Search

In this work, we show the feasibility of extreme ultraviolet (EUV) patterning on an HF-treated silicon (100) surface in the absence of a photoresist. EUV lithography is the leading lithography technique in semiconductor manufacturing due to its high resolution and throughput, but future progress in resolution can be hampered because of the inherent limitations of the resists. We show that EUV photons can induce surface reactions on a partially hydrogen-terminated silicon surface and assist the growth of an oxide layer, which serves as an etch mask. This mechanism is different from the hydrogen desorption in scanning tunneling microscopy-based lithography. We achieve silicon dioxide/silicon gratings with 75-nanometer half-pitch and 31-nanometer height, demonstrating the efficacy of the method and the feasibility of patterning with EUV lithography without the use of a photoresist. Further development of the resistless EUV lithography method can be a viable approach to nanometer-scale lithography by overcoming the inherent resolution and roughness limitations of photoresist materials., Science Advances, 9 (16), ISSN:2375-2548

ACS applied nano materials 6(9), 8004 - 8016 (2023). doi:10.1021/acsanm.3c01306, We investigated Cu nanoparticles (NPs) on vicinal and basal ZnO supports to obtain anatomistic picture of the catalyst’s structure under in situ oxidizing and reducing conditions.The Cu/ZnO model catalysts were investigated at elevated gas pressures by highenergy grazing incidence X-ray diffraction and ambient pressure X-ray photoelectronspectroscopy (AP-XPS). We find that the Cu nanoparticles are fully oxidized to Cu$_2$Ounder atmospheric conditions at room temperature. As the nanoparticles swell duringoxidation, they maintain their epitaxy on basal ZnO (000±1) surfaces, whereas on thevicinal ZnO (10$\bar{14}$) surface, the nanoparticles undergo a coherent tilt. We find thatthe oxidation process is fully reversible under H$_2$ flow at 500 K, resulting in predominantlywell-aligned nanoparticles on the basal surfaces, whereas the orientation of CuNPs on vicinal ZnO was only partially restored. The analysis of the substrate crystaltruncation rods evidences the stability of basal ZnO surfaces under all gas conditions.No Cu-Zn bulk alloy formation is observed. Under CO$_2$ flow, no diffraction signalfrom the nanoparticles is detected, pointing to their completely disordered state. TheAP-XPS results are in line with the formation of CuO. Scanning electron microscopyimages show that massive mass transport has set in, leading to the formation of largeragglomerates., Published by ACS Publications, Washington, DC

Occupancy of different structural positions in a crystal lattice often seems to play a key role in material prop-erties. Several experimental techniques have been developed to uncover this issue, all of them being mostly bulk sensitive. However, many materials including topological insulators (TIs), which are among the most intriguing modern materials, are intended to be used in devices as thin films, for which the sublattice occupancy may differ from the bulk. One of the possible approaches to occupancy analysis is X-ray Photoelectron Diffraction (XPD), a structural method in surface science with chemical sensitivity. We applied this method in a case study of Sb2(Te1-xSex)3 mixed crystals, which belong to prototypical TIs. We used high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) as a reference method to verify our analysis. We revealed that the XPD data for vacuum cleaved bulk crystals are in excellent agreement with the reference ones. Also, we demonstrate that the anion occupancy near a naturally formed surface can be rather different from that of the bulk. The present results are relevant for a wide range of compositions where the system remains a topological phase, as we ultimately show by probing the transiently occupied topological surface state above the Fermi level by ultrafast photoemission.

Magnetic topological insulators (MTIs) have recently become a subject of poignant interest; among them, Z

We investigated Cu/ZnO model catalysts for methanol synthesis to obtain an atomistic picture of activation and deactivation processes under in situ oxidizing and reducing conditions. We have investigated Cu nanoparticles with different shapes and aspect ratios grown epitaxially on basal and vicinal ZnO surfaces at elevated gas pressures by high energy grazing incidence X-ray diffraction and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We find that the Cu nanoparticles are fully oxidized to Cu2O under atmospheric conditions at room temperature. During oxidation, they maintain their epitaxy on basal ZnO (000-1) surfaces, whereas on the vicinal ZnO (10-14) surface, the nanoparticles undergo a coherent tilt. We find that the oxidation process is fully reversible under H2 flow at 500 K, resulting in predominantly well-aligned nanoparticles on the basal surfaces, whereas a random orientation is preferred for the (10-14) surface. Under CO2 flow, no diffraction signal from the nanoparticles is detected, pointing to their completely disordered state. The AP-XPS results are in line with the formation of CuO. The analysis of the substrate crystal truncation rods evidences the stability of basal ZnO surfaces under all gas conditions. No proof for Cu-Zn alloy formation is found. Scanning electron microscopy images show that massive mass transport has set in, leading to the formation of larger agglomerates, which is detrimental to the catalyst’s performance.

X-ray Absorption Spectroscopy (XAS) is used for measuring monolayer quantities of Ho3N@C80endofullerene molecules on graphene at a low flux bending magnet beamline. The total electron yield is measured with an Everhart- Thornley detector. In comparison to sample current measurements with the same noise level, our approach reduces data acquisition time and radiation dose by a factor of 25. As the first application of this set-up we report temperature- dependent measurements of the Ho M45 edge with per mille accuracy. This documents the advantages and capabilities of an Everhart-Thornely detector for XAS measurements under low x-ray flux.

Realization of stable spin states in surface-supported magnetic molecules is crucial for their applications in molecular spintronics, memory storage or quantum information processing. In this work, we studied the surface magnetism of dimetallo-azafullerene Tb

IrTe$_2$ undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate the atomic position changes undergone near the surface of $1T-$IrTe$_2$ in the first-order phase transition, from the $(1\times1)$ phase to the $(5\times1)$ phase. Comparison between experiment and simulation allows us to identify the consequence of the dimerization on the Ir atoms local environment. We report that XPD permits to unveil the break of symmetry of IrTe$_2$ trigonal to a monoclonic unit cell and confirm the occurence of the $(5\times1)$ reconstruction within the first few layers below the surface with a staircase-like stacking of dimers., Comment: This is the version of the article before peer review or editing, as submitted by M. Rumo to J. Phys.: Condens. Matter. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at DOI

The advent of on-surface chemistry under vacuum has vastly increased our capabilities to synthesize carbon nanomaterials with atomic precision. Among the types of target structures that have been synthesized by these means, graphene nanoribbons (GNRs) have probably attracted the most attention. In this context, the vast majority of GNRs have been synthesized from the same chemical reaction: Ullmann coupling followed by cyclodehydrogenation. Here, we provide a detailed study of the growth process of five-atom-wide armchair GNRs starting from dibromoperylene. Combining scanning probe microscopy with temperature-dependent XPS measurements and theoretical calculations, we show that the GNR growth departs from the conventional reaction scenario. Instead, precursor molecules couple by means of a concerted mechanism whereby two covalent bonds are formed simultaneously, along with a concomitant dehydrogenation. Indeed, this alternative reaction path is responsible for the straight GNR growth in spite of the initial mixture of reactant isomers with irregular metal–organic intermediates that we find. The provided insight will not only help understanding the reaction mechanisms of other reactants but also serve as a guide for the design of other precursor molecules., We acknowledge financial support from the Spanish Agencia Estatal de Investigación (AEI) and the European Regional Development Fund (FEDER) (Grants Nos. PID2019-107338RB-C63 and PID2019-107338RB-C64) and the Academy of Finland (project numbers 311012 and 314882).

In-plane heterostructures of hexagonal boron nitride (h-BN) and graphene (Gr) have recently appeared in the focus of material science research owing to their intriguing and tunable electronic properties. However, disclosure of the atomic structure and properties of one-dimensional heterojunctions between Gr and h-BN domains remains a largely unexplored and challenging task. Here, we report an approach to obtain a perfectly oriented and atomically thin hybrid h-BN–Gr heterolayer on the Co(0001) surface. A perfect matching of the lattice parameters ensures an epitaxial growth of both Gr and h-BN on the close-packed Co surface. High crystalline quality of the resulting interface allowed us to uncover the structural and electronic properties of the lateral h-BN/Gr heterojunctions by means of complementary microscopic and spectroscopic techniques. In particular, we established the coexistence of two types of zigzag boundaries made of B–C bonds, while the boundaries with N–C bonds were found to be unfavorable. ...

Inert single-layer boron nitride (h-BN) grown on a catalytic metal may be functionalized with quaternary ammonium compounds (quats) that are widely used as nonreactive electrolytes. We observe that the quat treatment, which facilitates the electrochemical transfer of two-dimensional materials, involves a decomposition of quat ions and leads to covalently bound quat derivatives on top of the 2D layer. Applying tetraoctylammonium and h-BN on rhodium, the reaction product is top-alkylized h-BN as identified with high-resolution X-ray photoelectron spectroscopy. The alkyl chains are homogeneously distributed across the surface, and the properties thereof are well-tunable by the choice of different quats. The functionalization further weakens the 2D material-substrate interaction and promotes easy transfer. Therefore, the functionalization scheme that is presented enables the design of 2D materials with tailored properties and with the freedom to position and orient them as required. The mechanism of this functionalization route is investigated with density functional theory calculations, and we identify the proximity of the catalytic metal substrate to alter the chemical reactivity of otherwise inert h-BN layers.

IrTe

The adsorption configurations of a technologically relevant model organic adsorbate on the silicon (001) surface were studied using energy scanned x-ray photoelectron diffraction (PhD). Previous work has established the existence of an interesting vertically-aligned (‘flagpole’) configuration, where the acetophenone attaches to Si(001) via the acetyl group carbon and oxygen atoms. Density functional theory calculations have predicted two energetically similar variants of this structure, where the phenyl ring is orientated parallel or perpendicular to the rows of silicon dimers on this reconstructed surface. However, previously published experimental measurements, including scanning tunnelling microscopy, x-ray photoelectron spectroscopy, and near-edge x-ray absorption fine structure investigations were unable to distinguish between these two configurations. Here, we apply the unique experimental capabilities of the PhD technique to this system and demonstrate that the dominant adsorption configuration has the phenyl ring parallel to the dimer rows (the end-bridge structure). This information in turn facilitates the determination of the dominant reaction pathway for acetophenone on Si(001), which has remained elusive until now. Information about subtle preferences in reaction pathways that affect the alignment and orientation of organic adsorbates such as acetophenone on technologically-relevant semiconductor surfaces such as Si(001) is critical for the fabrication of future atomically-precise atomic and molecular-scale electronic devices utilising the organic-silicon interface, and this work demonstrates the unique and complementary capabilities of PhD for providing this information.

Graphene growth and doping are well studied on flat surfaces of various materials. To further advance the technological implementation of graphene-based systems, fundamental studies need more appropriate model templates, whose surfaces would mimic substrates with non-trivial topography. Here, using electron and photoelectron diffraction and photoemission spectroscopy as well, we demonstrate how a curved tungsten crystal covered by a thin nickel film can properly be used as such platform, allowing the fabrication and comprehensive characterization of nitrogen-doped graphene. We show the way in which nitrogen impurities prefer to embed into the graphene matrix at different areas of the curved metallic surface with variable density of atomic steps. In particular, at atomically flat regions with a strong graphene-metal interaction, pyridinic configuration is the most abundant form of dopants, while graphitic nitrogen strongly dominates in places with a weak coupling of graphene to the substrate, i.e., in the vicinity of the surface irregularities. We recognize single crystals with curvilinear surfaces as versatile platforms for the studies of not only low-dimensional materials, but also molecular adsorption, chemical reactions and catalysis on surfaces with complex structure., The authors acknowledge RFBR for research Grants No. 18-29-19178 and No. 19-32-90013, and SPbU Grant No. 73028629. A. M. acknowledges BMBF Grant No. 05K19KER. F. S., D. V. V. and J. E. O. acknowledge the Spanish Ministry of Science and Innovation (Grants MAT-2017-88374-P), Consejo Superior de Investigaciones Cientificas (Grant 2020AEP178) and the Basque Government (Grant IT-1255-19). The authors thank the Helmholtz-Zentrum Berlin für Materialien und Energie for support within the bilateral Russian-German Laboratory program. This work was supported by the German-Russian Interdisciplinary Science Center (G-RISC: M-2019b-13_r) funded by the German Federal Foreign Office via the German Academic Exchange Service (DAAD).

The underlying mechanism behind the metal-to-insulator transition in ${\mathrm{VO}}_{2}$ is still a topic of intense debate. The two leading theoretical interpretations associate the transition with either electron-lattice or electron-electron correlations. Novel experimental results are required to converge towards one of the two scenarios. Here we report on a temperature-dependent angle-resolved photoelectron study of ${\mathrm{VO}}_{2}$ thin films across the metal-to-insulator transition. The obtained experimental results are compared to density functional theory calculations. We find an overall energy shift and compression of the electronic band structure across the transition while the overall band topology is conserved. The results demonstrate the importance of electron-electron correlations in establishing the insulating state.

Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. A broad spectral phonon line shape is often interpreted as a marker of strong electron-phonon coupling associated with Fermi surface nesting, i.e., parallel sections of the Fermi surface connected by the phonon momentum. Alternatively broad phonons are known to arise from strong atomic lattice anharmonicity. Here, we show that strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations to demonstrate this scenario in the highly anisotropic tetragonal electron-phonon superconductor YNi2B2C. This new scenario likely applies to a wide range of compounds., Phonon anomalies are commonly attributed to Fermi-surface nesting or phonon anharmonicity, but these mechanisms do not apply in all cases. Here, the authors propose a new mechanism based on electron-momentum-dependent electron-phonon coupling in the case of YNi2B2C, that could also apply to other materials.

We present and discuss the methodology for modeling $4f$ photoemission spectra, $4f$ photoelectron diffraction (PED) patterns, and magnetic dichroism effects for rare-earth-based materials. Using PED and magnetic dichroism in photoemission, we explore the electronic and magnetic properties of the near-surface region of the valence-fluctuating material ${\mathrm{EuIr}}_{2}{\mathrm{Si}}_{2}$. For the Eu-terminated surface, we found that the topmost Eu layer is divalent and exhibits a ferromagnetic order below 10 K. The valency of the next Eu layer, that is the fifth atomic layer, is about 2.8 at low temperature that is close to the valency in the bulk. The properties of the Si-terminated surface are drastically different. The first subsurface Eu layer (fourth atomic layer below the surface) behaves divalently and orders ferromagnetically below 48 K. Experimental data indicate, however, that there is an admixture of trivalent Eu in this layer, resulting in its valency of about 2.1. The next deeper lying Eu layer (eighth atomic layer below the surface) behaves mixed valently, but the estimated valency of 2.4 is notably lower than the value in the bulk. The presented approach and obtained results create a background for further studies of exotic surface properties of $4f$-based materials, and allow us to derive information related to valency and magnetism of individual rare-earth layers in a rather extended area near the surface.

Since the discovery of graphene, intensive efforts have been made in search of novel two-dimensional (2D) materials. Decreasing the materials dimensionality to their ultimate thinness is a promising route to unveil new physical phenomena, and potentially improve the performance of devices. Among recent 2D materials, analogs of graphene, the group IV elements have attracted much attention for their unexpected and tunable physical properties. Depending on the growth conditions and substrates, several structures of silicene, germanene, and stanene can be formed. Here, we report the synthesis of a Kagome lattice of silicene on aluminum (111) substrates. We provide evidence of such an exotic 2D Si allotrope through scanning tunneling microscopy (STM) observations, high-resolution core-level (CL) and angle-resolved photoelectron spectroscopy (ARPES) measurements, along with Density Functional Theory calculations., 13 pages, 6 figures

2D oxide quasicrystals (OQCs) are recently discovered aperiodic, but well-ordered oxide interfaces. In this topical review, an introduction to these new thin-film systems is given. The concept of quasicrystals and their approximants is explained for BaTiO3 - and SrTiO3 - derived OQCs and related periodic structures in these 2D oxides. In situ microscopy unravels the high-temperature formation process of OQCs on Pt(111). The dodecagonal structure is discussed regarding tiling statistics and tiling decoration based on the results of atomically resolved scanning tunneling microscopy and various diffraction techniques. In addition, angle-resolved ultraviolet photoemission spectroscopy and X-ray photoelectron spectroscopy results prove a metallic character of the 2D oxide.

Quantum devices depend on addressable elements, which can be modified separately and in their mutual interaction. Self-assembly at surfaces, for example, formation of a porous (metal-) organic network, provides an ideal way to manufacture arrays of identical quantum boxes, arising in this case from the confinement of the electronic (Shockley) surface state within the pores. We show that the electronic quantum box state as well as the interbox coupling can be modified locally to a varying extent by a selective choice of adsorbates, here C60, interacting with the barrier. In view of the wealth of differently acting adsorbates, this approach allows for engineering quantum states in on-surface network architectures.

A theoretically designed van der Waals ferromagnet Fe4GeTe2 is synthesized and shows the nearly room temperature ferromagnetism., In spintronics, two-dimensional van der Waals crystals constitute a most promising material class for long-distance spin transport or effective spin manipulation at room temperature. To realize all-vdW-material–based spintronic devices, however, vdW materials with itinerant ferromagnetism at room temperature are needed for spin current generation and thereby serve as an effective spin source. We report theoretical design and experimental realization of a iron-based vdW material, Fe4GeTe2, showing a nearly room temperature ferromagnetic order, together with a large magnetization and high conductivity. These properties are well retained even in cleaved crystals down to seven layers, with notable improvement in perpendicular magnetic anisotropy. Our findings highlight Fe4GeTe2 and its nanometer-thick crystals as a promising candidate for spin source operation at nearly room temperature and hold promise to further increase Tc in vdW ferromagnets by theory-guided material discovery.

Renewed interest in the ferroelectric semi-conductor 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 similar to 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 (similar to 68%) as we determined using photoelectron diffraction and XPS. The lateral dimensions of the surface domains are in the range of similar to 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.

Nanoparticle generation by aerosol methods, particularly spark ablation, has high potential for creating new material combinations with tailored magnetic properties. By combining elements into complex alloyed nanoparticles and controlling their size and structure, different magnetic properties can be obtained. In combination with controlled deposition, to ensure nanoparticle separation, it is possible to minimize interparticle interactions and measure the intrinsic magnetic property of the nanoparticles. Most magnetic materials are highly sensitive to oxygen, and it is therefore crucial to both understand and control the oxidation of magnetic nanoparticles. In this study, we have successfully generated oxidized, bimetallic FeCr and FeMn nanoparticles by spark ablation in combination with a compaction step and thoroughly characterized individual particles with aerosol instruments, transmission electron microscopy and synchrotron-based X-ray photoelectron spectroscopy. The generated nanoparticles had an almost identical transition-metal ratio to the electrodes used as seed materials. Further, we demonstrate how the carrier gas can be used to dictate the oxidation and how to alternate between self-passivated and entirely oxidized nanoparticles. We also discuss the complexity of compacting alloyed nanoparticles consisting of elements with different vapor pressures and how this will affect the composition. This knowledge will further the understanding of design and generation of complex alloyed nanoparticles based on transition metals using aerosol methods, especially for the size regime where a compaction step is needed. As a proof of concept, measurements using a magnetometer equipped with a superconducting quantum interference device were performed on samples with different particle coverages. These measurements show that the magnetic properties could be explored for both high and low surface coverages, which open a way for studies where interparticle interactions can be systematically controlled.

The Engineering and Physical Sciences Research Council (EPSRC) is acknowledged for the funding of FG (EP/M029077/1) and RM (EP/506631/1). The Paul Scherrer Institut, Villigen, Switzerland is acknowledged for provision of synchrotron radiation beamtime at the PEARL beamline of the SLS. Nicolas Bachellier, Daria Sostina and Patrick Ascher are thanked for assistance. DB thanks the Helmholtz Association Program MML for financial funding in this work. Geometrical structure and electronic characteristics of ultrathin metallic films of one metal on another are strongly influenced by factors such as lattice mismatch and formation of near-surface alloys. Doped systems often show modified chemical-physical properties which may be amenable to different reactivity routes. Copper-gold surface alloys have received the attention of several studies, only a few of which have been undertaken in an UHV environment, using surface sensitive techniques. In this contribution, systems produced upon room temperature deposition of copper on the (22×√3)-Au(111) surface, at various copper loadings and annealed to different temperatures, are investigated using scanning tunnelling microscopy and synchrotron radiation based techniques (X-ray photoelectron diffraction, photoemission), with comparison with some theoretical modelling. Overall the fcc lattice is essentially maintained on preparation. Upon thermal treatments structural and compositional changes, in favour of alloyed structures with tetragonal packing, are seen. Postprint

Structural, pressure-dependent resistivity, angle resolved photoemission spectroscopy (ARPES), x-ray photoelectron diffraction (XPD) and band structure by DFT calculation have been investigated for BiSbTe3 Topological insulator. It has been demonstrated that the Dirac point of the topological surface state (TSS) located exactly at the Fermi level. Additionally, superconductivity emerges under pressure of 8 GPa with a critical temperature of ∼2.5 K. With further increase of pressure, the superconducting transition temperature (Tc) increases and at 14 GPa it shows the maximum Tc (∼3.3 K). It has also been shown that the surface state remains unchanged under pressure and has been suggested that the origin of the superconductivity is due to the bulk state. The investigation indicates that the BiSbTe3 has robust surface states and becomes superconductor under pressure.

Endohedral fullerenes, such as Dy2ScN@C80, are single-molecule magnets with long relaxation times of their magnetization. An open and anisotropic 4f electron shell in the lanthanides (here Dy) imposes a magnetic moment that maintains its orientation at liquid-helium temperatures for macroscopic times. If these molecules shall be used as single-bit information storage elements or for quantum operations, the orientation of the endohedral units and the orientation of the magnetic moments has to be controlled. X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD)—with variation of the angle of x-ray incidence—allows for the detection of these two structural elements. We present XMCD data of Dy2ScN@C80 on an h−BN/Rh(111) nanomesh that display at 2 K a large hysteresis with a coercive field of 0.4 T. The angular dependence of the XAS data at the Dy M5 edge indicates partial ordering of the endohedral units. In order to quantify anisotropic orientation we introduce the “deviation” D as an operational quantity that measures differences between two spectra.

The temperature dependent dehydrogenation of naphthalene on Ni(111) has been investigated using vibrational sum-frequency generation spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory with the aim of discerning the reaction mechanism and the intermediates on the surface. At 110 K, multiple layers of naphthalene adsorb on Ni(111); the first layer is a flat lying chemisorbed monolayer, whereas the next layer(s) consist of physisorbed naphthalene. The aromaticity of the carbon rings in the first layer is reduced due to bonding to the surface Ni-atoms. Heating at 200 K causes desorption of the multilayers. At 360 K, the chemisorbed naphthalene monolayer starts dehydrogenating and the geometry of the molecules changes as the dehydrogenated carbon atoms coordinate to the nickel surface; thus, the molecule tilts with respect to the surface, recovering some of its original aromaticity. This effect peaks at 400 K and coincides with hydrogen desorption. Increasing the temperature leads to further dehydrogenation and production of H-2 gas, as well as the formation of carbidic and graphitic surface carbon. Published under license by AIP Publishing.

Germanium telluride features special spin-electric effects originating from spin-orbit coupling and symmetry breaking by the ferroelectric lattice polarization, which opens up many prospectives for electrically tunable and switchable spin electronic devices. By Mn doping of the α-GeTe host lattice, the system becomes a multiferroic semiconductor possessing magnetoelectric properties in which the electric polarization, magnetization and spin texture are coupled to each other. Employing spin- and angle-resolved photoemission spectroscopy in bulk- and surface-sensitive energy ranges and by varying dipole transition matrix elements, we disentangle the bulk, surface and surface-resonance states of the electronic structure and determine the spin textures for selected parameters. From our results we derive a comprehensive model of the α-GeTe surface electronic structure which fits to experimental data and first principle theoretical predictions and we discuss the unconventional evolution of the Rashba-type spin splitting upon manipulation by external B- and E-fields.

International audience; Ions inside of fullerene molecules are model systems for the study of the electrostatic interaction across a single layer of carbon. For TbSc2N@C80 on h-BN/Ni(111), we observe with high-resolution X-ray photoelectron spectroscopy a splitting of the C 1s core level. The data may be explained quantitatively with density functional theory. The correlation of the C 1s eigenvalues and the Coulomb potential of the inside ions at the corresponding carbon sites indicates incomplete screening of the electric field due to the endohedral ions. The screening comprises anisotropic charge transfer to the carbon atoms and their polarization. This behavior is essential for the ordering of endohedral single-molecule magnets and is expected to occur in any single-layer material.

Upon a core level excitation by circularly polarized light (CPL), the angular momentum of light, i.e. helicity, is transferred to the emitted photoelectron. This phenomenon can be confirmed by the parallax shift measurement of the forward focusing peak (FFP) direction in a stereograph of the atomic arrangement. The angular momentum of the emitted photoelectron is the sum of CPL helicity and the magnetic quantum number (MQN) of the initial state that define the quantum number of the core hole final state. The core hole may decay via Auger electron emission, where in this two electron process the angular momentum has to be conserved as well. Starting from a given core hole, different Auger decay channels with different final state energies and angular momenta of the emitted Auger electrons may be populated. Here we report the observation and formulation of the angular momentum transfer of light to Auger electrons, instead of photoelectrons. We measured photoelectron and Auger electron intensity angular distributions from Cu(111) and Cu(001) surfaces as a function of photon energy and photoelectron kinetic energy. By combining Auger electron spectroscopy with the FFP shift measurements at absorption threshold, element- and MQN-specific hole states can be generated in the valence band.

The valence band dispersion of the graphite $\ensuremath{\pi}$ band along the ${k}_{z}$ axis was measured using a sequence of photon energies between 60 and 230 eV. The interlayer interaction induces the well-known splitting of the $\ensuremath{\pi}$ band into two, a lower binding energy band ${\ensuremath{\pi}}_{\mathrm{L}}$ and a higher binding energy band ${\ensuremath{\pi}}_{\mathrm{H}}$. However, in the photoelectron spectra, the bands ${\ensuremath{\pi}}_{\mathrm{L}}$ and ${\ensuremath{\pi}}_{\mathrm{H}}$ appeared alternatively in the Brillouin zones along the ${k}_{z}$ direction. As a result, a $4\ensuremath{\pi}$-periodic oscillation in the binding energy as a function of photon energy was observed for the $\ensuremath{\pi}$ band dispersion. This is explained by considering the constructive and destructive interference of ${p}_{z}$ orbitals within the unit cell, called the photoelectron structure factor. We derived the analytic expression of the photoelectron structure factor for graphite. The resonance integrals of the ${p}_{z}$ orbitals were determined for the in-plane and interlayer. Furthermore, the inner potential of graphite from the vacuum level was determined to be 17.17 eV.

We present a time-resolved angle-resolved photoelectron spectroscopy study of IrTe$_2$, which undergoes two first-order structural and charge-ordered phase transitions on cooling below 270 K and below 180 K. The possibility of inducing a phase transition by photoexcitation with near-infrared femtosecond pulses is investigated in the charge-ordered phases. We observe changes of the spectral function occuring within a few hundreds of femtoseconds and persisting up to several picoseconds, which we interpret as a partial photoinduced phase transition (PIPT). The necessary time for photoinducing these spectral changes increases with increasing photoexcitation density and reaches timescales longer than the rise time of the transient electronic temperature. We conclude that the PIPT is driven by a transient increase of the lattice temperature following the energy transfer from the electrons. However, the photoinduced changes of the spectral function are small, which indicates that the low temperature phase is particularly robust against photoexcitation. We suggest that the system might be trapped in an out-of-equilibrium state, for which only a partial structural transition is achieved., 8 pages, 5 figures, accepted for publication in Phys. Rev. B

We report the investigation of the angular momentum transfer of light to photoelectrons and Auger electrons at the Ni ${L}_{3}$ absorption threshold. Upon core-level excitation by circularly polarized light, the angular momentum of light, or helicity, is transferred to the emitted photoelectron. The angular momentum of the emitted photoelectron (${m}_{\mathrm{f}}$) is the sum of the helicity ($\ensuremath{\sigma}$) and the orbital magnetic quantum number of the initial state (${m}_{\mathrm{i}}$). Here the quantization axis was defined as the direction along the incident light. This can be measured by the parallax shift of the forward focusing peak (FFP) direction in the photoelectron intensity angular distribution. At the absorption threshold, the excited core-level electron is promoted to a conduction-band state and the angular momentum of the light is partially transferred to Auger electrons. We measured photoelectron and Auger electron intensity angular distributions from the Ni(111) surface at the ${L}_{3}$ absorption threshold. We observed a significant angular circular dichroism of the $[101]$ FFP for the ${L}_{3}{M}_{4,5}{M}_{4,5}$ Auger electrons. Furthermore, we discovered non-negligible reversal angular circular dichroism contrasts for the triplet components in the case of the ${L}_{3}{M}_{2,3}{M}_{4,5}$ Auger electrons, suggesting that the angular momentum of light was transferred to the emitted electron in the antiparallel way.

Using photoelectron diffraction and spectroscopy, we explore the structural and electronic properties of the hexagonal boron nitride (h-BN) monolayer epitaxially grown on the Co(0001) surface. Perfect matching of the lattice parameters allows formation of a well-defined interface where the B atoms occupy the hollow sites while the N atoms are located above the Co atoms. The corrugation of the h-BN monolayer and its distance from the substrate were determined by means of R-factor analysis. The obtained results are in perfect agreement with the density functional theory (DFT) predictions. The electronic structure of the interface is characterized by a significant mixing of the h-BN and Co states. Such hybridized states appear in the h-BN band gap. This allows to obtain atomically resolved scanning tunneling microscopy (STM) images from the formally insulating 2D material being in contact with ferromagnetic metal. The STM images reveal mainly the nitrogen sublattice due to a dominating contribution of nitrogen orbitals to the electronic states at the Fermi level. We believe that the high quality, well-defined structure and interesting electronic properties make the h-BN/Co(0001) interface suitable for spintronic applications., L.V.Ya. acknowledges the RSF (Grant No. 16-42-01093). A.V.T., V.O.S., K.A.B., O.Yu.V., and D.Yu.U. acknowledge St. Petersburg State University for research Grant No. 11.65.42.2017. M.V.K. and I.I.O. acknowledge the RFBR (Grant No. 16-29-06410). C.L. acknowledges the DFG (Grant Nos. LA655-17/1 and LA655-19/1).

On-surface synthesis is a powerful route toward the fabrication of specific graphene-like nanostructures confined in two dimensions. This strategy has been successfully applied to the growth of graphene nanoribbons of diverse width and edge morphology. Here, we investigate the mechanisms driving the growth of 9-atom wide armchair graphene nanoribbons by using scanning tunneling microscopy, fast X-ray photoelectron spectroscopy, and temperature-programmed desorption techniques. Particular attention is given to the role of halogen functionalization (Br or I) of the molecular precursors. We show that the use of iodine-containing monomers fosters the growth of longer graphene nanoribbons (average length of 45 nm) due to a larger separation of the polymerization and cyclodehydrogenation temperatures. Detailed insight into the growth process is obtained by analysis of kinetic curves extracted from the fast X-ray photoelectron spectroscopy data. Our study provides fundamental details of relevance to the production of future electronic devices and highlights the importance of not only the rational design of molecular precursors but also the most suitable reaction pathways to achieve the desired final structures.

The properties of atomically thin materials essentially depend on their structures, including impurities, defects and interfaces with underlying substrates. Thus, the detailed structural information is relevant for creation of 2D materials with desired properties. Here, we explore the capabilities of photoelectron diffraction and holography for structural analysis of atomically thin layers using as examples such systems as h-BN, graphene, and modified graphene with boron impurities. We show that for planar 2D crystals with commensurate interface to the substrate, it is possible to visualize the interface and impurities with high spatial resolution, and to distinguish possible non-equivalent structural units. Our approach applied to B-doped graphene on Ni(1 1 1) and Co(0 0 0 1) surfaces has allowed to reveal asymmetry of boron concentrations in the two carbon sublattices and established its dependence on the applied synthesis procedure and chosen substrate. The obtained results suggest that such approach can be widely applied for studies of various 2D systems, where the structures of interfaces and defects are of remarkable importance.

The Photo-Emission and Atomic Resolution Laboratory is a new soft X-ray beamline at the Swiss Light Source for the study of surface structure using photoelectron diffraction and scanning tunneling microscopy., The Photo-Emission and Atomic Resolution Laboratory (PEARL) is a new soft X-ray beamline and surface science laboratory at the Swiss Light Source. PEARL is dedicated to the structural characterization of local bonding geometry at surfaces and interfaces of novel materials, in particular of molecular adsorbates, nanostructured surfaces, and surfaces of complex materials. The main experimental techniques are soft X-ray photoelectron spectroscopy, photoelectron diffraction, and scanning tunneling microscopy (STM). Photoelectron diffraction in angle-scanned mode measures bonding angles of atoms near the emitter atom, and thus allows the orientation of small molecules on a substrate to be determined. In energy scanned mode it measures the distance between the emitter and neighboring atoms; for example, between adsorbate and substrate. STM provides complementary, real-space information, and is particularly useful for comparing the sample quality with reference measurements. In this article, the key features and measured performance data of the beamline and the experimental station are presented. As scientific examples, the adsorbate–substrate distance in hexagonal boron nitride on Ni(111), surface quantum well states in a metal-organic network of dicyano-anthracene on Cu(111), and circular dichroism in the photoelectron diffraction of Cu(111) are discussed.

We report on the surface-assisted synthesis and spectroscopic characterization of the hitherto longest periacene analogue with oxygen-boron-oxygen (OBO) segments along the zigzag edges, that is, a heteroatom-doped perihexacene 1. Surface-catalyzed cyclodehydrogenation successfully transformed the double helicene precursor 2, i.e., 12a,26a-dibora-12,13,26,27-tetraoxa-benzo[1,2,3-hi:4,5,6-h'i']dihexacene, into the planar perihexacene analogue 1, which was visualized by scanning tunneling microscopy and noncontact atomic force microscopy. X-ray photoelectron spectroscopy, Raman spectroscopy, together with theoretical modeling, on both precursor 2 and product 1, provided further insights into the cyclodehydrogenation process. Moreover, the nonplanar precursor 2 underwent a conformational change upon adsorption on surfaces, and one-dimensional self-assembled superstructures were observed for both 2 and 1 due to the presence of OBO units along the zigzag edges.

Functionalization of surfaces has become of high interest for a wealth of applications such as sensors, hybrid photovoltaics, catalysis, and molecular electronics. Thereby molecule-surface interactions are of crucial importance for the understanding of interface properties. An especially relevant point is the anchoring of molecules to surfaces. In this work, we analyze this process for a zinc-porphyrin equipped with carboxylic acid anchoring groups on rutile TiO2 (110) using scanning probe microscopy. After evaporation, the porphyrins are not covalently bound to the surface. Upon annealing, the carboxylic acid anchors undergo deprotonation and bind to surface titanium atoms. The formation of covalent bonds is evident from the changed stability of the molecule on the surface as well as the adsorption configuration. Annealed porphyrins are rotated by 45 degrees and adopt another adsorption site. The influence of binding on electronic coupling with the surface is investigated using photoelectron spectroscopy. The observed shifts of Zn 2p and N 1s levels to higher binding energies indicate charging of the porphyrin core, which is accompanied by a deformation of the macrocycle due to a strong interaction with the surface.