Langmuir 2008, 24, 5981-5983
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Selective Surface Patterning for the Coadsorption of Self-Assembled Gold and Semiconductor Nanoparticles Gilad Gotesman and Ron Naaman* Department of Chemical Physics, The Weizmann Institute of Science, RehoVot, Israel 76100 ReceiVed January 20, 2008. ReVised Manuscript ReceiVed March 19, 2008 The predetermined patterned adsorption of two types of nanoparticles on the same substrate may be of considerable importance in various applications, among others, to enhance the absorption of semiconductor nanoparticles by the plasmonic effect of metal NPs. We describe here a simple method for self-assembling 2D lateral patterns in which both gold and semiconductor nanoparticles are adsorbed, each in a predesigned area. Our method is based on a one-step lithographic process and the adsorption of two distinct self-assembled monolayers that can selectively bind only one type of nanoparticle.
Nanoparticles (NPs) are promising building blocks in many futuristic applications because of their unique optical and electrical properties.1–3 Predetermined patterned adsorption of two types of nanoparticles on a single device may be of particular importance in various applications, among others, to enhance the absorption of semiconductor NPs by the plasmonic effect of metal NPs.4,5 To construct electro-optic devices that contain both metallic and semiconductor NPs, one must control their positioning on macroscale substrates. This letter focuses on the development of a process for patterning substrates for the coadsorption of two types of NPs on one substrate, each on a different predetermined area. Selective surface patterning is an important technological challenge in many nanotechnology-related applications; therefore, this subject has been addressed in numerous publications.6–8 Methods such as microcontact printing9 and the nanofountain probe10 are used to place a single type of NP directly on the desired spot on the substrate. Dip pen nanolithography,11 constructive nanolithography,12 and chemical e-beam lithography13 are used to chemically modify regions on the surface in order to bind NPs selectively. A technique based on block copolymers was developed for the purpose of depositing more than one type of NP.14 This technique yields uniform and organized layers of gold and iron oxide NPs, but it cannot be * Corresponding author. E-mail:
[email protected]. (1) Maier, S. A.; Kik, P. G.; Atwater, H. A; Meltzer, S.; Harel, E.; Koel, B. E.; Requicha, A. A. G. Nat. Mater. 2003, 2, 229. (2) Eustis, S.; El-Sayed, M. A. Chem. Soc. ReV. 2006, 35, 209. (3) Somers, R. C.; Bawendi, M. G.; Nocera, D. G. Chem. Soc. ReV. 2007, 36, 579. (4) Paltiel, Y.; Aharoni, A.; Banin, U.; Neuman, O.; Naaman, R. Appl. Phys. Lett. 2006, 89, 033108. (5) Pompa, P. P.; Martiradonna, L.; Della Torre, A.; Della Sala, F.; Manna, L.; De Vittorio, M.; Calabi, F.; Cingolani, R.; Rinaldi, R. Nat. Nanotech. 2006, 1, 126. (6) Dulcey, C. S.; Georger, J. H.; Krauthamer, V.; Fare, T. L.; Atenger, D. A.; Calvert, J. M. Science 1991, 252, 551. (7) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1. (8) Pace, G.; Petitjean, A.; Lalloz-Vogel, M.-N.; Harrowfield, J.; Lehn, J.-M.; Samorı`, P. Angew. Chem. 2008, 120, 2518. (9) Bodas, D.; Khan-Malek, C. Sens. Actuators, B 2007, 128, 168. (10) Wu, B.; Ho, A.; Moldovan, N.; Espinosa, H. D. Langmuir 2007, 23, 9120. (11) Liu, X.; Fu, L.; Hong, S.; Dravid, V. P.; Mirkin, C. A. AdV. Mater. 2002, 14, 231. (12) Liu, S.; Maoz, R.; Sagiv, J. Nano Lett. 2004, 4, 845. (13) Mendes, P. M.; Jacke, S.; Critchley, K.; Plaza, J.; Chen, Y.; Nikitin, K.; Palmer, R. E.; Preece, J. A.; Evans, S. D.; Fitzmaurice, D. Langmuir 2004, 20, 3766. (14) Sohn, B. H.; Choi, J. M.; Yoo, S. I.; Yun, S. H.; Zin, W. C.; Jung, J. C.; Kanehara, M.; Hirata, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 6368.
applied to the formation of predetermined patterns. Plutowski et al. used the recognition capability of DNA strands to specifically bind two sizes of gold nanoparticles in different spots.15 Vossmeyer et al. demonstrated a method for self-assembling patterns of various nanoparticles by the multistep light deprotection of an aminosilane monolayer.16 Recently, a simple optical lithography process (including the deposition of an aluminum layer as an etching mask) has been employed to deposit silica and polystyrene nanoparticles.17 However, this method is restricted to NPs that can be etched by reactive ion etching (RIE). We describe here a simple method for self-assembling 2D lateral patterns of gold and semiconductor nanoparticles (GNP and ScNP). Our method is based on a one-step lithographic process and the adsorption of two distinct self-assembled monolayers (SAMs) that can selectively bind only one type of NP. The method is schematically described in Figure 1. To pattern the surface, we used a photolithography process. The results were imaged with a scanning electron microscope and by a fluorescence microscope. For the first, Si/SiOx substrates were used, whereas for the second, the deposition was performed on glass. A thin film of a positive photoresist was spun on the substrate and then exposed to UV light through a mask with 40-µm-wide alternating chrome stripes. After the exposed resist in the developing solution was removed, a 100-nm-thick chromium layer was deposited. A lift-off process to remove the remaining resist revealed a pattern of 40 µm chromium stripes on the surface. In general, the resolution of the patterning is defined by the lithography methods, namely, about 1 µm for photolithography and at least order of magnitude smaller features for e-beam lithography. The samples were then cleaned and oxidized in a UV/ozone cleaning system (UVOCS) to form fresh oxide on the surface. A self-assembled monolayer (SAM) of octenyltrichlorosilane (OETS) was then adsorbed on the open (free of chromium) oxide surfaces (Figure 1A) from a 1 mM solution in bicyclohexyl (BCH). In the following step, the chromium pattern was etched by dipping the sample in a commercial Cr-etching solution containing perchloric acid and ceric ammonium nitrate (Figure 1B). Next, a SAM of aminopropyltrimethoxysilane (APTMS) (15) Plutowski, U.; Jester, S. S.; Lenhert, S.; Kappes, M. M.; Richert, C. AdV. Mater. 2007, 19, 1951. (16) Vossmeyer, T.; Jia, S.; Delonno, E.; Diehl, M. R.; Kim, S.-H.; Peng, X.; Alivisatos, A. P.; Heath, J. R. J. Appl. Phys. 1998, 84, 3664. (17) Hua, F.; Shi, J.; Lvov, Y.; Cui, T. Nano Lett. 2002, 2, 1219.
10.1021/la800184z CCC: $40.75 2008 American Chemical Society Published on Web 05/16/2008
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Figure 1. Schematic illustration of the patterning of silica substrates by different silane-based self-assembled monolayers. Specifically, gold and CdSe nanoparticles are adsorbed separately on the predetermined structures (not to scale). (A) Lithographic process for patterning Cr on the surface and the adsorption of OETS on the bare silicon oxide areas. (B) Removal of Cr by etching. (C) Adsorption of APTMS on the area exposed after the removal of Cr. (D) Oxidation of the OETS that result in the formation of carboxylic head groups. (E) Adsorption of gold nanoparticles on the APTMS. (F) Adsorption of CdSe nanoparticles on the oxidized OETS.
Figure 2. SEM images of monolayers of nanoparticles adsorbed on substrates coated with a single SAM. (A) Gold nanoparticles bound to a SAM of aminopropyltrimethoxysilane. (B) CdSe nanoparticles bound to a SAM of oxidized octenyltrichlorosilane. (C) Gold nanoparticles (one at each dotted circle) physically adsorbed on a SAM composed of oxidized octenyltrichlorosilane.
was adsorbed on the freshly exposed surfaces from a 1 vol % solution (Figure 1C). The silanized samples were then immersed for 20 h in an aqueous oxidation solution (containing 5 mM KMnO4, 200 mM NaIO4, and 20 mM K2CO3) to oxidize the vinyl functional group of the OETS, yielding a carboxylic acid headgroup.18 We chose to oxidize the OETS after the APTMS had been adsorbed in order to prevent reactions between the carboxylic acid and the amino groups. This procedure yielded a patterned surface with amino and carboxyl functional groups deposited at predetermined areas (Figure 1D). Gold nanoparticles (GNPs, 7 nm in diameter with citrate capping at pH ∼4.5) were synthesized by a cold procedure.19 These GNPs were adsorbed selectively, overnight, on the APTMS SAM (Figure 1E). The selectivity is achieved by the electrostatic repulsion between the citrate-capped GNP and the negatively (18) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (19) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353.
Figure 3. Contrast between strips coated with either gold or CdSe nanoparticles. (A, B) SEM images, on different scales, of the interface between the strips. The upper brighter regions are the strips coated with the gold nanoparticles, whereas the lower darker regions are the parts covered with the CdSe nanoparticles. (C) Fluorescence microscope image of the patterned substrate, as monitored by the emission wavelength of the CdSe nanoparticles. The strips covered with the CdSe NPs are brighter. (D) Photoluminescence profile along the black arrow in C, which reveals the sharpness of the interface between the stripes.
charged adsorbed carboxylic acid, on one hand, and the attraction to the positively charged adsorbed amino groups, on the other hand. Finally, the samples were immersed in a solution of CdSe NPs (∼3 nm diameter for photoluminescence measurements and ∼7 nm diameter for the SEM imaging) so that the NPs were adsorbed on the open SAM of carboxylic acid groups (Figure 1F). A high-resolution scanning electron microscope (HR-SEM) has been used to characterize the adsorbed NPs monolayers. As a first control for determining the selectivity of the adsorption, we used substrates covered with only one type of silane SAM. When GNPs were adsorbed on samples with APTMS SAMs, they formed a very dense monolayer with a coverage density of about 7500 NP/µm2 (Figure 2A). However, when we adsorbed CdSe on substrates coated with oxidized OETS SAMs alone, they formed a monolayer with a coverage of about 4500 NP/µm2 (Figure 2B). When we attempted to adsorb the GNP on the
Letters
oxidized OETS SAM, very few particles were actually attached to the surface (Figure 2C). This result validates the high selectivity of the oxidized OETS SAM against the adsorption of GNP. Samples of 40 µm stripes of alternating CdSe and GNP were prepared by the method described above. Figure 3A,B shows SEM images with a very sharp border between the two NP stripes. Because the two kinds of NPs are of about the same size (7 nm), differentiating between them by SEM is possible only because of the atomic number contrast. Bright white was observed for the GNPs, and dark gray was observed for the CdSe NPs. The GNP monolayer has micrometer-sized regions of lower coverage (darker regions - Figure 3A, top middle) and an average coverage of about 2200 NP/µm2, which is lower than the coverage in the control sample described above. This decrease may result from the partial oxidation of the SAM made from aminosilane. The CdSe monolayer has a density of about 2900 NP/µm2, which is almost exactly the same as in the control. Figure 3C presents a fluorescent microscope image of the patterned sample with two types of NPs, monitored at the CdSe emission wavelength. This image reveals the NPs’ stripe pattern (bright for CdSe NPs and dark for GNPs). This is empirical evidence for the presence of CdSe NPs only on alternating stripes, whereas the GNPs (dark) are present on the others. The graph in Figure 3D presents the
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photoluminescence profile of the sample along the black arrow in Figure 3C. The sharp steps in the profile are due to the very sharp border between the two kinds of NP stripes. The method presented is advantageous because it allows the adsorption of two different types of nanoparticles and it combines three important aspects: it requires very few stages for preparation, a large-scale surface can be prepared in one stage, and it is general so that practically any type of nanoparticle, to which the molecules can be attached, can be deposited. This method enables the assembly of electro-optical devices based on nanoparticles, such as plasmon waveguides containing both metal and semiconductor nanoparticles. The process is simple and ensures high selectivity in the adsorption. Its resolution depends only on the lithography step. Acknowledgment. We thank Aviad Baram for helping with the fluorescence microscope imaging measurements. We acknowledge partial support from the Israel Ministry of Science and the Grand Center at the Weizmann Institute. Supporting Information Available: Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org. LA800184Z
