Blue phosphorene on Au(111): theoretical, spectroscopic and diffraction analysis reveal the role of single Au adatoms.

In investigating the monoatomic layers of P, several stable two-dimensional (2D) allotropes have been theoretically predicted. Among them, single-layer blue phosphorus (BlueP) appears to deliver promising properties. After initial success, where the structure of BlueP triangular patches on Au(111) was conceived on the basis of scanning tunneling microscopy (STM) and density functional theory (DFT), the surface structure model was revisited multiple times with increasing accuracy and insight of theoretical calculations and experimental datasets. Interestingly, the quest for a reliable atomic structure model of BlueP on Au(111) turned out to be very contentious and challenging, particularly considering the possible incorporation of Au atoms in the 2D sheet of P. This article proposes an extended report on theoretical findings that can be extracted from DFT calculations of the orbital projected band structure and employed for an efficient comparison protocol between the calculations and experimental datasets obtained from angle-resolved photoemission spectroscopy (ARPES). The findings, together with experimental and simulated data from STM imaging and surface X-ray diffraction (SXRD), show a clear way to verify the presence and characterize the stabilizing effect of foreign atoms in 2D materials.

Growth and characterization of epitaxially stabilized ceria(001) nanostructures on Ru(0001).

We have studied (001) surface terminated cerium oxide nanoparticles grown on a ruthenium substrate using physical vapor deposition. Their morphology, shape, crystal structure, and chemical state are determined by low-energy electron microscopy and micro-diffraction, scanning probe microscopy, and synchrotron-based X-ray absorption spectroscopy. Square islands are identified as CeO2 nanocrystals exhibiting a (001) oriented top facet of varying size; they have a height of about 7 to 10 nm and a side length between about 50 and 500 nm, and are terminated with a p(2 × 2) surface reconstruction. Micro-illumination electron diffraction reveals the existence of a coincidence lattice at the interface to the ruthenium substrate. The orientation of the side facets of the rod-like particles is identified as (111); the square particles are most likely of cuboidal shape, exhibiting (100) oriented side facets. The square and needle-like islands are predominantly found at step bunches and may be grown exclusively at temperatures exceeding 1000 °C.

A LEEM/micro-LEED investigation of phase transformations in TiOx/Pt(111) ultrathin films.

A combined use of low energy electron microscopy (LEEM) and microprobe LEED (micro-LEED) allows the in-situ observation of dynamical processes at the TiOx/Pt(111) interface. The transformations between different surface-stabilized phases are investigated in the case of room temperature TiOx reactive deposition with subsequent post-annealing. For a coverage of 0.6 MLeq, UHV annealing to 400 degrees C leads to the formation of the zigzag-like z-TiO1.33 layer. At higher temperatures a rotated z-TiO1.33 phase is observed, its lateral distribution being strongly influenced by surface morphology. Concurrently, the z-TiO1.33 layer partially transforms into a kagomé-like TiO1.5 structure. The resulting oxygen enrichment of the interface is interpreted by invoking Ti interdiffusion into the substrate. At a coverage of 0.45 MLeq, UHV annealing at 500 degrees C transforms the z-TiO1.33 layer into a different zigzag-like z'-TiO1.25 layer. Post-annealing in oxygen of the reduced phases or direct reactive deposition at high temperature both produce the rect-TiO2 stoichiometric phase, showing characteristic needle-like domains aligned according to the rect-TiO2 unit cell orientation.

The (1 X 1) --> hexagonal structural transition on Pt(100) studied by high-energy resolution core level photoemission.

The (1 X 1) --> quasihexagonal (HEX) phase transition on a clean Pt(100) surface was investigated by monitoring the time evolution of the Pt4f(72) core level photoemission spectra. The spectral component originating from the atoms forming the (1x1) metastable unreconstructed surface was found at -570+/-20 meV with respect to the bulk peak. Ab initio calculations based on density functional theory confirmed the experimental assignment. At temperatures above 370 K, the (1 X 1) phase irreversibly reverts to the more stable HEX phase, characterized by a surface core level shifted component at -185 +/ -40 meV. By analyzing the intensity evolution of the core level components, measured at different temperatures in the range of 393 - 475 K, we determined the activation energy of the phase transformation, E = 0.76 +/- 0.04 eV. This value is considerably lower than the one previously determined by means of low energy electron diffraction. Possible reasons for this discrepancy are discussed.

Gold-catalyzed oxide nanopatterns for the directed assembly of Ge island arrays on Si.

The heteroepitaxial growth of Ge on Au-patterned Si(001) is investigated using in situ spectromicroscopy. Patterning of a hydrogen-terminated Si surface with a square array of Au dots followed by brief exposure to air leads to the spontaneous, local oxidation of Si. The resulting oxide nanopattern limits the surface migration of Au during annealing up to 600 degrees C, resulting in complete preservation of the Au pattern. Subsequent deposition of Ge induces a redistribution of Au across the surface even as the oxide nanopattern persists. As a result, the oxide pattern drives the growth of Ge islands into an ordered assembly, while Au decorates the surfaces of the Ge islands and modifies their shape.