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

Abstract

The exploration of emerging materials physics and prospective applications of twodimensional materials greatly relies on the growth control of their thickness, phases, morphologies and film-substrate interactions. Though substantial progresses have been made for the development of two-dimensional films from conventional layered bulky materials, particular challenges remain on 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 non-layered materials compared to conventional epitaxy. We also show how the bonding nature of the threedimensional material itself affects the kinetic energy landscape of ultrathin films 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 extra strategies such as surface passivation may be needed for the growth of monolayer and a few layers films. Material and Methods Chemical vapor deposition (CVD) synthesis of MAPbCl 3Powdered Lead(II) chloride(PbCl 2 , 99%, Sigma-Aldrich) was placed in the furnace heating center with the heating temperature controlled at 360-380°C, while MACl (99%,Merck KGaA), the second precursor, was placed about 6 cm away from PbCl 2 in the upper stream due to a lower melting point. Fresh cleavaged muscovite mica substrates (SPI Grade V-5) with (001) face exposed were placed in the downstream. Prior to deposition, the base pressure of the system was pumped to 0.3 Torr after which a 30 sccm of Argon was flowed to maintain the pressure at 120-160 Torr before deposition. The chamber temperature rose from room temperature to the deposition temperature rapidly in 5 min. The deposition process lasted for 20 minutes before finally the furnace was shut down. The perovskite(PVK) film was found, in most cases, present on the third substrate with a temperature at about 200°C. Schematic drawing of the experimental setup can be found in Fig S1. Characterization of Perovskite thin filmMorphology of perovskite thin film was characterized by a Nikon Eclipse Ti-S inverted optical microscope and JEOL JSM 6330F Field Emission Scanning Electron Microscope. Multimode TM Atomic Force Microscope is used to obtain the film thickness. Transmission Electron Microscope JEOL JEM-2010 is used to characterize the structural and epitaxial relation of the as-grown PVK thin film.

Two-Dimensional Halide Perovskites for Emerging New- Generation Photodetectors

“…Besides, the electrondiffusion length was estimated to be 210 ± 50 nm, [49] which can be attributed to the high quality perovskites prepared from vapor-phase method. Wang et al prepared crystalline 2D MAPbCl 3 perovskite thin platelets on muscovite mica using VDW epitaxy method, with the thickness of sub-10 nm and a few tens of micrometers in the lateral dimension (Figure 3c) [50]. Weak VDW interaction and delocalized bonding characters of ionic and metallic crystals play key roles in forming ultrathin and large-scale MAPbCl 3 platelets.…”

“…Besides solution-processed and vapor-phase methods, other methods have also been attempted to prepare 2D metal halide perovskites such as self-organization synthesis and mechanochemical [50]. The inset shows the thickness of the sheet by AFM measurement.…”

“…(f) DOS curves of the neutral lead vacancy and isodensity plot of the defect states in (EDBE)PbI 4 [73]. (g) 3D bar charts showing Monte Carlo simulation results for a square (left) and fractal (right) seeds [50]. In addition, Monte Carlo calculations were used to explain the growth mechanism of 2D sheets of MAPbCl 3 on mica [50].…”

“…(g) 3D bar charts showing Monte Carlo simulation results for a square (left) and fractal (right) seeds [50]. In addition, Monte Carlo calculations were used to explain the growth mechanism of 2D sheets of MAPbCl 3 on mica [50]. The VDW nucleation and growth model was proposed (Figure 8g) [50].…”

“…In addition, Monte Carlo calculations were used to explain the growth mechanism of 2D sheets of MAPbCl 3 on mica [50]. The VDW nucleation and growth model was proposed (Figure 8g) [50]. The competition between the VDW diffusion and cohesive energy of perovskites can in principle increase the thickness of perovskite sheets.…”