Patterning of Organic Monolayers on Silver via Surface-Induced

Patterning self-assembled monolayers. Rachel K Smith , Penelope A Lewis , Paul S Weiss. Progress in Surface Science 2004 75 (1-2), 1-68 ...
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Patterning of Organic Monolayers on Silver via Surface-Induced Photoreaction Sang Woo Han, Inhyung Lee, and Kwan Kim* Laboratory of Intelligent Interface, School of Chemistry and Molecular Engineering and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, Korea Received October 17, 2001 4-Nitrobenzenethiol (4-NBT) is readily converted on silver to 4-aminobenzenethiol by irradiation with a visible laser. This is clearly evidenced from the surface-enhanced Raman scattering and the X-ray photoelectron spectroscopy measurements as well as from a coupling reaction to form amide bonds. The source of hydrogen atoms in the surface-induced photoreduction is presumed to be water or solvent molecules trapped inside the 4-NBT monolayer. The present surface-induced photoreaction allows us to readily prepare patterned binary monolayers on Ag that will show different chemical reactivities. Using the binary monolayers as a lithographic template, we can conduct site-specific chemical reactions. We also demonstrate that a typical biomineral, that is, calcite, can be grown selectively on either the nitro group terminated regions or the amine group terminated regions by adjusting the crystal growth conditions. Since the amine group can be derivatized further to give, for instance, a thiol terminus, the present surface modification method must be as beneficial as the hitherto known methods specifically for the construction of twodimensional heterostructures consisting of different kinds of nanoparticles.

Introduction Nano- to mesoscale structures to be employed in laboratory studies and commercial applications may be realized by controlling chemical functionalities on a nanoscale. Site-selective alternations of surface chemistry provide a means of engineering surfaces that can be used in sensitive optoelectronic devices, sensors, and devices that mimic biological functions. Patterning of the selfassembled monolayers (SAMs) is an excellent strategy for preparing such templates that possess variable surface chemical properties.1-4 Direct chemical synthesis of nanostructures on a patterned solid template that is capable of defining the position and lateral dimensions of the objects to be grown offers a suitable route for the generation, precise positioning, spatial fixation, and lateral interaction of various types of nano-objects of interest.5 Modification of organic monolayers is usually carried out either by utilizing the conventional optical methods using UV light as an excitation source1,6 and other lithographic schemes based on state-of-the-art technologies3,4,7 or by destructive atomic-beam8 and proximalprobe9 lithographic processes relying on the local degra* To whom all correspondence should be addressed. Tel: +822-8806651. Fax: +82-2-8743704. E-mail: [email protected]. (1) Collins, R. J.; Bae, I. T.; Scherson, D. A.; Sukenik, C. N. Langmuir 1996, 12, 5509. (2) (a) Maoz, R.; Yam, R.; Berkovic, G.; Sagiv, J. In Thin Films; Ulman, A., Ed.; Academic Press: San Diego, CA, 1995; Vol. 20, p 41. (b) Duschl, C.; Liley, M.; Corradin, G.; Vogel, H. Biophys. J. 1994, 67, 1229. (3) Weiss, J.; Himmel, H.-J.; Fischer, R. A.; Wo¨ll, C. Chem. Vap. Deposition 1998, 4, 17. (4) Rieke, P.; Tarasevich, B. J.; Wood, L. L.; Engelhard, M. H.; Baer, D. R.; Fryxell, G. E.; John, C. M.; Laken, D. A.; Jaehnig, M. C. Langmuir 1994, 10, 619. (5) (a) Ahmed, H. J. Vac. Sci. Technol., B 1997, 15, 2101. (b) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (6) (a) Dressick, W. J.; Calvert, J. M. Jpn. J. Appl. Phys. 1993, 32, 5829. (b) Behm, J. M.; Lykke, K. R.; Pellin, M. J.; Hemminger, J. C. Langmuir 1996, 12, 2121. (c) Vossmeyer, T.; DeIonno, E.; Heath, J. R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1080. (7) (a) Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D. L. Appl. Phys. Lett. 1996, 68, 1504. (b) Delamarche, E.; Schmid, H.; Michel, B.; Biebuyck, H. Adv. Mater. 1997, 9, 741. (c) Gupta, V. K.; Abbott, N. L. Science 1997, 276, 1533. (8) Thywissen, J. H.; Johnson, K. S.; Younkin, R.; Dekker, N. H.; Berggren, K. K.; Chu, A. P.; Prentiss, M.; Lee, S. A. J. Vac. Sci. Technol., B 1997, 15, 2093.

dation of portions of a monolayer employed as a passive ultrathin resist. Recently, several nondestructive processes have been reported. For instance, chemical reactions could be successfully induced for the terminal groups of organosilane by a catalytically active platinum- or palladium-coated atomic force microscope (AFM) tip.10,11 Sagiv and co-workers have also reported the spatially defined electrooxidation of terminal groups of alkylsilane SAMs on Si surfaces.12 Lines with a width of only a few nanometers could be drawn with a conducting AFM tip. On the other hand, selective conversion of a terminal group of organothiol monolayers on gold with low-energy electron beams was also reported in the literature.13 Wollman et al.14 reported further the photochemical patterning of SAMs in a nondestructive way. Photosensitive aryl azide and manganese moieties that undergo photoreaction upon irradiation of UV light were used in the patterning process. We have recently demonstrated that patterned binary monolayers can also be prepared on silver by inducing a surface-induced photoreaction for a SAM of benzyl phenyl sulfide (BPS) on Ag.15 Irradiation of an argon ion laser (514.5 nm) onto a selected region of BPS SAMs on silver led to the formation of benzenethiolate monolayers. The unilluminated sulfides were thereafter replaced with thiols or carboxylic acids by a self-assembly process. The method could produce very robust binary monolayers, but there was one inherent difficulty in that the region of benzenethiolate layers had nanosized vacancies owing to the removal of the decomposition product, that is, benzyl radical. A new system thus needs to be developed that will preserve the overall structural integrity of the primary monolayer even after the surface-induced photoreaction. In our search for alternative systems, we have found (9) Liu, G.-Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457. (10) Mu¨ller, W. T.; Klein, D. L.; Lee, T.; Klarke, J.; McEuen, P. L.; Schultz, P. G. Science 1995, 268, 272. (11) Blackledge, C.; Engebretson, D. A.; McDonald, J. D. Langmuir 2000, 16, 8317. (12) Maoz, R.; Cohen, S. R.; Sagiv, J. Adv. Mater. 1999, 11, 55. (13) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Adv. Mater. 2000, 12, 805. (14) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395. (15) Lee, I.; Han, S. W.; Kim, C. H.; Kim, T. G.; Joo, S. W.; Jang, D.-J.; Kim, K. Langmuir 2000, 16, 9963.

10.1021/la0115684 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/02/2002

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that nitro groups on aromatic SAMs on silver are selectively converted to amino groups under irradiation with visible light. Using this photoreaction, we can prepare patterned binary monolayers on silver with selectable surface chemical properties. Site-specific chemical reactions and patterned crystal nucleation have been performed with these monolayers. This novel surface group modification method will surely open up new possibilities for chemical lithography that allow the inscription of chemical information on the surface of a functionalized organic monolayer. Experimental Section 4-Nitrobenzenethiol (4-NBT), 4-aminobenzenethiol (4-ABT), and 4-cyanobenzoic acid (4-CBA) were purchased from Aldrich. 1-Ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC) was obtained from Advanced ChemTech. These compounds were used as received. Other chemicals, unless specified, were reagent grade, and triply distilled water (resistivity greater than 18.0 MΩ cm) was used when preparing aqueous solutions. The surface-enhanced Raman scattering (SERS) active silver surface was prepared by immersing a Ag foil (Aldrich, 0.05 mm thick) in diluted (1:1) HNO3.15 SAMs of 4-NBT or 4-ABT were prepared by immersion of the silver substrate in 1 mM ethanolic solution of adsorbate for about 3 h. SERS spectra were obtained using a Renishaw Raman system 2000 spectrometer equipped with an integral microscope (Olympus BH2-UMA). The 514.5 nm radiation from an air-cooled, 20 mW Ar+ laser (Spectra Physics model 163-C4210) was used as the excitation source. When required, samples were spun at 3000 rpm by a homemade Raman spinning cell.16 When Raman images were recorded, the sample was placed below the object lens on an XY-stage (Newport M-462) equipped with a dc motor actuator (Newport 850F), which was controlled by a motion controller (Newport PMC200-P2).15 The sample surface was sequentially scanned in 2 µm steps. Integrated Raman intensities at each point in the 2D image were obtained by integrating the intensities over a 10 cm-1 range on either side of the wavenumber of interest. The SERS image presented here was background corrected by subtracting an image obtained from the spectroscopically silent region. The electrochemical reduction of 4-NBT on a silver surface was carried out in a three-electrode cell using a CH Instrument model 600A potentiostat; the potentiostat employs CHI 600A Electrochemical Analyzer software (version 2.03) running on an IBM-compatible PC. A silver substrate onto which 4-NBT was self-assembled served as a working electrode. The reference electrode was a saturated calomel electrode (SCE), and a platinum spiral wire was used as the counter electrode. The electrolyte was NaClO4, and all experiments were carried out at room temperature. The electrolyte solution was deaerated with highpurity N2 gas before initiating any electrochemical measurement. The modification reaction with 4-CBA was performed by overnight incubation of the laser-irradiated sample in 0.02 M 4-CBA solution in N,N-dimethylformamide (DMF) containing 0.02 M EDC as a coupling reagent.17 The calcite crystallization was conducted by supporting the laser-irradiated sample in a 10 mM CaCl2 aqueous solution in a closed desiccator with vials of solid (NH4)2CO3 at the bottom.18-20 All experiments were carried out at room temperature for 30 min. Crystallization of calcite results from the diffusion of carbon dioxide from (NH4)2CO3 into the CaCl2 solution. The use of (NH4)2CO3 rather than CO2 allowed us to maintain neutral pH in the crystallization solution.19 After (16) (a) Joo, T. H.; Yim, Y. H.; Kim, K.; Kim, M. S. J. Phys. Chem. 1989, 93, 1422. (b) Yim, Y. H.; Kim, K.; Kim, M. S. J. Phys. Chem. 1990, 94, 2552. (17) (a) Katz, E.; Itzhak, N.; Willner, I. Langmuir 1993, 9, 1392. (b) Checkik, V.; Crooks, R. M.; Stirling, C. J. M. Adv. Mater. 2000, 12, 1161. (18) (a) Addadi, L.; Moradian, J.; Shay, E.; Maroudas, N. G.; Weiner, S. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 2732. (b) Berman, A.; Addadi, L.; Weiner, S. Nature 1988, 331, 546. (c) Albeck, S.; Aizenberg, J.; Addadi, L.; Weiner, S. J. Am. Chem. Soc. 1993, 115, 11691. (19) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500. (20) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1999, 398, 495.

Figure 1. SERS spectra of 4-NBT on silver taken (a) under spinning at 3000 rpm and (b) under static conditions. (c) SERS spectrum of 4-ABT adsorbed on silver. (d) SERS spectrum of 4-NBT on silver after electrochemical reduction in 0.2 M NaClO4 (aq). the crystallization step, the substrate was rinsed with water to remove any weakly bound CaCO3 and then blown dry with N2. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a VG Scientific ESCALAB MK II spectrometer. The Mg KR X-ray (1253.6 eV) was used as the light source, and the peak positions were referenced to the Ag 3d5/2 peak at 368.3 eV.21 The base pressure of the chamber was ∼2 × 10-10 Torr, and the electron takeoff angle was 90°. Scanning electron microscopy (SEM) images were obtained by a JSM 840-A scanning electron microscope operating at 20 kV.

Results and Discussion SERS of 4-NBT on Ag. Several SERS studies have been reported in the literature regarding the physicochemical properties of aromatic nitro molecules on silver.22 In most cases, it has been observed that the SERS peaks of the original nitro molecules gradually lose intensities, and a new set of peaks appears. The new peaks rapidly increased in intensity as a function of the laser illumination time, suggesting that the nitro molecules were subjected to photoreaction on the silver surface. On the basis of these earlier observations, we have reexamined the SERS characteristics of 4-NBT on silver. Figure 1a shows the SERS spectrum of 4-NBT adsorbed on a silver foil. When obtaining the spectrum, the silver substrate was spun at 3000 rpm to minimize the occurrence of photoreaction.16 The SERS peaks observed thereby could be attributed entirely to 4-NBT on silver. The complete absence of the S-H stretching peak in the SERS spectrum, observable at 2548 cm-1 for pure 4-NBT, indicates that 4-NBT is adsorbed on silver as thiolate after the S-H bond cleavage. On the other hand, the appearance of a prominent SERS peak at 1346 cm-1 that can be assigned to the symmetric stretching vibration of the nitro group (νs(NO2)) indicates that the nitro group is not subjected to change upon the surface adsorption of 4-NBT on silver. Other distinct SERS peaks in Figure 1a can be (21) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992. (22) (a) Roth, P. G.; Venkatachalam, R. S.; Boerio, F. J. J. Chem. Phys. 1986, 85, 1150. (b) Sun, S.; Birke, R. L.; Lombardi, J. R.; Leung, K. P.; Genack, A. Z. J. Phys. Chem. 1988, 92, 5965. (c) Bercegol, H.; Boerio, F. J. J. Phys. Chem. 1995, 99, 8763. (d) Yang, X. M.; Tryk, D. A.; Ajito, K.; Hashimoto, K.; Fujishima, A. Langmuir 1996, 12, 5525. (e) Yang, X. M.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. B 1998, 102, 4933. (f) Han, H. S.; Han, S. W.; Kim, C. H.; Kim, K. Langmuir 2000, 16, 1149.

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attributed to either the C-C stretching mode, that is, at 1573 cm-1, or the in-plane C-H bending modes, that is, at 1110 and 1082 cm-1.23 Figure 1b shows the SERS spectrum of 4-NBT on Ag that has been obtained after irradiation with the 514.5 nm line of an Ar+ laser (5 mW) for 5 s in a static condition, that is, without spinning the silver substrate. Since the actual power density was 0.2 W/cm2, the photon flux was calculated to be 1 J/cm2 for this measurement. The SERS feature in Figure 1b is substantially different from that in Figure 1a. The νs(NO2) peak was no longer identifiable, and several new peaks appeared, for instance, at 1436, 1392, 1192, and 1143 cm-1. This indicates that photoreaction has taken place for 4-NBT on the silver surface. Since further illumination of the laser light does not affect the SERS spectral feature, the reaction must occur very rapidly. To confirm that the above SERS spectral change is solely due to surface-induced photoreaction, two control experiments were carried out. First, to know whether the same reaction could also occur thermally, a series of SERS spectra were taken after storing a 4-NBT-adsorbed Ag film in an oven for a while; the substrate was spun during the spectral measurement. The SERS spectrum obtained thereby was little different from that in Figure 1a except for a gradual decrease in intensity as a function of temperature. The decrease in SERS intensity is probably due to a change in Ag morphology and/or desorption of adsorbates. This suggests that the local heating by the laser line has nothing to do with the spectral change in Figure 1b. Second, a series of ordinary Raman spectra were taken for pure 4-NBT in the solid state as well as for a 4-NBT solution in ethanol. As expected, no spectral change was observed even after irradiation with a 5 mW Ar+ laser for several hours; the UV absorption maximum of 4-NBT in ethanol is 312 nm so that the photoreaction of 4-NBT necessitates the irradiation with UV light. On the other hand, an Ar+ laser induced surface reaction hardly takes place for 4-NBT on Au. This implies that the SERS spectral change observed in Figure 1b is closely associated with the inherent nature of the silver substrate. Although the exact nature of the species responsible for Figure 1b is a matter of conjecture, the SERS spectral feature is surprisingly coincident with that of 4-aminobenzenethiol (4-ABT) on Ag, shown in Figure 1c. This suggests that 4-NBT has been reduced on Ag to 4-ABT upon illumination with the visible laser. To obtain more firm evidence, we have acquired the SERS spectrum of electrochemically reduced 4-NBT on silver in a 0.2 M aqueous NaClO4 solution. Several potential cycles between -1.2 and 0.0 V resulted in complete reduction of the nitro group. The SERS spectrum thus obtained is shown in Figure 1d. The spectral feature is once again a close match with that in Figure 1b. In previous electrochemical studies in aqueous media, it has been found that reduction of the surface-immobilized 4-NBT occurs via a complex route but eventually forms amine functionality via a 6-e transfer process.24 In this light, the newly observed peaks in Figure 1b at 1436 and 1392 cm-1 can be assigned to the C-C stretching plus the in-plane C-H bending modes of 4-ABT while the 1192 and 1143 cm-1 peaks are due to the inplane C-H bending modes of 4-ABT.25 Although other molecules such as nitroso-, hydroxylamine-, and azocompounds would also be produced by reduction of (23) Skadtchenko, B. O.; Aroca, R. Spectrochim. Acta, Part A 2001, 57, 1009. (24) (a) Zuman, P.; Fijalek, Z.; Dumanovic, D.; Suznjevic, D. Electroanalysis 1992, 4, 783. (b) Futamata, M. J. Phys. Chem. 1995, 99, 11901. (c) Tsutsumi, H.; Furumoto, S.; Morita, M.; Matsuda, Y. J. Colloid Interface Sci. 1995, 171, 505.

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Figure 2. High-resolution XP spectra of the C 1s (upper panel) and N 1s (lower panel) regions for 4-NBT on Ag (a) before and (b) after the laser irradiation and (c) for 4-ABT on Ag.

aromatic nitro molecules,26 no peak due to any such compound was identified at all in our SERS spectra. XPS of 4-NBT on Ag. To obtain further information on the nature of the reaction product, we have taken XPS spectra of 4-NBT on Ag before and after laser irradiation. The condition of laser irradiation was the same as that employed to obtain Figure 1b. For reference, the XP spectrum of 4-ABT on Ag was also obtained. The expected XPS peaks from the Ag 3d, C 1s, and N 1s core levels were clearly detected; no trace of contamination was present in any of the XP spectra. Figure 2 shows the highresolution XP spectra of the C 1s (upper panel) and N 1s (lower panel) regions for 4-NBT on Ag before (a) and after the laser irradiation (b) and for 4-ABT on Ag (c). The C 1s peak due to aromatic carbons was invariably observed at 284.5 eV for those three samples. In contrast, the N 1s peak for 4-NBT on Ag before the laser irradiation appeared at 405.7 eV while its counterpart after the irradiation appeared at 399.1 eV. The latter peak position is, however, close to that of 4-ABT on Ag. Moreover, the overall XP spectral features of 4-NBT on Ag after the laser irradiation are much the same as those of 4-ABT on Ag (see spectra (25) Osawa, M.; Matsuda, N.; Yoshii, K.; Uchida, I. J. Phys. Chem. 1994, 98, 12702. (26) Tomilov, A. T.; Mairanovskii, S. G.; Fioshin, M. Y.; Smirnov, V. A. Electrochemistry of Organic Compounds; Halstead: New York, 1972.

Patterning of Organic Monolayers on Ag

Figure 3. SERS spectra taken of the laser-irradiated regions of the 4-NBT monolayer (a) before and (b) after the 4-CBA coupling reaction.

b and c of Figure 2). Also, the XPS peak areas of 4-NBT are nearly the same before and after the laser irradiation, implying that the surface-induced photoreaction of 4-NBT on Ag takes place without loss of any structural integrity. All of these results indicate that the nitro group of 4-NBT is converted on Ag exclusively to an amine group upon laser irradiation, as surmised from the SERS observation. The possibility of the presence of other products such as nitroso-, hydroxylamine-, and azo-compounds seemed quite low also from the XPS measurement. Reaction of 4-Cyanobenzoic Acid with LaserIrradiated 4-NBT on Ag. The conversion of nitro group to amine functionality was confirmed further by monitoring the reaction of the laser-irradiated 4-NBT on Ag with 4-CBA in DMF. In fact, the as-prepared 4-NBT on Ag did not react with 4-CBA, but the laser-irradiated 4-NBT on Ag readily reacted with 4-CBA. The latter reaction was revealed by SERS spectroscopy to be the formation of amide bonds between the amine and carboxyl groups. This can be evidenced from spectra a and b of Figure 3 which show the SERS spectra of the laser-irradiated 4-NBT on Ag before and after the coupling reaction with 4-CBA, respectively; the photon flux used herein was the same as that used to obtain Figures 1 and 2. The newly observable peaks at 2234 and 1608 cm-1 in Figure 3b can be attributed to the CN stretching and the aromatic CC stretching modes of 4-CBA, respectively.27 On the other hand, the appearance of the amide I and II bands at 1650 and 1529 cm-1, respectively, in Figure 3b unambiguously indicates the formation of amide bonds.28 Plausible Mechanism of Surface-Induced Photoreaction of 4-NBT on Ag. The SERS and XPS observations as well as the coupling reaction with 4-CBA have consistently indicated that 4-NBT is converted on Ag to 4-ABT when irradiated with an Ar+ laser. Then, two particular questions are immediately raised, namely, how the reaction proceeds and what the hydrogen source is. Although the detailed mechanism is a matter of conjecture, the reaction is presumably associated with the charge transfer from silver to the adsorbed molecule. If the energy difference between the Fermi level (EF) of the metal and the low-lying excited state of the charge-transfer complex (ECT) matches the energy of the excitation radiation, a resonant charge transfer from the metal to the excited state of the complex will take place.22b The electron(27) Han, S. W.; Han, H. S.; Kim, K. Vib. Spectrosc. 1999, 21, 133. (28) Bahng, M. K.; Cho, N. J.; Park, J. S.; Kim, K. Langmuir 1998, 14, 463.

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transfer step is supposed to be a direct, optically induced charge transfer from the Fermi level to the low-lying lowest unoccupied molecular orbital of the adsorbate-silver complex. Such a direct laser-induced electron transfer has been proposed for the photoreduction of metal amine complexes based on the photocurrent measurements.29 When photoreduction occurs for aromatic nitro molecules in a solution phase, a chemical species from which hydrogen atom(s) can be abstracted is needed.30 For a similar photoreduction to occur in air, water has been claimed to act as a hydrogen source; oxygen has been reported to preclude the reaction, however.31 We found in this work that the photoreduction of 4-NBT on Ag occurs several times faster in water or ethanol medium than in air. Similarly, we observed previously that the photoreaction of 4-nitrobenzoic acid occurs much faster when the molecule is coadsorbed on Ag with hydrophilic group terminated alkanethiols than with hydrophobic group terminated alkanethiols.22f All of these observations indicate that the source of hydrogen atoms in the surfaceinduced photoreduction of 4-NBT on Ag to 4-ABT must be water or solvent molecules trapped inside the 4-NBT monolayer rather than 4-NBT itself. Formation of Patterned Binary Organic Monolayers on Ag. The potential of the method to produce patterned organic monolayers with selectable surface chemical properties is demonstrated by irradiating the SAMs of 4-NBT on Ag with an Ar+ laser through a copper grid (600 mesh). For this experiment, the substrate was moved at a rate of 0.4 µm/s under exposure of laser light with a power density of 0.5 W/cm2. After removal of the grid, the surface was subsequently treated with 4-CBA. This process is schematically drawn in Figure 4 (left panel). A typical SERS image obtained from the patterned monolayers after the process is shown in Figure 5. For the production of the image, we selected the SERS peak at 2234 cm-1 (see Figure 3b) as the most appropriate marker band, indicative of the coupling of 4-CBA to amine functionalities. Light areas in Figure 5 are due to higher SERS intensity, and they correspond to the coupled areas. The dimensions of the pattern match closely the actual dimensions of the Cu grid used as a photomask. Thus, the present method can serve as a means of chemically defined lithography. Since the preparation scheme is very simple, the method must be beneficial particularly when amineterminated, patterned organic monolayers are needed. In fact, amine-terminated surfaces have been widely used in recent years specifically for the development of biosensors,32 enzyme-coated electrodes,32 DNA computers,33 and so forth. Biomimetic Growth of Inorganic Crystals on the Patterned Monolayers. Certain organic surfaces are known to play active roles in promoting the nucleation of inorganic minerals. In this light, functionalized organic surfaces have frequently been employed for the deposition of inorganic minerals onto them from aqueous solutions.19,20,34,35 On the basis of the fact that patterned monolayers should possess area-dependent chemical (29) Corrigan, D. S.; Weaver, M. J. J. Electroanal. Chem. 1987, 228, 265. (30) Barltrop, J. A.; Bunce, N. J. J. Chem. Soc. C 1968, 1467. (31) Tsai, W.-H. Ph.D. Dissertation, University of Cincinnati, Cincinnati, OH, 1991. (32) Bain, C. D.; Whitesides, G. M. Science 1988, 240, 62. (33) Liu, Q.; Wang, L.; Frutos, A. G.; Condon, A. E.; Corn, R. M.; Smith, L. M. Nature 2000, 403, 175. (34) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; McVay, G. L. Science 1994, 264, 48. (35) Ku¨ther, J.; Seshadri, R.; Knoll, W.; Tremel, W. J. Mater. Chem. 1998, 8, 641.

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Figure 4. Experimental scheme for the site-specific modification of the patterned SAMs.

Figure 5. SERS image of the patterned SAMs on silver after the 4-CBA coupling reaction. The image is drawn using the characteristic SERS peak of 4-CBA at 2234 cm-1. The scale bar represents the intensity of the Raman signal.

carbonate (calcite), one of the most abundant biominerals,36 which can be grown easily under laboratory conditions.37 To grow the crystal, the patterned substrates produced by laser irradiation through a Cu grid (the experimental condition was exactly the same as that in the previous section) were supported upside down in an aqueous calcium chloride solution, to ensure that only particles grown on the substrates would be bound to the surface, and then placed in a closed desiccator with vials of solid ammonium carbonate at the bottom (see the right panel of Figure 4).18-20 Crystallization of calcite results from the diffusion of carbon dioxide vapor from ammonium carbonate into the solution. The surfaces decorated with crystals were examined using a scanning electron microscope. Figure 6 is a typical SEM image showing the patterns of calcite crystals formed on a sample substrate. Crystallization is restricted to well-defined, nitro group terminated regions and does not occur on the amine group terminated areas, even though the crystal can be grown on the latter type of surface when fully covered with amine groups. The crystallization of calcite only on the nitro group terminated regions must be associated with the greater affinity of calcium ions for the nitro groups, in comparison with the amine groups. Once the calcite crystals nucleate at the nitro-terminated sites, due to the mass transport

reactivity, we have thus examined whether our patterned monolayers could be used as a template of selective crystal growth. The crystal chosen for this study was calcium

(36) Lowenstam, H. A.; Weiner, S. On Biomineralization; Oxford University Press: New York, 1989. (37) Lippmann, F. Sedimentary Carbonate Minerals; SpringerVerlag: Berlin, 1973.

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Figure 6. SEM image of calcite crystals formed on a patterned substrate.

effect the calcium and carbonate ions will be depleted over the amine-terminated regions to a point of undersaturation so that the crystallization hardly takes place in these regions.20 Aizenberg et al.19 reported that the preorganization of organic surfaces in the presence of counterions would be decisively important for the controlled, face-selective nucleation of calcite on ω-functionalized alkanethiol SAMs. When SAMs terminated with hydrophilic groups such as -CO2-, -SO3-, and -OH were exposed to carbonate ions and then calcium ions were added subsequently, there was no preferred orientation for the crystallized calcite, presumably as a result of nonspecific precipitation rather than surface-induced nucleation. Consulting these observations, we examined how the calcite would grow on our patterned substrate when the substrate was exposed at the beginning to carbonate ions. For this purpose, a patterned substrate made by laser irradiation was supported in pure water and left for a while in a closed desiccator (containing vials of solid ammonium carbonate at the bottom) prior to adding a calcium chloride solution into water. In this case, the crystallization is restricted to the amine-terminated regions and does not occur on the nitro group terminated regions. This is in contrast with the previous case shown in Figure 6. Considering the preferential binding of carbonate ions to amine groups (amine groups are protonated in our experimental conditions), the crystallization of calcite only on the amineterminated regions is not unreasonable. The present observation suggests that light-directed crystallization of calcite can be accomplished on silver at one’s pleasure. To demonstrate its feasibility, after self-assembly of 4-NBT on Ag a focused Ar+ laser was irradiated along parallel lines separated from each other by 10 µm (the scan rate and power density were the same as those employed to obtain Figures 5 and 6), and then calcite crystals were grown on the substrate, exposing them first to carbonate ions. In fact, as shown in Figure 7, the crystals were selectively grown along the two lines which had been exposed to the laser beam to produce amine functionality. The present light-directed patterning method to form amine group terminated monolayers will also be applied to the fabrication of two-dimensional nanoparticle as-

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Figure 7. SEM image of calcite crystals formed on two parallel amine-terminated lines that were generated previously using a focused Ar+ laser.

semblies. Gold nanoparticles, for instance, are known readily to anchor on the amine-terminated SAMs.38 Since the amine group can be derivatized further to give a thiol terminus, the present method will be applicable even to the fabrication of two-dimensional arrays of CdS, CdSe, and other semiconductor or metal particles.6c When the light-directed patterning is carried out stepwise, it will also be possible to construct two-dimensional heterostructures consisting of different kinds of nanoparticles and/or biomimetic crystals. Summary and Future Work We have observed the chemical transformation of 4-NBT to 4-ABT on Ag by visible laser light irradiation. On the basis of this, we demonstrated that patterned binary organic monolayers could readily be formulated on a silver substrate by surface-induced photoreaction, preserving the overall structural integrity of the primary monolayer. Such locally modified monolayer surfaces can be used to direct the site-selective self-assembly of a number of different organic and inorganic materials, according to a predefined geometric pattern and a preselected type of chemical functionality. Our future work will be directed toward the reduction of the patterns down to a submicron scale by using optical near-field or solid immersion lens techniques. We will also examine the prospects of applying the present nondestructive patterning process in various areas of chemical and biological investigations. Acknowledgment. This work was supported in part by the Korea Research Foundation (KRF, 042-D00073) and the Korea Science and Engineering Foundation (KOSEF, 1999-2-121-001-5). K.K. also acknowledges KOSEF for providing a leading-scientist grant (KOSEF, R032001-00021). S.W.H. was supported by KOSEF through the Center for Molecular Catalysis at Seoul National University. I.L. is a recipient of the BK21 fellowship. LA0115684 (38) (a) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (b) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846.