Pinning on impact

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When it comes to investigating phenomena at the nanometre scale, clusters are all the rage. These are minute particles, containing anything from tens of atoms to a few thousand, which exhibit size-dependent electrical, optical, catalytic and mechanical properties.

To study the behaviour of clusters, so-called 'nanoislands' — isolated clusters of nanometre dimensions on a supporting substrate — are often used. One approach is to form the clusters in a gas phase and then deposit them on the substrate. The trouble is getting the resulting nanoislands to stick. R. E. Palmer and colleagues have looked into the problem of pinning them down and have devised a system that involves smashing clusters of silver into a graphite substrate (J. Chem. Phys. 113, 7723–7727; 2000).

Palmer's group use a beam of charged and size-selected silver clusters, accelerated in an electric field and directed onto the graphite surface. The simulation snapshots shown here illustrate the process for a silver cluster containing 147 atoms. When accelerated in an electric field to an energy of 1,500 electron volts (a), the cluster simply flattens on impact without penetrating the graphite surface. But at 2,000 electron volts (b), the cluster disrupts the carbon lattice of the graphite, and silver atoms implant into the surface. The onset of pinning — defined as one or more carbon atoms being displaced from their lattice site — occurs between 1,625 and 1,750 electron volts.

Similar behaviour is observed with clusters of 50 to 200 atoms in both molecular dynamics simulations and experiments using scanning tunnelling microscopy. The pinning threshold increases linearly with cluster size, suggesting that the energy required to remove one carbon atom from the lattice is transferred in the initial elastic collision. Pinning through the creation of defects beneath the incident cluster is particularly effective in this system; whether the approach might be applied to creating practically useful nanostructures remains to be seen.

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