Due to the toxicity of lead, commercial application of lead halide perovskites will cause environmental pollution. Therefore, replacing lead with nontoxic elements has been a focus in this field. Tin-based halide perovskite has a near-infrared optical response, which can effectively solve the toxicity of lead-based perovskite and exhibit properties comparable to lead-based perovskite. High-quality CH₃NH₃SnI₃ nanoplatelet with a smooth surface, regular shape, and controllable size is of great significance for the development of micro- or nano-optoelectronic devices. Currently, CH₃NH₃SnI₃ is mainly synthesized through the solution method. The development of novel synthetic routes for high-quality CH₃NH₃SnI₃ is critical for high-performance lead-free optoelectronic devices. The chemical vapor deposition method without the use of solvents, which can prevent the evolution of grain boundaries and surface defects, has been proven in the preparation of high-quality micro- or nano-structured perovskite. In this paper, a two-step vapor deposition method is developed to prepare high-quality CH₃NH₃SnI₃ nanoplatelets. The dependence of sizes and compositions of the nanoplatelets on deposition time, H₂ flow rate, conversion time, and Ar flow rate is systematically studied through experiments combined with crystal nucleation theory. High-quality CH₃NH₃SnI₃ nanoplatelets with controllable sizes and uniform surfaces are achieved, where the sizes can be controlled between 8-41 μm. They show good near-infrared (920 nm) photoluminescence performance. In addition, to slow down the oxidation rate of tin-based perovskite, which is another big challenge, researchers have proposed various solutions such as solution doping. Unfortunately, the effect of doping will also change the overall structure. Therefore, we achieve high-stability (more than 48 h in N₂ atmosphere) CH₃NH₃SnI₃ nanoplatelets by passivating the surface of the prepared nanoplatelets through polymethyl methacrylate (PMMA) coating, which does not destroy the molecular structure of the perovskite. This lead-free perovskite nanomaterial with controllable size and composition can be applied to develop near-infrared optoelectronic devices in the future.

We employ a two-step chemical vapor deposition method. Firstly, SnI₂ nanoplatelet precursor with a smooth surface, regular shape, and controllable size (8-41 μm) is successfully prepared on a mica substrate by adjusting the H₂ flow rate and reaction time. Then, the SnI₂ nanoplatelets are converted into CH₃NH₃SnI₃ by using an Ar-driven reaction between CH₃NH₃I and the precursor. The surface morphology and chemical compositions of the nanoplatelets are analyzed through optical microscopy and X-ray diffraction. Based on crystal nucleation theory, the effects of H₂ flow rate and reaction time on the surface morphology and size of SnI₂ nanoplatelets are systematically studied. The effects of Ar flow rate and conversion time on the composition and photoluminescence properties of the prepared CH₃NH₃SnI₃ nanoplatelets are also studied. Then absorption and photoluminescence of CH₃NH₃SnI₃ nanoplatelets prepared under appropriately optimized conditions are measured to characterize their quality. Finally, stability tests are conducted on the prepared nanoplatelets to characterize their stability through the passivation effect of PMMA film in the N₂ atmosphere.

The prepared SnI₂ nanoplatelets have uniform colors and regular shapes. By adjusting the H₂ flow rate and reaction time, nanoplatelets with controllable sizes ranging from 8 to 41 μm are prepared (Fig. 2). We find that the driving force of flow rate will affect the uniformity of deposition. As the reaction time increases, the desorption rate of the substrate increases, and appropriate conditions are critical for fabricating nanoplatelets with controllable sizes. The X-ray diffraction images and absorption spectra of the prepared CH₃NH₃SnI₃ nanoplatelets show that the nanoplatelet is mainly composed of CH₃NH₃SnI₃ and have narrower bandgaps. Subsequently, the nanoplatelet also exhibits good near-infrared (920 nm) photoluminescence characteristics (Fig. 4). A PMMA thin film that is spin-coated on the surface of the prepared nanoplatelets is also demonstrated to prevent the oxidation characteristics of tin. X-ray diffraction images of PMMA-passivated CH₃NH₃SnI₃ at different time shows that the perovskite exhibits great stability under a N₂ atmosphere for more than 48 h [Fig. 5(b)].

In the paper, we prepare high-quality single crystal CH₃NH₃SnI₃ nanoplatelets with controllable sizes by using a two-step chemical vapor deposition method and systematically study the effects of flow rate, reaction time, and other factors on the crystal size and morphology of CH₃NH₃SnI₃ nanoplatelets. We find that when the H₂ flow rate ranges from 14 mL/min to 18 mL/min, and the reaction time is 35 min, the prepared nanoplatelets has a regular shape and smooth surface. The size of the nanoplatelets increases with the increase in flow rate, and the average side length increases from 8 μm to 41 μm. At the same time, the density of nanoplatelets also increases. In the second step, the Ar flow rate is tuned from 38 mL/min to 42 mL/min, and the content of CH₃NH₃SnI₃ in the nanoplatelets increases first and then decreases. As the reaction time increases, the CH₃NH₃SnI₃ content in the nanoplatelets show similar behavior. Full conversion into CH₃NH₃SnI₃ is achieved at an Ar flow rate of 40 mL/min and reaction time of 160 min. The study of absorption spectra and steady-state PL spectra shows that CH₃NH₃SnI₃ nanoplatelets have good near-infrared (920 nm) photoluminescence characteristics. In addition, we also present a passivation method for adding PMMA thin films on the surface of nanoplatelets by spin coating, which can effectively isolate the water oxygen problem of Sn²⁺ and has good stability in the N₂ atmosphere for over 48 h. With the further improvement of the stability of tin-based perovskite nanoplatelets, the photoluminescence properties of tin-based perovskite nanoplatelets will be improved. This work lays the foundation for the research on lead-free perovskite nanoplatelets and is expected to significantly improve the performance of optoelectronic devices.