Two-Dimensional van der Waals Epitaxy Kinetics in a Three- Dimensional Perovskite Halide

The exploration of emerging materials physics and prospective applications of two-dimensional materials greatly relies on the growth control of their thickness, phases, morphologies, and film−substrate interactions. Though substantial progress has been made for the development of twodimensional films from conventional layered bulky materials, particular challenges remain for obtaining ultrathin, single crystalline, dislocation-free films from intrinsically non-van der Waals-type three-dimensional materials. In this report, with the successful demonstration of single crystalline ultrathin large-scale perovskite halide material, we reveal and identify the favorable role of weak van der Waals film−substrate interaction on the nucleation and growth of the two-dimensional morphology out of nonlayered materials compared to conventional epitaxy. We also show how the bonding nature of the three-dimensional material itself affects the kinetic energy landscape of ultrathin film growth. By studying the formation of fractal perovskites assisted with Monte Carlo simulations, we demonstrate that the competition between the van der Waals diffusion and surface free energy of the perovskite leads to film thickening, suggesting that extra strategies such as surface passivation may be needed for the growth of monolayer and few-layer films.