Phys. Rev. Lett. 110, 185901 (2013)

Solid objects cannot usually get through a hole smaller than itself without modifying its physical state. Sinisa Coh and colleagues at the University of California, Berkeley and Lawrence Berkeley National Laboratory have now discovered an intriguing exception to this where an iron nanocrystal that completely fills a carbon nanotube with an inner diameter of 20 nm can squeeze through a 5-nm constriction without melting and recrystallizing on the other side.

The researchers examined the movement of the nanocrystal through the nanotube under the application of an electric current with the help of an in situ transmission electron microscope. Electron imaging and diffraction measurements showed that while passing through the constriction the nanocrystal remained essentially solid and crystalline. To explain these observations, the team carried out kinetic Monte Carlo simulations, which assumed that each iron atom experiences an electromigration force that is proportional to the current applied to the carbon nanotube. It turns out that only the atoms in contact with the nanotube move in the direction of the current; the atoms in the bulk remain stationary. According to the simulations, therefore, the surface atoms constantly move forward. The back edge of the nanoparticle disassembles and then reassembles at the front edge. This means that the front edge can adjust its size to match the constriction and can therefore lead the whole nanoparticle through without losing its crystallinity.