CP-AFM Molecular Tunnel Junctions with Alkyl Backbones Anchored Using Alkynyl and Thiol Groups: Microscopically Different Despite Phenomenological Similarity.

In this paper, we report results on the electronic structure and transport properties of molecular junctions fabricated via conducting probe atomic force microscopy (CP-AFM) using self-assembled monolayers (SAMs) of -alkyl chains anchored with acetylene groups (CA; = 8, 9, 10, and 12) on Ag, Au, and Pt electrodes. We found that the current-voltage (-) characteristics of CA CP-AFM junctions can be very accurately reproduced by the same off-resonant single-level model (orSLM) successfully utilized previously for many other junctions. We demonstrate that important insight into the energy-level alignment can be gained from experimental data of transport (processed via the orSLM) and ultraviolet photoelectron spectroscopy combined with ab initio quantum chemical information based on the many-body outer valence Green's function method. Measured conductance < < is found to follow the same ordering as the metal work function Φ < Φ < Φ, a fact that points toward a transport mediated by an occupied molecular orbital (MO). Still, careful data analysis surprisingly revealed that transport is not dominated by the ubiquitous HOMO but rather by the HOMO-1. This is an important difference from other molecular tunnel junctions with p-type HOMO-mediated conduction investigated in the past, including the alkyl thiols (CT) to which we refer in view of some similarities. Furthermore, unlike in CT and other junctions anchored with thiol groups investigated in the past, the AFM tip causes in CA an additional MO shift, whose independence of size () rules out significant image charge effects. Along with the prevalence of the HOMO-1 over the HOMO, the impact of the "second" (tip) electrode on the energy level alignment is another important finding that makes the CA and CT junctions different. What ultimately makes CA unique at the microscopic level is a salient difference never reported previously, namely, that CA's alkyne functional group gives rise to two energetically close (HOMO and HOMO-1) orbitals. This distinguishes the present CA from the CT, whose HOMO stemming from its thiol group is well separated energetically from the other MOs.