When Scanning Tunneling Microscopy Gets the Wrong Adsorption Site: H on Rh(100)

At low tunneling resistance, scanning tunneling microscopy (STM) images of a Rh(100) surface with adsorbed hydrogen reproducibly show protrusions in all bridge sites of the surface, leading to a naive interpretation of all bridge sites being occupied with H atoms. Using quantitative low-energy electron diffraction and temperature programmed desorption we find a much lower H coverage, with most H atoms in fourfold hollow sites. Density functional theory calculations show that the STM result is due to the influence of the tip, attracting the mobile H atoms into bridge sites. This demonstrates that STM images of highly mobile adsorbates can be strongly misleading and underlines the importance of additional analysis techniques.

Figure 1

Room-temperature STM images ($50\times 50{\AA }^2$) of the same area of a Rh(100) surface after exposure to 15 L of ${\mathrm{H}}_2$ at (a) 0.5 mV, 4 nA ($R_T=125\mathrm{k}\Omega$) and (b) 0.5 mV, 9 nA ($R_T=55.6\mathrm{k}\Omega$). Using the various defects and impurities as markers, one can easily determine that the protrusions in (b) are in bridge sites.

Figure 2

LEED results for a hydrogen covered Rh(100) surface at $5\times {10}^{-8}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$ ${\mathrm{H}}_2$ background pressure and room temperature (Pendrys $R$ factor for various models on the $x$ axis). The grey bar indicates the range of models compatible with the LEED data.

Figure 3

(a) Top view of ($2\times 2$) cells with different tip-hydrogen configurations considered by DFT. (b) Potential energies per H atom depending on the vacuum size (before relaxation) for all calculated models. Full symbols show the values for adsorption in hollow sites and open symbols in bridge sites; triangles depict results for seven-layer slab, circles for eight-layer slab.

Figure 4

DOS plots at $E_F\pm 50\mathrm{m}\mathrm{e}\mathrm{V}$ of a Rh(100) surface with H in bridge sites. (a) Geometry with large vacuum; (b) geometry with small gap, showing a pronounced outward relaxation of the hydrogen and an increased DOS at the H positions for small tip-sample distances.