Orientation Control of a Two-Dimensional Conductive Metal–Organic Framework Thin Film by a Pyridine Vapor-Assisted Dry Process

Metal-organic frameworks (MOFs) are attractive materials with periodic pore structures constructed by coordinating metal ions and organic ligands. Recently, Cu₃(HHTP)₂ (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), a two-dimensional conductive MOF, has attracted attention as a promising device material. Owing to the anisotropy of Cu₃(HHTP)₂ properties, oriented thin films of this MOF are desired for evaluating its physical properties and device integration. To date, wet processes have been used to fabricate Cu₃(HHTP)₂ films, whereas dry processes are essential for high-quality devices. However, oriented Cu₃(HHTP)₂ thin films have not yet been fabricated by using dry processes. In this study, we succeed in fabricating an orientation-controlled Cu₃(HHTP)₂ film on Al₂O₃ (001) by using a two-step dry process involving (1) the multilayer deposition of copper acetate and HHTP using a vapor deposition system and (2) pyridine vapor-assisted annealing. In-plane and out-of-plane X-ray diffraction patterns confirm the successful fabrication of the (001)-oriented Cu₃(HHTP)₂ films. The conductivity evaluated by four-probe measurements is 2.6 × 10^-2 S cm^-1, comparable to that of films fabricated by wet processes. This study provides a novel guideline for the orientation control of two-dimensional conductive MOF thin films via a dry process.