Experimental evidence has accumulated in the recent decade that nanoscale patterns can self-assemble on solid surfaces. A two-component monolayer grown on a solid surface may separate into distinct phases. Sometimes the phases select sizes about 10 nm, and order into an array of stripes, disks or other regular patterns. These behaviors are intriguing because they are absent in bulk phase separation. The ability of patterning nanostructures on a surface is very important for many modern technological applications, such as in microelectronics circuits and digital storage media. It also opens up the possibility of fabricating cheap, large area devices using non‑ lithographic techniques.
Why do atoms self-assemble? What sets the feature size? The answers differ for different material systems. A unifying concept, however, can be identified. For many reasons the free energy of a material system depends on its configuration (e.g., the composition of the phases and their spatial arrangement). When the configuration changes, the free energy also changes. This defines thermodynamic forces that drive the configuration change. The change is effected by mass transport processes, such as diffusion. To assemble a nanostructure, some of the forces must act over the scale comparable to the feature size, and are therefore much longer ranging than atomic bond length.
We developed a thermodynamic framework to study the remarkable phenomena. Based on our continuous phase field model, we developed the numerical technique and performed large-scale simulation of the whole process of formation and evolution of nanostructures on a solid surface. The simulation reveals remarkably rich dynamics and suggests a significant degree of experimental control in growing ordered nanoscale structures. Our work reveals how various parameters, including free energy of mixing, phase boundary energy, surface stress, average concentration, anisotropy etc, may influence the nanostructures. The model and simulation technique provides a powerful tool to conduct “numerical experiments” and investigate various behaviors associated with nanostructure evolution. We are now studying guided self-assembly, i.e. how to grow desired fine nanoscale structures by pre-patterning some coarse structures.
The following are some examples of our simulations.