Browsing by Author "Torma, Andrew Jonathon"

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    Structural Effects in 2D-Stabilized FAPbI3 Films and ToF-SIMS for Ultra-thin h-BN Fabrication
    (2024-08-06) Torma, Andrew Jonathon; Mohite, Aditya D
    The next generation of electronics, photonics, and optoelectronics are based on advancements in semiconductor materials. In such, it is vital to gain understanding of and cultivate solutions for degradation pathways and engineer effective synthesis methods. This work details two thrusts that take a view through the stability and fabrication lens: first, structural changes in a novel method to stabilize halide perovskites, and second, a method for forming ultra-thin van der Waals materials. Thrust I: The halide perovskite formamidinium lead iodide (FAPbI3) is a prime candidate for photovoltaics due to its excellent optoelectronic properties, but its application has been limited due to its structural instability. The large size of the FA cation results in metastability of the photoactive cubic phase and a facile degradation into the thermodynamically stable hexagonal phase at room temperature. Recently, the incorporation of 2D Ruddlesden-Popper halide perovskite seeds into a FAPbI3 precursor solution has been shown to template the growth of and stabilize cubic FAPbI3. Here, we investigate the nanoscale structural and optoelectronic mechanisms behind the observed bulk stabilization using synchrotron-based x-ray microscopies. Nanoprobe x-ray diffraction reveals 2D-templated FAPbI3 films exhibit an average compressive strain normal to the substrate of -3.4%, two-fold larger than that of MACl-stabilized FAPbI3. Further, this compression creates locally templated regions comprised of tetragonal-phase FAPbI3 distributed non-uniformly throughout the film with fewer crystalline defects than purely cubic regions. Scanning x-ray excited optical luminescence (x-ray analogue of photoluminescence) reveals that this local templating results in increased radiative recombination and redshifted emission. Our results help better understand the structural phenomena resulting from stabilization methods in FAPbI3 for engineering durable photovoltaics. Thrust II: In recent years, hexagonal boron nitride (h-BN) has become a promising candidate for next-generation electronics and photonics, such as a gate dielectric in field effect transistors. However, methods for fabrication of ultra-thin materials often lack spatial control or require harsh environment depositions. Here, we report a method to prepare ultra-thin h-BN using the combination of micromechanical cleaving (i.e. Scotch Tape Method) and ion beam etching through time-of-flight secondary ion mass spectrometry (ToF-SIMS). ToF-SIMS is further employed for 3D reconstruction of h-BN flakes.