Adsorption of ${\mathrm{O}}_2$ on Ag(111): Evidence of Local Oxide Formation

The atomic structure of the disordered phase formed by oxygen on Ag(111) at low coverage is determined by a combination of low-temperature scanning tunneling microscopy and density functional theory. We demonstrate that the previous assignment of the dark objects in STM to chemisorbed oxygen atoms is incorrect and incompatible with trefoil-like structures observed in atomic-resolution images in current work. In our model, each object is an oxidelike ring formed by six oxygen atoms around the vacancy in Ag(111).

Figure 1

STM image ($187\times 187{\AA }^2$, $U_s=1737\mathrm{mV}$, $I_t=0.5\mathrm{nA}$, 77 K) obtained after molecular oxygen exposure ($1\times {10}^{-1}\mathrm{Torr}\times 30\mathrm{s}$, 433 K) on Ag(111) demonstrating the trefoil-like objects on the background of the Ag(111) lattice (“atomic-resolution” state of the tip). The concentration of the objects corresponds to the coverage of 0.028 ML. The inset shows the STM frame of the same size as the main image but showing the objects as dark spots (“normal” state of the tip).

Figure 2

Atomic-resolution STM image ($30\times 30{\AA }^2$) showing two types of trefoil structures with a high magnification. Red lines indicate close-packed atomic rows of Ag(111). Fcc and hcp adsorption sites are indicated by red and blue stars, respectively.

Figure 3

(a)–(i) DFT modeling of the trefoil structures. The optimized geometry and the theoretical STM image (bias voltage of $+1.7\mathrm{V}$) are shown for each structural model. The adsorption energy is also indicated. Two favorable configurations, (d) and (e), are marked by the red rectangle.

Figure 4

The bias dependence of the experimental and theoretical STM images for the trefoil structure (model hcp-octa). (a) $U_s=+3\mathrm{V}$; (b) $U_s=+1.7\mathrm{V}$; (c) $U_s=-1.5\mathrm{V}$.