Direct writing of Cu-based flexible thermal detectors using femtosecond laser-induced reduction

Cu-based thermal detectors were directly fabricated on poly(ethylene) naphthalate (PEN) films using femtosecond laserinduced reduction metal oxide nanoparticles. Both CuO and CuO/NiO mixed nanoparticle solution films on PEN films were reduced and sintered to form Cu-based micropatterns. Femtosecond laser system operating with a pulse duration of 120 fs, wavelength of 780 nm, and repetition frequency of 80 MHz, was used for direct writing in air. P-type Cu₂O/NiO composite micropatterns were obtained only on the glass substrates, even though n-type Cu-Ni micropatterns were formed on the both glass and flexible PEN substrates. These results indicate that metal oxide nanoparticles were well-reduced on the polymer substrates because of their low thermal conductivities. In addition, the re-oxidization of the reduced metals were prevented on the polymer substrates due to their contribution as reductants. Both thermistor-type Cu-rich thermoelectric-type and Cu-Ni-rich thermal detectors were fabricated by reducing CuO nanoparticles and CuO/NiO mixed nanoparticles, respectively. The sensitivity of the detector was 5.3 μV/K. The devices can be used as various sensors in internet of things.