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Gas sensors are widely used in many industrial and home applications. There is therefore continued need to develop novel gas sensor substrates which provide good mechanical and electrical stability, and good flexibility in comparison with the conventional alumina and silicon-based materials. In this paper, we present the experimental results on flexible gas sensors based on the Kapton foil and alumina substrate covered by copper oxide as a gas-sensitive layer. These sensors exhibited good mechanical stability and gas-sensing characteristics. The Kapton-based CuO gas sensors were tested under exposure to acetone in the 0.05-1.25 ppm range (150 °C, 50%RH). The results confirmed that sensors deposited on the flexible substrate such as Kapton can be used in the exhaled breath analyzers dedicated to diabetes biomarker detection or other applications for which the elastic substrate is needed. Graphic Abstract

Copper oxide (CuO) ultra-thin films were obtained using magnetron sputtering technology with glancing angle deposition technique (GLAD) in a reactive mode by sputtering copper target in pure argon. The substrate tilt angle varied from 45 to 85° and 0°, and the sample rotation at a speed of 20 rpm was stabilized by the GLAD manipulator. After deposition, the films were annealed at 400 °C/4 h in air. The CuO ultra-thin film structure, morphology, and optical properties were assessed by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), X-ray reflectivity (XRR), and optical spectroscopy. The thickness of the films was measured post-process using a profilometer. The obtained copper oxide structures were also investigated as gas-sensitive materials after exposure to acetone in the sub-ppm range. After deposition, gas-sensing measurements were performed at 300, 350, and 400 °C and 50% relative humidity (RH) level. We found that the sensitivity of the device is related to the thickness of CuO thin films, whereas the best results are obtained with an 8 nm thick sample.

Among many chemical compounds synthesized for third-generation photovoltaic applications, quinoline derivatives have recently gained popularity. This work reviews the latest developments in the quinoline derivatives (metal complexes) for applications in the photovoltaic cells. Their properties for photovoltaic applications are detailed: absorption spectra, energy levels, and other achievements presented by the authors. We have also outlined various methods for testing the compounds for application. Finally, we present the implementation of quinoline derivatives in photovoltaic cells. Their architecture and design are described, and also, the performance for polymer solar cells and dye-synthesized solar cells was highlighted. We have described their performance and characteristics. We have also pointed out other, non-photovoltaic applications for quinoline derivatives. It has been demonstrated and described that quinoline derivatives are good materials for the emission layer of organic light-emitting diodes (OLEDs) and are also used in transistors. The compounds are also being considered as materials for biomedical applications.

The p-n heterostructures of CuO-Ga2O3 obtained by magnetron sputtering technology in a fully reactive mode (deposition in pure oxygen) were tested under exposure to low acetone concentrations. After deposition, the films were annealed at previously confirmed conditions (400 °C/4 h/synthetic air) and further investigated by utilization of X-ray diffraction (XRD), X-ray reflectivity (XRR), energy-dispersive X-ray spectroscopy (EDS). The gas-sensing behavior was tested in the air/acetone atmosphere in the range of 0.1–1.25 ppm, as well as at various relative humidity (RH) levels (10–85%). The highest responses were obtained for samples based on the CuO-Ga2O3 (4% at. Ga).