Langmuir 2007, 23, 10069-10073
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Formation of Nanowire Striations Driven by Marangoni Instability in Spin-Cast Polymer Thin Films Shih-Yuan Lu,* Hsin-Lung Chen, Kuen-Hua Wu, and Yen-Yi Chen Department of Chemical Engineering, National Tsing-Hua UniVersity, Hsin-Chu 30043, Taiwan, Republic of China ReceiVed NoVember 2, 2006. In Final Form: July 11, 2007 Parallel striations made of silver nanowires were formed through the Marangoni instability induced during spin casting of poly(2-vinyl pyridine)/silver nanowire/chloroform solutions. The striation patterns of the silver nanowires resembled those obtained from spin casting of the corresponding neat polymer solutions, indicating essentially the same driving mechanism (i.e., the Marangoni instability). The silver nanowires were found to concentrate in the valleys of the striation pattern to balance the nonuniform surface tension distribution in the polymer thin film. The resulting nanowire striation patterns were found to depend on polymer concentration, rotational speed, and nanowire loading. Interestingly, this nanowire striation phenomenon was found to be independent of the substrate characteristics, hydrophobic or hydrophilic.
Introduction A wide range of 1-D nanostructures, including semiconductor nanowires, metallic nanowires, and carbon nanotubes, have been successfully fabricated in recent years.1 These nanoscale objects are attractive building blocks for bottom-up fabrication of functional assemblies and devices. To achieve this goal, it is critical to be able to manipulate the movement and placement of these 1-D nanostructures. Some processes have been developed for this purpose, including solvent evaporation-induced selfassembly within microchannels,2 the Langmuir-Blodgett technique-assisted self-assembly of monolayers on plain or patterned substrates,3,4 deposition on chemically functionalized nanolithographic templates,5 and controlled interactions of suspended nanowires with patterned surfaces.6 In addition, ferromagnetic nanowires can be manipulated with magnetic fields,7-9 and semiconductor10 and metallic nanowires11 can be maneuvered with electric fields. Research concerning the self-assembly of nanocrystals has also made substantial advances, such as the demonstration of the formation of arrays with ordering extending from nanometer to micrometer length scales. Rings and hexagonal arrays made of Ag nanocrystals dispersed in hexane were observed.12 Pattern formation was attributed to the changes in surface tension, which (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (2) Messer, B.; Song, J.; Yang, P. J. Am. Chem. Soc. 2000, 122, 10232. (3) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P. Nano Lett. 2003, 3, 1229. (4) Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Nano Lett. 2003, 3, 1255. (5) Liu, J.; Casavant, 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. (6) Martin, B. R.; St. Angelo, S. K.; Mallouk, T. E. AdV. Funct. Mater. 2002, 12, 759. (7) Tanase, M.; Bauer, L. A.; Hultgren, A.; Silevitch, D. M.; Sun, L.; Reich, D. H.; Searson, P. C.; Meyer, G. J. Nano Lett. 2001, 1, 155. (8) Love, J. C.; Urbach, A. R.; Prentiss, M. G.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12696. (9) Hangarter, C. M.; Myung, N. V. Chem. Mater. 2005, 17, 1320. (10) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 409, 66. (11) Smith, P. A.; Nordquist, C. D.; Jackson, T. N.; Mayera, T. S.; Martin, B. R.; Mbindyo, J.; Mallouk, T. E. Appl. Phys. Lett. 2000, 77, 1399. (12) (a) Maillard, M.; Motte, L.; Pileni, M. P. AdV. Mater. 2001, 13, 200. (b) Maillard, M.; Motte, L.; Ngo, A. T.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 11871.
induced the Marangoni instability in the liquid films.12 Stowell and Korgel demonstrated the morphological evolution that occurred at decreasing starting Au nanocrystal concentration (in chloroform) prior to deposition. Their results revealed a sequence of morphological changes from rings to polygonal networks to ordered hexagonal networks and finally to structures without microstructural order. They ascribed this phenomenon to a transition from diffusive (dewetting and hole formation) to convective (induced by the Marangoni instability) fluid flow in the evaporating liquid films.13 In addition, Ge and Brus observed 2D CdSe nanocrystal assemblies during the drying of various liquid films and showed that the spatial patterns were associated with the kinetic stages in fluid-fluid spinodal nucleation and subsequent coarsening.14 Moriarty et al. showed that Au nanocrystals spin coated onto silicon from toluene formed cellular structures. They claimed that the mechanism based on the Marangoni convection alone could not account for the variety of patterns observed and argued that spinodal decomposition played an important role.15 Finally, the self-assembly of conductive nanocrystals within ink-jet-printed tracks has also been extensively studied.16-19 In this study, we demonstrated the formation of parallel striation patterns made of silver nanowires from spin casting the nanowire-containing polymer solutions. On the surface of the spin-cast films, so-called radiative striations, which are radially extended ridges, are often observed.20 This phenomenon occurs particularly when the polymer films are spin cast from highly volatile solvents, inducing a strong Marangoni instability.21,22 For most solutions, a balance is established between the viscous outward radial flow of the solution on the surface of the substrate (13) Stowell, C.; Korgel, B. A. Nano Lett. 2001, 1, 595. (14) Ge, G. L.; Brus, L. J. Phys. Chem. B 2000, 104, 9573. (15) Moriarty, P.; Taylor, M. D. R. Phys. ReV. Lett. 2002, 89, 248303. (16) Cuk, T.; Troian, S. M.; Hong, C. M.; Wagner, S. Appl. Phys. Lett. 2000, 77, 2063. (17) Cuk, T.; Troian, S. M.; Hong, C. M.; Wagner, S. In Materials DeVelopment for Direct Write Technologies; Chrisey, D. B., Gamota, D., Helvajian, H., Taylor, D. P., Eds.; Material Research Society: Pittsburgh, 2001; Vol. 624, p 267. (18) de Gans, B.-J.; Schubert, U. S. Langmuir 2004, 20, 7789. (19) Perelaer, J.; de Gans, B.-J.; Schubert, U. S. AdV. Mater. 2006, 18, 2101. (20) Du, X. M.; Orignac, X.; Almeida, R. M. J. Am. Ceram. Soc. 1995, 78, 2254. (21) Scriven, L. E.; Sternling, C. V. Nature 1960, 187, 186. (22) Pearson, J. R. A. Fluid Mech. 1958, 4, 489.
10.1021/la063199n CCC: $37.00 © 2007 American Chemical Society Published on Web 08/28/2007
10070 Langmuir, Vol. 23, No. 20, 2007
Lu et al.
and the evaporation of solvent from the coating solution. Meyerhofer treated this dual-action process by splitting the spincoating run into two stagessone controlled only by viscous flow and the other controlled only by evaporation.23 During the evaporation-dominated period, the rapid solvent evaporation near the solution surface leads to nonuniform temperature gradients and thus drives the convective flows. The occurrence of the instability is mainly governed by the interplay among surface tension, thermal diffusion, and viscosity, which can be characterized by the Marangoni number, Ma24
Ma )
- (∂γ/∂T)H2∇T µR
(1)
where (∂γ/∂T) is the temperature derivative of the surface tension, ∇T is the temperature gradient near the solution surface, H is the film thickness, and µ and R are the viscosity and thermal diffusivity of the solution, respectively. As the value of Ma exceeds 80, the instability is triggered. The instability wavelength, λ, is related to Ma, that is, a stronger Marangoni instability leads to denser striations in spin-cast films, and is given by22
λ2 )
16π2H2 Ma
(2)
For spin-coating processes, the film thickness may not be sufficiently large to give a high enough Ma to induce the stable thermocapillary convection according to the above equation. Nevertheless, Birnie et al.25 have pointed out that the solutocapillary flow induced by concentration gradients is more important than the thermocapillary flow caused by temperature gradients. In this case, the main driving force is the composition gradient, and Ma is reformulated as
Ma )
-(∂γ/∂C)H2∇C µD
(3)
where C is the relevant composition variable and D is the diffusion coefficient of the component that causes the change in the composition-dependent surface tension. The rapid evaporation of solvents during the spin-coating process generates large composition gradients at the solution surface that drive the necessary convective flow for striation formation. All of these instabilities induced by the differences in surface tension are called the Marangoni effect. Recently, we extended the Marangoni instability-induced striation phenomena from neat polymer systems to polymer blend systems.26 The resulting striation patterns were manifested not only in the thickness variation but also in the composition variation. In this work, we demonstrate that the striation phenomena can occur not only for the polymers but also for the suspended nanowires. The nanowire striation patterns were found to resemble those achieved from spin casting of the corresponding neat polymer solutions. Moreover, the nanowires were found to concentrate in the concave regions of the striation patterns. We will show that the Marangoni instability was responsible for the formation of the nanowire striation patterns. Unlike previous work concerning the pattern formation of nanocrystals, the present work deals with nanowires of much larger dimensions, with a diameter and length of 300 nm and 10 µm, respectively. (23) Meyerhofer, D. J. Appl. Phys. 1978, 49, 3993. (24) Haas, D. E.; Birnie, D. P. J. Mater. Sci. 2002, 37, 2109. (25) Birnie, D. P. J. Mater. Res. 2001, 16, 1145. (26) Wu, K. H.; Lu, S. Y.; Chen, H. L. Langmuir 2006, 22, 8029.
Surprisingly, the Marangoni instability was strong enough to produce striation patterns of nanowires of such dimensions. Experimental Section Fabrication of Silver Nanowires. The silver nanowires (300 nm in diameter and 10 µm in length) were fabricated via electrodeposition with the anodic aluminum oxide (AAO) membrane serving as the template. The AAO membranes (Anodizc, Whatman, Inc.) used in this study possessed a pore diameter of ca. 300 nm, as observed from SEM images. To obtain free-standing nanowires, a Ni buffer layer was deposited into the AAO pores first. The Ni layer can be easily detached from the later-grown silver nanowires with ultrasonic agitation. Detailed procedures for and relevant characterizations of the resulting free-standing nanowires can be found in our previous paper.27 Surface Treatment of Substrates. Glass slides (2.5 cm × 2.5 cm) were used as the substrates for the spin-coating process. The surfaces of the glass slides were treated with two different procedures to obtain either hydrophilic or hydrophobic substrates. Hydrophilic substrates were prepared by simply cleaning the glass slides with a H2SO4/H2O2 (piranha) mixture to remove the organic residues. To produce hydrophobic substrates, the piranha-cleaned glass slides were immersed overnight in a 6% (w/w) solution of trimethylchlorosilane (TMCS) in hexane. The static water contact angles of the two treated substrates were
