Realization of Multiple Charge-Density Waves in NbTe2 at the Monolayer Limit

Layered transition-metal dichalcogenides down to the monolayer (ML) limit provide a fertile platform for exploring charge-density waves (CDWs). Here, we experimentally unveil the richness of the CDW phases in ML-NbTe2 for the first time. Not only the theoretically predicted 4 x 4 and 4 x 1 phases but also two unexpected 28 x 28 and 19 x 19 phases are realized. For such a complex CDW system, we establish an exhaustive growth phase diagram via systematic efforts in the material synthesis and scanning tunneling microscope characterization. Moreover, the energetically stable phase is the larger-scale order ( 19 x 19 ), which is surprisingly in contradiction to the prior prediction (4 x 4). These findings are confirmed using two different kinetic pathways: i.e., direct growth at proper growth temperatures (T) and low -T growth followed by high -T annealing. Our results provide a comprehensive diagram of the "zoo" of CDW orders in ML-NbTe2.