Non-thermal crystallization and sintering of thin films and nanoparticles by using ultrashort laser technology

Sharif, Ayesha
Ultrashort laser pulses can have a transformative effect on materials processing, enabling new processes which cannot be achieved with laser pulses of longer duration or with other material processing technologies. This thesis investigates the application of ultrashort pulses to optimise the electrical conductivity of thin films, printed nanoparticle (NP) inks, and graphene-based materials. In processes which are described as non-thermal, the electrical conductivities of materials are enhanced using fluences that are low, typically much less than the fluence required for the onset of laser ablation. Ultrashort laser interactions with metals develop a two-temperature response; the timescale over which the temperatures of the electrons are high, and the lattice is low, is significantly extended at these low fluence ranges. The presented hypothesis suggests that in this period of non-equilibrium when the electronic system is heated up to thousands of degrees Kelvin and the lattice system is relatively unheated; the thin film is crystallized, the nanoparticle ink is sintered, and the graphene oxide can be extensively reduced. Each process gives rise to the improved electrical conductivity of thin films necessary for combining electrical conductivity with heat sensitive substrates. The two-temperature response of a metal upon ultrashort-pulse laser exposure is first introduced using molybdenum (Mo), a high temperature refractory metal. The high lattice temperature associated with this metal permits the ultrashort laser parameter space to be fully investigated. Precision non-thermal delamination of a thin film of Mo, on a thermally softer aluminium layer, is demonstrated by proposing a hypothesis based on thermionic emission of hot electrons at moderate fluence. Next, the melt-free laser induced crystallization of a thin Mo layer at low fluence is reported. The parameter space for ultrashort laser -molybdenum thin film interactions is then presented which places the regime for Mo crystallization in context with other processes for Mo nano structurization and ablation. The application of the ultrashort laser-based crystallization is then extended to other materials. The crystallization of a thin gold (Au) film is next presented. This process is also confirmed as a non-melt crystallization process. The application to a nanoparticle silver (Ag) ink, printed using a droplet-based printing tool follows. The results confirm how the structures of the silver nanoparticles are retained, but the interfaces between NPs are sintered without any evidence of macroscopic melting. Finally, low fluence ultrashort laser pulses are applied to a graphene oxide (GO) material in a separate process. The results confirm that extensive reduction of the oxide to form a highly functional graphene derivative material is achieved by a scanning laser beam which makes it viable for many different types of laser-enabled printable sensors. In summary the thesis presents a study of ultrashort laser material interaction, encompassing the selection of different materials and laser material modification. The proposed, non-thermal laser-based process has a great potential for nano-scale materials processing on heat sensitive materials which often present limitations for advanced manufacturing.
NUI Galway
Publisher DOI
Attribution-NonCommercial-NoDerivs 3.0 Ireland