Tunnel currents across silane diamines/dithiols and alkane diamines/dithiols: A comparative computational study

Two different first-principles methods, one based on density functional theory combined with Green's functions and the other on a configuration interaction method, are used to calculate the electronic transport properties of alkane and silane chains terminated by amine end groups in metal - molecule - metal junctions. The low- voltage conductance is found to decay exponentially with increasing length in both systems, and decay constants are obtained from the different methods. Both methods predict smaller conductance values and steeper decay in the alkane-bridged junctions compared with the silane-bridged junctions, but quantitative differences in the decay constants obtained from the two formalisms arise. These differences are attributed to the treatment of the energy-level alignments in the tunnel junctions as well as the treatment of correlation within the molecular chains. Additionally, end-group effects for both the alkane and the silane chains are studied using both a simple tunnel barrier model and complex band-structure calculations. These results are used to explain differences observed in conductance decay constants in amine- and thiol-linked junctions obtained from the two transport methods; the results further highlight the importance of accurate energy-level alignment between the electrode and molecular states.