Growth and electronic structure of alkali-metal adlayers on icosahedral ${\mathrm{Al}}_{70.5}{\mathrm{Pd}}_{21}{\mathrm{Mn}}_{8.5}$

We report x-ray photoelectron spectroscopy (XPS) study of Na and K adlayers on icosahedral ${\mathrm{Al}}_{70.5}{\mathrm{Pd}}_{21}{\mathrm{Mn}}_{8.5}$ $\left(i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}\right)$ quasicrystal. The Na $1s$ core-level exhibits a continuous linear shift of $0.8\mathrm{eV}$ towards lower binding energies (BE) with increasing coverage up to one monolayer (ML) saturation coverage. In the case of $\mathrm{K}∕i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}$, a similar linear shift in the K $2p$ spectra towards lower BE is observed. In both cases, the plasmon related loss features are observed only above 1 ML. The substrate core-level peaks, such as Al $2p$, do not exhibit any shift with the adlayer deposition up to the highest coverage. Based on these experimental observations and previous studies of alkali metal growth on metals, we conclude that below 1 ML, both Na and K form a dispersed phase on $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}$ and there is hardly any charge transfer to the substrate. The variation of the adlayer and substrate core-level intensities with coverage indicates layer by layer growth.

Figure 1

Na $1s$ core-level spectra for $\mathrm{Na}∕i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}$ as a function of coverage at $125\mathrm{K}$. The spectra have been staggered for clarity of presentation. The inset shows the shift in BE of the Na $1s$ core-level peak in an expanded scale.

Figure 2

K $2p$ core-level spectra for $\mathrm{K}∕i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}$ as a function of coverage at $125\mathrm{K}$. The inset shows the shift in BE of the K $2p_{3∕2}$ core-level peak in an expanded scale.

Figure 3

Variation in the BE as a function of coverage for (a) Na $1s$ and (b) K $2p$ for alkali metals grown on $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}$ at $125\mathrm{K}$ substrate temperature.

Figure 4

Al $2p$ core-level spectrum of $i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}$ as a function of Na coverage in ML. The inset shows the comparison of Al $2p$ peak normalized to same height for clean (0 ML) and 3 ML Na coverage.

Figure 5

Area under (a) Na $1s$ and (b) K $2p$ (filled circles) core-level peaks compared with area under Al $2p$ (unfilled circles) core-level peak for alkali metal/$i\text{−}\mathrm{Al}\text{−}\mathrm{Pd}\text{−}\mathrm{Mn}$ as a function of coverage. The data are fitted with exponential curves (dashed lines). The ratio of adlayer to substrate signal (unfilled squares) and the ratio of the two exponential curves (solid line) are also shown.