Importance of Transmission Electron Microscopy for Carbon Nanomaterials Research
Prakash R. Somani *, 1, 2, and M. Umeno 1 1Department of Electronics and Information Engineering, Chubu University, Matsumoto-cho 1200,
Kasugai-shi, Aichi-Ken, Japan 487– 8501. 2Applied 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 * Corresponding author: e-mail: [email protected], [email protected]
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 electron-optical 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
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 multi-walled 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,
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
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.
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
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 nano-thermometer; 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.
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 photo-resist, 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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