Carbon-Nanotube-Based (Gecko-Inspired) Adhesives

Bryan Schubert

March 3, 2008

Gecko Adhesion

• Notable features– Strength: 10 N/cm2 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 μm 5 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

R

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 layer2,3.

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: 20-30 nm

Length: 5-100 μm

Density: 1010-1011 cm-2

Carbon Nanotube Adhesives - Fabrication

• Vertically-aligned single-walled carbon nanotubes (VA-SWNTs)– Plasma-enhanced CVD on SiO2/Si wafer with iron layer as catalyst and

pre-coated aluminum adhesion layer. Fast heating4 was used to minimize catalytic particle aggregation, creating single-walled carbon nanotubes.

Nanotube diameter: 2-3 nm

Bundle diameter: 20-30 nm

Length: 5-100 μm

Bundle density: 1011 cm-2

4. Y. Q. Xu et al., J. Am. Chem. Soc. 128, 6560 (2006).

Carbon Nanotube Adhesives - Performance

• VA-MWNTs

Attachment Cycle

Ad

hes

ion

Str

eng

th (

N/c

m2 )

5. Y. Zhao et al., J. Vac. Sci. Technol. B 24, 331 (2006).

glass 4 mm2

glass 6 mm2

Au 4 mm2

parylene 7 mm2

Si 5 mm2

GaAs 7.8 mm2

Carbon Nanotube Adhesives - Performance

• VA-SWNTs– Normal Adhesion: 12-29 N/cm2

– Shear Adhesion: 4-16 N/cm2

– Performance decreases with use

6. L. Qu and L. Dai, Adv. Mater. 19, 3844 (2007).

4 mm x 4 mm

Before use After 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

Material Gecko VA-MWNTs VA-SWNTs

Maximum Adhesion

10 N/cm2 12 N/cm2 29 N/cm2

Surfaces Any Smooth (glass) Smooth (glass)

Reusable Yes Yes but degrades Yes but degrades

Self-cleaning Yes Maybe (uses Lotus effect)7 Maybe (using Lotus effect)

Directional Yes Probably not Probably not

Electrically Conductive

No Yes Yes

Thermally Conductive

No Yes Yes

7. S. Sethi et al., Nano Lett., 10.1021/nl0727765 (2008).