| 1. |
- Niu, Yuran, et al.
(author)
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MAXPEEM : a spectromicroscopy beamline at MAX IV laboratory
- 2023
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In: Journal of Synchrotron Radiation. - 0909-0495. ; 30:Pt 2, s. 468-478
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Journal article (peer-reviewed)abstract
- 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 ≥1015 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.
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| 2. |
- Armakavicius, Nerijus, et al.
(author)
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Resolving mobility anisotropy in quasi-free-standing epitaxial graphene by terahertz optical Hall effect
- 2021
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In: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 172, s. 248-259
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Journal article (peer-reviewed)abstract
- In this work, we demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-free-standing-bilayer epitaxial graphene on 4H–SiC(0001). Directionally independent free hole mobility parameters are found for the monolayer graphene. In contrast, anisotropic hole mobility parameters with a lower mobility in direction perpendicular to the SiC surface steps and higher along the steps in quasi-free-standing-bilayer graphene are determined for the first time. A combination of THz-OHE, nanoscale microscopy and optical spectroscopy techniques are used to investigate the origin of the anisotropy. Different defect densities and different number of graphene layers on the step edges and terraces are ruled out as possible causes. Scattering mechanisms related to doping variations at the step edges and terraces as a result of different interaction with the substrate and environment are discussed and also excluded. It is suggested that the step edges introduce intrinsic scattering in quasi-free-standing-bilayer graphene, that is manifested as a result of the higher ratio between mean free path and average terrace width parameters. The suggested scenario allows to reconcile existing differences in the literature regarding the anisotropic electrical transport in epitaxial graphene.
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| 3. |
- Boix, Virgínia, et al.
(author)
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Area-selective Electron-beam induced deposition of Amorphous-BNx on graphene
- 2021
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In: Applied Surface Science. - : Elsevier BV. - 0169-4332. ; 557, s. 149806-149806
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Journal article (peer-reviewed)abstract
- 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 amorphous-BNx 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 ~ 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.
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| 4. |
- Bokai, Kirill A., et al.
(author)
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Visualization of graphene grain boundaries through oxygen intercalation
- 2021
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In: Applied Surface Science. - : Elsevier BV. - 0169-4332. ; 565
-
Journal article (peer-reviewed)abstract
- Efficient control over the grain boundaries (GBs) is a vital aspect in optimizing the graphene growth conditions. A number of methods for visualization of GBs were developed for graphene grown on weakly interacting surfaces. Here, we utilize oxygen intercalation to reveal GBs and study their morphology for graphene strongly bound to the cobalt surface. We demonstrate that upon the intercalation of oxygen, GBs in polycrystalline graphene become easily detectable due to graphene cracking and selective oxidation of the substrate, thus giving a direct insight into the graphene micro- and nanostructure by means of different electron microscopy methods, including scanning electron microscopy, photoemission microscopy and low-energy electron microscopy.
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| 5. |
- Bouhafs, Chamseddine, et al.
(author)
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Synthesis of large-area rhombohedral few-layer graphene by chemical vapor deposition on copper
- 2021
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In: Carbon. - : Elsevier BV. - 0008-6223. ; 177, s. 282-290
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Journal article (peer-reviewed)abstract
- 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 ∼50 μm2. 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.
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| 6. |
- Forti, Stiven, et al.
(author)
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Semiconductor to metal transition in two-dimensional gold and its van der Waals heterostack with graphene
- 2020
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 11:1
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Journal article (peer-reviewed)abstract
- The synthesis of two-dimensional (2D) transition metals has attracted growing attention for both fundamental and application-oriented investigations, such as 2D magnetism, nanoplasmonics and non-linear optics. However, the large-area synthesis of this class of materials in a single-layer form poses non-trivial difficulties. Here we present the synthesis of a large-area 2D gold layer, stabilized in between silicon carbide and monolayer graphene. We show that the 2D-Au ML is a semiconductor with the valence band maximum 50 meV below the Fermi level. The graphene and gold layers are largely non-interacting, thereby defining a class of van der Waals heterostructure. The 2D-Au bands, exhibit a 225 meV spin-orbit splitting along the Γ K ¯ direction, making it appealing for spin-related applications. By tuning the amount of gold at the SiC/graphene interface, we induce a semiconductor to metal transition in the 2D-Au, which has not yet been observed and hosts great interest for fundamental physics.
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| 7. |
- Greco, Rossella, et al.
(author)
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Activation of 2D cobalt hydroxide with 0D cobalt oxide decoration for microplastics degradation and hydrogen evolution
- 2023
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In: Chemical Engineering Journal. - 1385-8947. ; 471
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Journal article (peer-reviewed)abstract
- The 2D semiconductors are important players in environmental and energy fields due to their unique catalytic and physical properties defined by their dimensionality. Versatile functionalities on one 2D matrix will enlarge its application scopes but require dedicated engineering paths. In this work, we present a cross-dimensional strategy by decorating 0D Co3O4 onto 2D Co(OH)2 to form a multifunctional photocatalyst. The one-pot hydrothermally synthesized Co3O4@Co(OH)2 composite is capable of degrading polystyrene microplastics with an efficiency of 40% under 0.495 W white LED illumination. In a separated experiment, H2 evolution reaction from water splitting was evaluated in absence of sacrificial agents leading to 43 μmol g−1 and to an apparent quantum efficiency of 3.48% at 420 nm. The study of the energy band diagrams by UV–Visible and ambient photoemission spectroscopy and the analysis of the radicals involved in the reaction of photocatalytic degradation allow to unveil the mechanisms for both the processes herein studied. Finally, we could confirm that the heterostructure benefits the redox potentials of 2D and 0D counterparts and facile electron transfers when crossing two different dimensions. These results provide guidelines and inspiration for cross-dimensional activations of low-dimensional materials for versatile functionalities.
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| 8. |
- Karakachian, Hrag, et al.
(author)
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One-dimensional confinement and width-dependent bandgap formation in epitaxial graphene nanoribbons
- 2020
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In: Nature Communications. - : NATURE RESEARCH. - 2041-1723. ; 11:1
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Journal article (peer-reviewed)abstract
- The ability to define an off state in logic electronics is the key ingredient that is impossible to fulfill using a conventional pristine graphene layer, due to the absence of an electronic bandgap. For years, this property has been the missing element for incorporating graphene into next-generation field effect transistors. In this work, we grow high-quality armchair graphene nanoribbons on the sidewalls of 6H-SiC mesa structures. Angle-resolved photoelectron spectroscopy (ARPES) and scanning tunneling spectroscopy measurements reveal the development of a width-dependent semiconducting gap driven by quantum confinement effects. Furthermore, ARPES demonstrates an ideal one-dimensional electronic behavior that is realized in a graphene-based environment, consisting of well-resolved subbands, dispersing and non-dispersing along and across the ribbons respectively. Our experimental findings, coupled with theoretical tight-binding calculations, set the grounds for a deeper exploration of quantum confinement phenomena and may open intriguing avenues for new low-power electronics. Here, the authors investigate armchair graphene nanoribbons by angle-resolved photoelectron spectroscopy, and show the development of a width-dependent semiconducting gap driven by quantum confinement effects, and an ideal one-dimensional electronic behaviour.
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| 9. |
- Karakachian, Hrag, et al.
(author)
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Periodic Nanoarray of Graphene pn-Junctions on Silicon Carbide Obtained by Hydrogen Intercalation
- 2022
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In: Advanced Functional Materials. - : Wiley-V C H Verlag GMBH. - 1616-301X .- 1616-3028. ; 32:18
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Journal article (peer-reviewed)abstract
- Graphene pn-junctions offer a rich portfolio of intriguing physical phenomena. They stand as the potential building blocks for a broad spectrum of future technologies, ranging from electronic lenses analogous to metamaterials in optics, to high-performance photodetectors important for a variety of optoelectronic applications. The production of graphene pn-junctions and their precise structuring at the nanoscale remains to be a challenge. In this work, a scalable method for fabricating periodic nanoarrays of graphene pn-junctions on a technologically viable semiconducting SiC substrate is introduced. Via H-intercalation, 1D confined armchair graphene nanoribbons are transformed into a single 2D graphene sheet rolling over 6H-SiC mesa structures. Due to the different surface terminations of the basal and vicinal SiC planes constituting the mesa structures, different types of charge carriers are locally induced into the graphene layer. Using angle-resolved photoelectron spectroscopy, the electronic band structure of the two graphene regions are selectively measured, finding two symmetrically doped phases with p-type being located on the basal planes and n-type on the facets. The results demonstrate that through a careful structuring of the substrate, combined with H-intercalation, integrated networks of graphene pn-junctions could be engineered at the nanoscale, paving the way for the realization of novel optoelectronic device concepts.
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| 10. |
- Kim, Kyung Ho, 1984, et al.
(author)
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Ambipolar charge transport in quasi-free-standing monolayer graphene on SiC obtained by gold intercalation
- 2020
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 102:16
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Journal article (peer-reviewed)abstract
- 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 °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.
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