Synthesis of two-dimensional indium oxide using liquid metal chemistry

2D metal oxides (2D MOXs) represent an exciting class of 2D materials and have attracted considerable interest due to their distinctive properties, such as excellent optical transparency, high carrier mobility, and good environmental stability. This family of 2D material has been attracting significant attention in various fields of electronics, optoelectronics, sensing, energy storage. Among several 2D MOXs, indium oxide (In2O3) is an important n-type semiconductor and has been intensively investigated and recognized as a promising candidate for the future of electronics and optoelectronics. However, the synthesis of large-area, uniform, and unit-cell-thick indium oxide nanosheets with minimal defects still remains a great technical challenge, limiting its use in transparent and flexible devices. The core of this PhD thesis involves exploring a new printing technique based on liquid metal chemistry that can directly deposit lateral-large and ultrathin indium oxide nanosheets onto desired substrates. This approach provides a low-temperature, low-cost, and vacuum-free synthesis route for realizing ultrathin In2O3 which does not rely on any bulky and expensive instrument. The author also investigates furnace-free doping methods to incorporate a small amount of impurities into the produced 2D In2O3 nanosheets. The doping process can be carried out at low temperature via either wet chemical reaction or selective migration of dopants in liquid metal alloys. The effect of anion and cation doping on the properties of 2D In2O3 was investigated for potential applications such as transistors and photodetectors. In addition, a thermal annealing process is proposed to improve the transparency and electron mobility of the synthesised 2D indium oxide, in which nanostructural changes within the oxide layers during annealing process was investigated. In the first part of this thesis, the author developed a wet chemical method to incorporate sulfur into the structure of 2D indium oxide nanosheets. The process consists of printing indium oxide skins out of molten indium metal and a subsequent sulfur insertion conducted in a trisulfur radical anion solution. The obtained 2D indium oxysulfide (In2O3−xSx) nanosheets with a thickness of ~2 nm were utilized for fabricating back-gated field-effect transistors (FETs), revealing a notably high electron mobility of ∼20 cm2 V−1 s−1. In addition, 2D In2O3−xSx based photodetectors exhibited an excellent performance in ultraviolet (UV) region, with a photoresponsivity of ∼3.4 × 103 A W−1 greatly surpassing that of many commercial materials. More importantly, the same reaction parameters were employed to obtain 2D bismuth oxysulfide and 2D tin oxysulfide, offering a furnace-free approach for 2D oxysulfide semiconductor fabrication. In the second stage of research, the author further explored a one-step synthesis of few-unit-cell-thick and laterally large antimony-doped indium oxide (IAO). The doping process occurs spontaneously when the oxide is grown on the surface of a molten Sb–In alloy and 2D IAO nanosheets can be easily printed onto desired substrates. With thicknesses at the atomic scale, the obtained 2D IAO sheets exhibited excellent transparency exceeding 98% across the visible and near-infrared range. Field-effect transistors based on low-doped IAO nanosheets revealed a high electron mobility of ≈40 cm2 V−1 s−1. Additionally, a notable photoresponse was observed in 2D IAO-based photodetectors under ultraviolet (UV) radiation. Photoresponsivities of low-doped and highly doped IAO at a wavelength of 285 nm were found to be 1.2 × 103 and 0.7 × 103 A W−1, respectively, identifying these materials as promising candidates for the fabrication of high-performance optoelectronics in the UV region. Following the successful investigation of anion and cation dopants for 2D In2O3 nanosheets, the third and final stage of this PhD thesis involved studying the effect of thermal annealing on the electronic and optoelectronic properties of 2D In2O3. A simple approach has been proposed to achieve 2-nm-thick indium oxide nanosheets from liquid metal surfaces by employing a squeeze printing technique and thermal annealing at 250 °C in air. The resulting materials exhibited a high degree of transparency (>99 %) and an excellent electron mobility of ≈96 cm2 V−1 s−1, surpassing that of pristine printed 2D In2O3 and many other reported 2D semiconductors. UV-detectors based on annealed 2D In2O3 also benefited from this process step, with the photoresponsivity reaching 5.2 × 104 and 9.4 × 103 A W−1 at the wavelengths of 285 and 365 nm, respectively. These values are an order of magnitude higher than for as-synthesized 2D In2O3. Utilizing transmission electron microscopy with in situ annealing, it was demonstrated that the improvement in device performances was due to nanostructural changes within the oxide layers during annealing process. This work highlights a facile and ambient air compatible method for fabricating high-quality semiconducting oxides, which will find application in emerging transparent electronics and optoelectronics. Overall, the author successfully demonstrated significant findings in the course of this PhD research, signifying the potential of liquid metal chemistry in the synthesis and applications of 2D materials. The author believes that the outcomes of this PhD research will not only benefit the advancement of nanotechnology, but also contribute to the development of high-performance electronic and optoelectronic devices.