Stability and current-induced local heating of single molecular junctions.

摘要

A fundamental and technologically significant issue in molecular electronics, how stable molecular junctions are in the operation, has received little attention. This dissertation presents a study of the stability and current-induced local heating of single molecule (n-alkanedithiol) junctions formed via gold-thiol (Au-S) bonds in toluene, using the scanning tunnelling microscopy (STM)/conducting atomic force microscopy (CAFM) break junction approach at room temperature.; First, the breakdown mechanism of single molecular junctions is studied by measuring the stretching distance and breakdown force with the stretching rate. The non-monotonic dependence is well described by a thermodynamic bond-breaking theory. Several comparative studies demonstrate that the breakdown most likely happens at a Au-Au bond near the Au-S contacts.; Second, by fitting the experimental data with the thermodynamic bond-breaking theory, the average natural lifetime is extracted as 0.05-0.1 seconds, shorter than the observed lifetime. This apparent contradiction is explained by the observation of the bond reformation, which prolongs the lifetime of molecular junctions. The distribution of the dissociation energy barrier is broad, owing to the diverse atomic contact geometries, which leads to an extremely broad distribution in the lifetime varying from nano seconds to several days. This finding indicates that although the molecular junctions are short lived on average, certain contact geometries are considerably more stable.; Third, current-induced local heating leads to a finite increase in the local temperature in the single molecular contact. The local temperature is evaluated by measuring the breakdown force and stretching distance as a function of bias and molecular length. It has been found that for a given molecule, the effective local temperature increases with applied bias, and then decreases after reaching a maximum. At a fixed bias, the effective temperature decreases with molecular length. These experimental findings are in good agreement with hydrodynamic predictions which include both electron-phonon and electron-electron interactions.