Charge transfer and energy level alignment at the interface between cyclopentene-modified Si(001) and tetracyanoquinodimethane

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^- 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.