Study of NBE emission enhancement with an absence of DL emission from ZnO nanorods through controlled growth on ultra-thin Ag films

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ZnO nanorods (NRs) exhibiting enhanced ultra-violet near band edge (UV-NBE) emission without a broad visible deep level (DL) emission has been investigated on catalytically grown ZnO/Ag hybrid nanostructure. The hybrid structure is fabricated in two steps, (1) Thermal evaporation of ultra-thin catalytic layer of Ag with mass thickness ∼1nm on glass substrate followed by annealing process from 50 to 250°C and (2) vertical growth of ZnO NRs by hydrothermal reaction process on all Ag films. The surface properties of Ag layer such as particle size, inter-particle distance, particles number density, surface roughness and surface coverage area were altered through annealing process. Annealing at 100°C modifies Ag from quasi-amorphous to nanocrystalline leading to high density growth and high aspect ratio of ZnO NRs where as a random and less density growth was realized at 250°C due to increase of both particle size and inter-particles distance in Ag layer. X-ray diffraction reveals a predominant growth of (002) plane at 100°C confirming the formation of wurtzite phase of ZnO NRs with highest texture coefficient of 2.35. Raman spectra verify the chemical structure of ZnO with very good crystallinity. Absorption spectra demonstrates the overlapping of surface plasmon resonance (SPR) and exciton bands up to 200°C while the excitonic absorption band is resolved at above 200°C because of the red shift in SPR due to change in surface properties of Ag layer. At 250°C, a broad optical absorption spectra from 300 to 800nm attributed to the dominant properties of SPR and exciton. Besides acting as a catalyst, Ag interlayer enhances the NBE emission at above 200°C through electrons transfer from Ag to ZnO which is quite possible because of the direct contact between them, explained by giving energy band diagram. The morphology is such that there is an increase in passage for light interaction due to less density and random growth of ZnO NRs leading to increase light scattering over absorption of Ag interlayer.