A characterization of crystalline ice nanoclusters using transmission electron microscopy

The preparation of crystalline ice nanoclusters from a mixture of 0.5-2.0% water vapor in N-2 carrier gas produces clusters that range in diameter from similar to10 to several hundred nanometers, with a mode of 15-30 rim. By varying the percent water vapor in the mixture or the aliquot size, the average cluster size and the size distribution of the clusters can be controlled. If a second strong hydrogen-bonding molecule such as methanol is added to the mixture, the size distribution of the clusters is changed. The spherical shape of the crystalline ice nanoclusters observed in transmission electron microscopy (TEM) bright field imaging suggests that the clusters form from liquid droplets that crystallize upon further cooling. TEM dark field imaging of individual clusters reveals a structure in which crystalline domains are oriented one to another in mosaic fashion. This structural relationship appears to propagate from the centers of the clusters, indicating that nucleation and growth proceed from the centers outward. Water ice nanoclusters prepared in this way are useful analogs for ices from a variety of natural environments, including cold interstellar molecular clouds, comets, and terrestrial polar stratospheric clouds. The ice nanoclusters have a large surface area relative to their bulk that allows for the rapid reaction of the ice with other species present in the water-carrier gas mixture or deposited on the clusters subsequent to their formation.