  Semiconducting transition metal dichalcogenides (TMDs) have emerged as
materials that can be used to realize two-dimensional (2D) crystals possessing
rather unique transport and optical properties. Most research has so far
focused on sulfur and selenium compounds, while tellurium-based materials
attracted little attention so far. As a first step in the investigation of
Te-based semiconducting TMDs in this context, we have studied MoTe$_2$ crystals
with thicknesses above 4 nm, focusing on surface transport and a quantitative
determination of the gap structure. Using ionic-liquid gated transistors, we
show that ambipolar transport at the surface of the material is reproducibly
achieved, with hole and electron mobility values between 10 and 30 cm$^2$/Vs at
room temperature. The gap structure is determined through three different
techniques: ionic-liquid gated transistors and scanning tunneling spectroscopy,
that allow the measurement of the indirect gap ($E_{ind}$), and optical
transmission spectroscopy on crystals of different thickness, that enables the
determination of both the direct ($E_{dir}$) and the indirect gap. We find that
at room temperature $E_{ind}$ = 0.88 eV and $E_{dir}$ = 1.02 eV. Our results
suggest that thin MoTe$_2$ layers may exhibit a transition to a direct gap
before mono-layer thickness. They should also drastically extend the range of
direct gaps accessible in 2D semiconducting TMDs.
