Site-Selective Assemblies of Gold Nanoparticles on an AFM Tip

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Langmuir 2003, 19, 166-171

Site-Selective Assemblies of Gold Nanoparticles on an AFM Tip-Defined Silicon Template Qiguang Li, Jiwen Zheng, and Zhongfan Liu* College of Chemistry and Molecular Engineering, Center for Nanoscale Science and Technology (CNST), Peking University, Beijing 100871, China Received May 6, 2002. In Final Form: August 13, 2002 This paper provides a convenient method for fabricating highly controlled arrays of gold nanoparticles on silicon by combining AFM nanolithography and self-assembly techniques. Our main methodology is to fabricate nanopatterns on silicon, covered with amino group and methyl group terminated self-assembled monolayers (SAMs), at different surface locations. These nanopatterns were then used to guide the assembly of gold nanoparticles according to their different affinities for amino and methyl groups. AFM was employed to induce a localized degradation of the methyl SAM on the silicon and therefore to control the amino SAM-covering area for nanoparticle adsorption, where the amino SAM was formed on the degraded regions by the conventional self-assembly technique. Gold nanoparticles (15 nm in diameter) selectively assembled at the AFM tip-defined amino regions, forming precisely positioned nanoparticle arrays. Such kinds of highly ordered nanoparticle assemblies are believed to contribute to the studies of nanoelectronic devices and mesoscopic phenomena.

Introduction Nanostructuring using colloidal nanoparticles is one of several promising approaches for the fabrication of nanoelectronic devices. Since the first report of Alivisatos,1 who used a bifunctional self-assembled monolayer (SAM) as molecular linker to immobilize colloidal CdS nanoparticles onto a metal surface to create two-dimensional nanoparticle assemblies, much work has been done to make position-controlled assemblies of nanoparticles on solid surfaces,2-9 which is essential for nanodevice studies. For instance, Ahmed and his colleagues2 fabricated a patterned assembly of gold nanoparticles by the combined use of electron beam lithography and SAM-assisted nanoparticle assembly. Photolithography3 and microcontact printing4,5 techniques have also been used, which led to patterned nanoparticle assemblies on a micrometer scale. A precise positioning of gold nanoparticles on a surface was demonstrated by Samuelson et al.6 using atomic force microscopy (AFM). Resch et al.7 have developed probe control software and constructed various patterns of nanoparticles using dynamic force microscopy. Important progress was made by Sugimura and his colleagues,8 who employed AFM to locally oxidize a silicon surface and constructed nanoparticle patterns. Following a similar methodology, we succeeded in creating quasione-dimensional arrays of gold nanoparticles on silicon.9 In this work, we demonstrate the combined use of AFM nanolithography and self-assembly techniques for real* To whom correspondence should be addressed. E-mail: lzf@ chem.pku.edu.cn. Telephone and fax: (86)-10-6275-7157. (1) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (2) Sato, T.; Hasko, D. G.; Ahmed, H. J. Vac. Sci. Technol., B 1997, 15, 45. (3) Liu, J. F.; Zhang, L. G.; Mao, P. S.; Chen, D. Y.; Gu, N.; Ren, J. Y.; Wu, Y. P.; Lu, Z. H. Chem. Lett. 1997, 11, 1147. (4) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375. (5) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (6) Junno, T.; Carlsson, S. B.; Xu, H.; Montelius, L.; Samuelson, L. Appl. Phys. Lett. 1998, 72, 548. (7) Resch, R.; Bugacov, A.; Baur, C.; Koel, B. E.; Madhukar, A.; Requicha, A. A. G.; Will, P. Appl. Phys. A 1998, 67, 265. (8) Sugimura, H.; Nakagiri, N. J. Am. Chem. Soc. 1997, 119, 9226. (9) Zheng, J. W.; Zhu, Z. H.; Chen, H. F.; Liu, Z. F. Langmuir 2000, 16, 4409.

izing positioning of gold nanoparticles on a silicon surface. The nanotemplate was fabricated by AFM tip-localized degradation and subsequent SAM deposition on the silicon surface, for guiding the nanoparticles to assemble on the desired surface regions. We have created various gold nanoparticle arrays on silicon, with the interparticle spacing being precisely controlled on the nanometer scale. Materials and Method 1. Materials. Octadecyltrichlorosilane (Cl3Si(CH2)17CH3, OTS, 95%) and HAuCl4‚3H2O (>98%) were purchased from Aldrich. Aminopropyltrimethoxysilane ((CH3O)3Si(CH2)2CH2NH2, APTMS, 97%) was purchased from PCR. Phenyltrichlorosilane (C6H5SiCl3, PTS) and 3-chloropropyltrichlorosilane (ClCH2CH2CH2SiCl3, CPTS) were purchased from ShinEtsu (Japan). The other reagents were of analytical grade. All reagents were used as received. Ultrapure water with a resistance of 17 MΩ‚cm was used throughout the experiments. 2. Preparation of Gold Nanoparticles. Colloidal gold nanoparticles with a diameter of 15 nm were synthesized by the Frens method.10 The pH value of the nanoparticle suspension was ∼6.5. 3. Preparation of OTS, PTS, and CPTS SAMs. The p-type Si(111) substrate, with a resistance of 0.015 MΩ‚cm, was cleaned in 90 °C Piranha solution (H2SO4/H2O2 ) 7:3, v/v), ultrasonicated in ultrapure water, and blown with dry N2. The cleaned substrate was immersed in a 1 mM OTS/hexadecane solution for 15 min for making an OTS self-assembled monolayer (SAM), followed by rinsing with CCl4, ethanol, and ultrapure water successively and finally blowing with dry N2. The OTS-coated silicon was immersed in a 1 mM PTS or CPTS solution of ethanol for 1 h, ultrasonicated in ethanol, and blown with dry N2. Before use, the sample was baked at 110 °C for 30 min and cleaned with ethanol and ultrapure water, respectively. 4. AFM Nanodegradation. AFM nanodegradation was performed with a Multimode Nanoscope III SPM (Digital Instruments, USA) in contact mode using a TiN-coated heavily doped silicon tip (NTMDT, Russia). The SAM-modified silicon substrate was subjected to a programmed voltage pulse (6-12 V, 10-500 ms) for nanodegradation via the internal Analog 2 signal line from the Nanocope controller. 5. Assembly of APTMS Monolayer and Nanoparticles. The nanodegraded sample was ultrasonicated in CCl4, ethanol, and ultrapure water for 15, 2, and 2 min, respectively, to remove the residual OTS molecules in the degraded regions. It was then (10) Frens, G. Nature Phys. Sci. 1973, 241, 20.

10.1021/la0259149 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/02/2002

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Langmuir, Vol. 19, No. 1, 2003 167 Scheme 1. Experimental Procedure for Fabricating Site-Selective Gold Nanoparticle Assemblies

Figure 1. Tapping AFM height images (range: 30 nm) of (a) APTMS-modified silicon and (b) OTS-modified silicon after immersion in a 15 nm gold nanoparticle suspension for 30 min. immersed in a 1 mM APTMS/ethanol solution for 25 min, followed by ultrasonication in ethanol and ultrapure water for 10 and 5 min, respectively. Finally, the sample was immersed into a gold nanoparticle suspension for 30 min. After cleaning with ultrapure water, it was characterized by tapping mode AFM.

Results and Discussion 1. Methodology for Site-Selective Nanoparticle Assembly. Our previous work has shown that colloidal gold nanoparticles can be assembled onto solid surfaces terminated with thiol,5 amino,5,11,12 or pyridyl groups,13 forming submonolayer nanoparticle assemblies. Figure 1a shows the tapping mode AFM image of a typical gold nanoparticle (15 nm in diameter) assembly on an APTMSmodified silicon surface. The gold nanoparticles were bound to the amino surface via electrostatic interaction.11 In contrast, such an assembly of gold nanoparticles cannot be formed on a methyl group terminated surface, as seen in Figure 1b, because of the lack of specific affinity between the gold and the surface. The different affinities of gold nanoparticles for amino (or thiol) groups and methyl groups create the basis for precisely controlling their assembly on solid surfaces. Scheme 1 shows the experimental strategy for realizing site-selective assembly of gold nanoparticles on silicon. First, the silicon substrate is modified with a densely (11) Zhu, T.; Fu, X. Y.; Mu, T.; Wang, J.; Liu, Z. F. Langmuir 1999, 15, 5197. (12) Wang, J.; Zhu, T.; Song, J. Q.; Liu, Z. F. Thin Solid Films 1998, 327-329, 591. (13) Zhu, T.; Zhang, X.; Wang, J.; Fu, X. Y.; Liu, Z. F. Thin Solid Films 1998, 327-329, 595.

packed OTS monolayer,14,15 which leads to a methyl group terminated surface. Second, the OTS monolayer is degraded with AFM by applying a programmed voltage pulse between the conductive AFM tip and the substrate at a controlled humidity. This process is followed by ultrasonication in CCl4, ethanol, and ultrapure water to remove the degraded OTS molecules, giving a nanosized surface region consisting of silicon oxide. Third, the exposed silicon oxide region is further modified with an APTMS monolayer, generating an amino group terminated surface. Finally, this prepared template is immersed into the suspension of gold nanoparticles for site-selective assembling. By controlling the size of the AFM-degraded area, we can realize the precise positioning of gold nanoparticles on the silicon surface, giving truly ordered nanoparticle arrays. 2. AFM Tip-Defined Template. The key for the siteselective nanoparticle assembly is to fabricate the guiding template. By applying a programmed voltage pulse between the AFM tip and the OTS-coated silicon, the OTS was degraded and the underlying silicon was oxidized into silicon oxide.16 This nanodegradation process can be explained with an electrochemical mechanism.16 The AFM tip, the silicon substrate, and the condensed water layer created a nanometer-sized electrochemical cell. Given enough voltage between the AFM tip and the substrate, an electrochemical reaction took place and H+ was generated by electrolysis of H2O, which was followed by a H+-catalyzed hydrolysis of the Si-O linkage, leading to the cleavage of Si-O bonds between the OTS monolayer and the silicon substrate. As a result, a hydroxyl group terminated region was formed on the silicon surface after removing the degraded OTS molecules by ultrasonication in CCl4, ethanol, and ultrapure water. Figure 2 shows the silicon oxide dot array created in such a way. The height of the oxide dot is ∼1.5 nm (see Figure 2a), resulting from the volume expansion from silicon (12.0 cm3/mol) to silica (19.4 cm3/mol).17 The AFM friction force image (Figure 2b) shows a remarkable difference between the oxidized (14) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (15) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (16) Sugimura, H.; Nakagiri, N. Langmuir 1995, 11, 3623.

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Figure 2. AFM images of the oxide dots array fabricated on an OTS-modified silicon surface by the AFM nanodegradation technique: (a) height image; (b) friction image. Voltage, 9 V; pulse duration, 50 ms; humidity, 55%.

region and the unoxidized region, suggesting the different chemical identities of surface groups. The higher friction force region corresponds to the hydrophilic hydroxyl groups while the lower friction force region corresponds to the hydrophobic methyl groups. The basic strategy for precise positioning of nanoparticles on the silicon surface is to confine the nanoparticles within the AFM tip-oxidized region. So the size of the oxidized area is a critical parameter. The smaller the oxidized area, the more precisely the position of the nanoparticle is controlled. When the oxidized area is so small that it can only accommodate single nanoparticle adsorption, the one-by-one positioning of gold nanoparticles on silicon can be achieved. Therefore, the first critical step is to make the oxide dots small enough. Since Dagata and his colleagues18 first explored STM to induce nanometer scale oxidation on silicon, several groups8,9,19-21 have devoted effort to such nano-oxidation techniques using both STM and AFM. Basically, the diameter and height of the oxide dot on hydrogen-passivated silicon are dependent on the humidity, the applied voltage, and the pulse duration.20 Garcia et al.21 reported a 10 nm oxide dot array made on a hydrogen-passivated silicon surface. Here we studied the dependence of oxide dot size on the pulse voltage, pulse duration, and the surrounding humidity on OTS monolayer-modified silicon. To our knowledge, no detailed study has been done on this system. Figure 3 shows the result. Compared with the AFM nanooxidation on hydrogen-passivated silicon, the size of the oxide dots is less dependent on humidity. As seen in Figure 3b, despite the change of humidity in a large range, the diameter of the oxide dots increased only from 31 to 40 nm. This is because the methyl surface is more hydrophobic than the hydrogen-terminated surface and thus it’s harder to form the condensed water column between the tip and the surface. The dependence of oxide dot size on the pulse voltage and duration was similar to that on hydrogen passivated silicon, though the voltage threshold was slightly larger under similar conditions. By optimizing the experimental parameters, we have obtained an oxide dot array with a dot diameter of as small as 15 nm (Figure (17) Sugimura, H.; Nakagiri, N. Jpn. J. Appl. Phys. 1995, 34, 3406. (18) Dagata, J. A.; Schneir, J.; Harary, H. H.; Evans, C. J.; Postek, M. T.; Bennett, J. Appl. Phys. Lett. 1990, 56, 2001. (19) Snow, E. S.; Campell, P. M. Appl. Phys. Lett. 1994, 64, 1932. (20) Fontaine, P. A.; Dubois, E.; Stievenard, D. J. Appl. Phys. 1998, 84, 1776. (21) Garcia, R.; Calleja, M.; Rohrer, H. J. Appl. Phys. 1999, 86, 1898.

4), where the dot diameter data were from the half-height width of the AFM image. After AFM nanodegradation treatment, the sample was immersed into an APTMS solution for modifying the degraded dot areas with an amino group terminated APTMS monolayer. It is found that the APTMS modification occurred not only at the dot areas but also at the pinhole areas of the OTS monolayer. This can be seen from the illustrative experimental result shown in Figure 5a. In this experiment, the silicon substrate was first modified with an OTS monolayer and then immersed into an APTMS solution for 25 min, followed by ultrasonication in ethanol and water, and finally immersed into a gold nanoparticle suspension for 30 min. We still could find many gold nanoparticles on the surface. Considering the strong bonding strength of the Si-O linkage, it is unlikely that the OTS molecules were replaced by APTMS molecules. So the reasonable explanation is that the OTS monolayer has many pinholes and the nanoparticleattracting APTMS molecules have been deposited on the pinhole areas as illustrated in Figure 5b. To realize the site-selective assembly of gold nanoparticles, we needed to find an effective way to remove the bad effect of the pinholes existing in the OTS monolayer. Here the small silane molecules, PTS or CPTS, were chosen to prevent APTMS assembly in the pinholes by filling up the pinholes, as illustrated in Figure 6a. The reason that we chose small molecules other than the original OTS molecules for secondary assembly is that small molecules are more likely to have access to the small pinholes than the big ones. Figure 6b demonstrates the feasibility of the “pinhole-filling” treatment, where the only experimental difference from Figure 5a is that the OTS monolayer was immersed into CPTS solution for 1 h before APTMS and nanoparticle adsorption. No gold nanoparticles were observed on the surface at all, indicating that the simple “pinhole filling” treatment is effective to fabricate a nanoparticle-inert surface. The 2-3 nm high bumps on the picture are believed to be due to the polymerization of the CPTS molecules, which put no effects on our following experiments. PTS molecules were found to have a similar “pinhole-filling” function. In the following, all the AFM tip-defined template was fabricated with this “pinhole-filling” treatment of the OTS monolayer. 3. Site-Selective Assembly of Gold Nanoparticles. Figure 7 shows the tapping mode AFM image of gold nanoparticles assembled on the above-fabricated AFM tipdefined template. The diameters of the oxide dots in the

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Figure 4. AFM height image (range: 5 nm) of the 5 × 5 oxide dot array with a dot diameter of 15 nm and an interspacing of 40 nm.

Figure 3. Relationship between oxide dot diameter and (a) voltage, (b) humidity, and (c) pulse duration.

template and the gold nanoparticles were ∼100 and 15 nm, respectively. The gold nanoparticles were found to be deposited only on the AFM-degraded areas, forming a highly regular 3 × 3 array on the silicon surface with an interspacing of ∼400 nm. Control experiments were conducted on the SiO2 surface to eliminate the possibility of nanoparticle deposition on the SiO2 surface. No gold nanoparticles were found after dipping the SiO2 substrate into the suspension of gold nanoparticles for 30 min. So the nanoparticle deposition on the oxidized area was due to the existence of APTMS, which, in reverse, proved the successful assembly of APTMS on the oxidized area.

Figure 5. (a) Tapping mode AFM height image (range: 30 nm) of the OTS-modified silicon surface after immersion into an APTMS solution for 25 min and further into a gold nanoparticle suspension for 30 min. (b) Schematic illustration of nanoparticle adsorption on the APTMS-filled pinhole sites.

The colloidal gold nanoparticles were negatively charged due to the adsorption of anions including citrate, chloride, gold chloride (AuCl4-), and hydroxide.22,23 The pH value of the colloidal suspension was ∼6.5, which is smaller than the pKa of the amino groups of the APTMS monolayer (∼7.524). Therefore, the amino groups on the silicon surface were positively charged when immersed into the colloidal (22) Horanyi, G.; Rizmayer, E. M.; Joo, P. J. Electroanal. Chem. Interfacial Electrochem. 1983, 152, 211. (23) Sandroff, C. J.; Herschbach, D. R. Langmuir 1985, 1, 131. (24) Zhang, H.; He, H. X.; Wang, J.; Mu, T.; Liu, Z. F.; et al. Appl. Phys. A 1998, 66, S269.

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Figure 6. (a) Schematic illustration of the “pinhole filling” treatment of an OTS monolayer by CPTS molecules. (b) Tapping mode AFM height image (range: 30 nm) of the OTS-modified silicon surface after immersion into a CPTS solution for 1 h, an APTMS solution for 25 min, and a gold nanoparticle suspension for 30 min in sequence.

Figure 7. Site-selective assembly of 15 nm gold nanoparticles on an AFM tip-defined silicon template (tapping mode AFM image; oxide dot diameter, 100 nm; height range, 30 nm).

suspension. The electrostatic attraction between the positively charged amino groups and the negatively charged gold nanoparticles was the driving force of nanoparticle adsorption, which led to the site-selective assembly of gold nanoparticles on the AFM tip-defined amino-terminated regions. From Figure 7, we found three to five nanoparticles on each AFM tip-defined dot area, suggesting that the dot area defined by the AFM tip was too large as compared to the nanoparticle size. We need to further reduce the oxidized dot size to realize one-by-one positioning of gold nanoparticles on silicon. There exists an electrostatic repulsion between the negatively charged gold nanoparticles on the silicon surface, which resulted in the

Figure 8. (a) 15 nm gold nanoparticle array on the AFM tipdefined assembling template. (b) Original oxide dot array fabricated by AFM nanodegradation of OTS-covered silicon, in which the oxide dot diameter was ∼70 nm. The section analysis data are also given for both images (tapping mode AFM image in height mode).

extremely low surface coverage even at the saturated adsorption state.11,12 The average interparticle spacing at saturated adsorption was ∼35 nm for 15 nm gold nanoparticles. This suggests that, when the diameter of the oxidized dot is less than 50 nm (35 + 15 nm), no more than one nanoparticle can be assembled onto the dot area because of the electrostatic repulsion of the subsequent nanoparticles. This is the simple estimate of the critical size of the oxidized dot for obtaining one-by-one positioning of gold nanoparticles. Figure 8a shows the one-by-one gold nanoparticle array on the AFM tip-defined silicon template. For comparison, the original oxide dot array fabricated by AFM nanodegradation is given in Figure 8b, where the average dot diameter is ∼70 nm. The nanoparticles in this case formed

Site-Selective Assemblies of Gold Nanoparticles

a very regular array, perfectly guided by the oxide dot array, with one oxide dot attracting only one nanoparticle. The remarkable change of dot height before and after nanoparticle assembling from the AFM section analysis clearly indicated the existence of nanoparticles on the oxide dot area. The height of the original oxide dots was ∼2.5 nm, which increased to 18.6 nm after nanoparticle adsorption. This corresponds to a nanoparticle having a diameter of ∼15.6 nm if the APTMS monolayer thickness is estimated to be ∼0.5 nm, consistent with the nanoparticle size used in the experiment taking account of the size distribution. This result demonstrates the successful control of the gold nanoparticle position on the silicon surface with nanometer scale precision by using the AFM tip-defined assembly guiding template. It seems that there is a discrepancy between the estimated critical oxide dot size (50 nm) and the actual dot size (70 nm) for obtaining a one-by-one nanoparticle assembly. This may arise from the following uncertainties. First, the oxide dot diameter was obtained from an AFM image, which may involve the tip convolution effect, leading to the increase of the observed feature size. Second, the tip-induced degradation may not be perfect and uniformly occur in the AFM nanodegradation process. The degradation took place first from the central region under the AFM tip and then expanded to the surroundings gradually. From center to edge, the completeness of OTS degradation may decrease, which may affect the quality of the subsequent assembled APTMS monolayer. This uncertainty may finally affect the deposition of gold nanoparticles. Moreover, the distributions of nanoparticle size and interspacing at saturated adsorption also make it difficult to estimate the exact critical size. By any means, the above result has shown that, with the assembly guiding template having a small enough feature size, we can realize the one-by-one site-selective assembly of gold nanoparticles on a silicon surface. 4. Assembling Efficiency of Nanoparticles on the Guiding Template. The adsorption probability of gold nanoparticles on the oxide dots is found to be not always 100%. For example, in Figure 8a, nanoparticles were found only on 70% of the oxide dots. In other words, the nanoparticles could not be assembled onto some oxide dots. The assembling efficiency showed a gradual decrease when the oxide dots were made smaller. In Figure 9 is shown the case when the oxide dot diameter was 28 nm, in which only 30% of the oxide dots were covered with gold nanoparticles. On the other hand, as shown in Figure 7, the assembling efficiency reached 100% when the oxide dot diameter was as large as 100 nm. A series of experimental results show that the smaller the oxide dots, the more difficult it is for the nanoparticles to adsorb. It is generally believed that there exists a minimum contact area for nanoparticles to assemble on the aminoterminated surface. In our experiments, though the geometrical area of the oxide dots was much larger than the nanoparticle size, the effective area to adsorb the nanoparticle was smaller than the geometrical area and, possibly, was not always bigger than the minimum contact area. The difference between the geometrical area and the effective area was due to the imperfectness of the AFM degradation process. Milder operation conditions were used to obtain smaller oxide dots in the AFM degradation process, which may result in the incompleteness of the OTS degradation. The smaller the oxide dots, the larger the portion of OTS remaining on the oxide dots and the fewer APTMS molecules assembling on the oxide dots. This may result in the low assembling efficiency on the smaller oxide dots. The nonplanar shape of the oxide dots may also increase the difficulty to stabilize the nano-

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Figure 9. Taping mode AFM height images of (a) an oxide dot array with a dot diameter of 28 nm (range: 10 nm) and (b) the corresponding gold nanoparticle array (range: 30 nm).

particle on the surface. We also found a slight height increase of the oxide dots without adsorbing gold nanoparticles after dipping into the gold nanoparticle suspension. This indicated that adsorption of the unfavorable species may have occurred on some oxide dots. Further studies are underway to clarify the actual reason for the low assembling efficiency. Conclusion By fabricating an assembly-guiding template using AFM nanolithography and self-assembly techniques, we have realized the site-selective assembly of gold nanoparticles on a silicon surface, with the position of the nanoparticles being precisely controlled on the nanometer scale. The size of the AFM degraded oxide dots and the quality of the OTS monolayer were found to be the key factors for realizing one-by-one positioning of nanoparticles on a surface. By controlling the oxide dot size and filling up the pinholes in the OTS monolayer with small silane molecules, we have successfully fabricated various highly aligned nanoparticle arrays on silicon. Such kinds of highly aligned nanoparticle arrays may find various applications, including fabrication of single-electron tunneling (SET) devices, investigation of the surface enhanced Raman scattering (SERS) mechanism, and so forth, which are being studied in this laboratory. Acknowledgment. The authors gratefully acknowledge the financial support from Ministry of Science and Technology of China, 973 program (001CB6105) and the National Science Foundation of China (29973001, 599101061982). Q.L. thanks Mr. B. J. Murray for correcting the grammatical errors. LA0259149