Two-dimensional semiconductors for future optical applications

Carey, B 2017, Two-dimensional semiconductors for future optical applications, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Two-dimensional semiconductors for future optical applications
Author(s) Carey, B
Year 2017
Abstract Field of two dimensional (2D) materials has experienced rapid development and received considerable attention in recent years. Although various methods of producing 2D materials have been presented with ample success across numerous families of materials and for many applications, there are still several factors limiting their performance in various fields. The field of 2D materials is mostly focused on stratified crystals, out of which ultra-thin (a single or few monolayers) planes can be readily extracted. There are however still many unexplored materials which would be suitable candidates in their 2D morphologies for future applications. However, due to their naturally occurring non-stratified crystals, their synthesis and implementation in 2D form have posed significant challenges to date.

Access to colloidal suspensions of 2D sheets provides a pragmatic avenue for numerous applications. The processes for obtaining colloidal suspensions can be scaled up to the industrial scales with relative ease. These processes can be as simple as applying ultrasound energy into a suitable solvent containing bulk layered materials. However, there are still several factors which affect the quality of the sheets within these suspensions and hence their suitability for use in various applications. This issue is highlighted by the fact that several solvents which assist high yield production can also inhibit potential applications due to surface bound solvent residues and breakdown products which can be difficult to remove.

Thus, the first objective of this thesis was to investigate the solvent effects within the ultrasound exfoliation process for obtaining colloidal suspensions of 2D materials. For this tungsten disulphide (WS2) was used as it is a well-known material with a high potential for future implementation across numerous fields. This objective is to attempt alternative strategies for WS2 exfoliation which provide high yield, while also providing less detrimental effects on the desired properties. In this PhD research, comparative evaluation of organic solvents used in grinding assisted ultrasonic exfoliation revealed the importance of the pre-exfoliation grinding step and that careful consideration of the solvent(s) used in both grinding and ultrasonication is necessary. It was shown that such alterations vastly affect the quality of the resultant suspension. Through these considerations it was found possible to provide comparatively high yields of unobstructed 2D WS2.

Although the outcomes of the previous objective provide insight into the solvent effects on production and application of colloidal suspensions of 2D semiconductors, the exfoliation method is limited to materials which exist naturally with a layered (stratified) crystal system. It is not possible to use this exfoliation process non-stratified materials to produce 2D sheets. Thus, the following objective was to develop the processes which may be utilized in the synthesis of 2D semiconductors from non-stratified materials.

In this PhD work, in order to achieve the synthesis of 2D semiconductors for non-stratified materials, the self-limiting oxide layer which exists at the interface of many liquid-metals was utilized. These surface skins are in essence a naturally occurring 2D material and therefore can offer utilization potential for creating large area atomically thin coatings. By manipulating the interfacial layer of a gallium based liquid metal alloy, this PhD research investigates the possibility of forming suspensions of ultrathin non-stratified semiconductors. The investigations shown, demonstrated that with relevant chemistry the interfacial layer could be utilized. A eutectic alloy of gallium, indium, tin and zinc was repeatedly reacted with a solution of sodium polysulphide to form 2D semiconducting zinc sulphide (ZnS).

This concept was extended further to the direct deposition of ultrathin metal oxides. It was shown within this thesis that liquid gallium could be efficiently used to deposit a 2D layer of gallium oxide. The oxide was then chemically treated in vapour phase reactions to form 2D semiconducting gallium sulphide (GaS). Using this concept GaS based optical and electronic devices were developed on the wafer-scale demonstrating the potential for this concept.

In conclusion, this Ph.D. research resulted in several new advances and novel synthesis paths in the field of 2D materials and the methods and outcomes presented herein will serve as launching points for many future investigations into the synthesis and application of 2D materials.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Synthesis of Materials
Optical Properties of Materials
Photonics and Electro-Optical Engineering (excl. Communications)
Keyword(s) Material science
Two-dimensional semiconductors
Two-dimensional materials
Liquid metals
Optoelectronic materials
Materials synthesis
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Created: Wed, 10 Jan 2018, 08:59:36 EST by Keely Chapman
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