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Carbon doped into Bi2Se3 as substitution for Se was reported inducing simultaneous magnetic and hole doping. In this work, based on theoretical and experimental approaches, we find that carbon doped Bi2Se3 is indeed spin polarized, while the magnetic moments are small and hole doping is little. Most carbon atoms energetically favor to be dimer- and trimer-substitutions for Se which induce neither magnetism nor charge doping. A few isolated carbon atoms doped at interstitial sites in the vdW gap or at Se vacancies result in spontaneous spin polarization and charge doping. The diffusion of a single carbon dopant into Bi2Se3 is difficult, while if dopants are close, carbon clusters can easily form, which means that the isolated, pairwise, and trimeric carbon dopants should coexist. These studies suggest that to obtain strong ferromagnetic C-Bi2Se3, experiments need careful design to dope single carbon atoms into bulk dispersedly. [ABSTRACT FROM AUTHOR]

Carbon doped into Bi2Se3 as substitution for Se was reported inducing simultaneous magnetic and hole doping. In this work, based on theoretical and experimental approaches, we find that carbon doped Bi2Se3 is indeed spin polarized, while the magnetic moments are small and hole doping is little. Most carbon atoms energetically favor to be dimer- and trimer-substitutions for Se which induce neither magnetism nor charge doping. A few isolated carbon atoms doped at interstitial sites in the vdW gap or at Se vacancies result in spontaneous spin polarization and charge doping. The diffusion of a single carbon dopant into Bi2Se3 is difficult, while if dopants are close, carbon clusters can easily form, which means that the isolated, pairwise, and trimeric carbon dopants should coexist. These studies suggest that to obtain strong ferromagnetic C-Bi2Se3, experiments need careful design to dope single carbon atoms into bulk dispersedly. [ABSTRACT FROM AUTHOR]