Strong Overtones Modes in Inelastic Electron Tunneling Spectroscopy with Cross-Conjugated Molecules: A Prediction from Theory

Abstract

Cross-conjugated molecules are known to exhibit destructive quantum interference, a property that has recently received considerable attention in single-molecule electronics. Destructive quantum interference can be understood as an antiresonance in the elastic transmission near the Fermi energy and leading to suppressed levels of elastic current. In most theoretical studies, only the elastic contributions to the current are taken into account. In this paper, we study the inelastic contributions to the current in cross-conjugated molecules and find that while the inelastic contribution to the current is larger than for molecules without interference, the overall behavior of the molecule is still dominated by the quantum interference feature. Second, an ongoing challenge for single molecule electronics is understanding and controlling the local geometry at the molecule-surface interface. With this in mind, we investigate a spectroscopic method capable of providing insight into these junctions for cross-conjugated molecules: inelastic electron tunneling spectroscopy (IETS). IETS has the advantage that the molecule interface is probed directly by the tunneling current. Previously, it has been thought that overtones are not observable in IETS. Here, overtones are predicted to be strong and, in some cases, the dominant spectroscopic features. We study the origin of the overtones and find that the interference features in these molecules are the key ingredient. The interference feature is a property of the transmission channels of the π system only, and consequently, in the vicinity of the interference feature, the transmission channels of the σ system and the π system become equally transmissive. This allows for scattering between the different transmission channels, which serves as a pathway to bypass the interference feature. A simple model calculation is able to reproduce the results obtained from atomistic calculations, and we use this to interpret these findings.

IETS and quantum interference: Propensity rules in the presence of an interference feature

“…Furthermore, the triple bond stretch mode is predicted to be one of the dominant modes for this molecule. 18 Conversely, in molecules dominated by destructive interference, we see that the observed magnitude of the stretch mode decreases when the anti-resonance approaches the Fermi energy. As the antiresonance is moved away from the Fermi energy, this overlap between the left and right channels gradually increases, which causes an increase in the peak height of the mode.…”

“…For ordinary linearly conjugated or saturated molecules, the channels are generally extended out all over the molecule so that no modes are strictly forbidden. How- ever, for molecules exhibiting destructive interference effects, there can be a very small overlap between the left and the right channels, 18 which can be shown directly by visualizing the elastic transmission channels.…”

Inelastic electron tunneling spectroscopy in molecular junctions showing quantum interference

“…Different bands in Fig. 2 might be attributed to overtones, which are predicted to be dominant spectroscopic features in molecular junction where quantum interference occurs [30]. Overtones of the =C-H deformation can be related to the band arounds 235 meV (1880 cm −1 ) and 250 meV (2100 cm −1 ).…”

“…However, for the case of molecules strongly coupled to the leads, such as in the quantum interference regime, IETS remains a better spectroscopic tool [20][21][22][23][24][25][26][27]. Recent theoretical works have calculated the inelastic contributions to the current in the presence of QI [28] and have explored propensity rules that can play a role in this case [29,30]. QI is not completely quenched by the decoherence due to the inelastic interaction of electrons with molecular vibrational modes; furthermore, the inelastic contributions to the current in the presence of QI is predicted to be larger than in molecules without interference meaning that QI can be exploited to investigate vibrational mode signatures on electronic transport.…”

Nonequilibrium transport through a disordered molecular nanowire

“…Unfortunately, the detailed mechanisms of charge transport in molecular systems driven out of equilibrium are still controversial, posing a complicated scenario for the theoretical description of experiments [14]. For example, it has been argued that, depending on the various energy scales involved (electron bandwidth, zero-point energy of molecular vibrations, thermal energy), electron-phonon coupling may not play a significant role on charge transport even at room temperature, as deduced from inelastic electron tunneling spectroscopy experiments [15][16][17][18]. When the charge carriers interact with low-energy intermolecular modes, they move in a slowly changing potential landscape that gives rise to the so-called transient localization [19].…”