Langmuir 1999, 15, 8577-8579
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Formation of Self-Assembled Monolayers of Alkanethiols on GaAs Surface with in Situ Surface Activation by Ammonium Hydroxide Theo Baum, Shen Ye, and Kohei Uosaki* Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Received August 19, 1999. In Final Form: October 12, 1999 The formation of self-assembled monolayers (SAMs) of alkanethiols on GaAs was studied using attenuated total reflection Fourier transform infrared spectroscopy. SAMs formed from an ethanol solution containing ammonium hydroxide were more ordered and stable than those obtained by the method previously reported in which surfaces were derivatized from neat molten alkanethiols at elevated temperatures. It is suggested that ammonium hydroxide etches the native surface oxide of GaAs during an initial step followed by the chemisorption of alkanethiols on the chemically reactive surface. The effect of ammonium hydroxide concentrations on the SAM coverage was investigated. Well-ordered layers with close packing were formed when the ammonium hydroxide concentration was 3 vol %. SAMs of alkanethiols with different chain lengths showed that the order increases with increasing chain length (from C12H25SH to C18H37SH).
Introduction The formation of self-assembled monolayers (SAMs) of organic molecules on metals has been the focus of numerous studies in recent years1-7 and a variety of applications have been suggested.8-10 The immersion of metals such as gold, silver, and copper in solutions and their functionalized derivatives readily results in the formation of well-ordered monolayers. SAMs formed on semiconductors have become of recent interest due to their possible applications utilizing semiconducting material properties and monolayer functionality.10,11 Potential applications in the fields of lithography, chromatography, biochemistry, tribology, chemical sensors, and microelectronic device fabrication are now under development.12-15 The assembly of organic layers on bare oxide-free semiconducting surfaces proves to be more difficult than that on metal surfaces due to the high tendency of oxide formation.16,17 To form monolayers on semiconducting materials, the chemical inertness has to * To whom correspondence should be addressed: TEL: +81-11-706-3812. Fax: +81-11-706-3440. E-mail: uosaki@ PCL.sci.hokudai.ac.jp. (1) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. J. Am. Chem. Soc. 1983, 105, 4481. (2) Rubinstein, I.; Steinberg, S.; Tor, Y.; Schanzer, A.; Sagiv, J. Nature 1988, 332, 426. (3) Bain, C. D.; Troughton, E. D.; Tao, Y.-T.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (4) Ulman, A. Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (5) Ulman, A. Chem. Rev. 1996, 96, 1533. (6) Zamborini, F. P.; Crooks, R. M. Langmuir 1997, 13, 122-126. (7) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855. (8) Whitesides, G. M.; Gorman, C. B. In The Handbook of Surface Imaging and Visualization; Hubbard, A. T., Ed.; CRC Press: Boca Raton, FL, 1995; p 713. (9) Amato, I. Science 1998, 282, 402. (10) Yates, J. T., Jr. Science 1998, 279, 335. (11) Bergerson, W. F.; Mulder, J. A.; Hsung, R. P.; Zhu, X.-Y. J. Am. Chem. Soc. 1999, 121, 454. (12) Buczkowski, A.; Radzimski, Z.; Rozgonyi, G. A.; Shimura, F. J. Appl. Phys. 1991, 69, 6495. (13) Lewis, N. S. Annu. Rev. Phys. Chem. 1991, 42, 543. (14) Lercel, M. J.; Redinbo, G. F.; Craighead, H. G.; Sheen, C. W.; Allara, D. L. J. Appl. Phys. 1994, 65, 974. (15) Hofmann H.; Mayer U.; Krischanitz A. Langmuir 1995, 11, 1304. (16) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (17) de Villeneuve, C. H.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415.
be overcome by harsh experimental conditions. In recent studies, SAMs on silicon with Si-C bond formation have been reported by several groups.16-20 The formation of SAMs on GaAs has attracted much less attention despite its predominant use as a material for high-frequency data transfer in the area of telecommunications. SAMs have been formed from molten alkanethiols at temperatures between 100 and 200 °C.21,22 Although this method has been successfully used for submicrometer patterning,14,23 the process is restricted to a small group of alkanethiols. By use of a different approach, SAMs are formed on GaAs by cleaving the sample in a thiol solution producing a fresh surface.24 The objective of the present work is to develop a universally applicable procedure to form SAMs with functionalized end groups on GaAs. To anchor the organic molecules to the GaAs surface, alkanethiols are used due to the high affinity of sulfur to GaAs. To overcome the chemical inertness of the GaAs surface, NH4OH, which is known to act as an etchant, is added to the solution to provide a fresh and oxide-free surface. Although the addition of base to a millimolar alkanethiol/ethanol solution for the derivatization of GaAs surfaces was previously suggested by Allara et al.,25 this approach was not further developed. The formation, coverage, and order of the SAMs on GaAs were investigated using IR spectroscopy with attenuated total reflection (ATR) geometry, which provides a high resolution (0.5 cm-1) and signal/ noise ratios due to multiple internal reflection in contrast to single reflection or transmission geometry. (18) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (19) Bansal A.; Li X.; Lauermann I.; Lewis, N. S. J. Am. Chem. Soc. 1996, 118, 7225. (20) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. J. Appl. Phys. 1999, 85, 213. (21) Nakagawa, O. S.; Ashok, S.; Sheen, C. W.; Ma¨rtensson, J.; Allara, D. L. Jpn. J. Appl. Phys. 1991, 30, 3759. (22) Sheen, C. W.; Shi, J.-X.; Ma¨rtensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514. (23) Lercel, M. J.; Rooks, M.; Tiberio, R. C.; Craighead, H. G.; Sheen, C. W.; A. N.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1995, 13, 1139. (24) Ohno, H.; Motomatsu, M.; Mizutani, W.; Tokumoto, H. J. Appl. Phys. 1995, 34, 1381. (25) Mars, C. K.; Allara, D. L. 216th ACS National Meeting, Boston, 1998, poster presentation Coll 158.
10.1021/la991124w CCC: $18.00 © 1999 American Chemical Society Published on Web 11/13/1999
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Langmuir, Vol. 15, No. 25, 1999
Letters Table 1. νCH2as Values (cm-1) of SAMs of Alkanethiols Formed on GaAs and Au n ) 18 GaAs/CnH2n+1SH 2917.6 GaAs/CnH2n+1SH (neat) 2917-2918 Au/CnH2n+1SH 2917-2918 “crystal-like” CH3(CH2)21SH “liquid-like” CH3(CH2)7SH a
Figure 1. ATR-FTIR spectra of GaAs(100) samples derivatized in (a-e) 5 mM ODT/ethanol for 8 h at 50 °C for different concentrations of aqueous ammonia solution and in (f) pure ODT for 5 h at 100 °C.
Experimental Section Nondoped, double-sided polished GaAs(100) wafers were obtained from Furukawa Electric and Hitachi Cable. Alkanethiols C18H37SH (ODT) (>97%), C15H31SH (>95%), and C12H25SH (>95%) were purchased from Tokyo Kasei Organic Chemicals. Ethanol (95%, Uvasol) and 30% aqueous ammonium hydroxide solution were obtained from Merck and Wako, respectively. All chemicals were used without further purification. A detailed description about the ATR-FTIR system was given elsewhere.26 GaAs crystals (23 × 15 0.6 mm) were prepared by polishing the edges to a 45° trapezoidal geometry corresponding to approximately 36 reflections. The sample manufacturing had to be carried out with great care due to the brittle material properties of GaAs. Samples were heated to 250 °C for 1 h, immersed in hot acetone, rinsed with ethanol and H2O, followed by a 60-s etch in concentrated HCl to remove the surface oxide, rinsed with H2O, and dried in a stream of nitrogen. Surface derivatization of GaAs was performed in 5 mM alkanethiol solutions with the addition of 30% aqueous ammonia solution at 50 °C for 8 h after purging with N2. Physisorbed ODT was removed by carefully rinsing with ethanol. Derivatization of the GaAs surfaces in neat ODT, which was purged with N2 prior to the derivatization, was carried out at 100 °C for 5 h followed by a thorough sample rinse with chlorobenzene, ethanol, and water and then drying with N2 as previously described.21 ATR-FTIR spectra were recorded using a BioRad FTS30 spectrometer equipped with a HgCdTe (MCT) detector with a resolution of 0.5 cm-1 and light with p-polarization. The absorbance is normalized with respect to the number of reflections. Reference and sample spectra were recorded before and after the derivatization process, respectively.
Results The formation of SAMs on GaAs, which is a covalent material and originally covered with oxides, proves to be more difficult than that of metals. The immersion of GaAs samples into a 5 mM ODT/ethanol solution for 8 h at elevated temperatures did not lead to the formation of stable organic films, which can be seen from the absence of the characteristic C-H stretching modes in the IR spectrum (Figure 1a). ODT attached to the surface was easily removed by a thorough rinse with ethanol. A variation in the ODT concentration as well as a change in bath temperature did not lead to the chemisorption of alkanethiols on GaAs. To overcome the chemical inertness of the GaAs surface, ammonia solution was added thus providing a reactive and oxide-free surface. Figure 1b shows the IR spectrum of a derivatized GaAs surface from an ODT solution with (26) Ye, S.; Ichihara, T.; Uosaki, K. Appl. Phys. Lett. 1999, 75, 1562.
n ) 15
n )12
2919.8 2921.6 2919a 2923 2918 2919-2922 2918 2924
ref this work 21, 22 30, 31 30 30
n ) 17.
the addition of 0.5 vol % ammonia solution. Three pronounced bands were observed in the C-H stretching region at energies of 2960.1, 2918.6, and 2850.5 cm-1, which have been previously assigned to the asymmetric CH3 (νCH3as), asymmetric CH2 (νCH2as), and symmetric CH2 (νCH2s) stretching vibration modes, respectively.27,28 The C-H bands are indicative of adsorbed alkanethiols since this is the only organic compound involved in the process. It is well-known that the νCH2 band positions are strongly affected by the order of the organic adsorbate, whereas the νCH3 band position is nearly unchanged. The νCH2 bands appear at higher and lower wavenumbers for “liquid-like” isotropic layers and for “crystal-like” anisotropic layers, respectively. The νCH2as bands are more intense and show a larger energy shift with a change in order than the νCH2s bands.27-29 Hence, we will only refer to the νCH2as band in the following discussion. The ammonium hydroxide concentration dependence has been studied in order to obtain ODT SAMs with monolayer coverage. With increasing ammonium hydroxide concentration, a shift to lower wavenumbers is observed (Figure 1c,d). For the addition of 3 vol % ammonia solution, the value of νCH2as is 2917.6 cm-1 indicating a densely packed assembly of rod-like chains with a low degree of gauche defects.28,30,31 A further increase to 10 vol % ammonia solution did not lead to a further shift in νCH2as to lower wavenumbers, and the band intensity did not change (Figure 1e). These results suggest that a maximum monolayer coverage is reached for the addition of 3 vol % ammonia solution. The anchoring group of ODT is covalently bonded to the GaAs surface, and all adsorption sites are occupied. Additional ODT can only physisorb onto the surface without forming covalent bonds between the anchoring group of ODT and the surface. A thorough rinse with ethanol should almost lead to the complete removal of physisorbed ODT. Figure 1f shows an organic SAM formed on GaAs from molten ODT at elevated temperatures which was prepared following the approach of Allara et al.22 The observed νCH2as value of 2918.5 ( 0.5 cm-1 compares to the reported values of 2917-2918 ( 2 cm-1.21,22 Allara et al. formed monolayers of high coverage with thicknesses of 14 ( 2 Å.22 The band maxima of the νCH2as stretching modes of alkanethiol SAMs of different chain lengths are listed in Table 1. A shift in νCH2as to higher wavenumbers with decreasing chain length is observed. This is in agreement with previous observations on Au.32 The stability of the ODT SAMs has been examined by immersion in deaerated CH2Cl2 and ethanol. Figure 2 shows the ATR-FTIR spectra of ODT-derivatized GaAs (27) Snyder, R, G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395. (28) Snyder, R, G.; Strauss, H. L.; Ellinger, C. A. J. Phys. Chem. 1982, 86, 5145. (29) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (30) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey; C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (31) Stole, S. M.; Porter, M. D. Langmuir 1990, 6, 1199. (32) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141.
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Langmuir, Vol. 15, No. 25, 1999 8579
Complementary support for the formation of stable SAMs on GaAs has been obtained by XPS measurements. GaAs samples etched in HCl showed a small amount of residual and regrown oxides. For samples derivatized in alkanethiol solutions containing NH4OH, these bands were of much lower intensity. This clearly indicates that the addition of NH4OH reduces the amount of residual oxide significantly and prevents the regrowth of surface oxides. Furthermore, the SAMs formed on GaAs inhibit the surface oxidation of GaAs during air exposure. Conclusion
Figure 2. Immersion of ODT derivatized GaAs samples (same as in Figure 1d) in CH2Cl2 for (a) 30 min, (b) 1 h, (c) 3 h, and (d) 12 h.
samples immersed in CH2Cl2. The νCH2as band shifts after 1, 3, and 12 h from the initial value of 2917.6 cm-1 to 2917.8, 2918.2, and 2918.6 cm -1, respectively. The shift of the peak position was very small, indicating that an ordered monolayer still remained on the GaAs surface. The intensities of the C-H bands decreased, however, with increasing immersion time in CH2Cl2. The intensity of the νCH2as band decreased to approximately 70% of its original intensity after 12 h of immersion. The loss in C-H band intensity is not necessarily due to the coverage decrease but can be due to the orientation change. To quantify the desorption, one needs to determine the amount of the adsorbed molecules after the immersion by other techniques such X-ray photoelectron spectroscopy (XPS). This experiment is under way. The stability of the SAMs in ethanol was similar to that in CH2Cl2.
Stable, well-ordered SAMs of alkanethiols have been formed on GaAs in ethanol. The addition of ammonia solution provided an in situ method to ensure the chemical activation and the removal of residual oxides from the GaAs surface. The major advantages of the method presented in this work are the high coverage and stability of the SAMs, low amounts of starting material, and mild conditions enabling the formation of a variety of SAMs of functionalized alkanethiols. It is suggested that the in situ surface activation by adding an etchant such as ammonium hydroxide providing oxide-free surfaces is also transferable to other semiconductors. Acknowledgment. This work was supported by Grant-in-Aid for Scientific Research on Priority Area of “Electrochemistry of Ordered Interfaces” (09237101) from the Ministry of Education, Science, Sports and Culture, Japan. T.B. thanks the JSPS and EU for a postdoctoral fellowship. Mr. Kashiwa of Hitachi Cable, Ltd., and Mr. Ushio of Furukawa Electric Co., Ltd., are acknowledged for the donation of the GaAs wafers. LA991124W
