Controlled Synthesis Of Bi₂Se₃Nanostructures:Spiral,Layer-by-Layer, And Dendritic Growth Modes

Topological insulators are recently discovered a new quantum materials with insulating in the bulk and conductive on the surface. These surface states originated from strong spin-orbit coupling, are nondegenerate and topologically protected against backscattering generated from defects or non-magnetic impurities. Therefore, the charge transport on the surface of topological insulator is dissipationless. Owing to the exotic surface electronic states, topological insulators have a great of potential applications for spintronics and quantum computing. Bi2Se3was known historically as a thermoelectric materials, but recently has been shown to be a promising candidate for topological insulator as it has a simple energy band of structure and large band gap, which is much larger than the room temperature energy scale. Compared with bulk materials, low-dimensional topological insulator nanostructures can effectively lower the concentration of carriers and thus highlight the contribution of surface states. Many physical methods have been successfully applied to synthesize Bi2Se3nanoplates and nanoribbons. However, the chemical synthesis of Bi2Se3nanostructures has not been developed deeply. In addition, incorporating a line defect of screw dislocation into Bi2Se3may create a pair of topologically protected one-dimensional helical states or a perfect conductive channel. However, the screw dislocation-driven growth by wet chemical methods has not been achieved for topological insulator. Base on the crystal growth theory, we focus on the controlled synthesis of Bi2Se3nanostructures with different morphologies and structures, and preliminarily survey the growth mechanisms and strategies. This dissertation contains four chapters and the contents are outlined as following.In chapter one, we briefly introduce the two-dimensional crystal structure of Bi2Se3as well as the basic properties of topological insulators. Then we focus on the synthesis methods of Bi2Se3and experimental studies and applications of topological insulators. Lastly, we briefly introduce the theory of crystal growth which is the basis of the synthesis of Bi2Se3nanomaterials.In chapter two, we synthesized spiral-type Bi2Se3nanoplates by a polyol synthesis. Screw dislocations on the nanoplates were substantiated by two sets of centrosymmetric helical fringes. Through TEM analysis, we identified herringbone contours related with the bipyramidal structure. By lowering the supersaturation value, we were able to obtain spiral-type nanoplates with smaller slopes. These products are characterized by spiral arms hollow cores that generated as a result of the relaxation of strain energy for dislocations. All of these evidence indicated that the growth of nanoplates are indeed driven by screw dislocation. By elevating the concentration of precursors, increasing pH value, or adding more AA, we obtained nanoplates with preliminarily developed dislocations and smooth nanoplates. The screw dislocation-driven growth process was distinguished on intermediate nanoplates with preliminarily developed dislocations. Understanding of the SDD bidirectional growth process of Bi2Se3nanoplates and explore the properties of the new structure, can hopefully be extended to other2D layered materials and will promote their design and practical applications.In chapter three, we mainly introduce the syntheses of Bi2Se3nanoplates and nanoflowers. Firstly, we studied the general strategies for the synthesis of Bi2Se3nanoplates. The process of layer-by-layer growth mode for Bi2Se3nanoplates were investigated. Secondly, we demonstrate a seeded growth approach to the synthesis of Bi2Se3nanoplates. By controlling the injection rates of precursors, we can control the layer-by-layer growth conducted on the lateral and vertical directions. With a fast injection, the growth of nanoplates was confined in the lateral direction. However, with a slower injection, the growth were proceeded on all dimensions. We also synthesized the Bi2Se3heterostructures by using seeded growth method. The Raman measurement for the two types of products showed the thickness-dependent Raman properties.In addition, homogeneous Bi2Se3thin films were assembled from as-synthesized Bi2Se3nanoplates for the construction of near infrared transparent flexible electrodes. Lastly, by manipulating the reaction kinetics, we syntheized Bi2Se3nanoflowers with different morphologies and sizes. We also surveyed the dendritic growth process of Bi2Se3nanoflowers, which is different from the growth of layer-by-layer modes.In chapter four, we prospect the future applications of spiral-type Bi2Se3nanoplate and demonstrate several challenges and development trends in the researches of topological insulator.