pubs.acs.org/Langmuir © 2009 American Chemical Society
Alkyl and Alkoxyl Monolayers Directly Attached to Silicon: Chemical Durability in Aqueous Solutions Hikaru Sano,† Hajime Maeda,† Takashi Ichii,† Kuniaki Murase,† Kei Noda,‡ Kazumi Matsushige,‡ and Hiroyuki Sugimura*,† †
Department of Materials Science and Engineering, Kyoto University Yoshida-hommachi, Sakyo-ku, Kyoto 606-8501, Japan, and ‡Department of Electronic Science and Engineering, Kyoto University Kyotodaigaku-katsura, Nishikyo-ku, Kyoto 615-8510, Japan Received December 11, 2008. Revised Manuscript Received February 4, 2009
For practical application of self-assembled monolayers (SAMs), knowledge of their chemical durability in acidic or basic solutions is important. In the present work, a series of SAMs directly immobilized on a silicon (111) surface through Si-C or Si-O-C covalent bonds without a native oxide layer were prepared by thermally activated chemical reactions of a hydrogen-terminated Si(111) substrate with linear molecules, i.e., 1-hexadecene, 1-hexadecanol, 1dodecanol, and n-dodecanal, to investigate the durability of the SAMs to HF and Na2CO3 solutions. While grazing incidence X-ray reflectivity measurements showed that all the as-prepared SAMs had almost the same film density and molecular coverage, keeping the original step and terrace structure of Si(111) as is observed by atomic force microscopy, they gave different degradation behaviors, i.e., pitting and concomitant surface roughening in both solutions. 1Hexadecene SAM was stable against immersion in both solutions, while the other SAMs were damaged within 60 min, most likely due to the difference in chemical bonding modes at the SAM/Si interface, i.e., Si-C and Si-O-C.
1. Introduction Organic thin films with a single molecular thickness are formed via self-integration and self-organization of molecules chemisorbing on solid surfaces. Such films are called self-assembled monolayers (SAMs). They bear not only fundamental scientific interest but also practical application potential. One characteristic feature of the SAMs is that the orientation and arrangement of the molecules is highly ordered.1 Hence, self-assembling has been recognized as a key process for bottom-up nanotechnology to integrate a set of minute elements and to fabricate novel materials and devices. In particular, SAMs on the surface of inorganic semiconducting materials such as silicon (Si) are of great interest for building up novel micro/nano electronic devices integrating a variety of functions based on organic molecules and semiconductor characteristics.2 It is well-known that SAMs are formed on Si substrates through silane coupling chemistry using a specific organosilane reagent as a precursor.3-5 In this case, the Si substrates are covered with an native oxide layer, which works as a reaction site with the silane groups. In other words, an insulator exists between the SAMs and the bulk Si substrate, and such an interfacial insulating layer works as an electron transport barrier only to prevent fusion of the electronic properties of the SAM and those of Si. Thus, a SAM directly attached to Si without an interfacial insulator is crucial for the development of novel micro/nano electronic devices where the organic monolayer and Si are effectively integrated.
*Corresponding author. E-mail:
[email protected]. (1) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (2) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074–1087. (3) Moses, P. R.; Murray, R. W. J. Am. Chem. Soc. 1976, 98, 7435–7436. (4) Osa, T.; Fujihira, M. Nature (London) 1976, 264, 349–350. (5) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92–98. (6) Buriak, J. M. Chem. Commun. 1999, 1051–1060. (7) Buriak, J. M. Chem. Rev. 2002, 102, 1271–1308. (8) Leftwicha, T. R.; Teplyakov, A. V. Surf. Sci. Rep. 2008, 63, 1–71. (9) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 2002, 2, 23–34.
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A large number of studies have been carried out on SAMs directly attached to Si.6-29 Linford et al. reported formation of such SAMs using a reaction between 1-alkene molecules and an H-terminated Si (H-Si) surface and revealed that Si-C linkages exist at the interface between the SAM and bulk Si.14 The reaction is a kind of hydrosilylation and is generally considered to be (10) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631– 12632. (11) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M. Langmuir 2000, 16, 7429–7434. (12) Niederhauser, T. L.; Lua, Y.-Y.; Jiang, G.; Davis, S. D.; Matheson, R.; Hess, D. A.; Mowat, I. A.; Linford, M. R. Angew. Chem., Int. Ed. 2002, 41, 2353– 2356. (13) Lua, Y.-Y.; Fillmore, W. J. J.; Linford, M. R. Appl. Surf. Sci. 2004, 231 232, 323–327. (14) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145–3155. (15) Bergerson, W. F.; Mulder, J. A.; Hsung, R. P.; Zhu, X.-Y. J. Am. Chem. Soc. 1999, 121, 454. (16) Faucheux, A.; Yang, F.; Allongue, P.; Henry de Villeneuve, C.; Ozanam, F.; Chazalviel, J.-N. Appl. Phys. Lett. 2006, 88, 193123. (17) Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 1999, 121, 11513– 11515. (18) Stewart, M. P.; Buriak, J. M. J. Am. Chem. Soc. 2001, 123, 7821–7830. (19) Hossain, Md.Z.; Kato, H. S.; Kawai, M. J. Phys. Chem. B 2005, 109, 23129–23133. :: (20) Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 1999, 15, 8288–8291. (21) Asanuma, H.; Lopinski, G. P.; Yu, H.-Z. Langmuir 2005, 21, 5013–5018. :: (22) Effenberger, F.; Gotz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. 1998, 37, 2462–2464. (23) de Smet, L.C.P.M.; Stork, G. A.; Hurenkamp, G. H. F.; Sun, Q.-Y.; Topal, H.; Vronen, P. J. E.; Sieval, A. B.; Wright, A.; Visser, G. M.; Zuilhof, H.; :: Sudholter, E. J. R. J. Am. Chem. Soc. 2003, 125, 13916–13917. (24) Sun, Q.-Y.; de Smet, L.C.P.M.; van Lagen, B.; Wright, A.; Zuilhof, H.; :: Sudholter, E. J. R. Angew. Chem., Int. Ed. 2004, 43, 1352–1355. (25) Eves, B. J.; Sun, Q.-Y.; Lopinski, G. P.; Zuilhof, H. J. Am. Chem. Soc. 2004, 126, 14318–14319. :: (26) Sun, Q.-Y.; de Smet, L.C.P.M.; van Lagen, B.; Giesbers, M.; Thune, P. C.; :: van Engelenburg, J.; de Wolf, F. A.; Zuilhof, H.; Sudholter, E. J. R. J. Am. Chem. Soc. 2005, 127, 2514–2523. (27) Fabre, B.; Hauquier, F. J. Phys. Chem. B 2006, 110, 6848–6855. (28) Tajimi, N.; Sano, H.; Murase, K.; Lee, K.-H.; Sugimura, H. Langmuir 2007, 23, 3193–3198. (29) Sano, H.; Maeda, H.; Matsuoka, S.; Lee, K.-H.; Murase, K.; Sugimura, H. Jpn. J. Appl. Phys. 2008, 47, 5659–5664.
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initiated or promoted by the use of a radical initiator,10,14 the application of heat,14 the irradiation of UV,22 visible light,23-26 and their combination. 1-Alcohols9,11 and n-aldehydes9,11,22 are also candidates for the precursor to form SAMs directly attached to Si by reacting with H-Si, and it is known that the SAMs from 1-alcohols and n-aldehydes have Si-O-C linkages at the interface.9 Recently, several articles have been devoted to the studies of electric characteristics of such SAMs. Our group has prepared a set of a vinylferrocene SAM with Si-C bonds and a ferrocenecarboxaldehyde SAM with Si-O-C bonds on n-type Si(111) substrate and has investigated their electrochemical behavior.28 The fundamental data on the chemical durability of the SAMs are of primary importance in order to put these SAMs to practical use. For example, when the SAMs are used as resist films or when the SAM surfaces are chemically functionalized, SAMs are required to resist etching solutions such as acid, basic solutions, and HF solution during immersion. Linford et al.14 studied the chemical disabilities of 1-alkene SAM directly attached to Si by comparing it with organosilane SAM formed on bulk Si with an interfacial oxide layer. In the research, they found that 1-alkene SAM has more durability to NH4F solution than organosilane SAM, while both SAMs showed strong durability to acid H2SO4 solution, and considered that the different durability to NH4F solution is due to the difference in the interfacial structure. Saito et al.30 also compared the durability of SAMs directly attached to Si with those of organosilane SAMs with an interfacial oxide layer. They concluded that, when immersed in HF and NH4F solutions, the SAMs directly attached to Si have more resistibility to the solutions than organosilane SAMs. This strong resistance of SAMs directly attached to Si was applied to the AFM lithography technique by Ara et al.31,32 Boukherroub et al.11 compared the n-aldehyde SAM and 1-alcohol SAM, both of which have Si-O-C linkages at the interface between the SAMs and Si, and they found that n-aldehyde SAM showed more durability to HCl and HF solutions due to the higher packing density of the molecules in the n-aldehyde SAM. Although several studies have been made on the chemical durability as mentioned above, little is known about the difference in durability between the SAMs with Si-C linkages and those with Si-O-C linkages. Moreover, there is no report on a detailed observation of the SAM surface during immersion in chemical solution such as HF and basic solution using, for example, atomic force microscopy (AFM), to elucidate temporal changes of decaying of the SAMs. In the present work, we prepared a set of the SAMs by the thermally activated chemical reaction of HSi substrate with 1-alkene, 1-alcohol, and n-aldehyde; the former molecule gives the SAM with Si-C linkages, while the latter two give SAMs with Si-O-C linkages. The resulting samples were confirmed to be monolayers using AFM, ellipsometric measurement, water contact angle measurement, and X-ray photoelectron spectroscopy (XPS). The structures of the SAMs were also examined using Fourier transform infrared spectroscopy (FTIR) and grazing incidence X-ray reflectivity (GIXR) data. Then, their chemical durabilities, such as temporal changes in HF and Na2CO3 solutions, were examined in detail using AFM, ellipsometric measurement, water contact angle measurement, and XPS.
2. Experimental Section A n-Si(111) wafer (phosphorus doped, resistivity 1-10 Ω cm, single polished) was used for the present experiments. All the (30) Saito, N.; Youda, S.; Hayashi, K.; Sugimura, H.; Takai, O. Chem. Lett. 2002, 31, 1194–1195. (31) Ara, M.; Tada, H. Appl. Phys. Lett. 2003, 83, 578–580. (32) Ara, M.; Graaf, H.; Tada, H. Appl. Phys. Lett. 2002, 80, 2565.
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substrates cut from the wafer were cleaned ultrasonically with ethanol and ultrapure water (>18.0 Ω cm) and then photochemically cleaned by exposing to vacuum ultraviolet light generated from an excimer lamp (λ = 172 nm, 10 mW cm-2) for 20 min.33 H-Si substrates were obtained by etching the cleaned samples in 5% HF solution (prepared from 50% HF solution, Morita Chemical Industry Co. Ltd.) for 5 min at room temperature under a light shield and subsequently in 40% NH4F solution (Daikin Industry Co. Ltd.) for 30 s at 80 °C;34 the heating of the 40% NH4F solution eliminates the dissolved oxygen. The native oxide layer on each sample was removed, and the surface Si atoms were terminated with hydrogen through these treatments. SAM formation was performed using a thermal activation method. A three-necked separable glass flask (capacity 360 cm3) was equipped with a thermometer, an Allihn condenser for refluxing, and a N2 gas inlet and was placed on a hot plate. 1Hexadecene (HD, Tokyo Chemical Industry, >90%), 1-hexadecanol (HDO, Tokyo Chemical Industry, 99%), 1-dodecanol (DDO, Tokyo Chemical Industry, >99%), and n-dodecanal (DDA, Fluka, g95%) were used as precursor molecules. About 200 cm3 of each precursor was put in the flask as neat liquid. Since HDO and DDO are solids at room temperature, they were preheated into liquid state. In order to suppress oxidation of the H-Si substrate surface with dissolved oxygen molecules, deaeration was carried out for at least 30 min before and after the freshly prepared H-Si was immersed in the liquid. The deaeration was continued during each reaction with the condition as follows: For the SAM preparation from HD molecules, the liquid was heated up to 180 °C and was kept for two hours. For the SAM preparation from HDO and DDO molecules, the liquid was heated up to 150 °C and was kept for 16 h. For the SAM preparation from DDA molecules, the liquid was heated up to 150 °C and was kept for 2 h. These conditions for thermal treatment were determined by optimization experiments (not shown). After the SAM formation reaction, the samples taken from HD and DDA were subsequently sonicated for 10 min each in hexane, methanol, and ultrapure water, in that order, to remove physisorbed molecules. The samples taken from HDO and DDO were subsequently rinsed in mesitylene at about 80 °C and sonicated for 5 min in mesitylene at 80 °C, for 10 min in methanol, and for 10 min in ultrapure water. The reason for the heating of mesitylene is to remove residual solid HDO and DDO effectively. The samples prepared from HD, HDO, DDO, and DDA are hereafter referred to as HD SAM, HDO SAM, DDO SAM, and DDA SAM, respectively. The chemical durability of the SAMs was examined by immersing the samples in 5% HF aqueous solution or 300 mM Na2CO3 solution for 1-60 min. The solvent of the Na2CO3 solution was a 2:5 vol mixture of methanol and water; the pH of the solution was 11.8. The mixture was used because removed organic molecules or decomposed substances from the SAMs seemed easier to be dissolved in the mixture. After the immersion in the HF solution for given time period, the samples were rinsed in ultrapure water twice, sonicated in ethanol for 5 min and in ultrapure water for 5 min. In contrast, after immersion in the Na2CO3 solution, the samples were sonicated in ethanol for 5 min and in ultrapure water for 5 min. The washed samples are evaluated as described in the next paragraph. The static water contact angles of the samples were measured with water droplets of fixed size, about 1.8 μL in volume. XPS analysis was carried out using an ESCA-3400 system (Kratos Analyical Ltd.), the background pressure of which was lower than 5 10-6 Pa. The X-ray source was Mg KR operated at 10 kV and 10 mA. The Si 2p, O 1s, and C 1s regions were scanned for all the samples under the following conditions: step width 0.1 eV, sampling time 298.5 ms, scan cycles 10 times. The obtained XPS (33) Sugimura, H.; Hozumi, A.; Kameyama, T.; Takai, O. Surf. Interface Anal. 2002, 34, 550–554. (34) Kurokawa, S.; Takei, T.; Sakai, A. Jpn. J. Appl. Phys. 2003, 42, 4655–4658.
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spectra were calibrated against Si-Si, i.e., bulk Si peaks from the Si substrates referenced to 99.6 eV,35 in order to cancel binding energy shifts due to the charge-up effect. To make the chemical state conspicuous, the intensity scales of the Si 2p spectra were standardized so that the main Si-Si peaks became the same intensity. Surface atomic compositions were determined using standard XPS cross sections, after making a Shirley-type background correction. The thicknesses of the organic film on the Si sample were measured with a spectroscopic ellipsometer (Otsuka Electronics Co., Ltd., FE-5000). The measured region was 400800 nm in wavelength, and the incident angle was set at 70°. The model of air/organic film/Si was used for the analysis of raw data. Refractive index data of SiO236 were used as alternative to the organic film for all the measured wavelengths. This alternative is usually used for the ellipsometry of self-assembled alkyl monolayers. Topographic images of the resulting samples with an area of 500 nm 500 nm were acquired by an AFM (Seiko Instruments Inc., SPA-300HV + SPI-3800N) with two types of Si probes: SI-DF20-Al (Seiko Instruments Inc., tip radius DDO SAM = DDA SAM. From the GIXR data, XPS analysis, and elipsometric data, the sequence of packing density is as follows: HD SAM = HDO SAM > DDO SAM = DDA SAM. These sequences of orientational order and packing density are the same. The sequences are accounted for as follows: The chain length of the molecules comprising HD SAM and HDO SAM are almost the same as that for hexadecane. Those for DDO SAM and DDA SAM are almost the same as that for dodecane. Considering the van der Waals force between the molecules, molecules with longer chain length would form SAMs with higher orientational order and greater packing density. Table 5 summarizes the results of each measurement of the durability test in HF solution. The results obtained by the four kinds of measurements did not give completely the same results due to the difference in sensitivity of each measurement. However, they mostly gave consistent results, and considering them, the sequences of durability of the four SAMs are as follows: HD SAM > HDO SAM > DDO SAM = DDA SAM. The reason HD SAM is more stable against HF etching than the other SAMs is that Si-C SAMs do not have the Si-O-C bond but Si-O-C SAMs have the Si-O-C bond, which is likely to dissociate in HF solution. For the Si-O-C SAMs, the sequence of durability is consistent with sequences of orientational order and of the packing density. When a SAM is densely packed, HF molecules are unlikely to reach to the bottom of an absorbed molecule to remove the molecule. Table 6 summarizes the results of each measurement of the durabilities test in Na2CO3 solution. They also gave consistent results, although the sensitivities are different among the four kinds of measurement. Considering the results, the sequence of durability of the four SAMs is as follows: HD SAM > DDA SAM > DDO SAM > HDO SAM. The reason HD SAM is more stable against Na2CO3 etching than the other SAMs is that Si-C SAMs do not have Si-O-C bond but Si-O-C SAMs have Si-O-C bond, which is likely to dissociate in basic solution. For the Si-O-C SAMs, the sequence of durability is not consistent with the sequences of orientational order and of packing density, unlike the case for the durability test in HF solution. When a SAM is densely packed, OH- ions would be unlikely to reach the bottom of an absorbed molecule to remove 5524
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Table 5. Durability Evaluation of Each SAMs to HF Solution by Each Measurement SAM samples HD SAM HDO SAM DDO SAM DDA SAM
contact carbon content angle (XPS) good poor poor poor
ellipsometric thickness
AFM topography
good good good good
good fair poor poor
good good good good
Table 6. Durability Evaluation of Each SAMs to Na2CO3 Solution by Each Measurement SAM samples HD SAM HDO SAM DDO SAM DDA SAM
contact carbon content angle (XPS) good poor poor fair
ellipsometric thickness
AFM topography
excellent poor fair fair
good poor fair fair
good poor poor good
the molecule. So, considering the orientational order and packing density, HDO SAM could be more stable than the other two SAMs, and DDA SAM could be as stable as DDO SAM. This inconsistency means that durabilities are not determined only by the interfacial bonds, orientational order, and packing density. There will be other factors which affect the durabilities of the SAMs.
5. Summary We successfully formed SAMs from three kinds of molecules, i.e., 1-alkene, 1-alcohol, and n-aldehyde on the hydrogen-terminated silicon (111) surface with high orientational orders of molecular arrangement, which were examined by water contact angle measurements, XPS, ellipsoemetry, and AFM. FT-IR measurement clarified that SAMs formed from 1-alcohol and naldehyde have an Si-O-C linkage at the SAM/Si interface, while the SAM from 1-alkene does not have the Si-O-C interfacial linkage, indicating it has Si-C linkage. FT-IR results indicated that the sequence of orientational order is as follows: HD SAM = HDO SAM > DDO SAM = DDA SAM, which is consistent with the sequence of packing density indicated by results from GIXR data, XPS, and ellipsometric data. The chemical durabilities of the SAMs are important in order to put these SAMs into practical use. We immersed these SAMs in 5% HF solution and 300 mM Na2CO3 solution. The sequence of durability of the SAMs to the HF solution was HD SAM > HDO SAM > DDO SAM = DDA SAM, and that to the Na2CO3 solution was HD SAM > DDO SAM = DDA SAM > HDO SAM. 1-Alkene SAM resisted both solutions, displaying no molecular removal, while SAMs formed from 1-alcohol or n-aldehyde suffer molecular removal. HD SAM showed greater durability than HDO SAM, although HD SAM have a same orientational order and packing density as HDO SAM. This is due to the difference in interfacial linkage, i.e., Si-C and Si-O-C. The sequence of durability to HF is accounted for from the viewpoints of orientational order, packing density, and the interfacial linkage, while the durability to Na2CO3 is not fully accounted for from these viewpoints. Acknowledgment. This work was partially supported by a Grant-in-Aid for the Global COE Program, “International Center for Integrated Research and Advanced Education in Materials Science”, Grant-in-Aid for Scientific Research (KAKENHI) No. 19049010 on Priority Areas, “Strong Photons-Molecules Coupling Fields (470),” and KAKENHI (B) No. 20360314, and Langmuir 2009, 25(10), 5516–5525
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Nanotechnology Support Project, from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Supporting Information Available: ATR FT-IR spectra of C-H vibrational stretching region of the SAM samples
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(Figure A), ATR FT-IR spectra of resulting SAM samples in Si-O vibrational stretching region (Figure B), and X-ray reflectivity profiles of resulting four SAM samples (Figure C). This material is available free of charge via the Internet at http://pubs.acs.org.
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