Large-area synthesis of ultra-thin, flexible, and transparent conductive metal-organic framework thin-films via microfluidic-based solution shearing process

Abstract

Iminosemiquinone linker-based conductive metal-organic frameworks (c-MOFs) have attracted much attention as next-generation electronic materials due to their high electrical conductivity combined with high porosity. However, the utility of such c-MOFs in high-performance devices has been limited to date by the lack of high-quality MOFs thin-film processing. Herein, we introduce a technique known as Microfluidic-Assisted Solution Shearing combined with Post-synthetic Rapid Crystallization (MASS-PRC) process to generate a high-quality, flexible, and transparent thin-film of $Ni_3(hexaiminotriphenylene)_2$ ($Ni_3(HITP)_2$) uniformly over a large-area in a high-throughput manner with thickness controllability down to tens of nanometers. The MASS-PRC process utilizes 1) micromixer-embedded blade to simultaneously mix and continuously supply metal-ligand solution towards the drying front during solution shearing to generate an amorphous thin-film, followed by 2) immersion in amine solution for rapid directional crystal growth. As-synthesized c-MOF film had transparency of up to 88.8% and conductivity as high as 37.1 S/cm. High uniformity in conductivity was confirmed over a 3,500 $mm^2$ area with an arithmetic mean roughness (Ra) of 4.78 nm. Our flexible thin-film demonstrated the highest level of transparency for $Ni_3(HITP)_2$ and the highest hydrogen sulfide ($H_2S$) sensing performance (2,085% at 5 ppm) amongst c-MOFs-based $H_2S$ sensors, enabling wearable gas sensing applications.