Revisiting the Structure of the $p(4\times 4)$ Surface Oxide on Ag(111)

Scanning tunneling microscopy (STM) and density-functional theory are used to reexamine the structure of the renowned $p(4\times 4)\mathrm{\text{−}}\mathrm{O}/\mathrm{Ag}(111)$ surface oxide. The accepted structural model [C. I. Carlisle et al., Phys. Rev. Lett. 84, 3899 (2000)] is incompatible with the enhanced resolution of the current STM measurements. An “${\mathrm{Ag}}_6$ model” is proposed that is more stable than its predecessor and accounts for the coexistence of the $p(4\times 4)$ and a novel $c(3\times 5\sqrt{3})\mathrm{rect}$ phase. This coexistence is an indication of the dynamic complexity of the system that until now has not been appreciated.

Figure 1

STM images and partial structural models of the $p(4\times 4)$ and $c(3\times 5\sqrt{3})\mathrm{rect}$ overlayers. (a) A $200{\AA }^2$ patch of Ag(111) covered by the $p(4\times 4)$ and $c(3\times 5\sqrt{3})\mathrm{rect}$ overlayers with inclusions of isolated $p(4\times 5\sqrt{3})\mathrm{rect}$ units (box), measured at $-0.51\mathrm{nA}$ and $-131.5\mathrm{mV}$. The inset displays a line scan along the dashed line (left) and the superposition of a $p(4\times 4)$ (green or light gray) with a $c(3\times 5\sqrt{3})\mathrm{rect}$ (black) feature (right). (b) The ${\mathrm{Ag}}_{1.83}\mathrm{O}$ model for the $p(4\times 4)$ proposed by Carlisle et al. [10]. Solid (open) large gray balls represent the overlayer (substrate) Ag atoms, small (red) balls the oxygen atoms. (c),(e) STM images of $35{\AA }^2$ patches of the $p(4\times 4)$ and $c(3\times 5\sqrt{3})\mathrm{rect}$ overlayers, respectively. (c) is at $-0.42\mathrm{nA}$ and $-21.7\mathrm{mV}$, and (e) is at $-0.40\mathrm{nA}$ and $-34.2\mathrm{mV}$. The inset in (e) (dotted lines) displays the six-atom structural element of the $c(3\times 5\sqrt{3})\mathrm{rect}$ phase measured with a tip state that allowed resolution of the central parts of the proposed ${\mathrm{Ag}}_6$ triangles ($-0.42\mathrm{nA}$, $-21.7\mathrm{mV}$). (d),(f) The distribution of the overlayer Ag atoms in the $p(4\times 4)$ and $c(3\times 5\sqrt{3})\mathrm{rect}$ phases, respectively, proposed here.

Figure 2

Structural models and Tersoff-Hamann simulated STM images for (a),(c) the $p(4\times 4)$ and (b),(d) the $c(3\times 5\sqrt{3})\mathrm{rect}$ phase. The dark (light) gray balls are Ag atoms in the overlayer (substrate), and the small dark (red) balls are O atoms. In all four images a circa $30{\AA }^2$ region of the surface is displayed.