Precise Synthesis, Assembly, And Optoelectric Properties Research Of Metal, Semiconductor Nanosheets

With the boom of graphene research, the metal and semiconductor two-dimension(2D) nanosheets have been extensively investigated in the fields of optoelectronics, energy and information materials due to their unique electrical, optical properties and the advantages of easily forming large area film by layer-by-layer(LBL) assembly. In this thesis, we have successfully synthesized some metallic and semiconductor nanosheets with different morphology and compositions by new liquid phase synthesis method. These nanosheets are further in situ assembled on different flexible/rigid substrates to perform optoelectronic devices applications. The Ag and Au-Ag alloy nanosheets are obtained in large scale(gram scale). By modulating the surface micro/nanostructure evolution of Au-Ag alloy nanosheets experimentally and theoretically, the surface plasmon resonance(SPR) can be tuned in the near infrared region from 800 to1400 nm. And then enables their promising applications in photothermal effect, surface-enhanced Raman scattering(SERS) and biological molecules trace detection.In colloidal-quantum-dot(CQD) optoelectronics, the long-chained capping ligands between CQDs produce insulating barriers that interfere with efficient carrier transport when processed into films. We devote our contributions to find a new method to solve this problem. We put forward the new strategy of cation exchange initiated three-dimensional(3D) oriented attachment growth(OA) of the CQDs on both rigid and flexible substrates under large lattice mismatch to obtain 2D microsized nanosheets, and then get centimeter sized thin films on rigid/flexible substrates through their hierarchical assembly. The close packing of these nanosheets with highly preferred orientation and good contact between nanosheets and substrate and among the nanosheets leads to improved electronic properties, which demonstrates promising applications in flexible electronic and photovoltaic device.The detailed research results are summarized as follow:1) We successfully synthesized ultrathin single-crystalline Ag nanosheets(reach to ~30 nm with different shapes and thickness by using the solvothermal method in the presence of N, N-dimethylformamide and PVP. By adjusting the ratio of PVP to silver nitrate, temperature and reaction time, silver nanobelts(6~7 μm) with high purity can be obtained through OA growth. The SPR of the silver nanosheets and nanobelts can be fine-tuned from visible to infrared region(500~2000 nm). These silver nanosheets or nanobelts can be further assembled on flexible/rigid substrates to form silver thin film with quasi single-crystal orientation. These results lay a solid foundation for their applications of optoelectronic devices.2) Based on these silver nanosheets, we synthesized Au-Ag alloy nanosheets, hollowed nanosheets and nanorings with different surface roughness by galvanic exchange and Kirkendall mechanism. The UV-Vis absorption spectra showed that their SPR can be tuned from 950 to 1400 nm. The finite element method(FEM) simulations demonstrated that as-prepared Au-Ag alloy nanosheets/nanorings with fine nanostructures on micro-frame provide a large amount of ―hotspots‖ for electromagnetic field(E-field) enhancement. The photothermal measurements showed that the photothermal conversion efficiency of roughened Au-Ag alloy nanosheets can reach to 78.5%. These roughened alloy nanosheets were chosen as SERS substrate to realize the trace detection of beta amyloid peptide(Aβ1-42). This work lays a theoretical and experimental foundation for the applications of roughened Au-Ag nanoplates in the photothermal therapies and biological detections.3) Centimeter scale semiconductor thin films from QDs to macroscopic optoelectronic devices have been achieved through synergistic control of components, morphology, assembly, and the hetero-interfaces between the nanosheets and the substrates. Ag-doped single-crystalline CdX(X=S, Te etc.) nanosheets were obtained through cation exchange from amorphous Ag2-δX nanoparticles aggregates. The centimetre-level uniform CdX nanosheets film on rigid substrate(ITO conductive glass) and flexible substrate(PET conductive film) were realized by hierarchical assembly of nanosheets. The single-crystallinity and Ag dopant concentration of CdX nanosheets were controlled by adjusting the concentration of Cd2+, phosphine, capping ligands as well as the different coordination activities between ligands and Cd2+, Ag2+ ions. The electrical, optoelectronic and mechanical stability properties of CdX nanosheets/nanobelts films on rigid/flexible substrates have been characterized by PW-600 probe station, CP-AFM and Keithley 2400 respectively. Compared with the films by dip-coating of CdX nanosheets, as-prepared CdX films were demonstrated good and uniform contact between CdX nanosheets and the substrate as well as between CdX nanosheets and enabled highly efficient electron transfer in film. Comparably, the films through dip-coating are unstable and easy to fall off and the resistance is more than 103 times. As-prepared films on flexible substrate showed stable electrical properties under different degree of bending. The distinct conductivity changes with or without white light illumination exhibit potential photoresponse switching behavior of as-prepared CdX nanosheet films. In summary, micrometer-sized Ag-doped quasi-sc CdX nanosheets/nanobelts by topotactic reaction on rigid/flexible substrates and their in-situ hierarchical assembly on substrates at a large scale effectuate enhanced electronic properties.4) Crystallized non-stoichiometric Ag2-δTe nanosheets can be synthesized through OA growth of amorphous Ag2-δTe nanoparticles aggregates. The electrical and magnetic properties by Hall effects and PPMS characterization have been done. The Hall voltage sensitivity of c-Ag2-δTe nanosheets film is up to 188% at room temperature, which showed their potential applications of Hall effects sensor. The weak anti-localization of the c-Ag2-δTe nanosheets film and the quantum oscillation behavior under low temperature showed that they have a topological insulator property.