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STM study of the initial stages of C60 adsorption on the Pt(1 1 0)-(1 × 2) surface
Applied Surface Science 252 (2006) 5534–5537 www.elsevier.com/locate/apsusc
STM study of the initial stages of C60 adsorption on the Pt(1 1 0)-(1 2) surface T. Orzali a,*, M. Petukhov a,b,1, M. Sambi a,b, E. Tondello a,b a
Dipartimento di Scienze Chimiche, University of Padova, Via Marzolo 1, Padova, Italy b Consorzio INSTM, Via Giusti 9, 50121 Firenze, Italy Available online 24 January 2006
Abstract We have studied the initial stages of adsorption of C60 on the Pt (1 1 0)-(1 2) surface by means of STM. At room temperature, fullerene molecules adsorb in the troughs between two adjacent Pt rows of the missing row reconstruction. Mobility over the terraces is negligible, denoting strong bonding with the surface, also testified by a well-defined orientation of fullerene monomers with respect to the substrate. Upon annealing at 750 K, molecular migration towards kinks and step edges occurs, where small islands nucleation begins. A commensurate registry with the substrate is maintained by small (5–10 molecules) C60 aggregates, leading to expanded nearest-neighbour distances with respect to those found in hexagonal close packed fullerene ad-islands grown on other metallic substrates. # 2006 Elsevier B.V. All rights reserved. PACS: 61.48.+c; 68.37.Ef Keywords: Fullerene; Platinum; Scanning tunnelling microscopy
1. Introduction Since the discovery of the most basic fullerene structure, the C60 molecule, great efforts are being made to understand its physical and chemical properties on various substrates. Since fullerenes consist in a large number of carbon atoms, several possible bonding configurations towards the substrate may occur, leading to interactions which can be quite complex: interesting phenomena are observed such as charge transfer [1], chemical reactions [2], orientational ordering and reconstruction of the substrate surface [3,4]. Much of the effort is then focused on determining how the C60–substrate interaction alters the local molecular electronic structure [5] and is an important step towards creating controllable single molecule technologies. In this field STM has been confirmed as a valuable tool for studying nucleation and growth processes of C60 films deposited on various metal single crystal substrates [3,4,6–8], where hexagonal close packed (hcp) islands at sub-monolayer and
* Corresponding author. Fax: +39 0498275161. E-mail address: [email protected] (T. Orzali). 1 Present address: IGNP, RRC ‘‘Kurchatov Institute’’, Kurchatov sq., 123182 Moscow, Russia. 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.12.149
monolayer coverage are frequently found. The interface geometry is the result of the balance between the intermolecular van der Waals (VdW) interactions, which are responsible for the hcp structure of the overlayer, and the nature of chemical bonding occurring at the ad-layer/substrate interface, which favours specific adsorption sites. Data reported in the literature [1,2,9–11] show that C60 interactions with Pt (1 1 1) are particularly strong and have been shown to have a predominantly covalent character [9], leading to immobile adsorbates bound to the surface via a hexagonal face [11]. Disordered films grown at room temperature (RT) can be turned into two ordered quasihexagonal domains through annealing at 600 K. We report here a set of preliminary results on the initial stages of deposition of C60 molecules on the Pt (1 1 0) surface, obtained by using UHV-STM. The Pt (1 1 0) surface undergoes a (1 2) reconstruction of the missing row type [12], which exhibits a one-dimensional corrugation in the [0 0 1] direction, resulting in a series of atomic ridges running along ½1 1¯ 0, ˚ . This separated by troughs at a mutual distance of 7.84 A surface geometry represents a strong constraint on the favoured formation of the C60 hcp overlayer structure with a nearestneighbour (NN) distance close to the VdW diameter of solid ˚ [13]. Previously reported results on C60 fullerene, a = 10.04 A
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deposited on the similar Au (1 1 0)-(1 2) reconstruction showed that the deposit actually rearranges the substrate surface into a new reconstruction pattern in order to form commensurate overlayers with a quasi-hcp structure [3,4]. The aim of the present study is to investigate the behaviour of C60 molecules during the first stages of growth on Pt (1 1 0)(1 2), whose anisotropic corrugation is expected to have strong effects on molecular diffusion and ordering. 2. Experimental The experiments were performed with an Omicron scanning tunnelling microscope (VT-STM) operating in ultra-high vacuum at a base pressure of 5 1011 mbar. The Pt (1 1 0) single crystal was prepared by standard sputtering (KE = 2 keV) and annealing (T = 970 K) cycles, followed by cooling in oxygen ( pO2 = 5 107 mbar) down to 700 K. C60 (99% purity) was sublimed at 820 K from a tungsten crucible. The Pt specimen was kept at room temperature during sublimation. 3. Results and discussion Fig. 1 shows the initial stage of adsorption of C60 molecules on the Pt (1 1 0)-(1 2) surface at room temperature. Almost all C60 molecules are adsorbed as isolated monomers set in the troughs between two Pt ridges. Step edges are not favoured as adsorption sites and no island nucleation is observed either on terraces or at steps and kinks. Rather, molecules are randomly distributed over the channels, as if they adsorb almost in the place where they land, after a transient needed to optimize their bonding to two adjacent Pt ridges. This adsorption behaviour is thus different than on Cu [7], Au [14] and Ag [7,15], and clearly indicates the existence of strong bonding with the substrate, already evidenced in the case of Pt (1 1 1) [9]. The average diameter of isolated C60 ˚ , a rather large value compared to molecules is equal to 17 A ˚ ): it is clearly a tip the VdW diameter of the molecule (10 A convolution effect over a high aspect-ratio feature. The apparent height of imaged molecules measured with respect to the Pt ridges (and calibrated against a Pt monoatomic step ˚ ) is 6.0 0.3 A ˚ . The value is largely height of 1.4 0.1 A independent on the applied bias and considerably smaller than the molecule’s VdW diameter, reflecting the effect of bonding to the substrate. Two interesting features are observed in Fig. 1: the first one is the C60 trimer in the lower right corner. Each of the three molecules is adsorbed on adjacent Pt troughs. Since the sum of their radii exceeds the channel spacing, adjacent molecules cannot align perpendicularly to the Pt ridges, but are forced to form an angle of 54 18 with the close packed Pt rows running along the ½1 1¯ 0 substrate azimuth. In this case the C60 NN ˚ , a large value if compared to the distance is approximately 12 A cubic close packed fullerene lattice parameter [13] and for hcp 2D islands found on more weakly interacting substrate metals ˚ ). Interestingly, molecules are not aligned within the [7] (10 A same Pt channel as one may expect, since ridge crossing is
Fig. 1. STM image of the Pt (1 1 0)-(1 2) surface after 1 min of C60 deposition at RT (VS = 0.21 V; It = 1.46 nA; 47 nm 47 nm). Left inset: a fullerene dimer aligned along the ½1 1¯ 0 direction (VS = 0.21 V; It = 1.46 nA; 6 nm 9 nm). Middle inset: STM image of clean Pt (1 1 0) (VS = 0.055 V; It = 1.37 nA; 48 nm 47 nm). Right inset: current image of a single fullerene molecule (VS = 0.76 V; It = 1.46 nA; 4.5 nm 5.5 nm).
hampered at RT by a too high activation barrier. Occasionally, some dimers and trimers are found to line up along the same channel (see the left inset in Fig. 1), in this case their mutual ˚. distance amounts to the expected 10 A The second interesting feature is the presence of two molecules (highlighted by white arrows in Fig. 1) which are ˚ apparently smaller than the others: their dimensions are 13.5 A ˚ in height from the Pt ridges. These in diameter and 4.0 0.2 A two molecules are adsorbed asymmetrically over a Pt ridge rather than symmetrically over a trough. C60 molecular species characterized by different shapes and brightness have been previously observed on Pd (1 1 0) [16] and on Si (1 0 0) surfaces [17], and the effect has been attributed to molecular chemisorption over surface defects, such as vacancies. The middle inset in Fig. 1 reports an STM image of the clean Pt (1 1 0) surface: defects identified as Pt vacancies or more extended vacancy islands are observed in several rows. In analogy with the Pd (1 1 0) case [16], we explain the existence of smaller C60 molecules by suggesting their preferential adsorption on the observed defects, with a resulting electron density redistribution due to the increased C60–Pt coordination. The right inset in Fig. 1 shows that the intramolecular structure of adsorbed fullerenes can be resolved at RT, confirming the strong interaction with the substrate that
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prevents molecular self-rotation. The discernible three-fold symmetry of the intramolecular fine structure points to bonding to the surface through a hexagonal ring [11]. After an annealing treatment at 750 K followed by cooling back to 300 K, molecular migration towards kinks and step edges is observed. Small islands nucleation occurs, whose size depends on the fullerene dose. The temperature-induced increase of the diffusion coefficient allows nucleation exclusively at step edges, since at this temperature the mobility of C60 molecules on Pt (1 1 0) is high enough for their mean free path to match the mean terrace width. Due to the anisotropy of Pt (1 1 0)-(1 2), the activation barrier for diffusion along the ½1 1¯ 0 close packed rows is expected to be considerably smaller than along the strongly corrugated [0 0 1] direction. Indeed, STM data show that diffusion paths along ½1 1¯ 0 are predominant, as deduced by the observation that molecules diffuse preferentially towards corner sites and step edges that are perpendicular to the Pt rows and never at step edges that are parallel to them. This is evident in the STM image reported in Fig. 2, where the effects of annealing over the so-called fish scale pattern [18,19] of the Pt (1 1 0)-(1 2) surface are shown. The high annealing temperatures (up to 850 K) reached for ordering deposited molecules without observing a significant desorption of C60 from the surface is another indication of the strong binding between fullerenes and the substrate. Fig. 3 reports an STM image of Pt (1 1 0)-(1 2) annealed at 850 K after 3 min of C60 deposition at RT. As a consequence of the annealing, fullerene molecules diffuse towards step edges where they coalesce into ordered islands. The inset of Fig. 3 displays the surface before annealing: the comparison shows the consequence of the thermal treatment in terms of segregation and coalescence. As already pointed out, nucleation occurs at [0 0 1] oriented kinks of the ½1 1¯ 0 steps, where
Fig. 2. STM image of the Pt (1 1 0)-(1 2) surface annealed at 750 K after 1 min of C60 deposition (VS = 0.14 V; It = 1.46 nA; 65 65 nm).
Fig. 3. STM image of the Pt (1 1 0)-(1 2) surface annealed at 850 K after 3 min of C60 deposition (VS = 0.18 V; It = 2.75 nA; 48 50 nm). Inset: STM image of the Pt surface before annealing (VS = 0.14 V; It = 1.50 nA; 76 78 nm).
small clusters of few atoms start to grow, with the occasional exception of somewhat larger aggregates, e.g. the 18 molecules island in the bottom-left corner of Fig. 3. Molecules decorate exclusively the lower level of step edges, indicating significant lateral attractive interactions between Pt atoms and C60 molecules. Fullerenes arrange with an incipient quasihexagonal geometry, as evidenced in the top part of Fig. 3, where some clusters show a characteristic zig-zag arrangement induced by the surface corrugation constraint, and even more clearly by the already mentioned 18 molecules fullerene adisland. Single molecules within a C60 cluster can be easily distinguished: the intermolecular distance, as in the case of isolated dimers and trimers on the as-deposited surface, is ˚ . Evidently, the strong constraint posed by the surface 12 A geometry on the favoured formation of the C60 hcp overlayer structure is maintained even in the presence of a high temperature treatment, at least at low C60 coverages. It should be mentioned, though, that the apparent dimensions of buckyballs after annealing are smaller than immediately after ˚ high deposition at RT: isolated molecules are now 5.0 0.3 A ˚ large while, when part of a cluster, they are and 14.5 A ˚ high and 13 A ˚ large (again considering the tip 4.5 0.3 A convolution effect). The latter effect seems to point to a reoptimization in bonding as a consequence of the thermal treatment. Since at low coverage clusters grow at kinks rather than over terraces, it is difficult to identify the substrate–adsorbate interface registry. However, it appears that for small fullerene clusters (5–10 molecules) the pseudo-hexagonal arrangement is commensurate to the Pt surface, which mainly means that fullerenes remain adsorbed in the troughs between two Pt rows.
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In the case of the larger island on the bottom-left side of the figure, the simple registry with the channel array is partially broken by the presence of a (1 4) line defect (see the dotted white arrow in Fig. 3), which forces some molecules in the right side of the aggregate to adsorb over Pt ridges in order to keep the quasi-hexagonal packing. Remarkably, the C60 ˚ nearest-neighbour distance is reduced to approximately 11 A in the area affected by the line defect. Further investigation at higher coverages is required to understand if the occurrence of line defects in the substrate is induced by the nucleation of close packed fullerene islands and if line defect formation is the first stage of a new substrate reconstruction, similar to that observed on Au (1 1 0), driven by the optimization of intermolecular bonding [3,4]. A hint in this direction is given by the fact that after annealing at high temperatures in the presence of adsorbed C60 molecules, some (1 3) missing row troughs (see white arrows in Fig. 3) appear in the Pt (1 1 0)-(1 2) terraces supporting C60 ad-islands, which coexist with small, not atomically resolved areas of presumably (1 1) periodicity (see areas marked as ‘‘A’’ in Fig. 3). Evidently, lateral C60–C60 interactions have a relevant role in determining the final and most stable cluster/substrate registry, even in the presence of strong bonding of the molecular overlayer to the surface. 4. Conclusions We have presented a preliminary STM analysis of the deposition of C60 on the Pt (1 1 0)-(1 2) surface. In the initial stages of adsorption at RT, C60 are bound over the troughs between successive Pt rows. The strong adsorbate–substrate interaction that freezes the deposited molecules in the place where they land is overcome upon thermal treatment at temperatures over 700 K: molecules migrate towards kinks and step edges where small island nucleation occurs. Due to the anisotropy of the Pt (1 1 0) surface, diffusion paths along the ½1 1¯ 0 direction are predominant. Molecules decorate the lower level of step edges and rearrange with short-range quasihexagonal geometry.
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Acknowledgements This work has been partially funded by MIUR through the FIRB research program ‘‘Nano- and micro-spectroscopy by synchrotron radiation integrated with advanced STM/AFM systems to study manmade atomic scale functional materials’’ (prot. N. RBNE0155X7) and by the University of Padua through the grant CPDA038285. References [1] M. Pedio, K. Hevesi, N. Zema, M. Capozi, P. Perfetti, R. Gouttebaron, J.-J. Pireaux, R. Caudano, P. Rudolf, Surf. Sci. 437 (1999) 249. [2] N. Swami, H. He, B.E. Koel, Phys. Rev. B 59 (1999) 8283. [3] J.K. Gimzewski, S. Modesti, R.R. Schlittler, Phys. Rev. Lett. 72 (1994) 1036. [4] S. Modesti, J.K. Gimzewski, R.R. Schlittler, Surf. Sci. 331–333 (1995) 1129. [5] K. Lu, M. Grobis, K.H. Khoo, S.G. Louie, M.F. Crommie, Phys. Rev. Lett. 90 (2003) 96802, and references therein. [6] C. Rogero, J.I. Pascual, J. Gomez-Herrero, A.M. Baro`, J. Chem. Phys. 116 (2002) 832. [7] T. Sakurai, X.D. Wang, T. Hashizume, V. Yurov, H. Shinohara, H.W. Pickering, Appl. Surf. Sci. 87–88 (1995) 405. [8] E. Giudice, E. Magnano, S. Rusponi, C. Boragno, U. Valbusa, Surf. Sci. 405 (1998) L561. [9] C. Cepek, A. Goldoni, S. Modesti, Phys. Rev. B 53 (1996) 7466. [10] H. He, N. Swami, B.E. Koel, Thin Solid Films 348 (1999) 30. [11] L. Giovanelli, C. Cepek, L. Floreano, E. Magnano, M. Sancrotti, R. Gotter, A. Morgante, A. Verdini, A. Pesci, L. Ferrari, M. Pedio, Appl. Surf. Sci. 212–213 (2003) 57. [12] K. Swamy, E. Bertel, I. Vilfan, Surf. Sci. 425 (1999) L369. [13] P.A. Heiney, J.E. Fisher, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley Jr., A.B. Smith III, D.E. Cox, Phys. Rev. Lett. 66 (1991) 2911. [14] E.I. Altman, R.J. Colton, Surf. Sci. 279 (1992) 49. [15] G. Costantini, S. Rusponi, E. Giudice, C. Boragno, U. Valbusa, Carbon 37 (1999) 727. [16] J. Weckesser, J.V. Barth, K. Kern, Phys. Rev. B 64 (2001) 161403. [17] X. Yao, R.K. Workman, C.A. Peterson, D. Chen, D. Sarid, Appl. Phys. A 66 (1998) S107. [18] T. Gritsch, D. Coulman, R.J. Behm, G. Ertl, Surf. Sci. 257 (1991) 297. [19] S. Speller, J. Kuntze, T. Rauch, J. Bo¨mermann, M. Huck, M. Aschoff, W. Heiland, Surf. Sci. 366 (1996) 251.
STM study of the initial stages of C60 adsorption on the Pt(1 1 0)-(1 2) surface T. Orzali a,*, M. Petukhov a,b,1, M. Sambi a,b, E. Tondello a,b a
Dipartimento di Scienze Chimiche, University of Padova, Via Marzolo 1, Padova, Italy b Consorzio INSTM, Via Giusti 9, 50121 Firenze, Italy Available online 24 January 2006
Abstract We have studied the initial stages of adsorption of C60 on the Pt (1 1 0)-(1 2) surface by means of STM. At room temperature, fullerene molecules adsorb in the troughs between two adjacent Pt rows of the missing row reconstruction. Mobility over the terraces is negligible, denoting strong bonding with the surface, also testified by a well-defined orientation of fullerene monomers with respect to the substrate. Upon annealing at 750 K, molecular migration towards kinks and step edges occurs, where small islands nucleation begins. A commensurate registry with the substrate is maintained by small (5–10 molecules) C60 aggregates, leading to expanded nearest-neighbour distances with respect to those found in hexagonal close packed fullerene ad-islands grown on other metallic substrates. # 2006 Elsevier B.V. All rights reserved. PACS: 61.48.+c; 68.37.Ef Keywords: Fullerene; Platinum; Scanning tunnelling microscopy
1. Introduction Since the discovery of the most basic fullerene structure, the C60 molecule, great efforts are being made to understand its physical and chemical properties on various substrates. Since fullerenes consist in a large number of carbon atoms, several possible bonding configurations towards the substrate may occur, leading to interactions which can be quite complex: interesting phenomena are observed such as charge transfer [1], chemical reactions [2], orientational ordering and reconstruction of the substrate surface [3,4]. Much of the effort is then focused on determining how the C60–substrate interaction alters the local molecular electronic structure [5] and is an important step towards creating controllable single molecule technologies. In this field STM has been confirmed as a valuable tool for studying nucleation and growth processes of C60 films deposited on various metal single crystal substrates [3,4,6–8], where hexagonal close packed (hcp) islands at sub-monolayer and
* Corresponding author. Fax: +39 0498275161. E-mail address: [email protected] (T. Orzali). 1 Present address: IGNP, RRC ‘‘Kurchatov Institute’’, Kurchatov sq., 123182 Moscow, Russia. 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.12.149
monolayer coverage are frequently found. The interface geometry is the result of the balance between the intermolecular van der Waals (VdW) interactions, which are responsible for the hcp structure of the overlayer, and the nature of chemical bonding occurring at the ad-layer/substrate interface, which favours specific adsorption sites. Data reported in the literature [1,2,9–11] show that C60 interactions with Pt (1 1 1) are particularly strong and have been shown to have a predominantly covalent character [9], leading to immobile adsorbates bound to the surface via a hexagonal face [11]. Disordered films grown at room temperature (RT) can be turned into two ordered quasihexagonal domains through annealing at 600 K. We report here a set of preliminary results on the initial stages of deposition of C60 molecules on the Pt (1 1 0) surface, obtained by using UHV-STM. The Pt (1 1 0) surface undergoes a (1 2) reconstruction of the missing row type [12], which exhibits a one-dimensional corrugation in the [0 0 1] direction, resulting in a series of atomic ridges running along ½1 1¯ 0, ˚ . This separated by troughs at a mutual distance of 7.84 A surface geometry represents a strong constraint on the favoured formation of the C60 hcp overlayer structure with a nearestneighbour (NN) distance close to the VdW diameter of solid ˚ [13]. Previously reported results on C60 fullerene, a = 10.04 A
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deposited on the similar Au (1 1 0)-(1 2) reconstruction showed that the deposit actually rearranges the substrate surface into a new reconstruction pattern in order to form commensurate overlayers with a quasi-hcp structure [3,4]. The aim of the present study is to investigate the behaviour of C60 molecules during the first stages of growth on Pt (1 1 0)(1 2), whose anisotropic corrugation is expected to have strong effects on molecular diffusion and ordering. 2. Experimental The experiments were performed with an Omicron scanning tunnelling microscope (VT-STM) operating in ultra-high vacuum at a base pressure of 5 1011 mbar. The Pt (1 1 0) single crystal was prepared by standard sputtering (KE = 2 keV) and annealing (T = 970 K) cycles, followed by cooling in oxygen ( pO2 = 5 107 mbar) down to 700 K. C60 (99% purity) was sublimed at 820 K from a tungsten crucible. The Pt specimen was kept at room temperature during sublimation. 3. Results and discussion Fig. 1 shows the initial stage of adsorption of C60 molecules on the Pt (1 1 0)-(1 2) surface at room temperature. Almost all C60 molecules are adsorbed as isolated monomers set in the troughs between two Pt ridges. Step edges are not favoured as adsorption sites and no island nucleation is observed either on terraces or at steps and kinks. Rather, molecules are randomly distributed over the channels, as if they adsorb almost in the place where they land, after a transient needed to optimize their bonding to two adjacent Pt ridges. This adsorption behaviour is thus different than on Cu [7], Au [14] and Ag [7,15], and clearly indicates the existence of strong bonding with the substrate, already evidenced in the case of Pt (1 1 1) [9]. The average diameter of isolated C60 ˚ , a rather large value compared to molecules is equal to 17 A ˚ ): it is clearly a tip the VdW diameter of the molecule (10 A convolution effect over a high aspect-ratio feature. The apparent height of imaged molecules measured with respect to the Pt ridges (and calibrated against a Pt monoatomic step ˚ ) is 6.0 0.3 A ˚ . The value is largely height of 1.4 0.1 A independent on the applied bias and considerably smaller than the molecule’s VdW diameter, reflecting the effect of bonding to the substrate. Two interesting features are observed in Fig. 1: the first one is the C60 trimer in the lower right corner. Each of the three molecules is adsorbed on adjacent Pt troughs. Since the sum of their radii exceeds the channel spacing, adjacent molecules cannot align perpendicularly to the Pt ridges, but are forced to form an angle of 54 18 with the close packed Pt rows running along the ½1 1¯ 0 substrate azimuth. In this case the C60 NN ˚ , a large value if compared to the distance is approximately 12 A cubic close packed fullerene lattice parameter [13] and for hcp 2D islands found on more weakly interacting substrate metals ˚ ). Interestingly, molecules are not aligned within the [7] (10 A same Pt channel as one may expect, since ridge crossing is
Fig. 1. STM image of the Pt (1 1 0)-(1 2) surface after 1 min of C60 deposition at RT (VS = 0.21 V; It = 1.46 nA; 47 nm 47 nm). Left inset: a fullerene dimer aligned along the ½1 1¯ 0 direction (VS = 0.21 V; It = 1.46 nA; 6 nm 9 nm). Middle inset: STM image of clean Pt (1 1 0) (VS = 0.055 V; It = 1.37 nA; 48 nm 47 nm). Right inset: current image of a single fullerene molecule (VS = 0.76 V; It = 1.46 nA; 4.5 nm 5.5 nm).
hampered at RT by a too high activation barrier. Occasionally, some dimers and trimers are found to line up along the same channel (see the left inset in Fig. 1), in this case their mutual ˚. distance amounts to the expected 10 A The second interesting feature is the presence of two molecules (highlighted by white arrows in Fig. 1) which are ˚ apparently smaller than the others: their dimensions are 13.5 A ˚ in height from the Pt ridges. These in diameter and 4.0 0.2 A two molecules are adsorbed asymmetrically over a Pt ridge rather than symmetrically over a trough. C60 molecular species characterized by different shapes and brightness have been previously observed on Pd (1 1 0) [16] and on Si (1 0 0) surfaces [17], and the effect has been attributed to molecular chemisorption over surface defects, such as vacancies. The middle inset in Fig. 1 reports an STM image of the clean Pt (1 1 0) surface: defects identified as Pt vacancies or more extended vacancy islands are observed in several rows. In analogy with the Pd (1 1 0) case [16], we explain the existence of smaller C60 molecules by suggesting their preferential adsorption on the observed defects, with a resulting electron density redistribution due to the increased C60–Pt coordination. The right inset in Fig. 1 shows that the intramolecular structure of adsorbed fullerenes can be resolved at RT, confirming the strong interaction with the substrate that
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prevents molecular self-rotation. The discernible three-fold symmetry of the intramolecular fine structure points to bonding to the surface through a hexagonal ring [11]. After an annealing treatment at 750 K followed by cooling back to 300 K, molecular migration towards kinks and step edges is observed. Small islands nucleation occurs, whose size depends on the fullerene dose. The temperature-induced increase of the diffusion coefficient allows nucleation exclusively at step edges, since at this temperature the mobility of C60 molecules on Pt (1 1 0) is high enough for their mean free path to match the mean terrace width. Due to the anisotropy of Pt (1 1 0)-(1 2), the activation barrier for diffusion along the ½1 1¯ 0 close packed rows is expected to be considerably smaller than along the strongly corrugated [0 0 1] direction. Indeed, STM data show that diffusion paths along ½1 1¯ 0 are predominant, as deduced by the observation that molecules diffuse preferentially towards corner sites and step edges that are perpendicular to the Pt rows and never at step edges that are parallel to them. This is evident in the STM image reported in Fig. 2, where the effects of annealing over the so-called fish scale pattern [18,19] of the Pt (1 1 0)-(1 2) surface are shown. The high annealing temperatures (up to 850 K) reached for ordering deposited molecules without observing a significant desorption of C60 from the surface is another indication of the strong binding between fullerenes and the substrate. Fig. 3 reports an STM image of Pt (1 1 0)-(1 2) annealed at 850 K after 3 min of C60 deposition at RT. As a consequence of the annealing, fullerene molecules diffuse towards step edges where they coalesce into ordered islands. The inset of Fig. 3 displays the surface before annealing: the comparison shows the consequence of the thermal treatment in terms of segregation and coalescence. As already pointed out, nucleation occurs at [0 0 1] oriented kinks of the ½1 1¯ 0 steps, where
Fig. 2. STM image of the Pt (1 1 0)-(1 2) surface annealed at 750 K after 1 min of C60 deposition (VS = 0.14 V; It = 1.46 nA; 65 65 nm).
Fig. 3. STM image of the Pt (1 1 0)-(1 2) surface annealed at 850 K after 3 min of C60 deposition (VS = 0.18 V; It = 2.75 nA; 48 50 nm). Inset: STM image of the Pt surface before annealing (VS = 0.14 V; It = 1.50 nA; 76 78 nm).
small clusters of few atoms start to grow, with the occasional exception of somewhat larger aggregates, e.g. the 18 molecules island in the bottom-left corner of Fig. 3. Molecules decorate exclusively the lower level of step edges, indicating significant lateral attractive interactions between Pt atoms and C60 molecules. Fullerenes arrange with an incipient quasihexagonal geometry, as evidenced in the top part of Fig. 3, where some clusters show a characteristic zig-zag arrangement induced by the surface corrugation constraint, and even more clearly by the already mentioned 18 molecules fullerene adisland. Single molecules within a C60 cluster can be easily distinguished: the intermolecular distance, as in the case of isolated dimers and trimers on the as-deposited surface, is ˚ . Evidently, the strong constraint posed by the surface 12 A geometry on the favoured formation of the C60 hcp overlayer structure is maintained even in the presence of a high temperature treatment, at least at low C60 coverages. It should be mentioned, though, that the apparent dimensions of buckyballs after annealing are smaller than immediately after ˚ high deposition at RT: isolated molecules are now 5.0 0.3 A ˚ large while, when part of a cluster, they are and 14.5 A ˚ high and 13 A ˚ large (again considering the tip 4.5 0.3 A convolution effect). The latter effect seems to point to a reoptimization in bonding as a consequence of the thermal treatment. Since at low coverage clusters grow at kinks rather than over terraces, it is difficult to identify the substrate–adsorbate interface registry. However, it appears that for small fullerene clusters (5–10 molecules) the pseudo-hexagonal arrangement is commensurate to the Pt surface, which mainly means that fullerenes remain adsorbed in the troughs between two Pt rows.
T. Orzali et al. / Applied Surface Science 252 (2006) 5534–5537
In the case of the larger island on the bottom-left side of the figure, the simple registry with the channel array is partially broken by the presence of a (1 4) line defect (see the dotted white arrow in Fig. 3), which forces some molecules in the right side of the aggregate to adsorb over Pt ridges in order to keep the quasi-hexagonal packing. Remarkably, the C60 ˚ nearest-neighbour distance is reduced to approximately 11 A in the area affected by the line defect. Further investigation at higher coverages is required to understand if the occurrence of line defects in the substrate is induced by the nucleation of close packed fullerene islands and if line defect formation is the first stage of a new substrate reconstruction, similar to that observed on Au (1 1 0), driven by the optimization of intermolecular bonding [3,4]. A hint in this direction is given by the fact that after annealing at high temperatures in the presence of adsorbed C60 molecules, some (1 3) missing row troughs (see white arrows in Fig. 3) appear in the Pt (1 1 0)-(1 2) terraces supporting C60 ad-islands, which coexist with small, not atomically resolved areas of presumably (1 1) periodicity (see areas marked as ‘‘A’’ in Fig. 3). Evidently, lateral C60–C60 interactions have a relevant role in determining the final and most stable cluster/substrate registry, even in the presence of strong bonding of the molecular overlayer to the surface. 4. Conclusions We have presented a preliminary STM analysis of the deposition of C60 on the Pt (1 1 0)-(1 2) surface. In the initial stages of adsorption at RT, C60 are bound over the troughs between successive Pt rows. The strong adsorbate–substrate interaction that freezes the deposited molecules in the place where they land is overcome upon thermal treatment at temperatures over 700 K: molecules migrate towards kinks and step edges where small island nucleation occurs. Due to the anisotropy of the Pt (1 1 0) surface, diffusion paths along the ½1 1¯ 0 direction are predominant. Molecules decorate the lower level of step edges and rearrange with short-range quasihexagonal geometry.
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Acknowledgements This work has been partially funded by MIUR through the FIRB research program ‘‘Nano- and micro-spectroscopy by synchrotron radiation integrated with advanced STM/AFM systems to study manmade atomic scale functional materials’’ (prot. N. RBNE0155X7) and by the University of Padua through the grant CPDA038285. References [1] M. Pedio, K. Hevesi, N. Zema, M. Capozi, P. Perfetti, R. Gouttebaron, J.-J. Pireaux, R. Caudano, P. Rudolf, Surf. Sci. 437 (1999) 249. [2] N. Swami, H. He, B.E. Koel, Phys. Rev. B 59 (1999) 8283. [3] J.K. Gimzewski, S. Modesti, R.R. Schlittler, Phys. Rev. Lett. 72 (1994) 1036. [4] S. Modesti, J.K. Gimzewski, R.R. Schlittler, Surf. Sci. 331–333 (1995) 1129. [5] K. Lu, M. Grobis, K.H. Khoo, S.G. Louie, M.F. Crommie, Phys. Rev. Lett. 90 (2003) 96802, and references therein. [6] C. Rogero, J.I. Pascual, J. Gomez-Herrero, A.M. Baro`, J. Chem. Phys. 116 (2002) 832. [7] T. Sakurai, X.D. Wang, T. Hashizume, V. Yurov, H. Shinohara, H.W. Pickering, Appl. Surf. Sci. 87–88 (1995) 405. [8] E. Giudice, E. Magnano, S. Rusponi, C. Boragno, U. Valbusa, Surf. Sci. 405 (1998) L561. [9] C. Cepek, A. Goldoni, S. Modesti, Phys. Rev. B 53 (1996) 7466. [10] H. He, N. Swami, B.E. Koel, Thin Solid Films 348 (1999) 30. [11] L. Giovanelli, C. Cepek, L. Floreano, E. Magnano, M. Sancrotti, R. Gotter, A. Morgante, A. Verdini, A. Pesci, L. Ferrari, M. Pedio, Appl. Surf. Sci. 212–213 (2003) 57. [12] K. Swamy, E. Bertel, I. Vilfan, Surf. Sci. 425 (1999) L369. [13] P.A. Heiney, J.E. Fisher, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley Jr., A.B. Smith III, D.E. Cox, Phys. Rev. Lett. 66 (1991) 2911. [14] E.I. Altman, R.J. Colton, Surf. Sci. 279 (1992) 49. [15] G. Costantini, S. Rusponi, E. Giudice, C. Boragno, U. Valbusa, Carbon 37 (1999) 727. [16] J. Weckesser, J.V. Barth, K. Kern, Phys. Rev. B 64 (2001) 161403. [17] X. Yao, R.K. Workman, C.A. Peterson, D. Chen, D. Sarid, Appl. Phys. A 66 (1998) S107. [18] T. Gritsch, D. Coulman, R.J. Behm, G. Ertl, Surf. Sci. 257 (1991) 297. [19] S. Speller, J. Kuntze, T. Rauch, J. Bo¨mermann, M. Huck, M. Aschoff, W. Heiland, Surf. Sci. 366 (1996) 251.