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Metal-molecule-metal junctions based on alkane thiol (C n T) and oligophenylene thiol (OPT n ) self-assembled monolayers (SAMs) and Au electrodes are expected to exhibit similar electrical asymmetry, as both junctions have one chemisorbed Au–S contact and one physisorbed, van der Waals contact. Asymmetry is quantified by the current rectification ratio RR apparent in the current–voltage ( I – V ) characteristics. Here we show that RR < 1 for C n T and RR > 1 for OPT n junctions, in contrast to expectation, and further, that RR behaves very differently for C n T and OPT n junctions under mechanical extension using the conducting probe atomic force microscopy (CP-AFM) testbed. The analysis presented in this paper, which leverages results from the previously validated single level model and ab initio quantum chemical calculations, allows us to explain the puzzling experimental findings for C n T and OPT n in terms of different current rectification mechanisms. Specifically, in C n T-based junctions the Stark effect creates the HOMO level shifting necessary for rectification, while for OPT n junctions the level shift arises from position-dependent coupling of the HOMO wavefunction with the junction electrostatic potential profile. On the basis of these mechanisms, our quantum chemical calculations allow quantitative description of the impact of mechanical deformation on the measured current rectification. Additionally, our analysis, matched to experiment, facilitates direct estimation of the impact of intramolecular electrostatic screening on the junction potential profile. Overall, our examination of current rectification in benchmark molecular tunnel junctions illuminates key physical mechanisms at play in single step tunneling through molecules, and demonstrates the quantitative agreement that can be obtained between experiment and theory in these systems. 

Conducting probe atomic force microscopy (CP-AFM) was employed to examine electron tunneling in self-assembled monolayer (SAM) junctions. A 2.3 nm long perylene tetracarboxylic acid diimide (PDI) acceptor molecule equipped with isocyanide linker groups was synthesized, adsorbed onto Ag, Au and Pt substrates, and the current–voltage ( I – V ) properties were measured by CP-AFM. The dependence of the low-bias resistance ( R ) on contact work function indicates that transport is LUMO-assisted (‘n-type behavior’). A single-level tunneling model combined with transition voltage spectroscopy (TVS) was employed to analyze the experimental I – V curves and to extract the effective LUMO position ε l = E LUMO − E F and the effective electronic coupling ( Γ ) between the PDI redox core and the contacts. This analysis revealed a strong Fermi level ( E F ) pinning effect in all the junctions, likely due to interface dipoles that significantly increased with increasing contact work function, as revealed by scanning Kelvin probe microscopy (SKPM). Furthermore, the temperature ( T ) dependence of R was found to be substantial. For Pt/Pt junctions, R varied more than two orders of magnitude in the range 248 K < T < 338 K. Importantly, the R ( T ) data are consistent with a single step electron tunneling mechanism and allow independent determination of ε l , giving values compatible with estimates of ε l based on analysis of the full I – V data. Theoretical analysis revealed a general criterion to unambiguously rule out a two-step transport mechanism: namely, if measured resistance data exhibit a pronounced Arrhenius-type temperature dependence, a two-step electron transfer scenario should be excluded in cases where the activation energy depends on contact metallurgy. Overall, our results indicate (1) the generality of the Fermi level pinning phenomenon in molecular junctions, (2) the utility of employing the single level tunneling model for determining essential electronic structure parameters ( ε l and Γ ), and (3) the importance of changing the nature of the contacts to verify transport mechanisms. 

The Stark effect plays a key role in understanding why, against expectation, alkane thiols are not high-performance molecular rectifiers.