Fig. 2.

Ice-VII-like molecular structure of water nanomeniscus. a Normal Raman spectrum of bulk liquid water (red curve, at 300 K) and ice-Ih (black curve, 253 K). The room-temperature signal shows the broad OH-stretching bands in the high-frequency bands (also shown in Fig. 1d) and the broad features in the low-frequency bands. For ice-Ih, on the other hand, there exists the intense lattice-vibration band near 230 cm⁻¹ as well as the strong DDAA peak (~3145 cm¹). b The ordered structure of ice-Ih originates from the enhanced tetrahedral order of HB. c The full SERS spectra of the nanomeniscus in ambient condition (black curve, 300 K) as well as at 393 K (red curve) are presented. The SERS peak (3340 cm⁻¹, blue shade) at 300 K is significantly blue-shifted from ice-Ih but with the similar bandwidth (~50 cm⁻¹), which implies a distorted tetrahedral HB configuration while preserving the ice-like crystalline structure. We observe two heating-induced effects: correlated intensity-decrease of the DDAA component in the high- and low-frequency bands, as well as selective intensity-decrease of only the DDAA peak within each band. d The results of heating show that the nanoconfined water molecules contribute to the DDAA mode (blue shade) as indicated by the blue-dashed arrow, while the tightly surface-adsorbed water molecules produce the DA mode (red shade) (see the red-dashed arrow). e Raman spectra of ice-VII from supercompressed water (green curve) and ice-VIII (orange curve). Unlike the ice-VII-like confined water obtained at room temperature, ice-VIII is formed at low temperature. f The molecular structure of ice-VII is known as the densest phase of ice with the body-centered-cubic symmetry. g Thorough investigation of the available Raman bands of all ice phases (see Supplementary Fig. 3 for their references) shows the unique route where the SERS position of ice-VII (3340 cm⁻¹, ~1 GPa) can be located only by a slight extrapolation of the ice-VII from supercompressed water (3335.5 cm⁻¹, 1.72 GPa) at 300 K. Note that all the Raman spectra reported in the references were performed by normal Raman spectroscopy, not by SERS