Decagonal Sn clathrate on $d$-Al-Ni-Co

TL;DR

This study demonstrates the formation of a decagonal Sn clathrate structure on d-Al-Ni-Co, showing persistent quasiperiodic ordering up to 10 nm thickness, with insights into nucleation, lattice matching, and stability from experimental and computational analyses.

Contribution

It reveals the decagonal Sn clathrate structure on d-Al-Ni-Co, combining STM, LEED, and DFT to understand nucleation, lattice matching, and stability, which was not previously established.

Findings

Decagonal quasiperiodic ordering persists up to 10 nm thickness.

Nucleation centers are identified as patches of the decagonal clathrate.

Lattice matching favors clathrate formation over crystalline Sn.

Abstract

Decagonal quasiperiodic ordering of Sn thin film on $d$-Al-Ni-Co, is shown based on scanning tunneling microscopy (STM), low-energy electron diffraction and density functional theory (DFT). Interestingly, the decagonal structural correlations are partially retained up to a large film thickness of 10 nm grown at a 165$\pm$10 K. The nucleation centers called 'Sn white flowers' identified by STM at submonolayer thickness are recognized as valid patches of the decagonal clathrate structure with low adsorption energies. Due to the excellent lattice matching (to within 1%) between columns of Sn dodecahedra in the clathrate structure and pentagonal motifs at the $d$-Al-Ni-Co surface, the interfacial energy favors clathrate over the competing Sn crystalline forms. DFT study of the Sn/Al-Ni-Co composite model shows good mechanical stability, as shown by the work of separation of Sn from Al-Ni-Co slab that is comparable to clathrate self-separation energy. The relaxed surface terminations of the R$_2$T$_4$ clathrate approximant are in self-similarity correspondence with the motifs observed in the STM images from monolayer to thickest Sn film.