Currents through single molecular junction of Au/hexanedithiolate/Au measured by repeated formation of break junction in STM under UHV: Effects of conformational change in an alkylene chain from gauche to trans and binding sites of thiolates on gold

The currents through single molecular bridges of 1,6-hexanedithiolate sandwiched between two gold protruded electrodes were measured by scanning tunneling microscopy (STM) under ultrahigh vacuum. The currents through the single molecules were measured by repeating formation of break junction between an Au(111) substrate covered with 1,6-hexanedithiolate and a gold STM tip, while current-separation (i-s) curves were repeatedly recorded. The gradual increase in the tunneling currents through the single molecules was observed almost every time (ca. 80%) during stretching of the molecular bridges. The increase in the tunneling currents can be attributed to the increase in the single molecular conductivity caused by the change in alkylene chains of 1,6-hexanedithiolate from gauche to trans conformations. The change from the gauche rich (4-5 gauche content in a single hexylene chain) to all-trans conformation resulted in one order of magnitude increase in the observed currents. Between the extreme gauche rich (5 gauche content) and all-trans (0 gauche content) conformations, there are many kinds of conformers (i.e., rotamers) with different gauche contents having different single molecular conductivities. Complexity of the observed currents due to such conformational changes made the study of the effect of Au-S contacts on single molecular conductivities difficult, although the effect was observed clearly for single molecular bridges with a previous rigid pi-conjugated system without the conformational effect (K. Ishizuka et al., Jpn. J. Appl. Phys., 2006, 45, 2037). To solve this problem, new methods are proposed and their usefulness is demonstrated.