Pan et al. Nanoscale Research Letters 2011, 6:97 http://www.nanoscalereslett.com/content/6/1/97 spin-polarized in the spin-up states. The coupling among the spin-polarized W-d, W-s, C-p and N-p elec- trons results in the alignment of magnetic moments and thus the ferromagnetism of W-functionalized graphitic carbon nitride nanotubes. The magnetic moment is about 3.82 μB per W (see Table 1). The calculated band structures and PDOSs suggest that the ferromagnetism in the W-functionalized nanotube can be explained by the carrier-mediated interaction because the impurity bands are close to the CBBs, which is one of important mechanisms for the ferromagnetism in dilute magnetic semiconductors [1,3,14-16,35]. The carriers, i.e., elec- trons, introduced by the functionalization of W, mediate the alignment of the magnetic moments via the hybridi- zation and coupling of spin-polarized p and d electrons (Figure 4). Figure 5 shows the calculated electronic structures of Ti-functionalized nanotube. The g-C3N4-zz4-Ti is also a heavily-doped n-type semiconductor with the direct gaps of approximately 1.4 and 1.6 eV in spin-up and spin-down band structures (Figure 5a,b). Similar to those of W-functionalized nanotube, the dispersion fea- tures in the both of the spin-up and spin-down band structures show the improvement of carrier mobility Page 5 of 7 due to the functionalization. The CBB of the spin-up and spin-down states of g-C3N4-zz4-Ti are also dispersion, except flat levels appear above the bottoms, indicating the coupling between the localized spin-polarization and carrier, which leads to the ferromagnetism of graphitic carbon nitride nanotubes. The ferromagnetism of the Ti-functionalized nanotube is further confirmed by the unsymmetrical TDOS (Figure 5c). The spin-polarized impurity states within the CBB are mainly attributed to the Ti-d electrons (see Figure 5d). Similar to g-C3N4- zz4-W, the coupling among the spin-polarized Ti-d, C-p, and N-p electrons results in the alignment of the magnetic moments, and thus the ferromagnetism of Ti- functionalized graphitic carbon nitride nanotube (Figure 5d). The magnetic moment is about 2.88 μB per Ti atom (Table 1). The ferromagnetism of Ti-functionalized nanotube is also attributed to carrier-mediated interac- tion because the impurity bands are close to the band edges and the Fermi level is within the conduction bands [15,35], In contrast to W- and Ti-functionalized nanotubes, the Cr-, Mn-, Co-, and Ni-functionalized nanotubes are anti-ferromagnetic, as indicated by the calculated elec- tronic structures (Figure 6). The PDOS analysis reveals Figure 6 Calculated TDOSs of graphitic carbon nitride nanotubes functionalized by metals. (a) Cr, (b) Mn, (c) Co, and (d) Ni. The Fermi level is shifted to 0 eV (dot-dash line).