Letter pubs.acs.org/JPCL
A Low-Energy Electron Beam Does Not Damage Single-Walled Carbon Nanotubes and Graphene Jae Hong Choi,§ Junghyun Lee,† Seung Min Moon,§ Yun-Tae Kim,§ Hyesung Park,*,†,‡ and Chang Young Lee*,†,§ †
School of Energy and Chemical Engineering, §School of Life Sciences, and ‡Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 44919, Republic of Korea S Supporting Information *
ABSTRACT: Scanning electron microscopy (SEM) is a principal tool for studying nanomaterials, including carbon nanotubes and graphene. Imaging carbon nanomaterials by SEM, however, increases the disorder mode (D-mode) in their Raman spectra. Early studies, which relied on ambiguous ensemble measurements, claimed that the D-mode indicates damage to the specimens by a low-energy electron beam (e-beam). This claim has been accepted by the nanomaterials community for more than a decade without thorough examination. Here we demonstrate that a low-energy e-beam does not damage carbon nanomaterials. By performing measurements on single nanotubes, we independently examined the following factors: (1) the e-beam irradiation itself, (2) the e-beam-deposited hydrocarbon, and (3) the amorphous carbon deposited during synthesis of the material. We concluded that the e-beam-induced D-mode of both carbon nanotubes and graphene originates solely from the irradiated amorphous carbon and not from the e-beam itself or the hydrocarbon. The results of this study should help minimize potential ambiguities for researchers imaging a broad range of nanomaterials by electron microscopy.
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the research community has accepted and heavily cited such claims for both SWNTs12−14 and graphene15−17 (Table S1). Other changes upon irradiation, such as metal-to-semiconductor transitions,18,19 quenching of photoluminescence,20 decreased electrical conductivity,21,22 and diminished Raman scattering23 have also been attributed to e-beam-induced damage. Previous studies, however, included ensemble measurements involving bundles and networks of nanotubes, without consideration of the effect of native amorphous carbon or the EBID of a hydrocarbon layer.24,25 Hence, the conclusions of such studies are largely ambiguous and should be re-evaluated. In this work, we demonstrate using Raman spectroscopy of individual nanotubes, thereby avoiding potential ambiguities arising from the bundled tube analysis, that the low-energy electron beam in SEM does not damage SWNTs or graphene. By carefully examining the effect of a low-energy e-beam, the EBID of hydrocarbon, and the amorphous carbon deposited during synthesis of SWNTs, we conclude that the e-beaminduced increase of the D-mode in Raman spectra originates primarily from the amorphous carbon, not from the structural damage to SWNTs. We show that this conclusion also applies to graphene. Our findings contradict the current consensus on e-beam-induced specimen damage during SEM observations and require immediate attention by the nanomaterials community.
lectron microscopy has served as a powerful tool for imaging nanoscale objects unresolvable with conventional optical microscopy because of Abbe’s diffraction limit. Offering convenient sample preparation, fast alignment of the electron beam (e-beam), and a relatively large scan area, scanning electron microscopy (SEM) has been particularly useful for imaging nanostructures, including carbon-based nanomaterials. For instance, SEM can readily image single-walled carbon nanotubes (SWNTs) with subnanometer diameters1 and also help the synthesis of high-quality graphene by enabling visualization of the grain boundaries.2 When performing electron microscopy, however, the operator must consider the effect of the e-beam on the specimen. The high-energy e-beams used in transmission electron microscopy cause knock-on displacement and ballistic ejection of atoms.3 Although such knock-on damage does not occur in SEM, where low-energy ebeams are used, it is still unclear whether other forms of damage exist. When using a scanning electron microscope to examine carbon nanomaterials, the most prominent and easily perceived changes are the following two: (1) Irradiation with a lowenergy e-beam deposits a layer of electrically insulating hydrocarbon contaminants on the specimen.4 (2) SEM imaging increases the disorder mode (D-mode) in the Raman spectra of carbon nanotubes and graphene. The former is well-understood, and researchers have even utilized the phenomenon for e-beam-induced deposition (EBID) to create surface patterns5 or glue materials together.6 The latter, however, is a matter of debate.7−9 Early studies interpreted the D-mode increase as the result of broken sp2 bonds and thus concluded that the lowenergy e-beam damaged SWNTs10 and graphene.11 Since then, © XXXX American Chemical Society
Received: September 24, 2016 Accepted: November 7, 2016 Published: November 7, 2016 4739
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The Journal of Physical Chemistry Letters Raman spectra and SEM images of an individual SWNT verify that the D-mode increases upon irradiation with a lowenergy e-beam and that this increase can be reversed by a thermal treatment, as shown in Figure 1. Horizontally aligned arrays of SWNTs were synthesized by chemical vapor deposition (CVD) on a silicon substrate as reported previously (Figure S1).26,27Figure 1a shows the SEM images of representative pristine SWNTs, SWNTs irradiated with an e-
Figure 2. Removal of the e-beam-induced D-mode without heat treatment. (a) SEM images of a pristine SWNT, a SWNT after ebeam-irradiation, and a SWNT after application and removal of a PMMA coating. (b) Raman D-maps corresponding to the rectangular areas in panel a. The D-mode of the irradiated SWNT disappeared after the application and removal of the PMMA coating (dotted areas). (c) The corresponding Raman spectra confirm that the e-beaminduced D-mode of SWNT can be eliminated by chemical treatment at room temperature. Figure 1. SEM and Raman spectroscopic analysis of an e-beamirradiated SWNT. (a) SEM images and (b) Raman D-maps show the presence of hydrocarbon in the irradiated rectangular area, which is removed after high-temperature annealing. (c) SWNT Raman spectra collected from the circled areas in panel a showing the increased Dmode intensity upon irradiation, followed by elimination of the Dmode by annealing.
interpreted as e-beam-induced damage and recovery from it, respectively.20,23,28 We were able to remove the e-beam-induced D-mode of SWNTs via a simple chemical treatment at room temperature, which strongly suggests that no structural damage was caused by the e-beam. The SEM images (Figure 2a) and the Raman Gmap (Figure S7) of a pristine SWNT (Figure S8) and the same SWNT irradiated with the e-beam at the same dosage described in Figure 1 are shown in Figure 2a and Figure S7, respectively; however, in this case, instead of a heat treatment, the nanotube was coated with poly(methyl methacrylate) (PMMA) and rinsed with acetone at room temperature. The treatment with PMMA did not completely remove the rectangular deposits (Figure 2a) but substantially reduced the intensity of the Dmode in the Raman spectra (Figure 2b,c). In the D-map of this particular nanotube (Figure 2b), a pronounced line is observed along the irradiated nanotube; this line disappears after the application and subsequent removal of the PMMA coating. These results directly contradict those of previous studies on the recovery of structural defects in SWNTs at high temperatures11,20 and thereby challenge the validity of the interpretation that e-beams damage SWNTs. In addition, our single-tube measurements show that there is little correlation between the e-beam-induced D-mode and the nanotube diameter (Figure S9). By examining the radial breathing mode (RBM) normalized by the G-mode, or RBM/G, we also found that the RBM decreased only when the D-mode increased upon irradiation (Figure S10).
beam at 1 kV for 1 min (dosage 2.3 × 1021 to 3.1 × 1021 cm−2), and SWNTs annealed in N2 at 1025 °C for 30 min. The irradiated area is clearly visible as a dark rectangle caused by the deposition of a hydrocarbon layer,5 as shown in the surface elemental analysis results (Figure S2). The hydrocarbon was removed when the sample was annealed, as confirmed by SEM images (Figure 1a and Figure S3), the Raman G-map (Figure S4), and the Raman D-map (Figure 1b). In the D-map, the irradiated regions are pronounced because the e-beamdeposited hydrocarbon increased the background signal in the wavenumber range from 100 to 3000 cm−1 (Figure S5). Raman spectra of the SWNT (Figure S6), corresponding to each condition (pristine, irradiated, and annealed), are shown in Figure 1c (left); a zoomed-in view of the shaded D-mode region is shown in the right panel. The pristine SWNT exhibits no D-mode (black), indicative of a defect-free sp2 carbon network; however, a D-mode emerged when the sample was irradiated with the e-beam (red) and disappeared after the specimen was annealed at high temperature (blue). The results in Figure 1 agree with previous ensemble measurements in which the appearance and disappearance of the D-mode were 4740
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Figure 4. Effect of irradiated amorphous carbon on the D-mode of a SWNT. (a) Raman spectra of amorphous carbon (left) and SWNT (right) after prolonged irradiation with an e-beam; both cases show enhanced D-modes. The histograms in the insets indicate that the extent of the increase in the intensity of the D-mode is similar in both samples. (b) SEM images and (c) corresponding Raman spectra of a SWNT subjected to various treatments. After the amorphous carbon has been removed by annealing (blue), e-beam irradiation no longer increases the D-mode intensity (green).
Figure 3. Detailed analysis of the effect of hydrocarbon on the increased D-mode intensity in the Raman spectrum of a SWNT. (a) Schematic showing the transfer of e-beam-deposited hydrocarbon onto a pristine SWNT and (b) the corresponding Raman spectra, which show that the enhanced D-mode arises only from directly irradiated SWNT. (c) Schematic showing the e-beam-induced deposition of hydrocarbon onto a PMMA-coated SWNT and (d) the corresponding Raman spectra. Lifting off the hydrocarbon layer from the irradiated SWNT does not eliminate the D-mode from the spectrum.
Because the e-beam exposure and the hydrocarbon deposition occur simultaneously, the effect of each must be ruled out to reveal the origin of the D-mode. Here, we examined the effect of e-beam-deposited hydrocarbon independently from the e-beam itself by depositing the hydrocarbon onto a bare silicon substrate and transferring the resulting layer onto pristine SWNTs (Figure S11) on another substrate (Figure 3a and Figure S12). Alternatively, we transferred SWNTs onto the layer of e-beam-deposited hydrocarbon (Figure S13). In both situations, the SWNTs directly contacted the hydrocarbon without being exposed to the e-beam. Neither specimen exhibited any increase in the Dmode, as shown in Figure 3b (black) and Figure S13c (black), whereas direct irradiation of the same nanotube increased the D-mode, as shown in Figure 3b (red) and Figure S13c (red). Moreover, we observed that the e-beam irradiation of SWNTs in the absence of direct contact with hydrocarbon also increased the intensity of their D-mode. As depicted in Figure 3c, SWNTs coated with a 50 nm-thick layer of PMMA
Figure 5. Effect of irradiated amorphous carbon on the D-mode intensity in the Raman spectra of graphene. (a) SEM images and (b) corresponding Raman spectra of graphene treated under conditions similar to those described in Figure 4b,c. The D-mode also does not appear upon exposure to the e-beam after the amorphous carbon has been removed.
were irradiated with the e-beam and subsequently rinsed with acetone. The corresponding Raman spectra are shown in Figure 4741
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the D-mode of the transferred graphene increased upon irradiation of the specimen. The extent to which the D-mode increased, however, was substantially reduced after the amorphous carbon was decomposed via annealing. The lowenergy e-beam therefore did not damage the graphene. In conclusion, we have verified that the increase in intensity of the D-mode upon e-beam irradiation of carbon-based nanomaterials, which for more than a decade has been accepted as a consequence of e-beam-induced damage to the materials, is in fact caused by a small amount of amorphous carbon that gives rise to the observed Raman D-mode. This study was designed to investigate single nanotubes and to independently examine the effects of e-beam irradiation, EBID of hydrocarbon, and amorphous carbon, thereby precluding the ambiguities present in the ensemble measurements of previous studies and ensuring that the results are applicable to a broad range of Raman-active low-dimensional nanomaterials.
3d. This procedure ensured that no direct contact occurred between the nanotube and the hydrocarbon; nonetheless, the intensity of the SWNT’s D-mode (Figure S14), which also shifted slightly here,29 still increased upon irradiation (Figure 3d, red). The acetone rinse lifted off the hydrocarbon but did not eliminate the D-mode (Figure 3d, blue). Similarly, the RBM exhibited a decrease when PMMA-coated and irradiated but did not recover when PMMA was removed (Figure S15). The procedure in Figure 3c overexposes the PMMA to the ebeam,30 which prevents its complete removal by acetone. Here, the D-mode is not from the pristine or the residual PMMA (Figures S16−S18). These analyses confirm that the hydrocarbon is not the main cause of the e-beam-induced D-mode. The remaining potential cause of the D-mode increase is the amorphous carbon (Figure S2), whose Raman modes overlap with the G- and D-modes24,25 of SWNTs and graphene. The deposition of amorphous carbon is unavoidable during the CVD of nanotubes and graphene. The experiments in Figure 3c also involve irradiation of the amorphous carbon, which is represented in the figure as changing from yellow to red after ebeam irradiation. Hence, understanding the effect of e-beamirradiated amorphous carbon on the Raman spectra is important. In this regard, we intentionally deposited amorphous carbon onto SWNTs and the substrate by increasing the synthesis time. For both the substrate (Figure 4a, left) and the nanotube (Figure 4a, right), we observed that the D-mode at 1350 cm−1 increased when the specimens were irradiated with the e-beam (Figure 4a and Figure S19). From the pixel-by-pixel comparison obtained from the Raman Dmap, histograms of the D-mode change (ΔD) were created for the substrate (31.54 ± 26.22) and the nanotube (42.67 ± 32.96) (Figure 4a, inset and Figures S20−S21). The statistically insignificant (p = 0.146) difference between the two led us to hypothesize that the increased D-mode intensity upon irradiation originates primarily from amorphous carbon deposited onto the nanotube during synthesis. Indeed, irradiating the carbon nanomaterials free of amorphous carbon did not increase the D-mode of the nanomaterials. Figure 4b,c shows the SEM images and Raman spectra of a pristine SWNT before (black) and after it was irradiated (red), annealed (blue), and irradiated again (green). All of the SEM images and Raman spectra were obtained from the same region of the SWNT. Upon the first irradiation, a dark rectangle was observed in the SEM image and the D-mode appeared in the Raman spectra, as previously mentioned. After the sample was annealed at 1025 °C for 3 h, which was a sufficiently high temperature to remove both the ebeam-deposited hydrocarbon and the native amorphous carbon,31 the D-mode disappeared. Furthermore, the second irradiation did not increase the D-mode. Thus, we conclude that the e-beam-induced D-mode originates from the irradiated amorphous carbon, not from the e-beam-induced structural damage of nanotubes. The conclusion is further supported by the similar trend observed from the RBM (Figure S22). The mechanism by which the D-mode increases upon irradiation of amorphous carbon has yet to be elucidated. Although we have drawn our conclusion primarily by examining SWNTs, our conclusion is valid for graphene as well. We repeated the experiments described in Figure 4b,c using single-layer graphene synthesized by CVD and transferred onto a silicon substrate.32 The heat treatment effectively removed the hydrocarbon deposits on the graphene (Figure 5a). As shown in the corresponding Raman spectra (Figure 5b),
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02185. Experimental details, a list of articles reporting that SWNTs and graphene are damaged by e-beam, and Figures S1−S22 (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Hyesung Park: 0000-0002-0222-0395 Chang Young Lee: 0000-0002-2757-8019 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A1A2073264, 2015R1D1A1A0105791) and by the Development Program of Internet of Nature System (1.150090.01) and the 2014 Research Fund (1.140065.01) funded by UNIST. This study contains results obtained using research equipment operated by UNIST Central Research Facilities (UCRF).
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