Devices and systems based on two dimensional MoO3 and MoS2
Creators: name: Balendhran, Shiva
Publisher: name: RMIT University
Year: 2024

description: Materials that are composed of atoms which are covalently bonded in two dimensional (2D) planar spaces are considered quasi 2D materials. Graphene is an excellent example of a 2D structure and of great interest due to its enhanced carrier mobilities. The enhancement in carrier mobility is due to the emergence of near massless fermions in 2D graphene. Such large carrier mobilities are of great value for the development of future transistors, as the current technologies for the fabrication of nanostructured silicon transistors are fast exceeding their limits in reducung size and enhancing their performance.The aim of this Ph.D. research is to synthesize and characterize the properties of 2D MoO<sub>3</sub> and MoS<sub>2</sub>, in order to realize 2D electronic devices with high carrier mobilities and understand the behaviour of these 2D materials upon small ion intercalation. The author of this thesis thoroughly reviewed the fundamental properties, as well as methods of synthesis and properties of layered MT&amp;DCs. Based on the review, the author recognized some of the niche areas to be further investigated regarding 2D MoS2. The author identified the lack of a simple and efficient process for the synthesis of layered MoS<sub>2</sub>. Additionally, the author identified 2D molybdenum trioxide (MoO<sub>3</sub>) as the core material suitable for the development of a new family of 2D electronic devices and systems. MoO<sub>3</sub> is an intrinsically high dielectric material with tuneable electronic properties. This tunability can be achieved via facile ion intercalation. In order to realize the aforementioned goals and to create new knowledge, the author implemented his research work in three distinct stages. The first stage involved realizing 2D MoS<sub>2</sub> flakes for the purpose of investigating the structural effects of ion intercalation in comparison to its bulk counterpart. The author developed a simple and effective method of synthesis involving the simultaneous co-evaporation of MoO<sub>3</sub> powder together with sulphur. The MoS<sub>2</sub> was characterized to be high purity layered nanostructures. Subsequently, the author realized 2D MoS<sub>2</sub> via mechanical exfoliation of the as obtained material. The author the conducted electrochemical lithium ion exposure studies and investigated the effects using Raman spectroscopy presenting a new insight regarding the vibrational properties of such systems. In the second stage, the author synthesized layered α-MoO<sub>3</sub> crystals by the thermal evaporation of MoO<sub>3</sub> powder. 2D MoO<sub>3</sub> layers were realized via micromechanical exfoliation of as obtained crystals. The author demonstrated various approaches of H<sup>+</sup> intercalation and partial reduction of MoO<sub>3</sub>, for the purpose of reducing the bandgap to be viable for FET applications. The author then developed FETs based on such 2D MoO<sub>3</sub> flakes with enhanced carrier mobilities exceeding 1100 cm<sup>2</sup>V<sup>–1</sup>s<sup>–1</sup>. Such mobility was significantly larger than the best of those reported for the 2D MoS<sub>2</sub> based FETs and en par with the best mobilities offered by silicon technology. Overall, the author strongly believes that the objectives achieved in this Ph.D. research work, have contributed significantly to the advancement of 2D electronic devices and systems as well as created exciting new knowledge. <br>