The growth and structure of ultrathin vanadium oxide films on Rh(111) has been studied by scanning tunneling microscopy, low-energy electron diffraction, high-resolution x-ray photoelectron spectroscopy, high-resolution electron energy-loss spectroscopy, and ab initio density-functional-theory calculations. For submonolayer coverages $[\Theta <0.6\mathrm{}\mathrm{MLE}$ (monolayer equivalents)], depending on the oxide preparation route (reactive evaporation vs postoxidation), two well-ordered V-oxide phases with $(\sqrt{7}\times \sqrt{7})R19.1\mathrm{}°$ and $(\sqrt{13}\times \sqrt{13})R13.8\mathrm{}°$ structures and similar electronic and vibrational signatures have been observed. The $\sqrt{7}$ and $\sqrt{13}$ phases are interface stabilized and exhibit high formal oxidation states $\left(\sim 5^{+}\right).\mathrm{}$ In the oxide coverage range $0.6<\mathrm{}\Theta <1.2\mathrm{}\mathrm{MLE},$ i.e., after the completion of the first oxide layer, the $\sqrt{7}$ and $\sqrt{13}$ structures are replaced by several coexisting V-oxide phases, where the oxidation state of the V atoms progressively decreases from $4^{+}$ to $2^{+}$ with increasing oxide coverage. For coverages exceeding 2 MLE a bulk-type ${\mathrm{V}}_2{\mathrm{O}}_3$ phase with corundum structure grows epitaxially on the Rh(111) surface. The observed growth mode is examined by assessing kinetic and energetic effects in the ultrathin oxide film growth. The importance of the oxide-free areas of the metal support for the formation of highly oxidized V-oxide layers at the initial stages of growth is discussed.

The reduction of vanadium oxide monolayer structures on Rh(111) has been investigated by variable-temperature scanning tunneling microscopy, low energy electron diffraction, photoelectron spectroscopy of the core levels, and the valence band, and by probing the phonon spectra of the oxide structures in high-resolution electron energy loss spectroscopy. A sequence of oxide phases has been observed following the reduction from the highly oxidized $(\sqrt{7}\times \sqrt{7})\mathrm{R}19.1°$ V-oxide monolayer: $(5\times 5)$, $(5\times 3\sqrt{3})\mathrm{rect}$, $(9\times 9)$, and “wagon-wheel” oxide structures are formed with decreasing chemical potential of oxygen ${\mu }_{\mathrm{O}}$. The structures have been simulated by ab initio density functional theory, and structure models are presented. The various V-oxide structures are interrelated by common $\mathrm{V}\mathrm{O}$ coordination units, and the reduction progresses mainly via the removal of $\mathrm{V}\mathrm{O}$ vanadyl groups. All oxide structures are stable at the appropriate ${\mu }_{\mathrm{O}}$ only in the two-dimensional V-oxide/Rh(111) phase diagram and are thus stabilized by the metal-oxide interface. The results demonstrate that oxides in ultrathin layer form display modified physical and chemical properties as compared to the bulk oxides.

Exposing vanadium oxide nanoparticles on a Pd(111) surface to reducing conditions is shown to cause a spreading of the oxide over the metal until a reduced oxide phase covers the entire surface. Reoxidation reverses this process and oxide island structures and bare metal patches are reestablished. The physical origin of this wetting-dewetting process is revealed at the atomic level by in situ variable-temperature scanning tunneling microscopy and in terms of a surface oxide phase stability diagram, as calculated by density functional theory as a function of the chemical potential of oxygen and the vanadium concentration.

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 (FeWO_x) 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 FeWO_x 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.

Two-dimensional manganese oxide layers have been grown on Pd(100) and have been characterized by means of scanning tunnelling microscopy, low energy electron diffraction and x-ray photoelectron spectroscopy (XPS). The complex surface phase diagram of MnO(x) on Pd(100) is reported, where nine different novel Mn oxide phases have been detected as a function of the chemical potential of oxygen μ(O). Three regions of the chemical potential of oxygen can be identified, in which structurally related oxide phases are formed, often in coexistence at the surface. The different regions of μ(O) are reflected in the oxidation states of the respective Mn oxide nanolayers as revealed by the Mn 2p and O 1s XPS binding energies. The MnO(x) nanolayers form two-dimensional wetting layers and it is speculated that they mediate the epitaxial growth of MnO on Pd(100) by providing structurally graded interfaces.