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

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 (TAnn) was varied from 150 to 350 °C. At TAnn = 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 TAnn = 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 TAnn = 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 TAnn = 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 TAnn =150 °C or 200 °C, but not at TAnn =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 TAnn = 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.