Ambipolar to Unipolar Conversion in C₇₀/Ferrocene Nanosheet Field-Effect Transistors

Organic cocrystals, which are assembled by noncovalent intermolecular interactions, have garnered intense interest due to their remarkable chemicophysical properties and practical applications. One notable feature, namely, the charge transfer (CT) interactions within the cocrystals, not only facilitates the formation of an ordered supramolecular network but also endows them with desirable semiconductor characteristics. Here, we present the intriguing ambipolar CT properties exhibited by nanosheets composed of single cocrystals of C₇₀/ferrocene (C₇₀/Fc). When heated to 150 °C, the initially ambipolar monoclinic C₇₀/Fc nanosheet-based field-effect transistors (FETs) were transformed into n-type face-centered cubic (fcc) C₇₀ nanosheet-based FETs owing to the elimination of Fc. This thermally induced alteration in the crystal structure was accompanied by an irreversible switching of the semiconducting behavior of the device; thus, the device transitions from ambipolar to unipolar. Importantly, the C₇₀/Fc nanosheet-based FETs were also found to be much more thermally stable than the previously reported C₆₀/Fc nanosheet-based FETs. Furthermore, we conducted visible/near-infrared diffuse reflectance and photoemission yield spectroscopies to investigate the crucial role played by Fc in modulating the CT characteristics. This study provides valuable insights into the overall functionality of these nanosheet structures.