Sharp blue emission of ZnO crystals by supercritical CO₂ processing

We report the synthesis of hierarchically structured ZnO films that exhibit primarily near band edge (NBE) emission at room temperature in response to UV light excitation (λ_excitation = 337 nm). The films are deposited by a scalable supercritical CO₂ (sc-CO₂) based nebulization process, under mild processing conditions (P = 145 bar; T = 110 °C). This process consists of evaporation of CO₂ enriched water micro-droplets (∼3 μm diameter) deposited from an aerosol onto heated substrates at 135 ± 5 °C. Deposition of aerosol onto silicon substrates results in the formation of porous scaffolds consisting of 1-5 μm sized pores. Micro-porous hydrozincite scaffolds are produced along with ZnO (hexagonal wurtzite type zincite) and zinc acetate in minor quantities as indicated by X-ray diffraction (XRD) and micro-Raman analysis. Band edge absorption (362 nm) and band gap values (3.27 eV) confirm ZnO optical characteristics. Treatment of deposited scaffolds with warm water at 80 °C for 2 h results in various ZnO morphologies such as, flowers, foams, disks and rods with nano and micro-scale features. This treatment doubles the NBE intensity but also results in slight increase in green photoluminescence (PL). The ability to fabricate varieties of zinc oxide nanostructures implies for the implementation of ZnO as multifunctional material with potential applications in diverse fields of nanotechnology. Moreover, with strong blue emission properties, these films have potential applications in the development of optoelectronic devices such as light-emitting diodes (LEDs), low threshold UV/sharp blue lasers and nanoscale lasing applications.