In this contribution, we report on CuO nanowire devices integrated on CMOS microhotplates working as miniaturized, low power consumption gas sensors. The CuO nanowires were synthesized by thermal oxidation of a microstructured Cu thin film by local heating of the CMOS microhotplates. Direct nanowire device integration could be achieved as CuO nanowires form an electrical connection between adjacent oxidized Cu microstructures. By this approach, functional gas sensors were realized which require a low power consumption of less than 20 mW in order to attain the operation temperature around 325°C. The gas sensing properties of the CuO nanowire devices were investigated during exposure to humidity pulses. Moreover, the devices were found to be suitable for detecting the toxic gas CO at concentrations down to 10ppm and thus are potential candidates for future integrated smart gas sensor devices for security applications.

The ppb level detection of the toxic gas H2S is of great importance for industrial and safety applications. We report on CuO nanowire gas sensors, which are capable to detect H2S concentrations as low as 10ppb in dry as well as humid atmosphere. In particular, measurements with different humidity levels up to 65% have been performed, which is of high practical relevance regarding H2S detection in ambient atmosphere. Three different types of conductometric gas sensors have been investigated: a single CuO nanowire configuration and two different multiple CuO nanowire configurations that have been developed in order to optimize sensor performance and to enable CMOS integration of CuO nanowire gas sensors in the future. All sensor devices employ suspended CuO nanowires as gas sensitive components. This is a highly favorable configuration because the nanowires are entirely surrounded by the gas atmosphere. The devices based on multiple CuO nanowires show enhanced H2S response in humid air compared to dry synthetic air. Furthermore, we have found that the sensor design, which employs CuO nanowires with the smallest average diameters around 20nm, has the highest gas response. The estimated detection limit approaches the ppt range, which shows the excellent sensor performance even in humid atmosphere. These results and the CMOS backend compatibility of the optimized CuO nanowire sensor design are of high importance for the realization of low power silicon integrated gas sensors for daily life applications.

We report on novel gas sensing devices based on cupric oxide (CuO) nanowires which are synthesized on-chip by thermal oxidation of electroplated copper microstructures. This technique enables the direct integration of a multitude of CuO nanowires, which bridge the electrical contacts of a conductometric gas sensor. The CuO nanowire bridges exhibit a huge surface-to-volume ratio and are entirely surrounded by the gas atmosphere, which is a highly favorable gas sensor configuration. As a result, the CuO nanowire gas sensor devices are able to detect carbon monoxide (CO) down to a concentration of 10ppm and exhibit extraordinary sensitivity to hydrogen sulfide (H2S) where concentrations down to 10ppb have been detected, even in the presence of humidity. For characterization of the CuO nanowires, X-ray diffraction measurements, transmission electron microscopy and electron energy loss spectroscopy are employed. As no process temperatures higher than 400°C are required for the fabrication of the CuO nanowire devices, our approach can be employed in a CMOS backend process enabling the realization of a fully silicon integrated CuO nanowire gas sensing device.

In this paper we report on conductometric gas sensors based on suspended CuO nanowire arrays for CO detection in humid atmosphere. CuO nanowires with very small diameters (10nm-30nm) are synthesized by a thermal oxidation process of Cu microstructures. Our approach allows efficient CuO nanowire integration using a CMOS backend compatible technology. We perform CO measurements in a concentration range between 25ppm and 150ppm and investigate humidity interference effects by comparing CO measurements in dry and humid atmosphere.

In order to optimize the performance of gas sensor devices, nanocrystalline SnO2 thin film and single crystalline SnO2 nanowire sensors have been characterized for the target gases CO, CH4, H2, CO2, SO2 and H2S. At optimum operating temperature – varying from 250°C to 400°C – the SnO2 thin film sensor detects CO, CH4, CO2 and SO2 with responses in the range of 23–34%, H2 and H2S with responses above 80%. The SnO2 nanowire sensor shows responses in the range of 1–8% for CO, H2 and SO2, 29% for H2S, while CH4 and CO2 are not detected. Taking into account that the exposed surface area of the thin film sensor is 800 times larger than that of the single nanowire, we have correlated the number of CO gas molecules impinging the sensors’ surface with the number of electrons, which are actually detected as sensors’ response for the target gas CO. In case of the thin film sensor a single detected electron requires ∼2760 gas molecules impinging the sensor's surface. For the nanowire sensor only ∼86 gas molecules are required for a single detected electron. The SnO2 nanowire sensor thus has a detection efficiency more than 30 times higher than the SnO2 thin film sensor, which we attribute to a lack of grain boundaries. From our measurements we conclude that single crystalline SnO2 nanowire sensors provide a higher sensitivity and an improved cross-sensitivity than their nanocrystalline counterpart.

The employment of nanowires is a very powerful strategy to improve gas sensor performance. We demonstrate a gas sensor device, which is based on silicon chip-to-chip synthesis of ultralong tin oxide (SnO2) nanowires. The sensor device employs an interconnected SnO2 nanowire network configuration, which exhibits a huge surface-to-volume ratio and provides full access of the target gas to the nanowires. The chip-to-chip SnO2 nanowire device is able to detect a H-2 concentration of only 20 ppm in synthetic air with similar to 60% relative humidity at room temperature. At an operating temperature of 300 degrees C a concentration of 50 ppm H-2 results in a sensitivity of 5%. At this elevated temperature the sensor shows a linear response in a concentration range between 10 ppm and 100 ppm H-2. The SnO2-nanowire fabrication procedure based on spray pyrolysis and subsequent annealing is performed at atmospheric pressure, requires no vacuum and allows upscale of the substrate to a wafer size. 3D-integration with CMOS chips is proposed as viable way for practical realization of smart nanowire based gas sensor devices for the consumer market.

We present a novel method for integrating single CuO nanowires as gas sensing components in a suspended configuration. In these devices, the nanowire surface area that is actually exposed to the surrounding gas atmosphere is increased, which is favourable when operating the nanowire as conductometric gas sensor. CuO nanowires bridging two adjacent Cu structures were contacted using a thermal oxidation process leading to growth of copper oxide around the nanowire. By this method, linear IV characteristics could be achieved showing the Ohmic contact properties. The sensitivity of the suspended CuO nanowire devices towards humidity and CO was investigated. High signal changes up to 60% were measured in the presence of humidity while small concentrations of CO down to 1ppm could be detected validating the excellent sensing performance of the presented devices.

Cupric oxide (CuO) nanowires were synthesized by thermal oxidation of resistively heated copper wires in ambient air conditions. Aspect ratios up to 1000 have been achieved with this easy growth method that requires only standard laboratory equipment. The as-synthesized nanowires were subsequently transferred to silicon substrates and contacted by a structured metallization layer. These devices were used for conductometric gas sensing as nanowire conductivity is strongly dependent on the surrounding gas atmosphere. The sensitivity of a single CuO nanowire towards water vapour and hydrogen sulfide (H2S) was investigated regarding a possible application in hydrogen fuel cells.

We present nanosensors based on ultrathin SnO2 films, which are very sensitive to the highly toxic gases SO2 and H2S. The SnO2-sensing films are fabricated by a spray pyrolysis process on Si substrates with a thickness of 50 nm. The sensor resistance is decreased in the presence of the toxic gases. Exposure to 50 ppm SO2 leads to a sensor resistance drop of ∼40% whereas a H2S gas concentration of only 2.5 ppm decreases the resistance by ∼85%, which demonstrates the extraordinary sensitivity of the nanosensors. With respect to further system integration a CMOS technology based micro-hotplate containing heating element and sensing layer has been simulated. Preliminary results show that the micro-hotplates can provide operating temperatures of 400 °C with a power consumption of less than 5 mW. A concept for 3D system integration of the nanosensor chip and a CMOS chip based on Through-Silicon-Via (TSV) technology is proposed as potential roadmap towards smart nanosensor systems for daily life applications.

We demonstrate a novel gas sensor device, which is based on silicon chip-to-chip synthesis of ultralong tin oxide (SnO2) nanowires. The sensor device employs an interconnected nanowire network configuration, which exhibits a huge surface-to-volume ratio and provides full access of the target gas to the nanowires. The chip-to-chip SnO2 nanowire device has an extraordinary sensitivity to the toxic gas H2S. A concentration of only 1.4 ppm decreases the resistance of the sensor by ~ 85%, which demonstrates a detection limit far in the ppb range. The SnO2-nanowire fabrication procedure based on spray pyrolysis and subsequent annealing is performed at atmospheric pressure, requires no vacuum and allows upscale of the substrate to a wafer size. 3D-integration with CMOS chips is proposed as viable way for practical realization of smart nanowire based gas sensor devices for the consumer market.

We have realized gas sensor devices, which are based on a single SnO2-nanowire or a multiple SnO2-nanowire network as gas sensing components and are very sensitive to the toxic gas H2S. The nanowires are fabricated in a two-step atmospheric pressure synthesis process directly on the Si-chip by spray pyrolysis and subsequent annealing. Exposure of the single SnO2-nanowire sensor H2S with a concentration of only 1.4 ppm decreases the resistance by ~ 30%, while the multiple SnO2-nanowire network sensor exhibits a resistance decrease by ~ 90%. The nanowire sensors have extraordinary sensitivity with resolution limit in the ppb range and are able to measure concentrations well below the threshold limit value of 10 ppm. Due to their high performance the nanowire based sensors are basically suited for the realization of smart gas sensing devices for personal safety issues as well as industrial applications.

In this paper supersensitive gas sensors incorporating single SnO2-nanowires integrated on Si for the detection of gases relevant to environmental and safety issues are presented. The SnO2-nanowires are produced by a spray pyrolysis process followed by subsequent tempering, which results in growth of single-crystalline SnO2-nanowires with diameters of 30 - 250 nm and lengths up to several 100 mum. TEM-analysis reveal the single crystalline character of SnO2-nanowires with two preferred growth directions. The nanowires are removed from their substrate and transferred to another Si-substrate coated with 750 nm SiO2. Evaporation of metal contact pads on both ends of single SnO2-nanowires enable their direct use as sensing elements. The devices are very sensitive and are able to detect concentrations of CO and CH4 as low as a few ppm.