Construction Of Ultrathin Two-dimensional Nanostructrues Through Metal Nanodots Self-assembly

In general, nanomaterials with atomic layer thickness and two-dimensional（2D） scalable feature are defined as ultrathin 2D materials. The unique structure can preserve functionality in maximum with a minimal size. The ultrathin 2D materials have been employed extensively in the fields of condensed matter physics, materials science, and chemistry for their novel electronic, optical, mechanical and biocompatible properties, among which the graphene is a typical success. Recently, the research about ultrathin 2D materials is mainly focused on the van der Waals（vd W） solids, whose 2D structure depends on the intra-layer vd W interactions while the inner layer structure is based on covalent bonds, ionicbonds, amd other strong interactions among atoms and polyhedrals. Although some new 2D materials including transition metal dichalcogenides（Mo S2, WS2） and promising black phosphorus（BP） have been concerned, the research progress around ultrathin 2D materials is just in the initial stage. For example, the main species of the ultrathin 2D materials are limited to graphene family and metal chalcogenides. Besides, due to the lack of the intrinsic driving force of 2D growth, how to endow non-layered materials with ultrathin 2D feature is still a big challenge. Based on these points, in this paper, we try to obtain the ultrathin 2D self-assembled architectures of ultrasmall metal nanodots（NDs） to enrich their diversity of composition and techniques. Meanwhile, as to metal NDs, we want to enhance their properties including catalytic and fluorescent performance, and so on for their feature of assemblies.In chapter 2, we first demonstrate the self-assembly of Au dots into single-dot-thick nanosheets in high-boiling solvents, which are miscible solvents with a slight polarity difference. With high temperature annealing treatment, the solvents will execute microphase separation and generate a lamellar interface, which acts as a soft template to direct the 2D self-assembly. In this case, the surface tension drives 2D orientation. The formation of nanosheets relates to both dot aggregation（nucleation） and reorganization（recrystallization） at the interface. Furthermore, from the view of nanoscale driving forces, we reveal the dynamic process of 2D assembly. In detail, the main driving forces of such self-assembly are the dipolar interaction originated and the vd W interaction contributed from DTs. Triggered by the anisotropic dipolar attraction, the initial assembly is 1D-oriented, which in return leads to the asymmetric spatial distribution of DTs and hence the vd W attraction. As a result, the followed self-assembly is conducted toward 2D model.In chapter 3, we extend the 2D self-assembly of metal NDs:（1） we demonstrate the enhancement of Cu NDs luminescence by forming compact and ordered self-assembly architectures. Such strategy reinforces the cuprophilic inter-action of NDs, and suppresses intra molecular vibration and rotation of the capping ligands, thus greatly improving the radiative relaxation of excited states. Self-assembly strategy also permits to tune the regularity of NDs in the assemblies, which produces polymorphic Cu NDs assemblies with various emission colors. The transformation of polymorphism leads to mechanochromic luminescence of NDs assemblies. Because the NDs assemblies exhibit strong emission, they are employed as the phosphors in color conversion layer for fabricating LEDs. A white-LED prototype is fabricated by combining the blue-green and yellow emission of Cu NDs assemblies and red emission of Au NDs assemblies.（2） We further focus the catalytic property of NDs. In the first quarter, we we demonstrate the 2D self-assembly method to improve the durability of catalytic Cu NDs. This method involves the successive formation of Cu12DT8Ac4 NDs, preassembly of the NDs into NWs, con-version of Cu12DT8Ac4 into Cu8DT8, and the production of thin ribbons through NC reorganization. The cooperation between the dipolar attraction between Cu NDs and the vd W attraction between DT ligands are key driving forces of the self-assembly process. The strong dipolar attraction between Cu12DT8Ac4 NDs induces their orientated self-assembly to form NWs, and the self-assembly architectures are reinforced by the inter-NC vd W attraction. Further annealing at higher temperature leads to the removal of the Ac ligand from Cu12DT8Ac4 to form the smaller Cu8DT8. The ribbons exhibit the best electrocatalytic activity and improved durability for oxygen reduction reaction.In chaper 4, we demonstrate a simple, novel, and scalable method to synthesize the ultrathin Co₉S₈ nanosheets with single-unit-cell thickness. This method involves the successive transfer from preassembly Co₁₄ NDs architectures by simple heating treatment in tube furnace, which beyond the limitation of wet-chemical synthesis for the first time as to synthesis methods of the non-layer structured 2D nanomaterials. During transfer process, high concentration of H2 S plays a key factor in driving the 2D OA growth of initial isolated small 2D Co₉S₈ nanoplates. Because they are prefer to adsorb on the corner sites and side surface sites of the small 2D nanoplates. Such ultrathin Co₉S₈ nanosheets exhibit excellent eclctrocatalytic activity and durability in oxygen evolution reaction.