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Ultralow-energy (ULE) ion implantation is increasingly being explored as a method to substitutionally dope graphene. However, complex implantation-related effects such as defect creation and surface contamination, and how they can be minimized by thermal annealing, remain poorly understood. Here, we address these open questions taking as the model case epitaxial graphene grown on Cu(111), which was subsequently ULE implanted with Mn at 40 eV and then studied as a function of annealing temperature under ultrahigh vacuum. While significant surface cleaning occurs at annealing temperatures as low as 200 °C, recovery from the implantation-induced disorder requires at least 525 °C. Upon high-temperature annealing, in the 600–700 °C range, the Mn atoms that were incorporated upon implantation as intercalated species (between graphene and the Cu surface) experience diffusion into the Cu layer, creating a subsurface alloy. Annealing at 700 °C restored implanted graphene to a nearly pristine state, with a well-ordered graphene lattice with substitutional Mn atoms and a well-defined Dirac cone. In addition to the insight into the complex physicochemical effects induced by thermal annealing, our results provide useful guidelines for future experimental studies on graphene that is modified (e.g., substitutionally doped) by using ULE ion implantation.

Impurity doping in silicon (Si) ultra-large-scale integration is one of the key challenges which prevent further device miniaturization. Using ultraviolet photoelectron spectroscopy and X-ray absorption spectroscopy in the total fluorescence yield mode, we show that the lowest unoccupied and highest occupied electronic states of ≤3 nm thick SiO2-coated Si nanowells shift by up to 0.2 eV below the conduction band and ca. 0.7 eV below the valence band edge of bulk silicon, respectively. This nanoscale electronic structure shift induced by anions at surfaces (NESSIAS) provides the means for low-nanoscale intrinsic Si (i-Si) to be flooded by electrons from an external (bigger, metallic) reservoir, thereby getting highly electron- (n-) conductive. While our findings deviate from the behavior commonly believed to govern the properties of silicon nanowells, they are further confirmed by the fundamental energy gap as per nanowell thickness when compared against published experimental data. Supporting our findings further with hybrid density functional theory calculations, we show that other group IV semiconductors (diamond, Ge) do respond to the NESSIAS effect in accord with Si. We predict adequate nanowire cross-sections (X-sections) from experimental nanowell data with a recently established crystallographic analysis, paving the way to undoped ultrasmall silicon electronic devices with significantly reduced gate lengths, using complementary metal–oxide–semiconductor-compatible materials.

We report the incorporation of substitutional Mn atoms in high-quality, epitaxial graphene on Cu(111), using ultralow-energy ion implantation. We characterize in detail the atomic structure of substitutional Mn in a single carbon vacancy and quantify its concentration. In particular, we are able to determine the position of substitutional Mn atoms with respect to the Moiré superstructure (i.e., local graphene-Cu stacking symmetry) and to the carbon sublattice; in the out-of-plane direction, substitutional Mn atoms are found to be slightly displaced toward the Cu surface, that is, effectively underneath the graphene layer. Regarding electronic properties, we show that graphene doped with substitutional Mn to a concentration of the order of 0.04%, with negligible structural disorder (other than the Mn substitution), retains the Dirac-like band structure of pristine graphene on Cu(111), making it an ideal system in which to study the interplay between local magnetic moments and Dirac electrons. Our work also establishes that ultralow-energy ion implantation is suited for substitutional magnetic doping of graphene. Given the flexibility, reproducibility, and scalability inherent to ion implantation, our work creates numerous opportunities for research on magnetic functionalization of graphene and other two-dimensional materials.

A flat energy dispersion of electrons at the Fermi level of a material leads to instabilities in the electronic system and can drive phase transitions. Here we show that the flat band in graphene can be achieved by sandwiching a graphene monolayer by two cesium (Cs) layers. We investigate the flat band by a combination of angle-resolved photoemission spectroscopy experiment and the calculations. Our work highlights that charge transfer, zone folding of graphene bands, and the covalent bonding between C and Cs atoms are the origin of the flat energy band formation. Analysis of the Stoner criterion for the flat band suggests the presence of a ferromagnetic instability. The presented approach is an alternative route for obtaining flat band materials to twisting bilayer graphene which yields thermodynamically stable flat band materials in large areas.

The electronic structure and the geometric arrangement of picene molecules adsorbed on Ag(111) were studied by means of photoemission and near-edge x-ray absorption fine-structure spectroscopies (NEXAFS). While the valence band of a picene monolayer shows a clear metallic state that evolves with alkali metal doping, in the case of a picene multilayer there is no evidence of metallic states evolving with doping. Our data suggest that the bulk ultra-high-vacuum films of Kxpicene are in an insulating phase and we attribute this behavior to a strong electron-electron coulomb interaction that is instead screened in the monolayer. The NEXAFS profiles of different picene layers show a coverage-dependent orientation, from flat molecules (monolayer) to an orientation of ~ 40˚ of the molecular long axis with respect to the substrate surface (multilayer). The observed molecular orientations are in disagreement with the expected crystal structure of the bulk material and may explain the presence of insulating states in strongly correlated doped picene multilayers.

Despite its broad potential applications, substitution of carbon by transition metal atoms in graphene has so far been explored only to a limited extent. We report the realization of substitutional Mn doping of graphene to a record high atomic concentration of 0.5%, which was achieved using ultralow-energy ion implantation. By correlating the experimental data with the results of ab initio Born–Oppenheimer molecular dynamics calculations, we infer that direct substitution is the dominant mechanism of impurity incorporation. Thermal annealing in ultrahigh vacuum provides efficient removal of surface contaminants and additional implantation-induced disorder, resulting in Mn-doped graphene that, aside from the substitutional Mn impurities, is essentially as clean and defect-free as the as-grown layer. We further show that the Dirac character of graphene is preserved upon substitutional Mn doping, even in this high concentration regime, making this system ideal for studying the interaction between Dirac conduction electrons and localized magnetic moments. More generally, these results show that ultralow energy ion implantation can be used for controlled functionalization of graphene with substitutional transition-metal atoms, of relevance for a wide range of applications, from magnetism and spintronics to single-atom catalysis.

The modifications of chemical and magnetic properties of hybrid ferromagnetic/organic interfaces composed of Co (Ni) as the top layer and iron phthalocyanine, FePc, as a thin film (deposited on Cu surfaces) are studied by means of X-ray photoemission and absorption spectroscopies. The bond formation between Co (Ni) and carbon and nitrogen atoms is indicated by the presence of additional features in C 1s and N 1s core level spectra. The interaction between Co (Ni) atoms and Fe within FePc induces an overall redistribution of 3d orbital population, as shown by the decrease of the a1gb1gratio with respect to one of the noninteracting FePc films. X-ray magnetic circular dichroism (XMCD) reveals in-plane magnetization of the Co (Ni) film on top of FePc, which appears at room temperature and in remanence for Co and Ni film thicknesses of 0.4 and 4 nm, respectively. In the case of the Co/FePc interface, we studied the magnetic response in the presence of a field of 6 T and in remanence observing the Fe XMCD intensity and line shape. The differences in XMCD spectra are related to the co-existence of two contributions to the chemical and magnetic interactions according to the distance between the molecules and the metal interface. The closest to the metal top layer chemically bind and align with its magnetization, whereas the farthest have no preferential bonding and magnetic alignment, except in the presence of a large external magnetic field. These findings are relevant to the understanding and the development of hybrid organic/inorganic spin devices.

We employ ultra-high vacuum (UHV) Raman spectroscopy in tandem with angle-resolved photoemission (ARPES) to investigate the doping-dependent Raman spectrum of epitaxial graphene on Ir(111). The evolution of Raman spectra from pristine to heavily Cs doped graphene up to a carrier concentration of 4.4 × 1014cm–2is investigated. At this doping, graphene is at the onset of the Lifshitz transition and renormalization effects reduce the electronic bandwidth. The optical transition at the saddle point in the Brillouin zone then becomes experimentally accessible by ultraviolet (UV) light excitation, which achieves resonance Raman conditions in close vicinity to the van Hove singularity in the joint density of states. The position of the Raman Gband of fully doped graphene/Ir(111) shifts down by ∼60 cm–1. The Gband asymmetry of Cs doped epitaxial graphene assumes an unusual strong Fano asymmetry opposite to that of the Gband of doped graphene on insulators. Our calculations can fully explain these observations by substrate dependent quantum interference effects in the scattering pathways for vibrational and electronic Raman scattering.

Graphene is one of the most promising materials for nanoelectronics owing to its unique Dirac cone-like dispersion of the electronic state and high mobility of the charge carriers. However, to facilitate the implementation of the graphene-based devices, an essential change of its electronic structure, a creation of the band gap should controllably be done. Brought about by two fundamentally different mechanisms, a sublattice symmetry breaking or an induced strong spin–orbit interaction, the band gap appearance can drive graphene into a narrow-gap semiconductor or a 2D topological insulator phase, respectively, with both cases being technologically relevant. The later case, characterized by a spin–orbit gap between the valence and conduction bands, can give rise to the spin-polarized topologically protected edge states. Here, we study the effect of the spin–orbit interaction enhancement in graphene placed in contact with a lead monolayer. By means of angle-resolved photoemission spectroscopy, we show that intercalation of the Pb interlayer between the graphene sheet and the Pt(111) surface leads to formation of a gap of ∼200 meV at the Dirac point of graphene. Spin-resolved measurements confirm the splitting to be of a spin–orbit nature, and the measured near-gap spin structure resembles that of the quantum spin Hall state in graphene, proposed by Kane and Mele [Phys. Rev. Lett.2005, 95, 226801]. With a bandstructure tuned in this way, graphene acquires a functionality going beyond its intrinsic properties and becomes more attractive for possible spintronic applications.

The two-dimensional electron system (2DES) located at the surface of strontium titanate (STO) and at several other STO-based interfaces has been an established platform for the study of novel physical phenomena since its discovery. Here we report how the interfacing of STO and tetracyanoquinodimethane (TCNQ) results in a charge transfer that depletes the number of free carriers at the STO surface, with a strong impact on its electronic structure. Our study paves the way for efficient tuning of the electronic properties, which promises novel applications in the framework of oxide/organic-based electronics.

Here, it is demonstrated, using high-resolution X-ray spectroscopy and density functional theory calculations, that 2H-tetraphenyl porphyrins metalate at room temperature by incorporating a surface metal atom when a (sub)monolayer is deposited on 3d magnetic substrates, such as Fe(110) and Ni(111). The calculations demonstrate that the redox metalation reaction would be exothermic when occurring on a Ni(111) substrate with an energy gain of 0.89 eV upon embedding a Ni adatom in the macrocycle. This is a novel way to form, viachemical modification and supramolecular engineering, 3d-metal–organic networks on magnetic substrates with an intimate bond between the macrocycle molecular metal ion and the substrate atoms. The achievement of a complete metalation by Fe and Ni can be regarded as a test case for successful preparation of spintronic devices by means of molecular-based magnets and inorganic magnetic substrates.

Scratching the surface: Formation of a monolayer of 2H‐tetraphenylporphyrins (2H‐TPP) on Ag(111), either by sublimation of a multilayer in the range 525–600 K or by annealing (at the same temperature) a monolayer deposited at room temperature, induces a chemical modification of the molecules (see figure). Rotation of the phenyl rings into a flat conformation is observed and tentatively explained, by using DFT calculations, as a peculiar reaction due to molecular dehydrogenation.

We present a combined experimental and theoretical study of the ultrathin film of Co-octaethylporphyrin (Co-OEP) molecules deposited on the Ag(110) surface. The morphological and electronic properties of this heterogeneous metallorganic interface were studied by means of scanning tunneling microscopy (STM), near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy, ultraviolet photoemission spectroscopy (UPS), and density functional theory calculations (DFT). The long-range self-ordered single layer of Co-OEP was obtained by thermal desorption of the molecular multilayer. The single-layer molecules were arranged in a noncommensurate rectangular lattice aligned with the substrate high-symmetry directions. The combination of experimental techniques and numerical simulations indicated that in this configuration each molecular macrocycle is tilted with respect to the metal surface of about 15°. The strong molecular interaction with the substrate leads to the electron transfer from the metal substrate to the molecule. The direct interaction with the substrate involves mostly the Co metal center, which modifies the valence state with respect to the free Co-OEP molecules due to the hybridization between Co states and Ag bands.

In-situ metalation of porphyrin molecules in ultrahigh vacuum (UHV) is of great interest for the characterization of pure species in a controlled environment. Here, we report the characterization of the electronic states and the moleculesʼ geometrical adaptation during the formation of pure 2H-5,10,15,20-tetraphenylporphyrin (2H-TPP) and Fe- tetraphenylporphyrin (Fe-TPP) layers on Ag(111) single crystal. Core level absorption spectra indicate the flat conformation of the monolayer suggesting an adatom hopping instead of a surface mediated dopant diffusion for the metalation process. Photoemission points out that the interaction between Fe dz-states and Ag bands increases the monolayer metallic character already induced by the charge transfer from the substrate.

Three distinct wet chemistry recipes were applied to hydrogen‐terminated n‐ and p‐Si(100) surfaces in a comparative study of the covalent grafting of two differently substituted 2,2′‐bipyridines. The applied reactions require the use of heat, or visible light under a controlled atmosphere, or a suitable potential in an electrochemical cell. In this last case, hydrogen‐terminated silicon is the working electrode in a cathodic electrografting (CEG) reaction, in which it is kept under reduction conditions. The resulting SiC bound hybrids were characterized by a combination of AFM, dynamic contact‐angle, and XPS analysis, with the help of theoretical calculations. The three distinct approaches were found to be suitable for obtaining ligand‐functionalized Si surfaces. CEG resulted in the most satisfactory anchoring procedure, because of its better correlation between high coverage and preservation of the Si surface from both oxidation and contamination. The corresponding Si–bipyridine hybrid was reacted in a solution of CH3CN containing CuIions coordinatively bound to the anchored ligands, as evidenced from the XPS binding‐energy shift of the N atom donor functions. The reaction gave a 1:2 Cu–bipyridine surface complex, in which two ligands couple to a single CuIion. The surface complex was characterized by the Cu Auger parameter and Cu/N XPS atomic‐ratio values coincident with those for pure, unsupported CuIcomplex with the same 2,2′‐bipyridine. Further support for such a specific metal–ligand interaction at the functionalized Si surface came from the distinct values of Cu2p binding energy and the Cu Auger parameter, which were obtained for the species resulting from CuIion uptake on hydrogen‐terminated Si(100).

Semiconducting O-doped polycyclic aromatic hydrocarbons constitute a class of molecules whose optoelectronic properties can be tailored by acting on the π-extension of the carbon-based frameworks and on the oxygen linkages. Although much is known about their photophysical and electrochemical properties in solution, their self-assembly interfacial behavior on solid substrates has remained unexplored so far. In this paper, we have focused our attention on the on-surface self-assembly of O-doped bi-perylene derivatives. Their ability to assemble in ordered networks on Cu(111) single-crystalline surfaces allowed a combination of structural, morphological, and spectroscopic studies. In particular, the exploitation of the orbital mapping methodology based on angle-resolved photoemission spectroscopy, with the support of scanning tunneling microscopy and low-energy electron diffraction, allowed the identification of both the electronic structure of the adsorbates and their geometric arrangement. Our multi-technique experimental investigation includes the structure determination from powder X-ray diffraction data for a specific compound and demonstrates that the electronic structure of such large molecular self-assembled networks can be studied using the reconstruction methods of molecular orbitals from photoemission data even in the presence of segregated chiral domains.