Probing Interfacial Processes of Lithium Ion Batteries

In the last decade, lithium ion batteries held a major role in the path towards personal electronics due to being lightweight and providing a high energy density. However, several problems have been identified with lithium ion batteries. Due to inherent instability, lithium ion batteries are known to have issues with safety and capacity loss. Our goal is to advance the understanding of the electrochemical processes, specifically the interfacial processes at the anode, to continue their advancement in our electronic age. At the interface of the electrolyte and anode, during the first several charging and discharging cycles, appears a protective layer by interaction of decomposed electrolyte at the electrode surface. This protective layer, termed the solid electrolyte interphase, is of particular importance as it increases the stability, impeding dendrite growth, and ultimately leading to improved capacity and safety. Our electrolyte is a lithium salt (LiClO4) with ethylene carbonate (EC) in a tetrahydrofuran (THF) solvent, leading, primarily, to one of the main SEI contributors, lithium ethylene dicarbonate (LiEDC). By spectroscopically probing the interface with sumfrequency generation and simultaneously scanning with cyclic voltammetry, we are able to see the SEI contribution formation in real time.Ope

Developing Spin and Polarization Order in 2D Hybrid Perovskites

Metal-halide perovskite (MHP) semiconductor materials have demonstrated a booming rise in optoelectronic performance, spanning IR to the ionizing radiation regimes. MHP crystals are largely enabled by avenues of multifunctionality, such as strong light absorption/emission, charge carrier conduction, and multiferroic properties, which are cooperatively engineered to break theoretical conversion efficiency limits. Without doubt, curiosity accumulates around the fundamental material physics of their broadly tunable optical, electronic, and magnetic properties. This work aims to impact the fundamental understanding of polarization and spin order in MHPs, while developing material solutions towards sensing and imaging from the IR to high energy (X-ray, gamma-ray) radiation. Bulk single crystals and single-crystalline like films of 2D-phase MHP are prepared with crystal chemistries that host multiferroic properties (ferroelectric-ferroelastic-ferromagnetic), exhibit strong light absorption/emission, and support carrier conduction. The broken inversion symmetry and strong orbital polarization of 2D-phase MHPs promote spatial ordering of excitonic states and spin-dependent Rashba band splitting, leading to excited state polarization and spin order. Several MHPs are developed as platforms to investigate the fundamental mechanisms involved in photo-ferroelectric and photo-ferromagnetic coupling. Ultra-fast spectroscopic measurements, such as transient absorption and time-resolved photoluminescence with temperature and light polarization dependence, reveals tunable coupling of photoexcited excitons. Further in-situ optical characterization uncovers extraordinary excited state dynamics in ferroelectric 2D-phase MHPs. Active control of the spin degree of freedom in ferroelectrics is shown using circularly polarized photoexcitation into the Rashba bands, where the resulting spin-polarized states are found to be spin-operability through varying interfacial spin. This optical-magnetic coupling behavior is further investigated through the magnetic and chemical depth sensitive polarized neutron reflectometry technique. Collectively, this work explores the microscopic structure-property behaviors of MHP to develop optical control and coupling with multifunctional properties toward breaking technical boundaries in optoelectronic devices

Ligand-induced self-assembly of twisted two-dimensional halide perovskites

Two-dimensional (2D) halide perovskites (HPs) are now an emerging materials system that exhibits intriguing optoelectronic functionalities. Conventionally, they have been synthesized with linear and/or planar molecular spacers, rendering nominal modifications in optoelectronic properties. In contrast, lower dimensional HPs (0D and 1D) have been accommodating to the incorporation of bulky molecular spacers, leaving fundamental insights to remain elusive in their application for unconventional 2D HP structures. Herein, by implementing a high-throughput autonomous exploration workflow, crystallization behaviors of 2D HPs based on bulky 3,3-diphenylpropylammonium (DPA) spacer are comprehensively explored. Counterintuitive to the conventional HP chemistry, synthesis of 2D DPA2PbI4 HP is indeed feasible when the steric hindrance is mediated by minute incorporation of 3D HPs. Furthermore, a Moiré superlattice is observed from the DPA2PbI4 flakes, indicating the spontaneous formation of twisted stacks of 2D HPs – the first time to the best our knowledge. We hypothesize that the unconventional van der Waals surface of DPA2PbI4 facilitates the self-assembly of the twisted stacks of 2D HPs. This work exemplifies how high-throughput experimentation can discover unconventional material systems where the synthetic principle lies beyond the conventional chemical intuition. Furthermore, these findings provide hints on how to ‘chemically’ manipulate the twist stacking in 2D HPs, thus rendering a straightforward way for bespoke realization of functionalities in exotic materials systems via bottom-up approach

Spin-orbital coupling and slow phonon effects enabled persistent photoluminescence in organic crystal under isomer doping

International audienceWhen periodically packing the intramolecular donor-acceptor structures to form ferroelectric-like lattice identified by second harmonic generation, our CD49 molecular crystal shows long-wavelength persistent photoluminescence peaked at 542 nm with the lifetime of 0.43 s, in addition to the short-wavelength prompt photoluminescence peaked at 363 nm with the lifetime of 0.45 ns. Interestingly, the long-wavelength persistent photoluminescence demonstrates magnetic field effects, showing as crystalline intermolecular charge-transfer excitons with singlet spin characteristics formed within ferroelectric-like lattice based on internal minority/majority carrier-balancing mechanism activated by isomer doping effects towards increasing electron-hole pairing probability. Our photoinduced Raman spectroscopy reveals the unusual slow relaxation of photoexcited lattice vibrations, indicating slow phonon effects occurring in ferroelectric-like lattice. Here, we show that crystalline intermolecular charge-transfer excitons are interacted with ferroelectric-like lattice, leading to exciton-lattice coupling within periodically packed intramolecular donor-acceptor structures to evolve ultralong-lived crystalline light-emitting states through slow phonon effects in ferroelectric light-emitting organic crystal