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Two-dimensional (2D) materials have become a fertile playground for theexploration and manipulation of novel collective electronic states. Recentexperiments have unveiled a variety of robust 2D orders in highly-crystallinematerials ranging from magnetism to ferroelectricity and from superconductivityto charge density wave (CDW) instability. The latter, in particular, appears indiverse patterns even within the same family of materials with isoelectronicspecies. Furthermore, how they evolve with dimensionality has so far remainedelusive. Here we propose a general framework that provides a unfied picture ofCDW ordering in the 2H polytype of four isoelectronic transition metaldichalcogenides 2H-MX$_2$ (M=Nb, Ta and X=S, Se). We first show experimentallythat whilst NbSe$_2$ exhibits a strongly enhanced CDW order in the 2D limit,the opposite trend exists for TaSe$_2$ and TaS$_2$, with CDW being entirelyabsent in NbS$_2$ from its bulk to the monolayer. Such distinct behaviours arethen demonstrated to be the result of a subtle, yet profound, competitionbetween three factors: ionic charge transfer, electron-phonon coupling, and thespreading extension of the electronic wave functions. Despite its simplicity,our approach can, in essence, be applied to other quasi-2D materials to accountfor their CDW response at different thicknesses, thereby shedding new light onthis intriguing quantum phenomenon and its underlying mechanisms.

Patterns and driving forces of dimensionality-dependent charge density waves in 2H-type transition metal dichalcogenides