Tailoring the emission spectrum of colloidal nanocrystals by means of lithographically-imprinted hybrid vertical microcavities

Abstract

Colloidally synthesized CdSe/ZnS core/shell semiconductor nanocrystals (NCs) show highly efficient, narrow-width and size-tunable luminescence. Moreover, they can be incorporated in polymer matrices and deposited on solid substrates by means of spin-coating techniques. When embedded between two mirrors a NCs/polymer blends microcavity is realised, thus allowing to tailor the photoluminescence spectrum of these emitters. By virtue of the quantized photonic and electronic density of states, colloidal quantum dots embedded in a single mode vertical microcavity are good candidates for the fabrication of high-efficiency emitting devices with high spectral purity and directionality. In this paper, we have applied a new organic-inorganic hybrid technology for the fabrication by imprint lithography (IL) of vertical microcavities that embed colloidal quantum dots. Two dielectric distributed Bragg reflectors (DBR) are evaporated on two different substrates. The active organic layer (NCs/polymer blend) is spin coated on the first DBR, whereas a lithographic pattern is realized on the second DBR, used as the IL mold. The two parts are then assembled together in an IL process in order to create a vertical microcavity. The fine control of the thickness of the active material waveguide layer can be achieved through the mold patterning depth and the IL process parameters. All the fabrication steps have been engineered in order to decrease the thermal stress of the active layer. The effectiveness of this technology is demonstrated by the room temperature photoluminescence (PL) spectra, recorded on the fabricated microcavity, which show a sharp emission peak with a line width of 4.15 nm.