Morphology of ultrathin lithium fluoride deposited on Ag(100): Dendrites versus islands

We have investigated lithium fluoride (LiF) in the monolayer range deposited by thermal sublimation onto the (100) surface of a silver single crystal in ultrahigh vacuum. Our study combines scanning tunneling microscopy, low-energy electron diffraction, and polarized x-ray absorption spectroscopy. The results reveal that LiF grows epitaxially on Ag(100), with a morphology strongly depending on the growth temperature: at room temperature, dendritic patterns are formed with branches along the [011] and $\left[0\overline{1}1\right]$ directions, exhibiting the same lattice orientation as the substrate. Conversely, at a substrate temperature of 500 K LiF forms square islands exhibiting a moiré pattern, with the islands’ borders oriented along the [001] and [010] directions. While the lattice constant of the surface deposited samples is close to the one of bulk fcc LiF, the appearance of a strong x-ray linear dichroism for dendritic LiF points to a tetragonal distortion arising from the LiF-substrate interaction. The less prominent dichroism and the presence of the moiré pattern on the square islands both suggest the relaxation of the LiF grown on the hot Ag substrate.

Figure 1

Constant-current $400\times 400\text{−}\mathrm{n}{\mathrm{m}}^2$ STM images of LiF/Ag(100) at different coverages deposited at room temperature, and corresponding LEED patterns at $E_{\mathrm{beam}}=60\mathrm{eV}$. (a), (b) $V_{\mathrm{b}}=+1\mathrm{V}, I_s=5\mathrm{pA}$; (c) $V_{\mathrm{b}}=+4.5\mathrm{V}, I_s=1\mathrm{pA}$; (d) $V_{\mathrm{b}}=+4.2\mathrm{V}, I_s=1\mathrm{pA}$. (e) Line profiles extracted along the blue and green lines in panels (b) and (c), respectively; (f) Fourier transforms extracted from panels (b) and (c).

Figure 2

Constant-current $400\times 400\text{−}\mathrm{n}{\mathrm{m}}^2$ STM images of LiF/Ag(100) at different coverages deposited at 500 K, and corresponding LEED patterns at $E_{\mathrm{beam}}=60\mathrm{eV}$. (a) $V_{\mathrm{b}}=+5.8\mathrm{V}, I_s=10\mathrm{pA}$; (b)–(d) $V_{\mathrm{b}}=+4.8\mathrm{V}, I_s=10\mathrm{pA}$. (e) Line profile extracted along the blue line in panel (b); (f) $120\times 120\text{−}\mathrm{n}{\mathrm{m}}^2$ zoom of a square LiF island, extracted from panel (b), and its Fourier transform. The gray square has a size of 13.9 nm and highlights the primitive cell of the modulation pattern.

Figure 3

(a) Normalized x-ray absorption spectra recorded on different coverages of LiF/Ag(100) at the fluorine $K$ edge as indicated in the plots. (b) Differences between the normalized absorption spectra of LiF powder and the XAS of LiF grown on Ag(100). (c) X-ray linear dichroism $({\mu }_{\mathrm{v}}–{\mu }_{\mathrm{h}})$. The spectra are vertically offset for better visibility.

Figure 4

Microscopic model of (a) the dendrites and (b) the square islands of LiF/Ag(100). The LiF lattice has the same orientation as the one of the substrate in both cases, with a partial relaxation of LiF in (b) (not visible). The relevant crystallographic directions are indicated along with the primitive unit cells of Ag (red) and LiF (blue). The orange arrow in (a) indicates the preferred direction of the surface diffusion of LiF units at room temperature. Color code: gray, Ag; purple, ${\mathrm{F}}^{–}$; green, ${\mathrm{Li}}^{+}$ [38].