MAXPEEM, a dedicated photoemission electron microscopy beamline at MAX IV Laboratory, houses a state-of-the-art aberration-corrected spectroscopic photoemission and low-energy electron microscope (AC-SPELEEM). This powerful instrument offers a wide range of complementary techniques providing structural, chemical and magnetic sensitivities with a single-digit nanometre spatial resolution. The beamline can deliver a high photon flux of >= 10(15) photons s(-1) (0.1% bandwidth)(-1) in the range 30-1200 eV with full control of the polarization from an elliptically polarized undulator. The microscope has several features which make it unique from similar instruments. The X-rays from the synchrotron pass through the first beam separator and impinge the surface at normal incidence. The microscope is equipped with an energy analyzer and an aberration corrector which improves both the resolution and the transmission compared with standard microscopes. A new fiber-coupled CMOS camera features an improved modulation transfer function, dynamic range and signal-to-noise ratio compared with the traditional MCP-CCD detection system.

Thin, stable and inert dielectric spacers are essential for manufacturing electronic devices based on 2D materials. However, direct synthesis on top of 2D materials is difficult due to their inert nature. In this work, we studied how an electron beam induces fragmentation of borazine and enables spatially confined synthesis of amorphousBNx on graphene at room temperature. Using a combination of X-ray Photoelectron Spectroscopy, Low Energy Electron Microscopy, and Scanning Tunneling Microscopy we studied the morphology of the heterostructure, its chemical composition, and finally its temperature evolution. We find that electron-beam induced deposition starts by the binding of heavily fragmentized borazine, including atomic boron, followed by the growth of a multilayer with a 1:0.7 B:N ratio. The final structure exhibits a thermal stability up to 1400 K and similar to 50 nm spatial control provided by the electron beam. Our studies provide surface science insight into the use of electron beams for synthesis and lateral control of stable and inert layers in 2D heterostructures.

Rhombohedral-stacked few-layer graphene (FLG) displays peculiar electronic properties that could lead to phenomena such as high-temperature superconductivity and magnetic ordering. To date, experimental studies have been mainly limited by the difficulty in isolating rhombohedral FLG with thickness exceeding 3 layers and device-compatible size. In this work, we demonstrate the synthesis and transfer of rhombohedral graphene with thickness up to 9 layers and areas up to similar to 50 mu m(2). The domains of rhombohedral FLG are identified by Raman spectroscopy and are found to alternate with Bernal regions within the same crystal in a stripe-like configuration. Near-field nano-imaging further confirms the structural integrity of the respective stacking orders. Combined spectroscopic and microscopic analyses indicate that rhombohedral-stacking formation is strongly correlated to the underlying copper step-bunching and emerges as a consequence of interlayer displacement along preferential crystallographic orientations. The growth and transfer of rhombohedral FLG with the reported thickness and size shall facilitate the observation of predicted unconventional physics and ultimately add to its technological relevance. (c) 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

The use of protective barriers to isolate a metal surface from an aggressive environment is a common-way to inhibit its degradation. We used ambient pressure x-ray photoelectron spectroscopy to assess in real time the evolution of the copper surface and the contextual protective action of 2D material coatings (graphene and hexagonal boron nitride) towards copper oxidation. In an isobaric experiment with 2 mbar of oxygen, the bare copper oxidizes near room temperature, while both 2D materials can retard the onset temperature for the first oxidation of copper by more than 120 degrees C. However, their protection mechanism is different: boron nitride behaves more straightforwardly, forming an effective barrier to copper oxidation until it is etched away at high temperatures, leading to a rapid oxidation to cuprous and then cupric oxide. On the other hand, graphene reveals to be a more interesting playground underneath oxygen intercalates and begins a slower undercover oxidation of copper. The coexistence between graphene and cuprous oxide, not observed in boron nitride, protects the copper from further oxidation to cupric oxide. (C) 2020 Elsevier Ltd. All rights reserved.

We present a study of quasi-free-standing monolayer graphene obtained by intercalation of Au atoms at the interface between the carbon buffer layer (Bu-L) and the silicon-terminated face (0001) of 4H-silicon carbide. Au intercalation is achieved by deposition of an atomically thin Au layer on the Bu-L followed by annealing at 850 degrees C in an argon atmosphere. We explore the intercalation of Au and decoupling of the Bu-L into quasi-free-standing monolayer graphene by surface science characterization and electron transport in top-gated electronic devices. By gate-dependent magnetotransport we find that the Au-intercalated buffer layer displays all properties of monolayer graphene, namely gate-tunable ambipolar transport across the Dirac point, but we find no observable enhancement of spin-orbit effects in the graphene layer, despite its proximity to the intercalated Au layer.

We performed spatially resolved near-ambient-pressure photoemission spectromicroscopy on graphene-coated copper in operando under oxidation conditions in an oxygen atmosphere (0.1 mbar). We investigated regions with bare copper and areas covered with mono- and bi-layer graphene flakes, in isobaric and isothermal experiments. The key method in this work is the combination of spatial and chemical resolution of the scanning photoemission microscope operating in a near-ambient-pressure environment, thus allowing us to overcome both the material and pressure gap typical of standard ultrahigh-vacuum X-ray photoelectron spectroscopy (XPS) and to observe in operando the protection mechanism of graphene toward copper oxidation. The ability to perform spatially resolved XPS and imaging at high pressure allows for the first time a unique characterization of the oxidation phenomenon by means of photoelectron spectromicroscopy, pushing the limits of this technique from fundamental studies to real materials under working conditions. Although bare Cu oxidizes naturally at room temperature, our results demonstrate that such a graphene coating acts as an effective barrier to prevent copper oxidation at high temperatures (over 300 °C), until oxygen intercalation beneath graphene starts from boundaries and defects. We also show that bilayer flakes can protect at even higher temperatures. The protected metallic substrate, therefore, does not suffer corrosion, preserving its metallic characteristic, making this coating appealing for any application in an aggressive atmospheric environment at high temperatures.

One of the ways to use graphene in field effect transistors is to introduce a band gap by quantum confinement effect [1]. That is why narrow graphene nanoribbons (GNRs) with width less than 50nm are considered to be essential components in future graphene electronics. The growth of graphene on sidewalls of SiC(0001) mesa structures using scalable photolithography was shown to produce high quality GNR with excellent transport properties [2-7]. Such epitaxial graphene nanoribbons are very important in fundamental science but if GNR are supposed to be used in advanced nanoelectronics, high quality thin (<50nm) nanoribbons should be produced on a large (wafer) scale. Here we present a technique for scalable template growth of high quality GNR on Si-face of SiC(0001) and provide detailed structural information along with transport properties. We succeeded to grow GNR along both [1-100] and [11-20] crystallographic directions. The quality of the grown nanoribbons was confirmed by comprehensive characterization with high resolution STM, dark field LEEM and transport measurements.

The advanced properties of modern materials originate from their nanoscale size and shape and from chemical modifications or doping. Special techniques that can measure the chemical state in the nanoscale are required for exploration and understanding the properties of these materials. While X‐ray photoelectron spectroscopy (XPS) can access the necessary chemical information, conventional setups have no spatial resolution. The scanning photoelectron microscope (SPEM) takes in advent the third generation synchrotron radiation facilities and uses a zone plate (ZP) focusing optics that allows spatially resolved XPS measurements in the submicron scale. Several recent examples of investigations of chemically modified or doped nanomaterials are given. The modification of suspended and supported graphene with nitrogen and fluorine is presented as well as the doping dependent position of the Fermi‐level in single GsAs nanowires and the Mott–Hubbard transition in Cr‐doped vanadium oxide. These examples show several peculiar SPEM abilities like a high surface and chemical sensitivity and a submicron spatial resolution proving the capability and importance of this technique to study materials at the nanoscale.

Modification of the surface electronic properties of vertically aligned and randomly distributed carbon nanotubes and the hydrophobic character after exposure to Ar:F-2 and CF4 plasma are exploited to optimize the sensing characteristics of these materials. The sensing properties of fluorinated carbon nanotubes are disclosed by probing their stability and responsiveness towards the detection of two selected pollutants such as nitrogen dioxide and ammonia (NO2 and NH3). The effects of both humidity level and geometry of the sensing layer are assessed. It is demonstrated that fluorination, by increasing the surface hydrophobicity, results in increased response reproducibility and enhanced sensor response towards NH3 when using vertically aligned carbon nanotubes.

The interaction between 2,2'-bithiophene-5-carboxylic acid (PT2) sublimed under ultra-high vacuum conditions and anatase (101) and rutile (110) TiO2 single crystal surfaces is investigated by studying the electronic spectral density near the Fermi level with synchrotron-based spectroscopy. The experimental results are compared to density functional theory calculations of the isolated PT2 molecule and of the molecule adsorbed on an anatase TiO2 (101) cluster. The relative concentrations of Ti, C, and S atoms indicate that the adsorbed molecule remains intact upon deposition, which is typical of a Stranski-Krastanov growth mode. The analysis of the O1s spectrum suggests a predominant bidentate geometry of the adsorption with both rutile and anatase surfaces, as supported by previous theoretical simulations. It is also theoretically and experimentally demonstrated that the PT2 adsorption causes the appearance of new electronic states in the gap near the TiO2 valence band. A pinning effect of the LUMO level of the dye is also theoretically predicted.

Carbon nanomaterials' reactivity towards oxygen is very poor, limiting their potential applications. However, nitrogen doping is an established way to introduce active sites that facilitate interaction with gases. This boosts the materials' reactivity for bio-/gas sensing and enhances their catalytic performance for the oxygen reduction reaction. Despite this interest, the role of differently bonded nitrogen dopants in the interaction with oxygen is obscured by experimental challenges and has so far resisted clear conclusions. We study the interaction of molecular oxygen with graphene doped via nitrogen plasma by in situ high-resolution synchrotron techniques, supported by density functional theory core level simulations. The interaction leads to oxygen dissociation and the formation of carbon-oxygen single bonds on graphene, along with a band gap opening and a rounding of the Dirac cone. The change of the N1s core level signal indicates that graphitic nitrogen is involved in the observed mechanism: the adsorbed oxygen molecule is dissociated and the two O atoms chemisorb with epoxy bonds to the nearest carbon neighbours of the graphitic nitrogen. Our findings help resolve existing controversies and offer compelling new evidence of the ORR pathway.

Hexagonal-shaped nanoplates (HNPs) of MoS2 on vertically aligned carbon nanotubes (CNTs) over a patterned area (a circular area of 1 cm(2) diameter) are produced by chemical vapor deposition technique. With an optimized initial Mo film thickness, a uniform coverage of MoS2 HNPs with a thickness around 20 nm is achieved. The results confirm that the CNT template plays an important role in the MoS2 HNPs growth. Each MoS2 HNP consists of abundant exposed edges, interesting for sensing and catalysis applications. High crystallinity and quality of the as-produced material are revealed by X-ray photoelectron and Raman spectroscopies. Furthermore, NO2 gas-sensing studies show better sensitivity and recovery for MoS2/CNT samples as compared to pristine CNTs. The detection of NO2 gas in a few tens of parts per million to a few hundreds of parts per billion range, at room temperature, is achieved. Density-functional theory calculation indicates that the exposed edges of MoS2 play a significant role in the NO2 sensing as compared to horizontally aligned MoS2 layers. The present report can promote the research toward the fabrication of efficient and reliable MoS2-based hybrid materials for toxic gas-sensing applications for air quality monitoring in various environments.

The surface chemistry of plasma fluorinated vertically aligned carbon nanotubes (vCNT) is correlated to the CF4 plasma chemical composition. The results obtained via FTIR and mass spectrometry are combined with the XPS and Raman analysis of the sample surface showing the dependence on different plasma parameters (power, time and distance from the plasma region) on the resulting fluorination. Photoemission and absorption spectroscopies are used to investigate the evolution of the electronic properties as a function of the fluorine content at the vCNT surface. The samples suffer a limited ageing effect, with a small loss of fluorine functionalities after two weeks in ambient conditions.

Suspended graphene is exposed to different fluorine-containing species produced by a plasma source fed with CF4 precursor gas. We investigate the fluorination process by selecting two different kinetic energies for the ions striking the graphene surface. The chemical-bonding environment is discussed, and the control of the graphene-fluorination homogeneity is investigated at the individual graphene sheets. The modifications of the electronic and structural properties are examined by scanning photoelectron microscopy, micro-Raman analysis, and scanning electron microscopy. The results are compared with those obtained for supported graphene on copper. Suspended graphene provides a quasi-ideal model for investigating the intrinsic properties of irradiated carbon nano-systems while avoiding damage due to backscattered atoms and recoil due to a supporting substrate.

Suspended graphene flakes are exposed simultaneously to fluorine and sulfur ions produced by the mu-wave plasma discharge of the SF6 precursor gas. The microscopic and spectroscopic analyses, performed by Raman spectroscopy, scanning electron microscopy and photoelectron spectromicroscopy, show the homogeneity in functionalization yield over the graphene flakes with F and S atoms covalently bonded to the carbon lattice. This promising surface shows potential for several applications ranging from biomolecule immobilization to lithium battery and hydrogen storage devices. The present co-doping process is an optimal strategy to engineer the graphene surface with a concurrent hydrophobic character, thanks to the fluorine atoms, and a high affinity with metal nanoparticles due to the presence of sulfur atoms. (C) 2017 Elsevier B.V. All rights reserved.

Plasma fluorination of graphene is studied using a combination of spectroscopy and microscopy techniques, giving insight into the yield and fluorination mechanism for functionalization of supported graphene with both CF4 and SF6 gas precursors. Ion acceleration during fluorination is used to probe the effect on grafting functionalities. Adatom clustering, which occurs with CF4 plasma treatment, is suppressed when higher kinetic energy is supplied to the ions. During SF6 plasma functionalization, the sulfur atoms tend to bond to bare copper areas instead of affecting the graphene chemistry, except when the kinetic energy of the ions is restricted. Using scanning photoelectron microscopy, with a 100 nm spatial resolution, the chemical bonding environment is evaluated in the fluorinated carbon network at selected regions and the functionalization homogeneity is controlled in individual graphene flakes.

The chemical modification of multiwalled carbon nanotubes (MWCNTs) with a long chain mercapto acid is reported as a way to improve sensitivity and response time of gas sensors for detecting alcohols, acetone and toxic gases such as DMMP. We have developed sensors employing MWCNTs decorated with gold nanoparticles and modified with a 16-mercaptohexadecanoic acid (MHDA) monolayer. Morphological and compositional analysis by Transmission Electron Microscopy (TEM), Fourier Transform Infra-red Spectroscopy (FTIR) and X-ray photoelectron spectroscopy were performed to characterize the gold nanoparticles and to check the bonding of the thiol monolayer. The detection of aromatic and non-aromatic volatiles and DMMP vapors by MWCNT/Au and MWCNT/Au/MHDA shows that the presence of the self-assembled layer increases sensitivity and selectivity towards non-aromatics. Furthermore, it ameliorates response dynamics, and significantly reduces nitrogen dioxide and moisture cross-sensitivity.

Highly nitrogen-doped graphene on copper has been obtained by post-synthesis low-energy ion implantation. Core level and angle resolved photoemission spectroscopies are correlated to link the actual charge carrier doping to the different nitrogen species implanted in the nanostructure. Indeed, we exploit the possibility of controlling the graphitic/pyridinic ratio through thermal heating to tune the charge carrier density; this implicates Dirac cone shifts that are directly correlated to the different doping contribution of the nitrogen species. Supported by density functional theory calculations, we identify graphitic nitrogen as being responsible for n-doping when the amount of counterbalancing pyridinic nitrogen species is reduced upon thermal heating.