Two-dimensional (2D) silica (SiO) and aluminosilicate (AlSiO) 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 SiO and AlSiO bilayers start ordering above 850 K in 2 × 10 Torr oxygen. Both bilayers show hexagonal LEED patterns with a periodicity approximately twice that of the Pd(111) surface. Importantly, the SiO bilayer forms an incommensurate crystalline structure whereas the AlSiO bilayer crystallizes in a commensurate structure. The incommensurate crystalline SiO structure on Pd(111) resulted in a moiré pattern observed with LEED and STM. Theoretical results show that straining the pure SiO 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 Al for Si is supplied by Pd creating a chemical bond to the AlSiO bilayer, whereas van der Waals interactions predominate for the SiO 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.

We investigated the growth and partial reduction of Sm 2 O 3 (111) thin films on Pt(111) using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We find that the Sm 2 O 3 (111) films are high quality and grow in a defective fluorite structure wherein the Sm cations adopt a hexagonal (1.37 × 1.37) lattice in registry with the Pt(111) surface, while oxygen vacancies are randomly distributed within the film. STM measurements show that Sm 2 O 3 (111) film growth on Pt(111) occurs by the Stranski-Krastanov mechanism, in which a single O−Sm−O trilayer initially forms, followed by the growth of well-defined, multilayer islands. The Sm 2 O 3 (111) films undergo partial reduction during annealing at 1000 K in ultrahigh vacuum. LEED and STM provide evidence that a fraction of the Sm 2 O 3 in the first layer, closest to the Pt(111) substrate, decomposes to produce well-ordered domains of rocksalt SmO(100) during reduction, and that Sm 2 O 3 from the third and higher layers concurrently spreads onto the first layer to form a more contiguous second layer of Sm 2 O 3 (111). We show that the SmO(100) and Sm 2 O 3 (111) lattices can form a coincidence structure with minimal strain to the Sm-atom sublattices, and that satellite features observed in the LEED patterns are consistent with the coexistence of SmO(100) and Sm 2 O 3 (111) domains as well as the proposed Sm 2 O 3 (111)/SmO(100) coincidence structure. Lastly, we find that reoxidation of the partially reduced films restores the original Sm 2 O 3 (111) crystal structure, and significantly improves the film quality, as reflected by a flatter film morphology and better connectivity among oxide domains. An implication from this study is that the formation of (100)-oriented monoxide structures is a general characteristic of the reduction of rare-earth oxide thin films on hexagonally close-packed metal surfaces.