Temperature dependent charge transport across tunnel junctions of single-molecules and self-assembled monolayers: a comparative study

Abstract

In this work we present a comparative study of the temperature behavior of charge current in both single-molecule transistors and self-assembled monolayer-based tunnel junctions with symmetrical molecules of alkanethiolates functionalized with a ferrocene (Fc) unit. The Fc unit is separated from the electrodes with two equal alkyl chains of enough length to ensure weak coupling of the Fc unit with the electrodes. These junctions do not rectify charge current and display exponential dependence with temperature with moderate slopes, which can be directly associated to the thermal broadening of the electronic occupation Fermi distribution in the electrodes. The capability to electrically gate the molecular frontier orbital of the Fc (here the highest occupied molecular orbital, HOMO) in the single-molecule transistor, not possible in the two-terminal SAM-based junctions, allows for a detailed comparative between the two classes of junctions. Our findings demonstrate that, although most transport characteristics are equivalent, collective effects arising from interactions between molecules in the self-assembled monolayer greatly affect the energetics of SAM-based junctions, resulting in a bias-independent tunnel current, contrary to the case of the single-molecule junction and as expected from the thermal broadening of the electronic occupation around the Fermi energy in the electrodes.