Femtosecond Laser Interaction with Ultrathin Metal Films: Modifying Structure, Composition, and Morphology

Femtosecond Laser;Laser Materials Interaction;Carbon Nanotube;Ultrafast Laser;Nanocrystalline;CNT;Materials Science and Engineering;Engineering;Materials Science and Engineering

Schrider, KeeganTorralva, Ben ;

University of Michigan

关键词: Femtosecond Laser;    Laser Materials Interaction;    Carbon Nanotube;    Ultrafast Laser;    Nanocrystalline;    CNT;    Materials Science and Engineering;    Engineering;    Materials Science and Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/138559/keegschr_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Femtosecond laser pulses allow for the study of materials in unique, and often extreme, conditions.As research regarding femtosecond laser interaction with materials progresses, the mechanisms for damage and modification are better understood. As a result, more complicated configurations of the target material can be explored often revealing new routes to modify materials. This work explores femtosecond laser irradiation of thin metal films and characterizes the resulting morphology, microstructure, and composition using optical microscopy, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and time-resolved pump-probe microscopy. Three areas are addressed; the removal of ultrathin metal films from non-absorbing substrates, mixing of multilayer Ni-W and Ni-Ag films to form thermally stable nanocrystalline alloys, and in-situ irradiation of Fe catalyst thin films to stimulate chemical vapor deposition of carbon nanotubes. A mechanistic understanding of the materials response was pursued for each case studied including the role of interfaces, and the interaction of solid, liquid, and gaseous phases formed by irradiation. The dynamics of femtosecond laser removal of ultrathin Ni films from glass substrates were measured using time-resolved pump-probe microscopy. Within 50 ps of irradiation the film-substrate interface separated and the removed layer accelerated to a constant velocity, faster than predicted by previous models. The Ni film was driven into extreme thermodynamic states after irradiation that caused the Ni film to rapidly decompose into a liquid-vapor mixture, similar to spinodal decomposition. Spinodal-like decomposition of a bulk metal after irradiation causes homogeneous nucleation of vapor; in this study spinodal-like decomposition caused heterogeneous nucleation at the Ni-substrate interface at relatively low fluences, broadening the range of temperatures to observe this process compared to previous studies. Studying the removal of ultrathin films provides a route to explore the extreme states of matter that occur after femtosecond laser irradiation.It has been predicted in previous studies that rapid cooling after femtosecond laser irradiation can drive homogeneous solidification in pure metals to form nanocrystalline material at the surface, however this is not generally observed. Ultrathin, multilayer films of Ni-W and Ni-Ag were irradiated with a single pulse and the resulting morphology and microstructure of the films were studied at a range of laser fluences. For Ni-W, a thermally stable nanocrystalline film was formed possibly due to solute stabilization of grain boundaries. The method for femtosecond laser mixing of multilayer metal thin films can be applied to create nanocrystalline surface layers for a wide range of alloy compositions that possess robust mechanical properties, increased corrosion resistance, unique magnetic properties, and are used as catalyst.Irradiation of carbon nanotube catalyst during Chemical Vapor Deposition stimulates the growth of aligned forests and in some cases increasing the terminal length of the aligned forest by up to 150X. It is shown that the femtosecond laser stimulates growth by combining Pulsed Laser Vaporization synthesis and Chemical Vapor Deposition, vaporizing the initial population of carbon nanotubes to jump start forest growth. This method provides a novel and simple route to efficient growth in systems where the static growth parameters are not optimized. Laser irradiation may also be used to pattern growth of aligned CNT forests without pre-pattering catalyst via photolithography, or post processing.