When growing a crystal from solution, the role of the solvent can be more than just a transport medium. It is well-known that the habit of a crystal can be very different for different solvents; for example NaCl crystals have {100} facets when growing from aqueous solution, but have {111} facets in water-formamide solutions. Another case is formed by protein crystals that are only stable when they contain large inclusions of (disordered) water. In order to unravel the role of such solvents at the molecular/atomic level, we have used X-ray diffraction (using synchrotron radiation sources) to determine the structure of various crystal-solution interfaces and in particular the ordering properties of the solution in contact with the crystal surface. We find that typically three layers are quite strongly ordered perpendicular to the surface (‘layering’) and that the first layer often shows significant lateral ordering as well.

A few examples will be discussed. When growing InP nanowires using the so-called vapour-solid-liquid (VLS) method, often the wurzite crystallographic structure is formed instead of the zinc-blende that is the most stable structure in the bulk. We found that the position of the solution atoms hampers the zinc-blende growth and thus provides a possible explanation for this [1]. At the interface of liquid In with Si(111), the In is found to have a higher density than in the bulk liquid, which seems to be caused by the mismatch between the surface lattice and the size of the In atoms [2]. In the case of wet chemical etching (‘negative growth’) of Si(111) in alkaline solutions, the solution provides not only the reactants but also the H-atoms to keep  the surface stable. The H-terminated, hydrophobic surface leads to only weak ordering of the liquid.

[1] R.E. Algra et al., Nano Lett. 11 (2011) 1259.

[2] V. Vonk et al., unpublished.

[3] I.A. Shah et al., Surf. Sci. 605 (2011) 1027.

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1.  Complete deracemisation of proteinogenic amino acids using Viedma ripening
2. High resolution protein crystals using an efficient convention-free growth geometry
3. Polymorph prediction by nucleation modeling
4. Complete deracemisation of proteinogenic amino acids using Viedma ripening