Gecko‐Foot‐Mimetic Aligned Single‐Walled Carbon Nanotube Dry Adhesives with Unique Electrical and Thermal Properties

The unusual ability of gecko lizards to climb on any vertical surface and hang from a ceiling with one toe has inspired scientific research for centuries. Recent studies revealed that the driving force for holding gecko lizards on a surface arises from the strong van der Waals (vdW) forces (∼ 10 N cm) induced by countless aligned microscopic elastic hairs called setae (3–130 lm in length) splitting into smaller spatulae (0.2–0.5 lm in diameter) on a gecko’s foot. This finding has prompted many researchers to fabricate microarrays of polymer pillars to mimic gecko feet. Due to the difficulty to synthetically mimic the fine structure of geckos’ setae and spatulae, however, these polymeric dry adhesives (with a maximum achievable adhesive force of ∼ 3 N cm) are not comparable to the gecko feet. Subsequent work on macroscopic arrays of vertically-aligned multi-walled carbon nanotubes (VA-MWNTs) set a new record of 11.7 N cm for these carbon nanotube dry adhesives, though independent atomic force microscopic (AFM) measurements on VA-MWNTs showed strong nanometer-scale adhesion forces 200 times higher than those observed for gecko foot-hairs. VA-MWNTs have been widely studied for many years, while it is a recent effort to synthesize vertically aligned single-walled carbon nanotube (VA-SWNT) arrays. Having an extremely high aspect ratio, exceptional mechanical strength, and excellent electronic and thermal properties, the VA-SWNTs show great potential for dry adhesion applications with additional electrical/thermal management capabilities. Compared with VA-MWNTs, the smaller nanotube diameter, higher packing density, and more perfect electronrich p–p conjugated carbon structure could also allow VASWNT arrays to have more contact points per unit surface area and a stronger vdW force for each of the contacts. As a result, an enhanced adhesion force can be expected for the VA-SWNT dry adhesives. As far as we are aware, however, the possibility of using VA-SWNTs to mimic gecko feet has not yet been exploited largely because it is still at the state of art to repeatedly synthesize high quality VA-SWNT arrays. A few of previous studies have demonstrated that VA-SWNT arrays can be produced by pyrolysis of certain hydrocarbons either in the presence of oxidants like H2O or O2 and/or activated by microwave, RF discharge or hot filament. Following the recent success in minimizing catalytic particle aggregation by fast heating, we have used a combined method of plasma-enhanced chemical vapor deposition (PECVD) and fast heating for the syntheses of VA-SWNTs. This method, as described in detail in the Experimental section, allowed us to readily grow SWNTs in the form of large scale aligned arrays without the need for any additional H2O or O2. We report here the strongest achievable macroscopic adhesion force (up to 29.0 N cm) among all of the synthetic and natural gecko feet for the VA-SWNT arrays thus prepared. Figure 1a shows a typical scanning electron microscope image of VA-SWNT arrays produced by the combined PECVD and fast heating method. As can be seen in Figure 1a, the assynthesized nanotubes aligned almost normal to the substrate surface and have a fairly uniform tubular length. The corresponding cross-sectional view under a higher magnification shows that the as-grown nanotubes were very clean and formed into loosely-packed “bundles” (Fig. 1b). A Raman spectrum of the as-synthesized sample recorded with a 785 nm laser (Fig. 1c) clearly shows the strong resonant radial breathing modes (RBM) of SWNTs in the range of 130– 280 cm. The clear separation of the G peaks at ca. 1570 and 1600 cm seen in Figure 1c is also a characteristic for SWNTs. The high value of ∼ 12 for the G-band to D-band intensity ratio suggests a high degree of graphitization for the as-synthesized VA-SWNT arrays. We have also performed Raman measurements on the as-synthesized SWNT sample using the 514 nm laser (Fig. 1c), which can probe both semiconducting and metallic SWNTs and is often used to estimate their relative contents. The much stronger RBM peaks in the range of ca. 150–210 cm, attributable to semiconducting nanotubes, than those over ca. 210–280 cm, characteristic of metallic nanotubes, indicate that the as-synthesized SWNT sample contains a high percentage of the semiconducting nanotubes. A transmission electron microscope (TEM, Hitachi H-7600) image of individual nanotubes dispersed from an ethanol solution also reveals that the nanotubes thus prepared are SWNTs almost free from amorphous carbon (Fig. 1d). We demonstrate below an exceptional dry adhesive performance for the as-synthesized VA-SWNT arrays. Figure 2a shows a stainless steel adapter of 473 g being held by a thin C O M M U N IC A TI O N