Microwell-directed self-assembly of vertical nanowire arrays

Vertical arrays of anisotropic particles are desired for many applications including solar cells, battery electrodes, and lasers. Partially etched nanowires (PENs) are hybrid silica nanotube/nanowires with partial metallic cores. These particles spontaneously form vertical arrays in which on average 70% of the particles are oriented perpendicular to the underlying substrate. Here, we perform PEN self-assembly on lithographically prepared substrates patterned with square microwells having dimensions comparable to the length of the particles. Particle self-assembly is observed both in the microwells and on the intervening surfaces. PENs both directly deposit into a well and diffuse across the surface between microwells until falling into a well. Assembly occurs as the local concentration of PENs in a well increases by these two mechanisms. Microwells provide a way to control array location on a surface and improve standing percentages up to 100% when the edge dimensions are decreased to a size approximately equal to the nanowire length. Microwells also protect against array disruption during sample drying.