  Superlattice engineering provides the means to reshape the fabric felt by
quasiparticles moving in a material. Here we argue that bandstructure
engineering with superlattices can be pushed to the extreme limit by stacking
gapped van der Waals (vdW) materials on patterned dielectric substrates.
Specifically, we find that high quality vdW patterned dielectric superlattices
(PDS) realize a series of robust flat bands that can be directly switched on
and off by gate voltage in situ. In contrast to existing superlattice
platforms, these flat bands are realized without the need for fine tuning.
Instead, the bands become flat as the gate voltage increases in magnitude. The
characteristics of PDS flatbands are highly tunable: the type of flatband
(single non-degenerate or dirac-cone-like), localization length, and
interaction energy are sensitive to the applied gate voltage. As a result,
electron-electron interactions in the PDS flatbands can become stronger than
both the bandwidth and disorder broadening, providing a setting for correlated
behavior such as flatband ferromagnetism. We expect PDS flatbands can be
experimentally realized in a range of readily available gapped vdW materials
such as monolayer transition metal dichalcogenides, e.g. WSe2.
