Charge Transport in Azobenzene-Based Single-Molecule Junctions

Abstract

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.

Rigidity of the conductance of an anchored dithioazobenzene optomechanical switch

“…[7,8,23], namely use of the anchoring methylene -CH 2 -groups added at the ends of the AB molecule. This is important as presence of the methylene groups was made responsible for the small SR found also experimentally [23]. The importance of the methylene groups on the transport has been experimentally studied on related systems by Danilov et.…”

“…Plane waves are deemed important for a better description of the electronic states delocalized over the metallic tips [31]. This is at variance with all previous theoretical modeling [7,8,23,24], which used localized basis sets that were not specifically designed for treatment of nano-junctions or surfaces [32].…”

“…However, high SRs are typically achieved by opening/closing of π-conjugated rings [22], which is not the case in AB. Moreover, up to now, low-temperature charge transport experiments using single-molecule junctions are scarce [22,23]. Hence, direct experimental verification could be provided only recently [23].…”

“…However, the same ABTM molecule was also considered in the theoretical treatment [7,8], and hence, this does not explain the discrepancies between theory and exper-iment. To make things even more involved, recent theoretical modeling of self-assembled ABTM junctions on Au(111) surfaces [24] yielded higher trans than cis conductance, fairly small SR, and transmissions significantly different from those corresponding to mechanically controlled break-junction (MCBJ) [25] type of treatment [23], suggesting that also coupling to electrodes may play an important role. In addition, other studies explored contacting the AB molecule with silicon [26] or carbon nanotube [27] and found even larger SRs of the cis/trans conductance than those reported in Refs.…”

“…The only experiment, also using a particular junction geometry, finds virtually identical conductances for the two junction conformation, albeit with very large error bars. This situation invites a new theoretical study which would sort out the questions, preferably using a different formulation from the previous localized basis set approach [7,8,23,24]. To this end we focus here on the conductance of the trans and cis conformers of embedded 4,4 -dithioazobenzene (DAB, C 12 H 8 N 2 S 2 ), i.e.…”