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 n -alkyl chains anchored with acetylene groups (C n A; n = 8, 9, 10, and 12) on Ag, Au, and Pt electrodes. We found that the current-voltage ( I - V ) characteristics of C n A 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 G Ag < G Au < G Pt is found to follow the same ordering as the metal work function Φ Au < Φ Au < Φ Pt , 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 (C n T) to which we refer in view of some similarities. Furthermore, unlike in C n T and other junctions anchored with thiol groups investigated in the past, the AFM tip causes in C n A an additional MO shift, whose independence of size ( n ) 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 C n A and C n T junctions different. What ultimately makes C n A unique at the microscopic level is a salient difference never reported previously, namely, that C n A's alkyne functional group gives rise to two energetically close (HOMO and HOMO-1) orbitals. This distinguishes the present C n A from the C n T, whose HOMO stemming from its thiol group is well separated energetically from the other MOs.