Nitrogen plasma surface modification of poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonate) films to enhance the piezoresistive pressure-sensing properties

A conductive polymeric film, poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS), is surface-modified by nitrogen plasma in order to enhance its piezoresistive characteristics. With an optimized 3 min nitrogen plasma surface modification, the piezoresistive sensitivity and response were significantly enhanced. Hall measurements and temperature-dependent conductance measurements are carried out to determine the electron-hopping behavior of nitrogen-plasma-modified PEDOT:PSS films, suppressing the horizontal carrier conducting pathway in the PEDOT:PSS piezoresistive pressure sensors. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy are applied to observe the PEDOT:PSS film surface after being modified with nitrogen plasma. The presence of sulfamate (SO₃-NH₂) and thiocyanate (S-C?N) groups indicates a breaking of the electrostatic bonding between PEDOT and PSS and a modification of the conductive PEDOT conjugated chain. At the film surface, the formation of thiocyanate groups of PEDOT oligomers without the electrostatic bonding of PSS makes the PEDOT:PSS more hydrophobic, changing the surface characteristics of the PEDOT:PSS film. The newly formed less-conductive film surface alters the piezoresistance of PEDOT:PSS pressure sensors, implying their potential applications for future high-performance tactile sensing.