In this paper, we investigated the effect of humidity on paper substrates and propose a simple and low-cost method for their passivation using ZnO nanoparticles. To this end, we built paper-based microdevices based on an interdigitated electrode (IDE) configuration by means of a mask-less laser patterning method on simple commercial printing papers. Initial resistive measurements indicate that a paper substrate with a porous surface can be used as a cost-effective, sensitive and disposable humidity sensor in the 20% to 70% relative humidity (RH) range. Successive spin-coated layers of ZnO nanoparticles then, control the effect of humidity. Using this approach, the sensors become passive to relative humidity changes, paving the way to the development of ZnO-based gas sensors on paper substrates insensitive to humidity.

This paper describes the fabrication and the characterization of an original example of a miniaturized resistive-type humidity sensor, printed on flexible substrate in a large-scale manner. The fabrication process involves laser ablation for the design of interdigitated electrodes on PET (Poly-Ethylene Terephthalate) substrate and a screen-printing process for the deposition of the sensitive material, which is based on TiO₂ nanoparticles. The laser ablation process was carefully optimized to obtain micro-scale and well-resolved electrodes on PET substrate. A functional paste based on cellulose was prepared in order to allow the precise screen-printing of the TiO₂ nanoparticles as sensing material on the top of the electrodes. The current against voltage (I–V) characteristic of the sensor showed good linearity and potential for low-power operation. The results of a humidity-sensing investigation and mechanical testing showed that the fabricated miniaturized sensors have excellent mechanical stability, sensing characteristics, good repeatability, and relatively fast response/recovery times operating at room temperature.

A novel UWB bandpass filter based on a grounded square patch resonator with slots is proposed. Owing to this specific geometry, three fundamental resonances of the resonator can be independently controlled and by their even distribution within the UWB range, an UWB passband can be formed. An UWB filter with in‐band insertion loss <0.9 dB, group delay variation of 0.25 ns, and very small size of only 0.26 λ _g × 0.26 λ _g has been designed and fabricated, and measurement results are in good agreement with simulations.

ZnO nanostructures, especially in the form of dense arrays of nanorods or belts have the ability to efficiently convert mechanical energy to electrical energy. One of the drawbacks though for the exploitation of nanorod technology for commercial devices is the ability to make the electrical contacts to these nanostructured piezoelectric converting elements. Although technologies have been developed that provide solutions for electrical contact issues, metal contact on uniform thin films are much simpler, and can readily be implemented to commercial mass-produced applications. At the same time it is known that high piezoelectric coefficients ZnO uniform films with columnar grains having their c-axis perpendicular to the substrate are required. In this work, we investigate the growth of uniform ZnO films, using a low temperature, low cost hydrothermal process typically used for the fabrication of ZnO nanorods. Under appropriate conditions coalescence of the nanorods occur resulting in uniform films with a columnar structure. The study focuses on understanding the role of the growth factors in order to be able to fully control the proposed process. Moreover, the hydrothermal method is further exploited for the fabrication of uniform ZnO nanostructures on patterned substrates with Au interdigitated electrodes (IDE) using standard lithography as a proof-of-concept of the applicability of the method to standard microfabrication techniques. The piezoelectric films with the IDEs are electrically characterized in order to assess the electrical properties of the grown films. From this analysis, process conditions have been identified for the growth of uniform nanostructured ZnO films, suitable for piezoelectric microgenerators.

A low-temperature hydrothermal process for the growth of ZnO nanostructures on patterned Si substrates was investigated with the aim of their future exploitation as functional cores of nanopiezotronic applications. The study focused on understanding the role of the growth factors in order to better control the suggested process and to introduce it as a low-cost, repeatable and reliable method for large-scale ZnO nanorod production. The parameters that were examined were: (a) the role of the substrate, and (b) the concentration of the metal precursor in conjunction with the growth temperature and time.