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Molecular electronics and molecules on surfaces

Published: 8 May 2013

Investigation of thiol binding of molecules to gold (Au) for molecular electronics by first-principles methods. In our study of thiol binding to Au for molecular electronics we pointed out the considerable changes in the Au-Au bonds in the metal, and thus the importance of not constraining the metal surface when obtaining equilibrium geometries. The molecules computed represent the now standard reference system in molecular electronics, the thiol terminated molecules bonded to gold contacts. Our cluster models of these systems, with full geometrical degrees of freedom, paved the way for a very tractable protocol for describing possible molecular devices, i.e. by not only looking at the properties of the molecule, but to also include parts of the metal leads as clusters, which has subsequently been used extensively. Our results showed how thiol molecules bonds as thiolates on gold, which is the basis for modern self-assembled monolayers (SAMs).

We also study phthalocyanines, porphyrines and fullerenes, their doping and bonding to metal surfaces. We simulate scanning tunneling microscopy (STM)  images and curves of these molecules bound on surfaces. We also do computations of single-molecule spectroscopy performed using STM-tips, such as scanning tunneling spectroscopy (STS) and in-elastic tunneling spectroscopy (IETS) for the study of individual molecule vibrations excited through electron-vibration interactions. We study how doping affects the molecular conductance and how molecular conformational changes can be used to switch a potential molecular device on and off, for molecular electronics applications.
For these simulations we use density functional theory (DFT), for which standard functionals have difficulties describing the weak van der Waals interactions in general, and the dispersion contribution in particular. Then we use fuctionals specially desiged for the purpose, or we correct for this by including dispersion in similar ways as is done in the force fields in Molecular Mechanics based Molecular Dynamics (MD), in DFT with dispersion (so called DFT-D). These effects are essential for molecules that are physisorbed on a substrate surface, and are also needed for quantitative results for large molecules that are chemisorbed but also have large contributions from physisorption.