Instead of drop casting, it would be better if you follow the dip coating method for thiols monolayer on the surface. Solution can be evaporated by keeping under vacuum for overnight.

Water will usually not evaporate completely from a surface even under UHV conditions. You will need to heat the sample slightly in vacuum. Organic solvents evaporate much easier in vacuum. So use organic solvents whenever possible.

The process is the dipcoating. The substrate was dip into a 1 mM~ 1μM thiol ethanol solution for 24h. The thiol molecules can arrange well onto the substrate, typically the epitaxial Au(111) on mica. The solvent can be dried by the nitrogen gas.

Use of organic solvents rather than water under high vacuum would be better I think.