4P027

Tip-enhanced Raman Spectroscopic study of local
structures of epitaxial graphene grown on SiC(Kwansei Gakuin University) Sanpon Vantasin, Toshiaki Suzuki, Yasunori Kutsuma,Koji Ashida, Tadaaki Kaneko, Yukihiro Ozaki

Epitaxial graphene synthesized by sublimation of silicon atom from SiC has been considered
as a promising material for electronic applications. It has many astonishing properties such as
unique band gap, high charge mobility, and large graphene sheet. However, local structureson graphene sheet such as ridges and cliffs can greatly affect these properties. In this study,
we used atomic force microscopy (AFM) and tip-enhance Raman spectroscopy (TERS) toinvestigate these local nanostructures. TERS is a technique that utilizes interaction between
laser and metal tip to give near-field enhancementofRamansignal.

With

spatialresolution

surpassing the diffraction limit of excitation laser,graphene local nanostructure can be precisely

near field enhancement~ 100 nm spatial resolution

studied.

local nanostructures< 100 nm size

The graphene samples used in this study weresynthesizedbyheatingC-terminated

4H-SiC(000 ̅ ) to 1800 °C for 15 minutes. TERS
measurement were done with top-illumination

Fig. 1 Schematic illustration of TERS
measurement on graphene localnanostructures.

cliff nanostructures with height
up to ~10 nm. In the areas apartfrom these structures, graphenesheet is extremely flat (< 1 nm
height variation). The non-flat

Height [nm]epitaxial graphene show ridge and100 150 200 250 300 350 400 450 500
Distance [nm]Height [nm]AFM images of synthesized

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Distance [nm]Height [nm]laser.

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Distance [nm]Height [nm]tip radius) and 514 nm excitation

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Distance [nm]Height [nm]top-collection setup using bulk Ag tip (~75 nm

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Distance [nm]Fig. 2 AFM image of graphene grown on SiC and line
profiles with corresponding paths.

structures arose from unequal shrinkage between SiC and graphene while cooling down fromsynthesizing temperature to room temperature [1]. This is due to the difference in thermal
expansion coefficient between SiC and graphene [2].
Fig. 3 demonstrates series of TERS measurements on graphene ridge nanostructure. Since
all G' peaks of TERS spectra has Gaussian shape and FWHM in 17 – 28 cm-1 range, we candetermine that graphene over this nanostructure is single layer. For the points on ridge
structure, the position of G' peaks in TERS spectra clearly shifts (~9 cm-1) to lower
wavenumber. This denotes tensile strain of graphene at those points [3]. In Raman spectra
this shift is far less obvious, because the distance between each point is too small for normal
Raman to resolve. Thus, signal from nearby area is averaged out as the spatial resolution isnot enough. We also studied other sub-micrometer size and nanometer size structures using
this scheme. The results are similar, as TERS can specifically probe each structure while
normal Raman only provides average spectra of the area.
From these results, we demonstrate that graphene grown on SiC has inhomogeneous
strain even in nanoscale and TERS is a powerful tool to investigate this characteristic.

a)G’ peak position (cm-1)

b)

Raman

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TERS

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Raman shift (cm )

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Raman

TERS

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Raman shift (cm )Fig. 3 (a) TERS measurement points shown in AFM image. (b) G’ peak of Raman and TERS
spectra acquired from points in (a).
[1] Ferralis, N. et al. C. Evidence of Structural Strain in Epitaxial Graphene Layers on 6H-SiC(0001). Phys.
Rev. Lett. 101, 156801 (2008).
[2] Röhrl, J. et al. Raman spectra of epitaxial graphene on SiC(0001). App. Phys. Lett. 92, 201918 (2008).
[3] Mohiuddin, T. M. G. et al. Uniaxial strain in graphene by Raman spectroscopy: G peak splitting, Grüneisenparameters, and sample orientation. Phys. Rev. B 79, 205433 (2009).