The synthesis of colloidal nanomaterials offers a vast potential for diverse applications, yet controlling the precise stoichiometry and crystal phase remains a significant challenge. Colloidal chemistry, while offering simplicity and mild reaction conditions, can lead to a mixture of products due to the multiple possible combinations of elements and structures. This is particularly relevant in the synthesis of complex inorganic nanomaterials where the formation of impurities or undesired phases is common. Organic chemistry has successfully addressed similar issues using reaction-directing groups that promote specific product formation and are later removed. This paper explores an analogous approach using inorganic nanocrystals as templates to guide the selective synthesis of desired phases. Specifically, the researchers aimed to synthesize two previously unexplored lead sulfochlorides: Pb3S2Cl2 and Pb4S3Cl2, leveraging the controlled formation and subsequent removal of a sacrificial CsPbCl3 perovskite template.

The synthesis of colloidal semiconductor nanocrystals has seen remarkable advancements, allowing for precise control over size, shape, and composition. Methods such as hot-injection synthesis have yielded monodisperse nanocrystals of various materials, including lead chalcogenides (PbS, PbSe, PbTe) and metal chalcogenides. However, the challenge of controlling the phase selectivity in the presence of multiple possible stoichiometries and structures remains. The use of epitaxial growth for creating heterostructures with tailored properties has gained traction, particularly with the advent of halide perovskite nanocrystals. These perovskites offer attractive optoelectronic properties and have been successfully integrated into heterostructures with other semiconductors. This work builds upon previous studies on lead chalcohalides (Pb4S3Br2, Pb4S3I2) which showed surprising similarities in optoelectronic properties despite variations in halide composition. This suggests that the electronic structure of these materials may be relatively insensitive to the specific halide involved.

The research employed a two-step process involving the synthesis of CsPbCl3 nanoclusters, followed by their use as templates for the phase-selective synthesis of lead sulfochlorides. CsPbCl3 nanoclusters were prepared using a modified procedure, optimizing the concentration of oleic acid in the Cs-oleate precursor. A lead oleate stock solution and a PbCl2 stock solution were also prepared for the subsequent reactions. The synthesis of Pb3S2Cl2 nanocrystals involved dissolving PbCl2 in a mixture of oleylamine, oleic acid, and octadecene (ODE), followed by the injection of a sulfur-ODE solution. The reaction was quenched rapidly, and the resulting nanocrystals were purified through centrifugation. Larger Pb4S3Cl2 nanocrystals were obtained via a seeded growth method using previously synthesized Pb4S3Cl2 nanocrystals as seeds. A stock solution containing PbCl2 and Pb(SCN)2 was added dropwise to a heated solution containing the seeds, promoting further growth. The synthesis of Pb4S3Cl2/CsPbCl3 heterostructures was achieved by reacting CsPbCl3 nanoclusters with sulfur in the presence of lead oleate and dodecanethiol at 200°C. The resulting heterostructures were purified and treated to restore their colloidal stability. A crucial step was the selective etching of the CsPbCl3 domain from the heterostructures to obtain phase-pure Pb4S3Cl2 nanocrystals. This etching process involved the use of dimethylformamide (DMF) to selectively dissolve CsPbCl3 while leaving Pb4S3Cl2 intact. The etched nanocrystals were purified and redispersed in toluene for characterization. A range of techniques were used for nanocrystal characterization. These include transmission electron microscopy (TEM) for imaging and analysis, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) for high-resolution imaging, X-ray powder diffraction (XRPD) for structural analysis, and three-dimensional electron diffraction (3D-ED) for crystal structure determination. Optical properties were characterized by UV-Vis absorption and photoluminescence (PL) spectroscopy, including time-resolved PL measurements. Density functional theory (DFT) calculations were employed to model the electronic structure of the lead sulfochlorides and the heterostructures. The DFT calculations helped in understanding the band alignment and charge carrier dynamics within the heterostructures.

The study successfully demonstrated the synthesis of two novel lead sulfochlorides, Pb3S2Cl2 and Pb4S3Cl2, using a phase-selective templating approach. Direct synthesis yielded Pb3S2Cl2 nanocrystals, while the presence of CsPbCl3 nanocrystals as a template promoted the formation of Pb4S3Cl2/CsPbCl3 heterostructures. A selective etching procedure using DMF successfully removed the perovskite template, producing phase-pure Pb4S3Cl2 nanocrystals. Detailed structural characterization using TEM, HAADF-STEM, XRPD, 3D-ED and PDF analysis revealed the crystal structures of both Pb3S2Cl2 and Pb4S3Cl2. The crystal structure of Pb3S2Cl2 showed that Cl can be coordinated by an octahedron of Pb2+ ions, unlike Br- and I-. Optical characterization revealed that both Pb3S2Cl2 and Pb4S3Cl2 exhibited similar absorption spectra with an indirect band gap of ~1.8 eV. Time-resolved PL studies showed a dominant sub-microsecond decay followed by a slower component. The PL intensity was significantly enhanced at lower temperatures. The Pb4S3Cl2/CsPbCl3 heterostructures displayed a type-I band alignment, resulting in efficient PL quenching of the perovskite domain due to photocarrier migration into the sulfochloride domain. DFT calculations confirmed the type-I band alignment and indicated that the band edge states were localized on the Pb4S3Cl2 domain. The optoelectronic properties of the lead sulfochlorides demonstrated remarkable similarity irrespective of the structure and stoichiometry, highlighting the dominant role of Pb2+ and S2- in defining their electronic properties.

The successful use of CsPbCl3 perovskite nanocrystals as disposable epitaxial templates for the phase-selective synthesis of lead sulfochloride nanocrystals represents a significant advancement in colloidal nanomaterial synthesis. The approach parallels the use of reaction-directing groups in organic chemistry, offering a pathway for controlling the outcome of competing reactions. The remarkable similarity in optoelectronic properties between Pb3S2Cl2 and Pb4S3Cl2 suggests a degree of structural and stoichiometric insensitivity in these materials. This finding contrasts with the behavior of other semiconductor nanocrystals, where variations in structure and stoichiometry significantly influence the optoelectronic properties. The type-I band alignment in the Pb4S3Cl2/CsPbCl3 heterostructures leads to efficient PL quenching in the perovskite domain, indicating potential applications in energy transfer and light harvesting systems. The discovery of Pb3S2Cl2 further expands the family of lead chalcohalides, providing valuable insights into the structure and properties of this class of materials. The findings of this study pave the way for exploring similar strategies for the controlled synthesis of other complex nanomaterials.

This research successfully employed CsPbCl3 perovskite nanocrystals as disposable templates for the phase-selective synthesis of two novel lead sulfochlorides, Pb3S2Cl2 and Pb4S3Cl2. The study revealed that the optoelectronic properties of these materials are largely insensitive to stoichiometry and structure. This phase-selective templating approach provides a powerful new strategy for overcoming challenges in colloidal nanomaterial synthesis, offering a route to create complex materials with predictable properties. Future research could explore the application of this method to other material systems and investigate the potential of lead sulfochlorides in various optoelectronic applications.

While the study provided a comprehensive investigation of the synthesis and properties of lead sulfochlorides, certain limitations exist. The synthesis procedures involved several steps and required careful control of reaction parameters. The use of DMF as an etching solvent introduced a potential for side reactions or changes in nanocrystal surface chemistry. Furthermore, the study focused on the optical properties in solution; further investigations are needed to determine how these properties are affected when integrated into solid-state devices. The DFT calculations were based on simplified atomistic models of the nanocrystals. Further theoretical calculations using more sophisticated models could provide a more accurate description of the electronic structure and properties.