[en] The electronic band structure of monolayer (4 × 4) silicene on Ag(111) is imaged by angle resolved photoelectron spectroscopy. A dominant hybrid surface metallic band is observed to be located near the bulk Ag sp-band which is also faintly visible. The two-dimensional character of the hybrid band has been distinguished against the bulk character of the Ag(111) sp-band by means of photon energy dependence experiments. The surface band exhibits a steep linear dispersion around the K^¯_Ag point and has a saddle point near the M^¯_Ag point of Ag(111) resembling the π-band dispersion in graphene

[en] Controlling the number of layers of graphene grown by chemical vapor deposition is crucial for large scale graphene application. We propose here an etching process of graphene which can be applied immediately after growth to control the number of layers. We use nickel (Ni) foil at high temperature (T = 900 °C) to produce multilayer-AB-stacked-graphene (MLG). The etching process is based on annealing the samples in a hydrogen/argon atmosphere at a relatively low temperature (T = 450 °C) inside the growth chamber. The extent of etching is mainly controlled by the annealing process duration. Using Raman spectroscopy we demonstrate that the number of layers was reduced, changing from MLG to few-layer-AB-stacked-graphene and in some cases to randomly oriented few layer graphene near the substrate. Furthermore, our method offers the significant advantage that it does not introduce defects in the samples, maintaining their original high quality. This fact and the low temperature our method uses make it a good candidate for controlling the layer number of already grown graphene in processes with a low thermal budget. (paper)

[en] Using molecular beam epitaxy, atomically thin 2D semiconductor HfSe₂ and MoSe₂/HfSe₂ van der Waals heterostructures are grown on AlN(0001)/Si(111) substrates. Details of the electronic band structure of HfSe₂ are imaged by in-situ angle resolved photoelectron spectroscopy indicating a high quality epitaxial layer. High-resolution surface tunneling microscopy supported by first principles calculations provides evidence of an ordered Se adlayer, which may be responsible for a reduction of the measured workfunction of HfSe₂ compared to theoretical predictions. The latter reduction minimizes the workfunction difference between the HfSe₂ and MoSe₂ layers resulting in a small valence band offset of only 0.13 eV at the MoSe₂/HfSe₂ heterointerface and a weak type II band alignment

[en] Electrical data on ZrO₂/GeO₂ stacks prepared by atomic oxygen beam deposition on Ge at 225 deg. C reveal a relatively weak dependence of the stack equivalent oxide thickness upon the ZrO₂ thickness. This trend points to a very high zirconia dielectric permittivity (k) value which is estimated to be around 44. This is indicative of zirconia crystallization into a tetragonal phase which is also supported by x-ray diffraction data. X-ray photoelectron spectroscopy analysis is in line with the assumption that due to a finite GeO₂ decomposition, Ge is incorporated into the growing ZrO₂, thus, stabilizing the high-k tetragonal phase

[en] Epitaxial thin films of 1T-HfTe₂ semimetal are grown by MBE on AlN(0001) substrates. The measured in-plane lattice parameter indicates an unstrained film which is also azimuthally aligned with the AlN substrate, albeit with an in-plane mosaic spread, as it would be expected for van der Waals epitaxy. Angle resolved photoemission spectroscopy combined with first principles electronic band structure calculations show steep linearly dispersing conduction and valence bands which cross near the Brillouin zone center, providing evidence that HfTe₂/AlN is an epitaxial topological Dirac semimetal. (paper)

[en] In this work, a structural analysis of Ge layers deposited by molecular beam epitaxy (MBE) on Ag(1 1 1) surfaces with and without an AlN buffer layer have been investigated by x-ray Absorption Spectroscopy (XAS) at the Ge-K edge. For the Ge layers deposited on h-AlN buffer layer on Ag(1 1 1) an interatomic Ge–Ge distance $R_{\text{Ge}-<!--\; ???\; -->\text{Ge}}=2.38$ Å is found, typical of 2-Dimensional Ge layers and in agreement with the theoretical predictions for free standing low-buckled Germanene presented in literature. First principles calculations, performed in the density functional theory (DFT) framework, supported the experimental RHEED and XAS findings, providing evidence for the epitaxial 2-D Ge layer formation on h-AlN/Ag(1 1 1) template. (paper)

[en] Few layer graphene (FLG) is grown on single crystal Cu(111) by Chemical Vapor Deposition, and the electronic valence band structure is imaged by Angle-Resolved Photo-Emission Spectroscopy. It is found that graphene essentially grows polycrystalline. Three nearly ideal Dirac cones are observed along the Cu Γ^¯K^¯ direction in k-space, attributed to the presence of ∼4° twisted three layer graphene with negligible interlayer coupling. The number of layers and the stacking order are compatible with Raman data analysis demonstrating the complementarity of the two techniques for a more accurate characterization of FLG

[en] Ultrathin (sub-monolayer to 12 monolayers) AlN nanosheets are grown epitaxially by plasma assisted molecular beam epitaxy on Ag(111) single crystals. Electron diffraction and scanning tunneling microscopy provide evidence that AlN on Ag adopts a graphite-like hexagonal structure with a larger lattice constant compared to bulk-like wurtzite AlN. This claim is further supported by ultraviolet photoelectron spectroscopy indicating a reduced energy bandgap as expected for hexagonal AlN

[en] A study into the optimal deposition temperature for ultra-thin La₂O₃/Ge and Y₂O₃/Ge gate stacks has been conducted in this paper with the aim to tailor the interfacial layer for effective passivation of the Ge interface. A detailed comparison between the two lanthanide oxides (La₂O₃ and Y₂O₃) in terms of band line-up, interfacial features, and reactivity to Ge using medium energy ion scattering, vacuum ultra-violet variable angle spectroscopic ellipsometry (VUV-VASE), X-ray photoelectron spectroscopy, and X-ray diffraction is shown. La₂O₃ has been found to be more reactive to Ge than Y₂O₃, forming LaGeO_x and a Ge sub-oxide at the interface for all deposition temperature studied, in the range from 44 °C to 400 °C. In contrast, Y₂O₃/Ge deposited at 400 °C allows for an ultra-thin GeO₂ layer at the interface, which can be eliminated during annealing at temperatures higher than 525 °C leaving a pristine YGeO_x/Ge interface. The Y₂O₃/Ge gate stack deposited at lower temperature shows a sub-band gap absorption feature fitted to an Urbach tail of energy 1.1 eV. The latter correlates to a sub-stoichiometric germanium oxide layer at the interface. The optical band gap for the Y₂O₃/Ge stacks has been estimated to be 5.7 ± 0.1 eV from Tauc-Lorentz modelling of VUV-VASE experimental data. For the optimal deposition temperature (400 °C), the Y₂O₃/Ge stack exhibits a higher conduction band offset (>2.3 eV) than the La₂O₃/Ge (∼2 eV), has a larger band gap (by about 0.3 eV), a germanium sub-oxide free interface, and leakage current (∼10⁻⁷ A/cm² at 1 V) five orders of magnitude lower than the respective La₂O₃/Ge stack. Our study strongly points to the superiority of the Y₂O₃/Ge system for germanium interface engineering to achieve high performance Ge Complementary Metal Oxide Semiconductor technology

[en] We discuss the use of resonant photoemission in the study of rare earth/transition metal perovskites and perovskite-related structures. We illustrate its application to the assignment of the RE and TM valence band density of states in Nd₂Ni_1-xCu _xO₄ and REBaCo₂O_5+δ. In both cases, the relative positions of the 4f and 3d contributions within the valence band density of states (DOS) relative to the Fermi energy provide a rationalisation of the redox behaviour of the system. We also apply the technique to the determination of the Co spin state in Co(III) perovskites such as LaCoO₃ and GdBaCo₂O_5+δ, where the observation of a delayed Co 3p → 3d resonance is a powerful diagnostic of the Co(III) low spin state. This reveals that while the Co(III) ions in LaCoO₃ increasingly populate the low spin state as the sample temperature is lowered, a significant proportion of the Co(III) species in GdBaCo₂O_5+δ are not in the low spin state at temperatures as low as 50 K