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To understand the optical and transport properties of graphene nanoribbons, an unambiguous determination of their electronic band structure is needed. In this work we demonstrate that the photoemission intensity of each valence sub-band, formed due to the quantum confinement in quasi-one-dimensional (1D) graphene nanoribbons, is a peaked function of the two-dimensional (2D) momentum. We resolve the long-standing discrepancy regarding the valence band effective mass (m∗VB) of armchair graphene nanoribbons with a width of N = 7 carbon atoms (7-AGNRs). In particular, angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy report m∗VB .0.2 and .0.4 of the free electron mass (me), respectively. ARPES mapping in the full 2D momentum space identifies the experimental conditions for obtaining a large intensity for each of the three highest valence 1D sub-bands. Our detail map reveals that previous ARPES experiments have incorrectly assigned the second sub-band as the frontier one. The correct frontier valence sub-band for 7-AGNRs is only visible in a narrow range of emission angles. For this band we obtain an ARPES derived effective mass of 0.4 me, a charge carrier velocity in the linear part of the band of 0.63 106 m s.1 and an energy separation of only .60 meV to the second sub-band. Our results are of importance not only for the growing research field of graphene nanoribbons but also for the community, which studies quantum confined systems.

We investigate the N = 9 atoms wide armchair-type graphene nanoribbons (9-AGNRs) by performing a comprehensive spectroscopic and microscopic characterization of this novel material. In particular, we use X-ray photoelectron, near edge X-ray absorption fine structure, scanning tunneling, polarized Raman and angle-resolved photoemission (ARPES) spectroscopies. The ARPES measurements are aided by calculations of the photoemission matrix elements which yield the position in k space having the strongest photoemission cross section. Comparison with well-studied narrow N = 7 AGNRs shows that the effective electron mass in 9-AGNRs is reduced by two times and the valence band maximum is shifted to lower binding energy by ∼0.6 eV. In polarized Raman measurements of the aligned 9-AGNR, we reveal anisotropic signal depending upon the phonon symmetry. Our results indicate the 9-AGNRs are a novel 1D semiconductor with a high potential in nanoelectronic applications.

The discovery of a square magnetic-skyrmion lattice in GdRu 2 Si 2 , with the smallest so far found skyrmion size and without a geometrically frustrated lattice, has attracted significant attention. In this work, we present a comprehensive study of surface and bulk electronic structures of GdRu 2 Si 2 by utilizing momentum-resolved photoemission (ARPES) measurements and first-principles calculations. We show how the electronic structure evolves during the antiferromagnetic transition when a peculiar helical order of 4f magnetic moments within the Gd layers sets in. A nice agreement of the ARPES-derived electronic structure with the calculated one has allowed us to characterize the features of the Fermi surface (FS), unveil the nested region along k z at the corner of the 3D FS, and reveal their orbital compositions. Our findings suggest that the Ruderman-Kittel-Kasuya-Yosida interaction plays a decisive role in stabilizing the spiral-like order of Gd 4f moments responsible for the skyrmion physics in GdRu 2 Si 2 . Our results provide a deeper understanding of electronic and magnetic properties of this material, which is crucial for predicting and developing novel skyrmion-based systems., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Collective spin excitations in magnetically ordered crystals, called magnons or spin waves, can serve as carriers in novel spintronic devices with ultralow energy consumption. The generation of well-detectable spin flows requires long lifetimes of high-frequency magnons. In general, the lifetime of spin waves in a metal is substantially reduced due to a strong coupling of magnons to the Stoner continuum. This makes metals unattractive for use as components for magnonic devices. Here, we present the metallic antiferromagnet CeCo 2 P 2 , which exhibits long-living magnons even in the terahertz (THz) regime. For CeCo 2 P 2 , our first-principle calculations predict a suppression of low-energy spin-flip Stoner excitations, which is verified by resonant inelastic X-ray scattering measurements. By comparison to the isostructural compound LaCo 2 P 2 , we show how small structural changes can dramatically alter the electronic structure around the Fermi level leading to the classical picture of the strongly damped magnons intrinsic to metallic systems. Our results not only demonstrate that long-lived magnons in the THz regime can exist in bulk metallic systems, but they also open a path for an efficient search for metallic magnetic systems in which undamped THz magnons can be excited., (© 2023. Springer Nature Limited.)

The orientation of the 4f moments offers an additional degree of freedom for engineering the spin-related properties in spintronic nanostructures of lanthanides. Yet, precise monitoring of the direction of magnetic moments remains a challenge. Here, on the example of the antiferromagnets HoRh 2 Si 2 and DyRh 2 Si 2 , we investigate the temperature-dependent canting of the 4f moments near the surface. We demonstrate that this canting can be understood in the framework of crystal electric field theory and the exchange magnetic interaction. Using photoelectron spectroscopy, we disclose subtle but certain temperature-dependent changes in the line shape of the 4f multiplet. These changes are directly linked to the canting of the 4f moments, which is different for the individual lanthanide layers near the surface. Our results illustrate the opportunity to monitor the orientation of the 4f-moments with high precision, which is essential for development of novel lanthanide-based nanostructures, interfaces, supramolecular complexes, and single-molecule magnets for various applications.

Magnetic topological insulators (MTIs) have recently become a subject of poignant interest; among them, Z 2 topological insulators with magnetic moment ordering caused by embedded magnetic atoms attract special attention. In such systems, the case of magnetic anisotropy perpendicular to the surface that holds a topologically nontrivial surface state is the most intriguing one. Such materials demonstrate the quantum anomalous Hall effect, which manifests itself as chiral edge conduction channels that can be manipulated by switching the polarization of magnetic domains. In the present paper, we uncover the atomic structure of the bulk and the surface of Mn 0.06 Sb 1.22 Bi 0.78 Te 3.06 in conjunction with its electronic and magnetic properties; this material is characterized by naturally formed ferromagnetic layers inside the insulating matrix, where the Fermi level is tuned to the bulk band gap. We found that in such mixed crystals septuple layers (SLs) of Mn(Bi,Sb) 2 Te 4 form structures that feature three SLs, each of which is separated by two or three (Bi,Sb) 2 Te 3 quintuple layers (QLs); such a structure possesses ferromagnetic properties. The surface obtained by cleavage includes terraces with different terminations. Manganese atoms preferentially occupy the central positions in the SLs and in a very small proportion can appear in the QLs, as indirectly indicated by a reshaped Dirac cone.

We show that graphene can be magnetized by coupling to a ferromagnetic Co film through a Au monolayer. The presence of dislocation loops under graphene leads to a ferrimagnetic ordering of moments in the two C sublattices. It is shown that the band gap of ∼80  meV in the K[over ¯] point has a magnetic nature and exists for ferrimagnetic ordering. Interplay between Rashba and exchange couplings is evidenced by spin splitting asymmetry in spin-ARPES measurements and fully supported by DFT calculation of a (9×9) unit cell. Owing to sign-opposite Berry curvatures for K[over ¯] and K[over ¯]^{'} valleys, the synthesized system is promising for the realization of a circular dichroism Hall effect.

To use efficiently the magnetic functionalities emerging at the surfaces or interfaces of novel lanthanides-based materials, there is a need for complementary methods to probe the atomic-layer resolved magnetic properties. Here, we show that 4f photoelectron spectroscopy is highly sensitive to the collective orientation of 4f magnetic moments and, thus, a powerful tool for characterizing the related properties. To demonstrate this, we present the results of systematic study of a family of layered crystalline 4f-materials, which are crystallized in the body-centered tetragonal ThCr 2 Si 2 structure. Analysis of 4f spectra indicates that the 4f moments at the surface experience a strong reorientation with respect to the bulk, caused by changes of the crystal-electric field. The presented database of the computed 4f spectra for all trivalent rare-earth ions in their different M J states will facilitate the estimation of the orientation of the 4f magnetic moments in the layered 4f-systems for efficient control of their magnetic properties.

A magnetic field modifies optical properties and provides valley splitting in a molybdenum disulfide (MoS 2 ) monolayer. Here we demonstrate a scalable approach to the epitaxial synthesis of MoS 2 monolayer on a magnetic graphene/Co system. Using spin- and angle-resolved photoemission spectroscopy we observe a magnetic proximity effect that causes a 20 meV spin-splitting at the Γ̅ point and canting of spins at the K̅ point in the valence band toward the in-plane direction of cobalt magnetization. Our density functional theory calculations reveal that the in-plane spin component at K̅ is localized on Co atoms in the valence band, while in the conduction band it is localized on the MoS 2 layer. The calculations also predict a 16 meV spin-splitting at the Γ̅ point and 8 meV K̅- K ' ¯ valley asymmetry for an out-of-plane magnetization. These findings suggest control over optical transitions in MoS 2 via Co magnetization. Our estimations show that the magnetic proximity effect is equivalent to the action of the magnetic field as large as 100 T.

The f-driven temperature scales at the surfaces of strongly correlated materials have increasingly come into the focus of research efforts. Here, we unveil the emergence of a two-dimensional Ce Kondo lattice, which couples ferromagnetically to the ordered Co lattice below the P-terminated surface of the antiferromagnet CeCo 2 P 2 . In its bulk, Ce is passive and behaves tetravalently. However, because of symmetry breaking and an effective magnetic field caused by an uncompensated ferromagnetic Co layer, the Ce 4f states become partially occupied and spin-polarized near the surface. The momentum-resolved photoemission measurements indicate a strong admixture of the Ce 4f states to the itinerant bands near the Fermi level including surface states that are split by exchange interaction with Co. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario. Our findings show how rich and diverse the f-driven properties can be at the surface of materials without f-physics in the bulk.

Discovered in 1962, the divalent ferromagnetic semiconductor EuS ( T C = 16.5 K, E g = 1.65 eV) has remained constantly relevant to the engineering of novel magnetically active interfaces, heterostructures, and multilayer sequences and to combination with topological materials. Because detailed information on the electronic structure of EuS and, in particular, its evolution across T C is not well-represented in the literature but is essential for the development of new functional systems, the present work aims at filling this gap. Our angle-resolved photoemission measurements complemented with first-principles calculations demonstrate how the electronic structure of EuS evolves across a paramagnetic-ferromagnetic transition. Our results emphasize the importance of the strong Eu 4f-S 3p mixing for exchange-magnetic splittings of the sulfur-derived bands as well as coupling between f and d orbitals of neighboring Eu atoms to derive the value of T C accurately. The 4f-3p mixing facilitates the coupling between 4f and 5d orbitals of neighboring Eu atoms, which mainly governs the exchange interaction in EuS.

Monolayer hexagonal boron nitride (hBN) is attracting considerable attention because of its potential applications in areas such as nano- and opto-electronics, quantum optics and nanomagnetism. However, the implementation of such functional hBN demands precise lateral nanostructuration and integration with other two-dimensional materials, and hence, novel routes of synthesis beyond exfoliation. Here, a disruptive approach is demonstrated, namely, imprinting the lateral pattern of an atomically stepped one-dimensional template into a hBN monolayer. Specifically, hBN is epitaxially grown on vicinal Rhodium (Rh) surfaces using a Rh curved crystal for a systematic exploration, which produces a periodically textured, nanostriped hBN carpet that coats Rh(111)-oriented terraces and lattice-matched Rh(337) facets with tunable width. The electronic structure reveals a nanoscale periodic modulation of the hBN atomic potential that leads to an effective lateral semiconductor multi-stripe. The potential of such atomically thin hBN heterostructure for future applications is discussed., (© 2021 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates.

Renewed interest in the ferroelectric semiconductor germanium telluride was recently triggered by the direct observation of a giant Rashba effect and a 30-year-old dream about a functional spin field-effect transistor. In this respect, all-electrical control of the spin texture in this material in combination with ferroelectric properties at the nanoscale would create advanced functionalities in spintronics and data information processing. Here, we investigate the atomic and electronic properties of GeTe bulk single crystals and their (111) surfaces. We succeeded in growing crystals possessing solely inversion domains of ∼10 nm thickness parallel to each other. Using HAADF-TEM we observe two types of domain boundaries, one of them being similar in structure to the van der Waals gap in layered materials. This structure is responsible for the formation of surface domains with preferential Te-termination (∼68%) as we determined using photoelectron diffraction and XPS. The lateral dimensions of the surface domains are in the range of ∼10-100 nm, and both Ge- and Te-terminations reveal no reconstruction. Using spin-ARPES we establish an intrinsic quantitative relationship between the spin polarization of pure bulk states and the relative contribution of different terminations, a result that is consistent with a reversal of the spin texture of the bulk Rashba bands for opposite configurations of the ferroelectric polarization within individual nanodomains. Our findings are important for potential applications of ferroelectric Rashba semiconductors in nonvolatile spintronic devices with advanced memory and computing capabilities at the nanoscale.

Spin-orbit interaction and structure inversion asymmetry in combination with magnetic ordering is a promising route to novel materials with highly mobile spin-polarized carriers at the surface. Spin-resolved measurements of the photoemission current from the Si-terminated surface of the antiferromagnet TbRh_{2}Si_{2} and their analysis within an ab initio one-step theory unveil an unusual triple winding of the electron spin along the fourfold-symmetric constant energy contours of the surface states. A two-band k·p model is presented that yields the triple winding as a cubic Rashba effect. The curious in-plane spin-momentum locking is remarkably robust and remains intact across a paramagnetic-antiferromagnetic transition in spite of spin-orbit interaction on Rh atoms being considerably weaker than the out-of-plane exchange field due to the Tb 4f moments.

We used x-ray photoemission and absorption spectroscopies to study the influence of thermal molecular oxygen exposure on the h-BN/Co(0001) and h-BN/Au/Co(0001) systems. The spectral analysis was supported by density functional theory calculations. It is shown that oxygen can intercalate h-BN on Co(0001) and also be embedded into its lattice, replacing the nitrogen atoms. Upon substitution, the structures containing one (BN 2 O) and three (BO 3 ) oxygen atoms in the boron atom environment are formed predominantly. In the case of gold-intercalated h-BN, only the (BN 2 O) structures are formed; the long-lasting oxygen exposures lead to etching of the h-BN layer.

Embedding foreign atoms in graphene and interchanging the underlying substrate are proved to be efficient methods for manipulating the properties of graphene. Combining ARPES experiments with DFT calculations we show that boron-doped graphene (B-graphene) grown on a Co(0001) substrate by chemical vapor deposition (CVD) becomes hole doped and its Fermi surface near the K-point reveals strongly spin-polarized states. The latter stems from the spin-polarized mini Dirac cone that is an intrinsic two-dimensional feature of the graphene/Co(0001) interface and is formed by a mixture of C 2p z and Co 3d states. Since the CVD method allows the achievement of up to 20 at% of incorporated B atoms, this provides a certain flexibility for handling the spin-polarized properties of the system. We also show that the bonding of the B-graphene layer to the Co(0001) substrate can be released by intercalation of Li into the interface. This allows the exploration of the doping effect in detail. Finally, our ARPES data indicate a gap opening in the Dirac cone as a result of the highly unbalanced boron concentrations in the two graphene sublattices.

We investigate the electronic and vibrational properties of bottom-up synthesized aligned armchair graphene nanoribbons of N = 7 carbon atoms width periodically doped by substitutional boron atoms (B-7AGNRs). Using angle-resolved photoemission spectroscopy and density functional theory calculations, we find that the dopant-derived valence and conduction band states are notably hybridized with electronic states of Au substrate and spread in energy. The interaction with the substrate leaves the bands with pure carbon character rather unperturbed. This results in an identical effective mass of ≈0.2 m 0 for the next-highest valence band compared with pristine 7AGNRs. We probe the phonons of B-7AGNRs by ultrahigh-vacuum (UHV) Raman spectroscopy and reveal the existence of characteristic splitting and red shifts in Raman modes due to the presence of substitutional boron atoms. Comparing the Raman spectra for three visible lasers (red, green, and blue), we find that interaction with gold suppresses the Raman signal from B-7AGNRs and the energy of the green laser (2.33 eV) is closer to the resonant E 22 transition. The hybridized electronic structure of the B-7AGNR-Au interface is expected to improve electrical characteristics of contacts between graphene nanoribbon and Au. The Raman fingerprint allows the easy identification of B-7AGNRs, which is particularly useful for device fabrication.

Recrystallization of bulk materials is a well-known phenomenon, which is widely used in commercial manufacturing. However, for low-dimensional materials like graphene, this process still remains an unresolved puzzle. Thus, the understanding of the underlying mechanisms and the required conditions for recrystallization in low dimensions is essential for the elaboration of routes towards the inexpensive and reliable production of high-quality nanomaterials. Here, we unveil the details of the efficient recrystallization of one-atom-thick pure and boron-doped polycrystalline graphene layers on a Co(0001) surface. By applying photoemission and electron diffraction, we show how more than 90% of the initially misoriented graphene grains can be reconstructed into a well-oriented and single-crystalline layer. The obtained recrystallized graphene/Co interface exhibits high structural quality with a pronounced sublattice asymmetry, which is important for achieving an unbalanced sublattice doping of graphene. By exploring the kinetics of recrystallization for native and B-doped graphene on Co, we were able to estimate the activation energy and propose a mechanism of this process.

Regardless of the widely accepted opinion that there is no Raman signal from single-layer graphene when it is strongly bonded to a metal surface, we present Raman spectra of a graphene monolayer on Ni(111) and Co(0001) substrates. The high binding energy of carbon to these surfaces allows formation of lattice-matched (1 × 1) structures where graphene is significantly stretched. This is reflected in a record-breaking shift of the Raman G band by more than 100 cm -1 relative to the case of freestanding graphene. Using electron diffraction and photoemission spectroscopy, we explore the aforementioned systems together with polycrystalline graphene on Co and analyze possible intercalation of oxygen at ambient conditions. The results obtained are fully supported by Raman spectroscopy. Performing a theoretical investigation of the phonon dispersions of freestanding graphene and stretched graphene on the strongly interacting Co surface, we explain the main features of the Raman spectra. Our results create a reliable platform for application of Raman spectroscopy in diagnostics of chemisorbed graphene and related materials.

The rapidly developing field of bionanotechnology requires detailed knowledge of the mechanisms of interaction between inorganic matter and biomolecules. Under conditions different from those in an aqueous solution, however, the chemistry of these systems is elusive and may differ dramatically from their interactions in vitro and in vivo. Here, we report for the first time a photoemission study of a metal silver-DNA interface, formed in vacuo, in comparison with DNA-Ag + and fluorescent DNA-Ag complexes formed in solution. The high-resolution photoelectron spectra reveal that in vacuo silver atoms interact mainly with oxygen atoms of the phosphodiester bond and deoxyribose in DNA, in contrast to the behavior of silver ions, which interact preferentially with the nitrogen atoms of the bases. This offers new insight into the mechanism of DNA metallization, which is of importance in creating metal-bio interfaces for nanotechnology applications.

The implementation of future graphene-based electronics is essentially restricted by the absence of a band gap in the electronic structure of graphene. Options of how to create a band gap in a reproducible and processing compatible manner are very limited at the moment. A promising approach for the graphene band gap engineering is to introduce a large-scale sublattice asymmetry. Using photoelectron diffraction and spectroscopy we have demonstrated a selective incorporation of boron impurities into only one of the two graphene sublattices. We have shown that in the well-oriented graphene on the Co(0001) surface the carbon atoms occupy two nonequivalent positions with respect to the Co lattice, namely top and hollow sites. Boron impurities embedded into the graphene lattice preferably occupy the hollow sites due to a site-specific interaction with the Co pattern. Our theoretical calculations predict that such boron-doped graphene possesses a band gap that can be precisely controlled by the dopant concentration. B-graphene with doping asymmetry is, thus, a novel material, which is worth considering as a good candidate for electronic applications.

The full exploration of the potential, which graphene offers to nanoelectronics requires its integration into semiconductor technology. So far the real-world applications are limited by the ability to concomitantly achieve large single-crystalline domains on dielectrics and semiconductors and to tailor the interfaces between them. Here we show a new direct bottom-up method for the fabrication of high-quality atomically precise interfaces between 2D materials, like graphene and hexagonal boron nitride (hBN), and classical semiconductor via Ge intercalation. Using angle-resolved photoemission spectroscopy and complementary DFT modelling we observed for the first time that epitaxially grown graphene with the Ge monolayer underneath demonstrates Dirac Fermions unaffected by the substrate as well as an unperturbed electronic band structure of hBN. This approach provides the intrinsic relativistic 2D electron gas towards integration in semiconductor technology. Hence, these new interfaces are a promising path for the integration of graphene and hBN into state-of-the-art semiconductor technology.

Embedding foreign atoms or molecules in graphene has become the key approach in its functionalization and is intensively used for tuning its structural and electronic properties. Here, we present an efficient method based on chemical vapor deposition for large scale growth of boron-doped graphene (B-graphene) on Ni(111) and Co(0001) substrates using carborane molecules as the precursor. It is shown that up to 19 at. % of boron can be embedded in the graphene matrix and that a planar C-B sp(2) network is formed. It is resistant to air exposure and widely retains the electronic structure of graphene on metals. The large-scale and local structure of this material has been explored depending on boron content and substrate. By resolving individual impurities with scanning tunneling microscopy we have demonstrated the possibility for preferential substitution of carbon with boron in one of the graphene sublattices (unbalanced sublattice doping) at low doping level on the Ni(111) substrate. At high boron content the honeycomb lattice of B-graphene is strongly distorted, and therefore, it demonstrates no unballanced sublattice doping.

Oxygen reduction reaction (ORR) plays a key role in lithium-air batteries (LABs) that attract great attention thanks to their high theoretical specific energy several times exceeding that of lithium-ion batteries. Because of their high surface area, high electric conductivity, and low specific weight, various carbons are often materials of choice for applications as the LAB cathode. Unfortunately, the possibility of practical application of such batteries is still under question as the sustainable operation of LABs with carbon cathodes is not demonstrated yet and the cyclability is quite poor, which is usually associated with oxygen reduced species side reactions. However, the mechanisms of carbon reactivity toward these species are still unclear. Here, we report a direct in situ X-ray photoelectron spectroscopy study of oxygen reduction by lithiated graphene and graphene-based materials. Although lithium peroxide (Li2O2) and lithium oxide (Li2O) reactions with carbon are thermodynamically favorable, neither of them was found to react even at elevated temperatures. As lithium superoxide is not stable at room temperature, potassium superoxide (KO2) prepared in situ was used instead to test the reactivity of graphene with superoxide species. In contrast to Li2O2 and Li2O, KO2 was demonstrated to be strongly reactive.