  Electron valleys in transition-metal dichalcogenide monolayers drive novel
physics and allow designing multifunctional architectures for applications. We
propose to manipulate the electron valleys in these systems for spin/valley
filter and valve devices through band engineering. Instead of the magnetic
proximity effect that has been extensively used in previous studies, in our
strategy, the electron valleys are directly coupled to the spin-polarized
states of the two-dimensional ferromagnets. We find that this coupling results
in a valley-selective gap opening due to the spin-momentum locking in the
transition-metal dichalcogenide monolayers. This physics gives rise to a
variety of unexpected electronic properties and phenomena including
halfmetallicity, electrically switchable valley polarization, spin/valley
filter and valve effects in the transition-metal dichalcogenide monolayers. We
further demonstrate our idea in MoTe$_2$/CoCl$_2$ and
CoCl$_2$/MoTe$_2$/CoCl$_2$ van der Waals heterojunctions based on
first-principles calculations. Thus, our study provides a way of engineering
the electron valleys in transition-metal dichalcogenide monolayers for
new-concept devices.
