In Situ Monitoring of the Deposition of Flame-Made Chemoresistive Gas-Sensing Films

Abstract

Flame-deposited semiconducting nanomaterials on microelectronic circuitry exhibit exceptional performance as chemoresistive gas sensors. Current manufacturing technology, however, does not monitor in situ the formation of such nanostructured films, even though this can facilitate the controlled and economic synthesis of these sensors. Here, the resistance of such growing films is measured in situ during fabrication to monitor the creation of a semiconducting nanoparticle network for gas sensors. Upon formation of that network, the film resistance drops drastically to an asymptotic value that depends largely on the film structure or morphology rather than on its thickness and size of nanoparticle building blocks. Precursor solutions of various concentrations enable the flame deposition of Sb-doped SnO sensing films of different morphologies, each of which exhibit a characteristic in situ resistance pattern. Low precursor concentrations (1 mM) lead to thin (ca. 0.16 μm) films with slender columnar structures of increasing diameter (up to 25 nm) after prolonged deposition (up to 6 min) and show an oscillating in situ resistance during their fabrication. On the other extreme, high precursor concentrations (100 mM) lead to thick (up to 80 μm) dendritic and porous films consisting of nanoparticles with relatively small primary particle diameter (around 7 nm) that remain invariant of deposition duration, which is in agreement with the stable in situ resistance. Such dendritic films exhibit a sensor recovery time that is an order of magnitude longer than that of those made at lower concentrations. The above understanding enables the rapid and economic flame synthesis of thin gas sensors consisting of minimal semiconducting nanomaterial mass possessing a tuned baseline resistance and exhibiting excellent response to ethanol vapor.

Rapid and Selective NH3 Sensing by Porous CuBr

“…Figure 1b shows the film resistance (solid line) and temperature (dashed line) during this dry conversion. At room temperature, the resistance of the CuO film is ≈90 MΩ indicating an interconnected network between the substrate‐mounted electrodes,19 in agreement with SEM (Figure 2d). This drops to 0.2 MΩ when heating the film to 150 °C for 1 h in N 2 (dashed line) due to thermal activation of intrinsic charge carriers23 in the semiconductive CuO.…”

“…The higher porosity can facilitate faster diffusion of NH 3 through the sensing film while the smaller crystal size can lead to shorter solid‐state diffusion length when forming upon interaction of NH 3 with CuBr 16. To accurately determine the reason for the better performance of dry‐converted sensors, however, a systematic investigation19 of the impact of film characteristics on sensor performance through its process synthesis variables is needed.…”

“…Such films are obtained by flame‐aerosol deposition18 of CuO nanoparticles onto interdigitated electrodes ( Figure a). Then, these films are reduced and brominated by a dry process monitored in situ19 through their resistance (Figure 1b). They are tested as room temperature sensors for NH 3 down to 5 ppb at realistic 90% RH.…”

Guiding Ketogenic Diet with Breath Acetone Sensors

“…Specifically, their sensing structure can be grown [ 20 ] or deposited (e.g., by thermophoresis from flame aerosols [ 21 ] or doctor-blading [ 22 ]) directly on micro-machined chips resulting in compact sensors for ready integration into hand-held devices [ 23 ]. Furthermore, their film morphology and deposited mass can be optimized during fabrication by in-situ resistance read-out [ 24 ]. When nanostructured, such sensors exhibit high sensitivity to detect even low ppb analyte concentrations [ 21 ].…”

Gas sensors using ordered macroporous oxide nanostructures

“…If combined with in situ resistance measurements during deposition, the formation of highly porous networks can be monitored with morphologydependent resistance patterns to systematically design metaloxide-based gas sensors with high performances. 69 Besides such 2DOM nanostructures, 3D ordered macroporous (3DOM) metal oxides have also been similarly synthesized using 3D CC templates. [70][71][72] When compared with a monolayer 2D counterpart, 3DOM thin lms tend to be endowed with strengthened continuous connection channels, as well as improved avenues for conducting electrical signals.…”