Density Functional Study of Fluorinated Single-Walled Silicon Carbide

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Density Functional Study of Fluorinated Single-Walled Silicon Carbide Nanotubes Xiao Wang and K. M. Liew* Department of Civil and Architectural Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR ABSTRACT: Structural, electronic, and magnetic properties of fluorine (F)-doped silicon carbide nanotubes (SiCNTs) are studied using density functional theory. It is found that F atoms prefer to adsorb on Si sites of both (8, 0) and (6, 6) SiCNTs. The chemisorption of the F atom on the Si site also induces a push down of the Fermi level for both types of SiCNTs, whereas the Fermi levels are lifted up when the F atom is attached on the C site of the tubes. Our calculation results show that fluorine adsorption for either a single silicon or a single carbon atom yields spontaneous magnetization, and the net magnetic moment is 1 μB. This may lead to a new approach to tune the electronic and magnetic properties of SiCNTs toward some nanoelectronics and metal-free magnetic materials.

1. INTRODUCTION Carbon nanotubes (CNTs), discovered by Ijima in 1991,1 are quasi-one-dimensional nanostructures with unique physical properties that make them ideal candidates for applications in nanoelectronics.2 4 To satisfy the requirements for actual applications, considerable experimental and theoretical efforts have been directed to tailoring and controlling their electronic properties through various methods.5 9 Among these methods, doping or functionalizing CNTs through chemical binding of atoms, molecules, or molecular groups is viewed as a viable approach to tailoring their electronic properties.7,10,11 Fluorination is one of the important reactions for CNTs which always serves as a route to further chemical modifications,12,13 and Touhara and Okino demonstrated that fluorine (F) atoms chemically absorbed onto the sidewall of CNTs can result in diverse electronic structures.14 Meanwhile, theoretical simulations have shown that the band gap of semiconductor zigzag carbon nanotubes is reduced by addition of the F atom on the walls of nanotubes. For metallic armchair nanotubes, the doubly degenerate states crossing the Fermi level were separated by the introduction of the F atom.15 Not only CNTs, fluorination also can increase the solubility of boron nitride nanotubes (BNNTs) and offers intermediates for further chemical modifications.16 Experimental and theoretical studies indicate that electronic properties of BNNTs can be controlled by F doping, involving both substitution and addition.17 19 Furthermore, ab initio calculations by Li et al. also found that the chemisorptions of F atoms on B atoms of BNNT can induce spontaneous magnetization.20 Thus, fluorination seems to be a promising way to control and adjust electronic properties of nanostructures and even induce magnetization. Silicon carbide nanotubes (SiCNTs), first synthesized in 2001,21 have many advantages over CNTs because they possess high reactivity of the exterior surface, facilitating sidewall decoration r 2011 American Chemical Society

and stability at high temperature.22 Due to their polar nature, SiCNTs can intrinsically be excellent sensors for detecting some harmful gases, such as CO, HCN,21 NO, NNO,23 NO2,24 and HCOH.25 Unlike CNTs, SiCNTs are semiconducting, regardless of chirality.22 When a series of transition metal atoms can be chemically adsorbed on the outer surface of SiCNTs, theoretical simulations have shown that they exhibit many interesting physical properties, such as metallic and magnetic properties.26 In addition, electronic properties of SiCNTs can be manipulated by adsorption of SiH3 and CH3 radicals which can form acceptor or donor levels, depending on their adsorption sites.27 Furthermore, F-based dry etching of bulk SiC has been extensively studied in recent years for widening their applications and fabricating a wide range of devices such as complementary metal oxide semiconductor field effect transistors and bipolar devices.28 30 Based on the first-principle calculation, it is found that the interstitial F introduces a rather shallow acceptor level and can be a novel p-type dopant for 4H-SiC.31 Thus, in view of the potential applications of fluorine-modified SiCNTs in nanoelectronic and spintronic devices, exploration of the novel properties of F-doped SiCNTs is likely to yield interesting results. Zhao et al. have shown that the adsorption of F on the Si atom of (8, 0) SiCNT can induce magnetization.32 However, detailed analyses of their electronic properties and other F-doped SiCNT types were not made. Hence, in this work, all structural, electronic, and magnetic properties of F adsorption on zigzag (8, 0) and armchair (6, 6) SiCNTs are systemically studied by the firstprinciple calculation. The spin-resolved electronic structures show that F adsorption on both SiCNTs, for either the carbon or silicon atom, can induce magnetization, while the in-gap states Received: August 19, 2011 Revised: December 19, 2011 Published: December 19, 2011 1702

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Figure 2. Band structure of (a) pristine (8, 0) SiCNT and (b) pristine (6, 6) SiCNT. The Fermi level is indicated with a black dotted line.

Figure 1. Schematic diagram of SiCNT (8, 0): yellow and gray spheres are silicon and carbon atoms. Two adsorption sites C and Si for the fluorine atom are indicated.

induced by F doping strongly depend on the doped site. Our results are likely to be useful for further studies related to SiCNT functionalization and construction of nanodevices.

2. COMPUTATIONAL DETAILS To address electronic and magnetic properties of F-doped SiCNTs, a typical zigzag (8, 0) and a typical armchair (6, 6) SiCNT are investigated by density functional theory (DFT) calculations, with spin polarization taken into account, as implemented in the Dmol3 package.33 35 Structure optimization and the corresponding total energy calculations of the most stable geometries are based on the generalized-gradient approximation (GGA) with the Perdew Burke Ernzerhof (PBE) correction.36 The all-electron calculations and a double numerical basis set plus polarization functional (DNP) are adopted.33 The k-point is set to 1 1 5 for all models, which bring out the convergence criterion of 2 10 6 a.u. on energy and electron density and that of maximum force of 0.001 Ha/Å. The selfconsistent field procedure is carried out with a convergence criterion of 10 6 a.u. on energy and electron density. Our supercells for SiCNTs (8, 0) and (6, 6) contain 64 and 72 atoms, respectively. Taking (8, 0) SiCNT as an example, two possible adsorption sites of the fluorine atom, adsorbing on either the carbon (F C Si configuration) or the silicon (F Si C configuration) atom, are considered. These two adsorption sites are labeled C and Si (Figure 1). The adsorption energy is defined as Eads = E(SiCNT+F) ESiCNT EF, where E(SiCNT+F) stands for the calculated total energy of the SiCNT with adsorbate on the tube wall. ESiCNT corresponds to the energy of a pristine SiCNT, and EF is the energy of a single adsorbate F atom. By definition, E < 0 corresponds to exothermic chemical bonding. 3. RESULTS AND DISCUSSION We first optimized the zigzag (8, 0) SiCNT. Similar to boron nitride nanotubes, after optimization of the nanotube, the more

electronegative atoms (C atoms) move radically outward, and the more electropositive (Si atoms) move inward, resulting in a rippled surface.37,38 The average Si C bond length is 1.787 Å, and the calculated average diameter is about 7.97 Å, in accordance with previously reported values.38 The calculated band structure of the (8, 0) SiCNT is shown in Figure 2(a). Like other previous studies,22,26,38 it is found that the zigzag SiCNT is a direct semiconductor, with a band gap of 1.415 eV. The minimum of the conduction band edge (CBM) and the maximum of the valence band edge (VBM) are at the Γ point. The structure of the optimized armchair (6, 6) SiCNT shows properties similar to those of the (8, 0) SiCNT. The average Si C bond length is about 1.798 Å, and the calculated average diameter is about 10.29 Å. An indirect semiconductor is obtained from the band structure of the (6, 6) SiCNT shown in Figure 2(b), where the CBM is at the X point while the VBM is along Γ X. However, it is noted that the X point in the Brillouin zone is projected onto the Γ point of the reduced Brillouin zone of this (6, 6) supercell, and therefore, the CBM shows at the Γ point here. The calculated indirect band gap is 2.03 eV, which is in good agreement with previous studies.38 Furthermore, it can be seen that all states of zigzag, as well as armchair SiCNT, are in 2-fold degeneracy, indicating there is no spin polarization in the pristine (8, 0) or (6, 6) nanotube. In addition, it is worth noting that GGA PBE results in a well-known and physically understood underestimation of the band gap. Various computational approaches can be used to correct the shortcomings of approximate DFT theories, including GW corrections39 or some exact Hartree Fock (HF) exchange in the modern hybrid density functional (B3LYP, PBE0, HSE, etc.)40 42 which can lead to substantially improved band gaps; however, they are significantly computationally demanding. Moreover, DFT predictions have proven useful for prediction of trends as shown by the numerous studies of the band gap of hydrogen passivated nanotubes,43,44 fluorine-doped nanotubes,45,46 and the first-row or different transition metal atom adsorbed SiCNTs.26,32 Hence, we believe it is reasonable to investigate the electronic and magnetic properties of F-doped SiCNT using the DFT theory here. Second, we examined a fluorine atom adsorption on C and Si sites of (8, 0) and (6, 6) SiCNTs. Our calculated adsorption energies (summarized in Table 1) indicate that fluorine adsorption on both C and Si sites is an exothermic process. When the adsorption is on the (8, 0) SiCNT, the value of Eads equals 1703

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to 1.996 and 4.734 eV for the F adatom adsorbed on C and Si sites, respectively. These results indicate that F atom adsorption on the Si site is more favorable by 2.738 eV compared with the F adatom on the C site in terms of energy. Compared with F-doped CNT, it is found that the binding energy for the most favorable fluorine chemisorption on the (8, 0) SiCNT (F doped on the Si site) is greatly larger than that of F attached to the C atom of the (10, 0) CNT (0.93 eV).15 It is known that SiCNTs are expected to have better reactivity than CNTs because of the sp3 hybrization and polar nature of silicon. Due to the difference in the electronegativity of Si and C atoms, the Si C bonds in the SiCNTs show the iconicity which localizes the electronic states.22 Hence, the strong Si F bond probably like B F in F-doped BNNTs47 is due to an ionic covalent resonance bond strengthening (i.e., either a double bond between two ions (Si+ and F ) or a single bond between Si and F atoms). In addition, as a typical chemical functionalization of CNTs, fluorine addition to CNTs forms new C F bonds; the C atom changes from sp2 to sp3 hybridization; and local structural deformation occurs. Similarly, F chemisorptions in SiCNTs also switch the tube surface atom bound to F from sp2 to sp3 hybridization and induce a local structural deformation. As shown in Figure 3, in both cases, the C or Si atom, on interaction with the F atom, is pulled outward from the tube wall, with the nearest-neighbor Si C bond’s length increasing from 1.771 and 1.790 Å of the pristine tube to 1.855 and 1.881 Å or 1.826 and 1.857 Å. Similar to adsorption of the F atom on the (8, 0) SiCNT, it is found that the adatom also prefers to adsorb on a Si atom of the (6, 6) SiCNT, with a much higher adsorption energy ( 4.775 eV) compared with that of the F atom adsorbed on a C atom of the tube ( 2.141). After adsorption, structural deformation is also found in the F-doped (6, 6) SiCNT due to the chemical Table 1. Optimized Adsorption Energies Eads and Charge Transfer of the F Atom on (8, 0) or (6, 6) SiCNTs adsorption site

Eads (eV)

Q (e)

binding between the F atom and C or Si atom. For instance, in the F Si C configuration, the Si F bond length is 1.636 Å, and one of the C Si bonds is stretched by 0.058 Å while the C Si distance of the other two near-neighbor atoms is elongated by 0.063 Å, as is found in the (8, 0) SiCNT. Further binding energy calculations indicate that the F C bond becomes a little stronger (by 0.145 eV) on the armchair (6, 6) SiCNT compared with the one on the (8, 0) zigzag SiCNT. The F Si binding energies are practically the same for both SiCNT chiralities. In general, the structures of fluorinated CNTs can be investigated by various experimental methods involving infrared (IR) and Raman spectroscopies, transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and X-ray photoemission spectroscopy (XPS).48 52 To give more information about the F-doped SiCNT which could be used to compare with experimental results, we also calculated the vibrational bands of these models. The vibrational bands of Si C bonds in the pristine (8, 0) SiCNT are concentrated in 820 cm 1 which is in agreement with experimental results.53 It broadens and shifts to 770 810 cm 1 as F is incorporated into the Si site of the nanotubes. These changes are due to the increasing presence of Si F vibrational modes that occur at 857 cm 1. When the F atom is doped into the C site of the nanotubes, the vibrational bands of Si C bonds also shift left to 760 cm 1, and 1208 cm 1 is assigned to the stretching of the C F bond. Similar results can be seen in the F-doped (6, 6) SiCNT which list in Table 2. The spin-polarized band structures of F doped on the C site and Si site of the (8, 0) SiCNT are given in Figure 4(a) and (b). It is clearly seen that the F atom adsorbed onto a C atom of the SiCNT results in a lift-up of the Fermi level of the system and emergence of a new band near the Fermi level, which is due to the fact that the dopant introduced by interacting with a carbon atom acts as an n-type defect. All bands with energies lower Table 2. Calculated Vibrational Frequency of F Si and F C Configurations in (8, 0) and (6, 6) SiCNTsa

SiCNT (8, 0)

C Si

1.996 4.734

0.181 0.208

mode

Si F

C F

(8, 0)

857

1208

SiCNT (6, 6)

C

2.141

0.187

(6, 6)

867

1199

Si

4.775

0.214

a

The units are in cm 1.

Figure 3. Two different configurations of the F-doped (8, 0) SiCNT. (a) One F atom adsorbed on the C site and (b) one F atom adsorbed on the Si site. 1704



Figure 4. Band structure of F adsorbed on (a) the C site and (b) the Si site of the (8, 0) SiCNT. The Fermi level is indicated with a black dotted line.

than 5.0 eV are fully occupied and thus do not contribute to spin polarization. However, the new band near the Fermi level is split into two branches. The spin-up branch is occupied, and the spindown branch is left empty, leading to a spontaneous polarization with a net magnetic moment of 1.0 μB. While the case of F doped onto the Si site of the (8, 0) SiCNT appears to have a different behavior, the Fermi level is being pushed down since the fluorine adatom forms a p-type defect here. Below the Fermi level, occupied bands of spin-up are more than those of spin-down, inducing a spontaneous polarization with a net magnetic moment of 1.0 μB. More interestingly, there is an apparent gap for the spin-up state and two bands crossing the Fermi level for the spin-down state, revealing half-metallic behavior. The band structure of F doped on the C site of the (6, 6) SiCNT also has a description similar to that of the (8, 0) SiCNT. As can be seen from Figure 5(a), adsorption of fluorine on the carbon atom also induces a lift up of the Fermi level. All bands well below 5.0 eV and above 3.6 eV are spin-degenerate and do not contribute to spin polarization. The most notable change takes place within the original band gap: the F doping induced one new pair of spin-polarized states which has almost dispersionless bands in the whole first Brillouin zone. The majority spin state is fully occupied, while the minority state is unoccupied, which leads to a strong spontaneous magnetization in the system. The net moment of F-doped SiCNT is 1.0 μB. However, the band structure of F doped on the Si site is slightly different from

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Figure 5. Band structure of F adsorbed on the (a) C site and (b) Si site of the (6, 6) SiCNT. The Fermi level is indicated with a black dotted line.

the (8, 0) SiCNT. The F atom acts as an acceptor, which induces the Fermi level shifts down. The two bands near the Fermi level are exchange split into two pairs of spin-polarized states with both majority spin states fully occupied, while only one of the two minority spin states is occupied, which magnetizes (1.0 μB) to the whole SiCNT. As compared to F-doped SiCNTs, the degenerate levels in the pure semiconducting and metallic CNTs also split by functionalization by the F atom. An acceptor level is found in the F-doped (10, 0) CNT, while the metallic nanotube remains metallic.15 In addition, to deal with the effect of the magnetic coupling between the adsorption-induced moments, we doubled the size of the supercell of calculated systems, and each supercell now contains two F atoms. For the case of F doped on the Si site of SiCNTs, we found that the magnetic moments of F-doped (8, 0) and (6, 6) are 1.7 and 1.1 μB, respectively. By evaluating the relative energies of ferromagnetic (FM) and antiferromagnetic (AFM) ordered states, we list the energy differences of these states in Table 3. It is found that in this case ferromagnetic alignment is always energetically favorable for both (8, 0) and (6, 6) SiCNTs. However, for configuration of F doped on the C site of SiCNTs, the obtained magnetic moments of F-doped (8, 0) and (6, 6) SiCNTs are zero, and the total energy of the states with antiferromagnetic coupling are found to be lower than that of the state with ferromagnetic coupling. That is to say, antiferromagnetic coupling is energetically favorable. The origin of magnetism in the F-SiCNTs is related to its electronic structures. Hence, we investigate the spin density of 1705



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states (DOSs) and the projected DOSs (PDOS) for the mostly influenced atoms of both configurations of the F-doped (8, 0) SiCNT to further understand the spontaneous magnetization of the F-doped SiCNT. Here, we focus on magnetic behavior of the F-doped (8, 0) SiCNT since the F-doped (6, 6) SiCNT has nearly the same magnetic behavior. The spin DOS and PDOS of the F Si C configuration are plotted in Figure 6(b). The spin-up peaks near the Fermi level are all below the Fermi level, whereas only one spin-down peak crosses over the Fermi level. According to the previous discussion, the spin-down peak includes two electronic states which are, however, not visible due to the Gaussian broadening applied to the DOS.20 It is found that the spin-down peak mainly arises from three C atoms (C1, C2, and C3) adjacent to the fluorinated Si atom (Si1), as shown in Figure 6(a), whereas contributions of F and Si1 to spin-down states are very small. To understand the charge distribution of the doped system, we also studied the occupied and unoccupied states of F-doped SiCNTs. It is well-known that the charge states of the pristine SiCNTs near the Fermi level are highly localized but have different characteristics. Typically, the highest occupied molecular orbital (HOMO) corresponds to isolated electron pairs localized at the C atoms and has the spindle-shaped scheme like pz orbitals, whereas the lowest unoccupied molecular orbital (LUMO) is present as π states localized at the Si C pair along the tube axis and is contributed by the Si-3p (major) and C-2p (minor) states. For the F-adsorbed (8, 0) SiCNT with F Si C configuration, parts of charge on the highest-energy occupied 2p orbitals of C atoms around Si1 move to the F atom, and its HOMO shown in Figure 7(a) mainly consists of the pz states of the F atom and the isolated electron pairs at C atoms. The total net magnetic moment is mainly contributed by the unpaired 2p

orbitals of the three C atoms bonding to the fluorinated Si atom, which is in good agreement with Zhao’s work.32 Contributions of these carbon atoms, C1, C2, and C3, to the magnetic moment are

Figure 7. HOMO band states for the (8, 0) SiCNT with the F adatom adsorbed on the (a) Si site and (b) C site. The isosurfaces of them with values of 0.02 and 0.02 a.u. are depicted in blue and yellow, respectively.

Table 3. Values of the Energy Difference Between the AFM and FM States (ΔE = EAFM EFM) and the Total Net Magnetic Moments (M), Calculated for the (8, 0) and (6, 6) SiCNTs with Two F Atoms Doping ΔE (meV) doping sites Si site C site

M (μB)

(8, 0)

(6, 6)

10.1 13.2

3.6 1.5

(8, 0)

(6, 6)

1.7 0

1.1 0

Figure 8. Isosurface of the difference spin density at the isovalue of 0.02 e/Å3 for the F Si C configuration of the (8, 0) SiCNT system.

Figure 6. (a) Local map of F Si C configuration of the (8, 0) SiCNT system near the adsorbing site. (b) DOS and PDOS (only the more interesting contributions) of the F Si C configuration. The Fermi level is indicated by dotted the line. 1706



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’ AUTHOR INFORMATION Corresponding Author

*Tel./Fax: +852 34426581/+852 34420426. E-mail address: [email protected].

’ ACKNOWLEDGMENT The work described in this paper was fully supported by Grants from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 9041674, CityU 118411), and the China National Natural Science Foundation (Grant No. 11172253). ’ REFERENCES

Figure 9. Isosurface of the difference spin density at the isovalue of 0.02 e/Å3 for th F C Si configuration of the (8, 0) SiCNT system.

0.093, 0.094, and 0.094 μB, respectively, which are calculated by Hirshfeld population.54 This is also consistent with the isosurfaces of the spin density of the F Si C configuration, as shown in Figure 8, where the net spin mainly localizes on the three C atoms, around Si1. On the contrary, in F C Si configuration, the HOMO is highly localized and composed of the remaining sp2 orbital of the F adatom and π orbitals of the adjacent Si and C atoms. The spin density of the F C Si configuration (Figure 9) also shows that the magnetization density has a very localized nature, mainly localized on adatom F and the atoms near it (including C1, Si1, Si2, and Si3), which shows great accordance with the results of its HOMO. The magnetic moments are 0.098, 0.075, 0.163, 0.164, and 0.105 μB for F, C1, Si1, Si2, and Si3, respectively.

4. CONCLUSION In summary, using first-principle calculations, we carried out a systematic investigation of fluorinated SiCNTs. We show that the adsorption of F on Si sites is more favorable than that on C sites due to the large electronegativity of F for both zigzag SiCNT (8, 0) and armchair SiCNT (6, 6). Our results also indicate that attachment of the F atom on walls of SiCNTs gives rise to significant changes in electronic and magnetic properties of SiCNTs. In the case of F adsorbed on the C site of the two types of SiCNTs, the Fermi level of the systems lifts up due to the n-type introduction of the fluorine atom. This induces spinpolarized in-gap states, and the total magnetization of the nanotube is 1 μB, mainly localized on the dopant F, bonded C, and three Si atoms interacted with C. When F is adsorbed on the Si site of both SiCNTs, an acceptor level is found after the F-functionalization, which comes mainly from the unpaired p orbitals of C atoms around the adatom. The interaction between the tubes and F atom makes the degenerate levels in the SiCNT split. For the band structure of the zigzag (8, 0) SiCNT, there is an apparent gap for the spin-up state and two bands crossing the Fermi level for the spin-down state, revealing half-metallic behavior, whereas the armchair (6, 6) SiCNT still shows semiconducting behavior. On the basis of the results of our calculations, it can be expected that the functionalization of silicon carbide nanotubes by the fluorine atom can be an effective way of modifying their electronic and magnetic properties.

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Density Functional Study of Fluorinated Single-Walled Silicon Carbide

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