The problem is related to the surfactant molecular shells, which are inherent to the colloidal particles. They usually survive the array assembly procedure and become incorporated into the nano-array structure, physically separating the individual nanoparticles and acting as very effective insulators. Extra technological steps are required to thin or remove these surfactant shells in order to raise the conductivity of the array to the required level. These steps not only add to the processing cost, but also pose a challenge in terms of preserving the original useful properties of the nanoparticles.

New strategy

Researchers at the University of Hull, UK, recently proposed a new strategy for tuning the conductivity in colloidal nanoparticle arrays. This technique, published in Nanotechnology, is based on compression of the arrays by means of photo-polymerisation of the host matrix. It was applied to magnetic Fe3O4 nanoparticles, which self-assemble into columnar arrays under an applied magnetic field as shown in the image above.

Each column of the array is enclosed within a polymerisable organic matrix. Polymerisation results in shrinking of the matrix inducing a hydrostatic-like pressure on the arrays, which opposes the steric repulsion of the surfactant shells and brings the particles closer together. The decreased interparticle separation results in an exponential rise in the interparticle tunnelling conductance, giving arrays with greatly enhanced conductivity. Preserving the original passivation effect of the shells allows a large magnetoresistance to be achieved in the compressed arrays.

Template up for grabs

This method is not limited to magnetic materials. It could provide a general approach for raising conductivity in assemblies of nanoparticles formed in a variety of different ways and of varying geometry. Its application to semiconductor quantum dots is currently under investigation by the research team.