Tip-Based Hybrid Simulation Study of Frictional Properties of Self-Assembled Monolayers: Effects of Chain Length, Terminal Group, Scan Direction, and Scan Velocity


The frictional properties of alkanethiol self-assembled monolayers (SAMs) on Au(111) at room temperature were studied by using hybrid molecular simulations at the atomic force/frictional force microscopy (AFM/FFM) experimental time scale. Various parameters influencing frictional properties were investigated, including chain length, terminal group, scan direction, and scan velocity. Simulation results show that frictional force decreases with an increase of chain length and that hydrophilic –OH terminated SAMs have higher frictional force than hydrophobic –CH3 terminated SAMs. Frictional force as a function of normal load exhibits different behaviors for –CH3 and –OH terminated SAMs. Simulation results further show friction anisotropy on SAMs at low and high temperatures. Frictional force is the smallest when scanned along the tilt direction, the largest when scanned against the tilt direction, and in-between when scanned perpendicular to the tilt direction. Finally, simulation results also show the dependence of friction on scan velocity. Friction exhibits a maximum for hydrophobic –CH3 terminated SAMs and decreases for hydrophilic –OH terminated SAMs as scan velocity increases. It approaches a constant value at high scan velocities for both surfaces.


Hybrid Method:

 

Figure 1. A schematic representation of the simulation system: AFM tip/cantilever assembly and alkanethiol SAMs on Au(111) and the governing equations for the motion of the tip.


Simulation Results:

The following figures show the frictional force is a function of chain length, terminal group, scan direction, and scan velocity.

Figure 2. Snapshots of (a) C7CH3 and (b) C11CH3 SAMs under different normal loads at 300K.

Figure 3. Frictional force vs. normal load for –CH3 and -OH terminated alkanethiol SAMs of different chain lengths at v=400 nm/s, T=300 K, and q=0º.

 

Figure 4. Frictional force as a function of scan direction from hybrid simulations for C11CH3 SAMs on Au(111) at 300 K.

Figure 5. Frictional force as a function of scan velocity for (a) C11CH3, and (b) C11OH SAMs at 300 K.