Growth and characterizations of two-dimensional metal-organic frameworks

Abstract

Metal-Organic Frameworks (MOFs) are a class of porous materials with a crystalline structure that can be designed based on extremely tunable building blocks of organic molecules and metal ions. They are typically insulators but making them [pi]-conjugated with two-dimensional structure results in high electrical conductivity. This makes the two-dimensional a-conjugated MOFs (2D [pi]MOFs) good candidates for applications that need porous conductors such as supercapacitors and batteries. More importantly, tunability of the crystal structure enables us to explore exotic physical properties, including topological protection. This great potential has inspired the synthesis of various 2D [pi]MOFs, but their crystal growth remains challenging, preventing the characterization of intrinsic electrical properties. In this thesis, I will explain the growth mechanisms of 2D [pi]MOFs and the limitations of conventional growth methods.

Based on the analysis, I developed a novel growth method that generates single-crystal plates of a 2D [pi]MOF, Ni₃(HHTP)₂ (HHTP= 2,3,6,7,10,11 hexahydroxytriphenylene), over 10 [mu]m in lateral dimension, two orders of magnitude larger than previous reports. The growth mechanism of the new method is also studied by varying multiple growth parameters. The properties of the single crystals are characterized by various spectroscopic techniques. Among assorted characteristics, the electrical properties are explored closely. The large single-crystal plates enable us to study in-plane properties of a 2D [pi]MOF for the first time. The in-plane conductivity of Ni₃(HHTP)₂ is up to 2 S/cm, two orders of magnitude higher than pressed pellet, and shows a clear temperature dependence. Hall measurements reveal that the origin of the high conductivity is a high charge carrier density rather than high charge carrier mobility.

We anticipate our demonstration will facilitate the discovery of fundamental properties of various 2D [pi]MOFs and further our realization of their potential as electronic materials.