Switching and Memory in Molecular Bundles

Conductivity of these OPE molecular scale wires and devices is hypothesized to arise from transfer of electrons through the it-orbital backbone that extends over the entire molecule. When the phenyl rings of the OPEs are planar, the Jt-orbital overlap within the molecules is continuous. Thus transfer over the entire molecule is achieved, electrons can freely flow between the two metal contacts, and conductivity is maximized. But if the phenyl rings become perpendicular with respect to each other, the n-orbitals between the phenylene rings likewise become perpendicular. The discontinuity of the 7t-orbital network in the perpendicular arrangement minimizes electron transport through the molecular system, thus conductivity is greatly decreased. 

Pulling of wire unUl cleavage results in metal tip formations 

Accordingly, the nitroaniline OPE was tested in the nanopore, in the absence of an orthogonal external electric field, to determine its electronic characteristics. A series of control experiments were first performed with alkanethiol-derived SAMs and systems containing no molecules. Both the Au-alkanethiolate-Au junctions and the Au-silicon nitride membrane-Au junctions showed current levels at the noise limit of the apparatus ( 1 pA) for both bias polarities at both room and low temperatures. The Au-Au junctions gave 

To further explore the mechanism of this molecular NDR (mNDR) and molecular DRAM (mDRAM) phenomenon, several related compounds have been synthesized such as a nitroacetamide rather than a free nitroaniline moiety. After testing in the nanopore, nitroacetamide exhibited the NDR effect, however, with a smaller PVR of 200 1 at 60 K. 

This memory can be rationalized based upon conduction channels that change upon charge injection as studied by density fimctional theory (DFT). These DFT studies further corroborate with the experimental results in that the unsubstituted OPE and the amine-substituted OPE would be inactive as deviees (having linear I(V) curves) while nitroaniline and nitro OPE would both have switching states (exhibited by sharp nonlinear I(V) characteristics) due to the accepting of electrons during voltage application. Furthermore, the DFT calculations showed that nitroaniline would need to receive one electron in order to become conductive whereas the nitro OPE would be initially conductive ( on in the mDRAM) and then become less conductive, off , upon receipt of one 

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