Nitrogen is one of the most important doping elements for carbon materials. N doping was reported to enhance the catalytic ability of carbon materials and can improve the oxygen reduction reaction efficiency. Besides, the N doped graphene are also anticipated for several applications such as n-type transistor, sensor or lithium battery. Distinct N configurations, i.e., graphitic N and pyridinic N, have been predicted to behave different electronic properties. Therefore, to precisely control the type of N doping is considered a critical issue to realize high performance graphene-based devices. Though X-ray photoelectron spectroscopy (XPS) analysis suggests distinct N 1s states for different N configuration, however, the direct link to the specific N defect is still limited by the spacial resolution. Here, we present systematic studies of atomic structures and the atomic EELS studies on graphitic N and pyridinic N defect in graphene by scanning transmission electron microscopy [1]. Figure 1a and 1b show the ADF images and the corresponding atomic models of graphitic and pyridinic N defect in graphene. The energy loss near-edge structures of the graphitic N and pyridinic N defects are shown in Figure 1c. The graphitic N K-edge shows two sharp peaks at 401.4 eV (π*) and 407.6  eV (σ*), while the π* and σ* peak of pyridinic N exhibits significant down shift to 398.0 eV and 406.6 eV.

Pyridinic N defects are also found highly reactive to attract individual single transition metals (TM) to the defect sites. The spin state of single TM atoms in graphene defect was studied by core-level electron spectroscopy. Figure 2a and 2b show the ADF images of single Fe atoms anchored at graphene divacancy and at the four pyridinic N defects. We found that the single Fe atom possess high spin at graphene divancacy, while the spin state can be altered to low spin when bonding to pyridinic N (Figure 2c). This work realize the controllable of spin state of an individual TM atom which can be regarded as the smallest component of spintronic devices [2].

Reference

1. Y. C. Lin et al., Nano Lett. 15, 7408-7413 (2015).
2. Y. C. Lin et al., Phys. Rev. Lett. 115, 206803 (2015).

Figures:

(a) An ADF image and the atomic model of graphitic N defect. (b) An ADF image and the atomic model of pyridinic N defect. (c) The corresponding EEL spectra of graphitic N and pyridinic N defects.

(a) An ADF image and the atomic model of Fe atom in graphene divacancy (Fe@DV). (b) An ADF image and the atomic model of Fe atom in four pyridinic N defect (Fe+4N). (c) The corresponding EEL spectra of the Fe@DV and Fe+4N.

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EMC Abstracts - https://emc-proceedings.com/abstract/discovery-of-pyridinic-nitrogen-defects-and-single-atom-spin-in-graphene/