Creation of metal–organic framework nanosheets by the Langmuir-Blodgett technique

Over the past decade, significant progress has been made in research on metal–organic frameworks (MOFs) – highly ordered microporous materials composed of metal ions and organic linkers – resulting in an increased understanding of the relationship between framework structure/components and performance in various applications in diverse fields such as electronics, energy creation/storage and biomaterials. For most potential applications, organizing MOFs into two-dimensional (2D) thin films – nanosheets – is necessary. In order to create MOF nanosheets, two experimental approaches have been developed: top-down and bottom-up and most review papers on MOF nanosheets deal with these two methods. In this review, I focus on the bottom-up approach for creation of MOF nanosheets, a simple process with one or few steps that potentially offers access to large nanosheets with macroscopic continuity. Especially, the Langmuir-Blodgett (LB) methodology utilizing air/liquid interfaces is sequentially discussed. Because of the ultrathin state of nanosheets, characterization methods of MOF nanosheets often differ from those employed for bulk MOF crystals conducted with conventional techniques. Therefore, representative and useful techniques to determine nanosheet characteristics are explained in detail. Following that, I discuss the structural and morphological development of porphyrin- and triphenylene-based MOF nanosheets that critically influences their chemical/physical properties. Finally, I describe challenges and perspectives related to the LB synthesis towards further understanding of the growth mechanism at the interfaces, leading to extensive applications of MOF nanosheets.