3766
Langmuir 2004, 20, 3766-3768
Gold Nanoparticle Patterning of Silicon Wafers Using Chemical e-Beam Lithography Paula M. Mendes,†,⊥ Susanne Jacke,§,⊥ Kevin Critchley,‡ Jose Plaza,§ Yu Chen,§ Kirill Nikitin,| Richard E. Palmer,§ Jon A. Preece,*,† Stephen D. Evans,‡ and Donald Fitzmaurice| School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom, Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom, Department of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom, and Nanochemistry Group, Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland Received January 21, 2004 This paper demonstrates a novel facile method for fabrication of patterned arrays of gold nanoparticles on Si/SiO2 by combining electron beam lithography and self-assembly techniques. Our strategy is to use direct-write electron beam patterning to convert nitro functionality in self-assembled monolayers of 3-(4nitrophenoxy)-propyltrimethoxysilane to amino functionality, forming chemically well-defined surface architectures on the 100 nm scale. These nanopatterns are employed to guide the assembly of citratepassivated gold nanoparticles according to their different affinities for amino and nitro groups. This kind of nanoparticle assembly offers an attractive new option for nanoparticle patterning a silicon surface, as relevant, for example, to biosensors, electronics, and optical devices.
Introduction Colloidal metal nanoparticles have received much attention in recent years due to potential applications in areas of electronics,1,2 photonics,3 sensors,4 and catalysis.5 Monodisperse nanoparticles, for example, have been proposed as a basis for single electron transistors.1 In this and many other examples, potential applications require precise spatial control over nanoparticle assembly on Si/ SiO2 surfaces. One approach is to modify nanoparticles to recognize and bind selectively a specific region of a substrate.6 Another approach is to modify a specific region of a surface to recognize and bind selectively nanoparticles. Both approaches, despite increased effort and significant advances, still offer the opportunity for innovation.7 Herein, we present a promising new approach to fabricate nanopatterned arrays of gold nanoparticles on silicon by the combined use of bottom-up self-assembly techniques and top-down chemical electron beam (e-beam) * To whom correspondence should be addressed. Telephone: +44 (0) 12 1 414 3528. Telefax: +44 (0) 121 414 4403. E-mail:
[email protected]. † School of Chemistry, University of Birmingham. § School of Physics and Astronomy, University of Birmingham. ‡ Department of Physics and Astronomy, University of Leeds. | Department of Chemistry, University College of Dublin. ⊥ These authors contributed equally to this work. (1) Feldheim, D. L.; Keating, C. D. Chem. Soc. Rev. 1998, 27, 1. (2) Sato, T.; Hasko, D. G.; Ahmed, H. J. Vac. Sci. Technol., B 1997, 15, 45. (3) McConnell, W. P.; Novak, J. P.; Brousseau, L. C., III; Fuierer, R. R.; Tenent, R. C.; Feldheim, D. L. J. Phys. Chem. B 2000, 104, 8925. (4) Liu, T.; Tang, J.; Zhao, H.; Deng, Y.; Jiang, L. Langmuir 2002, 18, 5624. (5) Li, H.; Luk, Y.-Y.; Mrksich, M. Langmuir 1999, 15, 4957. (6) (a) Fitzmaurice, D.; Rao, S. N.; Preece, J. A.; Stoddart, J. F.; Wenger, S.; Zaccheroni, N. Angew. Chem., Int. Ed. Engl. 1999, 38, 1147. (b) Ryan, D.; Rao, S. N.; Rensmo, H.; Fitzmaurice, D.; Preece, J. A.; Wenger, S.; Stoddart, J. F.; Zaccheroni, N. J. Am. Chem. Soc. 2000, 122, 6252. (7) Sander, M. S.; Tan, L.-S. Adv. Funct. Mater. 2003, 13, 393 and references therein.
lithography. The method utilizes chemical reactions initiated in self-assembled monolayers (SAMs) by directwrite8 e-beam nanopatterning.9-11 Three successive steps are involved (Scheme 1): (i) SAM formation on a SiO2 surface with a NO2-terminated silane; (ii) spatially selective reduction of the NO2 group to an NH2 group by e-beam irradiation; (iii) deposition of citrate-passivated gold nanoparticles onto the protonated NH2 surface-terminated regions. Results and Discussion Organic films were prepared by immersing a Si substrate with a 100 nm thermally grown SiO2 layer, thoroughly cleaned by a procedure described earlier,12 into a 5 mM solution of 3-(4-nitrophenoxy)-propyltrimethoxysilane (NPPTMS) in anhydrous THF under an Ar atmosphere, which was sonicated at 25 °C for 2 h. Details of the synthesis and analysis of NPPTMS have been published.12 The substrates were rinsed intensively with THF, CHCl3, and EtOH, sonicated twice in fresh THF, and rinsed once more with THF, CHCl3, and EtOH in order to remove any physisorbed material from the surface. Samples were then cured at 120 °C for 30 min under a vacuum. SAMs of NPPTMS on Si/SiO2 were characterized by contact angle, ellipsometry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Details of these measurements have been previously (8) (a) Bedson, T. R.; Palmer, R. E.; Jenkins, T. E.; Hayton, D. J.; Wilcoxon, J. P. Appl. Phys. Lett. 2001, 78, 1921. (b) Bedson, T. R.; Palmer, R. E.; Wilcoxon, J. P. Appl. Phys. Lett. 2001, 78, 2061. (9) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Adv. Mater. 2000, 12, 805. (10) Go¨lzha¨user, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Adv. Mater. 2001, 13, 806. (11) Jung, Y. J.; La, Y.-H.; Kim, H. J.; Kang, T.-H.; Ihm, K.; Kim, K.-J.; Kim, B.; Park, J. W. Langmuir 2003, 19, 4512. (12) Mendes, P.; Belloni, M.; Ashworth, M.; Hardy, C.; Nikitin, K.; Fitzmaurice, D.; Critchley, K.; Evans, S.; Preece, J. A. ChemPhysChem 2003, 4, 884.
10.1021/la049803g CCC: $27.50 © 2004 American Chemical Society Published on Web 04/01/2004
Patterning Silicon Wafers Using e-Beam Lithography
Langmuir, Vol. 20, No. 9, 2004 3767
Scheme 1. Schematic of the Strategy for Fabricating Designed Au Nanoparticle Structures on Silica Surface Monolayersa
a
Not to scale: the Au particle is actually larger (16 nm) and covers several NH3+ groups.
Figure 1. XPS N(1s) spectrum of a monolayer of NPPTMS on Si/SiO2 taken after 1 h of continuous exposure to X-ray irradiation.
described.12 The clean bare silicon substrates displayed low H2O contact angles (θa and θr < 10°), which increased to 54 ( 2° and 25 ( 2°, respectively. Ellipsometry showed that the average film thickness was 1.2 ( 0.2 nm, in good agreement with the calculated molecular length (1.1 nm). AFM showed that the NPPTMS monolayer is smooth (roughness ) 0.16 nm). XPS measurements using a monochromatic Al KR X-ray source confirmed the adsorption of NPPTMS on Si/SiO2 (Figure 1) by the observation of both the NO2 and NH2 N(1s) binding energies. As previously observed,12 the NO2 groups (N(1s) binding energy ) 405.6 eV) in SAMs of NPPTMS are gradually reduced to an NH2 group (N(1s) binding energy ) 399.6 eV) during X-ray irradiation initiated by the photoelectrons and secondary electrons emitted from the surface.13 Therefore, it can be concluded that NPPTMS forms a uniform molecular layer on Si/SiO2. An ISI DS-130 field emission gun scanning electron microscope (SEM) was used to convert nitro-terminated monolayers to NH2 surfaces using both 5 and 6 keV electrons with doses14 ranging from 50 to 100 µC cm-2, through a TEM grid held a few hundred micrometers above the surface. Additionally, direct-write e-beam lithography was employed (primary beam energy ) 5 and 6 keV, doses14 between 25 and 300 µC cm-2) to write patterns with sub200-nm resolution, using an electron beam lithography system (Elphy Quantum Raith GmbH, Dortmund, Germany). The SEM image in Figure 2 illustrates a pattern (13) Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G. M. Science 1991, 254, 981. (14) Dose values are based on Faraday cup measurements.
Figure 2. SEM image of a patterned SAM composed of digits (650 nm wide lines) written by e-beam lithography (5 keV and 290 µC cm-2).
by the bright and dark areas corresponding to unexposed and exposed regions of the NPPTMS monolayer surface, respectively. This contrast is consistent with previously reported work15 in SEM imaging of heterogeneous SAMs consisting of different terminally exposed chemical functionalities. Further analysis by XPS of the N(1s) binding energy region in the electron-exposed areas indicated only the presence of the NH2 group (399.6 eV). Additionally, chemical differentiation of the NO2 and NH2 groups was confirmed on the patterned surfaces by immersing the surface in a 10% trifluoroacetic acid anhydride (TFAA) solution in dry THF overnight. XPS analysis on the electron-irradiated (Figure 3a) and non-electron-irradiated (Figure 3b) areas revealed the F(1s) core level peak only on the electron-irradiated areas. This leads to the conclusion that the NH2 functionality reacted with the TFAA as expected and formed the amide, whereas the NO2 did not react with the TFAA. Finally, the viability of the new strategy for the siteselective assembly of gold nanoparticles on nanopatterned Si/SiO2 surfaces, created by direct-write chemical e-beam lithography, was demonstrated by the deposition of citratepassivated gold nanoparticles on to the patterned samples. Citrate-passivated gold nanoparticles with a diameter of 16 nm were synthesized by the Frens method.16 The (15) Lo´pez, G. P.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1513.
3768
Langmuir, Vol. 20, No. 9, 2004
Mendes et al.
Figure 4. Tapping-mode AFM images of citrate-passivated gold nanoparticles attached preferentially to the exposed areas of the NPPTMS monolayer.
of the lack of any strong affinity between the NO2 functionality and the surface of the citrate-passivated nanoparticles. Figure 3. XPS F(1s) spectra for the (a) electron-irradiated and (b) non-electron-irradiated NPPTMS monolayer on Si/SiO2 after amide bond formation with TFAA.
patterned monolayer sample (5 keV and 250 µC cm-2) was then immersed in the citrate-stabilized colloidal gold acidic solution (pH ∼ 4.5)17 for 2 h. After immersion, the substrate was rinsed with ultrapure water for ∼1 min and dried under argon. The AFM images of the regions terminated by the NH2 groups (Figure 4) clearly revealed a dense monolayer array of gold nanoparticles, while the unirradiated areas (NO2 groups) showed very little nanoparticle adsorption. The rational for this differentiation is that when immersed in the acidic gold colloidal solution the NH2 groups become protonated forming NH3+. The NH3+ groups are then able to bind electrostatically to the negatively charged citrate-passivated gold nanoparticles, thus immobilizing them on the substrate.17,18 In contrast, such an assembly of gold nanoparticles is not formed on the NO2-terminated regions (Figure 4) because (16) Frens, G. Nat. Phys. Sci. 1973, 241, 20. (17) Zhu, T.; Fu, X.; Mu, T.; Wang, J.; Liu, Z. Langmuir 1999, 15, 5197.
Conclusions We have presented a new approach for fabricating nanopatterned arrays of gold nanoparticles on a silicon wafer using both chemical e-beam lithography and molecular recognition. This approach, combining both topdown and bottom-up techniques, offers a new methodology for patterning complex nanostructures from nanoparticles on surfaces, with the position of the nanoparticles being precisely controlled on the nanometer scale. Investigations are in progress to reduce feature sizes and to explore other e-beam-initiated chemical reactions for subsequent immobilization. Acknowledgment. This work was supported by the European Community under Grant No. HPRN-CT2000-00028. LA049803G (18) (a) Sato, T.; Brown, D.; Johnson, B. F. G. Chem. Commun. 1997, 1007. (b) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (c) Mougin, K.; Haidara, H.; Castelein, G. Colloids Surf. 2001, 193, 231.
