Observation of strain-modulated topological insulator and semimetal states in monolayer 1T′-MoTe2

Two-dimensional topological insulators have attracted considerable attention due to their topological edge states (ESs), which originate from the nontrivial topology of the insulating bulk states. Monolayer 1T′-MoTe2 has been theoretically predicted to have topological ESs. However, the experimental verification of its topological characteristics (i.e., bandgap opening) with spatial information remains elusive due to its thermodynamic instability and semi-metallic characteristics. Here, we utilize molecular beam epitaxy to synthesize a series of high-quality monolayer 1T′-MoTe2 with different lattice deformations (strains) on graphitized SiC (Gr/SiC) and Au (111) substrates. Using scanning tunneling microscopy, we observe the topological insulator and semimetal states in monolayer 1T′-MoTe2. Furthermore, by combining experimental observation and density functional theory calculations, we confirm that the tensile strain along the Y-axis (lattice constant b) is a crucial factor in modulating the bandgap opening of the monolayer 1T′-MoTe2. This work not only deepens our understanding of the topological properties of monolayer 1T′-MoTe2 but also provides the potentiality for optimizing its topological characteristics through the regulation of local strain.