Mercury-mercury tunneling junctions. 1. Electron tunneling across symmetric and asymmetric alkanethiolate bilayers

Electron tunneling experiments involving Hg-Hg junctions incorporating two alkanethiolate monolayers are described. Formation of a symmetric junction(Hg-SCn-CnS-Hg) is accomplished by bringing in contact two small (3 x 10(-3) cm(2)) mercury drop electrodes in a 5-20% (v/v) hexadecane solution of an alkanethiol. Formation of asymmetric junctions (Hg-SCn-CmS-Hg) and junctions containing n-alkane-3-thiopropanamide bilayers are also described. Tunneling currents in the junctions were measured for voltage biases extending to +/-1.5 V. The currents decrease exponentially with the junction thickness yielding the tunneling decay constant, beta = 0.89 +/- 0.1 per CH2. The decay constant exhibits only a weak dependence on the voltage bias suggesting that electron tunneling follows a through-bond mechanism. Tunneling currents in the n-alkane-3-thiopropanamide bilayer junctions were larger than those in alkanethiolate junctions with the same number of atoms suggesting that introduction of an amide group increases the strength of the electronic coupling through these types of sigma-bonded systems.