Interface structure between graphene and metal substrates revealed by positron
diffraction

Y. Fukaya1), S. Entani1), S. Sakai1), I. Mochizuki2), K. Wada2), T. Hyodo2) and S. Shamoto1)
1)
R. G. for Nanoscale Structure and Function of Advanced Materials, ASRC, JAEA 2) KEK.

Graphene, a two-dimensional atomic sheet of carbon, iscurrently of great interest because of many promising features
such as extremely high carrier mobility originated from socalled Dirac cone in electronic band structure [1]. Moreover, thegraphene has a long spin diffusion length caused by the weak
spin-orbit interaction and thus is a good candidate material forspintronics.
The physical properties of graphene adsorbed on substratechange from those of freestanding graphene. Theoretical study
shows that the nature of the interaction between graphene andthe substrate depends on the actual substrate material and thisinteraction modulates the shape of the energy dispersion of theDirac cone [2]. The magnitude of the interaction is closely
related to the spacing between the graphene and the substrate.
Therefore, the determination of the spacing is important tounderstand the electronic property of graphene adsorbed on thesubstrate. In this study, we determined the spacing between thegraphene and, the Co and the Cu substrates using total-reflection
high-energy positron diffraction (TRHEPD) [3].
TRHEPD is a surface-sensitive tool owing to the positive
charge of positron. Fig. 1 shows schematic drawing ofexperimental setup for TRHEPD. A positron beam incident on acrystal surface at a grazing angle can undergo total reflection at
the surface. The penetration depth of the positron beam with anenergy of 10 keV is less than approximately 2 Å that
corresponds to the thickness of one atomic layer. When theincidence angle is slightly over critical angle for the totalreflection, the positron beam reaches the layer just below thetopmost layer. Therefore, TRHEPD is suitable for thedetermination of the atomic configuration of graphene adsorbed
onto a substrate. Experiments were carried out at the Slow
Positron Facility, KEK, Japan. The readers refer to reference 3
for the details of the experiment.
Figure 2 shows the intensities of the diffracted positrons for thesingle layer graphene on the Co and the Cu substrates as afunction of the incidence angle. The obtained curves for thegraphene on the two substrates exhibit different shapes. Thissuggests that the spacings between the graphene and the two
substrates are different. From the analysis of the intensities ofthe diffracted positrons based on the dynamical diffraction
theory, the spacing between the graphene layer and the Co andCu substrates are estimated as 2.06 Å and 3.35 Å, respectively,
as shown in Fig. 3. The spacing for the Co substrate is muchsmaller than that for the Cu substrate which is close to theinterlayer spacing in graphite. This indicates that the grapheneon the Co substrate interacts strongly with the substrate ascompared with that on the Cu substrate. This study will promotea better understanding of the interface properties of the grapheneand substrates.
References[1] A. K. Geim, Science 324, 1530 (2009).
[2] G. Giovannetti et al., Phys. Rev. Lett. 101, 026803 (2008).
[3] Y. Fukaya et al., Carbon 103, 1 (2016).

Fig.1 Schematic drawing of experimental setup for TRHEPD.

Fig.2 Intensities of diffracted positrons for graphene (Gr) on the Co(upper) and the Cu (lower) substrates. Open circles and solidlines indicate the experimental data and the calculated
intensities, respectively.

Fig.3 Schematic drawings of graphene on the Co (left) and Cu (right)
substrates. Gray circles indicate the carbon atoms. Blue andorange circles do the Co and Cu atoms, respectively.