Polycrystalline Transparent Al-Doped ZnO Thin Films for Photosensitivity and Optoelectronic Applications.

Thin nanocrystalline transparent Al-doped ZnO (1-10 at.% Al) films were synthesized by solid-phase pyrolysis at 700 °C. Synthesized Al-doped ZnO films were investigated by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM). All obtained materials were crystallized into the wurtzite structure, which was confirmed by XRD. The material crystallinity decreases with the introduction of aluminum. SEM and TEM showed that the films are continuous and have a uniform distribution of nanoparticles with an average size of 15-20 nm. TEM confirmed the production of Al-doped ZnO films. The transmittance of Al-doped ZnO films in the range of 400-1000 nm is more than 94%. The introduction of 1% Al into ZnO leads to a narrowing of the band gap compared to ZnO to a minimum value of 3.26 eV and a sharp decrease in the response time to the radiation exposure with a wavelength of 400 nm. An increase in aluminum concentration leads to a slight increase in the band gap, which is associated with the Burstein-Moss effect. The minimum response time (8 s) was shown for film containing 10% Al, which is explained by the shortest average lifetime of charge carriers (4 s).

Nanocomposite Co3O4-ZnO Thin Films for Photoconductivity Sensors.

Thin nanocomposite films based on zinc oxide (ZnO) added with cobalt oxide (Co3O4) were synthesized by solid-phase pyrolysis. According to XRD, the films consist of a ZnO wurtzite phase and a cubic structure of Co3O4 spinel. The crystallite sizes in the films increased from 18 nm to 24 nm with growing annealing temperature and Co3O4 concentration. Optical and X-ray photoelectron spectroscopy data revealed that enhancing the Co3O4 concentration leads to a change in the optical absorption spectrum and the appearance of allowed transitions in the material. Electrophysical measurements showed that Co3O4-ZnO films have a resistivity up to 3 × 104 Ohm∙cm and a semiconductor conductivity close to intrinsic. With advancing the Co3O4 concentration, the mobility of the charge carriers was found to increase by almost four times. The photosensors based on the 10Co-90Zn film exhibited a maximum normalized photoresponse when exposed to radiation with wavelengths of 400 nm and 660 nm. It was found that the same film has a minimum response time of ca. 26.2 ms upon exposure to radiation of 660 nm wavelength. The photosensors based on the 3Co-97Zn film have a minimum response time of ca. 58.3 ms versus the radiation of 400 nm wavelength. Thus, the Co3O4 content was found to be an effective impurity to tune the photosensitivity of radiation sensors based on Co3O4-ZnO films in the wavelength range of 400-660 nm.

High Gas Sensitivity to Nitrogen Dioxide of Nanocomposite ZnO-SnO2 Films Activated by a Surface Electric Field.

Gas sensors based on the multi-sensor platform MSP 632, with thin nanocomposite films based on tin dioxide with a low content of zinc oxide (0.5-5 mol.%), were synthesized using a solid-phase low-temperature pyrolysis technique. The resulting gas-sensitive ZnO-SnO2 films were comprehensively studied by atomic force microscopy, Kelvin probe force microscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, energy dispersive X-ray spectrometry, and X-ray photoelectron spectroscopy. The obtained films are up to 200 nm thick and consist of ZnO-SnO2 nanocomposites, with ZnO and SnO2 crystallite sizes of 4-30 nm. Measurements of ZnO-SnO2 films containing 0.5 mol.% ZnO showed the existence of large values of surface potential, up to 1800 mV, leading to the formation of a strong surface electric field with a strength of up to 2 × 107 V/cm. The presence of a strong surface electric field leads to the best gas-sensitive properties: the sensor's responsivity is between two and nine times higher than that of sensors based on ZnO-SnO2 films of other compositions. A study of characteristics sensitive to NO2 (0.1-50 ppm) showed that gas sensors based on the ZnO-SnO2 film demonstrated a high sensitivity to NO2 with a concentration of 0.1 ppm at an operating temperature of 200 °C.

A novel paper-based sensor for determination of halogens and halides by dynamic gas extraction.

A new chemosensor in the form of indicator paper, which is sensitive to chlorine and bromine concentrations, is proposed. A simple technique of sensor preparation in a laboratory without expensive reagents is described. The method provides uniform and reproducible distribution of the reagent. A possibility to use the sensor for chemical analysis when combining with dynamic gas extraction with the application of scanner technologies and color image processing is demonstrated. Sensitive and selective techniques of chlorine, bromine, chloride, and bromide determination have been developed. The limits of detection of bromine, chlorine, bromide, and chloride have been found to be 0.06, 0.28, 0.5 and 1.4 µmol L-1 respectively. A scheme of successive determination of these analytes in the same sample including selective halides oxidation is proposed. In the simple and available set-up used for dynamic gas extraction, sample decomposition, oxidation of halide-ions and extraction of the formed halogen take place simultaneously while the color reaction of halogen interaction with the sensor is conducted out of the analyzed solution with reagents. This approach provides high selectivity and good analytical characteristics of analysis of such complex samples as foods, pharmaceutical formulations, and different water objects. The method is easy-to-use, cost-effective, time-efficient and promising for determination of other volatile compounds.