Growth of two dimensional silica and aluminosilicate bilayers on Pd(111): from incommensurate to commensurate...

Two-dimensional (2D) silica (SiO₂) and aluminosilicate (AlSi₃O₈) bilayers grown on Pd(111) were fabricated and systematically studied using ultrahigh vacuum surface analysis in combination with theoretical methods, including Auger electron spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory. Based on LEED results, both SiO₂ and AlSi₃O₈ bilayers start ordering above 850 K in 2 × 10^-6 Torr oxygen. Both bilayers show hexagonal LEED patterns with a periodicity approximately twice that of the Pd(111) surface. Importantly, the SiO₂ bilayer forms an incommensurate crystalline structure whereas the AlSi₃O₈ bilayer crystallizes in a commensurate structure. The incommensurate crystalline SiO₂ structure on Pd(111) resulted in a moiré pattern observed with LEED and STM. Theoretical results show that straining the pure SiO₂ bilayer to match Pd(111) would cost 0.492 eV per unit cell; this strain energy is reduced to just 0.126 eV per unit cell by replacing 25% of the Si with Al which softens the material and expands the unstrained lattice. Furthermore, the missing electron created by substituting Al³⁺ for Si⁴⁺ is supplied by Pd creating a chemical bond to the AlSi₃O₈ bilayer, whereas van der Waals interactions predominate for the SiO₂ bilayer. The results reveal how the interplay between strain, doping, and charge transfer determine the structure of metal-supported 2D silicate bilayers and how these variables may potentially be exploited to manipulate 2D materials structures.