Credit: © 2008 AAAS

The properties of nanostructures are dictated by their size and morphology, so understanding and controlling their growth is crucial. Inorganic nanowires are usually synthesised using a metal nanoparticle catalyst that directs the growth in one dimension. Now Matthew Bierman and co-workers from the University of Wisconsin–Madison have made1 complex lead sulphide nanowires relying only on crystal defects to direct the growth — proving a 55-year-old theory in doing so.

The team slightly modified the hydrogen flow in a known chemical vapour deposition technique to grow lead sulphide 'nanotrees' with a trunk and nanowire branches attached in a helical pattern. The growth of the trunk occurs through a defect — called a screw dislocation — in the developing crystal. This creates a self-perpetuating spiral-like pattern at the tip of the trunk that enables the lead and sulphur atoms to deposit there preferentially. A theory from 1953 suggests2 that a screw dislocation within a rod causes torsional forces that twist the structure. This is known as an Eshelby twist, and Bierman and co-workers propose that this accounts for the helical rotation of the tree branches. They believe that their nanotrees provide the most conclusive evidence yet for Eshelby's theory.

This method could be applicable to the growth of other materials that are prone to screw dislocations, suggesting that understanding crystal defects in such systems may provide a new way for directing catalyst-free nanostructure growth.