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Au Nanoparticle Micropatterns Prepared from Self-Assembled Films Conghua Lu,† Fang Wei,‡ Nianzu Wu,† Lan Huang,† Xinsheng Zhao,‡ Xiaoming Jiao,§ Chuanqiou Luo,† and Weixiao Cao*,† College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871 China, State Key Laboratory of Molecular Dynamic and Stable Structure and Department of Chemical Biology, College of Chemistry, Peking University, Beijing, 100871 China, and The Institute of Chemical Reagent, Beijing, 100022 China Received July 30, 2003. In Final Form: November 10, 2003 A kind of hybrid multilayer film based on mercaptobenzoic acid-capped Au nanoparticles (MBA-Au-NPs) and photoreactive nitrodiazoresin (NDR) has been fabricated via electrostatic self-assembly. Upon exposure to UV light, the initial ionic bonds between the layers of the film convert into covalent bonds and the film stability toward polar solvents, salt, or surfactant solutions increases significantly. The micropatterned NDR/MBA-Au-NP film with the covalently linked architecture was formed by selecting exposure of the film through a photomask and later developed in sodium dodecyl sulfate (SDS) aqueous solution. The metallic Au-NP micropatterns, furthermore, are produced by sintering the micropatterned NDR/MBAAu-NP film at 550 °C, at which the organic components are removed completely. The well-defined micropatterns were characterized with atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), microscope with a charge-coupled device (CCD) camera, and X-ray photoelectron spectroscopy (XPS).
Introduction Fabrication of metallic nano- and micropatterns, especially those containing Au nanoparticles, has attracted increasing interest in modern material science, as a result of the potential application in the areas of microelectronics and miniaturized devices.1 Generally, two methodologies have been developed to realize the goal, i.e., micropatterning the metallic nanoparticle-containing film2-6 and site-selective deposition of the metal on the micropatterned surface.7-9 To construct the metallic nanoparticle-containing films, methods have been exploited such as electroless deposition,8,9 electrochemical deposition,7 chemical (or physical) vapor deposition,6,10 and so on. Meanwhile, a variety of techniques have been used to obtain the micropatterned surface including photolithography,9,11,12 * To whom correspondence should be addressed: Phone +8610-62757155; fax +86-10-62751708; e-mail
[email protected]. † College of Chemistry and Molecular Engineering, Peking University. ‡ State Key Laboratory of Molecular Dynamic and Stable Structure and Department of Chemical Biology, College of Chemistry, Peking University. § The Institute of Chemical Reagent. (1) Masuko, N., Osaka, T., Ito, Y., Eds., Electrochemical Technology: Innovations and New Developments; Gordon and Breach Publishers: Amsterdam, 1996. (2) Reetz, M. T.; Winter, M. J. Am. Chem. Soc. 1997, 119, 4539. (3) Stellacci, F.; Bauer, C. A.; Meyer-Friedrichsen, T.; Wenseleers, W.; Alain, V.; Kuebler, S. M.; Pond, S. J. K.; Zhang, Y.; Marder, S. R.; Perry, J. W. Adv. Mater. 2002, 14, 194. (4) Rolandi, M.; Quate, C. F.; Dai, H. Adv. Mater. 2002, 14, 191. (5) Klehn, B.; Kunze, U. J. Appl. Phys. 1999, 85, 3897. (6) Stutzmann, N.; Tervoort, T. A.; Bastiaansen, K.; Smith, P. Nature 2000, 407, 613. (7) Bailey, R. C.; Stevenson, K. J.; Hupp, J. T. Adv. Mater. 2000, 12, 1930. (8) Saito, N.; Haneda, H.; Sekiguchi, T.; Ohashi, N.; Sakaguchi, I.; Koumoto, K. Adv. Mater. 2002, 14, 418. (9) Shirahata, N.; Masuda, Y.; Yonezawa, T.; Koumoto, K. Langmuir 2002, 18, 10379. (10) Weinberger, D. A.; Hong, S.; Mirkin, C. A.; Wessels, B. W.; Higgins, T. B. Adv. Mater. 2000, 12, 1600.
soft lithography,7,13,14 electron beam lithography,2,3 scanning probe lithography,4,5 embossing,15 micromolding,16 and microcutting.6 However, most of the fabricated micropatterned hybrid films based on metallic components are unstable because of the existence of weak bonds (i.e., ionic bonds or H-bonds). Here, we report Au-NP micropatterned film with covalently linked architecture. It was built up from mercaptobenzoic acid-capped Au nanoparticles (MBA-Au-NPs) and a photoreactive -N2+ containing polymer of nitrodiazoresin (NDR) by electrostatic selfassembly, followed by selective exposure to UV light through a photomask and later development in SDS aqueous solution. The covalent bonds formed between the assembled layers come from the conversion of the ionic bonds upon UV irradiation. Furthermore, annealing of the micropatterned NDR/MBA-Au-NP film at 550 °C leads to complete removal of the organic components and the metallic Au-NP micropattern was obtained. The advantage of the method used in the present work is that the metalcontaining precursor film was fabricated by self-assembly, which is a simple and universal method with ease of regulating the film thickness and introducing a variety of functional components into the resultant films and the micropatterns.17 Therefore, it may be very interesting as a promising approach to build up metallic NP micropatterns. (11) Shi, F.; Dong, B.; Qiu, D.; Sun, J. Q.; Wu, T.; Zhang, X. Adv. Mater. 2002, 14, 805. (12) Doshi, D. A.; Huesing, N. K.; Lu, M. C.; Fan, H. Y.; Lu, Y. F.; Potter, K. S.; Jr., B. G. P.; Hurd, A. J.; Brinker, C. J. Science 2000, 290, 107. (13) Xia, Y.; Kim, E.; Zhao, X.-M.; Rogers, J. A.; Prentiss, M.; Whitesides, G. M. Science 1996, 273, 347. (14) Zheng, H. P.; Lee, I.; Rubner, M. F.; Hammond, P. T. Adv. Mater. 2002, 14, 569. (15) Stutzmann, N.; Tervoort, T. A.; Bastiaansen, C.; Feldmann, K.; Smith, P. Adv. Mater. 2000, 12, 557. (16) Kim, E.; Xia, Y.; Whitesides, G. M. Nature 1995, 376, 581. (17) Decher, G. Science 1997, 277, 1232.
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Materials and Methods 1. Materials. Nitrodiazoresin (NDR) was synthesized from 2-nitro-N-methyldiphenylamine-4-diazonium salt with paraformaldehyde in concentrated sulfuric acid according to ref 18; Mn ∼2100 g/mol. The preparation of 4-mercaptobenzoic acid- (MBA-) capped Au-NPs of ∼3 nm was followed the described procedure.19 The quartz slide and silicon wafer used as the substrate was treated in a boiling mixture of H2O2 (30%)-H2SO4 (98%) (v/v ) 3/7) for 0.5 h before use. 2. Self-Assembly of NDR/MBA-Au-NP Multilayer Film. The pretreated substrate (the quartz slide or silicon wafer) was alternately dipped into the aqueous solutions of NDR (0.5 mg/ mL) and MBA-capped Au-NPs (1 mg/mL, pH ≈ 7) for 5 and 10 min, respectively, with the interruption of deionized water rinsing and cool air-drying. After repetition of the procedure for n times in the dark, a composite multilayer film was constructed with n bilayers of NDR/MBA-Au-NPs on both sides of the substrate. 3. Micropatterning NDR/MBA-Au-NP Multilayer Film. The resulting NDR/MBA-Au-NP assembled films were selectively exposed to 360 nm UV light (3 mW/cm2, 2 min) from a mediumpressure Hg lamp through a photomask having images with different resolutions. After development in SDS solution (0.25 M) for 2 h at room temperature (∼25 °C) in the dark, the patterned film was sonicated in deionized water for 1 min and cleaned thoroughly by deionized water. 4. Sintering Micropatterned NDR/MBA-Au-NP Film. The above micropatterned NDR/MBA-Au-NP film was heated in a furnace at a heating rate of 2 °C/min and sintered at 550 °C for 2 h, then cooled naturally to room temperature. 5. Characterization. UV-vis spectra were recorded on a Shimadzu 2100 spectrophotometer after completion of each assembly cycle on the quartz slide with MBA-Au-NPs as the outermost layer of the film. The photoreaction occurring in the film upon UV irradiation was also determined spectroscopically. Atomic force microscopy (AFM), performed on a Nanoscope IIIA (Digital Instruments, Inc.) in the tapping mode with the commercial silicon probe (model TESP-1000), was used to visualize the surface morphology of the patterned films. The micropattern without sputtering gold was studied by fieldemission scanning electron spectroscopy (FE-SEM, AMERY 1900) operating at 5 kV. The image was also investigated by a Leika Microscope with a charge-coupled device (CCD, Cool Snap, Roper Scientific) camera and X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra spectrometer) with Al KR monochromatized X-ray source run at 250 W and the base pressure in the analytic chamber about 5 × 10-9 Torr. Except for the special designation, the micropatterned samples for the measurement of AFM, SEM, CCD, and XPS were built up on the silicon wafer.
Figure 1. UV-vis spectra of NDR/MBA-Au-NP multilayer film with different bilayers (bottom to top): 0, 2, 4, 6, 8, 10, and 12. The inset shows the absorbance at 384 nm as a function of the bilayers.
Figure 2. UV-vis spectra of (NDR/MBA-Au-NP)6 film changing under 360 nm UV irradiation (230 µW cm-2) with the irradiation time (minutes, top to bottom): 0, 0.5, 1, 1.5 2.5, 4.5, and 10. (Inset) Ln [(A0 - Ae)/(At - Ae)] vs the irradiation time (t, minutes). Scheme 1. Schematic Illustration of an Ionic Bond Converting to a Covalent Bond between the Layers of NDR/MBA-Au-NP Film upon Exposure to UV Light
Results and Discussion 1. Fabrication of (NDR/MBA-Au-NP)n Multilayer Film. Figure 1 shows the evolution of UV-vis spectra of NDR/MBA-Au-NP film in the self-assembly process. The absorbance at 384 nm, assigned to the characteristic π-π* absorption of diazonium group (-N2+-) of NDR, increases linearly with the bilayers (inset plot), indicating that NDR has been uniformly assembled into the multilayer film. The characteristic surface plasmon absorption band of Au-NPs at 520 nm is not observed in MBA-capped Au-NP films. It may be that MBA-capped Au-NPs have very weak surface plasmon absorption, similar to the results of Hao and Lian.19 The driving force of the assembly should be attributed to the electrostatic interaction between the diazonium group (-N2+) of NDR and the dissociated carboxylic anion of MBA-Au-NPs. The NDR/MBA-Au-NP multilayer film is photosensitive. Upon UV exposure, the -N2+ groups involved in the film will decompose and the absorbance at 384 nm decreases just as shown in Figure 2. The linear relation of ln [(A0 (18) Wang, R. X.; Chen, J. Y.; Cao, W. X. J. Appl. Polym. Sci. 1999, 74, 189. (19) Hao, E. C.; Lian, T. Q. Chem. Mater. 2000, 12, 3392.
- Ae)/(At - Ae)] with the irradiated time (inset plot) indicates that the decomposition of -N2+ groups in the film follows first-order reaction kinetics, where A0, At, and Ae represent the absorbance at 384 nm of the film before the irradiation (A0), after the irradiation of time t (At), and the end of the irradiation (Ae, 10 min), respectively. Meantime, the ionic bonds (-Ph-N2+ -OOC-) between the assembled layers convert into covalent ester bonds (-Ph-OOC-) upon UV irradiation. Referring to the previous studies,20,21 the nature of the bond conversion can be illustrated in Scheme 1. The irradiated film is very stable due to the formation of the covalently cross-linked structure and there is negligible change after immersion in SDS solution (0.25 (20) Luo, H.; Chen, J. Y.; Luo, G. B.; Chen, Y. N.; Cao, W. X. J. Mater. Chem. 2001, 11, 410. (21) Sun, J. Q.; Wu, T.; Liu, F.; Wang, Z. Q.; Zhang, X.; Shen, J. C. Langmuir 2000, 16, 4620.
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Figure 3. AFM image of the micropatterned (NDR/MBA-Au-NP)8 film: (a) flattened image; (b) detail of the square region in panel a.
Figure 4. CCD micrograph of the micropatterned (NDR/MBAAu-NP)8 film.
M) at ∼25 °C for 2 h, while the nonirradiated film is unstable and removed completely in the same etching conditions because it is linked by ionic bonds. This fact is very similar to previous reports.11,22,23 Thus, SDS solution can be used as the developing agent to obtain the micropatterned (NDR/MBA-Au-NP)n film after selective exposure to UV light through a photomask. 2. Characterization of the Micropattern. First the micropatterned (NDR/MBA-Au-NP)8 film before sintering was characterized by AFM as shown in Figure 3. The regularly arrayed squares with dimensions of 5 × 5 µm2 are clearly visualized in the flattened image (Figure 3a), and the thickness of the film is determined to be around 32 nm by AFM height analysis. Figure 3b demonstrates that the reserved (NDR/MBA-Au-NP)8 film has a rather flat surface with an average roughness of 0.8 nm and the closely packed Au nanoparticles are visualized clearly. Micropatterns with different resolutions were also taken by the Leika Microscope with a charge-coupled device (CCD) camera (Figure 4). The well-defined image with distinct dark and bright contrast should come from the different reflex against the incident light from a 50 W medium-pressure mercury lamp. Namely, it is more difficult for the irradiated parts of the film (dark regions of the micrograph) to reflect the incident light, compared with the nonirradiated parts (bright regions of the micrograph), which are the exposed silicon surface. To verify that the nonirradiated parts of the film have been fully removed in the developing process, X-ray photoelectron spectroscopy (XPS) was utilized to characterize micropatterned NDR/MBA-Au-NP film. XPS can provide plentiful information about chemical composition of the first few nanometers of the surface, especially for the top atomic layer.24 This technique has been used successfully to investigate the layer-by-layer (LBL) self(22) Cao, W. X.; Yang, L.; Luo, H. J. Appl. Polym. Sci. 1998, 70, 1817. (23) Lu, C. H.; Wu, N. Z.; Wei, F.; Zhao, X. S.; Jiao, X. M.; Xu, J.; Luo, C. Q.; Cao, W. X. Adv. Funct. Mater. 2003, 13, 548. (24) Swartz, W. E., Jr. Anal. Chem. 1973, 45, 789.
Figure 5. Survey-scan X-ray photoelectron spectra of the micropatterned (NDR/MBA-Au-NP)8 film fabricated on silicon wafer.
Figure 6. Elemental distribution images of the micropatterned (NDR/MBA-Au-NP)8 film: (a) Au 4f7/2; (b) Si2p3/2.
assembly films,25 but to our knowledge, there are few reports that use XPS to characterize the LBL assembled micropatterns. Figure 5 shows the XPS spectra of the micropatterned (NDR/MBA-Au-NP)8 film. The presence of Au 4f peaks, as well as O1s and C1s peaks, indicates that NDR and MBA-Au-NPs were incorporated into the pattern. The characteristic Si peaks between the binding energies of 99 and 160 eV originate from the signal of the exposed substrate of silicon. Meantime, the elemental distribution images of Au and Si with signals of Au 4f7/2 at 84.3 eV and Si2p3/2 at 99 eV are given in Figure 6, panels a and b, respectively. Obviously, the distribution of Au [bright on (a) and dark on (b)] matching well with that of corresponding Si [dark on (a) and bright on (b)] strongly supports the fact that the unirradiated regions of the film were removed in the developing process, whereas the irradiated regions of the film were kept on the silicon surface. The above-prepared NDR/MBA-Au-NP micropatterns without sputtering gold were also investigated by fieldemission scanning electron spectroscopy (FE-SEM) (Fig(25) Nakanishi, T.; Ohtani, B.; Shimazu, K.; Uosaki, K. Chem. Phys. Lett. 1997, 278, 233.
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Figure 7. (a) SEM image of the micropatterned (NDR/MBA-Au-NP)8 film on silicon wafer; (b) corresponding sintered Au-NP micropattern from panel a at 550 °C for 2 h.
electric conductivity.27 But on the other hand, the sintering process results in the loss of the nanoscale nature of film’s constituents to a certain extent and the obvious decrease in the stability of the film. All of these need to be further resolved.
Figure 8. AFM image of Au-NP micropattern on silicon wafer.
ure 7a). The gray and bright parts of the image correspond to the irradiated (i.e., the reserved film) and nonirradiated (i.e., the exposed silicon substrate) parts, respectively. The SEM image without sputtering gold should be produced from the different secondary electron contrast for the two parts.26 It is supported by the fact that sintering leads to the reversion of the contrast of SEM images. Figure 7b shows that the metallic Au-NP micropattern is realized by sintering the micropatterned NDR/MBA-Au-NP film at 550 °C for 2 h. No noticeable changes such as cracks or deformations have been observed in the sintering process (Figure 7b). The AFM image of the sintered micropattern is illustrated in Figure 8. It is rational that the thickness of the film decreases obviously from ∼32 nm (before sintering, Figure 3a) to ∼20 nm (after sintering, Figure 8) because of the removal of the organic components, and some AuNPs agglomerate into larger clusters, which may be favorable in some situation such as improvement of the
Conclusion A kind of self-assembly multilayer film from mercaptobenzoic acid-capped Au nanoparticles (MBA-Au-NPs) and nitrodiazoresin (NDR) has been built up via electrostatic self-assembly. The ionic bonds between the layers of the film were converted into covalent bonds by exposing the film to UV light. As the result the film stability toward polar solvents, etc., increases significantly. The covalently linked micropatterned NDR/MBA-Au-NP composite film then was prepared by the self-assembly film combined with the photolithography technique. After removal of the organic components of NDR and MBA from the micropatterned NDR/MBA-Au-NP film by sintering at 550 °C, a well-defined metallic Au-NP micropattern was obtained. Both the micropatterned NDR/MBA-Au-NP films and the metallic Au-NP micropatterns have been characterized by AFM, SEM, etc. The feasible method used in the present study may provide a promising approach to fabricate micropatterned metallic films with the convenience to fine-tune the composition and the structure of the films. Acknowledgment. We are grateful to the National Natural Science Foundation (Grants 20274002 and 50173002) and the Major State Basic Research Development Program of China (Grant G2000077503) for the financial support to this work. We also thank Professor Jiming Ma and Dr. Dongbai Zhang (Peking University) for their help in the sintering experiments. LA035395P
(26) Hayat, M. A. Principles and Techniques of Scanning Electron Microscopy: Biological Applications, Vol.6; Van Nostrand Reinhold Company: New York, 1974.
(27) Andreasen, G.; Schilardi, P. L.; Azzaroni O.; Salvarezza, R. C. Langmuir 2002, 18, 10430.
