Highly sensitive and flexible pressure sensors using position- and dimension-controlled ZnO nanotube arrays grown on graphene films

A facile and novel technique for the fabrication of pressure sensors is reported based on the hybridization of one-dimensional nanomaterials and two-dimensional graphene film. In particular, piezoelectric pressure sensors are fabricated by using vertically aligned and position- and dimension-controlled ZnO nanotube arrays grown on graphene layers. Graphene layers act not only as substrates for catalyst-free growth of high-quality ZnO nanotubes but also as flexible conduction channels connecting ZnO nanotubes and metal electrodes. Freestanding and flexible sensors have been efficiently obtained via mechanical lift-off of hybrid ZnO nanotube/graphene film structures and by exploiting the weak van der Waals forces existing between the graphene film and the original substrates. A prototype of such devices shows a high pressure sensitivity (−4.4 kPa−1), which would enable the detection of weak flows of inert gas. The relatively low wall thickness and large length of the ZnO nanotubes suggest a relatively high sensitivity to external pressures. The obtained nanotube sensors are attached to the philtrum and wrist of a volunteer and used to monitor his breath and heart rate. Overall, the prototype hybrid sensing device has great potential as wearable technology, especially in the sector of advanced healthcare devices.

CuO-Decorated ZnO Nanotube-based Sensor for Detecting CO Gas: A First-Principles Study

Understanding the effect of decorating of copper oxide (CuO) on Carbon monoxide (CO) adsorption at zinc oxide nanotube is crucial for designing a high performance CO gas sensor. In this work, CO sensing properties of copper oxide-decorated zinc oxide (CuO-ZnO) nanotube is studied theoretically by employing first-principles density functional theory for the first time. The stability, adsorption mechanism, density of states, and change in electrical conductivity are studied. The results of calculating the adsorption energy show strong chemical adsorption of CO on CuO-ZnO nanotubes. The adsorption energy of CO on CuO-ZnO nanotube is calculated as 7.5 times higher than that on ZnO nanotube. The results of the Mulliken charge analysis reveal that electron transfer occurs from CO molecules to CuO-ZnO nanotubes. Additionally, the electrical conductivity of CuO-ZnO nanotubes significantly changes after adsorption of CO at room temperature. According to these studies, CuO-ZnO nanotube sensors can be used for the detection of CO gas. The results are in excellent agreement with the reported experimental results.

Vertical monolithic integration of wide- and narrow-bandgap semiconductor nanostructures on graphene films

We report monolithic integration of indium arsenide (InAs) nanorods and zinc oxide (ZnO) nanotubes using a multilayer graphene film as a suspended substrate, and the fabrication of dual-wavelength photodetectors with the hybrid configuration of these materials. For the hybrid nanostructures, ZnO nanotubes and InAs nanorods were grown vertically on the top and bottom surfaces of the graphene films by metal-organic vapor-phase epitaxy and molecular beam epitaxy, respectively. The structural, optical, and electrical characteristics of the hybrid nanostructures were investigated using transmission electron microscopy, spectral photoresponse analysis, and current–voltage measurements. Furthermore, the hybrid nanostructures were used to fabricate dual-wavelength photodetectors sensitive to both ultraviolet and mid-infrared wavelengths.