Ionic polymer metal composites (IPMCs) are a class of materials with a rising appeal in biological micro-electromechanical systems (bio-MEMS) due to their unique properties (low voltage output, bio-compatibility, affinity with ionic medium). While tailoring and improving actuation capabilities of IPMCs is a key motivator in almost all IPMC manufacturing reports, very little efforts have been dedicated to sensing using IPMC thinner than 100 µm. Most reports on IPMC manufacturing and utilization rely on 180 µm-thick Nafion with platinum electrodes, too stiff for bio-MEMS applications. The same fabrication process on thinner membranes does yield in very poor electrodes and performance, and needs to be studied to increase flexibility and sensitivity in the microscale range. This study demonstrates an electroless Pt deposition method for fabricating bio-MEMS-suitable 50 µm-thick IPMC samples. First, we perform a comparative study on the platinum distribution within the Nafion backbone as well as on the surface for the standard electroless deposition recipe for thin (50 µm) and thick (180 µm) Nafion. We report strong differences in platinum distribution for thick and thin IPMC that experienced the same manufacturing process. By varying chemical concentrations from the standard recipe we obtain convenient platinum distribution on thin Nafion, with platinum mainly localized in proximity of surface, as well as electrodes with lower sheet resistance. We could measure the flexural rigidity as 3.43 × 10 − 8 N·m², 46 times lower than standard 180 µm-thick IPMC. The calculated sensitivity is 0.476 ± 0.02 mV mm⁻¹ and the limit of detection for our sensor is 500 ± 20 µm. This procedure sets a milestone for manufacturing 50 µm-thick IPMC for transducers and sensors in bio-MEMS applications.

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The fabrication and design of carbon-based hierarchical structures with tailored nano-architectures have attracted the enormous attention of the materials science community due to their exceptional chemical and physical properties. The collective control of nano-objects, in terms of their dimensionality, orientation and size, is of paramount importance to expand the implementation of carbon nanomaterials across a large variety of applications. In this context, porous anodic alumina (PAA) has become an attractive template where the pore morphologies can be straightforwardly modulated. The synthesis of diverse carbon nanomaterials can be performed using PAA templates, such as carbon nanotubes (CNTs), carbon nanofibers (CNFs), and nanodiamonds, or can act as support for other carbon allotropes such as graphene and other carbon nanoforms. However, the successful growth of carbon nanomaterials within ordered PAA templates typically requires a series of stages involving the template fabrication, nanostructure growth and finally an etching or electrode metallization steps, which all encounter different challenges towards a nanodevice fabrication. The present review article describes the advantages and challenges associated with the fabrication of carbon materials in PAA based materials and aims to give a renewed momentum to this topic within the materials science community by providing an exhaustive overview of the current synthesis approaches and the most relevant applications based on PAA/Carbon nanostructures materials. Finally, the perspective and opportunities in the field are presented. @en

Nitrogen dioxide (NO₂) is a potential hazard to human health at low concentrations, below one part per million (ppm). NO₂ can be monitored using gas sensors based on multi-layered graphene operating at ambient temperature. However, reliable detection of concentrations on the order of parts per million and lower is hindered by partial recovery and lack of reproducibility of the sensors after exposure. We show how to overcome these longstanding problems using ultraviolet (UV) light. When exposed to NO₂, the sensor response is enhanced by 290 % − 550 % under a 275 nm wavelength light emitting diode irradiation. Furthermore, the sensor's initial state is completely restored after exposure to the target gas. UV irradiation at 68 W/m2 reduces the NO₂ detection limit to 30 parts per billion (ppb) at room temperature. We investigated sensor performance optimization for UV irradiation with different power densities and target gases, such as carbon oxide and ammonia. Improved sensitivity, recovery, and reproducibility of UV-assisted graphene-based gas sensors make them suitable for widespread environmental applications.

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The precise control of patterned arrays of carbon nanofibers (CNFs) is an issue of interest in a wide range of applications. In the present work, we report the synthesis of CNFs grown by plasma-enhanced CVD (PECVD) from a Ni catalyst patterned via aerosol printing of nanoparticles (NPs) by a spark ablation technique. The printing speeds were adjusted to vary the catalyst density and characterize the CNFs distribution in the pre-patterned lines. Depending on the printing speed, different CNFs distributions are obtained, ranging from dense vertically-aligned CNFs (VACNFs) to sparse individual CNFs.@en

This work demonstrates a humidity sensor based on two multi-layered graphene (MLG) strips monolithically integrated onto a micro-hotplate (MHP). A transfer-free approach was adopted to release the graphene from the catalyst to reduce device variations and ensure scalability. The sensing performance of the device was tested by exposing the device to humidity levels in the range of 10% to 84% of relative humidity (RH). Both MLG strips present a linear response over this range. The micro-heater implementation is vital to fully desorb water molecules from the MLG surface. The micro-heater was powered with 31 mW to reach 140°C in the MHP zone. The sensitivity of the devices is of the order of 1000 pp/RH[%]. The developed device aims at providing a robust and reliable gas sensors platform based on MLG.@en

An ethanol gas sensor based on carbon nanofibers (CNFs) with various densities and nanoparticle functionalization was investigated. The CNFs were grown by means of a Plasma-Enhanced Chemical Vapor Deposition (PECVD), and the synthesis conditions were varied to obtain different number of fibers per unit area. The devices with a larger density of CNFs lead to higher responses, with a maximal responsivity of 10%. Furthermore, to simultaneously improve the sensitivity and selectivity, CNFs were decorated with gold nanoparticles by an impaction printing method. After metal decoration, the devices showed a response 300% higher than pristine devices toward 5 ppm of ethanol gas. The morphology and structure of the different samples deposited on a silicon substrate were characterized by TEM, EDX, SEM, and Raman spectroscopy, and the results confirmed the presence of CNF decorated with gold. The influence of operating temperature (OT) and humidity were studied on the sensing devices. In the case of decorated samples with a high density of nanofibers, a less-strong cross-sensitivity was observed toward a variation in humidity and temperature.@en

In this paper, tin oxidation (SnO x )/tin-sulfide (SnS) heterostructures are synthesized by the post-oxidation of liquid-phase exfoliated SnS nanosheets in air. We comparatively analyzed the NO2 gas response of samples with different oxidation levels to study the gas sensing mechanisms. The results show that the samples oxidized at 325 °C are the most sensitive to NO2 gas molecules, followed by the samples oxidated at 350 °C, 400 °C and 450 °C. The repeatabilities of 350 °C samples are better than that of 325 °C, and there is almost no shift in the baseline. Thus this work systematically analyzed the gas sensing performance of SnO x/SnS-based sensor oxidized at 350 °C. It exhibits a high response of 171% towards 1 ppb NO2, a wide detecting range (from 1 ppb to 1 ppm), and an ultra-low theoretical detection limit of 5 ppt, and excellent repeatability at room temperature. The sensor also shows superior gas selectivity to NO2 in comparison to several other gas molecules, such as NO, H2, SO2, CO, NH3, and H2O. After X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscope, and electron paramagnetic resonance characterizations combining first principle analysis, it is found that the outstanding NO2 sensing behavior may be attributed to three factors: The Schottky contact between electrodes and SnO x/SnS; active charge transfer in the surface and the interface layer of SnO x/SnS heterostructures; and numerous oxygen vacancies generated during the post-oxidation process, which provides more adsorption sites and superior bandgap modulation. Such a heterostructure-based room-temperature sensor can be fabricated in miniaturized size with low cost, making it possible for large-scale applications.

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2D and nanostructured metal sulfide materials are promising in the advancement of several gas sensing applications due to the abundant choice of materials with easily tunable electronic, optical, physical, and chemical properties. These applications are particularly attractive for gas sensing in environmental monitoring and breath analysis. This review gives a systematic description of various gas sensors based on 2D and nanostructured metal sulfide materials. Firstly, the crystal structures of metal sulfides are introduced. Secondly, the gas sensing mechanisms of different metal sulfides based on density functional theory analysis are summarised. Various gas-sensing concepts of metal sulfide-based devices, including chemiresistors, functionalized metal sulfides, Schottky junctions, heterojunctions, field-effect transistors, and optical and surface acoustic wave sensors, are compared and presented. It then discusses the extensive applications of metal sulfide-based sensors for different gas molecules, including volatile organic compounds (i.e., acetone, benzene, methane, formaldehyde, ethanol, and liquefied petroleum gas) and inorganic gas (i.e., CO2, O2, NH3, H2S, SO2, NOx, CH4, H2, and humidity). Finally, a strengths-weaknesses-opportunities-threats (SWOT) analysis is proposed for future development and commercialization in this field. This journal is

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In this paper, the compressive stress of pristine and coated vertically-aligned (VA) multi-walled (MW) carbon nanotube (CNT) pillars were investigated using flat-punch nano-indentation. VA-MWCNT pillars of various diameters (30–150 µm) grown by low-pressure chemical vapor deposition on silicon wafer. A conformal brittle coating of niobium-titanium-nitride with high superconductivity temperature was deposited on the VA-MWCNT pillars using atomic layer deposition. The coating together with the pillars could form a superconductive vertical interconnect. The indentation tests showed foam-like behavior of pristine CNTs and ceramic-like fracture of conformal coated CNTs. The compressive strength and the elastic modulus for pristine CNTs could be divided into three regimes of linear elastic, oscillatory plateau, and exponential densification. The elastic modulus of pristine CNTs increased for a smaller pillar diameter. The response of the coated VA-MWCNTs depended on the diffusion depth of the coating in the pillar and their elastic modulus increased with pillar diameter due to the higher sidewall area. Tuning the material properties by conformal coating on various diameter pillars enhanced the mechanical performance and the vertical interconnect access (via) reliability. The results could be useful for quantum computing applications that require high-density superconducting vertical interconnects and reliable operation at reduced temperatures.@en

Owing to their sensitive chemical-to-electrical transducer capabilities and compatibility with device miniaturization and low operation temperature, single-walled carbon nanotube field-effect transistor (SWCNT-FET) represents an attractive platform to provide solutions in the gas sensing field. In this work, SWCNT-FETs were fabricated and their performances for detecting low NO2 concentrations were evaluated. Outstanding devices response was obtained, which was shown to follow a 2 power law dependence between the response and the NO2 concentration in the range of 100 ppb up to 10 ppm. Such ultra-high response is attributed to an enhancement of the Schottky barrier modulation triggered by the specific device configuration. The device configuration is based on individual semiconducting SWCNTs directly connecting the interdigitated Source-Drain electrodes. To the best of our knowledge, the results reported here correspond to the most sensitive device among the devices based on non-functionalized carbon materials and operational at low temperatures. Furthermore, the obtained results are supported by a deep SWCNT characterization, and the changes in Schottky barrier's height induced by the presence of gas molecules are estimated and discussed. Overall, the present reported results provide useful information for establishing a robust process for the fabrication of the next generation of CNT based gas-sensing devices.@en

In this work, a thin-film transistor gas sensor based on the p-N heterojunction is fabricated by stacking chemical vapor deposition-grown tungsten disulfide (WS₂) with a sputtered indium-gallium-zinc-oxide (IGZO) film. To the best of our knowledge, the present device has the best NO₂ gas sensor response compared to all the gas sensors based on transition-metal dichalcogenide materials. The gas-sensing response is investigated under different NO₂ concentrations, adopting heterojunction device mode and transistor mode. High sensing response is obtained of p-N diode in the range of 1-300 ppm with values of 230% for 5 ppm and 18 170% for 300 ppm. On the transistor mode, the gas-sensing response can be modulated by the gate bias, and the transistor shows an ultrahigh response after exposure to NO₂, with sensitivity values of 6820% for 5 ppm and 499 400% for 300 ppm. Interestingly, the transistor has a typical ambipolar behavior under dry air, while the transistor becomes p-type as the amount of NO₂ increases. The assembly of these results demonstrates that the WS₂/IGZO device is a promising platform for the NO₂-gas detection, and its gas-modulated transistor properties show a potential application in tunable engineering for two-dimensional material heterojunction-based transistor device.

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The present work aims to become a standard step for the fabrication of the next generation porous anodic alumina (PAA) templates based devices and to provide new insights on the mechanism involved in the porous anodic alumina formation. The oxide barrier layer at the bottom of the pores has been successfully thinned by applying an exponential voltage decrease process followed by a wet chemical etching. The impact of the potential drop on the PAA structure has been deeply investigated, as well as the electrolyte temperature, the number of potential steps and the exponential decay rate. The presented results underline that a smart adjustment between the anodization conditions and exponential voltage decay parameters can simultaneously give PAA structures with straight pores and remove the dielectric layer in spite of applying the exponential voltage decay step. Additionally, the PAA structure has been tuned to fabricate hierarchically nanoporous templates with secondary pores ranging from 2 up to 10 branches.

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