Cunming Liu

Lead-free perovskites have emerged as environmentally benign alternatives to lead halide counterparts for photovoltaic and optoelectronic applications. Among them, the double perovskite Cs ₂ AgInCl ₆ family, with proper composition engineering, exhibits remarkable white-light emission characteristics enabled by strong electron-phonon coupling and the formation of self-trapped excitons (STEs). Despite these advantages, the fundamental photophysics and structural dynamics governing their excited-state behavior remain poorly understood. Here, we report a long-lived metastable phase in the Cs ₂ AgInCl ₆ double perovskite family and unravel this process and the concomitant electronic and structural evolution using a suite of tools including transient optical spectroscopy, time-resolved x-ray diffraction (TR-XRD), time-resolved x-ray absorption spectroscopy (TR-XAS), and inelastic x-ray scattering (IXS). We show that the photoinduced, transient metastable phase is associated with B-site [silver (Ag)–indium (In)] disorder, which induces a markedly reduced optical bandgap. Supported by TR-XRD and first-principles calculations, the Ag-In disorder drives the formation of Ag-rich and In-rich domains with millisecond lifetimes, with lifetimes increasing at lower temperatures. TR-XAS further reveals that the photogenerated STEs oxidize Ag ⁺ to Ag ²⁺ , which facilitates this highly temporally asymmetric order-disorder transition. Our findings demonstrate a previously unidentified mechanism, mediated by hole-localized STE formation, that enables prolongation of transient light-induced states to the multimillisecond regime in halide double perovskites, opening possibilities to harvesting the functional properties of metastable phases of these material systems.

Photoexcited dynamics of heteroleptic Cu(I) complexes as noble-metal-free photosensitizers are closely intertwined with the nature of their ligands. By utilizing ultrafast optical and X-ray transient absorption spectroscopies, we characterized a new set of heteroleptic Cu(I) complexes [Cu(PPh₃)₂(BPy^R)]⁺ (R = CH₃, H, Br to COOCH₃), with an increase in the electron-withdrawing ability of the functional group (R). We found that after the transient photooxidation of Cu(I) to Cu(II), the increasing electron-withdrawing ability of R barely affects the internal conversion (IC) (e.g., Jahn-taller (JT) distortion) between singlet MLCT states. However, it does accelerate the dynamics of intersystem crossing (ISC) between singlet and triplet MLCT states and the subsequent decay from the triplet MLCT state to the ground state. The associated lifetime constants are reduced by up to 300%. Our understanding of the photoexcited dynamics in heteroleptic Cu(I) complexes through ligand electronic tuning provides valuable insight into the rational design of efficient Cu(I) complex photosensitizers.

Stimulated Raman scattering (SRS) not only expands the spectral range of stimulated emission but also presents a way to delve into the intricate microscopic dynamics of matter. In this manuscript, SRS of O–O and O–H stretching vibrations in H2O–H2O2 solution is measured using 532 nm Nd:YAG laser. SRS peak intensities of O–O and O–H stretches are observed to engage in a competitive process, depending on the coupling direction of the excess electrons, which is determined by the recombination rate of H2O2 and H2O under laser-induced breakdown. Notably, a high-order (fifth order) SRS peak originating from H2O2 is detected, attributable to the Raman-active enhancement caused by surplus electrons and the amplification of vibration energy resulting from shockwaves. This experiment holds potential for advancing multi-wavelength selective regulation in terms of its contributions.

Spontaneous Raman scattering is an attractive optical technique for the analysis of gases and liquids; however, their low densities and notoriously weak scattering cross sections demand an enhancement of the spontaneous Raman scattering signal for detection. Here, we have developed a simple but highly effective and fast technique to enhance the signal of spontaneous Raman scattering from gases and liquids. The technique is developed based on the principle of an integrating sphere, which realizes the multiple pass actions of low-energy pump light and the collection of all Raman scattered light for a sample volume of 2 mL. By measuring the ambient air sample with an exposure time of 180 s, we found the experimental detection limit of our spontaneous Raman scattering setup can reach 3 ppm. CH₄ (<2 ppm) in air can be also examined by increasing the exposure time to 300 s. The performance of our setup used for the analysis of trace gases is further illustrated by characterizing ethane, propane, butane, and pentane in methane as well as isotopes of carbon dioxide. The results reveal that the detection limit of our setup for liquids can be improved by nearly 4 orders of magnitude compared to that of confocal Raman scattering spectroscopy with the same experimental conditions.

Nanosecond laser-induced breakdown (LIB) in liquids (e.g., water) can produce dynamic high pressure and high temperature. However, since high pressure needs to negate the effect of high temperature to some degree, it is only partially effective. As a result, it is difficult to directly measure the effective pressure due to the transient and complex LIB process. Here, we presented a simple method based on Raman spectroscopy to indirectly determine the effective pressure caused by LIB in liquid pure H 2 O and low concentration H 2 O − H 2 O 2 mixtures. By comparing the Raman shifts of the ice-VII mode for pure H 2 O and H 2 O − H 2 O 2 mixtures under laser pumping and static high pressure, the LIB effective pressure can be first estimated. The empirical equation was then derived base on the correlation of the LIB effective pressure to ice-VII-point stimulated Raman scattering thresholds for pure and mixture water solutions, which can be used to estimate the LIB effective pressures for other different mixture water solutions with the uncertainty of 0.14–0.25 Gpa. Hopefully, our study here would advance the measurements of effective pressure in the LIB process.

We report the light‐induced electronic and geometric changes taking place within a heteroleptic Cu^I photosensitizer, namely [(xant)Cu(Me₂phenPh₂)]PF₆ (xant=xantphos, Me₂phenPh₂=bathocuproine), by time‐resolved X‐ray absorption spectroscopy in the ps–μs time regime. Time‐resolved X‐ray absorption near edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) analysis enabled the elucidation of the electronic and structural configuration of the copper center in the excited state as well as its decay dynamics in different solvent conditions with and without triethylamine acting as a sacrificial electron donor. A three‐fold decrease in the decay lifetime of the excited state is observed in the presence of triethylamine, showing the feasibility of the reductive quenching pathway in the latter case. A prominent pre‐edge feature is observed in the XANES spectrum of the excited state upon metal to charge ligand transfer transition, showing an increased hybridization of the 3d states with the ligand p orbitals in the tetrahedron around the Cu center. EXAFS and density functional theory illustrate a significant shortening of the Cu−N and an elongation of the Cu−P bonds together with a decrease in the torsional angle between the xantphos and bathocuproine ligand. This study provides mechanistic time‐resolved understanding for the development of improved heteroleptic Cu^I photosensitizers, which can be used for the light‐driven production of hydrogen from water.

Complementary ultrafast techniques provide clear observation of charge hopping between metals in dinuclear complexes.