Key Parameters In Controlling The Crytal Phase Of Polymorphic Semiconductor Nanocrystals

In the past few years, semiconductor nanocrystals have been extensively investigated due to their unique physical properties and promising applications in many fields including optoelectronic devices, photocatalytic and biomedicine etc. Because of the physical properties of semiconductor nanocrystals are closely related to their phase structure, composition, size, morphology and surface, it is highly aspired to develop simple, green and universal synthetic techniques for high-quality semiconductor nanocrystals with precise control. Moreover, another significant problem is to understand the key factors in the controlling their phycial parameters colloidal nanocrysal.This work aims to develop the colloidal chemistry for pricse crystal control of Cu_2-xS and CsPbBr₃ semiconductor nanocrystals. The main achievements are summerized as follows.（1） Cu_2-xS nanocrystals were selective synthesized using a simple hot injection method. All the resulting samples(hexagonal CuS, monoclinic Cu_1.75 S and rhombohedral Cu_1.8S nanocrystals) have well-defined size and morphology with intense near-infrared absorption peaks. By considering the theoretical chemical kinetics and experimental results, we reveal that oleyamine plays a key role in indirectly controlling the transformation from Cu²⁺ to Cu⁺, from S₈ to S^2-, and from Cu_2-xS clusters to Cu_2-xS nanocrystals in the reaction system, which account for the phase control. We further explored the use of these Cu_2-xS nanocrystals as couter electrode in quantum-dot sensitized solar cells, the power conversion efficiency reach to 5.81%.（2） We extended the emulsion synthesis for the fabrication of colloidal CsPbBr₃ nanocrystals at room temperature. The resulting CsPbBr₃ nanocrystals exhibit a controlled phase structre with photoluminescence quantum yields up to 70%. By adjusting Cs/Pb ratio of the precursors, we obtained cubic green（512 nm） and blue（460 nm） emissive CsPbBr₃ nanocrystals. These high quality CsPbBr₃ nanocrystals may find wide applications for thin film displays, solid-state lighting, and other solutionprocessed optoelectronic devices.The insights into the fine chemical reaction synthesis and reaction mechanism play an important role in the precise control of crystal phase and are also helpful to guide the colloidal synthesis of other colloidal nanocrystals.