Screw Dislocation Drives Nanotree Growth

Researches from the University of Wisconsin - Madison have found a way of making lead sulfide nanowires that resemble pine trees. The unusual, twisted nanostructures have trunks that are up to hundreds of microns in length along the <100>crystallographic direction ,with diameters ranging from 40 - 350nm .These are surrounded by helical arrangements of branches that are commonly tens of microns long, becoming progressively shorter as they spiral upwards. The method of synthesis is simple and involves very similar reaction conditions to those reported for the synthesis of

hyper branched nanowires ,Chemical vapor deposition is performed using PbCl₂ and elements sulfur as precursors under argon flow with a co-flow of H2 at atmospheric pressure. Reaction temperatures are typically 600- 650 ^o C .the key difference is the hydrogen flow profile that very sutle change led to drastic difference in the synthetic product. To control these, and have now synthesized literally hundreds of batches of nanowires pine trees with good reproducibility.

Nanowire synthesis is typically driven by metal catalysts that promote growtn in one dimension to from long rods. The propose for that for these pine tree nanostructures, one dimensional growth is instead driven by the screw component of an axial dislocations .They confirmed the presence of screw dislocation using diffraction contrast transmission electron microscopy and scanning electron micrographs revealed that the angular twist matched.