  Strong evidence suggests that transformative correlated electron behavior may
exist only in unrealized clean-limit 2D materials such as 1T-TaS2.
Unfortunately, experiment and theory suggest that extrinsic disorder in free
standing 2D layers impedes correlation-driven quantum behavior. Here we
demonstrate a new route to realizing fragile 2D quantum states through
epitaxial polytype engineering of van der Waals materials. The isolation of
truly 2D charge density waves (CDWs) between metallic layers stabilizes
commensurate long-range order and lifts the coupling between neighboring CDW
layers to restore mirror symmetries via interlayer CDW twinning. The
twinned-commensurate charge density wave (tC-CDW) reported herein has a single
metal-insulator phase transition at ~350 K as measured structurally and
electronically. Fast in-situ transmission electron microscopy and scanned
nanobeam diffraction map the formation of tC-CDWs. This work introduces
epitaxial polytype engineering of van der Waals materials to access latent 2D
ground states distinct from conventional 2D fabrication.
