Self-assembled Monolayer - Kinetics

Kinetics

There is evidence that SAM formation occurs in two steps, an initial fast step of adsorption and a second slower step of monolayer organization. Adsorption occurs at the liquid-liquid, liquid-vapor, and liquid-solid interfaces. The transport of molecules to the surface occurs due to a combination of diffusion and convective transport. According to the Langmuir or Avrami kinetic model the rate of deposition onto the surface is proportional to the free space of the surface.

Where θ is the proportional amount of area deposited and k is the rate constant. Although this model is robust it is only used for approximations because it fails to take into account intermediate processes. Dual polarisation interferometry being a real time technique with ~10 Hz resolution can measure the kinetics of monolayer self-assembly directly.

Once the molecules are at the surface the self-organization occurs in three phases:

1. A low density phase with random dispersion of molecules on the surface.

2. An intermediate density phase with conformational disordered molecules or molecules lying flat on the surface.

3. A high density phase with close packed order and molecules standing normal to the substrate's surface.

The phase transitions in which a SAM forms depends on the temperature of the environment relative to the triple point temperature, the temperature in which the tip of the low density phase intersects with the intermediate phase region. At temperatures below the triple point the growth goes from phase 1 to phase 2 where many islands form with the final SAM structure, but are surrounded by random molecules. Similar to nucleation in metals, as these islands grow larger they intersect forming boundaries until they end up in phase 3, as seen below.

At temperatures above the triple point the growth is more complex and can take two paths. In the first path the heads of the SAM organize to their near final locations with the tail groups loosely formed on top. Then as they transit to phase 3, the tail groups become ordered and straighten out. In the second path the molecules start in a laying down position along the surface. These then form into islands of ordered SAMs, where they grow into phase 3, as seen below.

The nature in which the tail groups organize themselves into a straight ordered monolayer is dependent on the inter-molecular attraction, or Van der Waals forces, between the tail groups. To minimize the free energy of the organic layer the molecules adopt conformations that allow high degree of Van der Waals forces with some hydrogen bonding. The small size of the SAM molecules are important here because Van der Waals forces arise from the dipoles of molecules and are thus much weaker than the surrounding surface forces at larger scales. The assembly process begins with a small group of molecules, usually two, getting close enough that the Van der Waals forces overcome the surrounding force. The forces between the molecules orient them so they are in their straight, optimal, configuration. Then as other molecules come close by they interact with these already organized molecules in the same fashion and become a part of the conformed group. When this occurs across a large area the molecules support each other into forming their SAM shape seen in Figure 1. The orientation of the molecules can be described with 2 parameters, α and β. α is the angle of tilt of the backbone from the surface normal. In typical applications α varies from 0 to 60 degrees depending on the substrate and type of SAM molecule. β is the angle of rotation along the long axis of tee molecule. β is usually between 30 and 40 degrees. In some cases existence of kinetic traps hindering the final ordered orientation has been pointed out. Thus in case of dithiols formation of a "lying down" phase was considered an impediment to formation of "standing up" phase, however various recent studies indicate this is not the case.

Many of the SAM properties, such as thickness, are determined in the first few minutes. However, it may take hours for defects to be eliminated via annealing and for final SAM properties to be determined. The exact kinetics of SAM formation depends on the adsorbate, solvent and substrate properties. In general, however, the kinetics are dependent on both preparations conditions and material properties of the solvent, adsorbate and substrate. Specifically, kinetics for adsorption from a liquid solution are dependent on:

• Temperature – room temperature preparation improves kinetics and reduces defects.
• Concentration of adsorbate in the solution – low concentrations require longer immersion times and often create highly crystalline domains.
• Purity of the adsorbate – impurities can affect the final physical properties of the SAM
• Dirt or contamination on the substrate – imperfections can cause defects in the SAM

The final structure of the SAM is also dependent on the chain length and the structure of both the adsorbate and the substrate. Steric hindrance and metal substrate properties, for example, can affect the packing density of the film, while chain length affects SAM thickness. Longer chain length also increases the thermodynamic stability.