Metal-functionalized single-walled graphitic carbon nitride nanotubes: a first-principles study on magnetic property
© Pan et al; licensee Springer. 2011
Received: 19 September 2010
Accepted: 19 January 2011
Published: 19 January 2011
The magnetic properties of metal-functionalized graphitic carbon nitride nanotubes were investigated based on first-principles calculations. The graphitic carbon nitride nanotube can be either ferromagnetic or antiferromagnetic by functionalizing with different metal atoms. The W- and Ti-functionalized nanotubes are ferromagnetic, which are attributed to carrier-mediated interactions because of the coupling between the spin-polarized d and p electrons and the formation of the impurity bands close to the band edges. However, Cr-, Mn-, Co-, and Ni-functionalized nanotubes are antiferromagnetic because of the anti-alignment of the magnetic moments between neighboring metal atoms. The functionalized nanotubes may be used in spintronics and hydrogen storage.
Applications of spin-based devices are so far limited to information storage, and most of the spin-related materials and devices still rely primarily on the spontaneous ordering of spins in the form of different types of magnetic materials. This situation is expected to change with successful development of spin-based electronics, or spintronics, the new kind of electronics that seeks to exploit, in addition to the charge degree of freedom, the spin of the carriers . The primary requirement for spintronics is to have a system that can generate a current of spin-polarized electrons. To enable a host of new device applications, it is necessary to develop materials which should have (a) high Curie temperature, (b) controllable carrier density and mobility, and (c) easily magnetic doping . Ideally, a semiconductor can be made magnetic by including ions that have a net spin into a semiconductor . The doped semiconductors are referred as dilute magnetic semiconductors (DMSs) because only a small amount of magnetic ions is required to make the semiconductor magnetic. In recent years, considerable efforts have been devoted to the study of DMS materials. The search for DMSs has been focused on cation substitution of semiconductors with transition metal (TM) elements, where the Curie temperature of the doped semiconductor can be below or above room temperature, depending on the host materials, carrier concentration, and doping elements [4–11]. However, the clustering of transition metal and the formation of secondary phases in DMSs are obstacles to their practical applications in spintronics . Although the cation substitution with two different elements and anion substitution may overcome the clustering issue [7, 13–15], the observation of ferromagnetism in undoped semiconductor nanoparticles suggested that doping-induced defects also contributed to the magnetic moment [16–19]. Therefore, for the practical application of DMSs, a crucial prerequisite is to overcome the clustering of TM in DMS and control the magnetic property, which may be solved by cation-anion codoping method .
The first-principles calculation was carried out based on the density function theory (DFT) and the Perdew-Burke-Eznerhof generalized gradient approximation (PBE-GGA) [28, 29]. The projector augmented wave (PAW) scheme as incorporated in the Vienna ab initio simulation package (VASP) was used [30, 31]. The Monkhorst and Pack scheme of k-point sampling was used for integration over the first Brillouin zone . The geometry of the g-C3N4 monolayer was first optimized to obtain the lattice constants with a vacuum space of 12 Å used to minimize the inter-layer interaction. A 5 × 1 × 1 grid for k-point sampling and an energy cutoff of 400 eV were used for the bulk and monolayer. The g-C3N4 nanotube is obtained by rolling up the g-C3N4 monolayer into a cylinder along the axial (x) direction (Figure 1), which is defined as a zigzag tube, adopting a similar terminology used in other nanotubes [33, 34]. In our study, we chose a nanotube with an index of (4, 0), labeled as g-C3N4-zz4. A 1 × 1 × 3 grid for k-point sampling and an energy cutoff of 400 eV were consistently used in our calculations. Excellent convergence was obtained using these parameters, and the total energy was converged to 2.0 × 10-5 eV/atom. A large supercell dimension with a wall-wall distance of 10 Å in the plane perpendicular to the tube axis was used to avoid any interaction between the nanotube and its images in neighboring cells.
where E tot(tube + doping) and E tot(tube) are total energies of the g-C3N4 with and without doping, respectively; μ doping is the chemical potential of the functionalizing metal element, calculated from the metal bulk, and n is the number of the metal atoms.
Results and discussion
Calculated functionalization energies, exchange energies, and magnetic moments of the metal-functionalized g-C3N4 nanotubes
E f (eV)
E exch (meV)/pair
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 features in the both of the spin-up and spin-down band structures show the improvement of carrier mobility 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 interaction because the impurity bands are close to the band edges and the Fermi level is within the conduction bands [15, 35],
In summary, we studied the magnetic properties of metal-functionalized graphitic carbon nitride nanotubes based on the first-principles calculations. The results show that magnetic properties strongly depend on the coupling between the d electrons of metal atoms and p electrons of C or N atoms. The coupling and hybridization between the spin-polarized d electrons of W or Ti atoms and p electrons of C and N atoms result in the ferromagnetic W- and Ti-functionalized nanotubes. Although the d electrons of Cr, Mn, Co, and Ni atoms are spin-polarized, the functionalized nanotubes are anti-ferromagnetic because the spin-polarized d electrons are anti-parallel due to lack of coupling. The ferromagnetic nanotubes may be used in spintronics and other magnetic devices. The functionalized nanotubes may also find potential applications, such as hydrogen and energy storage [36–38].
conduction band bottom
dilute magnetic semiconductors
- g-C3N4 :
graphitic carbon nitride
projector augmented wave
partial density of states
Vienna ab initio simulation package
valence band top.
This work was supported by the Visiting Investigator Program "Size Effects in Small Scale Materials" hosted at the Institute of High Performance Computing of the Agency for Science, Technology and Research (A*STAR) in Singapore. The DFT calculations were performed at the A*STAR Computational Resource Center (A*CRC).
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