Electron transport across fractal-like nanocrystalline clusters in $N+$ ion-beam induced poly(phenylene oxide)

Nanocrystalline carbonaceous cluster evolution and electron transport in the $N+$ beam induced spin coated poly(2,6-dimethyl-1,4-phenylene oxide) thin films as a function of ion fluence has been investigated. Following Robertson’s model and electron diffraction, the narrow optical band gaps were explained in terms of polyaromatic, single crystalline graphitelike clusters. With a threshold fluence of $1×1015 ions/cm2$ for cluster growth, the size of the clusters ranged from 2 to 50 nm with the number of aromatic rings varying between 20 and 170 over the entire fluence range upto $8×1016 ions/cm2.$ A molecular reconstruction/self organization has been envisaged as a possible clue to the above structure evolution upon a critical energy density transferred to the 53 nm implanted layer. Transmission electron microscopy study of fractal scaling in the nanoparticle aggregates revealed a fractal dimension of $1.37±0.02$ with the growth process to follow a diffusion limited aggregation model. Electrical conductivity data are explained in terms of a phase transition from an insulating state to a trap controlled hopping conduction of charge carriers between localized states on the backbone cluster with a backbone fractal exponent $∼3.$