Low-Dimensionality and Epitaxial Stabilization in Metal ...



Low-Dimensionality and Epitaxial Stabilization in Metal Supported Oxide Nanostructures: Mn[pic]O[pic] on Pd(100) Mn[pic]O[pic]

Cesare Franchini† and Francesco Allegretti‡

†Faculty of Physics, Computational Materials Physics, Universität Wien and Center for Computational Materials Science A-1090, Wien, Austria

‡Institute of Physics, Surface and Interface Physics, Karl-Franzens University Graz, A-8010 Graz, Austria

We present a survey of the growth and structure of manganese oxide nanolayers on a Pd(100) substrate, investigated in two different thickness regimes through a plethora of surface science techniques (STM, AFM, LEED, SPA-LEED, XPS, XAS, and HREELS) and state-of-the-art theoretical tools (DFT and hybrid DFT approaches). In the high thickness regime, above 10-15 monolayers, depending on the preparation conditions different films with specific growth direction and stoichiometry are formed. At low and intermediate pressure ([pic] 1[pic]10[pic] mbar and 2-5[pic]10[pic] mbar, respectively) the oxide structures are already bulk-like in terms of their in-plane lattice constant and can be assigned to MnO(111) and MnO(100), respectively. At high pressures ([pic] 5[pic]10[pic] mbar), Mn[pic]O[pic] layers (001)-oriented are obtained by oxidation of MnO(100). To explore the epitaxial (geometric) relationships that favor the growth of the different oxide phases, we have investigated the atomistic details of different oxide/oxide and oxide/metal interfaces. In particular, we have addressed the issue of the stability of the Mn[pic]O[pic](001)/MnO(001) interface and determined the phase stability diagram of Mn[pic]O[pic]/Pd(100) phases at a Mn coverage of about 1 ML. In the latter low thickness regime we have identified nine different two-dimensional (2D) phases, which are novel in terms of their structural and electronic properties. These nanophases can be classified according to similar building block units and described either as O-Mn-O MnO(111)-like trilayers or in terms of metal-deficient MnO(100)-like monolayers, and therefore we argue that they mediate the epitaxial growth of MnO thicker films on Pd(100) by providing structurally graded interfaces. Moreover, the formation of O or Mn vacancies drives the transition between 2D phases with similar structural units but different lattice periodicity, indicating that ion vacancies, mixed valence states and substoichiometry lie at the basis of the architectural flexibility in the monolayer regime. Interestingly, the latter concepts play a major role in the more complex class of functional oxides such as the manganites, of which binary manganese oxides are the simplest parent compounds.

10.1 Introduction

Since several decades, the study of transition metal oxides represents a field of active and intense research. Thanks to the uniquely rich spectrum of structural, electronic and magnetic properties, which cover the range from metallic behavior to magnetic insulators and encompass spectacular phenomena as superconductivity, charge ordering and colossal magnetoresistance, these compounds have attracted the interest of physicists, solid state chemists and materials scientists. Renewed interest has been fuelled by the advances in the synthesis processes of oxide materials, which now allow control over the structure and stoichiometry at the level of a unit cell. This has opened up new far-reaching perspectives for both fundamental studies and technological applications, in particular in the field of functional and multi-functional oxides, many of which incorporate in their structure one or more transition metal atom species. Functional oxide materials have unique physical and chemical properties, which can be suitably controlled and modified by means of external stimuli, such as changes of temperature and pressure, electric or magnetic fields and the adsorption of foreign atoms or molecules, thereby enabling the development of new functionalities. As a result, a wide range of intriguing phenomena can be observed, ranging from superconductivity to ferroelectricity, piezoelectricity and exotic magnetic behaviors. The ability to tailor and tune the properties of these functional materials thus guarantees a high potential for applications in micro- and nano-electronics, spintronics, heterogeneous catalysis, gas sensing, energy harvesting, etc.

In addition to these developments, novel properties can also arise by the scaling down of the dimensions of oxide-based devices. At the nanoscale, in fact, effects arising from the reduced dimensionality, the geometric confinement, and the proximity of surfaces and interfaces as well as effects arising from the coupling of the oxide to the supporting material (so-called substrate-induced effects, such as electronic hybridization, charge transfer and elastic strain) may convey new physical and chemical properties and therefore new functionalities to oxide systems. This potential is indeed reflected in the enormous impulse given to fundamental and applied research in the fields of nanoscience and nanotechnology. Among low-dimensional nanostructured oxide systems, a prominent role is played by transition metal oxide ultrathin films supported on metal surfaces. Just to name a few technological applications, they are typically employed as gate dielectrics and tunnelling barrier layers in conventional and novel electronic devices [9, 63], as protective layers in corrosion prevention and inhibition [42], as gas sensor materials [34, 71], as support surfaces in the field of heterogeneous catalysis [22]. In fundamental scientific studies, on the other hand, thin films of transition metal oxides grown on metal single crystal surfaces constitute preferred model systems for the elucidation of emerging phenomena at the atomistic scale. Not only the presence of the metal substrate allows circumventing charging problems arising from the insulating character of many oxides, but also it may lead - through the active participation in the elastic and electronic coupling - to the stabilization of novel hybrid systems, whose structural, electronic and magnetic properties bear no correspondence to those of bulk oxides [60].

With this general frame in mind, in this chapter we focus on the study of ultrathin manganese oxide films epitaxially grown on Pd(100), providing a brief review of a combined theoretical and experimental investigation performed by our research groups at the University of Vienna and at the Karl-Franzens University of Graz. All calculations presented in the present chapter have been performed using the Vienna ab initio Simulation Package (VASP) [46, 47] in the framework of density functional theory (DFT) [45] and hybrid DFT [40].

Due to the ability of manganese to assume different oxidation states, ranging from +2 to +7 [39], Mn oxides in bulk compounds exhibit a number of stoichiometries with a complex phase diagram [28]. Their architecture is dictated by the ability of the Mn/O complexes to assemble by corner-sharing, edge-sharing or double corner-sharing, such that more than thirty Mn oxide mineral phases occur in nature [64]. As a consequence, Mn oxides display a richness of behaviors which render them attractive systems in applications of heterogeneous catalysis [10] (for example as electro-catalysts in fuel cells [72, 77]) as well as in applications as electrode materials in solid state batteries [8] and in the environmental waste treatment [21, 75]. Moreover, Mn oxides are parent compounds of a particular class of functional oxides, the manganites, the properties of which are determined by the complex interplay among spin, orbital and lattice degrees of freedom, which results in outstanding phenomena [66] such as giant magnetoresistance [76], metal-insulator transitions and orbital orderings[41]. These spectacular properties promise to find application in the fields of magnetic recording and novel spintronic devices. As to the study of Mn oxides in reduced dimensions, an increasing effort has been devoted in the last decade, which has revealed the richness of unusual behaviors and an even higher degree of complexity relative to bulk Mn oxides. For example, the ferromagnetic behavior of small MnO nanoparticles, anomalous with respect to the observed antiferromagnetic ordering in the bulk phase, has been first predicted by DFT [59] and then confirmed by subsequent experiments [50]. Recently, ferromagnetism of MnO and Mn[pic]O[pic] nanowires has also been observed [56], whereas unconventional exchange bias interaction has been reported for oxide coated Mn nanoparticles with a Mn[pic]O[pic] shell [73], the latter effect being attributed to an unusual spin alignment sequence at the interface.

To understand the unconventional properties of low-dimensional Mn oxides, fundamental studies on model systems are desirable, which require a detailed knowledge of the oxide structures at the atomistic level in the effort to elucidate the structure-properties relationship. So far, these studies on model systems have mainly focused on the growth of MnO thin films on the Ag(001) surface. Due to the relatively large in-plane lattice constant of MnO(001) ([pic]=3.14 Å), the template has been chosen to ensure a reasonable overlayer-substrate matching ([pic]=2.89 Å), which is expected to favor better epitaxial growth. Indeed, despite the still large mismatch (about 9%) films of good quality have been obtained [55, 74]. Further investigations cast light on the evolution of the structural and electronic properties with the oxide thickness due to the partial release of the epitaxial strain [57, 58, 19]. However, no detailed studies of the oxide-Ag(001) interface in terms of the Mn oxide phases formed at coverage below or about 1 monolayer (ML) are available in the literature. Interface-stabilized Mn[pic]O[pic] monolayers have instead been reported on two different substrates, Rh(100) [61] and Pt(111) [38], but the literature is still sparse and the unambiguous structural assignment of the oxide phases has not been yet accomplished. In this context, we have recently performed a joint experimental-theoretical investigation aimed to extensively characterize the growth and structure of Mn[pic]O[pic] ultrathin films on a Pd(100) substrate. The film thickness range extended from a few Å, corresponding to the interface-stabilized phases of the monolayer regime, up to 30-50 Å, where bulk-like behavior sets in. The detailed theory-experiment comparison for the Mn oxide systems, which will be presented in the next section, proved a decisive factor for the unambiguous assignment of the oxide phases, and it enabled to identify the interface-stabilized oxide structures that mediate the epitaxial growth. Moreover, it proved highly beneficial, in that it allowed assessing the reliability of density functional theory (DFT) and post-DFT approaches applied to the Mn oxides. This is a very important issue in the light of the well-known difficulties encountered by standard DFT in dealing with strongly correlated materials [1].

10.2 Growth of Mn[pic]O[pic] Layers on Pd(100)

The growth of Mn oxides on Pd(100) has been the subject of a series of recent papers [3, 16, 4, 51, 26, 27], which explore in a rather systematic way the phase stability diagram at different oxide thicknesses. In particular, it has been shown that below 1 ML a complex surface phase diagram with a multitude of novel Mn oxide structures develops, the oxide structures being characterized by specific structural building blocks and vibrational properties [51, 26, 27]. In the high thickness regime, upon deposition of 20-30 ML, MnO(100) films exhibiting good long range order can be grown epitaxially [3], despite the considerable lattice mismatch with the substrate ([pic]14%). The MnO(100) structure can be preferentially converted either into MnO(111) [3, 4] or Mn[pic]O[pic](001) [16] depending on the combination of temperature and applied oxygen pressure. The evolution of the physical properties of the manganese oxide layers on Pd(100) with respect to the changes in film thickness provides a unique method to separate surface (2D) and bulk (3D) effects. In fact, to a decrease of film thicknesses from the high coverage (multilayer) to the low coverage ((sub)monolayer) regime corresponds an increase of the surface to bulk ratio, and low dimensional effects become more distinguishable. The fundamental characteristics of the Mn[pic]O[pic]/Pd(100) system are discussed in details in Sec. 2.1 and 2.2 and can be summarized as follows:

Mn[pic]O[pic]/Pd(100)

Low coverage regime, [pic] 0.75 monolayer (see Sec. 2.1). Ultrathin layers of variable Mn[pic]O[pic] stoichiometry, only 1-2 ML thick, can be formed at the metal-MnO interface. At least nine different 2D Mn[pic]O[pic] phases on Pd(100) are found, which are novel in terms of the known Mn oxide bulk crystal structures and which are stabilized by the metal-oxide interface and by the confinement in the direction perpendicular to the surface. These low-dimensional interface-stabilized phases may mediate the epitaxial growth of thicker layers by providing structurally graded interfaces.

High coverage regime, [pic] 20-30 layers (see Sec. 2.2). MnO(100) with bulk-like in-plane lattice constant is stable in a wide range of pressure and temperature. Through the appropriate tuning of temperature and oxygen pressure the MnO(100) films can be transformed into MnO(111) (annealing at elevated temperatures or reactive evaporation at lower oxygen pressures [[pic] mbar]) or transformed into Mn[pic]O[pic](001) surfaces (high temperature oxidation at relatively high oxygen pressure [[pic] mbar]).

10.2.1 Low Coverage Regime

The experimental phase stability diagram of Mn oxides on Pd(100) below 1ML is depicted schematically in Fig. 1, where the various Mn oxide nanolayer phases are ordered as a function of the oxygen pressure [pic] and of the oxygen chemical potential [pic] during the preparation procedure, and they are represented by their corresponding scanning tunneling microscopy (STM) profile.

Fig. 1 Experimental schematic phase stability diagram of the interfacial Mn oxides, presented as a function of the oxygen pressure [pic] and of the oxygen chemical potential [pic]. The nominal coverage of Mn on Pd(100) is [pic] 0.75 ML. From Ref. [51] with permission

The manifold Mn oxide phase diagram comprises nine different nanophases which are characterized by specific windows in the parameter space of the thermodynamic variables temperature (in the total range: 600-800 K) and oxygen pressure (in the total range: 5[pic]10[pic]-5[pic]10[pic] mbar). We distinguish:

1. The oxygen-rich regime (5[pic]10[pic] mbar > [pic] < 5[pic]10[pic] mbar)

(a) Two hexagonal phases (HEX-I and HEX-II), which are both obtained at high [pic] pressures

2. The oxygen-intermediate regime (5[pic]10[pic] mbar > [pic] < 1[pic]10[pic] mbar)

(a) A c(4[pic]2) structure and a stripe phase described as a uni-axially compressed c(4[pic]2), which are both stabilized at intermediate [pic] pressures

(b) Two structures which were called chevrons (CHEV-I and CHEV-II), because of their STM appearance

3. The oxygen-poor regime: (1[pic]10[pic] mbar > [pic] < 5[pic]10[pic] mbar)

(a) Two reduced phases with complex structures, named waves and labyrinth

(b) At the most reducing conditions, a third hexagonal phase (HEX-III), commensurate with the Pd(100) substrate ([pic]2) along one of the two [pic] directions.

The extraordinary architectural flexibility of the interfacial Mn oxides on Pd(100) and the electronic properties of the novel 2D phases can be rationalized and understood through a synergic combination of experimental [STM, low energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoemission spectroscopy (XPS)] and computational (DFT and hybrid functionals) techniques. Indeed, in modern surface science theoretical calculations have become an efficient complementing tool to the experimental observations. Theoretical models based on educated guesses of possible structures can be tested and directly compared with the experiments in order to clarify the structural aspects and provide an atomistic interpretation of the measured properties. In the present case, the joint experimental-theoretical analysis reveals that the two oxygen-rich phases (HEX-I and HEX-II) can be described in terms of O-Mn-O trilayers with MnO(111)-like structures (see Sec.2.1.1), whereas the intermediate oxygen regime (c(4[pic]2) and chevrons) is based on a compressed MnO(100)-like monolayer model (see Sec.2.1.2). At low oxygen chemical potentials the waves and labyrinth structures show in STM very complex unit cells whose link to the other phases is less clear but seems still to be related to a MnO(100)-like wetting layer, while the additional hexagonal phase HEX-III is of uncertain attribution (see Sec.2.1.3).

Fig. 2 HREELS phonon spectra of the four Mn oxide submonolayer phases at high and intermediate oxygen partial pressures. From the top to the bottom: HEX-I, HEX-II, c(4[pic]2) and CHEV-I. The statistical uncertainty in the peak position is [pic] 0.5 meV. For every Mn oxide phase, measurements performed on samples freshly prepared in different days agree within 1 meV. Adopted from Ref. [51]

The recurrent MnO(100)- and MnO(111)-like structural features are reflected in the phonon-loss spectrum displayed in Fig. 3(b)@. As expected, the common building blocks shared between the different oxide phases result in similar phonon losses. All spectra exhibit a clear single peak structure. The phonon loss is centered at around 70 meV for the two HEX-I and HEX-II phases and shifts down to 44-45 meV for the c(4[pic]2) and CHEV-I phases. Interestingly, the phonon spectra (not shown) of the CHEV-II, waves and HEX-III phases, which are obtained by further lowering the O chemical potential, are also characterized by a single peak at 43-45 meV. These findings provide clear indication of two distinct regimes, a MnO(111)-like regime comprising phases with a higher energy phonon loss (70 meV) and a MnO(100)-like regime with a single phonon loss around 44 meV.

10.2.1.1 MnO(111)-like Phases (Oxygen-Rich Regime)

In this pressure regime two Mn oxide phases have been detected, HEX-I and HEX-II, which are characterized by hexagonal or quasi-hexagonal (i.e. distorted) symmetry linked to that of MnO(111), and which display a phonon spectrum with a single loss peak centered at around 70 meV.

HEX-I

For the sake of clarity, it is instructive to recall the structure of MnO(111), which is made out of alternating O and Mn hexagonal layers with ABC stacking sequence, each layer having in-plane lattice constant [pic]=3.14 Å [see Fig.(a)].

Fig. 3 (a) Real and (b) reciprocal lattice for a (1[pic]1) MnO(111)-like hexagonal structure on a Pd(100) surface. The lattice parameter of the overlayer is assumed to be the measured bulk value [pic]= 3.14 Å, whereas [pic]=2.75 Å. In (a) only one of the two symmetry domains is reported for clarity. (c) SPA-LEED two-dimensional pattern measured at E=90 eV, for the undistorted HEX-I phase. (d) LEED pattern of the distorted HEX-I phase, recorded at E=96 eV. (e) Real and (f) reciprocal lattice associated with the distorted hexagonal MnO(111)-like phase. Only one symmetry domain is shown in (e) for clarity, for which the lattice parameter in the direction of the distortion ([011][pic]) is [pic]=2.94 Å. In total, four symmetry domains contribute to the reciprocal lattice in (f): two are obtained from panel (e) with [pic] at either +60[pic] or -60[pic] from [pic], and the other two are obtained by rotating [pic] by 90[pic] relative to the substrate mesh. (g) Hard sphere model simulating in the real space the moiré pattern that originates from the interference of the quasi-hexagonal oxide lattice of panel (e) with the square mesh of the Pd(100) substrate. Figure adapted from Ref. [26] and Ref. [51] with permission

A perfect MnO(111) film would give rise to a hexagonal reciprocal space pattern with lattice parameter [pic]/3[pic]=2.30 Å[pic]. The latter value is very close to the reciprocal space lattice parameter of Pd(100) ([pic]/[pic]=2.28 Å[pic]). The resulting LEED pattern averaged over two hexagonal domains rotated by 90[pic], which account for the different symmetry of substrate and overlayer, is a circular array of 12 extra-spots superimposed on the (10) spots of the substrate, as illustrated in Fig. (b). Although such a LEED pattern has been indeed observed in the experiments, as demonstrated in Fig.(c), the structural details sensibly depend on the preparation conditions. It appears that this [pic] hexagonal structure is favored at slightly lower coverage (0.5-0.6 ML) or after repeated oxidation cycles. In contrast, at 0.75 ML and after a single oxidation step at high pressure the LEED pattern reported in Fig.(d) is reproducibly observed: it still reflects a hexagonal-like lattice, but the characteristic elongation of the overlayer spots indicates a clear distortion of the ``perfect'' (1[pic]1) structure into a [pic] hexagonal structure. With respect to the perfect hexagonal lattice, the [pic] LEED pattern is distinctive of an incommensurate overlayer with a contracted lattice vector [pic] = 2.94 Å along [011][pic] [Fig. (e)]. The simulated reciprocal space pattern of the [pic] structure, shown in Fig. (f), reproduces very well the experimental LEED pattern [Fig. (d)]. Interestingly, the comparison between the [pic] and [pic] real space lattices of Figs. (a) and (e) suggests that the better matching between overlayer and substrate might be the driving force for the distortion: the initial mismatch of 14% along the [011][pic] direction is in fact reduced to ([pic])/[pic]=7% upon distortion. This better epitaxial relationship is likely to be at the origin of the increased stability and reproducibility of the distorted HEX-I phase.

Fig. 4 (a-b) Top view of the geometrical models for [pic] (a) and [pic] (b) HEX-I MnO(111)-like trilayer phases: red spheres: O atoms; light gray spheres: Mn atoms. Dashed lines delimit the 2D unit cell. (c-e) Experimental (c,d) and simulated (e) STM images of the HEX-I phase; Experimental images: sample bias U = +0.5 V (c), +0.6 V (d); tunneling current I =0.13 nA (c), 0.15 nA (d). The simulated STM image has been calculated considering tunneling into empty states between 0 and +0.5 eV. Taken from Ref. [26] with permission

In passing, we note that the [pic] model explains also the moiré pattern observed experimentally in the STM images [Fig.2(c)]. The latter results from the interference of the quasi-hexagonal Mn oxide overlayer with the square mesh of the Pd(100) surface. Taking a ratio [pic]/[pic]16/15, a moiré pattern can be generated with a geometrical model that displays modulations in form of broad lines inclined with respect to the [011][pic] direction and with an average periodicity of about 22 Å parallel to [011][pic] [see Fig. (g)]. In this model, only one type of overlayer atoms is considered for simplicity, and a different color gradation (white, grey, black) is used to emphasize the different lateral registry of these overlayer atoms relative to the underlying matrix of substrate atoms. For on-top/bridge location, the white/black color of the overlayer atoms reflects the different height above the surface. The remarkable similarity between the modeled moiré pattern [Fig. (g)] and the experimental STM image [Fig.2(c)] suggests implicitly that the HEX-I phase may consist of alternately stacked layers with quasi-hexagonal symmetry containing only one single atom species, as it is realized in the MnO(111) structure, which is therefore the natural toy-model for the computational analysis.

The HEX-I model used in the DFT analysis is constructed by cleaving the MnO structure perpendicular to the [111] direction to form the smallest block containing 1 ML of Mn atoms in the O-Mn-O stacking, with a formal stoichiometry of MnO[pic]. A rigid sphere sketch of the MnO[pic] trilayer is given in Fig. 2(a), along with the corresponding [pic] trilayer [Fig. 2(b)] obtained by shrinking the lattice constant [pic] by 7%. Due to the free-standing (i.e.: unsupported trilayers) computational setup, which is unavoidable because of the incommensurate registry of the substrate/overlayer system, the optimized 2D lattice constants are significantly smaller than the measured one. Energetic considerations on the computed relative stability of the two HEX-I models reveal that although the perfect hexagonal structure is the most favorable, only a low energy cost of 70 meV/cell is required to convert it into a [pic] hexagonal structure. Therefore, the interaction with the substrate, neglected in the unsupported trilayer model, might easily provide the energy gain to reverse this energy sequence through the better matching discussed above.

The soundness of the MnO[pic] trilayer model is confirmed by a direct comparison of the HREELS and STM data with the computed counterparts. Panels (d) and (e) of Fig. 2 show the comparison between the measured and simulated atomically-resolved STM images. Despite the low resolution of the experimental map the agreement is acceptable, in particular in that the arrangement of large bright protrusions is fairly reproduced; moreover, the theoretical data allow assigning the bright spots to the topmost oxygen atoms. Further convincing support for the proposed model comes from the comparison between the experimental (70.5 meV) and computed ([pic] 73 meV) distinct single dipole-active phonon mode, which is associated to the vertical antiphase vibration of O and Mn layers [26]. Finally, it is also worth noting that this trilayer model is well compatible with the O [pic] core level photoemission data reported in Ref. [51], which show a strong component at 529.1 eV attributable to the surface O layer and a weaker line chemically shifted to higher binding energy presumably related to the O layer at the interface with the Pd(100) substrate. Indeed, a model consisting of a MnO(111)-like bilayer, with only one layer of chemically equivalent O atoms either at the interface or at the surface to yield a formal MnO stoichiometry, is not only incompatible with the photoemission data but it is also shown to be much less stable than the corresponding trilayer model in a broad window of the O chemical potential (Ref. [26]).

HEX-II

The O-Mn-O MnO(111) trilayer model is the crucial key for the structural understanding of the HEX-II phase, which is obtained from the HEX-I phase upon reduction of the oxygen partial pressure. Unlike HEX-I, the STM appearance of HEX-II is characterized by a well-ordered hexagonal array of triangularly shaped maxima [Fig. 3(c)], and the lattice constant derived from the STM and LEED measurements [Fig. 3(a)] is [pic] Å , corresponding to a compressed (2[pic]2) reconstruction of the MnO(111) termination. The DFT analysis reveals that the so-called O-terminated [pic] reconstruction displays the lowest surface energy in a wide range of oxygen partial pressures [26]. The [pic] structure is obtained by removing one topmost oxygen atom per (2[pic]2) unit cell from the HEX-I structure, resulting in a [pic] trilayer with in-plane hexagonal lattice constant of 5.95 Å, as shown in Fig. 3(b).

Fig. 5 (a) LEED pattern of the HEX-II phase (E = 80 eV). (b) Top view of the [pic]-[pic] HEX-II model: red spheres: O atoms; light gray spheres: Mn atoms. Dashed lines delimit the 2D unit cell, whereas triangles highlight the octopolar Mn-O[pic] pyramids and the O[pic] trimers of the [pic] termination. (c) Experimental and simulated STM images of the hexagonal [pic] HEX-II phase; sample bias U = +0.75 V; tunneling current I =0.1 nA. The simulated STM image has been calculated considering tunneling into empty states between 0 and +0.75 eV. Unit cells are indicated by full lines. Figure adapted from Ref. [26] and Ref. [51] with permission

Compelling evidence for the O-terminated [pic] model comes from the comparison between the simulated STM and HREELS data and the corresponding experimental results. The hexagonal pattern of triangular bright protrusions in the experimental STM picture is well reproduced by the simulated STM image [Fig. 3(c)], and the triangular protrusions are therefore ascribed to the oxygen trimers outlined in the sketch of Fig. 3(b). As for the HREELS spectrum, in analogy with the precursor HEX-I phase, the HEX-II [pic] phase displays a single phonon loss peak centered at 69.5 meV [Fig. 3(b)@], characteristic of the MnO(111)-like structures. The simulation reproduces well the position of the vibrational peak (within 1-2 eV) and ascribes it to the concerted movement of the surface oxygen trimers against the manganese atoms underneath.

10.2.1.2 MnO(100)-like Phases (Intermediate Oxygen Regime)

At intermediate oxygen pressures four different Mn[pic]O[pic] phases have been observed. At the high pressure end of this regime two phases coexist (see Fig. 1): a quasi-hexagonal [pic] phase characterized by bands of parallel stripes separated by [pic]5.5 Å (2[pic]), and the c(4[pic]2) structure, commensurate with the Pd(100) substrate as expressed by the transformation matrix M = [2,-1 / 2,1]. At the low pressure end, instead, the c(4[pic]2) phase is often observed in coexistence with the so-called chevron structures, chevron I and chevron II, corresponding to the transformation matrices M = [2,-1 / 3,2] and [2,-1 / 5,3]. The phase boundaries between the two chevrons structures and the c(4[pic]2) structure are smooth and continuous, suggesting that from the structural point of view these phases are closely related. Indeed, the unit cells of the chevrons structures can be generated from the c(4[pic]2) structure by simply extending the [pic] unit cell vectors to adjacent antiphase positions as illustrated in the geometric models of Fig. 2.1.2(b). In the following the structural analogies of these phases are rationalized in terms of MnO(100)-like monolayers.

Fig. 6 (a) LEED patterns, (b) real lattice models, and (c) reciprocal lattice patterns of the c(4[pic]2) (left panel), chevron I (middle panel) and chevron II (right panel) structures. The respective LEED energies in (a) are 116, 108 and 60 eV. From Ref. [51]

c(4[pic]2)-Mn[pic]O[pic]/Pd(100)

The LEED pattern of the c(4[pic]2) structure, the central phase in this pressure regime, is displayed in Fig. 2.1.2(a) and compared with the simulated reciprocal space pattern of panel (c), which corresponds to the real space periodicity given in panel (b). The structural model that explains the c(4[pic]2) Mn[pic]O[pic] phase is depicted in Fig. 4. It is constructed by placing a compressed MnO(100) monolayer on top of Pd(100) and by creating a regular rhombic array of Mn vacancies, in close analogy with the Ni/Co oxide monolayer phases, c(4[pic]2)-Ni[pic]O[pic]/Pd(100) and c(4[pic]2)-Co[pic]O[pic]/Pd(100), respectively, obtained upon reactive evaporation of nickel/cobalt on Pd(100) [2, 5]. It is, however, to be noted that the lattice mismatch between MnO (14%) and the substrate is much larger than that for NiO (7%) and CoO (9.5%). The lateral compression required to fit a single layer of the bulk MnO(100) structure onto the Pd(100) substrate and the consequent formation of the Mn vacancies dominate the structural rearrangement and determine the occurrence of two distinct types of Mn atoms with different environment, as illustrated in Fig.4(a): the Mn1 atoms are localized between two nearest neighbor vacancies, while the Mn2 atoms form zig-zag Mn-Mn chains along the [01[pic]] direction. Large relaxations take place in the Mn[pic]O[pic] layer, which is found to be well separated from the substrate by z[pic]2.3 Å, approximately 20% larger than the Pd bulk interlayer distance. The lateral compression is accompanied by a significant rumpling of the monolayer, yielding a corrugation of [pic] 0.23 Å, whereas the Mn vacancy breaks the local Mn-O bonds and induces a planar outward relaxation [pic] of the oxygen atoms, which move closer to Mn1, resulting in the waving short-ranged ordering highlighted in Fig.4(a).

The comparison with the experimental data, given in Fig.4(b)-(c), validates the proposed c(4[pic]2)-Mn[pic]O[pic] structural model. The measured STM image [inset of panel (b) in Fig.4] is characterized by a rhombic arrangement of bright spots and dark depressions connected by weaker segments. These features are very well reproduced by the simulated STM picture and can be interpreted in the following way: the black depressions reflect the network of Mn vacancies, the single bright spots can be assigned to the Mn1 species, whereas the light segments correspond to the zig-zag Mn2-Mn2 chains embedded in the regular array of oxygen atoms, which are not seen in the STM image. Further support for the c(4[pic]2)-Mn[pic]O[pic] model is provided by the comparison of the calculated phonon spectrum with the HREELS measurements [Fig.4(c)]. The single peak structure centered at [pic] 43 meV revealed by HREELS is very well reproduced by the hybrid functional calculations (standard PBE, instead, underestimates the phonon energy by [pic]7 meV). The diagonalization of the dynamical matrix enables to assign this specific phonon mode to the collective vertical vibration of the oxygen sublattice against the Mn sublattice, a vibrational mode typical of the ideal MnO(100) surface [26, 38].

Fig. 7 (a) Optimized structural model for the c(4[pic]2)-Mn[pic]O[pic]/Pd(100) system. The Pd(100) substrate is displayed with light gray large spheres, whereas Mn and O atoms are depicted with small blue and pink spheres. Two distinct Mn species are distinguishable: Mn1, sandwiched between two vacancies, and two Mn2 atoms forming the zig-zag arrangements highlighted by the horizontal lines. Atoms surrounding the Mn vacancies experience considerable strain (indicated by arrows). (b) Comparison between simulated and experimental (inset) STM images for the c(4[pic]2)-Mn[pic]O[pic]/Pd(100) phase. Experimental parameters: sample bias U = +0.8 V, tunneling current I= 0.25 nA.(c) Comparison between the measured HREELS phonon value (vertical bar) and PBE (dashed line) and HSE (full line) predicted dipole active modes. Adapted from Ref. [26] and Ref. [27]

Chevron structures

The chevron structures are adjacent to the c(4[pic]2) phase in the phase stability diagram and are obtained by lowering the O[pic] partial pressure. These phases, the name of which originates from the ``chevron-like'' STM motifs, can coexist locally with the c(4[pic]2) phase, as shown in Fig.5(a). Due to the close link with the c(4[pic]2) phase, presented in a pictorial fashion in Fig. 2.1.2 (b), the chevron structures can be rationalized in terms of structural models inspired by the c(4[pic]2) structural features and by invoking the concept of vacancy propagation. As discussed above, the periodicity of the c(4[pic]2) phase is given by the Mn vacancies distributed in a regular array with rhombic unit cell. Therefore, within this picture and based on Fig. 2.1.2(b), the chevron I and II structures may be naturally obtained from the c(4[pic]2) by propagating a Mn vacancy to a neighboring lattice site. Accordingly, the stoichiometry becomes [pic] and [pic], respectively, i.e. closer to that of MnO. Compared to the c(4[pic]2) phase, the chevron structures exhibit a much higher degree of long range order, with large defect-free areas extending over typical distances of 200 Å and interrupted by ordered domain boundaries, which convey the characteristic ``chevron-like'' appearance [see Fig. 5(a)].

Fig. 8 (a) STM image showing the characteristic appearance of the chevron structures and the local coexistence of the c(4[pic]2), chevron I and chevron II phases (140 Å [pic]100 Å ; U = +1.0 V; I = 0.2 nA). (b) Top view of the geometrical model for the Pd(100) supported chevron I phase: red spheres: O atoms; light gray spheres: Mn atoms; green small spheres: Pd atoms underneath. Dashed lines delimit the 2D unit cell and circles denote the position of the vacancies. (c) Experimental and simulated STM images of the chevron I phase; sample bias U = +1.0 V; tunneling current I =0.13 nA. The simulated STM image has been calculated considering tunneling into empty states between 0 and +1.0 eV. Unit cells are indicated by full lines. Figure adapted from Ref. [26]

The structural model for the chevron I structure, obtained by removing 14% of Mn atoms from the compressed MnO(100) monolayer, is depicted in Fig. 5(b). The geometrical optimization yields again an oxygen terminated surface with a corrugation of 0.33 Å, and in analogy with the c(4[pic]2)-[pic] structure a moderate displacement (0.13 Å) of Mn atoms towards the vacancy is found, indicated by the arrows in Fig. 5(b). Figure 5(c) shows a comparison between the experimental and simulated STM images. Although the experimental resolution does not allow a detailed atomic comparison between theory and experiment, the overall agreement is satisfactory. The dark circular depressions in the images represent the position of the Mn vacancies and determine the 2D unit cell of this phase, as outlined by the full lines. The bright protrusions are clearly due to manganese atoms, suggesting that electronic effects contribute predominantly to the STM topography. Two distinct bright features are observed in the experimental picture, one large spot in the center of the unit cell and weaker flecks at the edges. The simulated STM image together with the optimized geometrical data allows the atomically resolved identification of these two kinds of features. The manganese sublayer is itself slightly corrugated, due to a small upward shift (0.05 Å) of the Mn atoms laying closer to the vacancy and aligned in the [01[pic]] direction. These atoms, indicated by filled dots in Fig. 5(b), are responsible for the two large bright spots in the simulated STM image, which merge together in the experimental image thereby giving rise to the wavy lines of bright protrusions. The remaining spots (four per unit cell) arise from the lower manganese atoms. In the experimental STM image their intensity partially mixes up with the topmost Mn spots thus contributing to the large bright feature centered in the cell and partially providing the weaker spots along the edges. Finally, the DFT-based analysis establishes that the [pic] chevron I structure is characterized by a single dipole-active phonon mode at about 40 meV, in good agreement with the HREELS measurements (phonon loss at 44.5 meV). This peak reflects the vibrational fingerprint of the parent MnO(100) not only in terms of the phonon loss energy, very similar to the c(4[pic]2)-[pic] value (43.5 meV, see Fig. 3(b)@), but also in terms of the atomic displacements producing this vibration, which are associated to the vertical opposite movement of manganese and oxygen atoms.

10.2.1.3 The Reduced Phases (Oxygen-Poor Regime)

There is a smooth transition from the c(4[pic]2) and chevron structures of the intermediate oxygen pressure regime to the structures in the oxygen-poor regime denoted as labyrinth and waves. These two phases are named according to their STM pattern shown in Fig. 1. Although the complexity of these structures is a major obstacle for the computational machinery, some fundamental properties can be deduced from a careful analysis of STM and LEED data (not shown) [51]. The labyrinth structure is described by the lattice vectors [pic] = 13.5 [pic] 0.5 Å and [pic] = 11.0 [pic] 0.5 Å, [pic] = 85[pic] [pic] 2[pic] ([pic]: angle between lattice vectors [pic] and [pic]), the unit cell being rotated by [pic] = 21[pic] [pic] 2[pic] with respect to the Pd [011] direction. The unit cell of the waves structure is characterized by [pic] = 13.8 [pic] 0.2 Å, [pic] = 34 [pic] 1 Å, [pic] = 90[pic] [pic] 2[pic], which corresponds approximately to a (5[pic]12) superstructure with respect to the Pd(100) substrate. On the basis of the measured phonon loss spectrum and of the occurrence of ordered phase boundaries with the c(4[pic]2) and chevron structures, these two reduced phases with complex appearance are tentatively assigned to MnO(100)-like wetting layers.

Finally, at the most reducing conditions a different structure is observed, the HEX-III structure, shown on the left side of the phase stability diagram of Fig. 1. The HEX-III structure is described by the transformation matrix M = [0, 2 / [pic], 1] (or equivalently M = [[pic], -1 / [pic], 1]) with respect to Pd(100), but corresponds also to a ([pic])R30[pic] superstructure with respect to a MnO(111) surface. The last observation suggests that this phase might be related to a MnO(111)-like model, possibly a bilayer with 1/3 of the surface atoms removed. On the other hand, the matrix notation shows that the periodicity of the HEX-III phase can be also obtained upon a uni-directional 14%-compression of the c(4[pic]2) structure. As a result, a (7[pic]2) coincidence lattice relative to the Pd(100) surface is observed. This interpretation would be compatible with the very similar phonon loss spectra of the HEX-III and c(4[pic]2) phases, and with certain features of the STM appearance at particular bias voltages (indeed, the c(4[pic]2) periodicity can itself be regarded as due to a distorted hexagonal lattice of vacancies) [6]. This further compression could hypothetically produce a more pronounced rumpling of the corrugated monolayer, possibly resulting in a well-defined bilayer. The precise assignment of the HEX-III requires detailed structural investigations, and to this purpose DFT calculations are underway. It is, in any case, possible that the HEX-III structure is an interfacial phase that mediates the growth of MnO(111)-oriented films with hexagonal symmetry, which will be discussed in the next section.

10.2.2 High Coverage Regime

The reactive evaporation of 20-30 ML Mn on Pd(100) at moderate temperature ([pic]620 K) and at an oxygen pressure in the range between 2[pic]10[pic] and 5[pic]10[pic] mbar leads to the stabilization of (100)-oriented MnO films. The formation of well-ordered MnO(100) is evident from the spot profile analysis LEED (SPA-LEED) image presented in Fig. 2.2(a), which displays sharp reflexes arranged in a square pattern and superimposed on a very low background. The surface lattice constant determined from the separation of the LEED spots measures 3.14[pic]0.03 Å, identical to the (100) in-plane lattice parameter of bulk MnO. The frequency modulation atomic force microscopy (FM-AFM) image [Fig. 2.2(b)] shows that the MnO(100) surface is atomically flat and consists of terraces with lateral dimensions up to 500 Å [3]. The MnO stoichiometry of the oxide film has been confirmed by the characteristic fingerprint provided by core level photoemission spectroscopy (see Ref. [3] for details) and by the comparison between the valence band photoemission spectrum and the calculated [15] (PBE+Hubbard U method) density of states of MnO given in Fig. 2.2(c), which provides further evidence of the good quality of the MnO(100) multilayers. The main spectral features, namely the triple peak structure with maxima at 2.2, 3.5 and 4.9 eV, are reasonably well reproduced by the theory, although the calculated peak with highest binding energy is upshifted by about 1 eV with respect to the experimental one. Finally, in panel (d) of Fig. 2.2 the HREELS phonon spectrum exhibits a main phonon loss peak at [pic] meV and a weaker structure at [pic] meV, which can be connected with the bulk optically allowed phonons at 33.6-36.4 meV (Ref. [20]) and 62 meV (Ref. [37]). This finding is in line with the single peak structure at 71 meV detected in MnO(100) crystals [49]. The comparison of the phonon spectrum of the MnO(100) thick films with those of the MnO(100)-related monolayer structures discussed previously [c(4[pic]2)-Mn[pic]O[pic] and chevrons-Mn[pic]O[pic]/Mn[pic]O[pic]] clearly indicates that in the monolayer regime the dipole-active phonon mode is strongly redshifted by about 20 meV. A similar thickness dependence of the main phonon frequency has been also reported for the MnO(100)/Pt(111) system [38].

Fig. 9 30 ML MnO(100) on Pd(100) [3, 15]. (a) SPA-LEED pattern, recorded with an electron energy of 90 eV. (b) FM-AFM image of the MnO(100) surface (size 2500[pic]2500 Å[pic], detuning [pic]=[pic]5 Hz, bias U=[pic]0.2 V). (c) Valence band spectra excited by a photon energy of 120 eV (solid line) compared with the calculated density of states (dashed line). (d) HREELS spectrum of the MnO(100) film. Figure adapted from Ref. [3]

We have shown above that the number of oxidation states available to manganese in the solid state is reflected in the complex stoichiometric and architectural flexibility in the interface-dependent monolayer regime. Similarly, the manganese oxide surface can support a range of manganese oxidation states and oxide phases because of the similar electronic character of the simplest Mn oxides (MnO, Mn[pic]O[pic] and Mn[pic]O[pic]), which ultimately differ by a diverse electronic population and splitting of the partially filled [pic] shell. Langell et al. have shown that by proper adjustment of temperature and oxidizing/reducing conditions it is possible to select the dominant oxide forms, MnO, Mn[pic]O[pic] or Mn[pic]O[pic], present at the MnO(100) surface [49]. Within this context, it is not unexpected that under suitable preparation conditions (5[pic]10[pic]-2[pic]10[pic] mbar O[pic] and elevated temperature) the Pd(100)-supported MnO(100) structure can be converted into the Mn[pic]O[pic](001) surface [16]. It is nonetheless more surprising that either by annealing the MnO(100) films at elevated temperature ([pic]770 K) in vacuo or by reactive evaporation at relatively low pressure ([pic] 1[pic]10[pic] mbar) the transformation of the MnO(100) structure into a MnO(111) surface occurs [3]. The discussion of these two phase transitions will be addressed in the following two sections.

10.2.2.1 Formation of Mn[pic]O[pic] on MnO(001)

Although, like in other spinel-like structured compounds, [pic] terminations are formally considered to be unstable because of the uncompensated electrostatic potential perpendicular to the surface, it is reported that the cleavage of [pic] leads distinctly to a (001)-oriented surface [18]. Indeed, Noguera has rationalized that this formal instability is an artificial consequence of the oversimplistic ionic model and can be removed by a number of natural mechanisms involving structural and electronic reconstructions [62]. The epitaxial growth of [pic](001) has been achieved on MgO(001) [35, 36] and circumstantial evidence indicates that [pic]-like surfaces can be obtained by mild oxidation of MnO(001) single crystals [49]. In this section we discuss the formation of [pic](001) surfaces on Pd(100)-supported MnO(100) (denoted, equivalently, as MnO(001)/Pd(100)) by high temperature oxidation at intermediate and high molecular oxygen pressures.

Fig. 10 Comparison between the initial 20 ML thick [pic] film on Pd(100) and the [pic] phase prepared by oxidation of [pic]. (a) Mn 3[pic] core level spectra excited by a photon energy of 180 eV. The corresponding exchange spin splitting is indicated. (b) Mn L[pic]-edge X-ray absorption spectroscopy (XAS) spectra recorded in total yield mode. (c) HREELS spectra. The energies of the main energy losses are reported. (d) 2D SPA-LEED pattern of the 20 ML thick [pic] film and (e) of the [pic] phase. Both diffraction patterns were recorded with an electron energy of 125 eV. (f) Corresponding 1D SPA-LEED profiles centered on the (00) reflex and recorded along the [110] substrate direction at E[pic]=125 eV. Adapted from Ref. [16]

The Mn 3[pic] core level spectra shown in Fig. 6(a) indicate that exposing for 20 min Pd(100)-supported MnO(001) layers (20 ML) to molecular oxygen at a pressure of 2[pic] mbar with the sample kept at 770 K induces a marked decrease of the 3[pic] splitting down to 5.4 eV, a value that is in line with the measured exchange splitting for bulk [pic] (5.3-5.5 eV) and with the expected change in the electronic configuration of the [pic] shell. Further experimental evidence for the formation of [pic]-like films upon oxidation is provided by the Mn [pic]-edge X-ray absorption data reported in Fig. 6(b): as it is evident, the characteristic fingerprint of MnO (bottom spectrum) is completely lost upon oxidation (top spectrum), and the intensity ratio of the [pic] to [pic] lines is reduced from [pic]3.8 to [pic]2.6. Since the I([pic])/I([pic]) ratio can be related to the occupancy of the 3[pic] orbitals on the Mn ion [48], the overall changes are again consistent with the (partial) presence of Mn ions with 3[pic] configuration. In particular, the profile of the [pic] peak is considerably broadened and exhibits a triplet fine structure which is a distinctive signature of [pic] [29]. The [pic] stoichiometry of the oxidized film is also confirmed by the HREELS phonon spectrum [Fig. 6(c)]. While the MnO(001) film is characterized by a Fuchs-Kliewer phonon loss peak at [pic] 65 meV and a much weaker structure at [pic] 48 meV (the peak at 130 meV is a double loss of the 65 meV peak) as noted above, the oxidized film is dominated by a new phonon loss at 83 meV, very close to the analogous vibrational peak at 650-654 cm [pic] ([pic]81 meV) that has been detected by Raman spectroscopy measurements from mineral and synthetic hausmannite [pic] crystals [17, 43].

The issue of the surface symmetry and growth direction of the [pic] layers is successfully addressed by SPA-LEED measurements, see Fig. 6(d)-(e). Upon oxidation the sharp (1[pic]1) square pattern characteristic of well-ordered [pic] evolves into a weaker (2[pic]2)-like pattern with broader spots, which preserves the square symmetry of the unit cell but with a larger periodicity in the real space. The line scan along the < 110 > high symmetry direction [panel (f) of Fig.6] indicates that the spots appearing at roughly [pic]2 positions are the first order spots of [pic], which are located at 53[pic]1% of the [pic] Brillouin zone and in real space describe a square lattice with a unit cell parameter of 5.9[pic]0.1 Å. From the geometry of the bulk hausmannite spinel structure it is inferred that the [pic] film is (001)-oriented, with the [pic] and [pic] sides of the unit cell aligned along the < 110 > directions of the MnO(001) substrate. The data also demonstrate that due to the [pic]9% lattice mismatch with the [pic]2 supercell of the underlying MnO(001) the supported [pic] phase grows strained relative to the bulk phase (5.76 Å).

The DFT-based study of this system complements the experimental information, giving additional insight into the atomic-level characterization of the isolated [pic] surfaces and of the [pic] interface [16]. In line with the experimental observations, the (001) orientation of [pic] is found to be the energetically most favorable, with a surface energy more than 40 meV/Å[pic] lower than that of the (110) and (100) surfaces. Among the two possible as-cleaved (001) terminations, the mixed oxygen and manganese surface with stoichiometry Mn[pic]O[pic] is more stable than the manganese-terminated one by about 20 meV/Å[pic]. The structural modifications of the Mn[pic]O[pic]-terminated [pic] crystal induced by the presence of the surface are localized within the topmost two layers: they can be described in terms of a buckling ([pic]0.2 Å) of the outermost species and of the changes in the interlayer distances with respect to the bulk geometry (in particular, there is a very pronounced contraction of 26% of the first interlayer distance, partially compensated by an appreciable expansion of 9% of the second interlayer distance) [16].

Fig. 11 Model structure for the interface between [pic] and [pic] and calculated relative stability (PBE). Black and red circles indicate Mn and O atoms respectively. From Ref. [16]

Table 10.1: Calculated interlayer distances (d[pic]) for the [pic] optimized structure, expressed either in Å (first row) or in percent with respect to the bulk interlayer spacing of the strained Mn[pic]O[pic] structure (second row). d[pic] and d[pic] indicate the distances between the two [pic] layers and the first two [pic] layers at the interface, respectively, whereas d[pic] is the interfacial distance between the [pic] and [pic] contact planes.

| [pic] | [pic] | [pic] | [pic] | [pic] | [pic] | [pic] | [pic] | [pic] |[pic] |

| 0.74 | 1.26 | 0.95 | 0.95 | 1.02 | 1.00 | 1.10 | 1.02 | 2.19 |2.27 |

|-29.5 | +20.0 | -0.1 | -0.1 | - | -0.1 | +0.1 | 0.0 | 0.0 |- |

Based on the two distinct [pic] terminations and considering that the MnO(001) surface possesses a single termination with planar unit containing an equal number of manganese and oxygen atoms, two models of the interface emerge naturally: (1) interface A, which is constructed by placing the Mn-terminated (Mn-t) [pic] surface on the [pic] substrate and (2) interface B, which directly joins the MnO surface layer with the [pic]-t layer of [pic]. Different junctions can be constructed by changing the registry of the interfacial [pic] slice. The resulting models are sketched in Fig. 7: submodels A1 and B1 correspond to an atop site registry of the [pic] interfacial manganese atoms on the [pic] oxygen atoms underneath, whereas submodels A2 and B2 refer to the corresponding bridge site [pic]-[pic] registry. The calculated relative stability of the explored models, listed in Fig. 7, indicates that interface B1 has the lowest energy and represents by far the most favorable model.

The structural characteristics of the [pic] interface are summarized in Tab. 1 (optimized geometry) and Fig. 8 (layer by-layer decomposed charge density plot) [16]. The structural optimization was performed using a (2[pic]2) unit cell relative to the MnO lattice constant and preserving the bulk internal geometry in the three layers at the bottom of the slab. A large vertical contraction of 20% of the [pic] region of the slab is obtained, which compensates for the in-plane tensile strain. As a consequence of the latter strain, the distance between adjacent surface oxygen rows increases by 0.7 Å, in contrast with the contraction observed in the unstrained [pic] surface.

Fig. 12 Charge density plots for the [pic] interface: (a) layer-by-layer decomposition of the charge density along the [001] direction. The dashed lines A, B, C and D (only A and D labelings are given in the figure) are 2D projections of the (001) planes displayed in panel (b) and (c) showing vertical cuts of the charge density along the [100] and [010] directions, respectively. Dark gray (red) and light blue circles indicate the position of oxygen and manganese atom, respectively. The arrows highlight the most significant internal structural relaxations. The yellow line marks the separation between the [pic] and [pic] components, whereas the blue line indicates the unrelaxed starting position of the topmost [pic] layer. From Ref. [16]

In terms of interlayer distances it is found that the relaxations are well localized within the first three layers and do not affect the rest of the slab. By referring the changes of the interlayer distances to the bulk interlayer spacing corresponding to the bulk Mn[pic]O[pic] strained structure (i.e. with [pic]), a significant compression of the first interlayer distance ([pic] = -29.5 %) is observed, which is partially canceled out by the expansion of [pic] (+20.0 %). Deep down in the [pic] part of the slab the situation remains practically unchanged, including the distance [pic] between I2 and (S-7). At the interface, the distance [pic] between the contact [pic] and [pic] layers is found to be slightly larger than the [pic] interlayer distance. The formation of the interface leads to an upward shift of the manganese atoms placed in the MnO layer, thus inducing a relatively small buckling of 0.2 Å between these atoms and the interfacial MnO layer oxygens. It is worth noting that the bare [pic] surface exhibits a larger displacement of the topmost manganese atoms and an almost doubled surface buckling [15]. The rest of the MnO slab remains unaffected and reproduces the bulk-like behavior. In conclusion, the growth of strained [pic] films on a [pic](001)-oriented substrate inhibits the structural relaxations observed in the clean [pic] surface. Finally, because of the large vertical compression experienced in the [pic] side of the slab there is a general in-plane rearrangement of the atoms. As shown by the yellow arrows in Fig. 8 (b) and (c), the displacements mainly involve the movement of oxygens along the [010] direction, which leads to an important distortion of the octahedral O-Mn-O chain.

10.2.2.2 Epitaxial Stabilization of MnO(111) Overlayers

This conclusive section is devoted to the transformation of MnO(100) layers into a polar MnO(111) surface, which is observed upon annealing at high temperature in ultra high vacuum (UHV) [3, 4]. Keeping in mind the intrinsic thermodynamic stability of the neutral MnO(100), this is an interesting and somewhat unexpected result and therefore deserves an in-depth analysis. As already mentioned, the problem with the stability of polar ionic crystal surfaces has been long recognized, and recently its main lines have been discussed in a number of review articles [62, 33, 14]. However, while the (111) surfaces of rocksalt oxides such as MgO [31, 44, 7, 32], NiO [11, 12, 54, 13, 78], FeO [65, 68, 30], and CoO [52, 53] have been intensively investigated experimentally and theoretically, there is instead surprisingly little work done on the polar MnO(111) surface. Using the grazing incidence X-ray scattering technique, Renaud and Barbier have found that the MnO(111) single crystal surface is non-stoichiometric, containing a mixture of MnO(111) and Mn[pic]O[pic](111) phases [67]. Rizzi et al. have grown epitaxial MnO(111) films on a Pt(111) substrate and characterized their structure by X-ray photoelectron diffraction (XPD) and LEED, suggesting that the non-reconstructed MnO(111) surface can be stabilized by adsorption of OH groups [69, 70]. Finally, DFT-based thermodynamics has found that a (2[pic]2) octopolar reconstructed MnO(111) is the most stable surface over a wide range of the oxygen chemical potential [25].

LEED patterns corresponding to manganese oxide films of various thickness obtained by reactive evaporation of Mn at low oxygen pressures and high temperature are displayed in Fig. 2.2.2. The LEED reflexes now exhibit a pure hexagonal arrangement due to the MnO(111) overlayer, and the presence of a faint streakiness signals that faceting occurs to some extent. The phenomenon of the formation of facets becomes more pronounced at higher oxide coverage and high post-annealing temperatures, as visible in MnO films of about 30 ML [Fig. 2.2.2 (b)-(d)]. A detailed analysis [3] reveals that the facet rods are inclined with respect to the (00) rod of the MnO(111) surface by an angle of 53[pic] [pic], which is very close to the angle of 54.7[pic], expected between bulk (111) and (100) planes. This clearly demonstrates that the facets are of (100) type. Geometrical arguments, supported by FM-AFM profiles (not shown), indicate that the MnO(111) surface is covered by triangular pyramids exposing neutral (100) facets with the lowest surface energy. Therefore, the formation of the (100)-faceted pyramids on top of the MnO(111) surface provides a channel for minimizing the total energy. Similar (100)-faceted pyramids have been observed with AFM by Mocuta et al. on NiO(111) films supported on an [pic]-Al[pic]O[pic](0001) surface [54].

Fig. 13 (a) Conventional LEED picture of [pic]10 ML MnO(111) on Pd(100) evaporated at 670 K in [pic](O[pic])=1[pic]10[pic] mbar and annealed in UHV at 770 K. [(b)-(c)] SPA-LEED patterns of 30 ML MnO(111) evaporated at 670 K in [pic](O[pic])=1[pic]10[pic] mbar and annealed at 820 K, recorded with electron energies of 54 eV and 70 eV, respectively. (d) SPA-LEED pattern of the surface in (b) and (c) annealed at 920 K in UHV, recorded with an electron energy of 70 eV. Adapted from Ref. [3]

However, the replacement of the MnO(100) surface by the polar MnO(111) cannot be completely explained by the faceting argument alone. To understand this result we need to take into account the strain energy at the metal-oxide interface, which is determined by the lattice matching conditions. The MnO(100)-Pd(100) interface is characterized by a large lattice mismatch of [pic]14% along the [011] rows [Fig. 2.2.2(a)], whereas for the MnO(111)-Pd(100) interface in one direction the lattice mismatch is only [pic]1%, which results in an almost perfect row matching along the [01[pic]] rows, as illustrated in Fig. 2.2.2(b). This lowers the energy of the MnO(111)-Pd(100) interface and presumably stabilizes the formation of the MnO(111) overlayer. This finding is corroborated by the DFT results of Fig. 2.2.2(c) showing the pressure dependent variation of the energy of formation E[pic] for MnO (100) and (111) thin slabs. Although the (100) termination remains the most stable surface in the whole range of variation of the lattice constant [pic], the curves show that there is a clear tendency of MnO(111) to gain stability with respect to MnO(100) layers upon compression, i.e. when the MnO bulk lattice constant shrinks in order to match that of Pd(100) the energy difference [pic]E[pic] = E[pic](111) - E[pic](100) decreases by a factor of 2.5 with respect to that calculated for [pic]. This result shows that when considering the stabilization mechanism of polar surfaces of oxide films, the metal-oxide interface can play a crucial role. Indeed we have demonstrated above that well-defined and well-ordered MnO(111)-like interfaces are formed at monolayer coverage, which can provide structurally graded interfaces for the growth of thicker MnO(111) polar films. Importantly, the epitaxial stabilization mechanism may also allow the design of oxide surface orientations in thin films that are not stable in the bulk form. These oxide phases can have interesting implications in the application of ultrathin oxide films in diverse areas of the emerging nanotechnologies.

Fig. 14 Real-space model of the (a) MnO(100)-Pd(100) and (b) MnO(111)-Pd(100) interfaces. Note the row-matching condition along the [011] rows at the MnO(111)-Pd(100) interface. DFT+U derived plot of the formation energy of freestanding 3ML MnO(100) and MnO(111) layers as a function of the lattice constant. From Ref. [3]

Acknowledgements

We are very grateful to all coworkers mentioned in the references, in particular to F. Netzer, S. Surnev, R. Podloucky and G. Kresse for their scientific vision, the unfailing inspiration and the enthusiastic support, and to F. Li, G. Parteder and V. Bayer for their invaluable assistance during different stages of this work. Financial support by the Austrian Science Funds FWF, by the 6th Framework Programme of the European Community (GSOMEN and ATHENA), and by the 7th Framework Programme of the European Community (ERC Advanced Grant SEPON) is thankfully acknowledged.

References

1. Anisimov, Vladimir I (Ed.): Strong Coulomb Correlation in Electronic Structure Calculations: Beyond The Local Density Approximation. Gordon and Breach Science Publishers, the Netherlands (2000)

2. Agnoli, S., Sambi, M., Granozzi, G., Schoiswohl, J., Surnev, S., Netzer, F.P., Ferrero, M., Ferrari, A.M., Pisani, C.: Experimental and theoretical study of a surface stabilized monolayer phase of nickel oxide on Pd(100). J. Phys. Chem. B 109, 17197 (2004)

3. Allegretti, F., Franchini, C., Bayer, V., Leitner, M., Parteder, G., Xu, B., Fleming, B., Ramsey, M.G., Podloucky, R., Surnev, S., Netzer, F. P.: Epitaxial stabilization of MnO(111) overlayers on a Pd(100) surface. Phys. Rev. B 75, 224120 (2007)

4. Allegretti, F., Leitner, M., Parteder, G., Xu, B., Fleming, B., Ramsey, M.G., Surnev, S., Netzer, F. P.: The (100) [pic] (111) Transition in Epitaxial Manganese Oxide Nanolayers. In: Cat, D.T., Pucci, A., Wandelt, K. (eds.) Physics and Engineering of New Materials, pp. 163-170. Springer, Heidelberg (2009)

5. Allegretti, F., Parteder, G., Gragnaniello, L., Surnev, S., Netzer, F.P., Barolo, A., Agnoli, S., Granozzi, G., Franchini, C., Podloucky, R.: Strained c(4[pic]2) CoO(100)-like monolayer on Pd(100): experiment and theory. Surf. Sci. 604, 529(2010)

6. Allegretti, F., Parteder, G., Gragnaniello, L., Surnev, S., Netzer, F.P., Franchini, C., Podloucky, R.: Unpublished

7. Arita, R., Tanida, Y., Entani, S., Kiguchi, M., Saiki, K., Aoki, H.: Polar surface engineering in ultrathin MgO(111)/Ag(111): Possibility of a metal-insulator transition and magnetism. Phys. Rev. B 69, 235423 (2004)

8. Armstrong, A.R., Bruce, P.G.: Synthesis of layered LiMnO2 as an electrode for rechargeable lithium batteries. Nat. 381, 499 (1996)

9. Bachmann, K.J.: The Materials Science of Microelectronics. Wiley-VCH , New York (1994)

10. Baldi, M., Finocchio, E., Milella, F., Busca ,G.: Catalytic combustion of C3 hydrocarbons and oxygenates over Mn3O4. Appl. Catal. B 16, 43 (1998)

11. Barbier, A., Mocuta, C., Kuhlenbeck, H., Peters, K.F., Richter, B., Renaud, G.: Atomic structure of the polar NiO(111)- p(2[pic]2) surface. Phys. Rev. Lett. 84, 2897 (2000)

12. Barbier, A., Mocuta, C., Renaud, G.: Structure, transformation, and reduction of the polar NiO(111) surface. Phys. Rev. B 62, 16056 (2000)

13. Barbier, A., Mocuta, C., Neubeck, W., Mulazzi, M., Yakhou, F., Chesne, K., Sollier, A., Vettier, C., de Bergevin, F.: Surface and bulk spin ordering of antiferromagnetic materials: NiO(111). Phys. Rev. Lett. 93, 257208 (2004)

14. Barbier, A., Stierle, A., Finocchi, F., Jupille, J.: Stability and stoichiometry of (polar) oxide surfaces for varying oxygen chemical potential. J. Phys.: Condens. Matter 20, 184014 (2008)

15. Bayer, V., Franchini, C., Podloucky, R.: Ab-initio study of the structural, electronic, and magnetic properties of MnO(100) and MnO(110). Phys. Rev. B 75, 035404 (2007)

16. Bayer, V., Podloucky, R., Franchini, C., Allegretti, F., Xu, B., Parteder, G., Ramsey, M.G., Surnev, S., Netzer, F. P.: Formation of Mn3O4(001) on MnO(001): surface and interface structural stability. Phys. Rev. B 76, 165428 (2007)

17. Buciuman, F., Patcas, F., Craciun, R., Zahn, D.R.T.: Vibrational spectroscopy of bulk and supported manganese oxides. Phys. Chem. Chem. Phys. 1, 185 (1999)

18. Caslavska, V., Roy, R.: Epitaxial Growth of Mn3O4 Single-Crystal Films. J. Appl. Phys. 41, 825 (1970)

19. Chassé, A.,Langheinrich, Ch., Müller, F., Hüfner, S.: Growth and structure of thin MnO films on Ag(001) in dependence on film thickness. Surf. Sci. 602, 597 (2008)

20. Chung, E.M.L., Paul, D. McK, Balakrischnan, G., Lees, M.R., Ivanov, A., Yethiray, M.: Role of electronic correlations on the phonon modes of MnO and NiO. Phys. Rev. B 68, 140406(R) (2003)

21. de Rudder, J., Van de Wiele, T., Dhooge, W., Comhaire, F., Verstraete, W.: Advanced water treatment with manganese oxide for the removal of 17[pic]-ethynylestradiol (EE2). Water Res. 38, 184 (2004)

22. Ertl, G., Knötzinger, H., Schüth, F., Weitkamp, J. (Eds.): Handbook of Heterogeneous Catalysis (2nd Ed.), Vol. 1-8. Wiley-VCH, Weinheim (2008)

23. Franchini, C., Bayer, V., Podloucky, R., Paier,J., Kresse, G.: Density functional theory study of MnO by an hybrid functional approach. Phys. Rev. B 72, 045132 (2005)

24. Franchini, C., Bayer, V., Podloucky, Parteder, G., Surnev, S., Netzer, F. P.: Density functional study of the polar MnO(111) surface. Phys. Rev. B 73, 155402 (2006)

25. Franchini, C., Podloucky, R., Paier,J., Marsman, M., Kresse, G.: Ground-state properties of multivalent manganese oxides: Density functional and hybrid density functional calculations. Phys. Rev. B 75, 195128 (2007)

26. Franchini, C., Podloucky, R., Allegretti, F., Li, F., Parteder, G., Surnev, S., Netzer, F.P.: Structural and vibrational properties of two-dimensional MnxOy layers on Pd(100): Experiments and density functional theory calculations. Phys. Rev. B 79, 035420 (2009)

27. Franchini, C., Zabloudil, J., Podloucky, R., Allegretti, F., Li, F., Surnev, S., Netzer, F.P.: Interplay between magnetic, electronic and vibrational effects in monolayer Mn3O4 grown on Pd(100). J. Chem. Phys. 130, 124707 (2009)

28. Franke, P., Neuschütz, D. (Ed.): SpringerMaterials - The Landolt-Börnstein Database, Vol. IV/19B4 doi,: 10.1007/10757285[pic]37

29. Gilbert, B., Frazer, B. H., Belz, A., Conrad, P. G., Nealson, K. H., Haskel, D., Lang, J. C., Srajer, G., De Stasio, G.: Multiple scattering calculations of bonding and X-ray absorption spectroscopy of manganese oxides. J. Phys. Chem. A 107, 2839 (2003)

30. Giordano, L., Pacchioni, G., Goniakowski , J., Nilius, N., Rienks, E.D.L., Freund, H.-J.: Interplay between structural, magnetic, and electronic properties in a FeO/Pt(111) ultrathin film. Phys. Rev. B 76, 075416 (2007)

31. Goniakowski, J., Noguera, C.: Microscopic mechanisms of stabilization of polar oxide surfaces: Transition metals on the MgO(111) surface. Phys. Rev. B 66, 085417 (2002)

32. Goniakowski, J., Noguera, C., Giordano, L.: Using polarity for engineering oxide nanostructures: Structural phase diagram in free and supported MgO(111) ultrathin films. Phys. Rev. Lett. 93, 215702 (2004)

33. Goniakowski, J., Finocchi, F., Noguera, C.: Polarity of oxide surfaces and nanostructures. Rep. Prog. Phys. 71, 016501 (2008)

34. Graf, M., Gurlo, A., Bârsan, N., Weimar, U., Hierlemann, A.: Microfabricated gas sensor systems with sensitive nanocrystalline metal-oxide films. J. Nanoparticle Res. 8, 823 (2006)

35. Guo, L.W., Ko, H.J., Makino, H., Chen, Y.F., Inaba, K., Yao, T.: Epitaxial growth of Mn3O4 film on MgO(001) substrate by plasma-assisted molecular beam epitaxy (MBE). J. Cryst. Growth 205, 531 (1999)

36. Guo, L.W., Peng, D.L., Makino, H., Inaba, K., Ko, H.J., Sumiyama, K., Yao, T.: Structural and magnetic properties of Mn3O4 films grown on MgO(001) substrates by plasma-assisted MBE. J. Magn. Magn. Mater. 213, 321 (1999)

37. Haywood B.C.G., Collins, M.F.: Optical phonons in MnO. J. Phys. C 4, 1299 (1971)

38. Hagendorf, Ch., Sachert, S., Bochmann, B, Kostov, K., Widdra, W.: Growth, atomic structure, and vibrational properties of MnO ultrathin films on Pt(111). Phys. Rev. B 77, 075406 (2008)

39. Henrich, V.E., Cox, P.A.: The Surface Science of Metal Oxides. Cambridge University Press, Cambridge (1994)

40. Heyd, J., Scuseria, G.E., Ernzerhof, M.: Hybrid functionals based on a screened Coulomb potential. J. Chem. Phys. 118, 8207 (2003)

41. Imada, M., Fujimori, A., Tokura, Y.: Metal-insulator transitions. Rev. Mod. Phys. 70, 1039 (1998)

42. Jones, D.A.: Principles and Prevention of Corrosion (2nd ed.). Prentice Hall, Upper Saddle River, New Jersey (1996)

43. Julien, C.M., Massot, M., Poinsignon, C.: Lattice vibrations of manganese oxides: Part I. Periodic structures. Spectrochim. Acta A 60, 689 (2004)

44. Kiguchi, M., Entani, S., Saiki, K., Goto, T., Koma, A.: Atomic and electronic structure of an unreconstructed polar MgO(111) thin film on Ag(111). Phys. Rev. 68, 115402 (2003)

45. Kohn, W.: Nobel Lecture: Electronic structure of matter-wave functions and density functionals. Rev. Mod. Phys. 71, 1253 (1999)

46. Kresse G., Furthmüller, J.: Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6, 15 (1996)

47. Kresse G., Joubert, D.: From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 59, 1758 (1999)

48. Kurata, H., Colliex, C.: Electron-energy-loss core-edge structures in manganese oxides. Phys. Rev. 48, 2102 (1993)

49. Langell, M.A., Hutchings, C.W., Carson, G.A., Nassir, M.H.: High resolution electron energy loss spectroscopy of MnO(100) and oxidized MnO(100). J. Vac. Sci. Techol. A 14, 1656 (1996)

50. Lee, G.H., Huh, S.H., Jeong, J.W., Choi, B.J., Kim, S.H., Ri, H.-C.: Anomalous magnetic properties of MnO nanoclusters. J. Am. Chem. Soc. 124, 12094 (2002)

51. Li, F., Parteder, G., Allegretti, F., Franchini, C., Podloucky, R., Surnev, S., Netzer, F.P.: Two-dimensional manganese oxide nanolayers on Pd(100): Surface phase diagram. J. Phys.: Condens. Matter 21, 134008 (2009)

52. Meyer, W., Hock, D., Biedermann, K., Gubo, M., Müller, S., Hammer, L., Heinz, K.: Coexistence of rocksalt and wurtzite structure in nanosized CoO films. Phys. Rev. Lett. 101, 016103 (2008)

53. Meyer, W., Biedermann, K., Gubo, M., Hammer, L., Heinz, K.: Superstructure in the termination of CoO(111) surfaces: Low-energy electron diffraction and scanning tunneling microscopy. Phys. Rev. B 79, 121403(R) (2009)

54. Mocuta, C., Barbier, A., Renaud, G., Samson, Y., Noblet, M.: Structural characterization of NiO films on Al2O3(0001). J. Magn. Magn. Mater. 211, 283 (2000)

55. Müller, F., de Masi, R., Reinicke, D., Steiner, P., Hüfner, S., Stöwe, K.: Epitaxial growth of MnO/Ag(001) films. Surf. Sci. 520, 158 (2002)

56. Na, C.W., Han, D.S., Kim, D.S., Park, J., Jeon, Y.T., Lee, G., Jung, M.-H.: Ferromagnetism of MnO and Mn3O4 nanowires. Appl. Phys. Lett. 87, 142504 (2005)

57. Nagel, M., Biswas, I., Peisert, H., Chassé, T.: Interface properties and electronic structure of ultrathin manganese oxide films on Ag(0 0 1). Surf. Sci. 601, 4484 (2007)

58. Nagel, M., Biswas, I., Nagel, P., Pellegrin, E., Schuppler, S., Peisert, H., Chassé, T.: Ultrathin transition-metal oxide films: Thickness dependence of the electronic structure and local geometry in MnO. Phys. Rev. B 75, 195426 (2007)

59. Nayak, S.K., Jena, P.: Giant magnetic moments and magnetic bistability of stoichiometric MnO clusters. Phys. Rev. Lett. 81, 2970 (1998)

60. Netzer, F.P., Allegretti, F., Surnev, S.: Low-dimensional oxide nanostructures on metals: hybrid systems with novel properties. J. Vac. Sci. Technol. B 28, 1 (2010)

61. Nishimura, H., Tashiro, T., Fujitani, T., Nakamura, J.: Surface structure of MnO/Rh(100) studied by scanning tunneling microscopy and low-energy electron diffraction. J. Vac. Sci. Technol. A 18, 1460 (2000)

62. Noguera, C.: Polar oxide surfaces. J. Phys.: Condens. Matter 12, R367 (2000)

63. Ogale, S.B. (ed.): Thin Films and Heterostructures for Oxide Electronics. Springer, Boston (2005)

64. Post, J.E.: Manganese oxide minerals: Crystal structures and economic and environmental significance. Proc. Natl. Acad. Sci. USA 96, 3447 (1999)

65. Ranke, W., Ritter, M., Weiss, W.: Crystal structures and growth mechanism for ultrathin films of ionic compound materials: FeO(111) on Pt(111). Phys. Rev. B 60, 1527 (1999)

66. Rao, C.N.R., Raveau, B. (Eds.): Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides. World Scientific, Singapore (1998)

67. Renaud, G., Barbier, A.: The Chemical Physics of Solid surfaces. 9, 256, Elsevier, New York (2001)

68. Rienks, E.D.L., Nilius, N., Rust, H.-P., Freund, H.-J.: Surface potential of a polar oxide film: FeO on Pt(111). Phys. Rev. B 71, 2414048(R) (2005)

69. Rizzi, G.A., Zanoni, R., Di Siro, S., Perriello, L., Granozzi, G.: Epitaxial growth of MnO nanoparticles on Pt(111) by reactive deposition of Mn2 (CO)1O. Surf. Sci. 462, 187 (2000)

70. Rizzi, G.A., Petukhov, M., Sambi, M., Zanoni, R., Perriello, L., Granozzi, G.: An X-ray photoelectron diffraction structural characterization of an epitaxial MnO ultrathin film on Pt(111). Surf. Sci. 482-485, 1474 (2001)

71. Sahner, K., Tuller, H.L.: Novel deposition techniques for metal oxides: Prospect for gas sensing. J. Electroceram. (2008) doi:10.1007/s10832-008-9554-7

72. Samant P.V., Fernandes, J.B.: Nickel-modified manganese oxide as an active electrocatalyst for oxidation of methanol in fuel cells. J. Power Sources 79, 114 (1999)

73. Si, P.Z., Li, D., Lee, J.W., Choi, C.J., Zhang, Z.D., Geng, D.Y., Brück, E.: Unconventional exchange bias in oxide-coated manganese nanoparticles. Appl. Phys. Lett. 87, 133122 (2005)

74. Soares, E.A., Paniago, R., de Carvalho, V.E., Lopes, E.L., Abreu, G.J.P., Pfannes, H.-D.: Quantitative low-energy electron diffraction analysis of MnO(100) films grown on Ag(100). Phys. Rev. B 73, 035419 (2005)

75. Tourney, J., Dowding, C., Worrall, F., McCann, C., Gray, N., Davenport, R., Johnson, K.: Mn oxide as a contaminated-land remediation product. Mineral. Mag. 72, 513 (2008)

76. von Helmolt, R., Wecker, J., Holzapfel, B., Schultz, L., Samwer, K.: Giant negative magnetoresistance in perovskitelike La2/3Ba1/3MnOx ferromagnetic films. Phys. Rev. Lett. 71, 2331 (1993)

77. Yang, J., Xu, J.J.: Nanoporous amorphous manganese oxide as electrocatalyst for oxygen reduction in alkaline solutions. Electrochem. Commun. 5, 306 (2003)

78. Zhang, W.-B., Tang, B.-W.: Stability of the polar NiO(111) surface. J. Chem. Phys. 128, 124703 (2008)

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download