Dr Shunran Li

Abstract:

Lead-free perovskites have emerged as environmentally benign alternatives to lead halide counterparts for photovoltaic and optoelectronic applications. Among them, the double perovskite Cs₂AgInCl₆ 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 Cs₂AgInCl₆ 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 Ag²⁺, 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.

Abstract:

Colloidal lead halide perovskite nanocrystals (LHP NCs) are promising semiconductor materials for optoelectronic applications due to their strong quantum confinement, near-unity photoluminescence quantum yields, and tunable emission characteristics. However, their modest thermal stability remains a challenge, particularly at smaller core diameters due to enhanced phonon scattering at inorganic core-organic ligand interfaces. In this study, we directly quantify size-dependent thermal conductivity (κ) in lecithin-capped CsPbBr₃ NC thin films using a transducer-free, vibrational pump-visible probe (VPVP) spectroscopy technique. A mid-infrared pump thermally excites the ligand shell, while a broadband probe tracks transient reflectance change correlated to lattice temperature decay. Finite-element modeling of the decay dynamics yields κ values from 0.13 to 0.16 W·m^-1·K^-1 for NC films with sub-10 nm core diameter, significantly lower than those of its bulk counterpart. A steep κ suppression with decreasing NC size emphasizes the dominant role of ligand shells and boundary effects in thermal transport.

Abstract:

Self-trapped excitons (STEs), which have one or both carriers spatially trapped by a lattice distortion, are associated with broadband emission and a large Stokes shift that is desirable for many applications. The fundamental physical processes that lead to their formation are difficult to observe, mainly due to the ultrafast time scales involved and the low oscillator strength of STE transitions. Here, we employ ultrafast transient absorption spectroscopy with sub-20 fs temporal resolution in the ultraviolet to study the STE formation process in a pair of lead-free double perovskites, Cs₂AgInCl₆ and Cs₂(Ag_0.6Na_0.4)InCl₆. Using first-principles calculations, we assign a broad photoinduced absorption band in Cs₂AgInCl₆ to an intraband transition in the valence band that tracks the initial 70 fs hot-hole cooling step. Furthermore, exciton-phonon coupling calculations unravel the phonon modes that couple strongly with excitons in the lowest absorption peak to cause self-trapping. The transient absorption data shows the buildup of a stimulated emission band from the STE on a 200 fs time scale and long-lived coherent oscillations corresponding to the phonons of the lattice modified by the STE formation process.

Abstract:

Alkali metals are considered as a promising alternative to conventional noble metals for plasmonic applications, offering lower optical loss and significantly reduced material costs. The recent development of a thermo-assisted spin-coating process paired with phase-shift photolithography has enabled the creation of stable nanostructured sodium, which exhibits narrow resonances in the near-infrared (NIR) region and demonstrates free electron relaxation times comparable to noble metals. Through the control of nanostructure pitch and light incident angle, the surface plasmon polariton (SPP) resonance wavelength can be tuned throughout the visible and NIR regions, making nanostructured sodium particularly attractive for nanophotonics, surface-enhanced sensing, and photocatalytic applications. In this work, we investigate hot electron dynamics in nanostructured sodium thin films on polyurethane supports by leveraging the high sensitivity of SPPs to their metal's bulk properties. Through optical transient reflectance measurements, we probe the distinct signatures of electron-electron and electron-phonon interactions in sodium at ultrafast time scales. Our results show the unique early time response of sodium that differs from those observed in noble metals, providing key insight into sodium-based plasmonics. This comprehensive understanding of hot electron dynamics will enable more efficient design and implementation of sodium in next-generation plasmonic devices and applications where hot electron processes are critical considerations.

Abstract:

In two-dimensional (2D) chiral metal-halide perovskites (MHPs), chiral organic spacers induce structural chirality and chiroptical properties in the metal-halide sublattice. This structural chirality enables reversible crystalline-glass phase transitions in (S-NEA)₂PbBr₄, a prototypical chiral 2D MHP where NEA⁺ represents 1-(1-naphthyl)ethylammonium. Here, we investigate two distinct spherulite states of (S-NEA)₂PbBr₄, exhibiting either radial-like or stripe-like banded patterns depending on the annealing conditions of the amorphous film. Despite similarities in optical absorption and photoluminescence, the stripe-like, banded spherulite exhibits higher crystallinity and improved optical transparency compared to those of radial-like spherulite. X-ray nanoprobe measurements reveal tilting-angle modulations in the octahedral plane of stripe-like spherulites, correlating with the film's surface geometry. Transfer matrix calculations indicate that the optical contrast in stripe-like patterns, seen in bright-field optical microscopy, arises from optical interference effects, differing from the contrast mechanism observed in polymer spherulites. Ultrafast carrier dynamics experiments suggest that the stripe-like spherulites resemble single crystals more closely than radial-like spherulites, while electrical conductivity measurements show enhanced charge carrier transport in stripe-like spherulites. These findings offer insights into MHP spherulite states with a single composition but different morphologies, previously observed only in polymers, highlighting their potential for optoelectronic applications.