Atomically Thick Two-Dimensional Crystals:Design And Properties Study

Atomically thick two-dimensional (2D) crystals are brand new materials, which can be used for the design of high quality nano-devices. The atomically thick2D crystals can effectively combine the microscopic electronic, magnetic and optical properties with macroscopic ultrathin, transparent and flexible devices, guaranteeing a maximum functionality while keeping the size minimized. However, the study of atomically thick2D crystals were mainly restrict on the layered materials with weak van der Waals force between the layers, such as, hexagonal boron nitride (h-BN), layered transition metal chalcogenides, and so on. Expanding the study area to atomically thick2D crystals with nonlayered structure or quasi-layered structure with relatively strong bonds between the layers will not only enrich the family of2D atomic crystals, but also bring us unprecedented surprises.The goal of this dissertation is to design and controllable synthesize newly atomically thick2D crystals, as well as the study of their corresponding properties. In this dissertation, we highlight direct liquid exfoliation, substitutional-solid-solution based exfoliation, oriented attachment and in-plane coassembly strategies for the design of a series of new2D crystals with atomic thickness. Furthermore, the reaction mechanism, structures and properties of the atomically thick2D crystals are comprehensively studied by the combination of density functional theory (DFT) and experimental studies. The details are summarized briefly as follows:1. By detailed crystal structure analysis, for the first time, we proposed that the graphite-C3N4(g-C3N4) could be exfoliated into atomically thick nanosheets by direct liquid exfoliation strategy for its layered structure. Further study shows that water is the best solvent to exfoliate the g-C3N4for its highest polarity among the used solvents and matched surface energy with g-C3N4. DFT calculations indicate that the atomically thick g-C3N4nanosheets show enhanced photoresponsivity compared with corresponding bulk materials, which has been confirmed by detailed experiments, such as, intrinsic photocurrent and photocatalytic activity. Furthermore, the ultrathin g-C3N4nanosheets show extremely high photoluminescence (PL) quantum yield of about19.4%, which is four times higher than the corresponding bulk materials. Thus, benefiting from the inherent blue light PL with high quantum yields and high stability, good biocompatibility, and nontoxicity, the water-soluble ultrathin g-C3N4nanosheet is a brand-new but promising candidate for bioimaging application.2. The authors firstly developed a substitutional-solid-solution based exfoliation strategy for the preparation of the atomically thick2D crystals of MAX phases, in which the MX and A layers are covalented by relatively strong bonds and cannot be exfoliated into nanosheets by direct liquid exfoliation process. By taking Ti3SiC2for example, we substituted1/4of Si with Al atoms to form a Si-Al disordered substitutional-solid-solution Ti3Si0.75Al0.25C2(TSAC) to activate the A layers of Ti3SiC2, which would be exfoliatable. Compared to bulk counterparts, the atomically thick TSAC nanosheets show enhanced thermal and mechanical properties, and can be used as a promising candidate for the application in polymeric composites.3. The atomically thick Co9Se8nanosheets with unique lamellar stacking have been prepared by2D oriented attachment process, which show single layer just half unit cell thick of Co9Se8crystal of about0.52nm. Further density of states (DOSs) study indicate that the single layered Co9Se8nanosheet is half-metallic ferromagnetism, however, the corresponding bulk counterpart is a typical semiconductor. Meanwhile, the weak semiconducting and room-temperature ferromagnetic characteristics strongly supported the intrinsic half-metallic ferromagnetism of the atomically thick Co9Se8nanosheets. To the best of our knowledge, the ultrathin Co9Seg nanosheets with atomic thickness are the first artificial inorganic2D nanosheets with intrinsic half-metallic ferromagnetism, which will not only inspire scientific interest in the possible half-metallic ferromagnetism of low dimensional materials but also pave a practical way to achieve ultrathin, transparent, and flexible paperlike spintronic devices.4. For the first time, we highlight a universal pathway for the controlled synthesis of ultrathin transition metal chalcogenide-alkylamine (MxTy-alkylamine, where M=Co, Fe, Ni; T=S, Se; alkylamine means CnH2n+xNH2with n≥12) inorganic-organic hybrid nanosheets with atomic thickness by a unique in-plane coassembly strategy between small2D transition metal chalcogenide nanoplates and alkylamines with different lengths. By taking Co9S8-oleylamine (Co9S8-OA) hybrid nanosheets as example, we studied the structure and formation mechanism of the hybrid nanosheets in detail. The Co9S8-OA hybrid nanosheets with thickness of only about0.5nm and lateral size up to1μm, and the inorganic and organic parts alternately arrange with each other in a confined2D space forming a network structure. Theoretical studies show that the oleylamine molecules tend to adsorb on the corner and side surface Co sites of Co9S8nanoplates, however, adsorbing on the top surface Co sites is extremely unstable, which supports the2D in-plane coassembly strategy. Furthermore, the surface distortion of the Co9S8nanoplate revealed by X-ray absorption fine structure spectroscopy (XAFS) not only stabilizes the hybrid nanosheets structure but also influences their electronic structure and brings on novel physical properties. The as-obtained MxTy-alkylamine hybrid nanosheets reported here was the first case of artificial inorganic-organic hybrid nanosheets with atomic thickness, enriched the kinds of atomically thick nanosheets.