Tunneling conductivity of one- and two-component alkanethiol bilayers in Hg–Hg junctions

Electron tunneling (ET) through alkanethiol bilayers trapped between two small mercury drops (Hg–Hg tunneling junction, geometric area: 8×10−4 cm2) was investigated. Self-assembled monolayers were formed on Hg drops using one- or two-component solutions of n-alkanethiols (ranging from nonanethiol to hexadecanethiol) in hexadecane. Mercury drops covered by monolayers were brought into contact using micromanipulators. Current–voltage cyclic curves were used to measure the capacitance and tunneling current for the alkanethiol bilayer. The experimental current–voltage curves were compared with the following theoretical models: classical Simmons theory, Simmons theory modified by including effective electron mass (Lindsayʹs model), and a model for off-resonance tunneling through a molecular bridge (Ratnerʹs model). The classical Simmons theory does not fit our tunneling data while Lindsayʹs and Ratnerʹs models agree reasonably well with the tunneling characteristics of Hg–Hg junctions. The electrical properties of two-component bilayers, containing a mixture of hexadecanethiol and nonanethiol deposited on each Hg drop, were studied as a function of a monolayer composition. The thickness of the two-component monolayer on each Hg drop depends linearly on the mole-fraction of nonanethiol. ET through a two-component system is less efficient than ET through single-component bilayers. This result is rationalized in terms of diminished electronic coupling through van der Waals contacts.