Synthesis, investigation and application of two-dimensional nanosheets from liquid bismuth
Creators: name: MESSALEA, Kibret
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401804
subject: Nanoelectronics

description: This PhD project explores the synthesis and application of two-dimensional (2D) nanosheets derived from liquid bismuth metal and investigates their application for optoelectronic applications. Bismuth metal with a melting point of 271°C is one of the lower melting metals, and due to its environmental friendliness compared to other heavy metals it is increasingly utilised. In contrast to mercury which is liquid at room temperature liquid bismuth is non-toxic and as such may be a suitable alternative for processes at elevated temperatures, with applications in liquid metal facilitated chemistry being particularly intriguing.

Atomically thin, semiconducting transition and post-transition metal oxides are emerging as a promising category of materials for high-performance oxide optoelectronic applications. However, the wafer-scale synthesis of crystalline atomically thin samples has been a challenge, particularly for oxides that do not present layered crystal structures. As my first research contribution, I use a convenient, scalable method to synthesise ultrathin bismuth oxide nanosheets using a liquid metal facilitated synthesis approach. Monolayers of α-Bi2O3 featuring sub-nanometre thickness, high crystallinity, and large lateral dimensions could be isolated from the liquid bismuth surface. The nanosheets were found to be n-type semiconductors with a direct bandgap of ~3.5 eV and were suited for developing ultraviolet (UV) photodetectors. The developed devices featured a high responsivity of ~400 AW-1 when illuminated with 365 nm UV light and fast response times of ~70 µs. The developed methods and obtained nanosheets can likely be developed further towards the synthesis of other bismuth-based atomically thin chalcogenides that hold promise for electronic, optical, and catalytic applications.

As my second research contribution, the synthesised bismuth oxides converted to bismuth sulphides through tube furnace sulphurisation. Two-dimensional (2D) ultrathin stibnite's attracted a range of interest for a variety of emerging applications, including field-effect transistors (FETs), spintronic devices, and low-power electronics. Bi2S3 2D nanosheets as a member of tetradymites and due to quantum confinement effects of ultrathin thickness showed promise to be environmentally friendly optoelectronic nanomaterial.

Furthermore, I report the synthesis of monocrystalline, large-area, scalable 2D nanosheets with a thickness of up to two-unit cells using bottom-up approach liquid metal-based synthesis and tube furnace sulphurisation. The synthesised nanosheets have orthorhombic crystal structure, a direct bandgap of 2.3 eV, and strong optical absorption properties. The nanosheets are highly photoconductive with a photoresponsivity of 8 AW-1, and the designed FETs using Bi2S3 as channel material resulted in maximum hole mobility of 28 cm2V-1S-1. As my third research contribution, I report the stochiometric cubic polymorph of 2D Sb2O3 as an ideal high k dielectric nanosheet using a convenient liquid metal synthesis technique. For practical applications, new methods that enable the large-scale synthesis of 2D Sb2O3 are required. Here I also report a method that allows synthesizing mm-sized cubic Sb2O3 nanosheets with a tuneable thickness between 1.5 and 3 nm in a convenient printing process. A liquid metal-based strategy is deployed, exploiting surface oxidation phenomena that occur on liquid metal interfaces. Since the melting point of antimony is comparatively high (631℃) a bismuth-antimony alloy is used to lower the melting point to 350℃. Preferential oxidation causes the surface of the Bi-Sb melt to be dominated by antimony oxide. Ultrathin α-Sb2O3 could then be deposited onto desired surfaces via liquid metal print transfer. The obtained α-Sb2O3 exhibited high crystallinity and a wide bandgap of ~ 4.4 to 4.6 eV. The relative permittivity was determined via frequency-dependent capacitance measurements and conductive AFM revealing a maximum k of 84, while the breakdown electric field of ~ 10 MV/cm was observed. The isolated 2D α-Sb2O3 nanosheets were utilised in top-gated field-effect transistors that featured low leakage currents, highlighting that the obtained material is a promising gate oxide candidate for conventional and future van der Waals heterostructure based electronics.