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Ab initio study of an organic molecule interacting with a silicon-doped carbon nanotube
Diamond and Related Materials 12 (2003) 861–863
Ab initio study of an organic molecule interacting with a silicon-doped carbon nanotube ˆ Solange B. Fagana, R. Motaa,*, R.J. Baierleb, Antonio J.R. da Silvac, A. Fazzioc b
a ´ Departamento de Fısica, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil ˆ ´ Departamento de Ciencias Exatas, Centro Universitario Franciscano, 97010-032 Santa Maria, RS, Brazil c ´ ˜ Paulo, CP 66318, 05315-970 Sao ˜ Paulo, SP, Brazil Instituto de Fısica, Universidade de Sao
Abstract The electronic structure of an organic molecule interacting with a Si-doped single-wall carbon nanotube is studied through first principles calculations based on density functional theory. The silicon substitutional doping on (10, 0) semiconductor carbon nanotube introduces an empty level in the gap. Differently to the weak physisorption resulting of direct interaction of an organic molecule to the tube surface, the binding energy of the molecule through the Si-site is shown to be much stronger. The band structure for the resulting system presents one half-filled level in the gap. A study of one particular single organic molecule (S2C6H5) connected to the tube through a Si-site is presented. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Nanotubes; Chemisorption; Band structure; Electronic device structure
1. Introduction The possibility of modifying the properties of singlewall carbon nanotubes (SWCN) by doping has become a field of growing interest w1,2x. In a previous paper w1x, we have presented the electronic and structural properties of silicon-doped carbon nanotubes. It was predicted that the Si substitutional doping may impose changes in the chemical reactivity and hence in the interaction of the tube with foreign atoms and molecules through the Si site. This would permit another approach to create novel nanotube-related materials consisting in modifying the ‘functionality’ of carbon nanotubes by, first, substitutional doping of carbon by silicon atoms and, subsequently, trapping of foreign atoms or molecules at these Si sites. The doping of fullerenes with Si atom has been recently reported w3x through the synthesis of the C59Si hetero-fullerenes. This is an interesting result since silicon is known to strongly prefer sp3-like bonding. Another relevant feature is that this material is shown to be very reactive at its Si site. Although no experimental report of silicon-doped carbon nanotubes has *Corresponding author. Tel.: q55-559-972-7545; fax: q55-552208-032. E-mail address: [email protected] (R. Mota).
been published, we have many reasons to believe that ¨ it is perfectly feasible. For example, recently Gulseren w x et al. 4 have investigated the adsorption of single atoms on SWCN from first principles and they showed that the chemical reactivity of the nanotube could be modified by radial deformation. The binding energy on the high curvature sites of the deformed tube increases with increasing radial deformation. This is one possible mechanism to promote the Si substitutional process. Once inserted, the Si substitutional atom should be quite stable. Then, similarly to C59Si, the Si-doped SWCN at their Si sites should offer a preferential path towards the adsorption of atoms and molecules. Many papers have been published recently reporting effects on the electronic properties and modifications of the conductance of the nanotubes by direct adsorption of atoms or molecules on SWCN w5x. We must take into account that most atoms or molecules, when adsorbed directly on SWCN, present weak binding energies. In a recent work we have presented a proposal for altering the electronic properties of SWCN through chemical binding of atoms or molecules (F, Cl, H, CH3, and SiH3) on Si-doped SWCN w6x. We observed that, once the Si is inserted in the SWCN, the binding of these atoms or molecules to the Si atom is much stronger than to one of the SWCN C atoms.
0925-9635/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-9635(02)00394-1
862
S.B. Fagan et al. / Diamond and Related Materials 12 (2003) 861–863
we use 80 atoms for the carbon nanotubes. The distance along the nanotube axis between a Si atom and its ˚ The relaxed atomic image in the next cell is 8.52 A. structures are obtained by a minimization of the total energy using Hellmann–Feynman forces including Pullay-like corrections. Structural optimizations were performed using the conjugate gradient algorithm until the ˚ residual forces were below 0.05 eVyA. 3. Results and discussion
Fig. 1. Optimized structure for an organic molecule interacting with a silicon-doped SWCN.
Having in mind that both SWCN and organic molecules might be important components in future nanodevices, this feature that a substitutional Si in SWCN provides a strong binding site may be very useful in order to connect these device components. In this paper we present a study of one particular single organic molecule (S2C6H5) connected to the tube through a Sisite. This molecule was chosen due to its possible use as wire in nanodevices w7x. 2. Method The interaction of the organic molecule with a Sidoped SWCN is studied by means of first principles density-functional theory calculations. We have used the SIESTA code w8x, which performs this calculation solving the standard Kohn–Sham (KS) equations. The calculations are done using the local density approximation w9x for the exchange-correlation term, as parameterized by Perdew and Zunger w10x. The standard norm-conserving Troullier–Martins pseudopotentials w11x are used. The KS orbitals are expanded using a linear combination of numerical pseudoatomic orbitals, similar to the ones proposed by Sankey and Niklewski w12x. In all calculations we have used a split-valence double-zeta basis set with polarization function w13x. A cutoff of 120 Ry for the grid integration was utilized to represent the charge density. Our calculations were performed using a (10, 0) semiconductor SWCN (diam˚ We use periodic-boundary conditions eter of 8.14 A). and a supercell approximation with lateral separation of ˚ between tube centers to make sure that they do 25 A not interact with each other. We have used three Monkhorst-Pack k-points for the Brillouin zone integration along the tube axis, which is shown to give a good approximation for (8, 0) nanotube w14x. In the supercell
In a previous paper w1x, we have demonstrated that Si substitutional doping introduces an empty level in the gap, which is intimately related to an outward relaxation of the Si atom through which it acquires a more sp3 configuration. This increases the Si reactivity, since it prefers to become fourfold coordinated. Approximating the organic molecule (S2C6H5) (Fig. 1) to the tube an adsorption through the Si-site is studied for the optimized structure. From the figure we can observe that the Si atom presents an outward local distortion ˚ This along the radial direction of approximately 0.8 A. same feature has been observed for other atoms or molecules adsorbed through the same process w6x. The binding energy of the Si-tube system to the organic molecule, between Si-site and S atom, is 3.06 eV. It is remarkable that the substantial amount to create the substitutional Si defect (3.10 eV w1x) is compensated for this relatively strong binding energy. Fig. 2 shows the band structures for (a) a pure carbon nanotube, (b) Si-doped SWCN, and (c) the organic molecule interacting with the Si-doped system. The pure SWCN is a (10, 0) semiconductor with a gap approximately 0.8 eV. As pointed out before w1x when a C atom is replaced by a Si atom in the tube and the system is allowed to relax, an empty level close to the bottom of the conduction band is observed (Fig. 2b) with a strong
Fig. 2. Band structure for: (a) (10, 0) pure carbon nanotube; (b) (10, 0) silicon-doped carbon nanotube and (c) (10, 0) silicon-doped carbon nanotube interacting with an organic molecule.
S.B. Fagan et al. / Diamond and Related Materials 12 (2003) 861–863
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interactions between organic molecules and SWCN in chemisorption regime, resulting from the presence of the Si-sites acting as ‘bridges’. Acknowledgments We would like to thank the CENAPAD-SP for computational time. This research is supported by Brazilian agencies: CNPq, CAPES, FAPESP and FAPERGS. References
Fig. 3. Electronic charge density for the orbital at the Brillouin-zone boundary located at the Fermi level (dashed line in Fig. 2). (a) Frontal and (b) lateral view.
Si character. In Fig. 2c, a half-filled level is seen 0.27 eV above the top of the valence band. This half-filled level is a common characteristic observed for all other systems studied w6x. The electronic charge density of the half-filled level is located around the defect region in the nanotube, as can be seen in Fig. 3. 4. Conclusions Considering that both SWCN and organic molecules might be important components in future nanodevices, this feature, that a substitutional Si in SWCN provides a strong binding site, is demonstrated to be very useful in order to connect these device components. A particular single organic molecule (S2C6H5) connected to the tube through a Si-site was chosen due to its possible use as wire in nanodevices. Differently to the weak binding energy associated to the physisorption of the molecule directly on the SWCN surface, when the interaction of the molecule with the surface is through the Si-site, the binding energy is shown to be much stronger. The resulting band structure shows one halffilled level 0.27 eV above the top of the valence band. These results permit a better understanding of the
w1x R.J. Baierle, S.B. Fagan, R. Mota, A.J.R. da Silva, A. Fazzio, Phys. Rev. B 64 (2001) 85413. w2x (a) J. Yang, et al., Phys. Rev. B 64 (2001) 85420 (b) J. Zhao, A. Buldum, J. Han, J.P. Lu, Phys. Rev. Lett. 85 (2000) 1706. w3x (a) J.L. Fye, M.F. Jarrold, J. Phys. Chem. A 101 (1997) 1836 (b) T. Kimura, T. Sugai, H. Shinohara, Chem. Phys. Lett. 256 (1996) 269 ´ et al., Phys. Rev. Lett. 80 (c) C. Ray, M. Pellarin, J.L. Lerme, (1998) 5365 (d) I. Fuks, I.V. Kityk, J. Kasperczyk, J. Berdowsky, I. Schirmer, Chem. Phys. Lett. 353 (2002) 7. w4x O. Gulseren, ¨ T. Yildirim, S. Ciraci, Phys. Rev. Lett. 87 (2001) 116802. w5x (a) J. Kong, N.R. Franklin, C. Zhou, et al., Science 287 (2000) 622 (b) S. Peng, K. Cho, Nanotechnology 11 (2000) 57 (c) E.T. Mickelson, C.B. Huffman, A.G. Rinzler, R.E. Smalley, R.H. Hauge, J.L. Margrave, Chem. Phys. Lett. 296 (1998) 188. w6x S.B. Fagan, A.J.R. da Silva, R. Mota, R.J. Baierle, A. Fazzio, Phys. Rev. B 67 (2003) 33405. w7x (a) C. Joachim, J.K. Gimzewski, A. Aviram, Nature 408 (2000) 541 (b) M. Dorogi, J. Gomes, R. Osifchin, R.P. Andres, R. Reifenberger, Phys. Rev. B 52 (1995) 9071. w8x (a) P. Ordejon, ´ E. Artacho, J.M. Soler, Phys. Rev. B 53 (1996) 10441 ´ ´ E. Artacho, J.M. Soler, Int. (b) D. Sanchez-Portal, P. Ordejon, J. Quantum Chem. 65 (1997) 453. w9x D.M. Ceperley, B.J. Alder, Phys. Rev. Lett. 45 (1980) 566. w10x J.P. Perdew, A. Zunger, Phys. Rev. B 23 (1981) 5048. w11x N. Troullier, J.L. Martins, Phys. Rev. B 43 (1991) 1993. w12x O.F. Sankey, D.J. Niklewski, Phys. Rev. B 40 (1989) 3979. w13x E. Artacho, D. Sanchez-Portal, ´ ´ A. Garcia, J.M. P. Ordejon, Soler, Phys. Status Solidi B 215 (1999) 809. w14x D. Srivastava, M. Menon, K. Cho, Phys. Rev. Lett. 83 (1999) 2973.
Ab initio study of an organic molecule interacting with a silicon-doped carbon nanotube ˆ Solange B. Fagana, R. Motaa,*, R.J. Baierleb, Antonio J.R. da Silvac, A. Fazzioc b
a ´ Departamento de Fısica, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil ˆ ´ Departamento de Ciencias Exatas, Centro Universitario Franciscano, 97010-032 Santa Maria, RS, Brazil c ´ ˜ Paulo, CP 66318, 05315-970 Sao ˜ Paulo, SP, Brazil Instituto de Fısica, Universidade de Sao
Abstract The electronic structure of an organic molecule interacting with a Si-doped single-wall carbon nanotube is studied through first principles calculations based on density functional theory. The silicon substitutional doping on (10, 0) semiconductor carbon nanotube introduces an empty level in the gap. Differently to the weak physisorption resulting of direct interaction of an organic molecule to the tube surface, the binding energy of the molecule through the Si-site is shown to be much stronger. The band structure for the resulting system presents one half-filled level in the gap. A study of one particular single organic molecule (S2C6H5) connected to the tube through a Si-site is presented. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Nanotubes; Chemisorption; Band structure; Electronic device structure
1. Introduction The possibility of modifying the properties of singlewall carbon nanotubes (SWCN) by doping has become a field of growing interest w1,2x. In a previous paper w1x, we have presented the electronic and structural properties of silicon-doped carbon nanotubes. It was predicted that the Si substitutional doping may impose changes in the chemical reactivity and hence in the interaction of the tube with foreign atoms and molecules through the Si site. This would permit another approach to create novel nanotube-related materials consisting in modifying the ‘functionality’ of carbon nanotubes by, first, substitutional doping of carbon by silicon atoms and, subsequently, trapping of foreign atoms or molecules at these Si sites. The doping of fullerenes with Si atom has been recently reported w3x through the synthesis of the C59Si hetero-fullerenes. This is an interesting result since silicon is known to strongly prefer sp3-like bonding. Another relevant feature is that this material is shown to be very reactive at its Si site. Although no experimental report of silicon-doped carbon nanotubes has *Corresponding author. Tel.: q55-559-972-7545; fax: q55-552208-032. E-mail address: [email protected] (R. Mota).
been published, we have many reasons to believe that ¨ it is perfectly feasible. For example, recently Gulseren w x et al. 4 have investigated the adsorption of single atoms on SWCN from first principles and they showed that the chemical reactivity of the nanotube could be modified by radial deformation. The binding energy on the high curvature sites of the deformed tube increases with increasing radial deformation. This is one possible mechanism to promote the Si substitutional process. Once inserted, the Si substitutional atom should be quite stable. Then, similarly to C59Si, the Si-doped SWCN at their Si sites should offer a preferential path towards the adsorption of atoms and molecules. Many papers have been published recently reporting effects on the electronic properties and modifications of the conductance of the nanotubes by direct adsorption of atoms or molecules on SWCN w5x. We must take into account that most atoms or molecules, when adsorbed directly on SWCN, present weak binding energies. In a recent work we have presented a proposal for altering the electronic properties of SWCN through chemical binding of atoms or molecules (F, Cl, H, CH3, and SiH3) on Si-doped SWCN w6x. We observed that, once the Si is inserted in the SWCN, the binding of these atoms or molecules to the Si atom is much stronger than to one of the SWCN C atoms.
0925-9635/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-9635(02)00394-1
862
S.B. Fagan et al. / Diamond and Related Materials 12 (2003) 861–863
we use 80 atoms for the carbon nanotubes. The distance along the nanotube axis between a Si atom and its ˚ The relaxed atomic image in the next cell is 8.52 A. structures are obtained by a minimization of the total energy using Hellmann–Feynman forces including Pullay-like corrections. Structural optimizations were performed using the conjugate gradient algorithm until the ˚ residual forces were below 0.05 eVyA. 3. Results and discussion
Fig. 1. Optimized structure for an organic molecule interacting with a silicon-doped SWCN.
Having in mind that both SWCN and organic molecules might be important components in future nanodevices, this feature that a substitutional Si in SWCN provides a strong binding site may be very useful in order to connect these device components. In this paper we present a study of one particular single organic molecule (S2C6H5) connected to the tube through a Sisite. This molecule was chosen due to its possible use as wire in nanodevices w7x. 2. Method The interaction of the organic molecule with a Sidoped SWCN is studied by means of first principles density-functional theory calculations. We have used the SIESTA code w8x, which performs this calculation solving the standard Kohn–Sham (KS) equations. The calculations are done using the local density approximation w9x for the exchange-correlation term, as parameterized by Perdew and Zunger w10x. The standard norm-conserving Troullier–Martins pseudopotentials w11x are used. The KS orbitals are expanded using a linear combination of numerical pseudoatomic orbitals, similar to the ones proposed by Sankey and Niklewski w12x. In all calculations we have used a split-valence double-zeta basis set with polarization function w13x. A cutoff of 120 Ry for the grid integration was utilized to represent the charge density. Our calculations were performed using a (10, 0) semiconductor SWCN (diam˚ We use periodic-boundary conditions eter of 8.14 A). and a supercell approximation with lateral separation of ˚ between tube centers to make sure that they do 25 A not interact with each other. We have used three Monkhorst-Pack k-points for the Brillouin zone integration along the tube axis, which is shown to give a good approximation for (8, 0) nanotube w14x. In the supercell
In a previous paper w1x, we have demonstrated that Si substitutional doping introduces an empty level in the gap, which is intimately related to an outward relaxation of the Si atom through which it acquires a more sp3 configuration. This increases the Si reactivity, since it prefers to become fourfold coordinated. Approximating the organic molecule (S2C6H5) (Fig. 1) to the tube an adsorption through the Si-site is studied for the optimized structure. From the figure we can observe that the Si atom presents an outward local distortion ˚ This along the radial direction of approximately 0.8 A. same feature has been observed for other atoms or molecules adsorbed through the same process w6x. The binding energy of the Si-tube system to the organic molecule, between Si-site and S atom, is 3.06 eV. It is remarkable that the substantial amount to create the substitutional Si defect (3.10 eV w1x) is compensated for this relatively strong binding energy. Fig. 2 shows the band structures for (a) a pure carbon nanotube, (b) Si-doped SWCN, and (c) the organic molecule interacting with the Si-doped system. The pure SWCN is a (10, 0) semiconductor with a gap approximately 0.8 eV. As pointed out before w1x when a C atom is replaced by a Si atom in the tube and the system is allowed to relax, an empty level close to the bottom of the conduction band is observed (Fig. 2b) with a strong
Fig. 2. Band structure for: (a) (10, 0) pure carbon nanotube; (b) (10, 0) silicon-doped carbon nanotube and (c) (10, 0) silicon-doped carbon nanotube interacting with an organic molecule.
S.B. Fagan et al. / Diamond and Related Materials 12 (2003) 861–863
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interactions between organic molecules and SWCN in chemisorption regime, resulting from the presence of the Si-sites acting as ‘bridges’. Acknowledgments We would like to thank the CENAPAD-SP for computational time. This research is supported by Brazilian agencies: CNPq, CAPES, FAPESP and FAPERGS. References
Fig. 3. Electronic charge density for the orbital at the Brillouin-zone boundary located at the Fermi level (dashed line in Fig. 2). (a) Frontal and (b) lateral view.
Si character. In Fig. 2c, a half-filled level is seen 0.27 eV above the top of the valence band. This half-filled level is a common characteristic observed for all other systems studied w6x. The electronic charge density of the half-filled level is located around the defect region in the nanotube, as can be seen in Fig. 3. 4. Conclusions Considering that both SWCN and organic molecules might be important components in future nanodevices, this feature, that a substitutional Si in SWCN provides a strong binding site, is demonstrated to be very useful in order to connect these device components. A particular single organic molecule (S2C6H5) connected to the tube through a Si-site was chosen due to its possible use as wire in nanodevices. Differently to the weak binding energy associated to the physisorption of the molecule directly on the SWCN surface, when the interaction of the molecule with the surface is through the Si-site, the binding energy is shown to be much stronger. The resulting band structure shows one halffilled level 0.27 eV above the top of the valence band. These results permit a better understanding of the
w1x R.J. Baierle, S.B. Fagan, R. Mota, A.J.R. da Silva, A. Fazzio, Phys. Rev. B 64 (2001) 85413. w2x (a) J. Yang, et al., Phys. Rev. B 64 (2001) 85420 (b) J. Zhao, A. Buldum, J. Han, J.P. Lu, Phys. Rev. Lett. 85 (2000) 1706. w3x (a) J.L. Fye, M.F. Jarrold, J. Phys. Chem. A 101 (1997) 1836 (b) T. Kimura, T. Sugai, H. Shinohara, Chem. Phys. Lett. 256 (1996) 269 ´ et al., Phys. Rev. Lett. 80 (c) C. Ray, M. Pellarin, J.L. Lerme, (1998) 5365 (d) I. Fuks, I.V. Kityk, J. Kasperczyk, J. Berdowsky, I. Schirmer, Chem. Phys. Lett. 353 (2002) 7. w4x O. Gulseren, ¨ T. Yildirim, S. Ciraci, Phys. Rev. Lett. 87 (2001) 116802. w5x (a) J. Kong, N.R. Franklin, C. Zhou, et al., Science 287 (2000) 622 (b) S. Peng, K. Cho, Nanotechnology 11 (2000) 57 (c) E.T. Mickelson, C.B. Huffman, A.G. Rinzler, R.E. Smalley, R.H. Hauge, J.L. Margrave, Chem. Phys. Lett. 296 (1998) 188. w6x S.B. Fagan, A.J.R. da Silva, R. Mota, R.J. Baierle, A. Fazzio, Phys. Rev. B 67 (2003) 33405. w7x (a) C. Joachim, J.K. Gimzewski, A. Aviram, Nature 408 (2000) 541 (b) M. Dorogi, J. Gomes, R. Osifchin, R.P. Andres, R. Reifenberger, Phys. Rev. B 52 (1995) 9071. w8x (a) P. Ordejon, ´ E. Artacho, J.M. Soler, Phys. Rev. B 53 (1996) 10441 ´ ´ E. Artacho, J.M. Soler, Int. (b) D. Sanchez-Portal, P. Ordejon, J. Quantum Chem. 65 (1997) 453. w9x D.M. Ceperley, B.J. Alder, Phys. Rev. Lett. 45 (1980) 566. w10x J.P. Perdew, A. Zunger, Phys. Rev. B 23 (1981) 5048. w11x N. Troullier, J.L. Martins, Phys. Rev. B 43 (1991) 1993. w12x O.F. Sankey, D.J. Niklewski, Phys. Rev. B 40 (1989) 3979. w13x E. Artacho, D. Sanchez-Portal, ´ ´ A. Garcia, J.M. P. Ordejon, Soler, Phys. Status Solidi B 215 (1999) 809. w14x D. Srivastava, M. Menon, K. Cho, Phys. Rev. Lett. 83 (1999) 2973.