Time-resolved Laser Spectroscopy Research Articles

Laboratory Transition-rate Measurement of the Coronal Intercombination Line of Ar xv by Time-resolved Laser Spectroscopy

The extreme-ultraviolet emission line (424 Å) of the intercombination 1s 22s 2 1 S 0–1s 22s2p 3 P 1 transition of Ar xv can potentially characterize the electron temperature of astrophysical plasma. Various theoretical studies have investigated the intercombination transition rate, which is essential for the plasma diagnostics; however, experimental difficulties have prevented its measurement. We present here measurement of the lifetime of the 3 P 1 excited state of Ar xv, providing the experimental value of the intercombination transition rate. Employing time-resolved plasma-assisted laser spectroscopy, a method we recently demonstrated, enables us to measure this submicrosecond lifetime. The experimental result exhibits a 25%–43% higher transition rate than theoretical predictions.

Site-selective time resolved laser spectroscopy and DFT studies of Nd3+ optical centers in BaF2 doped crystals

Site-selective time-resolved laser spectroscopy techniques, which provide high spectral resolution at low temperatures (4.2 K and 6.5 K), were used to clarify the formation mechanism, spectral and kinetic properties of the Nd3+ ion single-site optical centers formed in the fluorite-type BaF2 single crystals doped with Nd3+. This formation occurs by heterovalent substitution of Ba2+ host cations with additional charge compensation by F− anions. To determine the lifetimes of the Stark 4F3/2(1) levels of the Nd3+ impurity centers, the fluorescence decay kinetics was measured using the gated photon counting technique. The data from spectroscopic experiments were combined with density functional theory (DFT) calculations to identify and analyze the local structure of Nd3+ cations in the Nd3+ ions doped BaF2 crystal. Our studies have shown that several types of single-site Nd3+ optical centers can be distinguished in BaF2:Nd3+. We confirmed the existence of an optical center with high C4v site symmetry and a long radiative lifetime of the 4F3/2(1) state τ = 14.3 ms. We also describe two other configurations of centers with local structure characterized by low site symmetry of Cs, which were identified and described for the first time. The decrease in the tetragonal site symmetry from C4v to Cs was explained by the fact that the Nd3+ dopants, distributed over the equivalent cationic positions of the fluorite-type crystal lattice, coordinate to anionic motifs that essentially differ in the position of the additional charge compensator F−. This structural variability induced in the local coordination environment of the optical center affects its spectroscopic properties through a shift in the energy of the 4F3/2(1) Stark level of the Nd3+ ion and results in a different radiative lifetime τ.

Tunable Photoinduced Charge Transfer at the Interface between Benzoselenadiazole-Based MOF Linkers and Thermally Activated Delayed Fluorescence Chromophore.

Structural modifications to molecular systems that lead to the control of photon emission processes at the interfaces between photoactive materials play a key role in the development of fluorescence sensors, X-ray imaging scintillators, and organic light-emitting diodes (OLEDs). In this work, two donor-acceptor systems were used to explore and reveal the effects of slight changes in chemical structure on interfacial excited-state transfer processes. A thermally activated delayed fluorescence (TADF) molecule was chosen as the molecular acceptor. Meanwhile, two benzoselenadiazole-core MOF linker precursors, Ac-SDZ and SDZ, with the presence and absence of a C≡C bridge, respectively, were carefully chosen as energy and/or electron-donor moieties. We found that the SDZ -TADF donor-acceptor system exhibited efficient energy transfer, as evidenced by steady-state and time-resolved laser spectroscopy. Furthermore, our results demonstrated that the Ac-SDZ-TADF system exhibited both interfacial energy and electron transfer processes. Femtosecond-mid-IR (fs-mid-IR) transient absorption measurements revealed that the electron transfer process takes place on the picosecond timescale. Time-dependent density functional theory (TD-DFT) calculations confirmed that photoinduced electron transfer occurred in this system and demonstrated that it takes place from C≡C in Ac-SDZ to the central unit of the TADF molecule. This work provides a straightforward way to modulate and tune excited-state energy/charge transfer processes at donor-acceptor interfaces.

Sensitizer-controlled photochemical reactivity via upconversion of red light.

By combining the energy input from two red photons, chemical reactions that would normally require blue or ultraviolet irradiation become accessible. Key advantages of this biphotonic excitation strategy are that red light usually penetrates deeper into complex reaction mixtures and causes less photo-damage than direct illumination in the blue or ultraviolet. Here, we demonstrate that the primary light-absorber of a dual photocatalytic system comprised of a transition metal-based photosensitizer and an organic co-catalyst can completely alter the reaction outcome. Photochemical reductions are achieved with a copper(i) complex in the presence of a sacrificial electron donor, whereas oxidative substrate activation occurs with an osmium(ii) photosensitizer. Based on time-resolved laser spectroscopy, this changeover in photochemical reactivity is due to different underlying biphotonic mechanisms. Following triplet energy transfer from the osmium(ii) photosensitizer to 9,10-dicyanoanthracene (DCA) and subsequent triplet-triplet annihilation upconversion, the fluorescent singlet excited state of DCA triggers oxidative substrate activation, which initiates the cis to trans isomerization of an olefin, a [2 + 2] cycloaddition, an aryl ether to ester rearrangement, and a Newman-Kwart rearrangement. This oxidative substrate activation stands in contrast to the reactivity with a copper(i) photosensitizer, where photoinduced electron transfer generates the DCA radical anion, which upon further excitation triggers reductive dehalogenations and detosylations. Our study provides the proof-of-concept for controlling the outcome of a red-light driven biphotonic reaction by altering the photosensitizer, and this seems relevant in the greater context of tailoring photochemical reactivities.

Novel Synthetic Dopamine Analogues: Carbon-13/Nitrogen-15 Isotopic Labeling and Fluorescence Properties

Dopamine (DA) and its analogues are compounds of interest in polymer science where their properties provide a multitude of applications. However, in elucidating polydopamine structure, there are ambiguities related to the types of bonds, the structure of monomeric units, their ratio, and influence of synthesis parameters. In order to minimize these variables, isotopic labeling presents an optimal alternative. In the diversification and understanding of these polymers, we aim to investigate analogues that by selective isotopic labeling provide possibilities for structure elucidation both of the polymerized analogues as well as polydopamine. In the present paper we report the synthesis of three new dopamine derivatives, labeled *CL ((Z)-3-(3,4-bis(benzyloxy)phenyl)-2-cyano-2-propenamide) and both labeled and unlabeled DL/*DL (2-aminomethyl-3-(3,4-dihydroxyphenyl)propionamide). The labeled *CL and *DL compounds contain selective 13C and 15N labeling in the alkyl chain. For these new compounds, NMR, FT-IR and fluorescence characterization, including by time-resolved laser spectroscopy, are reported.

Luminescence and Stability Enhancement of Inorganic Perovskite Nanocrystals via Selective Surface Ligand Binding.

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as one of the most promising light-emitting materials for optoelectronic devices with outstanding performance. However, the facile detachment of surface capping organic ligands from these NCs leads to very poor colloidal stability and durability. This is mainly due to the weak interfacial interactions between the inorganic perovskite core and ligands, high density of surface defect states, and aggregation of NCs. Here, using a combination of time-resolved laser spectroscopy and density functional theory (DFT) calculations, we explored the major impact of surface orientations and terminations for both CsPbBr3 and Cs4PbBr6 NCs not only on the interfacial binding affinities with organic ligands but also on surface defect formation and NC aggregation. By rationalizing that surface trap states are responsible for the decrease in photoluminescence (PL) upon fabrication and purification, we propose a powerful ligand-engineering strategy for eliminating these trap states and preventing the aggregation of CsPbBr3 and Cs4PbBr6 NCs. Interestingly, we find that the surface orientation and dimensionality determine the degree of interfacial interactions between the inorganic perovskite core and ligands and subsequently control the overall PL intensity and NC stability. Our results demonstrate that a treatment of as-synthesized CsPbBr3 NCs consisting of the addition of extra oleylammonium bromide (OAmBr) as a capping ligand, allows the CsPbBr3 NCs to retain their green emission with increased PL intensity and quantum yields and improves colloidal durability. On the other hand, the ultraviolet emissions of Cs4PbBr6 NCs are effectively increased upon addition of extra cesium oleate (CsOL) as the trap states induced by surface cesium ions are largely reduced by the formation of Cs-O bonds. Our work provides a robust and adequate ligand engineering approach to significantly enhance the optical behavior of perovskite NCs with different dimensionalities and various compositions and to achieve more efficient and stable light-harvesting devices.

Interfaces and Interfacial Carrier Dynamics in Perovskites

Metal halide perovskites are currently among the most promising materials to reshape our renewable energy future through photovoltaics. Nevertheless, they are also among the more complicated materials to understand and to engineer functional photovoltaics devices from. Their current performance efficiencies have not reached the highest predicted value of 30.06%. Many efforts have been dedicated to developing MHP materials, while fewer efforts were directed to understand and engineer the interfaces and interfacial properties in MHPs. Recently, the understanding and engineering of interfacial properties in MHPs have become a hot topic due to the vital role of interfaces, especially carrier dynamics, on device stability and efficiency. This perspective highlights the importance of focusing research on interfaces and interfacial carrier dynamics in metal halide perovskites. After introducing current challenges in MHPs interfaces and interfacial engineering, we provide a perspective on the contribution that different time-resolved laser spectroscopies add to the growing field of perovskite photovoltaics.

Case Studies on Structure-Property Relations in Perovskite Light-Emitting Diodes via Interfacial Engineering with Self-Assembled Monolayers.

Metal halide perovskites promise bright and narrow-band light-emitting diodes (LEDs). To this end, reliable understanding on structure-property relations is necessary, yet singling out one effect from others is difficult because photophysical and electronic functions of perovskite LEDs are interwoven each other. To resolve this problem, we herein employ self-assembled monolayers (SAMs) for interfacial engineering nanomaterials. Four different molecules that have the same anchor (thiol), different backbone (aryl vs alkyl) and different terminal group (amine vs pyridine vs methyl) are used to form SAMs at the interface with the thin film of a green-color perovskite, CH3NH3PbBr3. SAM-engineered perovskite films are characterized with X-ray diffraction (XRD), depth-profile X-ray photoelectron spectroscopy (XPS), Kelvin probe force microscopy (KPFM), scanning electron microscopy (SEM), time-resolved laser spectroscopy, and UV-vis absorption and emission spectroscopies. This permits access to how the chemical structure of molecule comprising SAM is related to the various chemical and physical features such as quality and grain size, cross-sectional atomic composition (Pb(0) vs Pb(II)), charge carrier lifetime, and charge mobility of perovskite films, leading to inferences of structure-property relations in the perovskite. Finally, we demonstrate that the trends observed in the model system stem from the affinity of SAM over the undercoordinated Pb ions of perovskite, and these are translated into considerably enhanced EQE (from 2.20 to 5.74%) and narrow-band performances (from 21.3 to 15.9 nm), without a noticeable wavelength shift in perovskite LEDs. Our work suggests that SAM-based interfacial engineering holds a promise for deciphering mechanisms of perovskite LEDs.

Fluorescence of Nd3+ optical centers close to cubic symmetry in a calcium fluoride crystal co-doped with Na+

Earlier, it was shown by the EPR method that at a very low concentration of the Nd3+ ion in the Nd3+:CaF2 crystal, highly symmetric cubic centers with the Oh symmetry are formed. In the present study, the optical centers of the Nd3+ ion with an anomalously long radiative lifetime τR = 13.6 ms of the 4F3/2(1) crystal-field (CF) level in the Nd3+ (0.05 at.%): Na+ (0.2 at.%): CaF2 single crystal at T = 7 K were detected by the method of time-resolved site-selective fluorescence laser spectroscopy. Its fluorescence excitation at the 4I9/2(1) → 4G5/2(1) transition of Nd3+ is in a longer wavelength spectral range compared to optical tetragonal (C4v) L-, orthorhombic (Cs) pair M-, and quartet N- centers. New optical centers exhibit inhomogeneous splitting and can be attributed to nearly cubic sites with symmetry close to Oh. Also numerous Nd3+ optical sites in the Nd3+ (0.03 at.%): CaF2 and Nd3+ (0.05 at.%): Na+ (0.2 at.%): CaF2 crystals at T = 7 K with τR = 240 μs ‒ 8.5 ms of the 4F3/2(1) CF level of Nd3+ were detected in a shorter wavelength spectral range compared to the nearly cubic sites. Among them, the sites with the longest τR = 8.5 ms except for the nearly cubic sites, which were identified as C2v symmetry sites. Their radiative lifetime is about six times that of the C4v site (τR = 1.4 ms). This means that, for optical sites without a center of inversion, the radiative lifetime of the excited state at sites with higher symmetry can be almost an order of magnitude less than at sites with lower symmetry. We explained this result by the different distance between the Nd3+ ion and the ion, which compensates for the excess of its charge. Thus, we have demonstrated the use of the Judd-Ofelt approach to explain the difference in the rates of emission of the excited state in different optical sites of the same rare-earth ion in the same crystal matrix.

Rovibrationally resolved radiative lifetime and collisional cross section measurements of the 6[formula omitted] state of molecular sodium

We report on radiative lifetime and collisional cross section measurements of the Na2 61Σg+(v=6, J=31) and 61Σg+(v=8, J=31) molecular levels by time-resolved laser spectroscopy. The excitation to the 61Σg+(3s+5s) electronic state was achieved using two synchronized pulsed lasers, directed to the sodium heatpipe oven and molecular fluorescence decay time as a function of buffer gas was recorded via time-correlated single photon counting technique. Radiative lifetimes were measured using a Stern-Volmer plots. From the lifetime measurements, collisional cross sections between the sodium molecules in the 61Σg+ state and the ground state argon atoms were extracted.

Photophysics of graphene quantum dot assemblies with axially coordinated cobaloxime catalysts.

We report a study of chromophore-catalyst assemblies composed of light harvesting hexabenzocoronene (HBC) chromophores axially coordinated to two cobaloxime complexes. The chromophore-catalyst assemblies were prepared using bottom-up synthetic methodology and characterized using solid-state NMR, IR, and x-ray absorption spectroscopy. Detailed steady-state and time-resolved laser spectroscopy was utilized to identify the photophysical properties of the assemblies, coupled with time-dependent DFT calculations to characterize the relevant excited states. The HBC chromophores tend to assemble into aggregates that exhibit high exciton diffusion length (D = 18.5 molecule2/ps), indicating that over 50 chromophores can be sampled within their excited state lifetime. We find that the axial coordination of cobaloximes leads to a significant reduction in the excited state lifetime of the HBC moiety, and this finding was discussed in terms of possible electron and energy transfer pathways. By comparing the experimental quenching rate constant (1.0 × 109 s-1) with the rate constant estimates for Marcus electron transfer (5.7 × 108 s-1) and Förster/Dexter energy transfers (8.1 × 106 s-1 and 1.0 × 1010 s-1), we conclude that both Dexter energy and Marcus electron transfer process are possible deactivation pathways in CoQD-A. No charge transfer or energy transfer intermediate was detected in transient absorption spectroscopy, indicating fast, subpicosecond return to the ground state. These results provide important insights into the factors that control the photophysical properties of photocatalytic chromophore-catalyst assemblies.

Exciton Coherence Length and Dynamics in Graphene Quantum Dot Assemblies.

Exciton size and dynamics were studied in assemblies of two well-defined graphene quantum dots of varying size: hexabenzocoronene (HBC), where the aromatic core consists of 42 C atoms, and carbon quantum dot (CQD) with 78 C atoms. The synthesis of HBC and CQD were achieved using bottom-up chemical methods, while their assembly was studied using steady-state UV/vis spectroscopy, X-ray scattering, and electron microscopy. While HBC forms long ordered fibers, CQD was found not to assemble well. The exciton size and dynamics were studied using time-resolved laser spectroscopy. At early times (∼100 fs), the exciton was found to delocalize over ∼1-2 molecular units in both assemblies, which reflects the confined nature of excitons in carbon-based materials and is consistent with the calculated value of ∼2 molecular units. Exciton-exciton annihilation measurements provided the exciton diffusion lengths of 16 and 3 nm for HBC and CQD, respectively.

Radiative lifetime measurements of Sc I levels by time-resolved laser spectroscopy

Radiative lifetimes of 39 excited levels of Sc I in the energy range 16,021.779–48,582.141 cm−1 were measured by time-resolved laser-induced fluorescence spectroscopy in a laser-ablation plasma. To our best knowledge, 32 lifetimes are reported for the first time. These new lifetimes will be important for extending the set of oscillator strengths available for Sc I.

Layer-Dependent Rashba Band Splitting in 2D Hybrid Perovskites

The strong spin–orbit coupling (SOC) in perovskite materials due to the presence of heavy atoms induces interesting electronic characteristics, such as Rashba band splitting. In spite of several recent reports on Rashba effects in 2D perovskites, the impacts of the nature of surface termination and of the number of inorganic layers on the extent of Rashba band splitting still remain to be determined. Here, using a combination of density functional theory (DFT) calculations and time-resolved laser spectroscopy, we provide a comprehensive understanding of the Rashba band splitting of the prototype 3D MAPbI3 and of 2D Ruddlesden–Popper (RP) hybrid perovskites. We demonstrate that significant structural distortions associated with different surface terminations are responsible for the observed Rashba effect in 2D perovskites. Interestingly, our theoretical and experimental data clearly indicate that the intrinsic Rashba splitting occurs in the perovskite crystals with an even number of inorganic layers (n = 2...

Excited Triplet State Interactions of Fluoroquinolone Norfloxacin with Natural Organic Matter: A Laser Spectroscopy Study.

In sunlit waters, the fate of fluoroquinolone antibiotics is significantly impacted by photodegradation. The mechanism of how natural organic matter (NOM) participates in the reaction has been frequently studied but still remains unclear. In this work, the interactions between the excited triplet state of the fluoroquinolone antibiotic norfloxacin (3NOR*) and a variety of NOM extracts were investigated using time-resolved laser spectroscopy. The observed transient absorption spectrum of 3NOR* showed a maximum at ca. 600 nm, and global fitting gave a lifetime of 1.0 μs for 3NOR* in phosphate buffer at pH = 7.5. Quenching of 3NOR* by Suwannee River hydrophobic acids (HPO), Beaufort River HPO, and Gartempe River HPO yielded rate constants of 1.8, 2.6, and 4.5 (×107 molC-1 s-1) respectively, whereas HPO from South Platte River unexpectedly increased the lifetime of 3NOR* with an as yet unknown mechanism. Concurrent photodegradation experiments of NOR (5 μM) in the presence of these NOM were also performed using a sunlight simulator. In general, the effects of NOM on the photodegradation rate of NOR were in agreement with observations from transient absorption studies. We suggest that adsorption of NOR to NOM is one of the major factors contributing to the observed quenching. These results yield a new insight into the likely role of NOM in sunlight-induced degradation of micropollutants.

Accelerated carrier recombination by grain boundary/edge defects in MBE grown transition metal dichalcogenides

Defect-carrier interaction in transition metal dichalcogenides (TMDs) plays important roles in carrier relaxation dynamics and carrier transport, which determines the performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we investigated the effect of grain boundary/edge defects on the ultrafast dynamics of photoexcited carrier in molecular beam epitaxy (MBE)-grown MoTe2 and MoSe2. We found that, comparing with exfoliated samples, the carrier recombination rate in MBE-grown samples accelerates by about 50 times. We attribute this striking difference to the existence of abundant grain boundary/edge defects in MBE-grown samples, which can serve as effective recombination centers for the photoexcited carriers. We also observed coherent acoustic phonons in both exfoliated and MBE-grown MoTe2, indicating strong electron-phonon coupling in this materials. Our measured sound velocity agrees well with the previously reported result of theoretical calculation. Our findings provide a useful reference for the fundamental parameters: carrier lifetime and sound velocity and reveal the undiscovered carrier recombination effect of grain boundary/edge defects, both of which will facilitate the defect engineering in TMD materials for high speed opto-electronics.

Mapping Charge Carrier Dynamics of Photoactive Material Surfaces in Space and Time

Understanding light-triggered charge carrier dynamics on photovoltaic-material surfaces and at interfaces has been a key element and one of the major challenges for the development of real-world energy devices. For this reason, studying charge carrier dynamics at photoactive material surfaces and interfaces have been among the most active areas of research in the last decade [1-4]. However, the ability to access carrier dynamics selectively on material surfaces with high spatial and temporal control in a photo-induced reaction is a particularly challenging task that can only be achieved by applying four-dimensional (4D) ultrafast electron microscopy (4D UEM) along with time-resolved laser spectroscopy. For this purpose, we established and developed the second generation 4D S-UEM and demonstrate the ability to take time-resolved secondary electrons images (snapshots) of material surfaces with 650 fs and ~ 4 nm temporal and spatial resolutions, respectively. In this method, the surface of the photoactive materials is excited by a clocking optical pulse and the photo-induced changes will be imaged using a pulsed primary electron beam as a probe pulse, generating secondary electrons, which are emitted from the top surface of the material in a manner that is extremely sensitive to the localization of the electron and hole on the surface and at the donor-acceptor interfaces. This method provides direct and controllable ultrafast dynamical information in many photoactive materials commonly used in solar cells and photo-catalysis. For instance, we have clearly demonstrated in space and time how the surface morphology, surface passivation, thickness of the absorber layer, grains, surface defects and nanostructured features can significantly impact the overall dynamical processes on the surface of absorber layers including nanomaterials and single crystals [5-8]. Finally, charge carrier dynamics in semiconductor quantum dots and perovskite single crystal using femtosecond laser spectroscopy will be also presented and discussed. References M. Bakr, O. F. Mohammed., Science 355 , 1260 (2017).O. El-Ballouli, E. Alarousu, M. Bernardi, S. M. Aly, A. P. Lagrow, O. M. Bakr, O. F. Mohammed., J. Am. Chem. Soc. 136 , 6952 (2014).Begum, M. R. Parida, A. L. Abdelhady, B. Murali, N. Alyami, G. H. Ahmed, M. N. Hedhili, O. M. Bakr, and O. F. Mohammed., J. Am. Chem. Soc. 139, 731 (2017).F. Mohammed, D.-S. Yang, S. Pal, A. H. Zewail, J. Am. Chem. Soc. 133, 7708 (2011).Sun, V. A. Melnikov, J. I. Khan, O. F. Mohammed, J. Phys. Chem. Lett. 6, 3884 (2015).Bose, J. Sun, J. I. Khan, B. S. Shaheen, A. Adhikari, T. K. Ng, V. M. Burlakov, M. P. Parida, D. Priante, A. Goriely, B. S. Ooi, O. M. Bakr, O. F. Mohammed, Adv. Mater. 28, 5106 (2016). S. Shaheen, J. Sun, D-S Yang, and O. F. Mohammed, J. Phys. Chem. Lett. 8, 2455 (2017).Bose, A. Bera, M. R. Parida, A. Adhikari, B. S. Shaheen, E. Alarousu, J. Sun, T. Wu, O. M. Bakr, O. F. Mohammed, Nano Lett. 16 , 4417 (2016).

Fluorescence quenching of 2,7-diaminoxanthone in alcohols by hydrogen bonding: An experimental and theoretical research

Using methods of stationary and time-resolved femtosecond laser spectroscopy, as well as time-dependent density functional theory, the spectral-kinetic properties of excited electronic states of synthesized 2,7-diaminoxanthone (DAX) have been studied in polar protic and aprotic solvents. The intramolecular charge transfer from amino-groups to the central oxygen-containing moiety manifests itself in appearance of a new intensive long-wavelength absorption band in DAX. Considerable bathochromic Stokes shift observed in DAX fluorescence spectra with increasing aprotic solvents polarity indicates growth of charge transfer degree and, respectively, of transition dipole moment between the ground to the first singlet-excited states. In alcohols, hydrogen bond formation between DAX and solvent results in an increase of Stokes shift and fluorescence quenching. The observed spectral dynamics reflects the solvent relaxations and is well reproduced with the dynamic parameters of the solvents measured and reported elsewhere.

Streptavidin as a Scaffold for Light-Induced Long-Lived Charge Separation.

Long-lived photo-driven charge separation is demonstrated by assembling a triad on a protein scaffold. For this purpose, a biotinylated triarylamine was added to a RuII -streptavidin conjugate bearing a methyl viologen electron acceptor covalently linked to the N-terminus of streptavidin. To improve the rate and lifetime of the electron transfer, a negative patch consisting of up to three additional negatively charged amino acids was engineered through mutagenesis close to the biotin-binding pocket of streptavidin. Time-resolved laser spectroscopy revealed that the covalent attachment and the negative patch were beneficial for charge separation within the streptavidin hosted triad; the charge separated state was generated within the duration of the excitation laser pulse, and lifetimes up to 3120 ns could be achieved with the optimized supramolecular triad.

Enhanced Size Selection in Two-Photon Excitation for CsPbBr3 Perovskite Nanocrystals.

Cesium lead bromide (CsPbBr3) perovskite nanocrystals (NCs), with large two-photon absorption (TPA) cross-section and bright photoluminescence (PL), have been demonstrated as stable two-photon-pumped lasing medium. With two-photon excitation, red-shifted PL spectrum and increased PL lifetime is observed compared with one-photon excitation. We have investigated the origin of such difference using time-resolved laser spectroscopies. We ascribe the difference to the enhanced size selection of NCs by two-photon excitation. Because of inherent nonlinearity, the size dependence of absorption cross-section under TPA is stronger. Consequently, larger size NCs are preferably excited, leading to longer excited-state lifetime and red-shifted PL emission. In a broad view, the enhanced size selection in two-photon excitation of CsPbBr3 NCs is likely a general feature of the perovskite NCs and can be tuned via NC size distribution to influence their performance within NC-based nonlinear optical materials and devices.