ARTICLE pubs.acs.org/JPCC
Are Azafullerene Encapsulated Single-Walled Carbon Nanotubes n-Type Semiconductors? Shuang Ni, Wei He, Zhenyu Li, and Jinlong Yang* Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026 China ABSTRACT: There is a controversy in the literature about the transport behavior of azafullerene encapsulated single-walled carbon nanotubes (SWCNTs). Both n-type and p-type semiconducting behaviors have been suggested experimentally. To clarify this issue, we study the electronic structure of C59N nanopeapods with density functional theory. It turns out that C59N doping in pristine SWCNTs does not change the carrier type, although it is possible to change the transport behavior from p-type to n-type for SWCNTs with defects via a new mechanism.
’ INTRODUCTION Molecule doping is a possible way to modify the electronic structure of single-walled carbon nanotubes (SWCNTs). In 1998, C60 molecules were inserted into SWCNTs for the first time.1 Later, larger fullerenes2 and metallofullerenes3 were also encapsulated into nanotubes, forming a large variety of “peapod” structures. Recently, researchers have become widely interested in azafullerene peapods,4,5 because it is suggested that azafullerene (C59N or C69N) doping makes SWCNTs show an n-type semiconducting behavior.6,7 In contrast, C60 or C70 fullerene peapods are typically p-type semiconductors. This difference is attributed to the charge transfer between azafullerenes and SWCNTs. However, based on UV vis-NIR absorption and photoluminescence spectroscopy, Iizumi et al. suggested that interaction between C59N and the outer SWCNT is very weak and C59N peapods maintain the p-type behavior.8 In this article, we investigate this discrepancy using density functional theory. We find that these experimental observations can be understood if defects are introduced to SWCNTs. A new mechanism to change the carrier type has been proposed. ’ COMPUTATIONAL DETAILS Calculations were carried out with the density functional theory implemented in the Vienna Ab-initio Simulation Package (VASP).9,10 The Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional within the generalized gradient approximation11 and the projector augmented-wave pseudopotential12,13 were adopted. A (16,0) SWCNT with a diameter of 12.4 Å was adopted in this study, with the lattice parameter along the tube axis (the z direction) optimized to be 4.28 Å. Each SWCNT was separated by an about 10 Å vacuum. A cubic 23.0 23.0 17.13 Å3 super r 2011 American Chemical Society
cell was used for C60 and C59N encapsulation in pristine SWCNTs. A 23.0 23.0 25.70 Å3 super cell was used in other cases. All geometry structures were fully relaxed until the forces on each atom are less than 0.01 eV/Å. Geometry optimizations were performed with 1 1 1 k-point sampling, while static calculations were done with a 1 1 5 or 1 1 3 Monkhorst Pack k-point grid14 for small and large super cells, respectively.
’ RESULTS AND DISCUSSION A C59N dimer with two azafullerene cages joining together through a single C C bond is more stable than the monomer.15 Therefore, we first consider a C59N dimer encapsulated SWCNT (Figure 1a). A semiconducting (16,0) SWCNT, which has a similar radius to those used in experiments, is chosen as a model system in this study. Our test calculation for a larger SWCNT gives similar results. It turns out that the interaction between the C59N dimer and the outer carbon nanotube is really weak. The resulting band structure is just a simple combination of two parts (Figure 2a,b), where those almost dispersionless horizontal energy bands come from the C59N dimer. These molecular bands are not in the band gap, and they do not strongly affect the transport properties. Although not as stable as the dimer, the C59N monomer has also been found in SWCNTs.8,16 With C59N monomer encapsulation, we obtain a new band appearing in the energy gap (the blue line in Figure 2c), which is half-occupied and spatially localized on C59N. Because this band is still far away from the edges of Received: February 23, 2011 Revised: June 1, 2011 Published: June 02, 2011 12760
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Figure 2. Band structures of (a) a pristine (16,0) SWCNT, (b) a C59N dimer encapsulated in a (16,0) SWCNT, (c) a C59N monomer encapsulated in a (16,0) SWCNT, and (d) C60 encapsulated in a SWCNT(16,0).
Figure 1. Geometric structures of (a) a C59N dimer encapsulated in a (16,0) SWCNT, (b) a C59N monomer encapsulated in a (16,0) SWCNT, (c) C60 encapsulated in a (16,0) SWCNT, (d) a (16,0) SWCNT adsorbed with an oxygen molecule, (e) a (16,0) SWCNT with a divacancy defect, (f) a C59N monomer encapsulated in a (16,0) SWCNT adsorbed with an oxygen molecule, (g) a C59N monomer encapsulated in a (16,0) SWCNT with a divacancy defect, (h) C60 encapsulated in a (16,0) SWCNT adsorbed with an oxygen molecule, (i) C60 encapsulated in a (16,0) SWCNT with a divacancy defect. Gray dots and lines represent carbon atoms and a C C bond, blue dots represent nitrogen atoms, violet dots represent oxygen atoms, and red dots and lines mark the divacancy defect area and also represent carbon atoms and C C bonds.
both conduction and valence bands, it is not expected to generate a p-type or n-type carrier effectively. The band structure of an SWCNT encapsulated with C60 molecules (Figure 2d) is similar to that of the C59N dimer peapod, with doping states outside the band gap. Therefore, in pristine SWCNTs, doping C59N/C60 molecules does not significantly modify the transport properties. Our calculations also support the experimental observations that interaction between C59N and the SWCNT is very weak.8 In experiment, the as-made SWCNT is usually a p-type semiconductor,17,18 which indicates that experimentally available SWCNTs are not perfect. To consider this effect, we introduce two typical defects, oxygen molecule adsorption and divacancy. In the first case, the oxygen molecule is set to be parallel to a certain C C bond (Figure 1d). The optimized distance between the oxygen molecule and the tube is about 3.2 Å. The band structure is shown in Figure 3a. Partially occupied molecular energy bands (violet bands in Figure 3a) appear in the middle of the gap. The oxygen bands are completely localized and close to
the valence bands. Therefore, the system is a p-type semiconductor indeed. The calculated band structures of SWCNTs with divacancy defects is similar to that obtained by Savas Berber et al.19 A new unoccupied energy band (red line in Figure 3b) appears in the energy gap. This defect band is close to the valence band edge. Therefore, electrons can be excited from the valence band to it. As shown in Figure 4, although the distance between two neighboring defects is already very large, electrons in this defect band are still delocalized. Of course, holes in the valence band have much smaller effective masses, and they thus are more conductive than electrons in the defect band. Therefore, the transport should still be p-type dominated. We note that both defects considered here lead to an overall p-type semiconducting behavior, but through different mechanisms. When C59N is inserted into an oxygen adsorbed SWCNT (Figure 1f), a half-occupied completely localized band is introduced into the gap (the blue band in Figure 3c). This new band is accidently degenerated with the oxygen bands, and the system remains a p-type semiconductor. However, when C59N is encapsulated in a nanotube with divacancies (Figure 1g), there is a half-occupied molecular band (blue line in Figure 3d) just below the defect band. Now, the system has two tendencies: One is that electrons can jump from the valence bands to the molecular band. This makes the system a p-type semiconductor, because the C59N band is completely localized. The other tendency is that electrons can also jump from the C59N band to the defect band, which makes the system show an n-type behavior, because the defect band is more conductive than the C59N band. The two tendencies will compete and finally determine the property of the system. In our case, the energy gap between the valence band and the C59N band is much larger than that between the C59N band and the defect band. Therefore, the system will be an n-type semiconductor. Now, with a defective SWCNT, the experimentally observed C59N doping induced transition from p-type to n-type can be understood. We note that C60 encapsulation in the oxygen adsorbed nanotube and the defective nanotube (Figure 1h,i) cannot lead to such a behavior. The C60 molecular bands are far away from both the oxygen band and the defect band (Figure 3e,f). 12761
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even enhances the p-type characteristics. These results provide an explanation for the controversy in experiment.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is partially supported by NSFC (20933006, 20873129); by the National Key Basic Research Program (2011CB921404); and by USTC-SCC, SCCAS, and Shanghai Supercomputer Center. ’ REFERENCES Figure 3. Band structures of (a) a (16,0) SWCNT with molecular oxygen adsorption, (b) a (16,0) SWCNT with a divacancy defect, (c) a C59N monomer encapsulated (16,0) SWCNT with oxygen adsorption, (d) a C59N monomer encapsulated (16,0) SWCNT with a divacancy defect, (e) a C60 encapsulated (16,0) SWCNT with oxygen adsorption, and (f) a C60 encapsulated (16,0) SWCNT with a divacancy defect.
Figure 4. Charge density isosurface of the defect band in Figure 3b; F = 0.001 e/Å3.
Thus, the system remains a p-type semiconductor. A slight energy gap decrease can be found when C60 was inserted into the defective nanotubes, which enhances the p-type behavior. Our results give a new method to change a system from a p-type to an n-type semiconductor: First, certain defects are introduced to form a slightly extended defect band; then, some electron donors are brought into the system, making a donor state just below the defect band. When the donor state and the defect band are close enough, there can be enough electron carriers in the defect band, which makes the system an n-type semiconductor.
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’ CONCLUSION In summary, electronic properties of SWCNTs encapsulating C59N molecules and C60 molecules are calculated. In pristine SWCNTs, doping C59N and C60 molecules cannot make the system an n-type/p-type semiconductor. With divacancies on the SWCNTs, C59N monomer doping can change the system from a p-type to an n-type semiconductor, while C60 molecule doping 12762
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