Polymer coatings on nano-structured semiconductor surfaces

We present a method of encasing a layer of freestanding nanostructures with an insulating photo-crosslinked polystyrene (PS) thin film that is flexible and transparent to visible light. In this simple and inexpensive procedure, polystyrene dissolved in toluene is applied by spin-coating in ambient air and immobilized by ultraviolet light. Because the film is crosslinked it is insoluble and will not intermix with other applied films that might compromise the insulation of the layer. We used the film to prepare a heterojunction UV light emitting diode from vertically aligned ZnO nanorods grown on the conductive surface of SnO2-coated glass. In this application overall film thickness was about one micrometer and at the nanorod tips it was in the order of tens of nanometers. The tips of the nanorods could be exposed by oxygen plasma and contacted directly by other films. The PS film may be a single layer or a composite built of many layers. We report relationships between layer thickness and solution concentration and viscosity for 280 kg/mol and 1800 kg/mol PS that allow for a general recipe for use on freestanding nanostructures of varying dimensions and packing densities.