Two-dimensional (2D) WO 3 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 WO are unclear. Here we fabricated, using MBE techniques, a crystalline 2D WO 3 overlayer on a Ag(100) surface and unveil its geometric, electronic and vibrational structure via a combination of state-of-the-art experimental (microscopic and spectroscopic) and computational techniques. The 2D WO 3 phase forms a bilayer with staggered arrangement of WO 6 octahedra, linked together by corner and edge sharing, which is significantly different from the cubic and monoclinic WO 3 bulk structures, but resembles a bilayer of the -MoO 3 layered bulk lattice. Such 2D WO 3 bilayer on Ag(100) is a robust non-polar structure, which is incommensurate in various rotational orientations, weakly coupled to the metal substrate, and according to the density functional theory (DFT) calculations should survive as a stable free-standing layer, i.e. as a nanosheet.
Doping of tungsten trioxide (WO 3 ) and molybdenum trioxide (MoO 3 ) 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) WO 3 and MoO 3 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 WO 3 /MoO 3 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.
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