  Abstract: Layered transition-metal dichalcogenides (TMDCs) down to the
monolayer (ML) limit provide a fertile platform for exploring charge-density
waves (CDWs). Though bulk NbTe2 is known to harbor a single axis 3*1 CDW
coexisting with non-trivial quantum properties, the scenario in the ML limit is
still experimentally unknown. In this study, we unveil the richness of the CDW
phases in ML NbTe2, where not only the theoretically predicted 4*4 and 4*1
phases, but also two unexpected sqrt(28)*sqrt(28) and sqrt(19)*sqrt(19) phases,
can be 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, we report that the
energetically stable phase is the larger scale order (sqrt(19)*sqrt(19)), which
is surprisingly in contradiction to the prior prediction (4*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
1T-NbTe2 for the first time and offer a new material platform for studying
novel quantum phases in the 2D limit.
