Langmuir 1999, 15, 1579-1583
Azobenzene-Incorporated Alkanethiol Monolayer Film on Au(111): Reflection-Absorption Infrared Spectroscopy and Atomic Force Microscopy Study Sang Woo Han, Chang Hwan Kim, Soon Hyeok Hong, Young Keun Chung, and Kwan Kim* Department of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, Korea Received June 24, 1998. In Final Form: November 24, 1998
1. Introduction In the past decade, self-assembled monolayers (SAMs) have received a great deal of attention for their fundamental importance in understanding interfacial properties as well as for their potential applications in molecular technologies.1 SAM films have been used to study important fundamental processes involving interfacial electron transfer,2 adhesion,3 surface wetting,4 lubrication, and catalysis.5 Such films have also been used in the design of various interfaces for chemical sensors, nonlinear optical materials, optical switches, and high-density memory devices.6 The principal ingredient for obtaining SAMs is a relatively strong interfacial binding asymmetry of the molecular constituents. For the most frequently studied alkanethiol SAMs on Au(111), this is obviously provided by the sulfur affinity for gold and a comparatively strong lateral interaction arising from the van der Waals forces between the chains.7 Even in this case, the actual packing structures and chemisorption mode of sulfur on Au(111) are still unclear, however. In this sense, a better understanding of the molecular details for various SAM systems is needed in order to engineer nanoscale surfaces. Recently, azobenzene-based alkanethiol SAMs have attracted much attention not only from the structural point of view but also from that of the photo- and electrochemical characteristics of the azobenzene group; azobenzene-based molecules have been proved to be a potential medium for high-density recording elements and molecular switches.8-12 Simulations and infrared experiments suggested that such a bulky aromatic unit within an alkanethiol SAM could change the packing density as well as the lattice parameter.13-15 For instance, for azobenzeneterminated alkanethiol SAMs on Au(111), highly ordered structures are usually resumed, regardless of the alkyl chain length, even though the packing densities of the azobenzene moieties are not commensurate with the * To whom all correspondence should be addressed. Fax: 822-8743704 and 82-2-8891568. E-mail:
[email protected]. (1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Murray, R. W. Molecular design of electrode surface; Techniques of Chemistry Series; John Wiley & Sons: New York, 1992; Vol. XXII. (3) (a) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 1230. (b) Wasserman, S. R.; Biebuyck, H.; Whitesides, G. M. J. Mater. Res. 1989, 4, 886. (4) (a) Bain, C. D.; Whitesides, G. M. Angrew. Chem., Int. Ed. Engl. 1989, 28, 506. (b) Whitesides, G. M.; Labinis, P. E. Langmuir 1990, 6, 87. (c) Labinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (5) (a) Adamson, A. W. Physical Chemistry of Surfaces; Wiley: New York, 1976. (b) Somorjai, G. A. Chemistry of Two Dimensions: Surfaces; Cornell University Press: Ithaca, NY, 1981. (6) Ulman, A. Adv. Mater. 1990, 2, 573. (7) Ulman, A. Chem. Rev. 1996, 96, 1533.
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underlying sulfur-bound (x3 × x3)R30° unit cells.9-11 This is attributed to a rather stronger interaction between the azobenzene moieties than between the alkyl chains. Herein we aim to report a model study performed to further increase our understanding of the structure of azobenzene-based alkanethiol SAMs on Au(111). Since most of the previous studies have dealt with azobenzeneterminated alkanethiol SAMs, we have synthesized an azobenzene-based alkanethiol molecule in which the azobenzene moiety is situated at the intermediate position of a long alkyl chain, i.e., 4-dodecoxy-4′-(6-mercaptohexoxy)azobenzene (C12AzoC6SH), and have investigated the effect of the azobenzene moiety on the overall adsobate structure on Au(111) by means of reflection-absorption infrared (RAIR) spectroscopy and atomic force microscopy (AFM). Considering that dodecanethiol has been reported to form a closed-packed structure on Au(111),1,7 the chain length of the dodecoxy group should be long enough for the end group of the target molecule to form a close-packed structure by the van der Waals type interchain interaction. If the packing structure of the dodecoxy group is found to be disordered, this must then occur due to the presence of the azobenzene moiety. 2. Experimental Section A thiol-functionalized azobenzene (C12AzoC6SH) was synthesized by following the procedure in the literature.10 4-Nitrophenol (purity 98%) purchased from Janssen Chimica and 1-bromododecane (purity 97%), 1,6-dibromohexane (purity 96%), and sodium thiosulfate (purity 99%) obtained from Aldrich were used as received. The final product was recrystallized from a 1:1 hexane/ethyl acetate solution. Its chemical structure, i.e., p-CH3(CH2)11OC6H4NdNC6H4O(CH2)6SH was confirmed from 1H NMR spectra taken in CDCl media; δ values were 0.86-0.90 3 (t, 3H, methyl), 1.27-1.84 (m, 28H, methylene), 2.52-2.57 (dd, 2H, -CH2S-), 4.01-4.05 (tt, 4H, -OCH2-), and 6.97-7.87 (m, 8H, phenyl ring). The thiol proton could not be detected due to traces of water present. Unless otherwise stated, all chemicals and gases were analytical grade. Gold substrates with a predominant (111) texture were prepared by the epitaxial growth of 100 nm gold films onto freshly cleaved mica sheets (Asheville-Schoonmaker, for AFM measurement) or precleaned glass slides (for RAIR and ellipsometry measurements). The deposition was carried out by resistive evaporation in a house-customized vacuum chamber at a pressure (8) (a) Tachibana, H.; Nakamura, T.; Matsumoto, M.; Komizu, H.; Manda, E.; Niino, H.; Yabe A.; Kawabata, Y. J. Am. Chem. Soc. 1989, 111, 3080. (b) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658. (c) Liu, Z. F.; Loo, B. H.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1991, 297, 133. (d) Freimanis, J.; Markava, E.; Matisova, G.; Gerca, L.; Muzikante, I.; Rutkis, M.; Silinsh, E. Langmuir 1994, 10, 3311. (e) Wang, R.; Jiang, L.; Iyoda, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. Langmuir 1996, 12, 2052. (f) Velez, M.; Mukhopadhyay, S.; Muzikante, I.; Matisova, G.; Vieira, S. Langmuir 1997, 13, 870. (9) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071. (10) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michel, B.; Gerber, C.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102. (11) Wang, R.; Iyoda, T.; Jiang, L.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1996, 1005. (12) (a) Campbell, D. J.; Herr, B. R.; Hulteen, J. C.; Van Duyne, R. P.; Mirkin, C. A. J. Am. Chem. Soc. 1996, 118, 10211. (b) Yu, H. Z.; Shao, H. B.; Luo, Y.; Zhang, H. L.; Liu, Z. F. Langmuir 1997, 13, 5774. (c) Yu, H.-Z.; Zhang, H.-L.; Liu, Z.-F.; Ye, S.; Uosaki, K. Langmuir 1998, 14, 619. (13) Shnidman, Y.; Ulman, A.; Eilers, J. E. Langmuir 1993, 9, 1071. (14) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121. (15) Chang, S.-C.; Chai, I.; Tao, T.-T. J. Am. Chem. Soc. 1994, 116, 6792.
10.1021/la9807457 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/15/1999
1580 Langmuir, Vol. 15, No. 4, 1999 of ∼1 × 10-6 Torr. Prior to deposition, the substrates were heated in a vacuum at 250 °C for 1-2 h. Gold (purity 99.99%) was deposited onto the heated substrates at a rate of ∼3 Å/s. Subsequently, the substrates were allowed to cool radiatively to below 60 °C, removed from the evaporation chamber, and immediately immersed in a 1 mM benzene solution of C12AzoC6SH. After ∼24 h, the substrate was removed from the solution and rinsed thoroughly with benzene and then ethanol, followed by drying in a N2 gas stream. The infrared spectra were obtained with a Bruker IFS 113v Fourier transform spectrometer equipped with a globar light source and a liquid-N2-cooled mercury cadmium telluride detector. The method for obtaining the RAIR spectra has been reported previously.16 Each spectrum was obtained by averaging 1024 interferograms at 4 cm-1 resolution, with p-polarized light incident on the gold substrate at 80°. To reduce the effect of water vapor rotational lines, the sample and reference interferograms were recorded alternately after every 32 scans. The Happ-Genzel apodization function was used in Fourier transforming all the interferograms. The RAIR spectra are reported as -log(R/Ro), where R and Ro are the reflectivities of the sample and the bare clean metal substrates, respectively. The ellipsometric thickness of the self-assembled C12AzoC6SH film was estimated using a Rudolph Auto EL II optical ellipsometer. The measurement was performed using an He/Ne laser 632.8 nm line incident upon the sample at 70°. The ellipsometric parameters, ∆ and Ψ, were determined for both the bare clean substrates and the self-assembled films. The socalled DafIBM program supplied by the Rudolph Technologies was employed to determine the thickness values. At least five different sampling points were considered in order to obtain an averaged thickness value. AFM images were obtained in air at room temperature by using a Digital Instruments Model Nanoscope IIIa scanning probe microscope. Using a V-shaped and 200 µm long Si3N4 cantilever with a nominal spring constant of 0.12 N/m (Nanoprobe, Digital Instruments), topography images were recorded in a conventional height mode (contact mode, normal AFM) at a scan rate of 2030 Hz.
3. Results and Discussion It is well-known that the surface morphology of metal films prepared by a thermal evaporation technique is generally dependent on the rate of deposition, the ultimate film thickness, the type of substrate and its temperature, and the pressure of the evaporation chamber.17 As will be described later, the gold films deposited in this work on the surfaces of glass slides as well as on the freshly cleaved mica were revealed by AFM to consist of atomically flat crystallites. The nearest neighbor spacings of gold atoms were 2.9 ( 0.2 Å, implying the formation of Au(111) lattices. The domain size of the gold crystallites was usually larger than 200 nm. 3.1. RAIR Spectral Features. As one might expect, C12AzoC6SH molecules chemisorbed on the gold surface very favorably. The RAIR spectra taken with a film prepared by a 30-min self-assembly were barely different from those taken with a film prepared by a 24-h selfassembly. Figure 1a shows the RAIR spectrum of C12AzoC6SH self-assembled on gold in 1 mM benzene solution for 24 h. For comparison, the transmission infrared (TIR) spectrum of neat C12AzoC6SH in a KBr matrix is shown in Figure 1b. All the peaks in these spectra can be readily assigned by consulting the data in the literature,8,18,19 and the results are summarized in Table 1. In the neat TIR spectrum, the S-H stretching peak appeared at 2560 cm-1, (16) (a) Son, D. H.; Ahn, S. J.; Lee, Y. J.; Kim, K. J. Phys. Chem. 1994, 98, 8488. (b) Lee, Y. J.; Jeon, I. C.; Paik, W.-K.; Kim, K. Langmuir 1996, 12, 5830. (c) Han, S. W.; Ha, T. H.; Kim, C. H.; Kim, K. Langmuir 1998, 14, 6113. (17) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45.
Notes
Figure 1. (a) RAIR spectrum of C12AzoC6SH on gold and (b) neat (solid) TIR spectrum of C12AzoC6SH. Table 1. Infrared Spectral Data and Vibrational Assignment of C12AzoC6SH in Neat Solid and Adsorbed States on Gold neat (cm-1)a
RAIR (cm-1)b
assgntc,d
2956 2937 2919 2874 2850 2560 1604 1581 1498 1473 1464 1394 1319 1298 1248 1151 1107 1024 1005 999 943 843 775
2964 2936 2922 2878 2852
νas(CH3) νs(CH3), FR νas(CH2) νs(CH3), FR νs(CH2) ν(SH) 8a/b 8a/b 19a δ(CH2) δ(CH2) δ(CH2) 14 3 νas(COC) ν(C-N) 18b νs(COC) 18a 12 17b(op) 10a(op) 11(op)
1605 1583 1500 1475 1464 1394 1317 1298 1252 1151 1109 1026 1007
a Taken in KBr matrix. b Reflection-absorption infrared spectrum on gold film. c Assigned based on refs 8e, 18, and 19. d FR ) Fermi resonance, and op ) out of plane.
but the counterpart was completely absent in the RAIR spectrum. This indicates that C12AzoC6SH molecules should chemisorb on gold as thiolates, as one would certainly expect. In the RAIR spectrum, one can readily identify the presence of C-H stretching modes of methyl and methylene groups as well as several benzene ring modes. This indicates that the species responsible for the RAIR spectra is in fact C12AzoC6S-. The distinct peaks at 1500, 1583, and 1605 cm-1 in Figure 1a can be assigned, respectively, to the benzene ring 19a, 8a/b, and 8a/b modes.18d Their transition dipoles are all aligned parallel to the long axis of the trans azobenzene moiety.18c It is noteworthy that the out-of(18) (a) Nakahara, H.; Fukuda, K. J. Colloid Interface Sci. 1983, 93, 530. (b) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990, 6, 672. (c) Katayama, N.; Ozaki, Y.; Seki, T.; Tamaki, T.; Iriyama, K. Langmuir 1994, 10, 1898. (d) Armstrong, D. R.; Clarkson, J.; Smith, W. E. J. Phys. Chem. 1995, 99, 17825. (e) Wang, R.; Iyoda, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. Langmuir 1997, 13, 4644. (19) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
Notes
Langmuir, Vol. 15, No. 4, 1999 1581
plane benzene ring 10a mode appears very distinctly at 843 cm-1 in the TIR spectrum of neat C12AzoC6SH (Figure 1b), but its counterpart is hardly seen in the RAIR spectrum (Figure 1a). If we recall the infrared surface selection rule that vibrational modes whose transition dipole moments are directed normal to the metal substrate are exclusively infrared active, the above observation dictates that the azobenzene moiety should assume a nearly perpendicular orientation with respect to the underlying gold substrate. The upright orientation of the azobenzene group may be further envisaged from the relative intensities of the symmetric and antisymmetric C-O-C stretching bands. The transition dipole of the former band should be aligned parallel to the axis bisecting the C-O-C angle while that of the latter should be perpendicularly aligned. With reference to the literature,18a,b the bands at 1026 and 1024 cm-1 in the RAIR and TIR spectra, respectively, can be assigned to the νs(COC) mode and the bands at 1252 and 1248 cm-1 in the RAIR and TIR spectra, respectively, can be attributed to the νas(COC) mode. Compared with the TIR spectrum, the νs(COC) band is seen to be far weaker than the νas(COC) band in the RAIR spectrum. This can be understood by invoking an upright orientation of the azobenzene moiety. If the theory of RAIR spectroscopy is recalled, the intensity ratio of the νs(COC) and νas(COC) bands in the RAIR spectrum can be related to that in the TIR spectrum as follows:20
IRs /IRas ) ITs cos2 θs/ITas cos2 θas
(1)
Here IRs and ITs are the intensities of the νs(COC) band in the RAIR and TIR spectra, respectively, IRas and ITas are the intensities of the νas(COC) band in the RAIR and TIR spectra, respectively, and θs and θas are the angles between the transition dipoles of the νs(COC) and νas(COC) modes and the surface normal, respectively. For an upright orientation of the azobenzene moiety, the values of θs and θas should be ca. 30 and 60°, respectively. Using these angles and substituting the intensity ratio observed in the TIR spectrum into eq 1, the intensity ratio of the νs(COC) and νas(COC) bands in the RAIR spectrum is estimated to be 0.12. On these grounds, the weak appearance of the νs(COC) band in the RAIR spectrum does not seem to be surprising. We are thus confident that the azobenzene moiety assumes a perpendicular orientation with respect to the metal substrate. The fact that peaks at low frequencies, i.e.,
