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Langmuir 2005, 21, 6596-6602
Investigations into the Mechanism of Adsorption of Carbon Nanotubes onto Aminopropylsiloxane Functionalized Surfaces T. P. Burgin,*,† J. C. Lewenstein,†,| and D. Werho‡ Physical Sciences Research Laboratories and Process & Materials Characterization Laboratory, Motorola Labs, Motorola, M.D. EL37, 2100 East Elliot Road, Tempe, Arizona 85284 Received February 28, 2005. In Final Form: April 16, 2005 The factors that control carbon nanotube (CNT) adsorption onto aminopropyl siloxane (APS)-derivatized surfaces were investigated using two distinct types of well-characterized films with significant differences in their detailed structures. Both types of APS films showed a marked increase in CNT adsorption relative to untreated SiO2 surfaces but differed in the amount of CNTs adsorbed. To gain insight into the factors governing adsorption, the surface coverage of the CNTs was monitored as a function of the pH during the deposition, the surfactant used to suspend the CNTs, and the type and amount of salt added to the deposition solution. The adsorption is shown to be governed by electrostatic and VDW forces. In the case of complimentarily charged surfaces, the adsorption is proposed to occur through an ion exchange mechanism.
Introduction Carbon nanotubes (CNTs) have generated a great deal of interest in the field of nanotechnology due to their unique physical, electrical, and mechanical properties.1 Practical utilization of CNTs in the preparation of nanoscale devices necessitates control over their placement and orientation on a surface. Burghard et al. first reported that CNTs suspended with sodium dodecyl sulfate (SDS) in water preferentially adsorbed onto aminopropylsiloxane (APS)derivatized SiO2 surfaces as compared to regions with a carboxylate-terminated surface.2 Lui et al. later demonstrated that CNT suspensions in dimethylformamide selectively deposited CNTs onto regions of a SiO2 surface derivatized with APS, relative to areas with a monolayer of an alkylsiloxane.3 Since that time, SiO2 surfaces have been patterned with polymeric resist and CNTs have been placed in the lithographic openings after functionalization with APS.4 Interestingly, the selectivity of these depositions was maintained even after the removal of the resist. The enhancement of the CNT adsorption to APS derivatized surfaces is dramatic, with carefully prepared monolayers of APS yielding a high density of CNTs and the passivated surfaces yielding no, to very few, CNTs under the same deposition conditions.3,4 This work describes investigations into the mechanism of the CNT adsorption onto APS-functionalized surfaces. It is hoped that a better understanding of the parameters involved in CNT ad* To whom correspondence should be addressed. E-mail:
[email protected]. † Physical Sciences Research Laboratories. ‡ Process & Materials Characterization Laboratory. | Currently at Freescale Semiconductor, Arizona Product Analysis Laboratories. (1) Dresselhaus, M.; Dresselhaus, G.; Avouris, P. Carbon Nanotubes: Synthesis, Structure, Properties and Applications; SpringerVerlag: Berlin, 2001. (2) Burghard, M. D. G.; Philipp, G.; Muster, J.; Roth, S. Adv. Mater. 1998, 10, 584-588. (3) Liu, J. C. M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Rimberg, A. J.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1999, 303, 125-129. (4) (a) Choi, K. H. B. J. P.; Auvaray, S.; Esteve, D.; Duesberg, G.; Roth, S.; Burghard, M. Surf. Sci. 2000, 462, 195. (b) Ahlskog, M. S. E.; Vullers, R. J. M.; Haesendonck, C. V. J. Appl. Phys. 1999, 85, 84328435. (c) Lewenstein, J. C.; Burgin, T. P.; Ribayrol, A.; Nagahara, L. A.; Tsui, R. K. Nano Lett. 2002, 2, 443-446.
sorption will allow the rational optimization of the process and facilitate investigation into other types of surfaces suitable for CNT adsorption. Results and Discussion The dramatic increase in adsorption of CNTs onto SiO2 surfaces after functionalization with APS could, in principle, be due to differences in either the thermodynamics or the kinetics of adsorption between the surfaces. Under equilibrium conditions, the relative amount of CNTs adsorbed is determined by differences in the free energy of adsorption of the CNTs on the surfaces.5 The cause of these differences could be a chemical interaction between the APS and the CNTs, charge-charge interactions, or dispersion forces. The mechanism of APS monolayer formation is complex, and several possible orientations of the molecule have been proposed in the literature, depending on the deposition conditions used.6 The molecules can orient themselves with the amine moiety facing away from the surface as expected for coupling of the polymerized siloxane network of the APS to the silicon oxide surface of the substrate. The APS can also couple to the surface via an electrostatic interaction between the protonated amine and the negatively charged silicon oxide surface. In disordered films, it is further possible for the APS to form loops to the surface. The degree of surface coverage is also dependent on the preparation method, ranging from submonolayer to multilayer films. Depending on the details of the monolayer formation and the conditions of the CNT deposition, the surface of the APS-derivatized surface could be positively or negatively charged or nearly charge neutral. The surface can also vary dramatically in the number of amines present at the monolayer-liquid interface. To help ascertain the mechanism of CNT adsorption, the detailed structures of APS films formed (5) (a) Giles, C. H.; Smith, D.; Huitson, A. J. Colloid Interface Sci. 1974, 47 (3), 755. (b) Giles, C. H.; D’Silva, A. P.; Easton, I. A. J. Colloid Interface Sci. 1974, 47 (3), 766. (6) Vandenberg, E. T. B. L.; Liedberg, B.; Uvdal, K.; Erlandsson, R.; Elwing, H.; Lundstrom, I. J. Colloid Interface Sci. 1991, 147, 103-118 and references therein.
10.1021/la050540f CCC: $30.25 © 2005 American Chemical Society Published on Web 05/27/2005
Adsorption of CNTs onto APS Functionalized Surfaces Table 1. Contact Angle and Ellipsometric Data for APS Films Prepared in CH2Cl2 and Water solvent
contact angle (°)
ellipsometric thickness (Å)
CH2Cl2 water
20 ( 7
