Preparation of metal selenide nanocrystals with hot-injection method

Nanomaterials have received extensive and growing research interests due to their unique properties and potential applications in various fields. They have contributed immensely to modern materials science and technology during the last three decades. Among various types of functional nanomaterials, metal selenide nanocrystals have attracted considerable attention because of their special properties. These nanomaterials have been applied in a wide range, such as medicine, electronics, photonics, energy conversion and storage devices, etc. Interestingly, when metals are chemically combined with selenide in the form of nanocrystals, they exhibit semiconductor-like electronic properties. These metal selenide nanocrystals have attracted significant attention in recent years due to their potential applications in solar energy conversion and storage, which are the most important challenges to be addressed to meet the world's rising demand for clean energy in the future. Hot-injection method is an effective method for the synthesis of metal selenide nanocrystals. The success of this approach lies in the use of non-ionic precursors in high-boiling organic solvents. This makes it possible to grow the nanocrystals relatively slowly at a high temperature, which yields defect-free, well-passivated nanocrystals. The anther important aspect of this method is the separation of the nucleation and growth stages. Due to this, a high degree of monodispersity can be achieved. The fundamental principle of hot-injection method with concrete steps has been introduced. This review surveys the recent progress of the preparation of metal selenide nanocrystals by hot-injection method in terms of binary metal selenide nanocrystals, ternary metal selenide nanocrystals, quaternary, and multinary metal selenide nanocrystals. For binary metal selenide nanocrystals, four important binary metal selenide nanocrystals including CdSe, ZnSe, PbSe, and Sb₂Se₃ nanocrystals are discussed. Then the achievement of synthesis for CuInSe₂ and CuGaSe₂ nanocrystals by hot-injection method and their application in photovoltaic field are systematically cataloged as well. And the progress in preparation for CuInGaSe₂ and Cu₂ZnSnSe₄ nanocrystals by hot-injection method has been clarified clearly. Organoselenide chemicals are generally toxic and unstable. The replacement of phosphine containing compounds in the synthesis process of metal selenide nanocrystals via hot-injection method has received considerable attention in recent years. Many attempts have been made to replace trioctylphosphine (TOP) as a medium for delivering selenium in these reactions, but each alternative possesses has significant limitations. This review mainly emphasizes on the recent advances toward the development of methods to achieve "green Se precursor" with high activity and "facile green synthesis" of metal selenide nanocrystals involved in various approaches. Two kinds of different schemes for synthesis and utilization "green Se precursor" are described. Finally, technical challenges and future trends are presented for synthesis of high quality metal selenide nanocrystals, in terms of composition-tunable and structure-controllable multinary metal selenide nanocrystals, green synthesis, industrial production and large-scale application.