934 Letters to the editor / Carbon 38 (2000) 929 –941

Does carbyne really exist? — carbynes in expanded graphitea,b ,1 a,b b a,b ,*Shu-You Li , Hui-Hua Zhou , Jia-Lin Gu , Jing Zhu

aElectron Microscopy Laboratory, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR ChinabDepartment of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China

Received 13 September 1999; accepted 12 February 2000

Keywords: A. Carbyne; C. Transmission electron microscopy (TEM), Electron energy loss spectroscopy (EELS), Raman spectroscopy

Carbyne is considered as a linear carbon allotrope with are some very thin lines perpendicular to the thick fringes.pure sp-hybridization. Due to its unique geometrical The line spacing of these thin lines is about 0.128 nm,structure, carbynes are expected to have many interesting which corresponds to the 006 reflection spot in Fig. 1b.properties as do fullerenes and carbon nanotubes [1–3]. Fig. 1b shows a selected area electron diffraction (SAED)However, in contrast to the discovery of fullerenes and pattern from the region shown in Fig. 1a. Taking thecarbon nanotubes, the existence of carbynes has been internal graphite layer as a reference, the diffractioncontinuously disputed since early the 1980’s [3–8]. As a patterns of carbyne were indexed and the hexagonal cellresult, studies on carbynes are far less than those on parameters were found to be a 50.82 nm and c 50.77 nm.0 0

fullerenes and carbon nanotubes. Although some evidence These values are quite close to the b form of carbynehas been presented in the literature, they were obtained reported in the literature [11]. Repeated observations ofseparately, so it is not convincing to interpret them such SAED patterns indicate that there is no preferentialinterdependently. Here we report an unambiguous identifi- directional relationship between the carbyne species andcation of such a nano scale species using a combination of the carbon matrix, although the c-direction of carbyne isseveral techniques associated with modern analytical elec- always parallel to the matrix graphite layer.tron microscopy (AEM). The geometric microstructure and Fig. 2 shows a K-edge of EELS spectrum of carbynethe electronic properties were obtained from the same compared with HOPG and diamond. The characteristic Ksample area of carbyne nano species. While the geometric edge of carbyne is evident in the figure. More than tenstructure of carbynes we observed is consistent with that such spectra were obtained from different areas on theobserved previously, unique feature of sp hybridization is same TEM sample. Disregarding other information fromreported for the first time in the world in present work. The the sample area, this feature may be interpreted from thesp hybridization of carbynes is distinct from other carbon orientation dependence of EELS in anisotropic materialsallotropes. [12]. However, it must be noticed that the EELS spectra

The carbynes were found in worm-like expanded and the diffraction patterns, as well as other information,graphite, or exfoliated graphite, which was obtained from are obtained from the same sample area simultaneously,graphite hydrogensulfate prepared by electrochemical oxi- thus they must be interpreted inter-dependently. If thedation of graphite flakes under concentrated sulfuric acid. large p* peak is from graphite with the electron beamThe fabrication details can be found in ref. [9] and [10]. perpendicular to the c-axis, the interplanar spacing ofCarbyne species usually exist at the edge of the expanded 0.128 nm can by no means be interpreted.graphite flake in this work, as shown in Fig. 1. An apparent In order to check the presence of impurities, EELS andcharacteristic of carbyne is that the contrast of the fringes EDX spectra were obtained on both the carbyne area andis somewhat discontinuous and the spacing of the fringes is the graphite matrix. Special attention was paid to the peakslightly larger than that of the normal C-interplanar positions of sulphur, oxygen and (in EELS only) hydrogenspacing of highly oriented pyrolytic graphite (HOPG). It [5,8]. No characteristic peaks other than carbon were foundcan also be seen in the HREM image (Fig. 1c) that there in the spectra, i.e., the impurities, if there are any, are

under the minimum detectable limit of EELS and EDX.Supposing the integrated EELS intensities I and Ip* s*

are proportional to the relative orbital number of p* and*Corresponding author. Tel.: 186-10-6277-3015; fax: 186-

s*, the average number of p orbitals and the fraction of10-6277-1160.

sp-hybridized carbon atoms within the sample of analyzedE-mail address: jzhu@mail.tsinghua.edu.cn (J. Zhu).1 area can be roughly estimated, as was done by Cuomo et¨Current address. Institut fur Physikalische Chemie, Johannes

al. and Kulik et al. for diamond-like carbon (DLC) films¨Guttenberg Universitat, Jakob-Welder-Weg 11, D-55099 Mainz,[13–15]. All of the p orbital numbers we obtained fromGermany. Tel: 149-6131-392-3148 (O); fax: 149-6131-392-

3768, e-mail: syli@mail.uni-mainz.de. the carbyne-like region are larger than the typical value 1

0008-6223/00/$ – see front matter 2000 Elsevier Science Ltd. All rights reserved.PI I : S0008-6223( 00 )00056-7

Letters to the editor / Carbon 38 (2000) 929 –941 935

Fig. 1. (a) Morphology of the carbyne-like species appearing at the edge of expanded graphite; (b) SAED pattern of this area; (c)Enlargement of the area marked by white square in (a). Taking the 100 reflection of graphite (marked ‘graphite’ in Fig. 1b, d50.213 nm) asa standard, we can calculate the interplanar distance of other spots. Note the diffraction spot marked by 006 corresponds to an interplanardistance of 0.128 nm, which is close to the theoretical prediction of r(C5C)50.1282 nm [6].

for pure HOPG. For example, one of the numbers we HOPG and diamond are at 19.5 eV, 23.6 eV and 34 eV,obtained is 1.48, which implies a ratio of carbyne to respectively. The main peak of carbyne is broadening,graphite of about 1:1 in the detected area of the sample. because here the carbyne is surrounded by the graphiteThe graphite component here is due to either the graphite matrix. There are two stronger peaks, compared with thatmatrix within the probing region or to electron beam from HOPG and diamond, at 4.85 eV and 9.35 eV in theirradiation damage of carbyne. spectrum from carbyne; they are characterized as p-sub-

The low energy loss regions of EELS from carbyne, band and p-subsubband transitions.HOPG and diamond are shown in Fig. 2b. The sp The effect of electron radiation damage on chemicalhybridization of carbyne arrives further confirmation. The bonding in the carbyne species is shown in the sequentialmain plasmon (s1p or s) peak positions of carbyne, EELS spectra in Fig. 3. It can be seen that the p* feature

936 Letters to the editor / Carbon 38 (2000) 929 –941

decreased gradually and an amorphous carbon was finallyproduced. This result is consistent with our continuousHREM observations, and also with the HREM resultsreported by Yamada et al. [16]. The longest tolerableelectron beam exposure time we have observed was 8 min,which is much longer than the 5 seconds reported byKudryavtsev [17] and 50 seconds reported by Yamada[16]. That is why we have enough time to get the spectrumfrom the undamaged carbyne.

For comparative purpose, Raman spectra were takenfrom the expanded graphite (Fig. 4). In addition to the

21normal Raman peaks of HOPG at 1580 cm and21 21867 cm [18], two strong peaks at 970 cm and211070 cm were observed. According to our knowledge of

Raman spectroscopy [19], these two peaks correspond tothe stretching frequency of 5C5C5. Thus the carbynespecies we observed is probably polycumulene, which isconsistent with the chemical bonding of b carbyne re-ported in the literature [11,20]. A small peak at about 1910

21cm was also observed. This peak may be assigned toC;C, but the two forms of carbyne should not coexist.This remains to be interpreted.

For the existence of polyyne form of carbyne, the mostconvincing evidence may be that by L. Kavan [21].However, this form of carbon was stable only enclosed in aLiF matrix. If this material is isolated by dissolution of thesalt matrix, it is highly reactive, and reacts with the oxygen

2from atmosphere and converts, at least partially, to sp -bonded carbon [22].

It is interesting to consider the transformation mecha-nism of the carbyne chains. Generally it is accepted thatFig. 2. (a) Carbon K-edge EELS spectra obtained from diamond, 2the sp -bonded form of the graphene layers is not brokenHOPG and carbyne, respectively. All these spectra were obtainedduring exfoliation of graphite. However, we can notunder the same experimental condition. (b) Low loss regions of

2exclude the possibility of breaking some sp bonds at theEELS from the same sample area of (a).

Fig. 3. Time evolution of carbyne EELS spectrum during high-energy electron irradiation.

Letters to the editor / Carbon 38 (2000) 929 –941 937

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