Design and characterization of nanocrystalline graphite/graphene based environmental sensors

Abstract

Carbon based thin films, such as graphene and graphite are promising materials for environmental sensing due to their chemical and mechanical stability, low-toxicity and selectivity towards certain environmental parameters. The scope of environmental sensing for this project will be limited to sensing of humidity and oxidizing gases for environmental monitoring applications. The most widely used methods to fabricate carbon based thin films rely on either physical transfer of CVD grown graphene films, or the deposition of graphene flakes onto substrates. However, such methods are less compatible with standard semiconductor manufacturing processes. The variations caused by the film transfer (wrinkling, defects, different flake sizes etc) could cause variance in sensing performance which is not ideal. Therefore, we proposed the use of nanocrystalline graphite/graphene (NCG) films deposited with plasma-enhanced CVD (PECVD) onto large area substrate as a potential solution. The PECVD deposited NCG has high grain boundary concentrations, which makes it ideal for sensing applications as the sensing effect is thought to lie in the grain boundaries. At the boundaries, charge transport barrier is formed between two or more conductive nanocrystals. Interactions between elements in the environment may alter the charge transport barrier significantly, thus, causing a measurable change in the conductivity of the film.

The PECVD method for NCG deposition was studied with NCG deposited at different substrate temperature to obtain the Arrhenius deposition rate. Electrical characterization and surface characterization were done on each sample to study the effect of deposition temperature on the fabrication of the film. The deposition rate between wafer scale deposition and sample scale deposition was studied, and it was found that on sample scale deposition, the deposition rate is slower than the wafer scale process due to the temperature difference introduced by a carrier wafer in the sample scale production process. The uniformity of the deposition was investigated using ellipsometry. AFM and FESEM images of the deposited films shown that the films consists of graphitic grains which are about 20-40 nm.

Here, the sensing behaviour of PECVD NCG towards different gas type is reported for the first time. The sensitivity of NCG towards 3 different gases, NO2, NH3, and CO was shown. The resistance of a strip (30 μm by 3000 μm, with thickness of 34 nm) decreases by 0.663% upon exposure to 10 ppm of NO2, increases by 0.14% upon exposure to CO and increases by 0.1% upon exposure to NH3. The desorption rate for the gas is observed to be slow. The characteristics of resistance change matches that of a p-type semiconducting gas sensor. Using Hall effect measurements, the majority carrier was found to be holes instead of electrons.

The humidity sensing performance of PECVD NCG was also demonstrated for the first time in this thesis. Upon exposure to humid air (95% relative humidity), the resistance of the sensor drops 3% (Sensor dimensions 10 μm by 20 mm meandered strip, with thickness of 200 nm). The effect of different geometrical designs on the sensing performance of the NCG humidity sensor was also investigated.

A potential application of this device is to incorporate such a sensor in a smart breath monitoring mask which monitors the wearer’s respiration patterns continuously for applications such as health monitoring during physical activities or rest. Tests were carried out to investigate the effects of bodily fluids such as human perspiration and saliva on the reliability of the sensor. The sensor shows great recovery and virtually no degradation to its sensing performance after exposure to KCl solutions.

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Identifiers

ORCID for Harold Chong: orcid.org/0000-0002-7110-5761

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