Enhancing the Layer-by-Layer Assembly of Indium Tin Oxide Thin Films by Using Polyethyleneimine

We demonstrate that polyethyleneimine (PEI) stabilizes indium tin oxide (ITO) colloidal suspensions to a wider pH range and enhances the growth rate of the nanoparticle thin film during a layer-by-layer (LBL) assembly process. The interaction between the PEI and the ITO nanoparticle surface was determined by Fourier transform infrared spectroscopy to be mainly due to the electrostatic force between the amine groups of PEI and the ITO particle surface. The enhanced growth rate of ITO thin films was evaluated using a quartz crystal microbalance, atomic force microscopy, impedance spectroscopy, and thermogravimetric analysis. Results were found to be dependent on the PEI/ITO ratio used. Using the PEI-modified ITO suspension at a PEI/ITO ratio of 0.5 wt %, more than twice the amount of ITO was deposited onto model surfaces, such as cellulose fibers, silicon wafers, and gold electrodes of quartz crystals, when compared against the unmodified suspension. A 2 orders of magnitude higher conductivity was obtained for conductive paper made from PEI-modified, ITO-coated cellulose fibers as compared with paper made from unmodified, ITO-coated cellulose fibers. This enhanced assembly rate is attributed to ( I) the relatively constant electrostatic attraction forces between the PEI-modified ITO and PSS maintained during the LBL assembly process, (2) the large number of interaction sites and strong polymer chain entanglement between the PEI-modified ITO and the PSS assembly layers, and (3) the relatively modest interparticle repulsive forces between the PEI-modified ITO nanoparticles.