High resolution imaging of few-layer graphene

Abstract

In this work, we successfully demonstrate how imaging ellipsometry can be applied to obtain high-resolution thickness maps of few-layer graphene (FLG) samples, with the results being thoroughly validated in a comparative study using several complementary techniques: Optical reflection microscopy (ORM), atomic force microscopy (AFM), and scanning confocal Raman microscopy. The thickness map, revealing distinct terraces separated by steps corresponding to mono-and bilayers of graphene, is extracted from a pixel-to-pixel fitting of ellipsometric spectra using optical constants (n ¼ 2.7 and k ¼ 1.2) derived by fitting slab model calculations to averaged W and D spectra collected in large homogenous sample areas. An analysis of reflection spectra and contrast images acquired by ORM confirm the results by quantifying the number of graphene layers and retrieving the FLG optical constants using a simple Fresnel-law-based slab model. The morphology results are further corroborated with AFM and Raman images, the latter unambiguously verifying that the thinnest part of the FLG consists of a graphene bilayer and providing additional information of electronic origin that might help identifying subtle FLG features, such as the presence of impurities, variations in stacking order, or rolling and folding at the FLG edges. V

Influence of transfer residue on the optical properties of chemical vapor deposited graphene investigated through spectroscopic ellipsometry

“…Coupling between the discrete excitonic state and the continuum of the conduction band affect the lineshape of the resonant peak, which can be phenomenologically described by the Fano resonant profile. 12,13 Recently, ellipsometry has been employed to study the optical properties of exfoliated graphene, 11,[14][15][16][17][18][19] graphene synthesized by chemical vapor deposition (CVD), [20][21][22] and graphene grown on SiC. [23][24][25] For graphene to reach its full potential, cost efficient growth methods need to be developed.…”

Large-area microfocal spectroscopic ellipsometry mapping of thickness and electronic properties of epitaxial graphene on Si- and C-face of 3C-SiC(111)

“…24 Imaging ellipsometry was used to determine thickness of small flakes of exfoliated graphene. [25][26][27] We have recently reported the visible-ultravacuum dielectric functions of EG grown on different polytypes of SiC, where we developed a parameterized dielectric function model for graphene. 21 frequencies combined with optical Hall effect measurements revealed and quantified conductive channels in EG on 6H-SiC.…”

Possibility of graphene growth by close space sublimation

“…Let us turn to the values of the optical constants of thin upper film. Its refractive index value (3.24) is higher and absorption index value (0.463) is lower than the reported values for bulk graphite, the film consisting of 8 to 9 graphene layers, and single-layer graphene ( n  = 2.73, k  = 1.42 are found at λ  = 633 nm for bulk graphite [16]; n  = 2.68, k  = 1.24 at λ  = 633 nm are found for the film consisting of 8 to 9 layers of graphene [17]; n  = 2.7 to 2.8, k  = 1.4 to 1.6 [18] and n  = 2.5 to 2.7, k  = 1.1 to 1.4 [19] have been reported for single-layer graphene). On the other hand, these values are very close to the values of the optical constants for a -C films deposited using pulsed laser deposition ( n  ~ 3.10, k  ~ 0.40 at λ  = 633 nm) [20].…”