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 structu ral analyses (for proteins) and better electronic properties (for semi conductors)(1-3). Here we study the effect of a microgravity environme nt on the crystallization of a class of materials-layered microporous tin(IV) sulphides(4-11)-whose crystal structure is determined by weak interlayer interactions (electrostatic, hydrogen-bonding and van der W aals) as well as strong intralayer covalent bonds. We find that the cr ystals grown in microgravity (on board the Space Shuttle Endeavour) sh ow improved crystal habits, smoother faces, greater crystallinity, bet ter optical quality and larger void volumes than the materials grown o n Earth. These differences are due at least in part to the profound in fluence of microgravity on the layer registry over length scales of ar ound 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 significa ntly 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 ope rate in crystallization on Earth.