How Do Electrons Cross Si-bound Alkyl Monolayers?

Electron flow through ultra-small wires made of single molecules is a fascinating, but poorly understood, area of study. Here, we investigate an "ideal" system consisting of a well ordered monolayer of alkyl chains (C_nH_2n+1, n=12,14,16, 18) attached to a Si wafer and contacted with a mercury droplet (Fig. a). The electronic structure of the system is determined by electron spectroscopy (Fig. b). Current-voltage (J-V) measurements (Fig. c) reveal two dominant transport mechanisms. At low bias (<0.5 V), the current is limited by thermionic emission over the barrier in the Si substrate, yielding a current that is independent of the molecular chain length. The inset shows a temperature-dependent measurement that confirms this mechanism. At high bias, the current shows an exponential dependence on chain length, signaling a current dominated by tunneling across the molecular layer. The combination of spectroscopic and detailed J-V data allows a realistic investigation of the meaning of charge-carrier effective mass in these systems.