Liquid Surface Synthesis of Ultrathin Two-Dimensional Metal Halide Perovskite

Abstract

Two-dimensional (2D) metal halide perovskite-like quantum wells can be obtained by slicing the inorganic perovskite lattices with large organic molecules, where the number of consecutive inorganic slabs (n) determines the quantum confinement. Synthesizing large-area and ultrathin 2D perovskite sheets is crucial to achieving heterostructures for future thin-sheet optoelectronic devices. In this work, we demonstrate a synthesis method in which perovskite precursor solutions are introduced on an antisolvent liquid surface. Well-defined n = 1 sheets with sub-10 nm thickness and up to 50 μm lateral size are obtained in a scalable manner. This is achieved through careful engineering of subphase and spreading phase compositions to encourage controlled perovskite crystallization at the antisolvent−air interface. Structural and spectroscopic characterizations reveal a high phase purity and a clean excitonic emission, with their overall optical properties comparable to those of the highly crystalline films fabricated by blade coating, highlighting the clear potential of this liquid surface synthesis strategy. M etal halide perovskites, 1 marked by their facile solution-based synthesis and flexible chemical design, have occupied in the last 12 years a major share of the attention of researchers interested in optoelectronics. 2,3 Most recently, next to the three-dimensional (3D) lattices belonging to the metal halide perovskite family, their two-dimensional (2D) homo-logues obtained by inserting large organic spacer molecules that break the 3D lattice into 2D perovskite-like slabs have also entered the optoelectronic arena, mostly because of their excitonic properties and fine control over the confinement degree. 4−6 Inserting monofunctional spacers along the ⟨100⟩ planes could yield the staggered slab arrangement or the so-called Ruddlesden−Popper (RP) phases with a general formulation of A′ 2 A n−1 B n X 3n+1 , where A is a small cation, A′ is large spacer molecule, B is a metal cation in the octahedron center, and X is a halogen anion. These material systems can crucially be characterized by the number of corner-sharing inorganic layers (n) hosting small A-site cations between the spacer interruptions in (00l) directions, a parameter that determines the quantum well properties (Figure 1a, top shows the case for n = 1, 2, and 3, respectively). This quantum confinement can, to an extent, be preserved regardless of the material's physical thickness, as the film can be viewed as a periodic arrangement of the "well" and the "barrier". Modern optoelectronics is deeply rooted in the ability to fabricate heterostructures of different semiconductors with the aim of directing carriers to specific regions of a device to favor charge recombination or separation. Traditionally, semiconductor heterostructures are fabricated by epitaxially growing different materials on top of each other, where the lattice matching represents a strong limitation for the materials that can be used. 7 In recent years, increased interest has also appeared in the fabrication of heterostructures that are not limited by lattice mismatch. While common examples are definitely the transition metal dichalcogenides and organic semiconductor heterostructure, 8,9 there is also a growing desire to form heterostructures using the soft RP perovskite phases with different bandgap, which could be simply obtained via altering halide compositions or n values. 10 For standard thin film fabrication techniques, there are however major bottlenecks for the sequential deposition of perovskite layers: (1) due to the highly dynamic and ionic nature of the halide perovskite lattice, deposition of heterolayers from solution or vapor could be destructive; (2) for n > 1 quasi-2D perovskites, chemical disproportionation would cause the formation of mixed-n phases with unwanted physical properties. 11,12 This seems to be different from transition metal dichalcogenides, where the so-called van der Waals assembly of these 2D