Pseudomorphic Growth of a Single Element Quasiperiodic Ultrathin Film on a Quasicrystal Substrate

An ultrathin film with a periodic interlayer spacing was grown by the deposition of Cu atoms on the fivefold surface of the icosahedral ${\mathrm{Al}}_{70}{\mathrm{Pd}}_{21}{\mathrm{Mn}}_9$ quasicrystal. For coverages from 5 to 25 monolayers, a distinctive quasiperiodic low-energy electron diffraction pattern is observed. Scanning tunneling microscopy images show that the in-plane structure comprises rows having separations of $S=4.5\pm 0.2\AA$ and $L=7.3\pm 0.3\AA$, whose ratio equals $\tau =1.618\dots$ within experimental error. The sequences of such row separations form segments of terms of the Fibonacci sequence, indicative of the formation of a pseudomorphic Cu film.

Figure 1

(a),(b) $500\times 500\mathrm{\AA }$ STM images of the fivefold surface of Al-Pd-Mn after Cu deposition at coverages of 0.09 and 3.8 ML, respectively. (c) $700\times 700\mathrm{\AA }$ STM image of the surface at a coverage of 5.5 ML. (d) The ratios of the intensities of the ${\mathrm{Cu}}_{LVV}$ (60 eV) and ${\mathrm{Al}}_{LVV}$ (68 eV) Auger peaks as a function of coverage of Cu (in monolayers).

Figure 2

(a) $400\times 400\mathrm{\AA }$ STM image of the fivefold surface of Al-Pd-Mn after deposition of 5.5 ML of Cu. (b) $100\times 100\mathrm{\AA }$ detail from (a). (c) A profile between the points marked with a cross in (b); it demonstrates that the sequence of Cu rows is quasiperiodic with spacings given by $LSLLSLSLL$, where $S=4.5\pm 0.2\mathrm{\AA }$ and $L=7.3\pm 0.3\mathrm{\AA }$. The ratio of these numbers equals the golden mean $\tau$ within experimental error. (d) LEED pattern (beam energy 50 eV) corresponding to this phase. The relationships between spot positions are indicative of $\tau$ scaling.

Figure 3

(a) $350\times 350\mathrm{\AA }$ STM image of the Al-Pd-Mn surface dosed with 8.7 ML of Cu. The correlation of the row structures across four atomic steps is indicated by the arrow. (b) $800\times 800\mathrm{\AA }$ image of the surface after adsorption of 11.7 ML. The features highlighted by arrows are discussed in the text.