Quality of graphene on sapphire: long-range order from helium diffraction versus lattice defects from Raman spectroscopy

Abstract

We report a new method to produce high-quality, transparent graphene/sapphire samples, using Cu as a catalyst. The starting point is a high-quality graphene layer prepared by CVD on Cu(111)/Al 2 O 3 . Graphene on sapphire is obtained in situ by evaporation of the Cu film in UHV. He-diffraction, atomic force microscopy (AFM), Raman spectroscopy and optical transmission have been used to assess the quality of graphene in a metal free area. We used helium atom scattering as a sensitive probe of the crystallinity of the graphene on sapphire. The observation of high reflectivity and clear diffraction peaks . The uniformity of the graphene has also been investigated by Raman mapping. Judging by the ratio of the 2D to G peaks, the quality of the graphene is not degraded by Cu removal. The high transparency (80%) measured in the visible range makes this system suitable for many applications that require hybrid properties commonly associated with metals (conductivity) and insulators (transparency).Our study shows that He-diffraction and Raman provide crucial information on quite different, complementary aspects of the same samples.

Suppression of intrinsic roughness in encapsulated graphene

“…The gap of the original ZO mode increases from a value of 8.55 meV to a value of 9.66 meV, while an extra ZO mode with a gap of 5.78 meV emerges. Similar ZO gaps have been found in bilayer graphene (10 meV) [37] and on graphene on Cu/sapphire (6 meV) [38,39] and are a general feature of graphene on weakly interacting substrates [34,35,40]. The new ZO mode is exclusively localized in the hBN layers.…”

Bending Rigidity of 2D Silica

“…(1): this gives a value of 6.1 Å. As discussed above, defects in graphene have been shown to strongly reduce the value of Young's modulus [45,46]. Using our calculated thickness with our experimental bending rigidity in Eq.…”

“…Furthermore, the sample may have defects in the form of holes. Recent experiments show a reduced bending rigidity of graphene deposited on sapphire compared to the theory [45]. The authors attribute this to defects.…”

Material properties particularly suited to be measured with helium scattering: selected examples from 2D materials, van der Waals heterostructures, glassy materials, catalytic substrates, topological insulators and superconducting radio frequency materials

“…30 HAS was recently used to obtain also the bending rigidity and coupling strength of a 2D silica bilayer weakly bound on Ru. 31 Furthermore the bending rigidity and coupling strength of graphene on sapphire 32 have been measured. The latter experiment illustrates how the defect density affects the bending rigidity of the graphene.…”