NANO LETTERS
Light-Patterned and Recognition-Directed Adsorption of Nanoparticles at a Silicon Wafer Substrate
2004 Vol. 4, No. 4 573-575
Declan Ryan, Lorraine Nagle, and Donald Fitzmaurice* Department of Chemistry, UniVersity College Dublin, Belfield, Dublin 4, Ireland Received December 5, 2003
ABSTRACT A silicon wafer was modified by chemisorption of a monolayer of a cation precursor and exposed to blue light through a mask. In the regions exposed to blue light, the cation precursor was converted to cation. These cations were recognized and bound selectively by nanoparticles modified by adsorption of crown. As a consequence, these crown-modified nanoparticles were adsorbed at only the desired regions and pattern transfer was affected.
Introduction. The demand for faster integrated circuits is bringing into focus the inherent limitations of current fabrication technologies.1 Consequently, a great deal of attention is focused on developing alternative technologies.2 A possible alternative technology is the recognitiondirected self-assembly in solution of active and passive circuit components from nanoscale molecular and condensed phase building blocks.3 It is further envisaged that individual circuit components will be organized at the surface of a patterned substrate.4 By this means, it will be possible to address and integrate these components in order to realize more complex function. In considering what molecular and condensed phase building blocks might be suitable for the self-assembly of circuit components,5 one is attracted to supermolecules formed by receptor-substrate pairs that recognize and selectively bind each other6 and to nanoparticles whose surfaces can be readily modified and whose magnetic and electronic properties can be tuned by controlling their size.7 In this context, it was Stoddart and co-workers who first described the pseudorotaxane formed by an electron-rich crown (dibenzo24crown-8) and an electron poor cation (dibenzylammonium cation).8 They established that the crown is threaded by the cation (Scheme 1) and measured the association constant. Building on these findings, it was possible to “program” a crown-modified silver nanoparticle to recognize and bind selectively the surface of a cationmodified silica nanoparticle and to form a crown-modified core-shell nanoparticle (Scheme 2).9-11 * Corresponding author. E-mail:
[email protected]. 10.1021/nl0351340 CCC: $27.50 Published on Web 03/18/2004
© 2004 American Chemical Society
Scheme 1 . Self-Assembly of Pseudorotaxane
Scheme 2 . Crown-Modified Silica-Silver Core-Shell Nanoparticle
Here we describe the preparation of a silicon wafer that has been modified by a precursor to the dibenzylammonium cation moiety (Scheme 3). The cation precursor can be converted to the corresponding cation by exposure to blue light. We also describe how crown-modified silica-silver core-shell nanoparticles are adsorbed selectively at the irradiated regions of the silicon wafer substrate, resulting in effective pattern transfer. We discuss the significance of these findings and conclude by considering how these and related
Scheme 3 . Irradiation of a Silicon Wafer Modified by Adsorption of Cation Precursor through a Suitable Mask Results in Pattern Transfer When the Same Wafer Is Exposed to a Dispersion of Crown-Modified Nanoparticles
Figure 1. Silver-silica core-shell nanoparticles.
findings can be built upon to develop a scalable approach to the self-assembly and self-organization of addressable arrays of functional nanoparticle assemblies on commercially relevant substrates. Results and Discussion. Crown-Modified Nanoparticles. It was determined by TEM that the crown-modified silver nanoparticles used in the present study possess an average diameter of 6.7 nm and a polydispersity of 1.09.12 It was determined by elemental analysis and 1H NMR that 15% of the long chain alkane thiols adsorbed at the surface of a silver nanoparticle incorporate a crown moiety in the terminal position. It was also determined by TEM that the cationmodified silica nanoparticles possess an average diameter of 180 nm and a polydispersity of 1.08. While the extent of coverage of cation moieties at the surface of the silica nanosphere is not known, it is sufficient to ensure a highdegree of coverage of crown-modified silver nanoparticles. As may be seen from Figure 1, the crown-modified silver nanoparticles recognize and bind selectively the surface of cation-modified silica nanoparticles.11 The driving force for assembly of these crown-modified silica-silver core-shell nanoparticles is pseudorotaxane formation.8 Photopatterning of Silicon Wafer Substrate. Silicon wafer substrates were modified by exposure to a silane-coupling agent in refluxing toluene.12,13 It has been established that use of the chosen agent minimizes silane polymerization at the surface, while also providing a sufficiently high number of binding sites for covalent linkage of the cation precursor.14 To generate the cation, the silicon substrate, which had been modified by adsorption of the cation precursor, was irradiated while immersed in an acidic solution.12 This approach was favored as photocleavage of the cation precursor results in the formation of an aldehyde-containing byproduct,15 which may react with the photogenerated amine moiety, but does not at low pH.14,16 Pattern Transfer to Crown-Modified Nanoparticles. A cation-modified silicon wafer is immersed in a dispersion of core-shell nanoparticles consisting of crown-modified 574
Figure 2. Patterned substrate.
silver nanoparticles adsorbed at the surface of a cationmodified silica nanoparticle. As may be seen from the SEM in Figure 2, these core-shell nanoparticles are adsorbed predominantly at the cation-modified regions of the silica substrate resulting in pattern transfer. By varying the immersion time and the concentration of the dispersion, it is possible to minimize nonspecific adsorption. Recent Related Findings. The self-organization of nanoparticles at a modified substrate has been the subject of a number of studies.17 Two general approaches have been adopted. In the first, a bifunctional molecule is adsorbed at a substrate to form a monolayer at which unmodified nanoparticles are subsequently adsorbed. In the second, nanoparticles are modified by adsorption of a bifunctional molecule and are subsequently adsorbed at an unmodified substrate. In all such studies the nanoparticles have either been electrostatically or covalently bound to the modified substrate. While the possibility was noted, none of the above reports described the adsorption of nanoparticles at a patterned substrate. A number of such reports have subsequently been published. These included reports that described use of the microcontact printing technique developed by Whitesides and co-workers,18-19 and an ink based on a suitable bifunctional molecule to pattern a substrate.20-22 Nanoparticles were electrostatically or covalently adsorbed at those regions of Nano Lett., Vol. 4, No. 4, 2004
the substrate modified by adsorption of the bifunctional molecule and resulted in successful pattern transfer. A related approach based on the hydrophobic effect has also been described.23 In a relatively recent development Alivisatos, Heath, and co-workers reported the findings of a study in which a silicon substrate was modified with a bifunctional molecule.24 One functional moiety was covalently adsorbed at the surface of the silicon wafer. The second functional moiety was the precursor to a moiety that was covalently adsorbed at the surface of a silica nanoparticle. This moiety was generated in situ by irradiating a photolabile protecting group on the precursor. By this means photopatterning of the substrate, with a moiety that covalently binds a nanoparticle proved possible, led to pattern transfer. What distinguishes the findings in the present report from, those in the report cited above, is the following: First, the fact that the bifunctional molecule adsorbed at the surface of the substrate can be irradiated to generate a molecular substrate that is recognized and noncovalently bound by a molecular receptor adsorbed at the surface of the nanoparticle; and second, that the noncovalent organization of nanoparticles may be reversed by immersion of the patterned wafer in a suitable solvent. Clearly, this light-based approach to organizing nanoparticles at a substrate has significant potential. For example, extending this approach to the use of an NSOM to write the pattern on the substrate would greatly increase the spatial resolution that may be achieved. Also for example, extending this approach to the use of more than one receptor-substrate pair would allow different nanoparticles be adsorbed at different regions on the substrate. Other developments of this approach can be envisaged. Conclusion. We have demonstrated the recognitiondirected noncovalent self-organization of silver-silica coreshell nanoparticles at a photopatterned silicon wafer substrate. The driving force for organization is pseudorotaxane formation between the cation-modified silicon wafer substrates and the crown-modified silver-silica core-shell nanoparticles. Acknowledgment. The authors acknowledge useful discussions with D. Cottell at the Electron Microscopy Centre, University College Dublin. This work was supported by a Strategic Research Grant from Enterprise Ireland and cosponsored by Loctite (Ireland).
Nano Lett., Vol. 4, No. 4, 2004
Supporting Information Available: A detailed description of the experimental methods employed in the present study. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) (2) (3) (4) (5) (6) (7) (8)
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(10)
(11) (12)
(13) (14) (15) (16) (17)
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(19) (20) (21)
(22) (23) (24)
Ito, T.; Okazaki, S. Nature 2000, 406, 1027. Chen, Y.; Pepin, A. Electrophoresis 2001, 22, 187. Service, R. Science 1997, 277, 1036. Mann, S.; Shenton, W.; Li. M.; Connolly, S.; Fitzmaurice, D. AdV. Mater. 2000, 12, 147. Niemeyer, C. Angew. Chem., Int. Ed. 2001, 40, 4128. Lehn, J.-M. Supramolecular Chemistry, Concepts and PerspectiVes; VCH: Weinheim, 1995. Shipway, A.; Katz, E.; Willner, I. ChemPhysChem 2001, 2, 21. Ashton, P.; Chrystal, E.; Glink, P.; Menzer, S.; Schiavo, C.; Spencer, N.; Stoddart, J. F.; Tasker, P.; White, A.; Williams, D. Chem. Eur. J. 1996, 2, 709. Fitzmaurice, D.; Rao, S. N.; Rensmo, H.; Preece, J.; Stoddart, J. F.; Wenger, N.; Zaccheroni, N. Angew. Chem., Int. Ed. Engl. 1999, 38, 1147. 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. Ryan, D.; Nagle, L.; Rensmo, H.; Fitzmaurice, D. J. Phys. Chem. B 2002, 106, 5371. A detailed description of the experimental methods employed in the present study is contained in the accompanying Supporting Information. van Blaarderen, A.; Vrij, A. J. Colloid Interface Sci. 1993, 156, 1. Vossmeyer, T.; Jia, S.; Delonno, E.; Diehl, M.; Kim, S.; Peng, X.; Alivisatos, A. P.; Heath, J. J. Appl. Phys. 1998, 84, 3664. Pillai, V. Synthesis 1980, 1. Patchornik, A.; Amit, B.; Woodward, R. J. Am. Chem. Soc. 1970, 92, 6333. Rizza, R.; Fitzmaurice, D.; Hearne, S.; Hughes, G.; Spoto, G.; Ciliberto, E.; Kerp, H.; Schropp, R. Chem. Mater. 1997, 9, 2969 and references (7 and 8) therein. Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Junker, D.; Kind, H.; Renault, J.-P.; Rothuizen, H.; Schmidt, H.; Schmidt-Winkel, P.; Stutz, R.; Wolf, H. IBM J. Res. DeV. 2001, 45, 697. Whitesides, G.; Ostuni, E.; Takayama, S.; Jiang, X.; Igber, D. Annu. ReV. Biomend. Eng. 2001, 3, 335. He, H.; Zhang, H.; Li, Q.; Zhu, T.; Li, S.; Liu, Z. Langmuir 2000, 16, 3846. Musick, M.; Keating, C.; Lyon, L.; Botsko, S.; Pena, D.; Holloway, W.; McEvoy, T.; Richardson, J.; Nathan, M. Chem. Mater. 2000, 12, 2869. Zhong, Z.; Gates, B.; Xia, Y.; Qin, D. Langmuir 2000, 16, 10369. Himmelhaus, M.; Tahei, H. Phys. Chem. Chem. Phys. 2002, 4, 496. Vossmeyer, T.; Jia, S.; Delonno, E.; Diehl, M.; Kim, S.-H.; Peng, X.; Alivisatos, P.; Heath, J. J. Appl. Phys. 1998, 84, 3664.
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