Friday, December 3, 2010

Self-Assembly of Nano-Rotors

I hate to call this a solution looking for a problem but it was already speculated on as a plausible technical objective decades ago.  Now we actually have something.

It is still a demonstration of rapidly expanding capability and the tricks learned will all be tossed into the great research pot presently boiling in this field.

At least we have a way to do a very complex bit of engineering at unheard of smallness.

NOVEMBER 23, 2010
A Nano-rotor in its honeycomb-cage

The confinement of molecular species in nanoscale environments leads to intriguing dynamic phenomena. Notably, the organization and rotational motions of individual molecules were controlled by carefully designed, fully supramolecular host architectures. Here we use an open 2D coordination network on a smooth metal surface to steer the self-assembly of discrete trimeric guest units, identified as noncovalently bound dynamers. Each caged chiral supramolecule performs concerted, chirality-preserving rotary motions within the template honeycomb pore, which are visualized and quantitatively analyzed using temperature-controlled scanning tunneling microscopy. Furthermore, with higher thermal energies, a constitutional system dynamics appears, which is revealed by monitoring repetitive switching events of the confined supramolecules’ chirality signature, reflecting decay and reassembly of the caged units.

First, the physicists built up an extensive nano lattice by allowing cobalt atoms and rod-shaped molecules of sexiphenyl-dicarbonitrile to react with each other on a silver surface. This results in a honeycomb-like lattice of extreme regularity with astonishing stability

When the researchers added further molecular building blocks, the rods spontaneously gathered, typically in groups of three, in a honeycomb cell while neighboring cells remained empty. The chummy molecules must have had a reason for organizing themselves in threesomes. Under a scanning tunneling microscope the scientists were able to recognize why. The three molecules oriented themselves in such a way that the nitrogen ends each faced a phenyl-ring hydrogen atom. This triple-bladed rotor arrangement is so energetically advantageous that the molecules maintain this structure even when thermal energy drives it to rotation.

Because the honeycomb-cell is not round, but hexagonal, there are two different positions for the rotors that can be distinguished as a result of the interactions between the outer nitrogen atoms and the hydrogen atoms of the cell wall. Furthermore, the three molecules arrange in a clockwise and a counter-clockwise manner. In experiments at various carefully controlled temperatures the physicists were able to “freeze” all four states and examine them closely. They could thus determine the energy of these thresholds from the temperature at which the rotation resumed.

“We hope that in future we will be able to extend these simple mechanical models to optical or electronic switching,” says Professor Johannes Barth. “We can set a specific cell size, we can specifically bring in further molecules and study their interaction with the surface and the cell wall. These self-organizing structures hold enormous potential.”

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