ACCESSING GROUP 6 TRANSITION METAL DICHALCOGENIDE NANOSTRUCTURES WITH COLLOIDAL SYNTHESIS

Book excerpt: Layered transition metal dichalcogenides (TMDs) are particularly intriguing targets due to unique anisotropic quantum confinement, phase engineering enabled by rich polymorphism, and favorable properties for optical, electronic, magnetic and catalytic applications. Colloidal synthesis has been developed as a powerful and scalable solution-phase tool to access free-standing TMD nanostructures. While several colloidal TMD nanostructures have been prepared, questions remain at establishing universal synthetic protocols to directly access well-defined nanostructures without post-annealing, delicate adjustment over chemical composition, structure and morphology associated with vertical thickness and lateral size, as well as understanding formation and transformation for the two-dimensional nanosheets. More challenges, from both synthesis and characterization perspectives, emerge for the colloidal synthesis of complex TMD species including atomically-mixed alloys and mixed-dimensional heterostructures. In this dissertation, I focus on the colloidal synthesis of group 6 TMD (MX2, M = Mo, W and X = S, Se, Te) nanostructures, alloys and heterostructures, as well as systematic study with a suite of spectroscopic and microscopic techniques. Moreover, I aim to extract novel fundamental insights through exploring the structure-property relationship, which can trigger a wider range of applications based on the dimension-confined TMD nanostructures.I start with the colloidal synthesis of few-layer 1T-MoTe2 nanostructures (Chapter 2). Uniform 1T-MoTe2 nanoflowers comprised of few-layer nanosheets form directly in colloidal solution, with approx. 1 % lateral lattice compression compared with the bulk analogue. It is interesting to directly obtain the metastable monoclinic (1T) polymorph at low temperatures where the 2H phase should be preferred. Besides a small energy difference between 1T- and 2H-MoTe2, and modification of the surface energy and formation barrier by organic ligands, grain boundary pinning facilitated by polycrystallinity and small domain size also contributes to the stabilization of the metastable 1T phase as revealed by computational studies. This study demonstrates the capability of colloidal approaches to obtain synthetically challenging TMD systems. I then target nanostructured TMD alloys to elucidate the relationship between continuous adjustment of elemental composition and tunable optical properties (Chapter 3). Few-layer TMD alloys, MoxW1-xSe2 and WS2ySe2(1-y), exhibiting tunable metal and chalcogen compositions spanning the MoSe2-WSe2 and WS2-WSe2 solid solutions, respectively, are directly synthesized in colloidal solution. Comprehensive structural characterization of the composition-tunable TMD samples are presented, together with instructive chemical synthetic guidelines. Importantly, we are able to identify a random distribution of the alloyed elements and various types of vacancy sites with high-resolution microscopic imaging. The A excitonic transition of the solution-dispersible TMD samples can be readily tuned between 1.51 and 1.93 eV via metal and chalcogen alloying, correlating composition modification with tunable optical properties. In Chapter 4, I further modify the colloidal synthetic approach to access tungsten ditelluride (WTe2), which exhibits exotic properties in magnetic and topological devices. Nanostructured WTe2 with the orthorhombic (Td) structure is directly synthesized in colloidal solution. Microscopic imaging monitors the anisotropic pathway by which the few-layer WTe2 nanoflowers grow, and captures the co-existence of multiple stacking patterns of the atomically-thin layers. In addition, nanostructured transition metal ditelluride alloys (MoxW1-xTe2) with 1T-MoTe2 and Td-WTe2 as end members are obtained. Using the variety of TMD nanostructures now accessible based on our studies and previous reports, we investigate the solution-phase deposition of noble metals (Au and Ag) on transition metal disulfides (1T- and 2H-WS2), diselenides (MoSe2 and WSe2) and ditellurides (1T-MoTe2 and WTe2) in Chapter 5. Au3+ and Ag+ are reduced on the surface of the TMD nanostructures at room temperature via a spontaneous charge transfer process, and the nucleation, growth, structure, and morphology of the deposited Au and Ag are highly dependent on the noble metal/chalcogen interface. In particular, efficient electron transfer and strong interactions between silver and tellurium through interfacial Ag-Te bonding lead to the deposition of single-atom-thick Ag layers on nanostructured 1T-MoTe2 and WTe2, producing unique monolayer coatings with distinct structural and energetic features. Construction of the interface-tunable hybrids indicates that colloidal TMD nanosheets provide a diverse platform to probe charge transfer as well as interfacial coupling at the atomic scale. In Chapter 6, I expand the knowledge gained from previous synthetic studies and exploit structure-property relationships of colloidal TMD nanostructures to identify new heterogeneous catalysts. Colloidally synthesized 2H-WS2 nanostructures are identified as active and robust catalysts to selectively hydrogenate nitroarenes to their corresponding anilines with molecular hydrogen. A broad scope of molecular substrates with reducible functional groups including alkynes, alkenes, nitriles, ketones, aldehydes, esters, carboxylic acids, amides, and halogens are tested to demonstrate the wide applicability of the 2H-WS2 nanostructures for chemoselective transformation of substituted nitroarenes. In addition, microscopic evidence indicates that the improved performance for the nanostructured 2H-WS2 compared with the inactive bulk counterparts is due to the existence of sulfur vacancies situated on the high surface area nanosheets.