Photon Factory Activity Report 2014 #32 (2015) B

SPF-A3/2014S2-004

Structure Determination of Graphene on Metal Substrates using Total-Reflection
High-Energy Positron Diffraction (TRHEPD)
Yuki Fukaya1,*, Shiro Entani1, Seiji Sakai1, Izumi Mochizuki2, Ken Wada2, Toshio Hyodo2, andShin-ichi Shamoto1
1
JAEA, Ibaraki 319-1195, Japan2
KEK-IMSS, Tsukuba 305-0801, Japan

2 Experiment
Experiments were carried out at the Slow Positron
Facility, KEK. The incident positron beam wasaccelerated to 10 keV. The diffraction pattern wasobserved with a microchannel plate with a phosphor
screen and a CCD camera. In order to measure therocking curve, the glancing angle of the incident positron
beam was varied by rotating the sample at a step of 0.1°.
All the measurements were conducted at roomtemperature.
Cu atoms were deposited on a α-Al2O3(0001) substrate
at room temperature, followed by annealing up to 1270 K
in H2 atmosphere to form a Cu(111) thin film. Then asingle layer of graphene was grown on the the film byexposing it to the gas mixture (Ar, H2, CH4) at 1270 K.
Co atoms were deposited on a α-Al2O3(0001) substrate at
620 K to form Co(0001) thin film. A single layer ofgraphene was grown on the film by an exposure toacetylene at 870 K.

3 Results and Discussion
The obtained TRHEPD rocking curves for thegraphenes on the Cu(111) and the Co(0001) substrates
exhibit different shapes, as shown by the open circles inFig. 1. We determined the spacing between the grapheneand the substrate by minimizing the difference between
the measured and calculated curves. From the analysis
based on the dynamical diffraction theory, the spacings
between the graphenes and the Cu(111) substrate and theCo(0001) substrate were estimated to be 3.32 Å and 2.24
Å, respectively. The results are consistent with thetheoretical calculations based on the density functional
theory [1]. The value of 3.32 Å for the Cu(111) substrate
is very close to the interlayer distance in graphite (3.35 Å).
The spacing for Co(0001) substrate is much lower than
this. Therefore, the graphene on the Co(0001) substrateinteracts strongly with the substrate as compared with thaton the Cu(111) substrate.

Intensity of 00 spot (arb. units)

1 Introduction
Graphene, a two-dimensional atomic sheet of carbon,
has attracted much attention because of its high electron
mobility and other promising properties such as highthermal conductivity and high rigidity. The synthesis ofgraphene has been successfully performed on various
metal substrates. The theoretical calculations demonstratethat the electronic band structure of the graphene, socalled Dirac cone, is modified by the interaction with thesubstrate materials [1]. The magnitude of the interaction
is closely related to the spacing between the graphene andthe metal substrate. Thus, the spacing between graphene
and metal substrate is the key factor to determine theelectronic property of graphene adsorbed on the substrate.
In this study, we determined the spacing between thegraphene and the Cu(111) and the Co(0001) substrates
using total-reflection high-energy positron diffraction(TRHEPD).
TRHEPD is a surface-sensitive tool owing to thepositive charge of the positron [2]. When the positron
beam is incident on a crystal surface at a grazing angle,
the total reflection takes place on the surface. The
penetration depth of the positron beam under the totalreflection condition is less than approximately 2 Å, whichcorresponds to the thickness of 1-2 atomic layers.
Therefore, the TRHEPD is suitable for the determinationof the atomic configurations of graphene adsorbed on thesubstrate.

: exp
: cal

G/Co

G/Cu

0

1

2

3

4

5

6

7

Glancing angle (deg)Fig. 1: TRHEPD rocking curves for the graphene on theCu(111) (lower) and the Co(0001) (upper) substrates.
Open circles and solid lines denote the experiment andcalculation, respectively.
References[1] G. Giovannetti et al., Phys. Rev. Lett. 101, 026803(2008).
[2] Y. Fukaya et al., Appl. Phys. Express 7, 056601(2014).
* fukaya.yuki99@jaea.go.jp