The HERMES beamline (High Efficiency and Resolution beamline dedicated to X‐ray Microscopy and Electron Spectroscopy), built at Synchrotron SOLEIL (Saint‐Auban, France), is dedicated to soft X‐ray ...microscopy. The beamline combines two complementary microscopy methods: XPEEM (X‐ray Photo Emitted Electron Microscopy) and STXM (Scanning Transmission X‐ray Microscopy) with an aim to reach spatial resolution below 20 nm and to fully exploit the local spectroscopic capabilities of the two microscopes. The availability of the two methods within the same beamline enables the users to select the appropriate approach to study their specific case in terms of sample environment, spectroscopy methods, probing depth etc. In this paper a general description of the beamline and its design are presented. The performance and specifications of the beamline will be reviewed in detail. Moreover, the article is aiming to demonstrate how the beamline performances have been specifically optimized to fulfill the specific requirements of a soft X‐ray microscopy beamline in terms of flux, resolution, beam size etc. Special attention has been dedicated to overcome some limiting and hindering problems that are usually encountered on soft X‐ray beamlines such as carbon contamination, thermal stability and spectral purity.
Abstract
Ptychography is a coherent diffraction imaging technique that measures diffraction patterns at many overlapping points on a sample and then uses an algorithm to reconstruct amplitude and ...phase images of the object and probe. Here, we report imaging, spectroscopy and linear dichroism ptychographic measurements at the carbon K-edge. This progress was achieved with a new generation of scientific Complementary Metal Oxide Semiconductor (sCMOS) X-ray cameras with an uncoated image sensor which has fast image transfer and high quantum efficiency at the carbon K-edge. Reconstructed amplitude and phase contrast images, C 1s spectral stacks, and X-ray linear dichroism of carbon nanotubes at the carbon K-edge were measured with ptychography. Ptychography and conventional Scanning Transmission X-ray Microscopy (STXM) are compared using results acquired from the same area. Relative to STXM, ptychography provides both improved spatial resolution and improved image quality. We used defocus ptychography, with an X-ray beam spot size of 1.0 micron, in order to reduce radiation damage and carbon deposition. Comparable spatial resolution was achieved to that of ptychography performed with a focused beam. Ptychography at the carbon K-edge offers unique opportunities to perform high resolution spectromicroscopy on organic materials important in medicine, biology, environmental science and energy materials.
Herein, we report the synthesis of a γ-Al2O3-supported NiCo catalyst for dry methane reforming (DMR) and study the catalyst using in situ scanning transmission X-ray microscopy (STXM) during the ...reduction (activation step) and under reaction conditions. During the reduction process, the NiCo alloy particles undergo elemental segregation with Co migrating toward the center of the catalyst particles and Ni migrating to the outer surfaces. Under DMR conditions, the segregated structure is maintained, thus hinting at the importance of this structure to optimal catalytic functions. Finally, the formation of Ni-rich branches on the surface of the particles is observed during DMR, suggesting that the loss of Ni from the outer shell may play a role in the reduced stability and hence catalyst deactivation. These findings provide insights into the morphological and electronic structural changes that occur in a NiCo-based catalyst during DMR. Further, this study emphasizes the need to study catalysts under operating conditions in order to elucidate material dynamics during the reaction.
Nitrogen doping of graphene is of great interest for both fundamental research to explore the effect of dopants on a 2D electrical conductor and applications such as lithium storage, composites, and ...nanoelectronic devices. Here, we report on the modifications of the electronic properties of epitaxial graphene thanks to the introduction, during the growth, of nitrogen-atom substitution in the carbon honeycomb lattice. High-resolution transmission microscopy and low-energy electron microscopy investigations indicate that the nitrogen-doped graphene is uniform at large scale. The substitution of nitrogen atoms in the graphene planes was confirmed by high-resolution X-ray photoelectron spectroscopy, which reveals several atomic configurations for the nitrogen atoms: graphitic-like, pyridine-like, and pyrrolic-like. Angle-resolved photoemission measurements show that the N-doped graphene exhibits large n-type carrier concentrations of 2.6 × 1013 cm–2, about 4 times more than what is found for pristine graphene, grown under similar pressure conditions. Our experiments demonstrate that a small amount of dopants (<1%) can significantly tune the electronic properties of graphene by shifting the Dirac cone about 0.3 eV toward higher binding energies with respect to the π band of pristine graphene, which is a key feature for envisioning applications in nanoelectronics.
Stacking various two-dimensional atomic crystals is a feasible approach to creating unique multilayered van der Waals heterostructures with tailored properties. Herein for the first time, we present ...a controlled preparation of large-area h-BN/graphene heterostructures via a simple chemical deposition of h-BN layers on epitaxial graphene/SiC(0001). Van der Waals forces, which are responsible for the cohesion of the multilayer system, give rise to an abrupt interface without interdiffusion between graphene and h-BN, as shown by X-ray Photoemission Spectroscopy (XPS) and direct observation using scanning and High-Resolution Transmission Electron Microscopy (STEM/HRTEM). The electronic properties of graphene, such as the Dirac cone, remain intact and no significant charge transfer i.e. doping, is observed. These results are supported by Density Functional Theory (DFT) calculations. We demonstrate that the h-BN capped graphene allows the fabrication of vdW heterostructures without altering the electronic properties of graphene.
Spectro‐ptychography offers improved spatial resolution and additional phase spectral information relative to that provided by scanning transmission X‐ray microscopes. However, carrying out ...ptychography at the lower range of soft X‐ray energies (e.g. below 200 eV to 600 eV) on samples with weakly scattering signals can be challenging. Here, results of soft X‐ray spectro‐ptychography at energies as low as 180 eV are presented, and its capabilities are illustrated with results from permalloy nanorods (Fe 2p), carbon nanotubes (C 1s) and boron nitride bamboo nanostructures (B 1s, N 1s). The optimization of low‐energy X‐ray spectro‐ptychography is described and important challenges associated with measurement approaches, reconstruction algorithms and their effects on the reconstructed images are discussed. A method for evaluating the increase in radiation dose when using overlapping sampling is presented.
First B‐edge (192 eV) soft X‐ray ptychography, challenges related to weak scattering, measurement approaches and reconstructions in energies as low as 180 eV are discussed.
The growth of large and uniform graphene layers remains very challenging to this day due to the close correlation between the electronic and transport properties and the layer morphology. Here, we ...report the synthesis of uniform large-scale mono- and bilayers of graphene on off-axis 6H-SiC(0001) substrates. The originality of our approach consists of the fine control of the growth mode of the graphene by precise control of the Si sublimation rate. Moreover, we take advantage of the presence of nanofacets on the off-axis substrate to grow a large and uniform graphene with good long-range order. We believe that our approach represents a significant step toward the scalable synthesis of graphene films with high structural qualities and fine thickness control, in order to develop graphene-based electronic devices.
The remarkable properties of graphene have shown promise for new perspectives in future electronics, notably for nanometer scale devices. Here we grow graphene epitaxially on an off-axis 4H-SiC(0001) ...substrate and demonstrate the formation of periodic arrangement of monolayer graphene on planar (0001) terraces and Bernal bilayer graphene on
nanofacets of SiC. We investigate these lateral superlattices using Raman spectroscopy, atomic force microscopy/electrostatic force microscopy (AFM/EFM) and X-ray and angle resolved photoemission spectroscopy (XPS/ARPES). The correlation of EFM and ARPES reveals the appearance of permanent electronic band gaps in AB-stacked bilayer graphene on
SiC nanofacets of 150 meV. This feature is confirmed by density functional theory (DFT) calculations. The charge transfer between the substrate and graphene bilayer results in an asymmetric charge distribution between the top and the bottom graphene layers opening an energy gap. This surface organization can be thus defined as self-organized metal-semiconductor graphene.
Heteroepitaxial growth is a process of profound fundamental importance as well as an avenue to realize nanostructures such as Ge/Si quantum dots (QDs), with appealing properties for applications in ...opto‐ and nanoelectronics. However, controlling the Ge/Si QD size, shape, and composition remains a major obstacle to their practical implementation. Here, Ge nanostructures on Si(111) were investigated in situ and in real‐time by low energy electron microscopy (LEEM), enabling the observation of the transition from wetting layer formation to 3D island growth and decay. The island size, shape, and distribution depend strongly on the growth temperature. As the deposition temperature increases, the islands become larger and sparser, consistent with Brownian nucleation and capture dynamics. At 550°C, two distinct Ge/Si nanostructures are formed with bright and dark appearances that correspond to flat, atoll‐like and tall, faceted islands, respectively. During annealing, the faceted islands increase in size at the expense of the flat ones, indicating that the faceted islands are thermodynamically more stable. In contrast, triangular islands with uniform morphology are obtained from deposition at 600°C, suggesting that the growth more closely follows the ideal shape. During annealing, the islands formed at 600°C initially show no change in morphology and size and then rupture simultaneously, signaling a homogeneous chemical potential of the islands. These observations reveal the role of dynamics and energetics in the evolution of Ge/Si QDs, which can serve as a step towards the precise control over the Ge nanostructure size, shape, composition, and distribution on Si(111).
Real‐time low energy electron microscopy observations reveal the growth dynamics and stability of Ge quantum dots formed on Si(111) at temperatures between 450°C and 600°C. A mix of metastable, flat islands and tall, faceted islands are produced at 550°C and below, whereas growth at 600°C yields uniform large, triangular islands.
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