Despite the abundance of materials synthesis techniques, producing an array of chemiresistive sensing units on the same platform chip remains challenging. Herein a dry printing system based on spark ablation is showcased, capable of depositing nanoporous layers (NPLs), consisting of metal/metal oxide semiconductor (MOS) materials directly on CMOS compatible substrates. In this work $\mathbf{S n O}_{\mathbf{2}}, \mathbf{Z n O}$, and NiO were used as MOS-base materials, and Ag and Au as metal catalysts. NPLs used as sensing units with up to four materials were explored and evaluated upon exposure to formaldehyde gas molecules in the range of $\mathbf{0. 1}-\mathbf{1 0} \mathbf{~ p p m}$. The $\mathbf{1 2}$ best performing devices show a response in the range of 40-70% at 0.1 ppm of formaldehyde. This work aims to highlight this robust and versatile approach to producing highly sensitive units with multiple elemental compositions that enable the fabrication of a sensor array.

This parameter study examines the impact of two distinct adhesion layers, chromium (Cr) and titanium (Ti), on the performance of CuO/Cu2O-based chemoresistive gas sensors by varying the layer thickness. The sensing material utilised on a Si-SiO2 sensor chip with Pt electrodes is an ultrathin CuO/Cu2O film fabricated through thermal deposition of Cu and subsequent oxidation. The sensors were evaluated by measuring the change in electrical resistance against a range of target gases, including carbon monoxide (CO), carbon dioxide (CO2) and a mixture of hydrocarbons (HCMix), in order to assess any potential cross-sensitivity issues. As the reactions occur at the surface, the surface was characterised by scanning electron microscopy (SEM) and the composition by grazing incidence X-Ray diffraction (GIXRD) measurement to gain further insight into the influence of the adhesion layer on the sensing performance.

This study examines the impact of Au nanoparticles (Au-NPs) on the chemoresistive gas sensing properties as a function of particle size. The sensing material is composed of ultrathin CuO/Cu2O films, which are fabricated by either thermal deposition technology or spray pyrolysis. These are used on a silicon nitride (Si3N4) micro hotplate (µh) chip with Pt electrodes and heaters. The gas sensing material is then functionalised with Au-NP of varying sizes (12, 20, and 40 nm, checked by transmission electron microscopy) using drop coating technology. The finalised sensors are tested by measuring the electrical resistance against various target gases, including carbon monoxide (CO), carbon dioxide (CO2), and a mixture of hydrocarbons (HCMix), in order to evaluate any cross-sensitivity issues. While the sensor response is markedly contingent on the structural surface, our findings indicate that the dimensions of the Au-NPs exert a discernible influence on the sensor’s behaviour in response to varying target gases. The 50 nm thermally evaporated CuO/Cu2O layers exhibited the highest sensor response of 78% against 2000 ppm CO2. In order to gain further insight into the surface of the sensors, a scanning electron microscope (SEM) was employed, and to gain information about the composition, Raman spectroscopy was also utilised.

Chemical sensors, relying on electrical conductance changes in a gas-sensitive material due to the surrounding gas, have the (dis-)advantage of reacting with multiple target gases and humidity. In this work, we report CMOS-integrated SnO2 thin film-based gas sensors, which are functionalized with mono-, bi-, and trimetallic nanoparticles (NPs) to optimize the sensor performance. The spray pyrolysis technology was used to deposit the metal oxide sensing layer on top of a CMOS-fabricated micro-hotplate (µhp), and magnetron sputtering inert-gas condensation was employed to functionalize the sensing layer with metallic NPs, Ag-, Pd-, and Ru-NPs, and all combinations thereof were used as catalysts to improve the sensor response to carbon monoxide and to suppress the cross-sensitivity toward humidity. The focus of this work is the detection of toxic carbon monoxide and a specific hydrocarbon mixture (HCmix) in a concentration range of 5–50 ppm at different temperatures and humidity levels. The use of CMOS chips ensures low-power, integrated sensors, ready to apply in cell phones, watches, etc., for air quality-monitoring purposes.

Gas sensors based on metal oxide semiconductors dominates part of the gas sensor market due to their relative high sensitivity towards various gas molecules, inexpensive production costs, and miniaturization compatibility [1]. However, to meet the requirements imposed in multiple sectors, many key figure of merits such as selectivity, limit of detection, power consumption, and baseline stability needs to be urgently improved [2]. Manipulating the sensing material at the nanoscale is a common adopted approach to outperform the gas sensing capabilities comparable to their bulk counterparts [3]. In this context, the generation of nanoparticle by spark ablation connected to a programable dry printing system based on inertial impaction aroused as an excellent route to nanomanufacturing gas sensing layers [4]. Spark ablation is a gas-phase nanoparticle (NP) synthesis method that takes place at ambient pressure when pulsed electric discharges are induced between two bulk electrodes. The energy liberated during the discharge locally heat the electrodes [5] ablating the electrode material, leading to the formation of a vapor cloud that is rapidly cool down due to the extinction of the spark and a flowing carrier gas. In this quenching process the vaporized material condensate into atomic clusters that evolve into NPs nucleation that finally forms larger agglomerates (see Figure 1.a). This is a scalable dry gas method to produce NPs with a well-defined size and composition, it is considerable a simple and versatile in view of the multiplicity of materials that can be formed, and environmental friendly since it does not require chemical precursors or binders [5]. The produced NPs can be collected and deposited using inertial impaction, in this case, the aerosol containing the NPs can be accelerated through a nozzle by means of pressure difference between the NP generator reactor and chamber where the target substrate is placed. The production of nanoporous layers (NPLs) or disperse NPs can be controlled depending on the sparking/printing conditions. Herein, ZnO based NPLs were printing on a Si-based platform chip [7] produced with different Ar flows during the spark ablation process. The dry printed NPLs were used as a gas sensing layers towards the detection of a gas mixture of hydrocarbons, named Hcmix (including, acetylene, ethane, ethene, and propene). Figure 1.b shows the NPLs resistance variation upon exposure of the Hcmix. As a result, the NPLs formed using a higher carrier flow leads to a faster response/recovery times but a higher drift baseline was detected. The difference in the gas response are attributed to different porosity and NPs sizes forming the NPLs. Higher carrier flow conducts to a faster quenching process leading to the formation of relative smaller NPs that can affect the agglomerate formation and subsequent NPL deposition. In this work, the impact of the carrier flow on the NPLs formation is discussed and their influence on the gas sensing performance. This works aims to demonstrate the high potential of the nano-printing technology in terms of flexibility for the nanomaterial synthesis to optimize chemical sensors. Figure 1. (a) Schematized of the spark ablation process and the evolution of the NPs formation. (b) Resistance variation upon exposure of Hcmix gas molecules of two NPLs based on ZnO printed by inertial impaction. During the spark ablation process for each NPL, the NPs were generated using different carrier flows (1 l/min and 5 l/min. References: [1] A. Staerzet et al , “Current state of knowledge on the metal oxide based gas sensing mechanism,” Sensors and Actuators B: Chemical, vol. 358, p. 131531, 2022. [2] K. Sivaperuman et al, “Binary and ternary metal oxide semiconductor thin films for effective gas sensing applications: A comprehensive review and future prospects,” Progress in Materials Science, vol. 142, p. 101222, 2024. [3] P. K. Panigrahi, et al “Recent Advances in Nanostructured Materials for Application as Gas Sensors,” ACS Omega, vol. 9, p. 3092–3122, 2024. [4] L. N. Sacco et al, “Multiarray Gas Sensors Based on Nanoporous Layers Produced à la carte by Spark Ablation Using Metal Oxides, Binary and Ternary Alloys,” Kobe, 2024. [5] R. Raymond and M. Akram, “Temporal investigation of a fast spark discharge in chemically inert gases,” Journal of Physics D: Applied Physics, vol. 30, p. 1125, 1997. [6] T. Pfeiffer et al “New developments in spark production of nanoparticles,” Advanced Powder Technology, vol. 25, no. 1, pp. 56-70, 2014. [7] L. Egger et al “Development of a Screening Platform for Optimizing Chemical Nanosensor Materials,” Sensors, vol. 24, p. 5565, 2024. Figure 1

We herein demonstrate the pulsed-mode temperature operation of chemical sensor devices based on thin SnO2 films, which were synthesized by magnetron sputtering. The gas-sensitive films were integrated on SiN-based micro-hotplate (µhp) chips, which enable operation temperatures up to 500 °C. We compared the gas sensor performance in constant temperature mode with pulsed temperature mode operation towards the test gases carbon monoxide and toluene. In contrast to constant temperature, the pulsed temperature mode operation reveals additional information about the type of test gas.

We demonstrate the systematic optimization of SnO2-based thin-film chemical sensors by using mono-, bi- and tri metallic nanoparticles (NPs) composed of Ag, Pd, and Ru, which are deposited via magnetron sputtering inert gas condensation. The ultrathin SnO2 films are integrated on CMOS-based micro-hotplate devices, where each chip contains 16 sensor devices in total. We found that the response of the sensor device can be significantly tuned to specific target gases, such as CO and VOCs, by using various types of NPs.

Chemiresistive gas sensing layers were produced by the generation of nanomaterials by spark ablation technique and subsequently deposition by inertial impaction. Spark ablation is a dry-nanomaterial fabrication approach that allows to mix multiple materials in an aerosol, and deposit them as nanoporous layers (NPL) on sensor structures. This highly flexible technology was employed to realize gas sensor multiarray based on pure metal oxides (MOx) layers (SnO2 and NiO), binary alloys (SnO2-Ag, NiO-Ag, and SnO2-NiO), and a ternary alloy (SnO2-NiO-Ag). The multi sensor array was used to detect the toxic gas formaldehyde in a concentration range from 0.1 ppm up to 10 ppm. The unique versatility of the spark ablation technology to deposit different sensing layers on multiarray sensor configurations, enables a very efficient engineering of the NPL. Key figures of merits like responsivity and dynamic range can be controlled by the NPL composition. For instance, the addition of the Ag catalyst to the pure MOx drastically increased the response to formaldehyde for 0.1 ppm concentration, for higher concentration of 10 ppm the response saturates. The ternary alloy has a lower response in the whole concentration range but exhibits no response saturation. While the present work is focused on the response and detectable range towards formaldehyde molecules, the spark ablation technique can be extended to a variety of other materials and target molecules. Therefore, this work is setting a highly promising basis for development of the next-generation chemiresistive sensor devices based on nanostructured materials.

Buildings worldwide are becoming more thermally insulated, and air circulation is being reduced to a minimum. As a result, measuring indoor air quality is important to prevent harmful concentrations of various gases that can lead to safety risks and health problems. To measure such gases, it is necessary to produce low-cost and low-power-consuming sensors. Researchers have been focusing on semiconducting metal oxide (SMOx) gas sensors that can be combined with intelligent technologies such as smart homes, smart phones or smart watches to enable gas sensing anywhere and at any time. As a type of SMOx, p-type gas sensors are promising candidates and have attracted more interest in recent years due to their excellent electrical properties and stability. This review paper gives a short overview of the main development of sensors based on copper oxides and their composites, highlighting their potential for detecting CO2 and the factors influencing their performance.

Chemical sensors based on metal oxides (MOx) are one of the most promising gas sensing devices due to their high sensitivity to numerous gases, fast response, miniaturization, and simple production. The detection principle of these sensors is a conductivity change of the MOx-sensing material due to the chemical reactions of gases with surface molecules. Cross sensitivities and interference to humidity, however, are still significant drawbacks of these sensors. The functionalization of MOx-sensing films with catalytic nanoparticles (NP) is a highly promising technology for optimizing sensor performance. The huge variety of potential MOx–NP combinations requires efficient screening technologies to find proper hybrid material mixtures which enable the controlled adjustment of the sensor response to specific target gases. This is of high importance for the realization of a multi-gas sensor device capable of the clear discrimination of single gas components from a gas mixture. In this work we introduce our approach for the efficient screening of hybrid MOx–NP material combinations. We have developed a specific Si-platform chip along with a gas measurement setup which enables the simultaneous characterization of 16 chemical sensor structures in parallel. The Si-chips feature an array of Ti/Pt electrodes for contacting ultrathin MOx-sensing films, which are deposited by spray pyrolysis, and structured by photolithography to a size of 50 × 100 µm2. On these platform chips we tested three different MOx (SnO2, ZnO, and CuO) before and after functionalization with mono- and bimetallic NPs (such as Au, Pt, Pd, and NiPt) on several test gases (CO, HCmix, toluene, CO2). Measurements were performed in a background gas of synthetic air at different relative humidity levels (25–75%) and at different operating temperatures up to 350 °C. We present the sensing performance results of various MOx-NP combinations, exhibiting an optimized response to specific target gases.

Combating the health effects of particulate matter (PM) pollution requires affordable and reliable real-time air quality monitoring. The potential for large-scale manufacturing of acoustic-wave-based sensors makes them an interesting option for low-cost, low-power particle sensing applications. This article demonstrates a solidly mounted resonator (SMR) PM sensor with improved sensitivity through thermal modulation of the device. A novel, complementary metal oxide semiconductor (CMOS)-compatible SMR with an integrated microheater was designed, manufactured, and tested. In simulations, it was found that particle deposition increases both the heat loss and the thermal time constant of SMR. The effect of this on the resonant frequency shift of the device caused by particle deposition is investigated closely in this work. The sensitivity of the devices to particle deposition was tested experimentally with and without temperature modulation by placing the device in a test chamber and allowing the randomized settling of aerosolized particles on its surface. The unmodulated sensor demonstrated a particle mass sensitivity of ~40 Hz/ng while the mass sensitivity of the temperature-modulated device was shown to improve by a factor of nearly $\times 5$ to 190 Hz/ng. Temperature modulation also improved the detection limit from 100 to 50 ng. Further experiments were conducted by adding an impactor mechanism to have a more controlled measurement setup. To this effect, a thermophoretic particle deposition mechanism was added to the device to enhance its performance. It was demonstrated that the repeatability of measurements was significantly improved, making the device a promising low-cost technology for air quality monitoring.

Introduction SnO2 nanowires (NWs) have been synthesized and implemented on CMOS-based gas sensors devices with gold inter-digital electrode structures (IDES). For the implementation process of NWs a polydimethylsiloxane-based stamping transfer was employed with help of a Condor Sigma bond tester. The multi-nanowire gas sensor has demonstrated sensitivity for toluene concentrations as low as 100 ppb. The transfer method applied in this work is one step further from previous manual transfer [1] to the industrial adaptation of the process. Materials and substrates A two-step synthesis was applied to prepare SnO2 nanowires, as described elsewhere [2]. The gas sensing substrate was a commercial chemical sensor platform provided by ams AG, Austria - a 2x2 mm micro-hotplate chip with a Pt-based microheater and Pt-IDES with an electrode width and the distance between the electrode fingers both 5 µm. SnO2 NWs were transferred on the IDES-structure with a PDMS stamp, prepared by 1:10 standard recipe and cut into 3x3mm pieces. Transfer of SnO2 NWs A Condor Sigma bond tester tool was employed to transfer SnO2 NWs. The PDMS-stamp was glued on a specific tool of the bond tester, which is usually employed for testing wire bond connections. The PDMS stamp was pressed with well controlled parameters (force, distance and position) onto the substrate with MOx NWs in order to collect the NWs with the stamp. Afterwards the NWs were transferred onto the gas sensing substrate with maximal force of 0.7N and 10s time of contact between the stamp and the micro-hotplate substrate, which resulted in NW transfer and interconnection to the electrodes. Gas measurement Gas measurements were performed by an automatized setup, with synthetic air (80% N2, 20% O2) as a background gas and a constant flow rate of 1000 sccm. Three different humidity levels of 25, 50 and 75% were investigated in the presence of 5 different toluene concentrations – 0.05, 0.1, 0.5, 1 and 5 ppm. The temperature of the microhotplate was kept constant at 300°C. Response was calculated as follows: R = (Rair -Rgas)/Rair *100% where Rair is a resistance of the sensor in synthetic air before the gas pulse and Rgas is a resistance of the sensor in the presence of a test gas at the end of a gas pulse. Two independent sensors were measured, prepared by the same technique. Results and Conclusions The Condor Sigma bond tester enabled precise manipulation of the PDMS NW transfer stamp not only in all 3 axis but also by using force sensor. Due to the high precision of the machine the transfer process could be performed with high precision and control without damage to the micro-hotplate. This demonstrates that such a NW transfer process is suited for realization of a NW-based chemical sensor devices. The response towards toluene is shown in Fig.1. The multi-nanowire based devices are able to sense a gas concentration as low as 100 ppb (where the threshold limit value for Switzerland is 50 ppm [3]). There is also visible dependence towards humidity – but it is not as high as for toluene – i.e. for 5 ppm of toluene the mean response for both sensors is 9%, 8% and 8% for 25, 50 and 75% rH levels, respectively. Presently the work is focused on the NWs transfer on CMOS-based microhotplate array chips. Acknowledgements This work was partly performed within the project “FunkyNano – Optimized Functionalization of Nanosensors for Gas Detection by Screening of Hybrid Nanoparticles” funded by the FFG - Austrian Research Promotion Agency (Project No. 858637). References [1] Sosada-Ludwikowska, R. Wimmer-Teubenbacher, M. Sagmeister, A. Köck, Transfer Printing Technology as a Straightforward Method to Fabricate Chemical Sensors Based on Tin Dioxide Nanowires, Sensors (2019), 19, 3049; doi: 10.3390/s19143049 [2] Köck, A. Tischner, T. Maier, M. Kast, C. Edtmaier, C. Gspan, G. Kothleitner, Atmospheric pressure fabrication of SnO2-nanowires for highly sensitive CO and CH4 detection, Sensors and Actuators B: Chemical (2009), 138 no. 1, pp. 160–167 (2009); doi: 10.1016/j.snb.2009.02.055 [3] Schweizerische Unfallversicherungsanstalt (SUVA): Grenzwerte – Aktuelle MAK- und BAT-Werte für Toluol. Available online: https://www.suva.ch/de-CH/material/Richtlinien-Gesetzestexte/grenzwerte-am-arbeitsplatz-aktuelle-werte (accessed on 29.11.2019) Figure 1

The current technological trends associated with Industry 4.0 and the Internet of Things (IoT) require an interconnected network of sensor nodes providing distributed information on the environment to enable intelligent action to be taken by control systems. Such sensors need to be wireless, self-powered and energy independent. In this work we provide an overview of possible strategies to realize a positive energy balance in autonomous sensor nodes without the use of batteries. We will first overview different sensors in terms of power consumption. We will then concentrate on energy harvesting and storage, showing state-of-the-art possibilities in both cases.

The current technological trends associated with Industry 4.0 and the Internet of Things (IoT) require an interconnected network of sensor nodes providing distributed information on the environment in order to enable intelligent action to be taken by control systems. Typical examples are the condition monitoring of machines or industrial equipment, or the detection of hazardous environmental conditions (e.g., in chemical plants). Such sensors need to be distributed in areas that are difficult to reach for wiring or to exchange batteries, and thus need to be self-powered and energy-independent. In this work, we provide an overview of possible strategies to realise a positive energy balance in autonomous sensor nodes without the use of batteries, focussing on gas sensors for air-quality monitoring as a use case. We will first present ways to reduce the power budget of sensing elements using self-heating nanowires made of CMOS-compatible metal oxides. We will then concentrate on energy harvesting and storage, showing state-of-the-art possibilities in both cases: broadband piezoelectric harvesters, perovskite-based photovoltaic elements, and high-energy density ceramic capacitors. Finally, we will discuss the possibility of integrating all sensor node elements in a single device using advanced interconnect technologies.

Metal oxide (MOx) gas sensor signals are mainly governed by adsorption and desorption processes of oxygen and its reaction with surrounding gas molecules. Different target gases exhibit different reaction rates leading to characteristic sensor responses for specific gas species and their concentrations. In this work, we compare temperature-modulated sensor operation (TMO) with sensor operation at a single temperature. Further, we explore if under specific TMO regimes, a simple signal processing allows for quantification of gas concentrations. We specifically investigate, if the relevant information can be captured in selected discrete wavelet coefficients. In addition, we compare the results received from this wavelet features to reaction rate evaluation features.

The response of single SnO2 nanowire gas sensors with different diameters between 20 and 140 nm are evaluated by calculating the nanowire conductivity as a function of the surface charge density. The procedure involves the numerical solution of the Poisson-Boltzmann equation for the electrostatic potential in cylindrical geometry in order to model the depletion region and band bending at the SnO2 nanowire surface. In the model we take into account varying surface charge densities sigma and bulk electron concentrations n(0) to calculate the electrical conductivity. Considering the fact that the surface charge density depends on the nanowire surface interactions with ambient gas, the model allows us to simulate the sensor response when the nanowire is employed as gas sensing component. We report a saturation in depletion length lambda versus surface charge density s which is the principal reason for limiting the sensor responses. The results also show that the conductivity is decreasing by increasing surface charge density, the smaller the nanowire diameter the steeper the decrease. As a result the nanowire response is proportional to 1/d where d is the nanowire diameter. Furthermore, we argue about the validity of the modeling results and their relevance to experimental findings on SnO2 nanowire based gas sensors reported in literature.

Cupric oxide (CuO) nanowires were produced by thermal oxidation of copper surfaces at temperatures up to 450 °C. Three different surfaces, namely a copper foil as well as evaporation deposited copper and an application relevant sputtered copper film on Si(100) substrates were characterized ex-situ before and after the experiment. The development of oxide layers and nanowires were monitored in-situ using grazing incidence small angle X-ray scattering. The number density of nanowires is highest for the sputtered surface and lowest for the surface prepared by evaporation deposition. This can be linked to different oxide grain sizes and copper grain boundary diffusions on the different surfaces. Small grains of the copper substrate and high surface roughness thereby lead to promoted growth of the nanowires.

A worldwide unique CMOS based chemical sensor device comprising an array of 8 microhotplates (µhps) for a total of 16 chemical sensors has been fabricated (Figure 1). [...]

Chemical gas sensors are operated at elevated temperatures and the actual temperature has a tremendous influence on sensitivity and selectivity. From that perspective, precise temperature control over the chip is an absolute requirement. Next to a stable heating system, a controlled gas flow in the test box is required. The test gases should not cool down the sensor surface too much and not be heated up by the heater. To make the material integration easy and reduce the costs for sensor platforms, often rather large sensor devices are fabricated. We demonstrate that a combined approach of thermal analysis and computational fluid dynamics enables the co-design of gas flow path and heater to archive precise temperature conditions at the sensor material and in the surrounding test gas atmosphere.

Integration of metal oxide nanowires in metal oxide gas sensors enables a new generation of gas sensor devices, with increased sensitivity and selectivity. For reproducible and stable performance of next generation sensors, the electric properties of integrated nanowires have to be well understood, since the detection principle of metal oxide gas sensors is based on the change in electrical conductivity during gas exposure. We study two different types of nanowires that show promising properties for gas sensor applications with a Scanning Probe Microscope—Scanning Electron Microscope combination. Electron Beam Induced Current and Kelvin Probe Force Microscopy measurements with a lateral resolution in the nanometer regime are performed. Our work offers new insights into the dependence of the nanowire work function on its composition and size, and into the local interaction between electron beam and semiconductor nanowires.