Comprehensive characterization of hybrid junctions comprised of a porphyrin monolayer sandwiched between a coinage metal overlayer and a Si(100) substrate

The promise of molecular electronics has stimulated a variety of approaches for making hybrid junctions wherein molecules are sandwiched between two metal contacts or a metal and a semiconductor contact. However, the fate of molecules subsequent to deposition of a top metal contact has generally not been well.Characterized. Toward this goal, the interaction of evaporated Cu, Ag, and Au films deposited in varying thicknesses (3, 5, and 8 nm) on a series of monolayer-coverage porphyrins covalently attached to Si(100) substrates was investigated. Each porphyrin contains a triallyl tripod attached to the porphyrin via a p-phenylene unit, which anchors the porphyrin to the Si(100) substrate via hydrosilylation of the allyl groups. All of the porphyrins are Zn chelates bearing meso p-tolyl substituents orthogonal (lateral) to the tripodal surface anchor, but contain different substituents in the meso position opposite (distal) to the surface anchor, p-tolyl, alpha,alpha,alpha-trifluoro-p-tolyl, pentafluorophenyl, or p-cyanophenyl, to test the potential for the distal meso substituents to impart different characteristics to the metal/molecule/Si junction. The methods of interrogation used include ellipsometry, atomic force microscopy, FTIR spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, and current-voltage measurements. The studies indicate that all of the porphyrin monolayers are robust under the conditions of metal deposition, experiencing no noticeable degradation, and that none of the distal functional groups reacts with the deposited metal, except for the nitrile group of the p-cyanophenyl-substituted porphyrin, which reacts/complexes with Cu. Neither Cu nor Ag penetrates through the monolayer to an appreciable extent to form electrically conductive filaments, whereas Au does penetrate through the porphyrin monolayer and contacts the Si substrate. Collectively, the studies provide a comprehensive assessment of the characteristics of the metal (Cu, Ag, Au)/porphyrin monolayer/Si junctions.