Preservation of Topological Surface States in Wafer-Scale Transferred Membranes*

Ultrathin topological insulator membranes have been proposed to be building blocks of exotic quantum matters. However, traditional epitaxial growth does not facilitate material stacking in arbitrary orders, while mechanical exfoliation from bulk topological insulator crystals is also challenging due to the non-negligible interlayer coupling therein. Here we liberate wafer-scale ultrathin films of topological insulator Bi₂Se₃, grown by molecular beam epitaxy, down to the thickness of 3 quintuple layers. We characterize the preservation of the topological surface states and quantum well states using angle-resolved photoemission spectroscopy. With this liberation method, we fabricate free-standing topological insulator membranes and reveal the local strain effect near the grain boundaries using scanning transmission electron microscopy. Our work delineates the essential material processing protocol to explore the fundamental properties and applications in topological insulator thin films and heterostructures.