As a new family of quasicrystals (QCs), ultra-thin oxide quasicrystal films on Pt(111) were discovered by Förster in 2013 [1]. Identifying the geometrical structure is an inevitable challenge not only for fundamental studies of this material’s unique chemical and physical properties, but also for industrial applications. In general, QC structures can be expressed as cluster tiling. The clusters are arranged quasi-periodically for a QC, while clusters are arranged periodically for a crystalline approximant. Structural analysis of the clusters in a crystalline approximant is the first step in determining the QC geometrical structure, as it is easier to perform density functional theory simulations and diffraction experiments because of the periodicity of the approximant. Recently, QCs, crystalline approximant, and QC related structure have been named as “hypermaterial”.
In this work, we report the growth of well-ordered, large-area, ultra-thin Ba-Ti-O films on Pt(111), changing from the oxide crystalline approximant (OCA) phase into the oxide quasicrystal (OQC) phase and a (2×2) periodic structure on annealing. We examined the elemental atomic density of Ba, Ti, and O by scanning tunneling microscopy (STM), low-energy electron diffraction, Auger electron spectroscopy, and X-ray photoelectron spectroscopy (XPS). For quantitative analysis, Rutherford backscattering spectroscopy was employed to obtain calibration XPS intensity curves as a function of elemental atomic density. STM images reveal a wide-scale ultra-thin OQC film without BaO islands, and magnified STM images display typical OQC clusters. The ultra-thin OQC film has been prepared on annealing an oxide crystalline approximant with decreasing Ti concentration. We also found that the ultra-thin Ba-Ti-O film forms a (2×2) superstructure on annealing the ultra-thin OQC film. For quantitative analysis, the XPS peak intensity was measured as a function of elemental atomic density. The elemental atomic densities for the ultra-thin OQC film were determined to be 8 × 1014, 4 × 1014, and 3 × 1014 atoms/cm2 for Ba, Ti, and O, respectively [2]. The structural models of OQC and OCA ultrathin Ba-Ti-O films have also been proposed [3].
Recently, the growth of ultra-thin Ce-Ti-O and Yb-Ti-O films on Pt(111) has also been studied in order to explore the rare earth oxide hypermaterials [4]. In both cases, OQC-related structures have been prepared. These results will also be shown in the talk.
REFERENCES
[1] S. Förster, K. Meinel, R. Hammer, M. Trautmann, W. Widdra, Nature 2013, 502, 215.
[2] J. Yuhara, K. Horiba, R. Sugiura, X. Li, T. Yamada, Phys. Rev. Mater. 2020, 4, 103402.
[3] X. Li, K. Horiba, R. Sugiura, T. Yamada, J. Yuhara, Appl. Surf. Sci. 2021, 561, 150099.
[4] X. Li et al., Phys. Chem. Chem. Phys. (2023) (in press).
