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This study reports the humidity sensing characteristics of aluminum (Al) and fluorine (F) co-doped zinc oxide (ZnO) nanostructured thin films with different Al concentrations. Sol–gel method was used in the synthesis of Al and F doped ZnO nanoparticles. The Al concentration was changed to 0.5 mol%, 1 mol% and 1.5 mol% while F concentration was kept constant at 2 mol%. Al and F co-doped ZnO (AFZO) nanostructured thin films for manufacturing relative humidity (RH) sensors were fabricated on glass substrates with a dip coating technique and then annealed at 500 °C for 1 h. The scanning electron microscopy (SEM) micrographs indicated that all AFZO films had uniform and homogeneous surfaces. The X-ray diffraction (XRD) patterns revealed that AFZO films were polycrystalline and have a hexagonal wurtzite structure with a preferential orientation along the (002) plane. The RH sensing characteristics of AFZO sensors was determined by electrical resistance measurements in the range of 40–90% RH at room temperature (RT). Both the electrical resistivity and RH sensing characteristics of AFZO sensors were found to be highly dependent on the Al concentration. All AFZO sensors exhibited high sensitivity, excellent stability, fast response and recovery times and a repeatable characteristic. The lowest electrical resistivity and best humidity sensor characteristics were obtained for AFZO sensor containing 1 mol% Al (AFZO–010). The sensitivity ratio between 40 and 90% RH is approximately 247 × for AFZO–010. This study showed that AFZO nanostructured thin films are promising for high performance humidity sensor applications.

In this study, the effect of fluorine (F) doping on the humidity sensing properties of F doped zinc oxide structures was investigated. Undoped zinc oxide (ZnO) and F doped zinc oxide (FZO) nanoparticles were synthesized by sol gel method. The synthesized nanoparticles were coated on glass substrates by dip coating method. In the coating process, the glass substrates were immersed in undoped ZnO and FZO solutions and withdrawn at a rate of 25 cm/s at room temperature. Then, the thin film coated glasses are annealed in a square furnace at 500 °C for 1 h. Structural analyzes of the produced thin films were performed using X-ray diffraction and scanning electron microscopy. The thin film nanostructures were placed in the homemade system in order to investigate the humidity sensing characteristics such as relative humidity dependent resistance change, sensitivity, hysteresis, stability, response and recovery times and reproducibility. As a result, it is predicted that the FZO structures can be used in the humidity sensing applications and it is determined that the optimal F additive amount for application is 2 mol%.

Undoped ZnO and Zn0.995−x Al0.005Y x O (x = 0.0 (AZO), 0.005, 0.01, and 0.02) solutions were prepared by sol–gel method and grown on soda–lime glass substrates by using dip coating method. In this study, the effects of dopant concentration on the morphology, crystal structure, electrical and optical properties of Al/Y co-doped ZnO (AZOY) thin films were investigated. Structural characterization of produced thin films were examined using Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray Diffractometer (XRD), respectively. SEM images indicated that the nanoparticles are formed on the thin films surface and the particle sizes of nanostructures decrease with increasing Yttrium (Y) doping concentration. XRD results revealed that undoped ZnO and Y/Al co-doped ZnO nanostructures have single phase hexagonal (wurtzite) structure with (002) orientation. Optical properties of thin films were investigated by Varian UV–Vis. Spectrophotometer in 350–800 nm range and the band gaps of nanoparticles were calculated from the transmission data using Tauc equation. We found that the optical transmittance of AZOY samples increases up to 95% and the band gaps (Eg) of samples decrease from 3.272 to 3.242 eV with increasing the Y doping concentration. The resistance measurements were employed to understand the DC electrical performance by Y amount of each Zn995−x Al0.005Y x O thin film in the 50–300 K temperature range in the voltage interval −10 to 10 V using four point method. This measurement showed the resistivity increases with increasing Y concentration and different types of conductivity mechanisms were observed for high and low temperature regions. Finally, humidity sensing properties of AZOY films were investigated at room temperature. It was observed that the humidity sensing properties increase with increasing Y doping ion in AZOY samples.

We measured the absorption/transmission performance of ZnO thin films doped with europium (Eu) and aluminum (Al). ZnO and Zn1−(y+0.01)(Al0.01, Euy)O (y = 0.00, 0.01, 0.03 and 0.05) nano particles were synthesized using sol–gel method and covered on a soda-lime glass surface by spin coating. After annealing at 500 °C, UV–Vis. measurements were performed in wavelength range of 300 and 800 nm to calculate the band gap of nano particles from the absorption/transmission performances. The resistivity measurements were employed to understand the relationship between band gap and DC electrical performance by Eu amount of each Zn1−(y+0.01)(Al0.01, Eu y )O thin film. As a conclusion, after reaching a minimum value with 1 mol% Eu doping, a monotonic increase in band gap was observed by enhancing Eu amounts in Zn0.99Al0.01O lattice.