Since the early 2000s, 1D nanostructures like semiconducting metal oxide (SMOX) nanowires (NW) or carbon nanotubes (CNT) are known as excellent materials in several applications, from gas sensors to solar cells.
As a wide-bandgap SMOX, ZnO turns out to be an attractive candidate for blue and UV optoelectronics. We present photoluminescence (PL), electrical and gas sensing properties of individual ZnO NW. NO2 gas effects on the electrical and PL response of ZnO single NW and ZnO microrod are investigated.
Several studies on the gas sensing mechanism in n-type and p-type SMOX have been carried out. Among them, some works reported the combination of the materials to form p-n junctions. However, a clear understanding of the mechanisms leading to gas sensing has not yet been reached. Here, we present the room temperature (RT) response of low-cost chemiresistor gas sensors based on p-type single-wall carbon nanotubes (SWCNT) functionalized with n-type indium tin oxide (ITO) nanoparticles (NP), prepared on different substrates, including flexible plastic foils. An enhanced sensitivity to ammonia and nitrogen dioxide is demonstrated in the functionalized sample, related to the formation of nano-heterojunctions at the interface between SWCNT bundles and ITO NP. Furthermore, the different response of the two devices towards humidity and upon NO2 exposure provides a way to tailor the sensor selectivity with respect to the most relevant interfering gases in outdoor environmental monitoring. In the light of the experimental results, we present a gas sensing mechanism based on space-charge layers in p-n junction .
Finally, we propose a combination of the functionalities of carbon nanotube (CNT)-Si hybrid heterojunctions as a novel method to steer the efficiency of the photovoltaic (PV) cells based on these junctions, and to increase the selectivity and sensitivity of the chemiresistor gas sensors operating with the p-doped CNT layer. The electrical characteristics of the junctions have been tracked by exposing the devices to oxidizing (NO2) and reducing (NH3) molecules. It is shown that when used as PV cells, the cell efficiency can be reversibly steered by gas adsorption, providing a tool to selectively dope the p-type layer through molecular adsorption. The chemiresistive response was improved, both in terms of selectivity and sensitivity, by operating the system under illumination, as the photo-induced charges at the junction increase the p-doping of CNTs making them more sensitive to NH3 and less to NO2 .
Federica Rigoni got her PhD in 2015 at the University of Milan. During the PhD years, her main research topic was the study of gas sensing properties of CNT and hybrid CNT/SMOX layers, focusing on the detection limits and response time scales of chemiresistor gas sensors for environmental monitoring application. Currently, the post-doc research of Dr. Rigoni at University of Brescia is focused on photoluminescence and Raman spectroscopy investigation of 1D nanostructures (mainly ZnO nanowires) and 2D materials (graphene, MoS2, SnS2) for gas sensing application.