Specific and Highly Sensitive Detection of Ketone Compounds Based on p-Type MoTe2 under Ultraviolet Illumination

Recent Advances in Engineering of 2D Layered Metal Chalcogenides for Resistive-Type Gas Sensor

2D nanomaterials have triggered widespread attention in sensing applications. Especially for 2D layered metal chalcogenides (LMCs), the unique semiconducting properties and high surface area endow them with great potential for gas sensors. The assembly of 2D LMCs with guest species is an effective functionalization method to produce the synergistic effects of hybridization for greatly enhancing the gas-sensing properties. This review starts with the synthetic techniques, sensing properties, and principles, and then comprehensively compiles the advanced achievements of the pristine 2D LMCs gas sensors. Key advances in the development of the functionalization of 2D LMCs for enhancing gas-sensing properties are categorized according to the spatial architectures. It is systematically discussed in three aspects: surface, lattice, and interlayer, to comprehend the benefits of the functionalized 2D LMCs from surface chemical effect, electronic properties, and structure features. The challenges and outlooks for developing high-performance 2D LMCs-based gas sensors are also proposed.

Anisotropic sensing based on single ReS2flake for VOCs discrimination

Selective and sensitive detection of volatile organic compounds (VOCs) holds paramount importance in real-world applications. This study proposes an innovative approach utilizing a single ReS2field-effect transistor (FET) characterized by distinct in-plane anisotropy, specifically tailored for VOC recognition. The unique responses of ReS2, endowed with robust in-plane anisotropic properties, demonstrate significant difference along thea-axis andb-axis directions when exposed to four kinds of VOCs: acetone, methanol, ethanol, and IPA. Remarkably, the responses of ReS2were significantly magnified under ultraviolet (UV) illumination, particularly in the case of acetone, where the response amplified by 10-15 times and the detection limit decreasing from 70 to 4 ppm compared to the dark conditions. Exploiting the discernible variances in responses along thea-axis andb-axis under both UV and dark conditions, the data points of acetone, ethanol, methanol and IPA gases were clearly separated in the principal component space without any overlap through principal component analysis, indicating that the single ReS2FET has a high ability to distinguish various gas species. The exploration of anisotropic sensing materials and light excitation strategies can be applied to a broad range of sensing platforms based on two-dimensional materials for practical applications.

Unique Photoactivated Time-Resolved Response in 2D GeS for Selective Detection of Volatile Organic Compounds

Volatile organic compounds (VOCs) sensors have a broad range of applications including healthcare, process control, and air quality analysis. There are a variety of techniques for detecting VOCs such as optical, acoustic, electrochemical, and chemiresistive sensors. However, existing commercial VOC detectors have drawbacks such as high cost, large size, or lack of selectivity. Herein, a new sensing mechanism is demonstrated based on surface interactions between VOC and UV-excited 2D germanium sulfide (GeS), which provides an effective solution to distinguish VOCs. The GeS sensor shows a unique time-resolved electrical response to different VOC species, facilitating identification and qualitative measurement of VOCs. Moreover, machine learning is utilized to distinguish VOC species from their dynamic response via visualization with high accuracy. The proposed approach demonstrates the potential of 2D GeS as a promising candidate for selective miniature VOCs sensors in critical applications such as non-invasive diagnosis of diseases and health monitoring.

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.

Challenges and Opportunities of Chemiresistors Based on Microelectromechanical Systems for Chemical Olfaction

Microelectromechanical-system (MEMS)-based semiconductor gas sensors are considered one of the fastest-growing, interdisciplinary high technologies during the post-Moore era. Modern advancements within this arena include wearable electronics, Internet of Things, and artificial brain-inspired intelligence, among other modalities, thus bringing opportunities to drive MEMS-based gas sensors with higher performance, lower costs, and wider applicability. However, the high demand for miniature and micropower sensors with unified processes on a single chip imposes great challenges. This review focuses on recent developments and pitfalls in MEMS-based micro- and nanoscale gas sensors and details future trends. We also cover the background of the topic, seminal efforts, current applications and challenges, and opportunities for next-generation systems.

Research status of gas sensing performance of MoTe2-based gas sensors: A mini review

Transition metal dichalcogenides (TMDs) have been widely explored for their excellent gas sensing properties, especially high sensitivity and stability at room temperature. MoTe2 exhibits good sensitivity and selectivity to some nitrogen-containing gases (i.e., NO2, NH3) and has received extensive attention in gas sensing. In addition, increasingly complex production environments place demands on high-quality gas sensors. Therefore, worldwide efforts are devoted to designing and manufacturing MoTe2-based gas sensors with faster response and recovery speed. This paper summarizes the research progress of MoTe2-based gas sensing, focuses on the practical measures to improve the response and recovery speed of MoTe2-based sensors, and discusses the mechanism. This provides guidance for exploring higher performance MoTe2 sensors.

The non-volatile electrostatic doping effect in MoTe2 field-effect transistors controlled by hexagonal boron nitride and a metal gate

The electrical and optical properties of transition metal dichalcogenides (TMDs) can be effectively modulated by tuning their Fermi levels. To develop a carrier-selectable optoelectronic device, we investigated intrinsically p-type MoTe₂, which can be changed to n-type by charging a hexagonal boron nitride (h-BN) substrate through the application of a writing voltage using a metal gate under deep ultraviolet light. The n-type part of MoTe₂ can be obtained locally using the metal gate pattern, whereas the other parts remain p-type. Furthermore, we can control the transition rate to n-type by applying a different writing voltage (i.e., − 2 to − 10 V), where the n-type characteristics become saturated beyond a certain writing voltage. Thus, MoTe₂ was electrostatically doped by a charged h-BN substrate, and it was found that a thicker h-BN substrate was more efficiently photocharged than a thinner one. We also fabricated a p–n diode using a 0.8 nm-thick MoTe₂ flake on a 167 nm-thick h-BN substrate, which showed a high rectification ratio of ~ 10⁻⁴. Our observations pave the way for expanding the application of TMD-based FETs to diode rectification devices, along with optoelectronic applications.

Gate-Tuned Gas Molecule Sensitivity of a Two-Dimensional Semiconductor

In this work, we develop a gate-tunable gas sensor based on a MoS2/hBN heterostructure field effect transistor. Through experimental measurements and numerical simulations, we systematically reveal a principle that relates the concentration of the target gas and sensing signals (ΔI/I0) as a function of gate bias. Because a linear relationship between ΔI/I0 and the gas concentration guarantees reliable sensor operation, the optimal gate bias condition for linearity was investigated. Taking NO2 and NH3 as target molecules, it is clarified that the bias condition greatly depends on the electron accepting/donating nature of the gas. The effects of the bandgap and polarity of the transition metal dichalcogenides (TMDC) channel are also discussed. In order to achieve linearly increasing signals that are stable with respect to the gas concentration, a sufficiently large VBG within VBG > 0 is required. We expect this work will shed light on a way to precisely design reliable semiconducting gas sensors based on the characteristics of TMDC and target gas molecules.

Light-Assisted Enhancement of Gas Sensing Property for Micro-Nanostructure Electronic Device: A Mini Review

In recent years, gas sensing electronic devices have always attracted wide attention in the field of environment, industry, aviation and others. In order to improve the gas sensing properties, many micro- and nano-fabrication technologies have been proposed and investigated to develop high-performance gas sensing devices. It is worth noting that light irradiation is an effective strategy to enhance gas sensitivity, shorten the response and recovery time, reduce operating temperature. In this review, firstly, the latest research advances of gas sensors based on different micro-nanostructure materials under UV light and visible light activation is introduced. Then, the gas sensing mechanism of light-assisted gas sensor is discussed in detail. Finally, this review describes the present application of gas sensors with improved properties under light activation assisted conditions and the perspective of their applications.

Exhaled Breath Analysis for Diabetes Diagnosis and Monitoring: Relevance, Challenges and Possibilities

With the global population prevalence of diabetes surpassing 463 million cases in 2019 and diabetes leading to millions of deaths each year, there is a critical need for feasible, rapid, and non-invasive methodologies for continuous blood glucose monitoring in contrast to the current procedures that are either invasive, complicated, or expensive. Breath analysis is a viable methodology for non-invasive diabetes management owing to its potential for multiple disease diagnoses, the nominal requirement of sample processing, and immense sample accessibility; however, the development of functional commercial sensors is challenging due to the low concentration of volatile organic compounds (VOCs) present in exhaled breath and the confounding factors influencing the exhaled breath profile. Given the complexity of the topic and the skyrocketing spread of diabetes, a multifarious review of exhaled breath analysis for diabetes monitoring is essential to track the technological progress in the field and comprehend the obstacles in developing a breath analysis-based diabetes management system. In this review, we consolidate the relevance of exhaled breath analysis through a critical assessment of current technologies and recent advancements in sensing methods to address the shortcomings associated with blood glucose monitoring. We provide a detailed assessment of the intricacies involved in the development of non-invasive diabetes monitoring devices. In addition, we spotlight the need to consider breath biomarker clusters as opposed to standalone biomarkers for the clinical applicability of exhaled breath monitoring. We present potential VOC clusters suitable for diabetes management and highlight the recent buildout of breath sensing methodologies, focusing on novel sensing materials and transduction mechanisms. Finally, we portray a multifaceted comparison of exhaled breath analysis for diabetes monitoring and highlight remaining challenges on the path to realizing breath analysis as a non-invasive healthcare approach.

Charge transfer driven by redox dye molecules on graphene nanosheets for room-temperature gas sensing

Special functional groups to modify the surface of graphene have received much attention since they enable the charge transfer enhancement, thus realizing gas-sensing at room temperature. In this work, three typical redox dye molecules, methylene blue (MB), indigo carmine (IC) and anthraquinone-2-sulfonate (AQS), were selected to be supramolecularly assembled with reduced graphene oxide (rGO), respectively. Remarkably, three graphene-based materials AQS-rGO (response = 3.2, response time = 400 s), IC-rGO (response = 4.3, response time = 300 s) and MB-rGO (response = 7.1, response time = 100 s) exhibited excellent sensitivity and short response time toward 10 ppm NO₂ at room temperature. The corresponding NO₂ sensing mechanism of the obtained materials was further investigated by cyclic voltammetry (CV) measurements. CV was conducted to measure the anodic peak potential (E_pa) of three redox dyes. Interestingly, it is obvious that the E_pa values were positively correlated with the gas sensitivity and response time of the three materials. To explore the mechanism, UV-vis spectroscopy was adopted to analyze the lowest unoccupied molecular orbitals (LUMOs) of three redox dye molecules. The results show that the oxidation abilities of three redox dyes were also positively correlated with the gas sensitivity and response time of three corresponding graphene-based materials.

Increased Active Sites and Charge Transfer in the SnS2/TiO2 Heterostructure for Visible-Light-Assisted NO2 Sensing

Tin disulfide (SnS₂) has been extensively researched as a promising sensing material due to its large electronegativity, suitable band gap, earth abundance, and nontoxicity. However, the poor conductivity and slow response/recovery speed at room temperature greatly hinder its application in high-performance practical gas sensors. Herein, to promote the study of SnS₂-based gas sensors, a hierarchical SnS₂/TiO₂ heterostructure was synthesized and used as a sensing material to detect NO₂ with the help of light illumination. Through the synergistic effect of the SnS₂/TiO₂ heterostructure and 525 nm light activation, the NO₂ sensor based on the SnS₂/TiO₂ heterostructure exhibited a high response factor of 526% toward 1 ppm NO₂ and a short response/recovery time of 43/102 s at room temperature due to the enhanced charge transfer and increased adsorption sites, which was superior to the vast majority of other NO₂ sensors. An obvious decrease in the surface-adsorbed oxygen content based on the X-ray photoelectron spectroscopy measurement further confirmed that light illumination was helpful to clear the surface of SnS₂/TiO₂ and thus increased active sites for NO₂ sensing. In addition, a flexible SnS₂/TiO₂ sensor was also fabricated to confirm its potential application in portable and wearable devices.

Advanced Strategies to Improve Performances of Molybdenum-Based Gas Sensors

Molybdenum-based materials have been intensively investigated for high-performance gas sensor applications. Particularly, molybdenum oxides and dichalcogenides nanostructures have been widely examined due to their tunable structural and physicochemical properties that meet sensor requirements. These materials have good durability, are naturally abundant, low cost, and have facile preparation, allowing scalable fabrication to fulfill the growing demand of susceptible sensor devices. Significant advances have been made in recent decades to design and fabricate various molybdenum oxides- and dichalcogenides-based sensing materials, though it is still challenging to achieve high performances. Therefore, many experimental and theoretical investigations have been devoted to exploring suitable approaches which can significantly enhance their gas sensing properties. This review comprehensively examines recent advanced strategies to improve the nanostructured molybdenum-based material performance for detecting harmful pollutants, dangerous gases, or even exhaled breath monitoring. The summary and future challenges to advance their gas sensing performances will also be presented.

Enhanced Selectivity in Volatile Organic Compound Gas Sensors Based on ReS2-FETs under Light-Assisted and Gate-Bias Tunable Operation

Using a single-device two-dimensional (2D) rhenium disulfide (ReS₂) field-effect transistor (FET) with enhanced gas species selectivity by light illumination, we reported a selective and sensitive detection of volatile organic compound (VOC) gases. 2D materials have the advantage of a high surface-area-to-volume ratio for high sensitivity to molecules attached to the surface and tunable carrier concentration through field-effect control from the back-gate of the channel, while keeping the top surface open to the air for chemical sensing. In addition to these advantages, ReS₂ has a direct band gap also in multilayer cases, which sets it apart from other transition-metal dichalcogenides (TMDCs). We take advantage of the effective response of ReS₂ to light illumination to improve the selectivity and gas-sensing efficiency of a ReS₂-FET device. We found that light illumination modulates the drain current response in a ReS₂-FET to adsorbed molecules, and the sensing activity differs depending on the gas species used, such as acetone, ethanol, and methanol. Furthermore, wavelength and carrier density rely on certain variations in light-modulated sensing behaviors for each chemical. The device will distinguish the gas concentration in a mixture of VOCs using the differences induced by light illumination, enhancing the selectivity of the sensor device. Our results shed new light on the sensing technologies for realizing a large-scale sensor network in the Internet-of-Things era.

Highly sensitive gas sensing platforms based on field effect Transistor-A review

Highly selective, sensitive and fast gas sensing has attracted increasing attention in the fields of environmental protection, industrial production, personal safety as well as medical diagnostics. Field effect transistor (FET) sensors have been extensively investigated in gas sensing fields due to their small size, high sensitivity, high reliability and low energy consumption. This comprehensive review aims to discuss the recent advances in FET gas sensors based on materials such as carbon nanotubes, silicon carbide, silicon, metal oxides-, graphene-, transition metal dichalcogenides- and 2-dimensional black phosphorus. We first introduce different types of sensor structures and elaborate the gas-sensing mechanisms. Then, we describe the optimizing strategies for sensing performances, response parameters, FET based dual-mode sensors and FET based logic circuit sensors. Moreover, we present the key advances of the above materials in gas sensing performances. Meanwhile, shortcomings of such materials are also discussed and the future development of this field is proposed in this review.

Recent Development of Gas Sensing Platforms Based on 2D Atomic Crystals

Sensors, capable of detecting trace amounts of gas molecules or volatile organic compounds (VOCs), are in great demand for environmental monitoring, food safety, health diagnostics, and national defense. In the era of the Internet of Things (IoT) and big data, the requirements on gas sensors, in addition to sensitivity and selectivity, have been increasingly placed on sensor simplicity, room temperature operation, ease for integration, and flexibility. The key to meet these requirements is the development of high-performance gas sensing materials. Two-dimensional (2D) atomic crystals, emerged after graphene, have demonstrated a number of attractive properties that are beneficial to gas sensing, such as the versatile and tunable electronic/optoelectronic properties of metal chalcogenides (MCs), the rich surface chemistry and good conductivity of MXenes, and the anisotropic structural and electronic properties of black phosphorus (BP). While most gas sensors based on 2D atomic crystals have been incorporated in the setup of a chemiresistor, field-effect transistor (FET), quartz crystal microbalance (QCM), or optical fiber, their working principles that involve gas adsorption, charge transfer, surface reaction, mass loading, and/or change of the refractive index vary from material to material. Understanding the gas-solid interaction and the subsequent signal transduction pathways is essential not only for improving the performance of existing sensing materials but also for searching new and advanced ones. In this review, we aim to provide an overview of the recent development of gas sensors based on various 2D atomic crystals from both the experimental and theoretical investigations. We will particularly focus on the sensing mechanisms and working principles of the related sensors, as well as approaches to enhance their sensing performances. Finally, we summarize the whole article and provide future perspectives for the development of gas sensors with 2D materials.

Nanoengineering Approaches Toward Artificial Nose

Significant scientific efforts have been made to mimic and potentially supersede the mammalian nose using artificial noses based on arrays of individual cross-sensitive gas sensors over the past couple decades. To this end, thousands of research articles have been published regarding the design of gas sensor arrays to function as artificial noses. Nanoengineered materials possessing high surface area for enhanced reaction kinetics and uniquely tunable optical, electronic, and optoelectronic properties have been extensively used as gas sensing materials in single gas sensors and sensor arrays. Therefore, nanoengineered materials address some of the shortcomings in sensitivity and selectivity inherent in microscale and macroscale materials for chemical sensors. In this article, the fundamental gas sensing mechanisms are briefly reviewed for each material class and sensing modality (electrical, optical, optoelectronic), followed by a survey and review of the various strategies for engineering or functionalizing these nanomaterials to improve their gas sensing selectivity, sensitivity and other measures of gas sensing performance. Specifically, one major focus of this review is on nanoscale materials and nanoengineering approaches for semiconducting metal oxides, transition metal dichalcogenides, carbonaceous nanomaterials, conducting polymers, and others as used in single gas sensors or sensor arrays for electrical sensing modality. Additionally, this review discusses the various nano-enabled techniques and materials of optical gas detection modality, including photonic crystals, surface plasmonic sensing, and nanoscale waveguides. Strategies for improving or tuning the sensitivity and selectivity of materials toward different gases are given priority due to the importance of having cross-sensitivity and selectivity toward various analytes in designing an effective artificial nose. Furthermore, optoelectrical sensing, which has to date not served as a common sensing modality, is also reviewed to highlight potential research directions. We close with some perspective on the future development of artificial noses which utilize optical and electrical sensing modalities, with additional focus on the less researched optoelectronic sensing modality.

Adsorption of gas molecules (NH3, C2H6O, C3H6O, CO, H2S) on a noble metal (Ag, Au, Pt, Pd, Ru)-doped MoSe2 monolayer: a first-principles study

In this paper, the effects of five noble metal (Au, Pt, Pd, Ag, Ru)-doped MoSe₂ on improving the gas sensing performance were predicted through density functional theory (DFT) based on first-principles.

Room-Temperature Gas Sensors Under Photoactivation: From Metal Oxides to 2D Materials

Room-temperature gas sensors have aroused great attention in current gas sensor technology because of deemed demand of cheap, low power consumption and portable sensors for rapidly growing Internet of things applications. As an important approach, light illumination has been exploited for room-temperature operation with improving gas sensor's attributes including sensitivity, speed and selectivity. This review provides an overview of the utilization of photoactivated nanomaterials in gas sensing field. First, recent advances in gas sensing of some exciting different nanostructures and hybrids of metal oxide semiconductors under light illumination are highlighted. Later, excellent gas sensing performance of emerging two-dimensional materials-based sensors under light illumination is discussed in details with proposed gas sensing mechanism. Originated impressive features from the interaction of photons with sensing materials are elucidated in the context of modulating sensing characteristics. Finally, the review concludes with key and constructive insights into current and future perspectives in the light-activated nanomaterials for optoelectronic gas sensor applications.

Environmental Analysis with 2D Transition-Metal Dichalcogenide-Based Field-Effect Transistors

Field-effect transistors (FETs) present highly sensitive, rapid, and in situ detection capability in chemical and biological analysis. Recently, two-dimensional (2D) transition-metal dichalcogenides (TMDCs) attract significant attention as FET channel due to their unique structures and outstanding properties. With the booming of studies on TMDC FETs, we aim to give a timely review on TMDC-based FET sensors for environmental analysis in different media. First, theoretical basics on TMDC and FET sensor are introduced. Then, recent advances of TMDC FET sensor for pollutant detection in gaseous and aqueous media are, respectively, discussed. At last, future perspectives and challenges in practical application and commercialization are given for TMDC FET sensors. This article provides an overview on TMDC sensors for a wide variety of analytes with an emphasize on the increasing demand of advanced sensing technologies in environmental analysis.