Letter pubs.acs.org/NanoLett
Atomic Configuration of Nitrogen-Doped Single-Walled Carbon Nanotubes Raul Arenal,*,†,‡ Katia March,§ Chris P. Ewels,∥ Xavier Rocquefelte,∥,# Mathieu Kociak,§ Annick Loiseau,⊥ and Odile Stéphan§ †
Laboratorio de Microscopias Avanzadas (LMA), Instituto de Nanociencia de Aragon (INA), Universidad de Zaragoza, Calle Mariano Esquillor, 50018 Zaragoza, Spain ‡ ARAID Fundation, Calle Mariano de Luna, 50018 Zaragoza, Spain § Laboratoire de Physique des Solides (LPS), CNRS UMR 8502, Université Paris Sud XI, Bâtiment 510, 91405 Orsay, France ∥ Institut des Matériaux Jean Rouxel (IMN), CNRS UMR6502, Université de Nantes, 44322 Nantes, France ⊥ Laboratoire d’Etude des Microstructures (LEM), UMR 104 CNRS-ONERA, 29 Avenue de la Division Leclerc, 92322 Châtillon, France # Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Université de Rennes 1, Rennes Cedex 7, 35708 Rennes, France S Supporting Information *
ABSTRACT: Having access to the chemical environment at the atomic level of a dopant in a nanostructure is crucial for the understanding of its properties. We have performed atomically resolved electron energy-loss spectroscopy to detect individual nitrogen dopants in single-walled carbon nanotubes and compared with first-principles calculations. We demonstrate that nitrogen doping occurs as single atoms in different bonding configurations: graphitic-like and pyrrolic-like substitutional nitrogen neighboring local lattice distortion such as Stone-Thrower-Wales defects. We also show that the largest fraction of nitrogen amount is found in poly aromatic species that are adsorbed on the surface of the nanotube walls. The stability under the electron beam of these nanotubes has been studied in two different cases of nitrogen incorporation content and configuration. These findings provide key information for the applications of these nanostructures. KEYWORDS: EELS-STEM, DFT calculations, nitrogen-doped carbon nanotubes, atomic configuration
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results remains complicated, as different local structures may give rise to similar features.7−11 In this sense, high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) combined with electron energy-loss spectroscopy (EELS) have provided very valuable and rich information at the atomic scale.12 Great progress has been made in (S)TEM due to the development of aberration-corrected microscopes.12−14 Recent work has demonstrated that annular dark-field imaging performed in a Cs-probe corrected STEM enables quantitative atom-by-atom analysis of 2D low-Z materials such as h-BN.13 Single atoms have also been investigated via STEM-EELS in these materials.12,14−21 Three recent works have demonstrated the possibility to identify by STEM-EELS the presence of N atoms in graphene.19−21 In addition, N atoms have been also detected in N-implanted multi-walled carbon nanotubes, although not at the atomic-resolved scale.22 However, none of these studies have unambiguously provided, via EELS
oped carbon nanotubes (NT), notably nitrogen-doped (CNx-NT), have attracted much attention because of their interesting physical and chemical properties.1−4 Knowledge of the atomic arrangement of the dopant atoms in such nanostructures is essential for a complete understanding of the material’s electronic properties, e.g., field emission5 or transport6 properties. This requires precision measurements, combining high spatial resolution and high spectroscopic sensitivity. Several techniques have been deployed with this aim, but until now, none of them have provided the required information in such nanostructures. Most characterization techniques, such as Raman spectroscopy and X-ray absorption spectroscopy (XAS), have relatively low spatial resolution, of the order of hundreds of nanometers, and in some cases only provide indirect information.1,2 Thus, local structural and analytical methods are needed. Scanning tunneling microscopy (STM) and spectroscopy (STS) are local techniques for studying the structural and electronic features/properties of materials. Indeed, some studies have been reported on CNxNT7−9 and recently also on N-doped graphene.10,11 However, chemical characterization cannot be unambiguously performed from the STM/STS analysis. Indeed, the interpretation of the © 2014 American Chemical Society
Received: May 2, 2014 Revised: August 16, 2014 Published: August 26, 2014 5509
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analysis, the local bonding configuration of these N atoms. Indeed, atomically resolved EELS can yield not only the elemental composition, but also chemical bonding/local environmental information.23,24 Single-walled nanotubes (SWNTs) pose significantly more complications for high resolution imagery and spectroscopy than graphene due to their high defect concentration and motional instability under the electron beam. Indeed, a clear and unambiguous proof of identification of individual dopants (N atoms in this case) in single-walled nanotubes has not yet been obtained, in contrast to the case of 2D materials,25 and the question remains whether electron microscopy is able to identify individual dopant atom configurations in such complex nonplanar and beam-sensitive architectures. Here, we have carried out such experiments on single-walled, nitrogencontaining carbon nanotubes (CNx-SWNTs) synthesized via a laser vaporization technique.8,26 We clearly demonstrate the feasibility of dopant identification in a nanotube at the singleatom level. Notably, we show that as well as discriminating between two individual atoms of low atomic number differing only by one electron (carbon Z = 6 and nitrogen Z = 7), this approach can also unambiguously identify the local chemical bonding of the atoms in topologically complex materials. More specifically, we discuss the characteristics of different possible conformations for individual nitrogen atoms incorporated in the host carbon network of the nanotubes. We also show that nitrogen can be found in adsorbed polyaromitic hydrocarbon molecules at the surface of the nanotubes. These findings question previous analyses (macroscopic and microscopic down the nanometer scale) performed on such samples and demonstrate the importance of atomically resolved analysis. Results and Discussion. The HRTEM micrographs in Figure 1 show the presence of localized defects in the SWNTs. These images are consistent with previous studies suggesting the incorporation of nitrogen atoms in the honeycomb carbon structure of a NT, which often implies the presence of defects.1−3 This is corroborated by the higher sensitivity of CNx-SWNTs under the electron beam (see Supporting Information Videos 1 and 2). Therefore, atomic-level (S)TEM studies on CNx-SWNTs require low voltages to avoid knock-on damage.27,28 They also require mechanically stable NTs, with two fixed extremities to reduce vibrations, near to which the measurements can be taken (see Supporting Information Figure S1). These important aspects of the NTs behavior under the electron beam limit the information that can be extracted from the HRTEM images. Thus, the only way to achieve these goals is via analytical STEM-EELS measurements. We have performed atomically resolved EELS experiments at low-voltage (60 kV) and in high-vacuum conditions (∼1.4 × 10−8 Torr) to avoid carbon contamination, comparing the resulting spectra to density functional simulations (details in Supporting Information). A high-angle annular dark field (HAADF) STEM image of an area containing three CNx-SWNTs forming a bundle is shown in Figure 2A (associated bright field image in Supporting Information Figure S2). Figure 2B shows a magnified image after filtering of the green rectangular region, where the atomic resolution of the lattice is clearly visible. One atom, labeled with an arrow, shows higher intensity than its neighbors (∼1.2 times more intense), see profile intensity plotted in Figure 2C. This intensity is consistent with the Z1.7 contrast expected between C and N (1.15 assuming a superposition of the nitrogen atom with an underlying carbon). A multislice HAADF-HRSTEM
Figure 1. Nitrogen-doped single-walled carbon nanotubes: highresolution transmission electron micrographs of different single-walled nitrogen containing carbon nanotubes (CNx-SWNTs), taken at 80 kV. The images show the presence of defects within the walls marked by white arrows: side and central section of the imaged NTs. These latest defects correspond to vacancies, which may be associated with the presence of nitrogen atoms. The scale bar is 2 nm for all the micrographs.
image simulation using QSTEM37 on a (22,0) SWCNT with a single substitutional nitrogen atom is shown in Figure 2E; as can be seen, this agrees well with the experimental image, supporting the potential assignment. Figure 2D displays the atomic model (red atom corresponds to the substitutional nitrogen atom), and Figure 2F shows the intensity profile obtained from Figure 2E. An EELS spectrum-image (SI)29,30 was recorded over the area of Figure 3A (marked in Figure 2A with a red dotted box), and the corresponding HAADF image captured simultaneously is shown in Figure 3B. Despite the coarse sampling (the pixel size was set to 0.5 Å so as to minimize the electron dose), the atomic structure of the nanotube is once again clearly visible. Ripples along the nanotube are evident during acquisition, reflecting its vibration under the electron beam. This effect may also be due to charging effects, which could explain the abrupt jumps seen in the image scan. Figure 3D displays three single EEL spectra extracted from this SI in the marked positions/ pixels of Figure 3B (spectra labeled (i), (ii) and (iii)). The carbon K edge is visible in the three spectra. In two of these spectra the nitrogen signal is also detectable, see Figure 3D ((i) and (ii)). Nitrogen was detected in only these two spectra out of the whole data set (1755 spectra). Their position corresponds to the higher intensity atom identified in the HAADF above, confirming that it is indeed a nitrogen atom as suggested from the analysis of Figure 2. The only two individual spectra in which nitrogen is detected are located in diagonally adjacent pixels. Considering the EELS signal delocalization, one would expect an effective spatial extension of the N−K EELS signal over a 2 × 2 grid of 4 pixels of the SI (pixel size is 0.5 Å). However, due to SWCNT vibration under the electron beam, only part of the atom signals is detected. Considering the dimensions of the NT, this means the nitrogen 5510
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Figure 2. HAADF-HRTEM identification of a single substitutional nitrogen atom. (A) HAADF image of a bundle of SWNTs. The red dotted rectangle indicates the position where the spectrum-image (SI) of Figure 3 has been recorded. (B) filtered image of the green dotted region in (A). Four atoms are labeled from 1 to 4. The red arrow points toward brighter atom 2. (C) Intensity profile at the location of the red rectangle from (B). The positions of atoms 1−4 are indicated. Brighter atom 2 is pointed by a red arrow. (D) Atomic model of a (22,0) nanotube including a single nitrogen atom in a substitutional configuration. (E) Multislice HRSTEM-HAADF image simulation using QSTEM37 of the model displayed in (D). (F) Intensity profile extracted from the red line plotted in (E). The red arrows of (E) and (F) indicate the position of the nitrogen atom.
concentration in the probed area can be estimated at ∼0.25 atom %. The carbon 1s (C−K) energy loss near edge structure (ELNES) in Figure 3D shows the peaks at ∼285.5 and ∼292 eV characteristic of sp2-carbon materials, attributed to transitions from an initial 1s state to unoccupied antibondinglike π* and σ* states above the Fermi level.2,14,17,18The nitrogen 1s (N−K) ELNES, expanded in Figure 3E, show a strong peak at ∼401 eV, with very little signal at energies above this. Comparing the spectra to density functional theory (DFT) ELNES calculations of possible single nitrogen defects, there is excellent agreement with the spectrum for substitutional nitrogen (Figure 3F(iii) and atomic model, Figure 3C) across the range of π* and σ* bands. This assignment is consistent with the HAADF observations (Figure 2A,B) and with recent works on nitrogen doped graphene.19−21 The spectrum shows a number of signature features, which would be lost in a signal averaged with other nitrogen configurations. There is no initial high intensity π* peak present as, for example, in graphitic carbon at the C K-edge. This can be understood from the projected density of p-states (p-DOS) on the nitrogen atom (Figure 3F(i),(ii)) and surrounding carbon neighbors. The additional nitrogen electron rests relatively localized and disrupts the local conjugation, reducing the local π- and π*contributions and resulting instead in a strong pz-signal at the Fermi level, which is not sampled (Supporting Information Figure S8). Instead, there is an exceptionally sharp “molecular like” signature at the start of the σ* band, as shown in the partial px−y-DOS; the rest of the σ* signal is significantly reduced compared to bulk N−K edge EELS measurements for nitrogen doped nanotube samples in the literature.26,31 We note that no apparent damage was observed after the acquisition of this SI (Supporting Information Figure S2). This is consistent with earlier calculations of electron beam knock-on cross sections,27 suggesting that substitutional nitrogen and its neighbors should be stable under a 60 kV beam. This suggests that this configuration of substitutional graphitic nitrogen is a minority species in these bulk
measurements carried out at higher acceleration voltages (100 instead 60 kV) and beam doses, with the probed defects demonstrating a much stronger σ* band at higher energies26,31 as discussed later. The simulated C K-edge for this defect is given in Figure 5 and discussed later below. To summarize thus far, this result demonstrates the capability of STEM-EELS to successfully detect individual nitrogen heteroatoms within the wall of a SWCNT, as confirmed by HAADF imaging, as well as unambiguously identify the associated bonding configuration in combination with DFT ELNES simulations (in this case, substitutional “graphitic” nitrogen). A more complex example is shown in Figure 4. Figure 4A shows an annular dark field image of a SWNT at the edge of the bundle (the corresponding bright field image is shown in Supporting Information Figure S4). A SI was collected from the rectangular area marked in Figure 4A. Three different EEL spectra extracted from this SI are shown in Supporting Information Figure S3, over the energy range covering the C−K, and O−K edges. In contrast with the previous example (Figures 2 and 3), nitrogen has been detected in 5 different positions, relatively far (a few angstroms) from each other, giving an estimated nitrogen content of ∼1.6 atom %. For clarity, due to the low signal-to-noise ratio in some cases, we have focused the analysis on two of the positions for which the N−K edge is better resolved (spectra (i) and (ii); we also show the sum of these 2 spectra). It is worth mentioning that, during spectral acquisition, even though we worked under so-called “gentle-STEM conditions”,28 obvious beam-induced damage was observed after measurement (Supporting Information Figure S4 acquired before, simultaneously and after the SI). We have observed the difference in the behavior of these two situations of CNx-SWNTs, low and high structural instability, via HRTEM (Supporting Information Videos 1 and 2), suggesting higher nitrogen concentration and associated intrinsic defect concentration. We note that out of more than 40 NTs investigated, nitrogen was detected in less than 10% of the total bare/clean probed areas (see also the discussion related 5511
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this confirms the lower structural stability of nitrogencontaining nanotubes and indicates a different nitrogen atomic configuration, see below. The C−K ELNES depicted in Figure 4B show the characteristic profile of sp2 carbon atoms in a hexagonal structure, with π* and σ* bands, as described above. This is the case for almost all the spectra in the SI. However, for most of the positions where nitrogen has been detected, there is an additional peak at ∼288 eV, see Figure 4B ((ii), and (i) + (ii)). This particular signature has previously been associated with several atomic configurations: CO, CC (from CH2 groups, after reduction by the electron beam), and C−N bonds.32 Its origin is revealed via analysis of the DFT ELNES simulations of the C−K edge (Figure 5) for the substitutional, nitrogen-vacancy, and Stone-Thrower-Wales defects (two carbon atoms rotated about their bond center, creating a patch of paired pentagons and heptagons in the lattice, see Figure 5C). In the case of substitutional nitrogen, the C−K edge of the surrounding atoms shows no significant deviation from those of carbon atoms in the pristine lattice far from the defect (see Figure 5A,C). This is consistent with Figure 3D, where we see no C−K peak at 288 eV associated with substitutional nitrogen. However, in the case of the nitrogenvacancy defect we see major variations, including a strong peak between the pristine π* and σ* bands, corresponding to our experimental observation at 288 eV. This is localized, not on the carbon atoms adjacent to the nitrogen, but primarily on the atoms labeled ‘a’ in the reformed pentagon on the opposing side of the vacancy (see Figure 5B,F). These atoms share a weak reconstructed bond (1.73 Å), whose relative instability compared to pristine C−C bonds is reflected in an energy drop in the corresponding σ* states. Thus, this additional peak at 288 eV corresponds to weakened C−C σ-bonds occurring through local strain, reconstruction and bond dilation. While these are typically found in the local environment of impurities, they are not directly linked to C-impurity bonding; hence, the general nature of this peak is irrespective of impurity type (N, O, H, etc.). This assignment is further supported by near edge X-ray absorption fine structure (NEXAFS) studies of graphenebased materials, where a peak at 288 eV was also observed in the proximity of folds,33 edges,34 and divacancy species,34 confirming that it represents a general signature of dilated C−C bonds associated with lattice damage, including edges, folds and grain boundaries. For similar reasons (asymmetric C−N bonds), we observe this peak at 288 eV for other defects, which will be discussed later, that are based on Stone-ThrowerWales rearrangement with nitrogen substituted at different locations. Figure 4C shows the N−K edge ELNES of the selected spectra (i) and (ii), along with their sum (i) + (ii). The spectral form is quite different to that of substitutional nitrogen, with a dual peak split by ∼3 eV. The instability of this sample under the electron beam precludes the possibility of using imagery to identify the defect configuration, and we rely on the spectroscopy in this case. The absence of a 288 eV C−K peak in the majority of the SI indicates that the nitrogen is not sitting in a largely amorphous carbon layer structure13 but is present in a local defect configuration within an otherwise largely pristine lattice. Just as the additional 288 eV peak was assigned to a local distortion in the σ* antibonding states for carbon, the peak splitting in the N−K signal can also be explained via asymmetric C−N bonding. This is because the peak at approximately the same energy for substitutional nitrogen has been identified above with σ* rather than π*
Figure 3. Atomically resolved EELS identification of a single substitutional nitrogen. (A) HAADF image displaying atomic resolution. The red dotted rectangle marks out the scanned area during the acquisition of an EELS spectrum-image (39 × 45 recorded spectra (1.95 nm × 2.25 nm), step size 0.05 nm, probe size ∼0.11 nm, acquisition time 100 ms/spectrum). (B) HAADF image acquired simultaneously with the spectra, showing the motional drift due to nanotube vibration. The only two pixels with N−K edge signal are outlined in blue and red. (C) DFT optimized structure of a substitutional nitrogen atom. Carbon and nitrogen atoms are in gray and blue, respectively. (D) Selection of EEL spectra extracted from the SI; blue, red, and green spectra correspond to the pixels outlined in blue, red, and green in (B). Each curve corresponds to a single spectrum from the SI, except the black, which is a sum of the blue and red spectra. (E) N−K edge fine structures (ELNES), extracted from the spectra of (D). To filter noise, spectra were smoothed using a Savitzky−Golay filter (second-order polynomial). (F) Simulated N−K ELNES (gray, (iii)) and nitrogen partial DOS calculations ((i) purple = pzπ*-states, (ii) green = px‑yσ* states) for substitutional nitrogen, compared to the experimental spectrum (iv), same as spectra (i) + (ii) of (E). The EELS simulations allow unambiguous assignment of the peak at ∼401 eV in the N−K edge to a substitutional configuration shown in (C).
with Figure 6 and other nitrogen-rich regions). Furthermore, among those only in 2 of these areas, including that of Figures 2 and 3, no damage was observed after EELS acquisition. Thus, 5512
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Figure 4. Atomically resolved EELS identification of “pyrrolic” nitrogen. (A) HAADF image obtained from a second bundle of single-walled nanotubes. An EELS SI was recorded from the rectangular area marked in the HAADF image (30 × 41 recorded spectra (1.5 nm × 2.05 nm), step size 0.05 nm with a probe size of ∼0.11 nm, acquisition time 100 ms/spectrum). The scale bar is 1 nm. A selection of EEL spectra from the pixels marked on (A) (red, green, and the 2 pixels in blue) are displayed in Supporting Information Figure S3 (each spectrum is associated with a single pixel, except for the black one, which corresponds to the sum of the red and blue spectra). Yellow crosses indicate extra locations where nitrogen was detected but spectra not displayed. (B) C−K edge fine structures extracted from the spectra. A prominent peak at ∼288 eV is observed in the spectra containing nitrogen (particularly in the red spectrum). (C) N−K ELNES obtained from each atomic position already mentioned. The N−K edge is observed in all these spectra, except in the green one, which has been selected as a reference. Peaks at ∼399.8 and ∼402.7 eV are observed in two of these spectra. To filter noise, spectra were smoothed using a Savitzky−Golay filter (second-order polynomial). (D) Simulated N−K ELNES (gray, (iii)) and nitrogen partial DOS calculations ((i) purple = pzπ*-states, (ii) green = px‑yσ* states) for structure displayed in Figure 5C, compared to the experimental spectrum (iv) from (C)i.
Figure 5. Simulated carbon 1s ionization edges for carbon atoms neighboring nitrogen, in a (A) “graphitic” substitutional, (B) “pyridinic” nitrogenvacancy, (C and D) “pyrrolic” substitutional with a Stone-Thrower-Wales defect configuration. (E−H) Calculated C−K spectra for the different C atoms marked in the models of (A)−(D). *Nitrogen atom marked in white. Experimental spectra from Figure 3D(i) and (ii) and Figure 4D(i) and (ii).
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seen on the HAADF images displayed in Figure 6, this material also incorporates a significant amount of atoms from the catalyst, observed as bright dots in the image. The EELS results acquired on this disordered material are summarized in Figure 6. Figure 6C(i) shows the N−K edge EELS spectrum resulting from the sum of a few hundreds of spectra acquired by quickly rastering the electron probe over the area indicated by the yellow rectangle in Figure 6A (typically, 3 spectra (acquisition time 500 ms) were acquired during the acquisition of one single 512 × 512 image single frame). At such a moderate electron dose (typically 4 × 108 e−·Å−2·s−1), there was no significant evolution of the spectrum with time. However, this sheath was found to be quite sensitive to the electron beam at lower scanning speeds. Under the standard conditions used for SI acquisitions (typically 50 ms per probe step), large sputtering effects were observed, leading to a complete “cleaning” of the SWNT surface in some areas of the rope. The sum of 10 spectra located at the top left corner of a spectrum image acquired on a partly cleaned area of the rope is shown in Figure 6C(ii). The N−K edge ELNES of both spectra ((i) and (ii)) are similar to those found in our previous nanometer-resolved STEM-EELS analyses also performed on CNx-SWNT,26,27,31 showing a broad σ* band above 405 eV, which is different to those recorded here on bare SWNT surfaces (not covered by any amorphous material) presented in Figures 3 and 4 and also displayed for comparison in this figure (Figure 6C(iii) and (iv)). This kind of signature (broad and triangular in shape σ* contribution) has also been observed in other CNx sp2 materials as films35 or in the carbon sheath covering nanodiamond grains.36 It is commonly admitted that such signature is associated with a so-called N pyridinic atomic conformation in the polymeric entities contained in those materials. Thus, we deduce that this observed sheath corresponds to some polyaromatic materials grown with the NT. The transformation of this sheath under the electron beam is reflected in the N−K edge (Figure 6C(ii)), in which the contribution of features in the 400−403 eV range is getting higher. As the sheath spectroscopic contribution decreases due to sputtering, the signature from substitutional N atoms incorporated in the SWNT carbon network is then detectable. Another significant aspect related to the N signals associated with these amorphous sheaths is that the probability to detect them is much higher than the probability to detect signals associated with N incorporated in the C network composing the SWNT. This a very important point because it means that these species are playing a crucial role on the properties of the samples containing CNx-SWNT when those studies are not investigated at the atomic or very local (subnanometer) scale. Conclusion. This study shows that, despite the technical challenges associated with motional and structural instability under the electron beam, we have resolved two types of individual nitrogen defects in single-walled carbon nanotubes, and determined their local chemical bonding environment using a combination of HAADF, EELS, and DFT simulations. This is the first time that such a combination of experimental and simulated EELS spectra have been obtained at this resolution for nitrogen dopants in SWNTs, demonstrating significant differences to earlier bulk studies. Substitutional (graphitic) nitrogen was unambiguously identified thanks to its relative stability under the 60 keV electron beam. The EELS spectroscopic signature shows a very weak π* peak, a sharp “molecular-like” σ* peak which is commonly misinterpreted as a π* peak, and then almost no σ*
Figure 6. Spatially resolved EELS of polyaromatic materials containing nitrogen covering SWNT walls. (A and B) HAADF images obtained from a bundle of single-walled nanotubes covered by an amorphous sheath where a cumulative EELS spectrum (over the area marked by the yellow rectangle) and an EELS SI (over the red dotted rectangle) were recorded. The SI contains about 1000 spectra, recorded with a step size 0.05 nm and with a probe size of ∼0.11 nm. The acquisition time was 50 ms/spectrum. (C) N−K ELNES obtained as (yellow, (i)) the sum of EEL spectra from the pixels marked on (A) and (magenta, (ii)) as the sum of 10 spectra located at the top left corner of the SI. Both spectra are compared with those of Figure 3F(iv) (black, C(iv)) and Figure 4D(iv) (blue, C(iii)), corresponding to a substitutional N atom in the C network and to a “pyrrolic” N atom, respectively. A broad peak centered around 407 eV is visible in both spectra collected on the amorphous sheath ((i) and (ii)). This peak corresponds to σ* contribution.
states. Figure 5C shows a Stone-Thrower-Wales defect, with nitrogen substituted at the defect edge. The nitrogen atom has asymmetric C−N bonds (1.357/1.410/1.448 Å as compared to 1.396 Å for substitutional nitrogen). In the associated simulated N−K spectrum, Figure 4D(iii), peak splitting in the N−K signal is clearly visible and corresponds well with the experimental data. Similarly to the carbon, the peak splitting is reflected primarily in the σ* states (Figure 4D(ii)). We note that the 288 eV C−K peak is also observable in some neighboring pixels in the SI that contain no N−K signal. This is consistent with the spatially extended variation in C−C bonds associated with the Stone-Thrower-Wales defect. This allows us to assign the N−K peak at 400 eV, commonly denoted “pyrrolic” nitrogen, to substitutional nitrogen in a locally asymmetric bonding configuration. The Stone-Thrower-Wales defect is an example, but there are clearly other point defect structures that could contain nitrogen with asymmetric bonding, such as the 555− 777 divacancy (various other defect structures are given in Supporting Information, only those with substitutional nitrogen in an asymmetric bonding configuration show this peak splitting). We note that this result also suggests that this peak at 400 eV should always occur simultaneously with a higher energy peak, which in bulk measurements would be difficult to distinguish from substitutional nitrogen. It is important to note that a large fraction of the N present in the sample was detected in external species to the SWNT C network. Indeed, we have observed that the SWNT can be partially covered by an “amorphous” sheath (Figure 6 and Supporting Information Figure S5), which incorporates a significant amount of nitrogen (typically a few percents). As 5514
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tail. Since bulk EELS N−K signals previously reported in the literature show a large σ* tail, it seems therefore likely that these are associated to the presence of other kinds of N (polyaromatic) containing materials, see below. Analysis of a second nitrogen defect reveals that asymmetry in local sigma bonding results in peak splitting in C−K and N− K signals. Peaks at 399.8 and 288.0 eV in the N−K and C−K signals, respectively, are attributed to this effect, resolving previous literature ambiguities as to the structural origin of these peaks. This is the source of “pyrrolic” nitrogen; an example is substitutional nitrogen in the presence of a StoneThrower-Wales bond rotation. The 288 eV C−K peak is explained as a σ* signal from dilated or reconstructed C−C bonds at damage sites in the lattice, for example, near to certain (but not all) impurity sites. This information could only be gained from such an atomic-resolution study. Furthermore, we have also shown that other kind of nitrogen response can be obtained from amorphous sheaths partially covering some of the CNx-SWNT. This sheath is associated with polyaromatic materials containing N, and it may influence the conclusions drawn on the properties probed on these samples. Materials and Methods. Materials: Description of the Synthesis of the Samples and Their Preparation for TEM. The CNx-SWNT sample was synthesized by laser vaporization of a graphite target mixed with Ni/Y catalyst powders in a 300 mbar N2 atmosphere.8,26 The raw powder was dispersed ultrasonically in ethanol and placed onto a copper grid coated with carbon film for the TEM-STEM studies. TEM and STEM Instruments. High-resolution transmission electron microscopy was performed using an imaging-side aberration-corrected FEI Titan-Cube microscope working at 80 kV, equipped with a Cs corrector (CESCOR from CEOS GmbH). The STEM-EELS-experiments were performed in a NION UltraSTEM 200 dedicated scanning transmission electron microscope, equipped with a cold field emission gun and operated at 60 kV (typical electron dose: 4 × 108 e−·Å−2·s−1) in order to minimize knock-on damage to the sample.15,27 The energy resolution was about 0.7 eV. The collection semiangle of the spectrometer was 50 mrad. The spectroscopic information was obtained using the spectrum-imaging acquisition mode.29,30 To filter the noise in the experimental data, the backgroundcorrected EEL spectra showed in Figures 3E,F, 4B,C, and 5 were smoothed using a Savitzky−Golay filter (second-order polynomial). To avoid contamination, the sample was heated under vacuum before being introduced into the microscope. Furthermore, the clean vacuum conditions at the sample’s position in the column of the STEM (∼1.4 × 10−8 Torr) minimize carbon contamination. The beam current was 75 pA and the probe size was around 1.1 Å. Three different STEM imaging detectors were employed: the high angle annular dark field (HAADF) detector (with inner and outer radii of 85 and 195 mrad, respectively), the medium angle annular dark field (MAADF) detector (with inner and outer radii of 55 and 195 mrad, respectively), and the bright field (BF) detector. HRSTEM simulations have been carried out using QSTEM37 software package. Simulations conditions were chosen to correspond closely to the experiments (including 60 kV, the geometry of the detectors, 30 mrad of convergence angle, and no significant aberrations of the probe).
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ASSOCIATED CONTENT
S Supporting Information *
Additional TEM studies; irradiation effects, and videos of a series of high-resolution TEM images recorder under different conditions. Complementary and extra HRSTEM images, EEL spectra, and DFT calculations are also included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS R.A. acknowledges funding from grants 165-119 and 165-120 from Universidad de Zaragoza, from the Spanish Ministerio de Economia y Competitividad (FIS2013-46159-C3-3-P) and from ARAID foundation. We acknowledge Marcel Tencé from the LPS (CNRS-U. Paris XI, Orsay (France)) for his ̈ support with some of the EELS data analyses. We thank Shaima Enouz-Véd renne and Jean-Lou Cochon from ONERA (Châtillon and Palaiseau, France) for sample synthesis. We acknowledge the useful comments of Mike Walls from the LPS (CNRS-Université Paris XI, Orsay (France)). Part of the microscopy work was conducted in the Laboratorio de Microscopias Avanzadas at the Instituto de Nanociencia de Aragon, Universidad de Zaragoza (Spain). The research leading to these results has received funding from the European Union Seventh Framework Program under Grant Agreement 312483 ESTEEM2 (Integrated Infrastructure Initiative − I3) and from the French CNRS (FR3507) and CEA METSA network. C.P.E. acknowledges the SPRINT project ANR-10-BLAN-0819 funded by the French ANR for funding. COST network MP0901 NanoTP is acknowledged. C.P.E and X.R. thank the IMN (Nantes) and the CCIPL (Centre de Calcul Intensif des Pays de la Loire) for computing facilities.
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