In situ spectroscopic studies to investigate uncharacteristic NH3 sensing behavior of polycarbazole Langmuir–Blodgett films

The polycarbazole films deposited by Langmuir–Blodgett (LB) technique exhibit a decrease in resistance on exposure to NH3, which is exactly opposite to the usual response (i.e. increase in resistance) observed for p-type conducting polymers. We have investigated the mechanism of this uncharacteristic NH3 sensing behavior of polycarbazole LB films by carrying out in situ Fourier transform infrared and ultraviolet–visible absorption spectroscopy. The results are compared with electropolymerized (EP) films. It is shown that EP film is p-doped, which gets reduced by interacting with NH3, and as a result, its resistance increases. On the other hand, LB film is found to be intrinsically undoped. Interaction with NH3 makes LB film n-doped, which results in a decrease of its resistance. The n-doping of LB film has independently been confirmed by electrochemical reduction process.