Controlled Synthesis Of Highly Luminescent All-Inorganic Halide Perovskite Nanocrystals

All-inorganic halide perovskite nanocrystals(NCs),as new semiconductor materials,have attracted wide attention,due to their potential applications in optoelectronic devices.However,how to obtain high-quality nanocrystals with excellent optical properties is always a great challenge.Among lead halide perovskite NCs,cesium lead chloride(CsPbC13)NCs have extremely poor optical properties because of their intrinsic atomic point defects.So far,the creation of highly luminescent violet-emitting CsPbCl3 NCs have mostly relied on doping of a limited number of small-sized metal ions or post-synthetic surface treatment of NCs.Alkaline-earth(AE)metals(e.g.,Ca2+,Sr2+,and Ba2+)have been proposed to be able to replace Pb2+ in halide perovskites,yet it remains incompletely understood whether AE metal ions can be incorporated into the perovskite lattice or can be merely situated at the surface.Moreover,lead halide perovskites have high-toxic lead that is harmful to the environment.To overcome this issue,many researches are trying to develop lead-free halide perovskite materials.Among lead-free halide perovskites,cesium tin iodide(CsSnI3)NCs have an appropriate bandgap that is suitable for solar cells.However,because of the extremely low-stability of Sn2+,it is really a great challenge to get high-quality CsSnI3 NCs.The reported highest photoluminescence quantum yield(PLQY)is only 0.06%for CsSnI3 NCs that is synthesized by a traditional hot-injection method.As a result,developing a new synthesis method to boost the PLQY of CsSnI3 NCs is urgently needed.Therefore,this thesis aims to:1)introduce AE metals into the reaction system via one-pot synthesis,and study the actual distribution of AE metals enabled by pyridine etching;2)develop an innovative synthesis method to obtain CsSnI3 NCs with a high PLQY,and study their structural and optical properties by various experimental characterizations coupled with theoretical calculations.Detailed results are as follows.Firstly,through a one-pot synthesis method,we introduced AE metals into the reaction system(AEC12,AE=Mg,Ca,Sr,or Ba),and successfully obtained AE ions-passivated CsPbCl3 NCs.Through the creation of a Ca2+/Sr2+-involved passivation layer,we reduced the Cl vacancies in CsPbCl3 NCs and boosted their PLQY as high as 77.1%.A wide range of optical and structural characterizations,coupled with first-principles calculations,aid in clarifying the underlying mechanism for the AE-metal-dependent passivation of CsPbCl3 NCs.Specifically,based on the experimental and theoretical results,a model is proposed for the observed abnormal incorporation phenomenon of AE2+ ions in NCs(i.e.,Ba2+ can be incorporated into the core of NCs,Ca2+/Sr2+ can only be at/near the surface,while Mg2+can neither be in the core nor at the surface).We believe that the knowledge gained here may not only offer a new perspective to obtain high-efficiency violet-emitting perovskite NCs but can also help elucidate the functions that AE2+ ions play in the optimization of perovskite optoelectronic devices.Secondly,we developed an innovative synthetic approach to synthesize CsSnI3 NCs:we chose Cs-oleate,benzoyl iodide(BI)and tin 2-ethylhexanoate(EHT)as three precursors to produce Cs+,I-and Sn2+ ions,respectively.Based on a hot-injection method,we successfully obtained high-quality CsSnI3 NCs with a 22.9%PLQY.By changing different ratios of Sn/I,we synthesized a series of CsSnI3 NCs.We then studied the properties of these CsSnI3 NCs with a wide range of optical and structural characterizations.Our results not only provide a new synthetic approach to get high-quality CsSnI3 NCs,but also may pave the way for their applications in various optoelectronic devices.