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Nanopatterned Assembling of Colloidal Gold Nanoparticles on Silicon Jiwen Zheng, Zihua Zhu, Haifeng Chen, and Zhongfan Liu* Center for Nanoscale Science & Technology (CNST) and College of Chemistry & Molecular Engineering, Peking University, Beijing 100871, China Received October 12, 1999. In Final Form: February 2, 2000 A quasi-one-dimensional (quasi-1D) Au nanocolloids array has been fabricated on silicon by combining the techniques of atomic force microscopy (AFM)-based nanooxidation and chemical assembling of colloidal nanoparticles. The silicon substrate, modified with an octadecyltrichlorosilane (OTS) monolayer, was first subjected to a localized chemical oxidation by using conductive AFM to form silicon oxide lines. After further modification of the oxidized region with an aminopropyltriethoxylsilane (APTES) monolayer via selective chemical adsorption, the substrate was exposed to a colloidal suspension of gold for deposition of gold nanoparticles. It is found that the Au nanoparticles can be selectively immobilized onto the AFM tip-defined amino-terminating regions of the silicon surface, forming quasi-1D gold nanoparticle arrays. The patterned structure is highly controllable and reproducible, which, we believe, will contribute to studies of nanodevices and mesoscopic phenomena.
Introduction Nanostructuring using wet colloids chemistry has received increasing attention since the first report of Alivisatos,1 who used a bifunctional self-assembled monolayer (SAM) as the molecular linker to immobilize colloidal CdS nanoparticles onto metal surfaces to create twodimensional (2D) nanoparticle monolayers. Typical works along this line were carried out by Natan,2 Liu,3 Willner,4 and Burmeister5 for the purposes of creating surfaceenhanced Raman scattering (SERS)-active substrates, fabricating double-barrier tunneling junctions for singleelectron tunneling, preparing nanoelectrode arrays, and nanofabrication. Great efforts have also been made on position-controllable assembling of colloidal nanoparticles on solid substrates.6-9 For instance, Ahmed7 reported the combined use of electron beam lithography and the colloids assembling technique to construct patterns of gold nanoparticles. We exploited the microcontact printing technique to create patterned assemblies of nanocolloids.8 Important progress was made by Sugimura and his colleagues,9 who employed ultrahigh-resolution atomic force microscopy (AFM) to locally oxidize the silicon surface and to perform spatially selective deposition of gold nanoparticles via specific chemical interaction. Despite the increasing efforts in the past few years, it is still a great challenge to develop an effective way to fabricate well-controllable nanostructures using colloidal quantum dots as the structural elements. In this Letter, we demonstrate the fabrication of onedimensional arrays of gold nanoparticles on silicon by the combined use of AFM-based nanooxidation and colloid chemistry, following Sugimura’s approach. AFM was used here to draw oxide lines by localized oxidation of silicon. The selective immobilization of gold nanoparticles on the * To whom correspondence should be addressed. Telephone and Fax: +86-10-6275-7157. E-mail:
[email protected]. (1) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (2) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (3) Jiang, P.; Liu, Z. F. Appl. Phys. Lett. 1999, 75, 3023. (4) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (5) Burmeister, F.; Schafle, C.; Matthes, T.; Bohmisch, M.; Boneberg, J.; Leiderer, P. Langmuir 1997, 13, 2983.
Figure 1. Experimental procedures for fabricating onedimensional arrays of gold nanoparticles on silicon by the combined use of AFM nanooxidation and chemical assembling techniques.
10.1021/la991332o CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000
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Figure 2. 2 µm × 2 µm tapping mode AFM images of OTS-covered (a) and APTES-covered (b) silicon surfaces after soaking in aqueous gold colloids for 12 h, showing the different affinities of CH3 and NH2 groups to gold nanoparticles.
oxide lines was achieved by the specific electrostatic interaction of charged nanoparticles with the amine functions of the monolayer chemisorbed on the locally oxidized lines. By optimizing the experimental conditions of nanooxidation and nanoparticle assembling, we have obtained well-defined quasi-one-dimensional (quasi-1D) arrays of gold nanoparticles on silicon. Experimental Section Octadecyltrichlorosilane (OTS), aminopropyltriethoxylsilane (APTES), and HAuCl4‚3H2O were purchased from Aldrich and used as received. An n-type Si(111) wafer with a resistivity of 0.025 Ω‚cm was used as the substrate. Ultrapure water (>17 MΩ‚cm) was used throughout the experiments. Other reagents were of analytical grade. The silicon substrate was pretreated following a slightly modified RCA method:10 (1) immersion in piranha solution (H2SO4/H2O2 ) 7:3 v/v) at 90° for 10 min; (2) etching in a buffered HF solution (40% HF/40% NH4F ) 1:7 v/v) for 60 s; (3) immersion in NH3H2O/H2O2/H2O (1:1:5 v/v) at 80 °C for 10 min; (4) immersion in HCl/H2O2/H2O (1:1:6 v/v) at 80 °C for 10 min. Finally, the wafer was thoroughly rinsed with water and dried with highpurity nitrogen. This pretreatment made the silicon surface rich in hydroxyl groups, which were used to link the OTS molecules. The OTS self-assembled monolayer (SAM) was formed on silicon by immersing the silicon substrate in a 1 mM OTS solution of hexadecane/tetrachlorocarbon/chloroform (10:1:1.5 v/v) for 1015 min,11 followed by successive rinsing in chloroform and ethanol to remove the polymeric residuals. The monolayer surface becomes hydrophobic, showing a water contact angle of approximately 105°. Localized oxidation of the OTS-covered silicon substrate was performed with a Nanoscope III atomic force microscope (Digital Instruments, Santa Barbara, CA) in contact mode by applying a programmed voltage pulse between a heavily doped silicon tip and the substrate (tip negative). On the basis of an electro(6) Klein, D. L.; McEuen, P. L.; Katari, J. E. B.; Roth, R.; Alivisatos, A. P. Appl. Phys. Lett. 1996, 68, 2574. (7) Sato, T.; Ahmed, H.; Brown, D.; Johnson, B. F. G. J. Appl. Phys. 1997, 82, 696. (8) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir, in press. (9) Sugimura, H.; Nakagiri, N. J. Am. Chem. Soc. 1997, 119, 9226. (10) Kern, W. RCA Rev. 1970, 31, 187. (11) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577.
chemistry-like mechanism,12 the OTS layer underneath the AFM tip was degraded and the underlying silicon was oxidized. The surrounding relative humidity was approximately 58% throughout the nanooxidation experiments. The amino-terminating APTES monolayer was selectively formed on the lithographically created silicon oxide regions by soaking the substrate in a 1 mM APTES solution of ethanol for 40 min, followed by rinsing successively in ethanol and water and blowing with high-purity nitrogen. Finally, the sample was baked at 120 °C for 30 min to complete the Si-O bond formation.13 The aqueous gold colloids, with an average particle size of 12 ( 1 nm (confirmed by TEM), were synthesized by the method proposed by Frens.14 The pH value of the colloidal suspensions was about 6.1. The Au nanoparticles were assembled on the chemically patterned silicon surface by immersing the substrate into the Au colloids suspension for 12 h, as previously described.15 The substrate was thoroughly rinsed with ultrapure water immediately after it was taken out of the gold suspension, and blown dry with high-purity nitrogen. Scanning electron microscopy (SEM, AMRAY-1910FE) was used to characterize the obtained nanoparticles’ pattern structures.
Results and Discussion Figure 1 shows our basic strategy to fabricate 1D arrays of gold nanoparticles on silicon surfaces. The experimental procedure involves four successive steps: (1) to coat the silicon surface with an OTS self-assembled monolayer; (2) to draw oxide lines on the silicon by AFM localized chemical oxidation; (3) to modify the oxidized regions with an amino-terminating APTES self-assembled monolayer; (4) to deposit gold nanocolloids selectively onto the amino-terminated regions. The specific affinity of the amino-terminating surface to colloidal gold nanoparticles is clearly shown in Figure 2, which exhibits the tapping mode AFM images of OTScovered (Figure 2a) and APTES-covered (Figure 2b) silicon surfaces after soaking in aqueous gold colloids for 12 h. No nanoparticles were observed on the methyl-terminated (12) Sugimura, H.; Nakagiri, N. Langmuir 1995, 11, 3623. (13) Sato, T.; Brown, D.; Johnson, B. F. G. Chem. Commun. 1997, 1007. (14) Frens, G. Nature Phys. Sci. 1973, 241, 20. (15) Zhu, T.; Zhang, X.; Wang, J.; Fu, X. Y.; Liu, Z. F. Thin Solid Films 1998, 327/329, 595.
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Figure 3. 1 µm × 1 µm AFM topographic image (a) and friction force image (b) of the locally oxidized silicon surface, where the bright lines correspond to the oxidized regions. The images were collected concurrently, and the scanning direction was from left to right. Oxidation pulse height: 8 V. Scan rate: 4 µm/s.
silicon surface, while a monolayer of gold nanoparticles was clearly observed on the amino-functionalized silicon surface. The lateral size of Au nanoparticles obtained from the AFM image falls into the range 35-40 nm, remarkably larger than the value determined from the SEM image (average 12 ( 1 nm). This can be attributed to the magnification effect of the AFM tip.16 As proved in our previous studies,17 at pH 6.1, the gold colloids are dominantly negatively charged due to the adsorption of anions, while the amino groups are mainly positively charged. So the specific affinity here originates from the strong electrostatic attractions between the negatively charged gold nanocolloids and the positively charged amino groups. The localized oxidation of the OTS-covered silicon surface was performed by applying a programmed voltage pulse (oxidation pulse height, 8 V; scan rate, 4 µm/s) between the conductive AFM tip and the silicon substrate. Figure 3 shows the AFM topographic image (Figure 3a) and friction force image (Figure 3b) of the locally oxidized silicon surface, where the bright lines correspond to the oxidized regions. In the topographic image, the oxidized regions showed protrusion from other areas, while, in the friction force image, the oxidized regions showed higher friction force, indicative of the different chemical nature. Sugimura and his colleagues have studied the AFM-based nanooxidation of a trimethylsilyl (TMS)-modified silicon surface.12 They suggested that the TMS monolayer exposed to a high-voltage pulse is degraded and the underlying silicon is oxidized following an electrochemical mechanism. This explanation is believed to hold true in the present case. The degradation of the OTS layer and the formation of silicon oxide have been confirmed by the etching experiment shown in Figure 4. When the AFM-oxidized silicon sample was immersed into a buffered HF solution for 5 min, the high-voltage-exposed regions were etched out, forming a row of grooves. This strongly suggests that the degraded OTS layer cannot function as an effective (16) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. J. Anal. Chem. 1995, 67, 735. (17) Zhu, T.; Fu, X. Y.; Mo, T.; Wang, J.; Liu, Z. F. Langmuir 1999, 15, 5197.
Figure 4. AFM image of the grooves formed on OTS-covered silicon by localized nanooxidation and subsequent etching in buffered HF solution. It suggests that the degradation of the OTS layer and the formation of silicon oxide have occurred after AFM nanooxidation.
resist, leading to the dissolution of the formed silicon oxide into the buffered HF solution. With an optimized experimental condition, the line width and the interline spacing of the AFM-derived oxide patterns on silicon could be made as small as 39 and 45 nm, respectively. After localized nanooxidation treatment, the silicon substrate was immersed into a 1 mM APTES solution of ethanol for 40 min for depositing an APTES monolayer onto the oxidized regions. The evidence of APTES monolayer formation on the oxide lines was coming from the etching experiment. We did not find any etching effect when the APTES-treated sample was immersed into a buffered HF solution for 1 min, indicating that the oxidized regions have been protected by the formed APTES layer. Figure 5 shows the SEM image of the above chemically patterned silicon substrate after immersion into a
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Figure 5. SEM image of the quasi-one-dimensional arrays of 12 nm gold nanoparticles formed on the chemically patterned silicon surface. Oxidation pulse height: 8 V. Scan rate: 8 µm/s.
12 nm gold colloids suspension for 12 h, in which the original line width and interline spacing of the aminoterminating oxidized regions were approximately 65 and 130 nm, respectively. Obviously, deposition of gold nanoparticles was well restricted to the predefined amino group-terminating oxidized regions, forming quasi-onedimensional dot arrays on the silicon surface. The adhesion between gold nanocolloids and amino groups is electrostatic, as mentioned above. This indicates that one can chemically position the nanoparticles on the silicon surface in a well-controlled fashion by combining AFM nanooxidation and chemical assembling techniques. As seen in Figure 5, most of the nanoparticles are well separated from each other, having a typical interparticle distance of approximately 35-80 nm. Compared with the case of gold nanoparticles assembling on a flat amino-terminating surface (see Figure 2b), the packing disorder and coagulation were more serious, possibly originating from insufficient degradation of monolayer and/or imperfect APTES modification. We preliminarily investigated the influence of the oxidized line width on the packing status of gold
nanoparticles and found that, with a line width < 75 nm, the nanoparticles easily form an array on the AFM tipdefined amino-terminating regions of a silicon surface. In summary, we have demonstrated the combined use of AFM nanooxidation and wet colloids chemistry for fabricating well-controlled nanostructures with colloidal nanoparticles being the structural elements. We have succeeded in creating quasi-one-dimensional arrays of gold nanoparticles on a silicon surface with an optimized experimental condition of localized oxidation and chemical assembling. This study may provide an experimental approach to examining various electrical phenomena in low-dimensional nanostructures such as single-electron tunneling, electrical coupling between quantum dots, and so forth. Relevant studies are in progress in this laboratory. Acknowledgment. This work was supported by the State Educational Committee and National Natural Science Foundation of China. LA991332O
