  Molecular self-assembly has been extensively used for surface modification of
metals and oxides for a variety of applications, including molecular and
organic electronics. One of the goals of this research is to learn how the
electronic properties of these surfaces can be modified by self-assembled
monolayers (SAM). Here, we demonstrate a new type of molecular self-assembly:
the growth of organosilane SAMs at the surface of organic semiconductors, which
results in a dramatic increase of the surface conductivity of organic
materials. For organosilane molecules with a large dipole moment, SAM-induced
surface conductivity of organic molecular crystals approaches 10^-5 S per
square, which is comparable to the highest conductivity realized in organic
field-effect transistors (OFETs) at ultra-high densities of charge carriers.
SAM-functionalized organic surfaces are fully accessible to the environment
which makes them very attractive for sensing applications. We have observed
that the interaction of vapors of polar molecules with SAM-functionalized
organic semiconductors results in a fast and reversible change of the
conductivity, proportional to the pressure of an analyte vapor.
