Volume 2

WPI-AIMR NEWS

April 30, 2008

World Premier International Research Center
Advanced Institute for Materials Research
Tohoku University

Produced by the WPI-AIMR Communication Ofﬁce
Director: Yoshinori Yamamoto
Editor, writer: Toshio Sakurai
Associate Editor, writer: Hiroshi Komatsu
Production coordinator: Yasunori Fujikawa
We welcome your comments, questions and involvement.
e-mail to: wpi-ofﬁce@bureau.tohoku.ac.jp, or
sakurai@imr.tohoku.ac.jp

Contents
Preface ………………………………………………………………………………………… 2
Research Prospect
1. A. Inoue (WPI-AIMR) and D. V. Louzguine-Luzgin (WPI-AIMR) ………………… 5
2. M. G. Lagally (WPI-AIMR and Univ. of Wisconsin-Madison) ………………… 21
3. K. Nakajima (WPI-AIMR) ………………………………………………………… 27
4. T. Hitosugi (WPI-AIMR) ………………………………………………………… 39
5. R. M. Tromp (WPI-AIMR and IBM Thomas J. Watson Research Center) ……… 47
The Salient Point of Research ……………………………………………………………… 65
Interviews
Yoshinori YAMAMOTO (WPI-AIMR) ……………………………………………… 95
Masashi KAWASAKI (WPI-AIMR) ………………………………………………… 101
Announcement
Staff/post-doctoral fellowship ……………………………………………………… 121
Ph. D. student scholarship …………………………………………………………… 122
International workshop guidelines …………………………………………………… 123
Appendix
WPI & IFCAM Joint Workshop snapshots ………………………………………… X-1

1

Prof. Yoshinori YAMAMOTO
機構長 山本 嘉則

Preface
The ﬁrst international workshop of WPI-AIMR
It is a pleasure for me to be able to deliver volume 2 of WPI-AIMR News to you. WPIAIMR was established on October 1, 2007, and about half a year passed so quickly. Already,excellent researchers in various disciplines including physics, chemistry, material sciences, andengineering are joining us from all over the world. Thirty PIs (principal investigators) of WPIAIMR together with those excellent researchers have started their activities toward our goal, thatis to say, through in-depth understanding and control at atomic and molecular level to promote
the development of new materials and substances, to invent innovative processes and devices,and ultimately to beneﬁt the society and mankind. In order to assist to gain access to the goal, weorganize the first international WPI workshop “Challenge of Interdisciplinary Materials Science
to Technological Innovation of the 21st Century”. A major purpose of this workshop is to findcollaboration partners within the WPI framework or even beyond the WPI framework, because
fusion and integration of the above disciplines are needed to make the goal feasible. We have four
major research groups, physical metallurgy (bulk metallic glasses), nano-physics, nano-chemistry,and device/system development, and in the workshop PIs of those four thrusts, together withresearchers and collaborators of WPI, present the state of the art of their accomplishments. I hopethat you will learn in-depth the researches going on now in the disciplines different from your own
specialty, and you will be able to ﬁnd partners and collaborators in order to pursue integration andfusion of your own research.
We plan that this kind of international WPI workshop including all the disciplines of WPI isheld around the ﬁrst week of March, every year. Of course, more specialized symposia of compact
size may be organized when they are needed. Finally I believe you will obtain rich fruits byparticipating in these future workshops.

2

Research Prospect

Prof. Akihisa INOUE
東北大学総長・主任研究者

井上明久

Prof. Dmitri V. Louzguine
教授 ルズギン・ドミトリ

SOLIDIFICATION, GLASS-FORMATION, STRUCTURAL FEATURES AND
CRYSTALLIZATION BEHAVIOR OF Cu-BASED GLASSY ALLOYS
Dmitri V. Louzguine-Luzgin and Akihisa Inoue
WPI Advanced Institute for Materials Research, Tohoku University,
Katahira 2-1-1, Aoba-Ku, Sendai 980-8577, Japan
Cu-based glassy alloys exhibit high strength, good glass-forming ability (GFA) andsigniﬁcantly high stability of the supercooled liquid. In the present work we describe solidiﬁcation,
structural features and crystallization behavior of recently discovered Cu-based bulk glassy alloys.
The resistance of the supercooled liquid to crystallization in these alloys has opened opportunities
to study the glass transition and crystallization kinetics in the supercooled liquid region. The
addition of a certain fraction of Ag to Cu-based alloy causes an increase in the reduced glasstransition temperature and γ parameter values in expense of lowering liquidus and increasing theircrystallization temperature. The structure of the Cu-based alloys is studied by real-space distributionfunctions. It is suggested that the medium-range order maintains up to about 2 nm distance. It isalso found that the structure of the Cu-Ag-Zr-Ti glassy alloy depends on cooling rate. It is glassy
in the case of ribbon samples while the nanoscale highly ordered clusters are formed in the bulk
samples. Nevertheless the studied Cu-based alloys crystallize by nucleation and 3-dimensional
crystal growth process. The cooling curve of the ingot cast in Cu-mould can be fit successfully
with an exponential decay equation with two exponents. This rather indicates that both processes:
the heat transfer through the melt as well as through the melt/mould interface inﬂuence the coolingrate observed. For a small ingot of 3 mm in diameter the average cooling rate in the region between
the liquidus and glass-transition temperature is about 103 K/s. Such a high value demonstrates thatequilibrium phase diagrams can be used for predicting the GFA only in a rough approximation.

5

Introduction
Since the synthesis of a metallic amorphous phase in the Au-Si system by a rapid
solidification technique [1], a great number of metallic amorphous and glassy alloys have been
produced in different alloy systems. Later the high glass-forming ability achieved at some alloy
compositions allowed to produce bulk metallic glasses [2] in the thickness range of 1-100 mm
by using various casting processes based on the principles for achieving high glass-forming
ability [2]. Bulk glassy alloys exhibit high strength, hardness, wear resistance and large elastic
deformation and high corrosion resistance . Glassy alloys being metastable undergo relaxationand devitriﬁcation (crystallization) processes on heating. The thermal expansion, glass-transitionand volume change on heating were studied in several glassy alloys by synchrotron beam X-ray
diffractometry. Recently these features were also studied for Cu-Hf-Ti-Pd glassy alloy using a realspace distribution function [3].
Cu-Zr-Ti bulk glassy alloys were reported to exhibit a high mechanical strength exceeding
2 GPa. A quaternary Cu55Zr30Ti10Co5 glassy alloy showed the highest among Cu-based glasses
ultimate compressive strength value of 2.3 GPa [4]. The formation of the nanoscale icosahedralphase was recently observed in the Cu-Zr-Ti alloys containing Pd [5] and Au while Ag- and Ptbearing alloys did not form the icosahedral phase.
1. Influence of cooling rate on the structure and properties of a Cu-Zr-Ti-Ag glassy
alloy
Solidiﬁcation behavior of Cu44Ag15Zr36Ti5 glassy alloy (the composition is given in nominal
atomic percentages) was studied using a Cu-mould casting technique [6]. The time-temperature
cooling curves upon casting into Cu mould were recorded by using a thin K-type thermocouple (asschematically is shown in Fig. 1.1) which was connected to an analog-to-digital signal converter.

Fig. 1.1. Schematic illustration of Cu mould andlocation of the thermocouple (TC).

6

As was shown by XRD experiments the structure of the as-solidified alloy in ribbon andbulk shapes was found to consist of a glassy phase. Fig. 1.2 represents the time-temperature
cooling traces for the Cu44Ag15Zr36Ti5 glassy alloy samples of 3 and 5 mm in diameter and theresulted cooling rate measured at Tg~700 K as a tangent to the graph is about 600 K/s and 200 K/s,
respectively. Upon casting the melt temperature decreases nonlinearly. The initial cooling rate of
the liquid in the center of the 3 mm ingot of the sample during initial 100 ms is about 2000 K/s. The
same value for the 5 mm sample is 600 K/s. The cooling rate upon melt spinning was estimated at105-106 K/s.

Fig. 1.2. Time-temperature cooling traces for the Cu44Ag15Zr36Ti5 glassy alloy samples of 3 and 5
mm in diameter. (a and b) represent different time scale for convenience.

The devitriﬁcation behavior of the alloy samples on heating was investigated using DSC. The
studied alloy undergoes a two-stage crystallization by 3-dimensional diffusion-controlled growth
process which is typical for primary crystallization mechanism.
The DSC traces of the as-solidiﬁed samples demonstrate an increment of the heat capacity(Cp) at the glass transition temperature (Tg) and subsequent exothermic peak owing to crystallizationof the supercooled liquid (see Fig. 1.3). Heat of structure relaxation of the ribbon, 3 mm and 5 mmsamples prior to glass-transition is 1.83 (100 %), 0.87 (48 %) and 0.64 (35 %) J/mol, respectively.

7

Fig. 1.3. The DSC traces of the as-solidiﬁed ribbon and bulk samples as wellas the exothermic heat release corresponding to both exothermic events.

The isothermal calorimetry trace allowed to produce the Avrami plot. The Avrami plot createdaccording to the following kinetic law for the volume fraction (x) transformed as a function of time(t): x(t) = 1–exp(–Ktn) was found to be reasonably linear and n=2.3 rather indicates 3-dimensional
diffusion-controlled growth process.
During the exothermic reactions (see peaks in Fig. 1.3) the supercooled liquid crystallizes
forming oC68 Cu10Zr7 and tP4 AgZr phases, though some weak peaks remain unidentiﬁed whichindicates possible existence of a third phase.
Vickers microhardness data indicate that the HV values of the bulk samples are found to beclose to each other but significantly different from that of the ribbon sample using a confidence
interval with the probability of 95 % (Fig. 1.4).

Fig. 1.4. Vickers microhardness as a function of the sample size, and thus, cooling rate.

8

As the Vickers microhardness of the ribbon and bulk samples is different we performed a
deep structural investigation of the as-solidiﬁed samples. As shown in Fig. 1.5 (a and c) the ribbon
sample has a uniform structure while the bulk sample exhibits some highly ordered regions Fig.
1.5 (b and d) of about 3 nm size within the glassy phase. The nanobeam diffraction (NBD) patterntaken from this region can be indexed according to the AgZr phase.

Fig. 1.5. The structure of (a) the ribbon sample and (b) bulk sample of 5 mm in diameter, TEM dark-ﬁeld. The inserts are typical
NBD patterns, respectively. Visible camera length is somewhat different in each case due to subsequent image processing. Some of
the NBD patterns taken from these regions can be indexed according to AgZr phase. (c) and (d) are the corresponding high-resolution
TEM images. The ribbon sample (c) has a uniform glassy structure while the bulk sample exhibits some highly ordered regions (d) of
about 3 nm in size within the glassy phase marked with arrows in (b). The structure of the ion-polished and electrolitically polishedsamples did not differ by sight.

It is found that the structure of the Cu44Ag15Zr36Ti5 glassy alloy depends on cooling rate. It
is glassy in the case of ribbon samples while the nanoscale clusters of the crystalline phase (highlyordered regions) are formed in the bulk samples. Such a change is reﬂected in the shift of the X-ray
diffraction peak, in the magnitude of the heat of structure relaxation and in the variation of theVickers microhardness observed in Fig. 1.4. A slight deviation of the cooling curve signal of the 5
mm bulk sample at 1030 K observed in Fig. 1.2 likely corresponds to the formation of the orderedregions.
We also studied the cooling rate as a function of casting temperature. The cooling curves inFig. 1.6 are plotted from a certain arbitrary temperature above the liquidus temperature for better
comparison. Fig. 1.7 shows that the cooling rate at Tg slightly changes with casting temperature.

9

However, the DSC, HV and XRD data of the samples cast from different temperatures did not show
any systematic variation as the cooling rate (Fig. 1.7) varied only by about 20 %.

Fig. 1.6. Cooling curves of the bulk sample (3 mm)

Fig. 1.7. Cooling rate R at Tg as a function ofrecorded from different casting temperature.

casting temperature.

The cooling rate at Tg depends upon the ingot size. At the same time the cooling rate alsoslightly depends upon the casting temperature (Fig. 1.6 and 1.7). It takes place likely owing todifferent amounts of heat which was transferred to the mould (depending upon casting temperature
which determines the heat content of the melt) and caused its heating.
Heat transfer through the melt, heat transfer through the melt-mould interface and heattransfer through the mould take place upon cooling of the melt. The interfacial heat transfer
coefﬁcient (h) indicates a heat transfer rate through the melt/mould interface. The Biot number (Bi)deﬁned as hL/λl, (where L is a characteristic length scale and λl is the thermal conductivity of theliquid being cast) indicates which process: heat transfer through the melt or heat transfer throughthe melt/mould interface dominates upon cooling. The value of the Biot number calculated for
Cu-based alloys is about 1 which implies that the temperature gradients inside the melt are nonnegligible.
As one can see in Fig. 1.8 the cooling curve of the 3 mm ingot cannot be fit well with
Newtonian equitation (violet line). However, it can be fit very well (Fig. 1.8) if an exponential
-t/τ1
-t/τ2
decay equation with two exponents is used:T(t)=T0+A1e +A2e where T0, A1, A2, τ1 and τ2 are theconstants at T0= 314 K, A1= 347 K, τ1= 0.205 s, A2= 429 K, τ2 = 1.209 s. This may indicate that bothprocesses: the heat transfer through the melt as well as through the melt/mould interface inﬂuence
the cooling rate obtained. The results suggest that upon bulk glass casting the thermal conduction
within the melt also plays an important role inﬂuencing the cooling rate.
The cooling rate drastically changes with temperature as shown in Fig. 1.8 (b) but theimportant values belong to the region between Tl and Tg. The average value in this region is close
to 1200 K/s. Such a high average value of 1200 K/s clearly demonstrates that casting conditions of10

Fig. 1.8. (a) Cooling curve of the studied alloy (black curve behind blue one) taken from the central part of the ingot of 3 mm indiameter. Violet line – the best ﬁt with the Newtonian cooling law. Blue line – ﬁt with second order exponential decay function. (b)The cooling rate as a function of time and temperature.

conventional bulk glassy alloys are far from equilibrium and equilibrium phase diagrams can beused for predicting the GFA only in a rough approximation.
Contrary to ribbon samples cast at about 106 K/s bulk glassy samples cast at a cooling rate
about three-four orders in magnitude lower contain highly ordered clusters up to 3-5 nm in size.
Such a difference in the structure causes the observed variation in microhardness as nanoscaleordered regions were found to cause strengthening of the glassy matrix owing to different atomic
structure of these regions or change in the matrix composition.
2. Real-space structural studies of Cu-Zr-Ti glassy alloy
The interference function Qi(Q) for Cu60Zr30Ti10 alloy calculated from the XRD intensity
profile after the necessary corrections is shown in Fig. 2.1 while RDF(r) obtained by Fourier
Transformation is shown in Fig. 2.
Taking into account large negative mixing enthalpy of the Cu-Zr interatomic pair one canassume that this pair is the most probable interatomic distance corresponding to left shoulder of
the PDF function in the present alloy. As the maximum of the left shoulder (0.273 nm) corresponds
to lower value than the predicted Cu-Zr interatomic distance (0.289 nm) one can assume that thisdistance in the studied glassy alloy is lower than that predicted by the Goldschmidt atomic radii due
to negative mixing enthalpy (attractive interaction) in this atomic pair.
Such effect is quite common in the intermetallic compounds. For example, oC68 Cu10Zr7
phase is a stable phase forming in the Cu60Zr30Ti10 alloy after crystallization of the glassy phase. It

11

Fig. 2.1. Scattered intensity i(Q) and Qi(Q) functions of Cu60Zr30Ti10

Fig. 2.2. Radial distribution function of Cu60Zr30Ti10 alloy.

alloy. The insert – high resolution TEM image (HRTEM) of assolidiﬁed sample.

is isomorphous to oC68 Ni10Zr7 one, has close lattice parameters and the Goldschmidt atomic radius
of Ni is close to that of Cu. In some clusters this phase shows Ni-Zr distances as small as 0.238
nm and typical Ni-Zr interatomic distances are about 0.26-0.28 nm which is less than 0.289 nm
predicted by the Goldschmidt atomic radii but corresponds well to the PDF(r) peak position.
These data are in good correspondence with the results of the PDF(r) function ﬁtting withtwo Gaussian peaks as shown in Fig. 2.3. The second peak corresponds well to Zr-Zr interatomic
distance while ﬁrst one can be a combination of the peaks produced by other atomic pairs. Different
Cu-Cu and Cu-Zr interatomic distances observed in the oC68 Cu10Zr7 compound varying in wide
ranges from about 0.24 to 0.31 nm for Cu-Zr pair and from about 0.22 to 0.31 for Cu-Cu pair
explain the single-peak shape of the main PDF(r) maximum (Fig. 3) responsible for the Cu-Cu, CuTi and Cu-Zr atomic pairs.
According to Fig. 2.4 a detectable medium-range order maintains until about 2 nm (Fig.
2.4 (a)) of the interatomic distance independently on the truncation distance of the Fourier
Transformation in the reciprocal space (Fig. 2.4 (b)). Such a high degree of medium range order iswell consistent with recent models which predict that metallic glasses are not random packing ofFig. 2.3. Fitting of the ﬁrst maximum of PDF(r) (baseline corrected) with 2 Gaussian peaks.

12atoms but dense packing of clusters. It is also deduced from the small density difference between
glassy and the corresponding crystalline phases.

Fig. 2.4. (a) Fitting of the PDF(r) maxima with the exponential decay function. (b) medium-range distance PDF(r) function.
Q100-160 PDF(r) traces represent truncation of the Fourier Transformation procedure of Qi(Q) at different Q values ranged from 100
to 160 nm-1. It is seen that the notwithstanding on some noise (inherited from the XRD patterns) the positions of the PDF(r) maximachange insigniﬁcantly.

A question may arise if the medium-range order maintains until about 2 nm why no clearlyordered zones are seen in the HRTEM image in Fig. 1. If one takes a Fast Fourier Transformation(FFT) of two parts of the HRTEM image like shown in Fig. 2.5 one can see that although the FFTimage shows a diffuse halo only from the area of about 4 nm2 it has some features if taken from thearea of about 2 nm2. At the same time an icosahedral-type medium range order which is expected to
be a dominant local conﬁguration in liquids and metallic glasses may be a good explanation why no
crystal-like atomic arrays are observed in the HRTEM images (Figs. 1 and 5) of these glasses.

Fig. 2.5. Two parts of the HRTEM image in Fig. 1:(a) about 4 nm2 and (b) about 2 nm2 in size. Area(b) is a part of (a). (c) and (d) the corresponding
FFT images.

13

3. Devitriﬁcation behavior and glass-forming ability of Cu-Zr-Ag alloys
Bulk glass formation in the Cu-Zr-Ag glassy alloys has been studied recently. Cu45Zr45Ag10
alloy exhibited the highest glass-forming ability (GFA) (the critical diameter is 6 mm) among CuZr-Ag glassy alloys and high mechanical strength of 1.8 GPa [7].
The structure of the as-solidiﬁed Cu-Zr-Ag alloys was found to consist of a glassy phase. The
devitriﬁcation behavior on heating was investigated using DSC. The DSC traces of the as-solidiﬁed
Cu55-xZr45Agx samples shown in Fig. 3.1 demonstrate an increment of heat capacity (Cp) at theglass transition temperature (Tg) (the temperature corresponding to the beginning of glass-transitionis shown in the ﬁgure) and subsequent exothermic peak due to crystallization of the supercooled
liquid starting at the onset temperature (Tx) (see Fig. 3.1). The Tg and Tx temperatures plotted inFig. 3.2 (a) as a function of Ag content were determined from the intersection of two tangents tothe curve below and above the onset temperature. The supercooled liquid region (∆Tx) also shownin Fig. 3.2 (a) is deﬁned as the difference between Tx and Tg. On heating above Tg a glassy phasebecomes a supercooled liquid which is still not crystallized below Tx.
The phase composition of the annealed samples determined using the XRD patterns is

Fig. 3.2. (a) Tg, Tx and ∆Tx as well as (b) Tg/Tl and γ

Fig. 3.1. DSC traces of the Cu55-xZr45Agx glassy alloys.

parameter values as a function of Ag content.

shown in Fig. 3.3. During the exothermic reactions (see peaks in Fig. 3.1) the supercooled liquid
crystallizes forming oC68 Cu10Zr7 phase in Cu55Zr45 alloy and a mixture of oC68 (Cu,Ag)110Zr7andtP4 (Ag,Cu)Zr solid solution phases in Ag-bearing alloys.
Upon the isothermal calorimetry in both Ag-bearing alloys the crystallization started after a14certain incubation period (about 350 s for Cu45Zr45Ag10 and Cu35Zr45Ag20 alloys at 717 and 722 K,
respectively) which indicates nucleation and growth transformation mechanism.
The linearity of the Avrami plot of ln(-ln(1-x)) vs. ln(t) for both Ag-bearing alloys shown

Fig. 3.3. X-ray diffraction patterns of the Cu 55-xZr 45Ag x

Fig. 3.4. The Avrami plots for Cu45Zr45Ag10 and Cu35Zr45Ag20

glassy alloys in a heat treated state.

glassy alloys constructed using the isothermal calorimetry data.

in Fig. 3.4 indicates that the crystallization of the alloys obeys the well-known kinetic law for thevolume fraction (x) transformed as a function of time (t): x(t) = 1–exp(–Ktn) provided that thenucleation and growth rates are time-independent which causes the linearity of the Avrami plot. The
values of the Avrami exponent obtained are close to 4 which indicates a steady-state nucleation and3-dimensional interface-controlled growth of nuclei.
XRD results (see Fig. 3.3) indicate that (Cu,Ag)10Zr7 phase is the main structure constituent
in the Cu45Zr45Ag10 alloy while the Cu35Zr45Ag20 alloy contains a large fraction of (Ag,Cu)Zr phase.
The phase composition was confirmed using TEM (Figs. 3.5 and 3.6) including selected-area
electron diffraction (SAED) patterns.
A small volume fraction (~10 vol.%) of a residual globular glassy phase is observed in the
structure of the Cu35Zr45Ag20 alloy (Fig. 3.6 (a)) while no phase separation was found in the caseof the as-solidiﬁed sample and the sample annealed at Tg. According to EDX analysis (Table 3.1)
the residual amorphous phase is depleted in Ag compared to the crystalline region. It contains only
about 14 at.% Ag.
A limited difference in the crystallization behavior of Cu55-xZr45Agx alloys (at least at x ≤ 10)

15

Fig. 3.5. The structure of the Cu45Zr45Ag10 alloy annealed at

Fig. 3.6. The structure of Cu35Zr45Ag20 alloy annealed at 722 K

730 K for 300 s, TEM. (a) – Bright-field image. Grains offor 1 ks, TEM. (a) – Bright-ﬁeld image. The insert-SAED pattern

both (Cu,Ag)10Zr7 and (Ag,Cu)Zr phases are observed. (b) andfrom the globular amorphous phase. (b) and (c) SAED patterns(c) SAED patterns taken from (Cu,Ag))10Zr7 and (Ag,Cu)Zr

taken from (Ag,Cu)Zr and (Cu,Ag)10Zr7 phases, respectively.

phases, respectively.
Table 3.1. Chemical composition of the thermodynamic phases observed in Cu35Zr45Ag20 alloy annealed at 722 K for 1 ks.

Phases\Elements

Cu, at.%Zr, at.%Ag, at.%oC68 (Cu,Ag)10Zr7

34

43

23

tP4 (Ag,Cu)Zr

20

54

26

Residual amorphous

34

52

14

was found. Ag-bearing and Ag-free alloys exhibit devitriﬁcation forming of a supercooled liquid
region and subsequent crystallization forming equilibrium crystalline phases. The simultaneous
formation of oC68 (Cu,Ag)10Zr7 and tP4 (Ag,Cu)Zr solid solution phases in the Ag-bearing alloys
by nucleation and 3-dimensional interface-controlled growth of nuclei indicates rather eutectic
crystallization. These phases exist in both studied Ag-bearing alloys at room temperature inaccordance with the equilibrium Ag-Cu-Zr phase diagram.
Tg/T1 ratio for the Cu45Zr45Ag10 alloy of 0.596 (Fig. 3.2 (b)) is larger than that 0.587 calculated
for the binary Cu55Zr45 alloy calculated taking into account its liquidus temperature of about 1183
K and 0.563 obtained for Cu35Zr45Ag20 alloy having Tl of 1203 K. γ parameter (Tx/(Tg+T1)) values
are 0.399 for Cu55Zr45, 0.413 for Cu45Zr45Ag10 and 0.404 for Cu35Zr45Ag20 alloy (Fig. 3.2 (b)). As
one can see the addition of 10 at.% Ag causes a signiﬁcant increase in Tg/T1 and γ parameter values
in expense of lowering T1 and increasing Tx. Cu45Zr45Ag10 alloy exhibits the highest GFA (critical
diameter of about 6 mm) while Cu35Zr45Ag20 alloy shows much lower GFA in accordance withlower Tg/T1 and γ parameter values.

16

Cu35Zr45Ag20 alloy exhibits possible phase separation upon heating within a supercooled
liquid region just prior to crystallization of a liquid because no phase separation is observed at Tg
and below it. This may be the reason for the existence of the residual globular amorphous phaseobserved in the sample heat treated at 722 K for 1 ks (Fig. 3.6 (a)). This residual amorphous phaserequires higher temperature for crystallization. Such a phase separation in the Cu-Zr-Ag system
can be expected according to a simple eutectic-type Cu-Ag phase diagram and a slightly negative
mixing enthalpy in Cu-Ag pair.
4. Inﬂuence of Al and Ag on the Devitriﬁcation Behavior of a Cu-Zr Glassy Alloy
Cu-Zr-Al-Ag alloys also showed high GFA [8]. The DSC traces of the as-solidiﬁed Cu50Zr50,
Cu50Zr45Al5 and Cu45Zr45Al5Ag5 samples are shown in Fig. 4.1.

Fig. 4.1. DSC traces of the studied alloys. The DSC trace of the
Cu50Zr50 alloy is also shown for comparison; P2 illustrates the positionof the broad second peak.

The phase composition of the annealed samples is determined using the XRD patterns
shown in Fig. 4.2. During the exothermic reactions (see peaks in Fig. 4.2) the supercooled liquid
crystallizes forming oC68 Cu10Zr7 phase and an unidentified phase in Cu50Zr45Al5 alloy while
Cu45Zr45Al5Ag5 alloy forms a mixture of oC68 Cu10Zr7, cF16 Cu2AlZr and tP4 AgZr phases. XRD
results (see Fig. 4.2) indicate that although the Cu10Zr7 phase is the main structure constituent in
the studied alloys other crystalline phases also precipitate simultaneously indicating rather eutectic
crystallization.
The isothermal calorimetry data showed that in both alloys the crystallization started after a17

Fig. 4.2. XRD patterns of the studied
alloys in a heat-treated state.

certain incubation period (about 180 and 230 s for Cu50Zr45Al5 and Cu45Zr45Al5Ag5 alloys at 745 and746 K, respectively) which indicates nucleation and growth transformation mechanism.
All of the Avrami plots of ln(-ln(1-x)) vs. ln(t) demonstrated a non-linear character whichindicates that the nucleation and growth rates are rather time-dependent. Nevertheless, an average
Avrami exponent for Cu45Zr45Al5Ag5 alloy is close to 4 which may indicate the nucleation and3-dimensional interface-controlled growth process.
The binary Cu50Zr50 alloy exhibits inharmonic shape of the DIC traces, and thus, the shape of
the Avrami plot is not linear indicating deceleration of the phase transformation with time (4.3). The
XRD analysis indicates that the crystallizing phase is rather a metastable monoclinic CuZr phase.
This is in consistent with the Cu-Zr phase diagram which indicatives that cubic cP2 CuZr phase isunstable below 988 K. The mP CuZr phase decomposes at higher temperature while Cu10Zr7 andCuZr2 phases are formed.

Fig. 4.3. (a) Isothermal calorimetry traces and fraction transformed asa function of time (without incubation period) of the Cu50Zr50 glassy
alloy and (b) the resulted Avrami plot. (c) XRD trace of the sample
annealed for 1.2 ks at 698 K.

18

The crystallization behavior of the studied alloys nearly follows the equilibrium phase
diagrams, that is, Cu50Zr45Al5 and Cu45Zr45Al5Ag5 glassy alloys exhibit rather eutectic crystallization
while the Cu50Zr50 forms a nanoscale primary metastable CuZr phase with a monoclinic structure.
The formation of such a phase may be initiated by the relatively low crystallization temperaturecompared to the temperature of eutectoid transformation in CuZr alloy of 988 K.
The eutectic compositions are suitable for BMG alloys as these alloys exhibit low liquidus
temperature, and thus, high stability of a liquid versus crystallization compared to Cu50Zr 50
alloy. For example, Cu50Zr50 glassy alloy has a Tg/Tl ratio and γ parameter of 0.555 and 0.387,
respectively while Cu45Zr45Al5Ag5 glassy alloy 0.617 and 0.429, respectively. Cu45Zr45Al5Ag5 alloy
also shows a large supercooled liquid region of 86 K which suggests that it can be deformed by
viscous ﬂow in this region.
Another important reason is effective filling of space by atoms of different sizes. Sucha feature may also be favored at eutectic compositions at which liquid shows high stability.
Nevertheless, as cooling conditions upon bulk glass casting are far from equilibrium one must
consider the effect of the metastable phase diagram and the shift of eutectic point. The fact that the
crystalline Cu45Zr45Al5Ag5 alloy does not show a single stage eutectic reaction on melting agrees
well with the proposition that the best glass-formers belong to slightly off- equilibrium eutectic
compositions and correspond to non-equilibrium eutectics. Solute partitioning upon eutectic
crystallization may also be the reason restricting the crystallization rate of Al- and Ag- bearing
alloys compared with Cu-Zr one.
EXPERIMENTAL DETAILSThe ingots were prepared by arc-melting mixtures of pure metals in an argon atmosphere.
From these ingots, ribbon samples of about 20 μm as well as bulk glassy samples having a diameterof 3 or 5 mm were prepared by rapid solidiﬁcation of the melt on a single copper roller at a roller
tangential velocity of about 40 m/s and a Cu-mould casting technique, respectively. The structureof the samples was examined by X-ray diffractometry (XRD) with monochromatic CuKα radiation.
Phase transformations were studied by differential scanning calorimetry (DSC) at a heating rateof 0.67 K/s and differential isothermal calorimetry. Transmission electron microscopy (TEM)
investigation was carried out using a JEM 2010 (JEOL) microscope operating at 200 kV equippedwith an energy dispersive X-ray spectrometer (EDX) of 0.1 keV resolution.
Synchrotron radiation X-ray diffraction in transmission was carried out using a high energy
monochromatic beam of the European Synchrotron Radiation Facility (ESRF) equipped witha nitrogen-cooled double-silicon monochromator. After correction the measured intensity wasconverted to electron units per atom. The total structure factor S(Q) and the interference function
Qi(Q) have been obtained from the coherent scattering intensity. The radial distribution RDF(r) andpair distribution PDF(r) functions have been obtained by the Fourier transformation of Qi(Q).

19

References[1] W. Klement, R. H. Willens, and P. Duwez, Nature 187 (1960) 869.
[2] A. Inoue, Acta Mater. 48 (2000) 279.
[3] D. V. Louzguine-Luzgin, A. Inoue, A. R. Yavari, and G. Vaughan, Appl. Phys. Lett. 88 (2006)
121926.
[4] D. V. Louzguine and A. Inoue, Phil. Mag. Lett. 83 (2003) 191.
[5] D. V. Louzguine, A. R. Yavari and A. Inoue, Philosophical Magazine 83 (2003) 2989.
[6] D. V. Louzguine-Luzgin, T. Saito, J. Saida, A. Inoue, Journal Materials Research 23 (2008) 515.
[7] W. Zhang and A. Inoue, J. Mater. Res. 21 (2006) 234.
[8] Q. Zhang, W. Zhang and A. Inoue, Mater. Trans. 48 (2007) 629.

20

Prof. Max LAGALLY主任研究者 ラガリー・マックス

Silicon Nanomembranes
Max G. Lagally, University of Wisconsin-Madison
Madison, WI 53706 USA
lagally@engr.wisc.edu
Introduction
Silicon-on-insulator (SOI) is the source for a new class of nanostructures, Sianomembranes(SiNMs). In SOI, a SiO 2 layer is interspersed between a thin crystalline top
Silayer and the bottom Si wafer: the ability to etch this buried oxide selectively creates
themembranes. When released from the oxide, this layered structure can form extremelyflexible
thin nanomembranes, with thicknesses from several 100 nm to less than 10nm,which can bestrain engineered and patterned to create various shapes (including sheets,tubes, spirals, andribbons), depending on how one defines a pattern before release. [1-2]The opportunity to create
nanomembranes exists for many other materials beyond Si andGe: all that is required is a releaselayer, a layer that can be preferentially removed withoutdamaging the material of the membrane.
The strain is introduced into membranes bycareful heteroepitaxial growth techniques.
Table 1 shows a summary of the novel features of Si nanomembranes and the propertiesthat
are or can be modiﬁed from those of bulk Si when Si is in the form of a thin membraneThe very
high interface-to-volume ratio makes
surfaces or interfaces potentiallyvery
important contributors to uniquemembrane properties. [3] The
consequence is awide variety of potential
applications in sensors, photonics andoptoelectronics, flexibleelectronics,
and thermoelectrics. Even biologicalapplications are likely. Furthermore,nanomembranes are possible inother materials
systems and mayprovide an entirely

21new platformfor nanomaterials applications.In the form of nanowires ornanoribbons (easily madeby selectivepattering before etching to releasethem) SiNMs may have a modified band structureand density of states if sufﬁcientlysmall. Combined with the strong dependence of the conductivity
on the surface,SiNMs in the form of ribbons may have signiﬁcant potential as nanowire sensors ordevices,in the same manner as proposed for carbon nanotubes and for nanowires producedby vaporliquid-solid (VLS) growth, but in a much more controlled fashion.

Figure 2. Si nanomembranes in various
shapes, clockwisefrom top left: as released,rumpled but single-crystal;transferred to a
new host substrate; patterned and etchedinto
arrays of nanoribbons; and strain engineered
and cutto make corkscrews or tubes.

Figure 2 shows optical images of SiNMs made from SOI(001) in various shapes – ﬂatsheets,
tubes, wires, and sheets that are warped, rumpled, or rippled; or smoothly curved.
All of them are single-crystal,with one, two, or three epitaxiallayers. Membranes withmorelayers have also been made, andsingle- or multilayer membraneshave been stacked ontopof each other. The stackedmembranes do not have an epi-taxial relationship to each other, ofcourse, but each membrane is a single crystal, with alllayers in that membrane having an epitaxial
relationship to each other.
Fabrication of Si nanomembranes
To fabricate elastically strained Si membranes from SOI(001) [4] a heteroepitaxial,strained
SiGe layer is grown on the Si template layer of SOI. Because Ge has a 4% largerlattice constant
than Si, a SiGe alloy layer grown epitaxially on unstrained Si will becompressed. The higher the Ge
alloy concentration, the higher the compressive strain becomesin this layer, but to avoid dislocation
formation, the layer must be thin. To createflat released membranes, an additional layer of Si isgrown to balance the effect of thetemplate Si layer when the structure is released. When this threelayer sandwich is stillattached to the oxide, only the middle (SiGe) layer is strained. It cannot relax
elasticallyuntil the structure is released from the oxide – at that stage the strain in the SiGe layer
isshared elastically with the two Si layers on either side. They become tensilely strainedwhile the
SiGe becomes less compressively strained.

22

To release the membrane, the oxide layer is selectively removed by HF. HF attacks SiO2
but not Si or Ge or their alloys. To increase the etch rate, access holes can be patterned inthe layer
sandwich before release. These etch holes are visible in the top two images in Fig. 2. The strain gets
divided among the layers, with the strain in the Si layers of oppositesign as that in the SiGe layer; i.e.,they are under tension. The SiGe cannot relaxcompletely. The degree of strain in the Si is related to
its thickness – the thinner, relativeto the SiGe layer, the more strain.
While the use of SiGe as the straining layer always produces tensile strain in the Si layersand
compressive strain in the SiGe layer, other materials combinations (e.g., a C-containing Si layer)
could produce compressively strained Si. In non-group-IV materials,e.g., III-V materials, bothoutcomes (compressive or tensile top layer) are also possible.
Characterization and applications
Thinness and strain show dramatic effects in Si nanomembranes. Thinness causeschanges
in conductivity that are surprising and dramatic under the proper circumstances.[3] Strain in
nanomembranes affects the electronic band structure. [5,6] Examples ofthese two phenomena are
brieﬂy described here.
When a doped semiconductor is made very thin, the total number of dopants per unit
areabecomes small. The sheet resistance thus naturally increases as the membrane gets thinner.
If, in addition, there exist surface or interface states, the nature of the conductivitycan change. For
example, the number of dopants in one cm2 of a 100nm thick Si membranedoped at 1015 cm-3 is 1010.
There are ~1011 cm-2 states at an oxide/Si interface thatcan trap charge, depleting this Si membraneof mobile charge carriers. The membrane(and all thinner ones), behaves as if it were intrinsic. Suchbehavior can be observed [3].Thus for thin enough oxide-covered Si membranes, the Fermi level
will be determined bya competition between interface trap density and membrane “bulk” dopant
concentration.This statement will be true for any materials system that has, relative to the dopant
concentration,a sufﬁcient density of interface states that can trap charge in the appropriate manner.
SiNMs with a surface oxide are therefore very good insulators at room temperature.
Theywould be very good insulatorseven if the top oxide were replaced withanother materialthat does not provide
traps, because the bottom interface
provides more than enough trapstates.
But if the top oxide layer is replacedwith a layer that provides surface bands
inappropriate positions in energy,
conductivity can not only be restored to23

what the valuewould have been for a bulk wafer doped at the same level,but it can be signiﬁcantly
enhanced. In Si(001) membranes,the simplest example is a clean surface. Si(001) surfacesforma 2x1 reconstruction to minimize the dangling bondson the surface, consisting of π* and π bands,
each with thedensity of states of about 1015 cm-2 eV-1. These bands arepositioned [7] approximately
such that it is easy to excite electronsthermally across the gap (variously determined to be 0.35to
0.6 eV) from the bulk valence band edge to the surfaceLUMO band than across the 1.1 eV bulk
gap. Two parallelconducting paths, electrons on the surface and holes in the“bulk” of the Si
membrane, can now contribute to the conductivity of the membrane system.This so-called surface
transfer doping restores conductivity via “bulk valence band”holes even if the surface conduction is
negligible (e.g., the surface electron mobility issmall). Figure 3 shows a schematic diagram of theband structure that is associated withsurface transfer doping. The number of states on the surface
is approximately the numberof atoms on the surface, hence 1014 to 1015/cm2. That is a high doping
level for a thinmembrane and stays this large no matter
how thin the membrane is made. In general,then, HOMO
(highest occupied molecular orbital) or LUMO (lowest
unoccupied molecularorbital) states localized in energy
and appropriately positioned in energy with respectto
the valence or conduction bands of the membraneprovide for the thermal creation ofhigh-mobility free
carriers in the membrane. The deposition or adsorptionof appropriatemolecular or atomic layers on the surface
could allow the same behavior outside a vacuumand create
the potential for a new class of chemicaland biologicalsensors. A schematic example of a possiblesensor isshown in Fig. 4. It has not yet been possible toachieve
the appropriate surface functionalization to providethe surface bands, but all elements of the idea
are present inFig. 4. If a surface band is created and charge can be eitherinjected or removed fromthe band by speciﬁc adsorptionof a biomolecule or toxin,
the conductivity along the ribbonwould change by manyorders of magnitude.
As a second example we show characterization of
the band structure changes that occurwhen Si is strained.
In particular, we show the situation corresponding to ourcurrentSiNMs, i.e., tensile biaxial strain in the plane. The
relevantparts of the Si band structure are shown in Fig.
5. Inparticular the conduction band minimum is affectedbystrain. Strain splits the 6-fold degeneracy of the24minimum,reducing the intervalley scattering and thus raisingtheelectron mobility by more that a factor if 2 if the strain
ishigh enough.
A theoretical determination of the change in band gap
with tensile or compressive strainis show in Fig. 6 [8]. The ∆4 and ∆2 valleys at the X point split with strain. Both movetolower energies with tensile strain, but by different amounts.
For compressive strain, they move to higher energies. The
valence band also shifts, so that the band gap becomessmaller
for both compressive and tensile strains. The availability of
uniformly strained,defect-free Si membranes makes it possible
to measure the strain dependence of featuresof the band
structure. The required resolution demands atunable source of x-rays with ~10meV line width. Suchxraysources require a synchrotron. We perform x-ray absorptionspectroscopy using the Universityof WisconsinSynchrotron to measure the shift in conduction band featureswith strain. Figure 7
shows an absorption edge andand the near-edge structurethat is observable. Making thesemeasurements as a functionof the strain in a membrane and usingthe derivative spectra
to identify the precise positionsof peaks allows us to obtainband shifts with strain. Wecan therefore predict the band
gap with strain.
Many other structural, chemical, mechanical,
andelectronic properties of membranes can be expected.
Ourresearch efforts are currently focused on exploring
suchproperties in detail.
These unique properties allow many potential
applications.We have already shown that we can make extremely fast ﬂexible electronics, [9,10]
stacked membranes for Bragg mirrors, [11] simple 2D photonic crystals, first-generationfast
photodetectors, thermoelectric nanowires, and membrane materials for hybridorientationand hybrid
composition electronics. The opportunities are far-reaching bothfor fundamental science and for
useful technology.
Summary
Single-crystal semiconductor membranes, at the dimensions discussed here, represent anew
class of nanosystems with exceedingly large potential. In particular, the ability to introducestrain
without defects, the ability to make thousands of identical structures usingtop-down approaches,

25

and the ability to integrate structures with different dimensionalities,different shapes, and different
compositions make these systems very versatile.Nanomembranes can be made from othersemiconductors and even non-semiconductors.The focus here has been on ﬂat SiNMs, because theirproperties are both easier to understandand more near-term in their potential applicability than thoseof curled SiNMs.
Acknowledgements
I have greatly beneﬁted from collaborations with and the efforts of G.K. Celler, M. M.Roberts,
D. E. Savage, F. Liu, P.P. Zhang, H.C. Yuan, I. Knezevic, Z.Q. Ma, P.G. Evans,M.A. Eriksson, R.
Blick, L. Klein, C. Ritz, F. Flack, S. A. Scott, and M.H. Huang. Aspectsof the work to which thisgroup of researchers contributed were variously supportedby the USDOE, USAFOSR, USNSF, andSoitec, Inc.
References[1] M. H. Huang, C. Boone, M. M. Roberts, D. E. Savage, M. G. Lagally, N. Shaji, H.Qin, R. Blick, J.
A. Nairn and F.Liu, Adv. Mater. 17 2860 (2005)[2] For a review of early work, see S A. Scott and M.G. Lagally, J. Phys. D. 40 R1 (2007)[3] P. P. Zhang, E. Tevaarwerk, B. N. Park, D. E. Savage, G. K. Celler, I. Knezevic, P. G.Evans, M. A.
Eriksson, and M. G. Lagally, Nature 439 703 (2006)[4] M.M. Roberts, L.J. Klein, D.E. Savage, M. Friesen, K.A. Slinker, G. K. Celler, M.A.Eriksson,and M.G. Lagally, Nature Materials 5, 388 (2006)[5] C. G. Van de Walle and R. M. Martin, Phys. Rev. B 34, 5621 (1986)[6] C. Euaruksakul, Z. W. Li, F. Zheng, F. J. Himpsel, C. S. Ritz, B. Tanto, D. E. Savage,X. S. Liu,and M. G. Lagally, Phys. Rev. Lett., in press.
[7] J. E. Northrup, Phys. Rev. B. 47 10032 (1993)[8] F. Liu, in preparation[9] E. Menard, K. J. Lee, D.-Y. Khang, R. G. Nuzzo, and J. A. Rogers, Appl. Phys. Lett.84 5398
(2004)[10] H.C. Yuan, Z.Q. Ma, M. M. Roberts, D. E. Savage, and M. G. Lagally, J. Appl. Phys.100
013708 (2006)[11] Weina Peng, M. M. Roberts, E. P. Nordberg, F. S. Flack, P. E. Colavita, R. J. Hamers,D. E.
Savage, M.G. Lagally, and M. A. Eriksson, Appl. Phys. Lett. 90 183107 (2007)26

Assoc. Prof. Ken NAKAJIMA
准教授 中嶋 健“Nanoﬁshing of a Single Polymer Chain”(Presentation at the 1st WPI & IFCAM Joint Workshop, Tohoku University)
Ken NAKAJIMA
Associate Professor, WPI - Advanced Institute for Materials Research
1. Introduction
Let’s open the ﬁrst page of the books for polymer physics or rubber physics. 1-3) These books
usually begin with the explanation of the origin of polymer chains’ elasticity. It is not originatedfrom energetic elasticity mainly possessed by solid-state materials such as a metallic spring
but from so-called entropic elasticity. The entropic elasticity is the consequence of an essential
property of polymer chains, i.e., a single polymer chain being composed of a large number ofunit constituents. Thus, even a single polymer chain can be treated by statistical mechanics. The
calculation with a boundary condition that these constituents are bonded to each other to form achain structure gives the ﬁnal formula of entropic elasticity. This is the story developed during theera when a great advocate of polymer science, Prof. H. Staudinger, proposed that a single polymer
takes the form of a linear string or chain molecule.
The entropic elasticity of a single polymer chain is treated similarly with the statisticalmechanical property of gas molecules. Now, let’s compare gas molecules to children movingaround freely (with random speed and direction) in a schoolyard even though this situation is twodimensional. Then, in the case of a single polymer chain, the children must move hand in hand as
in a schematic illustration in a famous textbook of polymer physics (Figure 1). 1) This is the only

Figure 1 Drawing to help explain entropic elasticity. 1)

27

restraint condition for the children who become constituents of a polymer chain. If two adult men
take the hands of the children at each end of the chain and they stand at ﬁxed point, they must feela very strong force. This is analogous to the restoring force of rubber, entropic elasticity. Higher
temperature is equivalent to the children’s cheerfulness. In fact, the entropic elasticity for a single
polymer chain is calculated to be proportional to absolute temperature using a statistical mechanicaltreatment, 2)

where kB is the Boltzmann constant and n and a are the number and size of the constituents(monomers, children), respectively. The front factor of x can be treated as the spring constant ofrubbery elasticity, which obeys Hooke’s law.
Some readers might be suspicious of the above description. Polymers are very small chain
molecules and are under intense thermal motion. By using statistical mechanics, the spring constantof a single polymer chain can be calculated. Furthermore, this spring is not like a metallic spring
but is entropic. However, who has observed it directly? Has anyone experienced pulling a single
polymer chain as in the case of macroscopic rubber? The actual situation is that nobody has seen
this theoretically equipped situation of the microscopic world. In any case, molecular-level rubber
elasticity has been “a house of cards” based on the single polymer chain entropic elasticity without
any experimental evidence.
The fact that rubber shows rubber elasticity was discovered more than hundred years
earlier than Prof. H. Staudinger’s proposal. The memory effect acquired by vulcanization, socalled Gough-Joule effect and its thermodynamic explanation were the great achievements in thenineteenth century. As seen in many textbooks, 1-3) this thermodynamic approach was the easiest
one to gain consistency between ever-performed experiments and theory. In fact, thermodynamics
of rubbery material can be treated in parallel with thermodynamics of gas. One could show
experimentally that rubber elasticity was mainly dominated by entropic elasticity, which at present
all of rubber researcher should know. However, the situation is almost the same with the case of asingle polymer chain; nobody has seen the movement of chain molecules in rubbery materials as in
the case of gas molecules.
In this report, I describe the experimental methods that can be used to answer this serious
question; whether a single polymer chain really has entropic elasticity. For the purpose, I introduce
the technique named as “single molecule force spectroscopy” or “nanofishing,” where a single
polystyrene chain as an example is stretched at its both ends in a solvent. The experiment was
realized by atomic force microscope (AFM) 4) that was invented in the late twentieth century. It
has been possible to perform a real experiment for the conceptual experiment that appeared in the
famous textbook written by Prof. P. G. de Gennes. 5)28

2. Entropic Elasticity of Single Polymer Chain
2.1 Brief Introduction
The use of AFM has enabled us to visualize a small world consisting of atoms and molecules.
Similar to reading Braille by tracing with a ﬁnger or tracking grooves with a needle in a phonographrecord, AFM uses a very sharp probe tip (ultimately consisting of a single atom) to trace the surface
of an object. For the reader’s imagination, think of a scale with a length of 10 cm. A sharp spike of
5 mm long is attached at the end of the scale. Take another end and push the spike onto a desk. The
stronger the force becomes, the larger the deﬂection of the scale. Then, move the assembly of thescale and the spike on the desk. The deﬂection changes in accordance with the surface topography.
Raster scans over the two-dimensional surface thus reproduce a surface topographic image, whichis the principle of AFM. By scaling down by a thousand times, the scale and the spike correspondto a cantilever and probe of AFM. The tip radius of the spike (10 µm) then corresponds to that ofthe probe (10 nm). If the AFM system detects a deﬂection of 10 nm (10 µm on the macroscopic
scale), it becomes possible to detect a weak force. Since typical values of the spring constants ofcantilevers become about 1 N/m or even less, the resolution of force detection reaches 1 nN.
By using AFM, we are able to visualize a single polymer chain, as many researchers havealready reported. In practice, a single polymer chain looked like a piece of string. 6) It does notresemble a metallic spring at all. The recent study has revealed the dynamic movements of suchstrings. 7) Furthermore, dozens of researchers have placed their attention on the expanded capabilityof AFM as single molecule force spectroscopy. With this technology, the relationship between
force and deformation of a single molecule sandwiched between a substrate and an AFM probe has
been measured. A technique “nanofishing” introduced in this section comes under this category.
Now, in the twenty-ﬁrst century, we have obtained a novel method for experimentally verifying thestatistical mechanics of a single polymer chain, which is indeed the basis of polymer physics. As
in the case of stress-strain (S-S) curves of rubber, we can now investigate such curves for single
polymer chains.
2.2 Static Nanoﬁshing
According to the literature, trials on stretching single polymer chains by means of AFM havebeen extensively performed by many researchers as schematically shown in Figure 2. However,these trials have been especially dedicated to studies of “single protein unfolding” events, 8-11)
physisorbed macromolecules. 12-15) In the former case, proteins with modiﬁed ends were sandwiched
via speciﬁc chemical bonds between a substrate and a cantilever and could be successfully stretchedin the course of force-distance curve measurement by AFM. Those successes were attributed to two
principal factors, the unique three-dimensional structures of proteins and the genetic engineering
techniques for modifying the ends of proteins. Thus, picking points were controllably fixed.

29

From a comparison of such cases, synthesizedpolymers have many disadvantages. For instance,
it is difficult to attach any reactive groups attheir ends in general. Thus, the force-distancecurve data obtained in uncontrolled experiments
contain many complicated factors such as thatthe adsorptions occur not only at the tails but also
at the loops. It is very difficult to judge whether
the stretching event is really for a single polymer
chain. The polydispersity also makes the situation
very complicated. As described, however,successful nanofishing results on polystyrene(PS) chains with chemically active termini havebeen reported and thus several statistical analyses
could be conducted to investigate the physical e
polymer chain. 16,17)
A thiol-terminated PS was used as asample in the experiment. It was based on a living
polymerized carboxyl-terminated PS with Mn = 93,800 and MW = 100,400. The polydispersity was
MW / Mn = 1.07. The degree of polymerization was about 900 and, thus, its contour length was
about 220 nm. The thiol groups were substituted for the carboxylic ends using 1,10-decanedithiolby means of thiolester bonding, anticipating the preferential interaction between thiol and gold.
No dimer formation was indicated from the result of gel permeation chromatography (GPC).
The polymer was dissolved in a Θ solvent, cyclohexane, at 20 µM/ml, and 10 µl of the solution
was cast on a Au(111) substrate. After 5 min of incubation, the surface was rinsed with a puresolvent to remove weakly physisorbed molecules. These sample preparation procedures brought
about well-dispersed (checked by AFM imaging, data not shown) chemisorption of polymer
chains that possibly took on a random-coil form just like advertising balloons, which assured thatthe subsequent nanofishing measurements were on a single-molecule basis. The apparent sizeof the random-coil obtained by AFM imaging (data not shown), W, was about 40 nm. Then, byconsidering probe-shape convolution effect, 18) the deconvoluted size Rs could be calculated, withthe formula of, to be about 5 nm assuming that a probe radius, Rt, of 20 nm. The value
was almost comparable with the radius of a single polymer chain. Indeed, the radius of a polymer
random coil with n = 900 and a = 0.25 becomes. It was very rare to ﬁsh “two ﬁsh” atthe same time. At this stage, it was speculated that, except for their modiﬁed termini, the interaction
between a gold substrate and polymer chains would be weak enough for them to not strongly adsorb
with collapsed or extended forms. Thus, this was the case of the adsorption of polymer chains to a30

repulsive wall. 5)
In addition to the use of cyclohexane,
dimethylformamide (DMF) was used as a good
solvent. So as to pick up the thiol- modified
terminal, gold-coated cantilevers were used. The
nominal values of their spring constant k1 were
30 or 110 pN/nm. A typical force-extension
curve measured in cyclohexane is shown in
Figure 3. 16) The solvent temperature was kept
at about 35°C, which corresponded to its Θtemperature for PS chains. Thus, a chain should
behave as an ideal chain. The slope at the lowest
extension limit (dashed line in Figure 3) was 1.20
× 10-4 N/m. The value should be that of single
polymer chain elasticity caused by entropic
contribution. At ﬁrst glance, the force data ﬂuctuated a great deal. However, this ﬂuctuation was
due to the thermal noise imposed on the cantilever. A simple estimation told us that the root-meansquare (rms) noise in the force signal ( ∆F ~ 15.6 pN) for an extension length from 300 to 350 nm
was comparable with the thermal noise, ∆F = (k1kBT)1/2 ~ 21.6 pN.
As for further analysis, curve fitting against the worm-like chain (WLC) model was
conducted and indicated as a solid line in Figure 3. The model describes single polymer chain
mechanics ranging from random-coil to fully extended forms, as follows:

where x is the extension length at an external load of F. lp and L are persistence and contour lengths,
respectively. The ﬁtting results were lp = 0.31 ± 0.01 nm and L = 284.5 ± 0.8 nm. The persistence
length almost corresponded to the length of a single monomer. The degree of polymerization wasthen calculated from these two intrinsic values to be about 918, which was in good agreement withthe expected value from the synthesis information. Actually, a histogram of contour length madeby dozens of successful ﬁshing results showed a single peak with good coincidence with the GPC
result. Another estimation was made with the following equation derived from Eq. (2) for condition
x << L:The substitution of lp and L resulted in a spring constant of 0.71 × 10-4 N/m for single31polymer chain elasticity, which was almost equivalent to the experimentally obtained slope value,
1.20 × 10-4 N/m, and the value obtained by the simplest form, 1.45 × 10-4 N/m (n = 918 and a =0.31 nm for Eq. (1)). These comparisons implied that the measurements were consistent with thetheoretical predictions. The deviation between the rupture length of 260.9 nm and the ﬁtted contour
length indicated that the polymer chain was not fully stretched at the rupture event. The reason forthis was that the rupture event was a stochastic process and was dependent on many factors such as
pulling speed, bond strength, and temperature. The validity of the freely jointed (FJC) model (dashedgreen line) was also checked:

where lK is the Kuhn length and (y) is the Langevin function. It was obvious that the FJC
model did not represent the experimental result well. Thus, nanoﬁshing could be used to judge theever-present basic theories of polymer physics.
Further examinations by changing solvent temperature and solvent type proved to be usefulin order to reveal statistical mechanical properties such as the second virial coefﬁcient on a single
polymer chain basis. 16,17) Such properties have normally been investigated by light scattering or
osmotic pressure as averaged information from an ensemble of many polymer chains. Detailed
discussions are given in the literature and readers are strongly recommended to refer them, while
differences in solvent qualities cause differences in the conformations of single polymer chains,resulting in observable changes in force-extension curves. The results seemed to be explained wellby Flory’s lattice model, at least as the ﬁrst approximation. 19)
In a poor solvent (cyclohexane at 5°C), a polymer chain takes on a condensed globular statebecause constituent molecules are repulsed by
the solvent molecules. Nanofishing of this chain
revealed a perfectly different force-extension
curve, as shown in Figure 4. It was observed that
constant force continued from about 30 to 130 nm
after non-specific adsorption between a substrate
and a probe. This phenomenon could be related
to the extraction of monomers one by one from acollapsed globule into a stretched strand, which
was numerically predicted by M. Wittkop andcoworkers. 20) Using Monte Carlo simulation, theyexamined the deformation behavior of a single
polymer chain below the Θ temperature, thus
the poor region. This type of measurement is not32possible by ensemble-averaged measurements, and nanoﬁshing is the only tool that can be used. As
seen, nanoﬁshing seems to have a very promising future. However, there are several drawbacks to
be solved. For example, it is difﬁcult to deﬁne the transverse section perpendicular to the extension
direction and therefore stress that is defined as force divided by section area. It is also difficult
to discuss Poisson’s ratio, though these deﬁnition themselves may be unnecessary. If an ultimate
goal is the measurement of single polymer chain elasticity inside a practical rubbery material, thecurrent status that the measurement must be performed in liquid condition would be severe problem
together with the above-mentioned difﬁculty for uncontrolled physisorption-based experiments.
Before closing this section, we would like to introduce an experiment that checks thevalidity of the hypothesis of whether nanoﬁshing is performed in a quasistatic condition. The forceextension curve in Figure 3 was taken at a pulling speed of 2 µm/s. Under this condition, no change
in force was observed if extension was stopped at a certain point before rupture. Thus, it could betentatively regarded that the condition was near equilibrium. In contrast, the curve in Figure 5 gave
an apparent deviation from the former result. 21) In this experiment, a single polymer chain was
exposed to a sudden, stepwise stress-increase of maximum 1500 pN with the speed of 500 pN/ms
for four times before rupture as in the case of themacroscopic creep measurement. In this case,the pulling speed corresponded to about 17 µm/s,
8.5 times faster than the former measurement.
A sudden increase in force with almost “no”
deformation of the polymer chain was observedfor each part. It was speculated that thisphenomenon was due to topological constraint,i.e., internal entanglement of a polymer chain. The
relaxation of this constraint was also found in eachsuccessive response. Further studies are necessary
to explain the observed result. Nevertheless, we
could conclude from this experiment that theresult obtained in Figure 3 was for a quasi-static condition because the curve merely shows mild,
gradual changes in contrast to that in Figure 5. In addition, we should note that the curve in Figure
5 might have a certain relationship with the hysteresis observed in the S-S curve for macroscopic
rubbery materials since we never observe any hysteresis for quasi-static nanoﬁshing.
2.3 Dynamic Nanoﬁshing
In the previous section, we showed that static nanofishing merely gave static information
about polymer chains. Therefore, viscoelasticity, which is a quite important property of polymers,
seldom appears. Why do polymer solutions have viscosity? How does an elastomeric isolator

33

absorb the energy created by earthquakes? It is possible to ascribe these qualities to the viscosity ofpolymers. Incidentally, there is a model to describe polymeric viscoelasticity, which is composed ofa single spring and a single dashpot (device composed of piston, cylinder, and viscous ﬂuid, which
gives resistance proportional to velocity). Let’s say that the model is an extremely simpliﬁed onethat can reproduce only a single relaxation event. Where are the spring and dashpot? What doesa dashpot look like? These are frequently asked questions by many students. The students do notunderstand that the model is just a phenomenological one. As explained, a single polymer chain
does not necessarily take the form of spring. In the same way, we do not ﬁnd dashpot structures
on a microscopic scale. The phenomena modeled by dashpots are several frictional contributions
such as internal friction between neighboring segments (monomers) during rotational movementwith respect to bonding axis, the friction between segments and solvent molecules, and the friction
between segments during cooperative movement in polymer “spaghetti.” These origins have theirown time constants. Thus, the use of the phenomenological model requires thinking about thekinds of interaction that take place. In any case, the frictional or energy-dissipative processes are ofgreat importance in applying polymers to realistic industrial applications. Thus, the origins must be
explained experimentally. Here, we introduce a
potential technique of revealing monomer-solvent
friction for a single polymer chain. 22)
The experimental setup was almost sameas that for the static measurement, while thecantilever was now vibrated at its resonant
frequency in liquidA
gold-coated cantilever was again used and its
spring constant was experimentally determinedby the thermal noise method (k1 = 29.6 pN/nm).
23)
The RMS oscillation amplitude was A = 6.0
nm when free oscillation (no interaction with a polymer or a substrate) was reached. In the case of
successful nanoﬁshing, the RMS amplitude decreased as in the case of the tapping-mode operation.
The phase shift between input and output signals also deviated from 90°, which was the value forfree resonance. The change in boundary condition by polymer chain attachment caused a change inthe free oscillation of the cantilever.
To obtain information about the mechanical properties of a single polymer chain, theseparation-dependent RMS amplitude and phase shift changes (data not shown) were analyzed
based on the phenomenological model depicted in Figure 6. Assuming sinusoidal drive of thecantilever basewith drive amplitude Ad and drive frequency , the deﬂection of thecantilever as a sinusoidal response can be written as
with amplitude A and phaseshift θ. The cantilever is represented by an effective tip mass together with a spring and dashpot

34

connected in parallel (Voigt element), which are characterized by mass m, spring constant k1, anddamping coefﬁcient η1. Here, the damping is due to the viscous drag by the surrounding medium
and the intrinsic damping caused by the deﬂection of the cantilever beam. A polymer chain is also
described as a Voigt element, which has a spring k2 and dashpot η2 connected in parallel; the massof a molecule is so small compared to the tip mass that it is ignored. The two parameters, k2 and η2,
are what we are interested in.
Analysis started from the following equation of motion: 24)

Parameters η1 and m were obtained from thespring constant of the cantilever k1 (= 29.6 pN/nm),
resonant frequency

1(= 2 f 1 = 21.2 kHz), andquality factor Q (= 25), as follows:

Note that these values were determined during
the cantilever tuning process and were therefore
independent of the stretching event. Although detailed
calculation is omitted, k2 and η2 could be calculated
from the solution of Eq. (5):where A1 is the oscillation amplitude without the polymer chain and ∆ is the difference in phaseshift with (θ) and without (θ1) the polymer chain (Δ= θ1-θ) Amplitude and phase shift in theapproaching process are regarded as A1 and θ1.
Figure 7 shows the obtained result. The stiffness of a single chain k2 (Figure 7a) increased
abruptly with stretching. This is commonly observed in conventional simple stretching experimentson polymers, as in Figure 3, i.e., an almost constant value for spring constant at low extension andan apparent increase due to the fully stretched effect caused not only by entropic contribution but
also by enthalpic contribution such as bond angle constraint. The average value of k2 in the low
extension region was 2.86 × 10-5 N/m. A new result, only available by dynamic nanoﬁshing for thefrictional coefﬁcient, which might be related to viscosity, was obtained as shown in Figure 7b. At

35

ﬁrst glance, almost zero viscosity was observed in the low extension region. However, viscosity hada certain value in this region. From Figure 7b, viscosity η2 in the region (30-150 nm) averaged 2.62
× 10-9 kg/s. Note that this value is the ﬁrst estimation of viscosity on a single polymer chain basis ina low extension region.
It was also found that the viscosity also increased in the same manner as in k2. There havebeen many theoretical descriptions for polymer-solvent friction. 1, 5, 19) As an extreme case, a single
polymer chain is treated as a rigid sphere with a certain hydrodynamic radius. The radius must be
correlated to Flory’s radius, RF. 25)

where ηs is solvent viscosity. In contrast, there is a theory called the free-draining model where all
constituents of a single polymer chain are exposed in a solvent ﬂow. 26)

where ζs and n are the frictional coefﬁcient between a constituent monomer and a solvent moleculeand the number of monomers, respectively. It seemed that the experimental result contradicted theassumption of the free-draining molecule, which ought to be treated as the ﬁrst approximation ofpolymer chains interacting with solvent molecules because the increase in viscosity in the higher
extension region was never explained by this simple additivity. The increase might be related to anincrease in the number of constituent molecules exposed to solvent friction. Further studies withimproved signal-to-noise ratios are necessary to explain this point including comparison with moresophisticated theories that can treat solvent ﬂow inside a random coil and an extended chain form. 5)
Up to here, the discussion has seemed to assume that viscosity η2 determined by dynamic
nanofishing is due to solvent friction, an idea that is not groundless. To verify this assumption,
the dependence of η 2 on solvent temperature and quality was investigated. There is a wellaccepted concept to describe polymer solution viscosity, intrinsic viscosity [η]. Values have been
accumulated for different polymer species, solvent qualities, and temperatures by conventional
techniques such as light scattering and osmotic pressure.

As is seen, [η] is the value of specific viscosity, ηsp =(η - ηs)/ ηs , is extrapolated to an
inﬁnitely dilute condition, where η, c, and M are solution viscosity, concentration, and molecularweight, respectively. K and α are constants depending on polymer species, solvent quality, or
temperature. 27) Although the units of [η] and of η2 were different, it was considered that the originof both parameters was the same to each other. This was because both parameters represented
viscosity in an infinite dilute polymer solution. Thus, the coefficient of the viscosity-molecularweight relationship was chosen as the property for comparison. Assumed that there exist a constant36

K' satisfying the following relationship:
To compute constant K' from the experimental data, molecular weight M was calculated
from the contour length obtained by WLC fitting against the force-extension curve (data notshown, however, obtained together with the curves in Figure 7), and a constant α was determinedby reference data. 27) The temperature dependence of the viscosity constant K' and correspondingreference data K are shown in Figure 8. Viscosity constants K' and K were divided by those inΘ solvent K'Θ and KΘ for scaling, respectively. Experimental and reference data showed good
agreement with each other. This agreement supports the idea that the viscosity measured here
clearly depends on solvent conditions as wellas intrinsic viscosity. Taking the increase inviscosity in the high extension region into
account, it would be concluded that viscosity
under 10 kHz perturbation was attributed tomonomer-solvent friction. The investigationof molecular weight dependence will also help
in checking the validity of the analysis. The
realization of such an experiment is expected.
3. Summary
To close the section, it is worth mentioning the possible future direction of anofishingexperiments. The leading person in this ﬁeld, Prof. H. E. Gaub and coworkers realized a kind ofheat engine using a photosensitive single polymer chain. 15) They used the cis-trans isomerizationof an azobenzene polymer to convert photochemical energy into mechanical energy. Of course,
industrial application of this technique is not simple, while the research gives us a kind of dreamthat nanofishing will open a promising future. In the past, studies on a single polymer chain
had been preceded by theoretical work. However, we believe that in the twenty-first century,
experimental studies will take the initiative in this ﬁeld.
References
1) Saito, N: “Polymer Physics (in Japanese)”, Syokabo, Tokyo, Japan (1967)
2) Kubo, R.: “Rubber Elasticity /the reprint of the ﬁrst edition (in Japanese)”, Syokabo, Tokyo,
Japan (1996)
3) Treloar, L.R.G.: “The Physics of Rubber Elasticity”, Oxford University Press, Oxford, UK

37

(1975)
4) Binnig, G., Quate, C. F., Gerber, Ch., Weibel, E.: Phys. Rev. Lett., 56, 930 (1986)5) de Gennes, P. G.: “Scaling Concepts in Polymer Physics”, Cornell University Press, New York,USA (1979)
6) Kumaki, J., Nishikawa, Y., Hashimoto, T.: J. Am. Chem. Soc., 118, 3321 (1996)
7) Ando, T., Kodera, N., Naito, Y., Kinoshita, T., Furuta, K., Toyoshima, Y.: Chemphyschem., 4,1196 (2003)
8) Mitsui, K., Hara, M., Ikai, A .: FEBS Lett., 385, 29 (1996)
9) Rief, M., Oesterhelt, F., Fernandez, J. M., Gaub, H. E.: Science, 276, 1109 (1997)
10) Dietz, H., Rief, M.: PNAS, 101, 16192 (2004)
11) Wang, T., Sakai, Y., Nakajima, K., Miyawaki, A., Ito, K., Hara, M.: Colloid Surf., B40, 183(2005)
12) Cortiz C., Hadziioannou, G.: Macromolecules, 32, 780 (1999)
13) Li, H., Zhang, W., Xu, W., Zhang, X.: Macromolecules, 33, 465 (2000)
14) Haupt, B. J., Senden, T. J., Sevick, E. M.: Langmuir, 18, 2174 (2002)15) Holland, N. B., Hugel, T., Neuert, G., Scholz, A. C., Renner, C., Oesterhelt, D., Moroder, L.,
Seitz, M., Gaub, H. E.: Macromolecules, 36, 2015 (2003)
16) Nakajima, K., Watabe, H., Nishi, T.: Polymer, 47, 2505 (2006)17) Nakajima, K., Watabe, H., Nishi, T.: Kautsch. Gummi Kunstst., 5-06, 256 (2006)18) Vesenka, J., Guthold, M., Tang, C. L., Keller, D., Delaine, E., Bustamante, C.:Ultramicroscopy,
42-44, 1243 (1992)
19) Flory, P. J.: “Principles of Polymer Chemistry”, Cornell University Press, New York USA (1953)20) Wittkop, M., Kreitmeier, S., Goeritz, D.: Phys. Rev. E, 53, 838 (1996)21) Nakajima, K., Watabe, H., Ohno, N., Nagayama, S., Watanabe, K., Nishi, T.: J. Soc. RubberInd., Jpn. (in Japanese), 79, 466 (2006)22) Watabe, H., Nakajima, K., Sakai, Y., Nishi, T.: Macromolecules, 39, 5921 (2006)23) Hutter, J. L., Bechhoefer, J.: Rev. Sci. Instrum., 64, 1868 (1993)24) Anczykowski, B., Gotsmann, B., Fuchs, H., Cleveland, J. P., Elings, V. B.: Appl. Surf. Sci., 140,
376 (1999)
25) Kirkwood, J. G., Riseman, J.: J. Chem. Phys., 16, 565 (1948)
26) Kramers, H. A.: J. Chem. Phys., 14, 415 (1946)27) Brandrup, J., Immergut, E. H.: “Polymer Handbook: 3rd ed.”, Wiley Interscience, New York,USA (1989)

38

Assoc. Prof. Taro HITOSUGI
准教授 一杉 太朗

Development of TiO2-based transparent conductor
Taro Hitosugi
WPI-AIMR, Tohoku University
ABSTRACT
We, here, report on the development of TiO2-based transparent conducting oxide (TCO)
which is a promising candidate for next-generation transparent electrode. Anatase Nb-doped TiO2
(Ti1-xNbxO2) epitaxial thin films with 0.03 ≤ x ≤ 0.06 shows resistivity ρ of 2-3 x 10-4 Ωcm andinternal transmittance of ~ 95% in the visible light region. Further, we demonstrate polycrystalline
ﬁlms deposited on glass with ρ = 4.6x10-4 Ωcm at room temperature. These values are comparable
to those of typical TCOs, In2-xSnxO3 (ITO). We also present the recent progress on sputter-deposited
TiO2-based TCO on glass substrate and discuss unique characteristics of this material, such as highrefractive index, high transmittance in infrared, and high stability in reduced atmosphere.
Introduction
Transparent conducting oxide (TCO) exhibits two properties at once, namely, high opticaltransmittance and high electrical conductivity. It has been an essential part of devices that requireFig. 1 Wide spread usage of transparent conducting oxides (TCO).

39

electrical contact and optical access, such as ﬂat panel displays (FPDs), light emitting diodes (LEDs),
and solar cells (Fig. 1).1,2 Currently, Sn-doped indium oxide (ITO) is the most widely used TCO,
because of its high transparent conducting properties3,4 and its easiness for preparation. There is,however, a strong demand to search for a new TCO from two backgrounds. One is that since TCO
is a key component in opto-electronic devices, various characteristics are required in emerging
devices. For example, if there is a TCO with high refractive index, brighter GaN-based LED would
be available. Solar cells and organic LED has similar problem, and if suitable TCO is developed,
more efﬁcient devices would be possible. The other background is the unstable supply of indium,
since its demand is continuously expanding. These backgrounds motivate us to develop a newtransparent conducting oxide. Although ZnO, SnO2 and many kinds of multicomponent materials
have been investigated as ITO alternatives,5 exploring new materials should be more encouraged inview of the increasing request for novel applications of TCOs.
In this article, we report on the development of a new TCO, anatase Ti1-xNbxO2 (TNO),6,7
which possess electrical and optical properties comparable to those of ITO. Anatase TiO2 is suitable
as a mother compound of next-generation TCO, because it is characterized by a wide band gap (3.2eV)8 and relatively low effective mass ~ m0 (m0: free electron mass).9 This TNO has properties that
ITO does not have, such as, high refractive index, high transmittance in infrared, and high stability
in reduced atmosphere. These properties would provide new optoelectrical devices, as witnessedin the role of ITO in FPDs. Moreover, titanium, being the major constituent of TNO is much moreabundant in the earth’s crust, approximately a factor of 105, than indium.10
Titanium dioxide (TiO2)
Titanium dioxide (TiO2) has been intensively
studied over the last decades because of its interesting
technological applications, such as for catalysis 1,
pigment 2, and sensors 3. More recently, TiO 2 hasbeen attracted researchers from the interest in theapplications for high-k materials, 11 resistive random
access memory (RRAM),12 and spintronics.13 Among
many polytypes of TiO2, technologically important
crystal structures are rutile and less dense structure,anatase (Figs. 2(a) and (b)). Of these phases, rutile
phase has been extensively studied. The reason is thatmost crystal growth techniques yield TiO2, basicallyin the rutile phase. Crystal growth of anatase isdifﬁcult because it is not the most stable structure in40

bulk form, and the ﬁrst report on the growth of single crystals were from Lévy et al.14 Both rutile
and anatase crystallizes in a tetragonal phase, and the structure of rutile and anatase crystals can be
described in terms of chains of TiO6 octahedra. The two structures differ by the distortion insideeach octahedron and by their network of octahedra. Each Ti4+ ion is surrounded by an octahedron of
six O2- ions. In an anatase structure, each octahedron is in contact with eight neighbors (four sharing
an edge and four sharing a corner) (Fig. 2(c)), while in a rutile structure, the coordination number is
10 (two sharing an edge and eight sharing a corner) (Fig. 2(d)).
A conventional method for controlling resistivity (ρ) of TiO 2 is to introduce oxygendeﬁciency δ, as expressed as TiO2- δ , by growing or annealing ﬁlms under reducing atmosphere.
In this way, the n-type carrier density, ne , of TiO2 is controllable by amount of δ. Empirically,
however, the obtained oxygen-deficient films tend to lose transparency, so that they are notapplicable to TCOs. Recently, we demonstrated a different and more effective way of carrier doping
into anatase TiO2, substitution of Nb for Ti.6,7 Nb is soluble to anatase TiO2 up to 20%, and Nbions, existing as Nb5+ , release conduction electrons with an efﬁciency of >90%. By using this Nbdoping technique, therefore, it is possible obtain highly conducting transition metal doped TiO2.
Further, the Nb-doping technique combined with the annealing procedure enables us to achieve
high electrical conductivity and high transparency at the same time even in polycrystalline ﬁlms on
glass substrate.
Nonmetal – metal transition & Transparent conducting properties
Figure 3(a) shows the ρ of Ti1-xNbxO2 epitaxial thin ﬁlms as a function of x. Undoped anatase
TiO2 shows ρ in the order of 10-1 Ωcm, and doping Nb result in the drastic drop of ρ, showing
minimum at x = 0.06. These anatase epitaxial thin ﬁlms were grown by pulsed laser deposition (PLD)
technique on SrTiO3(100) or LaAlO3(100) substrates,6, 7 and X-ray diffraction (XRD) conﬁrmed theFig. 3 (a) Resistivity and internal transmittance (@ 600 nm) of Ti1-xNbxO2. (b) Appearance of
Ti0.94Nb0.04O2 epitaxial thin ﬁlm on SrTiO3(100) substrate.

41

single phase growth of anatase. Fig. 3(a) also shows the internal optical transmittance at 600 nm
obtained from measuring extinction coefﬁcient. The transmittance gradually decreases on doping
Nb. What we found is that there is an optimum doping x that can achieve both high electricalconductivity and high optical transmittance. It should be emphasized here that the ρ values of
optimally doped ﬁlms, 0.03 ≤ x ≤ 0.06, are as low as 2 - 3 × 10-4 Ωcm, which are comparable tothose of conventional ITO ﬁlms.3,15 Fig. 3(b) shows the appearance of SrTiO3 substrate with andwithout ﬁlms.
These highly conductive ﬁlms are characterized by degenerate semiconductor with metallic
temperature dependence of ρ. Figure 4(a) plots carrier density, ne, against x. As can be seen, thereis good linear relation between ne and x up to x ~ 0.06, which can be formulated as ne ~ 0.9ncation,
where ncation is the density of Nb. This provides the evidence that Nb ions release conduction
electrons with an efﬁciency of ~90%. At higher doping level, x > 0.06, the ionization efﬁciency isslightly suppressed. The Hall mobility, μH, increases as temperature is lowered, and indicates theimportance of phonon scattering (Fig. 4(b)). The 0.03 ≤ x ≤ 0.06 ﬁlms with optimal electric andoptical properties possess μH (300 K) ~ 20 cm2V-1s-1, which is approximately half of that of ITO
with the same dopant concentration.

Fig. 4 (a) Carrier density at 300 K as a function of Nb concentration and x. (b) Hall mobility as afunction of temperature.

The above experimental results demonstrates that Nb-doped anatase TiO2 (Ti0.94Nb0.06O2 :TCO) in epitaxial thin ﬁlm form, is an excellent TCO with high conductivity, ρ ~ 2x10-4 Ωcm andinternal transmittance >95 % in visible. These values are comparable to those of typical TCOssuch
as indium tin oxide (ITO) and ZnO, indicating that TNO could be a good candidate for nextgeneration TCO.

42

Polycrystalline ﬁlms on glass substrate
From a practical viewpoint, TCO ﬁlms need to be coated on glass and plastics. In particular,considerable efforts have been devoted to the deposition of TCO films on glass because ofincreasing technological demands for flat panel displays and solar cells. Very recently, we haveattempted to grow TNO ﬁlms on glass by using PLD, and obtained ρ = 4.6 x 10-3 Ωcm (Fig. 5(a),(b)),16 which satisﬁes the practical requirement for TCOs of ρ ~10-4 Ωcm. These ﬁlms were preparedon glass via crystallizing from amorphous phase, and the crystallization process was found to cause
a substantial ρ drop by a factor of 106. Further, we applied this technique to sputtering and preparedhighly conducting TNO ﬁlms on glass. Our best ﬁlm shows 8x10-4 Ωcm at room temperature withabsorption less that 10 % in visible.

Figure 5 (a) X-ray diffraction patterns of the Ti0.94Nb0.06O2 ﬁlm before (as-grown, amorphous)and after H2 annealing at 500ºC.

Amorphous ﬁlm crystallizes on annealing. (b) Resistivity of as grown amorphous ﬁlm and annealed ﬁlm. The annealed ﬁlm shows
positive temperature coefﬁcient indicative of metallic behavior.

Preparation method is as follows. Amorphous Ti0.94Nb0.06O2 films were deposited using acommercial sputtering apparatus on unheated non-alkali glass (Corning 1737) substrates (Fig. 6(a)).
A sintered pellet with a nominal composition of Ti0.94Nb0.06O2 or Ti metal was used as a sputteringtarget, and deposition time was adjusted so as to obtain ﬁlms with thicknesses of ~200 nm. The asgrown ﬁlms were further annealed in vacuum using a rapid thermal annealing furnace, wherein theannealing temperature was raised to 500 °C within 5 minutes (Fig. 6(b)). The annealing furnace was
evacuated to 10-4 Torr, in order to eliminate residual oxygen in the furnace.
Appearance of the ﬁlms is shown in Figs 6(c) and (d).
The minimum ρ achieved in from this process using sputtering is 8 x 10-4 Ωcm, which isvery low ρ as transition metal oxide in polycrystalline form.17 Notably, annealing treatment in air
provides insulating ﬁlms with ρ >106 Ωcm, suggesting that oxygen vacancies play an important role
for lowering ρ of the polycrystalline TNO ﬁlms.
A typical film shows a metallic temperature dependence of ρ. Further, n e is almost

43

Fig. 6 Preparation process of TiO2-based TCO on glass substrate. (a) At ﬁrst, deposit amorphous TiO2 ﬁlm at room temperature by
sputtering technique. (b) Then, anneal the ﬁlm in vacuum at 300 – 600ºC. (c) Photograph of ﬁlms on glass. Films are colored due tointerference effect. (d) Demonstration of measurement of TiO2-based TCO ﬁlm.

independent of temperature, so that the present
polycrystalline film can be regarded as a degenerate
semiconductor. From the ne value of ~1.6 x 1021 cm-3,
it is estimated that doped Nb atoms are activated with
an efficiency of ~90%. This strongly suggests that the
Nb dopants are substituted for the Ti sites in anatase
TiO 2 without segregation. High activation efficiency
is a unique characteristic of TNO in both epitaxial andpolycrystalline ﬁlms, compared with other conventional
TCOs, typically <50 %.18,19,20
Transmittance (Tr) and reﬂectance (R) spectra of a
TNO ﬁlm after annealing (thickness ~ 200 nm) are shownin Fig. 7(a). The Tr value throughout the wavelength
region of 400-1500 nm is 60-80%, which is lower thanthose of typical ITO films. This is due to the relatively
high refractive index of anatase TNO, approximately
2.4 at 500 nm, which tends to enhance R and, thus tosuppress Tr. The absorbance (A) in the visible region,
evaluated from the formula A = 1-(Tr+R), is below 10%(Fig. 7(b)).

44

Fig. 7 (a) Transmittance, reflectance, and (b)absorbance, of a typical anatase Ti1-xNbxO2 on glass
substrate.

Fig. 8 Anatase Ti1-xNbxO2 is a d-electron-based new TCO.

Prospects toward application
It is very important to ﬁnd applications that utilize the unique characteristics of TiO2-based
TCO. As mentioned earlier, this TNO has several properties that ITO does not have, such as, highrefractive index, high transmittance in infrared, and high stability in reduced atmosphere. These
properties would provide new opto-electrical devices that would not compete with ITO. We are now
seeking applications for light emitting diodes and solar cells to increase their efﬁciency. Further,since this TNO system is based on d-electron system (Fig. 8), combining TCO properties withd-electrons may open up new spinelectronic devices or new perspectives utilizing d-electrons.
Lower ρ and higher transmittance is still required, and experiments are underway. By
introducing seed layer, we have obtained 6.5 x 10-4 Ωcm on glass substrate by using sputtering
method. Exploration of other dopant and to establish guiding principles to obtain a new TCO shouldbe very important research topics.
CONCLUSIONSWe reported on the development of anatase TiO2-based transparent conducting oxide. The
epitaxial film exhibits low ρ ~ 2×10-4 Ωcm and high optical transmittance in visible. We havealso succeeded in preparation of highly conducting anatase Ti0.94Nb0.06O2 polycrystalline ﬁlms on
glass substrates by sputtering technique. The polycrystalline ﬁlms was revealed to show excellent
transparency with absorption <10% in visible region. These results are encouraging for the
fabrication of wide-area polycrystalline TNO on glass with practical electrical and optical properties
as a TCO. We conclude that anatase Nb-doped TiO2 is a promising candidate for ITO alternatives.
This material is the ﬁrst transparent conducting oxide based on d-electrons, and the present results
would stimulate material explorations to search for new TCOs based on d-electrons.

45

Acknowledgments
I would like to express my sincere thanks to my collaborators, Prof. Tetsuya Hasegawa (Univ.
of Tokyo), Prof. Yutaka Furubayashi (Kanazawa Inst. of Tech.), and Dr. Naoomi Yamada (Kanagawa
Academy of Science and Technology). This work was supported by MEXT Elements Science andTechnology Project, the Global COE Program for Chemistry Innovation, and NEDO.

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46

Dr. Rudolf M. TROMP
主任研究者 トロンプ・ルドルフ

LOW ENERGY ELECTRON MICROSCOPY:
A 10 YEAR OUTLOOKRudolf M. Tromp
IBM T.J. Watson Research Center
1101 Kitchawan Road
P.O. Box 218
Yorktown Heights, NY 10598
USA
SUMMARY
Low Energy Electron Microscopy (LEEM), and its close cousin Photo Electron Emission
Microscopy (PEEM) have evolved from curiosities in the hands of physicists, to powerful tools for
dynamic materials analysis. Invented in 1962, and ﬁrst successfully realized in 1985, there are about
30 combined LEEM/PEEM instruments in the world today, in addition to about an equal number of
PEEM-only instruments. Most of these instruments follow a design that is now almost 20 years old.
The most advanced instruments are located at synchrotron radiation facilities, but they are relatively
few in number, and not readily available to the general user. In the meantime, the ﬁeld of electron
microscopy is undergoing revolutionary advances fueled by dual breakthroughs in electron energy
ﬁltering and aberration correction. Similarly, the ﬁeld of quantum optics is developing ever more
powerful light sources in the Vacuum Ultra Violet (VUV) and soft X-ray ranges that do not dependon massive investments in national infrastructure (i.e. synchrotrons), but deliver their photons ina standard laboratory. These advances from two very different ﬁelds will combine to set the stage
for the development of LEEM and PEEM over the next decade. Aberration correction will improve
LEEM resolution from 5-10 nm today, to 1.5-2 nm in the near future, enough to resolve individual
unit cells in the famous Si(111)-(7x7) surface, or –probably more important- sufﬁcient to resolve thestructure of nanoscale features such as magnetic domain walls. In PEEM the spatial resolution will
improve from ~ 20 nm today to 4-5 nm, while at the same time improving transmission by a factor~10. Energy filtering, today mostly used by synchrotron-based instruments, will be ubiquitous,
powerful, simple, and relatively inexpensive. For PEEM there will be a broad choice of light47

sources, not just the ever-present Hg discharge lamp of today. We will obtain not just the atomic
structure of surfaces using LEED-IV information with nm spatial resolution, nor just the spatial
arrangement of structural domains, quantum well structures, or other multi-phase arrangements,
as well as their evolution in time; but we will also learn the spatially resolved electronic structurefrom nano-ARUPS, Angle Resolved Ultraviolet Photoelectron Spectroscopy at the nanoscale, as
well as the chemical makeup of the surface with nano-XPS, X-ray Photoelectron Spectroscopy atthe nanoscale, all in one compact instrument, in a standard laboratory environment. Some of theingredients of this new paradigm are in place today, others need more work. This paper will give an
overview of where we are, and where we need more work and more investment.
A LITTLE BIT OF HISTORYLEEM and PEEM, while having reached adolescence only recently (not always wellbehaved, and with pimples here and there), are not exactly new kids on the block. Ernst Bauer1
invented LEEM in 1962 . But cathode-lens microscopy goes back to the early days of electron
microscopy, to Bruche2 , and other pioneers of the field. For many decades electron microscopy
was thrust forward by a single-minded imperative: resolution. As Oliver Wells often reminds me,
the Scanning Electron Microscope could have been built much earlier. But who was interested ina microscope that promised only 20 nm spatial resolution? In the 80’s and 90’s we witnessed theemergence of MeV electron microscopes that required the construction of multistory buildings tohouse them, trying to beat chromatic and geometric lens aberrations by sheer force. But faster, more
powerful computers caught up with them. First, computer-based reconstructions of aberrationcorrected images3 pushed resolution down below 0.1 nm. This was followed by computer-designed
and -controlled electron-optical multipole aberration correctors that deliver 70 pm spatial resolution
in a microscope that ﬁts inside a regular lab again4 . In transmission electron microscopy, aberration
correction is a commercial reality. The race for resolution will now be replaced by a race forusability and function. In LEEM and PEEM the driving force has never been resolution. The best
non-corrected resolution measured with LEEM is 4.1 nm, at 3.5 eV electron energy5 , not evenclose to 70 pm. But LEEM naturally has an exquisite surface sensitivity that TEM does not offer. It
can follow the evolution of surface structure in real time with an ease and grace that neither TEM,
nor SEM, nor STM can match. LEEM cannot see atoms, but it can see what atoms do, how they
make atomic steps ﬂuctuate at high temperature, how and where nucleation occurs during growth,
how critical ﬂuctuations mark two-dimensional phase transitions. With PEEM we can look at corelevels for chemical information, use magnetic dichroism to study magnetic structure and dynamics
with sub-nanosecond time resolution, probe valence band information for electronic structure. So,
while function has been the ﬁrst and foremost driving force in LEEM and PEEM, resolution has
taken a backseat. Even so, a few experiments have shown that aberration correction, based not on48

multipoles but on electron reflection mirrors, can work in LEEM/PEEM, and developments ofcommercial versions are well under way.
On the photon end of the spectrum, many of us will recall the Teller X-ray laser. Whilepotentially powerful, it never had much appeal for scientists who aspired to having a reliable laser
source in their own lab. How often do you get a chance to trigger the explosion of an atomic bomb?
Fortunately, we have also witnessed the renaissance of atomic physics. Thirty rears ago, this ﬁeld
seemed of interest only to scientists with an innate fondness of bookkeeping. Long, inscrutable
tables of spectral lines, labeled with orbital transition indices failed to inspire most of us. Now,
atomic fountains, laser cooling, Bose-Einstein condensation, atoms in strong fields, and Nobel
Prizes too, sing a siren song that casts a powerful spell. Soft X-ray lasers now exist6 and you don’t have to work for the government to own one. VUV lasers are rapidly improving7 ,8 . In the nextten years these light sources will become more mature, and will become a formidable competition
for synchrotron radiation light sources. High brightness, easily controlled polarization, tunability,
low cost, year-round availability to the individual researcher, photons@home. These are some of
the attributes that will drive the development of these sources, as well as compelling commercial
applications such as the several kW average power 13.5 nm laser-based light sources needed for
VUV lithography 9 , supported by powerful international semiconductor consortia.
Thus, out of these two seemingly unconnected research strands, low-resolution electron
microscopy with cathode lenses, and atomic physics, have emerged two exciting opportunities thatwill merge over the next 5-10 years, and create a new form of microscopy that will probe atomic,
geometric, electronic, chemical, and magnetic structure, on one sample, in one instrument, in realtime, with nanometer spatial resolution, and with (sub)nanosecond time resolution. Aberration
corrected, energy ﬁltered LEEM/PEEM plus photons@home will bring these powerful capabilities
to small scale research labs, at a cost that will be affordable to numerous institutions.
WHAT IS LEEM?
As the name suggests, LEEM uses low energy electrons to form an image of a sample.
Compared to any other form of electron microscopy, really low energy electrons, in the range of
0-100 eV, but most often below 10 eV. At these very low electron energies, the elastic reﬂectivityof the incident electron beam is very high, guaranteeing bright images that can be obtained at videorate (or faster) acquisition times. So LEEM can make real-time movies of surface processes such as
growth, phase transitions, step motion, etching, etc. Since the electrons are slow and reﬂect from thesurface, LEEM is also very surface sensitive, with probe depths ranging from sub-nm to several 10s
of nm. The electron energy can be kept almost arbitrarily low, even down to zero eV if we make theelectron turn around just a few nanometers in front of the sample in so-called mirror microscopy.

49

This makes LEEM extremely suitable for the study of highly radiation sensitive materials suchas thin ﬁlm organics and self-assembled monolayers, which can be observed without degradation
under the electron beam10.
To get the electrons onto the sample, we start with an gun that generates electrons with
15 keV kinetic energy (ﬁgure 1) 11. A rather standard condenser lens system transports the beam
down the optical column towards the objective lens. However, before reaching the objective lens,the electrons pass through a double-focusing magnetic 90° deﬂection system whose purpose willsoon become clear. The electrons are then refocused by a transfer lens into the backfocal plane ofthe objective lens, so that a collimated beam is directed at the sample. While the objective lens isgrounded, the sample is held at a potential close to the electron gun potential. Thus the electrons are
decelerated between the objective lens and the sample to an adjustable energy in the range of 0-100
eV, and they interact with the sample at this low electron energy. At such low energies, the electron
wavelength is on the same order of magnitude as the atomic spacing in the surface. If the sample iscrystalline, the reﬂected electrons will undergo Low Energy Electron Diffraction (LEED), and after
reﬂection from the sample, and after re-acceleration to 15 keV, a LEED pattern will be observedin the backfocal plane of the objective lens. The objective lens also forms a real-space image ofthe sample at a greater distance. The transfer lens then focuses the image onto the diagonal planeof the magnetic 90° deﬂection system, which directs the beam down into the projector column. As
we see, the incident and reflected paths of the electron beam between the 90° deflection systemand the objective lens coincide. The purpose of the 90° deﬂection system is to spatially separate
the illumination and projection systems of the electron microscopes so that we have independent
control over these essential functions. We will see later that the 90° deﬂection system can also serve
as a high quality, and very convenient energy ﬁlter that allows us to record energy ﬁltered images
and angular intensity distributions, as well spatially resolved energy spectra. But for now we will
ignore this feature.
The LEED pattern is again in focus in the center of the ﬁrst lens below the 90° deﬂection
system, where we can insert a contrast aperture to select the LEED beam that we want to use
for image formation, enabling bright field and dark field imaging conditions, as in a TEM. The
remaining lenses then project either the real space image, or the diffraction pattern onto thechannelplate intensiﬁed phosphor screen.
This arrangement allows us a variety of imaging modes. In bright field imaging the (0,0)
specular LEED beam is selected (ﬁgure 2a). In this case all areas on the surface will diffract into theaperture (assuming that all areas of the surface are crystalline), and contrast can arise in a variety
of ways. First, areas with different structures will have different structure factors, and thereby
different (0,0) beam intensities, at least at some electron energies. Second, contrast can arise at50

atomic steps when the path length difference between electrons reﬂected from the top and bottomof an atomic step is a non-integer fraction of the electron wavelength. By slight defocusing of theobjective lens Fresnel fringes will mark the locations of the atomic steps on the surface. Third,
thin ﬁlms present on the surface may have a thickness that locally matches the wavelength of theincident electron beam, giving rise to so-called quantum well contrast. Dark ﬁeld imaging uses a
LEED beam other than (0,0) (ﬁgure 2b). For instance, on the dimmer-reconstructed Si(001)(2x1)
we may select a (1/2,0) diffracted beam. As the dimmer reconstruction rotates by 90 degrees at
each atomic step, every other atomic terrace will diffract into this (1/2,0) beam, while intervening
terraces diffract into the (0,1/2) beam. Therefore, alternating terraces will have bright and dark
contrast. Finally, in mirror imaging the sample bias is more negative than the gun potential, so thatthe electrons never touch the sample but turn around just in front of it (ﬁgure 2c). The mirror image
is very sensitive to geometric features on the sample, as well as spatial variations in electrostatic
potential (such as workfunction variations), and even to magnetic fringe field emerging from amagnetic sample which easily deﬂect the electrons at the turning point where their kinetic energy is
momentarily reduced to zero. Finally, ﬁgure 2d shows a representative LEED pattern obtained on
the Si(111)(7x7) surface, obtained with the contrast aperture removed from the beam path.
As mentioned before, the electron reﬂectivity of the electrons at these low energies can bevery high (in mirror mode the reﬂectivity equals unity), and bright, high contrast images are easily
obtained at video acquisition rates, enabling dynamic imaging of surface process such as epitaxial
growth, interface formation, phase transitions, etcetera. Several reviews are available in theliterature that provide a broad background on the types of experiments that are possible12-15.
One feature we would like to highlight here, is that the bright-ﬁeld image intensity depends
strongly on the electron energy. Of course, the analysis of the LEED spot intensity as a function ofelectron energy (known as LEED-IV intensity analysis) is the oldest structural technique in surface
science16,17. Calculations of the intensity of the diffracted LEED beams with energy are not trivial.
Of course, the intensities depend on the structure of the surface. However, unlike electron scattering
at much higher energies, and unlike X-ray diffraction, multiple scattering events are extremely
important and must be taken into account in detail. Over the last 30 years such calculations havebeen used to determine hundreds if not thousands of surface structures17. The bright ﬁeld LEEM
image shows us the spatial distribution of the intensity of the (0,0) LEED beam. If we now vary the
sample potential, and thereby the electron energy, we can measure the spatially resolved LEEDIV information for each pixel in the image, with a spatial resolution that is limited by the resolutionof the microscope, between 5 and 10 nm. In a recent paper J. Hannon and coworkers18 have shownhow this information can be used to determine the atomic structure and composition of the Cu(001)Pd system with high accuracy, distinguishing and measuring the amount of Pd in the outer three
atomic layers of the crystal with 10 nm lateral resolution (ﬁgure 3). This highlights an important51

strength of LEEM: while the spatial resolution is not sufﬁcient to see individual atoms, we can stillobtain atomic resolution structure data by making use of the quantitative information contained in
the diffraction pattern. As the images in ﬁgure 2 and 3 show, we also obtain structural informationon larger scales, such as the locations and shapes of atomic steps, spatial distributions of phasedomains, etcetera.
AND WHAT IS PEEM?
In PEEM we illuminate the sample not with an electron beam, but with photons which –by
the photoelectric effect- emit electrons from the sample. The most universally used light source is a
Hg discharge lamp which emits photons with an upper energy of about 4.8 eV. If the workfunctionof the sample (energy separation between the Fermilevel of the solid and the vacuum level) is
smaller than the photon energy, then photoexcited electrons can leave the sample into the vacuum.
They are then accelerated into the objective lens, and used to form an image of the sample. This
image is very sensitive to local variations in the workfunction. If the sample is not perfectly ﬂat,the equipotential planes in front of the sample are distorted by the geometric features of the sample,
giving rise to localized focusing or defocusing effects. Thus, the image is also very sensitive tosample geometry, which is sometimes a blessing, sometimes a detriment.
Things get more interesting when the photon energy becomes higher. In the early days ofAngle-Resolved Ultraviolet Photoelectron Spectroscopy (ARUPS), the He discharge lamp was thelightsource of choice. It delivers an intense beam of UV light at a photon energy of 21.2 eV (HeIline), or a weaker beam at 40.8 eV (HeII line). This light source is compact, simple to operate,
highly monochromatic, and relatively inexpensive. Other photon energies are available by switching
to different gases such as Ne or H. At these higher photon energies, electrons are excited from alarger range of binding energies inside the valence band. For instance, on semiconductor surfaces
we can see the bulk valence bands, as well as electron states localized at the outermost atomic layer– surface states. By measuring the electron energy distributions as a function of emission angle, theenergy versus momentum distributions of the electrons can be measured, and the bandstructure of
the sample reconstructed and compared with theory. There is no doubt than ARUPS has contributed
more to our understanding of the electronic structure of materials than any other method19.
Presently, high resolution ARUPS plays a key role in building an understanding of the fundamental
science behind such materials as high temperature superconductors, and graphene.
At higher photon energies yet, electron emission from the deeper core levels becomes
possible. These core levels are to ﬁrst order a marker of the chemical identity of the atom whichemits the electron. But the detailed finestructure of the corelevel spectrum contains information
not only on the chemical identity of the atom, but also on its chemical environment, as the exact

52

binding energy depends sensitively on that environment. The angular emission pattern is affectedby the location of the atom’s neighbors, and this atomic geometry can be reconstructed from thephotoelectron diffraction pattern. Magnetic information is available by means of circular and linear
magnetic dichroism effects, enabling dynamic imaging of magnetic domain structure with time
resolutions below 1 nanosecond20. Thus, the use of corelevel information in PEEM adds a powerful
set of analytical tools to the experiment.
Unfortunately, this expansive power of PEEM imaging methods is today available only at
the few instruments coupled to synchrotron radiation sources. Of course, synchrotrons are in many
ways the ideal photon sources for such experiments, with high brightness, adjustable polarization,and broad energy tunability. Synchrotrons are also expensive, and not easily accessible to thegeneral user, with only small chunks of beamtime in any given year, and little opportunity tosystematically explore a given materials system over longer time. Also, it is hard to get accessfor routine (but tremendously important) routine sample analysis for things such as metrology
inspection, quality control, contamination analysis, etcetera. The promise of PEEM is therefore only
a distant vision for many potential applications, difﬁcult if not impossible to turn into reality.
ENERGY FILTERING
There are yet other technical hurdles. To take advantage of the full power of PEEM it isdesirable to analyze the photoelectrons in both energy and emission angle. This is absolutely
necessary if we want to obtain information on the electronic bandstructure of our sample. Even
many PEEM experiments performed at synchrotron rings don’t use an energy filter21,22. The
elemental sensitivity is obtained by scanning the photon energy across a corelevel excitationthreshold, and the contrast aperture serves to crudely select electrons over a bandwidth of several
eVs in the secondary electron band near zero kinetic energy. There is an important reason for doing
this: it simpliﬁes the setup (no energy ﬁlter), the transmission is high, and it is easier to scan thephoton energy than to analyze the electron energy. Many elegant experiments have been performedin this manner, but it also leaves many possibilities on the table. An energy ﬁlter is a rather tricky
piece of electron optics, and energy ﬁltering in transmission electron microscopes has only cometo maturity over the last 10-15 years, with signiﬁcant advances still being made23,24. In PEEM, themain approach explored so far is to install a hemispherical electrostatic analyzer, well known fromlab-based XPS experiments, in the projector column of the PEEM instrument25. The electrons are
deflected by a 180° spherical capacitor which exhibits a chromatic dispersion (displacement perunit of energy) given by D=2R/E, where R is the mean radius of the spherical capacitor, and E theenergy with which the electrons pass through the analyzer. To increase the energy dispersion (andthereby the energy resolution for a given slit size), one can make R larger, i.e. increase the physical53

size of the analyzer. This is expensive, and heavy, because the volume and mass of the analyzer
increase with R3, while the dispersion increases only with R. Alternatively, one can decrease thepass energy E by decelerating the electrons before they enter the analyzer to energies of a few 100
eV or less, far lower than the transport energy of the electron beam in the rest of the microscope. In
practice, both strategies are followed, often at the same time, so that the highest resolution electron
spectrometers combine large size with low pass energy. In an electron microscope, one must also
carefully control the optical properties of the analyzer, in reference to both the diffraction and theimage planes, which are widely separated in space. That is, the energy ﬁlter must produce stigmatic
images of both the image and the diffraction planes, preferably at unit magnification, withoutcompromising image quality. This requirement necessitates the addition of coupling lenses, as well
as retardation/acceleration lenses, resulting in a complex and expensive setup. Furthermore, oncethe energy ﬁlter is in place, its use can no longer be avoided. The electron beam must always passthrough the analyzer and all its associated optics, further complicating alignment and use of the
microscope, even when the energy ﬁltering function is not required for a certain experiment.
This is where we take advantage of a hitherto overseen or even avoided fundamental
property of the magnetic 90° deflection system, namely its chromatic dispersion. On their paththrough the 90° deflection system, electrons with higher energy will be deflected over a smaller
angle than electrons with a lower energy, unless speciﬁc optical design measures are taken to avoid
such dispersion29. In the simple 90° deﬂection system described here dispersion is present, and thefollowing discussion will apply to this speciﬁc design. If the mean radius of the deﬂector is given
by R, and the distance from the diffraction planes (situated symmetrically about the deflectionﬁeld) to the edge of the deﬂection ﬁeld by L, then the dispersion is given26 by D=(R+L)/2E. In thecase that R and L are of similar magnitude, we see that this dispersion is about half as large as thatof a hemispherical capacitor. Since the deﬂection angle is also half as large (90° instead of 180°),
this makes sense. Of course, dispersion occurs only in the plane of deﬂection, and not orthogonal
to it. So, a diffraction plane located in the diffraction entrance plane of the 90° deﬂection systemon the objective lens side is both imaged (1:1), and dispersed in the diffraction exit plane of the
90° deﬂection system. Importantly, the image plane located on the diagonal of the 90° deﬂection
system is achromatic, not affected by dispersion. This is a consequence of the symmetric locationof this speciﬁc plane. If we now place a slit in the diffraction entrance plane, oriented normal tothe deﬂection plane, we will see a dispersed image of this slit in the diffraction exit plane: energy
versus momentum along one of the two primary momentum directions, E-ky , as schematicallyshown in ﬁgure 4.
The vertical position of the slit in the diffraction entrance plane determines the value of kx.
This image can be projected directly on the channelplate intensiﬁed phosphor screen and recordedwith a suitable camera. By placing a contrast/energy selection aperture in this E-ky plane, we54can selected an energy window of interest and record an energy-ﬁltered real space image for thatchoice of E(kx,ky). By scanning the sample voltage, the energy window is conveniently scanned
across the energy spectrum, and we can record real space images for different energies across theenergy spectrum. Sometimes one is interested not only in energy ﬁltered images, but also in energy
ﬁltered angular distributions, i.e. not just E-ky, but E-k, the full 3-dimensional binding energy vs
wavevector distribution. This can be conveniently obtained by scanning the entrance slit acrossthe diffraction plane, in the kx direction. By recording an E-ky distribution for each kx, the full E-k
distribution is easily reconstructed27.
A few questions still need to be addressed. First, the magnetic deﬂection ﬁeld is rather smallin size, with R=68 mm and L=116 mm for our microscope, and the electrons are not decelerated
on their path through the 90° deﬂection system. Can we still get sufﬁcient energy resolution? At
15 keV these numbers translate in a dispersion of about 6 m/eV. Using an energy ﬁlter slit with awidth of 1 µm, fabricated by a Focused Ion Beam milling system, we measure an energy resolutionof 0.15 – 0.2 eV, in good agreement with this prediction. Thus, even without slowing the electrons
down we obtain an energy resolution E/∆E of 100,000, similar to spherical capacitor analyzers at apass energy of a few 100 eV. For energy-ﬁltered imaging we use a larger slit with a width of 10 µm,
giving a passband of 1.5 eV, well matched to the natural line width of typical spectroscopic features
and thus optimizing instrument transmission in imaging applications.
Second, how hard is it to operate this energy ﬁlter? This is one of the really nice features.
The 90° deﬂection system is an essential and integral part of the LEEM instrument, with or without
additional energy filter. Thus, utilizing it as an energy filter only requires the installation of ascanning slit in the entrance side diffraction plane, but no additional optics. The electrons are notdecelerated, but pass through the 90° deﬂection system at full 15 keV energy, eliminating the needfor retardation and acceleration optics. Already in unﬁltered LEEM/PEEM operation the electrons
pass through the 90° deﬂection system, and aligning it is a routine and simple part of microscope
setup. In order to make use of the energy ﬁltering function, all that is required is to insert the energy
filter slit in the entrance side diffraction plane. No other alignment or changes to the instrument
operational parameters are needed to use the energy ﬁltering function.
Finally, is it possible to reconstruct the full E-k distribution simply by scanning the entrance
slit? Indeed it is27. Figure 5 shows illustrative results obtained on a composite Ag(111)/Ag(001)island grown inside the LEEM on a Si(111) substrate. The sample was illuminated with a focused
HeI light source, running at a power of about 270 W. Imaging the sample with secondary electrons,
the Ag(111) area shows up in bright contrast, while the (001) area is dark (ﬁgure 5a). The crystal
orientations of each area were determined by selected-area LEED in the LEEM instrument. Usinga selected-area aperture in the image plane on the diagonal of the 90° deﬂection system, we canobtain E-ky spectra from either the (111) or (001) areas of the Ag crystallite. Placing the energy slit

55

across the center of the Ewald sphere, the Ag d-bands are clearly observed on both areas, but theirdispersions are quite different. By scanning the slit along the kx direction, the full E-k distribution
can be obtained. Energy spectra at selected kx,ky points are shown in figures 5c and 5d for both(111) and (001) areas. We can then extract the full kx,ky intensity distribution at a fixed binding
energy. Such distributions are shown at a binding energy of 6.1 eV, for both (111) and (001) areas
in ﬁgures 5e and 5f, reﬂecting the 3-fold and 4-fold symmetries of both areas. Both surfaces showsa strong photoemission resonance at the boundary of the surface Brillouin zone, corresponding
to the bulk L-point where resonant photoemission can occur from an occupied d-band to a highlying unoccupied band with an interband separation of 21.2 eV, just equal to the incident photon
energy. This is the ﬁrst time that this photoemission resonance has been observed on Ag in an offnormal direction. Acquisition time of the data sets shown in figure 5 was about 1 hour. Finally,
figure 5b shows an energy filtered image with the filter aperture selecting the Ag d-band. The
contrast difference between (111) and (001) has now disappeared, as both (111) and (001) areas
feature prominent d-band intensity. It turns out that the contrast seen in ﬁgure 5a is not related toa difference in workfunction between (111) and (001) areas (as usually assumed in such PEEM
images), but to a ﬁnal state effect on the (001) surface where no ﬁnal state bands are available justabove the vacuum level, and photoemission at those energies is therefore strongly reduced relativeto (111).
ABERRATION CORRECTION
To further improve the performance of the LEEM/PEEM instrument it is highly desirable toreduce or eliminate the chromatic and spherical aberrations of the objective lens. While aberration
coefficients of TEM instruments are measured in millimeters, the corresponding numbers fora cathode lens are measured in meters. In other words, these are really rather poor lenses. This
is not the result of inadequate design. The first image forming element in the microscope is thestrong accelerating field between the sample and objective lens, which forms a virtual image at
magniﬁcation 2/3 behind the sample. It is this unavoidable image forming electrostatic ﬁeld that
dominates the microscope aberrations. To ﬁrst order, the image resolution scales with 1/Eg, where
Eg is the strength of the electrostatic ﬁeld in the gap between sample and objective lens. Thus, tooptimize resolution it is advantageous to work at the largest ﬁeld strength practical, in the rangeof 100 keV/cm, close to the electrostatic discharge limit. It was first shown by Rempfer andcoworkers28, and further demonstrated in the SMART project by Rose and coworkers29, that thechromatic and spherical aberrations of the cathode objective lens can be corrected with a 4-element
electrostatic electron mirror. This mirror, containing 1 grounded lens element, and 3 negatively
biased lens elements has three degrees of freedom, i.e. the three bias voltages. There are also
three parameters that need to be controlled: the mirror focal length fm, the chromatic aberration56

coefficient Cc, and the spherical aberration coefficient Cs. It is a generous gift of nature that thethree independently controllable voltages are sufﬁcient to adjust all three parameters, fm, Cc and Cs ,so that the mirror creates a properly focused image of the sample, while at the same time cancelingthe chromatic and spherical aberrations of the objective lens. It even turns out that the mirror
reduces the leading 5th order geometric aberration term by a factor 5-10, and the ﬁnal resolution islimited by a combination of diffraction, 2nd order chromatic, and 5th order geometric aberrations.
The effects of aberration correction are demonstrated in ﬁgure 6 which shows how improvement in
LEEM resolution by a factor 3 is accompanied by an increase in transmission by a factor 9. Similar
numbers hold for PEEM experiments with energy pass bands of 1.5 – 3 eV. We predict that LEEM
resolution will be better than 1.5 nm, while PEEM resolution will be about 4 nm.
At present, the best resolution recorded with a non-corrected LEEM is 4.1 nm, while theaberration-corrected SMART machine has obtained a resolution of ~3 nm. There is still room
for improving resolution by another factor 2. We have completed the design of a new, compact
aberration-corrected and energy-ﬁltered LEEM/PEEM instrument, and this new instrument is underconstruction at the time of writing of this manuscript. It is projected to be operational by the end of
2008.
LIGHT SOURCES
The results in the previous chapter were obtained with a conventional HeI discharge lamp,
equipped with a focusing capillary to improve brightness. What other lab-based light sources arepresently available? First, in addition to the HeI line, we have also used the HeII line at 40.8 eVwith good results, for instance to map the bandstructure of 1.5 monolayers of graphene grown on
SiC(0001). In addition, we have a very bright pulsed laser (80 MHz, 12 ps pulse width) in which
355 nm laser light (SpectraPhysics Vanguard laser system) is doubled by a non-linear KBBF crystal(grown at the Institute for Crystal Growth and Development of the Chinese Academy of Sciencesin Beijing7) to 177 nm. This light is fully coherent, polarized, and focused onto the sample to a
spot size of ~35x100 µm2. With a photon energy of 6.994 eV, this laser light accesses the top 2-3
eV of the valence band of most materials, providing a very useful spectral range for a wide variety
of materials problems. We also have a compact 266 nm laser source. In the longer term we plan
to add light sources at signiﬁcantly shorter wavelengths, in the energy range of 100-250 eV where
many corelevels become accessible. Much higher photon energies are not necessarily advantageous,as the electron kinetic energy for a given corelevel increases linearly with photon energy. As kinetic
energy increases, the diameter of the Ewald sphere also increases, while only a small diameterarea near the optical axis can be accepted for image formation. The transmission therefore drops
off with 1/E where E is the kinetic energy of the core electron after excitation into the vacuum.
The standard light source used in XPS experiments, using Al or Mg Kα radiation will therefore not57

be very useful for energy filtered PEEM imaging and spectroscopy experiments, as the reduced
transmission at high electron energies will result in weak signal strengths.. Much more promising
are the efforts expended in developing 13.5 nm light sources for EUV lithography9. With a photon
energy of ~92 eV, a 13.5 nm source would give access to both valence bands and core levels fora fairly broad range of materials. At 200 eV photon energy much of the periodic table would beaccessible. Development of such light sources will require effort and investment, but will be well
worth the trouble. Already, high harmonic generation in gas-ﬁlled capillaries can generate photons
up to 250 eV6, although the short pulse lengths and low duty cycles are not yet attractive for PEEM
applications.
Of course, synchrotron sources will continue to provide excellent tunability and brightness,
and a small number of experiments will be able to take advantage of such facilities. But we foreseethat the center of gravity for spectroscopic LEEM/PEEM will shift from synchrotron to academic
and industrial laboratories, taking advantage of both improved electron optics and improved labbased VUV, EUV, and soft X-ray light sources. The latter may not provide the same level of
ﬂexibility as synchrotron light, but they will be available full time to the individual researcher, in anenvironment which iscompletely under his or her control. When new light sources become available, they can be
added to the instrument without major revision of the instrument layout. Figure 7 shows the present
conﬁguration of the spectroscopic LEEM/PEEM system at IBM, with HeI/HeII light sources, aswell as 262 nm and 177 nm laser sources installed on the system.
CONCLUSIONSLEEM and PEEM have become powerful tools in the study of surfaces and interfaces.
While LEEM primarily gives structural information, PEEM can complement LEEM by providing
insight into the electronic, chemical, and magnetic properties of the sample. Powerful energy
ﬁltering methods have already enabled the acquisition of energy ﬁltered valence band images, aswell as full E-k ARUPS data on selected sample areas with a diameter of a few µm, in a standard
lab environment using HeI and HeII gas discharge sources27. Combination of energy ﬁltering withelectron mirror-based aberration correction will improve resolution in LEEM to below 1.5 nm,
and in PEEM to 4-5 nm, while increasing transmission by an order of magnitude. New laser-based
VUV, EUV, and soft X-ray light sources are starting to become available, and will become evermore useful over the next 5-10 years. Twenty years ago Scanning Tunneling Microscopy was aphysicist’s dream toy, with limited practical utility. Today it is hard to imagine nanoscale science
without STM and AFM. Similarly, LEEM/PEEM instruments are not widely available today, and58mostly in the hands of physicists pursuing questions in basic science. Over the next decade wewill witness a broadening of the LEEM/PEEM user community into materials science, chemistry,
micro- and nano-electronics, MEMS and NEMS, spintronics, and even life sciences, driven by theconvergence of developments in electron optics and atomic physics/quantum optics brieﬂy reviewed
in this paper. We hope that WPI-AIMR will play a central role in, and take extensive advantage ofthese developments.
Acknowledgements:
I like to thank Dr. Jim Hannon for his many essential and deep contributions to the work
presented here, Dr. Y. Fujikawa (IMR, Tohoku University) for his critical role in the experiments
here reported, Prof. C-T.Chen and Dr. Guiling Wang (CAS, Beijing) for their generous and essential
support in developing the 177 nm laser facility at IBM, Arthur Ellis and Joe Santilli for theirexpert technical support, and Dr. Thomas Theis and Prof. T. Sakurai for their unfailing support andencouragement.
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Instr. 78 (2007) 083105 and references therein7) C.T. Chen, T. Kanai, X.Y. Wang, Y. Zhu, S. Watanabe, Optics Letter 33 (2008) 282 andreferences therein8) www.toptica.de9) M.C. Richardson, C.-S. Koay, K. Takenoshita, C. Keyser, Journal of Vacuum Science andTechnology B 22 (2004) 785-790
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5914) J.B. Hannon and R.M. Tromp; Annual Review of Materials Research, 33 (2003) 263
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Vvedensky, ‘Surface Crystallographic Information Service; A Handbook of Surface Structures’,
D. Reidel Publishing Company (1987)
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60Figure 1. LEEM Optical System.
15 keV electrons originating from the gun are deflected to thesample by the 90º magnetic deﬂector. The electrons are decelerated
between objective lens to an energy between 0and 100 eV. Afterreﬂection from the sample the same magnetic deﬂector steers theelectrons into the projector column. Either a real space image, adiffraction pattern, or an energy spectrum can be displayed on theviewing screen. The sample can also be illuminated by a UV lightsource from the side.

Figure 2. Basic LEEM Imaging Modes
a-Bright ﬁeld Si(111) image using the specular LEED diffracted beam. Atomic steps are visible.
b-Dark field Si(001)image using a (½,0) LEED diffracted beam. Alternating atomic terraces light up as thereconstruction rotates by 90º from terrace to terracec-Mirror mode Si(111) image at ~860 ℃ . The electrons turn around just before hitting the sample. Contrast is
visible between Si(111) (7x7) and (1x1) areas.
d-LEED pattern of the Si(111) (7x7) surface

61

Figure 3. Concentration of Pd in the ﬁrst three atomic layers of a Cu(001) crystal, shown in the colored circles superimposed on a
LEEM image obtained at 13.1 eV, after evaporation of 0.45 monolayer Pd at 200℃ . An atomic step runs from the lower left to theupper right. The lower atomic terrace is on the upper left.
The outermost atomic layer contains almost no Pd, as the Cu atoms have a lower surface energy. Pd atoms are buried by theadvancing atomic step, but Pd atoms in the second and third layer repel each other, explaining why a higher concentration in the thirdlayer corresponds with the reduced concentration in the second layer. These data were obtained by quantitative analysis of the LEED
spot intensities as a function of electron energy for each pixel of the real space image.

Figure 4. Basic Operation of the Prism Array as an Energy
FilterIn the diffraction entrance plane (D entrance ) the angular
distributions of electrons with increasing energy form
concentric discs with increasing diameter (light green, darkgreen, red). A (pink) slit is placed across these discs to selecta slice for a given value of kx, here shown across the center ofthe discs. The length of the slit is orthogonal to the plane of thedrawing.
On their path through the prism array electrons with different
energies are dispersed in angle. In the diffraction exit plane(Dexit) we then observe the dispersed E-ky distribution, which
can be projected directly onto the viewing screen, in the same
way that a diffraction pattern can be projected onto the viewing
screen.
Alternatively, an energy selection aperture can be used to selectan energy of interest in the spectrum, and a real space, energy
filtered image can be projected onto the viewing screen. The
dispersed rays, traced backward, converge in the diagonalplane of the prism array, which is therefore achromatic. The
objective lens system places an image in this diagonal plane, sothat dispersion does not negatively affect image quality.

62Figure 5. Energy ﬁltered imaging and angle resolved photoemission from a compound Ag island grown on Si(111)a-Energy ﬁltered secondary electron image. The lighter island area is Ag(111), while the darker area is Ag(001),
as determined by selected area Low Energy Electron Diffraction. The contrast difference is due to a difference
in ﬁnal states.
b-Energy ﬁltered d-electron image. Ag(111) and Ag(001) areas have similar intensities, while the Si substrate is
dark
c-Electron energy spectra obtained on a 3 µm diameter area on Ag(111)d-Electron energy spectra obtained on a 3 µm diameter area on Ag(001)e-Angular distribution of the electron intensity on Ag(111) at a binding energy of 5.7 eV. The surface Brillouinzone (SBZ) is shown in white outline. Strong resonances at the boundary of the surface Brillouin zone,
corresponding to the bulk L point, are seen with 3-fold symmetry.
f-Same as (e) for Ag(001), at a binding energy of 5.9 eV. Now the symmetry of the resonance is 4-fold.

63

Figure 6. Effect of aberration correction on resolution andtransmission.
The figure shows resolution as a function of aperture angle.
Blue lines are for an uncorrected microscope, with an electronenergy of 3 eV at the sample, and with energy spreads of0, 1.5, and 3 eV. The best resolution, for ∆E=0 is about 4
nm, at an aperture angle of ~0.0002 radians. Red curves are
obtained after correction of both chromatic and sphericalaberration. For ∆E=0 the resolution improves by a factor 3,
while the optimum aperture angle increases by a factor 3,
corresponding to an increase in transmission of a factor 9.
Similar improvements in resolution and transmission are seenfor ∆E=1.5 eV and ∆E=3 eV.

Figure 7. Conﬁguration of the spectroscopic LEEM/PEEM system at the IBM T.J.
Watson Research Center. The LEEM/PEEM system is on the left, coupled to a dedicated
XPS/UPS system on the right. The LEEM/PEEM is equipped with the in-line energy ﬁlter
described in this paper, a HeI/HeII discharge light source, as well as 262 nm and 177 nm
pulsed laser systems, and a Hg discharge lamp.

64

The Salient Point of ResearchList of the salient of presentation at “Challenge of Interdisciplinary Materials Science toTechnological Innovation of the 21st Century”
1. C. T. Liu (Univ. of Tennessee)“Shear-band dynamics and fracture strength of bulk metallic glasses (BMGs)”
2. A. R. Yavari (WPI-AIMR and Grenoble Institute of Technology)“Giga-strength micro- and nano-structures “chill-zone” alloys from glass-forming alloys
outside the glass-forming composition range”
3. T. Yamasaki (Univ. of Hyogo)“Viscous ﬂow behaviours of Zr55Cu30Al10Ni5 bulk metallic glasses over the entire temperature
range”
4. W. H. Wang (Chinese Academy of Sciences)“Unique properties of new rare-earth-based bulk metallic glasses”
5. J. H. Perepezko (Univ. of Wisconsin-Madison)“Bulk Liquid Undercooling and Nucleation Kinetics Analysis”
6. Y. Shibutani (Osaka Univ.)“Deformation-Induced Free Volume Evolution of Computational Binary Amorphous Alloys”
7. M. Tsukada (WPI-AIMR and Waseda Univ.)“Theory of Scanning Probe Microscopy and Transport for Nano- Structures”
8. X. F. Jin (Fudan Univ.)“Interfacial Spin Frustration at Mn/Ni/Cu(001)”
9. V. Crespi (Penn State Univ.)“Islands and roads at the nanoscale: Universal trends in the theory of two-dimensional growth (ofSAMS) and one-dimensional bisection (of nanotubes) on substrates”
10. A. Al-Mahboob (WPI-AIMR)“LEEM study of organic molecular ﬁlm growth”
11. W. Song (Chinese Academy of Sciences)“Nanostructured Metal Oxides: Facile Synthesis methods and Promising Applications”
12. P. Harrowell (Univ. of Sydney)“Characterizing the Geometry and Composition of Long-Lived Structural Fluctuations inSupercooled Alloys”
13. H. Teichler (Univ. of Goettingen)“Dynamical heterogeneity in glass forming NiZr bulk melts and nanosized two-dimensional
ﬁlms.-Molecular dynamics results”
14. Y. Hirotsu (Osaka Univ.)“Medium Range Order Structures and their Changes in the Course of Primary Crystallization insome Metallic Glasses”

6715. D. V. Louzguine-Luzgin (WPI-AIMR)“Nanocrystallisation of high-strength Al-based metallic glasses”
16. K. Fujita (Ube National College of Technology)“Fracture Toughness and Fatigue Strength in Bulk Metallic Glasses”
17. P. V. Sushko (WPI-AIMR)“Electronic excitations, charge trapping and defect formation in oxides and oxide
nanostructures”
18. T. Gessner (WPI-AIMR, Fraunhofer Institute for Reliability & Microintegration
and Chemnitz Univ. of Technology)“NEMS, MEMS - issues of smart systems integration”
19. J. Zi (Fudan Univ.)“Propagation of liquid surface waves in periodic structures”
20. T. Hashizume (WPI-AIMR, Hitachi, Ltd. and Tokyo Institute of Technology)“Interface of organic devices by Scanning probe microscopy”
21. K. Uosaki (Hokkaido Univ.)“Formation of Two-dimensionally Ordered Atomic/molecular Layers at Solid/liquid Interfaces
as New Functionality Materials”
22. T. Adschiri (WPI-AIMR)“Supercritical hydrothermal synthesis of organic inorganic hybrid nanocolloids”
23. Y. Fujikawa (Tohoku Univ.)“Five-Dimensional Spectromicroscopy Probing Slow Electrons”
24. K. Nakajima (WPI-AIMR)“Nanoﬁshing of a Single Polymer Chain”

68Shear-banding dynamics detected in bulk metallic glasses (BMGs)
Cang Fan, B. Yang, C. T. Liu*, P. K. Liaw, and T. G. Nieh
Department of Materials Science and Engineering
The University of Tennessee, Knoxville TN 37996 USA*Phone: 865-574-4459; Fax: 865-576-6298
E-mail: liuct@ornl.gov

Contrary to abrupt linear unloading spikes seen in conventional investigations of shear-band
deformation of BMGs, we surprisingly observed these stress relief spikes to be nonlinear in nature
over ﬁne time scales, recorded by a fast data collection (FDC) system. The dynamic behavior ofshear-band glide in BMGs involves ﬁve steps: initiation, acceleration, viscous glide, deceleration,
and arrest. The shear-band sliding velocity is estimated to be very low, around 0.5 mm per sec,
consistent with viscous gliding.

Shear load drop is nonlinear in nature as recorded by FDC•The shear load drop at 1.5% strain
Curve (I) by CDC
Curve (II) by FDC•100% increase inload drop by FDC•The FDC curve with an expanded Y axis that the
dynamic behavior of shearbanding includes fivestages:
Initiation, acceleration, fast glide, deceleration,and,arrest69

Giga-strength micro- and nano-structures “chill-zone” alloys from glass-forming
alloys outside the glass-forming composition range
A. R. Yavari(WPI-AIMR and Grenoble Institute of Technology)
E-mail:yavari@ltpcm.inpg.fr

X-ray micro-proﬁling using 90 keV synchrotron light in transmission on the cross-section of
a Cu90Zr5Hf5 copper mold cast slab with scratch resistant nanostructures surface layer.
The diffraction patterns from near the surface produce continuous diffraction rings typical ofsmall equiaxed crystals. Further into the surface layer the rings break up into discontinuous streaks
then spots [from “Chill-zone copper with strength of stainless steel and tailorable color”, A.R.
Yavari, K. Ota, K. Georgarakis, A. LeMoulec, F. Charlot, G. Vaughan, A.L. Greer and A Inoue, Acta
Mater. In press]

70Viscous Flow Behaviours of Zr55 Cu30 Al10 Ni5 Bulk Metallic Glasses over the
Entire Temperature Range
T. Yamasaki1), Y.Tanimoto1), T. Kikuchi1), Y. Yokoyama2),
T. Ishikawa 3) , H. M. Kimura2) and A. Inoue2)
1) University of Hyogo, Japan, 2) IMR, Tohoku University, Sendai, Japan,
3) JAXA, Ibaraki, Japan
E-mail:yamasaki@eng.u-hyogo.ac.jp

Viscosity of the Zr 55 Cu 30 Al 10 Ni 5 supercooled liquids was quiteindependent of thevarious applied loads. However, when the sample was slowly heated at the rate of 20 °C/min,viscosityexhibited relatively high values that may be mainly due to theskin effects of oxides. With
increasing the heating rate to200 °C/min and above, viscosity largely decreased indicatingthe
equilibrium viscosity may be measured in these heatingconditions.

71Unique properties of new rare-earth-based bulk metallic glasses
W. H. Wang (Chinese Academy of Sciences)E-mail:whw@aphy.iphy.ac.cn

Magnetic entropy changes as a function of temperature for Ho30Y26Al24Co20 , (triangle )and
Dy50Gd7Al23Co20 (square) BMGs under 2 T and 5 T (the inset is for Er50Al24Co20Y6 )72

Bulk Liquid Undercooling and Nucleation Kinetics Analysis
J. H. Perepezko (Univ. of Wisconsin-Madison)E-mail:perepezk@engr.wisc.edu

Annealing of Melt-Spun Al88Y7Fe5
Metallic glass formation requires a large liquid undercooling that is controlled byheterogeneous nucleation. Rapid cooling is often used to achieve metallic glasses, but a new
analysis of undercooling shows that nucleant refining can be effective. In addition, a number of
useful properties can be obtained by annealing metallic glass to develop nanocrystalline materials.

73

Deformation-Induced Free Volume Evolution of Computational Binary
Amorphous Alloys
Y. Shibutani (Osaka Univ.)
E-mail:sibutani@mech.eng.osaka-u.ac.jp

This ﬁgure indicates the multiple shear banding of binary amorphous alloy under the uniaxial
loading by molecular dynamics simulations. The localized deformation can be achieved by the 2-D
model with plane-stress condition, while the 3-D model as the small illustration realizes the mainband with the ﬁnite width.

74

Theory of Scanning Probe Microscopy and Transport for Nano-Structures
M. Tsukada (WPI-AIMR and Waseda Univ.)
E-mail:tsukada@cms.nano.waseda.ac.jp

Professor Masaru Tsukada
Theory of Scanning Probe Microscopy and Transport of NanoStructures

Theory predicts that a strong loop current is induced by a small bias voltage inside thetriangular graphene enclosing a wide area. The current strength is by more than ten times larger
than the source drain current. This type of the quantum current is predicted for various nano-scale
structures including the donuts of carbon nanotube, and some organic molecules.

75Interfacial Spin Frustration at Mn/Ni/Cu(001)
X. F. Jin (Fudan Univ.)
E-mail:xfjin@fudan.edu.cn

76Islands and roads at the nanoscale: Universal trends in the theory of
two-dimensional growth (of SAMS) and one-dimensional bisection(of nanotubes) on substrates
V. Crespi (Penn State Univ.)
E-mail:vhc2@psu.edu

A nanotube bisected by two opposing rows of chemisorbed hydrogen toms bisects the lowenergy electronic states and so generates a new physical regime wherein vibrationail energy scales
exceed electronic energy scales.

77

LEEM study of organic molecular ﬁlm growth
A. Al-Mahboob, J. T. Sadowski, Y. Fujikawa and T. Sakurai（WPI-AIMR）
E-mail:almahb-0@imr.tohoku.ac.jp

Fig.1. (a)-(c) LEEM images showing (001)-oriented Pn growth (bright contrast) on various substrates (dark contrast): (a) Fractal
growth on clean Si(111)-7x7 surface – disordered wetting layer is formed before nucleation of standing-up structure; Pn ﬁlm is
textured; (b)-(c) Step ﬂow-like growth of compact Pn island on Bi(0001)/Si(111) and α √ 3-Bi-Si(111) are observed; The ﬁrst layer
single crystalline domain size are exceeding 200 and 100 μ m in diameter respectively; Pn grown on Bi(0001)/Si(111) is the ﬁrst
reported (001)-oriented epitaxial ﬁlm.

F i g . 2 . ( a ) Ti l t e d - b r i g h t - f i e l d L E E M i m a g e s h o w i n g
microstructure within a Pn island on H-Si(111) and (b) tip of theone of the branches in Pn island with three distinctive contrasts(I, II, III) visible; Pn 2D crystal alignment depicted from µ-EED
patterns are outlined in (b), shorter and longer sides of eachrectangle corresponds to a- and b-axes respectively; A Pn island
is textured inside, even it evolved from a single nucleus.

Fig.3. LEEM images of Pn domains: (a) Tilted
dark-field LEEM image showing rotational Pn
domains on trigonal Bi(0001)/Si(111) substrate,
additional
contrast of chiral twin is visible within a rotational
domain; each domain is directed along their b
directions; (b) Pn (bright contrast) on self patternedBi(0001) substrate (grey contrast), separated bydisordered Bi region (dark contrast) which limits
Pn moleculeʼs diffusion across it (dark contrast),
having Pn b-axis closely parallel to strip direction
over the surface within the limit of epitaxial
relation; Outlined is the unit cell orientations
depicted from μ -LEED patterns.

78

Nanostructured Metal Oxides: Facile Synthesis methods andPromising Applications
W. Song (Chinese Academy of Sciences)E-mail:wsong@iccas.ac.cn

Nanostructured Metal Oxides:
Facile Synthesis methods and Promising Applications
Hierarchically structured metal oxides have two or more levels of structures that respectively
satisfy our requirements for desired materials. Facile synthesis method are developed for various
metal oxides, which show promising potenials in water treatment and nano catalysis.

79

The Geometry and Composition of Long-Lived Structural Fluctuationsin Supercooled Alloys: Simulations Studies
Peter Harrowell( University of Sydney)
E-mail:peter@chem.usyd.edu.au

Plots of the potential energy (after a steepest decent minimization) vs time for two different
simulations of a supercooled binary liquid mixture. On the left we see the steady drop in energy
associated with crystallization. On the right, we observe the supercooled liquid experiences manylarge ﬂuctuations in the quenched potential energy. These are associated with structural ﬂuctuations
characterized by imperfect crystal order.

80

Dynamical heterogeneity in glass forming NiZr melts. Molecular dynamics results.
H. Teichler(Institute for Materials Research and SFB 602, University of Goettingen, Germany)

Atomic arrangement in the bulk glass forming Al15Ni25Zr60 melt from computer modeling.
(M. Guerdane and H. Teichler, Phys. Rev. B, 014203 (2001))
The cross-section displays a marked medium range order in the structure, consisting of chainsof Al-atoms (red) within the Zr-matrix (green), and Ni-atoms (blue) immersed in between, the latteravoiding direct Ni-Ni contacts. The structure is the result of thermal relaxation processes taking
place as heterogeneous dynamics in form of correlated motions of whole chains of atoms.It means ahighly complex, rather stable amorphous arrangement signiﬁcantly different from crystalline nuclei.

81

“Medium Range Order Structures and their Changes in the Course of Primary
Crystallization in some Metallic Glasses” Y. Hirotsu(Osaka Univ.)
E-mail:yhirotsu@sanken.osaka-u.ac.jp

A: bcc Fe<100>Cluster
B: bcc Fe<111>Cluster

MRO structures extended in alloys of low-glass-forming–ability have strong structuralsimilarities to the primary phases in crystallization, and have potentials to become nuclei in thecourse of the primary crystallization. In Fe84Nb7B9 glass, MROs with the bcc-Fe structure areformed in the glass formation and are grown gradually in the course of annealing to become nucleiof the primary α-Fe crystals. After annealing at 773K, bcc-MROs are clearly observed by highresolution TEM(a). Occasionally, we observe α-Fe crystals among the bcc-MROs (b). Simulated
images of bcc-MROs along the beam directions of [100] and [111] are compared with MRO images
at A and B indicated in (a), respectively. Also shown are nano-beam diffraction patterns from thebcc-MROs.

82

Nanocrystallisation of high-strength Al-based metallic glasses
D. V. Louzguine-Luzgin (WPI-AIMR)
E-mail:dml@imr.tohoku.ac.jp

Fig. (a) and (b) high-resolution transmission electron microscopy images revealing
dislocations and other defects (encircled) observed in Al nanoparticles below 10 nm is size(with extremely high number density in the order of 1024m-3) which were crystallized on rapid
solidification of the Al85Y4Ni5Co2Pd4 glassy alloy. The sources of these dislocations and the
distortions of the crystalline lattice are believed to be impingement of the growing particles
and possibly the difference in the thermal expansion coefficients of crystalline Al andthe
multicomponent amorphous phase. Both factors lead to stresses on cooling in a solid state. The
observed dislocations are attracted to the grain boundaries but did not annihilate on them possibly
owing to a high cooling rate in the order of 106 K/s during melt spinning.

83

Fracture Toughness and Fatigue Strength in Bulk Metallic Glasses
Kazutaka Fujita1*, Yoshihiko Yokoyama2, Wei Zhang2, Baolong Shen2,
Kenji Amiya2,Nobuyuki Nishiyama3, Hisamichi Kimura2 and Akihisa Inoue4
1 Ube National College of Technology, Department of Mechanical Engineering,Ube 755-8555, Japan
2 Tohoku University, Institute for Materials Research, Sendai 980-8577, Japan
3 R&D Institute of Metals and Composites for Future Industries (RIMCOF),
Sendai 980-8577, Japan
4Tohoku University, Sendai 980-8577, Japan
E-mail:fujita@ube-k.ac.jp84Electronic excitations, charge trapping and defectformation in oxides and oxide nanostructures
P. V. Sushko(WPI-AIMR and Univ. College London)
E-mail:p.sushko@ucl.ac.uk

Schematics of photo-induced electronic excitations, charge trapping, and desorption of atoms
and ions in oxide nanoparticles exposed to ultra-violet light.

85

NEMS, MEMS - issues of smart systems integration
T. Gessner(WPI-AIMR, Fraunhofer Institute for Reliability &Microintegration and Chemnitz Univ. of Technology)
E-mail:thomas.gessner@che.izm.fraunhofer.de

Wafer level integration of MEMS/NEMS

The integration of MEMS/NEMS with electronics to smart systems requires new approaches
for wafer level packaging. This concerns material as well as technology aspects. The application ofnano structures for interconnects is described.

86

“Propagation of liquid surface waves in periodic structures”
Jian Zi(Fudan University)
Email: jzi@fudan.edu.cn
Collaborators: Jiong Yang, Xiaohan Liu, Xinhua Hu, and Yunfei Tang

Observation of superlensing in liquid surface waves:
A slab of arrays of copper cylinders, arranged in a square lattice, is placed in the middle of avessel. A point source is placed on the left side of the cylinder arrays. Top (experiment) and lower(theory) panels represent the snapshots of liquid surface wave patterns. At low frequencies (e.g.
4.5 Hz), the transmitted wave patterns are circular-like, with their centers located inside the slab,
forming a virtual image. At around 6.15 Hz, an image appears outside the slab, forming a real image(middle panels). This is caused by negative refraction at the slab surfaces. With the further increasein frequency, the image shifts further to the right and becomes elongated, eventually leading to anearly directive emission (right panels).

87Interface of organic devices by Scanning probe microscopy
T. Hashizume(WPI-AIMR, Hitachi, Ltd. and Tokyo Institute of Technology)
E-mail:tomihiro.hashizume.qb@hitachi.com

The effect of self-assembled monolayer (SAM) at the metal- semiconductor interfaces wascharacterized by examining voltage drops by Kelvin probe microscopy while the OTFTs were in
operation.

88Formation of Two-dimensionally Ordered Atomic/molecular Layers
at Solid/liquid Interfaces as New Functionality Materials
K. Uosaki (Hokkaido Univ.)
E-mail:uosaki@pcl.sci.hokudai.ac.jp

Structure of Au(111) surface during electrochemical atomic layer epitaxial growth of Pd
XRD patterns showed that Pd(111) and Pd(100) were deposited on Au(111) and Au(100),
respectively. STM measurements showed the adsorption of Pd complex on both the substrate goldand deposited Pd, inhibiting the three dimensional growth of the deposits.. Surface x-ray scattering89Supercritical hydrothermal synthesis of organic inorganic hybrid nanocolloids
Tadafumi Adschiri（WPI-AIMR)
E-mail:ajiri@tagen.tohoku.ac.jp

In supercritical water, organic-inorganic hybrid nanocrystals can be synthesized.

90

Five-Dimensional Spectromicroscopy Probing Slow Electrons
Y. Fujikawa (Tohoku Univ.)
E-mail:fujika-0@imr.tohoku.ac.jp

He-I photoelectron distributions in the 2D reciprocal space obtained from Ag microstructures
formed on Si substrate, selecting Ag(111) (left) and Ag(100) (right) areas with a 4-micron aperture.

91

Nanoﬁshing of a Single Polymer Chain
K. Nakajima (WPI - AIMR)
E-mail:knakaji@polymer.titech.ac.jp

Figure (upper) The concept of nanofishing, (b) the original nanofishing data for a single
polystyrene chain with high-speed data capter mode, and (c) the power spectral density calculated
from nanoﬁshing data. Four PDS signals are corresponding to four different conditions as indicatedin the legend.

92Interviews

Prof. Yoshinori YAMAMOTO
機構長 山本 嘉則Interview with Yoshinori Yamamoto, Institute Director of the WPI-AIMR
In "WPI-AIMR News Vol. 1" issued on December 24, 2007, the concept/background,purpose, personnel composition, and joint research system, etc. of this research institute wereclearly illustrated. Its main target is to create innovative substances or new materials throughcooperation between researchers from three different disciplines of materials science, physics, andchemistry. Thirty principal investigators (PIs) are to be scouted from both inside and outside thecountry, and in addition we will recruit associate professors, assistant professors, and post-doctoral
researchers. This will become an intelligence group where materials are researched (including
basic and applied research) from the atomic level to the molecular level by a total of around 200
researchers over a 10 to 15 year period. This research institute holds the lofty ideal of contributing
to the betterment of humankind by producing world-leading original and outstanding research
results.
Practically, the ﬁrst pillar is to achieve outstanding research results in materials scienceand technology, which receive attention from all over the world. The second pillar is to reform thenegative elements in Japan’s traditional promotion system, and to enable young researchers who
can submit excellent proposals to carry out research independently and freely. Generally, young
researchers in Japan have had to wait too long before pursuing their own researches independent
from their supervisors. The last pillar is to destroy the barrier to communication. Since there aremany participants from overseas countries, the ofﬁcial language we use is English. Even those of
you whose mother tongue is not English are requested to carry on all communication in English,
regardless of your English skills. To make a leap forward to the international stage, a precondition
for all of us is that we are able to communicate in English without hesitation.
Personal exchange within the research institution is very important outstanding research
results, because researchers may cooperate with each other while making the best use of theirown expertise. To discover new materials and new processes and to develop new systems it isnecessary to combine multidisciplinary ideas. It often happens that "common sense in specialty A
is not common sense in specialty B." There are many examples where a new discovery with goodprospects for the future has been produced by an approach that appeared to be contrary to common
sense. For example, the use of laser ionizing for measuring the mass of a protein would probably
not appear to be productive to an expert in this research ﬁeld, because he/she knows that a protein iseasily broken up. However, it was Koichi Tanaka, a newcomer to this research ﬁeld who combined95

laser ionizing of protein with TOF, an experiment which led to his winning a Nobel Prize.
It is increasingly found that specialists in their own fields are so fixated on standard
knowledge that they ﬁnd it difﬁcult to take a drastic leap. In making a leap forward, youth is often
the catalyst. There are a lot of examples when a productive combination of a newcomer and an oldtimer has produced signiﬁcant results.
This institute is encouraging daily teatime discussions, monthly meetings, and mini
symposia with outside joint researchers to promote exchange between persons of different
specialties. We want to improve creativity dramatically by cooperative research based on various
free, large and small discussion formats.
I think it will be better for us to remove fixed ideas by having open discussion withspecialists in different fields at least once a day. In particular, asking questions to PIs fromdifferent specialties may present opportunities for learning. And at the same time, it will present
opportunities for senior PIs to throw away preconceptions and to think through things from theirfoundations again.
The advantage this institute has from the personnel aspect is that it is free to employ
people from an international viewpoint. It will be good for young researchers to become aware of
their international rivals, and this may stimulate their research.
In the future, administrative staff will have to handle international correspondence. A
system to support administrative work in English has already been established at NIMS in Tsukuba,
and I have heard that the WPI at Tokyo University is also going to adopt the system.
We have nothing equivalent to a so-called faculty council, either. It is also important to
reduce the time spent on administrative work to ensure that as much time as possible is spent on
research.
This concludes my explanation of the internal operation of the center.
For external operation, the ﬁrst thing I have in my mind is dissemination of the research
results and external evaluation. It is normal to make research results available on the Internet. But
it is also important to periodically communicate research activities at this institution to people
on the outside through quarterly newsletters, etc. and to facilitate the feedback of information.
“Outside” here refers to three parties: universities, the private sector, and the general public. As
far as the general public is concerned, there is a trend for students to stay away from science, andwe'd like to make cutting edge information available to them, including high schools and liberalarts universities. Aside from deep specialist studies, receiving feedback and questions on aspects ofdaily life will surely be useful for researchers.
So that we can receive feedback from the outside at any time, the email address and fax
number of the WPI office are open to the public. We also welcome direct verbal feedback from
actual companies, etc. We want to expand our news editing to avoid one-way communication. We
must aim at reforming the development of materials science, while keeping in mind the tendency

96

of a university to conduct research for the sake of the research itself, and not let our researchers slip
into self-complacency.
We ask for your continued support and cooperation.
(Interviewed in the Institute Director’s Office of WPI on the Katahira Campus on January 23,
2008.)
by Hiroshi Komatsu

97原子分子材料科学高等研究機構（WPI – AIMR）の山本嘉則機構長にきく（Interview with Yoshinori Yamamoto – Japanese version）
2007 年 12 月 24 日発行の「WPI-AIMR News vol. 1」でこの研究機構の構想と背景、目的、人的構成、共同研究体制などがわかり易く図解されています。材料科学系、物理系、化学系の 3 つの異専門分野の研究者が共に協力し合って、革新的な材料や新物質を生み出すことが大目標です。30 人の主任研究者 (PI) をそれぞれ国内外からスカウトし、さらに准教授、助教、ポスドクを募集します。全体で 200 人ほどの研究者が 10 年ないし 15 年にわたって基礎から応用までにまたがり、材料を原子・分子レベルから研究する知能集団になります。本研究機構は世界をリードする独創的な研究成果を挙げ、人類社会に貢献するという高い理想を持っています。
具体的には、材料科学及び材料工学の分野で世界中から注目される卓越した研究成果をあげることが第一の柱です。
第二の柱は日本の旧来の閉鎖的な講座制がもっていた負の部分を改め、きちんとしたプロポーザルを出せる若手も独立して自由に研究を展開できるように計らいます。日本ではこれまで、若手が待機する時間が長すぎました。最後の柱はコミュニケイションのバリヤー破壊です。国外から参加する研究者も多勢いますので、公用語は英語になります。英語を母国語としない人たちも、broken English でかまいませんので、とにかく英語で物事をすすめます。全員が気後れすることなく、英語でコミュニケイションが出来るようになることは、今後、国際的な飛躍をする上でも必須条件になります。
研究機関内の人的交流は素晴らしい研究成果を達成するために重要なことです。なぜなら、互いの専門を活かしながら共同研究し協力することで融合研究を行い素晴らしい成果へと実を結ぶことが多いからです。新材料、新しいプロセスの発見、システムの開発などには多面的な考え方が組み合わされることが必要です。
「A 専門分野の常識は B 専門
分野の非常識」ということは多々あります。将来性豊かな新発見が、常識をはずれたところで生れた例はたくさんあります。たとえばタンパク質の質量測定をするのにレーザーによるイオン化を使うことなどは、タンパク質の壊れ易いことを知っているその道のベテランはまず手を出しません。ところが、レーザーによるタンパク質のイオン化と TOF を組
み合わせたのが、その道の新人だった田中耕一さんで、実験結果がノーベル賞のきっかけになっています。
その道の専門家の中には標準的な知識にしばられる余り、思い切った冒険をしない人が多くなっています。飛躍す
るには常に若いバネが必要です。新人とベテランが上手く組み合さって、大きな成果が出た例は多いです。
本機構では専門の異なる人同士の交流を促進するため、日常的なティータイム、月ごとの討論会、外部の共同研究
者とのミニシンポジュームなども推奨しています。大小様々な自由な討論を元にした研究協力により創造力を飛躍させたいです。
1 日に 1 回は異分野の専門家と自由討論して、頭の中の枠をはずすようなこともできればよいと思います。特に異専門の PI に質問をぶっつけることは、自分の勉強になると同時に、先輩の PI にとっても、ものごとを先入観なしで再び根本から考え直すチャンスになることもあります。
本機構の人事面のメリットは、かなり自由に国際的な見地から人を採用できることです。若手も良い意味で国際的なライバルを意識するようになり、研究のよい刺激になるでしょう。
これからは事務の方も国際対応ができることが望まれます。つくばの NIMS では既に英語による事務支援体制ができていますし、東京大学の WPI もその体制をとるようにすると聞きました。
いわゆる教授会に相当するものもありません。事務的な時間をすくなくし、研究する時間が出来るだけ潰れないよ
うに計らうことも大切です。

98

以上、内部の運営についてお話しました。
外部に関しては、研究成果の伝達や外部評価がまず思い浮かびます。研究成果をネットで見ることができるようにすることは当然ですが、季刊ニュースなどで定期的に本機構の研究活動状況を外部に伝え、情報のフィードバックを円滑にすることも大切です。外部とは「大学、企業、および一般」の 3 者を指します。
「一般」については理科離れが出ている現在、高校や文系の大学までも含めて、最先端の情報を送りたいです。専門を深く追求することはもちろんですが、同時に、日常生活の視点からコメントや質問をもらうことも研究者にとって有用になるはずです。
外部からの反響をいつでも受信できるように、WPI オフィスのメイルアドレスとファックス番号を公開します。企業などの現場の声が素直にはいってくることも歓迎します。一方通行にならないようにニュースの編集も工夫したいです。ともすれば研究のための研究となり、自己満足に陥りやすい大学の傾向を、常に意識して材料科学の革新的発展を目指すように心がけます。
皆様のご声援・ご支援をお願いします。

(2008 年 1 月 23 日

片平キャンパス WPI 機構長室にてインタービュー )
ひろし

小松

99

啓 記

Prof. Masashi KAWASAKI
主任研究者 川崎 雅司

Interview with Professor Masashi Kawasaki,
Principal Investigator , WPI-AIMR
Personal History, Encounter and Breakthrough—Three pillars: chemistry, solid state physics and electronics—
An epoch-making research achievement by Professor Masashi Kawasaki of the Institute ofMaterials Research at Tohoku University was released across Japan on Valentine’s Day in 2005.
It was presented under the title “A visionary light opens into the future—Impactful light-emitting
diode” on an infotainment program by NHK (Japan Broadcasting Corporation), “Today’s CloseUp.” The light-emitting diode (LED) that was the subject of the program was developed using zincoxide and gives out ultraviolet radiation. This ultraviolet light-emitting diode is said to be leading
the illumination revolution. Differing from its predecessor, the blue-light-emitting diode based on
GaN, the material for the new diode is found in abundance in the environment around us and thus
can be obtained at a low cost. Moreover, the LED has very low energy consumption and is expected
to have a semi-permanent life.
What brought about this world-leading research result, and what developments can be
expected from it? We asked Professor Kawasaki directly.
Readers will be interested to hear about the professor’s life history, and this may enable
us to see the motivations behind his research activities. Of course he was gifted with the intuition
needed to be a leading researcher, but at the same time it seems that his vigorous and unique
attitude which is characterized by a fearlessness and a tendency to push down the boundaries comesfrom the success that he experienced at an early stage in his research career. His background reveals
that since that time, he has maintained a proactive stance toward challenges, and has been fortunate
enough to have good instructors that encouraged him to establish his own original procedures
through actual experience. What is more, within his own realm of expertise, he attempted to engage
in joint research with top-level researchers in different fields. I hope that you can see what lies
behind his distinguished career, including the various painstaking efforts and attempts he made that
led to his successes.
Now let’s listen to Professor Kawasaki’s story.
“I encountered the energy crisis or so-called “Oil Shock” when I was an elementary101

schoolboy. This was the original experience that made me concerned about energy issues. When I
was a junior high school student, I read “Suiso Enerugi (Hydrogen Energy)” (Blue Backs, KodanshaLtd.) written by Professor Tokio Ohta of Yokohama National University and began to form thedesire to be a researcher who ﬁghts against future energy crises. In my high school days, I read a lotof books published by Simul Press Inc. that published books with shocking titles like “Netsuosenn(Thermal Pollution),” and began to think more and more about the mission of scientists including
energy issues. At the time, global warming had not emerged as a topic, but I naturally understoodthat once we had burned up all the oil and fossil fuels, the Earth would revert to the climate thatexisted at the time those fuels were being stored. When I had to decide my future career, I wrote
to Professor Ohta to ask for advice on which university I should attend in order to study hydrogenenergy. He told me that I could choose Yokohama National University; at this time Professor
Kenichi Honda of the Faculty of Engineering at the University of Tokyo was conducting active
research. I entered the Science I of the University of Tokyo in 1980 and found out that the scores inthe exams at the College of Arts and Science, Junior Division required to enter the Department ofIndustrial Chemistry were not high, because not many students wanted to get into the Department.
So I devoted myself to training both body and spirit by playing American Football and learning
about society through after-school jobs in liquor shops. I went on to the Department of Industrial
Chemistry in the Faculty of Engineering, where I actually visited laboratories and joined thelaboratory of Professor Kazuo Fueki who had been studying hydrogen energy.
In the summer of my fourth year, when I gave a presentation at one of the laboratory’s seminars on the theme of my undergraduate thesis and what I had found out about the future ofhydrogen energy, then associate professor of the Fueki Laboratory, Dr. Hideomi Koinuma, provoked
me by saying, “Hydrogen energy has no future. Instead, you had better study the amorphous siliconcell.” Docile as I was, I thought he was right after listening to his account in detail and decided tocommit myself to Dr. Koinuma for my postgraduate studies.
That was the time of new materials, with high hopes for amorphous silicon and many
companies entering the market. For Japan, which has no crude oil resources, an energy policy
directly linked to the survival of the state has always been a central issue, and government
projects on photovoltaic power generation such as the Sunshine Project were just being launched.
Amorphous silicon, in particular, had attracted people’s attention for thin film transistors fordisplays as well as for use in solar cells. Spin-off research from this currently contributes to Japan’s technical competitiveness and trade balance with the development of large screen televisions. At
the time, however, I had many opportunities to listen to young researchers giving presentations at
technical exhibitions on liquid crystal displays full of defects. This allowed me to learn about the
future and diversity of materials research from a wide point of view.
By the way, Dr. Koinuma, my postgraduate instructor, requested his students to ask
questions in a conference. My ﬁrst experience of an academic conference was when I attended a102

session on amorphous silicon at the Japan Society of Applied Physics. In the session, an engineer
from a company was giving a presentation on the characteristics of amorphous silicon created bysilicon electrolysis (anodic oxidation). Since I knew nothing about it, I plucked up the courage to
ask him what “anodic oxidation” meant. The researcher fobbed me off, saying, “The technique has
been around for many years.” Just as I was feeling embarrassed and frustrated, the chairman, Dr.
Kuwano (later president of SANYO Electric Co., Ltd.), said to him, “Since it appeared sometimearound the time the young man asking the question was born, he asked because he did not know.
Would you kindly answer his question in full? ” I was saved by those words. I continued to feel
deeply that it is important to answer questions even after I became an expert. Dr. Kuwano is the
person who commercialized amorphous silicon solar cells for use in devices such as electronic
calculators. I always keep in mind that I will attract ambitious young people if I respect that kind of
free-wheeling atmosphere.
At that time, we were making amorphous silicon by decomposing silane by discharging
ultraviolet rays or plasma. Silane is a gas generated by combining silicon and hydrogen. Inspired
by Dr. Junichi Nishizawa’s method to control reactions by light, I considered that repeatedly
and alternately discharging ultraviolet light and plasma in silane and methane made from a
combination of carbon and hydrogen would generate two substances; the reaction initiated by thelight would decompose silane into amorphous silicon, while the reaction initiated by the plasma
would decompose the methane as well into an amorphous semiconductor in which silicon andcarbon are mixed (silicon carbide). I noticed that by using this, a new superlattice of amorphoussilicon and silicon carbide could be generated. “Amorphous” indicates that the atoms do not showa regular alignment, but I created a structure in which irregularly-aligned atoms were forced into
regular alignment using nanoscale technology. Dr. Reona Ezaki proposed this concept for crystal
semiconductors and named it a “superlattice,” while I adopted the concept of the superlattice fornoncrystal substances and achieved it chemically. I published the results in the journal Nature, andlater developed them in my doctoral thesis [Nature Vol. 331, 153-155 (1998)].
In 1986, when I was continuing with my studies and had established the general directionof my thesis, an unexpected and life-changing event happened to me. An IBM researcher in
Switzerland discovered high-temperature superconductivity in oxide ceramics made mainlyof copper. A large number of scientists rushed into research related to the discovery. I haveunconsciously acquired the habit of being a top runner in major trends, and experienced both thejoys and severity of global cutting-edge research while leading a fairly busy life working day andnight in the frontiers of research at the Koinuma Laboratory. I gradually became fascinated by the
Bell Laboratory and IBM Laboratory, the key places for superconductivity studies, and developed
the desire to study abroad to try out my skills.
Thanks to an introduction by Dr. Koichi Kitazawa, then associate professor at the Fueki
Laboratory, I was fortunate enough to be accepted by Dr. Praveen Chaudhari (a senior of Dr.

103

Kitazawa’s in his MIT days) at the IBM Watson Research Center. Conversant in both chemistry andsolid physics, Dr. Kitazawa was a prominent ﬁgure in studies into a new material physics called
ceramics super conductivity. I vaguely began to think that this combination of chemistry and solid
physics had already been covered by him and that I would thus place myself on the borders ofchemistry and electronics. After taking a doctorate, I spent several months at the Hitachi Laboratory
where I learned things that were a bit different from what I had learned so far, such as writing
patents for electronic devices and how to use clean rooms. In the high-thermal conductivity groupat IBM, I succeeded in developing an ultrasensitive magnetic sensor called SQUID. I also studied
the physics of superconductive devices intensively for two years, freely handling the top-in-theworld device that I had developed myself.
Immediately after I returned home (1991), a new major project called “PRESTO (Precursory
Research for Embryonic Science and Technology)” was launched by the young technology projectgroup under the umbrella of the Science and Technology Agency. I applied for the project with myproposal for emitting light from a high-temperature superconductor, and was selected as a memberof the inaugural staff. I participated in the “light & material” project led by Dr. Kenichi Honda andagain had an important encounter.
One of the members in the project, who was then a young researcher at Mitsubishi Electric
Corporation, developed a theory predicting that the non-linear effect caused by excitons would
become extremely great due to the interference effect when the thickness of the thin ﬁlm and thewavelength of the light were arranged in a certain relationship. In order to verify this throughexperiment, young researchers of the “PRESTO” gathered and negotiated with Dr. Honda, thegeneral manager. As a result, it was decided that we would select ZnO as the material, buy a new
device for my laboratory to make thin films, and then Dr. Yuzaburo Segawa of RIKEN would
make an optical assessment. In the course of the experiment, by chance we discovered ultraviolet
laser oscillations at room temperature. [Proc. 23rd International Conference on the Physics of
Semiconductors, 1453-1456 (1996)]. Light was emitted, not from a superconductor but from ZnO,
a white powder used in cosmetics. In the “PRESTO” there was a climate of respecting unexpected
achievements more than promised ones, and so I preserved my pride even though I could not keep
my promise.
Immediately after our discovery, efforts to develop a light emitting diode (LED) by
combining n-type ZnO, which is negatively charged, with p-type ZnO, which is positively charged,
became active all over the world. While many dubious theses announced that they had succeededin making p-type ZnO, no one in the world was able to make an LED using ZnO. We were also
involved in painstaking efforts. The breakthrough was an idea based on chemistry to challenge how
far we could weaken the n-type instead of adding, without any ﬁrm conviction, additives that appear
to turn ZnO into p-type. ZnO naturally tends to turn into n-type. We succeeded in making the ﬁrst
p-type ZnO in the world through a procedure to emit ultraviolet light from a pn-joint LED. This was104

announced in December 2004 [Nature Materials Vol. 4, 42-46 (2005)]. Eight years had passed fromthe discovery of the ZnO laser, where we succeeded in synthesizing genuine and authentic p-type
ZnO, to creating the LED. This achievement was presented later in an NHK’s special program of“Today’s Close-up.”
The thesis in Nature Materials that introduced this success with the LED was ranked ﬁrst
in the world in the materials & science ﬁeld as the most frequently cited paper in the subsequent
two years. As a result of this, the reports on p-type Zn0 that had been published before virtuallydisappeared. Now, we are working jointly with businesses on studying applications for the LED
based on ZnO.
Compared with GaN, which uses materials that exist only in certain areas of China andsome other countries, ZnO is superior in that it can be obtained easily in any country. Moreover, theGaN is made via a special buffer layer using a sapphire substrate, since the GaN crystal substratecannot be synthesized easily. Differing from GaN, the process can be simpliﬁed for ZnO since alarge single-crystal substrate can be made easily. Since a substrate made of ZnO, unlike a sapphire
substrate, allows electricity to ﬂow, it does not have the weak point of GaN, which does not allow
electric poles to be created on the surface or reverse side of the LED.
I have just completed the basic research on the ZnO-based LED and am looking for mynext target. I have a dream to expand the potential of oxidative products. The textbooks say thatthe mobility of the electrons in ZnO is approximately 200 cm2/Vs, but when I investigated thenature of pure ZnO, I found that the mobility of the electrons in ZnO is approximately 500 cm2/Vs.
So I conﬁned the n-type ZnO electrons in a very thin space to generate a secondary electron gas
and measured the Hall resistance. As a result, I succeeded in monitoring the quantum Hall effect,a macro quantum effect [Science 315, 1338 (2007)]. This is the first macro quantum effect inoxidative products found besides superconductivity. I am glad that oxidative products have at last
reached this stage. In the background to this success lies the over ten years of joint research withProfessor Hideo Ohno of the Research Institute of Electrical Communication at Tohoku University
on transparent transistors and transparent ferromagnetic materials. I always feel stimulated when I
talk about my research with Professor Ohno, who is an expert in electronics. However, I am not sointerested in orthodox semiconductors. What attract me are non-orthodox things that do not attract
other researchers, such as amorphous and oxidative products. I also discovered that adding a little
bit of magnetic impurity to oxidized titanium would turn the semiconductor into a magnet [Science
291,845 (2001)], which I believe is an important achievement that clearly indicates a bright futurefor oxidative products. I now harbor the desire to contribute to bringing about an energy-saving,energy-creating and environmentally harmonized society by further expanding the range of myresearch targets and discovering new functional devices for strongly-correlated electronic oxidative
products related to high-temperature superconductivity.
The motivating force driving this research is, above all, our unique procedure for105

synthesizing oxide thin films. In fact, the synthesization procedure for amorphous superlattices
during my university days and the procedure for the later p-type ZnO are based on almost the same
concept. I control ZnO at the atomic level; to be more intuitive, I design processes while thinking
seriously from the viewpoint of the atom how I wish to freeze the solid. Nevertheless, it is not easy
to ﬁnd out the optimum conditions, and we have to search for the conditions on a “carpet-bombing”
basis when dealing with non-orthodox materials such as oxidative products. Here I have also
invented a unique procedure called the “combinatorial method” to synthesize 100 to 10000 different
samples in one experiment. I wish to add a modern touch to “manufacturing,” which is the essenceof chemistry, and use my unique skills to make further discoveries one after another. My futuretarget is to study how the three attributes of the electron (electrical charge, spin and orbit) behave
and create a paradigm for a new materials science via functional development using the new mutual
relationship between their impacts and responses (cross-correlation).
(Interviewed by Hiroshi Komatsu, in Professor Kawasaki’s ofﬁce at the Institute of Materials
Research, Tohoku University, February 8, 2008 )106

WPI-AIMRPI

川崎雅司教授にきく（Interview with Professor Masashi Kawasaki – Japanese version）「生い立ち・出会い・ブレイクスルー」−化学・固体物理学・電子工学の 3 本柱−
東北大学金属材料研究所の川崎雅司教授による画期的な研究成果が 2005 年のバレンタインデーに全国報道された。
NHK の「クローズアップ現代」で、「夢の光が未来をひらく〜発光ダイオードの衝撃〜」と題して。照明の革命を導くとされる紫外光を出す発光ダイオード（LED）を酸化亜鉛 (ZnO) で作製したのである。先行の窒化ガリウム (GaN) 青色
発光ダイオードとは異なり、原材料は身近に大量にあり、安く得られる。しかも LED のエネルギー消費はごく少なく、寿命も半永久的である。
このような、世界をリードする研究はいかにして生まれたか、そして今後どのように発展するのか。今回、川崎教授から直接お聞きした。
生い立ちからうかがうことで、研究活動の裏にある動きが見え、一般読者の方々も興味を持たれると思います。研究を進める上でのカンの良さには先生の天性もありますが、研究者として育つ初期に巡り会った成功体験を通して、物怖じしなくなり、いらぬ束縛から解放された面もあるように見えます。それ以後は困難に挑戦する姿勢を持ち続け、良い指導者にめぐまれ、実体験から独自の手法を確立してゆく姿が浮き彫りにされます。さらに、自分の得意分野を武器として、異分野の第一級の研究者と共同研究を展開するなど、成功に至るさまざまな苦心や工夫など隠れた部分を感じ取っていただければ幸いです。
では、早速、川崎先生の話を聞いてみましょう。

小学生の時、エネルギー危機、いわゆる “ 石油ショック ” に出会いました。これがエネルギー問題に関心を持った原体験です。中学生の頃、横浜国立大学の太田時男先生が書かれた「水素エネルギー」（講談社・ブルーバックス）を読んで、将来はエネルギー危機に立ち向かう研究者になりたいと思うようになりました。高校生時代には「熱汚染」などショッキングなタイトルの本を出すサイマル出版の本を数多く読み、ますますエネルギー問題など科学者の使命について考えるようになりました。当時は、地球温暖化など話題にはなっていませんでしたが、石油など化石燃料を燃やし尽くすと、それを蓄えたころの気候に戻ると自然と納得していました。将来の進路を考える頃、太田先生に手紙を出して、水素エ
ネルギーの研究をするにはどの大学へ進学するのが良いかについて助言をいただきました。横浜国大も良いが、東大工学部の本多健一先生が活発な研究を行っていると教えてもらいました。1980 年に東大へ入学して色々調べると、当時学科としては人気がなかった応用化学への進学には、教養学部での試験で大して点数が必要でないことがわかり、アメフトの部活動や水商売のアルバイトを通した社会勉強で徹底的に心身を鍛えました。工学部の応用化学へ進学し、実際に研究室を見学して、同じく水素エネルギーについて研究していた笛木和雄先生のところにお世話になることになりました。
大学 4 年の夏頃、卒業論文研究の構想と水素エネルギーの将来性について調べた内容を研究室のセミナーで発表した
ところ、笛木研の助教授であった鯉沼秀臣先生が「水素エネルギーに将来はない、それよりもアモルファスシリコン太陽電池の研究をやるべきだ」と挑発されました。素直な私は、鯉沼先生のお話を詳しくうかがって、「なるほど」と思い、

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大学院からは鯉沼先生の指導を頂くことになりました。
当時は新素材ブームで、アモルファスシリコンへの関心が高く、多くの企業が参入していました。原油資源を持たない日本にとって、国家の存続に直結するエネルギー政策は中心的な課題であり、サンシャインプロジェクトなどの太陽光発電に関する国家プロジェクトが推進されている最中でした。特にアモルファスシリコンは、太陽電池だけでなく、
ディスプレー用の薄膜トランジスタとして注目されるようになりました。副産物とも言えるこの研究が、今では大型テレビを可能にして日本の技術競争力と貿易収支に貢献していますが、当時は、欠陥だらけの液晶ディスプレーを若い研究者が技術展で発表するのを勉強して、材料研究の将来性と多様性について広い視野から学べる良い機会でした。
話は変りますが、大学院で指導を受けた鯉沼先生は、学会では毎日一つは質問をするようにと学生に命じていました。
初めて学会に出席したのは、応用物理学会のアモルファスシリコンのセッションでした。そのときのセッションで、ある企業の人がシリコンを電気分解（陽極酸化）して作製したアモルファスシリコンの特性について発表していました。
私は何も知らなかったので、勇気を出して「陽極酸化」はどういうことかと質問をしたところ、もう何年も前からわかっていることだ・・と、切って捨てるような返答を受けました。とても恥ずかしいような悔しいような気持ちになった瞬
間に、座長であった桑野さん（後の三洋電気社長）が『質問した若い人が生れた頃のことで、知らないからこそ質問している。きちんと説明すべきです。』と注意されました。このときは救われました。そして、どんなにえらくなっても質問にはきちんと答えることが大切だと身に沁みて感じました。桑野さんは、電卓などについているアモルファスシリ
コン太陽電池を事業化した方です。そういう闊達な雰囲気を尊重して、志のある若い人を惹きつけたいと常に思っています。
当時、シリコンが水素と結合してできるシランというガスを紫外線やプラズマ放電で分解してアモルファスシリコンを作っていました。西澤潤一先生の「光で反応を制御する方法」にヒントを得て、シランと炭素が水素と結合したメタンを入れて紫外光とプラズマ放電を交互にくりかえすと、光によって進行する反応ではシランが分解されて、アモルファ
スシリコンが生じ、プラズマだとメタンも分解してシリコンと炭素が混じったアモルファス半導体（シリコンカーバイ
ド）ができると考えました。これを使えば、アモルファスシリコンとシリコンカーバイドの新しい超格子が作製できることに気づきました。アモルファスというのは原子が規則的に並ばないことを示しているのですが、それを無理矢理ナノスケールで規則的に並べた構造を作ったのです。江崎玲於奈先生が結晶半導体でこの概念を提案され超格子と名付けられましたが、わたしは、結晶でない物質に超格子の概念を取り入れ、化学のセンスでそれを実現したのです。この結果を Nature 誌に発表し、後に博士論文にまとめました [Nature vol.331, 153-155 (1998)］。
研究も進み、博士論文の方向性も見えだした 1986 年に、突如として人生が変わるできごとが起こりました。銅を主成分とする酸化物セラミックスで、スイスの IBM の研究者が高温超伝導を発見したのです。どっと大勢の研究者がその関連研究に流れ込みました。大きな流れの中では、先頭の方に立とうという習性がいつからか身についていて、鯉沼研の核弾頭として昼も夜もない生活を送りながら世界の最先端研究の楽しさと厳しさを味わいました。当時、超伝導研究のメッカであったアメリカのベル研究所や IBM の研究所に心を惹かれるようになり、留学して腕試しをしてみたくなりました。
幸い、笛木研の助教授であった北澤先生の紹介で IBM ワトソン研究所のチャウドリーさん（北澤先生の MIT 時代の先輩）が採用してくれました。北澤先生は、化学と固体物理学の両方に精通しておられ、セラミックス超伝導という新しい材料物理の研究で存在感を持っておられ、何となくこの組み合わせは取られちゃったからわたしは化学と電子工学の境目で勝負しようと考え始めました。博士を取った後、出発までの数ヶ月を日立中研で過ごし、電子デバイスの特許
書きやクリーンルームの使い方など、これまでと少し違ったことも教わりました。IBM では、高温超伝導グループで

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SQUID という超高感度磁気センサーの開発に成功し、世界最高性能の自作デバイスを突っつき回して 2 年間ミッチリと超伝導デバイスの物理も勉強しました。
帰国直後 (1991 年 )、科技庁傘下の新技術事業団で “ さきがけ研究 ” という若手が主体で行う新しい大型プロジェク
トがスタートし、高温超伝導体から光を出すという提案を応募したところその第 1 期生に選ばれました。本多健一先生が主宰する「光と物質」プロジェクトに参画し、また新しい出会いに巡り会いました。
そのプロジェクトに参画した当時三菱電機の若手研究者であった理論家が、薄膜の厚さと光の波長をある関係にそろえると干渉効果で励起子による非線形効果が極度に大きくなると予言しました。それを実験で検証しようと “ さきがけ
研究 ” の若手が集まり、統括の本多先生に談判して、材料として ZnO を選び、わたしのところに新しい装置を買ってもらって薄膜を作って、理化学研究所の瀬川勇三郎さんが光学評価をすることになりました。実験するうちに、なんと室温での紫外レーザー発振を発見しました［Proc. 23rd International Conference on Physics of Semiconductor, 1453-1456(1996)］。超伝導体ではなく化粧品の白い粉である ZnO から光を出したわけですが、“ さきがけ研究 ” では約束通りの成果より思いもかけない成果を大切にする風土があり、約束を守れなかった割には偉そうにしていたと思います。
その直後からマイナスの性質を持つ n 型とプラスの性質を持つ p 型の ZnO を組み合わせて発光ダイオード (LED) を作る研究が世界中で活発に展開され、p 型化に成功したと主張するあやしげな論文は多数発表されるものの、世界中の誰も ZnO で LED が作れませんでした。われわれも大変苦労をしました。突破口は、天然に n 型になりやすい ZnO に闇雲に p 型になりそうな添加物を加えるのではなく、n 型の性質をどこまで弱めることができるかに挑戦しようという化学の発想でした。レーザーを使う新しいエピタキシー法で、世界ではじめてきちんとした p 型 ZnO 作りに成功し、pn接合の LED から紫外光を出すことに成功し、2004 年 12 月に発表しました。[Nature Materials vol.4, 42-46(2005)］。ZnO レー
ザーの発見から 8 年かけて正真正銘の p 型 ZnO の合成に成功し LED を作成したのです。この成果が後に NHK のクローズアップ現代の特別番組として紹介されました。
この LED の成功を伝えた Nature Materials の論文は、以後 2 年間の引用度数が多いホットペーパーとして材料科学分
野の世界ナンバーワンになりました。これにより、それ以前に発表されていた p 型 ZnO の報告は事実上消えてしまうことになりました。現在、企業と共同で ZnO の LED の実用化研究をすすめています。
中国など一部の国に偏在した素材を使う GaN に比べ、ZnO はどの国でも簡単に得られる点ですぐれています。さらに、
GaN 結晶基板が簡単には合成できないためにサファイア基板を使って特殊なバッファー層を介して作る GaN と違って、ZnO は単結晶の大型基板が簡単に作れるためにプロセスが単純化されます。また ZnO の基板はサファイアとは異なり電気が流れるため、LED の表と裏で電極が取れない GaN の弱点もありません。
ZnO の LED に関した基礎研究が終ったところで、今は次のターゲットを探しています。酸化物の可能性を拡げることに夢をもっています。ZnO は電子の移動度が 200cm2/Vs ぐらいと教科書に書いてありましたが、p 型をつくる研究の途中で純粋な ZnO の性質を丹念に調べたところ、電子の移動度が 500 cm2/Vs ぐらいあることがわかました。そこで、n型 ZnO の電子を非常に薄い空間に閉じこめた二次電子ガスを作製し、そのホール抵抗をしらべた結果、マクロな量子効
果である量子ホール効果の観測に成功しました［Science 315, 1338（2007）］。これは超伝導以外で見つかった、酸化物における最初のマクロな量子効果です。酸化物もいよいよここまで来たという感想です。この成功の背景には、東北大学
通研の大野英男先生との 10 年来の透明トランジスタや透明強磁性体に関する共同研究がありました。電子工学がご専
門の大野先生と研究の相談をするといつも刺激を受けるのですが、わたしは由緒正しい半導体には興味がなく、アモル
ファスだとか酸化物だとか、いわゆるゲテモノにしか興味がありません。酸化チタンに少し磁性不純物を加えることで半導体が磁石になることも発見しましたが［Science 291, 845（2001）］、酸化物の明るい未来を明示する貴重な成果だと

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思っています。これからは、さらに研究対象を広げて、高温超伝導体に関連する強相関電子酸化物でも新しい機能デバ
イスを発見して、省エネルギーやエネルギー創出・環境調和型の社会に貢献したいと夢を膨らませているところです。
この研究を推進する原動力は、なんと言っても独自の酸化物薄膜合成手法です。学生時代のアモルファス超格子と後の p 型 ZnO の合成方法は、実はコンセプトがほとんど一致しています。原子のレベルで制御して、もっと直感的には、自分が原子になった気持ちでどう固体として凍りつきたいか、真剣に考えてプロセスを設計しています。とは言え、最適な条件を見つけ出すのは簡単ではありませんし、酸化物というゲテモノが相手では、それこそ絨毯爆撃的に条件を洗い出すことになります。ここでも独自手法をあみだしていて、100 〜 10000 個ぐらいの異なる試料を一度の実験で合成する手法、「コンビナトリアル法」を実現しています。化学の神髄である「ものづくり」を現代風に味付けして、独自技術で次々に新しい発見をなしとげたいと思っています。今後の対象は、光・電圧・磁界などの環境下で、電子の電荷・スピン・軌道といった三つの属性がどう振る舞うかを調べ、その相互の新しい刺激と応答の関係（交差相関）を活用した機能開発を通じて新しい物質科学のパラダイムを構築したいと思っています。

（2008 年 2 月 8 日

110

東北大学金研川崎教授室にて

小松

啓

記）

Tc>400K (TiCo)O2

Science 2001
Nature Mater. 2004

Transparent Magnet Memory( ᡢଢᄬჽᡢଢȡȢȪ )Transparent Transistors, CircuitᲢᡢଢȈȩȳǸǹǿᡢଢᩓ‫ׅ܇‬ែᲣ

Adv. Mat. 2004
Science 2007

ZnO Ultraviolet LED, laser( ᣠ҄ʣᤲ LED ከ‫ٳ‬ȬȸǶ )Nature Mater. 2005

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112

Evaporate Oxides by Laser Pulses
࡟࡯ࠩߢ㉄ൻ‛ࠍ⫳⊒ߒߡ⭯⤑ࠍ૞ࠆ

Pulsed Laser Deposition: PLD

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Blue Light-Emitting Diodes Based on ZnO( ИỜềᣠ҄ʣᤲ pn ੗ӳầᩓ්ỂήụộẲẺ )Tsukazaki et al.
. (2005)

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Atomically Controlled Oxide Epitaxyᵆ ᣠ҄ཋỺἾἁἚἿἝἁἋỉἨἾὊἁἋἽὊỺἦἑỿἉὊỉች݅҄ ᵇ

Flat Surface of Oxide

Science (1994)115

Temperature Gradient Epitaxyίภࡇͼ૲ඥỂಏẉஇᢘ҄ὸ

116

117

118

Announcement

Junior Faculty/Post-doctoral Positions
Tohoku University
WPI-AIMR
Effective October 1, 2007, Tohoku University created a new Research Institute, the Advanced
Institute for Materials Research (AIMR), based on an initiative of the Japanese Department of
Education (MEXT) for World Premier International Research Center Initiative (WPI) to bring
together scientists involved in research on nano-science and technology.
In the 21st century, material science, broadly defined as the study of how complex/novel
properties arise in matters/materials from the interactions of individual components, will compriseof inter-discipline collaboration.
(HTTP://WWW.WPI-AIMR.TOHOKU.AC.JP).
Over the next few years, as many as one hundred new appointments at the levels of postdoctoral fellows and junior faculty will be available. All innovative researchers are welcome asactive promoters of basic/applied sciences in the ﬁelds of physical metallurgy, physics, chemistry,
precision mechanical engineering and electronic / informational engineering.
We are continuously looking for excellent applicants throughout the year.
Please submit
1) a curriculum vitae,
2) research proposal (<3,000 words),
3) summary of previous research accomplishments (<2,000 words),
4) copies of 5 signiﬁcant publications, and
5) 2 letters of recommendationby email to yoshi@mail.tains.tohoku.ac.jp,
sakurai@imr.tohoku.ac.jp, andwpi-ofﬁce@bureau.tohoku.ac.jp.
All ﬁles must be submitted electronically in pdf or Word format.
Applications from, or nominations of, women and minority candidates are encouraged.
Tohoku University WPI-AIMR is an afﬁrmative action / equal opportunity employer

121Graduate Student scholarship
In Materials Science/Engineering
WPI-AIMR
Graduate Student scholarship
Effective October 1, 2007, Tohoku University created a new Research Institute, the Advanced
Institute for Materials Research (AIMR), based on an initiative of the Japanese Department of
Education (MEXT) for World Premier International Research Center Initiative (WPI) to bring
together scientists involved in research on nano-science and technology.
In the 21st century, material science, broadly defined as the study of how complex/novel
properties arise in matters/materials from the interactions of individual components, will becomesan essential and most important research topics(HTTP://WWW.WPI-AIMR.TOHOKU.AC.JP).
TU WPI-AIMR is now looking for young motivated Ph.D. graduate student candidates
in the fields of physical metallurgy, physics, chemistry, mechanical engineering and electronic /informational technology. All innovative M. S. students are welcome as active promoters of basic/
applied sciences in these ﬁelds.
Applications are continuously screened throughout the year.
Please submit
1) a curriculum vitae,
2) research proposal (<1,000 words),
3) 2 letters of recommendation,by email toyoshi@mail.tains.tohoku.ac.jp,
sakurai@imr.tohoku.ac.jp, andwpi-ofﬁce@bureau.tohoku.ac.jp.
All ﬁles must be submitted electronically in pdf or Word format.

122

WPI-AIMR
Workshop Guideline
Tohoku University’s new Research Institute, the Advanced Institute for Materials Research(WPI-AIMR) solicits several applications per year for International Workshops in the field of“broadly deﬁned Materials Science.”
Guidelines:
1) Organizers
Qualiﬁed research staff of academic institutions and public or private research establishments
can submit the application for an international workshop to be held at WPI-AIMR or its Satellite
branches, jointly with the WPI-AIMR principal investigator(s) whose research interest overlaps
with the scope of the workshop.
2) Financial support
Under normal circumstances, WPI-IMR supports up to 2/3 of the workshop budget, while theorganizer is expected to cover the rest.
3) deadline
The application must be received at least four months in advancetoyoshi@mail.tains.tohoku.ac.jp,
sakurai@imr.tohoku.ac.jp, andwpi-ofﬁce@bureau.tohoku.ac.jp.
All ﬁles must be submitted electronically in pdf or Word format.

123Appendix
WPI & IFCAM Joint Workshop snapshots

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