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. In this work, 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

Integrated Strategy toward Self-Powering and Selectivity Tuning of Semiconductor Gas Sensors

Inorganic conductometric gas sensors struggle to overcome limitations in high power consumption and poor selectivi-ty. Herein, recent advances in developing self-powered gas sensors with tunable selectivity are introduced. Alternative general approaches for powering gas sensors were realized via proper integration of complementary functionalities (namely; powering and sensing) in a singular heterostructure. These solar light driven gas sensors operating at room temperature without applying any additional external powering sources are comparatively discussed. The TYPE-1 gas sensor based on integration of pure inorganic interfaces (e.g. CdS/n-ZnO/p-Si) is capable of delivering a self-sustained sensing response, while it shows a non-selective interaction towards oxidizing and reducing gases. The structural and the optical merits of TYPE-1 sensor are investigated giving more insights into the role of light activation on the modu-lation of the self-powered sensing response. In the TYPE-2 sensor, the selectivity of inorganic materials is tailored through surface functionalization with self-assembled organic monolayers (SAMs). Such hybrid interfaces (e.g. SAMs/ZnO/p-Si) have specific surface interactions with target gases compared to the non-specific oxidation-reduction interactions governing the sensing mechanism of simple inorganic sensors. The theoretical modeling using density functional theory (DFT) has been used to simulate the sensing behavior of inorganic/organic/gas interfaces, revealing that the alignment of organic/gas frontier molecular orbitals with respect to the inorganic Fermi level is the key factor for tuning selectivity. These platforms open new avenues for developing advanced energy-neutral gas sensing devices and concepts

Charge Transfer Characteristics of n-type In0.1Ga0.9N Photoanode across Semiconductor-Liquid Interface

Understanding the mechanisms of charge transfer across the semiconductor/liquid interface is crucial to realize efficient photoelectrochemical devices. Here, the interfacial charge transfer characteristics of n-type In0.1Ga0.9N photoanodes are investigated and correlated to their photo-activity properties measured in phosphate buffered saline solution (pH 7) under illumination conditions. Cyclic voltammetry measurements show evident photoactivity changes as the number of cycles increases. In particular, the photocurrent density reaches its maximum value after 49 voltammetric cycles; meanwhile, the photocurrent onset potential shifts toward more negative cathodic potentials. Electrochemical impedance measurements reveal that, first, the hole transfer process occurs mainly via localized states at the surface and the photocurrent onset potential is dependent on the energetic position of those states. Therefore, the observed initial photocurrent increase and cathodic shift of the photocurrent onset potential can be attributed to a decrease of the transfer resistance and partial passivation of the states at the surface. On the other hand, a gradual oxidation and corrosion of the InGaN surface arises, causing a consequential decrease of the photocurrent. At this point, the charge transfer process occurs predominantly from the valence band. This work provides a basic understanding of the charge transfer mechanisms across the InGaN/liquid interface which can be used to improve the overall photoanode efficiency

Burnout among surgeons before and during the SARS-CoV-2 pandemic: an international survey

Background: SARS-CoV-2 pandemic has had many significant impacts within the surgical realm, and surgeons have been obligated to reconsider almost every aspect of daily clinical practice. Methods: This is a cross-sectional study reported in compliance with the CHERRIES guidelines and conducted through an online platform from June 14th to July 15th, 2020. The primary outcome was the burden of burnout during the pandemic indicated by the validated Shirom-Melamed Burnout Measure. Results: Nine hundred fifty-four surgeons completed the survey. The median length of practice was 10 years; 78.2% included were male with a median age of 37 years old, 39.5% were consultants, 68.9% were general surgeons, and 55.7% were affiliated with an academic institution. Overall, there was a significant increase in the mean burnout score during the pandemic; longer years of practice and older age were significantly associated with less burnout. There were significant reductions in the median number of outpatient visits, operated cases, on-call hours, emergency visits, and research work, so, 48.2% of respondents felt that the training resources were insufficient. The majority (81.3%) of respondents reported that their hospitals were included in the management of COVID-19, 66.5% felt their roles had been minimized; 41% were asked to assist in non-surgical medical practices, and 37.6% of respondents were included in COVID-19 management. Conclusions: There was a significant burnout among trainees. Almost all aspects of clinical and research activities were affected with a significant reduction in the volume of research, outpatient clinic visits, surgical procedures, on-call hours, and emergency cases hindering the training. Trial registration: The study was registered on clicaltrials.gov "NCT04433286" on 16/06/2020

Fabrication of ZnO Nanorods on MEMS Piezoresistive Silicon Microcantilevers for Environmental Monitoring

In this study, a ZnO nanorods (NRs) patterned MEMS piezoresistive silicon micro-cantilever was fabricated as environmental monitor. The fabrication starts from bulk silicon, utilizing photolithography, diffusion, inductively coupled plasma (ICP) cryogenic dry etching, Zinc DC-sputtering, and chemical bath deposition (CBD) etc. This sensor shows a humidity sensitivity value of 6.35 ± 0.27 ppm/RH% at 25 °C in the range from 30% RH to 80% RH

Top-Down Fabrication of Arrays of Vertical GaN Nanorods with Freestanding Top Contacts for Environmental Exposure

Arrays of 1D-vertically arranged gallium nitride (GaN) nanorods (NRs) are fabricated on sapphire and connected to both bottom and freestanding top contacts. This shows a fully validated top-down method to obtain ordered arrays of high-surface-to-volume elements that can be electrically interrogated and used, e.g., for sensing applications. Specifically, these will be used as highly integrated heating elements for conductometric gas sensors in self-heating operation. Detailed fabrication and processing steps involving inductively coupled plasma reactive ion etching (ICP-RIE), KOH-etching, interspace filling, and electron-beam physical vapor deposition technologies are discussed, in which they can be well adjusted and combined to obtain vertical GaN NRs as thin as 300 nm in arbitrarily large and regular arrays (e.g., 1 × 1, 3 × 3, 9 × 10 elements). These developed devices are proposed as a novel sensor platform for temperature-activated measurements that can be produced at a large scale offering low-power, and very stable temperature control

Front contact optimization for rear-junction SHJ solar cells with ultra-thin n-type nanocrystalline silicon oxide

In this work, ultra-thin n-type hydrogenated nanocrystalline silicon oxide [(nc-SiOx:H (n)] film was used to replace amorphous silicon [a-Si:H (n)] as electron transport layer (ETL) in rear-junction silicon heterojunction (SHJ) solar cell to reduce front parasitic absorption. The contact resistivity between the transparent conductive oxide (TCO) and ultra-thin ETL interface plays an important role on the cell performance. A nanocrystalline silicon (nc-Si:H) contact or seed layer was introduced in the solar cell with ultra-thin nc-SiOx:H and the impact of the nc-Si:H thickness on the cell performance was investigated. To demonstrate scalability, bifacial solar cells with 10 nm ETL were fabricated on the M2 (244 cm2) wafer. The best cell performance is obtained by the solar cell with 5 nm nc-SiOx:H (n) and 5 nm nc-Si:H (n) contact layer and it exhibits open-circuit voltage (Voc) of 738 mV, fill factor (FF) of 80.4%, short-circuit current density (Jsc) of 39.0 mA/cm2 and power conversion efficiency (η) of 23.1% on M2 wafer. Compared to the one with nc-SiOx:H (n), an increase of 3%abs of FF and 0.5%abs of η and lower front contact resistivity is demonstrated for the solar cells with nc-Si:H (n) / nc-SiOx:H (n) double layer, which is caused by the lower energy barrier for electrons, according to the band diagram calculated by the AFORS-HET simulator. A simulation on the solar cell optical and electrical losses was done by the Quokka 3 simulator and shows much lower electrical transport loss and a bit higher front surface transmission loss for the one with double layer than nc-SiOx:H (n) single layer

Enhancement of the sub-band-gap photoconductivity in ZnO nanowires through surface functionalization with carbon nanodots

We report on the surface functionalization of ZnO nanowire (NW) arrays by attachment of carbon nanodots (C-dots) stabilized by polyethylenimine. The photoconductive properties of the ZnO NWs/C-dots devices were investigated under photoexcitation with photon energies below and above the ZnO band gap. The results indicate an increased photoresponse of the functionalized devices in the visible spectral range, as well as enhanced UV photoconductivity. This is attributed to the fast injection of photoexcited electrons from the C-dots into the conduction band of the ZnO NWs, and the subsequent slower desorption of molecular species from the NW surface, which reduces the surface depletion region in the NWs. The surface functionalization of the ZnO NWs with carbon nanodots also impacts the dynamics of the photocurrent decay, inducing a slower relaxation of the photogenerated charge carriers

Efficient light trapping in silicon heterojunction solar cells via nanoimprint periodic texturing

The surface patterning of Si heterojunction solar cells is of key importance to enhance the light trapping properties and enable high efficiencies of the emerging thin crystalline silicon solar cells. Herein, periodic inverted pyramids with different periodicity patterns were fabricated by nanoimprint lithography in combination with dry- and wetetching techniques. The impact of their periodicity on the light trapping properties was studied by measuring their reflectance and photoluminescence properties. The inverted pyramids with 700 nm periodicity show excellent anti-reflection and selective light-trapping properties