Structure-material property relationships and applications of low dimensional metal-chalcogenides nanostructures
Creators: name: Dhar, Nripen
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401605
subject: Functional materials

description: The separation and optimization of graphene in 2004 by Geim and Novoselov boosted the interest in low dimensional materials within the world of electronics. One specific class of low dimensional materials is the metal chalcogenides (MCs) (i.e MoSx, Bi2S3, Bi2Se3), which has drawn notable attention owing to their tuneable exotic properties and natural abundance. This PhD thesis emphasises on several major features of low dimensional metal chalcogenides: their exfoliation techniques, growth behaviour and ultimately augmenting their fundamental properties. Due to the dimension dependent properties of metal chalcogenides, it is essential to attain full control over their lateral shape and thickness during the exfoliation process. Regardless of many rcent improvements, there are still plenty of opportunities to develop enhanced exfoliation processes, which is one of the primary focuses of this work.<br><br>The main goals of this Ph.D. research are divided into two parts. First, developing a facile synthesis process for amorphous 1D inorganic polymeric materials such as amorphous molybdenum sulphide (a-MoSx) and investigate the fundamental properties of as synthesised nanostructures. The preparation route and synthesis of low dimensional materials play a significant role, that ultimately defines the material's properties. To date, many efforts have been invested in order to device an appropriate exfoliation procedure, resulting in the development of various techniques and processes which feature their respective benefits and flaws. In this work, a template acidification process was used to synthesize a-MoSx. The resulted synthesized nanostructures were found to be polymeric and one dimensional in nature. Sulphur rich molybdenum sulphides are an emerging class of inorganic coordination polymers which are predominantly utilised for their superior catalytic properties. Here the surface water dependent properties of sulphur rich a-MoSx and its interaction with water vapour were investigated. It has been observed that a-MoSx is a highly hygroscopic semiconductor. The presence of surface water is found to have profound influence on the semiconductor's properties, modulating the material's photoluminescence by over one order of magnitude, in transition from dry to moist ambient. Additionally, the conductivity of the synthesized film increases by several magnitudes as the surrounding humidity increases. As the core application, the newly discovered properties of a-MoSx has been utilized to develop an electrolyteless water splitting photocatalyst that relies entirely on the hygroscopic nature of MoSx as the water source, leading to efficient gas phase water splitting. Moreover, taking advantage of the hygroscopic nature of synthesized a-MoSx, a low energy dehumidification device has been designed that exhibits a-MoSx can be a prominent candidate for moisture sensing devices.<br><br>The second goal of the work was to develop as facile synthesis process for crystalline low dimensional material such as Bi2S3 and Bi2Se3. Here the focus of this work was a family of crystals constituting covalently bound strings, held together by van der Waals forces, which can be exfoliated into smaller entities, similar to crystals made of van der Waals sheets. Depending on the anisotropy of such crystals, and the spacing between their strings in each direction, van der Waals sheets or ribbons can be obtained after the exfoliation process. In this work, it has been demonstrated that ultra-thin nanoribbons of bismuth sulfide (Bi2S3) can be synthesized via a high power sonication process. As stated earlier, preparation and exfoliation process of low dimensional materials are very crucial as the quality and individual properties of the nanostructure depends on it. Here, N-methyl-pyrrolidine was used as a solvent that was proven to be the most effective solvent for this process. The thickness and width of these ribbons are governed by the van der Waals spacings around the strings within the parent crystal. The lengths of the nanoribbons are initially limited by the dimensions of the starting bulk particles. Interestingly, these nanoribbons change stoichiometry, composition and are elongated when the duration of agitation increases, due to Ostwald ripening. An application of the exfoliated van der Waals strings is presented for optical biosensing using photoluminescence of Bi2S3 nanoribbons where bovine serum albumin has been used a model protein. The concept of exfoliating van der Waals strings could be extended to a large class of crystals for creating bodies ranging from sheets to strings, with optoelectronic properties different from that of their bulk counterparts.<br><br>Finally, crystalline bismuth selenide (Bi2Se3) was exfoliated using liquid phase exfoliation technique. Generally, Bi2Se3 crystallizes in two polymorphs which are 1D van der Waals string (similar to Bi2S3) and another is two dimensional nanosheets. Depending upon the provided energy during the exfoliation process a phase transition from 2D to 1D might take place. In this work, the exfoliated films were found to be 2D nanosheets with various lateral dimensions. An expected opening in the bandgap due to quantum confinement effect has been observed. Moreover, a well-defined photoluminescence effect has been observed concluding that Bi2Se3 can be notable candidate for sensors and optoelectronic devices.<br><br>In summary, the author has demonstrated several significant findings during this Ph.D. research, exploring facile synthesis processes while revealing some exotic properties of low dimensional materials. It is expected that the findings of this Ph.D. research will have an ongoing impact on future electronics and optoelectronics industries.