Short-Pulsed Laser Processing of Metal-Oxide Nanomaterials: Role of Defects in Properties, Nanojoining and Sintering

Abstract

Metal-oxide nanomaterials are promising candidates for next-generation nano-electronic and optoelectronic devices due to their tunable band structures, multifunctionality, and chemical stability. However, their practical deployment is limited by intrinsic challenges such as low plasticity, brittleness, poor conductivity, and difficulties in reliable integration. Central to these limitations is the role of point, line, and interfacial defects, which govern charge transport, plastic deformation, diffusion, and bonding at multiple length scales. Controlling and engineering these defects without degrading structural integrity is therefore essential for unlocking the full potential of metal-oxide nanomaterials. This thesis investigates ultra-short (femtosecond) and short (nanosecond) pulsed laser irradiation as versatile strategies to manipulate defect landscapes and interfacial behavior. The highly localized, non-equilibrium energy delivery of pulsed lasers enables two complementary pathways: (i) defect engineering to enhance intrinsic mechanical and electrical properties, and (ii) defect-assisted diffusion to promote nanojoining and low-temperature and rapid sintering. Both nanosecond and femtosecond laser treatment can significantly influence the properties of individual CuO nanowires, but through fundamentally different mechanisms. Nanosecond laser pulses generate a moderate density of vacancies and dislocations that, with heat accumulation and partial annealing, reorganize into dislocation loops. This defect rearrangement converts a brittle, predominantly elastic response into elastic–plastic behavior, modestly increasing ductility and improving carrier mobility while maintaining structural stability. In contrast, femtosecond laser irradiation operates under a highly nonthermal regime, generating a supersaturated nonequilibrium defect density comprising abundant vacancies and densely interconnected dislocation networks. The excess vacancies enhance point-defect mobility, facilitating dislocation climb and cross-slip, while the internal stress fields within the dislocation network reduce nucleation barriers and promote glide and multiplication. Collectively, these mechanisms drive a brittle-to-plastic transition, resulting in stable plastic flow, greater deformability, and improved fracture resistance. By enhancing both the plasticity and electrical conductivity of CuO nanowires, laser treatment creates favorable conditions for single-nanowire device applications. Furthermore, it serves as an effective pre-treatment step before integration, mitigating the intrinsic brittleness of metal oxide nanowire and enabling reliable assembly and reuse in nanoscale systems. At the integration level, both lasers were used to fabricate functional nanodevices. Nanosecond laser treatment enabled precise cutting and rejoining of CuO nanowires by controlling defect network formation, leading to flexible, conductive CuO–CuO junctions suitable for strain-sensing applications. Similarly, it facilitated the fabrication of robust CuO–ZnO p–n junctions for photodetector devices, where thermal effects improved interfacial diffusion and bonding strength. In contrast, femtosecond laser treatment promoted non-thermal nanojoining through a two-step mechanism: first inducing localized plasticity via defect formation and then achieving strong joints without a heat-affected zone or phase transformation through shot-peening-assisted bonding. To assess the role of defects in diffusion, defect-assisted sintering was investigated using nanosecond laser pre-treatment of TiO₂ nanopowders. Optimizing the laser parameters led to partial annealing of laser-generated dislocations and recrystallization into ultrafine grains. These microstructural modifications increased the density of high-angle grain boundaries, creating short-circuit diffusion pathways that enhanced mass transport during sintering. Consequently, efficient densification was achieved at 750 °C, approximately 250 °C lower than conventional thermal furnace sintering (~1000°C). In contrast, femtosecond laser pre-treated TiO₂ nanoparticles generated a high concentration of oxygen vacancies that reorganized into dislocation-coupled vacancy channels, facilitating pipe-diffusion–dominated mass transport. This rapid and nonthermal diffusion pathway enabled fast neck growth and densification within only 10 minutes at 650 °C, demonstrating the crucial role of laser-induced defects in accelerating diffusion and achieving rapid low-temperature sintering of metal oxides. Overall, this work establishes pulsed-laser irradiation as a powerful platform for defect engineering in metal-oxide nanomaterials.