|M.Sc Student||Rotem Sagi|
|Subject||Light Controlled Self Assembly of Structures from Silicon|
|Department||Department of Physics||Supervisor||Professor Uri Sivan|
|Full Thesis text|
This research presents a technique for fabricating nonspherical colloidal particles using photolithography that has been developed. The fabricated colloids are silicon boxes with typical sizes of 5x5x10 µm. These anisotropic “bricks” are studied at an air-water interface, where they self assemble into open structures or chains. Each brick consists of a p-n junction, which is localized away from the brick's edges. The p-n junction is overall neutral and hardly affects the potential presented by the brick to the solution. However, the p and n edges are opposite charged and hence create an attractive dipole-dipole interaction with other bricks. In case of absence of light the bricks are expected to build a filament. However, light generates electrons and holes that drift in the built-in junction and tend to flatten the brick's band bending. This flattening is equivalent to forward biasing of a diode which tends to reduce the charge presented to the other brick and, hence, reduces the brick's dipole moment. Since the bricks are overall charged, the elimination of the dipole moment should lead to brick-brick repulsion resulting in the melting of the filaments.
The “bricks” experience strong, anisotropic, and long ranged attractive capillary interactions which greatly exceed thermal energy of kBT . ‘Capillary forces’ are interactions between particles mediated by fluid interfaces and originate in the overlap of the menisci formed around each separate particle. In the case of the anisotropic “bricks” the capillary force rises from irregular wetting at the particle surface.