Session: 02-03-01: Session #1: Nanomanufacturing: Novel Processes, Applications, and Process-Property Relationships

Paper Number: 99470

99470 - Photoinduced Additive Manufacturing of Metal at Micro/nanoscale 

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 of metals with submicron features is mainly developed based on photosensitive polymers or inorganic-polymer composites. A new strategy for direct 3D printing of metal nanoparticles without any organic additives is presented. Femtosecond laser with intensity in the range of 1x10¹⁰ to 1x10¹² W/cm² is used to induce ligand transformation enabling spontaneous fusion and local assembly of nanoparticles. To facilitate the current printing technique, oleylamine (OA) capped gold nanoparticles (3-5nm) have been synthesized in-house and dispersed into xylene as ink with desired concentration. A femtosecond laser with 1030nm wavelength, adjustable repetition rate (120KHz to 27MHz) and pulse width (700fs to 10ps) was focused by an oil immersion objective with high NA (NA=1.3) objective lens in a printing chamber containing (0.2 wt.%) OA capped gold nanoparticles ink. An interfacial layer (IL) of nanoparticle ink (25 wt.% spin-coated on glass and annealed at 100-200℃ for an hour) was employed to initialize printing/anchoring sites and promote adhesion of the printed structures on glass substrate. The sealed chamber was prepared on top of the IL to prevent the ink from drying out during the printing process. Next, an in-situ optical transmission feedback system has been introduced actively to control the size of the printed structures. When the laser was focused on the IL in the printing chamber, transmission signal drops (due to nanoparticle fusion at the laser voxel) detected from a photodetector controlled the laser shutter and enabled repeatable nanoscale (~200 nm) 2D features. The printed features are about 1/4^th of the diffraction limited spot size (~950 nm). Sub-diffraction limited printing capability by this technique was further investigated to understand the printing mechanism. It is hypothesized that laser induced ligand desorption and pyrolysis is the reason behind nanoparticle fusion at the laser voxel and the enabled printing. To determine the mechanism, laser pulse duration was varied in the range of electron-phonon relaxation time (from ~700 fs to ~10 ps) and its effects on laser induced printing rate was analyzed. The printing rate is considerably lower for ~10 ps pulses as compared to ~700 fs pulses at same laser powers. The lifetime of thermalized electron due to electron-phonon relaxation is ~1 ps for low excitation intensity and ~5 ps for higher intensity. The lattice thermal relaxation time was calculated to be ~33 ps. Since the range of pulse duration change overlaps with the electron lifetime and is shorter than lattice thermal relaxation time, it is then inferred that laser induced ligand transformation is mediated mainly by thermalized electron with  a possible contribution of lattice heating. Transmission electron microscope (TEM) images of the nanoparticles before and after laser irradiation confirmed the controlled/limited coalescence of individual nanoparticle without significant coalescence as conventional thermal sintering. To verify the metallic property of the printed features, 2D lines were fabricated connecting two gold pads to measure the conductivity. The highest conductivity attained from this printing method was ~2.5% of bulk. This technique was further employed to demonstrate ~340nm diameter vertical pillars and array of 3D micropillars using raster scan of the piezoelectric stage. Finally, direct printing of metallic structures with complex 3D geometries has been demonstrated for hierarchical micro/nanostructures and printed 3D devices for multiscale integration for future electronics, biomedical and photonics applications.

Presenting Author: Chinmoy Podder Texas A&M University

Presenting Author Biography: Chinmoy Podder is a PhD student in Mechanical Engineering department at Texas A&M University. His research focuses on ultrafast laser processing and nanomanufacturing, solvent-free Li-ion battery manufacturing.

Authors:

Chinmoy Podder Texas A&M University
Heng Pan Texas A&M University