  Transport studies of atomically thin 1T-TaS2 have demonstrated the presence
of intermediate resistance states across the nearly commensurate (NC) to
commensurate (C) charge density wave (CDW) transition, which can be further
switched electrically. While this presents exciting opportunities for the
material in memristor applications, the switching mechanism has remained
elusive and could be potentially attributed to the formation of inhomogeneous C
and NC domains across the 1T-TaS2 flake. Here, we present simultaneous
electrical driving and scanning photocurrent imaging of CDWs in ultrathin
1T-TaS2 using a vertical heterostructure geometry. While micron-sized CDW
domains form upon changing temperature, electrically driven transitions result
in largely uniform changes, indicating that states of intermediate resistance
for the latter likely correspond to true metastable CDW states in between the
NC and C phases, which we then explain by a free energy analysis. Additionally,
we are able to perform repeatable and bidirectional switching across the
multiple CDW states without changing sample temperature, demonstrating that
atomically thin 1T-TaS2 can be further used as a robust and reversible
multi-memristor material.
