Applied Physics

One... Two... Three... Push

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Science  17 Aug 2001:
Vol. 293, Issue 5533, pp. 1225
DOI: 10.1126/science.293.5533.1225b

How hard can you push on one corner of a table placed on a rug before the leg bends as much as it can, and the table starts to shuffle across the floor· Moresco et al. have looked at this type of problem, but on the molecular scale, by prodding a molecule with a scanning tunneling microscope (STM) under high vacuum. Four substituents on a Cu porphyrin complex act as legs, both supporting it and anchoring it above a flat copper (100) surface. Previous studies at room temperature showed that the legs are not square to the surface but tilt by 10° to 20°. Using the STM in constant-height mode at much lower temperature (12 K), they pushed on one of the legs and recorded changes in current with displacement. By comparing these measurements to calculations, they determined that the leg tilted an additional 20° and that the other three legs tilted by about 10°. Once the legs were sufficiently deformed, the molecule had enough internal energy to overcome the surface barriers, and it shuffled across the copper surface. — MSL

Phys. Rev. Lett., 10.1103/PhysRevLett.87.088302.

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