Self‐Assembly of Single‐Walled Carbon Nanotubes into a Sheet by Drop Drying

Single-walled carbon nanotubes (SWNTs) are currently the focus of extensive interdisciplinary studies because of their unique physical and chemical properties and potential electronic applications, for example, in making sensors and fieldemission devices. Processing of SWNT-based materials into engineered macroscopic materials is still in its infancy; the most successful methods so far have been based on adapting techniques that had been developed in other areas of material science such as colloids and polymers. Recent successes include preparing fibers and ribbons of SWNTs; films of pure SWNTs, polymers doped with SWNTs, and growth in situ of SWNT arrays. Evaporation of drops on substrates has been used for patterned deposition of solutes onto non-porous substrates, such as in DNA microarrays, nanolithography, protein crystallization, and stretching DNA for hybridization studies. Shimoda et al. prepared continuous selfassembled films of SWNT bundles on glass near a receding contact line by solvent evaporation. The moving contact line of a drying drop could be similarly used to form aligned patterns of SWNTs on substrates for making films or for nanofabrication. Drops of a solution on a substrate follow one of two drying mechanisms: either the drop maintains a constant contact angle by de-pinning the contact line (e.g., water on non-wetting substrates), or the contact line gets pinned and the drop maintains a fixed contact area (e.g., colloidal dispersions). Deegan and co-workers have studied the drying of drops of colloidal dispersions and found that the particles deposit in a ring at the periphery of the drop due to capillary flow in which the pinned contact line causes the solvent to flow towards the edge. Recent investigations have also shown the formation of a skin or crust at the free surface of drops of polymers and colloidal suspensions. Pauchard and Allain found that the crust may collapse and evolve into different shapes as the surface area remains constant while the drop volume decreases due to solvent evaporation. “Crusting” on the surface of spin-cast films is a well-known phenomenon. De Gennes suggested a transport model for crust formation in spin-cast films. Because the glass-transition or gelation temperature of a pure polymer/colloid is higher than that in solution, at any temperature below the glass transition there is a critical particle concentration at which the system transitions from fluid to glassy or gel-like. Evaporation of solvent from the free surface leads to a local increase in concentration of the polymer/suspension at the free surface, and a very thin glassy or gelled crust is formed at the free surface. Here, we investigated drying of a sessile drop of individually suspended SWNTs in an aqueous solution of F68 Pluronic. We found that, instead of assembling on the substrate, the SWNTs selfassemble into a crust at the free surface. This entangled meshlike crust was characterized by various microscopy techniques. The “crusting” phenomenon could be used as a potential route for making thin coatings and films of SWNTs. Video microscopy showed that the initial drying progressed by de-pinning of the contact line, i.e., the radius of the base decreased with time. Figure 1a shows the drop radius (normalized by the initial radius) as a function of time. The diameter of each drop was measured at four different angles and then averaged. After about 360 s the drop attained a fixed base radius and a foot started appearing. Drops of pure water on the same substrate dried by maintaining a fixed base radius, in agreement with the findings by Birdi et al. Assuming quasistationary conditions, if diffusion of water in air is rate controlling, then in a sessile drop receding with a constant contact angle the square of the base radius is linear with time. While the drop radius shrunk, the contact angle between the drop and the glass substrate was about 10–15 °C (inferred by video microscopy). Up to t ∼ 210 s, we find that the assumption that diffusion is rate controlling is fairly accurate (Fig. 1b). We recorded the variation of weight of the drop with time (Fig. 1c) and found that the initial evaporation rate J0 was J0 ∼ 2 × 10 cm s. As the drying progressed further, the drop attained a constant base radius at t∼ 360 s, and a surface undulation appeared at the top of the drop. A thin crust appeared at the free surface, with a convective flow toward the foot underneath. The formation of the crust slowed down the solvent loss from the initial evaporation rate J0 (Fig. 1c) by reducing the diffusion of water from the core of the drop to the free surface. Loss of the solvent decreased the volume enclosed by the thin crust, and the crust thus inverted, forming an undulation similar to a collapsing dome. The drying process is summarized schematically in Figure 2. C O M M U N IC A IO N S