- Email: [email protected]
micro-domain composed of alkyl- and amino-terminated self-assembled monolayer
Applied Surface Science 254 (2008) 7453–7458
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface characterization on binary nano/micro-domain composed of alkyl- and amino-terminated self-assembled monolayer S.H. Lee a, T. Ishizaki b, N. Saito c,*, O. Takai d a
Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan c Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagano 464-8603, Japan d EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 December 2007 Received in revised form 30 May 2008 Accepted 2 June 2008 Available online 7 June 2008
The binary alkyl- and amino-terminated self-assembled monolayers (SAMs) composed of nano/microsized domains was prepared though a self-assembly technique. In addition, the wetting and electrostatic property of the binary SAMs was investigated by the analysis of the static and dynamic water contact angle and zeta-potentials measurement. The binary SAMs were also characterized by atomic force microscope (AFM), Kelvin probe force microscope (KPFM) and X-ray photoelectron spectroscopy (XPS). The domains on the binary SAMs were observed in topographic and surface potential images. The height of domain and the surface potential between octadecyltrichlorosilanes (OTS)-domain and n-(6aminohexl)aminopropyl-trimethoxysilane (AHAPS)-SAM were about 1.1 nm and 30 mV. These differences of height and surface potential correspond to the ones between OTS and AHAPS. In XPS N 1s spectra, we confirmed the formation of binary SAMs by an amino peak observed at 399.15 eV. The dynamic and the static water contact angles indicated that the wetting property of the binary SAMs was depended on the OTS domain size. In addition, static water contact angles were measured under the conditions of different pH water and zeta-potential also indicated that the electrostatic property of the binary SAMs depended on OTS domain size. Thus, these results showed that the wetting and electrostatic property on the binary SAMs could be regulated by controlling the domain size. ß 2008 Published by Elsevier B.V.
Keywords: Binary self-assembled monolayer Wetting and electrostatic property Zeta-potentials Water contact angle
1. Introduction Self-assembled monolayers (SAMs) have attracted attention from the viewpoint of biological surface modification [1–3]. The behavior of bio-materials such as proteins, cells and DNA on SAM has been investigated to obtain the characteristics of flatness and to facilitate the adjustment of surface property. The wetting and electrokinetic properties of a SAM surface are considered as the most important factors. Therefore, the study of wetting and electrokinetic characterization of terminated groups on SAM from the viewpoint of biological interaction is important to apply SAMs to bio-materials [4,5]. A surface consisting of several SAMs is expected to have multiinteractions to biomolecules. Several studies have dealt with the fabrication of surfaces with binary functional groups, e.g., Fang
* Corresponding author. E-mail address: [email protected] (N. Saito). 0169-4332/$ – see front matter ß 2008 Published by Elsevier B.V. doi:10.1016/j.apsusc.2008.06.001
et al. and Finnie et al. In addition, binary organosilane SAMs were fabricated using the vacuum ultraviolet (VUV) lithography technique [6–8]. However, fabrication of SAMs at nanometer scale is difficult to achieve through conventional photolithography process. On the other hand, trichlorosilane and trimethoxysilane molecules of organosilane SAM groups are formed via the covalent bond in a two-dimensional siloxane network (Si–O–Si) [9–13]. Therefore, the domain phase of an organosilane SAM is formed at an early stage of formation by self-assembly under appropriate preparation condition [14–18]. In addition, the domain size depended on the preparation conditions, such as temperature, humidity, and solution age. We used this mechanism to fabricate binary SAMs. This assembling method is simpler than the conventional lithography method. In this study, we investigated if the wetting and electrokinetic characterization of a SAM surface could be controlled by fabricating the nano/micro-domain phase of binary SAMs. We confirmed that nano/micro-domains composed of binary methyl/ amino-SAMs were formed on a silicon oxide substrate using an
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S.H. Lee et al. / Applied Surface Science 254 (2008) 7453–7458
atomic force microscope (AFM), a Kelvin probe force microscope (KPFM) [6–8,19–21] and X-ray photoelectron spectroscopy (XPS). In addition, we investigated the wetting and electrokinetic characterization of OTS SAMs, binary SAMs with amino/methyl terminated groups, and AHAPS SAM by considering the dynamic water contact angles, static water contact angles by water droplets with different pH, and zeta-potential measurement. 2. Experimental procedure In this study, n-type Si (1 1 1) covered with an oxide layer was selected as the substrate and was cut into a suitable size (10 mm 10 mm 0.4 mm). The silicon substrates were cleaned by the UV/ozone cleaning method. The substrates were exposed to UV light (172 nm) from an excimer lamp for 30 min under atmospheric pressure and room temperature. Pollutants were simultaneously removed from the substrate surface and hydroxylated. Octadecyltrichlorosilanes [OTS: CH3(CH2)17Si(Cl)3] and n(6-aminohexl)aminopropyl trimethoxysilane [AHAPS: NH2(CH2)6NH(CH2)3Si(OCH3)3] were used as precursor molecules. Toluene (anhydrous, 99.8%) was used as the solvent. Fig. 1 shows schematic illustrations of experimental process as well as OTS and AHAPS molecules. First, OTS SAM domains were prepared on the substrates by the liquid phase method. To control the sizes of the OTS SAM domain structures, the deposition conditions were changed. Humidity, temperature, and solution age were changed from 20 to 50%, 10 to 20 8C, and 5 min to 1 h, respectively. After OTS domain preparation, the AHAPS SAM was prepared on the silicon oxide uncovered with OTS SAM. The humidity and temperature conditions for the formation of the AHAPS SAM were 50% and 20 8C, respectively. After immersion in the AHAPS SAM solution, all samples were cleaned in toluene, acetone, and ethanol and rinsed in ultrapure water. The domain structures of organosilane SAM were analyzed by AFM (Seiko Instruments Inc., SPA-300HV + SPI-3800N), KPFM (Seiko Instruments Inc., SPA-300HV + SPI-3800N), and XPS (ShimadzuKratos, AXIS). AFM was performed in the tapping mode. The AC bias voltage and frequency applied between the probe and sample in the KPFM measurement were 2 V and 25 kHz, respectively. The scan rate
of KPFM and AFM measurements was 0.3 and 1 Hz, respectively. A gold-coated silicon probe (Seiko Instruments Inc., Micro Cantilever, SI-DF3-A) was used. Force constant and resonance frequency of this probe were 0.17 N/m and 27.53 kHz, respectively. XPS was measured with Mg Ka (hn = 1253.6 eV) radiation operating at 10 mA and 12 kV. The take-off angle and spot size were 858 and 6 mm, respectively. Charge correction was calibrated by observing the Si 2p binding energy. The wetting and electrokinetic characterization of OTS SAM, binary SAM with amino/methyl terminated group, and AHAPS SAM were investigated by considering the dynamic water contact angles, static water contact angles by water droplets with different pH, and zeta-potential measurement. The static water contact angles by water droplets (Kruss, DSA10-Mk2) were measured in the pH range of 1–14 controlled by the addition of HCl or NaOH. The volume of water drop was 10 ml. Dynamic water contact angle measurement was performed using the drop shape analysis system (Kruss, DSA10-Mk2). The volume of water drop was changed from 6 to 10 ml in 5 s. The measurement was performed at every 0.5 s. Zeta-potential was measured by an electrophoretic light scattering spectrophotometer (ELSS; ELS-600, Otsuka Electronics). A solution containing 100 mM NaCl as a supporting electrolyte was used, and its pH was adjusted over the range of 1.5– 13 by adding HCl or NaOH. 3. Results and discussion First, we fabricated the OTS domains of nano/micro-size using the self-assembly mechanism of organosilane molecules. These domain sizes were controlled by changing the preparation conditions, such as temperature, humidity, and solution age. Table 1 shows the experimental conditions of each sample. Fig. 2 shows the topographic images, surface potential images, and the profile of OTS domain structures. In topographic images, white structures and dark background were observed on all samples. The domain heights were 1.9 0.2 nm. These heights corresponded to the length of the alkyl chain of OTS. These results indicated that white structures and dark background correspond to OTS domains and Si substrate, respectively. In surface potential images, dark and bright areas were observed on all samples. In topographic images, the dark and bright areas corresponded to the OTS domain and Si substrate, respectively. These results indicated that the methyl groups of OTS molecules had lower surface energy than the SiO2 substrate and, therefore, have stable bonds. The difference in surface potential between the OTS domain and SiO2 surface was about 300 mV. Next, we fabricated binary alkyl- and amino-terminated SAMs by a self-assembly technique that APAHS SAM was site-selectively deposited on the SiO2 surface of the OTS SAM sample. Fig. 3 shows the topographic images, surface potential images, and profiles of a binary SAM surface. The temperature, humidity, solution age, and immersion time during AHAPS SAM preparation were 20 8C, 50%, 1 min, and 1 h, respectively. In topographic images, white structures and dark background were observed in all samples. The domain height was 0.7 0.1 nm, which corresponded to the difference between the length of the alkyl chain of OTS (2.62 nm) and Table 1 Various conditions for micro/nano-domain formation of OTS SAMs The distribution of OTS
Fig. 1. Schematic illustrations of our experimental process and the chemical structures of OTS and AHAPS molecules.
Temperature (8C) Humidity (%) Immersion time (s) Solution age (min)
21.6%
26.3%
29.3%
46.4%
20 25 1 1
20 50 1 1
10 50 5 1
20 50 1 30
S.H. Lee et al. / Applied Surface Science 254 (2008) 7453–7458
that of AHAPS (1.43 nm). Therefore, white structures and dark background corresponded to OTS domains and AHAPS SAM, respectively. In surface potential images, dark and bright areas corresponded to OTS domains and AHAPS SAM, respectively. Methyl groups showed lower surface potential than amino surface and, therefore, have stable bonds. The difference in surface potential between OTS domains and AHAPS SAM was about 30 mV. Using XPS, we investigated the formation of binary alkyl- and amino-terminated SAMs. Fig. 4(a) and (b) shows XPS N 1s spectra for sample surfaces of the binary SAMs and OTS domains, respectively. An additional peak from –NH2 groups is observed at 399.15 eV in the N 1s spectrum of binary SAMs, but not in the N 1s spectrum of OTS domains. These XPS results indicate the formation of the amino functional groups on the Si substrate and the fabrication of binary SAMs. SPM and XPS results indicated the formation of binary alkyl- and amino-terminated SAMs. We investigated the effect of distribution of the OTS domain on surface characterizations of the binary SAM. The distributions of the OTS domain on the sample surfaces in KFM surface potential images were 21.6, 26.3, 29.3, and 46.4%, respectively. The effect of preparation conditions on the distributions of OTS SAM is observed. Therefore, we could control the distribution of alkyland amino-terminated groups on binary SAMs by changing the preparation conditions. Next, we investigated the wetting property of binary SAMs with different distributions of OTS SAM by considering the static water contact angles. Fig. 5 shows the static water contact angles of various binary SAMs with different distributions of the OTS molecule, the plots of Cassie–Baxter’s equation, and the contact angles when considering polarizability or dipole moment of SAM molecules. From the results, the water contact angles of the pure OTS and AHAPS SAM were obtained as 105 and 628. In Fig. 5, we observed that water contact angles of the binary SAMs increased in proportion to the increase in distribution of OTS. On the other hand, the wetting ability of binary SAMs depended on domain size because the change of OTS distribution is corresponded to that of domain size. Therefore, we could control the wetting ability of binary SAMs by changing OTS domain size.
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Cassie Eq. (1) gives the contact angle u of a liquid on a heterogeneous surface composed of a fraction f1 of type 1 chemical group and f2 of type 2 group, where u1 and u2 are the contact angles on the pure homogenous surface of 1 and 2, respectively. Cos u ¼ f 1 cos u 1 þ f 2 cos u2
ð f 1 þ f 2 ¼ 1Þ
(1)
When polarizability or dipole moment of macromolecules such as amino (NH2) and carboxyl (COOH) was considered, the water contact angle of the two heterogenous surfaces shown in Eq. (2) decreased compared to the plots of the Cassie equation, which only considered the distribution of composed groups [22]. 2
2
ð1 þ Cos uÞ ¼ f 1 ð1 þ cos u1 Þ þ f 2 ð1 þ cos u2 Þ
2
(2)
Water contact angles of the binary SAMs increased in proportion to the increase in distribution of OTS. However, more high water contact angle of the binary SAMs surface prepared in this study than the plots of Cassie equation was observed. The domains are composed of difference length molecules of OTS and AHAPS. The difference let to the effect of nano-roughness in wetting ability. From these reasons, it can be inferred that the domain water contact angle is greater than that obtained by the Cassie equation because the water–air–SAM interface enhances the hydrophobicity of the SAM surface. Fig. 6(a) shows the dynamic water contact angles of binary SAMs, OTS SAMs, and AHAPS SAMs. The measurement of dynamic water contact angle was performed for 300 s and the change in water contact angle in solution was obtained. The bio-property can be modified by changing the wetting property. The water contact angle of all samples was rhythmically measured at every 0.5 s, while the volume of water droplet was changed between 6 and 10 ml. The water contact angle at 6 and 10 ml corresponds to advancing contact angle (ua) and receding contact angle (ur), respectively. Fig. 6(b) shows the decrement of ua and ur for 300 s. The ua and ur values of OTS SAM at the starting point increased by about 2.58 compared to that at the ending point because of the hydrophobic interaction of methyl groups. On the other hand, in the case of AHAPS SAM, these contact angles decreased by about
Fig. 2. (a) Topographic images, (b) surface potential images, and (c) profile of OTS domain structures prepared at various preparation conditions.
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Fig. 3. (a) Topographic images, (b) surface potential images, and (c) profiles of an alkyl- and amino-terminated binary SAM surface.
108 because of hydrophilic interaction of amino groups. The change in ua and ur on binary SAMs were remarkable. The amount of decrement in ua and ur decreased in proportion to the increase of distribution of OTS. This could be attributed to the increase in hydrophobic interaction with OTS surface and decrease in hydrophilic interaction with AHAPS surface. These results of static and dynamic water contact angles indicated that the wetting
Fig. 4. (a) XPS N 1s spectra for sample surfaces of the binary SAMs and (b) that for the OTS domain surface.
property of binary SAMs could be controlled by changing the distribution of OTS. Next, we investigated the electrokinetic property of the binary SAM surface by considering the static water contact angles, water droplets with different pH, and the zeta-potential measurement. Fig. 7 shows the static water contact angles measured for water droplets with different pH. The values of water contact angle for the OTS SAM did not change with pH. The wetting property of the OTS SAM surface remains constant irrespective of the change in pH, resulting in the formation of stable bonds between methylterminated groups. However, the water contact angles of the AHAPS SAM decreased when pH was 4 because NH2 group was converted to NH3+. In addition, the water contact angle of the
Fig. 5. Static water contact angles of various binary SAMs with different distributions of the OTS molecule, plots of Cassie–Baxter’s equation, and the contact angles when considering polarizability or dipole moment of macromolecule.
S.H. Lee et al. / Applied Surface Science 254 (2008) 7453–7458
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Fig. 8. Zeta-potential of each sample against pH.
Fig. 6. (a) Dynamic water contact angle of binary SAMs, OTS SAM, and AHAPS SAM and (b) the decrement in advancing contact angle (ua) and receding contact angle (ur) in 300 s.
AHAPS SAM decreased when pH was 10. In the case of binary SAM, the water contact angles were obtained between the values of OTS and AHAPS. In addition, I observed that the water contact angle increased with an increase in OTS distribution.
Fig. 8 shows the zeta-potential of each sample against pH. Isoelectric point (iep) of OTS and AHAPS surfaces was observed at pH 10.3 and 7.2. However, the wetting property of OTS remained constant for any pH. In zeta-potential measurement, we measured the potential of slip plane. In the case of OTS, the conversion of zeta-potential depended on the increase in OH ions in alkali pH. The iep of each sample increased with an increase in OTS distribution. These results of static water contact angles obtained using water droplets with different pH and the zeta-potential measurement indicated that electrokinetic characterization on the binary SAM surface depended on OTS distribution. Therefore, we considered that the electrokinetic characterization of binary SAM could be controlled by fabrication of micro/nano-domains. 4. Conclusion We successfully fabricated binary nano/micro-sized domains composed of methyl/amino terminated SAMs though the selfassembly mechanism. We investigated the surface characterization of binary SAMs by analyzing the dynamic and static water contact angles measured by water droplets with different pH and zeta-potential measurements. Dynamic and the static water contact angles measured by water droplets with different pH indicated that wetting ability of the SAM surface depended on domain size. In addition, zeta-potential results indicated that the electrokinetic property of the SAM surface also depended on domain size. These results show that the wetting and electrokinetic properties could be controlled by the fabrication of micro/ nano-domains on binary SAM. References
Fig. 7. Static water contact angles measured by water droplets with different pH.
[1] W.C.W. Chan, S.M. Nie, Science 281 (1998) 2016. [2] H. Mattoussi, J.M. Mauro, E.R. Goldman, G.P. Anderson, V.C. Sundar, F.V. Mikulec, M.G. Bawendi, J. Am. Chem. Soc. 122 (2000) 12142. [3] P. Chirakul, V.H. Perez-Luna, H. Owen, G.P. Lopez, Langmuir 18 (2002) 4234. [4] M. Sastry, V. Patil, K.S. Mayya, J. Phys. Chem. B 101 (1997) 1164. [5] J. Li, K.S. Liang, G. Scoles, A. Ulman, Langmuir 11 (1995) 4418. [6] K. Hayashi, N. Saito, H. Sugimura, O. Takai, N. Nagagiro, Langmuir 18 (2002) 7469. [7] L. Hong, H. Sugimura, T. Furukawa, O. Takai, Langmuir 19 (2003) 1966. [8] N. Saito, Y. Wu, K. Hayashi, H. Sugimura, O. Takai, J. Phys. Chem. B 107 (2003) 664. [9] M. Pope, H.P. Kallmann, P. Magnante, J. Chem. Phys. 38 (1963) 2042. [10] F. Ebisawa, T. Kurosawa, S. Nara, J. Appl. Phys. 54 (1983) 3255. [11] J. Sagiv, J. Am. Chem. Soc. 102 (1980) 92.
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[18] T. Koga, M. Morita, H. Ishida, H. Yakabe, S. Sasaki, O. Sakata, H. Otsuka, A. Takahara, Langmuir 21 (2005) 905. [19] N. Saito, N. Maeda, H. Sugimura, O. Takai, Langmuir 20 (2004) 5182. [20] N. Saito, S.-H. Lee, I. Takahiro, J. Hieda, H. Sugimura, O. Takai, J. Phys. Chem. B 109 (2005) 11602. [21] H. Sugimura, N. Saito, S.-H. Lee, O. Takai, J. Vac. Sci. Technol. B 22 (2004) 44. [22] J.N. Israelachvili, M.L. Gee, Langmuir 5 (1989) 288.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface characterization on binary nano/micro-domain composed of alkyl- and amino-terminated self-assembled monolayer S.H. Lee a, T. Ishizaki b, N. Saito c,*, O. Takai d a
Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan c Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagano 464-8603, Japan d EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 December 2007 Received in revised form 30 May 2008 Accepted 2 June 2008 Available online 7 June 2008
The binary alkyl- and amino-terminated self-assembled monolayers (SAMs) composed of nano/microsized domains was prepared though a self-assembly technique. In addition, the wetting and electrostatic property of the binary SAMs was investigated by the analysis of the static and dynamic water contact angle and zeta-potentials measurement. The binary SAMs were also characterized by atomic force microscope (AFM), Kelvin probe force microscope (KPFM) and X-ray photoelectron spectroscopy (XPS). The domains on the binary SAMs were observed in topographic and surface potential images. The height of domain and the surface potential between octadecyltrichlorosilanes (OTS)-domain and n-(6aminohexl)aminopropyl-trimethoxysilane (AHAPS)-SAM were about 1.1 nm and 30 mV. These differences of height and surface potential correspond to the ones between OTS and AHAPS. In XPS N 1s spectra, we confirmed the formation of binary SAMs by an amino peak observed at 399.15 eV. The dynamic and the static water contact angles indicated that the wetting property of the binary SAMs was depended on the OTS domain size. In addition, static water contact angles were measured under the conditions of different pH water and zeta-potential also indicated that the electrostatic property of the binary SAMs depended on OTS domain size. Thus, these results showed that the wetting and electrostatic property on the binary SAMs could be regulated by controlling the domain size. ß 2008 Published by Elsevier B.V.
Keywords: Binary self-assembled monolayer Wetting and electrostatic property Zeta-potentials Water contact angle
1. Introduction Self-assembled monolayers (SAMs) have attracted attention from the viewpoint of biological surface modification [1–3]. The behavior of bio-materials such as proteins, cells and DNA on SAM has been investigated to obtain the characteristics of flatness and to facilitate the adjustment of surface property. The wetting and electrokinetic properties of a SAM surface are considered as the most important factors. Therefore, the study of wetting and electrokinetic characterization of terminated groups on SAM from the viewpoint of biological interaction is important to apply SAMs to bio-materials [4,5]. A surface consisting of several SAMs is expected to have multiinteractions to biomolecules. Several studies have dealt with the fabrication of surfaces with binary functional groups, e.g., Fang
* Corresponding author. E-mail address: [email protected] (N. Saito). 0169-4332/$ – see front matter ß 2008 Published by Elsevier B.V. doi:10.1016/j.apsusc.2008.06.001
et al. and Finnie et al. In addition, binary organosilane SAMs were fabricated using the vacuum ultraviolet (VUV) lithography technique [6–8]. However, fabrication of SAMs at nanometer scale is difficult to achieve through conventional photolithography process. On the other hand, trichlorosilane and trimethoxysilane molecules of organosilane SAM groups are formed via the covalent bond in a two-dimensional siloxane network (Si–O–Si) [9–13]. Therefore, the domain phase of an organosilane SAM is formed at an early stage of formation by self-assembly under appropriate preparation condition [14–18]. In addition, the domain size depended on the preparation conditions, such as temperature, humidity, and solution age. We used this mechanism to fabricate binary SAMs. This assembling method is simpler than the conventional lithography method. In this study, we investigated if the wetting and electrokinetic characterization of a SAM surface could be controlled by fabricating the nano/micro-domain phase of binary SAMs. We confirmed that nano/micro-domains composed of binary methyl/ amino-SAMs were formed on a silicon oxide substrate using an
7454
S.H. Lee et al. / Applied Surface Science 254 (2008) 7453–7458
atomic force microscope (AFM), a Kelvin probe force microscope (KPFM) [6–8,19–21] and X-ray photoelectron spectroscopy (XPS). In addition, we investigated the wetting and electrokinetic characterization of OTS SAMs, binary SAMs with amino/methyl terminated groups, and AHAPS SAM by considering the dynamic water contact angles, static water contact angles by water droplets with different pH, and zeta-potential measurement. 2. Experimental procedure In this study, n-type Si (1 1 1) covered with an oxide layer was selected as the substrate and was cut into a suitable size (10 mm 10 mm 0.4 mm). The silicon substrates were cleaned by the UV/ozone cleaning method. The substrates were exposed to UV light (172 nm) from an excimer lamp for 30 min under atmospheric pressure and room temperature. Pollutants were simultaneously removed from the substrate surface and hydroxylated. Octadecyltrichlorosilanes [OTS: CH3(CH2)17Si(Cl)3] and n(6-aminohexl)aminopropyl trimethoxysilane [AHAPS: NH2(CH2)6NH(CH2)3Si(OCH3)3] were used as precursor molecules. Toluene (anhydrous, 99.8%) was used as the solvent. Fig. 1 shows schematic illustrations of experimental process as well as OTS and AHAPS molecules. First, OTS SAM domains were prepared on the substrates by the liquid phase method. To control the sizes of the OTS SAM domain structures, the deposition conditions were changed. Humidity, temperature, and solution age were changed from 20 to 50%, 10 to 20 8C, and 5 min to 1 h, respectively. After OTS domain preparation, the AHAPS SAM was prepared on the silicon oxide uncovered with OTS SAM. The humidity and temperature conditions for the formation of the AHAPS SAM were 50% and 20 8C, respectively. After immersion in the AHAPS SAM solution, all samples were cleaned in toluene, acetone, and ethanol and rinsed in ultrapure water. The domain structures of organosilane SAM were analyzed by AFM (Seiko Instruments Inc., SPA-300HV + SPI-3800N), KPFM (Seiko Instruments Inc., SPA-300HV + SPI-3800N), and XPS (ShimadzuKratos, AXIS). AFM was performed in the tapping mode. The AC bias voltage and frequency applied between the probe and sample in the KPFM measurement were 2 V and 25 kHz, respectively. The scan rate
of KPFM and AFM measurements was 0.3 and 1 Hz, respectively. A gold-coated silicon probe (Seiko Instruments Inc., Micro Cantilever, SI-DF3-A) was used. Force constant and resonance frequency of this probe were 0.17 N/m and 27.53 kHz, respectively. XPS was measured with Mg Ka (hn = 1253.6 eV) radiation operating at 10 mA and 12 kV. The take-off angle and spot size were 858 and 6 mm, respectively. Charge correction was calibrated by observing the Si 2p binding energy. The wetting and electrokinetic characterization of OTS SAM, binary SAM with amino/methyl terminated group, and AHAPS SAM were investigated by considering the dynamic water contact angles, static water contact angles by water droplets with different pH, and zeta-potential measurement. The static water contact angles by water droplets (Kruss, DSA10-Mk2) were measured in the pH range of 1–14 controlled by the addition of HCl or NaOH. The volume of water drop was 10 ml. Dynamic water contact angle measurement was performed using the drop shape analysis system (Kruss, DSA10-Mk2). The volume of water drop was changed from 6 to 10 ml in 5 s. The measurement was performed at every 0.5 s. Zeta-potential was measured by an electrophoretic light scattering spectrophotometer (ELSS; ELS-600, Otsuka Electronics). A solution containing 100 mM NaCl as a supporting electrolyte was used, and its pH was adjusted over the range of 1.5– 13 by adding HCl or NaOH. 3. Results and discussion First, we fabricated the OTS domains of nano/micro-size using the self-assembly mechanism of organosilane molecules. These domain sizes were controlled by changing the preparation conditions, such as temperature, humidity, and solution age. Table 1 shows the experimental conditions of each sample. Fig. 2 shows the topographic images, surface potential images, and the profile of OTS domain structures. In topographic images, white structures and dark background were observed on all samples. The domain heights were 1.9 0.2 nm. These heights corresponded to the length of the alkyl chain of OTS. These results indicated that white structures and dark background correspond to OTS domains and Si substrate, respectively. In surface potential images, dark and bright areas were observed on all samples. In topographic images, the dark and bright areas corresponded to the OTS domain and Si substrate, respectively. These results indicated that the methyl groups of OTS molecules had lower surface energy than the SiO2 substrate and, therefore, have stable bonds. The difference in surface potential between the OTS domain and SiO2 surface was about 300 mV. Next, we fabricated binary alkyl- and amino-terminated SAMs by a self-assembly technique that APAHS SAM was site-selectively deposited on the SiO2 surface of the OTS SAM sample. Fig. 3 shows the topographic images, surface potential images, and profiles of a binary SAM surface. The temperature, humidity, solution age, and immersion time during AHAPS SAM preparation were 20 8C, 50%, 1 min, and 1 h, respectively. In topographic images, white structures and dark background were observed in all samples. The domain height was 0.7 0.1 nm, which corresponded to the difference between the length of the alkyl chain of OTS (2.62 nm) and Table 1 Various conditions for micro/nano-domain formation of OTS SAMs The distribution of OTS
Fig. 1. Schematic illustrations of our experimental process and the chemical structures of OTS and AHAPS molecules.
Temperature (8C) Humidity (%) Immersion time (s) Solution age (min)
21.6%
26.3%
29.3%
46.4%
20 25 1 1
20 50 1 1
10 50 5 1
20 50 1 30
S.H. Lee et al. / Applied Surface Science 254 (2008) 7453–7458
that of AHAPS (1.43 nm). Therefore, white structures and dark background corresponded to OTS domains and AHAPS SAM, respectively. In surface potential images, dark and bright areas corresponded to OTS domains and AHAPS SAM, respectively. Methyl groups showed lower surface potential than amino surface and, therefore, have stable bonds. The difference in surface potential between OTS domains and AHAPS SAM was about 30 mV. Using XPS, we investigated the formation of binary alkyl- and amino-terminated SAMs. Fig. 4(a) and (b) shows XPS N 1s spectra for sample surfaces of the binary SAMs and OTS domains, respectively. An additional peak from –NH2 groups is observed at 399.15 eV in the N 1s spectrum of binary SAMs, but not in the N 1s spectrum of OTS domains. These XPS results indicate the formation of the amino functional groups on the Si substrate and the fabrication of binary SAMs. SPM and XPS results indicated the formation of binary alkyl- and amino-terminated SAMs. We investigated the effect of distribution of the OTS domain on surface characterizations of the binary SAM. The distributions of the OTS domain on the sample surfaces in KFM surface potential images were 21.6, 26.3, 29.3, and 46.4%, respectively. The effect of preparation conditions on the distributions of OTS SAM is observed. Therefore, we could control the distribution of alkyland amino-terminated groups on binary SAMs by changing the preparation conditions. Next, we investigated the wetting property of binary SAMs with different distributions of OTS SAM by considering the static water contact angles. Fig. 5 shows the static water contact angles of various binary SAMs with different distributions of the OTS molecule, the plots of Cassie–Baxter’s equation, and the contact angles when considering polarizability or dipole moment of SAM molecules. From the results, the water contact angles of the pure OTS and AHAPS SAM were obtained as 105 and 628. In Fig. 5, we observed that water contact angles of the binary SAMs increased in proportion to the increase in distribution of OTS. On the other hand, the wetting ability of binary SAMs depended on domain size because the change of OTS distribution is corresponded to that of domain size. Therefore, we could control the wetting ability of binary SAMs by changing OTS domain size.
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Cassie Eq. (1) gives the contact angle u of a liquid on a heterogeneous surface composed of a fraction f1 of type 1 chemical group and f2 of type 2 group, where u1 and u2 are the contact angles on the pure homogenous surface of 1 and 2, respectively. Cos u ¼ f 1 cos u 1 þ f 2 cos u2
ð f 1 þ f 2 ¼ 1Þ
(1)
When polarizability or dipole moment of macromolecules such as amino (NH2) and carboxyl (COOH) was considered, the water contact angle of the two heterogenous surfaces shown in Eq. (2) decreased compared to the plots of the Cassie equation, which only considered the distribution of composed groups [22]. 2
2
ð1 þ Cos uÞ ¼ f 1 ð1 þ cos u1 Þ þ f 2 ð1 þ cos u2 Þ
2
(2)
Water contact angles of the binary SAMs increased in proportion to the increase in distribution of OTS. However, more high water contact angle of the binary SAMs surface prepared in this study than the plots of Cassie equation was observed. The domains are composed of difference length molecules of OTS and AHAPS. The difference let to the effect of nano-roughness in wetting ability. From these reasons, it can be inferred that the domain water contact angle is greater than that obtained by the Cassie equation because the water–air–SAM interface enhances the hydrophobicity of the SAM surface. Fig. 6(a) shows the dynamic water contact angles of binary SAMs, OTS SAMs, and AHAPS SAMs. The measurement of dynamic water contact angle was performed for 300 s and the change in water contact angle in solution was obtained. The bio-property can be modified by changing the wetting property. The water contact angle of all samples was rhythmically measured at every 0.5 s, while the volume of water droplet was changed between 6 and 10 ml. The water contact angle at 6 and 10 ml corresponds to advancing contact angle (ua) and receding contact angle (ur), respectively. Fig. 6(b) shows the decrement of ua and ur for 300 s. The ua and ur values of OTS SAM at the starting point increased by about 2.58 compared to that at the ending point because of the hydrophobic interaction of methyl groups. On the other hand, in the case of AHAPS SAM, these contact angles decreased by about
Fig. 2. (a) Topographic images, (b) surface potential images, and (c) profile of OTS domain structures prepared at various preparation conditions.
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Fig. 3. (a) Topographic images, (b) surface potential images, and (c) profiles of an alkyl- and amino-terminated binary SAM surface.
108 because of hydrophilic interaction of amino groups. The change in ua and ur on binary SAMs were remarkable. The amount of decrement in ua and ur decreased in proportion to the increase of distribution of OTS. This could be attributed to the increase in hydrophobic interaction with OTS surface and decrease in hydrophilic interaction with AHAPS surface. These results of static and dynamic water contact angles indicated that the wetting
Fig. 4. (a) XPS N 1s spectra for sample surfaces of the binary SAMs and (b) that for the OTS domain surface.
property of binary SAMs could be controlled by changing the distribution of OTS. Next, we investigated the electrokinetic property of the binary SAM surface by considering the static water contact angles, water droplets with different pH, and the zeta-potential measurement. Fig. 7 shows the static water contact angles measured for water droplets with different pH. The values of water contact angle for the OTS SAM did not change with pH. The wetting property of the OTS SAM surface remains constant irrespective of the change in pH, resulting in the formation of stable bonds between methylterminated groups. However, the water contact angles of the AHAPS SAM decreased when pH was 4 because NH2 group was converted to NH3+. In addition, the water contact angle of the
Fig. 5. Static water contact angles of various binary SAMs with different distributions of the OTS molecule, plots of Cassie–Baxter’s equation, and the contact angles when considering polarizability or dipole moment of macromolecule.
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Fig. 8. Zeta-potential of each sample against pH.
Fig. 6. (a) Dynamic water contact angle of binary SAMs, OTS SAM, and AHAPS SAM and (b) the decrement in advancing contact angle (ua) and receding contact angle (ur) in 300 s.
AHAPS SAM decreased when pH was 10. In the case of binary SAM, the water contact angles were obtained between the values of OTS and AHAPS. In addition, I observed that the water contact angle increased with an increase in OTS distribution.
Fig. 8 shows the zeta-potential of each sample against pH. Isoelectric point (iep) of OTS and AHAPS surfaces was observed at pH 10.3 and 7.2. However, the wetting property of OTS remained constant for any pH. In zeta-potential measurement, we measured the potential of slip plane. In the case of OTS, the conversion of zeta-potential depended on the increase in OH ions in alkali pH. The iep of each sample increased with an increase in OTS distribution. These results of static water contact angles obtained using water droplets with different pH and the zeta-potential measurement indicated that electrokinetic characterization on the binary SAM surface depended on OTS distribution. Therefore, we considered that the electrokinetic characterization of binary SAM could be controlled by fabrication of micro/nano-domains. 4. Conclusion We successfully fabricated binary nano/micro-sized domains composed of methyl/amino terminated SAMs though the selfassembly mechanism. We investigated the surface characterization of binary SAMs by analyzing the dynamic and static water contact angles measured by water droplets with different pH and zeta-potential measurements. Dynamic and the static water contact angles measured by water droplets with different pH indicated that wetting ability of the SAM surface depended on domain size. In addition, zeta-potential results indicated that the electrokinetic property of the SAM surface also depended on domain size. These results show that the wetting and electrokinetic properties could be controlled by the fabrication of micro/ nano-domains on binary SAM. References
Fig. 7. Static water contact angles measured by water droplets with different pH.
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