STM visualisation of counterions and the effect of charges on self-assembled monolayers of macrocycles.

Tibor Kudernac,Wael Mamdouh,Natalia Shabelina,Steven De Feyter,Sigurd Höger

doi:10.3762/bjnano.2.72

Abstract

Despite their importance in self-assembly processes, the influence of charged counterions on the geometry of self-assembled organic monolayers and their direct localisation within the monolayers has been given little attention. Recently, various examples of self-assembled monolayers composed of charged molecules on surfaces have been reported, but no effort has been made to prove the presence of counterions within the monolayer. Here we show that visualisation and exact localisation of counterions within self-assembled monolayers can be achieved with scanning tunnelling microscopy (STM). The presence of charges on the studied shape-persistent macrocycles is shown to have a profound effect on the self-assembly process at the liquid–solid interface. Furthermore, preferential adsorption was observed for the uncharged analogue of the macrocycle on a surface.

Highlights

Ordered monolayers formed by the self-assembly of molecular building blocks on solid surfaces have recently attracted considerable attention [1,2,3,4], due to their promising use as functional surfaces in nanotechnological applications [5,6]
It can be envisioned that counterions of charged molecules that are adsorbed at the surface could be used to control the structure of 2D crystals at the liquid–solid interface, and to control properties of the physisorbed molecules and to carry specific functionalities
We demonstrated that the direct scanning tunnelling microscopy (STM) visualisation of counterions is possible for charged molecules that are self-assembled at the liquid–solid interface

Summary

Introduction

Ordered monolayers formed by the self-assembly of molecular building blocks on solid surfaces have recently attracted considerable attention [1,2,3,4], due to their promising use as functional surfaces in nanotechnological applications [5,6]. Combining the ordering of self-organised, physisorbed monolayers with an additional functionality remains an important challenge. While counterions constitute an elegant way to modify electronic properties of molecules at the liquid–solid interface, their role in 2D self-assembly has been given little attention so far, mostly because they could not be visualised by STM [11,12,13,14]

Results

Discussion

Conclusion