The analysis, design, and simulation of molecular electronic devices using ab initio based methods: The negative differential resistance
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abstract
There are tough difficulties interpreting experiments performed to demonstrate the existence of important characteristics needed for the implementation of molecular electronics [I]. On the other hand, it is believed that atomistic systems can be used as electronic devices if they present highly nonlinear current-voltage (I-V) characteristics like negative differential resistance (NDR) [2]. This is because the interconnection of even two NDR devices in series yields multivalued I-V characteristics for the composed system, which can eventually be programmed to perform a controllable logic function. At this point the observation of NDR in atomistic systems can not be only associated to the molecules employed for that purpose but ir can also be associated to the contacts used to attach the molecules. In most cases it was assumed that the NOR is due to metal-molecule interface. Some experiments have confirmed that when molecules were eliminated from their settings that showed NDR, still NDR was found suggesting that NDR was most likcjv due to electromigration effects. This implies that earlier assignments of NDR to molecules might have been wrong and that NDR should be assigned to the creation and destruction of metallic filaments. The Figure on the left shows the I-V curve measured in our labs on a nanocell device provided by Tour at Rice University. This nanocell is a square chip with a substrate of SiO2where a discontinuouz gold film was deposited in Franzon labs at NCSU. Nitro molecules were self-assemble on gold nanorods which were deposited onto the box containing the gold islands. The nature of the ohservcd NDR is difficult to determine experimentally. It may be due to the molecules or the metals alone. Similar experiments have been carried out at Rice University without the niolecules and also NDR was observed. Regardless of whether NDR is due to a molecule, metal. insulator. semiconductor, or their interfaces, the mechanism of NDR may involve several procozes that allow switching due to changes of electronic, charge, conformational, or vibrational states. Several of these processes may involve chemical reactions yielding the creation and destruction of bonds.
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International Semiconductor Device Research Symposium, 2003
