With recent advances in the aligned growth of carbon nanotubes (CNTs), there are great interests in CNT-based field-emission and electronic applications. In conventional thermal chemical vapor deposition, substrates as well as chambers need to be globally heated to a sufficiently-high reaction temperature. In this paper, we report a method for direct synthesis of CNTs on pre-defined electrodes using laser-assisted chemical vapor deposition. A CW CO<sub>2</sub> laser (wavelength 10.6 μm, beam diameter 2 mm) was used to irradiate the pre-defined structures for CNT growth. The temperature of the substrate was measured by a pyrometer, ranging from 850-1000 °C. By varying catalysts and laser parameters, carbon nanostructures including carbon nanofiber, multi-walled and single-walled CNTs can be controllably synthesized.
Laser material interaction involved in laser-assisted microscale packaging is endowed with rapid and coupled optical, mechanical, and thermal processes. In-depth understanding of the underlying physics in these processes is instrumental for process optimization and functionality and dependability design of systems. This work is focused on the atomistic modeling of laser material interaction, particularly about the phase change, nanoparticle formation, stress generation and propagation, and formation and revolution of sub-surface structural damages. Large-scale parallel molecular dynamics simulation is conducted to model over 200 millions of atoms. The result reveals no clear interface when phase change occurs, but a transition region where the solid and liquid structures co-exist. The solid-liquid interface is found to move with a velocity up to the local sound speed. A vapor and droplet mixture is ejected from the surface with a high speed. The simulation reveals that nanoparticles originate from an intense vapor phase explosion after laser heating. The emerging time of larger particles is much later than that associated with smaller clusters. The resulting nanoparticles are characterized with a gas-like structure while characteristics of liquid are also preserved to a certain degree. In laser-assisted surface nanoscale structuring, visible sub-surface nanoscale structural damages are observed in the direction of 45 degrees with respect to the laser incident direction. Detailed study of the lattice structure reveals atomic dislocation in the damaged regions. Both temporary and permanent structural damages are observed in the material.
Steam laser cleaning of alumina and titanium carbide nanoparticles from silicon substrates is presented. A KrF excimer laser with a wavelength of 248 nm was used to irradiate the substrates in laser cleaning. A water layer of micrometer thickness was deposited on silicon substrates to improve the cleaning process. Cleaning efficiency was measured for different laser fluences ranging from 50 to 250 mJ/cm<sup>2</sup> and pulse numbers from 1 to 100. Research work was carried out to address the factors governing steam laser cleaning, during which thickness of water thin film and lift-off velocities of water films from Si substrate surfaces were monitored. In addition, one-dimensional simulations were employed to estimate the temperature increase on the material surfaces upon laser irradiation. Water layer thickness was measured using Fourier Transform Infrared Spectroscopy. Monitoring of both lift-off velocities and water thin film removal time were carried out by optical probing approaches using He-Ne laser of 632.8 nm wavelength.