Quasiperiodic gallium adlayer on i-Al-Pd-Mn

Using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT), we demonstrate the formation of quasicrystalline gallium adlayer on icosahedral ($i$)-Al-Pd-Mn. Quasiperiodic motifs are evident in the STM topography images, including the Ga white flower (GaWF) and $\ensuremath{\tau}$ inflated GaWF ($\ensuremath{\tau}$-GaWF), where $\ensuremath{\tau}$ is the golden mean. A larger and more complicated ring motif is also identified, comprised of a bright center and an outer ring of pentagons. The fast Fourier transform of the STM images exhibits distinct quasiperiodic spots, thereby establishing quasiperiodicity on a length scale of $\ensuremath{\sim}350\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$. Based on our DFT calculations, the preferred adsorption sites of Ga on $i$-Al-Pd-Mn are determined to be either the Mn atoms at the center of the Penrose P1 tile or the vertices of the P1 tile containing Pd atoms at the center of a cluster of five Al atoms (5-Al). The GaWF motif is modeled by an inner six-atom Ga cluster (6-Ga) around the central Mn atom and an outer ring of five Ga atoms adsorbed at the centers of the 5-Al clusters, both having pentagonal symmetry. The $\ensuremath{\tau}$-GaWF motif is modeled by the 6-Ga arranged on the $\ensuremath{\tau}$-P1 tiling, while the ring motif is modeled by Ga atoms adsorbed at the center of 5-Al clusters above a Pd atom. The side lengths and diameters of the GaWF, $\ensuremath{\tau}$-GaWF, and the ring motifs are $\ensuremath{\tau}$ scaled and show excellent agreement with the DFT-based models. An additional indication of the quasiperiodic characteristics of the Ga monolayer is the fivefold LEED patterns that were detected throughout the entire range of beam energy that was measured.