Screw Dislocation Generation by Inclusions in Molecular Crystals

Dislocations in crystals affect material properties and are essential for crystal growth near equilibrium, yet their genesis in the absence of external or internal stresses is unresolved. X-ray topography has revealed microscopic inclusions as dislocation sources, but the real-time creation of a dislocation by a particulate inclusion has not been reported. In situ atomic force microscopy (AFM) was used herein to visualize dislocation generation in an l-cystine crystal by a cube-like hematite particle embedded in, and slightly inclined with respect to, the l-cystine {0001} surface. The particle produced two pairs of heterochiral screw dislocations with opposing Burgers vectors. After overgrowth of the particle, dissolution in undersaturated solutions revealed the dislocations once again until the detachment of the particle exposed a flat basal plane devoid of dislocations, thereby corroborating the essential role of the particle. Hematite particles with their flat faces parallel or at high angle to the surface, as well as spherical poly(styrene) particles, did not produce dislocations, suggesting that shape and orientation of the particle with respect to the step train advancing across the growing crystal surface are critical features for dislocation generation.