The impact of ZnO configuration as an external layer on the sensitivity of a bi-layer coated polymer optical fiber probe

Abstract

Salinity magnitude changes are a critical factor for determining the chemistry of natural water bodies and biological processes. Label-free refractive index sensors are promising devices for detecting these changes. A polymer optical fiber (POF) sensor modified with cladding treatment and a bi-layer zinc oxide/silver (ZnO/Ag) nanostructure coating to determine sodium chloride concentration changes through refractive index variations in water is experimentally demonstrated. The use of three ZnO nanostructure shapes, nanoparticles and horizontally and vertically oriented nanorods, as an external layer and a broad spectrum light source from the visible (Vis) to the near infrared (NIR) region are investigated to achieve optimum sensitivity. The rms roughness, optical band-gap and zeta potential (ZP) value for the vertically oriented sample are 148 nm, 3.19 eV and 5.96 mV, respectively. In the NIR region the wavelength-intensity sensitivity values of probes coated with ZnO nanoparticles and horizontally and vertically oriented nanorods are 104 nm RIU^-1-12 dB RIU^-1, 63 nm RIU^-1-10 dB RIU^-1 and 146 nm RIU^-1-22 dB RIU^-1, respectively, and in the Vis area the values are 65 nm RIU^-1-14 dB RIU^-1, 58 nm RIU^-1-11 dB RIU^-1 and 89 nm RIU^-1-23 dB RIU^-1, respectively. The maximum amplitude sensitivity is obtained for the probe coated with vertically aligned ZnO nanorods in the NIR area due to the deeper penetration of evanescent waves, a higher surface-volume ratio, better crystallinity, more adhesive interactions with salt molecules, larger surface roughness and higher-order dispersion compared to the other coated ZnO nanostructures.

Fig. 1. A cross sectional FESEM image of deposited Ag nanoparticles on a partially unclad POF. The remaining cladding and Ag nanolayer are shown by arrows and their thicknesses are ∼100 nm and ∼20 nm, respectively.

Fig. 2. A schematic drawing of the experimental setup. The fabricated probe is immersed in saline with different concentrations from 0 to 20%. The two ends are connected to the light source and OSA.

Fig. 3. FESEM-EDX analysis. Cross sectional FESEM micrographs, EDX spectra and elemental mapping of (a and b) ZnO(NPs)/Ag/POF, (c and d) ZnO(NRH)/Ag/POF and (e and f) ZnO(NRV)/Ag/POF samples. The insets show corresponding top-view FESEM images.

Fig. 4. Histograms of ZnO nanostructure size distributions determined with ImageJ software: (a) nanoparticles, (b and c) horizontally oriented nanorods, and (d and e) vertically oriented nanorods. The size distribution is determined by measuring a sample consisting of 160 ZnO nanoparticles, 130 horizontal nanorods, and 70 vertical nanorods.

Fig. 5. The XRD patterns of the ZnO nanostructures deposited on the Ag/polymer substrate. The peaks observed at 2θ values of 31.7°, 34.4°, 36.2°, 47.5°, 56.6°,62.8°, 66.5°, 69.2° and 75.8° match perfectly with the (100), (002), (101), (102), (110), (103), (112), (201) and (202) crystalline planes of the hexagonal wurtzite structure of ZnO reported in JCPDS: 36-1451 with lattice parameters of a = b = 3.250 Å and c = 5.207 Å.

Fig. 6. AFM topographic 3D images of ZnO (a) nanoparticles, (b) horizontal nanorods, and (c) vertical nanorods deposited on Ag/POF.

Fig. 7. A schematic diagram of the shape dependent charge distribution on the ZnO nanostructure surface. Vertically oriented nanorods due to a higher surface area have better adhesion and interaction with saline molecules.

Fig. 8. Room temperature photoluminescence (PL) emission spectra for ZnO (a) nanoparticles, (b) horizontal nanorods, and (c) vertical nanorods deposited on an Ag/polymer substrate excited by 334 nm light.

Fig. 9. Schematic illustrations of changes in the band gap diagram of ZnO nanostructures due to alteration of the depletion layer width with saline concentration.

Fig. 10. Absorption spectra of saline solutions of different concentrations in water measured using a LAMBDA 1050 UV-VIS/NIR spectrophotometer in the (a) visible and (b) IR ranges. The insets summarize that the absorption range for specific selected wavelengths matches with the main propagating modes through the fiber.

Fig. 11. Transmission spectra of (a) ZnO(NPs)/Ag/POF, (b) ZnO(NRH)/Ag/POF and (c) ZnO(NRV)/Ag/POF fiber probes as a function of saline concentration. The insets show the Gaussian smoothed propagating mode centered at a wavelength of 770 nm.

Fig. 12. The wavelength and intensity changes as a function of refractive index in the propagating mode centered at a wavelength of 1072 nm for all samples.

Fig. 13. The (a) wavelength and (b) intensity changes of samples in the three main propagating modes.