Dislocation-Driven Nanoscale Surface Pattern Formation: Atomic Volcanoes and Whirl Pools in Crystals

Scientific Accomplishment

Progress in nanoscience and technology depends on the ability to systematically organize, manipulate, and characterize matter at the nanoscale, which can only be achieved through "bottom-up" processes (i.e. by self-organized assembly of atoms and molecules into desired nanostructures). We have discovered a new and promising approach for fabrication of spatially-ordered functional two-dimensional (2D) nanostructures on surfaces of crystalline solids.

Using time-resolved low-energy electron microscopy (LEEM), we show that dislocations -- the line defects bounding plastically-deformed regions in crystalline solids -- act as sources or sinks for atoms, even in the absence of crystal growth or etching, leading to nanoscale patterns. We observed in real-time the thermally-driven (1600-1750 K) nucleation and growth of spiral steps around cores of dislocations terminating on the (111) surface of titanium nitride (TiN), a technologically important material widely used in the microelectronics and hard coatings industries. This process, which occurs in the absence of deposition or evaporation, is distinctly different from conventional curvature-driven surface smoothening and spiral growth phenomena and is attributed to facile exchange of material between the bulk and the surface at the dislocation cores. Furthermore, an analysis of spiral step dynamics shows that the dislocation core behaves like an atom "volcano" or "whirlpool" exchanging atoms with a surface. Our studies demonstrate that surface-terminated dislocations provide a means to controllably direct the self-organization of spatially-periodic nanostructures.

Significance

Dislocations terminating on surfaces are known to strongly influence nanostructural and interfacial stability, physical, chemical, and electronic properties, crystal growth kinetics, and other surface processes. However, very little is known concerning their effects on surface dynamics and nanoscale pattern formation. This study, for the first time, provides atomic-level insights into mechanisms controlling nanostructural stability and spatially-periodic nanoscale pattern formation on surfaces. The novel phenomena of dislocation-mediated surface nanoscale morphological evolution, which occurs in the absence of deposition or evaporation i.e. without any additional processing, is a promising approach for the directed synthesis of self-organized nanostructures. Our investigations suggest that this is a general process that occurs in a wide variety of materials systems including metals, magnetic materials, polymers, semiconductors, and ceramics, offering potential applications in advanced materials, biosensors, thermal barriers, fuel cells, magnetic storage, and optoelectronic devices.all crystalline materials.

Performers

Senior FS-MRL PIs: J. E. Greene and I. Petrov