time-resolved photoluminescence spectroscopy revealing the complex character of electronic relaxation. A general statistical model involving normalized discrete distribution of decay rates was used for the analysis of luminescence decay curves. Different relaxation paths including the excitonic relaxation, donor–acceptor pair recombination and free-to-bound state recombination are discussed. The results indicate that optical properties of the CuInS2/ZnS and CuInS2 quantum dots are well suited for the steady-state as well as time-resolved single/multiphoton fluorescent techniques utilized commonly for the bio-sensing issues.

T4有机染料敏化薄膜的超快动力学特性

Excited-state processes in T4 organic dye-sensitized Al2O3and TiO2films were studied by femtosecond transient absorption spectroscopy and femtosecond time-resolved photoluminescence dynamics based on up-conversion method.By multiexponential analysis,we obtained the electron injection efficiency of 93% in T4 dye-sensitized TiO2film.In addition,we further got the electron injection rate of1.17 ps-1in the T4 dye-sensitized TiO2film by a static inhomogeneous electronic coupling model.