Femtosecond Laser Assisted Three-Dimensional Printing of Metal at Micro/nanoscale

Abstract

In the present era of Internet of Things, the need of miniaturized devices is growing rapidly for electronics, biomedical and photonics applications. Specific applications in these fields like specified sensitivity, energy density and miniaturized form factor often demand various two- and three-dimensional (3D) printed structures at micro and nanoscale. Currently, light-based 3D printing with sub-micron features is mainly developed based on photosensitive polymers or inorganic-polymer composite materials. To eliminate polymer/organic additives, a strategy for direct 3D assembly and printing of metallic nanocrystals without additives is presented. Ultrafast laser with intensity in the range of 1×10^10 to 1×10^12 W/cm^2 is used to non-equilibrium heat nanocrystals and induce ligand transformation, which triggers the spontaneous fusion and localized assembly of nanocrystals. The process is due to operation of hot electrons as confirmed by a strong dependence of printing rate on laser pulse duration varied in the range of electron-phonon relaxation time. The nonlinear dependence of sintering rate on laser fluence was exploited to print sub-diffraction-limited features in nanocrystal suspension. The smallest feature printed is ~200nm, which is ~¼ of the laser wavelength. Using the developed Laser Induced Ligand Transformation (LILT) process, direct printing of 3D metallic structures at micro and sub-micron scales is demonstrated. Facile integration with other microscale additive manufacturing for printing 3D device containing multiscale features is also demonstrated.