Substrate and Monolayer Preparation. Silicon substrates were
cleaned by immersion in piranha solution (conc. H2SO4 and 33%
H2O2 in a 3:1 volume ratio; Warning! Piranha should be handled
with caution; it can detonate unexpectedly!) for 15 min to form a
SiO2 layer on the surface. The substrates were then sonicated in
Milli-Q water and ethanol for 1 min, and dried with a stream of N2.
Amino-terminated SAMs (NH2-SAMs) were obtained by gas-phase
evaporation of APTES in a desiccator under vacuum for several
hours, and then carefully rinsed with ethanol and Milli-Q water.