Molecular Nanostructures at Surfaces: Tuning the Work Function and the Role of Substrate on 2D Metal-Organic Frameworks Properties


Molecular Nanostructures at Surfaces: Tuning the Work Function and the Role of Substrate on 2D Metal-Organic Frameworks Properties


J. Čechal

CEITEC – Central European Institute of Technology, Brno University of Technology,

Czech Republic.


In the seminar, I will introduce the scientific activities in our research group, focussing on two main directions: a molecular interface between organic semiconductors (OS) and electrodes and 2D metal-organic frameworks (2D MOFs) on graphene. In both topics, I will highlight the multimethod approach: a combination of low-energy electron microscopy (LEEM), scanning tunneling microscopy (STM), and X‑ray photoelectron spectroscopy (XPS) combined with ab initio calculations is used to get comprehensive insights in the system properties. 

Concerning the first topic, the energy level alignment between a metal electrode work function and the position of organic semiconductor frontier orbitals (HOMO or LUMO) is required to reduce contact resistance and enhance the efficiency of organic-semiconductor-based devices. I will present monolayer thick charge injection layers (CILs) based on aromatic carboxylic acids that can induce an energy level shift in the subsequent layers by up to 0.8 eV. By gradual on-surface deprotonation of both molecules, a highly tunable shift in the work function of the system is achieved. The work function and energy-level positions in the CIL increase linearly with the density of dipoles formed upon the deprotonation of carboxylic groups; the energy-level positions of the subsequent layers follow the changes in the CIL. I will also present robust dipolar layers that cover the substrate entirely.

To a lesser extent, I will present the second topic in which we elucidate the extent of the substrate effect by comparing Fe-TCNQ 2D MOFs on two weakly interacting supports: graphene and Au(111). I will show that the Fe-TCNQ on graphene partially retains its intrinsic properties, as it is non-planar with iron in quasi-tetrahedral sites. In contrast, when synthesized on Au(111), a popular weakly interacting support, the Fe‑TCNQ structure is planarized by stronger van-der-Waals interaction. The differences in physical and electronic structure result in distinct properties of the supported 2D MOFs. The dz2 center position associated with Fe-sites is shifted by 1.4 eV between the two supports, which implies dramatically different adsorption and catalytic properties of these seemingly similar “single-atom catalyst” sites.