Femtosecond laser-induced plasticity of copper oxide nanowires

It is reported that researchers from the University of Waterloo in Canada have reported a study on the plasticity of copper oxide nanowires induced by femtosecond laser. The related research was published in Applied Surface Science under the title "Femtosecond laser induced plasticity in CuO nanowires".

Metal oxide nanowires are ideal materials for manufacturing nanodevices, especially strain sensors. However, the inherent brittleness of these materials limits their applications. Researchers have proven that secondary treatment through femtosecond laser irradiation can solve this limitation.

Researchers have shown that the mechanical properties of copper oxide nanowires can be adjusted at room temperature to reduce brittle failure and enhance plasticity. Researchers used transmission electron microscopy images to study the microscopic mechanism of femtosecond laser induced plasticity. Under moderate laser irradiation intensity, this treatment method induces the plasticity of copper oxide nanowires by generating higher density oxygen vacancies, thereby changing the configuration of the nanowires from fracture to bending. In bent nanowires, due to high strain rates, vacancies migrate from the bending stretching zone to the compression zone, resulting in a localized laser-induced phase transition of copper oxide to oxygen deficient components. The condensation of oxygen vacancies induced by femtosecond laser paves the way for the activation of diffusion mechanism, which in turn promotes dislocation motion, leading to the development of substructures. In addition, nanoindentation analysis further confirms that femtosecond processed nanowires exhibit plastic behavior similar to that of metals. This result can be used to improve the performance of nanowires in future applications.

Highlights:
1. Femtosecond laser induces high-density oxygen vacancies in copper oxide nanowires.
2. The induced vacancies promote the formation of substructures and subgrains.
During the bending process, vacancies condense to form diffusion pathways.
4. Oxygen vacancies form a local hypoxic phase.
5. Femtosecond laser enhances the plasticity of copper oxide nanowires by generating defects.

Figure 1: a) Schematic diagram of femtosecond pulse laser irradiation device; b) SEM images of well oriented and vertically arranged nanowire films prepared; c. D) Bright field TEM images of the prepared single crystal monoclinic CuO nanowires, along with corresponding SAD patterns and EDS analysis.

Figure 2. Scanning electron microscopy images of copper oxide nanowires after laser processing at a) 80 mJ/cm2 for 15 seconds, b) 80 mJ/cm2 for 50 seconds, c) 120 mJ/cm2 for 15 seconds, d) 120 mJ/cm2 for 50 seconds, e) 180 mJ/cm2 for 15 seconds, and f) 180 mJ/cm2 for 50 seconds.

Figure 3. XPS analysis of copper oxide nanowires before preparation and after femtosecond irradiation.

Figure 4. XRD analysis tracks the structural changes of laser processed copper oxide nanowire samples relative to unprocessed samples.

Figure 5. Experimental setup and results of nanoindentation testing on raw and laser processed copper oxide nanowire samples.

Figure 6. TEM characterization of bent nanowires after 50 seconds of laser irradiation at a flux of 120 mJ/cm2.

Figure 7. Schematic diagram of substructure evolution in the necking region of CuO nanowires during bending after femtosecond laser irradiation.

Figure 8. TEM characterization of the cross-section of CuO nanowires after 50 seconds of laser irradiation at a flux of 120 mJ/cm2.

This article comprehensively analyzes the structural evolution of copper oxide nanowires under femtosecond laser irradiation, emphasizing the mechanism of enhancing the plasticity of brittle copper oxide nanowires. Research has shown that femtosecond lasers can be effectively used for in-situ engineering of the mechanical properties of copper oxide nanowires. Using optimized laser energy and irradiation time as processing parameters, a significant transition from brittleness to plasticity can be observed when the sample is irradiated with a pulse of 120 mJ/cm2 energy. Under these conditions, individual copper oxide nanowires remain unbroken under mechanical loading, but most of them will bend and become ductile. The analysis of XPS data indicates that this behavior corresponds to an increase in oxygen vacancy defect density. This indicates that high-density oxygen vacancies generated by femtosecond laser irradiation play a crucial role in accommodating strain and suppressing mechanisms leading to fracture during the bending process.

This process can be clearly traced in HRTEM micrographs, which originates from the activation of the diffusion mechanism generated by the condensation of oxygen vacancies. They make atoms mobile, forming different grains on the cross-section of single crystal nanowires, thereby promoting dislocation sliding within the nanochannels. The study also found that localized regions of irradiated copper oxide nanowires underwent a phase transition from copper oxide to oxygen deficient phase, thereby enhancing the inelastic response. The nano diffraction patterns obtained through XRD and HRTEM confirmed the presence of Cu2O phase in these nanowires. These structural modifications promote the transition from brittle fracture mode to more ductile metalloid behavior. The characteristic of this transformation is the appearance of obvious necking phenomenon, which confirms the transformation of material response properties in irradiated samples. According to the yield pressure law, the H/Py ratio obtained from nanoindentation testing increased from 2.2 to 3.2 for unprocessed brittle copper oxide nanowires in laser processed samples, indicating a transition from the brittle region to the plastic region.

Source: Yangtze River Delta Laser Alliance