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Surface-Guided Self-Assembly of Silver Nanoparticles on Edges of Heterogeneous Surfaces Weidong Ruan,† Chunxu Wang,† Nan Ji,† Zhicheng Lu,† Tieli Zhou,† Bing Zhao,*,† and John R. Lombardi‡ State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, P. R. China, and Department of Chemistry, the City College of New York, New York 10031 ReceiVed April 13, 2008. ReVised Manuscript ReceiVed July 6, 2008 A general method for the generation of two-dimensional (2D) ordered silver nanoparticles (av 45 nm) ring array has been demonstrated via controllable self-assembly. The selective self-assembly is conducted on the edges of a gold coated polyelectrolyte film. This film is fabricated using the monolayer polystyrene (PS) spheres (av 600 nm) on a substrate as template, followed by depositing a positively charged polyelectrolyte and gold colloids (av 17 nm) via the layer-by-layer (LbL) self-assembly technique, and finally by eliminating the PS monolayer. This gold coated polyelectrolyte film has a regular pattern of sharp edges, and those edges are composed of abundant polyelectrolyte. This heterogeneous surface is easily prepared and universal for site-selective absorption of nanoparticles (silver nanoparticles in this paper, av 45 nm). This surface-guided self-assembly is powerful for fabricating micro/nanostructures on the edges of prepatterns. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the products.
Introduction The past decade has witnessed much progress in the study of nanoparticles for their numerous applications in biological labeling,1,2 catalysis,3,4 sensor fields,5,6 and surface-enhanced Raman scattering.7,8 Recently, the most exciting prospect of nanoparticles is their implementation as building blocks for the generation of novel functional devices.9–11 To realize these goals, a general route is needed to accurately control the position of nanoparticles deposited on substrates and form a desirable pattern. Up to now, many effective methods have been developed including vapor deposition,12 soft lithography,13 and selfassembly14 to prepare subtle nanostructures. In this letter, we present a simple and generally applicable patterning methodology, which utilizes site-selective absorption on the edges of a heterogeneous surface (surface-guided selfassembly). Our technique is low-cost and is versatile for a lot of templates. Here, we adopt the nanosphere lithography (NSL) technique invented by Van Duyne et al.15 to prepare the active * To whom correspondence should be addressed. E-mail:
[email protected]. edu.cn. Telephone: +86-431-85168473. Fax: +86-431-85193421. † Jilin University. ‡ The City College of New York.
(1) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016–2018. (2) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013–2016. (3) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343–1348. (4) Zhou, K. B.; Wang, X.; Sun, X. M.; Peng, Q.; Li, Y. D. J. Catal. 2005, 229, 206–212. (5) Zhang, J. T.; Liu, J. F.; Peng, Q.; Wang, X.; Li, Y. D. Chem. Mater. 2006, 18, 867–871. (6) Liu, J. F.; Wang, X.; Peng, Q.; Li, Y. D. AdV. Mater. 2005, 17, 764–767. (7) Xie, X. S. Acc. Chem. Res. 1996, 29, 598–606. (8) Nie, S. M.; Emory, S. R. Science 1997, 275, 1102–1106. (9) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335– 1338. (10) Alivisatos, A. P. Science 1996, 271, 933–937. (11) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1992. (12) Shan, C. X.; Liu, Z.; Hark, S. K. Appl. Phys. Lett. 2008, 92, 073103. (13) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002–2004. (14) Li, X. L.; Xu, W. Q.; Zhang, J. H.; Jia, H. Y.; Yang, B.; Zhao, B.; Li, B. F.; Ozaki, Y. Langmuir 2004, 20, 1298–1304. (15) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599–5611.
absorbing sites on the surface. A heterogeneous surface film which has periodic circular vacancies is obtained. This film is composed of polyelectrolyte (bottom) and gold coverage (top). On the edge of the vacancy or well, an edge nucleation induced by the polyelectrolyte is observed. To our knowledge, this is the first report of the surface-guided self-assembly method on the edges of covered polyelectrolyte films. All these micro/nanostructures may have use in applications such as sensors, chips, and substrates for surfaceenhanced Raman scattering.
Materials and Methods Chemicals. HAuCl4 (99.8%), AgNO3 (99.5%), sodium citrate (98%), and poly(diallydimethylammonium chloride) (PDDA) solution (20 wt %) with medium molecular weight (200 000-350 000) were purchased from Sigma-Aldrich Chemical Co. and used without further purification. The other chemicals (acetone, 30% H2O2, 98% H2SO4) were all reagent grade and obtained from Beijing Chemical Plant. Deionized water was obtained with a Mini-Q system (Millipore). Preparation of Gold and Silver Colloids. Gold colloids with an average diameter of 17 nm were prepared by sodium citrate reduction of HAuCl4.16 Solutions of colloidal silver were prepared according to the standard procedure17 with some modifications. In a typical synthesis, 36 mg of AgNO3 was dissolved in 200 mL of water. This aqueous solution was then heated to a boil under stirring and reflux. Sodium citrate (4 mL, 1% (w/v)) was added to the solution, and a reacting time of 1.5 h was employed. The average diameter of these particles is 45 nm. Substrate Treatment and Surface-Guided Self-Assembly. Glass slides were cleaned by immersion in fresh piranha solution (30% H2O2/98% H2SO4, 3:7 v/v) and heated until no more bubbles evolved. After cooling, the slides were rinsed repeatedly with twice-distilled water. Polystyrene (PS) colloids were synthesized, and an ordered monolayer of PS spheres (av 600 nm) was deposited on the glass slides using the Langmuir-Blodgett (LB) self-assembly technique as we published previously.18 The substrates were then immersed (16) Frens, G. Nat. Phys. Sci. 1973, 241, 20–22. (17) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391–3395. (18) Ruan, W. D.; Lu, Z. C.; Ji, N.; Wang, C. X.; Zhao, B.; Zhang, J. H. Chem. Res. Chin. UniV. 2007, 23, 712–714.
10.1021/la8011537 CCC: $40.75 2008 American Chemical Society Published on Web 07/26/2008
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Figure 1. Schematic representation of surface-guided self-assembly via the NSL and LbL self-assembly technique: (1) PS spheres array, (2) deposition of polyelectrolyte, (3) assembly of colloidal gold, (4) prepattern of circular vacancy array, and (5) assembly of silver “necklace” array.
into a 0.5 wt % PDDA solution for 20 min to absorb a layer of positively charged polyelectrolyte. Colloidal gold was deposited on the chemically modified templates through electrostatic force by immersing the substrates into the gold colloidal solution for 6 h. Finally, the PS mask was removed by sonicating the entire sample in acetone solvent for 3 min. After drying by air flow, the substrates were immersed into the silver colloidal solution directly for 40 min. Characterization. Atomic force microscopy (AFM) images were obtained in contact mode at room temperature with a Digital Instruments Nanoscope IIIA instrument by a multimode using Si cantilevers purchased from DI and Nanosensor Co. Ltd. Scanning electron microscopy (SEM) micrographs were taken with a JEOL FESEM 6700F electron microscope with a primary electron energy of 3 kV. All samples of the Ag patterns on Au substrates were not sputtered.
Results and Discussion To clearly see the whole process, all the fabricating procedures have been illustrated in Figure 1. As a first step, a hexagon close-packed monolayer consisting of PS spheres was deposited on a glass substrate as described previously.18 The period of this film can be tuned readily from 200 or 300 nm to 1 µm by using different size spheres. In this work, we have patterned the PS array with an average diameter of 600 nm. In successive steps (steps 2 and 3), the positively charged polyelectrolyte of PDDA and negatively charged colloidal gold (av 17 nm) were deposited on the glass substrate and PS mask via layer-by-layer (LbL) self-assembly. The opposite charges of the building blocks guaranteed electrostatic absorption effectively. In the fourth and fifth steps, the PS template was removed and negatively charged colloidal silver (av 45 nm) was deposited at specific areas. Figure 2A shows the typical SEM image of a hexagon closepacked PS monolayer, indicating that a large-scale and ordered 2D monolayer consisting of PS spheres (av 600 nm) has been obtained successfully. Figure 2B shows the SEM image of the gold colloid (av 17 nm) coated PS film prepared in step 3 (Figure 1). It demonstrates that the deposition of gold nanoparticles is sufficient and uniform. Gold colloids have been chosen for their stable and robust properties. The gold structures are not easily destroyed in the post process. A periodic circular vacancy film has been prepared and characterized by SEM and AFM. Figure 3A shows the SEM image of this gold coated polyelectrolyte pattern on the glass
Figure 2. (A) SEM image of PS sphere (av 600 nm) array in a hexagon close-packed pattern on the glass substrate. (B) SEM image of colloidal gold (av 17 nm) coated PS sphere monolayer.
substrate. The gold film remained due to the fact that gold nanoparticles and polyelectrolyte do not disperse in acetone. Enhancing the magnification (inset in Figure 3A), uniform gold nanoparticles can be clearly seen. To determine the accurate structure of the product, AFM has been used to characterize the assembly (Figure 3B). From the side view and a corresponding line scan, the nanowell structure can be seen clearly. This gold coated polyelectrolyte film with discrete micrometer-sized holes has fairly sharp edges. The well diameter and depth are confirmed to be approximately 325 and 17 nm, respectively. One of the open questions with preparing nanodevices is how to deposit nanoparticles accurately at specific sites on substrates and form the desirable pattern. We proceeded with our experiments toward this goal. In our particular case, the edges of nanowells were developed for selective absorption of silver nanoparticles (av 45 nm). Figure 4 shows the SEM and AFM images of the product after the selective deposition of silver nanoparticles from a nanowell film (Figure 3). Due to the regular array, a network of “necklace” structures was formed along the edges of the nanowells. The diameter of the nanowell was reduced to 220 ( 20 nm from 325 nm because of the deposition of silver nanoparticles. The reason for the site-selective absorbing process is explored. In the gold coated polyelectrolyte film, due to the sharp edges of the periodic nanowells, a great deal of polyelectrolyte is left on the edges, which is very active and could be used as new nucleation sites for a secondary electrostatic assembly. This chemically modified heterogeneous surface is the basis of surfaceguided self-assembly. These specific areas can be used as surfaceguided centers for depositing desirable nanoparticles. To develop this unique method, we also prepared silver colloid coated polyelectrolyte films. On the edges of the silver nanowells, gold nanoparticles (av 17 nm) were selectively deposited. Thus,
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Figure 3. Typical SEM (A) and AFM (B) images of the nanowell array consisting of gold nanopartilcle (av 17 nm) on the glass slides. The diameter and depth of the wells are approximately 325 and 17 nm, respectively.
Figure 4. SEM and AFM images of a silver “necklace” array on prepatterned substrates. The prepattern is a gold nanowell array with a diameter and depth of approximately 325 and 17 nm, respectively; the silver nanoparticles are 45 nm in average diameter; and the silver “necklace” is 220 ( 20 nm in diameter.
Figure 5. SEM and AFM images of a gold colloid ring array on glass substrates. The ring array has a period of approximately 600 nm. The gold nanoparticles are 17 nm in average diameter.
compound micro/nanostructure gold ring arrays along the edges of silver nanowells were formed. It is interesting to obtain the inverse structure of silver on the edges of gold nanowells (Figure 4). More importantly, the silver covering can be removed by selectively dissolving in dilute nitric acid. Figure 5 shows the SEM and AFM images of gold colloid ring arrays on glass
substrates. From the side view and a corresponding line scan of AFM, it can been seen clearly that the nanoring array has a period of approximately 600 nm, which is in accordance with the period of PS templates. For future application, we think it is a general method to trap nanoparticles on the edges of a heterogeneous relief surface.
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Figure 6. Illustration of the surface-guided self-assembly technique: (1) Removable patterning template on a substrate. (2) Deposition of a positively charged polyelectrolyte. (3) Coverage or deposition of an oppositely charged layer to neutralize the polyelectrolyte. (4) Removal of the template. The rectangular gray area is still neutralized, but the edge is exposed to positive charges. (5) Surface-guided self-assembly of negatively charged nanoparticles on the edges. (6) Removal of the covering used in step 3. If the coverage layer used in step 3 is removed, a linelike array of nanoparticles can be obtained.
This surface-guided self-assembly technique provides the opportunity to modify the edges of micro/nanopatterns with any shape and size. Relief patterns on the substrate serve as a template to direct and control the nucleation and growth of the nanoparticles. Herein, we give a drawing to illustrate the general process (Figure 6). All these micro/nanostructures may have use in applications such as sensors, chips, and substrates for surfaceenhanced Raman scattering.
Conclusions In summary, a surface-guided self-assembly technique has been demonstrated by constructing site-selective centers on the edges of a micro/nanopatterns. The NSL technique was employed, and selective absorption was obtained successfully. The edges of the pattern can serve as the nucleation sites for forming compound necklace structures. This experiment tested the validity
of the surface-guided self-assembly technique as a general method. To our knowledge, this is the first report of the surface-guided self-assembly method on the edges of covered polyelectrolyte films. Acknowledgment. This work was supported by the National Natural Science Foundation (Grant Nos. 20473029, 20573041, 20773044) of P. R. China, Program for Changjiang Scholars and Innovative Research Team in University (IRT0422), Program for New Century Excellent Talents in University, Scientific Research Foundation for the Returned Overseas Chinese Scholars initiated by State Education Ministry, and the 111 project (B06009). This work was conducted while J.R.L. was a visiting professor at Jilin University. LA8011537
