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

Abstract

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.

Keywords: ambipolar to unipolar switch; charge transfer complex; field-effect transistors; fullerenes; nanosheets.

Figure 1

Transfer characteristics (drain current (I_D) vs. gate voltage (V_G)) of C₇₀/Fc nanosheets in the dark for positive and negative gate biases (a) without annealing, and (b) after annealing at 80 °C. The solid lines show I_D vs. V_G for various drain–source voltages (V_DS), and the open symbols show the square root of I_D (right vertical axis). Inset: optical microscopy image of the typical FET based on a hexagonal C₇₀/Fc nanosheet with long and short axes.

Figure 2

Transfer characteristics (I_D vs. V_G) of the C₇₀/Fc nanosheets in the dark after annealing.

Figure 3

Diffuse reflectance spectra in the visible and near-infrared regions (K/M stands for Kubelka–Munk function, which is a measure of the absorbance): (a) C₇₀/Fc nanosheets at room temperature; (b) C₇₀/Fc nanosheets after heating to 200 °C; (c) C₇₀/Fc nanosheets after heating to 250 °C; (d) C₇₀ powder; (e) subtraction of the normalized absorption spectra at 610 nm, i.e., (a)–(d).

Figure 4

(a) PYSA spectra of C₇₀/Fc nanosheets and C₇₀ powder; (b) energy level diagrams of C₆₀/Fc nanosheets, Fc powder, C₇₀/Fc nanosheets, and C₇₀ powder.