Polytype switching by super-lubricant van der Waals cavity arrays.

Expanding the performance of field-effect devices is a key challenge of the ever-growing chip industry at the core of current technologies. Non-volatile multiferroic transistors that control atomic movements rather than purely electronic distribution are highly desired. Recently, a field-effect control over structural transitions was achieved in commensurate stacking configurations of honeycomb van der Waals (vdW) polytypes by sliding boundary strips between oppositely polarized domains. This ferroelectric hysteretic response, however, relied on pre-existing dislocation strips between relatively large micron-scale domains, severely limiting practical implementations. Here we report the robust electric switching of single-domain polytypes in nanometre-scale islands embedded in super-lubricant vdW arrays. We etch cavities into a thin layered spacer and then encapsulate it with functional flakes. The flakes above/under the lattice-mismatched spacer sag and touch at each cavity to form islands of commensurate and metastable polytype configurations. By imaging the polarization of the polytypes, we observe nucleation and annihilation of boundary strips and geometry-adaptable ferroelectric hysteresis loops. Using mechanical stress, we further control the position of boundary strips, modify marginal twist angles and nucleate patterns of polar domain. This super-lubricant arrays of polytype (SLAP) concept suggests 'slidetronics' device applications such as elastic-coupled neuromorphic memory cells and non-volatile multiferroic tunnelling transistors and programmable response by designing the size, shape and symmetry of the islands and of the arrays.