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 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 half-metallicity, electrically switchable valley polarization, spin/valley filter, and valve effects in the transition-metal dichalcogenide monolayers. We further demonstrate our idea in ${\mathrm{MoTe}}_2/{\mathrm{CoCl}}_2$ and ${\mathrm{CoCl}}_2/{\mathrm{MoTe}}_2/{\mathrm{CoCl}}_2$ 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 contemporary devices.

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• First-principles calculations
• Valleytronics
• Transition metal dichalcogenides
• Electronic structure