Atomic-scale characterization of nitrogen-doped graphite: Effects of dopant nitrogen on the local electronic structure of the surrounding carbon atoms

We report on the local atomic and electronic structures of a nitrogen-doped graphite surface by scanning tunneling microscopy, scanning tunneling spectroscopy, x-ray photoelectron spectroscopy, and first-principles calculations. The nitrogen-doped graphite was prepared by nitrogen ion bombardment followed by thermal annealing. Two types of nitrogen species were identified at the atomic level: pyridinic-N (N bonded to two C nearest neighbors) and graphitic-N (N bonded to three C nearest neighbors). Distinct electronic states of localized π states were found to appear in the occupied and unoccupied regions near the Fermi level at the carbon atoms around pyridinic-N and graphitic-N species, respectively. The origin of these states is discussed based on experimental results and theoretical simulations.

Figure 1

(a) N 1s XPS spectra of the graphite surface after nitrogen ion bombardment at 200 eV. The results before and after annealing at 900 $\pm$ 50 K for 300 s are shown. Deconvoluted components are pyridinic-N (398.5 eV; before: 32.6$\%$, after: 31.4$\%$), pyrrolic-N (399.9 eV; before: 17.4$\%$, after: 7.6$\%$), graphitic-N (401.1 eV; before: 40.1$\%$, after: 54.3$\%$), and oxide-N (403.2 eV; before: 9.8$\%$, after: 6.8$\%$). (b) Typical STM topographic image of the N-doped graphite surface ∼5.3 K (scan size: 49.45 $\times$ 46.97 nm${}^2$, tunneling current $I_t$ $=$ 179 pA, sample bias $V_s$ $=$ 98.8 mV). (c) and (d) STM topographic images of regions A and B in Fig. 1b, respectively (in both cases, scan size: 9.88 $\times$ 9.26 nm${}^2$, $I_t$ $=$ 97.5 pA, $V_s$ $=-109$ mV).

Figure 2

(a) STM topographic image of region A in Fig. 1b (scan size: 4.81 $\times$ 4.61 nm${}^2$, $I_t$ $=$ 96.9 pA, $V_s$ $=-108$ mV). The simulated STM image ($V$ $=$ $-$0.1 V) is also shown for comparison. (b) STS spectrum measured at the position indicated by the arrow in (a). (c) Equilibrium geometry of the pyridinic-N defect calculated by DFT. (d) Simulated STS spectrum of pyridinic-N.

Figure 3

Simulated STM images (constant-current mode) for the pyridinic-N graphene defect: (a) $V$ $=$ $-$1.0 V and (d) $V$ $=$ $+$1.0 V. The N atom is placed at the center of the image. (b) DFT equilibrium geometry and isosurface plot of electron density at 1.5 $\times$ 10${}^{-3}$ electrons/Å${}^3$ integrated from $-$0.7 eV to the Fermi level (square modulus of the wave function up to the Fermi level from $-$0.7 eV). The N atom is shown in light gray. (c) Same image as Fig. 3b except for color near the nitrogen atom, where only the $xy$-plane DOS contribution near N is represented by blue–red gradients. (e) DFT equilibrium geometry and isosurface plot of electron density at 1.5 $\times$ 10${}^{-3}$ electrons/Å${}^3$ integrated from the Fermi level to $+$0.7 eV. (f) Schematic representation of $p_z$ orbitals in pyridinic-N.

Figure 4

(a) STM topographic image corresponding to the defect shown in region B in Fig. 1b (image taken from a different sample, scan size: 5.09 $\times$ 5.08 nm${}^2$, $I_t$ $=$ 39.0 pA, $V_s$ $=$ 500 mV). The simulated STM image (V $=$ $+$0.5 V) is also shown for comparison. (b) STS spectrum measured at the position indicated by the arrow in (a). (c) Equilibrium geometry of the graphitic-N defect calculated by DFT. (d) Simulated STS spectrum of graphitic-N.

Figure 5

Simulated STM images of the graphitic-N defect: (a) $V$ $=$ $-$0.5 V, and (c) $V$ $=$ $+$0.5 V. The N atom is placed at the center of the image. DFT equilibrium geometry and isosurface plot of electron (b) electron density at 2 $\times$ 10${}^{-4}$ electrons/Å${}^3$ integrated from $-$0.7 eV to the Fermi level (square modulus of wave function up to the Fermi level from $-$0.7 eV) and (d) electron density at 1.5 $\times$ 10${}^{-3}$ electrons/Å${}^3$ integrated from the Fermi level to $+$0.7 eV.