Extended Data Fig. 3: Detailed reaction conditions in our experiments.

Eight MIs powders were used as metallic compound precursors. They can be vaporized below melting points. Then, the deposited 2D MIs flakes will transform to 2D MCs in the environments with heated chalcogen (S, Se and Te) vapour. The blue color represents a layered structure, and the purple color represents a non-layered structure. The CH₃NH₃PbI₃ perovskites were obtained by replacing the chalcogen resource with CH₃NH₃PbI₃ by using the same low-temperature strategy. It is noticed that many difficulties exist in the I-to-Te conversion due to the higher melting point of tellurium powder and the lower activity of Te atoms with metals compared to sulfur or selenium atoms. Therefore, most of the tellurides were not successfully synthesized using this approach. Although the SnTe and Bi₂Te₃ were successfully synthesized, they need more reaction time than sulfurization and selenization. In addition, the 2D crystal morphology of converted PbS cannot be preserved from PbI2 precursors. We believe the quickly centralized release of strain easily leads to broken PbS. Thus, we think the epitaxial substitution is unsuitable for synthesizing 2D PbS.