Uniaxial structural flexibility of an anisotropic Br adlayer structure on Au(100) electrodes revealed by video-rate STM

communications materials Article A Nature Portfolio journal https://doi.org/10.1038/s43246-026-01195-w Uniaxial structural flexibility of an anisotropic Br adlayer structure on Au(100) electrodes revealed by videorate STM Check for updates 1 1234567890():,; 1234567890():,; Chaolong Yang , Falk Wendorff 2 2 2 1 , Sönke Buttenschön , Eckhard Pehlke & Olaf M. Magnussen The motion of surface species within a dense layer of coadsorbates, a common case in real-world interface systems, relies on structural flexibility of the coadsorbate phase. Here, we study by videorate scanning tunneling microscopy the lateral deformability of the pseudo-hexagonal pffiffiffi pffiffiffi cð 2 × 2 2ÞR45°-Br adlayer on Au(100). It is shown that the structural anisotropy of this adlayer phase allows strongly dynamic, uniaxial distortions via antiphase shifting of Br adsorbate rows along the [010] direction. The observed structural flexibility can be explained by rapid one-dimensional diffusion of fractional vacancies in the Br adlayer. This is supported by density functional theory calculations, which find low diffusion barriers for this process, and allows the adlayer adapting in a highly dynamic way to embedded surface species. Diffusion of adsorbates on surfaces covered by a high-coverage adlayer of another species has attracted increasing attention over the past decade due to its relevance to many real-world interface processes in industrial applications and natural environments. Such diffusion on ‘crowded’ surfaces is typical in interfacial processes occurring in ambient or high-pressure gas and liquid phases, such as heterogeneous catalysis, electrodeposition, corrosion, and organic self-assembly, where surface diffusion under complex conditions is an elementary and often determining step. It differs distinctly from the well-studied case of (tracer) surface diffusion under ultrahigh vacuum (UHV) conditions1, where adsorbed atoms or molecules diffuse at low density across a clean surface by hopping between neighboring binding sites. Although diffusion on crowded surfaces has been addressed up to now only in a rather small number of studies, a remarkable diverse range of mechanisms has been discovered that support a facile motion of adsorbates under these conditions. Previous extensive investigations of our group have addressed this phenomenon for the case of adsorbate surface diffusion at electrochemical interfaces, using in situ high-speed scanning tunneling microscopy (Video-STM)2–7. In particular, we studied the diffusion of sulfur adsorbates on Cu(100) and Ag(100) surfaces covered by a high-coverage adlayer of coadsorbed halide. In these systems, sulfur and the halide species reside both on the fourfold-hollow sites of the metal substrate and diffusion occurs on the same common lattice. According to these studies, the potential-dependent diffusion behavior depends strongly on the presence 1 and nature of the coadsorbate adlayer2–6. Density functional theory (DFT) calculations reveal different sulfur diffusion mechanisms on the halidecrowded surface for the two coadsorbates, involving a collective rotation of adsorbate/coadsorbates or an exchange via a subsurface position3. Henß et al. studied by UHV-STM the diffusion of oxygen atoms on CO-covered Ru(0001) surfaces8, where adsorbate and coadsorbate bind to different adsorption sites. Here, the oxygen atoms were observed not only to jump between the three-fold symmetrical sites within the cage formed by surrounding CO molecules but also to jump to other neighboring binding sites at surprisingly high rates. These jumps are facilitated by the natural dynamic fluctuations of the CO coadsorbate lattice, highlighting how its flexibility enables diffusion through a crowded surface. Further studies of the same system confirm that the mechanism remains valid at higher CO coverage and reveal the role of domain boundaries in facilitating the diffusion of oxygen atoms9,10. Furthermore, similar collective motion was found on Pt(111) electrodes covered by a (2 × 2)-CO adlayer, where a small number of point defects allowed the CO lattice to relax in a local (1 × 1) geometry with high quasi-collective mobility7. In all these cases, the coadsorbate layer is not rigid but structurally flexible and dynamic, which is the key reason why it affords a high mobility of embedded species even at high coverage. The above described previous studies investigated isotropic diffusion within coadsorbate adlayers with the same hexagonal or square order as the underlying substrate. However, in many adsorbate systems, especially also in anion adlayers on metal electrode surfaces, densely-packed anisotropic Institute of Experimental and Applied Physics, Kiel University, Kiel, Germany. 2Institute of Theoretical Physics and Astrophysics, Kiel University, Kiel, Germany. e-mail: Communications Materials | (2026)7:138 1 https://doi.org/10.1038/s43246-026-01195-w adlayer structures were observed11. A well-known example is adsorbed bromide on Au(100), in a 300 mV wide potential range a pseudopffiffiffi pwhere ffiffiffi hexagonal cð 2 × 2 2ÞR45 adlayer phase exists (Supplementary Fig. 1)12,13. In this phase, the Br adsorbates occupy bridge sites of the unreconstructed pffiffiffi Au(100) surface and exhibit along the [010] direction spacings of 2 d Au = 0.408 nm between nearest neighbor adsorbates; the spacing to the next-nearest pffiffiffiffiffiffiffi neighbor Br adsorbates along the [012] and [012] directions is 5=2 d Au = 0.456 nm (with Au surface atom spacing dAu = 0.289 nm). In this study, we present in situ Video-STM data and complementary DFT calculations demonstrating a highly anisotropic structural flexibility of this adlayer structure, allowing rapid uniaxial fluctuations. In particular, we find a strong interplay between these fluctuations and local boundary conditions imposed by domain boundaries in the Br adlayer steps of the Au surface, and embedded molecular adsorbates, such as the Au2Br6 surface complexes, recently reported by our group in this electrochemical system14. Results Video-STM observations Video-STM measurements at 277 K on Au(100) electrodes pffiffiffi pinffiffiffi solution containing 1 mM KBr reveal the presence of the cð 2 × 2 2ÞR45 -Br adlayer in the potential range 0.05–0.3 VSCE. Because of its anisotropic structure, rotational domains with two orientations exist, which can be clearly distinguished in the STM images, as shown in Fig. 1b (extracted from Supplementary Video S1, see the Supplementary information). As seen in this series of images, the boundary between the two domains is highly mobile and can move by several nanometers within the 0.2 s between subsequent images. Near the domain boundary, the Brad lattice becomes difficult to resolve. In particular, some atomic rows of Brad appear blurred, indicating these bromine atoms undergo rapid uniaxial positional fluctuations. A similar high mobility of domain boundaries has been reported in other electrochemical adlayer systems15. Clearer i (...truncated)