We present here a comprehensive study of the single molecule conductance of alkanes with carboxylic-acid terminal groups, investigated with the I(s) and I(t) scanning tunneling microscopy (STM) methods. These methods enable the formation of single molecular bridges between the gold substrate surface and gold STM tip without the need to first form a metallic break junction. The I(s) and I(t) methods have up until this study been used primarily with thiol head groups, and this current study shows that they can be extended to other contact groups. The single molecule conductance of a series of dicarboxylic acids, HOOC−(CH2)n−COOH, has been investigated as a function of length of the alkane chain (N = 4−12), temperature, pH, electrochemical potential, and contact gap separation. The latter parameter, which has been often neglected in single molecule conductance studies, is particularly noteworthy given the strong dependence of molecular conductance on the separation between the STM tip and the surface. In this study, particular emphasis is put on determining the contact gap spacing corresponding to the measured conductance values. Both molecular conductance and the temperature dependence of molecular conductance are shown to depend significantly on contact gap separation for these dicarboxylic acid-terminated molecular wires. Possible origins for this behavior are discussed. Values for the tunneling decay constant, βN, and the contact resistance, A, have been determined for the α,ω-dicarboxylic acid-terminated alkanes fully stretched in the contact junction (all-trans configuration). βN values determined for this stretched configuration agree well with literature expectations, and the high values for the contact resistance attest to a relatively poor coupling between the carboxylate end groups and the Au contacts.

This work was supported by EPSRC under grant EP/C00678X/1 (Mechanisms of Single Molecule Conductance), and by the European Union (Marie Curie Transfer of Knowledge, “Electropep”, MTKD-CT-2005-029864). S.M. acknowledges a postdoctoral fellowship from Ministerio de Educación y Ciencia of Spain. We are grateful for financial assistance from MEC (Spain) and fondos FEDER in the framework of the project CTQ2006-05236 as well as to DGA for its support through the interdisciplinary project PM079/2006 (P.C. and M.C.L.).

Peer reviewed