Study of the Photoluminescence Enhancement Observed in ZnO Nanowire Gratings Optimally Grown by the Hydrothermal Method

Abstract

An original ZnO nanowire (NW) architecture has been developed, entirely based on a soft chemistry approach, and thoroughly assessed through optical measurements and electromagnetic simulations. This architecture relies on the photoimprinting of a sol–gel ZnO‐based photosensitive seed layer combined with the subsequent localized hydrothermal growth of ZnO NWs. The optimization of the elaboration protocol has been shown to lead to uniform and reproducible linear and periodic gratings of ZnO NWs with a width/pitch of 2/4 µm. The NW gratings are compared with full‐covered samples (NWs coating) elaborated from a nonimprinted seed layer. A morphological study reveals that NW gratings present a peculiar hedgehog‐like profile. Standard and angle‐resolved photoluminescence studies demonstrate that the ZnO NWs visible emission is strongly modified by the presence of NW gratings and that its red part is directionally extracted and enhanced by a factor of up to 2. The electromagnetic simulations performed for both samples highlight the role of the gratings acting as coupled microcavities that boost the ZnO emission through light localization and diffractive mechanisms. It enables the extraction of the resonant photons at specific angles and wavelengths.

Enhanced Spontaneous Light Emission of ZnO Nanowire-Based Gratings

“…The configuration is the same as in our previous work. 24 A 425 nm long-pass filter was used to remove the excitation line. The signal was then focused on the slit of a 32 cm focal monochromator and detected with a CCD camera.…”

“…These resonances reveal that the fundamental guided mode is phasematched with the emitted plane wave corresponding to the fifth diffraction order of the grating. 24 In comparison, a 640 nm-thick NW coating presents an almost constant PL from 0 to 40°, indicating a diffuse emission due to the scattering extraction process. However, the resonant emission around 65°f or the grating nearly disappears for thinner gratings, 200 and 300 nm high.…”

“…These electromagnetic resonances for a unique ribbon have indeed been demonstrated to originate from Fabry−Perot interference. 24 The Purcell factor slightly decreases when a roughness of Δh = 200 nm is introduced at the upper interface of the ribbon. To determine the Purcell effect of the ZnO NWs forming the cavity, we calculate the Purcell factor weighted by the PL spectrum: g(λ) × F(λ).…”

“…■ FABRICATION METHOD OF ZNO NW-BASED STRUCTURES ZnO NW coatings and gratings have been elaborated using a two-step sol−gel/hydrothermal procedure that has been fully detailed in our previous paper and is briefly recalled hereafter. 24 A specifically developed sol−gel formulation is first used for the deposition by spin-coating of a ZnO-based resist thin film on 2.5 × 2.5 cm 2 quartz substrates. A crystallized ZnO film is then obtained after annealing the sample at 540 °C.…”

“…In order to merge both periodic and random texturing approaches, the microstructuring of ZnO NWs arranged in a one-dimensional grating or in photonic crystal membranes has been proposed to combine both Bragg and scattering effects. However, to our knowledge, a full study explaining the improvement of the photoluminescence quantum yield (PLQY), defined as the ratio of emitted photons to injected photons, is still lacking.…”