Researchers at the University of Hull, UK, recently proposed a new strategy for tuning the conductivity in colloidal nano-particle (CNPs) arrays. CNPs are addressed as device building blocks for electronic and optical applications because of their easy assembly into arrays using inexpensive solution-based methods; nevertheless the major obstacle limiting their wider application is that the CNPs arrays generally exhibit poor conductivity. 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 nano-particles 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.
The innovative technique proposed by Dr Sergey Rybchenko, Prof. Stephanie Haywood, Dr Igor Itskevich, Dr Vesselin Paunov (University of Hull, UK), and Dr Amro Dyab (now at Minia University), consists of the CNPs arrays compression by means of photo-polymerisation of the host matrix and on the employing magnetic Fe3O4 nano-particles, which self-assemble into columnar arrays under an applied magnetic field. 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 inter-particle separation results in an exponential rise in the inter-particle tunneling conductance, giving arrays with greatly enhanced conductivity. Preserving the original passivation effect of the shells allows a large magneto-resistance to be achieved in the compressed arrays. Although using magnetic fields, this method is not limited to magnetic materials only. It could provide a general approach for raising conductivity in assemblies of nano-particles 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.
Further information available in: Sergey I. Rybchenko et al 2009 Nanotechnology 20 425607