Pine-tree PbS nanowires, screw dislocations and Eshelby twist

This week’s item is Science Express paper “Dislocation-Driven Nanowire Growth and Eshelby Twist ” by Michael Bierman et al. (doi:10.1126/science.1157131). By growing PbS nanowires using chemical vapor deposition (CVD) they observe hyper-branched structures, similar to the pine trees with a trunk and multiple branches. The spiral growth pattern is due to the existence of a single screw dislocation within the trunk of the nanoscale “pine tree”. The authors test the theory of screw dislocations developed by Eshelby in 1950ies, in particular his prediction of the “Eshelby twist”, an angular twist in the lattice, with twist per unit length proportional to Burgers vector of the dislocation and inversely proportional to the radius of the structure. Because of small radii of the grown PbS nanostructures, nanowires present an excellent testing ground for expected Eshelby twist. The authors find that the fit to Eshelby theory produces Burgers vector on the order of 6 Angstroms, comparable to the expected value of a single unit cell, 5.94 Angstroms.

Giant molecules or tiny crystals?

Nature Materials has a News and Views article by Ian Robinson titled “Coherent diffraction: Giant molecules or tiny crystals?”, which reviews recent coherent electron diffraction results by Huang et al. featured here earlier. One of the interesting points made in this mini-review is the phase diagram on the left showing a transition from bulk cubic crystal to decahedral and icosahedral structures, including quasi-molten and liquid phases.