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Jon Meyers, Ph.D.

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Ph.D. in Chemistry
UNC-Chapel Hill, 2021

B.S., Chemistry, 2014
Brigham Young University - Idaho

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First Author

Driven by the exceptional optoelectronic performance and prospective applications of organic–inorganic hybrid perovskites (HPs), an array of methods to synthesize and process HPs has been developed. Although most studies focus on solution processing, a number of reports have examined vapor-phase effects, such as the unusual liquefaction of HP films when exposed to methylamine (MA0) vapor. Here, using in situ spectroscopy and microscopy, we examine the thermodynamics and kinetics of the liquefaction and recrystallization of methylammonium lead iodide (MAPbI3) films with MA0 and find that the phenomena are best described as amino-deliquescence and amino-efflorescence, respectively. By constructing a quantitative phase diagram, we show that amino-deliquescence is driven by the highly exothermic dissolution of MAPbI3 by MA0 with a heat of solution of approximately −96 kJ mol–1, which drives the condensation of MA0 at a pressure more than two orders of magnitude below the equilibrium vapor pressure. Surprisingly, the dissolution is accompanied by a decrease in entropy of ∼173 J mol–1 K–1, suggesting the formation of a liquid state with the semi-ordered MA0 solvent. Kinetic analysis of amino-efflorescence reveals nucleation and growth rates that decrease and increase, respectively, with increasing temperature, which together yield thin-film grain sizes that increase exponentially with temperature to produce millimeter-sized grains. The findings reveal amino-deliquescence as a highly driven thermodynamic process that is potentially a general effect for HP materials in the presence of amines. The apparently ordered nature of the liquid and large grain size after amino-efflorescence may provide a further pathway for control over morphology, crystallinity, and composition of HP systems.

Nano Letters

Lead halide perovskites (LHPs) have shown remarkable promise for use in photovoltaics, photodetectors, light-emitting diodes, and lasers. Although solution-processed polycrystalline films are the most widely studied morphology, LHP nanowires (NWs) grown by vapor-phase processes offer the potential for precise control over crystallinity, phase, composition, and morphology. Here, we report the first demonstration of self-catalyzed vapor–liquid–solid (VLS) growth of lead halide (PbX2; X = Cl, Br, or I) NWs and conversion to LHP. We present a kinetic model of the PbX2 NW growth process in which a liquid Pb catalyst is supersaturated with halogen X through vapor-phase incorporation of both Pb and X, inducing growth of a NW. For PbI2, we show that the NWs are single-crystalline, oriented in the ⟨1̅21̅0⟩ direction, and composed of a stoichiometric PbI2 shaft with a spherical Pb tip. Low-temperature vapor-phase intercalation of methylammonium iodide converts the NWs to methylammonium lead iodide (MAPbI3) perovskite while maintaining the NW morphology. Single-NW experiments comparing measured extinction spectra with optical simulations show that the NWs exhibit a strong optical antenna effect, leading to substantially enhanced scattering efficiencies and to absorption efficiencies that can be more than twice that of thin films of the same thickness. Further development of the self-catalyzed VLS mechanism for lead halide and perovskite NWs should enable the rational design of nanostructures for various optoelectronic technologies, including potentially unique applications such as hot-carrier solar cells.