Tooth and Nail

The architecture of many living creatures combines soft organic tissue with hard inorganic crystal. How do the hard parts develop while up against the soft parts?

To examine this issue, physicists at Northwestern University have grown an inorganic lattice (barium fluoride, BaF₂) directly beneath a two-dimensional crystalline array of organic molecules (a fatty acid).

Using the diffraction of synchrotron radiation from these planar arrays, the researchers observe the structure of the two lattices and also affirm that the two become commensurate (that is, they register with each other), the first time this has been done in an experiment.

Even though the lattice spacings of the BaF₂ and the organic monolayer are different, each contributes toward a compromise, the barium fluoride structure by contracting just a bit, and the molecules by expanding their spacing at one end: picture the molecules as a stack of pencils standing on end and then being tilted a bit, modifying the spacing of the pencil tips (see figure).

BaF₂ is not a biologically important mineral, but the Northwestern scientists (contact Pulak Dutta, 847-491-5465, pdutta@northwestern.edu) expect to look directly at biomineralization in an upcoming phase of their work.

Furthermore, since growing two or more incommensurate materials next to each other (an important operation in the microelectronics industry) is difficult because of the unequal atomic spacings, the new research might in the long run be able to lessen or end the currently stringent need for high vacuum to make epitaxially grown materials. (Kmetko et al., Physical Review Letters, 28 Oct)