Nano Lett. http://doi.org/zfw (2015)

A single ripple in a floor carpet is easily smoothed out by applying a bit of sideways pressure. A wrinkled carpet, though, is more difficult to flatten. Similar situations arise on the nanoscale, as Akihiro Kushima and colleagues discovered when studying surface defects in molybdenum disulphide (MoS2).

The surface of a stack of MoS2 layers is prone to structural defects because the layers are held together only by weak van der Waals forces. Narrow linear ripples, forming due to a line of excess atoms in the top layer, are particularly common. They are similar to dislocations in bulk crystals — Kushima et al. call them ripplocations. The authors probed the mobility of ripplocations following electrochemical lithiation or mechanical edge compression. In situ high-resolution transmission electron microscopy revealed that ripples can easily move around, merge or cancel out, whereas wrinkles don't migrate.

The experiments were complemented by density-functional theory calculations, confirming that it is energetically favourable for nearby ripplocations to combine into one. Further numerical work showed that the electrical conductance is reduced across a ripple, which acts as a barrier for electrons.