Two-Dimensional Iron Tungstate: A Ternary Oxide Layer With Honeycomb Geometry.

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.

Nucleation and 3D growth of para-sexiphenyl nanostructures from an oriented 2D liquid layer investigated by photoemission electron microscopy.

The deposition in an ultrahigh vacuum of prototypical linear para-sexiphenyl (6P) molecules onto the anisotropic reconstructed surface of Cu(110)2 × 1-O presents an ideal system with reduced symmetry for investigation. A dynamic photoemission electron microscopy (PEEM) study of the nucleation and growth of 6P, combined with data obtained from static techniques, is shown to facilitate our understanding of the requirements for 6P nuclei formation and self-assembly into long anisotropic needles. High-rate image acquisitions in PEEM are shown to reveal dynamic phenomena, such as meta-stable layer de-wetting and nanostructure growth in real time, that are the result of nucleation and self-assembly processes. Furthermore, time dependent studies of the relaxation of the meta-stable layer give insights into the molecular diffusion kinetics, whereas temperature dependent studies allow nucleation energies and molecular binding energies to be quantitatively measured. The deposition of the first monolayer of material is found to assemble without the formation of islands until full coverage (1 ML) is achieved. The second layer fills homogeneously and remains in a liquid smectic phase until a total deposition of 1.95 ± 0.07 ML is reached, whereupon critical nuclei of 6P crystallize out of the 2D liquid layer. The maximum of the diffusion coefficient is estimated to be 2 × 10(-9) cm(2) s(-1). The resulting de-wetting of the meta-stable second layer rapidly increases the size of the nuclei while maintaining the anisotropic needle nanostructure shape. Probing the de-wetting layer reveals that 6P diffusion is 1D up to 100 °C. The nucleation energy and intermolecular binding energy are measured to be 675 meV and 2.1 eV, respectively.

Experimental and theoretical study of a surface stabilized monolayer phase of nickel oxide on Pd(100).

A surface stabilized monolayer phase of nickel oxide, c(4 x 2)-Ni(3)O(4), has been found to grow epitaxially under reactive deposition conditions on Pd(100), in the presence of other adsorbed phases and in competition with them. High-quality scanning tunneling microscopy data are reported and discussed, including a detailed analysis of the defects and of the border morphology of this new phase. The data are discussed in the light of ab initio simulations of the electronic, energetic, and geometric properties of such a phase. A hybrid-exchange density functional theory approach has been used, and a slab model is adopted where palladium is simulated by a thin film covered on both sides by regular epilayers. A growth model has been developed that explains both the unusual stoichiometry of the phase and the observed defects.