Organic materials such as organic semiconductors or conducting polymers is
particularly attractive because the wide range of material choice. The gas sensitivity in
conventional organic-based resistors or transistors, however, is usually in parts-per-
million (ppm) regime . Improving the sensitivity to parts-per-billion (ppb)
regime is hence an important issue if the gas sensor is used in medical applications. Up
to now, significant progress has been achieved to improve gas sensor performance.
Incorporating various nanostructures into gas sensor is one of the powerful strategies to produce highly sensitive sensors by providing a high surface to volume ratio.

In the previous work, we reported a sensitive ammonia gas sensor with a 50-ppb
sensitivity to detect the breath ammonia of rats . Compared with conventional
transistor-based sensor, the vertical poly (3-hexylthiophene) (P3HT) diode with a porous
top electrode greatly improved the sensitivity to ammonia from ppm regime to ppb regime. Here, the sensor in our previous work is named as porous electrode sensor. The structure is shown in Figure 3(a). Vertical sensors with nano-structure exhibit a better sensitivity because of its higher surface to volume ratio in current distribution. Similarly, a 10-ppb detection limit to nitric dioxide was realized by using vertical silicon nanowire sensor with a porous top electrode . On the other hand, an ammonia sensor based on P3HT transistor in transverse configuration was reported . However, the sensitivity is only in ppm-regime. Sensors in transverse configuration usually rely on comb-like electrodes in nanoscale to increase the surface to volume ratio. In general, to forming the comb-like electrodes in nanoscale, an expensive advanced lithography process is needed.

Figure Schematic structures of sensors, (a) the porous electrode sensor, (b) the
cylindrical nano-pore sensor, and (c) the nanowire electrode sensor.

When vertical type gas sensor appears to deliver highly sensitive response, it should
be emphasized that a gas permeable top electrode is required to allow gases interact with underlying sensing layer. Besides, the structure should be applicable to other kinds of sensing materials when developing new sensors for detecting different kinds of gases. Our previous work fabricated the porous metal on top of the organic sensing layers , hence the process easily caused unwanted damage to some fragile organic materials. For example, when using phenyl-C61-butyric acid methyl ester (PCBM) or Copper(II) phthalocyanine (CuPC) as the sensing layers, they were washed out in this fabrication process. Accordingly, as shown in Figures (b) and (c), this work demonstrated two different kinds of alternative nano-structures containing gas permeable top electrodes and allowing the formation of various organic materials: (1) the cylindrical nano-pore sensor, and (2) the nanowire electrode sensor. It is noted that cylindrical nano-pore sensor in Figure(b) . is an extension version of the porous electrode sensor in Figure . The robustness is improved by modifying the fabrication process. The structure with porous electrode was formed prior to the coating of the sensing layer. Therefore, this new structure is applicable to a wide range of sensing materials.

Among various kinds of nanostructure, nanowire structure is also particularly
attractive because nanowires can easily form a mesh structure to facilitate the current
conduction. Zhang and co-workers demonstrated indium oxide (In₂O₃) nanowire devices for the detection of nitrogen dioxide (NO₂) down to parts-per-billion (ppb) levels . Wang et al. also utilized polyaniline nanowires to form a resistive sensor with polymer
nano-framework to successfully detect 0.5 ppm ammonia . When most reported
nanowire-based sensors utilized nanowire to serve as the active sensing layer, a different concept was introduced in this work. The metallic nanowire was used as a gas permeable top electrode to cover an organic sensing layer and form a vertical diode. The electrode with nanostructure allowed gas molecules to penetrate the electrode and to interact with the charges in the underlying sensing diode. Besides, the gas permeable top electrode was formed by a one-step coating of metallic nanowires. The underlying organic sensing film did not need to have any structure and no lithography process was required. As a result, both kinds of alternative nano-structures introduced in this work is promising and useful for developing new sensors.

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