Mechanism of electron conduction in self-assembled alkanethiol monolayer devices

Electron tunneling through self-assembled monolayers (SAMs) of alkanethiols was investigated using nanometer scale devices that allow temperature-dependent current-voltage, I(V, T), measurements. The I(V, T) measurement results show, for the first time, temperature-independent electron transport characteristics, proving direct tunneling as the transport mechanism in alkanethiol SAMs. The measured tunneling currents can be fitted with theoretical calculations using the modified rectangular barrier model of direct tunneling with a barrier height Phi(B) = 1.42 +/- 0.04 eV and a non-ideal barrier factor alpha = 0.65 +/- 0.01 (that may correspond to effective mass of 0.42 m). From the length-dependent conduction measurement on different alkanethiols of various lengths, the tunneling current exhibits exponential dependence on the molecular length, d, as I proportional, variant exp(-betad), where beta is a decay coefficient that was found to be bias-dependent and agrees with the existing theory of direct tunneling. A zero field decay coefficient beta(0) of 0.79 +/- 0.01 A(-1) was obtained.