© Copyright 2006 American Chemical Society
MAY 23, 2006 VOLUME 22, NUMBER 11
Letters Phase Separation of a Mixed Self-Assembled Monolayer Prepared via a Stepwise Method Inhee Choi,† Younghun Kim,‡ Sung Koo Kang,† Jeongjin Lee,† and Jongheop Yi*,† School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National UniVersity, San 56-1, Shillim, Kwanak, Seoul 151-742, Korea, and Department of Chemical Engineering, Kwangwoon UniVersity, Seoul 139-701, Korea ReceiVed July 19, 2005. In Final Form: March 26, 2006 Self-assembled monolayers (SAMs), a molecular-level assembly that forms spontaneously, provide a vehicle for investigating specific interactions at interfaces. This is particularly true for mixed SAMs that are composed of organosilanes with different chain lengths and/or chemical functionalities because they offer an adjustable surface for constructing 3D structures containing a variety of moieties. We recently observed that coadsorbed monolayers with different organosilanes on a Si wafer were separated into several tens or hundreds of nanometer domains that were rich in individual components. Several organosilanes, such as octadecyltrichlorosilane (OTS), octadecyltrimethoxysilane (OTMS), (3-mercaptopropyl)trimethoxysilane (MPTMS), and (3-aminopropryl)trimethoxysilane (APTMS), were used for regional separation. In this study, we propose a stepwise deposition method, namely, the deposition of a second siliane on a SAM substrate that creates intentional defects in the first silane. The surface morphologies were adjusted by the deposition sequence and immersion time of the silanes. As a result, a mixed SAM prepared by the proposed method showed effectively functionalized films compared to that prepared by the one-step method.
Introduction Self-assembled monolayers1 with well-ordered structures have been shown to be particularly useful for studies of surface phenomena such as wetting, adhesion,2 nucleation and growth,3 surface-initiated polymerization,4 protein adsorption,5 and DNA assembly.6 The characterization of organic thiolate SAMs on a * To whom all correspondence should be addressed. E-mail:
[email protected]. Phone: +82-2-880-7438. Fax: +82-2-885-6670. † Seoul National University. ‡ Kwangwoon University. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic Press: London, 1991. (2) Sethuraman, A.; Han, M,; Kane, R. S.; Belfort, G. Langmuir 2004, 20, 7779-7788. (3) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 12, 424-429. (4) Jeon, N. L.; Choi, I. S.; Whitesides, G. M.; Kim, N. Y.; Laibinis, P. E.; Harada, Y.; Finnie, K. R.; Girolami, G. S.; Nuzzo, R. G. Appl. Phys. Lett. 1999, 75, 4201-4203. (5) Robert, G.; Chapman, E. O.; Lin, Y.; Whitesides, G. M. Langmuir 2000, 16, 6927-6936.
gold substrate7 is well established and has been reported by many researchers. It is noteworthy, however, that the properties of organosilane SAMs on silicon wafers have not been extensively studied. In addition, the selection of appropriate functional groups for nano- or microstructured devices, such as microelectromechanical systems (MEMS),8 molecular switches,9 and biosensors,10 is strongly dependent on the interfacial properties of the SAMs. Most of these processes are conducted on a silicon wafer, and as a result, the modification of the silicon surface is a key issue in decreasing the dimensions of such devices. Among the (6) Chi, Y. S,; Jung, Y. H.; Choi, I. S.; Kim, Y. G. Langmuir 2005, 21, 46694673. (7) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103-1170. (8) Witvrouw, A.; Tilmans, H. A. C.; Wolf, I. D. Microelectron. Eng. 2004, 76, 245-257. (9) Lahann, J.; Mitragotri, S.; Tran, T.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 199, 371-374. (10) Boozer, C.; Ladd, J.; Chen, S.; Yu, Q.; Homola, J.; Jiang, S. Anal. Chem. 2004, 76, 6967-6972.
10.1021/la0519406 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/20/2006
4886 Langmuir, Vol. 22, No. 11, 2006 Chart 1. Organosilanes Used for Phase Separation via a Mixed SAM
variety of surface modification techniques, mixed SAMs are an effective method for the direct regulation of surface properties. Mixed SAMs composed of alkylsilanes of different chain lengths and functionalities offer an adjustable surface with different chemical functionalities. Therefore, prior to these applications of mixed SAMs, the development of reliable methods for analyzing the physical and chemical properties of phase-separated mixed SAMs was an important goal. In this study, a mixed silane layer was used to modify a silicon surface for regional separation. The mixed silane layer, which contain two types of compounds, are molecules with a nonionic/ nonactivated functional group and molecules with an affinity for other materials. One method for preparing a mixed SAM is to expose a substrate to a mixed solution of two adsorbates in a defined concentration ratio. However, the relationship between the surface concentration and the initial solution concentration ratio is not absolutely predictable because of the preferential adsorption of one of the components. Therefore, adjusting the surface morphology and composition is not an easy task. Frequently, higher affinity among adsorbates results in the formation of a mixed SAM with the two adsorbates that are not randomly distributed. In this case, such mixed SAMs are not regarded as mixtures at the molecular level. Therefore, we propose a stepwise deposition method, namely, the deposition of the second silane on the SAM substrate with intentional defects in the first silane. The direct observation of the morphology and properties of a phase-separated mixed SAM is possible by AFM, an ellipsometer, and a contact angle apparatus. Lateral force microscopy (LFM) was applied to discriminate the chemical functionalities of a binary system of mixed SAMs consisting of methyl (CH3-), mercapto (-SH), and/or amino (-NH2) silane groups. Experimental Section Materials. The organosilanes and anhydrous toluene were purchased from Sigma-Aldrich Chemical Co. and were used as received. Chemical structures of used organosilanes are shown in Chart 1. B-doped p-type Si(100) wafers were obtained from LG Siltron Co. Before using the Si wafer, it was treated with piranha solution (H2SO4/H2O2 ) 7:3 v/v) at 130 °C for 60 min, followed by sequentially rinsing with DI water, acetone, and ethanol, and then it was stored under absolute ethanol. One-Step Preparation of Mixed SAMs. Mixures of 5 mM organosilanes (OTS/MPTMS ) 0:1, 1:2, 1:4, 1:8, 1:12, 1:16, 1:0, 2:1, 4:1, 8:1, 12:1, 16:1) were dissolved in anhydrous toluene. As shown in Figure 1a, one-step mixed monolayers were prepared by immersing the Si substates in the 5 mM mixtures at room temperature (6 and 24 h), followed by sonication in pure toluene, acetone, and ethanol to remove excess molecules. This process was carried out in a sealed vial to minimize contact with moisture in the air. The mixed monolayer deposited substrate was treated by sonication in pure toluene, acetone, and ethanol to remove excess molecules. Stepwise Preparation of Mixed SAMs. OTS/MPTMS solutions of 5 mM were prepared separately in anhydrous toluene. As shown in Figure 1b, in the first deposition step, pretreated Si wafers were immersed in an OTS (or MPTMS) solution for 0.5, 1, 2 and 3 h to produce intentional defects, followed by thorough rinsing with pure toluene. In the next step, the defect-abundant predeposited substrate was immersed in an MPTMS (or OTS) solution for 24 h, followed by sequential sonication in pure toluene, acetone, and ethanol. The
Letters mixed monolayers deposited on the Si wafers were stored in glovebox filled with N2 gas. OTMS/APTMS mixed SAMs were also prepared by the same process. For site-selective deposition on the phaseseparated monolayers, a prepared OTMS/APTMS mixed SAM substrate was immersed in an aqueous colloidal solution of Au prepared by the Frens method11 for 30 min, followed by sonication in pure ethanol to remove the aggregated AuNPs. Characterizations. The thickness of the SAM was obtained as an average value measured at several points using an ellipsometer (L116B, Gaertner). At least five individual points were measured on each sample, with an assumption of a refractive index of 1.462 for the organic film. The results are in good agreement with theoretical predictions and results reported for perfect SAMs.12,13 The static contact angle was determined at room temperature by the sessiledrop method (DSA10, Kruss, Germany). The measured value is given as the average value of the advancing angle and the receding angle. The topographical and frictional properties of the mixed SAMs were measured with the tip deflection of an AFM apparatus (PSIA, XE-150, Korea), using a silicon cantilever with a typical tip curvature radius of ∼10 nm (NSC36 series, MikroMasch, Estonia). The force constant and resonance frequency were 0.45 N/m and 95 kHz, respectively. Simultaneous surface imaging was accomplished by topography and frictional forces between the tip and sample surfaces. Because the frictional forces were directly related to interactions between the functional groups on the tip and sample surfaces, a force/distance curve was also obtained from the AFM apparatus to distinguish between the deposited materials.
Results and Discussion The mixed SAMs in this study were prepared on a hydroxylated Si wafer. The thickness of several mixed SAMs was analyzed by ellipsometry. In the case of the one-step mixed SAMs, a monolayer prepared with one material showed a unique thickness, 0.8 ( 0.2 nm for MPTMS SAMs and 2.7 ( 0.2 nm for OTS SAMs, consistent with previously reported data.12,13 A fluctuation in the measured values, however, was noted for several mixing ratios. As shown in Figure 2a, the fluctuation found in the case of a 24 h immersion was much higher than that for a 6 h immersion. The thickness of the OTS/MPTMS mixed SAM prepared using a one-step method was thicker than that for the single long-chain material, OTS. It should be noted that self-aggregation and coaggregation between the two materials occurred readily. The OTS/MPTMS mixed SAMs prepared by the stepwise method showed small variations in thickness, as shown in Figure 2b, and the thickness corresponded to the thickness of the long-chain material used. Considering the spectral range of the ellipsometer, 250-800 nm, these values are matched with the thickness of coexisting mixed monolayers with a phase separation of several hundreds of nanometers. Results for the water contact angle of individual samples are summarized in Table 1. The contact angles for the mixed SAMs prepared by the one-step and stepwise methods were 104 ( 3° and 92 ( 3°, respectively. When the two components of a monolayer were to act independently, the contact angles would follow Cassie’s law (cos θobs ) q1 cos θ1+ q2 cos θ2), where q1 and q2 are the mole fractions of the two components in the monolayer (q1 + q2 ) 1) and θ1 and θ2 are the contact angles of the monolayers prepared from each individual component.14 The mixed monolayers prepared in this study, however, did not follow this relationship because of the mixture of polar (-SH) and nonpolar (CH3) components present. As a result, the two components of the monolayer acted dependently. From a simple (11) Frens, G. Nature 1973, 241, 20. (12) Hu, M.; Noda, S.; Okubo, T.; Yamaguchi, Y. Appl. Surf. Sci. 2001, 181, 307-316. (13) Wang, M.; Liechti, K. M.; Wang, Q.; White, J. M. Langmuir 2005, 21, 1848-1857.
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Figure 1. Schematic diagram of two methods used in preparing mixed SAM: (a) the one-step deposition method and (b) the stepwise deposition method.
Figure 2. Thickness variation of OTS/MPTMS mixed SAMs: (a) thickness of mixed SAMs prepared using the one-step method; (b) 6 h and (3) 24 h immersion mixed SAMs. (b) Thickness of mixed SAMs prepared using the stepwise method; (b) stepwise OTS/MPTMS mixed SAM, (9) OTS SAM, (3) stepwise MPTMS/OTS mixed SAM, and (]) MPTMS SAM. Table 1. Water Contact Angle Data sample
angle (deg)
hydroxylated surface OTS single-molecule SAM MPTMS single-molecule SAM OTS/MPTMS one-step mixed SAMs OTS/MPTMS stepwise mixed SAMs
38.7 107 ( 1 83 ( 1 104 ( 3 92 ( 3
thermodynamic viewpoint, it is predicted that the observed angle θ on a rough surface with a roughness factor (r) is related to the true angle θtrue on a smooth surface according to cos θ ) r cos θtrue. Consequently, contact angles that are greater than 90° increase because of the roughness of the surface. It should be noted that the former ones have a rough, hydrophobic surface derived from the self- or coaggregation of organosilanes compared to the latter ones. Self- or coaggregation gives rise to an increase in surface tension, which results in an increase in the water contact angles of the surfaces. These results are consistent with the thickness data. As a result, a mixed SAM prepared by the stepwise method contains more effectively functionalized thin films than that prepared by the one-step method. In a stepwise deposition method, surface morphologies are adjusted by the deposition sequence and the immersion time of the silanes. Before depositing the second silane, we collected topography and lateral force images over an area of 5 × 5 µm2. Figure 3 shows AFM images of the surface on which the first silane was deposited. Figure 3a and c showed that the bright region of the distributed domains corresponds to predeposited silane and the background region corresponds to intentional defects derived from the short immersion time, which are bare Si-OH domains to be positioned by the second silane. Figure
3b and d shows lateral images according to the different surface functionalities, which were collected simultaneously with the topographic images. These findings demonstrate that the different contrasts in the predeposited areas are due to silicon tip and surface functionality interactions. In the case of Figure 3d, because of the electron-pair repulsion force between the noncovalent electrons of sulfur atoms and the silicon tip, the SH-terminated surface is darker than the nondeposited surface. However, because of the absence of noncovalent electrons in the methyl groups of OTS, the OTS deposited area is relatively bright. This result was applied to the APTMS SAM with the noncovalent electrons of the nitrogen atoms. This observation plays an important role in the distinction of phase-separated regions of mixed SAMs.15-17 Most functional groups that have an affinity for metals and biomaterials have noncovalent electrons. Therefore, the active sites of a stepwise-deposited mixed SAM can be identified through AFM lateral force images. Figure 4 shows the topography of the second material deposited surfaces on the preproduced defects. The left image was obtained by MPTMS (second silane) deposition on the OTS (first silane) predeposited surface, and the right one corresponds to the inverse deposition process. The first material was treated for a short period to induce intentional defects, and the second material was deposited for the full saturation time. The morphologies showed a positive or negative surface arranged on the monolayer scale (14) Gupta, P.; Ulman, A.; Fanfan, S.; Korniakov, A.; Loos, K. J. Am. Chem. Soc. 2005, 127, 4-5. (15) Chambers, R. C.; Inman, C. E.; Hutchison, J. E. Langmuir 2005, 21, 4615-4621. (16) Klein, H.; Battaglini, N.; Bellini, B.; Dumas, P. Mater. Sci. Eng. C 2002, 19, 279-283. (17) Okabe, Y.; Akiba, U.; Fujihira, M. Appl. Surf. Sci. 2000, 157, 398-404.
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Figure 3. Topography and lateral images of a predeposited monolayer with numerous defects: (a) topography of the OTS deposited surface, (b) lateral image of part a, (c) topography of the MPTMS deposited surface, and (d) lateral image of part c (5 × 5 µm2 scan area).
Figure 4. Topography of stepwise-deposited OTS/MPTMS mixed SAMs: (a) MPTMS dominant surface and (b) OTS dominant surface (1 × 1 µm2 scan area).
according to the deposition sequence and the immersion time used. The properties of these surfaces make them amenable for immobilizing or trapping other materials, such as metal nanoparticles and proteins.18 It was possible to attach target material to a specific region, that is, the target material was selectively and spontaneously bound to an active site of the phase-separated region that was formed without an additional patterning process. As a result, the different properties of the target material-bound region and the nonbound region can be detected in only one scan. As a proof-of-concept test, AuNPs were deposited on the stepwise-deposited OTMS/APTMS mixed SAMs. As shown in Figure 5, their positive and negative surface morphologies were similar to those of the OTS/MPTMS mixed SAMs. These tunable surfaces were used to selectively deposit AuNPs (ca. 10-12 nm) prepared by the Frens method, which has good adhesion for the amine moiety. Figure 5a shows a surface image of OTMS deposited on an APTMS predeposited layer with numerous defects. Because of the insufficient immersion time for the APTMS, the APTMS deposited area appears as an aggregated island shape whereas OTMS is densely deposited around the APTMS islands in a sufficient immersion time, resulting in a (18) Nelson, K. E.; Gamble, L.; Jung, L. S, Boeckl, M. S.; Naeemi, E.; Golledge, S. L.; Sasaki, T.; Castner, D. G.; Campbell, C. T.; Stayton, P. S. Langmuir 2001, 17, 2807-2816.
Figure 5. AuNPs were site-selectively deposited on the phaseseparated OTMS/APTMS mixed SAMs: (a) topography of OTMS/ APTMS mixed SAMs, (b) topography of AuNPs site-selectively immobilized on the phase-separated OTMS/APTMS mixed SAMs, (c) lateral image corresponding to the part b, (d) topography of APTMS/OTMS mixed SAMs, (e) topography of AuNPs siteselectively trapped on the phase-separated APTMS/OTMS mixed SAMs, and (f) lateral image corresponding to part e (5 × 5 µm2 scan area).
depth difference of about 2 nm. Figure 5b is an image obtained from assemblies of AuNPs on a protrudent surface that contains an amine functionality. Before and after AuNP immobilization, their height difference was 10 nm, in good agreement with the size of the AuNPs. Figure 5d is an image of a surface produced by APTMS deposition on the OTMS predeposited layer. The deposition of OTMS was carried out for a sufficient length of time to cover the area almost completely, and APTMS was then positioned on the small area that is unoccupied by OTMS, resulting in a difference in the depth and length of two materials (2 nm). Figure 5e was obtained from assemblies of AuNPs on
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the amine functionality positioned on the negative site. They also show a 10 nm height difference before and after AuNPs assembly, indicating that the AuNPs were trapped by the hollowed surface. Parts c and f of Figure 5 are the corresponding lateral images of parts b and e of Figure 5, respectively. The AuNP immobilized areas are darker than the methyl-terminated surface because of the sulfur atoms around the Au colloidal particles prepared by the Frens method. Site-selectively deposited AuNPs were identified via the use of AFM force/distance curves and SEM EDS (energy-dispersive spectroscopy) analysis (these data are included in the Supporting Information). When the tip and gold-deposited area are in close proximitly, the adhesion force between the tip and surface is very weak compared to that of a nondeposited, methyl-terminated area. Therefore, the areas where the AuNPs are deposited are darker than the methyl-terminated background in lateral images. In an SEM EDS spectrum, a unique peak corresponding to gold was observed. Using these procedures, it is possible to achieve site-selective immobilization of phase-separated mixed SAMs.
Conclusions Mixed SAMs composed of alkylsilanes of different chain lengths and different functionalities offer an adjustable surface for constructing moieties containing different chemical functionalities. In the case of a one-step preparation of a mixed SAM, it was difficult to control the surface composition and morphology
because of the difference in immobilizing speeds with respect to terminal functionality and chain length, whereas when a stepwise method was used, phase separation was more easily controlled because of the ease of preparation of the desired surfaces according to the morphology of the predeposited monolayer. The first method was advantageous for use with two components with similar chemical properties, and the second one was more useful when the molecules differ in their chemical properties. Mixed monolayers obtained using the stepwise method were thinner and had a more effectively functionalized surface, which permits target materials to be immobilized or trapped. These tunable surfaces can be applied to the design of biologically and chemically modified surfaces. Acknowledgment. We are grateful to the Eco-Technopia-21 project of the Ministry of Environment, Korea, for financial support, and this research was conducted through the Engineering Research Institute (ERI) at Seoul National University, Korea. Supporting Information Available: Colloidal Au nanoparticles immobilized on organosilane films with one-component silanes (i.e., MPTMS or APTES or OTS). A comparison of AFM force/distance curve between the Au-immobilized area and the nonimmobilized surface area. SEM EDS (energy-dispersive spectroscopy) spectrum of Au nanoparticles immobilized on the phase-separated area. This material is available free of charge via the Internet at http://pubs.acs.org. LA0519406
