Gecko-Foot-Mimetic Aligned Single-Walled Carbon Nanotube Dry Adhesives with Unique Electrical and Thermal Properties**
By Liangti Qu and Liming Dai*
The unusual ability of gecko lizards to climb on any vertical surface and hang from a ceiling with one toe has inspired sci- entific 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-2) 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 poly- mer pillars to mimic gecko feet.[2,3] 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-2) are not comparable to the gecko feet. Subsequent work on macro- scopic arrays of vertically-aligned multi-walled carbon nano- tubes (VA-MWNTs) set a new record of 11.7 N cm-2 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 sin- gle-walled carbon nanotube (VA-SWNT) arrays.[7-13] Having an extremely high aspect ratio, exceptional mechanical strength, and excellent electronic and thermal properties,
the VA-SWNTs show great potential for dry adhesion appli- cations with additional electrical/thermal management capa- bilities. Compared with VA-MWNTs, the smaller nanotube diameter, higher packing density, and more perfect electron- rich p-p conjugated carbon structure could also allow VA- SWNT 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 ar- rays can be produced by pyrolysis of certain hydrocarbons either in the presence of oxidants like H2O or O2 and/or acti- vated by microwave, RF discharge or hot filament.[7-13] Fol- lowing the recent success in minimizing catalytic particle ag- gregation 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 sec- tion, allowed us to readily grow SWNTs in the form of large scale aligned arrays without the...