The Role of Oligomeric Gold–Thiolate Units in Single-Molecule Junctions of Thiol-Anchored Molecules

Abstract

Using the break junction (BJ) technique we show that 'Au(RS)2' units play a significant role in thiol-terminated molecular junctions formed on gold. We have studied a range of thiol-terminated compounds, either with the sulfur atoms in direct conjugation with a phenyl core, or bonded to saturated methylene groups. For all molecules we observe at least two distinct groups of conductance plateaus. By a careful analysis of the length behavior of these plateaus, comparing the behavior across the different cores and with methyl sulfide anchor groups, we demonstrate that the lower conductance groups correspond to the incorporation of Au(RS)2 oligomeric units at the contacts. These structural motifs are found on the surface of gold nanoparticles but they have not before been shown to exist in molecular-break junctions. The results, while exemplifying the complex nature of thiol chemistry on gold, moreover clarify the conductance of 1,4-benzenedithiol on gold. We show that true Au-S-Ph-S-Au junctions have a relatively narrow conductance distribution, centered at a conductance of log(G/G0) = -1.7 (± 0.4).

Cross-conjugation increases the conductance of meta-connected fluorenones

“…We find that the longest plateaus for thiol-terminated junctions tend to exceed the predicted maximum value by 1-2 Å, whereas the pyridyls tend to be shorter than this value by 1-2 Å. This behavior agrees with our previous observations, which for thiols 12 is a result of the strength of the Au-S bond which produces more significant deformation of the electrodes compared with weaker binding groups like pyridyl. For both para fluorene and fluorenone compounds with the same anchor group, the break-off histograms almost coincide (Fig.…”

“…Finally, for the thiol measurements we observed a faint low-conductance group after the main plateau, which we have previously shown arises from junctions involving gold adatoms and sulfur groups from neighboring molecules. 12 To calculate the conductance of each molecule connected to two gold electrodes, the optimal geometry and ground state Hamiltonian were obtained using the SIESTA 16 implementation of density functional theory (DFT) and the room-temperature electrical conductance was calculated using the Gollum 17 code (see computational methods in the ESI). Fig.…”

Doping hepta-alanine with tryptophan: A theoretical study of its effect on the electrical conductance of peptide-based single-molecule junctions

“…For instance, a previous study on homopeptides suggested that bonding could be established via physisorption of a tryptophan unit onto gold, 30 and other binding configurations for the thiolate or carboxylate groups have been explored. [51][52][53][54] In the experiments, variable SAM heights between 20 and 26 Å have been observed. This means that molecules may be slightly tilted or crumpled in the ensemble junctions.…”

Measuring Single-Molecule Conductance at An Ultra-Low Molecular Concentration in Vacuum

“…Substantial research efforts have been devoted to single molecule electronics and related fields since the first current-voltage (I-V) characteristics measurements of benzene-1,4-dithiol [ 1 ] using a mechanically controllable break junction (MCBJ). Various fabrication techniques and device structures were realized aiming to fulfill the requirements for the envisaged applications such as molecular field-effect transistors (FETs), diodes, and thermoelectric materials [ 2 , 3 , 4 , 5 , 6 ]. Nowadays, single molecule measurements have become more and more feasible by virtue of break junction methods wherein scanning probe microscope (SPM) set up is usually utilized to address electron transport through metal-molecule-metal structures in organic solvents [ 3 , 7 ].…”

“…Various fabrication techniques and device structures were realized aiming to fulfill the requirements for the envisaged applications such as molecular field-effect transistors (FETs), diodes, and thermoelectric materials [ 2 , 3 , 4 , 5 , 6 ]. Nowadays, single molecule measurements have become more and more feasible by virtue of break junction methods wherein scanning probe microscope (SPM) set up is usually utilized to address electron transport through metal-molecule-metal structures in organic solvents [ 3 , 7 ]. Meanwhile, from practical viewpoints, such measurements are preferably conducted in dry environments considering that any electronic components would be used in ambient conditions.…”