Ronan McGrath

Due to their diverse adsorption sites with distinct electronic, chemical, and geometrical properties, quasicrystal surfaces serve as a platform for growing novel overlayer structures. Here, we present the growth of C 60 molecules on a twofold surface of the decagonal Al-Ni-Co quasicrystal, which possesses both periodic and quasiperiodic order. Scanning tunneling microscopy (STM) reveals that the deposited C 60 preferentially grows along the periodic direction, producing chains of C 60 molecules arranged in a Fibonacci sequence, a representative of one-dimensional quasicrystalline order. By comparing STM images at submonolayer coverage with the substrate model structure, we identified that C 60 molecules preferentially adsorb at Ni/Co atomic sites matching the geometry of C 60 molecules. The C 60 molecules experience two competing processes: substrate-molecule interactions and molecule-molecule van der Waals interactions, resulting in chains of limited length. These findings provide insights into tailoring molecular assemblies on quasicrystalline templates, with potential applications in nanostructured materials and molecular electronics.

Intermetallic compounds have shown promising catalytic properties, exhibiting better selectivity and thermal stability than their metallic counterparts. Understanding their surface properties is crucial for optimizing catalytic performance. Here, we present an investigation of the atomic structure, chemical composition, and oxidation behavior of the previously unexplored binary intermetallic catalyst, Ni 2 In 3 , using various complementary techniques, including low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and low-energy ion scattering (LEIS), which provides elemental information from the topmost surface layer. We also calculated the surface energy of several possible bulk-terminated surfaces by density functional theory (DFT) to investigate surface stability. The Ni 2 In 3 (100) surface was prepared by ion bombardment and annealing under ultrahigh vacuum conditions. We find that ion bombardment preferentially removes indium (In), while annealing promotes In segregation to the surface. After annealing, the surface exhibits a ( 1 × 1 ) LEED pattern. The surface develops a three-domain c ( 2 3 × 4 ) rectangular structure upon exposure to ambient hydrogen present in the experimental chamber or hydrogen dosed from an external source, as evidenced by both STM and LEED. Both nickel (Ni) and In oxidize upon exposure to molecular oxygen. Based on STM and LEIS results along with surface energy calculations, we reveal that the surface planes contain In atoms bonded with Ni, the catalytically active constituent, suggesting that the surface may be stabilized by intermetallic bonding.

The intermetallic compound ZnPd has been found to have desirable characteristics as a catalyst for the steam reforming of methanol. The understanding of the surface structure of ZnPd is important to optimize its catalytic behavior. However, due to the lack of bulk single-crystal samples and the complexity of characterizing surface properties in the available polycrystalline samples using common experimental techniques, all previous surface science studies of this compound have been performed on surface alloy samples formed through thin-film deposition. In this study, we present findings on the chemical and atomic structure of the surfaces of bulk polycrystalline ZnPd studied by a variety of complementary experimental techniques, including scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), low energy electron microscopy (LEEM), photoemission electron microscopy (PEEM), and microspot low-energy electron diffraction ( μ -LEED). These experimental techniques, combined with density functional theory (DFT)-based thermodynamic calculations of surface free energy and detachment kinetics at the step edges, confirm that surfaces terminated by atomic layers composed of both Zn and Pd atoms are more stable than those terminated by only Zn or Pd layers. DFT calculations also demonstrate that the primary contribution to the tunneling current arises from Pd atoms, in agreement with the STM results. The formation of intermetallics at surfaces may contribute to the superior catalyst properties of ZnPd over Zn or Pd elemental counterparts. Published by the American Physical Society 2024

We outline an approach to fabricate nanoscale artificial quasiperiodic tilings with thermal‐scanning probe lithography. Quasiperiodic tilings such as the Ammann‐Beenker, Square Fibonacci tiling, and Penrose are fabricated and imaged with thermal‐conductance feedback microscopy, followed by electron microscopy. The design implementation, chemical, and physical challenges involved in fabricating such artificial systems using nanolithography are discussed. Additionally, the potential applications of fabricated quasiperiodic tilings are explored.

The growth of antimony (Sb) thin films on the fivefold surface of icosahedral Ag−In−Yb quasicrystal has been studied by scanning tunneling microscopy (STM) and x‐ray photoelectron spectroscopy (XPS). At low coverage, the deposited Sb yields a network of pentagons of different sizes and heights. These Sb pentagons can be mapped by a pentagonal tiling of the substrate and thus exhibit quasicrystalline long‐range order. Subsequent deposition of Sb yields a disordered film. XPS observations of the growth mode are consistent with the STM results.

We present a study of the adsorption of pentacene (Pn) molecules on the high symmetry (fivefold, threefold, and twofold) surfaces of the icosahedral (i) Ag–In–Yb quasicrystal. We also compare the results with adsorption of Pn on a surface of a periodic crystal related to this quasicrystal, the (111) surface of the Au–Al–Tb 1/1 approximant. Scanning tunneling microscopy reveals that Pn molecules on the quasicrystal surfaces are aligned along the high symmetry directions of the substrates and selectively adsorb on Yb atoms and thus exhibit quasicrystalline order. Pn molecules on the Au–Al–Tb approximant surface also preferably adopt Tb sites. The behavior of selective adsorption can be understood in terms of the geometry and electronic properties of the adsorbate and substrate. The Yb–Yb (Tb–Tb) separations are comparable to the C–C or H–H distances in a Pn molecule. Pn is an electron donor, whereas the unoccupied electronic states of the substrate are dominated by the rare earth atoms, suggesting that there is an electronic transfer between the Pn molecules and Yb (Tb) atoms.

Novel epitaxial quasicrystalline films can be grown using the surfaces of intermetallic quasicrystals as templates. Here, we present a study of Pb adsorption on the threefold i-Ag-In-Yb surface, where Pb grows in a manner contrasting with conventional thin-film growth modes. Pb atoms are found to adsorb at sites over a range of heights, which are explained by bulk atomic positions left vacant by surface truncation, producing three-dimensional, isolated quasicrystalline Pb structures. This finding is contrasted with the growth of Pb on the more commonly used fivefold surface of the same quasicrystal, where smooth epitaxial layers result. We suggest that this unique structure originates due to the lower atomic density of the threefold surface, compared to the fivefold surface. Similar atomic density can be found in lower symmetry planes of periodic systems, but these planes are often unstable and become facetted. This stable low-density quasicrystalline substrate provides a facile route to achieve this type of templated growth.

Quasicrystals differ from conventional crystals and amorphous materials in that they possess long-range order without periodicity. They exhibit orders of rotational symmetry which are forbidden in periodic crystals, such as five-, ten-, and twelve-fold, and their structures can be described with complex aperiodic tilings such as Penrose tilings and Stampfli–Gaehler tilings. Previous theoretical work explored the structure and properties of a hypothetical four-fold symmetric quasicrystal—the so-called Fibonacci square grid. Here, we show an experimental realisation of the Fibonacci square grid structure in a molecular overlayer. Scanning tunnelling microscopy reveals that fullerenes (C₆₀) deposited on the two-fold surface of an icosahedral Al–Pd–Mn quasicrystal selectively adsorb atop Mn atoms, forming a Fibonacci square grid. The site-specific adsorption behaviour offers the potential to generate relatively simple quasicrystalline overlayer structures with tunable physical properties and demonstrates the use of molecules as a surface chemical probe to identify atomic species on similar metallic alloy surfaces.