Superlattice structure of an Ar monolayer on Ag(111) observed by low-temperature scanning tunneling microscopy

Scanning tunneling microscope (STM) images of a carefully prepared monolayer of Ar on Ag(111) reveal patterns of bright spots, which exhibit perfect ordering over large areas. The dimensions of the slightly oblique unit cell are $1137\times 2507{\text{pm}}^2$, where the short side is aligned along the $\left[-110\right]$ high-symmetry direction of the substrate, and the long side shows a regular division into two parts. In total six domains are observed. Analysis of this pattern, taking into account the tendency of noble gas overlayers to form hexagonal structures, which might be slightly stretched or compressed, leads to a structure for the Ar overlayer, which reproduces all the observed features. Surprisingly, it turns out that the bright STM spots correspond to Ag atoms which are not directly covered by Ar but surrounded by three Ar atoms from the adsorbate layer. A possible electronic origin for this unusual appearance is discussed.

Figure 1

(a)–(c) STM images of Ar monolayer on Ag(111) at 5 K. The Ar superstructure unit cells of the observed bright spot row structure are indicated. The short side $d_1$ of the unit cell is aligned along a high-symmetry $\left[-110\right]$ axis of the Ag(111) surface; experimental values: $d_1=1137\text{pm}$, $d_2=2507\text{pm}$, and $u=1039\text{pm}$. (a), (b) $5\times 5{\text{nm}}^2$, $V_{\text{S}}=50\text{mV}$, $I_{\text{T}}=50\text{pA}$. (a) Notice the slight misorientation of the large unit-cell side $d_2$ of the Ar superstructure against the $\left[-1-12\right]$ direction yielding ${\alpha }_{\text{exper}}\approx 84°$. (b) A domain (reflection domain) symmetric with respect to $\left[-1-12\right]$ as compared to (a). (c) Large area image of the row structure showing another domain rotated by $60°$ against the domain in (a), $25\times 25{\text{nm}}^2$, $V_{\text{S}}=50\text{mV}$, $I_{\text{T}}=10\text{pA}$. No domain boundaries have been found on terraces. Image (d) shows an atomically resolved $5\times 5{\text{nm}}^2$ area of the clean Ag(111) surface ($V_{\text{S}}=20\text{mV}$, $I_{\text{T}}=1\text{nA}$), which could be used to determine the exact orientation of the Ar unit cell against the substrate. Relevant crystallographic directions as well as the substrate unit cell are indicated.

Figure 2

(Color online) Model of the adsorbed Ar monolayer (yellow/light gray) on Ag(111) (red/gray). The dashed circles indicate Ag atoms that are uncovered but surrounded by three Ar atoms. The Ag layer (red/gray) is a perfect hexagonal structure with a lattice constant of 289 pm and angles of $60°$. For the modeled Ar monolayer a stretching by a few percent of the nearest-neighbor distances as compared to the bulk value is found: $c_1=385.3\text{pm}$ and $c_2=412.9\text{pm}$. The angle between directions 1 and 2 is $60°$, and between 1 and 3 is $116.6°$. Thus, the directions 1 and 2 of the Ar monolayer are coincident with the closed packed directions of the Ag lattice beneath. Direction 3 is rotated by $\sim 3.4°$ with respect to the Ag lattice. Angle ${\alpha }_{\text{model}}=83.4°$ (see text).