Narrow bandgap p-type semiconducting metal oxide nanowires (NWs), such as copper oxide (CuO), have gained significant attention for their potential in the development of electrical nano-devices. Tailoring the mechanical and electrical properties of CuO NWs is crucial for optimizing their functionality in specific applications. In this study, we employ nanosecond laser irradiation to precisely modify the properties of individual CuO NWs by inducing point and line defects, including oxygen vacancies and dislocations. Through controlled laser irradiation, we observe a gradual enhancement in the concentration of oxygen vacancies within CuO NWs until reaching a saturation point. The accumulation of vacancies leads to a substantial residual stress, resulting in lattice distortion and misfit. This high residual stress serves as a catalyst for the nucleation of dislocations, subsequently leading to a meaningful enhancement in plasticity. Remarkably, the density of dislocations demonstrates a strong correlation with the duration of laser irradiation. Prolonged irradiation leads to a thermally activated restoration process, where the dislocation configuration transitions from a random distribution to ordered dislocation loops. Mechanical characterization tests indicate that pristine CuO NWs exhibit brittleness, while laser irradiation renders them ductile with improved plasticity. Furthermore, the laser processing of CuO NWs demonstrates an enhancement in their electrical conductivity and optical absorbance.
Metal oxides have emerged as promising alternatives to traditional materials such as silicon and gallium arsenide for the fabrication of components in nanodevices.1–5 One such example is copper oxide (CuO) nanowires (NWs), which exhibit p-type semiconductor behavior with a bandgap of 1.2 eV.6
CuO NWs exhibit unique electrical, optical, and catalytic properties due to their high surface-to-volume ratio and quantum confinement effects at the nanoscale.7 These properties make them suitable for integration into nanodevices such as sensors,8 actuators,9 transistors,10 and energy storage systems.11 For instance, CuO nanomaterials have been explored as efficient components in gas sensors for detecting toxic gases, as their large surface area enhances gas adsorption and sensitivity.12 Furthermore, their tunable bandgap and exceptional photovoltaic properties enable their utilization in next-generation solar cells.13 The utilization of CuO nanomaterials in nanodevices opens up exciting possibilities for developing high-performance, miniaturized devices with enhanced functionality and efficiency across a wide range of technological applications.
To fully harness the potential of CuO NWs, it is crucial to understand their mechanical and electrical properties. Particularly, enhancing the plastic deformation and mechanical properties of CuO NWs is of paramount importance in the field of nanodevice applications.14–16 By improving their ability to undergo plastic deformation, CuO NWs become more resilient and capable of withstanding mechanical stresses without failure. This enhanced durability ensures the longevity and reliability of nanodevices, such as nanosensors,17 nanoswitches,18 and nanoelectromechanical systems (NEMSs).19 For example, in nanosensors,17 robust CuO nanowires can withstand mechanical deformation caused by external forces, allowing them to accurately detect and respond to changes in their environment. In nanoswitches, the improved mechanical properties of CuO NWs enable them to withstand repeated mechanical actuation, ensuring reliable switching behavior.18
Additionally, exploring methods to modify these properties prior to assembly is of utmost importance. This knowledge not only facilitates a comprehensive understanding of CuO NWs but also enables their effective utilization as nanoscale interconnects, nanocomposite strengtheners, and active components in NEMS devices.14–16 By optimizing these properties, CuO NWs can realize their full functionalities and offer significant potential in a wide range of applications.19
In recent years, a diverse range of methodologies have been employed to conduct comprehensive investigations into the mechanical characteristics of NW systems.20–27
Due to the inherent brittleness of CuO (NWs) at room temperature, the majority of research studies have predominantly focused on the analysis of their elastic properties.28,29 During compression experiments, it has been observed that CuO NWs possess the remarkable ability to undergo significant bending in response to high mechanical stress. This exceptional behavior can be attributed to the intrinsic anelasticity inherent in CuO NWs, which arises from the cooperative motion of twin-associated atoms as observed through in situ transmission electron microscopy (TEM) characterization.29 Li et al.30 further reported that this anelastic strain is linked to a phase transformation occurring within an oxygen-deficient CuOx phase, which gradually reverts back to CuO following stress relaxation, ultimately resulting in the gradual recovery of the NW. These findings shed light on the unique mechanical response of CuO NW and highlight the underlying mechanisms responsible for their anelastic behavior and recovery dynamics. The electrical behavior of CuO NWs has also been extensively studied under various physical conditions in different applications.30
Zhou et al.31 introduced an ultrasensitive gas sensor architecture utilizing a vertically aligned array of CuO NWs for the detection of hydrogen sulfide (H2S) gas, revealing an impressively low detection threshold of 500 ppb. The heightened sensitivity was attributed to the formation of a highly conductive copper sulfide (CuS) layer upon exposure to H2S within the sensing chamber. In a separate investigation, Hansen et al.32 demonstrated that illuminating CuO aligned NWs with light at photon energies exceeding the bandgap energy can effectively modulate the surface density of photoinduced electron–hole pairs. This phenomenon has been leveraged to enhance the gas sensing capabilities of CuO NWs. An alternative utilization of CuO NWs was demonstrated by Zheng et al.,33 who developed an efficient catalyst system employing CuO NWs for the oxidation of carbon monoxide (CO). It is important to note that while most functional devices integrate such nanostructure arrays onto substrates, the electrical behavior of these devices essentially represents a composite outcome derived from the properties of individual NWs interfacing with respective electrodes.34 Therefore, a comprehensive comprehension of the intrinsic characteristics of individual NWs becomes imperative for enhancing the grasp of electrical transport mechanisms within such multifunctional devices.35 A selection of studies has delved into the investigation of current–voltage (I–V) characteristics exhibited by CuO NWs under varying experimental conditions.36 An analysis of the electrical behavior at room temperature shows that the current density in CuO NWs has different slopes in different regions of the applied bias, indicating distinct types of charge transport, which are characterized as near Ohmic (lower voltage), trap controlled, and space charge limited conduction (higher applied voltage).37 For example, Wu et al.38 investigated the intricacies of electrical transport in CuO NWs and identified mechanisms such as thermal activation, phonon scattering, and polaron hopping as contributors, with their prevalence contingent upon the applied bias and temperature conditions.
Steinhauer et al.39 showcased a substantial enhancement in the gas sensing response for a single CuO NW through the incorporation of size-selected palladium (Pd) nanoparticles. Furthermore, Luo et al.40 constructed a field-effect transistor employing an individual CuO NW as the active element, elucidating its p-type conductivity behavior.
Furthermore, recent research has identified defect engineering as a promising approach for modifying the electrical and mechanical properties of metal-oxide NWs.41 This technique involves introducing dopants or creating lattice vacancies at or near specific lattice sites. Such modifications have shown potential for enhancing the properties of CuO NWs.
In CuO and other metal oxide NWs, the presence of dislocations plays a decisive role in determining their low-temperature plasticity. The intrinsic density of dislocations in these materials is typically low, which limits their plastic behavior. Consequently, any increase in the dislocation density can significantly impact the plasticity of CuO NWs.42 Therefore, by modifying the concentration and distribution of defects in CuO NWs, their mechanical and electrical properties can be altered.30
Pulsed laser irradiation enables localized energy deposition, leading to the dissociation of atomic bonds within the metal oxide lattice and facilitating the generation of defects.43 In the context of defect engineering, laser processing offers advantages over conventional methods, as it allows in situ implementation using a compact stand-alone system.44,45 Consequently, laser processing technology has emerged as an effective approach for integrating nanomaterials into functional devices.46 However, there are a limited number of studies that have explored the influence of laser irradiation on the defect-dependent mechanical and electrical properties of CuO NWs.46,47
In this report, we demonstrate the efficacy of nanosecond laser pulses for controlling the concentration of point and line defects, including oxygen vacancies and dislocations, in the crystalline lattice of CuO NWs. Furthermore, we discuss how these laser-induced defects significantly impact the mechanical and electrical behavior of CuO NWs. Our findings shed light on the potential of laser processing as a means to tailor the defect characteristics and subsequent properties of CuO NWs.
CuO NWs were synthesized by washing a commercial copper foil (99.9% purity, thickness 0.1 mm) for 30 s in acetone; subsequently, the foil was subjected to a washing process using deionized water, followed by drying using nitrogen gas. To achieve the desired dimensions and density, the copper foil was then heated in a furnace under an air atmosphere at temperatures ranging from 500 to 700 °C for a duration of 4−6 h. The heating rate during this process was set at 8 °C per minute. The oxidized samples were then allowed to cool to room temperature at a rate of 5 °C/min in the furnace. This resulted in the growth of an array of well-aligned CuO NWs on the surface of the oxidized copper foil. The NWs exhibited diameters ranging from 50 to 300 nm, while their lengths spanned from 5 to 15 μm. A forest of CuO NWs was subjected to irradiation using a nanosecond laser operating at a wavelength of 1064 nm, specifically a neodymium-doped yttrium aluminum garnet (Nd: YAG) laser. The laser had a pulse duration of 4 ns, operated at a frequency of 30 Hz, and featured a beam spot diameter of 1.25 mm. The experimental setup utilized a laser fluence of ∼180 mJ/cm2, and the duration of irradiation varied between 5 and 1800 s. The laser power and polarization were adjusted using a combination of a half-wave plate and neutral density (ND) filters. Microstructural observation of specimens was carried out at different times of irradiation using a field emission scanning electron microscope (Hitachi SU5000 FE-SEM). The chemical states of oxygen in the irradiated CuO NWs were obtained using x-ray photoelectron spectroscopy (XPS, Thermo VG Microlab 350), and Casa XPS software was utilized for deconvolution fitting to analyze the data accurately.
X-ray diffraction (XRD) technique was employed to determine the dislocation density in the CuO film. XRD spectra were obtained using a Bruker D8-discover x-ray diffractometer equipped with a 2D detector. The XRD measurements utilized radiation with a wavelength (l = 1.5418 Å). The scanning range for was set from 20° to 160°.
To measure the residual stress, the XRD machine (Bruker D8-discover) equipped with a VÅNTEC-500 two-dimensional area detector was utilized. The operating conditions were set at 40 kV and 40 mA. The CuO  plane was probed using the line with a wavelength of 1.5418 Å. The method was employed on the CuO  plane at to determine the residual stress.48 The software utilized for computing residual stress based on XRD measurements is DIFFRAC.LEPTOS 7.
The dislocation structure in CuO NWs was captured using transmission electron microscopy (TEM, JEOL 2200FS), while the electrical properties of CuO NWs were measured at room temperature with source measure units (Keithley 4200A-SCS). Nanoindentation (Hysitron T1 980) was conducted to analyze the mechanical properties of CuO NWs. To prepare the sample for the nanoindentation test, the CuO NWs were first peeled off from the copper foil. They were then dispersed in an ethanol solution and left for a duration of one day. Subsequently, the dispersed NWs were deposited onto a silicon substrate. For the nanoindentation test, a Berkovich diamond indenter was utilized. The applied load during the test ranged from 0.2 to 50 μN. The indenter was used to create controlled indentations on the CuO NWs. To measure the optical absorption of the CuO NWs, ultraviolet-visible spectroscopy was employed. Specifically, a Shimadzu instrument was used to carry out the optical absorption measurements.
III. RESULTS AND DISCUSSION
Figures 1(a) and 1(b) present the characterization results of the unprocessed CuO NWs following the thermal oxidation process. Figure 1(a) shows the well-oriented and aligned growth of CuO NWs, which are observed to vertically emerge from the CuO substrate. To verify the chemical composition of the as-prepared NWs, Fig. 1(b) displays the energy-dispersive x-ray spectroscopy (EDS) analysis, indicating a uniform mixture of copper and oxygen in each CuO NW with an atomic ratio of 1.
In Fig. 1(c), the experimental setup for the Nd: YAG nanosecond laser irradiation of the CuO NW forest is depicted. The laser irradiation is conducted in a direction parallel to the long axis of the NWs. Taking into account previous findings47 that CuO NWs undergo fracture under high laser fluence, the samples were subjected to irradiation for durations ranging from 5 to 1800 s. To avoid unnecessary degradation or fragmentation, a laser fluence of 180 mJ/cm2 was chosen for the irradiation process.
Figure 2 illustrates the representative morphologies of the CuO NW array after laser processing for various time durations. The degradation, melting, and agglomeration of NWs gradually change when the irradiation time increases from 5 to 800 s. CuO NWs remain intact and inclined after short irradiation times [Figs. 2(a) and 2(b)], while exposure times between 180 and 410 s result in the fragmentation of some NWs and their detachment from the substrate as indicated by the red arrows in Figs. 2(c) and 2(e). [The inset in Figs. 2(c)–2(e) represents the regions where nanowires underwent fragmentation and detached from the substrate through laser processing, respectively.] It is noteworthy to mention that, based on approximate calculations using image software, the average percentages of fragmented nanowires with lengths less than 3 μm were found to be 10%, 13%, and 16% after laser processing for durations of 180, 300, and 410 s, respectively. Longer exposure times may result in heat accumulation, which, in turn, can lead to the fragmentation of certain NWs. However, no conclusive evidence of melting is observed, indicating negligible thermal effects and limited degradation of the NW structure within the investigated range of exposure times. The majority of NWs remained intact. Nevertheless, when the irradiation time was extended to 800, 1200, and 1800 s, evident degradation, melting, and agglomeration of the CuO NWs are observed [Figs. 2(f)–2(h)].
Our investigation revealed that the optimal laser irradiation exposure time for fine-tuning the mechanical and electrical properties of CuO NWs lies within the range of 5–410 s. It is worth noting that within this exposure range, the majority of NWs maintain their structural integrity and do not undergo melting. Instead, at low fluence levels, the primary effect of laser irradiation is the generation of defects, as previously documented.49,50 These defects have a well-established influence on the physical properties of metal oxides, and the presence of point defects, such as vacancies, specifically holds the potential to modify the mechanical strength and ductility of solid materials.35,50,51
The high-resolution XPS scans (Fig. 3) provide clear evidence of the introduction of oxygen vacancies in the CuO NW lattice following laser irradiation. Gaussian curve fittings were utilized to fit the O-1s XPS peaks in all samples.47 The fitting algorithm took the XPS data and initial parameters as inputs, iteratively optimizing the curve parameters to minimize the disparity between the fitted curve and the experimental data. The fitting process aimed to achieve a fitting error below 10% and ensure an R-squared (R2) value exceeding 0.9 for all the samples.
Prominent peaks observed at approximately 530 and 532 eV correspond to lattice oxygen and oxygen vacancies, respectively.47,52 The as-prepared sample [Fig. 3(a)] exhibits an oxygen vacancy-to-lattice oxygen ratio of 0.22, indicating that oxygen vacancies are inherent defects in CuO NWs and are formed during the synthesis process.53 After laser irradiation for 5 s, the ratio of oxygen vacancy sites to lattice oxygen sites increases by approximately 50%, and this trend continues until an irradiation time of 300 s, where it reaches a saturation point of approximately 0.5 for longer irradiation durations. These results indicate the ability to control the relative concentration of oxygen vacancy centers in CuO NWs through laser irradiation.
The presence of oxygen vacancies in oxide materials is known to induce atomic displacements, resulting in strains and anisotropic stresses.54–56 These local and residual stresses primarily arise from the anisotropic lattice distortions around the oxygen vacancy sites. It is well-established that such stresses can have a remarkable impact on the mechanical properties of oxide materials.56
The XRD measurement of residual stress involves selecting sensitive crystallographic planes, directing x rays at the material's surface, and measuring the diffraction angles. Lattice spacing is calculated using Bragg's law57 and compared to unstressed values to determine residual stress. Mathematical models relate the lattice spacing difference to residual stress, providing insights into stress distribution. (For a comprehensive understanding of the XRD measurement of residual stress, please refer Fig. S1 in the supplementary material.) The residual compressive stress is seen to increase from −12 to −65 MPa of the unprocessed sample and irradiated sample for 410 s, respectively. This indicates that the formation of defects during laser irradiation changes the average localized stress in CuO NWs for laser irradiation times up to 300 s. Subsequent increases in exposure time have little effect on the residual stress as shown in Fig. 4. The observed saturation in the concentration of oxygen vacancy sites after laser irradiation for 300 s is in agreement with the XPS data. This indicates that there is a limit to the generation of oxygen vacancies under the given irradiation conditions, and further prolonged irradiation does not result in a significant increase in their concentration.
At low temperatures, it has been observed that many metal oxides, including CuO, exhibit limited plasticity due to a low density of dislocations and the high stress required for their nucleation.42 This characteristic contributes to their inherent brittleness and hinders their ability to undergo significant plastic deformation. In general, plastic deformation in crystalline metallic NWs proceeds through the activity of dislocations at room temperature and/or diffusion creep at elevated temperature.42,58 However, it should be noted that metal oxide NWs typically do not exhibit room temperature plasticity due to the absence of dislocations required for accommodating plastic deformation.47,59 This inherent brittleness imposes limitations on their applications, particularly in environments with low or moderate temperatures.42 To analyze the effect of laser irradiation on the dislocation generation in CuO NWs, XRD analysis was carried out to measure the dislocation density of the sample after laser irradiation at different exposure times [Fig. 5(a)].
In the given context, K is a constant known as the shape factor, typically assigned a value of 0.9,60 representing the relationship between crystallite shape and the material, represents the x-ray wavelength, specifically 1.5418 Å for , and corresponds to the full width half maximum (FWHM) of the diffraction peaks. It is important to note that accurate calculations involved considering the average FWHM of seven sharp peaks for each sample. Last, denotes the position of the diffraction peaks.
Figure 5(b) expresses that the dislocation density has been calculated, which increases from 1.93 × 10−3 nm−2 in the unprocessed sample to 2.8 × 10−3, 3.5 × 10−3, and 3.9 × 10−3 nm−2 after exposure times of 5, 90, and 180 s, respectively. This density becomes 4.2 × 10−3 nm−2 after 300 s and then slightly reduces to 4.01 × 10−3 nm−2 after 410 s. In a study conducted by Sarkar,61 it was found that the dislocation density in CuO thin films reached to 2.4461 × 10−3 nm−2 from 2.1244 × 10−3 following gamma irradiation in the range of 20–100 kGy. These observed enhancements in dislocation density are in agreement with the XPS data presented in Fig. 3 and can be related to the presence of a high concentration of oxygen vacancies induced by the irradiation. These oxygen vacancies introduce localized stress within the CuO lattice, leading to the nucleation of dislocations and subsequent increases in the dislocation density.
This curve serves as a mechanical signature, reflecting the response of the CuO NW to deformation at various load levels. Three important parameters, namely, the peak load , the depth at peak load , and the initial loading stiffness (S), have been identified and labeled in Fig. 6. These parameters are decisive for characterizing the mechanical properties of the CuO NW and provide valuable information regarding its deformation behavior. Previous studies16,66,67 have highlighted the significance of these parameters in understanding the mechanical response of NW systems.
As shown in Fig. 6, the nanoindentation test is conducted in a load-control mode. This approach ensures a consistent loading rate and minimizes the impact of external factors on the test results. Additionally, to validate the materials' independent behavior concerning texture, multiple indents are performed in various directions, and the average outcomes are reported.
TEM has been used to observe the dislocation behavior and substructure evolution in CuO NWs. Figures 8(a)–8(c) show dislocation density was increased after irradiation, while Fig. 8(d) shows the configuration of these dislocations changes at higher irradiation times (410 s). Laser irradiation could create oxygen vacancies,69 which introduces a stress field around vacancy sites and enables the nucleation of dislocations.70 Therefore, increasing the irradiation time up to 300 s produces an enhancement in the accommodation of strain and incrementally increases the dislocation density while facilitating the glide/slip of dislocations. In addition, an increase in the density of oxygen vacancies is accompanied by the trapping of dislocations at these vacancy sites71 introducing the possibility of wavy slip.72 Some re-arrangement of dislocations can be seen in Fig. 8(c) after irradiation for 300 s, implying that longer irradiation time allows dislocations to reset their configuration to decrease the system energy. This process requires a longer irradiation time as the dislocation glide/slip process is time-dependent and also thermally activated. As no major change in the oxygen vacancy density is observed due to saturation as the irradiation time increases from 300 to 410 s (see Fig. 3), the observed modification in the configuration of dislocations can be attributed to a thermally activated restoration mechanism. This suggests that heat generated by laser irradiation for 410 s can provide the required driving force for the development of the substructure. This heating allows the re-arrangement and annihilation of dislocations together with the formation of dislocation loops. Figure 9 depicts a schematic of the dislocation configuration after laser irradiation. Evidence for the effect of thermal heating including partial dislocation pileups and complete dislocation loops is highlighted in this figure. The activation of a restoration mechanism in CuO NWs can lead to the development of a dislocation substructure, which enhances the strain accommodation capacity during mechanical loading.72–74 This restoration mechanism allows for stress relaxation and incremental dislocation slip, enabling the accommodation of strain without significant interactions between dislocations or dislocation barriers. As a result, the observed low resistance to plastic deformation in CuO NWs after laser irradiation for 410 s can be attributed to the activation of this restoration mechanism, which facilitates strain accommodation and reduces the barriers to dislocation motion.
The electrical characteristics of CuO NWs have been investigated using conventional two-probe methodologies. Figure 10(a) illustrates the representative I–V behaviors of CuO NWs subjected to various laser processing conditions, while the inset delineates the configuration of gold electrodes fabricated on a Si substrate via lithographic techniques. Furthermore, Fig. 10(b) depicts the schematic of the arrangement of CuO NWs deposited onto the aforementioned gold electrodes. The SEM image of the configuration of a single NW on electrodes is shown in Fig. S2 in the supplementary material.
In Fig. 10(c), the impact of diverse laser exposure durations on the resistivity of CuO NWs is elucidated. The resistivity of these NWs exhibited a diminishing trend following laser processing until the exposure time of 300 s. Concretely, the resistivity values underwent a reduction from an initial measurement of 3.01 KΩ m in the as-prepared sample to 2.85, 2.68, 2.1, 1.49, and 1.83 KΩ m subsequent to laser processing durations of 5, 90, 180, 300, and 410 s, respectively. The point of the lowest resistivity manifested following a 300 s exposure, coinciding with the sample showcasing the highest concentration of oxygen vacancies. The observed enhancement in conductivity within irradiated CuO NWs is ascribed to the presence of oxygen vacancies. The formation of defect centers, oxygen vacancies included, is acknowledged to contribute to heightened conductivity within the irradiated CuO NWs.75
The creation of an oxygen vacancy introduces localized energy levels within the semiconductor's bandgap. These energy levels frequently lie in close proximity to the valence band, responsible for the transport of positively charged carriers—holes—in p-type semiconductors.76 As a consequence, the density of mobile holes in the material is elevated, consequently amplifying the availability of charge carriers for conduction.77 These positively charged holes are capable of traversing the lattice. This phenomenon has been documented in prior investigations,47,75–78 as well as by Zheng in the context of single CuO NW.50 He reported that an escalation in the concentration of defect centers, along with a reduction in the potential energy barrier at the interfaces of Au/CuO due to laser irradiation, culminates in heightened electrical conductivity and augmented photoconductivity. On the other hand, the gradual rise is seen in resistivity from 300 to 410 s exposure time, which is indicative of partial annealing, brought about by heat accumulation post 300 s. A similar trend is reported by Zheng in relation to the concentration of oxygen vacancies in the ZnO nanofilm. He found that the concentration is proportionate to the laser fluence below 700 mJ/cm2; however, an elevation in laser fluence beyond this threshold results in thermal heating.52
The significant increase in the conductivity observed in CuO NW after laser processing holds potential for various practical applications and is vital for their successful integration into electronic devices, enabling efficient charge transport, improved device performance, and expanding their potential for future technological advancements.79–81
UV–Vis absorbance analyses are performed on samples of CuO NWs prior to and subsequent to laser irradiation with a duration of 410 s, as illustrated in Fig. S3 in the supplementary material. In order to standardize the spectra, a reference blank sample is selected to establish a baseline representation. Following the acquisition of absorption spectra for each sample, a normalization procedure is employed by dividing the absorption spectrum of each sample by that of the reference, facilitating coherent comparisons among various specimens. The observed spectrum reveals that the heightened intensity of an absorption band at 450 nm is linked to the augmentation of defect centers due to laser irradiation. The bandgap energy, as deduced from a Tauc plot, measures approximately 1.8 eV for the as-prepared CuO NWs and around 1.3 eV for the CuO NWs subjected to 410 s of laser irradiation [depicted in Fig. 10(d)]. This discernible reduction in bandgap energy signifies that laser-induced defects introduce supplementary energy levels within the bandgap, concurrently intensifying photo-absorption at extended wavelengths, as previously noted.82
In summary, our investigation underscores the effectiveness of controlled nanosecond laser irradiation as a potent technique for modulating both the mechanical and electrical characteristics of CuO NWs. By employing low-fluence laser irradiation, we successfully induce discrete point and line defects, encompassing oxygen vacancies and dislocations, within the CuO NW lattice. Notably, the precision manipulation of oxygen vacancy concentration is attainable by fine-tuning the duration of laser irradiation. This laser-induced augmentation of oxygen vacancy density yields a significant elevation in residual stress, with values escalating from −12 MPa in pristine CuO NWs to −65 MPa in samples subjected to 410 s of irradiation. This heightened residual stress profile acts as a facilitator for the nucleation of dislocation. Furthermore, the density of dislocations evinces a substantial reliance on the irradiation duration, achieving saturation as exposure times exceed 300 s. Mechanical evaluations reveal a conspicuous transformation in the behavior of CuO NWs, transitioning from inherent brittleness in their unprocessed state to a ductile demeanor characterized by enhanced plasticity subsequent to laser irradiation. This advantageous alteration can be attributed to the presence of laser-induced oxygen vacancies. Additionally, alterations in electrical conductivity and optical absorbance are observed following laser irradiation. The resistivity of individual NWs reaches a minimum of 1.49 KΩ m following the exposure time of 300 s, coinciding with the sample manifesting the highest oxygen vacancy concentration. The introduction of an oxygen vacancy engenders localized energy states within the semiconductor's bandgap, frequently proximal to the valence band, governing the migration of positively charged carriers—holes—within p-type semiconductors. This discernible contraction in bandgap energy underscores that laser-induced defects introduce supplemental energy levels within the bandgap, which leads to the reduction in bandgap energy from 1.8 to 1.3 eV. In conclusion, our findings underscore the considerable potential of nanosecond laser irradiation as a versatile tool for fine-tuning the mechanical and electrical attributes of CuO NWs, thereby paving new pathways for their diverse applications.
For the details of residual stress measurements and the calculation of mechanical properties from nanoindentation tests, please refer to the supplementary material.
This work was financially supported by the Natural Sciences and Engineering Research Council (NSERC) Discovery grants and the Canada Research Chairs (CRC) Programs.
Conflict of Interest
The authors have no conflict to disclose.
Maryam Soleimani: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Software (equal); Writing – original draft (equal); Writing – review & editing (equal). Walter Duley: Formal analysis (equal); Methodology (equal); Project administration (equal); Supervision (equal); Writing – review & editing (equal). Norman Zhou: Funding acquisition (equal); Project administration (equal); Supervision (equal); Writing – review & editing (equal). Peng Peng: Funding acquisition (equal); Methodology (equal); Project administration (equal); Supervision (equal); Writing – review & editing (equal).
The data that support the findings of this study are available from the corresponding authors upon reasonable request.