Adsorption of fullerenes on Cu(111) and Ag(111) surfaces

apolieO surface ELSEVIER

science

Applied Surface Science 87/88 (1995) 405-413

Adsorption of fullerenes on Cu(111) and

111) surfaces

T. Sakurai *, X.D. Wang, T. Hashizume, V. Yurov, H. Shinohara i H.W. Pickering 2 Institute for Materials Research (1MR), Tohoku University, Sendai 980-77, Japan

Received 11 July 1994; acceptedfor publication 23 September1994

Abstract We have studied the initial stage adsorption and film growth of C60 , C70 , and C60(x)C70(l_x) o n Cu(lll) and Ag(lll) surfaces using field-ion scanning tunneling microscopy. Fullerene molecules are mobile on the terrace of the metal surfaces and initially segregate to the step edges. A well-ordered two-dimensional overlayer forms with a close-packed arrangement upon annealing the fullerene covered surfaces.

1. Introduction Physical properties of fullerene films on semiconductor and metal surfaces have been intensively studied after the discovery of C60 by Kroto et al. [1] and the development of the method of extracting and purifying C60 and other fullerenes by Kr~itschmer et al. [2,3]. Scanning tunneling microscopy (STM) [4,5] has been used to investigate the surface and electronic structures of fullerene films on HOPG (highly oriented pyrolytic graphite) (0001) [6], GaAs(ll0) [7], Au(111) [8-12], (110) [12,13] and (100) [14], A g ( l l l ) [11], and Si(111) [15,16] and (100) [17-19] surfaces. On the inert surfaces such as HOPG(0001) [6] and GaAs(110) [7] where the interaction between the substrate and the adsorbates is not strong enough,

* Corresponding author. E-mail: [email protected];Fax: + 81 22 215 2020. 1 Permanent address: Chemistry Department, Nagoya University, Nagoya, Japan. 2 Permanent address: Department of Materials Science and Engineering, The PennsylvaniaState University, University Park, PA 16802, USA.

smeared-out round shape STM images of C60 molecules were observed, which were interpreted as rotating C60 molecules similar to those in the bulk fcc crystal. We observed striped intramolecular structures for the C60 adsorption on the S i ( l l l ) 7 X 7 and Si(100)2 X 1 surfaces [15,17,18], due to the strong chemical bonds between the dangling bonds of Si and the C60 molecules, which suppresses the freedom of the C60 molecules. The STM images were compared with theoretical calculations [20] and the intramolecular structures were interpreted as a three-dimensional (3D) mapping of the local density of states of the HOMO (highest occupied molecular orbital) band of the C60 monolayer film [18]. Realizing the strong interaction between C60 and Si surfaces, we have investigated the monolayer film formation on the C u ( l l l ) l X 1 [21,22] and A g ( l l l ) l × 1 [23] surfaces.

2. Experimental The apparatus we have used in this study is the FI-STM which is a high-performance STM equipped

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with a room-temperature field ion microscope (RTFIM) [24]. The atomic arrangement of the scanning tip can be monitored and an ideal single-atom tip can be fabricated by field evaporation by using the RTFIM [24]. The C u ( l l l ) and A g ( l l l ) samples were cut from a single crystal bar of 99.9999% pure copper and 99.999% pure silver with a miscut of less than 0.5 °. The surface was polished using a standard technique and was cleaned in situ by a combination of As sputtering. High-purity fullerene powders (99.95% and 99.5% for C60 and C70, respectively) [25] or mixed C60(x)C70(l_x) were placed in a small Ta dispenser, which was resistively heated to approximately 370°C (for C6o) to 500°C to evaporate the fullerene molecules on to the substrate placed at 1 cm away from the dispenser. The operation pressure is 4 X 10-11 Tort. The pressure during the fullerene adsorption was kept below 1 X 10 -1° Tort after outgassing the doser. For STM imaging, sample biases (V~) between - 3 . 5 V and 4-3.5 V were used, while the tunneling current I t was kept constant at 20 pA.

Fig. 2. STM image of the two-dimensional (2D) islands of C60 gorowing from the step edges of the Cu(111)-1X 1 surface (320 A×320 A, V~= -2.5 V).

Fig. 1 shows the initial stage adsorption of C60 molecules on the Cu(111)1 X 1 surface at room tem-

perature. The C60 molecules are mobile on the terrace and segregate to the steps and adsorb on top of the step edges. As the coverage increases, C60 molecules start to form two-dimensional (2D) islands and grow from the step edges in the direction of the ascending steps (Fig. 2) until they reach to the next step edges, resulting in a monolayer film of C6o (Fig. 3). In Fig. 3, the steps formed in the C60 monolayer film replicate those in the original C u ( l l l ) surface. The exception is the 2nd layer we observe in the center of Fig. 3 with very bright contrast. The mono-

Fig. 1. Typical STM image of the Cu(lll)-i X 1 surface covered with 0.01 monolayerof C60 molecules (640 A × 640 A, Vs = - 3.5 V).

Fig. 3. STM image of the monolayer film of C60 molecules deposited at room temperature (320 AX320 A, V~= -2.0 V).

3. Results and discussion 3.1. Cu(111)-C6o

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in the monolayer film is 10.2 A, which is 4 times the nnd of Cu atoms on the C u ( l l l ) surface (a 0 = 2.56 A). We observe the (4 X 4) LEED pattern for the monolayer film of C6o, consistent with a recent study using XPS and HREELS and other techniques [26]. Individual C60 molecules are imaged at bias voltages (V~) close to zero. This suggests that the gap states are induced by charge transfer from the substrate to the C60 overlayer, which enables us to image the C60 molecule with Vs within the H O M O (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) band gap. We note that individual C60 molecules appear in a three-leaf-clover shape at V~ = + 2.0 V (Figs. 4 and 5c). When the surface is imaged at a different bias voltage of Vs = - 2 . 0 V (Fig. 5a), each C60 molecule appears in a doughnut shape, almost round with a hole at the center. When bias voltages of close to zero are used, we observe a triangular shaped intramolecular structure (Fig: 5b). Similar to the case of the C60 adsorption on the Si(100)2 X 1 surface [17,18], the C60 molecules ratchet (the free rotation is suppressed) to a specific direction on the terraces of the C u ( l l l ) surface and intramolecular structures are observed, in order for the C60 molecule to display the three-fold symmetry, one of the hexagonal rings of a C60 molecule should face down on the Cu(111) surface. The atop site and three-fold hollow site are possible for the substrate adsorption position. If the C60 molecule occupies the three-fold hollow site, there can be two adsorption positions (the so-called hcp and fcc sites) and thus, the C60 adsorption may have

Fig. 4. STM image of the monolayer film of C60 molecules prepared by annealing at 290°C (230 A x 230 A, V~= + 2.0 V). layer film grown at room temperature shows less ordering and we can observe several domains existing on the surface. Fig. 4 shows an STM image of the C60 monolayer film formed after annealing at 290°C. The surface is highly ordered, except for point defects and domain boundaries. We find the steps are lined up along the (0~1) direction, which is the substrate atomic row direction, while the steps are not straight for the case of the C60 monolayer film formed by room temperature adsorption (Fig. 3). This suggests a significant mass transport of the surface Cu atoms while the surface is being annealed at 290°C and the stability of the :C60 chain in the (011) direction. The nearest-neighbor distance (nnd) of the C60 molecules Vs = -2,0V

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Fig. 5. Bias-dependent intramolecular structures of C60 molecules. (a) Filled states image (Vs - - 2 . 0 V) with doughnut shape intramolecular structure, (b) near the Fermi level (V~- -0.1 V) with triangular shape intramolecular structure and (c) empty states image (V~- + 2.0 V) with a three-lobe-clovershape intramolecular structure.

T. Sakurai et al. /Applied Surface Science 8 7 / 8 8 (1995) 405-413

408

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Fig. 6. Adsorption model for the C60 molecules on the C u ( l l l ) - I × 1 surface. Three pentagonal rings of the C60 molecules are shaded. The adsorption position of (a) or (b) is different from that of (c) or (d) in terms of the existence of the Cu atom in the second layer of the subst~:ate. Arrows (1) and (2) indicate the directions, which have a double bond and a pentagonal ring at the side of the molecule, respectively.

four types of domains with two orientationally nonequivalent configurations for the C60 molecule; 60 ° rotational freedom, in the case of the C u ( l l l ) surface (Fig. 4). If the C60 molecule occupies the

atop site, the adsorption position is unique except for the rotational freedom, which means there are only two domains. When we examine domain boundaries, the shift of the C60 molecules in the adjacent two different domains can be expressed by a two-dimensional vector ( m a o, na o) parallel to the surface. The numbers m and n are integers for the case of the atop site adsorption, and integer or integer _+ ~1 for the case of the three-fold hollow site adsorption if the molecule resides both on the hcp and the fcc site. We have analyzed many domain boundaries and observed four domains with m and n being integer 1 a n d / o r integer -+- 7. It is quite natural to conclude that the adsorption position should be the three-fold hollow site. Based on these analyses of the STM images, an adsorption structure model of C60 molecule on the C u ( l l l ) surface is derived as is shown in Fig. 6. The local density of states is calculated for the monolayer film of C60 molecules based on the adsorption model shown in Fig. 6 [21,27]. The fact that we can observe STM images at V~ close to zero indicates that there is a significant amount of charge transfer from Cu to C60, at least more than in the case of the Si surfaces [17,18]. We are able to calculate the simulated images of intermolecular structures by evaluating the 3D mapping of the local electron density of states near the Fermi level. When

Fig. 7. Simulated STM images of a C60 molecule for Vs --(a) - 2.0 V, (b) - 0 . 1 V and (c) +2.0 eV, obtained by calculating the local density of states near the Fermi level.

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we are observing the empty states, the 181 (LUMO band for a neutral C60 crystal), 182 and 183 levels should contribute to the image. When we are observing the filled density of states, the 180, 179, 178, 177 and 176 levels should contribute [21,27]. Figs. 7a, 7b and 7c show the calculated 3D map of the local density of states that simulates the STM images in Figs. 5a, 5b and 5c, respectively. The clover lobes we observe at V~ = +2.0 V correspond to three pentagonal rings surrounding the hexagonal ring at the top of the molecule. The filled states image appears in a doughnut shape.

A

3.2. Cu(111)-C60(x)C70¢_x) C The initial stage adsorption of the C70 molecules on the C u ( l l l ) surface is similar to the case o f C60 adsorption. We observe a high mobility of the C70 molecules on the terrace and segregation to the step edges. However, further increase in the coverage results in 2D island formation towards both upper and lower terrace, in contrast to the C60 case. Monolayer films formed with the mixture C60(x)C70(l_x) upon annealing the surface show several interesting phenomena. Fig. 8 shows the case of the C60(x)C70(1-x) (x = 0.06), obtained by annealing the surface at 320°C. We mainly observe two kinds of contrasts (dim and bright) for the individual C60 and C70 molecules. Based on the number density of the

Fig. 8. STM image of the C6o(x)C7o(l_x) (x = 0.06) monolayer film, showing the domain boundary segregation of C60 molecules (160 A×160 ~, V~= +2.0 V).

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Fig. 9. Structure model of the domain boundary observed in the center of Fig. 8.

dim molecules and the apparent height difference in the STM images between dim and bright molecules, the dim and bright molecules are assigned to be C60 and C70 molecules, respectively. A similar assignment has been used by Lang et al. for the case of C60(x)C7o(l_x) adsorption on the A u ( l l l ) surface [28]. The C70 molecules form a well-ordered hexagonal arrangement, displaying the C u ( l l l ) - 4 X 4-C70 phase, and the nnd is equal to 1 0 . 2 A (4a0), similar to the case of C60. The monolayer film shown in Fig. 8 is divided into two domains separated by a domain boundary running from top left of the image to right bottom, where we observe domain boundary segregation of the C60 molecules. The domain in the right half of the image is shifted upward by exactly one unit distance of the substrate (a 0) and individual C60 and C70 molecules occupy equivalent sites on the C u ( l l l ) surface, which are determined to be threefold hollow sites, the same as in the case of C60 adsorption (Fig. 9). We do not observe intramolecular structures both in C60 and C70 molecules, and individual molecules are imaged round. We attribute this fact to the rotation of the fullerene molecules. The long axis of

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the C70 molecule is 7.9 A long and the nnd of the bulk van-der-Waals-like crystal of C70 is 10.6/k. It is probable that the C70 molecules in the 4 X 4 phase are taking an upright orientation, the long axis being perpendicular to the C u ( l l l ) surface, since the bulklike structure (free rotation) or lying-down orientation, the long axis being parallel to the C u ( l l l ) surface, forms a monolayer film with too much stress. So the rotation of the C70 molecules should be along the long axis, similar to the bulk crystal in the moderate temperature range between 70°C and 10°C [29,30]. The apparent height difference between the C6o and C70 molecules observed in the STM images is consistent with this argument. When a C70 molecule takes the upright orientation, one of the pentagonal rings faces to the C u ( l l l ) surface, thus the positions of carbon atoms at the bottom of the C70 molecule do not fit to those of Cu atoms of the substrate. The C60 molecules in this film appear to rotate similarly to those near the defects in the C u ( l l l ) - 4 × 4-C60 phase. Fig. 9 shows the details of the domain boundary near the kink of the domain boundary observed at the center of Fig. 8. Because of the existence of the kink, two 4 X 4 phases form two kinds of domain boundaries, which are shown as the lines A - B and B-C. At the domain boundary A - B (the nnd is 4.6 a 0) we observe a large amount of C60 segregation (approximately 75%) to the domain boundary. At the domain boundary B - C (the nnd is 3.6a 0) we observe almost no segregation. Fig. 10 shows a nnd dependence of the density of C60 at the domain boundary. We clearly see that the degree of domain boundary segregation is zero when nnd is less than approximately 4a0, and it goes up rapidly as nnd becomes larger than 4a 0. If we assume that the difference in the substrate-adsorbate interactions is negligible between the C60 and C70 molecules, since all the molecules occupy the same adsorption site, this phenomenon may be treated as two-dimensional domain boundary segregation based on the adsorbate-adsorbate interactions. Although we are not fully understanding the mechanism of domain boundary segregation and its nnd dependence at this stage, we like to point out two possible explanations. We adapt the bond-breaking model to explain our observation. Each molecule has six nearest neighbors, except at the domain boundaries. The bond

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strength can be estimated from the bulk sublimation energy. Because the sublimation energy of C70 is slightly larger than that of C60, we estimate that the bond strength is slightly larger for C70 than C60. For the extreme case where we have an infinite nnd value at the domain boundary, each molecule has two broken bonds, and the degree of domain boundary segregation is determined by the energy difference between the C60 impurity in the 4 X 4-C70 phase and that at the domain boundary. Namely, it is energetically favorable when the C60 impurity is located at the domain boundary, because energy instability caused by bond breaking is less than in the case of C70 located at the domain boundary. We apply a strain energy model as the second possible explanation. We assume that the C60 impurity in the 4 X 4-C70 phase induces compressive stress. This may be understandable when we recall that the C 7 0 molecules are taking upright orientation in the 4 X 4-C70° phase and the size of the C70 molecules is 7.0 A along the short axis (along the 2D film in this case), slightly smaller than the diameter of the C60 molecule (7.1 A). In the strain energy model, the degree of segregation is determined by the strain energy difference between the C60 impurity in the 4 X 4-C70 phase and that at the domain boundary. It is energetically favorable when the C6o molecule segregates to the domain boundary with a more open area, i.e. larger nnd at the domain bound-

T. Sakurai et al. /Applied Surface Science 8 7 / 8 8 (1995) 405-413

ary. This explains not only the degree of segregation to the domain boundary with nnd larger than 4a0, but also the depletion of the C60 molecule at the domain boundary with nnd smaller than 4a 0 in Fig. 10. When the mixture C60(x)C70(1-x) (x = 0.3) monolayer film forms, we observe adsorption geometry different from the case of the C60(x)C70(l_x) (x = 0.06). Fig. 11 shows a typical empty states STM image. Individual fullerene molecules are again occupying the lattice sites similar to the case of the 4 X 4-C60 and 4 X 4-C70 phases and may be expressed as C u ( l l l ) - 4 X 4-C6o//C70 phase. We observe three types of images for individual fullerene molecules in Fig. 11. The C60 molecules are imaged dim and can be easily recognized by their unique three-lobe-clover shape. The brighter images are judged as the C70 molecules by analyzing the concentration and are categorized into two; one is brighter and round similar to the C70 molecules in the 4 × 4-C70 phase, the other is less bright showing two-fold symmetry. If we inspect the latter case closely, we find that the image of a molecule consists of two elongated protrusions, each of them consisting of two spots. From a symmetry argument similar to the case of C60 molecules in the 4 X 4-C60 phase, we attribute the latter as the C70 molecules with lying-down orientation. This assignment is supported by a stress argument as follows: when the

411

Fig. 12. Structure model of the lying-down C70 molecule in the Cu(111)-4×4-C6o/C7o phase.

concentration of the C6o molecule is low, the most favorable arrangement of the C7o molecules is the upright orientation since the long axis of the C70 molecule is 7.6 .A and much larger than in the C6o case. Since the nnd in the bulk fcc phase of C6o is 10.0 A, slightly smaller than the nnd in the Cu(111)4 x 4 phase (10.2 A), the co-adsorption of C70 with C6o reduces the stress surrounding the C70 molecules and C7o molecules are able to take the lying-down orientation in the 4 X 4-C60/C70 phase. Even in the 4 X 4-C6o/C70 phase, a C70 molecule surrounded by the C7o molecules, may have too much stress and may take the upright orientation, which is observed as the brightest round molecules in Fig. 11. In analogy to the intramolecular structure observed for the C6o molecules, we claim that the adsorption orientation shown in Fig. 1 2 i s probable, assuming that the pentagonal rings in the C70 molecules are observed bright in the empty states STM images. Although we do not have an exact calculation for the 4 X 4-C6o/C7o phase, recent results by Kawazoe et al. [27] show that the LUMO of the C70 molecule localizes at the four pentagon rings facing up the surface.

3.3. Ag(lll)-C6o

Fig. 11. STM image of the C60(x)C70(l_x) (X = 0.3) monolayer film, showing the intramolecularstructures of the C60 and C7o molecules (75 A × 75 A, V~= + 2.0 V).

We have also investigated monolayer films of C60 on the A g ( l l l ) surface. We observe several kinds of close packed hexagonM phases, which have almost equal nnd but have different rotational angles with respect to the substrate Ag atomic row direction. Fig. 13 shows an example f o r the case of a C60 monolayer film on the A g ( l l l ) surface after annealing at 300°C. We observe three phases in this image, which are consistent with the STM observation reported by

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Fig. 13. STM image of the C6o monolayer film on the A g ( l l l ) surface, showing three phases with almost equal nnd (300 A X 300 A, Vs = + 2.0 V).

Altman and Colton [11]. The nnd in all these phases is measured to be 10.0 __+0.1 A, essentially the same as 2v~-a 0 (10.0 .~; a 0 = 2.89 A for Ag). The phase at the right top in Fig. 13 has a rotation angle of 30 ° and is determined to be the A g ( l l l ) - 2 f 3 - X 2x/3-R30°-C60 phase. The phase in the lower part is rotated by 12 ° and the phase at the upper, left is rotated by 46 ° . We have examined many phases and found that the rotation angle can be grouped into two types, 11.5°-13.5 ° and 450-47 °. We have also found that a prolonged annealing at 300°C for up to 36 h

13 °



results in the 2v~- X 2v~R30 ° phase and the disappearance of other phases. This indicates that the 2v~- X 2V~-R30° phase is most energetically stable. As is proposed by Altman and Colton [11] and analogous to the structure of the C u ( l l l ) - 4 X 4-C60 phase, we estimate that the adsorption site of the 2x/3- × 2v/3-R30° phase is the three-fold hollow site of the A g ( l l l ) surface. By analyzing the nnd and rotation angles of other phases, we are able to propose two structure models for one of the two groups (with rotation angle of 11.5°-13.5°), which are shown in Fig. 14. We can obtain the other group of phases with rotation angle of 45°-47 ° by considering the mirror symmetry of these two models. To make these models, we have tried to draw as many C60 molecules as possible at the 3FH sites, because of stability considerations. The mechanism of the monolayer film formation of fullerenes on the A g ( l l l ) surface is modeled as follows. In comparison with the fullerenes adsorption on the Cu(111) surface, the interaction between fullerene molecules and the A g ( l l l ) surface is relatively weak and the interaction among fullerene molecules becomes dominant. Close-packed hexagonal monolayer films form because of rather weak van-der-Waals-type interaction among fullerene molecules and the monolayer film still have rotational freedom with respect to the substrate. The stability of the phases is closely related to fraction of the number of molecules per unit cell that occupy the most stable three-fold hollow sites.

14.6

Fig. 14. Structure models of the Ag(lll)-C60 phases.



T. Sakurai et aL /Applied Surface Science 8 7 / 8 8 (1995) 405-413

4. S u m m a r y W e h a v e studied the m o n o l a y e r f i l m f o r m a t i o n o f f u l l e r e n e s (C60 , C70 , and C60(x)C70(l_x)) on C u ( l l l ) l x 1 and A g ( l l l ) l X i surfaces by using F I - S T M . A t the initial stage adsorption, fullerene m o l e c u l e s are m o b i l e on the terraces and segregate to steps. A f t e r filling the adsorption positions at the step edges, 2 D islands f o r m f r o m steps and then c l o s e - p a c k e d h e x a g o n a l phases f o r m w i t h m o n o l a y e r c o v e r a g e . B e c a u s e o f the strong interaction b e t w e e n the Cu surface and fidlerene m o l e c u l e s and because o f the interaction a m o n g the orientationally ordered C60 m o l e c u l e s , the free rotation o f the C60 m o l e c u l e s is inhibited. S o m e o f the C70 m o l e c u l e s in the Cu(111)-4 X 4-C60(x)C70(l_x) ( x : 0.3) phase also stop rotation. T h e b i a s - d e p e n d e n t i n t r a m o l e c u l a r structures w i t h three-fold s y m m e t r y o f the C60 m o l e c u l e s are o b s e r v e d and interpreted as 3 D m a p pings o f the local electron density o f states corres p o n d i n g to the bias v o l t a g e condition. T h e interaction b e t w e e n fullerene m o l e c u l e s and the A g ( l l l ) substrate is relatively w e a k , in contrast to the C u ( l l l ) surface, and c l o s e - p a c k e d h e x a g o n a l m o n o l a y e r films f o r m w i t h nnd the s a m e as for the b u l k fcc crystal. T h e rotation angles o f the m o n o l a y e r films w i t h respect to the substrate are d e t e r m i n e d b y the adsorb a t e - s u b s t r a t e interactions.

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