Monodisperse Metal/Semiconductor Hetero-Nanocrystals:Assembly,Superstructres And Properties Research

With the rapid development of nanotechnology,a great number of integration and assembly technologies of functional nanocrystals(NCs)materials have come out to cater for the application of photoelectric and new energy fields in the past decades.By integrating multiple components within the individual NCs,the hetero-NCs(HNCs)are capable of coupling multi-componential advantages to exhibit novel synergistic properties.In particular,the HNCs assemblis enable to play a greater role in internal coordination among components and possess unique properties that are different from conventional discrete NCs or bulk materials.Such unique assembled structures are capable of opening up a new way for the“new structure and new energy”application of NCs.Remarkably,due to their versatile physical and chemical functions of plasmon-exciton,the metal/semiconductor HNCs have a great wide range of scientific significance and applicable value in photovoltaic cells,catalysis and other related fields.However,how to precisely control the preparation of monodisperse HNCs with well-defined interface and to achieve the controllable fabrication of their assemblies and superstructures are still the key scientific problems to be solved.In this thesis,based on a chemical conversion of nonepitaxial growth strategy,the monodisperse metal NCs(Au and Au@Ag NCs)and Au@semiconductor core/shell HNCs as well as their large-scale flexible assemblies are controllbally prepared.By the controllable solvent volatilization induced HNCs self-assembly and laser irradiation assisted superstrucures synthesis,the large-scale Au-based core/shell HNCs superlattices and plasmonic superstructures were successfully achieved.The main contents and results are summarized as follows:1.Based on the gradient centrifugation and modified nonepitaxial growth strategies,monodisperse Au NCs and Au@MS(M=Ag₂,Cd,Zn)core/shell HNCs with controlled crystallinity,shell thickness and varied compositions as well as well-defined hetero-interface were successfully prepared.In particular,the very first core/shell nanostructures with anisotropically shaped Au cores but isotropic overall morphologies were successfully obtained.Benifited from their monodispersity,the as-prepared HNCs all exhibited strong tendencies to self-assemble into ordered patterns.2.Based on an aqueous solution,the monodisperse Au@MX(MX=Ag₂S,Ag₃AuTe₂)core/shell HNCs with isotropically Au cores but anisotropic overall morphologies were successfully prepared.Besides,with the help of well-exposed facets of Au@Ag core/shell hetero-nanocubes,the HNCs could be connected via oriented attachment through the chemical synthesis process,that resulted in one-dimensional multi-Au-cores@semiconductor core/shell hetero-nanostructueres.3.The very first SLs with hetero-nanostrcutures were successfully obtained.By using a solvent-evaporation dip-coating technique with a controlled pulling speed,the as-prepared Au@MS HNCs with C18 acid/alkali co-capping surface ligands can self-assemble into large-scale two-dimensional monolayer and three-dimensional multilayer SLs on both flexible(graphene)and rigid(Si wafer or glass)substrates.Moreover,unprecedented binary nanocrystal SLs can be achieved through synergistic self-assembly of Au@MS HNCs together with Au NCs(or Cd S NCs)into different intricate patterns.Experimental evidence and finite-element method theoretical simulations indicate that the Au@Cd S HNC SL film taking up a closest-packing mode has a rate of photocurrent generation of 0.05 m A·cm^-2,which is 4.5 times of that of its disordered-assembled counterpart,probably because of the enhanced plasmon-exciton coupling permitted by the large-scale ordered nanopatterning.4.Instead of the ligands exchange strategies,we take advantage of continuous laser with optimal wavelength and power to irradiate the film-scale NCs SLs at solid-liquid interfaces.Owing to the Au-based NCs'surface plasmon resonance(SPR)effect,the gentle laser irradiation leads the Au NCs or Au@Cd S core/shell NCs to attach each other with controlled pattern at the interfaces between solid NCs phase and liquid ethanol/ethylene glycol.We used the continuous wave 532 nm laser to control Au-based SLs to form the monolayer with uniformly reduced interparticle distance followed by welded superstructures.Considering the size effect to Au NCs'melting,when decreasing the Au NCs size to?5 nm,stronger welding nanostructures were obtained with diverse unprecedented shapes which cannot be achieved by normal colloidal synthesis.With the help of facile scale-up and formation at solid-liquid interfaces and well connection of crystalline between NCs,the obtained Au superstructure films that could be facilely transferred onto different substrates exhibit a broad SPR absorption in the visible and near infrared regime(650?2620 nm),enhanced electric conductivities,and wide application as Surface Enhanced Raman Scattering(SERS)-active substrates.5.By using a high power laser irradiation,the Au@Ag core/shell superstructures with shell welding conection could be obtained in an aqueous solution.After the sulfuration and cation exchange reactions,we found that only the welded core/shell HNCs with convex morphologies are able to keep their connection structures.This phenomenon reveals that the connection structures of Au@semiconductor HNCs are sensitive to the change of composition and lattice of shells.