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Size Analysis of Single Fullerene Molecules by Electron Microscopy Anish Goel A,*, Jack B

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Size Analysis of Single Fullerene Molecules by Electron Microscopy Anish Goel A,*, Jack B Carbon 42 (2004) 1907–1915 www.elsevier.com/locate/carbon Size analysis of single fullerene molecules by electron microscopy Anish Goel a,*, Jack B. Howard a,*, John B. Vander Sande b a Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA b Department of Materials Science and Engineering, Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA Received 8 August 2003; accepted 10 March 2004 Available online 24 April 2004 Abstract Individual fullerene molecules were observed and sized using high resolution transmission electron microscopy. Fullerene C60 molecules were tethered by chemical bonding to carbon black particles to facilitate HRTEM imaging and sizing of known fullerenes. HRTEM analysis of soot samples from a fullerene-forming flame revealed the presence of a range of fullerenes from C36 to C176 and larger fullerene-like structures. The observation of fullerenes smaller than C60 is noteworthy in that such structures necessarily contain adjacent pentagons and hence are strained and expected to have interesting reactivity that may be useful in certain applications. HRTEM can be used to detect and size fullerenes in samples containing fullerenes but not in sufficient quantity, or not sufficiently removable, to be detectable by chemical analysis. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon black, Fullerene, Soot; C. Electron microscopy; D. Microstructure 1. Introduction permit easy removal for chemical analysis. Such species include fullerenes smaller than C60, all of which neces- An ability to observe and measure the size of an sarily contain adjacent pentagons in their structure and individual fullerene molecule by high resolution trans- are strongly curved and strained, and hence interactive, mission electron microscopy (HRTEM) is of interest for and much larger fullerenes whose size facilitates exten- several reasons. Such a technique would offer a possible sive contact and increases the opportunity for bonding means for extending the detection and analysis of interactions. Curved carbon structures seen in HRTEM fullerenes to lower limits of detection than can be at- images of soot from fullerene-forming flames exhibit tained by conventional chemical analysis. For example, radii of curvature which include not only values com- circular objects approximately the size of C60 and C70 parable to those of the prevalent fullerenes C60 and C70 fullerenes can be seen in HRTEM images of soot not but also smaller and larger values [1–3]. Carbon struc- only from certain low-pressure benzene/oxygen flames tures with radii of curvature less than that of C60 may be well known to contain fullerenes but also from atmo- indicative of adjacent pentagons, the presence of which spheric-pressure ethylene/air flames in which fullerenes in carbon layers would be of sufficient practical and could not be detected by state-of-the-art chemical scientific interest to merit confirmation and further analysis [1]. study by more quantitative HRTEM analysis. The HRTEM technique would be useful also for the Mass spectra which show the presence of C60,C70 and detection and characterization of fullerenes which are higher fullerenes in the vapor from graphite vaporiza- too strongly bound to, or within, the material with tion [4] and in low-pressure flames of certain hydrocar- which they are condensed in the synthesis process to bons [5] also exhibit mass peaks corresponding to C50 and other carbon species lighter than C60. Under some flame conditions the apparent abundance of C50 is even * Corresponding authors. Tel.: +1-202-736-4256; fax: +1-202-736- comparable to that of C60 and C70 [5,6]. However, C50 4259 (A. Goel), Tel.: +1-617-253-4574; fax: +1-617-324-0066 (J.B. Howard). fullerene has never been found in chemical analysis of E-mail addresses: [email protected] (A. Goel), [email protected] condensed material from fullerene-containing flames or (J.B. Howard). carbon vaporization systems, possibly reflecting the 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.03.022 1908 A. Goel et al. / Carbon 42 (2004) 1907–1915 difficulty of extracting this compound from the soot and of approximately the same diameter. Based on this other components of the condensate. The structure of observation the authors suggest it should be possible, C50 is expected to contain five pairs of adjacent penta- although they had not been able yet, to observe by TEM gons and hence to be highly strained and strongly at- more prevalent fullerenes such as C60,C70, and C84 tracted to the other molecules, including C50,and suspended on the tip or side of a carbon nanotube. surfaces in the condensed material. Fullerene C36 which, Fuller and Banhart [12] using HRTEM observed not even more than C50, is highly strained and prone to only fullerene cages but also the formation of the cages forming strong bonds with the condensed material, has during electron irradiation of carbon specimens in the been identified in material condensed from a fullerene- electron microscope. They observed tube-like structures forming graphite vaporization process using fraction- as well as spherical cages, and saw that tube-like struc- ation and analysis methods especially designed to deal tures viewed end-on resembled, but gave much more with the strong bonding [7]. The smallest possible ful- contrast than, spherical cages. In a HRTEM study of lerene, C20, consisting solely of 12 pentagons, has been fullerenic nanostructures from fullerene producing generated from a brominated hydrocarbon, dodeca- flames, Das Chowdhury et al. [13] observed on a mul- hedrane, by gas-phase debromination [8]. The lifetime of tiwalled carbon nanotube an ellipsoidal structure having the C20 cage under the experimental conditions used was a single closed-shell whose major and minor diameters at least 0.4 ms. Owing to the extreme curvature and were about 1.1 and 0.7 nm, corresponding approxi- reactivity of this structure, it would not be expected to mately to a C94 fullerene molecule. C94 fullerene has attain substantial gas-phase concentrations in most been isolated since and identified by solvent extraction methods of fullerene synthesis. Nevertheless, it is con- and HPLC analysis of condensable material collected ceivable that the C20 cage might be stabilized for longer from flames [14]. Burden and Hutchison [15] using lifetimes through strong bonding with soot or other HRTEM observed features consistent with fullerenes condensed species. A HRTEM capability to observe C60 and C70 on the surface of carbon black particles directly in the condensed phase C50,C36,C20 and other irradiated with an electron beam in the presence of he- highly interactive fullerenes, both smaller and much lium. Smith et al. [16] and Sloan et al. [17] obtained larger then C60, and to contribute to their character- HRTEM images of round, closed shells of carbon, ization would clearly be useful. apparently fullerenes, encapsulated within the hollow The observation of single fullerene molecules by central region of individual single-wall carbon nanotu- HRTEM has been addressed previously. Cox et al. [9] bes produced in carbon vaporization systems. The deposited C60 on MgO crystals supported on holey diameters of the apparent fullerenes correspond to C60 carbon films. Circular contrast patterns with about 0.8 [16,17] and various other fullerenes both smaller and nm diameter, consistent with that of C60, were observed larger than C60, ranging from 0.4 to 1.6 nm [17]. The on the MgO crystals and could be seen most clearly on estimated precision in the diameter measurements of the edges of crystals hanging over holes in the support Sloan et al. [17] was ±0.05 nm. film. The circular images were not seen on MgO crystals Although previous studies (mentioned above) provide without C60 deposition. Cox et al. [9] regarded it to be strong evidence that individual fullerene molecules can improbable that the circular patterns could represent be observed using HRTEM, such capability has been two or more C60 molecules aligned parallel to the elec- questioned [10]. There is need for unequivocal demon- tron beam axis. The strong inference is that the observed stration of the capability of HRTEM for observing and circular patterns were images of individual C60 cages. sizing fullerene molecules on or within condensed car- Ajayan and Iijima [10] observed with HRTEM a 0.8 nm bon material in order to establish the viability of the diameter circular image at the outside edge of the image technique for use in the detection and characterization of a multilayer truncated conical carbon nanostructure. of fullerenes which are below the detection limit, or are The authors point out that the image could correspond difficult to transfer to the gas or liquid phase, for to a single C60 structure but they argue against such an chemical analysis. Furthermore, there is need to apply interpretation on the grounds that the contrast exhibited the capability, once it is established, to the study of by the small circular image is comparable to that of the bound fullerenes in combustion-generated material, lattice fringes of the much larger supported multilayered which could be produced in whatever large quantities structure. Instead, they interpret the circular image to be might eventually be demanded by practical applications. an end-on view of a nanotube having the same diameter To these ends, the present study (1) tests the capability as a C60 molecule and a length of a few such units in the of HRTEM for observing individual C60 fullerene mol- direction perpendicular to the plane of the image. Saito ecules added in a controlled manner to a sample of et al. [11] present a HRTEM image of a round, hollow carbon material, (2) calibrates the HRTEM measure- single-layered particle of 1.3 nm diameter, which they ment of fullerene diameters against a known standard, suggest may be a fullerene having approximately 200 and (3) uses the established HRTEM technique to carbon atoms, held by a single-walled carbon nanotube identify and size interesting flame-generated fullerenes A.
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