Controllable Modulation of Morphology and Property of CsPbCl 3 Perovskite Microcrystals by Vapor Deposition Method

: As a direct wide bandgap semiconductor, CsPbCl 3 has great potential applications in the field of near-ultraviolet photodetectors, lasers and higher-order multiphoton fluorescent detectors. In this paper, we systematically explored the technology to synthesize CsPbCl 3 micro/nanocrystals by vapor deposition method with CsCl and PbCl 2 powders as the source materials. It was confirmed that the formation of CsPbCl 3 perovskite through the chemical reaction of CsCl with PbCl 2 occurred in the quartz boat before the source evaporation, not in vapor or on substrate surface. The evaporated CsPbCl 3 can form micro/nanocrystals on substrate surfaces in appropriate conditions. Various morphologies including irregular polyhedrons, rods and pyramids could be observed at lower temperature, while stable and uniform CsPbCl 3 single crystal microplatelets were controllably synthesized at 450 o C. Prolonging the growth time could modulate the size and density of the microcrystals, but could not change the morphology. Substrate types made little difference to the morphology of CsPbCl 3 crystals. The photoluminescence spectra indicated that the crystallinity and morphology of CsPbCl 3 micro/nanocrystals have significant effects on their optical properties. The above results are expected to be helpful to the development of optoelectronic devices based on individual CsPbCl 3 microcrystal.

Lead halide perovskites have been extensively studied in the past 10 years since Akihiro Kojima used CH3NH3PbBr3 and CH3NH3PbI3 as the visible-light sensitizer in photoelectrochemical cells [1]. Owing to their unique optical and electric properties, a series of lead halide perovskite materials have become research hotspots in the field of optoelectronic devices such as solar cells, photodetectors and lasers [2][3][4][5][6][7]. Considering the fact that organic-inorganic hybrid lead halide perovskites are unstable under ambient conditions because of the presence of organic cation [8], more studies have been focused on the all inorganic perovskites such as cesium lead halide (CsPbX3) perovskites which display improved thermal stability while reserving intriguing photoelectronic properties [9].
As a typical direct and wide bandgap semiconductor, CsPbCl3 perovskite has been paid much attention and found wide applications. The usage in fast scintillators make CsPbCl3 a promising material in high-energy physics and for near-ultraviolet optoelectronic detectors [10,11]. Chun Kit Siu has studied the nonlinear absorption and emission characteristics of single crystalline CsPbCl3 perovskite microcavities under multiphoton excitation, and revealed the prospect in high-performance upconversion lasing devices [12]. Tze Chien Sum et al. prepared single crystalline CsPbCl3 whispering-gallery-mode microcavities with low threshold and high spectral coherence by van der Waals epitaxy method [13]. In addition, Zhifeng Shi and co-workers reported a photodetector based on high quality horizontal CsPbCl3 microwire networks with high on/off photocurrent ratio of 2 × 10 3 and a photoresponsivity of 14.3 mAxW -1 , as well as a fast response speed [14]. Any way, it is expected that some types of morphologies of CsPbCl3 would be promising candidates for new kinds of optoelectronic applications. Therefore, the morphology-controlled synthesis of CsPbCl3 microcrystals is important for exploring the potentials of CsPbCl3.
In this work, we systematically investigated the modulation effect of some factors including evaporation temperature of source materials, growth temperature, growth time and substrate type on the morphologies and optical properties of CsPbCl3 microcrystals using one-step vapor deposition method. It is found that the growth temperature plays a crucial role in modulating the morphology and size of CsPbCl3 microcrystals. And the room temperature photoluminescence property of CsPbCl3 microcrystals grown at different temperatures significantly depends on the crystallinity and the morphology of CsPbCl3 crystals.

Ⅱ. EXPERIMENTS
The source materials including cesium chloride (CsCl, >99.9%) and lead chloride (PbCl2, >99.9%) were purchased from Alfa Aesar and used without any further purification. The Si (100) wafer with an oxide overlayer about 300 nm were bought from Suzhou Research Materials Micro-nano Technology Co., Ltd and was cleaved to 10 x10 mm 2 slices for using as substrate. At first, the SiO2/Si (100) substrate slices were ultrasonically cleaned in acetone, ethanol and deionized water for 15 min, respectively. Next, they were immersed in a fresh piranha solution (Concentrated sulfuric acid : hydrogen peroxide solution= 3:1) for 1 h at 90 o C in order to remove the residual acetone and other impurities on the surface. Finally, the substrates were rinsed with ultrapure water repeatedly and dried in a stream of N2 gas.
The CsPbCl3 microcrystals were prepared in a home-built vapor deposition system, a quartz tube furnace (inner diameter = 6 cm, length = 100 cm) with two temperature zones, equipped with a mass flow controller and pressure monitor. A quartz boat filled by the mixture of CsCl and PbCl2 powders with a molar ratio of 1:1 was placed in the center of upstream zone of the furnace. The cleaned SiO2/Si (100) substrate slices were placed in the center of downstream zone of the furnace. Prior to heating, the quartz tube was pumped down with high-purity Ar gas at a flow rate of 100 sccm for 30 min to purge out any residual air in the tube. Afterwards, the upstream zone was heated to 600 o C at a rate of 10 o C/min, and the downstream zone was heated to 390-470 o C in 45 min.
The inner pressure was maintained at about 380 Torr. The growth lasted for a duration of 5-70 min since the target temperatures were reached at almost the same time and then the furnace was naturally cooled to room temperature.
The crystal structure was characterized by X-ray diffraction (XRD) technique using the TTR-III diffractometer with Cu-Kα radiation. The morphology of assynthesized samples was obtained by the SU8220 cold field emission scanning electron microscope (FESEM). The photoluminescence (PL) spectra were measured on a Jobin Yvon-LabRamHR Evolution spectrometer with the excitation wavelength λex =325 nm. And, the X-ray diffraction (XRD) spectra and photoluminescence (PL) spectra all were obtained under ambient conditions.

A. Solid-State Reaction of the Source Materials
The formation route of CsPbCl3 perovskite through the sources of CsCl and platelets are stuck together tightly to form a dendritic shape with 500 µm in length, which may be due to the poor tolerance of perovskite materials when the temperature is out of an appropriate range [17].
The morphology evolution of the samples could be explained by the Gibbs-Curie-Wulff theorem. It is generally accepted that the surface free energy plays a vital part in nucleation as well as crystal growth, and determines the equilibrium crystal shape at a certain temperature. The facets with higher surface energy usually make up a very small fraction of the surface or even disappear in the final crystals due to instability, but the facets with lower surface energy will be preserved, determining the shape of the final crystals [18]. Hence, the morphology of CsPbCl3 microcrystals is irregular polyhedrons at low temperature but regular square platelets at high temperature. High temperature might promote crystal growth but inhibit crystal nucleation, so the size of  3(a, c)) but much weaker diffraction intensities (FIG. 3(k)), this behavior might be attributed to the multiple morphology and heterogeneity of the microcrystals. Although the tetragonal phase single crystal is still the main component at 390 o C and 410 o C, its diffraction peaks were not enhanced due to the existence of other crystal phases. The similar phenomenon also happens between the samples at 430 o C and 450 o C, the former has larger microcrystal concentration but lower diffraction intensity than the latter. In addition, it is interesting that the change trend in diffraction intensity may remain consistent with the evolution in crystal morphology and crystal quality of the CsPbCl3 microcrystals, which is similar to the CH3NH3PbBr3 crystals as Zhang reported previously [19]. The single crystal morphology and excellent crystallinity lead to strong XRD diffraction peaks. Combined with the above results, it's suggested that the growth time has an imperative effect on the size and particle density of CsPbCl3 microcrystals, while the substrate type makes little difference to the morphology of CsPbCl3 crystals.

C. Optical Properties of the CsPbCl3 Microcrystals
The CsPbCl3 has a wide bandgap with a large exciton binding energy about 75 meV [20], which suggests that stable excitons may exist in interior region of the crystal and this material is suitable for near-ultraviolet emission, so it's necessary to estimate the optical properties of the CsPbCl3 microcrystals .The However, when the temperature reaches 470 o C, the peak intensity decreases with the decrease of peak shape characteristics. All these results are consistent with the variation of XRD intensity in FIG.3(k). It demonstrates that the PL intensity of the characteristic peak get increased due to the improvement of crystal quality. Additionally, the main emission peaks happen obvious blueshift with lifted temperature. We can conclude that the difference of spectrum shape and blueshift of the emission peak may be due to the difference in the crystallinity and morphology of CsPbCl3 microcrystals grown at different temperatures. The nature of the optical emission features and transition mechanism needs further investigation and analysis in more detail, and has been under progress.
FIG.6 Steady-state PL spectra of the samples prepared at various growth temperatures measured using a 325 nm excitation wavelength at 300 K.

Ⅳ. CONCLUSION
In summary, controllable growth of microcrystals were systematically studied through CVD technique. At first, the CsPbCl3 perovskite phase is proved to form in the source boat before the source evaporation by the reaction of CsCl with PbCl2 on heating, not on the substrate surface. Through tuning the growth temperature, the micro-sized CsPbCl3 crystals with different morphologies including rods, frustum of pyramids, square platelets could be obtained.
Additionally, the growth time and temperature have a great effect on the size and particle density of CsPbCl3 microcrystals. The higher photoluminescence intensity is consistent with the higher crystal quality and uniformity remarkably. Importantly, the CsPbCl3 microplatelets grown at 450 o C show excellent luminous performance and the main photoluminescence characteristic peak occurs blue shift with increasing temperature. These interesting results are worthy of further exploring. This work can offer pivotal parameters for controlling