Nov 4, 2009
Nanoparticle array compressed to overcome conductivity hurdle
Colloidal nanoparticles are popular device building blocks for electronic and optical applications because of their easy assembly into arrays using inexpensive solution-based methods. Poor conductivity of colloidal nanoparticle arrays is perhaps the major obstacle limiting their wider application.
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.
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.
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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.
About the author
The work was performed by a cross-disciplinary team of researchers at the University of Hull and supported by the UK Engineering and Physical Sciences Research Council. Dr Sergey Rybchenko is a postdoctoral researcher in the Department of Engineering. Dr Amro Dyab used to work as a postdoctoral researcher in the Surfactant and Colloid Group, which is part of the Department of Chemistry, and now works at Minia University. Prof. Stephanie Haywood is the head of the Optoelectronic Nanotechnology Group within the Department of Engineering where Dr Igor Itskevich is also a lecturer. Dr Vesselin Paunov is a reader in physical chemistry.