Controlling the Properties of n-Alkanethiolate Self-Assembled Monolayers on Gold by Using Supercritical Carbon Dioxide−Ethanol Mixtures as Solvents

We report the use of ethanol−supercritical carbon dioxide mixtures as solvents in the formation of n-alkanethiolate self-assembled monolayers (SAMs) on gold. Results from reflectance−absorption infrared spectroscopy and electrochemical impedance spectroscopy reveal that the SAMs formed from these environmentally friendly solvent mixtures are highly crystalline and exhibit large resistances against the diffusion of redox probes. Increasing the percentage of ethanol in the solvent results in SAMs with larger average cant angles and lower charge-transfer resistances, consistent with a greater density of defect sites within the film. The ability to control the defect density within the SAM by simply altering the ethanol−carbon dioxide solvent composition represents a straightforward and environmentally friendly strategy for tuning the properties of these molecular films.