  Two-dimensional (2D) topological insulators (TIs) are promising platforms for
low-dissipation spintronic devices based on the quantum spin Hall (QSH) effect,
but experimental realization of such systems with a large band gap suitable for
room-temperature applications has proven difficult. Here, we report the
successful growth on bilayer graphene of a quasi-freestanding WSe$_2$ single
layer with the 1T' structure that does not exist in the bulk form of WSe$_2$.
Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling
microscopy/spectroscopy (STM/STS), we observed a gap of 129 meV in the 1T'
layer and an in-gap edge state located near the layer boundary. The system's 2D
TI characters are confirmed by first-principles calculations. The observed gap
diminishes with doping by Rb adsorption, ultimately leading to an
insulator-semimetal transition. The discovery of this large-gap 2D TI with a
tunable band gap opens up opportunities for developing advanced nanoscale
systems and quantum devices.
