Temperature Dependent Barrier Crossover Regime in Tunneling Single Molecular Devices Based on the Matrix of Isolated Molecules

Abstract

This paper reports analysis of direct (low bias regime that corresponds to Simmons model) and field emission, Fowler−Nordheim (FN) tunneling, and a crossover between these regimes in molecular nanojunction. We have fabricated molecular devices based on a heterogenius mixture of molecular wires of 2-[4-(2-mercaptoethyl)-phenyl]ethanethiol (Me-PET) as self-assembled monolayer (SAM) molecules incorporated into the matrix of molecular insulator spacers [penthane 1-thiol (PT)] at a concentration ratio of r = 10−6 wires/spacers. The monolayer is sandwiched between two gold (Au) electrodes. A temperature-depended conductivity in this structure was analyzed at both low and elevated biases using models of direct tunneling (Simmons model) and field emission (FN) regime. A crossover voltage, Vtrans, between these two regimes was determined at different temperatures. Comparison of temperature-dependent Simmons and FN barriers, (ΦBSim(T) and ΦBFN(T)), and transition bias (Vtrans(T)) reveals an anomaly in position of Vtrans(T) with respect to ΦBSim(T) and ΦBFN(T) at low temperatures (15−100 K). The change in slopes of ΦBSim(T), ΦBFN(T), and Vtrans(T) at different temperatures pointed to the switching of the transport mechanism in the system as well. Activated by temperature, the observed phenomenon can be attributed to the existence of an additional transport barrier, which operates in series with the molecular barrier. As candidate for this additional barrier, the injection barrier must be considered. In this context, the transport was controlled by the injection barrier or injection barrier regime (IBR) at low temperatures (15−100 K), and by molecular barrier or molecular barrier regime (MBR) at high temperatures (150−294 K). A molecular diode, with the same structure (Me-PET/PT r = 10−6), but with Al electrodes, was fabricated to check this assumption. While devices with Au electrodes have a low tunneling barrier (ΦBSim ≈ 1.2 eV), devices with Al electrodes have a high tunneling barrier (ΦBSim ≈ 3 eV). Only direct tunneling could be observed in measured bias range (V = ±2 V). Nevertheless the “crossover behavior” was observed in the device with Al electrodes at low temperatures (15−100 K). Therefore, observed “crossover” in the device with Al electrodes is related to the transport processes, which occurs at the electrode due to the injection barrier, rather than the transition from direct to FN tunneling in the molecule at low temperatures. In this case a transition point, Vtrans, characterizes the IBR at low temperatures, the MBR at high temperatures, and the BTR between these two regimes.