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We report on strategies for characterizing hexagonal coincidence phases by analyzing the involved spatial moiré beating frequencies of the pattern. We derive general properties of the moiré regarding its symmetry and construct the spatial beating frequency K ⃗ moir é as the difference between two reciprocal lattice vectors k ⃗ i of the two coinciding lattices. Considering reciprocal lattice vectors k ⃗ i , with lengths of up to n times the respective (1, 0) beams of the two lattices, readily increases the number of beating frequencies of the nth-order moiré pattern. We predict how many beating frequencies occur in nth-order moirés and show that for one hexagonal lattice rotating above another the involved beating frequencies follow circular trajectories in reciprocal-space. The radius and lateral displacement of such circles are defined by the order n and the ratio x of the two lattice constants. The question of whether the moiré pattern is commensurate or not is addressed by using our derived concept of commensurability plots. When searching potential commensurate phases we introduce a method, which we call cell augmentation, and which avoids the need to consider high-order beating frequencies as discussed using the reported ( 6 3 × 6 3 ) R 30 ° moiré of graphene on SiC(0001). We also show how to apply our model for the characterization of hexagonal moiré phases, found for transition metal-supported graphene and related systems. We explicitly treat surface x-ray diffraction-, scanning tunneling microscopy- and low-energy electron diffraction data to extract the unit cell of commensurate phases or to find evidence for incommensurability. For each data type, analysis strategies are outlined and avoidable pitfalls are discussed. We also point out the close relation of spatial beating frequencies in a moiré and multiple scattering in electron diffraction data and show how this fact can be explicitly used to extract high-precision data.

The intercalation of metals beneath graphene offers a powerful route to stabilizing and protecting novel 2D phases. The epitaxial growth of Pb monolayers on SiC(0001), combined with the relatively large spacing of the suspended graphene, makes this system particularly distinctive. Using low‐energy electron diffraction (LEED) and various microscopy techniques—including scanning electron microscopy (SEM), scanning tunneling microscopy (STM), and low‐energy electron microscopy (LEEM)—the intercalation process has been investigated across multiple length scales. The analysis reveals the formation of different 2D Pb monolayer phases, such as stripes and hexagons, which emerge due to the interplay between substrate pinning and strain within the Pb layer, depending on local coverage. These findings provide new insights into the strain‐driven stabilization of intercalated metal layers and highlight the potential of graphene as a versatile platform for engineering low‐dimensional materials. Intercalating Pb beneath epitaxial graphene on SiC(0001) stabilizes distinct 2D phases at the interface. Using LEED, LEEM, SEM, and STM, the formation of characteristic stripe and hexagonal Pb monolayer structures is observed, and atomistic models are provided. Their stability and coexistance are discussed in context of small coverage variations and a balance of substrate pinning and in‐plane strain.This work demonstrates graphene's potential for engineering strain‐stabilized, low‐dimensional materials.

The initial stages of room temperature growth of Pb overlayers on commercial Si(100) p ( 2 × 1 ) surface have been investigated using low-energy electron diffraction and low-energy electron microscopy (LEEM) techniques. A well-ordered reconstructed Si(100) p ( 10 × 2 ) surface phase has been observed for 0.5 monolayers of Pb deposition and is found to vanish for higher Pb coverages. We do not observe any island formation in our LEEM studies during the early stages of growth, unlike earlier studies on low-miscut substrates. Our dark-field LEEM experiments suggest the observed high step density with low terrace widths is responsible for this behaviour.

The results of a study of the crystal structure and surface conductivity of a Si(111) silicon substrate with a reconstructed surface after deposition of submonolayer doses of lithium are presented. We used the method of low energy electron diffraction to study changes in the structure of the crystal lattice of the surface, as well as the four-point probe metho d for measuring the conductivity of substrates under in situ conditions. As the initial surfaces, we used the Si(111)7 × 7 reconstruction of an atomically clean silicon substrate, and the reconstructions obtained by adsorption of 1 ML of gold, lead, and bismuth atoms: Si(111)β– × –Au, Si(111)1 × 1–Pb, and Si(111)β– × –Bi, respectively.

The growth of a high‐quality complete graphene layer is successfully achieved for Ir(111) and Ru(0001) substrates using liquid ethanol as a precursor. Metallic substrates, which are cleaned in ultra‐high vacuum conditions, were ex‐situ immersed in liquid ethanol followed by the controlled in situ thermal annealing. The process of graphene formation and its quality are carefully monitored using X‐ray photoelectron spectroscopy, low‐energy electron diffraction, and scanning tunneling microscopy methods. It is found that graphene formation starts at 400 °C via ethanol decomposition and desorption of oxygen from the surface leading to the formation of the high‐quality complete graphene layer at 1000 °C. The results of the systematic angular‐resolved photoelectron spectroscopy experiments confirm the high quality of the obtained graphene layer, and it concludes that such an approach offers an easy, quick, and reproducible method to synthesize large‐scale graphene on different metallic substrates.

Germanium is an excellent material candidate for various applications, such as field effect transistors and radiation detectors/multijunction solar cells, due to its high carrier mobilities and narrow bandgap, respectively. However, the efficient passivation of germanium surfaces remains challenging. Recently, the most promising results have been achieved with atomic-layer-deposited (ALD) Al2O3, but the obtainable surface recombination velocity (SRV) has been very sensitive to the surface state prior to deposition. Based on X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED), we show here that the poor SRV obtained with the combination of HF and DIW surface cleaning and ALD Al2O3 results from a Ge suboxide interlayer (GeOx, x < 2) with compromised quality. Nevertheless, our results also demonstrate that both the composition and crystallinity of this oxide layer can be improved with a combination of low-temperature heating and a 300-Langmuir controlled oxidation in an ultrahigh vacuum (LT-UHV treatment). This results in a reduction in the interface defect density (Dit), allowing us to reach SRV values as low as 10 cm/s. Being compatible with most device processes due to the low thermal budget, the LT-UHV treatment could be easily integrated into many future devices and applications.

Beyond the macroscopic perspective, this study microscopically investigates Si1−xGex(001)-2×1 samples that were grown on the epi Ge(001) and epi Si(001) substrates via molecular-beam epitaxy, using the high-resolution synchrotron radiation photoelectron spectroscopy (SRPES) as a probe. The low-energy electron diffraction equipped in the SRPES chamber showed 2×1 double-domain reconstruction. Analyses of the Ge 3d core-level spectra acquired using different photon energies and emission angles consistently reveal the ordered spots to be in a Ge–Ge tilted configuration, which is similar to that in epi Ge(001)-2×1. It was further found that the subsurface layer was actually dominated by Ge, which supported the buckled configuration. The Si atoms were first found in the third surface layer. These Si atoms were further divided into two parts, one underneath the Ge–Ge dimer and one between the dimer row. The distinct energy positions of the Si 2p core-level spectrum were caused by stresses, not by charge alternations.

The interlayer coupling can be used to engineer the electronic structure of van der Waals heterostructures (superlattices) to obtain properties that are not possible in a single material. So far research in heterostructures has been focused on commensurate superlattices with a long-ranged Moiré period. Incommensurate heterostructures with rotational symmetry but not translational symmetry (in analogy to quasicrystals) are not only rare in nature, but also the interlayer interaction has often been assumed to be negligible due to the lack of phase coherence. Here we report the successful growth of quasicrystalline 30° twisted bilayer graphene (30°-tBLG), which is stabilized by the Pt(111) substrate, and reveal its electronic structure. The 30°-tBLG is confirmed by low energy electron diffraction and the intervalley double-resonance Raman mode at 1383 cm−1. Moreover, the emergence of mirrored Dirac cones inside the Brillouin zone of each graphene layer and a gap opening at the zone boundary suggest that these two graphene layers are coupled via a generalized Umklapp scattering mechanism—that is, scattering of a Dirac cone in one graphene layer by the reciprocal lattice vector of the other graphene layer. Our work highlights the important role of interlayer coupling in incommensurate quasicrystalline superlattices, thereby extending band structure engineering to incommensurate superstructures.

Two-dimensional systems such as surfaces and molecular monolayers exhibit a multitude of intriguing phases and complex transitions. Ultrafast structural probing of such systems offers direct time-domain information on internal interactions and couplings to a substrate or bulk support. We have developed ultrafast low-energy electron diffraction and investigate in transmission the structural relaxation in a polymer/graphene bilayer system excited out of equilibrium. The laser-pump/electron-probe scheme resolves the ultrafast melting of a polymer superstructure consisting of folded-chain crystals registered to a free-standing graphene substrate. We extract the time scales of energy transfer across the bilayer interface, the loss of superstructure order, and the appearance of an amorphous phase with short-range correlations. The high surface sensitivity makes this experimental approach suitable for numerous problems in ultrafast surface science.

We report on mixed ordered monolayers of the electron acceptor-type molecule tetracyano - 2,6 - naphthoquinodimethane (TNAP) and the electron donor-type molecule hexathiapentacene (HTPEN). This investigation was motivated by the general question which type of mixed stoichiometric structures are formed on a surface by molecules that are otherwise typically used for the synthesis of bulk charge-transfer materials. The layers were obtained by vacuum deposition on the Ag(100) surface and analyzed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The formation of the mixed structure occurs spontaneously. An important motif for the structure formation is given by hydrogen bonds between the TNAP molecules. Both molecules, TNAP and HTPEN also form well-ordered monolayers on the Ag(100) surface on their own. In all structures, the molecules are adsorbed in a planar orientation on the surface. We discuss the influence of intermolecular charge transfer on the ordering in the mixed structure.