Charge
Transfer and Energy Level Alignment at the
Interface between Cyclopentene-Modified Si(001) and Tetracyanoquinodimethane
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Abstract
We
examine how the electronic structure (via synchrotron radiation
XPS, UPS, and NEXAFS) and the molecular orientation (via NEXAFS) of
a strong acceptor molecule, tetracyanoquinodimethane
(TCNQ), change as a function of thickness when it is deposited on
the cyclopentene-covered Si(001)-2×1 substrate. XPS shows
that the monomolecular cyclopentene layer acts as an efficient
chemical protective barrier. All spectroscopies indicate that anionic
TCNQ is formed at (sub)monolayer coverage. However, the transfer
should only concern those CN moieties pointing toward the Silicon
surface. At higher thicknesses, neutral TCNQ is observed. We do not
observe the upward bending of the silicon bands associated with electron
transfer from the substrate to the acceptor molecular that one would
expect for an unpinned Fermi level interface. In fact, donor levels
are likely created within the cyclopentene layer or at its interface
with silicon. The formation of TCNQ<sup>–</sup> is associated
with a strong increase in the work function. The attained value (∼5.7
eV) is independent of the work function of the cyclopentene-modified
Si(001) surface (that varies with Si doping), in agreement with the
integral charge transfer model. Therefore, ultrathin layers of TCNQ
can be used to improve the hole-injection properties of this alkene-modified
silicon surface