| 1. |
- Ng, May Ling, et al.
(författare)
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Effect of substrate nanopatterning on the growth and structure of pentacene films
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:11, s. 115449-
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Tidskriftsartikel (refereegranskat)abstract
- The effect of modulating the structure of thin pentacene (C22H14) films by a nanopatterned inert substrate, known as hexagonal boron nitride nanomesh, is reported. Films of different thickness are grown and characterized by x-ray absorption, core-level photoemission, low-energy electron microscopy, microbeam low-energy electron diffraction, and scanning tunneling microscopy. Initially the pentacene molecules adsorb with the molecular plane lying flat on the substrate but they tend to flip up with increasing coverage, forming well-ordered monolayer-thick islands of upright molecules with low nucleation density. The herringbone packing of the upright molecules is observed with scanning tunneling microscopy. The electronic structure of the adsorbed molecules is very similar to that of the gas-phase pentacene, implying weak interaction with the substrate and between the molecules. The periodic corrugation of the substrate surface causes the monolayer of upright pentacene molecules to form two different coincidence superstructures. The lattice parameters of the pentacene unit cell for each of these two substrate-induced domains are determined from the microdiffraction patterns. Both domains can occur in several equivalent configurations, thus resulting in a number of twins with a typical size of a few micrometers. The first monolayer grows in a layer-by-layer mode until it is completed while the second monolayer forms diffusion-limited fractal islands. Upon annealing, the pentacene films are thermally stable up to approximately 80 degrees C and thereafter the onset of desorption is observed.
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| 2. |
- Bokai, Kirill A., et al.
(författare)
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Visualization of graphene grain boundaries through oxygen intercalation
- 2021
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332. ; 565
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Tidskriftsartikel (refereegranskat)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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| 3. |
- Ivanov, A. A., et al.
(författare)
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Studying the effect of oxygen content on the electron structure of Nd1.85Ce0.15CuO4 by means of photoelectron spectromicroscopy
- 2007
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Ingår i: Journal of Experimental and Theoretical Physics. - 1063-7761. ; 105:1, s. 241-245
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Tidskriftsartikel (refereegranskat)abstract
- Changes in the electron structure of the surface layer of Nd1.85Ce0.15CuO4 (NCCO) epitaxial films, which were caused by variation of the oxygen content and modification of the crystal structure of samples as a result of Ar+ ion etching and annealing, have been studied by means of photoelectron spectromicroscopy. A method is proposed for the cleaning the surface of oxygen-containing superconductors, which includes sequential stages of deep ion etching, high-temperature annealing in an oxygen-containing atmosphere (for the structural recovery and saturation with oxygen), a short-term ion etching (for the removal of an adsorbed layer of the oxidizer), and the final vacuum annealing of radiation-induced effects. The application of this procedure to NCCO films allowed an electron structure to be obtained, which was identical to that inherent in the surface of single crystals cleaved in situ in the measurement chamber.
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| 4. |
- Krasnikov, Sergey A., et al.
(författare)
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Formation of extended covalently bonded Ni porphyrin networks on the Au(111) surface
- 2011
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Ingår i: Nano Reseach. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 4:4, s. 376-384
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Tidskriftsartikel (refereegranskat)abstract
- The growth and ordering of {5,10,15,20-tetrakis(4-bromophenyl)porphyrinato}nickel(II) (NiTBrPP) molecules on the Au(111) surface have been investigated using scanning tunnelling microscopy, X-ray absorption, core-level photoemission, and microbeam low-energy electron diffraction. When deposited onto the substrate at room temperature, the NiTBrPP forms a well-ordered close-packed molecular layer in which the molecules have a flat orientation with the porphyrin macrocycle plane lying parallel to the substrate. Annealing of the NiTBrPP layer on the Au(111) surface at 525 K leads to dissociation of bromine from the porphyrin followed by the formation of covalent bonds between the phenyl substituents of the porphyrin. This results in the formation of continuous covalently bonded porphyrin networks, which are stable up to 800 K and can be recovered after exposure to ambient conditions. By controlling the experimental conditions, a robust, extended porphyrin network can be prepared on the Au(111) surface that has many potential applications such as protective coatings, in sensing or as a host structure for molecules and clusters.
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| 5. |
- Sammelselg, V, et al.
(författare)
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Study of thin oxide films by electron, ion and synchrotron radiation beams
- 2002
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Ingår i: Microchimica Acta. - : Springer Science and Business Media LLC. - 1436-5073 .- 0026-3672. ; 139:1-4, s. 165-169
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Tidskriftsartikel (refereegranskat)abstract
- Titanium oxide and zirconium oxide thin films deposited on silicon substrates were characterised using electron probe microanalysis (EPMA), Rutherford backscattering spectroscopy (RBS), time-of-flight elastic recoil detection analysis (TOF-ERDA) and scanning photoelectron microscopy (SPEM). The composition and mass thickness of the films were determined and the results of different methods compared. It was revealed that the synchrotron radiation used for SPEM studies caused considerable modification of zirconia films grown at low temperatures.
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| 6. |
- Tsetlin, M. B., et al.
(författare)
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Investigation of Epitaxial Nd1.85Ce0.15CuO4-y Film Surface by Low Energy Electron Diffractometry
- 2008
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Ingår i: Journal Of Surface Investigation-X-Ray Synchrotron And Neutron Techniques. - 1027-4510. ; 2:6, s. 928-930
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Tidskriftsartikel (refereegranskat)abstract
- The surface of epitaxial Nd1.85Ce0.15CuO4-y(001) (NCCO) film has been studied by low energy electron diffractometry (LEED) and photoelectron spectroscopy. Ar+ ion etching of a surface with subsequent annealing in oxygen at atmosphere pressure has been found to lead to the ordered structure restoration of surface layers with the symmetry type and lattice parameters corresponding to the NCCO phase. Annealing in vacuum at temperatures close to the boundary of thermodynamic phase stability results in the formation of epitaxial Ce0.5Nd0.5O1.75 phase on a surface that is indicated in the LEED pattern as additional spots corresponding to the surface lattice (root 2 x root 2) R45 degrees.
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| 7. |
- Vinogradov, Nikolay, et al.
(författare)
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Formation and Structure of Graphene Waves on Fe(110)
- 2012
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 109:2
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Tidskriftsartikel (refereegranskat)abstract
- A very rich Fe-C phase diagram makes the formation of graphene on iron surfaces a challenging task. Here we demonstrate that the growth of graphene on epitaxial iron films can be realized by chemical vapor deposition at relatively low temperatures, and that the formation of carbides can be avoided in excess of the carbon-containing precursors. The resulting graphene monolayer creates a novel periodically corrugated pattern on Fe(110). Using low-energy electron microscopy and scanning tunneling microscopy, we show that it is modulated in one dimension forming long waves with a period of similar to 4 nm parallel to the [001] direction of the substrate, with an additional height modulation along the wave crests. The observed topography of the graphene/Fe superstructure is well reproduced by density functional theory calculations, and found to result from a unique combination of the lattice mismatch and strong interfacial interaction, as probed by core-level photoemission and x-ray absorption spectroscopy.
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| 8. |
- Wang, F., et al.
(författare)
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Silicon intercalation into the graphene-SiC interface
- 2012
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 85:16
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Tidskriftsartikel (refereegranskat)abstract
- In this work we use low-energy electron microscopy, x-ray photoemission electron microscopy, and x-ray photoelectron spectroscopy to study how the excess Si at the graphene-vacuum interface reorders itself at high temperatures. We show that silicon deposited at room temperature onto multilayer graphene films grown on the SiC(000 (1) over bar) rapidly diffuses to the graphene-SiC interface when heated to temperatures above 1020 degrees C. In a sequence of depositions, we have been able to intercalate similar to 6 ML of Si into the graphene-SiC interface.
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| 9. |
- Aristov, Victor Yu., et al.
(författare)
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Graphene Synthesis on Cubic SiC/Si Wafers. Perspectives for Mass Production of Graphene-Based Electronic Devices
- 2010
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 10:3, s. 992-995
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Tidskriftsartikel (refereegranskat)abstract
- The outstanding properties of graphene, a single graphite layer, render it a top candidate for substituting silicon in future electronic devices, The so far exploited synthesis approaches, however, require conditions typically achieved in specialized laboratories and result in graphene sheets whose electronic properties are often altered by interactions with substrate materials. The development of graphene-based technologies requires an economical fabrication method compatible with mass production. Here we demonstrate for the fist Lime the feasibility of graphene synthesis on commercially available cubic SiC/Si substrates of >300 mm in diameter, which result in graphene flakes electronically decoupled from the substrate. After optimization of the preparation procedure, the proposed synthesis method can represent a further big step toward graphene-based electronic technologies.
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| 10. |
- Baringhaus, J., et al.
(författare)
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Bipolar gating of epitaxial graphene by intercalation of Ge
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:26
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the ambivalent behavior of Ge intercalation is studied by means of scanning tunneling microscopy and spectroscopy as well as local 4-point probe transport measurements. In quantitative agreement with angle-resolved photoemission experiments, both p-and n-type doped graphene areas and their doping level were identified by local spectroscopy. The p-doped areas appear higher by 2 angstrom with respect to the n-doped areas suggesting incorporation of thicker Ge-layers accompanied by a modified coupling to the initial SiC-surface. Furthermore, the sheet resistance was measured on each of the patches separately. The intrinsic imbalance between the carrier types in the different areas is well reflected by the transport study. The process of intercalation does not affect the transport properties in comparison to pristine graphene pointing to a sufficient homogeneity of the decoupled graphene layer. Transport measurements across chemically gated pn-junctions reveal increased resistances, possibly due to enlarged tunneling barriers. (C) 2014 AIP Publishing LLC.
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| 11. |
- Bertolo, M, et al.
(författare)
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Photoemission spectromicroscopy study of a Bi2Sr2CaCu2O8+delta single crystal
- 2003
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Ingår i: Journal de Physique IV - Proceedings. - : EDP Sciences. - 1155-4339 .- 1764-7177. ; 104, s. 487-490
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Konferensbidrag (refereegranskat)abstract
- Using photoemission spectromicroscopy at sub-micron lateral resolution we studied two superconductor systems of high fundamental and practical importance, such as Bi2Sr2CaCu2O8+delta and MgB2. Our findings demonstrate the importance of considering, for this class of systems, the possible presence of spatial inhomogeneities in interpreting the results of conventional photoemission experiments, which typically probe an area of the order of 1 mm(2). In particular, in the case of MgB2, we report a measured density of states directly comparable to the theoretical predictions, thus rejecting the claim for the existence of strong correlation effects proposed by previous spatially averaged measurements.
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| 12. |
- Cavar, Elizabeta, et al.
(författare)
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A single h-BN layer on Pt(111)
- 2008
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 602:9, s. 1722-1726
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Tidskriftsartikel (refereegranskat)abstract
- The structure and formation of an ultrathin hexagonal boron nitride (h-BN) film on Pt(111) has been studied by a combination of scanning tunneling microscopy, low energy electron diffraction, low energy electron microscopy, X-ray absorption and high resolution core level spectroscopy. The study shows that a single boron nitride layer is formed on Pt(111), resulting in a coincidence structure. High resolution scanning tunneling microscopy (STM) images of the h-BN ultrathin film display only one of the atomic species in the unit cell. Probing the boron and nitrogen related local density of states by near edge X-ray absorption fine structure measurements we conclude that the nitrogen sublattice is visible in STM images. The growth of the single hexagonal boron nitride layer by vapourized borazine in the pressure range of 1 x 10(-6)-1 x 10(-8) at 800 degrees C is further studied by low energy electron microscopy, and reveals that the number of nucleation sites and the perfection of the growth is strongly pressure dependent. A model for the single, hexagonal, boron nitride layer on Pt(111) is proposed.
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| 13. |
- Coletti, C., et al.
(författare)
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Revealing the electronic band structure of trilayer graphene on SiC: An angle-resolved photoemission study
- 2013
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 88:15
-
Tidskriftsartikel (refereegranskat)abstract
- In recent times, trilayer graphene has attracted wide attention owing to its stacking and electric-field-dependent electronic properties. However, a direct and well-resolved experimental visualization of its band structure has not yet been reported. In this paper, we present angle-resolved photoemission spectroscopy data which show with high resolution the electronic band structure of trilayer graphene obtained on alpha-SiC(0001) and beta-SiC(111) via hydrogen intercalation. Electronic bands obtained from tight-binding calculations are fitted to the experimental data to extract the interatomic hopping parameters for Bernal and rhombohedral stacked trilayers. Low-energy electron microscopy measurements demonstrate that the trilayer domains extend over areas of tens of square micrometers, suggesting the feasibility of exploiting this material in electronic and photonic devices. Furthermore, our results suggest that, on SiC substrates, the occurrence of a rhombohedral stacked trilayer is significantly higher than in natural bulk graphite.
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| 14. |
- Cooil, Simon P., et al.
(författare)
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In Situ Patterning of Ultrasharp Dopant Profiles in Silicon
- 2017
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 11:2, s. 1683-1688
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Tidskriftsartikel (refereegranskat)abstract
- We develop a method for patterning a buried two-dimensional electron gas (2DEG) in silicon using low kinetic energy electron stimulated desorption (LEESD) of a monohydride resist mask. A buried 2DEG forms as a result of placing a dense and narrow profile of phosphorus dopants beneath the silicon surface; a so-called δ -layer. Such 2D dopant profiles have previously been studied theoretically, and by angle-resolved photoemission spectroscopy, and have been shown to host a 2DEG with properties desirable for atomic-scale devices and quantum computation applications. Here we outline a patterning method based on low kinetic energy electron beam lithography, combined with in situ characterization, and demonstrate the formation of patterned features with dopant concentrations sufficient to create localized 2DEG states.
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| 15. |
- Neckel, Itamar T., et al.
(författare)
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Unveiling Center-Type Topological Defects on Rosettes of Lead Zirconate Titanate Associated to Oxygen Vacancies
- 2021
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Ingår i: Annalen der Physik. - : Wiley. - 0003-3804 .- 1521-3889.
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the formation of topological defects in ferroelectric materials can provide access to their unusual physics, invaluable for possible applications of domain engineering in nanoelectronics. The present study shows the spontaneous formation of center-type topological defects in lead zirconate titanate (PZT) films, characterized by a strong in-plane radial electromechanical response on rosette-like structures, as revealed by piezoelectric response force microscopy. Multi-modal electron and X-ray based techniques provide a comprehensive set of tools for delving at atomic scale the local chemistry, electronic, and crystalline structures. The elemental surface-sensitivity of soft X-ray absorption shows chemical segregation and drastic changes in the titanium electronic structure, which have been associated to oxygen vacancies, leading to a mechanism that describe the polarization symmetry on the rosettes. X-ray nanofluorescence experiments revealed titanium segregation, corroborating with the absorption and low energy electron microscopy images. Taking advantage of a nanobeam, 2D nanodiffraction maps probed the complex microstructure of the domains, revealing its mosaicity and lattice strain at the nanoscale. This study highlights a complex topological structure and provides, by the simultaneous application of a set of analytical tools, a broad comprehension of the formation of center-type ferroelectric topological defects in PZT.
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| 16. |
- Shi, Yuchen, et al.
(författare)
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A patterning-free approach for growth of free-standing graphene nanoribbons using step-bunched facets of off-oriented 4H-SiC(0 0 0 1) epilayers
- 2020
-
Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 53:11
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Tidskriftsartikel (refereegranskat)abstract
- The tunable electronic structure of graphene nanoribbons (GNRs) has attracted much attention due to the great potential in nanoscale electronic applications. Most methods to produce GNRs rely on the lithographic process, which suffers from the process-induced disorder in the graphene and scalability issues. Here, we demonstrate a novel approach to directly grow free-standing GNRs on step-bunched facets of off-oriented 4H-SiC epilayers without any patterning or lithography. First, the 4H-SiC epilayers with well-defined bunched steps were intentionally grown on 4 degree off-axis 4H-SiC substrates by the sublimation epitaxy technique. As a result, periodic step facets in-between SiC terraces were obtained. Then, graphene layers were grown on such step-structured 4H-SiC epilayers by thermal decomposition of SiC. Scanning tunneling microscopy (STM) studies reveal that the inclined step facets are about 13-15 nm high and 30-35 nm wide, which gives an incline angle of 23-25 degrees. LEEM and LEED results showed that the terraces are mainly covered by monolayer graphene and the buffer layer underneath it. STM images and the analysis of their Fourier transform patterns suggest that on the facets, in-between terraces, graphene is strongly buckled and appears to be largely decoupled from the surface.
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| 17. |
- Ali-Löytty, Harri, et al.
(författare)
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The role of (FeCrSi)2(MoNb)-type Laves phase on the formation of Mn-rich protective oxide scale on ferritic stainless steel
- 2018
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Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 132, s. 214-222
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Tidskriftsartikel (refereegranskat)abstract
- Microalloying of stainless steel with reactive elements increases oxidation resistance but makes the alloy prone to microstructural changes. XPS results reveal changes in the initial oxidation mechanism on Ti-Nb stabilized ferritic stainless steel (EN 1.4521) after 120 h heat treatment at 650 °C. Age-precipitation of (FeCrSi)2(MoNb)-type Laves phase resulted in less pronounced surface segregation and oxidation of microalloying elements. Si oxidizes preferentially at the Laves precipitate locations via outward diffusion forming diffusion barrier for the other scale forming elements. Most significantly the diffusion of Mn and the formation of low volatile (Mn,Cr)3O4 spinel oxide at the surface was strongly suppressed.
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| 18. |
- Aprojanz, Johannes, et al.
(författare)
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Ballistic tracks in graphene nanoribbons
- 2018
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 9:1
-
Tidskriftsartikel (refereegranskat)abstract
- High quality graphene nanoribbons epitaxially grown on the sidewalls of silicon carbide (SiC) mesa structures stand as key building blocks for graphene-based nanoelectronics. Such ribbons display 1D single-channel ballistic transport at room temperature with exceptionally long mean free paths. Here, using spatially-resolved two-point probe (2PP) measurements, we selectively access and directly image a range of individual transport modes in sidewall ribbons. The signature of the independently contacted channels is a sequence of quantised conductance plateaus for different probe positions. These result from an interplay between edge magnetism and asymmetric terminations at opposite ribbon edges due to the underlying SiC structure morphology. Our findings demonstrate a precise control of transport through multiple, independent, ballistic tracks in graphene-based devices, opening intriguing pathways for quantum information device concepts.
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| 19. |
- Aprojanz, Johannes, et al.
(författare)
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Nanoscale imaging of electric pathways in epitaxial graphene nanoribbons
- 2019
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Ingår i: Nano Research. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 12:7, s. 1697-1702
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Tidskriftsartikel (refereegranskat)abstract
- Graphene nanoribbons (GNRs) are considered as major building blocks in future carbon-based electronics. The electronic performance of graphene nanostructures is essentially influenced and determined by their edge termination and their supporting substrate. In particular, semi-conducting, as well as metallic GNRs, can be fabricated by choosing the proper template which is favorable for device architecture designs. This study highlights the impact of microscopic details of the environment of the GNRs on the charge transport in GNRs. By means of lateral force, conductive atomic force and nanoprobe measurements, we explore the charge propagation in both zig-zag and armchair GNRs epitaxially grown on SiC templates. We directly image transport channels on the nanoscale and identify SiC substrate steps and nano-instabilities of SiC facets as dominant charge scattering centers. [Figure not available: see fulltext.].
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| 20. |
- Armakavicius, Nerijus, et al.
(författare)
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Resolving mobility anisotropy in quasi-free-standing epitaxial graphene by terahertz optical Hall effect
- 2021
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 172, s. 248-259
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Tidskriftsartikel (refereegranskat)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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| 21. |
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| 22. |
- Bouhafs, Chamseddine, et al.
(författare)
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Synthesis of large-area rhombohedral few-layer graphene by chemical vapor deposition on copper
- 2021
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223. ; 177, s. 282-290
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Tidskriftsartikel (refereegranskat)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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| 23. |
- Chaika, Alexander N., et al.
(författare)
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Rotated domain network in graphene on cubic-SiC(001)
- 2014
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 25:13
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Tidskriftsartikel (refereegranskat)abstract
- The atomic structure of the cubic-SiC(001) surface during ultra-high vacuum graphene synthesis has been studied using scanning tunneling microscopy (STM) and low-energy electron diffraction. Atomically resolved STM studies prove the synthesis of a uniform, millimeter-scale graphene overlayer consisting of nanodomains rotated by +/- 13.5 degrees relative to the < 110 >-directed boundaries. The preferential directions of the domain boundaries coincide with the directions of carbon atomic chains on the SiC(001)-c(2 x 2) reconstruction, fabricated prior to graphene synthesis. The presented data show the correlation between the atomic structures of the SiC(001)-c(2 x 2) surface and the graphene/SiC(001) rotated domain network and pave the way for optimizing large-area graphene synthesis on low-cost cubic-SiC(001)/Si(001) wafers.
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| 24. |
- Cooil, S. P., et al.
(författare)
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Controlling the growth of epitaxial graphene on metalized diamond (111) surface
- 2015
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 107:18
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Tidskriftsartikel (refereegranskat)abstract
- The 2-dimensional transformation of the diamond (111) surface to graphene has been demonstrated using ultrathin Fe films that catalytically reduce the reaction temperature needed for the conversion of sp(3) to sp(2) carbon. An epitaxial system is formed, which involves the re-crystallization of carbon at the Fe/vacuum interface and that enables the controlled growth of monolayer and multilayer graphene films. In order to study the initial stages of single and multilayer graphene growth, real time monitoring of the system was preformed within a photoemission and low energy electron microscope. It was found that the initial graphene growth occurred at temperatures as low as 500 degrees C, whilst increasing the temperature to 560 degrees C was required to produce multi-layer graphene of high structural quality. Angle resolved photoelectron spectroscopy was used to study the electronic properties of the grown material, where a graphene-like energy momentum dispersion was observed. The Dirac point for the first layer is located at 2.5 eV below the Fermi level, indicating an n-type doping of the graphene due to substrate interactions, while that of the second graphene layer lies close to the Fermi level. (C) 2015 AIP Publishing LLC.
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| 25. |
- Darakchieva, Vanya, et al.
(författare)
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Large-area microfocal spectroscopic ellipsometry mapping of thickness and electronic properties of epitaxial graphene on Si- and C-face of 3C-SiC(111)
- 2013
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 102:21, s. 213116-
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Tidskriftsartikel (refereegranskat)abstract
- Microfocal spectroscopic ellipsometry mapping of the electronic properties and thickness of epitaxial graphene grown by high-temperature sublimation on 3C-SiC (111) substrates is reported. Growth of one monolayer graphene is demonstrated on both Si- and C-polarity of the 3C-SiC substrates and it is shown that large area homogeneous single monolayer graphene can be achieved on the Si-face substrates. Correlations between the number of graphene monolayers on one hand and the main transition associated with an exciton enhanced van Hove singularity at ∼4.5 eV and the free-charge carrier scattering time, on the other are established. It is shown that the interface structure on the Si- and C-polarity of the 3C-SiC(111) differs and has a determining role for the thickness and electronic properties homogeneity of the epitaxial graphene.
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| 26. |
- Ericsson, Leif, 1964-, et al.
(författare)
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Characterization of nano-ZnO using SPELEEM and STM
- 2008
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Characterization of nano-ZnO using SPELEEM and STMLeif Ericsson1, Kjell Magnusson1 and Alexei Zakharov21) Karlstad University, Sweden, 2) MAX-lab, Lund University, SwedenFuture functional applications of nanostructured ZnO, a wide bandgap semiconductor, include optical, electronic, optoelectronic, photovoltaic and piezoelectric devices. Much of the previous research has mainly concerned synthesis and growth of ZnO in different forms. In a previous survey, we have studied commercially available nanoparticles (Alfa Aesar GmbH, Germany) with an average size of 70 nm, in dispersions spin coated on SiO2/Si(001) surfaces. Using 2-10 mg/ml in ethanol and using ultrasound, we have prepared surfaces with a varying number of separated individual nanoparticles. These particles have been characterized with SEM and AFM, to reveal variations in morphology and size, and degree of agglomeration.In the present study, we have used Low-Energy Electron Microscopy (LEEM) and Photoemission Electron Microscopy (PEEM), the latter using synchrotron radiation, to further characterize the ZnO nanoparticles in terms of crystallinity, chemical state and electronic properties. The experiments have been done at MAX-lab, Lund, Sweden, at Beamline I311, using a system supplied by ELMITEC GmbH, Germany. In addition to microscopy, X-ray Photoelectron Spectroscopy from selected areas down to 1,3 micrometer, micro-XPS, has been used, yielding detailed spectroscopic information with this degree of lateral resolution.After a low-temperature anneal at 500 °C, LEEM images from 25 micrometer show strong (0,0)-reflections from separated areas of sizes down to less than 100 nm, in accordance with expectations for the size of the ZnO particles. Different electron energies show reflections from different grains due to orientation of the nanoparticles. Further annealing at 650 °C improves the LEEM image, yielding evidence for a fair degree of crystallinity and surface cleanliness in accordance with peak intensities from micro-XPS.The chemical state and electronic properties of both ZnO particles and SiO2/Si(001) substrate were studied with micro-XPS and PEEM, yielding well-resolved peaks from Zn 3d and Si 2p, which were used to create PEEM images with a perfect complementarity between Zn 3d and Si 2p based images. micro-XPS from ZnO nanoparticle agglomerates, ZnO free SiO2/Si(001) surface as well as mixed areas has been collected, showing differences in binding energies.For characterization of atomic structure and electronic structure with yet higher lateral resolution, Scanning Tunneling Microscopy and Spectroscopy, STM-STS, has been applied and preliminary result will be presented.
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| 27. |
- Forti, Stiven, et al.
(författare)
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Semiconductor to metal transition in two-dimensional gold and its van der Waals heterostack with graphene
- 2020
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 11:1
-
Tidskriftsartikel (refereegranskat)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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| 28. |
- Goriachko, A., et al.
(författare)
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Hexagonal C and BN superstructures on Ru(0001) and Ge(111)
- 2013
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Ingår i: Materialwissenschaft und Werkstofftechnik. - : Wiley. - 0933-5137. ; 44:2-3, s. 129-135
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Tidskriftsartikel (refereegranskat)abstract
- A brief overview of hexagonal superstructures with a periodicity of similar to 3 nm formed on Ru(0001) and Ge(111) by graphene or hexagonal boron nitride with a thickness of just a single atomic layer is given. A periodic height corrugation of such layers due to the lattice mismatch with the substrate material is of the order of 0.1 nm. Selected examples of scanning tunneling microscopy (STM) and low energy electron microscopy/diffraction (LEEM/LEED) investigations of BN/Ru(0001), C/Ru(0001), (BN)(x)C-y/Ru(0001) and C/Ge(111) are presented. These systems can act as nanotemplates for metal nanoparticles growth, as well as strongly heterogeneous substrates for molecular adsorption.
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| 29. |
- Goriachko, A., et al.
(författare)
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Oxygen-etching of h-BN/Ru(0001) nanomesh on the nano- and mesoscopic scale
- 2008
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 112:28, s. 10423-10427
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Tidskriftsartikel (refereegranskat)abstract
- The stability of the recently discovered b-BN/Ru(0001) nanomesh is of crucial importance for potential applications as a nanotemplate. In particular, thermal stability in oxygen environment is important for nanocatalysis applications. We report here on the etching experiments of the h-BN layer by molecular oxygen exposure at elevated temperatures. This process is studied both by scanning tunneling microscopy (STM) on a microscopic scale and with in situ low energy electron microscopy (LEEM) on the mesoscopic scale. Temperature thresholds are determined for the microscopic (600 degrees C) and the mesoscopic (750 degrees C) etching processes for O-2 pressures up to 1 x 10(-6) mbar. Submonolayer amounts of An deposited on the h-BN/Ru(0001) nanomesh improve considerably the stability of the h-BN nanomesh against etching by O-2.
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| 30. |
- Hjort, Martin, et al.
(författare)
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Electronic and Structural Differences between Wurtzite and Zinc Blende InAs Nanowire Surfaces: Experiment and Theory
- 2014
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 8:12, s. 12346-12355
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Tidskriftsartikel (refereegranskat)abstract
- We determine the detailed differences in geometry and band structure between wurtzite (Wz) and zinc blende (Zb) InAs nanowire (NW) surfaces using scanning tunneling microscopy/spectroscopy and photoemission electron microscopy. By establishing unreconstructed and defect-free surface facets for both Wz and Zb, we can reliably measure differences between valence and conduction band edges, the local vacuum levels, and geometric relaxations to the few-millielectronvolt and few-picometer levels, respectively. Surface and bulk density functional theory calculations agree well with the experimental findings and are used to interpret the results, allowing us to obtain information on both surface and bulk electronic structure. We can thus exclude several previously proposed explanations for the observed differences in conductivity of Wz-Zb NW devices. Instead, fundamental structural differences at the atomic scale and nanoscale that we observed between NW surface facets can explain the device behavior.
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| 31. |
- Karakachian, Hrag, et al.
(författare)
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One-dimensional confinement and width-dependent bandgap formation in epitaxial graphene nanoribbons
- 2020
-
Ingår i: Nature Communications. - : NATURE RESEARCH. - 2041-1723. ; 11:1
-
Tidskriftsartikel (refereegranskat)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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| 32. |
- Karakachian, Hrag, et al.
(författare)
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Periodic Nanoarray of Graphene pn-Junctions on Silicon Carbide Obtained by Hydrogen Intercalation
- 2022
-
Ingår i: Advanced Functional Materials. - : Wiley-V C H Verlag GMBH. - 1616-301X .- 1616-3028. ; 32:18
-
Tidskriftsartikel (refereegranskat)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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| 33. |
- Khaustov, Vladislav O., et al.
(författare)
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Heterocontact-Triggered 1H to 1T′ Phase Transition in CVD-Grown Monolayer MoTe2 : Implications for Low Contact Resistance Electronic Devices
-
Ingår i: ACS Applied Nano Materials. - 2574-0970.
-
Tidskriftsartikel (refereegranskat)abstract
- Single-layer molybdenum ditelluride (MoTe2) has attracted attention due to the smaller energy difference between the semiconducting (1H) and semimetallic (1T′) phases with respect to other two-dimensional transition metal dichalcogenides (TMDs). Understanding the phenomenon of polymorphism between these structural phases is of great fundamental and practical importance. In this paper, we report a 1H to 1T′ phase transition occurring during the chemical vapor deposition (CVD) synthesis of single-layer MoTe2 at 730 °C. The transformation originates at the heterocontact between monoclinic and hexagonal crystals and progresses to either yield a partial or complete 1H to 1T′ phase transition. Microscopic and spectroscopic analyses of the MoTe2 crystals reveal the presence of Te vacancies and mirror twin boundaries (MTB) domains in the hexagonal phase. The experimental observations and theoretical simulations indicate that the combination of heterocontact formation and Te vacancies are relevant triggering mechanisms in the observed transformation. By advancing in the understanding and controlling of the direct synthesis of lateral 1T′/1H heterostructures, this work contributes to the development of MoTe2-based electronic and optoelectronic devices with low contact resistance.
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| 34. |
- Kim, Kyung Ho, 1984, et al.
(författare)
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Ambipolar charge transport in quasi-free-standing monolayer graphene on SiC obtained by gold intercalation
- 2020
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 102:16
-
Tidskriftsartikel (refereegranskat)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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| 35. |
- Laverock, J, et al.
(författare)
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Direct Observation of Decoupled Structural and Electronic Transitions and an Ambient Pressure Monocliniclike Metallic Phase of VO_{2}.
- 2014
-
Ingår i: Physical Review Letters. - 1079-7114. ; 113:21
-
Tidskriftsartikel (refereegranskat)abstract
- We report the simultaneous measurement of the structural and electronic components of the metal-insulator transition (MIT) of VO_{2} using electron and photoelectron spectroscopies and microscopies. We show that these evolve over different temperature scales, and are separated by an unusual monocliniclike metallic phase. Our results provide conclusive evidence that the new monocliniclike metallic phase, recently identified in high-pressure and nonequilibrium measurements, is accessible in the thermodynamic transition at ambient pressure, and we discuss the implications of these observations on the nature of the MIT in VO_{2}.
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| 36. |
- Li, Hao, et al.
(författare)
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Atomic-Scale Tuning of Graphene/Cubic SiC Schottky Junction for Stable Low-Bias Photoelectrochemical Solar-to-Fuel Conversion
- 2020
-
Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 14:4, s. 4905-4915
-
Tidskriftsartikel (refereegranskat)abstract
- Engineering tunable graphene-semiconductor interfaces while simultaneously preserving the superior properties of graphene is critical to graphene-based devices for electronic, optoelectronic, biomedical, and photoelectrochemical applications. Here, we demonstrate this challenge can be surmounted by constructing an interesting atomic Schottky junction via epitaxial growth of high-quality and uniform graphene on cubic SiC (3C-SiC). By tailoring the graphene layers, the junction structure described herein exhibits an atomic-scale tunable Schottky junction with an inherent built-in electric field, making it a perfect prototype to systematically comprehend interfacial electronic properties and transport mechanisms. As a proof-of-concept study, the atomic-scale-tuned Schottky junction is demonstrated to promote both the separation and transport of charge carriers in a typical photoelectrochemical system for solar-to-fuel conversion under low bias. Simultaneously, the as-grown monolayer graphene with an extremely high conductivity protects the surface of 3C-SiC from photocorrosion and energetically delivers charge carriers to the loaded cocatalyst, achieving a synergetic enhancement of the catalytic stability and efficiency.
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| 37. |
- Liu, Chaocheng, et al.
(författare)
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Probing the Néel-Type Antiferromagnetic Order and Coherent Magnon–Exciton Coupling in Van Der Waals VPS3
- 2023
-
Ingår i: Advanced Materials. - 0935-9648. ; 35:30
-
Tidskriftsartikel (refereegranskat)abstract
- 2D van der Waals (vdW) antiferromagnets have received intensive attention due to their terahertz resonance, multilevel magnetic-order states, and ultrafast spin dynamics. However, accurately identifying their magnetic configuration still remains a challenge owing to the lack of net magnetization and insensitivity to external fields. In this work, the Néel-type antiferromagnetic (AFM) order in 2D antiferromagnet VPS3 with the out-of-plane anisotropy, which is demonstrated by the temperature-dependent spin–phonon coupling and second-harmonic generation (SHG), is experimentally probed. This long-range AFM order even persists at the ultrathin limit. Furthermore, strong interlayer exciton–magnon coupling (EMC) upon the Néel-type AFM order is detected based on the monolayer WSe2/VPS3 heterostructure, which induces an enhanced excitonic state and further certifies the Néel-type AFM order of VPS3. The discovery provides optical routes as the novel platform to study 2D antiferromagnets and promotes their potential applications in magneto-optics and opto-spintronic devices.
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| 38. |
- Momeni Pakdehi, Davood, et al.
(författare)
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Silicon Carbide Stacking-Order-Induced Doping Variation in Epitaxial Graphene
- 2020
-
Ingår i: Advanced Functional Materials. - : Wiley. - 1616-301X .- 1616-3028. ; 30:45
-
Tidskriftsartikel (refereegranskat)abstract
- Generally, it is supposed that the Fermi level in epitaxial graphene is controlled by two effects: p-type polarization doping induced by the bulk of the hexagonal silicon carbide (SiC)(0001) substrate and overcompensation by donor-like states related to the buffer layer. The presented work is evidence that this effect is also related to the specific underlying SiC terrace. Here a periodic sequence of non-identical SiC terraces is fabricated, which are unambiguously attributed to specific SiC surface terminations. A clear correlation between the SiC termination and the electronic graphene properties is experimentally observed and confirmed by various complementary surface-sensitive methods. This correlation is attributed to a proximity effect of the SiC termination-dependent polarization doping on the overlying graphene layer. These findings open a new approach for a nano-scale doping-engineering by the self-patterning of epitaxial graphene and other 2D layers on dielectric polar substrates.
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| 39. |
- Mousavi, S. Fatemeh, et al.
(författare)
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Atomic Hydrogen Annealing of Graphene on InAs Surfaces and Nanowires : Interface and Morphology Control for Optoelectronics and Quantum Technologies
- 2022
-
Ingår i: ACS Applied Nano Materials. - : American Chemical Society (ACS). - 2574-0970. ; 5:12, s. 17919-17927
-
Tidskriftsartikel (refereegranskat)abstract
- Folding two-dimensional graphene around one-dimensional III-V nanowires yields a new class of hybrid nanomaterials combining their excellent complementary properties. However, important for high-quality electrical and optical performance, needed in many applications, are well-controlled oxide-free interfaces and a tight folding morphology. To improve the interface chemistry between the graphene and InAs, we annealed the samples in atomic hydrogen. Using surface-sensitive imaging, we found that the III-V native oxides in the interface can be reduced at temperatures that maintain the graphene and the III-V nanostructures. Transferring both single- and multilayer graphene flakes onto InAs NWs, we found that single layers fold tightly around the NWs, while the multilayers fold weakly with a decline of only a few degrees. Annealing in atomic hydrogen further tightens the folding. Together, this indicates that high-quality morphological and chemical control of this hybrid material system is possible, opening for future devices for quantum technologies and optoelectronics.
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| 40. |
- Nguyen, Thi Thuy Nhung, et al.
(författare)
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Topological Surface State in Epitaxial Zigzag Graphene Nanoribbons
- 2021
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 21:7, s. 2876-2882
-
Tidskriftsartikel (refereegranskat)abstract
- Protected and spin-polarized transport channels are the hallmark of topological insulators, coming along with an intrinsic strong spin-orbit coupling. Here we identified such corresponding chiral states in epitaxially grown zigzag graphene nanoribbons (zz-GNRs), albeit with an extremely weak spin-orbit interaction. While the bulk of the monolayer zz-GNR is fully suspended across a SiC facet, the lower edge merges into the SiC(0001) substrate and reveals a surface state at the Fermi energy, which is extended along the edge and splits in energy toward the bulk. All of the spectroscopic details are precisely described within a tight binding model incorporating a Haldane term and strain effects. The concomitant breaking of time-reversal symmetry without the application of external magnetic fields is supported by ballistic transport revealing a conduction of G = e2/h.
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| 41. |
- Rullik, Lisa, et al.
(författare)
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Surface oxide development on aluminum alloy 6063 during heat treatment
- 2019
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Ingår i: Surface and Interface Analysis. - : Wiley. - 0142-2421 .- 1096-9918. ; 51:12, s. 1214-1224
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the influence of oxygen partial pressure for the development of surface oxides covering the industrial aluminum alloy standard 6063 at temperatures ranging from room temperature to 500° C. Using an array of synchrotron-based techniques, we followed the change in oxide thickness, chemical composition, and the lateral distribution of alloying elements. The impact of the oxygen chemical potential is most visible at high temperatures where the oxide composition changes from mostly Al based to mostly Mg based. This is in stark contrast to the ultra-high vacuum (UHV) conditions where only a partial compositional transition is observed. The microscopy data demonstrate that in the UHV case, Mg segregation onto the surface occurs firstly at grain boundaries at 300° C and secondly at sites over the entire surface at 400° C. Further, the initial oxide thickness is 45 Å, as determined by XPS and XRR, decreases in all observed cases after heating to 300° C. At higher temperatures, however, the oxygen partial pressure highly influences the resulting oxide thickness as evident from our X-ray reflectivity data.
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| 42. |
- Shi, Yuchen, et al.
(författare)
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Elimination of step bunching in the growth of large-area monolayer and multilayer graphene on off-axis 3CSiC (111)
- 2018
-
Ingår i: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 140, s. 533-542
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Tidskriftsartikel (refereegranskat)abstract
- Multilayer graphene has exhibited distinct electronic properties such as the tunable bandgap for optoelectronic applications. Among all graphene growth techniques, thermal decomposition of SiC is regarded as a promising method for production of device-quality graphene. However, it is still very challenging to grow uniform graphene over a large-area, especially multilayer graphene. One of the main obstacles is the occurrence of step bunching on the SiC surface, which significantly influences the formation process and the uniformity of the multilayer graphene. In this work, we have systematically studied the growth of monolayer and multilayer graphene on off-axis 3CSiC(111). Taking advantage of the synergistic effect of periodic SiC step edges as graphene nucleation sites and the unique thermal decomposition energy of 3CSiC steps, we demonstrate that the step bunching can be fully eliminated during graphene growth and large-area monolayer, bilayer, and four-layer graphene can be controllably obtained on high-quality off-axis 3CSiC(111) surface. The low energy electron microscopy results demonstrate that a uniform four-layer graphene has been grown over areas of tens of square micrometers, which opens the possibility to tune the bandgap for optoelectronic devices. Furthermore, a model for graphene growth along with the step bunching elimination is proposed.
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| 43. |
- Shi, Yuchen, et al.
(författare)
-
Epitaxial Graphene Growth on the Step-Structured Surface of Off-Axis C-Face 3C-SiC(1¯1¯1¯)
- 2020
-
Ingår i: Physica Status Solidi (B) Basic Research. - : Wiley. - 0370-1972 .- 1521-3951. ; 257:6
-
Tidskriftsartikel (refereegranskat)abstract
- Graphene layers grown on the C-face SiC exhibit quite different structural and electronic properties compared with those grown on the Si-face SiC. Herein, the growth and structural properties of graphene on the off-axis C-face 3C-SiC((Formula presented.)) are studied. The as-grown 4° off-axis 3C-SiC((Formula presented.)) exhibits highly periodic steps with step height of ≈0.75 nm and terrace width of ≈50 nm. After annealing at 1800 °C under 850 mbar argon atmosphere, relatively uniform large graphene domains can be grown. The low-energy electron microscopy (LEEM) results demonstrate that one monolayer (ML) to four-ML graphene domains are grown over several micrometers square, which enables us to measure micro low-energy electron diffraction (μ-LEED) on the single graphene domain. The μ-LEED pattern collected on the monolayer domain mainly exhibits four sets of graphene (1 × 1) spots, indicating the presence of graphene grains with different azimuthal orientations in the same graphene sheet. Raman spectra collected on the graphene domains show rather small D peaks, indicating the presence of less defects and higher crystalline quality of the graphene layers grown on the C-face off-axis 3C-SiC((Formula presented.)).
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| 44. |
- Shtepliuk, Ivan, et al.
(författare)
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Understanding of the Electrochemical Behavior of Lithium at Bilayer-Patched Epitaxial Graphene/4H-SiC
- 2022
-
Ingår i: Nanomaterials. - : MDPI AG. - 2079-4991. ; 12:13
-
Tidskriftsartikel (refereegranskat)abstract
- Novel two-dimensional materials (2DMs) with balanced electrical conductivity and lithium (Li) storage capacity are desirable for next-generation rechargeable batteries as they may serve as high-performance anodes, improving output battery characteristics. Gaining an advanced understanding of the electrochemical behavior of lithium at the electrode surface and the changes in interior structure of 2DM-based electrodes caused by lithiation is a key component in the long-term process of the implementation of new electrodes into to a realistic device. Here, we showcase the advantages of bilayer-patched epitaxial graphene on 4H-SiC (0001) as a possible anode material in lithium-ion batteries. The presence of bilayer graphene patches is beneficial for the overall lithiation process because it results in enhanced quantum capacitance of the electrode and provides extra intercalation paths. By performing cyclic voltammetry and chronoamperometry measurements, we shed light on the redox behavior of lithium at the bilayer-patched epitaxial graphene electrode and find that the early-stage growth of lithium is governed by the instantaneous nucleation mechanism. The results also demonstrate the fast lithium-ion transport (~4.7–5.6 × 10−7 cm2·s−1) to the bilayer-patched epitaxial graphene electrode. Raman measurements complemented by in-depth statistical analysis and density functional theory calculations enable us to comprehend the lithiation effect on the properties of bilayer-patched epitaxial graphene and ascribe the lithium intercalation-induced Raman G peak splitting to the disparity between graphene layers. The current results are helpful for further advancement of the design of graphene-based electrodes with targeted performance.
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| 45. |
- Singh, Harishchandra, et al.
(författare)
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Unveiling interactions of non-metallic inclusions within advanced ultra-high-strength steel : A spectro-microscopic determination and first-principles elucidation
- 2021
-
Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462. ; 197
-
Tidskriftsartikel (refereegranskat)abstract
- Determining non-metallic inclusions (NMIs) are essential to engineer ultra-high-strength steel as they play decisive role on performance and critical to probe via conventional techniques. Herein, advanced Synchrotron X-ray absorption coupled with photoemission electron microscopy and first-principles calculations are employed to provide the structure, local bonding structure and electronic properties of several NMI model systems and their interaction mechanism within and the steel matrix. B K-, N K-, Ca L2,3- and Ti L2,3-edge spectra show that the additional B prefers to result in h-BN exhibiting strong interaction with Ca2+. Such Ca2+-based phases also stabilize through TiN, revealing the irregular coordination of Ca2+. Observed intriguing no interaction between TiN and BN is further supported with the first-principles calculations, wherein unfavorable combination of TiN and h-BN and stabilization of bigger sized Ca2+-based inclusions have been found. These observations can help to optimize the interaction mechanism among various inclusions as well as steel matrix.
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| 46. |
- Stanishev, Vallery, et al.
(författare)
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Critical view on buffer layer formation and monolayer graphene properties in high-temperature sublimation
- 2021
-
Ingår i: Applied Sciences (Switzerland). - : MDPI AG. - 2076-3417. ; 11:4, s. 1-16
-
Tidskriftsartikel (refereegranskat)abstract
- In this work we have critically reviewed the processes in high-temperature sublimation growth of graphene in Ar atmosphere using closed graphite crucible. Special focus is put on buffer layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding monolayer graphene on 4H–SiC. We show that by introducing Ar at higher temperatures, TAr, one can shift the formation of the buffer layer to higher temperatures for both n-type and semi-insulating substrates. A scenario explaining the observed suppressed formation of buffer layer at higher TAr is proposed and discussed. Increased TAr is also shown to reduce the sp3 hybridization content and defect densities in the buffer layer on n-type conductive substrates. Growth on semi-insulating substrates results in ordered buffer layer with significantly improved structural properties, for which TAr plays only a minor role. The free charge density and mobility parameters of monolayer graphene and quasi-freestanding monolayer graphene with different TAr and different environmental treatment conditions are determined by contactless terahertz optical Hall effect. An efficient annealing of donors on and near the SiC surface is suggested to take place for intrinsic monolayer graphene grown at 2000◦C, and which is found to be independent of TAr . Higher TAr leads to higher free charge carrier mobility parameters in both intrinsically n-type and ambient p-type doped monolayer graphene. TAr is also found to have a profound effect on the free hole parameters of quasi-freestanding monolayer graphene. These findings are discussed in view of interface and buffer layer properties in order to construct a comprehensive picture of high-temperature sublimation growth and provide guidance for growth parameters optimization depending on the targeted graphene application.
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| 47. |
- Stöhr, Alexander, et al.
(författare)
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Graphene Ribbon Growth on Structured Silicon Carbide
- 2017
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Ingår i: Annalen der Physik. - : Wiley. - 0003-3804. ; 529:11
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Tidskriftsartikel (refereegranskat)abstract
- Structured Silicon Carbide was proposed to be an ideal template for the production of arrays of edge specific graphene nanoribbons (GNRs), which could be used as a base material for graphene transistors. We prepared periodic arrays of nanoscaled stripe-mesas on SiC surfaces using electron beam lithography and reactive ion etching. Subsequent epitaxial graphene growth by annealing is differentiated between the basal-plane mesas and the faceting stripe walls as monitored by means of atomic force microscopy (AFM). Microscopic low energy electron diffraction (μ-LEED) revealed that the graphene ribbons on the facetted mesa side walls grow in epitaxial relation to the basal-plane graphene with an armchair orientation at the facet edges. The π-band system of the ribbons exhibits linear bands with a Dirac like shape corresponding to monolayer graphene as identified by angle-resolved photoemission spectroscopy (ARPES).
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| 48. |
- Stöhr, Alexander, et al.
(författare)
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Intercalation of graphene on SiC(0001) via ion implantation
- 2016
-
Ingår i: Physical Review B. - 1098-0121. ; 94:8
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Tidskriftsartikel (refereegranskat)abstract
- Electronic devices based on graphene technology are catching on rapidly and the ability to engineer graphene properties at the nanoscale is becoming, more than ever, indispensable. Here, we present a procedure of graphene functionalization on SiC(0001) that paves the way towards the fabrication of complex graphene electronic chips. The procedure resides on the well-known ion-implantation technique. The efficiency of the working principle is demonstrated by the intercalation of the epitaxial graphene layer on SiC(0001) with Bi atoms, which was not possible following standard procedures. The investigation of the obtained graphene system reveals no clear spin-orbit coupling enhancement expected by theory in addition to the presence of residual structural defects. Our graphene/SiC(0001) intercalation procedure puts forward the ion-beam lithography to nanostructure and functionalize desired graphene chips.
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| 49. |
- Wang, Weimin, et al.
(författare)
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Flat-Band Electronic Structure and Interlayer Spacing Influence in Rhombohedral Four-Layer Graphene
- 2018
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 18:9, s. 5862-5866
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Tidskriftsartikel (refereegranskat)abstract
- The stacking order of multilayer graphene significantly influences its electronic properties. The rhombohedral stacking sequence is predicted to introduce a flat band, which has high density of states and the enhanced Coulomb interaction between charge carriers, thus possibly resulting in superconductivity, fractional quantum Hall effect, and many other exotic phases of matter. In this work, we comprehensively study the effect of the stacking sequence and interlayer spacing on the electronic structure of four-layer graphene, which was grown on a high crystalline quality 3C-SiC(111) crystal. The number of graphene layers and coverage were determined by low energy electron microscopy. First-principles density functional theory calculations show distinctively different band structures for ABAB (Bernal), ABCA (rhombohedral), and ABCB (turbostratic) stacking sequences. By comparing with angle-resolved photoelectron spectroscopy data, we can verify the existence of a rhombohedral stacking sequence and a nearly dispersionless electronic band (flat band) near the Fermi level. Moreover, we find that the momentum width, bandgap, and curvature of the flat-band region can be tuned by the interlayer spacing, which plays an important role in superconductivity and many other exotic phases of matter.
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| 50. |
- Wang, Yao, et al.
(författare)
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Polaronic Trions Induced by Strong Interfacial Coupling in Monolayer WSe2
- 2023
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Ingår i: Advanced Electronic Materials. - : Wiley. - 2199-160X. ; 9:2
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Tidskriftsartikel (refereegranskat)abstract
- The weak dielectric screening in 2D semiconducting transition metal dichalcogenides give rise to strongly bound quasiparticles, which provides a platform to investigate the diverse excitonic phenomena and correlated physics. However, how to effectively control these quasiparticles is still a challenge for their applications in optoelectronic and valleytronic devices. Herein, by means of fabricating monolayer WSe2 and transition metal oxide (TMO) heterostructures, polaronic trion, that is a trion dressed with soft rotational optical (RO) phonons, is realized due to the strong interfacial coupling. This Fröhlich bound state of trion dramatically increases the trion binding energy (BE) from room temperature to 65 meV at 80 K in WSe2/LaAlO3 (LAO). However, the increase of the trion BE for WSe2/SrTiO3 (STO) occurs below the phase transition temperature. This work expands the possibilities of the TMDs/TMOs heterostructures and promotes the development of 2D van der Waals materials for quasiparticle-based devices.
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