Study On The Preparation And Optical Properties Of Topological Insulators

Topological insulators?TIs?has typical characteristics of two dimensional nanomaterials,with ultra-thin thickness and relatively large lateral dimension.Topological insulator materials are a new type of quantum matter.Unlike traditional insulators or metal materials,the topological insulators have a certain bulk band gap,and also have a metal surface state or an edge state without any band gap.Due to the special band structure,the topological insulators have good optical response in the range of visible light to infrared light.At the same time,the topological insulator materials have good nonlinear optical properties in the wide band range.The saturated absorption characteristics of the topological insulators have been proved in succession from visible to infrared and even in the terahertz band.Because of the unique optical properties of the topological insulators,it has important application value in the fields of transparent conductive film,solid laser,fiber laser,optical modulator and so on.The preparation of topological insulator materials can be achieved by many means.With the methods of stripping including mechanical peeling,liquid-phase stripping and electrochemical intercalation,ultrathin two-dimensional nanomaterials can be stripped from bulk materials.On the other hand,shape-controlled topological insulators can be obtained using chemical synthesis method such as chemical vapor deposition,molecular beam epitaxy and solution-based synthesis by controlling reaction conditions.As one of those chemical synthesis methods,solution-based synthesis is an effective way to prepare topological insulator materials.By controlling the reaction conditions including solvent,reaction temperature,reaction time,p H value,surfactant and so on,the shape,thickness and size of the product can be effectively controlled,and ultrathin two-dimensional nanomaterials can be obtained.The major contents are summarized as follows:1.Bi₂Te₃ nanosheets were prepared by two different methods.One is to use bismuth chloride and sodium tellurite as raw materials,ethylene glycol as solvent and PVP as surfactants in the high pressure reactor by solvothermal method under alkaline conditions.Ultrathin two-dimensional Bi₂Te₃ nanosheets with highly uniform hexagonal morphology have been successfully synthesized.These as-prepared nanosheets are 10-20 nm in thickness and 500-800 nm in width.Then the effects of reducing agent type,reaction time,reaction temperature,p H value and surfactant on the morphology of the reaction products were further discussed in detail.Another way to prepare Bi₂Te₃ nanosheets is the injecting method by using hotplate stirrer.Different morphologies of nanosheets and nanobelts were prepared by using different kinds of reductants,surfactants and p H regulators,and then the effects of the reaction conditions on the morphology of the products were studied.After comparing the morphologies of the Bi₂Te₃ nanosheets prepared by the two methods,the Bi₂Te₃ produced by the solvothermal method is selected as the optical medium,and the production of the ring-shaped beams based on the plasma channel effect is realized.When the pumping power reaches the ionization threshold of Bi₂Te₃ dispersion,the control beam will turn into a ring-shaped beam.Meanwhile,the change of the concentration of Bi₂Te₃ dispersion solution will lead to the change of nonlinear optical properties of the solution,thus the power of the pump beam required to produce the same ring-shaped beam will be different.The ring-shaped beam produced by the Bi₂Te₃ nanosheets as a nonlinear optical medium has very good stability and excellent spatial transmission characteristics.2.Bi₂Se₃ nanosheets were prepared in ethylene glycol solution using bismuth nitrate and sodium selenite as raw materials,PVP as surfactants and hydroxylamine as reducing agents by injection method.The Bi₂Se₃ nanosheets have very uniform shape and size,and almost all the nanosheets are of hexagonal shape.The Bi₂Se₃ nanosheets are 120 nm in edge width and 7 nm in thickness.Experimental research shows in detail that the morphology of the Bi₂Se₃ nanosheets can be controlled by choosing different reaction temperature,reaction time,reducing agent,surfactant and p H value.Meanwhile,Bi₂Se₃ nanosheets were prepared by solvothermal method,and the effects of different reaction conditions on the morphology of the products were discussed in detail.After comparing the morphologies of the Bi₂Se₃ nanosheets prepared by the two methods,the structure of a two-color all-optical switch based on the plasma channel effect was designed by using the Bi₂Se₃ nanosheets prepared by the injection method as the optical medium.The signal light can be modulated as two states including dot and ring shape by changing the intensity of control light.The modulated signal light exhibits excellent spatial propagation properties.3.Sb₂Te₃ nanosheets were prepared using antimony chloride and sodium tellurite as raw materials,PVP as surfactants and hydroxylamine as reducing agents by solvothermal method.These Sb₂Te₃ nanosheets exhibit hexagonal morphology and are 1.4?m in edge width and 60 nm in thickness.The effects of reaction temperature,reaction time,surfactant,reducing agent and p H value on the morphology of the product were studied in detail.Subsequently,Sb₂Te₃ nanosheets were prepared by another method of injection,and the influence of the above reaction conditions on the morphology of the product was also studied in detail.After comparing the morphologies of the Sb₂Te₃ nanosheets prepared by the two methods,the Sb₂Te₃ materials prepared by solvothermal method is used as the optical medium,and the generation of the ring-shaped beam based on plasma channel effect is realized.Due to the nonlinear effect of the Sb₂Te₃ materials,a plasma channel is generated in the Sb₂Te₃ dispersion when the solution is irradiated with pump beam.Then when the signal beam passes through the plasma channel,it turns into a hollow ring-shaped beam,just like passing through a gradient index lens.It was indicated that the dark spot size of the ring-shaped beam can be easily controlled by adjusting the power of the pump beam.The ring-shaped beam produced in the experiment keeps stable and displays well propagation properties in free space.4.Bi₂Se₃/Bi₂Te₃ heterojunction was prepared by two step method.Firstly,Bi₂Se₃ seed nanosheets were prepared by injection method.Then Bi₂Se₃/Bi₂Te₃ heterojunction was achieved by solvothermal method after adding proper amount of bismuth nitrate and sodium tellurite in the solution of Bi₂Se₃ seed nanosheets.Subsequently,the assynthesized Bi₂Se₃/Bi₂Te₃ heterojunction material was dispersed into ethanol solution as an optical medium to achieve the spatial self-phase modulation effect of 457 nm,532nm and 671 nm.At the same time,the two-color all-optical switching is realized innovatively based on the spatial self-phase modulation effect.By adjusting the light intensity of the control beam,the spatial light intensity distribution of the signal beam can be controlled conveniently,so as to achieve the purpose of controlling the beam switch.