  We grow nearly freestanding single-layer 1T'-WTe2 on graphitized 6H-SiC(0001)
by using molecular beam epitaxy (MBE), and characterize its electronic
structure with scanning tunneling microscopy / spectroscopy (STM/STS). We
demonstrate the existence of topological edge states at the periphery of
single-layer WTe2 islands. Surprisingly, we also find a band gap in the bulk
and the semiconducting behaviors of the single-layer WTe2 at low temperature,
which is likely resulted from an incommensurate charge density wave (CDW)
transition. The realization of two-dimensional topological insulators (2D TIs)
in single-layer transition metal dichalcogenide (TMD) thus provides a promising
platform for further exploration of the 2D TIs' physics and related
applications.
