Controlling the Optical and Electronic Properties of Polyaniline (PAni) Using Vapor Phase Infiltration of Titanium Tetrachloride

Abstract

This talk with discuss our use of vapor phase infiltration (VPI) to alter the optoelectronic properties of poly(aniline) (PAni). VPI is similar to atomic layer deposition (ALD), but instead of cycling viscous flows of co-reactants, VPI doses a single reactant vapor in a static atmosphere followed by a pump-purge cycle. ALD typically provides conformal surface coatings while VPI enables extensive sub-surface diffusion and reactions within organic substrates. In particular to our study, PAni was VPI treated with a single cycle of TiCl4­­-H2O, and the resulting hybrid films showed unique optoelectronic properties that have not been previously reported. As VPI hold time increased, PAni films turned from blue to nearly transparent to green. This color transition is because of the formation of polaronic charge carriers concomitant with the formation of TiOx; the polaronic carriers absorb the red-NIR-MIR spectra (700- 5000 nm) while TiO­x absorb the blue-UV region (450 - 350 nm). This combination of optical absorbances in PAni has not been observed before. Furthermore, these polaronic carriers enabled electrical conductivities as high at 0.2 S/cm- ca. 3 orders of magnitude higher than prior PAni VPI doping studies. By varying the TiCl4 exposure time and measuring ex-situ­ optical signatures, an effective diffusion coefficient of ca. 1·10-15 cm2/s was measured; this value is self-consistent with the measured film thicknesses and optical peak intensity evolution. Additionally, we demonstrated that the relative ratio of UV and IR absorbing species can be controlled by varying reaction temperature. Lastly, XPS measurements elucidated a reaction mechanism for the formation of TiO­x and PAni polarons. By better understanding this reaction mechanism and kinetics, ALD and VPI can be optimized for future organic semiconductor doping processes.