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THE beauty and unique properties of crystals are all down to their rigidly
uniform structure, identical units repeated millions upon millions of times in a
regular 3D pattern. To get a closer look, researchers have grown, for the first
time, what is in effect a one-dimensional crystal—a string of units inside
the world’s smallest test tube.

The crystals, lines of repeating units just two or three atoms wide, were
formed inside carbon nanotubes—minuscule honeycomb sheets of carbon atoms
rolled into tubes. “Very, very small crystals can’t exist without our help,”
says Malcolm Green of Oxford University. “To get them that small you have to
wrap them in something.”

Such a crystal is great for researchers because between 87 and 100 per cent
of the atoms are actually part of the surface. “In the chemistry of solids, the
interesting reactions take place on the surface,” says Green. Getting precise
data on surfaces has always been difficult, he explains, because it’s hard to
know where the surface ends and the bulk of the interior begins.

The team created the 1D crystals by mixing empty nanotubes, each just 1
nanometre wide and up to several millimetres long, with molten potassium iodide
or potassium chloride. About half the tubes filled spontaneously by capillary
action, as if someone was “sucking something up a straw”, says chemist Jeremy
Sloan, also of Oxford. In the confined space of the nanotube, the atoms
crystallise into a single line of units.

The Oxford team studied the crystals using a transmission electron
microscope. Angus Kirkland of Cambridge University, who collaborated in the
study, helped develop an imaging technique that eliminated enough distortion to
show all the atoms of the potassium iodide crystals
(see Diagram). Earlier
studies hadn’t been able to see potassium atoms in potassium iodide because they
scatter electrons less strongly than the heavier iodine.

“It’s really remarkable that they can see single atom distances between the
rows,” says Jan-Olov Bovin, a materials chemist at Sweden’s National Center for
High-Resolution Electron Microscopy in Lund. “It’s a very nice achievement.”

And the images provided a real surprise. Measuring the distances between
atoms, the researchers found that they were up to 10 per cent further apart than
those in the bulk salt. Theoreticians would expect a crystal that small to
contract, not expand, because removing the attraction of surrounding units would
allow the atoms to pull themselves closer together. Green says this unexpected
swelling is probably due to interactions between the crystal and the
nanotube.

Sloan thinks the altered lattice spacing could have a significant impact on
the crystal’s physical properties, though for now he can’t guess how they might
change. “Surely that will be very interesting future work,” says Bovin.