Ambient pressure x-ray photoelectron spectroscopy study of water formation and adsorption under two-dimensional silica and aluminosilicate layers on Pd(111).

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.

Tuning two-dimensional phase formation through epitaxial strain and growth conditions: silica and silicate on NixPd1-x(111) alloy substrates.

Two-dimensional (2D) materials can have multiple phases close in energy but with distinct properties, with the phases that form during growth dependent on experimental conditions and the growth substrate. Here, the competition between 2D van der Waals (VDW) silica and 2D Ni silicate phases on NixPd1-x(111) alloy substrates was systematically investigated experimentally as a function of Si surface coverage, annealing time and temperature, O2 partial pressure, and substrate composition and the results were compared with thermodynamic predictions based on density functional theory (DFT) calculations and thermochemical data for O2. Experimentally, 2D Ni silicate was exclusively observed at higher O2 pressures (∼10-6 Torr), higher annealing temperatures (1000 K), and more prolonged annealing (10 min) if the substrate contained any Ni and for initial Si coverages up to 2 monolayers. In contrast, decreasing the O2 pressure to ∼10-8 Torr and restricting the annealing temperature and time enabled 2D VDW silica formation. Amorphous 2D VDW silica was observed even when the substrate composition was tuned to lattice match crystalline 2D VDW silica. The trend of decreased O2 pressure favoring 2D VDW silica was consistent with the theoretical predictions; however, theory also suggested that sufficient Si coverage could avoid Ni silicate formation. The effect of epitaxial strain on 2D Ni silicate was investigated by modifying the solid solution alloy substrate composition. It was found that 2D Ni silicate will stretch to match the substrate lattice constant up to 1.12% tensile strain. When the lattice mismatch was over 1.40%, incommensurate crystalline domains were observed, indicating relaxation of the overlayer to its favored lattice constant. The limited epitaxial strain that could be applied was attributed to a combination of the 2D silicate stiffness, the insensitivity of its bonding to the substrate to its alignment with the substrate, and its lack of accessible structural rearrangements that can reduce the strain energy. The results demonstrate how the resulting 2D material can be manipulated through the growth conditions and how a solid solution alloy substrate can be used to maximize the epitaxial strain imparted to the 2D system.

Using ZnO-Cr2O3-ZnO heterostructures to characterize polarization penetration depth through non-polar films.

The ability to affect the surface properties of non-polar Cr2O3 films through polar ZnO(0001) and (0001[combining macron]) supports was investigated by characterizing the polarity of ZnO films grown on top of the Cr2O3 surfaces. The growth and geometric and electronic structures of the ZnO films were characterized with X-ray photoelectron spectroscopy, ultra-violet photoelectron spectroscopy, reflection high-energy electron diffraction, low-energy electron diffraction, and X-ray diffraction. The ZnO growth mode was Stranski-Krastanov, which can be attributed to the ZnO layers initially adopting a non-polar structure with a lower surface tension before transitioning to the polar bulk structure with a higher surface energy. A similar result has been reported for ZnO growth on α-Al2O3(0001), which is isostructural with Cr2O3. The polarity of the added ZnO layer was determined by examining the surface morphology following wet chemical etching with atomic force microscopy and by characterizing the surface reactivity via temperature-programmed desorption of alcohols, which strongly depends on the ZnO polarization direction. Consistent with prior work on ZnO growth on bulk Cr2O3(0001), both measurements indicate that thick Cr2O3 layers support ZnO(0001[combining macron]) growth regardless of the underlying ZnO substrate polarization; however, the polarization direction of ZnO films grown on Cr2O3 films less than three repeat units thick follows the direction of the underlying substrate polarization. These findings show that it is possible to manipulate the surface properties of non-polar materials with a polar substrate, but that the effect does not penetrate past just a couple of repeat units.

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

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.

Epitaxial NixPd1-x (111) Alloy Substrates with Continuously Tunable Lattice Constants for 2D Materials Growth.

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 NixPd1-x(111) alloy films with lattice constants between 3.61 and 3.89 Å on the hexagonal (0001) basal planes of α-Al2O3 and Cr2O3 (grown as epitaxial films on α-Al2O3 (0001)). The Cr2O3 acted as an adhesion layer, which not only improved the high-temperature stability of the films but also produced single-domain films with NixPd1-x [112̅] parallel to Cr2O3 [112̅0], in contrast to growth on α-Al2O3 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 SiO2, a two-dimensional van der Waals material, was grown on a NixPd1-x(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.

Controlling the physics and chemistry of binary and ternary praseodymium and cerium oxide systems.

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.

Methanol Adsorption and Reaction on Samaria Thin Films on Pt(111).

We investigated the adsorption and reaction of methanol on continuous and discontinuous films of samarium oxide (SmOx) grown on Pt(111) in ultrahigh vacuum. The methanol decomposition was studied by temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), while structural changes of the oxide surface were monitored by low-energy electron diffraction (LEED). Methanol dehydrogenates to adsorbed methoxy species on both the continuous and discontinuous SmOx films, eventually leading to the desorption of CO and H2 which desorbs at temperatures in the range 400-600 K. Small quantities of CO2 are also detected mainly on as-prepared Sm2O3 thin films, but the production of CO2 is limited during repeated TPD runs. The discontinuous film exhibits the highest reactivity compared to the continuous film and the Pt(111) substrate. The reactivity of methanol on reduced and reoxidized films was also investigated, revealing how SmOx structures influence the chemical behavior. Over repeated TPD experiments, a SmOx structural/chemical equilibrium condition is found which can be approached either from oxidized or reduced films. We also observed hydrogen absence in TPD which indicates that hydrogen is stored either in SmOx films or as OH groups on the SmOx surfaces.