Exploring the Tilt-Angle Dependence of Electron Tunneling across Molecular Junctions of Self-Assembled Alkanethiols

Electronic transport mechanisms in molecular junctions are investigated by a combination of first-principles calculations and current−voltage measurements of several well-characterized structures. We study self-assembled layers of alkanethiols grown on Au(111) and form tunnel junctions by contacting the molecular layers with the tip of a conductive force microscope. Measurements done under low-load conditions permit us to obtain reliable tilt-angle and molecular length dependencies of the low-bias conductance through the alkanethiol layers. The observed dependence on tilt-angle is stronger for the longer molecular chains. Our calculations confirm the observed trends and explain them as a result of two mechanisms, namely, a previously proposed intermolecular tunneling enhancement as well as a hitherto overlooked tilt-dependent molecular gate effect.
T.F., D.S.P. and A.A. acknowledge support from Basque Departamento de Educación, UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de Ciencia e Innovación (Grants No. FIS2007-6671-C02-01, FIS2007-66711-C02-02 and MAT2007-62732), the ETORTEK program funded by the Basque Departamento de Industria and the Diputación Foral de Guipuzcoa, and the EC under contract No. NMP4-CT-2006-032109 (STREP “SURMOF”). T.F. acknowledges support from the Danish FNU (Grant No. 272-07-0114).