Development of pulsed-laser sintering processes for metal nanomaterials

Abstract

With ever-increasing demand for flexible, high-resolution, lightweight, and miniaturized electronic devices, development of pulsed-laser sintering processes in micro/nanoscale has recently attracted substantial attention. In the sintering process, such metallic nanomaterials as metal nanoparticles (NPs) and nanowires (NWs) are widely employed owing to their unique characteristics. For example, as the size of a material is reduced to nanoscale, the melting/sintering temperature is remarkably lowered by the thermodynamic size effect. Accordingly, a number of laser sintering processes have been developed using various lasers. In laser sintering, CW (continuous-wave) lasers or pulsed lasers with a pulse width longer than nanoseconds have typically been employed. In this work, we analyzed and developed micro-/nanoscale metal sintering processes using femtosecond and nanosecond pulsed-lasers. In the first part, an ultrafast laser sintering process was developed using a femtosecond laser with a pulse width of 50 fs. In the study, the physical mechanisms and characteristics of femtosecond laser sintering of silver nanoparticles (Ag NPs) were analyzed and compared with those of nanosecond laser sintering. In the second part, a novel non-thermal, i.e., mechanical, sintering process using pulsed laser-induced shockwave was developed for potential application of the technology to highly heat-sensitive substrates. Finally, for various substrates, optimized sintering strategies were proposed to enhance the properties of sintered film. Fabrication of thin-film electrodes/patterns on various substrates, including polymer, silicon, glass, and metal substrates, by femtosecond laser irradiation was demonstrated, resulting in significantly better properties of sintered film than those obtained by nanosecond laser sintering. Femtosecond laser sintering occurred by solid-state atomic diffusion at low fluences and by liquid-state recrystallization, i.e., full-melting, at high fluences. Pulsed laser-induced shock pressing could sinter Ag NPs mechanically without severe thermal damages. Furthermore, the laser shock pressing method produced Ag films with substantially improved film properties, e.g., low surface roughness, reduced crack generation, increased adhesion strength, and low porosity. Comprehensive parametric studies were performed to optimize the femtosecond laser sintering process. For a given particle-substrate combination, matching the coefficient of thermal expansion was key to producing mechanically, and thus electrically, robust thin films.