**4. Conclusions**

The goal of the work that has been presented in this article is to access the potential of graphene nanoribbon based FET for the detection of propane and butane gases. Armchair graphene nanoribbons and zigzag nanoribbon have been used to develop FET device. In the absence of any target gas molecules, the IV-characteristics of the simulated device are similar to n-channel depletion mode FET. In the proximity of target gas molecules, a change in source-to-drain current of FET at di fferent gate voltages has been observed. This change is distinct for each specific target gas i.e., propane and butane. Our simulated FET device also exhibited a noticeable change in the density of states in the proximately of these target gases. The change in source-to-drain current of FET in the presence of target gas molecules has been used as a detection signal for the leakage detection of these gases. The influence of atmospheric factors like the presence of water and oxygen molecules on the proposed device have also been investigated. It has been observed that the presence of these molecules also affected the IV-characteristics of the device. A careful calibration of such physically fabricated device is required to nullify the e ffect of atmospheric factors and ge<sup>t</sup> the correct reading for desired target gases. Our proposed device could be a promising candidate to replace conventional solid-state gas sensors due to its exceptional electronic properties and compact size. Theoretically, it is possible to simulate such type of FET sensors. However, the physical fabrication of these kind of device could be a challenge due to extremely small dimensions of graphene nanoribbons that make it di fficult to handle at atomic levels. Moreover, during physical fabrication of our proposed FET, there is a need of some suitable substrate like SiC or Si for the deposition of graphene. Our nearest future intension is to develop graphene based FET device for the detection of propane and butane gases.

**Author Contributions:** M.H.R. and A.K. conceived the idea. M.H.R. performed the simulations and drafted the article. A.K. and T.R. reviewed the article and managed funding of the project. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Estonian Research Council through Research Projects PUT 1435, IUT 1911 and by the Horizon 2020 ERA-chair gran<sup>t</sup> "Cognitive Electronics COEL H2020-WIDESPREAD-2014-2" (agreement number 668995; project TTU code VFP15051).

**Acknowledgments:** Authors would like to acknowledge the courtesy of Poklonski, N.A. and SPIE publishers to give copyright permission to reprint a figure from article Poklonski, N.A.; Kislyakov, E.F.; Vyrko, S.A.; Bubel', O.N.; Ratkevich, S.V.; Electronic band structure and magnetic states of zigzag graphene nanoribbons: quantum chemical calculations. *J. Nanophotonics* **2012** *Volume 6*, 61712. doi:10.1117/1.JNP.6.061712.

**Conflicts of Interest:** The authors declare no conflict of interest.
