Multiplexed gas sensor based on heterogeneous metal oxide nanomaterial array enabled by localized liquid-phase reaction

Optimization of the Mixed Gas Detection Method Based on Neural Network Algorithm

Real-time mixed gas detection has attracted significant interest for being a key factor for applications of the electronic nose (E-nose). However, mixed gas detection still faces the challenge of long detection time and a large amount of training data. Therefore, in this work, we propose a feasible way to realize low-cost fast detection of mixed gases, which uses only the part response data of the adsorption process as the training set. Our results indicated that the proposed method significantly reduced the number of training sets and the prediction time of mixed gas. Moreover, it can achieve new concentration prediction of mixed gas using only the response data of the first 10 s, and the training set proportion can reduce to 60%. In addition, the convolutional neural network model can realize both the smaller training set but also the higher accuracy of mixed gas. Our findings provide an effective way to improve the detection efficiency and accuracy of E-noses for the experimental measurement.

Ligand-Free Direct Optical Lithography of Bare Colloidal Nanocrystals via Photo-Oxidation of Surface Ions with Porosity Control

Microscale patterning of colloidal nanocrystal (NC) films is important for their integration in devices. Here, we introduce the direct optical patterning of all-inorganic NCs without the use of additional photosensitive ligands or additives. We determined that photoexposure of ligand-stripped, "bare" NCs in air significantly reduces their solubility in polar solvents due to photo-oxidation of surface ions. Doses as low as 20 mJ/cm² could be used; the only obvious criterion for material selection is that the NCs need to have significant absorption at the irradiation wavelength. However, transparent NCs can still be patterned by mixing them with suitably absorbing NCs. This approach enabled the patterning of bare ZnSe, CdSe, ZnS, InP, CeO₂, CdSe/CdS, and CdSe/ZnS NCs as well as mixtures of ZrO₂ or HfO₂ NCs with ZnSe NCs. Optical, X-ray photoelectron, and infrared spectroscopies show that solubility loss results from desorption of bound solvent due to photo-oxidation of surface ions. We also demonstrate two approaches, compatible with our patterning method, for modulating the porosity and refractive index of NC films. Block copolymer templating decreases the film density, and thus the refractive index, by introducing mesoporosity. Alternatively, hot isostatic pressing increases the packing density and refractive index of NC layers. For example, the packing fraction of a ZnS NC film can be increased from 0.51 to 0.87 upon hot isostatic pressing at 450 °C and 15 000 psi. Our findings demonstrate that direct lithography by photo-oxidation of bare NC surfaces is an accessible patterning method for facilitating the exploration of more complex NC device architectures while eliminating the influence of bulky or insulating surfactants.

Noise Spectrum as a Source of Information in Gas Sensors Based on Liquid-Phase Exfoliated Graphene

Surfaces of adsorption-based gas sensors are often heterogeneous, with adsorption sites that differ in their affinities for gas particle binding. Knowing adsorption/desorption energies, surface densities and the relative abundance of sites of different types is important, because these parameters impact sensor sensitivity and selectivity, and are relevant for revealing the response-generating mechanisms. We show that the analysis of the noise of adsorption-based sensors can be used to study gas adsorption on heterogeneous sensing surfaces, which is applicable to industrially important liquid-phase exfoliated (LPE) graphene. Our results for CO2 adsorption on an LPE graphene surface, with different types of adsorption sites on graphene flake edges and basal planes, show that the noise spectrum data can be used to characterize such surfaces in terms of parameters that determine the sensing properties of the adsorbing material. Notably, the spectrum characteristic frequencies are an unambiguous indicator of the relative abundance of different types of adsorption sites on the sensing surface and their surface densities. We also demonstrate that spectrum features indicate the fraction of the binding sites that are already occupied by another gas species. The presented study can be applied to the design and production of graphene and other sensing surfaces with an optimal sensing performance.

Artificial Olfactory Neuron for an In-Sensor Neuromorphic Nose

A neuromorphic module of an electronic nose (E-nose) is demonstrated by hybridizing a chemoresistive gas sensor made of a semiconductor metal oxide (SMO) and a single transistor neuron (1T-neuron) made of a metal-oxide-semiconductor field-effect transistor (MOSFET). By mimicking a biological olfactory neuron, it simultaneously detects a gas and encoded spike signals for in-sensor neuromorphic functioning. It identifies an odor source by analyzing the complicated mixed signals using a spiking neural network (SNN). The proposed E-nose does not require conversion circuits, which are essential for processing the sensory signals between the sensor array and processors in the conventional bulky E-nose. In addition, they do not have to include a central processing unit (CPU) and memory, which are required for von Neumann computing. The spike transmission of the biological olfactory system, which is known to be the main factor for reducing power consumption, is realized with the SNN for power savings compared to the conventional E-nose with a deep neural network (DNN). Therefore, the proposed neuromorphic E-nose is promising for application to Internet of Things (IoT), which demands a highly scalable and energy-efficient system. As a practical example, it is employed as an electronic sommelier by classifying different types of wines.

Nanoparticle design and assembly for p-type metal oxide gas sensors

Metal oxide semiconductors have wide band gaps with tailorable electrical properties and high stability, suitable for chemiresistive gas sensors. p-Type oxide semiconductors generally have less sensitivity than their n-type counterparts but provide unique functionality with low humidity dependence. Among various approaches to enhance the p-type characteristics, nanostructuring of active materials is essential to exhibit high sensing performances comparable to n-type materials. Moreover, p-n heterojunction formation can achieve superior sensitivity at low operating temperatures. The representative examples are hollow and urchin-like particles, mesoporous structures, and nanowire networks. These morphologies can generate abundant active surface sites with a high surface area and induce rapid gas diffusion and facile charge transport. For growing interests in environmental and healthcare monitoring, p-type oxide semiconductors and their heterojunctions with well-designed nanostructures gain much attention as advanced gas sensing materials for practical applications. In addition to precise nanostructure design, the combination with other strategies, e.g. light activation and multiple gas sensing analysis using sensor arrays will be able to fabricate the desired gas sensors with exclusive gas detection at ultra-low concentrations operating even at room temperature.

Low-Concentration NOx Gas Analysis Using Single Bimodular ZnO Nanorod Sensor

Quantitative measurement of the nitrogen oxide mixture (NO_x, usually of NO and NO₂) usually relies on sophisticated, space-consuming, and expensive spectroscopy techniques such as gas chromatography (GC), Fourier-transform infrared spectroscopy (FTIR), and chemi-luminescence detection (CLD). The direct and portable measurement solutions are lacking in this regard. In this work, by utilizing the bimodular sensing strategy, we successfully demonstrated the differential measurement of NO_x with errors smaller than 8.3%, by correlating the sensor electrical and electrochemical responses. The effective detection is successfully displayed in the low-concentration ranges of 1-10 ppm for NO and 100 ppb-1 ppm for NO₂, where weak competitive gas co-adsorption mitigated the cross-sensitivities compared to the higher-concentration range. Based on the electron occupation with negligible competitive adsorption, the accurate theoretic prediction of the mixture responses versus component concentration relieves the reliance on repeated calibration and empirical functions. With the miniaturized size and simplified electrical feedthrough, the single bimodular nanorod sensor provides a promising solution for direct and portable NO_x analysis at low concentrations.

In Situ Assembly of Ordered Hierarchical CuO Microhemisphere Nanowire Arrays for High-Performance Bifunctional Sensing Applications

Seeking a facile approach to directly assemble bridged metal oxide nanowires on substrates with predefined electrodes without the need for complex postsynthesis alignment and/or device procedures will bridge the gap between fundamental research and practical applications for diverse biochemical sensing, electronic, optoelectronic, and energy storage devices. Herein, regularly bridged CuO microhemisphere nanowire arrays (RB-MNAs) are rationally designed on indium tin oxide electrodes via thermal oxidation of ordered Cu microhemisphere arrays obtained by solid-state dewetting of patterned Ag/Cu/Ag films. Both the position and spacing of CuO microhemisphere nanowires can be well controlled by as-used shadow mask and the thickness of Cu film, which allows homogeneous manipulation of the bridging of adjacent nanowires grown from neighboring CuO hemispheres, and thus benefits highly sensitive trimethylamine (TMA) sensors and broad band (UV-visible to infrared) photodetectors. The electrical response of 3.62 toward 100 ppm TMA is comparable to that of state-of-the-art CuO-based sensors. Together with the feasibility of in situ assembly of RB-MNAs device arrays via common lithographic technologies, this work promises commercial device applications of CuO nanowires.

Chemo-Mechanically Operating Palladium-Polymer Nanograting Film for a Self-Powered H2 Gas Sensor

This study proposes a reliable and self-powered hydrogen (H₂) gas sensor composed of a chemo-mechanically operating nanostructured film and photovoltaic cell. Specifically, the nanostructured film has a configuration in which an asymmetrically coated palladium (Pd) film is coated on a periodic polyurethane acrylate (PUA) nanograting. The asymmetric Pd nanostructures, optimized by a finite element method simulation, swell upon reacting with H₂ and thereby bend the PUA nanograting, changing the amount of transmitted light and the current output of the photovoltaic cell. Since the degree of warping is determined by the concentration of H₂ gas, a wide concentration range of H₂ (0.1-4.0%) can be detected by measuring the self-generated electrical current of the photovoltaic cell without external power. The normalized output current changes are ∼1.5%, ∼2.8%, ∼3.5%, ∼5.0%, ∼21.5%, and 25.3% when the concentrations of H₂ gas are 0.1%, 0.5%, 1.0%, 1.6%, 2%, and 4%, respectively. Moreover, because Pd is highly chemically reactive to H₂ and also because there is no electrical current applied through Pd, the proposed sensor can avoid device failure due to the breakage of the Pd sensing material, resulting in high reliability, and can show high selectivity against various gases such as carbon monoxide, hydrogen sulfide, nitrogen dioxide, and water vapor. Finally, using only ambient visible light, the sensor was modularized to produce an alarm in the presence of H₂ gas, verifying a potential always-on H₂ gas monitoring application.

Stretchable gas sensors for detecting biomarkers from humans and exposed environments

The recent advent of stretchable gas sensors demonstrates their capabilities to detect not only gaseous biomarkers from the human body but also toxic gas species from the exposed environment. To ensure accurate gas detection without device breakdown from the mechanical deformations, the stretchable gas sensors often rely on the direct integration of gas-sensitive nanomaterials on the stretchable substrate or fibrous network, as well as being configured into stretchable structures. The nanomaterials in the forms of nanoparticles, nanowires, or thin-films with nanometer thickness are explored for a variety of sensing materials. The commonly used stretchable structures in the stretchable gas sensors include wrinkled structures from a pre-strain strategy, island-bridge layouts or serpentine interconnects, strain isolation approaches, and their combinations. This review aims to summarize the recent advancement in novel nanomaterials, sensor design innovations, and new fabrication approaches of stretchable gas sensors.

Self-Powered Gas Sensor Based on a Photovoltaic Cell and a Colorimetric Film with Hierarchical Micro/Nanostructures

We report a new type of self-powered gas sensors based on the combination of a colorimetric film with hierarchical micro/nanostructures and organic photovoltaic cells. The transmittance of the colorimetric film with micro/nanostructures coated with N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) changes by reacting with NO₂ gas, and it is measured as a current output of the photovoltaic cell. For this purpose, materials for the organic photovoltaic cells were carefully chosen to match the working wavelength of the TMPD. Micropost arrays and nanowires increase the surface area for the gas reaction and thus improve the transmittance changes by NO₂ gas (6.7% change for the plain film vs 27.7% change for the film with hierarchical micro/nanostructures to 20 ppm of NO₂). Accordingly, the colorimetric device with the hierarchical structures showed a response of ΔI/I₀ = 0.27-20 ppm of NO₂, which is a 71% improvement compared to that of the plain sensing film. Furthermore, it showed a high selectivity against other gases such as H₂S and CO with almost negligible responses. Since the current output change of the photovoltaic cell is utilized as a sensor signal, no extra electrical power is required for the operation of gas sensors. We also integrated the sensor device with an electrical module and demonstrated a self-powered gas alarm system.

Joule-Heated and Suspended Silicon Nanowire Based Sensor for Low-Power and Stable Hydrogen Detection

We developed self-heated, suspended, and palladium-decorated silicon nanowires (Pd-SiNWs) for high-performance hydrogen (H₂) gas sensing with low power consumption and high stability against diverse environmental noises. To prepare the Pd-SiNWs, SiNWs were fabricated by conventional complementary metal-oxide-semiconductor (CMOS) processes, and Pd nanoparticles were coated on the SiNWs by a physical vapor deposition method. Suspended Pd-SiNWs were simply obtained by etching buried oxide layer and Pd deposition. Joule heating of Pd-SiNW (<1 mW) enables the detection of H₂ gas with a faster response and without the reduction of sensitivity unlike other Pd-based H₂ gas sensors. We proposed a H₂ sensing model using oxygen adsorption on the Pd nanoparticle-coated silicon oxide surface to understand the H₂ response of Joule-heated Pd-SiNWs. A suspended Pd-SiNW showed a similar transient sensing response with around four times lower Joule heating power (147 μW) than the substrate-bound Pd-SiNW (613 μW). The effect of interfering gas on the Pd-SiNW was investigated, and it was found that the Joule heating of Pd-SiNW helps to maintain the H₂ sensing performance in humid or carbon monoxide environments.

Gas Sensor by Direct Growth and Functionalization of Metal Oxide/Metal Sulfide Core-Shell Nanowires on Flexible Substrates

We have developed a novel fabrication method for flexible gas sensors for toxic gases based on sequential wet chemical reaction. In specific, zinc oxide (ZnO) nanowires were locally synthesized and directly integrated on a flexible polymer substrate using localized hydrothermal synthesis methods and their surfaces were selectively functionalized with palladium (Pd) nanoparticles using a liquid phase deposition process. Because the entire process is conducted at a low temperature in a mild precursor solution, it can be applied for flexible substrates. Furthermore, the surface of ZnO nanowires was sulfurized by hydrogen sulfide (H₂S) gas to form zinc oxide/zinc sulfide (ZnO/ZnS) core-shell nanowires for stable sensing of H₂S gas. The locally synthesized ZnO/ZnS core-shell nanowires enable an ultracompact-sized device, and Pd nanoparticles improve the sensing performance and reduce the operating temperature (200 °C). The device shows a high sensitivity [(G_gas - G_air)/G_air × 100% = 4491% to 10 ppm], fast response (response/recovery time <100 s) to hydrogen sulfide, and outstanding selectivity (>100 times) to other toxic gases (e.g., carbon monoxide, acetone, ethanol, and toluene). Moreover, vertically synthesized nanowires provide a long bending path, which reduces the mechanical stresses on the structure. The devices showed stable gas sensing performance under 9 mm positive radius of curvature and 5 mm negative radius of curvature. The mechanical robustness of the device was also verified by numerical simulations which showed dramatic decrease of maximum stress and strain to 4.2 and 5.0%, respectively.

Pd-loaded SnO2 hierarchical nanospheres for a high dynamic range H2S micro sensor

The sensitivity of Pd-loaded SnO₂ nanosphere sensor to H₂S gas: micro gas sensors based on Pd-loaded SnO₂ nanospheres have credible gas detection abilities down to 10 ppb and 4 orders of magnitude concentration detection ranges.

Selective Deposition of Multiple Sensing Materials on Si Nanobelt Devices through Plasma-Enhanced Chemical Vapor Deposition and Device-Localized Joule Heating

This paper describes a novel method, using device-localized Joule heating (JH) in a plasma enhanced atomic layer deposition (PEALD) system, for the selective deposition of platinum (Pt) and zinc oxide (ZnO) in the n^- regions of n⁺/n^-/n⁺ polysilicon nanobelts (SNBs). COMSOL simulations were adopted to estimate device temperature distribution. However, during ALD process, the resistance of SNB device decreased gradually and reached to minima after 20 min JH. As a result, thermal decomposition of precursors occurred during PEALD process. Selective deposition in the n^- region was dominated by CVD instead of ALD. Selective deposition of Pt and ZnO films has been achieved and characterized using atomic force microscopy, scanning electron microscopy, and transmission electron microscopy.

Localized Liquid-Phase Synthesis of Porous SnO2 Nanotubes on MEMS Platform for Low-Power, High Performance Gas Sensors

We have developed highly sensitive, low-power gas sensors through the novel integration method of porous SnO₂ nanotubes (NTs) on a micro-electro-mechanical-systems (MEMS) platform. As a template material, ZnO nanowires (NWs) were directly synthesized on beam-shaped, suspended microheaters through an in situ localized hydrothermal reaction induced by local thermal energy around the Joule-heated area. Also, the liquid-phase deposition process enabled the formation of a porous SnO₂ thin film on the surface of ZnO NWs and simultaneous etching of the ZnO core, eventually to generate porous SnO₂ NTs. Because of the localized synthesis of SnO₂ NTs on the suspended microheater, very low power for the gas sensor operation (<6 mW) has been realized. Moreover, the sensing performance (e.g., sensitivity and response time) of synthesized SnO₂ NTs was dramatically enhanced compared to that of ZnO NWs. In addition, the sensing performance was further improved by forming SnO₂-ZnO hybrid nanostructures due to the heterojunction effect.

Hierarchical Morphology-Dependent Gas-Sensing Performances of Three-Dimensional SnO2 Nanostructures

Hierarchical morphology-dependent gas-sensing performances have been demonstrated for three-dimensional SnO₂ nanostructures. First, hierarchical SnO₂ nanostructures assembled with ultrathin shuttle-shaped nanosheets have been synthesized via a facile and one-step hydrothermal approach. Due to thermal instability of hierarchical nanosheets, they are gradually shrunk into cone-shaped nanostructures and finally deduced into rod-shaped ones under a thermal treatment. Given the intrinsic advantages of three-dimensional hierarchical nanostructures, their gas-sensing properties have been further explored. The results indicate that their sensing behaviors are greatly related with their hierarchical morphologies. Among the achieved hierarchical morphologies, three-dimensional cone-shaped hierarchical SnO₂ nanostructures display the highest relative response up to about 175 toward 100 ppm of acetone as an example. Furthermore, they also exhibit good sensing responses toward other typical volatile organic compounds (VOCs). Microstructured analyses suggest that these results are mainly ascribed to the formation of more active surface defects and mismatches for the cone-shaped hierarchical nanostructures during the process of thermal recrystallization. Promisingly, this surface-engineering strategy can be extended to prepare other three-dimensional metal oxide hierarchical nanostructures with good gas-sensing performances.

A new method to integrate ZnO nano-tetrapods on MEMS micro-hotplates for large scale gas sensor production

A new method, which is easily scalable to large scale production, has been developed to obtain gas sensor devices based on zinc oxide (ZnO) nanostructures with a 'tetrapod' shape. The method can be easily extended to other kinds of nanostructures and is based on the deposition of ZnO nanostructures through polymeric masks by centrifugation, directly onto properly designed MEMS micro-hotplates. The micromachined devices, after the mask is peeled off, are ready for electrical bonding and sensing test. Sensor response has been successfully measured for some gases and volatile organic compounds with different chemical properties (ethanol, methane, nitrogen dioxide, hydrogen sulfide).

Interface engineering: broadband light and low temperature gas detection abilities using a nano-heterojunction device

Herein, we have designed a nano-heterojunction device using interface defects and band bending effects, which can have broadband light detection (from 365-940 nm) and low operating temperature (50 °C) gas detection abilities. The broadband light detection mechanism occurs because of the defects and band bending between the heterojunction interface. We have demonstrated this mechanism using CoSi2/SnO2, CoSi2/TiO2, Ge/SnO2 and Ge/TiO2 nano-heterojunction devices, and all these devices show broadband light detection ability. Furthermore, the nano-heterojunction of the nano-device has a local Joule-heating effect. For gas detection, the results show that the nano-heterojunction device presents a high detection ability. The reset time and sensitivity of the nano-heterojunction device are an order faster and larger than Schottky-contacted devices (previous works), which is due to the local Joule-heating effect between the interface of the nano-heterojunction. Based on the abovementioned idea, we can design diverse nano-devices for widespread use.