Supporting Information for. Structural and Chemical Dynamics of Pyridinic Nitrogen. Defects in Graphene

Transcription

1 Supporting Information for Structural and Chemical Dynamics of Pyridinic Nitrogen Defects in Graphene Yung-Chang Lin, 1* Po-Yuan Teng, 2 Chao-Hui Yeh, 2 Masanori Koshino, 1 Po-Wen Chiu, 2 Kazu Suenaga 1* 1 National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba , Japan 2 Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan *Corresponding Author: yc-lin@aist.go.jp, suenaga-kazu@aist.go.jp

2 Fig. S1. (a) A low-magnification ADF image of O 3 +N 2 treated graphene. Yellow circles indicate the existence of single vacancies where most of them contain pyridinic-n. All the brighter regions are contaminants. Red circles indicate the existence of individual heavier metal atoms been trapped in single vacancies. (b,c) Higher-magnification ADF images where N atoms appeared in brighter contrast and can be seen more clearly at the vicinity of graphene vacancies. A clearer example of DV+4N structure is highlighted in the inset of (b) where four pyridinic-n atoms show

3 brighter contrast. Fig. S2. (a) An ADF image of N-doped graphene without imaging process. (b) A Gaussian blurred ADF image from (a). The distance between two graphitic-n is 6.2Å. Scale bar is 5 Å.(c) An EELS color map acquired from the green selected area in (b). Green color represents the signal of carbon, while red color represents the signal of nitrogen. (d) The simultaneously collected ADF contrast during the EELS mapping. (e) The EEL spectra of graphitic-n and graphene extracted from the EELS map in (c) with 5x5 spectra pixels.

4 Fig. S3. (a) An ADF image of N-doped graphene. The green square shows the EELS scanned area where a SV+1N defect was selected. (b) A corresponding EEL spectrum image. The scan size includes 12x12 pixels, and the exposure time is 0.2sec for one pixel. (c) The EELS color map of carbon (red, ev) and nitrogen (green, ev) with an atomic structure superimposed on top. (d) An ADF image recorded simultaneously with the EELS mapping. (e,f) The extracted EEL spectra from the regions of interest 1 and 2 where only the region 2 exhibits pyridinic-n signal. Fig. S4. (a) An ADF image of N-doped graphene. The green square shows the EELS scanned area where a SV+2N defect was selected. (b) A corresponding EEL spectrum image. The scan size includes 40x40 pixels, and the exposure time is 0.01sec for one

5 pixel. (c) The EELS color map of carbon (red, ev) and nitrogen (green, ev) with an atomic structure superimposed on top. (d) An ADF image recorded simultaneously with the EELS mapping. A slight sample drifting causes the distortion to the ADF image. (e,f) The extracted EEL spectra from the regions of interest 1 and 2 where both regions show clear pyridinic-n signals. Fig. S5. (a) An ADF image of N-doped graphene. The green square shows the EELS scanned area where a DV+4N defect was selected. (b) A corresponding EEL spectrum image. The scan size includes 12x12 pixels, and the exposure time is 0.2 sec for one pixel. (c) The EELS color map of carbon (red, ev) and nitrogen (green, ev) superimpose on the ADF contrast (d). (d) An ADF image recorded simultaneously with the EELS mapping. (e-h) The extracted EEL spectra from the regions of interest 1, 2, 3, and 4 where all of these regions show the existence of pyridinic-n signals.

6 Fig. S6. (a) An ADF image of N-doped graphene taken right after the EELS mapping shown in Fig. S4. The green square shows the EELS scanned area where a DV+4N defect plus a heavier impurity was selected. (b) A corresponding EEL spectrum image. The scan size includes 12x12 pixels, and the exposure time is 0.2 sec for one pixel. (c) The EELS color map of carbon (red, ev) and nitrogen (green, ev) with an ADF contrast image (d) superimposed on top. (d) An ADF image recorded simultaneously with the EELS mapping. (e-h) The extracted EEL spectra from the regions of interest 1, 2, 3, and 4 where all of these regions show the existence of pyridinic-n signals. The impurity is recognized as a single Ca atom. Fig. S7. (a-c) Consecutive ADF images of the structural dynamics of graphitic-n. The corresponding atomic models are shown under the ADF images. Red and grey arrows indicate the inversion of C-N bond. See also Movie 2. (d) An ADF image of N-doped

7 graphene taken right after the sequential ADF images shown in Figure 2. The green square shows the EELS scanned area where a N@C defect was selected. (e) A corresponding EEL spectrum image. The scan size includes 20x20 pixels, and the exposure time is 0.2 sec for one pixel. (f) The EELS color map of carbon (red, ev) and nitrogen (green, ev) with an ADF contrast image (g) superimposed on top. (g) An ADF image recorded simultaneously with the EELS mapping. A slight sample drifting causes the distortion to the structure image. (h,i) The extracted EEL spectra from the regions of interest 1 and 2 where region 2 show clear graphitic-n signals. In our 60kV STEM observation, we have never found a pyridinic-n been created by removing a C atom next to the N@C. Fig. S8. (a-d) Sequential ADF images of the structural dynamics of N dopant in a defective graphene region. The raw images with marks are showing under the contrast enhanced images. See also the Movie 3. Yellow and Red circles highlight the position of N dopants. The red circled N defect exhibits as a N@C in 5-7 rings. (c) After sec, the red circled N@C was kicked out be e-beam. The other yellow circled N defect transformed from (a) a pyridinic-n to (b) an azocine-like N which is located in the middle of two octagons. The azocine-like N then transformed to (c) a N@C in 5-7 rings. (d) The yellow circled N@C was also displaced after sec and graphene reconstructed to ideal hexagonal lattice. Scale bar is 2.5Å.

8 Fig. S9. (a-c) A back-and-forth structure transform of double pyridinic-n defects between SV+1N+N@C and SV+2N by 120 rotation of a C-N bond (marked by green line). In the last half of the Movie 4, one can see the stability of SV+1N+N@C defect structure. (d) An ADF image of N-doped graphene taken right after the sequential ADF images. The green square shows the EELS scanned area where a SV+1N+N@C defect was selected. (e) A corresponding EEL spectrum image. The scan size includes 20x20 pixels, and the exposure time is 0.2 sec for one pixel. (f) The EELS color map of carbon (red, ev) and nitrogen (green, ev) with an atomic model superimposed on top. (g) An ADF image recorded simultaneously with the EELS mapping. (h,i) The extracted EEL spectra from the regions of interest 1 and 2 where region 1 shows N@C signal and region 2 shows SV+1N signal.

9 Supporting Method Carbon and Nitrogen K-edge ELNES simulations for various types of N-substituted graphene by pseudo potential method The structure of graphene was initially modelled with a hexagonal structure (a=b=246.4 pm, c=1.00 nm). A of supercell was created for a graphitic-n, maintaining three coordination with adjacent carbon atoms, so as an atom with a core-hole well separated, considering the periodic nature of the model. Four of supercells were created for N-substituted graphene models of SV+1N, SV+2N, SV+3N, and DV+4N, in which all nitrogen atoms maintain two coordination with adjacent carbon atoms. We performed a band-structure calculation based on pseudo-potentials method, using CASTEP module in Materials Studio ver. 7.0 (Accelrys Co.) for both geometry optimization (BFGS algorithm S1 ) and core spectroscopy simulation of carbon K-edge and nitrogen K-edge S2. Geometry optimization We performed the geometry optimization of four N-substituted graphene models, SV+1N, SV+2N, SV+3N, and DV+4N, by using the generalized gradient approximation (GGA) PBE functionals S3 with ultra-fine quality implemented in the CASTEP module of the Materials Studio ver. 7.0 (Accelrys Co.). The calculation parameters for ultra-fine quality were: the convergence threshold for the maximum energy change, maximum force, maximum stress and maximum displacement set to ev/atom, 0.01 ev/å, 0.02 GPa, and Å, respectively. An SCF tolerance smaller than ev/atom was regarded as convergence. The BFGS line search was adopted. We also set ev of Energy cutoff, FFT grid density, and a K-point set. Either ultrasoft or norm conserving Pseudopotentials represented in a reciprocal space were selected as an on the fly option. Density mixing of 0.5 charge was used for electronic minimizer. A proper electronic minimization parameters (20 % of empty bands and 0.1 ev of smearing), instead of assigning fixed occupancy, were required for SCF convergence. ELNES simulation The energy loss near edge structures (ELNES) of carbon and nitrogen K-edge from each atomic site are simulated by the core spectroscopy function in CASTEP after self-consistent field (SCF) iterations for the final state (core-hole) electron configurations. The SCF iterations were converged based on GGA PBE functionals

10 S3 with ultra-fine quality regarding ev/atom ev/atom as convergence similar to the geometry optimization. Typical parameters applied in the simulation are, for example, a (1 1 1) K-point set, 440 ev of cut-off energy in plane wave basis set and FFT grid density with 1.5 of an augmentation density scaling factor. A core-hole is introduced to either a carbon atom or a nitrogen atom by removing an electron from core level at the atom of interest in order to simulate carbon and nitrogen K-edge spectra. The obtained spectra are mainly originated from 2p component of unoccupied bands smeared with 0.3 ev of energy broadening (FWHM) with a consideration of lifetime effect, (ε k -ε threshold ) ev, where the FWHM increases with increasing energy differential between the threshold energy, ε threshold, and the energy above the threshold, ε k. We have attempted to estimate the accurate energy shift of carbon and boron K-edge spectra from the pseudo potential method S4-6, resulting in good agreement with the experiment. Although the total energy obtained from the pseudo potential method is not directly related to the absolute energy of EELS, it can be used to estimate the energy shift of certain atoms by following an appropriate procedure described before S4-6. For carbon K-edge, we assign a bulk carbon (far enough from vacancies or nitrogen) as a reference, having a π* peak at ev. For nitrogen K-edge, as we do not have a reference nitrogen atom in simulation, we manually aligned the energy as to fit the highest peak between simulation and experiment. Reference S1 Pfrommer, B. G., Côté, M., Louie, S. G. & Cohen, M. L. Relaxation of Crystals with the Quasi-Newton Method. Journal of Computational Physics 131, , (1997). S2 Gao, S.-P., Pickard, C. J., Payne, M. C., Zhu, J. & Yuan, J. Theory of core-hole effects in 1s core-level spectroscopy of the first-row elements Physical Review B 77, , (2008). S3 Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 77, 3865, (1996). S4 Warner, J. H., Lin, Y.-C., He, K., Koshino, M. & Suenaga, K. Atomic Level Spatial Variations of Energy States along Graphene Edges. Nano Lett. 14, , (2014). S5 Cretu, O. et al. Structure and Local Chemical Properties of Boron-Terminated Tetravacancies in Hexagonal Boron Nitride. Phys. Rev. Lett. 114, , (2015). S6 Tizei, L. H. G. et al. Single Molecular Spectroscopy: Identification of Individual Fullerene Molecules. Phys. Rev. Lett. 113, , (2014).