Non-monotonic Size Dependence of Electron Mobility in Indium Oxide Nanocrystals Thin Film Transistor

摘要

Indium oxide nanocrystals (In2O3 NCs) with sizes of 5.5 nm–10 nm were synthesized by hot injection of the mixture precursors, indium acetate and oleic acid, into alcohol solution (1-octadecanol and 1-octadecence mixture). Field emission transmission electron microscopy (FE-TEM), High resolution X-Ray diffraction (Xray), Nuclear magnetic resonance (NMR), and Fourier transform infrared spectroscopy (FT–IR) were employed to investigate the size, surface molecular structure, and crystallinity of the synthesized In2O3 NCs. When covered by oleic acid as a capping group, the In2O3 NCs had a high crystallinity with a cubic structure, demonstrating a narrow size distribution. A high mobility of 2.51 cm2/V·s and an on/off current ratio of about 1.0 × 103 were observed with an In2O3 NCs thin film transistor (TFT) device, where the channel layer of In2O3 NCs thin films were formed by a solution process of spin coating, cured at a relatively low temperature, 350 °C. A size-dependent, non-monotonic trend on electron mobility was distinctly observed: the electron mobility increased from 0.43 cm2/V·s for NCs with a 5.5 nm diameter to 2.51 cm2/V·s for NCs with a diameter of 7.1 nm, and then decreased for NCs larger than 7.1 nm. This phenomenon is clearly explained by the combination of a smaller number of hops, a decrease in charging energy, and a decrease in electronic coupling with the increasing NC size, where the crossover diameter is estimated to be 7.1 nm. The decrease in electronic coupling proved to be the decisive factor giving rise to the decrease in the mobility associated with increasing size in the larger NCs above the crossover diameter.