Electronics and Chemistry:  Varying Single-Molecule Junction Conductance Using Chemical Substituents

Abstract

Abstract:We measure the low bias conductance of a series of substituted benzene diamine molecules while breaking a gold point contact in a solution of the molecules.Transport through these substituted benzenes is by means of non-resonant tunneling or super-exchange, with the molecular junction conductance depending on the alignment of the metal Fermi level to the closest molecular level. Electron donating substituents, which drive the occupied molecular orbitals up, increase the junction conductance while electron withdrawing substituents have the opposite effect. Thus for the measured series, conductance varies inversely with the calculated ionization potential of the molecules.These results reveal that the occupied states are closest to the gold Fermi energy, indicating that the tunneling transport through these molecules is analogous to hole 2 tunneling through an insulating film.Understanding the transport characteristics of molecules bonded between metal electrodes is of fundamental importance for molecular scale electronics.1 It is well known that these transport characteristics are influenced by the intrinsic properties of the molecules, including their length, conformation, the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and the Figure 1A). These steps are due to conduction through a single molecule bridging the gap between the two Au pointcontacts. Repeated measurements give a statistical assessment of the junction properties presented as conductance histograms with the peak in the histogram representing the most probable measured conductance value for the molecular junction. Figure 1B shows the conductance histograms for two molecules: 2-chloro 1,4 diaminobenzene and 2-methoxy 1,4-diaminobenzene constructed from thousands of consecutively measured conductance traces without any data selection or processing 11,12 . These histograms show clear peaks both at multiples of G 0 , and at a molecule dependent value below G 0 .The shift in the peak positions in Figure 1B shows the change in the conductance of the two molecules, differing only in their substituents on the benzene ring. We have performed the same experiment with nine other substituted 1,4 diaminobenzenes (listed in Table 1) and find that their conductance depends on the nature of the substituent. For all the substituents tested, we have also performed control experiments on molecules with the substituent groups at the 1 and 4 positions (for example with 1,4 dicyano benzene) to ensure that the substituent groups does not bind to broken gold point-contacts. For all the substituents listed in Table 1, the histograms measured with the 1,4 disubstituted benzenes do not differ from the control histograms in solvent alone. However for carboxylic group (COOH) and nitro group (NO 2 ), we find that the histograms measured with the 1,4 disubstituted benzenes differ considerably from the control histograms in clean solvent indicating that these groups bind to gold and hence we do not include t...

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