To gain a better understanding of the role of the PNIPAM dielectric, OTFTs incorporating SiO2 and PMMA as dielectric materials were also fabricated for comparison. As displayed in Supplementary Fig. S3, the fabricated devices containing SiO2 and PMMA as dielectrics also exhibited increases in the values of IDS and shifts in the values of Vth, but they were not as significant as those observed for the OTFT incorporating the PNIPAM dielectric.
In the case of SiO2, the value of IDS increased from an initial current of –9.15 µA to a final current of –10.0 µA upon heating from 30 to 45 °C. A similar trend was observed for the OTFT incorporating the PMMA dielectric: here, the value of IDS increased correspondingly from –3.97 to –4.32 µA.
Thus, charge transport through the pentacene semiconductor was indeed improved upon increasing the temperature 31,32, with PNIPAM playing an important role in enhancing the temperature-sensitivity of the OTFT sensor. To confirm the influence of PNIPAM on the sensing performance, a ITO/PNIPAM/Au sandwich structure was fabricated; its capacitance was measured at various temperatures. Fig.
4a presents the dependency of the capacitance on temperature when the structure containing 600-nm-thick PNIPAM was heated from 25 to 45 °C and measured at frequencies in the range from 20 Hz to 1 MHz. An increase in temperature led to an increase in capacitance and, therefore, an increase in dielectric permittivity. Fig.
4b presents the capacitance of the PNIPAM film measured at 20 Hz while heating from 25 to 45 °C. The measured capacitance increased linearly with respect to temperature: from 657 to 688 pF upon increasing the temperature from 25 to 45 °C. The phenomenon of capacitance and permittivity increases may be related to the PNIPAM backbone chain collapsing as the temperature increased 33,34. PNIPAM contains both hydrophobic and hydrophilic groups that form hydrogen bonds with water molecules 26.
Increasing the temperature disrupts the hydrogen bonds between the water molecules and the hydrophobic moieties of PNIPAM, leading to a conformational change from a linear to a globular state 35-37. This transformation from linear to globular occurs with a shrinking of volume that also affects the thickness of a PNIPAM film 38,39. Our results confirmed that the enhanced sensing sensitivity of the OTFT sensor was governed by changes in the structure of the PNIPAM dielectric, as well as the improved thermal transport properties of the pentacene semiconductor.