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Multiscale investigation of graphene layers on 6H-SiC(000-1).

Tiberj A, Huntzinger JR, Camassel J, Hiebel F, Mahmood A, Mallet P, Naud C, Veuillen JY - Nanoscale Res Lett (2011)

Bottom Line: At the same scale, electron diffraction reveals a significant rotational disorder between the first graphene layer and the SiC surface, although well-defined preferred orientations exist.This is confirmed at the nanometer scale by scanning tunneling microscopy (STM).The most striking result is that the FLGs experience a strong compressive stress that is seldom observed for graphene grown on the C face of SiC substrates.

View Article: PubMed Central - HTML - PubMed

Affiliation: Groupe d'Etude des Semiconducteurs, UMR5650 CNRS-Université Montpellier II, cc074, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France. Antoine.Tiberj@univ-montp2.fr.

ABSTRACT
In this article, a multiscale investigation of few graphene layers grown on 6H-SiC(000-1) under ultrahigh vacuum (UHV) conditions is presented. At 100-μm scale, the authors show that the UHV growth yields few layer graphene (FLG) with an average thickness given by Auger spectroscopy between 1 and 2 graphene planes. At the same scale, electron diffraction reveals a significant rotational disorder between the first graphene layer and the SiC surface, although well-defined preferred orientations exist. This is confirmed at the nanometer scale by scanning tunneling microscopy (STM). Finally, STM (at the nm scale) and Raman spectroscopy (at the μm scale) show that the FLG stacking is turbostratic, and that the domain size of the crystallites ranges from 10 to 100 nm. The most striking result is that the FLGs experience a strong compressive stress that is seldom observed for graphene grown on the C face of SiC substrates.

No MeSH data available.


Related in: MedlinePlus

STM images of a 6H-SiC(000-1) sample after graphitization (same sample as in figure 1). (a) 300 × 300 nm2 STM image of few layers of graphene grown on 6H-SiC (000-1). The brightest areas correspond to multilayers grown on a step edge, the right part corresponds to monolayer graphene. (b) 150 × 150 nm2 zoom of the top right corner of image (a). The dark areas correspond to monolayer graphene grown on (3 × 3) SiC-reconstructed surface. MPs are seen on the monolayer and on the multilayers (the brighter area) indicating that the first graphene layer is disoriented compared to the SiC surface and that the multilayers are "twisted" (turbostratic stacking). (c) 50 × 50 nm2 STM images of a bilayer and a monolayer. The turbostratic stacking of the bilayer is revealed by a long-range MP with a wavelength of 4 nm. On the monolayer, one can only see the (3 × 3) SiC surface reconstruction pattern due to the high tunnel voltage (-2.5 V). It should be stressed that the top graphene plane is continuous between the mono and the bilayer. (d) 300 × 300 nm2 STM image showing the distribution of FLG grown ranging from the bare (3 × 3) SiC surface (0L), monolayers (1L), and bilayers (2L). On the left, a 1-nm high wrinkle can be seen on a bilayer. The bright horizontal line corresponds to a tip change.
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Figure 2: STM images of a 6H-SiC(000-1) sample after graphitization (same sample as in figure 1). (a) 300 × 300 nm2 STM image of few layers of graphene grown on 6H-SiC (000-1). The brightest areas correspond to multilayers grown on a step edge, the right part corresponds to monolayer graphene. (b) 150 × 150 nm2 zoom of the top right corner of image (a). The dark areas correspond to monolayer graphene grown on (3 × 3) SiC-reconstructed surface. MPs are seen on the monolayer and on the multilayers (the brighter area) indicating that the first graphene layer is disoriented compared to the SiC surface and that the multilayers are "twisted" (turbostratic stacking). (c) 50 × 50 nm2 STM images of a bilayer and a monolayer. The turbostratic stacking of the bilayer is revealed by a long-range MP with a wavelength of 4 nm. On the monolayer, one can only see the (3 × 3) SiC surface reconstruction pattern due to the high tunnel voltage (-2.5 V). It should be stressed that the top graphene plane is continuous between the mono and the bilayer. (d) 300 × 300 nm2 STM image showing the distribution of FLG grown ranging from the bare (3 × 3) SiC surface (0L), monolayers (1L), and bilayers (2L). On the left, a 1-nm high wrinkle can be seen on a bilayer. The bright horizontal line corresponds to a tip change.

Mentions: STM measurements were done at room temperature using mechanically cut PtIr tips. Typical STM images are gathered in Figure 2 in which a large diversity of graphene layers can be observed. First, on the edge of the SiC reconstructed steps, the growth rate is much higher, and small multilayers which are a few tens of nm width appear (Figure 2a). On the terraces, mono and bilayers cover the majority of the surface and are much wider (up to 100 nm). Few small areas are not graphitized, and the usual SiC(3 × 3) surface reconstruction can be observed (Figure 2d) [16]. The (3 × 3) is also seen on Figure 2c through the graphene monolayer because of the high sample bias (-2.5 V) [17,18]. In Figure 2b,c one can also clearly distinguish some Moiré patterns (MP) on graphene mono, bi, and multilayers. Such MPs have several origins. The MP observed on the monolayer graphene comes from disorientation between the first graphene plane and the SiC(3 × 3) surface. The disorientation angle determines the period of the MP based on a classical model previously described [18]. For instance, the MP for the island in the lower right part of Figure 2b corresponds to a rotation angle of 11.2°. The MPs observed on the multilayers come both from the interface (as above) and from rotational stacking faults between the different graphene planes, which is characteristic of a turbostratic stacking. Such disorientations between the graphene sheets and the SiC substrate confirm the weak coupling between the graphene planes, and also between the FLGs and the SiC substrate. This is corroborated by the presence of wrinkles seen in Figure 2d. Finally, the top graphene plane on Figure 2c is a continuous sheet between the mono and the bilayer graphenes.


Multiscale investigation of graphene layers on 6H-SiC(000-1).

Tiberj A, Huntzinger JR, Camassel J, Hiebel F, Mahmood A, Mallet P, Naud C, Veuillen JY - Nanoscale Res Lett (2011)

STM images of a 6H-SiC(000-1) sample after graphitization (same sample as in figure 1). (a) 300 × 300 nm2 STM image of few layers of graphene grown on 6H-SiC (000-1). The brightest areas correspond to multilayers grown on a step edge, the right part corresponds to monolayer graphene. (b) 150 × 150 nm2 zoom of the top right corner of image (a). The dark areas correspond to monolayer graphene grown on (3 × 3) SiC-reconstructed surface. MPs are seen on the monolayer and on the multilayers (the brighter area) indicating that the first graphene layer is disoriented compared to the SiC surface and that the multilayers are "twisted" (turbostratic stacking). (c) 50 × 50 nm2 STM images of a bilayer and a monolayer. The turbostratic stacking of the bilayer is revealed by a long-range MP with a wavelength of 4 nm. On the monolayer, one can only see the (3 × 3) SiC surface reconstruction pattern due to the high tunnel voltage (-2.5 V). It should be stressed that the top graphene plane is continuous between the mono and the bilayer. (d) 300 × 300 nm2 STM image showing the distribution of FLG grown ranging from the bare (3 × 3) SiC surface (0L), monolayers (1L), and bilayers (2L). On the left, a 1-nm high wrinkle can be seen on a bilayer. The bright horizontal line corresponds to a tip change.
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Figure 2: STM images of a 6H-SiC(000-1) sample after graphitization (same sample as in figure 1). (a) 300 × 300 nm2 STM image of few layers of graphene grown on 6H-SiC (000-1). The brightest areas correspond to multilayers grown on a step edge, the right part corresponds to monolayer graphene. (b) 150 × 150 nm2 zoom of the top right corner of image (a). The dark areas correspond to monolayer graphene grown on (3 × 3) SiC-reconstructed surface. MPs are seen on the monolayer and on the multilayers (the brighter area) indicating that the first graphene layer is disoriented compared to the SiC surface and that the multilayers are "twisted" (turbostratic stacking). (c) 50 × 50 nm2 STM images of a bilayer and a monolayer. The turbostratic stacking of the bilayer is revealed by a long-range MP with a wavelength of 4 nm. On the monolayer, one can only see the (3 × 3) SiC surface reconstruction pattern due to the high tunnel voltage (-2.5 V). It should be stressed that the top graphene plane is continuous between the mono and the bilayer. (d) 300 × 300 nm2 STM image showing the distribution of FLG grown ranging from the bare (3 × 3) SiC surface (0L), monolayers (1L), and bilayers (2L). On the left, a 1-nm high wrinkle can be seen on a bilayer. The bright horizontal line corresponds to a tip change.
Mentions: STM measurements were done at room temperature using mechanically cut PtIr tips. Typical STM images are gathered in Figure 2 in which a large diversity of graphene layers can be observed. First, on the edge of the SiC reconstructed steps, the growth rate is much higher, and small multilayers which are a few tens of nm width appear (Figure 2a). On the terraces, mono and bilayers cover the majority of the surface and are much wider (up to 100 nm). Few small areas are not graphitized, and the usual SiC(3 × 3) surface reconstruction can be observed (Figure 2d) [16]. The (3 × 3) is also seen on Figure 2c through the graphene monolayer because of the high sample bias (-2.5 V) [17,18]. In Figure 2b,c one can also clearly distinguish some Moiré patterns (MP) on graphene mono, bi, and multilayers. Such MPs have several origins. The MP observed on the monolayer graphene comes from disorientation between the first graphene plane and the SiC(3 × 3) surface. The disorientation angle determines the period of the MP based on a classical model previously described [18]. For instance, the MP for the island in the lower right part of Figure 2b corresponds to a rotation angle of 11.2°. The MPs observed on the multilayers come both from the interface (as above) and from rotational stacking faults between the different graphene planes, which is characteristic of a turbostratic stacking. Such disorientations between the graphene sheets and the SiC substrate confirm the weak coupling between the graphene planes, and also between the FLGs and the SiC substrate. This is corroborated by the presence of wrinkles seen in Figure 2d. Finally, the top graphene plane on Figure 2c is a continuous sheet between the mono and the bilayer graphenes.

Bottom Line: At the same scale, electron diffraction reveals a significant rotational disorder between the first graphene layer and the SiC surface, although well-defined preferred orientations exist.This is confirmed at the nanometer scale by scanning tunneling microscopy (STM).The most striking result is that the FLGs experience a strong compressive stress that is seldom observed for graphene grown on the C face of SiC substrates.

View Article: PubMed Central - HTML - PubMed

Affiliation: Groupe d'Etude des Semiconducteurs, UMR5650 CNRS-Université Montpellier II, cc074, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France. Antoine.Tiberj@univ-montp2.fr.

ABSTRACT
In this article, a multiscale investigation of few graphene layers grown on 6H-SiC(000-1) under ultrahigh vacuum (UHV) conditions is presented. At 100-μm scale, the authors show that the UHV growth yields few layer graphene (FLG) with an average thickness given by Auger spectroscopy between 1 and 2 graphene planes. At the same scale, electron diffraction reveals a significant rotational disorder between the first graphene layer and the SiC surface, although well-defined preferred orientations exist. This is confirmed at the nanometer scale by scanning tunneling microscopy (STM). Finally, STM (at the nm scale) and Raman spectroscopy (at the μm scale) show that the FLG stacking is turbostratic, and that the domain size of the crystallites ranges from 10 to 100 nm. The most striking result is that the FLGs experience a strong compressive stress that is seldom observed for graphene grown on the C face of SiC substrates.

No MeSH data available.


Related in: MedlinePlus