Fig. 3: FTIR metrology.
FTIR spectrum of nanodroplets in various pore dimensions. a For 150 nm nanodroplets, at the temperature of −11 °C, the OH stretch peak is narrowed and red-shifted to a wavenumber of ~3200 cm−1. This redshift indicates the water–ice phase change and provides TN value at this length-scale. b The redshift occurs at a temperature of −17 °C for 20 nm water droplets. c For 5 nm water droplets, the narrowing of OH stretch peak occurs at a temperature of −40 °C, while the redshift goes to lower wavenumbers compared to those of larger nanodroplets (150, 80, 40, 20, and 10 nm). This suggests a shorter O–H bond due to the confinement effect for the ice formed. d The TN value for 2 nm droplets becomes ~−44 °C. A similar confinement effect on the shortening of O–H bonds is observed for these droplets. That is for nanodroplets smaller than 5 nm, the formed ice phase has a different bond length compared to the bulk ice. e The nucleation temperature of nanodroplets measured through two independent metrologies is shown as a function of length-scale. For nanodroplets of 2 nm, the nucleation temperature drops below the limit of homogeneous bulk nucleation. In e, the horizontal error bars denote variations in the dimension of the water droplets, and the vertical errors bar denotes experimental errors in the measurement of nucleation temperature.
