Using Density Functional Theory to Correlate Charge Transport Properties with Gas Sensing by Organic Nanowires

Organic semiconductor based gas sensors have several advantages over their inorganic counterparts. However, despite the advantages organic gas sensors are far behind the inorganic ones in terms of applicability mainly due to lack of understanding of the fundamental processes involved. Hence, to understand the gas sensing mechanism in chemiresistor and transistor based organic sensors more deeply and to achieve better performances, it is essential to establish a relationship between device performance and intrinsic charge transport properties of an organic semiconductor (OSC) which is fundamentally different from the usual inorganic ones. In this work, we therefore designed a small organic molecule with push–pull architecture, trans-4-nitro-4′-dimethylamino-α-aminostilbene (NNDMNH2), which has been predicted to have one-dimensional (1D) π–π stacking of molecules, and we used it as a model nanowire to study the correlation between the sensing mechanism and basic structural and charge transport parameters. Our work shows how possibly the modulation of the coupling matrix element, one of the basic charge transport parameters, due to the change in molecular geometry during adsorption of gas molecules (nonlocal electron–phonon coupling), can decrease the intrinsic mobility and ultimately affect the sensing performance of the device. The difference in the gas sensing mechanisms of an OSC and its inorganic counterpart can be well understood from our study. On the basis of our findings we propose a materials design principle which shall help to achieve better performance in future. We also find NNDMNH2 as a potential sensor of oxidizing SOx (x = 2, 3) gases, which is important because OSC based gas sensors are still far behind from their inorganic counterparts in sensing of oxidizing gases.