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Xia, Chao
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Publications (10 of 16) Show all publications
Xia, C., Tal, A., Johansson, L., Olovsson, W., Abrikosov, I. & Virojanadara, C. (2018). Effects of rhenium on graphene grown on SiC(0001). Journal of Electron Spectroscopy and Related Phenomena, 222, 117-121
Open this publication in new window or tab >>Effects of rhenium on graphene grown on SiC(0001)
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2018 (English)In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 222, p. 117-121Article in journal (Refereed) Published
Abstract [en]

We study the effects of Rhenium (Re) deposited on epitaxial monolayer graphene grown on SiC(0001) and after subsequent annealing at different temperatures, by performing high resolution photoelectron spectroscopy (PES) and angle resolved photoelectron spectroscopy (ARPES). The graphene-Re system is found to be thermally stable. While no intercalation or chemical reaction of the Re is detected after deposition and subsequent annealing up to 1200 degrees C, a gradual decrease in the binding energy of the Re 4f doublet is observed. We propose that a larger mobility of the Re atoms with increasing annealing temperature and hopping of Re atoms between different defective sites on the graphene sample could induce this decrease of Re 4f binding energy. This is corroborated by first principles density functional theory (DFT) calculations of the Re core-level binding energy shift. No change in the doping or splitting of the initial monolayer graphene electronic band structure is observed after Re deposition and annealing up to 1200 degrees C, only a broadening of the bands. (C) 2017 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Rhenium; Graphene; Photoelectron spectroscopy; Core-level shift; Ab initio density functional theory
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-145152 (URN)10.1016/j.elspec.2017.07.006 (DOI)000423638100016 ()
Note

Funding Agencies|Knut and Alice Wallenberg Foundation [2012.0083]; Knut and Alice Wallenberg Foundation through CoTXS; Swedish Foundation or Strategic Research (SSF) program SRL Grant [10-0026]; Swedish Research Council (VR) [2015-04391]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Swedish Research Council [621-2011-4252]

Available from: 2018-02-13 Created: 2018-02-13 Last updated: 2018-03-19Bibliographically approved
Nuala, M., Johansson, L. I., Xia, C., Armiento, R., Abrikosov, I. & Jacobi, C. (2016). Structural and electronic properties of Li-intercalated graphene on SiC(0001). Physical Review B: covering condensed matter and materials physics, 93(19), 195421-1-195421-9
Open this publication in new window or tab >>Structural and electronic properties of Li-intercalated graphene on SiC(0001)
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2016 (English)In: Physical Review B: covering condensed matter and materials physics, ISSN 2469-9950, Vol. 93, no 19, p. 195421-1-195421-9Article in journal (Refereed) Published
Abstract [en]

We investigate the structural and electronic properties of Li-intercalated monolayer graphene on SiC(0001) using combined angle-resolved photoemission spectroscopy and first-principles density functional theory. Li intercalates at room temperature both at the interface between the buffer layer and SiC and between the two carbon layers. The graphene is strongly n-doped due to charge transfer from the Li atoms and two pi bands are visible at the (K) over bar point. After heating the sample to 300 degrees C, these pi bands become sharp and have a distinctly different dispersion to that of Bernal-stacked bilayer graphene. We suggest that the Li atoms intercalate between the two carbon layers with an ordered structure, similar to that of bulk LiC6. An AA stacking of these two layers becomes energetically favourable. The pi bands around the (K) over bar point closely resemble the calculated band structure of a C6LiC6 system, where the intercalated Li atoms impose a superpotential on the graphene electronic structure that opens gaps at the Dirac points of the two pi cones.

Place, publisher, year, edition, pages
American Physical Society, 2016
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-129161 (URN)10.1103/PhysRevB.93.195421 (DOI)000376248200006 ()
Note

Funding Agencies|Swedish Research Council (VR) [621-2011-4252, 621-2011-4426]; Swedish Foundation for Strategic Research (SSF) program [10-0026]; European Union Seventh Framework Programme, Graphene Flagship [604391]; Swedish Government Strategic Research Areas SeRC and in Materials Science on Functional Materials at Link oping University [2009 00971]; SRC VR Grant [621-2011-4249]; Linnaeus Environment at Linkoping on Nanoscale Functional Materials (LiLi-NFM) - VR; Grant of Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Tomsk State University Academic D. I. Mendeleev Fund Program [8.1.18.2015]

Available from: 2016-06-13 Created: 2016-06-13 Last updated: 2016-07-11Bibliographically approved
Xia, C. (2015). Characterizations of as grown and functionalized epitaxial graphene grown on SiC surfaces. (Doctoral dissertation). Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>Characterizations of as grown and functionalized epitaxial graphene grown on SiC surfaces
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The superior electronic and mechanical properties of Graphene have promoted graphene to become one of the most promising candidates for next generation of electronic devices. Epitaxial growth of graphene by sublimation of Si from Silicon Carbide (SiC) substrates avoids the hazardous transfer process for large scale fabrication of graphene based electronic devices. Moreover, the operation conditions can potentially be extended to high temperatures, voltages and frequencies. This thesis is focused on characterizations of as grown and functionalized epitaxial graphene grown on both Si-face and C-face SiC. Synchrotron radiation-based techniques are employed for detailed investigations of the electronic properties and surface morphology of as grown and functionalized graphene.

Large area and homogeneous monolayer (ML) graphene has been possible to grow on SiC(0001) substrates by sublimation, but efforts to obtain multilayer graphene of similar quality have been in vain. A study of the transport behavior of silicon atoms through carbon layers was therefore performed for the purpose to gain a better understanding of the growth mechanism of graphene on Si-face SiC. It showed that a temperature of about 800°C is required for Si intercalation into the interface to take place. Intercalation of Si was found to occur only via defects and domain boundaries which probably is the reason to the limited growth of multilayer graphene. Annealing at 1000-1100°C induced formation of SiC on the surface and after annealing above 1200°C Si started to de-intercalate and desorb/sublimate.

Different alkali metals were found to affect graphene grown in SiC quite differently. Li started to intercalate already at room temperature by creating cracks and defects, while K, Rb and Cs were found unable to intercalate into the graphene/SiC interface. Effects induced by the alkali metal Na on graphene grown on both Si-face and C-face SiC were therefore studies. For the Si-face, partial intercalation of Na through graphene was observed on both 1 ML and 2 ML areas directly after Na deposition. Annealing at a temperature of about 75°C strongly promoted Na intercalation at the interface. The intercalation was confirmed to start at domain boundaries between 1 ML and 2 ML areas and at stripes/streaks on the 1 ML areas. Higher annealing temperature resulted in desorption of Na from the sample surface. Also for C-face graphene, a strong n-type doping was observed directly after Na deposition. Annealing at temperatures from around 120 to 300 °C was here found to result in a considerable π-band broadening, interpreted to indicate penetration of Na in between the graphene layers and at the graphene SiC interface.

The thermal stability of graphene based electronic devices can depend on the choice of contact material. Studies of the stability and effects induced by two commonly used metals (Pt and Al) on Si-face graphene were carried out after deposition and after subsequent annealing at different temperatures. Both Al and Pt were found to be good contact materials at room temperature. Annealing at respectively ~400 ºC and ~ 800 ºC was found to trigger intercalation of Al and Pt into the graphene/SiC interface, and induce quasi-free-standing bilayer electronic properties. Contacts of Pt can thus withstand higher temperatures than Al contacts. For Al inhomogeneous islands of different ordered phases were observed to form on the surface during annealing, while this was not the case for Pt. The initial single π-band structure was in the Al case restored after annealing at ~1200 ºC although some Al remained detectable from the sample. For Pt, the bilayer graphene electronic properties induced by intercalation were thermally stable up to 1200ºC. In the case of Al the stability and effects induced on C-face graphene were also investigated for comparison, and significant differences were revealed. An ordered Al-Si-C compound was found to form after annealing at temperatures between ca. 500ºC and 700ºC. Formation of this compound was accompanied with a large reduction of graphene in the surface region. Annealing at temperatures above 800°C resulted in a gradual decomposition of this compound and regrowth of graphene. No Al signal could be detected after annealing C-face graphene at 1000°C.

Graphene grown on C-face SiC has attracted high interest since its mobility has been reported to be one order of magnitude higher compared to Si-face graphene. C-face graphene has moreover been claimed to be fundamentally different compared to Si-face graphene. A rotational disorder between adjacent graphene layers has been suggested that effectively decouples the graphene layers and result in monolayer electronic properties of multilayer C-face graphene. The domain/grain size is typically much smaller for C-face graphene and the number of graphene layers less uniform than on Si-face graphene. Using LEEM and micro-LEED we showed that there is no rotational disorder between adjacent layers within the domains/grains but that they had different azimuthal orientations. Using nano-APRES, we recently also revealed that multilayer Cface graphene show multiple π-bands and Bernal stacking, similar to multilayer Si-face graphene.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. p. 32
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1689
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-120893 (URN)10.3384/diss.diva-120893 (DOI)978-91-7685-998-8 (ISBN)
Public defence
2015-10-08, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2015-08-28 Created: 2015-08-28 Last updated: 2019-11-15Bibliographically approved
Xia, C., Johansson, L. I., Niu, Y., Hultman, L. & Virojanadara, C. (2015). Effects of aluminum on epitaxial graphene grown on C-face SiC. Journal of Applied Physics, 117(19), 195306
Open this publication in new window or tab >>Effects of aluminum on epitaxial graphene grown on C-face SiC
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2015 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 19, p. 195306-Article in journal (Refereed) Published
Abstract [en]

The effects of Al layers deposited on graphene grown on C-face SiC substrates are investigated before and after subsequent annealing using low energy electron diffraction (LEED), photoelectron spectroscopy, and angle resolved photoemission. As-deposited layers appear inert. Annealing at a temperature of about 400 degrees C initiates migration of Al through the graphene into the graphene/SiC interface. Further annealing at temperatures from 500 degrees C to 700 degrees C induces formation of an ordered compound, producing a two domain root 7 x root 7R19 degrees LEED pattern and significant changes in the core level spectra that suggest formation of an Al-Si-C compound. Decomposition of this compound starts after annealing at 800 degrees C, and at 1000 degrees C, Al is no longer possible to detect at the surface. On Si-face graphene, deposited Al layers did not form such an Al-Si-C compound, and Al was still detectable after annealing above 1000 degrees C.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:liu:diva-119250 (URN)10.1063/1.4921462 (DOI)000355005600036 ()
Note

Funding Agencies|Swedish Research Council [621-2011-4252]; Linnaeus Grant

Available from: 2015-06-12 Created: 2015-06-12 Last updated: 2017-12-04
Johansson, L. I., Xia, C. & Jacobi, C. (2015). Li induced effects in the core level and pi-band electronic structure of graphene grown on C-face SiC. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, 33(6), Article ID 061405.
Open this publication in new window or tab >>Li induced effects in the core level and pi-band electronic structure of graphene grown on C-face SiC
2015 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 33, no 6, article id 061405Article in journal (Refereed) Published
Abstract [en]

Studies of the effects induced in the electronic structure after Li deposition, and subsequent heating, on graphene samples prepared on C-face SiC are reported. The as prepared graphene samples are essentially undoped, but after Li deposition, the Dirac point shifts down to 1.2 eV below the Fermi level due to electron doping. The shape of the C 1s level also indicates a doping concentration of around 10(14) cm(-2) after Li deposition, when compared with recent calculated results of core level spectra of graphene. The C 1s, Si 2p, and Li 1s core level results show little intercalation directly after deposition but that most of the Li has intercalated after heating at 280 degrees C. Heating at higher temperatures leads to desorption of Li from the sample, and at 1030 degrees C, Li can no longer be detected on the sample. The single pi-band observable from multilayer C-face graphene samples in conventional angle resolved photoelectron spectroscopy is reasonably sharp both on the initially prepared sample and after Li deposition. After heating at 280 degrees C, the p-band appears more diffuse and possibly split. The Dirac point becomes located at 0.4 eV below the Fermi level, which indicates occurrence of a significant reduction in the electron doping concentration. Constant energy photoelectron distribution patterns extracted from the as prepared graphene C-face sample and also after Li deposition and heating at 280 degrees C look very similar to earlier calculated distribution patterns for monolayer graphene. (C) 2015 Author(s).

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-123832 (URN)10.1116/1.4927856 (DOI)000365503800032 ()
Note

Funding Agencies|Swedish Natural Research Council [621-2011-4252]; Swedish Natural Research Council (Linnaeus Grant)

Available from: 2016-01-11 Created: 2016-01-11 Last updated: 2017-12-01Bibliographically approved
Chen, J.-T., Pomeroy, J. W., Rorsman, N., Xia, C., Virojanadara, C., Forsberg, U., . . . Janzén, E. (2015). Low thermal resistance of a GaN-on-SiC transistor structure with improved structural properties at the interface. Journal of Crystal Growth, 428, 54-58
Open this publication in new window or tab >>Low thermal resistance of a GaN-on-SiC transistor structure with improved structural properties at the interface
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2015 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 428, p. 54-58Article in journal (Refereed) Published
Abstract [en]

The crystalline quality of AlGaN/GaN heterostructures was improved by optimization of surface pretreatment of the SiC substrate in a hot-wall metal-organic chemical vapor deposition reactor. X-ray photoelectron spectroscopy measurements revealed that oxygen- and carbon-related contaminants were still present on the SiC surface treated at 1200 °C in H2 ambience, which hinders growth of thin AlN nucleation layers with high crystalline quality. As the H2 pretreatment temperature increased to 1240 °C, the crystalline quality of the 105 nm thick AlN nucleation layers in the studied series reached an optimal value in terms of full width at half-maximum of the rocking curves of the (002) and (105) peaks of 64 and 447 arcsec, respectively. The improvement of the AlN growth also consequently facilitated a growth of the GaN buffer layers with high crystalline quality. The rocking curves of the GaN (002) and (102) peaks were thus improved from 209 and 276 arcsec to 149 and 194 arcsec, respectively. In addition to a correlation between the thermal resistance and the structural quality of an AlN nucleation layer, we found that the microstructural disorder of the SiC surface and the morphological defects of the AlN nucleation layers to be responsible for a substantial thermal resistance. Moreover, in order to decrease the thermal resistance in the GaN/SiC interfacial region, the thickness of the AlN nucleation layer was then reduced to 35 nm, which was shown sufficient to grow AlGaN/GaN heterostructures with high crystalline quality. Finally, with the 35 nm thick high-quality AlN nucleation layer a record low thermal boundary resistance of 1.3×10−8 m2 K/W, measured at an elevated temperature of 160 °C, in a GaN-on-SiC transistor structure was achieved.

Place, publisher, year, edition, pages
Elsevier, 2015
Keywords
Heat transfer; Metalorganic chemical vapor deposition; Nitrides; High electron mobility transistors
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117132 (URN)10.1016/j.jcrysgro.2015.07.021 (DOI)000360501200009 ()
Available from: 2015-04-17 Created: 2015-04-17 Last updated: 2017-12-04Bibliographically approved
Watcharinyanon, S., Xia, C., Niu, Y., Zakharov, A. A., Johansson, L. I., Yakimova, R. & Virojanadara, C. (2015). Soft X-ray Exposure Promotes Na Intercalation in Graphene Grown on Si-Face SiC. Materials, 8(8), 4768-4777
Open this publication in new window or tab >>Soft X-ray Exposure Promotes Na Intercalation in Graphene Grown on Si-Face SiC
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2015 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 8, no 8, p. 4768-4777Article in journal (Refereed) Published
Abstract [en]

An investigation of how electron/photon beam exposures affect the intercalation rate of Na deposited on graphene prepared on Si-face SiC is presented. Focused radiation from a storage ring is used for soft X-ray exposures while the electron beam in a low energy electron microscope is utilized for electron exposures. The microscopy and core level spectroscopy data presented clearly show that the effect of soft X-ray exposure is significantly greater than of electron exposure, i.e., it produces a greater increase in the intercalation rate of Na. Heat transfer from the photoelectrons generated during soft X-ray exposure and by the electrons penetrating the sample during electron beam exposure is suggested to increase the local surface temperature and thus the intercalation rate. The estimated electron flux density is 50 times greater for soft X-ray exposure compared to electron exposure, which explains the larger increase in the intercalation rate from soft X-ray exposure. Effects occurring with time only at room temperature are found to be fairly slow, but detectable. The graphene quality, i.e., domain/grain size and homogeneity, was also observed to be an important factor since exposure-induced effects occurred more rapidly on a graphene sample prepared in situ compared to on a furnace grown sample.

Place, publisher, year, edition, pages
MDPI AG, 2015
Keywords
graphene on Si-face SiC; intercalation of Na; soft X-ray exposure; electron exposure
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-121762 (URN)10.3390/ma8084768 (DOI)000360643900010 ()
Note

Funding Agencies|Swedish Research Council [621-2011-4252]

Available from: 2015-10-06 Created: 2015-10-05 Last updated: 2017-12-01
Xia, C., Johansson, L. I., Zakharov, A. A., Hultman, L. & Virojanadara, C. (2014). Effects of Al on epitaxial graphene grown on 6H-SiC(0001). Materials Research Express, 1(1), 1-13, Article ID 015606.
Open this publication in new window or tab >>Effects of Al on epitaxial graphene grown on 6H-SiC(0001)
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2014 (English)In: Materials Research Express, ISSN 2053-1591, Vol. 1, no 1, p. 1-13, article id 015606Article in journal (Refereed) Published
Abstract [en]

Aluminum was deposited on epitaxial monolayer-grown graphene on SiC(0001). The effects of annealing up to 1200 °C on the surface and interface morphology, chemical composition, and electron band structure were analyzed in situ by synchrotron-based techniques at the MAX Laboratory. After heating at around 400 °C, Al islands or droplets are observed on the surface and the collected Si 2p, Al 2p, and C 1s core levels spectra indicate Al intercalation at the graphene SiC interface. Also, the original single π -band splits into two, indicating decoupling of the carbon buffer layer and the formation of a quasi-free-standing bilayer-like electronic structure. Further heating at higher temperatures from 700 to 900 °C yields additional chemical reactions. Broader core level spectra are then observed and clear changes in the π -bands near the Dirac point are detected. More electron doping was detected at this stage since one of the π -bands has shifted to about 1.1 eV below the Fermi level. Different ordered phases of (7x7), (4x4), (1x1)Al , and (1x1)G were also observed on the surface in this temperature range. The original single π π-band was restored after heating at ~1200°C, although an Al signal was still able to be detected.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2014
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-120851 (URN)10.1088/2053-1591/1/1/015606 (DOI)
Available from: 2015-08-28 Created: 2015-08-28 Last updated: 2016-08-31Bibliographically approved
Xia, C., Johansson, L. I., Niu, Y., Zakharov, A. A., Janzén, E. & Virojanadara, C. (2014). High thermal stability quasi-free-standing bilayer graphene formed on 4H-SiC(0 0 0 1) via platinum intercalation. Carbon, 79, 631-635
Open this publication in new window or tab >>High thermal stability quasi-free-standing bilayer graphene formed on 4H-SiC(0 0 0 1) via platinum intercalation
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2014 (English)In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 79, p. 631-635Article in journal (Refereed) Published
Abstract [en]

Influences on electronic structure induced by platinum (Pt) deposited on monolayer graphene grown on SiC(0 0 0 1) are investigated by photoelectron spectroscopy (PES), selected area low energy electron diffraction (μ-LEED) and angle resolved photoelectron spectroscopy (ARPES) techniques at the MAX Laboratory. Stable monolayer graphene electronic properties are observed after Pt deposition and after annealing at temperatures below 600 °C. At ⩾600 °C platinum silicide forms at the graphene/SiC interface. Annealing at 900 °C results in an efficient decoupling of the carbon buffer layer from the SiC substrate and transformation into a second graphene layer. At this stage a quasi-free standing bi-layer graphene sample is obtained. The new superstructure spots then appearing in μ-LEED pattern suggest formation of an ordered platinum silicide at the interface. This silicide is found to be stable even after annealing at temperature up to 1200 °C.

Place, publisher, year, edition, pages
Elsevier, 2014
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-111494 (URN)10.1016/j.carbon.2014.08.027 (DOI)
Available from: 2014-10-20 Created: 2014-10-20 Last updated: 2017-12-05Bibliographically approved
Johansson, L. I., Armiento, R., Avila, J., Xia, C., Lorcy, S., Igor A., A., . . . Virojanadara, C. (2014). Multiple π-bands and Bernal stacking of multilayer graphene on C-face SiC, revealed by nano-Angle Resolved Photoemission. Scientific Reports, 4(4157)
Open this publication in new window or tab >>Multiple π-bands and Bernal stacking of multilayer graphene on C-face SiC, revealed by nano-Angle Resolved Photoemission
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2014 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 4, no 4157Article in journal (Refereed) Published
Abstract [en]

Only a single linearly dispersing π-band cone, characteristic of monolayer graphene, has so far been observed in Angle Resolved Photoemission (ARPES) experiments on multilayer graphene grown on C-face SiC. A rotational disorder that effectively decouples adjacent layers has been suggested to explain this. However, the coexistence of μm-sized grains of single and multilayer graphene with different azimuthal orientations and no rotational disorder within the grains was recently revealed for C-face graphene, but conventional ARPES still resolved only a single π-band. Here we report detailed nano-ARPES band mappings of individual graphene grains that unambiguously show that multilayer C-face graphene exhibits multiple π-bands. The band dispersions obtained close to the K-point moreover clearly indicate, when compared to theoretical band dispersion calculated in the framework of the density functional method, Bernal (AB) stacking within the grains. Thus, contrary to earlier claims, our findings imply a similar interaction between graphene layers on C-face and Si-face SiC.

Place, publisher, year, edition, pages
Nature Publishing Group, 2014
Keywords
Electronic properties and materials, Graphene, Stacking
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-105279 (URN)10.1038/srep04157 (DOI)000331885900004 ()
Funder
Swedish Research Council, 621-2011-4252Swedish Research Council, 621-2011-4249Swedish Foundation for Strategic Research , 10-0026
Available from: 2014-03-14 Created: 2014-03-14 Last updated: 2017-12-05Bibliographically approved
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