Fabrication of gas sensing devices & Synthesis of innovative sensing materials

Our research efforts are directioned towards the synthesis of innovative sensing materials as well as towards the development of reliable, cutting-edge gas sensing devices based on thin films. Such a fabrication approach favorizes both the miniaturization of the device as well as its cost of production. Furthermore, these devices are also amenable to exhibit low-power demands, hence their easy integration into other electronic circuitry and more complex systems such as wearables or consumer electronics.

We have successfully designed and fabricated miniaturized conductometric as well as TFT gas sensing devices using the pulsed laser deposition (PLD) technique in conjunction with specific shadow masks. While the masks for the TFTs were purchased from Ossila®, those for the chemiresistors were fabricated locally, at CETAL using the TruLaser Cell 3010 robotic equipment.

**LEFT**: A structure composed of five transparent TFTs based on metallic oxides. Each of the transistors features a bottom-top configuration, with an IZO70 gate, Nb2O5 insulator, IZO70 source and drain and a GIZO channel. The characteristic response of an individual TFT for 9 different voltages applied on the gate (1 to 9V) and for a drain-source voltage ranging between -20V and 20V is also shown.

**RIGHT**: A SnO2:ZnO thin film conductometric gas sensor fabricated on an oxidized Si substrate, with platinum electrodes deposited on top.

We fabricate our sensing devices using metal oxides and low-cost substrates such as glass or oxidized silicon. Semiconductor metallic oxides are globally among the most researched and most promissing materials for gas sensors, transparent and flexible electronics.

LEFT: A structure of five miniature conductometric gas sensors fabricated on oxidized silicon. RIGHT: A SEM image of the central area of one of the sensors in the structure. The film of metal oxides has an area of 4sq.mm, while the space between the Pt electrodes is around 40microns.

In the context of a global transparent electronics market estimated to reach 5.1 billion EUR in 2018, we are actively following the latest trends and have extended our research towards transparent TFT based gas sensing devices. TFT sensing devices exhibit important advantages over the conductometric sensors such as increased selectivity, better accuracy and precision in detection (higher sensitivity and lower detection limit due to the transistor effect), increased dynamic range and reversibility. These devices have higher signal to noise ratios than chemiresistors, longer lifetime, lower response times, diminished temporal drift and lower operating temperature range (closer to room temperature). TFT based gas sensors are mostly employed to detect reducing gases like hydrogen, CO, hydrocarbons and vapors of organic solvents.

Using our conductometric devices
we were able to accurately detect
CO concentrations as low as 1ppm.

According to the EU regulations, CO has an occupational limit value of 20ppm (TWA), hence sensing devices should address concentrations below this limit. Currently, the majority of commercial indoors CO sensors on the market are able to detect only concentrations of over 75ppm. Prerequisites for outdoor systems (pollution monitors) demand accurate measurements of concentrations below 5ppm as CO background levels for the outdoor environment normally fall within the range of 40 to 200ppb.

Growing 1D nanostructured sensitive layers

Nanostructured sensitive layers are known to boost the gas sensing performance of conductometric sensors. For this purpose we are using electrochemical anodization as a convenient and efficient way of obtaining a surface covered in metallic oxides nanotubes. Common metals such as Zn, Fe, Ni, Nb, Sn, Al and and their alloys can be anodized without any special requirements. We have successfully managed to grow nanotubular structures of TiO2 on Ti substrates as well as on Ti-6Al-4V grade 5 alloy (along with the corresponding aluminum and vanadium oxides) using the same Keithley 2450 SourceMeter SMU instrument. Alternatively, an OrigaStat OGS100 instrument (potentiostat, galvanostat and impedance meter) specially designated for electrochemistry is also available in our lab.