Ternary and Quaternary Nano-/Micro-structures of All-Inorganic Halide Perovskites (CsPbX3 (X = Cl, Br, I)): Growth, Properties and Applications

全無機三元與四元鹵化物鈣鈦礦 (CsPbX3 (X = Cl, Br, I)) 的納米/微觀結構:生長,性質和應用

Student thesis: Doctoral Thesis

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Award date4 Mar 2021


All-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskites (IHPs) have attracted much attention in recent years because of their distinctive intriguing optical and electronic properties including wide band gap tunability, strong optical absorption, long carrier diffusion length, and high carrier mobility, to name a few. Moreover, CsPbX3 perovskites also show excellent resistance against open air oxygen, moisture and heat while still maintaining most of the unique properties of their organic hybrid counterparts. These unique characteristics make IHPs potentially suitable for light-emitting and other linear-nonlinear optical and electronic applications. In order to accelerate the advent of IHP optoelectronics, a detailed understanding of the materials system, their synthesis, structure, and properties are required.

Morphology and composition tuning of IHPs are keys to control their optical and electronic properties which determines their functionalities in various applications. Therefore, detailed understandings of the various aspects of different IHP morphologies, both nano- and micro- structures are of fundamental as well as technological importance. This thesis covers a systematic study on the various aspects of CsPbX3 IHPs, including the development of chemical vapor deposition (CVD) synthesis of nano- and micro- structures; revealing the growth mechanism of IHP nanowires; detailed characterization of different IHP and their anion alloys. Results of this work will then be discussed in terms of the materials’ application prospects.

First, we developed the non-catalytic chemical vapor deposition (NC-CVD) method for the growth of intrinsically pure CsPbX3 (Cl, Br) nanowires (NWs). The NW growth at different stages was thoroughly studied by electron microscopy and spectroscopic measurements in order to reveal the underlying NW growth mechanism. Our results suggested IHP NWs were grown via a vapor phase self-assembly mechanism. Two- and multi-photon absorption (TPA and MPA) based below bandgap excitation, photoluminescence (PL) emission, and second harmonic generation (SHG) demonstrated very good nonlinear optical (NLO) application prospect of the self-assembled IHP NWs. Moreover, low temperature optical measurements of a typical NW showed excellent amplified spontaneous emission (ASE) with the highest ever quality factor value, Q ⁓ 4500, reported so far for CsPbCl3 materials.

Secondly, growths of high quality single crystalline CsPbX3 (X = Cl, Br, I) IHP and their anion quaternary (X = Cl1-xBrx, Br1-xIx) alloy microplates were carried out by the single-step CVD approach. Intense emissions with narrow line width from these alloy microplates were achieved. Furthermore, optical emissions tunable in the whole visible spectral range by controlling the alloy composition were demonstrated. These IHP quaternary alloy microplates also showed good nonlinear two-photon absorption (TPA) property. Surface photovoltage (SPV) response from a typical quaternary alloy microplate sample having bandgap in between 1.7 – 2.3 eV, further demonstrated their prospect for photovoltaic tandem solar cell applications.

Finally, an enhanced two-step CVD process was developed and implemented for the precise composition controlled growth of quaternary IHP alloys. This new two-step CVD strategy exploited the fast diffusion of halide anions and eliminated the complex optimization process of growing alloys with different compositions due to vapor chemistry issues. It also eliminated the problem of deviation of the precursor ratio and materials composition encountered in the conventional single-step CVD growth process. High quality alloy microplates grown by the two-step process also exhibited strong PL emissions covering the visible solar spectrum and exhibited two-photon (TP) excited amplified spontaneous emissions (ASE). A comparative study demonstrated the superiority of the newly designed deposition time controlled enhanced two-step CVD over the existing conventional single-step CVD growth route in terms of the precise composition tunability and material quality.