August 2009, Volume 2, Issue 8, pp 648-659,
Open Access This content is freely available online to anyone, anywhere at any time.
Date: 12 Aug 2009
Silicon-based molecular switch junctions
In contrast to the static operations of conventional semiconductor devices, the dynamic conformational freedom in molecular devices opens up the possibility of using individual molecules as new types of devices such as a molecular conformational switch or for molecular data storage. Bistable molecules—such as those having two stable cis and trans isomeric configurations—could provide, once clamped between two electrodes, a switching phenomenon in the non-equilibrium current response. Here, we model molecular switch junctions formed at silicon contacts and demonstrate the potential of such tunable molecular switches in electrode/molecule/electrode configurations. Using the non-equilibrium Green function (NEGF) approach implemented with the density-functional-based tight-binding (DFTB) theory, a series of properties such as electron transmissions, current-voltage characteristics in the different isomer conformations, and potential energy surfaces (PESs) as a function of the reaction coordinates along the trans to cis transition were calculated for two azobenzene-based model compounds. Furthermore, in order to investigate the stability of molecular switches under ambient conditions, molecular dynamics (MD) simulations at room temperature were performed and time-dependent fluctuations of the conductance along the MD pathways were calculated. Our numerical results show that the transmission spectra of the cis isomers are more conductive than trans counterparts inside the bias window for both model compounds. The current voltage characteristics consequently show the same trends. Additionally, calculations of the time-dependent transmission fluctuations along the MD pathways have shown that the transmission in the cis isomers is always significantly larger than that in their trans counterparts, showing that molecular switches can be expected to work as robust molecular switching components.
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