Understanding growth mechanisms of epitaxial manganese oxide (Mn₃O₄) nanostructures on strontium titanate (STO) oxide substrates

The role of substrate orientation on interface registry and nanocrystal shape has been investigated for epitaxial manganese oxide (Mn₃O₄) nanocrystals. Mn₃O₄ (101) nanoplatelets and (112)-orientated nanowires have been successfully deposited on (111) and (110) SrTiO₃ (STO) substrates, respectively. Under higher magnifications, the (101) platelets were found to exhibit step-like growth, spiraling outward from a local dislocation site at the Mn₃O₄-STO interface. Selected area electron diffraction analysis from transmission electron microscope (TEM) was carried out to determine the in-plane edge directionalities of (101) and (112) Mn₃O₄. We found the (101) Mn₃O₄ orientation to exhibit a complex in-plane epitaxial relation of Mn₃O₄//[100]STO and an out-of-plane relation of Mn₃O₄// STO. Furthermore, lattice misorientations of 58° in-plane and 35° out-of-plane have been calculated, attributed to the shear caused by the spiral growth. For the (112) Mn₃O₄ nanowires, the TEM diffraction pattern indicates pyramidal cross-sections based along STO. Subsequent calculations reveal that the (112) nanowires have their long axis (c-axis) such that [001]Mn₃O₄//[110]STO. Thus the nanowires grow preferentially along its longest axis giving rise to the observed shape and anisotropic nature.