Self-Assembly of Nickel Icosahedrons and Truncated Octahedral Nanocrystals on a SrTiO3 (111) Support

Nickel nanocrystals have received much attention for their ferromagnetic properties. The crystal properties are strongly dependent on their facets and therefore detailed study of their morphology, facets and orientation is critical for magnetic applications. In this work, equilibrium crystal shapes of self-assembled nickel nanocrystals on the (111) termination of strontium titanate (SrTiO3) at room temperature and under ultra-high vacuum (UHV) conditions have been investigated using scanning tunneling microscope (STM). SrTiO3 (111) substrate was sputtered (0.5 keV, 2.5 µA, 10 min) and annealed (900 °C, 1 h) under UHV conditions. Three different periodicities were observed: (2.21 ± 0.01) nm corresponding to (4 × 4) reconstruction, (3.31 ± 0.02) nm corresponding to (6 × 6) reconstruction, and (2.85 ± 0.05) nm, rotated at 30° with respect to (4 × 4) reconstruction, corresponding to (3√3 × 3√3)R30° reconstruction. Nickel (~1 ML) was deposited using an e-beam evaporator on the substrate preheated to 320 °C and the sample was post-annealed multiple times. Nickel took platonic shapes of supported icosahedron comprising of (111) facets and truncated octahedron comprising of (001) and (111) facets. Based on surface energy ratios of truncated octahedrons at equilibrium the work of adhesion was calculated to be (3.889 ± 0.167) J/m2.