Benchmark Study of Alkane Molecular Chains

Abstract

Alkanes serve an important role as benchmark system in molecular electronics. However, a large variation in the conductance values is reported in the literature. To better understand these fluctuations, in this study we measure large molecular data sets (up to 100 000 breaking traces) of a series of alkanes with different lengths and anchoring groups using the mechanically controlled break junction (MCBJ) technique. Employing an unsupervised learning algorithm, we investigate both the time evolution and the distance dependence of the measured traces. For all the molecules considered, we have been able to identify the single-molecule conductance value for the fully stretched molecular configuration. For alkanedithiols, the corresponding extracted β decay constant of 1.05 ± 0.08 per CH 2 group agrees well with literature values. In the case of the stronger thiol bonding, additional peaks in the conductance histograms are found, suggesting the formation of molecular junctions containing a single molecule plus additional gold/molecule unit(s). The results shine light on the dispersion in reported conductance values and show that the evolution of the molecule as a function of stretching and time contains crucial information in determining the molecular junction configuration in MCBJs.

Switching in Nanoscale Molecular Junctions due to Contact Reconfiguration

“…We also analyzed a set of MCBJ measurements on alkanes previously published by our group. 21 Alkanes show a much smaller difference between the conductance states before and after the switch compared to OPE3. This is evident from the 2Dhistograms shown in Supporting Information, Section S2.2 (compare, e.g., the histograms of OPE3-NH 2 in Figure S3 with the ones of C6 in Figure S8 and C8 in Figure S9).…”

“…The same behaviors have been reported by others. 5,7,21 In order to quantify the presence of such switching events, we developed an algorithm that works as follows: the derivative of the conductance is computed in a defined region of interest, and its peaks were identified as possible locations for switching events. Going in order of decreasing peak height, each peak was further analyzed by checking the difference in conductance before and after the switch.…”

“…When inspecting individual traces, most show a continuous plateau until the junction ruptures (blue curves in Figure b), but not all of them: breaking traces can exhibit switching either after an exponential decay (red curves in Figure b) or after a plateau with a different conductance (green curves in Figure b). The same behaviors have been reported by others. ,, …”

“…We also analyzed a set of MCBJ measurements on alkanes previously published by our group . Alkanes show a much smaller difference between the conductance states before and after the switch compared to OPE3.…”

“…For this reason, we analyze the presence of switching events in OPE3s derivatives and in two alkanes (hexane and octane). Both systems are well-studied benchmark molecules in molecular electronics, the former having a rigid conjugated backbone, − the latter having a flexible, non-conjugated one. ,− Additionally, we consider different anchoring groups for each molecule. For OPE3, we use amino (−NH 2 ), pyridine (−Pyr), thiomethyl (−SMe), and acetate-protected thiol (−SAc).…”