Experimental realization of the topologically nontrivial phase in monolayer Si2Te2.

Quantum spin Hall insulators, with dissipationless edge channels protected by time-reversal symmetry, are promising for low-power and quantum devices. However, experimentally realized examples with sizable nontrivial gaps remain scarce. Monolayer Si2Te2 has been theoretically predicted to host a room-temperature quantum spin Hall phase, but the absence of a bulk analog has hindered its experimental realization. Here we show that HfTe2 provides an ideal van der Waals template for the epitaxial growth of strain-free monolayer Si2Te2 while preserving its nontrivial topological phase. Scanning tunneling microscopy and spectroscopy reveal an intact (1×1) lattice and a bulk band gap of  ~ 300 meV, consistent with first-principles calculations. Moreover, pronounced edge states extending ~ 2.0 nm from the island boundary are observed in monolayer Si2Te2, exhibiting characteristics expected for topological edge modes. Our results establish monolayer Si2Te2 as a large-gap two-dimensional topological-insulator platform for topological phenomena and device concepts at elevated temperatures.