Nanosphere lithography (NSL) is a widely used and convenient technique for large-area patterning of solid surfaces with periodic arrays of nano-features. In NSL a mono- or doublelayer of hexagonally close-packed nanospheres from a colloidal suspension is used as a shadow mask in order to modify the substrate underneath at the openings between each triple of spheres. This can be done by either removing or depositing material, using a big variety of materials combinations, mask opening shape modifications and materials deposition or removal techniques in order to tailor surface nanopatterns.
Among the techniques applied to obtain the nanosphere mono- or double-layer masks, methods based on the convective self-assembly are most important. Convective self-assembly (CSA) of colloidal nanospheres into ordered thin sphere layers takes place at the solid-liquid-gas triple phase boundary of a drying droplet of a colloidal suspension on a solid substrate surface by evaporation of the liquid phase. CSA happens at the triple-phase boundary of any drying droplet as well as during dip coating or when a droplet is moved across a surface using a doctor blade.
Since the latter technique is particularly useful for creating nanomasks, the transport processes occurring in a colloidal suspension droplet pulled by a doctor blade were analyzed for the first time numerically. Both the in-plane and the out-of-plane arrangement of spheres are studied as a function of experimental conditions such as the doctor blade velocity (triple phase boundary velocity), suspension evaporation flux, colloidal particle concentration, colloidal sphere size fluctuations, variations of the flux direction and the presence and crystallographic orientation of nuclei at which the 2d growth of a colloidal mask can start. The resulting arrangements of colloidal spheres are statistically evaluated using Delaunay triangulation procedures.
The simulations reproduce well experimentally observable stripe patterns of mono- and double-layer areas and a method is derived to suppress double-layer formation. Grain boundaries in the 2d sphere monolayers are found parallel to the doctor blade direction of motion as in corresponding experiments using polystyrene spheres. In addition the simulations reveal that a certain finite width of the particle size distribution is beneficial for the crystalline quality of a monolayer mask. The simulations allow to identify stable and less stable growth directions on seed monolayers. Last but not least the mechanism of vacancy formation in monolayer sphere masks can be observed in the simulations. It is expected that the simulation results will largely contribute to bring the convective self-assembly of colloidal nanomasks to perfection.