Single-water-dipole-layer-driven Reversible Charge Order Transition in 1T-TaS₂

Water-solid interactions are crucial for many fundamental phenomena and technological processes. Here, we report a scanning tunneling microscopy study about the charge density wave (CDW) transition in 1T-TaS₂ driven by a single water dipole layer. At low temperature, pristine 1T-TaS₂ is a prototypical CDW compound with $$ × $$ charge order. After growing a highly ordered water adlayer, a new charge order with 3 × 3 periodicity emerges on water-covered 1T-TaS₂. After water desorption, the entire 1T-TaS₂ surface appears as localized $$ × $$ CDW domains that are separated by residual-water-cluster-pinned CDW domain walls. First-principles calculations show that the electric dipole moments in the water adlayer attract electrons to the top layer of 1T-TaS₂, which shifts the phonon softening mode and induces the $$ × $$ to 3 × 3 charge order transition. Our results pave the way for creating new collective quantum states of matter with a molecular dipole layer.