Electrically tunable dielectric function in glass with tree like percolating pathways of chargeable conductive nanoparticles

The design of nanostructured materials with specific physical properties is generally pursued by tuning nanoparticle size, concentration or surface passivation. An important step forward is to realize “active” systems where nanoparticles are vehicles for controlling, in situ, some specific, tunable features of a responsive functional material. For this reason, there is a growing interest in hierarchical assemblies of nanocrystal semiconductors, produced by both self- or assisted-assembly of colloidal nanoparticles and through selective segregation of nanophases in composite materials synthesized by solution-based processes. These approaches provide additional degrees of freedom for the design of functional nanocomposites as they allow for fine tuning of the electrical and optical properties of the NC building blocks, and for the simultaneous optimization of the overall response of the material system. In this perspective, we have focused on the rational design of a nanostructured glass with electrically tunable dielectric function obtained by injection and accumulation of charge on embedded conductive nanocrystals. This enables to achieve electrically controlled switching of semiconducting nanophases to charged polarisable states, which could lead to smart, field enhancement applications in nanophotonics and plasmonics. Here we show that such response switching can be obtained if a percolating charge transport mechanism is activated through a disordered tree-like network, as we demonstrate to be possible in SiO2 films where suitable dispersions of SnO2 nanocrystals, with conductive interfaces, are obtained as a result of a new synthesis strategy. The observed increase in the dielectric function, with respect to mean-field effects, is consistent with a polarization process of electrically charged nanocrystals. The quantitative analysis of the nanoparticle charging processes supports the co-existence of charge transport through the nanostructured material and nanoparticle dielectric polarization. This achievement represents a first step towards a new type of functional silicon-compatible nanostructured materials for electrically-responsive systems with switchable plasmonic properties