- Email: [email protected]
Scanning tunneling microscopy and spectroscopy of electron-irradiated thin films of C60 molecules
CARBON
4 9 ( 2 0 1 1 ) 1 8 2 9 –1 8 3 3
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Scanning tunneling microscopy and spectroscopy of electron-irradiated thin films of C60 molecules Masato Nakaya
a,* ,
Masakazu Aono a, Tomonobu Nakayama
a,b
a
International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan b Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
We have used scanning tunneling microscopy and spectroscopy to determine the effect of
Received 2 September 2010
electron irradiation on thin films of C60 molecules. Upon the irradiation of electrons with an
Accepted 5 January 2011
energy of 100 eV or 2 keV, clusters of molecules are created in the C60 films as the initial
Available online 9 January 2011
products. The molecules in the clusters exhibit different electronic structures from that of pristine C60 molecules; the molecule at the center of the cluster exhibits a metallic or semimetallic nature and the other molecules exhibit an additional density of states in the filled states. C60 thin films are covered with clusters upon continuous electron irradiation for a longer duration, changing the electronic structure of the films. We consider that the electron-induced polymerization between C60 molecules is responsible for the observed alteration of the C60 thin films. 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
The development of electronics using nanoscale architectures (nanoarchitectures) is an important task in current nanotechnology [1,2]. Toward this end, the key challenge is controlling the functionalities and properties of the created nanoarchitectures, which play a critical role in the design of electronics. From this viewpoint, the fullerene C60 is a promising component of nanoscale architectures because the electrical and electronic properties of C60 molecules can be altered depending on the intermolecular chemical bonds [3–6]. For example, an interconnection between C60 molecules via [2 + 2] cycloadditive four-membered rings (referred to as a [2 + 2] bond hereafter), which can be formed by photon irradiation [7], the application of pressure at high temperatures [8], tunneling carrier injection [9], or local electrostatic ionization [10,11], improves the conductivity of C60 films from insulating to semiconducting [6]. Recently, it has been reported that the electrical properties of C60 films can be further controlled to
become metallic upon the irradiation of electrons with an energy of 3 keV [12]. Although the origin of the metallic transport in electron-irradiated C60 films is regarded to be the formation of p-electron-conjugated polymers of C60 molecules [13–15] on the basis of spectroscopic studies and theoretical calculations [12,16,17], direct information about individual products is needed to obtain a more detailed understanding. In this study, we have investigated electron-irradiated thin films of C60 molecules using scanning tunneling microscopy (STM) and spectroscopy (STS). In the initial stage of electron irradiation, we found that clusters of C60 molecules were formed. The density of the clusters increases upon continuous electron irradiation for a longer duration, resulting in the C60 film becoming covered with clusters. STM/STS observations of individual clusters reveal that the C60 molecule at the center of the cluster covalently bonds with an adjacent molecule and exhibits a metallic or semimetallic nature in its electronic structure. In addition, the other C60 molecules
* Corresponding author: Fax: +81 29 860 4886. E-mail address: [email protected] (M. Nakaya). 0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.01.004
1830
CARBON
4 9 ( 2 0 1 1 ) 1 8 2 9 –1 8 3 3
in the cluster exhibit an additional density of states (DOS) in the filled states. We consider that the electron-induced polymerization between C60 molecules is responsible for the appearance of the clusters.
2.
Experimental
All the experiments were carried out in an ultrahigh-vacuum (UHV) chamber with a base pressure of approximately p p 1 · 108 Pa. C60 thin films grown on a Si(1 1 1) 3 · 3R30-Ag p (referred to as Si(1 1 1) 3-Ag hereafter) surface were used as p samples. The Si(1 1 1) 3-Ag surface [18] was prepared by depositing 1 monolayer (ML) of Ag atoms onto a Si(1 1 1)7 · 7 surface at 600 C. Subsequently, 5 ML of C60 molecules were p deposited onto the Si(1 1 1) 3-Ag surface at room temperature (RT) at a constant deposition rate of 0.03 ML/min. The weak p interaction between the C60 molecules and the Si(1 1 1) 3-Ag surface enables the formation of a C60 film with a well-ordered molecular arrangement from the early stages of growth [19]. To further improve the flatness and ordering of the C60 films, we carried out mild annealing at 170 C for 60 min. As a result of these procedures, C60 thin films with a thickness within 4–5 molecular layers were formed. After the preparation of the C60 films, electrons with an energy of 100 eV or 2 keV were irradiated at RT with a dose rate of 1.1 · 1015 cm2s1 using an electron-beam gun. The electrons were irradiated over a region of each sample surface with a size of about 8 · 4 mm without focusing. The C60 films before and after electron irradiation were characterized by UHV-STM/STS at RT using a platinum20% iridium tip.
3.
Results and discussion
Fig. 1 shows a series of STM images of C60 films taken before and after the irradiation of electrons with an energy of 100 eV. STM images were typically taken at a sample bias voltage (Vs)
of 2.0 V and a tunneling current (It) of 15 pA. Before electron irradiation (Fig. 1a), the C60 film is molecularly flat and homogeneous with a well-ordered hexagonal molecular arrangement. As shown in the magnified image (inset of Fig. 1a), all C60 molecules appear as smooth spheres owing to their free rotation at RT [20,21]. In contrast,Fig 1b–d shows the C60 films after irradiating electrons for 2, 5, and 30 h, respectively. After electron irradiation for 2 h, some of the C60 molecules are observed to be bright in the image contrast and to have formed clusters, as shown in Fig. 1b. As can be observed by comparing Fig. 1b and c, the aerial density of the clusters becomes higher with increasing duration of electron irradiation. At the same time, some neighboring clusters coalesce into an extended cluster, as indicated by the black arrows in Fig. 1c. After irradiating electrons for 30 h, the C60 film is mainly covered with clusters exhibiting bright image contrast, as shown in Fig. 1d. Interestingly, at this stage, most of the C60 molecules exhibit on the internal structure, as shown in the magnified image (inset of Fig. 1d). This clearly indicates that the free rotation of C60 molecules is inhibited by the formation of intermolecular chemical bonds, as discussed in the case of a photon-induced C60 polymer [22]. In other words, polymerization between C60 molecules is induced by the irradiation of electrons with an energy of 100 eV. We confirmed that the irradiation of electrons with a higher energy of 2 keV similarly creates molecules showing bright image contrasts and forming clusters (see Supplementary Data). We briefly discuss the thermal stability of the polymerized C60 molecules. Although C60 molecules chemically connected via a [2 + 2] bond completely decompose into individual molecules upon thermal annealing at 140–200 C [23], the polymerized molecules shown in Fig 1d were stable under thermal annealing at 220 C for 60 min. Thus, the chemical linkages between C60 molecules created by the irradiation of electrons are thermally more stable than the [2 + 2] bond. Note that the annealing temperature was accurately
Fig. 1 – Series of STM images showing C60 thin films before and after electron irradiation. (a) STM image taken before electron irradiation. A high-resolution image of C60 molecules is shown in the inset. (b), (c), and (d) show STM images taken after electron irradiation for 2, 5, and 30 h, respectively. The inset in (d) shows a high-resolution image taken at Vs = 1.5 V and It = 15 pA. Other images were typically taken at Vs = 2.0 V and It = 15 pA.
CARBON
4 9 ( 20 1 1 ) 1 8 2 9–18 3 3
measured using a thermocouple and an infrared thermometer calibrated using the desorption temperature (250–260 C) p of C60 molecules from a Si(1 1 1) 3-Ag surface [24,25]. To understand the effect of electron irradiation in more detail, we investigated the clusters of ‘‘bright molecules’’ observed at the initial stage. Fig. 2 shows high-resolution images of the clusters created by electron irradiation for 2 h. Hexagonal clusters, as shown in Fig 2a and on the left of Fig. 2b, were typically observed in our studies. Hexagonal clusters involve two types of molecules with different image contrasts: a brighter molecule (type A) and a slightly bright molecule (type B). A type A molecule exists at the center of each hexagonal cluster and is surrounded by type B molecules, as indicated in Fig. 2a. It is noteworthy that the type A molecule always exhibits an internal structure, namely, it corresponds to a polymerized molecule. In contrast, most of the type B molecules are observed as smooth hemispheres, indicating that they do not polymerize. In the initial stage of electron irradiation, many of the hexagonal clusters do not include molecules with an internal structure other than that of the type A molecule, suggesting that the type A molecule bonds with an adjacent molecule in the underneath layer. In addition, three type B molecules form a triangular cluster as a minor product, as shown on the right of Fig. 2b. Note that the type A and type B molecules are also created in C60 films on a Si(1 1 1)7 · 7 surface by electron irradiation, indicating that they are not products specific to electron-irradiated C60 films p on a Si(1 1 1) 3-Ag surface and that they are not formed by an admixture of Ag atoms and C60 molecules [26]. Fig. 3 shows schematic side and top views of the hexagonal and triangular clusters in the C60 film; the structures shown are proposed on the basis of our STM results and the following discussion. In the hexagonal cluster, the type A molecule mainly bonds with an adjacent molecule in the underneath layer, while most type B molecules do not form such bonds, as mentioned above. This implies that the C60 molecules around the polymer (represented by a dimer in Fig. 3a) are observed to be bright in STM imaging, as discussed later. In other words, the creation of type A molecules via polymerization is considered to be essential for the appearance of the hexagonal clusters. This model also provides a reasonable explanation for the triangular clusters. When the polymerized type A molecules are created in the molecular layer below the surface of the C60 film, their three neighboring
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Fig. 3 – Schematic illustrations of hexagonal and triangular clusters in the C60 film. Side and top views are shown in (a) and (b), respectively.
molecules on the surface are observed as type B molecules and a single triangular cluster appears in the STM image, as illustrated on the right of Fig. 3a and b. The observed bright image contrast of the type A and type B molecules strongly depends on the polarity of Vs. Fig. 4 shows the variation of tip height along the line intersecting the single hexagonal cluster shown in the inset. Five different curves were measured with different values of Vs at a fixed It of 20 pA. As indicated in the figure, the tip heights on the type A and type B molecules appear to be higher (corresponding to a brighter image contrast) for negative values of Vs, while no obvious increase in height is observed at positive values of Vs. These results strongly indicate that the bright image contrast of the type A and type B molecules can be ascribed to their electronic structures rather than the geometrical height difference. Fig. 5 shows STS spectra, (dI/dV)/(I/V), taken on the C60 films after irradiating electrons with an energy of 100 eV for 2 h. The spectrum shown in Fig. 5a is averaged over C60 molecules located far from a hexagonal cluster (filled circles in the inset), and clearly exhibits an energy gap around the Fermi level (EF). The same spectral features were predominantly observed for the C60 molecules before electron irradiation,
Fig. 2 – High-resolution STM images of two types of clusters: (a) hexagonal clusters, (b) hexagonal (left) and triangular (right) clusters. All the images were taken at Vs = 2.0 V and It = 15 pA.
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CARBON
4 9 ( 2 0 1 1 ) 1 8 2 9 –1 8 3 3
Fig. 4 – Tip height variation over a hexagonal cluster for five Vs of 2.0 V, 1.5 V, 1.0 V, 1.5 V, and 2.0 V.
Fig. 5 – Normalized conductance spectra taken on three types of C60 molecules after electron irradiation for 2 h: (a) C60 molecules located far from a hexagonal cluster, (b) type B molecules in a hexagonal cluster, and (c) type A molecule in a hexagonal cluster. Each set of (dI/dV)/(I/V) data was numerically derived from the respective tunneling I–V curve, which was measured on the C60 films by STM with the feedback loop disabled. Before all I–V measurements, Vs and It were set to 1.0 V and 200 pA, respectively.
indicating that the C60 molecules existing far from clusters remain pristine after electron irradiation. Although the spectrum taken on type B molecules (Fig. 5b) also exhibits an energy gap, a DOS in the filled states additionally appears as indicated by the arrow in Fig. 5b. Interestingly, the electronic structure of the type A molecules (Fig. 5c) is obviously different from those of the type B and pristine C60 molecules; a fi-
nite DOS appears near EF. As shown by Fig. 5a–c, the DOS in the filled states becomes larger upon approaching the type A molecule from the pristine C60 molecules, clearly indicating that the bright contrast of the type A and type B molecules in the filled-state image reflects such differences in the DOS. An important finding is that the type A molecules are not only polymerized with an adjacent molecule but also exhibit a metallic or semimetallic nature in terms of their electronic structure. Note that the irradiation of electrons with a higher energy of 2 keV also induces polymerization between C60 molecules and changes the electronic structure to metallic or semimetallic (see Supplementary Data). These results are consistent with those of photoelectron spectroscopy studies [27–29], which also indicate the metallic electronic structure of C60 films after the irradiation of electrons with an energy of 3 keV. In addition, the results of a previous study involving infrared spectroscopy and theoretical calculations suggest that C60 molecules interconnected via tubular linkage structures, namely, peanut-shaped C60 polymers, are created in electron-irradiated C60 films [12,16,17]. A comparison between previously reported results obtained by spectroscopy and the present STM/STS study suggests that the type A molecule is probably an elemental unit of the peanut-shaped polymer of C60 molecules. This is also consistent with the above-mentioned experimental result showing the thermal stability of type A molecules because the number of C–C bonds that bind to adjacent C60 molecules in the peanut-shaped dimer is apparently greater than that in the [2 + 2] dimer [12–15]. On the other hand, we also observed type B molecules with an additional DOS in the filled states compared with the pristine C60 molecules, although they did not polymerize in many cases. It is noteworthy that they always appeared at the nearest-neighbor positions to the type A molecule, suggesting that an interaction between the type A molecule and its neighbors is not a pure van der Waals force and that the transfer of a few charges occurs between them. This chemical interaction between type A and type B molecules is considered to be sufficiently weak to allow the free rotation of type B molecules at RT. This type of electronic interaction is also considered to contribute to the drastic improvement of the conductivity of C60 thin films upon the irradiation of electrons [12].
4.
Conclusion
We have investigated electron-irradiated C60 thin films using STM/STS. We found that the electron irradiation created clusters of molecules in the C60 films as the initial products. The molecule at the center of the cluster polymerizes with the adjacent molecule in the underneath layer and exhibits a metallic or semimetallic nature. We also found that C60 molecules adjacent to the polymerized molecule exhibit an additional DOS in the filled states owing to the chemical interaction with the polymerized molecule. The C60 films became covered with clusters upon continuous electron irradiation for a longer period. The present STM/STS study suggests that the electron-induced polymerization between C60 molecules causes the electronic and electrical alteration of C60 thin films at the single-molecular scale.
CARBON
4 9 ( 20 1 1 ) 1 8 2 9–18 3 3
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.01.004.
R E F E R E N C E S
[1] Meindl JD, Chen Q, Davis JA. Limits on silicon nanoelectronics for terascale integration. Science 2001;293(5537):2044–9. [2] Joachim C, Gimzewski JK, Aviram A. Electronics using hybridmolecular and mono-molecular devices. Nature 2000;408(6872):541–8. [3] Saito Y, Shinohara H, Kato M, Nagashima H, Ohkohchi M, Ando Y. Electric conductivity and band gap of solid C60 under high pressure. Chem. Phys. Lett. 1992;189(3):236–40. [4] Menon M, Richter E, Subbaswamy KR. Fused fullerenes and multiply connected carbon clusters: Proposed new forms of carbon. Phys. Rev. B 1998;57(7):4063–6. [5] Okada S, Saito S, Oshiyama A. New metallic crystalline carbon: Three dimensionally polymerized C60 fullerite. Phys. Rev. Lett. 1999;83(10):1986–9. [6] Onoe J, Nakayama T, Aono M, Hara T. Electrical properties of a two-dimensionally hexagonal C60 photopolymer. J. Appl. Phys. 2004;96(1):443–5. [7] Rao AM, Zhou P, Wang KA, Hager GT, Holden JM, Wang Y, et al. Photoinduced polymerization of solid C60 films. Science 1993;259(5097):955–7. [8] Iwasa Y, Arima T, Fleming RM, Siegrist T, Zhou O, Haddon RC, et al. New phases of C60 synthesized at high pressure. Science 1994;264(5165):1570–2. [9] Nakamura Y, Mera Y, Maeda K. In situ scanning tunneling microscopic study of polymerization of C60 clusters induced by electron injection from the probe tips. Appl. Phys. Lett. 2000;77(18):2834–6. [10] Nakaya M, Tsukamoto S, Kuwahara Y, Aono M, Nakayama T. Molecular scale control of unbound and bound C60 for topochemical ultradense data storage in an ultrathin C60 film. Adv. Mater. 2010;22(14):1622–5. [11] Nakaya M, Kuwahara Y, Aono M, Nakayama T. Reversibilitycontrolled single molecular level chemical reaction in a C60 monolayer via ionization induced by scanning tunneling microscopy. Small 2008;4(5):538–41. [12] Onoe J, Nakayama T, Aono M, Hara T. Structural and electrical properties of an electron-beam-irradiated C60 film. Appl. Phys. Lett. 2003;82(4):595–7. [13] Ueno H, Osawa S, Osawa E, Takeuchi K. Stone-Wales rearrangement pathways from the hinge-opened [2+2] C60 dimer to IPR C120 fullerenes. Vibrational analysis of intermediates. Fullerene Sci. Technol. 1998;6(2):319–38.
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[14] Ueda S, Ohno K, Noguchi Y, Ishii S, Onoe J. Dimensional dependence of electronic structure of fullerene polymers. J. Phys. Chem. B 2006;110(45):22374–81. [15] Koshino M, Niimi Y, Nakamura E, Kataura H, Okazaki T, Suenaga K, et al. Analysis of the reactivity and selectivity of fullerene dimerization reactions at the atomic level. Nat. Chem. 2010;2(2):117–24. [16] Hara T, Onoe J. Vibrational analysis of peanut-shaped C120 fullerenes. Eur. Phys. J. D 2003;24(1–3):389–92. [17] Takashima A, Onoe J, Nishii T. In situ infrared spectroscopic and density-functional studies of the cross-linked structure of one-dimensional C60 polymer. J. Appl. Phys. 2010;108(3):033514-1–5. p p [18] Takahashi T, Nakatani S. Refinement of the Si(111) 3· 3-Ag structure by surface X-ray diffraction. Surf. Sci. 1993;282(1– 2):17–32. [19] Nakaya M, Nakayama T, Kuwahara Y, Aono M. Fabrication of nanostructures by selective growth of C60 and Si on Si(111) substrate. Surf. Sci. 2006;600(13):2810–6. [20] Wilson RJ, Meijer G, Bethune DS, Johnson RD, Chambliss DD, de Vries MS, et al. Imaging C60 clusters on a surface using a scanning tunnelling microscope. Nature 1990;348(6302):621–2. [21] Li YZ, Patrin JC, Chander M, Weaver JH, Chibante LPF, Smalley RE. Ordered overlayers of C60 on GaAs(110) studied with scanning tunneling microscopy. Science 1991;252(5005):547–8. [22] Nakayama T, Onoe J, Nakatsuji K, Nakamura J, Takeuchi K, Aono M. Photoinduced products in a C60 monolayer on p p Si(111)( 3· 3)-Ag: An STM study. Surf. Rev. Lett. 1999;6(6):1073–8. [23] Wang Y, Holden JM, Bi XX, Eklund PC. Thermal decomposition of polymeric C60. Chem. Phys. Lett. 1994;217(4):413–7. [24] Upward MD, Moriarty P, Beton PH. Double domain ordering p p and selective removal of C60 on Ag/Si(111)-( 3· 3)R30. Phys. Rev. B 1997;56(4):R1704–7. [25] Nakayama T, Onoe J, Takeuchi K, Aono M. Weakly bound and p p strained C60 monolayer on the Si(1 1 1) 3 · 3R30 -Ag substrate surface. Phys. Rev. B 1999;59(19):12627–31. [26] Yoshikawa G, Tsuruma Y, Ikeda S, Saiki K. Noble metal intercalated fullerene fabricated by low-temperature codeposition. Adv. Mater. 2010;22(1):43–6. [27] Onoe J, Nakao A, Hida A. Valence photoelectron spectra of an electron-beam-irradiated C60 film. Appl. Phys. Lett. 2004;85(14):2741–3. [28] Onoe J, Ito T, Kimura S, Ohno K, Noguchi Y, Ueda S. Valence electronic structure of cross-linked C60 polymers: In situ highresolution photoelectron spectroscopic and densityfunctional studies. Phys. Rev. B 2007;75(23):233410-1–4. [29] Onoe J, Ito T, Kimura S. Time dependence of the electronic structure of an electron-beam-irradiated C60 film. J. Appl. Phys. 2008;104(10):103706-1–3.
4 9 ( 2 0 1 1 ) 1 8 2 9 –1 8 3 3
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Scanning tunneling microscopy and spectroscopy of electron-irradiated thin films of C60 molecules Masato Nakaya
a,* ,
Masakazu Aono a, Tomonobu Nakayama
a,b
a
International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan b Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
We have used scanning tunneling microscopy and spectroscopy to determine the effect of
Received 2 September 2010
electron irradiation on thin films of C60 molecules. Upon the irradiation of electrons with an
Accepted 5 January 2011
energy of 100 eV or 2 keV, clusters of molecules are created in the C60 films as the initial
Available online 9 January 2011
products. The molecules in the clusters exhibit different electronic structures from that of pristine C60 molecules; the molecule at the center of the cluster exhibits a metallic or semimetallic nature and the other molecules exhibit an additional density of states in the filled states. C60 thin films are covered with clusters upon continuous electron irradiation for a longer duration, changing the electronic structure of the films. We consider that the electron-induced polymerization between C60 molecules is responsible for the observed alteration of the C60 thin films. 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
The development of electronics using nanoscale architectures (nanoarchitectures) is an important task in current nanotechnology [1,2]. Toward this end, the key challenge is controlling the functionalities and properties of the created nanoarchitectures, which play a critical role in the design of electronics. From this viewpoint, the fullerene C60 is a promising component of nanoscale architectures because the electrical and electronic properties of C60 molecules can be altered depending on the intermolecular chemical bonds [3–6]. For example, an interconnection between C60 molecules via [2 + 2] cycloadditive four-membered rings (referred to as a [2 + 2] bond hereafter), which can be formed by photon irradiation [7], the application of pressure at high temperatures [8], tunneling carrier injection [9], or local electrostatic ionization [10,11], improves the conductivity of C60 films from insulating to semiconducting [6]. Recently, it has been reported that the electrical properties of C60 films can be further controlled to
become metallic upon the irradiation of electrons with an energy of 3 keV [12]. Although the origin of the metallic transport in electron-irradiated C60 films is regarded to be the formation of p-electron-conjugated polymers of C60 molecules [13–15] on the basis of spectroscopic studies and theoretical calculations [12,16,17], direct information about individual products is needed to obtain a more detailed understanding. In this study, we have investigated electron-irradiated thin films of C60 molecules using scanning tunneling microscopy (STM) and spectroscopy (STS). In the initial stage of electron irradiation, we found that clusters of C60 molecules were formed. The density of the clusters increases upon continuous electron irradiation for a longer duration, resulting in the C60 film becoming covered with clusters. STM/STS observations of individual clusters reveal that the C60 molecule at the center of the cluster covalently bonds with an adjacent molecule and exhibits a metallic or semimetallic nature in its electronic structure. In addition, the other C60 molecules
* Corresponding author: Fax: +81 29 860 4886. E-mail address: [email protected] (M. Nakaya). 0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.01.004
1830
CARBON
4 9 ( 2 0 1 1 ) 1 8 2 9 –1 8 3 3
in the cluster exhibit an additional density of states (DOS) in the filled states. We consider that the electron-induced polymerization between C60 molecules is responsible for the appearance of the clusters.
2.
Experimental
All the experiments were carried out in an ultrahigh-vacuum (UHV) chamber with a base pressure of approximately p p 1 · 108 Pa. C60 thin films grown on a Si(1 1 1) 3 · 3R30-Ag p (referred to as Si(1 1 1) 3-Ag hereafter) surface were used as p samples. The Si(1 1 1) 3-Ag surface [18] was prepared by depositing 1 monolayer (ML) of Ag atoms onto a Si(1 1 1)7 · 7 surface at 600 C. Subsequently, 5 ML of C60 molecules were p deposited onto the Si(1 1 1) 3-Ag surface at room temperature (RT) at a constant deposition rate of 0.03 ML/min. The weak p interaction between the C60 molecules and the Si(1 1 1) 3-Ag surface enables the formation of a C60 film with a well-ordered molecular arrangement from the early stages of growth [19]. To further improve the flatness and ordering of the C60 films, we carried out mild annealing at 170 C for 60 min. As a result of these procedures, C60 thin films with a thickness within 4–5 molecular layers were formed. After the preparation of the C60 films, electrons with an energy of 100 eV or 2 keV were irradiated at RT with a dose rate of 1.1 · 1015 cm2s1 using an electron-beam gun. The electrons were irradiated over a region of each sample surface with a size of about 8 · 4 mm without focusing. The C60 films before and after electron irradiation were characterized by UHV-STM/STS at RT using a platinum20% iridium tip.
3.
Results and discussion
Fig. 1 shows a series of STM images of C60 films taken before and after the irradiation of electrons with an energy of 100 eV. STM images were typically taken at a sample bias voltage (Vs)
of 2.0 V and a tunneling current (It) of 15 pA. Before electron irradiation (Fig. 1a), the C60 film is molecularly flat and homogeneous with a well-ordered hexagonal molecular arrangement. As shown in the magnified image (inset of Fig. 1a), all C60 molecules appear as smooth spheres owing to their free rotation at RT [20,21]. In contrast,Fig 1b–d shows the C60 films after irradiating electrons for 2, 5, and 30 h, respectively. After electron irradiation for 2 h, some of the C60 molecules are observed to be bright in the image contrast and to have formed clusters, as shown in Fig. 1b. As can be observed by comparing Fig. 1b and c, the aerial density of the clusters becomes higher with increasing duration of electron irradiation. At the same time, some neighboring clusters coalesce into an extended cluster, as indicated by the black arrows in Fig. 1c. After irradiating electrons for 30 h, the C60 film is mainly covered with clusters exhibiting bright image contrast, as shown in Fig. 1d. Interestingly, at this stage, most of the C60 molecules exhibit on the internal structure, as shown in the magnified image (inset of Fig. 1d). This clearly indicates that the free rotation of C60 molecules is inhibited by the formation of intermolecular chemical bonds, as discussed in the case of a photon-induced C60 polymer [22]. In other words, polymerization between C60 molecules is induced by the irradiation of electrons with an energy of 100 eV. We confirmed that the irradiation of electrons with a higher energy of 2 keV similarly creates molecules showing bright image contrasts and forming clusters (see Supplementary Data). We briefly discuss the thermal stability of the polymerized C60 molecules. Although C60 molecules chemically connected via a [2 + 2] bond completely decompose into individual molecules upon thermal annealing at 140–200 C [23], the polymerized molecules shown in Fig 1d were stable under thermal annealing at 220 C for 60 min. Thus, the chemical linkages between C60 molecules created by the irradiation of electrons are thermally more stable than the [2 + 2] bond. Note that the annealing temperature was accurately
Fig. 1 – Series of STM images showing C60 thin films before and after electron irradiation. (a) STM image taken before electron irradiation. A high-resolution image of C60 molecules is shown in the inset. (b), (c), and (d) show STM images taken after electron irradiation for 2, 5, and 30 h, respectively. The inset in (d) shows a high-resolution image taken at Vs = 1.5 V and It = 15 pA. Other images were typically taken at Vs = 2.0 V and It = 15 pA.
CARBON
4 9 ( 20 1 1 ) 1 8 2 9–18 3 3
measured using a thermocouple and an infrared thermometer calibrated using the desorption temperature (250–260 C) p of C60 molecules from a Si(1 1 1) 3-Ag surface [24,25]. To understand the effect of electron irradiation in more detail, we investigated the clusters of ‘‘bright molecules’’ observed at the initial stage. Fig. 2 shows high-resolution images of the clusters created by electron irradiation for 2 h. Hexagonal clusters, as shown in Fig 2a and on the left of Fig. 2b, were typically observed in our studies. Hexagonal clusters involve two types of molecules with different image contrasts: a brighter molecule (type A) and a slightly bright molecule (type B). A type A molecule exists at the center of each hexagonal cluster and is surrounded by type B molecules, as indicated in Fig. 2a. It is noteworthy that the type A molecule always exhibits an internal structure, namely, it corresponds to a polymerized molecule. In contrast, most of the type B molecules are observed as smooth hemispheres, indicating that they do not polymerize. In the initial stage of electron irradiation, many of the hexagonal clusters do not include molecules with an internal structure other than that of the type A molecule, suggesting that the type A molecule bonds with an adjacent molecule in the underneath layer. In addition, three type B molecules form a triangular cluster as a minor product, as shown on the right of Fig. 2b. Note that the type A and type B molecules are also created in C60 films on a Si(1 1 1)7 · 7 surface by electron irradiation, indicating that they are not products specific to electron-irradiated C60 films p on a Si(1 1 1) 3-Ag surface and that they are not formed by an admixture of Ag atoms and C60 molecules [26]. Fig. 3 shows schematic side and top views of the hexagonal and triangular clusters in the C60 film; the structures shown are proposed on the basis of our STM results and the following discussion. In the hexagonal cluster, the type A molecule mainly bonds with an adjacent molecule in the underneath layer, while most type B molecules do not form such bonds, as mentioned above. This implies that the C60 molecules around the polymer (represented by a dimer in Fig. 3a) are observed to be bright in STM imaging, as discussed later. In other words, the creation of type A molecules via polymerization is considered to be essential for the appearance of the hexagonal clusters. This model also provides a reasonable explanation for the triangular clusters. When the polymerized type A molecules are created in the molecular layer below the surface of the C60 film, their three neighboring
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Fig. 3 – Schematic illustrations of hexagonal and triangular clusters in the C60 film. Side and top views are shown in (a) and (b), respectively.
molecules on the surface are observed as type B molecules and a single triangular cluster appears in the STM image, as illustrated on the right of Fig. 3a and b. The observed bright image contrast of the type A and type B molecules strongly depends on the polarity of Vs. Fig. 4 shows the variation of tip height along the line intersecting the single hexagonal cluster shown in the inset. Five different curves were measured with different values of Vs at a fixed It of 20 pA. As indicated in the figure, the tip heights on the type A and type B molecules appear to be higher (corresponding to a brighter image contrast) for negative values of Vs, while no obvious increase in height is observed at positive values of Vs. These results strongly indicate that the bright image contrast of the type A and type B molecules can be ascribed to their electronic structures rather than the geometrical height difference. Fig. 5 shows STS spectra, (dI/dV)/(I/V), taken on the C60 films after irradiating electrons with an energy of 100 eV for 2 h. The spectrum shown in Fig. 5a is averaged over C60 molecules located far from a hexagonal cluster (filled circles in the inset), and clearly exhibits an energy gap around the Fermi level (EF). The same spectral features were predominantly observed for the C60 molecules before electron irradiation,
Fig. 2 – High-resolution STM images of two types of clusters: (a) hexagonal clusters, (b) hexagonal (left) and triangular (right) clusters. All the images were taken at Vs = 2.0 V and It = 15 pA.
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CARBON
4 9 ( 2 0 1 1 ) 1 8 2 9 –1 8 3 3
Fig. 4 – Tip height variation over a hexagonal cluster for five Vs of 2.0 V, 1.5 V, 1.0 V, 1.5 V, and 2.0 V.
Fig. 5 – Normalized conductance spectra taken on three types of C60 molecules after electron irradiation for 2 h: (a) C60 molecules located far from a hexagonal cluster, (b) type B molecules in a hexagonal cluster, and (c) type A molecule in a hexagonal cluster. Each set of (dI/dV)/(I/V) data was numerically derived from the respective tunneling I–V curve, which was measured on the C60 films by STM with the feedback loop disabled. Before all I–V measurements, Vs and It were set to 1.0 V and 200 pA, respectively.
indicating that the C60 molecules existing far from clusters remain pristine after electron irradiation. Although the spectrum taken on type B molecules (Fig. 5b) also exhibits an energy gap, a DOS in the filled states additionally appears as indicated by the arrow in Fig. 5b. Interestingly, the electronic structure of the type A molecules (Fig. 5c) is obviously different from those of the type B and pristine C60 molecules; a fi-
nite DOS appears near EF. As shown by Fig. 5a–c, the DOS in the filled states becomes larger upon approaching the type A molecule from the pristine C60 molecules, clearly indicating that the bright contrast of the type A and type B molecules in the filled-state image reflects such differences in the DOS. An important finding is that the type A molecules are not only polymerized with an adjacent molecule but also exhibit a metallic or semimetallic nature in terms of their electronic structure. Note that the irradiation of electrons with a higher energy of 2 keV also induces polymerization between C60 molecules and changes the electronic structure to metallic or semimetallic (see Supplementary Data). These results are consistent with those of photoelectron spectroscopy studies [27–29], which also indicate the metallic electronic structure of C60 films after the irradiation of electrons with an energy of 3 keV. In addition, the results of a previous study involving infrared spectroscopy and theoretical calculations suggest that C60 molecules interconnected via tubular linkage structures, namely, peanut-shaped C60 polymers, are created in electron-irradiated C60 films [12,16,17]. A comparison between previously reported results obtained by spectroscopy and the present STM/STS study suggests that the type A molecule is probably an elemental unit of the peanut-shaped polymer of C60 molecules. This is also consistent with the above-mentioned experimental result showing the thermal stability of type A molecules because the number of C–C bonds that bind to adjacent C60 molecules in the peanut-shaped dimer is apparently greater than that in the [2 + 2] dimer [12–15]. On the other hand, we also observed type B molecules with an additional DOS in the filled states compared with the pristine C60 molecules, although they did not polymerize in many cases. It is noteworthy that they always appeared at the nearest-neighbor positions to the type A molecule, suggesting that an interaction between the type A molecule and its neighbors is not a pure van der Waals force and that the transfer of a few charges occurs between them. This chemical interaction between type A and type B molecules is considered to be sufficiently weak to allow the free rotation of type B molecules at RT. This type of electronic interaction is also considered to contribute to the drastic improvement of the conductivity of C60 thin films upon the irradiation of electrons [12].
4.
Conclusion
We have investigated electron-irradiated C60 thin films using STM/STS. We found that the electron irradiation created clusters of molecules in the C60 films as the initial products. The molecule at the center of the cluster polymerizes with the adjacent molecule in the underneath layer and exhibits a metallic or semimetallic nature. We also found that C60 molecules adjacent to the polymerized molecule exhibit an additional DOS in the filled states owing to the chemical interaction with the polymerized molecule. The C60 films became covered with clusters upon continuous electron irradiation for a longer period. The present STM/STS study suggests that the electron-induced polymerization between C60 molecules causes the electronic and electrical alteration of C60 thin films at the single-molecular scale.
CARBON
4 9 ( 20 1 1 ) 1 8 2 9–18 3 3
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.01.004.
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