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Langmuir 1998, 14, 5656-5658
Determination of Thickness of a Self-Assembled Monolayer of Dodecanethiol on Au(111) by Angle-Resolved X-ray Photoelectron Spectroscopy Toshihiro Kondo,† Masatoshi Yanagida,† Katsuaki Shimazu,‡ and Kohei Uosaki*,† Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan, and Division of Material Science, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan Received May 4, 1998. In Final Form: July 7, 1998
The self-assembly (SA) technique, which makes use of a chemical bond between surface atoms of solid substrates and molecules and attractive lateral interaction between adsorbed molecules, has been widely employed to form ordered molecular layers. Self-assembled monolayers (SAMs) of alkanethiols on gold comprise the most wellstudied system.1 Many investigations on electrodes modified with molecules of various functionalities using the SA technique have been extensively reported.2 We have already reported the electrochemical and photoelectrochemical characteristics of gold electrodes modified with SAMs with various functional moieties, such as ferrocene,3-8 quinone,9,10 Ru(bpy)32+,11 azobenzene,12 and porphyrin.13-16 To understand in general the relation between the functionality and the surface structure of the SAM and electron-transfer property in particular, it is essential to determine the thickness of the SAM. Two methods are generally used to determine thickness, d, of the SAMs. The most popular technique is ellipsometry.1 For example, Porter et al. used this technique to determine the thickness of alkanethiol SAMs on gold.17 To determine the thickness by ellipsometry, the value of the refractive index of the monolayer is required. Because * To whom correspondence should be addressed. TEL: 81-11706-3812. FAX: 81-11-706-3440. E-mail:
[email protected]. † Physical Chemistry Laboratory. ‡ Division of Material Science. (1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic: New York, 1991. (2) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109. (3) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1170. (4) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385. (5) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 372, 117. (6) Ohtsuka, T.; Sato, Y.; Uosaki, K. Langmuir 1994, 10, 3658. (7) Kondo, T.; Takechi, M.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 381, 203. (8) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (9) Sato, Y.; Fujita, M.; Mizutani, F.; Uosaki, K. J. Electroanal. Chem. 1996, 409, 145. (10) Ye, S.; Yashiro, A.; Sato, Y.; Uosaki, K. J. Chem. Soc., Faraday Trans. 1996, 92, 3813. (11) Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 384, 57. (12) Yu, H. Z.; Zhang, H. L.; Liu, Z. F.; Ye, S.; Uosaki, K. Langmuir 1998, 14, 619. (13) Shimazu, K.; Takechi, M.; Fujii, H.; Suzuki, M.; Saiki, H.; Yoshimura, T.; Uosaki, K. Thin Solid Films 1996, 273, 250. (14) Kondo, T.; Ito, T.; Nomura, S.; Uosaki, K. Thin Solid Films 1996, 284, 652. (15) Kondo, T.; Yanagida, M.; Ito, T.; Nomura, S.; Uosaki, K. J. Electroanal. Chem. 1997, 438, 121. (16) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (17) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
it is very difficult to measure the refractive index of a SAM, the value of the bulk alkane crystal is usually used. The other technique is angle-resolved X-ray photoelectron spectroscopy (ARXPS), which is known to be one of the useful techniques to investigate the molecular orientation of thin layer materials on solid substrates.18-22 For example, Nakayama et al. determined the molecular orientation of azobenzene-containing ammonium amphiphiles in a vesicle by ARXPS.20 We also employed this technique to investigate the orientation of the ferrocenylalkanethiol molecule in the SAM21 and the layer-bylayer structure of composite thin films of cadmium sulfide nanoparticles and alkanedithiols.22 One can determine the absolute value of the thickness of a surface layer, if the photoelectron mean free path, λ,23 in the surface layer is known. Recently, He et al. determined the thickness of a SAM on germanium with the assumption that λ in the SAM is equal to that in the bulk layer.24 Thus, if one applies either of the methods just mentioned, one needs to assume that the values of the refractive index and photoelectron mean free path of SAM for ellipsometry and ARXPS, respectively, are same as those of the bulk crystals. This assumption is not necessarily valid, particularly when the coverage is low. The value of λ for the surface layer can be theoretically calculated if the energy-loss function in the region of photoelectron energy, 40-2000 eV, is known.25 It is, however, very difficult to measure the energy-loss function of the surface layer. Thus, Tanuma et al. proposed an equation to evaluate λ using empirical parameters26-28 and found that the values of λ calculated using the proposed equation for the thick layers of organic materials were in good agreement with those theoretically calculated using the experimentally obtained energy-loss functions.27 Bain et al.29,30 reported that the photoelectron mean free paths of various alkanethiol SAMs on metal substrates evaluated using the thickness measured by Porter et al.17 based on ellipsometry and those calculated using Tanuma’s equation27 were in good agreement. Although the paper by Bain et al.29,30 suggests that Tanuma’s equation27 is applicable to SAMs, one needs to know the thickness to determine λ or vice versa. In this report, we propose a novel way for the simultaneous determination of the thickness and the photo(18) Fraser, W. A.; Florio, J. V.; Delgass, W. N.; Tobertson, W. D. Surf. Sci. 1973, 36, 661. (19) Fadley, C. S.; Baird, R. J.; Siekhaus, W.; Novakov, T.; Bergstro¨m, S. Å. L. J. Electron Spec. 1974, 4, 93. (20) Nakayama, T.; Takahagi, T.; Soeda, F.; Ishitani, A.; Shimomura, M.; Kunitake, T. J. Colloid Interface Sci. 1988, 122, 464. (21) Shogen, S.; Kawasaki, M.; Kondo, T.; Sato, Y.; Uosaki, K. Appl. Organometallic Chem. 1992, 6, 533. (22) Nakanishi, T.; Ohtani, B.; Shimazu, K.; Uosaki, K. Chem. Phys. Lett. 1997, 278, 233. (23) In this paper, we use the term photoelectron mean free path, λ, as the thickness of material required to reduce the flux of the emitted photoelectrons by 1/e. (24) He, J.; Lu, Z.-H.; Mitchell, S. A.; Wayner, D. D. M. J. Am. Chem. Soc. 1998, 120, 2660. (25) Penn, D. R. Phys. Rev. B 1987, 35, 482. (26) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1988, 11, 577. (27) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1993, 21, 165. (28) Tanuma et al. used λ as an inelastic mean free path (IMFP), which is not identical to the mean free path except in the absence of elastic scattering. Because elastic scattering is negligible,30 however, we assumed that λ is nearly equal to IMFP in the present study. (29) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670. (30) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017.
S0743-7463(98)00517-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/27/1998
Notes
Langmuir, Vol. 14, No. 19, 1998 5657
electron mean free path of SAMs by ARXPS using Tanuma’s equation27 and the coverage of the SAM obtained by electrochemical reductive desorption and demonstrate the usefulness of this method by applying it to dodecanethiol (C12SH) SAM on Au(111). The photoelectron intensity from a thin film-covered substrate varies with the takeoff angle, θ, as given by18,19
ln (I) ) -d/(λ sin θ) + ln (I0)
(1)
where I0 and I are the intensities of the photoelectron from the clean substrate and from the substrate covered with the thin film of thickness d, respectively. According to eq 1, ln(I) should be linearly related to 1/(sin θ) with the slope of -(d/λ). Tanuma et al. proposed the following equation to calculate the λ (in Å) for organic thin films27
/[
{
Figure 1. Au4f XP spectra of the C12SH SAM on Au(111) measured at various values of θ.
λ ) [Ek] E2p β ln(0.191 F-0.5 Ek) -
}]
(1634 - 0.91Ep) (4429 - 20.8Ep) + 829.4Ek 829.4E2 k
(2)
where Ek (in eV) is the kinetic energy of the photoelectron, Ep (in eV) is the free electron plasmon energy, and β is an empirical parameter. Ep and β are given by the following two equations
Ep ) 28.8
( ) NvF M
0.5
(3)
β ) -0.10 + 0.944(E2p + E2g)-0.5 + 0.069F0.1
(4)
where M and Nv are the molecular weight and the number of valence electrons of the overlayer molecule, respectively, F (in g cm-3) is the density of the overlayer, and Eg (in eV) is the band gap energy. In the case of the alkanethiol SAM, F is given by:
F ) ΓM/NAd
(5)
where Γ is the adsorbed amount of the molecules on the gold substrate (molecules cm-3) and NA is Avogadro’s number. By insertion of eq 5 into eqs 2-4, we obtain the following equations
[[ { (
( ) )
ΓM d - ) d E2p β ln 0.191Ek λ NAd
-0.5
-
}]]/
1634 - 0.91Ep 4429 - 20.8Ep + 829.4Ek 829.4E2 Ep ) 28.8
( ) NvΓ NAd
k
[Ek] (6)
0.5
β ) -0.10 + 0.944(E2p + E2g)-0.5 + 0.069
(7)
( ) MΓ NAd
0.1
(8)
Because (d/λ) can be determined from the ARXPS measurement as the slope of the plot of ln(I) versus 1/(sin θ), as shown in eq 1, and Γ can be electrochemically determined by measuring the charge of the reductive desorption in a basic solution,31 d and λ of the alkanethiol SAM can be simultaneously calculated from eq 6 using (31) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335.
Figure 2. Relation between 1/sin θ and logarithm of integrated peak intensity of Au4f in XP spectrum of the C12SH SAM on Au(111).
the values of Ep and β that are calculated eqs 7 and 8, respectively. Reagent grade chemicals were obtained from Wako Pure Chemicals and were used without further purification. The Au(111)-oriented gold substrate was prepared by vacuum evaporation on a polycrystalline gold.32 The surface modification of the Au(111) was carried out by dipping the substrate in hexane solution containing 1 mM C12SH at 20 °C for 15 h under an N2 atmosphere. After the modification, the sample was sequentially washed with hexane, ethanol, and pure water and was transferred to an X-ray spectrometer (Rigaku, XPS7000). MgKR radiation (1253.6 eV) was used for excitation. The sample holder was rotated at the line intersecting the sample surface and the plane of X-ray incidence and reflection. The XP spectra were obtained at the takeoff angles, θ, of 25°, 30°, 40°, 50°, 70°, and 90° by rotating the sample holder. The Au4f signal was acquired for 4 scans. The adsorbed amount of C12SH on Au(111) was electrochemically determined by measuring the charge for the reductive desorption in 0.5 M KOH.31 Figure 1 shows the Au4f XP spectra of the C12SH SAM on Au(111) measured at various values of θ. Two peaks due to Au4f5/2 and Au4f7/2 at 87.5 and 83.8 eV, respectively,33 were observed. The intensity of the Au4f peaks decreased with the decrease in θ. Damage of the SAM by the X-rays was negligible over the present XPS measurements as indicated by the observation that the amount of the adsorbed molecules electrochemically determined did not change before and after the XPS measurements. The value of Γ was determined to be 4.7 × 1014 molecules cm-2 (32) Uosaki, K.; Ye, S.; Kondo, T. J. Phys. Chem. 1995, 99, 14117. (33) Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Ermer: Minnesota, 1978.
5658 Langmuir, Vol. 14, No. 19, 1998
which is in good agreement with the value when C12SH molecules adsorb with a (x3 × x3)R30° structure on Au(111). Figure 2 shows the relation between the logarithm of the integrated peak intensity of Au4f and 1/(sin θ). A linear relation expected from eq 1 was observed with the slope of -0.47 ( 0.02, which is equal to -d/λ. Using this value and Γ ) 4.7 × 1014 molecules cm-2, Ek ) 1169.8 () 1253.6-83.8) eV, M ) 201 (C12H26S), Nv ) 79 (C ) 4, S ) 6, and H ) 1), Eg ) 6 eV,26 one obtains d and λ of the C12SH SAM as 17 ( 1 Å and 36 ( 1 Å, respectively, from eqs 6-8. The thickness is slightly smaller than that determined using ellipsometry, 20 Å,17 and is in a good agreement with that calculated using the molecular structure model with a tilt angle of ∼30° normal to the gold surface of 17.6 Å. Porter et al.17 reported the same tilt angle of the alkanethiol SAM on the gold estimated by using infrared reflection absorption spectroscopy, which supports the present results. The value of λ is also in
Notes
good agreement with the previously reported values for the SAM of alkanethiol, 34 Å, reported by Bain et al.30 and for 26-n-paraffin, 39 Å, reported by Tanuma et al.27 In conclusion, we demonstrated that the combination of ARXPS and electrochemical reductive desorption measurements provides a powerful tool for the determination of the thickness and photoelectron mean free path of the organic thin layer on solid substrates. Acknowledgment. This study was partially supported by Grant-in-Aids for Scientific Research on Priority Area of “Electrochemistry of Ordered Interfaces” (no. 0937101) from the Ministry of Education, Science, Sports, and Culture, Japan. MY acknowledges the Japan Society for the Promotion of Science for the JSPS Research Fellowships for Young Scientists. LA980517C
