Layer-by-layer (LBL) assembly is a simple and versatile method that is often used for manufacturing thin film structures on various types of substrates. The method uses the electrostatic forces between oppositely charged species, such as polyelectrolytes, DNA and nanoparticles. However, the interconnection between the conducting nanoparticles and the electrical conduction had not been studied until now. In a recent study published in Nanotechnology, researchers from the Georgia Institute of Technology, US, have demonstrated a simple approach to coating cellulose fibres with ITO nanoparticles using the LBL method. In order to obtain conductive paths and promote better particle–particle connections, salt-free conditions were applied so that the thickness of the polyelectrolyte interlayer was as thin as possible.

Impedance spectroscopy (IS) was used to obtain the conductivity of the papers for frequencies ranging from 0.01 Hz to 1 MHz for the fabricated paper along two directions: the in-plane (IP) direction and the through-the-thickness (TT) direction. Researchers found that the anisotropic structure of the paper sheet contributes to the differences in the electrical conductivity measured along the two directions. The conductivity along the IP direction was found to be about two orders of magnitude higher than that along the IP direction, for all papers examined regardless of the number of LBL assembled layers.

Current atomic force microscopy imaging (I-AFM) was used to evaluate the microstructure and the electrical property of the papers simultaneously. As shown in the main figure above, a substantial amount of current can be detected by the AFM tip when scanning on top of a single cellulose fibre that was subjected to an applied voltage from below. The figure on the left shows the topography of the ITO coated cellulose fibre. The I-AFM image shows that the highest current can be obtained from the regions, where ITO nanoparticles cover the surface and are interconnected in-plane and through the thickness of the paper.

Due to the versatility of the LBL method, this technique can potentially contribute to a variety of industrial applications, such as the fabrication of flexible electrodes for solar cells, flexible ITO films for portable displays and flexible electronic devices.