Accurate determination of multiple sets of single molecular conductance ofAu/1,6-hexanedithiol/Au break junctions by ultra-high vacuum-scanning tunnelingmicroscope and analyses of individual current–separation curves

Abstract

The effect of the binding sites of the terminal groups−S on gold on currents through a single molecular junction (MJ) ofAu/1,6-hexanedithiol/Au was studied by measuring current–separation(i–s) curves during repeated formation of a break junction in UHV-STM.Three different single molecular conductance (SMC) values (i.e.Gm(HC),Gm(MC) and Gm(LC)) were found by a careful analysis of corrected current histograms for background tunnelingcurrents using a previously developed robust statistical analysis. Here, HC, MC and LCrepresent a single MJ with high, medium and low conductance, respectively. These threeSMC values are attributed to three different contact modes (i.e. strong–strong, strong–weak(or weak–strong) and weak–weak bindings at the two ends). In addition to these threeSMC values due to the different contacts, another lower SMC value was newlyobserved in the corrected histogram. The presence of the fourth SMC is specific toMJs of alkanedithiols and is attributable to LC of a single alkylene chain withgauche rich conformation, which has a lower SMC value than that of LC withall-trans conformation as proposed previously (Fujihira M et al 2006 Phys. Chem.Chem. Phys. 8 3876). Due to the effects of the contact and the conformationalchange, it was difficult to determine six different SMC values corresponding totwo different conformations (i.e. gauche-rich versus all-trans) with three differentcontacts (i.e. HC, MC and LC). In addition to this complexity, the current stepscorresponding to HC, MC and LC almost always appeared in this order in measuredi–s curves during separation. The current step observed here could not only be a contributionfrom a single molecule, but also contributions from a few groups of molecules that happento link gold atoms of the substrate with those of the tip apex. Therefore, the SMC valuefor HC obtained as a peak or a set of peaks in the current histogram could bebased upon the sum of the current of HC and those of MCs and LCs coexisting inparallel, unless every MJ would change successively from HC to MC and MCto LC. Namely, the currents through coexisting MCs and LCs would raise theintrinsic current observed for HC itself, while those through coexisting LCs wouldraise the intrinsic current for MC. To avoid such errors in determining the trueSMC, we demonstrate here a new method based upon analyses of individuali–s curves referred to as jump height analyses of individuali–s curves. By this method, the true SMC of LC(all-trans) was determined to be 1.6 nS (i.e.Gm (LC,all-trans) of 2.1 × 10−5Go) without ambiguity in spite of the possible presence of LCs(gauche rich) in parallel.