Carbon/molecule/metal and carbon/molecule/metal oxide molecular electronic junctions

Molecular junctions were fabricated on the basis of a 1.7-4.5 nm thick layer of fluorene (FL) or nitroazobenzene (NAB) covalently bonded to a graphitic pyrolyzed photoresist film (PPF) substrate. The junction was completed with a top contact consisting of metallic Cu, TiO2, or aluminurn(III) oxide (AlOx) and a final layer of Au. The current/voltage behavior of the junctions depended strongly both on the nature of the metal or metal oxide top layers and on the structure of the molecular layer. PPF/NAB/Cu/ Au and PPF/FL/Cu/Au junctions were highly conducting, with resistances of 0.3-1.7 Omega cm(2), depending on the identity and thickness of the molecular layer. Substitution of Cu with either AlOx or TiO2 caused a large increase in junction resistance by 2-4 orders of magnitude, but also yielded rectifying junctions in the case of PPF/NAB(4.5)/TiO2(3.1)/Au. For a positive bias (PPF relative to Au) above +2 V, the NAB(4.5)/TiO2(3.1) junction became highly conductive, apparently due to injection of electrons into the TiO2 conduction band. PPF/NAB(4.5)/AlOx(3.3)/Au junctions exhibited symmetric i/V responses with very low currents, and capacitances consistent with those expected for a parallel plate capacitor with two dielectric layers. However, Raman spectroscopy of the NAB/AlOx junctions showed structural changes under negative bias corresponding to reduction of NAB, despite the absence of significant current flow. The changes were reversible and repeatable provided the bias was between -1.5 and +1.0, but partially irreversible when the bias excursion was negative of -1.5 V. Combined with a previous spectroscopic study of PPF/NAB/TiO2/Au junctions, the results imply a rectification mechanism based on electron transport through the NAB LUMO and the TiO2 conduction band, and possibly a Coulombic barrier resulting from reduction of the NAB in the molecular junction.