Self-Assembly of CsPbBr Nanocubes into 2D Nanosheets.

All-inorganic metal halide perovskites have attracted considerable attention due to their high application potentials in optoelectronics, photonics, and energy conversion. Herein, two-dimensional (2D) CsPbBr nanosheets with a thickness of about 3 nm have been synthesized through a simple chemical process based on a hot-injection technique. The lateral dimension of CsPbBr nanosheets ranges from 11 to 110 nm, which can be tuned by adjusting the ratio of short ligands (octanoic acid and octylamine) over long ligands (oleic acid and oleylamine). The nanosheets result from the self-assembly of CsPbBr nanocubes with an edge length of about 3 nm, which possess the same crystal orientation. In addition, an amorphous region of about 1 nm in width is found between adjacent nanocubes. To investigate both the structure and the growth mechanism of these nanosheets, microstructural characterizations at the atomic scale are conducted, combined with X-ray diffraction analysis, H nuclear magnetic resonance (H NMR) measurement, and density functional theory (DFT) calculation, aiming to determine the configuration of different ligands adsorbed onto CsPbBr. Our results suggest that the adjacent nanocubes are mainly connected together by short ligands and inclined long ligands. On the basis of the DFT calculation results, a relationship is derived for the volume ratio of short ligands over long ligands and the lateral dimensions of CsPbBr nanosheets. Moreover, a physicochemical mechanism is proposed to explain the 2D growth of CsPbBr nanosheets. Such a finding provides new insights regarding the well-ordered self-arrangement of CsPbBr nanomaterials, as well as new routes to synthesize 2D CsPbX (X = Cl and I) nanosheets of suitable dimensions for specific and large-scale applications.