Langmuir 1999, 15, 2099-2102
2099
Energy Shift for Core Electron Levels of Chemisorbed Molecules Observed by X-ray Photoelectron Spectroscopy in the Course of Monolayer Growth on a Si(111) Surface Munehisa Mitsuya*,† Advanced Research Laboratory, Hitachi Limited, Hatoyama, Saitama 350-0395, Japan
Naoki Sato* Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Received March 27, 1998. In Final Form: January 21, 1999 Adsorption of fluorinated molecules from a liquid solution onto a fluorine-terminated Si(111) surface has been examined using X-ray photoelectron spectroscopy (XPS). The observed interchange of an F 1s peak from the terminal Si-F at the surface by the core electron peaks ascribed to the adsorbed molecule indicates the formation of an adsorbed monomolecular layer on the surface. In the case of molecules with a linear fluoroalkyl chain, the adsorbate core peaks are observed to shift to lower binding energies with increasing coverage of the Si surface by chemisorbed molecules: the maximum energy shifts, observed for 3-perfluorohexyl-1,2-epoxypropane, are -0.56 eV for C 1s and -0.35 eV for F 1s. This decrease in binding energy can be explained by the increasing relaxation shift caused by the increase of molecular density which permits a larger contribution of the van der Waals attraction among adsorbed molecules. This could be the first observation of a core-level shift due to lateral interchain interactions among ad-molecules, which shows that XPS is also useful for examining intermolecular interactions in the adsorbed layer.
Introduction Organic thin films show considerable technical promise, and further scientific studies of molecular interactions in the ordered molecular array will lead to an understanding and improvement of their bulk and surface properties.1 Coverage of a solid surface by the organic film is usually accomplished by one of three common procedures: forced interfacial transfer, condensation of molecules from a vapor environment, and adsorption of molecules from a liquid solution environment. Among the three procedures, the one that is currently most attractive is the third one, that is, the self-assembled adsorption of the monomolecular layer through a chemical reaction of the molecule with a solid surface. For example, structures of such monolayers deposited on gold surfaces have been examined using optical ellipsometry, infrared absorption spectroscopy, photoelectron spectroscopy, surface plasmon resonance spectroscopy, atomic beam or X-ray diffraction, contact angle measurements, scanning probe microscopy, and thermal desorption experiments.2-4 These techniques are usually applied to study molecular arrangements in the adsorbed layer which has reached the saturation coverage,2,3 but the current emphasis is devoted to in situ observation of the adsorption process.4 To prepare organic thin films of high quality, it is important to study the initial and intermediate stages of deposition for a fully extended monolayer. * To whom correspondence should be addressed. † Present address: Electron Tube & Devices Division, Hitachi Ltd., Hayano, Mobara, Chiba 297-8622, Japan. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachivili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (2) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87 and references therein. (3) Abbottt, N. L.; Kumar, A.; Whitesides, G. M. Chem. Mater. 1994, 6, 596 and references therein. (4) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855 and references therein.
From this perspective, we have studied the molecular arrangements on the surface of a Si single crystal by exploiting the high purity and well-defined nature of the Si surface. Here, our objective is to employ the silicon monofluoride (Si-F) bond5,6 as a chemisorption site on the surface. The Si-F bond is a minor species when a silicon surface is treated with a diluted or buffered HF solution, and its surface density increases with increasing HF concentration.5,6 This bond is so active that the F atom is easily replaced by an OH group upon contact with water.7,8 We therefore suppose that each Si-F bond works as a reactive site for molecular adsorption. In our previous studies,9-11 fluorinated molecules were used predominantly as adsorbates for efficient detection by X-ray photoelectron spectroscopy (XPS). The comparison of relative intensities of core (1s) level electrons ejected from atoms of the same element, i.e., fluorine, involved in different chemical bonds in various molecules has made possible the quantitative analysis of the adsorption behavior of such molecules. XPS and Fourier transform infrared attenuated total reflection (FT-IR ATR) studies have shown that carboxylic acids, epoxides, and alcohols adsorb on the Si surface. Quantitative analysis of F 1s intensities has revealed that (1) a surface Si-F species is stoichiometrically substituted for each molecule, (2) the substitution reaction proceeds until the steric hindrance caused by already adsorbed molecules prevents further (5) Weinberger, B. R.; Peterson, G. G.; Eschrich, T. C.; Krasinski, H. A. J. Appl. Phys. 1986, 60, 3232. (6) Takahagi, T.; Nagai, I.; Ishitani, A.; Kuroda, H. J. Appl. Phys. 1988, 64, 3516. (7) Gra¨f, D.; Grunder, M.; Schultz, R. J. Vac. Sci. Technol., A 1989, 7, 808. (8) Watanabe, S.; Nakayama, N.; Ito, T. Appl. Phys. Lett. 1991, 59, 1458. (9) Mitsuya, M. Langmuir 1994, 10, 1635. (10) Mitsuya, M. Appl. Phys. Lett. 1997, 70, 961. (11) Mitsuya, M.; Sugita, N. Langmuir 1997, 13, 7075.
10.1021/la980346b CCC: $18.00 © 1999 American Chemical Society Published on Web 02/25/1999
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deposition, and (3) a closely packed monolayer is formed with each compound.9-11 In these studies, besides the XPS peak intensities, we also have noticed the binding energy shift in the process of adsorption in the case of several kinds of molecules. In this work, we will study this energy shift in terms of the relaxation energy for core electron levels which could be attributed to the lateral interactions among molecules in the adsorbed layer being formed. Experimental Section Polished n-type (111) oriented Si wafers were used as substrates, because chemical oxidation followed by aqueous etching provides Si surfaces with flat terraces.12 The substrate was first oxidized in boiling HCl/H2O2/H2O () 1:1:13 in molar ratio) solution and dipped in 1% HF aqueous solution to remove the amorphous surface oxide. After this procedure was repeated several times, the surface-oxidized Si was immersed in 50% HF aqueous solution (semiconductor-use grade). The final immersion produced a Si surface terminated with hydrogen and fluorine.5,6 The fluorine population density as determined by XPS was about one-third of the monolayer coverage. Previous study has revealed that Si-F species are not randomly distributed but instead are segregated in islands on the Si surface.10 The following materials were purchased from commercial sources: 3-perfluorohexyl-1,2-epoxypropane 1 from Daikin; perfluorodecanoic acid 2 from PCR, Inc.; 2-(perfluorooctyl)ethanol 3, 2-(perfluorohexyl)ethanol 4, 2-(perfluorobutyl)ethanol 5 and 2,2-bis(trifluoromethyl)propionic acid 6 from Daikin; and 3,5bis(trifluoromethyl)benzoic acid 7 from Lancaster. These materials were used without further purification. A freshly opened dehydrated grade of hexane from Kanto Chemical was used as a solvent for the sample preparation. Its guaranteed and typical contents of H2O were less than 30 ppm and less than 5 ppm, respectively. Adsorption behavior of the above molecules was examined by XPS for different samples prepared using substrates cut from the same wafer. The samples were prepared by immersing substrates into a hexane solution of the adsorbates with a concentration of about 10-2 mol dm-3 at a temperature of 25 ( 0.5 °C. Nitrogen gas was bubbled into the solution prior to and during the immersion of a substrate to prevent the atmospheric oxidation of the substrate. After being rinsed with pure hexane several times, the sample was introduced into the air-lock chamber of a Vacuum Generator ESCA-Lab MARK-2 equipped with a hemispherical electron energy analyzer and a Mg KR X-ray source (1253.6 eV). The base pressure of the analysis chamber was 10-7 Pa. The irradiation and detection angles were 60° and 20°, respectively, with respect to the surface normal. Emitted electrons were collected at 0.05 eV intervals in their kinetic energy. Binding energies of the XPS spectra were referenced to Ag 3d5/2 at 367.9 eV using an Ag overlayer deposited onto a copper substrate. As all of the compounds used have fairly high vapor pressures even at room temperature, physisorbed multilayers were desorbed in vacuo before starting the measurement. This gives samples which are thin enough to get rid of surface charging. Thus, there were no spectral changes in intensity or width with acquisition time. The nominal energy resolution for the spectra in the energy region of C 1s and F 1s could be estimated to be about 0.5 eV because of the resolution of the energy analyzer and the line width of the X-ray source. However, their spectral analysis based on methodical deconvolution made it possible to resolve double peaks into two spectral features separated by much less than 0.5 eV. Further, because the measurements and data processing were carried out under the common conditions for all the materials, it was possible to determine binding energies for individual spectral components in the energy region of interest with a sufficient precision to permit relative comparison between different samples. (12) Itaya, K.; Sugawara, R.; Morita, Y.; Tokumoto, H. Appl. Phys. Lett. 1992, 60, 2534 and references therein.
Figure 1. XPS spectra in F 1s and C 1s regions, observed for the samples prepared by immersing substrates into a 10-2 mol dm-3 hexane solution of 1 at a temperature of 25 ( 0.5 °C. Immersion periods are indicated beside the corresponding spectra.
Results and Discussion Figure 1 shows XPS spectra in the F 1s and C 1s regions, observed for the samples prepared by immersing substrates into a 10-2 mol dm-3 hexane solution of 3-perfluorohexyl-1,2-epoxypropane 1 at a temperature of 25 ( 0.5 °C. The F 1s spectrum of the F-terminated Si surface (the bottom spectrum) in Figure 1 shows a peak at 685.5 eV in binding energy, with a shoulder on the high-energy side. The main peak is associated with the silicon monofluoride (Si-F) and the shoulder with the silicon difluoride (Si-F2).5 With increasing immersion time, the Si-F peak loses its intensity and a new peak at about 689 eV grows gradually. The binding energy of 1s electrons in a fluorine atom covalently bonded to a carbon atom ranges from 688.2 [for poly(vinylidene fluoride)] to 689.7 eV [for poly(tetrafluoroethylene)].13 The new peak is located in this energy region. Further, another new peak in the C 1s region grows at about 292 eV with increasing immersion time. This binding energy almost coincides with that of the C 1s level for poly(tetrafluoroethylene), i.e., 292.48 eV.13 Thus, these two new peaks can be assigned to the core electron levels in the adsorbed molecule. Their spectral intensities and the surface coverage by adsorbed molecules increase with increasing immersion time. Such spectral changes saturated for sample immersion time of more than 10 h, but the Si-F peak is still observable, as shown in Figure 1. Each F 1s spectrum in Figure 1 is deconvoluted into three components, assumed to correspond to Si-F at 685.5 eV, Si-F2 at 687.0 eV, and molecular C-F at 689.0 eV, by least-squares fitting. From the peak intensities, two Si-F species are regarded to be replaced by thirteen fluorine atoms covalently bonded to carbon atoms in the adsorbate. The small intensity of the C 1s spectrum (about one-tenth) relative to that of an F 1s can be explained by both the atomic composition ratio of 6/13 of carbon to fluorine in the perfluorohexyl group of the molecule and the relative atomic sensitivity of 0.21 for carbon versus fluorine.14 In addition, the contribution from Si-F2 species appears to remain constant in all of the observed spectra, so that Si-F2 does not appear to be involved in the reactive adsorption. (13) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; John Wiley & Sons: New York, 1992. (14) Handbook of X-ray Photoelectron Spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer Corporation; 1979.
Energy Shift for Chemisorbed Molecules
Langmuir, Vol. 15, No. 6, 1999 2101
Table 1. Binding Energies (eV) for F 1s and C 1s Levels of Molecules Chemisorbed on the Si(111) Surfacea F 1s
C 1s
compd
no.
a
b
b-a
a
b
b-a
3-perfluorohexyl-1,2-epoxypropane perfluorodecanoic acid 2-(perfluorooctyl)-ethanol 2-(perfluorohexyl)-ethanol 2-(perfluorobutyl)-ethanol 2,2-bis(trifluoromethyl)propionic acid 3,5-bis(trifluoromethyl)benzoic acid
1 2 3 4 5 6 7
688.94(10) 689.10(09) 688.94(13) 688.86(12) 688.70(10) 689.09(08) 688.31(08)
688.59(05) 688.79(06) 688.75(05) 688.65(06) 688.68(04) 689.12(09) 688.33(07)
-0.35 -0.31 -0.19 -0.21 -0.02 0.03 0.02
292.17(16) 292.17(17) 292.07(09) 291.83(14) 291.46(18)
291.61(06) 291.74(04) 291.65(04) 291.52(02) 291.43(03)
-0.56 -0.43 -0.42 -0.31 -0.03
293.20(12)
293.24(06)
0.04
a
Values in parentheses are estimated errors, and a and b denote the binding energy values evaluated by averaging the first and second stable values, respectively (see text). C 1s spectra observed for 6 could not be deconvoluted.
Figure 2. Immersion period dependence of the binding energies for C 1s and F 1s electrons ascribed to the adsorbed 1.
Furthermore, the peak positions for C 1s and F 1s levels in the molecules at various stages of adsorption also have been examined carefully. The binding energy values of those levels for 1, determined by spectral deconvolution, are plotted against the immersion time, as shown in Figure 2. In the figure, open circles represent F 1s peaks and filled circles represent C 1s peaks. This figure shows that binding energies of these core level electrons decrease with increasing immersion time, hence, with increasing surface coverage. When looking into the figure more closely, we notice that the binding energy for each peak remains constant (the first stable value), then decreases by several tenths of an electron volt, and again settles down to a constant value (the second stable value). The precise values of decrease in binding energy for the C 1s and F 1s peaks are -0.56 and -0.35 eV, respectively. Besides, the deconvolution of each spectrum in the F 1s region shows that the peak assigned to the Si-F species is always located at 685.49 ( 0.05 eV. Moreover, a C 1s peak due to hydrocarbon contamination in each spectrum of the C 1s region can constantly be observed at 284.63 ( 0.03 eV (not shown in Figure 1). These two peaks were used as references of the binding energy in the examination above. Table 1 summarizes the binding energies of C 1s and F 1s levels in adsorbed molecules of the seven compounds including 1; in this table, a and b denote the binding energy values evaluated by averaging the first and second stable values, respectively. These values were independent of reaction conditions such as solution concentration and temperature: when increasing the concentration or temperature of the solution, only the immersion time required for saturated adsorption was shortened. As can be seen from this table, notable binding energy shifts are observed for four compounds from 1 to 4. All of these compounds have long fluoroalkyl chains such as -(CF2)nCF3 with n larger than 5. In contrast, no appreciable shift was
observed for molecules having shorter chains in Table 1, that is, -(CF2)3CF3 for 5 and -CF3 for 6 and 7. It is well-known that the ionization energy for the valence electronic levels in an organic molecular compound decreases on going from the gas phase to condensed phases such as amorphous, polycrystalline, and single-crystalline states. This can be explained by the energetic stabilization effect working on a photoionized molecule by the electrostatic polarization induced on the surrounding molecules.15,16 The relaxation shift, defined as the decrease in ionization energy of a molecule moving from the gas phase to the condensed state, is about 1 eV for typical organic molecular solids in which the van der Waals force only supports attractive interaction among molecules.15,16 If the relaxation shift of the valence electronic level in an organic molecule is kept in mind, the energy shift of core electron levels observed for chemisorbed molecules will be discussed in terms of intermolecular interactions in the adsorbed layer as well as interactions between those molecules and the substrate surface. In the case of hexane adsorbed onto the Cu(100) surface, it is reported that the C 1s peak of hexane appears at 283.8 eV for a thick multilayer, whereas the peak is observed at 282.7 eV for a monolayer with the molecular axis oriented parallel to the substrate surface.17 Such a result indicates that a strong interaction between the adsorbate and the metal surface leads to the shift of the core level peak for the adsorbate toward the low binding energy. On the other hand, oxygen chemisorption on W(110) provides an example of the shift of adsorbate core peaks caused by the interaction among ad-molecules.18 In this system, core electron peaks of oxygen shift by up to 0.2 eV to the low binding energy with increasing coverage, which is explained as a “solvent shift” due to the increased number of ad-oxygen atoms surrounding the atom in question.18 At the topmost layer of the unreconstructed silicon (111) surface, the interatomic distance for the atomic arrangement with 3-fold symmetry is 0.385 nm,5,12 so it follows that one Si-F species is expected to occupy an area of 0.13 nm2 on the surface. In contrast, each molecule from 1 to 4 needs an area of about 0.30 nm2 when it stands vertically to the surface, because the cross section of a fluoroalkyl chain is estimated to be 0.30 nm2.19 If these cross sections are compared, it can be estimated that some parts (less than one-third) of the Si-F species on the surface remain unreacted in the process of chemisorption, even if two Si-F bonds disappear through the bidentate adsorption of a molecule. In practice, the Si-F peak is (15) Sato, N. Synth. Met. 1994, 64, 133 and references therein. (16) Sato, N. Electrical and Related Properties of Organic Solids; Munn, R. W., Miniewicz, A., Kuchta, B., Eds.; Kluwer Academic Publishers: Norwell, MA, 1997 and references therein. (17) Witte, G.; Wo¨ll, Ch. J. Chem. Phys. 1995, 103, 5860. (18) Fuggle, J. C.; Menzel, D. Surf. Sci. 1975, 53, 21. (19) Barton, S. W.; Goudot, A.; Bouloussa, O.; Rondelez, F.; Lin, B.; Novak, F.; Acero, A.; Rice, S. A. J. Chem. Phys. 1992, 96, 1343.
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still detectable in the F 1s spectrum for the sample with the longest immersion time in Figure 1. In the previous work, we already have shown that the more bulky molecules chemisorb on the fluorine-terminated Si(111) surface with a larger proportion of Si-F species unchanged.10 In contrast, the Si-F signal completely disappears in the bidentate adsorption of stearic acid glycidyl ester, which has a smaller cross section of 0.20-0.24 nm2.10 Thus, it can be concluded that a Si-F species remains unchanged because of steric protection by molecules already adsorbed at neighboring sites from the attack by a new molecule. This implies that the chemisorption reaction proceeds to give a closely packed monolayer where the steric hindrance by adsorbed molecules comes to inhibit further adsorption. In case of a Langmuir-Blodgett film, there is a strong attractive interaction among molecules with long alkyl chains assembled to form a layer because of the van der Waals force. Such an interaction could cause the electronic energy relaxation in the case of the O/W(110) system.18 Examining Table 1 again and taking account of these interactions, we notice that the magnitude of the energy shift, denoted by b-a, decreases on going from 3 to 5 among the three family compounds. This indicates that the energy shift observed for core electron levels in chemisorbed molecules can be attributed to the relaxation shift mentioned above. Accordingly, the rather abrupt decrease of binding energies shown in Figure 2 can be interpreted by a two-step self-assembly process, that is, the random adsorption followed by segregated coagulation or rearrangement of adsorbed molecules or both.4 On this basis, it will be explained that little energy shift is observed for the compounds 6 and 7, whose bulky structures prevent lateral interchain interactions among ad-molecules. On the other hand, the subtle difference of the relaxation
Mitsuya and Sato
shifts for the F 1s and C 1s levels shown in Table 1 could qualitatively be explained as follows. Because the binding energy of F 1s electrons is larger than that of C 1s electrons because of the difference in the charge of the respective atomic nuclei, electronic perturbation to core levels by valence electrons is less for F 1s than for C 1s so that chemical shifts for F 1s electrons are smaller than those for C 1s ones, as shown in the literature: chemical shifts of F 1s levels range from 683 to 689 eV; in contrast, those of C 1s from 280 to 292 eV.13 The similar differences in the electronic perturbation from the surrounding molecules could lead to the different relaxation energy shifts for the two kinds of core levels, as pointed out. Besides, the relaxation shifts for the core electrons as listed in Table 1 appear to be notably smaller than those for the valence electrons described above; further investigation will be necessary for quantitative discussion on this problem. In the present study, core level electrons of fluoroalkyl molecules have been examined by XPS in the process of chemisorption onto Si(111) surfaces from their liquid solutions. Core peaks from long perfluroalkyl chains are observed to shift to the low binding energy with increasing surface coverage. It could be concluded that such a result is explained by the relaxation shift of core electron levels derived from a close molecular packing in the adsorbate layer on the surface. Further, although XPS has been utilized for the analysis of chemical structure (chemical shift) as well as the atomic density on the surface in the research field of surface adsorption, the present result shows that this method is also useful for examining intermolecular interactions among ad-molecules. LA980346B
