Van der Waals Epitaxial Growth of Atomically Thin Bi₂Se₃ and Thickness-Dependent Topological Phase Transition

Two-dimensional (2D) atomic-layered heterostructures stacked by van der Waals interactions recently introduced new research fields, which revealed novel phenomena and provided promising applications for electronic, optical, and optoelectronic devices. In this study, we report the van der Waals epitaxial growth of high-quality atomically thin Bi₂Se₃ on single crystalline hexagonal boron nitride (h-BN) by chemical vapor deposition. Although the in-plane lattice mismatch between Bi₂Se₃ and h-BN is approximately 65%, our transmission electron microscopy analysis revealed that Bi₂Se₃ single crystals epitaxially grew on h-BN with two commensurate states; that is, the (1¯21¯0) plane of Bi₂Se₃ was preferably parallel to the (1¯100) or (1¯21¯0) plane of h-BN. In the case of the Bi₂Se₃ (2¯110) ∥ h-BN (11¯00) state, the Moiré pattern wavelength in the Bi₂Se₃/h-BN superlattice can reach 5.47 nm. These naturally formed thin crystals facilitated the direct assembly of h-BN/Bi₂Se₃/h-BN sandwiched heterostructures without introducing any impurity at the interfaces for electronic property characterization. Our quantum capacitance (QC) measurements showed a compelling phenomenon of thickness-dependent topological phase transition, which was attributed to the coupling effects of two surface states from Dirac Fermions at/or above six quintuple layers (QLs) to gapped Dirac Fermions below six QLs. Moreover, in ultrathin Bi₂Se₃ (e.g., 3 QLs), we observed the midgap states induced by intrinsic defects at cryogenic temperatures. Our results demonstrated that QC measurements based on h-BN/Bi₂Se₃/h-BN sandwiched structures provided rich information regarding the density of states of Bi₂Se₃, such as quantum well states and Landau quantization. Our approach in fabricating h-BN/Bi₂Se₃/h-BN sandwiched device structures through the combination of bottom-up growth and top-down dry transferring techniques can be extended to other two-dimensional layered heterostructures.