Ultrathin silver films on Ni(111)

The growth and atomic structure of ultrathin silver films on Ni(111) was investigated by low-energy electron microscopy and diffraction (LEEM/LEED) as well as intensity-voltage $\left[I\left(V\right)\right]$-LEEM in the growth temperature range between 470 and 850 K. We find that silver grows in a Stranski-Krastanov mode with a two monolayer thin wetting layer which takes on a $p\left(7\times 7\right)$ reconstruction at temperatures lower than 700 K and a $\left(\sqrt{52}\times \sqrt{52}\right)R13.9°$ reconstruction at higher temperatures. The occurrence of the two distinct reconstructions is shown to have profound implications for the growth characteristics of films exhibiting thicknesses of one and two monolayers. The nanoscale $I\left(V\right)$ characteristics of the films were analyzed by means of multiple-scattering calculations based on dynamical LEED theory. Furthermore, the vertical interatomic spacing at the interface between the Ag film and the Ni substrate was determined to $\left(2.8\pm 0.1\right)\text{Å}$ for all film thicknesses $\left(<13\text{ML}\right)$ while the uppermost silver layer relaxes by about $\left(4\pm 1\right)\mathrm{\%}$ toward the crystal.

Figure 1

Initial stages of submonolayer silver growth on Ni(111). In (a) and (b) the first silver layer (dark regions) grows on the flat terraces in single islands at 500 K. These images were taken at 2 eV electron energy and $6\mu \text{m}$ field of view (FOV). In (c) and (d), at 700 K and $10\mu \text{m}$ FOV, the silver islands (dark) nucleate at steps, which then spread across the terraces. At 850 K the silver (bright) grows preferentially along the step edges in thin lines [images (e) and (f)]. These images were taken at an electron energy of 12.4 eV with $10\mu \text{m}$ FOV. The dark round objects in (a)–(d) are insular mesas of the Ni surface. Depending on energy, the silver islands show up bright or dark in contrast.

Figure 2

Dendritical structure of the silver islands (bright) observed during growth on Ni(111) at 750 K and $30\mu \text{m}$ FOV in situ as visible in LEEM.

Figure 3

LEEM images of multilayer silver growth on Ni(111) at temperatures of 470, 600, 770, and 850 K. An increase in uniformity of silver film thickness is visible with increasing temperature. Local film thicknesses: (a) 2 ML, (b) 5 ML, (c) 6 ML, (d) 7 ML, (e) 9 ML, (f) 10 ML, and (g) 12 ML.

Figure 4

(Color online) Dewetting of the third silver layer. In (a) the growth of the third silver layer (dark) on the two monolayer silver film (bright) is mostly completed. At this point dewetting (b) sets in, resulting in three-dimensional island growth (islands labeled by 1, 2, and 3). At (c) the third layer is mostly gone and, in addition, island 2 shrinks to cover the thicker islands 1 and 3.

Figure 5

The LEED images (a)–(e) were taken after preparation of silver films of about 2 ML thickness at different growth temperatures. The dashed lines connect the ${\left(\overline{1}1\right)}_{\text{Ni}}$ and the ${\left(1\overline{1}\right)}_{\text{Ni}}$ reflexes. In (f) a schematic LEED image with the relevant reflections, the rotation angle $\alpha$ of the silver film relatively to the nickel substrate and the reconstruction angle $\theta$ according to the reconstruction nomenclature $\left(x\times y\right)R\theta$ is shown.

Figure 6

(Color online) The LEED image taken at 47.8 eV electron energy in (a) shows the moiré pattern of the superlattice structure around the (00) spot. (b) Bright-field LEEM image employing the (00) diffraction peak for imaging. By tilting the incident electron beam, also distinct superlattice spots can be selected by the contrast aperture for imaging. The corresponding LEEM images are presented in (c) (dark field, first domain), and (d) (dark field, second domain). Sample prepared at about 800 K.

Figure 7

LEEM image at $15\mu \text{m}$ field of view and 12.4 eV electron energy with local $I\left(V\right)$ spectrum of the clean Ni(111) surface. The dark round objects in the LEEM image are insular elevations on the Ni crystal.

Figure 8

Experimental $I\left(V\right)$ characteristics and theoretical calculations of thin silver films of $n=1$ to $n=10$ Ag layer thickness. The $R_2$ factors are calculated over the whole region shown here. For $n=5$ $\left({}^{\ast }\right)$ the $R_2$ factor has been determined for the range from 2 to 15 eV due to discontinuities in the experimental data set.

Figure 9

(Color online) The coherent Ag coverage over time has been extracted from the LEEM growth movies at constant electron energy and image focus. While the first monolayer grows, the apparent deposition rate decreases with increasing silver coverage due to strain effects.

Figure 10

Left: valence-band PEEM image $\left(E=13.5\text{eV}\right)$ taken after Ag growth at 850 K. Right: local valence-band spectra at regions indicated in PEEM image.

Figure 11

(Color online) (a) Composite LEEM image $\left(E=22.6\text{eV}\right)$ consisting of two dark-field LEEM images (blue and green) using superstructure spots as in Fig. 6 revealing the lateral distribution of rotational domains after Ag submonolayer growth at 750 K. (b) Bright-field LEEM image $\left(E=22.6\text{eV}\right)$ of the same area.