Effect of annealing temperature and capping ligands on the electron mobility and electronic structure of indium oxide nanocrystal thin films: a comparative study with oleic acid, benzoic acid, and 4-aminobenzoic acid

Abstract

The effect of annealing temperature and capping ligands on the electron mobility and electronic structure of indium oxide (In₂O₃) nanocrystals (NCs) was investigated using oleic acid (OA), benzoic acid (BA), and 4-aminobenzoic acid (4ABA). The NCs were deposited on SiO₂/Si wafers for electron mobility measurements using a field effect transistor device, and the annealing temperature (T_Ann) was varied from 150 to 350 °C. At T_Ann = 200 °C, the electron mobility of the BA-capped In₂O₃ NC thin film was greater than that of 4ABA-capped In₂O₃ NCs, while the opposite trend was observed at T_Ann = 250 °C. This difference can be attributed, at the lower annealing temperature, to the π–π interaction in the BA-capped In₂O₃ NC thin film, which is hindered in the ABA-capped In₂O₃ NC thin film owing to its –NH₂ group. At higher annealing temperature, NN bond formation in the ABA-capped In₂O₃ NC thin film confirmed by Raman spectroscopy plays a key role even after significant thermal decomposition of the ligands in the In₂O₃ NC thin films. At T_Ann = 250 °C, the reorganization energy of BA- or 4ABA-capped In₂O₃ NCs estimated in the framework of Marcus theory was very similar to each other, indicating that the ligands decompose almost completely, as confirmed by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The electronic structure was studied by energy-resolved electrochemical impedance spectroscopy (ER-EIS) after annealing the NCs on ITO electrodes at T_Ann = 150 °C, 200 °C, or 250 °C. The valence band peak was observed near −6.8 eV for the BA- or 4ABA-capped In₂O₃ NC films at T_Ann =150 °C or 200 °C, but not at T_Ann =250 °C. However, for the OA-capped In₂O₃ NCs, the peak near −6.8 eV was observed for all annealing conditions. Considering the exclusive perseverance of the carboxylate group in the OA-capped In₂O₃ NCs even at T_Ann = 250 °C, as confirmed by FT-IR and TGA, one attributes the peak at −6.8 eV to an electronic state formed by the electronic interaction between the In₂O₃ NC and the carboxylate groups.