  The attachment of redox active molecules to transition metal dichalcogenides
(TMDs), such as MoS2, constitutes a promising approach for designing
electrochemically switchable devices through the control of the material
charge/spin transport properties by the redox state of the grafted molecule and
thus the applied electrical potential. In this work, defective plasma treated
MoS2 is functionalized by a ferrocene derivative and thoroughly investigated by
various characterization techniques, such as Raman, photoluminescence, X-ray
photoelectron spectroscopies, atomic force microscopy (AFM) and
electrochemistry. Furthermore, in-plane and out-of-plane conductive-AFM
measurements (I-V and first derivative dI/dV-V curves) are measured to
investigate the effect of the chemical functionalization of MoS2 on the
electron transport properties. While the conduction and valence bands are
determined at +0.7 and -1.2 eV with respect of the electrode Fermi energy for
pristineMoS2, additional states in an energy range of ca. 0.45 eV below the
MoS2 conduction band are measured after plasma treatment, attributed to
S-vacancies. For ferrocene functionalized MoS2, the S-vacancy states are no
longer observed resulting from the defect healing. However, two bumps at lower
voltages in the dI/dV-V indicate a contribution to the electron transport
through ferrocene HOMO, which is located in the MoS2 band gap at ca. 0.4-0.6 eV
below the Fermi energy. These results are in good agreement with theoretical
density functional theory (DFT) calculations and UV photoelectron spectroscopy
(UPS) measurements.
