Elemental superdoping of graphene and carbon nanotubes

Doping of low-dimensional graphitic materials, including graphene, graphene quantum dots and single-wall carbon nanotubes with nitrogen, sulfur or boron can significantly change their properties. We report that simple fluorination followed by annealing in a dopant source can superdope low-dimensional graphitic materials with a high level of N, S or B. The superdoping results in the following doping levels: (i) for graphene, 29.82, 17.55 and 10.79 at% for N-, S- and B-doping, respectively; (ii) for graphene quantum dots, 36.38 at% for N-doping; and (iii) for single-wall carbon nanotubes, 7.79 and 10.66 at% for N- and S-doping, respectively. As an example, the N-superdoping of graphene can greatly increase the capacitive energy storage, increase the efficiency of the oxygen reduction reaction and induce ferromagnetism. Furthermore, by changing the degree of fluorination, the doping level can be tuned over a wide range, which is important for optimizing the performance of doped low-dimensional graphitic materials.

, respectively. Based on the XPS results, the N-doping levels defined as N/C  100 at% of total N and three N types of NG'' can be quantitatively obtained, which are shown in Supplementary Table 3.  The inset shows the galvanostatic charge/discharge curve for the sample in the first and final five cycles. As shown in Supplementary Figs. 9a,b, the C s of NG electrode at 5 A g -1 and 10 A g -1 were calculated to be ca. 390 and 354 F g -1 , and the C s at scan rate of 10-100 mV s -1 is 348.3, 303.4, 271.7 and 239.7 F g -1 , respectively. Moreover, one can find that (i) the capacity deterioration is less than 5.0% after 5000 cycles; and (ii) the shape of charge/discharge curve of the last 5 cycles is almost similar to that of the first 5 cycles ( Supplementary Fig. 9c), implying the high reversibility and long-term electrochemical stability of the NG electrode. found that NG-800 has the highest current density and onset potential, suggesting that the optimal temperature to prepare NG for oxygen reduction reaction (ORR) activity is 800 o C. As shown in Supplementary Fig. 13a, it is found that T C of NG-400 is ca. 76.4 K. The saturated paramagnetic magnetization is fitted to be 1.58 emu g -1 , and the saturated ferromagnetic magnetization is ca. 0.23 emu g -1 (Supplementary Fig. 13b). From saturated paramagnetic magnetization added with saturated ferromagnetic magnetization, one can calculate the M s of NG is 1.81 emu g -1 . It is found that T C of NG-600 is ca. 80.2 and 147.0 K (Supplementary Fig. 14a). The saturated paramagnetic magnetization is fitted to be 1.82 emu g -1 , and the saturated ferromagnetic magnetization is ca. 0.18 emu g -1 (Supplementary Fig. 14b). From saturated paramagnetic magnetization added with saturated ferromagnetic magnetization, one can calculate the M s of NG is 2.00 emu g -1 . As shown in Supplementary Fig. 15a, it is found that T C of NG-700 is ca. 126.2 and 225.2 K.
The saturated paramagnetic signal is fitted to be 1.72 emu g -1 , and the saturated magnetization of ferromagnetic signal is ca. 0.17 emu g -1 (Supplementary Fig. 15b). From saturated paramagnetic magnetization added with saturated ferromagnetic magnetization, one can calculate the M s of NG is 1.89 emu g -1 .  Fig. 16a). As shown by the fitting curve ( Supplementary Fig. 16b), the Brillouin function provides good fits with g = 2 and J = 2, and M s can be obtained which is 2.98 emu g -1 . Approximately, it suggests that the bandgap of NG can be tuned by changing the N-doping level.  As shown in Supplementary Fig. 11a, it is found that no ferromagnetism can be observed in graphene and NG, and only purely diamagnetism can be observed at 300 K. Shown in Supplementary Fig. 11b is the dependence of susceptibility  = M / H of graphene on temperature (T) and it fits well with the Curie law  = C / T. Inset is the corresponding 1/ -T curve, which demonstrates a linear, purely Curie-like paramagnetic behavior. To analyze the magnetic property of graphene and NG, the M -H curve is fitted by the Brillouin function. As shown by the fitting curve ( Supplementary Fig. 11c), the Brillouin function provides good fits for J = S = 1/2 for graphene, and the saturated paramagnetization is 0.14 emu g -1 . After high-level N-doping, the saturated paramagnetic signal is fitted to be ca. 1.99 emu g -1 for NG, and the saturated magnetization of ferromagnetic signal is ca. 0.31 emu g -1 (Supplementary Fig. 11d Fig. 11f), revealing further the magnetic transition at T C .
Interestingly, it is found that T c of the NG samples varies (Supplementary Table 7). It may attribute to the difference in the level and type of the N-doping. As reported previously, the N-doping can effectively introduce the magnetic moments 7,8 . As reported in diluted magnetic semiconductors, the ferromagnetic coupling between the magnetic moments may appear via non-localized interaction 9 . Theoretically, the coupling is proportional both to the ordinary Ruderman-Kittel-Kasuya-Yosida (RKKY) oscillation term F which increases with the increase of carrier concentrations, and to the exp(-r), where r is the distance between the magnetic moments. Additionally, N-doping can make the Fermi level shift upward due to the extra π electrons and, thus increase the carrier concentration 10 . In our N-superdoped NG samples, it is reasonable to assume that with the increase of the N-doping level, (i) F will increase with the increasing of carrier concentrations; and (ii) the distance r between the magnetic moments will decrease with the increase of the localized magnetic moments induced by N-doping. As a consequence, the ferromagnetic coupling may enhance and this may result in the increase in T C .
Furthermore, the bound magnetic polarons (BMPs) may also generate ferromagnetism in semiconductors. Therein, the ferromagnetic coupling can be mediated by BMPs formed by the shallow donor electrons, which overlap to create a spin-split impurity band 11 . Typically, ferromagnetism has been observed in C, Al, or Ca doped ZnO. It is considered that oxygen vacancies can act as BMPs and trap free electrons, and the electrons trapped in these BMPs tend to get easily polarized under the influence of the magnetic field, resulting in ferromagnetism 12 .
Note that T C is strongly related to the BMPs concentration 13 . Similarly, in our case of the N-superdoped NG, the N atoms may act as BMPs thanks to the strong electron affinity 7,14 Thus, 40 with the increase of the N-doping level, the ferromagnetic coupling may enhance and T C may increase. Unfortunately, the exact ferromagnetic coupling mechanism is not clear at present, and which need to be confirmed by more experimental and theoretical work.