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Fabrication and electron-beam-induced polymerization of C60 nanoribbon
Thin Solid Films 464 – 465 (2004) 327 – 330 www.elsevier.com/locate/tsf
Fabrication and electron-beam-induced polymerization of C60 nanoribbon Masato Nakaya a,b,*, Tomonobu Nakayama a, Masakazu Aono a,b a
b
Nanomaterials Laboratory, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Department of Material and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Available online 17 July 2004
Abstract We have presented a method of fabricating an array of C60 nanoribbons on a Si(111) surface and have investigated the electron-beaminduced polymerization of C60 nanoribbons using scanning tunneling microscopy (STM) and spectroscopy (STS). By selective growth of C60 multilayers on the terrace of a misoriented Si(111)M3 M3R30j-Ag surface, a uniformly oriented array of C60 nanoribbons having the dimensions of 40 nm 4 – 5 nm (width thickness) has been fabricated. The length of these nanoribbons exceeds several micrometers. The cluster-shaped C60 oligomer has been created on the nanoribbons after electron beam irradiation. In the case of the oligomer, we have observed a higher electron density near the Fermi level than in the case of the nonpolymerized C60 molecules. D 2004 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 61.48.+c; 68.37.Ef Keywords: Fullerene; C60; Polymerization; STM; STS; Nanoribbon
1. Introduction
2. Experimental
Fullerene C60 bulk phases exhibit semiconducting, metallic and superconducting properties depending on structural and electronic modifications by doping alkali metals or by polymerization [1 – 4]. Extensive studies have been conducted experimentally and theoretically on the polymerization [4– 9] induced by irradiating photons or electrons. In order to use the C60 molecule as a building block for nanoscale electronic devices, the fabrication of C60 nanostructures is important. For nanoscale electronics, not only the sizes and shapes but also the electrical properties of such nanostructures should be controlled. In this paper, we present a method for fabricating an array of self-organized C60 nanostructures, having the dimensions of 40 nm 4 – 5 nm several Am (width thickness length), namely, C60 nanoribbons, on the Si(111)M3 M3R30j-Ag [hereafter referred to as Si(111)M3-Ag] surface. Furthermore, we investigate the electron-beam-induced polymerization of C60 nanoribbons using scanning tunneling microscopy (STM) and spectroscopy (STS).
All experiments were carried out in an ultrahighvacuum chamber with a base pressure of approximately 1.0 10 10 Torr equipped with an STM/STS system. We used a p-type Si(111) wafer which has an off-angle of 4j toward the [112¯] direction, and the Si(111)M3-Ag surface was prepared by depositing Ag atoms onto the clean Si(111)7 7 surface [10]. In order to grow C60 films, the C60 molecules were thermally evaporated from a pyrolitic boron nitride crucible and were deposited onto the Si(111)M3-Ag surface kept at room temperature. The thickness of the C60 films was controlled by the deposition time. Electron beam irradiation to polymerize the C60 film was performed using an electron-shower gun. The energy of electrons was fixed at 100 eV in the present experiments.
* Corresponding author. Nanomaterials Laboratory, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. Tel.: +81-29-851-3354; fax: +81-29-860-4793. E-mail address: [email protected] (M. Nakaya). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.064
3. Results and discussion Fig. 1(a) shows an STM image acquired after depositing 0.5 monolayer (ML) of C60 molecules onto the substrate at room temperature followed by annealing at 200 jC for 60 min. Here, 1 ML represents 1.15 1014 molecules per 1 cm2, which corresponds to the density of C60 molecules on the (111) surface of solid C60. In Fig. 1(a), two kinds of
328
M. Nakaya et al. / Thin Solid Films 464 – 465 (2004) 327–330
Fig. 1. Fabrication process of C60 nanoribbons on the Si(111)M3-Ag surface. (a) Two kinds of C60 first layer (0.5 ML of C60). (b) C60 islands selectively grown on the terrace (additional 2 ML of C60). (c) C60 nanoribbons fabricated by depositing 3 ML of C60 onto (b) followed by annealing.
molecular arrangement were recognized, as indicated by black and white arrows. The molecular arrangement along the surface steps of the Si(111)M3-Ag surface (black arrow) was disordered, in other words, there was a random molecular arrangement. Every step on the substrate was a bunched step which was typically composed of 6 to 10 atomic steps of Si(111). The other type of molecular arrangement on the terraces (white arrow) was confirmed to be a well-ordered hexagonal arrangement according to
our STM observation at higher resolution. The random molecular arrangement is a consequence of the chemical bonding between C60 and Si at the steps [11– 13]. Since such strong chemical interaction overcomes the weak van der Waals interaction, C60 molecules are strongly bound to the substrate of which the lattice constant has a huge mismatch with that of the C60 solid. Fig. 1(b) shows an STM image obtained after depositing additional 2 ML of C60 molecules on the surface shown in Fig. 1(a) at room temperature. Many islands were grown on the terrace region. In contrast to this, as indicated by a black arrow, the surface steps were not covered with those islands. Fig. 1(c) shows an STM image acquired after further depositing 3 ML of C60 molecules on the surface shown in Fig. 1(b) at room temperature followed by annealing at 170 jC for 60 min. The C60 multilayers (white arrow) having thicknesses of 5 – 6 molecular layers were formed on the terraces, whereas none were formed on the steps. Therefore, the regions of the substrate steps were observed as valleys having widths of 5 – 10 nm, an example of which is indicated by a black arrow in Fig. 1(c). As a result of the selective growth of C60 multilayers on the terraces of the misoriented Si(111)M3-Ag surface, a uniformly oriented array of C60 nanoribbons having the dimensions of 40 nm 4 – 5 nm several Am (width thickness length) was fabricated. We now explain the mechanism of the selective growth of C60 multilayers on the terraces. It is known that the most stable intermolecular distance in solid C60 is 1.002 nm [1]. If we consider the two cases of using the random and the well-ordered first layers for stacking the second and the third layers, the use of the well-ordered first layer is energetically favorable. By using the first layer with a random molecular arrangement, it is impossible to realize the optimum intermolecular distance between every pair of adjacent molecules. Next, we investigated the electron-beam-induced polymerization of the C60 nanoribbons. Fig. 2(a) shows an occupied states STM image of C60 nanoribbons after electron beam irradiation with an energy of 100 eV at a dose rate of 1.1 1015 electrons/cm2s for 140 min. We observed the clusters of the plural C60 molecules with bright contrast in Fig. 2(a). Each bright cluster was mostly comprised of a very bright molecule (Type A) and less bright molecules (Type B), as seen in Fig. 2(a). The former molecule existed at the central part of the cluster and was surrounded by the latter molecules. On the other hand, we observed that the C60 molecules with dark contrast (Type C) were surrounding the bright clusters and the Type D molecules were observed in the region far away from the bright clusters. The observed contrast in Fig. 2(a) mainly reflects the density of electrons rather than the height of the molecule, as will be discussed later. The C60 molecules in the C60 nanoribbons before electron beam irradiation were observed as smooth spheres
M. Nakaya et al. / Thin Solid Films 464 – 465 (2004) 327–330
Fig. 2. (a) STM image (Vs = 1.5 V, It = 15 pA) of C60 nanoribbons after electron beam irradiation, and (b) high-resolution image (Vs = showing the C60 molecules with bright contrast.
whose internal structures could not be discerned. In contrast to this, the Type A and Type B molecules which appear after electron beam irradiation mostly exhibit their internal structures in high-resolution STM images such as shown in Fig. 2(b). This indicates that the rotational motion of molecules was forbidden by the interconnections between adjacent C60 molecules, in other words, the plural C60 molecules showing bright contrast are consid-
329
2 V, It = 15 pA)
ered to be C60 oligomers. However, the Type C and Type D molecules were observed as smooth spheres, which indicates that they are not interconnected with adjacent molecules. It is noted that the density of C60 oligomers was increased by applying electron beam irradiation for a longer period. The bright contrast of the C60 oligomers in the occupied states STM image indicates that the densities of states (DOS) of C60 oligomers are different from those of nonpolymerized C60 molecules. Fig. 3 shows STS data of the Types A – D molecules. As we can see clearly in the DOS of the Type A molecules, electrons near the Fermi level exist at the central part of the C60 oligomer. Density of electrons near the Fermi level decrease as the distance from the Type A molecule increases, as seen in the DOS for the Type B and C molecules. A further increase in distance results in the recovery of electrons in the occupied states (Type D). The present results show the characteristic electron distribution around the C60 oligomers, which is an important factor for inducing further polymerization by the electron beam irradiation and for stabilizing the oligomers.
4. Summary
Fig. 3. STS spectra of four kinds of C60 molecules on the nanoribbons after electron beam irradiation.
We presented the fabrication of an array of C60 nanoribbons on a Si substrate. By the selective growth of C60 multilayers on the terraces of the misoriented Si(111)M3Ag surface, C60 nanoribbons having the dimensions of 40 nm 4 – 5 nm several Am (width thickness length) have been fabricated. We also investigated the electronbeam-induced polymerization of the C60 nanoribbon. We observed C60 oligomers with a cluster shape on the nanoribbons. In the case of the oligomer, we have observed a higher electron density near the Fermi level than in the case of the nonpolymerized C60 molecules.
330
M. Nakaya et al. / Thin Solid Films 464 – 465 (2004) 327–330
References [1] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, CA, 1996. [2] Y. Saito, H. Shinohara, M. Kato, H. Nagashima, M. Ohkohchi, Y. Ando, Chem. Phys. Lett. 189 (1992) 236. [3] J. Nakamura, T. Nakayama, S. Watanabe, M. Aono, Phys. Rev. Lett. 87 (2001) 048301. [4] J. Onoe, T. Nakayama, M. Aono, T. Hara, Appl. Phys. Lett. 82 (2003) 595. [5] A.M. Rao, P. Zhou, K.-A. Wang, G.T. Hager, J.M. Holden, Y. Wang, W.-T. Lee, X.-X. Bi, P.C. Eklund, D.S. Cornett, M.A. Duncan, I.J. Amster, Science 259 (1993) 955.
[6] S. Okada, S. Saito, Phys. Rev., B 55 (1997) 4039. [7] S. Okada, S. Saito, Phys. Rev., B 59 (1999) 1930. [8] T. Nakayama, J. Onoe, K. Nakatsuji, J. Nakamura, K. Takeuchi, M. Aono, Surf. Rev. Lett. 6 (1999) 1073. [9] T. Hara, J. Onoe, H. Tanaka, Y. Li, K. Takeuchi, Jpn. J. Appl. Phys., Part 1 39 (2000) 1872. [10] T. Nakayama, S. Watanabe, M. Aono, Surf. Sci. 344 (1995) 143. [11] M.D. Upward, P. Moriarty, P.H. Beton, Phys. Rev., B 56 (1997) R1704. [12] T. Nakayama, J. Onoe, K. Takeuchi, M. Aono, Phys. Rev., B 59 (1999) 12627. [13] T. Nakayama, M. Nakaya, M. Aono, Riken Rev. 45 (2002) 3.
Fabrication and electron-beam-induced polymerization of C60 nanoribbon Masato Nakaya a,b,*, Tomonobu Nakayama a, Masakazu Aono a,b a
b
Nanomaterials Laboratory, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Department of Material and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Available online 17 July 2004
Abstract We have presented a method of fabricating an array of C60 nanoribbons on a Si(111) surface and have investigated the electron-beaminduced polymerization of C60 nanoribbons using scanning tunneling microscopy (STM) and spectroscopy (STS). By selective growth of C60 multilayers on the terrace of a misoriented Si(111)M3 M3R30j-Ag surface, a uniformly oriented array of C60 nanoribbons having the dimensions of 40 nm 4 – 5 nm (width thickness) has been fabricated. The length of these nanoribbons exceeds several micrometers. The cluster-shaped C60 oligomer has been created on the nanoribbons after electron beam irradiation. In the case of the oligomer, we have observed a higher electron density near the Fermi level than in the case of the nonpolymerized C60 molecules. D 2004 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 61.48.+c; 68.37.Ef Keywords: Fullerene; C60; Polymerization; STM; STS; Nanoribbon
1. Introduction
2. Experimental
Fullerene C60 bulk phases exhibit semiconducting, metallic and superconducting properties depending on structural and electronic modifications by doping alkali metals or by polymerization [1 – 4]. Extensive studies have been conducted experimentally and theoretically on the polymerization [4– 9] induced by irradiating photons or electrons. In order to use the C60 molecule as a building block for nanoscale electronic devices, the fabrication of C60 nanostructures is important. For nanoscale electronics, not only the sizes and shapes but also the electrical properties of such nanostructures should be controlled. In this paper, we present a method for fabricating an array of self-organized C60 nanostructures, having the dimensions of 40 nm 4 – 5 nm several Am (width thickness length), namely, C60 nanoribbons, on the Si(111)M3 M3R30j-Ag [hereafter referred to as Si(111)M3-Ag] surface. Furthermore, we investigate the electron-beam-induced polymerization of C60 nanoribbons using scanning tunneling microscopy (STM) and spectroscopy (STS).
All experiments were carried out in an ultrahighvacuum chamber with a base pressure of approximately 1.0 10 10 Torr equipped with an STM/STS system. We used a p-type Si(111) wafer which has an off-angle of 4j toward the [112¯] direction, and the Si(111)M3-Ag surface was prepared by depositing Ag atoms onto the clean Si(111)7 7 surface [10]. In order to grow C60 films, the C60 molecules were thermally evaporated from a pyrolitic boron nitride crucible and were deposited onto the Si(111)M3-Ag surface kept at room temperature. The thickness of the C60 films was controlled by the deposition time. Electron beam irradiation to polymerize the C60 film was performed using an electron-shower gun. The energy of electrons was fixed at 100 eV in the present experiments.
* Corresponding author. Nanomaterials Laboratory, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. Tel.: +81-29-851-3354; fax: +81-29-860-4793. E-mail address: [email protected] (M. Nakaya). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.06.064
3. Results and discussion Fig. 1(a) shows an STM image acquired after depositing 0.5 monolayer (ML) of C60 molecules onto the substrate at room temperature followed by annealing at 200 jC for 60 min. Here, 1 ML represents 1.15 1014 molecules per 1 cm2, which corresponds to the density of C60 molecules on the (111) surface of solid C60. In Fig. 1(a), two kinds of
328
M. Nakaya et al. / Thin Solid Films 464 – 465 (2004) 327–330
Fig. 1. Fabrication process of C60 nanoribbons on the Si(111)M3-Ag surface. (a) Two kinds of C60 first layer (0.5 ML of C60). (b) C60 islands selectively grown on the terrace (additional 2 ML of C60). (c) C60 nanoribbons fabricated by depositing 3 ML of C60 onto (b) followed by annealing.
molecular arrangement were recognized, as indicated by black and white arrows. The molecular arrangement along the surface steps of the Si(111)M3-Ag surface (black arrow) was disordered, in other words, there was a random molecular arrangement. Every step on the substrate was a bunched step which was typically composed of 6 to 10 atomic steps of Si(111). The other type of molecular arrangement on the terraces (white arrow) was confirmed to be a well-ordered hexagonal arrangement according to
our STM observation at higher resolution. The random molecular arrangement is a consequence of the chemical bonding between C60 and Si at the steps [11– 13]. Since such strong chemical interaction overcomes the weak van der Waals interaction, C60 molecules are strongly bound to the substrate of which the lattice constant has a huge mismatch with that of the C60 solid. Fig. 1(b) shows an STM image obtained after depositing additional 2 ML of C60 molecules on the surface shown in Fig. 1(a) at room temperature. Many islands were grown on the terrace region. In contrast to this, as indicated by a black arrow, the surface steps were not covered with those islands. Fig. 1(c) shows an STM image acquired after further depositing 3 ML of C60 molecules on the surface shown in Fig. 1(b) at room temperature followed by annealing at 170 jC for 60 min. The C60 multilayers (white arrow) having thicknesses of 5 – 6 molecular layers were formed on the terraces, whereas none were formed on the steps. Therefore, the regions of the substrate steps were observed as valleys having widths of 5 – 10 nm, an example of which is indicated by a black arrow in Fig. 1(c). As a result of the selective growth of C60 multilayers on the terraces of the misoriented Si(111)M3-Ag surface, a uniformly oriented array of C60 nanoribbons having the dimensions of 40 nm 4 – 5 nm several Am (width thickness length) was fabricated. We now explain the mechanism of the selective growth of C60 multilayers on the terraces. It is known that the most stable intermolecular distance in solid C60 is 1.002 nm [1]. If we consider the two cases of using the random and the well-ordered first layers for stacking the second and the third layers, the use of the well-ordered first layer is energetically favorable. By using the first layer with a random molecular arrangement, it is impossible to realize the optimum intermolecular distance between every pair of adjacent molecules. Next, we investigated the electron-beam-induced polymerization of the C60 nanoribbons. Fig. 2(a) shows an occupied states STM image of C60 nanoribbons after electron beam irradiation with an energy of 100 eV at a dose rate of 1.1 1015 electrons/cm2s for 140 min. We observed the clusters of the plural C60 molecules with bright contrast in Fig. 2(a). Each bright cluster was mostly comprised of a very bright molecule (Type A) and less bright molecules (Type B), as seen in Fig. 2(a). The former molecule existed at the central part of the cluster and was surrounded by the latter molecules. On the other hand, we observed that the C60 molecules with dark contrast (Type C) were surrounding the bright clusters and the Type D molecules were observed in the region far away from the bright clusters. The observed contrast in Fig. 2(a) mainly reflects the density of electrons rather than the height of the molecule, as will be discussed later. The C60 molecules in the C60 nanoribbons before electron beam irradiation were observed as smooth spheres
M. Nakaya et al. / Thin Solid Films 464 – 465 (2004) 327–330
Fig. 2. (a) STM image (Vs = 1.5 V, It = 15 pA) of C60 nanoribbons after electron beam irradiation, and (b) high-resolution image (Vs = showing the C60 molecules with bright contrast.
whose internal structures could not be discerned. In contrast to this, the Type A and Type B molecules which appear after electron beam irradiation mostly exhibit their internal structures in high-resolution STM images such as shown in Fig. 2(b). This indicates that the rotational motion of molecules was forbidden by the interconnections between adjacent C60 molecules, in other words, the plural C60 molecules showing bright contrast are consid-
329
2 V, It = 15 pA)
ered to be C60 oligomers. However, the Type C and Type D molecules were observed as smooth spheres, which indicates that they are not interconnected with adjacent molecules. It is noted that the density of C60 oligomers was increased by applying electron beam irradiation for a longer period. The bright contrast of the C60 oligomers in the occupied states STM image indicates that the densities of states (DOS) of C60 oligomers are different from those of nonpolymerized C60 molecules. Fig. 3 shows STS data of the Types A – D molecules. As we can see clearly in the DOS of the Type A molecules, electrons near the Fermi level exist at the central part of the C60 oligomer. Density of electrons near the Fermi level decrease as the distance from the Type A molecule increases, as seen in the DOS for the Type B and C molecules. A further increase in distance results in the recovery of electrons in the occupied states (Type D). The present results show the characteristic electron distribution around the C60 oligomers, which is an important factor for inducing further polymerization by the electron beam irradiation and for stabilizing the oligomers.
4. Summary
Fig. 3. STS spectra of four kinds of C60 molecules on the nanoribbons after electron beam irradiation.
We presented the fabrication of an array of C60 nanoribbons on a Si substrate. By the selective growth of C60 multilayers on the terraces of the misoriented Si(111)M3Ag surface, C60 nanoribbons having the dimensions of 40 nm 4 – 5 nm several Am (width thickness length) have been fabricated. We also investigated the electronbeam-induced polymerization of the C60 nanoribbon. We observed C60 oligomers with a cluster shape on the nanoribbons. In the case of the oligomer, we have observed a higher electron density near the Fermi level than in the case of the nonpolymerized C60 molecules.
330
M. Nakaya et al. / Thin Solid Films 464 – 465 (2004) 327–330
References [1] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, CA, 1996. [2] Y. Saito, H. Shinohara, M. Kato, H. Nagashima, M. Ohkohchi, Y. Ando, Chem. Phys. Lett. 189 (1992) 236. [3] J. Nakamura, T. Nakayama, S. Watanabe, M. Aono, Phys. Rev. Lett. 87 (2001) 048301. [4] J. Onoe, T. Nakayama, M. Aono, T. Hara, Appl. Phys. Lett. 82 (2003) 595. [5] A.M. Rao, P. Zhou, K.-A. Wang, G.T. Hager, J.M. Holden, Y. Wang, W.-T. Lee, X.-X. Bi, P.C. Eklund, D.S. Cornett, M.A. Duncan, I.J. Amster, Science 259 (1993) 955.
[6] S. Okada, S. Saito, Phys. Rev., B 55 (1997) 4039. [7] S. Okada, S. Saito, Phys. Rev., B 59 (1999) 1930. [8] T. Nakayama, J. Onoe, K. Nakatsuji, J. Nakamura, K. Takeuchi, M. Aono, Surf. Rev. Lett. 6 (1999) 1073. [9] T. Hara, J. Onoe, H. Tanaka, Y. Li, K. Takeuchi, Jpn. J. Appl. Phys., Part 1 39 (2000) 1872. [10] T. Nakayama, S. Watanabe, M. Aono, Surf. Sci. 344 (1995) 143. [11] M.D. Upward, P. Moriarty, P.H. Beton, Phys. Rev., B 56 (1997) R1704. [12] T. Nakayama, J. Onoe, K. Takeuchi, M. Aono, Phys. Rev., B 59 (1999) 12627. [13] T. Nakayama, M. Nakaya, M. Aono, Riken Rev. 45 (2002) 3.