Polyol Synthesis Of Topological Insulator Nanostructures And Mechanism Study

After the discovery of topological insulators (TIs), they gradually become the focus of materials science and condensed matter physics. The most studied topological insulator is bismuth selenide (Bi2Se3) and bismuth telluride (Bi2Te3), both of which possess metallic states on the surface with insulting bulk states. For TIs, there is a band gap at the Fermi level, which is similar to the band structure of insulators. However, there exists a massless Dirac electronic state on their surface, so as to show metallic character. Due to the protection of time reversal symmetry, backscattering caused by defects or impurities can be completely suppressed when electrons flow on the surface of TIs, making the charge transport in a dissipationless or low-dissipation state. The unique structure of TIs is of potential significance to the development of low-power electronics and error-tolerant quantum computing.Although lots of theoretical work has emerged and many novel properties have been predicted for TIs, the related experimental development is far behind owing to the following two factors. First, high-quality samples can hardly be prepared. The products obtained through conventional high-temperature sintering often have a high density of defects and are heavily doped, resulting in a high bulk carrier concentration to hinder the surface state. Second, it remains difficult to prepare TI materials into a low-dimensional, nanoscale, hetero-junction, or device structure, which greatly limits the development as well.Accordingly, preparation of TIs with a high quality plays a crucial role in the development of this field. Since morphological modulation can be used to optimize optical, electrical, and many other properties of TIs, controllable synthesis of TIs with different morphologies would obviously promote the sustainable improvement of this field. In this dissertation, we demonstrated polyol synthesis of Bi2Se3 and Bi2Te3 nanomaterials with a series of morphologies, and discussed their formation mechanism. Specifically, this dissertation contains four chapters in total, and the main contents are listed as follows:In chapter one, we briefly introduced the basic properties of TIs, and discussed common synthetic methods, related spectra, and applications in detail.In chapter two, based on the classical crystal growth theory, we synthesized three kinds of Bi2Se3 nanomaterials with different morphologies by manipulating the supersaturation level in the reaction system, including spiral-type nanoplates, smooth nanoplates, and dendritic nanostructures. The mechanism study indicates that at a low supersaturation condition, defects could cause a step edge, on which Bi2Se3 particles added continuously, leading to the formation of spiral-type nanoplates. At an intermediate supersaturation condition, the aggregation of amorphous Bi2Se3 particles and subsequent recrystallization resulted in the formation of smooth nanoplates. Furthermore, under a high supersaturation condition, polycrystalline Bi2Se3 cores were formed initially, on which anisotropic growth of Bi2Se3 occurred.In chapter three, referencing the chelating agent-assisted route, we introduced EDTA into the reaction system to prepare some new Bi2Se3 nanostructures with special edge sites. Due to the strong chelating ability of EDTA with Bi3+, the concentration of free Bi3 ions in the reaction process was decreased, which affected the subsequent growth process. Detailed analysis reveals that the dislocation source and a suitably low supersaturation were necessary for the spiral growth driven by dislocation. If a dislocation source existed under a relatively high supersaturation, the step edges in the initially formed seeds could serve as supporting sites for the growth of epilayers.In chapter four, we mainly introduced the synthesis of Bi2Te3 nanomaterials, and made a preliminary discussion regarding to the system of Tl-metal heterojunctions. It is found that NaOH plays a critical role in the preparation of Bi2Te3 nanomaterials. For the formation of TI-metal heterojunctions, we took Bi2Se3-gold heterogeneous as an example to illustrate the idea of seeded mediated growth.In chapter five, we summarized the keys of this work and prospected the furture development of this research field.