Acomputational-simulation study of two ferrocene-based molecular electronic devices was performed as part of a continuing effort to develop a capability for ab initio design of metallocene-based electronic devices in general. In addition to the obvious technological advantage for realization of the potential of molecular electronic devices, such a capability would afford an economic advantage by enabling avoidance of the cost of synthesis of many organic molecules that subsequent testing would show to be unpromising for electronic-device applications.
Each of the two devices studied consisted of a single sulfur-terminated ferrocene molecule between two infinitely long gold electrodes (see figure). In one device, the molecule was 1,3'-ferrocenedithiolate and contact with the electrodes was made via the sulfur atoms on different cyclopentadienyl rings. In the other device, the molecule was 1,3-ferrocenedithiolate and contact with the electrodes was made via the two sulfur atoms on the same cyclopentadienyl ring. The structures and the electronic and electronic-transport properties of the molecules, both in isolation and as incorporated into the devices, were simulated numerically by use of a computer program that implemented a combination of density-functional theory and a nonequilibrium-Green’s-function formalism of quantum transport.
The numerical results of the simulations were interpreted as revealing that the electrical conductance through a ferrocene molecule depends on the positions of sulfur atoms: at low applied bias voltage, the electrical conductance of the molecule in which the same cyclopentadienyl ring was connected to the electrodes via two sulfur atoms was found to exceed that of the molecule in which the connection was made via sulfur atoms on different cyclopentadienyl rings. The transmission coefficients of ferrocenedithiolate molecules were found to change with the applied bias voltage. These changes were attributed to shifts of energy levels and concomitant changes of molecular orbital shape induced by the applied electric field. The computed current-versus-voltage characteristics of the devices were further interpreted as signifying that the transport properties of the 1,3-ferrocenedithiolate molecule have metallic features.
This work was done by Kanichi Nakagawara of Nihon Gene Research Laboratories Inc. for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. AFRL-0006
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