  High-density phase change memory (PCM) storage is proposed for materials with
multiple intermediate resistance states, which have been observed in
1$T$-TaS$_2$ due to charge density wave (CDW) phase transitions. However, the
metastability responsible for this behavior makes the presence of multistate
switching unpredictable in TaS$_2$ devices. Here, we demonstrate the
fabrication of nanothick verti-lateral $H$-TaS$_2$/1$T$-TaS$_2$
heterostructures in which the number of endotaxial metallic $H$-TaS$_2$
monolayers dictates the number of resistance transitions in 1$T$-TaS$_2$
lamellae near room temperature. Further, we also observe optically active
heterochirality in the CDW superlattice structure, which is modulated in
concert with the resistivity steps, and we show how strain engineering can be
used to nucleate these polytype conversions. This work positions the principle
of endotaxial heterostructures as a promising conceptual framework for
reliable, non-volatile, and multi-level switching of structure, chirality, and
resistance.
