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1. Enhancing room-temperature gas sensing performance of metal oxide semiconductor chemiresistors through 400 nm UV photoexcitation

   https://doi.org/10.1016/j.snr.2024.100194  

   Paul, Suporna; Mendoza, Emily Resendiz; To, Dung_Thi Hanh; Stahovich, Thomas F; Schaefer, Jennifer; Myung, Nosang V (June 2024, Sensors and Actuators Reports)

   One of the most significant drawbacks of metal oxide (MOS) based chemiresistive gas sensors is the requirement of high operating temperature (250–450 °C), which results in significant power consumption and shorter lifetime. To develop room temperature (21±2 °C) MOS chemiresistive gas sensors, the sensing performance of different MOS nanostructures (i.e., tin (IV) oxide (SnO2) nanoparticles (NPs), indium (III) oxide (In2O3) NPs, zinc oxide (ZnO) NPs, tungsten trioxide (WO3) NPs, copper oxide (CuO) nanotubes (NTs), and indium tin oxide (In90Sn10O3 (ITO)) NPs) were systematically investigated toward different toxic industrial chemicals (TICs) (i.e., nitrogen dioxide (NO2), ammonia (NH3), hydrogen sulfide (H2S), carbon monoxide (CO), sulfur dioxide (SO2) and volatile organic compounds (VOCs) (i.e., acetone (C3H6O), toluene (C6H5CH3), ethylbenzene (C6H5CH2CH3), and p-xylene (C6H4(CH3)2)) in the presence and absence of 400 nm UV light illumination. Sensing performance enhancement through photoexcitation is strongly dependent on the target analytes. Under 400 nm UV photoexcitation at 76.0 mW/cm2 intensity, room temperature (21±2 °C) NO2 sensing was readily achieved where SnO2 NPs exhibited the highest sensor response (S = 474.4 toward 10 ppmm (parts per million by mass)) with good recovery followed by ZnO NPs > In2O3 NPs > ITO NPs. Meanwhile, indirect bandgap n-type WO3 NPs showed limited NO2 sensing performance under illumination, whereas p-type CuO NTs showed relatively good sensing response. The most significant improvements in SnO2 compared to other MOS nanoparticles might be attributed to the highest number of photogeneration electrons, which rapidly reacted with adsorbed species to enhance the reaction kinetics. WO3 NPs showed a unique sensing response toward aromatic compounds (e.g., ethylbenzene and p-xylene) under UV illumination, where maximum sensitivity was achieved under 36 mW/cm2 irradiation. Changing light intensity from 0.0 to 36.4 mW/cm2, WO3 showed 15.4-fold and 6.3-fold enhancement in sensing response toward 25 ppmm ethylbenzene and 100 ppmm p-xylene, respectively. 400 nm optical excitation has a limited effect on the sensing performance toward CO, SO2, toluene, and acetone. 

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2. Zinc Oxide/Molybdenum Disulfide as Nanocomposite for Multifunctional Sensor Prototype

   https://doi.org/10.3390/mi16040358  

   Palomera, Netzahualcóyotl; Feng, Peter (April 2025, Micromachines)

   Different materials are studied for environmental gas sensors as well as photodetection prototypes. A ZnO/MoS2 p-n junction was synthetized to act as a multifunctional sensor prototype. After the ZnO was prepared on a silicon substrate by using DC sputtering at room temperature, molybdenum disulfide layers were spin-coated on a nanostructured zinc oxide flake-shaped surface to form an active layer. The heterostructure’s composite surface was examined using scanning electron microscopy, energy-dispersed X-ray, and Raman spectroscopy. Responses to light frequencies, light intensities, and gas chemical tracing were characterized, revealing an enhanced multifunctional performance of the prototype. Characterizations of light-induced photocurrents indicted that the obtained response strength (photocurrent/illumination light power) was up to 0.01 A/W, and the response time was less than 5 ms. In contrast, the gas-sensing measurements showed that its response strength (variation in resistance/original resistance) was up to 3.7% and the response time was down to 150 s when the prototype was exposed to ammonia gas, with the concentration down to 168 ppm. The fabricated prototype appears to have high stability and reproducibility, quick response and recovery times, as well as a high signal-to-noise ratio. 

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3. Gallium Nitride (GaN) Nanostructures and Their Gas Sensing Properties: A Review

   https://doi.org/10.3390/s20143889  

   Khan, Md Ashfaque; Rao, Mulpuri V. (July 2020, Sensors)

   null (Ed.)

   In the last two decades, GaN nanostructures of various forms like nanowires (NWs), nanotubes (NTs), nanofibers (NFs), nanoparticles (NPs) and nanonetworks (NNs) have been reported for gas sensing applications. In this paper, we have reviewed our group’s work and the works published by other groups on the advances in GaN nanostructures-based sensors for detection of gases such as hydrogen (H2), alcohols (R-OH), methane (CH4), benzene and its derivatives, nitric oxide (NO), nitrogen dioxide (NO2), sulfur-dioxide (SO2), ammonia (NH3), hydrogen sulfide (H2S) and carbon dioxide (CO2). The important sensing performance parameters like limit of detection, response/recovery time and operating temperature for different type of sensors have been summarized and tabulated to provide a thorough performance comparison. A novel metric, the product of response time and limit of detection, has been established, to quantify and compare the overall sensing performance of GaN nanostructure-based devices reported so far. According to this metric, it was found that the InGaN/GaN NW-based sensor exhibits superior overall sensing performance for H2 gas sensing, whereas the GaN/(TiO2–Pt) nanowire-nanoclusters (NWNCs)-based sensor is better for ethanol sensing. The GaN/TiO2 NWNC-based sensor is also well suited for TNT sensing. This paper has also reviewed density-functional theory (DFT)-based first principle studies on the interaction between gas molecules and GaN. The implementation of machine learning algorithms on GaN nanostructured sensors and sensor array has been analyzed as well. Finally, gas sensing mechanism on GaN nanostructure-based sensors at room temperature has been discussed. 

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4. Recent Advances in Electrochemical Sensors for Detecting Toxic Gases: NO2, SO2 and H2S

   https://doi.org/10.3390/s19040905  

   Khan, Md; Rao, Mulpuri; Li, Qiliang (February 2019, Sensors)

   null (Ed.)

   Toxic gases, such as NOx, SOx, H2S and other S-containing gases, cause numerous harmful effects on human health even at very low gas concentrations. Reliable detection of various gases in low concentration is mandatory in the fields such as industrial plants, environmental monitoring, air quality assurance, automotive technologies and so on. In this paper, the recent advances in electrochemical sensors for toxic gas detections were reviewed and summarized with a focus on NO2, SO2 and H2S gas sensors. The recent progress of the detection of each of these toxic gases was categorized by the highly explored sensing materials over the past few decades. The important sensing performance parameters like sensitivity/response, response and recovery times at certain gas concentration and operating temperature for different sensor materials and structures have been summarized and tabulated to provide a thorough performance comparison. A novel metric, sensitivity per ppm/response time ratio has been calculated for each sensor in order to compare the overall sensing performance on the same reference. It is found that hybrid materials-based sensors exhibit the highest average ratio for NO2 gas sensing, whereas GaN and metal-oxide based sensors possess the highest ratio for SO2 and H2S gas sensing, respectively. Recently, significant research efforts have been made exploring new sensor materials, such as graphene and its derivatives, transition metal dichalcogenides (TMDs), GaN, metal-metal oxide nanostructures, solid electrolytes and organic materials to detect the above-mentioned toxic gases. In addition, the contemporary progress in SO2 gas sensors based on zeolite and paper and H2S gas sensors based on colorimetric and metal-organic framework (MOF) structures have also been reviewed. Finally, this work reviewed the recent first principle studies on the interaction between gas molecules and novel promising materials like arsenene, borophene, blue phosphorene, GeSe monolayer and germanene. The goal is to understand the surface interaction mechanism. 

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5. Intrinsic and atomic layer etching enhanced area-selective atomic layer deposition of molybdenum disulfide thin films

   https://doi.org/10.1116/6.0002811  

   Soares, Jake; Jen, Wesley; Hues, John D.; Lysne, Drew; Wensel, Jesse; Hues, Steven M.; Graugnard, Elton (September 2023, Journal of Vacuum Science & Technology A)

   For continual scaling in microelectronics, new processes for precise high volume fabrication are required. Area-selective atomic layer deposition (ASALD) can provide an avenue for self-aligned material patterning and offers an approach to correct edge placement errors commonly found in top-down patterning processes. Two-dimensional transition metal dichalcogenides also offer great potential in scaled microelectronic devices due to their high mobilities and few-atom thickness. In this work, we report ASALD of MoS2 thin films by deposition with MoF6 and H2S precursor reactants. The inherent selectivity of the MoS2 atomic layer deposition (ALD) process is demonstrated by growth on common dielectric materials in contrast to thermal oxide/ nitride substrates. The selective deposition produced few layer MoS2 films on patterned growth regions as measured by Raman spectroscopy and time-of-flight secondary ion mass spectrometry. We additionally demonstrate that the selectivity can be enhanced by implementing atomic layer etching (ALE) steps at regular intervals during MoS2 growth. This area-selective ALD process provides an approach for integrating 2D films into next-generation devices by leveraging the inherent differences in surface chemistries and providing insight into the effectiveness of a supercycle ALD and ALE process. 

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