Conductance through Carbosilane Cage Compounds: A Computational Investigation

Silicon is still the dominating material in microelectronics, yet primarily conjugated hydrocarbons are investigated in the field of single-molecule electronics even though linear oligosilanes are a-conjugated. A drawback with the latter is their high conformational flexibility which strongly affects conductance. Here we report on a first principles density functional theory investigation of a series of rigid [2.2.2]bicyclic carbosilanes with 3, 2, 1, or 0 disilanylene bridges, providing all-silicon paths for charge transport. It is explored if these paths can be seen as independent and equivalent current paths acting as parallel resistors. For high conductance through the carbosilanes they need to be anchored to the gold electrodes via groups that are matched with the a-conjugated paths of the oligosilane cage segment, and we find that silyl (SiH3) groups are better matched than thiophenol groups. Even for the carbosilane with three disilanylene bridges we find that the most transmitting conductance channel is not equally distributed on the three parallel bridges. In addition, there is significant communication between the various pathways, which results in destructive interference lowering the conductance. Taken together, the different disilanylene bridges in the cage compounds do not act as parallel resistors.