Contacting strategies for molecular electronics

Abstract

In the last twenty years, new sophisticated tools to contact single molecules were developed. However, a molecular junction (MJ) constituted by two atomic contacts and bridged by a single or a few molecules does not form a rigid system, in particular at room temperature. A major challenge consists therefore in understanding and optimizing the arrangement of stable and reproducible contacts. To get more insight into contact formation and rupture, new tools are needed. Conducting atomic force microscopy is an attractive approach enabling the correlation of mechanical and electrical properties in individual MJs. In the first part of this thesis we report on measurements of gold-gold and gold-octanedithiol-gold junctions. We introduce two-dimensional histograms in the form of scatter plots to better analyze the correlation between force and conductance. In this representation, the junction-forming octanedithiol compounds lead to a very clear step in the force-conductance data, which is not observed for control monothiol compounds. The conductance found for octanedithiols is in agreement with the idea that junction conductance is dominated by a single molecule. Until now, gold is the preferred electrode material within the field, as it allows a covalent or coordinative binding of the molecules for several binding groups and is easy to handle. Gold however also presents major disadvantages: the relative thick metal electrodes lead to a large screening of a backgate potential; and the mobility of surface atoms at room temperature strongly limits the junctions mechanical stability. A particularly promising approach to overcome these issues is based on using graphene as electrode material. In the second part of this thesis we demonstrate the controlled and reproducible fabrication of sub-5 nm wide gaps in single-layer graphene electrodes. The process is implemented for graphene grown via chemical vapor deposition (CVD) using an electroburning process at room temperature and in vacuum. A yield of over 95% for the gap formation is obtained. This approach allows producing single-layer graphene electrodes for molecular electronics at a large scale. Additionally, from Raman spectroscopy and electroburning carried out simultaneously, we can follow the heating process and infer the temperature at which the gap formation happens. Furthermore we briefly discuss the properties of graphene field effect transistors (FETs) and the first results of using our graphene electrodes to contact molecules.