Effect of microgravity on the crystallization of a self-assembling layered material

In microgravity, crystals of semiconductors and proteins can be grown with improved crystallinity, offering the prospect of improved structural analyses (for proteins) and better electronic properties (for semiconductors)1,2,3. Here we study the effect of a microgravity environment on the crystallization of a class of materials—layered microporous tin(IV) sulphides4,5,6,7,8,9,10,11—whose crystal structure is determined by weak interlayer interactions (electrostatic, hydrogen-bonding and van der Waals) as well as strong intralayer covalent bonds. We find that the crystals grown in microgravity (on board the Space Shuttle Endeavour) show improved crystal habits, smoother faces, greater crystallinity, better optical quality and larger void volumes than the materials grown on Earth. These differences are due at least in part to the profound influence of microgravity on the layer registry over length scales of around a nanometre, which is shown by X-ray and electron diffraction to be better in space than on Earth. Thus we can see a clear distinction between the covalent bonds in these materials, which are not significantly affected by microgravity, and the weaker forces (like those that determine the structure of proteins over length scales of around 0.3–0.4 nm) which are more susceptible to the dynamic disturbances that operate in crystallization on Earth.