Modern Research and Educational Topics in Microscopy.
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A. Méndez-Vilas and J. Díaz (Eds.)
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Importance of Transmission Electron Microscopy for Carbon
Nanomaterials Research
Prakash R. Somani *, 1, 2, and M. Umeno 1
1
Department of Electronics and Information Engineering, Chubu University, Matsumoto-cho 1200,
Kasugai-shi, Aichi-Ken, Japan 487– 8501.
2
Applied Science Innovations Private Limited, Maharashtra, India.
Importance of transmission electron microscopy (TEM) in the carbon nanomaterials research is discussed.
Carbon, is perhaps, the only element which has an infinite number of allotropes. Some of them can appear
very similar. For example, carbon nanofibers and multiwalled carbon nanotubes looks similar when
observed with scanning electron microscopy. It is almost impossible to distinguish between carbon
nanofibers and multiwalled carbon nanotubes unless and until one observes such materials by TEM.
Former is a solid filled 1D nanostructure while the later being hollow concentric 1D nano-tubules. TEM
gives direct insight in to the nanostructure of carbon materials. Absence of TEM observations can lead to
wrong conclusion in case of carbon nanomaterials. It is the most important and most reliable technique for
correctly identifying the nature and the form of carbon nanomaterials in academic research and in
industry. In addition, it provides lots of other valuable information which is discussed in detail by giving
suitable examples.
Keywords Carbon nanomaterials, Carbon nanotubes, Carbon nanofibers, transmission electron
microscopy.
1. Carbon and its allotropes
Carbon is an important element and exists in everything from crude oil to DNA. There are about sixteen
million compounds of carbon, more than for any other element. It has almost infinite number of
allotropes. Diamond (3D) and Graphite (2D) are two of the most famous allotropes. Discovery of C60 (0
D) [1] has opened a new family of carbon allotropes, known as “Fullerenes” which includes C60, C70,
C82, ……[2]. Most recent and notable discovery of carbon allotrope is of Carbon Nanotubes (1D) [3, 4].
It is again a family of carbon allotropes which includes multi-walled nanotubes, single–walled
nanotubes, double walled, few-walled, with open ends or closed ends; with either semiconducting or
metallic properties depending on the diameter and chirality; with either arm-chair, zig-zag or chiral
structures [5-8]. Each of these allotrope and sub-allotrope has unique properties. Other allotropes and / or
forms include diamond like carbon, amorphous carbon, glassy carbon, Planer graphenes, activated
carbon etc. Carbon allotropes / materials have found many industrial applications such as in water
purification, scratch resistant coatings, light-weight and high-strength composites, in electronic devices,
and so on [9-19]. A large area of chemistry deals with the interactions of carbon and is known as
“Carbon chemistry or Organic Chemistry”. Carbon is found in the fourteenth group in the periodic table
so its electronic structure is 1s2 2s2 2p2. Carbon can exist in interesting hybridizations (sp1, sp2 and sp3)
and can form sigma and / or pi- bonds. Carbon allotropes or carbon containing compounds can have pure
sp1 / sp2 / sp3 hybridizations or a mixture of them. Diamond possesses pure sp3 hybridized carbon
whereas graphite / planer graphenes have pure sp2 hybridized carbon (ignoring the atoms at the edges).
Fullerenes, carbon nanotubes, diamond like carbon, amorphous carbon possess a mixture of sp2 – sp3
carbon, with varying amounts. Properties of such materials are usually governed by the sp2 / sp3 ratio i.e.
amount of sp2 and sp3 hybridized carbon in the total material. Fullerenes are usually made of pentagons
and hexagons. Carbon nanotubes are mostly made of hexagons. However, carbon nanotubes can possess
pentagons as defects or at the close ends. Multi-walled carbon nanotubes, in general, can have more
*
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Corresponding author: e-mail: [email protected], [email protected]
Modern Research and Educational Topics in Microscopy.
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A. Méndez-Vilas and J. Díaz (Eds.)
_______________________________________________________________________________________________
pentagons – incorporated as defects. Single walled, double walled, triple walled or few walled carbon
nanotubes are mostly made of carbon hexagons and possess minimum defects in their lattice structures
which is usually reflected from the Raman spectra of these materials (absence or very small disorder (D)
peak as compared to very large intensity graphitic (G) peak). Figure (1) shows some of the carbon
allotropes and their structures.
Fig. 1 Some of the Carbon allotropes and their structures.
2. Transmission Electron Microscopy (TEM)
The transmission electron microscope is generally used to characterize the microstructure of materials
with very high spatial resolution. Information about the morphology, crystal structure and defects, crystal
phases and composition and magnetic microstructure can be obtained by a combination of electronoptical imaging (2.4 A point resolution), electron diffraction and small probe (20 A) capabilities.
The transmission electron microscope uses a high energy electron beam transmitted through a very
thin sample to image and analyse the microstructure of materials with atomic scale resolution. The
electrons are focussed with electromagnetic lenses and the image is obtained on a fluorescent screen, or
recorded on film or digital camera. The electrons are accelerated at several hundred kV, giving
wavelengths much smaller than that of visible light (For example : 200 kV electrons have a wavelength
of 0.025 A). The resolution of the optical microscope is limited by the wavelength of the light used. The
resolution of the electron microscope is limited by aberrations inherent in electromagnetic lenses, to
about 1 – 2 A. Even for very thin samples, the observer is looking through many atoms and one does not
usually see individual atoms. Rather the high resolution imaging mode of the microscope images the
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crystal lattice of the material as an interference pattern between the transmitted and diffracted beams.
This allows one to observe planer and line defects, grain boundaries, interfaces etc. with atomic scale
resolution. The bright field / dark field imaging modes of the microscope, which operate at intermediate
magnification, combined with electron diffraction are also invaluable for giving information about the
morphology, crystal phases and defects in a material. Modern microscopes can be equipped with a
special imaging lens allowing for the observation of micromagnetic domain structures in a field-free
environment.
The transmission electron microscope is also capable of forming a focussed electron probe, as small
as 20 A, which can be positioned on very fine features in the sample for micro-diffraction information or
analysis of X- rays for compositional information. The spatial resolution for this compositional analysis
in TEM is much higher, of the order of the probe size, as the sample used is very thin. Conversely the
signal is much smaller and hence less quantitative. The high brightness field-emission gun improves the
sensitivity and resolution of X-ray compositional analysis over that available with more traditional
thermionic sources. Detailed discussion on the TEM instrument, its operation and capabilities and its
usage is out of scope of this chapter and the readers are requested to see some of the dedicated books on
the transmission electron microscopy [20-23].
For carbon nanomaterials, TEM is the only tool which can identify the correct phase of the material
and can distinguish between the similar looking phases. For example, unless and until observations are
done using TEM, one may not be able to distinguish between a carbon nanotube and a carbon nanofiber;
which otherwise may look similar when observed by scanning electron microscope. High resolution
transmission electron microscopes can identify the defects in the carbon nanotube structures and can
differentiate between different types of carbon nanotubes (zig-zag, arm chair, chiral etc). This is the only
technique which can tell about the number of co-axial carbon nanotubes and their diameters in a multiwalled carbon nanotube. As a result of the invaluable information that TEM provides (which may not be
available from other characterization techniques), it has became the most important and a must technique
for the study, research and production of carbon nanomaterials.
3. Importance of TEM in Carbon Nanomaterials Study
Fig. 2 SEM photograph of (a) self vertically aligned conical carbon nanofibers (b) vertically aligned multiwalled
carbon nanotubes and (c) vertically aligned carbon nanofibers.
Let us see how important TEM observation is for the study of 1D carbon nanomaterials. Some of the
well known 1D carbon nanomaterials are : carbon nanofibers, multi-, single-, double- walled carbon
nanotubes, diamond nanowires etc. Carbon nanotubes can be considered as graphene sheets rolled in to a
cylinder. It’s a 1D hollow structure. When only one graphene sheet is involved, the tube structure thus
formed is known as single walled carbon nanotubes (SWCN). The tube in which two graphene sheets are
involved is called as a double walled carbon nanotube (DWCN) and so on. Carbon nanotube which has
many co-axial tubes is called as a multi-walled carbon nanotube (MWCN). Carbon nanotubes can have
different electronic properties depending on their diameter and chirality. Similar to carbon nanotubes,
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carbon nanofibers are also 1D material. However, in contrast to carbon nanotubes, nanofibers are
generally solid filled structures. It is really difficult to distinguish between carbon nanofibers and
MWCN – when observed by SEM.
Figure (2) shows the SEM pictures of some of the 1D carbon nanomaterials. Figure (2a) shows the
SEM picture of the self vertically aligned conical carbon nanofibers deposited on silicon substrates in
turn coated with a thin film of Co (100 nm) by pulsed discharge plasma chemical vapour deposition
method at about 400 – 450 C [16, 17]. Figure (2b) shows the SEM picture of the MWCN deposited by
thermal chemical vapour deposition using ferrocene as a catalyst (Fe – nanoparticles) source and
camphor as a carbon source at about 700 C. Figure (2c) shows the SEM picture of the vertically aligned
carbon nanofibers deposited by thermal chemical vapour deposition at about 800 C on a silicon coated
with a thin film of Co (100 nm). It is to be noted here that all these structures presented in Figure (2)
looks similar when observed by SEM. However, they are different materials, as their microstructures are
different. And these microstructures are evident from the TEM observations only.
Fig. 3 TEM micrograph of (a) conical carbon nanofibers (b) multiwalled carbon nanotubes and (c) carbon
nanofibers. SEM pictures of these materials are shown in Figure (2).
Figure (3) displays the corresponding TEM micrographs of the 1D carbon nanomaterials presented in
Figure (2). Figure (3a) shows the TEM picture of the vertically aligned conical carbon nanofiber. It is
observable that the material is completely amorphous in nature. XRD, visible Raman along with TEM
observation indicates that these conical carbon nanofibers are made of diamond like carbon (DLC).
Figure (3b) shows the TEM picture of the MWCNs. Inset shows the intensity pattern along the line
marked in the photograph. Central hollow portion is clearly observable which confirms that carbon
nanotubes are hollow 1D structure. The inter-planer separation i.e. the separation between walls of
adjacent nanotubes is estimated to be 0.34 nm (corresponding to the 002 separation of the graphite).
Outermost diameter of the MWCN is observed to be between 35 to 45 nm. TEM observation indicates
that the walls of the MWCN are well crystallized. In all, the present sample is identified as MWCN.
Figure (3c) presents the TEM picture of the vertically aligned carbon nanofibers deposited by thermal
chemical vapour deposition on silicon coated with a thin film of Co (100 nm). Inset shows the variation
of the intensity along the line marked in the photograph. It is observable that these 1D carbon
nanomaterilas are solid filled structures. Further they are amorphous as there is no long range order,
similar to that in carbon nanotubes. However, short range order is observable in these structures. Straight
lines in the photograph indicate graphite like structures. It can be safely concluded here that these carbon
nanofibers are made of sp2 and sp3 bonded carbon atoms. Now the difference in the microstructure of
conical carbon nanofibers (Presented in Figure 3a) and vertically aligned carbon nanofibers (presented in
Figure 3c) should be clear. Former material (conical carbon nanofibers) is an amorphous carbon material
predominately made of sp3 bonded carbon atoms i.e. diamond like carbon (DLC) whereas latter is also an
amorphous carbon 1D material (vertically aligned carbon nanofibers) containing sp2 and sp3 bonded
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carbon atoms. It is to be noted here that all these 1D carbon nanomaterials look similar when observed
with SEM and it is very difficult to identify which sample is a carbon nanotube sample and which is a
nanofiber sample. Further, MWCN having defects like pentagons, heptagons, etc. shows both graphitic
(G) peak and disordered (D) peak in their visible Raman spectra. Also, carbon nanofibers containing sp2
and sp3 bonded carbon atoms displays both G and D- peaks in their visible Raman spectra, similar to
MWCN having defects. Hence, it is almost impossible to correctly identify and distinguish between
carbon nanotubes and nanofibers; until TEM is applied. TEM gives direct insight into the microstructure
of these materials and definitely tells about the nature / form of the material [9-19].
Fig. 4 SEM photograph of carbon nanocapsules encapsulating Co nanoparticles (Note that the appearance of these
looks like metallic nanoparticles and looking at such a picture one can easily get a notion that these are metallic
nanoparticles only).
Let us discuss another example in order to understand the importance of TEM observations in carbon
nanomaterials research. Figure (4) shows the SEM photographs of some nanoparticles. By carefully
looking at the SEM photographs, a skilled person in the area of nanotechnology can suggest that these
nanoparticles might be of some metallic material. Indeed, these nanoparticles are of Co. However, these
nanoparticles are not simply Co nanoparticles but they are “Carbon Nanocapsules encapsulating Co
nanoparticles (CNC)”. CNC shown in Figure (4a) have more or less spherical or distorted spherical
shape whereas the CNC presented in Figure (4b) are polygonal in shape sometimes having sharp edges
and / or corners. Figure (5) shows the TEM photographs of the corresponding CNC shown in Figure (4).
These CNC are prepared by pulsed discharge plasma chemical vapour deposition method by sputter
induced growth. Graphitic ring like structures are easily observable around the central dark black Co
nanoparticles. The interlayer separation between the two adjacent carbon walls is estimated to be about
0.34 nm which corresponds to the 002 separation of the graphite. A careful observation suggests that the
overall shape of the CNC follows the shape of the inner Co nanoparticle. Presence of Co is verified by
the in-situ energy dispersive X-ray analysis (EDAX) measurements during TEM observations.
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Fig. 5 TEM micrograph of carbon nanocapsules encapsulating Co nanoparticle. Central dark/black portion
corresponds to the Co nanoparticle and graphite like structure with interlayer separation of about 0.34 nm is clearly
visible around the Co nanoparticles. Thus, TEM gives a direct proof that these structures are not only metallic
nanoparticles (as may appear from SEM images) but are Carbon nanocapsules encapsulating Co nanoparticle
(CNC).
However, studying this sample only by SEM can lead to a wrong conclusion that these might be
metallic nanoparticles only. Visible Raman spectra show the presence of G- and D- peaks indicating this
material should contain some carbon. However, it can not definitely tell us that these nanoparticles are
“Carbon Nanocapsules encapsulating Co nanoparticles”. TEM is the only technique which gives the
direct proof that these are “Carbon Nanocapsules encapsulating Co nanoparticles” [19].
4. Additional Information
Traditionally, TEM and high resolution TEM (HR-TEM) has been mainly used for imaging, electron
diffraction and chemical analysis of solid materials. Techniques like energy dispersive X-ray analysis
(EDAX) and electron energy loss spectroscopy (EELS) are coupled with HR-TEM and with such
facilities, it became a versatile and comprehensive analysis tool for characterizing the chemical and
electronic structure at nano-scales. In the recent years, new and novel developments have been made in
HR-TEM which became useful for in-situ microscopy for observing dynamic processes at the
nanoscales, nanomeasurments which directly correlates physical properties with structures, holographic
imaging of electric and magnetic fields, quantitative chemical mapping at sub-nanometer resolution and
ultrahigh resolution imaging techniques. For a more detailed review on these aspects, review by Wang is
recommended [24]. We will now discuss some of the interesting studies made on carbon nanomaterials
using transmission electron microscopy (TEM) and high resolution transmission electron microscopy
(HR-TEM) by other research groups.
HR-TEM is used for in-situ atomic observation of the formation of carbon nanofibers and single
walled carbon nanotubes (SWCN). Such studies are useful in understanding the nucleation points of
SWCN from the catalytic nanoparticles which are in turn useful for solving some of the fundamental
problems related to SWCN synthesis, such as, identifying the factors that influences the chiralities of
nanotubes during the growth of SWCN. SWCN synthesis with specific pre-determined chiralities is yet a
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dream to be realized. This should be possible only when the growth mechanism of SWCN is understood
fully. HR-TEM is proving to be a highly useful tool for this purpose [25].
Spring like behaviour of carbon nanotubes is studied with the help of TEM in an in-situ experiment.
MWCN are observed to bend when an energetic electron beam in the TEM hits the MWCN. On
removing the force which makes them to bend, they are observed to relax to the original straight shape.
Bending angle as high as 90 o is observed. Complete recovery of the original shape indicates that lattice
defects are not created in the MWCN by the incident energetic electron beam and that the bending
behaviour of carbon nanotubes is their inherent property (high flexibility). It is known that carbon
nanotubes have high strength. Such high strength together with high flexibility makes them an attractive
candidate for many applications. Owing to the very small size (i.e. in the nanometer range) of carbon
nanotubes, such bending behaviour is studied and imaged using TEM [26].
Fig. 6 (a) – (f) TEM images of a carbon nanotube based nano-thermometer containing a continuous Ga column : (a),
(b), and (c) before placing the nano-thermometer into a furnace heated to a high temperature in air; (d), (e), and (f)
after extracting the nano-thermometer from the furnace, i.e. after measuring its temperature; (a) and (d) low
magnification images of the nano-thermometer; (b) and (e) high magnification images of the closed tip of the nanothermometer; and (c) and (f) those of the open tip. All TEM micrographs were taken at 20 C. Reprinted from Ref.
No. [30].
Fracture of SWCN in the SWCN-Polymer composite under the application of tensile strength is
studied by real time in-situ TEM. Results indicate that stress is transferred from the external force field to
the nanotubes via the surrounding matrix. This suggests that the carbon nanotube-polymer interface is
not inert but significantly strong [27].
The dynamic behaviour and degradation of carbon nanotube field electron emitters was studied by
Seko et al. by an in-situ TEM experiment. Such a study allows to understand the reasons that contributes
to the degradation of field electron emission performance of carbon nanotubes with respect to time.
Since, for a good field electron emitter, a very stable performance with respect to time is expected; its
only after doing such experiments, one can overcome the causes for the degradation of performance.
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Hence, such an in-situ experiment using TEM gives fundamental understanding of the field electron
emission process [28].
Direct fabrication of nanowires with lateral sizes smaller than 10 nm is demonstrated by Hagan et al
[29] using a TEM. The conventional photolithiography based techniques have reached their fundamental
resolution limits, situated around 10 nm as directed by the interaction range of electrons with the photoresist, by the molecular size, and by the resist development mechanism. Hence, new approaches are
needed in order to develop / print / deposit electronic circuits in which the devices can have lateral
resolution smaller than 10 nm. This is very important for further improving the packing density of
electronic circuits in a chip. Initial experiments, such as demonstrated by Hagan et al. [29] using the
electron beam of a TEM are encouraging and can provide a path for further developments.
Final and most beautiful example to illustrate how TEM can be useful for nanodevices is that of a
nanothermometer demonstrated by Gao et al [30]. Nano-thermometer is made of Ga-filled carbon
nanotube with diameter < 150 nm and length about 12 microns. This nano-thermometer is first calibrated
and identification mark is made in a TEM. After which, it is kept in to an air-filled furnace whose
temperature is to be measured. This is followed by again observation and calibration in a TEM. The
difference between the initial and final mark can be calibrated as a temperature / temperature difference.
Mark on carbon nanotube originates from the fact that, at high temperature, the Ga column tip gets
exposed to the air through the open carbon nanotube end oxidizes, and a thin Ga-oxide layer sticks to the
nanotube walls upon cooling. It has been observed that the temperature according to such gradation mark
coincides closely with normal furnace temperature controlled by standard means. Such an experiment
and observation on a nano-device is possible only with a TEM.
5. Conclusions
Importance of transmission electron microscopy (TEM) in the carbon nanomaterials research is
discussed by giving suitable examples from authors own original research. TEM gives direct insight into
the structure of carbon nanomaterials and can help to identify the material / phase correctly. Without
observations by TEM, one may leads to wrong / incorrect conclusions. It is the most important and most
reliable technique for correctly identifying the nature and the form of carbon nanomaterials. It can be
used for imaging, electron diffraction and chemical analysis of solid materials. With added EDAX and
EELS facilities, it has became a versatile and comprehensive analysis tool for characterizing the
chemical and electronic structure at nano-scales. Further, with new developments, TEM has became
useful for in-situ microscopy for observing dynamic processes at the nano-scales, nano-measurments
which directly correlates physical properties with structures, holographic imaging of electric and
magnetic fields, quantitative chemical mapping at sub-nanometer resolution and for ultra-high resolution
imaging.
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