  The electronic bandgap of a material is often fixed after fabrication. The
capability to realize on-demand and non-volatile control over the bandgap will
unlock exciting opportunities for adaptive devices with enhanced
functionalities and efficiency. We introduce a general design principle for
on-demand and non-volatile control of bandgap values, which utilizes reversible
sliding-induced polarization driven by an external electric field to modulate
the irreversible background polarization in asymmetric two-dimensional (2D)
multilayers. The structural asymmetry can be conveniently achieved in
homobilayers of Janus monolayers and heterobilayers of nonpolar monolayers,
making the design principle applicable to a broad range of 2D materials. We
demonstrate the versatility of this design principle using experimentally
synthesized Janus metal dichalcogenide (TMD) multilayers as examples. Our
first-principles calculations show that the bandgap modulation can reach up to
0.3 eV and even support a semimetal-to-semiconductor transition. By integrating
a ferroelectric monolayer represented by 1T$'''$-MoS$_2$ into a bilayer, we
show that the combination of intrinsic ferroelectricity and sliding
ferroelectricity leads to multi-bandgap systems coupled to multi-step
polarization switching. The sliding-reversible bandgap modulation offers an
avenue to dynamically adjust the optical, thermal, and electronic properties of
2D materials through mechanical and electrical stimuli.
