Solution-Based Processing of Post-Transition Metal Dichalcogenide Layered Materials

Lam, David

doi:https://doi.org/10.21985/n2-k8wf-jq69

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Solution-Based Processing of Post-Transition Metal Dichalcogenide Layered Materials

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Van der Waals, or layered, materials offer a flexible platform to tune properties via exfoliation down to the single- or few-layer limit; they are at the forefront of cutting-edge materials science and engineering research because of the innumerable ways to tune materials as a function of thickness or composition. Due to quantum confinement effects, the band structures of many layered materials undergo drastic changes as they are thinned down, resulting in transitions from direct to indirect band gaps, modifications of the size of the band gap and emergence of exotic physics (e.g., spintronics, valleytronics, charge density waves, quantum spin liquids). Furthermore, exfoliation results in increased surface area, ideal for applications in areas like sensing, hydrogen evolution, or energy storage with the resulting nanosheets. In addition, due to their atomically flat nature, they are amendable to the formation of heterostructures, further diversifying the number of ways exfoliated materials can be utilized. Among various top-down techniques facilitating isolation of mono-, bi-, and multilayers of layered materials of interest, liquid-phase exfoliation is one of the most popular as it offers a relatively high-yield production and ease of further properties tailoring based on solution-phase post-processing (e.g., size selection), enabling large-scale industrial production. This thesis will focus on the liquid-phase exfoliation of two-dimensional materials as a whole before delving in detail into two post-transition metal dichalcogenide materials, germanium monosulfide (GeS) and ruthenium trichloride (RuCl3) that pose additional processing challenges compared to their transition metal dichalcogenide counterparts. Liquid-phase exfoliation thus provides researchers with a valuable tool in probing layered materials for both fundamental studies and engineering applications in the ultrathin limit.In the exfoliation of germanium monosulfide, careful processing of the ambient-reactive layered material in an anhydrous, anoxic environment is critical in achieving electrochemically pristine nanosheets. These nanosheets are utilized in the active material of a lithium-ion battery anode, resulting in superlative performance, with excellent cycling stability of up to 1,000 cycles and high specific capacities of 769 mA h g-1 at 1.5 C (C ~ 1,100 mA g-1), with high rate capabilities up to 10 C. In the exfoliation of ruthenium trichloride, a proximate Kitaev quantum spin liquid material, an electrochemical intercalation-assisted exfoliation approach is employed. This facile processing allows for the formation of electrically conductive percolating films, which are then studied via electrical, magnetic, and optoelectronic probes. For example, films of ruthenium trichloride show different behavior than the bulk ruthenium trichloride crystal, with the Néel temperature suppressed down to 2.8 K, and the structural phase transition temperature increased to 230 K. Furthermore, charge transfer arising from intercalation enables near-IR photodetectors based on Mott insulator ruthenium trichloride films stemming from intrinsic monomolecular processes. Finally, discussion on utilizing liquid-phase exfoliated materials as the precursor to subsequent synthesis will offer additional value to the field of two-dimensional materials by describing a possible avenue for obtaining materials that are not exfoliable. Under the paradigm of morphotaxy, liquid-phase exfoliated bismuth selenide is selectively oxidized, yielding signatures of the technologically relevant semiconductor bismuth oxyselenide. Overall, this work contributes to the greater field of two-dimensional materials by outlining several scientific insights into the exfoliation of post-transition metal dichalcogenide layered materials, while simultaneously laying the foundation for additional synthetic methods towards realizing non-van der Waals materials using exfoliated nanosheets.

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