- Email: [email protected]
Highly oriented pyrolitic graphite anomalous structures and fullerenes self-assembly
Applied Surface Science 255 (2009) 4512–4514
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Highly oriented pyrolitic graphite anomalous structures and fullerenes self-assembly L.S. Pinheiro *, J.A.K. Freire y Departamento de Fı´sica, Universidade Federal do Ceara´, Campus do Pici, Caixa Postal 6030, CEP 60455-900 Fortaleza, Ceara´, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 July 2008 Received in revised form 31 October 2008 Accepted 22 November 2008 Available online 30 November 2008
Scanning tunneling microscopy (STM) was used to look for unusual self-structures on highly oriented pyrolitic graphite (HOPG) that can mimic fullerenes assemblies. HOPG features that may be taken as C60 molecular structures were found on this surface. The HOPG self-structures have been presented earlier as anomalies of the bare HOPG surface in the literature. The experimental results are in agreement with earlier STM reports on bare and modified HOPG. ß 2008 Elsevier B.V. All rights reserved.
PACS: 68.43.h 68.35.bp 81.05.uw 68.37.Ef Keywords: Adsorption C60 HOPG Scanning tunneling microscopy
1. Introduction Highly oriented pyrolitic graphite (HOPG) is a well-known substrate for molecular deposition and subsequent imaging by scanning tunneling microscopy (STM). This substrate gives easy atomic resolution and can be used to calibrate the STM at the atomic level. However, it has to be handled carefully when studying surfaces modified by molecules. The literature contains several reports showing the surface defects that can be mistaken by molecular structures on HOPG and there is also the existence of unusual corrugation heights [1–4]. Reports on the adsorption of C60 on HOPG show different structures for the fullerene [5–7]. Recent studies of C60 on metals and semiconductors reveal different molecular arrangements on the surface [8–12]. High resolution STM disclosing the internal features of C60 was obtained in ultra high vacuum (UHV) [9,10]. Isolated C60 molecules have been observed with the STM inside a molecular domain on Au (1 1 1) [13]. Katsonis et al. performed scanning tunneling spectroscopy at room temperature for C60 on Au (1 1 1) finding a position
* Corresponding author. Tel.: +55 85 3366 9018; fax: +55 85 3366 9450. E-mail address: [email protected] (L.S. Pinheiro). y In memorian. 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.11.060
of the energy levels similar to that observed in UHV [14]. Recent studies of C60 and polycyclic aromatic hydrocarbons on Au (1 1 1) in electrochemical environment show that the fullerene sits on top of the aromatic molecule [15]. The aim of this work is to find out if the HOPG presents defects that are similar to C60 self-assembled structures. In this work, imaging was done on bare HOPG and after exposure to solvents and molecular solutions. 2. Experimental The STM was a Nanoscope IIIA equipped with scanners A and E. All images presented were obtained in constant current mode (tip tracks down the surface). Tips were made by clipping a 0.25 mm tungsten wire in diagonal. HOPG grade ZYH (10 mm 10 mm) supplied by Veeco was employed as substrate. Cleavage of HOPG was made using an adhesive tape. C60 99.5% and copper phthalocyanine (CuPc) 98% from Aldrich were used as received. Toluene P.A was employed as solvent for the pure C60 and the mixed molecular solution of 0.8 mM C60 + 2 mM CuPc. A concentrated molecular solution of C60 (7 mM) in toluene was sonicated for 60 min and afterwards left standing for 15 min before deposition on the cleaved substrate. The pure (0.8 mM) and mixed molecular solutions of C60 and C60 + CuPc were sonicated for 15 min to increase solubility. Pure toluene and the molecular
L.S. Pinheiro, J.A.K. Freire / Applied Surface Science 255 (2009) 4512–4514
4513
Fig. 1. Constant current (CC) STM image of bare HOPG. Chain is made of bead like features spaced by 1.9–2.5 nm. Scan area: 70 nm 70 nm.
solutions were deposited on HOPG using a micropipette. The solvent or molecular solutions were deposited on freshly cleaved HOPG surface and the solvent was left to evaporate in a fume hood. 3. Results and discussion Bare HOPG cleaved and imaged at different resolutions shows steps and structures with several shapes. Fig. 1A shows an image of bare HOPG, displaying on the flat background a chain structure made of bead like objects. The round features have a periodicity of 1.9–2.5 nm. The structure crosses the step and is several nanometers long. HOPG exposed to a mixed solution of C60 + CuPc showed the common features observed for this substrate, a very flat background with steps and several other features. A high corrugated structure formed by two chains closer to a broad ridge on the terrace was found as displayed in Fig. 2A. The apparent height of the chains ranges from 0.33 to 0.68 nm. Fig. 2B shows higher resolution of the chains, they are made of bead like objects with a periodicity of 1.4–1.6 nm. The mixed solution of CuPc and C60 was used to check if a mixed domain would form on the surface. C60 is known as an acceptor and CuPc is a donor, therefore cooperative molecular domains could take place on the surface. The surface of the HOPG exposed to the mixed molecular solution show a blue stain due to the deep color of CuPc. Tunneling was done on the stained surface, however no molecular structure that could be assigned to CuPc was found. CuPc has a distinct signature on some substrates and this was not found on the modified HOPG surface [16,17]. The high resolution images of the round features are not similar to that found for C60 in UHV [9,10]. The structure observed in Fig. 2A and B is thought to be due to features of the HOPG surface. After a few minutes scanning, a third chain that has a faint contrast in Fig. 2A, appeared with a higher corrugation height close to the double rows shown in Fig. 2A and B. This could be due to dirt that the tip picked up during scanning and started to act as a new tip enhancing that feature. The pictures presented by Chang and Bard for the bare and Nafion covered HOPG are very similar to the ones observed here and those presented by Klitgaard et al. [2,7]. Wang et al. [18], discuss the triangular and honeycomb atomic
Fig. 2. (A) CC STM images of HOPG exposed to C60 + CuPc, chains made of bead like features on the flat background. Scan area: 150 nm 150 nm. (B) Higher resolution image of (A). Surface chain is formed by two rows having bead like objects with periodicity 1.4–1.6 nm. Scan area: 20 nm 20 nm.
structure formed after exposure of HOPG to an organic solvent. They also detected the presence of a feature made of bead like objects after the exposure of HOPG to the solvent. In this work, features that look like chains or isolated bead like objects on the surface were observed several times for bare or modified HOPG. The presence of CuPC did not introduce new features and just a more pronounced bead like structure already seen for the bare HOPG was detected. The deposition of a very concentrated C60 solution (7 mM) did not change the surface structure of HOPG. It displayed the features already seen for imaging on the bare substrate. In this work, tunneling on bare and modified HOPG was reached with a tunneling gap resistance in the range of 2–4 GV. Images were collected with 400 mV bias voltage and tunneling current of 100–200 pA, to minimize tip sample interaction. Experiments using 10–50 mV bias voltage and 1 nA tunnel current
4514
L.S. Pinheiro, J.A.K. Freire / Applied Surface Science 255 (2009) 4512–4514
50 MV, as seen in Fig. 3B. The border of the steps is rougher than before. Bead like features protruding from the steps are now seen in the image of Fig. 3B. These structures are separated by 1.1– 2.5 nm, close to the spacing of C60 molecules. This behavior shows that low tunneling resistances influences HOPG imaging and should be avoided. Defects that can mimic molecular structures have been discussed by Clemmer and Beebe [1]. Some reports on the subject show a collection of defects revealed by the STM after the cleavage of HOPG [2,3]. The use of solvents has also been suggested to create some patterns on the surface different from the HOPG lattice [2,18]. Here, cleavage was made using an adhesive tape and this appears to enhance the formation of such defects that can be confused with molecular structures. The origin of these mimics of molecular structures is still being interpreted and the possible electronic and structural properties resulting in superperiodic structures have been recently discussed in the literature [4]. 4. Conclusions The presence of chains made of bead like features found on bare and modified HOPG in this work are assigned to intrinsic defects of the substrate based on the experimental images. The earlier literature on the subject also shows that these defects are commonly found on HOPG. The presence of chain like features does not appear to be due to a 1D structure of C60 on HOPG but rather to a surface feature formed during cleavage of the substrate. It is advisable to employ other technique, for instance Scanning Tunneling Spectroscopy to assess the chemical/electronic structure assigned to C60 when bead like patterns are found on the HOPG surface. Acknowledgments We thank Funcap and CNPq for financial support in the form of a DCR grant to LSP. LEM of IQ-USP for donating some materials useful to the development of this work. References [1] [2] [3] [4] [5] [6] Fig. 3. (A) CC STM image of HOPG exposed to a 7 mM C60 solution. Area: 100 nm 100 nm. Vt: 400 mV. It: 100 pA. (B) CC STM image of HOPG exposed to a 7 mM C60 solution at low gap resistance. Area: 100 nm 100 nm. Vt: 50 mV. It: 1 nA.
(10–50 MV), led to modifications of the structures that were not observed previously with a tunneling gap resistance of 4 GV. These surface modifications are seen in Fig. 3A and B acquired in constant current mode. Some drift took place from Fig. 3A and B, but the surface area is close to the former one being imaged. Some noise appears on the image of Fig. 3B due to switching from the high tunneling resistance condition to the lower one. New surface features appeared while scanning with tunneling resistance of
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
C.R. Clemmer, T.P. Beebe Jr., Science 251 (1991) 640. H. Chang, A.J. Bard, Langmuir 7 (1991) 1143. W.-T. Pong, C. Durkham, J. Phys. D: Appl. Phys. 38 (2005) R329. W.T. Pong, J. Bendall, C. Durkham, Surf. Sci. 601 (2007) 498. H. Liu, P.J. Reinke, J. Chem. Phys 124 (2006) 164707. R. Szuba, R. Czajka, A. Kasuya, A. Wawro, H. Rafii-Tabar, Appl. Surf. Sci. 144/145 (1999) 648. S.K. Klitgaard, K. Egeblad, L.T. Haahr, M.K. Hansen, D. Hansen, J. Svagin, C.H. Christensen, Surf. Sci. 601 (2007) L35. S. Guo, P.M. Nagel, A.L. Deering, S.M. Van Lue, S.A. Kandel, Surf. Sci. 601 (2007) 994. C. Silien, N.A. Pradhan, W. Ho, P.A. Thiry, Phys. Rev. B 69 (2004) 115434. J.I. Pascual, J. Go´mez-Herrero, C. Rogero, A.M. Baro´, D. Sa´nchez-Portal, E. Artacho, P. Ordejon, J.M. Soler, Chem. Phys. Lett. 321 (2000) 78. M. Feng, J. Lee, J. Zhao, J.T. Yates Jr., H. Petek, J. Am. Chem. Soc. 129 (2007) 12394. N. Ne´el, J. Kro¨ger, R. Berndt, Appl. Phys. Lett 88 (2006) 163101. S. Guo, D.P. Fogarty, P.M. Nagel, S.A. Kandel, J. Phys. Chem. B 108 (2004) 14074. N. Katsonis, A. Marchenko, D. Fichou, J. Photochem. Photobiol. A: Chem. 158 (2003) 101. S. Yoshimoto, E. Tsutsumi, R. Narita, Y. Murata, M. Murata, K. Fujiwara, K. Komatsu, O. Ito, K. Itaya, J. Am. Chem. Soc. 129 (2007) 4366. P.H. Lippel, R.J. Wilson, M.D. Miller, Ch. Wo¨ll, S. Chiang, Phys. Rev. Lett. 62 (1989) 171. X. Lu, K.W. Hipps, X.D. Wang, U. Mazur, J. Am. Chem. Soc. 118 (1996) 7197. Y. Wang, Y. Ye, K. Wu, Surf. Sci. 600 (2006) 729.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Highly oriented pyrolitic graphite anomalous structures and fullerenes self-assembly L.S. Pinheiro *, J.A.K. Freire y Departamento de Fı´sica, Universidade Federal do Ceara´, Campus do Pici, Caixa Postal 6030, CEP 60455-900 Fortaleza, Ceara´, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 July 2008 Received in revised form 31 October 2008 Accepted 22 November 2008 Available online 30 November 2008
Scanning tunneling microscopy (STM) was used to look for unusual self-structures on highly oriented pyrolitic graphite (HOPG) that can mimic fullerenes assemblies. HOPG features that may be taken as C60 molecular structures were found on this surface. The HOPG self-structures have been presented earlier as anomalies of the bare HOPG surface in the literature. The experimental results are in agreement with earlier STM reports on bare and modified HOPG. ß 2008 Elsevier B.V. All rights reserved.
PACS: 68.43.h 68.35.bp 81.05.uw 68.37.Ef Keywords: Adsorption C60 HOPG Scanning tunneling microscopy
1. Introduction Highly oriented pyrolitic graphite (HOPG) is a well-known substrate for molecular deposition and subsequent imaging by scanning tunneling microscopy (STM). This substrate gives easy atomic resolution and can be used to calibrate the STM at the atomic level. However, it has to be handled carefully when studying surfaces modified by molecules. The literature contains several reports showing the surface defects that can be mistaken by molecular structures on HOPG and there is also the existence of unusual corrugation heights [1–4]. Reports on the adsorption of C60 on HOPG show different structures for the fullerene [5–7]. Recent studies of C60 on metals and semiconductors reveal different molecular arrangements on the surface [8–12]. High resolution STM disclosing the internal features of C60 was obtained in ultra high vacuum (UHV) [9,10]. Isolated C60 molecules have been observed with the STM inside a molecular domain on Au (1 1 1) [13]. Katsonis et al. performed scanning tunneling spectroscopy at room temperature for C60 on Au (1 1 1) finding a position
* Corresponding author. Tel.: +55 85 3366 9018; fax: +55 85 3366 9450. E-mail address: [email protected] (L.S. Pinheiro). y In memorian. 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.11.060
of the energy levels similar to that observed in UHV [14]. Recent studies of C60 and polycyclic aromatic hydrocarbons on Au (1 1 1) in electrochemical environment show that the fullerene sits on top of the aromatic molecule [15]. The aim of this work is to find out if the HOPG presents defects that are similar to C60 self-assembled structures. In this work, imaging was done on bare HOPG and after exposure to solvents and molecular solutions. 2. Experimental The STM was a Nanoscope IIIA equipped with scanners A and E. All images presented were obtained in constant current mode (tip tracks down the surface). Tips were made by clipping a 0.25 mm tungsten wire in diagonal. HOPG grade ZYH (10 mm 10 mm) supplied by Veeco was employed as substrate. Cleavage of HOPG was made using an adhesive tape. C60 99.5% and copper phthalocyanine (CuPc) 98% from Aldrich were used as received. Toluene P.A was employed as solvent for the pure C60 and the mixed molecular solution of 0.8 mM C60 + 2 mM CuPc. A concentrated molecular solution of C60 (7 mM) in toluene was sonicated for 60 min and afterwards left standing for 15 min before deposition on the cleaved substrate. The pure (0.8 mM) and mixed molecular solutions of C60 and C60 + CuPc were sonicated for 15 min to increase solubility. Pure toluene and the molecular
L.S. Pinheiro, J.A.K. Freire / Applied Surface Science 255 (2009) 4512–4514
4513
Fig. 1. Constant current (CC) STM image of bare HOPG. Chain is made of bead like features spaced by 1.9–2.5 nm. Scan area: 70 nm 70 nm.
solutions were deposited on HOPG using a micropipette. The solvent or molecular solutions were deposited on freshly cleaved HOPG surface and the solvent was left to evaporate in a fume hood. 3. Results and discussion Bare HOPG cleaved and imaged at different resolutions shows steps and structures with several shapes. Fig. 1A shows an image of bare HOPG, displaying on the flat background a chain structure made of bead like objects. The round features have a periodicity of 1.9–2.5 nm. The structure crosses the step and is several nanometers long. HOPG exposed to a mixed solution of C60 + CuPc showed the common features observed for this substrate, a very flat background with steps and several other features. A high corrugated structure formed by two chains closer to a broad ridge on the terrace was found as displayed in Fig. 2A. The apparent height of the chains ranges from 0.33 to 0.68 nm. Fig. 2B shows higher resolution of the chains, they are made of bead like objects with a periodicity of 1.4–1.6 nm. The mixed solution of CuPc and C60 was used to check if a mixed domain would form on the surface. C60 is known as an acceptor and CuPc is a donor, therefore cooperative molecular domains could take place on the surface. The surface of the HOPG exposed to the mixed molecular solution show a blue stain due to the deep color of CuPc. Tunneling was done on the stained surface, however no molecular structure that could be assigned to CuPc was found. CuPc has a distinct signature on some substrates and this was not found on the modified HOPG surface [16,17]. The high resolution images of the round features are not similar to that found for C60 in UHV [9,10]. The structure observed in Fig. 2A and B is thought to be due to features of the HOPG surface. After a few minutes scanning, a third chain that has a faint contrast in Fig. 2A, appeared with a higher corrugation height close to the double rows shown in Fig. 2A and B. This could be due to dirt that the tip picked up during scanning and started to act as a new tip enhancing that feature. The pictures presented by Chang and Bard for the bare and Nafion covered HOPG are very similar to the ones observed here and those presented by Klitgaard et al. [2,7]. Wang et al. [18], discuss the triangular and honeycomb atomic
Fig. 2. (A) CC STM images of HOPG exposed to C60 + CuPc, chains made of bead like features on the flat background. Scan area: 150 nm 150 nm. (B) Higher resolution image of (A). Surface chain is formed by two rows having bead like objects with periodicity 1.4–1.6 nm. Scan area: 20 nm 20 nm.
structure formed after exposure of HOPG to an organic solvent. They also detected the presence of a feature made of bead like objects after the exposure of HOPG to the solvent. In this work, features that look like chains or isolated bead like objects on the surface were observed several times for bare or modified HOPG. The presence of CuPC did not introduce new features and just a more pronounced bead like structure already seen for the bare HOPG was detected. The deposition of a very concentrated C60 solution (7 mM) did not change the surface structure of HOPG. It displayed the features already seen for imaging on the bare substrate. In this work, tunneling on bare and modified HOPG was reached with a tunneling gap resistance in the range of 2–4 GV. Images were collected with 400 mV bias voltage and tunneling current of 100–200 pA, to minimize tip sample interaction. Experiments using 10–50 mV bias voltage and 1 nA tunnel current
4514
L.S. Pinheiro, J.A.K. Freire / Applied Surface Science 255 (2009) 4512–4514
50 MV, as seen in Fig. 3B. The border of the steps is rougher than before. Bead like features protruding from the steps are now seen in the image of Fig. 3B. These structures are separated by 1.1– 2.5 nm, close to the spacing of C60 molecules. This behavior shows that low tunneling resistances influences HOPG imaging and should be avoided. Defects that can mimic molecular structures have been discussed by Clemmer and Beebe [1]. Some reports on the subject show a collection of defects revealed by the STM after the cleavage of HOPG [2,3]. The use of solvents has also been suggested to create some patterns on the surface different from the HOPG lattice [2,18]. Here, cleavage was made using an adhesive tape and this appears to enhance the formation of such defects that can be confused with molecular structures. The origin of these mimics of molecular structures is still being interpreted and the possible electronic and structural properties resulting in superperiodic structures have been recently discussed in the literature [4]. 4. Conclusions The presence of chains made of bead like features found on bare and modified HOPG in this work are assigned to intrinsic defects of the substrate based on the experimental images. The earlier literature on the subject also shows that these defects are commonly found on HOPG. The presence of chain like features does not appear to be due to a 1D structure of C60 on HOPG but rather to a surface feature formed during cleavage of the substrate. It is advisable to employ other technique, for instance Scanning Tunneling Spectroscopy to assess the chemical/electronic structure assigned to C60 when bead like patterns are found on the HOPG surface. Acknowledgments We thank Funcap and CNPq for financial support in the form of a DCR grant to LSP. LEM of IQ-USP for donating some materials useful to the development of this work. References [1] [2] [3] [4] [5] [6] Fig. 3. (A) CC STM image of HOPG exposed to a 7 mM C60 solution. Area: 100 nm 100 nm. Vt: 400 mV. It: 100 pA. (B) CC STM image of HOPG exposed to a 7 mM C60 solution at low gap resistance. Area: 100 nm 100 nm. Vt: 50 mV. It: 1 nA.
(10–50 MV), led to modifications of the structures that were not observed previously with a tunneling gap resistance of 4 GV. These surface modifications are seen in Fig. 3A and B acquired in constant current mode. Some drift took place from Fig. 3A and B, but the surface area is close to the former one being imaged. Some noise appears on the image of Fig. 3B due to switching from the high tunneling resistance condition to the lower one. New surface features appeared while scanning with tunneling resistance of
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
C.R. Clemmer, T.P. Beebe Jr., Science 251 (1991) 640. H. Chang, A.J. Bard, Langmuir 7 (1991) 1143. W.-T. Pong, C. Durkham, J. Phys. D: Appl. Phys. 38 (2005) R329. W.T. Pong, J. Bendall, C. Durkham, Surf. Sci. 601 (2007) 498. H. Liu, P.J. Reinke, J. Chem. Phys 124 (2006) 164707. R. Szuba, R. Czajka, A. Kasuya, A. Wawro, H. Rafii-Tabar, Appl. Surf. Sci. 144/145 (1999) 648. S.K. Klitgaard, K. Egeblad, L.T. Haahr, M.K. Hansen, D. Hansen, J. Svagin, C.H. Christensen, Surf. Sci. 601 (2007) L35. S. Guo, P.M. Nagel, A.L. Deering, S.M. Van Lue, S.A. Kandel, Surf. Sci. 601 (2007) 994. C. Silien, N.A. Pradhan, W. Ho, P.A. Thiry, Phys. Rev. B 69 (2004) 115434. J.I. Pascual, J. Go´mez-Herrero, C. Rogero, A.M. Baro´, D. Sa´nchez-Portal, E. Artacho, P. Ordejon, J.M. Soler, Chem. Phys. Lett. 321 (2000) 78. M. Feng, J. Lee, J. Zhao, J.T. Yates Jr., H. Petek, J. Am. Chem. Soc. 129 (2007) 12394. N. Ne´el, J. Kro¨ger, R. Berndt, Appl. Phys. Lett 88 (2006) 163101. S. Guo, D.P. Fogarty, P.M. Nagel, S.A. Kandel, J. Phys. Chem. B 108 (2004) 14074. N. Katsonis, A. Marchenko, D. Fichou, J. Photochem. Photobiol. A: Chem. 158 (2003) 101. S. Yoshimoto, E. Tsutsumi, R. Narita, Y. Murata, M. Murata, K. Fujiwara, K. Komatsu, O. Ito, K. Itaya, J. Am. Chem. Soc. 129 (2007) 4366. P.H. Lippel, R.J. Wilson, M.D. Miller, Ch. Wo¨ll, S. Chiang, Phys. Rev. Lett. 62 (1989) 171. X. Lu, K.W. Hipps, X.D. Wang, U. Mazur, J. Am. Chem. Soc. 118 (1996) 7197. Y. Wang, Y. Ye, K. Wu, Surf. Sci. 600 (2006) 729.