Structural and Chemical Dynamics of Pyridinic-Nitrogen Defects in

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Structural and Chemical Dynamics of Pyridinic Nitrogen Defects in Graphene Yung-Chang Lin, Po-Yuan Teng, Chao-Hui Yeh, Masanori Koshino, Po-Wen Chiu, and Kazu Suenaga Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02831 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015

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Structural and Chemical Dynamics of Pyridinic Nitrogen Defects in Graphene Yung-Chang Lin,1* Po-Yuan Teng,2 Chao-Hui Yeh2, Masanori Koshino,1 Po-Wen Chiu,2 Kazu Suenaga1* 1

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan 2

Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

High density and controllable nitrogen doping in graphene is a critical issue to realize high performance graphene-based devices. In this paper, we demonstrate an efficient method to selectively produce graphitic-N and pyridinic-N defects in graphene by using the mixture plasma of ozone and nitrogen. The atomic structure, electronic structure, and dynamic behavior of these nitrogen defects are systematically studied at the atomic level by using a scanning transmission electron microscopy. The pyridinic-N exhibits higher chemical activity and tends to trap a series of transition metal atoms (Mg, Al, Ca, Ti, Cr, Mn and Fe) as individual atoms.

KEYWORDS: Graphene, Pyridinic Nitrogen, Graphitic Nitrogen, EELS, Chemical Dynamics

*Corresponding Author: [email protected], [email protected]

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Effective and controllable doping has been considered a crucial technique to govern the electronic and chemical properties of graphene. One of the most accessible approaches is nitrogen doping which can modulate the Fermi-level1-3 and enhance the local chemical activity.4 Therefore the nitrogen doped graphene can be anticipated for sensors,5,6 lithium battery,7 or oxygen reduction reaction applications.8,9 The substitutional N doping in the sp2 carbon network can be generally categorized into graphitic (three sp2 C-N bonds), pyridinic (two C-N bonds in a hexagon), and pyrrolic (two C-N bonds in a pentagon) bonding configurations. X-ray photoelectron spectroscopy (XPS) is the standard technique to investigate the chemical composition and distinguish between different N-functional groups in carbon nanomaterials.10 Numerous reports have mentioned that N doping can enhance the catalytic ability of carbon materials for energy conversion and storage applications, but the catalytic behaviors of distinct N configurations were still unclear.11-17 Though the distinct N 1s states have been suggested by XPS, the bonding information of the containing N cannot be directly connected to the defect structures due to the limited special resolution of the XPS technique.

Transmission electron microscopy (TEM)18,19 and scanning tunneling microscopy (STM)20-22 can directly visualize the atomic structure and the later can also present the local electronic information, but both of which are not fully suited to providing direct chemical information. Scanning transmission electron microscopy (STEM) with the combination of simultaneous electron energy-loss spectroscopy (EELS) has the advantage of not only distinguishing the element atom-by-atom via the contrast variation of the annular dark-field (ADF) images23 but also providing the


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chemical fingerprints and characterizing the local electronic states of the single atoms,24 defects,25 edge structures,26,27 and bonding configurations.28,29 Recently, individual N dopant atoms in graphene or carbon nanotubes have been visualized by using STEM with associated EELS studies.30-33 Most of the studies, however, reported only the graphitic-N. The N dopants in other defect configurations, such as pyridinic-N, has never been reported in experimental studies and believed unstable under the e-beam.31

In this paper, we present the systematic study of atomic structures, dynamics and the atomic EELS studies on pyridinic-N defect in graphene by using a low voltage (60 kV) STEM and compare its chemical activity with the graphitic-N (N@C) defect. The graphene specimen containing prolific N defects was prepared by employing the mixture plasma of O3 and N2 (1:1). The overall structure of the specimen is shown in low-magnification annular dark field (ADF) images in Supporting Information Fig. S1 where more than 60 single vacancies can be found in a 50 x 50 nm2 region. Most of the vacancies contain N atoms or trap individual impurity metal atoms at the defects.

We first show spectroscopic identification as well as the direct imaging of two nitrogen defects. Figure 1a-1f show the ADF images and the corresponding atomic models of various N defects in graphene where the N atom appears in brighter contrast.23 The N@C can be found with a simple N substitution for a carbon atom in graphene lattice forming sp2 bonding configuration as shown in Fig. 1a. The structure of single pyridinic-N in graphene (Fig. 1b) involves a nitrogen atom that occupies a site at a vicinity of single-vacancy (SV). We refer this single pyridinic-N defect as SV+1N. The C-N bond length in the SV+1N is about 1.3 Å, while the C-C bond length on the


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vacancy side of the pentagon is 1.9 Å. The nitrogen atom can hop into its next site (Fig. 1c) and often oscillates between the two identical sites. The hopping N@SV+1N is triggered by the high energy scanning electron beam. The e-beam with 60kV acceleration voltage is supposed to be below the displacement threshold of N@SV+1N,19 and it is found stable for several (5~10) frames of scanning (see Supplementary Movie 1). When the N atom is kicked out by e-beam and the graphene di-vacancy (DV) is left, then, the DV began to transform to a flower or a butterfly defect via Stone-Wales transformation.34 Apart from the single pyridinic-N defects, we have found the double, triple and quadruple pyridinic defects which can be noted as SV+2N, SV+3N and DV+4N as shown in Fig. 1d-1f, respectively. Such polymorphic structures are quite consistent with a theory that predicts the N atoms energetically favored to substitute the C atoms at the vicinity of SV.35 Recently, similar heterocyclic compounds which were claimed consisting of oxygen atoms were also reported.36

Note here that the concentration of graphitic-N doing in graphene has never exceeded a few at% because the N@C atoms cannot come closer to each other. Multiple graphitic-N atoms have never been found to co-exist in a hexagonal unit of graphene. The shortest distance between two graphitic-N we found is 6.2 Å in our N-doped graphene (See Supporting Information Fig. S2). On the other hand, pyridinic-N can easily exist with the higher local N density.19,35

We also discriminate the graphitic-N and pyridinic-N by the energy-loss near-edge structure (ELNES). Fig. 1g shows nitrogen K-edges extracted from graphitic-N and pyridinic-N structures. Two sharp peaks at 401.4 and 407.6 eV in


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graphitic-N K-edge can be assigned as, π*- and σ*-contributions. The graphitic-N introduces an additional electron into the graphene lattice which give rise to positively charged N atom and thus n-type doping to graphene. On the other hand, the π*- and σ*-peaks of K-edge for pyridinic-N largely shift down to 398.0 and 406.6 eV. This implies a lone pair in the pyridinic-N induces local negative charge to the N atom and raises π orbital toward Femi level in the valance band which results in the peak shift to lower energy. The π*-peak in K-edge is originated from the core level excitation of electron to unoccupied p states and the energy shift is therefore consistent to the chemical shift in N1s binding energy detected by XPS between graphitic-N (401.4±0.3 eV) and pyridinic-N (398.7±0.6 eV).10 More importantly, the ELNES reflects the electronic density of state at the local structure and can provide more detailed information about the unoccupied states according to distinct bonding situations. A larger energy split between N σ* and N π* in the pyridinic-N than in the graphitic-N is theoretically confirmed by a density functional theory (DFT) calculation using the CASTEP code (See Supporting Method), suggesting the larger energy splitting in the unoccupied 2p-orbital in the pyridinic-N. The increased split and lowered N1s state should be favorable eventually to hybridize with d orbital of transition metal (TM) atoms and then enhance the chemical reactivity of the pyridinic-N (See below).


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Figure 1.

ADF images and the atomic models of (a) graphitic-N (N@C), (b,c) single

pyridinic-N, 1N+SV and SV+1N, showing at two possible equivalent N sites, (d) SV+2N, (e) SV+3N, and (f) DV+4N. The graphitic-N is presented by a red ball while pyridinic-N by orange ball. Images are filtered by Gaussian blur using ImageJ. The four N atoms in the DV+4N show non-uniform contrast and may have difficulty to ensure the existence of four N atoms. Relative EELS maps, however, clearly shows the N signal at each corner site (Figure 2 and Supporting Information Fig. S5 and S6). Furthermore, another example of clear DV+4N is also presented in the inset of Supporting Information Fig. S1b. (g) The EEL spectra showing N K-edge of graphitic-N and pyridinic-N compared with the DFT calculation by using the CASTEP code. The spectrum of the graphitic-N is from a single N defect, while the one for pyridinic-N is extracted from a DV+4N structure (See Figure 2). Scale bar is 2 Å.

Figure 2 shows the detailed atom-by-atom spectroscopy regarding the C


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K-edge and N K-edge of the N@C and DV+4N structures. An EELS line-scan (100 pixels, 0.2 sec/pixel) was performed through a N@C defect on an indicated dotted line in Figure 2a and the corresponding model in Figure 2b. The EELS fine structures of each numbered atoms from Figure 2a extracted from the 2D spectral images for C Kand N K-edges are shown in Figure 2c and 2d, respectively. A significantly weaker C signal and an upshift of C π*-peak (285.5 eV) were detected on the N atom, which was contributed from three nearest neighboring C atoms.32 In order to prove the existence of the N polymorphic structures, we have also performed EELS analyses on SV+1N and SV+2N which are presented in Supporting Information Fig. S3 and S4, respectively. As one know that, the ADF contrast of N atoms often varies especially for pyridinic-N (such as Figure 1f) when a small inner angle (in comparison to the convergence angle) is used, therefore EELS chemical assignment is crucial to elucidate the authentic defect structures. Figure 2g and 2h show the EELS fine structures from the numbered atoms in Figure 2e. Each spectrum was extracted from one pixel of the EELS map (Figure 2f). The acquisition time for each spectrum is 0.2 sec. Four significant pyridinic-N signals can be clearly seen at the four corners in the DV. However the N defect structure might have suffered some structural changes induced during the EELS mapping, and therefore the EELS mapping may show some variations in intensity profiles. We performed several sets of EELS mapping on the same DV+4N in sequence (see Supporting Information Fig. S5 and S6), in which EELS results clearly show the N signals at the four corner sites. More interestingly, an additional impurity atom jumped into the center of DV+4N defect, which was further recognized as a Ca atom by a post EELS analysis.


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Figure 2. (a) An ADF image of N@C. The yellow dotted line indicates the EELS line scan. (b) An atomic model of N@C. (c) The 2D spectrum image and the extracted C-K edge spectra corresponding to the labeled atoms in (a). (d) The 2D spectrum image and the extracted N-K edge spectra of N@C. (e) An ADF image of DV+4N which is 90° rotated from Figure 1f. (f) A corresponding EELS color map of carbon (grey, 280-320 eV) and N (orange, 390-430 eV) with an atomic model of the DV+4N superimposed on top. (g,h) The atom-by-atom EEL spectra of C-K edge and N-K edge of the DV+4N. The spectra were processed by using weighted PCA analysis to enhance the S/N ratio. Scale bar is 2Å.


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Tracing the structural transformations of N defect among different atomic configurations is also important since it directly suggests the N defect structures with energy minima. We have observed that the N@C atom can exchange the atomic position with the neighboring carbon and migrate in the graphene lattice (see Supporting Information Fig. S7 and Movie 2). This N@C migration behavior is similar to the recent reported inversion of Si-C bond.37 Moreover, the N@C was found to be an e-beam displacing target in a defective carbon network, which also behaves like a graphene reknitting agent to assist the reconstruction of graphene network from defective to original hexagonal lattice (see Supporting Information Fig. S8 and Movie 3). In the case of multiple pyridinic-N dopants, for example SV+2N, we found a back-and-forth transformation between two theoretically suggested structures, namely, a pyridinic-N + graphitic-N complex defect (SV+1N+N@C) and a SN+2V defect (see Supporting Information Fig. S9 and Movie 4). This suggested that these polymorphs N defects are thermodynamically stable in both structures at elevated temperature (500°C). Note that the assignment of N atomic position by ADF contrast only is not fully straightforward because the contrast of N atom often varies during the fast movie acquisition. Additional STEM/EELS results for the polymorphs N defects are shown in the Supporting Information and their energetics are discussed in detail.

The N-doped graphene has been reported to enhance the chemical reactivity and has been demonstrated for molecular sensors,21,38 bio sensing,5 supercapacitors,39,40 lithium battery,7,41 and metal free oxygen reduction catalysis.8,9,42 However, there is no study to directly visualize the catalytic ability difference between the different N defects. Here, we demonstrate the high chemical reactivity of pyridinic-N defect which


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definitely attracts the TM atoms as shown in Figure 3. An example of TM atom (Cr) to jump into a SV+3N (Figure 3a,3b) during the EELS map acquisition is shown in Figure 3c,3d. Figure 3f shows the corresponding EEL spectra extracted from the numbered atoms in Figure 3e. It is quite frequent to find a TM atom anchored at a pyridinic-N defect during the STEM observation. Supplementary Information Fig. S5 and S6 also show the example of high reactivity of SV+4N defect that traps an alkaline metal ion (Ca) before and after the reaction, respectively. On the contrary, TM atoms have never been found at the graphitic-N site during our experiments. This result suggests that the reactivity of graphitic-N and pyridinic-N are different in attracting TM atoms (Figure 3g) possibly due to the distinct local charge distribution in the presence of lone pairs at the pyridinic-N as the above DFT study suggested.15


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Figure 3. (a,b) An ADF image and a model of a triple pyridinic-N defect with a single vacancy of carbon atom (SV+3N) before taking an EELS map. (c,d) A single Cr atom trapped at SV+3N defect structure after the EELS map. (e) The in situ ADF image during the EELS mapping. Scale bar is 2.5 Å. A Cr atom jumped into the SV+3N defect in the meantime of EELS scanning pointed by yellow arrow. (f) The EEL spectra from the corresponding numbered atoms from the EELS map in (e). Spectra shown are extracted as 7x7 pixels in EELS map. The acquisition time for each spectrum is 0.5 sec/pixel. (g) The schematic of a metal ion which is likely to be trapped at the pyridinic-N defect, but not at the graphitic-N defect.


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Such an attractive interaction with the pyridinic-N is equally observed for the other TM metals as well. Figure 4 shows the EEL spectra taken from a series of TM atoms. Individual TM can be intentionally doped to the single defect sites by thermal evaporation or solidification from liquid solution.43 However, several metal atoms are coming from atmosphere and most of the TM atoms in N defects can be found surprisingly as unexpected impurities. We have so far found in graphene layer Mg, Al, Ca, Ti, Cr, Mn, and Fe atoms, all of which are bonded to the N atoms. We are able to assign all the impurity atoms at the defect using atom-by-atom EELS. The other elements may be also present and could be further detected by either EELS or the other elemental analyses. The source of impurities cannot be identified yet, but no material will be perfectly pure under the chemical analysis with single atom sensitivity. The oxidation/spin states of these dopant atoms can be measured but will be described elsewhere.43


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Figure 4. (a) The EEL spectra for single atoms of various metal impurities, Mg, Al, Ca, Ti, Cr, Mn, and Fe, doped in O3+N2 plasma treated graphene layer. All of them are found at the N defect sites in graphene. (b)-(h) The corresponding ADF images of all the metal in N defect structures. Note that the N K-edge is always visible around 396 ~ 440 eV as well as the carbon K-edge at 285 eV. Scale bar is 2 Å. The weak N signal in the Ti+1N is attributed to the area of EELS line scan which may not completely hit the N atom.

In this experiment, we clearly show atomic resolution imaging and the ELNES fingerprints of the graphitic-N and the pyridinic-N in graphene. The N K-edge of the pyridinic-N exhibits a significant shift to lower energy than the graphitic-N, and the energy split between the N π* and σ*-edge of the pyridinic-N becomes wider than the graphitic-N, which refers to weaker binding energy of the pyridinic-N. Due to this weaker bond, the pyridinic-N is often observed oscillating between two equivalent


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single vacant sites under the electron beam. Higher chemical reactivity of the pyridinic-N than the graphitic-N is indeed visualized when trapping individual metal atoms to the defect sites. To synthesis graphene with specific type of N defects is of great technological importance, furthermore, to decorate such kind of N defect with selected metal ions can provide great potential in applications to energy conversion or biocompatible catalysts.44,45

Method Sample preparation Single layer graphene was synthesized on Cu foil by standard chemical vapor deposition of methane at 1050°C. Graphene films transferred onto SiO2/Si substrates were loaded in a reaction chamber, where ozone and nitrogen (1:1) was generated as gas source. When the ozone formation was ignited, additional ionic species such as N+ and N2+ and their excitations were also created. Ozone, along with the nitrogen-containing active species, reacts with graphene at a substrate temperature of 100°C, yielding high local defects and incorporating high-density of nitrogen into the graphene lattice. The graphene film was then transferred to SiN grid using the technique developed previously46,47 and heated to 500°C in a microscope.

STEM and EELS experiment condition STEM images were taken by JEOL 2100F microscope equipped with dodecaple


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correctors operating at 60 kV. The probe current is about 28-36 pA and the scanning time is about 64-77 µs. The convergence semi-angle and the inner semi-angle are 48 and 53 mrad. All the STEM images were filtered by Gaussian blur by using ImageJ. The EELS 2D maps and line scans were taken by using Gatan low-voltage quantum spectrometer with the spectrum pixel time 0.2-0.5 s. The EEL spectra were aligned by the sp2 C π* peak at 285 eV.

Acknowledgements Toma Susi and Jani Kotakoski are gratefully acknowledged for the discussion about the N atom dynamics. The authors from AIST acknowledge the support from JST Research Acceleration Programme. PWC appreciates the project support of National Tsing Hua University

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Supporting Information Low-magnification STEM images and description of the overall structure of the specimen. Additional STEM-EELS mapping of graphitic-N, SV+1N, SV+2N, DV+4N, DV+4N+Ca, SV+1N+N@C, and structural dynamics of graphitic-N. Description of computational method for geometry optimization and ELNES simulation.

References (1) Wang, X.; Li, X.; Zhang, L.; Yoon, Y.; Weber, P. K.; Wang, H.; Guo, J.; Dai, H.


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(11) Cho, Y. J.; Kim, H. S.; Im, H.; Myung, Y.; Jung, G. B.; Lee, C. W.; Park, J.; Park, M. H.; Cho, J.; Kang, H. S. Nitrogen-Doped Graphitic Layers Deposited on Silicon Nanowires for Efficient Lithium-Ion Battery Anodes. J. Phys. Chem. C 2011, 115, 9451-9457. (12) Rao, C. V.; Cabrera, C. R.; Ishikawa, Y. In Search of the Active Site in Nitrogen-Doped Carbon Nanotube Electrodes for the Oxygen Reduction Reaction. J. Phys. Chem. Lett. 2010, 1, 2622-2627. (13) Shao, Y.; Sui, J.; Yin, G.; Gao, Y. Nitrogen-Doped Carbon Nanostructures and Their Composites ad Catalytic Materials for Proton Exchange Membrane Fuel Cell. Appl. Catalys. B: Environmental 2008, 79, 89-99. (14) Luo Z.; Luo, Z.; Tian, Z.; Shang, J.; Lai, L.; Mac Donald, B.; Fu, C.; Shen, Z.; Yu, T.; Lin, J. Pyridinic N Doped Graphene: Synthesis, Electronic Structure, and Electrocatalytic Property. J. Mater. Chem. 2011, 21, 8038-8044. (15) Fujimoto, Y.; Saito, S. Hydrogen Adsorption and Anomalous Electronic Properties of Nitrogen-Doped Graphene. J. Appl. Phys. 2014, 115, 153701. (16) Niwa, H.; Horiba, K.; Harada, Y.; Oshima. M.; Ikeda, T.; Terakura, K.; Ozaki, J. –I.; Miyata, S. X-Ray Absorption Analysis of Nitrogen Contribution to Oxygen Reduction Reaction in Carbon Alloy Cathode Catalysts for Polymer Electrolyte Fuel Cells. J. Power Sources 2009, 187, 93-97. (17) Nagaiah, T.; Kundu, S.; Bron, M.; Muhler, M.; Schuhmann, W. Nitrogen-Doped Carbon Nanotubes as A Cathode Catalyst for the Oxygen Reduction Reaction in Alkaline Medium. Electrochem. Commun. 2010, 12, 338-341. (18) Meyer, J, C.; Kurasch, S.; Park, H. J.; Skakalova, V.; Künzel D.; Groß, A.; Chuvilin, A.; Algara-Siller, G.; Roth, S.; Iwasaki, T.; Starke, U.; Smet, J. H.; Kaiser, U. Experimental Analysis of Charge Redistribution Due to Chemical Bonding by High-Resolution Transmission Electron Microscopy. Nature Mater. 2011, 10, 209-215. (19) Susi, T.; Kotakoski, J.; Arenal, R.; Kurasch, S.; Jiang, H.; Skakalova, V.; Stephan, O.; Krasheninnikov, A. V.; Kauppinen, E. I.; Kaiser, U.; Meyer, J. C. Atomistic


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Description of Electron Beam Damage in Nitrogen-Doped Graphene and Single-Walled Catbon Nanotubes. ACS Nano 2012, 6, 8837-8846. (20) Zhao, L.; He, R.; Rim, K. T.; Schiros, T.; Kim, K. S.; Zhou, H.; Gutiérrez, C.; Chockalingam, S. P.; Arguello, C. J.; Pálová, L.; Nordlund, D.; Hybertsen, M. S.; Reichman, D. R.; Heinz, T. F.; Kim, P.; Pinczuk, A.; Flynn, G. W.; Pasupathy, A. N. Visualizing Individual Nitrogen Dopants in Monolayer Graphene. Science 2011, 333, 999-1003. (21) Lv, R.; Li, Q.; Botello-Méndez, A. R.; Hayashi, T.; Wang, B.; Berkdemir, A.; Hao, Q.; Elías, A. L.; Cruz-Silva, R.; Gutiérrez, H. R.; Kim, Y. A.; Muramatsu, H.; Zhu, J.; Endo, M.; Terrones, H.; Charlier, J. –C.; Pan, M.; Terrones, M. Nitrogen-Doped Graphene: Beyond Single Substitution and Enhanced Molecular Sensing. Sci. Rep. 2012, 2:586, 1-8. (22) Kondo, T.; Casolo, S.; Suzuki, T.; Shikano, T.; Sakurai, M.; Harada, Y.; Saito, M.; Oshima, M.; Trioni, M. I.; Tantardini, G. F.; Nakamura, J. Atomic-Scale Characterization of Nitrogen-Doped Graphite: Effects of Dopant Nitrogen on the Local Electronic Structure of the Surrounding Carbon Atoms. Phys. Rev. B 2012, 86, 035436. (23) Krivanek, O. L.; Chisholm, M. F.; Nicolosi, V.; Pennycook, T. J.; Corbin, G. J.; Dellby, N.; Murfitt, M. F.; Own, C. S.; Szilagyi, Z. S.; Oxley, M. P.; Pantelides, S. T.; Pennycook, S. J. Atom-by-Atom Structural and Chemical Analysis by Annular Dark-Field Electron Microscopy. Nature 2010, 464, 571-574. (24) Suenaga, K.; Sato, Y.; Liu, Z.; Kataura, H.; Okazaki, T.; Kimoto, K.; Sawada, H.; Sasaki, T.; Omoto, K.; Tomita, T.; Kaneyama, T.; Kondo, Y. Visualizing and Identifying Single Atoms Using Electron Energy-Loss Spectroscopy with Low Accelerating Voltage. Nat. Chem. 2009, 1, 415-418. (25) Suenaga, K.; Kobayashi, H.; Koshino, M. Core-Level Spectroscopy of Point Defects in Single Layer h-BN. Phys. Rev. Lett. 2012, 108, 075501. (26) Suenaga, K.; Koshino, M. Atom-by-Atom Spectroscopy at Graphene Edge. Nature

2010, 468, 1088-1090.


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Nano Letters

(27) Warner, J. H.; Lin, Y. C.; He, K.; Koshino, M.; Suenaga, K. Atomic Level Spatial Variations of Energy States Along Graphene Edges. Nano Lett. 2014, 14, 6155-1659. (28) Zhou, W.; Kapetanakis, M. D.; Prange, M. P.; Pantelides, S. T.; Pennycook, S. J.; Idrobo, J. –C. Direct Determination of the Chemical Bonding of Individual Impurities in Graphene. Phys. Rev. Lett. 2012, 109, 206803. (29) Ramasse, Q. M.; Seabourne, C. R.; Kepaptsoglou, D. M.; Zan, R.; Bangert, U.; Scott, A. J. Probing the Bonding and Electronic Structure of Single Atom Dopants in Graphene with Electron Energy Loss Spectroscopy. Nano Lett. 2013, 13, 4989-4995. (30) Nicholls, R. J.; Murdock, A. T.; Tsang, J.; Britton, J.; Pennycook, T. J.; Koós, A.; Nellist, P. D.; Grobert, N.; Yates, J. R. Probing the Bonding in Nitrogen-Doped Graphene Using Electron Energy Loss Spectroscopy. ACS Nano 2013, 7, 7145–7150. (31) Bangert, U.; Pierce, W.; Kepaptsoglou, D. M.; Ramasse, Q.; Zan, R.; Gass, M. H.; Van den Berg, J. A.; Boothroyd, C. B.; Amani, J.; Hofsäss, H. Ion Implantation of Graphene—Toward IC Compatible Technologies. Nano Lett. 2013, 13, 4902-4907. (32) Warner, J. H.; Lin, Y. -C.; He, K.; Koshino, M.; Suenaga, K. Stability and Spectroscopy of Single Nitrogen Dopants in Graphene at Elevated Temperatures. ACS Nano 2014, 8, 11806-11815. (33) Arenal, R.; March, K.; Ewels, C. P.; Rocquefelte, X.; Kociak, M.; Loiseau, A.; Stéphan, O. Atomic Configuration of Nitrogen-Doped Single-Walled Carbon Nanotubes. Nano Lett. 2014, 14, 5509-5516. (34) Banhart, F.; Kotakoski, J.; Krasheninnikov, A. K. Structural Defects in Graphene, ACS Nano 2011, 5, 26-41. (35) Fujimoto, Y.; Saito, S. Formation, stabilities, and electronic properties of nitrogen defects in graphene. Phys. Rev. B 2011, 84, 245446. (36) Guo, J.; Lee, J.; Contescu, C. I.; Gallego, N. C.; Pantelides, S. T.; Pennycook, S. J.; Moyer, B. A.; Chisholm, M. F. Crown Ethers in Graphene. Nature. Commun. 2014, 5, 5389.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(37) Susi, T.; Kotakoski, J.; Kepaptsoglou, D.; Mangler, C.; Lovejoy, T. C.; Krivanek, O. L.; Zan, R.; Bangert, U.; Ayala, P.; Meyer, J. C.; Ramasse, Q. Silicon–Carbon Bond Inversions Driven by 60-keV Electrons in Graphene. Phys. Rev. Lett. 2014, 113, 115501. (38) Pham, V. D.; Lagoute, J.; Mouhoub, O.; Joucken, F.; Repain, V.; Chacon, C.; Bellec, A.; Girard, Y.; Rousset, S. Electronic Interaction between Nitrogen-Doped Graphene and Porphyrin Molecules. ACS Nano 2014, 8, 9403-9409. (39) Jeong, H. M.; Lee, J. W.; Shin, W. H.; Choi, Y. J.; Shin, H. J.; Kang, J. K.; Choi, J. W. Nitrogen-Doped Graphene for High-Performance Ultracapacitors and the Importance of Nitrogen-Doped Sites at Basal Planes. Nano Lett. 2011, 11, 2472-2477. (40) Chen, L. –F.; Zhang, X. –D.; Liang, H. –W.; Kong, M.; Guan, Q. –F.; Chen, P.; Wu, Z, –Y.; Yu, S. –H.; Synthesis of Nitrogen-Doped Porous Carbon Nanofibers as an Efficient Electrode Material for Supercapacitors. ACS Nano 2012, 6, 7092-7102. (41) Reddy, A. L. M.; Srivastava, A.; Gowda, S. R.; Gullapalli, H.; Dubey, M.; Ajayan, P. M. Synthesis Of Nitrogen-Doped Graphene Films For Lithium Battery Application. ACS Nano 2010, 4, 6337-6342. (42) Fei, H.; Ye, R.; Ye, G.; Gong, Y.; Peng, Z.; Fan, X.; Samuel, E. L. G.; Ajayan, P. M.; Tour, J. M. Boron- and Nitrogen-Doped Graphene Quantum Dots/Graphene Hybrid Nanoplatelets as Efficient Electrocatalysts for Oxygen Reduction. ACS Nano 2014, 8, 10837-10843. (43) Lin, Y.-C.; Teng, P.-Y.; Chiu, P.-W.; Suenaga, K. Exploring the Single Atom Spin State by Electron Spectroscopy. Phys. Rev. Lett. 2015, in press. (44) Carrero-Sánchez, J. C.; Elías, A. L.; Mancilla, R.; Arrellín, G.; Terrones, H.; Laclette, J. P.; Terrones, M. Biocompatibility and Toxicological Studies of Carbon Nanotubes Doped with Nitrogen. Nano Lett. 2006, 6, 1609-1616. (45) Elías, A. L.; Carrero-Sánchez, J. C.; Terrones, H.; Endo, M.; Laclette Prof. J. P.; Terrones, M. Viability Studies of Pure Carbon- and Nitrogen-Doped Nanotubes with Entamoeba histolytica: From Amoebicidal to Biocompatible Structures. Small 2007, 3,


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1723-1729. (46) Lin, Y. –C.; Jin, C.; Lee, J. –C.; Jen, S. –F.; Suenaga, K.; Chiu, P. –W. Clean Transfer of Graphene for Isolation and Suspension. ACS Nano 2011, 5, 2362-2368. (47) Lin, Y. –C.; Lu, C. –C.; Yeh, C. –H.; Jin, C.; Suenaga, K.; Chiu, P. –W. Graphene Annealing: How Clean Can It Be? Nano Lett. 2012, 12, 414-419.


a

c Nano Letters g

b

π∗ σ∗

Page 22 of 26 Graphitic-N

N@C

1N+SV

e

SV+2N

SV+1N

Signal (a.u.)

1 2 3 4 5 6 7 8 d 9 10 11 12 13 14 15 16 17

f

SV+3N

(simulation)

Pyridinic-N

DV+4N (simulation)

ACS Paragon Plus Environment 380

400 420 Energy (eV)

440

1 2

1 2 N 3 3 4 5 6 7 8 9 10 11 12 13 14 15 16 DV+4N 17 1 2 10 18 N 1 N4 19 3 9 20 4 8 N2 21 N3 5 22 6 7 23 24 25 26 27 28 29 30 31 32

b

e

f

Nano Letters

c

d

285 Signal (a.u.)

a

1 2

285.5

401.4

N

407.6

3 280

g

300

Energy (eV)

320

380

340

h

285

400

420

440

420

440

Energy (eV) 406.6

398.0

1 2 N1 3 4 N2

Signal (a.u.)

Page 23 of 26 N@C

5 6 7 N3 8 9 N4 10

ACS Paragon Plus Environment 280

300

Energy (eV)

320

340

380

400

Energy (eV)

a

b

c

(c) Nano Letters

d

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Signal (a.u.)

1 2 3 e 4 285 C 5 f N 6 Cr 1 7 2 8 9 N 10 Cr 11 12 250 300 350 400 450 500 550 600 Energy (eV) 13 14 Metal ion 15 g Graphitic-N Pyridinic-N 16 17 18 19 20 ACS Paragon Plus Environment 21 22 23

1 2 Cr

N

aPage 25 of 26

Mg + 2N

b

f

Cr + 2N

N

Mg Al

Sinal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 180 19 20

Nano Letters

c

Ca

g

h

d

Cr Mn

e

Fe 200

Fe + 4N

Ca + 4N

Ti

100

4Mn + 7N

Al + 1N

300

400

Ti + 1N

Plus Environment 500 ACS 600 Paragon 700 800

Energy (eV)

Nano Letters

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85x34mm (300 x 300 DPI)


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Structural and Chemical Dynamics of Pyridinic-Nitrogen Defects in

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