Pan et al. Nanoscale Research Letters 2011, 6:97 http://www.nanoscalereslett.com/content/6/1/97 Page 3 of 7 Figure 2 (Color online) Geometries of (a) pure and (b) metal-functionalized g-C3N4-zz4 after optimization. Cr-, Mn-, Co-, and Ni-functionalized nanotubes appear to be anti-ferromagnetic because the energies of anti-ferro- magnetic state are lower than those of ferromagnetic state (see Table 1). Figure 3 shows the calculated band structures of W-functionalized g-C3N4-zz4. The non-symmetrical electronic structures between spin-up and spin-down states demonstrated the ferromagnetism of g-C3N4- zz4-W. The band structures of both spin-up and spin- down states show heavily doped n-type semiconducting behavior with a direct band gap of 0.75 and 0.95 eV, respectively (see Figure 3). The valence band top (VBT) in both of the spin-up and spin-down band structures have dispersion features, indicating the improvement of carrier mobility with the functionalization. However, the conduction band bottom (CBB) of the spin-up states of g-C3N4-zz4-W is almost flat, while that of the spin- down states keeps dispersion, revealing the coupling between the localized spin-polarization and carrier. The analysis of partial density of states (PDOS) further reveals the mechanism of ferromagnetism induced by the coupling. From the calculated PDOSs of W, N, and C (Figure 4), we can see that the conduction bottom Table 1 Calculated functionalization energies, exchange energies, and magnetic moments of the metal- functionalized g-C3N4 nanotubes Ef (eV) Eexch (meV)/pair Moment (μB/M) Ti 0.14 95 2.88 Cr 3.19 -98 - Mn 0.57 -7 - Fe 1.73 4 - Co 2.69 -21 - Ni 2.11 0 - W 4.05 180 3.82 states in both spin-up and spin-down bands are mainly attributed to the d electrons of W (Figure 4a), although the p electrons from C and N (Figure 5b, c, d, e, f) also partially contribute to these states. Interestingly, the s electrons of W only contribute to the spin-up CBB states (Figure 4a). The VBT states are mainly attributed to the d electrons of W and the p electrons of N (Figure 4a, d, e, f). The unsymmetrical features of the PDOSs reveal that W-d, W-s, C-p, and N-p electrons are Figure 3 Band structures of the W-functionalized g-C3N4-zz4. (a) spin-up and (b) spin down. The Fermi level is at 0 eV.