Synthesis Of Hybrid Nanocrystals With Controllable Morphology Via Seed-growth Method

Due to the quantum size effects, nanomaterials exibit new special properties and can be used innumerous potential applications, such as electricity, optics, magnetics, environmental protection, medicine,environmental protection, and new energy regeneration. The study on the preparation and properties ofnanomaterials is the most active and most important part of applicable nanotechnology and it alsoconstitutes the basic base of nanoscience. Hybrid-nanocrystals （HNCs） with core/shell, hetero, and alloystructures are emerging as more important materials than mono-nanocrystals （MNCs）. HNCs areanticipated to display not only the properties of distinct materials, but also new properties and capabilitiesdue to the combination of the two materials. More importantly, HNCs usually exhibit compositiondependent surface structures and atomic segregation behaviors, and therefore more potential in variousfields including electronics, light-emitting diode, and catalysis.Compared with MNCs, the preparation of HNCS is much more complicated. The choose of reactionparameters such as solvents, reaction temperature, reaction time and the use of surfactants play importantroles in the results of reactions during the synthesis and preparation of HNCs.The control of the key factorswill greatly help us to develop the new synthetic concepts toward efficiently controllable andscale-uppreparation of HNCs.To control the morphology, size, composition, and structure of HNCs as well as theexploration of their properties and applications, this dissertation focuses on the study of seed-growth routeand the effects of experimental parameters such as the reaction temperatures, reaction time, the choice ofsurfactants and precursors on the morphology, composition and properties of as-synthesized HNCs. Severaldifferent types of HNCs with metal-metal, metal-semiconductor, and semiconductor-semiconductorhetro-structures have been synthesized in organic solvents.Over the past decade, much attention has been paid to Au nanocrystals （NCs） due to their uniquephysicochemical properties and potential applications in catalysis, sensitive sensing, and drug delivery.Dueto the use of dodecanethiol as ligands which has a strong binding abilitycan reduce the activity of NCsNuclei, limited the development for the synthesis of Au NCs.We reported a facile one-step method forsynthesizing oleylamine （OAM, the binding ability is weaker than dodecanethiol） coated Au NCs with broad size range and different shapes by usingtworeducing agents （OAM and tert-butylamine-borane（TBB））. When TBB was used as the only active reducing agent and the reaction temperature was set below140°C, spherical-shaped Au NCs （3.9-8.7nm） were prepared. This growth process was that large numberof NCs formed at the beginning of the reaction and they neither grew nor aggregated with the reaction timewhen the reaction temperature was lower than140°C. If OAM instead of TBB was used as the reducingagent and the reaction temperature at180°C, different sized Au NCs with parallel hexahedron shape can beobtained with the reaction time lasted from2min to30min. Based on this work, we reported a facileseed-growth method to synthesize AgAu alloy and core/shell NCs, and found the effect of the seed size onthe growth of different shaped NCs. Spherical AgAu alloy NCs were obtained when pre-synthesized6.1and7.4nm Ag NCs were used as seeds. While, if9.6nm Ag NCs were used as seeds, cubic Ag/Aucore/shell NCs were finally obtained. The growth mechanism of alloy and core/shell NCs is discussed indetail, and found130°C was the proper reaction temperature to synthesize AuAg alloy NCs.Similarly,different structured metal-semiconductor HNCs were also formed after pre-synthesized semiconductor NCs,such as CdSNCs, CdSe/CdS/ZnSNCs, Ag₂S NCs and CdS nanorods （NRs） were used as seedfor thereduction of Au precursors.Among nanomaterials, semiconductor-based one-dimensional （1D） building blocks （such as nanorodsand nanowires（NWs）） have attracted intense research interest due to their tunable aspect ratio and carrierconfinement effect, which may result in novel optoelectronic properties and contributes potentially to awide range of applications. Our group introduces a method to prepare size-and shape-controlledhigh-quality, monodisperse, low-cost1D metal–semiconductor structured Ag–ZnS NRs and NWs. Basedon this work, our reported the synthesis of high-quality, size and shape controllableAg₂Se-ZnS andAg₂S-ZnS NRs and NWsfor the first time.For Ag₂Se-ZnS heterostructures, Ag₂Se NCswerepre-synthesized by using organometallic silver acetylacetonate and Se-octadecene as precursors, andtunable particle sizes could be easily obtained by adjusting the reaction temperatures. Ag₂Se NCs not onlyplayed a key role in the control of the shape of ZnS NCs but alsoinfluenced the crystal structure of ZnSNCs.By using Ag₂Se NCs （5-12nm）as seeds, high-quality colloidal Ag₂Se-ZnS nanorods and nanowireshaving purposefully controlled diameters in the range of5-12nm and with lengths of15-600nm weresynthesized. Moreover, the diameter and length of Ag₂Se-ZnS NRs and NWs were controlled by altering the experimental variables, such asprecursor concentration, reaction time, reaction temperature,diameter ofAg₂Se NCs.In additions, we also study the relationship between the Ag₂Se-ZnSNWS length and SPV orwork funciton, and consider NWs may be used in the field of solar cell in the future.In comparison with organic dyes and fluorescent proteins, quantum dots have unique optical andelectronic properties including narrow size-tunable light emission, improved signal brightness, resistanceagainst photobleaching, and simultaneous excitation of multiple fluorescence colors. Type-II CdTe/CdS andCdTe/CdSe core/shell NCs were synthesized by the use of green precursors. Through successive ion layeradsorption and reaction method, different-sized CdTe NCs were used as cores to synthesize CdTe/CdS andCdTe/CdSe core-shell, and finally formed Type-II structure. To the system of CdTe/CdS, a simple methodwas introduced to synthesize CdTe/CdS core/shell NCs by using a new kind of “green” tellurium precursor.Both absorption and photoluminescence （PL） spectra of CdTe/CdS NCs exhibited a discontinuous shift asfollow: slightly （<10nm）, big （³0nm）, and slightly （<10nm） red-shift during the growth of CdS shellthickness. Different sized CdTe cores were introduced to synthesize CdTe/CdS core/shell NCs and similardiscontinuous red-shift phenomena in the PL spectra were observed. These characteristics indicated that thestructure of CdTe/CdS NCs gradually evolved from Type-I to Type-II. For CdTe/CdSe core-shell NCs, ThePL can reach820nm and which can be used in near-infrared ranged, and the quantum dots’ QYs is between40-75%. Due to the special band gap alignment of type-II core-shell structure, holes and electrons can beseparated and delocalized into core materials and shell materials, and this facilitates the wide rangetunability of band gap of core-shell system by choosing suitable materials. The emission stability andpracticability of type-II structures is supposed to be much better than CdTe core.