Carbon-Nanotube-Based (Gecko- Inspired) Adhesives Bryan Schubert March 3, 2008

Gecko Adhesion Notable features –Strength: 10 N/cm 2 from van der Waals forces –Reusable: 30 days between molts –Directional –Adheres to most surfaces; smooth or rugged –Self-cleaning 1. K. Autumn et al., J. Exper. Biol. 209, 3569 (2006).

Gecko Adhesion 50 μm5 μm 1 μm 1. K. Autumn et al., J. Exper. Biol. 209, 3569 (2006). Basic principle of operation – Contact splitting –Density of contacts scales by contact radius as N ~ R -2 –Force per contact scales as F ~ R -1 (JKR: F = 3πRW) –Force per unit area scales as FN ~ R -1

Carbon Nanotube Adhesives - Fabrication Vertically-aligned multi-walled carbon nanotubes (VA- MWNTs) –Chemical vapor deposition (CVD) on silicon substrate using thin film iron as catalyst layer and aluminum film as adhesion layer 2,3. 2. L. Delzeit et al., J. Phys. Chem. B 106, 5629 (2002). 3. L. Delzeit et al., J. Phys. Chem. B 106, 5629 (2002). 1 μm Oxygen plasma etch 5 μm Top view Diameter: nm Length: μm Density: cm -2

Carbon Nanotube Adhesives - Fabrication Vertically-aligned single-walled carbon nanotubes (VA- SWNTs) –Plasma-enhanced CVD on SiO 2 /Si wafer with iron layer as catalyst and pre-coated aluminum adhesion layer. Fast heating 4 was used to minimize catalytic particle aggregation, creating single-walled carbon nanotubes. Nanotube diameter: 2-3 nm Bundle diameter: nm Length: μm Bundle density: cm Y. Q. Xu et al., J. Am. Chem. Soc. 128, 6560 (2006).

Carbon Nanotube Adhesives - Performance VA-MWNTs Attachment Cycle Adhesion Strength (N/cm 2 ) 5. Y. Zhao et al., J. Vac. Sci. Technol. B 24, 331 (2006). glass 4 mm 2 glass 6 mm 2 Au 4 mm 2 parylene 7 mm 2 Si 5 mm 2 GaAs 7.8 mm 2

Carbon Nanotube Adhesives - Performance VA-SWNTs –Normal Adhesion: N/cm 2 –Shear Adhesion: 4-16 N/cm 2 –Performance decreases with use 6. L. Qu and L. Dai, Adv. Mater. 19, 3844 (2007). 4 mm x 4 mm Before useAfter use

Carbon Nanotube Adhesives - Performance VA-SWNTs –Mostly semi-conducting nanotubes 6. L. Qu and L. Dai, Adv. Mater. 19, 3844 (2007). 4 mm x 4 mm 4.70 N 0.01 N

Comparison MaterialGeckoVA-MWNTsVA-SWNTs Maximum Adhesion 10 N/cm 2 12 N/cm 2 29 N/cm 2 SurfacesAnySmooth (glass) ReusableYesYes but degrades Self-cleaningYesMaybe (uses Lotus effect) 7 Maybe (using Lotus effect) DirectionalYesProbably not Electrically Conductive NoYes Thermally Conductive NoYes 7. S. Sethi et al., Nano Lett., /nl (2008).