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Ultrathin Transition Metal Oxide Films
Ultrathin Transition Metal Oxide Films P Luches, Istituto Nanoscienze, Consiglio Nazionale delle Ricerche, Modena, Italy © 2018 Elsevier Inc. All rights reserved.
Introduction Growth Characterization Properties Structure Electronic Properties Magnetic Properties Reactivity Perspectives References Further Reading Relevant Websites
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Glossary Coincidence cell In nonpseudomorphic epitaxial films, two-dimensional repeating unit which originates from the matching of the substrate and overlayer periodicities. Epitaxial growth Growth of a crystalline overlayer on a crystalline substrate with a well-defined reciprocal orientation of the crystal structures. Lattice mismatch Difference between the lattice parameters of two crystalline structures, typically expressed in percentual variation. Polar surface Surface with a finite electric dipole moment in the out-of-plane direction, generated by the charge unbalance in anions and cations. Pseudomorphic film Film which adopts the same structure and surface lattice parameter as the substrate on which it grows. Reducible oxide Oxide in which the cations can reversibly switch their oxidation state between two or more states under mild variations of the external conditions. Ultrathin film Film with a thickness below ten nanometers.
Abbreviations ALD Atomic layer deposition ML Monolayer ¼ single layer of closely packed atoms nm 10 9 m PVD Physical vapor deposition STM Scanning tunneling spectroscopy UHV Ultra-high vacuum XAS x-ray absorption spectroscopy XPS x-ray photoemission spectroscopy
Introduction Metal oxides are materials composed by atoms of at least one metallic element combined with oxygen. They generally have a solid aggregation state and they are very abundant on the earth crust. Transition metals, at variance with noble metals, are elements with a partially filled d or f subshell. The electrons in the open subshells can be easily shared with oxygen atoms forming transition metal oxides. Most transition metals, even in their ultrapure form, generally have a thin layer of oxide on their surface, which forms due to contact with the atmosphere. Indeed the thickness of this layer depends on the specific metal, and on the external conditions like temperature, exposure time, and pressure of the oxidizing gas to which it is exposed. The oxide layer sometimes protects, at least
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partially, the interior of the material from further oxidation or from chemical attacks. Oxides are typically much more inert; they have a better thermal stability and a lower electric conductivity than the corresponding metal. Indeed, transition metal oxide films can also be produced artificially for many different purposes. Oxide films are used for example as protective layers for metallic parts against corrosion, as insulating layers in solid-state electronic devices, as coatings which confer specific optical propertiesdlike for example transmission or reflection of specific wavelengthsdor as layers with taylored magnetic properties in magnetoelectronic devices.1 Transition metal oxides in the form of films are widely studied by material scientists to improve the functionality required for a specific application, as well as to identify or to deliberately induce new properties, which arise with vertical confinement.2 The superposition of films with different composition into multilayers or the combination with metallic nanoparticles or doping metal ions can give rise to a composite material, which exploits the interplay between its constituents to achieve a specific response.3 A further degree of freedom, which can be used to tune the properties of thin films, is given by the possibility to combine more than one metallic species with oxygen in ternary or multiple oxides. Due to the chemical and structural complexity, these materials often have a unique behavior, different from simple binary oxides. An outstanding example is represented by multiferroic materials, which are ternary oxides with a perovskite structure, which can simultaneously have two of the three possible ferroic orders: magnetic, electric, and elastic.4 When the film thickness is smaller than some characteristic length scale of the material, like for example the electron mean free path or the magnetic domain size, the behavior of the material presents unprecedented aspects. Typically, we refer to ultrathin films as layers with a thickness which is comparable with the interatomic distance, that is of a few or a few tens of nanometer. In these two-dimensional oxides the bond between the atoms is influenced by the reduced dimensionality and by the symmetry breaking, which often originate remarkably different atomic arrangement and properties, compared to thicker films. Moreover, since ultrathin films are typically grown in the form of epitaxial layers supported on a substrate, their properties are largely influenced also by the interaction with the support. This interplay, combined with the much higher structural flexibility, may lead to the stabilization of structural phases, considerably different from the bulk. A huge number of research studies document the peculiarities of ultrathin transition metal oxide films, for example in reactivity, magnetic couplings, optical response, mechanical and thermal properties. The progress of the research in the field is very effective also due to the development of very accurate theoretical models of the systems, which largely contribute to the atomic scale understanding of the observed properties and to the prediction of new and still unexplored ones.
Growth Many different methods have been used to grow transition metal oxide films for different purposes. Physical vapor deposition (PVD) techniques are usually required to obtain pure crystalline ultrathin overlayers. The samples are typically prepared and studied in clean conditions, that is in ultra-high vacuum or well-controlled oxidizing environments. The substrate plays a very important role in determining the structure, morphology, and electronic properties of the supported ultrathin oxide film. The growth mode and the resulting film structure and morphology are determined not only by the specific growth technique employed but also by the substrate material, by its surface structure and morphology, and by parameters like substrate temperature during growth, evaporation rate, pressure, and reactivity of the oxidizing gas. Oxide substrates typically offer a better lattice matching compared to metal substrates. However, the adatom mobility, usually larger on metal than on oxide surfaces, leads to the formation of ultrathin metal oxide films with excellent epitaxial quality also on substrates with a rather large (30%–40%) lattice mismatch. Pioneering studies investigated ultrathin oxide films obtained by simple oxidation of metal single crystal surfaces or of thin metal films.5,6 By changing the sample temperature, the partial pressure and/or the time of exposure to oxygen or other oxidizing gases, layers with tunable thickness and stoichiometrydwithin certain limitsdcan be obtained on most transition metal surfaces. Direct oxidation, however, does not typically induce the formation of layers with a well-defined thickness, and it often originates a gradient of oxygen concentration with a maximum at the surface and a decreasing trend toward the metal bulk, which depends on the oxygen diffusion rate within the considered material. The quality of oxide films obtained by thermal oxidation of metal surfaces is often limited to highly defective or polycrystalline films, due to the typically different structure and lattice parameter in metals and in the corresponding oxides. An interesting variant of this approach is represented by the oxidation of a surface alloy, which can be in turn prepared by deposition of a metallic layer on the substrate surface and by subsequent thermal annealing. This approach has been shown to be successful for example in the case titanium oxide,7,8 cerium oxide,9 zirconium oxide,5 and manganese oxide,10 leading to layers with a flat morphology and a good crystalline quality. More controlled overlayers, in terms of thickness, structure, and stoichiometry, can be obtained by depositing an oxide film on a support with a different composition. This can be achieved by PVD methods, which exploit processes like evaporation, sputtering, or ablation of oxide bulk targets. Using these techniques the thickness of the films can be controlled by the deposition time, the temperature of the thermal evaporator, or the sputtering rate. The oxide stoichiometry for transition metal oxides, in which the cation can typically adopt more than one oxidation state, may be tuned, to some extent, by changing the substrate temperature. An even better control of the oxide stoichiometry may be achieved by introducing an oxidizing gas during the growth, in the so-called reactive deposition methods, or by post-growth annealing treatments in oxidizing atmosphere. Magnetron sputtering, thermal evaporation, and pulsed laser ablation can be implemented in a variable pressure of oxygen or of different oxidizing gases, like for example NO2, ozone, or atomic oxygen. Thermal evaporation is widely used to obtain ternary or doped oxides with tunable composition, by using an independent metal evaporator for each constituent. However, complex functional oxides with good
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epitaxial quality are also very often produced by pulsed laser deposition or magnetron sputtering from targets with the desired stoichiometry, which is reproduced in the film.4 A different approach, namely the direct sublimation of monodispersed cyclic (WO3)3 trimers, has been used for the synthesis of tungsten oxide films.11 Transition metal oxides may also be obtained by purely chemical methods using gas- or liquid-phase precursors. chemical vapor deposition techniques typically offer a worse control of the film thickness, morphology, and composition compared to physical methods and they often induce a nonnegligible fraction of impurities and contaminants, which are left by the precursors. Among the chemical methods, atomic layer deposition (ALD) is gaining an increasing interest for the preparation of atomic scale controlled multilayers, containing also oxides.12 In ALD the growth proceeds in a cyclic manner by sequentially exposing the surface to two precursors, which react forming a monolayer with a specific composition. Chemical methods, compared to physical ones, are typically cheaper, more versatile, and often scalable to industrial requirements. Moreover, the possibility to use patterned functionalized substrates for the growth of oxide films from precursors allows to transfer the pattern to the oxide film without the need to use photoresist and chemical etching after the film growth. In some cases it is possible to isolate self-standing two-dimensional transition metal oxide layers by exfoliating them from three-dimensional layered crystals using soft-chemical processes, based on liquid or gas-phase techniques.13 This process leads to the formation of stable oxygen-terminated nanosheets, which allow to investigate the intrinsic properties of the materials without the influence of the support, but it is limited to specific materials, which can be handled without risks of mechanical damage.
Characterization To be suitable for the characterization of an ultrathin film, the available experimental techniques are required to have a certain degree of surface sensitivity. The analysis techniques can be distinguished into three general categories: microscopy techniques, spectroscopy techniques, and diffraction-based techniques. The charging problems, which limit the application of electron-based techniques to bulk oxides, are typically much less severe when the oxide ultrathin films are supported on metal substrates. The morphology, defectivity, and local structure of oxide films are usually investigated by techniques like scanning probe microscopies (e.g., scanning tunneling spectroscopy, atomic force microscopy), scanning or transmission electron microscopies. Spectroscopies, like for example x-ray photoemission spectroscopy, ultra-violet photoemission spectroscopy, x-ray absorption spectroscopy, Raman spectroscopy, are used to analyze the electronic properties of the material, to have information on the cation oxidation state, on the band structure, and on the magnetic and vibrational properties. Spectrophotometries and other spectroscopies can give information on the optical properties. Low energy electron diffraction is a diffraction-based technique, which is very sensitive to the outermost surface layers and suitable to determine the epitaxial orientation of ultrathin films and their surface long range crystalline order. Surface x-ray diffraction is sensitive also the structural details at different depths from the surface by changing the grazing angle, around the critical angle for total reflection. In this way not only the surface structure but also the interface between the film and the support can be investigated. The three categories identified for the characterization techniques overlap in many cases. Spectroscopic and diffraction techniques may be applied, using focused primary or detected beams, to obtain a spatially resolved image of the electronic properties or the structure of the investigated surface. Diffraction techniques at the adsorption edge of one of the elements can give chemically selected information on the specific structure of the individual constituents of a material. Synchrotron radiation facilities offer excellent opportunities for the analysis of low dimensional systems, by providing very intense, collimated photon beams with tunable energy within a very wide range, from hard X-rays to infrared and beyond. For an exhaustive treatment of surface science and synchrotron radiation-based techniques, we refer the reader to excellent textbooks in the respective fields.14,15
Properties Structure The structure of ultrathin transition metal oxide films is determined not only by the interaction with the substrate but also by the effects which intrinsically derive from reduced dimensionality. Oxide films with the most stable surface orientation and an almost bulk-like structure can be stabilized on surfaces with the same crystal symmetry and a low lattice mismatch compared to the chosen oxide. In pseudomorphic films some strain can be induced when the thickness is below a certain threshold, typically of the order of one nanometer. Above the threshold thickness the films release their structural strain through the introduction of defects like dislocations. In general, the structural flexibility of a film is higher than the one of the corresponding volume phase and larger strains can be sustained without the appearance of a significant density of defects. In some cases the dislocations introduced to relieve the strain above the critical thickness are arranged into an ordered network and they can be exploited to induce for example the self-assembly of regular arrays of supported nanoparticles.16 Novel structural phases can be obtained when considering less stable or polar surface orientations and substrates with a different symmetry compared to the bulk oxides. In transition metal oxides different structural motifs can be stabilized also as a function of the oxygen chemical potential, due to the stability of different oxidation states for the cations depending on the conditions. For example manganese oxide two-dimensional films supported on a Pd(100) substrate can show nine different MnOx phases at different values of the oxygen pressure and of the temperature during the growth, as schematized in Fig. 1.17
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Fig. 1 Schematic phase diagram of the two-dimensional Mn oxides, presented as a function of the oxygen pressure p(O2) and of the oxygen chemical potential mO. The nominal coverage of Mn on Pd(100) is 0.75 ML. Reproduced with permission from Li, F.; Parteder, G.; Allegretti, F.; Franchini, C.; Podloucky, R.; Surnev, S.; Netzer, F. P. Two-Dimensional Manganese Oxide Nanolayers on Pd(100): The Surface Phase Diagram. J. Phys. Condens. Matter 2009, 21 (13), 134008. All rights reserved. © IOP Publishing.
Also in the case of vanadium oxide ultrathin films different surface reconstructions are observed under variable oxidation conditions and at different thicknesses on different substrates.18,19 Some of the different structural phases observed have been proved to be stabilized by the interaction with the metal substrate. In cerium oxide the most stable oxidation state for Ce ions is 4þ at ambient conditions, but mild variations of the external conditions can stabilize also the 3þ state. By reduction of stoichiometric cerium oxide thin and ultrathin films with the bulk-like fluorite structure, a plethora of different reduced phases can be obtained.20–23 Theoretical studies largely contributed to the understanding of the mechanisms for the stabilization of the observed metastable structures.24,25 Fig. 2 shows an example of a surface reconstruction obtained by thermal reduction of films supported on Pt(111), highlighting the correlation between the new periodicities introduced and some specific sites in the coincidence cell. This evidence suggests that the oxygen vacancy formation energy may be modulated by the different bondings between substrate and overlayer in different areas of the coincidence cell, as reported also using other metal substrates.26 It is important to note that the original structure and stoichiometry of the film can be brought back to the original one by thermal treatments in oxidizing atmosphere.23,27 In titanium oxide films, the plethora of structural phases stabilized in the form of ultrathin films7,28,29 is even wider, since also in the stoichiometric phase different structural ploymorphs have a comparable stability. Moreover, unlike Ce ions, Ti ions can have the 2þ, as well as the 3þ and 4þ oxidation state, and consequently they can adopt many more coordinations and local geometries at reduced dimensionality. Polar surface orientations, unstable as bulk terminations, can be stabilized when the considered oxide is in the form of an ultrathin film. This gives the opportunity to address the behavior of these unique systems by surface science techniques and to eventually develop strategies to maximize such orientations also in real systems.30 In some cases a partial compensation of the polarity induces the stabilization of structures different from the bulk phases from which they are derived. This happens for example
Fig. 2 Left: Low energy electron diffraction pattern (Ep ¼ 80 eV) of a 2 ML cerium oxide film on Pt(111) after reduction induced by heating in ultra-high vacuum (UHV) at 1040 K for 15 min (cCe3 þ 60%, as measured by x-ray photoemission spectroscopy (XPS)), showing the (3 3) and the 9/4(O3 O3)R30 phase. Right: Model of the 3:4 coincidence cell at the interface between CeO2 and Pt(111), evidencing the vectors of the (3 3) supercell (solid line) and of the 9/4(O3 O3)R30 supercell (dashed lines), which define specific sites in the coincidence cell. Modified from Luches, P.; Pagliuca, F.; Valeri, S. Structural and Morphological Modifications of Thermally Reduced Cerium Oxide Ultrathin Epitaxial Films on Pt(111). Phys. Chem. Chem. Phys. 2014, 16 (35), 18848–18857, with permission from the PCCP Owner Societies.
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for monolayer thick FeO layers on Pt(111), which show a lateral lattice expansion, an oxygen surface termination, and a decreased vertical lattice parameter compared to bulk iron monoxide in the (111) orientation.31,32 In FeO films of this kind a locally different progressive oxidation to FeO2 at different sites of the coincidence supercell was observed. This effect allowed to prepare a patterned oxide film, with different iron oxidation states in different regions of the film.33 Another interesting example is the case of ZnO films with polar orientation, which can adopt a flat graphitic-like phase, rather than the bulk wurtzite-type phase, at specific values of the oxygen chemical potential and at the single layer regime.34 Also in cerium oxide (100) polar terminations have been stabilized at specific conditions and a (2 2) surface reconstruction with oxygen vacancies has been identified as the stabilizing mechanism.35,36 Among the structural properties specific of ultrathin oxide films, which induce a specific behavior, we underline their high structural flexibility. This property allows to balance external perturbations, like for example the presence of external charges or a mechanical stress, by introducing local atomic displacements much larger than the bulk phases. The so-called polaronic distortion is a local structural modification of the crystal structure associated with an excess charge. These deformations play an important role in charge transport mechanisms in oxide films, and they have been shown to be at the basis of the high-temperature superconducting behavior in copper oxide-based films.37
Electronic Properties In ultrathin films the reduced dimensionality in the out-of-plane direction, the interaction with the substrate, the structural strain or defectivity, and the different structural motifs stabilized are linked to a substantially different electronic structure compared to single crystalline oxide bulk phases. Transition metal oxides have typically intermediate electronic band gaps of 3–4 eV or below, although some of them are metallic (e.g., RuO2, ReO3, and some ternary perovskites) and some others are highly insulating (e.g., HfO2, BaTiO3, and other perovskites). The experimental measurement of the (optical or electronic) band gap of an ultrathin film is not always straightforward, due to the influence of the substrate electronic states. Complications arise also in the theoretical prediction of the band gap, since several methods underestimate electronic correlation effects and do not properly describe the filled and unfilled bands. Typically, in ultrathin oxide films the vertical confinement leads to a narrowing of the electronic bands and to an increase of the respective density of states. This often induces a decrease of the band gap with respect to the bulk value and the film becomes mildly conductive. In a NiO film on an Ag(001) substrate theoretical calculations have shown that the interfacial layer is conductive and that films with thickness up to 1 nm still do not reach the bulk electronic properties,38 in agreement with experimental results.39 The changes in the band structure are reflected in modifications of the work function when ultrathin films are considered. The work function, defined as the minimum energy required to remove an electron from the system, determines important properties like conductivity, reactivity, etc. The work function of a metal surface is modified by the absorption of an ultrathin oxide film. Its value is determined by different contributions. The first is the specific absorption geometry of the oxide on the metal. Moreover, the reciprocal electronic interactions also play a role. Some electrostatic charge compressions can be induced by the repulsion of the metal electrons after oxide absorption. In addition, some charge can be transferred between the metal and the oxide. In general the system work function is decreased if some charge is transferred from the oxide to the metal and vice versa. The surface relaxation and the geometric rumpling in the epitaxial films can also significantly affect the work function.40 Fig. 3 reports an example of the predicted variations of the work function for single layer thick MgO and transition metal oxide films along the first row, epitaxially supported on Ag(001)41. For MgO, CoO, and ZnO the work function of the system is smaller or comparable to the metal surface, while for NiO and CuO it is larger. The absorption geometry is very similar with negatively charged oxygen anions located on top of surface metal atoms, giving a comparable charge compression and a slight and progressive decrease of the work function from right to left due to the increased ionicity of the metal-oxygen bonds. On the contrary, the charge transfer changes significantly along the series, and it largely determines the work function changes. At variance with MgO, in transition metal
Fig. 3 Values of the work function for ultrathin transition metal oxide films adsorbed on Ag(100) evaluated by density functional theory calculations using different functionals. The value of the work function of Ag(100) is shown as a dotted line. Reproduced from Sementa, L.; Barcaro, G.; Negreiros, F. R.; Thomas, I. O.; Netzer, F. P.; Ferrari, A. M.; Fortunelli, A. Work Function of Oxide Ultrathin Films on the Ag(100) Surface. J. Chem. Theory Comput. 2012, 8 (2), 629–638, with permission.
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cations, the presence of empty d states in the conduction band favors a charge transfer from the metal to the oxide, which partially compensates the decrease of the work function due to charge compression. The balance between the two effects and the structural rumpling generates a maximum in the work function for Ni and Cu. In transition metal oxide films with polar orientation, dipole moment compensation can induce important modifications of the surface electronic structure, like for example the occupation of oxide surface states by charges coming from the metal substrate, as observed on cobalt and iron oxides.31,42 This mechanism generally leads to a decrease of the ionicity of the metal-oxygen bonds, and in some cases even to a surface metallization.
Magnetic Properties Transition metal oxides exhibit a rich variety of magnetic properties, which depend on the electronic configuration. At ambient conditions they can be paramagnetic (e.g., vanadium oxides, or reduced titanium or cerium oxide), antiferromagnetic (e.g., NiO, CoO, FeO, a-Fe2O3, and Co3O4), ferrimagnetic (e.g., Mn3O4 and g-Fe2O3), and diamagnetic (e.g., Y2O3, TiO2, CrO3, and AgO). The modifications of the band structure with out-of-plane confinement, discussed above, have indeed nonnegligible consequences also on the magnetic properties of transition metal oxide films. The band narrowing and the increase of the density of filled states, induced by vertical confinement, may cause an increase in the spin magnetic moment. This increase, combined with the symmetry breaking, may increase also the orbital moment. Additional magnetic anisotropies are typically introduced, and they compete with the magnetocrystalline anisotropy in determining magnetization reversal in two-dimensional systems. Among them we mention the shape anisotropy, induced by symmetry breaking, and the interface anisotropy. In addition, a growth-induced anisotropy, perpendicular to the growth direction, can be introduced. In general, ferromagnetic oxide ultrathin films have higher coercive films than thicker films and different reversal mechanisms due to the stabilization of domains with specific orientation and shape and because of the possibility of having some pinning of the magnetization at defect sites. Also the Curie temperature is typically dependent on the film thickness. In antiferromagnetic oxide films, such as for example NiO films, the most stable antiferromagnetic domains as well as the Néel temperature depend not only on the film thickness (see Fig. 4) but also on the epitaxial strain.43–45 The competition between the two effects allows to obtain ultrathin films with relatively high Néel temperatures43 or with selected orientation of the antiferromagnetic anisotropy.44,45 Very interesting and unprecedented magnetic properties arise when ultrathin ferromagnetic layers are coupled to antiferromagnetic layers, often in the form of oxide films. The magnetic anisotropy of the antiferromagnetic layer exerts a torque on the spins of the ferromagnetic layer. This interaction affects the magnetization reversal, inducing an increase in the coercive field, and a shift of the hysteresis loop with respect to the zero field by an amount called the exchange bias.46,47 The exchange bias and the coercivity increase depend not only on the specific materials but also on the thickness of the layers involved. It has to be pointed out that the interface between the two layers is often a unique two-dimensional phase with different magnetic properties, which have to be taken into account when describing the magnetic couplings in the system.48–50
Fig. 4 (A) Temperature and thickness dependence of the ratio of the two peaks in the Ni L2-XAS (x-ray absorption spectroscopy) of NiO/MgO(001) thin films at normal incidence. Néel temperatures of TN ¼ 295, 430, and 470 K are determined for the 5, 10, and 20 monolayer films, respectively. Reprinted from Alders, D.; Tjeng, L. H.; Voogt, F. C.; Hibma, T.; Sawatzky, G. A.; Chen, C. T.; Vogel, J.; Sacchi, M.; Iacobucci, S. Temperature and Thickness Dependence of Magnetic Moments in NiO Epitaxial Films. Phys. Rev. B 1998, 57 (18), 11623–11631, with permission. (B) Ratio of the two peaks in the Ni L2-XAS of NiO/Ag(001) thin films of different thickness at normal and grazing incidence angles. The inset shows the magnetic linear dichroism (MLD) obtained from the data. Reprinted from Krishnakumar, S. R.; Liberati, M.; Grazioli, C.; Veronese, M.; Turchini, S.; Luches, P.; Valeri, S.; Carbone, C. Magnetic Linear Dichroism Studies of In Situ Grown NiO Thin Films. J. Magn. Magn. Mater. 2007, 310 (1), 8–12, with permission.
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Multilayers, including oxide phases, with different magnetic and electric properties can also be used to achieve specific transport properties at different values of the external magnetic or electric field. Spin valves are devices used as magnetoresistive read/write heads for magnetic hard disks, exploiting the strong dependence of the resistance on the relative orientation of the magnetization in two ferromagnetic layers. Indeed the research on the properties of ultrathin oxide films has contributed largely to the optimization of the performances of devices of this kind. When extending the field to ternary oxides, the range of magnetic magnetoelectric, magnetoelastic, and magneto-transport properties and the possibilities for their modifications with dimensionality become incredibly wider and richer.4
Reactivity Ultrathin transition metal oxide films represent model catalysts, which can be used to determine the role of a specific surface orientation or site on the activity and selectivity in a chemical reaction. Moreover, the contribution of well-defined sites at the interface between an oxide and a metal can be understood by investigating epitaxial oxide nanostructures on single crystalline metal substrates or by using oxide films as supports for metal nanoparticles with specific size, shape, and composition. The information obtained allows in principle to design real catalysts with an optimized concentration of active or selective sites, in which the onset temperature and/or the rate of a chosen reaction can be modified to meet the specific requirement of the application. On the other hand, an accurate description of the properties of metal supported ultrathin transition metal oxide films can be very relevant to understand the performance of real catalysts made of metal nanoparticles on oxide supports. The interaction between the two materials may in fact induce a partial encapsulation of the metal nanoparticles by the oxide, forming an ultrathin oxide film on the metal surface. A representative example of the differences in reactivity between the bulk oxide phase and two-dimensional phases is given by platinum supported iron oxide films. The bilayer films were found to be inert, while trilayer films showed a substantial reactivity toward CO oxidation.51 The interface between iron oxide and platinum is claimed to be the active site for CO oxidation, as shown in Fig. 5,52 while the catalytic cycle is sustained by oxygen diffusion from the interior of an oxide island to the active edge sites.51 The low coordinated ferrous cations at the interface between the oxide and the metal support were identified also as active sites for O2 activation, of great importance for fuel cell operation.52
Fig. 5 (A) Scanning tunneling spectroscopy (STM) image (200 nm 200 nm) of a 0.25 ML FeO1 x/Pt(111) surface. (Inset) An atomic resolution STM image of FeO monolayer nanoislands (25 nm 20.8 nm). (B) Ratios of XPS O 1s to Fe 2p3/2 peak intensity from FeO1 x/Pt(111) surfaces with different periphery density of FeO nanoisland. Samples 1–3 are 0.25 ML FeO nanoislands prepared at 1.3 10 6 mbar O2 and annealed in UHV at 473, 573, and 673 K, respectively. Sample 4 is the full-monolayer FeO film on Pt(111). The STM images are all 100 nm by 100 nm. (C) Dependence of reactivity of CO oxidation on the periphery density at 0.25 ML FeO1 x/Pt(111) surfaces. Reprinted from Fu, Q.; Li, W.-X.; Yao, Y.; Liu, H.; Su, H.-Y.; Ma, D.; Gu, X.-K.; Chen, L.; Wang, Z.; Zhang, H.; Wang, B.; Bao, X. Interface-Confined Ferrous Centers for Catalytic Oxidation. Science 2010, 328 (5982), 1141, with permission.
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Epitaxial titanium, vanadium and tungsten oxide films have been been studied as model catalysts for the selective reduction of NO by NH3.53 Pd oxide epitaxial layers provided an important playground for the understanding CH4 catalytic combustion.53 Titania and ceria ultrathin epitaxial films provided valuable information on the role of metal-oxide sites and oxygen vacancy formation on the water gas shift reaction.54 Zinc oxide films on copper are studied in view optimizing the synthesis of methanol via the oxidation of CO.55 The number of further outstanding examples is indeed very large, and several dedicated reviews on the topic include an exhaustive treatment of the subject,56,57 pointing at the importance of having well-controlled systems and a proper theoretical modeling for a true atomic scale understanding of the functionality of metal/oxide systems.
Perspectives The recent research has largely demonstrated the potential of ultrathin transition metal oxide films as a new class of materials. Novel compositions and different architectures are being identified with the aim of obtaining compounds with new and multiple functionalities, designed on demand. The progress in this field requires parallel advances in the growth and synthesis methods, in the characterization tools, and in the theoretical modeling. Among the outstanding examples of promising research ways for the future we mention the possibilities introduced by the stabilization of aperiodic two-dimensional structures in transition metal-based ternary oxides.58 The possibility of obtaining new topological phases by changes in composition and in symmetry also offers a rich playground for research.59 A further still largely unexplored aspect is the time-scale description of the interactions and excitations occurring in these systems, encouraged by the development and the availability of ultrafast spectroscopies also available as shared facilities. As a final remark, we mention that oxides are promising materials also in view of the minimization and substitution of critical noble metals in catalysis and energy applications. Research on well-controlled systems is a mandatory step for a rational and sustainable design of the appropriate compounds.
See also: Characterization of ultrathin oxide films by LEEM/PEEM; Reactive Molecular Beam Epitaxy of Iron Oxide Films: Strain, Order, and Interface Properties; Ultrathin Oxide Films on Ferromagnetic Metallic Substrates.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
Pacchioni, G.; Valeri, S. Oxide Ultrathin Films, Wiley-VHC: Weinheim, 2012. Freund, H.-J.; Pacchioni, G. Oxide Ultra-Thin Films on Metals: New Materials for the Design of Supported Metal Catalysts. Chem. Soc. Rev. 2008, 37 (10), 2224–2242. Finazzi, M.; Duò, L.; Ciccacci, F. Magnetic Properties of Interfaces and Multilayers Based on Thin Antiferromagnetic Oxide Films. Surf. Sci. Rep. 2009, 64 (4), 139–167. Prellier, W.; Singh, M. P.; Murugavel, P. The Single-Phase Multiferroic Oxides: From Bulk to Thin Film. J. Phys. Condens. Matter 2005, 17 (30), R803. Bäumer, M.; Cappus, D.; Kuhlenbeck, H.; Freund, H. J.; Wilhelmi, G.; Brodde, A.; Neddermeye, H. The Structure of Thin NiO(100) Films Grown on Ni(100) as Determined by Low-Energy-Electron Diffraction and Scanning Tunneling Microscopy. Surf. Sci. 1991, 253 (1–3), 116–128. Klingenberg, B.; Grellner, F.; Borgmann, D.; Wedler, G. Low-Energy Electron Diffraction and X-ray Photoelectron Spectroscopy on the Oxidation of Cobalt (1120). Surf. Sci. 1997, 383 (1), 13–24. Matsumoto, T.; Batzill, M.; Hsieh, S.; Koel, B. E. Fundamental Studies of Titanium Oxide-Pt(100) Interfaces – I. Stable High Temperature Structures Formed by Annealing TiOx Films on Pt(100). Surf. Sci. 2004, 572 (2–3), 127–145. Antlanger, M.; Mayr-Schmölzer, W.; Pavelec, J.; Mittendorfer, F.; Redinger, J.; Varga, P.; Diebold, U.; Schmid, M. Pt3Zr(0001): A Substrate for Growing Well-Ordered Ultrathin Zirconia Films by Oxidation. Phys. Rev. B 2012, 86 (3), 035451. Berner, U.; Schierbaum, K. D. Cerium Oxides and Cerium-Platinum Surface Alloys on Pt(111) Single-Crystal Surfaces Studied by Scanning Tunneling Microscopy. Phys. Rev. B 2002, 65 (23), 235404. Knight, P. J.; Toomes, R.; Driver, S. M.; Woodruff, D. P. Oxidation of the Cu(100)c(22)-Mn Surface: Epitaxial Manganese Oxide. Surf. Sci. 1998, 418 (3), 521–528. Li, Z.; Zhang, Z.; Kim, Y. K.; Smith, R. S.; Netzer, F.; Kay, B. D.; Rousseau, R.; Dohnálek, Z. Growth of Ordered Ultrathin Tungsten Oxide Films on Pt(111). J. Phys. Chem. C 2011, 115 (13), 5773–5783. Miikkulainen, V.; Leskelä, M.; Ritala, M.; Puurunen, R. L. Crystallinity of Inorganic Films Grown by Atomic Layer Deposition: Overview and General Trends. J. Appl. Phys. 2013, 113 (2), 021301. Osada, M.; Sasaki, T. Exfoliated Oxide Nanosheets: New Solution to Nanoelectronics. J. Mater. Chem. 2009, 19 (17), 2503–2511. Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science, Cambridge University Press: Cambridge, 1994. Mobilio, S.; Boscherini, F.; Meneghini, C. Synchrotron Radiation Basics, Methods and Applications, Springer: Berlin, 2015. Torelli, P.; Soares, E. A.; Renaud, G.; Gragnaniello, L.; Valeri, S.; Guo, X. X.; Luches, P. Self-Organized Growth of Ni Nanoparticles on a Cobalt-Oxide Thin Film Induced by a Buried Misfit Dislocation Network. Phys. Rev. B 2008, 77 (8), 081409. Li, F.; Parteder, G.; Allegretti, F.; Franchini, C.; Podloucky, R.; Surnev, S.; Netzer, F. P. Two-Dimensional Manganese Oxide Nanolayers on Pd(100): The Surface Phase Diagram. J. Phys. Condens. Matter 2009, 21 (13), 134008. Guimond, S.; Sturm, J. M.; Gobke, D.; Romanyshyn, Y.; Naschitzki, M.; Kuhlenbeck, H.; Freund, H. J. Well-Ordered V2O5(001) Thin Films on Au(111): Growth and Thermal Stability. J. Phys. Chem. C 2008, 112 (31), 11835–11846. Schoiswohl, J.; Surnev, S.; Netzer, F. P.; Kresse, G. Vanadium Oxide Nanostructures: From Zero- to Three-Dimensional. J. Phys. Condens. Matter 2006, 18 (4), R1–R14. Eck, S.; Castellarin-Cudia, C.; Surnev, S.; Ramsey, M. G.; Netzer, F. P. Growth and Thermal Properties of Ultrathin Cerium Oxide Layers on Rh(111). Surf. Sci. 2002, 520 (3), 173–185. Duchon, T.; Dvorak, F.; Aulicka, M.; Stetsovych, V.; Vorokhta, M.; Mazur, D.; Veltruska, K.; Skala, T.; Myslivecek, J.; Matolinova, I.; Matolin, V. Ordered Phases of Reduced Ceria as Epitaxial Films on Cu(111). J. Phys. Chem. C 2014, 118 (1), 357–365. Hocker, J.; Krisponeit, J.-O.; Schmidt, T.; Falta, J.; Flege, J. I. The Cubic-to-Hexagonal Phase Transition of Cerium Oxide Particles: Dynamics and Structure. Nanoscale 2017, 9, 9352–9358. https://doi.org/10.1039/C6NR09760J. Luches, P.; Pagliuca, F.; Valeri, S. Structural and Morphological Modifications of Thermally Reduced Cerium Oxide Ultrathin Epitaxial Films on Pt(111). Phys. Chem. Chem. Phys. 2014, 16 (35), 18848–18857. Kozlov, S. M.; Demiroglu, I.; Neyman, K. M.; Bromley, S. T. Reduced Ceria Nanofilms From Structure Prediction. Nanoscale 2015, 7 (10), 4361–4366.
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Olbrich, R.; Murgida, G. E.; Ferrari, V.; Barth, C.; Llois, A. M.; Reichling, M.; Ganduglia-Pirovano, M. V. Surface Stabilizes Ceria in Unexpected Stoichiometry. J. Phys. Chem. C 2017, 121 (12), 6844–6851. Castellarin-Cudia, C.; Surnev, S.; Schneider, G.; Podlucky, R.; Ramsey, M. G.; Netzer, F. P. Strain-Induced Formation of Arrays of Catalytically Active Sites at the Metal-Oxide Interface. Surf. Sci. 2004, 554 (2–3), L120–L126. Gasperi, G.; Amidani, L.; Benedetti, F.; Boscherini, F.; Glatzel, P.; Valeri, S.; Luches, P. Electronic Properties of Epitaxial Cerium Oxide Films During Controlled Reduction and Oxidation Studied by Resonant Inelastic X-ray Scattering. Phys. Chem. Chem. Phys. 2016, 18, 20511. Matsumoto, T.; Batzill, M.; Hsieh, S.; Koel, B. E. Fundamental Studies of Titanium Oxide-Pt(100) Interfaces – II. Influence of Oxidation and Reduction Reactions on the Surface Structure of TiOx Films on Pt(100). Surf. Sci. 2004, 572 (2–3), 146–161. Sedona, F.; Rizzi, G. A.; Agnoli, S.; Xamena, F. X. L. I.; Papageorgiou, A.; Ostermann, D.; Sambi, M.; Finetti, P.; Schierbaum, K.; Granozzi, G. Ultrathin TiOx Films on Pt(111): A LEED, XPS, and STM Investigation. J. Phys. Chem. B 2005, 109 (51), 24411–24426. Noguera, C.; Goniakowski, J. Polarity in Oxide Nano-Objects. Chem. Rev. 2013, 113 (6), 4073–4105. Ritter, M.; Ranke, W.; Weiss, W. Growth and Structure of Ultrathin FeO Films on Pt(111) Studied by STM and LEED. Phys. Rev. B 1998, 57 (12), 7240–7251. Kim, Y. J.; Westphal, C.; Ynzunza, R. X.; Galloway, H. C.; Salmeron, M.; Van Hove, M. A.; Fadley, C. S. Interlayer Interactions in Epitaxial Oxide Growth: FeO on Pt(111). Phys. Rev. B 1997, 55 (20), R13448–R13451. Giordano, L.; Lewandowski, M.; Groot, I. M. N.; Sun, Y. N.; Goniakowski, J.; Noguera, C.; Shaikhutdinov, S.; Pacchioni, G.; Freund, H. J. Oxygen-Induced Transformations of an FeO(111) Film on Pt(111): A Combined DFT and STM Study. J. Phys. Chem. C 2010, 114 (49), 21504–21509. Weirum, G.; Barcaro, G.; Fortunelli, A.; Weber, F.; Schennach, R.; Surnev, S.; Netzer, F. P. Growth and Surface Structure of Zinc Oxide Layers on a Pd(111) Surface. J. Phys. Chem. C 2010, 114 (36), 15432–15439. Pan, Y.; Nilius, N.; Stiehler, C.; Freund, H. J.; Goniakowski, J.; Noguera, C. Ceria Nanocrystals Exposing Wide (100) Facets: Structure and Polarity Compensation. Adv. Mater. Interfaces 2014, 1 (9), 1400404. Höcker, J.; Duchon, T.; Veltruska, K.; Matolín, V.; Falta, J.; Senanayake, S. D.; Flege, J. I. Controlling Heteroepitaxy by Oxygen Chemical Potential: Exclusive Growth of (100) Oriented Ceria Nanostructures on Cu(111). J. Phys. Chem. C 2010, 120 (9), 4895–4901. Zhao, G.-m.; Hunt, M. B.; Keller, H.; Muller, K. A. Evidence for Polaronic Supercarriers in the Copper Oxide Superconductors La2 xSrxCuO4. Nature 1997, 385 (6613), 236–239. Thomas, I. O.; Fortunelli, A. Analysis of the Electronic Structure of Ultrathin NiO/Ag(100) Films. Eur. Phys. J. B. 2010, 75 (1), 5–13. Steurer, W.; Surnev, S.; Fortunelli, A.; Netzer, F. P. Scanning Tunneling Microscopy Imaging of NiO(100)(1 1) Islands Embedded in Ag(100). Surf. Sci. 2012, 606 (9–10), 803–807. Prada, S.; Martinez, U.; Pacchioni, G. Work Function Changes Induced by Deposition of Ultrathin Dielectric Films on Metals: A Theoretical Analysis. Phys. Rev. B 2008, 78 (23), 235423. Sementa, L.; Barcaro, G.; Negreiros, F. R.; Thomas, I. O.; Netzer, F. P.; Ferrari, A. M.; Fortunelli, A. Work Function of Oxide Ultrathin Films on the Ag(100) Surface. J. Chem. Theory Comput 2012, 8 (2), 629–638. Meyer, W.; Biedermann, K.; Gubo, M.; Hammer, L.; Heinz, K. Surface Structure of Polar Co3O4 (111) Films Grown Epitaxially on Ir(100)-(1 1). J. Phys. Condens. Matter 2008, 20 (26), 265011. Altieri, S.; Finazzi, M.; Hsieh, H. H.; Haverkort, M. W.; Lin, H. J.; Chen, C. T.; Frabboni, S.; Gazzadi, G. C.; Rota, A.; Valeri, S.; Tjeng, L. H. Image Charge Screening: A New Approach to Enhance Magnetic Ordering Temperatures in Ultrathin Correlated Oxide Films. Phys. Rev. B 2009, 79 (17), 174431. Alders, D.; Tjeng, L. H.; Voogt, F. C.; Hibma, T.; Sawatzky, G. A.; Chen, C. T.; Vogel, J.; Sacchi, M.; Iacobucci, S. Temperature and Thickness Dependence of Magnetic Moments in NiO Epitaxial Films. Phys. Rev. B 1998, 57 (18), 11623–11631. Krishnakumar, S. R.; Liberati, M.; Grazioli, C.; Veronese, M.; Turchini, S.; Luches, P.; Valeri, S.; Carbone, C. Magnetic Linear Dichroism Studies of In Situ Grown NiO Thin Films. J. Magn. Magn. Mater. 2007, 310 (1), 8–12. Nogués, J.; Schuller, I. K. Exchange Bias. J. Magn. Magn. Mater. 1999, 192 (2), 203–232. Berkowitz, A. E.; Takano, K. Exchange Anisotropy – A Review. J. Magn. Magn. Mater. 1999, 200 (1–3), 552–570. Luches, P.; Benedetti, S.; di Bona, A.; Valeri, S. Magnetic Couplings and Exchange Bias in Fe/NiO Epitaxial Layers. Phys. Rev. B 2010, 81 (5), 054431. Luches, P.; Pasquini, L.; Benedetti, S.; Bellini, V.; Valeri, S.; Manghi, F.; Rüffer, R.; Boscherini, F. Interfacial Magnetic Structure in Fe/NiO(001). Phys. Rev. B 2011, 83 (9), 094413. Luches, P.; Pasquini, L.; Benedetti, S.; Valeri, S.; Rüffer, R.; Boscherini, F. Depth-Dependent Magnetization Reversal and Spin Structure of Fe/NiO Exchange-Coupled Epitaxial Bilayers. Appl. Phys. Lett. 2012, 101 (8), 082412. Pan, Q.; Weng, X.; Chen, M.; Giordano, L.; Pacchioni, G.; Noguera, C.; Goniakowski, J.; Shaikhutdinov, S.; Freund, H.-J. Enhanced CO Oxidation on the Oxide/Metal Interface: From Ultra-High Vacuum to Near-Atmospheric Pressures. ChemCatChem 2015, 7 (17), 2620–2627. Fu, Q.; Li, W.-X.; Yao, Y.; Liu, H.; Su, H.-Y.; Ma, D.; Gu, X.-K.; Chen, L.; Wang, Z.; Zhang, H.; Wang, B.; Bao, X. Interface-Confined Ferrous Centers for Catalytic Oxidation. Science 2010, 328 (5982), 1141. Gao, W.; Li, M.; Klie, R.; Altman, E. I. Growth and Characterization of Model Oxide Catalysts. J. Electron Spectrosc. Relat. Phenom. 2006, 150 (2–3), 136–149. Rodriguez, J. A.; Ma, S.; Liu, P.; Hrbek, J.; Evans, J.; Perez, M. Activity of CeOx and TiOx Nanoparticles Grown on Au(111) in the Water-Gas Shift Reaction. Science 2007, 318 (5857), 1757–1760. Schott, V.; Oberhofer, H.; Birkner, A.; Xu, M.; Wang, Y.; Muhler, M.; Reuter, K.; Wöll, C. Chemical Activity of Thin Oxide Layers: Strong Interactions With the Support Yield a New Thin-Film Phase of ZnO. Angew. Chem. Int. Ed. 2013, 52 (45), 11925–11929. Gao, F.; Goodman, D. W. Model Catalysts: Simulating the Complexities of Heterogeneous Catalysts. Annu. Rev. Phys. Chem. 2012, 63, 265–286. Shaikhutdinov, S.; Freund, H.-J. Ultrathin Oxide Films on Metal Supports: Structure-Reactivity Relations. Annu. Rev. Phys. Chem. 2012, 63, 619–633. Forster, S.; Meinel, K.; Hammer, R.; Trautmann, M.; Widdra, W. Quasicrystalline Structure Formation in a Classical Crystalline Thin-Film System. Nature 2013, 502 (7470), 215–218. Moon, S. J.; Jin, H.; Kim, K. W.; Choi, W. S.; Lee, Y. S.; Yu, J.; Cao, G.; Sumi, A.; Funakubo, H.; Bernhard, C.; Noh, T. W. Dimensionality-Controlled Insulator-Metal Transition and Correlated Metallic State in 5d Transition Metal Oxides Srnþ1IrnO3nþ1 (n ¼ 1, 2, and Infinity). Phys. Rev. Lett. 2008, 101 (22), 226402.
Further Reading Chambers, S. A. Epitaxial Growth and Properties of Thin Film Oxides. Surf. Sci. Rep. 2000, 39, 105–180. Duò, L.; Ciccacci, F. Magnetic Properties of Antiferromagnetic Oxides, Wiley-VHC: Weinheim, 2010. Freund, H.-J.; Pacchioni, G. Oxide Ultra-Thin Films on Metals: New Materials for the Design of Supported Metal Catalysts. Chem. Soc. Rev. 2008, 37, 2224–2242. Giordano, L.; Pacchioni, G. Oxide Films at the Nanoscale: New Structures, New Functions, and New Materials. Acc. Chem. Res. 2011, 44, 1244–1252. Netzer, F. P.; Fortunelli, A. Oxide Materials at the Two-Dimensional Limit, Vol. 234: Springer Series in Materials Science, Springer: New York, 2016. Nilius, N. Properties of Oxide Thin Films and Their Adsorption Behavior Studied by Scanning Tunneling Microscopy and Conductance Spectroscopy. Surf. Sci. Rep. 2009, 64, 595–659.
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Noguera, C.; Goniakowski, J. Polarity in Oxide Nano-Objects. Chem. Rev. 2013, 113, 4073–4105. Pacchioni, G.; Valeri, S. Oxide Ultrathin Films, Wiley-VHC: New York, 2012. Prellier, W.; Singh, M. P.; Murugavel, P. The Single-Phase Multiferroic Oxides: From Bulk to Thin Film. J. Phys. Condens. Matter 2005, 17, R803. Surnev, S.; Fortunelli, A.; Netzer, F. P. Structure–Property Relationship and Chemical Aspects of Oxide–Metal Hybrid Nanostructures. Chem. Rev. 2013, 113, 4314–4372. Sementa, L.; Barcaro, G.; Negreiros, F. R.; Thomas, I. O.; Netzer, F. P.; Ferrari, A. M.; Fortunelli, A. Work Function of Oxide Ultrathin Films on the Ag(100) Surface. J. Chem. Theory Comput. 2012, 8 (2), 629–638.
Relevant Websites https://en.wikipedia.org/wiki/Multiferroics. https://en.wikipedia.org/wiki/Surface_properties_of_transition_metal_oxides.
Introduction Growth Characterization Properties Structure Electronic Properties Magnetic Properties Reactivity Perspectives References Further Reading Relevant Websites
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Glossary Coincidence cell In nonpseudomorphic epitaxial films, two-dimensional repeating unit which originates from the matching of the substrate and overlayer periodicities. Epitaxial growth Growth of a crystalline overlayer on a crystalline substrate with a well-defined reciprocal orientation of the crystal structures. Lattice mismatch Difference between the lattice parameters of two crystalline structures, typically expressed in percentual variation. Polar surface Surface with a finite electric dipole moment in the out-of-plane direction, generated by the charge unbalance in anions and cations. Pseudomorphic film Film which adopts the same structure and surface lattice parameter as the substrate on which it grows. Reducible oxide Oxide in which the cations can reversibly switch their oxidation state between two or more states under mild variations of the external conditions. Ultrathin film Film with a thickness below ten nanometers.
Abbreviations ALD Atomic layer deposition ML Monolayer ¼ single layer of closely packed atoms nm 10 9 m PVD Physical vapor deposition STM Scanning tunneling spectroscopy UHV Ultra-high vacuum XAS x-ray absorption spectroscopy XPS x-ray photoemission spectroscopy
Introduction Metal oxides are materials composed by atoms of at least one metallic element combined with oxygen. They generally have a solid aggregation state and they are very abundant on the earth crust. Transition metals, at variance with noble metals, are elements with a partially filled d or f subshell. The electrons in the open subshells can be easily shared with oxygen atoms forming transition metal oxides. Most transition metals, even in their ultrapure form, generally have a thin layer of oxide on their surface, which forms due to contact with the atmosphere. Indeed the thickness of this layer depends on the specific metal, and on the external conditions like temperature, exposure time, and pressure of the oxidizing gas to which it is exposed. The oxide layer sometimes protects, at least
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partially, the interior of the material from further oxidation or from chemical attacks. Oxides are typically much more inert; they have a better thermal stability and a lower electric conductivity than the corresponding metal. Indeed, transition metal oxide films can also be produced artificially for many different purposes. Oxide films are used for example as protective layers for metallic parts against corrosion, as insulating layers in solid-state electronic devices, as coatings which confer specific optical propertiesdlike for example transmission or reflection of specific wavelengthsdor as layers with taylored magnetic properties in magnetoelectronic devices.1 Transition metal oxides in the form of films are widely studied by material scientists to improve the functionality required for a specific application, as well as to identify or to deliberately induce new properties, which arise with vertical confinement.2 The superposition of films with different composition into multilayers or the combination with metallic nanoparticles or doping metal ions can give rise to a composite material, which exploits the interplay between its constituents to achieve a specific response.3 A further degree of freedom, which can be used to tune the properties of thin films, is given by the possibility to combine more than one metallic species with oxygen in ternary or multiple oxides. Due to the chemical and structural complexity, these materials often have a unique behavior, different from simple binary oxides. An outstanding example is represented by multiferroic materials, which are ternary oxides with a perovskite structure, which can simultaneously have two of the three possible ferroic orders: magnetic, electric, and elastic.4 When the film thickness is smaller than some characteristic length scale of the material, like for example the electron mean free path or the magnetic domain size, the behavior of the material presents unprecedented aspects. Typically, we refer to ultrathin films as layers with a thickness which is comparable with the interatomic distance, that is of a few or a few tens of nanometer. In these two-dimensional oxides the bond between the atoms is influenced by the reduced dimensionality and by the symmetry breaking, which often originate remarkably different atomic arrangement and properties, compared to thicker films. Moreover, since ultrathin films are typically grown in the form of epitaxial layers supported on a substrate, their properties are largely influenced also by the interaction with the support. This interplay, combined with the much higher structural flexibility, may lead to the stabilization of structural phases, considerably different from the bulk. A huge number of research studies document the peculiarities of ultrathin transition metal oxide films, for example in reactivity, magnetic couplings, optical response, mechanical and thermal properties. The progress of the research in the field is very effective also due to the development of very accurate theoretical models of the systems, which largely contribute to the atomic scale understanding of the observed properties and to the prediction of new and still unexplored ones.
Growth Many different methods have been used to grow transition metal oxide films for different purposes. Physical vapor deposition (PVD) techniques are usually required to obtain pure crystalline ultrathin overlayers. The samples are typically prepared and studied in clean conditions, that is in ultra-high vacuum or well-controlled oxidizing environments. The substrate plays a very important role in determining the structure, morphology, and electronic properties of the supported ultrathin oxide film. The growth mode and the resulting film structure and morphology are determined not only by the specific growth technique employed but also by the substrate material, by its surface structure and morphology, and by parameters like substrate temperature during growth, evaporation rate, pressure, and reactivity of the oxidizing gas. Oxide substrates typically offer a better lattice matching compared to metal substrates. However, the adatom mobility, usually larger on metal than on oxide surfaces, leads to the formation of ultrathin metal oxide films with excellent epitaxial quality also on substrates with a rather large (30%–40%) lattice mismatch. Pioneering studies investigated ultrathin oxide films obtained by simple oxidation of metal single crystal surfaces or of thin metal films.5,6 By changing the sample temperature, the partial pressure and/or the time of exposure to oxygen or other oxidizing gases, layers with tunable thickness and stoichiometrydwithin certain limitsdcan be obtained on most transition metal surfaces. Direct oxidation, however, does not typically induce the formation of layers with a well-defined thickness, and it often originates a gradient of oxygen concentration with a maximum at the surface and a decreasing trend toward the metal bulk, which depends on the oxygen diffusion rate within the considered material. The quality of oxide films obtained by thermal oxidation of metal surfaces is often limited to highly defective or polycrystalline films, due to the typically different structure and lattice parameter in metals and in the corresponding oxides. An interesting variant of this approach is represented by the oxidation of a surface alloy, which can be in turn prepared by deposition of a metallic layer on the substrate surface and by subsequent thermal annealing. This approach has been shown to be successful for example in the case titanium oxide,7,8 cerium oxide,9 zirconium oxide,5 and manganese oxide,10 leading to layers with a flat morphology and a good crystalline quality. More controlled overlayers, in terms of thickness, structure, and stoichiometry, can be obtained by depositing an oxide film on a support with a different composition. This can be achieved by PVD methods, which exploit processes like evaporation, sputtering, or ablation of oxide bulk targets. Using these techniques the thickness of the films can be controlled by the deposition time, the temperature of the thermal evaporator, or the sputtering rate. The oxide stoichiometry for transition metal oxides, in which the cation can typically adopt more than one oxidation state, may be tuned, to some extent, by changing the substrate temperature. An even better control of the oxide stoichiometry may be achieved by introducing an oxidizing gas during the growth, in the so-called reactive deposition methods, or by post-growth annealing treatments in oxidizing atmosphere. Magnetron sputtering, thermal evaporation, and pulsed laser ablation can be implemented in a variable pressure of oxygen or of different oxidizing gases, like for example NO2, ozone, or atomic oxygen. Thermal evaporation is widely used to obtain ternary or doped oxides with tunable composition, by using an independent metal evaporator for each constituent. However, complex functional oxides with good
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epitaxial quality are also very often produced by pulsed laser deposition or magnetron sputtering from targets with the desired stoichiometry, which is reproduced in the film.4 A different approach, namely the direct sublimation of monodispersed cyclic (WO3)3 trimers, has been used for the synthesis of tungsten oxide films.11 Transition metal oxides may also be obtained by purely chemical methods using gas- or liquid-phase precursors. chemical vapor deposition techniques typically offer a worse control of the film thickness, morphology, and composition compared to physical methods and they often induce a nonnegligible fraction of impurities and contaminants, which are left by the precursors. Among the chemical methods, atomic layer deposition (ALD) is gaining an increasing interest for the preparation of atomic scale controlled multilayers, containing also oxides.12 In ALD the growth proceeds in a cyclic manner by sequentially exposing the surface to two precursors, which react forming a monolayer with a specific composition. Chemical methods, compared to physical ones, are typically cheaper, more versatile, and often scalable to industrial requirements. Moreover, the possibility to use patterned functionalized substrates for the growth of oxide films from precursors allows to transfer the pattern to the oxide film without the need to use photoresist and chemical etching after the film growth. In some cases it is possible to isolate self-standing two-dimensional transition metal oxide layers by exfoliating them from three-dimensional layered crystals using soft-chemical processes, based on liquid or gas-phase techniques.13 This process leads to the formation of stable oxygen-terminated nanosheets, which allow to investigate the intrinsic properties of the materials without the influence of the support, but it is limited to specific materials, which can be handled without risks of mechanical damage.
Characterization To be suitable for the characterization of an ultrathin film, the available experimental techniques are required to have a certain degree of surface sensitivity. The analysis techniques can be distinguished into three general categories: microscopy techniques, spectroscopy techniques, and diffraction-based techniques. The charging problems, which limit the application of electron-based techniques to bulk oxides, are typically much less severe when the oxide ultrathin films are supported on metal substrates. The morphology, defectivity, and local structure of oxide films are usually investigated by techniques like scanning probe microscopies (e.g., scanning tunneling spectroscopy, atomic force microscopy), scanning or transmission electron microscopies. Spectroscopies, like for example x-ray photoemission spectroscopy, ultra-violet photoemission spectroscopy, x-ray absorption spectroscopy, Raman spectroscopy, are used to analyze the electronic properties of the material, to have information on the cation oxidation state, on the band structure, and on the magnetic and vibrational properties. Spectrophotometries and other spectroscopies can give information on the optical properties. Low energy electron diffraction is a diffraction-based technique, which is very sensitive to the outermost surface layers and suitable to determine the epitaxial orientation of ultrathin films and their surface long range crystalline order. Surface x-ray diffraction is sensitive also the structural details at different depths from the surface by changing the grazing angle, around the critical angle for total reflection. In this way not only the surface structure but also the interface between the film and the support can be investigated. The three categories identified for the characterization techniques overlap in many cases. Spectroscopic and diffraction techniques may be applied, using focused primary or detected beams, to obtain a spatially resolved image of the electronic properties or the structure of the investigated surface. Diffraction techniques at the adsorption edge of one of the elements can give chemically selected information on the specific structure of the individual constituents of a material. Synchrotron radiation facilities offer excellent opportunities for the analysis of low dimensional systems, by providing very intense, collimated photon beams with tunable energy within a very wide range, from hard X-rays to infrared and beyond. For an exhaustive treatment of surface science and synchrotron radiation-based techniques, we refer the reader to excellent textbooks in the respective fields.14,15
Properties Structure The structure of ultrathin transition metal oxide films is determined not only by the interaction with the substrate but also by the effects which intrinsically derive from reduced dimensionality. Oxide films with the most stable surface orientation and an almost bulk-like structure can be stabilized on surfaces with the same crystal symmetry and a low lattice mismatch compared to the chosen oxide. In pseudomorphic films some strain can be induced when the thickness is below a certain threshold, typically of the order of one nanometer. Above the threshold thickness the films release their structural strain through the introduction of defects like dislocations. In general, the structural flexibility of a film is higher than the one of the corresponding volume phase and larger strains can be sustained without the appearance of a significant density of defects. In some cases the dislocations introduced to relieve the strain above the critical thickness are arranged into an ordered network and they can be exploited to induce for example the self-assembly of regular arrays of supported nanoparticles.16 Novel structural phases can be obtained when considering less stable or polar surface orientations and substrates with a different symmetry compared to the bulk oxides. In transition metal oxides different structural motifs can be stabilized also as a function of the oxygen chemical potential, due to the stability of different oxidation states for the cations depending on the conditions. For example manganese oxide two-dimensional films supported on a Pd(100) substrate can show nine different MnOx phases at different values of the oxygen pressure and of the temperature during the growth, as schematized in Fig. 1.17
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Fig. 1 Schematic phase diagram of the two-dimensional Mn oxides, presented as a function of the oxygen pressure p(O2) and of the oxygen chemical potential mO. The nominal coverage of Mn on Pd(100) is 0.75 ML. Reproduced with permission from Li, F.; Parteder, G.; Allegretti, F.; Franchini, C.; Podloucky, R.; Surnev, S.; Netzer, F. P. Two-Dimensional Manganese Oxide Nanolayers on Pd(100): The Surface Phase Diagram. J. Phys. Condens. Matter 2009, 21 (13), 134008. All rights reserved. © IOP Publishing.
Also in the case of vanadium oxide ultrathin films different surface reconstructions are observed under variable oxidation conditions and at different thicknesses on different substrates.18,19 Some of the different structural phases observed have been proved to be stabilized by the interaction with the metal substrate. In cerium oxide the most stable oxidation state for Ce ions is 4þ at ambient conditions, but mild variations of the external conditions can stabilize also the 3þ state. By reduction of stoichiometric cerium oxide thin and ultrathin films with the bulk-like fluorite structure, a plethora of different reduced phases can be obtained.20–23 Theoretical studies largely contributed to the understanding of the mechanisms for the stabilization of the observed metastable structures.24,25 Fig. 2 shows an example of a surface reconstruction obtained by thermal reduction of films supported on Pt(111), highlighting the correlation between the new periodicities introduced and some specific sites in the coincidence cell. This evidence suggests that the oxygen vacancy formation energy may be modulated by the different bondings between substrate and overlayer in different areas of the coincidence cell, as reported also using other metal substrates.26 It is important to note that the original structure and stoichiometry of the film can be brought back to the original one by thermal treatments in oxidizing atmosphere.23,27 In titanium oxide films, the plethora of structural phases stabilized in the form of ultrathin films7,28,29 is even wider, since also in the stoichiometric phase different structural ploymorphs have a comparable stability. Moreover, unlike Ce ions, Ti ions can have the 2þ, as well as the 3þ and 4þ oxidation state, and consequently they can adopt many more coordinations and local geometries at reduced dimensionality. Polar surface orientations, unstable as bulk terminations, can be stabilized when the considered oxide is in the form of an ultrathin film. This gives the opportunity to address the behavior of these unique systems by surface science techniques and to eventually develop strategies to maximize such orientations also in real systems.30 In some cases a partial compensation of the polarity induces the stabilization of structures different from the bulk phases from which they are derived. This happens for example
Fig. 2 Left: Low energy electron diffraction pattern (Ep ¼ 80 eV) of a 2 ML cerium oxide film on Pt(111) after reduction induced by heating in ultra-high vacuum (UHV) at 1040 K for 15 min (cCe3 þ 60%, as measured by x-ray photoemission spectroscopy (XPS)), showing the (3 3) and the 9/4(O3 O3)R30 phase. Right: Model of the 3:4 coincidence cell at the interface between CeO2 and Pt(111), evidencing the vectors of the (3 3) supercell (solid line) and of the 9/4(O3 O3)R30 supercell (dashed lines), which define specific sites in the coincidence cell. Modified from Luches, P.; Pagliuca, F.; Valeri, S. Structural and Morphological Modifications of Thermally Reduced Cerium Oxide Ultrathin Epitaxial Films on Pt(111). Phys. Chem. Chem. Phys. 2014, 16 (35), 18848–18857, with permission from the PCCP Owner Societies.
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for monolayer thick FeO layers on Pt(111), which show a lateral lattice expansion, an oxygen surface termination, and a decreased vertical lattice parameter compared to bulk iron monoxide in the (111) orientation.31,32 In FeO films of this kind a locally different progressive oxidation to FeO2 at different sites of the coincidence supercell was observed. This effect allowed to prepare a patterned oxide film, with different iron oxidation states in different regions of the film.33 Another interesting example is the case of ZnO films with polar orientation, which can adopt a flat graphitic-like phase, rather than the bulk wurtzite-type phase, at specific values of the oxygen chemical potential and at the single layer regime.34 Also in cerium oxide (100) polar terminations have been stabilized at specific conditions and a (2 2) surface reconstruction with oxygen vacancies has been identified as the stabilizing mechanism.35,36 Among the structural properties specific of ultrathin oxide films, which induce a specific behavior, we underline their high structural flexibility. This property allows to balance external perturbations, like for example the presence of external charges or a mechanical stress, by introducing local atomic displacements much larger than the bulk phases. The so-called polaronic distortion is a local structural modification of the crystal structure associated with an excess charge. These deformations play an important role in charge transport mechanisms in oxide films, and they have been shown to be at the basis of the high-temperature superconducting behavior in copper oxide-based films.37
Electronic Properties In ultrathin films the reduced dimensionality in the out-of-plane direction, the interaction with the substrate, the structural strain or defectivity, and the different structural motifs stabilized are linked to a substantially different electronic structure compared to single crystalline oxide bulk phases. Transition metal oxides have typically intermediate electronic band gaps of 3–4 eV or below, although some of them are metallic (e.g., RuO2, ReO3, and some ternary perovskites) and some others are highly insulating (e.g., HfO2, BaTiO3, and other perovskites). The experimental measurement of the (optical or electronic) band gap of an ultrathin film is not always straightforward, due to the influence of the substrate electronic states. Complications arise also in the theoretical prediction of the band gap, since several methods underestimate electronic correlation effects and do not properly describe the filled and unfilled bands. Typically, in ultrathin oxide films the vertical confinement leads to a narrowing of the electronic bands and to an increase of the respective density of states. This often induces a decrease of the band gap with respect to the bulk value and the film becomes mildly conductive. In a NiO film on an Ag(001) substrate theoretical calculations have shown that the interfacial layer is conductive and that films with thickness up to 1 nm still do not reach the bulk electronic properties,38 in agreement with experimental results.39 The changes in the band structure are reflected in modifications of the work function when ultrathin films are considered. The work function, defined as the minimum energy required to remove an electron from the system, determines important properties like conductivity, reactivity, etc. The work function of a metal surface is modified by the absorption of an ultrathin oxide film. Its value is determined by different contributions. The first is the specific absorption geometry of the oxide on the metal. Moreover, the reciprocal electronic interactions also play a role. Some electrostatic charge compressions can be induced by the repulsion of the metal electrons after oxide absorption. In addition, some charge can be transferred between the metal and the oxide. In general the system work function is decreased if some charge is transferred from the oxide to the metal and vice versa. The surface relaxation and the geometric rumpling in the epitaxial films can also significantly affect the work function.40 Fig. 3 reports an example of the predicted variations of the work function for single layer thick MgO and transition metal oxide films along the first row, epitaxially supported on Ag(001)41. For MgO, CoO, and ZnO the work function of the system is smaller or comparable to the metal surface, while for NiO and CuO it is larger. The absorption geometry is very similar with negatively charged oxygen anions located on top of surface metal atoms, giving a comparable charge compression and a slight and progressive decrease of the work function from right to left due to the increased ionicity of the metal-oxygen bonds. On the contrary, the charge transfer changes significantly along the series, and it largely determines the work function changes. At variance with MgO, in transition metal
Fig. 3 Values of the work function for ultrathin transition metal oxide films adsorbed on Ag(100) evaluated by density functional theory calculations using different functionals. The value of the work function of Ag(100) is shown as a dotted line. Reproduced from Sementa, L.; Barcaro, G.; Negreiros, F. R.; Thomas, I. O.; Netzer, F. P.; Ferrari, A. M.; Fortunelli, A. Work Function of Oxide Ultrathin Films on the Ag(100) Surface. J. Chem. Theory Comput. 2012, 8 (2), 629–638, with permission.
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cations, the presence of empty d states in the conduction band favors a charge transfer from the metal to the oxide, which partially compensates the decrease of the work function due to charge compression. The balance between the two effects and the structural rumpling generates a maximum in the work function for Ni and Cu. In transition metal oxide films with polar orientation, dipole moment compensation can induce important modifications of the surface electronic structure, like for example the occupation of oxide surface states by charges coming from the metal substrate, as observed on cobalt and iron oxides.31,42 This mechanism generally leads to a decrease of the ionicity of the metal-oxygen bonds, and in some cases even to a surface metallization.
Magnetic Properties Transition metal oxides exhibit a rich variety of magnetic properties, which depend on the electronic configuration. At ambient conditions they can be paramagnetic (e.g., vanadium oxides, or reduced titanium or cerium oxide), antiferromagnetic (e.g., NiO, CoO, FeO, a-Fe2O3, and Co3O4), ferrimagnetic (e.g., Mn3O4 and g-Fe2O3), and diamagnetic (e.g., Y2O3, TiO2, CrO3, and AgO). The modifications of the band structure with out-of-plane confinement, discussed above, have indeed nonnegligible consequences also on the magnetic properties of transition metal oxide films. The band narrowing and the increase of the density of filled states, induced by vertical confinement, may cause an increase in the spin magnetic moment. This increase, combined with the symmetry breaking, may increase also the orbital moment. Additional magnetic anisotropies are typically introduced, and they compete with the magnetocrystalline anisotropy in determining magnetization reversal in two-dimensional systems. Among them we mention the shape anisotropy, induced by symmetry breaking, and the interface anisotropy. In addition, a growth-induced anisotropy, perpendicular to the growth direction, can be introduced. In general, ferromagnetic oxide ultrathin films have higher coercive films than thicker films and different reversal mechanisms due to the stabilization of domains with specific orientation and shape and because of the possibility of having some pinning of the magnetization at defect sites. Also the Curie temperature is typically dependent on the film thickness. In antiferromagnetic oxide films, such as for example NiO films, the most stable antiferromagnetic domains as well as the Néel temperature depend not only on the film thickness (see Fig. 4) but also on the epitaxial strain.43–45 The competition between the two effects allows to obtain ultrathin films with relatively high Néel temperatures43 or with selected orientation of the antiferromagnetic anisotropy.44,45 Very interesting and unprecedented magnetic properties arise when ultrathin ferromagnetic layers are coupled to antiferromagnetic layers, often in the form of oxide films. The magnetic anisotropy of the antiferromagnetic layer exerts a torque on the spins of the ferromagnetic layer. This interaction affects the magnetization reversal, inducing an increase in the coercive field, and a shift of the hysteresis loop with respect to the zero field by an amount called the exchange bias.46,47 The exchange bias and the coercivity increase depend not only on the specific materials but also on the thickness of the layers involved. It has to be pointed out that the interface between the two layers is often a unique two-dimensional phase with different magnetic properties, which have to be taken into account when describing the magnetic couplings in the system.48–50
Fig. 4 (A) Temperature and thickness dependence of the ratio of the two peaks in the Ni L2-XAS (x-ray absorption spectroscopy) of NiO/MgO(001) thin films at normal incidence. Néel temperatures of TN ¼ 295, 430, and 470 K are determined for the 5, 10, and 20 monolayer films, respectively. Reprinted from Alders, D.; Tjeng, L. H.; Voogt, F. C.; Hibma, T.; Sawatzky, G. A.; Chen, C. T.; Vogel, J.; Sacchi, M.; Iacobucci, S. Temperature and Thickness Dependence of Magnetic Moments in NiO Epitaxial Films. Phys. Rev. B 1998, 57 (18), 11623–11631, with permission. (B) Ratio of the two peaks in the Ni L2-XAS of NiO/Ag(001) thin films of different thickness at normal and grazing incidence angles. The inset shows the magnetic linear dichroism (MLD) obtained from the data. Reprinted from Krishnakumar, S. R.; Liberati, M.; Grazioli, C.; Veronese, M.; Turchini, S.; Luches, P.; Valeri, S.; Carbone, C. Magnetic Linear Dichroism Studies of In Situ Grown NiO Thin Films. J. Magn. Magn. Mater. 2007, 310 (1), 8–12, with permission.
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Multilayers, including oxide phases, with different magnetic and electric properties can also be used to achieve specific transport properties at different values of the external magnetic or electric field. Spin valves are devices used as magnetoresistive read/write heads for magnetic hard disks, exploiting the strong dependence of the resistance on the relative orientation of the magnetization in two ferromagnetic layers. Indeed the research on the properties of ultrathin oxide films has contributed largely to the optimization of the performances of devices of this kind. When extending the field to ternary oxides, the range of magnetic magnetoelectric, magnetoelastic, and magneto-transport properties and the possibilities for their modifications with dimensionality become incredibly wider and richer.4
Reactivity Ultrathin transition metal oxide films represent model catalysts, which can be used to determine the role of a specific surface orientation or site on the activity and selectivity in a chemical reaction. Moreover, the contribution of well-defined sites at the interface between an oxide and a metal can be understood by investigating epitaxial oxide nanostructures on single crystalline metal substrates or by using oxide films as supports for metal nanoparticles with specific size, shape, and composition. The information obtained allows in principle to design real catalysts with an optimized concentration of active or selective sites, in which the onset temperature and/or the rate of a chosen reaction can be modified to meet the specific requirement of the application. On the other hand, an accurate description of the properties of metal supported ultrathin transition metal oxide films can be very relevant to understand the performance of real catalysts made of metal nanoparticles on oxide supports. The interaction between the two materials may in fact induce a partial encapsulation of the metal nanoparticles by the oxide, forming an ultrathin oxide film on the metal surface. A representative example of the differences in reactivity between the bulk oxide phase and two-dimensional phases is given by platinum supported iron oxide films. The bilayer films were found to be inert, while trilayer films showed a substantial reactivity toward CO oxidation.51 The interface between iron oxide and platinum is claimed to be the active site for CO oxidation, as shown in Fig. 5,52 while the catalytic cycle is sustained by oxygen diffusion from the interior of an oxide island to the active edge sites.51 The low coordinated ferrous cations at the interface between the oxide and the metal support were identified also as active sites for O2 activation, of great importance for fuel cell operation.52
Fig. 5 (A) Scanning tunneling spectroscopy (STM) image (200 nm 200 nm) of a 0.25 ML FeO1 x/Pt(111) surface. (Inset) An atomic resolution STM image of FeO monolayer nanoislands (25 nm 20.8 nm). (B) Ratios of XPS O 1s to Fe 2p3/2 peak intensity from FeO1 x/Pt(111) surfaces with different periphery density of FeO nanoisland. Samples 1–3 are 0.25 ML FeO nanoislands prepared at 1.3 10 6 mbar O2 and annealed in UHV at 473, 573, and 673 K, respectively. Sample 4 is the full-monolayer FeO film on Pt(111). The STM images are all 100 nm by 100 nm. (C) Dependence of reactivity of CO oxidation on the periphery density at 0.25 ML FeO1 x/Pt(111) surfaces. Reprinted from Fu, Q.; Li, W.-X.; Yao, Y.; Liu, H.; Su, H.-Y.; Ma, D.; Gu, X.-K.; Chen, L.; Wang, Z.; Zhang, H.; Wang, B.; Bao, X. Interface-Confined Ferrous Centers for Catalytic Oxidation. Science 2010, 328 (5982), 1141, with permission.
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Epitaxial titanium, vanadium and tungsten oxide films have been been studied as model catalysts for the selective reduction of NO by NH3.53 Pd oxide epitaxial layers provided an important playground for the understanding CH4 catalytic combustion.53 Titania and ceria ultrathin epitaxial films provided valuable information on the role of metal-oxide sites and oxygen vacancy formation on the water gas shift reaction.54 Zinc oxide films on copper are studied in view optimizing the synthesis of methanol via the oxidation of CO.55 The number of further outstanding examples is indeed very large, and several dedicated reviews on the topic include an exhaustive treatment of the subject,56,57 pointing at the importance of having well-controlled systems and a proper theoretical modeling for a true atomic scale understanding of the functionality of metal/oxide systems.
Perspectives The recent research has largely demonstrated the potential of ultrathin transition metal oxide films as a new class of materials. Novel compositions and different architectures are being identified with the aim of obtaining compounds with new and multiple functionalities, designed on demand. The progress in this field requires parallel advances in the growth and synthesis methods, in the characterization tools, and in the theoretical modeling. Among the outstanding examples of promising research ways for the future we mention the possibilities introduced by the stabilization of aperiodic two-dimensional structures in transition metal-based ternary oxides.58 The possibility of obtaining new topological phases by changes in composition and in symmetry also offers a rich playground for research.59 A further still largely unexplored aspect is the time-scale description of the interactions and excitations occurring in these systems, encouraged by the development and the availability of ultrafast spectroscopies also available as shared facilities. As a final remark, we mention that oxides are promising materials also in view of the minimization and substitution of critical noble metals in catalysis and energy applications. Research on well-controlled systems is a mandatory step for a rational and sustainable design of the appropriate compounds.
See also: Characterization of ultrathin oxide films by LEEM/PEEM; Reactive Molecular Beam Epitaxy of Iron Oxide Films: Strain, Order, and Interface Properties; Ultrathin Oxide Films on Ferromagnetic Metallic Substrates.
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Electron Spectrosc. Relat. Phenom. 2006, 150 (2–3), 136–149. Rodriguez, J. A.; Ma, S.; Liu, P.; Hrbek, J.; Evans, J.; Perez, M. Activity of CeOx and TiOx Nanoparticles Grown on Au(111) in the Water-Gas Shift Reaction. Science 2007, 318 (5857), 1757–1760. Schott, V.; Oberhofer, H.; Birkner, A.; Xu, M.; Wang, Y.; Muhler, M.; Reuter, K.; Wöll, C. Chemical Activity of Thin Oxide Layers: Strong Interactions With the Support Yield a New Thin-Film Phase of ZnO. Angew. Chem. Int. Ed. 2013, 52 (45), 11925–11929. Gao, F.; Goodman, D. W. Model Catalysts: Simulating the Complexities of Heterogeneous Catalysts. Annu. Rev. Phys. Chem. 2012, 63, 265–286. Shaikhutdinov, S.; Freund, H.-J. Ultrathin Oxide Films on Metal Supports: Structure-Reactivity Relations. Annu. Rev. Phys. Chem. 2012, 63, 619–633. Forster, S.; Meinel, K.; Hammer, R.; Trautmann, M.; Widdra, W. Quasicrystalline Structure Formation in a Classical Crystalline Thin-Film System. Nature 2013, 502 (7470), 215–218. Moon, S. 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Further Reading Chambers, S. A. Epitaxial Growth and Properties of Thin Film Oxides. Surf. Sci. Rep. 2000, 39, 105–180. Duò, L.; Ciccacci, F. Magnetic Properties of Antiferromagnetic Oxides, Wiley-VHC: Weinheim, 2010. Freund, H.-J.; Pacchioni, G. Oxide Ultra-Thin Films on Metals: New Materials for the Design of Supported Metal Catalysts. Chem. Soc. Rev. 2008, 37, 2224–2242. Giordano, L.; Pacchioni, G. Oxide Films at the Nanoscale: New Structures, New Functions, and New Materials. Acc. Chem. Res. 2011, 44, 1244–1252. Netzer, F. P.; Fortunelli, A. Oxide Materials at the Two-Dimensional Limit, Vol. 234: Springer Series in Materials Science, Springer: New York, 2016. Nilius, N. Properties of Oxide Thin Films and Their Adsorption Behavior Studied by Scanning Tunneling Microscopy and Conductance Spectroscopy. Surf. Sci. Rep. 2009, 64, 595–659.
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Noguera, C.; Goniakowski, J. Polarity in Oxide Nano-Objects. Chem. Rev. 2013, 113, 4073–4105. Pacchioni, G.; Valeri, S. Oxide Ultrathin Films, Wiley-VHC: New York, 2012. Prellier, W.; Singh, M. P.; Murugavel, P. The Single-Phase Multiferroic Oxides: From Bulk to Thin Film. J. Phys. Condens. Matter 2005, 17, R803. Surnev, S.; Fortunelli, A.; Netzer, F. P. Structure–Property Relationship and Chemical Aspects of Oxide–Metal Hybrid Nanostructures. Chem. Rev. 2013, 113, 4314–4372. Sementa, L.; Barcaro, G.; Negreiros, F. R.; Thomas, I. O.; Netzer, F. P.; Ferrari, A. M.; Fortunelli, A. Work Function of Oxide Ultrathin Films on the Ag(100) Surface. J. Chem. Theory Comput. 2012, 8 (2), 629–638.
Relevant Websites https://en.wikipedia.org/wiki/Multiferroics. https://en.wikipedia.org/wiki/Surface_properties_of_transition_metal_oxides.