  One-dimensional organic heterostructures consisting of contiguous lines of
CF3- and OCH3-substituted styrene molecules on silicon are studied by scanning
tunneling microscopy and ab initio simulation. Dipole fields of OCH3-styrene
molecules are found to enhance conduction through molecules near
OCH3-styrene/CF3-styrene heterojunctions. Those of CF3-styrene depress
transport through the nearby silicon. Thus choice of substituents and their
attachment site on host molecules provide a means of differentially tuning
molecule and substrate transport at the molecular scale.
