Researchers from our researchers and Kyoto University have proposed a strategy to grow 'face-on' and 'edge-on' conductive metal-organic framework (cMOF) nanofilms on substrates by controlling the "stand-up" behaviors of ligands on various surfaces to overcome the difficulty in the orientation control of such film.

Key Takeaways

Scientists developed a method to control the orientation of conductive metal-organic frameworks (cMOF) nanofilms on substrates, addressing challenges in interface chemistry.

Using atomic force microscopy and X-ray, the team demonstrated the softness and unique conductive functions of these crystalline nanofilms.

The research revealed that structural softness, in addition to redox interactions, can modulate electrical conductivity in the cMOF nanofilm.

The Research

The team established operando characterization methodology using atomic force microscopy and X-ray to demonstrate the softness of the crystalline nanofilms and reveal their unique conductive functions.

The study was published in PNAS ("Growth mechanisms and anisotropic softness–dependent conductivity of orientation-controllable metal–organic framework nanofilms").

Schematic showing the preparation of the conductive MOF thin films with reversal orientations, the lattice image obtained from FM-AFM, and the anisotropic softness-conductivity revealed by the operando GIWAXS-Sensor. (Image: YAO Mingshui)

Electrically cMOFs have gradually emerged by unlocking their potential to conduct charges in porous crystals. cMOFs applied in electrical devices normally hybridize with other materials, especially substrates. Therefore, precisely controlling the interface between cMOF and a substrate is crucial.

However, the unexplored interface chemistry of cMOFs makes the controlled synthesis and advanced characterization of high-quality thin films particularly challenging. Specifically, in contrast to the anticipated "edge-on" alignment of the 2D planes stemming from the hydrophilic -OH edge and the hydrophobic triphenylene core, the orientation observed experimentally is, in fact, the "face-on" configuration of the 2D planes on the substrates.

"The challenge lies in inducing the necessary high surface pressure to achieve a 'standing up' configuration of the core," said Prof. YAO Mingshui from IPE, first author of the study.

It is commonly observed that, in the Langmuir-Blodgett (LB) technique, ligands with a hydrophobic core and a hydrophilic edge can adopt an upright orientation on hydrophilic surfaces when subjected to high surface pressure.

"Inspired by the 'standing up' behaviors, we employ ultra-high concentration, together with vigorous evaporation during spraying, to create a unique local high surface pressure that can induce the 'standing up' of HHTP (HHTP = 2,3,6,7,10,11-hexahydrotriphenylene) ligands. Accordingly, the 'face-on' and 'edge-on' thin films can be fabricated," said Prof. Kenichi Otake from Kyoto University, corresponding author of the study.

Various reliable analysis was conducted to verify the crystallinity and orientation of films with an ultra-thin thickness ranging from a few nanometers to tens of nanometers.

"The operando GIWAXS imaging and electrical monitoring revealed the anisotropic framework softness associated with electrical conductivity on the cMOF nanofilm. It answers the question whether the generally considered rigid Cu-HHTP can be soft," said Prof. Susumu Kitagawa from Kyoto University, corresponding author of the study. In addition to redox interactions, the structural softness has been confirmed to modulate the electrical conductivity in an anisotropic way.