1. Nanomaterials formation mechanism driven by screw dislocations.

We have discovered a nanowire (NW) growth mechanism driven by axial screw dislocations that is fundamentally different from the traditional vapor-liquid-solid (VLS) or analogous mechanisms using metal catalysts. The self-perpetuating steps of a screw dislocation spiral provide the fast crystal growth front under low supersaturation during crystal growth (Fig. B) to enable the anisotropic crystal growth of one-dimensional (1-D) NWs. The fast growing NWs driven by dislocation, when combined with epitaxial overgrowth of NW branches formed via the slower VLS mechanism, result in unprecedented “Christmas tree” nanostructures of PbS (Fig. A). These fascinating trees with rotating branches are the clearest demonstration of “Eshelby twist” — the rotation of a crystal lattice around a screw dislocation as the consequence of its associated stress.

Dislocation-driven NW growth is a general mechanism that has been greatly under appreciated in modern nanomaterial literature. We have further shown that this nanowire growth mechanism is applicable to the growth of other materials, both in vapor phase growth and solution phase growth.

Figure 1. A) “Christmas tree” nanowires of PbS; B) Scheme illustrating screw dislocation driven nanowire growth; C) Scheme showing the combination of screw dislocation driven nanowire growth and VLS branch growth.

We have further used dislocation mechanism and fundamental elasticity theory to generally explain the spontaneous formations of other exotic nanomaterials morphology and other intriguing crystal growth phenomena.  This discovery and our continuing study will create a new dimension in the rational design and synthesis of nanomaterials and could enable applications of such novel complex hierarchical nanostructures in solar energy conversion and 3-D electronics.