Surface structure of polar Co₃O₄(111) films grown epitaxially on Ir(100)-(1 × 1)

Abstract

Cobalt oxide films were prepared by oxidation of different amounts of cobalt deposited on Ir(100)-(1 × 1), where oxygen rich conditions were applied during deposition. The resulting oxide films with thicknesses of up to about 40 Å were investigated as regards their crystallographic structure and morphology, applying quantitative low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). It can be unequivocally shown that the spinel-type Co(3)O(4) phase develops, for which an excellent fit between measured and calculated LEED intensity spectra is achieved (Pendry R-factor R = 0.124). In spite of the quadratic unit cell of the substrate the oxide films are in the polar (111) orientation. Also, the native lattice parameter of the material is assumed, i.e. there is no pseudomorphic relation to the substrate. However, by means of orientational epitaxy, one of the unit-mesh vectors of the oxide and one of those of the substrate layer are aligned, leading to two mutually orthogonal domains in the oxide. The oxide is terminated by a sublayer of cobalt ions which in the bulk were tetrahedrally coordinated Co(2+) ions. There are drastic relaxations of layer spacings at and near the surface. As a consequence, the bond length between the surface terminating cobalt ions and oxygen ions below is considerably reduced, indicative of a substantial change of the ionicity of the cobalt and/or oxygen ions. This is interpreted as accounting for polarity compensation of the film, as surface reconstruction, oxygen vacancies and species adsorbed can be ruled out.

Iron oxide surfaces

“…This study, performed on 40 Å thick films on Ir(100), achieved a LEED IV R-factor of 0.124 [578]. More generally, the huge range of metal combinations that take the spinel structure combined with their high stability has seen a resurgence in tailoring these compounds for energy related applications.…”

“…It is crucial that quantitative structural methods are used to test the models and determine the structure of surfaces precisely. Previously it was assumed that LEED IV R-factors might be intrinsically limited by the ionic nature of the bonding in metal oxides, or by an inevitably high defect concentration, but recent LEED IV studies achieved excellent agreement (R P ≈ 0.12) even for complex metal oxide surfaces such as Fe 3 O 4 (100) [35], Co 3 O 4 (111) [578], and V 2 O 3 (0001) [353; 354]. Crucially, in each case the measured surface was imaged both before and after the LEED experiment to ensure the IV curves were acquired from a homogeneous clean surface, and that the electrons did not damage the surface.…”

Properties of oxide thin films and their adsorption behavior studied by scanning tunneling microscopy and conductance spectroscopy

“…The wurzite structure of bulk ZnO is, however, restored in thicker films, which is accompanied by a considerable roughening of the oxide surface in order to quench the reappearing surface dipole. The issue of polarity has been addressed also for spinel-like oxide structures, such as for bulk 293 294 In the latter case, polarity healing is achieved by a substantial decrease of the separation between the terminating Cr layer and the O plane below and a concomitant reduction of the ionicity of the surface species. Other polarityhealing mechanisms, such as changes of the stoichiometry and atomic structure at the surface or adsorption of charged species could be excluded with the help of STM measurements.…”

Epitaxy of polar semiconductor Co3O4 (110): Growth, structure, and characterization

“…This polarity further motivates study of polar Co 3 O 4 , as polar oxides are of prominent interest due to their increased surface reactivity as compared to the bulk 27,28 and as a mechanism for the formation of the two dimensional electron gas (2DEG). 29,30 Co 3 O 4 films have been grown by post-oxidation, 17,31 atomic layer deposition (ALD), [32][33][34] chemical vapor deposition (CVD), [35][36][37][38] pulsed laser deposition (PLD), 18,39 and molecular beam epitaxy (MBE). [40][41][42] A number of substrates, including MgO, 18,34,36,38,39 39 Yttria-stabilized zirconia, 39 SiO 2 / Si, 32,33 and iridium 31 have been studied for the growth of crystalline Co 3 O 4 films.…”