Oxide thin films are nowadays recognized as essential ingredients in many areas of modern technology. While binary oxides already exhibit a rich variety of structural, electronic, magnetic, and chemical properties, doping or mixing them with foreign cations allows further engineering to meet specific application requests. In this review, we first outline the fabrication and characterization techniques used to study mixed oxide thin films, both from an experimental and a theoretical viewpoint. We then provide a detailed overview of recent advances in the field, drawing on both experimental findings and theoretical insights. This section which is organized according to the dominant cation in doped films, focuses on systems with distinct thin film characteristics compared to the bulk, such as thickness-dependent properties and surface/interface effects. The subsequent section then explores the fundamental principles that govern cation mixing, addressing both solid solutions and ordered compounds within a unified framework. While these principles are relatively well understood in bulk materials, additional complexity exists in thin films due to finite size effects, as well as the interaction with the surrounding atmosphere (notably oxygen) and/or the substrate which supports the thin films. In several cases, these factors are shown to dramatically alter the mixing behavior, affecting the phase diagrams, the stability and composition of ordered phases, among others. The final section adopts a transversal perspective, and summarizes the benefits of cation mixing in oxide thin films for engineering their structural, electronic, magnetic, and reactivity properties, and expanding their potential for a use in a broad range of technological applications.

Abstract Due to the interesting properties of copper oxides, in particular for catalytic applications, ultrathin oxide layers formed by Cu oxidation have attracted sustained interest. Experimental observations of several surface phases, including the so-called “5–7”, “8”, “29” and “44” structures, have sparked ongoing debate. However, their precise atomic configurations remain incompletely resolved. Among these, the “8” phase is particularly challenging, as existing structural models rely on limited experimental evidence. Here, we combine surface X-ray diffraction, direct fitting of experimental data, and density functional theory (DFT) calculations, to determine both the composition and atomic structure of this phase. The resolved structure reveals a dense, interwoven network of four-membered Cu–O rings, comprising two distinct Cu cation species with markedly different structural and electronic characteristics. DFT results show that a substantial electron transfer from the Cu(111) substrate stabilizes its relatively oxygen-rich composition, Cu5O4. This “8” phase is energetically more stable than the most recent Cu8O4 model [Kim, H. J. et al. ACS Nano 2024, 18, 4559–4569] across the full range of oxygen chemical potential and is structurally and electronically distinct from the “29” and “44” surface oxides. Its bonding motifs and charge distribution resemble those of Pd5O4 and Ag5O4, formed on oxidized (111) surfaces of palladium and silver, but its formation requires less oxygen-rich conditions, reflecting the stronger affinity of Cu for oxygen.

Low-temperature tunneling spectroscopy has been employed to probe the workfunction ϕ of atomically flat Cr-oxide single- and double-stack films grown on Pt(111). While the single-stack Cr3O6 trilayer has a ϕ value of ∼7.0 eV, it decreases to ∼5.0 eV for a Cr6O11 double-stack. The charge redistribution underlying this workfunction drop has been analyzed by density functional theory. The remarkably high ϕ value of the Cr3O6/Pt trilayer, making it a highly electronegative substrate, originates from a massive electron transfer from the Pt(111) into empty Cr-states that reduces 2/3 of the oxide cations from their formal 4+ to a 3+ charge state. The negative surface dipole diminishes upon growing a Cr-O honeycomb layer on top of the trilayer, forming a Cr6O11 double-stack. The adlayer acts as electron donor, enabling the charge transfer from the Pt support to decrease substantially. The charge redistribution not only triggers the detected workfunction drop but also stabilizes the double-stack with respect to a single-stack geometry. A comparison of the observed CrOx/Pt behavior to that of hypothetical double-stack films, made of an interfacial TMO2 trilayer (TM = Ti, V, Mn, Fe) and a capping Cr-O honeycomb plane, allows us to correlate the charge transfer between the individual oxide layers and the substrate to the overall stability of the metal-oxide system.

The reduction of MoO3 profoundly influences its physical and chemical properties, making it a material of central importance across a wide range of applications. While bulk reduction processes and the resulting substoichiometric Magnéli phases─composed of extended shear planes─have been thoroughly investigated, the reduction chemistry of MoO3 nanoscale objects remains largely unexplored and nanoscale-specific mechanisms of oxygen deficiency accommodation are poorly understood. In this study, we employ a combination of atomic-resolution scanning tunneling microscopy (STM), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) modeling to systematically investigate the reduction of an ultrathin MoO3 bilayer supported on a Pd(100) surface. Our results reveal that the bilayer decomposes upon reduction into reduced monolayer and trilayer phases, with the trilayers consistently exhibiting ordered surface defect structures characterized by (2 × 3) and (2 × 4) periodicities. Through DFT modeling, we assign these defects to a peculiar type of surface oxygen vacancy, formed by a transformation of MoO3 octahedra configurations from corner-sharing to edge-sharing. Ordered assemblies of such defects form lattices of substoichiometric shear lines, which are the two-dimensional analogs of the bulk Magnéli crystallographic shear planes. Their genuinely nanoscale-specific character is proved by the instability of alike phases on surfaces of bulk MoO3. Their detection on MoO3 films suggests that the formation of similar two-dimensional (2D) Magnéli-type phases may be a more general phenomenon and occur also in other reducible oxide nanolayers, where enhanced structural flexibility enables the formation of phases not accessible in bulk crystals.

The mixing characteristics of oxide materials largely depend on the dimensionality of the system, and many oxide-alloy structures in three dimensions (3D) do not have a 2D analog. To unravel fundamental alloying mechanisms in 2D, V/Cr mixing into oxide thin films is investigated on Pt(111) by scanning tunneling microscopy and density functional theory. The experiments reveal flat, double-stack islands made of a compact bottom and a honeycomb top layer with a 4.5 Å total height. The energetically most favorable structure-match comprises an O-Cr-O trilayer at the interface to the Pt(111) capped by a mixed V/Cr honeycomb top layer. The structure is stabilized by strong interlayer adhesion, reinforced by a charge transfer toward the central trilayer from the metal support and the honeycomb plane. A negative V/Cr mixing enthalpy arises from the presence of two distinct surface sites that enable formation of tetrahedrally coordinated V5+ and octahedrally coordinated Cr3+ cations. The identified thin-film structure bears resemblance to a (111) cut of a hypothetical V/Cr spinel, a unique 2D configuration without bulk equivalent that is stabilized solely by its nanoscale thickness and a strong coupling to the Pt support.

The atomic structure of MoOx films formed upon a gradual thermal reduction of an ordered MoO3 monolayer on the Pd(100) substrate was explored via surface science characterization techniques and density functional theory (DFT) calculations. Two main reduction stages were identified. First, the initial oxygen excess was gradually eliminated by altering the domain boundary length, orientation, and atomic structure. The films nevertheless remained O-rich, with numerous terminal oxygen atoms (formation of Mo--O groups), and an elevated work function. Second, multiple ordered O-lean phases were formed, characterized by either very few or no terminal oxygen atoms, and a much smaller surface work function. According to calculations, the positive charging of the Pd substrate stabilizes the oxygen excess during the first stage, but during the second reduction stage, the substrate becomes negatively charged, stabilizing enhanced cation oxidation states. On their basis, the mechanisms underlying the oxygen release from the initial c(2 x 2) domains were disclosed. The experiments showed that the film reduction is perfectly reversible, which highlights the very promising properties of the MoO3/Pd system for heterogeneous catalysis.

The formation of chromium oxide thin films on Pt(111) was investigated by electron diffraction, X-ray photoelectron spectroscopy, and low-temperature scanning tunneling microscopy. Depending on the nominal Cr coverage and the oxygen chemical potential during preparation, two distinct oxide phases were identified. A (root 3 x root 3)R30 degrees phase emerges at submonolayer Cr exposure and O-rich oxidation conditions, while a (2 x 2) phase develops at higher coverage and after a vacuum-annealing step. For both phases, the atomic nature of cationic and anionic sublattices and the chemical composition were determined in detail. Guided by this experimental input, a comprehensive global structure optimization was performed by genetic algorithms, and key configurations were refined by density functional theory calculations in a second step. For the (root 3 x root 3)R30 degrees phase, good agreement was revealed for a Cr3O6 trilayer, comprising a mixture of Cr3+ and Cr4+ ions in the central cationic plane. The (2 x 2) phase matches well with a Cr6O11 film, in which an O-Cr-O trilayer is capped by a Cr2O3 honeycomb plane. The identified configurations not only reproduce the structure and symmetry deduced from experiments but also have the most favorable energetics at the employed oxygen chemical potential. Our work delivers reliable insights into the atomic nature of the two main CrOx phases on Pt(111) and clears up the conflicting models found in the literature.

The oxides of copper have attracted the attention of scientists already for more than hundred years. This fascination is fueled by many outstanding properties of the material, for example, a semiconducting behavior that led to the first diode fabricated in electronics, a pronounced excitonic response that stimulated an intense search for Bose-Einstein condensation, and a pivotal role in unconventional superconductivity. Despite this central position in past and present research activities, many aspects of copper oxides are not sufficiently understood to date. This applies in particular to their surface characteristics, where even fundamental questions, such as the energetically favored termination of low-index Cu2O and CuO planes, are still subject of debates. This review aims at addressing these deficiencies by compiling state-of-the-art knowledge of the surface science of copper oxides, and especially of cuprous oxide.A first focus of the article lies in the oxidation characteristic of copper as a means to prepare well-defined oxide surfaces. It demonstrates that low-pressure oxidation only results in the formation of ultrathin precursor oxides, with properties deviating substantially from those of the bulk material. Consequently, reliable pathways to produce high-quality and bulk-compatible surfaces, either of Cu2O thin films or bulk crystals, are presented. The following chapter provides a comprehensive introduction into the atomic structure of the most relevant Cu2O surfaces, i.e., the (111), (100) and (110) planes. It gives an overview of important diffraction and microscopy experiments on the most accessible Cu2O terminations, and complements this with state-of-the-art theoretical studies to develop corresponding atomistic models. The chapter closes by presenting the atomic configurations of the most relevant Cu2O surfaces at given thermodynamic conditions.Chapter four develops a surface-science view onto the unique optical response of cuprous oxide. After introducing the well-known bulk behavior, it highlights how optical properties can be probed on surfaces with high spectral and spatial resolution. The chapter discusses how optical near-field techniques are employed to analyze oxide excitons and their trapping at lattice defects in real-space experiments. The last chapter summarizes efforts to alter intrinsic Cu2O properties, e.g., the p-type conductivity, the width of the band gap and the exciton trapping and recombination behavior, via doping. It illuminates this topic from an experimental and theoretical viewpoint and highlights several unsolved questions related to the topic.Despite considerable efforts, this review can only present the current state of knowledge on Cu2O surfaces, a subject that continuously advances due to new scientific findings and innovations. We nonetheless hope that it provides a comprehensive and topical overview of the unusual properties of this fascinating oxide system.

We explore the structural evolution of two-dimensional (2D) MoO3 films beyond the monolayer (ML), which have been prepared by physical vapor deposition and post-oxidation onto a Pd(100) surface, and characterized by the tools of surface science and density functional theory (DFT) calculations. According to DFT, the most stable oxide layers are stoichiometric, and derive their energetic stability from the low cost of creating 2D freestanding layers from the orthorhombic bulk phase, good matching to Pd, and the particularly strong adhesion to the substrate. The observed 2D MoO3 layers are distinguished by well-ordered linear defects, such as domain boundaries in the ML, and misfit dislocations in the bilayer (BL). Applying reactive oxidation preparation conditions results in the formation of ordered arrays of nanostructures, nanowires and nanoclusters, in the MoO3 BL. The formation of such linear structures is accounted for in the DFT by models of missing row defects of various orientations and stoichiometries. Their relative stability is rationalized in terms of the number of broken Mo-O bonds, the polar character of the nanostructure edges and the interaction strength with the Pd substrate. Comparison with similar WO3 layers on Pd(100) is provided.

Low-coordinated atoms residing at the edges of oxide nanostructures play an important role for adsorption and reaction processes in heterogeneous catalysis. While elucidation of their local configuration is challenging already for binary oxides, hardly any information is available for ternary materials, for which the variable stoichiometry introduces an additional degree of freedom. In this study, low-temperature scanning tunneling microscopy is employed to analyze the edge configurations of V2-xFe x O3 honeycomb islands grown on Pt(111) as a function of film composition. The islands are delimited by zigzag and armchair edges, the latter with a tendency to reconstruct into sequences of four, five, six and seven-membered rings. Scanning tunneling spectroscopy is used to identify the chemical nature of the edge atoms to be either vanadium or iron. The thermodynamic forces for V or Fe segregation to the edges, both oxygen and cation terminated, are analyzed by density functional theory calculations. In an oxidizing environment, formation of vanadyl-terminated edges is energetically favorable, while Fe atoms segregate toward the edges at O-poor conditions. The observed behavior is explained by the significantly higher oxygen affinity of V versus Fe edge cations, and reflected in the higher stability of terminal vanadyl compared to ferryl groups. Our findings may indicate a pathway to tailor the chemical composition and thus the catalytic reactivity of oxide island edges via cationic mixing.

Planar oxide atomic clusters are of considerable scientific interest because of their potential for enhanced catalytic activity versus their three‐dimensional counterparts. This enhancement is the result of the substrate stabilizing novel planar configurations that have an extensive periphery where catalytic reactions can occur. A class of planar NbnOm atomic clusters that are synthesized by the evaporation of metallic Nb onto an Au(111) substrate in an ultrahigh vacuum environment and subsequent oxidation at elevated temperatures is reported. The atomic structures of the clusters are determined using a combination of scanning tunneling microscopy and density functional theory. The clusters are composed of structural units with four‐, five‐, and sixfold rotational symmetry and these units can assemble to form larger planar clusters. The theoretical comparison of supported structures with their hypothetical freestanding counterparts shows that the atomic and electronic structures of the oxide clusters are significantly altered by the interaction with the Au substrate. The substrate effects include interfacial charge transfer and structural relaxation to relieve the strain in the Nb‐O bonds. The substrate interactions also reduce the energy differences between clusters of different configurations and this enables the coexistence of a large variety of cluster configurations.

Cation mixing is a well-recognized means to obtain oxides of desired functionality with predetermined structure and stoichiometry, which yet has been only little analyzed at the nanoscale. In this context, we present a comparative analysis of the stability and mixing properties of O-poor and O-rich two-dimensional V-Fe oxides grown on Pt(111) and Ru(0001) surfaces, with the aim of gaining an insight into the role of substrate and oxygen conditions on the accessible Fe contents. We find that due to the high oxygen affinity of the Ru substrate, the mixed O-rich layers are highly stable while the stability of O-poor layers is limited to inaccessibly oxygen-poor environments. In contrast, on the Pt surface, O-poor and O-rich layers coexist with, however, a much lower Fe content in the O-rich phase. We show that cationic mixing (formation of mixed V-Fe pairs) is favored in all considered systems. It results from local cation-cation interactions, reinforced by a site effect in O-rich layers on the Ru substrate. In O-rich layers on Pt, Fe-Fe repulsion is so large that it precludes the possibility of substantial Fe content. These findings highlight the subtle interplay between structural effects, oxygen chemical potential, and substrate characteristics (work function and affinity towards oxygen), which governs the mixing of complex 2D oxide phases on metallic substrates.

We report successful synthesis of a mixed V/Fe ultrathin oxide phase on Ru(0001), in which mixing is governed by a self-organization scheme that selects V/Fe compositions close to 50%. Bias-dependent high-resolution STM imaging and DFT simulations enabled a thorough characterization of its honeycomb-like structure with additional oxygen anions at the metal/oxide interface as well as a convincing assignment of the two observed defect types. The mixed layer displays a unique structure and stoichiometry intermediate between those of pure vanadium and iron oxide films under the same experimental conditions. It results from a singular, geometry-driven mixing scenario in 2D which, because of the high interfacial oxygen content, is reminiscent of that known for bulk ternary oxides. Comparison with V/Fe mixed honeycomb layers on Pt(111) exemplifies how the choice of the metal substrate may induce largely different mixing behaviors. Such versatility opens a way toward a thorough fundamental understanding of the principles that underlie oxide alloying in 2D.

Grain boundaries (GBs) are ubiquitous in solids. Their description is critical for understanding polycrystalline materials and explaining their mechanical and electrical properties. A GB in a 2D material can be described as a line defect and its atomic structures have been intensively studied in materials such as graphene. These GBs accommodate the relative rotation of two neighboring grains by incorporating periodic units consisting of nonhexagonal rings along the boundary. Zero‐degree GBs, called domain boundaries (DBs), where there is only a lattice offset between two grains without any rotation, are rare in 2D van‐der‐Waals (vdW) bonded materials where the grains can easily move. However, this movement is not possible in 2D materials that have a strong epitaxial relationship with their substrate such as the M 2 O 3 (2 × 2) honeycomb monolayers on noble metal (111) supports. Involving experimental and theoretical investigations, four main DBs are observed here in a monolayer of Ti 2 O 3 supported on Au(111) and their atomic structures are solved. The DB formation energies explain why some DBs are more frequently observed than others. The strong epitaxial constraint from the Au(111) substrate stabilizes some unique Ti 2 O 3 monolayer DB structures that are not observed in vdW‐bonded 2D materials.

While cation replacement has long been used to tailor the functionalities of oxide compounds, the growing industrial demand for nanoscale materials nowadays requires precise atomic scale characterization and understanding of the cation distribution. We report a successful synthesis and characterization of the two-dimensional (V,Fe)(2)O-3 alloy supported on Pt(111) which is thermodynamically stable at realistic temperatures and for Fe contents up to 50%. Tight synergy between fine atomic resolution STM experiments and DFT calculations has revealed an unequivocal preference for formation of mixed V-Fe nearest-neighbor pairs, to a large extent driven by cation-cation electrostatic interactions. Moreover, Monte Carlo simulations have enabled an in-depth rationalization of the observed cation distribution in the honeycomb lattice. We show that the V-Fe mixing is restricted to 2D systems and not observed in the corresponding bulk systems, and that it is closely related to a change in vanadium oxidation state resulting from the interaction with the substrate. The flexibility of composition, of the distribution of cations and of their charge state provides levers to tune the properties of such two-dimensional oxide alloys and may thus enhance their potential for applications.

Considering the importance of sub-monolayer transition metal oxides supported on another oxide in many industrial processes, with the help of a DFT + U approach, we provide information on the structural and electronic properties of pure M 2O3 and mixed MM′O3 oxide monolayers (M, M′ = Ti, V, Cr, Fe) supported on an α-Al2O3(0001) support. With their structure in the prolongation of the alumina corundum lattice, the monolayers have non-equivalent surface and interface cations, which leads to two different cation configurations in the mixed oxides. In all cases, the interfacial charge transfer is weak, but strong cation–cation electron redistributions may take place as in TiVO3, TiFeO3, VFeO3, and TiCrO3 in which actual redox processes lead to cation oxidation states different from the expected +3 value. We show that the tendency to mixing relies on the interplay between two very different driving forces. Cation–cation redox reactions, in most cases, strongly stabilise mixed configurations, but preference for a given cation position in the monolayer, because of surface energy reasons, may strengthen, weaken or even block the mixing tendency. By comparison with results obtained in bulk ilmenite, in free-standing monolayers and in MLs deposited on transition metal substrates, we evidence the flexibility of their electronic structure as a function of size, dimensionality and nature of support, as a lever to tune their properties for specific applications.

Nanostructured oxide materials ultra-thin films, nanoparticles and other nanometer-scale objects play prominent roles in many aspects of our every-day life, in nature and in technological applications, among which is the all-oxide electronics of tomorrow. Due to their reduced dimensions and dimensionality, they strongly interact with their environment gaseous atmosphere, water or support. Their novel physical and chemical properties are the subject of this book from both a fundamental and an applied perspective. It reviews and illustrates the various methodologies for their growth, fabrication, experimental and theoretical characterization. The role of key parameters such as film thickness, nanoparticle size and support interactions in driving their fundamental properties is underlined. At the ultimate thickness limit, two-dimensional oxide materials are generated, whose functionalities and potential applications are described. The emerging field of cation mixing is mentioned, which opens new avenues for engineering many oxide properties, as witnessed by natural oxide nanomaterials such as clay minerals, which, beyond their role at the Earth surface, are now widely used in a whole range of human activities. Oxide nanomaterials are involved in many interdisciplinary fields of advanced nanotechnologies: catalysis, photocatalysis, solar energy materials, fuel cells, corrosion protection, and biotechnological applications are amongst the areas where they are making an impact; prototypical examples are outlined. A cautious glimpse into future developments of scientific activity is finally ventured to round off the treatise.

The novel physical and chemical properties and functionalities of two-dimensional (2-D) oxide materials are assessed. The synthesis of one unit-cell thick 2-D oxides poses particular challenges, since in contrast to other 2-D materials, which can be fabricated by exfoliation of layered bulk compounds, the majority of oxides do not occur in layered bulk structures. Most 2-D oxides are therefore prepared by thin-film deposition methods on substrates. However the fabrication of free-standing quasi-2-D oxide nanosheets, with less restrictive several monolayer thickness, has been successfully achieved by wet chemical procedures. New geometry concepts and electronic properties are observed in 2-D oxides, due to quantum confinement and interface proximity effects. Atomic geometries, electronic structure, ferroic properties and catalytic behaviour of 2-D oxides are discussed, together with promising prototypical proof-of-concept experiments for prospective applications. The edge states in oxide nanoribbons, 2-D objects of limited width, and their polarity aspects are discussed.

Surfaces of aluminum alloys are often coated with ultra-thin alumina films which form by self-limited selective oxidation. Although the presence of such films is of paramount importance in various applications, their structural and stability characteristics remain far from being known. In particular, on the NiAl(100) substrate, the observed structure has been tentatively assigned to a distorted θ-alumina polymorph, but the film stoichiometry, the nature of its surface and interface terminations, as well as the mechanisms that stabilize the θ phase remain unknown. Using a combined tight-binding/DFT genetic algorithm approach, we explicitly demonstrate that ultra-thin θ(100)-type films correspond to the structural ground state of alumina supported on the (2 × 1)-NiAl(100) substrate. Thus, experimentally observed θ-alumina films correspond to thermodynamic equilibrium, rather than being the result of kinetic effects involved in the alloy oxidation and film growth. They are favoured over other Al2O3 phases of dehydrated boehmite, pseudo-CaIrO3, γ, or bixbyite structures, which have recently been identified among the most stable free-standing ultra-thin alumina polymorphs. Moreover, our results prove that nonstoichiometry can be easily accommodated by the supported θ(100) film structure via an excess or deficiency of oxygen atoms at the very interface with the metal substrate. Dedicated DFT analysis reveals that the oxide-metal interaction at stoichiometric interfaces depends surprisingly little on the composition of the NiAl surface. Conversely, at oxygen-rich/poor interfaces, the number of additional/missing Al-O bonds is directly responsible for their relative stability. Finally the comparison between the experimental and theoretical electronic characteristics (STM and XPS) of supported θ(100)-type films provides clues on the detailed structure of the experimentally observed films.

By combining differential conductance (dI/dV) spectroscopy with a scanning tunneling microscope and hybrid density functional theory simulations we explore the electronic characteristics of the (1 × 1) and (√3 × √3)R30° terminations of the Cu2O(111) surface close to thermodynamic equilibrium. Although frequently observed experimentally, the composition and atomic structure of these two terminations remain controversial. Our results show that their measured electronic signatures, such as the conduction band onset deduced from dI/dVmeasurements, the bias-dependent appearance of surface topographic features, as well as the work function retrieved from field emission resonances unambiguously confirm their recent assignment to a (1 × 1) Cu-deficient (CuD) and a (√3 × √3)R30° nano-pyramidal reconstruction. Moreover, we demonstrate that due to a different localization of the screening charges at these Cu-deficient terminations, their electronic characteristics qualitatively differ from those of the stoichiometric (1 × 1) and O-deficient (√3 × √3) terminations often assumed in the literature. As a consequence, aside from the topographic differences recently pointed out, also their electronic characteristics may contribute to a radical change in the common perception of the Cu2O(111) surface reactivity.