Experimental and ab initio study of the structural and electronic properties of subnanometer thick Ag films on Pd(111)

We present a combined, experimental, and computational investigation of the growth mode and the valence-band structure of $\mathrm{Ag}∕\mathrm{Pd}\left(111\right)$, with the focus on the Ag $4d$ derived quantum well states. Low-energy electron diffraction and scanning-tunneling microscopy are used to determine epitaxial, layer-by-layer growth of silver on the palladium substrate. High-resolution (in both energy and angle) photoelectron spectra and ab initio density-functional band-structure calculations are compared for 1 and 2 ML silver films along the $\overline{\Gamma }-{\overline{M}}^{\prime }$ high symmetry line of the surface Brillouin zone. The observed $d$-derived electronic states and their dispersion are explained in terms of quantum well states. The interaction of the silver $4d$ electronic states with the palladium substrate is discussed.

Figure 1

Photoemission intensity gray-scale map of Pd(111) along $\overline{\Gamma }-{\overline{M}}^{\prime }$ high symmetry line. For each 5° of the polar angle, a corresponding energy distribution curve (EDC) is shown as well. Photon energy: He I line at $21.2\mathrm{eV}$.

Figure 2

Photoemission spectra of Pd(111) taken at a 35° polar angle (a) immediately after the cleaning procedure, (b) $15\min$ later, and (c) $30\min$ later.

Figure 3

STM images of Pd(111) covered with different amounts of silver. All images represent area of $116\times 116{\mathrm{nm}}^2$, obtained using bias voltage $U=0.5\mathrm{V}$ and tunneling current $\mathrm{I}=0.2\mathrm{nA}$. Images correspond to (a) clean Pd(111), (b) 0.25 ML Ag, (c) 0.75 ML Ag, (d) 1 ML Ag, (e) 1.25 ML Ag, (f) 1.5 ML Ag, (g) 2 ML Ag, and (h) 3 ML Ag.

Figure 4

(a) NE spectra of Pd(111) covered with silver films of different thickness (film thicknesses are indicated in the figure). The peaks associated with electron states in 1 and 2 ML films are indicated by letters $(a–k)$ in accordance with the spectra shown in Fig. 5b. (b) Same as (a) but only for submonolayer silver coverages: 0, 0.15, 0.30, 0.50, 0.70, and 1.0 ML.

Figure 5

Photoemission intensity gray-scale map of 1 ML (a,c) and 2 ML (b,d) of Ag on Pd(111) measured along $\overline{\Gamma }-{\overline{M}}^{\prime }$ high symmetry line. The spectra are taken with He I $\left(21.2\mathrm{eV}\right)$ (a,b) and Ne I $\left(16.8\mathrm{eV}\right)$ (c,d) excitation energy. For each 5° of the polar angle, a corresponding energy distribution curve (EDC) is shown as well.

Figure 6

(a) Calculated electron structure of bulk palladium: (a) Along $k_{111}$ direction and (b) projected onto (111) surface Brillouin zone, for several values of $k_{111}$.

Figure 7

Electronic eigenstates of unsupported single (solid line) and double (dashed line) hexagonal layer of Ag atoms, along the high-symmetry directions in the two-dimensional Brillouin zone. The symmetry of 1 ML states at the $\overline{\Gamma }$ point is indicated.

Figure 8

Electronic eigenstates of a (a) single and (b) double layer of Ag atoms adsorbed on a Pd(111) surface, along the high-symmetry directions in the surface Brillouin zone. The thickness of the lines shows the degree of localization of the states near the surface.

Figure 9

Structural model of $\mathrm{Ag}∕\mathrm{Pd}\left(111\right)$ for the nominal coverage of 2 ML. Schematics indicates the growth of the third silver layer before the second one is saturated.