Reduced dimensionality is generally expected to enhance charge density wave (CDW) order. However, experimental results in monolayer ${\mathrm{NbSe}}_2$ appear conflicting: while optical measurements reveal CDW signatures up to 145 K on oxide substrates, scanning tunneling microscopy (STM) studies on graphene-supported monolayers reveal no evidence of CDW above the bulk transition temperature (33 K), suggesting the absence of a dimensionality-driven enhancement. This discrepancy raises fundamental questions about the nature of CDW in the monolayer limit and the role of substrate interactions. Here, we use STM to investigate monolayer ${\mathrm{NbSe}}_2$ on graphene and directly resolve local $3\times 3$ CDW modulation persisting to 77 K, well above the bulk transition. Distinct from coherent long-range CDW orders, the observed CDW modulation is short-ranged, nucleates preferentially at atomic-scale defects, and exhibits defect-selective pinning analogous to bulk behavior. Compared to bulk ${\mathrm{NbSe}}_2$, the CDW correlation length is significantly enhanced but remains spatially inhomogeneous, varying across flakes with different twist angles relative to the graphene substrate. These results indicate that reduced dimensionality indeed promotes CDW correlation in monolayer ${\mathrm{NbSe}}_2$. However, the stronger dielectric screening provided by graphene, compared to insulating oxides, likely contributes to the suppression of phase-coherent, quasi-long-range CDW order. Overall, our findings highlight the roles of dimensionality and substrate in determining CDW behavior in the monolayer limit.

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• Charge density waves
• Monolayer films
• Molecular beam epitaxy
• Scanning tunneling microscopy