Titre de la dissertation: Photoluminescence and Surface-Enhanced Raman Scattering Analysis of Nanostructured Molybdenum and Tungsten Disulfide: Nanoflowers and Vertically Aligned Architectures

Promoteurs de thèse: Madame Carla Bittencourt et Monsieur Adrien Chauvin (ELI Prague)
Résumé de la dissertation: 
Transition-metal dichalcogenides (TMDs) are a class of materials that, despite their long-standing presence in scientific research, have recently attracted significant attention for their unique structural and electronic properties. Among the most widely studied TMDs are molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂), which exhibit both bulk and layered architecture. The layered nature of these materials aligns them with graphite and its groundbreaking derivative, graphene, which shares structural similarities with TMDs. In 2010, K.S. Novoselov and A.K. Geim were awarded the Nobel Prize for their pioneering work on graphene, catalyzing extraordinary advancements in two-dimensional (2D) physics. The success of graphene in enabling ultrathin devices has propelled monolayer TMDs to the forefront of solid-state research, owing to their distinctive band structures, characterized by large band gaps, degenerate valleys, and non-zero Berry curvature. Furthermore, the ability to tune their bandgap from indirect in bulk form to direct in monolayer form has opened new avenues for exploration in electronic and optoelectronic applications.
Various synthesis techniques have been employed to produce MoS₂ and WS₂ materials for applications in sectors such as semiconductors and optoelectronics. While certain methods have demonstrated the capability to yield high-quality TMD substrates, others have proven less effective. Monolayer TMDs are widely recognized as substrates with the highest performance; however, recent investigations into their morphology have revealed the advantages of three-dimensional (3D) substrates. Notably, many synthesis approaches documented in the literature are prohibitively expensive, such as micropatterning guided by laser beams.
This doctoral thesis investigates the utilization of 3D platforms, including nanoflowers and vertically aligned substrates, as alternatives to conventional 2D monolayer and bilayer TMDs. By leveraging the unique properties of these 3D platforms, this work demonstrates their ability to achieve performance comparable to or superior to that of conventional platforms in applications such as photoluminescence (PL) and surface-enhanced Raman scattering (SERS). Following synthesis, the substrates underwent a series of functionalization processes designed to maximize their potential, resulting in substantial improvements in both PL and SERS signals.
The functionalization methods adopted in this study were deliberately chosen to be environmentally sustainable and cost-effective. Ion irradiation emerged as a versatile technique for doping substrates with species such as hydrogen ions or inducing particle deformation within the substrate lattice. Depending on irradiation parameters, this method enables controlled modifications of material properties without causing significant damage. Parameters such as ion type, energy, fluence, angle of incidence, and substrate temperature were carefully
optimized to tailor the irradiation process, which operates under regimes dominated by nuclear stopping (energy transfer to atomic cores), electronic stopping (energy transfer to target electrons), or a combination of both mechanisms. While ion irradiation proved particularly effective for enhancing PL signals, an alternative technique was identified as more suitable for amplifying SERS signals.
Direct current (DC) magnetron sputtering, a physical vapor deposition method, was employed to deposit nanoparticles onto the surface of vertically aligned SERS platforms. This environmentally friendly technique offers significant advantages over other sputtering methods, including higher deposition rates and minimal substrate damage. Notably, deposition times were reduced to the order of seconds, and the efficiency of DC plasma deposition was comparable to that achieved with monolayer substrates. The findings presented in this thesis underscore the unprecedented potential of bulk-like 3D substrates for optoelectronic applications.
The thesis provides a comprehensive analysis of the morphological, structural, chemical, and elemental transformations observed in nanoflowers and vertically aligned SERS substrates following functionalization. Detailed characterization of the physical and chemical interactions between substrate elements and functionalization species is presented across Chapters 4, 5, 6, and 7. Each chapter adopts a tailored approach: Chapter 4 emphasizes theoretical insights, while Chapter 5 elaborates on experimental methodologies aligned with its objectives.
Given the growing interest in TMDs and their applications, this thesis serves as a resource for materials scientists seeking to explore novel pathways in solid-state research.