Effect of Anchoring Group Position on Formation and Conductance of a Single Disubstituted Benzene Molecule Bridging Au Electrodes: Change of Conductive Molecular Orbital and Electron Pathway

Abstract

We investigated the effect of anchoring group position on the formation and electric conductance of single molecule junctions for benzenedithiol and benzenediamine by the scanning tunneling microscope break junction technique. The conductances of the single 1,4-benzenedithiol, 1,3-benzenedithiol, 1,4-benzenediamine, and 1,3-benzenediamine molecules were 0.005 (±0.001) G 0 (G 0 = 2e 2/h), 0.004 (±0.001) G 0, 0.01 (±0.003) G 0, and 0.005 (±0.002) G 0, respectively. No 1,2-disubstituted benzene molecules formed junctions. While the 1,4-position provided larger conductance than the 1,3-position for both anchoring groups, the effect of the anchoring position on conductance was clearer for benzenediamine than benzenedithiol. The resulting anchoring position and its stability are discussed in consideration of the formation of the single molecular junction. The relationship between conductance and anchoring group (position) was analyzed based on ab initio transport calculations. The deformation and change of the energy alignment of the “conductive” molecular orbital give clearer insight to the anchoring position effect than to quantum interference.

Stretching of BDT-gold molecular junctions: thiol or thiolate termination?

“…5,7,[20][21][22][23][24][25][26][27][28][29] In the former, a gold nano-contact is created by stretching a gold wire and, just before rupture, a solution containing the target molecules is added to the system. Subsequently, the metallic contact is further stretched until rupture and in some cases the molecules remain trapped between the Au-Au tips forming the molecular junctions.…”

A quantum circuit rule for interference effects in single-molecule electrical junctions

“…Owing to their small size (on the scale of Angstroms) and the large energy gaps (on the scale of eV), transport through single molecules can remain phase coherent even at room temperature, and constructive or destructive quantum interference (QI) can be utilized to manipulate their room temperature electrical [10][11][12][13] and thermoelectrical 14,15 properties. In previous studies, it was reported theoretically and experimentally that the conductance of a phenyl ring with meta (m) connectivity is lower than the isomer with para (p) connectivity by several orders of magnitude [16][17][18][19][20][21][22][23][24][25] . This arises because partial de Broglie waves traversing different paths through the ring are perfectly out of phase leading to destructive QI in the case of meta coupling, while for para or ortho coupling they are perfectly in phase and exhibit constructive QI.…”

Orientation‐Controlled Single‐Molecule Junctions

“…The measured single-molecule conductance of Au/p/Au junctions is 0.125 G o (AE 0.006), consistent with calculations and two orders of magnitude higher than the measured conductance of a phenyl ring connected by standard anchoring groups. [14,[29][30] We attribute this conductance peak to charge transport perpendicular to the aromatic ring of the mesitylene molecule.…”

“…The single molecule conductance of this Au/p/Au junction is about 0.1 G o , 100-fold more conductive than junctions formed by benzene rings connected by typical standard linkers, for example, thiols or amines. [14,29,30] Our high-resolution STM images show that mesitylene forms a long-range ordered structure on Au(111) (Figure 1 a). A closer look at the STM images (Figure 1 b), as well as the corresponding cross sections (see Figure S1 in the Supporting Information), reveals that mesitylene lies flat on the gold surface.…”

“…Repeated experiments in mesitylene reproduced the % 0.1 G o peak with an average value of 0.125 G o (AE 0.006) as well as the G o peak (Table S1 and Figure S2). One may note that: 1) The conductivity of the benzene derivatives reported so far with Au electrodes are mostly in the range of 0.001-0.01 G o [14,[29][30] and 2) the methyl group is known as a poor anchoring group for gold electrodes. [28] Thus, the conductivity in the range of 0.1 G o cannot be attributed to a mesitylene molecule bridging between gold electrodes via gold-methyl contacts (unless these have remarkably LOW contact resistance).…”