  Scaling down materials to an atomic-layer level produces rich physical and
chemical properties as exemplified in various two-dimensional (2D) crystals
extending from graphene, transition metal dichalcogenides to black phosphorous.
This is caused by the dramatic modification of electronic band structures. In
such reduced dimensions, the electron correlation effects are also expected to
be significantly changed from bulk systems. However, there are few attempts to
realize novel phenomena in correlated 2D crystals. Here, we report memristive
phase switching in nano-thick crystals of 1T-type tantalum disulfide (1T-TaS2),
a first-order phase transition system. The ordering kinetics of the phase
transition was revealed to become extremely slow as the thickness is reduced,
resulting in an emergence of metastable states. Furthermore, we realized the
unprecedented memristive switching to multi-step non-volatile states by
applying in-plane electric field. The reduction of thickness is essential to
achieve such non-volatile electrical switching behavior. The thinning-induced
slow kinetics possibly makes the various metastable states robust and
consequently realizes the non-volatile memory operation. The present result
indicates that 2D crystal with correlated electrons is a novel nano-system to
explore and functionalize multiple metastable states which are inaccessible in
its bulk form.
