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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.


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Raman spectra of a HOPG sample (blue line), the thickest FLG (black line), and the thinnest FLG (red line). The FLG's spectra are extracted from the 16 × 16 μm2 Raman mapping collected in the center of the sample. D band can be seen around 1350 cm-1 indicating the presence of crystalline defects/disorder in the grown FLG. The 2D band around 2750 cm-1 has a single Lorentzian shape that is the fingerprint of a turbostratic stacking. Finally, both G and 2D bands are shifted to high energy compared to the HOPG spectrum. This up shift evidences that FLGs experience a high compressive stress.
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Figure 3: Raman spectra of a HOPG sample (blue line), the thickest FLG (black line), and the thinnest FLG (red line). The FLG's spectra are extracted from the 16 × 16 μm2 Raman mapping collected in the center of the sample. D band can be seen around 1350 cm-1 indicating the presence of crystalline defects/disorder in the grown FLG. The 2D band around 2750 cm-1 has a single Lorentzian shape that is the fingerprint of a turbostratic stacking. Finally, both G and 2D bands are shifted to high energy compared to the HOPG spectrum. This up shift evidences that FLGs experience a high compressive stress.

Mentions: To investigate the quality and thickness uniformity of the FLG, micro-Raman spectroscopy and microtransmission measurements were simultaneously performed. As has already been shown [20], these two techniques can be easily combined by inserting a low-noise photodiode between the SiC substrate and the XYZ piezoelectric stage. It is then possible to measure at the same time, during the acquisition of Raman spectra, using the same laser beam as a probe, the power transmitted through the sample. Raman spectra were collected at room temperature using a Jobin Yvon-Horiba T64000 spectrometer in the confocal mode, with a ×100 microscope objective. The 514-nm line of an Ar ion-laser was used for excitation. The spot size was 1 μm, with 1-mW incident power under the objective. Using this original combination of techniques, a 16 × 16 μm2 mapping of the FLG area located at the center of the sample was performed. The step size was 0.25 μm along both X and Y directions. Since no bare SiC surface could be found at the probe size, a SiC reference spectrum was collected by focusing the laser beam in the SiC substrate deeper than the confocal field depth. FLG's Raman spectra were obtained by subtracting the SiC reference spectrum from the experimental spectra. Typical spectra, collected on the thinnest and thickest FLG parts, are compared to the one of a highly oriented pyrolytic graphite (HOPG) sample in Figure 3. On these spectra, D, G, and 2D bands can easily be observed at 1380, 1610, and 2750 cm-1, respectively. These three bands are blue shifted compared with standard FLG and HOPG Raman spectra. As discussed later, this blueshift can only be explained by a high compressive strain of the graphene lattice. The 2D band has a single Lorentzian shape meaning that the FLG stacking is not Bernal but, rather, turbostratic. This first observation is in perfect agreement with the previous STM results. The D band around 1380 cm-1 comes from the breakdown of the wavevector selection rule and reveals the presence of crystalline defects inside or at the edges of FLG flakes. The in-plane size of the crystallites can be deduced from the ratio between the G and D band integrated intensities (ID/IG). Using the expression given by Pimenta et al. [22] the domain size map was extracted, as shown in Figure 4b.


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)

Raman spectra of a HOPG sample (blue line), the thickest FLG (black line), and the thinnest FLG (red line). The FLG's spectra are extracted from the 16 × 16 μm2 Raman mapping collected in the center of the sample. D band can be seen around 1350 cm-1 indicating the presence of crystalline defects/disorder in the grown FLG. The 2D band around 2750 cm-1 has a single Lorentzian shape that is the fingerprint of a turbostratic stacking. Finally, both G and 2D bands are shifted to high energy compared to the HOPG spectrum. This up shift evidences that FLGs experience a high compressive stress.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211224&req=5

Figure 3: Raman spectra of a HOPG sample (blue line), the thickest FLG (black line), and the thinnest FLG (red line). The FLG's spectra are extracted from the 16 × 16 μm2 Raman mapping collected in the center of the sample. D band can be seen around 1350 cm-1 indicating the presence of crystalline defects/disorder in the grown FLG. The 2D band around 2750 cm-1 has a single Lorentzian shape that is the fingerprint of a turbostratic stacking. Finally, both G and 2D bands are shifted to high energy compared to the HOPG spectrum. This up shift evidences that FLGs experience a high compressive stress.
Mentions: To investigate the quality and thickness uniformity of the FLG, micro-Raman spectroscopy and microtransmission measurements were simultaneously performed. As has already been shown [20], these two techniques can be easily combined by inserting a low-noise photodiode between the SiC substrate and the XYZ piezoelectric stage. It is then possible to measure at the same time, during the acquisition of Raman spectra, using the same laser beam as a probe, the power transmitted through the sample. Raman spectra were collected at room temperature using a Jobin Yvon-Horiba T64000 spectrometer in the confocal mode, with a ×100 microscope objective. The 514-nm line of an Ar ion-laser was used for excitation. The spot size was 1 μm, with 1-mW incident power under the objective. Using this original combination of techniques, a 16 × 16 μm2 mapping of the FLG area located at the center of the sample was performed. The step size was 0.25 μm along both X and Y directions. Since no bare SiC surface could be found at the probe size, a SiC reference spectrum was collected by focusing the laser beam in the SiC substrate deeper than the confocal field depth. FLG's Raman spectra were obtained by subtracting the SiC reference spectrum from the experimental spectra. Typical spectra, collected on the thinnest and thickest FLG parts, are compared to the one of a highly oriented pyrolytic graphite (HOPG) sample in Figure 3. On these spectra, D, G, and 2D bands can easily be observed at 1380, 1610, and 2750 cm-1, respectively. These three bands are blue shifted compared with standard FLG and HOPG Raman spectra. As discussed later, this blueshift can only be explained by a high compressive strain of the graphene lattice. The 2D band has a single Lorentzian shape meaning that the FLG stacking is not Bernal but, rather, turbostratic. This first observation is in perfect agreement with the previous STM results. The D band around 1380 cm-1 comes from the breakdown of the wavevector selection rule and reveals the presence of crystalline defects inside or at the edges of FLG flakes. The in-plane size of the crystallites can be deduced from the ratio between the G and D band integrated intensities (ID/IG). Using the expression given by Pimenta et al. [22] the domain size map was extracted, as shown in Figure 4b.

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