An overview is given of the advance of fabrication of two-dimensional (2D) ternary oxide layers supported on single crystal metal surfaces and the atomic level understanding of their novel structural properties. Specifically, new oxide preparation approaches for metal tungstates (MWOx), utilizing the on-surface solid-state chemical reaction between well-defined 2D binary metal oxide and tungsten oxide phases, are presented. The successful implementation of these fabrication methods is demonstrated for prototypical 2D ternary oxide layers, with different W-O coordination spheres. Structure models of 2D MWOx phases are discussed in the light of new building architecture concepts with no analogues in the bulk.

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.

Molybdenum trioxide (MoO3) is widely utilized as an interfacial layer in organic electronic devices due to its high work function and favorable energy level alignment with organic semiconductors. While its role in facilitating hole injection has been extensively studied, the microscopic mechanisms underlying charge transfer at MoO3/organic interfaces remain elusive. Here, we investigate the interaction between 2H-phthalocyanine (2H-Pc) and ultrathin MoO3 films grown on Pd(100) as a model system to explore the microscopic origin of charge transfer from the organic layer to the oxide substrate. Using a combination of scanning tunneling microscopy/spectroscopy, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure, work function measurements, and density functional theory, we find clear evidence for integer charge transfer from the molecules into the substrate, resulting in positively charged molecules in both upright and flat adsorption geometries. The electronic signatures of charging are accompanied by distinct SOMO-SUMO gaps, with upright molecules exhibiting a small gap (∼0.4 eV), while flat-lying molecules show a significantly larger gap (∼1.5 eV) owing to reduced electronic screening. These findings provide atomically resolved insight into charge transfer and highlight how adsorption geometry and local dielectric environment govern the electronic structure of hybrid interfaces.

Defects, that are irregularities in crystal lattices, create special sites or regions of particular geometry, electronic structure and activity that influence the physical and chemical behavior of the materials in a significant way. Here we examine published work on defects in two-dimensional (2D) oxide systems. The latter have attracted interest and significance in many areas of modern science and technology in recent years. The defects are classified according to their dimensionality: point defects, line defects and 2D specific topological defects are considered. Key features of the different defect types are illustrated and discussed with selected prototypical examples.

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.

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.

W- and Mo-oxides form an interesting class of materials, featuring structural complexities, stoichiometric flexibility, and versatile physical and chemical properties that render them attractive for many applications in diverse fields of nanotechnologies. In nanostructured form, novel properties and functionalities emerge as a result of quantum size and confinement effects. In this topical review, W- and Mo-oxide nanosystems are examined with particular emphasis on two-dimensional (2D) layers and small molecular-type clusters. We focus on the epitaxial growth of 2D layers on metal single crystal surfaces and investigate their novel geometries and structures by a surface science approach. The coupling between the oxide overlayer and the metal substrate surface is a decisive element in the formation of the oxide structures and interfacial strain and charge transfer are shown to determine the lowest energy structures. Atomic structure models as determined by density functional theory (DFT) simulations are reported and discussed for various interface situations, with strong and weak coupling. Free-standing (quasi-)2D oxide layers, so-called oxide nanosheets, are attracting a growing interest recently in the applied research community because of their easy synthesis via wet-chemical routes. Although they consist typically of several atomic layers thick-not always homogeneous-platelet systems, their quasi-2D character induces a number of features that make them attractive for optoelectronic, sensor or biotechnological device applications. A brief account of recently published preparation procedures of W- and Mo-oxide nanosheets and some prototypical examples of proof of concept applications are reported here. (MO3)3(M = W, Mo) clusters can be generated in the gas phase in nearly monodisperse form by a simple vacuum sublimation technique. These clusters, interesting molecular-type structures by their own account, can be deposited on a solid surface in a controlled way and be condensed into 2D W- and Mo-oxide layers; solid-state chemical reactions with pre-deposited surface oxide layers to form 2D ternary oxide compounds (tungstates, molybdates) have also been reported. The clusters have been proposed as model systems for molecular studies of reactive centres in catalytic reactions. Studies of the catalysis of (MO3)3clusters in unsupported and supported forms, using the conversion of alcohols as model reactions, are discussed. Finally, we close with a brief outlook of future perspectives.

Doping of tungsten trioxide (WO3) and molybdenum trioxide (MoO3) materials with alkali atoms, leading to the formation of the so-called sodium bronzes, is a viable approach to achieve a precise control of their electronic, optical, and magnetic properties via electron band structure engineering. Driven by the ongoing trend for thickness reduction and the resulting new functionalities at the nanoscale, using a combination of state-of-the-art experimental and computational techniques, we investigate here the interaction of two isostructural two-dimensional (2D) WO3 and MoO3 layers, grown epitaxially onto a Pd(100) surface, with Na dopants. We identify two interaction regimes as a function of the Na coverage: a low-coverage regime up to 0.3 ML, which we describe in terms of doping interactions, and a reaction regime, where at higher Na coverages, the 2D WO3/MoO3 lattices become destroyed and several ordered 2D bronze-type phases form upon thermal activation. In the doping regime, Na initially decorates the oxide domain boundaries and later adsorbs in a (2 × 2) superstructure, filling the regular adsorption sites within the oxide domains. Further Na accommodation in the 2D oxide lattice is unfavorable due to the poor lateral electrostatic screening and elastic strain increase. In the reaction regime, the most prominent and energetically stable phase is the hexagonal 2D bronze-like layer, whose atomic details are resolved in a density functional theory (DFT) analysis and compared with the structure of the bulk counterpart.

Two-dimensional (2D) WO3 nanosheets exhibit a range of novel properties and functionalities that render them attractive for advanced nanotechnologies. However, at the ultimate 2D limit of single-layer thickness, the structural properties of WO3 are unclear. Here, we fabricated, using molecular beam epitaxy techniques, a crystalline 2D WO3 overlayer on a Ag(100) surface and unveiled its geometric, electronic, and vibrational structure via a combination of state-of-the-art experimental (microscopic and spectroscopic) and computational techniques. The 2D WO3 phase forms a bilayer with a staggered arrangement of WO6 octahedra, linked together by corner- and edge-sharing, which is significantly different from the cubic and monoclinic WO3 bulk structures, but resembles a bilayer of the alpha-MoO3 layered bulk lattice. Such a 2D WO3 bilayer on Ag(100) is a robust nonpolar structure, which is incommensurate in various rotational orientations, weakly coupled to the metal substrate, and, according to the density functional theory calculations, should survive as a stable freestanding layer, that is, as a nanosheet.

Mixed CuO(2 × 1)-CuWO4 layers on a Cu(110) surface have been prepared by the on-surface reaction of the CuO(2 × 1) surface oxide with adsorbed (WO3)3 clusters. The adsorption and decomposition of methanol on these well-defined CuO-CuWO4 surfaces has been followed by high-resolution X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and temperature-programmed desorption (TPD) to assess the molecular surface species and their concentration, while the state of the surface oxide phases before and after methanol decomposition has been characterized by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and XPS. Surface methoxy species form the primary methanol decomposition products, which desorb partly by recombination as methanol at 200-300 K or decompose into CHx and possibly CO. The most reactive surfaces are mixed CuO-CuWO4 phase, with CuWO4 coverages 0.5-0.8 monolayer, thus pointing at the importance of oxide phase boundary sites. In a minority reaction channel, a small amount of formaldehyde is detected on the CuWO4 surface. The CuWO4 oxide phase becomes modified as a result of reduction and a morphology transition triggered by the methanol decomposition, but the pristine surface state can be recovered by a postoxidation treatment with oxygen.

The epitaxial growth of two-dimensional oxide layers on metal surfaces is examined in view of the 1949 van der Merwe proposition that epitaxy requires a pseudomorphic monolayer. It is argued that this limitation is relaxed in the 2-D case and that ordered oxide phases can grow out of a variety of interface scenarios, ranging from pseudomorphic to incommensurate. Prototypical examples of binary and ternary oxides supported on noble metal surfaces are presented, and the structural peculiarities of 2-D oxide phases are emphasized. The usually strong coupling at the oxide-metal interface leads to the stabilization of novel structure concepts that are not encountered in the, respective, bulk phases. The structural flexibility of 2-D lattices is discussed, and their ability to accommodate strain in generating novel 2-D oxide phases is emphasized. In the case of weakly coupled systems, it is reported that more subtle interactions at the interface can create periodic nanoscale morphologies and particular growth patterns in subsequent layers.

The structure and properties of ternary oxide materials at the nanoscale are poorly explored both on experimental and theoretical levels. With this work we demonstrate the successful on-surface synthesis of two-dimensional (2D) ternary oxide, MnWOx and FeWOx, nanolayers on a Pd(1 0 0) surface and the understanding of their new structure and phase behaviour with the help of state-of-art surface structure and spectroscopy techniques. We find that the 2D MnWOx and FeWOx phases, prepared under identical thermodynamic conditions, exhibit similar structural properties, reflecting the similarity of the bulk MnWO4 and FeWO4 phases with the wolframite structure. Structure models of prototypical 2D ternary oxide phases are proposed and discussed in the light of new structure architecture concepts which have no analogues in the bulk.

Despite the application potential of nickel tungstate (NiWO4) in heterogeneous catalysis, humidity and gas sensing, etc, its surfaces have essentially remained unexplored. In this work, NiWO4 nanoparticles and films with the wolframite structure have been grown via a solid-state reaction of (WO3)3 clusters and a NiO(100) film on a Ni(110) crystal surface and characterized by a variety of experimental techniques, including x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and x-ray diffraction (XRD), combined with ab-initio density functional theory (DFT) calculations. NiWO4 grows initially as three-dimensional (3D) crystalline nanoparticles displaying mainly two crystalline facets vicinal to the (100) surface, which merge with increasing the (WO3)3 coverage into a quasi-continuous epitaxial film. The DFT results provide an account of the energetics of NiWO4 low index surfaces and highlight the role of faceting in the stabilization of extended polar (100) terraces. These combined experimental and theoretical results show that interaction with a metal substrate and vertical confinement may stabilize oxide nano-objects with high energy facets, able to enhance their reactivity.

The exceptional physical properties of graphene have sparked tremendous interests toward two-dimensional (2D) materials with honeycomb structure. We report here the successful fabrication of 2D iron tungstate (FeWOx) layers with honeycomb geometry on a Pt(111) surface, using the solid-state reaction of (WO3)3 clusters with a FeO(111) monolayer on Pt(111). The formation process and the atomic structure of two commensurate FeWOx phases, with (2 × 2) and (6 × 6) periodicities, have been characterized experimentally by combination of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD) and understood theoretically by density functional theory (DFT) modeling. The thermodynamically most stable (2 × 2) phase has a formal FeWO3 stoichiometry and corresponds to a buckled Fe2+/W4+ layer arranged in a honeycomb lattice, terminated by oxygen atoms in Fe–W bridging positions. This 2D FeWO3 layer has a novel structure and stoichiometry and has no analogues to known bulk iron tungstate phases. It is theoretically predicted to exhibit a ferromagnetic electronic ground state with a Curie temperature of 95 K, as opposed to the antiferromagnetic behavior of bulk FeWO4 materials.

The novel method of so called “inverted fireballs” is employed to different surface processes important in carbon deposition. This new approach of PECVD offers the possibility to influence and vary the reactive plasma in a wide range. Details of the experimental setup required for a stable inverted fireball are presented and the advantages and limits of this novel method are discussed. Sputtering of carbon films, nucleation of carbon with negative biased substrate and growth of nano-diamonds are outlined together with characterizations by means of SEM, EDX, Raman, XRD and XPS. Also an ab-initio calculation of the Raman spectrum of a small diamond cluster is presented and discussed in comparison with obtained experimental results.

The atomic structures of two-dimensional oxide systems are investigated with the aim to unravel trends in their complex structural behavior and to identify the major structure-determining descriptors. Categories of oxide structures are introduced, and the important role of the various degrees of freedom of the metal-oxide interface parameters for 2-D structure formation and stability is highlighted with the help of prototypical examples.

The growth morphology and structure of iron oxide films, prepared by postoxidation of Fe monolayers on a Pd(100) surface, have been investigated in a multitechnique study, using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), high-resolution x-ray photoelectron spectroscopy (HR-XPS) and x-ray absorption spectroscopy (XAS), both using synchrotron radiation, and comprehensive density functional theory (DFT) analysis. A two-dimensional (2-D) hexagonal O–Fe–O trilayer phase has been generated at submonolayer Fe coverages, which converges into two different 2-D hexagonal Fe–O bilayer structures at one monolayer. One phase exhibits a c(8×2) coincidence structure and is associated with a stoichiometric FeO(111) bilayer. The second phase displays a superstructure of triangular loops, which is understood from DFT modeling as excess O ad-atoms in the terminating oxygen layer, thus corresponding to a FeO bilayer with a formal FeO1.125 stoichiometry. Annealing the latter in ultrahigh vacuum to 770K results in the pure c(8×2) wetting layer. The thermodynamic stability of the O–Fe–O trilayer and FeO bilayer phases is analyzed in the DFT framework and is found to be in good agreement with the experiment. The absence of a c(4×2)-Fe3O4 phase in the experimental phase diagram, which is found to be stable by DFT and is experimentally encountered for other transition metal oxide films, such as Ni-, Co-, and Mn-oxide on Pd(100), is ascribed to kinetic reasons.

A stoichiometric two-dimensional (2-D) WO3 layer has been fabricated by vapor-phase deposition of (WO3)3 clusters onto a Pd(100) surface and characterized by a combined experimental/theoretical multitechnique approach. The oxide forms a WO2 + O bilayer with a well-ordered c(2 × 2) structure, displaying at the full monolayer coverage a regular nanoscale pattern of antiphase domain boundaries, as revealed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) and rationalized by DFT analysis as a consequence of elastic strain relief. The stability of the WO2 + O bilayer is provided by polarity compensation via charge rearrangement at the WO3/Pd interface and allows for surface redox chemistry via reversible release and restoration of oxygen atoms of the tungstyl or W═O groups.

Tungsten oxide (WO3) is a key material in several applications including smart windows technology, photoelectrochemical water splitting, gas sensors and heterogeneous catalysis. In particular, tungsten oxides are important acid-base and redox catalysts, and they show excellent activity for many catalytic reactions, such as alcohol dehydrogenation, alkane hydrogenation and metathesis [1]. WO3 has been produced in single crystal form or as supported thin films with the bulk crystal structure. The latter is strongly temperature dependent, displaying a series of polymorphs, based on an idealized cubic ReO3 corner–sharing WO6 octahedral framework. Recently, the formation of an ordered two-dimensional (2D) tungsten oxide layer on Pt(111) has been reported, where however W atoms show a mixture of 5+ and 6+ oxidation states [2]. In principle, 2D oxide films may be regarded as so-called “oxide monolayer catalysts”, since they may exhibit higher catalytic activities for selective oxidation reactions than their bulk counterparts [3].

Metal-organic overlayer structures formed by 1,4-phenylene-diisocyanide (PDI) and Au adatoms on Au(111) in UHV, their stability in air, and the tip-induced Au nanoparticle formation on PDI-Au(111) surfaces in air were investigated using scanning tunneling microscopy (STM) and vibrational spectroscopy. This study reveals that the distribution of Au nanoparticles created during tip-induced release of Au atoms from molecule-Au adatom complexes shows strong dependence on the PDI coverage. Ordered Au nanoparticle arrays form in the medium-coverage regime, while more disordered distributions are observed at low and saturation coverages. The different distributions of Au nanoparticles are a direct consequence of the coverage-dependent assembly of (PDI-Au) chains, their different stability in air, and a templating effect of the Au(111) surface, which is most pronounced for medium coverage, where phases of densely packed (PDI-Au)n chains and disordered PDI-Au assemblies are confined, respectively, to the fcc and hcp regions of the (22 3) surface reconstruction of Au(111). The Au nanoparticles nucleate preferentially in the disordered or defective regions of the PDI-Au precursor overlayer, and their formation requires ambient air and high negative tip-bias, suggesting an electrochemical initiation of Au release from the molecule-Au adatom complexes.