Inelastic electron tunneling spectroscopy in molecular junctions showing quantum interference

Destructive quantum interference effect is implemented in large area molecular junctions to improve signatures of electron-phonon interaction. Vertical molecular junctions based on a cross-conjugated anthraquinone layer were fabricated and low-noise transport measurements were performed by acquiring simultaneously the current-voltage characteristics, its second derivative, and the differential conductance. Signatures of vibrational modes excited by inelastic events are present in the whole measured voltage range and superpose to the conductance suppression induced by destructive quantum interference. As a consequence vibrational modes have improved visibility in the low energy window $\left(<80\mathrm{meV}\right)$. Inelastic electron transport spectroscopy data are compared to infrared attenuated total reflection spectroscopy on Au/anthraquinone thin films. Common vibrational modes can be clearly identified, but inelastic electron tunneling spectroscopy reveals the existence of vibrational modes in a wider energy range $\left(0–400\mathrm{meV}\right)$ where infrared spectroscopy is lacking.

Figure 1

(a) Simplified picture of the AQ based molecular junction and scheme of the possible covalent bonds (Au-C and Au-N) between the molecular layer and the base electrode induced by electrografting together with azo-based molecule-molecule bonds in the layer revealed by transport measurements. (b) Current-voltage characteristics $I\left(V\right)$ (red) and differential conductance $G\left(V\right)$ (black) as a function of voltage in an AQ based junction measured at 5 K. The junction area is $\sim 20\times 100\mu {\text{m}}^2$ and the molecular layer thickness is $\sim 8$ nm. (c) Measurement of the second derivative of the current $d^{2}I/d{V}^2$ as function of the voltage $V$ for the same junction at 5 K (black curve). The same data after symmetrization is superposed (red curve).

Figure 2

Normalized second derivative of the current $(d^{2}I/d{V}^2)/(dI/dV)$ as a function of the voltage $V$ in the low (a), intermediate (b), and high (c) voltage ranges. The $x$ axis shows the energy dependence while the upper horizontal axis the wave number dependence.

Figure 3

FWHM as function of the rms ac voltage (black points) for the resonant peak measured at $V=-35$ mV in Fig. 1. The continuous red curve represents the fit of the experimental data following Eq. (1).

Figure 4

(a) IR ATR spectra on the AQ grafted gold surface. Information about $\mathrm{el}\text{−}\mathrm{ph}$ excited modes are associated to peaks. (b) Zoom of the spectra in the low energy region. Most evident vibrational bands are indicated by vertical black lines.