Role of metal contacts in the mechanical properties of molecular nanojunctions: Comparativeab initiostudy of Au/1,8-octanedithiol and Au/4,4-bipyridine

Abstract

A comparative study of the mechanical properties of Au/ 4,4Ј-bipyridine ͑4,4Ј BPD͒ and Au/1,8-octanedithiol ͑1,8 ODT͒ molecular nanojunctions is developed using different metal wires and small clusters to represent the metal contact. Rupture of the junction at different bonds is analyzed. While in the case of 1,8 ODT, rupture at Au-Au bonds is always found; in the case of 4 , 4Ј BPD, rupture of a N-Au bond also appears as possible. Comparison of rupture forces, maximum elongations and force constants with the experimental values lead to the conclusion that the most common geometrical arrangement in scanning tunneling microscopy break junctions should be that where the number of Au atoms is of the order of 4. Activation energies for the rupture of these structures are calculated at sample elongations.

A Reversible Molecular Switch Based on the Biphenyl Structure

“…The results obtained from the stretching of the molecular nanojunctions show that the proposed system has mechanical properties similar to those of other thiolate–gold nanojunctions. Bonding energies of ∼1.5 eV for thiolate-bonded systems are in good agreement with calculated values for thiol and thiolate adsorption on Au(111). − Rupture forces of ∼1.9 nN are also in excellent agreement with other calculated values from isotensional stretching of a octanedithiol/ate–gold nanojunction …”

Large-Scale Atomistic Simulations of Environmental Effects on the Formation and Properties of Molecular Junctions

“…Quantum mechanical (QM)-based methods, such as density functional theory (DFT), are capable of accurately resolving molecular-level bonding, but the high computational cost of QM methods may limit the system size, reduce the total number of independent statepoints, and require simplifications to the local junction environment ( e . g ., neglecting monolayer effects, employing ideal electrode geometries, and considering single-molecule junctions only). ,− ,− Additionally, energy minimizations often included in DFT calculations , may produce configurations that are not likely for thermal systems. Methods based on classical force fields have also been used to simulate molecular junctions − and related systems. − Classical force field (CFF) methods ( i .…”

Characterizing molecular junctions through the mechanically controlled break-junction approach

“…60,[66][67][68][69] Careful studies of similar chains occurring when a molecule is present in the junction have shown that metal chains of four atoms long can occur before the junction breaks. 70 Other simulation work has shown the effects of slipping gold-gold bonds ( Figure 3B and C) and molecule-electrode slipping (Figure 3D and E) and twisting ( Figure 3F and G) on conductance in MCBJs. 71,72 Studies like this reveal the molecular MCBJ to be flexible at the electrode junction, and this geometric flexibility has significant influence on molecular conductance as the junction evolves during the process of breaking.…”