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Formation of Confined C60 Islands within Octanethiol Self-Assembled Monolayers on Au(111) Fangsen Li,†,‡ Lin Tang,‡ Wancheng Zhou,† and Quanmin Guo*,‡ State Key Laboratory of Solidification Processing, Northwestern Polytechnical UniVersity, Xi’an 710072, China, and Nanoscale Physics Research Laboratory, School of Physics and Astronomy, UniVersity of Birmingham, Birmingham B15 2TT, United Kingdom ReceiVed: July 28, 2009; ReVised Manuscript ReceiVed: August 19, 2009
Confined C60 islands have been grown within octanethiol self-assembled monolayers (SAMs) on the Au(111) surface. The structure of the C60 islands is investigated by use of scanning tunneling microscopy (STM), and a single ordered phase of the close-packed C60 islands with a (23 × 23)R30° structure is found. In the vicinity of the close-packed C60 island there are loosely packed C60 molecules which are mixed with the thiolate species. Floating C60 on top of the SAM, in the form of either compact islands or individual molecules, is not found at room temperature and above. The diffusion of C60 molecules is strongly controlled by the structure of the SAM, and the (3 × 3)R30° phase of the SAM is not permissible to diffusing C60 molecules. 1. Introduction 1
Self-assembly is a widely observed phenomenon in nature and it has been extensively used as a bottom-up approach where atoms, molecules, nanoparticles, and other building blocks are self-assembled into many functional structures2 for potential technological applications in molecular electronics, biosensing, biomimetics, nanopatterning, and wetting.3-8 In the case of assembly on solid surfaces, there has been a lot of research effort in controlling the assembly process by introducing surface functionalities. An example of this is the controllable selfassembly of C60 molecules on well-designed surface templates,9-13 where the effects of the surface templates on electron transport between the C60 molecules and the substrate have been studied.14,15 Here we report findings from the adsorption of C60 molecules on Au(111) in the presence of an octanethiol selfassembled monolayer (SAM). We demonstrate the formation of compact C60 islands confined by the SAM as well as SAMregulated growth of the C60 islands. Alkanethiol self-assembled monolayers on gold surfaces have been studied extensively due to their interesting properties, ease of preparation, and high stability.16-18 Despite some of the unresolved issues around the SAM/Au system, such as the exact adsorption site19-21 and the nature of gold adatoms,22-25 alkanethiol SAMs have been successfully used for the fabrication of model molecular devices.26,27 Adsorption of C60 molecules on top of hexanethiol monolayers has been studied previously at 78 K,28 and it was shown that C60 molecules could form closepacked islands on top of the SAM at this temperature. The orientation of the SAM-supported C60 molecular islands appears to be unrelated to the Au(111) substrate. One-dimensional chains of C60 molecules were also found to form on top of a striped phase of the SAM.29 A recent study shows that when octanethiol molecules adsorb onto a Au(111) already covered by a fractional monolayer of C70, the C70 molecules are pushed aside by the expanding thiol monolayer.30 In this case the C70 molecules were deposited from a liquid, and no compact islands were formed. * Corresponding author: fax +44 121 414 7327; e-mail
[email protected]. † Northwestern Polytechnical University. ‡ University of Birmingham.
Similar to the SAM/Au system, the interaction between C60 molecules and clean metal surfaces has also been thoroughly studied since the pioneering work of Altman and Colton,31,32 and it is now known that C60 molecules would form closepacked C60 islands on the (111) surfaces of Au,31-35 Ag,36,37 Cu,38,39 Ni,40 and Pd41 at room temperature. Most of the research in this area is motivated by the potential application of C60 in solar cells and molecular electronic devices.42,43 On the Au(111) surface, C60 molecules can form several structures:31-35 in phase, (23 × 23)R30°, R14°, depending on the directions and orientations of close-packed C60 molecules in relation to the close-packed direction of the gold atoms. The (23 × 23)R30° phase is found to sit on top of (1 × 1)-Au(111), whereas underneath the in-phase C60 layer the herringbone reconstruction is often found to be retained.35 Thermal annealing of the C60/ Au(111) interface to 700 K has been found to be able to convert all phases to a single form, (23 × 23)R30°, of close-packed C60 islands.44 Lifting of the surface reconstruction of the Au(111) surface can also be achieved by alkanethiol adsorption.45-47 If C60 molecules are inserted into the SAM, they will interact with a (1 × 1)-like Au(111) substrate, rather than a herringbone reconstructed Au(111). Moreover, if the inserted C60 molecules are to condense into close-packed islands, they will likely form a single (23 × 23)R30° phase. On the bare Au(111) surface, C60 molecules can diffuse easily at room temperature and isolated molecules are in general not stable, except those trapped at the elbow sites. For C60 molecules embedded in the SAM, diffusion is expected to be slow since the molecules need to move through the adsorbed thiolate species. Our study aims to understand how C60 molecules diffuse through a SAM and how compact islands are formed inside the SAM layer. 2. Experimental Methods The Au(111) sample was a thin film (∼400 nm) on mica prepared by thermal evaporation in a BOC Edwards Auto 306 deposition system. The freshly prepared gold film was transferred into a 0.1 mM 1-octanethiol solution in ethanol (99.5%) and left for at least 24 h at room temperature for the completion of the 1-octanethiol monolayer. The sample with the SAM was then taken out of the solution and thoroughly rinsed with pure
10.1021/jp9071873 CCC: $40.75 2009 American Chemical Society Published on Web 09/17/2009
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Figure 1. STM images of the sample (a) before and (b) after deposition of C60 at RT, respectively. (a) Image (50 nm × 50 nm) showing the coexistence of close-packed and stripelike structures of the SAM (Vb ) 0.5 V; It ) 0.1 nA). (b) Image (100 nm × 100 nm) after the deposition of C60 (Vb ) 1.8 V; It ) 0.15 nA).
ethanol and dried in air. We subsequently annealed the sample at 353 K for 30 min in a vacuum of 10-6 mbar, leading to a significant reduction in the number of etch pits and the appearance of very large domains of an ordered SAM in the (3 × 3)R30° structure. C60 molecules were deposited onto the octanethiol SAMmodified gold sample surface at room temperature (∼293 K) from a homemade Knudsen cell in the form of a tantalum pocket with a pinhole aperture. The Ta cell is heated resistively with its temperature measured by a K-type thermocouple. The rate of deposition is approximately 0.1 ML/min at a cell temperature of 706 K, as estimated from the C60 coverage measured by scanning tunneling microscopy (STM). Imaging was performed at room temperature on an Omicrometer variable temperature scanning tunneling microscope (VT-STM) with electrochemically etched tungsten tips. 3. Results and Discussion Alkanethiol SAMs prepared in solution at room temperature always contain a high concentration of etch pits.16,48 Thermal annealing at a moderate temperature (∼353 K) can improve the quality of the SAM by removing the etch pits, leading to a well-ordered surface as shown in Figure 1a. The STM image in Figure 1a was from an octanethiol monolayer following thermal annealing at 353 K for 0.5 h. The SAM is seen to be free from etch pits and the molecules organize themselves into the well-known (3 × 3)R30° structure. The dark lines in the image are the domain boundaries, which are typical features of annealed SAMs.49,50 In addition to the dominant (3 × 3)R30° structure, stripelike phases were also found to coexist with the close-packed (3 × 3)R30° phase. The striped phases correspond to standing-up molecules with a slightly lower coverage as discussed in detail elsewhere.50 The areas inside the white rectangles in Figure 1a show that the transition from the striped phase to the (3 × 3)R30° phase is gradual and smooth without the presence of abrupt cracks in the SAM. C60 molecules are sublimed onto the SAM having a typical structure as that shown in Figure 1a. Figure 1b shows an STM image of the sample after C60 deposition at room temperature (RT), where the bright spots are individual C60 molecules. The structure of the SAM has not been altered upon C60 adsorption. It is clear from this image that the (3 × 3)R30° phase of the SAM is free from C60 molecules, and the C60 molecules are found only at the domain boundaries and regions where the octanethiol coverage is below saturation coverage. There is no C60 island of close-packed molecules, in contrast to that observed for C60 adsorption on
Li et al. bare Au(111).31-35 The individual C60 molecules in Figure 1b are not very stable and are observed to move under normal scanning conditions, as demonstrated by the presence of horizontal streaks around the molecules. The C60 molecules have a choice between sitting above the SAM or connecting directly to the Au(111) substrate. In the STM image, the C60 molecules appear 0.40 nm taller than the baseline level defined by the (3 × 3)R30° SAM. Considering that the adsorbed thiolates themselves are measured to be 0.18 nm above the bare gold substrate, the C60 molecules are thus 0.58 nm above the Au substrate. This height is consistent with the picture that the C60 molecules are embedded in the SAM and directly bonded to the gold substrate like golf balls in a patch of grass. The (3 × 3)R30° region consists of a defect-free dense layer of SAM, so the C60 molecules can diffuse over such an area but cannot penetrate the SAM. At the boundaries where the SAM is less dense the C60 molecules can squeeze into the SAM, by perhaps pushing some octanethiol molecules sideways. This then allows the C60 molecules to come into direct contact with the gold substrate. The octanethiol molecules standing in between the individual C60 molecules prevent the C60 from joining together to form a compact island. Within the oval shape in Figure 1b, there is a triangular region of striped phase where 34 C60 molecules are trapped. These molecules are found to be able to move around inside the triangle during scanning but are unable to move across the boundary into the (3 × 3)R30° region. The movement of the C60 molecules inside the triangle is likely to be mediated by the movement of the adsorbed thiolate species. The striped phase has thiolate coverage lower than the maximum coverage, and therefore, lateral displacement of thiolates species under the influence of the scanning tip is possible. Within the (3 × 3)R30° phase, the thiolate species are already packed with the saturation coverage, so there is no degree of freedom for translational movement of the thiolate and hence C60 molecules cannot move through the defect-free, dense SAM. This provides a simple but effective way to confine C60 molecules into a nanometer-sized area on the surface. At cryogenic temperatures, nucleation of compact C60 islands on top of the (3 × 3)R30° phase is possible as demonstrated previously.28 At room temperature with a low deposition flux, our experiments demonstrate that the C60 molecules are unable to form aggregates on top of the SAM. At room temperature, diffusion of the adsorbed octanethiolates on the Au(111) surface is very slow. Moreover, because C60 molecules cannot travel through the dense (3 × 3)R30° phase, they are thus confined within isolated patches where the C60 molecules and thiolates are randomly mixed. In order to form dense islands of C60 molecules, C60 need to be phaseseparated from the C60/SAM mixture. To achieve this, we annealed the sample to 413 K for 2 h. The sample was then cooled down to room temperature for STM imaging. This annealing process is found to give rise to a decrease in the thiolate coverage. The high coverage (3 × 3)R30° phase disappears completely after annealing and it is replaced by various striped phases. Accompanied by this structural change in the SAM, we found a significant number of compact C60 islands as shown in Figure 2a. In Figure 2a, we can see a dense C60 island near the top of the image. In the vicinity of this island, we observe C60 molecules loosely packed in a similar way as those shown in Figure 1b. By use of the striped SAM as a reference, the orientation of the dense C60 islands can be found. All the islands are found to have the same (23 × 23)R30° structure, which is also the structure for the most stable form of C60 islands found previously after C60 deposition directly onto
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Figure 2. STM images after annealing of the sample at 413 K for 2 h. (a) 120 nm × 120 nm, Vb ) 2 V; It ) 0.1 nA. Only striped phases of the SAM exist. (b) The same area as panel a but after 26 min of scanning: Vb ) 2 V; It ) 0.1 nA. (c) High-resolution STM image (45 nm × 36 nm) of the area inside the rectangle in panel b. (d) STM image (10 nm × 10 nm) showing the organization of the thiolate species within the striped phase of the SAM. Vb ) -0.71 V; It ) 0.02 nA.
the bare Au(111) surface.31 When C60 islands form on top of the SAM at 78 K, the azimuthal orientations of the islands are not related to each other, probably due to a rather weak interaction between the C60 molecules and the methyl-terminated SAM. Here we find a single well-defined orientation for all the islands observed, which is clear evidence for direct bonding between the C60 molecules and the gold substrate. Within the region where we find the loosely packed C60 molecules, there is no orientational ordering of the molecules. Molecules within the loose patches are much less stable than those within the compact islands. Under moderate scanning conditions (2.0 V, 0.1 nA), the positions of C60 molecules within the loose patches are constantly changing. Figure 2b shows an STM image from the same region as that of Figure 2a. In between the acquisition of the two images, the sample had been scanned continuously for 26 min. It can be seen that while the compact island remains more or less the same in the two images, the region containing the loose C60 molecules has changed so much that it is not possible to identify the same molecules in the two images. The transformation of the surface from that shown in Figure 1b to that in Figure 2a is described in the following. When the SAM is annealed, it becomes a melted layer accompanied by partial desorption of the thiolate species.51,52 Diffusion of C60 molecules through the liquidlike SAM becomes much easier and, as a consequence, compact C60 islands are nucleated. Once a stable island is formed, it grows by capturing diffusing C60 molecules. The final size of the compact island depends on the duration of annealing and the average flux of the diffusing C60
molecules at the annealing temperature. The loosely packed C60 patches are formed during the “solidification” of the SAM upon cooling down to room temperature. As the thiolate species lose their mobility, random diffusing C60 molecules get trapped within the SAM before they have any chance of attaching to the edges of the compact C60 islands. A high-resolution STM image of the C60 island is given in Figure 2c, where the dark spots are vacant sites. There are several domains within the large compact island, with each domain translated by half a unit cell dimension, 3a, relative to its neighbor. The striped appearance of the SAM following thermal annealing is a more complicated issue than it looks. Several striped phases have been observed in our experiments, and details will be published in a separate paper.50 However, for the benefit of curious readers, here we would like to show a high-resolution image (Figure 2d) of the striped phase related to the images in Figure 2a,b. According to the high-resolution image, the thiolate species are packed in the same way along the [11-2] direction as with the (23 × 23)R30° phase. However, along the [1-10] direction, the adsorbates shift their adsorption sites between the face-centered cubic (fcc) and the hexagonal close-packed (hcp) sites in a systematic way to give the striped appearance. There is thus an increase in the average molecular spacing along the [1-10] direction and the coverage of the striped phase is lower, by ∼20%, than that of the (23 × 23)R30° phase. The thiolates species in the striped phase shown in Figure 2d are still in the standing-up orientation. The
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Figure 3. (a) Schematic diagram illustrates the relationship between C60 molecules and the thiolate species in various locations. (b) STM image (120 nm × 120 nm) shows the structural changes after the tip has destroyed part of the close-packed C60 island (Vb ) 2 V; It ) 0.1 nA).
low coverage striped phase associated with lying-down thiolate has also been observed, but after annealing at a much higher temperature. Figure 3a shows a schematic diagram illustrating the relationship between C60 molecules and the thiolate species in various locations. Because the compact C60 islands are embedded in the SAM, molecular motion around the island edges is greatly suppressed. Figure 3b shows an STM image from the same area where images in Figure 2a,b are obtained. In Figure 3b, a part of the compact island is destroyed by the STM tip, giving rise to a much higher number of loose C60 molecules to the left side of the island. It is noted that the left edge of the remaining island appears very rough. This is in huge contrast to the behavior of C60 islands sitting on bare Au(111) without the SAM, where the edges of the island remain straight and parallel to the 〈112〉 directions even after the removal of some edge molecules by the STM tip, due to the rapid diffusion of edge molecules. Of course, in the absence of the SAM, loose C60 molecules are generally not stable at room temperature and will automatically reassemble into dense islands if not captured by an existing island. Therefore, the loose molecular patches as shown in Figures 2 and 3 are a unique feature of individual C60 molecules residing in the SAM. In comparison with C60 adsorbed on the bare Au(111) surface, the confinement of C60 islands by the SAM is expected to hinder molecular detachment from the island edges and thus enhance the overall stability of the C60 islands. The confinement also has a noticeable effect on the growth of the C60 islands, as demonstrated by Figure 4. The STM images shown in Figure 4a-d are obtained at 348 K from the same area of the sample. Under this temperature, the compact C60 islands remain stable. The streaks appearing in all the images are due to diffusing C60 molecules within the SAM. The streaks first appear at 333 K, which is the temperature at which SAM becomes to melt. The density of the streaks increases sharply with temperature as a direct result of faster molecular diffusion. In Figure 4a, there are several regions of streak-free SAM, where the usual striped feature suggests an ordered, not melted SAM. One such region is highlighted with
Figure 4. STM images obtained at (a-d) 348 K and (e) 363 K, all with the same tunneling parameters: Vb ) 1.8 V, It ) 0.1 nA. (a) Image size: 200 nm × 200 nm. The arrow highlights the growth direction. (b) After ∼3 min of scanning at 348 K, the C60 island becomes larger than that in panel a. Image size: 106 nm × 106 nm. (c) Image (106 nm × 106 nm) obtained 7 min after panel b. (d) Zoomed-out image (300 nm × 300 nm) from panel c. (e) Image (248 nm × 248 nm) obtained at 363 K; there were no streak-free areas left. C60 islands start to grow upward.
a rectangle. Under such situations where melting of the SAM is nonuniform, the growth of the C60 island is strongly anisotropic with the greatest growth rate occurring at step edges, which are frequently visited by diffusing C60 molecules. The time duration between collecting the images of Figure 4 panels b and c is 7 min.. Island growth during this time is clearly observable as demonstrated by the downward movement of the bottom step edge. After the image of Figure 4c was collected, a zoomed-out image was obtained immediately, and this is shown in Figure 4d. By comparison of panel d with panel a, it becomes very clear that the larger of the two compact islands has grown a substantial amount by expanding downward. There is hardly any upward growth for the same island because the upper step edge is connected to a “C60-free” region. Without the supply of the C60 molecules from the region above the island, growth in that direction is thus not possible. When the temperature is raised to 363 K, melting of the SAM is complete and there are no streak-free areas left (Figure 4e). By this time, the compact island has started to grow upward. The growth phenomenon exhibited by the sequence of images in Figure 4 points to the important role played by the SAM in the growth of compact C60 islands. The attachment of diffusing C60 molecules to the step edges of an existing C60 island is controlled
Confined C60 Islands within Octanethiol SAMs by the structure of the SAM, which surrounds the island. This offers the possibility of controlling the direction of growth via the SAM. The growth rate can also be controllable by varying the temperature of the sample. 4. Conclusions In summary, we have demonstrated the growth of C60 islands confined by octanethiol SAMs on Au(111). C60 molecules are unable to penetrate the dense phase, (3 × 3)R30°, of the SAM, but they can reach the Au(111) surface through defects within the SAM. A mixed C60/SAM layer can be formed where the C60 molecules are kept apart by the thiolate species. The formation of compact C60 islands requires partial melting of the SAM, and a single (23 × 23)R30° structure is found for all compact islands formed in this way. The growth of the C60 island in the presence of the SAM is very different from that on a bare gold surface. This is especially true for temperatures at which melting of the SAM is not complete. Anisotropic and directional growth can take place. Our findings suggest that it should be possible to purposefully modify the SAM around an existing C60 island to facilitate directional growth. Acknowledgment. We thank the EPSRC, U.K., for financial support. F.L. thanks the Chinese Scholarship Council for providing a studentship. References and Notes (1) Klug, A. Angew. Chem., Int. Ed. Engl. 1983, 22, 565. (2) Zhang, J. Z.; Wang, Z. L.; Liu, J.; Chen, S. W.; Liu, G. Y. SelfAssembled Nanostructures; Kluwer Academic/Plenum Publishers: New York, 2003. (3) Love, J.; Estroff, L.; Kriebel, J.; Nuzzo, R.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (4) Allara, D. L. Biosens. Bioelectron. 1995, 10, 771. (5) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550–1552. (6) Hatzor, A.; Weiss, P. S. Science 2001, 291, 1019. (7) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1. (8) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498–1511. (9) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (10) Stepanow, S.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Delvigne, E.; Lin, N.; Deng, X. B.; Cai, C. Z.; Barth, J. V.; Kern, K. Nat. Mater. 2004, 3, 229. (11) Spillmann, H.; Kiebele, A.; Stohr, M.; Jung, T. A.; Bonifazi, D.; Cheng, F. Y.; Diederich, F. AdV. Mater. 2006, 18, 275. (12) Yoshimoto, S.; Tsutsumi, E.; Narita, R.; Murata, Y.; Murata, M.; Fujiwara, K.; Komatsu, K.; Ito, O.; Itaya, K. J. Am. Chem. Soc. 2007, 129, 4366. (13) Li, M.; Deng, K.; Lei, S. B.; Yang, Y. L.; Wang, T. S.; Shen, Y. T.; Wang, C. R.; Zeng, Q. D.; Wang, C. Angew. Chem., Int. Ed. 2008, 47, 6717. (14) Glowatzki, H.; Broker, B.; Blum, R. P.; Koch, N.; et al. Nano Lett. 2008, 8, 3825.
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