Abstract
Transition metal dichalcogenides (TMDs) are layered van der Waals materials thathave been exploited in their bulk form in industrial applications for decades. The primary use of these materials was as solid-state lubricants and catalysts. Since the isolation of graphene in 2004, much attention has been drawn to the domain of two-dimensional (2D) materials and their potential applications in optoelectronics. In the last decade, the remarkable properties of atomically thin group VI-TMDs have inspired enormous amounts of research. The properties that are mainly being exploited are their direct band gaps in the visible to near-IR range which open a plethora of avenues for development into future optoelectrical devices. The primary materials that are studied in this thesis are MoS2, MoSe2, WS2, and WSe2.
Whilst fundamental research has been, and will continue to be, conducted on
exfoliated monolayers from bulk crystals, a reliable method to grow large-area high-quality thin films on suitable substrates remains elusive. Of the group VI-TMDs, WS2 presents as a promising candidate due to it best-in-class optical and electrical properties. Whilst there has been rapid progress in growing WS2 via chemical vapour deposition (CVD) using tungsten oxide (WO3) and sulphur (S) powders, film size is nominally limited to a few tens of microns due to poor control of the growth conditions.
In this work, we demonstrate a growth method to reliably deliver large-area highoptical quality thin films via CVD employing gaseous precursors. This has been demonstrated before, using metal-organic precursors such as tungsten hexacarbonyl (W(CO)6), and metal halides such as tungsten hexafluoride (WF6), but both are either highly toxic or produce dangerous waste products (HF). In this work, we employ the use of tungsten hexachloride (WCl6), which produces the much milder waste product, hydrochloric acid (HCl), which is safely dealt with within the CVD reactor.
Further, we investigate a second method of TMD monolayer isolation in the guise of laser treatment. Work has been published in the literature to show that MoS2 can be thinned from few-layer to monolayer by scanning a tuned laser across the surface. However, the literature is noticeably absent of an overarching piece of work that investigates laser processing in numerous different materials. In Chapter 6, we present the isolation of TMD monolayers via laser irradiation for MoS2, MoS2, WS2, and WSe2 in which we uncover some surprising and substantial differences mediated by the accompanying transition metal. Further, we use our laser-processing technique to fabricate lateral-homojunctions to investigate the junction energy band alignments via scanning photocurrent microscopy (SPCM). From the SPCM data, we can infer how the electronic bands align and influence the carrier dynamics at the junction interfaces.
| Date of Award | 11 Sept 2024 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | Oxford Instruments Plasma Technology |
| Supervisor | Simon Bending (Supervisor), Enrico Da Como (Supervisor) & Daniel Wolverson (Supervisor) |