Semiconductor gas sensors: dry synthesis and application

Ultrathin In2O3 Nanosheets with Uniform Mesopores for Highly Sensitive Nitric Oxide Detection

Nitric oxide (NO_x, including NO and NO₂) is one of the most dangerous environmental toxins and pollutants, which mainly originates from vehicle exhaust and industrial emission. The development of sensitive NO_x gas sensors is quite urgent for human health and the environment. Up to now, it still remains a great challenge to develop a NO_x gas sensor, which can satisfy multiple application demands for sensing performance (such as high response, low detection temperature, and limit). In this work, ultrathin In₂O₃ nanosheets with uniform mesopores were successfully synthesized through a facile two-step synthetic method. This is a success due to not only the formation of two-dimensional (2D) nanosheets with an ultrathin thickness of 3.7 nm based on a nonlayered compound but also the template-free construction of uniform mesopores in ultrathin nanosheets. The sensors based on the as-obtained mesoporous In₂O₃ ultrathin nanosheets exhibit an ultrahigh response (R_g/R_a = 213) and a short response time (ca. 4 s) toward 10 ppm NO_x, and a quite low detection limit (10 ppb NO_x) under a relatively low operating temperature (120 °C), which well satisfies multiple application demands. The excellent sensing performance should be mainly attributed to the unique structural advantages of mesopores and 2D ultrathin nanosheets.

Enhanced Sensitivity of MoTe2 Chemical Sensor through Light Illumination

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) semiconducting materials have recently attracted wide attention and been regarded as promising building blocks for chemical sensors due to their high surface-to-volume ratio. However, their low response hinders the realization of high-performance 2D TMDCs chemical sensors. Here, we demonstrate the improvement of sensing performance of molybdenum ditelluride (MoTe₂) gas sensor through continuous light illumination. The dependence of sensing performance on the energy of photons and light intensity is systematically studied. The response to NH₃ is dramatically enhanced by more than 25 times under 254 nm ultraviolet (UV) light illumination with intensity of 2.5 mW/cm². Moreover, a remarkable low detection limit of 3 ppb is achieved, which is improved by 80 times compared with that in dark condition. The results demonstrate that light illumination is a promising method to improve the sensitivity of 2D TMDCs chemical sensors.

Porous Ga-In Bimetallic Oxide Nanofibers with Controllable Structures for Ultrasensitive and Selective Detection of Formaldehyde

The design of appropriate composite materials with unique surface structures is an important strategy to achieve ideal chemical gas sensing. In this paper, efficient and selective detection of formaldehyde vapor has been realized by a gas sensor based on porous Ga_xIn_2-xO₃ nanofibers assembled by small building blocks. By tuning the Ga/In atomic ratios in the materials, crystallite phase, nanostructure, and band gap of as-obtained Ga_xIn_2-xO₃ nanofibers can be rationally altered. This further offers a good opportunity to optimize the gas sensing performances. In particular, the sensor based on porous Ga_0.6In_1.4O₃ nanofibers assembled by small nanoparticles (∼4.6 nm) exhibits best sensing performances. Toward 100 ppm formaldehyde, its highest response (R_a/R_g = 52.4, at 150 °C) is ∼4 times higher than that of the pure In₂O₃ (R_a/R_g = 13.0, at 200 °C). Meanwhile, it has superior ability to selectively detect formaldehyde against other interfering volatile organic compound gases. The significantly improved sensing performance makes the Ga_0.6In_1.4O₃ sensor very promising for selective detection of formaldehyde.

General fabrication and enhanced VOC gas-sensing properties of hierarchically porous metal oxides

A facile and general route to synthesize hierarchically porous metal oxides, along with their noble-metal modification, is reported. The lowest detection limit achieved for formaldehyde is 10 ppb, much less than the indoor limit (60 ppb).

Superwettability Strategy: 1D Assembly of Binary Nanoparticles as Gas Sensors

Binary 1D nanowires consisting of both SnO₂ nanoparticles and Au nanorods are fabricated through a "substrate-particle solution template" assembling method, which shows highly enhanced gas sensitivity toward acetone under ambient conditions.

In Situ Growth and Characterization of Metal Oxide Nanoparticles within Polyelectrolyte Membranes

This study describes a novel approach for the in situ synthesis of metal oxide-polyelectrolyte nanocomposites formed via impregnation of hydrated polyelectrolyte films with binary water/alcohol solutions of metal salts and consecutive reactions that convert metal cations into oxide nanoparticles embedded within the polymer matrix. The method is demonstrated drawing on the example of Nafion membranes and a variety of metal oxides with an emphasis placed on zinc oxide. The in situ formation of nanoparticles is controlled by changing the solvent composition and conditions of synthesis that for the first time allows one to tailor not only the size, but also the nanoparticle shape, giving a preference to growth of a particular crystal facet. The high-resolution TEM, SEM/EDX, UV-vis and XRD studies confirmed the homogeneous distribution of crystalline nanoparticles of circa 4 nm and their aggregates of 10-20 nm. The produced nanocomposite films are flexible, mechanically robust and have a potential to be employed in sensing, optoelectronics and catalysis.

Photoluminescent Metal-Organic Frameworks for Gas Sensing

Luminescence of porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) is sensitive to the type and concentration of chemical species in the surrounding environment, because these materials combine the advantages of the highly regular porous structures and various luminescence mechanisms, as well as diversified host-guest interactions. In the past few years, luminescent MOFs have attracted more and more attention for chemical sensing of gas-phase analytes, including common gases and vapors of solids/liquids. While liquid-phase and gas-phase luminescence sensing by MOFs share similar mechanisms such as host-guest electron and/or energy transfer, exiplex formation, and guest-perturbing of excited-state energy level and radiation pathways, via various types of host-guest interactions, gas-phase sensing has its unique advantages and challenges, such as easy utilization of encapsulated guest luminophores and difficulty for accurate measurement of the intensity change. This review summarizes recent progresses by using luminescent MOFs as reusable sensing materials for detection of gases and vapors of solids/liquids especially for O₂, highlighting various strategies for improving the sensitivity, selectivity, stability, and accuracy, reducing the materials cost, and developing related devices.

Low Working-Temperature Acetone Vapor Sensor Based on Zinc Nitride and Oxide Hybrid Composites

Transition-metal nitride and oxide composites are a significant class of emerging materials that have attracted great interest for their potential in combining the advantages of nitrides and oxides. Here, a novel class of gas sensing materials based on hybrid Zn3 N2 and ZnO composites is presented. The Zn3 N2 /ZnO (ZnNO) composites-based sensor exhibits selectivity and high sensitivity toward acetone vapor, and the sensitivity is dependent on the nitrogen content of the composites. The ZnNO-11.7 described herein possesses a low working temperature of 200 °C. The detection limit (0.07 ppm) is below the diabetes diagnosis threshold (1.8 ppm). In addition, the sensor shows high reproducibility and long-term stability.

Recent Developments in p-Type Oxide Semiconductor Materials and Devices

The development of transparent p-type oxide semiconductors with good performance may be a true enabler for a variety of applications where transparency, power efficiency, and greater circuit complexity are needed. Such applications include transparent electronics, displays, sensors, photovoltaics, memristors, and electrochromics. Hence, here, recent developments in materials and devices based on p-type oxide semiconductors are reviewed, including ternary Cu-bearing oxides, binary copper oxides, tin monoxide, spinel oxides, and nickel oxides. The crystal and electronic structures of these materials are discussed, along with approaches to enhance valence-band dispersion to reduce effective mass and increase mobility. Strategies to reduce interfacial defects, off-state current, and material instability are suggested. Furthermore, it is shown that promising progress has been made in the performance of various types of devices based on p-type oxides. Several innovative approaches exist to fabricate transparent complementary metal oxide semiconductor (CMOS) devices, including novel device fabrication schemes and utilization of surface chemistry effects, resulting in good inverter gains. However, despite recent developments, p-type oxides still lag in performance behind their n-type counterparts, which have entered volume production in the display market. Recent successes along with the hurdles that stand in the way of commercial success of p-type oxide semiconductors are presented.

Ultra-Porous Nanoparticle Networks: A Biomimetic Coating Morphology for Enhanced Cellular Response and Infiltration

Orthopedic treatments are amongst the most common cause of surgery and are responsible for a large share of global healthcare expenditures. Engineering materials that can hasten bone integration will improve the quality of life of millions of patients per year and reduce associated medical costs. Here, we present a novel hierarchical biomimetic coating that mimics the inorganic constituent of mammalian bones with the aim of improving osseointegration of metallic implants. We exploit the thermally-driven self-organization of metastable core-shell nanoparticles during their aerosol self-assembly to rapidly fabricate robust, ultra-porous nanoparticle networks (UNN) of crystalline hydroxyapatite (HAp). Comparative analysis of the response of osteoblast cells to the ultra-porous nanostructured HAp surfaces and to the spin coated HAp surfaces revealed superior osseointegrative properties of the UNN coatings with significant cell and filopodia infiltration. This flexible synthesis approach for the engineering of UNN HAp coatings on titanium implants provides a platform technology to study the bone-implant interface for improved osseointegration and osteoconduction.

Room temperature detection of individual molecular physisorption using suspended bilayer graphene

Detection of individual molecular adsorption, which represents the ultimate resolution of gas sensing, has rarely been realized with solid-state devices. So far, only a few studies have reported detection of individual adsorption by measuring the variation of electronic transport stemming from the charge transfer of adsorbate. We report room-temperature detection of the individual physisorption of carbon dioxide molecules with suspended bilayer graphene (BLG) based on a different mechanism. An electric field introduced by applying back-gate voltage is used to effectively enhance the adsorption rate. A unique device architecture is designed to induce tensile strain in the BLG to prevent its mechanical deflection onto the substrate by electrostatic force. Despite the negligible charge transfer from a single physisorbed molecule, it strongly affects the electronic transport in suspended BLG by inducing charged impurity, which can shut down part of the conduction of the BLG with Coulomb impurity scattering. Accordingly, we can detect each individual physisorption as a step-like resistance change with a quantized value in the BLG. We use density functional theory simulation to theoretically estimate the possible resistance response caused by Coulomb scattering of one adsorbed CO2 molecule, which is in agreement with our measurement.

Surface Superoxide Complex Defects-Boosted Ultrasensitive ppb-Level NO2 Gas Sensors

Sn(4+) -O2 (-•) centers are intentionally created in SnO2 nanoflowers by a thermodynamically instable synthetic process. The resulting SnO2 nanoflower-based sensor is confirmed to be the most sensitive ppb-level chemiresistor NO2 sensor to date. The Sn(4+) -O2 (-•) centers with strong gas-adsorbing and high eletron-donating capability towards NO2 molecules decisively determine the sensor sensitivity.

Hybrid Co3O4/SnO2 Core-Shell Nanospheres as Real-Time Rapid-Response Sensors for Ammonia Gas

Novel hybrid Co3O4/SnO2 core-shell nanospheres have been effectively realized by a one-step hydrothermal, template-free preparation method. Our strategy involves a simple fabrication scheme that entails the coating of natural cross-link agents followed by electrostatic interaction between the positive charges of Sn and Co ions and the negative charge of glutamic acid. The core-shell architecture enables novel flexibility of gas sensor surfaces compared to commonly used bulk materials. The highly efficient charge transfer and unique structure are key to ensuring the availability of high response and rapid-response speed. It demonstrates how hybrid core-shell nanospheres can be used as an advance function material to fabricate electrical sensing devices that may be useful as gas sensors.

An in situ self-assembled Cu4I4–MOF-based mixed matrix membrane: a highly sensitive and selective naked-eye sensor for gaseous HCl

A Cu₄I₄–MOF-based mixed matrix membrane which can be a highly sensitive and selective naked-eye sensor for gaseous HCl is reported.

Selectively enhanced sensing performance for oxidizing gases based on ZnO nanoparticle-loaded electrospun SnO2 nanotube heterostructures

In this work, we present gas sensors based on ZnO nanoparticle-loaded electrospun SnO₂ nanotube (ZnO/SnO₂) n–n heterostructures (HSs) synthesized by electrospinning combined with facile thermal decomposition.

MOF-templated controllable synthesis of α-Fe2O3 porous nanorods and their gas sensing properties

α-Fe₂O₃ porous nanorods (PNRs) with controlled morphologies were simply synthesized by thermolysis of Fe-based metal–organic framework (MIL-88A) at 500 °C.

A mesoporous Ni3N/NiO composite with a core–shell structure for room temperature, selective and sensitive NO2 gas sensing

Mesoporous Ni₃N/NiO composites with core–shell structure were synthesized by a template free method, demonstrate a significant improvements both in sensitivity and in selectivity for NO₂ gas sensing at room temperature.

Au-deposited porous single-crystalline ZnO nanoplates for gas sensing detection of total volatile organic compounds

Au-functionalized porous single-crystalline ZnO nanoplates via photodeposition for gas sensing detection of total volatile organic compounds.

Perspective on Nanoparticle Technology for Biomedical Use

This review gives a short overview on the widespread use of nanostructured and nanocomposite materials for disease diagnostics, drug delivery, imaging and biomedical sensing applications. Nanoparticle interaction with a biological matrix/entity is greatly influenced by its morphology, crystal phase, surface chemistry, functionalization, physicochemical and electronic properties of the particle. Various nanoparticle synthesis routes, characterization, and functionalization methodologies to be used for biomedical applications ranging from drug delivery to molecular probing of underlying mechanisms and concepts are described with several examples (150 references).

NiO/ZnO p–n heterostructures and their gas sensing properties for reduced operating temperature

The operating temperature of ZnO-based gas sensors has been decreased, which is attributed to the formation of NiO/ZnO p–n heterostructures.

Two-dimensional layered nanomaterials for gas-sensing applications

In this critical review, we mainly focus on the current developments of gas sensors based on typical 2D layered nanomaterials, including graphene, MoS₂, MoSe₂, WS₂, SnS₂, VS₂, black phosphorus (BP), h-BN, and g-C₃N₄.

Shape-controlled iron oxide nanocrystals: synthesis, magnetic properties and energy conversion applications

Iron oxide nanocrystals (IONCs) with various geometric morphologies show excellent physical and chemical properties and have received extensive attention in recent years.

In2O3-functionalized MoO3 heterostructure nanobelts with improved gas-sensing performance

A novel heterostructure with superior TMA-sensing performance were successfully obtained by dispersing In₂O₃ nanoparticles on the surfaces of MoO₃ nanobelts.

Generation of highly ordered nanoporous Sb–SnO2 thin films with enhanced ethanol sensing performance at low temperature

Highly ordered nanoporous Sb–SnO₂ sensing films synthesized through psHT treatment present high sensitivity to 50 ppm ethanol at low temperature.

Solution based CVD of main group materials

This critical review focuses on the solution based chemical vapour deposition (CVD) of main group materials with particular emphasis on their current and potential applications. Deposition of thin films of main group materials, such as metal oxides, sulfides and arsenides, have been researched owing to the array of applications which utilise them including solar cells, transparent conducting oxides (TCOs) and window coatings. Solution based CVD processes, such as aerosol-assisted (AA)CVD have been developed due to their scalability and to overcome the requirement of suitably volatile precursors as the technique relies on the solubility rather than volatility of precursors which vastly extends the range of potentially applicable compounds. An introduction into the applications and precursor requirements of main group materials will be presented first followed by a detailed discussion of their deposition reviewed according to this application. The challenges and prospects for further enabling research in terms of emerging main group materials will be discussed.

In situ synthesis of porous array films on a filament induced micro-gap electrode pair and their use as resistance-type gas sensors with enhanced performances

Resistance-type metal-oxide semiconductor gas sensors with high sensitivity and low detection limit have been explored for practical applications. They require both sensing films with high sensitivity to target gases and an appropriate structure of the electrode-equipped substrate to support the sensing films, which is still challenging. In this paper, a new gas sensor of metal-oxide porous array films on a micro-gap electrode pair is designed and implemented by taking ZnO as a model material. First, a micro-gap electrode pair was constructed by sputtering deposition on a filament template, which was used as the sensor's supporting substrate. Then, the sensing film, made up of ZnO porous periodic arrays, was in situ synthesized onto the supporting substrate by a solution-dipping colloidal lithography strategy. The results demonstrated the validity of the strategy, and the as-designed sensor shows a small device-resistance, an enhanced sensing performance with high resolution and an ultralow detection limit. This work provides an alternative method to promote the practical application of resistance-type gas sensors.

Application of principal component analysis to gas sensing characteristics of Cr0.8Fe0.2NbO4 thick film array

The transient changes in resistances of Cr0.8Fe0.2NbO4 thick film sensors towards specified concentrations of H2, NH3, acetonitrile, acetone, alcohol, cyclohexane and petroleum gas at different operating temperatures were recorded. The analyte-specific characteristics such as slopes of the response and retrace curves, area under the curve and sensitivity deduced from the transient curve of the respective analyte gas have been used to construct a data matrix. Principal component analysis (PCA) was applied to this data and the score plot was obtained. Distinguishing one reducing gas from the other is demonstrated based on this approach, which otherwise is not possible by measuring relative changes in conductivity. This methodology is extended for three Cr0.8Fe0.2NbO4 thick film sensor array operated at different temperatures.

Towards outperforming conventional sensor arrays with fabricated individual photonic vapour sensors inspired by Morpho butterflies

Combining vapour sensors into arrays is an accepted compromise to mitigate poor selectivity of conventional sensors. Here we show individual nanofabricated sensors that not only selectively detect separate vapours in pristine conditions but also quantify these vapours in mixtures, and when blended with a variable moisture background. Our sensor design is inspired by the iridescent nanostructure and gradient surface chemistry of Morpho butterflies and involves physical and chemical design criteria. The physical design involves optical interference and diffraction on the fabricated periodic nanostructures and uses optical loss in the nanostructure to enhance the spectral diversity of reflectance. The chemical design uses spatially controlled nanostructure functionalization. Thus, while quantitation of analytes in the presence of variable backgrounds is challenging for most sensor arrays, we achieve this goal using individual multivariable sensors. These colorimetric sensors can be tuned for numerous vapour sensing scenarios in confined areas or as individual nodes for distributed monitoring.

Individual vapour sensors often suffer from poor selectivity, which hinders their broad applicability. Here, Potyrailo et al. fabricate individual sensors inspired by the Morpho butterfly capable of selectively detecting vapours in mixtures and with a variable moisture background.

Recent developments in 2D layered inorganic nanomaterials for sensing

Two dimensional layered inorganic nanomaterials (2D-LINs) have recently attracted huge interest because of their unique thickness dependent physical and chemical properties and potential technological applications. The properties of these layered materials can be tuned via both physical and chemical processes. Some 2D layered inorganic nanomaterials like MoS2, WS2 and SnS2 have been recently developed and employed in various applications, including new sensors because of their layer-dependent electrical properties. This article presents a comprehensive overview of recent developments in the application of 2D layered inorganic nanomaterials as sensors. Some of the salient features of 2D materials for different sensing applications are discussed, including gas sensing, electrochemical sensing, SERS and biosensing, SERS sensing and photodetection. The working principles of the sensors are also discussed together with examples.

Hierarchical nanostructured WO3-SnO2 for selective sensing of volatile organic compounds

It remains a challenge to find a suitable gas sensing material that shows a high response and shows selectivity towards various gases simultaneously. Here, we report a mixed metal oxide WO3-SnO2 nanostructured material synthesized in situ by a simple, single-step, one-pot hydrothermal method at 200 °C in 12 h, and demonstrate its superior sensing behavior towards volatile organic compounds (VOCs) such as ammonia, ethanol and acetone. SnO2 nanoparticles with controlled size and density were uniformly grown on WO3 nanoplates by varying the tin precursor. The density of the SnO2 nanoparticles on the WO3 nanoplates plays a crucial role in the VOC selectivity. The responses of the present mixed metal oxides are found to be much higher than the previously reported results based on single/mixed oxides and noble metal-doped oxides. In addition, the VOC selectivity is found to be highly temperature-dependent, with optimum performance obtained at 200 °C, 300 °C and 350 °C for ammonia, ethanol and acetone, respectively. The present results on the cost-effective noble metal-free WO3-SnO2 sensor could find potential application in human breath analysis by non-invasive detection.

Ultraporous Electron-Depleted ZnO Nanoparticle Networks for Highly Sensitive Portable Visible-Blind UV Photodetectors

A hierarchical nano- and microstructured morphology for visible-blind UV photo-detectors is developed, which provides record-high milliampere photocurrents, nanoampere dark currents, and excellent selectivity to ultralow UV light intensities. This is a significant step toward the integration of high-performance UV photodetectors in wearable devices.

Template-free construction of hollow α-Fe2O3 hexagonal nanocolumn particles with an exposed special surface for advanced gas sensing properties

Hollow α-Fe2O3 hexagonal nanocolumn particles (HHCPs) with exposed (101[combining macron]0) and (112[combining macron]5) facets have been synthesized through a hydrothermal method in the absence of templates. The time-dependent experimental results demonstrate that the formation of HHCPs includes four main steps: (1) formation of nanowire precursors, (2) aggregation and conversion to Fe1.833(OH)0.5O2 solid ellipsoid particles (SEPs), (3) dehydration to form hollow ellipsoid particles (HEPs), and (4) recrystallization to HHCPs. Due to their advantages of the hollow structure and the exposed special external and internal surface on the pore structure, the HHCPs exhibit higher gas sensing ability than that of calcined SEPs (CSEPs) and HEPs.

Arc-melting to narrow the bandgap of oxide semiconductors

The bandgap of a series of oxide semiconductors is narrowed by a quick and facile arc-melting method. A defective structure is formed in the fast melting and cooling process without changing its phase structure. Enhanced optical and electrical properties are found in the arc-melted oxide, such as enhanced photocatalytic properties of the arc-melted ZnO under visible light.

Gas sensing properties and p-type response of ALD TiO2 coated carbon nanotubes

Amorphous titanium dioxide-coated carbon nanotubes (CNTs) were prepared by atomic layer deposition (ALD) and investigated as sensing layers for resistive NO2 and O2 gas sensors. By varying ALD process conditions and CNT structure, heterostructures with different metal oxide grain size, morphology and coating thickness were synthesized. Higher responses were observed with homogeneous and continuous 5.5 nm thick films onto CNTs at an operating temperature of 150 °C, while CNTs decorated with either discontinuous film or TiO2 nanoparticles showed a weak response close to the one of device made of bare CNTs. An unexpected p-type behavior in presence of the target gas was also noticed, independently of the metal oxide morphology and thickness. Based on previous works, hypotheses were made in order to explain the p-type behavior of TiO2/CNT sensors.

Facile fabrication and enhanced gas sensing properties of hierarchical MoO3 nanostructures

Hierarchical MoO₃ nanostructures, synthesized through oxidization conversion of hydrothermally synthesized MoS₂ precursors, show superior gas sensing performance toward ethanol.

3D porous α-Ni(OH)2 nanostructure interconnected with carbon black as a high-performance gas sensing material for NO2 at room temperature

The 3D porous α-Ni(OH)₂/carbon black nanostructure composites were fabricated via a simple refluxing method using SDBS as the template. The composites exhibited excellent sensing properties with fast response and low detection limit of NO₂ at room temperature.

An ultra-sensitive hydrogen gas sensor using reduced graphene oxide-loaded ZnO nanofibers

An extremely high response of about 866 at a low concentration of 100 ppb was obtained by developing a hydrogen sensor of reduced graphene oxide-loaded ZnO nanofibers.