Molecular structure of the ${\text{H}}_2\text{O}$ wetting layer on Pt(111)

The molecular structure of the wetting layer of ice on Pt(111) is resolved using scanning tunneling microscopy. Two structures observed previously by diffraction techniques are imaged for coverages at or close to completion of the wetting layer. At 140 K only a $\sqrt{37}\times \sqrt{37}R25.3°$ superstructure can be established while at 130 K also a $\sqrt{39}\times \sqrt{39}R16.1°$ superstructure with slightly higher molecular density is formed. In the temperature range under concern the superstructures reversibly transform into each other by slight changes in coverage through adsorption or desorption. The superstructures exhibit a complex pattern of molecules in different geometries.

Figure 1

(Color online) (a) STM topograph of the ${\text{H}}_2\text{O}$ wetting layer on Pt(111) formed by adsorption at 140 K $\left(\sqrt{37}\times \sqrt{37}R25.3°\right)$. The superstructure is characterized by a lattice of depressions spaced by $17\text{Å}$. Shading in false colors (dark, light) indicates two rotational domains (domain boundaries marked by black and light lines). The lines in the respective shadings are parallel to rows of depressions in the appropriate domain (also indicated by parallel thin lines). The angle bisector (white line) is parallel to the step edges of Pt vacancy islands, i.e., parallel to the dense packed $⟨1\overline{1}0⟩$ direction. The inset shows the respective Fourier transform. (b) Molecularly resolved STM image of the $\sqrt{37}$ with the supercell marked by a rhombus and associated supercell vectors ${\mathbf{s}}_1,{\mathbf{s}}_2$. Triangular depressions are marked by white triangles and lattice points associated with the dark spots are indicated by black dots. Primitive translations of the lattice partly populated by the dark spots are ${\mathbf{a}}_1$ and ${\mathbf{a}}_2$. (c) STM topograph of the ${\text{H}}_2\text{O}$ wetting layer on Pt(111) formed by adsorption at 130 K $\left(\sqrt{39}\times \sqrt{39}R16.1°\right)$. The superstructure is characterized by a lattice of depressions and additional protrusions. Color coding, construction of lines, and inset as in (a). (d) Molecularly resolved STM image of the $\sqrt{39}$ showing a periodic second layer decoration (white spots). Markings as in (b). Image parameters: width $710\text{Å}$ [(a) and (c)], $81\text{Å}$ (b), $68\text{Å}$ (d), $U=0.5\text{V}$ [(a)–(c)], $U=0.2\text{V}$ (d), $I=60\text{pA}$ (a), $I=100\text{pA}$ (b), $I=160\text{pA}$ (c), $I=300\text{pA}$ (d), and width of the Fourier transforms in the insets of (a) and (c) $1.26{\text{Å}}^{-1}$.

Figure 2

(Color online) Phase transition from (a) $T=138\text{K}$, $\sqrt{39}\times \sqrt{39}R16.1°$ symmetry to (b) $T=140\text{K}$, $\sqrt{37}\times \sqrt{37}R25.3°$ symmetry within 8 min. The white dots vanish, enabling the water layer to reorganize as indicated by the change of the orientation of the superstructure. Image width $820\text{Å}$, $U=0.5\text{V}$, and $I=160\text{pA}$.

Figure 3

(Color online) (a) STM topograph of the water layer after scanning with $U=-5\text{V}$. A $\sqrt{3}$ structure is visible as well as patches of bare Pt. (b) STM topograph next to an area, where the water layer was removed by prolonged scanning at $U=-5\text{V}$. In this transition region the Pt substrate, the dissociated and the intact water layer can be distinguished (see also the respective profile indicated by the black line, black arrows mark the depressions in the water layer). (c) STM topograph in inverted contrast imaging mode (see text) of the water layer after exposure to 1 MLE CO. The triangular depressions are now imaged as triangular protrusions rotated by $30°$. (d) With the triangular protrusions as nuclei the $\sqrt{3}$ grows by prolonged scanning in the inverted contrast mode. Image parameters: width $140\text{Å}$ (a), $320\text{Å}$ (b), $310\text{Å}$ (c), $180\text{Å}$ (d), $U=0.3\text{V}$ (a), $U=0.5\text{V}$ [(b), (c), and (d)], $I=220\text{pA}$ (a), $I=190\text{pA}$ (b), and $I=150\text{pA}$ [(c) and (d)].

Figure 4

(Color online) Sketches of the molecular structure (dismissing hydrogen atoms). Left: $\sqrt{37}\times \sqrt{37}R25.3°$ supercell. The ridge phase (circles) is an ice sheet in the H-down geometry whereas the depressions are segments of a $\sqrt{3}$ structure. The unit-cell vectors of the Pt $\left({\mathbf{c}}_1,{\mathbf{c}}_2\right)$, the ${\text{H}}_2\text{O}$ lattice $\left({\mathbf{a}}_1,{\mathbf{a}}_2\right)$, and the superstructure $\left({\mathbf{s}}_1,{\mathbf{s}}_2\right)$ are indicated. Right: model of the $\sqrt{39}\times \sqrt{39}R16.1°$ supercell. Symbols as in (a), in regions where the second-layer admolecules are found are marked by light circles.