Realizing radio-frequency epsilon-near-zero response via embedding cobalt nanoparticles into polyimide

Epsilon-near-zero (ENZ) materials have drawn considerable attention due to their enticing properties, such as large wavelength, quasi-static and tunneling. Most research on ENZ behaviors is conducted in the visible, microwave, and infrared bands, but there is seldom research in radio frequency (MHz ∼ GHz). In this work, polyimide (PI) embedded cobalt nanoparticles (NPs) were fabricated to realize ENZ behavior in MHz-bands. The percolation networks were constructed and the dielectric resonance frequency was observed for PI-35 wt% Co composite at 580 MHz. As Co content increased, the ENZ response was obtained due to dielectric resonance and plasma oscillation. Also, conduction mechanism of PI/Co composites undergo a change from hopping state to metal-like state. The reactance value changed from negative to positive with increasing frequency, corresponding to the transition of permittivity from positive to negative value.