A Photovoltaic Self-Powered Gas Sensor Based on All-Dry Transferred MoS2 /GaSe Heterojunction for ppb-Level NO2 Sensing at Room Temperature

Traditional gas sensors are facing the challenge of low power consumption for future application in smart phones and wireless sensor platforms. To solve this problem, self-powered gas sensors are rapidly developed in recent years. However, all reported self-powered gas sensors are suffering from high limit of detection (LOD) toward NO2 gas. In this work, a photovoltaic self-powered NO2 gas sensor based on n-MoS2 /p-GaSe heterojunction is successfully prepared by mechanical exfoliation and all-dry transfer method. Under 405 nm visible light illumination, the fabricated photovoltaic self-powered gas sensors show a significant response toward ppb-level NO2 with short response and recovery time and high selectivity at room temperature (25 °C). It is worth mentioning that the LOD toward NO2 of this device is 20 ppb, which is the lowest of the reported self-powered room-temperature gas sensors so far. The discussed devices can be used as building blocks to fabricate more functional Internet of things devices.

All-Dry Transferred ReS2 Nanosheets for Ultrasensitive Room-Temperature NO2 Sensing under Visible Light Illumination

For gas sensing applications, most of the reported two-dimensional (2D) materials are suffering from relatively low sensitivity and high limit of detection (LOD) at room temperature. In this work, we selected rhenium disulfide (ReS₂) nanosheets to fabricate ReS₂ transistor-based gas sensors (RTGSs) with ultrahigh sensitivity and low LOD toward nitrogen dioxide (NO₂). The ReS₂ nanosheets with different thicknesses were prepared via mechanical exfoliation and all-dry transfer method. Under 405 nm light illumination at room temperature (25 °C), the fabricated gas sensors showed a significant enhancement of the response with full reversibility toward ppb level NO₂ (response of 9.07 at 500 ppb, a LOD of 50 ppb). In particular, the total response and recovery time of the RTGS was revealed to be less than 4 minutes (55 and 180 s, respectively), which is one of the top three shortest response and recovery times toward ppb level NO₂ of the reported 2D material-based room-temperature gas sensors so far. Via Raman spectrometry, Kelvin probe force microscopy (KPFM), and X-ray photoelectron spectrometry (XPS), the structure and gas sensing mechanism of the materials were systematically investigated. It was confirmed that the electrons transfer from the ReS₂ surface to NO₂ molecules, inducing the hole doping of ReS₂, which consequently increased the sensor resistance. Moreover, the concentration of the photogenerated carriers in ReS₂ would accordingly be promoted by light illumination, which accounts for the substantial light enhancement of the gas sensing performance of RTGSs.

Bubble-Free Transfer Technique for High-Quality Graphene/Hexagonal Boron Nitride van der Waals Heterostructures

Bubbles at the interface of two-dimensional layered materials in van der Waals heterostructures cause deterioration in the quality of materials, thereby limiting the size and design of devices. In this paper, we report a simple all-dry transfer technique, with which the bubble formation can be avoided. As a key factor in the technique, a contact angle between a picked-up flake on a viscoelastic polymer stamp and another flake on a substrate was introduced by protrusion at the stamp surface. Using this technique, we demonstrated the fabrication of high-quality devices on the basis of graphene/hexagonal boron nitride heterostructures with a large bubble-free region. Additionally, the technique can be used to remove unnecessary flakes on a substrate under an optical microscopic scale. Most importantly, it improves the yield and throughput for the fabrication process of high-quality van der Waals heterostructure-based devices.