Charge Transport in Azobenzene-Based Single-Molecule Junctions

Azobenzene-derivative molecules change their conformation as a result of a cis-trans transition when exposed to ultraviolet or visible light irradiation and this is expected to induce a significant variation in the conductance of molecular devices. Despite extensive investigations carried out on this type of molecule, a detailed understanding of the charge transport for the two isomers is still lacking. We report a combined experimental and theoretical analysis of electron transport through azobenzene-derivative single-molecule break junctions with Au electrodes. Current-voltage and inelastic electron tunneling spectroscopy (IETS) measurements performed at 4.2 K are interpreted based on first-principles calculations of electron transmission and IETS spectra. This qualitative study unravels the origin of a slightly higher conductance of junctions with the cis isomer and demonstrates that IETS spectra of cis and trans forms show distinct vibrational fingerprints that can be used for identifying the isomer.

Figure 1

(a) Conductance traces of cis and trans isomers of AzoTM SMJs are obtained by applying 70 mV of dc voltage. The $I\mathrm{\text{−}}V$ curves of (b) cis and (c) trans isomers with fitting curves (black solid line). The curves are vertically offset for enhancing clarity. The single-level model fitting parameters are (from top to bottom) for cis (b) $\Gamma =0.21\mathrm{meV}$, $|E_0|=0.27\mathrm{eV}$; $\Gamma =0.24\mathrm{meV}$, $|E_0|=0.24\mathrm{eV}$; $\Gamma =0.21\mathrm{meV}$, $|E_0|=0.30\mathrm{eV}$ and for trans (c) $\Gamma =0.22\mathrm{meV}$, $|E_0|=0.55\mathrm{eV}$; $\Gamma =0.31\mathrm{meV}$, $|E_0|=0.62\mathrm{eV}$; $\Gamma =0.34\mathrm{meV}$, $|E_0|=0.59\mathrm{eV}$. In the inset of (b) and (c), the structures of AzoTM SMJs are presented.

Figure 2

Transmission curves for (a) cis and (c) trans isomers for different bridging distances. The dash-dotted lines in (a) and (c) indicate the representative single-level model and Fano resonance fits for cis and trans, respectively. The corresponding dominant transmission eigenchannels, i.e., isosurface of the scattering state incoming from the bottom side, for (b) cis ($L_2=17.72\AA$) and (d) trans ($L_2=19.48\AA$) isomers are shown.

Figure 3

(a) Experimental and (b) theoretical symmetrized IETS spectra for cis and trans isomers (averaged over several configurations) at temperature $T=4.2\mathrm{K}$, and 6 meV for experimental and 5 meV for theoretical ac modulation, respectively.