(Invited) Unexpected Molecular Interfaces: SAMs on Cobalt

Molecular-based materials are appealing for many applications due to their diverse composition, which in turn impacts physical and electronic properties. Often, the interaction between an organic layer and an inorganic substrate is unpredictable once the interface is formed. We use a holistic approach to investigate the molecular interfaces – photoelectron spectroscopies and electrical characterization to probe the interface and buried junction, respectively. Thus, molecular-based interfaces provide a unique platform to investigate the ensemble-scale impact molecules which provide basic physical insights and ideas for potential applications.  

An emerging field is to combine the flexibility of organic materials into spin-based electronics. Most research in this field implement organic semiconductors as the non-magnetic spacer in a spin-valve device configuration.  While there are many proposed mechanisms regarding spin-transport in organic materials, there is no doubt that the interface between the magnetic electrode and organic material is an important aspect for the success of an organic spintronic device. In addition to potentially altering the electronic structure at the interface, SAMs could serve as a tunneling barrier for spintronics due to their insulating nature and discrete thickness. Inorganic tunnel junctions have improved spin-injection into organic spin-valve devices.

We have explored self-assembled monolayers (SAM) on a template-stripped and oxidized cobalt surface to understand the molecular-metal interface from a structural, chemical, and electronic point of view. We find that the self-assembly of the bifunctional molecule is profoundly different than the thiol-alone species.  In particular, the cobalt surface exhibits a very different interface following self-assembly of a carboxylic acid-bearing species. We find that a thiol-carboxylic acid monolayer reacts with oxidized Co surface atoms in such a way that metallic Co atoms are detected at the surface. We propose two different models of the interface formation based on the results, and suggest that the MHA/ethanol removes the cobalt oxide during the self-assembly as the prevailing model. We also discuss the impact of this Co/molecule interface on electron transport through Co/molecule/Si molecular junctions. Our results provide insight into ex-situ modification and functionalization of ferromagnetic interfaces impacting an important aspect of spin-based devices. Interfaces are not always straight forward – they can have unforeseen impact and influence.