Substituent-Dependent Ordering of Adlayer Structures of Fullerene

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J. Phys. Chem. C 2010, 114, 3170–3174

Substituent-Dependent Ordering of Adlayer Structures of Fullerene Derivatives Adsorbed on Au(111): A Scanning Tunneling Microscopy Study Ting Chen,† Ge-Bo Pan,† Hui-Juan Yan,† Li-Jun Wan,*,† Yutaka Matsuo,*,‡,§ and Eiichi Nakamura*,‡,§ Institute of Chemistry, Chinese Academy of Sciences (CAS), and Beijing National Laboratory for Molecular Sciences, Beijing 100190, People’s Republic of China, Department of Chemistry, The UniVersity of Tokyo, and Nakamura Functional Carbon Cluster Project, ERATO, Japan Science and Technology Agency ReceiVed: December 1, 2009; ReVised Manuscript ReceiVed: January 18, 2010

The adsorption of four fullerene derivatives, including Re(C60Me5)(CO)3, Ru(C60Ph5)Cp, C60(C6H4C6H4COOH)5Me, and C60(C6H4-CC-SiMe2C12H25)5Me, on a Au(111) surface has been investigated by scanning tunneling microscopy (STM) and cyclic voltammetry. High-resolution STM images reveal that Re(C60Me5)(CO)3 forms a well-ordered (23 × 23)R30° structure. Different from C60 on Au(111), the so-called “in-phase” structure is not found with the attachment of methyl and Re(CO)3 groups. With the increase of substituent size, disordered adlayers have been observed for Ru(C60Ph5)Cp and C60(C6H4C6H4-COOH)5Me. However, the individual molecules could be distinguished in STM images, suggesting that the two molecules interact strongly with the Au(111) surface. When the substituent size is further increased, a multilayered structure is formed for C60(C6H4-CC-SiMe2C12H25)5Me. This is because of molecular aggregation in the bulk solution, which occurs at very low concentration. These results indicate that the structures of fullerene derivatives play an essential role in adlayer formation through adjusting molecule-substrate and molecule-molecule interactions. Introduction Fullerenes have attracted much attention since their discovery in 1985.1 They have been used in superconductors, optical devices, microsensors, and electronic devices. This is mainly due to their unique structural, chemical, and electronic properties. To pursue these applications, however, it is important to understand how fullerenes interact physically and electronically with each other and with their local environment. So far, a variety of studies have been carried out to understand the fullerene adsorption on various substrates by using different methods.2,3 As far as Au(111) is employed,4,5 two different structures, (23 × 23)R30° and the so-called “in-phase” (38 × 38), are observed both in UHV6,7 and in solution.8 However, it is still a challenge to construct high-quality fullerene nanostructured arrays. The use of patterned substrates can be considered as one of the promising ways. For instance, Uemura et al. have succeeded in preparing a well-ordered C60 adlayer on Au(111) in aqueous HClO4 by transfering the Langmuir film of C60 on iodine-modified Au(111).9 Besides, a large number of studies have been focused on the supramolecular assembly mainly through orbital interactions between porphyrins and fullerenes.10,11 The details can be referred to a recent review.12 Another kind of molecule used as a template monolayer to complex fullerenes is macrocyclic compounds, such as calixarenes13,14 and cyclothiophenes.15,16 It is reported that, on the calixarene-modified Au(111), fullerenes prefer to adsorb within the molecular cavities because of the steric match. However, * To whom correspondence should be addressed. E-mail: wanlijun@ iccas.ac.cn (L.-J.W.), [email protected] (Y.M.), nakamura@ chem.s.u-tokyo.ac.jp. (E.N.). Tel: +86-10-62558934. † Chinese Academy of Sciences (CAS) and Beijing National Laboratory for Molecular Sciences. ‡ The University of Tokyo. § Japan Science and Technology Agency.

Figure 1. Chemical structures of fullerene derivatives: (1) Re(C60Me5)(CO)3, (2) Ru(C60Ph5)Cp, (3) C60(C6H4C6H4-COOH)5Me, and (4) C60(C6H4-CC-SiMeC12H25)5Me.

as to cyclothiophene, STM images show that fullerenes are located at the conjugated rim rather than the intrinsic cavity of a macrocycle due to the strong donor-acceptor interaction between fullerene and thiophene groups. Recently, the use of electron-donating perylene,17 coronene,18 and pentacene19 as a template layer to control the assembly of fullerenes has been reported. It is supposed that the adlayer structure of fullerenes on such a molecule-modified gold surface is controlled by donor-acceptor interaction in the supramolecular assembly systems too. Note that previously, the assembly structure of fullerenes was mostly controlled by the modification of the substrate, which tunes the substrate-molecule interaction; the roles of fullerenes’ structure and intermolecular interactions in the self-organization process were rarely referred. In this article, we report results of a detailed investigation on the adlayer structures of a series of fullerene derivatives20-24 on Au(111) in HClO4. The chemical structures of these fullerene derivatives are shown in Figure 1. All the molecules used the fullerene molecule itself as the central element and the derivative group as the concial crown. The adlayer structures of these molecules were studied by STM and

10.1021/jp9114173  2010 American Chemical Society Published on Web 02/01/2010

Fullerene Derivatives Adsorbed on Au(111)

Figure 2. Cyclic voltammograms of bare and fullerene derivativemodified Au(111) electrodes in 0.1 M HClO4: (1) Re(C60Me5)(CO)3, (2) Ru(C60Ph5)Cp, (3) C60(C6H4C6H4-COOH)5Me, and (4) C60(C6H4CC-SiMe2C12H25)5Me. The scan rate is 50 mV/s.

electrochemical method. The results indicate that with the substituent size increase, the adlayers of fullerenes become less uniform. The structure of fullerene derivatives plays an essential role in the adlayer formation through adjusting moleculesubstrate and molecule-molecule interactions. Experimental Section Fullerene derivatives were synthesized according to the previous literature.20-24 Re(C60Me5)(CO)3, Ru(C60Ph5)Cp and C60(C6H4-CC-SiMe2C12H25)5Me were dissolved in toluene, whereas C60(C6H4C6H4-COOH)5Me was dissolved in ethanol. The average concentration of these molecules in toluene or ethanol is 10 µM. Electrolyte solution was prepared with ultrapure HClO4 (Cica-Merck, Japan) and Millipore (Milli-Q) water. Au(111) single-crystal electrodes were prepared by the Clavilier method.25 Before each measurement, the Au(111) electrode was further annealed in a hydrogen-oxygen flame and quenched in ultrapure water (Milli-Q) saturated with hydrogen. Molecular adlayers of fullerene derivatives were obtained by immersing the as-prepared Au(111) electrode into their corresponding solutions for a short time. Cyclic voltammetric measurement was carried out by the hanging meniscus method under a nitrogen atmosphere. A reversible hydrogen electrode (RHE) and a platinum wire were used as reference and counter electrodes, respectively. All the potentials were reported with respect to RHE. Molecular models were built and optimized with the HyperChem 6.0 package (Hypercube, Inc.). In situ STM experiments were carried out with a NanoScope E microscope (Digital Instrument Inc.). The tunneling tips were prepared by electrochemically etching W wire (0.25 mm in diameter) in 0.6 M KOH. The sidewalls of the tips were sealed with transparent nail polish to minimize Faradic currents. All the images were acquired in the constant-current mode to evaluate the corrugation heights of adsorbed molecules. Results and Discussion 1. Cyclic Voltammetry. Figure 2 shows typical cyclic voltammograms of bare and fullerene-modified Au(111) electrodes in 0.1 M HClO4. It can be seen that the cyclic

J. Phys. Chem. C, Vol. 114, No. 7, 2010 3171 voltammogram of bare Au(111) is consistent with the literature,26 indicating a well-defined (111) surface is exposed to solution. Line 1 is the cyclic voltammogram for the Re(C60Me5)(CO)3-modified Au(111) electrode. The adsorption of Re(C60Me5)(CO)3 is clearly shown in the double-layer potential region because the charging current decreases obviously and the reconstruction peaks of Au(111) disappear, suggesting that the Au(111) is covered with Re(C60Me5)(CO)3. No additional response is observed in the double-layer region, indicating that no electrochemical redox or adsorption/desorption process occurs. A similar cyclic voltammogram is obtained for the C60(C6H4C6H4-COOH)5Me-modified Au(111) electrode (line 3). However, for Ru(C60Ph5)Cp (line 2) and C60(C6H4CC-SiMe2C12H25)5Me (line 4), the cyclic voltammograms show a small cathodic current commencing at about 0.3 V. This is possibly ascribed to partial desorption of Ru(C60Ph5)Cp and C60(C6H4-CC-SiMe2C12H25)5Me from the Au(111) surface. Interestingly, repetitive potential cycles between 0 and 1.0 V cause no change in the cyclic voltammogram profile, suggesting a reversible adsorption/desorption process. 2. In Situ STM. A. Re(C60Me5)(CO)3 Adlayer. Figure 3A shows a top- and side-view of the molecular structure of Re(C60Me5)(CO)3 from the molecular single-crystal diffraction result. The derivative molecule keeps a nearly round shape and shows little change in dimension because the substituent groups are relatively small. Figure 3B represents a typical large-scale STM image of ReC60Me5(CO)3-modified Au(111) in 0.1 M HClO4. It is obvious that the atomically flat Au(111) terrace is covered by ReC60Me5(CO)3 molecules. Within the scan area are several small ordered domains, whose sizes are dependent on the specific surface preparation process. Each molecule is clearly recognized as a featureless bright spot with a diameter of about 0.7 nm. This is in good agreement with the molecular size of the C60 center. In the present experiment, no long-range ordered domains have been observed, though the concentration of Re(C60Me5)(CO)3 and the immersion time of the Au(111) electrode in solution are varied in a wide range. The structural details of the Re(C60Me5)(CO)3 adlayer is revealed by a higher-resolution STM image, as shown in Figure 3C. Several molecular defects can be clearly resolved (as outlined by the white arrows), which further confirms that each bright spot corresponds to a Re(C60Me5)(CO)3 molecule. The nearest distance between the centers of the bright Re(C60Me5)(CO)3 spots is 1.0 ( 0.1 nm. The molecular rows cross each other at an angle of 60° or 120° ( 2°. From the cross-sectional profile, the height of each spot is measured to be ca. 0.15 nm. A precise comparison between this image and that of the underlying Au(111) (see the inset in Figure 3C) reveals that the molecular rows are in parallel with the 〈112〉 direction of Au(111). Therefore, a (23 × 23)R30° structure is determined, and a unit cell is superimposed in Figure 3C. The above observation is similar to the adsorption of C60 on Au(111) both in air and in aqueous solution. A uniform monolayer of C60 has only been obtained by vapor deposition in vacuum. Note that the so-called “in-phase” structure for C60 is not found in the Re(C60Me5)(CO)3 adlayer. As reported previously, the formation of the (23 × 23)R30° structure is thermodynamically favored in comparison with the “in-phase” structure. In the case of Re(C60Me5)(CO)3, the addition of methyl and Re(CO)3 groups changes the geometrical and electronic structures of the C60 center. Consequently, the molecule-molecule and molecule-substrate interactions have also been changed. The (23 × 23)R30° structure becomes more stable than the ”in-phase” one.

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Figure 3. (A) Top- and side-view X-ray structures of Re(C60Me5)(CO)3. (B) Typical large-scale and (C) high-resolution STM images of Re(C60Me5)(CO)3 on Au(111) in 0.1 M HClO4. Tunneling conditions: (B) E ) 550 mV, Ebias ) -548 mV, Iset ) 517 pA; (C) E ) 550 mV, Ebias ) -281 mV, Iset ) 1.57 nA.

Figure 4. (A) Top- and side-view X-ray structures of Ru(C60Ph5)Cp. (B) Typical large-scale and (C) high-resolution STM images of Ru(C60Ph5)Cp on Au(111) in 0.1 M HClO4. Tunneling conditions: (B, C) E ) 550 mV, Ebias ) -610 mV, Iset ) 605 pA.

B. Ru(C60Ph5)Cp Adlayer. Figure 4A shows the top- and side-view X-ray structures of Ru(C60Ph5)Cp. Compared with Re(C60Me5)(CO)3, the methyls are replaced by phenyl and the Re(CO)3 group is changed to a buckyruthenocene in Ru(C60Ph5)Cp. However, with the size increase of these substituents, the adlayer structure of Ru(C60Ph5)Cp is completely different from that of Re(C60Me5)(CO)3. As shown in Figure 4B, a disordered adlayer is formed for Ru(C60Ph5)Cp on Au(111). It is clear that the Au(111) terrace is now covered with monodispersed spots. Each spot in the STM image is assigned to one Ru(C60Ph5)Cp molecule. Such a disordered structure is attributed mainly to the C5 symmetric structure shown in the space-filling models (Figure 4A) of Ru(C60Ph5)Cp from the X-ray data. The mismatch of C5 and hexagonal symmetry for the compound and substrate may cause this disordered monolayer structure. Careful inspection reveals that the spots are not simply round but show some internal features (Figure 4C). This may be an indication of the derivative part of the molecule. Similar phenomena have been observed for a ferrocene-cycloadducted C60 derivative (C60ONCFn)27 and a ferrocene-linked C60 derivative (C60Fc).28 Moreover, the brightness of the spots is not the same. Some of spots, marked by the black arrows in Figure 4C, are especially brighter than the others. This may be ascribed to the different molecular orientation of Ru(C60Ph5)Cp on Au(111). As reported previously, the ellipsoidal C70 adopts two different orientations, standing and lying-down, on Au(111).9 The adlayers of Ru(C60Ph5)Cp are stable in the potential range from 0.3 to 1.0 V. When the potential is more negative than 0.3 V, Ru(C60Ph5)Cp molecules start to desorb from the substrate (Figure S1, Supporting Information). It is interesting that the adlayers reappear when the potential is cycled back to 0.55 V. This phenomenon indicates that the adsorption/desorption is an electrochemically reversible process, consistent with the cyclic voltammetric result. Note that individual Ru(C60Ph5)Cp molecules can be continuously imaged, suggesting that the molecule interacts strongly with the Au surface.

Figure 5. (A) Typical large-scale and (B) high-resolution STM images of C60(C6H4C6H4-COOH)5Me adlayers on Au(111) in 0.1 M HClO4. The inset in (B) shows the image of a single C60(C6H4C6H4-COOH)5Me molecule. (C) Cross-sectional profile along the black line in (B). (D) Proposed adsorption model for the adlayer. Tunneling conditions: (A, B) E ) 550 mV, Ebias ) -677 mV, Iset ) 658 pA.

C. C60(C6H4C6H4-COOH)5Me Adlayer. Similar to the adsorption of Ru(C60Ph5)Cp, a disordered structure is formed for C60(C6H4C6H4-COOH)5Me on Au(111) too (Figure 5A). The molecules are randomly adsorbed on the Au terrace. However, their distribution is less uniform than that of Ru(C60Ph5)Cp. The surface is partially occupied by individual and aggregated molecules. This observation suggests a strong interaction between the molecule and the underlying Au(111) surface. Figure 5B shows a high-resolution STM image, revealing the internal molecular features of C60(C6H4C6H4-COOH)5Me. It is clear that each of the bright spots is separated into two parts rather than featureless round. The cross-sectional profile in Figure 5C clearly shows two protrusions for each C60(C6-

Fullerene Derivatives Adsorbed on Au(111)

Figure 6. (A) Typical large-scale and (B) high-resolution STM images of C60(C6H4-CC-SiMe2C12H25)5Me adlayers on Au(111) in 0.1 M HClO4. (C, D) Cross-sectional profiles along lines 1 and 2 in (B). (E) Proposed structural model of the multilayer. Tunneling conditions: (A, B) E ) 550 mV, Ebias ) -585 mV, Iset ) 1.38 nA.

H4C6H4-COOH)5Me. Shinohara et al.29 reported that the rotation of C60 is hindered and the electronic structure can be observed by STM when C60 molecules adsorbed on Si(100)-(2 × 1) surface. Similarly, C60 cages in a C60-C60 molecule are not simply round but show some internal features on Au(111) surface.30 In the present study, the rotational motion may also be inhibited on the Au surface. According to the literature, benzoic acid,31 isophthalic acid,32 and trimesic acid33 adsorb in a vertical or tilted manner on the Au(111) with a single deprotonated carboxylate group facing the electrode at positive potential. It is inferred that the C60(C6H4C6H4-COOH)5Me may interact with the Au substrate through the carboxylic groups. That is to say, the central C60 part faces the solution phase while the conical crown part contacts with the substrate through five carboxylic groups just like a molecular lunar landing module. Consequently, the interaction between neighboring molecules decreases due to the increase of the distance between the central C60 parts. This may partially explain why C60(C6H4C6H4COOH)5Me molecules cannot form ordered layers on the Au(111) surface. Furthermore, the height of each protrusion was found to be about 0.29 nm from the cross-sectional profile (Figure 5C). This value is much higher than that of Re(C60Me5)(CO)3, which may be another evidence supporting our supposition. Figure 5D shows a tentative structural model for the adsorption of C60(C6H4C6H4-COOH)5Me. D. C60(C6H4-CC-SiMe2C12H25)5Me Adlayer. Figure 6A shows a typical large-scale STM image of the C60(C6H4CC-SiMe2C12H25)5Me adlayer prepared by immersing a welldefined Au(111) surface in toluene solution containing 10 µM C60(C6H4-CC-SiMe2C12H25)5Me for 10 s. It is clear that C60(C6H4-CC-SiMe2C12H25)5Me molecules are inclined to form a multilayer on Au(111). Interestingly, such structures can be formed even at very low coverage. More structural details are revealed by a high-resolution STM image in Figure 6B. From the cross-sectional profiles along lines 1 and 2 in Figure 6B, we can see that the vertical distances between molecule a and b or molecule a’ and b’ are comparative to the height of

J. Phys. Chem. C, Vol. 114, No. 7, 2010 3173 the underlying monolayer. Careful inspection indicates that the first adlayer of C60(C6H4-CC-SiMe2C12H25)5Me is not absolutely disordered. The molecules are tightly arranged on the substrate one by one and create a uniform adlayer, which can be seen clearly in Figure 6D. The second layer then superposes on the first one. These observations may suggest a moderate molecule-substrate interaction and an extremely strong molecule-molecule interaction. The distance between two neighboring molecules is measured to be 1.3 nm, which is slightly larger than pure C60 and is comparable to those in crystals and liquid crystals.22 We have previously reported that the shuttlecock-shaped molecule C60(C6H4-CC-SiMe2C12H25)5Me can self-assemble into layered or lamellar structures in crystals and liquid crystals.22 Figure 6E shows a structural model for the ordered multilayer. It is shown that the structural ordering within the layer is primarily the result of the strong fullerene/fullerene interaction. On the other hand, the crystal can gain stabilization energy through the van der Waals interaction between the five side chains. These interactions can be used to explain the structure of the multilayer observed on the Au(111). After revealing the adlayer structure, the effect of potential on the adlayer was explored. Similar to Ru(C60Ph5)Cp, C60(C6H4-CC-SiMe2C12H25)5Me also desorbs from the Au(111) surface at negative potential, as shown in Figure S2 in the Supporting Information. This is consistent with the results of cyclic voltammetric measurements. Conclusions The adsorption of four fullerene derivatives, including Re(C60Me5)(CO)3, Ru(C60Ph5)Cp, C60(C6H4C6H4-COOH)5Me, and C60(C6H4-CC-SiMe2C12H25)5Me, on the Au(111) surface has been investigated by STM and cyclic voltammetry. Highresolution STM images reveal that Re(C60Me5)(CO)3 forms a well-ordered (23 × 23)R30° structure. Different from C60 on Au(111), the so-called “in-phase” structure is not found, owing to the addition of methyl and Re(CO)3 groups. With the increase of substituent size, disordered adlayers have been observed for Ru(C60Ph5)Cp and C60(C6H4C6H4-COOH)5Me. Nevertheless, individual molecules could be discerned in STM images, suggesting that the two molecules interact strongly with the Au surface. Further increasing the substituent size results in the formation of a multilayered structure for C60(C6H4CC-SiMe2C12H25)5Me. These results indicate that the structure of fullerene derivatives plays an essential role in adlayer formation through adjusting molecule-substrate and moleculemolecule interactions. Acknowledgment. The authors (L.J.W.) are thankful for financial support from the National Natural Science Foundation of China (Grant Nos. 20821003, 20905069, 20873160, and 746906), the National Key Project on Basic Research (Grant Nos. 2006CB806100 and 2006CBON0100), and the Chinese Academy of Sciences. Y.M. and E.N. thank MEXT (KAKENHI to E.N., No. 18105004) and the Global COE Program for Chemistry Innovation. Supporting Information Available: STM images of Ru(C60Ph5)Cp and C60(C6H4-CC-SiMe2C12H25)5Me adlayers at E ) 200 mV. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162.

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