Investigating novel synthesis, optical properties and applications of model 2D semiconducting nanocrystals
Creators: name: Atkin, Paul
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401806
subject: Nanomanufacturing
classificationCode: 401807
subject: Nanomaterials
classificationCode: 401810
subject: Nanoscale characterisation

description: The successful synthesis of graphene and other atomically thin materials in 2004 marked the beginning of a period of intense research into atomically thin materials that continues to this day. A wide range of these two dimensional (2D) materials have been explored that demonstrate many novel properties. Of this new family of materials, 2D semiconducting transition metal dichalcogenides (TMDs) such as molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are especially important, owing to their potential for applications including catalysis and novel electronic and optical devices. The transition of these materials from indirect to direct band gap with the change from bulk material to monolayer is just one example of the emergence of new properties. For these reasons, research into these 2D semiconducting materials has been a particularly active area of materials science in recent years. Similarly, transition metal oxides (TMOs) have proven potential for novel applications when reduced to the monolayer, and much research has been, and continues to be, conducted on these materials. The potential for production of large-area 2D stannous oxide (SnO) has been highlighted in recent works, inspiring investigations into viable synthesis methods for the material. 2D SnO promises great potential in sensing, catalysis and transistor technology, provided a successful method of production is available. With these challenges in mind, the author addresses several important aspects of model semiconducting 2D TMDs and TMOs in this Ph.D. thesis.<br><br>In the first stage of the thesis, the author addressed the existing ambiguity in reported explanations for the emergence of high-intensity emission at the edges of 2D WS2 flakes. The two main arguments were that these bright edges are the result of either differing exciton/trion concentration ratios across the 2D WS2 flake area, or the intercalation of H2O between the flake edges and the substrate. This work explains that this is the result of both ambient air H2O intercalation and exciton-dominated emission at these regions, rather than one of these factors alone. Additionally, in the process of this investigation, a previously not defined phenomenon of photoluminescence (PL) loss following high radiant exposure from an incident laser was discovered. The phenomenon was thoroughly investigated, and was found to occur only when laser exposure takes place in ambient conditions, resulting in the formation of sulphate groups on the 2D WS2 surface.<br><br>In the second stage, an investigation into methods for producing hybrid 2D WS2 based nanosheets was carried out. Although the enhancement of the photocatalytic properties of 2D WS2, when hybridised with graphitic carbon dots (CDs), had been investigated to a degree, no studies had yet examined the origin of the photocatalytic effect. Another issue was the synthesis that relied on liquid phase exfoliation, In the established techniques for liquid phase exfoliation of 2D WS2, the existence of solvent residues on the nanoflake surfaces was a main challenge. High quality hybridisation requires pristine, non-functionalised surfaces. A solution to this challenge was demonstrated by the liquid phase ultrasonic exfoliation of bulk WS2 powder in a 35% H2O/ethanol solution. By limiting exposure of WS2 to H2O and ethanol only, presence of solvent residues on the exfoliated 2D WS2 was kept to a minimum. Having achieved this, hybrid nanomaterial synthesis was able to proceed. Attempts to produce a hybrid nanomaterial by creating 2D WS2 and CDs separately and later combining them failed to achieve the necessary bonding between the two components. To solve this problem, the author developed a novel method of CD hybridisation by the addition of citric acid to the 35% H2O/ethanol solution, containing exfoliated 2D WS2, and microwaving this solution to grow CDs on 2D WS2 surfaces. The resulting hybrid nanomaterial was found to have a photocatalytic efficiency approximately 30% higher than that of pristine 2D WS2, and a comparison with similar recently produced photocatalytic 2D TMDs and their hybrid nanomaterials showed this hybrid to have a significantly higher turnover frequency.<br><br>In the final stage, the large-scale synthesis of 2D SnO using a novel method started from molten tin was explored. Recently reported methods of oxide exfoliation from liquid bulk metal were adapted for the synthesis of 2D SnO, and was shown to allow for the creation of very large scale 2D films of SnO. TMOs have been reported as having a superior stability in air relative to that of 2D TMDs, and could be useful in filling roles for which 2D TMDs are not optimally suited. 2D SnO , as a 2D TMO, is currently considered as a material with great potential in applications such as p-type semiconducting thin films, catalytic units, as the channel material in ferroelectric field-effect memories and complementary metal oxide semiconductor (CMOS) devices.<br>In summary, this Ph.D. research has yielded an extensive exploration of several important aspects of model semiconducting 2D TMDs and TMOs. Specifically, the PL properties of 2D WS2 have been explored, along with a novel approach to its enhancement as a photocatalyst via hybridisation and the practicalities thereof. This research has also contributed significantly to synthesis methodology for the production of the important semiconducting 2D TMO, SnO, at large scales.