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.     

Hybrid Films of Graphene and Carbon Nanotubes for High Performance Chemical and Temperature Sensing Applications

A hybrid composite material of graphene and carbon nanotube (CNT) for high performance chemical and temperature sensors is reported. Integration of 1D and 2D carbon materials into hybrid carbon composites is achieved by coupling graphene and CNT through poly(ionic liquid) (PIL) mediated‐hybridization. The resulting CNT/PIL/graphene hybrid materials are explored as active materials in chemical and temperature sensors. For chemical sensing application, the hybrid composite is integrated into a chemo‐resistive sensor to detect a general class of volatile organic compounds. Compared with the graphene‐only devices, the hybrid film device showed an improved performance with high sensitivity at ppm level, low detection limit, and fast signal response/recovery. To further demonstrate the potential of the hybrid films, a temperature sensor is fabricated. The CNT/PIL/graphene hybrid materials are highly responsive to small temperature gradient with fast response, high sensitivity, and stability, which may offer a new platform for the thermoelectric temperature sensors.     

Current understanding and emerging applications of 3D crumpling mediated 2D material-liquid interactions

Three dimensional (3D) crumpling of two dimensional (2D) materials provides new opportunities to modulate mechanical, optical, surface, and chemical properties. However, investigation of the effect of 3D crumpling on 2D material liquid interaction has been limited. In this perspective, we will review crumple/texture induced heterogeneous surface properties including chemical modification, energy corrugation, and electronic structure perturbation which may modulate fluid interaction. We will then describe simulations of fluid interaction in systems resembling 3D textured 2D materials, principally nanotubes, which have begun to substantiate perturbations to fluid structure driven by texture induced modification of the 2D material surface. Furthermore, we will detail current experimental understanding of how texture induced modulation of interactions with pure solvent affect macroscale wetting characteristics including textured driven transitions in water contact from Wentzel to Cassie Baxter states. Following this discussion of how texturing affects the interaction of 2D materials with pure solvent, we will detail cutting edge explorations of how texturing modifies interaction with ions and other chemical species dispersed in solvent phases. Particular focus will be placed on recent simulations of aqueous phase molecular interaction with crumpled 2D materials which show that crumpling increases the thickness of the electrical double layer (EDL) formed near a 2D material surface. This increased EDL thickness has allowed for the development of biomolecule sensors with gigantic sensitivity and the monitoring and templating of cells including neurons and myotubes. Sill, considerable work is needed to elucidate the effect of different crumpling geometries on the local properties of the full range of 2D materials, how these variation in local properties perturb fluid structure and molecular interaction, and how these tuned interactions enable diverse opportunities such as sensing, energy storage, and control of biological interaction.     

Shape morphing smart 3D actuator materials for micro soft robot

Stimuli-responsive materials have been used in major applications such as sensors, actuators, wearable devices, and biomedical devices owing to their ability to respond to external stimuli including heat, light, electricity, humidity, and chemicals. Strategies to trigger the stimuli-responsive, shape-morphing of two-dimensional (2D) sheets into three-dimensional (3D) shapes are of significant interest for a variety of smart applications including soft robotics. Stimuli-responsive properties can be designed by the selection of materials, structures, and processing methods. This review outlines seven broad categories of stimuli-responsive 2D soft materials for 3D smart actuator applications, namely (1) carbon nanomaterials, (2) metal nanomaterials, (3) shape memory polymers, (4) liquid crystal polymers and elastomers, (5) azobenzenes, (6) hydrogels, and (7) bio-hybrids, along with their basic mechanisms, processing methods, and applications.     

Chemical Preparation of Graphene‐Based Nanomaterials and Their Applications in Chemical and Biological Sensors

Graphene is a flat monolayer of carbon atoms packed tightly into a 2D honeycomb lattice that shows many intriguing properties meeting the key requirements for the implementation of highly excellent sensors, and all kinds of proof‐of‐concept sensors have been devised. To realize the potential sensor applications, the key is to synthesize graphene in a controlled way to achieve enhanced solution‐processing capabilities, and at the same time to maintain or even improve the intrinsic properties of graphene. Several production techniques for graphene‐based nanomaterials have been developed, ranging from the mechanical cleavage and chemical exfoliation of high‐quality graphene to direct growth onto different substrates and the chemical routes using graphite oxide as a precusor to the newly developed bottom‐up approach at the molecular level. The current review critically explores the recent progress on the chemical preparation of graphene‐based nanomaterials and their applications in sensors.     

Application of carbon nanotube in wireless sensor network to monitor carbon dioxide

Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. Over the years, new discoveries have led to new applications, often taking advantage of their unique electrical properties, extraordinary strength and efficiency in heat conduction. Since industrialisation, human activities have resulted in steadily increasing concentrations of the greenhouse gases. Excess amount of carbon dioxide (CO₂) in living environment is toxic and unsuitable for human consumption. Thus, a need exists for accurate, inexpensive, long-term monitoring of environmental contaminants using sensors that can be operated on site. Over the past decade, many wireless sensor network (WSN)-based monitoring applications have been proposed. This article reviews the developments of sensing elements to monitor CO₂ in the environment. The cylindrical carbon molecules have novel properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science, as well as potential sensing element in wireless sensor technology. They exhibit extraordinary strength and unique electrical properties, and are efficient thermal conductors. The unique properties of CNT makes it a potential sensing element in the WSN technology.     

Sol-gel-coated mid-infrared fiber-optic sensors

Application of organically modified sol-gels as novel recognition membranes for mid-infrared fiber-optic sensors is demonstrated for the first time by in situ detection of nitro-based aromatic compounds in aqueous media. Sol-gels were prepared by acid- and base-catalyzed copolymerization of alkyltrimethoxysiloxanes and applied onto the surface of silver halide (AgCl(0.3)Br(0.7)) fibers by drip coating. The coating process was monitored in situ using Fourier transform infrared (FT-IR) spectroscopy. Homogeneity of the layers was analyzed by scanning electron microscopy. Sol-gel-coated evanescent held sensors were investigated with respect to their capacity to suppress interfering water background absorptions, repeatability of dissolved analyte enrichment, and sensor response time. Nitrobenzene and parathion are the investigated analytes; figures of merit are derived from calibration curves determined to assess sensitivity and reproducibility of the developed sensor system. It can be concluded that sol-gel-coated infrared fiber-optic sensors enable reproducible detection of nitro-based aromatic compounds in the low ppm concentration range. Due to wide flexibility in tuning chemical properties of sol-gel films along with superior mechanical and chemical stability, organically modified sol-gels represent highly interesting coating materials for mid-infrared sensing applications.     

Emerging MXene-Based Flexible Tactile Sensors for Health Monitoring and Haptic Perception

Due to their potential applications in physiological monitoring, diagnosis, human prosthetics, haptic perception, and human–machine interaction, flexible tactile sensors have attracted wide research interest in recent years. Thanks to the advances in material engineering, high performance flexible tactile sensors have been obtained. Among the representative pressure sensing materials, 2D layered nanomaterials have many properties that are superior to those of bulk nanomaterials and are more suitable for high performance flexible sensors. As a class of 2D inorganic compounds in materials science, MXene has excellent electrical, mechanical, and biological compatibility. MXene-based composites have proven to be promising candidates for flexible tactile sensors due to their excellent stretchability and metallic conductivity. Therefore, great efforts have been devoted to the development of MXene-based composites for flexible sensor applications. In this paper, the controllable preparation and characterization of MXene are introduced. Then, the recent progresses on fabrication strategies, operating mechanisms, and device performance of MXene composite-based flexible tactile sensors, including flexible piezoresistive sensors, capacitive sensors, piezoelectric sensors, triboelectric sensors are reviewed. After that, the applications of MXene material-based flexible electronics in human motion monitoring, healthcare, prosthetics, and artificial intelligence are discussed. Finally, the challenges and perspectives for MXene-based tactile sensors are summarized.     

Vacuum technology applied to solid state chemical sensors, processing, characterization and applications

This chapter will review several solid state chemical sensors with focus on the importance of ultra high vacuum, UHV, for the development of this area. Examples of sensors will be given where processing of sensors and sensing layers as well as characterization of chemical sensors takes place in UHV as well as examples of sensors for operation in UHV. Applications of chemical sensors both already commercialized and still on the research level will be given. Sensor technologies will span from metal oxide sensors, field effect transistor sensors to surface plasmon resonance, SPR, sensors and microcalorimeters. Examples of new challenging novel sensor approaches like sensors based on indirect SPR sensing and ultra sensitive graphene-based sensors for NO₂ detection will also be given.     

Chemical sensors for portable, handheld field instruments

A review of three commonly used classes of chemical sensor technologies as applicable to implementation in portable, handheld field instruments is presented. Solid-state gas and chemical sensors have long been heralded as the solution to a wide variety of portable chemical sensing system applications. However, advances in optical sensing technology have reduced the size of supporting infrastructure to be competitive with their solid-state counterparts. Optical, solid-state, and hybrid arrays of sensors have application for portable instruments, but issues of insufficient selectivity and sensitivity continue to hamper the widespread introduction of these miniaturized sensors for solving chemical sensing problems in environments outside the laboratory. In this article, we evaluate three of the major classes of compact chemical sensors for portable applications: (solid-state) chemiresistors, (solid-state) CHEMFETs, and (optical) surface plasmon resonance sensors (SPR). These sensors are evaluated and reviewed, according to the current state of research, in terms of their ability to operate at low-power, small-size, and relatively low-cost in environments, with numerous interferents and variable ambient conditions     

Bimaterial Microcantilevers as a Hybrid Sensing Platform

Microcantilevers, one of the most common MEMS structures, have been introduced as a novel sensing paradigm nearly a decade ago. Ever since, the technology has emerged to find important applications in chemical, biological and physical sensing areas. Today the technology stands at the verge of providing the next generation of sophisticated sensors (such as artificial nose, artificial tongue) with extremely high sensitivity and miniature size. The article provides an overview of the modes of detection, theory behind the transduction mechanisms, materials employed as active layers, and some of the important applications. Emphasizing the material design aspects, the review underscores the most important findings, current trends, key challenges and future directions of the microcantilever based sensor technology.     

Recent development in 2D materials beyond graphene

Discovery of graphene and its astonishing properties have given birth to a new class of materials known as “2D materials”. Motivated by the success of graphene, alternative layered and non-layered 2D materials have become the focus of intense research due to their unique physical and chemical properties. Origin of these properties ascribed to the dimensionality effect and modulation in their band structure. This review highlights the recent progress of the state-of-the-art research on synthesis, characterization and isolation of single and few layer nanosheets and their assembly. Electronic, magnetic, optical and mechanical properties of 2D materials have also been reviewed for their emerging applications in the area of catalysis, electronic, optoelectronic and spintronic devices; sensors, high performance electrodes and nanocomposites. Finally this review concludes with a future prospective to guide this fast evolving class of 2D materials in next generation materials science.     

Organische Polymere als funktionelle Werkstoffe für chemische Sensoren

Organic polymers as functional materials for chemical sensors The function of many chemical sensors for measurements in liquids and in gases with ambient temperature is based on the combination of a transducer with organic membranes. These membrans determine essential sensor properties as selectivity, sensitivity and response characteristics. In addition they protect the detection system against external influences. Therefore the selection and synthesis of polymer membranes are an essential constituent of the sensor investigation and sensor development. Electrical, optical and biological properties of the polymers are important in this case. A survey of the materials used in the remote sensing is given. Of special interest to the sensor investigation are in last time intrinsic conducting polymers (ILP) whose properties opened new possibilities of the sensor development. With the help of an electrochemical pH glass electrode with inner solid contact it is shown that polypyrrole can be used as a material for a long‐lived inner solid contact and as substitute for inner secondary reference electrode. Practice tests confirm the suitability of this polymer material. Aspects of the transport mechanism of electrical charges through the boundary surface conducting polymer | glass are discussed.     

The influence of precursor powders and processing parameters on the properties of SnO2-based gas sensors

Semiconducting tin oxide precursor powders were synthesized via three different chemical processing routes. The influence of powder processing conditions on the physical properties, e.g., particle size, surface area and phase composition of both uncalcined and calcined materials, was investigated. These powders were used to fabricate gas sensors using thick-film screen-printing technology. The effect of precursor powders, sintering conditions, sensor temperature and Pd catalyst on the carbon monoxide, methane, propane and ethanol gas sensing characteristics of the sensors were investigated. Sensors were also fabricated using tin oxide powders obtained from a commercial source and their gas sensing properties were also investigated. The data indicates that the powder processing methodology, sensor fabrication conditions and Pd catalyst can profoundly influence the physical characteristics as well as the gas sensing properties of the sensors.     

Carbon and Nitrogen Based Nanosheets as Fluorescent Probes with Tunable Emission

2D carbon and nitrogen based semiconductors (CN) have attracted widespread attention for their possible use as low‐cost and environmentally friendly materials for various applications. However, their limited solution‐dispersibility and the difficulty in preparing exfoliated sheets with tunable photophysical properties restrain their exploitation in imaging‐related applications. Here, the synthesis of carbon and nitrogen organic scaffolds with highly tunable optical properties, excellent dispersion in water and DMSO, and good bioimaging properties is reported. Tailored photophysical and chemical properties are acquired by the synthesis of new starting monomers containing different substituent chemical groups with varying electronic properties. Upon monomer condensation at moderate temperature, 350 °C, the starting chemical groups are fully preserved in the final CN. The low condensation temperature and the effective molecular‐level modification of the CN scaffold lead to well‐dispersed photoluminescent CN thin sheets with a wide range of emission wavelengths. The good bioimaging properties and the tunable fluorescence properties are exemplified by in situ visualization of giant unilamellar vesicles in a buffered aqueous solution as a model system. This approach opens the possibility for the design of tailor‐made CN materials with tunable photophysical and chemical properties toward their exploitation in various fields, such as photocatalysis, bioimaging, and sensing.     

Advanced Soft Materials,Sensor Integrations,and Applications of Wearable Flexible Hybrid Electronics in Healthcare,Energy, and Environment

Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human–machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubtedly an attractive prospect in the sense that the successful device performance requires high degrees of mechanical flexibility, sensing capability, and user-friendly simplicity. Here, the most up-to-date materials, sensors, and system-packaging technologies to develop advanced WFHE are provided. Details of mechanical, electrical, physicochemical, and biocompatible properties are discussed with integrated sensor applications in healthcare, energy, and environment. In addition, limitations of the current materials are discussed, as well as key challenges and the future direction of WFHE. Collectively, an all-inclusive review of the newly developed WFHE along with a summary of imperative requirements of material properties, sensor capabilities, electronics performance, and skin integrations is provided.     

Synthesis of NiO nanofibers and their gas sensing properties

NiO nanofibers were synthesized by an electrospinning method with polyvinyl alcohol and nickel acetate tetrahydrate as precursor materials. Individual nanofibers consisted of nanograins. A gas sensor has been fabricated using these nanofibers. Its sensing properties to NO2 and benzene were investigated. The sensor exhibited good sensitivity and dynamic properties for the tested gases. All these results suggest that the electrospinning-synthesized NiO nanofibers hold promise for realizing sensitive and reliable gas sensors.     

An Ultrasensitive Silicon Photonic Ion Sensor Enabled by 2D Plasmonic Molybdenum Oxide

Silicon photonics has demonstrated great potential in ultrasensitive biochemical sensing. However, it is challenging for such sensors to detect small ions which are also of great importance in many biochemical processes. A silicon photonic ion sensor enabled by an ionic dopant–driven plasmonic material is introduced here. The sensor consists of a microring resonator (MRR) coupled with a 2D restacked layer of near‐infrared plasmonic molybdenum oxide. When the 2D plasmonic layer interacts with ions from the environment, a strong change in the refractive index results in a shift in the MRR resonance wavelength and simultaneously the alteration of plasmonic absorption leads to the modulation of MRR transmission power, hence generating dual sensing outputs which is unique to other optical ion sensors. Proof‐of‐concept via a pH sensing model is demonstrated, showing up to 7 orders improvement in sensitivity per unit area across the range from 1 to 13 compared to those of other optical pH sensors. This platform offers the unique potential for ultrasensitive and robust measurement of changes in ionic environment, generating new modalities for on‐chip chemical sensors in the micro/nanoscale.     

Emerging 0D Transition‐Metal Dichalcogenides for Sensors,Biomedicine, and Clean Energy

Following research on two‐dimensional (2D) transition metal dichalcogenides (TMDs), zero‐dimensional (0D) TMDs nanostructures have also garnered some attention due to their unique properties; exploitable for new applications. The 0D TMDs nanostructures stand distinct from their larger 2D TMDs cousins in terms of their general structure and properties. 0D TMDs possess higher bandgaps, ultra‐small sizes, high surface‐to‐volume ratios with more active edge sites per unit mass. So far, reported 0D TMDs can be mainly classified as quantum dots, nanodots, nanoparticles, and small nanoflakes. All exhibited diverse applications in various fields due to their unique and excellent properties. Of significance, through exploiting inherent characteristics of 0D TMDs materials, enhanced catalytic, biomedical, and photoluminescence applications can be realized through this exciting sub‐class of TMDs. Herein, we comprehensively review the properties and synthesis methods of 0D TMDs nanostructures and focus on their potential applications in sensor, biomedicine, and energy fields. This article aims to educate potential adopters of these excitingly new nanomaterials as well as to inspire and promote the development of more impactful applications. Especially in this rapidly evolving field, this review may be a good resource of critical insights and in‐depth comparisons between the 0D and 2D TMDs.     

Development of Low-cost Chemical and Micromechanical Sensors Based on Thick-film,Thin-film and Electroplated Films

Various films could be used as sensing materials or as constructional materials for the fabrication of chemical and micromechanical sensors. To illustrate this potential, three sensors fabricated by very different film deposition technologies are given as examples. The sensors are a humidity sensor in thickfilm technology, a multi-functional gas sensor in thin-film technology and a three-dimensional acceleration sensor chip manufactured by electroplating techniques. Design, fabrication and characterisation of these sensors are described in this paper.