  Two-dimensional (2D) topological insulators (TIs) with a large bulk band-gap
are promising for experimental studies of the quantum spin Hall effect and for
spintronic device applications. Despite considerable theoretical efforts in
predicting large-gap 2D TI candidates, only few of them have been
experimentally verified. Here, by combining scanning tunneling
microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we
reveal that the top monolayer of ZrTe5 crystals hosts a large band gap of ~100
meV on the surface and a finite constant density-of-states within the gap at
the step edge. Our first-principles calculations confirm the topologically
nontrivial nature of the edge states. These results demonstrate that the top
monolayer of ZrTe5 crystals is a large-gap 2D TI suitable for topotronic
applications at high temperature.
