Electronic Properties of Electroactive Ferrocenyl-Functionalized MoS₂

The attachment of redox-active molecules to transition metal dichalcogenides, such as MoS2, constitutes a promising approach for designing electrochemically switchable devices through the control of the material's 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, and 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 ∂I/∂V–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 to the electrode's Fermi energy for pristine MoS2, additional states in an energy range of ≈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 ∂I/∂V–V indicate a contribution to electron transport through ferrocene's highest occupied molecular orbital, which is located in the MoS2 band gap at ≈0.4/0.6 eV below the Fermi energy. These results are in good agreement with theoretical density functional theory calculations and UV photoelectron spectroscopy measurements.