pubs.acs.org/Langmuir © 2010 American Chemical Society
Formation and Superlattice of Long-Range-Ordered Self-Assembled Monolayers of Pentafluorobenzenethiols on Au(111) Hungu Kang,† Nam-Suk Lee,† Eisuke Ito,‡ Masahiko Hara,‡,§ and Jaegeun Noh*,† † ‡
Department of Chemistry, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea, Flucto-order Functions Asian Collaboration Team, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan, and §Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226-8502, Japan Received October 19, 2009. Revised Manuscript Received January 29, 2010
The formation and surface structure of pentafluorobenzenethiol (PFBT) self-assembled monolayers (SAMs) on Au(111) formed under various experimental conditions were examined by means of scanning tunneling microscopy (STM). Although it is well known that PFBT molecules on metal surfaces do not form ordered SAMs, we clearly revealed for the first time that the adsorption of PFBT on Au(111) at 75 C for 2 h yields long-range, well-ordered self√ assembled monolayers having a (2 5 13)R30 superlattice. Our results will provide new insight into controlling the structural order of PFBT SAMs, which will be very useful in precisely tailoring the interface properties of metal surfaces in electronic devices.
Introduction Self-assembled monolayers (SAMs) formed on noble metal surfaces offer a simple and powerful means of fabricating functional monomolecular films that can be applied to a variety of technological applications.1-14 The electron and hole injection in electronic devices could be greatly enhanced by tuning the work function of metal surfaces using organic SAMs.7-11 It has been reported that the structure of organic SAMs modified on metal surfaces strongly affects the 2D growth of organic semiconductor materials and the performance of organic field-effect transistors.12,13 Therefore, an understanding of the surface structure of organic SAMs on metal surfaces is essential to the development of organic electronic devices with enhanced performance. It is well known that pentafluorobenzenethiol (PFBT) with a strong electron-withdrawing group is one of the most popular molecules for modifying metal electrodes because PFBT SAM-modified devices display lower contact resistance and higher carrier mobility than electron-rich SAM-modified devices.7-10 *Corresponding author. Tel: þ82-2-2220-0938. Fax: þ82-2-2299-0762. E-mail:
[email protected]. (1) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Love, J. C.; Estroff., L. A.; Kriebel, J. K.; Nuzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (4) Noh, J.; Kato, H. S.; Kawai, M.; Hara, M. J. Phys. Chem. B 2006, 110, 2793. (5) Madueno, R.; R€ais€anen; Silien, C.; Buck, M. Nature 2008, 454, 618. (6) Yuan, Q.-H.; Yan, H.-J.; Wan, L.-J.; Northrop, B. H.; Jude, H.; Stang, P. J. J. Am. Chem. Soc. 2008, 130, 8878. (7) Hong, J.-P.; Park, A.-Y.; Lee, S.; Kang, J.; Shin, N.; Yoon, D. Y. Appl. Phys. Lett. 2008, 92, 143311. (8) Lim, J. A.; Lee, H. S.; Lee, W. H.; Cho, K. Adv. Funct. Mater. 2009, 19, 1515. (9) Hamadani, B. H.; Corley, D. A.; Ciszek, J. W.; Tour, J. M.; Natelson, D. Nano Lett. 2006, 6, 1303. (10) Khodabakhsh, S.; Poplavskyy, D.; Heutz, S.; Nelson, J.; Bradley, D. D. C.; Murata, H.; Jones, T. S. Adv. Funct. Mater. 2004, 14, 1205. (11) Boer, B. D.; Hadipour, A.; Mandoc, M. M.; Woudenbergh, T. V.; Blom, P. W. M. Adv. Mater. 2005, 17, 621. (12) Virkar, A.; Mannsfeld, S.; Oh, J. H.; Toney, M. F.; Tan, Y. H.; Liu, G.-Y.; Scott, C. S.; Miller, R.; Bao, Z. Adv. Funct. Mater. 2009, 19, 1962. (13) Asadi, K.; Wu, Y.; Gholamrezaie, F.; Rudolf, P.; Bolm, P. W. M. Adv. Mater. 2009, 21, 1. (14) Chen, D.; Kahn, A. Adv. Mater. 2003, 15, 271.
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Despite the technological importance of PFBT molecular films, there have been only a few reports on the formation and surface structure of PFBT SAMs on metal surfaces. It has been reported that although the adsorption of p-halo-substituted benzenethiols on Cu(111)15,16 or Au(111)17 yields ordered SAMs as a result of quadrupolar intermolecular interactions, PFBT on Cu(111) at a cryogenic temperature generates only disordered phases, even after an annealing step.15,16 High-resolution scanning tunneling microscopy (STM) revealed that the adsorption of benzenethiol (BT) onto Au(111) usually generates disordered phases,18-21 whereas ordered BT SAMs with a small lateral dimension of less than 15 nm were also observed.22,23 From these results, it was suggested that the lack of structural order in BT and PFBT molecules is due to their insufficient dipolar character.15,16 However, we recently found that BT applied to Au(111) at elevated solution temperatures forms long-range-ordered SAMs, which are driven by the high diffusion rate of BT molecules.24 However, there have been no reports to date regarding the formation and structure of well-ordered PFBT SAMs on Au(111). For the further application of PFBT SAMs, it is very important to understand the adsorption structure of SAMs and the ability to fabricate well-ordered 2D SAMs. Herein, we report the first molecular-scale adsorption structure of PFBT √ SAMs with long-range order, which adopts a (2 5 13)R30 superstructure. This result will provide new (15) Wong, K.; Kwon, K.-Y.; Rao, B. V.; Liu, A.; Bartels, L. J. Am. Chem. Soc. 2004, 126, 7762. (16) Wong, K. L.; Lin, X.; Kwon, K.-Y.; Pawin, G.; Rao, B. V.; Liu, A.; Bartels, L.; Stolbov, S.; Rahman, T. S. Langmuir 2004, 20, 10928. (17) Jiang, P.; Deng, K.; Fichou, D.; Xie, S.-S.; Nion, A.; Wang, C. Langmuir 2009, 25, 5012. (18) Yang, G.; Liu, G.-y. J. Phys. Chem. B 2003, 107, 8746. (19) Noh, J.; Park, H.; Jeong, Y.; Kwon, S. Bull. Korean Chem. Soc. 2006, 27, 403. (20) Dhirani, A. A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (21) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L. Langmuir 1997, 13, 4018. (22) Wan, L.; Terashima, M.; Noda, H.; Osawa, M. J. Phys. Chem. B 2000, 104, 3563. (23) K€afer, D.; Bashir, A.; Witte, G. J. Phys. Chem. C 2007, 111, 10546. (24) Kang, H.; Park, T.; Choi, I.; Lee, Y.; Ito, E.; Hara, M.; Noh, J. Ultramicroscopy 2009, 109, 1011.
Published on Web 02/04/2010
DOI: 10.1021/la903952c
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Kang et al.
Figure 1. STM images of PFBT SAMs on Au(111) formed after the immersion of Au(111) substrates in a PFBT 1 mM ethanol solution for 24 h at (a) room temperature, (b) 50 C, and (c) 75 C.
meaningful information on controlling the structural order of PFBT SAMs, which will be very useful in precisely tailoring the interfacial properties of metal surfaces in electronic devices.
Experimental Section PFBT was purchased from Aldrich and used without further purification. The Au(111) substrates were prepared by thermal evaporation onto mica as described previously.25 It has been revealed that the formation of organic SAMs is strongly influenced by solution concentration, temperature, and length of immersion time. In this work, we optimized the experimental conditions for SAM preparation to obtain ordered PFBT SAMs on Au(111). It was found that a diluted (0.02 mM) or concentrated (50 mM) solution is not suitable for ordered SAMs compared to 1 mM solution (Supporting Information). Therefore, the PFBT SAMs were prepared by dipping the Au(111) substrates into a PFBT 1 mM ethanol solution for 2 or 24 h as a function of solution temperature: room temperature (RT), 50, and 75 C. STM measurements were carried out using a NanoScope E (Veeco, Santa Barbara, CA) with a commercially available Pt/Ir tip. All STM images were acquired in air using constantcurrent mode at room temperature. Imaging conditions were in the range of 300-500 mV for the bias voltages and in the range of 0.30-0.60 nA for tunneling currents between the tip and the sample.
Results and Discussion The STM images in Figure 1 show surface structures of PFBT SAMs on Au(111) as a function of solution temperature. It was found that the adsorption of PFPT results in the formation of ordered SAMs containing molecular row structures with an interrow distance ranging from 0.6 to 0.7 nm. The molecular spacing in a row was measured to be 5.6 ( 0.2 A˚. PFBT molecules prefer to have row structures regardless of solution temperature, as revealed in the inset high-resolution STM images. It was reported that p-fluorobenzenethiol SAMs on Au(111) have similar row structures resulting from intermolecular π-π stacking.17 However, PFBT SAMs contained many structural defects: disordered phases, dislocation of rows, rows with different topographic heights, and molecular defects in the rows (bright spots). The STM image in Figure 1a shows gold adatom islands covered with PFBT molecules (bright islands), which are often observed with BT SAMs.4,20,24,26 As the solution temperature was increased to 50 or 75 C, the gold adatom islands disappeared and long-range-ordered domains were formed with lateral dimensions of more than 100 nm. The growth of ordered domains was limited by the size of the gold terraces. PFBT SAMs formed at (25) Choi, Y.; Jeong, Y.; Chung, H.; Ito, E.; Hara, M.; Noh, J. Langmuir 2008, 24, 91. (26) Kang, H.; Lee, H.; Kang, Y.; Hara, M.; Noh, J. Chem. Commun. 2008, 5197.
2984 DOI: 10.1021/la903952c
Figure 2. STM images of PFBT SAMs on Au(111) formed after the immersion of Au(111) substrates in a 1 mM ethanol solution of PFBT for 2 h at 75 C.
50 C have molecular rows with a higher actual topographic height, as shown in Figure 1b, and some of these rows were disconnected from each other. More uniform ordered domains were formed at 75 C compared to those formed at RT or 50 C, even though they have no periodic patterns between rows and have many small bright spots within rows (Figure 1c). We assume that these spots can be attributed to the structural instability of PFBT SAMs on the Au(111) surface, which may be due to the favorable desorption of adsorbed molecules as a result of the longer immersion at a high solution temperature. To reduce this negative contribution to structural order, we selected a shorter immersion time of 2 h at 75 C. Surprisingly, as shown in Figure 2a, very uniform PFBT SAMs with a large, ordered single domain were obtained using these preparation conditions. From this study, we suggest that PFBT molecules have a larger diffusion barrier than o- or p-halo-substituted benzenethiols. Hence, they need an elevated surface temperature to overcome this barrier, which results in the formation of wellordered SAMs as demonstrated in this work. The STM image in Figure 2b shows a row structure with a repeating unit. The structural details are displayed in Figure 3. However, the bright features, indicated by arrows on the image, were observed primarily in three ordered rows. We suggest that the molecules adsorbed in these regions probably have an unstable adsorption configuration because of the dynamic motion of PFBT molecules, similar to cyclohexanethiol SAMs with dynamic ring conversions,27 or an unstable adsorption condition of sulfur atoms due to the precipitation-dissolution equilibrium of PFBT molecules on the Au(111) surface during molecular self-assembly. Therefore, the tunneling current may be unstable during STM imaging and, as a result, may make it hard to visualize the individual molecules in these regions. (27) Joo, S.-W.; Chung, H.; Kim, K.; Noh, J. Surf. Sci. 2007, 601, 3196.
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Figure 3. (a) High-resolution STM image of PFBT SAMs on Au(111) formed for 2 h at 75 C and (b) proposed structural model of PFBT SAMs. Note that the yellow circles correspond to the gold atoms of Au(111) surfaces and the interatomic distance of the Au(111) lattice is 2.89 A˚ (ah).
The molecularly resolved STM image in Figure 3a clearly shows well-ordered PFBT SAMs with a repeating unit consisting of three bright rows separated by one dark row. On the basis of this STM observation, we extracted the lattice constants of a ˚ rectangular unit cell containing √ nine molecules: a = 5.6 ( 0.2 A = 2ah and b = 52 ( 2 A˚ = 5( 13)ah, where ah = 2.89 A˚ corresponding to the interatomic distance of the Au(111) lattice. Figure 3b shows a schematic structural model for PFBT SAMs on Au(111). The molecular packing structure for PFBT SAMs √ can be assigned as a (2 5 13)R30 superstructure, which is significantly different from BT or arenethiol SAMs.20,28,29 This means that the pentafluorinated molecular backbone greatly affects the 2D structure and order presumably caused by a difference in lateral interactions due to the size and shape of the molecular backbone. From the model, we assume that the sulfur atoms of PFBT molecules in the three bright rows (indicated by molecular rows B-D in Figure 3a,b) occupy bridge sites of the Au(111) lattice, whereas those in the dark rows (indicated by molecular row A in Figure 3a,b) occupy 3-fold hollow sites. We strongly suggest that nonequivalent adsorption sites of sulfur (28) K€afer, D.; Bashir, A.; Witte, G. J. Phys. Chem. C 2007, 111, 10546. (29) Yang, G.; Qian, Y.; Sita, L. R.; Liu, G.-y. J. Phys. Chem. B 2000, 104, 9059. (30) Li, B.; Zeng, C. G.; Li, Q. X.; Wang, B.; Yuan, L. F.; Wang, H. Q.; Yang, J. L.; Hou, J. G.; Zhu, Q. S. J. Phys. Chem. B 2003, 107, 972.
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atoms on Au(111) could be one of the reasons for STM imaging contrast, as has been suggested in previous work.30 Note that, from our X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS) results (Supporting Information), it was found that PFBT molecules adsorbed on Au(111) surfaces are present in the form of pentafluorobenzenethiolate (C6F5-S-, R-S-) without any structural decomposition. Therefore, it is reasonable to assume that STM imaging of PFBT SAMs reflects the adsorption sites of sulfur atoms of PFBT molecules adsorbed onto Au(111) surfaces, not the thiol group (R-SH) or pentafluorobenzene group (R). In addition, the relative clockwise orientations of the molecules in molecular rows A-D against the Æ110æ direction of the Au(111) lattice were measured to be 20, 41, 160, and 41, respectively.
Conclusions STM imaging revealed for the first time the formation of wellordered PFBT SAMs on Au(111) showing long-range order with lateral dimensions of a few hundred nanometers, which were mainly limited √ by the size of the Au(111) terraces. PFBT SAMs have a (2 5 13)R30 superstructure with a rectangular unit cell containing nine molecules. High-quality PFBT SAMs with a high degree of structural order could be obtained by controlling the solution temperature and immersion time during their preparation. We believe that these results will be very useful for the development of SAM-based electronic devices with controlled interfacial structures. Acknowledgment. This work was supported by the research fund of Hanyang University (HYU-2009-T), the Korea Science and Engineering Foundation through the Joint Research Program (grant no. F01-2008-000-10186-0), the International Research and Development Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) of Korea (grant no. K20901000006-09E0100-00610), and the Seoul R&BD Program (10919). Supporting Information Available: Additional STM images showing the concentration dependence, XPS, TDS, and CV results. This material is available free of charge via the Internet at http://pubs.acs.org.
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