The Formation and Conducting Mechanism of Imidazole‐Gold Molecular Junctions

Abstract

The formation mechanism and electronic transport properties of imidazole sandwiched between gold electrodes are investigated using the non‐equilibrium Green's function formalism combined with density functional theory. Our calculations show that the imidazole radical bridges the gold electrodes and the covalent Au−N bonds formed at the molecule‐electrode interfaces enhance the junction stability. In contrast, adding one more electron to the junction, which corresponds to the imidazole radical anion, will increase the antibonding orbital population and thus weaken the junction stability. The calculated low‐bias conductance values of junctions with a single imidazole radical and two imidazole radicals linked in series through one gold atom are in excellent agreement with the experimental measurements, which are mainly contributed by the σ‐type Au−N bonds. These findings deepen our understanding of the structure‐property relationship of imidazole‐gold molecular junctions and facilitate the design of future imidazole‐based molecular electronic devices.

In Situ Tuning of the Charge-Carrier Polarity in Imidazole-Linked Single-Molecule Junctions

“…19−22 Recently, we have reported that the Au−N bond is a donor−acceptor one when an imidazole molecule binds to the gold electrode via the pyridine-like nitrogen atom. 23 In this case, the density of states (DOS) projected onto the imidazole molecule indicates that the LUMO is much closer to E F than the HOMO. Interestingly, once the hydrogen atom bonded to the pyrrole-like nitrogen is removed, the Au−N bond becomes covalent, and now it is the HOMO that is closer to E F .…”

“…The hydrogen atom bonded to the latter can be removed either in basic solutions or by exposing it to oxygen. , Apart from the extensive studies in biology and electrochemistry, imidazole is winning growing attention as an aurophilic anchoring group in single-molecule junctions. It has been experimentally confirmed that both the nitrogen atoms in the imidazole moieties address reliable binding sites to form stable single-molecule junctions with gold electrodes. − Recently, we have reported that the Au–N bond is a donor–acceptor one when an imidazole molecule binds to the gold electrode via the pyridine-like nitrogen atom . In this case, the density of states (DOS) projected onto the imidazole molecule indicates that the LUMO is much closer to E F than the HOMO.…”

“…In this case, the density of states (DOS) projected onto the imidazole molecule indicates that the LUMO is much closer to E F than the HOMO. Interestingly, once the hydrogen atom bonded to the pyrrole-like nitrogen is removed, the Au–N bond becomes covalent, and now it is the HOMO that is closer to E F . In light of this feature, we expect that the hydrogen atom bonded to the pyrrole-like nitrogen can act as a switch to tune the polarity of the charge carriers in imidazole-linked single-molecule junctions.…”

“…Interestingly, once the hydrogen atom bonded to the pyrrole-like nitrogen is removed, the Au−N bond becomes covalent, and now it is the HOMO that is closer to E F . 23 In light of this feature, we expect that the hydrogen atom bonded to the pyrrole-like nitrogen can act as a switch to tune the polarity of the charge carriers in imidazole-linked singlemolecule junctions. Herein, we investigate the electronic transport properties of single-molecule junctions composed of imidazole-capped molecules and gold electrodes by employing the nonequilibrium Green function formalism combined with density functional theory (that is, the so-called NEGF+DFT approach).…”

“…As shown in Figure 1b, the bond lengths of the Au− N donor−acceptor bonds are optimized to be 2.25 Å. 23 Then the dehydrogenation reaction is applied: we remove the two hydrogens bonded to the pyrrole-like nitrogen atoms and reoptimize the atomic structure of the junction while keeping the electrode-to-electrode separation unchanged. Upon dehydrogenation, the unshared lone pairs will transfer from the two Au−bonded nitrogen atoms to the other two nitrogen atoms in the imidazole anchors by electronic relaxation.…”