Doctoral defence: Artjom Berholts "Light-enhanced sensors of oxidizing gases based on single-layer CVD graphene"

On 20 November at 14:15, Artjom Berholts will defend his doctoral thesis  "Light-enhanced sensors of oxidizing gases based on single-layer CVD graphene".

Supervisors: 
Raivo Jaaniso, Institute of Physics, University of Tartu 
Harry Alles, Institute of Physics, University of Tartu 

Opponents: 
Prof. Krisztián Kordás, University of Oulu

Summary

Gas sensors with improved functional properties (high sensitivity, selectivity, and stability; low power consumption) are needed to detect a wide variety of gases originating from both natural and anthropogenic sources. Graphene, a material consisting of a single layer of carbon atoms, has emerged as an ideal building foundation for gas sensors since its entire surface can act as a sensing area.

This thesis focuses on investigating the influence of ultraviolet (UV) light on the functional properties of graphene-based sensors towards two oxidizing gases – oxygen (O₂) and nitrogen dioxide (NO₂).

The first part of the thesis focuses on studying the light irradiation effect on pristine chemical vapor deposited (CVD) graphene. For that, the electrical properties of graphene were investigated, as well as the influence of UV light on O₂ sensing. The results demonstrated a highly beneficial effect of irradiation, which led to the activation of an initially inert sensor, making it sensitive to oxygen concentration changes and improving response and recovery speed.

As a next step, both pristine and modified CVD graphene were tested as sensors for NO₂, a highly toxic polluting gas causing millions of premature deaths per year in the whole world. For graphene modification, pulsed laser deposition was used to grow thin layers of oxides (ZrO₂, TiO₂) or metal (Ag) on top of graphene. The highest sensitivity was obtained with titania-coated graphene sensors under UV light. Gas concentrations down to 10 ppb (10 parts per billion) were investigated with the estimated level of detection of 0.03 ppb. Additionally, the excellent selectivity of the graphene/TiO₂ sensor was showcased, as NO₂ results significantly overperformed those for other polluting gases (CO, SO₂, and NH₃), as well as the humidity.

Overall, it was demonstrated that the irradiation with low-power UV light (365 nm) allowed a considerable increase in the sensitivity towards the studied gases and increased the response and recovery rates so that all functional parameters became within the range of real-world applications for the sensors operated at room temperature.