Hitachi Review Vol. 61 (2012), No. 6269

Low-energy Electron Diffractive Imaging for
Three‑dimensional Light-element Materials
Osamu Kamimura, Ph. D.
Takashi Dobashi

OVERVIEW: Hitachi has been developing a diffractive imaging techniquecapable of high-resolution imaging without causing serious damage tothe specimen in collaboration with Hokkaido University. The technique
has growing applications in the fields of environmental engineering,sustainable energy, and life science. Using a low-energy (30-keV) electron
beam, images of the atomic structure of SWCNTs with a resolution of0.12 nm were successfully reconstructed by digitally processing diffraction
patterns recorded with a new dedicated electron diffraction microscope(a scanning electron microscope with a diffraction recording function).
Using this technique, the microscope can obtain high-resolution images oflight-element materials with complex three-dimensional structures without
specimen damage. The technique is suitable for use in material and device
development in various fields.

INTRODUCTION
USE of electron microscopes for the development ofmaterials and devices has occurred most commonly inthe field of semiconductors, where they have frequently
been used to identify the structure of dislocations inmetals. Recently, demand for electron microscopy
has also been increasing in the fields of environmentengineering, sustainable energy, and life science.

Light-element materials such as the carbon electrodesof lithium-ion batteries are important in these fields.
The need to make observations of different
materials can be a driver of innovation in analysis
technology. While observations with atomic-scale
resolution are usually performed using transmission
electron microscopes (TEMs) with high-energy
electron beams, light-element materials tend to be

Iterative phase retrieval

Illumination beamSpecimen

Diffraction pattern

Reconstructed patternFig. 1—Schematic Diagram of Diffractive Imaging.
The microscope illuminates the specimen with a parallel electron beam and records the resulting diffraction pattern. The structure of
the specimen (reconstructed pattern) is then obtained by using iterative phase retrieval to retrieve the phase information lost when
the diffraction pattern was recorded. The diffraction pattern and reconstructed pattern shown here are actual patterns obtained fromobservation of a single-wall carbon nanotube.

Low-energy Electron Diffractive Imaging for Three‑dimensional Light-element Materials

sensitive to radiation damage. The level of knock-on
radiation damage caused by incident high-energy
electrons pushing atoms in the specimen out of place
is sufficiently severe that it is necessary to limit theobservation time when using a conventional 200-keV
TEM. Using an electron beam with lower energy is
one solution to this problem of short observation time,
but lens aberrations make it difficult to obtain atomicscale resolution when using low-energy electronmicroscopy. Accordingly, the growing demand foranalysis of light-element materials has created a needfor an imaging technique capable of atomic-scale
resolution that does not damage the specimen.
A new technique called diffractive imaging has
been attracting interest as a potential solution to thisproblem. Diffractive imaging reconstructs an imageof the specimen structure from its diffraction pattern.
Unlike conventional electron microscopy, in which
the resolution is restricted by lens aberrations, theresolution of diffractive imaging depends on the size ofthe diffraction pattern that can be acquired (the wider
the diffraction angle, the higher the resolution). Thismeans it is possible to obtain high-resolution images
without being limited by lens aberrations. In addition,
whereas conventional analysis of crystal structures
assumes a periodic structure, this technique can also
be used for imaging materials with a non-periodicstructure. Hitachi has developed an analysis techniquethat combines a low-energy electron beam withdiffractive imaging, and has tested its performance
when used for imaging with atomic-scale resolution.
This article describes this diffractive imaging
technique and how it can be used to make observationsof light-element materials with complex structures,with a low level of damage to the specimen.
DIFFRACTIVE IMAGING
Fig. 1 shows a schematic diagram of diffractive
imaging. The microscope illuminates the specimen
with a parallel beam and captures the resulting
diffraction pattern by locating a detector sufficiently
far from the specimen. Digital processing is used toapply an iterative phase retrieval algorithm to thediffraction pattern to retrieve the phase information
lost when the diffraction pattern was recorded. An
image of the specimen’s structure (reconstructedpattern) is obtained by combining the recorded
diffraction pattern amplitudes with the calculated
phase information.
Although this method was first suggested byD. Sayre in 1952(1), it was not successfully implemented

270

in practice until 1999 (using X-rays)(2). This was madepossible by advances in computer technology andthe high intensity of third-generation synchrotron
radiation. Since then, numerous examples of diffractive
imaging using X-rays have been published, including
the three-dimensional structures of nanoparticles(3),(a)(b)

Illumination optics

AperturesSecondaryelectron detector
SpecimenTransmission
electron detector

Film loader
system

CCD camera

CCD: charge coupled device SEM: scanning electron microscope

Fig. 2—Dedicated Electron Diffraction Microscope.
The photographs show the exterior (a) and interior (b) of theelectron diffraction microscope. A conventional SEM wasmodified to record diffraction patterns by the addition of a filmloader system under the SEM column (illumination optics).

Hitachi Review Vol. 61 (2012), No. 6(a)

271

220

200020

50 nm
(a)

(b)

(b)

(c)

STEM: scanning transmission electron microscope
BF-STEM: bright field scanning transmission electron microscope
MgO: magnesium oxideFig. 3—SEM Image (a) and BF-STEM Image (b) of MgONanoparticles.
Information about the surface morphology and inner structureof cubic nanoparticles was obtained from an SEM image andSTEM image respectively.

viruses (4), chromosomes (5), and cells (6). The firstsuccessful demonstration of diffractive imaging using
electrons was reported in 2002(7), and several groups
have reported examples of its use since then. However,most of these have involved the analysis of materialstructures using high-energy 200-keV electron beams.
With the aim of obtaining atomic-resolution images
without causing serious damage to the specimen,
Hitachi has been undertaking development workwith Hokkaido University to demonstrate the use ofdiffractive imaging with low-energy electron beams(8).
ELECTRON DIFFRACTION MICROSCOPE
The initial proof of principle for low-energy
electron diffractive imaging was conducted using
a prototype microscope (8) built by modifying aninstrument designed for reflection electron diffraction.
Subsequently, to overcome problems with thisprototype (including low resolution when used foroutline observation and the fact that loading film andoperating the shutter had to be performed manually),
and to make the technique more practical, a dedicatedlow-energy electron diffraction microscope wasdeveloped(9) based on a scanning electron microscope(SEM) (see Fig. 2). The microscope uses a film loader
system located under the conventional SEM column
to record the diffraction pattern. It also has a chargecoupled device camera to monitor the diffraction
pattern. In addition to conventional secondary electron(SE) detectors, a bright-field scanning transmission
electron microscope (BF-STEM) detector is attached

40 nm

Fig. 4—Diffraction Pattern (a) and BF-STEM Image (b) of
Isolated MgO Nanoparticle, and Magnified Image of 200
Diffraction Spots (c).
Fringes, which carry information about the specimen shape, canbe observed around the diffraction spots.

below the specimen (retractable). Fig. 3 shows
example SE and BF-STEM images of an array of
cubic magnesium oxide (MgO) nanoparticles. The
SEM image shows the surface morphology and theBF-STEM image shows the inner structure of theparticles. Fig. 4 shows the diffraction pattern for asingle isolated MgO nanoparticle. The BF-STEM
image in Fig. 4 (b) indicates that the size of thenanoparticle is about 30 nm. The diffraction patternshown in Fig. 4 (a) includes diffraction spots fora 0.21-nm lattice spacing, while the diffraction
fringes visible around each diffraction spot, shownin Fig. 4 (c), contain information about the shapeof the nanoparticle. These results demonstrate thatthe dedicated microscope can be used to observe theoutline and inner structure of a specimen as well as toperform a crystallization analysis from the diffraction
pattern.
VERIFICATION OF LOW-ENERGY ATOMICRESOLUTION IMAGING
Single-wall carbon nanotubes (SWCNTs) wereused as specimens to verify the performance of atomic
resolution imaging(10). The energy of the electron
beam was set at 30 keV, which is below the knock-on
voltages for SWCNTs (about 60 keV)(11) and carbon
atoms at the graphene edge (about 40 keV)(11).

Low-energy Electron Diffractive Imaging for Three‑dimensional Light-element Materials

Fig. 1 shows the recorded diffraction pattern andreconstructed image for a SWCNT. Fig. 5 (a) showsa magnified image of the reconstructed pattern, Fig. 5(b) shows the simulated pattern, and Fig. 5 (c) shows
the atomic structure model. SWCNT is a rolled
graphene sheet (a hexagonal lattice of carbon atoms).
The hexagonal lattice orientations in the upper andlower layers are different, and when the arrangements
of atoms in the two layers are combined, it results inthe complex pattern shown in Fig. 5 (c). A comparisonof these images indicates that the image reconstructedfrom the diffraction pattern contains information about
the atomic structure of the SWCNT with a resolutionof 0.12 nm. Based on the diffraction pattern, thediameter of the SWCNT was estimated to be 3.2 nm.
A feature of this technique is that it was able to achieve
this resolution when imaging two layers separated bymore than 3 nm. This is possible because it uses aparallel beam and produces an image without use ofa magnifying lens (see Fig. 1).
This work has demonstrated the ability ofdiffractive imaging to produce images of threedimensional materials such as SWCNTs with atomicscale resolution. Moreover, the bright regions indicatedby the gray and white arrowheads in the reconstructed272

image in Fig. 5 (a) correspond respectively to single
isolated carbon atoms and to overlapping atoms
in the upper and lower layers, as indicated in theatomic model in Fig. 5 (c). Thus, the reconstructed
image contains quantitative information sufficient
to divide between a single carbon atom and two
overlapping atoms. The quantitative accuracy of theimage intensity has become increasingly important inrecent times, with applications such as using the image
intensity to identify differences in atomic number(12).
The results of this work show that diffractive imaging
can be used for quantitative analysis.
CONCLUSIONSThis article has described the use of a diffractive
imaging technique that can make observations of lightelement materials with complex structures with only
a low level of damage to the specimen.
The work has demonstrated that diffractive imaging
can obtain images of radiation-sensitive materials withcomplex three-dimensional structures, such as carbon
nanotubes, with atomic-scale resolution using a lowenergy electron beam that causes little damage tothe specimen. In the future, Hitachi intends to utilize
this technology and the dedicated microscope in the(a)0.5 nm(b)(c)

SWCNT: single-wall carbon nanotube

Fig. 5—Reconstructed Pattern of SWCNT Obtained Using 30-keV Electron Beam (a), Simulated Pattern (b), and Model of SWCNT
Atomic Arrangement (c).
The resolution of the reconstructed pattern (a) is sufficient to resolve two atoms with a spacing of 0.12 nm (indicated by the grayarrowheads). In the figures (a)–(c), the white arrowheads indicate two overlapping carbon atoms and the gray arrowheads indicate
single isolated carbon atoms.

Hitachi Review Vol. 61 (2012), No. 6

development of new materials and devices that use
light-element materials.
This work was conducted in collaboration withProfessor Gohara’s group at Hokkaido University. The
SWCNTs were synthesized by Professor Shinohara’s
group at Nagoya University. Part of this work wassupported by the Japan Science and Technology
Agency.

REFERENCES(1) D. Sayre, “Some Implications of a Theorem due to Shannon,”
Acta Crystallographica 5, 843 (1952).
(2) J. Miao et al., “Extending the Methodology of X-ray
Crystallography to Allow Imaging of Micrometre-sized Noncrystalline Specimens,” Nature 400, pp. 342–344 (1999).
(3) Y. Takahashi et al., “Three-dimensional Electron Density
Mapping of Shape-controlled Nanoparticle by Focused Hard
X-ray Diffraction Microscopy,” Nano Letters 10, pp. 1922–1926 (2010).
(4) C. Song et al., “Quantitative Imaging of Single, Unstained
Viruses with Coherent X-rays,” Physical Review Letters 101,158101 (2008).

273(5) Y. Nishino et al., “Three-dimensional Visualization of a
Human Chromosome Using Coherent X-ray Diffraction,”
Physical Review Letters 102, 018101 (2009).
(6) H. Jiang et al., “Quantitative 3D Imaging of Whole, Unstained
Cells by Using X-ray Diffraction Microscopy,” Proceedingsof the National Academy of Sciences of the United States ofAmerica 107, pp. 11234–11239 (2010).
(7) U. Weierstall et al., “Image Reconstruction from Electronand X-ray Diffraction Patterns Using Iterative Algorithms:Experiment and Simulation,” Ultramicroscopy 90, pp. 171–195 (2002).
(8) O. Kamimura et al., “Diffraction Microscopy Using 20-kV
Electron Beam for Multiwall Carbon Nanotubes,” Applied
Physics Letters 92, 024106 (2008).
(9) O. Kamimura et al., “10-kV Diffractive Imaging Using
Newly Developed Electron Diffraction Microscope,”
Ultramicroscopy 110, pp. 130–133 (2010).
(10) O. Kamimura et al., “Low Voltage Electron Diffractive
Imaging of Atomic Structure in Single-wall Carbon
Nanotubes,” Applied Physics Letters 98, 174103 (2011).
(11) K. Suenaga et al., “Single Molecular Imaging by HR-TEM,”
Microscopy 45, pp. 31–36 (2010) in Japanese.
(12) O. L. Krivanek et al., “Atom-by-atom Structural and Chemical
Analysis by Annular Dark-field Electron Microscopy,” Nature464, pp. 571–574 (2010).

ABOUT THE AUTHORSOsamu Kamimura, Ph.D.

Takashi Dobashi

Joined Hitachi, Ltd. in 1994, and now works at theMeasurement Systems Research Department, Central
Research Laboratory. He is currently engaged inresearch into electron diffraction microscopes. Dr.
Kamimura is a member of The Japanese Society ofMicroscopy and The Japan Society of Applied Physics.

Joined Hitachi, Ltd. in 2006, and now works at theMeasurement Systems Research Department, Central
Research Laboratory. He is currently engaged inresearch into electron diffraction microscopes. Mr.
Dobashi is a member of The Japanese Society ofMicroscopy.