Liquid metal derived 2D post-transitional metal (In, Sn, Bi) compounds and their sensing properties
Creators: name: Guo, Xiangyang
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401605
subject: Functional materials
classificationCode: 401807
subject: Nanomaterials
classificationCode: 510404
subject: Electronic and magnetic properties of condensed matter; superconductivity

description: Among the post-transitional metals, 2D liquid metals of indium (In), tin (Sn), and bismuth (Bi) are representatives. These metals have melting points of 156.6℃, 231.93℃, and 271.3℃, respectively, and can be easily heated above their melting points using a laboratory hotplate. Liquid metals have emerged as a promising method for synthesizing semiconductor oxides and sulfides. Various compounds, such as SnS, In2O3, and TeO2, have been synthesized with outstanding properties, including piezoelectricity, photoelectricity, transistors and catalysts. Despite the significant progress made in the application of liquid metals, their potential in gas sensing and piezoelectricity remains largely unexplored, especially in 2D post-transitional metal (In, Sn, Bi) compounds. Therefore, the present study focuses on exploring the properties of 2D post-transitional metal compounds due to their promising potential outcomes. In this PhD thesis, the author establishes a new method to create gas sensor by liquid metal printing, this method can transfer oxide skin of molten indium to substrate in seconds. Various non-stratified 2D materials can be obtained from liquid metal surfaces that are not naturally accessible. Homogenous atomically flat formatted on the interfaces of liquid metals with air produces unprecedented high-quality oxide layers that can be transferred onto desired substrates. The atomically flat and large areas provide large surface-to-volume ratios ideal for sensing applications. Versatile crucial applications of the liquid metal-derived 2D oxides have been realized; however, their gas-sensing properties remain largely underexplored. The cubic In2O3 structure, which is nonlayered, can be formed as an ultrathin layer on the surface of liquid indium during the self-limiting CabreraMott oxidation process in the air. The morphology, crystal structure, and band structure of the harvested 2D In2O3 nanosheets from liquid indium are characterized. Sensing capability toward2 several gases, both inorganic and organic, entailing NO2, O2, NH3, H2, H2S, CO, and Methyl Ethyl Ketone (MEK) are explored. A high ohmic resistance change of 1974% at 10 ppm, fast response, and recovery times are observed for NO2 at an optimum temperature of 200 ℃. The sensing fundamentals are investigated for NO2, and its performances and cross-selectivity to different gases are analysed. The NO2 sensing response from room temperature to 300 ℃ has been measured and discussed, and stability after 24 hours of continuous operation is presented. The results demonstrate liquid metalderived 2D oxides as promising materials for gas sensing applications. 

After investigating the 2D indium oxide by liquid metal for gas sensing application, this PhD research is extended to explore other liquid metal oxide compounds which form on the surface of bismuth liquid metals. Atomically thin, mechanically flexible, memory-functional and power-generating crystals play a crucial role in the technological advancement of portable devices. However, the adoption of these crystals in such technologies is impeded by expensive and laborious synthesis methods, as well as the need for large-scale, mechanically stable, and air-stable materials. Here we present an instant-in-air liquid metal printing process utilizing liquid bismuth (Bi), forming naturally occurring, air-stable, atomically thin, mechanically flexible nanogenerators and ferroelectric oxides. Despite the centrosymmetric nature of the monoclinic P21/c system of achieved α-Bi2O3-δ the high kinetics of liquid metal synthesis leads to the formation of vacancies that disrupt the symmetry which was confirmed by density functional theory (DFT) calculations. The polarization switching was measured and utilized for ferroelectric nanopatterning. The exceptional attributes of these atomically thin multifunctional stable oxides, including piezoelectricity, mechanical flexibility, and polarizability, present significant opportunities for developing nano-components that can be seamlessly integrated into a wide range of devices. 

In the final parts, the author of this PhD thesis discovered the possibility to doping modification of 2D intrinsic oxide materials which have been synthesised before, gas–liquid reaction phenomena on liquid-metal solvents can be used to form intriguing 2D materials with large lateral dimensions, where the free energies of formation determine the final product. A vast selection of elements can be incorporated into the liquid metal-based nanostructures, offering a versatile platform for fabricating3 novel optoelectronic devices. While conventional doping techniques of semiconductors present several challenges for 2D materials. Liquid metals provide a facile route for obtaining doped 2D semiconductors. In this work, we successfully demonstrate that the doping of 2D SnS can be realized in a glove box containing a diluted H2S gas. Low melting point elements such as Bi and In are alloyed with base liquid Sn in varying concentrations, resulting in the doping of 2D SnS layers incorporating Bi and In sulphides. Optoelectronic properties for photodetectors and piezoelectronics can be tuned through the controlled introduction of selective migration doping. The structural modification of 2D SnS results in a 22.6% enhancement of the d11 piezoelectric coefficient. In addition, photodetector response times have increased by several orders of magnitude. Doping methods using liquid metals have significantly changed the photodiode and piezoelectric device performances, providing a powerful approach to tune optoelectronic device outputs. In conclusion, during the author's PhD research period, several novel discoveries were successfully verified, and new method for synthesizing piezoelectric materials based on liquid metals were proposed. Additionally, numerous devices with extraordinary properties were created. The results of this research have the potential to inspire new directions in various industries, including gas sensing, piezoelectricity, and photoelectricity.