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.

The ability to synthesize two-dimensional (2D) oxide layers with nonbulk structures on transition-metal surfaces has attracted attention as a method to expand the range of 2D structures and properties beyond those accessible by thinning known layered materials. In this paper, a combination of surface spectroscopies, scanning tunneling microscopy (STM), and density functional theory (DFT) show that the reaction of silicon and oxygen with a Ni–Pd alloy surface extracts Ni from the substrate to produce a non-bulk-structured 2D Ni silicate. The surface spectroscopies reveal that high-temperature annealing in an oxygen-rich environment causes substrate Ni to segregate toward the surface where it can oxidize to form a 2D crystalline layer that includes Ni–O–Si bonds. STM images of the 2D crystalline layer reveal a local honeycomb structure consistent with six-membered rings of corner-sharing SiO4 tetrahedra overlaid on a larger-scale moiré pattern that arises from the lattice mismatch between the 2D layer and the substrate. The DFT calculations identify a thermodynamically favorable 2D Ni silicate structure composed of a layer of six-membered rings of corner-sharing SiO4 tetrahedra bonded to an octahedrally coordinated Ni–O layer that can explain the observed STM contrast. The structure resembles a single layer of a dioctahedral clay with bonds to the substrate replacing O–H bonds in clays. The dioctahedral Ni clay stands in contrast to known bulk materials in which Ni is only observed in trioctahedral clays. Theory indicates that the formation of the 2D Ni silicate is not strongly dependent on the alloy composition while experiments suggest that its formation is self-limited; that is, once it forms, continued exposure to oxidizing conditions does not alter the structure. The results highlight the potential for using alloy substrates with a reactive component to form unique 2D surface layers with a broad processing window and without tight constraints on composition control.

The inherent properties of epitaxial oxide thin-film layers have attracted the intense interest of different research fields, such as catalysis and microelectronics. The focus of this work is the temperature-dependent oxygen release, oxygen vacancy formation, and lattice rearrangement of Ce1–x Pr x O2−δ thin films with systematic stoichiometry variation (x = 0–1) and oxygen deficiency (δ > 0) on Si(111). The mixed oxide layers were heteroepitaxially grown by coevaporating molecular beam epitaxy. To observe the oxygen release, temperature-programmed desorption was performed. Furthermore, laboratory-based X-ray diffraction measurements were carried out after several annealing steps to investigate the crystal structure rearrangement. The contribution of Ce4+/Ce3+ and Pr4+/Pr3+ redox systems to the oxygen release and lattice rearrangement was clarified by X-ray photoelectron spectroscopy. Finally, Raman spectroscopy was performed to detect structural defects in the oxide lattice (i.e., oxygen vacancies and MO8-complexes) and their temperature dependence, which thus provides microscopic insights into the atomic oxygen release mechanism. The oxygen-releasing temperature and the oxygen storage capacity in such Ce1–x Pr x O2−δ model thin films can be engineered by the Pr concentration.

Epitaxial strain can be a powerful parameter for directing the growth of thin films. Unfortunately, conventional materials only offer discrete choices for setting the lattice strain. In this work, it is demonstrated that epitaxial growth of transition metal alloy solid solutions can provide thermally stable, high-quality growth substrates with continuously tunable lattice constants. Molecular beam epitaxy was used to grow NiPd(111) alloy films with lattice constants between 3.61 and 3.89 Å on the hexagonal (0001) basal planes of α-AlO and CrO (grown as epitaxial films on α-AlO (0001)). The CrO acted as an adhesion layer, which not only improved the high-temperature stability of the films but also produced single-domain films with NiPd [112̅] parallel to CrO [112̅0], in contrast to growth on α-AlO that yielded twinned films. Surface characterization by electron diffraction and scanning tunneling microscopy (STM) as well as bulk X-ray diffraction analysis indicated that the films are suitable as inexpensive and stable substrates for thin-film growth and for surface science studies. To demonstrate this suitability, bilayer SiO, a two-dimensional van der Waals material, was grown on a NiPd(111) film tuned to closely match the film's lattice constant, with STM and electron diffraction results revealing a highly ordered, single-phase crystalline state.

Rare earth praseodymium and cerium oxides have attracted intense research interest in the last few decades, due to their intriguing chemical and physical characteristics. An understanding of the correlation between structure and properties, in particular the surface chemistry, is urgently required for their application in microelectronics, catalysis, optics and other fields. Such an understanding is, however, hampered by the complexity of rare earth oxide materials and experimental methods for their characterisation. Here, we report recent progress in studying high-quality, single crystalline, praseodymium and cerium oxide films as well as ternary alloys grown on Si(111) substrates. Using these well-defined systems and based on a systematic multi-technique surface science approach, the corresponding physical and chemical properties, such as the surface structure, the surface morphology, the bulk-surface interaction and the oxygen storage/release capability, are explored in detail. We show that specifically the crystalline structure and the oxygen stoichiometry of the oxide thin films can be well controlled by the film preparation method. This work leads to a comprehensive understanding of the properties of rare earth oxides and highlights the applications of these versatile materials. Furthermore, methanol adsorption studies are performed on binary and ternary rare earth oxide thin films, demonstrating the feasibility of employing such systems for model catalytic studies. Specifically for ceria systems, we find considerable stability against normal environmental conditions so that they can be considered as a "materials bridge" between surface science models and real catalysts.

Ambient pressure x-ray photoelectron spectroscopy (AP-XPS) supported by density functional theory (DFT) calculations was used to characterize the interaction of water with two-dimensional (2D) silica and aluminosilicate bilayers on Pd(111). Starting with oxygen adsorbed at the SiO2/Pd interface, exposure to water caused the SiO2-derived XPS peaks to shift to higher binding energy and the removal of an O 1s feature associated with interfacial adsorbed oxygen. These observations were attributed to the formation of a mixed water–hydroxyl interface, which eliminates the interfacial dipolar layer, and its associated electrostatic potential, created by adsorbed oxygen. Interfacial oxygen also reacted with H2 to produce adsorbed water which also caused an upward binding energy shift of the SiO2 peaks. Spectra recorded under 0.5 Torr water revealed additional water adsorption and a further shift of the overlayer peaks to higher binding energy. Incorporating Al into the 2D material caused the bilayer peaks to shift to lower binding energy which could be explained by electron donation from the metal to the bilayer. Although the stronger interaction between the bilayer and Pd substrate should restrict interfacial adsorption and reaction, similar trends were observed for water and hydrogen exposure to interfacial adsorbed oxygen. Less water adsorption was observed at the aluminosilicate interface which is a consequence of Al strengthening the bond to the metal substrate. The results reveal how the sensitivity of XPS to interfacial dipoles can be exploited to distinguish reactions taking place in confined spaces under 2D layers and how tuning the composition of the 2D layer can impact such reactions.

The combined experimental and theoretical results demonstrate the manipulation of 2D VDW silica and 2D Ni silicate through growth conditions, and the determination of the maximum epitaxial strain imparted to the 2D system through alloy substrate.