WEB Dielectric properties of nanostructured polymer-metal films
Dielectric layers are important parts of electronics on flexible substrates such as polymer foils. Here, we introduce a dielectric material that is easy to deposit from a liquid precursor. It is based on electrically conductive metal nanoparticles that are insulated by polymer shells and increase the polarizability of the material and the dielectric constant . In contrast to previous work, the hybrid films that we create here feature molecular insulating layers that are covalently attached to nanoscale gold cores.
We prepared an ink of hybrid nanoparticles based on gold (Au) nanospheres with a diameter of 5 nm prepared by solution chemistry that we covered with thiol-terminated polystyrene (PS) of different molecular weights (Mn=5000 Da or Mn=11000 Da). The resulting inks were dispersible in organic solvents. We prepared dielectric layers from the inks on metal covered silicon wafers by spin coating.
Small-Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM) on cross sections revealed that the spacing between Au cores depended on the polymer length (Figure 1a and 1b), supporting the covalent binding of the polymer component. We found homogenous distributions of the Au nanoparticles in the film. Thermogravimetric analysis indicated a gold volume fraction of 8 vol% for a PS molecular weight of 5000 Da. Our approach thus solves the problem of agglomeration that besets conventional nanocomposites at larger gold contents in previously reported materials [2,3].
Impedance measurements in the frequency range from 100 Hz up to 1 MHz indicated a dielectric constant of 3.5 for the short polystyrene at a dielectric loss of below 4%, while the longer polystyrene led to a dielectric constant of up to 6.1 with a dielectric loss of below 20% (Figure 1c). We believe that the covalent attachment of polymer ligands to metal cores is a promising approach to “molecular hybrid” films that can be deposited by wet processing without agglomeration. The strong attachment of the insulating prevents dielectric failure that otherwise besets nanocomposites at increased metal contents.