Enhanced NO2 Sensitivity in Schottky-Contacted n-Type SnS2 Gas Sensors

Band Alignment Transition and Enhanced Performance in Vertical SnS2/MoS2 van der Waals Photodetectors

The strong light-matter interaction and naturally passivated surfaces of van der Waals materials make heterojunctions of such materials ideal candidates for high-performance photodetectors. In this study, we fabricated SnS2/MoS2 van der Waals heterojunctions and investigated their photoelectric properties. Using an applied gate voltage, we can effectively alter the band arrangement and achieve a transition in type II and type I junctions. It is found that the SnS2/MoS2 van der Waals heterostructures are type II heterojunctions when the gate voltage is above -25 V. Below this gate voltage, the heterojunctions become type I. Photoelectric measurements under various wavelengths of incident light reveal enhanced sensitivity in the ultraviolet region and a broadband sensing range from 400 to 800 nm. Moreover, due to the transition from type II to type I band alignment, the measured photocurrent saturates at a specific gate voltage, and this value depends crucially on the bias voltage and light wavelength, providing a potential avenue for designing compact spectrometers.

Enhanced NO2 Sensitivity of Vertically Stacked van der Waals Heterostructure Gas Sensor and Its Remarkable Electric and Mechanical Tunability

Nanodevices based on van der Waals heterostructures have been predicted, and shown, to have unprecedented operational principles and functionalities that hold promise for highly sensitive and selective gas sensors with rapid response times and minimal power consumption. In this study, we fabricated gas sensors based on vertical MoS2/WS2 van der Waals heterostructures and investigated their gas sensing capabilities. Compared with individual MoS2 or WS2 gas sensors, the MoS2/WS2 van der Waals heterostructure gas sensors are shown to have enhanced sensitivity, faster response times, rapid recovery, and a notable selectivity, especially toward NO2. In combination with a theoretical model, we show that it is important to take into account created trapped states (flat bands) induced by the adsorption of gas molecules, which capture charges and alter the inherent built-in potential of van der Waals heterostructure gas sensors. Additionally, we note that the performance of these MoS2/WS2 heterostructure gas sensors could be further enhanced using electrical gating and mechanical strain. Our findings highlight the importance of understanding the effects of altered built-in potentials arising from gas molecule adsorption induced flat bands, thus offering a way to enhance the gas sensing performance of van der Waals heterostructure gas sensors.

Field Effect Transistor Gas Sensors Based on Mechanically Exfoliated Van der Waals Materials

The high surface-to-volume ratio and flatness of mechanically exfoliated van der Waals (vdW) layered materials make them an ideal platform to investigate the Langmuir absorption model. In this work, we fabricated field effect transistor gas sensors, based on a variety of mechanically exfoliated vdW materials, and investigated their electrical field-dependent gas sensing properties. The good agreement between the experimentally extracted intrinsic parameters, such as equilibrium constant and adsorption energy, and theoretically predicted values suggests validity of the Langmuir absorption model for vdW materials. Moreover, we show that the device sensing behavior depends crucially on the availability of carriers, and giant sensitivities and strong selectivity can be achieved at the sensitivity singularity. Finally, we demonstrate that such features provide a fingerprint for different gases to quickly detect and differentiate between low concentrations of mixed hazardous gases using sensor arrays.

1D and 2D Field Effect Transistors in Gas Sensing: A Comprehensive Review

Rapid progress in the synthesis and fundamental understanding of 1D and 2D materials have solicited the incorporation of these nanomaterials into sensor architectures, especially field effect transistors (FETs), for the monitoring of gas and vapor in environmental, food quality, and healthcare applications. Yet, several challenges have remained unaddressed toward the fabrication of 1D and 2D FET gas sensors for real-field applications, which are related to properties, synthesis, and integration of 1D and 2D materials into the transistor architecture. This review paper encompasses the whole assortment of 1D-i.e., metal oxide semiconductors (MOXs), silicon nanowires (SiNWs), carbon nanotubes (CNTs)-and 2D-i.e., graphene, transition metal dichalcogenides (TMD), phosphorene-materials used in FET gas sensors, critically dissecting how the material synthesis, surface functionalization, and transistor fabrication impact on electrical versus sensing properties of these devices. Eventually, pros and cons of 1D and 2D FETs for gas and vapor sensing applications are discussed, pointing out weakness and highlighting future directions.

Ultrasensitive Room-Temperature NO2 Detection Using SnS2/MWCNT Composites and Accelerated Recovery Kinetics by UV Activation

High performance with lower power consumption is one among the essential features of a sensing device. Minute traces of hazardous gases such as NO₂ are difficult to detect. Tin disulfide (SnS₂) nanosheets have emerged as a promising NO₂ sensor. However, their poor room-temperature conductivity gives rise to inferior sensitivity and sluggish recovery rates, thereby hindering their applications. To mitigate this problem, we present a low-cost ultrasensitive NO₂ gas sensor with tin disulfide/multiwalled carbon nanotube (SnS₂/MWCNT) nanocomposites, prepared using a single-step hydrothermal method, as sensing elements. Relative to pure SnS₂, the conductivity of nanocomposites improved significantly. The sensor displayed a decrease in resistance when exposed to NO₂, an oxidizing gas, and exhibited p-type conduction, also confirmed in separate Mott-Schottky measurements. At a temperature of 20 °C, the sensor device has a relative response of about ≈5% (3%) for 25 ppb (1 ppb) of NO₂ with complete recovery in air (10 min) and excellent recovery rates with UV activation (0.3 min). A theoretical lower limit of detection (LOD) of 7 ppt implies greater sensitivity than all previously reported SnS₂-based gas sensors, to the best of our knowledge. The improved sensing characteristics were attributed to the formation of nano p-n heterojunctions, which enhances the charge transport and gives rise to faster response. The composite sensor also demonstrated good NO₂ selectivity against a variety of oxidizing and reducing gases, as well as excellent stability and long-term durability. This work will provide a fresh perspective on SnS₂-based composite materials for practical gas sensors.

Light-Tunable Polarity and Erasable Physisorption-Induced Memory Effect in Vertically Stacked InSe/SnS2 Self-Powered Photodetector

van der Waals heterojunctions with tunable polarity are being actively explored for more Moore and more-than-Moore device applications, as they can greatly simplify circuit design. However, inadequate control over the multifunctional operational states is still a challenge in their development. Here, we show that a vertically stacked InSe/SnS₂ van der Waals heterojunction exhibits type-II band alignment, and its polarity can be tuned by an external electric field and by the wavelength and intensity of an illuminated light source. Moreover, such SnS₂/InSe diodes are self-powered broadband photodetectors with good performance. The self-powered performance can be further enhanced significantly with gas adsorption, and the device can be quickly restored to the state before gas injection using a gate voltage pulse. Our results suggest a way to achieve and design multiple functions in a single device with multifield coupling of light, electrical field, gas, or other external stimulants.

Contactless Photoelectrochemical Biosensor Based on the Ultraviolet-Assisted Gas Sensing Interface of Three-Dimensional SnS2 Nanosheets: From Mechanism Reveal to Practical Application

This work reports a contactless photoelectrochemical biosensor based on an ultraviolet-assisted gas sensor (UV-AGS) with a homemade three-dimensional (3D)-SnS₂ nanosheet-functionalized interdigitated electrode. After rigorous examination, it was found that the gas responsiveness accelerated and the sensitivity increased using the UV irradiation strategy. The effects of the interlayer structure and the Schottky heterojunction on the gas-sensitive response of O₂ and NH₃ under UV irradiation were further investigated theoretically by 3D electrostatic field simulations and first-principles density functional theory to reveal the mechanism. Finally, a UV-AGS device was developed to quantify the blood ammonia bioassay in a small-volume whole blood sample by alkalizing blood to release gas-phase ammonia with a linear range of 25-5000 μM with a limit of detection (LOD) of 29.5 μM. The device also enables a rapid immunoassay of human cardiac troponin I (cTnI) with a linear range of 0.4-25.6 ng/mL and an LOD of 0.37 ng/mL using a urease-labeled antibody as the immune recognition molecule. Both analyses showed satisfying specificity and stability, suggesting that the device can be applied to practical assays and is of great potential to increase the value of gas-sensitive sensors in chemical biosensing.

Two-Dimensional Field-Effect Transistor Sensors: The Road toward Commercialization

The evolutionary success in information technology has been sustained by the rapid growth of sensor technology. Recently, advances in sensor technology have promoted the ambitious requirement to build intelligent systems that can be controlled by external stimuli along with independent operation, adaptivity, and low energy expenditure. Among various sensing techniques, field-effect transistors (FETs) with channels made of two-dimensional (2D) materials attract increasing attention for advantages such as label-free detection, fast response, easy operation, and capability of integration. With atomic thickness, 2D materials restrict the carrier flow within the material surface and expose it directly to the external environment, leading to efficient signal acquisition and conversion. This review summarizes the latest advances of 2D-materials-based FET (2D FET) sensors in a comprehensive manner that contains the material, operating principles, fabrication technologies, proof-of-concept applications, and prototypes. First, a brief description of the background and fundamentals is provided. The subsequent contents summarize physical, chemical, and biological 2D FET sensors and their applications. Then, we highlight the challenges of their commercialization and discuss corresponding solution techniques. The following section presents a systematic survey of recent progress in developing commercial prototypes. Lastly, we summarize the long-standing efforts and prospective future development of 2D FET-based sensing systems toward commercialization.

Gate-controlled gas sensor utilizing 1D-2D hybrid nanowires network

Novel gas sensors that work at room temperature are attracting attention due to their low energy consumption and stability in the presence of toxic gases. However, the development of sensing characteristics at room temperature is still a primary challenge. Diverse reaction pathways and low adsorption energy for gas molecules are required to fabricate a gas sensor that works at room temperature with high sensitivity, selectivity, and efficiency. Therefore, we enhanced the gas sensing performance at room temperature by constructing hybridized nanostructure of 1D-2D hybrid of SnSe2 layers and SnO2 nanowire networks and by controlling the back-gate bias (Vg = 1.5 V). The response time was dramatically reduced by lowering the energy barrier for the adsorption on the reactive sites, which are controlled by the back gate. Consequently, we believe that this research could contribute to improving the performance of gas sensors that work at room temperature.

Selectivity in trace gas sensing: recent developments, challenges, and future perspectives

Selectivity is one of the most crucial figures of merit in trace gas sensing, and thus a comprehensive assessment is necessary to have a clear picture of sensitivity, selectivity, and their interrelations in terms of quantitative and qualitative views.

SnS2 with Flower-like Structure for Efficient CO2 Photoreduction under Visible-Light Irradiation

Photocatalytic CO₂ reduction using solar energy is a promising way to obtain renewable-energy sources for replacing fossil fuels. Through a hydrothermal process, we successfully designed and synthesized three-dimensional (3D) flower-like structured SnS₂ with a sheet-like structured quasi-hexagon as the building block. The 3D hierarchical structure is conducive to light capture and absorption, the sheet structure can shorten the transmission path and promote separation of the carriers, and the self-supporting effect can effectively prevent catalyst agglomeration during the catalytic reaction. Therefore, when used in photocatalytic CO₂ reduction, SnS₂ with a flower-like structure showed excellent photocatalytic performance compared with SnS₂ nanoparticles (NPs) under visible-light irradiation with a gas-solid reaction system.

Graphene/SnS2 van der Waals Photodetector with High Photoresponsivity and High Photodetectivity for Broadband 365-2240 nm Detection

The fabrication of graphene/SnS₂ van der Waals photodetectors and their photoelectrical properties are systematically investigated. It was found that a dry transferred graphene/SnS₂ van der Waals heterostructure had a broadband sensing range from ultraviolet (365 nm) to near-infrared (2.24 μm) and respective improved responsivities and photodetectivities of 7.7 × 10³ A/W and 8.9 × 10¹³ jones at 470 nm and 2 A/W and 1.8 × 10¹⁰ jones at 1064 nm. Moreover, positive and negative photoconductance effects were observed when the photodetectors were illuminated by photon sources with energies greater and smaller than the bandgap of SnS₂, respectively. The photoresponsivity (R) versus incident power density (P) follows the empirical law R ∝ P_in^β, with β > -1 for positive photoconductance effects and β < -1 for negative photoconductance effects. On the basis of the Fowler-Nordheim tunneling model and a Poisson and drift-diffusion simulation, we show quantitatively that the barrier height and barrier width of the heterostructure photodetector could be controlled by a laser and an external electrical field through a photogating effect generated by carriers trapped at the interface, which could be used to tune the separation and transport of photogenerated carriers. Our results may be useful for the design of high performance van der Waals heterojunction photodetectors.

Strategies for Improving the Sensing Performance of Semiconductor Gas Sensors for High-Performance Formaldehyde Detection: A Review

Formaldehyde is a poisonous and harmful gas, which is ubiquitous in our daily life. Long-term exposure to formaldehyde harms human body functions; therefore, it is urgent to fabricate sensors for the real-time monitoring of formaldehyde concentrations. Metal oxide semiconductor (MOS) gas sensors is favored by researchers as a result of their low cost, simple operation and portability. In this paper, the mechanism of formaldehyde detection by gas sensors is introduced, and then the ways of ameliorating the response of gas sensors for formaldehyde detection in recent years are summarized. These methods include the control of the microstructure and morphology of sensing materials, the doping modification of matrix materials, the development of new semiconductor sensing materials, the outfield control strategy and the construction of the filter membrane. These five methods will provide a good prerequisite for the preparation of better performing formaldehyde gas sensors.

A high-response formaldehyde sensor based on fibrous Ag-ZnO/In2O3 with multi-level heterojunctions

Timely detection of formaldehyde is pivotal because formaldehyde is slowly released from the indoor decorative materials, jeopardizing our healthy. Herein, a high-response formaldehyde gas sensor based on Ag-ZnO/In₂O₃ nanofibers was successfully fabricated. Compared with all the control samples, the hybrid exhibits superior sensitivity (0.65 ppm^-1), excellent selectivity (≥ 12.5) and durable stability (the deviation value ≤ 3%). Particularly, an ultra-high response value of about 186 towards 100 ppm of formaldehyde at 260 °C was achieved, heading the list of outstanding candidates. Additionally, the limit of detection is as low as 9 ppb. The enhanced gas sensing properties can be mainly attributed to multi-level heterojunctions (n-n heterojunction and Ohmic junction) and the "spill-over" effect of Ag, ultimately increasing the adsorption of gas molecules on the surface of sensing material. This work verifies that proper design of multi-level heterojunctions significantly upgrade the sensing performance.

Two-Dimensional Gallium Oxide Monolayer for Gas-Sensing Application

A two-dimensional (2D) Ga₂O₃ monolayer with an asymmetric quintuple-layer configuration was reported as a novel 2D material with excellent stability and strain tunability. This unusual asymmetrical structure opens up new possibilities for improving the selectivity and sensitivity of gas sensors by using selected surface orientations. In this study, the surface adsorptions of nine molecular gases, namely, O₂, CO₂, CO, SO₂, NO₂, H₂S, NO, NH₃, and H₂O, on the 2D Ga₂O₃ monolayer are systematically investigated through first-principles calculations. The intrinsic dipole of the system leads to different adsorption energies and changes in the electronic structures between the top- and bottom-surface adsorptions. Analyses of electronic structures and charge transport calculations indicate a potential application of the 2D Ga₂O₃ monolayer as a room-temperature NO gas-sensing device with high sensitivity and tunable adsorption energy using plenary strain-induced lattice distortion.

Large-Area Flexible Printed Thin-Film Transistors with Semiconducting Single-Walled Carbon Nanotubes for NO2 Sensors

Development of large-area, low-cost, low-voltage, low-power consumption, flexible high-performance printed carbon nanotube thin-film transistors (TFTs) is helpful to promote their future applications in sensors and biosensors, wearable electronics, and the Internet of things. In this work, low-voltage, flexible printed carbon nanotube TFTs with a large-area and low-cost fabrication process were successfully constructed using ultrathin (∼3.6 nm) AlO_x thin films formed by plasma oxidation of aluminum as dielectrics and screen-printed silver electrodes as contact electrodes. The as-prepared bottom-gate/bottom-contact carbon nanotube TFTs exhibit a low leakage current (∼10^-10 A), a high charge carrier mobility (up to 9.9 cm² V^-1 s^-1), high on/off ratios (higher than 10⁵), and small subthreshold swings (80-120 mV/dec) at low operation voltages (from -1.5 to 1 V). At the same time, printed carbon nanotube TFTs showed a high response (ΔR/R = 99.6%) to NO₂ gas even at 16 ppm with a faster response and recovery speed (∼8 s, exposure to 0.5 ppm NO₂), a lower detection limit (0.069 ppm NO₂), and a low power consumption (0.86 μW, exposure to 16 ppm NO₂) at a gate voltage of 0.2 V at room temperature. Moreover, the printed carbon nanotube devices exhibited excellent mechanical flexibility and bias stress stability after 12,000 bending cycles at a radius of 5 mm and a bias stress test for 7200 s at a gate voltage of ±1 V, which originated from the ultrathin and compact AlO_x dielectric and the super adhesion force between screen-printed silver electrodes and polyethylene terephthalate substrates.