Self Assembled Monolayers (SAMs)

Tutorial

SAMs: 2-D Self-Assembly at Atomic Level

• Self-assembly in liquid or vapor phase driven by amphiphilic character of the molecules

• Ordered molecular 2D assemblies formed spontaneously by the chemisorption of the head group to a substrate.

substrate

Head group

Tail

Functional group(e.g., -CH3, -COOH, -OH)

• Head group– Affinity to substrate to induce

chemisorbed surface reactions– High energy chemical bound (100 kJ/mol)

provides molecular stability (thermal, chemical, biological)

• Tail group– Closed-packed structure driven by Van

der Waals interaction between alkyl chains

• Functional group– Defines properties of monolayer, e.g.,

hydrophobicity/hydrophilicity, affinity to anchor with biological entities

SAMs precursors

• Key selection criteria– Head group determined by substrate

• Silanes for (trichlorosilane, alkylsilane) for oxides • Thiols (organosulfurs) for metals:

– Functional group• Non-polar hydrophobic: e.g., -CH3

• Polar hydrophilic: -OH, -COOH

Thiols for SAMs on Au surfaces Silanes for SAMs on oxide surfaces

DiBenedetto, S.A., et al., Adv. Mater., 2009. 21(14-15): 1407-1433.

Applications

• Permeation coating for flexible electronics• Nanostructure functionalization• Organic thin film transistor (gate dielectrics,

contacts)• Anti-stiction for MEMS and NEMS • Cell adhesion / protein adsorption• Large surface area surface coating for permeation

/ wettability control

Herrmann et al.,J.Micromech. and Microeng, 2005. 15(5): p. 984-992.

Teshima, K., et al., Langmuir, 2003. 19(20): p. 8331-8334.

Hydrophobic SAM on oxide

Biological assays Gas permeation barrier

Gurard-Levin, et al.. Annual Rev. of Analyt. Chem., 2008. 1: p. 767-800.

Asay and al., Tribol. Lett., 2008. 29(1): p. 67-74.

MEMS lubrication coating

Hydrophobic coating on Al2O3

85

90

95

100

105

110

0.25s 1s 5s

Cont

act a

ngle

[deg

res]

Pulse time [s]

60s

300s

600s

Source @ 105°CReactor @ 105°C

Expo time[s]

• Precursor: Dodecyltricholorosilane (DTS)• Source @ 100-120°C, Reactor 100-120°C

• Sample: Silicon with 200 nm thermal ALD Al2O3

• Sample prep: none• Result: 110° contact angle achieved with 1s pulse, 600s expo

Contact angles following DTS coatings on Al2O3 coated samples in Savannah 200

600 s DTS exposure

200 nm Al2O3 surface.

Combining ALD and SAMs

PEG hydrophilic coating on Nylon

• Precursor: 2Methoxy(polyethyleneoxy)propyl)trimethoxylsilane• Source @ 100°C, Reactor 50°C• Sample: Nylon12• Sample prep: Al2O3 ALD seed layer deposited at 50°C

• Result: nylon made more hydrophilic by combination of ALD+ polyethylene glycol coating

Tuning coating performance by combining ALD and SAMs

Oleophobic coatings

• Comment– Contact angle for oil increases

from 15 to 50 deg after 600 s expo on metal

• Precursor– Tridecafluoro 1,1,2,2

tetrahydrooxtyl)trichlorosilane

• Samples– Oxide:Si+25 nm Al2O3– Metal: Si+10 nm Ni

• Recipe– 75C source, 120C reactor– 1x 1s pulse, 600 s expo

2011.01.18_run 12 _DSC707

• Comment– Top view of samples from

previous slide– Notice oleophilicity of metal

surface and significant improvement after SAM expo

Oleophobic coating expo time

• Comment– 900 s expo appears optimal

• Samples– Si+10 nm Ni

• Recipe– 75C source, 120C reactor– 5x 1s pulses, 300 to 1800 s expos

Textiles

Wool fabric coated with ALD TiO2EDS X-ray map of the wool

Market for performance Textiles is large – domestic, to industrial

•Spill resistant fabrics – SAMs hydrophobic coatings•High moisture absorbancy fabric (sportswear) – SAMs Hydrophilic coating• Abrasion resistant fabrics – Al2O3 coatings