  Transition metal dichalcogenide (TMDC) monolayers present a remarkable
multifunctional material with potential to enable the development of a wide
range of novel devices. However, the functionalities observed often fall short
of the expectation, which hinders the device development. Here we demonstrate
that the optical, catalytic, and thermal functionalities of TMDC monolayers can
all be substantially enhanced by up to orders of magnitude with the
intercalation of water molecules or small cations (H+ and Li+) between the
monolayers and underlying substrates. In contrast, the same molecules or
cations adsorbed on top of the monolayers show negligible effects. We also
discover two major roles of the intercalated species in the enhancement: doping
the monolayers and modifying the interaction of the monolayers with the
substrate. The result points out a versatile and convenient strategy of using
the intercalation of molecules or ions to enhance the functionalities of TMDC
monolayers.
