Fully-gapped topological insulating states in a bilayer Bi(111) film grown on α-MoO₃

Fully-gapped topological insulators (TIs) integrated with semiconducting or insulating substrates are essential for non-dissipative electronics, where transport occurs through topologically protected edge channels. However, substrate-induced effects often increase bulk states within the gap or drive topological phase transitions, undermining device functionality. Here, we report the molecular beam epitaxy growth of bilayer Bi(111) on monolayer α-MoO₃, forming a high-quality TI-semiconductor heterostructure despite their distinct lattice symmetries. Scanning tunneling microscopy reveals atomically sharp interfaces, while spectroscopy demonstrates that the Bi(111) edge states remain intact and reside within a full bulk gap (∼+0.5 to +0.9 V). These edge states extend ∼2.5 nm from step edges, exhibit one-dimensional dispersion, and are resilient to variations in edge geometry and defects. First-principles calculations verified the experimental results and the topological origin of the edge states. Our findings establish a feasible platform for integrating fully-gapped TIs with van der Waals semiconductors, advancing the material foundation for ultra-thin-film, non-dissipative electronic devices.