Observation and impact of a “surface skin effect” on lateral growth of nanocrystals

We investigate the impact of a quasi-crystalline two-dimensional (2D) surface on the lateral epitaxy of one-dimensional (1D) nanocrystals. The quasi-2D surface was formed by locally conditioning a GaN crystalline lattice at and below surface using a low-dose focused Ga ion beam ranging from 9 to 354 ions/pulse. Short ion pulses/site are used to create the 2D arrays of sub-10-nm circular disks modulating the GaN lattice to a depth of about 30 nm. Impact of this localized lattice modulation was investigated on lateral epitaxy of 1D ZnO nanocrystal and electronic structure of formed heterojunctions. High resolution transmission electron microscopy (HRTEM) below the GaN surface reveals direct evidence of a “skin effect” that influences the surface epitaxy of low dimensional nanocrystals. We define this effect as the crystallinity of the top 10 nm of the substrate that is found to be a key factor in occurrence of lateral epitaxy. HRTEM shows that lateral epitaxy stops, if the thin skin layer is disrupted. However, if the lattice structure of this layer rebounds, the lateral epitaxy occurs. Results indicate that, beyond the skin depth of about 10 nm, the disorder of the subsurface lattice does not impact the structure of the overgrown nanocrystals. These findings suggest the possibility of surface engineering for enabling spatially controlled modulation of the electronic structure of low-dimensional nanocrystals on a scalable fashion.