Host-atom-driven transformation of a honeycomb oxide into a dodecagonal quasicrystal

Abstract

Dodecagonal oxide quasicrystals (OQCs) have so far been limited to a few elemental systems, with no general formation mechanism established. Here, we demonstrate a versatile approach to OQC formation via a host-atom-induced transformation of a metal-oxide honeycomb (HC) network. Adsorption of Ba, Sr, or Eu onto the HC layer triggers its reorganization into a dodecagonal tiling, as revealed by low-energy electron diffraction and scanning tunneling microscopy. Full conversion occurs when 73% of the honeycomb rings are occupied. Kelvin probe and UV photoelectron spectroscopy show a linear decrease in work function with increasing host coverage, followed by a sharp increase upon quasicrystal formation due to reduced host dipoles. This transformation mechanism enables the fabrication of structurally precise OQCs, including a new Eu-Ti-O phase that extends the field to lanthanide quasicrystals, forming a 2D grid of localized magnetic moments. The method offers a general route to explore lattice-matched substrates for epitaxial growth and may be adapted to other 2D honeycomb materials - such as graphene, hexagonal ice, and silica - paving the way for engineered aperiodic systems beyond transition metal oxides.

Figures (5)

• Figure 1: (a-c) Schematic model of a metal-oxide honeycomb (HC) structure (blue and red circles) with increasing atom decoration (green circles) with a HC ring coverages of 33;50;67. The host atoms considered here are Ba, Sr, or Eu. The atoms are set to scale in proportion to the atomic radii of O, Ti, and Ba for six-fold coordination Mueller_anorganische_2008. A ($\sqrt3\times\sqrt3)\textit{R30°}$ superstructure arises from (a) the occupied and (c) the empty HC pores. (b) Labyrinth phase formed at 50%. (d) Schematic model of the dodecagonal structure in which the host atoms decorate the vertices of a square-triangle-rhombus tiling. These host atoms are embedded in a Ti$_n$O$_n$ rings network with n=4, 7, and 10, in which 73% of the rings are occupied.
• Figure 2: (a) The decrease in the ring coverage-dependent workfunction of a titanium oxide HC layer grown on Pt(111) and Pd(111) substrates upon deposition of Ba (black squares), Eu (red circles) and Sr(blue triangles). The dashed line depicts the linear model describing the workfunction decrease for the Eu-Pd(111) series. The black arrow indicates the workfunction increase induced by the transformation of the Ba-decorated HC to the OQC network. Diffraction patterns of (b) the pristine HC, (c) the Ba-decorated HC before and (d) after conversion to the OQC.
• Figure 3: LEED (top) and STM data (bottom) of an Eu-covered Ti-O honeycomb layer on Pd(111) at ring coverages of (a) 33%, (b) 46%, (c) 53% and (d) 73%. The six intense outer spots in (a-c) correspond to the HC structure. The additional spots originate from a ($\sqrt3\times\sqrt3$)R30° superstructure at 33;67 coverage (Fig. \ref{['fig:Models']}(a,c)). (d) At 73% ring coverage, the network transforms to a dodecagonal square-triangle-rhombus tiling. The inset shows the FT of atom positions extracted from the STM image. STM images are 15$\times$15 nm² each. (a) 1V, 1nA. (b) -2V, 0.1nA. (c) 1V, 0.3nA. (d) 0.4V, 1nA.
• Figure 4: Eu3d$_{5/2}$ XPS data recorded for the OQC in Eu-Ti-O/Pd(111). The predominant oxidation state of Eu is +2. Hence Eu resides in a high-spin state in the aperiodic network.
• Figure 5: LEED (top) and STM data (bottom) for a Sr covered Ti$_2$O$_3$ honeycomb layer on Pd(111) at (a) 33%, (b) 46% and (c) 73% coverage. The six intense outer spots at the lowest coverage originate from the HC layer. The additional ($\sqrt3\times\sqrt3)\textit{R30°}$ superstructure spots relate to the Sr ions decorating 1/3 of the HC pores. (b) For higher coverage, a coexistence of the ($\sqrt3\times\sqrt3)\textit{R30°}$ decorated HC structure and the OQC is observed. (c) At 0.73 ring coverage, the HC almost completely disappears due to the conversion to the dodecagonal oxide quasicrystal. The inset shows the FT of the atom positions extracted from the STM image. STM images 15$\times$15 nm² each. (a) 1.5V, 0.3nA. (b) -0.4V, 0.3nA. (c) -0.4V, 0.7nA.