グループ名, Times New Roman, 14 point, bold
GL、メンバー名, Times New Roman, 12points

Research Group for Nanoscale Structure and Function of Advanced Materials
Group Leader : Katsuyuki Fukutani
Members : Wataru Higemoto, Yuki Fukaya, Takashi U. Ito, Satoshi Yasuda, Shin-ichi Shamoto,
Masahiro Yano, Koichiro,Yamakawa, Tomoo Terasawa, Hirokazu Ueta, Shinichi Machida, Yuto
Oda

Macroscopic properties of matter based on its local states could
be understood by investigating a nanoscale region of the sample.
An experimental technique that can be used to selectivelyinvestigate a surface, bulk, impurity/defect, or ultrathin film canextend the field of materials research. Existing technicaldevelopments enable us to efficiently produce high-intensity
beams, especially beams of synchrotron light, electrons,neutrons, muons, and positrons, which are useful for studying
various atomic structures and dynamics of functional materials.
In addition, experimental techniques that correspond to each
local state such as surface, interface, and impurity have beendeveloped and they can be used to obtain new scientific insights.
Our research objectives are to develop these new advanced
measurement technologies and study the nanoscale structures offunctional devices and materials to clarify the essential
properties of matter.
Growth Mechanism of Graphene on Au was Revealed
Graphene, a two-dimensional honeycomb lattice composed of
sp2 C atoms [1], shows fascinating mechanical, thermal, andelectronic properties and is promising for many applications
including high-temperature coating materials. Since graphene
applications require mass-production of graphene, chemical
vapor deposition (CVD) has been studied among other growthmethods for graphene. In CVD growth of graphene, small C
molecules (growth precursor of graphene) gather on a substrate
to form graphene lattice, resulting in the large-area monolayergraphene. Particularly, CVD growth of graphene on Cu fromCH4 and H2 has been the most popular system [2].
Recently, the growth of graphene on Au substrates has
attracted attention of researchers since graphene on Au wasknown to be applicable for electrodes of electrochemistry as
well [3]. Nevertheless, the studies focused on graphene on Au
have been made, but the basic growth mechanism has been still
unknown so far. In addition, although there is an advantage thatgraphene can be grown on Au without supplying explosive H2[3], how H2 supply affects the growth of graphene has not been
well investigated.
In the present study, we discussed the growth mechanism ofgraphene on Au foil substrates for the first time [4]. We
revealed the effect of H2 supply on the growth of graphene on
Au foil substrates. The results of scanning electron microscopy
and Raman spectroscopy showed that the coverage of graphenegrown without H2 supply was higher at the same growth time.
The Raman spectra of the graphene samples also indicated thatthe graphene grown without H2 supply had larger compressivestrain. The Au foil substrates showed (001) surfaces after thegraphene growth, which was confirmed by electron backscattering diffraction. We attributed the differences in thegrowth rate and compressive strain of graphene to theinteraction between the adsorbates (graphene and itsprecursors) and Au surface. Therefore, the Au foil surface wasreconstructed to form hex-Au (001) surface, enhancing theinteraction between the adsorbates and Au when H2 was notsupplied. The supply of H2 broke the interaction between them,lowering the growth rate and compressive strain of graphene.
Our results suggested that despite the inert surface of grapheneand Au, the supply of H2 can control the interaction between
graphene and Au, which is useful for controlling the growth ofgraphene to realize the high-quality graphene on Au substrates.
Asymmetric Structure of Single-Layer Superconductor
revealed by Positron Diffraction
We have investigated the atomic configuration of single-layer
superconductor of iron selenide (FeSe) using positron
diffraction technique [5]. FeSe material is a typical example ofFe-based superconductors, which shows the superconductiong
transition temperature (Tc) of 8 K. Recently, single-layer FeSe
has been successfully fabricated on an oxide substrate, andshowed the highest Tc (> 50 K) among the Fe-based
superconductors. Although various mechanisms have been
proposed to elucidate the origin of the highest Tc, the atomic
configuration remains unresolved. In this study, we performed
the structure analysis of single-layer FeSe by positron
diffraction, which is a surface sensitive technique owing to thepositive charge of the positron. As a result, we found that theheight of the top-Se layer from the Fe layer is larger than that of
the bottom-Se layer, leading to the asymmetric layer structure(Fig. 1). The averaged height is almost the same as the optimum
height, where the bulk-FeSe exibits the highest Tc underpressure. Consequently, the structure of single-layer FeSe isasymmetrically optimized for the highest Tc.

Fig.1 Asymmetrically optimized structure of single-layer FeSe on anoxide substrate, which was determined in this study. Se layer on
the top of Fe layer becomes taller as compared to Se layer on thebottom.

References[1] K. S. Novoselov, et al., Science. 306, 666 (2004).
[2] X. Li et al., Science. 324, 1312 (2009).
[3] S. Yasuda et al., J. Phys. Chem. Lett. 6, 3403 (2015).
[4] T. Terasawa et al., Jpn. J. Appl. Phys. 58, SIIB17 (2019).
[5] Y. Fukaya et al., J. Phys.: Cond. Mat 31, 055701 (2019).