Paper SS+AS-TuA2
W-oxide on Ag(100): a Flexible Decoupled 2-D Oxide Layer

Tuesday, November 8, 2016, 2:40 pm, Room 104E

Two-dimensional (2-D) transition metal oxide layers have attracted significant interest during the past decade due to their novel emergent properties and high potential for nanotechnology applications [1,2]. For practical reasons 2-D oxide layers are usually supported on metal surfaces. This leads to a coupling of the oxide overlayer to the metal substrate, thus creating a hybrid system with properties largely determined by the oxide-metal interface. Here, we report the formation of a 2-D W-oxide layer on a Ag(100) surface, where the oxide appears to be essentially decoupled from the substrate. The W-oxide has been prepared by vapor phase deposition of (WO₃)₃ clusters at 500°C substrate temperature. The WO_x grows as a well-ordered incommensurate 2-D wetting layer in large domains with variable orientation with respect to the substrate. This gives rise to a variety of oxide domains with different azimuthal orientation, which can easily be recognized in the STM by their different Moiré patterns. The overlayer lattice can be imaged with atomic resolution in the STM and analyzed using the Moiré formula, from which the square overlayer lattice constant can be accurately evaluated to a = 3.72 Å; this is close to the respective WO₃ bulk lattice constant. AES and XPS spectra indicate an overlayer stoichiometry close to WO₃, but the W 4f binding energy suggests a lower oxidation state than W⁶⁺. A structure model in terms of a 2-D WO_x sheet is discussed. It is conjectured that this WO_x sheet on Ag(100) behaves essentially like an isolated 2-D oxide layer.

[1] G. Pacchioni, Two-dimensional oxides: multifunctional materials for advanced technologies. Chem. Eur. J. 18(2012) 10144

[2] Oxide materials at the two-dimensional limit. F.P. Netzer, A. Fortunelli, Eds. (Springer Series in Materials Science, April 2016)