# Fluctuation-Enhanced Sensing (FES): A Promising Sensing Technique
## 6 Reducing Errors and Enhancing the Sensory Information Channel Capacity
### Photonic Excitation
The reduced thickness should result in other advantages, such as a lower price and lower energy consumption when operating at elevated temperatures. This result can be reached by spin coating technology, reducing the thickness of the sensing layer to the maximum of a few \(\upmu\)m only and ensuring more even surface and more repeatable sensing properties. Figure 7 presents results of 1/_f_ noise power spectral density generated in the thick gas sensing layer shown in Figure 6. The sensor was irradiated by UV LED, having maximal optical power at the wavelength of 394 nm. Both materials (graphene flakes, TiO\({}_{2}\) nanoparticles) are gas-sensitive and both exhibit the photocatalytic effect. The gas sensing layer has a very porous structure. A vivid change of power spectral density slope versus frequency was observed when UV light was applied. The intensified plateau of power spectral density was present about 10 Hz after irradiation. This effect can be explained by modulation of adsorption-desorption rate induced by UV light, making these processes faster and more intense. By increasing power of UV light (decreasing its wavelength), changing morphology of gas sensing layer (e.g., changing a graphene flakes/TiO\({}_{2}\) ratio of wt%), and reducing a thickness of the gas sensing layer it is possible to strengthen this effect. More visible changes of plateau can be observed at lower operating temperatures when the energy of UV light had a greater impact on adsorption-desorption process. The FES method can be applied not only in resistive gas sensing layers. The back-gated FET gas sensors, utilizing graphene in a channel, exhibit 1/_f_ spectra with a plateau at corner frequencies characteristic for selected ambient gases (e.g., chloroform, acetonitrile, methanol, ethanol) [87]. Figure 7: Power spectral density \(S_{\text{u}}(f)\) of voltage fluctuations across the gas sensor biased by DC voltage \(U\) and normalized to \(U^{2}\) versus frequency \(f\); UV LED, type OSVAYL5451B, made by OptoSupply was irradiating the sensing layer with a graphene flakes/TiO\({}_{2}\) ratio 5 of wt% (Figure 6) in the ambient atmosphere of synthetic air (S.A.) and at operating temperature \(T=120\)\({}^{\circ}\)C.  
This result was observed when a single-layer-graphene was used on the channel. Then, low-frequency noise was related to individual events of gas molecules adsorption-desorption. A corner frequency of the observed plateau was stable for different specimens of the single-layer-graphene FET sensors at an ambient atmosphere of acetonitrile [87]. This result suggests that the FES method can be very selective and repeatable when the FET channel is a two-dimensional material of the same physical structure. Another two-dimensional material, MoS\({}_{2}\), was applied successfully for gas sensing using the FES method in back-gated FET sensors [89]. These experiments, however, require further in-depth studies to confirm the result for other gases and other materials. We may suppose that in two-dimensional material the adsorbed molecules change the surface potential. Therefore, the observed low-frequency noise is modified by adsorption event more similarly for a given gas molecule than in a case of the porous gas sensing layer. In a bulk polycrystalline sensor, gas molecules modulate the potential barrier between the crystal grains, which have some size distribution. Any potential barrier modulation should depend on the size of the grains adsorbing the molecules. Thus, any 1/\(f\) noise change induced by the adsorbed molecule should be less characteristic in porous gas sensors, comprised of different grains, than in the FET sensor using two-dimensional material of repeatable structure in a channel. Moreover, we can suppose that the FES method in FET sensors can determine components of gas mixtures when each plateau in 1/\(f\) noise is related to the presence of different gas molecules. This conclusion is valuable for practical application when we have to consider gas mixtures and the effects of crossing gases.