1 Introduction

Carbyne is an allotrope of carbon that consists of an infinitely long chain of sp-hybridized carbon atoms. The structure of carbyne can take on two possible configurations: (1) cumulene, which has repeating double bonds throughout the chain, and (2) polyyne, which has alternating single and triple bonds. Cumulene may spontaneously transform into polyyne through a Peierls distortion [1, 2]. It has been theoretically predicted that carbyne possesses an exceedingly high stiffness and Young’s modulus [3,4,5], a potentially high thermal conductivity [6,7,8], and unusual electrical transport properties [9,10,11,12]. Moreover, carbyne has attracted great interest because it can potentially be used in many different applications, such as switching nanodevices [13, 14], spintronic nanodevices [15,16,17], and hydrogen storage [18].

The existence of carbyne was debated for many decades until the successful synthesis of a linear acetylenic carbon chain with end-capping groups [19]. The organic synthesis technique yielded an acetylenic carbon chain consisting of 44 atoms [20]. In addition, transmission electron microscopy has also identified short free-standing carbon chains directly forming from carbon nanotubes [21,22,23,24,25] and graphene [26,27,28,29,30,31,32,33,34,35,36] under electron-beam irradiation and application of electrical current. Furthermore, the electric current passing through carbon chains [25, 30, 32, 35] and their tensile force at fracture [23] were measured in situ.

Isolated long linear atomic carbon chains have not yet been observed under ambient conditions because of their high chemical reactivity and extreme instability. However, it has been reported that there is the possibility to grow such chains in multiwall carbon nanotubes (MWCNTs) by the arc discharge method in hydrogen gas [37], liquid nitrogen [38, 39], argon gas [40], or helium gas [41, 42]. Carbon chains have also been fabricated by the high-temperature fusion reaction of molecules such as C10H2 [43] and adamantane [44] within double-wall carbon nanotubes (DWCNTs), whose linear cavity can provide a space for encapsulating and stabilizing long linear carbon chains. Furthermore, Shi et al. [45] have reported long linear carbon chains consisting of more than 6000 atoms, synthesized within DWCNTs by high-temperature heat treatment under a high vacuum.

Recently, we have found out the presence of long linear carbon chains after electrical discharge during field electron emission from single-wall carbon nanotube (SWCNT) films [46], which is different from the previous method by arc discharge and high-temperature heat treatment. The present reproducible synthesis method may provide a new approach to the bulk production of long linear carbon chains. In this paper, we report the field emission properties accompanied by the electrical discharge of SWCNT films that lead to the formation of long linear carbon chains encapsulated within carbon nanotubes, and characterize these carbon chains by Raman spectroscopy and electron microscopy.

2 Experimental

SWCNT films synthesized by an enhanced direct injection pyrolytic synthesis method (Meijo Nano Carbon Co., Ltd., film thickness; approximately 40 μm, a nominal diameter of SWCNT; 1–3 nm) were used in the present study. In the as-prepared SWCNT films, DWCNTs and triple-wall carbon nanotubes were contained as minor constituents. Field electron emission experiments were conducted on these SWCNT films at room temperature under a vacuum pressure of 8.3 × 10−6 Pa in a parallel-plate configuration. The distance between the electrodes was 0.16 cm. The field emission specimens were prepared by cutting the SWCNT films into roughly 5 × 5 mm pieces, and attaching each piece onto the surface of an aluminum cathode using an Ag adhesive paste (Dotite D-500, Fujikura Kasei Co., Ltd.). The morphology and structure of the specimens were observed by scanning electron microscopy (SEM; FEI Quanta 200 FEG, 15 kV) and transmission electron microscopy (TEM; JEOL JEM-2010, 120 kV). The TEM specimens were prepared by dropping SWCNTs dispersed in isopropyl alcohol onto a microgrid. Raman characterization of the specimens was carried out with a Renishaw inVia Reflex Raman microscope using a laser excitation source with a wavelength of 532 nm at room temperature (measurement conditions; laser power 0.15 mW, exposure time 10 s).

3 Results and discussion

The field emission characteristics of the SWCNT films were measured at room temperature under a vacuum pressure of 8.3 × 10−6 Pa. Figure 1a plots current–voltage curves obtained during field emission from an SWCNT film. The first field emission experiment started with an emission current of more than 10 nA at a voltage of 354 V and increased up to 962 μA at 868 V (arrowhead a), at which point an electrical discharge occurred and the applied voltage was immediately made to decrease to zero. The process of field emission followed by electrical discharge was repeated eight times (see Fig. 1a). The voltages indicated by arrowheads a–h in Fig. 1a show, respectively, the discharge voltages from the first to eighth field emission experiment, which tend to increase with the number of repetitions. The turn-on voltage (defined here as the applied voltage to obtain an emission current of 10 nA) increased as the field emission experiment proceeded (see Table 1), indicating that it became increasingly more difficult to emit electrons during field emission under the same applied voltage, owing to morphological and destructive changes in the SWCNT film by the electrical discharge, as shown in Fig. 2b. According to the Fowler–Nordheim equation [47], the emission current I is given by

$$ I = 1.4 \times 10^{ - 6} \frac{{A\beta^{2} V^{2} }}{{d^{2} \phi }}\exp \left( { - 6.49 \times 10^{7} \frac{{d\phi^{3/2} }}{\beta V} + \frac{9.8}{{\phi^{1/2} }}} \right) $$

where A is the emitting area, β is the field enhancement factor, V is the applied voltage, d is the distance between electrodes, and \(\phi\) is the work function of the emitter. The graph obtained by plotting ln(I/V2) against 1/V is called the Fowler–Nordheim plot and is used to analyze the emission current–voltage curve. Figure 1b shows the Fowler–Nordheim plots obtained from the current–voltage curves in Fig. 1a, assuming \(\phi\) = 4.6 eV for graphite and using a d value of 0.16 cm. In all the Fowler–Nordheim plots in Fig. 1b, the slopes of the plots in the high-voltage regions deviate from those in the low-voltage regions, i.e., a reduction in emission current occurs at high voltages. The field enhancement factor for each field emission experiment is estimated from the slopes in the low-voltage regions in Fig. 1b (see results in Table 1). The enhancement factors decreased with each repetition of the field emission experiment owing to the morphological changes that the SWCNT film underwent due to the electrical discharge during field electron emission.

Fig. 1
figure 1

a Current–voltage curves measured during field emission from an SWCNT film. Arrowheads a–h indicate the discharge voltages from the first to eighth field emission experiment, respectively. b Fowler–Nordheim plots obtained from (a)

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Table 1 Discharge voltage, turn-on voltage, and field enhancement factor obtained from Fig. 1
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Fig. 2
figure 2

SEM images of a an as-prepared SWCNT film and b an SWCNT film after electrical discharge in the field emission experiment

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Figure 2a, b shows SEM images of an as-prepared SWCNT film and an SWCNT film after electrical discharge repeated eight times in the field emission experiment as shown in Fig. 1, respectively. The surface morphology of the as-prepared SWCNT film, which is composed of entangled SWCNTs, is homogeneous as shown in Fig. 2a, while after the electrical discharge, parts of the SWCNT film are damaged and torn up, resulting in the formation of holes, as seen in Fig. 2b. These significant morphological changes suggest that the electrical discharge during field emission causes structural transformation of the SWCNTs.

In order to determine the influence of the damage in the SWCNT films, they were examined by Raman spectroscopy. Figure 3a shows a Raman spectrum of the as-prepared SWCNT film, measured from the region indicated by arrowhead A in Fig. 2a. Two large peaks appear at 1593 and 2677 cm−1, which are assigned to the G and 2D bands for SWCNTs, respectively. The peaks in the range lower than ~ 400 cm−1 are the radial breathing modes (RBMs) for SWCNTs. At above 200 cm−1, the spectral noise is large as shown in Fig. 3 and the RBMs were hard to be assigned accurately. The D band for SWCNTs is observed at 1342 cm−1, and its peak intensity, ID, is much smaller than that of the G band, IG. The degree of structural disorder for carbon networks in SWCNTs, P, is estimated by \( P = {{I_{\text{D}} } \mathord{\left/ {\vphantom {{I_{\text{D}} } {\left( {I_{\text{D}} + I_{\text{G}} } \right)}}} \right. \kern-0pt} {\left( {I_{\text{D}} + I_{\text{G}} } \right)}} \times 100\% \). The P value for the as-prepared SWCNT film in Fig. 4a is 1.5%. Figure 3b, c shows Raman spectra from the non-damaged and damaged regions of an SWCNT film after electrical discharge, indicated by arrowheads B and C in Fig. 2b, respectively. The P values for the SWCNT films after electrical discharge are 4.5 (Fig. 3b) and 9.0% (Fig. 3c), suggesting that structural disorder is introduced into the carbon networks in the SWCNTs upon electrical discharge. In the Raman spectrum from the damaged region, a sharp peak and a broad peak appear at 1863 cm−1 and ~ 600 cm−1, respectively, as seen in Fig. 3c. These peaks do not appear in the Raman spectra of as-prepared SWCNT films. On the other hand, linear carbon chains in carbon nanotubes are characterized by a Raman band in the 1780–1870 cm−1 range, which has been termed the L band, and which is associated with C–C stretching-mode vibrations [37,38,39,40, 42,43,44,45, 48, 49]. The broad peak at around 600 cm−1 is also related to the linear carbon chains, because this peak is detected only when the L band is observed [40, 48, 49]. There may be a possibility that the lower frequency peak at 600 cm−1 is attributed to bending modes of sp linear structures [50]. The simultaneous appearance of the marked peaks at 1863 and 600 cm−1 suggests the formation of linear atomic carbon chains in the SWCNT film upon electrical discharge. Figure 4a, b shows, respectively, an enlarged image of the damaged region indicated by arrowhead C in Fig. 2b and the corresponding Raman mapping image using the L band. The regions where the L band signal is detected are colored red in Fig. 4b. The L band is detected exclusively in the damaged regions of the SWCNT film. The major peak position of the L band ranged from 1854 to 1865 cm−1, and its intensity and spectral shape differed, depending on the place in the damaged regions. The variation in the L band may be attributed to the length and the encapsulating environment of the carbon chains [51]. The present results demonstrate that field electron emission accompanied by electrical discharge leads to the formation of linear carbon chains in the SWCNT film. It was also confirmed using another film that linear carbon chains are formed even after first electrical discharge during field emission, indicating that the repetition process of electrical discharge is not essential for the formation of the linear carbon chains. However, the damaged area suffered by the first electrical discharge is local in the SWCNT film and the yield of the carbon chains is low. In the present study, therefore, the repetition process of electrical discharge was carried out to form a large amount of carbon chains.

Fig. 3
figure 3

Raman spectra measured from a the region indicated by arrowhead A in Fig. 2a for the as-prepared SWCNT film, b and c non-damaged and damaged regions indicated by arrowheads B and C in Fig. 2b for the SWCNT film after electrical discharge, respectively

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Fig. 4
figure 4

a SEM image of the damaged region indicated by arrowhead C in Fig. 2b, and b corresponding Raman mapping image using the L band

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To demonstrate the presence of linear atomic carbon chains in the SWCNT films after electrical discharge, the structure of the SWCNT films was investigated by high-resolution TEM. In addition to SWCNTs, the SWCNT films contained many DWCNTs that encapsulated linear atomic carbon chains. Figure 5 shows a typical high-resolution image of a DWCNT encapsulating a linear atomic carbon chain. The inner diameter of the DWCNT in Fig. 5 is 0.70 nm, which is consistent with the reported innermost tube diameter of MWCNTs [37, 38, 41] and DWCNTs [45] encapsulating a carbon chain. As seen in Fig. 5, the carbon chain is aligned along the central axis of the inner tube of the DWCNT, which is approximately at a van der Waals distance from the inner wall. The carbon chain as shown in Fig. 5 is longer than 21 nm. Assuming that the lengths of single, double, and triple bonds in the chain are, respectively, 0.1301, 0.1282, and 0.1265 nm [52] and the unit cell length of the carbon chain, which is twice the undistorted C–C bond (double bond) length, is approximately 0.256 nm, the carbon chain as shown in Fig. 5 is estimated to contain at least 160 contiguous carbon atoms.

Fig. 5
figure 5

High-resolution TEM image of a DWCNT with an inner diameter of 0.7 nm encapsulating a long linear atomic carbon chain along its central axis

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Long linear carbon chains encapsulated in MWCNTs and DWCNTs have been reported previously [37,38,39,40,41,42,43,44,45]. In the present study, carbon chains encapsulated not only in DWCNTs but also in SWCNTs were observed, as previously reported [46]. Figure 6 shows two SWCNTs encapsulating carbon chains. The diameters of both SWCNTs are approximately 1.0 nm, which is slightly larger than the inner diameter of the DWCNTs as shown in Fig. 5. Although the lengths of the carbon chains encapsulated in the SWCNTs in Fig. 6 are more than 14 nm, their shape is not completely straight, as shown in Fig. 5. The carbon chain in Fig. 6a is bent at several locations, and portions of the chain, e.g., in the region between arrows A and B, are positioned close to the nanotube wall, i.e., away from the central axis of the SWCNT. This deviation from the center probably results from the electronic π–π interaction between the carbon chain and the SWCNT wall. The image contrast of the carbon chain in the region between arrows C and D, as indicated in Fig. 6a, b, is as high as that of the nanotube wall, showing that the thickness of these carbon chains is not different from one-atom thickness of the carbon chain as shown in Fig. 5. In the region on the right-hand side of arrow D in Fig. 6a, the carbon chain branches off into two as indicated by arrowheads. The intensity profile between the nanotube walls indicated by arrows L and R in Fig. 6a is shown in Fig. 6c, showing that the SWCNT encapsulates at least two carbon chains. In Fig. 6b, the two carbon chains are also seen within the SWCNT in the region on the left-hand side of arrow C, as shown by arrowheads. Thus, the high image contrast of the carbon chain in the region between arrows C and D in Fig. 6a, b is thought to be due to the overlap of the carbon chains. The carbon chains may form cross-linking bonds between the chains, as previously reported [27, 34]. The formation of a single long linear carbon chain may require the sufficiently confined space of a carbon nanotube with a diameter of approximately 0.7 nm.

Fig. 6
figure 6

a and b High-resolution TEM images of SWCNTs with a diameter of 1.0 nm encapsulating more than one carbon chain that were not completely linear and in an off-center position close to the inner wall. c and d Intensity profiles between the nanotube walls indicated by arrows L and R in (a) and (b), respectively

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In the present study, DWCNTs and SWCNTs that encapsulated carbon chains were observed in the morphologically damaged regions of the SWCNT films by electrical discharge during field emission. Although such destruction process by electrical discharge may play some kind of a role in the formation of the carbon chains inside the carbon nanotubes, it would be difficult to explain the formation mechanism of the carbon chains by only the destruction process. During field emission, the nanotubes experience high temperature [53]. As a possible mechanism, it may be conceivable that residual carbon left inside and/or outside a carbon nanotube aggregates and polymerizes in a narrow cavity of the nanotube to form a carbon chain by high-temperature heating and electrical discharge.

Although the formation mechanism for long linear carbon chains under high electric fields is still unclear, it is a fascinating topic for further study. The present results concerning the strong Raman spectral feature at 1863 cm−1 can potentially pave the way to a novel approach to the bulk production of long linear carbon chains involving a method other than the conventional high-temperature heat treatment of carbon nanotubes and arc discharge.

4 Conclusion

We carried out field electron emission experiments at a vacuum pressure of 8.3 × 10−6 Pa using SWCNT films with a low degree of structural disorder for carbon networks and examined the structure of the SWCNT films after electrical discharge. The Raman spectroscopy and TEM results showed that field electron emission accompanied by electrical discharge from the SWCNT films induced the formation of long linear carbon chains, i.e., carbyne, within DWCNTs and SWCNTs. The present findings may lead to a new approach to synthesizing carbyne and promote carbyne research aimed at developing innovative applications.