  We present a study of the electronic and optical bandgap in layered TiS3, an
almost unexplored semiconductor that has attracted recent attention because of
its large carrier mobility and inplane anisotropic properties, to determine its
exciton binding energy. We combine scanning tunneling spectroscopy and
photoelectrochemical measurements with random phase approximation and
Bethe-Salpeter equation calculations to obtain the electronic and optical
bandgaps and thus the exciton binding energy. We find experimental values for
the electronic bandgap, optical bandgap and exciton binding energy of 1.2 eV,
1.07 eV and 130 meV, respectively, and 1.15 eV, 1.05 eV and 100 meV for the
corresponding theoretical results. The exciton binding energy is orders of
magnitude larger than that of common semiconductors and comparable to bulk
transition metal dichalcogenides, making TiS3 ribbons a highly interesting
material for optoelectronic applications and for studying excitonic phenomena
even at room temperature.
