The voltage is swept in both directions in the range of between -5 and +5 V using a Keithley 4200-SCS source measure unit. A compliance of 0.1-1 µA is set to limit the current, preventing damage to both the probe and the SAM. For each molecular length of alkanethiol, at least 10 junctions were formed (on different locations) with 40 sweeps per junction with a current compliance applied, giving a total sample size of N = 383-600 for the I-V or J-V curves. For each alkanethiol, 3-5 different droplets were used to check for consistency between droplets. See Supporting Figures S12, S13 and Supporting Table for details of the statistical analysis.