RESEARCH HIGHLIGHTS 2010
A publication of the WPI Advanced Institute for Materials Research

RESEARCH HIGHLIGHTS 2010

WPI Advanced Institute for Materials Research
The Advanced Institute for Materials Research (AIMR) at Tohoku University in Sendai,
Japan, is one of six World Premier International (WPI) Research Centers established withthe support of the Japanese Ministry of Education, Culture, Sport, Science and Technology(MEXT). Since its inauguration in 2007, the WPI-AIMR has been bringing together worldclass researchers from Japan and abroad to carry out cutting-edge research in materials
science through interdisciplinary collaboration among its four groups: Bulk Metallic
Glasses, Materials Physics, Soft Materials, and Device/Systems.
Led by chief scientist and institute director Yoshinori Yamamoto, the center promotes
fusion research across the different groups while fostering young researchers through afusion-research proposal system and the Global Intellectual Incubation and Integration
Laboratory (GI3 Lab), where international joint research is carried out in close cooperation
with high-profile researchers invited from countries throughout the world.
The WPI-AIMR is host to over 100 leading researchers from around the world including
32 principal investigators. In addition to the research hub at Tohoku University, the
WPI-AIMR collaborates with research centers in the UK, France, Germany, the USA
and China. Close ties with other leading foreign universities are maintained through its
international networks.

AIMResearch
AIMResearch is an online and print publication that highlights the scientific achievements
and activities of the WPI-AIMR. First published in June 2009, AIMResearch selects themost important papers from the wealth of research produced by WPI-AIMR scientists
throughout the year, distilling the essence of the achievements into timely, concise andaccessible research highlights that are easy to digest but retain all of the impact andimportance of the original research article. Published monthly on the AIMResearch website
in both English and Japanese, AIMResearch highlights bring the very best of WPI-AIMR
research to a global audience of specialists and nonspecialists alike. AIMResearch also
publishes a range of features articles introducing other activities of the WPI-AIMR’s researchgroups. Visitors to the website can register for monthly email alerts in either Japanese or
English to keep abreast of the latest developments and discoveries made at the WPI-AIMR.
research.wpi-aimr.tohoku.ac.jp© 2011 Tohoku University

MESSAGE FROM THE DIRECTOR
2Green materials
RESEARCH HIGHLIGHTS

in the SPOTLIGHT

6

Nanotechnology: metallic glass nanowires

28

Time to shake things up

7

Oxide electronics: A robust gate

29

MEMS technology makes the world

8

Organic gels: Dynamic duos

9Interfaces: An in-depth picture

31

Fusion research on the global stage

10

Spin dynamics: A nano-spinning top

32

Mixing the unmixable

11

Printable electronics: A little give and take34

From the bottom up

12Iron-based superconductors: Borrowing from graphene

36

Learning the difference

13

Nanomaterials: Supersizing at the nanoscale

14

Metallic glass: Go with the flow

15

Polymers: Atomic mapping

16

Photonics: Bright white light17

Atomic-scale imaging: Stepping up the pace

18

Clathrates: Rattling the cage

19

Single-molecule imaging: Finished in chrome

20

Oxides: Approaching perfection

21

Iron-based superconductors: Spin matters

22

Topological insulators: A large leap ahead

23

Superconductors: A sound solution

24

Nanostructured catalysts: Golden openings

25

Metals: Bringing glass to order

26

Oxides: Less is morea merrier place

Editorial
WPI-AIMR, Tohoku University
2-1-1 Katahira, Aoba-ku, Sendai,
Miyagi 980-8577, Japan

research.wpi-aimr.tohoku.ac.jp
AIMResearch is a publication of the World Premier International Advanced
Institute for Materials Research (WPI-AIMR), a Tohoku University institute fundedby the Ministry of Education, Culture, Sports, Science and Technology of Japan.
© 2011 WPI-AIMR, Tohoku University. This publication may be reproduced in its
original form for personal use only. Modification or commercial use without priorpermission from the copyright holder is prohibited.

© 2011 Tohoku University

Editor-in-Chief

Yoshinori Yamamoto

Managing Editor

Masashi Kawasaki

Editorial AssistantMariko Onodera

ISSN 1884-491X

Message from the director

Green materials
As the institute director of theWorld Premier International
Advanced Institute for Materials
Research (WPI-AIMR), I havegreat pleasure in welcoming you
to this, the second print editionof AIMResearch. In April, theAIMResearch website entered its
second year, and this print publication is a comprehensive collectionof the research highlights publishedon the website in 2010 along withlively journalistic features thatturn a spotlight on the people
and activities behind the researchcarried out at the WPI-AIMR.
The importance of materials
science to scientists and society
in general continues to grow onan annual basis — a point clearly
underlined by the awarding of the2010 Nobel Prize in Physics forthe discovery of graphene, one of
the most exciting discoveries inmaterials science in recent years.
When the WPI-AIMR was
inaugurated in 2007, its scientificobjectives were threefold. Firstly,
to create new substances andmaterials with innovative

functions by new methods of atomand molecular control; secondly,
to construct devices based upon
new fundamental research; andfinally, to contribute to society
by promoting applied research
projects on materials and systemarchitectures capable of delivering
concrete benefits to humankind.
In October 2010, the
WPI-AIMR celebrated its third
anniversary and the attainment of
this milestone provided an ideal
opportunity to consolidate thelong-term strategy of the instituteand to define more clearly its
research direction and vision.
Based on the experience andresearch outcomes garnered overthe past three years, it has become
clear that the future of the WPIAIMR lies in the development ofgreen materials.
While there are many areas inwhich green materials can play animportant role, we have identified
fields such as energy harvesting,energy saving and environmental
clean-up as high priorities. In
the arena of energy harvesting,© 2011 Tohoku University

the WPI-AIMR will target thecreation of new materials forphotovoltaic and thermoelectric
conversion, while research onenergy-conserving technologies
will concentrate on cutting-edge
materials for soft electronics,
superconductors, low friction, lowwear, lubricants and low energy
consumption logic and memory
circuits based on magnetic
materials. Environmental clean-up
is another topic of growing importance and the WPI-AIMR willturn its focus on green innovation
to the discovery of new catalysts,
porous materials, sensors, microelectromechanical systems andbiodevices for applications in thisarea. The basis of these component
technologies lies in the researchand development of green materials with functional properties that
fulfill key objectives.
The theme of green innovation
is a diverse and complex one
that requires comprehensive andimaginative use of component
technologies. It is not possible toconfront the demands placed ongreen materials and innovative
component technologies throughthe use of a single material. Rather,
it is necessary to investigate andexplore heteromaterials formed by
the combination of two or moredifferent materials and to use theirindividual characteristics synergistically to bring out new innovative
functional properties.
For example, ‘hard’ metallic
glasses and ‘soft’ polymers
constitute, in a static sense, very
different systems. However, lookedat dynamically, both systems are
non-equilibrium materials andthis common property makes
it easy to fabricate composite
materials from a combination of
metallic glasses and polymers,
which may find applications,for example, as a low-wear
rubber. Palladium catalysis forcross-coupling, the discovery forwhich was awarded the Nobel
Prize in Chemistry in 2010, is
another example. Although anincredibly powerful and versatiletechnique, the products of thesereactions are often contaminatedwith traces of the palladium metal
used in the catalysis, which limits
the approach’s usefulness inreactions demanding extremely
high levels of product purity, suchas those involved in the synthesisof pharmaceutical drugs. Our
research has shown that nanoporous palladium-based catalysts
containing a combination of
palladium and nickel may offer asolution via heterophase-interface
control, which is essential for therealization of the full power ofheteromaterial synergetics.
These are just two examples of
the ways in which the WPI-AIMR
is involved in developing new tools
and techniques for basic research
into green materials. By pursuingand deepening this kind of basic
research, I believe that the WPIAIMR will become established as
a visible and globally recognizedresearch center for green materials
in the coming years.
Yoshinori Yamamoto
Director
WPI-AIMR© 2011 Tohoku University

AIMResearch is an online publication that highlights the best of cutting-edge research carriedout at the World Premier International Advanced Institute for Materials Research (WPI-AIMR),
a leading interdisciplinary and internationally focused center of materials science researchbased at Tohoku University in Japan.

AIMResearch introduces the best of research from the WPI-AIMR in the formof concise, accessible research highlights and casts a spotlight on thescientists and laboratories behind one of the world's leading institutes forthe development of new and innovative functional materials.

research.wpi-aimr.tohoku.ac.jp
All articles published on AIMResearch can be accessed free of charge.
Register for monthly email alerts to get the latest research news from
WPI-AIMR delivered direct to your inbox!

© 2011 Tohoku University

RESEARCH HIGHLIGHTs
The WPI-AIMR advances research in bulk metallic glasses,
materials physics, soft materials and device/systems construction, and actively promotes collaboration among these divisions
toward the development of ground-breaking technologies thatcross the boundaries of conventional fields of study — bridging
the disciplines of materials science, physics, chemistry andprecision, mechanical, electronics and information engineering.

© 2011 Tohoku University

Research highlights

Nanotechnology

Metallic glass nanowires
Nanowires made from metallic glass are extremely tough and hold promise for alarge number of nanotechnology applications

Metallic glass (MG) is a promising classof material that could find potential
use in structural components for applications ranging from mobile phonecasings to biomedical implants. At thesame time, achieving MG structures on
the nanoscale, which could lead to novel
applications, has remained a challenge.
Researchers from Tohoku University’s
WPI-AIMR have now developed acontrolled method for producing MG
nanowires with very high mechanicalelasticity and strength1.
Metallic glasses are amorphous — likewindow glass except that they are madefrom metal alloys. Even though theirappearance is similar to that of crystalline metallic alloys, such as steel, theiramorphous structure leads to a numberof advantageous properties. For example,
in steels and other crystalline metals,
imperfections such as vacancies, dislocations and grain boundaries in the crystal
structure are the main cause of mechanical failure. Owing to their amorphous
structure, however, “MGs possess a combination of unique properties including
ultrahigh strength, high hardness andhigh resistance to wear corrosion,” according to Koji Nakayama, who leads the
WPI-AIMR fusion research team.
Remarkable progress has beenachieved in exploring alloy compositions
that are best suited for the fabrication of
MGs in bulk quantities — a requirement
for practical utilization of these unusualmaterials. However, while achievementsin this area have been significant, far lessattention has been paid to MG nanostructures, such as ultrathin wires.
Metallic glass nanowires were firstreported in 2008 by the same groupwhile studying the compression of

6

500 μm
A flexible metallic glass nanowire. The ribbon sample from which the nanowire is drawn is visible on the left.
The free end of the wire in the center oscillates with a sine-wave-like pattern.

bulk MGs. The surfaces of bulk MGs
typically get very hot during the fracture
process, which leads to the formation of
nanowires. Now, the researchers havedeveloped a mechanism to draw MG
ribbons in a much more controlled
fashion. Their method is based on superplastic deformation, following a simple
principle that glass blowers have beenusing for centuries. The MG is processed
when it is still in a viscous liquid state
and, with the help of a weight attached to
the ribbon, is drawn out into nanowires
by a custom-made machine.
The nanowires obtained by thismethod display excellent mechanicalproperties — they can be bent elastically (see image) while overall retaining
strength comparable to that of the bulk

material. Furthermore, the versatility of the drawing process ensures that
nanowires can be formed from MGs
with a broad range of compositions. “Interms of applications, palladium-based
MG nanowires could be used as sensors
with high hydrogen sensitivity. Ironbased MG, on the other hand, exhibits
soft magnetic properties that could be
useful in ultrahigh magnetic-field sensors for biomagnetic measurements,”
says Nakayama.

1. Nakayama, K.S., Yokoyama, Y., Ono, T., Chen,
M.W., Akiyama, K., Sakurai, T. & Inoue, A.
Controlled formation and mechanical characterization of metallic glassy nanowires. Advanced
Materials 22, 872–875 (2010).

AIMResearch 2010
© 2011 Tohoku University

Research highlights

Oxide electronics

A robust gate
High-quality transistors can be fabricated using an oxide interface anda polymer gate

Silicon and other covalent bonding semiconductors have been the
primary materials used in electronic
components for several decades, but
the need to improve performanceand develop novel functionalities has
spurred research efforts into new materials. Particularly promising are thehighly conductive ‘two-dimensional
electron gases’ that form at the interfaces between insulating oxides — forexample between LaAlO3/SrTiO3 and
MgxZn1–xO/ZnO — in which electrons
move freely in two dimensions.
Incorporating such interfaces into
electronic devices such as transistors
requires the ability to tune the carrier
concentration by applying a voltage between a gate electrode and the interface
itself. This is what Masashi Kawasaki
and colleagues at the WPI-AIMR havenow successfully demonstrated by constructing a transistor using a polymer
gate and a MgxZn1–xO/ZnO interface as
the active layer1.
When a metallic gate is deposited ona semiconductor, the difference in work
function — the energy required for anelectron to leave the surface — between
the two parts leads to the formation ofa Schottky junction, which allows thecarrier concentration in the semiconductor to be altered with minimal leakage current from the gate. As Kawasaki
explains, however, “Schottky junctions
involving ZnO and noble metals havenot been adequately reproducible andthe device characteristics are far fromexcellent. We suspect that the metals
react with oxygen in the ZnO somehow,
which induces defects.” The issue iseven more delicate in the case of interfaces, as any small defect creates a path

PEDOT:PSS

e
MgZnO
s

on

e e e e e e e e

ZnO

2D

tr
lec

e

100 μmTop view (left) of the transistor and schematic illustration (right) showing the two-dimensional electron gas
interface between ZnO and MgZnO.

for the current to leak from the gate,
which has a very damaging effect on
the properties of the two-dimensional
electron gas.
“Polymers are usually very stableagainst oxidation, and atoms in polymers are already saturated with chemicalbonds,” says Kawasaki. The team fabricated a transistors using a high-mobility
MgxZn1–xO/ZnO interface and a layer ofa conducting polymer for the gate (seefigure). The polymer, PEDOT:PSS, iscommercially available and widely usedin organic displays.
The two-dimensional electron gas
exhibits sharp resistance oscillations
under a magnetic field due to thequantum Hall effect, which confirms
the excellent quality of the device. In
addition, the oscillations are clearly
modulated by the application of a gate
voltage, demonstrating that the carrier

AIMResearch 2010
© 2011 Tohoku University

concentration can be tuned externally.
The results could be very significant
for practical applications. “This interface could be one of the important
ingredients for future transparent
circuitry. The fabrication method isextremely simple and the materials areof low cost,” says Kawasaki. There is also
no reason why the same route could
not be followed for other conducting
oxides: the team has already exploredthe possibility of using the same polymer as a gate for oxides such as SrTiO3
or TiO2.

1. Nakano, M., Tsukazaki, A., Ohtomo, A., Ueno, K.,
Akasaka, S., Yuji, H., Nakahara, K., Fukumura, T. &Kawasaki, M. Electric-field control of twodimensional electrons in polymer-gated–oxide
semiconductor heterostructures. Advanced
Materials 22, 876–879 (2009).

7

Research highlights

Organic gels

Dynamic duos
A two-part mixture of molecular helices proves to be a versatile route towards
functional soft materials

Ever since the realization that biomolecules such as DNA naturally assemble
into three-dimensional helix shapes,
researchers have tried to replicate thesestructures using different chemical
units. Helicenes — spiral-shaped molecules made up of several fused benzene
rings — have attracted much scientificinterest because their conjugated framework leads to unique optical, electronic
and aggregation properties.
Now, Masahiko Yamaguchi from the
WPI-AIMR at Tohoku University1 andcolleagues have used short chains of helicenes, known as oligomers, to produce
thermally reversible organic gels. The
methodology developed by the researchteam promises to improve control overthe properties of these hybrid liquid–
solid materials at the molecular level.
When helicenes are synthesized, they
form one of two chiral enantiomers— isomers of the same compound but arranged such that they are mirror imagesof one another. Most reactions produce
equal amounts of both enantiomers, butthese racemic mixtures usually form
crystals with well-ordered lattices, notgels. In pure solutions of enantiomers,
however, the tight packing needed forcrystallization is perturbed, and the helicenes tend to aggregate into long fibersthat trap solvent molecules in a gel.
Yamaguchi and his team developed
a new series of ‘pseudoenantiomers’,
which display better gel-forming properties than pure solutions. Composed of
two helicene enantiomers with slightly
different sizes, these substances spontaneously formed a yellow gel (pictured)
when mixed together at room temperature. Furthermore, this process wasthermally reversible: heating to 110 °C

80.5 μm
A new thermally reversible gel (inset) forms long fibrous aggregates (atomic force microscopy image) thanks
to a two-part mixture of pseudoenantiomers.

caused the gel to liquefy, but the gel reformed on cooling to 25 °C. Yamaguchisays that the pseudoenantiomers take on
properties that are intermediate between
those of the pure enantiomers and theracemic mixture, suppressing crystallization and enhancing fiber formation.
The need for soft gel-based materials
is expanding in diverse areas ranging
from bioengineering to consumer
packaging. Gels are suitable for theseapplications because of their ability
to mimic the networks found in living
systems and present properties distinct
from those of conventional materials.
Yamaguchi’s pseudoenantiomer method
has the potential to improve gel compounds by providing a reliable means ofjoining small molecules together.
“Precisely constructing gels withspecific functions, such as hardness,elasticity, flexibility or processability,
requires a delicate method to control thegel structure,” says Yamaguchi. “In thatsense, gel formation by small molecule
aggregation is interesting because thestructure of small molecules can bereadily controlled, and a diversity of gelscan be provided.”
The use of two-component gels offers
great promise in terms of tuning properties, as different enantiomers ratios
produce different gels. It also has other
implications, according to Yamaguchi:“For example, one compound can beon a surface, and the other in solution.
Such a system can provide control of
the interface structure between the solidand liquid.”

1. Amemiya, R., Mizutani, M. & Yamaguchi, M. Twocomponent gel formation by pseudoenantiomeric ethynylhelicene oligomers. Angewandte
Chemie International Edition49, 1995–1999 (2010).

AIMResearch 2010
© 2011 Tohoku University

Research highlights

Interfaces

An in-depth picture
Carbon atoms are important for creating efficient metallic contacts with widebandgap silicon carbide

Ti3SiC2

SiC© 2010 Z. Wang

The arrangement of atoms at the interface between a metal and a semiconductor plays a crucial role in determining
the efficiency of electronic devices. But
imaging atoms buried deep beneaththe surface is not an easy task. Yuichi
Ikuhara and co-workers from Tohoku
University and Tokyo University1
have now produced an atomic-levelpicture of these technologically important interfaces by combining stateof-the-art imaging techniques andnumerical simulations.
“Because we need to connect electricalcomponents together using wires, a key
issue that currently limits the efficiencyof electronic circuits is the manufactureof reliable, low-resistance contacts
between a metal and a semiconductor,”
explains Zhongchang Wang, a memberof the WPI-AIMR. A high-resistancebarrier formed at some metal–semiconductor interfaces, known as a Schottky
barrier, hinders the flow of electrons
and increases power dissipation.
A good choice of metal and a process
known as annealing — quickly heating
to a high temperature — can overcome
Schottky barriers. In the case of siliconcarbide (SiC), a wide-bandgap semiconductor with potential applications inhigh-power and high-temperature applications, a trial-and-error approach has
determined that titanium–aluminum(TiAl) alloys form such low-resistancecontacts with SiC. “Researchers havesuggested that a new metallic compound,
Ti3SiC2, is formed between the deposited
TiAl alloy and the SiC; however, it remained unclear how this interface could
reduce the contact resistance.”
Ikuhara and his team deposited alternating layers of titanium and aluminum3Å
Z-contrast STEM image of the position of atoms at the interface between SiC and Ti3SiC2. Titanium atoms arecircled in yellow, and silicon atoms in green. Carbon atoms, shown as white dots, are not evident in the image
but their position is inferred from a theoretical model.

on a SiC substrate and then annealed
them at 1,000 °C for a few minutes. They
then produced an image (pictured) of
the precise arrangement of the atoms at
the interface using a powerful techniqueknown as Z-contrast scanning transmission electron microscopy (STEM),
which is capable of distinguishing different atomic species. The titanium atoms
appeared as bright spots whereas thesilicon atoms were much darker.
Numerical calculations simulated 96
possible interface geometries; only twoof which matched the STEM image.
In both of these variations, the silicon
atoms in the metal (Ti3SiC2) sat in thespaces between silicon atoms in thesemiconductor (SiC). In one case,however, carbon atoms, not visible in

AIMResearch 2010
© 2011 Tohoku University

the STEM image because of their low
mass, were also present at the interface(SiCSi). Further calculations indicatedthat this SiCSi arrangement was morestable than the other candidate (anSiSi interface) and that the presenceof the carbon led to a much smaller
Schottky barrier. “Our results suggestthat an atomic layer of carbon plays animportant role in reducing the contact
resistance and provides a strategy
for producing efficient SiC contacts,”
says Wang.

1. Wang, Z., Saito, M., Tsukimoto, S. & Ikuhara, Y.
Interface atomic-scale structure and its impact
on quantum electron transport. Advanced
Materials 21, 4966–4969 (2009).

9

Research highlights

Spin dynamics

A nano-spinning top
Time-resolved experiments reveal details of spin motion in magnetic
nanostructures, contributing to the development of high-speed spintronic
devices with lower power consumption

10

1

S

N
S
N

0

Kerr signal(arb. unit)

The need for ever-increasing information
storage capacity is motivating researchers to investigate various systems for
reducing the size of the basic storage elements and increasing the speed at whichdata can be written and read. A structurethat has recently received considerable
attention is ‘spin transfer torque’ (STT)
magnetic random access memory(MRAM), which is based on arrays ofmagnetic tunneling junctions. In such a
device, the information is stored in themagnetization of a ferromagnetic film,
and is controlled by an electrical current
applied through the STT interaction.
Shigemi Mizukami and colleagues fromthe WPI-AIMR and Tohoku University1
have now observed the fast motion of
spins in a cobalt film just a few atomic
layers thick (0.5–4.0 nm) sandwiched
between layers of platinum. “Spinmoves constantly, like a bar magnet, in amemory cell, and it experiences ‘friction’,even though it’s a quantum mechanical object. This friction, called ‘Gilbert
damping’, influences power consumption
and speed in devices,” says Mizukami.
The researchers’ experiments allowed thefriction strength acting on the motion ofspin in the cobalt film to be determined
precisely for the first time (see figure).
In an ideal STT-MRAM element, theminimum current density needed toswitch the magnetization should be thelowest physically possible — a value thatis proportional to the Gilbert damping constant α, and the perpendicularmagnetic anisotropy Heff. However,changes in Heff also affect thermal stability. It is therefore crucial to understandhow α and Heff are related in orderto design the struc­tures that provide
optimal performance.

0.9 nm-thick cobalt
0100

Pump-probe delay time (ps)200

(Upper) Schematic diagram of an MRAM element and spin dynamics for writing from “0” to “1”. Spin is illustrated
by permanent ‘bar’ magnets (red and blue). (Lower) Spin motion observed in a 0.9 nm-thick cobalt layer.

Previous theoretical investigations
have suggested that there is a linear relationship between α and Heff, but this has
yet to be confirmed because it is difficult
to observe spin dynamics at high frequencies and in ultrathin films. Mizukami’s
team developed a special instrument
for performing ultrafast time-resolved
magneto-optical Kerr effect experiments,
which allowed the researchers to derivevalues for both α and Heff.
The results showed that Heff is inversely proportional to the thickness of thecobalt layer, whereas α increases linearly
up to a cobalt layer thickness of 1 nm before increasing more rapidly. Mizukami
considers that this behavior could reflect
the transition from perpendicular toparallel magnetization in the cobalt film.
“Our experiments demonstrated that
Gilbert damping in the cobalt/platinum
system can be tuned widely. It is crucial
for spintronics applications to optimize
Gilbert damping, while maintaining
high Heff, in order to achieve highspeed and low power consumption,”says Mizukami. The results therefore
represent an important step towards thedesign of applications based on STT.
1. Mizukami, S., Sajitha, E.P., Watanabe, D., Wu, F.,
Miyazaki, T., Naganuma, H., Oogane, M. & Ando,
Y. Gilbert damping in perpendicularly magnetized Pt/Co/Pt films investigated by all-opticalpump-probe technique. Applied Physics Letters96, 152502 (2010).

AIMResearch 2010
© 2011 Tohoku University

Research highlights

Printable electronics

A little give and takeA selective ‘ligand exchange’ reaction makes molecular-based inks easier toattach to surfaces

Nanocrystal

O O

O O
HN
Si
OO O

© 2010 D. Hojo

Innovative technologies that use
molecular-based ‘inks’ to print electronic
circuitry onto flexible plastic substrates
promise to usher in an era of futuristic
low-cost devices, such as smart clothingtags that can communicate instructions
to washing machines. Manufacturing
printable electronics remains challenging, however, because the high temperatures usually needed to bind the inks tosurfaces can damage the polymer materials used as substrates.
Researchers led by Daisuke Hojo andTadafumi Adschiri from the WPI-AIMRat Tohoku University have now developed
a simple and efficient way to attach an ink
made of metal oxide nanocrystals onto
surfaces at room temperature1. According
to Hojo and Adschiri, the diverse conditions needed to deposit extremely small
particles in an organized manner madethis breakthrough a tough one to achieve.
“For printable electronics, the nanoparticles have to be well-dispersed in solvents, but they also need to self-assemble
into films during the drying stage, andto subsequently become immobilized on
the substrate,” says Hojo. “For each step,
the required interactions between thenanoparticles, the solvent, and the surface
are different — making the fabrication
process difficult,” says Adschiri.
Researchers at Tohoku University led by
Adschiri had previously been successful incoating tiny particles of cerium oxide — arare-earth compound with catalytic andoptical capabilities — with a thin filmof organic molecules to prepare highly
soluble nanocrystals2. Using a process
known as ligand exchange, the researchers
demonstrated in their most recent study
that cerium oxide nanocrystals can be immobilized on a silica surface by replacing

Organic layer
Substrate
Nanocrystals bearing decanoic acid (left) can be fixed to a substrate bearing catechol groups by ligand exchange— swapping one molecule for another. This room-temperature ink ‘printing’ technique produces dense films ofmetallic nanoparticles (right, scanning electron microscopy image of nanocrystal film).

one molecule bound to the nanocrystal
with one bound to the substrate — achieving the effect of a chemical ‘glue’.
The scientists prepared the metal oxide
nanocrystals for the process by coating
the particles with decanoic acid, a longcarboxylic acid that binds to cerium
oxide via two oxygen atoms. They thendeposited the nanocrystals from solution at room temperature onto a siliconsurface that had been coated with a thinorganic layer of DHCA — a hydrocarbon
bearing a catechol group (a benzene ringwith two neighboring alcohol units).
Chemical equilibrium effects drove thenanocrystals to swap the decanoic acidfor the more favorable catechol groups
fixed on the silica surface (see figure),effectively binding the nanocrystals tothe substrate.
The researchers confirmed that thismethod produced dense films of surfacebound nanocrystals that were thermallystable up to 200 °C. They also found that

AIMResearch 2010
© 2011 Tohoku University

other carboxylic acid-coated nanocrystals, including crystals with iron andtitanium cores, could be trapped using
the same technique because of the broadaffinity of catechol groups. According to
Hojo and Adschiri, this ligand-exchange
method helps solve the critical problem
of immobilizing nanoparticles. “This isa definite step toward establishing technology for printed electronics, especiallyfor fabricating photovoltaic devices,”says Adschiri.
1. Hojo, D., Togashi, T., Iwasa, D., Arita, T., Minami,K., Takami, S. & Adschiri, T. Fabrication of
two-dimensional structures of metal oxide
nanocrystals using Si substrate modified with
3,4-dihydroxyhydrocinnamic acid. Chemistry ofMaterials 22, 1862–1869 (2010).
2. Zhang, J., Ohara, S., Umetsu, M., Naka, T.,
Hatakeyama, Y. & Adschiri, T. Colloidal ceria
nanocrystals: A tailor-made crystal morphology
in supercritical water. Advanced Materials
19, 203–206 (2007).

11

Research highlights

Iron-based superconductors

Borrowing from graphene
Researchers discover remarkable similarities between an iron-based superconductor
compound and graphene carbon

The discovery of iron-based superconductors in 2008 reenergized the field ofsuperconductivity research, all the more
so because the electronic properties ofthese new compounds have proved tobe complex and full of surprises. Few,
however, would have expected that thesesuperconductors would display features
similar to those of another material thathas recently attracted the excitement ofscientists — graphene.
A team of researchers led by Takashi
Takahashi of the WPI-AIMR had been
studying the iron-based compoundBaFe2As2 in order to gain new insights
into its magnetic properties, particularly with respect to the ‘spin-density
wave’ (SDW) order previously reported
for this material. The team discoveredthat the compound has a Dirac conelike electronic band structure1 — thesame feature that gives graphene its
exceptional properties.
BaFe2As2 is the parent compoundof one member of the iron-based
superconductor family. It is widely
accepted that superconductivity inthese iron-based compounds, as in the
cuprate superconductors, is present in
the parent compound but suppressed byanother property, usually magnetism.
The parent compound is induced into
its superconducting state by adding impurity atoms and cooling it to extremely
low temperature.
The researchers examined the parent
compound BaFe2As2 using a techniqueknown as angle-resolved photoemission
spectroscopy, which revealed two brightpoints of photon emission in momentum space below the SDW temperature(see figure). Focusing on these points
and studying emission as a function of

12

E
kx

ky

M
Λ

Ba
Fe
As
Schematic illustration of the Dirac cones in BaFe2As2 and the bright points (Λ) in momentum–energy (k–E)
space. The atomic structure of the compound is shown on the right.

energy and momentum, the team thendiscovered the Dirac cone — a band
structure that describes how electrons
behave like massless relativistic particles
at certain points in momentum space.
The similarity with graphene is notcomplete, however. While the Diraccone of graphene is symmetric withrespect to momentum, that for BaFe2As2
is distinctly asymmetric and displays
small pocket-like features and nodes,
features that present intriguing targets
for future research.
The team’s results have broad implications for many of the actively studied
topics in solid-state physics. Aside fromgraphene, Dirac cones have also been
observed in ‘topological insulators’, aclass of materials that are at once bothinsulating in the bulk and metallic at theedges. “Whoever would have thoughtthat iron-based superconductors, oneof the hottest materials in materials
science, would exhibit low-energy physics similar to graphene?” says Pierre
Richard, one of the lead researchers
involved in the discovery. “With ourfindings, people will have to think more
carefully on the connection between
all of these materials, which could leadto the development of new functional
materials with exotic properties.”

1. Richard, P., Nakayama, K., Sato, T., Neupane, M.,
Xu, Y.-M., Bowen, J.H., Chen, G.F., Luo, J.L., Wang,
N.L., Dai, X., Fang, Z., Ding., H. & Takahashi. T.
Observation of Dirac cone electronic dispersion
in BaFe2As2. Physical Review Letters
104, 137001 (2010).

AIMResearch 2010
© 2011 Tohoku University

Research highlights

Nanomaterials

Supersizing at the nanoscale
Polymer-based structures that bridge the nano- and microscopic scales can now bebuilt from the nanoscale up

In a range of advanced technologies thatdeal with mechanisms on the molecularscale, such as molecular electronics anddrug delivery, bridging the gap between
nanoscale objects and larger devices isof key importance. Constructing suitable structures that can fulfill the rolesof both nanoscale and bulk objects,
however, has generally been performedby miniaturizing larger structures.
A research team led by Masatsugu
Shimomura and Takeshi Higuchi fromthe WPI-AIMR1 has now had successwith the opposite approach — building
structures from the nanoscale up by
self-assembly.
Shimomura and Higuchi used an
approach known as self-organized precipitation (SORP) to produce nanoscale
aggregates from block copolymers —structures containing multiple distinct
polymer subunits. Polymer aggregatesor particles are typically prepared byemulsion polymerization, which involves the use of surfactant-stabilized
oil in water. “But with that method, it
is difficult to synthesize nanostructured
particles,” says Shimomura. “The SORP
method, on the other hand, can produce
block copolymer particles quite easily,”says Higuchi.
The SORP technique exploits differences in polymer solubility between
good and poor solvents. The researchers created polymer nanoparticles ofpolystyrene and polyisoprene by adding
water to a solution of these two microphases in an organic solvent. Two days
later, after the solvent had evaporated,
the team reacted the nanoparticles withan oxidant, causing the polyisoprene
to become insoluble while leaving thepolystyrene portion of the particles

300 nm

500 nmScanning transmission electron microscopy images of nanoparticles consisting of stacked layers (left) that aretransformed into nanodisks (right) by oxidation and dissociation. Schematic illustrations show the polystyrene(blue) and polyisoprene (green) components of the structures.

intact. The two microphases could thenbe dissociated by adding the nanoparticles once more to an organic solvent.
Using block copolymers containing
polystyrene and polyisoprene subunitsof similar length, the researchers obtained suprapolymer structures consisting of evenly stacked and alternating
polymer sheets. Imaging experiments
revealed that the block copolymers firstself-assembled into layered particles,
which then dissociated into nanometersized disks of polyisoprene sandwiched
between polystyrene layers (pictured).
In contrast, block copolymers with subunits of dissimilar length gave cylindrical polyisoprene nanoparticles coatedwith polystyrene chains.
Blends of the two individual polymers
produced particles with two distinct

AIMResearch 2010
© 2011 Tohoku University

hemispheres, known as ‘Janus’ particles,
which dissociated on oxidation into
the two constituent halves. “The Janus
particles can be applied as pigments
for electronic paper,” says Shimomura.
Mixtures of the polymer blend and theblock copolymers, on the other hand,gave porous nanostructures, “which areapplicable as carriers in drug delivery
systems,” says Higuchi. The team iscurrently working on assembling thenanometer-sized disks and cylinders
into higher-order structures.

1. Higuchi, T., Tajima, A., Motoyoshi, K., Yabu, H.
& Shimomura, M. Suprapolymer structuresfrom nanostructured polymer particles.
Angewandte Chemie International Edition
48, 5125–5128 (2009).

13

Research highlights

Metallic glass

Go with the flow
Simulating the movements of thousands of atoms reveals that stress can transform
glassy states into flowing liquids

Portable electronic devices of the future
may use a secret ingredient to guard
against dents and collisions — a toughnew type of alloy called metallic glass.
These materials, formed by rapidly cooling molten mixtures of metals, have a
non-crystalline, tightly packed internal
structure that makes them springier,stronger and more scratch-resistant
than conventional crystalline metals.
However, the random atomic distribution inside metallic glasses also makes
it difficult to optimize their mechanicalproperties, leading to problems such as
shattering and sudden failure.
Mingwei Chen, Pengfei Guan andTakeshi Egami from the WPI-AIMR at
Tohoku University have now performed
computer simulations that provide
new microscopic insights into howmechanical failure occurs in metallic
glasses1. The team found that external
stress — the act of pulling the material apart — induces a liquid-like ‘flow’of atoms similar to that caused by anincrease in temperature. Their findings
uncover the mechanism by which theseglassy states can form, with the promiseof greatly improving the reliability of
manufacture for these alloys.
Despite their rigidity, metallic glasses
actually exist in a ‘jammed’ phase of matter that falls between solids and liquids.
Forcing hot mobile metal atoms into amold and then cooling rapidly transforms the liquid into a ‘frozen’ glassy
state. It is know that expanding the volume or heating can make the atoms flow
once more. Until now, however, the roleof external stress on this jammed phase
had remained controversial.
Chen and his co-workers performed
their modeling using a computational

14

Al
Cu
Zr
Schematic illustration of the simulated metallic glass system under shear stress.

technique known as molecular dynamics simulation. They simulated theatomic movements in a zirconium–copper–aluminum metallic glass whensubjected to a shear force that gradually
stretched the glass apart under constantvolume and temperature conditions(see figure). After numerous simulations considering a range of shear rates,
the researchers observed that the metal
atoms entered a state of steady viscous
flow, similar to that of a liquid, before
snapping apart.
Remarkably, the viscosity of themetal under stress was found to be
similar to that associated with elevatedtemperature, and the researchers wereable to model the stress–temperature
conditions needed to form either the
jammed glassy state or flowing state.
The similarity of the effects of temperature and stress indicates that both effects
influence atomic flow in a like manner.
“This must reflect the simplicity of the
dynamics of glasses at a deep level,
hidden behind the apparent complexityof the phenomena and the structure,”says Chen.

1. Guan, P., Chen, M. & Egami, T. Stresstemperature scaling for steady-state flow inmetallic glasses. Physical Review Letters
104, 205701 (2010).

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© 2011 Tohoku University

Research highlights

Polymers

Atomic mapping
Atomic force microscopy reveals the true surface topography and nanomechanicalproperties of copolymer films

The physical properties of polymers areheavily dependent on their molecular
structure, and much research effort isexpended in obtaining a certain desired property, such as high strength
or elasticity. Molecular structures areoften tailored by preparing copolymers combining two or more different
monomer building blocks. In one kindof copolymer called a ‘block’ copolymer,
the different monomers form separate,
homogeneous blocks rather than combining in a random arrangement. This
block formation usually results in phaseseparation — the tendency for monomers to cluster with other monomersof the same type. Gaining insight into
the specific physical properties of block
copolymers therefore usually requires
an understanding of the compositionand properties of the separate blocks.
Dong Wang and co-workers fromthe WPI-AIMR at Tohoku University1
have now used atomic force microscopy (AFM) to map the mechanicalproperties of a block copolymer film.
“Other techniques such as transmission
electron microscopy have been used
for a long time,” explains Toshio Nishi
from the research group, “but such techniques only provide information on the
structure, not the physical properties.”
The radiation and staining techniques
necessary for electron microscopy canalso damage the polymer.
Atomic force microscopy involves
scanning an atomically sharp tip acrossa material surface. The deflection of
the tip provides a measure of the topography or surface shape with atomic
resolution. “Our technique allows us tosimultaneously measure the mechanical properties of the polymers, such as

250 nmA processed AFM image of a copolymer film showing the variation in the Young’s modulus (stiffness) from soft(red) to stiff (green). The soft and stiff regions can be assigned to separate phases of the block copolymer.

adhesiveness and the stiffness or Young’s
modulus, while at the same time mapping the topography of the surface,”
explains Nishi.
Earlier attempts to use AFM for thispurpose have run into problems becauseof the tendency of the phases in block copolymers to separate. As softer materials
like copolymers can be easily deformed,
different phases tend to react differently
to the AFM tip, giving a false impressionof the topography. Nishi and his team
were able to correct for this by measuring
both the deflection of the AFM tip andthe displacement of the scanner holding
the copolymer sample. In doing so, they
showed that the copolymer surfaces were

AIMResearch 2010
© 2011 Tohoku University

much smoother than originally thought.
As the mechanical properties of theblock copolymer components are wellknown, simultaneous mapping of thephysical properties allows the lightand dark regions in the AFM image(pictured) to be assigned to the different
phases of the block copolymers — an assignment that in the past has been madeon a rather arbitrary basis.

1. Wang, D., Fujinami, S., Nakajima, K. & Nishi, T.
True surface topography and nanomechanicalmeasurements on block copolymers with atomic
force microscopy. Macromolecules
43, 3169–3172 (2010).

15

Research highlights

Photonics

Bright white light
Making way for a new generation of light-emitting diodes based on zinc oxide

16Green phosphor© 2010 AIPWhite light-emitting diodes (LEDs) are
replacing incandescent light bulbs ina variety of applications, including car
headlights. An issue with the current
generation of white LEDs, however, isthat their color balance tends towarda bluish hue. Researchers from the
WPI-AIMR at Tohoku University incollaboration with other institutionsin Japan have now demonstrated that
LEDs constructed from zinc oxide(ZnO) compounds could solve thisproblem while also being relatively easy
to fabricate1.
“The solid-state lighting market has
been expanding rapidly, and now exceeds several billion dollars. I think there
is room for ZnO LEDs to penetrate thismarket, particularly because of their potentially very cheap fabrication process,”comments Masashi Kawasaki, the WPIAIMR researcher who led the team.
The best color balance is achieved inwhite LEDs when the light-converting
phosphors inside the diode are excitedwith ultraviolet (UV) light, rather thanthe blue light used in the LEDs available
on the market today. Shifting the blue
emission of gallium nitride (GaN), usedin conventional blue LEDs, to UV isdifficult to achieve because of a lack ofsuitable substrates that could be fabricated at practical cost. Zinc oxide, on theother hand, can be grown easily on commercially available, UV-compatible ZnO
substrates, but preparing high-quality
light-emitting films using standard
industrial growth processes has beenchallenging. For example, a laser deposition technique developed previously
for the fabrication of ZnO-based LEDs
requires extreme processing conditions
and large variations in temperature,p-MgZnO:N
n-ZnO
n-ZnO substrate
In

Schematic illustration of a ZnO-based UV-emitting LED with a green phosphor, and a photograph of theoperating LED.

making the approach unsuitable forlarge-scale production.
Instead, Kawasaki’s team used a‘molecular-beam epitaxy’ process — awidely used technique that grows
uniform atomic layers by exposing asubstrate to ultrapure atomic clouds ofcertain elements. The researchers deposited thin films of ZnO with magnesium
to shift the native blue emission of ZnO
to UV, and by optimizing the growth
process and structural details of thedevice, they were able demonstrate very
efficient UV emission.
For commercial use, the efficiency ofthe devices still needs to be improvedby a factor of about 100. While this may
seem substantial, the present device
is already 10,000 times more efficientthan earlier ZnO-based LEDs. “Theremaining efficiency gap comparedwith GaN LEDs could easily be closedwith improvements such as optimizing
the electric contacts to the LEDs,” says
Kawasaki. Once that happens, he is convinced that ZnO will not only challenge
the dominance of GaN but also lead tothe development of efficient UV lasers.

1. Nakahara, K., Akasaka, S., Yuji, H., Tamura, K.,
Fujii, T., Nishimoto, Y., Takamizu, D., Sasaki, A.,
Tanabe, T., Takasu, H., Amaike, H., Onuma, T.,
Chichibu, S. F., Tsukazaki, A., Ohtomo, A. &Kawasaki, M. Nitrogen doped MgxZn1−xO/ZnO
single heterostructure ultraviolet light-emitting
diodes on ZnO substrates. Applied Physics
Letters 97, 013501 (2010).

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© 2011 Tohoku University

Research highlights

Atomic-scale imaging

Stepping up the pace
An optical microscope that captures the dynamic motion of surface atomic layers
provides a new tool for the atomic study of electrode materials

Electrochemical cells rely on conductive
electrodes to convert chemical energy
into electrical energy, and any flaws inthese materials — such as etched or corroded metal surfaces — can detract fromdevice performance. A good understanding of electrochemistry at an atomic levelis needed to circumvent these issues, butsuch studies have always been challenging. Kingo Itaya and colleagues fromthe WPI-AIMR and the Department ofApplied Chemistry at Tohoku University1
have now developed an optical techniquethat makes it possible to visualize gold
electrodes in real time as they dissolve,
layer by atomic layer.
Although scanning probe microscopes have been used previously to
trace out atomically resolved imagesof metal electrode surfaces, these techniques have limitations. Imaging areas
are typically only a few tenths of a squaremicrometer in size, and each scan takes
several minutes, making it difficult toinvestigate the electrochemical reactions
that take place across an entire electrode.
The probe tip itself can also interferewith the surface processes.
To overcome these problems, Itaya
and his team turned to a system thatcombines a laser confocal microscope(LCM) with a differential interference
contrast microscope (DIM). In thismethod, a laser beam is passed througha series of prisms and polarizing filtersthat rejects all light except for that
focused on a very tiny portion of thesurface, producing an image with highdepth resolution. By using mirrors to
rapidly scan the beam over the sample,
the LCM-DIM technique can build athree-dimensional surface representation in very short time.

0 min

15 min

10 μm
Au + 4Cl–[AuIII Cl4]– + 3e–

10 μm
Cl–

AuCl4

(Upper) LCM-DIM images of an atomically terraced gold surface. Images on the right show the atomic etching of
the island and terrace edges while the surface is immersed in etchant. (Lower) Etching proceeds by the reactionof chlorine ions (Cl–) with gold atoms (Au) to produce free gold–chloride complexes (AuCl4) and electrons (e–).

The research team applied LCM-DIM
to the visualization of gold surface layers,which consist of small ‘terrace’ domains
with steps barely a quarter of a nanometer high. Resolving the individual
layers required a few innovations: theteam replaced the polymer coatings
on the LCM-DIM filters with silver
nanoparticle films to increase the lightintensity by an order of magnitude, andthey came up with a new method using
a hydrogen–oxygen flame and sphericalgold beads to create extremely smoothsurfaces with more distinct terraces.
In just seconds, the LCM-DIM
system provided clear pictures of thesurface steps and terraces on the gold
electrodes (pictured). The team then observed in real-time the retraction of theatomic terraces as they dissolved when

AIMResearch 2010
© 2011 Tohoku University

immersed in an acid chloride solution,
representing the first direct observational evidence of the electrochemical
etching mechanism.
“We believe that LCM-DIM could
become the main technique for investigating electrochemical processes
at solid/liquid interfaces,” says Itaya,“potentially leading to improvements insystems such as lithium-ion batteries andwet-chemical processes for large-scale
integrated circuits.”

1. Wen, R., Lahiri, A., Azhagurajan, M., Kobayashi,
S.-I. & Itaya, K. A new in situ optical microscopewith single atomic layer resolution for observation of electrochemical dissolution of Au(111).
Journal of the American Chemical Society
132, 13657–13659 (2010).

17

Research highlights

Clathrates

Rattling the cage
Atomic cage structures make excellent thermoelectric materials for converting heat
into electricity

18

Guest atom
(Sr or Ba)

Ge© 2010 AIPMany industrial processes, such as theinternal combustion engines in cars,
generate significant amounts of heat. If
some of this heat could be converted into
usable energy in the form of electricity,
energy consumption could be reduced
considerably. Thermoelectrics are a classof materials that convert heat to energy,
and unsurprisingly, they are the subjectof intensive scientific investigation.
Researchers from the WPI-AIMR at
Tohoku University in collaboration withresearchers from the university’s departments of physics and chemistry have
now investigated how the movement of
atoms in a particularly promising classof thermoelectrics called clathrates caninfluence their thermal properties1.
“Clathrates are one of the most promising candidates for efficient thermoelectric energy conversion,” says Katsumi
Tanigaki, who led the research team.
Clathrates are cage-like compounds
that enclose trapped guest atoms (pictured). The size of the guest atom in
relation to the host structure determines
how freely it can move and ‘rattle’ in its
cage. These rattling motions suppress
heat transmission and therefore have astrong influence on heat conductivity.
As thermoelectricity is based on heatdifferences within a material, lower heat
conductivity leads to better thermoelectric performance. The open structuresof clathrates are well-suited for this purpose, particularly when the guest atom
is small and therefore interacts less withthe surrounding cage as it vibrates.
In their systematic study, the researchers carefully investigated two similarclathratecompounds:
Sr8Ga16Ge30(SGG), which contains a strontium
guest; and Ba8Ga16Ge30 (BGG), whichGa
The structure of BGG and SGG clathrates. The clathrate cages are formed by gallium (Ga) and germanium (Ge)atoms, with strontium (Sr) or barium (Ba) atoms within the cages.

houses a barium guest. Strontium issmaller than barium, and consequently
SGG is the better thermoelectric.
An important contribution to theheat conductivity of clathrates is the
movement of the guest atom within aparticular cage. To reach a good understanding of this process, it is important
to distinguish the heat capacity contribution of atomic movements from thatof electron transport. Through careful
preparation of samples with different
electron concentrations, the researchers
were able to separate the influence ofelectrons on thermal properties fromthe effect of strontium and barium
atomic motion.
As the origin of SGG’s excellent thermoelectric properties, the researchers
identified that the off-center vibration ofstrontium atoms in the cage — as opposedto the centered vibration of the barium
guest — leads to enhanced coupling between strontium atom movement andelectrons in the crystal. This proved to be
the key to understanding the enhanced
thermoelectric performance of SGG,
says Tanigaki. “The anharmonic movement of the guest atoms is responsible
for their poor thermal conductivity, andcould provide a blueprint for enhanced
thermoelectric materials.”

1. Xu, J., Tang, J., Sato, K., Tanabe, Y., Miyasaka,
H., Yamashita, M., Heguri, S. & Tanigaki, K.
Low-temperature heat capacity of Sr8Ga16Ge30
and Ba8Ga16Ge30: Tunneling states and electronphonon interaction in clathrates. Physics Review
B 82, 085206 (2010).

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© 2011 Tohoku University

Research highlights

Single-molecule imaging

Finished in chrome

By combining high-resolution atomic
force microscopy (AFM) with extensive
theoretical modeling, Thomas Trevethan
and Alexander Shluger from the WPIAIMR at Tohoku University, alongwith co-workers from the University of
Hamburg in Germany and University
College London in the UK, have determined exactly where a magnetizable
metal–organic complex attaches to an
insulating surface — a critical step in thedevelopment of applications like ultradense molecular logic circuits1.
To use single molecules as tiny device
elements, scientists must first deposit
them on insulating surfaces, such as
sodium chloride, to ensure that they areelectrically isolated. These insulating
surfaces, however, make it exceptionallyhard to image the locations and orientations of adsorbed molecules — factorsthat can have a considerable influence on
overall device behavior.
One way to achieve atomic-scale
imaging of insulators is with AFM, atechnique that uses an atomically ‘sharp’oscillating tip to physically trace out
surface topography. Even so, achieving
simultaneous resolution of the structureof both the adsorbed molecules and thesubstrate underneath is no easy task, according to Trevethan. “Usually, when the
microscope tip is brought close enoughto the surface to resolve individual atoms,
the adsorbed molecules are pushed outof the way due to strong interaction withthe tip itself,” he explains.
The researchers solved this problem bycoating a supersharp AFM tip with a layerof chromium metal atoms that interact
favorably with surface chloride ions. The
resulting tip is able to resolve the topology of the substrate even while remaining© 2010 ACS

Atomic force microscopy using a chromium-coated tip makes it possible to image
molecular-scale devices

Atomic force microscopy image of a magnetizable cobalt–aromatic hydrocarbon molecule (yellow-red)
adsorbed on an insulating sodium chloride surface (blue) with a clearly resolved atomic lattice. The schematic
structure of the adsorbed molecule is superimposed.

quite far from the surface, and thus does
not disturb the adsorbed molecules.
With this new tip, the team studied
the adsorption of Co-salen — a cobalt–
aromatic hydrocarbon complex that has
interesting magnetic properties — on
sodium chloride surfaces. After thesystem was cooled to near absolute zero,
the chromium-coated tip produced exceptionally clear images (pictured) of theadsorbed molecules, which were found to
adopt up to sixteen different orientations
relative to the underlying ionic lattice.
To understand why Co–salen has
such a range of possible adsorption
geometries, Trevethan and colleagues
turned to quantum chemical calculations. “Co–salen is quite large and tomodel it accurately, we would normally

AIMResearch 2010
© 2011 Tohoku University

have to consider a very big system,” says
Trevethan. The researchers designed anovel method that treats only a smallpart of the surface. This approach revealed that the numerous orientations
arose from subtle interplay between theCo–salen complex and the atoms of theunderlying ionic surface.
The researchers plan to use theirchromium-coated tip to study insulating surfaces with interesting magnetic
properties, such as nickel oxide.

1. Lämmle, K., Trevethan, T., Schwarz, A., Watkins,
M., Shluger, A. & Wiesendanger, R. Unambiguous
determination of the adsorption geometry ofa metal–organic complex on a bulk insulator.
Nano Letters 10, 2965–2971 (2010).

19

Research highlights

OxidesApproaching perfection

Two-dimensional systems confine electrons to a single plane, and in doing so
produce a whole host of intriguing electrical phenomena. Masashi Kawasaki fromthe WPI-AIMR at Tohoku University
and co-workers from a variety of otherinstitutions in Japan have now observed
one such effect — the fractional quantum
Hall effect — in an oxide material1.
The generation of quantum transportphenomena requires extremely smoothinterfaces because such systems arenecessarily thin and therefore highly sensitive to any imperfections or roughness.
As oxide surfaces have a tendency to bequite rough, achieving such quantum
phenomena in oxide systems is particularly difficult.
Kawasaki and his team used a technique known as molecular-beam epitaxy
to slowly deposit a 300 nanometer-thicklayer of magnesium-doped zinc oxide
onto zinc oxide without introducing
any impurities. The electron mobility,a measure of how quickly electrons canmove under an electric field, of theirultraflat film was six times higher thanthat achieved previously for oxides.
“We made zinc oxide that is as clean
as advanced semiconductors so thatelectrons can move without scattering,”
explains Kawasaki.
The fractional quantum Hall effectarises because of the collective behaviorof the electrons confined in a two-dimensional system. The electrons behave as if
they are a composite particle composedof an electron and three magnetic-flux
quanta (see image). The tell-tale signatureof this behavior is that the Hall resistanceof the material — the transverse electrical resistance in the two-dimensional
plane — jumps in discrete steps as the20© 2010 M. Kawasaki

Precise material growth has now enabled observation of the fractional quantum
Hall effect in an oxide

Schematic illustration of the fractional quantum Hall effect. Electrons confined to two dimensions under amagnetic field behave like composite particles composed of an electron and three magnetic-flux quanta. Thiseffect has now been observed in a thin film of zinc oxide.

field is increased. These steps appear atvalues equal to the ‘von Klitzing’ constantdivided by either an integer or a fraction.
Integer steps, which have been seen inoxides before, are the result of different,
and somewhat simpler, underlying physics, producing what is known as the integer quantum Hall effect. The high qualityof the thin film produced by Kawasaki
and his team has enabled the observationof fractional steps in an oxide for the
first time.
The fractional quantum Hall effecthas already been seen in other materials such as silicon and gallium arsenide,
but Kawasaki hopes that such clean
zinc oxide films might soon enable theobservation of a new effect. “Electronsin zinc oxide behave as though they
are heavier than those in silicon or gallium arsenide. The heavier the electrons
are, the more they repulse each other,and this makes the correlation effects
more pronounced. It might enable thecreation of even-denominator statessuch as 5/2, which could be used forquantum computation.”

1. Tsukazaki, A., Akasaka, S., Nakahara, K., Ohno, Y.,
Ohno, H., Maryenko, D., Ohtomo, A. & Kawasaki,
M. Observation of the fractional quantum Hall
effect in an oxide. Nature Materials
9, 889–893 (2010).

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© 2011 Tohoku University

Research highlights

Iron-based superconductors

Spin matters

Superconductors were first discovered
almost 100 years ago, but much about
the origins of superconductivity itself
remain a mystery. It is known that below
a critical temperature, superconducting compounds enter a state in whichelectrons form pairs and the electricalresistance of the material vanishes. In
the classical, low-temperature class ofsuperconductors, this pairing is thought
to be induced by interactions between
electrons and resonant vibrations of
the atomic lattice, known as phonons.
Such a mechanism, however, is too
weak to account for the emergence of
pairing at the higher superconducting
transition temperatures of the recently
discovered class of cuprate and ironbased superconductors.
Mingwei Chen and co-workers fromthe WPI-AIMR in collaboration withresearchers in China1 have now obtained
strong evidence that the previously theorized role of spin in phonon–electroninteractions is responsible for electron
pairing in the high-temperature ironbased superconductor SmFeAsO.
SmFeAsO — an iron arsenide compound called a pnictide — only becomesa superconductor when ‘doped’ withfluorine and cooled to below 55 K. Usinga technique known as Raman spectroscopy, which can detect the various
types of resonances in an atomic system,
Chen and his team looked for differences
between doped and undoped SmFeAsO
as they varied the temperature. They
found that the Raman spectra for the twosystems were very similar above 130 K,
but below this temperature a new peak
emerged only in undoped SmFeAsO.
“The new Raman mode is closely correlated with the antiferromagnetic© 2010 ACS

Experimental evidence supports spin-dependent interactions between electrons andlattice vibrations as the origin of superconductivity in an iron-based superconductor

Crystallographic structure of SmFeAsO showing iron (red) and arsenic (purple) atoms. White arrows show
the spin orientation, yellow arrows show the new phonon mode, and the white and blue boxes outline themagnetic and crystallographic cells.

ordering in the material, in which thespins of neighboring atoms are aligned
in opposite directions,” says Ling Zhang,
who took part in the study.
Indeed, all the pieces fit together:
SmFeAsO is known to become antiferromagnetic below 130 K, and thetemperature-dependent change in the intensity of this peak can be well describedby a model involving a contribution fromspin. It is also known that fluorine doping
destroys the antiferromagnetic ordering,in good agreement with the observationthat the new Raman peak is absent fromfluorine-doped SmFeAsO.
To understand the details of the originof the new Raman peak, Pengfei Guan
from the research group calculated the

AIMResearch 2010
© 2011 Tohoku University

phonon dispersion of the material. His
results showed that the unusual mode
arose from the formation of a spin superstructure rather than a structural phasetransition (see image).
The importance of the results lies inthe solid confirmation of what many
theories have hypothesized before. “Ourstudy provides the first clear-cut evidenceof spin-dependent electron–phonon
coupling in the pnictides,” says Chen.

1. Zhang, L., Guan, P.F., Feng, D.L., Chen, X.H., Xie, S.S.
& Chen, M.W. Spin-dependent electron–phonon
interaction in SmFeAsO by low-temperature
Raman spectroscopy. Journal of the American
Chemical Society 132, 15223–15227 (2010).

21

Research highlights

Topological insulators

A large leap ahead
The discovery of a topological insulator with a large band gap suggests that roomtemperature operation is possible for these promising spintronic materials

22

ky

EF

kx
Bindnig energy (eV)

0.2

0.4

High
0.6

0.8
–0.10

Low
0.0

© 2010 APS

Spintronics is a promising alternative
to conventional electronics: the use ofan electron’s spin instead of its electrical charge offers a relatively easy andenergy-efficient way of manipulating
information. Unfortunately, these spins
don’t travel very far without the information they carry being destroyed. A newclass of materials known as topologicalinsulators could offer a solution to thisproblem by allowing almost unhinderedspin transport, but owing to fabrication and materials issues, none of the
topological insulators known so far
have clearly demonstrated this ability.
Researchers from the WPI-AIMR andthe Department of Physics at Tohoku
University, in collaboration with colleagues from Osaka University, have now
discovered a material that could alleviate
such problems1.
As their name suggests, topologicalinsulators have to be electrically insulating — that is, have an energy gap in theirelectronic states that electrons cannot
overcome. Their unique properties ariseon the surface, where they are electricallyconducting. These conducting surface
states are topologically protected, whichmeans that electron spins are protectedfrom degradation by, for example, thepresence of impurities in the material.
So far, only a limited number of topological insulators have been discovered,among them Bi2Se3. The problem with
virtually all of the existing compounds
is that their energy gap is too small, andso they are in fact electrically conducting
in the bulk. The researchers have now
discovered a new topological insulator, TlBiSe2. They found that the use ofthallium considerably widens the bandgap, leading to a topological insulator

0.10

ky (Å–1)
The tell-tale crossing of electronic states in TlBiSe2 is evidence of the protected surface states.
Left: experimental ARPES intensity data (k, electron momentum). Right: three-dimensional theoretical model.

with the largest known energy gap. The
existence of protected surface states was
confirmed by the tell-tale crossing of
surface electronic states (pictured). “Theenhanced material properties of thiscompound strengthen the foundation
for high-speed, low-power consumption devices powered by topologicalinsulators,” says Seigo Souma from theresearch team.
Like most other topological insulators, the samples fabricated fromthis compound are still electricallyconducting throughout. According toSouma, this most likely results frommismatches in sample composition, andcould potentially be solved by tuning
the ratio of thallium and bismuth in
the samples. The quality of the samples

prepared is sufficient to allow the precise
investigation of their properties using
the ultrahigh-resolution angle resolved
photoemission spectrometer (ARPES)
installed at Tohoku University. In any
case, the insight gained in this study
will also aid the search for new, related
topological insulators, says Souma. “Thediscovery of a topological insulator inthis TlBiSe2 ternary compound indicatesconsiderable potential for the discovery
of further topological insulators.”

1. Sato, T., Segawa, K., Guo, H., Sugawara, K., Souma,
S., Takahashi, T. & Ando, Y. Direct evidence forthe Dirac-cone topological surface states in the
ternary chalcogenide TlBiSe2. Physical Review
Letters 105, 136802 (2010).

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Research highlights

Superconductors

A sound solution
The vibrations of atoms in cage structures point to a strategy to increase thetransition temperature of superconductors

Superconductors are of significant
technological as well as scientific interest. Their ability to transport electrical current without resistance could
provide significant energy savings, yet
the known superconductors only display their superconducting properties
at temperatures close to absolute zero.
Although the recent discovery of a ‘hightemperature’ class of superconductors,
which despite their name only becomesuperconducting below 100 °C, suggeststhat room-temperature superconductivity might one day be possible, scientists
still do not understand their physics sufficiently well to raise the zero-resistancetransition temperature of these compounds significantly.
Returning to the classical, low-temperature superconductors, researchers fromthe WPI-AIMR in collaboration withcolleagues from Hiroshima University
and Yamaguchi University in Japan have
now unveiled the possibility of increasing
the transition temperature by judicious
design of the material’s crystal structure1.
There are fundamental differences between the mechanisms of superconductivity in classical and high-temperature
superconductors. In the classical case, superconductivity arises from the interaction between electrons and vibrations of
the crystal lattice, known as phonons. In
the high-temperature case, superconductivity is thought to arise through interactions between the electrons themselves,
mediated by their magnetic properties.
The classical superconductors studied
by the researchers have a clathrate crystal
structure (pictured) formed by a networkof silicon or germanium cages that encapsulate barium guest atoms. The barium
guests are free to ‘rattle’ within the cage,

The three-dimensional clathrate structure of Ba24Si100 and Ba24Ge100. Both clathrates have the same polyhedral
network characterized by a framework of pentagonal dodecahedra embedded in a three-dimensional channel
labyrinth. The larger cage of the Ge100 clathrate allows the guest barium atoms to rattle more freely.

producing a particular type of phononthat can interact strongly with electrons.
“The rattling motions in these cage materials are currently drawing attention as apossible way to increase superconducting
temperatures,” says Katsumi Tanigaki
from the research team.
The researchers revealed the influence of the cages on superconductivity
through a detailed comparison between
the properties of two clathrates, Ba24Si100
and Ba24Ge100, both of which are superconductors below their transition
temperature. Of the two compounds,
Ba24Ge100 has the larger cage and thus
the stronger rattling motion. It would
therefore be expected that it also has thehigher transition temperature — but thisis not the case. Its cage structure, it turns
out, is too weak and distorted, which

AIMResearch 2010
© 2011 Tohoku University

degrades its superconducting properties.
In principle, the findings point to thepossibility of enhanced properties. “Ourwork suggests that a higher superconducting temperature may be realized if
sufficiently strong cage materials can be
designed,” comments Tanigaki. While
the transition temperatures of clathrate
superconductors are low at present, tuning the design of their cage structure maylead not only to higher superconducting
temperatures in clathrates, but also to
new strategies to improve other classicalsuperconductors.
1. Tang, J., Xu, J., Heguri, S., Fukuoka, H., Yamanaka,
S., Akai, K. & Tanigaki, K. Electron–phonon
interactions of Si100 and Ge100 superconductors
with Ba atoms inside. Physical Review Letters105, 176402 (2010).

23

Research highlights

Nanostructured catalysts

Golden openings

Gold prices have skyrocketed in thepast few years, but a study by researchers from the WPI-AIMR and others at
Tohoku University1 promises to makethis precious metal even more valuable.
Naoki Asao, Yoshinori Yamamoto andtheir co-workers have found that nanoporous gold — a material produced by
selectively leaching silver atoms froma gold–silver alloy film — can catalyze
an important step in the production ofsilicon-based polymers under milder
and ‘greener’ conditions than those required in conventional methods.
In its bulk state, gold has a familiar
luster that doesn’t tarnish or discolor
because it is chemically unreactive. At
the nanoscale, however, the confinedelectrons of gold atoms can begin to oscillate together, imparting new colors to
the metal. “When we make nanoporous
gold with pores around 100 nanometersin size, it has a reddish gold color,” says
Asao. “As the pore size becomes smaller,
the color becomes deeper, clearly indicating a change in electronic properties
from the original thin gold film.”
This altered electronic behavior ofnanoscale gold also makes it an active
catalyst: gold nanoparticles supported
on a solid substrate are known to catalyze a variety of reactions, providing analternative to many toxic reagents in industrial processes. Unfortunately, thesematerials suffer from a limited lifespan:
eventually, gold nanoparticles agglomerate on the surface of the solid support
and lose their reactivity. In light of thisissue, attention has recently turned tonanoporous gold films.
The researchers investigated thecatalytic performance of nanoporous
gold for the oxidation of organosilanes,24© 2010 Wiley-VCH

Thin gold sheets with nanoscale pores act as durable and reusablegreen catalysts

A nanoporous gold substrate can catalyze the oxidation of organosilane compounds at room temperature,providing a promising alternative to existing reagents.

which contain silicon–hydrogen bonds(see image). On oxidation, a hydrogen
atom on the silicon center is replaced bya hydroxyl group, making the molecule
easier to combine into silicon-based
polymers. However, the reactions typically used to achieve this result do notoxidize certain substrate types, tend to
produce unwanted byproducts, or involve lengthy post-treatment processing.
The researchers found that immersing the nanoporous gold film in an
aqueous organosilane solution at room
temperature catalyzed the oxidation ofsilicon–hydrogen bonds with nearly
quantitative efficiency. Furthermore, thecatalyst was easily recovered and reused
several times with no change in either
reactivity or its nanopore structure.

The researchers are now investigating
the mechanisms of nanoporous gold
catalysis, and expect that this approach
will have a major impact in the fieldof sustainable chemistry. “We believethat nanoporous gold will be useful formolecular transformations of natural
gas and bioethanol for green chemistry,” says Asao. “Nanoporous materials
chemistry will surely develop much
more in the future.”

1. Asao, N., Ishikawa, Y., Hatakeyama, N., Menggenbateer, Yamamoto, Y., Chen, M., Zhang, W. &Inoue, A. Nanostructured materials as catalysts:
Nanoporous-gold-catalyzed oxidation oforganosilanes with water. Angewandte ChemieInternational Edition 49, 10093–10095 (2010).

AIMResearch 2010
© 2011 Tohoku University

Research highlights

MetalsBringing glass to order

In contrast to most metals in whichatoms are arranged in a repeated crystal
pattern, the atoms in metallic glasses do
not have long-range order. This gives
them certain desirable characteristics,
including resistance to wear and corrosion, and high strength. The continued
improvement of metallic glasses would
benefit greatly, however, from a detailed
understanding of their atomic structure.
Various atomic models for metallic
glasses exist, yet none have been confirmed because the experimental characterization of metallic glasses has been
limited to the acquisition of average dataover relatively large volumes. Now, in acollaboration between the WPI-AIMR
and the Institute for Materials Researchat Tohoku University, Osaka University
in Japan and the Grenoble Institute ofTechnology in France, Mingwei Chen,
Akihiko Hirata and colleagues havecharacterized the atomic structure ofmetallic glasses with unprecedented
resolution by combining ‘nanobeam’
electron diffraction with molecular dynamics simulations1.
The researchers began by characterizing the metallic glass Zr66.7Ni33.3 using
standard electron diffraction and electron microscopy tools. As expected, nodiffraction spots were observed, indicating a lack of regular atomic order, andno order could be detected by electron
microscopy. Unsatisfied with the finding, Chen, Hirata and their co-workers
began work to make the electron beam
narrower in order to resolve finer atomic
structures. By correcting for sphericalaberrations in their electron optics andusing a specially designed electron beam
condenser aperture, they were able toreduce the diameter of their electron

From Ref. 1. © 2010 M.W. Chen

Diffraction patterns obtained using an electron beam only a few angstroms wide
have revealed local order in otherwise amorphous metallic glasses

A

A typical narrow-beam electron diffraction pattern for the metallic glass Zr66.7Ni33.3. The distance to the spot A
can be used to model the short-range atomic order.

beam to about 3 angstroms (Å) or 0.3
nanometers, the narrowest coherent
electron beam demonstrated so far.
This much narrower beam produced adistinct set of diffraction spots (pictured)that could only result from a single crystal, confirming predictions that metallic
glasses are composed of small ordered
atomic clusters as fundamental structural units, even though these materials
are disordered on larger scales. By scanning their narrow beam around theirsample and using a low-power beam to
avoid damaging the atomic structure,
the team was able to sample hundredsof ordered clusters, each about 4–7 Å insize. The data showed that the average
atomic spacing inside an ordered atomic
cluster was 2.4 Å. A molecular dynamics

AIMResearch 2010
© 2011 Tohoku University

numerical model reproduced key features of the experimentally measured
diffraction pattern, confirming that local
order was indeed being observed.
According to Chen, this work could
have consequences in both research andapplications of metallic glasses. “We can
now begin to propose a reliable atomic
structure model for new metallic glass
design, aiming for better forming ability
and a better understanding of important
physical processes such as glass transition and deformation,” he says.

1. Hirata, A., Guan, P., Fujita, T., Hirotsu, Y., Inoue,
A., Yavari, A.R., Sakurai, T. & Chen, M.W. Direct
observation of local atomic order in a metallic
glass. Nature Materials 10, 28–33 (2011).

25

Research highlights

OxidesLess is more
Inserting insulating layers into an oxide can make it more conductive

Thin oxide films are used in applications
ranging from solar cells to transistors.
They are made useful in part by the ability to tune many of their key properties,
including conductivity and transparency. Some oxides can be changed fromhighly insulating to highly conductive byaltering their structure or ‘doping’ themwith foreign atoms. Zhongchang Wangand co-workers in the laboratories of
Yuichi Ikuhara and Masashi Kawasaki
at Tohoku University’s WPI-AIMR in
collaboration with researchers from the
University of Tokyo and the Japan Fine
Ceramics Center have now demonstrated a counter-intuitive way to induce
an oxide to become conductive1.
The research team studied the conductivity of a thin film of a layered oxide
composed of lanthanum, strontium andtitanium. Using pulsed laser deposition,
they were able to construct the film
one atomic layer at a time with control
over the nature of each layer. They
focused on examining the effects ofinserting insulating layers of SrTiO3 — aceramic commonly used in electronic
applications. Surprisingly, they foundthat the oxide underwent a transition
from insulating to conductive behavior
when a higher density of the insulating
layers was included. This conductivity appeared without any change in thedensity of charge-carrying electrons,
showing that the state was not the resultof doping — a standard technique forraising conductivity.
Wang and his colleagues investigatedthe effect of inserting the additional
insulating layers using a combination of
scanning electron microscopy (micrograph pictured) and numerical simulations. Their studies showed that the26

5Å
Scanning transmission electron microscopy image of the layered oxide film. Bright spots represent strontium,and dimmer spots correspond to titanium.

addition of the insulating layers reduced
distortion and bond strain in the oxidefilm. Distortion and strain can lead toinsulating states by affecting the interactions between the electrons and thelattice, as well as the interactions among
the electrons themselves. By allowing thematerial to relax to a less-strained state,
the researchers were able to increase
its conductance. Their data show that
this counter-intuitive behavior arises
from the creation of a conductive twodimensional layer inside the oxide.
The results represent a new route toobtaining metallic behavior in materials that are normally expected to be
insulators. The research team expects
their finding to accelerate the search forsuperconducting and thermoelectricmaterials in oxides and other insulating
materials. “Our first priority now is toconsider whether superconductivity
can be elicited in this particular oxideby reducing lattice disorder and structural defects so as to lower the barrier tocharge carrier motion,” says Wang. The
researchers believe that the material may
also find use in high-mobility transistor
and thermoelectric applications.

1. Wang, Z., Okude, M., Saito, M., Tsukimoto, S.,
Ohtomo, A., Tsukada, M., Kawasaki, M., Ikuhara, Y.
Dimensionality-driven insulator–metal transitionin A-site excess non-stoichiometric perovskites.
Nature Communications 1, 106 (2010).

AIMResearch 2010
© 2011 Tohoku University

IN THE SPOTLIGHT
The WPI-AIMR has grown rapidly since its inauguration in 2007,
now with over 100 leading researchers from all over the world,
including 32 internationally renowned Principal Investigators
who are charged with pioneering new and innovative breakthroughs in materials science. The institute is also active indeveloping young, promising researchers with a focus on strong
cross-disciplinary collaboration and creativity. AIMResearch
spotlights these talented researchers of the present and future,detailing their daily research activities and scientific ambitions.

© 2011 Tohoku University

IN THE SPOTLIGHT

DIRECTOR’S INTERVIEW

Published online 25 October 2010

Time to shake things up
As the WPI-AIMR celebrates its third anniversary in October 2010, Director Yoshinori Yamamoto shares his vision
for the institute’s future.

T

he WPI-AIMR first opened its
doors in 2007 as one of the first of
five World Premier International
research centers to be created under a new
initiative of the Ministry of Education,
Culture, Sports, Science and Technologyof Japan and administered by the Japan
Society for the Promotion of Science. The
WPI-AIMR has successfully navigated its
first three years of life. Now, having established itself as a leading center of materials
research in Japan and with a growinginternational reputation, attention within
the WPI-AIMR is turning to the next
phase of development. AIMResearchtalked to the institute’s director, Yoshinori
Yamamoto, an internationally renowned
chemist, about the successes and challenges of his three years at the helm andthe path that he has laid out for the future
direction of the institute.
The WPI-AIMR has a broad and interdisciplinary remit. What do you think stand out
as the institute’s biggest achievements to date?

The WPI-AIMR program has two main
objectives: one is to develop new innovative materials through the fusion ofresearch, and the other is to establish a
new management system that best suits
WPI-AIMR. However, achieving theseobjectives is not so easy.
We have research groups working inbulk metallic glasses, materials physics,
soft materials and devices/systems and allof them have made great progress over thelast three years. In bulk metallic glasses,
for instance, we have published a paper
about the controlled formation of metallic
glass nanowires in Advanced Materials1.
Nanowires are useful materials for a wide

28

range of applications and we were thefirst to make metallic glass nanowires.
In materials physics, we have published apaper concerning the mechanism behindsuperconductivity in Europhysics Letters2.

WPI-AIMR Director Yoshinori Yamamoto would like to
see more mixing among WPI-AIMR researchers.

We have the most powerful angleresolved photoemission spectrometerin the world and we used it to show thatelectron spin plays an important role insuperconductivity. The paper has so far
received more than 300 citations. In soft
materials, we are making nanoporous palladium catalysts. In devices/systems, weare investigating the use of bulk metallic
glasses as soldering materials in microelectromechanical systems.
Setting up the center and guiding it
through its initial phase could not havebeen easy. What were some of the challenges and how did you overcome them?
Asking researchers to jump outside
their own research discipline was quitea challenge. On the one hand, seniorresearchers prefer to stay in the same
discipline especially when they have established a presence in the community. On
the other hand, young researchers wantto establish their own discipline before
they venture into something new. I thinka good researcher should have a wide viewof science, no matter how challenging it
may be. At the WPI-AIMR, I often see
young researchers jumping between laboratories. There is extensive collaboration
going on and I think this is a very goodthing. However, the fusion of research
is much bigger than just collaboration. I
hope that in the near future, our researchers will go beyond collaboration and create
some entirely new science.
Reforming our management system
was another challenge. Most principal
investigators in Japan believe in equality— everyone should get the same area ofspace, hire the same number of researchers and receive the same salary. Here atthe WPI-AIMR, everything depends onachievement. If your results are excellent,
you can hire more people and get more
space. Such a management model is veryrare in Japan, but I think it suits the WPIAIMR very well.
A key part of the WPI Initiative is theglobalization of research. What plans do
you have for strengthening international
collaboration?
Internationally, we have been collaborating with the University of Cambridge inthe UK in bulk metallic glass research. We
have also started collaborating with theInstitute of Chemistry, Chinese Academyof Sciences (ICCAS) in materials research.

AIMResearch 2010
© 2011 Tohoku University

IN THE SPOTLIGHT

The WPI-AIMR is strong in materials
physics, while the ICCAS is strong in materials chemistry. I think this collaboration
will benefit us both.
Earlier this year the WPI-AIMR was evaluated by a follow-up committee to assess the
progress of the project. What were their key
findings and advice?
Every year, we need to report the progressof the WPI-AIMR program to a follow-up
committee. This year, the program director and members of the working groupof the follow-up committee carried out
a two-day site visit at the WPI-AIMR.
They submitted a very detailed report toupper committee members from Europe,
the United States and Japan. In July, we
had a meeting with the upper committee
members in Tokyo. We gave a 15-minutepresentation and the upper committee
members posed some very serious questions and gave some valuable advice. For

example, they said that our quality ofscience is excellent. However, they also
said that there is room for improvementin promoting the fusion of research andglobal visibility.
The WPI-AIMR will enter its second phasein 2012. What plans does the WPI-AIMR
have for the future?
We have set a roadmap for the WPIAIMR program. In April next year, theconstruction of the WPI-AIMR mainbuilding will be completed. Then in
October, we will have an interim evaluation. This is a very important time as we
will re-evaluate the directorship, the principal investigators and all researchers, andmake any changes that may be necessary.
We will also focus our research on the development of innovative materials, as wehave mentioned in our mission statement.
In 2017, the WPI-AIMR will enter its
third phase. By then, the university must

TECHNOLOGY FOCUS

establish a new organization based on innovative materials discovered through the
WPI-AIMR program.
How will the WPI-AIMR’s new building, tobe completed in April 2011, help with theresearch program?
Right now, some of our researchers areworking in Aobayama campus. They will
all move to the new building at Katahira
campus once the new building is ready. To
do fusion research, the researchers muststay in the same building. The building issort of like a beaker, and my job is to shakethings up.
1. 	Nakayama, K.S., Yokoyama, Y., Ono, T., Chen, M.W.,
Akiyama, K., Sakurai, T. & Inoue, A. Advanced
Materials 22, 872–875 (2010).
2. 	Ding, H., Richard, P., Nakayama, K., Sugawara,
K., Arakane, T., Sekiba, Y., Takayama, A., Souma,
S., Sato, T., Takahashi, T., Wang, Z., Dai, X., Fang,
Z., Chen, G.F., Luo, J.L. & Wang, N.L. Europhysics
Letters 83, 47001 (2008).

Published online 28 June 2010

MEMS technology makes the world amerrier place
Scientists at the WPI-AIMR have developed microelectromechanical systems for a wide variety of applications,
ranging from microphones for television studios to automobiles and airport surveillance equipment.

M

icroelectromechanical systems(MEMS) are miniature devices
containing both electrical andmechanical components. The technology
is an application of microfabrication for
integrated circuits and draws together
expertise in mechanics, optics and materials. “We call this heterogeneous integration,” says Masayoshi Esashi, a principal
investigator at Tohoku University’s WPIAIMR — he is also a global authority on
MEMS technology. “By merging different
technologies together, we can produce

value-added MEMS devices with important core functionality in many systems.”
Tiny devices, large applications
Esashi first encountered MEMS when he
was an engineering student at Tohoku
University in the early 1970s, and has
spent the last 40 years working in thefield. In his career, he has developed technologies for numerous MEMS devices thathave become integral parts of our daily
lives. Video game controllers, inkjet printers, digital cameras and automobiles are all

AIMResearch 2010
© 2011 Tohoku University

Esashi is the pioneer of MEMS technology and a global
authority in the field.

29

IN THE SPOTLIGHT

MEMS on a chipexamples of applications utilizing MEMS
technology in some form or another.
“For example, a Nintendo Wii remotecontroller has a MEMS accelerometerthat captures motion, which allows us tocontrol the user interface and enjoy thegame,” explains Esashi. “And one printerproduced by Canon incorporates a
MEMS print head with 6,000 nozzles foremitting small ink droplets, which allows
it to print high-resolution images.”
Some of Esashi’s latest MEMS creations include an angular motion sensor
developed in collaboration with Toyota
researchers that detects any rotation of
the car and compares it with the expected
speed and angle of steering. When thesensor detects an unexpected spin situation, it automatically applies the brakes.
Other transport-related applications
for Esashi’s MEMS sensors included a
two-dimensional optical scanner that isused in security surveillance equipment
at Tokyo’s Haneda International Airport
and at railway stations.
Advantest, the world’s largest semiconductor testing company, is also using
Esashi’s MEMS switches in their latest
large-scale integration (LSI) testers as areplacement for conventional switches
based on electronic transistors, which aresusceptible to electrostatic damage. The
new MEMS switches are not affected byelectrostatic discharge, making the LSI
testers much more reliable. Elsewhere,
the Japanese television channel NHK
is using Esashi’s MEMS microphones
in environments with particularly highhumidity, such as in television studios forrecording cooking programs.

30

Another important potential application for MEMS technology is in magnetic resonance imaging (MRI). “Wehave developed a thin cantilever holding
an iron particle for detecting extremely
weak magnetic resonance signals. It is
used by the Japanese instrument maker
JEOL,” explains Esashi. “They are using
it for electron spin resonance imaging
on micrometer-sized samples, which will
enable the real-time visualization of theeffects of medicine on cells.”
Small has its advantage
In addition to their small size, MEMS
offer many advantages over conventional
electromechanical devices. Esashi canmake very small devices that provide
extremely high sensitivity or spatial resolution, using extended microfabrication
technology and at a fraction of the cost of
conventional LSI devices.
“LSI fabrication typically involves theuse of a photomask for transferring transistor patterns onto the silicon wafer. The
mask can be very expensive, up to ¥500
million [US$5 million],” notes Esashi.
“Therefore, such advanced technology is
only viable for mass-production applications. For small-volume production, weare developing a MEMS-based masklesselectron-beam exposure system, inwhich an electron beam is used to draw
the transistor pattern on the wafer. The
technology reduces costs and increases
controllability, accuracy and versatility.”

Acting as the bridge
Esashi believes that the future of MEMS
lies in collaboration among academics
and multiple commercial stakeholders — what he terms ‘company + company+ university collaboration’ — and refocusing efforts on practical applications.
“We would like to support the industry by establishing a platform on whichpeople can come together to work. We
educate the researchers from companies.
They learn, and then they go back to theircompanies to commercialize the technology, in many cases through inter-company collaboration. This is our way tocontribute to society indirectly,” he says.
Esashi’s laboratory currently has 40
full-time researchers, including researchers from six companies of the likes ofFuji Film. His aim is to bridge the gap
between pure research and application,
while retaining the challenge and curiosity of scientific investigation. Within
the WPI-AIMR, Esashi promotes joint
research with the institute’s bulk metallic glasses group on MEMS bonding
technology with metallic glass and nano­structured metals.
“Our facilities are like toys. They are
home-made instruments, giving researchers the chance for a fundamental
experience not replicable in a company.
If the researchers succeed in making devices using such equipment, they find itvery rewarding and a lot of fun. This isvery important for the researchers.”

Esashi at his MEMS fabrication unit. His distinctive red cleansuit was given to him by his students to celebrate his
60th birthday.

AIMResearch 2010
© 2011 Tohoku University

IN THE SPOTLIGHT

INTERNATIONAL WORKSHOP

Published online 26 April 2010

Fusion research on the global stage
25–27 March 2010: The World Premier International Advanced Institute for Materials Research (WPI-AIMR) held its
third WPI-AIMR Annual Workshop in Sendai, Japan.

T

he WPI-AIMR Annual Workshop
is a key date in the yearly calendarof the WPI-AIMR when themembers of the center join together withother notable researchers from around theglobe to discuss the latest developments inmaterials science, exchange informationand ideas, and look towards the future.
The 2010 Annual Workshop — thethird in the series — was held from March
25 in the city of Sendai in Japan’s northerly
Miyagi prefecture, the base of the WPIAIMR. The 2010 meeting was attended
by more than 220 researchers from 17
countries, including many senior and internationally renowned materials scientists
from Asia, Oceania, Europe and North
America, who braved the unseasonably
low temperatures to engage in three daysof wide-ranging discussion and debate.
In his welcoming address, WPI-AIMR
Director Yoshinori Yamamoto encouraged
all participants to attend as many different
sessions as possible over the duration of
the workshop. He also pointed out thatthe guiding principle of the WPI-AIMR is‘fusion research’, and that the organizationof the lecture program into several parallelsessions dealing with a mixture of topics
including bulk metallic glasses, nanophysics, nanochembio and devices & systems
was a conscious effort by the organizers toreflect the center’s mission and encouragecross-disciplinary thinking.
The wide variety of parallel sessions
were designed to supplement a programof 18 plenary talks given by leading international materials science researchers,
including WPI-AIMR members based at
institutions outside Japan. In addition, theposter session held on the afternoon of
Friday March 26 drew close to 80 poster

WPI-AIMR Director Yoshinori Yamamoto welcomes
participants to the 2010 WPI-AIMR Annual Workshop

presentations on a wide variety of topics
that were the subject of vigorous debate
among the participants.
The workshop also witnessed thepresentation of the 2010 Acta Materialia
Gold Medal to Akihisa Inoue, presidentof Tohoku University and an International
Advisory Board member at the WPIAIMR. First awarded in 1974, the prize isgiven annually by Acta Materialia Inc. tohonor a researcher who has made a contribution of particular merit to materials
science. Inoue is only the third Japanese
researcher to receive the award, and thesecond from Tohoku University. The
award, which was made in recognition ofInoue’s pioneering work on bulk metallic
glasses and outstanding achievements
and leadership in materials science, waspresented by Ted B. Massalski (Carnegie
Mellon University), the Executive
Secretary of Acta Materialia, at a ceremonyin the Sendai International Center.
One of the focal points of the workshop

AIMResearch 2010
© 2011 Tohoku University

was a special Nobel Laureate Lecture inwhich Georg Bednorz, who was awarded
the 1987 Nobel Prize for Physics for his
work on high-temperature superconductivity, gave a wide-ranging talk on the
topic of ‘Exploring the Nanotechnology
Landscape’, drawing on his work at
the IBM Zurich Research Laboratory.
Speaking to AIMResearch, Bednorz, amember of the WPI-AIMR International
Advisory Committee, praised the workof the center: “Last year I attended the[WPI-AIMR] workshop and I found theactivities here to be quite amazing,” hesaid, particularly noting that the work of
the center encompasses “quite a broad
spectrum of activities.” Bednorz also
described the fusion research approachas being “very important” and praised
the WPI-AIMR policy of giving young
researchers a free hand in directing theirresearch. “I think it’s important that young
people take the initiative … they getfinancial support and take over a common
project. I think this is important,” he said.
Yamamoto also spoke to AIMResearch

Akihisa Inoue (right) receives the 2010 Acta
Materialia Gold Medal

31

IN THE SPOTLIGHT

about the importance of fusion research,and confirmed his commitment to keep
it at the heart of the scientific activities of
the center. “The WPI-AIMR is now in its
third year, and we feel that we have finallyestablished the right organizational andresearch structure that will allow us to go
forward,” he said. “We have already madea number of interesting discoveries and
now we can accelerate the pace of delivery. The concept of fusion research willbe critical in realizing this goal.”
Himself a longstanding proponentof increased participation of overseas
researchers in Japanese institutions,
Yamamoto commented favorably on theinternational diversity of the workshop.
This is reflected in the makeup of the
WPI-AIMR personnel, in which onethird of the academic staff and almost
two-thirds of the research associates are
non-Japanese, a fact that Yamamoto identified as being one of the key strengthsof the center. This success is a result

The audience at the third WPI-AIMR Annual Workshop

of a conscious and sustained policy of
proactively recruiting talented young researchers from overseas to work alongside
Japanese counterparts, he noted. “We are
now in the happy position of actually
getting unsolicited enquiries from young
foreign scientists asking us about research
opportunities at the WPI-AIMR,” commented Yamamoto. “We expect that in thenear future, young people from all over

TECHNOLOGY FOCUS

the world will come to see the WPI-AIMR
as a natural choice for advancing their research career in materials science.”
The very high level of science on display at the 2010 WPI-AIMR Annual
Workshop and the obvious enthusiasmof the many young international participants would seem to suggest that this ambition may well be realized sooner rather
than later.

Published online 30 August 2010

Mixing the unmixable
The synergy of different expertise and ideas helps advance the development of ‘super’ hybrid materials.

M

any scientists focus theirres e arch on ma k ing ne w
materials, but tend to lose
sight of novelty and the environment.
Supercritical fluid technology opens a
new world of possibilities in making
hybrid materials with multiple functions
without relying on hazardous chemicals.
Every fluid has a critical point — aspecific combination of temperature andpressure above which the liquid and gas
phases disappear and are replaced with a‘supercritical fluid’ phase.
Supercritical fluids have many unusual properties, such as high density
and low viscosity, which make them ideal

32

for use as solvents, as Tadafumi Adschiri— a principal investigator at Tohoku
University’s WPI-AIMR and a global
leader in supercritical fluids — points out.
“We can tune the density of a supercritical fluid through small changes in temperature or pressure,” he says. “Because
solubility and phase behavior are adirect function of density, we could mix
a supercritical fluid, such as water, withmany things that are normally immiscible
or insoluble, such as oil or hydrogen gas.”
Supercritical fluids have many uses
Adschiri has been using supercriticalfluids for the synthesis of inorganic

Tadafumi Adschiri is a global leader insupercritical fluids

AIMResearch 2010
© 2011 Tohoku University

IN THE SPOTLIGHT

Tadafumi Adschiri and his co-workers use supercriticalfluid technology to create new hybrid materials.

nanocrystals, which have a vast numberof potential uses in industry but which
are usually prepared using harsh chemicals. So far he has made nanocrystals of
various metal oxides, including titanium
dioxide, zirconium oxide and seleniumoxide, as well as double-metal oxides
such as barium titanate and barium ferrite. “There are many methods for synthesizing nanocrystals, but there areseveral problems. One problem is the useof organic solvents, which are hazardous
to the environment. Another problem is
low productivity.”
Adschiri has been able to increase
the productivity of synthesis by growing
the nanocrystals in supercritical water,
avoiding the use of hazardous chemicals.
Furthermore, as supercritical water mixes
with oil, Adschiri can synthesize novel
organic–inorganic hybrid nanocrystals— a feat that could not be achieved using
conventional methods.
“Many people can make inorganic
nanocrystals, but they have difficulty
dispersing these nanocrystals in polymers or organic solvents. We can makenanocrystals with organic molecules on
the surface, which have high affinity withplastic polymers and organic solvents,
and then transplant them into polymer
films. We can also tune the properties ofthe polymers very easily.”

A super filmmaker
Using his unique nanocrystals, Adschiri
is developing a range of nanocrystal–polymer hybrids, including a transparent film with high refractive index. The
plastic film rolls up just like any normal
plastic film, but its high refractive index
could be useful for manufacturing thinner optical lens and anti-reflective coatings. “You cannot see the particles, butthey give the film a very high refractive
index,” says Adschiri.
Another hybrid film he is developing
is a plastic film consisting of over 90%
boron nitride particles. The hybrid material has high thermal conductivity but
low electrical conductivity — normally incompatible properties — and can be used
as a heat-transfer material for cellular
phones, computers and other electronic
devices. In collaboration with other WPIAIMR researchers, Adschiri is also working on a plastic film containing magnetic
particles, which he believes could be useful for making polarizers and photonic
tuners. “We call these novel hybrid materials ‘super hybrid materials’ because they
have multiple, normally incompatible
properties,” he says.
A mix of ideas
Adschiri opened his laboratory eight years
ago, and at that time, he already foresaw
the importance of working with researchers from completely different fields. “I wasworking in the field of supercritical fluids,
but to explore new materials, I neededpeople who had different viewpoints. I
had three positions in the laboratory, so
I invited a biochemist, a material scientist
and a computer scientist to join,” he says.
In the beginning, the four original
members of the team did not have acommon scientific background. But theyspent half a year explaining their ideas
to each other from a basic graduate leveland eventually found a direction they
all could follow — the combination of
nanoparticles, polymers and biomaterials,
brought together with simulations of theunderlying physics.
Adschiri now has 30 researchers,
including ten postdoctoral researchers,
working in his laboratory. His uniquephilosophy of fusing different expertiseand ideas is still evident in the people heemploys, and he remains undaunted byany perceived risk of working with people
with such diverse backgrounds: “It’s agamble, but I like a gamble,” he says.
Many companies are now interested
in using organic and inorganic materials
to make super hybrid materials. Looking
forward, Adschiri has a clear vision
for where he wants to take his group’s
research on super hybrid materials: He
intends to support the growth of thisunique technology while exploring thecreation of new materials through thefusion of ideas.

A transparent, high-refractive-index film based on nanocrystal–polymer hybrid technology

AIMResearch 2010
© 2011 Tohoku University

33

IN THE SPOTLIGHTTECHNOLOGY FOCUS

Published online 28 February 2011

From the bottom up
Researchers in the Bulk Metallic Glasses Group at the WPI-AIMR are showing how fusion research can be used tounderstand the structure and properties of ‘atomic’ materials.

W

hen asked to describe thetoughest challenge facing him
in his work, Mingwei Chen,group leader and principal investigator of
the Bulk Metallic Glasses Group, does nothesitate in reply: “Bulk metallic glass is anatomic-scale material. We still don’t know
how the individual atoms are arranged.
Understanding structure at the atomic
level is the most important topic in thisresearch,” he says.
First reported in the early 1960s,
metallic glasses are a special type of metalalloy that promise to revolutionize thematerials industry. Unlike conventional
metals, which are produced by allowing
the molten metal to cool slowly to afforda material with a well-ordered crystalline
structure at the microscopic level, bulk
metallic glasses (BMGs) are formed by
rapid cooling of the bulk sample such thatit solidifies without crystallization. The
resulting metals are locked into a disordered ‘liquid-like’ structure, which meansthat they soften and flow at comparatively
low temperatures allowing them to bemolded and processed much more easily than conventional metals. At the sametime, the irregular, tightly packed internalarrangement of the atoms in BMGs gives
them higher density, strength and resistance to wear and corrosion compared tomany crystalline metals. Early BMGs had
critical cooling rates — the rate of coolingrequired to create the characteristic amorphous structure — on the order of one
million degrees per second. More recent
systems, some of which were discoveredin the ‘80s and ‘90s at Tohoku University,
the home of the WPI-AIMR, have much
lower critical cooling rates, although thereasons behind this are still not clear. “Part34

of our work focuses on some of the mostfundamental issues in BMG research, suchas the mechanism of BMG formation,”says Chen. “Why, for example, do somealloys have very low critical cooling rates
and such good glass-forming ability? Thisis a big question in this area and we spend
a lot of time and effort to understand thisissue in my group.”
Deformation without crystallization
Another important topic is the question of how BMGs can be plastically
deformed. Although BMGs are typicallyseveral times stronger than conventional
crystalline metals, they are not ductile atnormal temperatures and have a tendency
to fail abruptly when stretched, with noindication that failure is imminent. This
limits their usefulness in load-bearing
applications where structural reliability
is critical. “Crystalline materials can beplastically deformed by the motion of dislocations — a line of defects in the crystal
lattice,” explains Chen. “BMGs are glassy
and do not have a crystalline structure,
yet we still observe plastic deformationin these systems.” To account for thisphenomenon, Chen and his co-workers
utilize and develop the theory of shear
transformation zones (STZs), nano-sized
pockets that undergo plastic deformation
resulting in a build-up of stress and theformation of localized shear bands, which
can eventually lead to mechanical failureof the BMGs. Examination of a series ofdifferent BMG systems using a new technique called ‘rate-jump nanoindentation’developed at the WPI-AIMR has madeit possible to correlate the volume of
the atomic clusters involved in the STZs
with the ductility of the BMGs. “Unlike

Mingwei Chen, leader of the BMG Group at
the WPI-AIMR

conventional metals where we can gain
information about the bulk properties ofa sample by looking at its microstructure
through a microscope, in the case of
BMGs we have to examine the structure
at the atomic scale in order to understandthe material’s bulk mechanical properties.
Once we have a clear understanding of
this we can begin to design BMG systems
with the desired atomic arrangement for
improved mechanical properties,” he says.
Looking at the small picture
Getting down to this level of structuraldetail requires the deployment of cuttingedge experimental techniques such as
extended X-ray absorption fine structure
spectroscopy, which is carried out using
the synchrotron radiation facility at the
RIKEN SPring-8 center located at theHarima institute west of Kobe in central
western Japan. Closer to home, Chen and

AIMResearch 2010
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IN THE SPOTLIGHT

his co-workers have also developed an
angstrom beam electron diffraction technique using a coherent electron beam witha width of approximately 0.3 nanometers
to observe the arrangement of individual
atomic clusters in a BMG sample. The
high levels of precision required for theexperiment were only possible thanks toa state-of-the-art, custom-made spherical aberration-corrected transmission
electron microscope, which was built incollaboration with a major Japanese scientific instrument manufacturer and which
incorporates a condenser aperture designed by researchers at the WPI-AIMR.
The electron microscope, purchased withstrategic funding from the WPI-AIMR, is
one of only a handful of such instruments
in Japan and has generated interest fromother researchers both inside and outside
the WPI-AIMR.
Although he considers BMG as his
main field and notes its position as
one of the mainstays of the work of the
WPI-AIMR, Chen is keen to stress that
the Advanced Functional Materials
Laboratory is concerned with much morethan just BMG research. The remit of
the section is broad — nanostructured
materials, nanocomposites and nanocrystalline metals for high-strength materials
are all included in the portfolio, alongwith research into metal colloids andthe theoretical studies and simulations
that underpin concepts such as STZs.
“We carry out a wide range of research
here, but BMG research will always be
a key topic for the WPI-AIMR, not justbecause of its inherent importance but
also because Tohoku University has madea special contribution in this area,” says

Chen works on a number of topics in addition to
BMG research

Using the spherical aberration-corrected transmission electron microscope

Chen referring to the more than 40-year
involvement of the university in the fieldof BMG research.
Fusion research, green innovation
A key member of the original committeethat proposed and subsequently set up the
WPI-AIMR, Chen began his academic
career as an associate professor at theprestigious Shanghai Jiaotong University
in China before joining the Institute forMaterials Research at Tohoku University
as a guest researcher from 1997 to 1999.
Following a stint in the US where heworked as an associate research professor
at Johns Hopkins University, he returned
to Sendai in 2003 to join the International
Frontier Center for Advanced Materials
as one of the first non-Japanese researchers to be hired as a professor at Tohoku
University. “One of the reasons I cameback to Japan is because the equipment
here is so good. Plus the research funding
is much more stable than in other countries such as the US,” he says. Chen also
cites the strong teamwork ethic in Japan,
which sees graduate students working
closely with associate, assistant and full
professors, as another positive point ofworking at Japanese institutions such as
the WPI-AIMR.
As might be expected for one who
has been intimately involved with the
WPI-AIMR since its inception, Chen isan enthusiastic supporter of the conceptof fusion research being pioneered at

AIMResearch 2010
© 2011 Tohoku University

the institute. Discoveries made by Chenand his co-workers in the Advanced
Functional Materials Laboratory are
already being applied in other groups ofthe WPI-AIMR. Nanoporous metal–ionicliquid composites, for example, which arebeing developed for electrocatalysis andsuper capacitors, have their origins in
Chen’s laboratory, as do the nanoporous
gold catalysts being used in chemical
synthesis by the Soft Materials Group.
Other products of BMG research arebeing taken up by microelectromechanical systems (MEMS) researchers in the
Devices/Systems Group as materials for
micro-mirrors in applications such as
virtual retinal displays, virtual keyboards
and mobile phone projectors. In addition
to these applications, Chen foresees anincreased emphasis on the developmentof functional applications for materials in the arena of green innovation, an
endeavor in which fusion research will
be even more important. “I come from abackground of traditional physical metallurgy, but now I am doing research inso many areas, from physics, chemistry
and electrochemistry to energy-storage
devices and biosensors. My research team
all come from different backgrounds
too: applied chemistry, applied physics,
mechanics — the lot. Fusion research is
absolutely essential for the work of the
WPI-AIMR, and when you think about
it, we’re actually doing fusion researchwithin my own group,” he says.

35

IN THE SPOTLIGHTFEATURED RESEARCHERS

Published online 27 December 2010

Learning the difference
At the WPI-AIMR, researchers are beginning to understand each other’s strengths but there are still hurdles on theroad to finding new functional materials.

S

ince the WPI-AIMR first started
in 2007, the institute has beenbringing together researchers
from all over the world to work on new
fields in materials science. AIMResearch
speaks to four researchers who have madethe plunge and moved to Japan to join
the WPI-AIMR.
How did you discover the WPI-AIMR at
Tohoku University here in Sendai, and whatkind of research are you now involved in?
Ken Nakajima: After finishing my PhD
in polymer physics at the University ofTokyo, I took a postdoctorate position
at RIKEN and then joined the Tokyo
Institute of Technology as an assistantprofessor. I was actually born in Sendai
though, and Tohoku University is known
as one of the best universities in Japan.
I joined the Soft Materials ResearchGroup at the WPI-AIMR and am now anassociate professor.
“The WPI-AIMR has aunique organizational
structure that gives me
many new challenges.”
Ken Nakajima

Are you glad to be back in Sendai?
Nakajima: I am honoured to be here. The
WPI-AIMR has a unique organizational
structure that gives me many new challenges. I get to work with many foreign researchers and they give me broad insights
into science. I am now studying polymers
and their mechanical properties using
scanning probe microscopy (SPM).

36

Limei Xu: I came here by coincidence. I
discovered the WPI-AIMR when someone passed me an advertisement for the
institute while at a conference in Europe. I
knew the WPI-AIMR was the world’s leading research institute in materials science.
I thought it might be good chance to shift
my research direction to materials science,
so I immediately applied.
“I knew the WPI-AIMR
was the world’s leading
research institute inmaterials science.”
Limei Xu

Was the transition difficult?
Xu: My PhD at the Center of Polymer
Studies at Boston University involved
using statistical physics to study criticalphenomena in glasses and polymers. Now
I am an assistant professor working in
the Bulk Metallic Glass Research Groupat the WPI-AIMR. Materials science
was new to me and the transition frompolymers to bulk metallic glasses (BMGs)
was not straightforward. Before, my study
was more fundamental and the models
I developed were for general materials.
Here at the WPI-AIMR, researchers
work with very specific materials. It isvery difficult for me as a theoretician tocome up with a model that incorporates
the properties of all the different kindsof materials.
Kazunori Ueno: I did my doctorate study
at the National Institute of Advanced
Industrial Science and Technology (AIST)in Tsukuba. Professor Masashi Kawasaki,

my boss now at the WPI-AIMR, was oneof the principal investigators at AIST atthat time. Because Tohoku University’s
Institute for Materials Research is very
famous for research in material sciences,
I decided to join Kawasaki’s group at
Tohoku University and do my postdoctorate here. Five years later, the WPI-AIMR
was established, and I didn’t hesitate
to join.
What was the attraction of the WPI-AIMR?
Ueno: The new Materials Science
Research Group was very attractive forme as a young scientist. I am an assistantprofessor working in the Materials Physics
Research Group, where I study oxidematerials and ceramics, particularly theirapplications in semiconductor devices, superconductors and field-effect transistors.
I am currently trying to combine ceramics
with organic materials to create materials
with new properties.
Doctor Caron, you were born in France?
Why did you come to Japan?
Arnaud Caron: Yes, I was born in France. I
came to Japan partly because of curiosity. I
went to Germany to do postdoctoral study
at the University of Ulm. Professor HansJörg Fecht, my supervisor at that time,
happened to be an adjunct professor at the
WPI-AIMR. He suggested that I should
apply for a position here. As a young
researcher, I try to broaden my scope, and
the WPI-AIMR is a very international
institute where I get to interact with people
from different fields and nationalities.
I wanted to see something different toEurope and so I applied.

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IN THE SPOTLIGHT

What kind of research do you do at the
WPI-AIMR?

Doctor Xu, could your model be applied to
Dr Nakajima’s work?

Caron: I am a research associate working
in the BMG Research Group. I use acoustic
methods, such as ultrasound and near-fieldsound, to study the mechanical properties,
particularly frictional properties, of BMGs.
I did my PhD at Saarland University anda postdoc at the University of Ulm, bothin Germany. Since I moved to the WPIAIMR, I have been using SPM to study themechanical properties of BMGs.

Xu: That is the dream! My model works
well in theory, but I do not know how good
it works for real materials. The advantageof my model is that it is quite general.
However, there are some specific questions
that I still need to answer. One of the biggest questions in materials science rightnow is how do we make stable BMGs — orin other words, BMGs that do not crystallize easily. My job is to provide the theoretical basis for finding such stable BMGs. My
model shows that it is possible to makestable BMGs through polymorphisms, or
glass–glass transitions.

“The WPI-AIMR is a very
international institutewhere I get to interact
with people from different
fields and nationalities.”
ArnAud CAron

What was the application process like?
Caron: The first thing I did was to send a
CV and a cover letter to the WPI-AIMR.
Then I met Professor Dmitri Louzguine,
my present boss, in Sendai and we discussed what we could do together. He got
quite interested. Three months after myapplication, I came to Japan.
Does the WPI-AIMR provide assistance forforeign researchers?
Caron: Yes. They provide support foradministrative issues, such as funding applications, and for our daily life.
Whenever we have problems, we go to
see Director Yoshinori Yamamoto or the
institute’s secretariat.
Doctor Nakajima, could you tell me more
about your current projects?
Nakajima: Polymers cannot be used as
single-component materials. You need tomix a polymer with, say, another polymer
or an inorganic material. In this case, theinterface between the two materials becomes important. Doctor Xu and I are now
studying the interfacial properties of polymers. I am also working with researchers
on BMGs. We are planning to mix materials to make new functional materials. I am
not sure if it will work, but this is our goal.

Doctors Ueno and Caron, could you tell me
about your current projects? Who are you
collaborating with at the WPI-AIMR?
Ueno: I am collaborating with Dr Akagi ofProf. Tsukada’s group, who performs theoretical simulations of the interface between
organic polymer and other materials. My
device applications rely on the interface
between ceramic and organic materials, so
the collaborative study can provide invaluable information.
“The good thing about
the WPI-AIMR is thatthe laboratory doors
are always open.”
KAzunori ueno

Caron: I am collaborating with Dr
Nakajima to study friction in BMGs. BMGs
have the potential to be used in microdevices, such as microelectromechanicalsystems (MEMS), so their properties on the
microscale are very interesting. We study
the frictional properties between silicon
and BMGs and try to understand the role ofthe oxide layer, as well as the relaxation stateof the BMG. Doctor Nakajima is bringing
his experience in SPM to the experiment.
How are BMGs used in MEMS?
Caron: BMGs can be processed in anundercooled liquid state. You can heat

AIMResearch 2010
© 2011 Tohoku University

them up above the glass transition, and in
the undercooled liquid state, they are reallyeasy to form into devices such as microgears and cantilevers.
What are your goals in the short term?
What are some of the challenges?
Nakajima: One of WPI-AIMR’s missions
is to bridge conventional disciplines, suchas materials science, physics, chemistry
and engineering, to create new functional
materials — the so-called ‘fusion research’.
I think this is very important. We havepeople from the BMG group, semiconductor group, polymer group and soft
materials group working together. Thisis a unique structure. The WPI-AIMR
holds seminars regularly and we arestarting to understand each other and thedifferent disciplines. However, we are now
beginning to realize the difficulty in doing
fusion research. I think we will create
something new in the future, but this is
not something that happens overnight.
Xu: Yes, I know what you mean. It takes
time. For me, I find that I have a very
different background from most people
here — they are more like engineers and I
am more theoretical. There is a big gap. If
I go to the seminar, I will get something,but it will not be on the same level as when
I used to go to seminars in my field. The
information I am receiving is huge. As a
newcomer, it is difficult to get to the essential part immediately.
Ueno: The good thing about the WPIAIMR is that the laboratory doors arealways open. If I need to find someone
to discuss something, I can reach themeasily. In addition, my contacts at the
WPI-AIMR are not limited to physics,
but also include other fields such as biology and chemistry. This provides a good
opportunity for me to broaden my scopeand grow.
Caron: Even with institute members
overseas, communication is still quite
easy. There is Skype and email, and they
visit several times a year. It is quite easy to
discuss our research.

37

Advanced Institute for Materials Research
Tohoku University

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