The design and synthesis of three organic nonlinear optical crystals is presented for terahertz (THz) generation that incorporates the optimal characteristics of known organic nonlinear optical crystals 4‐(4‐(dimethylamino)styryl)‐1‐methylpyridinium 4‐methylbenzenesulfonate (DAST) and 2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 2,4,6‐trimethylbenzenesulfonate (HMQ‐TMS). The three crystals feature the 4‐((4‐(dimethylamino)phenyl)ethynyl)‐1‐methylpyridin‐1‐ium (4DEP) cation and three different anion combinations. Gas phase ab initio calculations of the cation show that 4DEP has a larger hyperpolarizability than cations from both DAST and HMQ‐TMS. To obtain the molecular packing needed for large nonlinear optical susceptibility and efficient THz generation, three anions are tested that result in noncentrosymmetric crystals: 4‐methylbenzenesulfonate, 3‐nitrobenzenesulfonate, and napthalene‐2‐sulfonate. After synthesis and crystallization, the crystal structure is characterized via X‐ray diffraction (XRD) analysis, and the relative THz generation efficiency for ideal crystals is predicted. 4DEP‐N2S shows a roughly 30% larger relative second‐order nonlinear optical susceptibility compared to HMQ‐TMS and DAST and, therefore, shows promise as a new advanced THz generation crystal.

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 (MA 0 ) vapor. Here, using in situ spectroscopy and microscopy, we examine the thermodynamics and kinetics of the liquefaction and recrystallization of methylammonium lead iodide (MAPbI 3 ) films with MA 0 and find that the phenomena are best described as amino-deliquescence and amino-efflorescence, respectively. By constructing a quantitative phase diagram, we show that aminodeliquescence is driven by the highly exothermic dissolution of MAPbI 3 by MA 0 with a heat of solution of approximately −96 kJ mol −1 , which drives the condensation of MA 0 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 MA 0 solvent. Kinetic analysis of aminoefflorescence 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.

Motivated by the extraordinary physical properties and potential optoelectronic applications of organic–inorganic hybrid perovskites (HPs), a variety of methods to synthesize and design high-quality HP structures has been developed. Nevertheless, the soft, organic nature of HP materials such as methylammonium lead iodide (MAPbI3) poses limitations to existing techniques, particularly for patterning the materials. Here, we demonstrate a hybrid top-down/bottom-up approach to patterning single-crystal HP microstructures. First, top-down lithography and chemical surface functionalization are used to prepare patterns with specific surface characteristics. Next, solid MAPbI3 powder is deposited on the patterns and liquefied by amino-deliquescence with methylamine (MA0) vapor, causing liquid MA0(MAPbI3) n to flow into the patterns while dewetting the remaining substrate. Lastly, MAPbI3 recrystallizes in the patterns by amino-efflorescence. By controlling the nucleation and growth conditions during amino-efflorescence, the characteristic grain size during recrystallization is orders of magnitude larger than the feature sizes of the patterns, thus causing the patterned microstructures to be single crystals. Contact angle measurements between liquid MA0(MAPbI3) n and a variety of organic and inorganic surfaces with and without chemical functionalization show that the wettability of surfaces can be tuned over a large range, providing flexibility in the choice of substrate and lithographic resist. The microstructures are free of exogenous solvents and suitable for optoelectronic device integration. As proof of concept, we demonstrate a photodetector that exhibits performance metrics consistent with single-crystal MAPbI3. The results provide a process for photonic and optoelectronic device design that can likely be extended to other potentially amino-deliquescent HP materials.