Understanding growth mechanisms of epitaxial manganese oxide (Mn 3 O 4 ) nanostructures on strontium titanate (STO) oxid
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esearch Letter
Understanding growth mechanisms of epitaxial manganese oxide (Mn3O4) nanostructures on strontium titanate (STO) oxide substrates Jia Yin Liu, Xuan Cheng, and Valanoor Nagarajan, School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia Huo Lin Xin, Brookhaven National Laboratory, Center for Functional Nanomaterials, Upton, New York 11973, USA Address all correspondence to Valanoor Nagarajan at [email protected] (Received 11 August 2014; accepted 11 February 2015)
Abstract 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 [231]Mn3O4//[100]STO and an out-of-plane relation of [101]Mn3O4//[111]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 [011] 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.
A fundamental issue that has attracted attention in recent years is the role of morphology and interface-driven shape transitions in oxide-supported nanocrystals.[1–3] Understanding these morphological changes is important as they determine what crystallographic orientations bind the nanocrystals. Given the dominance of surface controlled properties in nanomaterials, it is these outer facets that take part in physio-chemical interactions. This has had a profound influence on the development of electrochemical and catalyst technologies.[4–6] It follows that in order to fully understand the role of particular surfaces in such physio-chemical applications, we must first establish a detailed understanding of how a particular crystallographic surface may be formed. Simply put, the specific equilibrium shape of a particular nanocrystal on a substrate is governed primarily by three factors; namely, the surface energy of a particular crystal facet, the interface energy between the crystal and its substrate, and the surface energy of the substrate itself.[2] Furthermore, it is shown that the particular crystallographic orientation can be tuned by controlling the substrate temperature and hence the thermodynamics of
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