When Surface Dynamics Fakes Symmetry -- Oxygen on Rh(100) Revisited

Abstract

Heating a long-range ordered adsorbate phase beyond its stability temperature does not necessarily result in a disordered phase, it can also break up into heavily fluctuating ordered domains. Temporal and/or spatial averaging over these fluctuations may give the impression of both a wrong periodicity and a false local symmetry. This can happen even below liquid-nitrogen temperature, so that the true nature of the phase might remain undetected. We demonstrate this scenario at the catalytically active Rh(100) surface covered by 1/2 monolayer of oxygen, using quantitative low energy electron diffraction (LEED), variable-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). Using the example of CO adsorption, we show that local symmetry can have a decisive influence on the binding energy and thus the chemical reactivity.

Figures (3)

• Figure 1: (a) LEED pattern of the Rh(100) surface covered with 0.5 ML oxygen taken at $100\,\textrm{K}$. The pattern suggests a (2$\times$2) unit cell (blue) with two orthogonal glide planes that cause extinction of the $(m+1/2,~ 0)$ and $(0,~n+1/2)$ beams (the position of one is marked by a yellow circle). (b) Top view of the (2$\times$2)-2O structure resulting from the LEED analysis of Ref. Baraldi1999. This model possesses one glide plane (yellow) only. (c) and (d) Two proposed variants of the model (b) that enlarge the unit cell to a Rh(100)-$(2\sqrt{2} \times 2\sqrt{2})R45^\circ$-4O structure (black). The structures are obtained from (b) by letting the lower right (c) and also the upper right (d) oxygen atoms of the "$2\sqrt{2}$" cell hop to an equivalent site of the reconstructed Rh(100) surface. In all models (b)--(d) half of the Rh atoms in the top-layer are singly coordinated with oxygen (colored light green) and the other half are doubly coordinated (dark green). The arrows in (b--d) indicate the lateral relaxation pattern of the top layer Rh atoms.
• Figure 2: STM images (3.6 nm$\times$3.6 nm; $\pm20$ mV; 1.5 nA) of the 0.5 ML oxygen phase on Rh(100) taken at 6 K (a,b) and 78 K (c) with the apparent translational symmetry indicated by yellow squares. (a) Single domain of the $2\sqrt{2}$ phase together with the STM simulation (colored online, framed by a white line) of the model depicted in Fig. \ref{['LEED_Models']}(d). (b) A domain boundary between two mirror domains, where oxygen atoms are arranged according to the model Fig. \ref{['LEED_Models']}(c). (c) Apparent (2$\times$2)-periodicity with two glide symmetry planes (dashed yellow lines) with some $2\sqrt{2}$-like residues in the lower part. Overlayed (colored online, framed by a white line) is an STM simulation for quasi-simultaneous site occupation. (d) DFT calculation of the energy barrier between the two relaxed structures according to figures \ref{['LEED_Models']}(c) and \ref{['LEED_Models']}(d) that differ in the position of one oxygen atom. The blue solid line is a guide to the eye.
• Figure 3: (a) At 100 K, LEED will detect a momentary domain configuration as schematically shown. The gray $2\sqrt{2}$ areas are filled by the domain boundary structure Fig. \ref{['LEED_Models']}(c). The coherent superposition of the domains in (a) will produce a LEED pattern corresponding to the effective surface structure displayed in (b). This structure has a (2$\times$2) unit cell (blue square) with two orthogonal glide planes (green dashed lines).