Combining Optical Lithography with Rapid Microwave Heating for the

306, Yuanpei Street, Hsinchu 300, Taiwan. Received September 27, 2004. In Final Form: December 6, 2004. We demonstrate a novel approach for the ...
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Langmuir 2005, 21, 2519-2525

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Combining Optical Lithography with Rapid Microwave Heating for the Selective Growth of Au/Ag Bimetallic Core/Shell Structures on Patterned Silicon Wafers Fu-Ken Liu,*,† Pei-Wen Huang,‡ Yu-Cheng Chang,§ Fu-Hsiang Ko,† and Tieh-Chi Chu| National Nano Device Laboratory, 1001-1 Ta Hsueh Road, Hsinchu 300, Taiwan, Department of Nuclear Science and Department of Materials Science and Engineering, National Tsing Hua University, 101, Section 2 Kuang Fu Road, Hsinchu 300, Taiwan, and Department of Radiological Technology, Yuanpei University of Science and Technology, No. 306, Yuanpei Street, Hsinchu 300, Taiwan Received September 27, 2004. In Final Form: December 6, 2004 We demonstrate a novel approach for the production of patterned films of nanometer-sized Au/Ag bimetallic core/shell nanoparticles (NPs) on silicon wafers. In this approach, we first self-assembled monodisperse Au NPs, through specific Au‚‚‚NH2 interactions, onto a silicon substrate whose surface had been modified with a pattern of 3-aminopropyltrimethoxysilane (APTMS) groups to form a sandwich structure having the form Au NPs/APTMS/SiO2. These Au NPs then served as seeds for growing the Au/Ag bimetallic core/shell NPs: we reduced silver ions to Ag metal on the surface of Au seeds under rapid microwave heating in the presence of sodium citrate. Energy-dispersive X-ray analysis confirmed that the Au/Ag bimetallic core/shell NPs grew selectively on the regions of the surface of the silicon wafer that had been patterned with the Au seeds. Scanning electron microscopy images revealed that we could synthesize well-scattered, high-density (>82%) thin films of Au/Ag bimetallic core/shell NPs through the use of this novel strategy. The patterned structures that can be formed are simple to produce, easily controllable, and highly reproducible; we believe that this approach will be useful for further studies of nanodevices and their properties.

Introduction The ability to fabricate materials and structures that have submicrometer-scale features is critical to many areas of science and technology.1,2 Many applications in nanotechnology require the ability to arrange nanocrystals into larger-scale patterns with precise lateral control. Recently, great effort has been expended toward realizing the position-controlled assembly of colloidal NPs onto solid substrates.3,4 For instance, Ahmed and co-workers reported the construction of patterns of Au NPs through a combination of electron beam lithography and colloid assembly techniques.3 Liu and co-workers employed atomic force microscopy (AFM) to locally oxidize a silicon surface and to perform spatially selective deposition of Au NPs through specific chemical interactions.4 Interest in this area has arisen because these nanostructured materials often exhibit propertiessfor example, quantumconfined size-tunable luminescence behavior,5 Coulomb staircase effects,6 and photonic band gap responses7sthat * To whom correspondence should be addressed. Fax: 886-35713403. E-mail: [email protected]. † National Nano Device Laboratory. ‡ Department of Nuclear Science, National Tsing Hua University. § Department of Materials Science and Engineering, National Tsing Hua University. | Yuanpei University of Science and Technology. (1) Novak, J. P.; Feldheim, D. L. J. Am. Chem. Soc. 2000, 122, 3979. (2) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (3) Sato, T.; Ahmed, H.; Brown, D.; Johnson, B. F. G. J. Appl. Phys. 1997, 82, 696. (4) Zheng, J.; Zhu, Z.; Chen, H.; Liu, Z. Langmuir 2000, 16, 4409. (5) Alivisatos, A. P. Science 1996, 271, 933. (6) Ingram, R. S.; Hostetler, M. J.; Murray, R. W.; Schaaff, T. G.; Kohourt, J. T.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279.

are remarkably different from those exhibited by the corresponding materials that have macroscopic dimensions. It is well-known that Ag NPs have superior properties, relative to other metal NPs, when considering their electrical conductivities,8 antimicrobial effects,9 optical properties,10 and applications in oxidative catalysis.11 For example, surface-enhanced Raman scattering (SERS), which occurs on roughened metal NPs, is a powerful tool for microanalysis because it provides vibrational information on local areas and, as such, it has the potential for applications in bio-sensing.12 The enhancement factor of SERS when using Ag NPs has been estimated experimentally13,14 to be 106-1015. It is expected that the SERS technique, which has extraordinary sensitivity, may become a practical technique for the detection of single molecules.13 Although Ag nanostructures and solid support-based substrates have been used widely for SERS measurements, to the best of our knowledge no effective methods have been reported for the fabrication of “wellcontrolled” or “patterned” Ag nanostructures through the use of Ag NPs as structural elements. In particular, it is curious that, despite the increased efforts to effect the position-controlled assembly of Au NPs onto solid sub(7) Charlton, M. D. B.; Parker, G. J. J. Micromech. Microeng. 1998, 8, 172. (8) Chang, L. T.; Yen, C. C. J. Appl. Polym. Sci. 1995, 55, 371. (9) Feng, Q. L.; Cui, F. Z.; Kin, T. N. J. Mater. Sci. Lett. 1999, 18, 559. (10) Fritzsche, W.; Porwol, H.; Wiegand, A.; Bornmann, S.; Kohler, J. Nanostruct. Mater. 1998, 10, 89. (11) Shiraishi, Y.; Toshima, N. Colloids Surf., A 2000, 169, 59. (12) Cao, Y. C.; Jin, R.; Mirkin, C. A. Science 2002, 297, 1536. (13) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (14) Seitz, O.; Chehimi, M. M.; Cabet-Deliry, E.; Truong, S.; Felidj, N.; Perruchot, C.; Greaves, S. J.; Watts, J. F. Colloids Surf., A 2003, 218, 225.

10.1021/la047611f CCC: $30.25 © 2005 American Chemical Society Published on Web 02/12/2005

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Liu et al. Scheme 1. Procedure for Fabricating Patterned Au NPs on the Silicon Wafer

Figure 1. SEM image of Au NPs prepared by heating an aqueous solution (100 mL) containing 1 mM HAuCl4 to its boiling point under vigorous magnetic stirring, injecting 35 mM sodium citrate solution (10 mL), and then heating the resulting solution under reflux for 30 min.

strates in recent years,3,4 it remains a great challenge to develop similarly effective methods for fabricating wellcontrolled Ag nanostructures. Currently, microwave (MW) heating is used as a highly efficient method for preparing colloids and it has many applications in colloid chemistry.15 The main advantages of MW-assisted reactions over conventional synthetic methods are that (a) the kinetics of the reaction are increased by 1-2 orders of magnitude, (b) the initial heating process is rapid, and (c) the MW irradiation induces the generation of localized high temperatures at reaction sites, which enhances reaction rates. Although free suspensions of Ag NPs may be prepared in aqueous solutions by using a highly efficient rapid MW heating approach,16 no studies have reported the selective growth of Ag nanostructures onto silicon wafers through the use of this method. Clearly, it is worthwhile to investigate the use of MW-mediated synthetic methods for the rapid preparation of patterned Ag NPs as a means to promote studies in colloidal physics and chemistry, especially those that combine nanotechnology with biotechnology, a subject that has received considerable attention recently.17 We believed that the fabrication of selectively patterned Ag NP films on silicon wafers would be feasible through the use of a combination of optical lithography and rapid MW heating. A previous article has demonstrated that it is possible to pattern Au NPs selectively onto a specified area through the use of lithographic approaches.3 Because Ag and Au NPs share the same crystal structure (i.e., face-centered cubic structures of space group Fm3m) and their lattice parameters match closely,18 we believed that it would be relatively easy to prepare a Au NP-based film and then introduce the Ag components onto that surface. Such an approach may be a very promising one for depositing Ag nanostructures onto micrometer-sized patterns through the intermediacy of Au seeds. Experimental Section Apparatus. Silicon oxide (SiO2) films were deposited onto silicon wafers by employing a conventional high-density-plasma chemical vapor deposition (HDP-CVD) system (Duratek Inc., (15) Komarneni, S.; Li, D.; Newalkar, B.; Katsuki, H.; Bhalla, A. S. Langmuir 2002, 18, 5959. (16) Yin, H. B.; Yamamoto, T.; Wada, Y.; Yanagida, S. Mater. Chem. Phys. 2004, 83, 66. (17) Haes, A. J.; Hall, W. P.; Chang, L.; Klein, W. L.; Van Duyne, R. P. Nano Lett. 2004, 4, 1029. (18) Chen, D.; Gao, L. J. Cryst. Growth 2004, 264, 216.

Scheme 2. Procedure for Fabricating Patterned Au/Ag Bimetallic Core/Shell NPs on the Silicon Wafer

Taiwan). An optical lithography (I-line i5+ stepper, Canon, Japan) was used for patterning the Au NPs. Au/Ag bimetallic core/shell NPs were synthesized within a MW heating system (MARS-5, CEM Corporation, Matthews, NC). The MW instrument was operated in the range from 0 to 100% of its full power (1200 W). To ensure high accuracy, an optical fiber was used to monitor and control the temperature through a feedback system. The temperature sensor provided accurate in-vessel readings for optimum control of reactions. Samples were synthesized at 100 °C over a desired period of time. After cooling to room temperature, the products were collected for further characterization. SEM (JEOL JSM-6500F, Tokyo, Japan) and TEM (JEOL JEM4000EX, Tokyo, Japan) systems were used to characterize the

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Figure 2. SEM images of structures patterned (a) in the absence and (b) in the presence of Au NPs. EDS spectra of structures patterned (c) in the absence of Au NPs (as prepared in part a) and (d) in the presence of Au NPs (as prepared in part b). morphology of the Au/Ag bimetallic core/shell NPs. The particle size was determined by two-dimensional-grain analysis after digitizing the photographic image obtained from the SEM and TEM. X-ray diffraction (XRD) was performed on a Philips X′Pert Pro X-ray diffractometer (Philips, The Netherlands) using Cu KR radiation (λ ) 0.154 nm). The XRD patterns were recorded from 30 to 138° (2θ) at a scanning step of 0.02°. Reagents. All chemical materials were of G.R. grade. Sodium citrate was purchased from Merck (Darmstadt, Germany). Hydrogen tetrachloroaurate (HAuCl4), 3-aminopropyltrimethoxysilane (APTMS), and silver nitrate (AgNO3) were purchased from Acros (Acros Organics, Geel, Belgium). THMR-ip3650 HP resists (TOK, Tokyo, Japan) were employed for I-line exposure. After exposure, the resists could be removed completely after developing with 2.38% tetramethylammonium hydroxide (TMAH, TOK, Tokyo, Japan). Deionized water (>18 MΩ‚cm-1) was used throughout the preparation of the NPs. Preparation of Au NPs. The first step of this work involved the formation of Au NPs by using citrate to reduce HAuCl4; the synthetic procedure is similar to that described in a previous

paper.19 Briefly, 1 mM HAuCl4 (100 mL) was brought to its boiling point under vigorous magnetic stirring and then 35 mM sodium citrate solution (10 mL) was injected. The resulting solution was heated under reflux for 30 min. The color of the solution changed from yellow to brownish-red upon the chemical reduction of HAuCl4 mediated by citrate; this color change indicates the successful synthesis of the Au NPs. An SEM image (Figure 1) indicates that the Au NPs we synthesized were spherically shaped; we estimated their particle diameters to be 17.8 ( 1.0 nm.

Results and Discussion Prior to the deposition of Au NPs, we deposited the SiO2 films (10 nm) on silicon wafers by employing an HDPCVD system. The silicon substrates were cleaned in a boiling piranha solution [a mixture of H2O2 and H2SO4 (19) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735.

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Figure 3. (a, b) SEM images of patterned Au/Ag bimetallic core/shell NPs. (c, d) EDS spectra recorded at points 1 and 2 of the sample presented in Figure 3b.

(3:7, v/v)] for 10 min and then rinsed with deionized water and ethanol. In this study, we coated an APTMS layer, an adhesion agent for Au NPs, onto the surface of the SiO2 film by immersing the substrate into an ethanol solution containing 5 mM APTMS and then heating it under reflux for 4 h. After rinsing with ethanol and deionized water, the substrates were dried under a N2 purge. Scheme 1 presents the procedure we used to pattern a film of Au NPs onto a silicon wafer. The resist was coated on the APTMS layer for I-line exposure (Scheme 1a), after which these resists can be removed completely by developing with 2.38% TMAH aqueous solution (Scheme 1b). The patterned structure was characterized using SEM (Figure 2a), which revealed that we had fabricated uniform patterns having line widths of about 500 nm. Next, we employed the spin coating strategy (Scheme 1c) to selfassemble the Au NPs onto the APTMS surface-modified

silicon wafer.20 After dipping the wafer into the Au NP solution and spinning out the waste solution using a spin coater speed of 1000 rpm, the substrate was immediately rinsed with deioniized water and dried under a N2 purge. We then cut the wafer into pieces and used ethanol to strip the residual resists from the surface (Scheme 1d). Figure 2b displays a SEM image of the structure patterned with Au NPs. This image reveals that the Au NPs were scattered well and had a deposition density of about 60% on the surface of the patterned APTMS-coated silicon substrate; the Au NPs/APTMS/SiO2 sandwich structures are stabilized through specific Au-NH2 interactions.20 We characterized the surface compositions of the products by using EDS. Figure 2c reveals that we observed no Au signal when the electron beam was focused on a (20) Liu, F. K.; Chang, Y. C.; Ko, F. H.; Chu, T. C.; Dai, B. T. Microelectron. Eng. 2003, 67-68, 702.

Growth of Au/Ag Bimetallic Core/Shell Structures

spot of the patterned structure not coated with Au NPs (as shown in Figure 2a). In contrast, when we focused the electron beam on a spot coated with Au NPs (as shown in Figure 2b), we observe (Figure 2d) peaks at 9.6 and 11.3 keV that are associated with Au. These results demonstrate that we had grown Au NPs onto selective areas of the silicon wafer. In the Au seed-assisted synthesis of Au/Ag bimetallic core/shell NPs, we placed the Au NP-patterned substrate (the same as that depicted in Scheme 1c, before stripping the residual resists) into a MW reaction chamber. As Scheme 2a indicates, we placed a substrate (2 cm × 2 cm) of preadsorbed Au NPs, having a line width of about 700 nm, into an aqueous solution (10 mL) of 1 mM AgNO3 and 3.5 mM sodium citrate within the MW heating chamber. The reaction temperature was controlled at 100 °C for a reaction time of 2.5 min. After rapid MW heating (Scheme 2b), we used ethanol to strip the residual resists from the surface (Scheme 2c) and obtained the patterned Au/Ag bimetallic core/shell NPs (Figure 3a). This SEM image reveals that the Au/Ag bimetallic core/shell NPs (43.6 ( 7.1 nm) were grown on the surface of the original Au NPpatterned structures; the surface coverage was about 82.6%. We also employed EDS to characterize the surface compositions of the thin films, in this case using a sample having a larger-dimension pattern (line width: ca. 7 µm; Figure 3b). Figure 3c reveals that when the electron beam was focused on the NP pattern (point 1 in Figure 3b), we observed the Ag signal clearly through the appearance of a peak at 3.0 keV. Beyond the NP-patterned region (point 2 in Figure 3b), we detected no Ag signal (Figure 3d). From the results presented in Figure 3, we confirm that the Ag nanostructures grew on the sites that were patterned with the Au NPs (point 1). To examine whether a core/shell structure was obtained in the microwave reaction, we measured a TEM image of the suspension materials that were prepared using a method similar to the one described in this paper. The preparation of the suspended NPs in this study involved two steps. The first step of this synthesis involved the formation of gold NPs by the reduction of HAuCl4 with citrate. Briefly, a solution of HAuCl4 (1 mM, 100 mL) was brought to a boil under vigorous magnetic stirring, and then sodium citrate solution (35 mM, 10 mL) was injected. The resulting solution was heated under reflux for 30 min. In the second step, a mixed aqueous media solution (10 mL) consisting of AgNO3 (1 mM), sodium citrate (3.5 mM), and gold seeds (0.2 mL) was reacted in a microwave heating system. The reaction temperature was controlled at 100 °C for 3 min. Finally, the product was centrifuged (10 000 rpm) once with the mother liquid and then a few times with water. The TEM image in Figure 4a clearly indicates that the microwave-prepared NPs have core/ shell structures. Both the TEM observations and the EDS measurements were conducted using an electron beam of 25-nm size. The results presented in Figure 4b,c clearly displays that the composition of each individual particle is that of a core that is populated with Au and a shell that is composed of Ag. On the basis of these findings, we conclude that the NPs we produced have core/shell Au/Ag structures. For further characterization of the structures of the patterned materials, we recorded the XRD pattern (Figure 5a) of the Au NP-modified film on the silicon wafer depicted in Figure 2b. We observe resolved diffraction peaks (2θ) at 38.10, 44.39, 64.58, 77.55, 81.72, 98.13, 110.80, 115.26, and 135.42°, which correspond to the (111), (200), (220), (311), (222), (400), (331), (420), and (422) planes of metallic

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Figure 4. (a) TEM image of the core/shell structure that was obtained from the microwave reaction. The scale bar has a length of 5 nm. EDS spectra recorded at the (b) core and (c) shell of the sample presented in part a.

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Figure 6. SEM image of the Ag NPs patterned in the absence of the Au NPs.

Figure 5. XRD patterns of the synthesized NPs: (a) Au NPs as prepared in Figure 2b; (b) Au/Ag bimetallic core/shell NPs as prepared in Figure 3b.

Au. All of the peaks in this X-ray diffraction pattern suggest that the sample can be readily indexed to facecentered cubic Au belonging to the Fm3m [225] space group (JCPDS file No. 04-0784). Figure 5b, which displays the XRD pattern of the product presented in Figure 3b, reveals that the same diffraction peaks observed in Figure 5a also appear resolved in this pattern. This finding suggests that the core/shell NPs synthesized under these conditions can be indexed to face-centered cubic Ag belonging also to the Fm3m [225] space group (JCPDS file No. 04-0783). The similar diffraction peaks presented in Figure 5a,b suggest that the Au and Ag nanostructures that we synthesized through this approach have the same crystal

structures. In the growth of Au/Ag bimetallic core/shell NPs, the Ag+ was added to allow the nucleation and growth of Au/Ag bimetallic core/shell on the Au NP surface. Ag can nucleate through two different paths: heterogeneous and homogeneous nucleation. In our present case, heterogeneous nucleation was probably the major path; that is, the pregrown Au NPs serve as nuclei for epitaxial growth of Ag, because we observed the Ag and Au particles to have the same crystal structures (face-centered cubic structures; space group: Fm3m) and their lattice parameters match closely. The “seeding” Au NPs tethered onto the silicon wafer play a crucial role during the fabrication of the Au/Ag bimetallic core/shell NPs. Figure 3a displays the typical SEM image of the gradual growth of Au/Ag bimetallic core/shell NPs on the silicon wafer; in this case, sodium citrate is a reductant. Such a reducing agent was a prerequisite for the growth of Au/Ag bimetallic core/shell NPs: only the Au seeds grew in size upon the reduction of silver nitrate in the solution; that is, no new nucleation centers were introduced, which ensured that a minimal number of Ag NPs in the solution accompanied the growth of Ag NPs on the silicon wafer. Figure 6 displays the composite particles we prepared in the absence of the Au seeds: fewer Ag NPs were located on the patterned silicon wafer. In addition, we detected many Ag NPs dispersed in the solution. These experiments confirm that the Au seeds are necessary for the formation of a film of Au/Ag bimetallic core/shell NPs: they provide nucleation sites for the growth of the Au/Ag bimetallic core/shell NPs. We tested the electrical continuity of these Ag lines by measuring the resistance of individual Ag lines at room temperature using the four-point probe method. The Au/ Ag bimetallic core/shell NPs of 44-nm diameter were patterned on the silicon oxide surface. We calculated the electrical conductivity to be 1.6 × 105 S/cm. This value indicates that the Au/Ag bimetallic core/shell nanostructures synthesized using the present approach are electrically continuous (the conductivity of bulk silver is 6.2 × 105 S/cm). Figure 7 presents SEM images of two samples that we removed from the mixture after the Au NP-patterned structure, silver ions, and citrate ions had reacted under MW heating for 2 and 10 min, respectively. These images indicate that the Au/Ag bimetallic core/shell nanostructures have different morphologies after different reaction times. When the silver ions underwent reduction by citrate, the initial products (t ) 2 min) in the solution were small spherical particles (Figure 7a). Upon increasing the

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the smaller Au/Ag bimetallic core/shell NPs, which have a larger surface activity, probably dissolved into the solution and grew onto the larger Au/Ag bimetallic core/ shell NPs through a process known as Ostwald ripening.21 We believe that this situation occurred in our system because, as the reaction time increased from 2 to 10 min, the corresponding size of the Au/Ag bimetallic core/shell NPs increased from 41.5 ( 5.7 to 103.9 ( 68.0 nm. Hence, the short process time for rapid MW heating is adequate for the preparation of the patterned Au/Ag bimetallic core/ shell NPs. In these preliminary studies, we have demonstrated the potential that MW heating providess through its ability to heat solutions quicklysto the effective and rapid preparation of Au/Ag bimetallic core/ shell NPs. Conclusions Through a combination of a reliable lithography patterning technology and rapid MW heating, we have demonstrated that Au/Ag bimetallic core/shell NPs can be grown selectively onto specific locations of a silicon wafer. Moreover, our preliminary studies suggest that the successful formation of patterned Au/Ag bimetallic core/shell NP films on silicon wafers is controlled mainly by two factors: (i) the use of patterned Au NPs as seeds and (ii) an adequate reaction time. Although we used a very simple pattern in this study, it is clear that the strategy we have demonstrated here can be extended readily to other patterns and to other materials. Our procedure may be useful for the design of optical devices by patterning figures. In addition, by incorporating particular functional groups that display high affinity for the desired biologically functional molecules, such as proteins, we may be able to generate patterns that recognize biomolecules; that is, this approach may have potential for application toward the fabrication of biosensors and molecular sensor arrays. Progress toward these goals is currently underway in our laboratory. Figure 7. SEM images of the Au/Ag bimetallic core/shell NPs obtained after MW reaction times of (a) 2 and (b) 10 min. MW operation conditions: applied energy, 1200 W; reaction temperature, 100 °C.

Acknowledgment. This work was supported by the National Applied Research Laboratories and the National Science Council, Taiwan (NSC 93-2113-M-492-003). LA047611F

reaction time to 10 min, the particles underwent an obvious change in size (Figure 7b). Under these reaction conditions,

(21) Roosen, A. R.; Carter, W. C. Physica A 1998, 261, 232.