Tailoring Auger Recombination Dynamics in CsPbI3 Perovskite Nanocrystals via Transition Metal Doping.

Auger recombination is a pivotal process for semiconductor nanocrystals (NCs), significantly affecting charge carrier generation and collection in optoelectronic devices. This process depends mainly on the NCs' electronic structures. In our study, we investigated Auger recombination dynamics in manganese (Mn2+)-doped CsPbI3 NCs using transient absorption (TA) spectroscopy combined with theoretical and experimental structural characterization. Our results show that Mn2+ doping accelerates Auger recombination, reducing the biexciton lifetime from 146 to 74 ps with increasing Mn doping concentration up to 10%. This accelerated Auger recombination in Mn-doped NCs is attributed to increased band edge wave function overlap of excitons and a larger density of final states of Auger recombination due to Mn orbital involvement. Moreover, Mn doping reduces the dielectric screening of the excitons, which also contributes to the accelerated Auger recombination. Our study demonstrates the potential of element doping to regulate Auger recombination rates by modifying the materials' electronic structure.

Constructing ZnTe Spherical Quantum Well for Efficient Light Emission.

ZnTe colloidal semiconductor nanocrystals (NCs) have shown promise for light-emitting diodes (LEDs) and displays, because they are free from toxic heavy metals (Cd). However, so far, their low photoluminescence (PL) efficiency (∼30%) has hindered their applications. Herein, we devised a novel structure of ZnTe NCs with the configuration of ZnSe (core)/ZnTe (spherical quantum well, SQW)/ZnSe (shell). The inner layer ZnTe was grown at the surface of ZnSe core with avoiding using highly active and high-risk Zn sources. Due to the formation of coherently strained heterostructure which reduced the lattice mismatch, and the thermodynamic growth of ZnTe, the surface or interface defects were suppressed. A high PL efficiency of >60% was obtained for the green light-emitting ZnSe/ZnTe/ZnSe SQWs after ZnS outer layer passivation, which is the highest value for colloidal ZnTe-based NCs. This work paves the way for the development of novel semiconductor NCs for luminescent and display applications.

Facile Synthesis of Tetraphenylethene (TPE)-Based Fluorophores Derived by π-Extended Systems: Opposite Mechanofluorochromism, Anti-Counterfeiting and Bioimaging.

Although remarkable progresses are achieved in the design and development of the mono-shift in photoluminescence for mechanofluorochromic materials, it is still a severe challenge to explore the opposite mechanofluorochromic materials with both blue- and red-shifted photoluminescence. Herein, two unprecedented 4,5-bis(TPE)-1H-imidazole fused pyridine or quinoline-based fluorophores X-1 and X-2 were designed and synthesized, and X-1 and X-2, exhibit completely opposite mechanofluorochromic behavior. Under UV lamp, the color of pristine X-1 changed from blue to green with reversible redshifted 27 nm in fluorescence emission spectra after ground, while the color of pristine X-2 changed from red to yellow with reversible blue-shifted 74 nm after ground. The detailed characterizations (including PXRD, SEM and DSC) confirmed that this opposite mechanofluorochromism was attributed to the transformation of order-crystalline and amorphous states. The crystal structure analysis and theoretical calculation further explain that opposite mechanofluorochromic behavior take into account different π-π stacking mode by induced π-extended systems. In addition, these TPE-based fluorophores (X-1 and X-2) exhibited excellent bio-compatibility and fluorescence properties for bio-imaging, writable data storage and anti-counterfeiting materials.

Visualizing Light-Induced Microstrain and Phase Transition in Lead-Free Perovskites Using Time-Resolved X-Ray Diffraction.

Metal halide perovskites have emerged as promising materials for optoelectronic applications in the last decade. A large amount of effort has been made to investigate the interplay between the crystalline lattice and photoexcited charge carriers as it is vital to their optoelectronic performance. Among them, ultrafast laser spectroscopy has been intensively utilized to explore the charge carrier dynamics of perovskites, from which the local structural information can only be extracted indirectly. Here, we have applied a time-resolved X-ray diffraction technique to investigate the structural dynamics of prototypical two-dimensional lead-free halide perovskite Cs3Bi2Br9 nanoparticles across temporal scales from 80 ps to microseconds. We observed a quick recoverable (a few ns) photoinduced microstrain up to 0.15% and a long existing lattice expansion (∼a few hundred nanoseconds) at mild laser fluence. Once the laser flux exceeds 1.4 mJ/cm2, the microstrain saturates and the crystalline phase partially transfers into a disordered phase. This photoinduced transient structural change can recover within the nanosecond time scale. These results indicate that photoexcitation of charge carriers couples with lattice distortion, which fundamentally affects the dielectric environment and charge carrier transport.

Asymmetric Core-Expanded BOPYOs: Facile Synthesis and Optical Properties.

N,O-bidentate BF2 complexes with five- and six-membered core rings have been thoroughly investigated. However, the development of seven-membered N,O-boron complexes is still an area to be explored. We have developed BF3 ⋅ OEt2 -induced self-condensation and coordination reactions based on a single starting material, which had been elucidated by experiment and calculation. This parent asymmetric core-expanded borondifluoride-(Z)-1,3-di(1H-pyrrol-2-yl)but-2-en-1-one (BOPYO) showed reactivity with a wide range of aldehydes, thus providing a series of conjugation BOPYOs. Moreover, a BOPYO derivative with a dimethylamino group was developed as a new NIR dye that responds to acid with favorable photophysical properties based on intramolecular charge transfer effect.

Beating signals in CdSe quantum dots measured by low-temperature 2D spectroscopy.

Advances in ultrafast spectroscopy can provide access to dynamics involving nontrivial quantum correlations and their evolutions. In coherent 2D spectroscopy, the oscillatory time dependence of a signal is a signature of such quantum dynamics. Here, we study such beating signals in electronic coherent 2D spectroscopy of CdSe quantum dots (CdSe QDs) at 77 K. The beating signals are analyzed in terms of their positive and negative Fourier components. We conclude that the beatings originate from coherent LO-phonons of CdSe QDs. No evidence for the QD size dependence of the LO-phonon frequency was identified.

Phonon-Assisted Hot Carrier Generation in Plasmonic Semiconductor Systems.

Plasmonic materials have optical cross sections that exceed by 10-fold their geometric sizes, making them uniquely suitable to convert light into electrical charges. Harvesting plasmon-generated hot carriers is of interest for the broad fields of photovoltaics and photocatalysis; however, their direct utilization is limited by their ultrafast thermalization in metals. To prolong the lifetime of hot carriers, one can place acceptor materials, such as semiconductors, in direct contact with the plasmonic system. Herein, we report the effect of operating temperature on hot electron generation and transfer to a suitable semiconductor. We found that an increase in the operation temperature improves hot electron harvesting in a plasmonic semiconductor hybrid system, contrasting what is observed on photodriven processes in nonplasmonic systems. The effect appears to be related to an enhancement in hot carrier generation due to phonon coupling. This discovery provides a new strategy for optimization of photodriven energy production and chemical synthesis.

Highly Efficient and Ultralong Afterglow Emission with Anti-Thermal Quenching from CsCdCl3 : Mn Perovskite Single Crystals.

Triplet exciton-based long-lived phosphorescence is severely limited by the thermal quenching at high temperature. Herein, we propose a novel strategy based on the energy transfer from triplet self-trapped excitons to Mn2+ dopants in solution-processed perovskite CsCdCl3 . It is found the Mn2+ doped hexagonal phase CsCdCl3 could simultaneously exhibit high emission efficiency (81.5 %) and long afterglow duration time (150 s). Besides, the afterglow emission exhibits anti-thermal quenching from 300 to 400 K. In-depth charge-carrier dynamics studies and density functional theory (DFT) calculation provide unambiguous evidence that carrier detrapping from trap states (mainly induced by Cl vacancy) to localized emission centers ([MnCl6 ]4- ) is responsible for the afterglow emission with anti-thermal quenching. Enlightened by the present results, we demonstrate the application of the developed materials for optical storage and logic operation applications.

Ultrafast hot-hole injection modifies hot-electron dynamics in Au/p-GaN heterostructures.

A fundamental understanding of hot-carrier dynamics in photo-excited metal nanostructures is needed to unlock their potential for photodetection and photocatalysis. Despite numerous studies on the ultrafast dynamics of hot electrons, so far, the temporal evolution of hot holes in metal-semiconductor heterostructures remains unknown. Here, we report ultrafast (t < 200 fs) hot-hole injection from Au nanoparticles into the valence band of p-type GaN. The removal of hot holes from below the Au Fermi level is observed to substantially alter the thermalization dynamics of hot electrons, reducing the peak electronic temperature and the electron-phonon coupling time of the Au nanoparticles. First-principles calculations reveal that hot-hole injection modifies the relaxation dynamics of hot electrons in Au nanoparticles by modulating the electronic structure of the metal on timescales commensurate with electron-electron scattering. These results advance our understanding of hot-hole dynamics in metal-semiconductor heterostructures and offer additional strategies for manipulating the dynamics of hot carriers on ultrafast timescales.

A Novel Strategy to Design and Construct AIE-active Mechanofluorochromic Materials via Regulation of Molecular Structure.

In this work, we first designed and synthesized tetraphenylene-fused aryl-imidazole derivatives TM-1-4 via regulation of molecular structure, which were consisted of 1H-imidazo[4,5-f][1,10]phenanthroline, 1H-phenanthro[9,10-d]imidazole, 4,5-diphenyl-1H-imidazole, 3,3'-(1H-imidazole-4,5-diyl)dipyridine moieties and AIE-active tetraphenylethene units, respectively. The results illustrated that TM-1-4 exhibited clear AIE characteristics. Meanwhile, TM-2 and TM-3 show excellent solid emission properties (ΦTM-2 =13.73 % and ΦTM-3 =36.21 %), whereas TM-1 and TM-4 exhibit the opposite properties (ΦTM-1 =1.48 % and ΦTM-4 =4.83 %). The multiple rotors (pyridine and benzene ring) causes twisted conformations of the molecule that prevents π-π stacking and enhances solid emission(ΦTM-2<ΦTM-3, ΦTM-1<ΦTM-4). Significantly, TM-2 and TM-3 also exhibited reversible mechanochromic behavior (Emission red shifts: ΔλTM-2 =43 nm and ΔλTM-3 =41 nm) with color changes between blue and green emissions. The powder X-ray diffraction (PXRD) suggested the disordered state of ground sample could be readily returned to an ordered crystalline. Therefore, the mechanochromisms of TM-2 and TM-3 are ascribable to the phase transformation between crystal and amorphous structure. The single crystal X-ray analysis of TM-2 reveals a twisted conformation for TPE moiety and the absence of π-π intermolecular stacking. These excellent optical properties of TM-2 and TM-3 make them potentially applications in mechanochromic materials and imaging agents.

Direct Observation of a Plasmon-Induced Hot Electron Flow in a Multimetallic Nanostructure.

Plasmon hot carriers are interesting for photoredox chemical synthesis but their direct utilization is limited by their ultrafast thermalization. Therefore, they are often transferred to suitable accepting materials that expedite their lifetime. Solid-state photocatalysts are technologically more suitable than their molecular counterparts, but their photophysical processes are harder to follow due to the absence of clear optical fingerprints. Herein, the journey of hot electrons in a solid-state multimetallic photocatalyst is revealed by a combination of ultrafast visible and infrared spectroscopy. Dynamics showed that electrons formed upon silver plasmonic excitation reach the gold catalytic site within 700 fs and the electron flow could also be reversed. Gold is the preferred site until saturation of its 5d band occurs. Silver-plasmon hot electrons increased the rate of nitrophenol reduction 16-fold, confirming the preponderant role of hot electrons in the overall catalytic activity and the importance to follow hot carriers' journeys in solid-state photosystems.

Highly Stable Perovskite Supercrystals via Oil-in-Oil Templating.

Inorganic perovskites display an enticing foreground for their wide range of optoelectronic applications. Recently, supercrystals (SCs) of inorganic perovskite nanocrystals (NCs) have been reported to possess highly ordered structure as well as novel collective optical properties, opening new opportunities for efficient films. Here, we report the large-scale assembly control of spherical, cubic, and hexagonal SCs of inorganic perovskite NCs through templating by oil-in-oil emulsions. We show that an interplay between the roundness of the cubic NCs and the tension of the confining droplet surface sets the superstructure morphology, and we exploit this interplay to design dense hyperlattices of SCs. The SC films show strongly enhanced stability for at least two months without obvious structural degradation and minor optical changes. Our results on the controlled large-scale assembly of perovskite NC superstructures provide new prospects for the bottom-up production of optoelectronic devices based on the microfluidic production of mesoscopic building blocks.

Tuning the excited-state intramolecular proton transfer (ESIPT) process of indole-pyrrole systems by π-conjugation and substitution effects: experimental and computational studies.

A series of amino (NH)-type hydrogen-bonding (H-bonding) compounds, BNDAB-1-4, containing π-enlarged indole and ß-ethoxycarbonyl-substituted pyrrole units were designed and synthesized. BNDAB-1 and BNDAB-3 exhibited dual emission and BNDAB-2 and BNDAB-4 exhibited a single emission with a large Stokes shift in dichloromethane, methanol, DMSO and toluene except for a dual emission for BNDAB-4 in toluene. Inspired by their photophysical properties, the ESIPT process was speculated and further investigated by theoretical calculations including geometry and thermodynamic analyses. The results showed that the ester substitution on the proton donor unit and π-conjugation on the proton acceptor unit by structural modification can regulate the ESIPT behaviors of these compounds. First, a strong electron-withdrawing group promoted the ESIPT process according to the comparison of the ESIPT processes of NDAB-H and NDAB-6, BNDAB-1 and BNDAB-2, and BNDAB-3 and BNDAB-4. Second, π-conjugation in different positions ([g]- and [e]-position) of the indole unit decreased the speed of the ESIPT process irrespective of whether ethoxycarbonyl was substituted on the pyrrole ring based on the ESIPT process of Series 1 and 2. Finally, this work elucidated that the ESIPT process can be rationally tuned by π-conjugation and substitution, which is in good agreement with the experimental results.

Exploring the light-induced dynamics in solvated metallogrid complexes with femtosecond pulses across the electromagnetic spectrum.

Oligonuclear complexes of d4-d7 transition metal ion centers that undergo spin-switching have long been developed for their practical role in molecular electronics. Recently, they also have appeared as promising photochemical reactants demonstrating improved stability. However, the lack of knowledge about their photophysical properties in the solution phase compared to mononuclear complexes is currently hampering their inclusion into advanced light-driven reactions. In the present study, the ultrafast photoinduced dynamics in a solvated [2 × 2] iron(II) metallogrid complex are characterized by combining measurements with transient optical-infrared absorption and x-ray emission spectroscopy on the femtosecond time scale. The analysis is supported by density functional theory calculations. The photocycle can be described in terms of intra-site transitions, where the FeII centers in the low-spin state are independently photoexcited. The Franck-Condon state decays via the formation of a vibrationally hot high-spin (HS) state that displays coherent behavior within a few picoseconds and thermalizes within tens of picoseconds to yield a metastable HS state living for several hundreds of nanoseconds. Systematic comparison with the closely related mononuclear complex [Fe(terpy)2]2+ reveals that nuclearity has a profound impact on the photoinduced dynamics. More generally, this work provides guidelines for expanding the integration of oligonuclear complexes into new photoconversion schemes that may be triggered by ultrafast spin-switching.

Cation-Dependent Hot Carrier Cooling in Halide Perovskite Nanocrystals.

Lead halide perovskites (LHPs) nanocrystals (NCs), owing to their outstanding photophysical properties, have recently emerged as a promising material not only for solar cells but also for lighting and display applications. The photophysical properties of these materials can be further improved by chemical engineering such as cation exchange. Hot carrier (HC) cooling, as one of the key photophysical processes in LHPs, can strongly influence performance of LHPs NCs based devices. Here, we study HC relaxation dynamics in LHP NCs with cesium (Cs), methylammonium (MA, CH3NH3+), and formamidinium (FA, CH(NH2)2+) cations by using femtosecond transient absorption spectroscopy. The LHP NCs show excitation intensity and excitation energy-dependent HC cooling. We investigate the details of HC cooling in CsPbBr3, MAPbBr3, and FAPbBr3 at three different excitation energies with low excitation intensity. It takes longer time for the HCs at high energy to relax (cool) to the band edge, compared to the HCs generated by low excitation energy. At the same excitation energy (350 nm, 3.54 eV), all the three LHP NCs show fast HC relaxation (<0.4 ps) with the cooling time and rate in the following order: CsPbBr3 (0.39 ps, 2.9 meV/fs) > MAPbBr3 (0.27 ps, 4.6 meV/fs) > FAPbBr3 (0.21 ps, 5.8 meV/fs). The cation dependence can be explained by stronger interaction between the organic cations with the Pb-Br frameworks compared to the Cs. The revealed cation-dependent HC relaxation process is important for providing cation engineering strategies for developing high performance LHP devices.

Asynchronous Photoexcited Electronic and Structural Relaxation in Lead-Free Perovskites.

Vacancy-ordered lead-free perovskites with more-stable crystalline structures have been intensively explored as the alternatives for resolving the toxic and long-term stability issues of lead halide perovskites (LHPs). The dispersive energy bands produced by the closely packed halide octahedral sublattice in these perovskites are meanwhile anticipated to facility the mobility of charge carriers. However, these perovskites suffer from unexpectedly poor charge carrier transport. To tackle this issue, we have employed the ultrafast, elemental-specific X-ray transient absorption (XTA) spectroscopy to directly probe the photoexcited electronic and structural dynamics of a prototypical vacancy-ordered lead-free perovskite (Cs3Bi2Br9). We have discovered that the photogenerated holes quickly self-trapped at Br centers, simultaneously distorting the local lattice structure, likely forming small polarons in the configuration of Vk center (Br2- dimer). More significantly, we have found a surprisingly long-lived, structural distorted state with a lifetime of ∼59 µs, which is ∼3 orders of magnitude slower than that of the charge carrier recombination. Such long-lived structural distortion may produce a transient "background" under continuous light illumination, influencing the charge carrier transport along the lattice framework.

Nonconfinement Structure Revealed in Dion-Jacobson Type Quasi-2D Perovskite Expedites Interlayer Charge Transport.

Dion-Jacobson (DJ) type 2D perovskites with a single organic cation layer exhibit a narrower distance between two adjacent inorganic layers compared to the corresponding Ruddlesden-Popper perovskites, which facilitates interlayer charge transport. However, the internal crystal structures in 2D DJ perovskites remain elusive. Herein, in a p-xylylenediamine (PDMA)-based DJ perovskite bearing bifunctional NH3 + spacer, the compression from confinement structure (inorganic layer number, n = 1, 2) to nonconfinement structure (n > 3) with the decrease of PDMA molar ratio is unraveled. Remarkably, the nonconfined perovskite displays shorter spacing between 2D quantum wells, which results in a lower exciton binding energy and hence promotes exciton dissociation. The significantly diminishing quantum confinement promotes interlayer charge transport leading to a maximum photovoltaic efficiency of ≈11%. Additionally, the tighter interlayer packing arising from the squeezing of inorganic octahedra gives rise to enhanced ambient stability.

Compressive imaging of transient absorption dynamics on the femtosecond timescale.

Femtosecond spectroscopy is an important tool used for tracking rapid photoinduced processes in a variety of materials. To spatially map the processes in a sample would substantially expand the method's capabilities. This is, however, difficult to achieve, due to the necessity of using low-noise detection and maintaining feasible data acquisition time. Here, we demonstrate realization of an imaging pump-probe setup, featuring sub-100 fs temporal resolution, by using a straightforward modification of a standard pump-probe technique, which uses a randomly structured probe beam. The structured beam, made by a diffuser, enabled us to computationally reconstruct the maps of transient absorption dynamics based on the concept of compressed sensing. We demonstrate the setup's functionality in two proof-of-principle experiments, where we achieve spatial resolution of 20 µm. The presented concept provides a feasible route to imaging, by using the pump-probe technique and ultrafast spectroscopy in general.

A Simple Central Seven-Membered BOPYIN: Synthesis, Structural, Spectroscopic Properties, and Cellular Imaging Application.

A two-step, one-flask synthesis of central seven-membered borondifluoride-3,3-dimethyl-2-[2-(2-pyrrolyl)ethenyl] indole (BOPYIN) ligands has been developed by using the unexplored 3,3-dimethyl-2-[2-(2-pyrrolyl)ethenyl] indole. The simple synthetic approach has enabled modification of the electronic structure by changing the substituents on the indole unit. X-ray analysis indicated that conformations of the seven-membered BF2 complex including BOPYIN and diazaborepin differ from that of the five- and six-membered organoboron complexes. Interestingly, the bond angle of the N⋅⋅⋅B-N bond increases with the number of atoms in the core ring, based on Baeyer strain theory. These unsymmetric BOPYIN derivatives have excellent photophysical properties, including high fluorescence quantum yields, except for BOPYIN-4 in the solution state, large Stokes shifts, and good molar absorptivity. The dipole moment of BOPYIN-3 in the first excited singlet state and ground state was demonstrated by a linear Lippert-Mataga plot. The absorption and emission spectra were not mirror images for BOPYIN-1-3 and 5, in contradiction to Kasha's rule, as determined by TDDFT. The synthesized BOPYINs have been shown to be biocompatible fluorophores in cell bioimaging.

Insights into charge carrier dynamics in organo-metal halide perovskites: from neat films to solar cells.

Organo-metal halide perovskites have recently obtained world-wide attention as promising solar cell materials. They have broad and strong light absorption along with excellent carrier transport properties which partially explain their record power conversion efficiencies above 22%. However, the basic understanding of the underlying physical mechanisms is still limited and there remain large discrepancies among reported transport characteristics of perovskite materials. Notably, the carrier mobility of perovskite samples either in thin films or within solar cells obtained using different techniques can vary by up to 7-8 orders of magnitude. This tutorial review aims to offer insights into the scope, advantages, limitations and latest developments of the techniques that have been applied for studying charge carrier dynamics in perovskites. We summarize a comprehensive set of measurements including (1) time-resolved laser spectroscopies (transient absorption, time-resolved photoluminescence, terahertz spectroscopy and microwave conductivity); (2) electrical transient techniques (charge extraction by linearly increasing voltage and time-of-flight); and (3) steady-state methods (field-effect transistor, Hall effect and space charge limited current). Firstly, the basics of the above measurements are described. We then comparatively summarize the charge carrier characteristics of perovskite-based neat films, bilayer films and solar cells. Finally, we compare the different approaches in evaluating the key parameters of transport dynamics and unravel the reasons for the large discrepancies among these methods. We anticipate that this tutorial review will serve as the entry point for understanding the experimental results from the above techniques and provide insights into charge carrier dynamics in perovskite materials and devices.