Electrostatic design of polar metal-organic framework thin films

TL;DR

This paper proposes a quantum-mechanical simulation-based electrostatic design strategy for polar MOF thin films, enabling tuning of their electronic properties through controlled electrostatic potential gradients and non-centrosymmetric packing.

Contribution

It introduces a novel approach for creating electrostatic potential gradients in MOF thin films using polar linkers, guiding experimental design for tailored electronic properties.

Findings

Electrostatic potential gradients can close the energy gap in MOFs.

Non-centrosymmetric packing induces an energy staircase similar to a pin-photodiode.

Design guidelines for polar linkers to achieve uniform dipole orientation.

Abstract

In recent years, optical and electronic properties of metal-organic frameworks (MOFs) have increasingly shifted into the focus of interest of the scientific community. Here, we discuss a strategy for conveniently tuning these properties through electrostatic design. More specifically, based on quantum-mechanical simulations, we suggest an approach for creating a gradient of the electrostatic potential within a MOF thin film exploiting collective electrostatic effects. With a suitable orientation of the polar apical linkers, the resulting non-centrosymmetric packing results in an energy staircase of the frontier electronic states reminiscent of the situation in a pin-photodiode. This 1-D gradient of the electrostatic potential causes a closure of the global energy gap and also shifts core-level energies by an energy equaling the size of the original band gap. The realization of such assemblies could be based on so-called pillared layer MOFs, grown in an oriented fashion on a solid substrate employing layer by layer growth techniques. In this context, the simulations provide guidelines regarding the design of the polar apical linker molecules that would allow the realization of MOF thin films with the (vast majority of the) molecular dipole moments pointing in the same direction.