Investigation Of The Growth Mechanism And Properties Of Some Branched Nanostructures Prepared By A Two-stage Method

Recently, branched nanostructures based on one-dimensional nanostructures have become the focus of intensive research owing to their unique applications in mesoscopic physics and nanoscale devices. They are expected to play an important role as functional units in fabricating electronic, optoelectronic, electrochemical, and electromechanical devices with nanoscale dimensions. Base on the frontier and hot topic of this area, we devote most of our attention to the preparation, growth mechanism and some properties of branched nanostructures in this disseration. This work is composed of five parts.Part I:Preparation, growth mechanism and optical properties of ZnS nanocombsZnS nanocombs were synthesized by the self-evaporation of ZnS nanobelts. We found that the growth direction of the backbone of the ZnS nanocombs is [210], consisting with that of ZnS nanobelts. The growth direction of the branches of the ZnS nanocombs is [001], which is epitaxially grown on the (00±1) planes of the ZnS nanobelts. Because the (00±1) planes is the polar planes and have different chemical activity, the densities of the nanorods grown on the (00±1) planes are different. By careful analysizing, we believe the formation of ZnS nanocombs includes two processes:self-evaporation of ZnS nanobelts and the growth of ZnS nanorods on ZnS nanobelts via VLS mechanism. Room temperature PL measurements showed that ZnS nanocombs have different emissions compared with ZnS nanobelts, which is caused by its microstructures and impurities introduced during the synthesis.Part II:Preparation, growth mechanism and photoluminescence of comb-like CdS/ZnO nano-heterostructuresComb-like CdS/ZnO nano-heterostructures were synthesized by a two-stage thermal evaporation method. CdS nanobelts were synthesized in the first stage by vapor transport and consendation method. The formation of CdS nanobelts conformed VLS mechanism and Au was used as catalyst. ZnO nanorods were grown on the CdS nanobelts by VS mechanism. The growth direction of the backbone CdS nanobelts is [210], and the growth direction of ZnO nanorods is [001]. The ZnO nanorods epitaxially grew on the (001) planes of the backbone CdS nanobelts. Because the polar surfaces (001) and (00-1), which were terminated by Cd and S atoms respectively, have different chemical activities, ZnO nanorods only grew on the (001) of the CdS nanobelts. The room temperature photoluminescence of the comb-like CdS/ZnO heterostructures is also studied.Part III:Synthesis, growth mechanism and optical properties of SnO2/ZnO hierarchical nanostructuresSnO2/ZnO hierarchical nanostructures were synthesized by a two-stage method. SnO2 nanowires were synthesized in the first stage by vapor transport and consendation method. The SnO2 nanowires were grown by VLS mechanism and Au was used as catalyst. The ZnO braches were synthesized by using the as-synthesized SnO2 nanowires as substrate and thermal evaporation of ZnO/C mixture. During the growth of ZnO nanowires, ZnO and C will reacted with each other and reduced gases can be generated, such as Zn vapor and CO. The reduced gas will be transported to the downstream and reacted with SnO2 nanowires. Thus Sn droplets will be generated on the surfaces of SnO2 nanowires, which will act as catalyst to catalyze the growth of ZnO brached via VLS mechanism. We investigated the phases and microstructures of the SnO2/ZnO hierarchical nanostructures by XRD, SEM and TEM. We discussed the conditions for the synthesis of ZnO branches. Room temperature PL measurements showed that the emissions from SnO2 nanowires were constrained when excited by short wavelength laser, while the emission of SnO2 nanowires can be observed when excited by long wavelength laser. The phenomenon was casued by the screen effect from ZnO layer.Part ?:Preparation, growth mechanism and optical properties of SnO2/CdS hierarchical nanostructuresSnO2/CdS hierarchical nanostructures were synthesized via a two-stage nanocluster-catalysed VLS approach. The phases and microstructures of the SnO2/CdS hierarchical nanostructures were investigated by XRD, SEM and TEM. The fabrication conditions affecting the morphology of the products were discussed. Room temperature PL measurements showed that both of the emissions from SnO2 and CdS were observed.Part V:Preparation, growth mechanism and magnetical properties of branched Sn/CNT core/shell structuresBranched Sn/CNT core/shell structures were synthesized by a two-stage method. SnO2 nanowires were synthesized first by thermal evaporation of SnO2/C mixture and using Au as catalyst. Branched Sn/CNT core/shell structures were obtained by employing the catalytic chemical vapour deposition method over SnO2 nanowires. The phases and microstructures of the branched Sn/CNT core/shell structures were investigated by XRD, SEM and TEM. The backbones of the branched structures are CNTs with some Sn particles in their tunnels. The braches have a core/shell structure with a Sn nanowire as the core and a CNT as the shell. A growth model is proposed to explain the growth of the branched structures. Magnetization measurements revealed that the branched nanostructures showed diamagnetic behaviour below 3.7 K. Due to the coexistence of micro Sn particles and Sn nanowires, the critical magnetic field of bulk Sn and nanowires were observed.