  In this work, we demonstrate the tunability of electronic properties of
Si/SiO2 substrate by molecular and ionic surface modifications. The change in
the electronic properties such as the work function (WF) and electron affinity
(EA), were experimentally measured by contact potential difference (CPD)
technique and theoretically supported by DFT calculations. We attribute these
molecular electronic effects mainly to the variations of molecular and surface
dipoles of the ionic and neutral species. We have previously showed that for
the alkylhalide monolayers, changing the tail group from Cl to I decreased the
work function of the substrate. Here we report on the opposite trend of WF
changes, i.e. increase of the WF, obtained by using the anions of those halides
from Cl$^{-}$ to I$^{-}$. This trend was observed on self-assembled
alkylamonium halide (-NH3$^{+}$ X$^{-}$, where X$^{-}$=Cl$^{-}$, Br$^{-}$,
I$^{-}$) monolayers modified substrates. The monolayer' formation was supported
by Ellipsometry measurements, X-Ray Photoelectron Spectroscopy and Atomic Force
Microscopy. Comparison of the theoretical and experimental data suggests that
ionic surface dipole depends mainly on the polarizability and the position of
the counter halide anion along with the organization and packaging of the
layer. The described ionic modification can be easily used for facile tailoring
and design of the electronic properties Si/SiO2 substrates for various device
applications.
