In a recent paper published in Nanotechnology, the electronic transport properties of silicon/self-assembled monolayer/silicon interfaces were studied from a microscopic point of view by means of fully first-principles calculations, based on the plane-wave pseudopotential density functional theory approach and using maximally localized Wannier functions. The electronic structure of the interface and the quantum transport properties of the molecules in a lead/conductor/lead model device were directly calculated. The adopted theoretical scheme is based on the Landauer formalism, as implemented in the "WanT" code (developed by three of the authors in collaboration with Professor Marco Buongiorno Nardelli at NCSU, Raleigh, NC, US).
A prototype system was designed, comprised of an organic monolayer between two silicon surfaces, symmetrically bonded, through two different covalent bridges, Si-O-C and Si-S-C, with the aim of identifying a silicon-based interface as a good candidate for having efficient transport properties.
The molecule–surface bonding is found to be the key parameter to controlling transport, and oxygen bridges emerge as very efficient contacts. While substrate oxidation is known to be detrimental for electronic transport, the hydroxyl functional group of the molecule is shown to lead to a stable structure with very good transport characteristics within proper (realistic) substrate doping conditions. The stability of these results is probed with respect to different molecular bridges at the interface (sulphur), different orientations of the molecules, different compressions and coverages of the monolayer, and different doping concentrations of the substrate.