Selective fabrication of p-type and n-type thermoelectric micropatterns by the reduction of CuO/NiO mixed nanoparticles using femtosecond laser pulses

Abstract

p-type and n-type thermoelectric micropatterns were selectively fabricated via the reduction and reoxidation of CuO/NiO mixed nanoparticles using femtosecond laser pulses. The micropatterns were formed by raster scanning focused femtosecond laser pulses on a solution film containing CuO and NiO nanoparticles, ethylene glycol, and polyvinylpyrrolidone, followed by the removal of the non-irradiated nanoparticles. Cu–Ni was generated by reductive sintering of the CuO/NiO mixed nanoparticles at laser scanning speeds ranging from 5 to 20 mm/s and a laser fluence of 0.055 J/cm². In contrast, intense peaks corresponding to Cu₂O and NiO were observed in the X-ray diffraction spectrum of the micropattern formed at a scanning speed of 1 mm/s, indicating that Cu₂O and NiO were generated via the reoxidation of the reduced metals. The Seebeck coefficients of the micropatterns formed at a fluence of 0.055 J/cm² and scanning speeds of 5–20 mm/s were between − 32 and − 16 µV/K, whereas that of the micropattern formed at a fluence of 0.055 J/cm² and scanning speed of 1 mm/s was ~ 250 µV/K. These results suggest that the Seebeck coefficient depends on the generated n-type Cu–Ni and p-type Cu₂O and NiO phases. A thermoelectric couple was fabricated by selectively fabricating p-type and n-type thermoelectric elements. The thermoelectric couple exhibited a thermoelectric voltage of 0.25 mV/K when a temperature gradient was applied between its hot and cold sides. The generated voltage was nearly consistent with the estimated voltage based on the Seebeck coefficient. The developed process for selective fabrication is expected to be useful for the direct writing of thermoelectric-type sensors.