Single-Molecule Conductance Measurements

Conductance measurements for ferrocene-based molecular junctions were done using a customized STM-BJ setup that is described in detail before^{45 (link)}. A piezo actuator, used to drive a Au tip, is pushed to a Au substrate, forming a Au–Au contact with a conductance greater than 1G₀ (1G₀= 2e²/h, the quantum of conductance). Subsequently, the Au tip is retracted rupturing the contact, allowing a molecule to bridge the gap between two Au electrodes, forming a single-molecule junction at a rate of 20 nm s⁻¹. A bias voltage is applied and the resulting current is measured to yield a conductance (= current/voltage) trace as a function of relative tip-substrate displacement at an acquisition rate of 40 kHz. This process is repeated thousands of times to obtain statistically reproducible data that is presented as conductance histograms. For the measurements reported here, we use solutions of the molecules in propylene carbonate (polar) and tetradecane (non-polar) solvents under ambient conditions at room temperatures. In polar solvents, the measurements generate capacitive and Faradaic background currents. The STM tip is therefore coated with wax to reduce the exposed surface area to under ∼10 μm^{246 (link)}. Additionally, due to the large difference between the exposed surface areas of the coated tip and bare Au substrate, the voltage drop across the molecular junction is asymmetric, allowing in situ control of the redox state of the ferrocene derivatives^{47 (link)}. The standard deviation calculated from the histogram peak positions generated from sets of 100 traces is 2–6%.