Measurements of Single-Molecule Conductance by Conductive Atomic Force Microscopy with Tactile Feedback: the Effect of the Cantilever Force Constant

Abstract
In the field of molecular electronics, c-AFM BJ (conductive atomic force microscopy break junction) is one of the methods employed to create molecular junctions of metal–molecule–metal configurations through which single-molecule conductance can be measured. Reported in this thesis work is, for the first time, the acquisition of single-molecule conductance can be correlated with the applied stretching force at the molecular junction by taking advantage of a tensile sensor integrated in the c-AFM. Alkanedithiols (HS(CH2)nSH, n = 6, 8, and 10) are the model compounds and the consistence of their conductance values with literature reports validates this methodology. We construct two-dimensional (2D) histograms from the conductance and force traces using three kinds of force constant of tip (k = 40, 0.3, 0.03 N/m). We have found that (1) The conductance is independent of stretching force at k = 40 N/m. (2) The distribution of conductance become narrow with increasing stretching force at k = 0.3, 0.03 N/m. (3) Multi sets of conductance values are found at k = 0.3, reflecting the reduction of thermal fluctuation by using smaller force constant of tip.
A force feedback system, PicoAngler, for fixing tip-substrate spacing to monitor the conductance of single molecular junction with a long lifetime (≈ 450 s) that enables detailed nature of the conductance of single molecules to be gained. When the stretching force is small (0.2 (± 0.1) nN), multi sets of conductance in I-t and I-V curves are produced due to change of conformation in an alkylene chain and binding sites (bridge or the hollow site) of thiolates on gold. On the contract, when the stretching force is large (0.8 (± 0.1) nN), conductance is centralized, resulted from only elongated conformers and the atop binding site.