Fig. S4.
Control experiments demonstrating growth at the ice/water interface. (A) Schematic of the layered films. All three experiments have a 75-ML H2O CI film grown on the Pt(111). For film 1, the 25-ML ASW is composed entirely of 9.5% HDO in H2O. For film 2, the ASW layer is composed of a 10-ML thick layer of 9.5% HDO capped with 15 ML of H2O. For film 3, the ASW is composed of a 15-ML H2O layer capped with 10 ML of 9.5% HDO. The black arrows indicate ice growth at the ice/water interface. B and C show the amount of the initially ASW layer that has crystallized ice as a function of number of pulses as measured from the IRAS in the OH-stretching and OD-stretching regions, respectively. (B) As measured in the OH-stretch region, the growth rates for the three films are the same (within the experimental uncertainty associated with setting Tmax). (C) When the 9.5% HDO film is in direct contact with the CI template, crystallization measured via the OD-stretch starts immediately (red line), whereas it is delayed when the 9.5% HDO layer is above the 15-ML H2O layer (purple line).