Anchoring Group Effects

As can be seen from equation 1.1, the nature of the metal-molecule coupling is one of the fundamental factors that determine the coherent transport in a molecular junction. The strength of this coupling can be tuned chemically, to a certain degree, by using appropriate anchoring groups. To date thiol groups are the anchoring groups most often used to form connections to metal leads, such as gold electrodes, due to their high affinity and strong covalent bonding to gold. Lately, however, it has been argued that the variability in the bonding between thiol groups and gold may be detrimental for the reliability of measurements of single molecule conductance. Venkataraman and coworkers have investigated the potential use of amine groups to bind molecules to gold.33 They measured the conductance of benzenediisonitrile, benzenedithiol and benzenediamine using the STM BJ method and reported that the latter gave a strikingly well defined peak in the conductance histogram, whilst the other two gave no well defined trend and a broad peak respectively. However, an estimation of the contact resistance for two amine-Au bonds show it to be a factor of ten times larger than the value for alkanethiols.

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Figure 1.21 Normalised conductance histograms of over 3000 traces measured in the presence of 1,4-benzenediamine (blue), 1,4-benzenedithiol (red) and 1,4- benzenediisonitrile (green) on a log-log plot. The yellow line represents a control

experiment where no molecules were present. Reprinted with permission from Venkataraman et al.33 Copyright (2006) American Chemical Society.

The lack of a well defined peak for dithiols and diisonitriles in this work was attributed to variation in the binding to gold, as well as oxidative disulfide formation for thiols and ease of oligomerization for diisonitriles. Lörtscher and coworkers have since reported conductance measurements of dithiols using acetyl protected precursors, which are hydrolysed at the electrode surface thus avoiding disulphide formation.34

Tao et al. have systematically studied the effect of anchoring groups on the conductance of single molecules; they used the STM BJ method to measure the conductance of alkanes terminated with dithiol, diamine and dicarboxylic acid groups.35 These anchoring groups bind to gold surfaces with differing strength. The Au-S bond is known to be a strong covalent bond, whilst the Au-NH2 binding is thought to be a weak covalent interaction. The Au-COOH bond is not yet fully understood, but it is thought to involve both ionic and coordination interactions. The strength of these interactions is demonstrated by the variation in their experimental stretching lengths, which suggest that the S-Au contact is hardest to break and COOH-Au the easiest, whilst NH2-Au is in between.

Since tunnelling is the expected mechanism for these molecules, their length dependence can be described by equation 1.5. A systematic study of the length dependence was used to determine values of Gcon, which reflect the contact

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diamine and dicarboxylic-acid terminated alkanes respectively. Thus, the contact resistance appears to correlate with the binding strengths of the three contacts; stronger binding gives lower contact resistance. One factor that might affect the strength of the electronic coupling between the molecule and the contact is the size of the anchoring group. However, the variation in size in these molecules is much smaller than the corresponding variation of contact resistance. Thus, size alone does not seem to explain the observation. It is noteworthy that no issue was reported in this work regarding the measurement of single molecule conductance of dithiols.

Figure 1.22 Logarithmic plots of single molecule conductance vs. molecular length. These plots allowed the determination of both β and Gcon. Reprinted with permission

from Tao et al.35 Copyright (2006) American Chemical Society.

Tao et al. also observed differences in the magnitude of β, which is a measure of the decay with increasing length. They found that the anchoring groups differed with the trend β(dithiol) > β(diamine) ≥ β(dicarboxylic acid). Since both experimental evidence and theoretical calculations indicate that β depends on the alignment of the molecular energy levels relative to the Fermi energy of the electrodes, it can be deduced that anchoring groups change the energy level alignment.

The inherent nature of the amine and carboxylic acid anchoring groups mean that their binding to gold is expected to be dependent on the pH in which the measurement is carried out. For both diamine- and dicarboxylic acid- terminated alkanes the pH has a significant effect on either molecular junction formation or the conductance value; this is due to the protonation/deprotonation of the anchoring groups. Dithiol-terminated alkanes, contrastingly, show no significant pH dependence of conductance.

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Kim and coworkers have shown that the variation in contact resistances between anchoring groups is caused by differences in the offset between the energies of the HOMO and the Fermi level of gold.36 They used a conducting probe atomic force microscope to measure transport, and ultraviolet photoemission spectroscopy to probe the electronic structure of conjugated oligoacenes terminated with thiols or isocyanides. They found that the Au-CN contact was more resistive than Au-S, and that this difference correlates with the HOMO of the isocyanide series being lower in energy (relative to the Fermi level of gold) than the HOMO of the thiol series.

Venkataraman and coworkers have recently demonstrated a method of creating junctions with a direct Au-C covalent bond, thus removing the inherent resistance of the anchoring group.37 They synthesised a series of SnMe3-terminated polymethylene chains and measured their conductance using the STM BJ method. The conductance of a molecular junction formed using 1,4-bis- trimethylstannylbutane was shown to be about 0.1G0, which is approximately 100 times larger than that achieved with 1,4-diaminobutane. The results of this study strongly suggested that the SnMe3 groups are cleaved in situ, leading to the formation of covalent Au-C bonds. These new direct Au-C contacts have since been employed to measure the single molecule conductance of a series of oligophenyl molecules, with the backbone containing two to four phenyl units.38 The conduction mechanism of these molecules was shown to be tunnelling, and the decay constant shown to be 1.9 per phenyl unit.