A nanolithography method based on the use of a Dual Beam focused-ion-beam (FIB) equipment to perform electrical contacts on either individual metal oxide nanowires or three-dimensional (3-D) nanocrystal networks is reported. Both advantages and disadvantages of using this nanolithography process compared with other more conventional techniques are discussed. The possibility of using these FIB bottom-up devices in gas sensing application is also presented showing the performances of the gas sensors based on single nanowires and 3D nanocrystal networks of metal oxides. For it, two- and four-probe electrical measurements have been used determining the features associated to the contact resistances. To eliminate any influence of the contact values, AC Impedance Spectroscopy techniques have been adapted on individual nanowires facilitating the analysis of the gas sensing mechanisms in single metal oxide nanocrystal.

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A new kind of polystyrene and surfactant, didodecyldimethylammonium, co-intercalated MoO₃ nano hybrids was prepared by chemical vapor deposition combined with wet process of simple immersion in an organic solution. The hybrid thin films have a characteristic structure of polystyrene and surfactant co-intercalation into MoO₃ with b-axis orientation on the LaAlO₃ (100) single crystal substrate. The hybrid thin films show a distinct response to acetaldehyde vapor by decreasing the electrical resistance. We discuss the possible sensing mechanism for acetaldehyde gas.

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During the recent years, it has been widely recognized that the use of semiconductor nanoparticles for the fabrication of chemical gas sensors by screen-printing technology definitely improves the sensor performance. In this paper, we review the possibilities offered by Fourier transform infrared (FTIR) spectroscopy for the study of semiconductor nanoparticles. Thanks to FTIR spectroscopy, it is possible to identify the surface chemical groups, to characterize the surface reactivity, to monitor the surface functionalization, to investigate the surface reactions at the origin of the gas detection, and to evaluate the gas sensing potentiality of the nanoparticles before the device fabrication. All these steps are critical for the optimization of nanoparticle-based gas sensors because they ensure i) the reproducibility of the surface chemical composition and of the surface chemistry, ii) the control of the surface modifications to decrease cross-sensitivity, particularly to humidity, iii) the investigation of the gas detection mechanism to properly tailor the surface structure, iv) the selection of the best nanoparticles batches for further processing. Examples of tin oxide and titanium oxide nanoparticles are discussed with regards to CO and NO_x detection.

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Nanoscience and nanotechnology involve materials and systems with at least one dimension in the range of 1-100nm that exhibit novel and size-dependent properties. This article reviews the main processing techniques used to fabricate nanomaterials, ranging from sol-gel to vapor deposition to electrospinning. Emphasis is given to metal oxide nanostructures, such as SnO₂ nanoribbons and MoO₃ nanowires. Their electronic properties and their gas detection behavior are presented and discussed. The high surface-to-volume ratio of metal oxide nanostructures has been exploited to develop gas sensors with higher stability, faster response, and higher sensitivity to the analytes of interest than their conventional counterparts. Commonly observed p-n type transition phenomena in nanostructured metal oxides are explained on the basis of the sensing mechanism involved in gas detection by semiconducting oxides and the size-related effects of nanomaterials in the concentration of free charge carriers. Insights are given on the future challenges in resistive gas sensing and how nanotechnology may be helpful in overcoming these burdens

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In this paper, we investigate the properties of platinum heater micro-hotplates built on a thin dielectric membrane and used for the catalytic combustion of hydrogen in a micro-thermoelectric hydrogen gas sensor. The response of the micro-hotplates was studied with different working temperatures and a response time below 95 ms was determined at a working temeprature of 350°C. It's shown that the fabricated micro-hotplates can bear temperature as high as 600°C. Temperature measurements were carried out with an infrared camera and show an extremely sharp response of the catalyst to the temperature raise with a response time of less than 1s. The response of the hydrogen gas sensor fabricated with these micro-hotplates is reported at different temperature and for 1 v/v % hydrogen in air. The performance of the gas sensor is also examined with and without a micro-hotplate heat spreader located below the membrane. For a 70-μm-thick heat spreader, the heat dispersion from the membrane is enhanced. However, the hydrogen response of the gas sensor with the heat spreader is reduced 1/3 of the sensor without the heat spreader though the temperature distribution is more uniform below the catalyst.

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The effect of gap size on the gas sensitivity of semiconductor gas sensor was evaluated in the NO₂ sensing using WO₃ nanosensor, the Cl₂ sensing using In₂O₃ nanosensor and the H₂S sensing using SnO₂ nanosensor. The nano-gap effect was markedly observed in the NO₂-WO₃ system and the Cl₂-In₂O₃ system (resistance increase), while the H₂S-SnO₂ system showed the weak nano-gap effect. This difference resulted from the ratio (S_i/S_gb) of sensitivity at semiconductor oxide-electrode interface (S_i) to at grain boundary (S_gb). The NO₂-WO₃ and the Cl₂-In₂O₃ systems showed the large S_i/S_gb ratio (32-43), while the small ratio (9.7) was obtained in the H₂S-SnO₂ system at the gas concentration of 0.5-1 ppm. It was found that the clearer nano-gap effect was obtained for the system having the larger S_i/S_gb ratio. In the system having large S_i/S_gb ratio, the nano-design of electrode structure like nano-gap electrode was important for high sensitivity gas sensors.

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