Quantitative Measurements of Nanoscale Friction of Thin Films on Si(111) by Scanning Force Microscopy

1. Introduction:

Friction is a crucial factor that controls the efficiency and durability of moving mechanical assemblies in micro- or nano-electromechanical systems (MEMS/NEMS). It is affected by environmental conditions, such as humidity. Self-assembled monolayers (SAMs) are one of strategies used for minimizing stiction and reducing adhesion and friction in MEMS/NEMS. SAMs on silicon traditionally rely on siloxane chemistry on oxidized surfaces. The alternative approach is the direct reaction between alkene and H-terminated silicon surface to form thin organic films via Si-C linkage. The major advantage of the monolayers over conventional SAMs of thiols on gold is their high stability. Although various techniques have been used to characterize alkyl monolayers on Si surface, the frictional properties of the monolayers are not reported. 

In this study, quantitative nanoscale frictional and pull-off forces were measured by scanning force microscopy for various C12 alkyl monolayers with different terminal groups ranging from hydrophobic (terminated by -CH3) to hydrophilic (terminated by -COOH) on atomically flat Si(111) surfaces at a variety of relative humidities. These monolayers were prepared and characterized by the techniques, such as scanning probe microscopy, contact angle goniometry, and X-ray photoelectron spectroscopy. The magnitude of the frictional force was found to decrease in this order: SiO2/Si > –COOH,Si > –COOCH3,Si > –CH3,Si ~ –CH3,Au. Comparing to the native oxide silicon surface, friction coefficient decreases dramatically when the surface is coated with different monolayers: by 20 times for hydrophobic monolayers and by 3 times for hydrophilic monolayers. Pull-off forces are generally larger for hydrophilic than hydrophobic surfaces. For hydrophobic surfaces, frictional and pull-off forces are not sensitive to relative humidity. For hydrophilic surfaces, however, friction coefficient decreases while pull-off force increases with increase of relative humidity.


2. Monolayer preparation and characterization:




3. Scanning force microscopy


4. Nanoscale friction in Ambient condition:

5. Nanoscale friction and pull-ff forces in humidity air:

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January 21, 2002.