Bottom-up fabrication:Self-assembled monolayers

Self-assembled monolayers (SAM) of organic molecules are molecular assemblies formed spontaneously on surfaces by adsorption and that are organized into relatively large ordered domains. In some cases, the molecules that form the monolayer do not interact strongly with the substrate. In other cases, the molecules possess a functional group that has a strong affinity to the substrate that anchors the molecule. Such a SAM consisting of a head group, tail and functional end group is depicted in Figure 1. Common head groups include thiols, silanes, and phosphonates.

Figure 1: Self-assembly of monolayers. The head groups are anchored onto the substrate while the tail groups assemble together far from the substrate. Image courtesy of UF/IFAS.

SAMs are formed by the chemisorption of head groups onto a substrate from a vapor or liquid phase, followed by the slow organization of tail groups. Initially, at small molecular density, adsorbate molecules form either a disordered mass of molecules or an ordered two dimensional "lying down phase". At higher molecular coverage, over a period of minutes to hours, the molecules begin to form three dimensional crystalline or semicrystalline structures on the substrate surface. The head groups assemble together on the substrate, while the tail groups assemble far from the substrate. Areas of close-packed molecules nucleate and grow until the surface of the substrate is covered in a single monolayer.

Thin film SAMs can be placed on nanostructures, functionalizing the nano-structure. This is advantageous because the nanostructure can now selectively attach itself to other molecules or SAMs. This technique is useful when designing biosensors or other devices that need to separate one type of molecule from its environment.

Following is a video demonstrating the self-assembly of lithographically patterned 3D micro/nanostructures.