Femtosecond Time-resolved Transient Absorption Spectroscopy Research Articles

Modulation of Photoinduced Phase Segregation and Stress-Driven Nanoscale Cracking in Hybrid Halide Perovskite Solar Cells.

The negative effect of photoinduced halide segregation (PIHS) on the properties of hybrid halide perovskites poses a major obstacle for its future commercial application. Therefore, the in-depth understanding of halide-ion segregation and its causes is an urgent and intractable problem. When PIHS reaches a certain threshold, it will aggravate the deterioration of the film surface morphology and form nanoscale cracks. Herein, the formation mechanism and types of cracks are revealed by exploring the stress distribution in the film. Using the femtosecond time-resolved transient absorption spectroscopy, the ultrafast formation of the iodine rich phase is observed, which appears earlier than the bromine rich phase. In addition, the introduction of organic ligand didodecyldimethylammonium bromide can significantly inhibit PIHS and improve the surface morphology of the film, which can promote the device efficiency from 9.63 to 11.20%. This work provides a novel perspective for the exploration of the PIHS.

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.

Charge Trapping in Semiconductor Photocatalysts: A Time- and Space-Domain Perspective.

Harnessing solar energy to produce value-added fuels and chemicals through photocatalysis techniques holds promise for establishing a sustainable and environmentally friendly energy economy. The intricate dynamics of photogenerated charge carriers lies at the core of the photocatalysis. The balance between charge trapping and band-edge recombination has a crucial influence on the activity of semiconductor photocatalysts. Consequently, the regulation of traps in photocatalysts becomes the key to optimizing their activities. Nevertheless, our comprehension of charge trapping, compared to that of well-studied charge recombination, remains somewhat limited. This limitation stems from the inherently heterogeneous nature of traps at both temporal and spatial scales, which renders the characterization of charge trapping a formidable challenge. Fortunately, recent advancements in both time-resolved spectroscopy and space-resolved microscopy have paved the way for considerable progress in the investigation and manipulation of charge trapping. In this Perspective, we focus on charge trapping in photocatalysts with the aim of establishing a direct link to their photocatalytic activities. To achieve this, we begin by elucidating the principles of advanced time-resolved spectroscopic techniques such as femtosecond time-resolved transient absorption spectroscopy and space-resolved microscopic methods, such as single-molecule fluorescence microscopy and surface photovoltage microscopy. Additionally, we provide an overview of noteworthy research endeavors dedicated to probing charge trapping using time- and space-resolved techniques. Our attention is then directed toward recent achievements in the manipulation of charge trapping in photocatalysts through defect engineering. Finally, we summarize this Perspective and discuss the future challenges and opportunities that lie ahead in the field.

Solid-State Emissive meso-Aryl/Alkyl-Substituted and Heteroatom-Mixed BOPPY Dyes: Syntheses, Structures, Physicochemical, Redox Properties, and Computational Analysis.

A series of solid-state emissive meso-aryl/alkyl-substituted and heteroatom-mixed bisBF2-anchoring fluorophore incorporating pyrrolyl-pyridylhydrazone (BOPPY) dyes have been developed by a one-pot condensation of ketonized or formylated pyrroles and 2-heterocyclohydrazine as well as the subsequent borylation coordination. Interestingly, the BOPPY dyes with meso-alkyl-substituted groups or oxygen-substituted pyridine moieties exhibit high fluorescence quantum yields (QYs) of up to 79%, the highest solid QY of 74%, and long lifetimes independent of polarity in the available BOPPYs. On the other hand, the BOPPYs with meso-aryl or N-substituted moieties display a high solution QY of up to 93% and slight emission wavelength maxima. However, the S-substituted BOPPY dye exhibited weak fluorescence in all studied solvents, which was attributed to the structural flexibility of the N-C-S bond and different from those BOPPYs with O or N substitution, indicated by quantum calculations. And the significant excited-state structural rearrangement in a polar solvent is further confirmed by femtosecond time-resolved transient absorption spectroscopy. More importantly, those novel and barely fluorescent BOPPYs in acetonitrile show advantageous aggregation-induced enhanced emission and viscosity-dependent activities. These advancements in the photophysical and electrochemical properties of BOPPY dyes offer valuable insights into their further development and potential applications.

Difference of carrier dynamics in a semiconductor saturable absorber mirror with and without B+ ion-implantation.

Three samples whose growth temperatures were 450°C, 500°C, and 560°C for S E S A M 1, S E S A M 2, and S E S A M 3, respectively, were tested by femto-second time-resolved transient absorption spectroscopy. The results indicate that the carrier dynamics of excited state absorption were dominant, and the lifetimes of carriers trapped by defect levels were about tens of pico-seconds. To further study the influence of carrier dynamics and recovery time of samples by ion-implantation, B + ions of 80 and 130 KeV were implanted into the samples with dose of 1014/c m 2. The modified samples showed a dominance of ultra-fast carrier dynamics of ground-state bleaching and direct recombination, which lasted for hundreds of femto-seconds, over excited state absorption. Additionally, carrier fast trapping was observed to be competitive with the excited state absorption process. After ion-implantation, the carrier dynamics of carrier trapping were enhanced, which contributed to forming an ultra-short laser, while the carrier dynamics of absorption of the excited state were suppressed. The conclusion that defect levels were partially eliminated by B + ion-implantation can be drawn.

Is 1-methylcytosine a faithful model compound for ultrafast deactivation dynamics of cytosine nucleosides in solution?

1-Methylcytosine (1mCyt) is the base for nucleoside N1-methylpseudodeoxycytidine of Hachimoji nucleic acids and a frequently used model compound for theoretical studies on excited states of cytosine nucleosides. However, there is little experimental characterization of spectra and photo-dynamic properties of 1mCyt. Herein, we report a comprehensive investigation into excited state dynamics and effects of solvents on fluorescence dynamics of 1mCyt in both water and acetonitrile. The study employed femtosecond broadband time-resolved fluorescence, transient absorption, and steady-state spectroscopy, along with density functional theory and time-dependent density functional theory calculations. The results obtained provide the first experimental evidence for identifying a dark-natured ∼5.7 ps lifetime nπ* state in the ultrafast non-radiative deactivation with 1mCyt in aqueous solution. This study also demonstrates a significant effect of the solvent on 1mCyt's fluorescence emission, which highlights the crucial role of solute-solvent hydrogen bonding in altering structures and reshaping the radiative as well as nonradiative dynamics of the 1mCyt's ππ* state in the aprotic solvent compared to the protic solvent. The solvent effect exhibited by 1mCyt is distinctive from that known for deoxycytidine, indicating the need for caution in using 1mCyt for modelling the ultrafast dynamics of Cyt nucleosides in solvents with varying properties. Overall, our study unveils a deactivation mechanism that confers a high degree of photo-stability for 1mCyt in solution, shedding light on the molecular basis for solvent-induced effects on the excited state dynamics of nucleobases and derivatives.

Persistent Charging of CsPbBr3 Perovskite Nanocrystals Confined in Glass Matrix.

Manipulation of persistent charges in semiconductor nanostructure is the key point to obtain quantum bits towards the application of quantum memory and information devices. However, realizing persistent charge storage in semiconductor nano-systems is still very challenge due to the disturbance from crystal defects and environment conditions. Herein, the two-photon persistent charging induced long-lasting afterglow and charged exciton formation are observed in CsPbBr3 perovskite nanocrystals (NCs) confined in glass host with effective lifetime surpassing one second, where the glass inclosure provides effective protection. A method combining the femtosecond and second time-resolved transient absorption spectroscopy is explored to determine the persistent charging possibility of perovskite NCs unambiguously. Meanwhile, with temperature-dependent spectroscopy, the underlying mechanism of this persistent charging is elucidated. A two-channel carrier transfer model is proposed involving athermal quantum tunneling and slower thermal-assisted channel. On this basis, two different information storage devices are demonstrated with the memory time exceeding two hours under low-temperature condition. These results provide a new strategy to realize persistent charging in perovskite NCs and deepen the understanding of the underlying carrier kinetics, which may pave an alternative way towards novel information memory and optical data storage applications.

Carrier transfer in quasi-2D perovskite/MoS2 monolayer heterostructure

Abstract Two-dimensional layered semiconductors have attracted intense interest in recent years. The van der Waals coupling between the layers tolerates stacking various materials and establishing heterostructures with new characteristics for a wide range of optoelectronic applications. The interlayer exciton dynamics at the interface within the heterostructure are vitally important for the performance of the photodetector and photovoltaic device. Here, a heterostructure comprising two-dimensional organic-inorganic Ruddlesden–Popper perovskites and transition metal dichalcogenide monolayer was fabricated and its ultrafast charge separation processes were systematically studied by using femtosecond time-resolved transient absorption spectroscopy. Significant hole and electron transfer processes in the ps and fs magnitude at the interface of the heterostructure were observed by tuning pump wavelengths of the pump-probe geometries. The results emphasize the realization of the exciton devices based on semiconductor heterostructures of two-dimensional perovskite and transition metal dichalcogenide.

In-Plane Palladium and Interplanar Copper Dual Single-Atom Catalyst in Bulk-Like Carbon Nitride for Cascade CO2 Photoreduction.

Dual single-atom catalysts (DSACs) are promising for breaking the scaling relationships and ensuring synergistic effects compared with conventional single-atom catalysts (SACs). Nevertheless, precise synthesis and optimization of DSACs with specific locations and functions remain challenging. Herein, dual single-atoms are specifically incorporated into the layer-stacked bulk-like carbon nitride, featuring in-plane three-coordinated Pd and interplanar four-coordinated Cu (Pd1-Cu1/b-CN) atomic sites, from both experimental results and DFT simulations. Using femtosecond time-resolved transient absorption (fs-TA) spectroscopy, it is found that the in-plane Pd features a charge decay lifetime of 95.6ps which is much longer than that of the interplanar Cu (3.07ps). This finding indicates that the in-plane Pd can provide electrons for the reaction as the catalytically active site in both structurally and dynamically favorable manners. Such a well-defined bi-functional cascade system ensures a 3.47-fold increase in CO yield compared to that of bulk-like CN (b-CN), while also exceeding the effects of single Pd1/b-CN and Cu1/b-CN sites. Furthermore, DFT calculations reveal that the inherent transformation from s-p coupling to d-p hybridization between the Pd site and CO2 molecule occurs during the initial CO2 adsorption and hydrogenation processes and stimulates the preferred CO2-to-CO reaction pathway.

Insights into the Semiconductor SERS Activity: The Impact of the Defect-Induced Energy Band Offset and Electron Lifetime Change.

The significant boost in surface-enhanced Raman scattering (SERS) by the chemical enhancement of semiconducting oxides is a pivotal finding. It offers a prospective path toward high uniformity and low-cost SERS substrates. However, a detailed understanding of factors that influence the charge transfer process is still insufficient. Herein, we reveal the important role of defect-induced band offset and electron lifetime change in SERS evolution observed in a MoO3 oxide semiconductor. By modulating the density of oxygen vacancy defects using ultraviolet (UV) light irradiation, SERS is found to be improved with irradiation time in the first place, but such improvement later deteriorates for prolonged irradiation even if more defects are generated. Insights into the observed SERS evolution are provided by ultraviolet photoelectron spectroscopy and femtosecond time-resolved transient absorption spectroscopy measurements. Results reveal that (1) a suitable offset between the energy band of the substrate and the orbitals of molecules is facilitated by a certain defect density and (2) defect states with relatively long electron lifetime are essential to achieve optimal SERS performance.

Impact of π–bridge unit in fully nonfused ring electron acceptor on the photovoltaic performance of organic solar cells

Due to the advantages of simple synthesis and low cost, fully nonfused ring electron acceptors (FN-FREAs) have been developed in recent years. Almost all reported FN-FREAs use thiophene derivatives as π-bridging unit, while other π-bridging units have not yet been developed. Here, an FN-FREA M01 with 2,5-bis(alkyloxy)phenylene as the π-bridge unit is designed and synthesized to compare its photovoltaic property with an analogue M05 which uses 3,4-bis(alkyloxy)thiophene as the π-bridge unit. Although the only variation between M01 and M05 is the π-bridge unit, these two FN-FREAs demonstrate significantly different photovoltaic performances. Organic solar cells (OSCs) based on M01 exhibit a high power conversion efficiency of 13.70%, while M05 based one shows a much lower efficiency of 3.96% with a markedly reduced short-circuit current density. Fluorescence, Raman, electron paramagnetic resonance, and femtosecond time-resolved transient absorption spectroscopy prove that M05 tends to be oxidized to form radical cations which aggravates exciton quenching and charge carrier recombination in the active layer, while in M01 such radical cations are absent. All in all, we have demonstrated that the π-bridging unit of FN-FREAs is pivotal to the photovoltaic performance and provided a useful design strategy to low-cost, high-efficiency, and chemically stable acceptor materials.

Coherent Vibrational Dynamics of Au144(SC8H9)60 Nanoclusters.

The coherent vibrational dynamics of Au144(SC8H9)60, obtained from femtosecond time-resolved transient absorption spectroscopy, are described. Two acoustic modes were identified and assigned, including 2.0 THz breathing and 0.7 THz quadrupolar vibrations. These assignments are consistent with predictions using classical mechanics models, indicating that bulk models accurately describe the vibrational properties of Au144(SC8H9)60. Coherent phonon signals were persistent for up to 3 ps, indicating energy dissipation by the nanocluster was the primary dephasing channel. The initial excitation phases of the breathing and quadrupolar modes were π-phase-shifted, reflecting differences in the displacive nuclear motion of the vibrations. The combined agreement of the vibrational frequencies, relative phases, and decoherence times supported predictions based on classical models. The vibrational frequencies were insensitive to silver substitution for gold but did show increased inhomogeneous damping of the coherent phonons. The ability to predict the vibrational properties of metal nanoclusters can have an impact on nanoresonator and mass sensing technologies.

Excited-State Dynamics of All the Mono-cis and the Major Di-cis Isomers of β-Apo-8'-Carotenal as Revealed by Femtosecond Time-Resolved Transient Absorption Spectroscopy.

Cis isomers of carotenoids play important roles in light harvesting and photoprotection in photosynthetic bacteria, such as the reaction center in purple bacteria and the photosynthetic apparatus in cyanobacteria. Carotenoids containing carbonyl groups are involved in efficient energy transfer to chlorophyll in light-harvesting complexes, and their intramolecular charge-transfer (ICT) excited states are known to be important for this process. Previous studies, using ultrafast laser spectroscopy, have focused on the central-cis isomer of carbonyl-containing carotenoids, revealing that the ICT excited state is stabilized in polar environments. However, the relationship between the cis isomer structure and the ICT excited state has remained unresolved. In this study, we performed steady-state absorption and femtosecond time-resolved absorption spectroscopy on nine geometric isomers (7-cis, 9-cis, 13-cis, 15-cis, 13'-cis, 9,13'-cis, 9,13-cis, 13,13'-cis, and all-trans) of β-apo-8'-carotenal, whose structures are well-defined, and discovered correlations between the decay rate constant of the S1 excited state and the S0-S1 energy gap, as well as between the position of the cis-bend and the degree of stabilization of the ICT excited state. Our results demonstrate that the ICT excited state is stabilized in polar environments in cis isomers of carbonyl-containing carotenoids and suggest that the position of the cis-bend plays an important role in the stabilization of the excited state.

Impact of cation modification on phonon-dressed exciton dynamics in a prototype two-dimensional hybrid organic-inorganic perovskite system.

Organic-cation engineering has recently proven effective in flexibly regulating two-dimensional hybrid organic-inorganic perovskites (2D HOIPs) to achieve a diversity of newly emerging applications. There have been many mechanistic studies based on the structural tunability of organic cations; nevertheless, those with an emphasis on the effect solely caused by the organic cations remain lacking. To this end, here we deliberately design a set of 2D HOIPs in which the inorganic layers are kept nearly intact upon cation modification, i.e., the precursor phenethylammonium lead iodide and its four derivatives with the phenyl group's para-position H being replaced by CH3, F, Cl, and Br. By means of femtosecond time-resolved transient absorption spectroscopy and temperature-dependent/time-resolved photoluminescence spectroscopy, we interrogate the subtle impact of cation modification on phonon dynamics, coherent phonon modes, phonon-dressed exciton dynamics, and excitonic emissions. A concerted trend for phonon lifetimes and exciton relaxation lifetimes regulated by cation modification is revealed, evidencing the existence of strong exciton-phonon coupling in this 2D HOIP system. The observed mass effect can be ascribed to the change in moment of inertia of organic cations. In addition, we observe an interesting interplay of exciton kinetics pertinent to population transfers between two emissive states, likely linked to the subtle variation in crystal symmetry induced by cation modification. The mechanistic insights gained from this work would be of value for the 2D HOIPs-based applications.

Transient “cis-Peak” Is Present in Excited-State Absorption of Central-cis Isomers of Carotenoids

Application of femtosecond time-resolved transient absorption spectroscopy on all-trans and central-cis isomers of two exemplary carotenoids, lutein and spirilloxanthin, performed at room and cryogenic temperatures in the spectral range expanded toward UV revealed new spectroscopic transient features for the cis isomers. Notably, particularly for the central-cis spirilloxanthin, a very distinct additional transient absorption band is observed on the short wavelength side of the main excited-state absorption band of the S1 state, having the same temporal characteristics as the latter one. This band is absent in transient absorption spectra of all-trans isomers, suggesting it could be assigned to "transient cis-peak." Overall, the results show that the "transient" counterpart of the spectral marker of cis-isomers of carotenoids, or the so-called cis-peak, is detectable in the excited-state absorption spectrum and could be attributed to electronic transition from the S1 state. This new transient spectral band could be very useful for spectroscopic experiments targeting the dynamics of carotenoid isomers and their severely distorted forms.

π-Conjugated In-Plane Heterostructure Enables Long-Lived Shallow Trapping in Graphitic Carbon Nitride for Increased Photocatalytic Hydrogen Generation.

The relatively short-lived excited states, such as the nascent electron-hole pairs (excitons) and the shallow trapping states, in semiconductor-based photocatalysts produce an exceptionally high charge carrier recombination rate, dominating a low solar-to-fuel performance. Here, a π-conjugated in-plane heterostructure between graphitic carbon nitride (g-CN) and carbon rings (Crings ) (labeling g-CN/Crings ) is effectively synthesized from the thermolysis of melamine-citric acid aggregates via a microwave-assisted heating process. The g-CN/Crings in-plane heterostructure shows remarkably suppressed excited-state decay and increased charge carrier population in photocatalysis. Kinetics analysis from the femtosecond time-resolved transient absorption spectroscopy illustrates that the g-CN/Crings π-conjugated heterostructure produces slower exciton annihilation (τ1 = 7.9ps) and longer shallow electron trapping (τ2 = 407.1ps) than pristine g-CN (τ1 = 3.6ps, τ2 = 264.1ps) owing to Crings incorporation, both of which enable more photoinduced electrons to participate in the photocatalytic reactions, thereby realizing photoactivity enhancement. As a result, the photocatalytic activity exhibits an eightfold enhancement in visible-light-driven H2 generation. This work provides a viable route of constructing π-conjugated in-plane heterostructures to suppress the excited-state decay and improve the photocatalytic performance.

Observation of carrier transfer in a vertical 0D-CsPbBr3/2D-MoS2 mixed-dimensional van der Waals heterojunction.

Two-dimensional transition metal dichalcogenides with outstanding properties open up a new way to develop optoelectronic devices such as phototransistors and light-emitting diodes. Heterostructure with light-harvesting materials can produce many photogenerated carriers via charge and/or energy transfer. In this paper, the ultrafast dynamics of charge transfer in zero-dimensional CsPbBr3 quantum dot/two-dimensional MoS2 van der Waals heterostructures are investigated through femtosecond time-resolved transient absorption spectroscopy. Hole and electron transfers in the ps and fs magnitude at the interfaces between MoS2 and CsPbBr3 are observed by modulating pump wavelengths of the pump-probe configurations. Our study highlights the opportunities for realizing the exciton devices based on quantum dot/two-dimensional semiconductor heterostructures.

Ultrafast proton coupled electron transfer between tryptophan and tyrosine in peptides Trp-Pron-Tyr

A series of model peptides (Trp-Pron-Tyr, WPnY, n=O, 1, 2, 3, 5), which contain tryptophan (Trp, W), tyrosine (Tyr, Y), and proline (Pro, P), have been studied under three typical pH conditions (3, 7, and 10) by steady-state absorption and fluorescence spectroscopy, nanosecond time-resolved fluorescence and femtosecond time-resolved transient absorption spectroscopy. When the peptide’s chain length is increased, Trp fluorescence quenching is expected to be gradually weakened. However, Trp fluorescence in WPY is strongly quenched and reveals even stronger quenching with increasing pH values, whose hypochromicity is clearly different from other model peptides. Transient absorption spectra also demonstrate that the excited state decay of WPY is much faster than that of other model peptides, especially at pH = 10. It is attributed to the efficient proton coupled electron transfer (PCET) between Trp and Tyr. Moreover, due to the very short distance between Trp and Tyr in WPY, this PCET process could be achieved by “direct transfer”, contrasted with the slow and long-range PCET process in other model peptides. Our results of the dipeptides WY and WP further suggest that Trp may also have more complex interactions with the peptide backbone or proline in those peptides. This work provides an experimental evidence for the electron transfer mechanism in WY dyads, which can help ones to understand how to reduce Trp radicals in proteins.

Exploring Solvent Effects on the Proton Transfer Processes of Selected Benzoxazole Derivatives by Femtosecond Time-Resolved Fluorescence and Transient Absorption Spectroscopies.

Excited-state intramolecular proton transfer (ESIPT) is of great importance due to the large Stokes shift emission that can be observed in some ESIPT molecules. Although steady-state spectroscopies have been employed to study the properties of some ESIPT molecules, their excited-state dynamics have not been examined directly with time-resolved spectroscopy methods yet for a number of systems. Here, an in-depth investigation of the solvent effects on the excited-state dynamics of two prototypical ESIPT molecules, 2-(2'-hydroxyphenyl)-benzoxazole (HBO) and 2-(2'-hydroxynaphthalenyl)-benzoxazole (NAP), have been accomplished by using femtosecond time-resolved fluorescence and transient absorption spectroscopies. Solvent effects affect the excited-state dynamics of HBO more significantly than that of NAP. Particularly in the presence of water, the photodynamics pathways of HBO are changed, while only small changes can be found in NAP. An ultrafast ESIPT process that occurs within our instrumental response is observed for HBO, and this is followed by an isomerization process in ACN solution. However, in aqueous solution, the obtained syn-keto* after ESIPT can be solvated by water in about 3.0 ps, and the isomerization process is totally inhibited for HBO. The mechanism of NAP is different from HBO and is determined to be a two-step excited-state proton transfer process. Upon photoexcitation, NAP is deprotonated first in the excited state to generate the anion*, which can transfer to the syn-keto* form followed by an isomerization process.

Photophysics of Anionic Bis(4H-imidazolato)CuI Complexes.

In this paper, the photophysical behavior of four panchromatically absorbing, homoleptic bis(4H-imidazolato)CuI complexes, with a systematic variation in the electron-withdrawing properties of the imidazolate ligand, were studied by wavelength-dependent time-resolved femtosecond transient absorption spectroscopy. Excitation at 400, 480, and 630 nm populates metal-to-ligand charge transfer, intraligand charge transfer, and mixed-character singlet states. The pump wavelength-dependent transient absorption data were analyzed by a recently established 2D correlation approach. Data analysis revealed that all excitation conditions yield similar excited-state dynamics. Key to the excited-state relaxation is fast, sub-picosecond pseudo-Jahn-Teller distortion, which is accompanied by the relocalization of electron density onto a single ligand from the initially delocalized state at Franck-Condon geometry. Subsequent intersystem crossing to the triplet manifold is followed by a sub-100 ps decay to the ground state. The fast, nonradiative decay is rationalized by the low triplet-state energy as found by DFT calculations, which suggest perspective treatment at the strong coupling limit of the energy gap law.