Gate-Modulated Conductance of Extended Conjugation in Atomically Arrayed Molecular Assemblies

Within molecular electronics, the molecular-scale transistor provides a compelling and central device. While substantial efforts have been expended on this subject, current embodiments typically involve cumbersome gating with nonintuitive routes for integration. In this theoretical study, we examined the efficacy of combining a new molecular architecture with the well-established atomic resolution of the Si(100)­2 × 1 hydride-terminated surface to provide a molecular-scale modulation scheme that is conceptually easier to integrate. A series of alkyl-substituted carbazoles: ethylcarbazole, butylcarbazole, hexylcarbazole, and decylcarbazole, operating in the σ–π motif provided the transport conduit through extended conjugation of π–π stacking upon assembly along the Si(100)­2 × 1 dimer row. It was found that alkyl substituent lengths greater than four methylene units (butylcarbazole) effectively isolated the extended π-conjugation from the underlying substrate by preventing tunneling due to breakdown at terminal alkyl chains as well as coupling of eigenstates between the π-stack and silicon crystal. These findings were corroborated by systematically stepping through the alkyl substitution length and noting the distribution of eigenstates for all peaks in the corresponding transmission spectrum of π-stacked wires along with plotting the zero-bias resistance against the wire length. The resistance plots demonstrated a single, molecularly isolated, tunneling-type scaling factor β for hexyl through decylcarbazole. In contrast, an inflection point was observed for the shorter ethyl and butylcarbazole, indicating a transition to dual, substrate routed, conduction pathways in these cases. Further investigation of device response to localized gate potentials demonstrated that substituent lengths greater than six methylene units (hexylcarbazole) could block eigenstate coupling between the π-stack and substrate for gate potentials in the range of −4 to 1.5 V. This degree of isolation supported a modulation factor of over a 106× in conductance. These results suggest that elongating the σ group in crystalline organized σ–π assemblies may support transistor modulation by exploiting the underlying substrate as an easily integrated gate.