Solar Diodes: Novel Heterostructured Materials for Self-Powered Gas Sensors

The integration and correlation of multiple nanomaterial components and junctions in a singular device can open exciting new avenues for more advanced functionalities in nanodevices. One of the key challenges is to achieve controlled and reproducible synthetic protocols of such complex heterostructures with optimal material combinations and geometries. Due to the current global challenges including growing energy demand, limitation of natural resources, as well as envi-ronmental issues, research efforts have been devoted to the development of self-powered nanodevices that are capable of harvesting renewable energies such as solar and mechanical energies. Nevertheless, the current concept of self-powered nanodevices is based on coupling an external energy harvesting unit, such as a solar cell or piezo-electric nanogenerator, with the functional nanodevices. In this work, an innovative approach, named solar diode sensor (SDS), has been developed to realize an autonomously operated gas sensor with no additional need of coupling it to a powering devices. The SDS based on a CdS@n-ZnO/p-Si nanosystem unifies gas sensing (CdS@n-ZnO) and solar energy harvesting (n-ZnO/p-Si) functionalities in one single device. A novel sensing mechanism (change of open circuit voltage, Voc), in comparison to the well-known conductometric sensors (R), was demonstrated. It was explained in terms of modulated polarization of the nanoparticles/nanowire interface, gas-material surface interactions and the subsequent changes in the donor density of ZnO (ND), which is manifested in the varia-tion of Voc in CdS@n-ZnO/p-Si. The fabricated sensors were capable of detecting oxidizing (e.g. oxygen) and reducing gases (such as ethanol and methane) with reproducible response at room temperature and with no need of any other energy source except solar light illumination to deliver a self-sustained gas sensor signal. The generality of the new concept was demonstrated by extending the approach to other nanomaterial geometries including radial heterojunctions of CdS@ZnO/p-Si nanowires and thin-film planar heterojunction.

Additionally, the fabrication of stand-alone single nanowire devices was employed to study the inherent intrinsic electrical and functional properties of single coaxial heterostructures. The electrical characterization, the photovoltaic and gas sensing performances of a heterojunction device based on a single coaxial n-ZnO/p-Si nanowire were preliminary assessed.