Superstructure in the termination of CoO(111) surfaces: Low-energy electron diffraction and scanning tunneling microscopy

Abstract

The surface structures of CoO͑111͒ films epitaxially grown on Ir͑100͒ − ͑1 ϫ 1͒ are investigated by means of quantitative low-energy electron diffraction and scanning tunneling microscopy. A ͑ ͱ 3 ϫ ͱ 3͒R30°superstruc-ture is revealed for the films' ground states. It appears for film thicknesses Ն10 Å both for strained and unstrained films and so most likely applies also to the ͑111͒ surface of a bulk CoO crystal. The superstructure is interpreted as a stress-relieving reaction to the switch from rocksalt-type to wurtzite-type stacking below the surface which has been detected earlier.As is well known, transition-metal oxides play an important role in catalysis and modern material science. Concerning the latter the importance holds in particular for magnetoelectronic applications as, for example, spin valves and giant magnetoresistance devices as well as for beating the superparamagnetic limit of small clusters of a ferromagnetic material. Thereby, the phenomenon of exchange bias 1,2 is exploited, i.e., the shift of the magnetic hysteresis loop along the field axis and enhancement of the coercive field. This is observed in coupled ferromagnetic and antiferromagnetic thin films and is an interfacial phenomenon, i.e., interface properties have an essential influence on the device properties. These properties include the crystallographic structure of the interface as well as the interface roughness, that is, deviations from an atomically smooth interface. 3 It is common knowledge that the structure and roughness of catalyst surfaces are also essential for their catalytic activity.In this Rapid Communication we address the surface of CoO͑111͒ films. CoO plays an important role in the application and research of magnetoelectronic devices 3-8 and catalysts ͑e.g., Ref. 9͒. Recently we have shown that films grown on an unreconstructed Ir͑100͒ surface are terminated by a wurtzite-type stacking of layers in contrast to the rocksalttype stacking in native CoO͑111͒ and leading to a metallic surface. 10 Here we show that this termination in its low-temperature ground state ͑below 50°C͒ exhibits a ͑ ͱ 3 ϫ ͱ 3͒R30°superstructure with respect to the ͑111͒ bulk layer unit cell of CoO. We regard this structure to be of importance as it should influence both catalytic and interfacial properties. Both the wurtzite-type termination and the ͑ ͱ 3 ϫ ͱ 3͒R30°superstructure prevail in the thickness regime investigated, i.e., from 10 Å to values as large as 150 Å. Consequently, this special surface structure must be regarded as typical for CoO͑111͒ thin films and most likely also for the ͑111͒ surface of a CoO crystal. It possibly represents a new kind of stoichiometry-saving polarity compensation so far not known from the literature ͑for a review, see Ref. 11͒.Quantitative low-energy electron diffraction ͑LEED͒ and scanning tunneling microscopy ͑STM͒ were used to investigate the films within a two-stage ultrahigh vacuum apparatus as described earlier. 10 STM images were taken for the sample at room temperature. LEED intensity vs energy s...

Substrate-induced structural modulation of a CoO(111) bilayer on Ir(100)

“…Only four of the oxygen ions ͑O high ͒ are located, as expected, i.e., in or near hollow sites formed by the underlying Co ions. The Co-O high bond lengths are with an average of 1.85 Å ͑variational range 1.81-1.87 Å͒ identical to that found for the outermost Co-O bonds in thicker CoO films, 4 where the local oxygen coordination is the same as in the present case. The other five oxygen ions ͑O low ͒, however, are pushed into the surface by almost 1 Å with respect to the other ones.…”

“…On this background and as the structures of nanosized solids frequently differ from their bulk structures, we have studied ultrathin cobaltoxide films in the recent past whereby the Ir͑100͒-͑1 ϫ 1͒ surface has been used as support. 2 In fact, we have found that the surfaces of CoO films grown in ͑111͒ orientation deviate substantially from the rocksalt structure 3,4 ͓the large lattice misfit between CoO and Ir of about 10% prohibits pseudomorphic growth in ͑100͒ orientation͔.…”

Thermal evolution of cobalt deposits on Co3O4(111): atomically dispersed cobalt, two-dimensional CoO islands, and metallic Co nanoparticles

“…33,34 Here, various other factors were considered, in particular the presence of oxygen vacancies and surface Co 2+ ions. [38][39][40][41][42][43][44][45][46][47][48] In a unique fashion, these films allow varying the stoichiometry, the surface orientation, the film thickness, and the defect density. 29 The present work aims at the development of new cobaltoxide-based model systems which allow us to study their surface chemistry under well-controlled ultrahigh vacuum (UHV) conditions.…”

The two-dimensional cobalt oxide (9 × 2) phase on Pd(100)

“…Cobalt oxides have found interesting applications in many diverse fields of the advanced technologies, 8 and the epitaxial growth of Co oxide thin films has been reported on various metal substrates, such as Ag(100), [9][10][11][12][13] Ir(100), [14][15][16][17][18] and Pd(100). 8 have been described as a function of film thickness: while for thicker films (>10 ML) the oxide overlayers converge to the known bulk phases of CoO and/or Co 3 O 4 , the films in the ultrathin limit of 2D structures depend on the particular metal substrate and the thermodynamic and kinetic parameters of the deposition.…”