Two-dimensional (2D) self-assembly monolayers of 4-n-alkyloxybenzoic acids (4-OBA) on Au(111) were investigated by scanning tunneling microscopy (STM) and semiempirical modeling. The study reveals that the supramolecular organization of these molecules is governed by the interplay between directional intermolecular hydrogen and dipole bonding and nondirectional van der Waals (vdW) interactions with the metallic substrate. We observe a clear odd/even effect: the parity (even or odd) of the carbon number in the alkyloxy chain leads to distinct packing geometries. This behavior is attributed to a compromise between optimal intermolecular bonding, maximization of molecular packing, and substrate-adsorbate interaction. Both assemblies exhibit weak interactions with the substrate, as indicated by the preservation of the Au(111) herringbone reconstruction and molecular mobility. By integrating STM experiments with semiempirical modeling, we correlate real-space imaging with molecular-level conformational analysis, thereby offering a comprehensive picture of the structural determinants of chain-parity effects. These findings highlight the subtle balance of intermolecular and molecule-substrate forces in directing 2D crystal formation and offer a platform for engineering tunable supramolecular monolayers via chain-length control.

We present an experimental investigation of the Pb-induced changes on the electronic structure of selected topological insulators, namely, TlBiSe 2 and Bi 2 Se 3 . Using spatially resolved and surface-sensitive techniques, including micro-angle-resolved photoemission ( μ -ARPES) and micro-core-level photoemission ( μ -PES), we track the evolution of the electronic states during adsorbate growth. For low Pb coverage on TlBiSe 2 , we identify the formation of a disordered interface. For thicker films, μ -ARPES measurements clearly resolve well-defined electron confinement (quantum well) states within the metallic Pb layers and no migration of the topological surface state onto the Pb film is observed. Furthermore, μ -PES results on both Pb/ TlBiSe 2 and Pb/ Bi 2 Se 3 heterostructures can be rationalized in terms of a significant chemical interaction between Pb and Se, leading to substantial surface degradation and Bi atom segregation. The modeling performed on the adsorption of lead on topological insulator surfaces, which includes the consideration of the swapping of Pb adsorbates with Bi substrate atoms, validates the scenario derived from experimental observations. It also suggests that the development of a disordered interface is due to the formation of PbSe nuclei, whose size is constrained by the compressive stresses exerted by the TI substrate.

The investigation of intricate alignments in two-dimensional materials has garnered significant attention, fueled by the growing ability to precisely manipulate and engineer electronic properties through controlled stacking and orientation. Recent observations of spontaneous arrangements in multilayer materials with stochastic twist angles offer new opportunities to elucidate the unique physics governing moir & eacute; superstructures, and the critical role of the electronic coupling between layers. In this context, the present study focuses on the atomic-scale characterization of nanoporous graphene, with particular emphasis on regions formed by misoriented graphene layers that assemble into crumpled, continuous patterns. Despite the absence of flat domains at the submicron scale, scanning tunneling microscopy is employed to identify dominant twisted multilayer structures in the samples. To complement the experimental observations, a simplified, yet, effective exponential parametric model is introduced, merging independent analytic electron densities to estimate the number of misaligned layers in the scanned areas. This approach is further refined through density-functional theory calculations of projected electron densities from graphene, which are rotated and stacked at adjusted interlayer distances. The synergy between the two models provides a robust framework for distinguishing between twisted bilayer and twisted trilayer domains, as observed in constant-current imaging. Finally, a full density-functional theory analysis is conducted on simple few-layer graphene structures, assessing the role of interlayer correlations. The present study provides an atomic-level description of porous graphene, while also offering accessible tools for simulating twisted layered materials beyond graphene, which may be beneficial for the STM community.

Cement composites containing different carbon nanomaterials, namely graphene technical grade, graphene super grade, and graphene oxide, up to 1.0% by weight of cement, were prepared. Ultrasonic, chemical, and thermochemical treatments were applied to improve the stability of the dispersions containing the graphene-based nanomaterials. Their exfoliation was analyzed using Raman spectroscopy, and the stability of the dispersions was quantitatively investigated by means of the static multiple light scattering (SMLS) technique. The sonication process enhanced the intensity of the 2D band of graphene technical grade, suggesting a partial degree of exfoliation, while the hydrothermal treatment with sodium cholate significantly promoted the stability of its dispersion. The effect of the addition of selected graphene-based nanomaterials in mortars was evaluated in terms of fresh state properties, mechanical strength, capillary water absorption, and pore size distribution. Workability decreased with the increase in the amount of carbon nanomaterials. Field emission scanning electron microscopy (FESEM) was also employed to characterize the microstructure of pristine graphene-based nanomaterials and their inclusion within the cement matrix. Our results suggest that mechanical properties are only moderately affected by the inclusion of all additives, whereas the introduction of graphene significantly influences the coefficient of capillary water absorption. Specifically, a reduction of about 20% in the capillary water absorption coefficient was observed at the concentration of 1.0 wt% of graphene technical grade, which is ascribed to a refinement of the porosity.

Herewith, we propose a comprehensive study of the vibrational response of chemical doping of free-standing graphene (Gr). Complementary insights on the increased metallicity have been demonstrated by the emerging plasmon excitation in the upper Dirac cone, observed by inelastic electron scattering and core-level photoemission. The electron migration in the pi* upper Dirac band unveils an electron-phonon coupling of contaminant-free K-doped Gr, as evidenced by advanced micro-Raman spectroscopy in ultrahigh vacuum ambient. The vibrational response of potassium-doped Gr correlated with the charge injected in the upper Dirac cone, and the Fermi level shift unravel a notable electron-phonon coupling, which is stronger than that observed for gate voltage-doped Gr.

The doping mechanism of fully suspended nanoporous graphene with alkali metals unveils the charge redistribution between the donors and graphene. The evolution of the electronic charge induced in the graphene upper Dirac cone has been correlated with the electron-phonon interaction by employing complementary electronic and vibrational spectroscopic techniques in ultra high vacuum ambient. K and Cs doping induces a significant rigid band shift and Dirac cone filling correlated with the neat formation of a pi*-plasmon mode and a clear signature of electron-phonon interaction in the Raman spectra, while Na doping slightly influences the graphene electronic and vibrational response, fostered by adatom Na clustering.

The synthesis and design of two-dimensional supramolecular assemblies with specific functionalities is one of the principal goals of the emerging field of molecule-based electronics, which is relevant for many technological applications. Although a large number of molecular assemblies have been already investigated, engineering uniform and highly ordered two-dimensional molecular assemblies is still a challenge. Here we report on a novel approach to prepare wide highly crystalline molecular assemblies with tunable structural properties. We make use of the high-reactivity of the carboxylic acid functional moiety and of the predictable structural features of non-polar alkane chains to synthesize 2D supramolecular assemblies of 4-(decyloxy)benzoic acid (4DBA;C17H26O3 ) on a Au(111) surface. By means of scanning tunneling microscopy, density functional theory calculations and photoemission spectroscopy, we demonstrate that these molecules form a self-limited highly ordered and defect-free two-dimensional single-layer film of micrometer-size, which exhibits a nearly-freestanding character. We prove that by changing the length of the alkoxy chain it is possible to modify in a controlled way the molecular density of the "floating" overlayer without affecting the molecular assembly. This system is especially suitable for engineering molecular assemblies because it represents one of the few 2D molecular arrays with specific functionality where the structural properties can be tuned in a controlled way, while preserving the molecular pattern

We report on the electronic properties of an artificial system obtained by the intercalation of equiatomic FeCo layers under graphene grown on Ir(111). Upon intercalation, the FeCo film grows epitaxially on Ir(111), resulting in a lattice-mismatched system. By performing density functional theory calculations, we show that the intercalated FeCo layer leads to a pronounced corrugation of the graphene film. At the same time, the FeCo intercalated layers induce a clear transition from a nearly undisturbed to a strongly hybridized graphene π-band, as measured by angle-resolved photoemission spectroscopy. A comparison of experimental results with the computed band structure and the projected density of states unveils a spin-selective hybridization between the π band of graphene and FeCo-3d states. Our results demonstrate that the reduced dimensionality, as well as the hybridization within the FeCo layers, induces a narrowing and a clear splitting of Fe 3d-up and Fe 3d-down-spin bands of the confined FeCo layers with respect to bulk Fe and Co.

Porphyrins are a versatile class of molecules, which have attracted attention over the years due to their electronic, optical and biological properties. Self-assembled monolayers of porphyrins were widely studied on metal surfaces in order to understand the supramolecular organization of these molecules, which is a crucial step towards the development of devices starting from the bottom-up approach. This perspective could lead to tailor the interfacial properties of the surface, depending on the specific interaction between the molecular assembly and the metal surface. In this study, we revisit the investigation of the assembly of zinc-tetraphenylporphyrins on Au(111) in order to explore the adsorption of the molecular network on the noble metal substrate. The combined analysis of scanning tunneling microscopy (STM) imaging and core levels photoemission spectroscopy measurements support a peculiar arrangement of the ZnTPP molecular network, with Zn atoms occupying the bridge sites of the Au surface atoms. Furthermore, we prove that, at few-layers coverage, the interaction between the deposited layers allows a relevant molecular mobility of the adlayer, as observed by STM and supported by core levels photoemission analysis.

The dispersion of the electronic states of epitaxial graphene (Gr) depends significantly on the strength of the bonding with the underlying substrate. We report on empty electron states in cobalt-intercalated Gr grown on Ir(111), studied by angle-resolved inverse photoemission spectroscopy and x-ray absorption spectroscopy, complemented with density functional theory calculations. The weakly bonded Gr on Ir preserves the peculiar spectroscopic features of the Gr band structure, and the empty spectral densities are almost unperturbed. Upon intercalation of a Co layer, the electronic response of the interface changes, with an intermixing of the Gr π* bands and Co d states, which breaks the symmetry of π/σ states, and a downshift of the upper part of the Gr Dirac cone. Similarly, the image potential of Ir(111) is unaltered by the Gr layer, while a downward shift is induced upon Co intercalation, as unveiled by the image state energy dispersion mapped in a large region of the surface Brillouin zone.

The moire superstructure of graphene grown on metals can drive the assembly of molecular architectures, such as iron phthalocyanine (FePc) molecules, allowing for the production of artificial molecular configurations. A detailed analysis of the Gr/Co interaction upon intercalation (including a modeling of the resulting moire pattern) is performed here by density functional theory, which provides an accurate description of the template as a function of the corrugation parameters. The theoretical results are a preliminary step to describe the interaction process of the FePc molecules adsorption on the Gr/Co system. Core level photoemission and absorption spectroscopies have been employed to control the preferential adsorption regions of the FePc on the graphene moire superstructure and the interaction of the central Fe ion with the underlying Co. Our results show that, upon molecular adsorption, the distance of C atoms from the Co template mainly drives the strength of the molecules-substrate interaction, thereby allowing for locally different electronic properties within the corrugated interface.

By means of angle-resolved photoemission spectroscopy (ARPES) measurements, we unveil the electronic band structure of three-dimensional PbBi6Te10 topological insulator. ARPES investigations evidence multiple coexisting Dirac surface states at the zone-center of the reciprocal space, displaying distinct electronic band dispersion, different constant energy contours, and Dirac point energies. We also provide evidence of Rashba-like split states close to the Fermi level, and deeper M- and V-shaped bands coexisting with the topological surface states. The experimental findings are in agreement with scanning tunneling microscopy measurements revealing different surface terminations according to the crystal structure of PbBi6Te10. Our experimental results are supported by density functional theory calculations predicting multiple topological surface states according to different surface cleavage planes.

The opening of a gap in single-layer graphene is often ascribed to the breaking of the equivalence between the two carbon sublattices. It is shown by angle-resolved photoemission spectroscopy that Ir- and Na-modified graphene grown on the Ir(111) surface presents a very large unconventional gap that can be described in terms of a phenomenological massless Dirac model. The consequences and differences of this model are discussed in comparison of the standard massive gap model, and the conditions under which such anomalous gap can arise from a spontaneous symmetry breaking are investigated.

Graphene grown on crystalline metal surfaces is a good candidate to act as a buffer layer between the metal and organic molecules that are deposited on top, because it offers the possibility to control the interaction between the substrate and the molecules. High-resolution angular-resolved ultraviolet photo electron spectroscopy (ARPES) is used to determine the interaction states of iron phthalocyanine molecules that are adsorbed onto graphene on Ni(111). The iron phthalocyanine deposition induces a quenching of the Ni d surface minority band and the appearance of an interface state on graphene/Ni(111). The results have been compared to the deposition of iron phthalocyanine on graphene/Ir(111), for which a higher decoupling of the organic molecule from the underlying metal is exerted by the graphene buffer layer.

We report on the structural and electronic properties of an artificial graphene/Ni(111) system obtained by the intercalation of a monatomic layer of Ni in graphene/Ir(111). Upon intercalation, Ni grows epitaxially on Ir(111), resulting in a lattice-mismatched graphene/Ni system. By performing scanning tunneling microscopy measurements and density functional theory calculations, we show that the intercalated Ni layer leads to a pronounced buckling of the graphene film. At the same time, an enhanced interaction is measured by angle-resolved photoemission spectroscopy, showing a clear transition from a nearly undisturbed to a strongly hybridized graphene pi-band. A comparison of the intercalation-like graphene system with flat graphene on bulk Ni(111), and mildly corrugated graphene on Ir(111), allows us to disentangle the two key properties which lead to the observed increased interaction, namely lattice matching and electronic interaction. Although the latter determines the strength of the hybridization, we find an important influence of the local carbon configuration resulting from the lattice mismatch. DOI: 10.1103/PhysRevB.87.035420

The interface between graphene and the substrate plays a vital role for graphene based applications. It influences the electronic properties concerning the position and form of the Dirac cone, furthermore spin-filtering capabilities are predicted for magnetic substrates [1]. Apart from direct growth of graphene on a small number of substrates, a variety of metals can be intercalated between graphene and the substrate interface [2,3] and allow for the production of graphene on a large number of materials. In this work, we present the investigation of Ni intercalation underneath graphene on Ir(111) and Rh(111). The atomic structure and electronic properties were investigated for samples with intercalated Ni ranging from a submonolayer to few monolayers.

By combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy we reveal the structural and electronic properties of multilayer graphene on Ru(0001). We prove that large ethylene exposure allows the synthesis of two distinct phases of bilayer graphene with different properties. The first phase has Bernal AB stacking with respect to the first graphene layer and displays weak vertical interaction and electron doping. The long-range ordered moiré pattern modulates the crystal potential and induces replicas of the Dirac cone and minigaps. The second phase has an AA stacking sequence with respect to the first layer and displays weak structural and electronic modulation and p-doping. The linearly dispersing Dirac state reveals the nearly freestanding character of this novel second-layer phase.

We investigate the effects of Na adsorption on the electronic structure of bare and Ir cluster superlattice-covered epitaxial graphene on Ir(111) using angle-resolved photoemission spectroscopy and scanning tunneling microscopy. At Na saturation coverage, a massive charge migration from sodium atoms to graphene raises the graphene Fermi level by ~1.4 eV relative to its neutrality point. We find that Na is adsorbed on top of the graphene layer, and when coadsorbed onto an Ir cluster superlattice, it results in the opening of a large band gap of Δ(Na/Ir/G) = 740 meV, comparable to the one of Ge and with preserved high group velocity of the charge carriers.